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Lecture 2: Introduction to Material Science and Engineering ENSC 20062 Material Science and Engineering Kaycee B. Victorio, REE, RME Department of Electrical Engineering College of Engineering Polytechnic University of the Philippines Manila  Discussion Outline  What is Materials Science a...

Lecture 2: Introduction to Material Science and Engineering ENSC 20062 Material Science and Engineering Kaycee B. Victorio, REE, RME Department of Electrical Engineering College of Engineering Polytechnic University of the Philippines Manila  Discussion Outline  What is Materials Science and Engineering?  Classification of Materials  Functional Classification of Materials  Classification of Materials Based on Structure  Environmental and Other Effects  Materials Design and Selection  What is Materials Science and Engineering?  Materials Science and Engineering  Composition means the chemical make-up of a material.  Structure means a description of the arrangements of atoms or ions in a material.  Synthesis is the process by which materials are made from naturally occurring or other chemicals.  Processing means different ways for shaping materials into useful components or changing their properties. © 2003 Brooks/Cole Publishing / Thomson Learning  © 2003 Brooks/Cole Publishing / Thomson Learning Application of the tetrahedron of materials science and engineering to ceramic superconductors. Note that the microstructure-synthesis and processing-composition are all interconnected and affect the performance-to- cost ratio  © 2003 Brooks/Cole Publishing / Thomson Learning Application of the tetrahedron of materials science and engineering to sheet steels for automotive chassis. Note that the microstructure-synthesis and processing-composition are all interconnected and affect the performance-to-cost ratio  © 2003 Brooks/Cole Publishing / Thomson Learning Application of the tetrahedron of materials science and engineering to semiconducting polymers for microelectronics  Classification of Materials ❑ Metals and Alloys ❑ Ceramics, Glasses,and Glass-ceramics ❑ Polymers (plastics), Thermoplastics and Thermosets ❑ Semiconductors ❑ Composite Materials  Representative examples, applications, and properties for each category of materials Example of Applications Properties Metals and Alloys Gray cast iron Automobile engine blocks Castable, machinable, vibration damping Ceramics and Glasses SiO2-Na2O-CaO Window glass Optically transparent, thermally insulating Polymers Polyethylene Food packaging Easily formed into thin, flexible, airtight film  Representative examples, applications, and properties for each category of materials Example of Applications Properties Semiconductors Silicon Transistors and integrated Unique electrical circuits behavior Composites Carbide cutting tools for High hardness, yet Tungsten carbide machining good shock resistance -cobalt (WC-Co)  © 2003 Brooks/Cole Publishing / Thomson Learning Representative strengths of various categories of materials A section through a jet engine. The forward compression section operates at low to medium temperatures, and titanium parts are often used. The rear combustion section operates at A variety of complex ceramic high temperatures and nickel- components, including based superalloys are required. impellers and blades, which The outside shell experiences low allow turbine engines to temperatures, and aluminum and operate more efficiently at composites are satisfactory. higher temperatures. (Courtesy of GE Aircraft (Courtesy of Certech, Inc.) Engines.)  © 2003 Brooks/Cole Publishing / Thomson Learning Polymerization occurs when small molecules, represented by the circles, combine to produce larger molecules, or polymers. The polymer molecules can have a structure that consists of many chains that are entangled but not connected (thermoplastics) or can form three-dimensional networks in which chains are cross- linked (thermosets)  Integrated circuits for computers and other electronic devices rely on the unique electrical behavior of semiconducting Polymers are used materials. The X-wing for in a variety of (Courtesy of Rogers advanced helicopters electronic devices, Corporation.) relies on a material including these composed of a computer dip carbon-fiber- switches, where reinforced polymer. moisture resistance (Courtesy of Sikorsky and low Aircraft Division— conductivity are United Technologies required. (Courtesy Corporation.) of CTS Corporation.)  Functional Classification of Materials ❑ Aerospace ❑ Biomedical ❑ Electronic Materials ❑ Energy Technology and Environmental Technology ❑ Magnetic Materials ❑ Photonic or Optical Materials ❑ Smart Materials ❑ Structural Materials © 2003 Brooks/Cole Publishing / Thomson Learning Functional classification of materials. Notice that metals, plastics, and ceramics occur in different categories. A limited number of examples in each category is provided Classification of Materials-Based on Structure ❑ Crystalline material is a material comprised of one or many crystals. In each crystal, atoms or ions show a long-range periodic arrangement. ❑ Single crystal is a crystalline material that is made of only one crystal (there are no grain boundaries). ❑ Grains are the crystals in a polycrystalline material. ❑ Polycrystalline material is a material comprised of many crystals (as opposed to a single-crystal material that has only one crystal). ❑ Grain boundaries are regions between grains of a polycrystalline material.  Environmental and Other Effects Effects of following factors must be accounted for in design to ensure that components do not fail unexpectedly: ❑ Temperature ❑ Corrosion ❑ Fatigue ❑ Strain Rate  Increasing temperature normally reduces the strength of a material. Polymers are suitable only at low temperatures. Some composites, special alloys, and ceramics, © 2003 Brooks/Cole Publishing / Thomson Learning have excellent properties at high temperatures  © 2003 Brooks/Cole Publishing / Thomson Learning Figure 1.13 Skin operating temperatures for aircraft have increased with the development of improved materials. (After M. Steinberg, Scientific American, October, 1986.)  Materials Design and Selection ❑ Density is mass per unit volume of a material, usually expressed in units of g/cm3 or lb/in.3 ❑ Strength-to-weight ratio is the strength of a material divided by its density; materials with a high strength- to-weight ratio are strong but lightweight. Course Assignment  Pick a movie you haven’t watched from the The 150 Greatest Science Fiction Movies of All Time by Rolling Stones (https://www.rollingstone.com/tv-movies/tv-movie-lists/best- sci-fi-movies-1234893930/). Watch the movie and take note of a technology or material that interests you. Discuss how does material science and engineering plays a role in the development of technology or material that you find interesting in the movie. Make an inference what is the technology or material made of and its structure, and how can it be process and synthesis. Also, discuss how is the performance of the technology/material is important and estimate how much does it cost in Philippine peso. Course Assignment  Submission should be made online through our MS Teams in the Assignments tab. Submission should be in uneditable, readable, and comment-able format (PDF is recommended!).  File should be named as follows: CAXX SurnameFNMI (e.g. CA01 VictorioKCB). Do not forget to write your name, student number as header in the upper left portion of your submission. Copying the question/problem will be highly appreciated. DO NOT FORGET TO HIGLIGHT YOUR FINAL ANSWER. Page number in the following format Page X of Y (and centered) is appreciated. Further Reading  Ashby, MF, et-al (2013). Materials: Engineering, Science, Processing and Design (3rd ed). Butterworth- Heinemann.  