The structure of NaZr2(P4)3 was studied by L0Hagman and P. Kierkegaard et al. and determined as a hollow, relaxed and stable three-dimensional network composed of P4 tetrahedron and ZrO octahedron connected in a common vertex, with cations in the structure. Can be replaced by a variety of ions, derived a series of structural derivatives, collectively referred to as family compounds. The NZP material prepared by using a compound as a powder material has been extensively studied for its low thermal expansion properties and the scatterability of the coefficient of thermal expansion. By selecting and adjusting the chemical composition, a material having zero-expansion characteristics can be obtained. The type of NZP material has good thermal shock resistance, and therefore has a higher application value, and has a good application prospect as a coating in catalyst carriers, automotive engine components, small heat exchangers, and aerospace technologies.
In this study, chemically co-precipitation method was used to synthesize the NZP binary solid solution CanBaxZr4(P04)6(:c=0,0.25,0.5,0.75,1.0) series powders, and add different sintering additives to further prepare. Out of the CBZP series ceramics, the average coefficient of linear expansion of 201000X: was determined, and the influence of different sintering aids on the thermal expansion characteristics of CBZP ceramics was emphatically studied.
Visible, the same composition of the CBZP body, adding ZnO and MgO sintered ceramics, the thermal expansion coefficient is different, the use of ZnO as a sintering aid, except: r = 0.75 composition at 20100CTC the average coefficient of linear expansion is slightly greater than 2X1 (T6/ Outside the °C, the average coefficient of linear expansion of the other four components is less than 2X1 (T6/°C), which is a low thermal expansion material. This is because ZnO promotes densification and also promotes grain growth, resulting in more micro cracks. Therefore, the coefficient of thermal expansion is reduced.
This result shows that when ZnO promotes densification, it also greatly promotes the growth of crystal grains. If ZnO is added at the same time, adding a grain growth inhibitor (such as active Si02) is expected to obtain low thermal expansion materials. At the same time, microcracks are reduced to improve the thermal shock resistance of the material.
3.2 Effect of Sintering Temperature on Thermal Expansion Properties of CBZP Ceramics The green compacts with 3% ZnO as additives were sintered at 120 CTC for 2 h to obtain CBZP ceramics. The average linear expansion coefficient of 20KKKTC was measured by the same method, and it was at 1100° with the same additives. The average linear expansion coefficient of this series of ceramics sintered at 2 hours C was compared, and the results are shown in Table 2.
Table 2 shows the average coefficient of linear expansion of CBZP series ceramics obtained at different sintering temperatures. When MgO is added, the coefficient of thermal expansion increases significantly, and the average coefficient of linear expansion of 20100CTC is greater than 2X1 (TV°C (except for the composition of x=0). In medium-expansion materials, the coefficient of thermal expansion of the series is due to the formation of a second phase in the sintering process, which can be explained by the mechanism of MgO-enhanced densification, that is, the combination of MgO and PO in CBZP produces a low melting point. Mg3(PO4)2 (melting point is about 1186 DEG C.). At 1300X: sintering, since the melting point of Mg3(P04)2 is exceeded and the liquid phase generation promotes densification, the second phase exists in the sintered body.
From the results of Table 2, it can be seen that under the same additive, changing the sintering temperature has no significant effect on the thermal expansion coefficient of the CBZP series ceramics. (a) shows the sample with x = 0.75 fired at 1200°C. The microstructure, compared with (a), shows that the samples sintered at these two temperatures have similar grain sizes, and the thermal expansion curve shows that the hysteresis loops are also close in size, see (b) and (a). Therefore, it can be considered that the particle size has a certain influence on the thermal expansion coefficient, and the same additive is consistent in the degree and mechanism of promoting grain growth, and the obtained product has a close thermal expansion coefficient.
As can be seen from the figure, the ZnO-added product has a large hysteresis loop, indicating that there may be more microcracks inside the material, and therefore the coefficient of thermal expansion is very low. The microstructure of the sample shows that the grain size is larger (see a). When MgO is added, although the coefficient of thermal expansion is increased, the hysteresis loop on the thermal expansion curve is much smaller than that of ZnO, which indicates that there are few micro-cracks. From the microstructure of the sample, uniform small-grain structure can be seen. 3.3 Sintering Time vs. Thermal Expansion The effect of the characteristic is to add 3% Mg to the composition of x= 0 and: c=0.5, and sinter at 1300° C. for 30 min and 120 min, respectively, and the same method is used to determine the ceramic samples of the two compositions made at different sintering times. The average coefficient of linear expansion, the results are shown in Table 3. The composition of the sintering time min average linear expansion coefficient aXl6/.7-0.8 can be seen from the data in the table, for the same additives, sintered in 30min and 120min obtained CBZP ceramic thermal expansion coefficient size is very Similar, but the hysteresis loop on the thermal expansion curve is significantly increased with the extension of the sintering time, the composition of: r = 0.5, for example, the thermal expansion curve as shown.
The thermal expansion curve of CBZP (x = 0.5) ceramics sintered at different sintering times after sintering for 30 min has a small thermal hysteresis loop, which indicates that the internal micro-cracks are less, and as the sintering time is extended to 120 min, due to grain growth Microcracks occur, showing a large hysteresis loop on the thermal expansion curve. However, their thermal expansion coefficients are very small, and it can be shown that the growth of crystal grain size in this sintering time range has not become a major factor affecting the thermal expansion coefficient.
3.4 Effect of material composition on thermal expansion characteristics From the test results of the average coefficient of linear expansion (see Table 1), it can be seen that the coefficient of thermal expansion of the ceramic samples of both additive series shows negative to positive as the composition of the material changes. The trend of value changes. From the perspective of the thermal expansion curve, the same addition of 4% MgO at 1300'C sintered for 120 min to porcelain, the composition of x = 0.5 exhibited a larger hysteresis loop than the composition of = 0.75, see (b) and (b) In comparison with (b) and the microstructures shown, it can also be seen that the grain sizes of the two are different, and the crystal grain size of the composition: = 0.5 is significantly larger than that of the grain size when x = 0.75. Its thermal expansion curve shows a larger hysteresis loop. It can be seen that under the same sintering conditions, the thermal expansion characteristics of the CBZP series ceramics are greatly related to their composition.
4 Conclusions The thermal expansion coefficient of the series of ceramics of the CaiBarPCUdOxCl) series is greatly affected by the sintering aids. When ZnO is added (sintering temperature is 110CTC), the thermal expansion coefficient of the ceramics is lower than that of MgO (sintering temperature is 1300'C) and is a low thermal expansion material. When MgO is added, due to the formation of a second phase in the sintering process, the thermal expansion coefficient is increased beyond the range of low thermal expansion.
The composition of this series of ceramics has a large relationship with the coefficient of thermal expansion. With the change of x from 0 to 1, the average linear expansion coefficient of the corresponding composition shows a trend of change from negative to positive values, that is, through the composition adjustment, it can be realized. The coefficient of thermal expansion is continuously adjustable.
For the same additive and the same composition, the grain growth caused by the sintering temperature (1100120 (TC) and sintering time (30120 min)) in this study did not become the main factor affecting the thermal expansion coefficient.
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