Chemical and radiolytical characterization of some perfluorocarbon fluids used as coolants for LHC experiments Part one

Chemical and radiolytical characterization of some perfluorocarbon fluids used as coolants for LHC experiments Part one: Chemical characterization Part two: Radiation induced effects and purification Sorin ILIEa, Radu SETNESCUa,b aEuropean Organization for Nuclear Research (CERN) CH-1211 Geneva 23, Switzerland bNational Institute in Electrical Engineering - Advanced Research (ICPE CA) 313 Spl. Unirii, Bucharest, Romania

SUMMARY - Perfluorocarbon fluids were characterized by applying different methods: GC, FT-IR, UV-Vis, potentiometry, distillation, etc. - The first aim of this work was the quality control, the identification and the quantification of different impurities which could increase the radiation sensitivity of these fluids. - The procedures settled-up in this work are sensitive to the presence of disturbing impurities and were used for the analyses of the as received perfluorocarbons and for the irradiated fluids. - The second aim of this work was to assess the radiation induced modifications on different fluids irradiated gamma Co 60: acidity (HF), polymer & pre-polymers, new molecules, etc. - Cleaning tests were carried out on the as received fluids and on the irradiated ones to assess the efficiency of such purification treatments.

The main perfluorocarbon fluids studied in this work

 Typical GC-TCD and GC-MSD spectra from the analyzed fluids are shown in Figs. 1 and 2. • The purities of the received fluids were found to be higher as 98.5 %, conform to the CERN Technical Specification. • The chemical nature of some impurities present in n-C6F14 was found to be different from that evidenced in iso-C6F14. The perfluorinated isomers and their homologous compounds, commonly present, are not foreseen to be detrimental to the behavior of the cooling fluids during irradiation. Fig. 2 GC-TCD chromatograms for various perfluorocarbons: (1) C3F8; (2) n-C6F14; (3) iso-C6F14 Fig. 1 GC-MSD chromatograms for the studied perfluorocarbons: (1) C3F8; (2) n-C6F14; (3) iso-C6F14

In contrast, the presence of the H-containing molecules (such as hexane, hydrofluoroalkanes, etc.) or of the double bonds containing ones (alkenes, hydrofluoroalkenes, perfluoroalkenes, etc.) could result in an increased radiation sensitivity of the cooling fluids. (see Fig. 3 curve 1).  The introduction of known and controlled impurities confirmed the assignment of the peaks (see Fig. 4 a and b). The detection limit found for these impurities was around 30 ppm owing the used analytical parameters. The results of the purification tests, illustrated in Figs. 3 and 4, confirmed a promising way to the removal of the undesired impurities. Fig. 3 – GC-MSD chromatograms for the as received n-C6F14 (1) and the purified (2) one;

C6H14 Fig. 4 Zoom on the GC-MSD chromatograms of n-C6F14 impurified with: (a) hexane: 1 – 300 ppm; 2 – 30 ppm; 3 – purified; (b) 1-HC6F13: 1 - 300 ppm; 2 - 30 ppm; 3 – purified.

Fig. 5 (a) FT-IR spectra in the region of 2900 cm-1 of n-C6F14 containing different amounts of hexane: 1 - reference (purified n-C6F14; 2 - 5 ppm; 3 - 10 ppm; 4 - 20 ppm; 5 - 50 ppm; 6 - C6F14 initial, as received (b) The FT-IR calibration curve as a function of the hexane concentration and of the hexane like compounds (c) Comparative spectra in the region of 3100 - 2800 cm-1 from controlled impurified C6F14 : 1 - reference (pure C6F14); 2 - 1-HC6F13 300 ppm; 3 - hexane 50 ppm

Fig. 6 • FT-IR spectra in region of 1780 cm-1 from C6F14 samples impurified with C6F12: • 1 - reference (pure C6F12); 2 - 20 ppm; 3 - 50 ppm; 4 - 100 ppm; 5 - 200 ppm; • 6 - 386 ppm; 7 - 1000 ppm • (b) FT-IR calibration curve for determination of C6F12

UV-Vis results are also consistent to the GC data, the optical absorption in the spectral range 190 - 220 nm being increased when perfluorohexene or 1-H perfluorohexane were added to pure C6F14. All as received samples exhibited a certain optical absorption in UV spectral range, which was strongly decreased following the purification treatment (Fig. 7 a, b). • Fig. 7 • UV spectra in region of 190 – 240 nm from C6F14 samples impurified with C6F12: • 1 - reference (pure C6F12); 2 - 20 ppm; 3 - 50 ppm; 4 – 100 ppm; 5 - 200 ppm; • 6 - 400 ppm; 7 - 1000 ppm. • (b) The UV calibration curve at 195 nm for the C6F12dosage.

