High
Temperature
Chemistry
of
the
Binary Compounds of Boron
Downloaded by COLUMBIA UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch007
PAUL W. GILLES Department of Chemistry, University of Kansas, Lawrence, Kan.
The refractory binary compounds of boron with some of the transition metals have many attractive properties. They are high melting, high boiling, good conducting, chemically inert, and thermodynamically very stable compounds. Their methods of preparation, their thermodynamic, chemical, and physical properties, and their uses are listed, described, and discussed. Their phase relationships are unusual, for there are frequent cases of extensive solid solubility and narrow temperature ranges of stability. The present status of the field is assessed.
The high melting points, the high boiling points, the extreme hardness, the chemical inertness, the high electrical and thermal conductivity, the high thermal neutron cross section for boron-10, and the low cross section for boron-11 make the metallic borides materials of great current interest. M o s t of the metals form binary compounds with boron, and many of them are of high temperature interest. This discussion is concerned with the properties of these refractory borides, most of which contain the transition elements. They are sometimes called "refractory hard metals," and are frequently discussed along with carbides, nitrides, suicides, sulfides, and sometimes oxides. Boron carbide and boron nitride have refractory properties and frequently are discussed with the metallic borides. A s one examines the literature, he finds that the refractory borides have been studied i n a systematic fashion only i n the past two decades and that many of their properties are not known very well. F o r example, the compositions of many of the phases may not be known even at-the present time, and many of the melting points may be uncertain b y several hundred degrees. Reviews of the preparation and properties of the refractory borides have been written b y Schwarzkopf and Kieffer (25), Powell (21), Powell, Campbell, and Gonser (22), and Samsonov and Markovskiï (24). The important phase relationships and crystallographical data have been collected b y Hansen (8) and b y Pearson (17). T h e present discussion reviews some of the important chemistry of the metallic borides and indicates areas of present and future work.
General Features The characteristic properties of these refractory borides, i n addition to those mentioned earlier, are that they are good electron emitters ; they are thermodynamically very stable compounds; they are generally not soluble i n acids but are soluble i n 53 In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
54
ADVANCES IN CHEMISTRY SERIES
alkaline or fused alkaline media; they are fairly resistant to oxygen attack; and the systems show a wide variety of phases with occasional wide solid solubility. The phases that have been reported are indicated i n Table I , and some of the properties are listed i n Table I I . Phases of Metallic Borides (8, 16, 21, 24, 25)
Table I. MiB
M i ß
Be
MaB MIBJ x
M B
MB«
M1B4
Downloaded by COLUMBIA UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch007
Mg Ca Sr Ba
M B x 2
MB4
f
x
Al Y La RE Ao TJ
M Be x x? x x x
x
Ti Zr Hf
x
x x
V Nb Ta
x x
Cr Mo W
x
Mn Re
x
x x x
x x
x x x
x
x
x x
x x x x x
x
x x
Other
x?
x x x
x
MBit
x x
x x x
x x x
x x x
x x x
x
x
x x x
x
Cr»Bi x x
x x?
Fe Ru Os
x x
Co Rh Ir
x
x x x
x
x?
RusBj O84B9?
x x? x
Ni Pd Pt
x x
x
x
x x? x?
x
x
Ni B,? 2
x?
Cu
C11B21
Preparation The metallic borides are almost always prepared i n the laboratory b y synthesis from the elements or b y the reduction of the metal oxide with boron. M o s t generally, laboratory preparations of pure material from the elements are accomplished by heating pressed mixtures of powdered metal and boron. Considerable care must be exercised i n the preparation of borides i n this fashion. T h e reaction is strongly exothermic, and at temperatures ranging from 900° to 1200°C. reaction begins, and there is a very rapid rise i n temperature. V e r y frequently the materials are scattered throughout the apparatus, and the crucible is destroyed. The purity of samples prepared i n this way varies with the purity of the starting materials. M a n y of the more volatile impurities, such as iron, which is present i n commercially available pure boron, will distill out of a sample i f the product is heated to 2000°C. O n the other hand, other impurities, such as carbon, cannot be removed simply b y evaporative techniques. Another way of preparing pure borides i n the laboratory is b y the boron reduction of oxides, provided that there are no ternary compounds-. T h e reaction proceeds smoothly with the loss of B 0 ( g ) or B 0 ( g ) . Boron carbide may be used as the reducing agent, b u t contamination b y carbon probably occurs. Commercial preparation of metallic borides is very well summarized and described by Powell (21). The methods include the reduction of metal oxide-boric oxide m i x tures b y carbon or active metals; the vapor plating of the boride on a hot metallic surface from a boron-containing gas; the vapor plating of the boride on a hot surface 2
2
2
3
In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
GILLES
Binary Boron Compounds Table II. Phase
Properties of Refractory Borides (8, 21, 24, 25) Melting Point, ° C .
MgBs MgBu CaB« SrB. BaBc
