Showing papers on "Sintering published in 1998"

Theory of sintering: from discrete to continuum

Abstract: Theoretical concepts of sintering were originally based upon ideas of the discrete nature of particulate media. However, the actual sintering kinetics of particulate bodies are determined not only by the properties of the particles themselves and the nature of their local interaction with each other, but also by macroscopic factors. Among them are externally applied forces, kinematic constraints (e.g. adhesion of the sample's end face and furnace surface), and inhomogeneity of properties in the volume under investigation (e.g. inhomogeneity of initial density distribution created during preliminary forming operations). Insufficient treatment of the questions enumerated above was one of the basic reasons hindering the use of sintering theory. A promising approach is connected with the use of continuum mechanics, which has been successfully applied to the analysis of compaction of porous bodies. This approach is based upon the theories of plastic and nonlinear-viscous deformation of porous bodies. Similar ideas have recently been embodied in a continuum theory of sintering. The main results of the application of this theory for the solution of certain technological problems of sintering are introduced including their thermo–mechanical aspects.

High-strength alkali-resistant sintered SiC fibre stable to 2,200 °C

Abstract: The high-temperature stability of SiC-based ceramics has led to their use in high-temperature structural materials and composites1,2,3 In particular, silicon carbide fibres are used in tough fibre-reinforced composites Here we describe a type of silicon carbide fibre obtained by sintering an amorphous Si–Al–C–O fibre precursor at 1,800 °C The fibres, which have a very small aluminium content, have a high tensile strength and modulus, and show no degradation in strength or change in composition on heating to 1,900 °C in an inert atmosphere and 1,000 °C in air — a performance markedly superior to that of existing commercial SiC-based fibres such as Hi-Nicalon Moreover, our fibres show better high-temperature creep resistance than commercial counterparts We also find that the mechanical properties of the fibres are retained on heating in air after exposure to a salt solution, whereas both a representative commercial SiC fibre and a SiC-based fibre containing a small amount of boron were severely degraded under these conditions4 This suggests that our material is well suited to use in environments exposed to salts — for example, in structures in a marine setting or in the presence of combustion gases containing alkali elements

Ceramic suspensions suitable for stereolithography

Abstract: Ceramic three dimensional parts have been fabricated by a Stereolithography (SL) process using a ceramic slurry containing alumina powder, UV curable monomer, diluent, photoinitiator and dispersant, subsequent removal of organic components and sintering. The SL process consists of fabricating parts with complex shapes layer by layer by laser polymerization of a ceramic/resin mixture. The effects of each component on the rheology of the ceramic suspension were investigated. Both, the addition of dispersant and diluent to the curable monomer and the increase in temperature decrease the viscosity down to suitable values for tape casting of the layers and for SL. The homogeneous and stable high ceramic concentration suspensions (53 vol%) exhibited a shear thinning behavior, which is favorable for casting the layers. Adequate cured depth (above 200 μm) and width were obtained even at high scanning speeds with an argon ionized laser.

Powder Metallurgy of Iron and Steel

Abstract: Iron and Steel. The Powder Metallurgy Process. Iron and Steel Powders. Powder Shaping. Compaction. Sintering Behavior. Production Sintering Operations. Full--Density Processing. Heat Treatment. Finishing Operations. Properties. Design Considerations. Products. Appendices. Index.

The role of microstructure and processing on the proton conducting properties of gadolinium-doped barium cerate

Abstract: The influence of grain boundary conductivity and microstructure on the electrical properties of BaCe0.85Gd0.15O3–δ have been examined. Grain sizes were varied by sintering at various temperatures. Impedance data were analyzed using the brick layer model, and some new consequences of this model are presented. The specific grain boundary conductivity exhibits an activation energy of ~0.7 eV, and for similar processing routes, is independent of grain size. An isotope effect was observed, indicating that protons (or deuterons) are the mobile species. TEM investigations showed the intergranular regions to be free of any glassy phase that could account for the differences in bulk and grain boundary properties. Single-crystal fibers, grown by a modified float zone process, were notably barium deficient, and exhibited a low conductivity, comparable to that of polycrystalline Ba0.96Ce0.85Gd0.15O3–δ.

