NMC NOTES Dillon Pretorius Disclaimer: This is a summary of the theory. There are no diagrams or formulae in this. Go learn those. I might have also left out some detail you might need, or done-goofed and got something wrong, so probably a good idea to sit with the class notes parallel to this. INTRODUCTION TO MATERIALS AND FAMILIES OF MATERIALS 1. 2. 3. 4. 1. 2. 3. 4. 5. Mechanical Properties Elastic Modulus (E) – Stiffness Yield Strength (Pa) – Stress level before permanent deformation Tensile Strength (Pa) – Highest stress before necking (breaking) Fracture Toughness (KIc) – Resistance to fracturing/cracking Thermal Properties Maximum service temperature (Tmax) – Limiting temperature Thermal expansion coefficient (α) – Change in size per change in temperature Thermal conductivity (λ) - Rate of heat flow Heat Capacity (J/K) – Heat taken to raise temperature Thermal diffusivity (m2/s) – Measure of heat transfer when temperature isn’t steady-state. Classifying Materials 1. Metals – Stiff, tough, high fracture toughness, reactive 2. Ceramics – Non-metallic inorganic solids. Stiff, hard, abrasion resistant, insulators, fracture easily. Glasses are transparent and corrosion resistant. 3. Polymers – Organic solids. Low density, high elasticity, and easy to shape. 4. Composites – Combination of materials. Expensive and difficult to form. Either matrix (continuous) or dispersed phase (particulates, fibres) 5. Advanced materials – Above, manipulated for specific application. Process and Classification • • • • Shaping, joining and surface treatment (finishing) Choice of material limits choice of process. Primary process (Creates shapes) ◦ Casting ◦ Molding Deformation o o • o o • • • • Powder methods Composite forming Special methods Secondary process (modify shapes and properties) Machining Heat treatment Joining Surface treatment Processes affect and interact with material properties The Structure of Crystalline Solids Space Lattice and Unit Cells • • • Crystal Structure – Atoms or ions arranged in repeating 3D pattern. Space Lattice – Points creating a network of lines in 3D. Every point is centre of repeating pattern, with identical surroundings. Unit Cell – Smallest “building block” of space lattice. ◦ Lattice constants (axial lengths and inter-axial angles) define size and shape of cell. ◦ Relationship between parameters define cell geometries. Principal Metallic Crystal Structures • • • • • • • Metals – simple and densely packed structures ◦ Same size and electropositive nature of metals. Important Crystal Structures ◦ FCC (Face-Centered Cubic) – Atom at each face of cube ◦ BCC (Body-Centered Cubic) – Single atom in centre ◦ HCP (Hexagonal Close-Packed) - Hexagonal unit cell. Two hexagonal planes, and one triangular plane. ◦ BCT (Body-Centered Tetragonal) Coordination Number - Number of nearest neighbours to each atom in lattice. (Eg. FCC: CN=12) Atomic Packing Factor (APF) – Volume of atoms per unit cell. Indication of packing density. Density Polymorphism and Allotropy – Having more than one crystal structure, depending on temperature and external pressure. Crystal Systems – Classification based on geometry of unit cell (lattice parameter relationships). Closed-packed Crystal Structures • • • Density of 0.74 – most efficient. HCP – ABABABAB FCC – ABCABC Solidification and Crystalline Imperfections in Solids Solidification of Metals • • • Many small nuclei form. Each separate crystal represented by a grain in the material. Lattice orientations of crystals differ, forming grain boundaries. Forms polycrystalline structure (large number of grains) Solidification of Single Crystals • • Electronic components ◦ Eg. Transistors from semiconducting ionic materials. ◦ Eg. Silicon wafers for microchips. Grown using Chzokralski process ◦ Seed crystal in contact with molten metal. ◦ Crystal slowly withdrawn with rotating movement. ◦ Careful control of temperature required. Crystalline Imperfections • • • Affects following properties ◦ Mechanical ◦ Ductility ◦ Electrical and heat conductivity ◦ Atomic Diffusion ◦ Corrosion Resistance Typical Imperfections ◦ Micro defects ◦ Macro defects Micro