Niobium-containing catalysts—the state of the art

This review article is devoted to the materials containing niobium, which have been discovered or developed in the past few years and exhibit the potential application in heterogeneous catalysis. Niobium oxides and mixed oxides as well as sulfides, nitrides (oxynitrides), carbides (oxycarbides), and phosphates are considered. Among the catalytic processes in which Nb-containing materials were tested, liquid and gas phase oxidation is described in details, and the role of niobium in the prevention of the catalyst from SO₂ poisoning is mentioned.     

Influence of the Preheat Treatment on the Microstructure and Properties of X37CrMoV5-1 Steel

The technological properties of the hot-work tool steels depend on their microstructural quality induced by the production process and the heat treatment. One of the frequently applied methods to improve microstructure is preheat treatment. The treatment of carbonitriding X37CrMoV5-1 steel has been investigated in fluidized bed furnaces. Thickness and phase composition have been conducted using optical microscopy Neophot 32 and scanning microscopy, JEOL 5400, after etching in nital.     

热轧卷温度场模拟在CSP生产HP295钢相变分析中的应用

采用有限元模拟软件Ansys，模拟了3mmHP295气瓶钢热轧卷温度场的变化，作出不同节点的温降曲线，并与该钢连续冷却转变(CCT)曲线结合，分析了HP295钢卷材不同部分冷却过程的组织转变。模拟结果表明，卷取温度为660-680℃时卷材心部平均冷却速度仅为0．04℃/s，远远低于出现碳化物离异共析的临界冷却速度0.1℃/s，使卷材深冲性能急剧降低。避免碳化物离异共析谚碳体沿铁索体晶界分布的有效措施是将卷取温度降至600～620℃。     

Mechanical properties of submicronic and nanometric boron carbides obtained by Spark Plasma Sintering: Influence of B/C ratio and oxygen content

Boron carbide samples exhibiting nanometric and submicronic microstructure were sintered by Spark Plasma Sintering to investigate the effect of grain size on mechanical properties. The mechanical properties of sintered monoliths were characterized at the grain and macroscopic scales. Although nanostructured material exhibits finer grains than the submicronic material (i.e. mean diameter of 82 vs. 474?nm), its apparent rigidity and hardness are found to be reduced by 6.8% and 8.4% respectively. This contradiction with the Hall-Petch law is linked to the chemical compositions of both materials, which show significant difference in terms of B/C ratio and higher structural oxygen content especially for nanostructured material.     

Experimental and thermodynamic investigations regarding the effect of chromium on the carbides population in cast {Ni(bal.)-0.4C-6Ta-xCr} alloys with x varying from 0 to 50 wt%

Involving a particularly strong carbide-former metallic element, the tantalum carbides are potentially very stable at elevated temperatures in term of volume fraction and morphology. The TaC phase represents a major strengthening way to allow cast chromium-rich superalloys resisting mechanical stresses at elevated temperatures. They are exploited in recent high performance cobalt-based superalloys but seemingly not in nickel-chromium refractory alloys. Earlier studies showed that the stability of TaC in Ni-Cr alloys is not so good as in the Co-Cr ones, and they evidenced that chromium carbides may compete with TaC in the formation of the carbides population. A possible way to optimize the presence of TaC in Ni-Cr alloy may consist in rating the chromium content to an ideal value but preliminary knowledge about the TaC dependence on the Cr content is compulsory. The aim of this work is precisely the investigation of the effect of the content in chromium on the appearance and stability of the TaC phase in Ni-Cr alloys, by the means of thermodynamic calculations and real experiments in parallel. A global system Ni(bal.)-xCr-0.4C-6Ta compositions (with x varying from 0 to 50 wt%) was chosen. Thermodynamic calculations were performed to know the theoretic metallurgical states inside the considered x range. These theoretic results being dependent on the suitability of the used database, real experiments of verifications were also carried out for a selection of six alloys (x = 0, 10, 20, 30, 40 and 50 wt%). The alloys prepared by respecting these compositions were cast and isothermally exposed at high temperature (1400 and 1510 K), then subjected to metallographic characterization. For the used database the calculated results showed that no TaC should never appear whatever their Cr content, while TaC were really observed in the as-cast and aged versions of the alloys containing 20 wt%Cr and more, but never alone since chromium carbides were systematically also present. When the Cr content in the alloy is too low, the TaC are rare or even no present. This allowed concluding first that the database used for the calculations needs serious improvements, followed by tests with, as first criteria of quality, a good correspondence with the present experimental results. Second, the presence of Cr in quantity high enough is surprisingly compulsory to obtain TaC carbides in quantity high enough, but it is no possible to avoid the appearance of chromium carbides. Obviously, other ways than Cr adjustments must be found to obtain TaC in nickel-based alloys as the single carbide phase and in quantity high enough to achieve high mechanical properties at high temperature.     

