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

No.Product name Supplier Composition

Name usedin this report

1 PF 5060 3M Chemicals, USAmainly n-C6F14

(n-perfluorohexane)PF 5060

2 PF 5060 "purified"3M Chemicals, USA; tentatively

" treated" (purified) in CERN Chemistry Laboratory

mainly n-C6F14

(n-perfluorohexane)purified PF 5060

3 PF 5060 DL 3M Chemicals, USAmainly n-C6F14

(n-perfluorohexane)PF 5060 DL

4 Flutec PP1 F2 Chemicals Ltd., UKmainly iso-C6F14

(iso-perfluorohexane)PP1

5C3F8

(R218)ASTOR, Russia

C3F8

perfluoropropane

C3F8

perfluoropropane

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. 1 GC-MSD chromatograms for the studied perfluorocarbons: (1) C3F8; (2) n-C6F14; (3) iso-C6F14

Fig. 2 GC-TCD chromatograms for various 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;

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.

23.4 23.5 23.6 23.7 23.8

Time (minutes)

Abu

ndan

ce 1

C6F14

2

3

r.t. = 23.57 min.(a)

C6H14

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

2800290030003100

0

0.005

0.01

Wavenumber (cm-1)

Abs

orba

nce

(abs

. uni

ts)

3

2

1

(c)

Fig. 6 (a) 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 (a) 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 C6F12 dosage.

Characterization of the initial fluids

Main results of the chemical characterization of the C6F14 fluids: = test passed; test not passed

Parameter Units

Required value Fluid type

Lower limit

Upper limit

PP1PF 5060 as

receivedDL

Purified PF 5060

C6F14 purity by GLC

(area %)*% 98 - 99.259 98.964 99.023 99.052

Other PFC* % - 2 0.441 1.019 0.977 0.948

C6F14 double bonded

moleculesppm

weight- 10

< detection limit

detection limit

< detection limit

< detection limit

H containingmolecules(equiv hexane)+

ppm weight

- 10 < detection limit

722 < detection limit

< detection limit

Free fluoride(equiv. HF)

ppm weight

- 1 0.05 0.06 0.03 0.02

Density g/mL 1.660 1.700 1.721++ 1.689 1.694 1.689

Boiling point °C 51 59 56 56 56 56x

*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 C3F8 fluids: = test passed; test not passed

Parameter Unit Lower limit Upper limit C3F8 (as analysed)

C3F8 organic purity* % volume 99.96 - 99.964 Other perfluorocarbons* ppm weight - 400 360 (pefluorobutane)

C3F8 double bounded

molecules*

ppm weight - 10 not detected

H containing molecules* ppm weight 10 not detected Free fluoride ppm weight 0.1 0.08

*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

Bottle no. Content

PFC weight

(g)Dose (kGy)

Aluminum Inox

No. m/S x 105

(g/cm2) No. m/S x 105

(g/cm2)

C1 PP1 ca. 5 000 28 - -

C2 PF 5060 (as received) ca. 5 000 28 - -

A01 PF 5060 + air 908 28 43 * 42 *A02 PP1 + air 611 28 39 * 40 *A03 DL 924 28 40 * 31 *A04 Purified PF 5060 902 28 41 +9.82 ? 41 + 1.75A05 PF 5060 + hexane (1500 ppm) 896 28 33 +5.26 36 +1.75A06 PP1 + 1-C6F12 (1500 ppm) 868 28 37 * 44 *A07 PF 5060 + air (appl. 4 bar) 900 28 46 * 39 *A08 PF 5060 + air + H2O 905 28 44 * 38 *A09 PP1 + air (appl. 4 bar) 609 28 38 -2.46 27 +0.70

A10 PP1 + air + H2O (0.5 mL) 904 28 30 * 33

A11 PF 5060 as received (degassed 5' Ar) 892 56 32 * 45 *A12 PP1 (degassed 5' Ar) 803 56 45 -1.05 32 0A13 DL 899 56 28 +1.05 28 +0.70A14 Purified PF 5060 918 56 31 * 34 *A15 PF 5060 + hexane (1500 ppm) 878 56 27 +5.96 30 +1.75A16 PP1 + 1-C6F12 (1500 ppm) 998 56 35 * 35 *A17 PF 5060 as received (degassed 5' Ar) 900 56 36 * 37 *A18 PF 5060 + air + H2O 895 56 26 * 43 *A19 PP1 (degassed 5' Ar) 903 56 42 * 29 *A20 PP1 + air + H2O 907 56 34 * 26 *

