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Observation of the methyl radical CH3. in irradiated, high-purity synthetic fused silica
Journal of Non-Crystalline Solids 57 (1983) 167-175 North-Holland Publishing Company
167
O B S E R V A T I O N O F T H E M E T H Y L R A D I C A L CH~ IN I R R A D I A T E D , H I G H - P U R I T Y S Y N T H E T I C F U S E D SILICA E.J. F R I E B E L E a n d D.L. G R I S C O M Naval Research Laboratory, Washington, DC 20375, USA K. RAU Heraeus Quarzschmelze GmbH, D-6450 Hanau, Fed. Rep. Gerrnar~v
Received 2 November 1982 Revised manuscript received 10 January 1983
Stable methyl radicals CH3 have been observed in irradiated high-purity synthetic silicas by electron spin resonance techniques. Analysis of the dependence of the linewidth on the nuclear spin quantum number of the ESR spectrum demonstrates that the radicals tumble at a frequency of - 11 MHz. We suggest that this small tumbling rate as compared with the "free" rotation rate of 10 GHz is due to interaction of the radical with the glass network. Radicals were observed in concentrations of - 3 x 1013/g SiO 2 in samples fused in a hydrocarbon oxygen flame, but no radical concentration could be measured in samples fused in either an electric arc or hydrogen-oxygen flame. The methyl radical is presumed to form by radiodissociation of either CH 4 or CO dissolved in the silica during the fusion process and subsequent reaction of C with radiolytic H.
1. Introduction I r r a d i a t i o n of fused n a t u r a l quartz or high-purity synthetic silica is k n o w n to result in the f o r m a t i o n of p a r a m a g n e t i c defect centers by charge trapping. A n u m b e r of these defect centers have been observed a n d elucidated by electron spin resonance (ESR) spectroscopy [i]. In general, those previously studied have been ascribed to either intrinsic defects or to inorganic i m p u r i t y or d o p a n t - r e l a t e d defects. Examples of the former include the E' center (a hole trapped in an S i - S i b o n d at the site of an oxygen vacancy), the n o n b r i d g i n g oxygen hole center ( N B O H C ) , a n d the peroxy radical (a hole trapped by an S i - O - O - S i excess oxygen linkage); impurity-related defects include the a l u m i n u m - o x y g e n hole center, atomic hydrogen, and other hydrogen-related centers. Recently, G r i s c o m et al. [2] have reported the identification a n d analysis of the formyl radical H C O , which is the first organic defect center to be observed in irradiated synthetic silica. I n this paper we report a second organic defect center, the methyl radical CH3, which has been observed in irradiated synthetic silica; we relate its observation to the m e t h o d of preparation of the silica. 0 0 2 2 - 3 0 9 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
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2. Experimental Samples of various synthetic (Type IIl) silicas and fused natural quartz (Types I and II) were irradiated at room temperature in a 6°Co source to doses greater than 10 7 rad at a dose rate of 10 4 r a d / m i n . Several samples were irradiated with 100 kV X-rays at a dose rate of 5 × l 0 4 r a d / m i n . Typical total doses were 4.6 x 10 7 rad. in some cases the samples were annealed at 300°C for a period of 10 min to minimize the interference of the E' center and N B O H C spectra with that of the methyl radical [3]. ESR measurements were made on a Varian E-9 spectrometer equipped with a V-4540 nitrogen flow-through accessory, which was used to maintain the sample temperature at - 1 7 0 ° C during the measurement. Sample geometries consisted of both 4 m m rods and powders; the latter were loaded into high-purity synthetic silica sample tubes, evacuated and sealed prior to irradiation. The methyl radical concentrations were determined by double numerical integration of the third (best resolved) hyperfine line of the 1 : 3 : 3 : 1 quartet, multiplication by 8 / 3 and then comparison with the integrated intensity of a calibrated pitch sample. Signal averaging with a Nicolet 1080 data acquisition system was used to enhance the signal-to-noise ratio of samples with low concentrations. Correlations were sought between the methyl radical concentration determined by ESR and the methane concentration measured by gas extraction. Approximately 60 g of powder in a size fraction 400-1000/.tm was heated at a linear rate of 2 0 ° C / m i n from room temperature to 1200°C under vacuum in a fused silica tube. The gas extracted from the sample was collected and delivered to a gas chromatograph in periods of 3 min for analysis. Measurements were made of the H 2, C H 4, C O , and N 2 concentrations; no analysis of H 2 0 or CO 2 was possible with the system. The 02 concentration could also have been measured, but was not. Viscosity measurements were made on selected samples by the beam bending method. The fictive temperature of the silica or quartz was set to 1150°C by heating the samples to this temperature for a period of 1 h; the viscosity measurement was then made at 1150°C.
3. Results and discussion
3.1. ESR spectral analysis The presence of the methyl radical CH~ was first recognized in irradiated synthetic silica samples of Suprasil and Dynasil 1000 by its characteristic ESR spectrum consisting of four isotropic hyperfine (hf) lines with peak heights in the ratio 1 : 3 : 3 : 1 and 2.26 m T coupling constant. (Methyl radicals were not observed in irradiated Spectrosil, Suprasil W or Spectrosil WF). This intensity
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r a t i o can only be e x p l a i n e d by a hf i n t e r a c t i o n of an electron ( S = ½) with three e q u i v a l e n t spin-½ nuclei; the purity of these silicas suggests that the only c a n d i d a t e is a p r o t o n . T h e 2.26 m T c o u p l i n g c o n s t a n t is in close a g r e e m e n t with p u b l i s h e d values [4] for the methyl radical CH3 leading to the identification of this radical in the synthetic silica samples of the present study. T h e p r e l i m i n a r y E S R m e a s u r e m e n t s were m a d e with a m o d u l a t i o n amplitude of 0.05 m T a n d a m i c r o w a v e p o w e r of 50 m W , b u t it was later recognized that these p a r a m e t e r s h a d the effect of slightly o v e r m o d u l a t i n g a n d substantially p o w e r s a t u r a t i n g the lines, causing artificial b r o a d e n i n g of all the lines to the same linewidth. Since the intensity ratio of the lines is 1 : 3 : 3 : 1 a n d the linewidths were m a d e equal, the peak heights followed the same characteristic ratio. W h e n the m e a s u r e m e n t s were r e p e a t e d with p a r a m e t e r s which had been e x p e r i m e n t a l l y d e t e r m i n e d not to o v e r m o d u l a t e or saturate the lines (0.01 mT, 1.0 mW), a m a r k e d a s y m m e t r y of the lines a n d a d e p e n d e n c e of the linewidth on nuclear spin q u a n t u m n u m b e r M I was observed, as shown in fig. 1. A l t h o u g h the p e a k heights are no longer in the ratio 1 : 3 : 3 : 1, the i n t e g r a t e d intensities d e t e r m i n e d b y d o u b l e integration of each first derivative hf line do follow this ratio within - 4%. The identification of this center as the methyl radical is therefore valid.
