High Gravity Field Research Articles

Motion of Submicron Particles in a Supersonic Laminar Boundary Layer under Hypergravity

The process of submicron particle movement in laminar boundary layers has many applicable backgrounds including high-speed centrifugal devices. Although a big progress has been made on this problem during the last decades, many mechanisms in this process still remain unclear. Here, we developed a theoretical model to understand how submicron particles will behave when they are in a supersonic laminar boundary layer above an adiabatic plate along with the main stream under both zero gravity and hypergravity. In this model, we applied the Lagrangian method to track the particles and calculate their trajectories, and the Eulerian method was used to calculate the flow field. Because of the large velocity and temperature gradient near the wall and the small size of the particle in this question, and the high gravity field, five forces (e.g., drag force, Saffman lift force, thermophoretic force, Brownian force and gravitational force) acting on the particle are considered, and the role of each force is studied. The effects of entering position, intensity of gravity field, the size and density of particles are investigated. As a result, we discovered that there are three particle movement patterns when they enter the supersonic boundary layer under regular gravity and other patterns under high gravity. This research gives a better understanding of the particle movement process in the supersonic laminar boundary layer, which can be a useful instruction for the industrial processes relating to this phenomenon.

Microstructures and Toughening of TiC-TiB2Ceramic Composites with Cr-Based Alloy Phase Prepared by Combustion Synthesis in High-Gravity Field

Micro-nanocrystalline microstructures which are characterized by TiB2platelets of the average thickness close to or smaller than 1 μm can be achieved in nearly full-density solidified TiC-TiB2ceramic composites with Cr-based alloy phases by combustion synthesis in ultra-high gravity field of 2500 g. The filler phases in ceramic composites are actually Cr-based alloy with a little solidified solution of Ni atoms and Al atoms. The hardness, flexural strength, and fracture toughness of the materials are 18.5 ± 1.5 GPa, 650 ± 35 MPa, and 16.5 ± 1.5 MPa⋅m0.5, respectively. The improved fracture toughness of TiC-TiB2ceramic composites results from crack deflection, crack bridging, and pull-out by a large number of fine TiB2platelets and plastic deformation with some Cr-based alloy phases.

A 10 km-resolution synthetic Venus gravity field model based on topography

A high resolution gravity field model is extremely important in the exploration of Venus. In this paper, we present a 3-dimensional Venus gravity field VGM2014 constructed by using the latest gravity and topography models, residual terrain model (RTM) and the Airy–Heiskanen isostatic compensation model. The VGM2014 is the first 10km scale Venus gravity field model; the final results are representations of the 3-dimensional surface gravity accelerations and gravity disturbances for Venus. We found that the optimal global compensation depth of Venus is about 60km, and the crustal density is potentially less than the commonly accepted value of 2700–2900kgm−3. This model will be potentially beneficial for the precise orbit determination and landing navigation of spacecraft around Venus, and may be utilized as a priori model for Venus gravity field simulation and inversion studies. The VGM2014 does not incorporate direct gravity information beyond degree 70 and it is not recommended for small-scale geophysical interpretation.

Numerical Analysis on Droplet Deformation and Breakup in High Gravity Field

Research slope stability under rainfall condition in geotechnical centrifuge is an ideal test method, however, the influence of high centrifugal force field produced by running geotechnical centrifuge cannot be neglected. Droplet deformation and breakup under different gravity and of different diameters were studied with VOF method, the results shows that the process of droplet deformation and breakup is similar under condition of different g-value and diameters, droplet breakup in a very short time in high gravity field, and with the increase of g-value, the breakup time of droplet became shorter, with the increase of droplet diameter, the breakup time of droplet became longer under same gravity acceleration. Studies in this paper have important significance in developing geotechnical centrifuge artificial rainfall equipment.

Preparation of YAG glass-ceramic by combustion synthesis under high gravity

YAG glass-ceramic with a designed composition of 24Y2O3∙40Al2O3∙36SiO2 was prepared by a method which combines combustion synthesis with centrifugation. High temperature glass melt was produced from a highly-exothermic combustion reaction in Al/CuO system. Centrifugation induced high gravity field facilitates the crystallization of YAG from the glass melt. The effects of high gravity field on phase formation, composition distribution and morphology evolution of prepared YAG glass-ceramic were investigated. Morphology of YAG crystals changed gradually from dendritic to polygonal, and finally to columnar shape along high gravity direction. The density increased from 3.49 to 4.12gcm−3, and the hardness increased from 7.8 to 9.7GPa along high gravity direction.

