Ceramic fibre and process therefor

A process is described for producing ceramic fibres composed of titanium boride, zirconium boride or hafnium boride. The boride fibres are obtained by reacting a boron oxide precursor fibre with a titanium halide, zirconium halide or hafnium halide gas in the presence of hydrogen. Ceramic titanium, zirconium or hafnium nitride fibres may also be produced by the process, by means of the additional presence of nitrogen gas in the gas phase. The process is conducted at temperature higher than 500.degree. C.

FIELD OF THE INVENTION 
This invention relates to the field of fibres, more specifically the field 
of predominantly non-oxide ceramic fibres. 
BACKGROUND TO THE INVENTION 
Fibres are often combined with other materials to form composites thereby 
increasing the mechanical strength of such materials. Fibres may be made 
of inorganic or organic material or sometimes a combination of the two. 
Whiskers are sometimes considered to be synonymous with fibres, but it is 
generally understood that whiskers have smaller diameter and shorter 
lengths than fibres. 
In the following discussion fibres are considered to be elongated 
substances having chemically substantially homogeneous composition, and 
having an average diameter and an average length. The ratio of the average 
diameter of the type of fibre under consideration, to its average length 
is usually substatially greater than 5. Such ratio is sometimes referred 
to as aspect ratio. It is considered that a fibre may be a single 
elongated substance forming a relatively long continuous thread, or it may 
consist of several shorter fibres spun or stuck together to provide a 
fibre of more substantial length. 
Ceramic materials are characterized by having high melting points, are 
often refractory and are generally resistant to oxidation and corrosion. 
Ceramic fibres and whiskers have desirable properties such as high melting 
point, substantial physical strength in relation to their weight, 
relatively high modulus, good shape retention, resistance to oxidation, 
moreover ceramic fibres may often be obtained from relatively inexpensive 
materials. There are many structural applications where ceramic fibres can 
be usefully incorporated. For example, ceramic fibres such as alumina 
fibres, are frequently used for reinforcing materials when properties such 
as those listed above, are required. The average diameter or core 
dimension of desirable ceramic fibres range from a few microns, or even a 
fraction of a micron, to as wide as a millimeter. 
In some instances of commercial utilization of fibres high strength 
combined with low electrical resistivity are required. Some transition 
metal borides and nitrides such as titanium boride, hafnium boride, 
zirconium boride, as well as titanium nitride, hafnium nitride and 
zirconium nitride, are ceramics known to have low electrical resistivity, 
and hence the above transition metal boride and nitride fibres are 
suitable and desirable in such commercial applications. 
There are conventional methods for obtaining oxide, carbide and nitride 
fibres, by extruding or spinning ceramic oxide, carbide or nitride based 
particles carried in a viscous solution or by a low melting point organic 
substance. The extruded or spun fibres containing ceramic particles are 
subsequently subjected to heat treatment to evaporate the solvent and/or 
decompose the organic carrier. The extruded or spun fibres prior to the 
heat treatment are sometimes referred to as precursor fibres. It is 
generally observed that the elimination of the carrier substance leaves 
voids in the ceramic fibres so obtained, thus the coherence of the ceramic 
fibres produced by conventional methods is usually low, and consequently 
the mechanical strength and modulus of such fibres are low or only 
moderate. In order to increase the strength and coherence of ceramic 
fibres obtained by conventional methods, high temperature sintering, such 
as in excess of 1700.degree. C., is required. High temperature sintering 
process steps are likely to increase the cost of production of 
conventional ceramic fibres substantially. 
It is also known to grow ceramic fibres between electrodes in an electrical 
field, but such methods are unlikely to produce ceramic fibres in lengths 
and quantities which are required in commercial utilization. 
Pyrolysis of organic fibres or similar carbon rich filaments to provide 
carbon fibres has been practised for several decades. It is known to 
obtain silicon carbide fibres, for example, by a process in which 
polycarbosilanes are subjected to pyrolysis. 
Yoshiharu Kimura in U.S. Pat. No. 5,061,469, describes a process for 
producing boron nitride fibres by reacting an amine with a borazine 
compound. It is also known to obtain a composite fibre by providing a 
coating of titanium boride on tungsten fibres. 
There is a need for a method to produce coherent and substantially 
pore-free transition metal nitride and boride fibres without the 
application of expensive high temperature sintering steps, which could be 
utilized in obtaining the fibres in commercially required lengths and 
quantities. 
