Showing papers on "Sintering published in 1997"
TL;DR: In this article, a self-consistent, single-valued representation of the major physical, mechanical, and thermal properties of a sintered α-SiC is presented by focusing on a narrowly defined material specification in which boron and carbon are used as sintering aids to produce a dense ceramic (⩾98% of the theoretical maximum density) with a grain size of (6±2) μm.
Abstract: A self-consistent, single-valued representation of the major physical, mechanical, and thermal properties of a sintered α-SiC is presented. This comprehensive set of properties is achieved by focusing on a narrowly defined material specification in which boron and carbon are used as sintering aids to produce a dense ceramic (⩾98% of the theoretical maximum density) with a grain size of (6±2) μm. Such a representation is highly desirable in applications of concurrent engineering practices and for the increasing use of electronic processing of product specifications.
377 citations
TL;DR: In this article, the post-sintering density and the permeability of the sintered ferrite are strongly affected by the size of the starting oxide powders and the pre-sinting temperature, and the most effective method of preparing high permeability ferrite is to utilize fine particles of iron oxide and to calcine at about 800°C.
293 citations
TL;DR: In this paper, XRD and TEM were used to measure the particle sizes of the various phases of the CeO 2 -Zr oxide solid solution and found that the particle size of the supported particulate phase decreased with increasing zirconium loading, but a discrepancy was noted between the XRD results and the TEM results.
241 citations
TL;DR: Several sintering additives for hydroxyapatite (HA) have been tested in order to enhance its sinterability without decomposing the HA and/or decreasing bioactivity and biocompatibility, additionally providing a weak interface for HA ceramics.
192 citations
TL;DR: In this paper, the authors investigated the interface microstructure of the state-of-the-art cathode material for solid oxide fuel cells, SrxLa1-xMnO3 (SLM), with respect to its electrochemical performance.
Abstract: The interface microstructure of the state-of-the-art cathode material for solid oxide fuel cells, SrxLa1–xMnO3 (SLM), was investigated with respect to its electrochemical performance. The interface microstructure was characterized by grain size and coverage of SLM on the electrolyte surface. Variation of the grain size was obtained by using three different sintering temperatures, whereas variation of the coverage was obtained by using two powders with a different morphology. This resulted in a set of six cathode/electrolyte samples with different combinations of grain size and SLM coverage at the interface. The cathode overpotential, as a measure for the electrochemical performance, could not be related to the length of the three-phase boundary. Based on the constriction resistance occurring in the electrolyte a model was developed which provides an estimate for the width of the active three-phase boundary zone.This zone is most likely to extend outside the cathode particle across the zirconia surface. The width calculated in this way was found to vary in the range of 0.03 to 0.07 µm for the different electrode microstructures. It is argued that the actual values may be smaller by one or two orders of magnitude.
180 citations
TL;DR: In this paper, a new sintering mechanism based on particle repacking concurrent with particle coarsening was proposed, resulting in a higher packing factor for very fine, surface-active powders that coarsen rapidly.
Abstract: Conventional and new sintering mechanisms have been investigated using fine powders of CeO 2 and Y 2 O 3 of excellent sinterability. We have verified the validity of Herring's scaling law for 60%-84% relative density and found that it is consistent with grain-boundary-diffusion control. At lower densities, we have found that pores larger than the critical size, in the sense of Kingery and Francois, can still be sintered readily. This is rationalized by a new sintering mechanism based on particle repacking concurrent with particle coarsening, resulting in a higher packing factor. Very fine, surface-active powders that coarsen rapidly are uniquely capable of taking advantage of this new sintering mechanism, which along with their propensity to homogenization, accounts for their remarkable sinterability even at very low green densities.
171 citations
TL;DR: In this article, the structure, thermal expansion and ionic conductivity of solid electrolytes based on samarium doped cerium oxide, Ce1−xSmxO2−x2 (x = 0−0.30), prepared by the sol-gel method were systematically investigated in a wide range of temperature of 200-650 °C.
162 citations
TL;DR: In this paper, the authors used pressureless sintering, hot pressing, and hot isostatic pressing (HIP) to obtain a fracture toughness of 1.4-2.0 MPam1/2.
