Technology

Understanding Thermal Conductivity Methods

Thermal analysis and the resulting data can provide invaluable information to researchers and manufacturers in many sectors in the development advancement of new materials. However, before the introduction of C-Therm's patented approach, traditional technologies for such analysis have been expensive and/or destructive, as well as very time consuming in both sample preparation and testing.

This section will briefly explain and compare several techniques for measuring thermal conductivity, including the modified transient plane source technology developed by C-Therm. 

For an at-a-glance comparison of the various methods, click here.

Video copyright of TAL (Thermal Analysis Labs).

Guarded Hot Plate / Guarded Heat Flow

(ASTM E1530, ASTM C-177, ASTM C1044, ASTM C518, ISO 8302, ISO 8301)

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

Transient Line Source 

(ASTM D5334, ASTM D5930, IEEE 442-1981)

This transient technique involves placing an electrically heated wire into a material. This intrusive method is limited to testing foams, fluids and melted plastics. The heat flows out radially from the wire into the sample and the temperature of the wire is measured. By plotting the temperature of the wire versus the logarithm of time, thermal conductivity can be calculated. 

 

 

 

Modified Transient Plane Source

Conforms to ASTM D7984

(ASTM WK50791, ASTM WK43689, ASTM WK43374)

Developed by C-Therm (formerly Mathis Instruments), this approach is a transient technique that uses heat reflectance, similar to Hot Wire testing. The modification is that the heating element is supported on a backing, thus allowing a one-directional heat flow. This allows the testing to be non-intrusive and permits the testing of solids without the need to be melted. Therefore, the temperature of the heating element versus the time function is used to calculate the thermal conductivity and thermal effusivity.

Laser Flash Diffusivity

(ASTM E1461, DIN EN 821)

This is a transient method in which a short pulse of heat, supplied by a laser flash, is applied to the front face of a sample. On the rear face of the sample, an infrared (IR) detector is used to measure the temperature change. This temperature data is monitored as a function of time and used to determine thermal diffusivity. If the density and heat capacity of the sample are also known, the thermal conductivity can be calculated.

 

Transient Plane Source

The older Transient Plane Source (TPS) method utilizes a hot disc sensor and a patented mathematical model (Gustafsson 1991) combined with eletronics to derive the thermal conductivity. This technique requires two identical samples which sandwich the sensor, compressing heavily the sample in the process. The user is responsible for developing the necessary timing and power parameters and selecting the appropriate data so as to build a linear regression in satisfying the model's requirements.