Important Engineering Materials

Transcription

1

2 Important Engineering Materials i) Nanomaterials: Structure, properties, applications of CNTs, Fullerenes, Graphite. ii) Liquid Crystals: Definition, classification, properties with applications. Nanomaterials: Materials whose sizes of individual building blocks are less than 100 nm, at least in one dimension are called nanomaterials. Creation of nanostructures, functional materials, devices and components through control of matter on the nanometer length scale is nanotechnology. Nanomaterials are defined as a set of substances where at least one dimension is < 100 nm. 1 nanometer = 10-9 meter; 1 billionth of a meter which is analogous to 10,000X smaller than the diameter of human hair. Nanomaterials are of interest because at this scale unique optical, magnetic, electrical, and other properties emerge. Three types of Nanomaterials are described: a) Zero dimensional: metallic, semiconducting and ceramic nanoparticles (Spheres and Clusters); b) One dimensional: nanowires, nanotubes, nanorods; c) Two dimensional: Thin films (plates, and networks); and 4) 3-D: Nanomaterials. Structural Features of nanomaterials: Nanomaterials have structural features in between of those of atoms and bulk materials. While most microstructured materials have similar properties to the corresponding bulk materials, the properties of materials with nanometer dimensions are significantly different from those of atoms and bulk materials. This is mainly due to the nanometer size of materials which render them: (i) large fraction of surface atoms; (ii) high surface energy; (iii) spatial confinement; and (iv) reduced imperfections Novel applications of nanomaterials rose from these properties as shown in the table below: Properties at nanoscale Higher surface to volume ratio with enhanced reactivity Increased hardness with decreasing grain size Narrower band gap with decreasing grain size Light in weight with great strength Applications Catalysis, solar cells & batteries Hard coatings & thin protection layers Electronics Sports goods 2) Structural features and properties of Nanomaterials (CNTs, Graphite, Fullerenes) A) Carbon nanotubes (CNTs): In 2004, Geim, Novoselov and co-workers delicately cleaved a sample of graphite with sticky tape. They produced something that was long considered impossible: a sheet of crystalline carbon just one atom thick, known as graphene. Single-layered honeycomb structure of graphene makes it the mother of all carbon-based systems: Graphite in our pencils is simply a stack of graphene layers; Carbon nanotubes are made of rolled-up sheets of graphene; and buckminsterfullerene molecules, or buckyballs, are nanometer size spheres of wrapped-up graphene. Honeycomb atomic structure 1

3 of graphene cause electrons moving in the material to behave as if they have no mass. Electrons in graphene move at an effective speed of light 300 times less than the speed of light in a vacuum. Electrons in graphene can travel large distances without being scattered, making it a promising material for fast electronic components. CNTs are rolled up crystalline sheets of graphene thousand times thinner than a human hair. Their large length (up to several microns) and small diameter (a few nanometers) result in a large aspect ratio. They can be seen as one-dimensional form of fullerenes. Therefore, these materials are expected to possess additional interesting electronic, mechanical and molecular properties. Single Walled Nanotubes (SWCNTs) are long wrapped graphene sheets. These nanotubes generally have a length to diameter ratio of 1000 so they can be considered as nearly onedimensional structures. Multi Walled Nanotubes (MWCNTs) consists of concentric SWCNTs with different diameters with an interlayer spacing of 3.4 A 0. Length and diameter of these structures differ a lot from those of SWCNTs and their properties are also very different. Properties of Carbon Nanotubes: 1. Atomic arrangement determines mechanical and electronic properties of CNTs. 2. They have outstanding electrical properties surpassing standard conductors & semiconductors 3. Tubes with helical twists in their structures have semiconducting properties; achiral tubes are metallic. 2

4 4. CNTs have structural & electronic defects which allow SWCNTs to act as transistors. Conductivity measurements on aligned MWCNTs show that the material behaves as a nanoconductor. 5. They are so strong that it can act as satellite tethers & space elevators. 6. Exhibit superconductivity below 20 0 C. CNTs are extraordinarily flexible & do not break upon mechanical deformation. B) Graphite: Graphite is a stack of graphene layers. Graphite consists of network of hexagonal carbon rings arranged over each other held by Vander Waals forces separated by 3.35 Å, C-C distance is 1.42 Å. Each sp 2 hybridized carbon atom is linked by covalent bonds to three other carbon atoms. Distance from fourth carbon is more resulting in flexible fourth valency, thereby weakening the bonds between different layers. This results in soft and lubricating property of graphite. Properties: High electrical conductivity, High strength, chemically stable, High thermal conductivity, High resistance to thermal shock, Good lubricant. Applications: i) It is used in heating elements for electrical furnaces high temperature refractories and insulators, in chemical reactor vessels. ii) It can be used as electrode for arc welding, in metallurgical crucibles, in casting moulds for metal alloys and ceramics. iii) It is used for electrical contacts and resistors. iv) It can be used as electrodes in batteries and in air purification devices. C) Fullerenes: Allotrope of carbon, which is conceptually graphene sheet rolled into spheres called as fullerene. It is named after architect Buckminster Fuller resembling geodesic domes, also known as bucky ball. It consists of perfect hollow spherical cages of 60 carbon atoms arranged in interlocking 20 hexagons & 12 pentagons. Number of hexagonal faces can vary. Each carbon is bounded to their other carbons in pseudo-spherical arrangement consisting of alternating pentagonal and hexagonal rings similar to a soccer ball as shown in the figure. Properties: Fullerenes are extremely strong, able to resist great pressures-- they bounce back to original shape after subjected to extreme pressures (< 3000 Atm). Fullerenes do not bond to each other chemically rather they stick together thro' weaker Vander-Waals forces. They exhibit superconductivity & ferromagnetic-- Intercalation of alkali metal atoms leads to its metallic behavior. They are aromatic molecules which are stable yet not totally unreactive. Fullerenes are sparingly soluble in most solvents giving marvellous colors. 3

