222 Exam 1 notes - study guide PDF

Preview

Summary

study guide...

Description

TRUE/FALSE Radicals don’t change their identity during chemicals reactions. Melting T of polymers is typically increasing with their molecular weight Atomic number is equal to the number of protons in nucleus atom Valence electrons determine most of properties The atoms in the same row (period) on the periodic table have the same quantum number The atoms n the same column (group) on the periodic table have the same number of valence electrons Thermal expansion coefficient is inversely proportional to atomic bonding energy Non crystalline solids are called as an amorphous material The close – packed direction of FCC is , of BCC , HCP , simple cubic The close packed planes of FCC is {111}, BCC {110}, HCP {0001}, simple cubic {100} “n” is the principal quantum number and it is related to the radius of subshells BCC (68%) has a lower atomic packing density factor than FCC (74%) crystal structure A mer is the basic repeating unit in the polymer structure A slip plane contains the dislocation line and its burger’s vector The burger’s vector is parallel to the dislocation line for screw dislocation and perpendicular for edge dislocation Materials with higher melting point typically have a higher Young’s modulus Dislocation is one type of linear defects Crystals with hexagonal close packed (HCP) structure has 6 atoms in one unit cell Point defects appear in pair in ceramics because they need to achieve neutrally Fick’s 2nd second law describes non steady- state diffusion Ionic bond is non directional while metallic bonding is non-directional Thermoplastic has linear or branched polymer structures Most polymers have amorphous structure and most metals have crystalline structure Interstitial diffusion is much faster than that of vacancy diffusion Second phase particle is a 3D defect Material science focuses on investigating the relationship between the structures and the properties of materials. Two electrons in an atom can have the same four quantum number. (F) A material of which melting temperature is high has typically low elastic modulus. (F)

TERMS & CONCEPTS 6 Classifications of the properties of the materials: mechanical, electrical, thermal, optical, magnetic, biocompatible, and corrosion resistance. Bohr Atomic Model: describes the position and energy of the electrons. It arranges electrons according to energy levels. Wave Mechanical Model: electrons are treated as particles + waves. Describes electrons with 4 quantum numbers (l, n, ml, ms) Electronegativity: is the tendency of an atom to share or gain electrons. The ability of an atom in a molecule to attract electrons to itself. EN higher from L to R and from B to T (atomic radius gets smaller because R has more protons and shrink energy) Pauli Exclusion Principle: electrons in the same atom or electrons from different atoms which interact can have the same four quantum number (different spin orientation). Every electron is unique. Solid solution: is when you have your “perfect” structure; example: NACL looking like 000000 perfect lattice and no diffusion occurred 2nd Phase solution: once diffusion occurs, the boundary still has ‘perfect’ part left in it, maintains neutrality. 0 00000000000 Slip plane: a plane containing the burger’s vector and dislocation line Burger’s vector: describes the magnitude and direction of the dislocation Isomerism: when 2 or more polymer molecules have the same composition, but different structure and properties. 2 compounds have the same chemical formula (C4H10 ALWAYS draw the diagram) Polymorphism: a solid material that has more than one crystal structure, determined by pressure and temperature (iron, glycine, carbon  graphite & diamond)

ATOMIC STRUCTURE DESCRIBED “n” related to the distance of an electron from the nucleus related to the radius “l” related to the shape o the electron subshell (s, p, d, f)  the number of the shell is restricted by the magnitude of n “ml” number of energy state of each subshell “ms” spin moment number, related to electron spin, which must be oriented to either up or down THREE TYPES OF PRIMARY BONDING: Ionic: electrons are transferred, not directional. Examples: MgO Metallic: good conductor, all atoms share electrons, ‘sea of electrons’. Examples: Fe, FeCrNi Covalent: electrons are shared, highly directional. *Graphite (carbon)  Covalent + Van der Walls; H2O  Covalent + Hydrogen bonding Na+: 1s2, 2s2, 2p6, Cl-: 1s2, 2s2, 2p6, 3s2, 3p6 Ionic radius of Na+ is smaller than Cl-, Na+ is more electronegative than Cl-, more EN results in bringing the electrons closer together resulting in a smaller radius. For 29: 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10

-WHY ARE COVALENTLY BONDED MATERIALS GENERALLY LESS DENSE THAN IONICALLY OR METALLICALLY BONDED ONES? .Because ionic and metallic bondings are none directional and covalent bonds are directional .Covalent bonds have stronger bonding -WHY DOES THERMAL EXPANSION COEFFICIENT VARY REVERSELY CORRELATED WITH MELTIN TEMPERATURE? .The larger the interatomic bonding energy, the lower the coefficient thermal expansion .In order to higher Tm, strong bonds (interatomic bonds are needed) which means harder to stretch and less expansion -WHY DOES MgO HAVE A HIGHER MELTING TEMPERATURE THAN NACL? M has complete and higher energy level, so it takes more energy to break up a stable energy level. -EXPLAIN THE RELATIONSHIP B/W MELTING POINT AND MODULUS OF ELASTICITY (YOUNG”S MODULUS, Y) BY USING THE ENERGY (E) vs. INTERATOMIC DISTANCE (r) DIAGRAM. .Tm higher  Y higher .As TEC decreases, TM increases or vice-versa .Potential Energy is higher, the thermal expansion coefficient becomes smaller. Temperature is higher on EA than in EB , and the expansion coefficient is higher on EB than EA. (See graph) .Lower TEC means higher TM because high Y resulting in less interactive increase as fodded to substance, while vice-versa. Example: Iron has higher Y while higher TEC means lower TM because lower Y resulting in more interatomic increase as Fadded to substance. Example: Butter .Higher amount of E required to break bond, for “r” to reach infinity, also needed for Tm to change states .More force (F) required when Y higher to break bond, for “r” to reach infinity, also needed to reach Tm to change states, forced required to make interatomic radius bigger.

