Porous SiC ceramics prepared via freeze-casting and solid state sintering

Porous SiC ceramics were prepared by freeze-casting process. In order to enhance the mechanical properties of the porous SiC, poly(vinyl alcohol) (PVA) was added as binder and pore morphology controller in this work. The results indicated that high porosity (>60%) SiC ceramics was obtained although the sintering temperature was over 2000 °C. The pore structure could be divided into two kinds: macropores generated by sublimation of large ice crystals, and micropores in the ceramic matrix caused by sublimating of small ice crystals, stacking of SiC particles, and burning out of PVA. With the increase of the sintering temperature, the specimens exhibited higher density, thus resulted in higher strength. Porous SiC ceramics sintered at 2100 °C showed a good flexural strength of 11.25 MPa with an open porosity as high as 66.46%.     

Microstructure and micromechanical properties of GaV4S8 ceramics prepared by single-step solid state synthesis

Despite the current focus on the electronic properties of GaV₄S₈ lacunar spinel, the microstructural and mechanical characterization of this material is scarce in the literature. In this work, we propose an effective GaV₄S₈ ceramics production method and provide a detailed microstructural and micromechanical characterization. Light microscopy, scanning electron microscopy and X-ray diffraction are used to describe the microstructure of the ceramic targets that can be used for thin film deposition. V₂O₃ was found to be the main impurity in ceramic targets and its content is discussed with respect to the sintering atmosphere control. Nanoindentation and microcantilever bending were employed to provide estimates of the indentation modulus, hardness and fracture stress of individual grains. The values of these parameters have been determined as E_r = 130 ± 2 GPa, H = 8.9 ± 0.2 GPa and σ_c = 600 ± 57 MPa, respectively.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Pure-phase BiFeO3 ceramics with enhanced electrical properties prepared by two-step sintering

A two-step sintering method was employed to fabricate BiFeO₃ ceramics and the influences of sintering conditions on the structural, dielectric, ferroelectric, and piezoelectric properties were investigated. It was found that high-density and pure-phase BiFeO₃ ceramics could be successfully achieved through the two-step sintering method by optimizing the sintering conditions. Furthermore, ferroelectricity and piezoelectricity were also improved by the two-step sintering method. The pure-phase BiFeO₃ ceramics obtained at T₁=850 °C and T₂=750 °C exhibited high resistivity (1.57×10⁶ Ω.m), large remanent polarization (P_r=0.80μC/cm²) and strong piezoelectric activity (d₃₃=42pC/N). All these results indicated that the two-step sintering method was an effective way to improve the properties of the BiFeO₃ ceramics.     

助烧剂和造孔剂对真空烧结SiC多孔陶瓷性能的影响

以粒度≤0.063mm的SiC为主要原料,分别加入30%(质量分数)的Al2O3-Y2O3与10%的Al2O3-高岭土复合助烧剂,并外加不同量(分别为12.8%、26.3%、30.0%和36.4%)的造孔剂羧甲基纤维素钠(CMC),制样后首先在空气炉中经过300℃2h或1100℃4h的预烧,然后在真空炉中于1550℃4h真空烧结而制备成SiC多孔陶瓷,并研究了助烧剂种类以及造孔剂CMC外加量对SiC多孔陶瓷显微组织、显气孔率及抗折强度的影响。结果显示:采用Al2O3-Y2O3作为助烧剂的SiC多孔陶瓷比Al2O3-高岭土作助烧剂的具有较高的抗折强度,显气孔率稍有减小;随着羧甲基纤维素钠量的增加,加入两种助烧剂的SiC多孔陶瓷均表现为显气孔率增加,抗折强度降低。     

Coarse-grain CeO2 doped ZrO2 ceramic prepared by spark plasma sintering

In the present work, coarse grain cerium stabilized zirconia bulk ceramic was prepared by spark plasma sintering technique. The relatively high temperature of 2000 °C used for sintering led to enormous grain growth up to approximately 100 μm. Sintering at high temperatures and in the vacuum caused oxygen depletion and thus transformation from tetragonal to cubic phase during the sintering process. The tetragonal phase was recovered by annealing at 1400 °C in air. This led to a change in fracture behavior. Mostly transgranular fracture of the cubic phase was changed to intergranular fracture after recovering the tetragonal phase. On the intergranular fracture surface, twinning-like structure and structures similar to antiphase domain were observed.Mechanical properties represented by indentation hardness of prepared samples were evaluated.     

