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# Glossary of Terms

### Thermal Conductivity

Thermal conductivity (often denoted by k, λ, or κ) refers to the intrinsic ability of a material to transfer or conduct heat.

Thermal conductivity is quantified using the International Systems of Unit (SI unit) of W/m•K (Watts per meter per degree Kelvin), and is the reciprocal of thermal resistivity, which measures an objects ability to resist heat transfer.

The rate at which heat is transferred by conduction through a unit cross-section area of a material when a temperature gradient exits perpendicular to the area.

### Thermal Diffusivity

Thermal diffusivity (with a unit mm2/s) is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end.

### Thermal Effusivity

Thermal effusivity of a material is defined as the square root of the product of the material’s thermal conductivity and its volumetric heat capacity.

e = √k∙ρ∙Cρ

A material’s thermal effusivity is a measure of its ability to exchange thermal energy with its surroundings. Although, the quantity can be expressed in bulk property terms, it is not measured in bulk property terms. If material is not a solid, then other heat transfer mechanisms exist, modifying the “k” expressed in thermal effusivity to thermal conductance.

See Thermal Effusance

### Thermal Effusance

Thermal Effusance of a material is defined as the square root of the product of the material’s thermal condustance and its volumetric heat capacity.

e = √k∙ρ∙Cρ

A material’s thermal effusance is a measure of its ability to exchange thermal energy with its surroundings, when the heat transfer mechanism is not thermal conductivity, typically found in measurement of a non-solid.

See Thermal Effusivity

### Thermal Resistance

The measurement of a material temperature difference by which it resists heat flow.

### Guarded Heat Flow Meter (GHFM)

Steady-state device used to measure thermal resistance and thermal conductivity of smaller, more conductive materials using guarding to reduce lateral heat loss.

### Steady-State

Properties do not change with time. The system has finished evolving, now the properties when measured do not change.

### Transient

Short time, changing with time.

### Contact Transient Method (CTM)

Class of Contact type transient thermal conductivity methods.

### Particle Thermal Shunting

Composites, where the filler has a significant difference in thermal conductivity to the matrix. The transient heat signal can shunt through or around the particle or filler (via matrix), resulting in an overestimation of the materials’ thermal conductivity. This can be a significant source of error, when comparing results to a steady-state measurement, where the system is completely evolved.