Oxidation behavior of Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2)C-M_(x)C(M=Ti,Zr,Hf,Nb,Ta) composite ceramic at high temperature

Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2)C-M_(t)C composite ceramic was prepared by hot press sintering,with the Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2)C high-entropy carbide as the main phase.Secondary phase M_(x)C(M=Ti,Zr,Hf,Nb,Ta) was found to be distributed relatively uniform in the composite ceramic.The oxidation behavior of the ceramic was examined after exposure to 923 K and 1173 K.Morphology of the surface and cross sections of all oxidation samples were observed.The characteristics of the oxidation behavior of the high-entropy carbide and the secondary phase M_(x)C were compared and analyzed.The secondary phases(such as Ti-rich carbide or Hf-rich carbide) in the material were seriously oxidized at 923 K and 1173 K,which reflects the superior oxidation performance of the high-entropy carbide.The nano high-entropy oxides with Ti,Zr,Hf,Nb,Ta,and O elements were discovered by oxidation of the composite ceramic.This research will help deepen the understanding of the oxidation mechanism of high-entropy carbide and composite ceramic.

Effects of Co_(2)O_(3)Addition on Microstructure and Properties of SiC Composite Ceramics for Solar Absorber and Storage

SiC composite ceramics for solar absorber and storage integration are new concentrating solar power materials.SiC composite ceramics for solar absorber and storage integration were fabricated using SiC,black corundum and kaolin as the raw materials,Co_(2)O_(3)as the additive via pressureless graphite-buried sintering method in this study.Influences of Co_(2)O_(3)on the microstructure and properties of SiC composite ceramics for solar absorber and storage integration were studied.The results indicate that sample D2(5wt%Co_(2)O_(3))sintered at 1480℃exhibits optimal performances for 119.91 MPa bending strength,93%solar absorption,981.5 kJ/kg(25-800℃)thermal storage density.The weight gain ratio is 12.58 mg/cm2after 100 h oxidation at 1000℃.The Co_(2)O_(3)can decrease the liquid phase formation temperature and reduce the viscosity of liquid phase during sintering.The liquid with low viscosity not only promotes the elimination of pores to achieve densification,but also increases bending strength,solar absorption,thermal storage density and oxidation resistance.A dense SiO_(2) layer was formed on the surface of SiC after 100 h oxidation at 1000℃,which protects the sample from further oxidation.However,excessive Co_(2)O_(3)will make the microstructure loose,which is disadvantageous to the performances of samples.

Evaluation of Dielectric Properties of CCTO-BT/Epoxy Composites for Electronic Applications

In the current study,the calcium copper titanate(CCTO)/epoxy,barium titanate(BT)/epoxy and CCTO-BT/epoxy composite samples with variable volume fractions of CCTO and BT are fabricated using hand lay-up and compression moulding process. The composite samples are characterized for the frequency dependence on dielectric properties,conductivity,impedance spectroscopy and electrical modulus.X-ray diffraction(XRD)representation of CCTO-BT/epoxy composite samples confirmed the presence of both CCTO and BT ceramic samples separately. The dielectric characteristics of hybrid CCTO-BT/epoxy composite samples with CCTO∶BT ratio of 40∶60, 60∶40,and 50∶50 was found relatively better than those of single ceramic filler reinforced epoxy composites. AC conductivity analysis shows improvement in the results of hybrid filler-filled CCTO-BT/epoxy composites in comparison with single filler-filled epoxy composite.50∶50 CCTO-BT/epoxy composite shows the best AC conductivity value of~ 2.2 ×10^(-5) ohm^(-1)·m^(-1) at a higher frequency of 1MHz. The impedance analysis confirms the higher insulating properties for hybrid 40∶60 and 60∶40 CCTO-BT/epoxy composites with respect to the single and other hybrid ceramic epoxy composites. The analysis suggests the hybrid CCTO-BT/epoxy composites to be adopted as a potential dielectric material for energy storage devices and other electronic applications.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF SiC_W/BAS (BaAl_2Si _2O_8) GLASS-CERAMIC COMPOSITE

BAS (BaAl 2Si 2O 8) glass ceramic was prepared by a sol gel process and the SiC W/BAS composites were fabricated by hot pressing. The transformation from hexacelsian to celsian, the microstructure and mechanical properties of the composites was investigated. The results show that the transformation promoted by adding celsian seeds is retarded in the composite by the presence of SiC whisker. SiC whisker has a good effect of improving the mechanical properties of BAS glass ceramic matrix. The toughening mechanisms are crack deflection and whisker fracture. The strengthening mechanism is loading transition. The amorphous phase at SiC W/BAS matrix interface damages the fracture toughness and high temperature strength of the composites.