Abstract: Hydroxyapatite/0%-30% hydroxyapatite-whisker (HAp/0%-30%HAp(w)) composites have been fabricated by pressureless sintering, hot pressing, and hot isostatic pressing (HIP). Composites that were HIPed at 1000°-1100°C (2 h, 190 MPa) exhibited the best properties: relative densities of 97.0-99.5%, fracture toughness of 1.4-2.0 MPa·m1/2 (as compared with 1.0 MPam1/2 for the nonreinforced HAp matrix). Compressive pre-stressing and crack deflection contributed mostly to the increase of fracture toughness. Depending on processing conditions, grain growth in the HAp matrix and/or Rayleigh instability of the HAp whiskers were probably responsible for microstructural changes in the composites. The HAp/HAp(w) composites exhibited improved toughness, attaining the lower fracture-toughness limit of bone without a decrease of bioactivity and biocompatibility.
143 citations
TL;DR: In this article, a model for the sintering densification of prealloyed particles that form internal liquids when heated over the solidus temperature is derived, which considers the powder size, composition, and microstructure, as well as the processing conditions of green density, heating rate, maximum temperature, hold time, and atmosphere.
Abstract: A model is derived for the sintering densification of prealloyed particles that form internal liquids when heated over the solidus temperature. The model considers the powder size, composition, and microstructure, as well as the processing conditions of green density, heating rate, maximum temperature, hold time, and atmosphere. Internal liquid forms and spreads to create an interparticle capillary bond that induces densification during sintering. Densification is delayed until the particles achieve a mushy state due to grain boundary wetting by the internal liquid. This loss of rigidity and concomitant densification of the semisolid particles depends on the grain size and liquid quantity. Viscous flow is the assumed densification mechanism, where both viscosity and yield strength vary with the liquid content and particle microstructure. Densification predictions are compared to experimental data, giving agreement with previously reported rapid changes in sintered density over narrow temperature ranges. The model is tested using data from steels and tool steels of varying carbon contents, as well as boron-doped stainless steel, bronze, and two nickel-based alloys.
136 citations
TL;DR: The substrate is a site for heterogeneous nucleation, and crystallization makes densification more difficult, so the competition between sintering and crystallisation is particularly important for films as discussed by the authors.
Abstract: The sintering of films differs from that of bulk gels in several ways. The initial state of a film is generally denser and less crosslinked than a bulk gel made from the same sol, and these factors enhance the densification rate of the film. The substrate constrains the shrinkage of the film, leading to high stresses that retard densification and can influence phase changes. The substrate is a site for heterogeneous nucleation, and crystallization makes densification more difficult, so the competition between sintering and crystallization is particularly important for films. Fast heating favors densification over crystallization, so rapid thermal annealing usually produces denser films. The high surface to volume ratio of a thin film makes it susceptible to degradation by reaction with the substrate and the atmosphere, so choosing compatible materials and avoiding over-firing is essential.
133 citations
TL;DR: In this paper, the microwave and conventional heating methods were used to synthesize barium titanate, which showed advantages of shortened crystallization time, increased c/a ratio, and upon sintering led to a larger degree of increased c /a ratio.
Abstract: Hydrothermal synthesis of barium titanate was done by both microwave heating and conventional heating. The synthesis process was evaluated by examining samples at different processing times by X-ray diffraction. For samples produced by both heating methods, the metastable cubic phase crystallized first and gradually transformed to a tetragonal phase. Microwave synthesis showed advantages of shortened crystallization time, increased c/a ratio, and upon sintering led to a larger degree of increased c/a ratio. Elemental analysis showed a smaller Ti/Ba ratio and larger relative amount of Cl- ion in the microwave-treated sample than those in a conventionally treated sample. Sintering at 850 °C for both samples resulted in the samples exhibiting a clear transition temperature of around 125 °C as measured by DSC.
TL;DR: In this paper, the amount of Yb2O3 was used as a sintering aid to enhance the mechanical properties of silicon nitride (Si3N4) ceramics.
