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1.
《Ceramics International》2022,48(22):33151-33159
The thermal stability and selective nitridation of Cr2AlC under a high–temperature nitrogen atmosphere were studied. Cr2AlC began to decompose beyond 1073 K in N2, and the activation energy was estimated to be 108.93 kJ·mol?1. Initially, the selective nitridation led to the generation of an AlN layer, which shielded the underlying Cr2AlC and the process was dominated by a surface reaction. The final products were AlN, Cr7C3 and Cr3C2, and the weaker Cr–Al bonds in Cr2AlC facilitated the rapid diffusion of aluminum from the interior outwards. As the reaction proceeded, micropores were observed on the grain surface, as well as a loose structure, which facilitated the diffusion of N2 and thus accelerated the reaction. Finally, the intensive reaction involving Cr2AlC and N2 could be attributed to the gas diffusion channels caused by improved temperature and continuous escape of vaporized aluminum. 相似文献
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A flow-through potentiometric technique utilizing an yttria-stabilized zirconia (YSZ) pH sensor has been employed to elucidate the effects of NiSO4 and MgSO4 on pH of H2SO4 solutions at temperatures of 200 °C and 250 °C. Solution pH was found to increase with increasing NiSO4 and MgSO4 concentrations at both temperatures. This trend is attributed to the dissociation of NiSO4 and MgSO4 where the SO4 2?(aq) released reacts with H+(aq) to form HSO4 ?(aq). The conversion of measured potentials into pH values was based on the mixed-solvent electrolyte (MSE) model, which is a speciation model of the new OLI Systems® software. Both the Henderson equation and exact definition of the diffusion potential were employed in treating the obtained experimental data. Experimental pH values calculated using the diffusion potentials evaluated by either approach are similar, suggesting that the Henderson equation can be effectively used. In addition, LiCl was found to be a suitable alternative to NaCl as the reference electrode solution. 相似文献
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Juanjuan Chen Jixin Chen Guorui Zhao Minmin Hu Yiming Lei Yueming Li Xi Zhang Jialin Li Meishuan Li 《Journal of the European Ceramic Society》2018,38(15):4867-4873
High-temperature mechanical and thermal properties of h-BN based composite containing amorphous silica and Yb-riched silicate glass phases were systematically investigated in this work. Owing to anisotropic microstructure of h-BN matrix, the obtained composite demonstrates anisotropic mechanical and thermal properties. The composite possesses higher elastic modulus at 1673?K than that at room temperature and presents excellent high-temperature stiffness. Flexural strengths in parallel and perpendicular directions reach 496?±?22 and 258?±?21?MPa at?1073?K, respectively, and increases by 74 and 66% compared with the room-temperature strengths of 285?±?4?and 155?±?5?MPa. The composite containing dual glass phases shows lower coefficients of thermal expansion in the temperature range of 473–900?K, the values are ?1.4?×?10?6 and 0.3?×?10?6 ?K?1 for the perpendicular and parallel directions, respectively. Thermal conductivities in the perpendicular and parallel directions at 373?K are 24.8 and 14.8?W?m?1?K?1, respectively, and then decrease to 14.9 and 9.3?W?m?1?K?1 at 1473?K. 相似文献
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《Journal of the European Ceramic Society》2019,39(15):4595-4601
MAX-phase materials have shown great potential for different technical applications due to their mechanical properties. If the main group element is aluminium their excellent oxidation resistance also makes them attractive for several high temperature applications. As an example the thermodynamically stable MAX-phase Ti2AlC forms a thin, protective alumina layer in oxidising atmospheres at elevated temperatures. This alumina layer is formed due to the high Al activity within the material and prevents further attack by the environment. However, high temperature oxidation tests at 900 °C in air of “technical” Ti2AlC which is not pure single-phase Ti2AlC led to the formation of a non-continuous alumina scale which is intersected by a mixed TiO2/Al2O3 scale. Furthermore, internal oxidation was observed. This “technical” material consists of two phases namely Ti2AlC plus γ-TiAl due to the manufacturing route. Such γ-TiAl-grains are preferentially oxidised. This type of internal attack can be suppressed by a preceding fluorine treatment. 相似文献
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《Journal of the European Ceramic Society》2020,40(1):197-201
In an effort to overcome the property degradation of Ti2AlC MAX phase coating used in harsh environments, we fabricated a solid solution Ti2(Al0.6Sn0.4)C coating with amount of Ti5Sn3 (20 wt.%) by a combined technique composing of magnetron sputtering and post-heat treatment. The cracks induced by Vickers indentation on coating surface were self-healed at 700 ℃, which is the lowest self-healing temperature among the Al-based MAX phase coatings till now. The structural evolution and kinetic diffusion revealed that the formation of SnO2 is the key factor to achieve the crack self-healing at such a low temperature for Al-based MAX phase coatings. Additionally, the self-healed Ti2(Al0.6Sn0.4)C coating exhibited better oxidation resistance compared to the unhealed one at 800 ℃. The results provide a novel and facile strategy to develop the protective MAX phase coatings with high performance at high temperature by partially substituting Al with Sn. 相似文献
