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141.
Templated Grain Growth of Textured Bismuth Titanate 总被引:8,自引:0,他引:8
Jeffrey A. Horn S. C. Zhang U. Selvaraj Gary L. Messing Susan Trolier-McKinstry 《Journal of the American Ceramic Society》1999,82(4):921-926
Textured bismuth titanate, Bi4 Ti3 O12 (BiT), was produced by templated grain growth (TGG). Molten-salt-synthesized BiT platelets were dispersed in a matrix of 200 nm BiT powder and aligned by tape casting. Highly textured BiT was obtained with the use of only 5 vol% template particles by sintering at 1000°C for 1 h. The uniformity of the through-thickness texture is much higher than reported in the literature for BiT tapes cast with 100% platelets. Initial platelet alignment is shown to increase because of frequent interaction with the fine powder particles during tape casting. By avoiding pressure densification techniques and using only a small portion of anisometric particles, TGG is a low-cost option for fabricating textured ceramics. 相似文献
142.
Tobias Frueh Elizabeth R. Kupp Charles Compson Joe Atria Gary L. Messing 《Journal of the American Ceramic Society》2016,99(7):2267-2272
To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na2O and 603 ppm SiO2 and heated at 1525°C up to 8 h. Na2O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al2O3, Na2O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO2 concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na2O/SiO2 ratios, densification of Bayer Al2O3 at 1525°C is controlled by diffusion of Al3+ through the grain‐boundary liquid, whereas for high Na2O/SiO2 ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al2O3 or diffusion of Al3+. Increasing Na2O in SiO2‐doped samples increases diffusion of Al3+ and Al2O3 solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al2O3 densification can be manipulated by adjusting the Na2O to SiO2 ratio. 相似文献
143.
Ender Suvaci George Simkovich Gary L. Messing 《Journal of the American Ceramic Society》2000,83(2):299-305
The effect of attrition milling on the solid-state oxidation of aluminum powder is important for the reaction-bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m2 /g versus 1.2 m2 /g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11–13 nm thick amorphous plus γ-Al2 O3 layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ∼70% versus ∼9% for the as-received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid-state oxidation and to prevent thermal runaway. The critical temperatures for the as-received (1.24 m2 /g), the 6 h (14.4 m2 /g), and 8 h (20.2 m2 /g) attrition-milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented. 相似文献
144.
Tobias Frueh Cassie Marker Elizabeth R. Kupp Charles Compson Joe Atria Jennifer L. Gray Zi‐Kui Liu Gary L. Messing 《Journal of the American Ceramic Society》2018,101(7):2739-2751
In this work, we investigate the effects of powder chemistry on the sintering of MgO‐doped specialty alumina. The stages at which MgO influences densification of Al2O3 were identified by comparing dilatometry measurements and the sintering kinetics of MgO‐free and MgO‐doped specialty alumina powders. MgO is observed to reduce the grain boundary thickness during densification using TEM. We show that MgO increases the solubility of SiO2 in alumina grains near the boundaries using EDS. First‐principles DFT calculations demonstrate that the co‐dissolution of MgO and SiO2 in alumina is thermodynamically favored over the dissolution of MgO or SiO2 individually in alumina. This study experimentally demonstrates for the first time that removal of SiO2 from the grain boundaries is a key process by which MgO enhances the sintering of alumina. 相似文献
145.
Densification of alumina coated with a MgO-Al2 O3 -SiO2 (MAS) glass was investigated from 1400° to 1460°C. Spinel was observed to form at the liquid-alumina interface, whereas mullite crystallized uniformly in the liquid. Spinel and mullite crystallization kinetics were accelerated by smaller alumina particle size. Mullite and spinel crystallization retarded densification by forming a percolating network. Boron doping suppressed spinel formation, and thus alumina sintered to higher densities at low temperature. The concept of glass basicity is proposed as a useful guide for selecting dopants for low-temperature sintering. 相似文献
146.
