High electrocaloric effect in hot‐pressed Pb0.85La0.1(Zr0.65Ti0.35)O3 ceramics with a wide operating temperature range

The temperature stability of the electrocaloric effect (ECE) in relaxor ferroelectric Pb_0.85La_0.1(Zr_0.65Ti_0.35)O₃ (PLZT) prepared by the hot‐press sintering method has been investigated. Compared to the PLZTs prepared via the conventional sintering process, the hot‐pressed PLZTs exhibit larger ECE and superior temperature stability. The hot‐pressed sample with an appropriate content of excess PbO presents a high ΔT of 2.4°C and ΔS of 2.3 J kg^?1·K^?1, both of which are 30% greater than those of the conventionally sintered samples measured at 100 kV·cm^?1. More importantly, the hot‐pressed specimens display great stable electrical properties, including the dielectric breakdown strength and electrical resistivity in the temperature range from 0°C to 100°C, whose ECE instability, especially, is only one‐half that of the samples prepared by the conventional solid‐state method. In addition, the ECE and its stability of the hot‐pressed sample can be further enhanced by increasing the operating electric field to a relatively high level of 200 kV·cm^?1. This work demonstrates hot‐press sintering is an effective method to fabricate ferroelectric ceramics with high ECE as well as desirable temperature stability.     

Influence of Applied Pressure Load and Holding Time on Microstructure and Thermoelectric Properties of p‐Type Bi2Te3 Alloys

In this research, the effects of applied pressure load and holding time on microstructure and thermoelectric properties of p‐type Bi₂Te₃ + 75%Sb₂Te₃ alloys were studied. Besides this, the microstructural changes during the heat treatment of gas‐atomized (GA) powders were examined. The grain growth (12.5–25 μm) was observed with increasing of the holding time of the heat‐treated GA powders. The hot‐pressed bulks showed grain refinement behavior with increasing pressure load, whereas grain size was found to slightly increase with increasing holding time. The peak power factor value, 3.48 W/mK², was obtained for the hot‐pressed sample at 420°C with pressure load of 530 MPa.     

Elevated Temperature Strength Enhancement of ZrB2–30 vol% SiC Ceramics by Postsintering Thermal Annealing

The mechanical properties of dense, hot‐pressed ZrB₂–30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot‐pressed or after hot‐pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot‐pressed ZrB₂–SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot‐pressed ZrB₂–SiC. Neither ZrB₂ grain size nor SiC cluster size changed for these heat‐treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB₂‐based ceramics.     

Colloidal Processing of Zirconium Diboride Ultra‐High Temperature Ceramics

Colloidal processing of the Ultra‐High Temperature Ceramic (UHTC) zirconium diboride (ZrB₂) to develop near?net‐shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB₂ suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex‐shaped components that can be densified at lower sintering temperatures without pressure.     

Fabrication and laser performances of Yb:Sc2O3 transparent ceramics from different combination of vacuum sintering and hot isostatic pressing conditions

In this paper we report on the preparation and laser performance of transparent 3at.% Yb:Sc₂O₃ ceramics by reactive sintering of commercially available powders under vacuum followed by hot isostatic pressing (HIP). Combinations of different vacuum sintering temperatures (1650 °C and 1750 °C) and different HIP treatments (1700 °C and 1800 °C at 200 MPa) were tested in order to understand how these steps influence the microstructure and thus the optical and lasing properties of the ceramic samples. All the samples showed a good optical quality. The microstructure analysis and the laser tests showed that the vacuum pre-sintering temperature is the key factor determining the quality of the samples and the laser performances. The best values of slope efficiency i.e. η_L = 50 % and output power i.e. P_out = 6.62 W were obtained for the sample pre-sintered under vacuum at 1650 °C and hot isostatically pressed at 1800 °C.     