B01 C3F8 (Astor) as recd. 423 56 25 * 52 *

B02 C3F8 (Astor) + air 418 56 15 * 50 *

B03 C3F8 (Astor) + air + H2O 514 56 20 * 48 *

B04 C3F8 (Astor) + hexane 1500 ppm 516 56 3 * 49 *

B05 C3F8 (Astor) + 1-C6F12 1500 ppm 505 56 7 * 51 *

Irradiated perfluorocarbon samples

* To be measured

Expected radiation induced chemical effects

General processes

C6F14 C6F13, F, C6F14*, C6F14+, e

-

Low molecular weight productsi.e. homologous PFCs (C1 - C5),fluorinated molecules containing double bonds, O, H (if oxygen, water

e.g. alcohols, acids, carbonylor other O or H sources are present),

compounds, alkenes etc.

Acidity increaseHF, F and perfluoroacid formation

High molecular weight productspre-polymers (soluble in C6F14);volatile residues)

non-polymers (insoluble;

deposits over surfaces)

Similar proceses are expected for C3F8

-

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.

F +C C-F

F3C F3C CF2+ cage

F+

CF4 + CF2

recombination to form low molecular weight PFCs:

HF formation in presence of H sources:

H+F HF +

*

recombination to result the initial molecule:

Expected radiation induced chemical effects: PFC's radicals fate

CF3 + CF3(CF2)3CF2

C2F5 + CF3(CF2)2CF2

CF3CF2CF2 + CF3CF2CF2

F + CF3(CF2)4CF2

F + CF3(CF2)3CFCF3

F + CF3(CF2)2CFCF2CF3

n-C6F14

Radiation induced radicals

CF3 CF2 3CF2 +CF3 CF2 2CF2 CF3 CF2 7 CF3

(n-perfluoro nonane)CF3CF2CF2 + CF3 CF2 4CF2

(n-perfluoro nonane) CF3 CF2 7CF3

+ CF3CF2CF2 CF3 CF2 3CFCF3 CF3 CF2 2CF

CF3

CF2 3CF3

(perfluoro-4-methyl octane)

CF3CF2CF2 CF3 CF2 2CFCF2CF3+ CF3CF CF2 4CF3

CF2CF3

(perfluoro-2-ethyl heptane)

High molecular weight PFCs (e.g. C9F20) isomers

Low molecular weight products

CF3 + CF3

CF3 + C2F5 C3F8

C2F6

CF3 + C3F7 C4F10

C2F5 + C3F7 C5F12

CF3 + C6F14 CF4 + C6F13

C2F5 + C6F14 C2F6 + C6F13

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 of Carbonyl fluoride (CF2O) or to other perfluorocarbonyl compounds

C6F13 + O2 C6F13 OO

OOC6F132 C6F13O2 + O2

OC6F13 C5F11 + CF2O

CF3CFO+ C4F9C6F13O

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

Event Time scale per s

H2O

H2O* H2O+ + e

-

H + OH H2 + O

H2O

OH + H3O+

e-aq

formation of molecular products in the spurs and diffusion of the radicals out of the spurs

e-aq, H, OH, H2, H2O2, H3O 10

-7+

10-13

10-14

10-16

Water radiolysis

C6F13 + H C6F13H

C6F13 + OH C6F13OH

C6F14 + H C6F13 + HF

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) Parent compound C3F8 n-C6F14 2-CF3C5F11

Absorbed dose (kGy) 1430 1350 1450

Product Molecules/ 100 eV

CF4 1.20 0.59 0.88

C2F6 0.85 0.29 0.28

C3F8 -3.74 0.31 0.21

n-C4F10 0.43 0.24 0.049

i-C4F10 0.40 - 0.18

n-C5F12 0.17 0.20 0.13

i-C5F12 0.27 0.019 0.085

n-C6F14 0.056 -2.88 0.050

2-CF3C5F11 0.13 - -2.71

3-CF3C5F11 0.012 0.024 -

Total C7F16 0.0861 0.0108 0.3377

Total C8F18 - 0.16 0.19

Total C9F20 - 0.32 0.28

Total C10F22 - 0.21 0.12

Total C11F24 - 0.15 0.18

Total C11F24 - 0.042 0.069

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

Parent compound C3F8 n-C6F14 2-CF3C5F11

Absorbed dose (kGy) 1430 1350 1450

Product Molecules/ 100 eV

CO2 0.024 0.084 0.083

C2F6O 0.0057 - -

C3F8O - - -

(C2F5)2O - - -

Unknown products (fluorinated ethers and alcohols) (%)