Controlled exponential growth in layer-by-layer multilayers using high gravity fields

The high gravity technique is a strongly intensified process used for rapid mass and heat transfer, which has rarely been applied for the construction of layer-by-layer (LbL) self-assembled films. Herein, we used a poly(acrylic acid) (PAA)/poly(allylamine hydrochloride)–porphine complex (PAH–Por) multilayer to investigate the assembly behaviors between exponential and linear growth, and introduced strong shear stress using a high gravity field to reduce the accumulated surface roughness in exponentially growing films. The results demonstrated that strong shear stress in a high gravity field could transform film growth from exponential to linear. This phenomenon is thought to be the result of reducing surface roughness through shear forces, which was supported by stepwise characterization of ultraviolet-visible spectra and atomic force microscopy. Moreover, the intensified LbL deposition to achieve a transformation from exponential to linear growth could be applied to other systems, including PAH–PAA complex/poly(sodium-p-styrenesulfonate) (PSS) and PAH/PSS multilayers.

Microstructure modification and fracture behavior of solidified TiC–TiB2 ceramic prepared by combustion synthesis in ultra-high gravity field

A series of solidified TiC–TiB2 were fabricated by combustion synthesis integrated with ultra high gravity, and the ceramics were comprised of TiB2 primary phases, irregular TiC secondary phases, a few of Al2O3 inclusions and Cr-based metallic phases. Obviously, the increased high gravity field promoted phase separation of Al2O3 droplets from the molten reaction products, which resulted in not only the rapidly reduced Al2O3 inclusions but also the refined microstructure and the improved homogeneity. When high gravity acceleration reached 2500 g, the ultrafine-grained microstructure with the average thickness of TiB2 platelets smaller than 1 μm was achieved. More and more fine TiB2 platelets with aspect ratio of 4–6 participated in the toughening mechanism of crack bridging and subsequent pullout, the maximum fracture toughness could amount to 16.5 ± 1.0 MPa m0.5. Because of rapidly reduced Al2O3 inclusions, the refined microstructure and the improved homogeneity were obtained in the near full-density composite, the maximum flexural strength of 982 ± 20 MPa was also simultaneously achieved in the solidified TiC–TiB2 composite at 2500 g.

Preparation of tungsten fiber reinforced-tungsten/copper composite for plasma facing component

W fiber reinforced-W/Cu composite is designed as a transition layer between CuCrZr heat sink material and W plasma facing material. A novel method was developed for the preparation of W fiber reinforced-W/Cu composite by combining combustion synthesis with centrifugal infiltration. Cu melt with a transient temperature over 2000°C produced by the thermite reaction was infiltrated into the W powder and fiber bed with the assistance of a high gravity field. It was found that the W particles were sintered and bonded to the W fibers due to the high temperature produced by the thermite reaction. The bending strength of W/Cu composite improved 12.7% through W fibers reinforcement.

Effects of Ball Milling Processing on Solidified TiC-TiB<sub>2</sub> Composites Prepared by Combustion Synthesis in High Gravity Field

By combining ball milling processing with combustion synthesis in ultrahigh gravity field, the solidified TiC-TiB2 composite was achieved with fine-grained and ultrafine-grained microstructure. It is considered that taking balling milling processing to mechanically activate powder blend promotes thermal explosion to occur in ultrahigh gravity field by reducing ignition temperature, increases sharply the actual temperature of full-liquid product, thereby not only accelerating liquid-liquid separation of TiC-TiB2 liquid and Al2O3 droplets, but also refining the solidified microstructure by enhancing undercooling of TiC-TiB2 liquid, finally, brings about a series of dramatic improvements in densification and mechanical properties of the ceramic.

Large Scale Al<sub>2</sub>O<sub>3</sub>-ZrO<sub>2</sub> (Y<sub>2</sub>O<sub>3</sub>) Eutectic and Hypereutectic Achieved by Combustion Synthesis in High Gravity Field

By introducing ZrO2 (4Y) powder into the thermit, the solidified Al2O3-ZrO2 (4Y) ceramic composites with eutectic and hypereutectic microstructures were prepared via combustion synthesis in high gravity field, and the microstructures and mechanical properties of the solidified ceramic composites were discussed. XRD, SEM and EDS showed that the Al2O3-33%ZrO2 (4Y) as the eutectic were composed of random-orientated rod-shaped colonies consisting of a triangular dispersion of orderly submicron-nanometer t-ZrO2 fibers, surrounded by inter-colony regions consisting of spherically-shaped micrometer t-ZrO2 grains, whereas Al2O3-45%ZrO2 (4Y) as the hypereutectic were comprised of spherically-shaped micron-meter t-ZrO2 grains, surround by irregularly-shaped α-Al2O3 grains and a few colonies. Compared to the directionally solidified Al2O3-ZrO2 (Y2O3), the increase in hardness and flexural strength of the eutectic obtained in current experiment was due to high densification, small-size defect and high fracture toughness induced by residual stress toughening and transformation toughening mechanisms; meanwhile, in despite of the moderate decrease in hardness, high flexural strength of the hypereutectic was considered to be a result of small-size defect and high fracture toughness induced by transformation toughening and microcrack toughening mechanisms.

