Enhanced mechanical properties of SiC reticulated porous ceramics via adjustment of residual stress within the strut

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi‐layered struts were fabricated by polymer replica technique with SiC slurry, followed by infiltrating alumina slurries containing andalusite under vacuum condition. The effects of andalusite addition on the microstructure and mechanical properties of SiC RPCs were investigated, also the residual stress within the multi‐layered strut was predicted. Theoretical calculations showed that the residual tensile stress generated in the outer layer of SiC RPCs because of its larger thermal expansion coefficient of infiltration slurry than that of SiC slurry at elevated temperature. Furthermore, the addition of andalusite reduced the thermal expansion coefficient and Young's modulus of infiltration slurries, thereby significantly reducing the residual stress of the outer layer in multi‐layered struts. The reduced residual tensile stress within the outer layer was beneficial to eliminate surface cracks on the struts, thus improving the mechanical properties and thermal shock resistance of SiC RPCs.     

Preparation and Performance Study of Sialon‐Si3N4‐SiC Composite Ceramics for Concentrated Solar Power

For lowering the sintering temperature of silicon carbide ceramics used for solar thermal energy storage technology, O'‐Sialon and silicon nitride were employed as composite phases to construct Sialon‐Si₃N₄‐SiC composite ceramics. The composite ceramics were synthesized using SiC, Si₃N₄, quartz, and different alumina sources as starting materials with noncontact graphite‐buried sintering method. Influences of alumina sources on the physical properties and thermal shock resistance of the composites were studied. The results revealed that the employment of O'‐Sialon and silicon nitride could decrease the sintering temperature greatly to 1540°C. The optimum formula G2 prepared from mullite as alumina source achieved the best performances: 66.7 MPa of bending strength, 10.0 W/(m·K) of thermal conductivity. The composition parameter x = 0.4 of O'‐Sialon decreased to 0.04 after 30 cycles thermal shock, and the bending strength increased with a rate of 11.0% due to the increase of O'‐Sialon grain size, and the optimization of microstructure caused by the transformation of O'‐Sialon grains and densification within the samples. The good thermal shock resistance makes the composites suitable for the use as thermal storage materials of concentrated solar power generation.     

Preparation and thermal stability investigation of Al2O3–mullite–ZrO2–SiC composite ceramics for solar thermal transmission pipelines

To obtain composite ceramics with excellent thermal shock resistance and satisfactory high?temperature service performance for solar thermal transmission pipelines, SiC additive was incorporated into Al₂O₃?mullite?ZrO₂ composite ceramics through a pressureless sintering process. The effect of the SiC additive on thermal shock resistance was studied. Also, the variations in the microstructure and physical properties during thermal cycles at 1300 °C were discussed. The results showed that both thermal shock resistance and thermal cycling performance could be improved by adding 20 wt% SiC. In particular, the sample with 50 wt% Al₂O₃, 35 wt% Coal Series Kaolin (CSK), 15 wt% partially yttria?stabilized zirconia (PSZ), and 20 wt% SiC additional (denoted as sample A2) exhibited the best overall performance after firing at 1600 °C. Furthermore, the bending strength of sample A2 increased to 124.58 MPa, with an increasing rate of 13.63% after 30 thermal shock cycles. The increase in thermal conductivity and the formation of mullite were the factors behind the enhancement of thermal shock resistance. During the thermal cycles, the oxidation of SiC particles was favorable as it increased the microstructure densification and also facilitated the generation of mullite, which endowed the composite ceramics with a self?reinforcing performance.     

A Novel Si3N4–SiC Ceramic Used for Volumetric Receivers

Si₃N₄–SiC composite ceramics used for volumetric receivers were fabricated by pressureless sintering of micrometer SiC, Si₃N₄, andalusite, and other minor additions powders. Mechanical, thermal expansion, thermal conductivity, and thermal shock resistance properties were tested at different sintering temperatures. The best sintering temperature of optimum formula A2 is 1360°C, and the bending strength reaches 79.60 Mpa. And moreover, its thermal expansion coefficient is 6.401 × 10^?6/°C, thermal conductivity is 7.83 W/(m K), and no crack occurs even subjected to 30 cycles thermal shock with a bending strength increase rate of 4.72%. X‐ray diffraction results show that the phase constituents of the sintered products mainly consist of SiC, Si₃N₄, mullite, and quartz. Microstructure that is most appropriate and exhibits maximal thermal shock resistance was detected using SEM. The porosity of Si₃N₄–SiC ceramic foam prepared from formula A2 is 95%, which provides a rapid and steady action for the receiver. The evaluation of the present foam shows that Si₃N₄–SiC ceramic composite is a good candidate for volumetric receivers.     

