How to judge whether the thermal conductivity material is good or not

With the development of industrial production and science and technology, higher requirements are put forward for materials. In addition to the excellent comprehensive properties of the material, some occasions also require the material to have thermal conductivity and excellent electrical insulation properties. Such as electromagnetic shielding, electronic information, thermal measurement technology widely used in the field of power tubes, integrated blocks, heat pipes, etc.; Heat exchangers, heat conduction tubes, solar water heaters, coolers for storage batteries and other thermal insulating materials used in chemical production and waste water treatment; With the rapid development of microelectronics integration technology and assembly technology, the volume of electronic components and logic circuits has shrunk tens of thousands of times, electronic instruments have become thinner and shorter, and the working frequency has increased sharply, the semiconductor thermal environment has changed rapidly in the direction of high temperature, and the heat generated by electronic devices has accumulated and increased rapidly. In order to make electronic components can still work normally with high reliability under the use of ambient temperature, timely heat dissipation ability has become an important factor affecting the reliability of its operation and service life, so it is urgent to develop thermal insulation materials with high reliability and high heat dissipation.
　　The thermal conductivity K is an inherent property parameter of the material itself and is used to describe the thermal conductivity of the material. This characteristic has nothing to do with the size, shape, and thickness of the material itself, but only with the composition of the material itself. Therefore, the thermal conductivity of similar materials is the same, and will not change because of different thicknesses.
　　By combining the two formulas above, we get K=d/R. Since the value of K is constant, it can be seen that the value of thermal resistance R is proportional to the thickness of the material d. In other words, the thicker the material, the greater the thermal resistance.
　　However, if you look carefully at the data of some thermal conductivity materials, you will find that the thermal resistance value R of many thermal conductivity materials is not completely proportional to the thickness d. This is because thermal conductivity materials are mostly not composed of a single component, and there will be nonlinear changes accordingly. As the thickness increases, the thermal resistance value must increase, but it is not necessarily a completely proportional linear relationship, and it may be a steeper curve relationship.

　　According to the formula R=A△T/Q, it is theoretically possible to test and calculate the thermal resistance of a material R. But this formula is only a basic idealized formula, he set the condition is: the contact surface is completely smooth and flat, all the heat through the way of heat conduction through the material and to the other end. In practice this is an impossible condition. Therefore, the thermal resistance value tested and calculated is not entirely the thermal resistance value of the material itself, it should be the thermal resistance value of the material itself + the so-called contact surface thermal resistance value. Because the flatness, smoothness or roughness of the contact surface, and the pressure of the installation fastening are different, different contact surface thermal resistance values will be generated, and different total thermal resistance values will be obtained.
　　Therefore, the international high society recognizes the setting of a standard test method and conditions, which is often seen in the data of ASTM D5470. This test method will indicate how much contact area A, how much heat value Q, and how much pressure is applied to the contact surface when performing the thermal resistance test. Everyone uses the same method to test different materials, and the results are comparable. The thermal resistance R value obtained by the test is not completely the real thermal resistance value. Physical science is like this, many parameters can not be truly quantified, just a "fuzzy" mathematical concept. Through such "fuzzy" data, one can quantify some data for practical applications. The term "fuzzy" here is a mathematical term, and "fuzzy" means an approximation to reality.
　　Similarly, according to the thermal resistance value and thickness, the calculated thermal conductivity K value is not completely the real thermal conductivity value. Fourier's equation is a completely idealized formula. We can use it to understand the principle of heat-conducting materials. However, the practical application and thermal resistance calculation are complex mathematical models, and there will be many correction formulas to perfect all the possible problems.
　　1, the same material, the thermal conductivity is a constant value, the thermal resistance value will change with the thickness.
　　2, the same material, the greater the thickness, can be simply understood as the heat passed through the material to go more distance, the more time is consumed, the worse the performance.
　　3, for thermal conductivity materials, the selection of the appropriate thermal conductivity, thickness is very related to the performance. Choose a material with a high thermal conductivity, but the thickness is large and the performance is not good enough. The ideal choice is: high thermal conductivity, thin thickness, perfect contact pressure to ensure good interface contact.
　　4, what thermal conductivity material to use, theoretically speaking, is a very difficult thing. It's hard to really calculate exactly what kind of material to use with some simple data. It's more about testing and comparison, and experience. The test can achieve the ideal effect of the product requirements, is for the right material.