~]"'
FREQ. = 9.093 GHz f:103 K
~'x CENTER/ 0.0028)
O. 5 h
j
?
3/2 t/2-1/2-3/2 MI I
I
321
I
I
I
323
I
1
325 MAGNETIC FIELD
1
327
I
I
329
(mT)
Fig. 1. ESR spectrum of a y-irradiated sample of high-purity synthetic silica (Type IlI) fused in a hydrocarbon-oxygen flame. The spectrum shown here was taken 2 y after irradiation to a dose of 4.6 × 10v fad. The comb above the magnetic field scale shows the hf splittings of the methyl radical. The insert contains the experimental and calculated linewidths of the four hf lines (see text).
E.J. Friebele et aL / Methyl radical in silica
170
The lineshapes of the methyl radical lines shown in fig. 1 are suggestive of a powder pattern in which anisotropic g and hf interactions are averaged over all angles due to the random orientation of the radical in the glass with respect to the applied magnetic field. However, the temperature dependence of the ESR spectrum of the radical in irradiated silica indicates that it is tumbling at a rate which is sufficiently rapid to at least partially average these anisotropic effects (see below). Kazanskii and Pariiskii [5] have shown that the linewidths of a spectrum characterized by incompletely averaged g tensor anisotropy will depend on M/z, i.e. the broadening should be symmetric about the center of the spectrum. This is clearly not the case in fig. 1. The tumbling is then presumed to be sufficiently rapid to average the anisotropies completely, and the observed M;-dependent broadening must be due to a different mechanism./ Similar variations in linewidth with nuclear spin quantum numbe/" have been previously reported by Gardner and Casey [6] for the methyl radical adsorbed on silica gel; the tumbling frequency of the radical was sufficient to average the angular effects of the g and hf anisotropy, but these anisotropies gave rise to a relaxation mechanism that was dependent on M z. Following their analysis, the linewidth can be expressed as (T2) -' = a o + a,M;
(1)
+ azMi 2
if one neglects both the nonsecular part and pseudosecular contributions to the broadening [7]. The former is a valid assumption; the latter is good to within 10% for the methyl radical. The constants of eq. (1) have been determined to be [6] a 0 = A(
vB)2
c + K,
a, =
a2 =
(2)
where % is the correlation time, b is the hyperfine anisotropy, B is the magnetic field, Ay is the g-tensor anisotropy, and K contains other unspecified contributions to the linewidth that are independent of M. The linewidths of the four hf lines of the methyl radical were measured on an expanded magnetic field scan and least-squares fit to eq. (1), yielding a 0 = 0.847 MHz, a 1 = 0.473 M H z and a 2 = 0.373 MHz. The experimental and calculated linewidth data are shown in the inset in fig. 1; the fit is somewhat degraded by the interference of the N B O H C with the second hf line. From the expression for a 2, the correlation time can be calculated as 9.2 × 10 -8 s, using the value b = 4.5 M H z determined by Heller [8] for the methyl group in the radical C H 3 C ( C O O H ) v The calculated correlation time indicates a tumbling frequency of 1.1 × l 0 7 s-~ for the methyl radical trapped in irradiated silica. This value can be compared with 2.0 × 10 7 S-! determined by Gardner and Casey [6] for the radical on silica gel and frequencies on the order of 10 ~° s J for "free" rotation. It is obvious that the presence of the silica network substantially hinders the rotation of the methyl radical, as compared with the radical tumbling "freely" or adsorbed on a silica gel surface. One further feature of interest in the methyl radical spectrum of fig. 1 is the
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171
double line structure of the third line of the quartet. This structure is likewise suggestive of a powder pattern of anisotropic g and hf tensors. However, it has been established above that the angular anisotropy of the methyl radical in silica is completely averaged by the tumbling. Similar double line structure has been observed by Garbutt and Gesser [9] in methyl radicals stabilized on porous Vycor glass and has been explained as arising from second-order hf interactions removing the degeneracy of the M 1 = - ~ states contributing to this absorption [10]. As the sample is warmed from - 170°C, the linewidth of the third line broadens owing to increased tumbling frequency so that by - 7 5 ° C the second-order line is no longer resolved. The measured splitting of 0.022 mT at - 170°C for this line is in good agreement with that measured for methyl radicals trapped in Vycor glass [9], as well as the theoretical prediction of 0.0234 m T [10]. An identical splitting is predicted [10] for the second hf line in fig. 1, but it is not observed because the much larger linewidth of this component (Fig. 1 inset) obscures the second-order splitting. It is of interest to speculate why the methyl radical has not been reported in previous ESR studies of irradiated fused silica. The radical is quite stable at room temperature; we measured the integrated intensities of the spectrum in several silica samples two years after irradiation and found them to be virtually identical to those measured only a few days after the irradiation (although the N B O H C and E' center had substantially decayed). Fig. 1 is typical of such a spectrum. The quartet is discernable at room temperature when measured at high power and fairly large modulation amplitude (200 mW, 0.1 mT) even though the peak height is reduced by a factor of 9 from that measured at - 170°C. There is considerable overlap of the N B O H C with the second line of the radical, however, and unless this center is thermally annealed, the lines might remain unrecognized. Most previous studies of the hole center, which employed the broad magnetic field scans of 10-20 m T necessary to observe all four lines of the methyl radical, employed high microwave powers that would have substantially saturated the radical spectrum. Finally, certain preparation procedures appear to be required for the radical to be present in the silica, and it is not known whether these were used in the preparation of the synthetic silica samples previously studied. Although the four-line spectrum was most certainly observed in previous studies, it seems that it was identified as the methyl radical in the present work because of the fortuitous coincidence of low temperature, moderate microwave power, relatively low modulation amplitude and a 20 m T field sweep initially used in studying an irradiated silica sample that had been annealed sufficiently to remove the NBOHC.