Thermoelectric properties of A-site deficient La-doped SrTiO3 at 100–900 °C under reducing conditions

Lanthanum doped strontium titanate is a potential n-type thermoelectric material at moderate and high temperatures. (La_0.12Sr_0.88)_0.95TiO₃ ceramics were prepared by two different routes, conventional sintering at 1500 °C and spark plasma sintering at temperatures between 925 and 1200 °C. Samples with grain size between 40 nm and 1.4 μm were prepared and characterized with respect to their thermoelectric transport properties at temperatures between 100 and 900 °C under reducing conditions (H₂/H₂O-buffer mixtures). The thermal conductivity was significantly reduced with decreasing grain size reaching a value of 1.3 W m^−1. K⁻¹ at 600 °C for grain size of 40 nm and porosity of 19%. Electrical conductivity increased with increasing grain size showing a maximum of 500 S cm⁻¹ at 200 °C for a grain size of 1.4 μm. The highest figure-of-merit (zT) was measured for samples with 1.4 μm average grain size reaching 0.2 at 500 °C.     

Facile Fabrication of Highly Transparent Yttria Ceramics with Fine Microstructures by a Hot‐Pressing Method

In this study, we report highly transparent yttria ceramics fabricated by a facile hot‐pressing method with tantalum foil shielding which effectively prevents the ceramic samples from carbon contamination caused by the graphite mold used during the process. The hot‐pressed sample was already highly transparent without a post‐annealing step or hot isostatic pressing. For a 2‐mm‐thick specimen doped with 1 at.% ZrO₂, the in‐line transmittance reaches 74.4% at 400 nm and 81.1% at 1100 nm. The sample shows a very fine microstructure with an average grain size of about 1 μm owing to the low sintering temperature of only 1600°C. The study results indicate that it is possible to produce transparent yttria ceramics with excellent optical transparency using the economical and convenient hot‐pressing method.     

Mechanical properties,microstructure and grain orientation of hot pressed WB2 ceramics with Co as a sintering additive

Tungsten diboride (WB₂) ceramics were hot pressed using Co as a sintering additive at relatively low temperatures (<1600 °C). This paper aimed to clarify the microstructure-mechanical properties relationships for the Co-doped WB₂ ceramics. With the addition of Co into the WB₂ matrix, the densification of WB₂ ceramic could be achieved with a high relative density of 92.144% at a temperature of 1500 °C. The sample also exhibited excellent mechanical properties with the flexural strength of 512.7 ± 43.5 MPa, Vickers hardness of 22.4 ± 1.0 GPa and fracture toughness of 4.9 ± 0.3 MPa m^1/2. The effects of sintering temperature and Co content on the densification, mechanical properties and microstructure development of the ceramic composites were systematically investigated. It suggested that the samples demonstrated a dramatic improvement and then a slight decrease in mechanical properties with the increasing temperature. Additionally, the mechanical properties of the samples decreased gradually with the increase in the mass fraction of Co. Based on the XRD and EDS analysis, Co reacts with WB₂ to form binary and ternary phases during the sintering process, contributing to the densification to a large extent. Besides, the formation of Co₂B and W₃CoB₃ also strengthened the binding force at grain boundaries, resulting in the high strength of the samples. Crack propagation path evolved from tortuous to straight with the increasing Co addition, indicating the fracture toughness of the samples decreased gradually. Additionally, the fracture behaviours and toughening mechanisms of the Co-doped WB₂ ceramic composites were also discussed. It is noteworthy that the hot pressed composites exhibited an anisotropy structure to some extent, and the growth and alignment mechanism of hexagonal WB₂ grains was illustrated in this paper.     

