IEEE TRANSACTIONS ON MAGNETICS, VOL. 24, NO. 2, MARCH 1988 1105

EFFECT OF MULTILAYER INSULATION ON RADIATION HEAT TRANSFER AT CRYOGENIC TEMPERATURES

T.R. Gathright and P.A. Reeve TRIUMF, Physics Department, University of Victoria, Victoria, B.C., Canada V8W 2Y2

Abstract: We have continued to measure the heat transfer rate, as a function of the number of layers of different types of multilayer insula- tion, for temperature differences of 284K to 77K and 77K to 4.2K. We found that the insulation reduced heat transfer between the higher tempera- tures but generally had a detrimental effect at the lower temperatures. An exception was double aluminum coated NRC2, which also gave a slight reduction in heat transfer for up to ten layers, at the lower temperatures. Attempts to explain the results using the proximity effect were not successful and we still think the results may be explained by outgassing effects. We have also measured the effect of polishing and plating the surfaces; the results are encourdging for the lower temperature range.

Introduction

At the 9th International Conference on Magnet Technology, we presented a paper which showed that several types of multilayer superinsulation were ineffective between temperatures of 77K and 4.2K El ] . Here we present some measured.data on some further types of superinsulation and again show that, except for double aluminum coated NRC2, they are also ineffective. We also give some analyses and data to try to explain why superinsulation does not work. Our equipment and procedures were described in detail in our first paper 111 and are not further discussed here. Except where mentioned in the text, the high temperature surface for our measurements was clean stainless steel. The spreads of the results in the figures are one standard deviation.

Results 77K to 4.2K

In Fig. 1 are shown two sets of measurements. The results marked KEK were for an aluminum foil with a thin nylon mesh insulation which was supplied to us for testing by the KEK laboratory. The heat transfer for this foil appeared to be essentially independent of the number of layers. Also shown in Fig. 1 and marked Al.+Zr. are some results for an aluminum foil which was covered with a fine powder of ZrOz for insulation. This insulation is interesting because it is completely inorganic and therefore nuclear radiation hard. Although there was some initial improvement with two layers, there was no further improvemenc for four layers.

We did measurements with just 3M aluminised adhesive tape on both surfaces, which gave .0125 f .003 w/mZ. inner copper surface and electropolished stainless steel on the outer surface; this gave .015 ? .001 w/mz, which is nearly as good as for the double 3M surfaces.

Because the multilayer insulation results pre- sented above and those in our previous paper were unexpected, we decided to check our equipment by repeating the measurements reported in [2]. In our first paper c11 we reported on measurements done using the same NRC2 superinsulation as had been used in c21. We subsequently discovered that the material that we tested then was not NRC2, but

We also measured with 3M tape on the

an equivalent superinsulation manufactured by National Metalizing. The foil tested in 121 was not a standard NRC2 but a special double aluminum coated NRC2, which is not normally available commercially. Fortunately we were able to obtain a sample from the Fermi Laboratory. Our results for this foil, which has a 500A thick aluminum coating, are shown in Fig. 2 labeled flTRIUMFTT, along with the results from [ l ] marked "FERMI". For these measurements the hot outer surface was covered with self adhesive 3M aluminised tape. It can be seen that, whilst the shape of the distribution of our results is similar to those of c21 (results labeled "TRIUMF NORMALISED"), the absolute values are about twice as big. We can find no reason to reduce our results, but the results in C21 included some large corrections. Apparently the conduction loss correction equalled or exceeded the final results; also there was a pressure drop correction which was of a similar magnitude. It is possible, therefore, that the results in 121 could be adjusted upwards. Our results for 3M aluminised adhesive tape are very similar to those in 121.

The checks discussed above suggest that our earlier conclusions were correct when we found that the effect of multilayer insulation is to worsen or to only slightly improve radiation heat transfer. One possible explanation for this is that, with more layers of mylar or other organic materials, more outgassing occurs. The outgassing materials are cryopumped into the spaces between the layers and frozen, causing thermal shorts, so that in effect the many layers become one laminated layer. the outer aluminum surface, so that the effective emissivity worsens. Eventually the thickness of the cryopumped material becomes thick enough for it to have the bulk material emissivity and no further deterioration occurs.

