High conductivity and mechanical loss due to mobile fluoride ions in PbF2-MnF2-Al(PO3)3 glasses

Mat. Res. Bull., Vol. 21, pp. 1115-1121, 1986. Printed in the USA. 0025-5408/86 $3,00 + .00 C o p y r i g h t (c) 1986 Pergamon J o u r n a l s Ltd.

HIGH CONDUCTIVITY AND MECHANICAL LOSS DUE TO MOBILE FLUORIDE IONS IN PbF2-MnFz-AI(PO3)3 GLASSES

A. R. Kulkarni and C. A. Angell Department of Chemistry Purdue University West Lafayette, Indiana 47907 (Received June 16, 1986; Communicated by R. A. Huggins) ABSTRACT New fluoride glasses in the system PbF2-MnF2-AI(PO3)a have been synthesised and foundo to have high d.c. conductivities, presumably due t 6 m o b i l e F ions. A.C. electrical data (25-200 C) have been analyzed by impedance spectroscopy. A glass with the composition 85PbF2-5MnF 210AI(PO3)3 yields a record value for conductivity in an anionic conducting glass, c200 = 1.1 × 10~(f~-cm) -1, nearly two orders of magnitude higher than the best values so far reported. Electrical and mechanical relaxation data have been compared in the isofrequency-inverse temperature representation. As seen previously for cation conductors, the mechanical relaxation is less exponential (broader) than the electrical relaxation. However, in the present case the most probable mechanical relaxation time is shorter than the most probable electrical relaxation time. MATERIALS INDEX - Fluoride glasses

INTRODUCTION There has been a great deal of interest in fluorophosphate glasses for their practical applications in multispectral optical components, IR fibre optics and domes, laser windows and laser host materials (1-3). The majority of such systems have used, as a key component, aluminum metaphosphate. Recently, Tick (4) has shown that interesting water-durable ultra low-melting fluorophosphate glasses can be made using Sn(PO3)2 and SnF2 as major components. These glasses also have excellent optical properties. Phosphate-free heavy metal fuoride glasses under study for IR optical applications have also proved to have good electrical conductivities, the conductivity being due to fluoride ions. For instance, the fluorozirconate glasses prepared by Poulain et al (5) have conductivities in the range of 1-5 × 104 (f2-cm) -I at 200°C. Leroy et al (6) reported dc values of conductivity ff200Oc= 4 × 104 (f2-cm) -1 in the ZrF4-BaF2-ThF 4 and ZrF4-BaF2-LF3 where L = La, Pr or Nd. It has been shown by Tubandt's method and the sodium amalgam method that the mean ionic transport number in these glasses is around 0.999-I--0.004. Recently it was found that even higher conductivities could be obtained in Pb(PO3)2-PbX2 glasses where X = C1- or Br" (7). Pb(PO3)2-PbF2 glasses, however, could only be formed with low mole fractions of PbF2 and their conductivity was poor. Since the highest anionic conductivity previously found had been in PbF2-PbO-SiO 2 glasses (Sa,b) and since crystalline PbF2 itself is an excellent F- conductor (9), it seemed probable to us that great improvements in vitreous F- conductivity would be made by developing glass-forming systems with larger mole fraction of PbF 2. The highest mole fraction of PbF 2 so far reported in a glass-forming system is 50%, in the system PbP2-MnF2-FeF 3 (10) developed because of interest in the magnetic dS-ds spin 1115

1116

A.R.

KULKARNI, et al.