Porous materials : process technology and applications

Abstract: Preface. 1: Introduction. 1.1. What are porous materials? 1.2. Classification of porous materials. 2: Powder compacts and green bodies for porous materials. 2.1 .Sintering as a process for producing porous materials. 2.2. The raw powder. 2.3. Green bodies. 2.4. Pore forming agents. 3: Sintering mechanisms and advanced sintering methods for porous materials. 3.1. Theory of sintering for porous materials. 3.2. Conventional sintering processes for producing porous materials. 3.3. Advanced production methods for porous materials. 4: Sol-gel processing, designing porosity, pore size and polarity, and shaping processes. 4.1. Introduction. 4.2. Factors affecting porosity, pore size and plarity. 4.3. Xerogels. 4.4. Aerogels. 5: Applications of porous materials. 5.1. Filters. 5.2. Catalysts. 5.3. Bioreactors. 5.4. Cells. 5.5. Grinding wheels. 5.6. Gas sensors. 5.7. Gas separators. 5.8. Electrodes. 5.9. Porous metal bearings. 5.10. Thermal insulators. 5.11. Capacitors. 5.12. Impact energy absorbers. 5.13. Heaters and heat exchangers. 5.14. Molds. 5.15. Surgical implants. 6: Properties and evaluation techniques for porous materials. 6.1. Density and porosity. 6.2. Pore size. 6.3. Specific surface area. 6.4. Fluid permeability. 6.5. Mechanical properties. 6.6. Entropy (surface state of porous materials). Appendix A: Entropy of porous materials. A.1. Properties of entropy. A.2. Absolute temperature. A.3. Potential energy. A.4. Basic thermodynamics. A.5. Entropy. A.6. More on potential theory and thermodynamics. A.7. Proof of nonpotentiality of the gradient of energy functions. A.8. Conclusions. Index.

Molten salt synthesis of lead-based relaxors

Abstract: The molten salt synthesis (MSS) of lead-based relaxors which have a perovskite structure, A(BIBII)O3 where BI is Mg2+, Fe3+, Zn2+, Ni2+ or Co2+, and BII is Nb5+, has been reviewed with regard to the formation of the perovskites, phase stability and morphology characteristics. Two relaxor materials, Pb(Mg1/3Nb2/3)O3 and Pb(Fe1/2Nb1/2)O3 were found to be successfully synthesized at a low temperature in a very short time by the MSS method. Using the example of Pb(Mg1/3Nb2/3)O3, the phase stability has been discussed on the basis of thermal and chemical analyses. The influences of the processing parameters, such as temperature, time, type and amount of salt, and non-stoichiometry, on the formation and the powder characteristics of the perovskite phase were investigated with possible explanations for the observed differences which were induced by changing the parameters. Finally, densification behaviour and dielectric properties resulting from the MSS powder were examined and compared to those of powders obtained by using the conventional mixed oxides (CMO) method. © 1998 Kluwer Academic Publishers

Sintering and Creep Behavior of Plasma-Sprayed Zirconia and Hafnia Based Thermal Barrier Coatings

Abstract: The sintering and creep of plasma-sprayed ceramic thermal barrier coatings under high temperature conditions are complex phenomena. Changes in thermomechanical and thermophysical properties and in the stress response of these coating systems as a result of the sintering and creep processes are detrimental to coating thermal fatigue resistance and performance. In this paper, the sintering characteristics of ZrO2-8wt%y2O3, ZrO2-25wt%CeO2-2.5wt%Y2O3, ZrO2-6w%NiO- 9wt%Y2O3, ZrO2-6wt%Sc2O3-2wt%y2O3 and HfO2-27wt%y2O3 coating materials were investigated using dilatometry. It was found that the HfO2-Y2O3 and baseline ZrO2-Y2O3 exhibited the best sintering resistance, while the NiO-doped ZrO2-Y2O3 showed the highest shrinkage strain rates during the tests. Higher shrinkage strain rates of the coating materials were also observed when the specimens were tested in Ar+5%H2 as compared to in air. This phenomenon was attributed to an enhanced metal cation interstitial diffusion mechanism under the reducing conditions. It is proposed that increased chemical stability of coating materials will improve the material sintering resistance.

Synthesis and characterization of carbonate hydroxyapatite.

