Biomaterials Exam 1

(E₀) the energy required to separate two atoms to an infinite separation

bonding energy

r₀ = __________ spacing (about 0.3 nm for many atoms)

equilibrium

the greater E₀, the stronger atomic bonds are → a _______ material (higher elastic modulus) and ______ T_M

stiffer, higher

_______ bonds = higher bonding energy and melting point

primary

what are four examples of primary bonds?

ionic, covalent, metallic, and covalent-ionic mixed

what are two examples of secondary bonds?

van der Waals and hydrogen bonding

metals = primarily _______ bonds

metallic

ceramics = covalent + _____

ionic

polymers = covalent + _________ (+entanglement)

secondary

type of bonding

• strong, non-directional

• all + ions must have - ions as nearest neighbors in 3D, and vice versa: densely packed

• predominantly found in ceramics

ionic bonding

common characteristics of ionic materials:

• _____, ______ → initial failure in structure causes catastrophic propagation due to electrostatic instability

• ________ and thermally ________ due to tightly held electrons within the locality of the bond (can’t be charge carriers)

• _____ melting point increases with increasing bond energy

• ____ chemical reactivity

hard, brittle, electrically, insulating, high, low

• stable electron configurations are obtained by the ‘sharing’ of electrons between adjacent atoms

• observed between non-metallic atoms

• observed between elemental solids and other elements located on the right side of the periodic table

covalent bonding

common characteristics of covalent materials:

• can be ______ (diamond) or _____ (bismuth) depending on the arrangement, number of free electrons, etc.

• properties vary widely: can be strong or weak, ______ or ______

• tend to be insulators or ___________

• depends on if the material wants to give or take electrons

strong, weak, brittle, ductile, semiconductors

_______ electronegativity

• electropositive elements: readily give up elements to become + ions

• on pt ←

smaller

______ electronegativity

• electronegative elements: readily acquire electrons to become - ions

• on pt →

larger

mixed bonding

• most common type is ______-____ mixed bonding

covalent-ionic

higher % ionic character, _____ bonding more prevalent

ionic

lower % ionic character, _______ bonding more prevalent

covalent

high % ionic character

• ____ melting and boiling temperature

• not conductive in solid form (ions locked in place) but conductive when _______ or molten

• more likely soluble in _____ solvents (e.g., water)

high, dissolved, polar

low % ionic character:

• not ________ in any form except for graphite

• more soluble in ___-_____ solvents (e.g., benzene)

conductive, non-polar

found in metals and their alloys

• free-floating ‘sea’ of valence electrons within structure

• remaining non-valence electrons and atomic nuclei form ion cores (net positive charge)

metallic bonding

_____ _______ shield positively charge cores from repulsive electrostatic forces, and act as a glue to hold ion cores together

free electrons

common characteristics of metals:

• good conductors of ______ and ____

• ______- due to the nature of the bond that allows slipping and ‘sharing’ of electron sea

electricity, heat, ductile

• typically weak

• arises due to atomic/molecular dipoles (charge separation)

• present between all atoms/molecules, but effect is obscured if any of the primary bonding types is present

secondary bonding

secondary bonds are mainly observed in:

• inert ____

• covalently bonded molecules - _______

• ________

gases, polymers, adhesives

True/False: Materials with a higher bonding energy tend to have lower melting temperatures.

False

True/False: Hydrogen bonding is a type of primary atomic bond that is stronger than covalent bonding

False

the equilibrium spacing between two atoms (r₀) occurs where the net force is ______

zero

________: consists predominantly of covalent bonding and secondary bonding

polymers

energy and packing

• non dense, ______ packing

  • deviated from the min value

• dense ________ packing

  • close to the min energy

random, ordered

_________ materials

• atoms arranged in periodic, 3D arrays

• typical of:

  • metals

  • many ceramics

  • some polymers

crystalline

__________ materials

• atoms have no periodic arrangement

• “amorphous”

• occurs of:

