Gas permeability in polymeric membranes at low temperatures

1. TE-VSC Seminar 11.04.2017 Antonio Baldanza Gas permeability in polymeric membranes at low temperatures

2. Content • Introduction: Motivation, models • Methods of measurement • He and H2 permeation by dynamic and accumulation method at room temperature • He and H2 permeation at low temperature • Energies of activation: Permeation, diffusion, solution • Conclusions Baldanza Antonio TE/VSC-SCC

3. Motivation • Gas permeation of polymers at low temperature in particle physics experiments (Compass experiment) • Possible applications of polymer in cold vacuum system Compass hydrogen target lay out Baldanza Antonio TE/VSC-SCC 1

4. Introduction Permeability indicates the ability of a material to allow gas to pass through it. Permeation process consists of: The phenomenon is ruled by diffusion Fick’s laws. Model: membrane with constant thickness and area (assuming the gas flows only along one direction). Steady-state solution : (1) (2) [torr l s-1] = gas flow rate [l cm-3] = solubility coefficient [cm2 s-1] = diffusion coefficient [l cm-1s-1] = permeability coefficient Sample php plp J J A Intermediate temperature plate Baldanza Antonio TE/VSC-SCC 2

5. Permeation measurement Helium and hydrogen (as Compass experiment) have been chosen as permeant gas Two systems have been used: System 1 (RT measures) System 2 (LT measures) Measurements have been made on polyethylene terephthalate (PET) membrane (DN 40 copper gasket, thickness 0.036 mm) Baldanza Antonio TE/VSC-SCC 3

6. Dynamic method • V1, V3 (or V2, V4) and Vp1 are closed • Gas is injected in high pressure side • CP3 measures the pressure in HP side • RGA, previously calibrated, follows mass evolution in low pressure side • Measurement ends when the steady-state approaches Intermediate temperature plate Steady-state approaches when the curve is flat Baldanza Antonio TE/VSC-SCC 4

7. Example of dynamic measurement: He for helium is 42 l s-1 * The background signal has been subtracted ** The final result of the measurement is the average k Baldanza Antonio TE/VSC-SCC 5

8. Accumulation method • Gas is injected in high pressure side, measured by CP3 • V3, V4 and Vp1 are closed • CP1 or CP2 (both of them in two samples case) follow the pressure evolution in accumulation chamber • A first run, without gas injection, is necessary to estimate the outgassing chamber and to subtract it by the measured values • Measurement ends when the steady-state approaches Intermediate temperature plate Steady-state approaches when the curve is a straight line Baldanza Antonio TE/VSC-SCC 6

9. Example of accumulation measurement: He * The effect of the background is irrelevant Baldanza Antonio TE/VSC-SCC 7

10. RT permeability coefficient Intermediate temperature plate Baldanza Antonio TE/VSC-SCC 8

11. Low temperature experimental setup • AC: Accumulation chamber (Volume 0.22 l) • CP1: Capacitive pressure gauge • CP2: Piezo gauge • Plp: Penning gauge • RGA: Residual gas analyzer • TMP1, TMP2: Turbo molecular pumps • TS: Thermal shield System 2 schematic representation Baldanza Antonio TE/VSC-SCC 9

12. Cooling system 1 Cold head in insulation vacuum Intermediate temperature plate Thermalized sample Thermal shield Braids 5 2 3 4 Cryocooler system Baldanza Antonio TE/VSC-SCC 10

13. Background of accumulation chamber The outgassing at room temperature is dominated by hydrogen (after 2-3 months of pumping) Background measurements as a function of temperature have been made, in order to check the dependence Weak dependence from temperature has been shown It has been possible to estimate the minimum measurable permeation value : kmin= 1.8E-15 l cm-1s-1 Baldanza Antonio TE/VSC-SCC 11

14. RT data comparison: He Intermediate temperature plate Baldanza Antonio TE/VSC-SCC 12

15. LT permeability coefficient Accumulation method has been chosen (more stable values) and 3 measured values have been averaged Minimum measurable value Minimum measurable value Baldanza Antonio TE/VSC-SCC 13

16. Diffusivity measurement: Time Lag method The complete solutions of Fick’s laws for the previous model: (3) At long times (→): (4) t [s] = time [Torr l cm-2] = amount of gas passed in time t Assuming accumulation method condition: (5) (6) is called TIME LAG Baldanza Antonio TE/VSC-SCC 14

17. Diffusivity measurement: Time Zero At room temperature for the helium the time lag is very brief and it is influenced by the time of valve opening and gas injection The most important issue is the choice of the time zero The time lag increases strongly when the temperature decreases Baldanza Antonio TE/VSC-SCC 15

18. Diffusivity measurement: Results He As mean value has been chosen an average of three measurements Baldanza Antonio TE/VSC-SCC 16

19. Diffusivity measurement: Results H2 As mean value has been chosen an average of three measurements Baldanza Antonio TE/VSC-SCC 17

20. He Activation Energies Hs>0 the process is endothermic Baldanza Antonio TE/VSC-SCC 18

21. H2 Activation Energies Hs<0 the process is exothermic Baldanza Antonio TE/VSC-SCC 19

22. Conclusions and Perspective • Conclusions: • Performed measurements of permeation, diffusivity and solubility for He and H2 in the T range [157K, 294K] in PET • He permeation is larger than H2 permeation • He diffusivity is larger than H2 diffusivity • He and H2 solubility have shown two different behaviours (endothermic and exothermic, respectively) • Perspective: • Kaptonform Compass target • Push the system to lowest temperature • Develop a different method to estimate the diffusion coefficient using the RGA curve Baldanza Antonio TE/VSC-SCC 20

23. Thank you for your attention!