Process for the preparation of finely divided carbides and nitrides from ceramic precursor-compounds

Carbides, nitrides or carbonitrides of elements from the main groups III and IV and sub-groups III, IV, V and VI of the periodic system of elements are prepared by PA0 (i) reacting compounds of the formula MX.sub.m or R.sub.n MX.sub.m-n with a reactive hydrocarbon-containing compound or a mixture of compounds which is polymerizable and which contains a reactive compound with one C--OH-group in which PA1 M is an element of the main group III or IV or sub-group of III, IV, V or VI of the periodic system of elements, PA1 X is a halogen, PA1 R is hydrogen or alkyl or aryl, PA1 m is an integer corresponding to the valency stage of M, PA1 n is an integer from 1 to one less than the velency stage of M, and PA0 (ii) thermally decomposing the resulting product from (i) to the corresponding carbide or to the corresponding nitrides or carbonitrides with further nitridation.

This invention relates to a process for the preparation of finely divided 
carbides and/or nitrides and/or carbonitrides of metals and/or metalloids 
of elements of main groups III and IV and subgroups III, IV, V and VI of 
the periodic system of elements from compounds of the formula MX.sub.m 
and/or R.sub.n MX.sub.m-n, where X is a halogen and R stands for hydrogen, 
alkyl or aryl and n is an integer which may have a value from 1 to one 
less than the maximum valency stage of the element M by a reaction with a 
reactive compound containing hydrocarbon and/or a reactive mixture of 
compounds containing hydrocarbon and thermal decomposition of the 
resulting product to form the corresponding carbides or corresponding 
nitrides and/or carbonitrides with concomitant nitridation. 
BACKGROUND OF THE INVENTION 
When carbides, nitrides or carbonitrides are prepared from metals or 
metalloids, the educts are reacted in homogeneous distribution in order to 
give rise to finely divided, high quality products. 
In some carbothermal processes, homogeneous distribution is achieved by 
starting with mixtures of the corresponding oxides and organic hydrocarbon 
compounds (in most cases as liquids) which have a high residue of carbon 
when decomposed by heat and are therefore available as a finely divided 
source of carbon for the reduction of metal oxides and oxides of 
non-metals. 
An additional improvement is obtained if the oxidic compound is also used 
in an extremely finely divided form. Thus according to Ceramic Bulletin 
Vol. 63, No. 8 (1984), a finely divided or liquid metallic or non-metallic 
component is put into the process with colloidal SiO.sub.2 or methyl 
trimethoxy silane. The colloidal SiO.sub.2 is, however, prepared by an 
expensive and complicated method of flame hydrolysis and the alcoholate is 
prepared by a method of alcoholysis, which render this process 
uneconomical. Another disadvantage is that in addition to an organic 
hydrocarbon compound, water is used as solvent or gelling agent, which is 
withdrawn from the gel by a lengthy process of freeze drying at 
-50.degree. C. 
The gels are subsequently converted into SiO.sub.2 and C at temperatures 
from 500.degree. to 800.degree. C. and reacted for 4 to 16 hours at a 
temperature of 1600.degree. C. to form SiC. The products obtained are 
still contaminated with unreacted SiO.sub.2 in spite of this elaborate 
process. 
According to U.S. Pat. No. 3,085,863, SiCl.sub.4 is used as starting 
material instead of the alkoxide or colloidal SiO.sub.2 for the 
preparation of the SiC powder. Owing to the aqueous sugar solution used as 
source of carbon, however, a time consuming process of distillation (24 
hours at 200.degree. to 300.degree. C.) must subsequently be carried out 
for dehydrating the silica gel before the conversion to SiC can be carried 
out at 1800.degree. C. Since the hydrolysis reaction is in any case 
vigorous, the introduction of SiCl.sub.4 must be carried out extremely 
slowly. The process is further complicated by the fact that blockages are 
liable to occur in the part of the apparatus where the SiCl.sub.4 is 
introduced. 
According to Advanced Ceramic Materials 2 (3A) (1987) 253-56, alkoxides are 
again used for the preparation of Si.sub.3 N.sub.4 and AlN but these 
alkoxides are precipitated as gels on lamp black by a process of 
hydrolysis. Apart from the cost intensive starting materials used, the 
dehydration at low pressure which the process requires make it 
questionable whether the process can be carried out economically on an 
industrial scale. 
Moreover, the powders prepared at 1500.degree. C. have unacceptably high 
oxygen contents, amounting to 2% O for Si.sub.3 N.sub.4 and 3.multidot.1% 
O for AlN. 