defects ◦ Point defects ▪ Vacancy – Missing atom ▪ Self-interstitial defect – Extra atom (In Ceramics, which are composed of larger Anions and smaller Cations, the Anion interstitials rarely form.) ▪ Schottky defect – Vacancy of both cation and anion in ceramic (Maintains electroneutrality) ▪ Frenkel defect – Cation moved from usual position (Maintains electroneutrality) ▪ Impurity Metallic solid solution – alloying element dispersed within structure of parent metal. Single phase microstructure. ◦ Most engineering materials composed of more than single metal – Alloys ◦ Alloy ▪ Primary/Parent metal (solvent) ▪ Minority metal(s)/non-metal(s) or Alloying elements (solute) ◦ Substitutional Solid Solution ▪ Some of parent metal atoms substituted by alloying element atoms. ▪ Crystal structure remains the same, but distorted. This increases strength and hardness. ▪ Solubility limit – Maximum atomic or mass % of alloying element which can be dissolved into lattice of parent metal while maintaining a single phase structure. ▪ Requirements for solid solubility • Atomic diameters cannot differ more than 15% • Same crystal structure of metals • Small differences in electronegativity • Same valence ◦ Interstitial Solid Solution ▪ Solute atoms much smaller than solvent ▪ Solute atoms fit into interstitial void. ▪ If void is smaller than solute atoms, hardening distortion occurs. ▪ Origination of point defects • Vacancies ◦ Solidification of material ◦ Material rapidly cooled (vibrating atoms caught off-position) ◦ Cold working ◦ Radioactive bombardment • Self-interstitial (does not regularly occur) ◦ Radioactive bombardment • Impurities ◦ Solid solution and diffusion of foreign atoms ◦ Line Defects (Dislocations) – only edge dislocations considered ▪ Formed in crystals • Solidification • Plastic deformation or cold working • Vacancy condensation ▪ Plastic deformation is facilitated by slip planes. – Movement of dislocations on close packed crystal planes. ◦ Surface Defects ▪ Grain boundaries • 2D imperfections in polycrystalline materials separating grains of different orientations. • Grain boundaries have higher energy than grains (caused by atomic mismatch). • Lower APF at boundaries due to mismatch. ▪ Twin boundary • Plain across which mirror image of structure is observed • Forms ◦ Plastic deformation (mechanical twins) ◦ Annealing/heating (annealing twins) Macro defects ◦ Cracks ◦ Pores or blow holes ◦ Inclusions (they’re impurities of slag or dross left in the metal after casting) • • Grain Size Measurement Important, as amount of grain boundary surface affects properties of material. Grain boundaries restrict plastic flow. Difficult for dislocations to move from one grain to another. N – number of grains per inch2 at 100X magnification n – ASTM grain size number (inversely proportional to grain size) A - area M - magnification Mechanical Properties of Metals Introduction • • • • Mechanical properties = Elastic and plastic reactions of material to force. ◦ Yield strength ◦ Toughness ◦ Tensile Strength ◦ Fatigue Strength ◦ Ductility ◦ Creep Resistance ◦ Hardness ◦ Stiffness They rely on internal atomic structure ◦ Type of atomic structure ◦ Crystal structure ◦ Dislocation density ◦ Alloying elements ◦ Microstructure Mechanical properties can be modified by processing Mechanical properties under different circumstances ◦ Static application of loads ◦ Dynamic application of loads (varying over time) ◦ High temperature properties ◦ Fracture mechanics properties Stress and Strain • Engineering stress N/m2 • Engineering strain mm/mm • Shear stress and Shear strain – Deformation caused by shear force couple Tensile Test – Engineering stress-strain diagram • • • • • • • Strength – ability to resist applied tensile strength. Modulus of Elasticity – slope of stress/strain curve in elastic range. Yield Strength – Stress at which plastic deformation begins. Sharp point, or 0.2% offset yield strength. Ultimate Tensile Strength – Maximum strength reached, where specimen starts to neck. Ductility – ability to deform plastically ◦ Elongation – percentage gain in length ◦ Reduction – percentage loss in