Effects of water vapor on the oxidation and the fracture strength of SiC layer in TRISO fuel particles

Steam oxidation of silicon carbide (SiC) layer in nuclear fuel particles were performed in flowing argon‐water vapor mixture with a total pressure of 1 bar at 1173‐1673 K. Both the phase composition and the microstructure of the oxide scale on the SiC layer varied with the oxidation temperature. Reaction rates of water vapor with the SiC layer were determined by measuring the oxide scale thickness. It was found that the oxidation of SiC layer follows the parabolic law. The activation energy was calculated to be 103±11 kJ/mol. It is proposed that the rate determine step of the oxidation is the diffusion of water vapor molecules in the oxide scale. The fracture strength of SiC shell after steam oxidation was evaluated using a crush test. The fracture strength decreased with the increase in the oxidation temperature due to the thinning of the SiC layer.     

Comparison of hydrothermal corrosion behavior of SiC with Al2O3 and Al2O3 + Y2O3 sintering additives

Fully dense SiC bulks with Al₂O₃ and Al₂O₃ + Y₂O₃ sintering additives were prepared by spark plasma sintering and the effect of sintering additives on the hydrothermal corrosion behavior of SiC bulks was investigated in the static autoclave at 400°C/10.3 MPa. The SiC specimen with Al₂O₃ sintering additive exhibited a higher weight loss and followed a linear law. However, the SiC specimen with Al₂O₃ + Y₂O₃ additive exhibited a lower weight loss and followed a parabolic law, indicating that the corrosion kinetic and mechanism were different for these two SiC bulks. Further examination revealed that, a deposited layer was formed on the surface of SiC specimen with Al₂O₃ + Y₂O₃ sintering additive after corrosion, which can effectively protect the SiC specimen from further corrosion, and thereby improved the corrosion resistance of the SiC specimen with Al₂O₃ + Y₂O₃ sintering additive.     

Na2SO4 deposit-induced hot corrosion of SiC fibers relevant for SiC CMCs

Hi Nicalon, Hi Nicalon S, Sylramic, and Sylramic iBN SiC fibers were exposed to ~60 μg/cm² of Na₂SO₄ in a 0.1% SO₂/O₂ gaseous environment for times between 0.75 and 24 h at 1000°C. After exposure, the corrosion products were characterized using SEM, EDS, ICP-OES, TEM, and EFTEM to determine their high-temperature resistance to Na₂SO₄ and key reaction mechanisms. All SiC fiber types tested in this work exhibited little resistance to Na₂SO₄ deposit-induced attack relative to their behavior in dry O₂ environments. It was found that Hi-Nicalon displayed the least resistance to Na₂SO₄ deposit-induced attack due to excess carbon content resulting in the formation of a highly porous crystalline oxide and promotion of basic corrosion conditions. All fiber types formed a crystalline SiO₂ reaction product, either cristobalite or tridymite. Sylramic and Sylramic iBN formed a crystalline SiO₂ reaction layer containing TiO₂ needles due oxidation of TiB₂ particles. Additionally, Na₂SO₄ deposits resulted in pitting of all fiber surfaces.     

SiC fiber strength after low pO2 oxidation

Hi‐Nicalon?‐S SiC fiber was heat treated for 1 hour at 1300°C, 1400°C, and 1500°C in argon with pO₂ of 3.7, 10, 20, 50, 100, and 200 ppm. Fiber strengths were measured by 30 single‐filament tensile tests. Fiber microstructure and surface morphology were characterized by TEM. Active oxidation occurred in all cases except at 1500°C with 200 ppm pO₂, 1400°C with 100 ppm pO₂ or higher, and 1300°C with 50 ppm pO₂ or higher. When active oxidation did not occur, a glass SiO₂ scale formed at 1300°C and 1400°C, and a cristobalite scale formed at 1500°C. The thickness of these scales was much larger than that predicted by linear dependence of oxidation rate on pO₂. Fiber strengths were lowest after heat treatment at 1300°C and a pO₂ of 3.7 ppm, 1400°C and a pO₂ of 20 ppm, and 1500°C and a pO₂ of 200 ppm. Active oxidation caused fiber surface roughening, but no obvious changes to the internal fiber microstructure. Decreased fiber strength correlated with increased fiber surface roughness, but roughness magnitudes were not large enough to explain the amount by which strength was degraded. Fiber strengths, surface roughness, scale thicknesses, and the passive‐active oxidation transition for SiC are compared with previous observations. Possible strength degradation mechanisms are discussed.     