  • complex structures

  • rapid cooling

noncrystalline

metals are described using the ______ hard-sphere model

• spheres represent nearest-neighbor _____ that touch one another

atomic, atoms

repeat entities used to describe crystal structures

unit cells

crystal structures - metals

• _____-centered cubic (FCC)

• _____-centered cubic (BCC)

• _________-close-packed

face, body, hexagonal

3D array of points coinciding with atom positions

lattice

crystal structures: basics:

• each sphere represents an ____ core

ion

atoms at corners and centers of all the cube faces

FCC

atoms at all corners and a single atom at the cube center

BCC

unit cell with hexagonal shape. Top and bottom faces have 7 atoms making a hexagon, middle plane with 3 atoms

HCP

number of nearest-neighbor or touching atoms **not limited to unit cell**

coordination number

atomic packing factor (APF) is never going to be greater than ____ or equal to one

one

crystal structure

• centers of atoms located at the eight corners of a cube

• rare due to low packing density (only Po has this structure)

• closed-packed directions are cube edges

simple cubic (SC) crystal structures

what is the coordination # for a simple cubic crystal structure?

6

crystal structure

• atoms located at 8 cubes corners and at the centers of the 6 faces

face-centered cubic crystal structure

what is the coordination number of FCC crystal structure?

12

what is the coordination number of BCC crystal structure?

8

BCC is _____ dense than FCC

less

crystal structure

• top and bottom faces of the unit cell consist of six atoms that form regular hexagons and surround a single atom in the center

hexagonal close packed structure

what is the coordination number of HCP?

12

density comparisons for four material types

• metals have

  • close-packing (________ bonding)

  • often _______ atomic masses

• ceramics have

  • often _______ elements

• polymers have

  • low packing density (often ________)

  • lighter elements (C, H, O)

• composites have

  • moderate to low densities

metallic, large, lighter, amorphous

• when the periodic/repeated arrangement is perfect and extends throughout the entirety of the specimen

• crystal will assume a regular geometric shape that sheds light on its structure

• important in the microelectronics industry (silicon wafers)

single crystals

most crystalline solids are a collection of many small crystals or _____

grains

the different grain extremities _______ on each other as the solidification process approaches completion

impinge

the atomic mismatch within the region where two grains meet is called a _____ ________ (affects material properties)

grain boundary

grain boundaries are areas of high energy and are more ________ _______ than their surroundings

chemical reactive

grain boundaries

• they also help improve mechanical properties by inhibiting ________ ______; the smaller the grains (meaning _______ grain boundary area), the _______ the metal is

dislocation motion, higher, stronger

heat treatments cause grains to grow bigger by ________ boundary energy

reducing

some metals and nonmetals can have more than one crystal structure = ____________ or allotropy (elemental solids)

• prevailing structure depends on temperature, pressure

polymorphism

usually true because even if material is composed of many grains their directions are totally random → overall behavior is _______

isotropic

materials with less structural symmetry (e.g., deformed grains have preferential crystallographic orientation) are highly ________

anisotropic

True/False: Materials whose atoms are arranged in a periodic, 3D array are known as amorphous or noncrystalline

False

True/False: Stainless steel’s corrosion resistance is primary due to the formation of a thin film of chromium oxide on its surface

True

True/False: Of the common biodegradable metals, iron has the fastest corrosion rate, making it suitable for applications that require rapid degradation

False

True/False: Nitinol, a shape-memory allow, is composed of a nearly even mix of nickel and titanium

True

True/False: Surface atoms are in a higher energy state than interior atoms because they are not bonded to the maximum number of nearest neighbors

True

vacant atomic sites

vacancies

host atoms positioned in interstitial positions between atoms

self-interstitials

a ____-_______ is an atom from the crystal that is crowded into an interstitial site

self-interstitial

an interstitial site: a small _____ space usually not occupied

void

usually, the void space is much _______ than the atom that is trying to fit it

• large distortion into the surrounding ______

• not very thermodynamically favorable → much _____ than vacancies

• probability of self-interstitial _ vacancies

smaller, lattice, rarer, <

impurities occur when there is a ________ or interstitial positioning of a foreign atom (addition of impurity will be like a solid solution alloying)

substitution

alloy terminology:

