Papers by Keyword: High Gravity

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Authors: Chuan Zeng Pan, Long Zhang, Zhong Min Zhao, Zhen Sheng Qu, Quan Yang, Xue Gang Huang
Abstract: Based on preparation of Al2O3/ZrO2 (4Y) by combustion synthesis in high-gravity field, the microstructure transformation and properties of the materials are investigated through adjusting the ZrO2 (4Y) content in the composites. As the content of ZrO2 changed from 37% to 40%, the microstructures of the ceramics transformed the sphere-like tetragonal ZrO2 crystals from the rod-shaped colonies with nanocrystalline structures. Al2O3/33%ZrO2 (4Y) had the maximum relative density, hardness and flexural strength due to the low solidification temperature, the highest volume fraction of the colonies, small-size defect and high fracture toughness, whereas Al2O3/44%ZrO2 (4Y) was somewhat weakened in strength in despite of its highest fracture toughness.
Authors: Takashi Nishiyama, Takashi Kajiwara, Kunihito Nagayama
Abstract: A compositionally graded thin film of Fe/Si was fabricated by a gravity-assisted pulsed laser ablation (GAPLA) system. By this method, a compositionally graded structure along gravity direction was successfully produced under a gravity field of 5,400 G. Systematic experiments were conducted by several parameters, including gravity, distance between target and substrate, and laser fluence in case of typical target material of iron disilicide (FeSi2). We demonstrate that the atomic fraction of Fe, the heavier component of the thin film, showed increasing spatial distribution with the direction of gravity. Relatively high laser fluence as well as a very narrow space between the target and the substrate are found to be essential to the compositionally gradient of thin film.
Authors: Guang Hua Liu, Jiang Tao Li, Zeng Chao Yang
Abstract: Both single-phase and multiphase bulk ceramics were prepared by high-gravity combustion synthesis via melt solidification instead of conventional powder sintering. The synthesis process included three steps, namely, combustion reaction, phase separation, and melt solidification. In these steps, phase separation played a key role in producing bulk ceramic materials with high purities and low porosities. It was demonstrated that, in a high-gravity field the phase separation was greatly accelerated, compared with the case under common gravitational condition. In comparison with powder sintering, high-gravity combustion synthesis requires no furnace and can reduce the processing time for the fabrication of bulk ceramic materials.
Authors: Tao Ma, Zhong Min Zhao, Long Zhang, Xue Gang Huang, Liang Xiang Liu
Abstract: Large-bulk TiB2-TiC composite ceramics were prepared by combustion synthesis under high gravity. XRD, SEM and EDS results showed TiB2-TiC composites were mainly composed of the fine-grained microstructures of TiC matrix in which a large number of the fine TiB2 platelet grains were dispersed uniformly, whereas there discontinuously dispersed the ε-carbides with the enrichment of Ti atoms, and a few of isolated, irregular α-Al2O3 grains and Al2O3-ZrO2 colonies were also observed at the boundaries of the eutectic microstructures. The results of properties indicate that with increasing mass fraction of B4C+Ti+C in combustion systems, the relative density and fracture toughness of TiB2-TiC composites are all among 97%~99% and 6.5~7.1 MPa·m1/2, respectively, and the Vickers hardness and flexural strength are increased gradually to the maximum values of 28.6GPa and 615MPa, respectively. The achievement of full-density TiB2-TiC composites benefited from the design of full-liquid SHS products and the introduction of high-gravity field, and high hardness of the composite ceramics resulted from the absence of intermediate borides and the achievement of stoichiometric TiC phases due to rapid solidification, whereas high flexural strength of the composite ceramics benefited from the homogenization and refinement of the microstructures due to the rapid separation of the liquid oxides and the rapid coupled growth of TiB2-TiC.
Authors: Quan Yang, Zhong Min Zhao, Long Zhang, Chuan Zeng Pan, Zhen Sheng Qu, Min Quan Wang
Abstract: Based on preparing large bulk Al2O3/ZrO2 (4Y) eutectics, SiO2 additive is used for controlling the microstructures, densification and mechanical properties. XRD pattern showed SiO2 additive in the ceramics was in the form of glass phase. SEM images showed that the microstructure morphologies transformed from the cellular eutectics to the rod-shaped colonies with increasing content of SiO2 additive. With increasing content of SiO2 additive, the relative density of the ceramics increased whereas the hardness of the ceramics decreased. As the content of SiO2 additive reached 4%, fracture toughness of the ceramics had the maximum value due to the coupled toughening mechanisms of crack-pinning, crack- bridging and crack-deflection by rod-shaped colonies. As content of SiO2 additive reached 6%, the highest flexural strength of the ceramics was achieved due to high fracture toughness and small-size critical defect.