A process for the preparation of boron nitride and boron carbide by 
pyrolysis of a polymeric ester of boric acid and glycerol is described in 
Chemistry Letters (1985) 691-692. The disadvantage of the boron-containing 
ceramic powders prepared from these organoborate esters is the poor 
reaction to the end product. The boron nitride obtained is heavily 
contaminated with boron carbide. 
Patent Application EP-A No. 0 239 301 also describes a process for the 
preparation of nitrides and carbides by the thermal decomposition (up to 
1800.degree. C.) of esters of polyhydric alcohols. 
In contrast to the process described above, the starting materials used for 
esterification are not oxides but, as mentioned in all the Patent 
examples, the cost intensive metal alkoxides or alkoxides of non-metals. 
The alcohol component used is a compound containing at least two or 
preferably more than two hydroxyl groups, by means of which the 
cross-linking to a polymer takes place. 
The polyhydric alcohols, ethylene glycol and glycerol, undergo 
esterification with the alkoxides of metals or non-metals with liberation 
of alcohol. This alcohol must be distilled off by a lengthy process in 
order to shift the equilibrium to the side of the product. 
A small quantity of an organic compound, e.g. furfuryl alcohol, is 
optionally added to increase the carbon content. 
The process is therefore uneconomical on account of the expensive starting 
materials (alcoholates) and the costly and time consuming steps of the 
process. 
It is an object of the present invention to provide a process for the 
preparation of nitrides, carbides and carbonitrides in which inexpensive 
and highly pure chemicals may be used as starting materials and which does 
not have the disadvantages of the processes described above. Furthermore, 
the starting materials should be capable of being converted into the 
carbides, nitrides or carbonitrides without the costly and time consuming 
process steps described above. 
A process which fulfills these requirements has now surprisingly been 
found. In this process, inexpensive halides may be directly used as 
starting materials without the elaborate preliminary step of 
esterification or hydrolysis. 
BRIEF DESCRIPTION OF THE INVENTION 
Carbides, nitrides or carbonitrides of elements from the main groups III 
and IV and sub-groups III, IV, V and VI of the periodic system of elements 
are prepared by 
(i) reacting compounds of the formula MX.sub.m or R.sub.n MX.sub.m-n with a 
reactive hydrocarbon-containing compound or a mixture of compounds which 
is polymerizable and which contains a reactive compound with one 
C--OH-group in which 
M is an element of the main group III or IV or sub-group of III, IV, V or 
VI of the periodic system of elements, 
X is a halogen, 
R is hydrogen or alkyl or aryl, 
m is an integer corresponding to the valency stage of M. 
n is an integer from 1 to one less than the valency stage of M, and 
(ii) thermally decomposing the resulting product from (i) to the 
corresponding carbide or to the corresponding nitrides or carbonitrides 
with further nitridation. 
The metal halide or metalloid halide reacts by a spontaneous, hitherto 
unclarified reaction with a C--OH-functional polymerizable compound or a 
mixture of such compounds to undergo polymerization to a ceramic 
preliminary product which contains M--O--C and which can be converted by a 
simple heat treatment into the desired highly pure refractory metal 
compound or metalloid compound. 
DETAILED DESCRIPTION OF THE INVENTION 
This invention thus relates to a process for the preparation of finely 
divided carbides and/or nitrides and/or carbonitrides of metals and/or 
metalloids of elements of main groups III and IV and subgroups III, IV, V 
and VI of the periodic system of elements from compounds of the formulae 
MX.sub.m and/or R.sub.n MX.sub.m-n where X is a halogen and R stands for 
hydrogen, alkyl or aryl and n represents an integer with a value from 1 to 
one less than the maximum valency stage of the element M, by a reaction 
with a reactive compound containing hydrocarbon and/or a reactive mixture 
of compounds containing hydrocarbons and thermal decomposition of the 
resulting product to the corresponding nitrides and/or carbonitrides with 
concomitant nitridation, characterised in that the reactive compound 
containing hydrocarbon and/or the reactive mixture of compounds containing 
hydrocarbons contains a C--OH group and is polymerizable. 
