1 physical properties
1.1 mechanical
1.2 thermal
1.3 electrical
1.3.1 electro-mechanical
1.4 optical
1.4.1 opto-electronic
physical properties
physical properties of elements , compounds provide conclusive evidence of chemical composition include odor, color, volume, density (mass per unit volume), melting point, boiling point, heat capacity, physical form , shape @ room temperature (solid, liquid or gas; cubic, trigonal crystals, etc.), hardness, porosity, index of refraction , many others. section discusses physical properties of materials in solid state.
mechanical
granite rock formation in chilean patagonia. inorganic minerals formed oxidation in earth s atmosphere, granite consists of crystalline silica sio2 , alumina al2o3.
the mechanical properties of materials describe characteristics such strength , resistance deformation. example, steel beams used in construction because of high strength, meaning neither break nor bend under applied load.
mechanical properties include elasticity , plasticity, tensile strength, compressive strength, shear strength, fracture toughness, ductility (low in brittle materials), , indentation hardness. solid mechanics study of behavior of solid matter under external actions such external forces , temperature changes.
a solid not exhibit macroscopic flow, fluids do. degree of departure original shape called deformation. proportion of deformation original size called strain. if applied stress sufficiently low, solid materials behave in such way strain directly proportional stress (hooke s law). coefficient of proportion called modulus of elasticity or young s modulus. region of deformation known linearly elastic region. 3 models can describe how solid responds applied stress:
elasticity – when applied stress removed, material returns undeformed state.
viscoelasticity – these materials behave elastically, have damping. when applied stress removed, work has done against damping effects , converted heat within material. results in hysteresis loop in stress–strain curve. implies mechanical response has time-dependence.
plasticity – materials behave elastically when applied stress less yield value. when stress greater yield stress, material behaves plastically , not return previous state. is, irreversible plastic deformation (or viscous flow) occurs after yield permanent.
many materials become weaker @ high temperatures. materials retain strength @ high temperatures, called refractory materials, useful many purposes. example, glass-ceramics have become extremely useful countertop cooking, exhibit excellent mechanical properties , can sustain repeated , quick temperature changes 1000 °c. in aerospace industry, high performance materials used in design of aircraft and/or spacecraft exteriors must have high resistance thermal shock. thus, synthetic fibers spun out of organic polymers , polymer/ceramic/metal composite materials , fiber-reinforced polymers being designed purpose in mind.
thermal
normal modes of atomic vibration in crystalline solid.
because solids have thermal energy, atoms vibrate fixed mean positions within ordered (or disordered) lattice. spectrum of lattice vibrations in crystalline or glassy network provides foundation kinetic theory of solids. motion occurs @ atomic level, , cannot observed or detected without highly specialized equipment, such used in spectroscopy.
thermal properties of solids include thermal conductivity, property of material indicates ability conduct heat. solids have specific heat capacity, capacity of material store energy in form of heat (or thermal lattice vibrations).
electrical
video of superconducting levitation of ybco
electrical properties include conductivity, resistance, impedance , capacitance. electrical conductors such metals , alloys contrasted electrical insulators such glasses , ceramics. semiconductors behave somewhere in between. whereas conductivity in metals caused electrons, both electrons , holes contribute current in semiconductors. alternatively, ions support electric current in ionic conductors.
many materials exhibit superconductivity @ low temperatures; include metallic elements such tin , aluminium, various metallic alloys, heavily doped semiconductors, , ceramics. electrical resistivity of electrical (metallic) conductors decreases gradually temperature lowered, remains finite. in superconductor however, resistance drops abruptly 0 when material cooled below critical temperature. electric current flowing in loop of superconducting wire can persist indefinitely no power source.
a dielectric, or electrical insulator, substance highly resistant flow of electric current. dielectric, such plastic, tends concentrate applied electric field within property used in capacitors. capacitor electrical device can store energy in electric field between pair of closely spaced conductors (called plates ). when voltage applied capacitor, electric charges of equal magnitude, opposite polarity, build on each plate. capacitors used in electrical circuits energy-storage devices, in electronic filters differentiate between high-frequency , low-frequency signals.
electro-mechanical
piezoelectricity ability of crystals generate voltage in response applied mechanical stress. piezoelectric effect reversible in piezoelectric crystals, when subjected externally applied voltage, can change shape small amount. polymer materials rubber, wool, hair, wood fiber, , silk behave electrets. example, polymer polyvinylidene fluoride (pvdf) exhibits piezoelectric response several times larger traditional piezoelectric material quartz (crystalline sio2). deformation (~0.1%) lends useful technical applications such high-voltage sources, loudspeakers, lasers, chemical, biological, , acousto-optic sensors and/or transducers.
optical
materials can transmit (e.g. glass) or reflect (e.g. metals) visible light.
many materials transmit wavelengths while blocking others. example, window glass transparent visible light, less of frequencies of ultraviolet light cause sunburn. property used frequency-selective optical filters, can alter color of incident light.
for purposes, both optical , mechanical properties of material can of interest. example, sensors on infrared homing ( heat-seeking ) missile must protected cover transparent infrared radiation. current material of choice high-speed infrared-guided missile domes single-crystal sapphire. optical transmission of sapphire not extend cover entire mid-infrared range (3–5 µm), starts drop off @ wavelengths greater approximately 4.5 µm @ room temperature. while strength of sapphire better of other available mid-range infrared dome materials @ room temperature, weakens above 600 °c. long-standing trade-off exists between optical bandpass , mechanical durability; new materials such transparent ceramics or optical nanocomposites may provide improved performance.
guided lightwave transmission involves field of fiber optics , ability of glasses transmit, simultaneously , low loss of intensity, range of frequencies (multi-mode optical waveguides) little interference between them. optical waveguides used components in integrated optical circuits or transmission medium in optical communication systems.
opto-electronic
a solar cell or photovoltaic cell device converts light energy electrical energy. fundamentally, device needs fulfill 2 functions: photo-generation of charge carriers (electrons , holes) in light-absorbing material, , separation of charge carriers conductive contact transmit electricity (simply put, carrying electrons off through metal contact external circuit). conversion called photoelectric effect, , field of research related solar cells known photovoltaics.
solar cells have many applications. have long been used in situations electrical power grid unavailable, such in remote area power systems, earth-orbiting satellites , space probes, handheld calculators, wrist watches, remote radiotelephones , water pumping applications. more recently, starting used in assemblies of solar modules (photovoltaic arrays) connected electricity grid through inverter, not act sole supply additional electricity source.
all solar cells require light absorbing material contained within cell structure absorb photons , generate electrons via photovoltaic effect. materials used in solar cells tend have property of preferentially absorbing wavelengths of solar light reach earth surface. however, solar cells optimized light absorption beyond earth s atmosphere well.
Comments
Post a Comment