Effect of LaB6 content on the gas evolution and structure of ZrC coating for carbon/carbon composites during ablation

ZrC coatings with different contents of LaB₆ for SiC-coated carbon/carbon composites were fabricated by supersonic atmosphere plasma spraying. The mechanism of gas evolution during oxidation at ultra-high temperatures and the oxide scale structure evolution of the coating after addition of LaB₆ with different contents were studied. Results showed that rare earth borate rather than rare earth oxide mainly reduced the melting resistance of the oxide scale. The oxide scale for ZrC coating with 10% LaB₆ was unfavorable to the gas escape. After addition of 20% LaB₆, the oxide scale was favorable to the gas escape but disadvantageous to the coating applied in the environment containing scouring because of the reduced melting resistance. High content of LaB₆ also could cause evident boiling of B₂O₃, leading to producing big pores in the oxide scale during ablation. Appropriate content of LaB₆ for ZrC coating was found to be 15%.     

Infiltration mechanism and factors influencing carbon/carbon–Zr–Ti–C composites prepared by liquid metal infiltration

The effects of fibre architecture, reaction temperature and holding time on the infiltration performance of carbon/carbon (C/C)–Zr–Ti–C composites prepared by liquid metal infiltration were investigated. The results indicated that samples with a chopped-web needled preform and low initial density had a high final density. Increasing the reaction temperatures resulted in a decrease of the final density of samples. Additionally, increasing the initial holding time appeared to obviously result in a high final density, but its effectiveness was not obvious in later observations. An analysis of the infiltration kinetics and mechanisms indicated that the diffusivity of carbon in the carbide, the open-pore sizes and their distribution in C/C composites were the essential characteristics that controlled the height of infiltrating melts.     

Reaction coupling preparation of high sintering activity boron carbide nano-powders

Large scale B₄C nano-powders were synthesized via a novel ball milling assisted reaction coupling self-propagating high temperature synthesis method using Mg, B₂O₃ and CH₂H₃Cl as the starting materials. The XRD, FTIR, Raman, EDX, FSEM, TEM, HRTEM and SAED were used to characterize the B₄C samples. The optimum endothermic rate was 35%, when the samples presented fine and uniform regular morphology with an average particle diameter of about 100 nm. In addition, the reaction coupling principle, possible chemical reaction mechanism and the effects of the endothermic reaction rate were also discussed. Moreover, the commercial B₄C (C-B₄C) and homemade B₄C (H-B₄C) ceramics were prepared by spark-plasma sintering method at 1700 °C under 30 Mpa. Compared with the C-B₄C ceramic, the values of relative density, vickers hardness and fracture toughness of the H-B₄C ceramic were increased by 2.1%, 9.2% and 20.1%, respectively, demonstrating high sintering activity of the homemade B₄C nano-powders.     

Fabrication and microstructure of 3D Cf /ZrC-SiC composites: through RMI method with ZrO2 powders as pore-making agent

3D C_f/ZrC–SiC composites were prepared by a combination process of slurry infiltration and reactive melt infiltration. ZrO₂ powders and ZrSi₂ alloy, both of which reacted with amorphous carbon, were used as pore-making agent and infiltrator, respectively. After carbothermal reduction at 1650 °C, X-ray diffraction analysis revealed that ZrO₂ powders were completely converted into ZrC by reacting with amorphous carbon, and an in-situ formed submicron porous configuration was observed at the areas containing ZrO₂. Results showed that the matrix in composites mainly consisted of SiC, ZrC and a small quantity of residual metal. SEM and TEM images revealed the formation of ZrC or SiC intergranular particles in the matrix and the characteristic around the residual resin carbon. The composites had a bending strength of 94.89±16.7 MPa, fracture toughness of 11.0±0.98 MPa m^1/2, bulk density of 3.36±0.01 g/cm³, and open porosity of 4.64±0.40%. The formation mechanisms of ZrC–SiC dual matrix and intrabundles׳ structure were discussed in the article.