1 Introduction to Materials Chemistry Module 06517 Prof. Paul Fletcher Chemistry Building C216 Tel: 5433 E-mail: P.D.Fletcher@hull.ac.uk Lectures: LRC, Fridays 12.15 for weeks 7 and 8 Assessment Essay handed out by Prof. Kelly Notes: Re-written from originals by Dr. in het Panhuis, see Dept. website for electronic copy Text W.D. Callister, Materials Science and Engineering, 6th Ed., Wiley, 2003. (Brynmor Jones Library TA403 C1) Materials: composed of matter or concerned with matter. Materials Chemistry fits into Materials Science which operates at the interfaces of Engineering, Physics, Chemistry and Biology • Introduction to Materials Chemistry • Materials Chemistry in the world around us • Mechanical, electrical and optical properties of materials • Some new Materials Introduction to concepts Current and future applications Topics and learning objectives2 1. What is Materials Chemistry? • Materials are probably more deep-seated in our culture than most of us realise. Transportation, housing, clothing, communication, recreation and food production – virtually every segment of our everyday lives is influenced to one degree or another by materials. • The development of many technologies that make our existence so comfortable has been intimately associated with the accessibility of suitable materials. An advancement in the understanding of a material type is often the forerunner to the stepwise progression of a technology into a product on the market. • The discipline of Materials Science or Materials Chemistry in the context of these two lectures involves investigating the relationship that exists between the structures and properties of materials. • The structure of a material relates to the arrangement of its internal components; molecules, atoms, electrons. • The properties of a material relate to their response to an external stimulus. For example a material subjected to forces will experience deformation, or a polished surface might reflect light. Most properties of (solid) materials can be grouped into six different categories: • mechanical: strength, modulus • electrical: conductivity • optical: refraction, reflectivity • thermal: heat capacity, thermal conductivity • magnetic: response to magnetic field • deteriorative: chemical reactivity of materials, corrosion It is important to realise that the properties of materials depend on its structure, which in itself depends on the processing of materials. The relationship between processing and performance is as follows: The performance of material depends on the properties of the materials. Thus for materials with multifunctional application such as polymers, different processing techniques can be used to achieve different performance and different applications with the same material. For example, think of the differences in processing when using polymers to make plastic boxes and to make a sensor. This completes learning objective: 1. What is Materials Chemistry?3 2. Materials Chemistry around us The house of the future •Glazing and skylights •Cold lighting systems •Paints, coatings and textiles •Energy sources •Heating and cooling systems •Better surfaces Assessment essay (1000-1500 words, include proper referencing) “Describe how new materials might improve the house of the future, in terms of performance, cost and environmental impact. Include in your account at least two examples of materials for which you compare numerical values for relevant properties of current and new materials.” Medical materials, molecular electronics, space exploration, new battlefield materials, etc. This completes learning objective: 2. Materials Chemistry around us.4 3. Some Basic Materials Properties Mechanical properties Many materials, when in service, are subjected to forces or loads; examples include the aluminium alloy from which an airplane wing is constructed and steel in an automobile axle. In such situations it is necessary to know the characteristics of the material and use this in the design of the material to avoid fracture of the material under deformation. The mechanical behaviour of a material reflects the relationship between its response or deformation to an applied load or force. Important mechanical properties are strength, hardness, and stiffness. Now we introduce how loading a material causes deformation and strain and explain some important relationships which allows us to define mechanical properties. A useful unit is the pascal (Pa). 1 Pa = 1 N m-2 Tensile strain Compressive strain Shear strain Torsional strain5 Tensile stress and strain (also works for compressive stress) The applied load F is applied perpendicular to the specimen cross section. Engineering stress = F /A0 units = Pa = N m-2 Engineering strain = (– )/unitless For low strain (less than about 0.005 for many metals), materials are perfectly elastic, i.e. stress and strain are proportional to each other according to Hooke’s Law. = E The constant of proportionality E is the modulus of elasticity or Young’s modulus and may be thought of as stiffness or a material’s resistance to elastic deformation. The greater the modulus, the stiffer the material. Typical metals have values of E between 45 GPa for magnesium to 407 GPa for tungsten. Elastic deformation is non-permanent which means that, when the applied load is released, the piece returns to its original shape. At higher strain, where elastic deformation ends, plastic deformation or yielding begins. Plastic deformation is permanent, which means when the applied load is released the piece does not return to its original shape. After yielding, the stress necessary to continue plastic deformation increases to a maximum, and then, as the sample piece thins, decreases to a point of fracture. The stress at the maximum on the stress-strain curve is called the tensile strength of the material6 Poisson’s ratio Under tensile load, the sample stretches along the direction of the force (the axial direction) and shrinks in the lateral direction. Poisson’s ratio v is the negative ratio of the lateral and axial strains v = -elateral/eaxial