Showing papers on "Sintering published in 1992"

Densification of nanostructured titania assisted by a phase transformation

Abstract: NANOPHASE materials, characterized by an ultrafine grain size, have stimulated much interest in recent years1–11 by virtue of their unusual mechanical, electrical, optical and magnetic properties. Nanophase ceramics are of particular interest because they are more ductile at elevated temperatures than are coarse-grained ceramics11—an important property for the fabrication of ceramic components. Preparing materials that are both dense and fine-grained, however, has proved difficult: the high sintering temperatures generally required to obtain high densities can also lead to exaggerated grain growth, resulting in coarse-grained, nonuni-form materials. Sintering at lower temperatures gives a much finer grain size, but does not in general result in high-density materials. We show here that dense nanostructured titania, with density >99% of the theoretical maximum and an average grain size of less than 60 nm, can be prepared by sintering a titanium oxide sol gel near the anatase–rutile phase transformation temperature (about 600 °C). The increased mobility of the atoms during the phase transformation enhances the sintering rate at lower temperatures, suggesting that this method could be used more generally to produce nanophase materials with near theoretical densities.

Synthesis of dense silicon-based ceramics at low temperatures

Abstract: THE conventional preparation of advanced ceramic parts based on silicon carbide or nitride involves pressureless sintering, hot pressing or hot isostatic pressing of appropriate ceramic starting powders1. Owing to the covalent nature of the Si–C and Si–N bonds and hence the low diffusion coefficients in SiC and Si3N4, high sintering temperatures and the addition of sintering aids are normally used to enhance densification. During densification, the sintering additives form second phases located at grain boundaries, which commonly impair the mechanical and physical properties of the material, especially at higher temperatures. New processing routes that overcome these problems are therefore desirable. Here we report the direct transformation of a metallorganic precursor into non-oxide silicon-based ceramics with relative densities of up to 93%. This process can be used to make ceramic components and matrix composites at unusually low temperatures (1,000 °C) and without the addition of sintering aids.

Relationships between dopants, microstructure and the microwave dielectric properties of ZrO2-TiO2-SnO2 ceramics

Abstract: Ceramics with compositions in the solid solution region of the ZrO2-TiO2-SnO2 equilibrium diagram are finding wide application as dielectrics in filters for communications and radar systems operating at microwave frequencies. Commercially available compositions often incorporate sintering aids and dopants to reduce processing temperatures and modify the dielectric properties. However, the mechanism through which these additives influence dielectric loss is not obvious. The role of zinc oxide as a sintering aid and lanthanum and niobium as dopants, their effect upon microstructural development and their correlation with dielectric loss at microwave frequencies were investigated. For specimens of density greater than 90% theoretical, the influences of defect chemistry upon dielectric loss appear to dominate those of the microstructure. Properties close to those which might be considered intrinsic were attained through sintering for periods of up to 128h. Doping with lanthanum is detrimental to the dielectric loss, particularly after long sintering times.

Combined-Stage Sintering Model

Abstract: By focusing on the similarities between the three stages of sintering, a single equation is derived that quantifies sintering as a continuous process from beginning to end. The microstructure is characterized by two separate parameters representing geometry and scale. The dimensionless geometry parameter, denoted T, comprises five scaling factors that relate specific microstructural featuers (e.g., surface curvature) to the scale (grain diameter). Calculations of T from experimental data show (a) agreement with computer simulations of initial-stage sintering, (b) the effect of surface diffusion on T, and (c) changes in T with microstructural evolution during sintering. Application of the model to the design of firing schedules and the study of microstructural geometry effects on sintering is discussed.

Microwave Sintering of Solid Oxide Fuel Cell Materials: I, Zirconia‐8 mol% Yttria

Abstract: This paper reports that the successful microwave sintering of zirconia demonstrates the necessity to understand both the materials and electromagnetic field aspects of microwave processing. It was difficult to produce crack-free parts in the multimode microwave furnace employed in this investigation. Nonuniformities in the microwave field, and dielectric properties that increased rapidly with temperature, produced hot spots in the parts, which led to differential sintering and subsequent cracking. To produce crack-free sintered parts, an indirect heating method was developed that eliminated the severe differential heating. Using this indirect heating method, it was demonstrated that the sintering temperature of zirconia could be lowered from 1375{degrees} to 1200{degrees}C by microwave processing and that the resulting grain size was finer.

