Dielectric losses of alkali metaphosphate glasses in the low
temperature range 4-200 K, and their relation to the glass
composition
Citation for published version (APA):
Metselaar, R., Nelissen, R. G. W. J., & Vogel, D. L. (1981). Dielectric losses of alkali metaphosphate glasses in the low temperature range 4-200 K, and their relation to the glass composition. Journal of Non-Crystalline Solids, 44(2-3), 315-320. https://doi.org/10.1016/0022-3093(81)90034-X
DOI:
10.1016/0022-3093(81)90034-X
Document status and date: Published: 01/01/1981
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Journal of Non-Crystalline Solids 44 ( 1981) 315- 320 315 North-Holland Publishing Company
D I E L E C T R I C L O S S E S O F ALKALI M E T A P H O S P H A T E G L A S S E S IN T H E L O W T E M P E R A T U R E RANGE 4-200 K, AND T H E I R R E L A T I O N T O T H E G L A S S C O M P O S I T I O N
R. M E T S E L A A R , R.G.W.J. N E L I S S E N * and D.L. V O G E L
Department of Chemist~, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, the Netherlands
Received 10 July 1980
Revised manuscript received 20 February 1981
Dielectric losses of Li, Na, K, Rb and Cs metaphosphate glasses have been studied in the temperature range 4-200 K. In the sodium containing glasses the ratio of linear to cyclic phosphates was varied by adding cyclic tri or tetrametaphosphate. The height of the loss peak, tan 8 (max), increases with an increasing amount of cyclic metaphosphates in the glass.
I. Introduction
Dielectric losses have been studied extensively in silicate, borate and phos- phate glasses in the temperature range 200-700 K [1]. Dielectric losses of borate glasses in the low temperature range, 4 - 2 0 0 K, were studied by van G e m e r t and Stevels [2,3] in our institute. It seemed appropriate to extend their investigations to phosphate glasses.
Mainly through the work of W e s t m a n [4], it is known that alkali m e t a p h o s p h a t e glasses contain linear as well as cyclic metaphosphate mole- cules in greatly varying amounts. U p o n changing the ratio of the n u m b e r of cyclic molecules to the n u m b e r of linear molecules, a change in the mobility of the glass network is expected, and, consequently, a correlation between this ratio and the dielectric losses of the glasses. A fortunate circumstance is that a quantitative analysis of phosphate glasses is not too difficult since the latter m a y be dissolved in water without significantly changing their composition.
2. Experimental
Alkali (Li, Na, K, Rb and Cs) metaphosphate was made by heating a mixture of alkali carbonate (Merck) and a m m o n i u m mono-hydrogen ortho-
* Present address: Central Laboratories, N.V. Koninklijke Sphinx, Maastricht, the Netherlands. 0 0 2 2 - 3 0 9 3 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © N o r t h - H o l l a n d
316 R. Metselaar et al. / Dielectric losses of alkali metaphosphate glasses
phosphate with a molar ratio 1 : 2 in a platinum crucible in an electric furnace at 900°C. After three hours, no further weight loss occurred and the molar ratio alkali to phosphor remained equal to one. The melt was poured out onto a steel plate and immediately flattened with a cold flat-iron into a sheet, approximately 2 mm thick.
In order to obtain a series of phosphate glasses with a varying cyclic to linear ratio but containing the same alkali metal, varying amounts of cyclic sodium trimetaphosphate (Ventron), cyclic sodium tetrametaphosphate (pre- pared according to Brauer [5] and Pascal [6]), sodium orthophosphate (Merck), and sodium pyrophosphate (Merck, respectively, were added to sodium metaphosphate at 900°C in the furnace. After these additions the melts were held for two minutes in the furnace, and then processed as described above.
Cylindrical samples of 25 mm diameter were machined out of the glass sheets, ground to a thickness of 1 to 2 mm, and polished. The samples were mounted between blocking electrodes in a cryostat which was specially de- signed by Oxford Instruments Co., and is described in ref. 3. The samples between the electrodes were a part of a Wheatstone's bridge, equipped with a Rhode and Schwarz RC generator (Type SRM BN 4085), a dielectric test bridge (Type VRB BN 12 121/2), and a tunable indicator amplifier (Type VRB BN 3520). The loss measurements were carried out between 5 and 200 K at frequencies ranging from 0.1 to 100 kHz.
