Introduction to Material Science and Engineering PDF

Introduction to Material Science and Engineering  Introduction Materials Science and Engineering - a field of engineering that encompasses the spectrum of materials types and how to use them in manufacturing  Introduction Materials Science and Engineering...

Introduction to Material Science and Engineering  Introduction Materials Science and Engineering - a field of engineering that encompasses the spectrum of materials types and how to use them in manufacturing  Introduction Materials Science and Engineering is subdivided into two: 1. Materials Science 2. Materials Engineering  Introduction Material Science – involves investigating the relationships that exist between the structures and properties of materials – the role of a materials scientist is to develop or synthesize new materials  Introduction Materials Engineering involves, on the basis of these structure–property correlations, designing or engineering the structure of a material to produce a predetermined set of properties materials engineer is called upon to create new products or systems using existing materials and/or to develop techniques for processing materials  Introduction Materials Generic categories of materials ¤ Metals ¤ Ceramics ¤ Plastics ¤ Semiconductors ¤ Composites  Introduction Metals Metals are materials that are normally combinations of "metallic elements". These elements, when combined, usually have electrons that are non-localized and as a consequence have generic types of properties. Metals usually are good conductors of heat and electricity. They are also quite strong but deformable and tend to have a lustrous look when polished.  Introduction Ceramics Ceramics are generally compounds between metallic and nonmetallic elements and include such compounds as oxides, nitrides, and carbides. Typically they are insulating and resistant to high temperatures and harsh environments.  Introduction Plastics Plastics, also known as polymers, are generally organic compounds based upon carbon and hydrogen. They are very large molecular structures. Usually they are low density and are not stable at high temperatures.  Introduction Semiconductors Semiconductors have electrical properties intermediate between metallic conductors and ceramic insulators. Electrical properties are strongly dependent upon small amounts of impurities.  Introduction Composites Composites consist of more than one material type. Fiberglass, a combination of glass and a polymer, is an example. Concrete and plywood are other familiar composites. Many new combinations include ceramic fibers in metal or polymer matrix.  Introduction Four Elements of the Field Processing Structure Property Performance  the structure of a material depends on how it is processed structure of a material usually relates to the arrangement of its internal components – Microscopic – cannot be seen by naked eye – Macroscopic – large enough to be seen by naked eyes a material’s performance is a function of its properties all materials are exposed to external stimuli that evoke some type of response  Example These are ingots. Both are made from Silicon which were extracted from sand. The two materials are used in different ways. Ingot A is usually used in calibration while Ingot B is usually used in making semiconductor materials such as integrated circuits. Performance of materials will depend on the properties it has. The properties will depend on the atomic structure formulated and how it was processed.  Introduction Processing Solidification Processing Powder processing Deposition processing Deformation Processing  Introduction Solidification Processing Most metals are formed by creating an alloy in the molten state, where it is relatively easy to mix the components. This process is also utilized for glasses and some polymers. Once the proper temperature and composition have been achieved, the melt is cast.  Introduction Powder Processing Powder processing involves consolidation, or packing, of particulate to form a `green body'. Densification follows, usually by sintering. There are two basic methods of consolidating powders: - dry-pressing - dry powder can be compacted in a die, - slip-casting or filter pressing - the particles can be suspended in a liquid and then filtered against the walls of a porous mold  Introduction Powder Processing Bulk ceramics are usually processed in powder form since their high melting points and low formability prohibit other types of processing. Metals and polymers can also be processed from powders.  Introduction Deposition Processing Deposition processing modifies a surface chemically, usually by depositing a chemical vapor or ions onto a surface. It is used in semiconductor processing and for decorative or protective coatings. Vapor source methods require a vacuum to transport the gaseous source of atoms to the surface for deposition.  Introduction Deposition Processing Common vapor sources are thermal evaporation (similar to boiling water to create steam), sputtering (using energetic ions to bombard a source and create the gas state), or laser light (ablates, or removes, atoms from surface to create the gaseous state). Other sources use carrier media such as electrochemical mixtures (ions in a solution transported by an electrical field to the surface for depositions) or spray coating (ions or small particles transported by gases, liquids, and/or electrical field).  Introduction Deformation Processing One of the most common processes is the deformation of a solid to create a desired shape. A large force is generally used to accomplish the deformation, and many techniques heat the material in order to reduce the force necessary to deform it. Sometimes a mold is used to define the shape. Forging, an old method that heated the metal and deformed the metal by hammer blows is still used today, albeit with multi-ton hammers.  Introduction Deformation Processing Rolling to reduce the thickness of a plate is another common process. Some glasses when heated can be formed with tools or molds. Other common methods, like drilling to make holes, or milling, are machining versions of the deformation process.  Introduction Structure Structure refers to the arrangement of a material's components from an atomic to a macro scale. Understanding the structure of a substance is key to understanding the state or condition of a material, information which is then correlated with the processing of the material in tandem with its properties. Understanding these relationships is an intrinsic part of materials science engineering, as it allows engineers to manipulate the properties of a material.  Introduction Properties Materials engineers must frequently reconcile the desired properties of a material with its structural state to ensure compatibility with its selected processing.  