Glass formation and conductivity studies in silver boromolybdate systems

Materials Science and Engineering, B 7 ( 1991 ) 267- 273

267

Glass Formation and Conductivity Studies in Silver Boromolybdate Systems R. V. G. K. SARMA and S. RADHAKRISHNA Department of Physics, Indian Institute of Technology, Madras 600 036 (India) (Received January 8, 1990, in revised form August 1, 1990)

Abstract

2. Experimental details

The glass formation region was determined m the quaternary system AgI-Ag20-B203-MoO 3. The total electrical conductivity and its dependence on composition was studied. The total conductivity increased with an increase in Agl content. The variation of conductivity on mixing two glassforming oxides B203 and M o O 3 was investigated for the boromolybdate glasses. The electronic conductivity was measured by Wagner's polarization method and the transport number of silver ions was found to be 0.999 and larger. The glass transition temperature Tg and its dependence on composition were discussed.

AnalaR grade AgI, A g 2 0 , B203 and M o O 3 were used as the starting materials for sample preparation. Three-gram batches of a mixture of the required amounts of AgI, Ag20, B203 and MoO3 were melted in quartz crucibles at 750 °C over a period of two hours. The melt was stirred constantly to ensure the homogeneity of the compounds. The melt was then quenched on a plate kept at liquid-nitrogen temperature. X-ray diffractograms were recorded on the pulverized samples using a Philips PW-1140 X-ray generator with Cu K a radiation. The glass transition temperature of the samples was determined using the differential thermal analysis (DTA) technique. These measurements were carried out using a Stanton Redcroft 60 A series instrument. Pulverized samples were pressed to form cylindrical pellets of diameter 1.3 cm of the configuration silver/sample/silver, together with the electrode mixture of silver powder and sample powder in a 1:1 ratio by weight using a PerkinElmer handpress. The mixture of sample and silver powder was used to reduce the interracial resistance. Ionic conductivity measurements were performed on these pellets using a General Radio bridge model 1650 B and a null detector. Electronic conductivity measurements were carried out on a Wagner's polarization cell of the type ( - )Ag/sample/graphite( + ) [6]. Various d.c. potentials, smaller than the decomposition potential of the electrolyte, were applied and the corresponding currents after steady state was achieved were measured using a Keithley 610 C electrometer. Thermoelectric Power measurements were carried out on the cell configuration Ag(T)/

1. Introduction

The search for the high-conductivity solid electrolytes has progressed at a tremendous rate after the miniaturization of electronics. These electrolytes can be studied in ~ingle crystal, polycrystalline, glassy and thin-film forms. Normally glasses have more advantages over polycrystalline materials, for example, ease of preparation, a wide range of selection of composition and thereby a wide range of property control. Apart from these advantages, glasses have higher conductivities than the corresponding polycrystalline samples. Generally glasses have three components: (1) glass former, (2)glass modifier and (3) a doping salt. Though a vast literature is available on ternary superionic glasses, few reports are available in the direction of mixing two glass formers [1-5]. In the present paper we discuss the effect of mixing two glass formers B 2 0 3 and MoO 3 on the conductivity characteristics, glass formation and glass transition temperature in the system A g I - A g E O - B E O a - M o O 3. 0921-5107/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

268

electrolyte/Ag( T + A T ) with A T equal to 10 K. The ambient temperature was raised by an external heater and the temperature gradient was maintained using a small subheater. The temperature of the sample was taken as the average of the two readings. Silver wires were connected from silver discs in order to reduce the inhomogeneous contribution to the thermoelectromotive force. The following parameters have been defined for the system: (1) the ratio of boron oxide to total glassforming oxide content

I

F]

M~3

Y= B 2 0 3 / B 2 0 3 + M o O 3 (2) the ratio of silver oxide to total glassforming oxide content

g

N = Ag20/B203 + MoO 3

3. Results and discussion

The amorphous nature of the compounds has been confirmed by the X-ray diffraction technique. Figure 1 shows the diffraction patterns of AgI, Ag20, B203, M o O 3 and a typical composition in the silver boromolybdate system. The peak-flee diffraction pattern confirms the amorphous nature of the compound.

3.1. Glass-forming regions

60

50

40

30 2edeg

20

10

Fig. 1. X-ray diffraction patterns of AgI, Ag20, B203, M003 and a typical composition in the silver boromolybdate system.

