Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

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.     

Fabrication and microstructure evolution of monolithic bridged polysilsesquioxane-derived SiC ceramic aerogels

SiC aerogels are representative high-temperature ceramic aerogels that have demonstrated extensive potential utility in the thermal insulation field under extreme conditions. Although the efficient fabrication of monolithic and highly crystalline SiC aerogels is crucial, it remains highly challenging. Herein, a one-step pyrolysis strategy of a bridged polysilsesquioxane (BPSi) aerogel precursor is reported. The preceramic BP aerogel was prepared via a sol-gel method followed by vacuum drying, and the subsequent pyrolysis process conveniently converted the BPSi aerogel precursor into a SiC aerogel. This circumvents the need for harsh synthetic conditions, high-cost noble metal catalysts, special drying, and additional calcination processes. Furthermore, a competitive mechanism between the “gas-escape” caused by the volatilization of low molecular compounds and carbothermal reduction reaction, and the “volume-shrinkage” resulting from high-temperature sintering was proposed to explain the evolution of phase composition and pore hierarchy of BPSi-derived ceramic aerogels.     

Real and Ideal Relationships between Selectivity,Reactivity and Equilibrium

Reactivity and selectivity are discussed in terms of quadratic rate—equilibrium relationships for reaction series sharing a common intrinsic barrier, notably Marcus' equation. R.D. Levine's emphasis on such reactions as forming a “Brønsted Series” is suggested as a basis for defining “ideal” reactivity behavior. Ideal relationships between selectivity and reactivity and selectivity and equilibrium are derived based on reaction schemes drawing a formal distinction between reagent and substrate. In a simple Brønsted relationship, the reagent varies and the substrate is held constant. In an extended relationship, in which both substrate and reagent vary, the Brønsted series becomes a selectivity “network” or matrix characterized by direct and cross quadratic coefficients reflecting curvature of a free energy relationship and the dependence of selectivity upon rate or equilibrium respectively. In the ideal case, the quadratic coefficients are equal but, in practice, they generally differ. Inspection of the relevant reaction schemes shows that in evaluating the coefficients, there is greater cancellation of experimental and chemical scatter arising from non-systematic variations in reagent structure for the cross than the direct coefficient; this phenomenon accounts in part for the wider observation of reactivity—selectivity than non-linear free energy relationships. New measurements are reported for reactions of carboxylic acids with enolate and imine substrates. The results are combined with literature data and plotted as reactivity—selectivity and extended Brønsted relationships. The correlations are satisfactorily considered in the context of departures of real from ideal behaviour.     

Study of SiC–nickel alloy bonding for high temperature applications

We have studied the formation of metal/ceramic joints by solid state bonding technique for applications at temperatures >600 °C. The bonding is obtained between silicon carbide (SiC) and Ni-based super-alloy (HAYNES^® 214?) via metallic foils (Ni, Ag). In some cases a thin coating on the ceramic or the alloy by the electroless JetMétal? process has been used. Often used in brazing, nickel, when added to silicon carbide, usually give silicides. These reactions yield the “Pest Effect” (“pesting”) that induces a catastrophic brittleness of this type of assembling. To minimize the reaction of these metals with silicon carbide, addition of elements limiting the “Pest Effect” on the one hand and, diffusion barriers on the other hand, have been performed. Indeed, the choice of the thin Ni_0.93 B_0.07 coating is based on the ability of boron of improving the mechanical properties of silicides, thus avoiding the “Pest Effect”. However, we demonstrate that boron does not allow one to suppress the joint brittleness. Another new joining method employing a thin Ag coating or a Ag foil was tested. This process revealed the absence of chemical reaction at the Ag/SiC interface, thus proving the beneficial role of silver, which acts as an effective diffusion barrier for nickel beyond a certain thickness. This method has led to fabrication of joints presenting high shear resistance (>40 MPa).     

