Untitled Flashcards Set

Thermal Properties

Heat Capacity

Heat capacity is the amount of heat energy required to raise the temperature of a material by a specific amount, indicating how much energy a material can absorb before its temperature increases.

• Heat capacity is influenced by the energy that atoms or molecules can store in vibrations or motion.

• In solids, this involves atomic vibrations within a lattice.

• Heavier atoms or complex molecular structures generally have higher heat capacities due to increased energy absorption capability.

• In metals, free electrons contribute to heat capacity, while in insulators, it primarily comes from atomic vibrations.

Thermal Expansion

Thermal expansion refers to the increase in size or volume of a material as its temperature rises. It is crucial for materials exposed to temperature changes, where unaccounted expansion and contraction can cause stress and failure.

• Thermal expansion occurs as atoms vibrate more vigorously with temperature increases, pushing them farther apart.

• Weaker atomic bonds (e.g., in polymers) lead to higher thermal expansion, allowing greater atomic movement when heated.

• Strongly bonded materials (e.g., ceramics) exhibit lower thermal expansion due to tightly bound atoms.

Thermal Conductivity

Thermal conductivity is the ability of a material to transfer heat through its structure. High thermal conductivity allows heat to move easily, while low conductivity means the material acts as an insulator.

• Thermal conductivity is driven by energy transfer between atoms and, in metals, also by free electrons.

• Metals have high thermal conductivity due to efficient energy movement from free electrons.

• In non-metals, heat transfer occurs through atomic lattice vibrations (phonons).

• Ordered, tightly packed atomic structures (e.g., metals, crystalline ceramics) result in higher thermal conductivity, while amorphous materials with disordered structures exhibit lower conductivity.

Thermal Shock

Thermal shock refers to the stress and potential cracking or failure of a material undergoing rapid temperature changes. Poor thermal shock resistance leads to cracking under sudden fluctuations.

• Resistance to thermal shock depends on a material’s ability to withstand rapid atomic vibration changes without accumulating stress.

• Materials with low thermal expansion and high thermal conductivity generally resist thermal shock well by distributing heat quickly.

• Ceramics, which have strong but brittle atomic bonds, are prone to thermal shock due to their inability to dissipate heat and low strain tolerance.

Insulation

Insulation describes a material’s ability to resist the flow of heat or electricity. Thermal insulators resist heat transfer, while electrical insulators prevent the flow of electrical current.

• Heat or electrical insulation relies on the lack of free-moving particles (electrons or phonons) that carry energy.

• Thermal insulators have atomic structures that limit heat movement due to voids, disordered structures, or weak bonding.

• Electrical insulators (e.g., ceramics, polymers) have tightly bound electrons that restrict movement, preventing electrical current flow.

Electrical Properties

Electrical Conductivity

Ability to conduct electric current; most ceramics are insulators, but some can be conductive or semiconductive.

• Low conductivity results from ionic/covalent bonds with tightly bound electrons and fixed ions in the lattice, restricting electron movement.

• Conductive ceramics include doped zirconia, titanium nitride, and indium tin oxide; defects/doping create pathways for electron/ion movement.

Ionic Conductivity

Some ceramics conduct electricity via ions (e.g., yttria-stabilized zirconia in fuel cells allows oxygen ion movement).

Piezoelectricity

Ability to generate electric charge under mechanical stress; deformation occurs under an electric field.

• Found in non-centrosymmetric crystals like quartz and PZT; stress shifts ion positions, creating a net dipole moment and generating charge.

• Applications: Used in sensors, actuators, and transducers (e.g., accelerometers, ultrasonic generators).

• Structure Dependency: Requires non-centrosymmetric structures for electric polarization; centrosymmetric structures do not exhibit piezoelectricity.

Dielectric Properties

Ability to store electrical energy in an electric field, characterized by dielectric constant and dielectric loss.

• Occurs when the electric field overcomes bond strength, freeing electrons from the lattice, leading to conductivity and potential damage.

• Non-conducting ceramics form dipoles under electric fields, storing energy; polarization depends on structure and atom/molecule polarizability.

• High Dielectric Constant Ceramics: Materials like barium titanate (BaTiO₃) store energy well due to significant ion displacement in the lattice, making them ideal for capacitors.

• Ferroelectric Ceramics: Exhibit reversible spontaneous polarization under external electric fields, useful for high dielectric constants and energy storage (e.g., PZT).

