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Is CF3I a good gaseous dielectric? A comparative swarm study of CF3I and SF6

This paper deals with the measurement of the electron drift velocity, the longitudinal diffusion coefficient and the effective ionisation coefficient in pure CF3I and CF3I mixtures. The E/N range covered was 100-850 Td (1 Td=10-17 V cm2). The present results were derived from a pulsed Townsend experiment. For pure CF3I, the values of the electron drift velocity and of the effective ionisation coefficient were found to increase steadily with E/N. The E/N value at which ionisation equals attachment, commonly referred to as the limiting field strength, was found to be E/Nlim=437 Td, which is higher than that of SF6 (360 Td), a widely used insulating gas. Moreover, the curve for the mixture of 70% CF3I with N2 is very similar to that of SF6 in the neighbourhood of E/Nlim. Thus, from the point of view of these coefficients, CF3I would seem to be superior in dielectric behaviour to SF6. Besides, CF3I is an environmentally friendly gas as opposed to SF6. Certainly, many more tests should be performed to regard CF3I as a good dielectric and possible substitute to SF6.


Introduction
Sulphur hexafluoride is probably one of the most extensively used gases because of its many industrial and scientific applications. SF 6 is chemically inert, non-toxic, non-flammable, non-explosive, and thermally stable at temperatures less than 500 C. The outstanding properties of SF 6 make it suitable as an insulating medium in the equipment used for the transmission and distribution of electric power. SF 6 is strongly electronegative, with a high dielectric strength, and a breakdown voltage nearly three times higher than that of air at atmospheric pressure. On the other hand, SF 6 forms highly toxic and corrosive compounds (e.g., S 2 F 10 , SOF 2 ) when it is subjected to electrical discharges. Sulphur hexafluoride is an efficient infrared (IR) absorber and, due to its chemical inertness, it is not readily removed from the earth's atmosphere. Both these latter properties make SF 6 a potent greenhouse gas, although benign with regard to stratospheric ozone depletion, since it is chemically inert [1]. Since the very first measurements in 1970 [0.03 pptv (1 ppt=1 part in 10 -12 per volume of atmospheric air)], the purely anthropogenic greenhouse gas SF 6 increased by two orders of magnitude to a global mean value of 2.8 pptv in 1992. While the uncertainties in these numbers make extrapolations difficult, it is clear that the atmospheric concentration of SF 6 is increasing and could reach 10 pptv by the year 2010 and 65 pptv by the year 2100, depending upon the assumptions of release rates [2]. SF 6 has a global warming potential (GWP) of nearly 24,000 times greater than that of CO 2 , the predominant contributor to the greenhouse effect, and its atmospheric lifetime in the environment has been estimated around 3,200 years.
The above concerns have moved scientists and engineers into finding possible substitutes for this otherwise formidable gas. Mixtures of SF 6 with fluorocarbon gases, rare gases and atmospheric gases have been tried. Of these, the SF 6 -N 2 mixture has been found industrially useful for some high voltage applications. Other fluorocarbons, such as c-C 4 F 8 have a higher dielectric strength than that of SF 6 , although a GWP of 11,200 and a residence time in the atmosphere of 3,200 years of the former would discard it as a viable substitute gas. . Trifluoroiodomethane (CF 3 I), currently used as an etching gas [3], has been found very recently to be a potential high voltage insulator, both on its own and mixed with N 2 and CO 2 [4,5]. In constrast to SF 6 , the global warming potential of CF 3 I is less than that of CO 2 (GWP=1), and its lifetime in the atmosphere is very short, estimated to be less than two days, thereby ensuring that CF 3 I does not deplete the ozone layer [6]. CF 3 I has also found applications as an alternative refrigerant to commonly used fluorocarbons such as CF 4 , for example [7]. In spite of the potential importance of CF 3 I, there is still a substantial lack of information regarding this molecule. Recent reviews on electron interactions with CF 3 I [8,10] indicate that even the cross sections for total electron scattering, attachment and ionization are scarce, and that further research on these matters is called for to either confirm or revise the available data, and also to extend the energy ranges thus far studied. Moreover, one of these recent review [8] states that no data for electron transport, ionization and attachment coefficients are available for this molecule.
This paper presents previously published measurements [11] on the electron drift velocity v e , the density-normalized longitudinal diffusion coefficient ND L , and the density-normalized effective ionization coefficient (α−η)/N for pure CF 3 I and its mixtures (1%-70%) with N 2 (α and η are the electron impact ionization and attachment coefficients, respectively). From the point of view of a possible application of CF 3 I as a gaseous dielectric, we compare the above swarm coefficients with those of SF 6 , including their limiting field strength.

