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D.SUNEEL, PROFESSOR CHAPTER-4 SPECIAL NANO MATERIALS, CHARACTERIZATION AND TOOLS Characterization and tools: Introduction: Materials in the nanometer scale, such as colloidal dispersions and thin films, have been studied over many years and many physical properties related to the nanometer size, such as coloration of gold nanoparticles, have been known for centuries. One of the critical challenges faced currently by researchers in the nanotechnology and nanoscience fields is G Y the inability and the lack of instruments to observe measure and manipulate the materials at the nanometer level by manifesting at the macroscopic level. In the past, the studies LO have been focused mainly on the collective behaviors and properties of a large number of O nanostructured materials. The properties and behaviors observed and measured are N typically group characteristics. A better fundamental understanding and various potential H applications increasingly demand the ability and instrumentation to observe measure and C manipulate the individual nanomaterials and nanostructures. Characterization and TE manipulation of individual nanostructures require not only extreme sensitivity and accuracy, but also atomic level resolution. It therefore leads to various microscopies that O will play a central role in characterization and measurements of nanostructured materials AN and nanostructures. Miniaturization of instruments is obviously not the only challenge; the new phenomena, physical properties and short-range forces, which do not play a N noticeable role in macroscopic level characterization, may have significant impacts in the nanometer scale. The development of novel tools and instruments is one of the greatest challenges in nanotechnology. Characterization, when used in materials science, refers to the use of external techniques to probe into the internal structure and properties of a material. Characterization can take the form of actual materials testing, or analysis, for example in some form of microscope. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 1 D.SUNEEL, PROFESSOR Analysis techniques are used simply to magnify the specimen, to visualize its internal structure, and to gain knowledge as to the distribution of elements wit hin the specimen and their interactions. Magnification and internal visualization are normally done in a type of microscope, such as: Optical Microscope Scanning Electron Microscope (SEM) Transmission Electron Microscope (TEM) Y Field Ion Microscope (FIM) LO Atomic Force Microscope (AFM) G Scanning Tunneling Microscope (STM) O In this chapter, various structural characterization methods those are most widely used in N characterizing nanomaterials and nanostructures are discussed. These include X- ray and transmission electron microscopy (TEM), and scanning probe C (SEM) H diffraction, various electron microscopy (EM) including scanning electron microscopy TE microscopy(SPM). Scanning tunneling microscopy and atomic force microscopy comes O under the family of scanning probe microscopy. AN Electron Microscopy: N W ith the new found knowledge the metallurgist obtained about phases of the materials through the optical microscope, the desire came for more knowledge. The optical microscope was only able to resolve up to 1/125,000 of an inch, and scientists wanted to do better than that. This desire was satisfied with the invention of the electron microscope. Instead of using a light beam, the electron microscope focuses a beam of e lectrons which were accelerated from a thermionic triode electron gun. The electron beam travels directly down the microscope column and collides with the surface of the specimen. The best optical microscopes can resolve image up to 2000 angstroms while the electron microscope can resolve an image to 2 angstroms. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 2 D.SUNEEL, PROFESSOR Types of electron microscopy: SEM (Scanning Electron Microscopy): SEM is one of the most widely used techniques used in characterization of nanomaterials and nanostructures. The resolution of the SEM approaches a few nanometers, and the instruments can operate at magnifications that are easily adjusted from ~10 to over 300,000. Not only topographical information SEM also provides chemical composition N AN O TE C H N O LO G Y information near the surface. Figure: Schematic diagram of Scanning Electron Microscope. In a typical SEM, a source of electrons is focused into a beam, with very fine spot s ize of ~5 nm and having energy ranging from a few hundred eV to 50 KeV , which is rastered over the surface of the specimen by deflection coils. As the electrons strikes and penetrate the surface, a number of interactions occur that result in the emission of e lectrons and photons from the sample, and SEM images are produced b y collecting the DEPT OF MECHANICAL ENGG, KL UNIVERSITY 3 D.SUNEEL, PROFESSOR e mitted electrons on a cathode ray tube (CRT). Various SEM techniques are differentiated on the basis of what is subsequently detected and imaged, and the principle images produced in the SEM are of three types: secondary electron images, back scattered electron images and elemental X-ray maps. When a high energy primary electron interacts with an atom, it undergoes either inelastic scattering (deflecting the electrons with loss of energy) with atomic electrons or e lastic scattering (deflecting the electrons with no loss of energy) with the atomic Y nucleus. G In an inelastic collision with an electron, the primary electron transfers part of its LO energy to the other electron. When the energy transferred is large enough, the other e lectron will e mit from the sample. If the emitted electron has the energy of less than 50 O eV, it is referred to as secondary electron. Back scattered electrons are the high energy N e lectrons that are elastically sca ttered and essentially possess the same energy as the H incident or primary electrons. The probability of backscattering increases with the atomic C number of the sample material. Although backscattering images can not be used for TE e lemental identification, useful contrast can develop between regions of the specimen t hat O differ widely in atomic number, Z. AN An additional electron interaction in the SEM is that the primary electron collides with and ejects a core electron from an atom in the sample. The excited atom will decay N to its ground state by emitting either a characteristic X- ray photon or an Auger electron, both of which have been used for chemical characterization. Combining with chemical analytical capabilities, SEM not only provides the image of the morphology and microstructures of bulk and nanostructured materials and devices, but can also provide detailed information of chemical composition and distribution. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 4 D.SUNEEL, PROFESSOR TEM (Trans mission Electron Microscopy): The transmission electron microscope (TEM) forms an image b y accelerating a beam of e lectrons that pass through the specimen. I n TEM, electrons are accelerated to 100 KeV or higher (up to 1MeV), projected onto a thin specimen (less than 200 nm) by means of the condenser lens system, and penetrate the sample thickness either undeflected or deflected. The greatest advantages that TEM offers are the high magnification ranging from 50 to 106 and its ability to provide both image and diffraction information from a Y s ingle sample. G The scattering processes experienced by electrons during their passage through LO the specimen determine the kind of information obtained. Elastic scattering involves no energy loss and gives rise to diffraction patterns. Inelastic interactions between primary O e lectrons and sample electrons at heterogeneities such as grain boundaries, disloca tions, N second phase particles, defects, density variations, etc., cause complex absorption and H scattering effects, leading to a spatial variation in the intensity of the transmitted C e lectrons. In TEM one can switch between imaging the sample and viewing its diffraction N AN O TE pattern by changing the strength of the intermediate lens. Fig: Schematic diagram of TEM DEPT OF MECHANICAL ENGG, KL UNIVERSITY 5 D.SUNEEL, PROFESSOR One short coming of TEM is its limited depth resolution. Electron scattering information in a TEM image originates from a three- dimensional sample, but is projected onto a two dimensional detector. Therefore, structure information along the electron beam direction is superimposed at the image plane. Although the most difficult aspect of the TEM technique is the preparation of samples, it is less so for nanomaterials. In addition to the capability of structural characterization and chemical analyses, TEM also has been explored for other applications in nanotechnology. Examples include the determination of melting points of nanocrystals, in which, an electron beam is used to Y heat up the nanocrystals and the melting points are determined by the disappearance of G e lectron diffraction. Another example is the measurement of mechanical and electrical LO properties of individual nanowires and nanotubes. This technique allows a one-to- one H N STM (Scanning Tunneling Microscopy): O correlation between the structure and properties of the nanowires. C The s canning tunneling microscope (STM) is a non- optical microscope that TE scans an electrical probe o ver a surface to be imaged to detect a weak electric current flowing between the tip and the surface. The STM allows scientists to visualize regions O of high electron density and hence infer the position of individual atoms and molecules AN on the surface of a lattice. Previous methods required arduous study of diffraction patterns and required interpretation to obtain spatial lattice structures. The STM is N capable of higher resolution than its somewhat newer cousin, the atomic force microscope (AFM). Both the STM and the AFM fall under the class of scanning probe microscopes. The STM can obtain images of conductive surfaces at an atomic scale 2 10 10 m or 0.2 nanometer, and also can be used to manipulate individual atoms, trigger chemical reactions, or reversibly produce ions by removing or adding individual electrons from atoms or molecules. In a typical STM, a conductive tip is positioned above the surface of a sample. When the tip moves back and forth across the sample surface at very small intervals, the DEPT OF MECHANICAL ENGG, KL UNIVERSITY 6 D.SUNEEL, PROFESSOR height of the tip is continually adjusted to keep the tunneling current constant. The tip positions are used to construct a topographic map of the surface. For a current to occur