Papers by Keyword: Liquid Phase

Abstract: MnZn ferrites have been widely used as magnetic core materials. It is well known that Ca addition is effective to obtain homogeneous microstructure of fine grains and highly resistive grain boundaries. However, the behaviors of calcium as one of the main additives at different temperature ranges during sintering process are not completely understood yet. In this study, the influence of CaCO3 content and sintering temperature on the microstructure was investigated in 1473-1623 K. It was found that there existed a critical temperature around 1550 K. The grain size decreased with the increase of Ca content when the sintering temperature was lower than the critical temperature, but completely opposite result was observed at higher temperatures range. Possible mechanisms were discussed. When the sintering temperature was lower than the critical temperature, Ca content greatly affected the grain boundary mobility and dominated the grain growth. At higher temperatures, however, formation of liquid phases might be the main cause for the grain growth.

Abstract: Al-Sn alloys for tribological applications are industrially important alloys which have attracted little attention over their history. Being cold rolled directly from thin cast slabs and consisting of two ductile phases, their processing and physical behaviour are distinct from classical Al-Alloys. During cold rolling, the coarse-grained, random texture of the slab is transformed into the classical rolling texture of a fine-grained Al-alloy, with elongated Al-grains delimited by thin Sn-ribbons. During annealing at 300°C, the interior of the Al-grains recrystallises rapidly while the liquid Sn-phase migrates toward Al-grain triple lines to form a reticular structure. A weak texture, dominated by Goss and P is produced. Grain growth within the original cold-rolled grains is fast, but once the recrystallised grain size reaches the length scale of the second-phase distribution, it slows down and both phases coarsen simultaneously, accompanied by a significant texture change.

Abstract: We investigate the jamming transition observed in vibrated granular systems composed of millimeter size glass beads. When a granular system is submitted to vibrations with decreasing intensity, it evolves in a way similar to glass-forming liquids: from a low viscosity, liquid-like state, it evolves into an amorphous jammed state. This evolution is observed by the means of an immersed oscillator acting as a torsion pendulum in forced mode. The complex susceptibility of the oscillator is measured as a function of the probe forcing frequency and of the vibration intensity. Focusing on the strongly vibrated states, we observe that there are two different dynamic regions. The first is a high fluidization regime, where the internal friction is found to be proportional to the ratio between the pulsation and the vibration intensity: . In this region, the system shows an apparent viscous friction . In the second, low fluidization, regime, we observe a more complex behavior, and the measured internal friction appears to be well described by a relation of the form: . In this second case, the key role is played by a critical breakaway stress, σ_cr, needed to break the network of chains of forces that form between the grains. Finally, if vibration intensities are still reduced, we also observe that onset of jamming is clearly distinguishable: we observe a sharp increase in the apparent dynamic modulus together with a peak in internal friction. This transition presents important similarities to those observed in glasses, and it leads to the second (low vibrations) regime, where the key role is played by the square root of the vibration intensity.

Abstract: Highly aligned carbon nanotube arrays (HACNTAs) were synthesized on a stainless steel substrate from a methanol solution of Co(Ⅲ) acetylacetonate by the one-step liquid-phase synthesis, and effects of H2O addition on the HACNTA growth were examined. The growth rate was considerably accelerated, and the lifetime of the catalysts was prolonged by addition of a small amount of H2O. HACNTAs with over 400 m thickness were formed on the side surfaces of the substrate by resistance-heating for one hour. This suggests that the added H2O removes deposited amorphous carbons from the catalyst surfaces and consequently accelerates the growth rate.

Abstract: Elemental powders of tungsten, nickel, iron and cobalt of compositions corresponding to (W-3.2Ni-0.8%Fe), (W-3.5Ni-1.5%Fe), and (W-4.5Ni-1.0Fe-1.5%Co) were mechanically alloyed in a tumbler rod mill for 2 hrs. Mechanically alloyed powders were liquid phase sintered at 1500oC for 90 min in vacuum. The sintered materials were heated up to 1150-1200oC in vacuum atmosphere, followed by quenching in water to suppress the impurity segregated at grain boundary. The sintered materials were subjected to cold-working by swaging from 8-30% reduction in area. The swaged specimens were age-hardened at 700oC. Full characterization for both the elemental powders and the sintered tungsten alloys were performed using optical microscopy, SEM analysis, EDS quantitative analysis, X-ray diffraction, hardness and compression testing. This paper will discuss the effects of the elemental powders characterization and the liquid phase sintering parameters on the microstructure and strength of these three tungsten heavy alloys.

