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Concentration-dependent electrical conductivity of phosphate glasses containing zinc oxide
Materials Chemistry
and Physics 41 ( 1995) 299-301
Materials Science Communication
Concentration-dependent
electrical conductivity of phosphate glasses containing zinc oxide
M. Ashraf Chaudhry, Shakeel Bilal Departmenr of Physics, Buhauddin Zakuriya University. Multan 60800, Pakistan Received 30 November 1994; revised 31 March 1995; accepted 10 April 1995
Abstract
A range of glass samples of x% ZnO-( 100-x)% P205 with 20
glasses; Concentration
dependence;
Electrical conductivity
1. Introduction
Recent theoretical studies linking the properties of materials with the structure have provided a renewed interest in the preparation, characterization, processing, testing and manufacturing of new types of materials [ 1 I. Phosphate glasses are the subject of an increasing number of investigations. Particular attention has been paid to the phosphate glasses, which contain transition metals (TMs), owing to their potential applications [ 2,3]. The phosphate glasses have a large value of electrical conductivity in comparison with other glass systems such as silicates and borates and, thus, are the prime candidates for superionic conductors and electrolytes [ 41. Most of the properties such as electrical, optical, thermal, dielectric, etc., of phosphate glasses are greatly modified by the addition of alkali oxides (Na,O, K20, etc.), alkaline earth oxides (CaO, MgO, BaO, etc.) and many other divalent metal oxides such as ZnO, PbO, etc. [ 51. The activalion energy for electrical conduction of binary cadmium phosphate glass is found to decrease with increasing Cd0 content [6]. The optical bandgap of the zinc borate binary glass system increases with an increase in ZnO concentration [ 71. Similar results have been reported by the present authors [ 8-l 1] for zinc-containing binary and ternary phosphate glasses. The concentration dependence of electrical conductivity of x% ZnO-( 100-x) % P205 with 20 G x G 40 and x% ZnO(50-x) % CdO-50% P,O, with 30
presented in this communication. The main objective of this work is to establish the role of ZnO concentration as regards to the electrical conductivity of these glasses. To the best of our knowledge such type of work has not been reported in the literature for the phosphate glasses.
2. Experimental
The binary and ternary phosphate glass samples were prepared by the melt quench technique at 1200 “C by using analytical grade ZnO, Cd0 and P205. The relevant amounts of constituents were calculated for 20 g samples for each composition reported in this paper. After weighing the required amounts the mixed materials were put into the alumina crucible which was initially heated at 500 “C for half an hour to avoid possible fuming of phosphorus pentoxide. The crucible was then placed into another furnace at 1200 “C for at least 3 h. The contents were stirred with a stirrer to ensure the maximum homogeneity of the materials. Finally, the melt was quenched to form disc-type samples in a stainless-steel die. All the samples were annealed at 300 “C for 3 h to eliminate mechanical and the&al stresses produced during heating and casting of the samples. Each sample represents its starting batch composition. The amorphous nature of the glass samples was established by X-ray diffraction (XRD) analysis and optical absorption studies [ 8,101. The d.c. electrical conductivity of these spec-
300
M.A. Chaudhry. S. Bilal /Materials
Chemistry and Physics 41 (1995) 299-301
imens was measured by the two-probe method using springloaded copper electrodes which were 4 mm in diameter. A regulated d.c. power supply (Ortec model 456) was employed with a Keithley 610C electrometer to collect the required data for the estimation of the electrical conductivity of various samples.
1:520
3. Results and discussion
ZnO
Electrical conductivity of the glassy samples was calculated by using the relation:
30 40 CONCENTRATION
50
Fig. 2. Electrical conductivity vs. ZnO concentration for cadmium zinc phosphate (CdO-ZnO-P,O,) ternary glasses at two fixed bias voltages.
