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Ind. Eng. Chem. Prod. Res. Dev. 1903, 22, 594-599
Zirconia Membrane pH Sensors Leonard W. Niedrach' and Wlillam H. Stoddard General Nectric Corporate Research and Development, Schenectady, New York 1230 I
The usefulness of zirconia membrane sensors for the determination of the pH of high-temperature solutions is now well-established. Unlike sensors involving electron transfer couples, the membrane is insensitive to changes in the redox environment; e.@, changes in the molecular oxygen activity have no effect. Such sensors may also be useful for lower temperature measurements in highly alkaline media because of the absence of the "alkaline error" associated with the classical glass electrode. While present membranes suffer some degradation in response time upon aging, it appears likely that improvement will be possible with additional attention to purity of materials and detail of fabrication.
Introduction The use of zirconia ceramics in the fabrication of fuel cells and oxygen sensors has been well-known for years. More recently it has been shown that membrane electrodes fabricated from stabilized zirconia can be used for the measurement of pH (Niedrach, 1980a). This behavior, which resembles that of the classical glass electrode, has subsequently been investigated over a range of temperatures from ambient to about 285 "C with pressurized water (Niedrach, 1980b; Tsuruta and Macdonald, 1982) and simulated geothermal brines (Niedrach and Stoddard, 1983). At the higher temperatures few difficulties have been encountered and the response of the sensors has been close to theoretical. At lower temperatures-ambient to 150 OC-sluggish (Niedrach and Stoddard, 1981) and subtheoretical (Tsuruta and Macdonald, 1982) responses have been reported. The present paper addresses these lower temperature anomalies, which appear to reflect impurities and imperfections in the ceramic, and our approaches to overcoming them. In addition, an improved structure is described in which a brazed junction between the ceramic and a metal support replaces an earlier, less reliable compression seal. Description of the Sensor In the initial work sensors were fabricated in a fashion somewhat akin to that of the conventional glass electrode in that a buffered saline solution in contact with a chlorided silver wire served as the internal element withir the membrane sheath (Niedrach, 1980a,b). Since then we have adopted an internal element similar to those often employed in zirconia oxygen sensors, viz., a dry mix of a metal and its oxide or two oxides of the same metal in different valence states. For tests at high temperatures we have employed the copper-cuprous oxide couple, while mercury-mercuric oxide has been used in much of our work at 95 "C or lower temperatures. For measurements at the lower temperatures, when pressurization is not required, we prepare sensors by sealing the ceramic to lime or lead glass. This can be readily accomplished if care is taken to use a diffuse flame and to protect the ceramic with the aid of an intervening quartz shield. Mercury is then added to a depth of 1-2 in. and contact is made with a platinum wire. Attempts to establish a stable potential by adding mercuric oxide have been only moderately successful. A better approach has been to immerse the sensor and a platinum counterelectrode in a hot aqueous solution of dilute acid or base and electrolyze for several hours with the internal electrode of the sensor 30 to 100 V positive to the counterelectrode. Under these conditions a film of mercuric oxide evidently
forms at the ceramic-mercury interface to aid in the establishment of stable potentials. Alternatively, one may simply add the mercury and accept a slow drift in rest potential as oxide slowly forms over a longer period of time. This has been the approach used in much of the present work since the drift in the rest potential is slow compared with the responses of interest. The general structure of high-temperature sensors containing the copper/cuprous oxide internal element is shown in Figure 1. The nominal 'I4-in. 0.d. zirconia tube is retained in a Conax Type EG-125 Gland with a seal consisting of Teflon, Vespel, silver, and alumina elements as shown. The dry mix of copper and cuprous oxide powders'(1:l by weight) is packed around a copper wire in a 1-2 in. long section near the closed end of the zirconia tube. This defines the active region of the sensor. A backing of glass wool is then added to help confine the powder. Finally, the seal is assembled and tightened with a torque of about 50 in.-lb. We presently prefer this type of internal junction over the earlier aqueous system for several reasons: (i) we have found it to be readily prepared by simple mixing of the powders and packing into the tube; (ii) when prepared in this fashion it has been found to be extremely stable and reproducible; (iii) in contrast to aqueous internals, it permits ready designation of the active region of the sensor because it does not wet the wall with a conducting film; and (iv) in the absence of an internal aqueous phase, seal fabrication is simplified. This arrangement has proven highly satisfactory for laboratory work, but because of the thermal properties of the polymeric elements and their tendencies to flow and/or relax, care must be taken to keep them thoroughly confined. Even then, leaks are often encountered during cool-down unless the system pressure is reduced or care is taken to maintain the torque on the seal. For this reason we have developed a new design that should be more generally useful, particularly for field applications. A photograph of a sensor of this type appears in Figure 2, which also shows an optional protective shroud. Details of the brazed section and other aspects of the new structure are shown in the diagram in Figure 3. In this design the polymeric seal of the ceramic to the metal fitting has been replaced by a metallic braze. To maintain insulation, a Teflon compression seal has been retained where the copper wire emerges, but this is relatively small, it bears against the wire instead of a fragile ceramic, and it is in a relatively cool region. We have found that the thermal expansion coefficients of yttria stabilized zirconia and CP titanium are sufficiently well matched to tolerate the brazing conditions. As brazes we have used Ticusil and Ticuni (active metal
0 196-4321/83/1222-0594$01.50/00 1983 American Chemical Society
Ind. Eng. Chem. prod. Res. Dav.. Vol. 22. No. 4, 1983 595
l-- ~
~
,OXYGEN
ELECTR%MEMBRANE
w z
g o e
w -200
START PURGING ACID F E E D W I T H NITROGEN
AUTOCLAVE L
I HR
H
TIME
-
Figure 7. Behavior of a sensor, the oxygen electrode, and the autoclave as the redox potential of the environment is changed at 285 "C.
