Metal-nitrides prepared by photolytic/pyrolytic decomposition of metal-amides

This invention is directed to a method for preparing materials comprising metal nitrides by the photolytic/pyrolytic decomposition of metal amides. The metal amides may be zirconium and/or niobium amides, which may additionally contain titanium amides.

FIELD OF THE INVENTION 
This invention is directed to a method for preparing metal nitrides which 
involves the photolytic/pyrolytic decomposition of metal amides. More 
particularly, the metal nitrides are prepared from amide consisting 
essentially of zirconium amides and/or niobium amides, which may 
additionally include titanium amides. 
BACKGROUND OF THE INVENTION 
Metal nitrides are powders generally prepared from high temperature 
treatment of a source of metal and a source of nitrogen. Preparation of 
metal nitride films is usually carried out by chemical vapor deposition 
methods which require expensive instrumentation. If films are to be 
localized on the small area of a substrate, the rest of the area needs to 
be protected which adds to the cost of preparation of localized films. 
Clearly, new methods need to be developed which are convenient and 
economically acceptable. It would be desirable if localized coatings of 
metal nitrides could be produced by a method involving photolytic 
decomposition which would allow only the desired area of a substrate which 
has been covered with a metal nitride precursor to be converted to the 
metal nitride. According to such a process, a substrate could be covered 
with a metal nitride precursor and then the desired area could be 
irradiated with a suitable light which would result in the decomposition 
of the precursor. The undecomposed precursor could subsequently be removed 
and then the substrate processed further to convert the decomposed 
precursor to a metal nitride coating. 
Narula and Maricq, the present inventors, in an article entitled "1.064 
.mu.m Multiphoton Laser-induced Fluorescence and Dissociation of 
Tetrakis-(dimethylamino) Titanium(IV)", Chemical Physics Letters, Volume 
198, number 3, 6, December, 1991, disclose that titanium amide when 
subjected to a particular laser light decomposes primarily to an insoluble 
residue metal amide oligomer and dimethylamine. In articles entitled 
"Laser Induced Decomposition of Precursors Containing M-N Bonds for the 
Preparation of Metal-Nitride Preceramics, Powders and Coatings", Am. Chem. 
Soc. Pol. Div. 32,499 (1991), and "Preparation of Composite Particles by 
Pulsed Nd-TAG Laser Decomposition of [(CH.sub.3).sub.2 N].sub.4 Ti to TiN 
Coat Al.sub.2 O.sub.3, TiO.sub.2 O.sub.2 Si.sub.3 N.sub.4 Powders" J. Am 
Ceram. for. (1993) 2727 Narula and Maricq disclose that after subjecting 
the titanium amide to photolytic decomposition by means of a laser, the 
resulting material can be pyrolyticly decomposed to form titanium nitride. 
Both of these article disclose that materials similar to the titanium 
amide, i.e., boron amide and silicon amide, in contrast, were found 
unexpectedly to not decompose when exposed to the laser. 
BRIEF DESCRIPTION OF THE INVENTION 
This invention is directed to a method of preparing a material product 
comprising metal nitride by photolytic/pyrolytic decomposition of 
particularly claimed metal amides. The method comprises providing metal 
amides selected from the group consisting of (a) Zr[R.sub.2 N].sub.4 ; (b) 
Nb[R.sub.2 N].sub.4 ; (c) mixtures of (a) and (b); and (d) mixtures of any 
of (a) , (b) , or (c) with Ti[R.sub.2 N].sub.4 ; wherein R is an organic 
group; irradiating said metal amides for a time sufficient to decompose at 
least a portion of said metal amides to an metal-organic residue; removing 
undecomposed metal amides from said metal-organic residue; and pyrolyzing 
said metal-organic residue in an inert or an ammonia atmosphere at an 
elevated temperature to form a material comprising metal nitrides. For 
example, the residue, if pyrolyzed in an inert atmosphere such as 
nitrogen, will provide a metal nitride/metal carbide composite or, if 
pyrolyzed in an ammonia/nitrogen atmosphere, will provide substantially 
metal nitride.

DETAILED DESCRIPTION OF FURTHER EMBODIMENTS 
This invention is directed to a method of preparing metal nitrides by 
photolytic/pyrolytic decomposition of metal amides as disclosed above. The 
metal amides are selected from the group consisting of zirconium amide, 
niobium amide, and mixtures of them; in addition the metal amide may 
include titanium amide. As characterized by chemical formula, these amides 
are selected from zirconium Zr[R.sub.2 N].sub.4, Nb[R.sub.2 N].sub.4, and 
Ti[R.sub.2 N].sub.4. R is an organic group which may be, but is not 
limited to, methyl, ethyl, n-proryl, i-propyl, n-butyl, sec-butyl, 
t-butyl, etc. Preferably R is the ethyl group. We found that other similar 
amides, such as Y([(CH.sub.3).sub.3 Si].sub.2 N).sub.3, in contrast did 
not provide a nitride when subjected to the photolytic/pyrolytic method of 
this invention. 
