## Materials Exam 1

 What is atomic number
 # of protons in nucleus of atom
 What is the coefficient “A”
 atomic mass unit, amu, 1/12 mass of Carbon 12 1 amu/atom = 1 g/mol
 What is atomic weight
 the weight of 6.022E23 molecules or atoms units: g/mol
 4 properties that valence electrons determine
 1. Chemical2. Electrical 3. Thermal 4. Optical
 Electron changes correspond to quantum [number?] jumping to: higher level _____ energy, lower level ____ energy
 Jumping to higher level ABSORBS energy Jumping to lower level EMITS energy
 Describe the wave-mechanical model of an atom
 – orbitals not discrete– Position defined by probability of electron at various locations around nucleus
 Heisenberg Uncertainty Principle states:
 both particle momentum and position cannot be determined simultaneously
 Schrodinger’s equation defines:
 a wave function that can be used to determine the probability of finding an electron at a certain location
 electrons have ____-like and ____-like properties
 electrons have WAVE-like and PARTICLE-like properties
 Each orbital is described by a discrete energy level defined by ______.
 Each orbital is described by a discrete energy level defined by QUANTUM NUMBERS.
 List the quantum numbers and their designation
 n = principal (energy level-shell) | K, L, M, N, O (1, 2, 3, etc.)l = subsidiary (orbitals) | s, p, d, f (0, 1, 2, 3,…n-1) ml = magnetic 1, 3, 5, 7 (-l to +l) ms = spin 1/2, -1/2
 Pauli’s Exclusion Principle
 No two electrons can have the same quantum numbers. This leads to being able to have all the elements of the periodic table
 Electrons have discrete ______; they are ____.
 Electrons have discrete ENERGY STATES; they are QUANTIZED.
 Electrons tend to occupy _____ available energy state.
 Electrons tend to occupy LOWEST available energy state.
 Each electron state can accommodate ___ electrons
 Each electron state can accommodate TWO electrons (m = 1/2, -1/2)
 Electron orbitals (l = 0, 1, 2, etc. or s, p, d, f) describe:
 probabilistic location of electrons at a certain energy
 True of false: most elements have unstable electron configurations
 TRUE, most elements have unstable electron configurations
 What are the electrons called in unfilled shells
 Valence electrons
 Filled shells are [more or less] stable than unfilled shells
 Filled shells are MORE stable than unfilled shells
 Valence electrons are most available for bonding and tend to control the _____ properties.
 Valence electrons are most available for bonding and tend to control the CHEMICAL properties.
 What is the electron configuration of C (atomic number = 6)put the valence electrons in parentheses
 1s2(2s22p2)
 What is the electron configuration of Fe (atomic number = 26)put the valence electrons in parentheses
 1s2 2s2 2p6 3s2 3p6 (3d6 4s2)
 Columns have similar valence structure. Electropositive elements readily ____ electrons to become ____ ions. Electronegative elements readily ____ electrons to become ____ ions.
 Columns have similar valence structure. Electropositive elements readily GIVE UP electrons to become POSITIVE ions. Electronegative elements readily ACQUIRE electrons to become NEGATIVE ions.
 Electronegativity ranges from __ to __. Large values tend to ____ electrons. The bottom right of the periodic table has [smaller/greater] electronegativity The top left of the periodic table has [smaller/greater] electronegativity
 Electro negativity ranges from 0.7 to 4.0. Large values tend to ACCEPT electrons. The bottom right of the periodic table has SMALLER electronegativityThe top left of the periodic table has GREATER electronegativity
 Which bonds are PRIMARY bonds (usually STRONG): Ionic, metallic, van der waals (or induced dipoles), covalent, hydrogen bond (or permanent dipoles)
 Ionic, covalent, metallic bonds are primary bonds and usually strong
 Which bonds are SECONDARY bonds (usually much weaker): Ionic, metallic, van der waals (or induced dipoles), covalent, hydrogen bond (or permanent dipoles)
 van der waals and hydrogen bonds are secondary bonds and are usually much weaker
 In ionic bonding, a ____ donates electrons and a _____ accepts electrons
 In ionic bonding, a METAL donates electrons and a NONMETAL accepts electrons
 Ionic bonding is between elements with _____ electronegativities.
 Ionic bonding is between elements with DISSIMILAR electronegativities.