Bio‐Inspired Microstructure Design to Improve Thermal Ablation and Oxidation Resistance: Experiment on SiC

We propose a method to improve the ablation and oxidation resistance by designing specified microstructures on the surface of silicon carbide (SiC) based ceramic. Three different types of micropatterns were designed and simulations were carried out to compare their effects. Experiments on SiC ceramic at temperature of 1700°C–1750°C were conducted to validate the design. Results showed a favorable correlation between the pattern design, simulation, and experiments. These specifically designed microstructures can modify the local flow fields, change the local oxygen concentration and chemical reaction rate due to the structural influence on the fluids dynamics of high‐temperature gas flow. They can also tune the adhesion property between the liquid silica and SiC substrate by “locking in” the liquid silica on the surface to prevent further oxygen diffusion.     

Decomposition de l'acetylene,de l'ethylene et du benzene sur le carbone aux tres hautes temperatures et sous de basses pressions

At high temperatures (1000–2000°C) and low pressures (10^?5?10^?2 Torr) ethylene, acetylene and benzene decompose helerogeneously on pyrolytic carbon giving mainly hydrogen and deposited carbon, with collision yields of the order of 10^?4. The kinetics of these carbon deposition reactions show some striking similarities with carbon removal reactions by oxygen or oxygenated compounds.The true reaction order of these decomposition reactions is one above 1400°C, but becomes smaller at lower temperatures. This behaviour, common in gas-solid reactions, is generally interpreted as an inhibition due to chemisorption of some intermediate or reaction product. Evidence is also obtained that decomposition of the hydrocarbon molecules only occurs on peculiar sites of the carbon surface, i.e. the decomposition is not a purely thermal process, but involves a specific chemical interaction with the surface.Moreover, the behaviour of the pyrocarbon surface in carbon deposition reactions is similar to that observed in gasification reactions, i.e. the reactivity of the surface accommodates itself to the temperature and pressure conditions, as revealed by the observation of “transitory” and “stationary rates”. Transitory rates show that the surface deactivates with increasing temperatures (Figs. 4 and 5) [from which a maximum in the stationary rate results (Figs. 1–3)] and decreasing pressures (Figs. 7 and 8). The interpretation assumes that reaction sites are continuously created as an effect of carbon atoms deposition, but also deactivated by a thermal healing process.A main difference between carbon deposition reactions from hydrocarbons and carbon gasification reactions concerns the temperature range where reactivity is temperature dependent: in carbon deposition reactions, deactivation of the pyrocarbon surface is still effective up to much higher temperatures (Fig. 12).     

Synthesis of a silicon carbide coating on carbon fibers by deposition of a layer of pyrolytic carbon and reacting it with silicon monoxide

A method for preparing a SiC coating on carbon fibers is presented. The SiC coating was generated from the reaction of silicon monoxide (SiO) with a pyrolytic carbon (PyC) layer deposited on the fibers. The influence of holding time on the microstructure of the SiC layer was discussed. The oxidation behaviors of the uncoated and SiC coated carbon fibers were compared. The formation mechanism of the SiC coating was evaluated. With increased reaction time, the SiC coating becomes thicker and its surface becomes rough. The oxidation resistance of the carbon fiber was improved by the SiC coating. The initial oxidation temperature of the SiC coated carbon fiber is about 200 °C higher than that of the uncoated carbon fiber. The growth of the SiC coating is mainly attributed to the indirect reactions of SiO with PyC in the SiO/SiC/C system, in which silicon is considered a critical intermediate product.     

Thermally safe operation of a semibatch reactor for liquid-liquid reactions–fast reactions

Accumulation of the reactant supplied to a cooled semibatch reactor (SBR) will occur if the mass transfer rate across the interface is insufficient to keep pace with the supply rate. Then, due to a low starting temperature or supercooling, the reaction temperature does not rise fast enough to the desired value. This accumulation may eventually lead to a temperature runaway. We investigated the possibility of such an event for reactions of the type “chemically enhanced mass transfer” or “fast” and found that only low distribution coefficients, i.e. 10^?4 or lower, can lead to accumulation. At higher distribution coefficients, the mass transfer rate across the interface of a well-mixed dispersion is generally sufficient to prevent accumulation. A thermal runaway in the fast regime exerts a moderate effect, because the effective activation energy is halved. Calculations for the “instantaneous” reaction regime, regarded as a special case of fast reactions, show that there is no runaway possible.     