Microstructural Development During Gas‐Pressure Sintering of α‐Silicon Nitride

Abstract: Gas-pressure sintering of α-Si3N4 was carried out at 1850 ° to 2000°C in 980-kPa N2. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si3N4 powder was similar to that in the materials from β-Si3N4 powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si3N4 ceramics.

Nanoindentation of nanocrystalline ZnO

Abstract: A number of nanocrystalline ceramics have been fabricated by the gas phase condensation technique. The mechanical properties of one of the first ceramics produced by this method, nanophase TiO{sub 2},have been discussed in an earlier study. This paper reports a similar study undertaken to examine the properties of nanocrystalline ZnO. Nanoindenter techniques are used to determine hardness, Young's modulus, and strain rate sensitivity in ultra-fine grained ZnO. Significant properties variations are experienced within a given sample, indicating a large degree of microstructural inhomogeneity. Nevertheless, a distinct evolution in properties can be observed as a function of sintering temperature. Young's modulus and hardness values increase almost linearly with increasing sintering temperature, and, in addition, there also appears to be a linear correlation between the development of the two materials properties. In contrast, strain rate sensitivity is shown to have an inverse dependence on sintering temperature. This dependence appears to be linked to the strong influence of grain size on strain rate sensitivity, so that the lower sintering temperatures, which provide the finer grain sizes, tend to promote strain rate sensitivity. The results of this study are strikingly similar to those obtained earlier for nanophase TiO{sub 2}, and they indicate that themore » earlier results could probably be generalized to a much broader range of nanocrystalline ceramics.« less

Slip-casting alumina ceramics for crown and bridge restorations.

Abstract: The In-Ceram technique uses alumina ceramics and glass in a two-step firing procedure to create a high-strength core material for single-tooth restorations as well as small fixed partial dentures. Fine-grain alumina particles are sintered to form a porous substructure, which is infiltrated with molten glass. The combination of these two processes gives the material its outstanding properties. The sintering process is almost without shrinkage, providing an excellent fit, while the glass infiltration leaves practically no porosities, resulting in high strength.

Development and characterization of electroconductive Si3N4-TiN composites

Abstract: Dense Si3N4-TiN composites, with the second phase ranging from 20 to 40 vol.%, were produced by hot pressing, gas-pressure sintering and pressureless sintering under nitrogen gas atmosphere. The influence of densification technique and parameters and of amount and grain size of TiN particles on microstructure, mechanical properties, electrical resistivity and oxidation resistance was evaluated. The addition of TiN particles increases the stiffness and the fracture toughness of the base material. For TiN content higher than 30 vol.% the electrical resistivity of the composites is less than 10−3 ωcm. An evident effect of the grain size distribution of TiN powders on some mechanical properties was ascertained. The thermal stability of the composites is strongly affected by the amount of the second phase.

Plasma activated sintering of additive-free AlN powders to near-theoretical density in 5 minutes

Abstract: AlN powders (particle size = 0.44 ± 0.08 μm) containing no deliberate sintering additives were consolidated to near theoretical density in 5 min at 2003 K (1730 °C) using a Plasma Activated Sintering (PAS) process. PAS is a novel consolidation method that combines a very short time at high temperature with pressure application in a plasma environment. The in situ cleaning ability of powder particle during plasma activated densification leads to enhanced particle sinterability. The densities of undoped AlN specimens that were PAS consolidated at 2003 K for 5 min under 50 MPa pressure ranged from 97.5 to 99.3% of theoretical. The initial submicron particle size of AlN powders was retained in the final microstructure that consisted of polycrystalline grains with an average size of ≍0.77 ± 0.1 μm.

