Microwave sintering and grain growth behavior of nano-grained BaTiO3 materials

In this paper, we investigated the effect of microwave sintering parameters on the development of the microstructure of nano-grained BaTiO₃ materials co-doped with Y and Mg species. It is observed that the materials can not only be sintered densely at a lower temperature (1150 °C) and a shorter soaking time (20 min), but also the grain growth can be suppressed by 2.45 GHz microwave heating process. However, the grain growth exhibits a unique tendency in some processing conditions such as microwave sintering for longer intervals (≧60 min) or at higher temperatures (1200 °C). The grain growth behavior after densification was investigated in terms of the phenomenological kinetics, and the activation energy for grain growth using microwave sintering (59.4 kJ/mol) is considerably less than that of the conventionally sintered ones (96.0 kJ/mol), which indicates that microwave sintering process can accelerate the densification rate of the BaTiO₃ materials comparing with the conventional sintering process.     

Microstructural evolution of nanocrystalline Al2O3 sintered at a high heating rate

Experimental sintering studies on Al₂O₃ powder (200 nm and 600 nm) were done at a heating rate of 1600 °C/min. The microstructural changes of specimens were examined and corresponding detailed data on the densification and grain size as a function of sintering time were presented. The grain-growth transition behavior during sintering was discussed. The results showed that the neck growth caused principally by surface diffusion could be negligible within 2 min. With subsequent increases of sintering time, grain growth promoted by grain boundary and lattice diffusion occurred.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Yb3+ doped Lu2O3 transparent ceramics by spark plasma sintering

Transparent ceramics of 10% Yb doped Lu₂O₃ was fabricated by spark plasma sintering. The operating vital parameters in yielding transparency and mutual effects of sintering, pressure, dwell time, heating rate and annealing temperature on microstructure have been investigated. Fully compacted specimens were obtained at 1250 °C and the average grain size increased from few nm up to 5 μm until 1700 °C, above which abnormal grain growth was witnessed. The post-annealing of sintered ceramics at 1200 °C removes discoloration and improves transparency. The ceramics prepared at 1700 °C with dwell time of 5 min and heating rate at 50 °C/min shows the maximum transmittance with a thickness of 2 mm of 55% at a wavelength of 2 μm.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.     

Densification behaviour and microstructural development in undoped yttria prepared by flash-sintering

Conventional sintering of undoped Y₂O₃ requires temperatures above 1400 °C for a few hours. We show that it can be sintered nearly instantaneously to nearly full density at furnace temperature of 1133 °C under a DC applied field of 500 V/cm. At 1000 V/cm sintering occurs at 985 °C. The FLASH event, when sintering occurs abruptly, is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behaviour differs from earlier work on yttria-stabilized zirconia where all shrinkage occurred in the flash mode. The microstructure of flash-sintered specimens indicated that densification was accompanied by rapid grain growth. The single-phase nature of flash-sintered Y₂O₃ was confirmed by high-resolution transmission electron microscopy. The non-linear rise in conductivity accompanying the flash led to Joule heating. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field.     

Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 °C at a heating rate of 10 °C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 °C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 µm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 °C, attributed mainly to the enhance densification of the sintered bodies.     

Additive-free superhard B4C with ultrafine-grained dense microstructures

A unique combination of high-energy ball-milling, annealing, and spark-plasma sintering has been used to process superhard B₄C ceramics with ultrafine-grained, dense microstructures from commercially available powders, without sintering additives. It was found that the ultrafine powder prepared by high-energy ball-milling is hardly at all sinterable, but that B₂O₃ removal by gentle annealing in Ar provides the desired sinterability. A parametric study was also conducted to elucidate the role of the temperature (1600–1800 °C), time (1–9 min), and heating ramp (100 or 200 °C/min) in the densification and grain growth, and thus to identify optimal spark-plasma sintering conditions (i.e., 1700 °C for 3 min with 100 °C/min) to densify completely (>98.5%) the B₄C ceramics with retention of ultrafine grains (∼370 nm). Super-high hardness of ∼38 GPa without relevant loss of toughness (∼3 MPa m^1/2) was thus achieved, attributable to the smaller grain size and to the transgranular fracture mode of the B₄C ceramics.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Effects of heating rate on microstructure and transparency of spark-plasma-sintered alumina

Commercial alumina powder was densified by spark plasma sintering (SPS) at 1150 °C. During SPS processing, the effects of the heating rate were examined on microstructure and transparency. With decreasing heating rate, the grain size and the residual porosity decreased, while the transparency increased. At a heating rate of 2 °C/min, the grain size was 0.29 μm, and the in-line transmission was 46% for a wavelength of 640 nm. The mechanisms for the fine microstructure and low porosity at slow heating, which are conflicting with some existing results, were explained by considering the role of defect concentration and grain-boundary diffusion during densification.     

Athermal and thermal mechanisms of sintering at high heating rates in the presence and absence of an externally applied field

In order to establish the relative contributions of thermal and athermal mechanisms to densification in the absence of an extrinsic sintering pressure, nanometric powder compacts were sintered with and without applied fields using varied heating rates from 50 °C/min up to 800 °C/min. The relative contribution of the thermal and athermal mechanistic contributions to the densification behavior of two model dielectric ceramics, hydroxyapatite and zinc oxide, is evaluated in the context of the current leading theories of field-assisted sintering mechanisms. The effects of elevated heating rates in nanometric, dielectric ceramics are found to be minimal in the absence of a field. However, in the presence of an applied field there appears to be a synergistic effect with heating rate.     