Sam E. Schoenberg David J. Green Gary L. Messing 《Journal of the American Ceramic Society》2006,89(8):2448-2452
The thermomechanical properties of a commercial barium titanate were experimentally or theoretically determined for samples with green densities ranging from 45% to 55%. For stresses less than 300 kPa, sample deformation was determined to be linear viscous for all three stages of sintering. The shrinkage rates at a given temperature can differ by up to ∼25% as the green density changes from 45% to 55%, and the maximum shrinkage rate increased with decreasing green density. The increase in shrinkage rate with lower green density samples persisted through the final sintering stage. The viscosity was determined by cyclic loading dilatometry to range from 5 to 6 GPa·s in the initial stage of sintering, to 2 GPa·s in the intermediate stage, and to increase to 10–20 GPa·s for all specimens in the final stage of sintering. Differences in the final-stage viscosity were attributed to grain size differences. Relaxation times for the sintering body were estimated to be less than 1 s, indicating that viscous behavior is dominant throughout the sintering process. 相似文献
147.
T. K. Nandy Rebecca M. Messing J. Wayne Jones Tresa M. Pollock D. M. Walukas R. F. Decker 《Metallurgical and Materials Transactions A》2006,37(12):3725-3736
A new procedure for blending die-cast Mg−Al alloys by semisolid processing to achieve controlled variations in microstructure
and properties has been investigated. Granules of AM6-B and AZ91D have been blended in varying proportions and Thixomolded
at nominal solid fractions of 0.1 and 0.3, respectively. As-molded microstructures and the role of interdifussion during processing
have been analyzed in detail by scanning electron microscopy, transmission electron microscopy, and electron microprobe analysis.
Tensile properties and failure modes have been analyzed and a strength model that considers solid solution strengthening of
Al in the unmelted particles and a rule-of-mixtures behavior for microstructural components is proposed. 相似文献
148.
A new allele of the 27-kD zein locus in maize has been generated by interchromosomal recombination between chromosomes of two different inbred lines. A continuous patch of at least 11,817 bp of inbred W64A, containing the previously characterized Ra allele of the 27-kD zein gene, has been inserted into the genome of A188 by a single crossover. While both junction sequences are conserved, sequences of the two homologs between these junctions differ considerably. W64A contains the 7313-bp-long retrotransposon, Zeon-1. A188 contains a second copy of the 27-kD zein gene and a 2-kb repetitive element. Therefore, recombination results in a 7.3-kb insertion and a 14-kb deletion compared to the original S+A188 allele. If nonpairing sequences are looped out, 206 single base changes, frequently clustered, are present. The structure of this allele may explain how a recently discovered example of somatic recombination occurred in an A188/W64A hybrid. This would indicate that despite these sequence differences, pairing between these alleles could occur early during plant development. Therefore, such a somatically derived chimeric chromosome can also be heritable and give rise to new alleles. 相似文献
149.
Sang-Ho Lee Gary L. Messing Masanobu Awano 《Journal of the American Ceramic Society》2008,91(2):421-427
We describe a series of experiments to learn how to produce flat LaSrMnO3 /YSZ/NiO-YSZ multilayer packages typically used for solid oxide fuel cells, in a single cosintering cycle, by applying a load during cosintering using sintering arches. We demonstrate that the key step is to apply the load gradually after the multilayer components are stronger and at their minimum viscosity of ∼6 GPa·s during sintering. Alumina sintering arches, which support an alumina loading plate above the multilayer laminate, were designed to yield slowly at the intermediate stage of cosintering and thus apply the loading plate to the multilayer package. The primary design features of sintering arches are the arch material viscosity behavior, and the arch width, thickness, and curvature. 相似文献
150.
Bengt O. Meuller Maria E. Messing David L. J. Engberg Anna M. Jansson Linda I. M. Johansson Susanne M. Norlén 《Aerosol science and technology》2013,47(11):1256-1270
In the growing field of nanotechnology there is an increasing need to develop production methods for nanoparticles, especially methods that provide control and reproducibility. The spark discharge generator (SDG) is a versatile device for the production of nanoparticle aerosols. It can produce aerosol nanoparticles in the entire nanometer range (1–100 nm), and beyond. Depending on requirements, and the system used, these nanoparticles can be completely contamination free and composed of one or more materials. This provides a unique opportunity to create new materials on the nanoscale. Already in use in semiconductor, materials, health and environmental research, the SDG shows promise for yet more applications. If needed, particle production by the SDG could be scaled up using parallel generators facilitating continuous high-volume production of aerosol nanoparticles. Still, there is a surprisingly low knowledge of fundamental processes in the SDG. In this article we present a thorough review of the most common and relevant SDGs and the theory of their operation. Some possible improvements are also discussed. Copyright 2012 American Association for Aerosol Research 相似文献