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Dielectric relaxation in iron-containing borate glasses
Journal of Non-Crystalline Solids 27 (1978) 51-58 © North-Holland Publishing Company
DIELECTRIC RELAXATION IN IRON-CONTAINING BORATE GLASSES M.H. OMAR * and J.M. STEVELS
Department of lnorganic Chemistry, Eindhoven University of Technology, Emdhoven, The Netherlands Received 4 September 1975 Revised manuscript received 18 May 1977
The ac and dc conductivities, dielectric constant and dielectric loss of a series of calcium borate glasses containing Fe203 up to 10 mol% were measured as a function of temperature (300-700 K) and frequency (102-105 c/s). Glasses melted in platinum cructbles were found to show slightly higher dielectric constants and conductivities than glasses melted in zirconia crucibles under the same melting conditions Relaxation effects with a distribution of relaxation times were observed for all the glasses of this system. The data show that the pre-exponential factor r 0 is about 10 -11 s. Peaks in tan ~ were found to shift to higher temperatures when the frequency of measurements was increased. Activation energy for dc conduction was found to be equal to that of ac conduction. No polarization effects were observed in these glasses, indicating that conduction is predominantly electronic.
1. Introduction The electrical properties of borate glasses containing iron were not studied in detail so far as is the case of iron-containing phosphates [1--9] and silicate glasses [ 1 0 - 1 7 ] . It has been shown [18] that the dc resistivity of calcium borate glasses decreased by several orders of magnitude when Fe20 3 was increased above 20 mol%. The activation energy for dc conduction also decreased sharphly above this concentration. Devitrification of the glasses containing more than 20 mol% Fe20 3 was observed by optical microscopy and was confirmed by X-ray analysis. Therefore, it is not unlikely that the extensive changes in electrical properties observed can be attributed to the devitrification of the glass. In many cases electrical conduction in glasses containing transition metal oxides has been proved to be electronic in nature. The conduction process is believed to occur by electron hopping between the ions existing in different valence states in the glass. For example, if a glass contains iron oxide hopping will take place
* On sabbatical leave, Department of Materials Engineering and Physical Sciences, The American University in Cairo, Egypt. 51
52
M.H. Omar, J.M. Stevels / Dielectric relaxation in borate glasses
between the ferrous and ferric ions: Fe 2+ ~ Fe 3+ + F . Among the different factors which influence the conductivity in the glass are the following: (a) the average distance separating ion sites (total oxide concentration); (b) ferrous/ferric ratio (number of carriers); (c) thermal history or the degree of ordering (charge mobility). Reviewing previous work on the electrical properties of iron borate glasses [1,18], it was noticed that there is little information on their ac and dc properties for iron oxide concentrations below 10 mol%. In this concentration range glasses were found to be more homogeneous and free from microcrystalline structures [ 19] *. Iron-borate glasses studied earlier were ussually melted in zirconia crucibles. It seemed of some interest, therefore, to study the effect of melting in platinum on the electrical properties of these glasses.
2. Experimental methods Glasses were prepared by melting high-purity chemicals (B20 a, CaCOa and Fe203) in platinum or zirconia crucibles for 3 h at 1200°C in air. The melt was stirred at regular intervals before pouring into preheated molds. Annealing was carried out at 600°C and the glasses were cooled down overnight to room temperature. Glasses with the molar composition ( 7 0 - x)B203 • 30CaO .xFe203 were prepared with a total iron oxide composition x = 1,3, 5, 7, 9 and 10 tool%. Glasses of this series with 15% Fe20 a or more showed devitrification and were not investigated in this work. This is in agreement with previous work done by Osman [18]. Analysis of the glasses were carried out by means of atomic absorption spectroscopy in order to determine the amount of boron, calcium and total iron. Spectrophotometric methods were used to determine the ferrous/ferric ratio. This ratio was determined for all the samples and was found to be almost constant, namely 0.15 + 10%, again in agreement with the work of Osman. The results of the analysis are shown in table 1, the analysis indicating that there is no significant difference between the batch composition and the final composition. Figure 1 shows the variation of total iron indicated by analysis as a function of batch composition. All the electrical measurements were made on samples in disc form of thickness * X-ray diffraction analysis were carried on samples with different iron concentrations, and below 10 tool% no peaks were observed in the diffraction patters. When Fe203 was increased above 10 mol% some peaks started to appear, indicating the presence of crystals in the glass. This was also conf'trmed by optical microscopy which showed the presence of some small crystals in the upper surface of the samples. This crystalline phase is probably calcium ferrite.
