Process for preparing a titanium carbide film

A process for preparing a titanium carbide film characterized by vaporizing metallic titanium from a metallic titanium vaporizing source placed in an atmosphere containing a dilute acetylene gas and depositing titanium carbide onto the surface of base set up facing said source.

BACKGROUND OF THE INVENTION 
In general, a first wall (a wall directly facing on plasma) of nuclear 
fusion apparatus interacts with heat radiation from plasma and plasma 
constituent particle leaked out from plasma confinement region, and as the 
result the first wall is eroded to release first wall constituent 
materials as an impurity in the plasma. Therefore, the first wall is 
regarded as a main source of plasma impurity. On the other hand, in order 
to elevate a plasma temperature as higher as possible in a nuclear fusion 
reaction, it is necessary to lower an energy loss by impurity from plasma 
to the utmost. It is known that the energy loss by impurity increases with 
atomic number of impurity element under the same content. Accordingly, it 
is now planned to use a material of small atomic number such as carbide, 
boride, etc. as a first wall constituent material of nuclear fusion 
apparatus, and among them, titanium carbide having a stoichiometric 
composition, i.e. Ti/C=1 is the most hopeful material proposed. However, 
from a view of mechanical processability and mechanical strength., etc., 
it is difficult to constitute a first wall with titanium carbide only. 
Thereby, a material excellent in thermal and mechanical properties such as 
molybdenum material is used as a base and a titanium carbide film of 
several tens .mu.m in thickness is deposited onto the surface of base, and 
is used as a titanium carbide deposited first wall. However, since the 
first wall is eroded during the operation of nuclear fusion apparatus due 
to the cause as described above, eroded portions have to be repaired with 
a fresh titanium carbide film on all occasions. In this case, if the first 
wall of eroded portion is removed from the apparatus and repaired on all 
occasions, the operation and maintenance of nuclear fusion apparatus is 
very troublesome and the operation cost becomes high. Therefore, it is 
very desirable in operation and maintenance of nuclear fusion apparatus 
using a titanium carbide deposited first wall to develop such a method 
(i.e. in situ coating method) that the eroded portion can be repaired with 
a fresh titanium carbide film in vacuo without removing the first wall out 
of the apparatus. Now then, in a nuclear fusion apparatus which is now 
under contemplation, when a titanium carbide film is deposited onto the 
surface of first wall, a distance that a deposition particle (metallic 
titanium particle) flying out from a titanium vaporizing source has to fly 
to reach the surface of first wall is about 1 m, and this value increases 
more and more with a size of nuclear fusion apparatus to grow larger. 
In general, the flying distance of deposition particle is decided by a rate 
of collision and scattering of deposition particle and atmospheric gas 
particle in space. 
The average distance of particle flying from a collision to the next 
collision (mean free path) can be found by the kinetic theory of gasses. 
According to the theory, it is necessary to make an atmospheric gas 
pressure below 5.times.10.sup.-5 Torr in order that the mean free path is 
above 1 m. Therefore, if it is intended to fly a deposition particle above 
1 m for the purpose of depositing a titanium carbide film onto the surface 
of first wall of the above mentioned nuclear fusion apparatus, it is 
necessary to maintain an atmospheric gas pressure below 5.times.10.sup.-5 
Torr during deposition. 
Up to now we have a cathodic sputtering method, a reactive ion plating 
method, a gas phase reaction method and a reactive vacuum deposition 
method as a method of depositing out a titanium carbide film on a base. 
However, in the cathodic sputtering method, the reactive ion plating 
method and the gas phase reaction method, it is necessary to make the 
pressure of atmospheric gas such as reaction gas, operation gas, etc. 
above 10.sup.-3 Torr in time of performing deposition, and so these 
methods cannot be used as an in situ coating method for the first wall of 
nuclear fusion apparatus for the above mentioned reason. 
On the other hand, the reactive vacuum deposition method which has hitherto 
been known is a method of using ethylene as a reaction gas but, in 
practice, is very difficult to be put to practical use since the 
depositing velocity of titanium carbide film onto a base becomes very low 
when lowering the ethylene gas pressure below 5.times.10.sup.-5 Torr. 
SUMMARY OF THE INVENTION 
This invention relates to a process for preparing a titanium carbide film. 
More particularly, this invention relates to a process for preparing a 
titanium carbide film by directly reacting a metallic titanium vapor and a 
acetylene gas at low pressure on a base. 