Theoretically, v = ¼ for isotropic materials. Shear stress and strain The applied load F is applied parallel to the specimen cross section. Shear stress t = F /A0 units = Pa = N m-2 Shear strain g = tan q unitless For low strain (less than about 0.005 for many metals), materials are perfectly elastic, i.e. shear stress and shear strain are proportional to each other according to t = G g The constant of proportionality G is the shear modulus and is another measure of stiffness or a material’s resistance to elastic deformation. Other points •Torsion is a variation of pure shear. •Shear and elastic moduli and Poisson’s ratio are related for isotropic materials E = 2G(1+ v) and so G = 0.4 E theoretically •Single crystal solids with non-cubic symmetry are anisotropic and will have different elastic constants along different crystal planes. Polycrystalline materials with random grain orientation and glasses will behave isotropically. •Low strain (< about 0.5% change in dimensions) and reversible, elastic behaviour corresponds to bond stretching. Irreversible, plastic yielding occurs when bonds break.7 From W.D. Callister, Materials Science and Engineering The straight lines correspond to assuming that the tensile strength is proportional to (density)3/2 Optical properties Materials can interact with electromagnetic radiation (covering radio, micro-, infra-red, visible, UV, X-ray and higher energy radiation) by refraction, reflection, diffraction, scattering and absorption. Two of the main optical properties governing these processes are: Refractive index Light travels more slowly in matter than in vacuum. The refractive index n is the ratio n = cvacuum/cmatter •Light refraction in going from one phase to another is controlled by the difference in n according to Snell’s Law. Varies with wavelength to give rainbows •The amount of light reflection at a surface is proportional to (difference in n)2. •The amount of light scattering by a particle immersed in a medium is proportional to (difference in n)2 Material Refractive index for visible light vacuum 1 (exactly) air 1.00 (tiny fraction more than 1) Water 1.33 glass 1.45-14.8 diamond 2.42 silicon 3.5 8 Very high n values (e.g.10 with microwaves) make some surprising things theoretically possible Light absorption occurs when the energy of the incoming photon matches an energy level transition in the material. When incident light of intensity I0 enters an absorbing material, the intensity I decreases exponentially according to Lambert’s Law (half of the Beer-Lambert Law for absorbing solutions). I = I0 exp(-ax) where a is the extinction or absorption coefficient (equal to absorbance in natural log units per unit path length through the material) and x is the optical path length. The usual units of a are m-1 or cm-1. Optical absorption goes from nothing to high at a wavelength such that the light energy first exceeds the band gap in the semiconductor.9 Electrical properties One of the most important electrical characteristics of a solid material is the ease with which it transmits an electric current. Ohm’s law relates the current I to the applied voltage V according to V = I R, where R is the resistance of the material through which the current is passing. The electrical resistivity of a material is related to the resistance and the geometry of the material according to: = RA /L where L is the distance between the two points at which the voltage is measured and A is the cross-sectional area perpendicular to the direction of the current. The units of resistivity are Ohm-meters (m). Sometime the electrical conductivity k is used to specify the electrical character of a material; it is simply the reciprocal of the resistivity = 1 /and is indicative of the ease with which a material is capable of conducting an electric current. The units are -1 m-1 or Siemens per meter S m-1. •Good conductors typically have conductivities of the order of 107 -1 m-1 •Insulators have values between about 10-10 and 10-20 -1 m-1. •Semi-conductors have values between about 10-6 and 104 -1 m-1. This completes learning objective: 3. Some Basic Material Properties (Mechanical, Optical and Electrical) We have covered only the tip of the iceberg in these lecture. Remember! 1. There are other important material properties: •thermal: heat capacity, thermal conductivity •magnetic: response to magnetic field •deteriorative: chemical reactivity of materials, corrosion 2. The processing of materials can strongly affect their properties.10 4. Some new materials and potential applications In order to displace an existing material (or product) in its market a new material should either have a unique function or it should have a high (at least 10x typically) performance advantage. For example, “write-once” CDs based on a laser bleachable dye layer, have been largely superseded by re-writable CDs based on laser triggered phase change between amorphous and crystalline forms of a germanium-antimony-tellurium material (see www.ovonic.com and related links). There is also growing interest in Smart and/or Intelligent Materials. Such materials have increased functionality compared with passive materials such as wood or metals. They may be externally activated in some way or be able to sense changes in their environments and may also then be able to respond to these changes in predetermined manners. For example, electro-or photo-triggered actuators based on polymer materials have been demonstrated. We will continue our discussion with a look towards new and future materials; in particular we will consider materials based on electroactive polymers and carbon nanotubes. Electroactive polymers have been an area of intense interest over the past 30 years, culminating with the award of 2000 Nobel Prize in Chemistry to Alan MacDiarmuid, Alan Heeger and Hideki Shirakawa (the three scientists who discovered polyacetylene, the first conductive polymer). Carbon nanotubes have been an area of intense interest over the past 14 years. They can be classified as fullerenes (an allotrope of carbon), and Robert Curl, Harry Kroto