M.H. Omar, J M. Stevels / Dielectric relaxation in borate glasses
53
Table 1 Composition of glasses investigated. Batch (mol %) analysis
B20 3 CaO Fe20 3
69 30 1
67 30 3
65 30 5
63 30 7
61 30 9
60 30 10
B20 3 CaO Fe20 3 FeO
69.09 29.97 0.80 0.24
65.90 31.83 1.82 0.55
64.73 29.81 4.20 1.26
64.51 27.65 6.03 1.81
61 20 28.21 8 14 2.45
56.53 31.60 9.13 2.74
varying between 0.5 and 3 mm, the disc area being about 2 cm 2. Two sets of measurements were carried out' (1) on the glasses melted in platinum; (2) on those melted in zirconia. Two types of electrodes were used: (1) Conducting electrodes which were sputttered in vacuum on opposite surfaces of the samples. Gold, platinum and silver were used for that purpose. Contacts in all cases were found to be ohmic. The same values for the resistivities were obtained with gold and silver electrodes, but slightly lower values with platinum (cf. fig. 7). However, it was found that silver diffuses into the glass when the samples were heated to high temperatures during measurement. Reproducible results were always obtained with gold electrodes, and therefore these were used exclusively in our measurements. (2) Blocking electrodes similar to those used by Tomozawa and Doremus [20]. These were used to study the dielectric behaviour of the glass samples. This type of
15
~g nl
10
5
I
I
S
10 - -
Fe203
15
m o l e */, a d d e d - . - )
Fig. 1. Comparison between analyzed and added Fe20 3 in the glasses investigated.
54
M.tL Omar, J.M. Stevels / Dtelectric relaxation in borate glasses
10~3
I
1013
I
7
9 (rno~ */,Fe203)
3
1012
mole % Fe203 _
/
io11
1011
u 1010
ioI~
109
109
108
IO~
/ !
lO8
2b
3b 104/T
~
10
~'0
2'o 10~/T
Fig. 2. Resistivity curves for calcium borate glasses melted in platinum crucibles. Fig. 3. Resistiwty curves for calcium borate glasses melted in zirconia crucibles.
electrode was obtained by pressing stainless steel discs against the polished surfaces of the glass samples by a constant pressure high enough to cause appreciable dc conduction. Using this technique it was possible to obtain reproducible peaks in tan with much greater accuracy than those calculated from measurements with conducting electrodes after eliminating the dc loss contribution. All the elctrical measurements were carried out in vacuum. Three different bridges were used for measurement of the capacitance and the dissipation factor. (1) General Radio impedance bridge, type 1650A was used and its frequency range was extended by an external oscillator and tunes-frequency detector. The other bridges were (2) Rhode and Scharz, type VKB * and (3) Marconi precision capacitance bridge, type TF1313A. DC resistances were measured using a Kiethely electrometer, type 610C. The current in all samples was found to be a linear function of the applied voltage up to about 50 V. Measurements were usually carried out at an applied constant voltage of 10 V.
* Results obtained with this bridge were carried out in air and were found to be in good agreement with data obtained in vacuum except around room temperature, probably because of surface contamination due to humidity.