Vol. 21, No. 9

interactions it manifests. The conductivity was not investigated. In the present work we show it is possible to obtain glasses with 80 mol% PbF 2 by turning again to Al(PO3) 3 to promote glass formation. As expected, these glasses, and ternary glasses in the system PbF2-MnF2-Al(PO3) 3 containing even less Al(PO3) 3, exhibit record high conductivities. By mixing PbF_~with 80LiF-20Al(PO3) 3 glass which is known to be also an excellent ionic conductor due to Li , (11) the new glasses offer the interesting possibility of continuous transition from fast anion to fast cation conductivity. The results of such a study will be presented separately. EXPERIMENTAL Starting materials PbF 2 (Pfaltz and Bauer 99%), M n F 2 (Cerac Inc. 99.5%) and AI(PO3) 3 (ICN, pharmaceuticals, Inc. 99%) mixed in appropriate proportions (each batch weighing around 10 gm) were melted in a covered platinum crucible at a temperature of 600°C for 5 minutes. The melts were cast between two brass plates which were preheated to 160°C to avoid thermal stress-induced shattering of the samples, and the 0.5-2 mm thick discs obtained were annealed for a few hours before slowly cooling to room temperature. Since there is a possibility of PF 5 volatilization from these melts, weight checks were made on each preparation. These never showed more than 2 wt.% weight loss over the batch weight. Glass transition temperatures were measured on 20mg samples in sealed aluminum pans using a Perkin-Elmer DSC-2 differential scanning calorimeter at a heating rate of 20°C/min. For conductivity measurements, the glass discs were gold-coated in a three electrode configuration to provide a guard ring. The sample was mounted in a sealed metal vessel between ~pring loaded stainless steel electrodes. The a.c. electrical conductivity was determined using a computer controlled digital impedance measuring facility (12). A Genrad Digibridge 1689 scanned the frequency range 0.012 - 100KHz averaging 20 measurements at each frequency, while the computer stored the data on disc and then incremented the temperature 10 C. After stabilization at the new tem P~orature was confirmed, the procedure was repeated to cover the temperature range ambient-210 C. The ac data were then treated using a non-linear least squares fitting routine which gives semicircular plots of real and imaginary parts of the complex impedance, the co-ordinates of the center, and the intercept on the real axis. The conductivities were obtained from these real axis intercepts, and the predetermined cell constant. Mechanical relaxation studies were carried out on glass fibers pulled from quenched glass samples reheated to the softening temperature. 5cm-long fibers were subjected to sinusoidal mechanical stresses at 110Hz, using the Toyo Instruments Co. Rheovibron with high temperature sample fixture. The samples were studied in tensile configuration. To avoid extra losses at the grips, the fibers were fused at the ends to make a bead of nearly = l m m diameter on which the grips fastened. The temperature was measured with an accuracy of + l ° C by a Cu-constantan thermocouple positioned very close to the fiber. The theory used in analysis of our results has been described elsewhere in detail (11). R E S U L T S AND DISCUSSION The glass-forming region in the ternary PbF2-MnF2-AI(PO3)3 systems obtained under our experimental conditions, are shown in Fig. 1, with open circles. Note the small binary PbF 2AI(PO3)3 glass forming range. It is interesting to note that the glass formation is not found in MnF2-rich binary MnF/-AI(PO3) 3 melts under our experimental conditions though glass

Vol. 21, No. 9

PbF2-MnF2-AI(PO3) 3 GLASSES

~/Yv_~/~GLASSYcRY ESTA N LI

1117

MnF 2

28ol

I

o

260 0 AI(P03) 3

240 o

p-

220 PbFz

AI(P03) ~ 200

Figure 1. Glass-forming composition range in ternary system PbF2-MrlF 2 AI(PO3)3.

180

I0

20

r~ol

30

410

50

°/o M n F 2

Figure 2. Dependence of T on MnFz content at constant AI(PO3)3 and constan~ PbFz.

4

I

2OO

150

I

l

[ I I

II • []

80PbF2-O5MnF2-15Al(P03)3 / 80PbF2-1OMnF2-1OAI(P03)3 I 80PbFz- 12 MnFZ-OgAI (P03)3 /

i

~ 0

70PbF2-20 MnF2-10 A I [P03)3 | 80PI3 F2--20AI (P03) 3 /

I

I ~ ~--~'~*~ \~b,~.~

E

°,

',

o

ioo i

~

~o ~

25

~C

'

,

'

.E 0

(..9

~"~1 ~. I""

+

i

50"C

t,

"~.

