Process for producing transition metal boride fibers

A process for producing fibers of a transition metal boride, which comprises reacting a mixture of a vapor of an evaporable boron compound an a vapor of an evaporable transition metal compound in the presence of a catalyst composed of at least one metal selected from the group consisting of Pt, Pd, Cu, Au and Ni.

This invention relates to a process for producing transition metal boride 
fibers. 
Fibrous transition metal borides are useful as a heat-resistant 
high-tenacity fibrous material and fibers for reinforcing heat-resistant 
alloys. They can also be used as an anticorrosive material because of 
their chemical stability and as an electrically conductive material 
because of their high electrical conductivity. 
Processes have previously been known for producing thin films, bulk 
crystals and powders of transition metal borides. 
The transition metal boride films are produced by a vapor-phase reaction 
method, which, for example, comprises introducing a vapor of a transition 
metal halide, a vapor of a boron halide, and hydrogen gas into an open 
tube provided with an alumina or graphite plate as a substrate, and 
simultaneously heating the substrate to reduce the transition metal halide 
vapor and the boron halide vapor with hydrogen gas to deposit a thin film 
of the transition metal boride on the substrate [see, for example, H. O. 
Pierson, E. Randrich and D. M. Mattox: J. Less-Common Met., 67 (1979) 381; 
and J. Bouix, H. Vincent, M. Boubehira and J. C. Viala: J. Less-Common 
Met., 117 (1986) 83]. 
The transition metal boride bulk crystals are produced by the reaction of a 
transition metal with boron in a molten Fe, Co, Ni or Al flux containing 
the transition metal and boron with a stoichiometric composition [see, for 
example, A. A. Abdel-Hamid: J. Cryst. Growth, 71 (1985) 744; and K. Nakano 
and H. Hayashi: J. Cryst. Growth, 24/25 (1974) 679]. This method gives 
single crystals or polycrystals having a size of several mm at a maximum. 
The transition metal boride powders are produced by the direct reaction 
method which comprises heating a transition metal and boron mixed in a 
stoichiometric proportion to at least 1000.degree. C. in vacuum to react 
them directly [see, for example, K. Miyata and A. Sawaoka: Journal of the 
Ceramic Industry Association, 90 (1982) 610; and Japanese Laid-Open Patent 
Publication No. 270348/1986]. 
The above methods cannot produce transition metal boride fibers. 
It is an object of this invention to provide a process for producing fibers 
of a transition metal boride which have not been obtained heretofore. 
The present inventors made extensive investigations in order to achieve the 
above object, and found that when in the production of a transition metal 
boride by vapor-phase reaction of an evaporable boron compound and an 
evaporable transition metal compound, at least one metal selected from the 
group consisting of Pt, Pd, Cu, Au and Ni is used as a catalyst, the 
transition metal boride can be grown into a fibrous form. 
The present invention thus provides a process for producing fibers of a 
transition metal boride, which comprises reacting a mixture of a vapor of 
an evaporable boron compound and a vapor of an evaporable transition metal 
compound in the presence of a catalyst composed of at least one metal 
selected from the group consisting of Pt, Pd, Cu, Au and Ni. 
Examples of the evaporable boron compound are boron halides, diborane and 
trialkylboranes. 
Examples of the evaporable transition metal compound are halogen compounds 
of transition metals such as Ti, Zr, Hf, V, Cr, Nb, Mo, W, Sc, and Ta. 
Halogen compounds of Ti, Zr and Hf are preferred. Other transition metal 
compounds may be used so long as they can be evaporated. 
Preferably, the metal catalyst is in the form of fine particles with a 
diameter of 10 .ANG. to 100 .mu.m. The diameter of the resulting metal 
boride fibers depends upon the diameter of the fine particles of the metal 
catalyst. Thus, when it is desired to obtain fibers with a diameter of 
several hundred .ANG., it is preferred to use fine particles having a 
particle diameter of several tens of .ANG. as the catalyst. The metal 
catalyst may be in the form of a thin film if it has such a thickness that 
at the temperature of the vapor phase reaction, fine molten metal droplets 
can be formed. Preferably, the metal catalyst is supported on a substrate 
such as graphite or alumina, and provided in a reaction chamber. The 
supporting may be effected by simply placing the fine particulate metal 
catalyst on the substrate, or depositing it in fine particles or a thin 
film on the substrate by vacuum evaporation, sputtering, plasma CVD, etc. 
