Sintering behaviour of the diamond-super invar alloy system at high temperature and pressure

In the diamond-super invar alloy system, effect of the grain size of the starting diamond powder on the sintering behaviour was investigated at 5.8 G Pa and 1300 to 1500 ° C for 30 to 60 min. Abnormal grain growth was often observed in the sintered diamond using fine diamond powder, but a homogeneous sintered diamond was easily synthesized using 5 to 10m diamond. By the rigorous control of sintering conditions, a 3m grain size homogeneous sintered diamond was synthesized under conditions of 5.8 G Pa and 1350 ° C for 30 min. The Vickers hardness and thermal resistance of the fine grained sintered diamond was examined.     

The influence of high sintering temperatures on the mechanical properties of hydroxylapatite

The kinetics of the thermal decomposition of stoichiometric hydroxylapatite (HA) has been studied up to 1500°C for the purpose of determining the maximum admissible combinations of temperature and time for sintering HA. The influence of the sintering temperature on shrinkage, density and grain growth is then investigated in the temperature range from 1000 to 1450°C. Nearly theoretical density was achieved above 1300°C. A maximum fracture toughness is obtained for the samples sintered at 1300°C whereas hardness increases up to a sintering temperature of 1400°C. These results are discussed in terms of the roles of porosity and grain size.     

Microstructure of cBN-diamond sintered compact prepared by reaction sintering

cBN-diamond composite sintered compacts (diamond content 15–70 wt %) were prepared by reaction sintering at 7–7.5 GPa and 1400–1700 °C for 10–30 min from the starting powder of the hBN-diamond system in the presence of 1 wt % NH₄NO₃ as a volatile catalyst. A fully dense sintered compact with 99% conversion from hBN to cBN was obtained at 7 GPa and 1700 °C after 30 min. An induced transformation from hBN to cBN seemed to occur on the surface of the added diamond seed crystals. Diamond seed crystals (about 30 wt %, grain size 0.2–1.5 m) were found to be well-dispersed in the reaction-bonded cBN matrix. The Vickers microhardness of the sintered compact was 5100 kg mm^–2. The contacts between diamond grains were observed in the sintered compacts containing diamond seed grains of more than 70 wt %. The toughness of the sintered compact tended to increase with decreasing diamond content and the grain size of seed crystals.     

Formation of bridges between diamond particles during sintering in molten cobalt matrix

Large diamond particles of about 700 m size mixed with 70 wt% fine cobalt powder have been sintered at 5.25 GPa and 1500 °C after wrapping in zirconium foil. During the sintering treatment faceted inter-particle bridges form, and their pale yellow colour in contrast to the dark yellow colour of the original diamond particles indicates that the bridge is a new phase reduced in nitrogen content because of the gettering effect of the zirconium foil. The morphological characteristics of the bridge and the adjacent particles indicate that the bridge is formed by migration of the inter-particle grain boundary and solution-reprecipitation through the liquid cobalt matrix. It is possible that these processes are induced by the coherency strain energy produced by nitrogen diffusion from the region ahead of the retreating boundaries, as has been observed in metal systems.Presented to 2nd International Conference on New Diamond Science and Technology in poster session, held on September 23–27 1990 in Washington, USA.     

Effect of sintering temperature on the microstructure and high-frequency magnetic properties of Ni0.467Zn0.07Co0.015Fe0.511O4 ferrite

In this work, an attempt is made to study the effects of sintering temperature on the microstructure and high-frequency (HF) magnetic properties of a nickel zinc ferrite compound of very low ZnO content of Ni_0.467Zn_0.07Co_0.015Fe_0.511O₄ composition. Samples were prepared by a conventional ceramic route and sintered for 2 h at 1150, 1200, 1250, and 1300 °C. It was shown that the higher the sintering temperature the higher the saturation magnetization and the measured initial permeabilities, and the lower was the H _c of the samples. This was related to the increased sintered densities and grain sizes. The magnitudes of the electrical resistivity of the samples sintered at 1300 °C compared to those of the samples sintered at 1150 °C and 1200 °C showed four orders decrease. This is thought to be due to the grain-size increase and possibly the formation of higher Fe²⁺/Fe³⁺ concentration. The lowest measured quality factor (Q-factor) obtained in the range of 60–210 MHz, corresponds to the samples sintered at 1300 °C. The highest Q-value in the frequency range of 125–210 MHz was obtained for the samples sintered at 1150 °C, which has also shown the highest electrical resistivity.     

