Spark plasma sintering of TiC–SiCw ceramics

Silicon carbide whiskers (SiC_w) in TiC had impressive impacts on the properties and made it possible for special applications which generally would not be conceivable with TiC alone. In the present work, SiC_w reinforced TiC based composites were prepared by spark plasma sintering (SPS) technique, at the temperature of 1900 °C under the pressure of 40 MPa for sintering time of 7 min. To test out the effects of different amount of SiC whisker (0, 10, 20 and 30 vol%) on the characteristics of TiC, the sintered samples were investigated about sinterability and physical-mechanical properties. Microstructure observations and density measurements confirmed that the composites were dense with uniformly distributed reinforcement, and the specimen doped with higher than 10 vol% SiC_w could attain higher relative density (>100%) than pure TiC and TiC–10 vol% SiC_w. Also, the highest values for hardness (29.04 GPa) and thermal conductivity (39.2 W/mK) were achieved in specimen containing 30 vol% SiC_w, whereas the optimum bending strength (644 MPa) was recorded in material containing 20 vol% SiC_w. It seems that one of the reasons which contributes to this trend of properties variation is the generation of near-stoichiometric TiC_x phase and new Ti₃SiC₂ compound.     

Sintering properties and homogeneity of Ca2+-doped .65Y2O3–.35YCr0.5Mn0.5O3 NTC ceramics

Ceramics suitable for use over a wide temperature and having a negative temperature coefficient (NTC) based on .65Y₂O₃–.35YCr_0.5Mn_0.5O₃-doped with CaO were prepared by applying a solid-state reaction at different temperatures (1500–1650°C). The physical properties and scanning electron microscopy results revealed that dense NTC ceramics could be obtained by sintering at >1550°C. The effect of two different sintering methods on the properties of the NTC ceramics was studied, and the results indicated that the NTC ceramics obtained by employing the two-step sintering method exhibited better properties. The contents of Cr and Mn oxides in the NTC ceramic discs prepared by applying two-step sintering (1600°C) exhibited a decreasing trend from inside to outside. To quantify the diffusion rate, Fick's second law was used, and the diffusion coefficients of Cr and Mn oxides in the NTC ceramics were found to be 5.20 × 10^–5 and 2.36 × 10^–5 cm²/s, respectively. Resistivity and temperature analyses indicated that the resistivity (ρ₂₅), B_25/100, and B_700/1000 of the NTC ceramics were 1.61 × 10⁶ Ω cm, 2367 K, and 2697 K, respectively, which are suitable for a wide temperature range.     

Spark plasma sintering and high-temperature strength of B6O–TaB2 ceramics

Tantalum diboride – boron suboxide ceramic composites were densified by spark plasma sintering at 1900 °C. Strength and fracture toughness of these bulk composites at room temperature were 490 MPa and 4 MPa m^1/2, respectively. Flexural strength of B₆O–TaB₂ ceramics increased up to 800 °C and remained unchanged up to 1600 °C. At 1800 °C a rapid decrease in strength down to 300 MPa was observed and was accompanied by change in fracture mechanisms suggestive of decomposition of boron suboxide grains. Fracture toughness of B₆O–TaB₂ composites showed a minimum at 800 °C, suggestive a relaxation of thermal stresses generated from the mismatch in coefficients of thermal expansion.Flexural strength at elevated temperatures for bulk TaB₂ reference sample was also investigated.Results suggest that formation of composite provides additional strengthening/toughening as in all cases flexural strength and fracture toughness of the B₆O–TaB₂ ceramic composite was higher than that reported for B₆O monoliths.     

Improved dielectric properties of Ba0.5Sr0.5TiO3–ZnAl2O4 composite ceramics using double sintering

Ba_0.5Sr_0.5TiO₃–ZnAl₂O₄ composite ceramics were prepared by double sintering and conventional sintering. The results show that the double sintering can effectively reduce the ion diffusion between Ba_0.5Sr_0.5TiO₃ and ZnAl₂O₄ phases. The double sintered samples exhibit higher density and more uniform grain size distribution than the conventional sintered samples. The dielectric permittivity of double sintered samples is lower than that of conventional sintered samples. Impedance spectrum analysis shows that the oxygen vacancy content and grain boundary resistance of the double sintered samples is lower than that of the conventional sintered samples, which indicates that the Q value of the double sintered samples is higher than that of the conventional sintered samples. The optimum dielectric tunability and Q value of double sintered 60 wt%Ba_0.5Sr_0.5TiO₃-40 wt%ZnAl₂O₄ sample are 23.4% at 30 kV/cm and 276 at 2.257 GHz, respectively. Therefore, double sintering is a strategy that can effectively adjust the dielectric tunability and Q value of BST-ZA composite ceramics.     

