Showing papers on "Sintering published in 1986"
TL;DR: In this paper, the effect of the grain boundary effect on dc ionic conductivity was studied for ceria ceramics doped with trivalent dopants (Y3+, Gd3+, and La3+).
Abstract: The “grain-boundary effect,” which leads to a greatly reduced dc ionic conductivity due to the presence of a blocking layer in the vicinity of the grain boundaries, is studied in detail for ceria ceramics doped with various trivalent dopants (particularly Y3+, Gd3+, and La3+). The effects of porosity, of sintering time, and of dopant size and dopant concentration are investigated. Finally, it is shown that the grain-boundary effect virtually disappears when nearly silicon-free starting materials are used.
288 citations
TL;DR: In this article, the effect of MgO as a solid-solution additive in the sintering of Al2O3 was studied and the separate effects of the additive on densification and grain growth were assessed.
Abstract: The effect of MgO as a solid-solution additive in the sintering of Al2O3 was studied. The separate effects of the additive on densification and grain growth were assessed. Magnesia was found to increase the densification rate during sintering by a factor of 3 through a raising of the diffusion rate. The grain-size dependence of the densification rate indicated control primarily by grain-boundary diffusion. Magnesia also increased the grain growth rate during sintering by a factor of 2.5. The dependence of the grain growth rate on density and grain size suggested a mechanism of surface-diffusion-controlled pore drag. It was argued, therefore, that MgO enhanced grain growth by raising the surface diffusion coefficient. The effect of MgO on the densification rate/grain growth rate ratio was, therefore, found to be minimal and, consequently, MgO did not have a significant effect on the grain size/density trajectory during sintering. The role of MgO in the sintering of alumina was attributed mainly to its ability to lower the grain-boundary mobility.
194 citations
01 Jan 1986
Abstract: Processing and Fabrication of Multiphase Ceramics.- Sintering of Multiphase Ceramics.- The Morphological Stability of Continuous Intergranular Phases.- Role of Shear in the Sintering of Composites.- HIP of Liquid Phase Sintered Ceramic Composites.- Sinterable Yttria-Doped Zirconia Powders Chemically Coprecipitated in Non-Aqueous Medium.- Microstructural and Chemical Aspects of a Strontia Sintering Aid on Mg-PSZ.- Phase Relationships in Y-Si-A1-O-N Ceramics.- The Fabrication of Composite O'-B' Sialon Ceramics.- Ceramic Eutectic Composites.- Nickel Oxide-Based Aligned Eutectics.- Imperfections in the Directionally Solidified Structure of NiO-CaO Eutectic.- CVD-Processing of Ceramic-Ceramic Composite Materials.- CVD Fabrication of In-situ Composites of Non-oxide Ceramics.- Preparation of Boron Nitride/Boron Carbide Ceramics by Pyrolysis of Boric Acid-Glycerin Condensation Product.- Porous and Dense Composites for Sol-Gel.- Non-Equilibrium Surface Conditions and Microstructural Changes Following Pulsed Laser Irradiation and Ion Beam Mixing of Ni Overlayers on Sintered Alpha-SiC.- Structure-Property Relations in Multiphase Ceramics.- Quantitative Microstructural Characterization and Description of Multiphase Ceramics.- Displacive Transformation Mechanisms in Zirconia Ceramics and Other Non-Metals.- On Precipitate Morphology in ZrO2 ?-Al2O3, FeTiO3 Matrices.- Particle Toughening in Partially Stabilized Zirconia Influence of Thermal History.- Fabrication and Properties of Transformation-Toughened Sodium Beta"-Alumina.- Phase Transformation and Toughening in MgO Dispersed with ZrO2.- Effect of Impurities on Microstructure and Mechanical Properties of Si3N4-TiC Composites.- On the Microstructure and Hardness Characteristics of Composite Ceramics for Tool Applications.- Mechanical Properties and Wear Resistance of Silicon Nitride-Titanium Carbide Composites.- The Structure and Properties of Interfaces in Reaction-Bonded Silicon Carbides.- Some Factors Affecting Mechanical and Microstructural Anisotropy in Reaction-Bonded Silicon Carbides.