Sintering process optimization for multi-layer CGO membranes by in situ techniques

The sintering of asymmetric CGO bi-layers (thin dense membrane on a porous support; Ce_0.9Gd_0.1O_1.95?δ = CGO) with Co₃O₄ as sintering additive has been optimized by combination of two in situ techniques. Optical dilatometry revealed that bi-layer shape and microstructure are dramatically changing in a narrow temperature range of less than 100 °C. Below 1030 °C, a higher densification rate in the dense membrane layer than in the porous support leads to concave shape, whereas the densification rate of the support is dominant above 1030 °C, leading to convex shape. A flat bi-layer could be prepared at 1030 °C, when shrinkage rates were similar. In situ van der Pauw measurements on tape cast layers during sintering allowed following the conductivity during sintering. A strong increase in conductivity and in activation energy E_a for conduction was observed between 900 and 1030 °C indicating an activation of the reactive sintering process and phase transformation of cobalt oxide.     

Microwave sintering of AlN powder synthesized by a SHS method

Sintering of the AlN powder synthesized by a combustion synthesis method, which was developed recently by the present authors, was studied by using a microwave sintering technique. A single mode microwave cavity was used and an insulation package with a simple configuration was developed. A high sintering temperature (1900 °C or higher) and a stable and uniform heating were readily achieved. A temperature measurement technique using a thermocouple with extrapolation was established to obtain the sintering temperature. A percent theoretical density of 99.5% and a thermal conductivity of 186 W/m K were obtained for a specimen which was sintered at 1900 °C with a soaking time of 30 min and 3 wt.% of Y₂O₃ added. The effects of sintering aid (i.e., Y₂O₃) and sintering temperature on densification, microstructure and thermal conductivity of the sintered specimens were investigated.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Comparison among different sintering routes for preparing alumina-YAG nanocomposites

Al₂O₃-YAG (50 vol.%) nanocomposite powders were prepared by wet-chemical synthesis and characterized by DTA-TG, XRD and TEM analyses. Amorphous powders were pre-heated at different temperatures (namely 600 °C, 800 °C, 900 °C and 1215 °C) and the influence of this thermal treatment on sintering behavior, final microstructure and density was investigated. The best performing sample was that pre-calcined at 900 °C, which yields dense bodies with a micronic/slightly sub-micronic microstructure after sintering at 1600 °C. A pre-treatment step to induce controlled crystallisation of the amorphous powder as well as a fast sintering procedure for green compacts, were also performed as a comparison.Finally, the previously stated thermal pre-treatment of the amorphous product was coupled to an extensive mechanical activation performed by wet planetary/ball milling. This procedure was highly effective in lowering the densification temperature, so that fully dense Al₂O₃-YAG composites, with a mean grain size smaller than 200 nm, were obtained by sintering in the temperature range 1370–1420 °C.     

SPS processing of bismuth-layer structured ferroelectric ceramics yielding highly textured microstructures

The spark plasma sintering (SPS) behaviour of nano-sized Bi₄Ti₃O₁₂ (BIT) and micron-sized CaBi₂Nb₂O₉ (CBNO) powders is described. The densification process of both powders is very rapid, i.e. the densification occurs within a very narrow time interval (2–3 min using a heating rate of 100 °C min⁻¹ and a pressure of 50 MPa). The BIT powder exhibits a lower densification onset temperature (∼650 °C) and higher maximum shrinkage rate (8.9 × 10⁻³ s⁻¹ at 780 °C) than that of the CBNO powder (∼825 °C and 4.5 × 10⁻³ s⁻¹ at 950 °C). Isothermal compaction studies revealed that fully dense nano-sized BIT compacts could be obtained within the temperature region 750 °C < T_iso < 850 °C while for T_iso > 850 °C compacts containing elongated platelet grains are formed. A new preparation route to produce highly textured compacts is described in detail. Appropriate pre-forms are prepared by spark plasma sintering (SPS) and these fully dense compacts are subject to superplastic deformation in the SPS unit to achieve a total compressive strain of ∼60%. This strain was achieved within a period of 1.5 min and with a maximum strain rates of 1.1 × 10⁻² s⁻¹ achieved at ∼840 °C and 1.3 × 10⁻² s⁻¹ at 1020 °C for the BIT and CBNO compacts, respectively. The X-ray studies showed that the Lotgering orientation factors of grains in the deformed BIT and CBNO compacts are 99% and 70%. The formation of highly textured compacts is suggested to be governed by a superplastic deformation-induced directional dynamic ripening mechanism.     

Microstructure and mechanical properties of the spark plasma sintered Ta2C ceramics

Single phase hexagonal α-Ta₂C ceramics were synthesized by spark plasma sintering and using TaC and Ta as the starting powders. Effects of sintering temperatures and holding times on the densification process, phase formation, microstructure development, and mechanical properties of the α-Ta₂C ceramics were investigated. Densification occurred in the temperature range of 1520–1675 °C in less than 2.5 min. But completion of the Ta₂C formation took about 40 min at 1500 °C, and 5 min at 1900 °C. The materials sintered at 1500 °C consisted of fine equiaxed grains. The Ta₂C grains grew anisotropic to form an elongated self-toughening microstructure at 1700 °C. At 1900 °C, the neighboring Ta₂C individual crystals coalesced to form large Ta₂C blocks to entrap the residual pores. Although higher flexural strength and fracture toughness were reached at 1700 °C, the unstable microstructures of the Ta₂C materials indicated limited applications at high temperatures.