Injection molding of ultra-fine Si_3N_4 powder for gas-pressure sintering

The ceramic injection molding technique was used in the gas-pressure sintering of ultra-fine Si3N4 powder. The feedstock＇s flowability, debinding rate, defect evolution, and microstructural evolution during production were explored. The results show that the solid volume loading of less than 50vol% and the surfactant mass fraction of 6wt% result in a perfect flowability of feedstock; this feedstock is suitable for injection molding. When the debinding time is 8 h at 40°C, approximately 50% of the wax can be solvent debinded. Defects detected during the preparation are traced to improper injection parameters, mold design, debinding parameters, residual stress, or inhomogeneous composition distribution in the green body. The bulk density, Vickers hardness, and fracture toughness of the gas-pressure-sintered Si3N4 ceramic reach 3.2 g/cm^3, 16.5 GPa, and 7.2 MPa·m^1/2, respectively.

Effects of Na/K ratio on the phase structure and electrical properties of Na_xK_(1-x)NbO_3 lead-free piezoelectric ceramics

Lead-free piezoelectric NaxK1-xNbO3（x = 0.3-0.8）（NKN） ceramics were fabricated by normal sintering at 1060°C for 2 h.Microstructures and electrical properties of the ceramics were investigated with a special emphasis on the influence of Na content.The grain size of the produced dense ceramic was decreased by increasing Na content.A discontinuous change in the space distance was found at the composition close to Na0.7K0.3NbO3 ceramic, which indicates the presence of a transitional composition between two different orthorhombic phases, which is similar to the behavior of morphotropic phase boundary（MPB） in NaxK1-xNbO3 ceramics.Such MPB-like behavior contributes to the enhanced piezoelectric coefficient d33 of 122 pC/N, planar-mode electromechanical coupling coefficient kP of 28.6%, and dielectric constant εr of 703, respectively for the Na0.7K0.3NbO3 ceramic.Cubic temperature TC and the transitional temperature TO-T from orthorhombic to tetragonal phase are observed at around 420°C and 200°C, respectively.

Fast Welding of Chromium Carbide and Nickel Alloy by Spark Plasma Sintering

Spark plasma sintering（SPS） was used to weld the ceramics,eg,Cr3C2 and metal,Ni in this paper.It is found that the SPS can weld the Cr3C2 and Ni plates at lower temperatures and shorter holding time comparing with that of hot-pressing（HP）.The binding strength was 113 MPa when the temperature was 1000 ℃ by SPS,compared with 10 MPa by HP at the same temperature.SPS remarkably enhances the atom diffusion in welding.Thermodynamics analysis at different welding temperatures and holding times of SPS or HP shows that the local temperature gradient,different from the past effects of by-passing current,is the dominative mechanism of the SPS welding.

Dielectric and Magnetic Properties of (1-x)CaTiO3-xNi0.5Zn0.5Fe2O4 Composite Ceramics

（1-x）CaTiO3-xNi0.5Zn0.5Fe2O4（0 x 1.0） composite ceramics were synthesized by a conventional solid state reaction method.The phase formation,microstructure,and dielectric and magnetic properties were investigated by X-ray diffraction,scanning electron microscopy,precision impedance analysis,and vibrating sample magnetometry,respectively.The results indicate that the composite ceramics are composed of both perovskite phase Ca TiO3 and spinel phase Ni0.5Zn0.5Fe2O4.The maximal relative density for 0.5CaTiO3-xNi0.5Zn0.5Fe2O4 composite ceramics reaches 97.8%,as it has been sintered at the temperature of 1260 ℃ for 3 h.Dielectric constant and loss tangent of（1-x）CaTiO3-xNi0.5Zn0.5Fe2O4 composite ceramics show dispersion in the low frequency range.Their phase transition temperature of the dielectric constant shifts to lower temperatures with the increase of Ni0.5Zn0.5Fe2O4 content.This phenomenon is attributed to that the phase transition temperature of CaTiO3 is higher than that of Ni0.5Zn0.5Fe2O4.The saturation magnetization of （1-x）CaTiO3-xNi0.5Zn0.5Fe2O4 composite ceramics increases with the Ni0.5Zn0.5Fe2O4 ferrite content.