Alternatively, it is possible to introduce a decomposable metal compound 
containing the element of the catalyst together with a carrier gas into a 
reaction tube and form a fine particulate metal catalyst concurrently with 
the vapor-phase reaction. 
In the present invention, a binary or ternary alloy composed of one metal 
selected from Ni, Pd and Pt and one or more constituent elements of the 
metal boride to be produced may be used as catalyst. Examples of the 
binary or ternary alloys include Ni-B, Pd-B, Pt-B, Ni-Ti, Pd-Ti, Ni-Ti-B, 
Pd-Ti-B and Pt-Ti-B. 
Preferably, these alloys have such a composition that the alloy lump is 
mechanically brittle and easily crushable and can be easily converted to a 
fine powder by a crushing method or a grinding method. The Ni-B alloy is 
30-90 atomic % Ni-B, preferably 50 atomic % Ni-B. The Pd-B alloy is 3-90 
atomic % Pb-B, preferably 70 atomic % Pd-B. The Pt-B alloy is 20-80 atomic 
% Pt-B, preferably 60 atomic % Pt-B. The Ni-Ti alloy is 13-92 atomic % 
Ni-Ti, preferably 25 atomic % Ni-Ti. The Pd-Ti alloy is 10-70 atomic % 
Pd-Ti, preferably 33 atomic % Pd-Ti. The Pt-Ti alloy is 10-40 atomic % 
Pt-Ti, preferably 16 atomic % Pt-Ti. 
Preferably, ternary alloys such as Ni-Ti-B, Pd-Ti-B and Pt-Ti-B have a 
composition obtained by mixing the above binary alloys. 
These alloys are mechanically brittle and easily crushable. Hence, the 
alloys are converted into fine or ultrafine particles, and placed on a 
substrate which is placed in a reaction tube or in a vessel. They may 
alternatively be fed into a reaction tube as an aerosol together with the 
reaction gas or a carrier gas.

In FIG. 1, the reference numeral 1 represents an evaporator for evaporating 
the transition metal compound; 2, an evaporator for evaporating the boron 
compound; and 3 and 4, carrier gas introducing openings. H.sub.2, Ar, 
N.sub.2 or He may, for example, be used as the carrier gas. The reference 
numeral 5 represents a constant-temperature vessel for controlling the 
vapor pressure of the starting gas. 
Vapors of the transition metal compound and the boron compound obtained at 
the evaporators 1 and 2 are mixed, and the gaseous mixture is introduced 
into a reaction tube 6. The reference numeral 7 represents a metal 
catalyst. 
A substrate 8 for supporting the metal catalyst, such as graphite or 
alumina, is provided in the reaction tube 6. A heating device 9 is 
provided for heating the reaction tube 6. The heating is carried out by an 
electrical resistance heating method, an infrared heating method or an 
electromagnetic induction heating method, for example. The heating 
temperature is about 800.degree. to 1,500.degree. C. 
The exhaust gas from the vapor-pase reaction is discharged from a discharge 
port 11 through a trap 10. A gas discharge device 12 is provided for 
purging of the reaction tube 6. It is used to replace the inside 
atmosphere of the reaction tube 6 by the reaction gas at the start of the 
vapor-phase reaction and replace the residual exhaust gas in the reaction 
system by Ar, N.sub.2 or He gas at the end of the reaction. 
The mechanism by which the transition metal boride fibers grow by the 
process of this invention is presumed to be as follows. As shown in FIG. 
