containing W03 up to 5 mol % exhibited dipolar relaxation effects. These effects
are ... system with the aid of data on IR spectra and differential thermal analysis of
these glasses. The study of ... glasses, zinc oxide has the ability to get into
phosphate network ... Appropriate amounts (all mol %) of Analar grade reagents
of ZnO ...
Indian Journal of Engineering & Materials Sciences Vol. 8, October 2001 , pp. 275-284
Influence of W0 3 on dielectric properties of zinc phosphate glasses P Subbalakshmi & N Veeraiah Department of Physics, Nagarjuna University P G Centre, Nuzvid 521 20 I, Indi a
Received 19 March 2001; accepted 4 July 2001 Dielectric constant E', loss tan 8 and a.c conductivity cra.c of 40 ZnO-xWOr (60-x) P20 5 (with 0 :5 x :5 15) glasses are studied as a function of frequency (in the range I 0 2-10 5 Hz) and temperature (range 30-300°C) . The dielectric breakdown strengths of these glasses are also measured in the air medium. All the dielectric parameters (viz., E', tan 8 and cra.c) are found to decrease with the increase in the concentration of W0 3_Dielectric loss variation with the temperature for the glasses containing W0 3 up to 5 mol % exhibited dipolar relaxation effects. These effects are analysed by a pseudo Cole-Cole plot method. The dielectric breakdown strength, the activation energy for a.c conduction are found to decrease with increase in concentration of W0 3_These results have been used to £hrow some light on the structural change in ZnO-WOrP 20 5 glass system with the ai d of data on IR spectra and differential thermal analysis of these glasses.
The study of dielectric properties, such as dielectric constant £', loss tan8 and a.c conductivity cra.c. over a wide range of frequency and temperature and also the dielectric breakdown strength of the glasses helps in assessing their insulating character; these studies may also help in understanding the structure of the glass to some extent. Work along these lines was carried out in recent years on a variety of inorganic glasses by a number of researchers yielding valuable information1 ·5. P20 5 glasses have several advantages over conventional silicate and borate glasses due to their superior physical properties such as high thermal expansion coefficients, low melting and softening temperatures and high ultra-violet transmission 6"10 . However, the poor chemical durability, high hygroscopic and volatile nature of phosphate glasses prevented them from replacing the cdnventional glasses in a wide range of technological applications. In recent years, there has been an enormous amount of research on improving the physical properties and the chemical durability of phosphate glasses by introducing a number of glass formers and modifiers such as Ah0 3, Mo0 3, Cr0 3. Ta20 5. Sb20 3, As20 3 etc., . p 2O5 g Iass networ kll -15 . mto Among various phosphate glass systems, zinc phosphate glasses are considered as interesting glasses, zinc oxide has the ability to get into phosphate network in both metaphosphate and pyrophosphate glasses forming Zn0 4 tetrahedra which link adjacent phosphate chains together through
bridging oxygens 16 . The transition metal ions such as tungsten, dissolved in Zn0-P20 5 glass matrix influence the insulating character of these glasses very strongly because tungsten oxide participates in the glass network forming, with different structural units like W0 4 and W0 611 "12' 17 . Further, it is expected that, tungsten oxide groups form a si ngle tungsten phosphorus - oxygen framework with the phosphorus tetrahedrons, strengthen its structure and raises the chemical resistance of the glass. Most of the studies available on phosphate glasses are on opticaJ 8·12 ·18-21 (such as Raman spectra, IR spectra, optical absorp. 1ununescence · · properttes · 22 ·23 tlon, etc., ) an d e 1asttc though few studies on electrical properties (mostly on d.c. conductivity especially of some binary systems like CdO-P20 5, Pb0-P20 5 etc., 24 -26 ) of some phosphate glasses exists in literature, much devoted studies particularly on dielectric properties such as dielectric relaxation, a.c. conductivity and breakdown strength of ZnO-W0 3-P20 5 system are not available. The aim of the present investigation is to have a comprehensive understanding of the influence of tungsten ions on the insulating character of zinc phosphate glasses from a systematic study on dielectric constant E', loss tan 8, a.c. conductivity cra.c of Zn0-W03-P2 0 5 glasses in the frequency range 102105 Hz and temperature range of room temperature to 300°C and also the dielectric breakdown strength in air medium. Further in this study, it is intended to throw some light on the structural modifications (with the help of IR spectral studies and differential thermal
INDIAN J. ENG. MATER. SCI., OCTOB ER 2001
276
temperature in air medium using a high a.c. voltage breakdown tester (ITL Model AAH-55, Hyderabad) operated with an input voltage of 250 V at a frequency of 50 Hz; it was ensured that all the glasses used for this study were of almost identical thickness. Infrared transmission spectra of these glasses was recorded using a FT/IR-5300 Fourier Transform infrared spectrophotometer (Jasco make) in the freq uency range 400-4000 cm· 1 by KBr pellet method. The glass transition temperature (T8 ), the crystallisation temperature (Tc) and melting temperature (T111 ) were measured using Seiko TG/DT A 32 balance, at a heating rate of 20°C/min.