Authors: Zhong Min Zhao, Long Zhang, Chuan Zeng Pan, Quan Yang, Zhen Sheng Qu, Guan Ling Su, Xue Gang Huang, Min Quan Wang
Abstract: The large bulk Al2O3/ZrO2 (Y2O3) eutectics were achieved by combustion synthesis in high-gravity field. With increasing high-gravity level, the matrix of eutectics transformed the rod-shaped colonies from the cellular ones, and the nanocrystalline microstructures came into existence as the high-gravity level was larger than 200g. The relative density, hardness, flexural strength and fracture toughness increased simultaneously with increasing high-gravity level, and reached the maximum values of 98.6%, 18.6GPa, 1248MPa and 15.6MPa•m1/2 respectively as the high-gravity level was 250g.
Authors: Guan Ling Su, Zhong Min Zhao, Long Zhang, Xue Gang Huang, Chuan Zeng Pan
Abstract: By introducing self-pressure processing into combustion synthesis under high gravity, Al2O3-ZrO2(4Y) eutectic composite ceramics without shrinkage cavities and holes were achieved, and the self-pressure processing also made the macro-crack be controlled and the ceramic densification be promoted evidently. XRD results showed the eutectic ceramics were composed of α-Al2O3, t-ZrO2 and a few m-ZrO2 phases, and the volume fraction of the transformable t-ZrO2 increased with self-pressure force increasing. SEM images showed that with increasing self-pressure force, the rod-shaped eutectic colonies were refined and the volume fraction of the colonies increased. Meanwhile, the irregular ZrO2 crystals around eutectic colonies transformed to be fine t-ZrO2 spherical crystals gradually, and thickness of eutectic colonies decreased. With the changes in microstructures of the ceramics, the mechanical properties of the ceramics were improved greatly.
Authors: Zhong Min Zhao, Long Zhang, Yi Gang Song, San Qun Li, Wei Guo Wang
Abstract: Based on preparing the large-bulk solidified Al2O3-ZrO2 (Y2O3) ceramic composites by combustion synthesis under high gravity, the solidification behavior and microstructure transformation of the ceramics as well as their effects on the ceramic properties were investigated. XRD, SEM and EDS analyses showed the ceramic microstructures were composed of the irregular eutectics at the surface and the tetragonal ZrO2 micrometer spherical crystals in the center of the ceramics respectively. By combining with irreversible nucleation theory, it is considered that the formation of irregular eutectics with micro-nanocrystalline microstructures is a result of the leading nucleation of Al2O3 high-fusion-entropy phases followed by the coupled growth of Al2O3-ZrO2 phases, whereas the presence of the tetragonal ZrO2 spherical microstructures in the center of the ceramics results from the leading nucleation of ZrO2 cubic phases followed by the independent growth of Al2O3-ZrO2 phases. As a result, it is just the unique microstructure transformation during the solidification process that the highest hardness at the surface and the highest fracture toughness in the center of the ceramics are achieved respectively.
Authors: Yi Tao Liu, Bao Yin Song, Qiu Ping Yao
Abstract: In order to follow the technical progress in the filed of aeronautics and astronautics, a numerical investigation into flow characteristic of two-phase flow under high gravity (hi-g) condition is performed. Using the CFD code CFX, the two-phase flow in orthogonal pipe under high gravity condition has been evaluated, and the effects of hi-g on two-phase flow characteristic have been analyzed. Compared with the static condition, the flow pattern, volume fraction, velocity and pressure distributions, pressure drop through the pipe are quite different under hi-g, depending on the magnitude and direction.
Authors: Xue Gang Huang, Long Zhang, Zhong Min Zhao, Chun Yin
Abstract: By using combustion synthesis under high gravity, TiC-TiB2 fine-grained composite ceramics with hypoeutectic, eutectic and hypereutectic microstructures were prepared through rapid solidification. XRD, FESEM and EDS results show that with increasing TiB2 content, TiC-TiB2 composite ceramics transform the microstructures consisting of fine TiB2 platelets from ones composed of fine TiC spherical grains, whereas when TiB2 content reaches 50mol%, the ceramics develop the eutectic microstructures that small aspect-ratio TiB2 platelets were embedded in TiC matrix. Mechanical properties show that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB2 all reach the maximum values (respectively as 98.6 % ,18.4 GPa, 840 MPa) due to eutectic reaction during solidification, meanwhile TiC-50mol%TiB2 has the maximum fracture toughness of 11.5 MPa∙m0.5 due to the cooperative action of crack deflection, crack-bridging and pull-out toughening by fine TiB2 platelets.
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