Enhancement in thermoelectric properties using a P‐type and N‐type thin‐film device structure

In this study, we have fabricated thermoelectric devices with p‐type and n‐type conducting polymers and research the effect of device structure with the thermoelectric properties. It was found that the p‐type and n‐type structure greatly enhances the device's electrical conductivity due to separated charge carrier channels, but the Seebeck coefficient was reduced due to the increase of charge density by doping. Photoexcitation can improve the device's thermoelectric properties and can increase the Seebeck coefficient and electrical conductivity with increasing doping concentration simultaneously. The increases in both properties are due to the phonon–electron coupling effect: the concentration of electrons and holes are increased under illumination, and the phonon component of the heat flux can be reduced by phonon scattering. Consequently, the thermoelectric device structure can improve the efficiency of thermoelectric conversion. The P3HT:PCBM devices demonstrate a significant enhancement in the power factor (PF = S²σ), with a maximum value of ZT = 0.5 at 147°C, in which the PF value (34.8 μV/cm K²) is bigger than Bi₂Te₃/Sb₂Te₃ superlattice devices at room temperature. POLYM. COMPOS., 34:1728–1734, 2013. © 2013 Society of Plastics Engineers     

Low‐Temperature Spark Plasma Sintering of Pure Nano WC Powder

For the first time we have demonstrated the densification of high‐purity nanostructured (d_avg ≈ 60 nm) tungsten carbide by High Pressure Spark Plasma Sintering (HPSPS) in the unusually low temperature range of 1200°C–1400°C. The high‐pressure sintering (i.e., 300 MPa) produced dense material at a temperature as low as 1400°C. In comparison with more conventional sintering techniques, such as SPS (80 MPa) or hot isostatic pressing, HPSPS lowered the temperature required for full densification by 400°C–500°C. High Pressure Spark Plasma Sintering, even in absence of any sintering aid or grain growth inhibitor, retained a very fine microstructure resulting in a significant improvement in both hardness (2721 HV₁₀) and fracture toughness (7.2 MPa m^1/2).     

Phase reactions in a hot pressed TiC/Si powder mixture

This work investigated the possibility of producing dense Ti₃SiC₂ by hot pressing TiC/Si powders. A hot press with graphite heating elements was used for densification and the phase reactions of some hot pressed samples were further evaluated by pressureless heating in a dilatometer. The density and phase composition of the heat treated samples were evaluated using Archimedes principle and by X-ray diffractometry, respectively. Hot pressing resulted in a low Ti₃SiC₂ yield; the main phases were TiC and TiSi₂ regardless of starting powder composition, temperature, holding time or pressure. A second heating without pressure resulted in Ti₃SiC₂ formation, but only in samples initially hot pressed at 1300 °C or lower. At higher hot pressing temperatures, thin oxide layers on particle surfaces were locked into the structure. Acting as diffusion barriers, they prevented the Ti₃SiC₂ forming reaction. In hot pressed samples the density was significantly higher than in samples sintered without pressure.     

Zirconium Diboride with High Thermal Conductivity

Thermal properties were characterized for zirconium diboride produced by reactive hot pressing and compared to ZrB₂ ceramics that were hot pressed from commercial powders. No sintering additives were used in either process. Thermal conductivity was calculated from measured values of heat capacity, thermal diffusivity, and density for temperatures ranging from 298 to 2273 K. ZrB₂ produced by reactive hot pressing achieved near full density, but had a small volume fraction of ZrO₂, whereas hot‐pressed ZrB₂ contained porosity and carbon inclusions. Reactive hot pressing produced a ceramic with higher thermal diffusivity and heat capacity, resulting in thermal conductivities of 127 W·(m·K)^?1 at 298 K and 80 W·(m·K)^?1 at 2273 K, which were up to ~30% higher than typically reported for hot‐pressed ZrB₂.     

