Showing papers on "Sintering published in 2000"

Sintering dense nanocrystalline ceramics without final-stage grain growth

Abstract: Sintering is the process whereby interparticle pores in a granular material are eliminated by atomic diffusion driven by capillary forces. It is the preferred manufacturing method for industrial ceramics. The observation of Burke and Coble that certain crystalline granular solids could gain full density and translucency by solid-state sintering was an important milestone for modern technical ceramics. But these final-stage sintering processes are always accompanied by rapid grain growth, because the capillary driving forces for sintering (involving surfaces) and grain growth (involving grain boundaries) are comparable in magnitude, both being proportional to the reciprocal grain size. This has greatly hampered efforts to produce dense materials with nanometre-scale structure (grain size less than 100 nm), leading many researchers to resort to the 'brute force' approach of high-pressure consolidation at elevated temperatures. Here we show that fully dense cubic Y2O3 (melting point, 2,439 degrees C) with a grain size of 60 nm can be prepared by a simple two-step sintering method, at temperatures of about 1,000 degrees C without applied pressure. The suppression of the final-stage grain growth is achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. Such a process should facilitate the cost-effective preparation of other nanocrystalline materials for practical applications.

Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS)

Abstract: The graphite die set in spark plasma system (SPS) is heated by a pulse direct current. Weak plasma, discharge impact, electric field and electric current, which are based on this current, induce good effects on materials in the die. The surface films of aluminum and pure WC powders are ruptured by the spark plasma. Pure AlN powder is sintered without sintering additives in the electric field. The spark plasma leaves discharge patterns on insulators. Organic fibers are etched by the spark plasma. Thermosetting polyimide is consolidated by the spark plasma. Insoluble polymonomethylsilane is rearranged into the soluble one by the spark plasma. A single crystal of CoSb3 is grown from the compound powders in the electric field by slow heating. Coupled crystals of eutectic powder are connected with each other in the electric field.

Effect of metal particle size on coking during CO2 reforming of CH4 over Ni–alumina aerogel catalysts

Abstract: CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts prepared with various Ni loadings. The preparation of alumina supported Ni catalysts via sol–gel synthesis and subsequent supercritical drying led to the formation of very small metal particles, which are evenly distributed over the alumina support. The activity of the aerogel catalysts increased along with increasing metal loading, and eventually, the SAA25 (0.25 in Ni/Al mole ratio) catalyst exhibited the high activity comparable to that of a 5 wt.% Ru/alumina catalyst (ESCAT44, Engelhard). Compared to the alumina-supported Ni catalyst prepared by conventional impregnation method, Ni–alumina aerogel catalysts showed a remarkably low coking rate due to highly dispersed metal particles. From TEM micrograph studies, it was observed that the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7 nm. The loss of catalytic activity at 973 K was mainly caused by coke deposition and sintering.

Absorptance of powder materials suitable for laser sintering

Abstract: The normal spectral absorptance of a number of metal, ceramic and polymer powders susceptible to be utilised for selective laser sintering (SLS) technique was experimentally determined. The measurements were performed with two laser wavelengths of 1.06μm and 10.6μm obtained by using two lasers – Nd‐YAG and CO2 respectively. The change in the powder absorptance with time during laser processing was also investigated. The effect of the absorptance characteristics on the sintering process is discussed.

Sintering activation by external electrical field

Abstract: Field assisted sintering technique (FAST) is a non-conventional powder consolidation method in which densification is enhanced by the application of an electrical discharge combined with resistance heating and pressure. Interest in FAST is motivated by its ability to consolidate a large variety of powder materials to high densities in short times. Full densification of metal and ceramic powders has been achieved within minutes, with a reduced number of processing steps, no need for sintering aids and more flexibility in powder handling. Although the electrical discharge effects have not been completely elucidated, distinct surface effects created by micro-discharges have been noticed in FAST consolidated specimens such as atomically clean grain boundaries and new resistivity peaks in superconductors. On-going experimental and theoretical studies to provide more quantitative insight into the relevant FAST mechanisms are presented.