Laminated Composite of Solidified TiC-TiB<sub>2</sub> Ceramic to 1Cr18Ni9Ti Stainless Steel Achieved by Combustion Synthesis in High-Gravity Field

By combining mechanical activation and combustion synthesis in ultrahigh gravity field, the laminated composite of TiB2based ceramic to stainless steel was achieved in continuously-graded composition and microstructure, and within the Fe-Cr based intermediate Ti-Fe enriched carbides and fine TiB2platelets decreased gradually in size and volume fraction from the ceramic to stainless steel. Because of the sequent presence of thermal explosion, the dissolution of the molten stainless steel to TiC-TiB2liquid, the formation of diffusion-controlled concentration gradient from the ceramic liquid to the alloy liquid, the rapid sequent solidification of the ceramic and the alloy, the laminated composite is achieved in multilevel, scale-span hybrid microstructure that the different-size, different-morphology Fe-Cr alloy phases alternate with TiB2platelets and irregular TiC grains in size from micrometer to micro-nanometer.

Microstructures and Properties of Solidified TiC-TiB<sub>2</sub> Composites Prepared by Combustion Synthesis in Enhanced High-Gravity Field

By taking combustion synthesis to prepare a series of solidified TiC-TiB2composites under the increased high-gravity acceleration from 500g to 2500g, then in the near-full-density composite the ultrafine-grained microstructure with the matrix of TiB2platelets smaller than 1 μm in thickness was achieved, and all TiB2 platelets almost shared in the contributions of bridging and pullout to toughening ceramic, thereby presenting fracture toughness of 16.5 ± 1.0 MP · m0.5and flexural strength of 982 ± 20 MPa.

Preparation of Nano t-ZrO<sub>2 </sub>Particle by the Integrated Process of High-Gravity Field and Hydrothermal Crystallization

ZrO2nano-crystals have been prepared through the integrated process of high-gravity field and hydrothermal crystallization by using ZrOCl2•8H2O solution added scale 3mol% yttrium nitrate and precipitator ammonia. Precursor of ZrO2powders was prepared by high-gravity field co-precipitation reaction. Then, the precursor crystallized by the hydrothermal method to obtain ZrO2powders. The influence of hydrothermal crystallization temperature and hydrothermal crystallization time on crystal structure, morphology and grain size has been investigated. The structure, morphology and grain size of ZrO2were characterized by X-ray diffraction(XRD) , transmission electron microscopy( TEM).The sizes of pure tetragonal ZrO2nano-crystallites prepared by hydrothermal crystallizing at 175°C-200°C for 1-8h are observed to be typically in the rang of 4-10nm. What’s more, ZrO2powders prepared from crystallizing at 200°C for 6h shows great crystallinity , uniformly particle size and low aggregate.

The effects of ultra-high-gravity field on phase transformation and microstructure evolution of the TiC–TiB2 ceramic fabricated by combustion synthesis

By applying combustion synthesis in high-gravity field, the TiC–TiB2 ceramic composites were prepared under different gravitational conditions from 500g to 2500g. XRD, FESEM and EDS results showed that there is no significant change in phase composition of TiC–TiB2, but a remarkable decrease of Al2O3 inclusions with the high-gravity field increased. Meanwhile, the well-developed faceted TiB2 grains with narrow size distribution were observed. Therefore, it can be proposed that the ultra-high-gravity field not only enhances the Al2O3 liquid separation from TiC–TiB2 liquid during the combustion synthesis process, but also promotes the microstructure refinement of the reaction products. The results of the mechanical test showed that the mechanical performance of TiC–TiB2 ceramic can be improved by increasing high-gravity field, especially its relative density, fracture toughness and bending strength.

Multiscale and Multilevel Composite of Solidified TiC-TiB<sub>2</sub> with Ti-6Al-4V Achieved by Fusion Bonding in High-Gravity Field

By putting Ti-6Al-4V plate at the bottom of the blend of (Ti+B4C) and the high-energy thermit, a novel laminated composite of TiC-TiB2 ceramic to Ti alloy is achieved by combustion synthesis in high-gravity field, and within the joint there presents multiscale and multilevel microstructures characterized by size and distribution of TiB2 and TiB phases, thereby showing microstructure evolution rather than a clear interface from the ceramic to Ti alloy. The reasons for the unique microstructures in the joint came is considered a result of fusion bonding and atomic interdiffusion between liquid TiC-TiB2 and liquid Ti in thermal vacuum circumstances, followed by a series of metallurgical reaction including peritectic reaction of TiB2 solid and Ti liquid, direct growth of TiB from Ti liquid and subsequent eutectic reaction of TiB and Ti, so a continuously-graded microstructure consisting of the multiphases in size from micrometer to nanometer is eatablieshed in the joint.