Experimental
The pulsed Townsend apparatus that was used to measure the above parameters has been described in detail elsewhere [12]. A schematic of the apparauts is shown in Fig. 1a. Briefly, the method relies on the time-resolved observation of the total displacement current (electrons, positive and negative ions) moving through a parallel-plate capacitor (12 cm in diameter) filled with the research gas, to which a voltage has been applied to produce a very homogeneous electric field over a central portion of 2 cm diameter. The initial photoelectrons are released by a UV flash from the third harmonic (355 nm) of a Nd-YAG laser. In this experiment the gap distance was kept fixed at 3.1 cm, set to within an accuracy of 0.025 mm.
Prior to filling it with the gas or its mixture, the vacuum vessel was evacuated down to 10 -6 torr. The range of working pressures was 0.2-20 torr. The gas pressure and mixture composition was measured with an absolute capacitance manometer to an accuracy of 0.01%, while the gas temperature was measured to an accuracy of 0.2% over the range 293-300 K. The CF 3 I and N 2 samples both had a quoted purity of 99.9% and 99.999%, respectively, and were introduced into the discharge vessel without further purification. The displacement current was measured with a 40 MHz transimpedance amplifier, and recorded on a 100 MHz digital oscilloscope.
In the presence of electron drift, longitudinal diffusion, ionization and attachment, the temporal development of the electron current within the gap is described by the expression [11] ( ) ( ) (2) is the error function of argument u. Equation (1) was derived on the assumption of a simultaneous release of the photoelectrons from the cathode at time t=0. Because of the finite laser pulse duration and of the instrumental bandwidth, the rise of the measured electron avalanche is not sharp, yet it displays a Gaussian-like shape resembling that of the laser pulse.
On the other hand, the fall is affected by longitudinal diffusion effects which, for fixed E/N, become more apparent as the gas pressure is reduced.
When diffusion is either negligible or absent, equation (1) reduces to A result that had been quoted previously [14].
Initial values of v e and (α−η)/N were derived from equation (3) to fit the electron transients. Briefly, the electron transit time T e is that elapsed between the midpoints of the rising and falling edges of the pulse, from which v e =d/T e is calculated, and the density normalized effective ionization coefficient (α−η)/N is obtained from a least-squares fitting procedure applied to the rising (α >η) or falling (α <η) exponential part of the avalanche. At this point, we would like to note that both v e and of (α−η)/N are obtained simultaneously from the same electron transient. Under some conditions of E/N and N, when α  and η are large, the ionic currents during the electron transit may contribute to the    total, measurable current; even though its contribution is relatively small, this can be readily subtracted from the total current by approximating their exponential rise to a straight line, thereby obtaining an even closer measurement of the electron current (see Fig. 2, I excess ). When the above procedure was completed, then full use of equation (1) was made to obtain the longitudinal diffusion coefficient. Usually, corrections to the drift velocity rendered values slightly larger (1-2%), although the effective ionization coefficient remained essentially unchanged. These final values are the ones to be reported below.

Measurement of electron swarm coefficients in CF 3 I
The swarm coefficients reported in this section correspond to the measurements of electron avalanches over the pressure range 0.4-20 torr, and room temperatures between 293 and 301 K. The uncertainties reported here are the result of averaging a series of measured coefficients for the same E/N value and different pressures. Normally, we used two to five different pressurees. The average uncertainties are 1% for the electron drift velocity, 8% for the longitudinal diffusion coefficients, 9% for the ratio D L /K e , where K e is the electron mobility, and 5% for the effective ionization coefficient, respectively.

Electron drift velocities
The electron drift velocities, v e , for CF 3 I, CF 3 I-N 2 and N 2 are shown plotted in Figure 3 over the range of E/N from 100 to 850 Td. The CF 3 I-N 2 mixtures correspond to CF 3 I shares of 5%, 10%, 20%, 50% and 70%. The E/N range covered for each CF 3 I-N 2 mixture was limited, on the lower part, by the strong attachment of this gas, which precluded the observation of the falling portion of the electron avalanche, while the upper part of the E/N range was limited by the very low pressures used (<0.5 torr), which produced avalanches with substantial contributions from longitudinal diffusion. The drift velocity curves for the 1% and 2% CF 3 I-N 2 mixtures have also been measured, but their values over the E/N range used were very similar to those of pure N 2 , and thus were omitted for the sake of clarity in the figures presented.  Figure 3. The electron drift velocity in CF 3 I and in the CF 3 I-N 2 mixtures with 5%, 10%, 20%, 50% and 70% CF 3 I as a function of E/N [11]. The crosses are the recommended electron drift velocities for SF 6 [9,15]. The SF 6 data are shown only for order-of-magnitude comparison and trend with those of CF 3 I and the CF 3 I-N 2 mixtures. For the purpose of comparison with another potent gaseous dielectric, we have also plotted Aswchwanden's values for the drift velocity of electrons in SF 6 [9,15]. It is noted that the values of v e for SF 6 are higher than those for CF 3 I for E/N<700 Td. The calculated momentum transfer cross section for CF 3 I [8] is approximately an order of magnitude smaller than the measured cross section of SF 6 over a wide electron energy range [15]. This means that, even considering the larger relative mass of CF 3 I (194 Da) with respect to that of SF 6 (146 Da), the electron drift velocity in CF 3 I should still be greater than that in SF 6 . In view of this, we suggest that the relatively small values of the electron drift velocities in CF 3 I with respect to those in SF 6 for E/N<700 Td may be due, additionally, to the stronger polar character of CF 3 I as compared to that of SF 6 , with dipole polarizabilities of 9.36 Ǻ 3 and 6.54 Ǻ 3 , respectively [16,17]. The electron drift velocities in N 2 were also measured in this study, and found to be in good agreement with previously published data [18]. It is important to note that the 5% and 10% CF 3 I-N 2 curves deviate from the N 2 curve, and also that their v e values become systematically smaller than those of N 2 , due to the influence of CF 3 I. For CF 3 I contents greater than 10%, the v e curves bear a round curvature over their initial region of E/N, departing from the N 2 curve, and resembling the typical curves of negative differential conductivity [19,20].
To the best of our knowledge, no previous data for the electron drift velocities in either CF 3 I or the CF 3 I-N 2 mixture have been published before.