Fabrication of a transversal multilayer thermoelectric generator with substituted calcium manganite

The sintering behavior and thermoelectric performance of Ca_0.99Gd_0.01Mn_0.99W_0.01O₃ was studied, and a multilayer thermoelectric generator was fabricated. The addition of CuO as sintering additive was found to be effective for the reduction in the sintering temperature from 1300°C to about 1000°C‐1050°C. Dense samples were obtained after firing at 1050°C, whereas some porosity remained after firing at 1000°C. Samples sintered at reduced temperature exhibit lower electrical conductivity, whereas the Seebeck coefficient S = ?150 μV/K at 100°C is not affected by lowering the sintering temperature. The figure of merit is ZT = 0.12 at 700°C for samples sintered at 1300°C; ZT = 0.08 and 0.03 were obtained for multilayer laminates sintered at 1050°C and 1000°C, respectively. A transversal multilayer thermoelectric generator (TMLTEG) was built by stacking layers of substituted CaMnO₃ green tapes, and printing AgPd conductor stripes onto the thermoelectric layers at an angle of 30° relative to the direction of the heat flow. The multilayer stack was co‐fired at 1000°C. The TMLTEG has a power output of 2.5 mW at ?T= 200 K in the temperature interval of 25°C‐300°C. A meander‐like generator with larger power output comprising six TMTEGs is also presented.     

Effect of compaction on the sintering of borosilicate glass/alumina composites

The effect of initial compaction on the sintering of borosilicate glass matrix composites reinforced with 25 vol.% alumina (Al₂O₃) particles has been studied using powder compacts that were uniaxially pressed at 74, 200 and 370 MPa. The sintering behaviour of the samples heated in the temperature range 850–1150 °C was investigated by density measurement, axial and radial shrinkage measurement and microstructural observation. The density of the sintered composites increased continuously with temperature for compacts pressed at 74 MPa, while for compacts pressed at 200 and 370 MPa it reached the maximum value at 1050 °C and at higher temperatures it decreased slightly due to swelling. The results showed anisotropic shrinkage behaviour for all the samples, which exhibited an axial shrinkage higher than the radial shrinkage, and the anisotropic character increased with the initial compaction pressure.     

Synthesis,properties and thermal decomposition of the Ta4AlC3 MAX phase

The present work describes a synthesis route for bulk Ta₄AlC₃ MAX phase ceramics with high phase purity. Pressure-assisted densification was achieved by both hot pressing and spark plasma sintering of Ta₂H, Al and C powder mixtures in the 1200–1650 °C range. The phases present and microstructures were characterized as a function of the sintering temperature by X-ray diffraction and scanning electron microscopy. High-purity α-Ta₄AlC₃ was obtained by hot pressing at 1500 °C for 30 min at 30 MPa. The β-Ta₄AlC₃ allotrope was observed in the samples produced by SPS. The Young’s modulus, Vickers hardness, flexural strength and single-edge V-notch beam fracture toughness of the high-purity bulk sample were determined. The thermal decomposition of Ta₄AlC₃ into TaC_x and Al vapour in high (˜10⁻⁵ mbar) vacuum at 1200 °C and 1250 °C was also investigated, as a possible processing route to produce porous TaC_x components.     

Microstructure and Li‐Ion Conductivity of Hot‐Pressed Cubic Li7La3Zr2O12

The effect of hot‐pressing temperature on the microstructure and Li‐ion transport of Al‐doped, cubic Li₇La₃Zr₂O₁₂ (LLZO) was investigated. At fixed pressure (62 MPa), the relative density was 86%, 97%, and 99% when hot‐pressing at 900°C, 1000°C, and 1100°C, respectively. Electrochemical impedance spectroscopy showed that the percent grain‐boundary resistance decreased with increasing hot‐pressing temperature. Hot pressing at 1100°C resulted in a total conductivity of 0.37 mS/cm at room temperature where the grain boundaries contributed to 8% of the total resistance; one of the lowest grain‐boundary resistances reported. We believe hot pressing is an appealing technique to minimize grain‐boundary resistance and enable correlations between LLZO composition and bulk ionic conductivity.     