PERIODIC TABLE (FROM TOP-LEFT TO BOTTOM-RIGHT) Atomic radius: increases from right to left and top to bottom because the more electrons you have, the attraction into the center (nucleus) increases. Electronegativity: increases form left to right and bottom to top because that’s more likely to accept an electron for an empty space in their outer shells Electronegativity: is the tendency of an atom to share or gain electrons. The ability of an atom in a molecule to attract electrons to itself. EN higher from L to R and from B to T (atomic radius gets smaller because R has more protons and shrink energy) *ROWS = Periods (same value for the highest quantum number); COLUMNS = Groups (same number of valence, similar chemical property due to similar bonding)

FOUR TYPICAL MOLECULAR STRUCTURES OF POLYMERS 1. Linear: flexible (PV, PE, NYLON) 2. Branched: series boundaries, lower density (LDPE, THRMOSET) 3. Crosslinked: covalent b/w chains (Vulcanized Rubber) 4. Networked: tri functional mer units (EPOXY) 14: increasing in strength THERMOPLASTIC: linear or branched structure; soften when treating and harden when cooling (or reversible) THERMOSET: highly crosslinked or networked, harden when heating up ELATOMER: highly coiled, soft, capable of large elastic deformation EXPLAIN THE DECREASE IN VOLUME BY A FEW PERCENT AS IRON UNDERGOES A PHASE TRANSITION FROM BCC STRUCTURE TO AN FCC STRUCTURE AT 912 C. Due to the increase in open gas spaces with more atoms and bonds per cube over all volume. Also because BCC (68% & 2 atoms per cube) and FCC (74% & 4 atoms per cube) have different APF so the temperature is different.

DIFFUSION: Material transport by atomic motion Interdiffusion: or impurity diffusion. In an alloy, atoms tend to migrate from regions of large concentration Self-diffusion: migration of atoms in an elemental solid. Interstitial diffusion: small in purity atoms migrate among interstitial sites. It is from interstitial site to an adjacent interstitial site.* Vacancy diffusion: atomic motion from one lattice to an adjacent vacancy. * Interstitial diffusion is normally more rapid than vacancy diffusion because interstitial atoms are smaller, thus more mobile. Also because the probability is bigger for empty adjacent interstitial than for vacancy diffusion to a host atom DIFFUSION FASTER FOR: open crystal structures, lower melting T materials, materials with secondary bonding, smaller diffusion atoms, cation, lower density higher diffusion. DIFFUSION SLOWER FOR: closed packed structures, higher melting T materials, materials with covalent bonding, anions, higher density. STRATEGY TO MEASURE EXPERIMENTALLY THE ACTIVATION ENERGY OF DIFFUSION Is based on using the Nv=Ne^(-Qv/kT) formula. Identical substances, however heat then to different temperatures, record mass and use that as “Nv”. Vacancies which you can calculate for the diffusion that occurs for the substances when heated. Then compare data and solve backwards for a Qv, which should be about same for all samples. STRATEGY TO SLOW DOWN THE DIFFUSION OF HELIUM INTO STEEL A simple solution to slow down diffusion is to replace the steel walls with a denser material. Example: lead, a higher density wall, results in slower diffusion of Helium into that wall. *From table: WHY THE DIFFUSION OF FE IN A alpha-FE IS MUCH HIGHER THAN THAT OF IN gamma-FE? Because of the lower APF in alpha-FE (BCC) compared to the higher APF of gamma-FE.

DEFECTS: 0D- point defects: vacancy interstitial 1D – line defects: edge, screw, mixed 2D – planar defects: - EXTERNAL SURFACES: surface atoms have higher energy because they are not bounded to the max number of nearest neighbors in this case. Along surface boundary anywhere that crystal terminates. -GRAIN BOUNDARY: in boundary region, there is mismatch in transition from the crystalline orientation of one grain to that of an adjacent one. They separate grains with different crystallographic -PHASE BOUNDARY: exists in multiphase materials where in each side of the boundary a phase exists; different characteristics on phases. -TWIN BOUNDARIES: special of grain boundary across which there is a specific lattice symmetry, atoms located in mirror-imagine positions of the atoms on the other side. 3D- Volume defects

TYPES OF DISLOCATIONS: Edge dislocation: place of atoms that terminates within crystal has dislocation line, Burger’ vector is perpendicular to the dislocation line. It can be generated by sharing crystals. Screw dislocation: thought of as caused by shear stress, Burger’s vector is parallel to the dislocation line. Atomic distortion is linear and along dislocation line, causing screw line effect. Mixed dislocation: has both edge and screw dislocation. The difference b/w them is that edge dislocation has a dislocation line in which the edge is perpendicular to the plane of the edge. Screw dislocation is when is formed by shear stress, and mixed dislocation is when it has two or more components....