Helium permeation in composite materials for cryogenic application

PII: S0011-2275(97)00124-0

Cryogenics 38 (1998) 135–142  1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0011-2275/98/$19.00

Helium permeation in composite materials for cryogenic application S. Disdier*, J.M. Rey*, P. Pailler* and A.R. Bunsell† ` *Commissariat a l’Energie Atomique, Saclay, DSM/DAPNIA/STCM, F-91191 Gif sur Yvette, France †Ecole Nationnale Superieure des Mines de Paris Centre, P.M. Fourt, F-91003 Evry, France

Received 2 September 1996; revised 7 July 1997 Glass fibre composites are attractive alternatives to metals for cryogenic applications because of their high specific strength and stiffness and their low electrical and thermal conductivity. A disadvantage is the ease of permeability of He through the material at room temperature, and after damage accumulation at 4.2 K. For this study, a special experimental leak detector was built based on a controlled helium flow through a diaphragm and mass spectroscopic measurement. Temperature cycles between room temperature (RT) and 77 K have no effect on permeation rates at 300 K. Tensile tests producing damage at room temperature showed different effects on permeation flow of He, caused by cracking. At 4.2 K, the limit for using this material is given by matrix cracking. The glass fibre volume fraction is preponderant in controlling the coefficient of He permeation. It has been found that composite materials such as glass fibre reinforced epoxy can be used effectively for cryogenic applications using a vacuum as thermal insulation.  1998 Elsevier Science Ltd. All rights reserved Keywords: gas permeation; helium gas; fibre glass composite

Glass fibre composites are widely used for non-metallic He cryostats1 (FRP cryostat). This material shows small thermal and electrical conductivities and high specific mechanical properties. This type of cryostat has been used in various fields because no eddy current losses can be induced. The main difficulty with this type of cryostat is the high permeability of helium through the FRP material at room temperature which causes a reduction of vacuum and an associated increase in heat transfer. Several studies2,3 have shown that He permeation can be expressed by an Arrhenius equation. The data extrapolated to liquid helium temperature would give a negligible permeation rate. Consequently, if an FRP shows gas leakage at low temperatures, it is attributed to cracking and/or thermal stresses. Previous studies4,5 have reported that these materials did not show leaks at any temperature and no significant changes of permeation performance after several thermal cycles between liquid nitrogen temperature and room temperature. Moreover, an increase of glass fibre content leads to an increase of mechanical properties and decrease of gas permeation at room temperature. We chose to study the permeation performance at room temperature before and after damage caused by thermal shock and tensile loading. We propose a qualitative interpretation of FRP cracking. We developed special equipment to measure the permeation rate and damage developed by thermal cycling in the range of room tempera-

ture to 77 K and by tensile tests at RT and 4.2 K. This study reports the effects of glass fibre content on permeation rate.

Materials tested The following commercially available glass/epoxy composites cured around 120°C have been investigated. Vetronite 64120 (manufactured by UDD FIM), samples reference U11. Vetronite 64120 satisfies the NEMA G11 classification. It is a DGEBA epoxy resin reinforced by E-glass fibres in the form of a taffeta weave cloth with a weight of 200 or 85 g m−2. The resulting fibre weight content was measured to be 57%. The thickness of samples was 0.56 mm, the density 1.78 g cm−3 and the porosity less than 2%. Permaglass TE 630 (manufactured by Permali), samples reference PLI. Permaglass TE 630 satisfies the NEMA G11 classification. It is composed of epoxy resin reinforced by E-glass fibres. Samples have been tested with weight fraction of glass fibre ranging from 41 to 78% and with a thickness of 1 mm. Prepreg Brochier 1452/50%/1664, samples reference PrP. This composite is an epoxy resin reinforced by E-glass satin cloth. The density of composite is 1.93 g cm−3.

