Showing papers on "Sintering published in 1990"

Boron carbide ― a comprehensive review

Abstract: Boron carbide, which has a high melting point, outstanding hardness, good mechanical properties, low specific weight, great resistance to chemical agents and high neutron absorption cross-section (10BxC, x>4) is currently used in high-technology industries—fast-breeders, lightweight armors and high-temperature thermoelectric conversion. The contents of this review are: (1) introduction; (2) preparations—industrial preparative routes, powders, sintering (additives, pressureless, hot pressing, HIP); laboratory methods of synthesis (CVD, PVD, plasma, crystal growth); (3) analytical characterization; (4) phase diagram—a peritectic, nearly pure boron, and a wide phase homogeneity range (B4C-B10·5C); (5) rhombohedral crystal structure—a comprehensive model of the whole solid solution is proposed; (6) chemical properties; (7) physical properties—density, mechanical (strength, hardness, toughness) and thermo-electrical properties; (8) main industrial applications; (9) conclusion.

A simple way to make tough ceramics

Abstract: THE major problem with the use of ceramics as structural materials is their brittleness. One way of overcoming this problem is to introduce weak interfaces which deflect a growing crack1. Polymer composites of this sort can be easily prepared by surrounding fibres with liquid plastic. To make similar structures with ceramic matrices and fibres is difficult and expensive, however, because traditional ceramic processing techniques of powder compaction and sintering prevent densification and cause cracking2–4. Here we describe a simple, inexpensive way of preparing a ceramic material that contains such weak interfaces. Silicon carbide powder is made into thin sheets which are coated with graphite to give weak interfaces and then pressed together and sintered without pressure. Relative to the monolithic material, the apparent fracture toughness for cracks propagating normal to the weak interfaces is increased more than fourfold, and the work required to break the samples increases by substantially more than a hundredfold. The technique should be readily applicable to other ceramics.

Development of Superplastic Structural Ceramics

Abstract: Superplastic structural ceramics (Y-TZP, A1203, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or a phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain-growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second-phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain-boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain-boundary cracking during large strain deformation. Existing evidence suggests that a lower grainboundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by R. E. Newnham-contributing editor

Sintering characteristics of nanocrystalline TiO2

Abstract: The microstructural development of compacted nanocrystalline TiO2 powder was studied as a function of sintering temperature up to 1000°C. Grain growth was monitored using x-ray diffraction and scanning electron microscopy. The specific surface area and total porosity were determined quantitatively using the nitrogen adsorption BET. The density was measured by gravimetry using Archimedes principle. The green body density of compacted n-TiO2 with average grain size of 14 nm is as high as 75% of theoretical bulk density. Low temperature surface diffusion leads to the disappearance of small pores, while noticeable densification commences at 600°C and reaches near theoretical values at 900°C. Grain growth also begins at 600°C, accelerating rapidly by 1000°C. Hot isostatic pressing is observed to enhance densification while suppressing grain growth.

Estimate of the activation energies for boundary diffusion from rate-controlled sintering of pure alumina, and alumina doped with zirconia or titania

Abstract: Sintering experiments at constant heating rates were employed to estimate the activation energy for sintering in alumina and in alumina containing 5 vol% zirconia or 5 vol% titania. Grain growth, which can complicate the analysis of sintering kinetics data, was suppressed by using uniformly and densely packed grain compacts prepared by colloidal processing. Grain-boundary diffusion is believed to have been the dominant sintering mechanism. The activation energies were 440 {+-} 40 kJ/mol for pure alumina, 585 {+-} 40 kJ/mol for alumina (titania), and 730 {+-} 60 kJ/mol for alumina (zirconia). The alumina and alumina (titania) results are in agreement with the values reported in the literature. The possibility that the higher activation energies for doped alumina reflect a stronger bonding at alumina interfaces in the presence of zirconium and titanium is discussed.

Composite abrasive compact having high thermal stability

Abstract: An abrasive compact with a substantially solid body is provided from a mass of abrasive particles which are bonded together on a particle-to-particle basis. A network of interstices is formed within the body by removing the metallic second phase by-product of a solvent catalyst sintering aid. The network of interstices is filled with the carbide by product of a non-catalyst sintering aid forking a solid body. A substrate is bonded to some of the particles and to some of the carbide filling the network of interstices.