Thermal‐Shock Fracture and Damage Resistance Improved by Whisker Reinforcement in Alumina Matrix Composite

Fracture resistance of SiC‐whiskers‐reinforced Al₂O₃‐matrix composite under thermal shock was examined. Equibiaxial tensile thermal stress in the composite was significantly reduced before fracture, because the whiskers made percolation paths that increase heat flux and thereby reduced the temperature gradient. The thermal‐shock fracture resistance (R′) of the composite is thus much higher than that of monolithic Al₂O₃. Thermal‐shock damage resistance (R″″) was estimated from the thermal‐shock stress when a surface crack propagates. R″″ of the composite is also much higher than that of monolithic Al₂O₃ owing to an increment of work‐of‐fracture due to crack‐face bridging of the whiskers.     

High‐temperature tensile strength and fracture behavior of ZrB2‐SiC‐graphite composite

The tensile behavior of ZrB₂‐SiC‐graphite composite was investigated from room temperature to 1800°C. Results showed that tensile strength was 134.18 MPa at room temperature, decreasing to 50.34 MPa at 1800°C. A brittle‐ductile transition temperature (1300°C) of ZrB₂‐SiC‐graphite composite was deduced from experimental results. Furthermore, the effect of temperature on the fracture behavior of ZrB₂‐SiC‐graphite composite was further discussed by microstructure observations, which showed that tensile strength was controlled by the relaxation of thermal residual stress below 1300°C, and was affected by the plastic flow during 1300°C and 1400°C. At higher temperature, the tensile strength was dominated by the changes of microstructures.     

Preparation and Performance Study of Mullite/Al2O3 Composite Ceramics for Solar Thermal Transmission Pipeline

For lowering sintering temperature of mullite/Al₂O₃ composite ceramics for solar thermal transmission pipeline, kaolin, potassium feldspar, quartz, and γ‐Al₂O₃ were used as raw materials to in situ synthesize the composite ceramics with pressureless sintering method. Densification, mechanical properties, thermal expansion coefficient, thermal shock resistance, phase composition, and microstructure were investigated. The experiment results demonstrated that the introduction of potassium feldspar and quartz decreased the lowest sintering temperatures greatly to 1300°C. The optimum sample A3 sintered at 1340°C obtained the best performances. The water absorption, apparent porosity, bulk density, bending strength, and thermal expansion coefficient of A3 were 0.04%, 0.12%, 2.71 g/cm³, 94.82 MPa, and 5.83 × 10^?6/°C, respectively. After 30 thermal shock cycles (wind cooling from 1100°C to room temperature), no cracks were observed on the surfaces of the sample, and the bending strength increased by ?7.96%. XRD analysis indicated that the main phases of samples before and after 30 thermal shock cycles were consistently mullite, corundum, and α‐cristobalite, while the content of mullite increased after thermal shock. SEM micrographs illustrated that the mullite grains growth and micro‐cracks appeared after thermal shock endowed the composite ceramics with excellent thermal shock resistance.     

Fabrication of two-layer SiC nanowire cladding tube with high thermal conductivity

Among the various concepts of SiC-based accident-tolerant fuel cladding, duplex SiC cladding, consisting of an inner composite layer and an outer monolithic SiC layer, is considered an optimal design due to its low load failure probability. In this study, SiC nanowires (SiCnw) were introduced on the substrate graphite rod to decrease the diameter of architectural valley-regions of SiC fiber (SiC_f) tubular preform. By avoiding the architectural valley-voids, a dense two-layer SiCnw tube consisting of an inner SiC fiber-reinforced SiC matrix (SiC_f/SiC) composite layer deposited by chemical vapor infiltration with a smooth inner surface was obtained. The microstructure and thermal properties of as-obtained two-layer SiCnw tubes were studied. Results showed that the thermal conductivity of the whole tube was highly sensitive to variations in thermal conductivity of the inner composite layer. By improving the thermal conductivity of the inner composite layer, the two-layer SiCnw tube exhibited a thermal conductivity of 23.8 W m⁻¹ K⁻¹ at room temperature, which had an improvement of 71 % compared to the two-layer SiC tube (13.9 W m⁻¹ K⁻¹). Moreover, the thermal transport properties of the two-layer SiCnw tube were significantly improved by a reduction in roughness of the inner surface.     

A new route to fabricate SiB4 modified C/SiC composites

In order to improve the oxidation and thermal shock resistance of 2D C/SiC composites, dense SiB₄–SiC matrix was in situ formed in 2D C/SiC composites by a joint process of slurry infiltration and liquid silicon infiltration. The synthesis mechanism of SiB₄ was investigated by analyzing the reaction products of B₄C–Si system. Compared with the porous C/SiC composites, the density of C/SiC–SiB₄ composites increased from 1.63 to 2.23 g/cm³ and the flexural strength increased from 135 to 330 MPa. The thermal shock behaviors of C/SiC and C/SiC–SiB₄ composites protected with SiC coating were studied using the method of air quenching. C/SiC–SiB₄ composites displayed good resistance to thermal shock, and retained 95% of the original strength after being quenched in air from 1300 °C to room temperature for 60 cycles, which showed less weight loss than C/SiC composite.     

