THermal properties

Question 1

What can represent the bonds between atoms in solids?

Answer 1

springs

Question 2

If the spring obeys Hooke’s law……

Answer 2

Then the energy versus interatomic distance curve is a quadratic (x2)
curve

Question 3

What does the vibrational energy create?

Answer 3

This vibration energy is quantised into something called phonons so to accommodate heat the solid responds by increasing the numbers of phonons.

Question 4

Why does electrical conduction in metals decrease as temperature increases?

Answer 4

Phonons scatter electrons that’s why electrical conduction in metals deceases as temperature increases

Question 5

What is the process of thermal runaway?

Answer 5

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

Question 6

What is hooke’s law

Answer 6

The force on the spring varies as the displacement of the atom (sphere) from its equilibrium position which is (r - ro).

Question 7

What is the Debye-petit model?

Answer 7

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.

Question 8

If the spring is stiff then the f max and θd is…..

Answer 8

large fmax large θd

Question 9

Why does impurity reduce the thermal conductivity?

Answer 9

electrons are scattered by impurities

Question 10

what happens when a material is heated in a non-uniform way?

Answer 10

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

Question 11

How can you avoid thermal stress?

Answer 11

sing materials with low coefficients of expansion

Question 12

How do thermal stresses occur? (2)

Answer 12

restrained thermal expansion/contraction
– temperature gradients that lead to differential
dimensional changes

Question 13

What happens to the equilibrium when you go up in energy sources?

Answer 13

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

Question 14

Why is it negative in the thermal conductivity formula?

Answer 14

Heat flows away from the hottest parts, therefore -ve

Question 15

What is the best thermal conductor?

Answer 15

Diamond is the best thermal conductor due to its strong and stiff bonds

Question 16

How is heat transported in solids?

Answer 16

by both lattice vibrations (phonons) and free electrons

Question 17

Thermal diffusivity in metals.

Answer 17

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

Question 18

Thermal diffusivity in semiconductors

Answer 18

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.

Question 19

What kind of scale is a intrinsic carrier concentration graph?

Answer 19

Intrinsic carrier concentration (logarithmic scale) as a function of temperature for germanium and silicon.