Askeland, DR (2010). The Science and Engineering of Materials (6th ed). Cengage Learning.  Askeland, DR (2013). Essentials of Material Science and Engineering (3rd ed). Cengage Learning.  Askeland, DR, et-al (2011). The Science and Engineering of Materials, SI Edition (6th ed). Cengage Learning.  Callister, WD (2004). Fundamentals of Material Science and Engineering: An Integrated Approach (2nd ed). Wiley & Sons. Further Reading  Chung, YW (2006). Introduction to Materials Science and Engineering (1st ed). CRC Press.  Hashemi, J & Smith, W (2009). Foundations of Material Science and Engineering (5th ed). McGraw-Hill.  Newell, JA (2008). Essentials of Modern Material Science and Engineering (1st ed). Wiley & Sons.  Shackelford, JF (2008). Introduction to Material Science for Engineers (7th ed). Prentice Hall.  Lecture 3: Chemistry of Materials Structure of Atom Periodic Table Next Atomic Bonding Binding Energy and Interatomic Spacing Meeting Atomic and Ionic Arrangements Imperfections Movements Lecture 2: Introduction to Material Science and Engineering ENSC 20062 Material Science and Engineering Kaycee B. Victorio, REE, RME Department of Electrical Engineering College of Engineering Polytechnic University of the Philippines Manila Lecture 3 | Chemistry of Materials ENSC 20062 Material Science and Engineering Kaycee B. Victorio, REE, RME Department of Electrical Engineering | College of Engineering | Polytechnic University of the Philippines Manila 1 Discussion Outline  The Structure of Materials  The Structure of the Atoms  The Electron Structure of the Atom  The Periodic Table  Atomic Bonding  Binding Energy and Interatomic Spacing  Short-Range Order versus Long-Range Order 2 2 Discussion Outline  Amorphous Materials: Principles and Technological Applications  Lattice, Unit Cells, Basis, and Crystal Structures  Allotropic or Polymorphic Transformations  Points, Directions, and Planes in the Unit Cell  Interstitial Sites 3 3 Discussion Outline  Crystal Structures of Ionic Materials  Covalent Structures  Diffraction Techniques for Crystal Structure Analysis  Point Defects  Other Point Defects  Dislocations, Observing Dislocations, and Significance of Dislocations 4 4 Discussion Outline  Schmid’s Law  Influence of Crystal Structure  Surface Defects  Importance of Defects  Applications of Diffusion  Stability of Atoms and Ions  Mechanisms for Diffusion  Activation Energy for Diffusion 5 5 Discussion Outline  Rate of Diffusion (Fick’s First Law)  Factors Affecting Diffusion  Permeability of Polymers  Composition Profile (Fick’s Second Law)  Diffusion and Materials Processing 6 6  The Structure of Materials: Technological Relevance  Nanotechnology  Micro-electro- mechanical (MEMS) systems-Airbag sensors  Nanostructures 7 7  Table 1 Levels of Structure Level of Structure Example of Technologies Atomic Structure Diamond – edge of cutting tools Atomic Arrangements: Lead-zirconium-titanate Long-Range Order [Pb(Zrx Ti1-x )] or PZT – (LRO) gas igniters Atomic Arrangements: Amorphous silica - fiber Short-Range Order optical communications (SRO) industry 8 8  Table 1 Levels of Structure Level of Structure Example of Technologies Nanostructure Nano-sized particles of iron oxide – ferrofluids Microstructure Mechanical strength of metals and alloys Macrostructure Paints for automobiles for corrosion resistance 9 9  The Structure of the Atom  The atomic number of an element is equal to the number of electrons or protons in each atom.  The atomic mass of an element is equal to the average number of protons and neutrons in the atom.  The Avogadro number of an element is the number of atoms or molecules in a mole.  The atomic mass unit of an element is the mass of an atom expressed as 1/12 the mass of a carbon atom. 10 10  The Electronic Structure of the Atom  Quantum numbers are the numbers that assign electrons in an atom to discrete energy levels.  A quantum shell is a set of fixed energy levels to which electrons belong.  Pauli exclusion principle specifies that no more than two electrons in a material can have the same energy. The two electrons have opposite magnetic spins.  The valence of an atom is the number of electrons in an atom that participate in bonding or chemical reactions.  Electronegativity describes the tendency of an atom to gain an electron. 11 11  © 2003 Brooks/Cole Publishing / Thomson Learning The atomic structure of sodium, atomic number 11, showing the electrons in the K, L, and M quantum shells 12 12  © 2003 Brooks/Cole Publishing / Thomson Learning The complete set of quantum numbers for each of the 11 electrons in sodium 13 13  14 14  © 2003 Brooks/Cole Publishing / Thomson Learning The electronegativities of selected elements relative to the position of the elements in the periodic table 15 15  Example 1 Comparing Electronegativities Using the electronic structures, compare the electronegativities of calcium and bromine. SOLUTION The electronic structures, obtained from Appendix C, are: Ca: 1s22s22p63s23p6 4s2 Br: 1s22s22p63s23p63d10 4s24p5 Calcium has two electrons in its outer 4s orbital and bromine has seven electrons in its outer 4s4p orbital. Calcium, with an electronegativity of 1.0, tends to give up electrons and has low electronegativity, but bromine, with an electronegativity of 2.8, tends to accept electrons and is strongly electronegative. This difference in electronegativity values suggests that these elements may react readily to form a compound. 16 16  The Periodic Table  III-V semiconductor is a semiconductor that is based on group 3A and 5B elements (e.g. GaAs).  II-VI semiconductor is a semiconductor that is based on group 2B and 6B elements (e.g. CdSe).  Transition elements are the elements whose electronic configurations are such that their inner “d” and “f” levels begin to fill up.  Electropositive element is an element whose atoms want to participate in chemical interactions by donating electrons and are therefore highly reactive. 17 17  18 18 The Periodic Table of Elements © 2003 Brooks/Cole Publishing / Thomson Learning © 2003 Brooks/Cole Publishing / Thomson Learning  Atomic Bonding  Metallic bond, Covalent bond, Ionic bond, van der Waals bond are the different types of bonds.  Ductility refers to the ability of materials to be stretched or bent without breaking  Van der Waals interactions: London forces, Debye interaction, Keesom interaction  Glass temperature is a temperature above which many polymers and inorganic glasses no longer behave as brittle materials  Intermetallic compound is a compound such as Al3V formed by two or more metallic atoms 19 19 © 2003 Brooks/Cole Publishing / Thomson Learning The metallic bond forms when atoms give up their valence electrons, which then form an electron sea. The positively charged atom cores are bonded by mutual attraction to the negatively charged electrons 20 20  © 2003 Brooks/Cole Publishing / Thomson Learning When voltage is applied to a metal, the electrons in the electron sea can easily move and carry a current 21 21  © 2003 Brooks/Cole Publishing / Thomson Learning Covalent bonding requires that electrons be shared between atoms in such a way that each atom has its outer sp orbital filled. In silicon, with a valence of four, four covalent bonds must be formed 22 22  © 2003 Brooks/Cole Publishing / Thomson Learning Covalent bonds are directional. In silicon, a tetrahedral structure is formed, with angles of 109.5° required