GB2435918A

GB2435918A - Thermal barriers

Info

Publication number: GB2435918A
Application number: GB0604836A
Authority: GB
Inventors: Paul Beasley; Adrian Mark Thomas
Original assignee: Siemens Magnet Technology Ltd
Current assignee: Siemens Magnet Technology Ltd
Priority date: 2006-03-10
Filing date: 2006-03-10
Publication date: 2007-09-12
Anticipated expiration: 2026-03-10
Also published as: GB0604836D0; WO2007105011A1; GB2435918B

Abstract

A thermal barrier is disclosed, comprising by a porous material impregnated with a fluid. The fluid is preferably helium, the porous material may be an epoxy resin impregnated cloth. An anisotropic thermal barrier between two regions 2 and 3 is also disclosed, wherein the thermal conductivity in a direction parallel to the interface between the barrier and one of the regions is greater than the thermal conductivity in a direction substantially normal to the barrier. This may be achieved with layers of two materials 4 and 5, preferably a metal and a polymer. A further thermal barrier is disclosed, comprising a solid state composite that undergoes a magnetic phase change at temperatures below 10K, the composite is preferably gadolinium oxysulphide. These thermal barriers are preferably used in a MRI scanner. Conduits for carrying cooling fluid through the barrier may also be supplied.

Description

Thermal Diffusion Barrier The invention is concerned with barriers to

thermal diffusion and has particular utility in superconducting coils.

Medical equipment such as Magnetic Resonance Imaging (MRI) scanners employ superconducting coils to generate the strong magnetic fields necessary for operation of the instrument. In order to achieve the superconducting state, the coil is maintained at a low temperature (typically about 4K) by means of a cryogenic refrigerant.

Failure to control the coil temperature can lead to quenching, where the coil enters a normal conducting state and the magnetic field collapses. This can occur as a result of localised heating in the coil which causes a localised increase in resistance. The increased resistance gives rise to further temperature rises which in turn gives rise to further increased resistance and so on. Thus, a localised temperature rise can give rise to complete quenching of the coil in a very short time. Typically a temperature increase of 0.Skelvin for more than 10 ms will be sufficient to initiate the event Electromagnetic forces generated within superconducting coils induce small discrete wire or bulk movements, particularly circumferential expansion of the coil structure. In multi-coil systems, the coils experience whole body forces which are restrained through interaction with a mechanical structure. Movement at the interface between the coil and the restraining former is a mechanism for energy release. Since materials (particularly solids) generally have low specific heat capacity at low temperature, the energy released at this interface is capable of locally raising the temperature of the superconductor above its critical temperature and inducing a quench.

Energy release at the interface has previously been limited through a number of techniques. Bonding the coil to the support structure can be employed but is often unreliable due to weaknesses in the bond, and the effect of stress transferred into the coil structure.

More generally, a low friction interface has been created through the use of flouropolymer materials to minimise energy dissipation.

In another approach, a layer of low thermal Conductivity material bonded to the coil structure acts as a thermal barrier. This is normally a fibre composite. Although fibre composites offer better specific heat capacity than many homogeonous materials, their properties are still poor at low temperature and significant thicknesses must be used to achieve any benefit.

This introduces a further problem of delamination if the thermal and mechanical properties of the composite are not well matched to those of the coil.

There are examples were thermally and mechanically matched materials have also been employed with limited success (e.g. copper or aluminium rings).

The invention provides a thermal diffusion barrier which achieves improved thermal isolation between, for example, a coil and an interface plane without use of excessive material thickness.

According to a first aspect of the invention, a thermal diffusion barrier comprises the features set out in claim 1 attached hereto.

According to a second aspect of the invention, a thermal diffusion barrier comprises the features set out in claim 8 attached hereto.

According to a third aspect of the invention, a thermal diffusion barrier comprises the features set out in claim 17 attached hereto.

Referring to figure 1, a thermal diffusion barrier 1 is located between a superconducting coil 2 and a support structure 3. Relative movement of the barrier 1 against the support structure 3 gives rise to heat 0 which, to some extent passes through the barrier and causes a local temperature rise in the coil 2.

In one embodiment, a porous layer loaded with fluid serves as the thermal diffusion barrier 1 between the coil 2 and the support structure 3.

Most solid state materials have very low specific heat capacities (for example Copper has a volume specific heat of 8.1 x i0 J/cc/K) which renders them unsuitable as thermal insulators for superconducting coils: only a small amount of heat is required to raise the temperature of the material and consequently that of the coil.

In this embodiment the porous layer is loaded with a fluid that has a high specific heat capacity, for example helium (volume specific heat of 3.7 x 1 0 J/cc/K), to provide a barrier having a greater capacity to absorb heat generated at the barrier/former interlace, thus protecting the coil.

The porous layer could be realised as multiple layers of epoxy impregnated cloth. An open weave cloth would be suitable for this purpose, that is a woven fibre material wherein the gaps between the material strands are of a similar size to, or greater than, the size of the strands themselves.

In another embodiment of the invention, the thermal barrier 1 comprises a high specific heat capacity solid state composite.

The composite takes the form of a filled resin comprising a material with a low temperature magnetic phase change causing it to have a high specific heat capacity in the temperature range where the phase change occurs. Such materials are typically compounds based on the heavy transition elements, Holmium,Ertjum or Gadolinium. The preferred material for this application would be Gadolinium Oxysulphide (GOS) having a phase change in the 4-5.5K temperature range and being of reasonable cost.