Effect of particle size and dopant on properties of SnO2-based gas sensors

Abstract: The effect of composition, microstructure, and defect chemistry on sensing performance of gas sensors based on CuO-doped SnO2 is investigated using sol–gel derived nano-sized powders (about 20 nm). The particle size of copper oxide doped tin oxide is varied by annealing at different temperatures and a significant grain growth is observed at temperatures above 1000°C due to the liquid phase sintering effect of copper oxide. The reduction of particle size to nanometers, or to the dimension comparable to the thickness of charge depletion layer, leads to a dramatic improvement in sensitivity and speed of response. It appears that the substitution of Sn by Cu in the cassiterite structure increases the concentration of oxygen vacancies and decreases the concentration of free electrons. In particular, the existence of cuprous ions (Cu+), due to partial reduction of Cu2+ during sintering, plays an important role in enhancing the sensor response to nitric oxide (NO) and CO2.

The dependency of microstructure and properties of nanostructured coatings on plasma spray conditions

Abstract: In this paper, Al2O3-13 wt.% TiO2 coatings formed via a plasma spray approach using reconstituted nanosized Al2O3 and TiO2 powder feeds are described. Effects of various plasma spray conditions on the microstructure, grain size, phase content and microhardness of the coatings have been evaluated. It is found that phase transformation of nanosized Al2O3 and TiO2 during heat treating, sintering and thermal spraying is, in general, identical to that of micrometer-sized counterparts. Furthermore, the particle temperature during thermal spray could be divided into three regimes, i.e. low, intermediate and high temperature regimes, according to the characteristics of the coating produced from the nanopowder. The hardness and density of the coating increase with the spray temperature. The phase content and grain size of the coating also exhibits a strong dependency on the spray temperature. The coating sprayed using nanopowder feed displays a better wear resistance than the counterpart sprayed using commercial coarse-grained powder feed. The observed phenomena are discussed in terms of physics of thermal spraying, mechanisms of coating growth and phase transformation of the oxides.

Low-Temperature Sintering of TiO2 Colloids: Application to Flexible Dye-Sensitized Solar Cells

Abstract: Colloidal TiO2 films have been prepared in a manner suitable for use with flexible substrates. Sol−gel particles were sintered under various conditions of temperature and initial presence/absence of organic surfactants. The physicochemical properties of the resulting films are reported. Independent of the sintering temperature, films that did not initially contain organic surfactants adhered more strongly to the underlying F:SnO2 substrate. The amount of sensitizing dyes adsorbed by a film is also sensitive to the initial presence or absence of organic surfactants and to the sintering temperature. Used as a dye-sensitized anode in photoelectrochemical cells, an open-circuit photovoltage of 647 mV and a short-circuit photocurrent density of 2 mA/cm2 were obtained for 1 μm thick TiO2 films sintered at 100 °C; normalized to the same thickness, similar results were obtained with films initially containing surfactant and sintered at 450 °C.

Preparation of mullite by the reaction sintering of kaolinite and alumina

Abstract: In the present study, mullite specimens and mullite/alumina composites are prepared by reaction sintering kaolinite and alumina at a temperature above 1000°C. The phase and microstructural evolution of the specimens and their mechanical properties are investigated. Primary mullite appears at a temperature around 1200°C. The alumina particles are inert to the formation of primary mullite. Alumina starts to react with the silica in glassy phase to form secondary mullite above 1300°C. The formation of secondary mullite decreases the amount of glassy phase. Furthermore, the addition of alumina reduces the size of mullite grains and their aspect ratio. The strength and toughness of the resulting mullite increase with the increase of alumina content; however, the mechanical properties of the mullite and mullite/alumina composites are lower than those of alumina for their relatively low density.

