Microstress in the matrix of a melt‐infiltrated SiC/SiC ceramic matrix composite

Microstress in the SiC: Si matrix of a ceramic matrix composite (CMC) has been characterized, using Raman spectroscopy. The matrix of the composite was manufactured using liquid melt infiltration, and has about 20% unreacted free silicon. During the processing of the composite, the unreacted free silicon expands 11 vol% when transforming from liquid to solid. This crystallization expansion creates compressive microstress in the silicon phase of the matrix, which ranges from 2.4 to 3.1 GPa, and tensile microstress in the SiC of the matrix which ranges from 0.24 to 0.75 GPa. The microstress varies significantly with position, following a normal distribution.     

反应烧结碳化硅的显微组织及其导电性的研究

研究了液态硅参与下的反应烧结碳化硅的工艺参数、显微组织对其电阻率的影响.随着烧结气氛压力和成型压力增加,反应烧结碳化硅中游离硅量减少,电阻率增加.其烧结机理以碳的溶解及碳化硅的淀析过程为主.     

Residual stress measurements in melt infiltrated SiC/SiC ceramic matrix composites using Raman spectroscopy

Raman spectroscopy was utilized to characterize the chemical composition and residual stresses formed in melt infiltrated SiC/SiC CMCs during processing. Stresses in SiC fibers, in SiC chemical vapor (CVI) infiltrated matrix, in SiC melt infiltrated matrix, and in free silicon were measured for two different plates of CMCs. Stresses in the free silicon averaged around 2?GPa in compression, while stresses in the matrix SiC were 1.45?GPa in tension. The SiC CVI phase had stresses ranging between 0.9?GPa and 1.2?GPa in tension and the SiC fibers experienced stresses of .05–0.7?GPa in tension. These results were validated with the proposed model of the system. While the mismatch in the coefficients of thermal expansion between the constituents contributes to the overall residual stress state, the silicon expansion upon solidification was found to be the major contributor to residual stresses within the composite.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

燃烧合成制备Si3N4-TiN-SiC陶瓷的显微结构

以TiSi2为反应原料,SiC作稀释剂,燃烧合成制备Si3N4-TiN-SiC陶瓷.利用燃烧波"淬熄"法使反应各个阶段的物相得以保留,通过X射线衍射及扫描电镜分析TiSi2在燃烧合成中的反应过程及显微组织转化.结果表明:完全反应后产物的主相为Si3N4,其余为TiN和SiC.在燃烧过程中,TiSi2首先受热熔化,包覆于SiC颗粒表面,随后与N2反应生成TiN和Si.Si在高热作用下发生熔化、汽化,液态Si与未反应的TiSi2互溶.生成的Si与氮气发生反应,形成Si3N4晶核,并不断长大.燃烧合成反应过程中,Si3N4晶须的生长十分复杂,由气-液-固机制、气-固机制及蒸发凝聚的气相生长机制共同作用.     

An Original Way to Investigate Silver Migration Through Silicon Carbide Coating in TRISO Particles

Extensive release of the metastable silver nuclide Ag^110m from fully intact tristructural‐isotropic (TRISO) particles raises concerns over safety of advanced nuclear reactors. In this study, we propose a new model to interpret the silver migration mechanism in SiC based on experimental observations from both Ag/SiC composite pellets and TRISO particles with an entrapped silver layer. For the Ag/SiC composite pellets heat treated at 1450°C, silicon was detected in the silver phase, amorphous carbon was found, and new β‐SiC had formed at the Ag/SiC interface. The results indicate that Ag in its liquid state reacts with SiC by forming a Ag–Si alloy. Carbon precipitates as a second phase or reacts with the Ag–Si alloy to form new SiC. Results from the heat‐treated TRISO particles trapping Ag show that Ag penetrates through the SiC layer and is present in either “finger‐shape” or “wedge shape” at the SiC grain boundaries. Ag was also found inside abnormally large SiC grains at the trailing edge between Ag and SiC, indicating the recrystallization of SiC. A dissolution‐reaction model was proposed to explain Ag migration through SiC, and this model is supported by thermodynamic calculations.     