The material is used in powder form dispersed within an epoxy resin (bisphenol epoxy). This filled resin may be further combined with a filamentary material such as glass or polymer fibres to create mechanical properties which match those of the coil.

Referring to figure 2 another thermal diffusion barrier la according to the current invention comprises a barrier having anisotropic thermal conductivity, and is arranged so that the thermal conductivity K in a direction substantially parallel with the interlace between the barrier 1 and the coil 2 is greater than the thermal conductivity K in a direction substantially normal to the interlace.

This arrangement provides an increased tendency for heat to dissipate along the thermal diffusion barrier 1 rather than passing through to cause localized heating of the coil.

Referring to figure 3, the anisotropic barrier necessary for one embodiment of the invention could be realised as a plurality of layers of material wherin alternate layers have high (layers 4) and low (layers 5) thermal conductivity.

Multiple layer composite comprising alternate layers of high and low thermal conductivity material produces bulk thermal conductivity which is higher in the in-layer directions than the perpendicular one, This has the effect of dispersing a transient energy pulse liberated at a point on one side of the layer resulting in a significantly lower energy density at the coil. High conductivity layers would typically be metallic in nature while low conductivity layers would be polymer based.

The presence of high conductivity material between the former and coil would increase the thermal conductivity and so for a steady state distributed heat input would produce a poorer condition than the low conductivity material alone. However the nature of the energy input is both transient and localised so that the ratio of the conductivity in the perpendicular directions reduces the energy density at the coil surface. It can be shown that the ratio of conductivities in the two directions is approximately equal to the ratio of the two material conductivities. Typically, conductivity ratios of 10 can be achieved between metallic and composite materials.

The embodiments described above may be combined to provide a thermal diffusion barrier having a plurality of layers wherein alternate layers are materials of high thermal conductivity (for example metal foil) and porous material impregnated with helium.

Use of a filled resin bonding agent for the porous layers has the additional advantage of providing the coil against mechanical damage during assembly and wiring of the magnet system.

These structures bonded into the coil block can also include mechanisms for direct cooling of the coils and interface. In this configuration, cooling tubes, carrying liquid cryogens, can also be embedded, improving their efficiency.

Claims

Claims 1. A thermal barrier comprising a porous material impregnated

with a fluid.

2. A thermal barrier according to claim 1, arranged between a superconducting coil and a support structure for the coil.

3. A thermal barrier according to claim 2, wherein the porous material comprises an open weave, epoxy resin impregnated cloth.

4. A thermal barrier according to claim 3 wherein the epoxy resin serves to bond the barrier to the superconducting coil and to the support structure.

5. A thermal barrier according to claim 4, where the fluid is helium.

6. A thermal barrier according to claim 5, further including at least one conduit, suitable for carrying fluid throughout the barrier.

7. A thermal barrier according to claim 6, incorporated in a Magnetic Resonance Imaging (MRI) scanner.

8. A thermal barrier between two regions, said barrier having anisotropic thermal conductivity and being arranged such that the thermal conductivity of the barrier in a direction substantially parallel with an interface between the barrier and one of the regions is greater than the thermal conductivity in a direction substantially normal to said barrier.

9. A thermal barrier according to claim 8, comprising at least one layer of a first material and one layer of a second material, the first material having a higher thermal conductivity than the second material.

10. The thermal barrier of claim 9, comprising a plurality of layers of the first material arranged alternately with a plurality of layers of the second material.

11. The thermal barrier of claim 10 where first material comprises a metal.

12. The thermal barrier of claim 11 where the second material comprises a polymer.

13. The thermal barrier of claim 11 wherein the second material comprises a porous material impregnated with fluid.

14. The thermal barrier of claim 13 wherein the fluid is helium.

A thermal barrier according to claim 14, arranged between a superconducting coil and a support structure for the coil.

16. A thermal barrier according to claim 15, incorporated in a Magnetic Resonance Imaging (MRI) scanner.

17. A thermal barrier comprising a solid state composite, which composite includes a material that undergoes a magnetic phase change at temperatures below 10K.

18. The thermal barrier of claim 17, wherein the material comprises a compound of holmium, erbium or gadolinium.

19. The thermal barrier of claim 18 where the material comprises gadolinium Oxysulphide 20. A thermal barrier according to claim 17, 18 or 19 arranged between a superconducting coil and a support structure for the coil.

21. A thermal barrier according to claim 20, incorporated in a Magnetic Resonance Imaging (MRI) scanner.

Amendments to the claims have been filed as follow Claims 1. A thermal barrier, arranged between a superconducting coil and a support structure for the coil, said barrier comprising a porous material impregnated with a fluid.

2. A thermal barrier according to claim 1 wherein the porous material comprises an open weave, epoxy resin impregnated cloth.

3. A thermal barrier according to claim 2 wherein the epoxy resin serves to bond the barrier to the superconducting coil and to the support structure.

4. A thermal barrier according to claim 3, where the fluid is helium.

5. A thermal barrier according to claim 4, further including at least one Conduit, suitable for carrying fluid throughout the barrier.

-S 6. A thermal barrier according to Claim 5, incorporated in a Magnetic Resonance *5 * *,* Imaging (MRI) scanner. S * * I * . SI'.. * I.

I II * -I.