- Email: [email protected]
High temperature pH sensitivities of stabilized zirconia films and ceria ceramics
__ /.__ l!iB
2%
SOLID STATE
ELSEVIER
Solid State Ionics 86-88
High temperature
of Industrial
Chemistry,
Faculty
Yamashitaa’*, of Engineering,
of Chemistry,
Rutgers,
Takao Umegaki”,
Tokyo Metropolitan
192-03, ‘Department
IONICS
1121-1124
pH sensitivities of stabilized zirconia films and ceria ceramics
Yasushi Inda”, Kimihiro aDepartment
(1996)
University,
Martha Greenblattb 1-I Minami-Ohsawa,
Hachioji,
Tokyo
Japan
the State University of New Jersey,
Piscataway,
NJ 08855-0939,
USA
Abstract Yttria-stabilized zirconia (YSZ) thick films and bulk ceria (CeO,) ceramics used as membrane electrodes are shown to work as high temperature pH sensors. The YSZ film electrode shows a nearly ideal Nemstian response of the potential as a function of pH with mV/pH = - 56.8 even at 353 K. The YSZ film electrode responded to pH changes within 30 s, while the response time of the samaria-doped ceria ceramic membrane electrode was 3 min. Complex impedance analysis revealed that the bulk conductivity of the ceria ceramics is higher than that of YSZ film, whereas the grain-boundary conductivity of the former was 10’ times lower than that of the latter. Based on these results, it appears that at temperatures below 600 K, the pH sensory characteristics of these materials are dominated by the grain-boundary conductivity. Keywords:
High temperature
pH sensor; YSZ film; Ceria ceramics;
1. Introduction Measurement of pH at high temperatures is of great importance in the fields of oil-, geothermal-, and nuclear-electric power generation plants, and food processing industries [l]. Monitoring of pH in building construction, plumbing, and in the evaluation of corrosion is important. Glass electrodes or ion selective sensors, because of surface corrosion in the severe environment of acidic and alkaline solutions at high temperatures, do not work as pH sensors. Recently the well-known fast oxide ion conductor YSZ, applicable for oxygen gas sensors and fuel cells, has been shown to work as a high temperature pH sensor as well [2]; a YSZ ceramic membrane *Corresponding
author
0167-2738/96/$15.00 Copyright PIZ SO167-2738(96)00279-2
Impedance
conductivity
grain boundary
electrode showed Nernstian response at high temperatures, above 523 K [3]. These electrodes, however, have been generally recognized to be unsuitable for pH measurements at lower temperatures, because of their high impedance. The primary purpose of the present work was to investigate the applicability of YSZ films as pH sensors at lower temperature. Some yttria-stabilized ceria ceramics, which were prepared from commercial powders, have been shown to follow Nernstian behaviour [4]. Moreover, since the conductivities of yttria-stabilized ceria ceramics are ten times higher than those of YSZ [5,6], we also studied the pH sensory characteristics of stabilized ceria ceramic membrane electrodes. At lower temperatures the electronic conductivity of ceramics, comprised of grains and grain boundaries, generally depends on the microstructure
0 1996 Elsevier Science B.V. All rights reserved
1122
Y. Inda et al. I Solid State Ionics 86-88
[7]. In order to obtain highly homogeneous ceria ceramics, we prepared ceria powders by hydrothermal synthesis [8,9]. Because we have not yet been able to prepare films of stabilized ceria, the present work employed bulk ceramic ceria samples.
(1996) 1121-1124
where soln.( 1) = 0.1 M KC1 as the intimal solution of the membrane electrode and soln.(2) =Sat. KC1 as the internal solution of the reference glass electrode.