Hydrothermal ageing of yttria-stabilised zirconia, sintered at 1300°C–1325°C: the effects of copper oxide doping and sintering time variations

Co-precipitated yttria-stabilised zirconia powders were doped with 0.05% wt of copper oxide (CuO). Samples were sintered at 1300°C–1325°C with holding times varying from 12 mins up to 4 hrs. The material was stable when exposed to hydrothermal ageing at 180°C, with only minimal signs of degradation in many cases for in excess of 3000 hrs. Excellent ageing resistance was observed for samples doped with 0.05 wt% of CuO, sintered at 1315°C for 12 mins which developed less than 1% monoclinic phase content, after 5000 hrs. Samples doped with 0.05 wt% of CuO, sintered at 1325°C for 12 mins, developed more than 50% of surface monoclinic phase content after 800 hrs of ageing, maintaining the same amount of monoclinic phase content for in excess of 3000 hrs. Plain samples sintered at 1325°C for 2 hrs developed more than 30% monoclinic phase content after 750 hrs. Doping with CuO improved the ageing resistance of the samples sintered at 1300°C–1315°C for 2 hrs and 12 mins holding times respectively.     

Sintering of tin-doped indium oxide (Indium-Tin-Oxide, ITO) with Bi2O3 additive

Tin-doped indium oxide (Indium-Tin-Oxide, ITO) is known as a poorly sinterable material. Densification of ITO powders with relatively large particle size (1–2 m) was enhanced remarkably by the additive (Bi2O3) whose melting point is lower than the sintering temperature. The maximum bulk density of 6.75 g/cm3 (relative density; 95%) was obtained when pressurelessly sintered in air at 1500°C for 5 hours using the starting material containing 2.0 mass % i2O3, while the density was approximately equal to the green density when sintered using the starting material without Bi2O3. Increase of electrical resistivity caused by the additive was suppressed successfully when a small amount of Bi2O3 (1.0 mass %) was added and heated at 1500°C. The bismuth was eliminated from the sintered body to achieve the low resistivity (8.1 × 10–4 ohm · cm) which was approximately equal to that of the pure ITO.     

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in softening particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min^–1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.     

Effect of the sintering temperature on the properties of Ce0.85La0.10Ca0.05O2−δ electrolyte material

Rare earth and alkaline earth co-doped Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte material with the powder obtained by solid-state reaction method was sintered at 1300, 1400, 1500 and 1600 °C respectively. The results showed that the ionic conductivity of the sample sintered at 1400 °C was slightly lower compared to that sintered at 1500 °C in the temperature range of 300-550 °C, while the sample sintered at 1400 °C showed the highest ionic conductivity in all the samples above 550 °C. The ionic conductivity of ∼0.021 S/cm at 600 °C and the relative density of 98.2% were observed for the sample sintered at 1400 °C. In addition, the highest flexural strength with 145 MPa was also obtained for the sample sintered at 1400 °C. It suggested that the sintering temperature for Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte may be reduced to as low as 1400 °C with desired properties.     

Microstructure and mechanical properties of B6O-B4C sintered composites prepared under high pressure

Sintered composites in the B₆O-xB₄C (x = 0–40 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1500–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available B₄C powder. Relationship among the formed phases, microstructures and mechanical properties of the sintered composites was investigated as a function of sintering conditions and added B₄C content. Microhardness of the sintered composite was found to increase with treatment temperature up to 1800°C, while fracture toughness decreased slightly. Maximum microhardness of Hv 46 GPa was obtained from B₆O-30vol%B₄C sintered composite under the sintering conditions of 4 GPa, 1700°C and 20 min.     