By one aspect of the invention described hereinbelow, a method is provided 
whereby polycrystalline titanium boride, hafnium boride and zirconium 
boride fibres are obtained by reacting continuous boron trioxide precursor 
fibres at moderately high temperature, in a non-oxidizing atmosphere, with 
a transition metal halide and hydrogen, which are optionally carried by an 
inert gas. 
By another aspect of the present invention a method is described whereby 
polycrystalline titanium nitride, zirconium nitride and/or hafnium nitride 
fibres are obtained by reacting continuous boron trioxide precursor fibres 
at moderately high temperature, in a non-oxidizing atmosphere, with a 
transition metal halide and nitrogen gas, in the presence of hydrogen.

DETAILED DESCRIPTION OF THE INVENTION 
The preferred embodiment of the present invention will now be described by 
reference to the Figures. 
The present method for obtaining a titanium boride (TiB.sub.2) ceramic 
fibre utilizes a boron oxide (B.sub.2 O.sub.3) precursor fibre. It is to 
be understood that a similar method may be practised in order to obtain 
zirconium and hafnium boride ceramic fibres by the substitution of the 
appropriate transition metal halide in the subsequent reaction with the 
precursor fibre. 
A precursor fibre is defined in the scope of the present invention, as that 
which has similar shape and dimension, more particularly similar diameter 
and length, as the boride fibre to be obtained. The precursor fibre serves 
as a temporary substrate during the formation, and is converted 
substantially in its entirety to the desired transition metal boride or 
transition metal nitride, as the case may be. The precursor fibre utilized 
by the present process contains no organic component to be eliminated in a 
subsequent process step. 
Boron oxide of suitable purity is melted by appropriate means and kept 
above its melting point (460.degree. C.). Fibres of boron oxide are then 
obtained by pulling a continuous thread from the molten boron oxide, or 
forcing the melt through a small aperture, or similar conventional 
techniques producing coherent and usually continuous fibres of boron 
oxide. The diameter may be as small as a fraction of a micron (10.sup.-6 
m), ranging to a diameter of a millimeter. The length of the fibre is 
determined by convenience. 
The boron oxide fibre so obtained may be allowed to cool and solidify, or 
alternatively, is immediately reacted with a gaseous substance which forms 
an additive compound on the surface of the boron oxide. The requirement of 
the additive compound formed temporarily, is that it has a melting point 
which is notably higher than the melting point of the boron oxide, thereby 
encasing the molten boron oxide and stabilizing the shape of the molten 
oxide above its melting point. The gaseous substance is preferably ammonia 
gas or an ammonia derivative, or a mixture of nitrogen and hydrogen. The 
compound preferably forms a skin of sub-microscopic thickness around the 
precursor fibre. The thickness of the skin temporarily encasing the boron 
oxide is of no significance, however it is important that the skin made of 
the temporary additive compound be continuous. 
The boron oxide fibre formed as a continuous filament may be encased while 
still hot, or the cooled fibre, either as a long continuous filament or in 
the shape of shorter pieces of fibre may be reheated, and encased by a 
skin of an additive compound formed at a temperature initially below the 
melting point of B.sub.2 O.sub.3. 
In one of the preferred embodiments ammonia gas is passed over the boron 
oxide fibre at a slightly lower temperature than its melting point. The 
ammonia gas may be carried by an inert gas or, as mentioned above, it may 
also be an appropriate mixture of nitrogen and hydrogen. 
The boron oxide fibre which is now encased, that is its shape has been 
stabilized, is subsequently reacted with a titanium halide bearing gas at 
a temperature above the melting point of boron oxide. The preferred halide 
is titanium tetrachloride (TiCl.sub.4), but titanium bromide (TiBr.sub.4), 
titanium iodide (TiI.sub.4) may also be used. The reaction between the 
stabilized boron oxide precursor fibre and titanium tetrachloride takes 
place in the presence of hydrogen to form titanium boride, hydrogen 
chloride and water. The reaction may be represented by the following 
equation: 
EQU B.sub.2 O.sub.3 +TiCl.sub.4 +5H.sub.2 .fwdarw.TiB.sub.2 +4HCl+3H.sub.2 O(1) 
The reacting gases may be carried by an inert gas such as for example, 
argon or helium. The reaction temperature may conveniently be 
1000.degree.-1200.degree. C., but it may be as low as 500.degree. C., or 
as high as 1400.degree. C., dictated by convenience only. 