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Ultra-high molecular weight polyethylene (UHMWPE) is extensively used in frictional applications due to its advanced wear resistance. This advanced polymer is reinforced with hard particulate fillers for further developments against wear conditions. Since elevated temperatures prevail in the service conditions, wear behavior of UHMWPE composites is an important issue for the engineering applications. In the present work, UHMWPE-based composites including silicon carbide (SiC) fillers were fabricated in a compression molding chamber. In the specimen preparation stage, molding pressure, filler amount, and filler particle size were varied to investigate the influence of these variables. Upon deciding the optimum parameters from the wear tests conducted at room temperature, the wear experiments were repeated for the optimum specimen at elevated temperatures, such as 40 and 60°C. According to the results, the wear behavior of the SiC/UHMWPE composites is heavily changed by the effect of elevated temperature. Adhesive effect is pronounced at elevated temperatures while the wear characteristics possess the abrasive effect in the sliding path. In addition, the composites exhibit an accelerated material loss as temperature increases during the frictional system. 相似文献
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《Ceramics International》2021,47(18):25520-25530
(Ti0·8Mo0.2)3AlC2 solid solutions were successfully synthesized from Ti, Al, TiC, and Mo powders using the in situ hot-pressing sintering method. The tribological properties of (Ti0·8Mo0.2)3AlC2 and the reference Ti3AlC2 in the temperature range 25–800 °C were evaluated in ambient air with the counterpart of Al2O3 balls. The results show that (Ti0·8Mo0.2)3AlC2 has improved lubricating properties and wear resistance above 400 °C compared with Ti3AlC2. This can be contributed to the formation of tribo-oxidation films containing MoO3 and MoO3-x. Structural characterization of the tribo-oxidation films was conducted using SEM, EDS, Raman spectroscopy, and XPS to evaluate the effect of Mo doping on the wear mechanisms of Ti3AlC2 in detail. 相似文献
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The features of electrochemical formation process of porous silicon (PS) at the temperatures above the room temperature have been studied. It was found that besides electrochemical dissolution, chemical etching takes part in the formation process of PS even for concentrated HF electrolyte. The role of chemical etching increases with temperature causing an increase of the porosity and the crater depth. The temperature dependence of chemical etching rate has been established. Obtained results enable to conclude that OH− ions play a major role in the chemical etching. Electrochemical etching allows to fabricate PS with good surface quality at the temperatures at least below 65 °C provided that HF electrolyte is concentrated. 相似文献
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Thiswork provides amethod to explore the transport property of the electrolyte aqueous solutionswith one or two ionic liquids, especially focus on their electrical conductivity. The conductivities were measured for the ternary systems NaCl–[C6mim][Cl] (1-hexyl-3-methylimidazolium chloride)–H2O, [C6mim][BF4]–[C6mim][Cl]–H2O, NaNO3–[C6mim][BF4](1-hexyl-3-methylimidazolium tetrafluoroborate)–H2O, and [C4mim][BF4] (1-butyl-3- methylimidazolium tetrafluoroborate)–[C6mim][BF4]–H2O, and their binary subsystems NaNO3–H2O, NaCl–H2O, [C6mim][BF4]–H2O, [C6mim][Cl]–H2O, and [C4mim][BF4]–H2O, respectively. The conductivities of the ternary systems were also determined using generalized Young's rule and semi-ideal solution theory in terms of the data of their binary solutions. The comparison showed that the two simple equations provide good predictions for conductivity of mixed electrolyte solutions and the mixed ionic liquid solutions based on the conductivity of their binary subsystems. 相似文献
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《Journal of the European Ceramic Society》2023,43(8):3122-3130
A novel 312-type MAX-phase solid solution series in the Zr-Ti-Si-C system has been synthesized by the vacuum carbosilicothermic reduction method using mixtures of TiO2, ZrO2, SiC, and Si powders as starting materials. The upper limit for Zr content in metal sublattice of the synthesized (Zr,Ti)3SiC2 MAX phase solid solutions was found to be as much as approximately 66 at%, closely corresponding to a hypothetical quaternary Zr2TiSiC2 MAX phase. A wide miscibility gap inside the interval of Zr content in metal sublattice ranging between 22 at% and 55 at% was found. Crystal structure of the synthesized MAX-phase solid solutions was studied by HR-STEM/HAADF and XRD Rietveld analyses. The lattice constants were determined to be linearly correlated with Zr content as predicted by Vegard's law. A significant inhomogeneity in distribution of metal atoms similar to that of out-of-plane ordered quaternary MAX phases has been established for both Ti-rich and Zr-rich MAX-phase solid solutions. 相似文献