Effect of CeO_2 on low temperature pressureless sintering of porous Si_3N_4 ceramics

Porous silicon nitride（Si_3N_4） ceramics were fabricated by low temperature pressureless sintering, using carbonized rice husk and α-Si_3N_4 powders as raw materials, and MgO-CeO_2 as sintering additives. The effects of CeO_2 concentration and sintering temperature on phase composition, microstructure, porosity and flexural strength of the sintered products were investigated by X-ray diffraction（XRD）, scanning electronic microscopy（SEM）, Archimedes＇ displacement method and three-point bending strength, respectively. The results suggested that MgO-CeO_2 was a much more effective sintering additive for Si_3N_4 than MgO alone. When CeO_2 concentration was 4 wt.%–5 wt.% and sintering temperatures were in a range of 1500 to 1550 oC, the obtained porous silicon nitride ceramics had the porosity of 45.78%–42.81% and flexural strength of 49.44–84.09 MPa. Moreover, when sintering temperature was 1550 oC and CeO_2 concentration was 5%, large elongated β-Si_3N_4 grains were well developed.

Sintering characteristics and microwave dielectric properties of 0.5Ca_(0.6)La_(0.267)TiO_3-0.5Ca(Mg_(1/3)Nb_(2/3))O_3 ceramics prepared by reaction-sintering process

0.5 Ca（0.6La0.267TiO3-0.5 Ca（Mg1/3Nb2/3）O3（5 CLT-5 CMN） ceramics were prepared by a reaction-sintering process and their sintering characteristics, microwave dielectric properties were investigated in detail.Without any calcination stage involved,a mixture of CaCO_3, La_2 O_3, TiO_2, MgO and Nb_2 O_5 was pressed and sintered directly. Pure phase 5 CLT-5 CMN ceramics with high density and dense microstructure can be obtained after sintered at 1400 ℃ for 4 h. Compared with those prepared by the conventional ceramic route, 5 CLT-5 CMN ceramics produced by the reaction-sintering process exhibit slightly higher dielectric constant and Q×f value. Fine microwave dielectric properties of ε_r= 56.4, Q×f= 48,550 GHz and T_f = ＋8.7 ppm/℃ for 5 CLT-5 CMN ceramics sintered at 1400 ℃ for 4 h are obtained, suggesting reactionsintering process is a simple and efficient method to produce pure phase 5 CLT-5 CMN ceramics as a potential candidate for the fabrication of microwave devices.

Reaction Mechanism and Microstructure Evolution of Reaction Sintered h-BN

Hexagonal boron nitride ceramic（h-BN） based on the nitridation of B powders was obtained by reaction sintering method. The effects of sintering temperature on the mechanical properties and microstructure of the resultant products were investigated and the reaction mechanism was discussed. Results showed that the reaction between B and N2 occurred vigorously at temperatures ranging from 1 000 ℃ to 1 300 ℃, which resulted in the generation of t-BN. When the temperature exceeded 1 450 ℃, transformation from t-BN to h-BN began to occur. As the sintering temperature increased, the spherical particles of t-BN gradually transformed into fine sheet particles of h-BN. These particles subsequently displayed a compact arrangement to achieve a more uniform microstructure, thereby increasing the strength.

Effect of Temperature Schedule on the Morphology of SiC Made From Graphite and Silicon Powder

SiC whiskers were synthesized in a coke bed using a two-step heat treatment process.The temperature was held for 3 h at 1 200 ℃ before heating to 1 430 ℃ and holding for 3 h.The morphologies of SiC synthesized by the two-step heat treatment method were much more different from those produced by direct heating to 1 430 ℃.SiC whiskers formed at 1 200 ℃ firstly and grew at1 430 ℃ to obtain a mean diameter about 326 nm,while the SiC grains with a size range from 0.70 to2.30 μm were obtained by direct heating to the target temperature of 1 430 ℃.This was explained by the different formation mechanisms.The result proposes a promising alternative technical process for whisker-reinforced ceramic/refractory composites in-situ during sintering.