Influence of Composite Phosphate Inorganic Antibacterial Materials Containing Rare Earth on Activated Water Property of Ceramics

Antibacterial ceramic was prepared by doping enamel slurry with composite phosphate inorganic antibacterial materials containing rare earth (inorganic antibacterial additives), and then the mechanisms for activating water and improving seed germinative property were tested by nuclear magnetic resonance (NMR) and the method of testing oxygen dissolved in activated water. Results show that the half peak width of (()^(17)O-NMR) for tap water activated by the antibacterial ceramic drops from 115.36 to 99.15 Hz, and oxygen concentrations of activated water increase by 20%, germinate rate of horsebean and earthnut seeds increases by 12.5% and 7.5%, respectively. Therefore antibacterial ceramic doped enamel slurry with inorganic antibacterial additives containing rare earth can reduce the volume of clusters of water molecules, improve activation of tap water, and promote plant seeds germinate.

ELASTIC BEHAVIOR ANALYSIS OF 3D ANGLE-INTERLOCK WOVEN CERAMIC COMPOSITES

A micromechanical model for elastic behavior analysis of angle-interlock woven ceramic composites is proposed in this paper. This model takes into account the actual fabric structure by considering the fiber undulation and continuity in space, the cavities between adjacent yarns and the actual cross-section geometry of the yarn. Based on the laminate theory, the elastic properties of 3D angle-interlock woven ceramic composites are predicted. Different numbers of interlaced wefts have almost the same elastic moduli. The thickness of ceramic matrix has little effect on elastic moduli. When the undulation ratio increases longitudinal modulus decreases and the other Young＇s moduli increase. Good agreement between theoretical predictions and experimental results demonstrates the feasibility of the proposed model in analyzing the elastic properties of 3D angle-interlock woven ceramic composites. The results of this paper verify the fact that the method of analyzing polyester matrix composites is suitable for woven ceramic composites.

Dielectric and piezoelectric properties of(Li,Ce) modified NaBi_5Ti_5O_(18) composite ceramics

Nominal （Li0.5Ce0.5）x（Na0.5Bi0.5）（1-x）Na0.5Bi4.5Ti5O18 composite ceramics were fabricated using conventional solid-state reaction method. The coexistence of bismuth layer-structured phase and perovskite phase was determined in these ceramics using XRD technique. At room temperature, the x=0.11 sample showed the largest piezoelectric constant, d33, of about 26.5 pC/N and the largest electromechanical coupling factor, kt, of about 30%. Even after annealing at 500 ℃, the value of d33 was still about 19 pC/N, in x=0.08-0.11 samples. Moreover these composite ceramics showed low temperature coefficients of dielectric constant and high electrical resistivity in the temperature region of 450-550 ℃. These results indicated that （Li, Ce） modified NaBi5Ti5O18 composite ceramics were promising piezoelectric materials for high-temperature applications.

Study on Microstructure of Alumina Based Rare Earth Ceramic Composite

Analysis techniques such as SEM, TEM and EDAX were used to investigate the microstructure of rare earth reinforced Al2O3/(W, Ti)C ceramic composite. Chemical and physical compatibility of the composite was analyzed and in-terfacial microstructure was studied in detail. It is found that both Al2O3 and (W, Ti)C phases are interlaced with each other to form the skeleton structure in the composite. A small amount of pores and glass phases are observed inside the material which will inevitably influence the physical and mechanical property of the composite. Thermal residual stresses resulted from thermal expansion mismatch can then lead to the emergence of dislocations and microcracks. Interfaces and boundaries of different types are found to exist inside the Al2O3/(W, Ti)C rare earth ceramic composite, which is concerned with the addition of rare earth element and the extent of solid solution of ceramic phases.

Thermal shock fatigue behavior of TiC/Al_2O_3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference （ΔT） and number of thermal cycles （Ⅳ） on fatigue crack growth （Δa）. The mechanical properties and thermal fatigue resistance of TiC/Al203 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a ＂true＂ cycling effect （thermal fatigue）. Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.