氧化钙活性对固相烧结反应的影响

为了了解不同煅烧条件下氧化钙的活性对固相烧结反应的影响,通过实验用新生态氧化钙与砂岩在 1 250、1 300、1 350 ℃ 3个不同的温度下煅烧,通过探讨氧化钙活性对游离氧化钙的影响,来分析氧化钙活性对后续固相反应的影响。实验结果表明,对石灰石进行预煅烧,获得较高活性氧化钙参与固相反应,对减少产物中的游离氧化钙含量有较明显的作用,氧化钙活性越高,游离氧化钙含量越低,固相反应越完全。     

Sintering behaviour and properties of zirconia ceramics prepared by pressureless sintering

Improvements in the sintering process and powder quality can lead to wider application of zirconia in ceramics. In this study, the effects of different temperatures on the stability, relative content of the tetragonal phase, and composition of Al₂O₃–ZrO₂ ceramic powders were explored using pressureless-assisted sintering. The crystallinity of the sintered Al₂O₃–ZrO₂ samples was significantly improved. The content of the tetragonal-phase ZrO₂ in sintered ceramic powders was 52.07%, 52.46%, 56.16%, 63.99%, and 64.90%, respectively, which was significantly higher than those of the raw materials. The average particle size of the sintered samples decreased from 1.07 μm to 0.17 μm with an increase in temperature, indicating that the ceramic powder particles were refined. The sample that was subjected to pressureless-assisted sintering at 1200 °C and held for 1 h exhibited the best stability and more uniform particle distribution compared to other samples. The particle size distribution data were closer to the standard line, satisfying the requirements of the normal distribution law. The results revealed that a high temperature was more favourable to the solid solution, and the formation of an Al₂O₃–ZrO₂ solid solution can diminish the influence of the volume expansion of ceramic powders on the sample properties during sintering. Therefore, the addition of the sintering aid Al₂O₃ significantly promotes the densification of the powders, and the pressureless sintering technique reduces the sintering temperature of the solid solution, thus imparting a crystalline structure and excellent mechanical properties to the material.     

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.     

Structural,dielectric, vibrational and magnetic properties of Sm doped BiFeO3 multiferroic ceramics prepared by a rapid liquid phase sintering method

Rare earth Sm substituted Bi_1−xSm_xFeO₃ with x=0, 0.025, 0.05, 0.075 and 0.10 polycrystalline ceramics were synthesized by a rapid liquid phase sintering method. The effect of varying composition of Sm substitution on the structural, dielectric, vibrational, optical and magnetic properties of doped BiFeO₃ (BFO) ceramics have been investigated. X-ray diffraction patterns of the synthesized rare earth substituted multiferroic ceramics showed the pure phase formation with distorted rhombohedral structure with space group R3c. Good agreement between the observed and calculated diffraction patterns of Sm doped BFO ceramics in Rietveld refinement analysis of the X-ray diffraction patterns and Raman spectroscopy also confirmed the distorted rhombohedral perovskite structure with R3c symmetry. Dielectric measurements showed improved dielectric properties and magnetoelectric coupling around Néel temperature in all the doped samples. FTIR analysis establishes O–Fe–O and Fe–O stretching vibrations in BiFeO₃ and Sm-doped BiFeO₃. Photoluminescence (PL) spectra showed visible range emissions in modified BiFeO₃ ceramics. The magnetic hysteresis measurements at room temperature and 5 K showed the increase in the magnetization with the increase in doping concentration of Sm which is due to the structural distortion and partial destruction of spin cycloid caused by Sm doping in BFO ceramics.     

SiC-based ceramics with remarkable electrical conductivity prepared by ultrafast high-temperature sintering

SiC-based ceramics with high electrical conductivity are applied widely as electrode materials and semiconductor materials. In this study, a SiC-based ceramic with relative density of 96% was prepared by ultrafast high-temperature sintering (UHS) at 2000 ℃ (with a heating rate of 1000 ℃/min) for 40 s. The resistivity of as UHS-ed SiC-based ceramic was 1/15 of that prepared by the pressureless sintering. We found that the components of as-sintered body (SiC, Si and Y₃Si₅) by UHS were different from those (SiC and YAG) prepared by the pressureless sintering. The reason for the remarkable increase of the electrical conductivity of UHS-ed body was that the Si with higher electrical conductivity than SiC had emerged. Besides, the reaction mechanism was proposed and the unusual composition of the SiC-based ceramic sintered by UHS may also provide new reference for the application of SiC in specific fields.     