Figures 2 and 3 show the glass formation in the system A g I - A g 2 0 - B 2 0 3 - M o O 3. Figure 2 represents the glass formation in the silver boromolybdate system at a constant mol.% of AgI. Figure 3 represents the glass formation at a constant value of Y (0.1). From the figures the limits for the glass-forming region in the system can be given as follows:

/L ~2o

6O

N =0.75

O~
3.2. Glass transition temperature Network glass formers are (B203, SiO2, P205, WO3) oxides of trivalent, tetravalent, pentavalent

B203

20

40

60

80(

MoO3 Y:01

Fig. 2. Glass formation in the boromolybdate system for constant AgI (66.67) mol.%: • glass; x polycrystalline.

and hexavalent elements where the M - O bond is covalent enough to create a local structure (unit) and ionic enough to allow the deformation of bond angles, thus destroying long-range order. Addition of a glass modifier involves incorporation of negative charge (oxygen) into the covalent

269 Acjl

-I

'E 7 b -2

Ag20

40

20

60

80

0.IB203- 0.9 MoO3

Fig. 3. Glass formation in the boromolybdate system for constant Y(0.1B203-0.9MoO3): • glass; x polycrystalline. 230

-3

190

MoO3

I 0.4

I 0.6

I

08

Fig. 5. Variation of room temperature conductivity with the ratio of boron oxide to total glass-forming oxide content (moi.% AgI = 66.67, N = 0.75).

150

I

I

30

50

I 70

% Agl

140

13o I

I

05

I

1.0

N

I

1.5

2.0

140

130

I 02

•.

•

•

.

•

•

•

I

I

I

I

I

0.2

0.4

0.5

0.8

1.0

Y Fig. 4. C o m p o s i t i o n dependence o f glass transition temperature (Tg) in the boromolybdate system.

network. For example, in B203-based glass addition of oxygen helps conversion of BO 3 units into bridging BO4 l- first and then into BO3"- units, with non-bridging oxygens (NBO). In the case of MoO 3 glass, in addition to MoO42- and I-, other species such as MoO3I- and/or M02072- ions are present [7]. In boroaluminate glasses, spectroscopic evidence shows that the modifier oxygens are attached to AIO4 units [8, 9]. Recently, Raman studies on molybdophosphate glasses revealed the presence of MoO42- and PO43-

units in the matrix [10]. It is reported that the mixing of trivalent and pentavalent glass formers enhance the cationic diffusion, while tetravalent formers mixed with trivalent/pentavalent formers results in a phase separation [11]. In the case of silver borophosphate the glass transition temperature does not follow any rule. Tg decreased, increased or remained unchanged depending upon the composition of the mixed network [12]. The variation of glass transition temperature with the composition was given in Fig. 4. It is clear that Tg remains constant for constant AgI tool.% (with a slight deviation), and that it decreases as AgI content increases. Tg in the system decreased from 210°C to 130°C when the AgI content was increased from 30% to 66.67%. In the case of a single glass former, there are a few units present corresponding to different oxidation states (BO4-, BO3). When more than one glass former is present, the structure of the glass is complex. Thus, in the presence of two glass formers, the connectivity between the local structural units is higher, showing a higher value of Tg. These Tg values are compared with those values from conductivity measurements, and found to be in good agreement. 3.3. Ionic conductivity studies Figure 5 shows the variation of room temperature conductivity (303K) with the ratio

270

between two oxide formers B203 and MOO3, for the system 66.67AgI-14.29Ag20-19.04[xB203(1-x)MoO3]. Conductivity goes through a maxima when Y=0.1 and Y=0.8. It is reported in silver borophosphate glasses [12], silver molybdotungstate glasses [4] and silver phosphovanadate glasses [5] that the conductivity goes through two maxima. According to weak-electrolyte theory if one anion is progressively substituted by another, then the conductivity goes through a maximum even if the salts have the same ion dissociation constant [13]. The conductivity maximum depends on the bonding between Ag + and the boromolybdate group. It was assumed that this may consist of BO4 and MoO4 units sharing the negative charge supplied by the oxygen of the modifier. The enhancement in conductivity in the quaternary glasses when compared with the ternary glasses is because of the modification of glass structure, which causes an enhancement in the ionic mobility. The room temperature conductivity as a function of N ( A g 2 0 / B 2 0 3 + MOO3) is shown in Fig. 6. With the increase of the glass modifier content

the conductivity increases and attains a maximum at N = 0.75. Afterwards the conductivity decreases and attains a minimum at N = 1.00. This clearly shows that the glass modifier affects the conductivity. The conductivities of the glasses at room temperature are shown in Fig. 7 as a function of Agl content. The conductivity increases linearly on a logarithmic scale with the AgI content. The conductivity increased up to 10 -2 for 66.67% Agl and then decreased. This might be the result of the crystallization of AgI in the glassy matrix. In addition, from X-ray studies, a small degree of polycrystalline nature has been observed in the compounds for AgI contents of more than 66.67 mol.%. The fact that the limiting value of the ionic conductivity 10 -2 S cm-~ is often interpreted [14-16] as evidence that the ionic conductivity of doped glasses is essentially the result of the presence of the "a-AgI" phase within the glassy network. Figure 8 shows the temperature dependence of the conductivity of the glasses in the system

5-

-1 h-

iE -2

th 3~ ~o

5

2

S

0.25

0.50

I

I

I

0.75

1.00

1.25

I 1.50

N

Fig. 6. Variation of room temperature conductivity with the ratio of glass modifier (Ag20) to total glass-forming oxide content (mol.% of AgI = 66.67, Y= 0.1 ).

