Silicon nitride ceramics (Si3N4) have excellent flexural strength, thermal shock resistance, acid and alkali corrosion resistance and thermal conductivity, and are key materials in aerospace, medical equipment, electric vehicles and other fields. Studies have shown that silicon nitride ceramics have high theoretical thermal conductivity. Silicon nitride is a strong Silicon Nitride Ceramic Tube covalent bond compound, and its thermal conductivity is dominated by lattice thermal vibration. The key to affecting the thermal conductivity of ceramics is the content of the second phase and the crystallinity. Lattice defects, especially oxygen defects in the lattice, have a great influence on the thermal conductivity of silicon nitride ceramics.
Oxidation Behavior of Porous and Powdered Silicon Nitride
Dynamic oxidizing atmosphere, porous and powdery samples can make silicon nitride more severely oxidized.
There are two forms of oxygen in silicon nitride powder, one is to form a silicon dioxide oxide layer on the surface, and the other is to enter the silicon nitride lattice to form oxygen defects. During the powder preparation process, the oxygen adsorbed inside the lattice and on the surface of the powder particles is about 1 wt%. At high temperature, oxygen will dissolve in the crystal lattice and replace nitrogen atoms to generate silicon vacancies, forming scattering centers in the process of phonon propagation, affecting the thermal conductivity of silicon nitride. The lower the oxygen content of the powder, the better the comprehensive performance of the prepared ceramics.
Wang Yuelong et al. selected silicon nitride powder with an initial oxygen Silicon Nitride Parts content of 1.21wt%, and carried out oxidation at different temperatures at 573K-1273K in flowing air.
Variation of oxygen content in silicon nitride powder with temperature
The results show that the silicon nitride powder has good oxidation resistance, the oxygen content of the powder below 1073K hardly increases, the oxygen content increases slowly between 1073K-1273K, and the oxygen content increases sharply at 1273K. After holding at 1273K for 5h and 10h, the oxygen content of silicon nitride powder increased to 2.01wt% and 3.26wt%, respectively, and the thickness of the surface oxide layer increased from 0.45nm to 1.05nm and 2.31nm. And through theoretical calculation and XPS detection, the lattice oxygen content of silicon nitride powder is about 0.5wt%.
He Fengmei's research on porous Si3N4 found that the oxidation reaction of porous Si3N4 is very weak under normal pressure and static air atmosphere below 800 °C, and obvious oxidation reaction can be seen above 800 °C. It preferentially occurs at the Advanced Ceramic Parts surface and the outer pore walls, and then occurs at the inner pores of the sample. The oxidation reaction is controlled by the chemical kinetics at the interface. In addition, at the same temperature, the dynamic oxidizing atmosphere will accelerate the oxidation of Si3N4, especially for porous and Powder samples.
Oxidative mechanism
Similar to the silicon carbide material, the oxidation mechanism of silicon nitride is divided into active oxidation and passive oxidation with the difference of oxygen partial pressure and temperature. Active oxidation refers to the reaction of silicon nitride and oxygen to generate silicon monoxide and nitrogen. The passive oxidation mechanism is the basis for the Ceramic Disc transition temperature analysis, so it is necessary to have a clear understanding of the passive oxidation mechanism of silicon nitride. The reaction formula is as follows:
The reaction of silicon nitride under the active oxidation mechanism is mainly (1), and the reaction under the passive oxidation mechanism is mainly (2). Some researchers have found that there may be reactions in the passive oxidation mechanism (3 ) occurs. In addition, the reaction formula (4) may occur at the interface of SiO2 and Si3N4.
Reaction mechanism under passive oxidation Machinable Ceramic mechanism
Chen Siyuan et al. studied the proportion of the reaction (3) in the passive oxidation mechanism at a given temperature and pressure through thermodynamic calculations, and found that the ratio of NO to N2 was very small through experiments, which can be considered as the passive oxidation mechanism of silicon nitride. The reaction is only reaction (2) formula. The increase of temperature and the increase of oxygen partial pressure at the interface will increase the pressure of NO, that is, the possibility of reaction (3) will increase.
Reaction mechanism for switching between active and passive oxidative mechanisms
Under high temperature and low oxygen partial pressure environment, silicon nitride changes from passive oxidation mechanism to active oxidation mechanism, forming SiO and N2, the oxide film is destroyed, the anti-oxidation mechanism fails, and the material begins to ablate. The oxidation resistance of silicon nitride fails after ablation, and the wave transmission properties of the material are seriously affected. Therefore, the region where the oxidation mechanism of silicon nitride is transformed is very important to study its anti-oxidation properties and wave-transmitting properties.
At the same temperature, when the concentration of oxygen becomes smaller, the oxidation mechanism of silicon nitride changes to active oxidation. When the oxygen partial pressure is constant and the surface temperature increases, the oxidation mechanism changes from Macor Ceramics passive oxidation to active oxidation.
Chen Siyuan et al. obtained the transition temperature curve of silicon nitride under different oxygen partial pressures through experiments, and the curve divided the oxidation area into passive oxidation area and active oxidation area.
Transition temperature of silicon nitride under different oxygen partial pressures
Epilogue
Silicon nitride ceramics have high theoretical thermal conductivity, and the second phase content and lattice defects, especially the oxygen defects in the lattice, have a great influence on the thermal conductivity of silicon nitride ceramics. Therefore, the anti-oxidation performance of the powder is studied. And the existence form of oxygen in silicon nitride and its oxidation mechanism are very important.
Is the oxidation resistance of silicon nitride ceramics good?
2022 08/10