Low‐Density Open Cellular Silicon Carbide Foams from Sucrose and Silicon Powder

Open cellular SiC foams with low densities were prepared by thermo‐foaming and setting (130°C–150°C) of silicon powder dispersions in molten sucrose followed by pyrolysis and reaction sintering at 1500°C. The bubbles generated in the dispersion by water vapor produced by the –OH condensation was stabilized by the adsorption of silicon particles on the air‐molten sucrose interface. The composition of a sucrose‐silicon powder mixture for producing SiC foam without considerable unreacted carbon was optimized. The sucrose in the thermo‐foamed silicon powder dispersion leaves 24 wt% carbon during the pyrolysis. The sintering additives such as alumina and yttria promoted the silicon‐carbon reaction. SiC nanowires with diameters in the range of 35–55 nm and length >10 μm observed on the cell walls as well as in the fractured strut region were grown by both vapor–liquid–solid and vapor–solid mechanisms. Large SiC foam bodies without crack could be prepared as the total shrinkage during pyrolysis and reaction sintering was only ~30 vol%. The relatively low compressive strength (0.06–0.41 MPa) and Young's modulus (14.9–24.2 MPa) observed was due to the large cell size (1.1–1.6 mm) and high porosity (93%–96%).     

Porous eco-ceramics of low thermal conductivity and high EMI shielding effectiveness from sawdust and sucrose by paste molding

A facile method for making porous SiC ceramics from sawdust avoiding high-temperature liquid or vapor phase silicon infiltration has been outlined. Cylindrical bodies prepared by hand pressing of aqueous pastes produced from sawdust, silicon powder, sucrose, and sintering additives are subjected to freeze-drying followed by carbonization and ceramization heat-treatments to produce porous SiC. The carbon-silicon reaction is completed at a minimum temperature of 1550 °C as verified from TGA and XRD analysis. The SiC produced from sawdust particles is firmly held together by the sucrose-derived SiC. The pore channels inherited in the sawdust particle remain in the SiC ceramics. A large population of nanowires is observed throughout the sucrose-derived SiC as well as on the pore channel surfaces of SiC formed from sawdust particles. The porosity decreases from ~88 to 75 vol%, the compressive strength rises from 0.5 to 12.5 MPa, and thermal conductivity rises from 0.23 to 0.98 W/mK when the sucrose to sawdust weight ratio rises from 0.5 to 2.5. The reflection contribution (1.9–3 dB) to the total EMI shielding effectiveness (16 To 31 dB) of the porous SiC ceramics is very low compared to the absorption contribution (15–28 dB). The carbonized bodies are amenable to machinating enabling the production of near-net-shape before ceramization. The presence of large excess of silicon observed in biomorphic SiC made by molten liquid infiltration is avoided in this method.     

A study of the residual microstress at the matrix interface in filled epoxy resins

Using the technique of photoelasticity, we have studied in detail the magnitude of the residual microstress at the matrix interface between epoxy resin and rubber particles or glass beads. A spheric stress-optic equation applicable to these composites was derived. The photoelastic figures induced by the interfacial residual microstresses and the factors affecting the intensity of light are discussed. The experimental results show that the residual microstresses at the matrix interface are independent of the particle size of rubbers or glass beads, but depend on the nature of fillers, which have different thermal expansion coefficients and mechanical properties. Thermal history and interfacial chemical bonding of filled epoxy resins have distinct effects on the residual interfacial microstresses and the matrix internal stresses.     

Relaxation of residual microstress in reaction bonded silicon carbide

High temperature annealing reduces the residual microstress in the silicon phase and silicon carbide phase in monolithic reaction bonded silicon carbide and in the matrix of melt-infitrated composites of silicon carbide reinforced with silicon carbide fibers. Stress relaxation is related to creep of the silicon carbide with power-law creep exponents similar to tensile creep in reaction bonded silicon carbide.     

Interfacial and thermomechanical characterization of reaction formed joints in silicon carbide-based materials

The microstructure and mechanical properties of reaction formed joints of Refel^TM reaction bonded SiC (RB-SiC) and Hexoloy^TM sintered SiC were studied in order to achieve a better understanding of the influence of base materials and joining process parameters on the high temperature strength of reaction formed joints. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical microscopy were used to characterize the joints prior to mechanical tests. The microstructural analysis indicated that the joints consist of silicon carbide (SiC) grains (with grain sizes ranging from 0.1 to 2 μm) and crystalline silicon as an intergranular phase. Most of the silicon carbide grains in the joint have hexagonal crystal structure with certain preferential orientations related to the silicon matrix. The high temperature strength of joints was measured by constant strain rate experiments in compression where joints were forming 45° with the compression axe. The strength of the joined Refel RB-SiC has been found to be at least equal to that of the bulk materials (550 MPa at 1235°C and 400 MPa at 1385°C). The joined Hexoloy specimens had strengths (1.4 GPa at 1290°C and 750 MPa at 1420°C) lower than the bulk material but higher than the joints of RB-SiC.     

