Chapters 1, 2, 3 (intro, atomic structure and bonding, crystal structure) Flashcards
What is a property?
the response of a material to an external effect
Examples of properties
Mechanical, electrical, deteriorative, optical, magnetic, thermal
what are the classes of materials
Metals, ceramics, polymers
what is a metal
inorganic metallic elements
name 2 properties of metals
- Thermal and electrical conductors 2. Strong and ductile
why are metals good conductors
because they have free electrons
what is an alloy
a combination of 2 or more metals and non-metals
What is a ceramic
a compound of metals and non-metals
name 5 properties of ceramics
- Hard and strong 2. Brittle 3. Resist high temps 4. Low density 5. electrical insulators
common example of a polymer
plastic
what is a polymer
a long chain molecule and network usually based on carbon
name 3 properties of polymers
- electrical insulators. 2. low density 3. low operating temperatures
what are composites
a mixture of 2 or more materials
what is unique about semi conductors
they share properties of both insulators and conductors
what types of materials are used in “Bio-Materials”
all
order of density of materials
metals
ceramics
polymers & composites
order of stifness
metals and ceramics
composites
polymers
order of strength
metals
composites
ceramics
polymers
order of electrical conductivity
metals
semi conductors
ceramics and polymers
what do properties of materials depend on
bonds between atoms
atomic packing
what is an atom comprised of
Proton + neutron + electron
where is the mass of the atom located
nucleus
where is the volume of the atom located
electron cloud
what does the atomic # dictate
of protons
what type of electrons determine important properties
valence electrons
what are electroPOSITIVE elements
metallic elements that GIVE UP outer electrons to become CATIONS
what are electroNEGATIVE elements
non metals that ACCEPT electrons to become ANIONS
electronegativity
the tendency to attract electrons
essentially, why do atoms bond together?
to reduce overall energy
when is equilibrium reached
when Fa+Fr=0 OR when the overall energy is at a minimum
what is bonding energy
the energy required to break the bonds
what would a higher bonding energy do
stronger bonds=higher strength=higher melting temp
types of primary bonds
- Ionic
- Covalent
- Metallic
types of secondary bonds
- Van der Waals
2. Dipole fluctuations
Primary, Secondary; Physical Chemical. MATCH THEM
Primary=Chemical, Secondary=physical
between what types of elements are ionic bonds typically found
metals and non metals
what type of material are ionic bonds usually found in
Ceramics
why do ions pack together
to maintain neutrality
materials which are ionically bonded are what? (3 things)
- very strong
- brittle
- insulators
Covalent bonds
sharing of electrons, they try to attain noble gas configs
degrees between sp^3 orbitals
109.5
%Ionic Character formula
[1-exp(-0.25(Xa-Xb)^2)]x100
what are ‘ion cores’
closely packed metal ions in ordered arrangements
what type of bonding are SEAs of electrons found
Metallic
Metallic bonds are insulators or conductors
conductors (free electrons)
Secondary bonding
low energy-weak bonds
what are secondary bonds based on
dipoles(separation of charge)
Where are Van der Waals forces found
between 2 adjacent polar molecules
What does Amorphous mean?
NO REPEATING ORDER or pattern in bonds, aka short range order
What are crystals?
Atoms bonded together in a REGULAR 3D patter, aka long range
what materials are crystalline Patterns found in?
Ceramics, Metals, Some polymers
What are space lattices
the arrangement of atoms
What is a unit cell
the smallest repetitive unit which is part of the lattice pattern (it must display the minimum # of features but never repeat any)
name the 3 types of densely packed structures
- Body centered cubic(BCC)
- Face centered cubic (FCC)
- Hexagonal close packed (HCP)
formula for atomic packing factor
volume of unit cell occupied by atoms/ total unit cell volume
where do atoms touch each other in BCC
along the cube diagonals
where do atoms touch each other in FCC
along the FACE diagonals
Coordination # of BCC, FCC & HCP respectively
8, 12, 12
Atoms per unit cell of BCC, FCC, HCP respectively
2, 4, 6
crystal density formula
density=number of atoms in unit cell * Molar Mass/volume of unity cell * Avogadros #
what is polymorphism/allotropy
elements in which more than 1 crystal form can exist depending on temperature
which is the most & least symmetric crystal structure
cubic & tricrilic respectively
linear desnity formula
number of atoms/ unit length of direction vector
Planar density formula:
(atoms/2D repeat unit)/(area/2D repeat unit)
Single crystal
when a piece of material is made of 1 single crystal
Polycrystals
Many different crystals, with varying orienations
what is a grain boundary
the boundary line at which crystal grains meet
If a property varies with direction, it is said to be…
Anisotropic
If a property doesn’t vary with direction, it is said to be…
isotropic
describe creep
the tendency for a material to lengthen over time under load
The degree to which a material will creep is dependant on what?