cross-sectional area Work hardening rate – Slope of curves in plastic range. True stress and strain ◦ Stress and strain calculated with continuously decreasing area. Hardness and hardness testing • • • • • Measure of resistance to plastic deformation ◦ Brinell Hardness (BHN - Steel ball) and Vickers Hardness (VHN – Diamond pyramid) kg/mm2 ◦ Rockwell Hardness (R) – inversely proportional to depth of indentation Requirements ◦ Surface must be clean and oxide free ◦ Sample surface must be firm and perpendicular to indenter ◦ Thickness of sample >= 10X depth of indentation ◦ Distance between measurements = 3-5X indentation diameter Hardness directly related to strength Strength of products can be evaluated for quality control. Extensively used for surface hardened components. Dynamic, high-temperature and fracture mechanics properties • • • Two types of metal fracture ◦ Ductile fracture ◦ Brittle fracture Toughness – measure of energy that can be absorbed before fracturing. Resistance to propagation of crack. ◦ Toughness evaluated by impact test eg. Charpy V-notch ◦ Ductile-to-brittle transition temperature (BTT) ▪ Test temperature coinciding with specific value of impact energy. Fracture mechanics – relates material properties, stress levels, size of crack producing flaws, and crack propagation mechanisms in materials. ◦ Stress concentration ▪ Defect in material specimen under load causes local increase in stress. ▪ Microscopic flaws cause toughness measured to always be less than theoretically calculated toughness. Stress raisers. ▪ Cracks, notches, vacancies, discontinuities (sharp corners) ▪ Crack will propagate when applied stress exceeds tensile strength. ◦ Fracture toughness ▪ Surface energy. Twice this multiplied by the area of the surface is required to fracture it. ▪ If K1 exceeds K1c, the crack will propagate. ▪ Unless stated otherwise, assume y=1 ▪ Ductile materials have higher fracture toughness than brittle materials. Fatigue of Metals • • • • • • • • • • • Fatigue failure occurs under cyclic (repetitive) stress. Fatigue failure can occur at stress levels below tensile or yield strength. Results due to slow propagation of a fatigue crack. There is also sometimes random cyclic stress (not a regular sinusoid) Initiation controlled fatigue – Fatigue occurring after cracks are initiated Propagation controlled fatigue – Fatigue occurring in material in which there are already cracks. Low cycle fatigue – Applied stress higher than yield strength High cycle fatigue – Applied strength lower than yield strength Fatigue test ◦ Stress amplitude – Cycles to failure (on logarithmic scale) curve Factors affecting fatigue strength ◦ Stress concentrations (stress raisers, positions where cracks originate) ◦ Surface roughness (creates stress raisers) ◦ Surface condition (hard surfaces inhibit initiation and fatigue cracks) ◦ Environment (corrosion enhances propagation of fatigue cracks) At higher temperatures, time dependant plastic deformation or creep can result. Creep and Stress Rupture of Metals • • • • Creep is time dependant plastic deformation at lower stresses than the yield point, which occur at higher temperatures. Sometimes creep is desirable ◦ Extrusion, hot rolling, hot pressing, forging ◦ Power law creep plays dominant role in these deformation based processes. To test creep, a constant stress is applied at a high enough temperature that creep will occur, and the strain over time is recorded. Three stages of creep – balance of work hardening and recovery ◦ Primary creep phase – slope decreases with time ◦ Secondary creep – slope constant • • • • • • ◦ Tertiary creep – rate increases Diffusional flow ◦ Section where n=1 ◦ Grains elongate in direction of tensile strength ◦ Caused by stress gradient at grain boundary ◦ Vacancies and atoms move to grain boundaries Vacancies – Compressive stresses Atoms – Tensile stresses Dislocation climb and power law creep ◦ 3<n<8 ◦ Micro-structural processes combined with diffusional flow ◦ (This paragraph in the notes is screwed) Deformation Diagrams ◦ Diagram showing type of deformation and creep rate at combinations of stress and