Seal for a joint or juncture – Seal between relatively movable parts – Close proximity seal
Reexamination Certificate
1998-12-01
2001-06-19
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Close proximity seal
Reexamination Certificate
active
06247701
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnet proof magnetic fluid sealing device, and particularly to a magnet proof magnetic fluid sealing device which is used, for example, as a bearing portion of a crystal lift apparatus for preparing the single crystal of a semiconductor material.
2. Description of the Prior Art
Recently, there have been strong demands toward the increase in diameter of a silicon single crystal prepared by a crystal lift apparatus. To meet these demands, for example, there has been reported a technique of controlling the convection of molten silicon by applying a magnetic field to the molten silicon, thereby adjusting the concentration of oxygen in the molten silicon. This technique is based on the fact that oxygen regarded as one of impurities to be contained in the silicon single crystal is eluted from quartz, an essential component of a crucible, kept at high temperatures and poured into the molten silicon, and then oxygen thus eluted travels on the surface of the growing single crystal by thermal convection and eventually it is diffused and absorbed in the single crystal.
Oxygen adsorbed on the surface of the single crystal is almost evaporated; however, in the case where rapid thermal convection occurs, oxygen is not perfectly evaporated and is possibly absorbed in the single crystal.
Meanwhile, in the molten silicon applied with a magnetic field, a current is induced to thereby suppress the degree of the thermal convection; however, if the degree of thermal convection is excessively suppressed, the molten silicon is undesirably filled with oxygen. Accordingly, the degree of thermal convection must be suitably controlled.
In this regard, there has been reported a method in which a magnetic field applied to the molten silicon is desired to be in a range of about 300 to 500 mT (mili-tesla) in order to solve the above-described problem, that is, to suitably control the degree of thermal convection.
In a magnetic fluid sealing device used as a bearing portion of a rotating feed-through device of the above crystal lift apparatus for preparing the silicon single crystal, the sealing is realized using a magnetic fluid magnetized by a magnetic field which is quite independent from the external magnetic field applied for controlling the convection of molten silicon.
The magnetic fluid sealing device used under the above circumstances may cause a problem that it no longer keeps the sealing characteristic by interference from a strong external magnetic field.
Referring to
FIG. 4
, there will be described a prior art magnetic fluid sealing unit.
FIG. 4
is a view illustrating an essential portion of a related art magnetic fluid sealing unit.
A magnetic fluid sealing unit
10
a
includes, as shown in
FIG. 4
, a shaft
3
made of a magnetic material, a permanent magnet
1
, and a pair of magnetic pole pieces
2
disposed on both sides of the permanent magnet
1
in such a manner as to hold the permanent magnet
1
therebetween. The permanent magnet
1
and the magnetic pole pieces
2
encircle the shaft
3
with micro gaps &dgr; put therebetween, so as to form a magnetic circuit. And, in the magnetic circuit, the micro gaps &dgr; are filled with a magnetic fluid
5
.
Referring to
FIG. 5
there will be described in detail another prior art magnetic fluid sealing unit.
FIG. 5
is an enlarged view illustrating an essential portion of another prior art magnetic fluid sealing unit.
The magnetic fluid
5
in the micro gaps &dgr; keeps the air-tightness of a vacuum region while withstanding a difference between a pressure in the vacuum region and a pressure which is substantially equal to atmospheric pressure is applied at each end of the shaft
3
. The prior art magnetic fluid sealing device having such a function is, for example, used as a bearing portion of a rotating feed-through mechanism for introducing rotation to an enclosed chamber in a vacuum environment.
The magnetic sealing function of the magnetic fluid
5
is given by a magnetic field generated by the permanent magnets
1
a
,
1
b
as shown in FIG.
5
. Accordingly, when being applied with an external large magnetic field different from the above magnetic field given by the permanent magnet
1
a
,
1
b
, the magnetic fluid sealing unit is largely affected by the external magnetic field.
When the magnetic fluid sealing unit having the magnetic circuit shown in
FIG. 5
is applied with an external magnetic field of about 10 mT, it may lose the magnetic fluid sealing function and often cannot keep the necessary degree of vacuum. That is to say, the sealing unit shown in
FIG. 5
does not keep its sealing function even when being applied with a very weak external magnetic field.
As shown in
FIG. 5
, a plurality of sets of permanent magnets
1
a
and
1
b
are arranged in such a manner that the permanent magnets
1
a
and
1
b
of each set are spaced from each other with the sides thereof having the same polarity opposed to each other; and a plurality of magnetic pole pieces
2
a
and
2
b
are alternately arranged in such a manner that each of the magnetic pole pieces
2
a
and
2
b
is placed between the associated set of the permanent magnets
1
a
and
1
b
, wherein magnetic poles of the same polarity are mutually faced in said associated set. Each of cavities O
1
, O
2
, . . . is formed in the central portion of an end portion, on the shaft
3
side, of the associated one of the magnetic pole pieces
2
a
and
2
b
, and each of acute-angled peaks P
1
, P
2
, P
3
, P
4
, . . . is formed on each of both the sides of the associated one of the magnetic pole pieces
2
a
and
2
b
. In each of the magnetic pole pieces
2
a
and
2
b
, a magnetic flux is divided into two components. The divided components of the magnetic flux which have the same polarity repel one another and are spread toward both side ends of the magnetic pole piece, respectively. In this way, in the case of the magnetic pole piece
2
b
, for example, there is formed a closed loop of the magnetic flux passing through the peaks P
3
and P
4
.
Accordingly, there is formed a magnetic field distribution in which magnetic fluxes are extremely acutely concentrated on the surface of the shaft
3
. With such a magnetic field distribution, the magnetic fluid
5
is significantly strongly held. It is confirmed that the magnetic fluid sealing unit in which the magnetic fields for holding the magnetic fluid
5
are strong as described above is allowed to keep the above sealing function even if an external magnetic field of about 300 mT is applied to the sealing unit in the direction where the external magnetic field directly interferes with the magnetic circuit of the sealing unit. In this way, it is proved that the magnetic fluid sealing unit shown in
FIG. 5
in which sets of the magnets are arranged such that each set is arranged with the sides thereof having the same polarity opposed to each other exhibits a good sealing function.
However, when an external magnetic field as strong as 300 mT or more is applied to the prior art magnetic fluid sealing unit shown in
FIG. 5
, the sealing unit is affected by the external magnetic field and is made difficult to keep its sealing function.
Referring to
FIG. 6
, there will be described a magnetic fluid sealing device for a rotating machine, which uses the magnetic fluid sealing unit shown in FIG.
5
.
FIG. 6
is a view illustrating the magnetic fluid sealing device for a rotating machine, which uses the magnetic fluid sealing unit shown in FIG.
5
.
A magnetic fluid sealing device
50
includes a case
20
for accommodating the entire magnetic fluid sealing unit; and a shaft
3
, made of a magnetic material, for transmitting rotational motion from one end
3
b
(atmospheric pressure side) to the other end
3
a
(vacuum side), which shaft is inserted into the case
20
. In the cavity of the case
20
is arranged a magnetic circuit of the magnetic fluid sealing unit
10
encircling the shaft
3
inserted into the case
20
; bearings
6
a
and
6
b
, arranged
Kitada Masakatsu
Shimazaki Yasuyuki
Antonelli Terry Stout & Kraus LLP
Knight Anthony
Rigaku Corporation
Williams Mark
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