 What kind of bond is found in MgO? After bonding, Mg has the electron structure of? What about O?
 MgO is ionic, both Mg and O have the electron structure of Ne.
 In NaCl, which is the cation and which is the anion? What kind of bond is this?
 NaCl, Na is + so it is the cation and Cl is – so it is the anion. It is a ionic bond.
 In ionic bonding, for stability, nearest neighbors must have ____ charges
 In ionic bonding, for stability, nearest neighbors must have opposite charges
 Coulomb’s Law describes the ______ between two charges
 Coulomb’s Law describes the energy of interaction between two charges
 For Coulomb’s Law, if z1 and z2 are of the same sign, Epot is [+ or -] meaning [attractive or repulsive]
 For Coulomb’s Law, if z1 and z2 are of the same sign, Epot is + meaning repulsive
 For Coulomb’s Law, if z1 and z2 are of opposite signs, Epot is [+ or -] meaning [attractive or repulsive]
 For Coulomb’s Law, if z1 and z2 are of opposite signs, Epot is – meaning attractive
 Coulomb’s Law for a cation and anion will describe anegative potential energy which is attractive. However, as the ions come closer, electrons from theions interact and ____ each other.
 Coulomb’s Law for a cation and anion will describe anegative potential energy which is attractive. However, as the ions come closer, electrons from theions interact and REPEL each other.
 Ionic bonding is predominant in ____.
 Ionic bonding is predominant in CERAMICS.
 ____ range attraction, ___ range repulsion.
 LONG range attraction, SHORT range repulsion.
 Covalent bonding has similar ____ and ____ electrons. _ & _ orbitals dominate bonding.
 Covalent bonds have similar ELECTRONEGATIVITY and SHARE electrons. s & p orbitals dominate bonding.
 Covalent bonds are _____, meaning between specific atoms participating in electron sharing
 Covalent bonds are DIRECTIONAL, meaning between specific atoms participating in electron sharing
 # of possible covalent bonds for an atom = 8 – N’ N’ is the number of _____
 # of possible covalent bonds for an atom = 8 – N’ N’ is the number of VALENCE ELECTRONS
 Metallic bond – delocalized as _____ _____. Is it directional or nondirectional?
 Metallic bond – delocalized as ELECTRON CLOUD. It is NONDIRECTIONAL.
 Ionic-Covalent Mixed Bonding- ____ gives the percentage of bonds that are ionic
 %IC, %Ionic Character
 Secondary bonding arises from interaction between _____. Is it directional or nondirectional?
 Secondary bonding arises from interaction between DIPOLES. It is DIRECTIONAL.
 Relatively weak van der Waals bonds result from attractive forces between electric dipoles, which may be ____ or _____.
 Relatively weak van der Waals bonds result from attractive forces between electric dipoles, which may be INDUCED or PERMANENT.
 For hydrogen bonding, highly ____ molecules form when hydrogen covalently bonds to a nonmetallic element such as fluorine.
 For hydrogen bonding, highly POLAR molecules form when hydrogen covalently bonds to a nonmetallic element such as fluorine.This is the STRONGEST SECONDARY BONDING type.
 Tm (melting temperature) is larger if Eo (bond energy) is _____.
 Tm (melting temperature) is larger if Eo (bond energy) is LARGER.
 Coefficient of thermal expansion is larger if Eo (bond energy) is _____.
 Coefficient of thermal expansion is larger if Eo (bond energy) is SMALLER.
 Ceramics have _____ and _____ bonding. Metals have _____ bonding. Polymers have ____ and _____ bonding.
 Ceramics have IONIC and COVALENT bonding. Metals have METALLIC bonding. Polymers have COVALENT and SECONDARY bonding.
 Thanks to ______ forces, geckos can walk on walls.
 Thanks to VAN DER WAALS forces, geckos can walk on walls.
 Dense, ordered packed structures tend to have _____ energies compared to non dense, random packing.
 Dense, ordered packed structures tend to have LOWER energies compared to non dense, random packing.
 Crystalline materialsatomics pack in periodic, ___ arrays, typical of: – metals – many ceramics – some polymers
 Crystalline materialsatomics pack in periodic, 3D arrays, typical of: – metals – many ceramics – some polymers
 Noncrystalline materials…• atoms have no periodic packing– occurs for: 1. 2.