The process according to the invention give rises to a polymeric product 
which is obtained either as a black powder or as a black brittle solid, 
depending on the metal or metalloid compound used. Molecularly disperse 
distribution of the compounds in the ceramic preliminary product is 
obtained if the process is suitably carried out. A finely divided ceramic 
powder of high specific surface area may be obtained from the subsequent 
thermal decomposition. The use of nitrogen as gaseous atmosphere during 
tempering and the maintenance of a particular limiting temperature, which 
varies according to the element, results in the formation of a nitride 
whereas carbides may be obtained if suitably higher temperatures are 
employed or if an inert protective gas or gases such as hydrogen or CO are 
used. If the formation of carbide/nitride mixed phases or carbonitrides is 
desired, this may easily be achieved by varying the temperature and 
composition of the atmosphere. 
The process according to the invention results in molecularly disperse 
distribution of the above mentioned compounds in the preliminary product 
so that, in contrast to the carbothermal processes in which oxide/carbon 
mixtures are used as starting materials, only a small excess of carbon is 
required for calcination. Any unreacted carbon still remaining may be 
removed by a further process step. 
The reactive mixture of compounds containing hydrocarbons may be, for 
example, phenol or phenol derivatives with formaldehyde. In this example, 
polymerisation proceeds as an acid catalysed polycondensation reaction. 
Another hydrocarbon-containing compound used is a polycondensable 
monohydric alcohol or monohydric derivatives thereof, preferably furfuryl 
alcohol or its derivatives. When acid is added to furfuryl alcohol, this 
alcohol reacts spontaneously to undergo polycondensation or a vigorous 
decomposition with evolution of fumes, depending on the strength of the 
acid. 
In order to keep down the quantity of hydrocarbon compound used, it is 
advisable to use aromatic compounds such as those mentioned above or 
mixtures thereof, since these often have a high coking residue when the 
product is subjected to thermal decomposition after polymerization. Most 
other hydrocarbon compounds have only a small coking residue after 
pyrolysis. 
The metals and/or metalloids used in the form of compounds of formula 
MX.sub.m (in which X is a halogen ion such as chloride or bromide) are 
preferably Ti, Hf, Zr, V, Nb, Ta, Cr, Mo and/or W and the main group 
elements B, Al and/or Si. These compounds may easily be prepared in a 
highly pure form. 
For economical reasons, chlorides such as TiCl.sub.4, ZrCl.sub.4, 
BCl.sub.3, AlCl.sub.3 or SiCl.sub.4 are preferably used for the 
preparation of the carbides, nitrides or carbonitrides. 
The metal organyl compound used is preferably a compound or a mixture of 
compounds of the formula R.sub.n MX.sub.m-n in which R denotes hydrogen 
and/or an alkyl and/or aryl group and n has the value of an integer from 1 
to one less than the valency stage of the element M. R preferably stands 
either for hydrogen or for identical or different C.sub.1 to C.sub.6 
groups, in particular methyl or phenyl groups. These compounds may easily 
be brought to a high degree of purity by distillative processes. Compounds 
of the formula R.sub.n MX.sub.m-n could in principle be used as mixtures 
with other compounds of the formula R.sub.n MX.sub.m-n or also with 
compounds of the formula MX.sub.m but no positive influence on the product 
quality or the ease of carrying out the process is observed when such 
mixtures are used so that their use is only purposeful when they are 
formed as the result of the synthesis, in which case the cost of 
separation by distillation can be saved. 
The chloride function is preferred as functional group X in the compounds 
R.sub.n MX.sub.m-n on account of its low cost, e.g. in the compounds 
CH.sub.3 SiCl.sub.3, (CH.sub.3).sub.2 SiCl.sub.2, (CH.sub.3).sub.2 HSiCl 
and CH.sub.3 HSiCl.sub.2. Another advantage of using halides is that they 
are strong Lewis acids and therefore bring about spontaneous 
polymerization in many of the reactive mixtures by their catalytic 
activity. If the metal compound or metalloid compound is not a strong 
Lewis acid or if the reactive organic compound cannot be polymerised with 
the aid of acid catalysis, the conversion of the compounds to a polymeric 
product is brought about according to the invention by the addition of a 
catalytic quantity of acid or by the addition of another polymerization 
catalyst to the compounds. 
The conversion of the compounds to the desired polymeric product 
advantageously takes place in a solvent as this simplifies the process. 
Although the reaction may in principle be carried out without solvent, 
this may give rise to difficulties in the preparation of a homogeneous 
mixture of starting components or in the removal of the heat of reaction. 
Solvents such as toluene, acetone or methylene chloride not only simplify 
the preparation of a homogeneous mixture of reactants by stirring but also 
facilitate removal of the heat of reaction by means of the evaporating 
solvent. 