2-a, the transition metal boride 13 predominantly deposits on the fine 
particulate metal catalyst 7, and diffuses on the surface or interior of 
the fine particulate metal catalyst. It is accumulated between the fine 
particles and the substrate to form a layer of single crystals. Repetition 
of this process pushes the fine metal particles upwards, and transition 
metal boride fibers 14 are formed beneath them (FIG. 2-b). The reference 
numeral 15 represents a hollow area. It will thus be understood that the 
diameter of the fibers corresponds to the diameter of the catalyst 
particles. 
FIGS. 3 and 4 show another embodiment of the process of this invention. In 
these figures, the reference numeral 19 represents a heating device for 
heating a substrate 18 for supporting a metal catalyst 17. The heating 
device may be, for example, an infrared heating furnace or an 
electromagnetic induction heating furnace, or the heating may be carried 
out by directly passing an electric current through the substrate. A 
reaction tube 16 may be made of a suitable material selected depending 
upon the method of heating. Specifically, when the infrared heating method 
is used, it is made of a material easily permitting transmission of 
infrared rays, such as transparent quartz. For the electromagnetic 
induction heating method, a material which easily permits transmission of 
electromagnetic waves, such as transparent quartz and alumina may be used. 
The reaction tube 16 is preferably a cylindrical hollow tube, but may be 
of the bell-jar type. Its shape is not limited. The substrate 18 
supporting the metal catalyst is set up so as to be thermally insulated 
from the reaction tube 16. The insulating method is to place the substrate 
18 on a supporting stand 21 of a material having a low thermal 
conductivity, such as quartz or alumina, or to thrust a rod made of a 
material having a low thermal conductivity from the reaction tube 16 and 
causing it to support the substrate 18, or to use a thermocouple 22 as the 
supporting rod. These are only illustrative, and any method which effects 
thermal insulation may be used. The reaction tube 16 is cooled from 
outside by a cooling device 23. The method of cooling may be, for example, 
water cooling or gas jet cooling. When the infrared heating method or the 
electromagnetic induction heating method is used, it is necessary not to 
obstruct the incidence of the energy of infrared rays or electromagnetic 
waves. 
The reaction product is thus prevented from adhering to and building up on 
the inside wall of the reaction tube by thermally insulating the substrate 
supporting the metal catalyst in the inside of the reaction tube from the 
reaction tube, providing a heating source for heating only the substrate 
selectively, and providing a cooling device for forcibly cooling the 
reaction tube. Consequently, the proportion of the starting material 
utilized increases, and the reaction tube can be operated for a long 
period of time without the need to clean it. As a result, the efficiency 
of producing the metal boride fibers increases. 
The process of this invention yields the transition metal boride fibers, 
which have not heretofore been produced, with good efficiency. Transition 
metal boride fibers having a diameter of several tens of .ANG. to several 
microns, and a length of several microns to several millimeters, can be 
obtained by the process of this invention. The resulting fibers are useful 
as a heat-resistant high-tenacity fibrous material and fibers for 
reinforcing heat-resistant alloys. They can also be used as an 
anticorrosive material because of their chemical stability and as an 
electrically conductive material because of their high electrical 
conductivity. 
The binary or ternary alloy composed of the catalytically active metal and 
the constituent elements of the metal boride to be used as the catalyst in 
the process of this invention can be easily converted into fine particles 
by pulverization. The fine particulate catalyst can be used by placing it 
on a catalyst supporting substrate or in a vessel, or by feeding it into 
the reaction tube as an aerosol together with the reaction gas or a 
carrier gas. Accordingly, it is not necessary to deposit the catalyst on 
the substrate by vapor deposition, sputtering or plasma CVD. Accordingly, 
no expensive depositing device nor complex operation is required, and the 
cost of production can be curtailed. 
The following examples illustrate the present invention in greater detail. 
EXAMPLE 1 
The apparatus shown in FIG. 1 was used. TiCl.sub.4 was fed into evaporator 
1, and BBr.sub.3 was fed into evaporator 2. Constant temperature vessel 5 
was maintained at 30.degree. C. The resulting starting gases were 
introduced into reaction tube 6 together with H.sub.2 gas introduced from 
gas introducing inlet 4. 