analysis) that take place in zinc phosphate network due to varying concentrations of wo3.
Experimental Procedure Five 40 ZnO-xW0 3 -(60-x)P 2 0 5 glasses with x=O, 3, 5, 10 and 15 were sy nthesized. These compositions are referred to as glasses A, B, C, D and E. Glass with 20 mol% W0 3 did not form . Appropriate amounts (all mol %) of Analar grade reagents of ZnO, W0 3 and P2 0 5 were thoroughly mixed in an agate mortar and melted in a platinum crucible (tungsten free glass, glass A, at 840°C and the rest of the glasses between 950 and l050°C in a PID temperature controlled furnace for about lh h until a bubble free liquid was formed. The resultant melt was then cast in a brass mould and subsequently annealed at about 300°C. Transparent light blue glasses were obtained. It may be noticed here that when the concentration of wo3 is increased beyond 15 % the glasses became opaque with thick blue colour and crystalline nature was observed. The amorphous nature of the glasses was confirmed by X-ray diffractograms recorded on Seifert Diffractometer Model SO DEBYE FLUX-2002 instrument. The samples were then ground and finely polished. The final dimensions of the samples used for the present measurements were about I em x 1em x 0.2 em. The density (d) of the glasses was determined to an accuracy of 0.00 1 following Archimedes' principle usi ng xylene (99.99% pure) as the medium of flotation . A thin coating of silver paint was applied (to the larger area faces) on either side of the glasses to serve as electrodes for dielectric measurements. The dielectric measurements were made on LCR Meter (Hewlett-Packard Model-4263B) in the frequency range 102 -105 Hz. Dielectric breakdown strength for all the glasses was determined at room
Results From the measured values of the density (d) and calculated average molecular weight ( M ), various physical parameters such as W 6+ ion concentration N;. mean W 6+ ion separation distance (R;) polaron radius (Rp) and the fie ld strength (F1), which are useful for understanding dielectric properties of these glasses are evaluated using standard formu lae 27 and presented in Table 1. This table also contains the values of glass transition temperature evaluated from DT A traces. Fig. 1a presents the differential thermal analysis traces of the glasses under investigation. It is observed that the glass transition temperature (T8 ) increases as the concentration of W0 3 increases (Table 1). Further, an exothermic peak (Tc) due to the crystallisation followed by another endothermic peak due to the remelting of the glass (T are also observed for these glasses. The variations in the parameters T/[ (Tc-T8 )!T8 , (Tc- T8 )!Tm and Kg1= (Tc- T8 )/(T Tc), (which gives the information of the glass forming ability) wi th W0 3 concentration are presented in Fig.! b; these val ues are found to increase with increase in the concentration of wo). 111 )
111 ,
111 -
Table !-Various physical properties of 40 ZnO-xWOr (60-x) P 20 5 glasses Property Density (d) (glcm 3 ) Average molecular weight ( M ) W6 + ion concentration N; (I 0 20 ions/cm 3 ) Inter- ionic di stance of W6+ ions (R 1) (nm) Polaron radius (R") (nm) Field strength (F;) ( 10 15 cm- 2 ) Breakdown Strength (kV/cm) Glass Transition temperature (Tg) ( 0 C)
Glass A X =0
Glass B X =3
Glass C X =5
Glass D X =10
Glass E X =1 5
2.749 117.71
2.918 120.4 1
3.011 122.21
3.1 62 126.71
3.311 131.20
4.38
7.42
15.03
22.38
1.32
1.10
0.87
0.76
0.53 2.13 22. 1 475
0.45 3.03 22.3 503
0.35 4.85 22.7 523
0.31 6.40 23. 1 547
16.0 449
SUBBALAKSHM I & VEERAIAH : DIELECTRIC PROPERTIES OF ZINC PHOSPHATE GLASSES
50
277
,.--------------------------------~. 7
A •.