a=LIRA where L and A are thickness and cross-sectional area of the samples, respectively. The electrical resistance R was obtained from the current-voltage data. The measured conductivity of all the binary and ternary specimens is of the order lo-” (ohm cm)-‘. The dependence of electrical conductivity on ZnO concentration for the zinc phosphate binary glass system is illustrated in Fig. 1 for two fixed bias voltages. The klectrical conductivity shows a linear decrease with increase in ZnO concentration. The different gradients are indicative of voltage dependence of the electrical conductivity. Similarly, the electrical conductivity of cadmium zinc phosphate ternary glasses indicates a decrease with growing concentration of ZnO, as shown in Fig. 2. This decrease in electrical conductivity of zinc phosphate and cadmium zinc phosphate glass systems with the enhancement of ZnO concentration is probably due to the increase of non-bridging oxygen atoms. Depolymerization in phosphate glasses is known to occur with the addition of many divalent metal oxides such as ZnO, PbO, etc. [ 51. This causes the partial opening of the 3-d network structure and shortens the average chain length by breaking those oxygen bonds which form bridges between the corners of the PO4 tetrahedra. In this way, the amount of non-bridging oxygen atoms grows to a high concentration due to the change in oxygen bonding. It has been reported that the optical bandgap of zinc borate [7], zinc phosphate and cadmium zinc phosphate ternary glass systems (see Fig. 3) increases with increasing ZnO concentration. The reduction in electrical conductivity of zinc phosphate and cadmium zinc phosphate glasses can be COT-
3.40 (a)
(b)
20
40
60
ZnO Concentration (mot%)
ZnO Concentratcon(molX)
Fig. 3. Variation of optical bandgap with ZnO concentration for (a) zinc phosphate binary glasses [S] and (b) cadmium zinc phosphate ternary glasses [ 91 ( P,O, fixed at 50 mol%)
related to the optical bandgap which is found to increase with ZnO concentration. A linear relationship between optical and electrical bandgaps exists [ 121 for phosphate glasses. According to this the electrical bandgap increases with the optical bandgap. Consequently, the electrical conductivity will show a decrease with increasing amount of ZnO in the zinc-containing phosphate glasses. Ghauri et al. [131have reported an increase in the activation energy of electrical conduction for x% CdO-( 40-x) % Zn0-60% P20, systems and have linked it to the increase in Zn ions which grows with ZnO concentration. The present results support the general conclusion that the increasing concentration of ZnO reduces the electrical conductivity of binary and ternary phosphate glasses in which ZnO is present as one of the components.
Acknowledgement ZnO
CONCENTRATION
Fig. 1. Electrical conductivity vs. ZnO concentration (ZnO-PzOs) binary glasses at two fixed bias voltages.
for zinc phosphate
The authors are grateful to the N.S.R.D. Board for providing funds for the present work under Research Grant No. UNML&PH-3( 19) / 154.
M.A. Chaudhry, S. Bilal/ Materials
References [ 11 SF. Anvary, C.A. Hogarth and G.H. Moridi, 1. Muter. Sci., 26 ( 199 1) 3639. [2] A. Ghosh, Phys. Rev. B, 42 ( 1990) 5665. [3] L. Murawski, C.H. Chung and J.D. Mackenzie, J. Non-Cryst. Solids, 32 (1979) 91. [4] D.S. Rao, P.P. Karat and B. Parvatti, .I. Mater. Sci. L&t., 9 (1990) 748. [5] S. Chakraborty and A. Paul, J. Mafer. Sci. Lett., 8 (1989) 1358. [6] M.A. Ghauri and CA. Hogarth, Thin Solid Films. 68 (1980) L13.
Chemistry and Physics 41 (1995) 299-301
301
[7] M.M. Ahmed and CA. Hogarth, J. Mater. Sci. Letr, 2 (1983) 254. [S] M.A. Chaudhry, M.S. Bilal, M. Altaf and M.A. Ahmed, submitted for publication. [9] M.S. BilaI, M.A. Chaudhry, M. Altaf and M.A. Ahmed, Mod. Phys. Lett8,8(1994)461. [lo] M.A. Chaudhry, MS. Bilal, M. Altaf, M.A. Ahmed and A.M. Rana, J. Mater. Sci. Len.. accepted for publication. [ 111 M.S. Bilal, M.A. Chaudhry, M. Altaf, M.A. Ahmed and M.Y. Nadeem, submitted for publication [ 121 C.A. Hogarth anh A.A. Hosseini, J. Mater. Sci., 18 (1983) 2697. [13] M.A. Ghauri, A.R. Mir. C.A. Hogarth and M.M. Ahmed, Inr. J. Electron., 53 (1982) 311.