Properties of solid materials Important properties of solid materials may be grouped into six different categories: 1. Mechanical – relate deformation to an applied load or force include elastic modulus (stiffness), tensile strength, fracture toughness, fatigue strength, creep strength, hardness 2. Electrical – such as electrical conductivity and dielectric constant, the stimulus is an electric field – Conductivity or resistivity, ionic conductivity, semiconductor conductivity (mobility of holes and electrons)  Properties of solid materials 3. Thermal – can be represented in terms of heat capacity, thermal conductivity, and coefficient of thermal expansion 4. Magnetic – demonstrate the response of a material to the application of a magnetic field – Magnetic susceptibility, Curie Temperature, Neel Temperature, saturation magnetization  Properties of solid materials 5. Optical – the stimulus is electromagnetic or light radiation; index of refraction and reflectivity are representative optical properties – Polarization, capacitance, permittivity, refractive index, absorption 6. Deteriorative – relate to the chemical reactivity of materials – corrosion behavior, wear behavior  Properties of Materials 1. Chemical Properties 2. Physical Properties 3. Mechanical Properties 4. Dimensional Properties  Chemical Properties These are properties of a material that refer to the structure of a material and its formation from our elements. The chemical properties of a material include composition, corrosion resistance, crystal structure, microstructure, and stereospecificity  Chemical Properties Composition - is the elemental or chemical components of the material and the relative proportion of these components In metals, composition usually means the percentage of the various elements that make up the metal. Corrosion resistance - is the ability of the material to resists deterioration by chemical or electro-chemical reaction with its environment.  Chemical Properties Crystal structure - is the ordered, repeating arrangement of atoms and molecules in a material. Microstructure - is the structure of polished and etched materials as revealed by microscope magnifications greater than ten diameters. Stereospecificity - is the tendency for polymers and molecular materials to form with an ordered, spatial, three-dimensional arrangement of monomer molecules.  Physical Properties These are properties of materials that refer to the interaction of materials with various forms of energy and with other forms of matter. The physical properties of a material include Curie point, density, dielectric strength, electrical resistivity, heat distortion temperature, melting point, Poisson’s ratio, refractive index, specific gravity, thermal conductivity, and thermal expansion.  Physical Properties Curie point - is the temperature at which ferromagnetic materials can no longer be magnetized by outside forces. Density - is the mass per unit volume. Dielectric strength - is the maximum potential difference that an insulating material of given thickness can withstand for a specified time without occurrence of electrical breakdown through its bulk.  Physical Properties Electrical resistivity - is the electrical resistance of a material per unit length and cross-sectional area or per unit length and unit weight. Heat distortion temperature - is the temperature at which a polymer under a specified load shows a specified amount of deflection. Melting point - is the point at which a material liquefies on heating or solidifies on cooling.  Physical Properties Poisson's ratio - is the absolute value of the ratio of the transverse strain to the corresponding axial strain in a body subjected to uniaxial stress. Refractive index - is the ratio of the velocity of light in a vacuum to its velocity in another material. Specific gravity - is the ratio of the mass or weight of a solid or liquid to the mass or weight of an equal volume of water. Specific gravity of water is 1.000 at 4C.  Physical Properties Thermal conductivity - is the rate of heat flow per unit time in a homogeneous material under steady-state conditions, per unit area, per unit temperature gradient in a direction perpendicular to area. Thermal expansion - is the rate at which a material elongates when heated.  Mechanical Properties These are properties of a material that are displayed when a force is applied to the material. These include compressive strength, creep, creep strength, endurance limit, flexural strength, hardness, modulus of elasticity, percent elongation, percent reduction in area, shear strength, and yield strength.  Mechanical Properties Compressive strength - is the maximum compressive stress that a material is capable of withstanding. Creep - is the permanent strain under strain. Creep strength - is the constant nominal stress that will cause a specified quantity of creep in a given time of constant temperature. Endurance limit - the maximum stress below which a material can theoretically endure an infinite number of stress cycles.  Mechanical Properties Flexural strength - is the outer fiber stress developed when a material is loaded as a simple supported beam and deflected to a certain value of strain. Hardness - is the resistance of a material to plastic deformation. Modulus of elasticity - is the ratio. of stress to strain in a material loaded within its elastic range. It is a measure of rigidity. It is also known as Young's Modulus. The modulus of elasticity of steel is 200,000 MPa..  Mechanical Properties Percent reduction in area - is the difference (expressed as a percentage of original area) between the original cross-sectional area of a tensile test specimen and the minimum cross-sectional area measured after fractures. Shear Strength – is the stress required to fracture a shape in a cross-sectional plane that is parallel to the force application Yield Strength – is the stress at which a material exhibits a specified deviation from proportionality of stress and strain.  Dimensional Properties Camber - is the deviation from edge straightness. It is usually the maximum deviation of an edge from a straight line of given length. Lay - is the direction of a predominating surface pattern. It is usually after a machine operation. Out of flat - is the deviation of a surface from a flat plane, usually over a macroscopic area.  Dimensional Properties Roughness - is relatively finely spaced surface in regularities, the height, width and direction of which establish a definite surface pattern. Surface finish - is the microscopic and macroscopic characteristics that describe a surface. Waviness - a wavelike variation from a perfect surface.  Introduction Performance The evaluation of performance is an integral part of the field. The analysis of failed products is often used to obtain feedback on processing and its control as well as to assist in the initial selection of the material and in the stages of processing. Testing also ensures that the product meets performance requirements. In many products the control of its processing is closely associated with some property test and/or a structural characterization.  Material Selection Criteria need in material selection 1. in-service conditions to which the material will be subjected 2. any deterioration of material properties during operation 3. economics or cost of the fabricated piece END Classification of Solid Materials  Classification of Solid Materials Solid materials have been conveniently grouped into three basic categories: metals, ceramics, and polymers, a scheme based primarily on chemical makeup and atomic structure. Most materials fall into one distinct grouping or another. In addition, there are the composites that are engineered combinations of two or more different materials.  