3. Results and discussion 2. Experimental Dense YSZ thick films with 400 pm thickness were supplied by Mitsui Engineering and Shipping. Stabilized ceria ceramics were obtained by doping CeO, with 10 mol% Sm,O,. The polycrystalline samples were prepared hydrothermally, according to a method previously reported [9], which used reagent-grade Ce(NO,), .6H,O and SmCl, * 6H,O or SmCl, .6H,O powders. The hydrothermally synthesized powders were dried and calcined,and thereafter pressed into pellets with a diameter of 13 mm and a thickness of 0.6 mm. Sintering was performed at 1773-1863 K for 6 h. Powder X-ray diffraction analysis of all of the stabilized ceria preparations indicated single-phase cubic specimens. The densities of all specimens were above 95% of the theoretical value. The conduction properties of stabilized YSZ thick film and ceria ceramic specimens were measured by a 2-probe ac complex impedance method. The measurements were carried out in the frequency range 10 Hz to 10 MHz at 298 to 1273 K. The activation energies (E,) for ionic conduction were calculated from the Errhines plots for grain (bulk) and grain-boundary conduction. The pH sensing characteristics of the electrodes investigated were estimated by the measurements of the potential difference (AV) between the membrane electrodes and a reference electrode in various standard solutions. The membrane electrode consisted of a ceramic plate, or film adhered to a polypropylene tube, which was filled with an intimal solution of 0.1 M KCl, and Ag/AgCl internal electrode. A glass electrode with saturated KC1 as intimal solution was used as a reference electrode. The measurement system is summarized by the following cell: Ag/AgCl/soln.( soln.(2)/Ag/AgCl
l)/membrane/testsoln./Ref./ (I)
Fig. 1, shown as the time dependence of AV for pH =4.16, 6.86, and 8.88 at 353 K, demonstrates the response characteristics of YSZ thick film and ceria ceramic membrane electrodes. These results indicate the rapid pH response time of YSZ film electrode; the AV of YSZ film reached a constant value within 30 s in solutions of various pH. The ceria ceramic electrode also rapidly responded to pH changes within 3 min, and its AVvalue deviated only slightly from that of the YSZ-film electrode at higher and lower pH. In order to verify the validity of both membrane electrodes as pH sensors, the equilibrium constant values of V were measured in the range of pH=3 to 11 at temperatures 298, 340 and 353 K. Fig. 2 shows the relationship between AVand pH for the YSZ film membrane electrode. Although the change of AV is almost insensitive to changes of pH (pH) at room temperature, the slope of the line of AV vs pH increased with temperature, and the response at 353 K was ideally Nemstian; the measured value of AV/pH is -56.8 mV/pH, very close to the theoretical value of -59.0 mV/pH. This result
200
150
-150 012345678
Time (min)
Fig. 1. pH response as a function of time for YSZ thick film and 10 mol% Sm,O,-doped ceria ceramics at 353 K (YSZ: 0 0 a, ceria: n, 0, A, pH=4.16: 0, n, 6.86: 0, 0, 8.88 A A).
Y. Inda et al. I Solid State Ionics 86-88
(1996)
1121-1124
1123
moo
mooo
4oooo
3oooo
Z’ (ohm)
PH Fig. 2. pH response of YSZ thick film electrode temperatures (0 298 K, 0 343 K, 0 353 K).
at various
indicates that the working temperature of YSZ pH sensors is much lowered by thinning the YSZ ceramics, for the lowest temperature for bulk YSZ ceramic sensors has been reported to be 520 K [3]. Fig. 3 compares the pH response of a 10 mol% Sm,O,-doped ceria ceramic, which was sintered at 1863 K for 6 h, with that of a YSZ thick film at 353 K. Although the slope, AV/pH of the ceria electrode is -45.2 mV/pH, smaller than that of the YSZ film electrode, the pH response is linear and Nernstian. The response of ceria membrane electrodes was successfully improved by changing the sintering condition; for a 10 mol% Sm,O,-doped ceria sintered at 1863 K for 24 h, the value of AV/pH increased to -2.5 mV/pH. Thus the sensitivity of a ceramic membrane electrode is dependent upon the sintering conditions. This result explains the difference between the present observation and the previ-
Fig. 4. Typical Cole-cole doped ceria ceramics.