- Microstructure-Mechanical Property Relationships in 94% Alumina Ceramics.- New Low Expansion Magnetic Materials-A Composite Approach.- Multiphase Electroceramics.- Electroceramic Composites.- Finite Element/Difference Modeling of Electroceramics.- Multiphase Interaction for Seeking Exotic Phenomena.- Processing of Heterogeneous Ceramics for Dielectric Applications.- Dielectric and Electrical Properties of BaTiO3 Composites.- Composite Piezoelectric Sensors.- Wave Absorption in Piezoceramic-Polymer Composites.- Structured Macrovoids in Ceramic PZT.- Composite Thermistors.- Grain Resistivity and Conduction in Metal Oxide Varistors.- Influence of Microstructure and Chemistry on the Electrical Characteristic of ZnO Varistors.- Influence of Chemical Composition on the Barrier Height in Zn Varistors.- Polycrystalline H3O+-?/?" Alumina: A Designed Composite for Steam Electrolysis.- Fiber and Whisker Reinforced Composites.- The Development of Fiber Reinforced Glasses and Glass Ceramics.- Interfacial Characterization of Glass and Glass-Ceramic Matrix/Nicalon SiC Fiber Composites.- Fatigue Behavior of Silicon Carbide Fiber Reinforced Lithium-Alumino-Silicate Glass-Ceramics.- Fiber Reinforced Composites Via the Sol/Gel Route.- Fiber-Matrix Interactions in Carbon Fiber/Cement Matrix Composites.- Morphological and Mechanical Characterization of Ceramic Composite Materials.- Toughness Anisotropy of a SiC/SiC Laminar Composite.- Whisker Reinforced Ceramic Composites.- Whisker-Reinforced Zirconia-Toughened Ceramics.- Si3N4-SiC Whisker Composite Material.- Mechanical Properties of SiC Fiber-Reinforced Reaction-Bonded Si3N4 Composites.- Surface Energy as an Indicator of Interfacial Mechanical Response.- High Temperature Multiphase Ceramics.- Prospects for Ultra-High-Temperature Ceramic Composites.- Microstructural Engineering of Ceramics for High-Temperature Application.- Tailoring of the Thermal Transport Properties and Thermal Shock Resistance of Structural Ceramics.- Creep Rupture of Siliconized Silicon Carbide.- High Temperature Mechanical Properties of Siliconized Silicon Carbide Composites.- Contributors.
192 citations
TL;DR: In this article, a tritagonal zirconia polycrystals were produced from high purity powders containing 1.5 to 5.0 mol% Y/sub 2/O/sub 3/ by cold isostatic pressing and sintering.
Abstract: Tetragonal zirconia polycrystals were produced from high purity powders containing 1.5 to 5.0 mol% Y/sub 2/O/sub 3/ by cold isostatic pressing and sintering, hot-pressing, and hot isostatic pressing. The mechanical properties and microstructures of these resulting materials were examined, with emphasis on the relation between strength and fracture toughness.
171 citations
TL;DR: In this article, the effect of small uniaxial stresses on the sintering of CdO powder compacts was studied using a loading dilatometer, where compacts of two different green densities were sintered at 1123 K and subjected to stresses between 0 and 0.25 MPa.
Abstract: The effect of small uniaxial stresses on the sintering of CdO powder compacts was studied using a loading dilatometer. Compacts of two different green densities were sintered at 1123 K and subjected to stresses between 0 and 0.25 MPa. Densification and creep occur simultaneously, and the effects of these two processes can be separated. Between relative densities of 0.5 and 0.9, the dependence of the uniaxial creep rate on density can be described in terms of a stress intensification factor which depends exponentially on the porosity but is independent of the grain size. Comparison of the densification and creep rates permits definition of the sintering stress, which is found to decrease with increasing density, and verification of the Zener relation. The stress and grain size dependence of the creep rate, and the grain size dependence of the densification rate, support grain-boundary diffusion as the rate-controlling step in both processes.
157 citations
TL;DR: In this article, effective sintering aids for SiC ceramics are discussed on the basis of their reactivities with various sinterings aids around sinter-ing temperatures (2300 to 2400 K).