Metals Metals are composed of one or more metallic elements, and often also nonmetallic elements in relatively small amounts Atoms in metals and their alloys are arranged in a very orderly manner and are relatively dense in comparison to the ceramics and polymers  Metals With regard to mechanical characteristics, these materials are relatively stiff and strong, yet are ductile, and are resistant to fracture, which accounts for their widespread use in structural applications. Metallic materials have large numbers of nonlocalized electrons—that is, these electrons are not bound to particular atoms.  Ceramics Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. Common ceramic materials include: – aluminum oxide (or alumina, Al2O3) – silicon dioxide (or silica, SiO2) – silicon carbide (SiC), – silicon nitride (Si3N4) – traditional ceramics those composed of clay minerals as well as cement and glass.  Ceramics ceramic materials are relatively stiff and strong— stiffness and strengths are comparable to those of the metals they are typically very hard ceramics have exhibited extreme brittleness (lack of ductility) and are highly susceptible to fracture  Ceramics Ceramic materials are typically insulative to the passage of heat and electricity and are more resistant to high temperatures and harsh environments than are metals and polymers. With regard to optical characteristics, ceramics may be transparent, translucent, or opaque, and some of the oxide ceramics exhibit magnetic behavior.  Polymers Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (i.e., O, N, and Si). Furthermore, they have very large molecular structures, often chainlike in nature, that often have a backbone of carbon atoms.  Polymers Some common and familiar polymers are: polyethylene (PE) Nylon poly(vinyl chloride) (PVC) polycarbonate (PC) polystyrene (PS) silicone rubber.  Polymers These materials typically have low densities whereas their mechanical characteristics are generally dissimilar to those of the metallic and ceramic materials Polymers are extremely ductile and pliable, which means they are easily formed into complex shapes. One major drawback to the polymers is their tendency to soften and/or decompose at modest temperatures, which, in some instances, limits their use. They have low electrical conductivities and are nonmagnetic.  Composites The design goal of a composite is to achieve a combination of properties that is not displayed by any single material and also to incorporate the best characteristics of each of the component materials. A large number of composite types are represented by different combinations of metals, ceramics, and polymers.  Composites One of the most common and familiar composites is fiberglass, in which small glass fibers are embedded within a polymeric material. Another is the carbon fiber–reinforced polymer (CFRP) composite— carbon fibers that are embedded within a polymer. – CFRP composites are used in some aircraft and aerospace applications, as well as in high-tech sporting equipment and recently in automobile bumpers. END Advanced Materials  Advanced Materials Materials utilized in high-technology (or high- tech) applications are sometimes termed advanced materials. Includes: – Semiconductors – Biomaterials – Smart materials – Nanomaterials  Semiconductors Semiconductors have electrical properties that are intermediate between the electrical conductors and insulators. Furthermore, the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms, for which the concentrations may be controlled over very small spatial regions.  Semiconductors Semiconductors have made possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries over the past Integrated Circuit (IC) three decades.  Biomaterials Biomaterials are employed in components implanted into the human body for replacement of diseased or damaged body parts. Artificial total hip These materials must not replacement produce toxic substances and must be compatible with body tissues  Smart Materials These are a group of new and state-of-the-art materials now being developed that will have a significant influence on many of our technologies. The adjective “smart” implies that these materials are able to sense changes in their environments and then respond to these changes in predetermined manners Components of a smart material (or system) include some type of sensor (that detects an input signal), and an actuator (that performs a responsive and adaptive function).  Smart Materials Four types of materials are commonly used for actuators: – shape memory alloys – piezoelectric ceramics – magnetostrictive materials – electrorheological/magnetorheological fluids.  Smart Materials Shape memory alloys Are metals that after having been deformed, revert back to their original shapes when temperature is changed. Piezoelectric ceramics Piezoelectric ceramics expand and contract in response to an applied electric field (or voltage); conversely, they also generate an electric field when their dimensions are altered  Smart Materials Magnetostrictive materials is analogous to that of the piezoelectrics, except that they are responsive to magnetic fields. Electrorheological and magnetorheological fluids are liquids that experience dramatic changes in viscosity upon the application of electric and magnetic fields, respectively.  Nanoengineered Materials Materials that are not distinguished on the basis of their chemistry but rather their size The nano prefix denotes that the dimensions of these structural entities are on the order of a nanometer (10-9 m)—as a rule, less than 100 nanometers (nm; equivalent to the diameter of approximately 500 atoms). END Material Science and Engineering Atomic Structure  Atomic Structure Each atom consists of a very small nucleus composed of protons and neutrons and is encircled by moving electrons Electron Charge: p and e = ± 1.602x10-19 C n = neutral charge  Atomic Structure Each chemical element is characterized by the number of protons in the nucleus, or the atomic number (Z) Example: Hydrogen = 1 Uranium = 92  Atomic Structure The atomic mass (A) of a specific atom may be expressed as the sum of the masses of protons and neutrons within the nucleus. ≅ + Where: Proton & Neutron mass = 1.67x10-27 kg A = Atomic Mass Electron mass = 9.11x10-31 kg Z = Atomic Number N = Number of Neutrons  Atomic Structure Number of protons is the same for a given element but number of neutrons may vary. Atoms of some elements have two or more different atomic masses, which are called isotopes.  Atomic Structure The atomic weight of an element or the molecular weight of a compound may be specified on the basis of amu per atom (molecule) or mass per mole of material In one mole of a substance, there are 6.022x1023 (Avogadro’s number) atoms or molecules. 1 amu/atom (or molecule) = 1 g/mol Example: atomic weight of iron is 55.85 amu/atom, or 55.85 g/mol  Atomic Structure Example Cerium has four naturally occurring isotopes: 0.185% of 136Ce, with an atomic weight of 135.907 amu; 0.251% of 138Ce, with an atomic weight of 137.906 amu; 88.450% of 140Ce, with an atomic weight of 139.905 amu; and 11.114% of 142Ce, with an atomic weight of 141.909 amu. Calculate the average atomic weight of Ce. Ans. 140.115 amu  Atomic Structure Example: Silicon has three naturally occurring isotopes: 92.23% of 28Si, with an atomic weight of 27.9769 amu; 4.68% of 29Si, with an atomic weight of 28.9765 amu; and 3.09% of 30Si, with an atomic weight of 29.9738 amu. Calculate the average atomic weight of Si. Ans. 28.0854 amu Material Science and Engineering Atomic Models  Electrons in Atoms During the latter part of the nineteenth century, it was realized that many phenomena involving electrons in solids could not be explained in terms of classical mechanics.  