New Approach for Macro Porous RB‐SiC Derived from SiC/Novolac‐type Phenolic Composite

A novel fabrication route to make macroporous silicon carbide (SiC) has been proposed in this study. The route is composed of the following two steps: the fabrication of porous α‐SiC/novolac‐type phenolic composite using hexamethylenetetramine (HMT) as a curing/blowing agent for the novolac monomer and a conventional reaction‐bonded (RB) sintering of the composite. The α‐SiC/novolac‐type phenolic composite was carbonized at 800°C for 2 h in N₂ gas and then reacted with the molten silicon at 1450°C for 30 min under vacuum, resulting in the macroporous RB‐SiC with an open porosity of 48% and relatively large pore size of ~110 μm. The compressive strength of the macroporous RB‐SiC was 113 MPa, which is relatively high compared to those reported for macroporous SiC of equivalent porosities and pore sizes.     

Determination of Density Variation and Microstructure in Reaction‐sintered SiC Ceramics Using Ultrasonic Time‐of‐Flight

Ultrasonic time‐of‐flight (TOF) is investigated as a predictor of density variation across reaction‐sintered silicon carbide (SiC) ceramics. The importance of this research is heightened by the fact that the reaction‐sintered SiC ceramic tiles being investigated are manufactured using the reaction sintering process that involves the infusion of liquid silicon into a porous ceramic preform. This can potentially lead to the formation of islands of free silicon, small closed areas of un‐sintered silicon material as well as conventional porosity. All these defects can result in local variations of density which cannot be detected by conventional bulk density measurement techniques. To study the microstructural differences, the porosity dependence of ultrasonic TOF of the reflected signals was investigated to establish a correlation between the velocity and density across the ceramic tile that aids in characterizing the material. The data suggest that for these ceramic tiles, TOF C‐scan mapping surfs as a much better indicator of sample homogeneity than the amplitude of the back wall echo. At the current time, ceramic tiles are inspected offline and this takes time and effort and is very expensive. In particular, this study contains the procedure followed to establish a quantitative method to quantify density variation across selected ceramic tile.     

Biomorphic silicon/silicon carbide ceramics from birch powder

A novel process has been developed for the fabrication of biomorphic silicon/silicon carbide (Si/SiC) ceramics from birch powder. Fine birch powder was hot-pressed to obtain pre-templates, which were subsequently carbonized to acquire carbon templates, and these were then converted into biomorphic Si/SiC ceramics by liquid silicon infiltration at 1550 °C. The prepared ceramics are characterized by homogeneous microstructure, high density, and superior mechanical properties compared to biomorphic Si/SiC ceramics from birch blocks. Their maximum density has been measured as 3.01 g/cm³. The microstructure is similar to that of conventional reaction-bonded silicon carbide. The Vicker's hardness, flexural strength, elastic modulus, and fracture toughness of the biomorphic Si/SiC were 19.6 ± 2.2 GPa, 388 ± 36 MPa, 364 ± 22 GPa, and 3.5 ± 0.3 MPa m^1/2, respectively. The outstanding mechanical properties of the biomorphic Si/SiC ceramics are assessed to derive from the improved uniform microstructure of the pre-templates made from birch powder.     

Thermal microstress measurements in Al2O3/SiC nanocomposites by Cr3+ fluorescence microscopy

Alumina/SiC ‘nanocomposites’ show significant property improvements compared with pure alumina. The improvements are thought to stem at least in part from the microstresses caused by the thermal expansion mismatch between alumina and SiC. These microstresses have been measured previously by neutron and X-Ray diffraction. This paper reports stress measurements using Cr³⁺ fluorescence microscopy of the alumina matrix. The results show that although fluorescence microscopy is less powerful than the diffraction techniques in terms of the range of information provided, it does provide an alternative method of measuring subsurface microstresses in these materials which is quicker, cheaper and higher in spatial resolution.     

Polymer‐derived ceramic aerogels as sorbent materials for the removal of organic dyes from aqueous solutions

Polymer‐derived SiC and SiOC aerogels have been synthesized and characterized both from the microstructural point of view and as sorbent materials for removing organic dyes (Methylene Blue, MB, and Rhodamine B, RB) from water solutions. Their adsorbent behavior has been compared with a polymer‐derived SiC foam and a commercial mesoporous silica. The aerogels can efficiently remove MB and RB from water solution and their capacity is higher compared to the SiC foams due to the higher surface area. The SiOC aerogel remains monolithic after the water treatment (allowing for an easy removal without the need of a filtration step) and its maximum capacity for removing MB is 42.2 mg/g, which is higher compared to the studied mesoporous silica and many C‐based porous adsorbents reported in the literature. The reason for this high adsorption capacity has been related to the unique structure of the polymer‐derived SiOC, which consists of an amorphous silicon oxycarbide network and a free carbon phase.     

