Papers by Keyword: High Gravity

Abstract: The study of materials processed in centrifuges improves the understanding of the acceleration influence in the convection behavior in materials processing. This work aims to study the influence of high gravity in PbSn eutectic alloy solidification using a small centrifuge designed and built in the Associate Laboratory of Sensors and Materials of the Brazilian Space Research Institute (LAS/INPE). The samples were analyzed by densitometry and scanning electron microscopy (SEM).

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.

Abstract: A new synthetic technology of Strontium carbonate with high gravity is introduced in this paper. We got ultrafine SrCO3 powders with Sr(NO3)2 and NaCO3 or CO2 as raw material by high gravity method. We studied flows, the rotating speed, additives and CO2 as the reactants on the morphology of strontium carbonate. The results show that the presence of additive-free, we got rod-like structure of strontium carbonate. Added EDTA, we got a good dispersion of spherical particles of narrow particle size distribution. The presence of EDTA, the average particle size of spherical particles decreases with the speed increases. CO2 alternative Na2CO3 as the reactants, the presence of additive-free, we got the bundle structure of strontium carbonate. The presence of EDTA, we got spherical particles, but the particle dispersion is not well, gathered together.

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.

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.

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·m^1/2, 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.

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∙m^0.5 due to the cooperative action of crack deflection, crack-bridging and pull-out toughening by fine TiB₂ platelets.

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.

Abstract: Large bulk solidified TiC-TiB2 eutectic composite ceramics were prepared by combustion synthesis under high gravity, and the WO3 was introduced into the combustion synthesis as one of oxidants in thermit to obtain the Ti-W-Cr-C-B liquid, so near-full-density TiB2-(Ti,W)C eutectic composite ceramics without the macrocracks were achieved. The ceramic matrix was mainly composed of TiB2-(Ti,W)C eutectic microstructures, and a few of Al2O3 and Al2O3-ZrO2 eutectic structures were also detected in between Ti-W carbides. Due to the introduction of the high gravity field, Stocks immigration of the immiscible liquids took place due to their density differences, resulting in float-up of oxide liquid and settle-down of Ti-W-Cr-C-B liquid, and the layered melt consisting of oxide liquid and Ti-W-Cr-C-B liquid was formed, finally, TiB2-(Ti,W)C eutectic composite grown from the melt. Due to the mutual solubility of W-Ti, the W atom diffused into the TiC, leading to the formation of (Ti,W)C solid solution as same as crystal lattice structure of TiC. The relative density, Vickers hardness and fracture toughness of the composite ceramics measured 98.4%, 26.4 GPa and 7.6±0.5 MPa•m1/2, respectively.

Abstract: (Ti, W)C matrix metal ceramics were prepared through combustion synthesis under high gravity, structure formation mechanism of the ceramics was discussed and the properties of the ceramics were also measured. XRD and FESEM images show the matrix of metal ceramics was mainly composed of TiC and (Ti, W)C1-x. The formation mechanism mainly involves two stages: firstly, combustion reaction is advanced rapidly under high gravity, and the layered melt consisting of Ti-W-Fe-C-B liquid at the bottom and oxide liquid at the top is formed due to rapid liquid-liquid separation under high gravity, subsequently, TiC solids as the primary phase precipitates from Ti-W-Fe-C-B liquid due to the higher concentration and faster diffusion of C relative to B in the alloy liquid, whereas (Ti, W)C1-x solid nucleates and grows on the surface of TiC solids. The Vickers hardness, flexural strength and fracture toughness of the (Ti, W)C matrix Metal ceramics measured 25.6 GPa, 1060 MPa and 8.5 MPa•m1/2, respectively.