Showing papers on "Sintering published in 1999"

Full sintering of powdered-metal bodies in a microwave field

Abstract: The use of microwaves to process absorbing materials was studied intensively in the 1970s and 1980s, and has now been applied to a wide variety of materials1,2,3,4. Initially, success in microwave heating and sintering was confined mainly to oxide and some non-oxide ceramics5,6,7,8,9,10,11; but recently the technique has been extended to carbide semimetals12,13,14 used in cutting tools. Here we describe the microwave sintering of powdered metals to full density. We are able to sinter a wide range of standard powdered metals from commercial sources using a 2.45-GHz microwave field, yielding dense products with better mechanical properties than those obtained by conventional heating. These findings are surprising in view of the reflectivity of bulk metals at microwave frequencies. The ability to sinter metals with microwaves should assist in the preparation of high-performance metal parts needed in many industries, for example, in the automotive industry.

Thermal barrier coating systems and materials

Abstract: A ceramic material has particular utility as a thermal insulating or thermal barrier coating on metallic substrates. The ceramic material includes gadolinia and zirconia, preferably forming gadolinia-zirconia oxide. The material may include fluorite and pyrochlore structure. This material exhibits chemical stability, thermal stability and thermal insulating properties superior to those of currently used thermal barrier ceramics, and also provides resistance to sintering and erosion comparable to currently used ceramics. A preferred material has between about 5-60 mol. % gadolinia.

Development of Large-Size Ceramic/Metal Bulk FGM Fabricated by Spark Plasma Sintering

Abstract: Large-size ceramic/metal bulk FGMs have been fabricated on a recently developed and world's largest Spark Plasma Sintering (SPS) system. According to the development program for practical production processes and machines for FGMs by SPS, the processes, mechanical properties, dimensional size and shape effects, and production machine systems were investigated. A disk-shaped sintered compact with a diameter of 100mm and thickness of approximately 17mm ZrO 2 (3Y)/stainless steel FGM was homogeneously consolidated in a shorter sintering time, while maintaining high quality and repeatability by utilizing temperature gradient sintering method. The SPS heating-up and holding time totalled less than one hour.

Surface oxide and the role of magnesium during the sintering of aluminum

Abstract: The effect of trace additions of magnesium on the sintering of aluminum and its alloys is examined. Magnesium, especially at low concentrations, has a disproportionate effect on sintering because it disrupts the passivating Al2O3 layer through the formation of a spinel phase. Magnesium penetrates the sintering compact by solid-state diffusion, and the oxide is reduced at the metal-oxide interface. This facilitates solid-state sintering, as well as wetting of the underlying metal by sintering liquids, when these are present. The optimum magnesium concentration is approximately 0.1 to 1.0 wt pct, but this is dependent on the volume of oxide and, hence, the particle size, as well as the sintering conditions. Small particle-size fractions require proportionally more magnesium than large-size fractions do.

Origin of Solid‐State Activated Sintering in Bi2O3‐Doped ZnO

Abstract: Activated sintering in Bi2O3-doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic-level microstructures and bismuth solute distributions in doped powders have been investigated using high-resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi2O3 concentrations as low as 0.58 mol%. Transmission electron microscopy of as-calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm thickness, terminating in wetting menisci at sinter-necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free-surface counterpart to equilibrium-thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, equilibrium-thickness intergranular films, with the amorphous phase also providing capillary pressure.

Nanostructured pure and Nb-doped TiO2 as thick film gas sensors for environmental monitoring

Abstract: Thick films of nanostructured TiO 2 and Niobium-doped TiO 2 have been fabricated by screen-printing technology starting from pure Titania and Niobium-doped Titania powders; the powders were prepared by laser pyrolysis method which provides nanosized particles The laser powders are crystalline with anatase structure and an average specific surface area of approximately 100 m 2 g −1 ; their grain size ranges from 10 up to 25 nm, the particles are spherical monocristalline and without internal porosity In this work we evidentiated that TiO 2 -based thick film sensors exhibit a suitable sensitivity to atmospheric environmental monitoring provided that the microstructural properties of the materials are suitably correlated to the required electrical features Moreover nanostructured particles were obtained at high firing temperature by adding a proper metal ions to inhibit the grain sintering Finally, prototype sensors based on pure titania sensing film have been prepared and tested in field for environmental monitoring application

Synthesis and sintering of rare-earth-doped ceria powder by the oxalate coprecipitation method