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Abstract: Grain growth in a high-purity ZnO and for the same ZnO with Bi2O3 additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: Gn—Gn0=K0t exp(—Q/RT). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+ lattice diffusion mechanism. Additions of Bi2O3 to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2O3 content. The preexponential term K0 was also independent of Bi2O3 content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi2O3 ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2O3-rich liquid phase.

The Role of Excess Magnesium Oxide or Lead Oxide in Determining the Microstructure and Properties of Lead Magnesium Niobate

Abstract: Near-phase pure perovskite lead magnesium niobate (PMN) with MgO or PbO additives was produced by reacting PbO with MgNb2O6 at 800°C and sintering at 1200°C. Dense ceramics were characterized by scanning electron microscopy, X-ray diffraction, and dielectric measurements. The microstructural studies showed that excess MgO exists as micrometer spherical particles either in the grain boundary as a discrete particle or in the perovskite grain as an inclusion. The pyrochlore phase exists in large isolated grains in the microstructure. The 10 mol% MgO excess composition had a peak dielectric constant of 19 500 at 100 Hz, which suggests very “clean” or uninhibiting grain boundaries. The excess addition of PbO did not improve the yield of perovskite PMN phase and decreased the dielectric constant. PMN grain boundaries are the dominant path of fracture. This paper, to a certain degree, explores the chemistry and characteristics of these grain boundaries.

Grain growth in nanocrystalline TiO2 and its relation to vickers hardness and fracture toughness

Abstract: This paper reports on scientific interest in ultra-fine grained powders for processing of ceramic components motivated by the possibilities for the enhancement of sintering rates, reduction in flaw sizes and low-temperature superplastic deformation. Previous works have developed a technique, which combines the methods established of inert gas condensation of small particles and in situ powder compaction, for synthesizing materials with grain sizes {lt}10 nm. It has been shown that this method can be adapted for the production of ceramic nanocrystalline particles. Subsequent work has demonstrated that enhanced sintering and superplastic deformation is possible in nanocrystalline ceramics (TiO{sub 2}), but not without significant grain growth. Control of grain growth, however, is necessary if the capability for synthesizing nanoscale powders is to have benefit for structural applications. It is well known that the initial particle size in green body compacts is not always indicative of the final grain sizes in fully sintered ceramic bodies. This study was initiated to examine grain growth kinetics in nanocrystalline materials. TiO{sub 2} was selected for this initial study since sintering, deformation and diffusion data are available.

Selective laser sintering of parts by compound formation of precursor powders

Abstract: A method and apparatus for selectively sintering a layer of powder to produce a part comprising a plurality of sintered layers. The apparatus includes a computer controlling a laser to direct the laser energy onto the powder to produce a sintered mass. The computer either determines or is programmed with the boundaries of the desired cross-sectional regions of the part. For each cross-section, the aim of the laser beam is scanned over a layer of powder and the beam is switched on to sinter only the powder within the boundaries of the cross-section. Powder is applied and successive layers sintered until a completed part is formed. Preferably, the powder comprises a plurality of materials having different dissociation or bonding temperatures. The powder preferably comprises blended or coated materials, including precursor materials which are formed into a mass at the irradiated locations, and which either react due to the laser thermal energy or in a later heat treatment to form a compound with properties different than the precursors. Examples are disclosed wherein a compound is formed which has a significantly higher melting point than that of one or more of the precursor powders.

Liquid‐Phase Reaction‐Bonding of Silicon Carbide Using Alloyed Silicon‐Molybdenum Melts

Abstract: The authors have investigated reaction-forming of silicon carbide by the infiltration of carbonaceous preforms using alloyed silicon melts, in order to synthesize composite materials free of the residual silicon phase that has previously limited mechanical properties and upper use temperatures. In this approach, rejection of the alloying component(s) from the primary silicon carbide phase into the remaining melt results in the formation of a secondary refractory phase, such as a silicide, in place of residual free silicon. Experiments conducted in the Si-Mo melt system show that relatively dense ({gt}90%) silicon carbide-molybdenum silicide materials free of residual silicon and residual carbon can be obtained. A model for reactive infiltration based on time-dependent permeabilities is proposed. Processing variables important for control of the reaction rate relative to the infiltration rate, and associated processing flaws, are discussed.