Microstructures and Properties of TiC-TiB<sub>2</sub> Composites Prepared by Combustion Synthesis in Enhanced High-Gravity Field

By taking combustion synthesis in enhanced high gravity field with increasing acceleration from 500 g to 2500 g at intervals of 500 g, a series of TiC-66.7 mol%TiB2composites were prepared through liquid forming and rapid solidification. The ceramics were composed of TiB2primary phases, TiC secondary phases and a few of Al2O3inclusion and Cr metallic phases. Increasing high gravity acceleration not only promoted the combustion mode transfer from steady SHS mode to explosive one through enhancing the deposition of liquid product toward unreacted blend, but also accelerated liquid-liquid separation of TiC-TiB2/ Al2O3droplets and subsequent formation of layered melt through enhancing Stokes flow in mixed liquid products, thereby reducing sharply Al2O3inclusions and shrinkage cavities in solidified TiC-TiB2composite. Meanwhile, the enhanced Stokes flow brought about constitutional uniformity through enhancing atomic diffusion in TiC-TiB2liquid, not only refining the microstructure of the solidified ceramic, but also improving the homogeneity of the ceramic by inducing TiB2primary phases to grow at similar velocity. As a result, hardness, flexural strength and fracture toughness of the composite increased greatly in enhanced high-gravity field.

Microstructure Homogeneity, Crack Propagation and Self-Toughening of Solidified TiC-TiB<sub>2</sub> Composite Prepared by Combustion Synthesis in Enhanced High-Gravity Field

A series of solidified TiC-TiB2were prepared by combustion synthesis in enhanced high-gravity field, and the ceramics were comprised of TiB2primary phases, irregular TiC secondary phases, a few of Al2O3inclusions and Cr-based metallic phases. Because of the accelerated phase separation of Al2O3droplets from the molten reaction products, the enhanced high-gravity field not only caused both Al2O3inclusions and shrinkage cavities to be sharply reduced, but also brought about the refined microstructure and the improved homogeneity in the solidified ceramic. As high gravity acceleration reached 2500 g, the ultrafine-grained microstructure with the thickness of TiB2platelets smaller than 1 μm was achieved, and FESEM fractographs showed a whole of ultrafine TiB2platelets almost accomplished the pullout process, and the maximum fracture toughness of 16.5 ± 1.0 MP · m0.5was achieved, whereas the maximum flexural strength of 982 ± 20 MPa was also simultaneously obtained as a coupled result of sharply-reduced defects, the refined microstructure, the improved homogeneity and the high defect tolerance achieved in the near-full-density composite.

Fast fabrication of W–Cu functionally graded material by high-gravity combustion synthesis and melt-infiltration

W–Cu functionally graded material (FGM, 75wt%W+25wt%Cu—40wt%W+60wt%Cu) has been prepared by a method of high-gravity combustion synthesis and melt-infiltration in a short time (∼5min). The infiltration mechanism in the high-gravity field was investigated. The W–Cu FGM showed an overall relative density of∼97% and gradually-varying properties in terms of density, micro hardness, coefficient of thermal expansion. Especially, the W–Cu FGM exhibited a coefficient of thermal expansion between those of W and Cu, and thus could be used as a transition layer between W and Cu to relax the thermal stresses.

ChemInform Abstract: Review of Melt Casting of Dense Ceramics and Glasses by High Gravity Combustion Synthesis

AbstractReview: 63 refs.

Effects of Ultra-High Gravity Field on the Microstructure and Mechanical Properties of Solidified TiC-TiB2 Ceramic Prepared by Combustion Synthesis

Based on increasing the acceleration of high-gravity field from 500 g to 2500 g, a series of TiC-TiB2 composites are prepared by combustion synthesis in high-gravity field, and microstructure modification, microstructure homogeneity and mechanical properties of TiC-TiB2 composites are discussed in terms of liquid-liquid separation of TiC-TiB2 and Al2O3 in high gravity field. XRD, FESEM and EDS results showed that by increasing the acceleration of high gravity field, Al2O3 inclusions in the ceramic decreased both in volume fraction and size due to the enhanced separation of TiC-TiB2 liquid and Al2O3 liquid, while the enhanced Stokes flow induced by the enhanced liquid-liquid separation also promoted the constitutional homogeneity of TiC-TiB2 melt, thereby bringing about the refinement of the solidified microstructures, finally, making Vickers hardness and fracture toughness of the composites increase with the acceleration of high gravity field.