i
40
c
0 w
D
X
30
0
•···· ·
·w
0
z
w
20
~ 10
30
630
430
230
1030
830
1230 100
10000
1000
Temperature ,("C)
100000
Frequency, Hz
Fig. I a- DT A traces of Zn0-P 20 5 glasses containing W0 3
Fig. 2-Variation of dielectric constant (solid lines) £and loss tan
8 (dotted lines) with frequency at room temperature for ZnOWOr P20 5 glasses. 50 ,.----- -- -- - - -- - - - - - - - - - - - - - - - ,
"~
1K
~1 . 1
40
0.0
0.8
0
30
10 15 5 Concentration of W 0 3(mol%)
T,ITm
0
I
(T,-T0)/T0
~ 0.4 0:::
20
10
~Tg)/Tm
OL--_.__
~
0
50
_..__ 100
_
~--'----'---'---'
150
200
Temperature ,
0.0 0
5
10
15
Concentration of W0 3 (mol%)
300
350
°C
Fig . 3a- Variation of dielectric constant with temperature for Zn0-P20 5 glass containing 3% W0 3 for different frequencies (glass B).
Fig. lb-- Variation of KR, ,T/[,' (Tc·T.)I r. and (Tc·T.)I T, with WO, concentration in the Zn0-P 20 5 glass system.
The variation of dielectric constant £' and loss tan 8 at room temperature (30°C) of Zn0-P20 5 glasses with concentration of W0 3 is represented in Fig. 2. With the introduction of W0 3 , the values of dielectric constant and loss are found to decrease gradually with increase in the concentration of W0 3 ; further for any particular concentration of W0 3, the values of £' and tan 8 are fo und to decrease considerably with increase in frequency (Fi g. 2). The temperature dependence of £' at different frequencies for glass B (containing 3% W0 3 ) is shown in Fig. 3a and for different concentrations of WO., at I kHz is in Fig. 3b; £' is found to exhibit
250
50
r-- - - - - - - - - - -- - . -A - ,
40
c 30
.
-
D
20
10
0
L___
0
_.__~--~-_.__
50
100
150 200 Temperature,"c
_..__ _~-~
250
300
350
Fig. 3b-- Compari son plot of variation of dielectric constant wi th the temperature ofZnO-WOr P20 5glasses at I kHz.
INDIAN J. ENG. MATER. SCI., OCTOBER 200 1
278
considerable increase at higher temperatures. This increase is more pronounced at lower frequencies; in addition it is also observed that the value of E' (at fixed temperature and frequency) decreases with increase in the concentration of W0 3 . The variation of dielectric loss tan 8 with temperature at different freq uencies of glass B (containing 3% of W0 3) is shown in Fig. 4a; these curves have distinct maxima, with increase in the frequency, the temperature maximum of tan 8 shifts towards higher temperatures and with increase in the temperature, the frequency maximum shifts towards higher frequencies, indicating the relaxation character of dielectric losses in these glasses. The effect of W0 3 concentration on the relaxation strength of these glasses can be clearly understood from Fig. 4b where tan 8 at 10 kHz is plotted against temperature for different concentrations of W0 3; the glass A, which is free from W0 3 , has exhibited a relaxation peak with the highest value of (tan 8)max · With successive increase in the concentration of W0 3, the temperature region of relaxation shifted towards higher temperatures with decreasing values of
(tan 8)max; further, it seems, there has been an absence of such relaxation effects for the glasses containing high concentration of wo3 (> 5% ). Using the relationship:
!= fo
exp (-Wd IKn,
. .. (1)
where .f-=frequency, f 0 =constan,t. the effective activation energy Wd for the dipoles is calculated for these glasses and is presented in Table 2 along with the other pertinent data. The activation energy wd is found to increase with increase in wo3 content. The a.c. conductivity cra.