Grain‐Boundary‐Phase Crystallization and Strength of Silicon Nitride Sintered with a YSlAlON Glass

Abstract: Densifying silicon nitride with a YSiAlON glass additive produced 99% dense materials by pressureless sintering. Subsequent heat-treating led to nearly complete crystallization of the amorphous intergranular phase. Transmission electron microscopy revealed that for heat treatments at 1350°C, only β-Y2Si2O7 was crystallized at the grain boundaries. At a higher temperature of 1450°C, primarily YSiO2N and Y4Si2O7N2 in addition to small amounts of Y2SiO5 were present. Al existed only in high concentrations in residual amorphous phases, and in solid solution with Si3N4 and some crystalline grain-boundary phases. In four-point flexure tests materials retained up to 73% of their strengths, with strengths of up to 426 MPa, at 1300°C. High-strength retention was due to nearly complete crystallization of the intergranular phase, as well as to the high refractoriness of residual amorphous phases.

Processing and Sintering of Ultrafine MgO-ZrO2 and (MgO, Y2O3)-ZrO2 Powders

Abstract: This paper reports chemical coprecipitation used to produce ultrafine and easily sinterable MgO-stabilized and (MgO,Y{sub 2}O{sub 3})-stabilized ZrO{sub 2} powders. The sintering behavior is very sensitive to post-precipitation washing because hard agglomerates form when the precipitated gels are washed with water, whereas soft agglomerates form when they are washed with ethanol. The soft agglomerates pack uniformly, resulting in homogeneous shrinkage of powder compacts to near-theoretical density. The hard agglomerates result in compacts which have regions of localized densification and a significant fraction of residual porosity.

Computer Simulation of Final‐Stage Sintering: I, Model Kinetics, and Microstructure

Abstract: A Monte Carlo model for simulating final-stage sintering has been developed. This model incorporates realistic microstructural features (grains and pores), variable surface difusivity, grain-boundary diffusivity, and grain-boundary mobility. A preliminary study of a periodic array of pores has shown that the simulation procedure accurately reproduces theoretically predicted sintering kinetics under the restricted set of assumptions. Studies on more realistic final-stage sintering microstructure show that the evolution observed in the simulation closely resembles microstructures of real sintered materials over a wide range of diffusivity, initial porosity, and initial pore sizes. Pore shrinkage, grain growth, pore breakaway, and reattachment have all been observed. The porosity decreases monotonically with sintering time and scales with the initial porosity and diffusivity along the grain boundary. Deviations from equilibrium pore shapes under slow surface diffusion or fast grain-boundary diffusion conditions yield slower than expected sintering rates.

Diffusion-Controlled Processes in Microwave-Fired Oxide Ceramics

Abstract: Processing oxide-based ceramics using microwave heating leads to a number of unexpected results, which can only be interpreted in terms of enhanced diffusion Enhanced sintering has been observed in alumina and zirconia Accelerated grain growth in dense, hot-pressed alumina has been demonstrated Increased diffusion coefficients have been observed for diffusion of oxygen in sapphire As yet, a satisfactory theory to account for these phenomena has not been developed This paper reviews the experimental work conducted at the Oak Ridge National Laboratory during the past four years on the processing of oxides in both 245 and 28 GHz microwave furnaces

Grain Growth During Gas‐Pressure Sintering of β‐Silicon Nitride

Abstract: The microstructures of gas-pressure-sintered materials from β-Si3N4 powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si3N4 powder despite the smaller aspect ratio of the grains

Powder metallurgy repair technique

Abstract: To repair or join sections of a metal article, matching metal powder is sintered in the solid state to form a porous structure in the joint. A layer of matching powder, modified to incorporate melting point depressants such as boron and silicon is subsequently added to the surface of the sintered region. The joint is processed at a temperature where the modified layer melts while the sintered layer and base metal remain solid. The modified material flows into the sintered layer by capillary action and promotes liquid phase sintering, resulting in a dense joint with properties approaching those of the base metal.