Ablation effects and mechanism of sintered silicon carbide ceramics by an ArF excimer laser

The ablation of sintered silicon carbide ceramics by an ArF excimer laser was studied. Three zones are generated： the ablation zone that presented molten morphology and was composed by the Si and C phase; the condensation zone formed by vaporized SiC; and the oxidation zone that showed the characteristics of thermal oxidation. The ablation depth and oxidation range increase linearly with fluence and pulses within 0.5-4 J/cm2, but the normalized ablation efficiency is constant （3.60± 0.60 μm · mm2/J）. The theoretical photochemical ablation depth supplies 25% of the total depth at 1 J/cm2 but decreases to 16% at 4 J/cm2. The ablation is dominated by the photothermal effect and conforms to the thermal evaporation mechanism.

Preliminary Investigation of Preparing High Burn-Up Structure in Nuclear Fuel by Flash Sintering Using CeO_(2) as a Surrogate

The high burn-up structure(HBS)is characterized by the grain size of 100-300 nm and a porosity of up to 20%,which is formed at the rim of the nuclear fuel pellet due to 2-3 times higher local burn-up during the in-pile irradiation.HBS is considered a new potential structure for high-performance fuels.However,it is difficult to prepare HBS by conventional sintering methods.In this study,flash sintering was used to prepare HBS using CeO_(2)as a surrogate for a preliminary investigation.A new experimental configuration for rapid sintering of CeO_(2)pellets was provided,in which the green body can be rapidly preheated and pressure-assisted by the induction heating electrodes.An insulated quartz tube was used as the die for the flash sintered samples,allowing the current to flow through the sample and providing a stable condition for applying an external pressure of approximately 5.3-7.0 MPa during flash sintering process.Using an initial electric field of 141 V cm-1 and holding for 1-7 min at the maximum current density of~98 mA mm^(-2),CeO_(2)ceramics with a grain size of 114-282 nm and a relative density of 75.4-99.7%were prepared.The densification and microstructure evolution behaviors during flash sintering in this new experimental configuration have been discussed in detail.This new experimental configuration may provide a promising approach for preparing UO_(2)ceramics and their HBS.

Electron tomography for sintered ceramic materials by a neural network algebraic reconstruction technique

The missing wedge effect in electron tomography introduces various types of artifacts in the tomograms and lowers the reconstruction resolution and quality.The artifacts produced in tomographic reconstruction of bulk materials can be very severe,particularly for sintered bulk ceramic materials in which there are often nano-pores or pore-like microstructure features.Here,we report a neural network algebraic reconstruction algorithm with no prior knowledge to perform electron tomography for a sintered SiC material with nano carbon zones.The results show that the proposed algorithm has a great suppressive effect on the missing wedge artifacts and a high tolerance for noise.The information in the missing wedge can be partly recovered by this technique.Thus,both the shape of the bulk SiC specimen and its irregular inner pore-like features are correctly retrieved in the obtained 3D image.Our study shows the effectiveness of the neural network algorithm for improving the reconstruction accuracy of electron tomography,in order to reveal sophisticated 3D microstructures generally existing in sintered ceramic materials.

A sol-gel method to synthesize indium tin oxide nanoparticles

Transparent conductive indium tin oxide （ITO） nanoparticles were synthesized by a novel sol-gel method. Granulated indium and tin were dissolved in HNO3 and partially complexed with citric acid. A sol-gel process was induced when tertiary butyl alcohol was added dropwise to the above solution. ITO nanopartides with an average crystallite size of 18.5 nm and surface area of 32.6 m^2 ]g were obtained after the gel was heat-treated at 700 ℃, The ITO nanoparticles showed good sinterability, the starting sintering temperature decreased sharply to 900 ℃, and the 1400 ℃ sintered pellet had a density of 98.1% of theoretical density （TD）.