plot of complex impedance
for Sm,O,-
ously reported ideal Nernstian response of bulk ceria ceramics sintered at a higher temperatures, -2000 K 141. In order to understand the effect of the thickness of the membrane electrodes as well as the difference of YSZ film and ceria ceramic electrodes on pH sensitivity, the analysis was carried out at lower temperatures. The complex impedance loci of both YSZ film and ceria ceramic consisted of two arcs (Fig. 4). The impedance due to crystalline grains and the grain boundaries were calculated from the analysis of arc 1 and 2 in the figure. Based on these data, the Errhines plots of YSZ thick film and 10 mol% Sm,O,-doped ceria ceramic were made in Fig. 5. The conductivities (c) and activation energies (E,) of YSZ film and ceria ceramic at representative tem4 2OP d -2 c z -4 4 -6 -8 -
0.5
2
4
6
8
10
PH Fig. 3. pH response of YSZ thick film electrode ceramics at 353 K (YSZ: W ceria: 0).
1
2.5
3
12 and doped ceria
Fig. 5. Arrhenius plots of YSZ thick film and 10 mol% Sm,O,doped ceria ceramics (ceria grain: n, ceria grain boundary: 0, YSZ grain: 0, YSZ grain boundary: 0).
Y. Linda et al. ! Solid State Ionics
1124 Table 1 Conductivity
(S/cm)
YSZ
of YSZ thick film and Sm,O,-doped Temperature
(“C)
1124
Grain boundary
5.3x 2.6X
1om4 10mh
8.7X
lo-”
80 energy (kJ/mol) (“C)
500 300 80 Activation
1121-
Grain
300
Temperature
(1996)
ceria
500
Activation Ceria
86-88
energy (kJ/mol)
peratures are summarized in Table 1. At temperatures as low as 350 K, the total conductivity of the specimen is dominated by that of the grain boundaries. The conductivity due to grain boundaries is almost lo2 times higher in YSZ film than in ceria ceramics at such low temperatures. However, ceria exhibited higher conductivity through the grains than the YSZ film (Fig. 5). This result could explain the thickness effect mentioned above; i.e., the total impedance of a thinner ceramic specimen is lower, because of a smaller number of resistive grain boundaries.
4. Conclusions YSZ and ceria bulk ceramics and an YSZ film were shown to be excellent pH sensors; these membrane electrodes exhibited almost ideal Nernstian response at high temperature (-80°C). The pH sensory characteristics of YSZ film electrode were superior to those of ceria ceramic electrode. This result was attributed to differences in the grainboundary conductivities of YSZ film and bulk ceria ceramic; the grain-boundary conductivity of the YSZ film is two orders of magnitude larger than that of the ceria ceramic at temperatures as low as 300 K.
1.2x
lo-3
Total 3.9x
101
1.0x lo-” 105
lomJ lo-” 5.0x lo-‘* 102
Grain
Grain boundary
Total
8.1 X10-j 2.6~ 10-l 5.0x lo-”
3.3x 1om4 8.0X lo-’ 3.6X lo-” 116
2.8 x 10m4 1.0x 10-h 1.2x lo-l2 108
69
5.5 x lo-6
2.0x
Acknowledgments This work was partly supported by the Ministry of Education, Culture and Science of Japan, and by the Center for Advanced Food Technology (CAFT) of the New Jersey Commission on Science and Technology. One of the authors (K.Y.) also expresses his thanks to Dr. K. Murata of Mitsui Engineering and Shipping for the supply of YSZ ceramic thick films.
References [l] S. Hettiarachchi, K. Makela, H. Song, D.D. Macdonald, J. Electrochem. Sot. 139 (1992) 139. [2] L.W. Niedrach, Science 207 (1980)1200. [3] T. Tsuruta, D.D. Macdonald, J. Electrochem. Sot. 129 (1982) 1221. [4] Y. Sugie, R. Kanda, S. Fujii, J. Ceram. Sot. Jpn. 10 (1992) 1216. [5] Da Yu Wang, D.S. Park, J. Griffith, A.S. Nowick, Solid State Ionics 2 (1981) 95. [6] G. Bryan Balazs, R.S. Glass, Solid State Ionics 76 (1995) 155. [7] R. Gerhardt, A.S. Nowick, J. Am. Ceram. Sot. 69 [9] (1986) 641. [8] Y.C. Zhou, M.N. Rahaman, J. Mater. Res. 8 (1993) 1680. 191 K. Yamashita, M. Greenblatt, K. Ramanujachary, Solid State Ionics 81 (1995) 53.