Abstract: Doped ceria, which has a higher oxygen ion conductivity than yttria-stabilized zirconia, is one of the possible electrolytes for solid oxide fuel cell at low temperatures. This study concerns powder preparation and densification of rare-earth-doped ceria. Rare-earth-doped ceria powders with a composition of Ce0.8R0.2O1.9 (R = Yb, Y, Gd, Sm, Nd, and La) were prepared by heating the oxalate coprecipitate when a mixed rare earth/cerium nitrate solution was added to an oxalic solution. The oxalate and derived-oxide powders were characterized by x-ray diffraction (XRD), thermogravimetry differential thermal analysis (TG-DTA), particle size analyzer with laser diffraction, inductively coupled plasma (ICP), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). This method provided the oxalate solid solutions containing Ce and R, which were calcined to form the oxide solid solutions at 600 °C in air. The lattice parameter of oxide powders increased linearly with increasing ionic radius of doped rare earth. The size of platelike particles of oxalates and oxides depended on the concentration of oxalic acid and showed a minimum at 0.4 M oxalic acid. Dry milling of oxide powder with α–Al2O3 ball was effective in reducing the size and aspect ratios of particles with little contamination of Al2O3. These rare-earth-doped ceria powders with various sizes were formed by uniaxial pressing (49 MPa) followed by cold isostatic pressing (294 MPa), and sintered at 900–1600 °C in air for 4 h. The micrometer-sized-doped CeO2 powders were densified above 95% of the theoretical density at 1200 °C. The grain size of rare-earth-doped ceria after sintering at 1600 °C was larger in the samples with the larger rare-earth element.

Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals.

Abstract: Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO3)2 + 6(NH4)2HPO4 + 8NH4OH), could produce dense coatings when sintered at 875-1000degreesC. Single EPD coatings cracked during sintering owing to the 15-18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the "valleys" in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be approximately 12 MPa on a titanium substrate and approximately 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since -316L (20.5 microm mK(-1)) > alpha-HAp (approximately 14 microm mK(-1)), resulting in residual compressive stresses in the coating, whereas alpha-titanium (approximately 10.3 microm mK(-1)) < alpha-HAp, resulting in residual tensile stresses in the coating.

Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering

Abstract: Dense BaTiO3 ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO3 (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X-ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000-1200°C, because of the short sintering period (5 min). Cubic-phase BaTiO3 coexisted with tetragonal BaTiO3 at room temperature in the SPS pellets, even when well-defined tetragonal-phase BaTiO3 powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent100-106 Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.

Molybdenum silicide based materials and their properties

Abstract: Molybdenum disilicide (MoSi2) is a promising candidate material for high temperature structural applications. It is a high melting point (2030 °C) material with excellent oxidation resistance and a moderate density (6.24 g/cm3). However, low toughness at low temperatures and high creep rates at elevated temperatures have hindered its commercialization in structural applications. Much effort has been invested in MoSi2 composites as alternatives to pure molybdenum disilicide for oxidizing and aggressive environments. Molybdenum disilicide-based heating elements have been used extensively in high-temperature furnaces. The low electrical resistance of silicides in combination with high thermal stability, electronmigration resistance, and excellent diffusion-barrier characteristics is important for microelectronic applications. Projected applications of MoSi2-based materials include turbine airfoils, combustion chamber components in oxidizing environments, missile nozzles, molten metal lances, industrial gas burners, diesel engine glow plugs, and materials for glass processing. In this paper, synthesis, fabrication, and properties of the monolithic and composite molybdenum silicides are reviewed.

Microstructure and Fracture Toughness of Si3N4 Ceramics: Combined Roles of Grain Morphology and Secondary Phase Chemistry

Abstract: Silicon nitride materials that contained different mixtures of sintering aids were investigated with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted in order to coarsen the respective microstructures. Although constant processing conditions were used, a marked variation in fracture toughness of the Si3N4 materials was evaluated. With a larger grain diameter of the Si3N4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent aspect ratio could not be established. The observed changes in microstructure were in fact caused by the difference in secondary-phase chemistry. Si3N4 grain growth was dominated by diffusion-controlled Ostwald ripening and was hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sintering additives employed and was found to influence fracture toughness by altering the crack-propagation mode as a consequence of local residual microstresses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a variation in interface chemistry changes the glass network structure on the atomic level, which can promote transgranular fracture, i.e., can result in a low fracture resistance even in the presence of favorable large Si3N4 matrix grains. Therefore, secondary-phase chemistry plays a dominant role with respect to the mechanical behavior of liquid-phase-sintered Si3N4. Fracture toughness is, in particular, influenced by (i) altering the residual glass network structure, (ii) affecting the secondary-phase crystallization at triple pockets, and (iii) changing the Si3N4 grain size/morphology by affecting the diffusion rate in the liquid. The first two effects of secondary-phase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.