Abstract: Effective sintering aids for SiC ceramics are discussed on the basis of their reactivities with various sintering aids around sintering temperatures (2300 to 2400 K). The free energy consideration of the reactions suggests that metals and metal oxides which do not decompose SiC in the sintering process are effective as sintering aids for SiC ceramics.
152 citations
TL;DR: In this paper, a method is described and used to evaluate the transient stresses in a sintering compact of ZnO containing a hard, dense dispersion of SiC. The effect of these stresses on microstructural development is considered.
146 citations
TL;DR: In this article, the authors used transmission electron microscopy to investigate platinum particle sintering in specimens of a Pt Al 2 O 3 catalyst heat-treated under oxidising conditions.
91 citations
TL;DR: The reaction of Dy2O3 or Tb4O7 with Nd15Fe77B8 powder during the sintering increases the coercivity of the sintered magnets as discussed by the authors.
Abstract: The reaction of Dy2O3 or Tb4O7 with Nd15Fe77B8 powder during the sintering increases the coercivity of the sintered magnets. These oxides are reduced to metal during the sintering process, resulting in magnets with properties similar to those prepared from the Nd13.5Dy1.5Fe77B8 alloy.
87 citations
TL;DR: In this article, a method for producing uniform mixtures of silicon nitride powders and additive precipitates was formulated and used to produce green compacts of improved compositional homogeneity.
Abstract: Interactions between silicon nitride particles and hydroxide precipitates were investigated using electrophoresis measurements Conditions under which stable suspensions of silicon nitride particles and flocculation and heteroflocculation of silicon nitride/hydroxide mixtures occur were Identified On the basis of the observations, a method for producing uniform mixtures of silicon nitride powders and additive precipitates was formulated and used to produce green compacts of improved compositional homogeneity The effect of the mixing process on the sintering of green silicon nitride compacts was investigated and compared to the sintering behavior of conventionally prepared green compacts The results show that the improved homogeneity obtained using the precipitation mixing process leads to enhanced sintering of the green compacts
84 citations
TL;DR: In this article, the activation energies of viscous flow, crystal growth, and sintering behavior of a standard NBS 710 glass and two crystallizable glass powders in the calciaaluminosilicate system were investigated.
Abstract: Viscometry, crystal growth kinetics, and glass powder sintering were used to determine the activation energies of viscous flow, crystal growth, and sintering behavior of a standard NBS 710 glass and two crystallizable glass powders in the calciaaluminosilicate system. In a system which sintered successfully to zero porosity, the activation energy for sintering was comparable to the activation energy of viscous flow. When the apparent activation energy for the sintering process was lower than that of viscous flow, sintering was found to proceed at a slower rate because the precipitated crystalline phase retarded viscous flow and the shrinkage of the pores. Initial crystallization appeared to occur on the surfaces of coalesced particles. A chemical treatment was partially successful in suppressing the onset of surface nucleation of the precursor powder.
Patent•
25 Jul 1986
TL;DR: In this article, a low-density boron carbide-aluminum composites with a homogeneous microstructure possessing desired mechanical properties are produced. But the composites do not have the same properties as the high-density composites.
Abstract: Low-density composites are produced consisting chiefly of boron carbide and aluminum, or aluminum alloy, and minor amounts of ceramic material. The method allows control of the rate of reaction between boron carbide and metal so that the final components of the composite, and hence the mechanical properties, are controlled. The invention includes modification of the carbon content of the boron carbide composition, dispersion of boron carbide and formation of a compact, infiltration of the compact by aluminum or aluminum alloy, and heat treatments. The invention produces low-density boron carbide-aluminum composites with a homogeneous microstructure possessing desired mechanical properties.
TL;DR: In this paper, the pore surface area (Sp) and grain boundary area (Sgb) were measured during sintering of undoped and doped (100 ppm MgO) alumina compacts.
Abstract: The pore surface area (Sp) and grain-boundary area (Sgb) were measured during the sintering of undoped and doped (100 ppm MgO) alumina compacts. Since the presence of the additive affects only Sgb (raising it at a given value of the density), pinning of the boundaries by solid-solution drag is the only additive function evidenced by the results. The importance of such pinning even at densities as low as 75 percent of theoretical is linked to the existence of microstructural inhomogeneities.
