Lecture 3 - Ceramics Flashcards
(34 cards)
What type of bonding is predominantly found in ceramics?
Ionic bonding
Ionic bonding involves the transfer of electrons from metals to non-metals, creating cations and anions.
What is the coordination number in ceramics?
The number of anion nearest neighbors for a cation (and vice versa)
This indicates how many oppositely charged ions surround a given ion in a lattice.
Define polymorphism in the context of ceramics.
When a single material can exist in more than one crystal structure
Different atomic arrangements of the same substance lead to different properties.
What is allotropy?
Polymorphism that occurs in elemental solids
Example: Carbon exists as both graphite and diamond.
What characterizes ionic bonding?
Electrons are transferred, forming cations and anions that create a tight electrostatic lattice
This results in strong, non-directional bonds with high melting points but brittleness.
Explain covalent bonding in ceramics.
Atoms share valence electrons in a directional manner, forming networks
Covalent bonds are very strong, as seen in diamond or SiC.
What is the purpose of the transverse bending test?
To test brittle materials by bending them until they fracture
Unlike tensile tests, bending tests are used because ceramics are too brittle.
Describe brittleness in ceramics.
Ceramics tend to fracture under tensile load without plastic deformation at room temperature
This is a key characteristic of both crystalline and amorphous ceramics.
What is the definition of flexural strength?
The stress at which a ceramic fractures during bending
It is measured using a transverse bending test.
What is viscous flow in amorphous ceramics?
A deformation process where atoms slide past each other under stress
This occurs without dislocations, similar to thick syrup.
What is the glass transition temperature (Tg)?
The temperature at which a glassy ceramic transitions from brittle to rubbery
Below Tg, the material is stiff glass; above Tg, it starts to flow.
What is sintering?
Heating a compacted ceramic powder below its melting point to fuse particles
This process is used to strengthen and densify ceramics.
What is the equation for flexural strength?
Flexural strength = 3FL/2bd
Variables: F = load at fracture, L = distance between support points, b = width, d = height.
How does porosity affect ceramics?
Porosity reduces flexural strength by acting as stress concentrators
This makes it easier for cracks to initiate and propagate.
What are typical crack configurations in ceramics?
Impact or point loading, bending, torsion, internal pressure
These configurations indicate how ceramics may fail under different stresses.
What are slip systems in ceramics?
Specific crystallographic planes and directions where dislocations can move
Motion of dislocations is the main mechanism for plastic deformation.
What is heat capacity (C)?
The amount of heat required to produce a unit rise in temperature for 1 mole of a substance
It indicates the material’s ability to absorb heat.
What does the linear coefficient of thermal expansion (αl) indicate?
The extent to which a material expands upon heating
It is a material property relevant to thermal behavior.
What is electrical conductivity in ceramics?
Often low due to the unavailability of free electrons for conduction
This is a typical characteristic of traditional ceramics.
What characterizes corrosion in ceramics?
A process involving oxidation and reduction reactions in an aqueous environment
Ceramics can be resistant to corrosion but some can dissolve in water.
List the properties of ceramics.
- High melting temperatures
- Low thermal and electrical conductivity
- High flexural strength
- Brittle
- Chemically resistant
These properties make ceramics suitable for various applications.
What are glasses in ceramics?
Non-crystalline, often silicate-based materials containing other oxides
Soda-lime glass is a common example, consisting of SiO2 and other oxides.
What are the properties of glass-ceramics?
Fine-grained, polycrystalline materials made by crystallization of glass
They undergo high-temperature heat treatment to form crystals.
What are clays?
Naturally found, often silicate-based layered materials
Clays are inexpensive and easy to fabricate.