2%
SOLID STATE
ELSEVIER
Solid State Ionics 86-88
High temperature
of Industrial
Chemistry,
Faculty
Yamashitaa’*, of Engineering,
of Chemistry,
Rutgers,
Takao Umegaki”,
Tokyo Metropolitan
192-03, ‘Department
IONICS
1121-1124
pH sensitivities of stabilized zirconia films and ceria ceramics
Yasushi Inda”, Kimihiro aDepartment
(1996)
University,
Martha Greenblattb 1-I Minami-Ohsawa,
Hachioji,
Tokyo
Japan
the State University of New Jersey,
Piscataway,
NJ 08855-0939,
USA
Abstract Yttria-stabilized zirconia (YSZ) thick films and bulk ceria (CeO,) ceramics used as membrane electrodes are shown to work as high temperature pH sensors. The YSZ film electrode shows a nearly ideal Nemstian response of the potential as a function of pH with mV/pH = - 56.8 even at 353 K. The YSZ film electrode responded to pH changes within 30 s, while the response time of the samaria-doped ceria ceramic membrane electrode was 3 min. Complex impedance analysis revealed that the bulk conductivity of the ceria ceramics is higher than that of YSZ film, whereas the grain-boundary conductivity of the former was 10’ times lower than that of the latter. Based on these results, it appears that at temperatures below 600 K, the pH sensory characteristics of these materials are dominated by the grain-boundary conductivity. Keywords:
High temperature
pH sensor; YSZ film; Ceria ceramics;
1. Introduction Measurement of pH at high temperatures is of great importance in the fields of oil-, geothermal-, and nuclear-electric power generation plants, and food processing industries [l]. Monitoring of pH in building construction, plumbing, and in the evaluation of corrosion is important. Glass electrodes or ion selective sensors, because of surface corrosion in the severe environment of acidic and alkaline solutions at high temperatures, do not work as pH sensors. Recently the well-known fast oxide ion conductor YSZ, applicable for oxygen gas sensors and fuel cells, has been shown to work as a high temperature pH sensor as well [2]; a YSZ ceramic membrane *Corresponding
author
0167-2738/96/$15.00 Copyright PIZ SO167-2738(96)00279-2
Impedance
conductivity
grain boundary
electrode showed Nernstian response at high temperatures, above 523 K [3]. These electrodes, however, have been generally recognized to be unsuitable for pH measurements at lower temperatures, because of their high impedance. The primary purpose of the present work was to investigate the applicability of YSZ films as pH sensors at lower temperature. Some yttria-stabilized ceria ceramics, which were prepared from commercial powders, have been shown to follow Nernstian behaviour [4]. Moreover, since the conductivities of yttria-stabilized ceria ceramics are ten times higher than those of YSZ [5,6], we also studied the pH sensory characteristics of stabilized ceria ceramic membrane electrodes. At lower temperatures the electronic conductivity of ceramics, comprised of grains and grain boundaries, generally depends on the microstructure
0 1996 Elsevier Science B.V. All rights reserved
1122
Y. Inda et al. I Solid State Ionics 86-88
[7]. In order to obtain highly homogeneous ceria ceramics, we prepared ceria powders by hydrothermal synthesis [8,9]. Because we have not yet been able to prepare films of stabilized ceria, the present work employed bulk ceramic ceria samples.
(1996) 1121-1124
where soln.( 1) = 0.1 M KC1 as the intimal solution of the membrane electrode and soln.(2) =Sat. KC1 as the internal solution of the reference glass electrode.