M.H. Omar, J.M. Stevels / Dielectric relaxation in borate glasses
55
3. Results and discussion The dc resistivity o f the glass series melted in Pt crucibles is shown in fig. 2. All glasses show a decrease in resistivity as the temperature o f the glass is increased. The activation energy is obviously dependent on the iron content in the glass. These results were reproducible even when the samples were recycled several times between the extemes o f the range o f the measurements. Glasses with the same composition were melted as in zirconia crucibles and the results are shown in fig. 3. In general, the last-mentioned glasses were found to show slightly higher resistivities (fig. 4) and activation energies (fig. 5) than glasses melted in platinum crucibles. Experimental data obtained b y Osman [18] on borate glasses (melted in zirconia) in this concentration range are included and are in good agreement with our results. Measurements of the dielectric constant e' at room temperature and at a frequency o f 1 kc/s are shown in fig. 6. Glasses melted in platinum show a higher dielectric constant than those melted in zirconia. 18
,3\, 12
~
1-600 K
~6 h
\,
I
E u
\\\\\ 08
06
5
10
mote % Fe203
04 mote */© Fe203
Ftg. 4. Dependence of resistivity (at 600 K) on Fe203 content in the glass. - o - glasses melted in platinum crucibles, and - -e- - glasses melted in zirconia crucibles, this work; ~ from measurements of Osman [18] on similar glasses melted in zirconia crucibles. Fig. 5. Dependence of activation energy £~E on Fe203 content in the glasses. - o - glasses melted in platinum crucibles, and this work; - e - glasses melted in zirconia crucibles, $ from measurements of Osman (18) on similar glasses melted in zirconia crucibles; • reported by Mackenzie on calcium borate base glass.
M.H Omar, .I.11,I. Stevels / Dielectric relaxation in borate glasses
56
The observed decrease in resistivity and actwation energy, and increase of the dielectric constant may be due to the presence of dissolved platinum in the glasses which acts as donors of charge carriers. A qualitative analysis of the glasses using an emission spectrograph indicated the presence of platinum in the glasses melted in platinum crucibles. No zirconium was detected in the glasses melted in zirconia crucibles. As an example of the ac behaviour of the glasses studied here, the total ac resistwlty of a 5 mol% Fe20 3 sample melted in a Pt crucible ]s shown in fig. 7. The results obtained with sputtered electrodes (Ag, Pt and Au) are shown for comparison. The ac conductivity of iron borate glasses was found to increase with temperature and with frequency. This behaviour is similar to other transition metal oxide glasses. The ac conductivity Oac of this sample was found to vary with angular frequency co according to Oac .. 6o0.75 at a temperature of about 350 K. All samples studied in this work show a dependence of the form o :: 60n, where 0.1 < n < 1, depending on the temperature of the sample.
T(K) 1013
~
600 i
I
I
400 I
i
250 I
I
DC
lO12
~
1011
~r 01Kc/s
j/ / / ,/
/
/
/
/
/
~10
10
109
-w
lOKds
i// ~.._:.~I,,ooK.s /
10L --
mole % Fe203
104/] "
Fig. 6 Variation of the dielectric constant e' (at 300 K and for 1000 c/s) with Fe203 content in the glass. - e - glasses melted in platinum crucibles; - o - glasses melted in zirconia crucibles. Fig. 7. Dc and ac reslstivities of a glass containing 5 mol% Fe203. - o - with sputtered gold electrodes, - o - with sputtered platinum electrodes, dc measurements Ac measurements at different frequencies were carried out on samples with sputtered gold electrodes.
M.H. Omar, J.M. Stevels / Dielectric relaxation in borate glasses
300cls
O~
57
lO00cts
"7 30
(11
T(K)
Fig. 8. Dependence of tan 6 and capacitance with temperature (at 300 K and for 1000 c/s) for a glass containing 10 mol% Fe20 3.
Tan 8 and capacitance o f all samples were measured using blocking electrodes. As an example, the results for one composition, namely a sample with 10 mol% F e 2 0 3, are shown in fig. 8. Other compositions show similar behaviour. The actwation energies for dc conduction were found to be in good agreement with those obtained from dielectric relaxation measurements (cf. table 2). This suggests that the conduction mechanisms are the same in b o t h cases. From the measured relaxation times, linear plots were obtained between log r and 1/T, showing that r = ro exp(E/kT); ro obtained from the measurements was found to be o f the order ro ~ 10 -11 s. This value is the same as that reported b y Anderson and MacCrone [16] for a silicate glass containing up to 10 mol% Fe203. Further work will be carried out on this system in order to investigate the influence o f different Fe2+/Fe 3+ ratios on the electrical properties o f the glasses.