 Noncrystalline materials…• atoms have no periodic packing– occurs for: 1. complex structures 2. rapid cooling
 Amorphous means
 Amorphous means Noncrystalline
 Metallic Crystal Structures:– Have the simplest crystal structures – Tend to be densely packed, because: 1. typically only 1 element is present, so all atomic radii are ____. 2. Metallic bonding is ____. 3. Nearest neighbor distances tend to be ___ in order to lower bond energy. 4. ______ shields cores from each other.
 Metallic Crystal Structures:– Have the simplest crystal structures – Tend to be densely packed, because: 1. typically only 1 element is present, so all atomic radii are THE SAME. 2. Metallic bonding is NON DIRECTIONAL. 3. Nearest neighbor distances tend to be SMALL in order to lower bond energy. 4. ELECTRON CLOUD shields cores from each other.
 Small repeat units which describe the crystalline structure of a solid are called _____ _____ and they extend in all directions.
 Small repeat units which describe the crystalline structure of a solid are called UNIT CELLS and they extend in all directions.
 What are the four 3D arrangements found in metals:
 What are the four 3D arrangements found in metals: – SC: Simple Cubic– BCC: Body-centered cubic – FCC: Face-centered cubit– HCP: Hexagonal closed-packed
 Describe Simple Cubic Structure (SC)
 SC: – Rare, due to low packing density (only Po has this structure)– Close-packed directions are cube edges – Coordination # = 6 – APF = .52 – R = .5a – 1 atom / unit cell
 Coordination # is
 Coordination # is the number of nearest neighbors
 Describe Body Centered Cubic Structure (BCC)
 BCC: – Atoms touch each other along cube diagonals – Coordination # = 8 – 2 atoms / unit cell – APF = .68 – 4R = sqrt(3)a
 Describe Face Centered Cubic Structure (FCC)
 FCC:– atoms touch each other along face diagonals – MAXIMUM achievable APF, = .74– 4 atoms / unit cell – 4R = sqrt(2)a – coordination # = 12– ABCABC stacking sequence
 Describe Hexagonal Close-Packed Structure (HCP)
 HCP:– atoms touch each other along multiple directions – coordination # = 12 – 6 atoms / unit cell – APF = .074– ABAB stacking sequence
 What is the relationship between theoretical density, p, for metals, ceramics, and polymers. Why?
 In general: p metals > p ceramics > p polymers Metals have: – close-packing (metallic bonding) – often large atomic masses Ceramics have: – less dense packing – often lighter elements Polymers have:– low packing density (often amorphous) – lighter elements (C, H, O) Composites/fibers have intermediate values
 Some properties of crystalline materials often related to _____ _____.
 Properties of crystalline materials often related to CRYSTAL STRUCTURE.
 For a crystalline solid, when the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption, the result is a ____ ____.
 For a crystalline solid, when the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption, the result is a SINGLE CRYSTAL. Some real life applications: – turbine blades – electronic microcircuits – diamond single crystals for abrasives
 anisotropy
 directionality of properties is termed anisotropy, and it is associated with the variance of atomic or ionic spacing with crystallographic direction. a single crystal is anisotropic bc its material properties vary with crystal orientation GRAINS TEXTURED
 isotropic
 Substances in which measured properties are independent of the direction of measurement are isotropic GRAINS RANDOMLY ORIENTED
 Single crystals have properties that vary with direction so they are
 Single crystals have properties that vary with direction so they are ANISOTROPIC
 Polycrystals (majority of engr materials) may/may not vary with direction– each “grain” is a single crystal – if gains are randomly oriented: – if grains are textured:
 Polycrystals (majority of engr materials) may/may not vary with direction– each “grain” is a single crystal – if gains are randomly oriented: isotropic – if grains are textured: anisotropic
 How many possible lattice parameters
 3 possible lattice parameters
 What is polymorphism (or allotropy)
 Polymorphism is when there are two or or more distinct crystal structures for the same material ex. carbon, diamond, graphite
 What is solidification
 Solidification: result of casting of molten material – 2 steps: 1. Nuclei form 2. Nuclei grow to form crystals – grain structure – start with a molten material, all liquid, crystals grow until they meet each other
 Grain Boundaries• Regions between crystals• Transition from ____ of one region to that of theother• Slightly disordered• Low density in grain boundaries – ____ mobility– ____ diffusivity– ____ chemical reactivity
 Grain Boundaries• Regions between crystals• Transition from LATTICE of one region to that of theother• Slightly disordered• Low density in grain boundaries – HIGH mobility– HIGH diffusivity– HIGH chemical reactivity
 Grains can be: – _____ (roughly same size in all directions)– _____ (elongated grains)
 Grains can be: – EQUIAZED (roughly same size in all directions)– COLUMNAR (elongated grains)
 Grain Refiner can be added to make _____, more ____, ______ grains.