temperature. Creep fracture ◦ Grain boundaries slide in relation to one another ◦ Internal cracks form at grain boundaries - reduces load carrying surface and stress increases to fracture ◦ Small grain size usually higher strength, but can be detrimental for creep. Plastic Deformation, Strengthening and Recrystallisation of Metals Slides for this are weirdly good. Plastic deformation of metal single crystals • Tensile stress can be resolved into tensile and shear stress components. • Crystals slip due to resolved shear stress – caused by applied tension. Max shear stress at 45 degrees to applied tensile strength. • Slip along slip planes. • Slip occurs due to movement of dislocations, like a caterpillar. • Crystal orientation helps or impedes dislocations movement. • Plastic deformation increases dislocation density, which results in work hardening, an increase in yield strength and hardness. Slip Systems • Slip is favoured in close-packed planes, as the atoms are not set as deeply into one another. ◦ HCP – basal plan ◦ FCC – various intersecting octahedral planes ◦ BCC – diagonal planes not close-packed but is the most densely packed. • Slip system is combination of slip plane and direction. • Number of slip systems: (no. of slip planes) * (no. of slip directions) Plastic deformation in polycrystalline metals • Most engineering materials are polycrystalline as grain boundaries increase strength of materials. • Plastic deformation starts in grains where the slip systems are parallel to maximum resolved shear stress. • Dislocation movement is halted by grain boundaries. Dislocations pile up at boundaries. Mechanisms for strengthening in metals • Any mechanism inhibiting dislocation movement increases strength and hardness. • Reducing grain size ◦ Increases total grain boundary area. More energy therefore needed to move dislocations. ◦ Barrier strength increases with increasing angle of misorientation. ◦ Hall-Petch equation • Solid solution strengthening of metals ◦ Strengthening by adding alloying elements to parent metal (substitutional or interstitial) ◦ Stress fields form around the solute. Solutes migrate to dislocations to relieve the stress, impeding their movement. ◦ Factors affecting degree of strengthening ▪ Relative size factor ▪ Short-range order ▪ Solute concentration • Work or strain hardening – metals strengthening and hardening with plastic deformation. ◦ Dislocations pile up at grain boundary ◦ Dislocation density increase with cold working ◦ Interaction between stress fields of dislocations inhibit further movement ◦ With more plastic deformation, crystals need to rotate more to maintain coherency. Recovery and Recrystallisation of cold-worked metals Condition and properties of metal can be restored to the same as before it was cold-worked. This requires annealing heat treatment at elevated temperatures. Recrystallisation temperature is the temperature at which a cold worked material will recrystallise in one hour. • Recovery ◦ Occurs just below temperature range of recrystallisation. Also at a short heating time at high temperature. ◦ Reduction in internal energy occurs by relieving internal stresses ▪ Point defects reduced ▪ Dislocations eliminate each other ▪ • Remaining dislocations form sub-grain structure with low-angle grain boundaries (Polygonisation) Recrystallisation ◦ New grains begin growing in the cold-worked structure. Eventually the whole structure is replaced by new recrystallised grain structure. ◦ Caused by a decrease in lattice energy • Grain growth ◦ If annealing continues beyond recrystallisation, for more time and/or temperature, grains will grow at expense of others. ◦ Total grain boundary energy decreases, as total grain boundary area decreases. • Recrystallisation process affected by following factors ◦ Minimum amount of cold work is required ◦ The less cold work, the higher temperature is needed. ◦ If the annealing temperature is increased, less time will be required. ◦ Final grain size depends on degree of cold work. ◦ The larger the original grain size, the greater the amount of cold work required to produce an equivalent recrystallisation