Cryogenics 1998 Volume 38, Number 1 135

Helium permeation in composite materials for cryogenic application: S. Disdier et al. Sample

Mass quadrupol p1

GHe

p2 Recorder Diaphragm Vacuum Pumps

Figure 1 Apparatus for helium permeation measurement

Experimental Permeation tests An apparatus has been developed for measuring low helium leakage rate6. A schematic view is shown in Figure 1, and the permeation cell is detailed in Figure 2. The helium flow was determined by measuring the pressure of helium across a diaphragm which imposes the speed of the pump6, depending only on the temperature (T), mass of gas (m) and diaphragm diameter (Ad ). We used a mass spectrometer BALZERS QMA 120. In high vacuum the leak expression is given by: FHe =

冪2␲m(p kT

1

− p2 )Ad

where p1 and p2 are the pressure of helium measured across the diaphragm. To simplify the apparatus and the measurement, Ad has been chosen so as to be able to neglect the pressure near the pump (p2 ). The new expression of helium leak becomes FHe = CHepHeK where K is a special constant, determined experimentally, pHe the pressure of helium measured by the mass spectrometer calibrated on the mass of helium (pHe = p1 ) and Che as: CHe =

冪2␲mA kT

FHee A⌬p

where e is the thickness and A the area of the sample; ⌬p is the difference of the He gas pressure. The permeation tests were conducted on disks of 84 mm diameter and 0.56, 1 or 1.5 mm thickness. For damage studies, the samples were rectangular, 84 mm × 150 mm. The surface area was 43 cm2. During the test, the temperature of the sample was regulated by water circulation with a cryothermostat LAUDA 25. The experimental procedure was to introduce the disk into the cell and develop a vacuum (range 10−5 to 10−6 mb) so as to be able to connect the mass spectrometer. This process allows elimination of the outgassing problem during the measurement. After the helium was introduced, permeation was measured until steady state conditions. All data (temperatures, helium pressure, spectrometer helium intensity) were recorded on a computer provided with a data acquisition card for future treatment. Typical measurements of helium gas permeation are shown later in Figure 9. The curve shows a transitory state and a steady state where the permeation rate is calculated. The precision for the permeation rates is given to within about ± 15%.

Mechanical test and damage Mechanical tests were conducted on samples defined in NFT51-034 with overall lengths of 150 mm, widths of 10 mm and thicknesses of 0.56 and 1.5 mm. These tests were carried out using an INSTRON 4206 tensile machine fitted with 4 K cryostat made by TBT. For cryogenics tests the samples were immersed in liquid helium during experiments. The deformation of the samples was measured with an INSTRON extensometer over a gauge length of 50 mm. All the tensile tests were conducted at 0.2 mm min−1 with a recording sampling speed of 5 data s−1. For tensile tests, to provoke mechanical damage, the specimens were rectangular in shape. The tensile testing procedure consisted of a tensile test to a limited stress after an initial permeation test. This was followed by subsequent tensile tests in which the sample was stressed again to higher levels and the permeation measured again.

d

The sample divides the cell into two volumes. One volume is filled with gaseous helium at 1 atm and the other was pumped and connected to the leak detection system. After waiting to establish a constant flow rate through the sample, it becomes possible to express the permeability P by:

Figure 2 Detail of permeation cell

136

P=

Cryogenics 1998 Volume 38, Number 1

Experimental results Mechanical tests The stress/strain curves of the tensile tests at RT and 4.2 K on the U11 specimens are shown in Figure 3. The results are summarized in Table 1. The curve obtained at 4.2 K

Helium permeation in composite materials for cryogenic application: S. Disdier et al. 500 4.2 K

400

Stress in MPa

300

RT 200

100

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Strain in %

Figure 3 Stress/strain curves for composite material (type U11) at RT and 4.2 K Table 1 Average mechanical properties of composite materials at RT and 4.2 K (reference U11) Temperature (K) Young’s modulus (GPa) Failure stress (MPa) Failure strain (%) Number of tested samples

298 21.1 250 1.6 5

4 26.7 528 4 3

reveals a change of slope at 200 MPa and an increase of tensile properties at cryogenic temperatures. The tests were realized in the warp direction of the weave.