Abstract: Ytterbium oxide (Yb2O3) was used as a sintering aid to enhance the mechanical properties of silicon nitride (Si3N4) ceramics. The amount of Yb2O3 had significant effects on microstructural evolution and the composition of secondary phases at the grain boundary. When the Yb2O3 added was less than 8 wt%, small homogeneous grains were formed. At the grain boundary, crystalline Yb2Si2O7 was formed along with a glassy phase. As the amounts of Yb2O3 were increased to higher than 8 wt%, large elongated grains were developed in the fine matrix. In those cases, the grain boundary crystalline phase was changed from Yb2Si2O7 to Yb4Si2O7N2. Mechanical properties were influenced by these changes in microstructure and grain boundary phase. The fracture toughness increased with the Yb2O3 content up to 8 wt% and decreased slightly thereafter. The increase in fracture toughness was apparently due to the formation of the large elongated grains. When more than 8 wt% of Yb2O3 was added, interfacial debonding energy between the elongated grains and grain boundary phase became too large, resulting in a decrease in the fracture toughness. The room-temperature flexural strength was not significantly affected by the Yb2O3 content or the microstructure, other than in the case of 2 wt% addition. The high-temperature strength in nitrogen, however, increased steadily with Yb2O3 content. The highest strength, 870 MPa at 1400°C, was observed when 16 wt% of Yb2O3 was added. The increase in the high-temperature strength with Yb2O3 content was attributed to the formation of crystalline Yb4Si2O7N2 phase at the grain boundary.
TL;DR: In this article, a model to predict the densification and grain growth rates of n-TiO2 during high pressure/low temperature sintering has been developed and validated by experiments.
TL;DR: In this article, the Ni0.65Zn0.35Fe2O4 resistivity and dielectric properties of the nickel-zinc ferrite composition were studied near different sintering conditions to optimize the Sintering schedule for a specific application.
Abstract: Resistivity and dielectric properties of the nickel-zinc ferrite composition, Ni0.65Zn0.35Fe2O4, have been studied near different sintering conditions to optimize the sintering schedule for a specific application. Variable sintering parameters in the study include sintering temperature in the range from 1150 to 1300°C and sintering time with 1 h, 2 h and 4 h durations at each sintering temperature. The results indicate that the decrease of resistivity is linear and less rapid upto a sintering schedule of 1250°C/2 h, and thereafter any increase either in sintering temperature or in sintering time causes a rapid decrease of resistivity due to the increased volatilization of zinc from the samples at these temperatures/times. Possible mechanisms contributing to conduction processes in ferrites are discussed in explaining the obtained results.
TL;DR: In this paper, the sinterability of nano-size CeO2 compacts was investigated by continuous monitoring of the shrinkage kinetics, and microstructural features of the sintered specimens were observed by SEM.
TL;DR: In this article, a novel Ni-YSZ cermet anode for solid oxide fuel cell application has been developed in order to solve problems with the previous anode, which was prepared from a powder mixture of NiO and precalcined YSZ.
Abstract: A novel Ni-YSZ cermet anode for solid oxide fuel cell application has been developed in order to solve problems with the previous anode, which was prepared from a powder mixture of NiO and precalcined YSZ. In contrast to homogeneously sized YSZ in the previous anode, the new material has a YSZ frame made of coarse (average size of 27.0 {micro}m) and fine (average size of 0.6 {micro}m) powders to sustain the network of Ni and pore. Unlike the previous anode, no significant changes in shrinkage and microstructure of the new anodes were detected during sintering in air and upon transformation of NiO to Ni metal in a reducing atmosphere. The influence of the content of coarse YSZ powder was examined by fixing the Ni volume ratio at 40%. It has been found that with an increasing amount of coarse YSZ powder, the electrical conductivity increased by three orders of magnitude around 20 weight percent (w/o) coarse YSZ powder and also that the anode with an 80 w/o one was found to be the most stable during cell fabrication and cell operation. It is concluded that the consolidated framework made from two distinctly sized particles leads to a more stable YSZmore » frame and Ni network.« less
TL;DR: The effect of milling atmosphere on the end products was also studied in this article, where the mixtures 2Mg+Fe and Mg+Co were milled in a planetary ball mill under hydrogen and argon atmospheres.