The method for preparing the material of the invention comprising metal 
nitride may be carried out, according to one embodiment, using an 
apparatus of the type shown in FIG. 1. Using such as apparatus, the tube 
is charged with metal amide and then partially evacuated. According to 
this embodiment, the metal amide could be kept in gas or liquid phase or 
dissolved in an inert organic solvent such as pentane, hexane, heptane 
etc. Preferably, according to such an enbodiment of making the metal 
nitride, the solvent is hexane. The amide is then irradiated, in this 
embodiment, with a laser light, for a time sufficient to decompose at 
least a portion of the metal amide to form an organo-metallic residue. The 
irradiation is then stopped and undecomposed metal amide removed from the 
residue. This can be done, for example, by distillation or by extraction 
with hexane. 
In addition, should it be desired to form a material comprising metal 
nitride on a particular substrate, the process would be carried out by 
providing the desired amide (or combination of amides) on the substrate. 
The substrate may comprise particles of the substrate. Preferably, the 
substrate would comprise a refractory material such as magnesium oxide, 
aluminum oxide, silicon nitride, and titanium oxide. Decomposition of the 
metal amide, in such an instance, could then be carried out as described 
above. The decomposition of the metal amide may be carried out by 
irradiation with a laser including but not limited to a 1.064 micrometer 
frequency ND-YAG laser. Other laser frequencies which may also suitably be 
used include the 351 nanometer pulses from an excimer laser, 308 nanometer 
pulses, 248 nanometer pulses and 193 nanometer pulses or other intense 
visible/UV lasers. We observed that when employing the 1.064 micrometer 
laser, the radiation initially passed through the metal amide sample with 
little loss of intensity. Over the course of a few minutes, at a laser 
intensity of 1.6.multidot.10.sup.8 Wcm-2, the initially clear pale yellow 
zirconium amide (Zr[(C.sub.2 H.sub.5).sub.2 N].sub.4) began to turn brown. 
The color change was not noticeable in the case of the corresponding 
niobium amide, due to its initial dark brown color. Concurrently, in the 
particular example described above, an intense laser-induced fluorescence 
developed along the entire length of the sample tube. 
FIGS. 2A and 2B and 3A and 3B show emission spectra at various laser 
intensities after the laser decomposition; the spectra of 2A and 3A being 
generated by an unfocused laser beam and 2B and 3B being generated by a 
focused laser beam. At lower intensities, broad band emission is observed 
over the range of 300-650 nanometers; higher intensity irradiation 
produces structure in the emission spectrum corresponding to known Zr and 
Nb emission lines. The laser-induced fluorescence spectra shown is the 
result of multi-photon absorption by zirconium amide (Zr[(C.sub.2 
H.sub.5).sub.2 N].sub.4) or niobium amide (Nb[(C.sub.2 H.sub.5).sub.2 
N].sub.4 and/or other metal and nitrogen containing molecular fragments 
which have been decomposed and volatilized by a 1.064 um according to 
embodiments of the present invention laser. The sharp features show that 
atomic metal is among the decomposition products. The observation of 
atomic emission lines from the metal atoms (zirconium or niobium) and the 
formation of a black coating along the sides of the sample cell imply that 
not only multi-photon absorption, but also multi-photon dissociation 
occurred. 
The major dissociation products obtained after 1.064 micrometer radiation 
of zirconium amide and niobium amide are diethylamine, 
tetraethylhydrazine, and ethylenylene amine, (as revealed by GC-mass 
spectrometry) and an insoluble, gray-black, residue. The IR spectrum of 
the residue indicates that it contains organic groups. The residue could 
be either an oligomer, a polymer, or a mixture of species. 
After removal of the undecomposed metal amide from the metal-organic 
residue, the residue is pyrolyzed in an inert atmosphere (including, but 
not limited to nitrogen and argon and mixtures thereof) or in an ammonia 
comprising atmosphere, the latter generally including nitrogen, and at an 
elevated temperature to form a product comprising metal nitrides. 
Depending on the pyrolysis conditions, the residue will convert to a 
product which is (1) a composite of metal nitrides and metal carbides or 
(2) substantially metal nitrides. That is, at lower pyrolysis temperatures 
about 600.degree. C., the metal nitrides/metal carbide composite will be 
obtained while at higher pyrolysis temperatures above about 1000.degree. 
C., the product is substantially metal nitride in the presence of ammonia. 
If the nitride/carbide composite is desired, the inert atmosphere employed 
is preferably nitrogen. On the other hand, when attempting to form 
substantially metal nitride product, the preferred atmosphere employed is 
ammonia/nitrogen. If a mixture of amides has been employed in the 
photolytic/pyrolitic decomposition method of this invention under 
conditions to form substantially nitrides, the final product will be a 
composite of the various nitrides such as TiN/Nb.sub.4 N.sub.3, 
ZrN/Nb.sub.4 N.sub.3, and TiN/ZrN, with a uniform distribution of 
components. In addition, as disclosed above, according to the method of 
the invention metal nitride or metal nitride/metal carbide materials can 
be deposited on any substrate or substrate particle to result in, e.g., 
substrate particles having a coating of such materials thereon. 