Electrons in Atoms Quantum mechanics A set of principles and laws that govern systems of atomic and subatomic entities An understanding of the behavior of electrons in atoms and crystalline solids necessarily involves the discussion of quantum-mechanical concepts.  Electrons in Atoms Two atomic models 1. Bohr Atomic Model 2. Wave – Mechanical Model  Bohr Atomic Model Electrons are assumed to revolve around the atomic nucleus in discrete orbitals, and the position of any particular electron is more or less well defined in terms of its orbital  Wave-mechanical model Electron is considered to exhibit both wavelike and particle-like characteristics. With this model, an electron is no longer treated as a particle moving in a discrete orbital; rather, position is considered to be the probability of an electron’s being at various locations around the nucleus (electron cloud)  Quantum Numbers In wave mechanics (quantum mechanics), every electron in an atom is characterized by four parameters called quantum numbers. Bohr energy levels separate into electron subshells, and quantum numbers dictate the number of states within each subshell  Quantum Numbers Shells are specified by a principal quantum number (n) – Designated by letter K, L, M, N, O, etc.. – Which also corresponds to n = 1, 2, 3, 4, etc.. This quantum number is related to the size of an electron’s orbital or its average distance from the nucleus.  Quantum Numbers The second quantum number is angular quantum number (l). It is related to the shape of the electron subshell. In addition, the number of these subshells is restricted by the magnitude of n. Values of l are integer values that range from l = 0 to l = n-1  Quantum Numbers Each subshell is denoted by a lowercase letter— an s, p, d, or f—related to l values as follows  Quantum Numbers The angular quantum number is very important since it specifies the shape of an atomic orbital and strongly influences chemical bonds and bond angles.  Quantum Numbers The third (magnetic) quantum number describes the energy levels available within a subshell and yields the projection of the orbital angular momentum along a specified axis. The values of mℓ range from − to ℓ, with integer steps between them. Third (or magnetic) quantum number (ml) integer values between - ℓ to + ℓ, When ℓ = 0; then ml=0 = 0 ℓ = 1; then ml=1 = -1,0,+1 ℓ = 2; then ml=2 = -2,-1,0,+1,+2 ℓ = 3; then ml=3 = -3,-2-1,0,+1,+2,+3  Quantum Numbers Fourth Quantum Number is the Spin Moment It describes the spin (intrinsic angular momentum) of the electron within that orbital and gives the projection of the spin angular momentum (s) along the specified axis. + 1/2 (for spin up) and – 1/2 (for spin down) Example: Find the Spin of Neon Ans. - 1/2  Example Find the quantum number of: a) Sodium(Na) b) Cobalt(Co) Ans. Na = (3,0,0,1/2) Co = (3,2,-1,-1/2) Material Science and Engineering Atomic Bonding in Solids  Atomic Bonding in Solids Bonding Energy – the energy required to separate two atoms that are chemically bonded to each other. It may be expressed on per atom basis or per mole of atoms  Atomic Bonding in Solids Primary Bond – Three different types of primary or chemical bond are found in solids ─ ionic, covalent and metallic. Each of these three types of bonding arises from the tendency of the atoms to assume stable electron structures, like those of the inert gases, by completely filling the outermost electron shell.  Primary Interatomic Bonds Ionic Bonding – it is always found in compounds that are composed of both metallic and non metallic elements, elements that are situated at the horizontal extremities of the periodic table  Primary Interatomic Bonds Covalent Bonding - stable electron configurations are assumed by the sharing of electrons between adjacent atoms. Two atoms that are covalently bonded will each contribute at least one electron to the bond, and the shared electrons may be considered to belong to both atoms  Primary Interatomic Bonds Metallic Bonding – the final primary bonding type, is found in metals and their alloys. Metallic materials have one, two, or at most, three valence electrons. With these model, these valence electron are not bound to any particular atom in the solid and are more or less free to drift throughout the entire metal.  Secondary Bonding or Van der Waals Bonding Secondary Bonding – are weak in comparison to the primary or chemical ones; bonding energies are typically on the order of only 10 kJ/mol (0.1 eV/atom) Dipole – a pair of equal yet opposite electrical charges that are separated by a small distance Hydrogen Bonding – a special type of secondary bonding, is found to exist between some molecules that have hydrogen as one of the constituents Polar Molecule – a permanent dipole exist in some molecules by virtue of an asymmetrical arrangement of positively and negatively charged regions  Bonding Energies and Melting Temperature for Various Substances Bonding Type Substance kJ/mol eV/atom, Ion, Melting molecule Temperature (degree C) Ionic NaCl 640 3.3 801 MgO 1000 5.2 2800 Covalent Si 450 4.7 1410 C (Diamond) 713 7.4 > 3550 Metallic Hg 68 0.7 -39 Al 324 3.4 660 Fe 406 4.2 1538 W 849 8.8 3410 Van der Waals Ar 7.7 0.08 -189 31 0.32 -101 35 0.36 -78 Hydrogen 51 0.52 0 END Material Science and Engineering Structures of Crystalline Solids A. Atomic Structure in Solids B. Crystal Structures  UNIT CELL - The unit cell is building block for crystal. - Repetition of unit cell generates entire crystal.  Metallic Crystal Structures Tend to be densely packed Several reasons for dense packing: -Typically, only one element is present, so all atomic radii are the same. -Metallic bonding is not directional. -Nearest neighbor distances tend to be small in order to lower bond energy. Have the simplest crystal structures. 74 elements have the simplest crystal structures – BCC, FCC and HCP  Metallic Crystals Three simple lattices that describe metals are - Face Centered Cubic (FCC) - Body Centered Cubic (BCC) - Hexagonal Close Packed (HCP)  Simple Cubic Structure Rare due to poor packing (only Po has this structure) Coordination # = 6 Close-packed directions are (# nearest neighbors) cube edges.  BODY CENTERED CUBIC STRUCTURE (BCC) Atoms touch each other along cube diagonals. Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. ex: Cr, W, Fe (a), Tantalum, Molybdenum Coordination # = 8  FACE CENTERED CUBIC STRUCTURE (FCC) Atoms touch each other along face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. ex: Al, Cu, Au, Pb, Ni, Pt, Ag Coordination # = 12  HEXAGONAL CLOSE-PACKED STRUCTURE (HCP) ABAB... Stacking Sequence 3D Projection 2D Projection A sites B sites A sites Coordination # = 12 APF = 0.74  DENSITIES OF MATERIAL CLASSES rmetals rceramics rpolymers Why? Metals have... close-packing (metallic bonding) large atomic mass Ceramics have... less dense packing (covalent bonding) often lighter elements Polymers have... poor packing (often amorphous) lighter elements (C,H,O) Composites have... intermediate values Data from Table B1, Callister 6e. 13  DEMO: HEATING AND COOLING OF AN IRON WIRE The same atoms can Demonstrates "polymorphism" have more than one crystal structure. 