 Grain Refiner can be added to make SMALLER, more UNIFORM, EQUIAXED grains.
 Turbine Blade Casting 1. _________Uses a ceramic mold and develops an equiaxed grain structure 2. _____Ceramic mold with direction heating to produce elongated grains 3. _____Strict control of crystallization leads to structure which is creep and thermal shock resistant
 Turbine Blade Casting 1. CONVENTIONAL Uses a ceramic mold and develops an equiaxed grain structure 2. DIRECTIONALLY SOLIDIFIED Ceramic mold with direction heating to produce elongated grains 3. SINGLE CRYSTAL Strict control of crystallization leads to structure which is creep and thermal shock resistant
 Most common amorphous material that you come in contact with on a daily basis
 GLASS is the most common amorphous material that you come in contact with on a daily basis
 – Although more challenging, metals can also be madeamorphous. • No long range order• No grain boundaries• Formed from rapid solidification techniques• Alloys contain Fe, Ni, Cr with C, P, B, Al, Si • PROPERTIES: Excellent corrosion resistance, goodductility, high strength • Applications include face-plate inserts on golf clubheads and thermally-sprayed composite amorphousmetal coated baseball bats
 There is no such thing as a ____ crystal. Many of the important properties of materials are due to the presence of _____.
 There is no such thing as a PERFECT crystal. Many of the important properties of materials are due to the presence of IMPERFECTIONS.
 Vacancy atoms, interstitial atoms, and substitutional atoms are all _____ defects.
 Vacancy atoms, interstitial atoms, and substitutional atoms are all POINT defects.
 Dislocations are _____ defects.
 Dislocations are LINE defects.
 Grain boundaries are _____ defects.
 Grain boundaries are AREA defects.
 Vacancies: vacant atomic sites in a structure – point defect – all crystalline solids contain vacancies – presence of vacancies increases ______ of the crystal.
 Vacancies: vacant atomic sites in a structure – point defect – all crystalline solids contain vacancies – presence of vacancies increases ENTROPY (DISORDER) of the crystal.
 Each _____ is a potential vacancy site.
 Each LATTICE SITE is a potential vacancy site.
 Equilibrium vacancy concentration varies with _____.
 Equilibrium vacancy concentration varies with TEMPERATURE.
 Arrhenius equations are used to describe many ____ activated processes. What are some examples from class?
 Arrhenius equations are used to describe many THERMALLY activated processes. What are some examples from class? – The equilibrium vacancy concentration equation
 ______ temperature causes surface island of atoms to grow. The equivalent vacancy concentration ______ via atom motion from the crystal to the surface, where the atoms join the island.
 INCREASING temperature causes surface island of atoms to grow. The equivalent vacancy concentration INCREASES via atom motion from the crystal to the surface, where the atoms join the island.
 _________ are “extra atoms” position between atomic sites. This is a _____ defect.
 SELF-INTERSTITIALS are “extra atoms” position between atomic sites Let us write you a custom essay sample on Materials Exam 1 For Only \$13.90/page ORDER NOW This is a POINT defect.
 Which point defect, vacancy or self-interstitial, is more likely? Why?
 Vacancy is MORE likely than self-interstitial. Self interstitial atoms are large relative to interstitial space and therefore much less likely than vacancy.
 There is no such thing as a ____ metal. Most metals are ____.
 There is no such thing as a PURE metal. Most metals are ALLOYS.
 Impurity atoms deliberately added to modify _____.
 Impurity atoms deliberately added to modify PROPERTIES. e.g.: adding 7.5% Cu to silver to make Sterling Silver:– Pure Ag = very soft– Cu improves mechanical strength w/o reducingcorrosion resistance
 Adding impurity atoms results in solid ___ or second ___ formation.
