Glossary

ASTM
American Society for Testing and Materials.

Density
The density of a material is defined as its mass per unit volume.

Diffusivity
Thermal diffusivity is the measure of the way heat flows through a material to the other side. It is also often expressed as the rate of change of temperature in a transient heat transfer process. The higher the thermal diffusivity of a material, the higher the rate of temperature propagation.

Guarded Hot Plate
Guarded hot plate is a steady state technique that involves placing a solid sample of fixed dimension between two temperature controlled plates. One plate is heated while the other is cooled and temperatures of the plates are monitored until they are constant. The steady state temperatures, the thickness of the sample and the heat input to the hot plate are used to calculate the thermal conductivity.

Specific Heat Capacity
Specific heat capacity is by definition the quantity of heat required to raise the temperature of a unit of mass of a substance by a unit change in temperature.

Hot Wire
Hot wire is a transient technique that involves inserting an electrically heated wire into a material. This intrusive method is possible for foams and fluids and melted plastics. The heat flows out radially from the wire into the sample and the temperature of the wire is measured. The plot of the wire temperature versus the logarithm of time is used to calculate thermal conductivity.

Laser Flash Diffusivity
Laser flash diffusivity is a transient method that involves applying a short pulse of heat to the front face of a specimen using a laser flash, and measuring the temperature change of the rear face with an infrared (IR) detector. The resulting temperature rise of the other face of the test specimen is monitored as a function of time and used, together with the sample thickness, to determine the thermal diffusivity. This can be combined with density and heat capacity data to calculate thermal conductivity.

Modified Hot Wire
Modified hot wire is a transient heat reflectance technique similar to hot wire. The modification is that the heating element is supported on a backing, which provides a rectangular one dimensional heat flow. This eliminates the intrusive nature and allows for solids to be tested without being melted. The plot of the wire temperature versus the square root of time is used to calculate thermal conductivity, provided that density and heat capacity are known.

Modulated Differential Scanning Calorimetry (DSC)
Modulated or dynamic differential scanning calorimetry (M or DDSC) is a technique that involves conducting two DSC measurements on samples of differing thickness. The difference in apparent heat capacity is the result of the lag in the heat penetrating through the sample during testing. The DSC results and the thicknesses of the two samples are used to calculate thermal conductivity.

Thermal Conductivity
Thermal conductivity is a physical property of a material that characterizes the ability of that substance to transfer heat. The value of thermal conductivity determines the quantity of heat passing per unit of time per unit area at a temperature drop of 1-degree C per unit length. In the limit of infinitesimal thickness and difference in temperature, the fundamental law of heat conductity.

Thermal Resistivity
Thermal resistivity is an index of a materials' resistance to the flow of heat. It is the reciprocal of the thermal conductivity.

Transient Plane Source (TPS)
Transient plane source is a type of modified hot wire. It is a transient reflectance technique for solids and liquids in which the sample surrounds a heating element configured as a planar circle. With the constant current heat input to the sample, a 3 dimensional heat flow results and the temperature of the interface is monitored and plotted against a time function. The results are thermal conductivity and diffusivity – both independent of each other, without requiring sample geometry information, density or heat capacity.

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