0.0202 0.2920 0.0822

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

Bottle no. Content PFC Weight (g)

Dose (kGy)

pre-polymer FT-IR spectrumContent (%) Total weight (g)

C1 PP1 ca.5 000 28 0.007 0.35 565, 586,

588

C2 PF 5060 (as received) ca.5 000 28 0.147 7.35 557-560

A01 PF 5060 + air 908 g 28 0.067 0.61 711A02 PP1 + air 611 28 * * *A03 DL 924 28 0.021 0.19 634A04 Purified PF 5060 902 28 0.031 0.28 643A05 PF 5060 + hexane (1500 ppm) 896 28 0.074 0.66 644A06 PP1 + 1-C6F12 (1500 ppm) 868 28 * * *A07 PF 5060 + air (appl. 4 bar) 900 28 * * *A08 PF 5060 + air + H2O 905 28 0.103 0.93 710A09 PP1 + air (appl. 4 bar) 609 28 0.0015 0.009 623, 630A10 PP1 + air + H2O (0.5 mL) 904 28 <6∙10-4 -

A11 PF 5060 as received (degassed 5' Ar) 892 56 0.314 2.80 713

A12 PP1 (degassed 5' Ar) 803 56 <5.5∙10-4 - 685-688A13 DL 899 56 0.016 0.14 642A14 Purified PF 5060 918 56 0.097 0.89

A15 PF 5060 + hexane (1500 ppm) 878 56 * * *A16 PP1 + 1-C6F12 (1500 ppm) 998 56 * * *

A17 PF 5060 as received (degassed 5' Ar) 900 56 * * *

A18 PF 5060 + air + H2O 895 56 * * *A19 PP1 (degassed 5' Ar) 903 56 * * *A20 PP1 + air + H2O 907 56 * * *

B01 C3F8 (Astor) as recd. 423 56 0.0014 0.006 632

B02 C3F8 (Astor) + air 418 56 * * *

B03 C3F8 (Astor) + air + H2O 514 56 * * *

B04 C3F8 (Astor) + hexane 1500 ppm 516 56 * * *

B05 C3F8 (Astor) + 1-C6F12 1500 ppm 505 56 * * *

* To be measured

Radiation induced chemical effects - pre-polymer content– laboratory measurements -

(a)(b)

(c)

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)

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)

Radiation induced chemical effects - other molecules/GC-MS

(b)

(a)

Radiation induced chemical effects - other molecules/GC-MS

(a) (b)

(c)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

Radiation induced chemical effects - COF2 (FT-IR)

IR characteristic bands: 1944 cm-1 and 1925 cm-1

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

COF2 + H2O CO2 + 2HF

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

200 220 240 260-0.04

-0.03

-0.02

-0.01

0

Wavelength (nm)

Abso

rban

ce (a

.u.)

13

456

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.

Purification (cleaning) experiments on irradiated PP1 fluid (UV-VIS measurements)

200 220 240 260-0.04

-0.03

-0.02

-0.01

0

Wavelength (nm)

Abso

rban

ce (a

.u.)

1

4

3

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.

200 220 240 260 280 3000

1

2

3

Wavelength (nm)

Abs

orba

nce

(u.a

.)

1

2

3

4

5

Purification (cleaning) experiments on irradiated PF 5060 fluid (UV-VIS measurements)

200 250 300 350

0

1

2

3

4

Wavelength (nm)

Abs

orba

nce

(a.u

)

1

2

3

45

67

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.

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.

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 (cleaning) experiments on irradiated PF 5060 fluid (UV-VIS measurements)

(a) (b)

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

Purification experiments on irradiated as received PF 5060 fluid (FT-IR measurements)

- 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.