19 C. CRYSTALLOGRAPHIC POINTS, DIRECTIONS, AND PLANES Recall: Three-dimensional Graph Recall: Three-dimensional Graph Crystallographic Planes & Directions  Crystallographic Planes & Directions It is often necessary to be able to specify certain directions and planes in crystals. Many material properties and processes vary with direction in the crystal. Directions and planes are described using three integers - Miller Indices Crystallographic Direction Crystallographic Planes D. CRYSTALLINE AND NON- CRYSTALLINE MATERIALS  CRYSTALLINE MATERIALS - the state of a solid material characterized by a periodic and repeating three-dimensional array of atoms, ions, or molecules.  NON-CRYSTALLINE MATERIALS - the solid state wherein there is no long-range atomic order. - also known as Amorphous Solids.  CRYSTALLINE vs NON-CRYSTALLINE CRYSTALLINE: NON-CRYSTALLINE: They have characteristic In these solids particles are geometrical shape. randomly arranged in three They have highly ordered three- dimension. dimensional arrangements of They don’t have sharp melting particles. points. They are bounded by PLANES or Amorphous solids are formed FACES due to sudden cooling of Planes of a crystal intersect at liquid. particular angles. Amorphous solids melt over a They have sharp melting and wide range of temperature boiling points.  CRYSTALLINE Copper Sulfate (CuSO4) Diamond Graphite Nickel Sulfate (NiSO4)  NON-CRYSTALLINE Glass Plastic Rubber Paraffin  MATERIALS AND PACKING Crystalline materials... atoms pack in periodic, 3D arrays typical of: -metals -many ceramics -some polymers crystalline SiO2 Adapted from Fig. 3.18(a), Callister 6e. Noncrystalline materials... atoms have no periodic packing occurs for: -complex structures -rapid cooling "Amorphous" = Noncrystalline noncrystalline SiO2 Adapted from Fig. 3.18(b), Callister 6e. 26  GLASS STRUCTURE Basic Unit: Glass is amorphous 4- Amorphous structure Si04 tetrahedron occurs by adding impurities Si4+ (Na+,Mg2+,Ca2+, Al3+) O2- Impurities: interfere with formation of crystalline structure. Quartz is crystalline SiO2: (soda glass) Adapted from Fig. 12.11, Callister, 6e. 28  CRYSTALLINE MATERIAL: SINGLE CRYSTAL Also called MONOCRYSTALLINE solid. A crystalline material in which there is long-range order. Such a material has no grain boundaries, so it is completely uniform throughout the entire crystal, regardless of its size. Can be produced naturally and artificially.  CRYSTALLINE MATERIAL: SINGLE CRYSTAL Occurs when the periodicity in crystal pattern extends throughout a certain piece of materials. Many types of single crystal exhibit ANISOTROPY (Properties of a material that depends on the direction) A single crystal is formed by the growth of a crystal nucleus without secondary nucleation or impingement on other crystals.  CRYSTALLINE MATERIAL: POLYCRYSTALLINE A collection of many small crystals or grains. Has the characteristic called POLYMORPHISM (The ability of a solid material to exist in more than one form or crystal structure) Occurs when the periodicity in the crystal structure is interrupted at so-called grain boundaries.  CRYSTALLINE MATERIAL: POLYCRYSTALLINE The size of the grains or crystallites is smaller than the size of the pattern unit which forms the periodicity. In general, the grains in such a solid are not related in shape to the crystal structure, the surface being random in shape rather than well defined crystal planes POLYCRYSTALS Most engineering materials are polycrystals. Adapted from Fig. K, color inset pages of Callister 6e. (Fig. K is courtesy of Paul E. Danielson, Teledyne Wah Chang Albany) 1 mm Nb-Hf-W plate with an electron beam weld. Each "grain" is a single crystal. If crystals are randomly oriented, overall component properties are not directional. Crystal sizes typ. range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers). SINGLE VS POLYCRYSTALS Single Crystals -Properties vary with direction: anisotropic. -Example: the modulus of elasticity (E) in BCC iron: Polycrystals -Properties may/may not 200 µm vary with direction. -If grains are randomly oriented: isotropic. (Epoly iron = 210 GPa) -If grains are textured, anisotropic. END Crystalline and Noncrystalline Materials part 1  Introduction Previous discussion was concerned primarily with the various types of atomic bonding, which are determined by the electron structures of the individual atoms. This discussion is devoted to the next level of the structure of materials, specifically, to some of the arrangements that may be assumed by atoms in the solid state.  Introduction Two concepts in the arrangement of atoms: 1. Crystalline Structure 2. Noncrystalline Structure  Crystalline Structure crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances. That is, long-range order exists, such that upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest neighbor atoms.  Noncrystalline Structure Also called amorphous long-range atomic order is absent  Crystalline Structure Atomic hard-sphere model - atoms are thought of as being solid spheres having well-defined diameter. Lattice - a three-dimensional array of points coinciding with atom positions  Crystalline Structure Unit Cell - the smallest group of atoms of a substance that has the overall symmetry of a crystal of that substance, and from which the entire lattice can be built up by repetition in three dimensions.  Metal Crystal Structure Three crystal structure found on metals: 1. Face-centered cubic 2. Body-centered cubic 3. Hexagonal close-packed Continue at part 2 Crystalline and Noncrystalline Materials part 2  Face-centered cubic crystal structure Atoms located at each corners and the center of all the cube faces Cube edge length “a” and the atomic radius R are related through 𝑎 = 2𝑅 2  Face-centered cubic crystal structure The number of atoms per unit cell may computed using the following formula: 𝑁𝑓 𝑁𝑐 𝑁 = 𝑁𝑖 + + 2 8 Where: Ni = no. of interior atoms Nf = no. of face atoms Nc = no. of corners of atom  Face-centered cubic crystal structure Two other important characteristics of a crystal structure: 1. Coordination number 2. Atomic packing factor  Face-centered cubic crystal structure Coordination number For metals, each atom has the same number of nearest-neighbor or touching atoms For FCC, the coordination number is 12.  Face-centered cubic crystal structure Atomic packing factor is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic hard-sphere model) divided by the unit cell volume 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑎𝑡𝑜𝑚𝑠 𝑖𝑛 𝑎 𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙 𝐴𝑃𝐹 = 𝑡𝑜𝑡𝑎𝑙 𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙 𝑣𝑜𝑢𝑙𝑚𝑒 For FCC, atomic packing factor is 0.74 which is the maximum packing possible for spheres all having the same parameters  Face-centered cubic crystal structure Example: Calculate the volume of an FCC unit cell in terms of the atomic radius R. Ans. 16R3 2  Face-centered cubic crystal structure Example: Show that the atomic packing factor for the FCC crystal structure is 0.74 Ans. 16 3 𝑉𝑠𝑝ℎ𝑒𝑟𝑒 𝜋𝑅 3 𝐴𝑃𝐹 = = = 0.74 𝑉𝑐𝑢𝑏𝑒 16𝑅3 2 Body-Centered Cubic Crystal Structure has a cubic unit cell with atoms located at all eight corners and a single atom at the cube center unit cell length a and atomic radius R are related through 4𝑅 a= 3 Body-Centered Cubic Crystal Structure The number of atoms per BCC may computed using the following formula: 𝑁𝑓 𝑁𝑐 𝑁 = 𝑁𝑖 + + 2 8 Where: Ni = no. of interior atoms Nf = no. of face atoms Nc = no. of corners of atom Body-Centered Cubic Crystal Structure The coordination number for the BCC