Hydrogen Storage Alloys with PuNi3 ‐ Type Structure as Metal Hydride Electrodes

Abstract: A powder sintering method was used to synthesize the intermetallic compounds , , , and . The microstructure and primary phases were observed by scanning electron microscopy and X‐ray diffraction. The pressure‐composition isotherms showed that all alloys could reversibly absorb and desorb up to hydrogen at and a hydrogen pressure of . The sintered samples were employed as the active materials of metal hydride electrodes. The hydride stability and electrochemical performance, combined with low cost raw materials, make these compounds attractive for metal hydride electrodes. ©2000 The Electrochemical Society

Selective laser sintering of an amorphous polymer : simulations and experiments

Abstract: Thermal and powder densification modelling of the selective laser sintering of amorphous polycarbonate is reported. Three strategies have been investigated: analytical, adaptive mesh finite difference and fixed mesh finite element. A comparison between the three and experimental results is used to evaluate their ability reliably to predict the behaviour of the physical process. The finite difference and finite element approaches are the only ones that automatically deal with the non-linearities of the physical process that arise from the variation in the thermal properties of the polymer with density during sintering, but the analytical model has some value, provided appropriate mean values are used for thermal properties. Analysis shows that the densification and linear accuracies due to sintering are most sensitive to changes in the activation energy and heat capacity of the polymer, with a second level of sensitivities that includes powder bed density and powder layer thickness. Simulations of ...

Factors Affecting the Electrical Conductivity of Donor‐Doped Bi4Ti3O12 Piezoelectric Ceramics

Abstract: High-temperature piezoelectric ceramics based on W6+-doped Bi4Ti3O12 (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800° and 1150°C in air. A rapid densification, >99% of the theoretical density (rhoth) at 900°C/2 h, took place in the chemically prepared W6+-doped Bi4Ti3O12 ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of rhoth, at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W6+-doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO2 produced a further decrease of the electrical conductivity.

Mechanical Properties and Microstructure of Nano-SiC–Al2O3 Composites Densified by Spark Plasma Sintering

Abstract: Heterogeneous precipitation method has been used to produce 5 vol% SiC–Al2O3 powder, from aqueous suspension of nano-SiC, aqueous solution of aluminium chloride and ammonia. The resulting gel was calcined at 700°C. Nano-SiC–Al2O3 composites were densified using spark plasma sintering (SPS) process by heating to a sintering temperature at 1350, 1400, 1450, 1500 and 1550°C, at a heating rate of 600 °/min, with no holding time, and then fast cooling to 600°C within 2–3 min. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 vol% SiC–Al2O3 densified by SPS at 1450°C reached as high as 1000 MPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al2O3 grains and the fracture mode of the nanocomposites was mainly transgranular fracture.

Titanium carbide based composites for high temperature applications

Abstract: Titanium carbide based composites with nickel alloys and iron alloys are currently used in high performance applications where wear and corrosion are the main sources of material failure. For high temperature critical applications, however, the metallic binders nickel and iron limit the use of TiC-based composites. Hence, new binder systems which have good high temperature properties need to be developed in order to extend the use of TiC-based composites. Silicides and aluminides are potential binder systems with their good high temperature corrosion and mechanical properties. In this study, two binder systems, Fe–25 at% Si and Fe–40 at% Al have been selected, and were processed with reaction sintering of elemental Fe and Si, or Fe and Al powders, with 65 wt% TiC and 80 wt% TiC powders at temperatures at around 1410–1430°C under vacuum. X-ray diffraction analysis show TiC and Fe 3 Si phases in the TiC-iron silicide composites, whereas TiC, Fe 3 Al 2 and Fe 3 AlC 0·5 phases were observed in TiC–iron aluminide composites. Differential thermal analysis of the samples shows that liquidus temperatures of the iron-silicide and iron-aluminide binders were around 1265 and 1425°C, respectively. Vickers microhardness values of 1100–1470 kg mm −2 and 3-point bending strengths of 600–775 MPa were obtained in these high density reaction sintered TiC–iron silicide and TiC-iron aluminide composites.