Metal-organic residues of the preferred niobium amide (Nb[(C.sub.2 
H.sub.5).sub.2 N].sub.4 and corresponding zirconium amide, when fired at 
800.degree. C. in a nitrogen or ammonia atmosphere, yielded powders which 
were identified by X-ray powered diffraction to be Nb.sub.4 N.sub.3 /NbC 
and ZrN, respectively. These powders were converted to pure nitrides by 
firing in an ammonia atmosphere at 1100.degree. C. for two hours. The 
yields of ZrN and Nb.sub.4 N.sub.3 were quantitative based on decomposed 
zirconium amide (Zr[(C.sub.2 H.sub.5).sub.2 N].sub.4), 40% decomposition, 
and niobium amide (Nb[(C.sub.2 H.sub.5).sub.2 N].sub.4, 70% decomposition. 
Undecomposed zirconium amide and niobium amide were distilled out of the 
reaction vessel to obtain an accurate value for decomposed zirconium amide 
and niobium amide, respectively. As shown in FIG. 4, the position and 
intensity of diffraction peaks matches with those reported for Nb.sub.4 
N.sub.3. More particularly, the X-ray diffraction pattern of the obtained 
Nb.sub.4 N.sub.3 powder showed deffraction peaks at 
2.THETA.(Intensity)=35.6(100), 41.2(100), 41.8(60), 59.6(40), 60.1(60), 
71,4(20), 72.5(20), which is in complete agreement with that reported for 
niobium nitride. 
The following examples detail materials comprising metal nitrides made 
according to the method of the present invention. The method was carried 
out using the apparatus shown in FIG. 1. 
Example 1 
Precautions were taken to exclude air and moisture from the apparatus. 
Amides comprising, Zr[N(C.sub.2 H.sub.5).sub.2 ].sub.4, and Nb[N(C.sub.2 
H.sub.5).sub.2 ].sub.4 were prepared according to D. C. Bradley, I. M. 
Thomas, "Metallo-Organic Compounds Containing Metal-Nitrogen Bonds. Part 
I. Some Dialkylamino-Derivatives of Titanium and Zirconium." J. Chem Soc., 
3857 (1960). The amides were charged in the quartz tube and irradiated by 
an unfocused (beam diameter-7 mm) 1.064 .mu.m output of a Nd:YAG laser 
operating at 10 Hz with an energy output of approximately 600 mJ/pulse and 
a pulse width of 9 ns. Over the course of several minutes, the metal amide 
samples turned opaque black. The irradiation was stopped and excess amides 
and volatile decomposition products were distilled out of the reaction 
vessel under vacuum. The remaining dark grey-black free-flowing powders 
were fired in a flow of ammonia at 1100.degree. C. 
Example 2 
The quartz tube was charged with Zr[N(C.sub.2 H.sub.5).sub.2 ].sub.4 [0.948 
g] and connected to a vacuum system. After evacuation, the tube was filled 
back with sufficient nitrogen to keep the sample at 50 torr. The sample 
was irradiated with 1.064 .mu.m light from a Nd-YAG laser. Excess 
Zr[N(C.sub.2 H.sub.5)].sub.4 (0.77 g) was recovered by distillation and 
the residue was pyrolyzed at 1100.degree. C. under an ammonia atmosphere 
for 60 minutes. The product was ZrN in a yield of 90%. 
Example 3 
The preparation of niobium nitride powders was carried out from 
Nb[N(C.sub.2 H.sub.5).sub.2 ].sub.4. In a quart tube, 1.04 g of 
Nb[N(C.sub.2 H.sub.5).sub.2 ].sub.4 was placed and exposed to 1.064 .mu.m 
laser radiation. Undecomposed Nb[N(C.sub.2 H.sub.5).sub.2 ].sub.4 (0.29 g) 
was distilled out and the residue was pyrolyzed at 1100.degree. C. in an 
ammonia atmosphere to form Nb.sub.4 N.sub.3. The yield of Nb.sub.4 N.sub.3 
85%. 
Example 4 
A mixture of Nb[N(C.sub.2 H.sub.5).sub.2 ].sub.4, Zr[N(C.sub.2 
H.sub.5).sub.2 ].sub.4, and/or Ti[N(C.sub.3).sub.2 ].sub.4 is placed in a 
quartz tube and irradiated with 1.064 .mu.m laser light. Undecomposed 
metal amides are removed by distillation and residue is fired in an 
ammonia atmosphere at 1100.degree. C. to obtain composites. Firing in the 
nitrogen atmosphere at this temperature results in composites containing 
metal carbide also.