Dense gypsum ceramics prepared by room-temperature cold sintering with greatly improved mechanical properties

Dense gypsum (CaSO₄·2H₂O) ceramics were successfully fabricated by a simple room-temperature cold sintering process with 5 wt% water. The relative density of gypsum ceramics increased from 89.6% to 96.8% with increasing the applied uniaxial pressure from 100 to 400 MPa during cold sintering. The relative density changed slightly for higher pressure, and microcracks were observed as well as abnormal grain growth. Both the compressive and flexural strengths reached the peaks at 98.5 MPa and 26.5 MPa for the uniaxial pressure of 400 MPa, which were improved by 2.6 and 2.0 times, respectively comparing with the bulk gypsum prepared by traditional method from α-plaster. Furthermore, the dry-pressed gypsum compacts were very fragile, and had relative densities 5–12 % lower than the cold-sintered ceramics, indicating that the slight solubility of gypsum in water (0.2 g/100 g) played a critical role in the densification, microstructural evolution and greatly improved mechanical properties of cold-sintered gypsum ceramics.     

Rutile TiO2 microwave dielectric ceramics prepared via cold sintering assisted two step sintering

In this work, a sintering route named cold sintering assisted two step sintering process (CSP-TS) is presented to prepare rutile TiO₂ ceramics with submicron grain sizes. Cold sintering process at 300 °C with tetrabutyl titanate and water as the liquid phase yields a ‘green body’ with a relatively high density of ～80 %, and finally dense (98.5–99.8 %) rutile TiO₂ ceramics with grain sizes of ～600 nm can be obtained in the second sintering process at 950?1000 °C. The microstructural analysis with SEM and TEM indicates that the CSP-TS samples sintered at 950 °C have an obvious phenomenon of recrystallization, accompanying by a decrease of amorphous phases and a formation of clear grain boundaries. Besides, the rutile TiO₂ ceramics prepared by CSP-TS possess excellent microwave dielectric properties with relative permittivity of 92.0–98.4 and Q × f values of 27,800?31,900 GHz. Therefore, it is feasible to utilize CSP-TS to prepare ceramics with small grain sizes at low sintering temperatures.     

Phase and structural evolution of zirconolite ceramics prepared by solid-state reaction sintering

The evolution of phase assemblage and microstructure of stoichiometric zirconolite (CaZrTi₂O₇) ceramics, prepared by a solid-state reaction sintering route, was systematically investigated as a function of sintering temperature. Using powder XRD and quantitative phase analysis data, it was determined that the formation of zirconolite was a one-step reaction, without formation of intermediate phases. The accompanying fractions of secondary CaTiO₃ and ZrO₂ phases were reduced to approximately 2 wt % each after sintering at 1200 °C, with zirconolite formed as the major phase (> 99 wt%) after reaction at 1300 °C. Notable product densification only occurred at T ≥ 1400 °C, at which it was possible to achieve a relative density of 96.97% which is highly desirable for applications as a nuclear wasteform. The zirconolite-2M polytype structure (space group: C2/c) was formed in all products as expected, confirmed by combined high resolution TEM-ED analyses.     

Optical properties of YMASG:Ce fluorescent ceramics prepared by hot-pressure sintering

Cerium-doped yttrium aluminum garnet (YAG:Ce) based transparent ceramics have been widely used in fluorescent lighting as high-quality inorganic fluorescent conversion materials. This paper further explores the Mg²⁺-Si⁴⁺ ions doped YAG:Ce transparent ceramics by combining the solid-phase reaction method with vacuum hot-pressure sintering and implementing protection measures against hot-pressure mold contamination, and also investigates the effect of different Mg²⁺-Si⁴⁺ doping contents on the structure, transmittance and luminescence properties of the ceramics under hot-pressure sintering. In this work, pure-phase YMASG:Ce transparent fluorescent ceramics with a grain size of about 3-6 μm and clear and clean grain boundaries were obtained with an In-line transmittance of 67% at 800 nm. Under the excitation at 460 nm, the emission peak was red-shifted by 26 nm and the full width at half maxima (FWHM) was broadened with the increase of Mg²⁺-Si⁴⁺ content, which shows that the Mg²⁺-Si⁴⁺ ion pair effectively complements the absence of the red light component in the YAG:Ce emission spectrum. The optimized YMASG:Ce ceramics obtained high-quality warm white light with a low correlated color temperature (CCT) and a high color rendering index (CRI) under the excitation of the blue LED chip. This work proved the feasibility of vacuum hot-pressure sintering to prepare YMASG:Ce transparent fluorescent ceramics, and provided a new approach for studying YMASG:Ce-based ceramics, which was significant for the application of high-power visible laser illumination.     

