Showing papers on "Sintering published in 1993"

Core/Rim Structure of Liquid-Phase-Sintered Silicon Carbide

Abstract: Plasma etching in conjunction with scanning and analytical transmission electron microscopy was used to characterize the microstructure and microchemistry of silicon carbide sintered with yttrium-aluminum garnet (YAG). The SiC grains comprise a core/rim structure with small amounts of excess yttrium, aluminum, and oxygen being present in the rim while these elements are missing in the core. The core/rim interface was found to be coherent, and both the core and the rim are composed of the same polytype, predominantly the 6H structure. These results suggest that Ostwald ripening by solution and reprecipitation controls the sintering mechanism in this system.

Sintering behaviour and mechanical properties of hydroxyapatite and dicalcium phosphate

Abstract: The sintering behaviour of powders of two calcium phosphates, namely hydroxyapatite (HA) and dicalcium phosphate (DCP), were studied at various temperatures and in various environments. The density, flexural strength and Knoop hardness of HA sintered in air for 4 h initially increased with the sintering temperature, reaching maxima at around 1150°C, and then decreased due to decomposition of HA into tri- (TCP) and tetracalcium phosphates. Sintering in vacuum caused decomposition of HA at lower temperatures, and consequently the mechanical properties were poorer than those of HA sintered in air. The densification and mechanical properties of DCP sintered in air and vacuum showed similar behaviour to those of HA. In air DCP underwent phase transformation from γ- to β- and to α-phases. In vacuum DCP started to decompose into tricalcium phosphate at 1000°C. To reduce dehydroxylation, HA powder was sintered in moisture at various temperatures up to 1350°C and X-ray diffraction study did not indicate any decomposition at the highest sintering temperature. The density, flexural strength and hardness of HA sintered in moisture increased with the sintering temperature and eventually reached plateaux at about 1300°C, but below 1200°C they were lower than those of HA sintered in air at corresponding temperatures. Thus, it is seen that dehydroxylation did not hinder sintering of HA. On the other hand, decomposition obstructed sintering of both HA and DCP.

Hydrothermal synthesis and sintering of ultrafine CeO2 powders

Abstract: Undoped CeO2 and Y2O3-doped CeO2 powders, with particle sizes of almost-equal-to 10-15 nm, were prepared under hydrothermal conditions of 10 MPa at 300-degrees-C for 4 h. The compacted powders were sintered freely in air or in O2 at constant heating rates of 1-10-degrees-C/min up to 1350-degrees-C. The undoped CeO2 started to sinter at almost-equal-to 800-900-degrees-C and reached a maximum density of 0.95 of the theoretical at 1200-degrees-C, after which the density decreased slightly. Isothermal sintering at 1150-degrees-C produced a sample with a relative density of almost-equal-to 0.98 and an average grain size of almost-equal-to 100 nm. The samples sintered above 1200-degrees-C exhibited microcracking. The decrease in density and the microcracking above 1200-degrees-C are attributed to a redox reaction leading to the formation of oxygen vacancies and the evolution Of O2 gas. Doping with Y2O3 produced an increase in the temperature at which measurable sintering commenced and an increase in the sintering rate, compared with the undoped CeO2. Sintered samples of the doped CeO2 showed no microcracks. The CeO2 doped with up to 3 mol % Y2O3 was sintered to almost full density and with a grain size of almost-equal-to 200 nm at 1400-degrees-C.

Model of the selective laser sintering of bisphenol-A polycarbonate

Abstract: Selective laser sintering (SLS) is a new additive process for rapidly preparing prototype parts by selectively fusing portions of layers of polymer powder with a rastered, modulated, CO 2 laser beam. Layer to layer fusion is important for achieving good mechanics properties. Generally, the fusion depth is found to be a function of laser beam overlap on adjacent scans, scan speed, and intensity. It is also a function of the kinetic rates of powder sintering and of heat transfer. A one-dimensional model of the SLS thermal process is presented that can accurately predict, to within 20%, the effect of these process and material parameters on experimentally observed fusion depths in bisphenol-A polycarbonate powders