3. Results and discussion 2. Experimental Dense YSZ thick films with 400 pm thickness were supplied by Mitsui Engineering and Shipping. Stabilized ceria ceramics were obtained by doping CeO, with 10 mol% Sm,O,. The polycrystalline samples were prepared hydrothermally, according to a method previously reported [9], which used reagent-grade Ce(NO,), .6H,O and SmCl, * 6H,O or SmCl, .6H,O powders. The hydrothermally synthesized powders were dried and calcined,and thereafter pressed into pellets with a diameter of 13 mm and a thickness of 0.6 mm. Sintering was performed at 1773-1863 K for 6 h. Powder X-ray diffraction analysis of all of the stabilized ceria preparations indicated single-phase cubic specimens. The densities of all specimens were above 95% of the theoretical value. The conduction properties of stabilized YSZ thick film and ceria ceramic specimens were measured by a 2-probe ac complex impedance method. The measurements were carried out in the frequency range 10 Hz to 10 MHz at 298 to 1273 K. The activation energies (E,) for ionic conduction were calculated from the Errhines plots for grain (bulk) and grain-boundary conduction. The pH sensing characteristics of the electrodes investigated were estimated by the measurements of the potential difference (AV) between the membrane electrodes and a reference electrode in various standard solutions. The membrane electrode consisted of a ceramic plate, or film adhered to a polypropylene tube, which was filled with an intimal solution of 0.1 M KCl, and Ag/AgCl internal electrode. A glass electrode with saturated KC1 as intimal solution was used as a reference electrode. The measurement system is summarized by the following cell: Ag/AgCl/soln.( soln.(2)/Ag/AgCl
l)/membrane/testsoln./Ref./ (I)
Fig. 1, shown as the time dependence of AV for pH =4.16, 6.86, and 8.88 at 353 K, demonstrates the response characteristics of YSZ thick film and ceria ceramic membrane electrodes. These results indicate the rapid pH response time of YSZ film electrode; the AV of YSZ film reached a constant value within 30 s in solutions of various pH. The ceria ceramic electrode also rapidly responded to pH changes within 3 min, and its AVvalue deviated only slightly from that of the YSZ-film electrode at higher and lower pH. In order to verify the validity of both membrane electrodes as pH sensors, the equilibrium constant values of V were measured in the range of pH=3 to 11 at temperatures 298, 340 and 353 K. Fig. 2 shows the relationship between AVand pH for the YSZ film membrane electrode. Although the change of AV is almost insensitive to changes of pH (pH) at room temperature, the slope of the line of AV vs pH increased with temperature, and the response at 353 K was ideally Nemstian; the measured value of AV/pH is -56.8 mV/pH, very close to the theoretical value of -59.0 mV/pH. This result
200
150
-150 012345678
Time (min)
Fig. 1. pH response as a function of time for YSZ thick film and 10 mol% Sm,O,-doped ceria ceramics at 353 K (YSZ: 0 0 a, ceria: n, 0, A, pH=4.16: 0, n, 6.86: 0, 0, 8.88 A A).
Y. Inda et al. I Solid State Ionics 86-88
(1996)
1121-1124
1123
moo
mooo
4oooo
3oooo
Z’ (ohm)
PH Fig. 2. pH response of YSZ thick film electrode temperatures (0 298 K, 0 343 K, 0 353 K).
at various
indicates that the working temperature of YSZ pH sensors is much lowered by thinning the YSZ ceramics, for the lowest temperature for bulk YSZ ceramic sensors has been reported to be 520 K [3]. Fig. 3 compares the pH response of a 10 mol% Sm,O,-doped ceria ceramic, which was sintered at 1863 K for 6 h, with that of a YSZ thick film at 353 K. Although the slope, AV/pH of the ceria electrode is -45.2 mV/pH, smaller than that of the YSZ film electrode, the pH response is linear and Nernstian. The response of ceria membrane electrodes was successfully improved by changing the sintering condition; for a 10 mol% Sm,O,-doped ceria sintered at 1863 K for 24 h, the value of AV/pH increased to -2.5 mV/pH. Thus the sensitivity of a ceramic membrane electrode is dependent upon the sintering conditions. This result explains the difference between the present observation and the previ-
Fig. 4. Typical Cole-cole doped ceria ceramics.
plot of complex impedance
for Sm,O,-
ously reported ideal Nernstian response of bulk ceria ceramics sintered at a higher temperatures, -2000 K 141. In order to understand the effect of the thickness of the membrane electrodes as well as the difference of YSZ film and ceria ceramic electrodes on pH sensitivity, the analysis was carried out at lower temperatures. The complex impedance loci of both YSZ film and ceria ceramic consisted of two arcs (Fig. 4). The impedance due to crystalline grains and the grain boundaries were calculated from the analysis of arc 1 and 2 in the figure. Based on these data, the Errhines plots of YSZ thick film and 10 mol% Sm,O,-doped ceria ceramic were made in Fig. 5. The conductivities (c) and activation energies (E,) of YSZ film and ceria ceramic at representative tem4 2OP d -2 c z -4 4 -6 -8 -
0.5
2
4
6
8
10
PH Fig. 3. pH response of YSZ thick film electrode ceramics at 353 K (YSZ: W ceria: 0).