TL;DR: In this paper, a transmission electron microscopy study of sintered Nd•Fe•B magnets after annealing above 650°C shows the formation of a thin intergranular Nd−rich layer extending to pockets of polycrystalline fcc Nd crystals.
Abstract: A transmission electron microscopy study of sintered Nd‐Fe‐B magnets after annealing above 650 °C shows the formation of a thin intergranular Nd‐rich layer extending to pockets of polycrystalline fcc Nd crystals The grain boundary phase acts as a pinning site for the magnetic domain walls and is necessary for good magnetic hardening On the other hand, the Nd1+eFe4B4 phase is an unavoidable product of the sintering process but need not be present for good coercivity
TL;DR: In this paper, it was shown that pure zirconia fired in HCl shows greatly reduced shrinkage compared to the same material fired in air, and that the grain growth does not follow any simple growth law over the temperature range investigated, and the grains grow up to a limiting particle size close to the initial agglomerate size.
Abstract: Pure zirconia fired in HCl shows greatly reduced shrinkage compared to the same material fired in air. One of the main microstructural differences is the much more rapid grain growth in the porous zirconia fired in HCl. The observed grain growth does not follow any simple growth law over the temperature range investigated. The grains grow up to a limiting particle size close to the initial agglomerate size and appear to stop.
TL;DR: In this paper, the shrinkage rate of large pores within a fine-grained body has been derived based on a shrinkage potential derived from the surface and grain-boundary tensions and viscous response determined by the creep characteristics of the polycrystal.
Abstract: A viscoelastic model that describes the shrinkage rate of large pores within a fine-grained body has been developed. The concept is based on a shrinkage potential derived from the surface and grain-boundary tensions and viscous response determined by the creep characteristics of the polycrystal. Pore removal rates are also derived and used to predict pore removal times during sintering and hot isostatic pressing. I. Introduction HE mechanical properties of many sintered ceramics are limT ited by the presence of large pores. ’ Such pores occur because of entrapped organics, rearrangement, local melting, etc. Removal of large pores by either prolonged sintering or hot isostatic pressing should thus improve the mechanical performance of the material, provided that appreciable grain growth is suppressed. This paper provides an analysis of pore removal rates pertinent to this phenomenon. When the pores are large compared with the grain diameter, pore removal can be analyzed by regarding the polycrystal as a continuum subject to viscoelastic deformation. The removal rate is thus dictated by the stresses created by the surface and grainboundary tensions and the viscosity of the polycrystal. The analysis has some similarity with the analysis of shrinkage in amorphous solids, previously conducted by Mackenzie and Shuttleworth.’ However, it is noted that, when the pores reduce in size to be of the order of the grain size, the continuum analysis is inadequate and consideration of local diffusive fluxes into the pores is needed to deduce the shrinkage rate.3 The viscoelastic properties of ceramic polycrystals have been extensively characterized, by means of creep studies4 Such studies provide a basis for describing matrix deformation rates during sintering. For present purposes, linear behavior is assumed, with the viscosity, 7, given by5
TL;DR: In this article, the sintering behavior of amorphous and semicrystalline polymeric powders was studied and the dominating factor in their coalescence, unaccounted for by the Frankel expression, is the internal particle morphology which increases the total particle surface energy.
Abstract: The sintering behaviour of several amorphous and semicrystalline polymeric powders was studied. The coalescence of particles of polystyrene, PMMA and polyethylene of various molecular weights was photographically followed. The coalescence of the amorphous materials was found to depend on the common parameters affecting sintering including particle size and polymer viscosity. However, unexpectedly fast sintering was observed for the crystalline polyethylenes. The dominating factor in their coalescence, unaccounted for by the Frankel expression, is the internal particle morphology which increases the total particle surface energy. The polyethylene particles were actually found to be aggregates of small nodules, less than 1μm in diameter, interconnected by a very fine fibrillar network. It is concluded that some semicrystalline high viscosity polymers, known to be unprocessable by common methods, do sinter due to their highly developed internal particle morphology.