3.2. Origin of the trapped methane There appear to be at least two potential mechanisms by which the methyl radical could be formed by irradiation in high-purity synthetic fused silica. Griscom et al. [2] have inferred the existence of dissolved CO in silicas fused in h y d r o c a r b o n - o x y g e n flames; the radiation could break the C = O double
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bond, allowing the carbon to combine with radiolytic hydrogen (derived e.g. from Si-OH) to form CH~. Alternatively, methane could be dissolved in the glass network during fusion; the radiation could result in radiodissociation of the methane molecules to form the radical. To test these hypotheses, 4 mm rod samples of silica fused in a hydrocarbon-oxygen flame were irradiated in either a 6°Co gamma ray (1.1 MeV) source or by a 100 kV X-ray machine. Since the efficiency of bond breaking is expected to be correlated with the photon energy, reduced methyl radical formation is anticipated during 100 kV X-ray irradiation if the methyl is derived from CO. Experimentally, methyl radicals were observed after X-ray exposure in a concentration approximately 4.5 times less than that following gamma-ray irradiation to the same approximate dose. This result suggests that at least some methyl radicals in these samples result from radiolysis of dissolved CO, although the issue is somewhat beclouded by uncertainties in the X-ray dosimetry and questions concerning • / the complex kinetic scheme which would pertain to such a formation process. Samples of Type III synthetic silica were prepared by flame fusion using two different fuels to further test the hypothesis that methane or CO was trapped in the silica network during the fusion process. Boules were formed by flame hydrolysis in either a hydrocarbon-oxygen or hydrogen-oxygen flame. Bulk samples were prepared by core drilling 4 mm rods from the boule. Powdered samples were obtained by grinding sections of the boule, sizing into two fractions: < 4 0 0 ~ or 400-1250 btm, and drying at 150°C to remove adsorbed water. The rods and the powders were irradiated, thermally annealed (in some cases), and measured. Methyl radical concentrations similar to those observed in Suprasil and Dynasil rods were measured in the samples prepared by fusion in the hydrocarbon-oxygen flame, but no methyl radicals were measured (within the limits set by signal-to-noise of the ESR spectrometer) in the sample fused in the hydrogen-oxygen flame (table 1). Various attempts were made to diffuse methane into the special sample fused in the hydrogen-oxygen flame. Powders in the 400-1250 /~m size fraction were loaded into a bomb and soaked at 550°C for 3-½ h under a methane pressure of 250 psi; a similar experiment was conducted for 24 h at 650°C. ESR measurements of these samples after irradiation and thermal annealing revealed no evidence of methyl radicals. A subsequent experiment was conducted in which rods of silica prepared in a hydrocarbon-oxygen flame were subjected to intense heating in a methane-oxygen flame for an hour to attempt to increase the methyl radical concentration. As shown in table 1, the results were ambiguous: although the methyl radical concentration was greater in the flamed rods in one measurement, when the experiment was repeated on another section of the rods, no difference was found between the flamed and unflamed rods. It is unclear at this time whether the flaming actually increased the methane concentration or whether there is an uneven distribution of methane in the rods; either explanation (or perhaps a combination of both) can apparently account for the observed results.
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E.J. Friebele et al. / Methyl radical in silica
Table 1 Methyl radical concentrations of various SiO2 samples measured by ESR spectroscopy and viscosity of selected samples at 1150°C CH~ (1013/g SiO2) I. Synthetic silica A. Powder samples 1. H 2 - O 2 fused 2. H 2-C-O2 fused B. Rod samples 1. Experiment ~ I Unflamed Flamed 2. Experiment :~2 Unflamed Flamed 3. H 2 - O 2 fused 4. H 2 - C - O 2 fused II. Fused quartz A. H 2 - O 2 fused 1. No anneal 2. Annealed 3h at 950°C B. Electrically fused
< 0.01 2.63
"O (1012 Poise)
0.94 1.23
1.88-1.97 2.54-3.07 2.90-3.01 2.66-3.11 < 0.01 2.96
< 0.01 < 0.01 < 0.01
30.1
F i n a l l y , s a m p l e s of n a t u r a l q u a r t z fused in e i t h e r a h y d r o g e n - o x y g e n f l a m e o r an electric arc w e r e e x a m i n e d . T h e lack o f m e t h y l r a d i c a l s in these s a m p l e s is c o n s i s t e n t w i t h the h y p o t h e s i s t h a t C H 4 o r C O is t r a p p e d in the silica n e t w o r k o n l y d u r i n g f u s i o n in a h y d r o c a r b o n - o x y g e n f l a m e . T h e results of the gas e x t r a c t i o n e x p e r i m e n t are s h o w n in table 2, a n d it is a p p a r e n t t h a t m e a s u r a b l e q u a n t i t i e s o f b o t h C H 4 a n d C O are o b s e r v e d . T o use this f i n d i n g to f i r m l y c o n c l u d e t h a t o n e or the o t h e r of the m o l e c u l e s is p r e s e n t in the silica as a p r e c u r s o r to t h e m e t h y l r a d i c a l is p r o b l e m a t i c a l , h o w e v e r , for s e v e r a l r e a s o n s : (1) s u b s t a n t i a l a m o u n t s o f b o t h C H 4 a n d C O are m e a s u r e d w h e n the e x t r a c t i o n is p e r f o r m e d on the s a m p l e fused in a h y d r o g e n - o x y g e n f l a m e , w h e r e a s n e i t h e r m e t h y l n o r f o r m y l [2] r a d i c a l s are d e t e c t e d b y E S R in
Table 2 Gas concentrations (1016/g SiO2) measured by extraction in unannealed silica samples and a sample vacuum annealed for 3 h at 950°C H2 1. If.