The speed of the reaction depends on the concentration of the starting 
compounds, their reactivity and the temperature. When, for example, 
methylene chloride is used as solvent with AlCl.sub.3 and furfuryl 
alcohol, the reaction, including the polycondensation is immediately 
completed as soon as the components have been mixed at room temperature, 
regardless of the diluent used. 
One of the advantages of the process according to the invention, in 
contrast to the state of the art described above, is that the preparation 
of a high molecular weight preliminary product is not carried out by a 
tedious esterification reaction of a polyfunctional alcohol such as 
glycerol with an acid or an acid derivative such as tetraethoxy silane or 
an acid anhydride such as B.sub.2 O.sub.3, which has all the disadvantages 
described above. The rapid acid catalyzed polymerization results in highly 
cross-linked polymers which have a high coking residue and which are often 
obtained as brownish black powders or brittle solids after removal of the 
solvent. They are not fusible and can therefore be pyrolysed without 
difficulty. 
The polymer product containing metal and/or metalloid may be heated to a 
temperature of 1000.degree. C. in a first heat treatment stage carried out 
in a vacuum or an inert or reducing atmosphere of N.sub.2, H.sub.2, CO or 
Ar or mixtures thereof. Decomposition generally proceeds in several 
stages, the cracking process proper setting in at a temperature of 
300.degree. C. with evolution of gas. Before that temperature is reached, 
HCl, H.sub.2 O or alcohol may be evolved, depending on the starting 
compounds used. The evolution of gas is substantially completed at a 
temperature of up to 700.degree. C. although higher temperatures, up to 
1000.degree. C., may be necessary in individual cases. 
According to a preferred embodiment of the process according to the 
invention, the hydrogen halide may be completely removed if the gas 
atmosphere during the first stage of heat treatment contains steam at a 
partial pressure of from 10 to 1000 mbar. In order to avoid a reaction of 
the steam with carbon, a temperature of 850.degree. C. should not be 
exceeded when the steam is introduced. Complete removal of halogen is 
advisable if evolution of halogen or of hydrogen halide during the 
subsequent high temperature stage is to be avoided. The halogen compounds 
would limit the choice of material for the furnaces and crucible used for 
the high temperature treatment on account of their corrosiveness. 
The products prepared in the first stage of pyrolysis are black, 
pulverulent or granular solids which have a radiologically amorphous to 
partially crystalline structure and a high specific surface area. They 
consist of the metal or metalloid element, oxygen and carbon and also 
contain small amounts of halogen if the above described treatment with 
steam is not carried out. 
For the production of the carbide from the metal or metalloid, the 
thermally pretreated product is annealed by a second heat treatment at a 
temperature of from 1000.degree. C. to 1800.degree. C. in an inert or 
reduced atmosphere. Noble gases, carbon monoxide, hydrogen or mixtures of 
these gases are suitable for this purpose. 
The process according to the invention has the advantage that substantially 
lower temperatures, shorter calcining times and smaller quantities of 
carbon are used than, for example, in the carbothermal process. The cause 
for this must lie in the as yet unclarified structure of the polymeric 
preliminary products formed in the process according to the invention. 
For production of the nitrides or carbonitrides from the metal or 
metalloid, the thermally pretreated product is subjected to second heat 
treatment at a temperature from 1000.degree. to 1800.degree. C. in a gas 
atmosphere of gaseous nitrogen or ammonia gas or gaseous mixtures of 
ammonia and nitrogen. As for the formation of carbides so this nitridation 
is achieved at lower temperatures and shorter residence times than in 
other processes. In addition, the finely divided nitride or carbonitride 
powders generally have significantly lower oxygen contents. 
The product may advantageously be compressed before the second heat 
treatment so as to increase the volume/time yield and render the process 
more economical. Compression may be carried out, for example, in a roller 
compressor, a vacuum roller compressor or an edge runner mixer. 
The carbides, nitrides and carbonitrides prepared from the metal or 
metalloid compounds may be subjected to a thermal after treatment in an 
oxygen-containing atmosphere at temperatures of up to 800.degree. C. to 
remove residues of excess carbon. 
The process according to the invention gives rise to finely divided, sinter 
active powders with high specific surface areas. The loose agglomerate 
obtained under certain process conditions can be broken down by briefly 
grinding them. The powders obtained are chemically pure and are 
surprisingly found to have unexpectedly low halogen contents inspite of 
the use of metal halides or metalloid halides, even if the residual 
halogen is not removed by steam treatment after polymerization.