Longitudinal diffusion coefficients for electrons and the ratio D L /K e
The values of the density-normalized longitudinal diffusion coefficient, ND L , for electrons in pure CF 3 I and in the 5%, 10%, 20%, 50% and 70% CF 3 I-N 2 mixtures, have been plotted in Figure 4 as a function of E/N. The ND L values for the 1% and 2% CF 3 I-N 2 mixtures, since these are very close to that of N 2 .  Figure 4. The density-normalized longitudinal diffusion coefficients ND L for electrons in CF 3 I and in the CF 3 I-N 2 mixtures with 5%, 10%, 20%, 50% and 70% CF 3 I as a function of E/N. For the solid lines through the points, see the curve fittings in Ref. [11]. The crosses are the recommended ND L values for SF 6 [9,15]. These data are shown only for order-of-magnitude comparison and trend with those of CF 3 I and the CF 3 I-N 2 mixtures. A comparison with the ND L values for SF 6 , shows that those for pure CF 3 I are smaller than the ones for SF 6 by a factor of about two over the whole E/N range of overlap. Again, for the sake of comparison, the longitudinal diffusion coefficients of Aschwanden [9] for SF 6 are also plotted in this figure.. The different mixtures of CF 3 I-N 2 present a similar trend as in the pure case. Moreover, the percentages corresponding to 50% and 70% of CF 3 I have values smaller that those for SF 6 . To the best our knowledge, no previous values of ND L have been published.
A coefficient that provides a measure of the translational energy of the charge carriers as a function of E/N is the ratio between the density-normalized longitudinal diffusion coefficient and the electron mobility, D L /K e , with K e =v e /E. This ratio is shown plotted in Figure 5, together with the corresponding values for the mixtures as a function of E/N. The D L /K e values for SF 6 shown in this figure for the purpose of comparison are, as in the case for ND L , higher than those for CF 3 I by a factor between 1.5 and 2.

