Movement mechanism.
Yunnan Provincial Science and Technology Commission Youth Fund Project, 96B060Q 1 Foreword The composition and structure of compounds and solid solutions with similar properties, such as the typical low expansion to zero expansion thermal expansion coefficient of continuous adjustable special properties, can be used in harsh environments with rapid temperature changes, such as Aerospace and other high-tech fields are used as excellent heat-resistant impact materials, which have high research and application value. The composite potassium zirconium phosphate (KSZP) is one of the derivatives. However, in order to make the material have excellent thermal shock resistance, it also depends on high strength in addition to low thermal expansion. In order to meet the requirements for the mechanical and thermal properties of such materials, it is necessary to strictly control the material properties and process of the material. In this study, KSZP (x = 0, 0.5, 1.0) ceramic powders were synthesized by wet chemical method. The particle size was 0.81.8. The corresponding ceramics were obtained by dry compression molding and pressureless sintering.
It can be seen that the general trend of the curve change is that the compressive strength first increases with increasing temperature, but after reaching a certain temperature, the intensity change tends to be gentle and begins to decline.
The relationship between the sintering temperature and the volumetric shrinkage of the ceramic sample is shown. More similarly, the compressive strength of the sintered body is closely related to the densification of the sintering process. This is because the sintering process is a reaction process mainly based on solid-phase sintering, and it needs to be sintered at a certain temperature for a certain time, and the densification process can be fully completed. If the sintering time is fixed, the temperature is too low to complete (b) sintering temperature and KSZP (x=0 compressive strength of the relationship between temperature CC) sintering degree and volume shrinkage of the relationship between the densification, the temperature is too high then the grain growth too fast Causes local microstructural defects and reduces the strength of the material.
3.2 Relationship between sintering time and compressive strength According to the previous studies, sintering was chosen at 1300 to examine the effect of sintering time on the performance of the ceramic sample, as shown.
(mass fraction) of ZnO, ceramic compressive strength of up to 155 MPa. In the experiment found that without additives, the sintered body is almost not dense, not into porcelain. At the same time, if the amount of additives is too small, as described in (a), when adding 1% Mg, the strength of KZP is only 76.4 MPaD even if sintered at 1500 and when the addition amount is properly increased, the resistance is the same under the same sintering conditions. The compressive strength is greatly increased, indicating that the additive has a significant effect on promoting ceramic densification. The reason for this is that due to liquid phase sintering, the melting point of Zn3(P4)2 is 1000 Y and Mg3(P04):2 is 1186:. When the temperature rises, the melting point of the low-melting phase first melts and wets the solid phase. At high temperature, the protruding part of the solid phase particles or the entire particle with smaller size can be dissolved in the liquid phase, and can also be precipitated from the liquid phase again and deposited on The other parts of the solid phase particles undergo a dissolution-deposition process. When the liquid-phase sintering occurs, the pores in the sintered body are eliminated by the mass transfer under the action of the surface tension of the liquid phase. In addition to the viscous flow of the liquid phase, there are still particles moving or migrating, including particle rearrangement, in the mass transfer mechanism. Dissolving precipitation, etc. W. Thus, the sintering temperature is greatly reduced due to the presence of a low-melting phase, and the densification rate is greatly accelerated. Upon cooling, the liquid phase layer can be absorbed by the lattice of the main crystalline phase of the human or react with the main crystalline phase to form a solid solution, and if the added amount is too much, that is, the liquid phase amount is excessive, it exceeds the solid solution amount in the main crystalline phase. The excessive liquid phase crystallizes between the solid particles to form an additional crystalline phase or exists as an amorphous grain boundary glass phase, which changes the composition of the material, thereby reducing the compressive strength of the material. Therefore, the amount of additives must be moderate. The experimental results show that in the above materials, the compressive strength is the highest when the amount of additives is about 6% (mass fraction), indicating that the volume shrinkage is also the largest. In addition, it was also found in the experiment that the sintering temperature must be 1300 sintered bodies to be dense when adding Mg, and ZnO can be dense when ZnO is added, which is due to the melting point of M(P04)2 is higher than that of Zn3(P04)2. As a result, the existence of liquid phase sintering was explained again.
The microstructure of the KZP sample (for example) shows that: if MgO is too small (a), the pores of the grains are large and the degree of densification is low; when the MgO is suitable (b), the microstructure is very good, which further confirms the previous analysis.
3.4 Grain growth and densification of the material migration mechanism The above experimental results show that: sintering temperature, sintering time, the amount of additives 3 parameters have an impact on the compressive strength of NZP ceramics. Sintering temperature is too high or too low, sintering time is too long or too short, too much or too little additive, is not conducive to the improvement of compressive strength, all three parameters have a suitable value, this is because of grain growth and densification The generation of the process relies on the same mechanism of the mass transfer and grain growth leads to and promotes the densification process. In the main stage of the sintering densification, that is, the middle stage of sintering, both the grain growth mechanism and the densified material migration mechanism are surface diffusion. Particles move from the surface of the small particles along the surface of the particles to the large particles through the grain boundary-stomatal boundary, resulting in the growth of the crystal pores. The volume of the sintered body is reduced.
Grain growth at the later sintering stage is completed by grain boundary migration. Regardless of whether the pores migrate along with the grain boundaries or are trapped in the grains, the grain growth itself will not contribute to the densification process. In the later stage of sintering, the pores are closed pores and the grain boundaries are continuous phases. Therefore, the surface diffusion pathway that causes grain growth and densification at the same time in the middle stage is cut off. Therefore, the diffusion path of the densified particles at the later sintering stage can only be grain boundary or volume diffusion. . Close to the theoretical density, due to the disappearance of pores at the junction of many smaller grain sizes, the grain growth is accelerated, and the densification rate is slowed down, the grain size-density relationship will be more and more biased towards the side of the large grain size.
Because the mechanism of the material migration in the process of grain growth and densification is the same, the additive promotes grain growth while increasing the densification rate. Conversely, if the grain growth is inhibited, the densification rate is also suppressed. The results of this study indicate that the sintering time is prolonged and the degree of densification is not significantly improved. On the contrary, the crystal grains are larger and the compressive strength is decreased. The above analysis is proved, so after the introduction of the additive that promotes densification, the post-sintering time should be as short as possible, since the post-sintering phase is mainly the growth of crystal grains.
In addition, compared with the Sol-Gel method or the solid phase synthesis method, the powder prepared by the co-precipitation method in this study can obtain a higher degree of densification in a shorter sintering time at the same sintering temperature. The SZP compressive strength obtained by the Gel method is only 80 ~ lOOMPa, and the sintering time is 16h, the solid phase synthesis method sintering time is also more than 24h, but this study only in the addition of 6% (mass fraction) MgO1300 SZP only sintering 1.5h test The sample can reach compressive strength of 244.0MPa, which is far greater than the value, indicating that the coprecipitation method has high sintering activity.
4 Conclusions (1) The optimum conditions for the sintering of composite zirconium-barium-potassium ceramics are: the amount of additive MgCKZnO is 6% (mass fraction); the sintering temperature is 1300 when MfeO is added; 1200 is added when ZnO is added; and the sintering time is both 1.5h. Additives can significantly improve the sintering performance of the powder, and the principle of promoting the densification of the sintered body is liquid phase sintering.
The mechanism of grain migration and densification of the material is the same, that is, while promoting densification, it also promotes grain growth.
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