TL;DR: In this paper, high density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4.5 GPa, at room temperature.
Abstract: High density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4.5 GPa, at room temperature. The starting material was used three days after synthesis by hydrolysis of alkoxides without additional treatment. Fourier transform infrared spectroscopy (FTIR), using the KBr technique and a high vacuum cell at temperatures up to 450°C, and thermogravimetric analysis (TGA) up to 900°C was conducted. A reduction up to 60% in the adsorbed water content of the compacted silica gel was observed. Changes in the 3000 to 3800 cm−1 region indicate that the surface silanol groups became bridged, which promotes condensation reactions of dehydroxylation. Those results suggest that the mechanism for consolidation under high-pressure, is ‘cold sintering’ process, where silanol groups at the surface of the nanoparticles condense to form siloxane bonds between the particles and water, resulting in a stiff body with closed nanopores containing trapped water.
Patent•
06 Oct 1997TL;DR: The composite oxide and the composite oxide carrier are composed of a composite oxide in which at least one of cerium and zirconium, and aluminium disperse with extremely high homogeneity as discussed by the authors.
Abstract: The composite oxide and the composite oxide carrier are manufactured by the precursor forming step and firing step. The precursor forming step includes high speed mixing means. The composite oxide catalyst is obtained by preparing a composite of catalytic components simultaneously with the formation of the precursor of composite oxide in the step of forming the precursor of composite oxide. The composite oxide and the composite oxide carrier are composed of a composite oxide in which at least one of cerium and zirconium, and aluminium disperse with extremely high homogeneity. With this structure, the heat resistance of the carrier is improved and consequently, enlargement of particles of the composite oxide defining the carrier, and sintering of adjacent particles of the composite oxide can be restrained, whereby the catalyst using the composite oxide carrier in accordance with the present invention is excellent in heat resistance. With the present invention, the carrier is not limited to a general catalyst carrier. The carrier may be interpreted to indicate general formed bodies. For example, the carrier with the present invention can be also used as materials for sensors and electrodes, optical materials, semiconductors and structure materials. Furthermore, the carrier can be used for a three-way catalyst, NOx catalyst or oxidation catalyst or a part thereof, and a promoter.
TL;DR: In this paper, the authors report on two different new advances: sintering of WC-Co composite tool bits and other similar objects in under 15 min, while retaining extremely fine microstructures, without any grain growth inhibitors, using reduced TiO2 or Ta2O5 for the synthesis of phases such as BaTiO3, Ba3MgTa2O9, and Pb(Zr.Ti)O3.
Abstract: The innovations in microwave processing of ceramics have been dominated to date by serendipitous discovery, because the interaction between such radiation as delivered via available tools and the materials of widely varying properties, sizes, and shapes is so complex that it has defied quantitative analysis. For over 10 years a wide variety of inorganic ceramic and semiconducting materials have been synthesized, sintered, and reacted in our own labs, including microwave hydrothermal synthesis of metals, ferrites, and electroceramic phases. These local results are summarized and used as the reference point for reporting on two different new advances: sintering of WC-Co composite tool bits and other similar objects in under 15 min, while retaining extremely fine microstructures, without any grain growth inhibitors; using reduced TiO2 or Ta2O5 for the synthesis of phases such as BaTiO3, Ba3MgTa2O9, and Pb(Zr.Ti)O3 in a few minutes in a 2.45 GHz field at the astonishing temperatures of 300–700 oC.
TL;DR: The optimal thermal treatment required to Prepare pure β-TCP powder from the precursors (HA and DCP) was determined and the sintering method required to prepare fully dense β- TCP completely free from α-form, was identified.