Possible Electrical Double‐Layer Contribution to the Equilibrium Thickness of Intergranular Glass Films in Polycrystalline Ceramics

Abstract: The plausibility of the entropic repulsion of electrical double layers acting to stabilize an equilibrium thickness of intergranular glass films in polycrystalline ceramics is explored. Estimates of the screening length, surface potential, and surface charge required to provide a repulsive force sufficiently large to balance the attractive van der Waals and capillary forces for observable thicknesses of intergranular film are calculated and do not appear to be beyond possibility. However, it has yet to be established whether crystalline particles in a liquid-phase sintering medium possess an electrical double layer at high temperatures. If they do, such a surface charge layer may well have important consequences not only for liquid-phase sintering but also for high-frequency electrical properties and microwave sintering of ceramics containing a liquid phase.

Determination of Critical Concentrations of Silica and/or Calcia for Abnormal Grain Growth in Alumina

Abstract: Minimum amounts of SiO2 and CaO required for inducing abnormal grain growth in alumina were determined using ultrapure alumina (>99.999%) and sintering at 1900°C for 1 h in a contamination-free condition. The critical concentrations of silicon in cationic mole fractions in alumina were 300 ppm without calcium, 200 ppm with 10 ppm calcium, and 150 ppm with 20 ppm calcium. The critical concentration of calcium alone was 30 ppm. These concentrations seemed to match approximately the solubility limits reported in the literature, which suggested that the abnormal grain growth in commercially pure alumina was indeed related with the formation of a small amount of liquid phase during sintering.

Crack Growth and Damage in Constrained Sintering Films

Abstract: The constrained sintering of films on substrates leads to a reduction in densification rate and may lead to processing flaws. This paper reports on a study of damage and cracking in sintering films, with particular emphasis on the growth of preexisting cracks. Experiments have been conducted with glass and polycrystalline Al2O3 films on various substrates. The effect of important variables (viz., film thickness, crack length, and friction with the substrate) on crack growth is reported. The experiments with glass films show that cracking occurs above a critical film thickness which is in good quantitative agreement with a recent analysis for this problem. In the case of Al2O3 films, we observe a diffuse damage zone ahead of cracks. Crack growth occurs by the coalescence of microcracks with each other and with the main crack. Some possible reasons for this difference between the glass and Al2O3 films are presented. As a model for diffuse damage, the stability of a sintering film under spatial variations in constitutive parameters is analyzed. It is shown that the film is unstable to small perturbations only in the early stages of densification, and that for viscous sintering the films are usually kinetically stable.

Abrasive grain containing alumina and zirconia.

Abstract: A method of preparing preferred abrasive grain includes: preparing a dispersion including alpha alumina oxide monohydrate and zirconia sol therein; converting the dispersion to base grit material; and sintering the base grit to form abrasive grains. The presence of the zirconia sol within the dispersion from which the base grits are formed provides an improved abrasive grain. The improved abrasive grains are useful in abrasive products.

Selective laser sintering using nanocomposite materials

Abstract: A method of fabricating three-dimensional objects in a layerwise fashion, and having high structural strength and high density, is disclosed. Methods are disclosed by which nanocomposite powders of ceramic-ceramic systems, ceramic-metal systems, ceramic-polymer systems, and metal-polymer systems are produced. Disclosed examples utilize solution chemistry approaches, such as sol-gel processing, by way of which a gel is produced which is then fired and milled to form a powder suitable for selective laser sintering, where a laser fuses selected portions of layers of the powders according to a computer-aided-design data base. The ultraheterogeneity of the powder results in larger surface area and grain boundaries of the constituents, which enhances the solid state diffusion mechanism, and thus reduces the time and temperature required for sintering to occur. In addition, the higher stored metastable energy of the nanocomposite powder is believed to enhance densification during solid-phase sintering. Objects of high density of complex shape may thus be obtained directly from CAD data base design representations.