5 Applications: i) Fullerenes & related substances have shown considerable potential as catalysts e.g, Conversion of ethylbenzene into styrene. ii) Fullerene can be used in LEDs in different electronic equipments and computing. iii) It is used as rocket fuel. iv) Due to its non-reactive behavior, radioactive material can be introduced inside, thus avoiding dangerous leaks. v) Ultra-thin layers of fullerenes act as data storage devices in flexible organic solar cells, photodetectors for X-rays. Applications of Nanomaterials in Medicine and Catalysis Medicine: 1. Nanorobots carry out a very specific function and are just several nanometers wide. Nanorobots can also be used to prevent heart-attacks. 2. Quantum dots are nanomaterials that glow brightly when illuminated by UV light. Quantum dots bind themselves to proteins unique to cancer cells, literally bringing tumors to light and killing it. 3. Ferromagnetic nanoparticles have been developed and optimized for targeted delivery of therapeutic drugs, genes or radionuclides. Catalysis: Nanomaterial-based catalysts are heterogeneous catalysts. Nanomaterials are more effective than conventional catalysts due to their extremely small size (10-80 nm) thereby having a huge surface area-to-volume ratio. 1. One dimensional nanomaterials like nanowires, nanotubes, nanorods and nanocubes exhibit excellent catalytic activity. 2. Nano-TiO 2 in photocatalysis, the one containing more defects exhibits higher photocatalytic activity. Liquid Crystals: The study of liquid crystals began in 1888, when an Austrian botanist named Friedrich Reinitzer observed that the material known as cholesteryl benzoate had two distinct melting points. He increased the temperature of a solid sample and watched the crystal change into a hazy liquid. As he increased the temperature further, material changed again into clear transparent liquid. Reinitzer is credited with discovering a new phase of matter- liquid crystal phase. Cholesteryl benzoate (C 6 H 5 COOC 27 H 45 ) when heated undergoes two sharp phase transformations one after the other. It fuses sharply at 145 C to give a turbid liquid which on further heating changes suddenly in to clear liquid at 178 C. These changes reversed on cooling. 145 C 178 C p-cholesteryl benzoate p-cholesteryl benzoate p-cholesteryl benzoate (solid) (liquid crystal) (liquid) Mesomorphic state This turbid liquid show anisotropy (direction dependent-tendency to point along a common axis-properties of material depends on direction in which they are measured). True liquid, on the contrary are isotropic. Since anisotropic properties are associated with crystalline state, the turbid liquids are called Liquid crystals. Liquid crystal is an intermediate state of matter, in between the liquid & a crystal. It must possess some typical properties of a liquid (eg, fluidity, formation & coalescence of droplets) as well as crystalline properties (anisotropy in optical, electrical, magnetic properties, periodic arrangement). Temperature is a measure of randomness of the molecules and therefore the 4

6 higher the temperature, lesser the order exists and increasing temperature will cause transition from solid to liquid state through the intermediate liquid crystal state. A liquid crystal may flow like a liquid, but have the molecules arranged and/or oriented in a crystal-like way. When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have a distinct texture. Liquid crystal materials have several common characteristics. Among these are rod-like molecular structure, rigidness of the long axis, and strong dipoles and/or easily polarizable. Liquid Crystal Phases (a) Positional order (whether molecules are arranged in any sort of ordered lattice) and (b) Orientational order (whether molecules are mostly pointing in the same direction). Mesogen:It is the fundamental unit of a liquid crystal that induces structural order in the crystals. Liquid crystals (LCs) are orientationally ordered liquids or positionally disordered crystals that combine the properties of both crystalline (optical and electrical anisotropy) and liquid (molecular mobility and fluidity) states. Classification of Liquid Crystals: (A) Thermotropic LCs :Thermotropic LCs exhibit a variety of phases (smectic or nematic) as temperature is changed. 5

