Showing papers on "Sintering published in 1982"

Formation, Packing, and Sintering of Monodisperse TiO2 Powders

Abstract: Monodisperse TiO2 powders were synthesized by the controlled hydrolysis of dilute alcoholic solutions of titanium alkoxides. The state of powder aggregation in dispersions and powder packing in the green bodies strongly affected the sintering behavior. The sintering of uniformly packed powder compacts resulted in finegrained microstructures with >99% of theoretical density nt temperatures much lower than those required to sinter conventional TiO2 powders.

Pressureless Sintering of SiC

Abstract: Pressureless sintering of SiC was accomplished at 2100°C with oxide additives. These additives were the products of the reaction of Al(OH)3 with HCl and of Y(OH)3 with HCOOH. These reaction products were dissolved in water and mixed with submicrometer β-SiC. A mixture of equal weights of these additives was effective for the sintering of SiC.

Effect of impurities on sintering and conductivity of yttria-stabilized zirconia

Abstract: The effect of low concentrations of Fe2O3, Al2O3 and Bi2O3 on the sintering behaviour of (ZrO2)083 (YO15)017, made by alkoxide synthesis, has been investigated The best results are achieved with Bi2O3 as a sinter agent and a relative density of 95% is obtained at 1200 K The effects of these impurities on the electrical conductivity of the bulk and the grain boundaries has been investigated using frequency dispersion analysis (101-106 Hz) All investigated impurities have a negative influence on both the bulk and grain-boundary conductivity For Fe2O3 and Al2O3 grain-boundary segregation factors of about two are calculated

Considerations of Inhomogeneity Effects in Sintering

Abstract: Inhomogeneous regions of powder particle size or of particle packing result in nonuniform sintering rates. This nonuniformity induces stresses which may, in turn, create microstructural defects. A method for calculating these stresses is described and some aspects of defect development are discussed.

Effect of Variations in Polymerized Oxides on Sintering and Crystalline Transformations

Abstract: This work shows that molecular structural variations can be introduced in oxide systems, including AI2O3, ZrO2, TiO2, and SiO2, by controlled hydrolysis and polymerization reactions during the formation of oxides from metal alkoxides. The chemical and stoichiometric makeup of hydroxides and oxides is not fixed and changes rapidly as a function of molecular size in the region where the size is extremely small. It was also observed that internal structural variations significantly alter the sintering behavior of oxide powders and affect their subsequent crystalline transformations.

Microstructural Analysis of Sintered High-Conductivity Zirconia with Al203 Additions

Abstract: Transmission electron microscopy (at 100 and 1000 kV potential) and analytical scanning transmission electron microscopy were used to study α-Al203 second-phase particles and their interactions with grain boundaries in two high-conductivity Y203/Yb203 stabilized zirconia ceramics containing deliberate additions of the alumina as a sintering aid. Most of the Al203 particles were intragranular and microanalysis showed that they contained inclusions rich in Zr or Si plus Zr. Al2O3 particles at grain boundaries were frequently associated with amorphous cusp areas rich in Si and Al. The results suggest that the Al203 acts as a scavenger for SiO2, removing it from grain-boundary localities. A model is proposed whereby this process occurs as the boundaries meet the second-phase particles, assisted by rapid grain-boundary diffusion. Such an ZrO2-Al2O3-SiO2 interaction and partitioning is predicted thermodynamically and offers a possible explanation for the improvements in ionic conductivity brought about by Al2O3 additions, as reported in the literature.

Resistivities of Thin Film Transition Metal Silicides

Abstract: Resistivities of the transition metal silicides have been determined. The silicides were formed either by reacting thin metal films with silicon or polysilicon substrates or by sintering cosputtered (metal + silicon) films on silicon, polysilicon, or oxide substrates. The sheet resistance of the film was determined as a function of the sintering temperature, in the range of 300°–1100°C. At the same time, the intermetallic compound formation due to sintering at different temperatures was investigated by the use of x‐ray diffraction technique. It was found that the initial interaction between the metallic film and the substrate was slower for polysilicon substrates compared to silicon substrates; the end product, however, was identical. The resistivity of the refactory metal silicides was lowest for the disilicide and increased with increasing atomic number in a given period of the periodic table. The spread in the resistivities of the 4th period silicides was found to be larger than that of the 6th period silicides. Similarities with the borides, carbides, and nitrides of the same metals are pointed out. On the other hand, for VIII group metal silicides, resistivities were low and no correlation was observed.