 Adding impurity atoms results in SOLID SOLUTION or SECOND PHASE formation. solvent = majority component or “host” solute = minority concentration
 Two outcomes of impurity added to host: 1. Solid Solution of impurity in host, can be _______ or ______. 2. Solid solution of impurity in host plus particles of a new phase (usually for a _____ amount of impurity)
 Two outcomes of impurity added to host: 1. Solid Solution of impurity in host, can be SUBSTITUTIONAL or INTERSTITIAL 2. Solid solution of impurity in host plus particles of a new phase (usually for a LARGER amount of impurity)
 What are the conditions for substitutional solid solution (S.S)? What is the name of the rule?
 W. Hume – Rothery rule – 1. difference in r (atomic radius) < 15%– 2. Proximity in periodic table • i.e., similar electronegativities– 3. Same crystal structure for pure metals – 4. Valency• All else being equal, a metal will have a greater tendencyto dissolve a metal of higher valency than one of lowervalency
 Dislocations: – are ____ defects– slip between crystal planes result when dislocations move– produce _____ deformation.
 Dislocations: – are LINE defects– slip between crystal planes result when dislocations move– product PERMANENT (PLASTIC) deformation.
 Linear Defects (Dislocations) are __-dimensional defects around which atoms are misaligned.
 Linear Defects (Dislocations) are ONE-dimensional defects around which atoms are misaligned.
 Edge dislocation: – _____ inserted in crystal structure – b (burger’s vector) is _____ to dislocation line
 Edge dislocation: – EXTRA HALF-PLANE OF ATOMS inserted in crystal structure – b (burger’s vector) is PERPENDICULAR to dislocation line
 burger’s vector (b) measures _____.
 burger’s vector (b) measures lattice distortion.
 In dislocation motion, bonds across the slipping planes are ______ and ______ in succession.
 In dislocation motion, bonds across the slipping planes are BROKEN and REMADE in succession.
 _____ dislocation: – spiral planar ramp resulting from shear deformation. – b _____ to dislocation line.
 SCREW dislocation: – spiral planar ramp resulting from shear deformation. – b PARALLEL to dislocation line.
 (it would be really weird if he asked this but…)In _____ (a program):– a region of crystal containing a dislocation can be rotated in 3D– dislocation motion may be animated
 (it would be really weird if he asked this but…)In VMSE (a program):– a region of crystal containing a dislocation can be rotated in 3D– dislocation motion may be animated
 Dislocations are present in virtually all ____ materials.
 Dislocations are present in virtually all CRYSTALLINE materials.
 Dislocations are visible in ____ _____.
 Dislocations are visible in ELECTRON MICROGRAPHS.
 Crystal structures prefer _____ planes & directions. Compare the planes among the crystal structures: FCC: HCP:BCC:
 Crystal structures prefer CLOSE-PACKED planes & directions. Compare the planes among the crystal structures: FCC: many close-packed planes/directions HCP:only one plane, 3 directions BCC: none
 Planar defects in solids: Surfaces• Termination of the crystal structure.• Surface atoms are not bonded with the ____ number ofnearest neighbors (bonding ____ than coordination number).• Because the surface atoms don’t have all the bonds they would like, they are in a ____ energy state due to surface energy (J/m?).• There is a driving force to reduce this energy in a material by reducing the total ____ ____.• Easily accomplished in liquids, but not so easy in solids.
 Planar defects in solids: Surfaces• Termination of the crystal structure.• Surface atoms are not bonded with the MAXIMUM number of nearest neighbors (bonding LESS than coordination number).• Because the surface atoms don’t have all the bonds they would like, they are in a HIGHER energy state due to surface energy (J/m?).• There is a driving force to reduce this energy in a material by reducing the total SURFACE AREA.• Easily accomplished in liquids, but not so easy in solids.