Vaporization of mixed SiC powders. Partial pressures and grain morphology changes under vacuum conditions

The high temperature heat treatment of pure, different grain size mixtures and oxidized SiC powders has been studied under vacuum using the Knudsen cell mass spectrometric method in order to determine their vapor pressures and to study their morphology change with time. Various characterization methods – grain size distribution analysis, X-ray diffraction, Raman spectra and scanning electron microscopy – were used before and after heat treatment in the mass spectrometer in order to observe the growth of any possible connections between SiC grains. The present study shows that as long as silica was present as a layer on the SiC grains, connections growth could not be observed while during the active oxidation step obtained just after silica disappearance by vaporization as attested by mass spectrometry, the growth of SiC “like-necks” between the SiC grains occurred. Under active oxidation conditions the observed growth efficiency was better at the SiC–C phase limit.     

Molecular weight distribution control in a semibatch “living” anionic polymerization. I. Theoretical study

A method to produce homopolymers of preestablished molecular weight distribution (MWD), through “living” anionic polymerizations carried out in homogeneous semibatch reactors is proposed and theoretically justified. In the direct form of the technique, very fast reactions are assumed, and the monomer and “killing” agent feed flows are obtained from the knowledge of the desired MWD, the system spreading function, the total reaction time, the initial reaction volume, and the reagent concentrations. Alternatively, by controlling a reactor outlet flow instead of the “killing” agent feed, an external deactivation of the “living” ends can be implemented. The method can be extended to slow reactions, to more elaborated mechanisms, and to take into account the unwanted deactivation of “living” ends by impurities in the monomer solution feed.     

Effect of SiC whiskers on thermal cycling performance of YSZ-Based sealing coating

Two types of Y₂O₃ partially stabilized ZrO₂(YSZ)-based agglomerated powders with and without SiC whiskers were prepared by spray granulation, and the flow ability, apparent density and particle size distribution of them were investigated. The thermal cycling performance and failure mechanism of conventional high temperature sealing coating and modified one with additional layer with whiskers, which were sprayed by air plasma spraying (APS), were comparatively analyzed. The results showed that, compared to the powder without whiskers, the flow ability of that with whiskers reduced by 2.32%, and the apparent density and the proportion of 45–150 μm agglomerated powder increased by 1.53% and 2.29%, respectively. The thermal cycling failure mode of conventional high temperature sealing coating was the overall spalling of ceramic coating, and the spalling position originated from the interface of thermally grown oxide (TGO)/ceramic coating. Microstructure observation indicated that the structure integrity of SiC whiskers in the additional layer sprayed by APS was still retained. The whiskers were uniformly distributed and theinterface between bonding coating and ceramic coating exhibited excellent bonding. With the additional layer containing whiskers, the thermal cycling life of the coating was increased by 102.53%. In the thermal cycling process, the “bridging” and “pulling-out” effects of whiskers located at the additional layer consumed considerable energy, which could reduce the driving force of crack growth. Besides, a porous structure of the additional layer after thermal cycling was formed due to “bridging” and “pulling-out” of whiskers, further improving the thermal cycling life of coating with the additional layer.     

A study of reaction between palladium,palladium silver alloy and silicon carbide ceramics at high temperature

The reactions between palladium (Pd), palladium-silver alloy (Pd-Ag) and silicon carbide (SiC) from 1200 °C to 1400 °C have been studied to investigate the impact of liquid phases on reaction products formation. The liquid phases were generated in Pd/SiC reactions at 1400 °C and in Pd-Ag/SiC reactions above 1300 °C. An increase in the amount of liquid associated with higher temperatures or Ag presence strongly affected the reaction mechanism and was responsible for the inhomogeneous dissolution of SiC grains and grain boundaries by Pd-rich phase at reaction front. This study helps understand Pd’s role on Ag release through SiC layer in Tri-structural isotropic (TRISO) fuel particles.     