H 2 -02 fused H 2 - C - O 2 fused Unannealed Annealed
CH 4
CO
N2
7.02
0.29
4.35
0
10.5 3.64
0.34 0.30
7.73 3.36
0 0
174
E.J. Friebele et al. / Methyl radical in silica
this glass following irradiation; (2) the concentration of CO measured after vacuum annealing of the sample fused in the hydrocarbon-oxygen flame is less than that in one fused in the hydrogen-oxygen flame; and (3) both the C H 4 and CO concentrations measured by gas extraction are - 100 times the methyl or formyl [2] radical content determined by ESR. The evidence suggests that most of the C H 4 and CO measured during the gas extraction experiment was formed by the reaction of hydrogen or oxygen evolving from the silica with the C present in the vacuum system. (No C H 4 w a s measured when the extraction procedure was carried out without a silica sample in the furnace.) The fact that vacuum annealing of the silica did reduce the measured amount of C H 4 and CO (table 2) can be attributed to the hydrogen (and presumably oxygen) that was removed during the anneal resulting in less C H 4 and CO formation during a subsequent extraction. Further evidence against the vacuum anneal removing these molecule's/ from the silica can be found in the diffusion behavior. If one assumes that the diffusion coefficient scales approximately with molecular size, it can be concluded that C H 4 should diffuse somewhat more rapidly than N 2 or OH in silica, but more slowly than Ne. In this case, a period of 1 2 days would be required for C H 4 t o diffuse 100 /~m, so that little would be extracted during the hour of the extraction measurement. A similar result obtains for CO. Thus, the methyl radical concentrations measured by ESR are evidence of methane (or a precursor, such as CO) dissolved in the silica, whereas the C H 4 and CO measured by gas extraction unfortunately appears to be an artifact. Nevertheless, the H 2 measured during extraction (table 2) is presumed to have evolved from the glass since hydrogen is known to diffuse rapidly, and vacuum annealing has been shown to be effective in reducing the hydrogen content in silica [11]. The results of the viscosity measurements on several samples are shown in table 1. Although the slight increase in viscosity noted between the synthetic silica sample fused in hydrogen-oxygen and that fused in hydrocarbon-oxygen flames correlates with the methyl radical concentration, it seems unlikely that such a small amount (2.6 ppb) could affect the viscosity. Rather, it is more likely that there is a greater OH content in the sample fused in the hydrogen-oxygen flame than that fused in the hydrocarbon-oxygen flame. The greater viscosity of the latter and the much larger viscosity of the fused quartz sample are in accord with the known relationship between O H content and viscosity in silica [12].
4. Conclusions
The present study has identified the methyl radical in irradiated synthetic silica, and its characteristic ESR spectrum has been analyzed to determine that the radical is substantially hindered in its rotational freedom in the interstitial sites in the glass. The radical is apparently formed by radiodissociation of C H 4
E.J. Friebele et al. / Methyl radical in silica
175
molecules or radiodissociation of CO molecules followed by reaction of the C with radiolytic H. The precursor C H 4 o r CO are dissolved in the silica during flame hydrolysis in a hydrocarbon-oxygen flame. The remarkable stability of the radicals is presumed to result from the hydrogen atom diffusing away from the methane (or the oxygen atom separating from the C derived from CO) after dissociation and subsequently being trapped in a deep well.
Acknowledgements The authors would like to thank M. Stapelbroek for the ESR measurements taken in the early stages of this research.
References [1] E.J. Friebele and D.L. Griscom, in: Treatise on Materials Science and Technology, eds. M. Tomozowa and R. Doremus (Academic Press, New York, 1979), Vol. 17, Glass II, P. 257. [2] D . L Griscom, M. Stapelbroek and E.J. Friebele, J. Chem. Phys. 78 (1983) 1638. [3] M. Stapelbroek, D.L. Griscom, E.J. Friebele and G.H. Sigel, Jr., J. Non-Crystalline Solids 32 (1979) 313. [4] T. Cole, H.O. Pritchard, N.R. Davidson and H.M. McConnell, Mol. Phys. 1 (1958)406. [5] V.B. Kazanskii and G.B. Pariiskii, Proc. 3rd Int. Congr. Catalysis (Amsterdam, 1964) Vol. 1 (1965) p. 367. [6] C.L. Gardner and E.J. Casey, Can. J. Chem. 46 (1968) 207. [7] J.H. Freed and G.K. FraenkeL J. Chem. Phys. 39 (1963) 326. [8] C. Heller, J. Chem. Phys. 36 (1962) 175. [9] G.B. Garbutt and H.D. Gesser, Can. J. Chem. 48 (1970) 2685. [10] R.W. Fessenden, J. Chem. Phys. 37 (1962) 747. [11] J.E. Shelby, in: Treatise on Materials Science and Technology, Eds. M. Tomozawa and R. Doremus (Academic Press, New York, 1979) vol. 17, Glass II, p. 1; J.E. Shelby, J. Appl. Phys. 51 (1980) 2589. [12] R. BrOckner, J. Non-Crystalline Solids 5 (1971) 177.
