Phase borides

The two-phase boride/carbide layer described in item 1 extends downward by reaction as the boron-rich liquid below it is drawn into the porous boron carbide by capillary action. Boron and carbon rapidly diffuse through this thin B-rich liquid layer, causing the two-phase layer to thicken as the directed reaction proceeds. 

Five phases of titanium boride have been reported. TiB2 [12405-65-35] Ti2B [12305-68-9] TiB [12007-08-8] [ 2447-59-5] and TiB 2 [51311-04-7]. 

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. 

Preparation. The simplest method of preparation is a combination of the elements at a suitable temperature, usually ia the range of 1100—2000°C. On a commercial scale, borides are prepared by the reduction of mixtures of metallic and boron oxides usiag aluminum, magnesium, carbon, boron, or boron carbide, followed by purification. Borides can also be synthesized by vapor-phase reaction or electrolysis. 

Figure 6.S Frequency of occurrence of various stoichiometries among boride phases ...

The structures of metal-rich borides can be systematized by the schematic arrangements shown in Fig. 6.6, which illustrates the increasing tendency of B atoms to catenate as their concentration in the boride phase increases the B atoms are often at the centres of trigonal prisms of metal atoms (Fig. 6.7) and the various stoichiometries are accommodated as follows ... 

Schrey, F., Gallagher, P., and Levy, R., Selective Silicide or Boride Film Formation by Reaction of Vapor Phase TiC with Silicon or Boron, J. Electrochem. Soc., 1137(5) 1647-1649 (May 1990)... 

The present status of information about phase equilibria and the formation of borides is summarized in Table 2 (binary borides) and in Figure 2a, b (ternary systems) ". ... 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

Figure 2. Formation of ternary borides and phase equilibria within ternary boride systems of the type M-M-B or M-Y-B (M = metal, Y = honmetal). , complete isothermal section established B, part of a diagram only. Numbers in the lower part of each square correspond to the refs, to the ternary section. The number of ternary compounds observed is indicated in the right upper corner of each square.

Compounds isotypic with the k phases arc found among intcrmetallics, borides, carbides and oxides and also with silicides, germanides, arsenides, sulfides and sclcnides no nitrides, however, are found. The mode of filling the various voids in the metal host lattice of the k phases follows the schemein Ref. 4 and is presented in Table 1 for all those compounds for which the atom distribution is well known from x-ray or neutron diffraction. Accordingly, B atoms in tc-borides, Zr, Mo, W, Re)4B and Hfy(Mo, W, Re, Os)4B , occupy the trigonal prismatic interstices within the parent metal framework of a Mn, Aln,-type structure (see Table 1 see also ref. 48). Extended solid solutions are found for (Hf, Al)[Pg.140]

Considering the mode of filling the voids in the metal framework of rj phases with the Ti2Ni type (see Ref. 1) (Table 1), Re3Al2B is the only boride member of this group, with B atoms entering the large icosahedral center in 16d, occupied by metal... 

Formation of tetrahedral My4 clusters is the structural unit common to all known MRgMT4B4-type phases with strong M.,—B, but weak B-M g interaction in accord to the characterization of borides. Involving two remote Mre atoms, the B... 

Despite the occurrence of binary AIB2 borides (see also Fig. 2), no ternary representatives are known (Mn, Mo)B2 found from isothermal sections is a stabilized high-T phase by conversion to lower T by a statistical ( ) metal-metal substitution. Both MnB2 and M0B2 are high-T compounds stable above 1075°C and 1517°C respectively WB2 is claimed but is either metastable or impurity stabilized. Similar examples are observed with (W, Pd>2B5 (M02B5 type) as well as (Mo, Rh),, (B3 and (W, Ni), B3 (Mo,., 83 type). The phase relations in the B-rich section of the Mo(W)-B binaries, however, are not known precisely. 

Little information is available on homogeneity ranges and defect structures in the dodecaborides. The only variation from stoichiometry in these borides is for YB,2i the limiting phase determined by density measurements is Yq92B,2. This result can be attributed to the size of Y which is the maximum for metals that form the dodecaborides. Attempts to prepare DyB,2 with a nonstoichiometric composition are conclusive. ... 

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