MEASUREMENT OF THE OXYGEN PERMEABILITY OF A COMPOSITE TEST STRUCTURE FOR THE ALICE TPC FIELD CAGE

Transcription

1 CERN ORGANISATION EUROPÉENNE POUR LA RECHERCHE NUCLÉAIRE EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH Laboratoire Européen pour la Physique des Particules European Laborat ory for Part icle Physics ALICE NOTE 00-xx MEASUREMENT OF THE OXYGEN PERMEABILITY OF A COMPOSITE TEST STRUCTURE FOR THE ALICE TPC FIELD CAGE J. Bächler, M. Mast, T. C. Meyer CERN, 1211 Geneva 23, Switzerland ABSTRACT This note reports on measurements of oxygen permeation through a sample of a low-mass composite structure proposed for the manufacture of the TPC field cage vessels. The investigated test structure was commercially fabricated of three separate pieces glued together via two solid splices. The specimen was produced by Fischer Advanced Composite Components G.m.b.H. in Ried/Austria in cooperation with CERN. CERN, April 26,

2 1.) Introduction Owing to their large size, the low-mass TPC field cage vessels will most likely be fabricated from several 5 m long arched segments of composite material, each individually manufactured in an autoclave process. Thus, the procedure of gluing these shells together to form the cylindrical vessel structure bears the risk of potential leaks along the seam. The design and structure of the field cage vessels are described in detail in the TPC Technical Design Report (TDR) [1]. Various methods of providing sufficient mechanical stability and leak tightness along the glue joints have been discussed in the past with commercial manufacturers of composite materials. In the following test, three pieces of a sandwiched honeycomb structure were joined by solid splices, and the seams covered on either side with an additional Tedlar foil to provide the required gas tightness. The residual permeability of the test sample to oxygen was determined by measuring the oxygen content of an inert gas volume (argon) enclosed by the test sample and a base container, both being exposed to air. In addition, a helium leak test was performed for comparison. 2.) Test Setup The test sample was manufactured by Fischer Advanced Composite Components. It was in the form of a disc made from three flat segments (Fig. 1) with two parallel glue joints, called seams, across the disc (Fig. 2). The porous rim of the disc was sealed with a metal support ring (RTV silicon rubber and Araldite epoxy resin) to avoid out-gassing of air from the exposed honeycomb web into the test volume. The disc with the metal ring was then glued with RTV silicon rubber on a circular metal base container (recipient) to form the test gas volume (Fig. 1). Argon was flushed through the system at atmospheric pressure and the oxygen content measured at the outlet of the system. Table 1 describes the dimensions of the test system, and Table 2 lists for Tedlar and RTV silicon rubber the permeability to oxygen. Fig. 1: Schematic of the test setup 2

3 ALICE NOTE 00-xx Fig. 2: Picture of the Tedlar disc (white) with two seams on its aluminum recipient Test sample diameter: 41 cm Test sample surface: 1320 cm 2 Test sample thickness: 3 cm Thickness of Tedlar foil: 100 µm Seal 1: 128 cm Seal 2: 130 cm Seam length of Tedlar : 60 cm Gas volume: 1452 cm 3 Table 1: Basic geometrical parameters of the test system DU PONT Tedlar of 100 µm: DOW CORNING RTV 3145: cm 3 /m 2 h cm 3 /m h Table 2: Oxygen permeability of Tedlar and RTV silicon exposed to a partial pressure of 200 mbar of oxygen. 3

4 3.) Measurements Three different measurements were made. For the first two measurements the flow of argon is kept at 10 l/h: (i) Measurement of the oxygen concentration at the outlet of the gas volume with a continuous argon flow as a function of time with only RTV sealing used. (ii) Same as (i), but with additional sealing, i.e. Araldite epoxy resin covering the original RTV seal. (iii) Detection of helium escaping from the test volume when flushing the recipient with helium. 4.) Results The time rate of change of the oxygen concentration in the test gas typically proceeds exponentially for constant gas flow. This is shown in figure 3 for the case of measurement 3 (ii), where the optimum sealing technique (RTV plus epoxy) was applied. Fig. 3: Exponential decay of the oxygen concentration in argon at constant gas flow. 4

5 ALICE NOTE 00-xx The lower curve of figure 3 shows that the oxygen concentration of the test gas reaches a residual level that is given by the oxygen contamination of the input test gas itself (2 ppm) and the penetration of atmospheric oxygen through the recipient walls. Table 3 summarizes the results of measurements 3 (i) and 3 (ii), comparing measured and calculated leak rates. In both cases, the oxygen impurity of the test gas was subtracted from the measured, asymptotic, value, i.e. 4 ppm in test run 3 (i) and 2 ppm in run 3 (ii). Expected Values Measured Values [ppm] [cm 3 /h] [ppm] [cm 3 /h] Measurement 3 (i) Measurement 3 (ii) Table 3: Comparison of calculated leak rates with measured results It should be noted that no estimates of the leak rates for the splice-glued seams themselves could be made. Since, however, the seams are covered with the same Tedlar foil as the entire surface of the disc, the sample surface was treated in the calculations as a continuous Tedlar membrane. Furthermore, it is known from previous measurements of this type [2] that the coating of RTV glue with epoxy resin leads to sealing properties comparable to the quality of welding joints. Thus, for the calculations of leak rates in scenario 3 (ii), only the permeability of the Tedlar surface was computed, neglecting any contribution of the double silicon-epoxy seal. Case 3 (i), on the other hand, takes the permeability of the RTV seal to the total leak rate into account. Helium measurements: In this test, a standard helium leak sensor (sniffer) was moved across the sample surface while the recipient was filled with helium. Neither along the seams nor at any other location of the sample surface did the leak rate exceed 8 x 10-9 atm/s. 5.) Conclusions The results of all three measurements which match the computed values to a very high degree demonstrate the excellent quality of the splice-gluing technique employed by Fischer Advanced Composite Components. Along the splice seam, the leak rate was no higher than at any other point on the Tedlar surface. Acknowledgement The authors wish to thank the technical assistance of the CERN TA2 group, in particular D. Fraissard and M. Mast. References: [1] TPC Technical Design Report, CERN/LHCC , January 7, 2000 [2] D. Vranic, Internal Note, unpublished [3] J. Bächler, Proposal for a Leak Test Procedure and Estimates of Leak Rates for the ALICE TPC Field Cage, Internal Note ALICE/