Flux-sintered BaTiO3 dielectrics

Abstract: BaTiO3 modified with BaZrO3 or SrZrO3 was sintered at 1100° C with the aid of various fluxes. The objective was to identify compositions in the system CdO-Bi2O3-PbO-B2O3 that would permit growth of the ceramic grains during sintering to produce dielectric constants greater than 5000 at room temperature. By studying binary, and then ternary flux systems initially, the roles of the flux components were identified and classified according to their solubility and mode of substitution in the titanate crystal lattice. This, together with close control of cation stoichiometry, resulted in the discovery of flux systems with other oxides substituting for CdO, Bi2O3 or B2O3 that produced similar results.

Sintering of Mullite-Containing Materials: I, Effect of Composition

Abstract: Sintering behavior of mullite-containing powders was studied over a range of chemical compositions (Al{sub 2}O{sub 3}/SiO{sub 2} ratio). Densification measurements were made for both liquid phase-containing and solid state systems. Small amounts of liquid phase were observed to have a significant effect on densification rate. A linear relationship was obtained between the percent of theoretical density and the logarithm of time for compositions in the range 73-75 wt% Al{sub 2}O{sub 3}. Currently available models for intermediate stage sintering kinetics were considered to be inadequate for these systems. Grain boundary transport 0r diffusion appeared to be the primary mechanism of densification.

Rare-earth-doped yttria-gadolinia ceramic scintillators

Abstract: Rare-earth-doped, polycrystalline yttria-gadolinia ceramic scintillators with high density, optical clarity, uniformity, cubic structure and which are useful in the detection of X-rays, include one or more of the oxides of rare-earth elements Eu, Nd, Yb, Dy, Tb, and Pr as activators. The oxides of elements Zr, Th, and Ta are included as transparency-promoting densifying agents. Any decrease in scintillator light output, due to the addition of transparency promoting additives, may be partially restored by the addition of either calcium oxide (CaO) or strontium oxide (SrO). Sintering, sintering combined with gas hot isostatic pressing, and hot pressing methods for preparing the ceramic scintillators are also described.

Kinetics of grain coarsening during sintering of Co-Cu and Fe-Cu Alloys with low liquid contents

Abstract: The coarsening of grains immersed in varying amount of liquid matrix is investigated in Fe-Cu and Co-Cu alloys at the liquid phase sintering temperatures. Specimens containing 20, 30, and 50 wt pct Cu have been prepared by compacting and sintering mixtures of fine powders. With 50 wt pct of Cu, spherical grains are dispersed in the liquid matrix. With 20 wt pct of Cu, anhedral grains are in contact with the neighbors across grain boundaries or thin liquid films, and the liquid matrix forms continuous prisms along the three grain contacts. The form of the rate law for grain coarsening at all compositions agrees with predictions of the diffusion controlled Ostwald ripening theories of Lifshitz, Slyozov, Wagner, and others. The coarsening rate also increases with decreasing matrix content. The activation energy for grain coarsening does not vary with specimen composition. Therefore, the rate controlling mechanism for coarsening of the anhedral grains in contact with each other appears to be the solution and reprecipitation of solute atoms by diffusion through the liquid matrix.

Process for making sintered glasses and ceramics

Abstract: Glass or ceramic products including glass optical waveguides are produced by a vapor phase oxidation process wherein β-diketonate complexes of selected metals having significant vapor pressures at some temperature below their decomposition temperatures are vaporized, transported to an oxidation site in the vapor phase, and reacted in the vapor phase to form particulate metal oxide soot After capture, this soot can be consolidated by sintering to form eg clear glass of a purity suitable for drawing into glass optical waveguide fiber

Sintering of Mullite-Containing Materials: II, Effect of Agglomeration

Abstract: The sintering behavior of mullite powder compacts which contained soft and hard agglomerates was studied, The maximum density achieved depended on the size and packing of the agglomerates. Although the initial % total pore volume was kept constant, the presence of larger pores in the green compact, due to larger agglomerates, resulted in lower final densities after sintering. Densification rates were enhanced by the breakdown of agglomerates by grinding. The particle and agglomerate packing arrangements caused densification substages to occur. A schematic model is presented which agrees well with the observed experimental behavior.