Experimental based full process simulation of alumina selective laser processed parts densified by cold isostatic pressing and solid state sintering

The compound process of cold isostatic pressing (CIP) of alumina selective laser processed (SLP) parts and solid state sintering (SSS) and its full process simulation were realized in this paper, focusing on studying the overall deformation, relative density distribution, grain growth and sintering stress variation during the process. Especially, correlation was established between the macroscopic deformation and microscopic evolution. Model parameters for alumina are presented, which were optimized in accordance with the experimental results. CIPed part still exhibited density inhomogeneity, of which SSS tended to increase the overall density and homogenize density distribution. The sintering behavior was studied with the employment of dilatometer experiments. Furthermore, compared with conventional heating strategy, fast firing turned out to decrease sintering production time as well as drive the matter diffusion and densification process. The master sintering curve (MSC) moves upward a little under the condition of fast firing.     

Phase evolution and microwave dielectric properties of BaTi4O9 ceramics prepared by reaction sintering method

BaTi₄O₉ microwave dielectric ceramics were prepared by reaction sintering method using BaCO₃ and TiO₂ as raw materials. The phase evolution and the chemical reactions were proposed based on the X-ray diffraction results with sintering temperature. The microstructure characteristics were observed using scanning electron microscopy and energy dispersive spectrometer. The compact ceramics with a single phase of BaTi₄O₉ could be prepared successfully by reaction sintering method, exhibiting optimum microwave dielectric properties: a dielectric constant of 36.9, a high quality factor of 52 735 (at 7.5GHz), and a near zero temperature coefficient of resonant frequency of 5.8 ppm/°C, after sintering at 1200°C for 6 hours.     

Flash sintering of ceramics

Flash sintering is a novel densification technology for ceramics, which allows a dramatic reduction of processing time and temperature. It represents a promising sintering route to reduce economic, energetic and environmental costs associated to firing. Moreover, it allows to develop peculiar and out-of-equilibrium microstructures.The flash process is complex and unusual, including different simultaneous physical and chemical phenomena and their understanding, explanation and implementation require an interdisciplinary approach from physics, to chemistry and engineering. In spite of the intensive work of several researchers, there is still a wide debate as for the predominant mechanisms responsible for flash sintering process.In the present review, the most significant and appealing mechanisms proposed for explaining the “flash” event are analyzed and discussed, with the aim to point out the level of knowledge reached so far and identify, at least, possible shared theories useful to propose future scientific activities and potential technological implementations.     

High-strength Ti2AlN ceramics prepared by pulse electric current sintering based on powders synthesized by molten salt method

Ti₂AlN powders were synthesized through molten salt method and re-calcination process using TiH₂, Al and TiN powders as raw materials at 1100 ℃. The composition of final composite was directly influenced by the initial Al and TiH₂ content in the starting mixture. The purity of the synthesized Ti₂AlN powder could reach 97.1 wt% when the Al molar ratio was 1.05. Then high strength Ti₂AlN ceramics were successfully prepared in different modes, including two forms of pulse electric current sintering (PECS/SPS) and hot-pressing sintering (HP). A record-high flexural strength of 719 MPa was obtained for the PECS/SPS with an electrical insulating die (PECS/SPS II) sintered sample, based on the synthesized powder in which the initial molar ratio of Al was 1.1. The sintering behaviors in various modes were analyzed, confirming the shrinkage of particles starting at lower temperature in PECS/SPS II. The density, microstructure, Vickers hardness and elastic modulus of sintered ceramics were also investigated. Therefore, the present work provided the new methods about powder preparation and ceramic sintering of Ti₂AlN, making it possible to be used as high strength structural ceramics.     

Low-temperature solid state synthesis of Eu-doped Ca2SnO4 ceramics under CO-CO2 atmosphere

High temperature solid state is reported the most common synthesis technique for Ca₂SnO₄ ceramics, nevertheless it requires high thermal treatment temperature and long synthesis time. Our previous studies indicated that Ca₂SnO₄ was much easier to form if SnO₂ and CaCO₃ were roasted under CO-CO₂ atmosphere at relatively low temperatures. In this study, the reaction mechanism between SnO₂ and CaCO₃ under 15 vol% CO/(CO+CO₂) atmosphere and air atmosphere were firstly investigated by using XRD, SEM, XPS, etc. In addition, the Eu-doped Ca₂SnO₄ was synthesized by low-temperature solid state reaction under CO-CO₂ atmosphere. The results indicated that the crystal structure and photoluminescence properties of the products were similar to those of samples synthesized under air atmosphere, whereas the synthetic temperature was reduced by more than 300 °C.