Main results of the chemical characterization of the C6F14fluids:  = test passed; test not passed Characterization of the initial fluids *From GC-MSD measurements +The content in H-substitutes PFC was below the detection limit in all cases as shown both FT-IR, GC-MSD and GC-TCD measurements ++The CERN imposed density value refers to n-C6F14; the value found in literature for pure perfluoro 2-methyl pentane (iso-C6F14) is 1.723 [http://www.chemexper.com/]; that given by the supplier (F2) is 1.725 g/cm3 [Certificate of Analysis Batch No 0294/ Ref. No. P51917]. Thus, the measured density value corresponds to a pure fluid.

Characterization of the initial fluids Main results of the chemical characterization of the C3F8fluids:  = test passed; test not passed *GC-MSD

Conclusions on the chemical characterization of the perfluorocarbons • The aim of Part I of the work was successfully accomplished using the chemistry laboratory analytical techniques and methods, which were sensitive to the parameters or the properties of interest: these techniques should be used for the future characterization and analyses.  The following perfluorocarbon fluids supplied to CERN: PP1, PF 5060 DL, and C3F8 were found compliant to the CERN quality requirements and to the analysis certificates delivered by the respective producers. PF 5060 as received and FC 72 were not compliant in all points, as expected, to our quality requirements, mainly concerning the hydrogen content.  The acidity (HF content) of the all received fluids was below the imposed CERN limits. • The preliminary purification tests on less pure fluids were very promising. The work will continue and the tests will be adapted and optimized, thus making other perfluorocarbon fluids quality grades to meet the CERN quality requirements. In such a way the potential suppliers and the range of the available fluids, possibly less expensive, may be advantageously enlarged. A purified PF 5060 fluid was prepared in the laboratory and used in irradiation tests.

IRRADIATED PERFLUOROCARBON FLUIDS • Expected doses: max. 30 kGy • Irradiation: two gamma 60Co doses: 28 kGy and 56 kGy • Irradiation configuration: 60Co source 150 kCi (IRASM) • Irradiated fluids • Radiation induced modifications: • - acidity (HF) • - polymers & pre-polymers • - new molecules • Cleaning (purification) tests of irradiated fluids • Conclusions

The types of irradiation containers

Irradiated perfluorocarbon samples * To be measured

Expected radiation induced chemical effects General processes

Expected radiation induced chemical effects: F radical fate F atoms recombine with other radicals, resulting in a decrease of the final yield of radiolysis; CF4, C2F6, etc. formation is due to F reactions (F atoms have a great mobility; part of them can escape to recombination)  For thermodynamic reasons, F2 does not appears  Higher molecular weight products may result as it is described below for C9F20 isomers resulting from the radiolysis of n-C6F14.

CF3 + CF3(CF2)3CF2 C2F5+ CF3(CF2)2CF2 CF3CF2CF2 + CF3CF2CF2 F + CF3(CF2)4CF2 F+ CF3(CF2)3CFCF3 F + CF3(CF2)2CFCF2CF3 n-C6F14 Expected radiation induced chemical effects: PFC's radicals fate Radiation induced radicals High molecular weight PFCs (e.g. C9F20) isomers Low molecular weight products

Expected radiation induced chemical effects Water and oxygen complicate the reaction mechanism by their interference with the perfluorocarbon radical reactions Oxygen is a radical scavenger  leads to the formation ofCarbonyl fluoride (CF2O) or to other perfluorocarbonyl compounds

Water is a relative simple substance. However, it leads to many reactive species in a radiation field Possible reactions of the water radicals with perfluorocarbon or its radicals

Expected radiation induced chemical effects - G values for products formation; in red, there are the G values for radiochemical consumption of the parent compounds (literature data)

Expected radiation induced chemical effects - G values for other molecules resulted as radiolysis products (literature data)

Radiation induced chemical effects - pH, HF and F- ions content – laboratory measurements -

Radiation induced chemical effects - pre-polymer content– laboratory measurements - * To be measured

Radiation induced chemical effects - other molecules/GC-MS (a) (b) Details of the chromatograms (GC-MSD) of PP1 fluid initial and after irradiation (gas phase analysis): (a) focus on the initial portion, up to the main peak of CF3C5F11 (not shown); (b) focus on the first peaks region; (c) focus on the last part, after the main peak (partly shown) (c)

Radiation induced chemical effects - other molecules/GC-MS Details of the chromatograms (GC-MSD) of PP1 fluid initial and after irradiation and purification (liquid phase analysis): (a) focus on the initial portion, up to the main peak of CF3C5F11 (not shown); (b) focus on the last part, after the main peak (partly shown) (a) (b)

Radiation induced chemical effects - other molecules/GC-MS (b) (a) Comparison of the chromatograms of PP1, DL, and PF 5060 fluids irradiated at 56 kGy (gas phase analysis): (a) initial portion, up to the main peak; (b) zoom on the first portion to better see the small peaks of PF 5060/ 56 kGy (c) last portion, after the main peak (c)