the substance being scanned must be conductive (or semiconductive). Insulators cannot be scanned through the STM, as the electron has no available energy state to tunnel into or out of due to the band gap structure in insulators. AN O TE C H N O LO G Y The following figure shows the schematic diagram of STM. Figure: schematic diagram of STM. N An extremely sharp tip usually made of metals or metal alloys, such as t ungsten is mounted on to a three dimensional positioning stage made of an array of piezo electrics. Such a tip would move above the sample surface in three dimensions accurately controlled by the piezoelectric arrays. Typically the distance between the t ip and the sample surface falls between 0.2 and 0.6 nm, thus a tunneling current in the scale of 0.1 10 nA is commonly generated. The scanning resolution is about 0.01 nm in XY direction and 0.002 nm in Z direction, offering true atomic resolution three dimensional image. STM can be operated in two modes. In constant current imaging, a feed back mechanism is enabled that a constant current is maintained while a constant bias is DEPT OF MECHANICAL ENGG, KL UNIVERSITY 7 D.SUNEEL, PROFESSOR applied between the sample and tip. As the tip scans over the sample, the vertic al position of the tip is altered to maintain the constant separation. An alternate imaging mode is the constant height operation in which constant height and bias are simultaneously maintained. A variation in current results as the tip scans the sample surface because a topographic structure varies the tip- sample separation. The constant current mode produces a contrast directly related to e lectron charge density profiles, where as the constant height mode permits faster scan rates. Y AFM (Atomic Force Microscopy): G In spite of atomic r esolution and other advantages, STM is limited to an electrically LO conductive surface since it is dependent on monitoring the tunneling current between the sample surface and the tip. AFM was developed as a modification of STM for dielectric O materials. A variety of tip- sample interactions may be measured by an AFM, depending N on the separation. At short distances, the vander walls interactions are predominant. Van H der walls force consists of interactions of three components: perm anent dipoles, induced C dipoles and electronic polarization. Long range forces act in addition to short- range TE forces between the tip and sample, and become significant when the tip- sample distance increases such that the van der walls forces become negligib le, examples of such forces O include electrostatic attraction or repulsion, current induced or static- magnetic N and tip. AN interactions, and capillary forces due to the condensation of water between the sample In AFM, the motion of a cantilever beam with an ultra small mass is measured, and the force required to move this beam through measurable distance (10 -4 A0 ) can be as s mall as 10-18 N. The following figure shows a schematic diagram of AFM. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 8 LO G Y D.SUNEEL, PROFESSOR Figure: schematic diagram of AFM. O The instrument consists o f a cantilever with a nanoscale tip, a laser pointing at the end of N a cantilever, a mirror and a photodiode collecting the reflected laser beam, and a three H dimensional positioning sample stage which is made of an array of p iezoelectrics. Similar C to STM, t he images are also generated by scanning the tip across the surface. However, TE instead of adjusting the height of the tip to maintain a constant distance between the tip O and the surface, and thus a constant tunneling current as in STM, the AFM measures the AN minute upward and downward deflections of the tip cantilever while maintaining a N constant force of contact. SPM (Scanning Probe Microscopy): A combination of STM and AFM is also commonly referred to as scanning probe microscope (SPM). SPM is unique among imaging techniques in that it provides threedimensional (3-D) real- space images and among other analysis techniques in that it a llows spatially localized measurements of structure and properties. Under optimum conditions subatomic spatial resolution is ac hieved. SPM is a general term for a family of microscopes depending on the probing forces used. Two major members are STM and DEPT OF MECHANICAL ENGG, KL UNIVERSITY 9 D.SUNEEL, PROFESSOR AFM. The limitation of STM, which is restricted to electrically conductive sample surface, is complemented by AFM, which does not require conductive sample surface. Therefore, almost any solid surface can be studied with SPM: insulators, semiconductors and conductors, magnetic, transparent and opaque materials. In addition, surface can be studied in air, in liquid, or in ultra high vacuum. In addition, sample preparation for SPM analysis is minimal. SPM has been developed to a wide spectrum of techniques using various probe and sample surface interactions. The interaction force may be the interatomic forces Y between the atoms of the tip and those of a surface, short range van der walls force, or G long range capillary forces, or stick- slip processes producing friction forces. Modifying LO the tip chemically allows various properties of the sample surface to be measured. Depending on the type o f interactions between the tip and the sample surface used for the O characterization, various types of SPM have been developed. Electrostatic force N microscopy is based on local charges on the tip or surface, which lead to electrostatic H forces between tip a nd sample, which allow a sample surface to be mapped, i.e., local C differences in the distribution of electric charge on a surface to be visualized. In a similar TE way, magnetic forces can be imagined if the tip is coated with a magnetic material, e.g. iron, that has been magnetized along the tip axis, which is M agnetic force microscopy. O The tip probes the stray field of the sample and allows the magnetic structure of the AN sample to be determined. When the tip is functionalized as a thermocouple, temperature N distribution on the sample surface can be measured, which is Scanning thermal microscopy. The capacity change between tip and sample is evaluated in Scanning capacitance microscopy. The tip can be driven in an oscillating mode to probe the e lastic properties of a surface, which is referred to as Elastic modulus microscopy. SPM has proved its suitability in various fields of applications. First, SPM is capable of imaging the surface of all kinds of solids virtually under any kind of environment. Secondly, with various modifications of tips and operating conditions, SPM can be used to measure local chemical and physical properties of sample surface. Thirdly, SPM has been explored as a useful tool in nano- manipulation and nanolithography in DEPT OF MECHANICAL ENGG, KL UNIVERSITY 10 D.SUNEEL, PROFESSOR fabrication and processing of nanostructures. Fourthly, SPM has also been investigated as various nanodevices, such as nanosensors and nanotwizers. X-ray diffraction (XRD): XRD is a very important experimental technique that has long been used to address all issues related to the crystal structure of solids, including lattice constants and geometry, identification of unknown materials, orientation of single crystals, preferred orientation of polycrystals, defects, stresses, etc. in XRD, a collimated beam of X- rays, Y with a wavelength typically ranging from 0.7 to 2 A0 , is incident on a specimen and is G diffracted by the crystalline phases in the specimen according to Bragg s law: LO = 2d sin O Where d is the spacing between atomic planes in the crystalline phase and is the X- ray N wave length. The intensity of the diffracted X- rays is measured as a function of the H diffraction angle 2 and the specimen s orientation. This diffraction pattern is used to TE C identify the specimen s crystalline phases and to measure its structural p roperties. XRD is nondestructive and does not require elaborate sample preparation, which partly explains O the wide usage of XRD method in materials characterization. AN Diffraction peak positions are accurately measured with XRD, which makes it the N best met hod for characterizing homogeneous and inhomogeneous strains. Homogeneous or uniform elastic strain shifts the diffraction peak positions. From the shift in peak positions, one can calculate the change in d- spacing, which is the result of the change of lattice constants under a strain. Inhomogeneous strains vary from crystallite to crystallite or within a single crystallite and this causes a broadening of the diffraction peaks that increase with sin . Peak broadening is also caused by the finite size of cry stallites, but here the broadening is independent of sin . When both crystallite size and inhomogeneous strain contribute to the peak width, these can be separately determined by careful analysis of peak shapes. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 11 D.SUNEEL, PROFESSOR I f there is no inhomogeneous strain, the cr ystallite size, D, can be estimated from the peak width with the Scherrer s formula: D K B cos B Where is the X- ray wave length, B is the full width of height maximum of a diffraction peak, B is the diffraction angle, and K is the Scherrer s constant of the order of unity for usual crystal. However, one should be altered to the fact that nanoparticles often form Y twinned structures: therefore, Scherrer s formula may produce results different from the G true particle sizes. In addition, X-ray diffraction only provides the collective information LO of the particle sizes and usually requires a sizeable amount of powder. It should be noted that since the estimation would work only for very small particles, this technique is very O useful in characterizing nanoparticles. Similarly, the film thickness of epitaxial and H N highly textured thin films can also be estimated with XRD. C One of the disadvantages of XRD, compared to electron diffraction, is the low TE intensity of diffracted X-rays, particularly for low- Z materials. XRD is more sensitive to high Z materials, and for low Z- materials, neutron or electron diffraction is more O suitable. Typical intensities for electron diffraction are ~10 8 t imes larger than for XRD. AN Because of small diffraction intensities, XRD generally requires large specimens and the information acquired is an average o ver a large amount of material. The following figure N shows the powder XRD spectra of a series of nanoparticles with different sizes. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 12 AN O TE C H N O LO G Y D.SUNEEL, PROFESSOR Special nanomate rials: N Introduction: In the previous chapters, we have introduced the fundamentals and general methods for the synthesis and fabrication of various nanostructures and nanomaterials including nanoparticles, nanowires and thin films. How ever, there are a number of important nanomaterials not included, since their synthesis is unique and difficult to group into previous chapters. Examples of such nanomaterials are carbon fullerenes and nanotubes. In addition, bulk materials with nanosized building blocks, such as nanograined ceramics and nanocomposites have not been discussed so far. In this chapter, we will discuss the synthesis of these special nanomaterials. Most of these nanomaterials are unique, do not exist in nature, and are truly man- made relatively recently, therefore, a brief DEPT OF MECHANICAL ENGG, KL UNIVERSITY 13 D.SUNEEL, PROFESSOR introduction to the materials, such as their peculiar structures and properties, has also been included in this chapter. Carbon is a unique material, and can be a good metallic conductor in the form of graphite, a wide band gap semiconductor in the form of diamond, or a polymer when reacted with hydrogen. Carbon provides examples of materials showing the entire regime of intrinsic nanometer scaled structures from fullerenes, which are zero- dimensional nanoparticles, to carbon nanotubes, one- dimensional nanowires to graphite, a two dimensional layered anisotropic material, to fullerene solids, a three dimensional bulk Y materials with the fullerene molecules as the fundamental building block of the LO G crystalline phase. Carbon fullerenes: O Carbon fullerene commonly refers to a molecule with 60 carbon atoms, C 60 , and with an N icosahedral symmetry, but also includes larger molecular weight fullerenes C n ( n>60). H Examples of larger molecular weight fullerenes are C 70 , C76, C78, C80, and higher mass C fullerenes, which possess different geometric structures, e.g. C 70 has a rugby- ball shaped N AN O TE symmetry. The following figure shows the structure and geometry of C60 molecule. Figure: The icosahedral C 60 molecule. The name of fullerene was given to t his family of carbon molecules because of the resemblance of these molecules to the geodesic dome designed and built by R. Buckminster Fuller, where as the name of Buckminster Fullerene or buckyball was specifically given to the C 60 molecules, which are most widely studied in the fullerene family and deserve a little more discussion on its structure and properties. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 14 D.SUNEEL, PROFESSOR Properties : The 60 carbon atoms in C 60 are located at the vertices of a regular truncated icosahedron and every carbon site on C 60 is equivalent to every other site. The average nearest neighbor C-C distant in C 60 (1.44 A0 ) is almost identical to that in graphite (1.42A0 ). Each carbon atom in C 60 is trigonally bonded to other carbon atoms, the same as that in graphite, and most of the faces on the regular truncated isosahedron are hexagons. There are 20 hexagonal faces and 12 additional pentagonal faces in each C 60 molecule, which has a molecule diameter of 7.10A0 . Some selective properties of C 60 molecule were given in the following table. value Cage diameter 0.7 nm Vander walls diameter 1.0 nm LO G Y Property 0.1404 nm Bond distances: six- six bonds 0.1448 nm symmetry isosahedral N O Bond distances: five- six bonds C H Electron affinity TE First ionization potential Cohesive energy 7.58 ev 7.4 ev/atom O Synthesis : 2.65 ev AN There are four methods for the production of fullerenes. They are radio frequency thermal plasma method, laser vaporization, RF- inductive coupled plasma discharge N method and flame combustion method. Fullerenes are usually synthesized by using an arc discharge ( flame combustion method) between graphite electrodes in approximately 200 torr of He gas . The heat generated at the contact point between the electrodes evaporates carbon to form soot and fullerenes, which condense on the water cooled walls of the reactor. This discharge produces carbon soot that contains up to ~15% fullerenes: C 60 (~13%) and C 70 (~2%). The fullerenes are next separated from the soot, according to their mass, by use of liquid chromatography and using a so lvent such as toluene. However, there is no definite understanding of the growth mechanism of the fullerenes. The following figure shows the schematic diagram of fullerene soot production chamber. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 15 N O LO G Y D.SUNEEL, PROFESSOR H Applications : Speculation and some hard work on potential applications began almost C immediately after the discovery of buckyballs. There are some potential applications of TE fullerenes as listed below. O 1. As fullerenes are very large graphitic systems, they can easily accommodate extra electrons. When we add three electrons to C 60 we get ionic solids of the general formula AN A3 C60 , where A is any metal in Group I (lithium, sodium, potassium, rubidium, N cesium). These materials are actually metals, and display superconductivity at some what low temperatures. Current research is aimed at getting the maximum superconducting temperature (or Tc) to higher values. 2. C60 is just the right size to fit into the active cavity of HIV Protease, an enzyme important to the activity of the virus which causes AIDS. Cramming a buckyball into the active cavity would deactivate the enzyme and kill the virus. Ways of getting the molecule to the enzyme are under investigation 3. Possible applications of interest to industry include optical devices; chemical sensors and chemical separation devices; production of diamonds and carbides as cutting tools or DEPT OF MECHANICAL ENGG, KL UNIVERSITY 16 D.SUNEEL, PROFESSOR hardening agents; batteries and other electrochemical applications, including hydrogen storage media; drug delivery systems and other medical applications; polymers, such as new plastics; and catalysts. Catalysts, in fact, appear to be a natural application for fullerenes, given their combination of rugged structure and high reactivity. Experiments suggest that fullerenes which incorporate alkali metals possess catalytic properties resembling those of platinum. 4. The C60 molecule can also absorb large numbers of hydrogen atoms-- almost one hydrogen for each carbon-- without disrupting the buckyball structure. This property Y suggests that fullerenes may be a better storage medium for hydrogen than metal G hydrides, the best current material, and hence possibly a key factor in the development of LO new batteries and even of non-polluting automobiles based on fuel cells. O 5. A t hin layer of the C70 fullerene, when deposited on a silicon chip, seems to provide a H N vastly improved template for growing thin films of diamond. C Carbon nanotubes: TE A promising group of nanostructured materials is the nanotubes, which are currently fabricated from various materials such as boron nitride, molybdenum, carbon O (carbon nanotube), etc. However, at the moment, carbon nanotubes seem to be superior AN and most important due to their unique structure with interesting properties, which suit them to a tremendously diverse range of applications in micro or nanoscale electronics, N biomedical devices, nanocomposites, gas storage media, scanning probe tips, etc. Definition: Carbon nanotubes are a new form of carbon made by rolling up a single graphite sheet to a narrow but long tube closed at both sides by t wo hemispheres (1/2 section of fullerene carbon) like end caps. In 1991, while experimenting on fullerene and looking into soot residues sumio lijima invented two types of nanotubes namely single walled carbon nanotubes (SWNTs) and multi walled carbon nanotubes (MWNTs). SWNT consists only of a s ingle graphene DEPT OF MECHANICAL ENGG, KL UNIVERSITY 17 D.SUNEEL, PROFESSOR sheet with one atomic layer in thickness, while MWNT is formed from 2 to several tens of graphene sheets arranged concentrically into tube structures. They are promising one dimensional periodic structure along the axis of the tube with high aspect ratio TE C H N O LO G Y ( length/diameter). O Properties : N nanotubes. AN The following table shows selected electrical and mechanical properties of carbon Characteristics Measure Electrical conductivity Metallic or semi conducting Electrical transport Ballistic, no scattering Energy gap (semi conducting) E g (ev)=1/d (nm) Maximum current density 1010 A/cm2 Maximum strain 0.11% at 1 V Thermal conductivity 6 KW/Km Diameter 1- 100 nm DEPT OF MECHANICAL ENGG, KL UNIVERSITY 18 D.SUNEEL, PROFESSOR Length Gravimetric surface >1500 m2 /g E- modulus Up to millimeters 1000 Gpa, harder than steel Nanotubes can be either electrically conductive or semi conductive, depending on their helicity. These one- dimensional fibers exhibit electrical conductivity as high as copper, thermal conductivity as high as diamond. failure. Current length limits are about one millimeter. Synthesis (production of carbon nanotubes): LO Y Strength 100 times greater than steel at one sixth the weight, and high strain to G O The growth of carbon nanotubes during synthesis and production is believe d to N commence from the recombination of carbon atoms split by heat from its precursor. H Although a number of newer production techniques are being invented, three main C methods are the laser ablation, electric arc discharge and the chemical vapor deposition. TE Chemical vapor deposition is becoming very popular because of its potential for scale up O production. AN Che mical vapor deposition: In this technique, carbon nano tubes grow from the decomposition of N hydrocarbons at temperature range of 500 to 1200 0 C. They can grow on substrates such as carbon, quartz, silicon, etc or on floating fine catalyst particles, e.g. Fe, Ni, Co, etc from numerous hydrocarbons e.g. benzene, xylene, natural gas, acetylene, to mention but few. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 19 D.SUNEEL, PROFESSOR Y The above figure shows the schematic d iagram of a typical catalytic chemical vapor G deposition system. It is equipped with a horizontal tubular furnace as the reactor. The LO tube is made of quartz, 30 mm in diameter and 1000 mm in length. Ferrocene and Benzene vapors acts as the catalyst (Fe) and carbon atom precursors respectively were O transported either by argon, hydrogen or mixture of both into the reaction chamber, and N decomposed into respective ions of Fe and carbon atoms, resulting into carbon H nanostructures. The growth of the nanostructure s occurred in either the heating zone, C before or after the heating zone, which is normally operated between 500 0 C and 11500 C O Arc discharge: TE for about 30 min. 200ml/min of hydrogen is used to cool the reactor. AN The arc discharge method produces a number of c arbon nanostructures such as fullerenes, whiskers, soot and highly graphitized carbon nanotubes from high temperature N plasma that approaches 37000 C. The first ever produced nanotube was fabricated with the DC arc discharge method between two carbon electro des, anode and the cathode in a noble gas (helium or argon) environment. Schematic representation of a typical arc discharge unit is presented in figure below DEPT OF MECHANICAL ENGG, KL UNIVERSITY 20 Y D.SUNEEL, PROFESSOR G Figure: Schematic of Arc discharge method. LO Relatively large scale yield of carbon nanotubes o f about 75% was produced by Ebbesen and Ajayan with diameter between 2 to 30nm and length 1 m deposited on the cathode at O 100 to 500 Torr He and about 18 V DC. It has conveniently been used to produce both N SWNTs and MWNTs as revealed by Transmission Electron Microscope (TEM) analysis. H Typical nanotubes deposition rate is around 1mm/min and the incorporation of transition C metals such as Co, Ni or Fe into the electrodes as catalyst favors nanotubes formation TE against other nanoparticles, and low operating temperature. The arc discharge unit must O be provided with cooling mechanism whether catalyst is used or not, because overheating structure. AN would not only results into safety hazards, but also into coalescence of the nanotube N Laser ablation: Laser ablation technique involves the use of laser beam to vaporize a target of a mixture of graphite and metal catalyst, such as cobalt or nickel at temperature approximately 12000 C in a flow of controlled inert gas (argon) and pressure, where the nanotube deposits are recovered at a water cooled collector at much lower and convenient temperature. This method was used in early days to produce ropes of SWNTs with remarkably uniform narrow diameters ranging from 5-20 nm, a nd high yield with graphite conversion grater than 70-90%. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 21 D.SUNEEL, PROFESSOR The bundles entangled into a 2- D triangular lattice via the van der walls bonding to achieve lattice constant equal to 1.7 nm. The metal atom (catalyst) due t o its high e lectronegativity, deprived the growth of fullerenes and thus a selective growths of carbon nanotubes with open ends were obtained. Changing the reaction temperature can control the tubes diameters, while the growth conditions may be maintained o ver a higher volume and time, when two laser pulses are employed. However, by the virtue of relative operational complexity, the laser ablation method appears to be economically disadvantageous, which in effect hampers its scale up Y potentials as compared to the CVD method. The following figure shows the schematic of O TE C H N O LO G laser ablation method. AN Curre nt and future applications: N Currently, carbon Nanotubes are exte nding our ability to fabricate devices such as Molecular probes, Pipes, Wires, Bearings, springs, Gears, Pumps, Molecular transistors. In future we can find some more applications such as Field emitters, Building b locks for bottom- up electronics, Smaller, lighter weight components for next generation spacecraft and also enable large quantities of hydrogen to be stored in small low pressure tanks. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 22 O LO G Y D.SUNEEL, PROFESSOR N AN O TE C H N X-RAY DIFFRACTOMETER SAMPLE HOLDER IN XRD DEPT OF MECHANICAL ENGG, KL UNIVERSITY 23 G Y D.SUNEEL, PROFESSOR N AN O TE C H N O LO Convoluted Graph of Intensity profile for SiC Nanoparticles Deconvoluted Graph of Intensity profile for SiC Nanoparticles. Measured Values of characteristic peaks Peak Peak angle (2 in degrees) FWHM (degrees) % Area Under the peak Crystallite Size, nm 1 33.8732443 0.83440353 10.1126055 9.952 2 35.0094788 0.91961899 19.4767052 9.0583 3 35.6281511 0.34131587 70.4106893 24.448 4 36.2315103 0.92877212 16.1348898 8.999 DEPT OF MECHANICAL ENGG, KL UNIVERSITY 24 D.SUNEEL, PROFESSOR Scherrer formula is used to calculate average crystallite size from the above data and tabulated.Average Crystallite size (cs) = K FWHM COS Where K= Shape factor (0.9 for spherical particles) = Wave length of copper = 1.5405981 A0 = Diffraction angle for the maximum peak in degrees O TE C H N O LO G Y FWHM=Full width half maximum of the peak in radians N AN TEM Image of SiC nanopowder SEM Image of A356 matrix reinforced with 0.5wt% of nano SiC. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 25 LO G Y D.SUNEEL, PROFESSOR N AN O TE C H N O EDS analysis of nanocomposite. Scanning Electron Microscope DEPT OF MECHANICAL ENGG, KL UNIVERSITY 26 D.SUNEEL, PROFESSOR Carbon is an extraordinary element and of great importance to the scientist as well as for technologist. Its unique characteristics made it versatile for many applications areas ranging from house holds as well as major industries on one hand in high tech aerospace, defence, nuclear energy and new energy source program on the other hand. Although the carbon exist in the nature in the uncombined form as diamond and graphite, the principal source of carbon are coal, petroleum and natural gas. The greater part of the world conventional fuel resources consist of carbon compounds. All forms of plant and animal life involves carbon compounds both as component part and in their nutrition process. Although the carbon does not exist in the scale of abundance on and in the earth crust, the importance of this element can scarcely be overemphasized. Y Carbon nanotubes have unique mechanical and electronics properties which makes them a G prime candidate for many electronics and other engineering industries. Recently, these carbon LO nanotubes were also found to be very attractive reinforcing materials for development of high performance advanced composites. Based on these properties many exotic applications have been O predicted. Depending on process parameters single wall carbon nanotubes (SWNT) and multi-wall N carbon nanotubes (MWNT) can be produced. Various techniques have been developed for synthesis H of Carbon nanotubes with unique attributes. Among all the techniques, arc-discharge techniques and C CVD techniques are considered the best techniques for producing high quality carbon nanotubes. In TE arc discharge technique, graphite is evaporated by generating arc between two graphite electrodes. Deposits on cathode c onsist of mainly MWNT along with many unwanted carbonaceous materials. O These materials are to be separated out to get high purity CNT s. Various techniques exist for AN purification of carbon nanotubes. Composite films was developed using MWNT and dispersing it into the polymer matrix. Different properties were evaluated for these composite films. In CVD N technique hydrocarbon gas is cracked on the suitable substrate in the presence of metal catalyst to grow carbon nanotubes. Various parameters affects the grow th mechanism of carbon nanotubes. Results of these evaluation discussed along with some applications of Carbon nanotubes. Apart from some of applications of various carbon and graphite products for different application are also discussed. INTRODUCTION The Science of nanomaterials has become the flavor the day, with research being driven both by academic curiosity and the promise of useful applications. The discovery of the fullerenes and carbon nanotubes, interest in nanomaterials has increases remarka bly. Carbon is an extraordinary element and of great importance to the scientist as well as for technologist. Its unique characteristics made it versatile for many applications areas ranging from house holds as well as major industries on one DEPT OF MECHANICAL ENGG, KL UNIVERSITY 27 D.SUNEEL, PROFESSOR hand in high tech aerospace, defence, nuclear energy and new energy source program on the other hand. Although the carbon exist in the nature in the uncombined form as diamond and graphite, the principal source of carbon are coal, petroleum and natural gas. The greater part of the world conventional fuel resources consist of carbon compounds. All forms of plant and animal life involves carbon compounds both as component part and in their nutrition process. Although the carbon does not exist in the scale of abundance on and in the earth crust, the importance of this element can scarcely be overemphasized. Products derived from the carbon element are found in most facet of every day life, from grimy soot in the chimney to the diamonds in the jewelry box. Carbon has an extra-ordinary broad range of applications. Some of them are listed below: Y Natural graphite for lubricant and Shoe Polish G Carbon black reinforcement essential to every automobile tire LO Carbon black and lampblack found in all printing inks Acetylene black in the conductive rubber O Vegetable and bone chars for decolorize and purify sugar & other food N Activated Charcoal for gas purifications and catalytic support H High Strength Carbon Fibers for Advance Composite Materials C Carbon-Carbon Composites for air craft brakes and space shuttle components TE Very large Graphite electrodes for metal industries Carbon Black for Photo-copy machine O Graphite Brushes and contacts for electrical machinery AN Diamond optical window for space-crafts P olycrystalline diamond coating for cutting tools N Exfoliated Graphite Based products for gaskets & Sealing application CARBON NANOTUBES AND THEIR STRUCTURE Many exotic structures of fullerenes exist: regular spheres, cones, tubes and also more complicated and strange shapes. Here we will describe some of the most important and bestknown structures. Single Walled Nanotubes (SWNT) can be considered as long wrapped graphene sheets. As stated before, nanotubes generally have a length to diameter ratio of about 1000 so they can be considered as nearly one-dimensional structures. More detailed, a SWNT consists of two separate regions with different physical and chemical properties. The first is the sidewall of the tube and the second is the end cap of the tube. The end cap structure is similar to or derived from a smaller fullerene, such as DEPT OF MECHANICAL ENGG, KL UNIVERSITY 28 D.SUNEEL, PROFESSOR C60. C-atoms placed in hexagons and pentagons form the end cap structures. It can be easily derived from Euler s theorem that twelve pentagons are needed in order to obtain a closed cage structure which consists of only pentagons and hexagons. The combination of a pentagon and five surrounding hexagons results in the desired curvature of the surface to enclose a volume. A second rule is the isolated pentagon rule that states that the distance between pentagons on the fullerene shell is maximised in order to obtain a minimal local curvature and surface stress, resulting in a more