_

:

Pbrz

I

-8

oo - I0

0

\ I

0.5

I

I

I

;

"N

IO00/T

(K -I)

1.5

2

2,5

~

3

\",k

3.5

~.~'N

4

Figure 3. Arrhenius plot of d.c. conductivity of fluoride ion conducting glasses specified in legend. Insert shows typical complex impedance plots from which o was obtained.

formation in the NaPO3-MnF 2 system has been observed over wide composition ranges (14). There must be an additional small region near Al(PO3)3, which is itself a glass former (15). Fig. 1 shows that PbF 2 binary glass formation occurs in the range 75 - 85% PbF 2. In the ternary system as much as 30 mol% PbF z can be replaced by MnF 2 at a fixed concentration of AI(PO3) 3. Similarly, when holding the PbF 2 concentration constant at 80% we could incorporate 12%

A.R.

1118

KULKARNI, et al.

Vol. 21, No. 9

MnF 2 in place of Al(PO3)y Fig. 2 shows the variation of the glass transition temperature as MnF 2 is added to the glass at fixed PbF 2 content (80%) (decrease of Tg) and as MnF 2 is added at fixed (10%) Al(PO3) 3 content (increase of Ts). A selection of such data is collected in Table 1. T A B L E 1.

Composition in mol% PbF 2

MnF 2

0200oc

E a

AI(PO3) 3

(f~-cm)"1

Rx*

T

g

(kcal/mole)

at Tg

(°C)

1.

80

00

20

1.24x10 -8

23.00

3.07x10 7

281

2.

80

05

15

5.01x10 -7

17.86

8.68x107

212

3.

80

i0

10

4.16x10 -5

14.86

4.51x109

217

4.

80

12

08

7.94x10 -5

14.54

8.43x109

205

5.

85

5

10

1.12x10 -4

14.44

1.09x1010

203

6.

70

20

10

1.38x10 -5

15.00

6.30x10 -6

237

7.

60

30

10

6.30x10 -6

16.00

2.10x109

259

R <'Cs> foe** ] [ x= <'-~-~o>; < ' ~ o > = - ~ Tz ,

1

DC electrical conductivities, derived from real axis intercepts of the semi-circles in complex impedance plots, have been plotted against T 1 in Fig. 3. The inset to the figure shows complex impedance plots for one of the glasses at different temperatures. Included in Fig. 3 for comparison are data for 13-PbF2 and PbSnF4, the best known fluoride ion conducting crystalline materials, and also the highest vitreous state F- conductor known previous to this study (16). The electrical conductivity increases with temperature for all the compositions in accordance with Arrhenius law. The values of activation energies Ea, and cr200oc are included in Table 1. A glass with nominal composition 85PbF2-05MnF2-10Al(PO3) 3 exhibits a conductivity of --1.4 × 10 4 (f~-cm)q at 200°C (activation energy of 14 kcal/mole), about 2 orders of magnitude above the previous highest (G200Oc=7x10 -6 (f2cm)q for fluorozirconate glasses (16). The glass conductivity is, however, well below that of the crystalline fluoride ion conductors ~-PbF2 and PbSnF 4. The activation energy is also considerably higher than in the latter cases. The effect of MnF 2 content on conductivity can be seen from Table 1. At constant PbF2 (80%) the conductivity increases with increase of MnF z or with decrease in concentration of AI(PO3)y At fixed AI(PO3) 3 content the conductivity decreases with increase of MnF 2. The conductivity thus varies inversely as Tg though only because of the way each property is influenced by the PbF 2 content. The association of high conductivity with low T is a frequent, though by no means general, phenomenon• Comparison of glasses 4 and 5 in Table 1 makes clear that the fluoride introduced with (presumably coordinated to) Pb 2+ in the glass is the mobile element At ,T g measured by DSC at 20°/min, the structure relaxation is =i00 sec_ ° The conduc• . • . . . . . tlVity relaxation tune xo (see below) which reflects the residual mobility of the F subset of ions

