Microstructure and properties tailoring of liquid-phase sintered SiC

Silicon carbide (SiC) ceramics of the composition SiC-10 vol% (AlN–Y₂O₃) were liquid-phase sintered without application of external pressure in a graphite resistance furnace in Ar atmosphere at 1950°C to high density (up to 98.8%). Contents of -SiC and β-SiC, as well as the granulometry of the -SiC used, were varying parameters of the initial compositions, the influence of which on densification, microstructure and phase formation during sintering and post-sintering heat treatments was studied. Evolution of microstructure, in particular of the grain morphology occurring due to transformation-controlled grain growth was followed by SEM. The degree of β-SiC to -SiC phase transformation was measured by means of quantitative XRD using internal standard technique. Fracture toughness of sintered and annealed materials has been determined by the Vickers indentation method and varied in the range 3.6–5.9 MPa m^1/2. Mechanisms of material toughening are discussed in terms of known toughening mechanisms with consideration of residual porosity variation.     

Strengthening of transparent spinel/Si3N4 nanocomposites

In response to a need for improving the mechanical properties of optically transparent ceramics, the nanocomposites approach is used to strengthen transparent magnesium-aluminate spinel with Si₃N₄ nanodispersoids. The as-processed nanocomposites are found to be >70% transparent in the critical infrared wavelength range of 3-4.5 μm. Mie scattering combined with absorption by the Si₃N₄ nanodispersoids explains quantitatively the IR transmission behavior of these nanocomposites. The nanocomposites are also found to be transparent in the visible region. Upon heat treatment (1000 °C for 4 h in air), the optical properties of the nanocomposites remain unchanged. However, the heat treatment results in a 29% increase in the average strength, accompanied by almost doubling of the Weibull modulus, and an 85% increase in the indentation toughness. The improvements in mechanical properties after the heat treatment in these nanocomposites are explained qualitatively, based on generally accepted arguments involving surface-oxidation-induced surface compression and flaw-healing. While further work is needed to fully understand and exploit these effects, this first report on transparent nanocomposites could have broad implications for the creation of mechanically robust, transparent ceramics of the future.     

Study of (1 − x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xCa0.61Nd0.26TiO3 microwave dielectrics

The microwave dielectric properties and the microstructures of the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.61Nd_0.26TiO₃ has a large positive temperature coefficient of resonant frequency. (Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃ possesses a negative temperature coefficient of resonant frequency. By appropriately adjusting the x value in the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system, a near-zero τ_f value can be obtained. A new microwave dielectric material of 0.8(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–0.2Ca_0.61Nd_0.26TiO₃ possesses the excellent dielectric properties of a dielectric constant of 28.6, a Q × f value of 80,600 GHz and a temperature coefficient of resonant frequency of 4.1 ppm/°C and has a lower sintering temperature of 1250 °C.     

Microstructure and thermoelectric properties of β-FeSi2 ceramics fabricated by hot-pressing and spark plasma sintering

The microstructure and thermoelectric properties of β-FeSi₂ ceramics by hot pressing (HP) and spark plasma sintering (SPS) are investigated. With increasing hot-pressing temperature, the density, electronic conductivity and thermal conductivity of the samples increase significantly, the thermoelectric figure of merit is improved slightly. The microstructure study indicates that the sizes of the β-FeSi₂ and ?-FeSi phases in the sample sintered by the SPS process are smaller than that by the HP process. The SPS sample shows excellent thermoelectric performance due to the low thermal conductivity.     

Sintering-modified mixed Ni-Co-Cu oxymanganospinels for NTC electroceramics

Mixed Ni-Co-Cu oxymanganospinels of Cu_0.1Ni_0.8Co_0.2Mn_1.9O₄ composition with improved functional reliability are first developed for possible application as high-precise NTC thermistors. It is established the amount of additional rock-salt NiO phase in these ceramics, which was not externally introduced at the initial stages of ceramics processing, but extracted during sintering route occurs a decisive role to inhibit the parasitic degradation caused by thermal storage at the elevated temperatures. This effect is well revealed only in ceramics having a character fine-grain microstructure obtained due to injection of small amount of thermally transferred energy, while structural monolithization caused by great value of thermally transferred energy into ceramics bulk reveals an opposite influence. The fact, the ceramics with fine-grain microstructure and large content of rock-salt NiO extractions demonstrate the best suitability for stretched-exponential relaxation kinetics (the most appropriate one for describing degradation kinetics in structurally dispersive solids like ceramics) serves as additional confirmation to the above conclusion.     

Sintering effect on electrical properties of ZnO-V2O5-MnO2-Nb2O5 ceramics

The microstructure and electrical properties of quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics were investigated at different sintering temperature (875-950 °C). The average grain size increased from 4.4 μm to 9.6 μm with increasing sintering temperature. The breakdown field decreased from 6991 V/cm to 943 V/cm with increasing sintering temperature. Proper sintering for quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics led to surprisingly high nonlinear coefficient (50). The donor concentration increased from 3.33 × 10¹⁷ cm⁻³ to 7.64 × 10¹⁷ cm⁻³ with increasing sintering temperature and the barrier height exhibited the maximum value (1.07 eV) at 900 °C.     

