ESR detection of immiscibility in K2OCaOB2O3 glasses

Journal of Non-CrystallineSolids 29 (1978) 249-252 © North-Holland Publishing Company

LETTER TO THE EDITOR ESR DETECTION OF IMMISCIBILITY IN

K20-CaO-B203 GLASSES

H. KAWAZOE, H. HOSONO and T. KANAZAWA Department of Industrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Fukazawa, Setagaya-ku, Tokyo 158, Japan Received 6 March 1978

Immiscibility in a glass is usually detected by visual inspection, X-ray diffraction measurement and electron- or optical-microscopic observation. However, the composition of the separated phases in an immiscible glass can be determined only with great difficulty by these techniques. Line profile and spin hamiltonian parameters of a magnetic resonance spectrum for spins in a glass matrix are strikingly sensitive to a local fluctuation of their environments [1]. This suggests possibilities for detecting an immiscibility in which the separated phases are interlocking in microscopic scale, and for determining the approximate compositions of the separated phases by magnetic resonance spectroscopy. The purpose of the present paper is to show the applicability of ESR to the study of immiscibility in a glass. Shartsis et al. [2] and Imaoka [3] showed independently that the immiscibilities in CaO-B20 3 glasses diminish by substituting K20 for a certain def'mite fraction of CaO, as shown in fig. 1. The composition region where phase separation is pronounced is shaded in the figure. In the present work, the x K 2 0 - ( 1 0 - x)CaO-90B203 glass system was adopted, where x shows K20 mol%. Two reference systems, K 2 0 - B 2 0 3 and CaO-B203, were also studied. The required characteristics of an ESR probe are as follows: (1) intense absorption, (2) sensitiveness to local environmental fluctuations, and (3) a relatively moderate site preference, that is, having affinity with the separated phases. Copper(II), being expected to meet the conditions, was selected as an ESR probe. The guaranteed grade reagents of K2CO3, CaCO3, H3BO 3 and Cu(II)o were used as starting materials. The glass batches containing 0.2 wt% CuOI)O were melted in Pt-10% Rh crucibles at 1100°C for 30 rain. A quenched glass was obtained by pouring a melt onto one stainless steel plate and pressing it with another. Electron spin resonance spectra of Cu(II) ions in oxide glasses can be analysed by assuming the spin hamiltonian with axial g- and hyperfine-tensors. Line prof'des and spin hamiltonian parameters of ESR spectra of Cu(II) ions in the reference potassium borate glasses depend on K20 content [4]. The spectrum for one of the reference glasses with x = 10 (glass K) is shown in fig. 2. On the other hand, line profiles and parameters for other reference samples of 249

250

H. Kawazoe et al. / Immiscibility in K 2 0 - C a O - B 2 0 3 glasses PARALLEL HYPERFINE SHOULDERS

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Fig. 1. Glass formation region in the K20-CaO-B203 system. Opalescence is pronounced in the shaded area [2,3]. Filled circles represent the compositions where immiscibilities are observed by ESR spectroscopy. Homogeneous glass formations were recognized for the specimens whose compositions were denoted by open circles. Fig. 2. ESR spectra of Cu(II) in xK20-(10 -- x)CaO-90B203 glasses, where x represents K20 mol%. The signal gain for the parallel hyperfine shoulders is ten times larger than that for the perpendicular hyperfine divergences. The symbol m denotes nuclear spin quantum numbers I z. calcium borate, the CaO content of which were, 5, 10, 15, 20, 23, 27 and 30 mol%, were found to be substantially identical, irrespective o f the lime content. This is due to the fact that the immiscible region in the system C a O - B 2 0 3 ranges from ~ 0 to ~ 2 7 tool% CaO, as shown in fig. 3. The compositions o f the two separated phases are ,-~B203 (phase A) and ,~27CaO • 73B203 (phase B). Since the Ca ion concentration in A is much lower than that in B, most o f the Cu(II) ions will be distributed in B. Therefore, the line profile for the samples o f x = 5 - 2 3 is expected to be identical with that for the samples o f 30 ~>x >~ 27 which is recognized as homogeneous b y ESR and visual inspection. The spectrum o f another reference glass with x = 0 (glass c) is illustrated in fig. 2. This glass is heterogeneous and expressed as C=f~.A+f~-B,

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where f ~ and fB represent fractions of the A and B phases, respectively, and can be determined from the glass composition. The compositions of the A and B phases are, of course, approximate values. In fig. 2 clear distinctions can be recognized in the line profiles for the two reference glasses, K and C: four parallel hyperfine shoulders can be seen for glass K, while three of them are recognizable for glass C, the reverse obtaining in the perpendicular divergences. The mixed cation glass of x = 2 yielded opalescence, and on visual inspection glasses of x = 5, 7 and 9 were found to be transparent, in accordance with tile results reported by other authors [2,3] (fig. 1). The spectra for these glasses are also shown in fig. 2. It should be noted that not only the spectrum for opal glass (x = 2) but also those for transparent glasses (x = 5 and 7) are the superpositions of the two reference spectra (x = 0 and 10). The relative contributions of two extreme spectra change with the KzO/CaO tool ratio in the mixed cation glasses. This is remarkable in the parallel hyperfine shoulders, with m = - 1 / 2 and +1/2, and in the perpendicular hyperfine divergences. When 90% of the CaO is replaced by K20 the contribution of the spectrum of glass C becomes negligible. Therefore, the transparent glasses of x = 3 ~ 8 have heterogeneous structures which are contained in the opal glass of x = 2. ESR results show that the immiscible region given in fig. 1 should be enlarged to a higher K20 content. The glasses of x = 3 - 8 (glass G) include three phases and may be expressed as G = fK . K + fc . C = fK . K + fA . A + fB . B ,

where fK and f c denote fractions of the phases K and C, respectively. Again it may be noted that the compositions of the respective phases are approximate values. Electron spin resonance spectroscopy has been found suitable for detecting micro-heterogeneity in a glass and for determining approximate compositions of separated phases, if used together with a microscopic observation. The authors wish to thank Professor Sumio Sakka of Mie University for help in the preparation of this manuscript.

252

H. Kawazoe et al. /Immiscibility in K 2 0 - C a O - B 2 0 3 glasses

References [1] P.C. Taylor, J.F. Baugher and H.M. Kriz, Chem. Rev. 75 (1975) 203. [2] L. Shartsis, H.F. Shermer and A.G. Bestul, J. Am. Ceram. Soc. 41 (1958) 507. [3] M. Imaoka, Advances in Glass Technology, Proc. 6th Int. Congress on Glass, Washington, DC 1962 (Plenum Press, New York, 1962) p. 149. [4] H. Kawazoe, J. Hosono and T. Kanazawa, J. Non-Crystalline Solids, this issue, p. 173. [5] E.T. Carlson, Bur. Stand. J. Res. 9 (1932) 830.