Abstract: Substituted apatite ceramics are of clinical interest as they offer the potential to improve the bioactive properties of implants. Carbonate hydroxyapatite (CHA) has been synthesized by an aqueous precipitation method and precipitates with two different levels of carbonate, processed as powders. Sintering experiments were performed to establish the influence of carbonate in significantly reducing the temperature required to prepare high-density ceramics when compared with stoichiometric hydroxyapatite (HA). High-temperature X-ray diffraction was used to characterize the phase stability of the apatites on sintering. Increasing carbonate content was shown to reduce the temperature at which decomposition occurred, to phases of CaO and beta-TCP. Mechanical testing, performed using biaxial flexure, showed that the CHA specimens had strengths similar to stoichiometric HA.

The role of viscoelasticity in polymer sintering

Abstract: An experimental study for polymer sintering has been carried out using pairs of powder particles. Although in many cases Newtonian sintering models successfully describe polymer sintering, they predict a faster coalescence rate than that observed with the polypropylene copolymer resins used in this study, indicating that factors other than the surface tension and the viscosity play a role in polymer sintering. Observations of coalescence under the microscope and rotational molding experiments suggest that melt elasticity slows down the process. Based on these findings, a mathematical model describing the complete polymer sintering process for viscoelastic fluids has been developed. The approach was similar to that of Frenkel (1945) and the convected Maxwell constitutive equations were used together with the quasi-steady state approximation. The proposed viscoelastic sintering model is capable of predicting the sintering rate slowdown observed in this study.

Nanoparticle sintering simulations

Abstract: Molecular dynamics techniques were used to simulate Cu and Au nanoparticle arrays at different temperatures to study surface energies, grain boundary mobility and sintering. Preliminary results of the studies on multi-particle arrays several hundred degrees below the melting point ( T m ) show unexpectedly strong contributions from plastic deformation, mechanical rotations, amorphisation and ultra-rapid atomic force driven diffusion effects. Evidence is offered that the basic processes and kinetics of nanoscale sintering are fundamentally different from that of normal micronscale sintering.

Sintering Model for Mixed‐Oxide‐Derived Lead Zirconate Titanate Ceramics

Abstract: The properties of lead zirconate titanate (PZT) ceramics are determined by the microstructure and chemical homogeneity of Zr, Ti, and dopants within the grains as well as the presence of secondary grain boundary phases. Stoichiometric 53/47 PZT and compositions with 3 mol% PbO excess were prepared by the mixed-oxide process, and were densified by pressureless sintering in oxygen. The influence of PbO content and different La concentrations on the densification behavior was analyzed by dilatometric measurements. Quantitative image analysis showed a different relative density and grain size dependence for samples containing >0.5 mol% additives compared to samples with <0.5 mol% La. On the basis of a model experiment and by using different analytical methods (microprobe analysis, HRTEM, STEM, and Auger spectroscopy) three types of inhomogeneities could be detected in conventionally prepared PZT ceramics: the existence of Ti and La enrichment in the core of PZT grains, and PbO-rich secondary phases in triple junctions as well as in grain boundary films. The results of the microstructural characterization and the analysis of the densification behavior were finally combined to deduce a sintering model based on a Pb-vacancy concentration gradient within the PZT grains.

Processing and mechanical properties of boron carbide sintered with TiC

Abstract: The suitability of TiC as a sintering aid for boron carbide is investigated. The in situ reaction of TiC with boron carbide generates elemental carbon and TiB 2 which both aid the sintering process and permit pressureless sintering at temperatures between 2150 and 2200 °C. Relative densities of as-sintered materials exceed 93% of theoretical, but can be increased to nearly full density by subsequent hot isostatic pressing. The grain size of both B 4 C and TiB 2 increases with sintering temperature and decreases with the amount of sintering aid. The presence of TiB 2 causes a slight increase in Young's modulus and a small decrease of hardness as compared to single phase B 4 C materials. The flexural strength of B 4 C-TiC is found to decline with increasing fracture toughness. This behavior is related to crack length dependent toughness, particularly to the slope and steady state value of the R -curve. The dependence of R -curve properties on the grain size of the B 4 C matrix is discussed.

