Guarded Heat Flow Meter Heat Flow Meter Modified Transient Plane Source Transient Line Source Transient Hot Wire Laser Flash Apparatus Transient Plane Source Thermal Effusivity Thermal Diffusivity Thermal Conductivity Thermophysical Property

What are thermophysical properties?

Thermophysical properties are, in a broad sense, any property that depends on temperature. Thermophysical properties describe how materials change, or act, when the temperature of the material is altered, so long as the chemical identity of the material remains the same. In industry, thermophysical properties are generally restricted to those characteristics related directly to heat absorbance and conductance.

Thermal Conductivity, Diffusivity and Effusivity

Thermal conductivity, thermal diffusivity, and thermal effusivity are of particular relevance to many industries, for example the electronics industry. Respectively, these properties measure the ability of a material to: take heat from a point of high temperature to one of low temperature, spread heat throughout itself, and exchange heat with outside systems. Thermal resistivity is the inverse of thermal conductivity, and measures the ability of a material to resist heat flow. Like the name suggests, the exact values of thermophysical properties depend on temperature, and, unless normalized, volume and mass.

Specific heat capacity

The most basic thermophysical property is heat capacity; the amount of heat necessary to change the temperature of an object, or system, by one degree Celsius. Heat capacity is useful for applications in chemistry, calorimetry, and many other sciences. Specific heat capacity also changes with mass (or volume), and therefore, is used to readily compare materials.

Thermal expansion coefficients (volumetric and linear), coefficients of thermal stress, and shock are other thermophysical properties of interest. These determine how the configuration of an object will change in relation to temperature.

Factors affecting thermophysical properties

All thermophysical properties are influenced by the microscopic structure of the material. For example, the thermal expansion coefficient depends on the well of potential energy that the individual molecules of a solid sit in. If this well is asymmetric, then the material will expand or contract when temperature is increased. Thermal conductivity is determined by the microscopic movement of electrons and the vibration of the molecules. However, it is not practical to try to look at these microscopic changes to determine the thermophysical properties of a material.

Thermal Conductivity vs Temperature
The graph above shows how temperature influences thermal conductivity, using NIST 1450d (the standard reference material) Source

Mathematical determination of thermal properties

Thermal conductivity, diffusivity, and effusivity are all closely linked, as in the following the equation.

\[\text{effusivity}=\frac{\text{conductivity}}{ \sqrt{\text{diffusivity}} }\]

This makes intuitive sense if one considers that the properties are all related to how well a material absorbs and distributes heat. Using volumetric heat capacity and thermal conductivity, thermal diffusivity and effusivity can be easily calculated. In fact, measuring any two of these properties will give way to the other one, after some algebra and calculations are applied.

Measurement of thermal properties

Thermophysical properties can be measured with specialized steady state or transient equipment. A heat flow meter (HFM) is an example of a steady-state method, whereas the laser flash apparatus (LFA) and transient plane source techniques are transient methods. It is often the case that these methods measure more than one property. For example, the laser flash analysis and the transient plane source method each measure thermal conductivity as well as thermal diffusivity.

The measurement of thermophysical properties with transient and steady state methods may follow national or international standards, as well as proven testing methods not yet standardized. A standard is a protocol that is developed to ensure commercial materials, services, and systems are produced consistently and safely.  Without these globally recognized standards, international trade would not be possible.

laser flash apparatus and transient plane source
Used to measure thermal diffusivity, thermal conductivity, and specific heat, to the left is a laser flash apparatus method (source) and to the right is a transient plane source method (source).

astm-iso-standards-logosInternationally recognized standards

The American Society for Testing and Materials (ASTM) and the International Organization for Standardization (ISO) are two of the most commonly recognized organizations that produce international standards. Together, they have developed more than 30,000 standards for a variety of applications. IEEE standards are also internationally recognized, while DIN standards are acknowledged on a national scale, but are being replaced by European (EN) and international standards (ISO). The ASTM subcommittee E37.05 is a collection of the ASTM standards pertaining to the proper measurement of thermophysical properties using various methods. Similarly, the ISO technical committee ISO-TC61-SC5 is a collection of ISO standards which describes the requirements for measuring the thermal conductivity and thermal diffusivity of polymeric materials. Visit the Standards Category in the Information section.

Conferences and trade shows

Internationally and nationally recognized thermophysical property trade shows and conferences are useful resources for those interested in thermophysics. The most well-known conferences and trade shows include the International Thermal Conductivity Conference (ITCC), and the North American Thermal Analysis Society (NATAS) conference, which are both held within North America. The Central and Eastern Europe Conference on Thermal Analysis and Calorim (CEEC TAC), the European Conference on Thermophysical Properties (ECTP), and Thermophysics meetings take place approximately every two years in various locations throughout Europe.

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