What is thermal conductivity?The written term refers to the amount of heat(w/(m.K))passing through a unit of time when there is a difference of 1℃in unit area,thickness,and temperature,which is an important parameter for the conduction of heat energy by a substance.In plain English,it can be said:"The measurement standard used to transfer heat out of materials
There are many ways to test thermal conductivity,and according to the heat flow method,it can generally be divided into two types:steady-state and non-stationary;Others have divided it into three kinds:steady state method,Metastability method and unsteady state method.
Steady state testing method:
The sample to be tested is located in a temperature field that does not change with time.Based on the measured heat flux rate over the sample area,the temperature gradient in the direction of the sample heat flux,and the geometric size of the sample,the thermal conductivity is directly measured according to Fourier's law.
(Q is the heating power,which is the heat transferred to the outside when you heat the inner wall at a temperature of 140 degrees,K is the thermal conductivity,h is the thickness,and S is the area.In this case,you can only assume that the heat is completely conducted,know the heating power,calculate△t,and then calculate the temperature at the other end.)
When using the steady-state method to test the thermal conductivity of materials,there are two fundamental issues that need to be addressed:1.determining the heat flow rate of the sample to be tested;2.To obtain a heat flow image that is consistent with the assumptions made during the establishment of the physical model.There are usually several ways to solve the first problem:
①Determine the heat flow rate of the sample to be tested;
②Determine the amount of heat used to heat the sample;
③The standard sample,which causes heat to pass equally through the sample to be tested and the known thermal conductivity value in series with it,actually serves as a calorimeter and is commonly used.
There are usually several ways to solve the second problem:
①Design a device to constrain the heat flow in a specified direction;
②Design samples of various shapes and specifications to obtain heat flow images that are easy to describe mathematically;
③,the corresponding Formula is derived for the heat flow image in the prepared sample;
Therefore,using the different approaches mentioned above,steady-state methods are often divided into two categories:radial heat flux method and longitudinal heat flux method.In the radial heat flux method,a specimen that can completely surround the heat source is used.If the heat source is properly designed,the heat flow will flow out from the center in a symmetrical image.This system often consists of a cylindrical specimen with a large diameter ratio,surrounded by a heat source;Alternatively,there may be a hollow sphere or return ellipsoid sample that surrounds a heat source of the same shape.The characteristics of this method facilitate mathematical description of heat flow images.No matter which method is used in the steady state test,all the heat generated by the main heater must pass through the sample,the heat flow line should be perpendicular to the cross section of the sample,and the whole system should reach a stable Thermal equilibrium state.After the sample reaches thermal stability,measure the heat and temperature gradient flowing through the sample,and then determine the thermal conductivity of the sample material.
At present,most of the uniform polymer and inorganic materials used for electrical insulation testing in China use ASTM D5470-95 and ASTM D5470-2006 standards.This standard applies to the thermal resistance test of homogeneous materials with a thickness between 0.02 and 10mm,and calculates the thermal conductivity data.
Source:Standard Test Method for Thermal Transmission
Non steady state testing method:The temperature of the sample varies with time.During testing,it is common to cause a sudden or periodic change in the temperature of a certain part of the sample,while the other part of the sample measures the rate of temperature change over time,and then directly tests the thermal diffusion coefficient of the sample.The thermal conductivity value is obtained through the following calculation formula.
K=a*cp*p
(a is the thermal diffusion coefficient;cp is the Specific heat capacity;p is the density)
The thermal conductivity of materials is related to material structure,temperature,and humidity.There are significant differences in the experimental results obtained by different experimental testing methods and instruments,so it is necessary to indicate which instrument is used and the measurement method when testing.The principles and methods of testing vary depending on the type of material,so having multiple corresponding measurement methods for different materials is a key point for us to collect and break through.
Non steady state methods mainly include periodic heat flux method(including longitudinal heat flux method and radial heat flux method)and transient heat flux method.Transient heat flow method includes longitudinal heat flow method,flash method(laser pulse method),radial heat flow method,linear heat source method and probe method,movable heat source method and Comparative law.The non steady state heat flux method has two significant advantages:firstly,the testing cycle is short,and it usually takes only a few minutes or even seconds to measure an experimental result.Secondly,many testing methods in non steady state testing methods often have simultaneous thermal diffusion coefficient,thermal conductivity,and specific heat data,which cannot be achieved by steady state testing methods.
The TPS method,which has been popular in recent years,is a type of non steady state testing method.TPS technology uses a thin disc shaped temperature dependent resistance as the sample probe,which is formed by treating conductive nickel metal to form a continuous double helix structure thin sheet.The outer layer is a double layer Kapton protective layer,with a thickness of only 0.025mm.Its probe has a certain mechanical strength while maintaining electrical insulation between the probe and the sample.
There is another point that needs to be clarified regarding the testing of thermal conductivity of polymer thermal conductive materials: currently, there are many methods and equipment for testing thermal conductivity, and the reported thermal conductivity data must indicate the instrument and model used, as well as the testing method used. Because the data tested by different methods cannot be compared, and even the differences are very large, it is a headache problem. For example, in the thermal conductivity copper clad plate industry that I often venture into, thermal conductivity is a very important indicator for this type of product in the industry. The thermal conductivity announced by various manufacturers is very chaotic, and the copper clad plate industry has not yet developed a standard for testing thermal conductivity and designated testing instruments that meet this standard. Here, we welcome professionals in the copper clad panel industry to contact me for corresponding discussions.