Tungsten carbide (WC) synthesis from novel precursors

Abstract: This paper deals with the production and properties of tungsten carbide (WC) powders from novel carbon coated precursors. The process has two steps in which the oxide powders were first coated with carbon by cracking of a hydrocarbon gas, propylene (C 3 H 6 ), secondly mixed with a substantial amount of carbon black, and finally treated at temperatures in the range of 600-1400°C for 2 h in flowing Ar or 10% H 2 -Ar atmosphere to synthesize WC. The produced powders were characterized using TEM, BET surface area analyzer, X-ray diffraction and chemical analysis (oxygen and carbon). The results obtained for various types of precursors treated in different atmospheres indicated that the coated precursors produced high quality powders. Single phase, submicron WC powders were synthesized at temperatures as low as 1100°C. WC powders produced at 1400°C for 2 h in flowing 10%H 2 -Ar gas mixture were submicron (3–5 m 2 /g), single phase, and had low oxygen content (0.2–0.5 wt%). The sintering tests demonstrated that these powders can be densified to near theoretical density using 20 wt% Co binder at 1500°C for 2 h in flowing 10%H 2 -Ar atmosphere.

Synthesis and sintering of TiB2 and TiB2–TiC composite under high pressure

Abstract: TiB2 and TiB2x2013;TiC composite compacts with 98x2013;99% density are prepared by high-pressure sintering (HPS) of premixed powders and by high-pressure self-combustion synthesis (HPCS) from the elemental constituents. The sintering and synthesis experiments are carried out at 3 GPa in the temperature and time ranges 2250x2013;2750 K and 5x2013;300 s, respectively. A high sintering temperature (2750 K) is required to obtain dense monolithic TiB2 compacts (98% density) by HPS. Compacts with a similar density are obtained at lower sintering temperature (2250 K) when 15 mol% TiC is added to TiB2. The composite compacts have marginally better fracture toughness than that of monolithic compacts. TiB2 and TiB2x2013;TiC compacts (99% density) are also prepared by HPCS from elemental constituents. A minimum ignition temperature of 2250 K is required to make the reaction self-sustaining.The compacts prepared by HPCS have superior fracture toughness to those prepared by HPS. The microstructures and the properties of the compacts prepared by HPS and HPCS are compared. A possible sequence of reaction during the HPCS of TiB2x2013;TiC is proposed.

Bending strength and microstructure of Al2O3 ceramics densified by spark plasma sintering

Abstract: Al2O3 ceramics were superfast densified using spark plasma sintering (SPS) by heating to a sintering temperature between 1350 and 1700°C at a heating rate of 600°C/min, without holding time, and then fast cooling to 600°C within 3 min. High-density Al2O3 ceramics could be achieved at lower sintering temperatures by SPS, as compared with that by conventional pressureless sintering (PLS). The bending strength of Al2O3 superfast densified by SPS in the range of sintering temperature between 1400 and 1550°C reached values as high as 800 MPa, almost twice that obtained by the PLS. SEM observations indicated that intragranular fracture was the preponderant fracture mode in these samples, resulting in these excellent bending strength values.

Sintering and evaporation characteristics of gas-phase synthesis of size-selected PbS nanoparticles

Abstract: Gas-phase synthesis of nanoparticles is advantageous because of higher purity due to absence of liquid solvents and the possibility of reaching high temperatures, thereby extending the range of materials which can be synthesized. The synthesis of semiconducting material (PbS) via the evaporation–condensation route is here undertaken in combination with the control techniques provided by aerosol science such as size fractionation and on-line particle size measurements. Particle sizes obtained range from 3 to 50 nm with a relative standard deviation of 1.1. By means of an on-line measurement of the agglomerate diameter the evaporation and sintering characteristics of PbS nanoparticles are investigated in combination with a theoretical description of the evaporation and sintering process.

Densification of Al2O3 Powder Using Spark Plasma Sintering

Abstract: Al2O3 powders with four different particle sizes were densified using a spark plasma sintering (SPS) apparatus under three different sintering conditions: holding time, heating rate, and mechanical pressure. The Al2O3 powder compact sintered at a higher heating rate produced a sample with a higher density and a fine-grained microstructure, while abnormal grain growth and a lower density resulted when a lower heating rate was applied, though the sintering temperature and holding time were the same in both cases. This revealed that rapid sintering by SPS was effective for promoting the densification of the powder. However, the powder with a coarse particle size was hard to sinter at a higher heating rate. Microstructural observation revealed that the edge part was denser than the inside of the sample when the holding time was short. Increasing the holding time made it possible for the inside to be sintered almost as dense as the edge part. Mechanical pressure was found to enhance densification of the Al2O3 powder. On the basis of these results, the SPS process is discussed.

