Study On Texture Mechanism Of Ceramic Materials In A Strong Magnetic Field

The texturing of ceramics has been one of research hotspots because it is of great significance to obtain the excellent properties in advanced ceramic materials. In recent years, with the development of superconducting magnet technology, magnetic orientation as a new technology has received abroad attention in the application of ceramic materials. When ceramic materials are placed in the magnetic field, the magnetization energy can be transferred into the ceramic materials without a contact.Consequently, the preferred orientation of grains can be controlled in ceramic materials. There are two processes in preparation of textured ceramic materials using the strong magnetic field technology. One is that the grains in the green bodies are oriented during casting in the strong magnetic field. The other one is that the textured green bodies are sintered without the strong magnetic field. The two processes play an important role in the texturing formation of ceramic materials. Additionally,the degree of orientation in the green body has a great influence on the grain growth during subsequent sintering. Therefore, in this paper, the mechanisms of texturing formation and grain orientation in ceramic materials are researched systematically based on two processes. Then the experiments about textured ceramic materials are conducted to further verify and investigate these mechanisms. The major research results of this paper are as follows:1. Study on the model of forming crystal orientation in ceramics in a strong magnetic field. Firstly, the effect of the interaction force between grains on the orientation of many grains is analyzed. Futhermore, after analyzing the characteristics of the system composed of many grains, the rotational model of grain in the magnetic field is further developed. Results show that the interaction force between grains has a great effect on the orientation behavior of many grains.When the applied magnetic field intensity is same, the time achieving the saturation degree of orientation increases. With the increase of magnetic field intensity, the saturation degree of orientation increases and the time decreases. The experiments about the orientation of?-Si3N4 grains verify the law. The time of grain orientation calculated by improved model gets close enough to the experiment value. By comparison, improved model can accurately describe the law of many grains orientation.2. Study on the preparation of textured green bodies in a strong magnetic field.Based on the theoretical analysis involved in the above section, two kinds of grains with different magnetic axis are researched by experiment, including the Si3N4 with biax grain orientation and SiC grains with one-axis grain orientation. Results show that in the study of Si3N4 grain orientation,when a-Si3N4 powders as the matrix and?-Si3N4 as the seed particles are used to prepare the textured green bodies in a strong magnetic field of 6T,the degree of orientation of ?-Si3N4 grains gradually increases with increasing the seed particles. That is, the number of oriented grains increases in the textured green bodies. On the contrary, the used magnetic condition has no evident effect on the orientation of ?-Si3N4 grains. The lotgering orientation factors of green bodies are about 0.01-0.03. When textured green bodies are prepared by using the?-Si3N4 powders as the matrix, the high textured green bodies can be prepared in the magnetic field of 6T. The degree of texture can reach 0.72. In the above two kinds of textured green bodies,a- or b-axis of ?-Si3N4 grains is oriented parallel to the direction of the magnetic field. And its c-axis is aligned on the plane perpendicular to the magnetic field. The biax textured green bodies are prepared in a strong magnetic field. Under the same condition of magnetic field, ?-Si3N4 grain has the stronger ability to form the orientation than that of ?-Si3N4 grain. In the study of SiC grain orientation, when textured green bodies are prepared by using the ?-SiC powders as the matrix, c-axis of SiC grain is oriented parallel to the direction of the magnetic field. That is, SiC grain can form the one-axis orientation along its c-axis. The degree of texture can reach 0.84.3. Effect of sintering process with no phase transformation on texturing of ceramic materials. Duirng the liquid-phase sintering of textured Si3N4 ceramics prepared using P-Si3N4 powders as the matrix, the increasing sintering temperature without the magnetic field promotes the increase of the degree of texture in the sintered bodies. Owing to the occurrence of no phase transformation during sintering,the microstructure of sintered samples consists of near-spherical grains and short columnar grains. Therefore,although the degree of texture reaches 0.81 in the sintered sample at 1800?, the evident microstructure anisotropy can not be observed on the planes perpendicular and parallel to the magnetic field. During the liquid-phase sintering of textured SiC ceramics, the degree of texture of sintered sample is lower than that of green body, showing that sintering process without the magnetic field inhibits the texturing formation of sintered samples. The increasing sintering temperature and the prolong holding time at 1950? slightly promotes the texturing formation in the sintered bodies by the grain growth process. In the textured sample sintered at 1950? for 6h, the degree of texture reaches 0.83 and the sample shows the evident microstructure anisotropy. A large number of equiaxal grains exist on thesurface perpendicular to the direction of the magnetic field. A large number of elongated grains exist on the surface parallel to the direction of the magnetic field.The short-axis (c-axis) of SiC is oriented parallel to the direction of the magnetic field.The microstructure in SiC samples consists of fine equiaxal grains and big elongated grains. Based on the liquid-phase sintering mechanism without phase transformation,it is thought that the texturing formation in sintered sample is mainly affected by the rearrange process of grains and grain growth process. Among, the rearrange process of grains inhibits the texturing formation in sintered samples. The grain growth process plays a positive effect on the texturing formation in sintered samples.4. Effect of sintering process with phase transformation on the texturing of ceramic materials. Textured Si3N4 ceramics prepared by using the a-Si3N4 powders as the matrix and ?-Si3N4 as the seed particles are researched here. The results show that under the used sintering condition in this research, the increasing P-Si3N4 seed particles content promotes the increase of the degree of texture in sintered samples.This change trend is consistent with that in textured green bodies. The increasing sintering temperature without the magnetic field improves the degree of texture in the sintered samples. At 1750?, the prolong holding time has no evident effect on the degree of texture. When the textured Si3N4 ceramics are sintered at 1800?,?-Si3N4 grains transfer into ?-Si3N4 grains completely. In the sample with Owt% seed particles,the lotgering orientation factor of green body is about 0.01 and the lotgering orientation factor of sintered body is 0.19. In the sample with 9wt% seed particles, the lotgering orientation factor of green body is about 0.03 and the lotgering orientation factor of sintered body reaches 0.76. The textured ceramics with biax orientation microstructure are prepared. The phase transformation of a to ? during solution-precipitation process and the Ostwald ripe growing of grain have an important role in the texturing formation of sintered ceramics. The number of ? phase grains as the crystal nucleus plays a decisive role in the degree of texture in sinteredsample. Compared with sintering process with no phase transformation, phase transformation process improves the degree of texture.