and Physics 41 ( 1995) 299-301
Materials Science Communication
Concentration-dependent
electrical conductivity of phosphate glasses containing zinc oxide
M. Ashraf Chaudhry, Shakeel Bilal Departmenr of Physics, Buhauddin Zakuriya University. Multan 60800, Pakistan Received 30 November 1994; revised 31 March 1995; accepted 10 April 1995
Abstract
A range of glass samples of x% ZnO-( 100-x)% P205 with 20
glasses; Concentration
dependence;
Electrical conductivity
1. Introduction
Recent theoretical studies linking the properties of materials with the structure have provided a renewed interest in the preparation, characterization, processing, testing and manufacturing of new types of materials [ 1 I. Phosphate glasses are the subject of an increasing number of investigations. Particular attention has been paid to the phosphate glasses, which contain transition metals (TMs), owing to their potential applications [ 2,3]. The phosphate glasses have a large value of electrical conductivity in comparison with other glass systems such as silicates and borates and, thus, are the prime candidates for superionic conductors and electrolytes [ 41. Most of the properties such as electrical, optical, thermal, dielectric, etc., of phosphate glasses are greatly modified by the addition of alkali oxides (Na,O, K20, etc.), alkaline earth oxides (CaO, MgO, BaO, etc.) and many other divalent metal oxides such as ZnO, PbO, etc. [ 51. The activalion energy for electrical conduction of binary cadmium phosphate glass is found to decrease with increasing Cd0 content [6]. The optical bandgap of the zinc borate binary glass system increases with an increase in ZnO concentration [ 71. Similar results have been reported by the present authors [ 8-l 1] for zinc-containing binary and ternary phosphate glasses. The concentration dependence of electrical conductivity of x% ZnO-( 100-x) % P205 with 20 G x G 40 and x% ZnO(50-x) % CdO-50% P,O, with 30
presented in this communication. The main objective of this work is to establish the role of ZnO concentration as regards to the electrical conductivity of these glasses. To the best of our knowledge such type of work has not been reported in the literature for the phosphate glasses.
2. Experimental
The binary and ternary phosphate glass samples were prepared by the melt quench technique at 1200 “C by using analytical grade ZnO, Cd0 and P205. The relevant amounts of constituents were calculated for 20 g samples for each composition reported in this paper. After weighing the required amounts the mixed materials were put into the alumina crucible which was initially heated at 500 “C for half an hour to avoid possible fuming of phosphorus pentoxide. The crucible was then placed into another furnace at 1200 “C for at least 3 h. The contents were stirred with a stirrer to ensure the maximum homogeneity of the materials. Finally, the melt was quenched to form disc-type samples in a stainless-steel die. All the samples were annealed at 300 “C for 3 h to eliminate mechanical and the&al stresses produced during heating and casting of the samples. Each sample represents its starting batch composition. The amorphous nature of the glass samples was established by X-ray diffraction (XRD) analysis and optical absorption studies [ 8,101. The d.c. electrical conductivity of these spec-
300
M.A. Chaudhry. S. Bilal /Materials
Chemistry and Physics 41 (1995) 299-301
imens was measured by the two-probe method using springloaded copper electrodes which were 4 mm in diameter. A regulated d.c. power supply (Ortec model 456) was employed with a Keithley 610C electrometer to collect the required data for the estimation of the electrical conductivity of various samples.
1:520
3. Results and discussion
ZnO
Electrical conductivity of the glassy samples was calculated by using the relation:
30 40 CONCENTRATION
50
Fig. 2. Electrical conductivity vs. ZnO concentration for cadmium zinc phosphate (CdO-ZnO-P,O,) ternary glasses at two fixed bias voltages.