Glasses from aerogels

Abstract: Silica glasses are obtained by the densification of aerogels. The transformation of the material into a glass is followed by differential thermal analyis, thermo-gravimetric analysis, dilatometry and by the evolution of the structural, textural and mechanical properties of the material. The organic species and the hydroxyl groups are removed by oxidation and chlorination heat treatments in such a way as to avoid bloating and crystallization phenomena during sintering. Densification is obtained by heat treatment at a low temperature (1100 to 1300 ° C). The densified aerogel shows physical properties identical to those of molten silica. Moreover, this material is very pure and its water content is low. The same process can be extrapolated to multicomponent glasses and composite materials.

Conventionally Prepared Submicrometer Lead-Based Perovskite Powders by Reactive Calcination

Abstract: Submicrometer powders of various Pb-based perovskites, including PbTiO3, PbZrO3, Pb(Zr0.53Ti0.47)O3, and Pb(Mg1/3Nb2/3)O3 were prepared by a reactive calcination process. Using only reagent-grade raw materials and conventional processing techniques, highly reactive powders were produced by reacting the materials near the temperature of maximum volumetric expansion. At this point, the morphological development results in a skeletal-type structure consisting of ultrafine particulates that can be readily broken down further by milling. Powder sizes less than 0.3 μm and as small as 70 nm generally only achievable using chemical processing techniques were achieved. The highly reactive powders allowed densification to occur at temperatures as low as ∼900°C with correspondingly small grain sizes. A model describing the physiochemical behavior and associated morphological development of Pb-based perovskites was herein proposed.

Simulation of the Viscous Sintering of Two Particles

Abstract: Problems associated with the sintering of mixtures of hard and soft particles can be obviated by coating the hard particles (e.g., alumina) with a soft phase (e.g., glass). An analysis of the sintering kinetics of such materials based on finite-element simulations of the sintering of a pair of rigid spheres coated with a viscous material is presented. Until the contact radius is greater than the thickness of the coating, the sintering kinetics are very similar to those obtained if the rigid core is not present. The sintering rate decreases as the rigid cores approach each other and is limited by flow in the gap between the core particles. If the coating thickness is in excess of 20%, the simulations predict that a packing of such particles sinters to full density at a rate comparable to that of particles without a rigid core.

Tribological Properties of PM212: A High-Temperature, Self-Lubricating, Powder Metallurgy Composite

Abstract: This paper describes a research program to develop and evaluate a new high temperature, self-lubricating powder metallurgy composite, PM212. PM212 has the same composition as the plasma-sprayed coating, PS212, which contains 70 wt percent metal-bonded chromium carbide, 15 wt percent silver and 15 wt percent barium fluoride/calcium fluoride eutectic. The carbide acts as a wear resistant matrix and the silver and fluorides act as low and high temperature lubricants, respectively. The material is prepared by sequential cold press, cold isostatic pressing and sintering techniques. In this study, hemispherically tipped wear pins of PM212 were prepared and slid against superalloy disks at temperatures from 25 to 850 C in air in a pin-on-disk tribometer. Friction coefficients range from 0.29 to 0.38 and the wear of both the composite pins and superalloy disks was moderate to low in the 10(exp -5) to 10(exp -6) cubic mm/N-m range. Preliminary tests indicate that the material has a compressive strength of at least 130 MPa over the entire temperature range of 25 to 900 C. This material has promise for use as seal inserts, bushings, small inside diameter parts and other applications where plasma-sprayed coatings are impractical or too costly.