Table 2 Fe203 (tool %)
AEdc (eV)
AEac (eV)
3 5 7 10
1.15 0.94 0.81 0.68
1.18 0.95 0.80 0.69
+ 0.05 ± 0.05 ± 0.05 ± 0.05
-+ 0.05 ± 0.05 ± 0.05 ± 0.05
58
M.H. Omar, J M. Stevels /Dielectric relaxation in borate glasses
4. Conclusions (1) The observed increase in conductivity and dielectric properties o f calcium borate glasses containing iron melted in platinum crucibles, as compared to zirconia crucibles, is probably due to dissolved platinum in the glass. (2) Measurements with blocking electrodes were found to provide useful information on the behaviour o f tan ~ and e' for the glasses investigated here. (3) The activation energies for ac and dc conduction are nearly equal, indicating the same mechanism o f conduction in both cases. (4) The conduction in the glass systems investigated is predominantly electronic.
Acknowledgement The help o f Mr. A. van der Lee who prepared the glass samples and Mrs. M. Duran who analyzed the samples, is acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
J.D. Mackenzie, J. Am. Ceram. Soc. 47 (1964) 211. K.W. Hansen, J. Electrochem. Soc. 112 (1965)994. K.W. Hansen and M.T. Splann, J. Electrochem. Soc. 113 (1966) 895. A. Bishay and L. Makkar, J. Am. Ceram. Soc. 52 (1969) 605. D.L. Klnser, J. Electrochem. Soc. 117 (1970) 546. M. Sayer and A. Mansingh, Phys. Rev. 6 (1972) 4629. T. Tsuchiya and T. Moriya, Yogyo-Kyokai-Shi(Japan) 81 (1973) 303. M.S. Mizzoni, J. Electrochem. Soc. 120 (1973) 1592. L. Muruvski and O. Gzowski, Phys. Stat. Sol. 19 (1973) K125. O.V. Mazurin, G.A. Pavlova, E.Ia. Lev and E.K. Leko, Soy. Phys. Teeh. Phhys. 2 (1957) 2511. 11 ] G.O. Karapetyan, V.A. Tsekhomskl and D.M. Ydin, Soy. Phys. Solid State 5 (1963) 456. 12] H.J.L. Trap and J.M. Stevels, Phys. Chem. Glasses 4 (1963) 193. 13] M.P. O'Horo and R. Steinitz, Mater. Res. Bull. 3 (1968) 117. 14] T. Takamori, H. Seo and M. Tshiro, J. Am. Ceram. Soc. 9 (1969) 513. 15] M.P. O'Horo, Am. Ceram. Soc. Bull. 52 (1973) 701. 16] R.A. Anderson and R.K. MacCrone, J. Non-Crystalline Solids 14 (1974) 112. 17] L.A. Orechanik, E.A. Fainberg and I.N. Zertsalova, Soy. Phys. Solid State 4 (1962) 331. 18] A. Osman, Ph.D. Thesis, Cairo University, 1974. 19] R.A. Gdula and R.F. Tompkins, Glass TechnoL 11 (1970) 164. 20] M. Tomozawa and R.H. Doremus, J. Non-Crystalline Solids 14 (1974) 101.
DIELECTRIC RELAXATION IN IRON-CONTAINING BORATE GLASSES M.H. OMAR * and J.M. STEVELS
Department of lnorganic Chemistry, Eindhoven University of Technology, Emdhoven, The Netherlands Received 4 September 1975 Revised manuscript received 18 May 1977
The ac and dc conductivities, dielectric constant and dielectric loss of a series of calcium borate glasses containing Fe203 up to 10 mol% were measured as a function of temperature (300-700 K) and frequency (102-105 c/s). Glasses melted in platinum cructbles were found to show slightly higher dielectric constants and conductivities than glasses melted in zirconia crucibles under the same melting conditions Relaxation effects with a distribution of relaxation times were observed for all the glasses of this system. The data show that the pre-exponential factor r 0 is about 10 -11 s. Peaks in tan ~ were found to shift to higher temperatures when the frequency of measurements was increased. Activation energy for dc conduction was found to be equal to that of ac conduction. No polarization effects were observed in these glasses, indicating that conduction is predominantly electronic.