Investigation of Fireball Temperatures in Confined Thermobaric Explosions

New composite metalized explosives were studied. The explosives consisted of two different types of macroscopic granular multi‐component RDX‐based formulations. In a 0.15 m³ explosion chamber, fireball temperature histories for numerous cylindrical pressed and layered charges made from the composites were determined using optical spectroscopy. For comparison, charges consisting of simple mixtures instead of the composites as well TNT and RDX phlegmatized (RDXph) charges were also studied. The influence of the structure of the macroscopic granular composite, the charge type (pressed charge or layered charge with an RDXph core), oxygen availability (air or argon atmosphere) and aluminum particle size on the fireball temperature and the combustion of the aluminum powder were determined. The measured temperatures were compared with the theoretical ones calculated by assuming different activity of the aluminum fuel.     

High‐temperature mechanical behavior of ZrB2‐based composites with micrometer‐ and nano‐sized SiC particles

Using micrometer‐ and nano‐sized SiC particles as reinforcement phase, two ZrB₂‐SiC composites with high strength up to 1600°C were prepared using high‐energy ball milling, followed by hot pressing. The composite microstructure comprised finer equiaxed ZrB₂ and SiC grains and intergranular amorphous phase. The temperature dependency of flexure strength related to the initial particle size of SiC. In the case of micrometer‐sized SiC, the high‐temperature strength was improved up to 1500°C compared to room‐temperature strength, but the strength degraded at 1600°C, with strength values of 600‐770 MPa. In the case of nano‐sized SiC, the enhanced high‐temperature strength was observed up to 1600°C, with strength values of 680‐840 MPa.     

Influence of ZrO2 Content on the Performances of BN‐ZrO2‐SiC Composites for Application in the Steel Industry

BN‐ZrO₂‐SiC composites were fabricated with different ZrO₂ contents ranging from 0 to 40 vol%. The mechanical properties and corrosion resistance against molten steel increase with increasing ZrO₂ content, while thermal shock resistance decreases gradually. When ZrO₂ content is 40 vol%, the flexural strength is 346 MPa and corrosion depth is 254 μm. The improved corrosion resistance is attributed that the corrosion layer formed by residual ZrO₂ hinders molten steel penetration. Nevertheless, composite with 40 vol% ZrO₂ shows a sharp decline in residual flexural strength when temperature difference is more than 400°C and critical temperature difference is 520°C.     

Thermal Shock Behavior of an Alumina-SiC Whisker Composite

Thermal shock testing of an alumina-20 vol% SiC whisker composite showed no decrease in flexural strength with temperature differences up to 900°C. Alumina, on the other hand, normally shows a significant decrease inflexural strength with a temperature change of >400°C. The improvement in the thermal shock resistance of the composite is believed attributable to the increased fracture toughness of this material.     

Opportunities for Advanced Ceramics and Composites in the Nuclear Sector

Ceramics have played a crucial role in the development of fission based nuclear power, in glass & glass composite high level wasteforms, in composite cements to encapsulate intermediate level wastes (ILW) and also for oxide nuclear fuels based on UO₂ and PuO₂/UO₂ mixed oxides. They are also used as porous filters with the ability to absorb radionuclides (RN) from air and liquids and are playing a key role in the cleanup at Fukushima. Non‐oxides also find current fission applications including in graphite moderators and B₄C control rods. Ceramics will continue to be significant in the near‐term expansion of nuclear power via next‐step developments of fuels with inert matrices or based on thoria and in wasteforms using alternative composite cements or single or multiphase ceramics that can host Pu & other difficult RN. Longer term advances for Generation IV reactors, which will operate at higher temperatures & with higher fuel burn‐up require innovative fuel developments potentially via carbides & nitrides or composite fuel systems. Novel non‐thermal (cement‐like) and thermal techniques are currently being developed to treat some of the difficult legacy wastes. Non‐thermally derived wasteforms developed from geopolymers, composite cements, hydroceramics, and phosphate‐bonded ceramics and thermally derived wasteforms made by Hot Isostatic Pressing and fluidized bed steam reforming (FBSR) as well as vitrification techniques based on cold crucible melting (CCM), Joule‐heater in‐container melting and plasma melting (PM) are described. Future developments in waste treatment will be based on separation technologies for partitioning individual RN along with design & construction of RN‐containing ceramic targets for inducing transmutation reactions. Near demonstration actinide‐hosting ceramic wasteforms including multiphase Synroc systems are described. Opportunities also exist for ceramics in structural applications in Generation IV reactors such as composite SiC/SiC and C/C for fuel cladding and control rods and MAX phases and ultrahigh‐temperature ceramics (UHTCs) may find near core fuel coating and cladding applications. Uses of ceramics in fusion reactor systems will be both functional (ceramic superconductors in magnet systems for plasma control and in Li silicate breeder blankets in tokamaks) and structural including as sapphire diagnostic windows, graphite diverters, and plasma facing C and UHTCs. In all these cases, performance is limited by poorly understood radiation damage and interface controlled processes, which demands a combined modeling/experimental approach.     

