Melting borides

The crystal growth of metal borides by gas-phase methods permits preparation of products at moderate T (1000-1500°C). This is an important advantage since most borides melt at high T (ca. 3000°C), which makes their crystal growth from melts difficult. In addition, the gas-phase methods lead to the formation of single crystals and solid films of incongruently melting borides. 

It is impossible to find inert containers owing to both the high mp of borides (> 2000°C) and their reactivity with refractory materials. The problem is solved by melting borides either in water-cooled boats or pedestals. Water-cooled, metal... 

V. V. Svistunov, Investigation of Preparing and Oxidation Processes of Transition Metals Melted Borides, Thesis, Sverdlovsk, 1975. 

AH of the alloys Hsted in Tables 4 and 5 are austenitic, ie, fee. Apart from and soHd-solution strengthening, many alloys benefit from the presence of carbides, carbonitrides, and borides. Generally the cubic MC-type monocarbides, which tend to form in the melt, are large and widely spaced, and do not contribute to strengthening. However, the formation, distribution, and soHd-state reactions of carbides are very important because of their role... 

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

Borides have metallic characteristics such as high electrical conductivity and positive coefficients of electrical resistivity. Many of them, particularly the borides of metals of Groups 4 (IVB), 5 (VB), and 6 (VIB), the MB compounds of Groups 2(11) and 13(111), and the borides of aluminum and siUcon, have high melting points, great hardness, low coefficients of thermal expansion, and good chemical stabiUty. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Table 1 fists many metal borides and their observed melting points. Most metals form mote than one boride phase and borides often form a continuous series of solid solutions with one another at elevated temperatures thus close composition control is necessary to achieve particular properties. The relatively small size of boron atoms facilitates diffusion. 

Silicides are usually prepared by direct fusion of the elements hut coreduction of Si02 and a metal oxide with C or A1 is sometimes used. Heats of formation are similar to those of borides and carbides but mps arc substantially lower e.g. TiC 3140°, TiBj 2980°, TiSij 1540° and TaC 3800°, TaB2 3100°, TaSi2 1560°C. Few silicides melt as high as 2000-2500°, and above this temperature only SiC is solid (decomp 2700°C). 

Boride Density g/cm Melting Point Point°C Hardness Kg/mm (VHN50) Electrical Resistivity pohm-cm Thermal Conduc. w/cm °C Thermal Expans. 10-6/°C (300- 1000°C)... 

Borides are relatively inert, especially to non-oxidizing reagents. They react violently with fluorine, often with incandescence. Reaction with other halogens is not as violent and may require some heat. Resistance to oxidation, acids, and alkalis is summarized in Table 17.5. In oxidation conditions, a layer of boric oxide is formed on the surface which passivates it to some degree. Boric oxide melts at 450°C and vaporizes at 1860°C. It offers good protection up to 1500°C in a static environments but it has low viscosity at these temperatures and tends to flow under stress and the protection it offers is limited.f k l... 

Only Gd2B5 and Nd2Bg have the SmjBj-type structure . These borides melt peritectieally to give the tetraboride plus a metal-rich liquid. 

A pellet is pressed of an intimate mixture of finely divided reactants and reaction induced either by arc melting and high-T annealing or by solid-state sintering in an electrical or high-frequency furnace. Isolating the borides from reactive container components can be a problem. The use of boron nitride liners has proved effective. In some cases the protective liner is made of sintered boride containing the same elements as the boride in preparation. 

In this method " - the melt eontains boric oxide and the metal oxide in a suitable electrolyte, usually an alkali or alkaline-earth halide or fluoroborate. The cell is operated at 700-1000 C depending on electrolyte composition. To limit corrosion, the container serving as cathode is made of mild steel or of the metal whose boride is sought. The anode is graphite or Fe. Numerous borides are prepared in this way, e.g., alkaline-earth and rare-earth hexaborides " and transition-metal borides, e.g, TiBj NijB, NiB and TaB... 

Crystal growth is relatively difficult for borides because they have high melting points and sometimes low thermal stability, as indicated in 6.7.2. 

In a crystal-pulling procedure using a tri-arc furnace (Fig. 2), a resistor box, a d.c. power supply (300 A, 80/40 V) and a set of water-cooled power cables are used to bring power and water to the electrodes. The upper part of the furnace is equipped with three equally spaced copper cathodes, to which are fixed W-Rh electrodes. The upper part (cathode) is separated from the lower part (anode) by a transparent quartz glass tube. In the bottom of the furnace there is a tapered opening for a water-cooled copper hearth containing the boride melt. All parts of the furnace are also water... 

The production of boride single crystals from their own melt presents difficulties because they have high mp and melt ineongruently. The crystallization of borides... 

Owing to the high sintering temperatures employed, losses of material (by volatilization of boron or boride) and grain growth are observed. In order to limit these losses, the part to be sintered can be embedded in a powder of the same boride. Sintering of pure refractory borides requires > 0.7T, (Tj = absolute melting temper-... 

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