Multimodal grain size distribution and high hardness in fine grained tungsten fabricated by spark plasma sintering

Preparation of fine grained, hard and ductile pure tungsten for future fusion reactor applications was tested using the bottom-up approach via powder consolidation by spark plasma sintering (SPS) at different temperature (1300-1800 °C) and pressure (90-266 MPa) conditions. Pure tungsten powders with an average particle size of about 1 μm were sintered to high density (about 94%) with almost no grain growth at a temperature below 1400 °C and an applied pressure up to 266 MPa. These samples had a multi-modal grain size distribution (resembling the size distribution of the initial powder) and a very high Vickers hardness (up to 530 kg/mm²). Above 1500 °C fast grain growth occurred and resulted in a drop in hardness. XRD on the surface of bulk samples showed a small amount of tungsten oxides; however, XPS and EDS indicated that these oxides were only surface contaminants and suggested a high purity for the bulk samples. The results demonstrate that SPS can lead to ultrafine and nanocrystalline tungsten if used to consolidate pure nano tungsten powders.     

Processes involved during the production of Fe–W–Mo alloys by powder metallurgy

Sintering of Fe–W–Mo systems under a dynamic atmosphere of argon has been studied by different experimental techniques, including dilatometric trials, X-ray diffraction, optical microscopy and electron probe microanalysis. It has been found that sintering at 1300°C for only 10 min allows compacts to be obtained with a relative density close to 80%. The results also show an improvement in the final density by molybdenum addition. On the other hand, the structure of the sintered samples was, mainly, constituted of a soft matrix of (Fe–M) solid solutions (300 Hv) in which hard intermetallic compounds (1200 Hv) are dispersed. Furthermore, perturbing expansions have been observed during the sintering cycle. The first expansion occurred at about 620°C as a consequence of a Kirkendall effect. The second one arose at about 890°C in relation to Fe7M6 (where M is tungsten or molybdenum) formation. The latter expansion occurred at about 1200°C as a consequence of an increase in tungsten diffusion from indiffused areas to (Fe–M)a solid solutions and intermetallic compounds. © 1998 Chapman & Hall     

Diamond formation from glassy carbon under high pressure and temperature conditions

The process of the formation of diamond from the glassy carbon with its characteristic bond nature was investigated in the diamond stable region at high pressures (up to 10 GPa) and temperatures (up to 3000° C), without any intentional addition of metals as solvent. The process of diamond formation was found to obey Ostwalds's step rule as follows: amorphous glassy carbon crystallized to form fully well-crystallized graphite prior to diamond formation and then the graphite crystals were converted to diamond by further heat treatment at pressures above 9 GPa. The many trigons formed are considered to be essentially a record of growth failure in the growth period. As a result of heat treatment for a longer time and/or at a higher temperature close to the diamond—graphite stability boundary, the diamond tended to grow with the (111)-face composed of the thin growth layers.     

The crystallization behavior and thermal expansion properties of β-eucryptite prepared by sol–gel route

Lithium–aluminosilicate glass–ceramics in the form of eucryptite were synthesized through sol–gel technique by mixing boehmite sol, silica sol and lithium salt and sintering at different temperatures for further analysis. Thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), IR analysis and dilatometry were done to study sintering characteristics, phase transformation and thermal expansion behavior on the sintered specimens. XRD and FTIR results confirmed that crystallization of β-eucryptite took place at about 600 °C, substantial increase of β-eucryptite was observed in the specimens sintered at temperatures from 800 to 1300 °C. Trace amount of cristobalite also emerged at 600 °C and disappeared at 1300 °C. The thermal expansion behavior characteristics were found to be strongly influenced by crystalline phases in the specimens which depended on the sintering temperatures.     

Processing behaviour of hydroxyapatite powders with contrasting morphology

Three synthetic hydroxyapatite powders (designated A, B and C), supplied by British Charcoals and MacDonalds and Merck GmbH were chemically and physically characterized with X-ray diffraction (XRD), infrared spectroscopy (IRS), gravimetric analysis (GA), inductively coupled plasma spectroscopy (ICPS), surface area analysis (BET), particle size analysis and scanning electron microscopy (SEM). The powders were pressed at 80 MPa and fired at a range of sintering temperatures between 1190°C and 1300°C for 12 h. The sintering characteristics of the four powders were assessed by density measurement, hardness testing and scanning electron microscopy. Infrared spectroscopy and X-ray diffraction results indicated that the powders showed no evidence of decomposition into tricalcium phosphate after sintering at any of the temperatures tested. However, the marked differences in morphology between the three powders led to contrasting sintering characteristics. While two of the powders sintered to near full density, the other showed very little change in density over that of the green compact even after sintering at 1300°C. Measurement of the hardness versus sintering temperature indicated that the mechanical properties of only the two samples which sintered to full density would make them suitable for any load-bearing applications.     