Co-enhanced SiO2-BN ceramics for high-temperature dielectric applications

Abstract: Two typical high-temperature dielectric materials, fused silica and BN, have been used to form a composite with an attempt to overcome their own drawbacks. In the resultant BN–SiO 2 composites, BN platelike grains were preferentially orientated by hot pressing and homogeneously distributed in the fused silica matrix. An evident co-operative enhancement has been achieved by the combination of the constituents. The sinterability and the thermal shock resistance of the BN materials were increased and the ablation surface temperature was decreased by the involvement of the fused silica. On the other hand, the strength, fracture toughness, and flame ablation resistance of the fused silica were increased due to the addition of BN. Furthermore, an amorphous Si–B–O–N structure was identified in the surface layer of the ablated composites, to which attention should be further paid in the development of new elevated temperature dielectric materials.

Liquid-phase sintering of PZT ceramics

Abstract: Lead zirconate titanate (abbreviated as PZT) ceramics are of considerable commercial importance for a host of piezoelectric and pyroelectric applications. Conventionally, many PZT ceramics are sintered at temperatures above 1250°C. Such extreme temperatures are undesirable due to the increased energy consumption, limitation of electrode material and evaporation of volatile components. A liquid-phase sintering aid incorporating Cu2O and PbO is presented which demonstrates a reduction in the required sintering temperature of these ceramics. This new aid is described with particular reference to a commercial PZT, termed Pz26, used industrially for its optimised piezoelectric properties. Pz26 has a composition near the morphotropic phase boundary and possesses a tetragonal crystalline structure. Typically this material is sintered between 1260 and 1300°C for 1 h to achieve the required densification. With the inclusion of sintering aid, sintered densities comparable to those obtained by conventional sintering are achieved at only 800°C. The optimum weight percentage of sintering aid varies for different ceramic materials, particle sizes, morphology and the desired sintering temperature. However, with standard “mixed-oxide” produced Pz26 powder and with a median particle size in the range 1.6–1.7 μm, a value of 5 wt.% allows sintering at 800°C, according to densification, dielectric and piezoelectric measurements (ϵ=873, tan δ=1.13 %, kp=43.1%). When finer grained powder is used (d0.5=1.1 μm), improved properties (ϵ=960, tan δ=1.04%, kp=51.7%) are obtained for an addition of 3 wt.% sintering aid and a sintering temperature of 850°C.

Protein forming — a novel shaping technique for ceramics

Abstract: A new consolidation method for the forming of ceramics using globular proteins as consolidators/binders was developed. Various protein products: bovine serum albumin (BSA), albumen (egg white powder) and whey protein concentrate (WPC), were shown to work as gelling agents, which was confirmed by rheological measurements and forming experiments. Suspensions of Si3N4, Al2O3 and ZrO2 powders with various solids loadings (44–55 vol.%) were prepared, proteins were added and the resulting slips were poured into simply shaped moulds and consolidation (gelling) took place through heating at 80°C. Through an optimisation of the process, it was possible to sinter to high density values (>99% of theoretical) using gas pressure sintering (GPS) of Si3N4 materials, and using pressureless sintering in air of the oxides. Among the globular proteins studied, WPC showed the most favourable properties with less serious slip thickening, limited foam formation, faster gelling, sufficiently high green body strength to make demoulding in wet state possible and less cracking during drying. However, a larger amount of WPC was needed to achieve a proper gelling compared with the other proteins. This negatively influenced the sintering conditions by lower density of shaped bodies, which required ceramic powders that sinter more easily to full density. The most critical steps in the processing were to efficiently remove air, to break foam formations, and to avoid cracking during the demoulding and drying of the cast bodies. None of the proteins gave rise to critical stresses during the burnout operation and sintering was carried out without deformation or cracking.