between each covalent bond 23 23  © 2003 Brooks/Cole Publishing / Thomson Learning The tetrahedral structure of silica (Si02), which contains covalent bonds between silicon and oxygen atoms 24 24  Example 2 Design of a Thermistor A thermistor is a device used to measure temperature by taking advantage of the change in electrical conductivity when the temperature changes. Select a material that might serve as a thermistor in the 500 to 1000oC temperature range. Photograph of a commercially available thermistor. (Courtesy of Vishay Intertechnology, Inc.) 25 25 Example 2 Design of a Thermistor The resistance of a thermistor can be made to increase or decrease with increasing temperature. These are known as positive temperature coefficient of resistance (PTCR) or negative temperature coefficient of resistance (NTCR) thermistors, respectively.The fact that a thermistor changes its resistance in response to a temperature change is used to control temperature or switch (turn ‘‘on’’ and ‘‘off ’’) the operation of an electrical circuit when a particular device (i.e., a refrigerator, hairdryer, furnace, oven, or reactor) reaches a certain temperature. 26 26 Example 2 Design of a Thermistor Two design requirements must be satisfied. First, a material with a high melting point must be selected. Second, the electrical conductivity of the material must show a systematic and reproducible change as a function of temperature. Covalently bonded materials might be suitable. They often have high melting temperatures, and, as more covalent bonds are broken when the temperature increases, increasing numbers of electrons become available to transfer electrical charge. The semiconductor silicon is one choice: Silicon melts at 1410oC and is covalently bonded. A number of ceramic materials also have high melting points and behave as semiconducting materials. Silicon will have to be protected against oxidation. We will have to make sure the changes in conductivity in the temperature range are actually acceptable. Some thermistors that show a predictable decrease in the resistance with increasing temperature are made from semiconducting materials. 27 27 Example 2 Design of a Thermistor Polymers would not be suitable, even though the major bonding is covalent, because of their relatively low melting, or decomposition, temperatures. Many thermistors that can be used for switching applications make use of barium titanate (BaTiO3) based formulations. Many useful NTCR materials are based on Fe3O4-ZnCr2O4, Fe3O4-MgCr2O4, or Mn3O4, doped with Ni, Co, or Cu. In almost any design situation, once the technical performance criteria are met we should always pay attention to and take into account the cost of raw materials, manufacturing costs, and other important factors such as the durability. In some applications, we also need to pay closer attention to the environmental impact including the ability to recycle materials. 28 28  © 2003 Brooks/Cole Publishing / Thomson Learning An ionic bond is created between two unlike atoms with different electronegativities. When sodium donates its valence electron to chlorine, each becomes an ion; attraction occurs, and the ionic bond is formed 29 29  © 2003 Brooks/Cole Publishing / Thomson Learning When voltage is applied to an ionic material, entire ions must move to cause a current to flow. Ion movement is slow and the electrical conductivity is poor 30 30  © 2003 Brooks/Cole Publishing / Thomson Learning Illustration of London forces, a type of a van der Waals force, between atoms 31 31  © 2003 Brooks/Cole Publishing / Thomson Learning The Keesom interactions are formed as a result of polarization of molecules or groups of atoms. In water, electrons in the oxygen tend to concentrate away from the hydrogen. The resulting charge difference permits the molecule to be weakly bonded to other water molecules 32 32  (a) In polyvinyl chloride (PVC), the chlorine atoms attached to the polymer chain have a negative charge and the hydrogen atoms are positively charged. The chains are weakly bonded by van der Waals bonds. This additional bonding makes PVC stiffer, (b) When a force is applied to the polymer, the van der Waals bonds are broken and the chains slide past one another © 2003 Brooks/Cole Publishing / Thomson Learning 33 33  Example 3 Design Strategies for Silica Optical Fibers Silica is used for making long lengths of optical fibers. Being a covalently and ionically bonded material, the strength of Si-O bonds is expected to be high. Other factors such as susceptibility of silica surfaces to react with water vapor in atmosphere have a deleterious effect on the strength of silica fibers. Give n this, what design strategies can you think of such that silica fibers could still be bent to a considerable degree without breaking? 34 34 Example 3 Design Strategies for Silica Optical Fibers Based on the mixed ionic and covalent bonding in silica we know that the Si-O bonds are very strong. We also know that covalent bonds will be directional and hence we can anticipate silica to exhibit limited ductility. Therefore, our choices to enhance ductility of optical fibers are rather limited since the composition is essentially fixed. Most other glasses are also brittle. We can make an argument that silica fibers will exhibit better ductility at higher temperatures. However, we have to use them for making long lengths of optical fibers (most of which are to be buried underground or under the sea) and hence keeping them at an elevated temperature is not a practical option. 35 35 Example 3 Design Strategies for Silica Optical Fibers Therefore, we need to understand, beyond what the nature of bonding consideration can offer us, why glass fibers exhibit limited ductility. Is this a property that is intrinsic to the glass or are there external variables that are causing a change in the chemistry and structure of the glass? Materials scientists and engineers have recognized that the lack of ductility in optical glass fibers is linked to the ability of the silica surface to react with water vapor in the atmosphere. They have found that water vapor in the atmosphere reacts with the surface of silica leading to micro-cracks on the surface.When subjected to stress these cracks grow rapidly and the fibers break quite easily! They have also tested silica fibers in a vacuum and found that the levels to which one can bend fibers are much higher. 36 36  Binding Energy and Interatomic Spacing  Interatomic spacing is the equilibrium spacing between the centers of two atoms.  Binding energy is the energy required to separate two atoms from their equilibrium spacing to an infinite distance apart.  Modulus of elasticity is the slope of the stress-strain curve in the elastic region (E).  Yield strength is the level of stress above which a material begins to show permanent deformation.  Coefficient of thermal expansion (CTE) is the amount by which a material changes its dimensions when the temperature changes. 