Vol. 21, No. 9

PbF2-MnF2-AI(PO3)3 GLASSES

1119

much smaller. A comparison of the two times indicates the extent to which the subset motions have been decoupled from the viscous modes during cooling to Tg. The decoupling index (17) defined as Rx = /, has been determined for each of these glasses at T and has been included in Table 1. The values are typical of fast ion conductors. R~ shows systematic changes as the conductivity and Tg changes. The activation energy for conductivity lies in the range 2314 kcal/mole and decreases as the conductivity increases. We now turn to the matter of how the glass responds to different types of stress. Specifically we compare the electrical response with the response to oscillating mechanical stresses. In interpreting the effects of electrical and mechanical stress in the present low temperature regime in which fluoride ion motions are being probed selectively, an important difference in system responses must be noted, viz., the system polarizes in response to the mechanical stress. This means that after a certain deformation a terminal state is reached and flow of ions ceases. In other words, there is no mechanical equivalent of the d.c. conductivity except in the viscoelastic regime at much higher temperatures (near Tg). Nevertheless, fluoride ions do move in response to each type of stress and it is desirable to compare the two responses. It is theoretically preferable to compare the a.c. electrical response with the a.c. mechanical responses isothermally as a function of frequency. Unfortunately the frequency range available to the present mechanical relaxation technique is too limited to yield data suitable for an isothermal comparison. However, we may take advantage of the finding that the conductivity relaxation spectrum is independent of temperature (18) to cast data obtained at constant frequency in a form identical to the isothermal spectrum. This is done by combining Z' and Z" data of Fig. 3 insert to obtain real and imaginary parts of the electrical modulus M*, (19) and plotting M" versus 1/T together with the imaginary part of the complex tensile modulus E* obtained from the mechanical response experiment. The results of these data manipulations, which are described in more detail elsewhere, (11) are presented in Figure 4 in the following sequence. Part (a) shows an isothermal spectrum, M" versus log f, with the frequency scale chosen so that the spectrum has the same halfwidth as the solid curve in Figure 4(b). Part Co) shows M", obtained in the constant (110 Hz) frequency scan, plotted versus 1/T as discussed above. The two plots may be superposed within experimental error, deomonstrating the temperature independence of the spectral width. The dotted curve is the normalized response defined as N"elec = M"/NI**. P a r t (c) shows the imaginary part of the tensile modulus E" and also the mechanical loss spectrum normalized by the total dispersion E'300K - E'150K to give a dimensionless quantity N"m~ h suitable for comparison with the normalised electrical modulus. The important observations from Figure 4 are that (i) the peak frequency for the mechanical response is a little higher than for the electrical response (ii) the whole spectrum for the mechanical response is much broader i.e. a much broader range of modes can be excited by the mechanical stress and (iii) the shape of the mechanical "spectrum" is different from that of the electrical spectrum. Based on the half width alone, an attempt to describe the response with a transform of the commonly invoked KWW relaxation function ~(t) = e -(u~)~

(2)

would require 13= 0.26. But in this case the spectrum would be very asymmetric to the high frequency side. Fig. 4(c), on the other hand, is more symmetric than the electrical spectrum. In fact it can be described quite well with a log Gaussian distribution of relaxation times, as found earlier in an ullyasonic study of a fast Ag + conducting glass (20).

1120

A.R.

KULKARNI, et a l .

Vol. 21, No. 9

We conjecture that the mechanical response represents the complete spectrum of mobile Frelaxation modes and that the non-polarizing electrical response is less symmetrical because the nature of the electrical response permits the by-pass or short-circuiting of many of the modes. The bypassed modes would be predominantly the slow ones, which are activated as the system approaches its equilibrium deformation (fully polarized state) under mechanical stress. We note that the non-polarizing viscoelastic response seen under mechanical stress at much higher temperatures (near Tg), like the picosecond non-polarizing electrical response, conforms to the K W W description. Clearly there are also more high frequency relaxation modes activated under mechanical stress, though we offer no interpretation of their nature.

0.012

elec °

0 0.008 0.004

-5

-2

-1! 0 +1 Lbg ( f / f max') I

0.00~if elec

/i

~,,,

I I I

+2

+3

I

-10.3

t I I

O. b, 0.,{ ~J

,~ 0.012

-mech

[

c) ~0.3

®o × 0.008

t.