Processing nanocrystalline ceramics for applications in superplasticity

Abstract: The production of nanocrystalline ceramics for subsequent use in superplastic forming operations requires that the ceramics be made in large quantities, with high densities, and under stringent grain growth control. To make large amounts of nanocrystalline starting powders, two popular wet chemical techniques (precipitation from salt solutions and alkoxide hydrolysis) can be used and are described in this paper. Unfortunately, pressureless sintering of these powders does not typically lead to the high densities and ultrafine grain sizes desired in the final product. Sintering data suggest that pore shrinkage occurs only when grains reach a critical size with respect to the pore size; thus, if the ceramic contains large pores, densification can require significant grain growth. Separation of large pores from grain boundaries may also occur and lead to incomplete densification, even at extremely large grain sizes. In all cases the pressureless sintering behavior of the nanocrystalline ceramics appears to adhere to well established theories used to explain the sintering of conventional, larger-grained ceramics. During both pressureless sintering and sinter-forging experiments, the grain size of a nanocrystalline ceramic is identical to the average spacing between open pores in the sample. Pressureless sintering results in the closure of these pinning pores by about 90% density and thus3leads to a substantial grain growth at densities greater than 90%. Sinter-forging, however, often allows one to maintain a stable population of small open pores (for pinning purposes) throughtout sintering, while preferentially eliminating the large pores which detract from the sample density. The deformation regime in which sinter-forging is performed has a decided effect on whether large pores or small pores are eliminated preferentially and, consequently, on whether a high density and fine grain size combination is achieved or not.

The Electrical Properties of Ceramic Electrolytes for LiM x Ti2 − x ( PO 4 ) 3 + yLi2 O , M = Ge , Sn , Hf , and Zr Systems

Abstract: The electrical properties of systems of , were examined in detail. The conductivity and the sinterability increased with the amount of excess lithium oxide in the phosphate. The secondary phase acts as a flux to accelerate the sintering process and to obtain high conductivity grain boundaries. The conductivity decreased and the activation energy of the bulk component for Li+ migration increased by the partial substitution of Tr4+for M4+ in systems of . A minimum activation energy of 0.28–0.30 eV, was obtained for the sample with ca. 1310 A3 in the cell volume. has the most suitable tunnel size for a Li+ migration through the NASICON‐type network structure.

Sintering kinetics and microstructure development of nanoscale Y-TZP ceramics

Abstract: Zirconia samples doped with 3 mol% yttria were prepared by gel precipitation from a metal chloride solution and their sintering behaviour compared with that of a commercial powder. Dense (relative density 97%) nanoscale ceramics with a mean grain size of 60 nm are obtained after sintering at 1050°C for 7h. Important densification mechanisms in the initial sintering stage are grain boundary sliding and grain boundary diffusion. Grain growth in the final sintering stage seems to be impurity drag controlled. Extremely low activation energies are obtained for both densification and grain growth in the initial sintering stages. Special attention has been paid to the effect of aggregate size of the precursor powder on the final grain size.

Oxygen storage capacity of promoted Rh/CeC 2 catalysts. Exceptional behavior of RhCu/CeO 2

Abstract: The effect of various additives (V, Cr, Mn, Fe, Co, Ni, Cu and Pb) on the oxygen storage capacity (OSC) of CeO2 and Rh/CeO2 catalysts was investigated Copper is an excellent promoter of OSC conferring to Rh a very high resistance to sintering (900°C, 2% O2)

Sintering of Sol—Gel-Prepared Submicrometer Particles Studied by Transmission Electron Microscopy

Abstract: We studied the sintering process of sol-gel-prepared monodisperse submicrometer CeO2 spheres by examining the microstructural changes of single spherical particles after successive heat treatment in oxygen up to 850°C using transmission electron microscopy. Steps of organic phase removal, CeO2 crystallization, grain growth, and particle condensation were clearly illustrated. Grain-size and sphere-diameter changes were measured quantitatively using this technique. CeO2 particles were found to be highly porous until collapse, which occurred at~850°C.