crystal structure is 8 the atomic packing factor is also lower for BCC is 0.68 Body-Centered Cubic Crystal Structure It is also possible to have a unit cell that consists of atoms situated only at the corners of a cube. This is called the simple cubic (SC) crystal structure The only simple-cubic element is polonium, which is considered to be a metalloid (or semi-metal) Crystalline and Noncrystalline Materials part 3  Hexagonal Close-Packed Crystal Structure The top and bottom faces of the unit cell consist of six atoms that form regular hexagons and surround a single atom in the center. Another plane that provides three additional atoms to the unit cell is situated between the top and bottom planes. Hexagonal Close-Packed Crystal Structure The number of atoms per HCP may computed using the following formula: 𝑁𝑓 𝑁𝑐 𝑁 = 𝑁𝑖 + + 2 6 2 12 𝑁 =3+ + 2 6 𝑁=6  Hexagonal Close-Packed Crystal Structure c/a ratio should be 1.633 coordination number and the atomic packing factor for the HCP crystal structure are the same as for FCC: 12 and 0.74, respectively Hexagonal Close-Packed Crystal Structure Example: Calculate the volume of an HCP unit cell in terms of its a and c lattice parameters. Now provide an expression for this volume in terms of the atomic radius, R, and the c lattice parameter  Density Computation A knowledge of the crystal structure of a metallic solid permits computation of its theoretical density 𝜌 through the relationship 𝑛𝐴 𝜌= 𝑉𝑐 𝑁𝐴 Where: n – no. of atoms associated with each unit cell A – atomic weight Vc – volume of unit cell NA – Avogadro’s number (6.022x1023 atom/mol)  Density Computation Example: Copper has an atomic radius of 0.128 nm, an FCC crystal structure, and an atomic weight of 63.5 g/mol. Compute its theoretical density, and compare the answer with its measured density. Ans. 8.89 g/cm3 The literature value for the density of copper is 8.94 g/cm3, which is in very close agreement with the foregoing result.  Polymorphism and Allotropy phenomenon on metals, as well as nonmetals to have more than one crystal structure is known as polymorphism. When found on solids, the condition is termed as allotropy Graphite is the stable polymorph at ambient temperature Diamond formed at extremely high pressure Pure iron has a BCC crystal structure at room temperature which changes to FCC at 912℃.  Crystal Systems Because there are many different possible crystal structures, it is sometimes convenient to divide them into groups according to unit cell configurations and/or atomic arrangements. One such scheme is based on the unit cell geometry The unit cell geometry is completely defined in terms of six parameters: the three edge lengths a, b, and c, and the three interaxial angles 𝛼, 𝛽, and 𝛾 which is sometimes termed as lattice parameters. END 3.1 Impurities in Solids Why study Imperfections in Solids? The properties of some materials are profoundly influenced by the presence of imperfections. Consequently, it is important to have a knowledge about the types of imperfections that exist and the roles they play in affecting the behavior of materials. For example, the mechanical properties of pure metals experience significant alterations when the metals are alloyed (i.e., when impurity atoms are added)—for example, brass (70% copper/30% zinc) is much harder and stronger than pure copper. Also, integrated-circuit microelectronic devices found in our computers, calculators, and home appliances function because of highly controlled concentrations of specific impurities that are incorporated into small, localized regions of semiconducting materials. For this module, we are going to deal with the imperfections and defects. But what really are these? It has been tacitly assumed that perfect order exists throughout crystalline materials on an atomic scale. However, such an idealized solid does not exist; all contain large numbers of various defects or imperfections. As a matter of fact, many of the properties of materials are profoundly sensitive to deviations from crystalline perfection; the influence is not always adverse, and often specific characteristics are deliberately fashioned by the introduction of controlled amounts or numbers of particular defects. A crystalline defect refers to a lattice irregularity having one or more of its dimensions on the order of an atomic diameter. Classification of crystalline imperfections is frequently made according to the geometry or dimensionality of the defect. Several different imperfections, including point defects (those associated with one or two atomic positions); linear (or one-dimensional) defects; and interfacial defects, or boundaries, which are two-dimensional. Impurities in solids are also discussed, because impurity of atoms may exist as point defects. Let's start with impurities! A pure metal consisting of only one type of atom just isn't possible; impurity or foreign atoms are always present, and some exist as crystalline point defects. It is difficult to refine metals to a purity in excess of 99.9999%. At this level, on the order of 10 22 to 1023 impurity atoms are present in 1 m3 of material. Alloys They are not highly pure metal in which impurity atoms have been added intentionally to impart specific characteristics. Alloying is used in metals to improve mechanical strength and corrosion resistance. The addition of impurity atoms to a metal results in the formation of a solid solution and/or a new second phase. Solvent is the element or compound that is present in the greatest amount; on occasion, solvent atoms are also called host atoms. Solute is used to denote an element or compound present in a minor concentration. Solid Solutions A solid solution is compositionally homogeneous; the impurity atoms are randomly and uniformly dispersed within the solid. Impurity point defects are found in solid solutions, of which there are two types: –substitutional (solute or impurity atoms replace or substitute for the host atoms) –interstitial Here's a picture of the substitutional and interstitial: In impurities of solids, there is one rule named Hume-Rothery Rule: 1. Atomic size factor –Appreciable quantities of a solute may be accommodated in this type of solid solution only when the difference in atomic radii between the two atom types is less than about 15%. Otherwise, the solute atoms create substantial lattice distortions and a new phase forms. 2. Crystal Structure –For appreciable solid solubility, the crystal structures for metals of both atom types must be the same. 3. Electronegativity factor –The more electropositive one element and the more electronegative the other, the greater the likelihood that they will form an intermetallic compound instead of a substitutional solid solution. 4. Valences –Other factors being equal, a metal has more of a tendency to dissolve another metal of higher valency than to dissolve one of a lower valency. 3.3 Diffusion Mechanisms Many reactions and processes that are important in the treatment of materials rely on the transfer of mass either within a specific solid (ordinarily on a microscopic level) or from a liquid, a gas, or another solid phase. This is necessarily accomplished by diffusion, the phenomenon of material transport by atomic motion. This section discusses the atomic mechanisms by which diffusion occurs, the mathematics of diffusion, and the influence of temperature and diffusing species on the rate of diffusion. The phenomenon of diffusion may be demonstrated with the use of a diffusion couple, which is formed by joining bars of two different metals together so that there is intimate contact between the two faces. The phenomenon of material transport by atomic motion. Copper atoms have migrated or diffused into the nickel, and that nickel has diffused into copper.  