The partial pressures of the reaction gases in the reaction tube were as 
follows: PTiCl.sub.4 =15.8 torr, PBBr.sub.3 =89.l torr, PH.sub.2 =655 
torr. The total gas flow rate was maintained at 200 ml/min. 
A graphite plate was used as substrate 8, and fine Pt particles having a 
particle diameter of 50 to 1000 .ANG. were used as metal catalyst 7. An 
electrical resistance furnace was used as heating device 9. By this 
heating device, the substrate 8 was heated to 870.degree. to 1100.degree. 
C., and the chemical vapor-phase reaction was carried out for 90 minutes. 
Titanium boride fibers grew on the substrate 8. The resulting fibers had a 
diameter distributed within the range of about 100 to 4000 .ANG., and a 
length of several .mu.m (maximum 20 .mu.m, average 8 .mu.m). 
EXAMPLE 2 
The same chemical vapor-phase reaction as in Example 1 was carried out 
except that a Pt thin film vacuum deposited on a graphite plate to a 
thickness of about several hundred .ANG. was used as the metal catalyst, 
and the substrate was heated to 1110.degree. to 1120.degree. C. The 
resulting titanium boride fibers had a diameter distributed within the 
range of 10 .ANG. to 200 .ANG. and a length of several tens of .mu.m 
(maximum 30 .parallel.m, average 20 .mu.m). 
EXAMPLES 3-5 
The same chemical vapor-phase reaction as in Example 1 was carried out 
except that fine particles of Cu, Au or Pd having an average particle 
diameter of 500 .ANG. supported on a graphite plate were used as the metal 
catalyst and the temperature for heating the substrate was changed to 
900.degree. to 950.degree. C. The resulting titanium boride fibers and a 
diameter of about 2000 .ANG. and a length of several .mu.m. 
The frequency of occurrence of fibers was less than in the case of using 
the Pt catalyst. 
EXAMPLE 6 
Powders of Ni, Ti and B were vacuum-sealed in a quartz tube, and heated at 
1000.degree. C. for 24 hours to prepare a 24.5 atomic % Ni-TiB.sub.2 
ternary alloy. The alloy was pulverized into fine particles having a 
particle diameter distributed within the range of 1000 .ANG. to several 
.mu.m. The fine particles as a catalyst were sprinkled on a graphite plate 
and the graphite plate was set up in a transparent quartz tubular reactor 
and heated to 1100.degree. C. by a heating furnace. As materials for 
chemical vapor-phase, TiCl.sub.4 and BBr.sub.3 were used. They were placed 
into evaporators and maintained at 0.degree. C. H.sub.2 gas was passed 
through the evaporators at a rate of 60 cc/min. and 40 cc/min. 
respectively to form saturated gases. The saturated gases were diluted 
with H.sub.2 gases at 100 cc/min. and introduced into the reaction tube. 
The reaction was carried out for 50 minutes under these conditions at 
atmospheric pressure. As a result, TiB.sub.2 fibers having a diameter of 
several hundred .ANG. to several microns and a length of 1 to 2 mm were 
obtained on the graphite plate. When about 1 mg of the catalyst was used, 
the total amount of the resulting fibers was about 300 mg. 
EXAMPLE 7 
By the same method as in Example 6, a fine particulate catalyst composed of 
24.5 atomic % Ni-Ti alloy was prepared, and by using this catalyst, TiB2 
fibers were produced by the same method as in Example 6. 
EXAMPLE 8 
TiB.sub.2 fibers were produced by the same method as in Example 6 except 
that a fine particulate catalyst composed of 50 atomic % Ni-B alloy was 
used as the catalyst, and the temperature of synthesizing the fibers was 
changed to 800.degree. C. The resulting fibers included B fibers having a 
rhombic structure. 
EXAMPLE 9 
TiB fibers were produced by the same method as in Example 6 using each of 
fine particulate catalysts of 33 atomic % Pd-TiB.sub.2 alloy, 33 atomic % 
Pd-Ti alloy and 70 atomic % Pd-B alloy. The resulting fibers had a uniform 
diameter of as small as several hundred .ANG.. 