In order to determine the amounts of the various components in the phosphate glasses (cyclic tri and tetrametaphosphate, and orthophosphate) we followed the chromatographic method of Crowther [7]. However, instead of paper, we used Merck 20 × 20 cm plates covered with cellulose F. In the elution liquid we used trichloro-acetic instead of formic acid. We did not analyse for cyclic penta- and higher phosphates, since it is known from the work of Westman that these are only present in small amounts.
3. Results and discussion
The results of our measurements on the sodium glasses are shown in figs. 1 and 2.
It can be seen from these figures that the curve for tan ~ versus temperature is a superposition of the tail of the loss curve which is due to the migration of the alkali ions (having its maximum far above 200 K), and of the loss curve for a process which causes dielectric losses to occur in the low temperature region. Stainless steel blocking electrodes were used to separate the two loss peaks as far as possible. We assume that the contribution of the migration peak to the low-temperature losses becomes negligible below 140 K for the Li and Na glasses, and below 200 K for the K, Rb and Cs glasses, and that the tail of the alkali migration peak has an exponential shape [8,9]. We may then fit an
R. Metselaar et aL / Dielectric" losses of alkali metaphosphate glasses 317 I I I 1 - - a - - 2 [ o - - 6 , - 7 i. 7 /;( _ _.,~, J / ~ - y . . - - ~ /
,~ i I . j J
":I
I , / , J \i '/*1 1 7
\x 2 0 '#~
\ \ 7 .I /
,
fi \\ I 0 n I I \~ I l 60 80 120 160 t e m p e r a t u r e (K)Fig. 1. Dielectric loss curves for sodium phosphate glasses. Sample 2, Na2OP2Os; samples 6, 7 and the same 8 with the addition of cyclic trimetaphosphate (cf. tables 1 and 2). Curve 7* shows the peak of sample 7 after the subtraction of the exponential background due to the migration loss peak.
Table I
Overall composition, analyzed cyclic metaphosphate contents, and tan 3 (max) at 1 kHz for a series of alkali metaphosphate glasses. The concentrations of the cyclic phosphates were determined from five observations, standard deviation ~ 0.6, except for sample 5 where it was as large as 2.5. The orthophosphate concentration was less than l mo1.% in all cases
Sample Overall composition No.
Cyclic Cyclic Total cyclic tan 3 (max) trimeta- tetrameta- Metaphosphate × 10 4 phosphate phosphate (mol.%) (at I kHz) (mol.%) (mol.%) I Li2OP205 11.5 2 Na2OP205 12.2 3 K2OP205 14.4 4 Rb2OP205 10.6 5 CSEOP205 6.2
Na 2OP205 with addition of:
6 5 mol.% cyclic trimetaphosphate 17.0 7 10 mol.% cyclic trimetaphosphate 22.5 8 15 mol.% cyclic trimetaphosphate 26.9 9 5 mol.% cyclic tetrametaphosphate 12.2 10 10 mol.% cyclic tetrametaphosphate 12.2 11 15 mol.% cyclic tetrametaphosphate 12.7
7.6 19.1 20.0 12.5 24.7 20.0 20.3 34.7 36.4 < 1 10.6 16.4 < 1 6.2 16.4 12.6 29.6 22.0 12.8 35.3 28.2 12.5 39.4 28.5 17.9 30.1 21.5 22.5 34.7 28.2 27.4 40.1 28.4
318 40 30 % ~t 2 0 - IC 0
R. Metselaar et al. / Dielectric losses of alkali rnetaphosphate glasses
I I I I - - . . , m 2 i o m 9 - ,
_ , o
/ / /
- - . - - 1 1 i.///,~/'/
/ /
11/
/ x 10 x ~ . ~ x ~/'/
o~ o~"~°
/ ' / / * ~/2,/?
7
4 / I 1 I I 4 0 80 120 160 t e m p e r a t u r e ( K ) PFig. 2. Dielectric loss curves for sodium phosphate glasses. Sample 2, Na2OP2Os; samples 9, 10 and 11 the same with the addition of cyclic tetrametaphosphate (cf. table'l and 2).
