Conventional Pressureless Sintering Research Articles

Effect of grain size on wear behavior in Y-TZP ceramics

The influence of grain size on the wear behavior of zirconia ceramics was investigated when slided against steel in dry unlubricated conditions. Fully densified Y-TZP ceramics (≥98% of theoretical density) with a wide range of grain size from 75 to 1470 nm were developed using spark plasma sintering (SPS), microwave sintering (MS), and conventional pressureless sintering (CS) methods. SPS was effective in producing fully densified nanocrystalline zirconia at low temperature and short sintering time. The steady state coefficient of friction (COF) varied in a range of 0.35–0.44, but the wear rate reduced from 3.5 to 0.88 × 10 −6 mm 3/Nm with decreasing grain size. The plastic deformation and microcracking lead to a mild wear for the nanocrystalline zirconia, whereas the delamination/spalling results in the increased wear for the coarser zirconia.

Liquid Phase Sintering of SiC-Ceramics

In view of considerable attention in the development of liquid phase sintered SiC, a comprehensive study of the data on processing, structure and properties seems highly relevant. This article provides a detailed and critical overview of liquid phase sintered silicon carbide ceramics with primary emphasis of grain-boundary/secondary phase evolution, their structure, distribution on the final properties of the sintered materials. The roles of individual additives in developing boundary microstructures will be identified and demonstrated to be critical in optimizing the mechanical properties, including fracture toughness, flexural strength and creep resistance. Numerous methods of structure-properties modification, like in-situ-toughening, -SiC phase transformation, volume of liquid phase, partial/full crystallization of grain-boundary and/or secondary phases are conclusively discussed. Apart from conventional pressureless sintering of SiC, enhanced spark plasma sintering with different oxide and non-oxide sintering additives are also discussed in terms of phase evolution, microstructure and their structure mechanical properties are correlated.

Development of [formula omitted] composite ceramics for high-power millimeter-wave applications

Al 2 O 3 – TiO 2 composite dielectric ceramics have been prepared by different sintering techniques including high-temperature, high-pressure sintering (2.5 GPa, T = 1000 °C) and conventional pressureless sintering ( T = 1350 °C). Formation of the Al 2 TiO 5 secondary phase has been completely suppressed by optimization of the sintering and annealing temperatures. Dielectric properties were measured in the 10–11 GHz range using the cylindrical resonant cavity technique and in the 40–92 GHz range using the open resonator whispering gallery mode technique. At 10 GHz, the optimized composite material ( 0.895 Al 2 O 3 – 0.105 TiO 2 ) exhibited Q × f = 210 THz, ε ′ = 12.5 and τ f = + 2.0 ppm K - 1 at room temperature. As an evidence of an additional, low-frequency (possibly Debye-type), extrinsic contribution to the dielectric loss, the Q × f value gradually increased with frequency and reached a plateau of Q × f ≈ 340 THz at approximately 80 GHz. It is demonstrated that Al 2 O 3 – TiO 2 composites have considerable potential as dielectric resonators in the output multiplexers of communication satellites.

Preparation of nano-grained zirconia ceramics by low-temperature, low-pressure spark plasma sintering

Ce- and/or Y-doped zirconia nanopowders having average particle sizes ranging 12–18 nm have been synthesized by a technique based on mechanochemical processing (MCP). Despite their small particle size, the powders had excellent compactibility with green densities exceeding 50% achieved under a moderate uniaxial pressure of 150 MPa. Nearly fully dense ceramics having grain sizes of around 100 nm were successfully produced from these powders by spark plasma sintering (SPS) at temperatures of 1,050–1,150 °C for 5 min under pressures of 50–80 MPa; these temperatures and pressures are considerably lower than those required for achieving near full density with conventional nanopowders. Hardness and fracture-toughness measurements showed that the ceramics prepared by SPS had superior mechanical properties to those prepared by conventional pressureless sintering. It is argued that the high sinterability of the MCP nanopowders is ascribed to their ability to form uniform powder compacts under relatively low pressure, and that that ability in turn originates in two features of the MCP powders: absence of hard agglomeration and pseudo-spherical particle morphology.

High-pressure sintered yttria stabilized zirconia ceramics

Fast densification of 8YSZ ceramics under a high pressure of 4.5 GPa was carried out at different temperatures (800, 1000, 1450 °C), by which a high relative density above 92% could be obtained. FT-Raman spectra indicate that the 8YSZ underwent a phase transition from partially tetragonal to partially cubic phase as temperatures increase from 1000 to 1450 °C when sintering under high pressure. The electrical properties of the samples under different high-pressure sintering conditions were measured by complex impedance method. The total conductivity of 0.92 × 10−2 S cm−1 at 800 °C has been obtained for 8YSZ under high pressure at 1450 °C, which is about 200 °C lower than that of the samples prepared by conventional pressureless sintering.