The diameter of the titanium fibre obtained by the present process is 
governed by the diameter of the boron oxide precursor fibre. The length of 
the titanium boride fibres is decided by convenience. Irrespective of the 
diameter and length, the fibres will be polycrystalline and substantially 
pore-free. 
The above process may be conducted as a continuous process or as a batch 
process. 
Zirconium boride fibre may be obtained by reacting the encased boron oxide 
precursor fibre with a zirconium halide gas in the presence of hydrogen 
optionally diluted with an inert gas. The by-product gases will be 
hydrogen halide gas and water, as in the above reaction (1). 
Similarly, hafnium boride fibre may be obtained by reacting a hafnium 
halide gas with boron oxide precursor fibre in the presence of hydrogen 
and optionally in an inert gas, under conditions similar to those 
described hereinabove. 
Essentially the same process is used to obtain ceramic nitride fibres, 
however, the additional presence of nitrogen gas in the reacting gases is 
required. Thus titanium nitride is obtain by reacting the encased boron 
oxide precursor fibre at a temperature above the melting point of boron 
oxide with a titanium halide gas, such as titanium tetrachloride, in the 
presence of hydrogen and nitrogen, according to the equation summarizing 
the reactions: 
EQU B.sub.2 O.sub.3 +2TiCl.sub.4 +4H.sub.2 +N.sub.2 .fwdarw.2TiN+2BCl.sub.3 
+2HCl+3H.sub.2 O (2) 
The reacting titanium tetrachloride, hydrogen and nitrogen may be carried 
by argon or a similar inert gas, such as helium, but this is not 
essential. 
As it has been referred to hereinabove, zirconium nitride fibres or hafnium 
nitride fibres may be obtained from a shape-stabilized boron oxide 
precursor fibre by utilizing a zirconium halide or a hafnium halide gas in 
the presence of hydrogen and nitrogen. 
It is preferable that the gases reacting with the boron oxide fibre are 
flowing, in order to diminish the likelihood of hydrolysis of the reaction 
by-products. The presence of an oxygen containing gas in the reaction zone 
is preferably kept at a minimum, so that titanium dioxide formation/or 
zirconium dioxide or hafnium dioxide as the case may be/as a side-product 
is avoided and the direction of the reaction is not reversed, resulting in 
boron trioxide reformation. 
In the following, working examples of the process described hereinabove, 
are provided. 
EXAMPLE 1 
About 500 grams of pre-dried technical grade boron oxide (B.sub.2 O.sub.3) 
was heated above its melting point, that is above 460.degree. C. 
Continuous strands of fibres were pulled by means of a take-up wheel from 
the molten boron oxide and the obtained fibres were allowed to cool below 
the melting point. The strands of fibres obtained were about 4-6 .mu.m in 
diameter, and were cut up to form bundles of about 30 cm (12") long 
fibres. 
The bundle of boron oxide precursor fibre was then suspended and slowly 
heated to 700.degree. C. in an atmosphere of ammonia in a vertical 
furnace. When the temperature of 700.degree. was reached, the atmosphere 
of the furnace was changed to a flowing atmosphere of hydrogen gas diluted 
with argon and saturated with titanium tetrachloride. The partial pressure 
of hydrogen in the mixture was close to 1 atm. 
The bundle of fibres was kept in the above atmosphere at around 
1000.degree. C. for the period of 4-5 hours to complete the conversion of 
boron oxide to titanium boride. The completion of the reaction was 
indicated by the colour of the fibres changing to black. The fibres were 
subsequently allowed to cool in the flowing gas atmosphere of the same 
composition. 
FIG. 1 shows a SEM photograph of the titanium boride fibres so obtained. 
The SEM photograph of FIG. 2 shows the cross-section of a titanium boride 
fibre broken in a plane perpendicular to its length, indicating that the 
fibre has homogeneous composition through its cross-section. 
It is to be noted that the above example utilized hydrogen at close to 1 
atm. partial pressure in the reaction, but the partial pressure of the 
hydrogen may be reduced to 0.5 atm. by dilution with an inert gas, such as 
argon or helium. 