As the result of having made researches on developing a process for 
preparing a titanium carbide film having such a deposition velocity as 
capable of being sufficiently put to practical use at a low pressure, the 
present inventors have found a practically very useful fact that a 
titanium carbide film can be prepared with such a deposition velocity as 
capable of being sufficiently put to practical use even at a low pressure 
(below 5.times.10.sup.-5 Torr) by depositing a metallic titanium vapor 
onto a base in an atmosphere of acetylene gas at a low pressure and 
directly chemical reacting the metallic titanium vapor and acetylene gas 
on the base, and that, at the step that the chemical composition of 
deposited film comes up to the most stable composition (stoichiometric 
composition) of titanium carbide, the reaction terminates spontaneously, 
since the titanium carbide film is formed directly by a chemical reaction, 
and so actually it is not necessary to control the chemical composition of 
titanium carbide film artificially. 
The present invention has been made on the basis of this knowledge. 
That is, the process of the present invention comprises depositing a 
titanium carbide film onto a base by generating a metallic titanium vapor 
under an atmosphere of acetylene gas at a low pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As a method of vaporizing metallic titanium in the present invention, any 
of a heating method by an electron beam, a heating method by sending an 
electric current directly to metallic titanium, a side heating method, a 
high-frequency heating method, a method by a laser beam and a method by 
the irradiation of arc image spot may be used. 
If necessary, in order to improve the property of deposited film, an 
electrical voltage may be applied between a metallic titanium generating 
source and a base. The temperature range that a titanium carbide film can 
be deposited in the present invention may be any temperature above room 
temperature although a temperature above 200.degree. C. is more effective 
from the viewpoint of intending the improvement of adhesion between the 
film and the base. As a material capable of being used for a base in the 
present invention, any material which can bear up under the above 
mentioned deposition temperature of titanium carbide film may be used. 
The thickness of titanium carbide film capable of being deposited onto the 
base according to the present invention is not specially limited. And the 
deposition velocity of titanium carbide film depends upon the pressure of 
acetylene gas and the amount of metallic titanium vapour generated. For 
example, about 1.5 .ANG./second of deposition velocity can be easily 
obtained with 5.times.10.sup.-5 Torr of acetylene gas pressure. 
The process for preparing a titanium carbide film of the present invention 
will be explained with drawing. 
In FIG. 1, a vacuum container 1 provided with a metallic titanium 
vaporizing source 2 and a base 3 with a heater 4 in the interior is 
evacuated via a vacuum valve 5 by means of a vacuum pump 6. And then the 
base 3 is heated by the heater 4 to the desired temperature and, if 
necessary, a definite voltage is applied between the base 3 and the ground 
by an electric source. After the above operations, an acetylene gas is 
introduced into the vacuum container 1 through a flow variable vacuum 
valve 8 and a metallic titanium vapor is generated from the metallic 
titanium vapor source 2 controlling the pressure to deposit a titanium 
carbide film onto the base 3. 
FIG. 2 shows an example of the reaction characteristic curve of titanium 
carbide film prepared by the process of the present invention. The 
abscissa represents an acetylene gas pressure and the ordinate represents 
a chemical composition of deposited film when the acetylene gas pressure 
is changed under the base temperature of 300.degree. C. and the film 
deposition velocity of 1.5 .ANG./second. When the acetylene gas pressure 
becomes above 5.times.10.sup.-5 Torr, a stable film of Ti/C=1 
(stoichiometric composition of titanium carbide) is formed without 
depending on the acetylene gas pressure. This means that a reaction of 
taking more carbon (C) into the film, i.e. a reaction of becoming Ti/C&lt;1, 
does not proceed even in the presence of excess acetylene gas at a stage 
that a stoichiometric composition of titanium carbide film is formed 
because the film of Ti/C=1 which is a stoichiometric composition of 
titanium carbide is chemically most stable. Therefore, according to the 
process for preparing a titanium carbide film of the present invention, it 
is not necessary to control the acetylene gas pressure strictly for 
preparing a stoichiometric composition of titanium carbide film and so the 
operation is very easy. 
The advantages of the present invention can be given as follows: 
(1) A titanium carbide film can be deposited onto a base which is further 
away because of a preparation process of titanium carbide film under low 
pressure. 
(2) The reaction does not proceed over the stoichiometric composition of 
titanium carbide so that the preparation condition of film is easily 
established because of a preparation process utilizing a direct chemical 
reaction. 
(3) The preparation operation is simple because of a preparation process 
based on a simple principle, and the gas load for a vacuum pump is reduced 
because of being under low pressure, and so the preparation cost is 
reduced. 
(4) Various kinds of material can be used for a base because a titanium 
carbide film can be prepared at a low temperature.