FR2600073A1 - Thermal contact with high transfer coefficient and applications - Google Patents

Abstract

Thermal contact with high transfer coefficient between two materials which can have different expansion coefficients, consisting of compressed expanded graphite inserted between the said materials. The two materials can be chosen from carbon-containing materials, ceramics and metals or metal alloys. Expanded graphite can be inserted in the compressed state in the form of a laminated or compressed sheet, or can be compressed in situ. Application especially to thermonuclear fusion reactors and to dies and moulds for continuous casting.

Description

THERMAL CONTACT WITH HIGH TRANSFER COEFFICIENT
AND APPLICATIONS
The present invention relates to a high coefficient of thermal contact
transfer between two materials that may have
different dilation.

These materials are selected from carbon materials, ceramics
and metals or metal alloys.

Carbon materials are essentially carbon and
industrial graphites, carbon-carbon composites.

In general, it is known that such materials, in particular
if they have different coefficients of expansion, can difficult
to be assembled with good thermal contact, which is harmful
ble for many applications.

So for example, in devices where one needs to cool
an element made of carbon material by a circulation of water, the fragility
and the porosity of the carbonaceous material generally prevents cooling
directly and it is necessary to fix the element of carbon material a
metal element. Then arise the problems of fixation
and thermal contact.

To have an acceptable thermal contact, we must exert
clamping pressures greater than 100 kPa. In the best cases,
gets transfer coefficients of the order of 9 x 103 Wm-2.K-1.

These transfer coefficients are very sensitive to surface conditions
elements and little reproducible, which is obviously very embarrassing.

When the heat flows to be transmitted are very important, like,
for example, in nuclear fusion reactors of the TOKOMAK type or in continuous casting, because of the expansions the contact
conductivity is degraded and heat exchange is only
radiation. There is then a heating of the parts important eyelash
may possibly be redhibitoioiie for l lifespan.

One way to solve this problem is soldering. This solution, very effective with certain materials, is expensive and imposes a lower operating temperature than the melting temperature of the solder.

In addition, for materials having very different expansion coefficients, it is possible, in some cases, to braze them by interposing a metal sheet which accommodates the stresses. It is then necessary to use expensive metals and delicate to implement such as molybdenum, zirconium, etc ... Finally, ceramics such as carbide and silicon nitride are extremely difficult to solder especially if they are sintered at a density close to the theoretical density.

The main object of the invention is to have a thermal contact simpler to implement, more economical and can be used at high temperature - (greater than 20000C if the materials to assemble permit-entj.

According to the invention, the thermal contact with a high transfer coefficient between two materials that can have different expansion coefficients is characterized in that it consists of compressed expanded graphite, inserted between said materials.

The materials to be thermally bonded are chosen from: - carbon materials: carbon and artificial graphites such as
vitreous carbon, polycrystalline graphites, etc ..., composites
carbon-carbon, - ceramics such as silicon carbide, silicon nitride,
boron carbide, tantalum carbide, - metals and metal alloys
According to the invention, the thermal contact may be, for example, inserted between two different carbonaceous materials, a carbon material and a metal, a ceramic and a metal.

Compressed expanded graphite has excellent thermal conductivity in the plane of compression and much lower thermal conductivity in the perpendicular direction. But, it also has good elasticity. As a result, it keeps conductive contact even for strong thermal deformations of the materials.

It is recalled that this type of graphite is obtained by abrupt heating of lamellar graphite at a temperature up to 10000C, thus giving an exfoliated graphite whose density is of the order of 0.01. This graphite can then be compressed into blocks of density ranging from 0.1 to 0.2 or rolled into sheets of 0.5 to 1 mm thick, with a density of about 1.

Various tests show that the thermal transfer coefficient of two surfaces of materials which can have different expansion coefficients is always improved when compressed expanded graphite is inserted between said surfaces. This gain depends on the temperature, the clamping pressure of the materials, their surface condition and their nature.

Expanded graphite can, according to the invention, be inserted compressed in the form of rolled or compressed sheet. It can also be inserted uncompressed and compressed in situ when placing the materials. This last variant is advantageously used when the surfaces of the materials are not flat and / or very rough.

Expanded graphite may also comprise a filler such as, for example, a metal which improves its conductivity.

The following examples, given by way of indication and not limitation, illustrating the invention.

Example 1

A cylindrical disk of diameter 50 and thickness 15 mm is applied to a metal surface by central support.

The graphite (grade 1346 and the applicant) has a coefficient of expansion of 5.5 to 6 × 10 -6 K -1.

The metal is 316L stainless steel with a coefficient of expansion of 16 x 10-6K-1.