Sintering and grain growth of ultrapure alumina

Abstract: The kinetics of densification and grain growth of ultrapure alumina (> 99.999%) were measured for clean sintering conditions in a pure-sapphire tube, and compared with kinetics measured during normal sintering conditions in an alumina crucible of 99.8% purity. For the clean condition, the microstructure of sintered alumina remained homogeneous and only normal grain growth was observed up to 1900°C for 5 H. However, under the normal sintering condition, both normal and abnormal grain growth were observed depending on the sintering temperature and time. Thus, abnormal grain growth in alumina could be effectively suppressed without introducing sintering aids (such as MgO) by using an ultrapure powder and by preventing the introduction of any impurities throughout the sintering process. This result strongly suggests that abnormal grain in commercially pure alumina (⩽ 99.99%) is not an intrinsic property of alumina but an extrinsic property controlled by minor constituents that can be present in the original powder or introduced during powder processing and subsequent sintering.

Method for making abrasive grain containing alumina and ceria

Abstract: A method of preparing abrasive grain includes: preparing a dispersion including alpha alumina oxide monohydrate and ceria sol therein; converting the dispersion to base grit material; and sintering the base grit to form abrasive grains. The grain are useful in abrasive products.

Kinetics of β-Si3N4 Grain Growth in Si3N4 Ceramics Sintered under High Nitrogen Pressure

Abstract: The kinetics of anisotropic β-Si3N4 grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si3N4 grains follows the empirical growth law Dn– D0n=kt, with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si3N4 growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time.

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

Abstract: The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr0.2Ba0.8)TiO3 materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al2O3–SiO2–TiO2 (AST). The ultimate PTCR properties with ρht/ρrt as great as 105.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO3 is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth.

Phase equilibria effects on the enhanced liquid phase sintering of tungsten-copper

Abstract: The sintering behavior and mechanical properties of W-Cu are improved by the addition of elements that have solubility for W,e.g., Co, Ni, Fe, and Pd. The degree of enhancement with small concentrations of additive is dependent on specific phase diagram features, and the ranking of effectiveness does not follow the trend observed for the activated solid-state sintering of W. These observations are explained through a combination of liquid phase sintering and activated sintering theories that considers the combined W, Cu, and activator phase equilibria effects. In small concentrations, Ni and Pd have little effect on densification because they go into solution with Cu, resulting in only a slight increase in the solubility of W in the liquid phase. In this case, the sintered density, strength, and hardness increase with increasing additive concentration due to enhanced densification through solution-reprecipitation. Cobalt and Fe are the most ef-fective activators due to their limited solubility in Cu and the formation of a stable intermetallic phase with W at the sintering temperature. This promotes the formation of a high-diffusivity interboundary layer which enhances solid-state sintering of the tungsten grains at temperatures at which a liquid phase is present. With Co and Fe additions, the sintered density, strength, and hardness peak with activator concentrations of 0.35 to 0.5 wt pct. An evaluation of models for activated solid-state sintering and liquid phase sintering indicates a substantial solid-state contribution to densification when a high-diffusivity interboundary layer is present and the sol-ubility of W in the liquid phase is small.

Thermal stability of vanadia–titania catalysts

Abstract: The thermal behaviour of seven different ‘pure’ TiO2(anatase) preparations, as well as of materials obtained by doping them with potassium sulfate and carbonate, silica, and the oxides of tungsten, molybdenum and vanadium has been investigated by TG-DTA, XRD, FTIR and surface-area measurements. Vanadia–titania catalysts prepared by impregnation of these supports have also been investigated by the same techniques. The temperature at which anatase sintering and phase transformation to rutile occur strongly depends on the morphology of the TiO2 preparation. The anatase phase is much less stable in high-area, highly porous materials than in low-area powders. Vanadium oxide speeds up the anatase-to-rutile transition. However, common catalyst additives like silica, tungsten oxide and alkali-metal carbonates and sulfates strongly slow down both anatase sintering and its transformation to rutile. These phenomena influence the formulation of additives for vanadia–titania (anatase) catalysts for both selective oxidation and reduction of NOx.