黄杨木制备SiC木质陶瓷的性能与表征

SiC木质陶瓷是近年来应用前景广阔的新型陶瓷材料,以绿色可再生的木材为原材料,通过反应烧结工艺制备出的陶瓷具有优良的高温力学性能。为探究影响生物质陶瓷性能的因素,将黄杨木在800 ℃氮气保护下热解形成生物质炭坯,然后在1 650 ℃和1 900 ℃两种高温下进行熔融渗硅制备SiC木质陶瓷。利用X射线衍射仪(XRD)和扫描电子显微镜(SEM)研究SiC木质陶瓷的物相组成和微观结构,采用阿基米德排水法和三点弯曲法测定陶瓷的开孔率、密度和弯曲强度,再使用维氏硬度计测定SiC木质陶瓷的显微硬度。研究结果表明:在1 650 ℃下通过熔融渗硅可以得到微观结构均匀的SiC木质陶瓷,在1 650 ℃比在1 900 ℃下熔融渗硅制备陶瓷的力学性能更优异,陶瓷的密度更大,为2.27 g/cm³,此时弯曲强度为192.45 MPa;游离Si会提高SiC木质陶瓷的密度,增强陶瓷的弯曲强度。     

空间望远镜用C/SiC镜面研制综述

综述了空间望远镜的主镜用高强度、高表面精度、低热膨胀系数的低温（约4K）用镜面的制备和检测过程.日本将Φ710mm的高强度反应烧结SiC材料已用于红外望远镜镜面.在短切炭纤维增强C/C复合材料毛坯的基础上进行液相硅渗透（LSI）而制备的C/SiC复合材料在光学镜面方面具有更大的优势.通过提高C/C复合材料毛坯中沥青基炭纤维体积分数及控制硅化速度,可有效地提高LSI-C/SiC复合材料的机械性能和表面光学精度;通过不同规格的炭纤维的混杂化,可使C/SiC复合材料热膨胀系数的各向异性降低至小于4％的差异.SiC、Si-SiC浆料涂层处理可有效地提高表面精度至2 nm rms的极高要求.     

Reactive Wetting in the Liquid-Silicon/Solid-Carbon System

The wettability of glassy carbon by liquid silicon has been investigated at 1430°C in argon by using techniques of both in situ formation and capillary formation of sessile drops. Analyses of the results showed that there are three distinct contributions of reaction to wetting: (a) dissolution of solid substrate carbon in liquid silicon; (b) formation of a continuous SiC layer at the solid side of the interface, and (c) a contribution of the free energy released by the reaction localized at the interface between liquid silicon and solid carbon.     

Micromechanics of ITZ–Aggregate Interaction in Concrete Part I: Stress Concentration

At the macroscopic scale, concrete appears as a composite made of a cement paste matrix with embedded aggregates. The latter are covered by interfacial transition zones (ITZs) of reduced stiffness and strength. Cracking in the ITZs is probably the key to the nonlinear stress–strain behavior in the prepeak regime. For a deeper understanding of this effect triggered by tensile microstress peaks, we here employ and extend the framework of continuum micromechanics, as to develop analytical solutions relating the macroscopic stresses acting on a piece of concrete, to microtractions at the aggregates' surfaces and to three‐dimensional stress states within the ITZs. In the latter context, a new aggregate‐to‐ITZ stress concentration tensor is derived based on the separation‐of‐scale principle, which implies that ITZs may be modeled as two‐dimensional interfaces at the concrete scale, but as three‐dimensional bulk phases at the scale of a few micrometers. Microtensile peaks occur both under uniaxial macroscopic tension and compression. To describe the respective microtraction and microstress fields, it is suitable to define aggregate's “poles” and “equator” by an “axis” through the aggregate center, directed in the uniaxial macroscopic loading direction. Accordingly, tensile microtraction peaks, induced by macro‐tension and macro‐compression, respectively, occur at the “poles” and at the “equator”, respectively. The largest tensile ITZ‐microstresses occur at an offset of about π/8 from the “poles” and the “equator”, respectively. These fields of microtractions and ITZ microstresses are prerequisites for upscaling ITZ‐related strength to the macroscopic concrete level, as presented in the companion paper (Part II).