stable structure. The smallest stable structure that can be made this way is C60 the one just larger is C70 and so on. Another property is that all fullerenes are composed of an even number of C-atoms because adding one hexagon to an existing structure means adding two C-atoms. The other structure Y of which a SWNT is composed is a cylinder. It is generated when a graphene sheet of a certain size G that is wrapped in a certain direction. As the result is cylinder symmetric we can only roll in a LO discreet set of directions in order to form a closed cylinder. Two atoms in the graphene sheet are chosen, one of which servers the role as origin. The sheet is rolled until the two atoms coincide. The O vector pointing from the first atom towards the other is called the chiral vector and its len gth is equal N to the circumference of the nanotube. The direction of the nanotube axis is perpendicular to the chiral H vector. SWNTs with different chiral vectors have dissimilar properties such as optical activity, C mechanical strength and electrical conductivity. TE Structure The bonding in carbon nanotubes is sp , with each atom joined t o three neighbours, as in graphite. O The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual AN graphite layer). There are three distinct ways in which a graphene sheet can be rolled into a tube. The first two of these, known as armchair (top left) and zig-zag ( middle left) have a high degree of N symmetry. The terms "armchair" and "zig-zag" refer to the arrangement of hexagons around the circumference. The third class of tube, which in practice is the most common, is known as chiral, meaning that it can exist in two mirror-related forms. An example of a chiral nanotube is shown at the bottom left. The structure of a nanotube can be specified by a vector, (n,m), which defines how the graphene sheet is rolled up. This can be understood with reference to figure on the right. To produce a nanotube with the indices (6,3), say, the sheet is rolled up so that the atom labelled (0,0) is superimposed on the one labelled (6,3). It can be seen from the figure that m = 0 for all zig-zag tubes, while n = m for all armchair tubes. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 29 N AN O TE C H N O LO G Y D.SUNEEL, PROFESSOR DEPT OF MECHANICAL ENGG, KL UNIVERSITY 30 D.SUNEEL, PROFESSOR S YNTHESIS OF CARBON NANOTUBE Carbon nanotubes are generally produced by three main techniques, arc discharge, laser ablation and chemical vapour deposition. Though scientists are researching more economic ways to produce these structures. In arc disc harge, a vapour is created by an arc discharge between two carbon electrodes with or without catalyst. Nanotubes self-assemble from the resulting carbon vapour. In the laser ablation technique, a high-power laser beam impinges on a volume of carbon containing feedstock gas (methane or carbon monoxide). At the moment, laser ablation produces a small amount of clean nanotubes, whereas arc discharge methods generally produce large quantities of impure material. In general, chemical vapour deposition (CVD) results in MWNTs or poor quality SWNTs. The SWNTs Y produced with CVD have a large diameter range, which can be poorly controlled. But on the other LO G hand, this method is very easy to scale up, what favours commercial production. Arc discharge Method O The carbon arc discharge method, initially used for producing C60 fullerenes, is the most common N and perhaps easiest way to produce carbon nanotubes. It is a technique that produces a mixture of H components and requires separating nanotubes from the soot and the catalytic metals present in the C crude product. This method creates nanotubes through arcvaporisation of two carbon rods placed end TE to end, separated by approximately 1mm, in an enclosure that is usually filled with inert gas (helium, argon) at low pressure (between 50 and 700 mbar). A direct current of 100 A to 500 A driven by O approximately 30V 50V creates a high temperature discharge between the two electrodes. The AN discharge vaporises one of the carbon rods and forms a small rod shaped deposit on the other rod. Producing nanotubes in high yield depends on the uniformity of the plasma arc and the temperature N of the deposit form on the carbon electrode. Insight in the growth mechanism is increasing and measurements have shown that different diameter distributions have been found depending on the mixture of helium and argon. These mixtures have different diffusions coefficients and thermal conductivities. This implies that single-layer tubules nucleate and grow on metal particles in different sizes depending on the quenching rate in the plasma and it suggests that temperature and carbon and metal catalyst densities affect the diameter distribution of nanotubes. Depending on the exact technique, it is possible to selectively grow SWNTs or MWNTs. Two distinct methods of synthesis can be performed with the arc discharge apparatus. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 31 G Y D.SUNEEL, PROFESSOR LO Laser Ablation Technique In 1995, Smalley's group at Rice University reported the synthesis of carbon nanotubes by laser O vaporisation. The laser vaporisation apparatus used by Smalley's group. A pulsed, or continuous laser N is used to vaporise a graphite target in an oven at 1200 C. The main difference between continuous H and pulsed laser, is that the pulsed laser demands a much higher light intensity (100 kW/cm2 C compared with 12 kW/cm2). The oven is filled with helium or argon gas in order to keep the pressure TE at 500 Torr. A very hot vapour plume forms, then expands and cools rapidly. As the vaporised species cool, small carbon molecules and atoms quickly condense to form larger clusters, possibly O including fullerenes. The catalysts also begin to condense, but more slowly at first, and attach to AN carbon clusters and prevent their closing into cage structures. Catalysts may even open cage structures when they attach to them. From these initial clusters, tubular molecules grow into single - N wall carbon nanotubes until the catalyst particles become too large, or until conditions have cooled sufficiently that carbon no longer can diffuse through or over the surface of the catalyst particles. It is also possible that the particles become that much coated with a carbon layer that they cannot absorb more and the nanotube stops growing. The SWNTs formed in this case are bundled together by van der Waals forces. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 32 LO G Y D.SUNEEL, PROFESSOR Chemical Vapor Deposition Technique O Chemical vapour deposition (CVD) synthesis is achieved by putting a carbon source in the gas phase N and using an energy source, such as a plasma or a resistively heated coil, to transfer energy to a H gaseous carbon molecule. Commonly used gaseous carbon sources include methane, carbon C monoxide and acetylene. The energy source is used to crack the molecule into reactive atomic TE carbon. Then, the carbon diffuses towards the substrate, which is heated and coated with a catalyst (usually a first row transition metal such as Ni, Fe or Co) where it will bind. Carbon nanotubes will O be formed if the proper parameters are maintained. Excellent alignment, as well as positional control AN on nanometer scale, can be achieved by using CVD. Control over the diameter, as well as the growth rate of the nanotubes can also be maintained. The appropriate metal catalyst can prefere ntially grow N single rather than multiwalled nanotubes. CVD carbon nanotube synthesis is essentially a two -step process consisting of a catalyst preparation step followed by the actual synthesis of the nanotube. The catalyst is generally prepared by sputtering a transition metal onto a substrate and then using either chemical etching or thermal annealing to induce catalyst particle nucleation. Thermal annealing results in cluster formation on the substrate, from which the nanotubes will grow. Ammonia may be used as the etchant. The temperatures for the synthesis of nanotubes by CVD are generally within the 650 900 C range. Typical yields for CVD are approximately 30%. These are the basic principles of the CVD process. In the last decennia, different techniques for the carbon nanotubes synthesis with CVD have been developed, such as plasma enhanced CVD, thermal chemical CVD, alcohol catalytic CVD, vapour phase growth, aero gel-supported CVD and laser assisted CVD. When growing carbon nanotubes on a Fe catalytic film by thermal CVD, the diameter range of the carbon nanotubes DEPT OF MECHANICAL ENGG, KL UNIVERSITY 33 D.SUNEEL, PROFESSOR depends on the thickness of the catalytic film. By using a thickness of 13 nm, the diameter distribution lies between 30 and 40 nm. When a thickness of 27 nm is used, the diameter range is N O LO G Y between 100 and 200 nm. The carbon nanotubes formed are multiwalled. C H POTENTIAL APPLICATIONS OF CARBON NANOTUBES some of them are listed below : O Energy storage TE There are numerous potential applications of the carbon nano materials especially carbon nanotubes AN Graphite, carbonaceous materials and carbon fibre electrodes are commonly used in fuel cells, batteries and other electrochemical applications. Advantages of considering nanotubes for energy N storage are their small dimensions, smooth surface topology and perfect surface specificity. The efficiency of fuel cells is determined by the electron transfer rate at the carbon electrodes, which is the fastest on nanotubes. Hydrogen storage The advantage of hydrogen as energy source is that its combustion product is water. In addition, hydrogen can be easily regenerated. For this reason, a suitable hydrogen storage system is necessary, satisfying a combination of both volume and weight limitations. The two commonly used means to store hydrogen are gas phase and electrochemical adsorption. Because of their cylindrical and hollow geometry, and nanometre-scale diameters, it has been predicted that carbon nanotubes can store a liquid or a gas in the inner cores through a capillary effect. As a threshold for economical storage, the Department of Energy has set storage requirements of 6.5 % by weight as the minimum level for DEPT OF MECHANICAL ENGG, KL UNIVERSITY 34 D.SUNEEL, PROFESSOR hydrogen fuel cells. It is reported that SWNTs were able to meet and sometimes exceed this level by using gas phase adsorption (physisorption). Yet, most experimental reports of high storage capacities are rather controversial so that it is difficult to assess the applications potential. What lacks, is a detailed understanding of the hydrogen storage mechanism and the effect of materials processing on this