Refrigeration – Storage of solidified or liquified gas – Including cryostat
Reexamination Certificate
1999-08-26
2001-08-21
Capossela, Ronald (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Including cryostat
C062S383000, C165S276000
Reexamination Certificate
active
06276144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a thermal switch. More particularly, the present invention is directed to a device for alternately switching, on command, between having a high thermal conductivity and a low thermal conductivity.
2. Background Information
A thermal switch is a device for selectively conducting heat energy. In its on state, a thermal switch readily conducts heat energy. When switched into an off state, the thermal switch is a very poor conductor of heat energy. An ideal thermal switch has an infinite thermal conductivity in the ON state and a zero conductivity in the OFF state.
In cryogenic applications (e.g., below 60K), a suitable thermal switch does not exist in the prior art. An acceptable on-off ratio (ratio of ON state thermal conductance to OFF state thermal conductance) for cryogenic applications would be 1000:1. No prior art cryogenic thermal switch meets this criterion.
Referring to
FIG. 1
, a gas gap thermal switch is illustrated in cross-section. In this thermal switch, the hot side element
101
is in thermal contact with a heat source (not shown). The hot side element
101
is separated from cold side element
103
by a gap
105
. The gap
105
is the space formed between the frustoconical member
106
projecting from the hot side element
101
and the frustoconical cavity
104
in the cold side element
103
. A hermetically sealed container
107
surrounds the gap
105
and is used to selectively contain a thermally conductive gas.
When it is desired for the gas gap thermal switch to conduct in the ON state, the thermally conductive gas is placed inside
109
the hermetically sealed container
107
via the gas supply shown. When the thermally conductive gas is placed inside the hermetically sealed container
107
, heat energy is allowed to migrate from the hot side element
101
, across the gap
105
by means of the thermally conductive gas, to the cold side element
103
. When it is desired to switch the gas gap thermal switch to the OFF state, the thermally conductive gas is evacuated from the hermetically sealed chamber
107
, leaving a vacuum inside the void
109
and the gap
105
. When chamber
107
is evacuated, only a very reduced amount of heat energy is transmitted from the hot side
101
to the cold side
103
and conducted away by the cryocooler (not shown).
The gas gap thermal switch of the prior art has two major disadvantages. One is that it is a very complex device (because of the necessary gas handling structures and control system therefor, not shown) and, thus, is prone to failure. It requires a perfect hermetic seal in which the thermally conductive gas is contained. If the seal fails, then the switch will not properly work in an ON condition. Another disadvantage is the critical alignment required that cannot be verified after integration.
A failed attempt has been made to construct a thermal switch based on the principle of differential coefficients of thermal expansion, using the geometry of the gas gap switch shown in FIG.
1
. According to this prior art attempt, the hot side member
101
and the cold side member
103
are constructed of different materials which have different coefficients of thermal expansion, C
TE
. The hot side member
101
is chosen to have a coefficient of thermal expansion which is much smaller than the coefficient of thermal expansion of the material used for the cold side member
103
. As the temperature of the switch elements
103
and
101
rise, the dimensions of the cold side element
103
would expand at a faster rate than the dimensions of the hot side element
101
. Thus, as temperature rises, a gap would open up between
103
and
101
. This would cause the transition of the switch from the ON state to the OFF state. In order to reverse the state from OFF back to ON, the cold side
103
would need to be cooled down, thereby shrinking its dimensions so as to come into contact with the hot side element
101
.
The reason that this prior art attempt at a differential coefficient of thermal expansion type of thermal switch failed is because it is impossible to solve the problem of being able to make the surfaces of the two elements
101
and
103
meet together in a good thermal conducting relationship with good contact on a reliable basis. The problem goes beyond merely the challenges of how to machine and polish the surface of the frustoconical member
106
to sufficiently match the shape of the surface of the frustoconical cavity
104
. The primary problem is how to align the member
106
and the cavity
104
so that they reliably contact one another so as to produce positive engagement across substantially all of their opposed surface area. The smallest misalignment results in only point contacts at a few places, which is unacceptable because such minimal contact points produce very poor thermal conductivity for the ON state. Because of this mechanical mismatch of the surfaces, the two surfaces cannot reliably mate to one another and provide a reliable ON state. In addition, the proper alignment of the frustoconical member
106
and frustoconical cavity
104
is not easily verified when the switch is assembled. Consequently, the gap
105
is difficult to maintain and the switch does not work properly in the OFF condition.
Accordingly, what is needed is a cryogenic thermal switch that provides an adequate on-off ratio and a reliable ON state conductance. What is also needed is a cryogenic thermal switch which has a simple construction and which operates reliably.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thermal switch that solves the above-noted problems of the prior art.
It is a further object of the present invention to provide a thermal switch that operates reliably at cryogenic temperatures.
It is another object of the present invention to provide a thermal switch that operates with a high on-off ratio.
It is yet another object of the present invention to provide a thermal switch that operates based on differential coefficients of thermal expansion of materials.
It is still another object of the present invention to provide a thermal switch that operates with high reliability.
To achieve the above objects of the invention, the inventor has discovered a mechanical geometry and combination of elements for exploiting the differences in coefficients of thermal expansion of materials so as to construct a thermal switch which operates reliably and effectively at cryogenic temperatures.
To achieve the above objects, a thermal switch is provided for modulating heat conductance between a thermal load and a cooling element. The thermal switch has a hot side contact (in thermal communication with the thermal load), a cold side contact (in thermal communication with the cooling element), and a differential expansion member. The differential expansion member is disposed inside the hot side contact and is connected at a first end to the hot side contact and at a second end connected to the cold side contact.
It is important to note that this invention provides for an on-off ratio on the order of 1000:1.
REFERENCES:
patent: 3717201 (1973-02-01), Hosmer et al.
patent: 4770004 (1988-09-01), Logamos
patent: 5535815 (1996-07-01), Hyman
patent: 5842348 (1998-12-01), Kaneko et al.
Marland Brian
Stouffer Charles J.
Capossela Ronald
Roberts Abokhair & Mardula
Swales Aerospace
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