Properties of ZnO Varistors Prepared with Nano-Sized Powders

ZnO varistors with higher voltage gradients are expected in the power system, and the key to this property is to diminish the grain size. In this paper, nano-sized ZnO particles are used as the raw material in order to obtain small grains. Pellets are fabricated by two sintering methods: the conventional pressureless sintering and the spark plasma sintering (SPS). For the samples fabricated by the conventional sintering, the average grain size varies from 1 μm to more than 10 μm when the sintering temperature changes from 900 oC to 1200 oC. The voltage gradient can reach as high as 2020 V/mm. The grain size of the SPS samples is less than 1 μm, and the voltage gradients can reach 963 V/mm.

Densification behaviour of nanocrystalline hydroxyapatite bioceramics

The sinterability of nanocrystalline hydroxyapatite (HA) particles by microwave sintering was compared with conventional pressureless sintering. The results revealed that microwave heating was effective in producing a useful HA body in a very short sintering cycle without disrupting the HA phase stability. The maximum hardness of 7.21 GPa and 6.38 GPa was obtained for HA sintered at 1050 °C by the conventional method and 1150 °C by microwave sintering, respectively. The maximum fracture toughness measured for the microwave-sintered and conventional-sintered HA was 1.45 MPam 1/2 at 1050 °C and 1.22 MPam 1/2 at 1000 °C, respectively. Although the relative density of microwave-sintered HA was slightly lower than the conventional-sintered HA throughout the sintering regime employed, taking into account of the heating and soaking periods, the time taken by microwave sintering to achieve a relative density of 96.5% was about 3% of the time consumed for samples sintered by the conventional heating. Microwave heating was found to be an effective technique to produce a useful HA body for clinical applications without causing grain coarsening.

VC- and Cr3C2-doped WC–NbC–Co hardmetals

This study compares the microstructure and mechanical properties of plain and 0.9 or 3.6 wt% VC- or Cr3C2-doped WC–12 wt% Co hardmetals with 40 wt% NbC, prepared by pulsed electric current sintering (PECS) in the solid state for 4 min at 1240 °C and conventional pressureless liquid phase sintering (CS) for 1 h at 1420 °C. The addition of VC or Cr3C2 was found to inhibit grain growth of the residual WC grains, whereas the size of the solid solution (Nb,W,V/Cr)C grains was hardly influenced. The type of grain growth inhibitor and densification temperature however, strongly influenced the composition of the NbC solid solution formed, which was thermodynamically and experimentally assessed.

Pressure-less spark plasma sintering of alumina

The spark plasma sintering (SPS), also known as plasma activated sintering, is a method applicable for rapid sintering of metals and ceramics. The technique is thought of a type of rapid hot pressing, implying that the application of a uniaxial pressure is essential for enhanced densification kinetics. In this study, the efficiency of heat transfer in the SPS process was examined in the absence of mechanical pressure. Conditions for dynamic isotropic heating were established, and the densification and microstructure development sequences of alumina ceramics during pressure-less SPS process were investigated. The influence of early neck formation on the grain morphology and fine structure is also discussed. The study reveals that high heating rate plays an important role in enhancing densification kinetics. The observed principle bears a general implication for increasing the efficiency of conventional pressure-less sintering procedures that are nowadays widely applied in industry.

PMN–PT Ceramics Prepared By Spark Plasma Sintering

(1−x)Pb(Mg1/2Nb2/3)O3−xPbTiO3 (PMN–PT) ceramics of stoichiometric composition were fabricated by conventional pressureless sintering (CS) and spark plasma sintering (SPS). The CS ceramics exhibited a change from relaxor to normal ferroelectric behavior (FE) with increasing PT content. However, low dielectric constants, frequency dispersion, and diffuse phase transition behavior typical for relaxors were obtained for all SPS ceramics. FE and piezoelectric measurements further demonstrated low remanent polarization and strain, high coercive field, and low electromechanical response from SPS materials. Normal dielectric and enhanced FE performance appeared following high‐temperature heat treatment after SPS. The effects of grain size, microstructure, and chemical heterogeneity formed during fast sintering are considered.