Abstract: HA and beta-TCP-based ceramics were prepared using commercial powders. Powder characteristics were defined and the processing parameters studied, aimed at the production of samples with improved microstructural and mechanical properties. The behaviour of HA powder subjected to various thermal treatments was investigated in order to control the formation of secondary phases (alpha- and beta-TCP) during sintering. The optimal thermal treatment required to prepare pure beta-TCP powder from the precursors (HA and DCP) was determined and the sintering method required to prepare fully dense beta-TCP completely free from alpha-form, was identified. Translucent hot-pressed beta-TCP ceramics with potential applications in aesthetic restorative prostheses were prepared and characterized. The interval of existence of alpha-TCP and alpha-TCP as secondary products was also defined. Crystallographic analysis was carried out on the imperfectly known low-temperature alpha-TCP phase, and a proper monoclinic unit cell determined.
TL;DR: In this paper, the effects of corn starch addition on the microstructure and CO gas response were studied by sintering ZnO at 600∼900°C for 3 h in air.
Abstract: Effects of corn starch addition on the microstructure and CO gas response were studied by sintering ZnO at 600∼900°C for 3 h in air. The addition of 5 wt% corn starch as the fugitive phase decreased both the grain size and the sintered density of ZnO at all sintering temperatures and thus increased the sensitivity to 200 ppm CO. The increasing relative density and grain size with sintering temperature was accompanied by a decreasing CO gas sensitivity after maximum at 700°C. The temperature showing the maximum CO gas sensitivity decreased with decreasing grain size. Humidity decreased both the hysteresis of electrical conductivity and CO gas sensitivity.
TL;DR: In this article, the shape memory behavior of equiatomic Ni-Ti shape memory alloy was investigated and characterised by compaction and sintering of elemental Ti and Ni powders.
Abstract: The near net shape production of homogeneous Ni–Ti intermetallic displaying good shape memory behaviour is attractive, particularly for potential biomaterials applications. This study investigates and characterises equiatomic Ni–Ti shape memory alloy prepared by the compaction and sintering of elemental Ti and Ni powders. Phase development during sintering was determined using X–ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis, and dimensional integrity ,of the compacts was monitored during sintering. The shape memory behaviour of the Ni–Ti compacts was assessed using differential scanning calorimetry. Employment of a combined solid state and liquid phase sinter cycle produced homogeneous Ni–Ti intermetallic containing 35% porosity and displaying shape memory transformation enthalpy similar to that of commercially produced wrought Ni–Ti material. PM/0746
TL;DR: In this article, the change in size distribution of agglomerates in a heated pipe is measured using a differential mobility analyzer and a condensation nucleus counter, and the population balance equation for sintering is solved numerically.
Abstract: In order to evaluate the sintering characteristics of polydisperse TiO2 and SiO2 fractal agglomerates consisting of nm-sized primary particles (nanoparticles), the change in size distribution of agglomerates in a heated pipe is measured using a differential mobility analyzer and a condensation nucleus counter. Change in the structure and the primary particle size of the agglomerates collected by a thermophoretic aerosol sampler is also measured by the transmission electron microscopy as a function of furnace temperature. Coalescence of the agglomerates and the growth of primary particles due to sintering are observed at temperatures corresponding to 50%–100% of the bulk melting points of the particle material. To model the changes in the size distribution of agglomerates and primary particles due to sintering, two-dimensional sectional representation of the size distribution is employed, and the population balance equation for sintering is solved numerically. The calculated results, considering a...
TL;DR: In this paper, the relationship between processing parameters and morphology was studied using optical microscopy and transmission electron microscopy (TEM), and the magnitude of the PTC effect was found to be inversely proportional to the dimension of the carbon black channels in the composites.
Abstract: Conductive polymer composites were prepared by sintering a mixture of ultrahigh molecular weight polyethylene (UHMWPE) powder and carbon black. Two processing parameters-time and temperature-were shown to have a notable effect on the resistivity of the composites. The relationships between the processing parameters and morphology were studied using optical microscopy and transmission electron microscopy (TEM). The results of the optical microscopy studies indicate that the carbon black is distributed in the interfacial regions between the UHMWPE particles. The dimension of the carbon black channels increases with the carbon black concentration. TEM micrographs show that a high degree of intermixing between the carbon black and the polymer occurs at higher temperatures and longer processing times, resulting in higher resistivities. A positive temperature coefficient (PTC) effect was observed for these materials. A mechanism for the PTC effect in this system is proposed. The magnitude of the PTC effect is found to be inversely proportional to the dimension of the carbon black channels in the composites. The dimension is directly related to the carbon black concentration. The PTC effect is a result of the polymer volume expansion caused by melting of the crystallites. A large PTC effect is observed for the composites with a low carbon black concentration and vice versa. No negative temperature effect (NTC) is observed at temperatures substantially above the melting point of the polymer.