1
2.5
3
12 and doped ceria
Fig. 5. Arrhenius plots of YSZ thick film and 10 mol% Sm,O,doped ceria ceramics (ceria grain: n, ceria grain boundary: 0, YSZ grain: 0, YSZ grain boundary: 0).
Y. Linda et al. ! Solid State Ionics
1124 Table 1 Conductivity
(S/cm)
YSZ
of YSZ thick film and Sm,O,-doped Temperature
(“C)
1124
Grain boundary
5.3x 2.6X
1om4 10mh
8.7X
lo-”
80 energy (kJ/mol) (“C)
500 300 80 Activation
1121-
Grain
300
Temperature
(1996)
ceria
500
Activation Ceria
86-88
energy (kJ/mol)
peratures are summarized in Table 1. At temperatures as low as 350 K, the total conductivity of the specimen is dominated by that of the grain boundaries. The conductivity due to grain boundaries is almost lo2 times higher in YSZ film than in ceria ceramics at such low temperatures. However, ceria exhibited higher conductivity through the grains than the YSZ film (Fig. 5). This result could explain the thickness effect mentioned above; i.e., the total impedance of a thinner ceramic specimen is lower, because of a smaller number of resistive grain boundaries.
4. Conclusions YSZ and ceria bulk ceramics and an YSZ film were shown to be excellent pH sensors; these membrane electrodes exhibited almost ideal Nernstian response at high temperature (-80°C). The pH sensory characteristics of YSZ film electrode were superior to those of ceria ceramic electrode. This result was attributed to differences in the grainboundary conductivities of YSZ film and bulk ceria ceramic; the grain-boundary conductivity of the YSZ film is two orders of magnitude larger than that of the ceria ceramic at temperatures as low as 300 K.
1.2x
lo-3
Total 3.9x
101
1.0x lo-” 105
lomJ lo-” 5.0x lo-‘* 102
Grain
Grain boundary
Total
8.1 X10-j 2.6~ 10-l 5.0x lo-”
3.3x 1om4 8.0X lo-’ 3.6X lo-” 116
2.8 x 10m4 1.0x 10-h 1.2x lo-l2 108
69
5.5 x lo-6
2.0x
Acknowledgments This work was partly supported by the Ministry of Education, Culture and Science of Japan, and by the Center for Advanced Food Technology (CAFT) of the New Jersey Commission on Science and Technology. One of the authors (K.Y.) also expresses his thanks to Dr. K. Murata of Mitsui Engineering and Shipping for the supply of YSZ ceramic thick films.
References [l] S. Hettiarachchi, K. Makela, H. Song, D.D. Macdonald, J. Electrochem. Sot. 139 (1992) 139. [2] L.W. Niedrach, Science 207 (1980)1200. [3] T. Tsuruta, D.D. Macdonald, J. Electrochem. Sot. 129 (1982) 1221. [4] Y. Sugie, R. Kanda, S. Fujii, J. Ceram. Sot. Jpn. 10 (1992) 1216. [5] Da Yu Wang, D.S. Park, J. Griffith, A.S. Nowick, Solid State Ionics 2 (1981) 95. [6] G. Bryan Balazs, R.S. Glass, Solid State Ionics 76 (1995) 155. [7] R. Gerhardt, A.S. Nowick, J. Am. Ceram. Sot. 69 [9] (1986) 641. [8] Y.C. Zhou, M.N. Rahaman, J. Mater. Res. 8 (1993) 1680. 191 K. Yamashita, M. Greenblatt, K. Ramanujachary, Solid State Ionics 81 (1995) 53.