Microwave sintering of IR-transparent Y2O3–MgO composite ceramics

A method for preparation of dense Y₂O₃–MgO composite ceramics by the microwave sintering was developed. The initial powders were obtained by glycine-nitrate self-propagating high-temperature synthesis (SHS) with different oxidant-to-fuel ratio. Density and IR-transmission of microwave sintered Y₂O₃–MgO ceramics increase with respect to dispersity of the SHS-powders and reach its maximum values for the powder prepared in a 20% fuel excess. The sintering behavior of Y₂O₃–MgO compacts was investigated by optical dilatometry and measuring an electric conductivity upon heating. Significant microwave radiation power surges at temperatures of 900–1000 °C, caused by the decomposition of magnesium carbonate, have been found. As a result of matching the conditions for the synthesis of powders and sintering modes, a transmission of composite ceramics of 78% at a wavelength of 6 μm was achieved at a maximum processing temperature of 1500 °C.     

Spark plasma sintering of ZrC–SiC ceramics with LiYO2 additive

Spark plasma sintering (SPS) of ZrC–SiC composite powders in the presence of LiYO₂ sintering additive was studied. The starting powders were obtained by a carbothermal reduction (CTR) of natural mineral zircon (ZrSiO₄), which provided an intimate mixing of in-situ created ZrC and SiC powders. This composite powder and LiYO₂ as additive were densified by spark plasma sintering. Microstructural features of the composite were investigated by XRD, SEM/EDS and AFM analysis. The sintered composite material possesses promising mechanical properties and excellent cavitation resistance which was observed with a cavitation erosion test. The values of Vickers microhardness and fracture toughness of the composite material are 20.7 GPa and 5.07 MPam^1/2, respectively.     

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.     

Analysis of complex impedance and electrical conductivity of YCr0.5Mn0.5O3–CaCu3Ti4O12 negative temperature coefficient ceramics

YCr_0.5Mn_0.5O₃(YCM)-CaCu₃Ti₄O₁₂(CCTO) ceramics were prepared using a solid processing method, and their properties were studied. XRD measurements showed that all prepared ceramics had pure perovskite structures. With increasing YCM content, grain size in ceramics decreased, and the resistivity of materials was found to be very sensitive to grain size and grain size distribution. Moreover, ceramics had NTC characteristics, and values of ρ₂₅, B_25/75, and E_a ranged from 2.21 × 10⁶–7.30 × 10⁶ Ω cm, 5797–6300 K, and 0.5390–0.5800 eV, respectively. The presence of heterovalent ion pairs in samples suggested that conduction mechanism may have been related to electron hopping. Impedance spectroscopy results showed that excessive doping with YCM resulted in greater contribution from grains to overall resistance of the material. Temperature-dependent electrical relaxation behavior was dominated by short-range movement of charge carriers.     

Spark plasma sintering of Al-doped ZrB2–SiC composite

This research presents the influence of Al addition on microstructure and mechanical behavior of ZrB₂–SiC ultra-high temperature ceramic matrix composite (UHTCMC) fabricated by spark plasma sintering (SPS). A 2.5?wt% Al-doped ZrB₂–20?vol% SiC UHTCMC was produced by SPS method at 1900?°C under a pressure of 40?MPa for 7?min. The microstructural and phase analysis of the composite showed that aluminum-containing compounds were formed in-situ during the SPS as a result of chemical reactions between Al and surface oxide films of the raw materials (i.e. ZrO₂ and SiO₂ on the surfaces of ZrB₂ and SiC particles, respectively). The Al dopant was completely consumed and converted to the intermetallic Al₃Zr and Al₄Si compounds as well as Al₂O₃ and Al₂SiO₅. A relative density of 99.8%, a hardness (HV5) of 21.5?GPa and a fracture toughness (indentation method) of 6.3?MPa?m^1/2 were estimated for the Al-doped ZrB₂–SiC composite. Crack bridging, branching, and deflection were identified as the main toughening mechanisms.     

Spark plasma sintering of TiN–TiB2 composites

TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–2573 K. Effects of TiN and TiB₂ content on relative density, microstructure, and mechanical properties were investigated. Above 2373 K, TiN–TiB₂ composites exhibited relative densities over 95%. A high density of 99.7% was obtained at 2573 K with 20–30 vol% TiB₂. Shrinkage of the TiN–70 vol% TiB₂ composite was the highest at 1573–2473 K. For the TiN–70 vol% TiB₂ composite prepared at 1973–2373 K, TiN grains were small, while at 2573 K, TiB₂ became a continuous matrix, in which irregular-shaped TiN dispersed. hBN was formed in the TiN–TiB₂ composite containing 50–60 vol% TiB₂ above 2373 K. The maximum Vickers hardness and fracture toughness obtained for the TiN–80 vol% TiB₂ composite sintered at 2473 K was 26.3 GPa and 4.5 MPa m^1/2, respectively.     

Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite

Materials designed in the high-alumina region of Al₂O₃–MgO–CaO system have been widely used in many technological fields. However, their further applications are limited by the high sintering temperatures necessary to achieve densification due to the poor sintering ability of calcium hexaluminate (CaAl₁₂O₁₉) and spinel (MgAl₂O₄). Considering this aspect, the present work investigated the effect of TiO₂ addition on the sintering densification and mechanical properties of MgAl₂O₄–CaAl₄O₇–CaAl₁₂O₁₉ composite by solid state reaction sintering. The results showed that the CA₆ grains presented a more equiaxed morphology instead of platelet structure by incorporating Ti⁴⁺ into its structure, which greatly improved the densification after heating at 1600 °C. The flexural strength was greatly enhanced with increasing addition of TiO₂ due to the significant decrease in porosity and improvement in uniformity of grain size as well as the absence of microcracks in the presence of Al₂TiO₅. The increased content of TiO₂ also played an active role in toughening this composite attributed to the increase in resistance to crack initiation and propagation.     

Spark plasma sintering of ZrO2–Al2O3 nanocomposites at low temperatures aided by amorphous powders

In present work, ZrO₂-5 wt% Al₂O₃ and ZrO₂-10 wt% Al₂O₃ nanocomposites are fabricated through spark plasma sintering. Al₂O₃–ZrO₂ amorphous powders and polycrystal Al₂O₃ powders and are doped in the polycrystalline ZrO₂ powders, respectively. When doped with amorphous powders, the sintering of ZrO₂–Al₂O₃ nanocomposites is promoted, and ZrO₂-5 wt% Al₂O₃ and ZrO₂-10 wt% Al₂O₃ nanocomposites with relative densities of 99% are obtained after spark plasma sintering at 1200 °C; however, when sintering of polycrystalline ZrO₂ and polycrystalline Al₂O₃ powders, the relative densities are merely 93%. The enhanced sinterability is due to the metastability and phase transformation of the amorphous powders, which act as sintering aids. The nanocomposites with near-theoretical density show refined microstructure with homogenous mixture of ZrO₂ and Al₂O₃ grains, which further leads to excellent mechanical properties. This article provides new ideas for low-temperature sintering of nanocomposites via using doping amorphous powders.     

Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites

Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB₂–SiC–ZrC–C_f ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB₂:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB₂–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (C_f: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:C_f components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% C_f had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.     

Spark plasma sintering of TaC–HfC UHTC via disilicides sintering aids

Ta_0.8Hf_0.2C ceramic has the highest melting point among the known materials (4000 °C). Spark plasma sintering is a new route for consolidation of materials, specially ultra high temperature ceramics (UHTCs), which are difficult to be sintered at temperatures lower than 2000 °C.The purpose of this study is to consolidate Ta_0.8Hf_0.2C by spark plasma sintering at low temperature using MoSi₂ and TaSi₂ as sintering aid. In this regard, effect of different amounts of sintering aids and carbides ratio on densification behavior and mechanical properties of Ta_1?xHf_xC were investigated.Fully consolidation of Ta_0.8Hf_0.2C was achieved in presence of 12 vol.% sintering aid after sintering at 1650 °C for 5 min under 30 MPa. The first stage of sintering was due to plastic deformation of sintering aids particles and consequent rearrangement. The second stage was occurred via Ta_1?xHf_xC solid solution and liquid phase formation.     

Joining MgAl2O4 ceramics using ZnO–Al2O3–SiO2 glass ceramic with precipitated ZnAl2O4

In this work, crystallization, thermal expansion and wetting behavior of ZnO–Al₂O₃–SiO₂ (ZAS) glass were first investigated. The results showed that ZnAl₂O₄ was precipitated from ZAS glass after crystallization treatment. Crystallization increased the coefficient of thermal expansion (CTE) of ZAS glass ceramic due to the high CTE of ZnAl₂O₄. In addition, ZAS glass exhibited good wettability on the surface of MgAl₂O₄ substrate. On this basis, ZAS glass was used to join MgAl₂O₄ ceramic, and the microstructure and mechanical properties of joints obtained with different cooling methods were investigated. The flexural strength of joints was related to the content of ZnAl₂O₄ crystals in the brazing seams. Additional nucleation and crystallization treatment during cooling process improved the crystallinity of brazing seam, resulting in better matching of the CTE of brazing seam with that of MgAl₂O₄ ceramic. The maximum flexural strength of joints reached 201 MPa, which was equivalent to the strength of MgAl₂O₄ ceramic.     