Effect of Mismatched Sintering Kinetics on Camber in a Low-Temperature Cofired Ceramic Package

Abstract: Linear shrinkage profiles of an unconstrained gold thick film material and a low -tempearture cofireable glass-ceramic (LTCC) green tape were measured using a noncontact optical technique. A laser beam scans across a sample, at various times during the sintering process. The unconstrained sintering kinetics of the gold film were found to differ significantly from those of the LTCC tape. The densification of the gold film was nearly completed before the LTCC began to densify. The development of camber (warpage) during consintering of a gold/LTCC composite structure was monitored and recorded with a video camera. This camber development is analysed based on the viscous constitutive relations for porous based on the viscous constitutive relations for porous sintering bodies. The mismatched sintering kinetics of the two materials lead to the development of in-plane stresses in the two constituents of the composite structure as one material being constrained from composite structure as one material being constrained of the composite structure as one material being constrained from sintering by the other. The resulting camber during the consintering process is explained by the development of these stresses.

Zirconia/Alumina Functionally Gradiented Composites by Electrophoretic Deposition Techniques

Abstract: An electrophoretic deposition and sintering route was used to prepare YSZ/Al2O3 composites with a compositional gradient. The YSZ content was continuously decreased from the YSZ-rich surface to the Al2O3-rich surface, Microstructural and Vickers hardness (16–24 GPa) evidence tracked the compositional development, and the indentation fracture toughness was found to vary across the section (10–3 MPa·m1/2).

Liquid‐Phase‐Assisted Sintering of Calcium‐Doped Lanthanum Chromites

Abstract: Investigations have been made on the low-temperature sinterability of calcium-doped lanthanum chromites which are to be used as interconnectors in solid oxide fuel cells (SOFCs). Nominally chromium deficient lanthanum calcium chromites (La[sub 0.7]Ca[sub 0.3]Cr[sub 1[minus]y]O[sub 3], y = 0.02) were found to be sinterable to 94% theoretical density at 1573 K in air, whereas no densification was observed for samples with y = 0. The two-step shrinkage process suggests a liquid-phase sintering mechanism with calcium oxychromates playing an important role as the liquid phase. After sintering at 1573 K, calcium-rich substances remained at grain boundaries.

Target and its production

Abstract: PURPOSE: To obtain a transparent electrically conductive film more excellent in moisture resistance than an ITO film and having the same electrical conductivity and light transmissivity as the ITO film by using a sintered compact of an oxide containing hexagonal laminar compound mainly containing In and Zn and expressed by the general formula In 2 O 3 (ZnO) m ((m) is 2-20). CONSTITUTION: The In compound and Zn compound can be the oxides or a materials to be oxided after sintering. A solution (solution A) made by dissolving both compound in a solvent (e.g. water) is prepared. The conc. of each metal in the solution A is 0.01-10mol/l. On the other hand, a solution (solution B) made by dissolving a precipitation forming agent (e.g. alkali) is prepared. The temp. at the time of forming the precipitate can be above the m.p. and below the b.p. of the solvent. The precipitate is aged for 1-50hr after forming the precipitate. The obtained precipitate is dried at 40-200°C for 0.1-100hr, and next, calcined at 800-1200°C and is pulverized, reduced in a reducing gas atmosphere at 100-800°C and, after molded, sintered at 800-1700°C. COPYRIGHT: (C)1994,JPO&Japio

Structural Ceramic Components by 3D Printing

Abstract: The Three Dimensional Printing (3DP) Process hasbe~nadapted for processing of fine ceramic powders to •• prepare structllraLceramic components. Our preliminary study was designed to reveal those aspects ofthe.3DPprocesswhichmust be modified for use with fine ceramic powders. The basic elements of the modified process are to spread submicron alumina powder and printJatex binder. Several methods were used to spread thin layers of submicron powders. Gre.enparts are isostaticaUypressed followed by thermal decomposition prior to sintering to remove the polymer. The fired alumina components are greater than 99.2% dense and have·average flexural strength of324 MPa. This is lower than the best conventionally prepared alumina, but we believe that the strength results will improve as we learn more about the relationship between strength limiting flaws and the 3DP build process.