Sintering Mechanisms and Kinetics for Reaction Hot‐Pressed ZrB2

Sintering mechanisms and kinetics were investigated for ZrB₂ ceramics produced using reaction hot pressing. Specimens were sintered at temperatures ranging from 1800°C to 2100°C for times up to 120 min. ZrB₂ was the primary phase, although trace amounts of ZrO₂ and C were also detected. Below 2000°C, the densification mechanism was grain‐boundary diffusion with an activation energy of 241 ± 41 kJ/mol. At higher temperatures, the densification mechanism was lattice diffusion with an activation energy of 695 ± 62 kJ/mol. Grain growth exponents were determined to be ~4.5, which indicated that a grain pinning mechanism was active in both temperature regimes. The diffusion coefficients for grain growth were 1.5 × 10^?16 cm⁴/s at 1900°C and 2.1 × 10^?15 cm⁴/s at 2100°C. This study revealed that dense ZrB₂ ceramics can be produced by reactive hot pressing in shorter times and at lower temperatures than conventional hot pressing of commercial powders.     

Laser sintering of Al2O3-based green ceramics prepared by different pre-sintering temperatures

Al₂O₃-based green ceramics are prepared by isostatic cold pressing technology. The prepared green ceramics are pre-sintered at the temperature from room temperature to 1100°C, and then Al₂O₃ ceramics are prepared by laser sintering. The effects of pre-sintering temperatures and laser parameters on mechanical properties and the sintering quality are analyzed. The results show that good crystallinity of Al₂O₃ particles is obtained at a higher pre-sintering temperature. The flexural strength and density of green ceramics increase with the temperature of heat treatment. The flexural strength decreases slightly at ∼200°C due to the paraffin binder disintegration. The pre-sintering temperature and laser processing parameters have a significant influence on the sintering quality. With the increase of laser power and laser frequency, dynamic grain growth occurs, and then grains are refined. The majority of plate-like grains are transformed into long cylindrical-like grains in the severe densification process. However, porous flocculation microstructures are generated on the samples pre-sintered at 1100°C after laser sintering, which is due to the material gasification in atmospheric environment during sintering by infrared laser. More uniform microstructure and better sintering quality of samples pre-sintered at 500°C can be achieved after laser sintering with a relatively narrower grain size distribution.     

Physicomechanical and thermal properties of moldings made from liquefied wood‐based novolak PF resins under various hot‐pressing conditions

The wood powder of Cryptomeria japonica (Japanese cedar) was liquefied in phenol, with H₂SO₄ and HCl as a catalyst. The liquefied wood was used to prepare the liquefied wood‐based novolak phenol formaldehyde (PF) resins by reacting with formalin. Furthermore, novolak PF resins were mixed with wood flour, hexamethylenetetramine, zinc stearate as filler, curing agent, and lubricating agent, respectively, and hot‐pressed under 180 or 200°C for 5 or 10 min to manufacture moldings. The results showed that physicomechanical properties of moldings were influenced by the hot‐pressing condition. The molding made with hot‐pressing temperature of 200°C for 10 min had a higher curing degree, dimensional stability, and internal bonding strength. The thermal analysis indicated that using a hot‐pressing temperature of 180°C was not sufficient for the liquefied wood‐based novolak PF resins to completely cure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Densification,microstructure and mechanical properties of hot pressed tantalum carbide

Monolithic tantalum carbide (TaC) ceramics were prepared by hot pressing in order to investigate the effect of hot pressing temperature on the densification behavior, microstructure and mechanical properties of TaC. Monolithic TaC sample hot pressed at 2000 °C for 45 min under 40 MPa, with relative density value above 97%, Vickers hardness of 15.7 GPa and fracture toughness of 4.1 MPa m^1/2 was obtained. Fracture surfaces investigations of the samples, which were carried out using the SEM analysis, showed a significant grain growth by increasing the hot pressing temperature from 1700 to 2000 °C. Also, based on the X-ray diffraction pattern, a decrease in the lattice parameter of hot pressed TaC sample was observed.