Processing and characterization of particles reinforced SiOC composites via pyrolysis of polysiloxane with SiC or/and Al fillers

Polysiloxane loaded with SiC as inert filler, and Al as active filler, was pyrolyzed in nitrogen to fabricate SiOC composites, and the processing and properties of the filled SiOC composites were investigated. Adding SiC fillers could reduce the linear shrinkage of filler-free cured polysiloxane in order to obtain monolithic SiC/SiOC composites. The flexural strength of SiC/SiOC composites reached 201.3 MPa at a SiC filler content of 27.6 vol.%. However, SiC/SiOC composites exhibited poor oxidation resistance, thermal shock resistance and high temperature resistance. Al fillers could react with hydrocarbon generated during polysiloxane pyrolysis at 600 °C and N₂ at 800 °C to form Al₄C₃ and AlN, respectively. The volume expansions resulting from these two reactions were in favor of the reduction in linear shrinkage and the improvement in flexural strength of SiC/SiOC composites. The flexural strength of Al-containing SiC/SiOC composites was 1.36 times that of SiC/SiOC composites without Al at an Al filler content of 20 vol.%. The addition of Al fillers remarkably improved the high temperature resistance and oxidation resistance of SiC/SiOC composites, but not thermal shock resistance.     

Comparative investigation of ultrafast thermal shock of Ti3AlC2 ceramic in water and air

In this work, ultrafast thermal shock of Ti₃AlC₂ ceramic was evaluated in water and air by utilizing the induction heating method. First, the annealed samples were heated to the set temperature in tens of seconds and dropped into the cooling water within 0.1 s which is rather short not to degrade the sample temperature. Compared to the traditional thermal shock method when quenching in water, the abnormal thermal shock phenomenon did not occur, which is owing to that no dense oxide layers were formed on the samples’ surface to act as the thermal barrier. The continuous decrease in residual flexural strength when quenched in water is associated with water infiltration, chemical reaction, and large surface tensile stress. The residual strength has 27.25 MPa upon 1250°C. Second, at the same testing temperature, the residual flexural strength when quenched in air maintains a high value of 388 MPa up to 1400°C. Dense oxide scales existed on the quenched surface of Ti₃AlC₂ samples. The results exhibit that Ti₃AlC₂ ceramic possesses excellent thermal shock resistance in water and air, suitable to be applied in extreme environments.     

Thermal Shock Resistance of Laminated ZrB2–SiC Ceramic Evaluated by Indentation Technique

In this work, the thermal shock behavior of laminated ZrB₂–SiC ceramic has been evaluated using indentation‐quench method based on propagation of Vickers cracks and compared with the monolithic ZrB₂–SiC ceramic. The results showed that the laminated ZrB₂–SiC ceramic exhibited better resistance to crack propagation and thermal shock under water quenching condition, and the critical temperature difference (ΔT_c) of laminated ZrB₂–SiC ceramic (ΔT_c ≈ 590°C) was much higher than that of monolithic ceramic (ΔT_c ≈ 290°C). The significant improvement in thermal shock resistance was attributed to residual stresses enhancing the resistance to crack growth during thermal shock loading.     

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.     

Ti_3SiC_2可加工材料的研究现状

Ti3SiC2陶瓷具有很好的高温强度、热稳定性和耐腐蚀性能,同时它还具有很好的导电、导热能力,优良的可加工性,又具备金属良好的高的抗氧化性、抗热震性和高温塑性、良好的自润滑性。本文对其结构、性能以及制备方法和应用前景进行了综合评述。     

High temperature thermal physical performance of SiC/UO2 composites up to 1600 °C

SiC was introduced to UO₂ matrix by spark plasma sintering (SPS) to improve the thermal conductivity. The microstructure evolution and thermal physical properties up to 1600?°C were firstly reported. The grain growth and the formation of equiaxed grain structure were inhibited by the addition of SiC. The critical SPS sintering temperature, above which SiC was positive on improving thermal conductivity, was discovered to be 1300?°C. Two equations were proposed to calculate the thermal diffusivity and thermal conductivity of SiC/UO₂ sintering at 1500?°C. Each percent of SiC fraction brought about 3% increment in thermal conductivity. The coefficient of thermal expansion (CTE) was decreased after SiC addition. Such improvement in thermal conductivity and decrease in CTE were beneficial to the fuel safety in accident condition.