A high-power high-stability Q-switched green laser with intracavity frequency doubling using a diode-pumped composite ceramic Nd:YAG laser

We successfully obtain a high-average-power high-stability Q-switched green laser based on diode-side-pumped composite ceramic Nd：YAG in a straight piano-concave cavity. The temperature distribution in composite ceramic Nd：YAG crystal is numerically analyzed and compared with that of conventional Nd：YAG crystal. By using a composite ceramic Nd：YAG rod and a type-II high gray track resistance KTP （HGTR-KTP） crystal, a green laser with an average output power of 165 W is obtained at a repetition rate of 25 kHz, with a diode-to-green optical conversion of 14.68%, and a pulse width of 162 ns. To the best of our knowledge, both the output power and optical-to-optical efficiency are the highest values for green laser systems with intracavity frequency doubling of this novel composite ceramic Nd：YAG laser to date. The power fluctuation at around 160 W is lower than 0.3% in 2.5 hours.

Dielectric properties of spark plasma sintered AlN/SiC composite ceramics

In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a second conductive phase. All ceramic samples were pre-pared by spark plasma sintering （SPS） under a pressure of 30 MPa. AlN composite ceramics sintered with 30wt%-40wt%SiC at 1600℃ for 5 min exhibited the best dielectric loss tangent, which is greater than 0.3. In addition to AlN and β-SiC, the samples also contained 2H-SiC and Fe5Si3, as detected by X-ray difraction （XRD）. The relative densities of the sintered ceramics were higher than 93%. Experimental results indicate that nano-SiC has a strong capability of absorbing electromagnetic waves. The dielectric constant and dielectric loss of AlN-SiC ce-ramics with the same content of SiC decreased as the frequency of electromagnetic waves increased from 1 kHz to 1 MHz.

Microstructure and oxidation resistance of reactive plasma clad Cr_7C_3/γ-Fe ceramic composite coating

A new type oxidation resistance in situ Cr7 C3/γ-Fe ceramic composite coating was fabricated on hardened and tempered grade C steel by reactive plasma clad with Fe-Cr-C alloy powders. The oxidation resistance of the ceramic composite coating was investigated under the test condition of 900 ℃ and 50 hours. The results indicate that the coating has a rapidly solidified microstructure consisting of blocky primary Cr7 C3 and the inter-blocky Cr7 C3/γ-Fe eutectics and is metallurgically bonded to the hardened and tempered grade C steel substrate. The high temperature oxidation resistance of the coating is up to 1.9 times higher than that of grade C steel. The oxidation kinetics curve of the coating is conforming to the parabolic-rate law equation. The excellent oxidation resistance of the coating is mainly attributed to the continuous oxide films which consist of Cr203 and Fe203. The continuous oxide films can prevent the inner part of the coating from being further oxidized.

Direct deposition of graphene nanowalls on ceramic powders for the fabrication of a ceramic matrix composite

Uniform mixing of ceramic powder and graphene is of great importance for producing ceramic matrix composite. In this study, graphene nanowalls(GNWs) are directly deposited on the surface of Al2 O3 and Si3 N4 powders using chemical vapor deposition system to realize the uniform mixing. The morphology and the initial stage of the growth process are investigated. It is found that the graphitic base layer is initially formed parallel to the powder surface and is followed by the growth of graphene nanowalls perpendicular to the surface. Moreover, the lateral length of the graphene sheet could be well controlled by tuning the growth temperature. GNWs/Al2 O3 powder is consolidated by using sparking plasma sintering method and several physical properties are measured. Owing to the addition of GNWs, the electrical conductivity of the bulk alumina is significantly increased.

FRACTURAL PROCESS AND TOUGHENING MECHANISM OF LAMINATED CERAMIC COMPOSITES

Based on the model of multi-layer beam and the assumption of micro-inhomogeneity of material, the 3D fractural characteristics of laminated ceramic composites have been studied with numerical simulation. Under three-point bending load, crack initiation, coalescence, propagation, tuning off in the weak interface and final rupture have been simulated. The spatial distribution and evolution process of acoustic emission are also presented in the paper. The simulation verifies the primary mechanism of the weak interface inducing the crack to expand along there and absorbing the fractural energy. The disciplinary significance of the effect of strength and properties of material on the toughness and strength of laminated ceramic composites is, therefore, discussed in this paper.

Mechanical Behaviors of ZrO_2-Al_2O_3 Ceramic Composites with Y_2O_3 as Stabilizer

The ZrO2-Al2O3 ceramic composites were prepared by appropriate techniques with commercial ZrO2 and Al2O3 powders as raw materials and Y2O3 as stabilizer. The results indicate that with the introduction of Al2O3 into the ZrO2 matrix where the quantity of additive Y2O3 is 3.5% (mole fraction), the growth of ZrO2 grains is efficiently inhibited, which helps the ZrO2 grains exist in a metastable tetragonal manner; thus higher strength and toughness are acquired. When the content of alumina is 20% (mass fraction), the bending strength and fracture toughness of the composites are 676.7 MPa and 10 MPa·m1/2 respectively, the mechanical behaviors are close to those prepared with ZrO2 and Al2O3 powders synthesized through wet chemical approach. The mechanical behaviors of the composites are well improved owing to the dispersion toughening of alumina grains and phase transformation toughening of zirconia grains.

