The thermal properties of Alumina tubes, such as melting point, specific heat capacity, thermal conductivity, thermal expansion coefficient, etc., are of great significance to the preparation of the ceramic material and directly affect its application in engineering.
- Melting point
Compared with metals and polymer materials, high-temperature resistance is one of the excellent characteristics of ceramic materials. The heat resistance of a material is generally judged by factors such as high-temperature strength, oxidation resistance, and ablation resistance, but to become a heat-resistant material, the melting point must be high first. The melting point Tm reflects the strength of the interatomic bonding force that maintains the crystal structure. The stronger the bond, the more stable the thermal vibration of the atoms, the more the crystal structure can be maintained to a higher temperature, and the higher the melting point. The melting point of alumina is 2054℃.
2. Molar heat capacity is the most basic property of the thermal performance of materials. It refers to the amount of heat absorbed (or released) when the temperature of the material increases (or decreases) by 1K. The unit is J/(mol·K). There are constant pressure molar heat capacity Cp and constant volume molar heat capacity Cv. At high temperatures, the Cv of most ceramic materials tends to a certain value, Cv=3R=24.9J/(mol·K).
3. Thermal expansion
the volume of any crystal increases with increasing temperature, and the crystal tends to become more symmetrical. The increase in volume with temperature mainly depends on the increase in the amplitude of the atoms vibrating around an average position. The repulsion between atoms changes faster than the gravitational force as the atomic spacing changes. Therefore, the minimum energy valley is asymmetric. As the lattice energy increases, the amplitude of the non-harmonic vibration between the equilibrium energy positions increases, resulting in a larger atomic spacing, which corresponds to the expansion of the lattice. The thermal expansion coefficient is directly related to the magnitude of the repulsion and gravitation between atoms in the substance and the bond energy between atoms. Materials with strong bonds generally have smaller thermal expansion coefficients, and vice versa. The melting point of a substance is one of the characteristics of bonding strength. It can be seen that materials with high melting points have smaller thermal expansion coefficients. The volume change caused by lattice vibration is closely related to the increase in the material’s energy. Experimental results show that at almost all temperatures, the ratio of thermal expansion coefficient α to heat capacity Cv is close to a constant. The thermal expansion coefficient of alumina varies with purity, ranging from 6.5 to 7.5 10-6K-1.
4. Thermal conductivity
Thermal conductivity refers to the rate heat flows through a material, and its unit is generally expressed in W/mK. The thermal conductivity of alumina ceramics varies with purity, ranging from 20 to 32W/mK.
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