Refractory Metal Borides Research Articles

Soft Chemistry of Hard Materials: Low-Temperature Pathways to Bulk and Nanostructured Layered Metal Borides.

Conspectus"Synthesis by design" is often considered to be the primary goal of chemists who make molecules and materials. Synthetic chemists usually have in mind a target they want to make, and they want to be able to design a pathway that can get them to that target as quickly and efficiently as possible. Chemists who synthesize refractory solids, which have melting points above 1000 °C and are often chemically inert at these high temperatures, have access to only a small number of synthetic strategies due to the need to overcome solid-state diffusion, which is the rate-limiting step in such reactions. The use of extremely high temperatures to facilitate diffusion among two or more refractory solids, which precedes any chemical reaction that must occur, generally drives the system to form only the product that is the most thermodynamically stable-the global minimum on an energy landscape-for a certain combination of elements. When trying to target a different product in the same system, one generally cannot rely on thermally driven reactions. Lower-temperature reactions that side step this diffusion limitation can succeed where high temperatures fail by providing access to local minima on an energy landscape. These local minima represent metastable phases that are primed for synthesis, but only if an appropriate pathway and set of reactions can be identified. It is therefore important to develop and understand low-temperature, or "soft", chemical reactions in "hard" refractory systems. These reactions allow us to apply the retrosynthetic framework that molecular chemists rely on to systems where chemists have not previously had such control over reactions, reactivities, and metastable product formation.In this Account, we discuss the development of soft chemical reactions of hard materials in the context of a class of layered, refractory metal borides that are precursors to an emerging family of two-dimensional nanomaterials. Layered ternary metal boride phases such as MoAlB have layers of metal borides, which are chemically unreactive, interleaved with layers of aluminum, which are reactive. Some of the interlayer aluminum can be deintercalated at room temperature in dilute aqueous sodium hydroxide, transforming stable MoAlB into destabilized MoAl1-xB. Mild thermal treatment of submicrometer grains of this destabilized MoAl1-xB sample allows it to traverse the energy landscape and crystallize as Mo2AlB2, a metastable compound. Further thermal treatment transforms Mo2AlB2 into a Mo2AlB2-alumina nanolaminate and ultimately mesoporous MoB, all through continued traversing of the energy landscape using mild chemical and thermal treatments. Similar topochemical manipulations, which maintain structure but change composition, are emerging for other MAB phases and are opening the door to new types of metastable compounds and nanostructured materials in traditionally refractory systems.

Fe-Al-Si-Type Iron Aluminides: On the Strengthening by Refractory Metals Borides.

The effect of boron addition into Fe–28Al–5Si–X (X = -, 2Mo, or 2Ti) on the structure and high-temperature yield stress was investigated. Generally, the alloying of binary Fe3Al-type iron aluminides by silicon significantly improves high-temperature mechanical properties by solid-solution strengthening. On the other hand, the workability and ductile properties at room or slightly elevated temperatures get worse with the increasing silicon content. Boron alloying together with titanium or molybdenum alloying is one of the ways to improve the workability of this type of alloy and, at the same time, ensure the formation of a sufficient amount of secondary phase particles required for effective strengthening. In this paper, the influence of 1 at. % of boron on high-temperature yield stress is evaluated in response to structural changes and compared with results obtained previously on the same type of alloy (Fe–28Al–5Si–2X, X= -, Mo, or Ti) but without boron alloying. It can be concluded that the network structure of borides of refractory metals formed due to boron alloying works more effectively for alloy hardening at higher temperatures than a mixture of silicides and carbides present in the boron-free alloy of the same composition.

High-Temperature Heat-Resistant Coatings for Protection of Refractory Metals and Their Alloys (Overview)

Scientific publications on the methods of applying heat-resistant coatings to refractory metals, such as molybdenum, tungsten, niobium, tantalum, and their alloys, are analyzed. Vacuum-activated diffusion hardening is considered to be one of the most optimal methods of depositing protective coatings. For niobium and tantalum, a molybdenum or tungsten layer is required to be preliminary applied by one of the three methods: vapor-phase deposition, condensation, or ion bombardment. The materials used for the production of protective coatings are studied: silicides, refractory metal borides, high-temperature oxides, and enamels.