Constitutive models for the sintering of ceramic components—I. Material models

Abstract: A number of possible two state variable material models for the sintering of fine grained ceramic compacts are described. The two state variables employed in the models relate to the relative density of the material and the mean grain size. Constitutive relationships for the strain-rate of a material element and evolution laws for the two state variables are presented which reflect the different stages of sintering. The predictions of the model are in good agreement with experimental studies involving pressureless sintering and HIPing of green compacts.

DiC12: Magnesium titanate microwave dielectric ceramics

Abstract: Magnesium titanate ceramics have been prepared by a chemical route (Pechini method) and by the conventional mixed oxide route. Dielectric properties of sintered samples were measured at 8 GHz by the Hakki and Coleman technique. Microstructures were examined by optical microscopy and scanning electron microscopy. The use of the chemical method enabled the sintering temperature to be lowered and good quality, single phase, dense ceramics were obtained at temperatures down to 1150°C. Dielectric constants were approximately 17.5 and Q values up to 21200 were obtained at 8 GHz. The effect of powder preparation processes, sintering temperature and cooling rate on the densification, microstructure and microwave dielectric properties are discussed.

Preparation of MgAl2O4 Spinel Powders via Freeze-Drying of Alkoxide Precursors

Abstract: High-sinterability MgAl2O4 powder has been produced from alkoxide precursors via a freeze-drying method. Clear alumina sol and magnesium methoxide were used as starting materials in the process. The spinel powders were characterized by various techniques, such as thermal analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The tap density and sinterability of the spinel power are affected by the ball-milling techniques. Highly dense, transparent, polycrystalline MgAl2O4 has been obtained from these powders by sintering and hot isostatic pressing. Bimodal grain-size microstructure is observed in a HIPed sample.

Method of producing high-temperature parts by way of low-temperature sintering

Abstract: A method of producing high temperature parts by way of relatively low temperature sintering is disclosed A powder is produced, for example by spray drying a slurry, in which particles of a high temperature material such as a ceramic are partially coated with a low temperature material such as a polymer The powder is subjected to selective laser sintering, or another heat-based layerwise additive process, to form a green part where the ceramic particles are bound by the polymer The green part is permeated with a cementing agent having high temperature properties when cured; the cementing agent binds the exposed ceramic surfaces in the green part The polymer is then removed, leaving the high temperature part Additional cementing agent, or another high temperature material, may then be added to the part after removal of the polymer

Grain Growth of ZnO during Bi2O3 Liquid-Phase Sintering

Abstract: Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi2O3 ceramics has been studied for Bi2O3 contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi2O3 system. For the Bi2O3 contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi2O3. Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi2O3-content region of nearly constant activation energy values of about 150 kJ/mol, further Bi2O3 additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi2O3 changes from one of a phase-boundary reaction at low Bi2O3 levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi2O3 level and above.

Carbide/metal composite material and a process therefor

Abstract: A sintered product useful for abrasion- and impact-resistant tools and the like is provided comprising an iron-group metal binder and refractory metal carbide particles, e.g. tungsten carbide, formed in situ during sintering by the exothermic reaction of a carbide-forming refractory metal powder with a carbon source mixed therewith. The sintering is carried out at an elevated temperature and at a pressure at which diamond is stable in the presence of a liquid phase comprised substantially of an iron-group binder metal, refractory metal, and dissolved carbon. The product may optionally contain diamond, up to about 95% in volume, enabling its properties to be tailored for a wide range of applications.

Sintering time and atmosphere influences on the microstructure and mechanical properties of tungsten heavy alloys

Abstract: The mechanical properties of tungsten heavy alloys are sensitive to the processing cycle and are adversely affected by residual porosity. Sintering times greater than 2 hours usually result in pore growth with degraded properties. The development of an optimized sintering atmosphere has allowed exploration of long sintering times without significant property degradation due to pore growth. The optimal cycle was used to sinter two heavy alloy compositions (88 and 95 wt pct W) for times up to 600 minutes at 1480 °C. The 88 pct W samples slumped, but the 95 pct W samples were fully densified and suitable for tensile testing. At long sintering times, the tungsten grains flattened and the tungsten contiguity decreased, indicating a transition to low-energy configurations for the solid-liquid interfaces. The cube of the mean grain size varied linearly with the isothermal sintering time. This allowed determination of grain size effects on mechanical properties, showing a decreasing yield strength with increasing time in agreement with the Hall-Petch behavior. The tensile strength and elongation were highest for sintering times from 30 to 90 minutes, reflecting a minimum in the residual porosity.