Another explanation comes from Hargreaves C31, who points out that a theory due to Polder and Van Hove C41 on heat transfer between closely spaced bodies may have a significant effect at low temperatures. The theory, in effect, describes the transition phenomenon that occurs between radiation heat transfer and pure conduction. They point out that, for radiation heat transfer, standing waves must be present and that, if the spacing between two surfaces is smaller than a wavelength, then radiation heat transfer cannot occur. However, another transfer phenomenon appears, the "proximity effect", which increases at a rate that is inversely proportional to the fourth power of the separation distance. From Planck's law it can be shown that the wavelength of the dominant radiation is inversely proportional to temperature, so that this effect is likely to be important at low temperatures.

Hargreaves suggests that, to avoid this effect at 4.2K, surfaces should be separated by more than 1 mm. To test this we did some measurements with aluminum foils separated by 1 mm thick fibreglass sheets. The results are shown in Fig. 3 (AL.+ FIBRE), where it can be seen that the performance is similar to the other measurements discussed above, which does not support the Hargreaves

Also they are cryopumped onto

0018-9464/88/0300-1105$01.0001988 IEEE

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suggestion. the fibreglass was particularly bad at outgassing, so we did some additional measurements where aluminum foils were suspended 1 mm apart on very thin nichrome wires. It is very difficult to suspend thin foils in this way and be sure that they are not touching, so the number of layers we could test was restricted but, as can be seen from Fig. 3 (Al.ONLY), the results appeared independent of the number of layers, to within two standard deviations. These results indicate either that the theory does not apply or that the separation distance of 1 mm was too large for the effect to be noticed.

The radiation component of the heat transfer rate Q (w) between two surfaces of area A (in2) at temperature T1 (K) and T2, with n layers of multilayer insulation between them, can be theoretically estimated from

However, we thought it might be that

Q 5.7 x lo-* e (T: - T?) - = ( 1 ) A (n + 1 )

It can be shown from Eq. 1 that the temperature of the ith layer of n layers is given by

where Ta = temperature of fixed hot surface Tb = temperature of fixed cold surface

The results of using Eq. 2 for hot and cold temperatures of 77K and 4.2K are shown in Fig. 4 for various values of llill and rtnn. It can be seen that, where heat transfer is by radiation only, the temperature of the coldest floating surface is quite high, typically 30K to 77K, and not close to 4K as thought by Hargreaves. It should be noted that, because the outer layer of insulation in our measurements was never closer to the outer hot surface than 1 cm, heat transfer to it must be by radiation only. Also, if the inner layers of insulation were effectively shorted to each other, because of the proximity effect, then the outer layer would be at the same temperature as the first layer, i.e. at about 77K. It would be expected, therefore, that the heat transfer would be the same for many layers as for one layer and would correspond to that at a temperature of about 7OK. In fact we find that the heat transfer can increase with number of layers. It seems, therefore, that the theory does not explain our results.

The only superinsulation that partially worked was the double coated NRC2. Because of its thick layer of aluminum it appears to be less transparent than the other types we have tested. However, as discussed in 111, its performance is much worse than theoretically expected, and above about five layers the performance flattens out.

At this time it is not clear why regular types of superinsulation do not work at 4.2K. We may be experiencing a combination of all these effects, including that from Eq. 1. In any case, our measurements indicate that multilayer insulation should not be used between temperatures of 4.2K and 77K. The lowest radiation heat transfer is achieved by using low emissivity surfaces such as 3M aluminised adhesive tape, or electropolishing on each surface.

Results for 284K to 77K

The results for 284K t o 77K are shown in Figs. 5 to 7. In all cases the heat transfer rate decreased initially with number of layers and then plateaued out. they followed the theory quite well, as can be seen from Fig. 7. For the results in Figs. 5 and 6 the outer hot surface was clean stainless steel; for the results In Fig. 7 the surface was 3M tape. We also measured the rate with 3M tape on the inner surface and electropolished stainless steel on the outer surface; this gave 6.2 w/mz.