37 37  Atoms or ions are separated by and equilibrium spacing that corresponds to the minimum inter- atomic energy for a pair of atoms or ions (or when zero force is acting to repel or attract the atoms or ions) © 2003 Brooks/Cole Publishing / Thomson Learning 38 38  39 39  © 2003 Brooks/Cole Publishing / Thomson Learning The force-distance curve for two materials, showing the relationship between atomic bonding and the modulus of elasticity, a steep dFlda slope gives a high modulus 40 40  © 2003 Brooks/Cole Publishing / Thomson Learning The inter-atomic energy (IAE)-separation curve for two atoms. Materials that display a steep curve with a deep trough have low linear coefficients of thermal expansion 41 41  Example 4 Design of a Space Shuttle Arm NASA’s space shuttles have a long manipulator robot arm, also known as the Shuttle Remote Manipulator System or SRMS, that permits astronauts to launch and retrieve satellites. It is also used to view and monitor the outside of the space shuttle using a mounted video camera. Select a suitable material for this device. NASA’s Shuttle Remote Manipulator System: SRMS. Courtesy of Getty Images) 42 42 Example 4 Design of a Space Shuttle Arm Let’s look at two of the many material choices. First, the material should be stiff so that little bending occurs when a load is applied; this feature helps the operator maneuver the manipulator arm precisely. Generally, materials with strong bonding and high melting points also have a high modulus of elasticity,or stiffness. Second, the material should be light in weight to permit maximum payloads to be carried into orbit; a low density is thus desired. It is estimated that it costs about US $100,000 to take the weight of a beverage can into space! Thus, the density must be as low as possible. 43 43 Example 4 Design of a Space Shuttle Arm Good stiffness is obtained from high-melting-point metals (such as beryllium and tungsten), from ceramics, and from certain fibers (such as carbon). Tungsten, however, has a very high density, while ceramics are very brittle. Beryllium, which has a modulus of elasticity that is greater than that of steel and a density that is less than that of aluminum, might be an excellent candidate. However, toxicity of Be and its compounds must be considered. The preferred material is a composite consisting of carbon fibers embedded in an epoxy matrix. The carbon fibers have an exceptionally high modulus of elasticity, while the combination of carbon and epoxy provides a very low- density material. Other factors such as exposure to low and high temperatures in space and on earth must also be considered. The current shuttle robot arm is about 45 feet long, 15 inches in diameter and weighs about 900 pounds. When in space it can manipulate weights up to 260 tons. 44 44  Short-Range Order versus Long-Range Order  Short-range order - The regular and predictable arrangement of the atoms over a short distance - usually one or two atom spacings.  Long-range order (LRO) - A regular repetitive arrangement of atoms in a solid which extends over a very large distance.  Bose-Einstein condensate (BEC) - A newly experimentally verified state of a matter in which a group of atoms occupy the same quantum ground state. 45  Levels of atomic arrangements in materials: (a) Inert monoatomic gases have no regular ordering of atoms: (b,c) Some materials, including water vapor, nitrogen gas, amorphous silicon and silicate glass have short-range order. (d) Metals, alloys, many ceramics and some polymers have regular ordering of atoms/ions that extends through the material. (c) 2003 Brooks/Cole Publishing / Thomson Learning 46  Basic Si-0 tetrahedron in silicate glass. (c) 2003 Brooks/Cole Publishing / Thomson Learning 47  Tetrahedral arrangement of C-H bonds in polyethylene. (c) 2003 Brooks/Cole Publishing / Thomson Learning 48 (a) Photograph of a silicon single crystal. (b) Micrograph of a polycrystalline stainless steel showing grains and grain boundaries (Courtesy Dr. M. Hua, Dr. I. Garcia, and Dr. A.J. Deardo.) 49 Liquid crystal display. These materials are amorphous in one state and undergo localized crystallization in response to an external electric field and are widely used in liquid crystal displays. (Courtesy of Nick Koudis/PhotoDisc/Getty Images.) 50  (c) 2003 Brooks/Cole Publishing / Thomson Learning Classification of materials based on the type of atomic order. 51  Amorphous Materials: Principles and Technological Applications  Amorphous materials - Materials, including glasses, that have no long-range order, or crystal structure.  Glasses - Solid, non-crystalline materials (typically derived from the molten state) that have only short- range atomic order.  Glass-ceramics - A family of materials typically derived from molten inorganic glasses and processed into crystalline materials with very fine grain size and improved mechanical properties. 52  (c) 2003 Brooks/Cole Publishing / Thomson Learning This figure shows a schematic of the blow-stretch process used for fabrication of a standard two-liter PET (polyethylene terephthalate) bottle from a preform. The stress induced crystallization leads to formation of small crystals that help reinforce the remaining amorphous matrix. 53  (c) 2003 Brooks/Cole Publishing / Thomson Learning Atomic arrangements in crystalline silicon and amorphous silicon. (a) Amorphous silicon. (b) Crystalline silicon. Note the variation in the inter-atomic distance for amorphous silicon. 54  Lattice, Unit Cells, Basis, and Crystal Structures  Lattice - A collection of points that divide space into smaller equally sized segments.  Basis - A group of atoms associated with a lattice point.  Unit cell - A subdivision of the lattice that still retains the overall characteristics of the entire lattice.  Atomic radius - The apparent radius of an atom, typically calculated from the dimensions of the unit cell, using close-packed directions (depends upon coordination number).  Packing factor - The fraction of space in a unit cell occupied by atoms. 55  The fourteen types of Bravais lattices grouped in seven crystal systems. (c) 2003 Brooks/Cole Publishing / Thomson Learning 56 57  Definition of the lattice parameters and their use in cubic, orthorhombic, and hexagonal crystal systems. (c) 2003 Brooks/Cole Publishing / Thomson Learning 58  (a) Illustration showing sharing of face and corner atoms. (b) The models for simple cubic (SC), body centered cubic (BCC), and face-centered cubic (FCC) unit cells, assuming only one atom per lattice point. (c) 2003 Brooks/Cole Publishing / Thomson Learning 59  (c) 2003 Brooks/Cole Publishing / Thomson Learning Illustration of coordination in (a) SC and (b) BCC unit cells. Six atoms touch each atom in SC, while the eight atoms touch each atom in the BCC unit cell. 60  (c) 2003 Brooks/Cole Publishing / Thomson Learning The hexagonal close-packed (HCP) structure (left) and its unit cell. 61 62  Allotropic or Polymorphic Transformations  Allotropy - The characteristic of an element being able to exist in more than one crystal structure, depending on temperature and pressure.  Polymorphism - Compounds exhibiting more than one type of crystal structure. 63 Oxygen gas sensors used in cars and other applications are based on stabilized zirconia compositions. (Image courtesy of Bosch © Robert Bosch GmbH.) 64 Points, Directions, and Planes in the Unit Cell  Miller indices - A shorthand notation to describe certain crystallographic directions and planes in a material. Denoted by [ ] brackets. A negative number is represented by a bar over the number.  Directions of a form - Crystallographic directions that all have the same characteristics, although their ‘‘sense’’ is different. Denoted by h i brackets.  Repeat distance - The distance from one lattice point to the adjacent lattice point along a direction.  Linear density - The number of lattice points per unit length along a direction.  Packing fraction - The fraction of a direction (linear- packing fraction) or a plane (planar-packing factor) that is actually covered by atoms or ions. 65 Coordinates of selected points in the unit cell. The number refers to the distance from the origin in terms of lattice parameters. 66  (c) 2003 Brooks/Cole Publishing / Thomson Learning Equivalency of crystallographic directions of a form in cubic systems. 