The invention is described in more detail below with the aid of examples 
which, however, are not to be regarded as limiting the invention. 
EXAMPLE 1 
A solution of 130.multidot.0 g of furfuryl alcohol in 700 ml of methylene 
chloride was added dropwise within 30 minutes to a suspension of 
266.multidot.7 g of aluminium chloride in 800 ml of methylene chloride 
with stirring and reflux cooling. After removal of the methylene chloride 
by distillation, 372 g of a brownish black, pulverulent solid were left 
behind. 
This powder was heated to 600.degree. C. within 6 hours in a first heat 
treatment in a nitrogen atmosphere and slowly cooled after it had been 
kept at this temperature for 6 hours. 
192 g of a black, finely divided, radiologically amorphous powder having 
the following chemical composition were obtained: Al=26.multidot.9%; 
O=23%; C=36%; Cl=4%. This corresponds to a ratio of Al:O:C of 
1:1.multidot.45:3. The specific surface area is 62 m.sup.2 g.sup.-1. 
15.multidot.79 g of the substance from the first heat treatment were heated 
at 1650.degree. C. for 5 hours in a graphite crucible under a nitrogen 
atmosphere. 9.multidot.04 g (=57%) were left as a finely divided, black 
solid after cooling. This product consisted of crystalline AlN (X-ray 
diffraction analysis) and radiologically amorphous carbon. Chemical 
analysis revealed a carbon content of 28% and an oxygen content of 
0.multidot.39%. 
7.multidot.93 g of the substance from the second heat treatment were 
tempered in air at 700.degree. C. for 5 hours in a corundum crucible to 
remove excess carbon. 
The finely divided, light grey AlN powder (5.multidot.70 g=72% residue) 
could be prepared with a carbon content of 0.multidot.18%, an oxygen 
content of 0.multidot.90% and a Cl content of 0.multidot.005%. The 
specific surface area was found to be 3.multidot.1 m.sup.2 g.sup.-1. The 
primary particle size, determined from Raster Electron 
Microscope (REM) photographs, was less than 0.multidot.3 .mu.m. The yield 
of AlN, based on the quantity of AlCl.sub.3 put into the process, was 96%. 
EXAMPLE 2 
3100 g of AlCl.sub.3 were dissolved in 10 liters of methylene chloride and 
1.multidot.5 liters of acetone, and a solution of 1250 g of furfuryl 
alcohol in 5 liters of methylene chloride was added as in Example 1. The 
solvent was distilled off. The pulverulent reaction product was then 
heated under N.sub.2 in a quartz bulb at 200.degree. C. for 2 hours, at 
400.degree. C. for a further 2 hours and then at 900.degree. C. for 6 
hours. 1950 g of pulverulent, partially crystalline solid 
(.alpha.-Al.sub.2 O.sub.3) having the following chemical composition were 
obtained: C=44%; O=30%; Cl=0.multidot.20%; BET=468 m.sup.2 g.sup.-1 
(one-point measurement with N.sub.2). 
Part of the powder (500 g) was converted into crystalline AlN (340 g) by a 
second heat treatment carried out at 1550.degree. C. for up to 10 hours. 
The removal of carbon from the powder (10.multidot.2 g) was carried out as 
in Example 1. An AlN powder (6.multidot.6 g) having the following chemical 
composition was obtained: N=33%; C=0.multidot.088%; O=0.multidot.93%; 
Cl&lt;0.multidot.005%; BET=5.multidot.1 m.sup.2 * g.sup.-1 ; primary particle 
size (according to REM photographas)&lt;0.multidot.3 .mu.m. 
EXAMPLE 3 
44 g of furfuryl alcohol in 700 ml of methylene chloride were added to a 
solution of 95 g of TiCl.sub.4 in 1000 ml of methylene chloride within 
2.multidot.5 hours as in Example 1. The black solution was then stirred 
for 1 hour and the solvent was finally distilled off. 
The first heat treatment of the brownish black powder was carried out at 
500.degree. C. under a nitrogen atmosphere for 10 hours, at the end of 
which time 71 g of a radiologically amorphous powder were left behind. 
In a second heat treatment, 20 g of this powder were converted into 
titanium nitride (10 g residue) by heating to 1400.degree. C. for 15 hours 
under a nitrogen atmosphere. A TiN powder having a carbon content of 23% 
and an oxygen content of 1.multidot.42% was obtained.