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Questions and Answers
What is polymorphism in crystalline materials?
What is polymorphism in crystalline materials?
- The transformation of a solid from crystalline to amorphous state.
- The ability of a material to exist in multiple crystal structures under different conditions. (correct)
- The process of crystal growth at high temperatures.
- The presence of multiple chemical compositions in a material.
Which of the following forms of silica has a cubic crystal structure?
Which of the following forms of silica has a cubic crystal structure?
- Cristobalite (correct)
- Tridymite
- Quartz
- Fused quartz
Which transformation type involves a breakdown of atomic bonding?
Which transformation type involves a breakdown of atomic bonding?
- Structural transformation
- Displacive transformation
- Reconstructive transformation (correct)
- Lattice transformation
At what temperature does low quartz transform to high quartz?
At what temperature does low quartz transform to high quartz?
What is a key characteristic of displacive transformations?
What is a key characteristic of displacive transformations?
Among the following, which type of silica is in an amorphous state?
Among the following, which type of silica is in an amorphous state?
What describes the cubic crystal system among the silica forms?
What describes the cubic crystal system among the silica forms?
What happens to the volume during displacive transformations of silica?
What happens to the volume during displacive transformations of silica?
What does the atomic packing factor (APF) indicate in a crystal structure?
What does the atomic packing factor (APF) indicate in a crystal structure?
Which type of defect is characterized by vacant atomic sites in a structure?
Which type of defect is characterized by vacant atomic sites in a structure?
In cubic crystal systems, how many atoms are generally contained within one unit cell of a face-centered cubic (FCC) structure?
In cubic crystal systems, how many atoms are generally contained within one unit cell of a face-centered cubic (FCC) structure?
Which defect type involves atoms that are positioned between atomic sites?
Which defect type involves atoms that are positioned between atomic sites?
What is the main goal of controlling defects in materials science and engineering?
What is the main goal of controlling defects in materials science and engineering?
What defines a grain boundary in a polycrystalline solid?
What defines a grain boundary in a polycrystalline solid?
Which of the following best describes line defects in crystal structures?
Which of the following best describes line defects in crystal structures?
How does a lower amount of void space affect the packing fraction of a crystal?
How does a lower amount of void space affect the packing fraction of a crystal?
What is the glass-transition temperature (Tg)?
What is the glass-transition temperature (Tg)?
What defines a space lattice in crystalline solids?
What defines a space lattice in crystalline solids?
How many atoms are there in a simple cubic (SC) unit cell?
How many atoms are there in a simple cubic (SC) unit cell?
Which statement correctly describes the body-centered cubic (BCC) unit cell?
Which statement correctly describes the body-centered cubic (BCC) unit cell?
In a face-centered cubic (FCC) unit cell, where are the atoms located?
In a face-centered cubic (FCC) unit cell, where are the atoms located?
What is the smallest repeating unit in a crystal called?
What is the smallest repeating unit in a crystal called?
Which of the following best differentiates between crystalline and amorphous solids?
Which of the following best differentiates between crystalline and amorphous solids?
Which crystal system is characterized by atoms located at all corners and the center of the cube?
Which crystal system is characterized by atoms located at all corners and the center of the cube?
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Study Notes
Characteristics of Materials
- The glass-transition temperature (Tg) is the critical temperature where materials switch from hard to soft states.
Space Lattice
- The space lattice refers to a three-dimensional network formed by imaginary lines linking atoms in a crystal.
- A crystal consists of a repeating arrangement of atoms or molecules in three dimensions.
- The unit cell is the smallest repeating unit in a crystal structure.
Cubic Systems
Simple Cubic System (SC)
- The SC unit cell appears as a cube with atoms situated at each corner.
- There is only one atom per unit cell, calculated as 8 corners x 1/8 of each atom = 1 atom.
Body-Centered Cubic System (BCC)
- BCC unit cell features atoms at all corners and a single atom in the center.
- It contains 2 atoms per unit cell and is a common structure for metals.
Face-Centered Cubic System (FCC)
- FCC unit cell includes atoms at each corner and on each face of the cube.
- This structure has 4 atoms per unit cell essential for the mechanical behavior of materials.
Polymorphism
- Polymorphic materials exhibit multiple crystal structures under varying physical conditions.
- These forms have identical chemical compositions but distinct physical properties.
- Silica has four allotropic forms: Quartz, Tridymite, Cristobalite, and Fused Quartz, all chemically represented as SiO2.
Transformation of Silica
- Reconstructive Transformation: Involves breaking and reconstructing atomic bonding; requires high thermal energy.
- Quartz to Tridymite at 870ºC
- Tridymite to Cristobalite at 1470ºC
- Cristobalite to Fused Silica at 1710ºC
- Displacive Transformation: Entails atomic displacement without bond breaking; requires less energy.
- α-Quartz to β-Quartz at 575ºC
- α-Tridymite to β-Tridymite at 220ºC
- α-Cristobalite to β-Cristobalite transformation.
Atomic Packing Factor (APF)
- APF indicates the space occupied by atoms in a structure versus void space.
- A higher packing fraction corresponds to fewer voids and is computed as volume of atoms divided by volume of the unit cell.
Crystal Imperfections
- All crystals possess defects; controlling these is essential in materials science.
- Types of defects include:
- Point Defects: Vacancies (empty atomic sites) and interstitials (extra atoms in between lattice sites).
- Line Defects: Linear misalignment of atoms in the crystal lattice.
- Plane Defects: Grain boundaries separate different crystalline orientations within polycrystalline solids.
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