Deformation and Grain Growth of Low‐Temperature‐Sintered High‐Purity Alumina

Abstract: 9 = 0.6 I / s ' 7 = 6.0 0 7 = 2 4 i ti 3 = 6 0 Through close control over green-state powder processing, pure alumina ceramics of 0.5-pm grain size were obtained by sintering at 1250°C. The static grain growth of this material was modest at temperatures below 1300°C. However, dynamic grain growth occurred rapidly during superplastic deformation. Therefore, although the ultrafine-grained alumina exhibited rather low initial flow stress at relatively low deformation temperatures, dynamic grain-growth-induced strain hardening gave rise to high flow stress causing cavitation and cracking. As a result, superplastic deformation could not be achieved for the ultrafine-grained pure alumina. [

Lightweight oil and gas well proppants

Abstract: A lightweight oil and gas well proppant made by simultaneously mixing and compacting a milled calcined kaolin clay powder to form green pellets and then drying, screening and sintering the pellets to form proppant pellets having a specific gravity of 3.0 or less and a conductivity of at least 4,200 millidarci-feet and preferably at least 5,100 md-ft as measured by the Stim-Lab method after 50 hours at 8,000 psi and 275° F. in the presence of deoxygenated aqueous 2% solution of KCl.

Particle and grain size effects on the dielectric behavior of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3

Abstract: The role of particle and grain size on the dielectric behavior of the perovskite relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 [PMN] was investigated. Ultrafine powders of PMN were prepared using a reactive calcination process. Reactive calcination, the process by which morphological changes take place upon reaction of the component powders, produced particle agglomerates less than 0.5 μm. Through milling, these structures were readily broken down to ∼70 nanometer-sized particulates. The highly reactive powders allowed densification as low as 900 °C, but with corresponding grain growth in the micron range. Such grain growth was associated with liquid phase sintering as a result of PbO–Nb2O5 second phase(s) pyrochlore. Sintering, assisted by hot uniaxial pressing, below the temperature of liquid formation of 835 °C, allowed the fabrication of highly dense materials with a grain size less than 0.3 μm. The dielectric and related properties were determined for samples having grain sizes in the range of 0.3 μm to 6 μm. Characteristic of relaxors, frequency dependence (K and loss) and point of Tmax were found to be related to grain and/or particle size and secondarily to the processing conditions. Modeling of particle size/dielectric behavior was performed using various dielectric properties of 0–3 composites comprised of varying size powder in a polymer matrix. An intrinsic-microdomain perturbation concept was proposed to interpret observed scaling effects of the relaxor dielectric behavior in contrast to normally accepted extrinsic grain boundary models.