硅液相浸渍反应制备TiC/SiC和TiN/SiC复相陶瓷

以滤纸、酚醛树脂和氧化钛为原料,经过模压成型、固化、碳化及不同条件下渗硅制备了TiC/SiC和TiN/SiC复相陶瓷。通过X射线衍射和扫描电子显微镜研究了TiC/SiC和TiN/SiC复相陶瓷的微观结构和物相组成,测量了复相陶瓷的弯曲强度和断裂韧性。结果表明:真空条件下液态渗硅获得的TiC/SiC复相陶瓷具有多孔的微观结构,其弯曲强度和断裂韧性较小。氮气气氛下液态渗硅制备的TiN/SiC复相陶瓷结构致密,有较高的弯曲强度和断裂韧性。不同反应生成的TiC,TiN陶瓷颗粒对液态硅的润湿性不同,使得生成的复相陶瓷具有不同的微观结构。TiN/SiC复相陶瓷中TiN颗粒的引入,在基体与第二相颗粒间的界面上产生拉应力和压应力,使达到这一区域的裂纹偏转,从而获得增韧效果。     

Model for SiC fiber strength after oxidation in dry and wet air

The strengths of oxidized SiC fibers were modeled from the effects of SiO₂ scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO₂-SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal-Grove oxidation kinetics for glass and crystalline scales. Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600° and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed.     

Stress rupture of SiC/SiC composite tubes under high-temperature steam

SiC-fiber–reinforced SiC matrix composite cladding for light water reactor fuel elements must withstand high-temperature steam oxidation in a loss-of-coolant accident scenario (LOCA). Current composite designs include an outer monolithic SiC layer, in part, to increase steam oxidation resistance. However, it is not clear how such a structure would behave under high-temperature steam in the case when the monolithic layer cracks and carbon interphases and SiC fibers are exposed to the environment. To fill this knowledge gap, stress-rupture tests of prototypic SiC composite cladding at 1000°C under steam and inert environments were conducted. The applied stress was ∼120 MPa, which was beyond the initial cracking stress. The failure lifetime under steam was 400–1300 s, while 75% of the composite specimens did not fail after 3 h of total exposure under inert gases. Microstructural observations suggest that steam oxidation activated slow crack growth in the fibers, which led to failure of the composite. The results from this study suggest that stress rupture in steam environments could be a limiting factor of the cladding under reactor LOCA conditions.     

High-temperature flexural strength of SiC ceramics prepared by additive manufacturing

Silicon carbide (SiC) ceramics, as a kind of candidate material for aero-engine, its high-temperature performance is a critical factor to determine its applicability. This investigation focuses on studying the high-temperature properties of SiC ceramics fabricated by using additive manufacturing technology. In this paper, SiC ceramics were prepared by combining selective laser sintering (SLS) with precursor infiltration and pyrolysis (PIP) technique. The microstructure, phase evolution, and failure mechanism after high-temperature tests were explored. SiC ceramic samples tested at room temperature (RT), 800°C, 1200°C, 1400°C, and 1600°C demonstrated bending strengths of 220.0, 226.1, 234.9, 215.5, and 203.7 MPa, respectively. The RT strength of this material can be maintained at 1400°C, but it decreased at 1600°C. The strength retention at 1400°C and 1600°C were 98% and 92%, respectively. The results indicate that the mechanical properties of SiC ceramics prepared using this method have excellent stability. As the temperature increases, the bending strength of the specimens increased slightly and reached the peak value at 1200°C, and dropped to 203.7 MPa at 1600°C. Such an evolution could be mainly due to the crack healing, and the softening of the glassy phase.     

Electrochemical investigation of the codeposition of SiC and SiO2 particles with nickel

The use of electrochemical impedance spectroscopy (EIS) for the in situ control of the electrolytic codeposition of Ni/SiO₂ and Ni/SiC was investigated. An attempt was made to clarify why silica particles hardly codeposit in comparison to silicon carbide particles. It was found that the presence of SiO₂ and SiC particles influences the metal deposition process in different ways. SiC particles that are being embedded in the growing metal layer cause an apparent decrease in the electrode surface area, probably due to blocking off a part of the surface by partly engulfed particles. In the case of SiO₂ particles, which embed in the metal matrix to a very limited extent, no blocking was observed. It was found that the presence of particles in the solution causes an apparent increase in the electrode surface area, probably due to increased surface roughness.     

