The nature and influencing factors of thermal expansion coefficient

    The phenomenon that the volume or length of an object increases with the increase in temperature is thermal expansion. The coefficient of thermal expansion is a physical quantity used to describe the degree of expansion or contraction of a material when the temperature changes. The linear (body) expansion coefficient refers to the relative increase in the length (volume) of an object when the temperature increases by 1K.

    The essence of thermal expansion is the phenomenon that the average distance between the particles in the lattice structure increases with the increase of temperature. The lattice vibration is generally regarded as the simple harmonic vibration of the particle. In fact, the force between adjacent particles in the lattice vibration is nonlinear, and the force is not simply proportional to the displacement.

    There are many factors that affect the thermal expansion of materials, in addition to temperature, there are alloy composition and composition phase, crystal defects, crystal structure, etc.

    The solute elements and content of the alloy have a significant effect on the thermal expansion of the alloy. For most alloys, if the alloy forms a uniform single-phase solid solution, the thermal expansion coefficient of the alloy is generally between the thermal expansion coefficients of the components, and can be calculated with a simple addition ratio relationship. If transition elements are added to the metal solid solution, the expansion coefficient of the solid solution will have no regularity.

    When metals and alloys undergo phase changes, the amount of expansion and coefficient of thermal expansion also change. For the first-order phase transition, with the change of specific heat capacity, the corresponding thermal expansion coefficient changes discontinuously at the phase transition point and becomes infinite at the transition point. For the second-order phase transition, the expansion coefficient curve has an inflection point at the phase transition point.

    Crystal defect vacancies have a relatively large impact on the thermal expansion coefficient. Vacancies can be generated by ionizing radiation or high-temperature quenching. The generation of vacancies can lead to an increase in the coefficient of thermal expansion.

    Substances with the same crystal structure and composition but different structures have different expansion coefficients. In general, crystals with tight structure have a large expansion coefficient; while similar to amorphous glass, they often have a small expansion coefficient. The tightly structured polycrystalline binary compounds all have a larger expansion coefficient than glass.

    The crystal is anisotropic, and the direction with a higher elastic modulus will have a smaller expansion coefficient. For example, the expansion coefficient of corundum in the direction perpendicular to the C axis is 8.3×10^-6, and in the direction parallel to the C axis, the expansion coefficient is 9.0×10^-6.

    For ferromagnetic alloys, there is an abnormality due to ferromagnetic transformation, and there are expansion peaks and anti-expansion peaks. The reason for the abnormality is that the magnetically induced shrinkage offsets the normal thermal expansion of the alloy. Using this feature, it is possible to obtain Kovar alloy with a constant thermal expansion coefficient within a certain temperature range.

    In precision instruments, the requirements for dimensional changes are very high. Therefore, the size change with temperature is very small. For this reason, people developed low-expansion alloy Invar alloy. The main component of Invar alloy is Fe-Ni alloy. The low expansion effect of Invar alloy comes from the magnetostrictive effect. The volume of the alloy expands significantly when the spontaneous magnetization reaches the saturation state; and when the temperature rises, the spontaneous magnetization of the alloy weakens, the above effect is weakened, and the volume will inevitably shrink; normal thermal expansion increases the volume, and the interaction between the two makes the thermal expansion maintain at A lower level.

    Failure analysis of vacuum electronic components and development of constant expansion alloys In the electronics and telecommunications industries, the use of vacuum electronic components is large. However, in the process of preparation and use, there are often accidents such as loose sealing and vacuum damage. Failure analysis shows that insulating materials such as glass, ceramics, mica and conductive alloy materials are used in the components. The thermal expansion characteristics of the two types of materials do not match, or the trend of temperature changes is different, which will cause the difference in expansion and deformation of the two materials, leading to the occurrence of the above-mentioned accidents. People have prepared fixed expansion alloys to fix the coefficient of thermal expansion within a certain range and match the glass. Constant expansion alloys are divided into two categories: ① alloys that use the Invar effect to achieve a specific thermal expansion coefficient; ② high melting point metals and alloys that use their low expansion coefficient to meet the requirements, such as Ni-Mo alloys.

(Huiputech)