1. Introduction The electrical properties of borate glasses containing iron were not studied in detail so far as is the case of iron-containing phosphates [1--9] and silicate glasses [ 1 0 - 1 7 ] . It has been shown [18] that the dc resistivity of calcium borate glasses decreased by several orders of magnitude when Fe20 3 was increased above 20 mol%. The activation energy for dc conduction also decreased sharphly above this concentration. Devitrification of the glasses containing more than 20 mol% Fe20 3 was observed by optical microscopy and was confirmed by X-ray analysis. Therefore, it is not unlikely that the extensive changes in electrical properties observed can be attributed to the devitrification of the glass. In many cases electrical conduction in glasses containing transition metal oxides has been proved to be electronic in nature. The conduction process is believed to occur by electron hopping between the ions existing in different valence states in the glass. For example, if a glass contains iron oxide hopping will take place
* On sabbatical leave, Department of Materials Engineering and Physical Sciences, The American University in Cairo, Egypt. 51
52
M.H. Omar, J.M. Stevels / Dielectric relaxation in borate glasses
between the ferrous and ferric ions: Fe 2+ ~ Fe 3+ + F . Among the different factors which influence the conductivity in the glass are the following: (a) the average distance separating ion sites (total oxide concentration); (b) ferrous/ferric ratio (number of carriers); (c) thermal history or the degree of ordering (charge mobility). Reviewing previous work on the electrical properties of iron borate glasses [1,18], it was noticed that there is little information on their ac and dc properties for iron oxide concentrations below 10 mol%. In this concentration range glasses were found to be more homogeneous and free from microcrystalline structures [ 19] *. Iron-borate glasses studied earlier were ussually melted in zirconia crucibles. It seemed of some interest, therefore, to study the effect of melting in platinum on the electrical properties of these glasses.
2. Experimental methods Glasses were prepared by melting high-purity chemicals (B20 a, CaCOa and Fe203) in platinum or zirconia crucibles for 3 h at 1200°C in air. The melt was stirred at regular intervals before pouring into preheated molds. Annealing was carried out at 600°C and the glasses were cooled down overnight to room temperature. Glasses with the molar composition ( 7 0 - x)B203 • 30CaO .xFe203 were prepared with a total iron oxide composition x = 1,3, 5, 7, 9 and 10 tool%. Glasses of this series with 15% Fe20 a or more showed devitrification and were not investigated in this work. This is in agreement with previous work done by Osman [18]. Analysis of the glasses were carried out by means of atomic absorption spectroscopy in order to determine the amount of boron, calcium and total iron. Spectrophotometric methods were used to determine the ferrous/ferric ratio. This ratio was determined for all the samples and was found to be almost constant, namely 0.15 + 10%, again in agreement with the work of Osman. The results of the analysis are shown in table 1, the analysis indicating that there is no significant difference between the batch composition and the final composition. Figure 1 shows the variation of total iron indicated by analysis as a function of batch composition. All the electrical measurements were made on samples in disc form of thickness * X-ray diffraction analysis were carried on samples with different iron concentrations, and below 10 tool% no peaks were observed in the diffraction patters. When Fe203 was increased above 10 mol% some peaks started to appear, indicating the presence of crystals in the glass. This was also conf'trmed by optical microscopy which showed the presence of some small crystals in the upper surface of the samples. This crystalline phase is probably calcium ferrite.