exponential line through the origin and the experimental points above 140 K (or 200 K) and subtract this background from the experimental curve to obtain a line which is due to the low-temperature process only (cf. curve 7* in fig. 1 as an example). Although this a a rather rough procedure, we may determine in this manner the temperature of the maximum in the low-temperature loss
Table 2
Activation energies Q ± 0.01 eV, and temperatures Tma x at a frequency of 1 kHz of the low- temperature loss peak for alkali metaphosphate glasses
Sample no. Overall composition Q (eV) Tm~ x (K)
1 Li2OP20~ a 0.07; 0.17 43; 81
2 Na2OP205 0.09 52
3 K2OP205 0.21 120
4 Rb2OP205 0.25 120
5 Cs2OP205 0.21 100
6 - I 1 Na 2OP205 plus cyclic 52-56
R. Metselaar et aL / Dielectric losses of alkali metaphosphate glasses 319 30
I
I
I
zx
I
7 C ) x 2 0 x E 10 0 0qm
V • e N a AK o Rb [] CsI
I
I
I
10 20 30 40 total cyclic m e t a p h o s p h a t e c o n t e n t (moLe*/,)Fig. 3. Plot of tan 8(max) versus the total cyclic metaphosphate content for the low-temperature dielectric loss peaks of the alkali metaphosphates.
curve, Tma~, and the value for tan $ (max). These maxima are shown in table 1 together with the results of the chemical analysis (for the Li glass two peaks are observed).
It is clear from fig. 3 where tan 8 (max) is plotted versus the total cyclic metaphosphate content that the dielectric losses increase with the number of cyclic molecules per mole of metaphosphate glass.
On the other hand, addition of sodium ortho- and pyrophosphate gives no increase in tan ~ (max) for the sodium metaphosphate glass.
Though the peaks do not show the Debye shape it may be assumed that the relaxation process is thermally activated and that the Arrhenius equation T = T O
exp(Q/kT)
is valid. It is then possible to determine a mean activation energy for the process from the shift of Tma x as a function of frequency. The resulting Q-values, together with Tma x at a frequency of 1 kHz, are given in table 2. A typical value T O ,.~ 5 × 10 t4 s is observed, for the relaxation time, i.e. T O ] is of the order of a vibrational frequency. The activation energies of 0.07-0.2 eV strongly suggest a relaxation due to the so-called inherent local motions [3,10]. The observed correlation between tan ~ (max) and the amount of cyclic molecules indicates that the process is related to motions in the phosphate rings. This supposition is corroborated by the following calculation. The value of the relaxation strength, at Tmax, Ac can be estimated from the area under the curve of tan ~ versus T - i [3]. F r o m Ae, a value o f p 2 N can be obtained, where320 R. Metselaar et al. / Dielectric losses of alkali metaphosphate glasses
p is an a v e r a g e d d i p o l e m o m e n t a n d N is the c o n c e n t r a t i o n o f dipoles. F o r instance, for s a m p l e 3, we f i n d p 2 N , - ~ 5 × 10 20 D e b y e 2 c m - 3 (45 ×
10 4 C 2 m - l ) . A s s u m i n g t h a t d i p o l e s o f the o r d e r of a D e b y e unit a r e involved, in a c c o r d a n c e with e x p e r i m e n t a l results for v a r i o u s glasses [10,11] we o b t a i n N--~ 10 20 c m -3. T h i s value is well w i t h i n the range e x p e c t e d for m o t i o n s of the r i n g system.
W e t h a n k Prof. Dr. J.M. Stevels for h a v i n g s t i m u l a t e d this investigation.
References
[1] H.M.J.M. van Ass, Thesis, Eindhoven University of Technology, Eindhoven, the Netherlands (1975).
[2] W.J.Th. van Gemert, H.M.J.M. van Ass and J.M. Stevels, J. Non-Crystalline Solids 16 (1974) 282.
[3] W.J.Th. van Gemert, Thesis, Eindhoven University of Technology, Eindhoven, the Nether- lands (1977).
[4] A.E.R. Westman and R. Beatty, J. Am. Ceram. Soc. 49 (1966) 63; A.E.R. Westman, Glastech. Bet. 35 (1962) 500.
[5] G. Brauer, Handbuch der Praparativen Anorganische Chemie, Erster Band, 2. Auflage, Enke Verlag, Stuttgart (1960).
[6] P. Pascal, Nouveau Trait6 de Chimie Min~rale, Tome 2, Li/Na, Masson et Cie., Paris (1966) p. 772.
[7] J. Crowther, Anal. chem. 26 (1954) 1383.
[8] W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to ceramics, (J. Wiley and Sons, New York, 1976).
[9] J. Wong and G.A. Angell, Glass structure by spectroscopy (Marcel Dekker, New York, 1976). [10] J.M. Stevels and J. Volger, Philips Res. Rep. 17 (1962) 283.