Effect of cerium addition on the thermal stability of gamma alumina support

Abstract: Transition alumina, such as 7-A1203, 0-A1203 and 6-A1203, has been widely used as a catalyst support. The surface area of the support significantly decreases at high temperature operations, for example in purifying automotive exhaust gas [1]. In this case, it is important to maintain the support with high surface area free of sintering or phase transition, otherwise precious metals (platinum, rhodium etc.) supported on alumina will consequently sinter. Cerium and other rare earth elements have been used as a promoter to improve the catalytic activities for purifying carbon monoxide, nitrogen oxides and hydrocarbons emitted from automotive engines [2-4]. It has been suggested that the addition of cerium plays serveral roles, including the catalysis of the water-gas shift reaction, the oxygen storage in the lattice for oxidation catalysis under rich air-fuel conditions, and the inhibition of the growth of noble metal particles. The present letter describes the influence of the addition of cerium to 7-A1203 catalyst supports on the phase transition of 7-A1203 to ~-A1203 and the sintering of transition alumina below the transition temperature. A series of cerium-added alumina was prepared by the impregnation technique. Powdered 7-A1203 with surface area 170 m 2 g ~ was used as a support, and an aqueous cerium nitrate was used as an impregnation solution. The samples were then dried at 110 ° C for 8 h and calcined at 600°C for 3 h. The starting alumina contained 0.05 wt % impurities, mainly iron and trace silicon, calcium and sodium. The cerium contents of the samples were 0.5, 1, 2, 3, 5 and 10mol% by the CeO2 to the total moles of oxides (CeO2 + A1203). The samples were further calcined in air at a temperature between 1 000 ° C and 1 200 ° C for 5 h in order to examine the thermal stability of the alimina support. The surface area of the samples was derived by application of the BET equation, from nitrogen adsorption data obtained at 77K by a standard volumetric procedure. The phase transition temperature was measured with a DTA apparatus at heating rates of 2.5, 5, 10 and 20Kmin ~ using 0¢-A1203 powder as a reference material. Kissinger's plots [5] were applied to the evaluation of the activation energy of the phase transition from active alumina to c¢-A12 03. Phase analysis was done by the powder X-ray diffraction (XRD) technique. The amount of ~-A1203 in the samples was determined by the XRD intensity of the (1 00) diffraction line of 0~-Al203. The standard data were obtained from the XRD data of the mixtures of u-A1203 powder and 0-A1203 powder which were formed by calcining 7-A1203 at 1 300°C and l 000°C for 10 h. The samples containing 0.01 mol % gadolinium were prepared by the same procedure as above, but using an aqueous solution of gadolinium nitrate. Fig. 1 shows the Surface area of alumina supports containing cerium with different molar concentrations, calcined in air at 1 000 ° C, l 100°C and 1 200°C for 5 h. The thermal stability of alumina was improved by the addition of only 0.5 to 2 mol % cerium, and a gradual decline in surface area was found with increasing cerium content. XRD analysis of the samples calcined in air at 1 200°C indicated that 0.5 mol % Ce-A1203 consisted of 0-, ~and ~-A1203, and that pure A1203 completely transformed to ~-A1203 (Fig. 2). CeO2 phase was found in the samples with more than 2 mol % cerium. These results clearly show that the addition of cerium improves the thermal stability of alumina at 1 200°C by preventing the transformation to ~-A1203. However, the overloading of cerium decreases the surface area of the alumina because of the growth of CeO2 particles. The XRD data of the samples calcined at 1 100°C indicated the formation of 0and ~-A12 03. The decrease in surface area was inhibited by the cerium loading at 1 100 ° C. Kissinger's plots of the data obtained by DTA for both pure AI203 and 0.5 tool % Ce-AI203 and given in Fig. 3 as the relation of ln[(dT/dt)Tn72] with l/Tm. (dT/dt) is the heating rate, and Tm represents the maximum rate temperature of the phase transition. The activation energy of the phase transition was calculated at 581kJmol ~ for pure AI203 and 582 kJ tool ~ for 0.5 mol % Ce-A1203. Although these values are rather high compared with those in the literature [6, 7], it can be found that the alumina in this study has the constant activation energy of the phase transition regardless of the cerium addition. Scanning electron microscopy (SEM) observation of the samples calcined at ! 200 ° C revealed the formation of ~-A1203 particles, 0.1 to 0.3 #m in diameter. The particle size was independent of the content of cerium in the samples. These results can be explained by assuming that the cerium addition decreased the ~-A1203 transformation by influencing the nucleation process of ~-A1203 but not its growth process. Fig. 4 compares the 1 250 ° C isothermal transformation data for pure A1203 with those for 0.5mol % Ce-A1203. The data were analysed using the following empirical kinetic equation

Isothermal sintering of SiO2-aerogels

Abstract: Isothermal sintering has been studied for two base-catalyzed SiO2-aerogels with initial densities ρ0 = 122 kg/m3 and 256 kg/m3, covering a temperature range from 750 to 950 °C and time periods up to 200 h; the highest density achieved was 339 kg/m3. The change in density with time at a given temperature is compared with predictions of sintering theories. The elastic constants determined by ultrasonic measurements reveal scaling with the density achieved upon sintering. Microscopic structural changes during sintering have been studied by means of small angle X-ray scattering (SAXS). Characteristic structural parameters are the mean diameter of the particles, i.e. the building blocks of the gel network, the mean size of the macropores within the tenuous structure and the specific surface area. The major finding of the investigations is the observation of two processes with different time scales: a fast reduction of the size of the macropores accompanied by a slow decrease in specific surface area as well as a slight increase in particle diameter. For both samples investigated all parameters can be unequivocally correlated with the achieved density upon sintering - independent of the initial density.

Effects of Carbon as a Sintering Aid in Silicon Carbide

Abstract: Self-diffusion data are collected from the literature in an attempt to better understand the strong effects of carbon as a sintering aid in SiC. These data indicate that the presence of excess carbon, in addition to reducing the native SiO2 layer of the SiC, probably enhances the rate-controling bulk self-diffusion rate of SiC by a factor of about 100.