Microwave Plasma Sintered Nanocrystalline Bi2O3−HfO2−Y2O3 Composite Solid Electrolyte

Processing and characterization of nanocrystalline Bi2O3−HfO2-Y2O3 composite solid electrolyte having high density and conductivity has been investigated and reported in this paper. Nanopowders of mixed bismuth oxide, hafnia, and yttrium oxide in Bi2O3:HfO2:Y2O3 = 3:2:1 molar ratio have been prepared by a reverse titration chemical coprecipitation from Bi3+, Hf4+, and Y3+ containing aqueous solution. The high density, nanocrystalline Bi2O3-HfO2−Y2O3 solid electrolyte has been synthesized by microwave plasma and pressureless sintering. From the XRD results, the growth behavior indicates that growth of both δ-Bi2O3 and c-HfO2 crystallites obeys the parabolic rate law, expressed as (D − D0)2 = kt, during the sintering process. After the samples were sintered in microwave plasma at 700 °C for 30 min, the relative density was found to be greater than 96%. Moreover, the sintered specimens exhibit considerably finer microstructure and greater densification compared to that of samples sintered by conventional pressureless sintering. Results of AC impedance spectroscopy of the solid electrolyte indicates that the conductivity of nanocrystalline Bi2O3−HfO2−Y2O3 electrolyte was more than 10-6 S cm-1 at 350 °C and 10-2 S cm-1 at 550 °C, which is significantly higher than that of microcrystalline HfO2-based solid electrolyte.

Preparation of dense nanocrystalline Bi 2O 3-HfO 2-Y 2O 3 ceramic by microwave plasma sintering

Processing of nanocrystalline Bi 2O 3-HfO 2-Y 2O 3 ceramic having high density has been investigated and reported in this paper. Nanopowders of mixed bismuth oxide, hafnia and yttrium oxide have been prepared by a reverse titration chemical coprecipitation from Bi 3+, Hf 4+ and Y 3+ containing aqueous solution. The high density, nanocrystalline Bi 2O 3-HfO 2-Y 2O 3 ceramic has been synthesized by microwave plasma sintering. The XRD results of grain growth behavior indicates that growth of both δ-Bi 2O 3 and c-HfO 2 crystallites obeys the parabolic rate law, expressed as ( D– D 0) 2 = Kt, during sintering process. After sintering at 700 °C for 60 min, the relative density of the samples sintered by microwave plasma has been found to be greater than 97%, and the samples exhibit considerably finer microstructure with an average size between 60 and 70 nm and equiaxed morphology and better density comparing with that of samples sintered by conventional pressureless sintering. In addition, mechanical properties of nanocrystalline Bi 2O 3-HfO 2-Y 2O 3 ceramic has been improved greatly compareing with nanocrystalline Bi 2O 3-Y 2O 3 ceramic.

Dynamic shock compaction of a ZrO2–RuO2 electronic nanocomposite: toward functionally graded materials

AbstractAn electronic ZrO2–RuO2 nanocomposite was fabricated by dynamic compaction (DC) at 1.5 GPa resulting in a maximum relative density of 88% in the material. The DC process formed pristine elongated conical‐shaped compacts 3 cm in length. The compacts retained their original nanometer‐sized grains (∼20 nm) of the original powder from which it was prepared, marking the densest RuO2‐based nanocomposite to date. A change by a remarkable factor of 107 in electrical conductance was observed for two separate specimens, one processed by DC (σ = 1.1 × 10–4 S cm–1) and the other by conventional pressureless sintering (σ = 2.1 × 103 S cm–1) both obtained from the same starting nanopowder and thus with a similar concentration of conductor phase at 10 mol% RuO2. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microwave Characteristics of MgTiO3–CaTiO3 Dielectric Ceramics Fabricated Using Spark Plasma Sintering

Spark plasma sintering (SPS) was used in preparing dense 0.93MgTiO3–0.07CaTiO3 (MCT) ceramics, and their microstructure and microwave dielectric properties were investigated. Compared with conventional pressureless sintering, SPS was found to be effective in obtaining dense MCT ceramics with uniform microstructures at low temperatures, without any degradation of dielectric properties. The MCT specimen SPS-fabricated at 1150°C for 10 min exhibited excellent dielectric properties: a dielectric constant of 23 and a quality factor (Qf) of 70,000 GHz.

Electrical and Mechanical Properties of Sr-Doped LaPO4 Prepared by Spark Plasma Sintering

The spark plasma sintering (SPS) method was applied to prepare dense compacts of proton conducting Sr-doped at relatively low sintering temperatures. A relative density higher than 98% could be achieved with the SPS method when sintering temperatures above 1323 K were employed, whereas no more than 94% was achieved with the conventional pressureless sintering (PLS) method at 1473-1573 K. Mechanical properties, e.g., three-point bending strength and Vickers hardness, of the SPS samples were considerably improved compared with those of the PLS samples. The SPS samples prepared at 1323-1373 K exhibited a bending strength of 240 MPa and a Vickers hardness of 6.7 GPa. However, higher SPS temperatures than 1373 K diminished the mechanical properties probably because of the grain growth with increased sintering temperature. Conductivity measurements under various and conditions showed that the Sr-doped sintered with the SPS method exhibited dominant protonic conduction regardless of the sintering conditions and that the conductivities were comparable with those of the PLS samples. Throughout this work, the SPS method was demonstrated as an effective tool to prepare proton conducting phosphate-based ceramics with a high density and excellent mechanical properties while maintaining the feature of protonic conduction.

