What Is Thermal Conductivity?

What Is Thermal Conductivity?

Whether it is the new energy automobile industry, electronic electrician, or LED industry, the application of thermal conductive materials is relatively common. In thermal management, thermal conductivity is an important parameter reflecting the thermal conductivity of materials, and it is also the most concerned technical indicator for users.

Thermal conductivity is a physical quantity that characterizes the thermal conductivity of a material. Its definition means that under stable heat transfer conditions, a 1m thick material has a temperature difference of 1 degree (K, ℃) on both sides of the material, within 1 second (1s), through the heat transferred by the area of 1 square meter, the unit is watt/meter·degree (W/(m·K), where K can be replaced by °C). ( Fourier’s law : The heat flux density at each point in a homogeneous material body is proportional to the temperature gradient. The direction of heat transfer is from high temperature to low temperature and the direction of temperature gradient is opposite from low temperature to high temperature. )

The thermal conductivity of a material is not only related to the material type of the material, but also closely related to its microstructure and filler content. In scientific experiments and engineering design, the thermal conductivity of the materials used needs to be accurately determined by experimental methods. There are many methods for measuring thermal conductivity, and they have different application fields, measurement ranges, precision, accuracy and sample size requirements. Different methods may have large differences in the measurement results of the same sample. , so choosing the appropriate test method is the first and foremost.

At present, the measurement methods of thermal conductivity are divided into two categories: steady-state method and non-steady-state method, with different test principles. In the thermal conductive silicone industry, the common test method is the steady-state hot plate method (reference standard: ASTM D5470)

Standard Test Method for Heat Transport Properties of Thin Thermal Conductive Solid Electrical Insulating Materials: ASTM D5470

This method uses the so-called steady-state heat flow method. The test principle is to place a sample of a certain thickness between the upper and lower plates, apply a certain heat flow and pressure to the sample, and use a heat flow sensor to measure the heat flow through the sample, Test the thickness of the sample and the temperature gradient between the hot plate and the cold plate, and then obtain the corresponding thermal resistance data under different thicknesses and fit a straight line to obtain the thermal conductivity of the sample.

The advantages of this approach are:

  1. It can test the thermal resistance and thermal conductivity of the product;
  2. It is especially suitable for simulating the use state of the product under actual working conditions.

Weakness is:

  1. There are certain requirements for the thickness of the product;
  2. Contact thermal resistance will affect the test results;
  3. In order to reach a steady state, the test takes a long time.

Thermal conductivity is a parameter of the material itself and has nothing to do with shape or size. At present, there are a large number of thermal conductivity test methods, but no one method can be applied to all products and all occasions. Product characteristics, test standards, test environments, etc. will all affect the results of thermal conductivity. The thermal conductivity of materials cannot be compared using data obtained from different test methods. To get accurate and informative results, it is necessary to select a suitable test method for measurement.

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