M.H. Omar, J M. Stevels / Dielectric relaxation in borate glasses
53
Table 1 Composition of glasses investigated. Batch (mol %) analysis
B20 3 CaO Fe20 3
69 30 1
67 30 3
65 30 5
63 30 7
61 30 9
60 30 10
B20 3 CaO Fe20 3 FeO
69.09 29.97 0.80 0.24
65.90 31.83 1.82 0.55
64.73 29.81 4.20 1.26
64.51 27.65 6.03 1.81
61 20 28.21 8 14 2.45
56.53 31.60 9.13 2.74
varying between 0.5 and 3 mm, the disc area being about 2 cm 2. Two sets of measurements were carried out' (1) on the glasses melted in platinum; (2) on those melted in zirconia. Two types of electrodes were used: (1) Conducting electrodes which were sputttered in vacuum on opposite surfaces of the samples. Gold, platinum and silver were used for that purpose. Contacts in all cases were found to be ohmic. The same values for the resistivities were obtained with gold and silver electrodes, but slightly lower values with platinum (cf. fig. 7). However, it was found that silver diffuses into the glass when the samples were heated to high temperatures during measurement. Reproducible results were always obtained with gold electrodes, and therefore these were used exclusively in our measurements. (2) Blocking electrodes similar to those used by Tomozawa and Doremus [20]. These were used to study the dielectric behaviour of the glass samples. This type of
15
~g nl
10
5
I
I
S
10 - -
Fe203
15
m o l e */, a d d e d - . - )
Fig. 1. Comparison between analyzed and added Fe20 3 in the glasses investigated.
54
M.tL Omar, J.M. Stevels / Dtelectric relaxation in borate glasses
10~3
I
1013
I
7
9 (rno~ */,Fe203)
3
1012
mole % Fe203 _
/
io11
1011
u 1010
ioI~
109
109
108
IO~
/ !
lO8
2b
3b 104/T
~
10
~'0
2'o 10~/T
Fig. 2. Resistivity curves for calcium borate glasses melted in platinum crucibles. Fig. 3. Resistiwty curves for calcium borate glasses melted in zirconia crucibles.
electrode was obtained by pressing stainless steel discs against the polished surfaces of the glass samples by a constant pressure high enough to cause appreciable dc conduction. Using this technique it was possible to obtain reproducible peaks in tan with much greater accuracy than those calculated from measurements with conducting electrodes after eliminating the dc loss contribution. All the elctrical measurements were carried out in vacuum. Three different bridges were used for measurement of the capacitance and the dissipation factor. (1) General Radio impedance bridge, type 1650A was used and its frequency range was extended by an external oscillator and tunes-frequency detector. The other bridges were (2) Rhode and Scharz, type VKB * and (3) Marconi precision capacitance bridge, type TF1313A. DC resistances were measured using a Kiethely electrometer, type 610C. The current in all samples was found to be a linear function of the applied voltage up to about 50 V. Measurements were usually carried out at an applied constant voltage of 10 V.
* Results obtained with this bridge were carried out in air and were found to be in good agreement with data obtained in vacuum except around room temperature, probably because of surface contamination due to humidity.
M.H. Omar, J.M. Stevels / Dielectric relaxation in borate glasses
55
3. Results and discussion The dc resistivity o f the glass series melted in Pt crucibles is shown in fig. 2. All glasses show a decrease in resistivity as the temperature o f the glass is increased. The activation energy is obviously dependent on the iron content in the glass. These results were reproducible even when the samples were recycled several times between the extemes o f the range o f the measurements. Glasses with the same composition were melted as in zirconia crucibles and the results are shown in fig. 3. In general, the last-mentioned glasses were found to show slightly higher resistivities (fig. 4) and activation energies (fig. 5) than glasses melted in platinum crucibles. Experimental data obtained b y Osman [18] on borate glasses (melted in zirconia) in this concentration range are included and are in good agreement with our results. Measurements of the dielectric constant e' at room temperature and at a frequency o f 1 kc/s are shown in fig. 6. Glasses melted in platinum show a higher dielectric constant than those melted in zirconia. 18
,3\, 12
~
1-600 K
~6 h
\,
I
E u
\\\\\ 08
06
5
10
mote % Fe203
04 mote */© Fe203
Ftg. 4. Dependence of resistivity (at 600 K) on Fe203 content in the glass. - o - glasses melted in platinum crucibles, and - -e- - glasses melted in zirconia crucibles, this work; ~ from measurements of Osman [18] on similar glasses melted in zirconia crucibles. Fig. 5. Dependence of activation energy £~E on Fe203 content in the glasses. - o - glasses melted in platinum crucibles, and this work; - e - glasses melted in zirconia crucibles, $ from measurements of Osman (18) on similar glasses melted in zirconia crucibles; • reported by Mackenzie on calcium borate base glass.