The controlled formation of hybrid structures of multi-walled carbon nanotubes on SiC plate-like particles and their synergetic effect as a filler in poly(vinylidene fluoride) based composites

Vertically aligned carbon nanotubes (CNTs) grown on plate-like SiC microparticles as nano/micro hybrid structures were produced by floating catalytic chemical vapor deposition. Acetylene and ferrocene were used as carbon source and catalyst precursor, respectively. The effect of experimental conditions on the structure of the CNT-SiC multi-scale hybrids produced was investigated. The results indicated that the organization mode of CNTs on SiC particles could be effectively tuned by changing the hydrogen content in the carrier gases. The effect of the substrate on the hybrid structures was also studied and their formation mechanism was discussed. According to X-ray diffraction and Raman spectra, the asymmetric surface properties of 6H-SiC tended to produce “single-direction” growth of CNTs on SiC particles, while the competition between the nature of the substrate and the experimental conditions can result in a “multi-direction” hybrid structure. The resultant well-organized CNT-SiC hybrid structures can be further used as conductive filler to prepare percolative poly(vinylidene fluoride) composites. The composites containing the “single-direction” hybrid structures exhibited a much lower percolation threshold than those with “multi-direction” hybrid structures. The percolation behavior of the composites can be tuned by controlling the structure of CNT-SiC hybrids.     

炭／炭复合材料SiC／Mo（Six，Al1-x）2涂层形成机理与抗高温氧化性能

采用高温包渗技术在炭／炭复合材料表面制备了SiC／Mo（Six,Al1-x）2复合涂层,采用两步反应法研究了复合涂层的生成机理。发现复合涂层是由Si、Al2O3、SiC、MoSi2原始粉末材料与基体炭材料经过复杂化学反应生成的SiC、Mo（SixAl1-x）2以及微量Mo4．8Si3C0．6固溶体组成。在较低温度下（〈1750℃）,单质硅与基体碳的液-固相反应,经过2小时后可以在炭／炭复合材料表面和内部孔隙表面生成致密的SiC过渡涂层;在较高温度下（≤2000℃）,SiC、Al2O3和MoSi2间的反应较为复杂,其主要过程为SiC与Al2O3间生成液体硅、液体铝和气态SiO、Al2O的多相反应,该反应生成的液体铝能够与MoSi2颗粒发生置换反应,生成熔点降低的Mo（Six,Al1-x）2转移涂层;同时,生成的液体硅与CO反应生成晶须状β—SiC,并与Mo（Six,Al1-x）2形成增强型复合涂层。本文还研究了过量单质Si和SiC对Mo（Six,Al1-x）2的还原反应,化学反应推论与实验结果相吻合。以新提出的涂层生成机理为指导,以粉末原料质量组成为Si10％,Al2O3 10％,SiC54％和MoSi226％时所制得了致密并且无粘结的复合涂层材料,并研究了封孔处理后复合材料的抗氧化性能。     

Long-time sulfenamide-accelerated sulfur vulcanization of natural rubber/chlorobutyl rubber compounds

Sulfenamide-accelerated sulfur vulcanization of natural rubber/chlorobutyl rubber compounds has been investigated at temperatures from 155 to 175°C over 0.1 to 400 min. Continuous measurements in a Cone Rheometer were used to estimate the extent of crosslinking, which was plotted against cure time. On the basis of a kinetic analysis, two first-order vulcanization reactions, crosslinking and degradation, have been evaluated. Over the temperature range studied, there is no significant difference between the values of activation energy for these reactions. The rate of the degradation is slower by a factor of 20 than the rate of crosslinking. The degradation reaction can be limited by increasing the “efficiency” of the vulcanizing system.     

Unerwünschte Reaktionen organischer Stoffe als Gefahren in Chemieanlagen

Undesired reactions of organic substances as sources of danger in chemical plant . In chemical production processes, danger can arise both from the intended exothermal reaction and from unintended exothermal reactions. Such “undesired” reactions may be exothermal reactions of the substances themselves (e.g. decomposition, polymerization) or reactions of the substances involved with one another (e. g. of a reactant with the solvent). Differential thermal analysis and warm storage have been successfully used in various modifications as experimental methods for the study of such reactions. For exothermal decomposition reactions, correlations can be made between the chemical constitution of the substances and the energy released on decomposition, as well as the temperature of incipient decomposition. In mixtures with other substances a modified decomposition behaviour must be expected, usually of such a kind that the temperature range of decomposition is lowered. There are many possible exothermal reactions of substances with one another; only a few have hitherto been studied in detail.     