Densification Behavior in Microwave-Sintered Silicon Nitride at 28 GHz

Si3N4 powders were sintered using a 28 GHz gyrotron source, with Y2O3, Al2O3, and MgO as sintering aids, in an attempt to investigate the effect of microwave radiation on densification behavior. The microwave-sintered samples were compared with identical samples produced by conventional pressureless sintering. The effect of sintering on the microstructural development and grain growth of the samples was assessed using scanning electron microscopy. Phase transformation behavior was assessed using X-ray diffractometry. In the microwave-sintered samples, densification and α→β transformation occurred at temperatures ∼200°C lower than those of the conventionally sintered samples. More importantly, at comparable stages of densification, the microstructures of the microwave-sintered and conventionally sintered samples were significantly different, with the microwave-sintered samples showing the development of elongated β grains at a much earlier stage of the α→β transformation. It was concluded that the effect of microwave radiation on sintering was not simply a decrease in sintering temperatures, but in possibly a different sintering mechanism, clearly related to localized heating within the grain-boundary phase.

電池材料 放電プラズマ焼結法によるプロトン伝導性Ｓｒ添加ＬａＰＯ４焼結体の作製

The spark plasma sintering (SPS) method was employed to prepare sintered specimens of proton conducting Sr-doped LaPO4 ceramics. The relative densities of the sintered Sr-doped LaPO4 were around 99% when higher sintering temperatures than 1050°C were applied for the SPS process, while only 93% of relative density was achieved with the conventional pressure-less sintering (PLS) method at 1200°C. Electrical conduction properties of the sintered samples with the SPS method at 1200°C were evaluated by using conductivity measurements at 500-930°C under 0.4-5kPa of p(H2O) or p(D2O) and 0.01-100kPa of p(O2). The specimen prepared with the SPS method showed similar conductivities and conduction properties as that prepared with the PLS method. These results suggested that the SPS method was a useful sintering technique to prepare proton conducting phosphate-based ceramics with a high density while keeping a feature of protonic conduction.

Microstructure and fracture toughness of a spark plasma sintered Al2O3-based composite with BaTiO3 particulates

Dense Al2O3-based composites with dispersed BaTiO3 particulates were fabricated by using spark plasma sintering (SPS) at sintering temperatures between 1100 and 1500 °C with a heating rate of 100 °C/min. High-density of BaTiO3–Al2O3 with various compositions could be achieved by SPS at lower sintering temperatures compared to those by conventional pressure-less sintering (PLS). However, full densification was prevented by the onset of abnormal grain growth in Al2O3–BaTiO3 composites sintered by SPS. It was found that SPS process could accelerate the abnormal grain growth due to the activation and purification of particle surface. The addition of BaTiO3 in Al2O3 matrix improved fracture toughness of the composites fabricated by both SPS and PLS. The highest fracture toughness of 6.04 MPa.m1/2 was achieved in the composite sintered by SPS process with 5 mol% BaTiO3, while that of the monolithic Al2O3 was about 4.0 MPa.m1/2.

Pressureless sintering and characterization of cordierite/ZrO2 composites

Cordierite/ZrO2 composites with 5 to 25 wt% ZrO2 were fabricated by conventional powder mixing and pressureless sintering method. Their densification behavior, microstructure, mechanical and thermal properties were studied. By dispersing 25 wt% (9.57 vol%) ZrO2, densified cordierite/ZrO2 composite with a relative density of 98.5% was obtained at an optimum sintering condition of 1440 °C and 2 h. ZrO2 particles were homogenously dispersed within matrix grains and at the grain boundaries. The intragranular particles were finer than 100 nm and the intergranular particles were coarser. Both fracture strength and toughness could be enhanced more than two times higher, compare to those of monolithic cordierite, by dispersing 25 wt% ZrO2 into the cordierite matrix. The toughening mechanism in the present composites was mainly attributed to martensitic transformation due to ZrO2 dispersion.

The load dependence of hardness in alumina–silver composites

The room temperature Knoop micro-hardness properties of three alumina-silver composites vis-a-vis a monolithic alumina have been investigated at different indentation loads. The composites have been developed using conventional pressureless sintering at 1550°C and these consisted of 1.6, 3.38 and 5.01% by volume of silver particles. It has been shown that the measured Knoop hardness values of these materials sensitively depend on the applied load of indentation. But following an emerging analytical technique, the composites can be assigned some unique true hardness values, which are functions of their silver content.