 Grain Boundaries• Regions between crystals• Transition from lattice of one region to that of the other• Atomic bonding is less regular along the grain boundary leading to a grain boundary ______• ____ ____ tend to segregate here• Crystallographic misalignment exists
 Grain Boundaries• Regions between crystals• Transition from lattice of one region to that of the other• Atomic bonding is less regular along the grain boundary leading to a grain boundary ENERGY• IMPURITY ATOMS tend to segregate here• Crystallographic misalignment exists
 Grain Boundaries: • ______ ______ can form when edge dislocations line up.• ______ ______ are between two different phases in an alloy• ______ ______ is essentially a reflection of atom positions across the twin plane.• ______ _____– For FCC metals an error in ABCABC packing sequence– Ex: ABCABABC
 Grain Boundaries: • TILT BOUNDARIES can form when edge dislocations line up.• PHASE BOUNDARIES are between two different phases in an alloy• TWIN BOUNDARY is essentially a reflection of atom positions across the twin plane.• STACKING FAULTS– For FCC metals an error in ABCABC packing sequence– Ex: ABCABABC
 A catalyst ______ the rate of a chemical reactionwithout being consumedActive sites on catalysts are normally _____ defects
 A catalyst INCREASES the rate of a chemical reactionwithout being consumedActive sites on catalysts are normally SURFACE defects
 Optical Microscopy• Useful up to _____X magnification.• _____ removes surface features (e.g., scratches)• _____ changes reflectance, depending on crystalorientation. Grain boundaries are susceptible to this.
 Optical Microscopy• Useful up to 2000X magnification.• POLISHING removes surface features (e.g., scratches)• ETCHING changes reflectance, depending on crystalorientation. Grain boundaries are susceptible to this.
 Polarized light – metallographic scopes often use polarized light to increase ____ – Also used for transparent samples such as polymers
 Polarized light – metallographic scopes often use polarized light to increase CONTRAST – Also used for transparent samples such as polymers
 The _______ of the light determines its color.It also determines the resolution of what we observe
 The WAVELENGTH of the light determines its color.It also determines the resolution of what we observe
 Scanning Electron Microscopy• Surface is scanned by an electron beam and reflectedelectrons are collected to produce an image.• Surface must be electrically _______.• Magnifications of _______X possible with gooddepth of field.
 Scanning Electron Microscopy• Surface is scanned by an electron beam and reflectedelectrons are collected to produce an image.• Surface must be electrically CONDUCTIVE.• Magnifications of 10,000-100,000X possible with gooddepth of field.
 Transmission Electron Microscopy• Electron beam passes through the specimen• Image is created due to contrast from beam scattering from the elements in the material.• Specimen must be ___ ___ to allow for transmission• Magnification up to _______X.
 Transmission Electron Microscopy• Electron beam passes through the specimen• Image is created due to contrast from beam scattering from the elements in the material.• Specimen must be VERY THIN (<100nm) to allow for transmission• Magnification up to 1,000,000X.
 Scanning Probe Microscopy• Neither ____ nor ____ are used.• A tiny probe with a very sharp tip is brought into close contact with the specimen surface.• Electronic and other interactions between the tip and the specimen atoms to generate 3-D topography.– Nanometer scale resolution with ________X magnification.
 Scanning Probe Microscopy• Neither LIGHT nor ELECTRONS are used.• A tiny probe with a very sharp tip is brought into close contact with the specimen surface.• Electronic and other interactions between the tip and the specimen atoms to generate 3-D topography.– Nanometer scale resolution with 1,000,000,000X magnification.
 ________ _______ Microscopy• A sharp tip (usually tungsten or platinum/iridium) brought within 0.5-1 nm of surface.• Interactions between the tip and the surface lead to electron transfer and a measurable current. • ***can be used to manipulate atoms***
 SCANNING TUNNELING Microscopy• A sharp tip (usually tungsten or platinum/iridium) brought within 0.5-1 nm of surface.• Interactions between the tip and the surface lead to electron transfer and a measurable current. • ***can be used to manipulate atoms***
 Diffusion is mass transport by _____ _____.
 Diffusion is mass transport by ATOMIC MOTION.
 Mechanisms for diffusion in solids include _____ diffusion or ______ diffusion.
 Mechanisms for diffusion in solids include VACANCY diffusion or INTERSTITIAL diffusion.
 Interdiffusion: In an alloy, atoms tend to migrate from regions of ____ concentration to regions of ____ concentration. _______: In an elemental solid, atoms can also migrate.
 Interdiffusion: In an alloy, atoms tend to migrate from regions of HIGH concentration to regions of LOW concentration. SELF-DIFFUSION: In an elemental solid, atoms can also migrate.
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