EXAMPLE 2 
Boron oxide fibres were heated and encased in a composition having a 
melting point notably higher than that of boron trioxide, as described in 
Example 1, by slowly heating the precursor fibres to 700.degree. C. in an 
ammonia gas containing atmosphere. The furnace atmosphere was then changed 
to a flowing gas mixture containing hydrogen and nitrogen in a ratio of 
2:1 which has been saturated with titanium tetrachloride (TiCl.sub.4). The 
furnace temperature was further increased to about 1200.degree. C. and the 
treatment continued for 4-5 hours, until the colour of the fibres turned 
to a golden metallic colour, which is the colour of TiN. The fibres were 
then allowed to cool. The appearance of the fibres under 
electronmicroscope was similar to that of TiB.sub.2 shown on FIGS. 1 and 
2. 
The fibres obtained by the process described in Example 1 and 2 illustrate 
the preparation of titanium boride and titanium nitride, respectively. It 
should be clear to a person skilled in the art, that titanium 
tetrachloride gas may be replaced by TiBr.sub.4 and/or TiI.sub.4 and the 
products obtained would be the same. 
Ceramic fibres containing a mixture of titanium boride and titanium nitride 
may be obtained by adjusting the ratio of hydrogen and nitrogen in the 
mixture appropriately. 
In Examples 1 and 2, the process was conducted in a batch mode. The process 
could be adapted to a continuous mode by using appropriate equipment and 
technology known to a skilled person. 
EXAMPLE 3 
The identification of titanium boride obtained in Example 1 was conducted 
by X-ray diffraction. The X-ray diffraction was carried out by a Philips 
X-ray machine type PW 1120/60, at 40 kV, at 1/4.degree./min. rate, 
utilizing copper K.alpha. radiation. The X-ray diffractrogram obtained, 
indicating the most characteristic peaks of TiB.sub.2, is shown on FIG. 3. 
The d-values calculated correspond to the d-values listed as those of 
TiB.sub.2 in the ASTM Index. 
The electrical resistivity of the TiB.sub.2 fibres obtained in Example 1 
was measured by conventional means. The resistivity of bundles of 
approximately 100 strands of fibres were measured having an average 
diameter of 5 .mu.m and having lengths of 5.2 cm. It was found that the 
resistivity of the above bundle of fibres was 0.5 ohm cm. This value 
compares very well with the bulk resistivity of titanium boride, having 
taken into consideration the fibrous nature of the material on which the 
measurement was conducted. 
The modulus of the titanium boride fibres obtained by the process of this 
invention ranged between 35-68 GPa. The average tensile strength of the 
fibres was found to be 400 MPa. 
Thus it can be shown that strong, high conductivity ceramic fibres, made 
substantially of TiB.sub.2 can be obtained by the process of the present 
invention. 
The above modulus and tensile strength values were measured on the titanium 
boride fibres obtained in Example 1. By modifying the reaction 
temperature, extending duration of the reaction period and varying other 
preparative conditions such as gas flow rate, the physical characteristics 
of the fibres may be adjusted to suit different commercial requirements. 
EXAMPLE 4 
The titanium nitride fibre obtained as described in Example 2 was subjected 
to X-ray diffraction analysis conducted on the same Philips equipment as 
used in Example 3. The diffractogram indicating that the fibre is composed 
of substantially titanium nitride, is shown on FIG. 4. The calculated 
d-values were found to correspond to those listed in the ASTM Index as 
TiN. 
The electrical resistivity of a bundle of 100 fibres with average diameter 
of 5.mu., and having average length of 1.5 cm, was measured to be 0.05 
ohm.cm. 
The modulus of the fibres ranged between 27-46.5 GPa and the average 
tensile strength of the fibres was found to be 310 MPa. 
The physical characteristics of the titanium nitride fibres may be adjusted 
to requirements in some measure by modifying the preparative conditions 
with respect to temperature, duration of the reaction, flowrate of the 
gases, as a person skilled in the art will readily understand. 
The titanium, zirconium and hafnium boride and nitride fibres made in 
accordance with the present process are homogenous, dense, coherent, and 
by virtue of being produced by reacting a liquid with gaseous reactants, 
substantially pore-free. The reactions are conducted at moderately high 
temperatures. The fibres are obtained in the absence of separate high 
temperature sintering process steps. The present process for obtaining 
nitride and boride fibres may be readily adapted to production on a 
commercial scale at relatively low production costs. 
The ceramic fibres obtained by the process of the present invention may be 
utilized in composite materials, in the reinforcement of materials capable 
of resisting high temperatures wherein electrical conductivity is an 
additional requirement; and in similar applications in which ceramic 
fibres may be utilized. 
Although the present invention has been described with reference to the 
preferred embodiment, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit and scope 
of the invention, as those skilled in the art readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the invention and the appended claims.