The coefficient of thermal expansion is one of the main physical properties of the material, and it is an important indicator of the thermal stability of the material.
The phenomenon that the volume or length of an object increases with the increase in temperature is called thermal expansion.
So what is the coefficient of thermal expansion?
The coefficient of thermal expansion refers to the coefficient of regularity that the geometric properties of a substance change with the change of temperature under the effect of thermal expansion and contraction.
For materials used in the electronics industry, mechanical assembly, aerospace, new material development, construction engineering and other fields, it is necessary to fully consider the index of thermal expansion. Therefore, the thermal expansion coefficient of the material is measured during the production, processing and use of the material. It is of very important significance. Let's use examples to understand the influence of thermal expansion coefficient on the application of materials.
1. Can improve the thermal stability of the product
Choosing a material with a lower coefficient of linear expansion can improve the thermal stability of the product and improve the safety of the product.
2. Can improve the strength of the product
If the layer is formed by superimposing and connecting two materials, when the temperature changes, the expansion values of the two materials are different. Stress and the same magnitude of tensile stress is generated in another material. Appropriate use of this characteristic can increase the strength of the product.
3. It is an important reference index for welding or welding
When two different materials are welded or welded to each other, the two materials are required to have similar expansion coefficients. If the expansion coefficients of the selected materials are quite different, due to the different expansion speeds during welding, stress will be generated at the weld, which will reduce the mechanical strength and air tightness of the product, and in severe cases will cause the weld to fall off, burst, leak, or leak oil.
4. Provide a basis for us to select suitable materials
For the selection of materials for large-scale processing machinery and precision instruments and equipment, it is necessary to consider the selection of materials with a small expansion coefficient.
Therefore, it is of great significance to measure the thermal expansion coefficient of a material.
Material thermal expansion coefficient expression
The thermal expansion coefficient of the material has the linear expansion coefficient α, the surface expansion coefficient β and the volume expansion coefficient γ. In practical applications, people pay more attention to the linear expansion coefficient α and the volume expansion coefficient γ:
①Linear expansion coefficient
Indicates the degree of expansion or contraction of the material, which refers to the ratio of the elongation per unit length of the material to its original length at room temperature for every 1°C increase in temperature.
Strictly speaking, in a larger temperature range, the coefficient of linear expansion is usually not constant, and its value is closely related to temperature. Therefore, in practical applications, the average linear expansion coefficient is measured. The average linear expansion coefficient refers to the average elongation per unit length of the material in a certain temperature range for every 1°C increase in temperature.
Expressed as: α=ΔL/(L*ΔT)
②Coefficient of body expansion
When the temperature of an object changes by 1°C, the ratio of its volume change to its volume at room temperature. Expressed as: γ=ΔV/(V*ΔT)
Material thermal expansion coefficient detection
The methods for measuring thermal expansion coefficient of materials include dial gauge method, optical mechanical method, electromagnetic induction thermomechanical method, TMA static thermomechanical analysis method, etc. Among them, TMA static thermomechanical analysis method is currently the most accurate test method. How to test with TMA? Let's take a look together below.
Anytester (Hefei) Co., Ltd. received a sample of a metal material from the customer. As shown in the reference diagram below, We test the the sample with HY8120 Linear Thermal expansion Tester and HY8130 Coefficient of Linear Thermal Expansion Tester.
(The picture is for reference only, source Veer)
est standard: ASTM E813-19 Test method for measuring linear thermal expansion of solid materials by thermomechanical analysis
Detection conditions: under the environment of N₂, at a rate of 5°C/min from -100°C to 200°C
Get the test curve diagram of the material:
Finally, the linear thermal expansion coefficient of the submitted sample is shown in the following table:
Reference standard for linear thermal expansion coefficient of common materials
ISO 11359-1:2014 Plastics Thermomechanical Analysis (TMA) Part 1: General Rules
ISO 11359-2: 1999 Plastics-Thermomechanical Analysis (TMA) Part 2: Test Method for Determination of Linear Thermal Expansion Coefficient and Glass Transition Temperature
ASTM E831-19 Test method for measuring linear thermal expansion of solid materials by thermomechanical analysis
GB/T 4339-2008 Determination of characteristic parameters of thermal expansion of metallic materials
IPC-TM-650 2.4.24C TMA method to test glass transition temperature and Z-axis thermal expansion, etc.
Different standards have different requirements for samples and the heating rate of the test. Therefore, the test needs to be carried out in accordance with the specific requirements of different standards when testing.
Influencing factors of thermal expansion coefficient
The measurement process of the coefficient of linear expansion is affected by many factors. The difference in the processing method and shape and size of the sample is the main reason for the large deviation of the coefficient of thermal expansion. In addition, during the measurement process, the influence of environmental factors, including room temperature, vibration, noise, radiation, etc., will change the measurement results.
Therefore, only by comprehensively considering the influencing factors of the thermal expansion coefficient can the accuracy of the measurement of the linear expansion coefficient be effectively improved.