Radiation induced chemical effects - COF2 (FT-IR) IR characteristic bands: 1944 cm-1 and 1925 cm-1 COF2 + H2O  CO2 + 2HF Evidencing of COF2 in liquid PP1: 1 - PP1 + air/28 kGy; 2 - PP1 + air/ 28 kGy after water extraction; 3 - PP1/ 56 kGy-inert atmosphere; 4 - Reference unirradiated PP1

Radiation induced chemical effects – formation of COF2 (GC method) Evidencing COF2 presence in gaseous phase of the PP1 + air/ 28 kGy by GC-TCD analysis Evidencing of COF2 in liquid phase of the PP1 + air/ 28 kGy by GC-TCD analysis

Purification (cleaning) experiments on irradiated PP1 fluid (UV-VIS measurements) 1 - PP1 as received (reference);2 - PP1 28 kGy (C1);C1 + cartridge with 3 components (2809): 3 - 5 min; 4 - 30 min; 5 - 45 min; 6 - 75 min. 1 - PP1 as received (reference);2 – PP1 + air 28 kGy (A02); A02 + cartridge with 3 components (2809): 3 – 75 min; 4 – 120 min. 1 - Ref. PP1 as received (référence); 2 – A19 PP1 56 kGy.

Purification (cleaning) experiments on irradiated PF 5060 fluid (UV-VIS measurements) 1 - PF 5060 as received (reference); 2 – PF 5060 56 KGy (A11); A11 + treatment by cartridge with 3 components: 3 - 30 min; 4 – 60 min; 5 – 90 min; A11 + 90 min cartouche 3 composants + SiO2: 6 – 30 min; 7 – 60 min. 1 - PF 5060 as received (reference); 2 - PF5060 28 kGy (C2); C2 + treatment by activated carbon vegetal: 3 – 30 min; 4 – 60 min; 5 – 120 min.

Purification (cleaning) experiments on irradiated PF 5060 fluid (UV-VIS measurements) (b) (a) GC-MSD spectra of the PF 5060 as received, irradiated (28 kGy) and cleaned after irradiation: the first part of the chromatogram (a); the second part of the chromatogram (b). • The GC-MSD spectra of the as received and of the irradiated and subsequently purified PF 5060 are • quite similar. • Most of the supplementary peaks were identified as different perfluorocarbons. • The peaks of hexane or of other hydrogen containing compounds (e,g, C2F5H) traces existing in the • as received fluid were removed by the purifiers. • A very low content of 3-HC6F13 and other fluorocarbons containing oxygenated groups (ethers, carbonyl, • acid, etc.) was observed in the second part of the chromatograms and can be responsible of the residual • UV absorption after purification.

Purification experiments on irradiated as received PF 5060 fluid (FT-IR measurements) FT-IR spectra of as received PF 5060 fluid in the spectral range 3400 - 2800 cm-1(a) and 2000 - 1500 cm-1 (b): 1-reference air; 2 - as received; 3 - irradiated at 28 kGy; 4 - cleaned after irradiation • - All FT-IR spectra of the initial, irradiated and cleaned PF 5060 fluid were similar, excepting the regions • of 3400 - 2800 cm-1 (a) and 2000 - 1500 cm-1(b). • The cleaning treatment eliminates almost completely the H - containing molecules (3400 - 2800 cm-1). • Other O – containing molecules (carbonyl, carboxyl, ether...), structurally similar to the main component, are responsible for the small absorption increase at 2000 - 1500 cm-1 and were removed also in the purification process. • - It can be concluded, based on GC-MSD and FT-IR data, that the cleaning treatments are efficient even for the irradiated PF 5060 fluid; the observed UV absorption is probably due to traces of molecules with very high optical absorption coefficients.

CONCLUSIONS • the Part I of the work was successfully accomplished; for the Part II a part of the work is still ongoing; • the chemistry laboratory analytical techniques and methods were sensitive to the parameters of interest and will be used for the future analyses of perfluorocarbon fluids; • the following perfluorocarbon fluids supplied to CERN: PP1, PF 5060 DL and C3F8 were found compliant to the CERN quality requirements, while FC-72 and PF 5060 were not; • the purification tests on less pure fluids were very promising; the potential suppliers and the acceptable quality of the fluids may be advantageously enlarged; • various perfluorocarbon fluids, as received and/or modified in laboratory were irradiated using gamma 60Co at 28 kGy and 56 kGy doses; • radiation induced acidity, the presence of polymers or pre-polymers, the appearance of new chemical species, etc. were evidenced and measured; • it was evidenced the higher radiation hardness of PP1 fluid (i-C6F14) as compared to PF 5060 DL (n-C6F14) • the cleaning tests of irradiated fluids have shown a very good efficiency for the as received PP1 fluid (i-C6F14) and an acceptable efficiency for the n-C6F14.

Chemical and radiolytical characterization of some perfluorocarbon fluids used as coolants for LHC experiments Part one