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Calcium silicate hydrate is a porous hydrate that is sensitive to temperature and readily loses strength at elevated temperatures. Mechanical and chemical changes in the microstructure, due to escaping water, can significantly affect the mechanical properties, but these changes occur over different temperature ranges. By measuring Young's modulus as a function of temperature using the dynamic mechanical analyzer, the temperature range in which the greatest change in stiffness occurs can be identified. Additional mineralogy, pore size distribution, and composition analysis from high temperature X-ray diffraction, nitrogen sorption, and thermogravimetric analysis will demonstrate the changes in the microstructure. The results demonstrate that over 90% of the loss in stiffness occurs below 120 °C. Therefore, the damage is due to microcracking caused by pore water expansion and evaporation and not the change in mineralogy or composition. More damage, as indicated by greater loss in stiffness, occurs in stiffer and less permeable samples where higher stresses can develop. 相似文献
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Taiheiyo coal has been reacted in an autoclave with sodium hydroxide in ethanol. The extractability of the product with pyridine and ethanol increased remarkably as either the reaction temperature or the sodium hydroxide concentration rose. Under the same reaction conditions, the younger the coal the greater the extraction. The main reaction is hydrolysis, but other subsidiary reactions are also taking place. From a blank test, ethanol was found to react almost quantitatively with sodium hydroxide and water to produce sodium acetate and hydrogen gas. In the case of coal the weight of hydrogen produced was smaller than in the blank test, and this difference is ascribed to the consumption of hydrogen by coal on hydrogenation. In the i.r. spectra, bands of methyl and carbonyl absorption appear, which may possibly be explained by the addition of -COCH3, or by hydrogenation of phenyl nuclei to produce a cyclohexanone-type compound. 相似文献
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In this study, the effect of elevated temperatures on chemical composition, microstructure and mechanical properties of high strength mortars with nano alumina was investigated. Mortars with 1, 2 and 3% nano alumina as cement replacement were prepared and then exposed to 100 °C, 200 °C, 300 °C, 400 °C, 600 °C, 800 °C and 1000 °C. XRD, DSC and SEM tests were carried out to identify chemical composition and microstructure changes in the cement matrix after being exposed to elevated temperatures. Residual compressive strength, relative elastic modulus and gas permeability coefficient of samples were also obtained. A brittleness index was defined to monitor changes in brittleness of samples after being exposed to elevated temperatures. Nano alumina enhanced compressive strength of samples up to 16% and improved residual compressive strength. An increase in the relative elastic modulus, higher energy absorption and lower permeability were also observed when 1% nano alumina was added. 相似文献
15.
Fracture toughness of random fibrous materials with ultrahigh porosity at elevated temperatures 总被引:1,自引:0,他引:1
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Datao Li Wenshan Yu Wei Xia Qinzhi Fang Shengping Shen 《Journal of the American Ceramic Society》2018,101(3):1323-1332
The fracture toughness of three‐dimensional random fibrous (3D RF) material was investigated from room temperature to 1273 K by virtue of experimental method, theoretical model and Finite Element Method (FEM) in the through‐the‐thickness (TTT) and in‐plane (IP) directions. The experiments showed that the fracture toughness in the TTT and IP directions increases (from 0.0617 to 0.0924 Mpa·m1/2 and from 0.2958 to 0.3982 Mpa·m1/2 for the TTT and IP directions, respectively) as the temperature until reaching a transition temperature (1123 K and 1223 K for the TTT and IP directions, respectively), then the fracture toughness decreases from 0.0924 to 0.0393 Mpa·m1/2 and from 0.3982 to 0.3106 Mpa·m1/2 for the TTT and IP directions, respectively. The fracture behavior was related to the bulk microstructures, the mechanical properties of fibers and the blunting of crack tip. The crack tip blunting affected the fracture toughness at elevated temperatures which was verified using the theoretical model. A FEM model with a single edge crack where special attention was drawn to the influence of the morphological characteristic was developed to simulate the fracture behavior of 3D RF material. Numerical results from the FEM modeling along with a theoretical model with crack tip blunting mechanism incorporated agreed well with the experimental results. 相似文献
16.