It can be seen that results here do not correlate with those at 4.2K. In particular, the use of 3M tape is not so advantageous, and the order of performances is different. In general, the multi- layer insulation was effective in reducing heat transfer for up to about five to ten layers, beyond which there was little or no improvement. The best results were achieved using the double coated NRC2 (Fig. 7). Unfortunately this material is not available commercially.

While the rates were decreasing

Conclusions

Our results indicate that multilayer insulation has a beneficial effect at LNz temperatures for less than ten layers. Up to between five and ten layers the reduction in radiated heat transfer follows the predictions of Eq. 1. More than ten layers causes the heat transfer rate to remain the same or to increase. At LHe temperatures however, in all cases the effect of multilayer insulation was to increase or only slightly decrease the heat transfer rate, and none of them gave a decrease a3 predicted by Eq. 1. We must conclude, therefore, that the types of insulation we have tested are not useful for application at 4.2K. The best re- sults, confirming the result in [2], were obtained when we used just the 3M aluminised adhesive tape on the two surfaces. However, we did notice evidence of delamination after repeated pump and cooldown cyles. If extensive delamination were to occur, it could cause thermally conducting shorts between the surfaces. Therefore, it may in practice be better to electropolish the surfaces.

We are not sure why the multilayer insulation does not work, but we suspect it is because outgassing components were caught on the layers of insulation and frozen there when the LHe temperature was reached. The fact that cryopumping is observed in cryogenic systems when the helium cooldown is done supports this hypothesis. Scurlock and Sauli [SI report seeing evidence of the effect of outgassing in their measurements at LNz temperatures. The theory of Polder and Van Hove does not appear to give an explanation of our results. However, having determined from our results that multilayer insulation should not be used, the evacuated insulated space could, in principle, be made as small as possible; then the smallest separation distance of the two surfaces may be determined by the Polder and Van Hove theory. We plan to repeat the measurements that Hargreaves [31 did but between temperatures of 77K and 4.2K instead of at room temperature.

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The authors would like to thank their colleages at TRIUMF for their help, particularly T.A. Hodges and D. Healey for much useful discussion and advice, and T. Inglis and D. Evans for their patient help with the cryogens.

REFERENCES

[l] T.R.Gathright and P.A.Reeve, "Effect of Multi- layer Insulation on Radiation Heat Transfer from 77K to 4.2K," in Proceedings of 9th International Conference on Magnet Technology, Zurich, 1985, pp. 696-700.

[2] E.M. Leung et al., "Techniques for Reducing Radiation Heat Transfer between 77K and 4.2K" in Advances in Cryogenic Engineering, Vol. 25. Plenum Press, New York, 1979, pp. 489-499.

C31 C.M. Hargreaves, "Electromagnetic Proximity Effects and their Conseauences for Radiation Shielding", Proceedings of 5th International Cryogenic Engineering Conference, p. 574.

[4] D. Polder and M. Van Hove, "Theory of Radiative Heat Transfer between Closely Spaced Bodies", in Physical Review 8, Vol. 4, No.10, 15 November 1971.

[5] R.G. Scurlock and B. Sauli. "Development of Multilayer Insulations with Thermal Conductivities below 0.1 W em K", in Cryogenics 16, May 1979, p. 303.

00 r 0 1 2 3 4 5 6 7 8 9 1 0

# Layers

Fig. 1. Heat transfer 77K to 4.2K

03

<-TRIUMF

I - F Y

.oo 0 16 32 48 64 8

# Layers Fig. 2. Heat transfer 77K to 4.2K

0 1 2 3 4 5 # Layers

Fig. 3. Heat transfer 77K t o 4.2K

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8 0 1 10 I I I I

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Layer # F i g . 4 . M u l t i l a y e r t e m p e r a t u r e s between s u r f a c e s

a t 77K and 4.2K

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20

15 w/m‘

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\ u <-Al+Zr

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F i g . 5. Heat t r a n s f e r 284K t o 77K

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w/m’

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0 5 10 15 20 25 # Layers

F i g . 7 . Heat t r a n s f e r 284K t o 77K