EXAMPLE 10 
TiB.sub.2 fibers were obtained by the same method as in Example 6 using 
each of fine particulate catalysts of 16 atomic % Pt-TiB.sub.2 alloy, 16 
atomic % Pt-Ti alloy and 70 atomic % Pt-B alloy. 
EXAMPLE 11 
The apparatus shown in FIGS. 3 and 4 was used. An infrared ray image 
furnace adapted for elliptical reflective tubular light-converging and 
having a heating length of 265 mm and a total length of 361 mm was used as 
heating device 19, and a transparent quartz tube having an inside diameter 
of 42 mm and a length of 1000 mm was used as reaction tube 16. A substrate 
18 of graphite was supported on a transparent quartz supporting stand 21 
and thermally insulated from reaction tube 16. A fine particulate Pt 
catalyst 17 was placed on the substrate 18. The measurement and control of 
the reaction temperature were effected by embedding a thermocouple 22 in a 
hole formed in the substrate 18. The outside of the reaction tube was 
cooled with a low temperature N.sub.2 gas jet 23. 
Using this apparatus, vapors of TiCl.sub.4 and BBr.sub.3 were reduced at 
900.degree. C. using H.sub.2 gas as a carrier. The partial pressures of 
the starting materials were as follows: PBBr.sub.3 =89.l torr, PTiCl.sub.3 
=l5.8 torr, and PH.sub.2 =655 torr. The total gas flow rate was maintained 
at 200 ml/min. At this time the stable current in the infrared image 
furnace was 19 A, and the power was 3.8 KW. The pressure at the inlet of 
the low temperature N.sub.2 gas jet was adjusted to 4.0 kg/cm.sup.2. 
Under the above conditions, the chemical vaporphase reaction of the 
starting materials was carried out for 50 minutes to give TiB.sub.2 fibers 
at a rate of about 0.2 g/cm.sup.2 -hour on the substrate. After the end of 
the reaction, no TiB.sub.2 film or reaction product were seen to adhere to 
the inside wall of the reaction tube. 
EXAMPLE 12 
Example 11 was repeated except that 5 mole % of B.sub.2 H.sub.6 diluted 
with H.sub.2 gas was used as the boron compound, a 24.5 atomic % Ni-Ti 
alloy powder was used as the catalyst, and the reaction was carried out at 
1100.degree. C. TiB.sub.2 fibers were obtained which had a length of 
several tens of microns at a maximum and a diameter of several hundred to 
several thousand .ANG.. 
EXAMPLE 13 
By the same method as in Example 6, a fine particulate catalyst composed of 
24.5 atomic % Ni-Zr alloy was prepared, and using this catalyst, ZrB.sub.2 
fibers were produced in the same apparatus as used in Example 11. 
ZrCl.sub.4 was used as the evaporable Zr compound. About 5 g of ZrCl.sub.4 
was placed in the reaction tube about 20 cm away from the inlet of the 
image furnace, and a nichrome wire was wound up over a width of 5 cm on 
the outside wall of the reaction tube at a part corresponding to the part 
of ZrCl.sub.4 to maintain the temperature of the outside wall of the 
reaction tube at about 120.degree. C. BBr.sub.3 as a material for B was 
put in an evaporator kept at 0.degree. C., and H.sub.2 gas was passed 
through the evaporator at 40 ml/min. to form a saturated gas. The 
saturated gas was diluted with 100 ml/min. of H.sub.2 gas and introduced 
into the reaction tube. 
Under these conditions, the reaction was carried out at a temperature of 
1150.degree. to 2000.degree. C. for 120 minutes to give ZrB.sub.2 fibers 
on the graphite plate. 
EXAMPLE 14 
Example 12 was repeated except that 5 mole % of B(CH.sub.3).sub.3 diluted 
with H.sub.2 gas was used as the boron compound, and a 24.5 atomic % Ni-Ti 
alloy powder was used as the catalyst. TiB.sub.2 fibers the same as in 
Example 12 were obtained.