Inherent safer design for chemical process of 1,4-dioldiacetate-2-butene oxidized by ozone

Abstract

Oxidation reaction is the typical thermal runaway reaction, and the reaction of 1, 4-dioldiacetate-2-Butene oxidized by ozone was investigated in this study. Firstly, the thermal hazards of the oxidative exothermic reactions were identified and evaluated combined process risk assessment method. The Qualitative Assessment for Inherently Safer Design (QAISD) was used to identify the risk of the reaction process. Meanwhile, the Reaction Calorimeter (RC1^e) was used to obtain the thermal properties of the oxidation reaction. Then the inherent safer designs (ISD) were proposed according to the risk assessment results to increase the level of safety of chemical industry technique. 1) ISD I: reaction temperature was improved to –5?°C, and ventilation rate was improved to 200?L?h^?1. 2) ISDII: using a tubular reactor as reaction vessel. The results indicated that the severity of the reaction hazard was reduced by 43%, and safety was improved significantly via two ISDs. Moreover, the inherent safety level of the reaction was increased by 63% and 43.4% via ISD Iand II, respectively. The reaction process get closer to inherent safety theories of “minimize,” “substitute,” and “moderate”.     

Temperature Effects on Thermodestructive Processes

Temperature effects on thermodestructive processes in polymers are considered. The logarithm of lifetime of a linear polymer thin film on 1/T (inverse temperature) was found to be a smooth increasing curve tending to a limit at definite values of 1/T, characteristic of each polymer. Temperatures above which avalanche-like decomposition of polymers occurred were called “limiting.” To explain the dependence i t was assumed that associated and nonassociated monomer links are destroyed with different rate constants, the sequence of chemical acts in the process being the same. The rate constant of destruction of nonassociated monomer links is higher than that of associated links. Equilibrium of concentrations of associated and nonassociated monomer links is established much more quickly than the chemical reactions run. At rather low temperatures the reaction presumably runs at chain ends. As the temperature increases, the number (concentration) and the length of blocks consisting of nonassociated monomer links increase, also. When the length of such a block reaches a definite value, a break reaction occurs, the break constant increasing with the increase of the block length. The dependence of the process rate on temperature is determined both by concentrations of blocks of various length and by the temperature dependence of break rate constants. At a definite temperature the reactions presumably at the chain ends are substituted by chance reactions. At temperatures when intermolecular interactions can be neglected, thermodynamically unstable “polymer” gas appears which decomposes in an avalanche-like manner.     

醋酸酐与水在化学反应量热方法确认中的应用

热流法化学反应量热测试获取的目标反应热（ΔH）、反应体系比热容（c_p）和最大热累积度（X_ac）等热安全参数是精细化工反应安全风险评估中不可或缺的关键参数,其测试结果的准确度（即正确度与精密度）直接影响化学反应的工艺危险度分级。目前关于测得的化学反应热安全参数准确度和有效性的研究未见报道。为解决这一问题,提出了以醋酸酐和水作为参考物质,采用“变异系数（CV值）法” 和“道格拉斯（Grubbs）检验法”分别验证热流法反应量热测试醋酸酐水解反应的ΔH、C_p 、X_ac等参数的精密度（即重复性和再现性偏差）,采用“t验证法”评估热流法反应量热测试水的比热容的正确度。研究结果表明：热流法反应量热测试醋酸酐水解反应的ΔH、C_p 、X_ac等参数在同一实验室内的重复性标准偏差和与其他4家实验室比对的再现性标准偏差均满足要求;采用热流法量热测试得到25℃水的比热容数据与标准比热容值t验证结果不存在实质性偏倚。因此,测试结果的精密度和正确度均满足要求。