TiNi shape memory alloys prepared by normal sintering

Abstract: This paper covers the use of conventional press and sinter of titanium and nickel equiatomic blends, involving transient liquid phases. A notable development has been the fabrication of homogeneous compacts from the elemental powders. Results indicate that employment of a double-stage sintering cycle is effective to sustain the geometrical integrity of the sintered sample. The TiNi alloy prepared in this manner exhibits a recovery strain of 4.6% by generating a restoring stress of 230 MPa, with a martensitic transformation point Ms of 34 °C.

Sintering of ash during fluidized bed combustion

Abstract: Agglomeration of bed material and fuel ash may sometimes cause problems during fluidized bed combustion. In this paper a laboratory test method has been applied on different coal ashes to predict how they behave in temperatures typical for circulating fluidized bed boilers. The method is also useful when the influence of the different bed compounds on the sintering is studied or when the effect of the surrounding gas phase is investigated. The method is based on compression strength measurements of sintered cylindrical pellets and has been used earlier when slagging and fouling tendencies of different coal ashes in pulverized coal fired boilers have been studied. The results showed clear differences in sintering tendencies between the five different coal ashes studied. Temperatures where the sintering was initiated could vary between 500 and 900 [degrees]C, depending on the ash. The sintering tendency seemed also to correlate well with the experiences achieved from full-scale and pilot-scale operation. Addition of limestone decreased sintering of one coal ash when the amount exceeded the Ca/S ratio of 1.3. The decrease was even greater when an Al-Si-based clay mineral was used.

Sintering Behaviors and Consolidation Process for Al2O3/SiC Nanocomposites

Abstract: Al2O3/SiC nanocomposites exhibited the high fracture toughness and strength at room temperature and good high-temperature mechanical properties. These significant improvements of mechanical properties were contributed to the dispersion of nanometer-size SiC particle within Al2O3 matrix grains. In this paper, the sinterability of Al2O3/SiC nanocomposites containing 5vol%SiC particles was compared to the sinterability of monolithic Al2O3. Especially, the densification and grain growth behaviors of Al2O3 with nanometer-size SiC particles were investigated in a temperature range of 1000° to 1800°C during hot-pressing to clarify the role of nanometer-size SiC particle for Al2O3/5vol% SiC nanocomposites. The addition of SiC particle remarkably delayed the densification rate and grain growth of Al2O3 matrix grain. In addition, TEM observation showed that SiC particles were dispersed within Al2O3 matrix grains for Al2O3/SiC nanocomposite hot-pressed above 1200°C. The consolidation process will be discussed in detail for these Al2O3/SiC nanocomposites.

Sintering of in-Situ Synthesized Sic–TiB2 Composites with Improved Fracture Toughness

Abstract: TiB2-particle reinforcement is one of the most successful methods for improving the fracture toughness of SiC ceramics.1–3 Commercially available TiB2 powders, however, have a large particle size and/or are highly reactive so that they are not favorable as a starting powder. In the present work, TiB2 particles are formed by an in situ reaction between TiC and boron. The reaction takes place during sintering between 1000° and 1600°C and is accompanied by a large volume expansion. Under optimum conditions, dense composites (> 98% of theoretical) can be obtained by pressureless sintering using B and C as sintering additives. The in situ reaction method enables, for the first time, a complete densification of SiC-particulate composites by pressureless sintering. The fracture toughness of the composites was approximately 30% higher than that of the monolithic SiC ceramic.

Effect of Pore Distribution on Microstructure Development: III, Model Experiments

Abstract: Model experiments have been conducted on a series of alumina samples in which the microstructures have been tailored to conform to the classical configuratins depicted in the models of final-stage sintering. Simultaneous measurements of sintered density, grain size, pore number density, and pore size distribution were made as a function of sintering time at constant temperature (1850°C). The data supported a model of grain-boundary-diffusion-controlled densification and surface-diffusion-controlled grain growth. An atom flux equation for grain-boundary diffusion transport was deduced from the data. The kinetics analysis highlights the importance of incorporating the number of pores per grain as an independent variable in mechanistic studies of final-stage sintering. The number of pores per unit volume was identified as a critical factor influencing densification kinetics. The effect of pore distribution on microstructure development was simulated for comparison with the data obtained from the model experiments.