Defect reactions and the controlling mechanism in the sintering of magnesium aluminate spinel

Abstract: Pressureless sintering studies have been conducted for excess Al2O3, stoichiometric, and excess MgO compositions of MgAl2O4 at 1500-1625°C. Initial powders of various compositions are prepared by solid-state reaction of MgO and Al2O3. A Brouwer defect equilibrium diagram is constructed that assumes intrinsic defects of the Schottky type. The densification rate derived from sintering kinetics is compared with the compositions investigated when the concentration is converted to the activity of the two oxide components in MgAl2O4. The grain-size exponent of p similar/congruent 3 suggests that densification takes place by a lattice-diffusion mechanism in the solid state. Determined activation enthalpies of 489-505 kJmol-1 are close to those obtained from oxygen self-diffusion derived in previous sintering studies. It is, therefore, proposed that oxygen lattice diffusion through vacancies is the rate-controlling mechanism for the sintering of nonstoichiometric MgAl2O4 compositions. The discrepancy between densification-rate ratios in experimental results and oxygen vacancy concentration in the Brouwer diagram is accounted for by the defect associates formed in the nonstoichiometric compositions.

Elastic properties of high alumina cement castables from room temperature to 1600°C

Abstract: High-alumina refractory castables with compositions in the systems CaO–Al2O3 and CaO–Al2O3–SiO2 were studied using an ultrasonic technique. The technique allows in-situ, non-destructive measurement of Young's modulus from room temperature to 1600°C. Elastic and dilatometric properties were investigated in relation to phase changes (followed by XRD) and sintering phenomena. The conversion of CAH10, the hydration of still-anhydrous cement phases, and the dehydration of C3AH6 and AH3 are related with events in Young's modulus evolution. Addition of 1 wt% of silica fume strongly decreases the high-temperature mechanical properties.

Microwave properties of zinc, barium and lead borosilicate glasses

Abstract: The trend to fabricate mulitlayer structures of microwave dielectric ceramics cofired with low melting conductors results in the demand of additives for low temperature sintering. Low-melting glasses are good candidates for low temperature sintering aids. When the concentration of the added low-melting glasses reaches a high content of about 10–20 mol%, their microwave properties will certainly affect the performance of the final multilayer devices. It is the object of this study to investigate the microwave properties of low-melting ZnO–B 2 O 3 –SiO 2 , BaO–B 2 O 3 –SiO 2 , and PbO–B 2 O 3 –SiO 2 glasses. The parallel plate dielectric resonator rod method was employed for measurements of relative dielectric constants, quality factors, and temperature coefficients of frequency in the microwave frequency range. A thermo-mechanical analyzer was used to determine deformation temperatures of glasses. Properties of these three glass systems are chiefly determined by the amount of structural modifier oxides (ZnO, BaO, and PbO) and the ionic size of the modifying cations (Zn 2+ , Ba 2+ , Pb 2+ ). In contrast, the effect of the ratio of B 2 O 3 to SiO 2 is relatively small. The frequency constants of these three glass systems range from 500 to 3400, which are equivalent to tan δ of 0.02–0.003 at 10 GHz. The temperature coefficients of frequency range from −3 to −155 ppm/°C.

Microwave processing and properties of ceramics with different dielectric loss

Abstract: Microwave sintering behaviors of three kinds of ceramics with different dielectric loss [Al 2 O 3 , Ce–Y–ZrO 2 and lead-based relaxor ferroelectrics (PMZNT)] in 2·45 GHz microwave furnace were described. Measurement of sample densities showed an enhancement of the sintering processing for all materials studied. For PMZNT and Ce–Y–ZrO 2 with high dipolar loss or ionic conductive loss, the associated microstructure examined using scanning electron microscopy showed that microwave-sintered compacts produced much finer grain sizes at near theoretical density compared to conventional sintering. Resulting material properties, such as flexure strength and breakdown strength, were also increased due to developed microstructure in microwave processing. However, a comparable grain size and properties were observed for high pure Al 2 O 3 with low dielectric loss in microwave and conventional methods. ©

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

Abstract: The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from β to α-SiC reveals a dependency on the initial β-content and the sintering atmosphere. The transformation rate decreases with an increasing β-content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure β-SiC starting powders when the additive system consists of 10.34 wt % Y2O3 and 2.95 wt % AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial α/β-ratio (> 1 : 9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.

Optimization of powder layer density in selective laser sintering

Abstract: An important parameter for the overall quality of SLS parts is the density of powder layers before sintering. Previous studies have shown that the control of powder particle shape and size distribution can increase the density of non-packed powder beds. However, these studies concerned beds several orders of magnitude larger than the SLS layers. The purpose of this study is to determine if, and to what extent, the density of thin powder layers can be increased. Experiments show that the density of thin layers increases from 53% to 63% when adding 30% fine powder to the coarse powder, with a coarse-to-fine ratio of 1:10. Compared with the bulk experiments, this density improvement method is less efficient, because the particles do not arrange as efficiently, and the wall effects can become predominant.