67 68  Determining the repeat distance, linear density, and packing fraction for direction in FCC copper. (c) 2003 Brooks/Cole Publishing / Thomson Learning 69 70  Miller-Bravais indices are obtained for crystallographic planes in HCP unit cells by using a four-axis coordinate system. (c) 2003 Brooks/Cole Publishing / Thomson Learning 71  (c) 2003 Brooks/Cole Publishing / Thomson Learning Typical directions in the HCP unit cell, using both three-and-four-axis systems. The dashed lines show that the direction is equivalent to a direction. 72 73  The ABABAB stacking sequence of close- packed planes produces the HCP structure. (c) 2003 Brooks/Cole Publishing / Thomson Learning 74  (c) 2003 Brooks/Cole Publishing / Thomson Learning The ABCABCABC stacking sequence of close-packed planes produces the FCC structure. 75  Interstitial Sites  Interstitial sites - Locations between the ‘‘normal’’ atoms or ions in a crystal into which another - usually different - atom or ion is placed. Typically, the size of this interstitial location is smaller than the atom or ion that is to be introduced.  Cubic site - An interstitial position that has a coordination number of eight. An atom or ion in the cubic site touches eight other atoms or ions.  Octahedral site - An interstitial position that has a coordination number of six. An atom or ion in the octahedral site touches six other atoms or ions.  Tetrahedral site - An interstitial position that has a coordination number of four. An atom or ion in the tetrahedral site touches four other atoms or ions. 76  (c) 2003 Brooks/Cole Publishing / Thomson Learning The location of the interstitial sites in cubic unit cells. Only representative sites are shown. 77 78 Crystal Structures of Ionic Materials  Factors need to be considered in order to understand crystal structures of ionically bonded solids: ▪ Ionic Radii ▪ Electrical Neutrality ▪ Connection between Anion Polyhedra ▪ Visualization of Crystal Structures Using Computers 79  (c) 2003 Brooks/Cole Publishing / Thomson Learning Connection between anion polyhedra. Different possible connections include sharing of corners, edges, or faces. In this figure, examples of connections between tetrahedra are shown. 80  (c) 2003 Brooks/Cole Publishing / Thomson Learning (a) The cesium chloride structure, a SC unit cell with two ions (Cs+ and CI-) per lattice point. (b) The sodium chloride structure, a FCC unit cell with two ions (Na+ + CI-) per lattice point. Note: Ion sizes not to scale. 81  (c) 2003 Brooks/Cole Publishing / Thomson Learning (a) The zinc blende unit cell, (b) plan view. 82  (c) 2003 Brooks/Cole Publishing / Thomson Learning (a) Fluorite unit cell, (b) plan view. 83 Learning (c) 2003 Brooks/Cole Publishing / Thomson The perovskite unit cell showing the A and B site cations and oxygen ions occupying the face-center positions of the unit cell. Note: Ions are not show to scale. 84 Crystal structure of a new high Tc ceramic superconductor based on a yttrium barium copper oxide. These materials are unusual in that they are ceramics, yet at low temperatures their electrical resistance vanishes. (Source: ill.fr/dif/3D-crystals/superconductor.html; © M. Hewat 1998.) 85 (c) 2003 Brooks/Cole Publishing / Thomson Learning Corundum structure of alpha-alumina (α-AI203). 86  Covalent Structures  Covalently bonded materials frequently have complex structures in order to satisfy the directional restraints imposed by the bonding.  Diamond cubic (DC) - A special type of face-centered cubic crystal structure found in carbon, silicon, and other covalently bonded materials. 87  (c) 2003 Brooks/Cole Publishing / Thomson Learning (a) Tetrahedron and (b) the diamond cubic (DC) unit cell. This open structure is produced because of the requirements of covalent bonding. 88  Learning (c) 2003 Brooks/Cole Publishing / Thomson The silicon-oxygen tetrahedron and the resultant β-cristobalite form of silica. 89  The unit cell of crystalline polyethylene. 90 (c) 2003 Brooks/Cole Publishing / Thomson Learning  Diffraction Techniques for Crystal Structure Analysis  Diffraction - The constructive interference, or reinforcement, of a beam of x-rays or electrons interacting with a material. The diffracted beam provides useful information concerning the structure of the material.  Bragg’s law - The relationship describing the angle at which a beam of x-rays of a particular wavelength diffracts from crystallographic planes of a given interplanar spacing.  In a diffractometer a moving x-ray detector records the 2y angles at which the beam is diffracted, giving a characteristic diffraction pattern 91  (a) Destructive and (b) reinforcing interactions between x- rays and the crystalline material. Reinforcement occurs (c) 2003 Brooks/Cole Publishing / Thomson Learning at angles that satisfy Bragg’s law. 92  Photograph of a XRD diffractometer. (Courtesy of H&M Analytical Services.) 93  (a) Diagram of a diffractometer, showing powder sample, incident and diffracted beams. (b) The diffraction pattern obtained from a sample of gold powder. (c) 2003 Brooks/Cole Publishing / Thomson Learning 94 Photograph of a transmission electron microscope (TEM) used for analysis of the microstructure of materials. (Courtesy of JEOL USA, Inc.) 95 A TEM micrograph of an aluminum alloy (Al-7055) sample. The diffraction pattern at the right shows large bright spots that represent diffraction from the main aluminum matrix grains. The smaller spots originate from the nano-scale crystals of another compound that is present in the aluminum alloy. (Courtesy of Dr. JÖrg M.K. Wiezorek, University of Pittsburgh.) 96  Point Defects  Point defects - Imperfections, such as vacancies, that are located typically at one (in some cases a few) sites in the crystal.  Extended defects - Defects that involve several atoms/ions and thus occur over a finite volume of the crystalline material (e.g., dislocations, stacking faults, etc.).  Vacancy - An atom or an ion missing from its regular crystallographic site.  Interstitial defect - A point defect produced when an atom is placed into the crystal at a site that is normally not a lattice point.  Substitutional defect - A point defect produced when an atom is removed from a regular lattice point and replaced with a different atom, usually of a different size. 97 97  (c) 2003 Brooks/Cole Publishing / Thomson Learning Point defects: (a) vacancy, (b) interstitial atom, (c) small substitutional atom, (d) large substitutional atom, (e) Frenkel defect, (f) Schottky defect. All of these defects disrupt the perfect arrangement of the surrounding atoms. 98 98  Other Point Defects  Interstitialcy - A point defect caused when a ‘‘normal’’ atom occupies an interstitial site in the crystal.  Frenkel defect - A pair of point defects produced when an ion moves to create an interstitial site, leaving behind a vacancy.  Schottky defect - A point defect in ionically bonded materials. In order to maintain a neutral charge, a stoichiometric number of cation and anion vacancies must form.  KrÖger-Vink notation - A system used to indicate point defects in materials. The main body of the notation indicates the type of defect or the element involved. 99 99 Learning (c) 2003 Brooks/Cole Publishing / Thomson When a divalent cation replaces a monovalent cation, a second monovalent cation must also be removed, creating a vacancy. 100 100  Dislocations  Dislocation - A line imperfection in a crystalline material.  Screw dislocation - A dislocation produced by skewing a crystal so that one atomic plane produces a spiral ramp about the dislocation.  