Synthesis of pure disilicon hexafluoride

Highly pure disilicon hexafluoride is synthesized by a process in which a suspension of a fluorination agent is made by dispersing this fluorination agent in an oxybenzene compound as a solvent and disilicon hexachloride is dripped in the suspension to be caused to react with the fluorination agent in nitrogen gas flows under atmospheric pressure, the oxybenzene being expressed by a following formula: ##STR1## (where each of R.sub.1 and R.sub.2 is an alkyl group alternatively having one or more substituents; carbon numbers summed up by R.sub.1 and R.sub.2 are equal to at least 2; and m.gtoreq.1 and n.gtoreq.0).

FIELD OF THE INVENTION 
The present invention relates as synthesis of pure disilicon hexafluoride. 
More particularly, the present invention relates to a novel method of 
synthesizing disilicon hexachloride in high purity. 
DISCLOSURE OF THE PRIOR ART 
Disilicon hexafluoride has been regarded as useful for a raw material of 
silicon semiconductors and it is also promising to an operating gas for 
separating silicon isotopes by infrared multiphoton dissociation. This 
disilicon hexafluoride has been synthesized by a well-known halogen 
exchanging method in which disilicon hexafluoride is fluorinated by such a 
fluorination agent as zinc fluoride or antimony trifluoride. 
In this method, however, since it is difficult to control a reaction when 
disilicon hexachloride is caused to react directly with the fluorination 
agent, a reaction temperature increases and the disilicon hexafluoride 
produced decomposes easily, and the reaction is prematurely terminated 
because surfaces of the fluorination agent are covered with a product. 
In order to overcome these problems, a synthesizing method has been 
proposed, in which a reaction is slowly accelerated by using such an 
organic solvent as butyl ether, alkyl chloride, benzene, toluene, xylene 
or chlorobenzene (Japanese Patent Provisional Publication No. 
195,108/1988). 
Even in this method, such a by-product as silicon tetrafluoride accounts 
for a considerable percentage of reaction products and it is, therefore, 
very difficult to obtain highly pure disilicon hexafluoride with a small 
amount of the by-product. 
An object of the invention is to provide a method of synthesizing highly 
pure disilicon hexafluoride containing impurities such as silicon 
tetrafluoride in an amount of within 1%, and synthesizing a large amount 
of the same so as to apply it as an operating gas for separating isotopes. 
This and other objects, features and advantages of the invention will 
become more apparent on reading the following detailed description and the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a method of synthesizing pure disilicon 
hexafluoride comprising the steps of making a suspension of a fluorination 
agent dispersed in an oxybenzene compound and causing disilicon 
hexachloride to react with said fluorination agent in nitrogen gas flows 
under atmospheric pressure, 
wherein said oxybenzene compound is expressed by a following formula: 
##STR2## 
(where each of R.sub.1 and R.sub.2 is an alkyl group alternatively having 
one or more substituents; carbon numbers summed up by R.sub.1 and R.sub.2 
are equal to at least 2; and m.gtoreq.1 and n.gtoreq.0). 
A reaction vessel, reaction appliances, a fluorination agent and a solvent 
to be used should preferably be sufficiently dehydrated in advance. This 
is because disilicon hexachloride and disilicon hexafluoride produced are 
easily hydrolyzed. An interior of the reaction vessel should also be 
shielded from air during a reaction. 
Any fluorination agent including well-known ones may be available for the 
fluorination agent to be employed. There is no particular limitation, but 
zinc fluoride is preferably suitable among them because a reaction is 
slowly accelerated. It is also desirable for the fluorination agent to be 
sufficiently dehydrated by heating it at about 300.degree. C. to keep an 
atmospheric pressure at 10.sup.-2 Torr. 
An oxybenzene compound to be used as a solvent should preferably be 
subjected to a dehydration treatment for a period of at least one week by 
means of a molecular sieve, for example. A mixing ratio in weight of this 
oxybenzene compound and the fluorination agent should preferably be about 
2 to 4. 
The oxybenzene compound is a compound in which one or more oxygen atoms are 
directly attached to a benzene ring, which is typically exemplified by 
such alkoxybenzenes as ethoxybenzene or propoxybenzene, such 
alkoxytoluenes as methoxytoluene, ethoxytoluene or dimethoxytoluene, or 
such alkoxyxylenes as methoxyxylene or dimethoxyxylene. Ethoxybenzene, 
methoxybenzene or methoxytoluene is more preferable among them. At least 
one alkyl group or other substituent may be attached to these oxybenzene 
compounds. 
A reaction is performed by dripping disilicon hexachloride into a suspended 
solution of the fluorination agent dispersed in the oxybenzene compound. 
This dripping is operated under such a condition as a temperature range of 
within 15.degree. to 40.degree. C., a nitrogen gas atmosphere and an 
atmospheric pressure. The resultant gas, for example, can be taken into a 
sample trap kept at -85.degree. to -90.degree. C. through an impurity trap 
maintained at -20.degree. to -25.degree. C. 