TL;DR: In this article, a theory for sintering of bimodal powders is applied to a composite where the matrix is either a glass or a ceramic, and the filler phase is assumed to be rigid and nonsintering.
Abstract: A recently developed theory for sintering of bimodal powders is applied to a composite where the matrix is either a glass or a ceramic. The filler phase is assumed to be rigid and nonsintering. It is shown that glass-matrix composites should have predictable sintering behavior which follows the rule of mixtures, but ceramic-matrix composites are expected to be more difficult to sinter, and their sintering behavior may deviate considerably from the rule of mixtures, even at low concentrations of the filler phase.
Journal Article•
TL;DR: In this article, the dielectric properties of PbMg/sub 1/3/Nb/sub 2/3//O/sup 3/-type ceramics can be improved both by using a preferred sintering cycle, which prevents the formation of a pyrochlore layer at the surface of the pellets, and by adding the correct amount of excess PbO.
Abstract: Dielectric properties of PbMg/sub 1/3//Nb/sub 2/3//O/sup 3/-type ceramics can be improved both by using a preferred sintering cycle, which prevents the formation of a pyrochlore layer at the surface of the pellets, and by adding the correct amount of excess PbO, which results in dense ceramics with a nearly pure perovskite phase at a low sintering temperature (900/sup 0/C).
Patent•
24 Apr 1986TL;DR: Injection molding a kneaded mixture of a metal or alloy powder and an organic binder comprising a special methylcellulose, a plasticizer, a lubricant and water was used in this paper.
Abstract: Metal or alloy articles having complicated shapes can be produced with high precision by injection molding a kneaded mixture of a metal or alloy powder and an organic binder comprising a special methylcellulose, a plasticizer, a lubricant and water, followed by removal of the organic binder and sintering.
Patent•
26 Aug 1986TL;DR: A sintered zirconia material containing at least 50 mol % tetragonal zirconsia and 1.5-5 mol % yttria, from which carbon is substantially absent, has high strength and high toughness as mentioned in this paper.
Abstract: A sintered zirconia material containing at least 50 mol % tetragonal zirconia and 1.5-5 mol % yttria, from which carbon is substantially absent, the porosity of which is not greater than 0.6%, the pore size of which is not greater than 0.1 μm and the pores of which exist mainly at triple points of zirconia grain boundaries. The sintered zirconia material can be obtained by sintering the raw material zirconia powder, the green body or the presintered material containing tetragonal zirconia and yttria in a required amount and in an oxidizing atmosphere, by hot isostatic pressing. The sintered material thus obtained has a high strength and high toughness, and particularly excellent properties for use under conditions of higher than 600° C. and can be used as a material of various industrial machines or tools.
Patent•
01 Aug 1986
TL;DR: Ceramic oxide diphasic xerogels of structurally dissimilar phases can be converted to crystalline ceramic oxide products having enhanced densification when subjected to substantially lower sintering temperature as discussed by the authors.
Abstract: Ceramic oxide diphasic xerogels of structurally dissimilar phases (crystalline and amorphous, semicrystalline or noncrystalline) but compositionally similar phases will be converted to crystalline ceramic oxide products having enhanced densification when subjected to substantially lower sintering temperature.
TL;DR: In this paper, the effects of several processing variables, such as green density, particle size, composition, heating rate, sintering temperature, and sinter time, were examined for Fe-Ti powder mixtures.
Abstract: Transient liquid phase sintering was examined for Fe-Ti powder mixtures. The experimental plan included the effects of several processing variables, such as green density, particle size, composition, heating rate, sintering temperature, and sintering time. During heating, pores form at the Ti particle sites. At the first eutectic temperature (1085 °C), liquid spreading is inhibited by a surrounding intermetallic envelope, leading to swelling. At the second eutectic temperature (1289 °C), the liquid penetrates along the iron grain boundaries and provides densification. The amount of densification depends on the amount of liquid formed at the second eutectic temperature and its duration as determined by the titanium content and heating rate.
TL;DR: In this article, the sintering rate of a powder compact containing heterogeneities was calculated and the results reveal a strong retardation of sinteration in the presence of nonsinterable agglomerates because of the development of a hydrostatic tensile stress.