Permeation tests Figure 4 shows the importance of detecting the permeation rate by using a mass spectrometer. The mass spectrum shows the large contribution of outgassing (nitrogen, car-

bonic gas, oxygen, hydrogen) in residual vacuum and the low proportion of helium leakage compared to the other gases. With this detection, the time to obtain a steady state ranged from 3 h for the thinnest samples to 20 h for the thickest. The helium permeation rate was calculated for RT, and the solubility and diffusion rates calculated according to the method discussed in reference 6. The permeation rates are in agreement with published values on glass FRP, which are mainly in the range 10−14 to 10−17 mol s−1 m−1 Pa−1.

5 4.5 4 Mass intensity in 10-11A

H2O 3.5 3 2.5 2

OH N2

H2

1.5

H He

1

O2 0.5 CO2 0 0

5

10

15

20

25

30

35

40

45

M/ e (mass/ charge)

Figure 4 Mass spectrum during permeation tests

Cryogenics 1998 Volume 38, Number 1 137

Helium permeation in composite materials for cryogenic application: S. Disdier et al. Table 2 Permeation dependence on weight fraction effects in PLI specimens Volume fraction of glass 25 (%) Density (g cm−3 ) 1.57 Permeability 1.1 × 10−16 (mol s−1 m−1 Pa−1 )

35

53

65

1.87 6.7 × 10−17

1.95 2.5 × 10−17

2.1 1.6 × 10−17

Table 3 Temperature dependence on the permeability, diffusion and solubility Temperature (°C) 100/( T + 273) (K−1 ) Solubility (mol m−3 Pa−1 ) Diffusion (m2 s−1 ) Permeability (mol s−1 m−1 Pa−1 )

− 15 0.39

− 10 0.38

25 0.33

35 0.032

45 0.31

80 0.28

2.6 × 10−6

2.4 × 10−6

2.5 × 10−6

2.3 × 10−6

—

3.5 × 10−6

4.9 × 10−12 1.3 × 10−17

6.6 × 10−12 1.6 × 10−17

2.2 × 10−11 5.5 × 10−17

3.0 × 10−11 7.1 × 10−17

— 1.1 × 10−16

1.0 × 10−10 3.4 × 10−16

Table 4 Results of permeation test after mechanical damage at RT on U11 and PrP Specimen % Failure stress Solubility (mol m−3 Pa−1 ) Diffusion (m2 s−1 ) Permeability (mol s−1 m−1 Pa−1 )

U11 0 2.7 × 10−6

76 2.2 × 10−6

84 2.1 × 10−6

PrP 0 8.4 × 10−6

35 1.2 × 10−5

44 1.4 × 10−5

2.1 × 10−11 5.6 × 10−17

2.4 × 10−11 5.3 × 10−17

3.0 × 10−11 6.4 × 10−17

7.4 × 10−12 5.6 × 10−17

5.6 × 10−12 6.5 × 10−17

5.0 × 10−12 6.7 × 10−17

Table 5 Results of permeation test after mechanical damage at 4.2 K on U11 specimen 5 Loading level (MPa) % Failure stress Solubility (mol m−3 Pa−1 ) Diffusion (m2 s−1 ) Permeability (mol s−1 m−1 Pa−1 )

0 0 2.3 × 10−6 2.3 × 10−11 5.2 × 10−17

157 340 2.7 × 10−6 1.9 × 10−11 5.3 × 10−17

230 44 — — 1.2 × 10−14

Glass content effects

Discussion

The permeation tests at RT were conducted in the range of samples with different weight fraction of glass fibre. The results are shown in Table 2.

Effect of the glass content The results obtained with the reference PLI specimens have shown that increasing volume fraction of the glass fibres decreases the helium permeability rate. The lowest permeability is given when the specimen contains 100% glass or if the gas must pass through a region of high glass concentration.

Temperature effects The temperature dependencies of the permeation, diffusion and solubility rates were measured in the range from 260 to 350 K on U11 specimens. The results are shown in Table 3. Each value is the average of three permeation tests versus time.