Abstract: In this paper, BaPbO3(5Vol.%)/2024Al composite was fabricated by powder metallurgy method. Nanosized Pb particles were formed from the in-situ reaction between BaPbO3 and 2024Al, and mainly distributed homogeneously inside grains. The hot compression deformation of the composite at different temperatures was performed. The microstructure evolution of the 2024Al matrix and dynamic mechanical analysis (DMA) of the composite were made to investigate the effect of liquid Pb on the hot deformation. The results show that the presence of the liquid phase reduces deformation resistance by decreasing dislocation pileups, and simultaneous decrease in Young’s modulus of the composite.

Abstract: In this study, SnO2-based ceramics, with CuO as sintering aid and Sb2O3 as activator of the electrical conductivity, was obtained by pressure-less sintering at 1100°C ~ 1470°C. Addition of antimony leads to a higher densification temperature. Densification behavior and microstructure development are strongly dependant on CuO and Sb2O3. CuO gives rise to a liquid phase; Sb2O3 retards the formation of liquid phase and hinders the growth of grain. The electrical resistivities of SnO2-based ceramics vary in a wide range from 10-2 to 107 Ω•cm, depending on starting compositions and processing conditions. The electrical resistivities of samples with different amounts of CuO and Sb2O3 show different trends with the increasing of sintering temperature. The addition of antimony rapidly promotes electrical conductivity of SnO2-based ceramics containing CuO as the solid solution reaction of Sb2O3-SnO2. As the additions of CuO and Sb2O3 are the same, the electrical resistivity arrives the minimal value of 4.72×10-2 Ω•cm for 99%SnO2+0.5%CuO +0.5%Sb2O3 at 1470°C. More content of Sb2O3 than CuO causes the degression of density and the rising of electrical resistivity of ceramics.

Abstract: Like on 6H-SiC substrates, 3C-SiC islands precipitation was found to be the initial stage of the VLS growth of 3C-SiC layers on 4H-SiC surfaces. This precipitation happens between 1100 and 1200°C with a heating rate of 2.8°C.s-1, without addition of propane. The islands size increases in a similar manner whether the final temperature increases (for a given heating rate) or the heating rate decreases (for a given final temperature). This enlargement can give rise to a complete cubic layer for the highest temperatures or the slowest heating rates. It is suggested that the carbon atoms involved in the enlargement process (after the nucleation) come from the graphite crucible.

Abstract: Twin-free 3C-SiC layers were recently obtained by Vapour-Liquid-Solid mechanism on a a-SiC(0001) substrate using Si-Ge melt. The formation of cubic layers is rather unexpected since growth from the melt is known to promote lateral growth and should thus give homoepitaxial layers. The study of the early stage of such growth, after a simple contact between the melt and the substrate (without adding propane), reveals the precipitation of 3C-SiC elongated islands upon the substrate surface. The chemical interactions inside the Ge-Si-C ternary phase diagram suggest an initial dissolution of the SiC seed in contact with a Ge-rich melt (below 1200°C). When the Si content of the melt subsequently increases upon heating, the dissolved carbon atoms precipitate on the seed surface under the form of 3C-SiC islands. When propane is added, these islands enlarge and coalesce to form a complete 3C layer.

Abstract: The growth kinetics of 3C-SiC heteroepitaxial layers on α-SiC substrates by Vapour-Liquid-Solid (VLS) mechanism in Ge-Si melts was investigated. Various parameters were studied such as temperature, melt composition, propane flux and substrate nature (polytype, polarity and misorientation). It was found that the growth rate increases with increasing temperature, propane flux, Si content of the melt and misorientation of the substrate. The calculated activation energy (from 4.7 to 6.6 kcal/mole depending on the substrate type) is very small suggesting that the limiting process is the diffusion of the dissolved carbon inside the melt. The carbon solubility inside the melt mainly affects the carbon dissolution kinetics from the gas phase. The results also suggest that surface effects are important through the layer polarity and crystalline quality.