20

1

I

t

30

40

50

I

I

I

60 % A01

70

80

Fig. 7. Room temperature conductivityvariation with mol.% of AgI for the glasses AgI-Ag20-B203-MoO3 (N=0.75, Y=O.1).

271 TABLE 1 Room temperature conductivities and activation energies of the various compositions studied in the silver boromolybdate system (with variations in Y, N and tool.% of Agl) Parameter

Conductivity

E ~ (eV)

(S cm- i)

l.O

o

0.5

1.0

I 3.0

I 31

I 3.2

" ~ 1

3.3

IO~T ( R -1 )

Fig. 8. Plots of log o T against 1000/T for various mol.% of Agl (o, 30; o, 40; n, 50; m, 60; zx, 66.67; x, 75; • (80).

below the glass transition temperature for various mol.% of AgI at constant N and Y. All the glasses followed the Arrhenius behaviour given by the equation

Y=0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

8.9×10 3 4.5 x 10 -2 1.5 x 10 2 9.6 x 10 3 8.6 × 10 -3 4.1 x 10 -3 5.4 x 10 -3 8.7×10 3 9.6 × 10 -3 3.8 x 10 -3 5.5 × 10 -3

0.25 0.24 0.26 0.25 0.25 0.28 0.26 0.26 0.25 0.27 0.24

N = 0.25 0.50 0.75 1.00 1.25 1.50

3.1 1.0 4.5 8.5 9.1 1.0

× × x x x ×

10 -3 10 -2 10 2 10 -3 10 -3 10 -2

0.34 0.30 0.24 0.24 0.25 0.27

tool.% A gl = 20 30 40 50 60 66.67 70 75 80

8.9 x 4.2x 2.0 × 8.8 X 3.0x 4.5 x 9.0 × 5.1 x 3.7 x

10 -5 10 4 10 -3 10 -3 10 2 10 2 10 .3 10 -3 10 -3

0.45 0.32 0.26 0.28 0.25 0.24 0,25 0.25 0.28

AgI-Ag20-B203-MoO3,

oT = (70 exp( - Ea/kT ) where the symbols have their usual meanings. The activation energy is calculated from the above plots. The activation energy increased for Y= 0.1 to Y= 0.5 and then decreased from there up to Y= 0.8. Similarly with an increasing ratio of glass modifier to total glass-forming oxide, the activation energy decreased up to N = 0.75 and then increased from there. The activation energy values varied typically from 0.24 eV to 0.45 eV when AgI is changed from 66.67 to 20 mol.%. Table 1 gives the room temperature conductivities and activation energies of the various compositions studied with changes in Y, N and mol.% of AgI. The increase in the content of Ag20 breaks the weak bonds and weakens the glassy matrix so facilitating the easy migration of conducting species. Hence there is an optimum limit on the ratio of glass modifier to total glass-forming

oxide. As the content of Ag20 is increased further the glassy network breaks and the randomness of the glassy matrix reduces the motion of the silver ions, hence the low conductivity and high activation energy.

3.4. Electronic conductivity studies Wagner's d.c. polarization cell of the type (-)Ag/glass/graphite(+) was used to study the electronic contribution to the total conductivity. In the above cell, the graphite electrode is maintained as a positive electrode, whereas the silver electrode is maintained at negative potential [6]. Graphite acts as a blocking electrode and is polarized by applying a d.c. potential in the range 10 to 150 mV. In the glass, silver ions will migrate towards the silver electrode and the electrons will move towards the graphite electrode. After some time the ionic conductivity is suppressed, because silver ions are not supplied by the graphite electrode. Then the conduction will be the result of migration of electrons or holes across the electrolyte. According to the Wagner's d.c. polarization analysis, the total electronic current (I) is

272

4_

,o

~

O

O 0.5

L0 rr rr

m

04

1]

I

I

20

40

J

1

I

60

80

100

VOLTAGE (mY)

Fig. 9. Voltage and current polarization curves for the best conducting sample.

