THermal properties Flashcards
What can represent the bonds between atoms in solids?
springs
If the spring obeys Hooke’s law……
Then the energy versus interatomic distance curve is a quadratic (x2)
curve
What does the vibrational energy create?
This vibration energy is quantised into something called phonons so to accommodate heat the solid responds by increasing the numbers of phonons.
Why does electrical conduction in metals decrease as temperature increases?
Phonons scatter electrons that’s why electrical conduction in metals deceases as temperature increases
What is the process of thermal runaway?
high data rates means more energy per second (power) –> more electrical current –> more heat ->more phonons poor electrical conduction -> even more heat. This process is known as thermal runaway and leads to microprocessor failure
What is hooke’s law
The force on the spring varies as the displacement of the atom (sphere) from its equilibrium position which is (r - ro).
What is the Debye-petit model?
The important features are the Debye temperature, θd – also the R –the gas constant – the Debye temperature is the temperature at which the highest energy phonons with frequency fmax are present in the material.
If the spring is stiff then the f max and θd is…..
large fmax large θd
Why does impurity reduce the thermal conductivity?
electrons are scattered by impurities
what happens when a material is heated in a non-uniform way?
If material is heated in a non-uniform way then different parts of the material will expand by different amounts and cause the material to be stressed – if the stress exceeds the yield stress then the material’s shape is permanently changed
How can you avoid thermal stress?
sing materials with low coefficients of expansion
How do thermal stresses occur? (2)
restrained thermal expansion/contraction
– temperature gradients that lead to differential
dimensional changes
What happens to the equilibrium when you go up in energy sources?
Energy in bond varies,
as you go up in energy sources as they absorb more energy and the equilibrium position moves a little bit creating the asymmetry
Why is it negative in the thermal conductivity formula?
Heat flows away from the hottest parts, therefore -ve
What is the best thermal conductor?
Diamond is the best thermal conductor due to its strong and stiff bonds
How is heat transported in solids?
by both lattice vibrations (phonons) and free electrons
Thermal diffusivity in metals.
Copper has the largest value and Brass, an alloy of copper, has the lowest generally alloys have low values of conductivity. Aluminum even though it has a lower value of conductivity has almost as large a thermal diffusivity as copper because of its low density
Thermal diffusivity in semiconductors
Silicon has the largest value of the commonly used semiconductors and so it easier to extract heat from silicon chips compared to GaAs chips. Diamond has the largest value and there’s a lot of research currently on diamond electronics because it’s very easy to extract heat from diamond chips –also in some applications diamond is used as a heat spreader.
What kind of scale is a intrinsic carrier concentration graph?
Intrinsic carrier concentration (logarithmic scale) as a function of temperature for germanium and silicon.