Edge dislocation - A dislocation introduced into the crystal by adding an ‘‘extra half plane’’ of atoms.  Mixed dislocation - A dislocation that contains partly edge components and partly screw components.  Slip - Deformation of a metallic material by the movement of dislocations through the crystal. 101 101 (c) 2003 Brooks/Cole Publishing / Thomson Learning The perfect crystal (a) is cut and sheared one atom spacing, (b) and (c). The line along which shearing occurs is a screw dislocation. A Burgers vector b is required to close a loop of equal atom spacings around the screw dislocation. 102 102 The perfect crystal in (a) is cut and an extra plane of atoms is inserted (b). The bottom edge of the extra plane is an edge dislocation (c). A Burgers vector b is required to close a loop of equal atom spacings around the edge dislocation. (Adapted from J.D. Verhoeven, Fundamentals of Physical Metallurgy, Wiley, 1975.) 103 103 A mixed dislocation. The screw dislocation at the front face of the crystal gradually changes to an edge dislocation at the side of the crystal. (Adapted from W.T. Read, Dislocations in Crystals. McGraw- Hill, 1953.) 104 104 Schematic of slip line, slip plane, and slip (Burgers) vector for (a) an edge dislocation and (b) for a screw dislocation. (Adapted from J.D. Verhoeven, Fundamentals of Physical Metallurgy, Wiley, 1975.) 105 105 (a) When a shear stress is applied to the dislocation in (a), the atoms are displaced, causing the dislocation to move one Burgers vector in the slip direction (b). Continued movement of the dislocation eventually creates a step (c), and the crystal is deformed. (Adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill, 1976.) (b) Motion of caterpillar is analogous to the motion of a dislocation. 106 106  107 107  Observing Dislocations  Etch pits - Tiny holes created at areas where dislocations meet the surface. These are used to examine the presence and number density of dislocations.  Slip line - A visible line produced at the surface of a metallic material by the presence of several thousand dislocations.  Slip band - Collection of many slip lines, often easily visible. 108 108 (c) 2003 Brooks/Cole Publishing / Thomson Learning A sketch illustrating dislocations, slip planes, and etch pit locations. (Source: Adapted from Physical Metallurgy Principles, Third Edition, by R.E. Reed-Hill and R. Abbaschian, p. 92, Figs. 4-7 and 4-8. Copyright (c) 1992 Brooks/Cole Thomson Learning. Adapted by permission.) 109 109 Optical image of etch pits in silicon carbide (SiC). The etch pits correspond to intersection points of pure edge dislocations with Burgers vector a/3 and the dislocation line  1 1 20direction along (perpendicular to the etched surface). Lines of etch pits represent low angle grain boundaries (Courtesy of Dr. Marek Skowronski, Carnegie Mellon University.) 110 110  Learning (c) 2003 Brooks/Cole Publishing / Thomson Learning (c) 2003 Brooks/Cole Publishing / Thomson (c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Electron photomicrographs of dislocations in Ti3Al: (a) Dislocation pileups (x26,500). (b) Micrograph at x 100 showing slip lines and grain boundaries in AI. (c) Schematic of slip bands development. 111 111  Significance of Dislocations  Plastic deformation refers to irreversible deformation or change in shape that occurs when the force or stress that caused it is removed.  Elastic deformation - Deformation that is fully recovered when the stress causing it is removed.  Dislocation density - The total length of dislocation line per cubic centimeter in a material. 112 112  Schmid’s Law  Schmid’s law -The relationship between shear stress, the applied stress, and the orientation of the slip system— that is,  =  cos  cos   Critical resolved shear stress - The shear stress required to cause a dislocation to move and cause slip. 113 113  (c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. (a) A resolved shear stress τ is produced on a slip system. (Note: (ø + λ) does not have to be 90°.) (b) Movement of dislocations on the slip system deforms the material. (c) Resolving the force. 114 114  Influence of Crystal Structure  Critical Resolved Shear Stress  Number of Slip Systems  Cross-slip - A change in the slip system of a dislocation. 115 115  116 116  Example 5 Ductility of HCP Metal Single Crystals and Polycrystalline Materials A single crystal of magnesium (Mg), which has a HCP crystal structure, can be stretched into a ribbon-like shape four to six times its original length. However, polycrystalline Mg and other metals with a HCP structure show limited ductilities. Use the values of critical resolved shear stress for metals with different crystal structures and the nature of deformation in polycrystalline materials to explain this observation. 117 117  118 118 Example 5 Ductility of HCP Metal Single Crystals and Polycrystalline Materials From Table 4-2, we note that for HCP metals such as Mg, the critical resolved shear stress is low (50–100 psi). We also note that slip in HCP metals will occur readily on the basal plane—the primary slip plane. When a single crystal is deformed, assuming the basal plane is suitably oriented with applied stress, a very large deformation can occur. This explains why single crystal Mg can be stretched into a ribbon four to six times the original size. When we have a polycrystalline Mg, the deformation is not as simple. Each crystal must deform such that the strain developed in any one crystal is accommodated by its neighbors. In HCP metals, there are no intersecting slip systems, thus dislocations cannot glide over from one slip plane in one crystal (grain) onto another slip plane in a neighboring crystal. As a result, polycrystalline HCP metals such as Mg show limited ductility. 119 119  Surface Defects  Surface defects - Imperfections, such as grain boundaries, that form a two-dimensional plane within the crystal.  Hall-Petch equation - The relationship between yield strength and grain size in a metallic material—that is, y =  0 + Kd −1/ 2  ASTM grain size number (n) - A measure of the size of the grains in a crystalline material obtained by counting the number of grains per square inch a magnification  100.  Small angle grain boundary - An array of dislocations causing a small misorientation of the crystal across the surface of the imperfection. 120 120 (a) The atoms near the boundaries of the three grains do not have an equilibrium spacing or arrangement. (b) Grains and grain boundaries in a stainless steel sample. (Courtesy Dr. A. Deardo.) 121 121  (c) 2003 Brooks/Cole Publishing / Thomson Learning The effect of grain size on the yield strength of steel at room temperature. 122 122 Microstructure of palladium (x 100). (From ASM Handbook, Vol. 9, Metallography and Microstructure (1985), ASM International, Materials Park, OH 44073.) 123 123  The small angle grain boundary is produced by an array of dislocations, causing an angular mismatch θ between lattices on either side of the (c) 2003 Brooks/Cole Publishing / Thomson Learning boundary. 124 124 (c) 2003 Brooks/Cole Publishing / Thomson Learning Application of a stress to the perfect crystal (a) may cause a displacement of the atoms, (b) causing the formation of a twin. Note that the crystal has deformed as a result of twinning. 125 125 (c) 2003 Brooks/Cole Publishing / Thomson Learning A micrograph of twins within a grain of brass (x250). 126 126  127 127 Domains in ferroelectric barium titanate. (Courtesy of Dr. Rodney Roseman, University of Cincinnati.) Similar domain structures occur in ferromagnetic and ferrimagnetic materials. 