Diborides of transition metals: Properties, application and production. review. Part 2. Chromium and zirconium diborides

The second part of the review considers properties, application and methods for producing chromium and zirconium diborides. These diborides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Chromium and zirconium diborides exhibit significant chemical resistance in aggressive environments. They have found application in modern technology because of these reasons. Chromium diboride is used as a sintering additive to improve the properties of ceramics based on boron carbide and titanium diboride. Zirconium diboride is a component of advanced ultra-high temperature ceramics (UHTC) ZrB2 –SiC used in supersonic aircrafts and in gas turbine assemblies. Ceramics B4C–CrB2 and B4C–ZrB2 have high-quality performance characteristics, in particular, increased crack resistance. The properties of refractory compounds depend on the content of impurities and dispersion. Therefore, to solve a specific problem associated with the use of refractory compounds, it is important to choose the method of their preparation correctly, to determine the admissible content of impurities in the starting components. This leads to the presence of different methods for the borides synthesis. The main methods for their preparation are: a) synthesis from elements; b) borothermal reduction of oxides; c) carbothermal reduction (reduction of mixtures of metal oxides and boron with carbon; d) metallothermal reduction of metal oxides and boron mixtures; e) boron-carbide reduction. Plasma-chemical synthesis (deposition from the vapor-gas phase) is also used to obtain diboride nanopowders. Each of these methods is described.

One-Pot Reduction of Metal Oxides/Salts with Carbohydrate Biomass Under Mild Hydrothermal Conditions

<p></p><p>Development of novel strategies to make full use of highly functionalized biomass molecules to expand its application fields is crucial to biomass utilization. Due to possessing diverse reducing functional groups such as hydroxyl and aldehyde, carbohydrate biomass can be employed as reductant for metallic material preparation. Particularly, the reducing ability of carbohydrate biomass is enhanced under hydrothermal conditions. The reported studies focused on reduction of metal ions in acidic solution with the aid of biomass. However, we found alkali hydrothermal conditions are favorable to metal compounds reduction, even direct conversion metal oxides to metals. Meanwhile, low molecular weight organic acids were obtained from biomass oxidation. <a>Based on our previous research on direct reduction of CuO and NiO into the corresponding metals, herein, we investigated metal salts reduction with carbohydrates and compared the reduction performance on glucose and cellulose which are the two most abundant carbohydrates with and without alkali addition. Moreover, conversion of other metal oxides (</a>Fe₂O₃, MnO₂, Co₃O₄, PbO₂) with glucose were studied to illustrate the feasibility to direct reduction of metal oxides with carbohydrates under alkali hydrothermal conditions. The reduction pathway study showed not only carbohydrates but also decomposed intermediates can reduce metal oxides. This study may provide an alternative approach to metal preparation in hydrometallurgy.</p><br><p></p>

One-Pot Reduction of Metal Oxides/Salts with Carbohydrate Biomass Under Mild Hydrothermal Conditions

<p></p><p>Development of novel strategies to make full use of highly functionalized biomass molecules to expand its application fields is crucial to biomass utilization. Due to possessing diverse reducing functional groups such as hydroxyl and aldehyde, carbohydrate biomass can be employed as reductant for metallic material preparation. Particularly, the reducing ability of carbohydrate biomass is enhanced under hydrothermal conditions. The reported studies focused on reduction of metal ions in acidic solution with the aid of biomass. However, we found alkali hydrothermal conditions are favorable to metal compounds reduction, even direct conversion metal oxides to metals. Meanwhile, low molecular weight organic acids were obtained from biomass oxidation. <a>Based on our previous research on direct reduction of CuO and NiO into the corresponding metals, herein, we investigated metal salts reduction with carbohydrates and compared the reduction performance on glucose and cellulose which are the two most abundant carbohydrates with and without alkali addition. Moreover, conversion of other metal oxides (</a>Fe₂O₃, MnO₂, Co₃O₄, PbO₂) with glucose were studied to illustrate the feasibility to direct reduction of metal oxides with carbohydrates under alkali hydrothermal conditions. The reduction pathway study showed not only carbohydrates but also decomposed intermediates can reduce metal oxides. This study may provide an alternative approach to metal preparation in hydrometallurgy.</p><br><p></p>

Synthesis of YFeO3-δ and LaFeO3-δ Perovskites with High Catalytic Activity in Carbon Oxidation Reactions

YFeO3-δ (δ = 0.26) and LaFeO3-δ (δ = 0.5) perovskites with a high specific surface and oxygen non-stoichiometry was firstly synthesized by pyrolysis of polymer-salt compositions. It was shown that the catalytic oxidation of carbon in the presence of these complex oxide systems proceeds in the range of 400 - 700 °С, with a maximum temperature at 556 °С for YFeO3-δ; and 380 - 620 °С ,with a maximum temperature at 501 °С for LaFeO3-δ, in one-stage mode for both. By means of thermal analysis and diffractometry, it was shown that there is no contribution to the soot oxidation mechanism by cyclic perovskite surface transformations, due to the reduction of metal oxides by the soot and their subsequent reoxidation. It has been established that the basis of the catalytic reaction mechanism for both perovskites is the presence of oxygen vacancies on the surface of complex oxides.

Thermal Techniques for the Production of Fe-M Alloys

The present manuscript is designed to investigate the possibility of manufacturing iron-metal alloys (Fe-M) via thermal techniques. These techniques are mainly depends on simultaneous reduction-sintering reactions of metal oxides. The reduction of metal oxides is an important property in metallurgical processes. It can be applied to M-Fe-O systems and also is used to develop inter-metallic alloys with specific properties. The produced metallic materials have wide range of applications and are characterized by unique physical and mechanical properties. The composition of the produced alloys is often a key element in optimizing their properties. Iron oxide doped another metal oxide such as nickel oxide is used as starting materials to produce metallic materials containing iron contaminated with nickel metal using thermal techniques. The sintering-reduction reactions of the composite oxide materials are investigated under different operation conditions. The experimental results show that the reduction-sintering thermal techniques are economic and promising routes for the production of different Fe-M alloys. The different factors affecting the rate of reduction such as temperature and ratio of doping materials are investigated. The results obtained are used to demonstrate the kinetics and mechanisms of reduction of metal oxides.