Sinterability of commercial 8 mol% yttria-stabilized zirconia powders and the effect of sintered density on the ionic conductivity

Abstract: The sintering behaviour of a number of commercially produced 8 mol% yttria-stabilized zirconia powders has been studied. The effect of different sintering regimes on the density and microstructure of the sintered ceramic was determined using density measurements, scanning electron microscopy (SEM) and dilatometry. The chemical homogeneity, particle size and the morphology of the as-received powder were related to the sintering behaviour of the different commercial powders. Powders prepared via a route which involved a spray-drying step sintered more readily than those prepared without a spray-drying step. Plasma-derived powders did not sinter to as high an apparent density as co-precipitated powders. The effect of sample density on the ionic conductivity of sintered YSZ ceramics was studied using a.c. impedance spectroscopy. This technique allowed separation of the bulk and grain-boundary components, enabling clear intepretation of the effects of sample porosity of the conduction pathways. Ceramics prepared from the three different powders achieved a bulk ionic conductivity of ∼16 S cm-1 at 1000 °C for sintered densities of 95% or greater. The results obtained are compared to values reported for a variety of other commercial powders. © 1998 Kluwer Academic Publishers

High surface area silicon carbide as catalyst support characterization and stability

Abstract: High surface area silicon carbide (SiC) of 30 m 2 /g has been synthesized by the catalytic conversion of activated carbon. The stability of this SiC in aqueous hydrogen fluoride and a boiling nitric acid solution is shown to be excellent. No corrosion is encountered by treatment with boiling HNO 3 , HF treatment causes the dissolution of the silica surface layer present on the SiC while the SiC remains intact. Oxidation in air at elevated temperatures has been analyzed by thermal gravimetric analysis, diffuse reflectance infrared spectroscopy, nitrogen adsorption, and X-ray diffraction. The thermal stability in non-oxidizing environments is shown to be excellent; no significant sintering has been observed after ageing in nitrogen for 4 h at 1273 K. The presence of 2 v% steam at 1273 K results in partial SiC oxidation into SiO 2 and considerable sintering. Air oxidation at 1273 K of pure SiC, SiC loaded with 5 wt.% nickel, and HNO 3 treated SiC is shown to cause substantial SiC conversion, viz. 60% to 70% after 10 h. Air oxidation at 1080 K will result in complete conversion in about 100 days. This rate of oxidation agrees with reports on the oxidation of non-porous Acheson SiC and SiC coatings formed by Chemical Vapour Deposition. It is concluded that at high surface area SiC cannot be used as a catalyst support in processes operating in oxidizing environments and temperatures above 1073 K. SiC based catalysts are very well suited for (1) high-temperature gas-phase reactions operating in the absence of oxidizing constituents (O 2 or H 2 O) and (2) strong acidic liquid-phase processes.

Electrical and co gas sensing properties of zno-sno2 composites

Abstract: ZnO–SnO 2 composites of 0–100 mol% composition range were fabricated in the form of pellet by sintering at 900°C and their electrical conductivity and CO gas sensitivity were measured between 70 and 500°C. SnO 2 -rich composites showed higher sensitivity values than pure SnO 2 . ZnO-rich composites have more porous microstructure and thus are more sensitive to CO gas at all temperatures than pure ZnO. The effects of microstructure and composition on the gas sensitivity were discussed. SnO 2 –ZnO grain boundary was also proposed to be responsible for the gas sensitivity.

Porous NiTi alloy prepared from elemental powder sintering

Abstract: An elemental powder sintering (EPS) technique has been developed for the synthesis of porous NiTi alloy, in which Ni and Ti powders are used as the reactants and TiH2 powder is added as a pore-forming agent and active agent. Effects of various experimental parameters (sintering temperature, sintering time, and TiH2 content) on the porosity, pore size, and pore distribution as well as phase composition in experimental alloys are investigated. It is found that in order to avoid the formation of carcinogenic pure Ni phase, the porous NiTi alloy should be synthesized over a temperature of 1223 K. This gives NiTi as the main phase without any elemental phase. Substitution of Ti by TiH2 is more economic and more favorable to obtain homogeneous porous NiTi alloy. A proper selection of initial powders, ball-milling, pressing, and sintering process makes it possible to achieve the porous NiTi alloy with desired properties.