Dwight L. Myers Michael J. Kulis Joseph P. Horwath Nathan S. Jacobson Dennis S. Fox 《Journal of the American Ceramic Society》2017,100(6):2353-2357
Tantalum pentoxide (Ta2O5) and its solid solution phases are candidate coatings for components to be used in combustion environments. Thus, it is important to understand the response of Ta2O5 to high‐temperature water vapor, a product of combustion. Thermogravimetric methods are used to examine the oxide in reactant streams of controlled water vapor contents at 1250°C‐1450°C. The observed weight loss indicates a reaction of the general form ½ Ta2O5(s) + x H2O(g)=TaOy(OH)x(g). Methodical variation in the water vapor pressure suggests the products are a mix of TaO(OH)3(g) and Ta(OH)5(g). Evidence of TaO(OH)3(g) was observed with a sampling mass spectrometer. The measured hydroxide and oxyhyroxide vapor fluxes from Ta2O5 are compared with calculated vapor fluxes from SiO2 and Al2O3. Ta2O5 exhibits fluxes similar to those from SiO2 due to gaseous metal hydroxide formation. 相似文献
17.
Free radicals are formed upon irradiation of polymers. The annealing of γ-ray irradiated syndiotactic polystyrene (sPS) with doses 10, 23 and 36 kGy was studied with electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra are possibly attributed to three types of radicals, (a) the benzyl radical Ra, (b) resonance structure of the phenyl radical Rb, and (c) carbon-superoxide-centered radical Rc with three-, four-, and single-line features, respectively. Radical Ra can be easily formed by the removal of the proton from the tertiary carbon; radical Rb is created from delocalization of the spin of the radical Ra onto the benzene ring; and radical Rc may be generated from the interaction of a carbon-centered radical with dioxygen from the air which forms a carbon-superoxide center. By comparing EPR spectra of the radicals with a DPPH standard, the spin numbers of the radicals can be calculated. The spin number of all radicals decreases exponentially with time in the temperature range of 60-90 °C regardless of dose of the irradiation. The annealing of Ra, Rb, and Rc follows first-order kinetics. The activation energies of the annihilation are determined to be 15.8-19.0, 16.0-19.5, and 23.2-26.6 kJ/mol for radicals Ra, Rb, and Rc, respectively. The kinetic study presented herein can serve as a criterion for γ-ray irradiation process in various applications, such as sterilization of polymer materials and devices. 相似文献
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The National Institute of Standards and Technology (NIST) and the Center for Better Living have formed a collaboration to assess the performance and failure mechanisms of gypsum wall assemblies under real fires/furnace conditions. These measurements are being used to compile an experimental database necessary to validate models that could be used to predict their performance and ultimate failure under various design fires. A critical component of the database is thermal property data of gypsum board. The present paper describes the results of an effort to quantify thermal properties of gypsum board. The thermal conductivity specific heat mass loss and linear contraction for gypsum board types widely used in the U.S.A. and Japan were measured both at room temperature and at elevated temperatures. The gypsum board types tested include Type X and Type C from the U.S.A. and Type R and Type F from Japan. Results indicate that the difference in thermal properties of all gypsum board samples tested in the present study is not significant particularly at elevated temperatures. A large difference in linear contraction among gypsum board samples was observed at elevated temperatures, implying a significant difference in mechanical behavior at fire temperatures. The experimental data set provides valuable information that can be used to model the behavior of gypsum board at elevated temperatures. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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《Ceramics International》2023,49(8):12034-12041
As a good candidate for high-temperature uses in aerospace vehicles, the thermal behaviors of Cr2TiAlC2 in three different circumstances were studied. It exhibited good thermal-structural stability up to 1450 °C in flowing argon atmosphere, while heat treated to 1300 °C in Ar + CO/CO2 atmosphere, it decomposed to Cr2AlC resulting from the Ti consumption. The thermal stability in vacuum was also studied by in-situ X-ray diffraction methods, which showed a new decomposition mechanism based on the multi-scale bonding characteristics in its crystal structure. To deep understand the relationship between the crystal structure, chemical composition, and high-temperature behaviors, the hierarchical bonding characteristics were revealed. The out-plane ordering of M atoms together with their apparent negativity differences may result in the large M − C bonding strength, i.e., increased Cr–C strength and decreased Ti–C strength. The present work based on Cr2TiAlC2 helps better understand the composition-structure-property relationship of Mn+1AXn compounds. 相似文献