temparature. ◦ Recrystallisation temperature is lower, the purer the metal. Diffusion in Solids Atomic Diffusion In Solids • Solid State Diffusion – Process whereby atoms and ions move/migrate through crystal lattice. • Occurs as a result of vibrational energy ◦ Assists with jumping from one position to another ◦ Higher temperature = higher jump rate • Used in processing of materials ◦ Heat treatment of steel ◦ Doping of micro-electronic components ◦ Creep ◦ Recrystallisation Diffusion Mechanisms • Vacancy/substitutional diffusional ◦ Atoms moving from one atomic site to another. ◦ Move into vacancies ◦ Activation energy provided by thermal vibration. • Interstitial diffusional ◦ Small atoms can move between interstitial positions without vacancies needing to be created. ◦ No permanent displacement. ◦ Needs less activation energy and therefore is more rapid. Steady State Diffusion • Diffusion will occur if a concentration gradient of solute atoms exist. • Steady state diffusion is when the concentration difference between two planes being considered remains constant, therefore keeping a constant concentration gradient. • Follows Fick’s First Law (Formula to study) • D = Diffusivity Non-steady State Diffusion • Concentration of solute atoms at any point in material changes over time. • Fick’s Second Law Factors Influencing D the • Types of Diffusion Mechanism • Temperature lol ◦ Formula • Crystal structure of solvent • Types of crystal defects present ◦ Less densely packed atoms at grain boundaries ◦ Higher vacancy concentration • Concentration of diffusing species (lolwut I don’t know what that is either) Industrial Applications of Diffusion Processes • Heat treatment of engineering components. ◦ All types require re-arrangement of atoms by diffusion. • Case hardening of steel components by carburizing • Silicon wafer doping ◦ Illegally enhancing the performance of silicon. ◦ Diffuse impurities into silicon wafers for integrated micro-electronic circuits. Phase Diagrams/Transformations and Heat Treatment Please refer to class notes for diagrams. There are many important ones. Figures and explanations of figures not in these summaries. Also detail like the many, many percentages and temperatures. I couldn’t be bothered copying those over because, damn, there are so many I want to cry. • Heat treatment is heating a material to a specific temperature and then cooling it in order to obtain a micro-structural property or stress state in the material. • Phase – atomic structural state. ◦ Dependant on composition, temperature and pressure • Equilibrium phase diagrams ◦ Phase areas bordered by lines for alloys, or line intercepts for pure elements ◦ Either composition or time (x) to temperature (y) ◦ Liquidus and solidus lines. Border phase areas. • Use of phase diagrams ◦ Phases present are those in whose area the alloy composition and temperature lines intercept. ◦ Compositions of these phases are obtained from intercept of temperature with borderlines. ◦ Relative amounts of phases present obtained by applying lever rule ▪ Apply SWK moments stuff (lol I’m not kidding, literally mechanics) ▪ Actually not that hard. Just… this: http://bit.ly/244TztR • Iron – Iron Carbide phase diagram ◦ Used for predicting response of steel to heat treatment. ◦ Ferrite (α) – Fe with BCC ◦ Austenite (γ) – Fe with FCC, more soluble with C ◦ Cementite – Fe3C is intermediate compound with fixed ratio of Fe to C (6.7% C) ◦ Ledeburite – Austenite and cementite mixture. ◦ Pearlite – Ferrite and cementite mixture ◦ Peritectic reaction – Ferrite to Austenite. BCC to FCC ▪ 1493°C ◦ Eutectic reaction – Liquid to leduburite. ◦ Eutectoid – Austenite to pearlite • Equilibrium cooling of steel ◦ Depending on the cooling rate, the composition of the carbon steel changes. ◦ Hypo eutectoid – less than 0.76% C ◦ Hyper eutectoid – more than 0.76% C ◦ This whole section is cray. • Non-equilibrium cooling of steel – cooling rate too rapid for equilibrium phases ◦ Bainite transformation ▪ Fine dispersion of cementite in a matrix of ferrite. ▪ Feathery needle-like structure. ▪ Upper/feathery bainite – 350°C to 550°C – cementite