167
O B S E R V A T I O N O F T H E M E T H Y L R A D I C A L CH~ IN I R R A D I A T E D , H I G H - P U R I T Y S Y N T H E T I C F U S E D SILICA E.J. F R I E B E L E a n d D.L. G R I S C O M Naval Research Laboratory, Washington, DC 20375, USA K. RAU Heraeus Quarzschmelze GmbH, D-6450 Hanau, Fed. Rep. Gerrnar~v
Received 2 November 1982 Revised manuscript received 10 January 1983
Stable methyl radicals CH3 have been observed in irradiated high-purity synthetic silicas by electron spin resonance techniques. Analysis of the dependence of the linewidth on the nuclear spin quantum number of the ESR spectrum demonstrates that the radicals tumble at a frequency of - 11 MHz. We suggest that this small tumbling rate as compared with the "free" rotation rate of 10 GHz is due to interaction of the radical with the glass network. Radicals were observed in concentrations of - 3 x 1013/g SiO 2 in samples fused in a hydrocarbon oxygen flame, but no radical concentration could be measured in samples fused in either an electric arc or hydrogen-oxygen flame. The methyl radical is presumed to form by radiodissociation of either CH 4 or CO dissolved in the silica during the fusion process and subsequent reaction of C with radiolytic H.
1. Introduction I r r a d i a t i o n of fused n a t u r a l quartz or high-purity synthetic silica is k n o w n to result in the f o r m a t i o n of p a r a m a g n e t i c defect centers by charge trapping. A n u m b e r of these defect centers have been observed a n d elucidated by electron spin resonance (ESR) spectroscopy [i]. In general, those previously studied have been ascribed to either intrinsic defects or to inorganic i m p u r i t y or d o p a n t - r e l a t e d defects. Examples of the former include the E' center (a hole trapped in an S i - S i b o n d at the site of an oxygen vacancy), the n o n b r i d g i n g oxygen hole center ( N B O H C ) , a n d the peroxy radical (a hole trapped by an S i - O - O - S i excess oxygen linkage); impurity-related defects include the a l u m i n u m - o x y g e n hole center, atomic hydrogen, and other hydrogen-related centers. Recently, G r i s c o m et al. [2] have reported the identification a n d analysis of the formyl radical H C O , which is the first organic defect center to be observed in irradiated synthetic silica. I n this paper we report a second organic defect center, the methyl radical CH3, which has been observed in irradiated synthetic silica; we relate its observation to the m e t h o d of preparation of the silica. 0 0 2 2 - 3 0 9 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
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E.J. Friebele et al. / Methyl radical in silica
2. Experimental Samples of various synthetic (Type IIl) silicas and fused natural quartz (Types I and II) were irradiated at room temperature in a 6°Co source to doses greater than 10 7 rad at a dose rate of 10 4 r a d / m i n . Several samples were irradiated with 100 kV X-rays at a dose rate of 5 × l 0 4 r a d / m i n . Typical total doses were 4.6 x 10 7 rad. in some cases the samples were annealed at 300°C for a period of 10 min to minimize the interference of the E' center and N B O H C spectra with that of the methyl radical [3]. ESR measurements were made on a Varian E-9 spectrometer equipped with a V-4540 nitrogen flow-through accessory, which was used to maintain the sample temperature at - 1 7 0 ° C during the measurement. Sample geometries consisted of both 4 m m rods and powders; the latter were loaded into high-purity synthetic silica sample tubes, evacuated and sealed prior to irradiation. The methyl radical concentrations were determined by double numerical integration of the third (best resolved) hyperfine line of the 1 : 3 : 3 : 1 quartet, multiplication by 8 / 3 and then comparison with the integrated intensity of a calibrated pitch sample. Signal averaging with a Nicolet 1080 data acquisition system was used to enhance the signal-to-noise ratio of samples with low concentrations. Correlations were sought between the methyl radical concentration determined by ESR and the methane concentration measured by gas extraction. Approximately 60 g of powder in a size fraction 400-1000/.tm was heated at a linear rate of 2 0 ° C / m i n from room temperature to 1200°C under vacuum in a fused silica tube. The gas extracted from the sample was collected and delivered to a gas chromatograph in periods of 3 min for analysis. Measurements were made of the H 2, C H 4, C O , and N 2 concentrations; no analysis of H 2 0 or CO 2 was possible with the system. The 02 concentration could also have been measured, but was not. Viscosity measurements were made on selected samples by the beam bending method. The fictive temperature of the silica or quartz was set to 1150°C by heating the samples to this temperature for a period of 1 h; the viscosity measurement was then made at 1150°C.
3. Results and discussion
3.1. ESR spectral analysis The presence of the methyl radical CH~ was first recognized in irradiated synthetic silica samples of Suprasil and Dynasil 1000 by its characteristic ESR spectrum consisting of four isotropic hyperfine (hf) lines with peak heights in the ratio 1 : 3 : 3 : 1 and 2.26 m T coupling constant. (Methyl radicals were not observed in irradiated Spectrosil, Suprasil W or Spectrosil WF). This intensity
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E.J. Friebele et al. / Methyl radical in silica
r a t i o can only be e x p l a i n e d by a hf i n t e r a c t i o n of an electron ( S = ½) with three e q u i v a l e n t spin-½ nuclei; the purity of these silicas suggests that the only c a n d i d a t e is a p r o t o n . T h e 2.26 m T c o u p l i n g c o n s t a n t is in close a g r e e m e n t with p u b l i s h e d values [4] for the methyl radical CH3 leading to the identification of this radical in the synthetic silica samples of the present study. T h e p r e l i m i n a r y E S R m e a s u r e m e n t s were m a d e with a m o d u l a t i o n amplitude of 0.05 m T a n d a m i c r o w a v e p o w e r of 50 m W , b u t it was later recognized that these p a r a m e t e r s h a d the effect of slightly o v e r m o d u l a t i n g a n d substantially p o w e r s a t u r a t i n g the lines, causing artificial b r o a d e n i n g of all the lines to the same linewidth. Since the intensity ratio of the lines is 1 : 3 : 3 : 1 a n d the linewidths were m a d e equal, the peak heights followed the same characteristic ratio. W h e n the m e a s u r e m e n t s were r e p e a t e d with p a r a m e t e r s which had been e x p e r i m e n t a l l y d e t e r m i n e d not to o v e r m o d u l a t e or saturate the lines (0.01 mT, 1.0 mW), a m a r k e d a s y m m e t r y of the lines a n d a d e p e n d e n c e of the linewidth on nuclear spin q u a n t u m n u m b e r M I was observed, as shown in fig. 1. A l t h o u g h the p e a k heights are no longer in the ratio 1 : 3 : 3 : 1, the i n t e g r a t e d intensities d e t e r m i n e d b y d o u b l e integration of each first derivative hf line do follow this ratio within - 4%. The identification of this center as the methyl radical is therefore valid.