Dielectric Loss: Energy lost as heat due to dipole or defect movement; low loss is desirable in high-frequency devices (e.g., microwave, radio).

Dielectric Breakdown: The failure point when a material becomes conductive under high electric fields; free electrons move, causing breakdown.

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Thermal Properties

Heat Capacity

• Amount of heat energy required to raise the temperature of a material by a specific amount.

• Influenced by energy storage in vibrations or motion of atoms/molecules.

• Higher heat capacities in heavier atoms or complex molecules.

• Metals: heat capacity contributed by free electrons; insulators mainly from atomic vibrations.

Thermal Expansion

• Increase in size or volume of a material with temperature rise.

• Atoms vibrate more and push farther apart when heated.

• Weaker atomic bonds (e.g., in polymers) lead to higher thermal expansion.

• Strongly bonded materials (e.g., ceramics) exhibit lower thermal expansion.

Thermal Conductivity

• Ability to transfer heat through a material.

• High conductivity = easy heat transfer; low conductivity = insulator.

• Metals: high thermal conductivity due to free electrons; non-metals: heat transfer through lattice vibrations.

• Ordered atomic structures yield higher conductivity; amorphous materials have lower conductivity.

Thermal Shock

• Stress and potential failure from rapid temperature changes.

• Resistance depends on the ability to handle rapid atomic vibration changes.

• Low thermal expansion and high conductivity materials resist thermal shock well.

Insulation

• Ability to resist heat or electricity flow.

• Thermal insulators prevent heat transfer; electrical insulators prevent current flow.

• Effective insulation relies on lack of free-moving particles.

Electrical Properties

Electrical Conductivity

• Ability to conduct electric current; many ceramics are insulators.

• Low conductivity from ionic/covalent bonds with fixed ions and tightly bound electrons.

• Some ceramics like doped zirconia can conduct due to defects/doping.

Ionic Conductivity

• Some ceramics conduct electricity via ion movement.

Piezoelectricity

• Generates electric charge under mechanical stress; requires non-centrosymmetric structure.

• Used in sensors and actuators.

Dielectric Properties

• Ability to store electrical energy, characterized by dielectric constant and loss.

• Non-conducting ceramics can store energy via dipoles under electric fields.

• High dielectric constant ceramics like barium titanate are ideal for capacitors.

Dielectric Breakdown

• Failure point under high electric fields; leads to conductivity and breakdown.

Thermal Properties

Heat Capacity

• Amount of heat energy required to raise the temperature of a material by a specific amount.

• Influenced by energy storage in vibrations or motion of atoms/molecules.

• Higher heat capacities in heavier atoms or complex molecules.

• Metals: heat capacity contributed by free electrons; insulators mainly from atomic vibrations.

Thermal Expansion

• Increase in size or volume of a material with temperature rise.

• Atoms vibrate more and push farther apart when heated.

• Weaker atomic bonds (e.g., in polymers) lead to higher thermal expansion.

• Strongly bonded materials (e.g., ceramics) exhibit lower thermal expansion.

Thermal Conductivity

• Ability to transfer heat through a material.

• High conductivity = easy heat transfer; low conductivity = insulator.

• Metals: high thermal conductivity due to free electrons; non-metals: heat transfer through lattice vibrations.

• Ordered atomic structures yield higher conductivity; amorphous materials have lower conductivity.

Thermal Shock

• Stress and potential failure from rapid temperature changes.

• Resistance depends on the ability to handle rapid atomic vibration changes.

• Low thermal expansion and high conductivity materials resist thermal shock well.

Insulation

• Ability to resist heat or electricity flow.

• Thermal insulators prevent heat transfer; electrical insulators prevent current flow.

• Effective insulation relies on lack of free-moving particles.

Electrical Properties

Electrical Conductivity

• Ability to conduct electric current; many ceramics are insulators.

• Low conductivity from ionic/covalent bonds with fixed ions and tightly bound electrons.

• Some ceramics like doped zirconia can conduct due to defects/doping.

Ionic Conductivity

• Some ceramics conduct electricity via ion movement.

Piezoelectricity

• Generates electric charge under mechanical stress; requires non-centrosymmetric structure.

• Used in sensors and actuators.

Dielectric Properties

• Ability to store electrical energy, characterized by dielectric constant and loss.

• Non-conducting ceramics can store energy via dipoles under electric fields.

• High dielectric constant ceramics like barium titanate are ideal for capacitors.

Dielectric Breakdown

• Failure point under high electric fields; leads to conductivity and breakdown.