a=LIRA where L and A are thickness and cross-sectional area of the samples, respectively. The electrical resistance R was obtained from the current-voltage data. The measured conductivity of all the binary and ternary specimens is of the order lo-” (ohm cm)-‘. The dependence of electrical conductivity on ZnO concentration for the zinc phosphate binary glass system is illustrated in Fig. 1 for two fixed bias voltages. The klectrical conductivity shows a linear decrease with increase in ZnO concentration. The different gradients are indicative of voltage dependence of the electrical conductivity. Similarly, the electrical conductivity of cadmium zinc phosphate ternary glasses indicates a decrease with growing concentration of ZnO, as shown in Fig. 2. This decrease in electrical conductivity of zinc phosphate and cadmium zinc phosphate glass systems with the enhancement of ZnO concentration is probably due to the increase of non-bridging oxygen atoms. Depolymerization in phosphate glasses is known to occur with the addition of many divalent metal oxides such as ZnO, PbO, etc. [ 51. This causes the partial opening of the 3-d network structure and shortens the average chain length by breaking those oxygen bonds which form bridges between the corners of the PO4 tetrahedra. In this way, the amount of non-bridging oxygen atoms grows to a high concentration due to the change in oxygen bonding. It has been reported that the optical bandgap of zinc borate [7], zinc phosphate and cadmium zinc phosphate ternary glass systems (see Fig. 3) increases with increasing ZnO concentration. The reduction in electrical conductivity of zinc phosphate and cadmium zinc phosphate glasses can be COT-
3.40 (a)
(b)
20
40
60
ZnO Concentration (mot%)
ZnO Concentratcon(molX)
Fig. 3. Variation of optical bandgap with ZnO concentration for (a) zinc phosphate binary glasses [S] and (b) cadmium zinc phosphate ternary glasses [ 91 ( P,O, fixed at 50 mol%)
related to the optical bandgap which is found to increase with ZnO concentration. A linear relationship between optical and electrical bandgaps exists [ 121 for phosphate glasses. According to this the electrical bandgap increases with the optical bandgap. Consequently, the electrical conductivity will show a decrease with increasing amount of ZnO in the zinc-containing phosphate glasses. Ghauri et al. [131have reported an increase in the activation energy of electrical conduction for x% CdO-( 40-x) % Zn0-60% P20, systems and have linked it to the increase in Zn ions which grows with ZnO concentration. The present results support the general conclusion that the increasing concentration of ZnO reduces the electrical conductivity of binary and ternary phosphate glasses in which ZnO is present as one of the components.
Acknowledgement ZnO
CONCENTRATION
Fig. 1. Electrical conductivity vs. ZnO concentration (ZnO-PzOs) binary glasses at two fixed bias voltages.
for zinc phosphate
The authors are grateful to the N.S.R.D. Board for providing funds for the present work under Research Grant No. UNML&PH-3( 19) / 154.
M.A. Chaudhry, S. Bilal/ Materials
References [ 11 SF. Anvary, C.A. Hogarth and G.H. Moridi, 1. Muter. Sci., 26 ( 199 1) 3639. [2] A. Ghosh, Phys. Rev. B, 42 ( 1990) 5665. [3] L. Murawski, C.H. Chung and J.D. Mackenzie, J. Non-Cryst. Solids, 32 (1979) 91. [4] D.S. Rao, P.P. Karat and B. Parvatti, .I. Mater. Sci. L&t., 9 (1990) 748. [5] S. Chakraborty and A. Paul, J. Mafer. Sci. Lett., 8 (1989) 1358. [6] M.A. Ghauri and CA. Hogarth, Thin Solid Films. 68 (1980) L13.
Chemistry and Physics 41 (1995) 299-301
301
[7] M.M. Ahmed and CA. Hogarth, J. Mater. Sci. Letr, 2 (1983) 254. [S] M.A. Chaudhry, M.S. Bilal, M. Altaf and M.A. Ahmed, submitted for publication. [9] M.S. BilaI, M.A. Chaudhry, M. Altaf and M.A. Ahmed, Mod. Phys. Lett8,8(1994)461. [lo] M.A. Chaudhry, MS. Bilal, M. Altaf, M.A. Ahmed and A.M. Rana, J. Mater. Sci. Len.. accepted for publication. [ 111 M.S. Bilal, M.A. Chaudhry, M. Altaf, M.A. Ahmed and M.Y. Nadeem, submitted for publication [ 121 C.A. Hogarth anh A.A. Hosseini, J. Mater. Sci., 18 (1983) 2697. [13] M.A. Ghauri, A.R. Mir. C.A. Hogarth and M.M. Ahmed, Inr. J. Electron., 53 (1982) 311.