1- Which of these processes will lower the glass transition temperature of a glass?
Increase the cooling rate
Decrease the cooling rate
Increase amount of network modifier
Decrease amount of network modifier
Quench the glass to cool the surfaces quickly
Decrease the number of defects
2- Which of the following statements describes why ion exchange of 99% SiO2 - 1% B2O3 in a salt bath of molten postassium chloride (KCl) is not an effective way to increase the surface strength of the glass sample?
The ion exchange works correctly, however both air-cooling and water-quenching are more efficient ways to induce a surface strengthening effect.
The ion exchange works correctly, however both air-cooling and water-quenching are more efficient ways to induce a surface strengthening effect.
The ion exchange works correctly, however both air-cooling and water-quenching are more efficient ways to induce a surface strengthening effect.
The sample described above is composed of SiO2 and B2O3. Neither of these components are network modifiers, so ion exchange does not take place to strengthen the surface of the glass.
The surfaces of the glass sample are not cooled quickly enough to create a high internal strain energy that leads to strong surfaces.
3- In which of the following ways would you change the glass sample to make the ion exchange more efficient?
Change the composition of the glass to increase the proportion of B2O3.
Change the composition of the glass to add Na2O, Li2O, or MgO.
Change the composition of the glass to add CaO.
Change the composition of the salt bath (KCl) to an ion smaller than potassium, such as sodium chloride (NaCl).
Lower the temperature of the salt bath to cool the sample more quickly.
4-Please mark below all of the statments that correctly describe the physical properties of glass.
Glasses do not have slip systems.
Glasses consist of both covalent and ionic bonding.
Glasses are brittle at room temperature.
Glasses are amorphous and have no long range symmetry.
The volume of a sample of glass depends on its cooling rate.
Glasses must consist of network formers and modifiers.
5- You have a single crystal of 100% pure gold. Identify which of the following defects you would expect to be present at room temperature.
Free surface
Grain boundaries
Vacancies
Inclusions
Substitutional impurity atoms
Interstitial impurity atoms
6- At 10°C below the melting point of aluminum, 0.08% of the atom sites are vacant. At 484°C only 0.01% are vacant. Given this information, determine the energy of vacancy formation (ΔHv) for aluminum.
Energy of vacancy formation (in joules / vacancy)
X=.......
7- Determine the radius of the largest atom that can be accommodated in the interstices of BCC iron (Fe) without stress. (Hint: the center of the largest site is at ½, ¼, 0; draw a unit cell - it helps.)
Radius (in centimeters):......
8-An activationenergy of 2.0 eV is required to form a vacancy in a metal. At 800°C there is one vacancy for every 10-4 atoms. At what temperature will there be one vacancy for every 1000 atoms?
Temperature (in degrees Kelvin):......
1- To lower the glass transition temperature of a glass, there are a few processes that can be done:
- Increase the cooling rate: When a glass is cooled quickly, the molecules do not have enough time to rearrange and form the strong structural bonds, resulting in a lower glass transition temperature. So, increasing the cooling rate will lower the glass transition temperature.
- Decrease the cooling rate: Conversely, when a glass is cooled slowly, the molecules have more time to rearrange and form stronger bonds, resulting in a higher glass transition temperature. So, decreasing the cooling rate will raise the glass transition temperature.
- Increase the amount of network modifier: Network modifiers are added to the glass composition to disrupt or weaken the strong bonds between the glass-forming atoms, which helps in lowering the glass transition temperature. By increasing the amount of network modifier, the glass transition temperature can be lowered.
- Decrease the amount of network modifier: On the other hand, decreasing the amount of network modifier will strengthen the bonds between the glass-forming atoms, resulting in a higher glass transition temperature.
- Quench the glass to cool the surfaces quickly: Quenching is a process of rapidly cooling the glass by immersing it in a cooling medium like water or oil. This rapid cooling process can also lower the glass transition temperature.
- Decrease the number of defects: Defects in the glass structure can act as weak points, lowering the glass transition temperature. By decreasing the number of defects, the glass transition temperature can be raised.
2- The correct answer is: The sample described above is composed of SiO2 and B2O3. Neither of these components are network modifiers, so ion exchange does not take place to strengthen the surface of the glass.
In order for ion exchange to occur, the glass composition should contain network modifiers that can be exchanged with the ions in the salt bath. However, in this case, SiO2 and B2O3 are not network modifiers, so the ion exchange process does not take place. Both air-cooling and water-quenching are more effective ways to induce a surface strengthening effect.