the number of grain boundaries
state bragg’s law formula
n(lambda)=2d(hkl)sin(theta)
what can be said about the position of atoms which are covalently bonded?
their elements will be close on the periodic table
Basically, why are metals ductile?
Non-directional bonding–>fewer restrictions–>can withstand larger forces before bonds break.
ALSO, since metals have free electrons, this allows their structures more freedom, promoting ductility.
Which is the only Non-Crystalline ceramic? And what is another word for this category?
Glass. Amorphous
what are space lattices?
the patterns in which atoms prefer structuring themselves in
when we refer to lattice constant, which variables are we talking about?
a, b, c
List the properties of BCC (3)
- 4R=sqrt3*A
- coordination # =8
- 2 atoms/unit cell
- APF=0.68
List the properties of FCC (4)
- 4R=sqrt2*A
- coordination # = 12
- 4 atoms/unit cell
- APF=0.74
List the properties of HCP(2)
- 6 atoms/ unit cell
2. coordination #=12
what crystal SYSTEM is common for all CUBIC crystal STRUCTURES
a=b=c and usually, alpha =beta=gamma=90deg
how is a crystallographic direction denoted?
[xyz]
what does the family of directions denote?
all edges of a cube
what does family denote?
all internal diagonals
what does the family denote?
all face diagonals
what can be said about directions and planes with the same indices in cubic systems
the direction is perpendicular to the plane
if the electronegativity value of an element is high (ie near 4), what will it typically do to electrons
acquire/attract them
A balance of [these] forces determines the inter-atomic spacing
Repulsive and attractive forces
H2 would have which type of bonding and why
covalent, similar electronegativity, same number of valence electrons require to fill first orbital
describe fluctuating dipoles
when the atomic nucleus and electron cloud are not symmetrical, secondary bonding occurs between two adjacent molecules due to the charge separation
briefly describe why when water freezes, it expands
when water freezes, there are 4 hydrogen bonds found between 2 h20 molecules. these 4 bonds are relatively open and spaced apart, causing this expansion
chapter 2 question
how and what do alpha, melting temperature and bond length depend on?
they depend on Eo(bonding energy). the larger Eo, the large those constants.
is energy higher or lower for denser packed structures?
lower
which materials have the simplest crystal structures
metals
what happens if you change the sign of all the indices of a direction?
it would give the exact opposite direction
what can be said about two parallel planes
they are equal and will have the same miller indices
how do we denote a crystallographic plane?
(hkl)
how do we denote a family of planes?
{hkl}
what does the {100} family of planes denote?
all faces of a cube
In FCC, what are the close packed planes?
{111}
In HCP, what are the close packed planes?
(0001)
Although BCC is not as close packed, which is its most densely packed plane?
{110}
what is the stacking sequence of FCC
ABCABCABC
what is the stacking sequence of HCP
ABABABABABAB
give a well-known example of an Anisotropic material
wood
how might one increase the Creep on a material
increase the temperature
what effect does increasing the # of grain boundaries have on creep?
the material will creep more
would properties in metal typically be iso or anisotropic?
isotropic
why is the diffusion coefficient greater C in alpha-iron BCC rather than C in gamma-iron FCC
since the atomic packing factor is less in BCC than it is in FCC, this means there is slightly more interstitial space for the atoms to move about in, making it faster to diffuse
are covalent bonds directional
yes, they are directional
true or false and why? when ro (distance) of 2 atoms is at equilibrium, the binding energy is 0
FALSE! the change in binding energy is 0, not the actually value, the binding energy will be at a minimum