Production and characterization of a three-dimensional cellular metal-filled ceramic composite

Silicon carbide in the form of a foam network was vacuum infiltrated with aluminum alloy A356 to produce a new Interpenetrating Composite material. The foam, once infiltrated with a second phase transforms into a composite where two distinct, continuous, three-dimensional network structures are formed. The advantage of this metal matrix composite is its high strength-to-weight ratio for use in lightweight applications such as electronic packaging materials. The electroless nickel coating and vacuum infiltration procedures are developed. Materials characterization of the composite is evaluated by microstructural and compositional analysis, and density, porosity, and nano-indentation measurements. Selected experimental mechanical and thermal property measurements are performed to understand its properties and compare against theoretical models. Results show the final composite to have lower density than conventional electronic base plate packaging materials with low porosity. The composite has an increased Young's modulus and flexural strength to that of the unreinforced alloy and comparable impact toughness to composites with 50–70 vol% SiC particles but with only 12 vol% SiC. The fracture surface of the matrix illustrates conventional fibrous fracture and brittle cleavage whilst the reinforcement struts show signs of layer de-bonding from their SiC layered structure.     

Effect of sintering temperature on the structure and properties of cerium-doped 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 piezoelectric ceramics

Phase structure, microstructure, dielectric and piezoelectric properties of 0.4 wt% CeO₂ doped 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (Ce-BNT6BT) ceramics sintered in the temperature range from 1120 to 1200 °C have been investigated as a candidate for lead-free piezoelectric ceramics. Tetragonal phase played an important role in improvement of electrical properties and the density of the ceramics. Dielectric constant decreased slightly with the increase of sintering temperature in ferroelectric region but a reverse phenomenon occurred in antiferroelectric and paraelectric regions, suggesting that interfacial polarizations were improved with the increase of sintering temperature and domain walls of ferroelectricity became active after depolarization. At room temperature, Ce-BNT6BT ceramics sintered at 1180 °C showed good performances: dielectric constant was 914 at 1 kHz, thick coupling factor k_t was 0.52, and the ratio of k_t/k_p was 2.3. The ceramics were suitable for narrowband filters and ultrasonic transducers in commercial applications.     

Low-temperature sintering and dielectric properties of high-permittivity microwave (Ca, Nd)TiO3 ceramics

The effect of H₃BO₃-CuO-Li₂CO₃ combined additives on the sintering temperature, microstructure and microwave dielectric properties of (Ca_0.61Nd_0.26) (Ti_0.98Sn_0.02)O₃ (CNTS) ceramics was investigated. The H₃BO₃-CuO-Li₂CO₃ combined additives lowered the sintering temperature of CNTS ceramics effectively from 1300 to 950 °C. This may be due to the interim liquid-phase of Li₂O-CuO-B₂O₃, which were formed in the sintering process. (Li_0.5Nd_0.5)TiO₃ (LNT) demonstrated an effective compensation in τ_f value of the low-fired CNTS ceramics. The 0.4CNTS-0.6LNT ceramics with 5 wt% (H₃BO₃-CuO)-0.5 wt% Li₂CO₃ sintered at 900 °C for 2 h shows excellent dielectric properties: ?_r = 90.6, Q × f = 3400 GHz, and τ_f = 9 ppm/°C. Also, the LTCC material is compatible with Ag electrode.     

On the development of ice-templated silicon carbide scaffolds for nature-inspired structural materials

The processing of ceramic scaffolds using the ice-templating, or freeze casting, technique provides a relatively simple means to mimic the hierarchical design of natural materials such as nacre. In the present study, we investigated the architecture of silicon carbide (SiC) scaffolds produced by this technique over a range of cooling rates and suspension characteristics to demonstrate its versatility and effectiveness for fabricating unidirectional porous bodies with controlled lamella thickness, porosity fraction and morphology. An array of microstructures was generated specifically to examine the role of the suspension solid load and cooling rate on the pore morphology and final ceramic fraction. With respect to the morphology of the pores, a transition from lamellar to dendritic structure was found to be triggered by an increase in cooling rate or in suspension concentration. Similarly, the freezing condition and suspension characteristics were seen to influence the transition between particle rejection and entrapment by the ice. Based on this study, the specific processing parameters that result in distinct scaffold morphologies, namely lamellar, dendritic or isotropic morphology (the latter corresponding to particle entrapment), are identified and presented in the form of a “morphology map” to establish the regions of the different architectures of freeze-cast SiC scaffolds.     