~]"'
FREQ. = 9.093 GHz f:103 K
~'x CENTER/ 0.0028)
O. 5 h
j
?
3/2 t/2-1/2-3/2 MI I
I
321
I
I
I
323
I
1
325 MAGNETIC FIELD
1
327
I
I
329
(mT)
Fig. 1. ESR spectrum of a y-irradiated sample of high-purity synthetic silica (Type IlI) fused in a hydrocarbon-oxygen flame. The spectrum shown here was taken 2 y after irradiation to a dose of 4.6 × 10v fad. The comb above the magnetic field scale shows the hf splittings of the methyl radical. The insert contains the experimental and calculated linewidths of the four hf lines (see text).
E.J. Friebele et aL / Methyl radical in silica
170
The lineshapes of the methyl radical lines shown in fig. 1 are suggestive of a powder pattern in which anisotropic g and hf interactions are averaged over all angles due to the random orientation of the radical in the glass with respect to the applied magnetic field. However, the temperature dependence of the ESR spectrum of the radical in irradiated silica indicates that it is tumbling at a rate which is sufficiently rapid to at least partially average these anisotropic effects (see below). Kazanskii and Pariiskii [5] have shown that the linewidths of a spectrum characterized by incompletely averaged g tensor anisotropy will depend on M/z, i.e. the broadening should be symmetric about the center of the spectrum. This is clearly not the case in fig. 1. The tumbling is then presumed to be sufficiently rapid to average the anisotropies completely, and the observed M;-dependent broadening must be due to a different mechanism./ Similar variations in linewidth with nuclear spin quantum numbe/" have been previously reported by Gardner and Casey [6] for the methyl radical adsorbed on silica gel; the tumbling frequency of the radical was sufficient to average the angular effects of the g and hf anisotropy, but these anisotropies gave rise to a relaxation mechanism that was dependent on M z. Following their analysis, the linewidth can be expressed as (T2) -' = a o + a,M;
(1)
+ azMi 2
if one neglects both the nonsecular part and pseudosecular contributions to the broadening [7]. The former is a valid assumption; the latter is good to within 10% for the methyl radical. The constants of eq. (1) have been determined to be [6] a 0 = A(
vB)2
c + K,
a, =
a2 =
(2)
where % is the correlation time, b is the hyperfine anisotropy, B is the magnetic field, Ay is the g-tensor anisotropy, and K contains other unspecified contributions to the linewidth that are independent of M. The linewidths of the four hf lines of the methyl radical were measured on an expanded magnetic field scan and least-squares fit to eq. (1), yielding a 0 = 0.847 MHz, a 1 = 0.473 M H z and a 2 = 0.373 MHz. The experimental and calculated linewidth data are shown in the inset in fig. 1; the fit is somewhat degraded by the interference of the N B O H C with the second hf line. From the expression for a 2, the correlation time can be calculated as 9.2 × 10 -8 s, using the value b = 4.5 M H z determined by Heller [8] for the methyl group in the radical C H 3 C ( C O O H ) v The calculated correlation time indicates a tumbling frequency of 1.1 × l 0 7 s-~ for the methyl radical trapped in irradiated silica. This value can be compared with 2.0 × 10 7 S-! determined by Gardner and Casey [6] for the radical on silica gel and frequencies on the order of 10 ~° s J for "free" rotation. It is obvious that the presence of the silica network substantially hinders the rotation of the methyl radical, as compared with the radical tumbling "freely" or adsorbed on a silica gel surface. One further feature of interest in the methyl radical spectrum of fig. 1 is the
E,J. Friebele et al. / Methyl radical in silica
171
double line structure of the third line of the quartet. This structure is likewise suggestive of a powder pattern of anisotropic g and hf tensors. However, it has been established above that the angular anisotropy of the methyl radical in silica is completely averaged by the tumbling. Similar double line structure has been observed by Garbutt and Gesser [9] in methyl radicals stabilized on porous Vycor glass and has been explained as arising from second-order hf interactions removing the degeneracy of the M 1 = - ~ states contributing to this absorption [10]. As the sample is warmed from - 170°C, the linewidth of the third line broadens owing to increased tumbling frequency so that by - 7 5 ° C the second-order line is no longer resolved. The measured splitting of 0.022 mT at - 170°C for this line is in good agreement with that measured for methyl radicals trapped in Vycor glass [9], as well as the theoretical prediction of 0.0234 m T [10]. An identical splitting is predicted [10] for the second hf line in fig. 1, but it is not observed because the much larger linewidth of this component (Fig. 1 inset) obscures the second-order splitting. It is of interest to speculate why the methyl radical has not been reported in previous ESR studies of irradiated fused silica. The radical is quite stable at room temperature; we measured the integrated intensities of the spectrum in several silica samples two years after irradiation and found them to be virtually identical to those measured only a few days after the irradiation (although the N B O H C and E' center had substantially decayed). Fig. 1 is typical of such a spectrum. The quartet is discernable at room temperature when measured at high power and fairly large modulation amplitude (200 mW, 0.1 mT) even though the peak height is reduced by a factor of 9 from that measured at - 170°C. There is considerable overlap of the N B O H C with the second line of the radical, however, and unless this center is thermally annealed, the lines might remain unrecognized. Most previous studies of the hole center, which employed the broad magnetic field scans of 10-20 m T necessary to observe all four lines of the methyl radical, employed high microwave powers that would have substantially saturated the radical spectrum. Finally, certain preparation procedures appear to be required for the radical to be present in the silica, and it is not known whether these were used in the preparation of the synthetic silica samples previously studied. Although the four-line spectrum was most certainly observed in previous studies, it seems that it was identified as the methyl radical in the present work because of the fortuitous coincidence of low temperature, moderate microwave power, relatively low modulation amplitude and a 20 m T field sweep initially used in studying an irradiated silica sample that had been annealed sufficiently to remove the NBOHC.