128 128  Importance of Defects  Effect on Mechanical Properties via Control of the Slip Process  Strain Hardening  Solid-Solution Strengthening  Grain-Size Strengthening  Effects on Electrical, Optical, and Magnetic Properties 129 129  (c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. If the dislocation at point A moves to the left, it is blocked by the point defect. If the dislocation moves to the right, it interacts with the disturbed lattice near the second dislocation at point B. If the dislocation moves farther to the right, it is blocked by a grain boundary. 130 130  Example 6 Design/Materials Selection for a Stable Structure We would like to produce a bracket to hold ceramic bricks in place in a heat-treating furnace. The bracket should be strong, should possess some ductility so that it bends rather than fractures if overloaded, and should maintain most of its strength up to 600oC. Design the material for this bracket, considering the various crystal imperfections as the strengthening mechanism. SOLUTION In order to serve up to 600oC, the bracket should not be produced from a polymer material. Instead, a metal or ceramic would be considered. 131 131 Example 6 Design/Materials Selection for a Stable Structure In order to have some ductility, dislocations must move and cause slip. Because slip in ceramics is difficult, the bracket should be produced from a metallic material. We might add carbon to the iron as interstitial atoms or substitute vanadium atoms for iron atoms at normal lattice points. These point defects continue to interfere with dislocation movement and help to keep the strength stable. Of course, other design requirements may be important as well. For example, the steel bracket may deteriorate by oxidation or may react with the ceramic brick. 132 132 Microstructure of iron, for Problem 4-54 (x500). (From ASM Handbook, Vol. 9, Metallography and Microstructure (1985), ASM International, Materials Park, OH 44073.) 133 133 The microstructure of BMT ceramics obtained by compaction and sintering of BMT powders. (Courtesy of H. Shirey.) 134 134  Applications of Diffusion  Nitriding - Carburization for Surface Hardening of Steels  p-n junction - Dopant Diffusion for Semiconductor Devices  Manufacturing of Plastic Beverage Bottles/MylarTM Balloons  Sputtering, Annealing - Magnetic Materials for Hard Drives  Hot dip galvanizing - Coatings and Thin Films  Thermal Barrier Coatings for Turbine Blades 135 135 Furnace for heat treating steel using the carburization process. (Courtesy of Cincinnati Steel Treating). 136 136  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Schematic of a n-p-n transistor. Diffusion plays a critical role in formation of the different regions created in the semiconductor substrates. The creation of millions of such transistors is at the heart of microelectronics technology 137 137  Thomson Learning is a trademark used herein under license. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Schematic of the microstructure of the Co-Pt-Ta-Cr film after annealing. Most of the chromium diffuses from the grains to the grain boundaries after the annealing process. This helps improve the magnetic properties of Hot dip galvanized parts and structures the hard disk prevent corrosion. (Courtesy of Casey Young and Barry Dugan of the Zinc Corporation of America) 138 138 A thermal barrier coating on nickel-based superalloy. (Courtesy of Dr. F.S. Pettit and Dr. G.H. Meier, University of Pittsburgh.) 139 139  Stability of Atoms and Ions  Arrhenius equation  Activation energy -The energy required to cause a particular reaction to occur. 140 140  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. 141 reaction required for a determine the can be used to activation energy 141 in (rate) versus 1/T The Arrhenius plot of  Mechanisms for Diffusion  Self-diffusion - The random movement of atoms within an essentially pure material.  Vacancy diffusion - Diffusion of atoms when an atom leaves a regular lattice position to fill a vacancy in the crystal.  Interstitial diffusion - Diffusion of small atoms from one interstitial position to another in the crystal structure. 142 142 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Diffusion of copper atoms into nickel. Eventually, the copper atoms are randomly distributed throughout the nickel 143 143  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Diffusion mechanisms in material: (a) vacancy or substitutional atom diffusion and (b) interstitial diffusion 144 144  Activation Energy for Diffusion  Diffusion couple - A combination of elements involved in diffusion studies ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used A high energy is required to squeeze atoms past one another herein under license. during diffusion. This energy is the activation energy Q. Generally more energy is required for a substitutional atom than for an interstitial atom 145 145  146 146  Rate of Diffusion (Fick’s First Law)  Fick’s first law - The equation relating the flux of atoms by diffusion to the diffusion coefficient and the concentration gradient.  Diffusion coefficient (D) - A temperature-dependent coefficient related to the rate at which atoms, ions, or other species diffuse.  Concentration gradient - The rate of change of composition with distance in a nonuniform material, typically expressed as atoms/cm3.cm or at%/cm. 147 147  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. 148 diffusion is per unit time defined as the The flux during 148 number of atoms plane of unit area passing through a  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. 149 gradient concentration Illustration of the 149  Example 7 Semiconductor Doping One way to manufacture transistors, which amplify electrical signals, is to diffuse impurity atoms into a semiconductor material such as silicon (Si). Suppose a silicon wafer 0.1 cm thick, which originally contains one phosphorus atom for every 10 million Si atoms, is treated so that there are 400 phosphorous (P) atoms for every 10 million Si atoms at the surface (Figure 5.16). Calculate the concentration gradient (a) in atomic percent/cm and (b) in atoms /cm3.cm. The lattice parameter of silicon is 5.4307 Å. 150 150  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Silicon wafer showing variation in concentration of P atoms (for Example 7) 151 151 Example 7 Semiconductor Doping a) Calculate the initial and surface compositions in atomic percent. 1Patom ci = 7  100 = 0.00001at % P 10 atoms 400 Patom cs = 7  100 = 0.004at % P 10 atoms c 0.00001 − 0.004at % P at % P = = −0.0399 x 0.1cm cm 152 152 Example 7 Semiconductor Doping b) The volume of the unit cell: Vcell = (5.4307  10-8 cm)3 = 1.6  10-22 cm3/cell The volume occupied by 107 Si atoms, which are arranged in a diamond cubic (DC) structure with 8 atoms/cell, is: V = 2  10-16 cm3 The compositions in atoms/cm3 are: 1Patom atoms ci = = 0.005  10 P ( 18 −16 ) 2  10 cm 3 3 cm 400 Patoms atoms cs = −16 = 2  1018 P ( ) 2  10 cm 3 3 cm atoms 0.005  10 − 2  10 P ( 18 18 ) c cm 3 = x 0.1cm atoms = −1.995  10 P 19 cm3.cm 153 153  Factors Affecting Diffusion  Temperature and the Diffusion Coefficient (D)  Types of Diffusion - volume diffusion, grain boundary diffusion, Surface diffusion  Time  Dependence on Bonding and Crystal Structure  Dependence on Concentration of Diffusing Species and Composition of Matrix 154 154 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. The diffusion coefficient D as a function of reciprocal temperature for some metals and ceramics. In the Arrhenius plot, D represents the rate of the diffusion process. A steep slope denotes a high activation energy 155 155  Diffusion coefficients for different dopants in silicon. (Source: From ‘‘Diffusion and Diffusion Induced Defects in Silicon,’’ by U. GÖsele. In R. Bloor, M. Flemings, and S. Mahajan (Eds.), Encyclopedia of Advanced Materials, Vol. 1, 1994, p. 631, Fig. 2. Copyright © 1994 Pergamon Press. Reprinted with permission of the authors.) 