• jo.,g

0.004 1.6

2.4

5.2 lO00/Y

4.0

=

I |

4.8

(K) -I

Figure 4(a) Imaginary part of the electrical modulus M", versus reduced frequency for 0 tr • ~ , • pp • 75PbF2"10MnF2"15Al(PO3) 3 glass at 100 C. (b) M /M o = tanS, (sohd hne, LH scale) and M /M (~ = N" (dashed line, RH scale) as a function of I/T at 110 Hz. Scales are chosen so that spectral halfwidth remains the same in each plot, to facilitate comparisons with tensile modulus plots (part c). (c) Dissipative part of the complex tensile glass, E" (LH scale) and normalized tensile modulus N"mech ~, E /AE, versus I/T with same 1/T scale as for part b. Note the greatly increased half width and lower N relative to the conductivity relaxation, and the displacement of the peak of the mechanical spectrum to lower T (or higher frequency).

ACKNOWLEDGEMENTS Financial support to one of US (ARK), by Government o f India, Ministry of Education and Culture is gratefully acknowledged. The work was otherwise supported by the Department of Energy under Grant No. $9028884ER45102

Vol. 21, No. 9

PbF2-MnF2-AI(PO3) 3 GLASSES

1121

1. 2. 3.

REFERENCES M. Poulain, J. Lucas and P. Brun, Mat. Res. Bull, 10, 243 (1975). M. Poulain, J. Lucas, Verres Refract, 32, 505 (1978). M . G . Drexhage, C. T. Moynihan, M. Saleh-Boules and K. P. Quilan, Adv. in Ceramics, Vol. II, Physics of Fibre Optics, Be. Bendow and J. Mitra, eds. (Am. Ceram. Soc., Columbus, OH 1981) p. 57.

4. 5. 6. 7.

P.A. Tick, Phys. Chem. Glasses 25, 149 (1984). M. Poulain, M. Chantanasinh and J. Lucas, Mat. Res. Bull, 12 151 (1977). D. Leroy, J. Lucas, M. Poulain and D. Ravaine, Mat. Res. Bull. 13, 13 1125 (1978). H . G . K . Sundar, S. W. Martin and C. A. Angell, J. Solid State Ienics (in press).

8

a.P.C. Shultz and M. S. Mizzoni, J. Amer. Ceram. Soc. 56, 65 (1972). b. J.E. Shelby, J. Am. Ceram. Soc. 68, 551 (1985). 9. J . M . Reau and J. Portier, in Solid Electrolytes, eds P. Hagenmuller and W. Van Gool (Academic Press, New York, 1978) p. 313. 10. J.P. Miranday, C .Jacoboni and R. DePape, J. Non-Cryst Sol. 43, 393 (1981). 11. A.R. Kulkami and C. A. Angell (to be published). 12. S.W. Martin and C .A .Angell, J. Non-Cryst. Sol. (submitted). 13. Changle Liu and C. A. Angell, J. Non-Cryst. Sol. (in press). 14. M. Matecki, M. Poulain, r. Lucas, D. R. McFarlane and C. A. Angell Mat Res Bull 18, 293 (1983). 15. N.J. Kreidl in Glass Science & Technology vol. 1, eds. D. R. Uhlmann and N. J. Kreidl, (Academic Press 1983) p. 192 16. D. Ravaine and D. Leroy, J. Non-Cryst Sol. 38, 353 (1980). 17. C.A. Angell, Sol. State Ionics. 9&10, 3 (1983). 18

A. Kulkarni, H. Senapati, Changle Liu, and C. A. Angell, Proc 14th Intern. Congr. Glass, New Dehli 1986 (in press).

19. P.B. Macedo, C. T. Moynihan, and R. Bose Phys. Chem. Glasses, 13, 171 (1972). 20. G. Carini, M. Cutrc;.i, M. Federico, G. Galli and G. Tripodo, J. Non-Cryst. Sol. 56, 393 (1983).