Synthesis of transition metal borides layers under pulsed electron-beams treatment in a vacuum for surface hardening of instrumental steels

The saturation of the surface layers of metals and alloys with boron is conducted for increasing their surface hardness, wear resistance, etc. Multicomponent layers containing in its composition borides of refractory metals, as a rule, are formed by the methods of chemical- thermal processing in the interaction of boriding component with refractory one or by the method of saturation of refractory metal impurities or alloy with boron. In this work, we studied the features of vanadium and iron borides formation on the surface of instrumental steels U8A and R18 under the influence of intense electron beams in continuous and pulse modes.

Layered titanium diboride: towards exfoliation and electrochemical applications.

Layered transition metal diborides (TMDB), amongst other refractory metal borides, are commonly employed for material fabrication such as wear- and corrosion-resistant coatings due to their impressive chemical stability and thermal conductivity. In spite of the wide scope of studies carried out on TMDB in the physical field, investigations on its electrochemistry remain limited. Since the physical properties play a vital role in any material's electrochemical behaviour, we explore the viability of the most popular form of titanium boride, layered TiB2, as catalysts for electrochemical energy reactions, including hydrogen evolution and oxygen reduction. Three types of TiB2 were compared in this work - TiB2 separately modified with sodium naphtalenide and butyllithium in an attempt to exfoliate TiB2 and unmodified TiB2. The electrocatalytic activity displayed by all three TiB2 materials provides a wider range of opportunities for the application of TiB2 in material studies.

Ideal SiC Schottky barrier diodes fabricated using refractory metal borides

Several refractory metal borides were used as Schottky contacts to fabricate n-type 4H-SiC Schottky diodes. The boride contacts were deposited on SiC substrates held at 20 and 600 °C. Contacts deposited on substrates held at 600 °C produced better diodes with smaller ideality factors from 1.05 to 1.10, barrier heights from 0.94 to 1.15 eV, smaller leakage current densities at a reverse bias of 15 V, and smaller specific on-resistances compared to contacts deposited on substrates held at 20 °C, and these values remained essentially unchanged after annealing at 600 °C for 20 min. The Rutherford backscattering spectroscopy spectra of one of these boride contacts revealed a systematic decrease in oxygen with an increase in the deposition temperature. The improved electrical properties and thermal stability make these boride contacts attractive for high power and high temperature device applications.

Synthesis of boride, carbide, and carbonitride whiskers

A chemical route for synthesizing whiskers of refractory metal borides, carbides, and carbonitrides will be presented. Examples of whiskers are TiB 2, B 4C, TiC, TaC, NbC, and Ti x Ta 1− x (C,N). The synthesis technique used for the preparation is carbothermal reduction (CTR) of a starting oxide in combination with a vapor–liquid–solid growth mechanism (VLS) in the temperature range 1150–1800 °C in argon or nitrogen. Synthesis of e.g. TaC whiskers was made with Ta 2O 5 as a source for Ta. Carbon black was used to reduce the oxide, Ni to catalyse the whisker growth. NaCl was used as a source of Cl for vapor phase transportation of Ta-oxochlorides to the catalyst. The catalyst metal was recycled several times during the synthesis and was transported as NiCl 2(g) according to thermodynamic calculations. The rate of whisker formation was found to depend on many parameters e.g. the synthesis temperature, the choice of catalyst, and the atmosphere. Many of the different whisker types were obtained in a yield of 70–90 vol.% with a diameter of 0.5–1 μm and a length of 10–30 μm. The intended use for the whiskers is as reinforcing materials in ceramic composites. Application areas are e.g. ceramic cutting tools.

Plasma spray deposition and high temperature characterization of ZrB 2–SiC protective coatings

Refractory metal borides are the object of special interest for aerospace applications requiring properties of chemical and mechanical resistance in ultra high temperature, such as nose and leading edges of re-entry space vehicles. The main objective of the research is the fabrication and characterization of plasma sprayed zirconium diboride–silicon carbide composite coatings and free-standing components for high temperature applications. High and low pressure plasma spray in a controlled atmosphere were selected as manufacturing techniques for the deposition of ceramic coatings. Fine ZrB 2 and SiC precursors were agglomerated and preconsolidated into spherical, hollow powders for better flowability and silicon carbide thermal protection during the interaction with the plasma. Coatings and free-standing tubular specimens were fabricated and tested for high temperature behaviour. Thermogravimetric analysis, surface morphology investigation and high temperature X-ray diffraction showed that the addition of approximately 25% SiC induces a mechanism of self-protection of the ceramic material during heat treatment in oxidizing environments up to approximately 2100 K.