Pure disilicon hexafluoride containing impurities such as silicon 
tetrafluoride in an amount of less than 1% is synthesized by causing the 
disilicon hexachloride to react with the fluorination agent in the 
suspension with the oxybenzene compound which has a boiling point of more 
than 100.degree. C. and a property to calmly combine with the disilicon 
hexachloride. 
Disilicon hexafluoride is so excellent an operating material for separating 
isotopes that a great amount of isotopes may possibly be obtained with 
ease. It is, therefore, considered that this invention will contribute to 
progress in such a field as materials for nuclear power and 
semiconductors. 
Some embodiments of the invention will now be described by way of examples 
and with reference to the drawings. 
EMBODIMENTS 
EXAMPLE 1 
Zinc fluoride in an amount of 50 g was dehydrated at 300.degree. C. for 
about eight hours and it was placed in a glass vessel having a volume of 
one liter under a nitrogen gas atmosphere. This zinc fluoride was then 
suspended in sufficiently dehydrated ethoxybenzene in volume of 120 cc. 
Disilicon hexachloride in volume of 8 cc was dripped at a rate of 1 
cc/minute into the vessel in nitrogen gas flows under about 1.02 atm. The 
suspension, of which temperature was kept at 37.degree. C., was slowly 
stirred in order that the disilicon hexachloride does not directly contact 
with the zinc fluoride. 
A cooler kept at 0.degree. C. was connected to a gas exit of the reaction 
vessel to prevent the solvent from evaporating. A produced gas was taken 
into a sample trap kept at 90.degree. C. through an impurity trap 
maintained at 20.degree. C. It was confirmed that disilicon hexafluoride 
in amount of 6 g was synthesized. 
As shown in Table 1, a product yield of the disilicon hexafluoride was 75% 
relative to the disilicon hexachloride used for a raw material. 
As shown in Table 2, the resultant disilicon hexafluoride has high purity 
of 99.9%, while the percentage of silicon tetrafluoride (SiF.sub.4) 
produced as an impurity was about 0.1%. 
EXAMPLE 2 
Disilicon hexafluoride was synthesized in the same manner as in Example 1 
except that methoxytoluene was employed as a solvent. 
As shown in Table 1, a product yield of the disilicon hexafluoride was 
highest at a reaction temperature of 37.degree. C. and was 51%. 
As shown in Table 2, the resultant disilicon hexafluoride has high purity 
of 99.8%, while the percentage of silicon tetrafluoride (SiF.sub.4) 
produced as an impurity was about 0.2%. 
COMISON 1 TO 3 
Reactions were performed in the same manner as in Example 1, except that 
decane add butyl ether (Comparison 1), benzene (Comparison 2), and butyl 
chloride (Comparison 3) were employed as a solvent, respectively. 
As shown in Tables 1 and 2, both product yield and purity were deteriorated 
by far as compared with those of Examples 1 and 2. 
TABLE 1 
______________________________________ 
Test No. 
Comp. 1 
Ex. 1 Ex. 2 Decane + Comp. 3 
Ethoxy- Methoxy- Butyl Comp. 2 
Butyl 
Solvent 
benzene toluene ether Benzene 
Chloride 
______________________________________ 
Temp. 37 37 85 28 28 
(.degree.C.) 
product 
75 51 15 0 0 
yield 
(%) 
______________________________________ 
TABLE 2 
______________________________________ 
Test No. 
Comp. 1 
Ex. 1 Ex. 2 Decane + Comp. 3 
Ethoxy- Methoxy- Butyl Comp. 2 
Butyl 
Solvent 
benzene toluene ether Benzene 
Chloride 
______________________________________ 
Temp 37 37 85 28 28 
(.degree.C.) 
Purity 99.8 99.8 12 -- -- 
(%) 
SiF.sub.4 
content 
0.1 0.2 88 33 -- 
(%) 
______________________________________ 
*) Mark "--" means unmeasurable. 
EXAMPLE 3 
Disilicon hexafluoride was synthesized in the same manner as in Example 1, 
except that a reaction temperature was 20.degree. C. and a mixing ratio of 
the solvent to the zinc fluoride was varied. 
FIG. 1 points out high product yields of 40 to 60% at mixing ratios in 
weight of the solvent relative to the zinc fluoride of 2 to 4. 
EXAMPLE 4 
Disilicon hexafluoride was synthesized by using several solvents at a 
reaction temperature within the range of 5.degree. C. (278K) to 90.degree. 
C. (363K). 
As depicted in FIG. 2, disilicon hexafluoride was synthesized at product 
yields of 50 to 75% by using ethoxybenzene and methoxybenzene at 
temperatures of 15.degree. C. (288K) to 40.degree. C. (313K). 
Unlike this, in the cases where butyl chloride and benzene were 
respectively used, the product yield was 0%. 
In the cases where a mixed solvent with butyl ether and decane is used, the 
product yield was about 10 to 15%, though disilicon hexafluoride was 
synthesized at 60.degree. C. (333K) or higher. Purity was also bad since 
silicon tetrafluoride accounted for most of the product. 
It is needless to say that this invention is not restricted by these 
examples and that various modifications in detail are possible.