Effect of sintering temperature and holding time on the properties of 3Y-ZrO2 microfiltration membranes

Sintering behavior of supported and unsupported microfiltration membranes prepared from 3 mol% yttria doped zirconia powder was investigated as a function of temperature and holding time in non-isothermal and isothermal densification. Shrinkage that started at 1000°C showed the highest rate between 1200°C and 1300°C although the rate decreased above 1300°C. The activation energy of sintering was calculated at 735 kJ/mol, assuming the grain boundary diffusion mechanism for mass transport. Mean pore size decreased in unsupported membranes and increased in supported ones as the sintering temperature increased up to 1200°C. Dimensional shrinkage of unsupported membrane slabs showed an increase in shrinkage first in the lateral dimension and then in the thickness as the sintering temperature increased. Pore growth and lower hardness in supported membranes, can be explained due to the restricted lateral shrinkage in the supported membranes. Removal of porosity was pronounced above 1100°C and the density increased linearly as a function of holding time. Microhardness of membranes sintered above 1100°C increased as a function of sintering temperature and was higher in unsupported membranes. Samples sintered above 975°C had a100% tetragonal phase structure. Permeability of supported membranes increased as a function of sintering temperature due to pore growth despite a decrease in porosity.     

Sintering temperature, microstructure and resistivity of polycrystalline Sm0.2Ce0.8O1.9 as SOFC's electrolyte

In this work, near-completely soft-agglomerated Sm_0.2Ce_0.8O_1.9 powders have been prepared. The pellets were formed and sintered at various sintering conditions of temperature and time. It was found that the sintering conditions have significant effects on the pellet resistivity. By the measurements with the DC four-probe method, it was found that the overall resistivity of the polycrystalline Sm_0.2Ce_0.8O_1.9 material sintered at 1500°C increases linearly with the reciprocal of the average grain size. The AC impedance spectroscopy has been used to distinguish the grain resistivity and the apparent grain boundary resistivity. The brick layer microstructural model has been used to provide an estimate of the apparent grain boundary resistivity and to relate the electrical properties to the microstructural parameters. By lowering the sintering temperature to 1100–1200°C, the true grain boundary resistivity was nearly two orders lower than that sintered at 1500°C, and thus the overall resistivity decreases to about 31 ohm-cm at 700°C measurement. This makes the Sm_0.2Ce_0.8O_1.9 material capable of working as SOFC's electrolyte at temperatures lower than 700°C.     

Effect of temperature on nonlinear electrical behavior and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO₂ ceramics doped with 0.75 mol% Ca and 2.5 mol% Ta were sintered at different temperatures ranging from 1300 to 1450°C. The effects of sintering temperature on the microstructure, nonlinear electrical behavior, and dielectric properties of the ceramics were studied. The sample sintered at 1300°C exhibits the highest nonlinear coefficient (5.5) and a comparatively lower relative dielectric constant.     

High pressure consolidation of B6O-diamond mixtures

Sintered composites in the B₆O-xdiamond (x= 0–80 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1400–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5–3, and 5–10 m). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B₆O-diamond mixed phases when using diamond with grain sizes greater than 0.5 m, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 m. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H _v57 GPa was measured in the B₆O-60 vol% diamond (grain size < 0.25 m) sintered composite under the sintering conditions of 5 GPa, 1700°C and 20 min.     

Nanosized hydroxyapatite powders derived from coprecipitation process

Nanosized hydoxyapatite (Ca₁₀(PO₄)₆(OH)₂ or HA) powders were prepared by a coprecipitation process using calcium nitrate and phosphoric acid as starting materials. The synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific area measurment techniques. Single phase HA, with an average grain size of about 60 nm and a BET surface area of 62 m²/g, was obtained. No grain coarsening was observed when the HA powders were heated at 600°C for 4 hours. HA ceramics were obtained by sintering the powders at temperatures from 1000°C to 1200°C. Dense HA ceramics with a theoretical density of 98% and grain size of 6.5 m were achieved after sintering the HA powders at 1200°C for 2 hours. HA phase was observed to decompose into tricalcium phosphate when sintered at 1300°C. The microstructure development of the sintered HA ceramics with sintering temperature was also characterized and discussed.