Influence of additives on the γ-to-α transformation of alumina

Abstract: Some metal ions are known to affect the γ to α transformation temperature. Comparison of the influence of a variety of additives , particularly those forming a liquid phase, at relatively low concentrations (1 and 5 mol%), by means of differential thermal analysis, X-ray diffraction and sintering dilatometry studies.

Pressureless sintering and related reaction phenomena of Al2O3-doped B4C

Abstract: The effects of alumina on the densification of boron carbide and related reaction phenomena in alumina-doped B4C were studied. Pressureless sintering was conducted at various temperatures for 15 min in a flowing Ar atmosphere. The addition of alumina improved the densification of boron carbide. Maximum density of 96% theoretical was obtained with the 3 wt % alumina-doped B4C sintered at 2150°C. Abnormal (or exaggerated) grain growth was observed in the specimen containing more than 4 wt % alumina. In the B4C-Al2O3 reaction couples, good wetting of the liquid phase on the boron carbide grains was observed. X-ray diffraction and Auger electron spectra showed that the AlB12C2 phase was formed by the reaction between boron carbide and alumina. It is suggested that these phenomena promote the densification of boron carbide.

Reaction-sintered hot-pressed TiAl

Abstract: Titanium aluminide intermetallic alloys and composites were formed from elemental titanium and aluminium powders by self propagating, high-temperature synthesis in an induction-heated hot-press. The crystal phases, density, transverse rupture stress, and hardness of the reaction-sintered compacts, were observed to be controlled by hot-pressing conditions. The principal phase formed was TiAl together with a significant second-phase concentration of Ti3AI. The transverse rupture strength (TRS) of the intermetallic composites was observed to vary directly with compact density. Under selected high-temperature synthesis hot-pressing conditions, TRS values were comparable to those obtained for fully dense TiAl. Titanium aluminide composites were formed by adding boron, carbon, silicon and Al2O3, and SiC powders and whiskers to the Ti-Al powders before reaction sintering. Changing the alloying additions did not have as strong an effect on properties of the composite compacts as did varying hot-pressing conditions.

Thermal conduction mechanism of aluminium nitride ceramics

Abstract: Extremely large grain size AIN ceramics were produced by HIP sintering at an ultra-high temperature of 2773 K without reducing the oxygen content in order to determine experimentally whether the factor controlling thermal conductivity is either grain boundaries or the internal structure of the grains. The room-temperature thermal conductivity of the HIPed AIN with a grain size of ∼40 μm was 155 Wm−1 K−1, and was almost equal to that of the normally sintered AIN with a grain size of 4 μm. Therefore, thermal conductivity at room temperature is independent of AIN grain size, or the number and amount of grain-boundary phase for reasonably well-sintered AIN ceramics. The calculated phonon mean free path of sintered bodies was 10–30 nm at room temperature, which is too small to compare with the AIN grain size. Consequently, it is shown that the thermal conductivity of sintered AIN is controlled by the internal structure of the grains, such as oxygen solute atoms.

Rheology of porous alumina and simulation of hot isostatic pressing

Abstract: Rheology of a porous alumina was studied using sinter-forging, hot-pressing, and sintering tests. The results are analyzed using constitutive equations for porous materials. The deformation and densification rates are found to follow Coble creep behavior with an eventual control by interface reactions. The effect of porosity on shear and densification behavior is studied and compared to results obtained on metal powders showing similar trends for shear and a stronger influence in the case of densification. Large pores are likely to buckle at low densities when external forces are applied. The sintering pressure is also estimated and lies in the range 0 to 3 MPa. Finally, the constitutive equations are used to simulate hot isostatic pressing of test shapes, showing that the proposed model correctly predicts the deformation of the ceramic preforms.