Thermal damage effects Samples reference U11 and PrP were exposed to thermal cycles. The permeation was measured at RT before and after 50 and 100 thermal shocks from RT to 77 K. The effects of thermal damage caused by the differences of thermal expansion rates between the resin and fibres can break the bond and cause a formation of microcracks. At cryogenic temperatures this effect is enhanced.

Mechanical damage effect The results of permeation tests, with and without mechanical damage, are shown in Table 4 for U11 and PrP specimens at RT and Table 5 for U11 specimen at 4.2 K.

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Cryogenics 1998 Volume 38, Number 1

Temperature dependence of gas permeation The results obtained on U11 specimens can be expressed by an Arrhenius equation (Figures 5–7). By extrapolation, the composites used in this study can be impermeable to helium at about 250 K. The low variation of solubility with temperature shows that the permeability process is governed by diffusion. An activation energy of permeation of 25 kJ mol−1 was calculated for this specimen, in agreement with the published values5. Damage effects on permeation rates A qualitative interpretation of damage, caused by thermal shocks or mechanical stress, can be proposed. Three types of cracking can be defined and their effect on helium permeation rate discussed. Type I – Enclosed damage: has the effect of two thinner plates arranged in series.

Helium permeation in composite materials for cryogenic application: S. Disdier et al.

Permeability in mol.m-1.s-1.Pa-1

1,0e-15

1,0e-16

1,0e-17

1,0e-18 0,0027

0,0029

0,0031

0,0033

0,0035

0,0037

0,0039

1/T in K-1

Figure 5 Temperature dependence of He gas permeability

Diffusion in m2/ s

1.0e-09

1.0e-10

1.0e-11

1.0e-12 0.0027

0.0029

0.0031

0.0033

0.0035

0.0037

0.0039

1/ T in K-1

Figure 6 Temperature dependence of He gas diffusion

Soluble in mol.m-3.Pa-1

1.0e-05

1.0e-06 0.0027

0.0029

0.0031

0.0033

0.0035

0.0037

0.0039

1/ T in K-1

Figure 7 Temperature dependence of He gas solubility

Type II – Damage on one surface increases the permeation locally. Type III – Through damage: results in the gas penetrating the specimen without any diffusion mechanism. Figure 8 shows a schematic specimen with these three

Type III

Type I

Type II

Figure 8 Schematic specimen with three types of damage

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Helium permeation in composite materials for cryogenic application: S. Disdier et al. TYPE I DAMAGE

types of damage. The modification of helium flow is proposed in Figure 9. With this interpretation, it becomes possible to explain the modifications of test permeation.

He leak

Thermal shocks at liquid nitrogen temperature Ref

Time

The thermal shocks at 77 K had no noticeable effects on permeation, solubility and diffusion rates at RT. Therefore, microcracking caused by thermal expansion rate differences of the constituents has no detectable effect and the specimens conserve their permeation performance.

TYPE I DAMAGE

Mechanic damage at RT and 4.2 K

He leak

Ref

Time TYPE I DAMAGE

He leak

Ref

Time

Figure 9 Schematic qualitative modification with three types of damage

For damage provoked at RT, at 84% of failure stress, there is an increase of permeation rate. For lower stresses, no effect has been detected on the helium flow. With the qualitative interpretation expressed above, this modification could be attributed to Type II damage. Furthermore, cracking was observed by electronic microscopy investigations. In supposing that only the surfaces have been modified by the accumulation of damage, it is possible to assure that the diffusion and solubility rates have not changed. In this way it is possible to calculate an equivalent surface area, larger than the original surface, and an equivalent average thickness which is smaller than the specimen thickness. Using these calculated values and the original diffusion and solubility rates, it was possible to model the behaviour of the damage of materials. Numerical simulation agreed with tests on the damaged specimens, confirming that the damage can be interpreted as a modification of the surface area of material. On the other hand, Figure 10 shows particular behaviour due to Type I damage. The modification of the permeation rates appeared at very low stress levels. The composite damage is reflected by an apparent increase of solubility rates and an apparent decrease of diffusion (reference U11, Table 4). The results confirm that for imperfect materials, with voids for example, the permeation rate is not sufficient