and Richard Smalley were awarded the 1996 Nobel Prize in Chemistry for their discovery of C60 fullerene. R. Curl H. Kroto R. Smalley11 Electroactive polymers Most polymers are electrical insulators. However, some polymers with extended delocalised bonds can have energy levels like a semi-conductor with valence bands and conduction bands separated by a band gap. Adding charge carriers into the conduction or valence bands increases the electrical conductivity dramatically. This is done in silicon by pdoppin (removing an electron from the valence band or ndoppin (adding electrons to the conduction band. In addition to charge-transfer complexes, conductive polymers include polyacetylene, polyaniline, polypyrrole,polythiophene and their derivatives. The biological pigment malanin is a mixture of some of these polymers. Polacetylene ..C=C-C=C-C=C-…. Oxidation with iodine (p-doping): [CH]n + 3x/2 I2 ® [CH]nx+ + x I3-Reduction with alkali metal (n-doping): [CH]n + x Na ® [CH]nx-+ x Na+ Reduction (n-doping) is not used because air will oxidise it back again. p-doping can increase the conductivity by a factor of up to 100,000,000 from a typical semiconductor value to metallic levels.12 Some applications of conductive polymers. In electroluminescence, light is emitted from a thin layer of the polymer when excited by an electrical field. In photodiodes inorganic semiconductors such as gallium phosphide are traditionally used, but now one can also use semiconductive polymers. Electroluminescence from semiconductive polymers has been known for about ten years. Today there is extensive commercial interest in photodiodes and in light-emitting diodes (LEDs). A LED can consist of a conductive polymer as an electrode on one side, then a semiconductive polymer in the middle and, at the other end, a thin metal foil as electrode. When a voltage is applied between the electrodes, the semiconductive polymer will start emitting light. The process can also be reversed to make devices which produce electricity when illuminated -photovoltaic devices Other applications -current and potential Electrochromics. Polymers such as polythiophene can be switched from red (oxidised) to blue (reduced), while polyaniline transcends a spectrum of colours as different potentials are applied to it to oxidise it to different extents. This has possible applications in advertising displays, smart windows, etc. (useful for the House of the Future). Antistatic films -uses conducting property Corrosion inhibitor films Polymer electronic circuit components -e.g.transistors printed using inkjet printers13 Single-walled carbon nanotubes (SWNT) consist of a single graphite sheet seamlessly wrapped into a cylindrical tube. Double walled (DWNT) and multi-walled carbon nanotubes (MWNT) comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk. Carbon Nanotubes Carbon nanotubes are usually made by carbon-arc discharge, laser ablation of carbon, or chemical vapour deposition. Nanotube diameters range from ~ 0.4 to > 3 nm for SWNT and from ~1.4 to at least 100 nm for MWNT. Nanotubes are normally mixtures of metallic and semiconducting tubes. It is helpful to imagine how we would have to roll up a sheet of graphite if we were to transform it into such a tube. The sheet of graphite has rows of conjoined hexagons, separated by horizontally running zigzza lines. In the vertical direction, however, the hexagons are zig-zagging, and their arrays are separated by lines that are called armchair lines for obvious reasons. When the sheet is rolled up in the horizontal direction (along the zig-zag line, such that the cylinder axis is parallel to the armchair lines), the majority of the tubes will be metallic. If, in contrast, they are rolled up in a different direction (all angles are possible), neither parallel to a zig-zag nor to an armchair line, the tubes are most likely to be semiconducting. These tubes will also be chiral, because the zig-zag line will wind around the cylinder axis as a helix, which can be left-or righthannded Nanotubes come in bundles which can be hard to separate.14 Mechanical Properties (reinforcement) Carbon nanotubes have exceptional mechanical properties, they are extremely strong and flexible due to sp2 bonded carbon. The density normalised values for SWNT are even more impressive: E.g. Young’s modulus is 20× and tensile strength 56× that of steel wire, respectively. This allows for the fabrication of exceptionally strong and lightweight materials, e.g. “space elevator”. BMC bike frame reinforced with CNTs weighs less than 1 kg. Other potential and actual applications of CNTs. Super-strong fibres have been made using a spinning process involving carbon nanotubes, surfactant and polyvinylalcohol. These composite fibres are as long as 100 m (diameter 50 m), contain 60% SWNT and have atensile strength of about 1.8 GPa, which is similar to diamond and spider silk, but less than the calculated value (37 GPa). From Dalton et al, Nature 2003. Electronic components -unique electrical properties including conductivity to semiconductor behaviour and the highest current carrying capacity of any material. Currently, still no good way to manipulate CNTs to make e.g. “nanowires”. Applications of thermal properties. CNTs have very high thermal conductivity along the tube but are thermal insulators across the tube. Applications other than reinforcement applications are currently blocked by the lack of methods to make pure CNTs in bulk..15 This completes learning objective: 4. Some new materials -current and future applications We have covered only the tip of the iceberg in these lectures. Now start to think how you might use these ideas in preparing your essay assessment. Some additional reading • See Wikipedia articles on Carbon nanotubes and conducting polymers • http://nobelprize.org for 2000 for good article on conducting polymers • http://webpages.charter.net/dmarin/coat/for article on conducting polymer coatings • www.ovonic.com Interesting website of high-tech invention company • http://www.housesofthefuture.com.au/Start here for your assessment essay.