Sintering behaviour of the diamond-cobalt system at high temperature and pressure

Abstract: A powder mixture of diamond—8.9 vol % Co was consolidatedin situ on a WC-10 wt % Co base at temperatures of 1300 to 1500° C under a pressure of 5.8 GPa. The sintered body obtained at 1300° C, which is below the diamond—cobalt eutectic point, was not hard, and the surface of the diamond particle was partially graphitized. On the other hand, the sintered body obtained at 1400 to 1500° C was fairly hard. A strong correlation was also observed between hardness and the cobalt content found in the sintered body. The cobalt content in the harder sintered body was clearly lower compared with that of the softer one. The surface graphitization of the diamond particles is necessary to the transfer of cobalt during the sintering of diamond. In sintering the diamond-cobalt system, the sinterability of diamond was closely related to the feasibility of transformation from diamond to graphite.

Effects of oxidation and oxidation under load on strength distributions of Si3N4

Abstract: The room-temperature strength distributions of a sintered and a hot-pressed Si3N4 were examined in the as-machined condition, after oxidation at 1370 C and after oxidation under load at 1370 C. The strength-controlling flaw populations were highly transient in nature. Both the duration of oxidation and the magnitude of the applied load were observed to effect changes in strength. This dynamic situation is related to both strengthening and weakening processes, which at times may occur simultaneously in the same strength distribution.

The effect of Bi2O3 on the electrical and mechanical properties of ZrO2-Y2O3 ceramics

Abstract: ZrO2-Y2O3 ceramics with varying Bi2O3 content have been prepared and their microstructure, electrical conductivity and mechanical properties investigated. The sintering rate is strongly increased at low temperatures (1270 to 1420 K) due to the occurrence of a reactive phase the amount of which decreases during the sintering process. The main fluorite phase has an approximately constant composition at 0.85 ZrO2-0.13 Y2O3-0.02 Bi2O3 while the amount of second phase depends on overall composition and sintering procedure. The electrical conductivity is also approximately constant but about a factor of five lower than in ZrO2-Y2O3 ceramics. Typical strength and fracture toughness values are 160 MPa and 1.8 MPa m1/2, respectively. However, the non-reproducibility of the second phase content and morphology seriously influences both mechanical properties.

Volatilization Associated with the Sintering of Polyphase Si3N4 Materials

Abstract: Weight loss which occurs while sintering composited Si3N4 powders requires the loss of one or more of the end-member constituents through a volatilization reaction By plotting the direction of the compositional change on the appropriate equivalence phase diagram, the principal volatilization reaction can be determined For a particular composition in the system Si-Y-N-O sintered at 1750°C, the principal volatilization reaction was Si3N4(s) +3SiO2(s)→6SiO(g) +2N2(g)

Sintered bodies of aluminum nitride

Abstract: Sintered bodies of aluminum nitride There is disclosed a sintered body of aluminum nitride comprising a sintered body of powder mixture containing (a) A?N powder : 100 parts by weight, (b) at least one compound selected from CaO, BaO, SrO and a compound capable of being converted into one of these oxides by sintering : 0.05 to 6 parts by weight, and (c) carbon powder or powder of a compound capable of being converted into carbon by sintering : more than 0 to not more than 7 parts by weight. The sintered bodies of aluminum nitride according to this invention have high density and excellent properties such as high thermal conductivity.

Properties of the Solid Electrolyte Gadolinia‐Doped Ceria Prepared by Thermal Decomposition of Mixed Cerium‐Gadolinium Oxalate

Abstract: Fine-grained powder of the mixed oxide (CeO2)0.9(Gd2O3)0.1, which is an ionic conductor for oxygen ions, was prepared by coprecipitation of the corresponding oxalates followed by calcination. The powder was used to prepare pellets sintered at a relatively low temperature of 1000°C compared with the usual sintering temperature of 1700° to 1800°C. The size of the powder grains was determined from BET surface area (SBET) measurements. The effect of precipitation conditions and calcination temperature on Sbet was examined. The largest surface area measured was 88 m2/g. Decomposition of the oxalate powder was followed using an optical dilatometer. The decomposition was indicated by a large shrinkage and it was completed below 300°C (for a heating rate of 3.3°C/min). The formation of the oxide was verified by X–ray diffraction analysis. It shows that the product of decomposition is the oxide and that decomposition can be carried to completion at 250°C if the heating lasts for 1 h. The pellets had a density of 83% of theoretical, small grains (0.5 μm), and a conductivity which, at 900°C, is two–thirds of the conductivity of dense samples obtained from the same raw material, but calcined and fired at much higher temperatures.