7 At high temperatures, thermal motion destroys delicate cooperative ordering, pushing the material into a conventional liquid phase. At much low temperature, most LC materials will form a crystal. Eg, p-azoxy anisole (B) Lyotropic LCs: They exhibit phase transitions as a function of concentration of the mesogen in a solvent (typically water) as well as temperature. It consists of a flexible hydrophobic chain (the tail) and a polar, hydrophilic (ionic or non-ionic) head group Amphiphilic molecules. Liquid crystals which are prepared by mixing two or more substances, of which one is a polar molecule, are known as lyotropic liquid crystals. Eg. Soap in water, biological and cell membranes Smectic LCs: Smectic liquid crystals have layered structure but a variety of molecular arrangements are possible within each layer. Inter layer attractions are weak as compared to lateral forces of attraction between molecules. When a stress is applied or allowed to flow, layers slide over one another like soap (e.g. Ethyl p-azoxy benzoate) but still retain their parallelism. There is a very large number of different smectic phases, all characterized by different types and degrees of positional and orientational order. In the Smectic A phase, mesogen are oriented along the normal layer In the Smectic C phase they are tilted away from the layer. 6

8 Nematic LCs: Thread-like, parallel or nearly parallel arrangement to each other along the axis. They are mobile in 3 directions & rotate in one direction. Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field. An aligned nematic has the optical properties of a crystal and this makes them extremely useful in liquid crystal displays (LCD). On heating they loose periodicity and long range order, retain orientation. The molecules can move parallel to each other. (Smectic LCs move in layers). Nematic liquid crystal have more fluidity than smectic types. When these crystals viewed along lines of force in a strong magnetic field, the turbidity disappears and if field is removed they appear again turbid. e.g. p-azoxy phenetole ( C), dibenzal benzidine ( C). Rod like molecules tend to align parallel to each other with their long axes all pointing roughly in same direction. Fluidity of mesophase is due to the ease with which the molecules can slide past one another and still remaining parallel. It has dielectric anisotropy i.e. different dielectric constant in the different direction of orientation. Cholesteric LCs: This phase is called the cholesteric because it was first observed for cholesterol derivatives These are optically active and similar to nematic kind in arrangement but show strong colour effect in polarised light. Optical activity of these crystal is many times higher than of its solid crystalline variety. As in nematic phase, there is no long range order in cholesteric phase. Only those chiral molecules that lack inversion symmetry, can give rise to such a phase. Molecules are twisted about an axis. The twist may be right / left handed depending upon the conformation. 7

9 Liquid Crystalline Behavior and Chemical Structure Molecular structures play an important role in determining the phase, transition temperatures, optical and electro-optical properties of liquid crystals. Liquid crystal structure have weak intermolecular forces of attraction, hence when an electric field is applied, they exhibit various patterns and textures. In general, liquid crystals have chemical structure represented as R is the side chain group, Z is the linking group and X is terminal group. R: It can be alkyl, alkoxy or alkenyl groups. The length and flexibility of side chain affect the phase transition temperature and the type of liquid crystal phase. If number of carbon atoms are 3 to 7, nematic phase occurs. If carbon atoms are 8 or higher in R, then smectic phase appears. A and B : Aromatic ring A and B may be same or different. The substitution over the rings by CN, F, Cl polar groups change the dielectric properties of liquid crystals. Z : Linking group makes contribution to phase transition temperature and other physical properties. Linking group can be like, Ester ( CO O ), Ethylene ( CH 2 CH 2 ), azo N = N etc. The linking groups like N = N, CH = CH help for delocalization or resonance at longer length and the electronic transitions take place at longer wavelength. X : Terminal group X, contributes to dielectric anisotropy. The X may be like CN, OR, R, CNO, CF 3, Cl etc. The stronger electron attracting ( I effect) groups increase the dielectric property to larger extent. 8

10 Applications of Liquid Crystals: 1. Orientation of nematic liquid crystal is easily changed by electric field or pressure and the changed orientation have different light transmission and reflection. 2. When an electric field is applied, on a thin LC film with the help of electrodes, the patterns of molecules becomes visible. This principle is used for in LCD in calculators, reading displays, computer and mobile screen etc. 3. Cholesteric liquid crystals are used for detecting tumors in human body. 4. Liquid-crystals are used as solvents for spectroscopic study of anisotropic solids. 5. Liquid crystal memory units with extensive capacity were used in Space Shuttle navigation equipment. 6. Recording /Sensing temperature changes: Thermotropic chiral LCs whose pitch varies strongly with temperature can be used as crude thermometers, since colour of the materials will change as the pitch is changed. Liquid crystal color transitions are used on many aquarium and pool thermometers. 7. Liquid crystal materials change color when stretched or stressed. Thus, liquid crystal sheets are often used in industry to look for hot spots, map heat flow, measure stress distribution patterns, and so on. 9

11