3.2. Origin of the trapped methane There appear to be at least two potential mechanisms by which the methyl radical could be formed by irradiation in high-purity synthetic fused silica. Griscom et al. [2] have inferred the existence of dissolved CO in silicas fused in h y d r o c a r b o n - o x y g e n flames; the radiation could break the C = O double
172
E.J. Friebele et al. / Methyl radical in silica
bond, allowing the carbon to combine with radiolytic hydrogen (derived e.g. from Si-OH) to form CH~. Alternatively, methane could be dissolved in the glass network during fusion; the radiation could result in radiodissociation of the methane molecules to form the radical. To test these hypotheses, 4 mm rod samples of silica fused in a hydrocarbon-oxygen flame were irradiated in either a 6°Co gamma ray (1.1 MeV) source or by a 100 kV X-ray machine. Since the efficiency of bond breaking is expected to be correlated with the photon energy, reduced methyl radical formation is anticipated during 100 kV X-ray irradiation if the methyl is derived from CO. Experimentally, methyl radicals were observed after X-ray exposure in a concentration approximately 4.5 times less than that following gamma-ray irradiation to the same approximate dose. This result suggests that at least some methyl radicals in these samples result from radiolysis of dissolved CO, although the issue is somewhat beclouded by uncertainties in the X-ray dosimetry and questions concerning • / the complex kinetic scheme which would pertain to such a formation process. Samples of Type III synthetic silica were prepared by flame fusion using two different fuels to further test the hypothesis that methane or CO was trapped in the silica network during the fusion process. Boules were formed by flame hydrolysis in either a hydrocarbon-oxygen or hydrogen-oxygen flame. Bulk samples were prepared by core drilling 4 mm rods from the boule. Powdered samples were obtained by grinding sections of the boule, sizing into two fractions: < 4 0 0 ~ or 400-1250 btm, and drying at 150°C to remove adsorbed water. The rods and the powders were irradiated, thermally annealed (in some cases), and measured. Methyl radical concentrations similar to those observed in Suprasil and Dynasil rods were measured in the samples prepared by fusion in the hydrocarbon-oxygen flame, but no methyl radicals were measured (within the limits set by signal-to-noise of the ESR spectrometer) in the sample fused in the hydrogen-oxygen flame (table 1). Various attempts were made to diffuse methane into the special sample fused in the hydrogen-oxygen flame. Powders in the 400-1250 /~m size fraction were loaded into a bomb and soaked at 550°C for 3-½ h under a methane pressure of 250 psi; a similar experiment was conducted for 24 h at 650°C. ESR measurements of these samples after irradiation and thermal annealing revealed no evidence of methyl radicals. A subsequent experiment was conducted in which rods of silica prepared in a hydrocarbon-oxygen flame were subjected to intense heating in a methane-oxygen flame for an hour to attempt to increase the methyl radical concentration. As shown in table 1, the results were ambiguous: although the methyl radical concentration was greater in the flamed rods in one measurement, when the experiment was repeated on another section of the rods, no difference was found between the flamed and unflamed rods. It is unclear at this time whether the flaming actually increased the methane concentration or whether there is an uneven distribution of methane in the rods; either explanation (or perhaps a combination of both) can apparently account for the observed results.
173
E.J. Friebele et al. / Methyl radical in silica
Table 1 Methyl radical concentrations of various SiO2 samples measured by ESR spectroscopy and viscosity of selected samples at 1150°C CH~ (1013/g SiO2) I. Synthetic silica A. Powder samples 1. H 2 - O 2 fused 2. H 2-C-O2 fused B. Rod samples 1. Experiment ~ I Unflamed Flamed 2. Experiment :~2 Unflamed Flamed 3. H 2 - O 2 fused 4. H 2 - C - O 2 fused II. Fused quartz A. H 2 - O 2 fused 1. No anneal 2. Annealed 3h at 950°C B. Electrically fused
< 0.01 2.63
"O (1012 Poise)
0.94 1.23
1.88-1.97 2.54-3.07 2.90-3.01 2.66-3.11 < 0.01 2.96
< 0.01 < 0.01 < 0.01
30.1
F i n a l l y , s a m p l e s of n a t u r a l q u a r t z fused in e i t h e r a h y d r o g e n - o x y g e n f l a m e o r an electric arc w e r e e x a m i n e d . T h e lack o f m e t h y l r a d i c a l s in these s a m p l e s is c o n s i s t e n t w i t h the h y p o t h e s i s t h a t C H 4 o r C O is t r a p p e d in the silica n e t w o r k o n l y d u r i n g f u s i o n in a h y d r o c a r b o n - o x y g e n f l a m e . T h e results of the gas e x t r a c t i o n e x p e r i m e n t are s h o w n in table 2, a n d it is a p p a r e n t t h a t m e a s u r a b l e q u a n t i t i e s o f b o t h C H 4 a n d C O are o b s e r v e d . T o use this f i n d i n g to f i r m l y c o n c l u d e t h a t o n e or the o t h e r of the m o l e c u l e s is p r e s e n t in the silica as a p r e c u r s o r to t h e m e t h y l r a d i c a l is p r o b l e m a t i c a l , h o w e v e r , for s e v e r a l r e a s o n s : (1) s u b s t a n t i a l a m o u n t s o f b o t h C H 4 a n d C O are m e a s u r e d w h e n the e x t r a c t i o n is p e r f o r m e d on the s a m p l e fused in a h y d r o g e n - o x y g e n f l a m e , w h e r e a s n e i t h e r m e t h y l n o r f o r m y l [2] r a d i c a l s are d e t e c t e d b y E S R in
Table 2 Gas concentrations (1016/g SiO2) measured by extraction in unannealed silica samples and a sample vacuum annealed for 3 h at 950°C H2 1. If.