The process by which atoms of one metal diffuse into another is termed interdiffusion, or impurity diffusion. –Interdiffusion - there is a net drift or transport of atoms from high- to low-concentration regions. Diffusion also occurs for pure metals, but all atoms exchanging positions are of the same type; this is termed self-diffusion. Two mechanisms for diffusion are possible: 1. Vacancy diffusion - occurs via the exchange of an atom residing on a normal lattice site with an adjacent vacancy. 2. Interstitial diffusion - an atom migrates from one interstitial position to an empty adjacent one  In most metal alloys, interstitial diffusion occurs much more rapidly than diffusion by the vacancy mode, because the interstitial atoms are smaller and thus more mobile. Furthermore, there are more empty interstitial positions than vacancies; hence, the probability of interstitial atomic movement is greater than for vacancy diffusion. Imperfections in Solid  Crystal Defects - imperfection in the regular geometrical arrangement of the atoms in a crystalline solid - imperfections result from deformation of the solid, rapid cooling from high temperature, or high-energy radiation (X-rays or neutrons) striking the solid. - located at single points, along lines, or on whole surfaces in the solid - defects influence its mechanical, electrical, and optical behaviour. Point defects are where an atom is missing or have an irregular position in the lattice structure. Self interstitial – an atom from the crystal that is crowded into an interstitial site, a small void space that under ordinary circumstances is not occupied. Substitution Impurity – there is an atom of a different type than the bulk atoms which has replaced one of the bulk atoms in the lattice. They are usually close in size, about 15% to the bulk atom. Interstitial Impurity – atoms are much smaller than the atoms in the bulk matrix. It fits into the open space between the bulk atoms of the lattice structure. Vacancies – one normally occupied from which an atom is missing Frenkel – a cation–vacancy and a cation–interstitial pair.  Line Defects Line defects/dislocations atoms are out of position in the crystal structure. It is generated and move when a stress is applied. Edge dislocation is an extra half plane of atoms “inserted” into the crystal lattice. Due to the edge dislocations metals possess high plasticity characteristics: ductility and malleability. Screw dislocation forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part. It is called screw as atomic planes form a spiral surface around the dislocation line.  Surface Defects It arise at the boundary between two grains or small crystals within a larger crystal. Grain Boundaries – the boundary separating two small grains or crystals having different crystallographic orientations in polycrystalline materials. Tilt Boundaries – occurs in between of two slightly misaligned grains that appears as an array of edge dislocations. Twin Boundaries – appears as an array of screw dislocations. Stacking Faults – formed by fault in the stacking sequence of atomic planes in crystals.  Volume Defects (or Bulk defects) - It is a three-dimensional aggregates of atoms or vacancies. - Much larger defects compared to the previous ones - Common examples are cracks, pores, and other phases  Volume Defects (or Bulk defects) Inclusions – varies in size from a few microns to macroscopic dimensions; relatively large, entered the system as a dirt and usually formed through precipitation.  Volume Defects (or Bulk defects) Voids – holes in the solid formed by trapped gases or by the accumulation of vacancies.  Microscopic Examination - The grains of many crystals have diameters in order of micros (10-6 meters) - Microstructure – Structural features subject to observation under a microscope - Microscopy – Use of a microscope in studying crystal structure - Photomicrograph – A picture taken by a microscope Microscopic Techniques  Optical Microscopy “Light” microscope Uses a series of lenses to magnify images Has three basic lenses (4x, 10x, and a third ranging from 20x – 100x) Has a magnification limit of 2000x Electron Microscopy Uses focused beam of electrons to magnify target Magnification up to 2,000,000x Has 4 main types: TEM, SEM, REM, STEM  Transmission Electron Microscopy (TEM) Original form of electron microscope Utilizes an electron gun with a tungsten filament Image projected unto a phosphor viewing screen  Scanning Electron Microscopy (SEM) Scans rectangular area by using a focused beam of electrons Electrons give off differing energies based on structure of target Microscope reads these energies and produces a visual representation  Reflection Electron Microscopy (REM) Like TEM, uses a beam of electrons to develop a picture of the target Reads the reflected beam of electrons to form visual representation  Scanning Transmission Electron Microscope (STEM) A type of Transmission Electron Microscope Electrons focus on a small area of specimen Electrons pass through the sample, and a visual is formed END Material Science and Engineering Diffusion in Solids Diffusion Mechanisms Diffusion is the stepwise migration of atoms from lattice site to lattice site. Conditions for the atom to make such move: There must be an empty adjacent site. the atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement.  Two Types of Metallic Diffusion 1. Vacancy Diffusion 2. Interstitial Diffusion  Vacancy Diffusion Involves the interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy. Since diffusing atoms and vacancies exchange positions, the diffusion of atoms in one direction corresponds to the motion of vacancies in the opposite direction.  Interstitial Diffusion Involves atoms that migrate from an interstitial position to a neighboring one that is empty. This mechanism is found for interdiffusion of impurities such as hydrogen, carbon, nitrogen, and oxygen, which have atoms that are small enough to fit into the interstitial positions. In most metal alloys, interstitial diffusion occurs much more rapidly than diffusion by the vacancy mode, since the interstitial atoms are smaller and thus more mobile.  