5.0E-07

4.0e-07

He leaks in mb.l/s

Relative stress levels 3.0e-07 0%

2.0e-07

44%

35%

1.0e-07

0 0

50000

100000

150000

200000

Time in seconds

Figure 10 Evolution of He permeation rates with different stress levels at RT on PrP specimens

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Cryogenics 1998 Volume 38, Number 1

250000

300000

Helium permeation in composite materials for cryogenic application: S. Disdier et al. 1.5 Y = 1 + 2.23e-2X + 6.39e-4X2

R2 = 0.998

Normalised He flow (Y)

1.4

1.3

1.2

1.1

1.0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

"enclosed damage" in % of volume

Figure 11 Evolution of the steady state level with fraction of ‘enclosed damage’

to evaluate the damage. Compared to the first case discussed above, the numerical simulation for this damage is more difficult. We must consider not only the normal diffusion rates but the three directions of diffusion because of the existence of a pressure gradient caused by enclosed defects. An initial qualitative approach to model the behaviour has been developed using finite element theory in two dimensions. It is shown that steady state depends on the square of the quantity of ‘enclosed damage’ (Figure 11). The second result is that the evolution of the transitory part of the curve is similar to that proposed for Type I damage and also corresponds to the results obtained with the PrP specimens. Figure 12 shows the calculated permeation corresponding to 0% and 14% of ‘enclosed damage’. For damage provoked at 4.2 K, Figure 13 shows the level of loading. The ACK theory7 developed for UD composites explains the change of slope as the beginning of matrix cracking. This theory was used to describe the low temperature tensile behaviour of our samples. An increase of the permeation rate can be seen to have appeared at a

stress of 230 MPa. The composite damage is reflected by the absence of a transitory zone during the measurement and the leakage is three times bigger than for the other tests. This could be attributed to Type III damage which is characterised by ‘through damage’. A visual examination of the samples confirmed the existence of many cracks normal to the loading direction. These results confirmed the complete cracking of the matrix and that the material could not be used to contain helium with this damage. Permeation rates have been measured after 20 tensile loading cycles at 4.2 K up to 180 MPa, corresponding on the curve to the maximum stress before the change of slope. This result shows that the permeation rate was magnified about three times compared to the reference value. Visual observation has revealed several cracks which can be related to ‘through damage’.

Conclusion Thermal shocks on fibre glass composites between RT and 77 K had no effect on the helium permeation, solubility and

1.4 14% of enclosed damage

Normalised He flow

1.2 Reference

1 0.8 0.6 0.4 0.2 0 4.0E+10

8.0E+10

1.2E+10

1.6E+10

Time in seconds

Figure 12 Numerical calculation curve for 0% and 14% of ‘enclosed damage’

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Helium permeation in composite materials for cryogenic application: S. Disdier et al.

Stress in MPa

400

230 MPa 200 157 MPa

105 MPa 50 MPa

0

1

2

3

4

Strain in %

Figure 13 Loading levels at 4.2 K

diffusion rates at RT. These composite materials have been shown to conserve their properties even after 100 thermal shocks. The glass fibre composites have shown different tensile behaviours at RT and 4.2 K and also develop different damage processes and helium permeation behaviour. For damage caused at RT, no modification of helium permeation appeared except for stresses near failure. A Type II damage is postulated and gives good agreement with numerical simulation and surface observations. For some specimens, containing defects could appear under loading and change the shape of the helium permeation curves. These effects appear at very low stress levels. For damage caused at 4.2 K, modifications of permeation rates did not appear until the load exceeded the stress at which the slope of the stress/strain curve changed. Above this level, microcracking was reinduced in the specimens. This change in slope must be considered to be a limit for composites used to contain helium. This study has shown that the exploitation of transitory zones of the test permeation is essential in the understanding of the composite damage mechanisms. Glass reinforced epoxy resin composites are shown to be suitable candidates for structures storing helium at low temperatures.

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Cryogenics 1998 Volume 38, Number 1

Acknowledgements This work was supported by GEC Alsthom and L’Air Liquide. The authors wish to thank G. Lemiere and D. Thomas for their assistance in this study.

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