H 2 -02 fused H 2 - C - O 2 fused Unannealed Annealed
CH 4
CO
N2
7.02
0.29
4.35
0
10.5 3.64
0.34 0.30
7.73 3.36
0 0
174
E.J. Friebele et al. / Methyl radical in silica
this glass following irradiation; (2) the concentration of CO measured after vacuum annealing of the sample fused in the hydrocarbon-oxygen flame is less than that in one fused in the hydrogen-oxygen flame; and (3) both the C H 4 and CO concentrations measured by gas extraction are - 100 times the methyl or formyl [2] radical content determined by ESR. The evidence suggests that most of the C H 4 and CO measured during the gas extraction experiment was formed by the reaction of hydrogen or oxygen evolving from the silica with the C present in the vacuum system. (No C H 4 w a s measured when the extraction procedure was carried out without a silica sample in the furnace.) The fact that vacuum annealing of the silica did reduce the measured amount of C H 4 and CO (table 2) can be attributed to the hydrogen (and presumably oxygen) that was removed during the anneal resulting in less C H 4 and CO formation during a subsequent extraction. Further evidence against the vacuum anneal removing these molecule's/ from the silica can be found in the diffusion behavior. If one assumes that the diffusion coefficient scales approximately with molecular size, it can be concluded that C H 4 should diffuse somewhat more rapidly than N 2 or OH in silica, but more slowly than Ne. In this case, a period of 1 2 days would be required for C H 4 t o diffuse 100 /~m, so that little would be extracted during the hour of the extraction measurement. A similar result obtains for CO. Thus, the methyl radical concentrations measured by ESR are evidence of methane (or a precursor, such as CO) dissolved in the silica, whereas the C H 4 and CO measured by gas extraction unfortunately appears to be an artifact. Nevertheless, the H 2 measured during extraction (table 2) is presumed to have evolved from the glass since hydrogen is known to diffuse rapidly, and vacuum annealing has been shown to be effective in reducing the hydrogen content in silica [11]. The results of the viscosity measurements on several samples are shown in table 1. Although the slight increase in viscosity noted between the synthetic silica sample fused in hydrogen-oxygen and that fused in hydrocarbon-oxygen flames correlates with the methyl radical concentration, it seems unlikely that such a small amount (2.6 ppb) could affect the viscosity. Rather, it is more likely that there is a greater OH content in the sample fused in the hydrogen-oxygen flame than that fused in the hydrocarbon-oxygen flame. The greater viscosity of the latter and the much larger viscosity of the fused quartz sample are in accord with the known relationship between O H content and viscosity in silica [12].
4. Conclusions
The present study has identified the methyl radical in irradiated synthetic silica, and its characteristic ESR spectrum has been analyzed to determine that the radical is substantially hindered in its rotational freedom in the interstitial sites in the glass. The radical is apparently formed by radiodissociation of C H 4
E.J. Friebele et al. / Methyl radical in silica
175
molecules or radiodissociation of CO molecules followed by reaction of the C with radiolytic H. The precursor C H 4 o r CO are dissolved in the silica during flame hydrolysis in a hydrocarbon-oxygen flame. The remarkable stability of the radicals is presumed to result from the hydrogen atom diffusing away from the methane (or the oxygen atom separating from the C derived from CO) after dissociation and subsequently being trapped in a deep well.
Acknowledgements The authors would like to thank M. Stapelbroek for the ESR measurements taken in the early stages of this research.
References [1] E.J. Friebele and D.L. Griscom, in: Treatise on Materials Science and Technology, eds. M. Tomozowa and R. Doremus (Academic Press, New York, 1979), Vol. 17, Glass II, P. 257. [2] D . L Griscom, M. Stapelbroek and E.J. Friebele, J. Chem. Phys. 78 (1983) 1638. [3] M. Stapelbroek, D.L. Griscom, E.J. Friebele and G.H. Sigel, Jr., J. Non-Crystalline Solids 32 (1979) 313. [4] T. Cole, H.O. Pritchard, N.R. Davidson and H.M. McConnell, Mol. Phys. 1 (1958)406. [5] V.B. Kazanskii and G.B. Pariiskii, Proc. 3rd Int. Congr. Catalysis (Amsterdam, 1964) Vol. 1 (1965) p. 367. [6] C.L. Gardner and E.J. Casey, Can. J. Chem. 46 (1968) 207. [7] J.H. Freed and G.K. FraenkeL J. Chem. Phys. 39 (1963) 326. [8] C. Heller, J. Chem. Phys. 36 (1962) 175. [9] G.B. Garbutt and H.D. Gesser, Can. J. Chem. 48 (1970) 2685. [10] R.W. Fessenden, J. Chem. Phys. 37 (1962) 747. [11] J.E. Shelby, in: Treatise on Materials Science and Technology, Eds. M. Tomozawa and R. Doremus (Academic Press, New York, 1979) vol. 17, Glass II, p. 1; J.E. Shelby, J. Appl. Phys. 51 (1980) 2589. [12] R. BrOckner, J. Non-Crystalline Solids 5 (1971) 177.