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Materials structure

QuestionAnswer
Glasses are often grouped with ceramics because they have similar crystal structures. False, Glasses are not crystalline by definition
The stored elastic energy per unit volume is given by 0.5s2 /E, where s is stress and E is the elastic modulus True, This is also equal to 0.5se
The coordination number of magnesium ions in magnesium oxide (MgO) is 6 True, The coordination number is this case refers to the oxygen ions around the Mg ion
The density of a metal is mostly determined by the atomic weight of the metal. True
The rule of mixtures can be used to calculate the lower bound for composite stiffness for a given volume fraction of reinforcement. False, It is for the upper bound
The BCC unit cell has 2 atoms, and the FCC unit cell has 4 atoms. True
The FCC crystal structure contains 8 tetrahedral interstices per unit cell. True
Thermoplastic and thermoset polymers are often grouped together because they have similar crystal structures False, they are generally not crystalline
Thermoplastic polymers deform permanently by stretching the intermolecular bonds. False, They permanently deform by molecular sliding, not by stretching bonds
The coordination number of carbon in diamond is 6. False, it is 4
Elastomers show large reversible deformations below the glass transition temperature False, Elastomers become stiff below the glass transition temperature
The rule of mixtures can be used to calculate the density of a composite for a given volume fraction of reinforcement. True
The coordination number for silicon in silicon carbide is 4. True
The coordination number of iron atoms (BCC) is 8. True
The stored elastic energy per unit volume is given by 0.5sE, where s is stress and E is the elastic modulus. False, The stored elastic energy per unit volume is given by 0.5se
The density of metals is mostly determined by the crystal structure. False, It is mostly determined by atomic weight
Silicon has the diamond cubic structure, so there are 4 silicon atoms per unit cell. False, Diamond cubic has 8 atoms per unit cell
The elastic modulus of a thermoplastic polymer is directly related to the stiffness of carbon-carbon bonds in the polymer chains. False, It is related to intermolecular bond strength
Most polymers are amorphous or semicrystalline. True
The FCC crystal structure contains 8 octahedral interstices per unit cell. False, There are 4 octahedral and 8 tetrahedral interstices in the unit cell
The density of a composite can be estimated by using the rule of mixtures. True
The stress is always proportional to the strain for any material. False, This is true only in the elastic range
If a rod of an elastomer is loaded in uniaxial compression below the elastic limit, the elastic modulus of the elastomer is defined as the force applied to the material divided by its change in length. False, Elastic modulus is slope of stress-strain curve in elastic range, and the units are in terms of force per unit area
The stored elastic energy in a piece of material cannot be calculated using only the elastic modulus of the material and the applied stress if the material is loaded below the elastic limit. False, If elastic modulus and applied stress are known, then strain is also known and the stored elastic energy per unit volume can be calculated
The coordination number for carbon in silicon carbide is 4. True
The crystal structure of zirconium oxide (ZrO2 ) can be described using an FCC unit cell and there are 4 oxygen atoms per unit cell. False, There are 8 oxygen atoms per unit cell, twice as many as the number of zirconium atoms
The density of a pure element can be determined knowing only the atomic mass and the radius of the atom False, The crystal structure must also be known
Poisson's ratio is positive because generally stretching in one direction leads to contraction in the other two. True, The Poisson ratio is defined as –e t/e
The elastic modulus is a property that is independent of the applied stress or strain of the material. True
FCC and HCP crystals have largest packing with 100% packing of the unit cell. false, Packing is only 74% in close-packed structures
Crosslinks in thermoset polymers are due to weak hydrogen-hydrogen bonds that can be easily removed by heating. False, Crosslinks are strong bonds that are not easily removed
There are three directions along which atoms, represented by hard spheres, touch in a close-packed plane of the crystal structure of copper. True
Ceramics have relatively high values of elastic modulus because the atoms/ions are close-packed. False, The high values of elastic modulus are due to the strong ionic or covalent bonds. Ceramics are generally not close-packed)
Most polymers are crystalline solids below their glass transition temperature. False, Most polymers are not crystalline
The elastic modulus of a foam can be estimated by using the rule of mixtures. False, There is only one solid phase, so the rule of mixtures cannot be used. There is a separate formula for elastic modulus for foams
Thermoplastic polymers are commonly amorphous. True
The stored elastic energy per unit volume of a material is proportional to the elastic strain applied. False, It is proportional to e^2
The elastic modulus of a composite containing continuous uniaxially aligned fibers depends on fiber orientation relative to the direction of an applied uniaxial tensile stress. True
There are 8 zirconium ions per unit cell in the crystal structure of ZrO2 . False, There are 4
Elastomers are lightly crosslinked to give elastic moduli that are greater than the elastic moduli of thermoplastic polymers. false, Elastomers have the lowest moduli among true solids
The coordination number of oxygen ions in MgO is 4. False, It is 6
Cubic close-packed metals have crystal structures with higher packing factors than the packing factors of hexagonal close-packed metals. False, They are equal
Poisson’s ratio of a material is usually a negative number. False
In elastic deformation of metals, stress is proportional to strain. True
Elastomers is the material class with the lowest elastic moduli of any true solids (without porosity). True
The coordination number of atoms in aluminum (FCC) is 12. True
Metallic bonding involves shared pairs of electrons between atoms. False, Electrons are delocalized and are not shared between specific atoms
The coordination number of oxygen ions in magnesium oxide (MgO) is 4. False, It is 6. The coordination number is this case refers to the magnesium ions around the O ion
Face-centered cubic metals have crystal structures with the same packing factor as hexagonal close-packed metals. True
The coordination number of atoms in zinc (HCP) is 12. True
The density of a closed cell aluminum foam measured using the double weighing method will be higher than the density of an open cell aluminum foam if the volume fraction of pores is the same in both cases. False, The open cell foam will have a higher density using the double weighing method because the fluid can enter the open pores
The repeat unit of polystyrene (PS) contains equal numbers of carbon atoms and hydrogen atoms. True
The elastic modulus of a foam can be estimated by using the rule of mixtures False, There is a separate formula to calculate the elastic modulus of a foam.
Zirconium oxide (ZrO2), with the fluorite crystal structure, can be described as an FCC arrangement of zirconium ions with all of the tetrahedral interstices filled with oxygen ions. True
HCP and FCC both have a packing fraction of .74 True
BCC has a packing fraction of .70 False, BCC has a packing fraction of .68
FCC has 4 atoms per unit cell True
HCP has 6 atoms per unit cell True
BCC has 8 atoms per unit cell False, BCC has 2 atoms per unit cell
The coordination numbers for BCC, FCC, and HCP are 8,12,12 True
Iron is an example of BCC True
Tungsten is an example of FCC False, Tungsten is BCC, an example of an FCC metal would be copper,Aluminum, silver, and gold
What metals have HCP arrangement Titanium, Zirconium, and magnesium
FCC length a*sqrt(2)
BCC length a*sqrt(3)
Polyethylene (PE) CH2
Polypropylene (PP) CH2-CH-CH3
Polystyrene (PS) CH2-CH-C6H5
Polyvinyl Chloride (PVC) CH2-CH-Cl
Polytetrafluoroethylene (PTFE) CF2
Non-Cross linked amorphous-thermoplastic
partially crystalline semi-crystalline thermoplastic
slightly cross linked elastomers
highly cross linked thermoset polymers
Tetrahedron 4 faces and 4 apices
Octahedron 8 faces and 6 apices
diamond cubic coordination number 4
Created by: jacobbeggarly
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