Effects of various additives on sintering of aluminum nitride

Abstract: Effects of thirty additives on sintering A/N were investigated. The addition of alkali earth oxides and rare earth oxides gave fully densified aluminum nitride. This is due to the formation of nitrogen-containing aluminate liquid in the system aluminum nitride-alkali earth oxides or rare earth oxides. Microstructural studies of the sintered specimens with the above two types of additives suggested that the densification was due to the liquid phase sintering. Additions of silicon compounds resulted in poor densification by the formation of highly refractory compounds such as A/N polytypes.

Influence of Interdiffusion on Solid Solution Formation and Sintering in the System BaTiO3-SrTiO3

Abstract: Formation of BaTiO3-SrTiO3 solid solution during sintering of powder mixtures is characterized by preferential diffusion of Ba2+ ions. As a consequence, several nonequilibrium phases are temporarily formed; they were identified by X-ray and microprobe analysis. Eutectic liquid appears below 1300°C, which may explain exaggerated grain growth during sintering of BaTiO3-SrTiO3 mixtures. Disturbance in neck growth and Kirkendall-type porosity hamper densification in the heterogeneous system as compared with the pure titanates.

Ceramic honeycomb catalyst support coated with activated alumina

Abstract: A catalyst support provided with an activated alumina layer which is formed on the surface of the ceramic honeycomb structure is disclosed. The catalyst support of the present invention is formed of ceramic such as cordierite, of which coefficient of thermal expansion is lower than alumina and is provided with microcracks which occur when the honeycomb structure is formed. The activated alumina layer is formed on the surface of the catalyst support without filling the microcracks occurred therein. For example, the activated alumina layer is formed by adhering the activated alumina particulates to the surface of the honeycomb structure and firing the structure together with the activated alumina particulates after the microcracks occurred in the honeycomb structure are filled with such a material as to be burnt away at a temperature lower than the sintering temperature of activated alumina.

Fabrication of Grain-Oriented Bi2WO6 Ceramics

Abstract: Grain-oriented Bi2WO6 ceramics were fabricated by normal sintering techniques. Platelike crystallites were initially synthesized by a fused salt process using an NaCl-KCI melt. When calcined at 850°C, =100 μm. After dissolving away the salt matrix, the Bi2WO6 particles were mixed with an organic binder and tapecast to align the platelike crystallites. Large particles were easily oriented by tapecasting but the sinterability of the tape was poor. Preferred orientation of small particles was increased by tapecasting and grain growth during sintering further improves the degree of orientation. Sintering above the 950°C phase transition, however, results in discontinuous grain growth and low densities. Optimum conditions for obtaining highly oriented ceramics with high density occur at sintering temperatures of 900°C using fine-grained powders which yield orientation factors of =0.88 and densities of 94% theoretical.

Method for sintering high density yttria-gadolinia ceramic scintillators

Abstract: A method for preparing high density yttria-gadolinia ceramic scintillators by cold-pressing multicomponent powder to form powder compacts and then sintering the compacts to form transparent-to-translucent ceramic scintillator bodies. The powder compacts are formed by either die pressing or die pressing followed by isostatic pressing to further increase green density. The powder compacts are sintered in vacuum or a reducing atmosphere at a temperatue of between 1800° C. and 2100° C. The preferred heating sequence includes a holding period at a temperature lower than the final sintering temperature. The finished scintillator includes Y2 O3, Gd2 O3, and one or more of Eu2 O3, Nd2 O3, Yb2 O3, Dy2 O3, Pr2 O3, and Tb2 O3 rare earth activator oxides. At least one of the oxides of elements Zr, Th, and Ta is included as a transparency promoting densifying agent. At least one of CaO and SrO may be included as a light output restorer.