plates parallel to ferrite needles ▪ Lower bainite – 250°C to 350°C- cementite plates at 60° to ferrite needles ◦ Martensite transformation ▪ Disruption of Austenite to Ferrite cooling ▪ BCT structure. Carbon trapped in solution. ▪ Straw-like. Highly distorted. High hardness. Low ductility. • Some heat treatments of plain carbon steel. ◦ Dependant on specific temperature, speed of cooling and time left at temperature ◦ Annealing – steel heated to Austenite and allowed to cool slowly. Grain size increases and internal stresses are relieved. ▪ Slow cooling (Usually in the furnace) ▪ Renders a course pearlite ◦ Normalising – harder and less ductile than annealed steel. ▪ Intermediate cooling (Usually in air). ▪ Renders a finer pearlite ◦ Hardening – rapidly cooled (quenched) to form martensite ◦ Tempering – Toughness and ductility of martensite improved by keeping heated to relieve residual stresses. Decreases hardness and strength. ▪ Spheroidising (see below) occurs if left at temperature for time. ◦ Stress relieving – Heating to below eutectoid temperature for time. ▪ Relieve stresses introduced by cold working, quenching, welding, machining and grinding. ◦ Spheroidising ▪ Prolonged heating (+-24 hours at 600°C) ▪ Cementite particles take spherical shape, instead of rods. ▪ Softer, ductile, more machinable. ▪ Driving force – minimisation of interphase energy and smallest surface are between phases. • Factors affecting hardenability ◦ Chemical composition ◦ Austenite grain size Magnetic Properties of Metals Magnets. How do they work? Basic Concepts Magnetic Field Strength • Magnetism is dipolar • Field lines extend from one pole to the other • EIR FORMULAE Origin of Magnetic Moments • Electrons rotating around atom core and spinning on axis. • Magnetic moments only possible if there are unpaired electrons, as otherwise opposite spins will neutralise each other. Types of Magnetism • Diamagnetism ◦ μr < 1 ◦ Magnetic field created opposes applied magnetic field. • Paramagnetism ◦ μr > 1 ◦ Atomic dipole moments allign with external applied magnetic field. (Like when you put a paperclip on a magnet and then things can stick to the paperclip itself also) • Ferromagnetism ◦ Permanent magnetisation ◦ Dipole moments maintain orientation even after removal of external field. ◦ Iron (Fe), Cobalt (Co) and Nickel (Ni) can be magnetised ▪ They have unpaired electrons that align themselves with external fields. • Antiferromagnetism ◦ Elements that have small existing dipole moments at room temperature ◦ Align anti-parallel to external field. ◦ Cr and Mn are examples • Ferrimagnetism ◦ Ionic compounds with different dipole moments ◦ Ions align anti-parallel to applied field, but shows net magnetic dipole moment like ferromagnetism. Effect of Temperature on Ferromagnetism • At Curie Temperature, ferromagnetic behaviour is lost (now showing paramagnetic behaviour) • Energy from temperature decreases parallel alignment of unpaired 3d-electrons Domains and Magnetic Hysteresis • Magnetic Domains ◦ Small regions in crystal structures of ferro- and ferri- magnetic materials in which atomic dipole moments are aligned parallel ◦ When randomly oriented, there is no net magnetisation ◦ When oriented in one direction, material is macroscopically magnetised. • Magnetisation ◦ Domains which are parallel to the applied field grow at expense of others. ◦ In strong fields, dipole moments rotate ◦ Domain wall moves due to re-orientation of of less favourable orientated domains. ◦ Magnetised sample will remain magnetised after field is removed but will lose some magnetism. (domains rotate slightly back to original position) • Demagnetisation ◦ Application of an opposite directed magnetic field ◦ Heating to above Curie temperature and allowed to slow cool. ◦ Cold work or impact • Hysteresis loop ◦ Graph of application of field to magnetism exhibited. ◦ Positive and negative fields applied to create and oppose magnetism, magnetism gained and lost. ◦ Internal area = Energy loss • Soft Magnetic Materials ◦ Easily magnetised and demagnetised ◦ Requirements ▪ High saturation induction (Bs) and permeability (μ). ▪ Minimum