Fabrication and mechanical properties of 5 vol% copper dispersed alumina nanocomposite

Abstract: An optimum route to fabricate the Al 2 O 3 Cu nano-composite with sound microstructure and desired mechanical properties was investigated. Two methods for developing a uniform dispersion of Cu particles in Al 2 O 3 were compared on the basis of the resulting microstructures and mechanical properties. SEM and TEM analyses for the composites fabricated by reduction and sintering process using Al 2 O 3 CuO powder mixture showed that the nano-sized Cu particles were well distributed and situated on the grain boundaries of the Al 2 O 3 matrix. The composite, hot-pressed at 1450 °C, exhibited the maximum fracture strength and enhanced toughness compared with monolithic Al 2 O 3 . The strengthening was mainly attributed to the refinement of Al 2 O 3 matrix grains. The toughening mechanism in Al 2 O 3 Cu composite was discussed by the observed microstructural features and theoretical predictions based on crack bridging model and thermal residual stress effect.

Analysis of Microwave Sintering of Ceramics

Abstract: The microwave heating of ceramic materials has been analyzed by solving the equations for grain growth and porosity (Svoboda and Riedel, 1992) during the late stages of sintering, coupled with the heat conduction equation and electric field equations for 1-D slabs. Microwave power absorption and heating profiles have been calculated for $Al_2O_3$ and Sic in the absence of sintering, and calculations have been cam'ed out to study the effect of increasing dielectric loss of $Al_2O_3$ as a function of temperature. A comparison of the densification and grain growth for $Al_2O_3$ during microwave and conventional sintering indicates that within the framework of the present model, there is no difference between the two heating modes during the late stages of sintering.

High temperature mechanical behaviour of liquid phase sintered silicon carbide

Abstract: Silicon carbide was liquid phase sintered using Y 2 O 3 and AlN as sintering additives. A globular grained microstructure was obtained from α-SiC powder whereas a mixture of β-SiC, with a small amount of α-SiC seeds, revealed platelet shaped grains with an aspect ratio of eight. Beside this difference in grain morphology of the α-SiC grains, the phase content of the grain boundary phase is different. The influence of both aspects onto the mechanical behaviour, i.e. four-point bending strength between room temperature and 1400°C and fracture toughness between room temperature and 1100°C, are investigated.

Consolidation of MA amorphous NiTi powders by spark plasma sintering

Abstract: Spark plasma sintering, as a recent innovation in rapid powder consolidation was used to sinter amorphous NiTi alloy obtained from mechanical alloying. Densified samples with Ni3Ti, NiTi and NiTi, phases were produced. The grain sizes were retained to within 500 nm at the sintering temperature of 900 degrees C and 1 mu m for the sintering temperature of 1100 degrees C. Based on the results of SEM and TEM, it is found that the formation of crystalline structure is controlled by solid state reaction when the sample was sintered at 900 degrees C, but in the sample compacted at 1100 degrees C, liquid was introduced for the low melting point phase NiTi2 melt. The products after sintering associated with the advantage of spark plasma sintering and the property of amorphous NiTi alloys after mechanical alloying is discussed. (C) 1998 Elsevier Science S.A.

Microwave sintering of hardmetals

Abstract: Application of microwave radiation as a heat source for sintering of hardmetal is described. Sintering of hardmetal with microwaves leads to a finer microstructure because of lower sintering temperatures and shorter processing times. A further variant is the microwave reaction sintering of a powder mixture of metallic tungsten, carbon and cobalt to obtain finer microstructures than by the conventional route. Moreover, this process offers a great potential for simplifying and shortening the process sequence in hardmetal production. The in-situ formation of WC-platelets during microwave reaction sintering was observed.

Preparation and characterization of spinel-type Mn–Ni–Co–O negative temperature coefficient ceramic thermistors

Abstract: Powders of ternary Mn–Ni–Co oxide negative temperature coefficient thermistors were synthesized by a low-temperature alternative route. The procedure allowed straightforward preparation without the addition of any binder, of highly densified Mn–Ni–Co–O semiconducting ceramics as cubic single-phase spinels, at relatively moderate temperature (≈1000 °C). A tentative cation distribution for the Mn1.5Ni0.6Co0.9O4 spinel oxide has been proposed, and its variation with temperature has also been considered. The dilatometric and X-ray powder diffraction studies carried out for Mn1.5Ni0.6Co0.9O4 showed that sintering takes place in a single stage between 900 and 1000 °C, and yields highly densified ceramic with an apparant density larger than 96% of the calculated X-ray density. Scanning electron microscopy showed different microstructures for the Mn1.5Ni0.6Co0.9O4 spinel oxide, depending on sintering conditions. The value of the sensitivity index, β=3068 K, indicates a good technological thermistor performance for this material. © 1998 Chapman & Hall