Laser cladding of SiC/Si composite coating on Si-SiC ceramic substrates

Pieces of silicon infiltrated silicon carbide (Si-SiC) were coated with a SiC particles reinforced Si matrix composite (SiC/Si) obtained from mixtures of SiC + SiO₂ and SiC + Si by laser cladding. A Nd:YAG pulsed laser delivering an average power of 920 W was used to apply such coatings using the powder blowing technique. The results demonstrate that the use of the SiC + SiO₂ powder mixture produces a severe damage on the base material, whereas the use of the SiC + Si mixture leads to the formation of sound coatings without substrate damage. XRD and nanoindentation measurements corroborate the production of silicon carbides surrounded by a metallic silicon matrix. This method could be used for repairing surface defects of silicon infiltrated silicon carbide ceramics (Si-SiC).     

SiC颗粒增强铝基复合材料制造工艺及性能

将２０－４０μｍ的ＳｉＣ颗粒（ＳｉＣｐ）经预处理后在真空中与铝共熔，然后冷至铝合金固液两相区搅拌，可明显改善ＳｉＣｐ／基体间的润湿性和增强相分布的均匀性；加镁也可改善其润湿性，该材料的强度是铝合金的二倍，弹性模量也提高两倍以上。     

Molten salt synthesis of tungsten carbide powder using a mechanically activated powder

A novel method for synthesizing ceramic powders in molten salt was introduced in this paper. Unlike traditional mechanochemical process, the novel method is based on mechanical alloying followed by chemical reaction in liquid phase (molten salt). The reaction temperature to synthesize tungsten carbide in molten salt was 1000 °C. Results in this paper showed that W₂C and WC powder could be successfully synthesized by the method. Under raised reaction conditions, there was a transformation from W₂C to WC. The mechanism of the reaction in molten salt was discussed finally.     

Synthesis and optical properties of Yb0.6Y1.4O3 transparent ceramics

Transparent yttrium oxide ceramics doped with 30 at.% ytterbium were successfully produced by a combination of pre-sintering and hot isostatic pressing. The influence on the final transparency of different densification states and porosity configurations obtained by varying pre-sintering conditions were investigated by optical and electron microscopy. Our results show that densification prior to the final stage of sintering is necessary to limit final porosity. Samples showing open porosity were found to lead to pore entrapment because of the diffusion of argon inside the glass capsule during hot isostatic pressing. Transmittance measurements showed that the valence charge of the ytterbium ions was 3+ at the end of the process, indicating no effect of reduction when pre-sintering in vacuum was employed.     

Effects of alumina additions on sintering behavior of Ce0.8Sm0.2O1.9 ceramics synthesized by Pechini method

The effects of alumina additions on the densification behavior and grain growth of Ce_0.8Sm_0.2O_1.9 ceramics were investigated by the use of powders synthesized by Pechini method. Both the sintered density and grain size were found to increase rapidly up to 1 mol% Al₂O₃, and then to decrease with further additions. A pinning effect on grain growth was found at a higher Al₂O₃ content above 2 mol%. In the dopant content range of 0–5 mol%, 1 mol% Al₂O₃ was the optimum doping level in promoting densification and grain growth of Ce_0.8Sm_0.2O_1.9 ceramics.     

Low temperature sintering of ZrC-SiC composite

High-energy ball milling and spark plasma sintering were adopted to prepare ZrC-SiC composite. Zirconium carbide, silicon, and graphite powders were used as raw materials. ZrC-30 vol.%SiC was sintered to a relative density of >96.1% at 1800 °C. The composite showed a fine microstructure. The fracture strength reached up to 523.4 MPa, Vickers’ hardness 18.8 GPa, fracture toughness 4.0 MPa m^1/2, and elastic modulus 390.5 GPa.     

The research on the binder phase for the (W0.5Al0.5)C0.5 system

Cemented carbide hard alloys (W_0.5Al_0.5)C_0.5-M (Fe, Co, and Ni) were successfully produced by mechanical alloying and hot-pressing. The density, microhardness and bending strength of the samples were also tested. The relative density of the bulk samples can reach over 98% under the hot-pressing sintering. Comparison of sintering behavior, microstructure and mechanical property of the hard alloys (W_0.5Al_0.5)C_0.5 with different binders (Fe, Co, and Ni) has been made. It found that no η-type phases formed in all the three kinds of hard alloys (W_0.5Al_0.5)C_0.5-Co, (W_0.5Al_0.5)C_0.5-Ni and (W_0.5Al_0.5)C_0.5-Fe during the sintering process. The results also showed that, in (W_0.5Al_0.5)C_0.5-M (Fe, Co, and Ni) hard alloys with constant grain size and binder phase content, the hardness of (W_0.5Al_0.5)C_0.5-Fe is similar to (W_0.5Al_0.5)C_0.5-Co hard alloys and the bending strength of (W_0.5Al_0.5)C_0.5-Ni is a little lower than (W_0.5Al_0.5)C_0.5-Co hard alloys.