Advanced and new grades of WC and binder powder – their properties and application

Abstract: This paper deals with properties and applications of advanced WC and new (Fe/Ni/Co) powders. Special attention is focused on (1) fine-grained and highly sinteractive (Fe/Ni/Co) powder and (2) ultrafine WC powder with linear WC intercepts averaging 100–130 nm in structures of WC–10 wt% Co hardmetals. The quality, individual features and properties of the new powders are shown to be a key factor in achieving excellent and improved properties and performance of hardmetals and diamond tools. A number of correlations are established between powder properties, sintering behaviour and hardmetal structures and properties. In addition to other well-known parameters, the lattice microstructure, as well as the mixed-crystal phases of the new powders, are shown to be essential for optimized sintering, structure formation and improved properties of the tool materials.

Synthesis of nanocrystalline zirconia powders for TZP ceramics by a nitrate-citrate combustion route

Abstract: A nitrate–citrate combustion route to synthesise nanocrystalline yttria-doped zirconia powders for tetragonal zirconia polycrystal (TZP) ceramics is presented. This route is based on the gelling of nitrate solutions by the addition of citric acid and ammonium hydroxide, followed by an intense combustion process due to an exothermic redox reaction between nitrate and citrate ions. X-ray diffraction characterisation of powders showed the stabilisation of the tetragonal phase at room temperature because of their small crystallite size (about 10 nm). Dense ceramic samples prepared by uniaxial pressing and sintering in air were also studied.

Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide

Abstract: Dense SiC ceramics were obtained by hot pressing of β-SiC powders using Al2O3-Y2O3 and La2O3-Y2O3 additive systems. The effect of the addition of an amount of ultrafine SiC to commercial silicon carbide powder was evaluated. Sintering behaviour and microstructure depended on type and amount of liquid phase, as densification proceeded via a classical solution-reprecipitation mechanism. A core/rim structure of SiC grains indicated that reprecipitation of a solid solution of SiC containing Al and O occurred on pure SiC nuclei. Grain boundary phase was constituted of crystalline YAG and amorphous silicates. Values of flexural strength up to ∼750 MPa at RT and up to ∼550 MPa at 1000 °C were measured. At 1300 °C a strong degradation of strength was attributed to softening of the amorphous portion of grain boundary phase. In highly dense materials toughness ranged from 2.95 to 3.17 MPa·m1/2 and hardness from 21 to 23 GPa.

Fundamentals of liquid phase sintering for modern cermets and functionally graded cemented carbonitrides (FGCC)

Abstract: Metallurgical reactions and microstructure developments during sintering of modern cermets and functionally graded cemented carbonitrides (FGCC) were investigated by modern analytical methods such as mass spectrometer (MS), differential thermal analysis (DTA), differential scanning calorimeter (DSC), dilatometer (DIL), microscopy and analytical electronic microscopy with energy dispersive spectrometer (EDS). The complex phase reactions and phase equilibria in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni were studied. The melting behavior models in the systems of TiC–WC/MoC–Ni/Co, TiC–TiN–WC–Co and TiCN–TaC–WC–Co have been established. By an in-depth understanding of the mechanisms that govern the sintering processing and metallurgical reactions, new cermets and different types of FGCC with desired microstructures and properties were developed.

Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering

Abstract: SPS (spark plasma sintering) process was used to sinter nanocrystalline ZrO 2 (3Y). It was found to be different with the usual rapid sintering method, the density of the samples kept increasing with the rising of the sintering temperature. A higher density could be reached at a lower temperature and shorter dwelling time than that by hot-pressing under the similar pressures. In contrast to the samples with a differential densification from edge to center prepared by a rapid hot-pressing, no obvious densification gradient could be found in the samples sintered by SPS. The grain sizes of the Y-TZP obtained by SPS were smaller than those by the pressureless sintering method, while the grain growth speed was much higher under SPS conditions. All these unique sintering behaviors were explained by the special sintering process of SPS.