Comparative tests are carried out at different application pressures with the same transmitted power equal to 75 watts, using or not a thermal contact consisting of a sheet of compressed expanded graphite of density 1 and thickness 0.2 mm.

The results are summarized in Table I below.

Table 1

<tb>: <SEP> Pressure <SEP>: <SEP> 10 <SEP>: <SEP><SEP> 80 <SEP>: <SEP> 120
<tb>: <SEP> in <SEP> KPa
<tb> A <SEP> A <SEP> in
<tb><SEP> W.m2K1 <SEP> 1.7-x <SEP> 10 <SEP>: <SEP> 1.4 <SEP> x <SEP> 10 <SEP>: <SEP> 2.2 <SEP> x
<tb><SEP> B <SEP> in
<tb>: <SEP> Wm-2K-1 <SEP> 2.7 <SEP> x <SEP> 104 <SEP> 3.4 <SEP> x <SEP> 104 <SEP> 3.6 <SEP> x <SEP> 104 <SEP>
<Tb>
A: Thermal transfer coefficient without thermal contact in
expanded graphite tablet.

B: Thermal transfer coefficient with thermal contact according to
the invention.

Example 2

Example 2 is identical to the example: with the difference that the graphite is replaced by another graphite (grade 5890 of the applicant) having a coefficient of dilatation: 4.5 × 10 -6 K -1 Table 2 below similarly groups the results of the tests made under the same conditions as those of Example 1.

Table 2

<tb> Pressure <SEP>: <SEP> 10 <SEP>: <SEP><SEP> 80 <SEP>: <SEP><SEP> 120
<tb>: <SEP> in <SEP> kPa
<tb>: <SEP> A '<SEP> in
<tb><SEP> Wm-2.K-1 <SEP>: <SEP> 5 <SEP> x <SEP> 10 <SEP>. <SEP> 6 <SEP> x <SEP> 10 <SEP>: <SEP> 9 <SEP> x
<tb>: <SEP> B '<SEP> in
<tb><SEP> Wm-2.K-1 <SEP><SEP> 104 <SEP>: <SEP> 2 <SEP> x <SEP> 104 <SEP>: <SEP> 3 <SEP> x
<Tb>
A ': Thermal transfer coefficient without thermal contact in
expanded graphite tablet.

B 'Thermal transfer coefficient with thermal contact according to
the invention.

From these tests, it is found that the heat transfer coefficients with a contact according to the invention are of the order or greater than 104W.m.-2.K-1. With other material couples and / or different conditions, they reach values of 6 x 104W.m. -2.

These high values show the interest of the invention.

The invention thus makes it possible to produce various thermal assemblies by choosing the pair of materials that can have different expansion coefficients. These assemblies may in particular constitute: - protection tiles, limiters or diverteurs for nuclear fusion reactors, - dies or shells for continuous casting.

 Finally, it should be noted the last advantage of the invention: the thermal contact is only inserted between the materials, the assemblies can be easily dismounted, which may: oour applications have some interest.

Claims (14)

1. Thermal contact with a high transfer coefficient between two materials that can have different expansion coefficients characterized in that it consists of compressed expanded graphite inserted between said materials. 2. Thermal contact according to claim 1 characterized in that the two materials are selected from carbon materials, ceramics and metals or metal alloys. 3. Thermal contact according to claim 2 characterized in that the two materials are both carbonaceous materials. 4. Thermal contact according to claim 2 characterized in that the two materials are a carbon material and a metal or a metal alloy.  5. Thermal contact according to claim 2 characterized in that the two materials are a ceramic and a metal or a metal alloy. 6. Thermal contact according to any one of claims 1 to 5 characterized in that the expanded graphite is inserted compressed in the form of laminated sheet or compressed. 7. Thermal contact according to any one of claims 1 to 5 characterized in that the expanded graphite is compressed in situ. 8. Thermal contact according to any one of claims 1 to 7 characterized in that the expanded graphite. tablet contains a charge. 9. Thermal contact according to claim 8 characterized in that la- cha. e is a metal. 10. Thermal assembly of materials that can have different expansion coefficients characterized in that it comprises a thermal contact as claimed in any one of the preceding claims. 11. Thermal assembly according to claim 10 characterized in that it constitutes a protective tile for nuclear fusion reactors. 12. Thermal assembly according to claim 10 characterized in that it constitutes a limiter or a diverteur for nuclear fusion reactors. 13. Thermal assembly according to claim 10 characterized in that it constitutes a die for continuous casting. 14. Thermal assembly according to claim 10 characterized in that it constitutes a shell for continuous casting.