mechanism. Another possibility for hydrogen storage is electrochemical storage. In this case not a hydrogen molecule but an H atom is adsorbed which is called chemisorption. Lithium intercalation The basic principle of rechargeable lithium batteries is electrochemical intercalation and Y deintercalation of lithium in both electrodes. An ideal battery has a high-energy capacity, fast G charging time and a long cycle time. The capacity is determined by the lithium saturation LO concentration of the electrode materials. For Li, this is the highest in nanotubes if all the interstitial sites (inter-shell van der Waals spaces, inter-tube channels and inner cores) are accessible for Li O intercalation. SWNTs have shown to possess both highly reversible and irreversible capacities. N Because of the large observed voltage hysteresis, Li-intercalation in nanotubes is still unsuitable for battery application. This feature can potentially be reduced or eliminated by processing, i.e. cutting, H the nanotubes to short segments. Lithium intercalated graphite and other carbonaceous materials are C commercially used in Li-ion batteries. In this case, the specific energy capacity is partially limited by TE the thermodynamically determined equilibrium saturation composition of LiC6. During de- O intercalation (charging) the process is reversed. In graphite this adsorption could be visualised as in AN Figure 7. Carbon nanotubes are interesting intercalation hosts because of their structure and chemical bonding. Nanotubes might have a higher saturation composition than graphite as guest species can N intercalate in the interstitial sites and between the nanotubes. Therefore, carbon nanotubes are expected to be suitable high energy density anode materials for rechargeable Li-ion batteries Electrochemical supercapacitors Supercapacitors have a high capacitance and potentially applicable in electronic devices. Typically, they are comprised two electrodes separated by an insulating material that is ionically conducting in electrochemical devices. The capacity of an electrochemical supercap inversely depends on the separation between the charge on the electrode and the counter charge in the electrolyte. Because this separation is about a nanometre for nanotubes in electrodes, very large capacities result from the high nanotube surface area accessible to the electrolyte. In this way, a large amount of charge injection occurs if only a small voltage is applied. This charge injection is used for energy storage in nanotube supercapacitors. Generally speaking, there is most interest in the double-layer supercapacitors and DEPT OF MECHANICAL ENGG, KL UNIVERSITY 35 D.SUNEEL, PROFESSOR redox supercapacitors with different charge-storage modes. In the future, supercapacitors might become an excellent means of certain kinds of energy storage. These electrochemical capacitors have a long durability (over 106 cycles), don t suffer from short circuit conditions, have a complete discharge and possess a high power density. Loading of a supercap can be performed at high current densities, which decreases the loading time needed. However, their energy density is lower than for conventional batteries, which is a possible drawback for possible applications. Typical electrochemical accumulators, in which compounds only take place in redox reactions, cannot fulfil these good characteristics that electrochemical capacitors have. Supercapacitors have already been applied in small-scale energy storage devices, such as in memory backup devices. Now the capability Y of supercapacitors with a high power density is increasing, potential applications extend to hybrid G battery/supercapacitor systems. Carbon in general, and especially nanotubes, form an attractive LO material for electrochemical applications as they have a large active surface area. In addition, carbon is a relatively cheap, low density, environmentally friendly and highly polarisable material which O makes application even more attractive. At first this section deals with basic processes in N supercapacitors. Then, the determination of supercapacitor properties is explained briefly. Thereafter, H supercapacitors based on CNTs are investigated after which attention is given to the mod ification of C CNTs in supercapacitors to improve their properties. TE Field emitting devices If a solid is subjected to a sufficiently high electric field, electrons near the Fermi level can be O extracted from the solid by tunnelling through the surface potential barrier. This emission current AN depends on the strength of the local electric field at the emission surface and its work function (which denotes the energy necessary to extract an electron from its highest bounded state into the vacuum N level). The applied electric field must be very high in order to extract an electron. This condition is fulfilled for carbon nanotubes, because their elongated shape ensures a very large field amplification. For technological applications, the emissive material should have a low threshold emission field and large stability at high current density. Furthermore, an ideal emitter is required to have a nanometer size diameter, a structural integrity, a high electrical conductivity, a small energy spread and a large chemical stability. Carbon nanotubes possess all these properties. However, a bottleneck in the use of nanotubes for applications is the dependence of the conductivity and emission stability of the nanotubes on the fabrication process and synthesis conditions. Examples of potential applications for nanotubes as field emitting devices are flat panel displays, gas discharge tubes in telecom networks, electron guns for electron microscopes, AFM tips and microwave amplifiers DEPT OF MECHANICAL ENGG, KL UNIVERSITY 36 D.SUNEEL, PROFESSOR COMPOSITE MATERIALS Because of the stiffness of carbon nanotubes, they are ideal candidates for structural applications. For example, they may be used as reinforcements in high strength, low weight, and high performance composites. Theoretically, SWNTs could have a Young s Modulus of 1 TPa. MWNTs are weaker because the individual cylinders slide with respect to each other. Ropes of SWNTs are also less strong. The individual tubes can pull out by shearing and at last the whole rope will break. This happens at stresses far below the tensile strength of individual nanotubes. Nanotubes also sustain large strains in tension without showing signs of fracture. In other directions, nanotubes are highly flexible. One of the most important applications of nanotubes based on their properties will be as Y reinforcements in composite materials. However, there have not been many successful experiments G that show that nanotubes are better fillers than the traditionally used carbon fibres. The main problem LO is to create a good interface between nanotubes and the polymer matrix, as nanotubes are very smooth and have a small diameter, which is nearly the same as that of a polymer chain. O Secondly, nanotube aggregates, which are very common, behave different to loads than individual N nanotubes do. Limiting factors for good load transfer could be sliding of cylinders in MWNTs and H shearing of tubes in SWNT ropes. To solve this problem the aggregates need to be broken up and C dispersed or cross-linked to prevent slippage. A main advantage of using nanotubes for structural TE polymer composites is that nanotube reinforcements will increase the toughness of the composites by absorbing energy during their highly flexible elastic behaviour. Other advantages are the low density O of the nanotubes, an increased electrical conduction and better performance during compressive load. AN Another possibility, which is an example of a non-structural application, is filling of photoactive polymers with nanotubes. The composites show a large increase in conductivity with only a little loss N in photoluminescence and electro-luminescence yields. Another benefit is that the composite is more robust than the pure polymer. Of course, nanotube-polymer composites could be used also in other areas. For instance, they could be used in the biochemical field as membranes for molecular separations or for osteointegration (growth of bone cells). However, these areas are less explored. The most important thing we have to know about nanotubes for efficien t use of them as reinforcing fibres is knowledge on how to manipulate the surfaces chemically to enhance interfacial behavior between the individual nanotubes and the matrix material. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 37 D.SUNEEL, PROFESSOR NANOCOMPOSITES NANOCOMPOSITES W ith the evolution of nanomaterials (materials having their largest dimension in between 0 to 100 nm), new era in the field of Composite materials has been started on the name of nanocomposites. Nanocomposites can be considered as solid structures with nanometer scale dimensional repeat distances between different phases that constitute the structure. The field of nano composites is burgeoning. A brief look at new, common commercial Y uses reveals automotive panels for sports utility vehicles, polypropylene nanocomposites for G furniture, appliances and bulletin board substrates. Advanced technologies implemented include LO magnetic media, bone cement, filter membranes, aerogels, and solar cells. O While traditional composites use over 40% by weight of reinforcement, nanocomposites N may show improvements at less than 5%. More importantly traditional theories do not account H for meaningful change in properties when so little material is replaced. Nanoparticles can not C reinforce differently than micro particles. But continuum theories do not include size TE dependence. Thus, the mechanics must be understood as arising from load transfer as much as O from load bearing. AN On the other hand, research, describing structures containing nanoparticles seems to rely on methods that are being pushed to the limit of resolution. Few systematic studies have focused N on improvements in properties at size scales from the micron scale down to the nanoscale. Producing nanocomposites also poses processing challenges. The list of questions about the fabrication, characterization and use of nanocomposites is long-despite massive financial and intellectual investment. The most convincing examples of such are naturally occurring structures such as bone, which a hierarchical nanocomposite is built from ceramic tablets and organic binders. Since the constituents of a nanocomposite have different structures and compositions and hence properties, they serve various functions. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 38 D.SUNEEL, PROFESSOR An excellent example of the use of synthetic nanocomposites in antiquity is the recent LO G Y discovery of the constitution of Mayan paintings developed in the Mesoamericas. Mayan Painting Sample N O atural nanocomposite-Bone H Nanocomposites are classified into four types: Ceramic matrix nanocomposites, Metal C matrix nanocomposites (MMNCs), Polymer nanocomposites and natural nano biocomposites. TE MMNCs are the materials in which reinforcements of nanoscale are embedded in a ductile metal O or alloy matrix. AN 1. Polyme r Matrix Nanocomposites: Polymer composites are important commercial materials with applications that include N filled elastomers for damping, electrical insulators, thermal conductors, and high-performance composites for use in aircraft. Materials with synergistic properties are chosen to create composites with tailored properties; for example, high modulus but brittle carbon fibers are added to low- modulus polymers to create a stiff, light weight composite with some degree of toughness. In recent years, however, we have reached the limits of optimizing composite properties of traditional micro-meter-scale composite fillers, because the properties achieved usually involve compromises. Stiffness is traded for toughness, or toughness is obtained at the cost of optical clarity. In addition, macroscopic defects due to regions of high or low volume fraction of filler often lead to break down or failure. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 39 D.SUNEEL, PROFESSOR Recently, a large window of opportunity has opened to overcome the limitations of traditional micrometer-scale polymer composites. Nanoscale filled polymer composites- in which the filler is less than 100nm in at least one dimension. Although some nanofilled composites have been used for more than a century, research and development of nanofilled polymers has greatly increased in recent years for several reasons. First, unprecedented combinations of properties have been observed in some polymer nanocomposites. For example, the inclusion of equi-axed nanoparticles in thermoplastics, and particularly in semicrystalline thermoplastics, increases the yield stress, the tensile strength and young modulus compared to pure polymer. A volume fraction of only 0.04 mica-type silicates in epoxy increase the modulus Y below the glass transition temperature by 58% and modulus in the rubbury region by 450%. In G addition, the permeability of water in poly(e-caprolactone) decreases by an order of magnitude LO with the addition of 4.8% silicate by volume. A second reason for the large increase in research O and development efforts was the discovery of carbon nanotubes in the early 1990s. Third, significant development in the chemical processing of nanoparticles and in situ processing of N nanocomposites has led to unprecedented control over the morphology of such nanocomposites. C H It has also created an almost unlimited ability to control the interface between the matrix and the PROCESSING OF POLYM ER NANO COMPOSITES: O 1.1. TE filler. AN Polymer matrix composites are also known as FRP - Fiber Reinforced Polymers (or Plastics). These materials use a polymer-based resin as the matrix, and a variety of fibers such as glass, N carbon and aramid as the reinforcement. A very large number of polymeric materials, both thermosetting and thermoplastic, are used as matrix materials for the composites. The advantages of these being low densities, corrosion resistance, low thermal conductivity, low electrical conductivity, translucence, aesthetic color effects with few disadvantages like low transverse strength and limited operational temperatures. In general, the resinous binders (polymer matrices) are selected on the basis of adhesive strength, fatigue resistance, heat resistance, chemical and moisture resistance etc. The resin must have mechanical strength commensurate with that of the reinforcement and must be easy to use in the fabrication process selected and also stand up to the service conditions. Apart from these properties, the resin matrix must be capable of wetting and penetrating into the bundles of DEPT OF MECHANICAL ENGG, KL UNIVERSITY 40 D.SUNEEL, PROFESSOR fibers which provide the reinforcement, replacing the dead air spaces therein and offering those physical characteristics capable of enhancing the performance of fibers. Shear, chemical and electrical properties of a composite depend primarily on the resin. Again, it is the nature of the resin that determines the functioning of the laminates in a corroding environment. Generally, it can be assumed that in composites, even if the volume fraction of the fiber is high (of the order of 0.7), the reinforcement is completely covered by the matrix material; and when the composite is exposed to higher temperatures it is the matrix, which should withstand the Y hostile environment. The strength properties of the composite also show deterioration, which G may be due to the influence of the temperature on the interfacial bond. Thus, the high LO temperature resistant properties of the composites are directly related more to the ma trix, rather than to the reinforcement. The search for polymers which can withstand high temperatures has O pushed the upper limit of the service temperatures to about 300-3500 C. This range of N operational temperatures can be withstand by polyamides, which are the state-of-the-art high H temperature polymers for the present. Table indicates the approximate service temperature C ranges for the resins. It may be emphasized that there is no compromise with regard to the TE nature of the matrix material, particularly when it comes to the applied temperature of the O composite. If the application temperature exceeds 300- 3500 C, metal matrix appears to be the AN only alternative, at least for the present. N Application te mperatures of some matrix materials Limit of M atrix material Long term e xpos ure,0 c Short term exposure,0 c Unsaturated 70 100 Epoxies polyesters Phenolics 125 250 200 1600 Polymides 315 400 DEPT OF MECHANICAL ENGG, KL UNIVERSITY 41 D.SUNEEL, PROFESSOR There are three general ways of dispersing nanofillers in polymers. The first is direct mixing of the polymer and the nanoparticles either as discrete phases or in solution. The second is in-situ polymerization in the presence of the nanoparticles, and the third is both in situ formation of the nanoparticles and in-situ polymerization. The later can result in composites called hybrid nanocomposites because of the intimate mixing of the two phases. 1.1.1. Direct Mixing : Direct mixing takes advantage of well established polymer processing techniques. For example, polypropylene and nanoscale silica have been mixed successfully in a two-roll mill. But samples Y with more than 20 wt% filler could not be drawn. This is typical and is a limitation of this kind G of processing method. Nanoscale silica/PP composites have been processed in a twin screw LO extruder, but the dispersion was successful only after modification of the silica interface to make it compatible with the matrix. A brabender high shear mixer has been successfully used to mix O nanoscale alumina with PET, LDPE. Thermal spraying has also been successful in processing N nanoparticles filled nylon. When these traditional melt mixing or elastomeric mixing methods H are feasible, they are the fastest method for introducing new products to market, because the C composites can be produced by traditional methods. This has been successful in many cases, but TE for some polymers, the viscosity increases rapidly with the addition of significant volume O fractions of nanofiller, which in turn can limit the viability of this processing method. AN In addition to viscosity effects, nanoparticles can neither enhance nor inhibit polymer degradation. One method for measuring degradation is to place the polymer in a high shear mixer N and measure the torque as a function of time and temperature. As the material crosslinks, the torque begins to increase (at a constant speed), and when chain scission begins, the torque decreases. This leads to a peak in the torque, whose position is often used as a measure of the degradation time. Whether degradation is inhibited or enhanced depends on the particle surface activity and the increased interfacial area (particle size). 1.1.2. Solution Mixing: Some of the limitations of melt- mixing can be overcome if both the polymer and the nanoparticles are dissolved or dispersed in solution. This allows modification of the particle surface without drying, which reduces particle agglomeration. The nanoparticles/polymer DEPT OF MECHANICAL ENGG, KL UNIVERSITY 42 D.SUNEEL, PROFESSOR solution can then be cast into a solid, or the nanoparticles/polymer can be isolated from solution by solvent evaporation or precipitation. Further processing can be done by conventional techniques. 1.1.3. In-Situ Polyme rization: Another method is in-situ polymerization. Here, nanoparticles are dispersed in the monomer or monomer solution, and the resulting mixture is polymerized by standard polymerization methods. One fortunate aspect of this method is the potential to graft the polymer onto the particle surface. Many different types of nanocomposites have been processed by in-situ Y polymerization. A few examples are silica/Nylon6, alumina/polymethylmethacrylate and G CaCo3 /PMMA. The key to in-situ polymerization is appropriate dispersion of the filler in the LO monomer. This often requires modification of the particle surface because, although dispersion is O easier in a liquid than in a viscous melt, the settling process is also more rapid. N 2. Metal Matrix Nanocomposites: H Metal matrix composites in general, consist of at least two components, the metal matrix C and the reinforcement. The matrix is defined as a metal in all cases, but a pure metal is rarely TE used as the matrix. It is generally an alloy. Metal matrix composites are increasingly found in the O aerospace and automotive industry. These materials use a metal such as aluminum as the matrix, AN and reinforce it with fibers, particulates or whiskers such as silicon carbide. Metal- matrix composite reinforcements can be generally divided into five major categories: continuous fibers (each tow consisting of many individual fibers of diameters typically in N i. the range of 3 to 30 m.) ii. discontinuous fibers (cylindrical reinforcement or composite ingredient with a ratio of length to diameter greater than 5 (but typically greater than 100), and with a diameter typically greater than 1 m. iii. whiskers (elongated single crystals, typically produced with a length to diameter ratio greater than 10 and with a diameter typically less than 1micron.) iv. wires v. particulates (equiaxed reinforcement of various shapes (spherical, angular, plate- like) and are typically greater than 1 m in diameter, usually of aspect ratio less than about 5. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 43 D.SUNEEL, PROFESSOR The typical reinforcements used in metal- matrix composites are presented in Table. Silicon carbide, boron carbide and aluminum oxide are the key particulate reinforcements and can be obtained in varying levels of purity and size distribution. The Typical Reinforcements used in Metal Matrix Composites Reinforcement M atrices Boron, fiber (including coated) Aluminum, titanium Alumina fiber Aluminum, magnesium Silicon carbide fiber Aluminum, titanium Alumina- silica fiber Aluminum Silicon carbide whisker Aluminum, magnesium N O LO G Y Graphite fiber Aluminum, magnesium, copper Aluminum, magnesium H Silicon carbide particulate Aluminum, magnesium TE C Boron carbide particulate Several metallic systems have been considered for use as a matrix material for metal O matrix composites viz: Aluminum, Titanium, Magnesium, Copper, Bronze, Nickel, Lead, Silver, AN Super alloys, Niobium etc. Thus, entire families of light weight composites, though considered impossible just a few N years ago, are either available now or hovering on the brink of commercialization. For example, a series of Aluminium matrix composites reinforced with silicon carbide particulates have been developed by Duralcan USA, Div. Alcan Aluminium corp., San Diego, California. A high temperature creep resistant titanium alloy has been developed as matrix material for the National Aerospace plant by Timet for McDonnell Douglas. Titanium alloy Ti-6Al-4V, reinforced with continuous silicon carbide filaments, is hot isostatically pressed by Textron for turbine engine shafts. C ERAMTEC AG (Germany) is currently utilizing Al Si9 Cu3 standard alloy as matrix material for MMC products. Apart from being fairly inexpensive in comparison with other light metals (e.g., magnesium and titanium), it has delivered outstanding results in many automotive and aerospace applications and is noted for its uncomplicated processing properties. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 44 D.SUNEEL, PROFESSOR NEED FOR THE REINFORCEMENT OF NANOMATERIALS IN METAL MATRIX Metal matrix composites reinforced with ceramic micro particles are attractive materials for many structural applications. However, large ceramic micro particles often act as stress concentrators in the composites during mechanical loading, giving rise to failure of materials via particle cracking. Normally, micro-ceramic particles are used to improve the yield and ultimate strength of the metal. However, the ductility of the MMCs deteriorates with high ceramic particle concentration. It is of interest to use nano-sized ceramic particles to strengthen the metal matrix, Y so-called metal matrix nano-composite (MMNC), while maintaining good ductility. G There is much interest in producing metal matrix nanocomposites that incorporate LO nanoparticles and nanotubes for structural applications, as these materials exhibit even greater improvements in their physical, mechanical and tribological properties as compared to O composites with micron-sized reinforcements. The incorporation of carbon nanotubes in N particular, which have much higher strength, stiffness, and electrical conductivity as compared to H metals, can significantly increase these properties of metal matrix composites. C It is well recognized that nanoparticles tend to agglomerate into large clusters during TE composite processing even under low loading levels of reinforcement. In this respect, appropriate processing procedures are needed to improve the dispersion of nanoparticles in matrix. Recently, O Yang et al. used high- intensity ultrasonic waves to assist the dispersion of SiC nanoparticles AN (average size 30nm) in molten aluminum alloy A356 to cast Al-based nanocomposite with better dispersion of ceramic nanoparticles. This technique yielded 50 % improvement in yield strength. N strength with 2 weight percentage of nanoparticles for a little change in elongation and ultimate Research on Metal Matrix Nanocomposites with ultrasonic assisted cavitation is still at its infancy. A few people are doing research on this area. In the works reported so far, only impact on tensile properties, hardness by varying weight/volume percent of SiC nanoparticles in alloys like A356 and magnesium alloys were studied. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 45 D.SUNEEL, PROFESSOR 3. Ceramic Matrix composites: Ceramic matrix composites are used in very high temperature environments. These materials use a ceramic as the matrix and reinforce it with short fibers, or whiskers such as those made from silicon carbide and boron nitride. CMCs have received a great deal of interest since the 1980s for their potential as hightemperature structural materials. This new class of composites has been the subject of many research programs aimed at ultimately producing components of turbine engines for aerospace and power generation and components for space applications and a variety of industrial uses. Y Currently, these materials are considered to be expensive relative to available alternatives. G Progress on development of design methods and evaluation of full-scale components has been LO slow. This may be due to the wide variety of chemistries for both matrices and reinforcements and also the process methods used for fabrication. Other factors limiting CMCs use are cost and Natural nanocomposites H 4. N O the poor oxidation resistance of interfacial coatings. C Self-organization and directed assembly of biological macromolecules and inorganic materials TE plays an important role in the creation of the nanostructured and nanocomposite materials so O commonly found in biology. Over the past 10 years, materials scientists and chemists have made AN considerable efforts to create synthetic analogs of biological materials by attempting to mimic biology and, importantly, to learn the design rules of biological systems. Considerable efforts N have also been made to use what has been learned from biology to create new materials with properties not found in biological systems. All this effort has been made because natural materials unquestionably have exquisite properties not found in synthetic materials. And additionally, biological systems can produce these exquisite materials at or near room temperature in aqueous environments, whereas most synthetic schemes that produce materials often inferior to natural biomaterials require elevated temperature and pressure and harsh chemicals. Biological nanocomposite materials can be entirely inorganic, entirely organic, or a mixture of inorganic and organic materials. Even where the final material may be entirely one class of a material, multiple classes of materials may have bee n involved in the synthetic process, DEPT OF MECHANICAL ENGG, KL UNIVERSITY 46 D.SUNEEL, PROFESSOR which may or may not remain in the final structure. A good example of a biological nanocomposite in which the organic material does not remain in the final product is the enamel of the mature human tooth, which is 95% by weight hydroxyapatite. During tooth formation, enamel consists of a composite of proteins and hydroxyapatite; however, the proteins are removed as the tooth develops. The presence of the proteins, and the self-assembled structures they form with other biological macromolecules, do however help generate the mineral cross-ply structure of the enamel, which plays a large part in its observed toughness. The best known example of an inorganic/organic structural composite or which both phases remain in the final product is the aragonitic nacreous layer of the abalone shell, which is exceptionally strong Y because of its organic/inorganic layered nanocomposite structure, in which crystalline ceramic LO G layers are separated by highly elastic organic layers. Many more examples of attempts to copy biology have not been successful in generating O engineering materials as one would have hoped. In hindsight, this is not surprising. Biological N materials generally form over a period of days to years, use a limited set of elements, and are H designed to be used within a limited temperature range. Practical engineering materials must be C made rapidly (hours or minutes) and generally must operate over a wide range of temperature TE and other environmental conditions. The disconnect between the needs of engineering materials and biological materials has led many scientists to conclude that, rather that attempting to O directly copy to biology, a much better philosophy is to learn from biology and use this AN knowledge to create synthetic materials. This may or may not involve the use of some biological N molecules, but no attempt is made to copy specific biological processes. In biology, examples abound of structures with nanoscale dimensions, and virtually all the functionality provided by these materials is a direct consequence of the nanoscale dimensions of the structure. A few examples of nanoscale materials in biology are lipid cellular membranes, ion channels, proteins, DNA, actin, spider silk, and so forth. In all these structures, the characteristic dimension, atleast in 1D, and often in 3D, is on the order of a few nanometers. Although a materials scientist does not typically consider such materials to be composites, in truth, the properties of many biological materials are driven by structure on the nanoscale and, in the sense that the larger material is composed of discrete nanoscale building blocks, most of these biological materials can be considered nanocomposite materials. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 47 D.SUNEEL, PROFESSOR Spider silk is one completely organic nanocomposite with outstanding properties. Dragline spider silk, which makes up the spokes of a spider web, is five times tougher than steel by weight and can stretch 30%-40% without breaking. Although the work to failure is greater for silk than steel, the elastic modulus of steel is significantly less than that of steel. For applications in which flexibility and toughness are the primary need, a synthetic route to creating a material with properties equivalent to spider silk would be exceedingly valuable. Spiders cannot be kept in close quarters and harvested, because they eat one another; thus the N AN O TE C H N O LO G Y only route to creating quantities of spider silk sufficient for application will be synthetic. DEPT OF MECHANICAL ENGG, KL UNIVERSITY 48

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