• ______: element present in greatest amount, also called host atoms

• _____: element present in minor concentration

solvent, solute

Hume-Rothery rules

• ______ ____: difference in atomic radii between the solute and solvent should not be more than ___%

atomic size, 15

Hume-Rothery rules

• crystal structure: solutes and solvents should have _____ type of structure for appreciable solubility (FCC & FCC)

same

Hume-Rothery rules

• electronegativity: the _______ the difference, the more likely that intermetallic compound will form instead of a substitutional solid solution

greater

Hume-Rothery rules

• valences: metal tends to dissolve another metal of ______ valency. same valency can give complete solubility

higher

interstitial spaces are small, so interstitial atoms must be small and ___ concentrations, <___%

low, 10

substitutional impurities

• Fe + Cr → the larger chromium atoms places lattice ________ strains on the surrounding iron atoms - this also restricts lattice slip and increases the materials strength

compressive

interstitial impurities

• Fe + C → the smaller carbon atoms places lattice ______ strains on the surrounding iron atoms - this restricts crystal lattice slip and increases material strength

tensile

linear defects are also termed _________

dislocations

linear defects are caused by _________ of plane of atoms in the middle of crystal

termination

• forms due to shear/rotational stresses applied on material

• upper half is shifted one crystal unit to the right

screw dislocation

most dislocations found in materials are neither pure edge or screw but instead a ____ of the two

mix

Mixed dislocations

• key concept: ____ occurs along dislocation lines that exist around linear defects

• presence of linear defects/dislocations influences material ________

• dislocations are involved in _______ (permanent) deformation of materials

slip, properties, plastic

why/how do linear defects emerge?

• rapid ______ during solidification does not give atoms enough tome to arrange perfectly, creating dislocation arrays (vacancy clustering)

• local stress from ________

• cracks, _____ _______, and other pre-existing imperfections

• ________ damage; knock out atoms, create point defects that can nucleate into linear defects

cooling, impurities, grain boundaries, irradiation

boundary separating two small grains or crystals with different crystallographic orientations

grain boundaries

• atoms are bonded ____ regularly at a grain boundary

• the orientation mismatch can be _____ (small-angle) or ______ (high-angle)

• grain boundary energy is similar to ______ energy

less, slight, large, surface

grain boundaries are also more chemically _______ and act as sites for impurities and other defects because of their high energy state

reactive

grain boundaries impede ____

slip

• mass transport by atomic motion

• stepwise migration of atoms from one lattice site to another

diffusion

diffusion mechanisms

• gases & liquids: ______ (Brownian) motion

• solids: _______ diffusion and _______ diffusion

random, vacancy, interstitial

diffusion of atoms of one material into another material

inter-diffusion

atomic migration in a pure metal

self-diffusion

two conditions for diffusion:

1. there must be an _____ adjacent site

2. the atom must have sufficient energy to both:

   1. _____ bonds with its neighbor atoms

   2. cause ______ distortion during the displacement

empty, break, lattice

at a give T, a small % of atoms can undergo diffusion using ________ ______ (this fraction increases with T)

activation energy

• example of interstitial diffusion

• outer surface selectively hardened by diffusing carbon atoms into surface

• presence of C atoms makes iron (steel) harder: carburizing (CH₄ gas)

case harding

the diffusion coefficient ________ with increasing T

increases

the most practical diffusion are ___-______ ____

non-steady state

assumptions in solving non-steady state diffusion:

1. before diffusion, any of the diffusing solute atoms in the solid are _________ distributed with concentration of C₀

2. the value of position x at the surface is ____ and increases with distance into the solid

3. the time is taken to be zero the instant ______ the diffusion process begins

uniformly, zero, before