Effect of MnO2 on Properties of SiC-mullite Composite Ceramics for Solar Sensible Thermal Storage

For improving the properties of SiC-mullite composite ceramics used for solar sensible thermal storage, MnO2 was introduced as sintering additive when preparing. The composite ceramics were synthesized by using SiC, andalusite, a-Al2O3 as the starting materials with non-contact graphite-buried sintering method. Phase composition and microstructure of the composites were investigated by XRD and SEM, and the effect of MnOz on the properties of SiC composites was studied. Results indicated that samples SM1 with 0.2 wt% MnO2 addition achieved the optimum properties： bending strength of 70.96 MPa, heat capacity of 1.02 J.（g.K）-1, thermal conductivity of 9.05 W-（m.K）-1. Proper addition of MnO2 was found to weaken the volume effect of the composites and improve the thermal shock resistance with an increased rate of 27.84% for bending strength after 30 cycles of thermal shock （air cooling from 1 100 ℃ to RT）. Key words： SiC-mullite composite ceramics; MnO2; solar sensible thermal storage; non-contact graphite-buried sintering; thermal shock resistance

Ceramic-Metal Composite Coating by Laser Cladding

Four kinds of ceramics (silicon carbide, boron carbide, aluminum oxide and tungsten carbide) were added into Ni base self-fluxing alloy as reinforcing materials in order to obtain metalceramic composite coating by means of laser cladding. A lot of experiments have been carried out to test the processability of the four kinds of ceramics with different sizes and contents. The microstructures of sintered tungsten carbide (S-WC) and cast tungsten carbide (C-WC) reinforced Ni base alloy coatings were analysed, the distortion regulation of laser clad specimens was revealed. The wear resistance of the composite coating has been tested.

Preparation and Mechanical Properties of Al_2O_3/Al Laminated Ceramic Matrix Composites

Three series of Al2O3/Al laminated ceramic matrix composites,named SPA,SPV and HP,were fabricated by different methods.SPA and SPV were prepared using Al2O3 slices and Al slurry via screen printing and subsequent heat treatment in air or vacuum.HP samples were made by hot pressing the layered stack of Al foils and Al2O3 slices.SEM and XRD were applied to analyze the microstructure and the interlayer crystal phase.The bending strength,fracture toughness and fracture work of the samples made by the three methods were measured and compared.The results show that the composites have much better toughness and higher fracture work than the Al2O3 slice.Among the samples made by the three methods,the samples made by hot pressing have the optimum mechanical performance.The displacement-load curves and fracture mechanism were analyzed.

Effect of Nano-ZrO_2 on Microstructure and Thermal Shock Behaviour of Al_2O_3/SiC Composite Ceramics Used in Solar Thermal Power

The Al2O3-ZrO2（3Y）-SiC composite ceramics used in solar thermal power were prepared by micrometric Al2O3,nano-ZrO2 and SiC powders under the condition of pressureless sintering.The bulk density and bending strength of samples with 10vol% nano-ZrO2 sintered at 1480℃ were 3.222 g/cm3 and 160.4MPa,respectively.The bending strength of samples after 7 times thermal shock tests （quenching from 1000℃ to 25℃ in air medium） is 132.0MPa,loss rate of bending strength is only 17%.The effect of nano-ZrO2 content on the microstructure and performance of Al2O3-ZrO2（3Y）-SiC composite ceramic was investigated.The experimental results show that the bending strength of samples with above 10vol% nano-ZrO2 content has decreased,because the volume expansion resulting from t-ZrO2 to m-ZrO2 phase transformation is excessive;Adding proper nano-ZrO2 would be contributed to improve the thermal shock resistance of the composite ceramics.The Al2O3-ZrO2（3Y）-SiC composite ceramic has promising potential application in solar thermal power.

EVOLUTION OF THE MICROCRACKS IN WHISKER TOUGHENING CERAMIC MATRIX COMPOSITES

The modified equivalent inclusion theory by the authors and the internal variable theory are employed to investigate the evolution of the microcracks in whisker toughening ceramics and the influence of the microcracks on the mechanical properties of the material. The effect of residual thermostrain, whisker content and aspect ratio is considered. The modulus, initial nonlinear load, strength and nonlinear constitutive relation are calculated and some important conclusions are given.