M.H Omar, .I.11,I. Stevels / Dielectric relaxation in borate glasses
56
The observed decrease in resistivity and actwation energy, and increase of the dielectric constant may be due to the presence of dissolved platinum in the glasses which acts as donors of charge carriers. A qualitative analysis of the glasses using an emission spectrograph indicated the presence of platinum in the glasses melted in platinum crucibles. No zirconium was detected in the glasses melted in zirconia crucibles. As an example of the ac behaviour of the glasses studied here, the total ac resistwlty of a 5 mol% Fe20 3 sample melted in a Pt crucible ]s shown in fig. 7. The results obtained with sputtered electrodes (Ag, Pt and Au) are shown for comparison. The ac conductivity of iron borate glasses was found to increase with temperature and with frequency. This behaviour is similar to other transition metal oxide glasses. The ac conductivity Oac of this sample was found to vary with angular frequency co according to Oac .. 6o0.75 at a temperature of about 350 K. All samples studied in this work show a dependence of the form o :: 60n, where 0.1 < n < 1, depending on the temperature of the sample.
T(K) 1013
~
600 i
I
I
400 I
i
250 I
I
DC
lO12
~
1011
~r 01Kc/s
j/ / / ,/
/
/
/
/
/
~10
10
109
-w
lOKds
i// ~.._:.~I,,ooK.s /
10L --
mole % Fe203
104/] "
Fig. 6 Variation of the dielectric constant e' (at 300 K and for 1000 c/s) with Fe203 content in the glass. - e - glasses melted in platinum crucibles; - o - glasses melted in zirconia crucibles. Fig. 7. Dc and ac reslstivities of a glass containing 5 mol% Fe203. - o - with sputtered gold electrodes, - o - with sputtered platinum electrodes, dc measurements Ac measurements at different frequencies were carried out on samples with sputtered gold electrodes.
M.H. Omar, J.M. Stevels / Dielectric relaxation in borate glasses
300cls
O~
57
lO00cts
"7 30
(11
T(K)
Fig. 8. Dependence of tan 6 and capacitance with temperature (at 300 K and for 1000 c/s) for a glass containing 10 mol% Fe20 3.
Tan 8 and capacitance o f all samples were measured using blocking electrodes. As an example, the results for one composition, namely a sample with 10 mol% F e 2 0 3, are shown in fig. 8. Other compositions show similar behaviour. The actwation energies for dc conduction were found to be in good agreement with those obtained from dielectric relaxation measurements (cf. table 2). This suggests that the conduction mechanisms are the same in b o t h cases. From the measured relaxation times, linear plots were obtained between log r and 1/T, showing that r = ro exp(E/kT); ro obtained from the measurements was found to be o f the order ro ~ 10 -11 s. This value is the same as that reported b y Anderson and MacCrone [16] for a silicate glass containing up to 10 mol% Fe203. Further work will be carried out on this system in order to investigate the influence o f different Fe2+/Fe 3+ ratios on the electrical properties o f the glasses.
Table 2 Fe203 (tool %)
AEdc (eV)
AEac (eV)
3 5 7 10
1.15 0.94 0.81 0.68
1.18 0.95 0.80 0.69
+ 0.05 ± 0.05 ± 0.05 ± 0.05
-+ 0.05 ± 0.05 ± 0.05 ± 0.05
58
M.H. Omar, J M. Stevels /Dielectric relaxation in borate glasses
4. Conclusions (1) The observed increase in conductivity and dielectric properties o f calcium borate glasses containing iron melted in platinum crucibles, as compared to zirconia crucibles, is probably due to dissolved platinum in the glass. (2) Measurements with blocking electrodes were found to provide useful information on the behaviour o f tan ~ and e' for the glasses investigated here. (3) The activation energies for ac and dc conduction are nearly equal, indicating the same mechanism o f conduction in both cases. (4) The conduction in the glass systems investigated is predominantly electronic.
Acknowledgement The help o f Mr. A. van der Lee who prepared the glass samples and Mrs. M. Duran who analyzed the samples, is acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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