STEADY-STATE DIFFUSION Diffusion flux does not change in time  STEADY-STATE DIFFUSION FICK’S FIRST LAW where: Diffusion coefficient or Diffusivity (D) – constant of proportionality (m2/s) dC/dx – gradient of concentration NON STEADY-STATE DIFFUSION Diffusion flux vary with time  NON STEADY-STATE DIFFUSION FICK’S SECOND LAW - If the diffusion coefficient is independent of composition (which should be verified for each particular diffusion situation)  Factors Affecting Diffusion Diffusing Species - Self diffusion and interdiffusion are generally slower than interstitial diffusion. Diffusion Mechanism - Interstitial is usually faster than vacancy Microstructure - Diffusion is faster in polycrystalline vs single crystal materials because of the rapid diffusion along grain boundaries and dislocation cores. Temperature - Diffusion is a thermally activated process which occurs faster at higher temperatures. END  Material Science and Engineering Electronic Structures and Processes  CONDUCTORS, SEMICONDUCTORS and INSULATORS ► One way of classifying solid materials is according to the ease with which they conduct an electric current through this classification scheme of three groups: ► Conductors ► Semiconductors ► Insulators A diagram showing the valence and conduction bands of insulators, metals, and semiconductors. The Fermi level is the name given to the highest energy occupied electron orbital at absolute zero. Conductors In a conductor the valence band is full of electrons, while the conduction band has some free electrons and many empty energy levels. The addition of a very small amount of energy will allow electrons to move within the conduction band, some rising to a higher level and others returning to lower levels. This movement of electrons is electrical conduction. Semiconductors In a semiconductor, the gap between the valence band and conduction band is smaller and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band allowing some conduction to take place. An increase in temperature increases the conductivity of a semiconductor. Insulators In the insulator the valence band is full once again, but in these substances the energy gap between this and the empty conduction band is very large. It would take a great deal of energy to make an electron jump the gap and to cause the insulator to break down. At very high temperatures or under very large electric fields breakdown will occur, and like semiconductors the greater the temperature the greater the conduction.  ELECTRONIC AND IONIC CONDUCTION An electric current results from the motion of electrically charged particles in response to forces that act on them from an externally applied electric field. Within most solid materials a current arises from the flow of electrons, which is termed electronic conduction. For ionic materials a net motion of charged ions is possible that produces a current, such is termed ionic conduction.  ENERGY BAND STRUCTURES IN SOLIDS The electrical properties of a solid material depends upon its electron band structure – that is, the arrangement of the outermost electron bands and the way in which they are filled with electrons Four different types of band structures possible at 0 Kelvin (K): 1. THE FIRST BAND STRUCTURE ❖In this band structure, one outermost band is only partially filled with electrons. The energy corresponding to the highest filled state at 0 K is called the Fermi energy (Ef). ❖ This energy band structure is typified by some metals, in particular those that have a single s valence electrons (e.g. copper). 2. THE SECOND BAND STRUCTURE ❖This structure is also found metals. ❖In this structure, there is an overlap of an empty band and a filled band. ❖Magnesium has this band structure. 3. THE FINAL TWO BAND STRUCTURES ❖ this two band structure are similar. ❖ one band (the valence band) that is completely filled with electrons is separated from an empty conduction band and an energy gap lies between them. ❖ the difference between the two band structures lies in the magnitude of the energy gap. ❖for insulators, the band gap is relatively wide while for semiconductors it is narrow. ❖the Fermi energy for these band structures lies within the band gap near its center. Figure 2 The various possible electron band structures in solids at 0 K CONDUCTORS Conductors have vacant energy states adjacent to the highest filled state at Ef. Thus, very little energy is required to promote electrons into the low-lying empty states. Generally, the energy provided by an electric field is sufficient to excite large numbers of electrons into these conducting states. Figure 3 For a metal, occupancy of electron states (a) before and (b) after an electron excitation.  Insulators and Semiconductors For insulators and semiconductors, empty states adjacent to the top of the filled valence band are not available. To become free, therefore, electrons must be promoted across the energy band gap and into empty states at the bottom of the conduction band.  Insulators and Semiconductors The number of electrons excited thermally (by heat energy) into the conduction band depends on the energy band gap width as well as temperature. At a given temperature, the larger the gap the lower is the probability that a valence electron will be promoted into an energy state within the conduction band; this results in fewer conduction of electrons.  Insulators and Semiconductors Increasing the temperature of either a semiconductor or an insulator results in an increase in the thermal energy that is available for electron excitation. Thus, more electrons are promoted into the conduction band, which gives rise to an enhanced conductivity. Figure 4 For an insulator or semiconductor, occupancy of electron states (a) before and (b) after an electron excitation  ELECTRICAL RESISTIVITY OF METALS Since crystalline defects serve as scattering centers for conduction electrons in metals, increasing their number raises the resistivity (or lowers the conductivity). Matthiessen’s rule - the total resistivity of a metal is the sum of the contributions from thermal vibrations, impurities, and plastic deformation; that is, the scattering mechanisms act independently of one another. Which represent the individual thermal, impurity, and deformation resistivity contributions, respectively. Table 1 Room-Temperature Electrical Conductivities for Nine Common Metals and Alloys  Semiconductivity The electrical conductivity of the semiconducting materials is not as high as that of the metals; nevertheless, they have some unique electrical characteristics that render them especially useful. The electrical properties of these materials are extremely sensitive to the presence of even minute concentrations of impurities.  Semiconductivity Intrinsic semiconductors are those in which the electrical behavior is based on the electronic structure inherent in the pure material. When the electrical characteristics are dictated by impurity atoms, the semiconductor is said to be extrinsic.  INTRINSIC SEMICONDUCTOR - also called an undoped semiconductor or i-type semiconductor - a pure semiconductor without any significant dopant species present. (dopant is a trace impurity element that is inserted into a substance to alter the electrical or optical properties of the substance) - Pure silicon is therefore an example of an intrinsic semiconductor.  INTRINSIC SEMICONDUCTOR Properties: 1. In intrinsic semiconductor, the number density of electrons is equal to the number density of holes. i.e., ne=nh. 2. The electrical conductivity is low. 3. The electrical conductivity of intrinsic semiconductors depends on their temperatures  EXTRINSIC SEMICONDUCTOR - are semiconductors when a trivalent or pentavalent impurity is added to a pure semiconductor - conductivity of extrinsic semiconductor is quite large as compared to the conductivity of intrinsic semiconductor.  EXTRINSIC SEMICONDUCTOR - When some impurity is added in the intrinsic semiconductor, extrinsic semiconductors can be produced. - an improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity.  EXTRINSIC SEMICONDUCTOR Properties: 1. In extrinsic semiconductor, the number density of electrons is not equal to the number density of holes. 2. The electrical conductivity is high. 3. The electrical conductivity depends on the temperature and the amount of impurity added in them. END