156 156  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Diffusion in ionic compounds. Anions can only enter other anion sites. Smaller cations tend to diffuse faster 157 157 Diffusion coefficients of ions in different oxides. (Source: Adapted from Physical Ceramics: Principles for Ceramic Science and Engineering, by Y.M. Chiang, D. Birnie, and W.D. Kingery, Fig. 3-1. Copyright © 1997 John Wiley & Sons. Adapted with permission.) 158 158  159 159  160 160  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. The activation energy for self-diffusion increases as the melting point of the metal increases 161 161 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. The dependence of diffusion coefficient of Au on concentration. (Source: Adapted from Physical Metallurgy Principles, Third Edition, by R.E. Reed-Hill and R. Abbaschian, p. 363, Fig. 12-3. Copyright © 1991 Brooks/Cole Thomson Learning. Adapted with permission.) 162 162  Permeability of Polymers Permeability is expressed in terms of the volume of gas or vapor that can permeate per unit area, per unit time, or per unit thickness at a specified temperature and relative humidity. 163 163  Example 8 Design of Carbonated Beverage Bottles You want to select a polymer for making plastic bottles that can be used for storing carbonated beverages. What factors would you consider in choosing a polymer for this application? 164 164 Example 8 Design of Carbonated Beverage Bottles  First, since the bottles are to be used for storing carbonated beverages, a plastic material with a small diffusivity for carbon dioxide gas should be chosen.  The bottles should have enough strength so that they can survive a fall of about six feet. This is often tested using a ‘‘drop test.’’  The surface of the polymer should also be amenable to printing of labels or other product information.  The effect of processing on the resultant microstructure of polymers must also be considered. 165 165  Composition Profile (Fick’s Second Law)  Fick’s second law - The partial differential equation that describes the rate at which atoms are redistributed in a material by diffusion.  Interdiffusion - Diffusion of different atoms in opposite directions.  Kirkendall effect - Physical movement of an interface due to unequal rates of diffusion of the atoms within the material.  Purple plague - Formation of voids in gold-aluminum welds due to unequal rates of diffusion of the two atoms; eventually failure of the weld can occur. 166 166  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Diffusion of atoms into the surface of a material illustrating the use of Fick’s second law 167 167  168 168  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. 169 second law error function Graph showing the encountered in Fick’s 169 argument and value of  Diffusion and Materials Processing  Sintering - A high-temperature treatment used to join small particles.  Powder metallurgy - A method for producing monolithic metallic parts.  Dielectric resonators -Hockey puck-like pieces of ceramics such as barium magnesium tantalate (BMT) or barium zinc tantalate (BZN).  Grain growth - Movement of grain boundaries by diffusion in order to reduce the amount of grain boundary area.  Diffusion bonding - A joining technique in which two surfaces are pressed together at high pressures and temperatures. 170 170  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Diffusion processes during sintering and powder metallurgy. Atoms diffuse to points of contact, creating bridges and reducing the pore size 171 171 Particles of barium magnesium The microstructure of BMT tantalate (BMT) (Ba(Mg1/3 ceramics obtained by Ta2/3)O3) powder are shown. This compaction and sintering of ceramic material is useful in BMT powders. (Courtesy of making electronic components H. Shirey.) known as dielectric resonators that are used for wireless communications. (Courtesy of H. Shirey.) 172 172  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Grain growth occurs as atoms diffuse across the grain boundary from one grain to another 173 173 Grain growth in alumina ceramics can be seen from the SEM micrographs of alumina ceramics. (a) The left micrograph shows the microstructure of an alumina ceramic sintered at 1350oC for 150 hours. (b) The right micrograph shows a sample sintered at 1350oC for 30 hours. (Courtesy of I. Nettleship and R. McAfee.) 174 174  ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. The steps in diffusion bonding: (a) Initially the contact area is small; (b) application of pressure deforms the surface, increasing the bonded area; (c) grain boundary diffusion permits voids to shrink; and (d) final elimination of the voids requires volume diffusion 175 175 Course Assignment  For your homework, answer the questions in the Course Assignment Section.  Submission should be made online through our MS Teams in the Assignments tab. Submission should be in uneditable, readable, and comment-able format (PDF is recommended!).  File should be named as follows: CAXX SurnameFNMI (e.g. CA02 VictorioKCB). Do not forget to write your name, student number as header in the upper left portion of your submission. Copying the question/problem will be highly appreciated. DO NOT FORGET TO HIGLIGHT YOUR FINAL ANSWER. Page number in the following format Page X of Y (and centered) is appreciated. 176 For Further Reading  Amato, I., Stuff—The Materials The World Is Made Of, Basic Books, New York, 1997.  Barrett, Craig R., W. D. Nix, and A. S. Tetelman, The Principles of Engineering Materials, Prentice-Hall, New York, 1973.  Jastrzebski, Z., The Nature and Properties of Engineering Materials, 2nd ed., John Wiley & Sons, New York, 1976.  Materials Chemistry, L. V. Interrante, L. A. Casper, and A. B. Ellis, eds., ACS Advances in Chemistry Series, Volume 245, American Chemical Society, Washington, D.C., 1995.  Ralls, Kenneth M., T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley & Sons, New York, 1976.  Wyckoff, R. W. G., Crystal Structures, 2nd ed., Interscience, New York, 1963. 177 177 Next Discussion  Lecture 4 Heat Treatment of Steels and Cast Irons ◼ Designation and Classification of Steels ◼ Simple Heat Treatments ◼ Isothermal Heat Treatments ◼ Quench and Temper Heat Treatments ◼ Effect of Alloying Elements ◼ Application of Hardenability ◼ Specialty Steels ◼ Surface Treatment ◼ Weldability of Steel ◼ Stainless Steels ◼ Cast Irons 178 178 Lecture 3 | Chemistry of Materials ENSC 20062 Material Science and Engineering Kaycee B. Victorio, REE, RME Department of Electrical Engineering | College of Engineering | Polytechnic University of the Philippines Manila 179 Lecture 4 FERROUS ALLOYS ENSC 20062 Material Science and Engineering Kaycee B. Victorio, REE, RME Department of Electrical Engineering College of Engineering Polytechnic University of the Philippines Manila 1  Discuss how to use the eutectoid reaction to control the structure and properties of steels through heat treatment and alloying. Discussion Objectives Examine two special classes of ferrous alloys: stainless steels and cast irons. 2 Discussion  Designations and  Application of Hardenability Outline Classification of  Specialty Steels Steels  Simple Heat  Surface Treatments Treatments  Isothermal Heat  Weldability of Treatments Steel  Quench and  Stainless Stee