given by I = I ~ + 1h = (RTA/LF)[oe{exp( - E F / R T )}

+ oh{exp(EF/Rr)-

1}]

where 1c and I h are the currents arising from electrons and holes, respectively, R is the gas constant, T the absolute temperature, F the Faraday constant, E the applied voltage, ac and oh the conductivities owing to the electrons and holes respectively, A the area of cross section and L the thickness of the electrolyte. If cr~,> o h the second term will be negligible and I becomes independent of the applied voltage. On the other hand, if oe'~ o h the first term will be negligible and 1 increases exponentially with the applied voltage. The electronic conductivity measurements showed that the observed current in the glasses arises from the electrons. Figure 9 shows the plot of current against voltage for the glass with the highest conductivity. From the saturation of current at higher voltages it can be inferred that the current is essentially the result of electrons. The plot of current against voltage and 1 - e x p ( - E F / R T ) is a straight line through the origin, the slope of which gives ( R T A / L F ) o e, from which the electronic conductivity was calculated as 10-9 S cm-t at room temperature.

I 27

1 2.9

!~zg-rcK-~)

I 3.1

l 3,3

Fig. 10. Plot of - S against 1 0 0 0 / T for the best conducting sample.

open-circuit voltage (OCV) of the cell found to be 685 mV at 303 K. From the thermodynamic calculations, a silver-based cell should give a value of 687 mV. From the above, the transport number was calculated to be 0.997, which agrees with that from the conductivity calculations. 3.6. Thermoelectric power

Thermoelectric power (TEP) measurements have been carried out on the best conducting sample, in order to investigate the transport properties further. For the thermocell configuration already discussed, the expression for TEP has been given by ref. 17: S = -qAg-*/eT+H

where qAg'* is the heat of transport for Ag ÷ ions, e the charge on the mobile species, and H the correction term from the electrode contact potential effects. For thermocells constructed with reversible electrodes, H is almost temperature independent and constant. Figure 10 shows the plot of TEP ( - S ) against 1000/T. The expression for the straight line is given by -S=0.235(lO00/T)-O.228mVK

~

with a slope 0.235 eV, which is almost equal to the activation energy for the migration of silver ions. For materials possessing average structure the heat of transport is generally equal to the activation energy for the ion migration.

3.5. Transport number measurement

The transport number of the silver ions calculated from the total conductivity and electronic conductivity, was found to be 0.999. An Ag/ glass/I2+carbon cell was constructed and the

4. Conclusions The mixing of two glass formers is effective in improving the ionic conductivity of silver boro-

273

molybdate glasses. The conductivity attained a maximum of 10 -2 S cm -1 in these glasses. Electronic conductivity studies showed that in these glasses electronic conductivity is almost negligible (10 -9 S cm-1). TEP studies confirmed that silver ions are a mobile species. The transp o ~ number of silver ions was calculated to be near to unity. References 1 A. Magistris, G. Chiodelli and M. Duclot, Solid State

lonics, 9/10(1983)611. 2 K. Rajiv and K. Hariharan, Solid State Commun., 63 (1987) 925. 3 R. V. G. K. Sarma and S. Radhakrishna, Solid State lonics, 28-30(1988) 808. 4 P. S. S. Prasad and S. Radhakrishna, Solid State lonics, 28-30(1988) 814. 5 P. S. S. Prasad and S. Radhakrishna, J. Solid State Chem., 76(1988)7.

6 A. V. Joshi and J. B. Wagner, J. Phys. Chem. Solids, 33 (1972) 205. 7 S. Hemlata, P. R. Sarode and K. J. Rao, J. Non-Cryst. Solids, 54(1983)313. 8 W. L. Konijnendijk and J. M. Stevels, J. Non-Cryst. Solids, 18(1975) 307. 9 W. Muller-Warmuth and H. Eckert, Phys. Rep., 88(1982) 91. 10 B. V. R. Chowdari, R. Gopalakrishnan, S. H. Tang and M. H. Kuok, Solid State lonics, 28-30(1988) 704. 11 R.H. Doremus, J. Am. Ceram. Soc., 67(1984) C 150. 12 G. Chiodelli and A. Magistris, Solid State lonics, 18/19 (1986) 356. 13 B. Carette, M. Ribes and J. L. Souquet, Solid State lonics, 9/10(1983) 735. 14 T. Minami and M. Tanaka, J. Non-Cryst. Solids, 38/39 (1980) 289. 15 T. Minami, Y. Takuma and M. Tanaka, J. Electrochem. Soc., 124(1977) 1659. 16 A. Schiraldi, G. Chiodelli and A. Magistris, J. Appl. Electrochem., 6 (1976) 251. 17 K. Shahi and S. Chandra, Phys. Status Solidi A, 28(1975) 653.