Effective ionization coefficients and rates
The effective ionization coefficients (α−η)/N in CF 3 I and in the mixtures of this gas with N 2 are plotted in Figure 6 for the mixtures with low concentration of CF 3 I in N 2 , and in figure 7 for the 10%, 20%, 50%, 70% CF 3 I-N 2 mixtures, all as a function of E/N. Again, the lower limit of our measurements of (α−η)/N was due to the impossibility of assessing the electron transit time and hence determine both v e and (α−η), since the strong attachment produced a fast decaying signal that precluded us from measuring the electron transit time. The values of (α−η)/N in N 2 and 1%, 2% and 5% CF 3 I-N 2 have been plotted separately, in figure 6, to emphasize the increase in (α−η)/N for E/N>230 Td with respect to the values of α/N for pure N 2 . This effect is mainly due to the fact that the ionization potential of CF 3 I is lower (10.23 eV) than that of N 2 (15.58 eV). On the other hand, Figure 7 shows the values of (α−η)/N for pure CF 3 I and for the 10%, 20%, 50%, 70% CF 3 I-N 2 mixtures.  The ratio between the longitudinal diffusion coefficient and the electron mobility, D L /K e , for electrons in CF 3 I and in the CF 3 I-N 2 mixtures. The solid lines through the points represent a curve fitting described in Ref. [11]. The crosses are the D L /K e values for SF 6 [9,15], shown here for the purpose of comparing the relative magnitudes of this parameter in SF 6 with those of CF 3 I and the CF 3 I-N 2 mixtures. Interestingly, the trend of the (α−η)/N values for CF 3 I runs almost parallel to that of SF 6 . An interesting feature of this curve is the value of the limiting (or critical) field strength, E/N lim , which is defined as that value of E/N at which ionization equals attachment, that is, (α−η)/N=0. A value of E/N lim =437 Td is measured for CF 3 I, which turns out to be significantly higher than that of E/N lim =361 Td for SF 6 , an important and widely used high voltage gas insulator [15]. From this point of view, it would then be likely that CF 3 I be considered as a viable substitute for SF 6 , since it bears the good insulating properties of the latter, besides its non-ozone depleting effects and significantly low global warming potential, in contrast to those of SF 6 . Moreover, note that the 70% CF 3 I-N 2 curve is essentially the same as that for SF 6 Figure 6. The density-normalized effective ionization coefficients (α−η)/N for N 2 and for the CF 3 I-N 2 mixtures with 1%, 2% and 5% CF 3 I as a function of E/N.
Our experiment could only render effective ionization coefficients. At this stage, we have not been able to assess any particular ionization or attachment coefficients. However, we would like to mention that the only measurements of Jiao et al on the electron impact ionization of CF 3 I [21] indicate that the ions formed are CF 3 I + , CF 2 I + , CF 3 + , CF 2 + , CF + , and I + , with CF 3 I + as the predominant positive ion species for electron energies up to 70 eV, followed by I + , CF 3 + , CF 2 I + , CF 2 + , CF + , the cross sections of these latter two being a factor of ten smaller than the former three.
As regards negative ions from CF 3 I, the strongly electronegative character of this gas stems from the fact that electron attachment to CF 3 I proceeds dissociatively via the formation of I -, Fand CF 3 and FIvia two dissociating negative ion states at ~0 eV and 3.8 eV [8]. According to our D L /K e data, which provide a rough measure of the average energy of the electron swarm, it is likely that in our experiment both Iand Fare formed. It is known that Iis formed by the zero-eV process via the reaction [8] ( ) Very recently, using several variants of the laser photoelectron attachment method, Marienfeld et al [22] have measured and calculated the dissociative electron impact attachment cross sections to CF 3 I molecules leading to I − formation over the energy range 0.5-500 meV.
Moreover, the mixture with 70% of CF 3 I in N 2 runs almost over the corresponding values of SF 6 . The practical value of this situation is that an environmentally friendly mixture of CF 3 I in N 2 may have the same efficiency as an insulating gas mixture as compared to that of the much less environmentally friendly case of SF 6 . To the best of our knowledge, no measurements of the effective ionization coefficient have been published previously. Figure 6 presents the so called limiting or critical field strength values of E/N lim at which α=η for all the CF 3 I-N 2 mixtures studied, including that of pure CF 3 I. A comparison with the well known E/N lim values for the SF 6 -N 2 mixture reveals that for CF 3 I concentrations in CF 3 I-N 2 the mixture lower than 60%, the SF 6 -N 2 mixture is superior, although this limit the CF 3 I-N 2 mixture is superior to that of SF 6 -N 2 . In fact, as judged from these swarm properties, the 70%CF 3 I-N 2 mixture is even superior to that of pure SF 6 .  Figure 7. The density-normalized effective ionization coefficients (α−η)/N for CF 3 I and in the CF 3 I-N 2 mixtures with 5%, 10%, 20%, 50% and 70% CF 3 I as a function of E/N. The solid line through the N 2 curve represents a fitting according to Ref. [11]. The crosses are the recommended (α−η)/N values for SF 6 [9,15]. These latter data are shown only for the purpose of comparing the more electronegative character of CF 3 I and its mixtures with N 2 with respect to that of SF 6 .

Conclusion
We have used previously published data [11] of the electron transport and ionisation coefficients in CF 3 I and CF 3 I-N 2 mixtures, covering a wide range of E/N. In this paper we have presented a compariosn of such coefficients with those of SF 6 , in the aim to evalute them from the point of view of assessing CF 3 I as a possible substitute of SF 6 . In particular, we have found that the limiting field strength of pure CF 3 I (E/N lim = 437 Td) is higher than that of SF 6 (E/Nlim = 361 Td). An encouraging aspect of the measurements for the mixtures of CF 3 I in N 2 presented here is that CF 3 I or its mixture with N 2 both have the good properties to be regarded as viable gaseous dielectrics, in consonance with recent research in this direction indicating that CF 3 I or its mixture with N 2 would be viable gaseous dielectrics since the mixture with 70% CF 3 I presents a very similar behaviour to that of pure SF 6 . Admittedly, many other chemical, thermal and economical studies are needed to consider CF 3 I or its mixtures with N 2 as real substitutes for SF 6 in high voltage and switchgear applications. In view of the scarcity of data on the interaction of these gases with electrons, we hope the present data will be useful for testing the available relevant cross sections, or even to attempt deriving the first sets.