车用甲醇燃料金属腐蚀抑制剂的研究

通过分析甲醇燃料对金属部件的腐蚀原理,提出了抑制腐蚀的措施,以及通过气相色谱-质谱联用仪器、电化学工作站和金属腐蚀仪三种实验方法,分析了几种典型物质的缓释效果,从而得出甲醇燃料腐蚀抑制剂的主要组分。     

The effect of the third invariant on the strength and failure of boron carbide and silicon carbide

This article presents new test data to assess the effect the third invariant has on the strength and failure of two ceramic materials: boron carbide and silicon carbide. Two experimental techniques are used: the Brazilian test that produces a biaxial state of stress, and a new technique that uses a high-pressure confinement vessel to load a specially designed dumbbell specimen in triaxial extension. The dumbbell geometry provides two important advantages over the typically used cylindrical specimen: no adhesive is required to bond the specimen to the load cell because the dumbbell geometry naturally takes the specimen into tension, and any loading asymmetries are essentially eliminated due to the axisymmetric geometry. The results show that when the stress state is on the tensile meridian the equivalent stress at failure is constant, independent of the hydrostatic pressure. The average equivalent stress at failure is for boron carbide and for silicon carbide. The Brazilian test was only performed on boron carbide and failed at , much higher than when on the tensile meridian () indicating that the effect of the third invariant is significant (because of the difference in the failure strength) and must be accounted for to accurately predict when failure will occur.     

Effect of Carbon Containing Materials on Pure Carbon Reaction-bonded SiC

Petroleum coke, graphite, gas carbon and lower sulfur carbon black were used to prepare reaction-bonded silicon carbide. The influences of different carbon containing materials on properties of carbonaceous precursors, sintering process, and microstructure of the prepared SiC were researched. The results show that ： （ 1 ） With the density of carbon containing materials increasing, the porosity of carbonaceous precursors decreases and the infiltrating process of liquid silicon is more difficult. （2） The reaction between carbon containing materials and liquid silicon, the volume effect is more obvious with the density of carbon containing materials increasing. （3） As the carbon containing materials density decreasing, residual carbon in reaction bonded SiC also decreases.     

Effect of Interfacial Reaction on the Thermal Conductivity of Al–SiC Composites with SiC Dispersions

The effect of interfacial reactions between Al and SiC on the thermal conductivity of SiC-particle-dispersed Al-matrix composites was investigated by X-ray diffraction and transmission electron microscopy (TEM), and the thermal barrier conductance ( h _c) of the interface in the Al–SiC composites was quantified using a rule of mixture regarding thermal conductivity. Al–SiC composites with a composition of Al (pure Al or Al–11 vol% Si alloy)–66.3 vol% SiC and a variety of SiC particle sizes were used as specimens. The addition of Si to an Al matrix increased the thermal barrier conductance although it decreased overall thermal conductivity. X-ray diffraction showed the formation of Al₄C₃ and Si as byproducts in addition to Al and SiC in some specimens. TEM observation indicated that whiskerlike products, possibly Al₄C₃, were formed at the interface between the SiC particles and the Al matrix. The thermal barrier conductance and the thermal conductivity of the Al–SiC composites decreased with increasing Al₄C₃ content. The role of Si addition to an Al matrix was concluded to be restraining an excessive progress of the interfacial reaction between Al and SiC.     

不同粒度碳化硅对莫来石基浇注料性能的影响

以莫来石、铝矾土为主要原料,铝酸钙水泥为结合系统,分别研究了不同粒度的碳化硅经过不同热处理温度后对莫来石基浇注料性能的影响。试样自然干燥24h脱模后,再经110℃烘干24h,分别于1000℃,1300℃和1500℃热处理3h。检测各温度热处理后试样的线变化率、体积密度、抗折强度、耐压强度以及试样的线胀系数。结果表明,不同粒度的碳化硅对调整浇注料中温的线变化率无明显作用。本实验中,含有粒度75μm SiC的浇注料的体积密度最大,并且含有该粒度SiC的浇注料的力学性能最好。     

Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

不同铝源对SiC/堇青石复相多孔陶瓷的制备及性能影响

以碳化硅、氮化铝、层析氧化铝、氢氧化铝、氟化铝、滑石为主要原料,石墨为造孔剂通过原位反应烧结技术制备碳化硅/堇青石复相多孔陶瓷.研究了含铝化合物种类、烧结温度、石墨含量对SiC/堇青石复相多孔陶瓷相组成、微观结构、气孔率和抗折强度的影响,同时对S0组在1200℃烧结温度下制得的SiC/堇青石复合多孔陶瓷的孔径分布进行了测试分析.结果表明:以AlN为铝源在1200℃下烧结,石墨含量在15％时,堇青石结合SiC多孔陶瓷的抗弯强度和气孔率两项综合性能达到最优,气孔率为31.99％,相应的弯曲强度86.20 MPa.S0组的平均孔径大小在3.0191 μm.     

Small scale fracture and strength of high-entropy carbide grains during microcantilever bending experiments

The fracture behaviour of (Hf-Ta-Zr-Nb)C high-entropy carbide grains was investigated by microcantilever bending experiments, and fracture related properties (e.g. strength, toughness) were determined using linear beam theory. Microcantilevers were FIB-milled from large grains with {001} and {101} orientations and were subjected to micro-bending experiments. SEM based fractographic analysis of the broken cantilevers revealed that approximately half of them fractured at the fixing position at FIB-induced surface cracks, while the rest of the beams failed at small cracks located at submicron size pores or inclusions. In all cases, fracture occurred on the {001} cleavage plane. The fracture strength of beams fractured at the fixing position was 11.8 ± 0.2 GPa, while the strength of the beams that failed at submicron defects was in the range of 3.8-8.9 GPa. The calculation of stress concentration in the vicinity of pores revealed that local stress field exceeded the value that induced cracking in ‘defect free’ beams.     

水热氧化预处理对棉秆成型颗粒理化性质的影响

以棉秆为研究对象,经180～280℃水热氧化预处理后热压制备成型颗粒。利用热重分析、X射线衍射(XRD)及傅里叶变换红外光谱(FTIR)等分析手段,考察了水热氧化预处理温度对棉秆成型颗粒理化性能及燃烧性能的影响。结果表明:随着水热氧化温度的升高,棉秆的固相产物产率降低,半纤维素于180℃之前完全分解,无定形纤维素于200℃完全分解,结晶纤维素于260℃完全分解,水热氧化固相产物的纤维素结晶度呈现逐渐减小的趋势,而木质素相对含量增加;棉秆成型颗粒的固定碳含量、热值和能量密度增加,但燃烧性能变差。水热氧化预处理后棉秆成型颗粒的表观密度保持在1300kg/m³左右,抗压强度则随着水热氧化温度的增加而下降,其中180℃水热氧化预处理后棉秆成型颗粒的抗压强度相比原料成型燃料增加了183.33%。本研究范围内,经180℃水热氧化预处理后获得的棉杆成型颗粒的燃烧性能和物理性能最佳,其热值为17.76MJ/kg,能量密度为23.44GJ/m³,抗压强度达11.90MPa,可作为优质生物质成型燃料使用。     

结合体系对氮化后碳化硅基浇注料冷/热态强度的影响

本工作研究了二氧化硅微粉+铝酸钙水泥(MS+ CAC),二氧化硅微粉+水合氧化铝(MS+ HA),二氧化硅溶胶三种不同结合体系对添加9％硅粉的碳化硅基浇注料1420℃氮化后的冷态耐压强度,冷态和800～1400℃热态抗折强度的影响,应用SEM,XRD和EDAX,分析了不同试样中的物相及显微结构.结果表明,无水泥的结合体系结合的试样氮化后由于基质不含玻璃相,因而高温下更稳定,原位形成的氮化物呈网络状,使1200℃以上强度保持率高;含水泥的结合体系结合的试样基质中有CAS2相,形成的氮化物为粒状或柱状,使热态强度降低显著.     

Oxidation kinetics strength of Hi‐NicalonTM‐S SiC fiber after oxidation in dry and wet air

Hi‐Nicalon?‐S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO₂ scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO₂ scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal‐Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi‐Nicalon?‐S fibers are compared with previous studies on SiC fibers, bulk SiC, and single‐crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.