Microwave diodes with contact metallization systems based on silicides, nitrides and borides of refractory metals

Microwave diodes with contact metallization systems based on silicides, nitrides and borides of refractory metals

Electrode Reactions of Borides of Refractory Metals in Fluoroborate Melt

Electrode Reactions of Borides of Refractory Metals in Fluoroborate Melt

Properties of Refractory Metal Borides and Carbides Deposited by Electrochemical Synthesis from Molten Salts

Properties of Refractory Metal Borides and Carbides Deposited by Electrochemical Synthesis from Molten Salts

The oxidation of dispersed refractory metal compounds and their behavior as carbon oxidation catalysts

The oxidation behavior of a number of finely dispersed refractory metal borides, carbides, nitrides and silicides has been studied in flowing air at temperatures up to 1000°C. Most of the materials investigated were rapidly converted to oxides under these conditions, but the carbide, nitride and silicide of chromium were more oxidation resistant. Several of these compounds (particularly those of V and Mo), were very active catalysts for the oxidation of graphite in this temperature range, but the presence of boron oxide reduced gasification rates in most cases. Compounds of Zr, Si, Al, Ti, Hf and Nb had little or no catalytic activity for graphite oxidation, whereas the borides of Zr, Si, Cr and Al exerted an inhibiting effect.

The application of refractory metal borides as electrodes in electrospark machining

The properties (the phase composition, the strength, the hardness, the wear resistance and the oxidation resistance) of alloys based on the borides W 2B 5 and Mo 2B 5 using Permalloy or Nichrome as the binder were investigated. The test results of the alloys used as electrodes in electrospark alloying of a Y-8 steel surface are described. The optimum binder percentage for alloy electrodes was determined. The coating obtained was subjected to an oxidation resistance test at 1000 °C.

Thermally stable composite materials based on borides of refractory metals

Thermally stable composite materials based on borides of refractory metals

Reactions of lanthanum hexaboride with carbides and borides of refractory metals

1. A study was made of the solid-phase reactions of lanthanum hexaboride with the monocarbides (TiC, NbC, WC) and diborides (TiB2, NbB2) of the d transition metals. 2. It was established that the reactions of LaB6 with me carbides are accompanied by the destruction of the Me-C bonds and formation of the thermodynamically more favorable Me-B bonds. In the system LaB6-NbC, the niobium monocarbide fully decomposes and the resultant structure consists of two phases, based on LaB6 and NbB2. The structure in me system LaB6-TiC is composed of LaB6, TiB2, and TiC base phases, and that in the system LaB6-WC, of LaB6, W2C, andα-WB base phases. 3. The increased microhardness of the LaB6-base phases in all the systems investigated is a consequence of dissolution of the d transition metals in the lanthanum hexaboride.

Intensity of recombination of charged particles on some borides of refractory metals in a weakly ionized hydrogen plasma

Intensity of recombination of charged particles on some borides of refractory metals in a weakly ionized hydrogen plasma

Low temperature specific heat study of the electron transfer theory in refractory metal borides

Low temperature heat capacities of eleven 4th, 5th and 6th group transition metal borides were measured from 1.5–18°K to test current theories about bonding and band structure of borides. The electron transfer theory helps to interpret γ values, but the experiments also suggest certain modifications to that theory.

Measurement of the abrasive capacity of powders of carbides and borides of refractory metals

The abrasive capacity of carbides and borides of titanium, zirconium, hafnium, vanadium, chromium, tantalum, molybdenum, and tungsten was investigated.

Porous materials from carbides and borides of the transition metals

As a result of an investigation, procedures were developed for producing porous materials from carbides and borides of refractory metals by sintering freely poured spheroidized powders and precompacted blanks. Process parameters for the production of parts of the required porosity and permeability were determined. A study was made of the permeability of porous materials from carbides and borides of refractory metals.