hysteresis losses – increased by: • impurities in solid solution • high dislocation density • small grain size • precipitates limiting domain boundary movement ▪ Low eddy current losses • Hard Magnetic Materials ◦ High coercive force (Hc) and remnant induction (Br) ◦ Larger reverse field required to demagnetise (equal to Hc) ◦ Energy required for demagnetisation – largest rectangle in second quadrant of hysteresis loop. Electrical Properties of Metals Finally, something vaguely applicable to our degree Electrical Conduction in Metals • Metals are good conductors because electrons are loosely shared between all atoms, and they can move freely. • Without potential difference, they all move randomly and there isn’t any net current flow. • When electric potential is applied, electrons attain a directed drift velocity, and electrons flow in a direction (current) Resistivity and Conductivity • Resistance and resistivity are not the same thing. Resistivity is the true material property, while resistance is dependant on the shape of the wire also. • R – resistance • Conductivity (σ) is the inverse of resistivity. ρ – resistivity ◦ Conductors, semiconductors, and insulators have high to low conductivity respectively. Factors Affecting Resistivity and Conductivity • Resistivity is composed of three components. Total resistivity is the sum of the three. ◦ thermal ◦ impurity ◦ deformation • Temperature ◦ Thermal resistivity increases almost linearly with temperature ◦ Vibration of atoms scatters electrons, decreasing their free paths. • Alloying Elements ◦ Deformation of parent metals scatters conduction electrons. • Crystal defects ◦ Dislocations and grain boundaries (ie. by cold working) scatter electrons and increase resistivity. ◦ Recrystallisation can restore conductivity Energy-band Model for Electrical Conduction • The ability of a material to conduct electrons • There are different bands of electrons, their energy level. ◦ The valence band – highest occupied energy levels ◦ The conduction band – lowest unoccupied energy levels • To conduct electricity, electrons must jump from the valence band to the conduction band. • There is an energy gap between the valence and conduction bands, this is the activation energy required for conduction. • The easier it is for electrons to be bumped up to the energy level of the conduction band (the smaller the energy gap) the better the material conducts. • Insulators have electrons tightly bound to bonding atoms, not free to conduct without high energy levels. Intrinsic Semiconductors • Intrinsic semiconductors – Pure semiconducting materials (Silicon and Germanium) • Extrinsic semiconductors – Dilute solid solutions which have semiconducting properties (Phosphorous-doped silicon) • They only conduct when at higher temperature, the thermal vibration excites electrons to jump to the conduction band. Unlike metals, conductivity of semiconductors increases with temperature. • When an electron jumps to the conduction band it leaves a positively charged “hole”. The holes and electrons both move to conduct electricity, with both contributing to current flow. Extrinsic Semiconductors • Impurity (solute) atoms, “dopants”, have different valence characteristics to solvent (parent) atoms. This difference allows for charge carrying electrons or holes to be introduced. • Introduction of free electrons or holes decreases energy gap • n-type ◦ In bonding, an excess electron is provided. ◦ Majority charge carriers are electrons • p-type ◦ In bonding, there is one missing valence electron, forming a positive hole. ◦ Majority charge carriers are holes • Doping ◦ Introducing small amounts of impurity elements (dopants) into areas of silicon wafers. ◦ A special diffusion process at +-1000°C Dielectric (insulation) Behaviour Examples in class notes • Ceramics (ionic materials) and polymers (covalent materials) are electrical insulators (dielectrics). • Some ceramics are used for semiconductors • Applications like: ◦ High and low voltage insulators ◦ Inserts for capacitors Piezoelectric ceramics – Ceramics which can convert electrical pulses into vibrations and vice-versa.