Abstract: The sintering rate of a powder compact containing heterogeneities was calculated. The calculations reveal a strong retardation of sintering in the presence of nonsinterable agglomerates because of the development of a hydrostatic tensile stress. The variation in sintering rate with volume fraction is shown to be highly nonlinear, such that appreciable effects obtain even for 1% agglomerated powder. Specific results are calculated using constitutive laws pertinent to Al2O3.
TL;DR: In this paper, the aging of partially stabilized zirconia (Y-PSZ) materials with 2·5−5·0 mol% yttrium oxide was investigated and it was shown that the materials showed little degradation in strength after 2000 h of aging at 200°C with no changes in monoclinic/tetragonal ratio and density, when the grain size of the sintered material was less than 0·5 μm and the density was greater than 6·07 Mg m−3.
TL;DR: Sintering of UO2 under oxidative conditions enables more easily than conventional sintering the control of the density, of the open porosity and of the grain structure as discussed by the authors.
TL;DR: In this paper, the effect of solid solution of Al2O3 with hematite on strain of magnetite was estimated by line-broadenings of peaks of X-ray diffraction.
Abstract: Direct observation of lattice images by TEM of magnetite reduced from hematite made clear ultramicroscopic partial distortion due to solid solution of Al2O3. Furthermore, the effect of solid solution of Al2O3 with hematite on strain of magnetite, which had reduced grains of the hematite with a mixture gas of CO-CO2, was estimated by line-broadenings of peaks of X-ray diffraction. It revealed that increasing amounts of Al2O3 increased the strain of magnetite.Fracture strength of the minerals in sintered ore was estimated quantitatively by Vickers indentation, indicating crack propagation-arrest characteristics. Fracture toughness of both glassy silicate and calcium ferrite resulted in values of the same order with a little effect of Al2O3. Calcium ferrite, however, had the lowest value for “critical load”, indicating crack initiation characteristic among all the minerals tested.An experimental study of the crystallization mechanism of skeletal hematite indicated that magnetite coexisting with liquid silicate decomposed, melted and was oxidized at the falling stage of sintering.
TL;DR: In this article, the added SrO effectively neutralizes the detrimental effect of SiO/sub 2/ contaminant by forming a glass phase which is effected from the bulk of the ceramic during sintering.
Abstract: Improvements in the mechanical properties of magnesia-partially-stabilized zirconia are obtained by the addition of SrO. Evidence is presented which indicates that the added SrO effectively neutralizes the detrimental effect of SiO/sub 2/ contaminant by forming a glass phase which is effected from the bulk of the ceramic during sintering. this combined effect results in the retardation of the subeutectoid decomposition reaction while minimizing retention of glass phases at the grain boundaries.
TL;DR: In this paper, high-field ZnO varistors were made by sintering uniaxially pressed pellets in the range of 675°-740 °C in air.
Abstract: Chemical preparation methods were developed for high-field ZnO varistors which used precipitation techniques to prepare precursor powders. Varistors were made by sintering uniaxially pressed pellets in the range of 675°–740 °C in air. Properties of these varistors included electric fields (E) in the 10–100 kV/cm range at current densities (J) of 5 A/cm2, nonlinearity coefficients (α) greater than 30 for 2.5≤J≤5.O A/cm2, and densities in the range of 65%-99% of theoretical depending both on sintering temperature and composition.
TL;DR: In this article, the technique of zone sintering was applied to three BaTiO3 source materials; studies were made in air, oxygen and vacuum, and the optimum firing conditions suggested for the zone Sintering of these materials were a temperature of 1450° C and a time in the hot zone of 5 min.
Abstract: The technique of zone sintering was applied to three BaTiO3 source materials; studies were made in air, oxygen and vacuum The optimum firing conditions suggested for the zone sintering of these materials were a temperature of 1450° C and a time in the hot zone of 5 min The temperature dependence of the dielectric constant could be closely related to the firing conditions and material microstructure Lower room-temperature dielectric constants and lower Curie temperatures were found as the holding time in the hot zone was increased, yielding larger grained structures