TL;DR: In this paper, an optimized sintering program was designed to promote densification and suppress the decomposition of the hydroxyapatite matrix and the evaporation of silver.
Abstract: Calcium hydroxyapatite bioceramic was toughened by preparing composites with silver particles as reinforcements. The composites were fabricated from hydroxyapatite and silver oxide raw powders. The sintering behaviour was investigated using dilatometry. An optimized sintering programme was designed to promote densification and to suppress the decomposition of the hydroxyapatite matrix and the evaporation of silver. High density was achieved on both small cylinder samples and large block samples by pressureless sintering. The density of the composites is over 92.4% theoretical with silver inclusions up to 30 vol%. The strength of the composites is greater than 80 MPa as tested by four-point bending. Silver inclusions improve the toughness consistently, from 0.70 MPa m1/2 for the monolithic hydroxyapatite to 2.45 MPa m1/2 at 30 vol% silver. Studies on the toughening mechanism indicate that crack bridging and subsequent plastic work of silver are mostly responsible for the toughening, whereas crack deflection also makes some contribution.
TL;DR: In this paper, high-resolution transmission electron microscopy and X-ray diffraction line profile analysis was used to study the crystallite growth in inert gas condensation using thermal evaporation.
TL;DR: In this article, 12 different silicon nitride-disilicate ceramics have been fabricated by hot-pressing Si3N4 with the oxides of Y, Yb, Ho, Dy, Er, Sm, Ce, Lu, La, Pr, Gd, and Sc that are used as sintering additives.
Abstract: Twelve different silicon nitride-disilicate ceramics have been fabricated by hot-pressing Si3N4 with the oxides of Y, Yb, Ho, Dy, Er, Sm, Ce, Lu, La, Pr, Gd, and Sc that are used as sintering additives. The high temperature strength and oxidation behaviour of the hot-pressed ceramics were investigated and correlated with the cationic radii of the oxide additives. The flexural strength at 1200°C increased, from 666 MPa for Si3N4-La2Si2O7 to 965 MPa for Si3N4-Sc2Si2O7 which is correlated with a decreasing cationic radius of the oxide additive. The weight gain during oxidation at 1400°C for 192 h in air decreased, from 0.8732 mg cm-2 for a Si3N4-Sm2Si2O7 ceramic to 0.1089 mg cm-2 for a Si3N4-Sc2Si2O7 ceramic, which is a function of the decreasing cationic radius of the oxide additive.
TL;DR: In this paper, powder and ceramics of barium titanate prepared by the citric process were studied by x-ray photoelectron spectroscopy (XPS).
Abstract: Powder and ceramics of barium titanate prepared by the citric process were studied by x-ray photoelectron spectroscopy (XPS). Spectra of C1s, O1s, Ti2p, Ba3d, and Ba4d levels are analyzed in powder and ceramics immediately after the sintering step and after several months of exposure in the air. Ar-ion etching allowed one to characterize the material intrinsic carbon. The results are discussed in comparison with works previously published on oxide single crystals.
TL;DR: In this paper, the whole sintering process is divided into three sub-processes as powder preparation, compaction, and Sintering, and each subprocess is discussed in terms of its possible contribution to the reducing of sinterings temperature.
Abstract: Theories and applications associated with sintering of ceramics and reducing of sintering temperature are reviewed. The whole sintering process is divided into three sub-processes as powder preparation, compaction and sintering, and each sub-process is discussed in terms of its possible contribution to the reducing of sintering temperature. New approach for practical optimization of sintering process is investigated. The application of above in the Low Temperature Co-fired Ceramics (LTCC) is discussed. Meanwhile, many successful applications in reducing the sintering temperatures are presented.