3- To make the ion exchange more efficient, you need to change the composition of the glass or the salt bath. The options given are:
- Change the composition of the glass to increase the proportion of B2O3: This can help by introducing more network modifiers that can undergo ion exchange with the salt bath.
- Change the composition of the glass to add Na2O, Li2O, or MgO: These components are known as network modifiers and can increase the efficiency of ion exchange.
- Change the composition of the glass to add CaO: CaO is another network modifier that can enhance ion exchange.
- Change the composition of the salt bath (KCl) to an ion smaller than potassium, such as sodium chloride (NaCl): The size of the ions in the salt bath can affect the efficiency of ion exchange. Using smaller ions can promote more effective exchange.
- Lower the temperature of the salt bath to cool the sample more quickly: Lowering the temperature can increase the cooling rate and improve the efficiency of ion exchange.
4- The correct statements that describe the physical properties of glass are:
- Glasses do not have slip systems: Unlike crystalline materials, glasses do not have well-defined crystal planes or slip systems that allow for plastic deformation.
- Glasses consist of both covalent and ionic bonding: The atomic structure of glass is composed of a combination of covalent and ionic bonds.
- Glasses are brittle at room temperature: Glass materials are known for their brittleness, meaning they fracture easily under stress.
- Glasses are amorphous and have no long-range symmetry: Unlike crystalline materials, glasses lack long-range order and have an amorphous structure.
- Glasses must consist of network formers and modifiers: Glass compositions typically include both network formers (e.g., SiO2) and network modifiers (e.g., Na2O) to control the properties of the glass.
5- In a single crystal of 100% pure gold at room temperature, the following defects would not be present:
- Free surface: A single crystal does not have any free surface as it is one continuous piece of material.
- Grain boundaries: Grain boundaries are interfaces between different crystalline grains, which are not present in a single crystal.
- Vacancies: Vacancies are empty lattice sites within a crystal structure, which are not present in a perfect single crystal.
- Inclusions: Inclusions are foreign particles or impurities within a material, which are not present in a pure single crystal.
The defects that could be present in a single crystal of 100% pure gold at room temperature are:
- Substitutional impurity atoms: These are impurity atoms that replace some of the gold atoms, causing lattice distortion.
- Interstitial impurity atoms: These are impurity atoms that occupy interstitial sites within the crystal lattice, causing lattice distortion.
6- To determine the energy of vacancy formation (ΔHv) for aluminum, we can use the following equation:
ΔHv = (RT^2/2) * ln(X2/X1)
Given the information:
T1 = 10°C below the melting point = Melting point - 10
T2 = 484°C
X1 = 0.08%
X2 = 0.01%
First, we need to convert the temperatures to Kelvin:
T1 = Melting point - 10 + 273.15
T2 = 484 + 273.15
Then, we can substitute the values into the equation and solve for ΔHv:
ΔHv = (R * T2^2/2) * ln(X2/X1) / (T2 - T1)
Where R is the gas constant (8.314 J/(mol·K)).
7- To determine the radius of the largest atom that can be accommodated in the interstices of BCC iron (Fe) without stress, we need to consider the atomic radii and the crystal structure of BCC.
In the body-centered cubic (BCC) structure, the largest interstitial site is located at the center of the unit cell (½, ¼, 0). This site can be occupied by an atom with a radius that fits without causing stress or distortion.
The radius of the largest atom can be calculated using the following formula:
Radius of atom = (sqrt(3) / 4) * Lattice parameter
The lattice parameter for BCC iron (Fe) is known to be approximately 2.866 Å.
Substituting this value into the formula, we can calculate the radius:
Radius of atom = (sqrt(3) / 4) * 2.866 Å
8- To determine the temperature at which there will be one vacancy for every 1000 atoms, we can use the equation for the concentration of vacancies:
Nv/N = exp(-Qv/RT)
Where Nv is the number of vacancies, N is the total number of atoms, Qv is the activation energy for vacancy formation (2.0 eV), R is the gas constant (8.314 J/(mol·K)), and T is the temperature in Kelvin.
Given that at 800°C there is one vacancy for every 10^-4 atoms, we can calculate the value for Nv/N and solve for the temperature.
Nv/N = 10^-4/1 = exp(-Qv/(RT1))
Then, we can rearrange the equation and solve for T2:
T2 = -Qv / (R * ln(Nv/N))
Substituting the given values and solving the equation, we can find the temperature.