CaMn0.05Zr0.95O3–NiMn2O4 composite ceramics with tunable electrical properties for high temperature NTC thermistors

A series of novel high temperature negative temperature coefficient (NTC) composite ceramics based on (1-x)CaMn_0.05Zr_0.95O₃-xNiMn₂O₄ (x = 0, 0.1, 0.2, 0.3) were fabricated by using the solid-state method. The Rietveld refinement method and backscattered electron (BSE) image confirmed the absence of any other phase. The conduction mechanism of the composite ceramics was determined by analyzing the complex impedance and resistance-temperature characteristics, which could be related to the formation of a continuous Mn³⁺-O-Mn⁴⁺ network. The effect of spinel content on the high temperature stability was analyzed using aging tests. The ρ_400°C, ρ_700°C and B_400/700 values of the NTC thermistors were determined to be 4.9 × 10⁶–1.2 × 10⁴, 1.4 × 10⁵–0.8 × 10³ Ω cm and 12739-5712 K, respectively. This indicated that the electrical properties could be tuned by adjusting the weight ratio of CaMn_0.05Zr_0.95O₃ and NiMn₂O₄. The findings obtained in this study reveal that the composite NTC thermistor exhibits a good application potential at high temperatures and under harsh environments.     

Dielectric tunable properties of high dielectric breakdown Ba0.5Sr0.5TiO3–Zn2P2O7 composite ceramics

Dielectric properties of Ba_0.5Sr_0.5TiO₃–xZn₂P₂O₇ (x = 1, 3, 5, 10, 15 wt%) composite ceramics, which were prepared by solid-state reaction process, were intensively investigated. The results showed that the Curie temperature (T_c) of composites gradually shifted to lower temperature (?140 °C) with increasing the content of Zn₂P₂O₇, and the dielectric constant were tuned effectively from 2020 to 107, while maintaining a relatively high tunability. Zn₂P₂O₇ additions remarkably inhibited the grain growth of Ba_0.5Sr_0.5TiO₃ phases, and improved the breakdown strength of samples up to 385 kV/cm. The sample with x = 10 wt% exhibited good dielectric properties (?_r = 290, tg δ = 0.0006, T = 20.5%, BDS = 297 kV/cm). Meanwhile Zn₂P₂O₇ addition also made the T_c far away from the room temperature, which reduced the sensitivity of the dielectric constant to temperature change and simultaneously improved the stability of materials.     

Gelcasting and pressureless sintering of silicon carbide ceramics using Al2O3–Y2O3 as the sintering additives

In this paper, silicon carbide ceramics were prepared by aqueous gelcasting and pressureless sintering using Al₂O₃ and Y₂O₃ as the sintering additives. In order to develop well dispersed SiC slurries in the presence of sintering additives, the Al₂O₃ and Y₂O₃ powder was treated in the citric acid solution in advance. Zeta potential measurement showed that the isoelectric point (IEP) of Al₂O₃ and Y₂O₃ powder moved toward low pH region after treatment. Rheological measurement confirmed that the addition of as-treated powder showed very limited influence on the slurry properties as compared to that of untreated powder. SiC slurries with solid content of 54 vol% and enough fluidity can be developed. After gelcasting and pressureless sintering, SiC ceramics with nearly full density, fine grained and homogeneous microstructure can be obtained. Results showed that the surface treatment of Al₂O₃ and Y₂O₃ with citric acid is effective for the gelcasting process of SiC.     

Spark plasma sintering of Al2O3–Ni nanocomposites using Ni nanoparticles produced by rotary chemical vapour deposition

Al₂O₃–Ni nanocomposites were fabricated by spark plasma sintering (SPS) using Ni nanoparticle produced by rotary chemical vapour deposition. Carbon-free Ni nanoparticles were prepared by reacting NiCp₂ with O₂ to form NiO and then reducing to Ni by H₂ for 30 min at 823 K. The highest Ni content and grain size were 7.8 wt.% and 47.7 nm, respectively, using a NiCp₂ supply rate (R_s) of 1.67 × 10⁻⁶ kg s⁻¹. At a sintering temperature (T_SPS) of 1573 K, the hardness of Al₂O₃–3.8 wt.% Ni was 20.5 GPa, around 1 GPa higher than that of monolithic Al₂O₃ sintered at the same temperature. The tensile strength of Al₂O₃–4.6 wt.% Ni was 170 MPa, 60 MPa higher than that of Al₂O₃ sintered at 1573 K.     

BaMg1/3Nb2/3O3–Mg4Nb2O9 composite microwave ceramics with high Q-factor and low sintering temperature

Revised thermodynamic equilibrium in the BaO–MgO–Nb₂O₅ pseudo-ternary system has lead to development of a novel composite dielectric material with dielectric constant, ?′ = 25.5, efficacy factor, Q × f = 160 THz, and temperature coefficient of the resonant frequency, τ_f = +0.5 ppm/K. The material shows one of the highest Q-factors among the Ta-free microwave dielectric resonators. It also does not contain volatile Zn and Co elements. Other important property of the title compound is low sintering temperature of 1320 °C which significantly reduces the processing cost.