Microstructure and Oxidation Behavior of ZrB_(2)-SiC Ceramics Fabricated by Tape Casting and Reactive Melt Infiltration

ZrB_(2)-based ceramics typically necessitate high temperature and pressure for sintering,whereas ZrB_(2)-SiC ceramics can be fabricated at 1500℃using the process of reactive melt infiltration with Si.In comparison to the conventional preparation method,reactive synthesis allows for the more facile production of ultra-high temperature ceramics with fine particle size and homogeneous composition.In this work,ZrSi_(2),B4C,and C were used as raw materials to prepare ZrB_(2)-SiC via combination of tape casting and reactive melt infiltration herein referred to as ZBC ceramics.Control sample of ZrB_(2)-SiC was also prepared using ZrB_(2) and SiC as raw materials through an identical process designated as ZS ceramics.Microscopic analysis of both ceramic groups revealed smaller and more uniformly distributed particles of the ZrB_(2) phase in ZBC ceramics compared to the larger particles in ZS ceramics.Both sets of ceramics underwent cyclic oxidation testing in the air at 1600℃for a cumulative duration of 5 cycles,each cycle lasting 2 h.Analysis of the oxidation behavior showed that both ZBC ceramics and ZS ceramics developed a glassy SiO_(2)-ZrO_(2) oxide layer on their surfaces during the oxidation.This layer severed as a barrier against oxygen.In ZBC ceramics,ZrO_(2) is finely distributed in SiO_(2),whereas in ZS ceramics,larger ZrO_(2) particles coexist with glassy SiO_(2).The surface oxide layer of ZBC ceramics maintains a dense structure because the well-dispersed ZrO_(2) increases the viscosity of glassy SiO_(2),preventing its crystallization during the cooling.Conversely,some SiO_(2) in the oxide layer of ZS ceramics may crystallize and form a eutectic with ZrO_(2),leading to the formation of ZrSiO_(4).This leads to cracking of the oxide layer due to differences in thermal expansion coefficients,weakening its barrier effect.An analysis of the oxidation resistance shows that ZBC ceramics exhibit less increase in oxide layer thickness and mass compared to ZS ceramics,suggesting superior oxidation resistance of ZBC ceramics.

Design and preparation of an ultra-high temperature ceramic by in-situ introduction of Zr_(2)[Al(Si)]_(4)C_(5) into ZrB_(2)-SiC:Investigation on the mechanical properties and oxidation behavior

Novel ZrB_(2)-matrix composites were designed and prepared by in-situ introducing SiC and Zr_(2)[Al(Si)]_(4)C_(5) simultaneously for the first time.The obtained composites were dense and showed good mechanical properties,especially the strength and toughness,706 MPa and 7.33 MPa·m^(1/2),respectively,coupled with high hardness of 21.3 GPa,and stiffness of 452 GPa.SiC and Zr_(2)[Al(Si)]_(4)C_(5) constituted a reinforcing system with synergistic effects including grain refinement,grain pull-out as well as crack branching,bridging,and deflection.Besides,the oxidation results of the composites showed that the oxidation kinetics followed the parabolic law at 1600℃,and the oxidation rate constants increased with the increase of Zr_(2)[Al(Si)]_(4)C_(5) content.The formation and evolution model of the oxidation structure was also investigated,and the oxide scale of the composite exhibited a three-layer structure.

Thermal shock behavior of ZrB2-SiC ceramics with different quenching media

The thermal shock behavior of ZrB2-SiC ceramics was studied with water, air and methyl silicone oil as quenching media, respectively. The temperature of all coolants was room temperature （25℃） and the residual strength of the ceramics after quenching was tested. The strength of the ceramics after water quenching had an obvious drop when the temperature difference, AT, was about 275℃, while the residual strength of the specimens quenched by air and silicone oil only varied a little and even increased slightly when the temperature difference was higher than 800℃. The different thermal conductive coefficient of the coolants and surface heat transfer coefficient resulted in the differences in the thermal shock behavior. The formation of oxidation layer was beneficial for improving the residual strength of the ceramics after quenching.

Ce and W co-doped CaBi_(2)Nb_(2)O_(9) with enhanced piezoelectric constant and electrical resistivity at high temperature

Ce and W co-doped CaBi_(2)Nb_(2)O_(9) ceramics with chemical formula Ca_(0.96)Ce_(0.04)Bi_(2)Nb_(2-x)W_(x)O_(9)(CCBNW100x,x=0-0.07)are fabricated via conventional solid state sintering method,to investigate the effect of W addition on the structure,electrical resistivity,dielectric and piezoelectric properties.A piezoelectric constant d33 of 13.4 pC/N is obtained in CCBN-W2 ceramics,>100% higher than that of pure CaBi_(2)Nb_(2)O_(9)(d_(33)=5.8e6.4 pC/N).Of particular significance is that the electrical resistivity of CCBN-W2 ceramics(r=3.7×109 U cm at 500℃)is three orders of magnitude higher than pure CaBi_(2)Nb_(2)O_(9)(r=2.9×10^(6) U cm at same temperature).All these properties,together with its low dielectric loss(tandδ0.13%)and excellent d33 thermal stability up to 800℃,merit the CCBN-W2 ceramics for high temperature piezoelectric sensing applications.