hours a more rapid drop occurred in the potentials of the autoclave and the platinum electrode. This is attributed to a change in the passivation reaction for the stainless steel autoclave. Instead of the passive film being maintained by the reduction of oxygen to water, water is reduced to liberate hydrogen. The platinum flag then clearly showed its sensitivity to this change in the redox environment. The oxygen ion conducting ceramic membrane, however, retained an essentially constant potential throughout the transients. The small, transient increase in potential is believed to have been a response to a real transient in pH associated with the change in the passivation reaction on the stainless steel. Looked at in another light, if the potential of the platinum electrode were referred to that of the pH sensor the response would have been identical with that of an oxygen sensor having a platinum electrode applied directly to the outside of the zirconia tube. Behavior at 95 "C. Because our original interest in the new sensor stemmed from our desire to measure the pH of water in nuclear reactor applications our attention has focused on performance at 285 "C with a minimum amount of attention being given to other temperatures. More recently we have attempted comparisons with commercial glass electrodes in the temperature range of overlapping capability. For this purpose 95 "C has been a convenient temperature because it permits operation a t ambient pressure without interference from boiling even when dilute solutions are used. Data comparing the response of a zirconia sensor with that of a general purpose (GP) glass electrode in the presence of 1 m sodium chloride are shown in Figure 8. They were obtained by slowly titrating an added increment of sodium hydroxide with hydrochloric acid over a period of 15 min using a motor driven syringe. The two curves were normalized in the acidic region because of the well-
Y
O I/d
-100
-400
-300 -200
-100 0 100 200 REFERENCE - m V
300
400
E G ~ ~ SvsS
Figure 9. Response of a sensor relative to that of a glass electrode at 95 "C.
known "alkaline error" associated with the general purpose type of glass electrode. A direct comparison of the voltage responses of a zirconia sensor and a glass electrode (this time a high alkalinity (HA) type) is shown in Figure 9. Here increments of 5 m sodium hydroxide were added a t approximately 5-min intervals to a solution that was initially about 0.0625 m in hydrochloric acid. Upon reaching the alkaline side-approximately 0.0875 m excess base-increments of acid were added to retrace the curve. It is seen that the correlation is excellent over the entire range of pH. This reflects the facts that a large excess of NaCl was not present and the HA type glass electrode is less subject to the alkaline error than the GP type. The behavior of this zirconia sensor is in complete agreement with earlier results that we have obtained at 95 OC (Niedrach, 1980b). In no case have we ever encountered an "acid error" as reported by Tsuruta and Macdonald (1982). Further data indicating that long life can be obtained with zirconia electrodes at 95 "C are summarized in Table I. These too were obtained by adding aliquots of base and acid but in increments of sufficient size to swing from acidic to basic pH and vice versa. Between increments the system was allowed to digest for several days in the presence of either acid or base. Conditions are indicated in the table. Although an HA type electrode was employed in this test it is seen that over a period of time, as sodium accumulated in the solution, a slight loss of response was observed in contrast to the behavior of the zirconia electrode. Upon replacing the solution with fresh, the full
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
Table I. Comparison of Sensor SP-4 with a Glass Electrode (HA Type) over a 30-Day Period of Continuous Operation at 9 5 "C (Run AP-25) standby conditions
response ( m V ) to pH changes a
day
solution
time
1
0.025 m HC1
-1h
A+B+A
2
0.025 m HCl
overnight
A+B-+A
3
0.025 m HCI
overnight
A+&A
7
0.025 m HCI
4 days
A+B-+A+B
10
0.1 m NaOH
3 days
BtA+B+A
13
0.025 m HCl
3 days
A-+B-+A+B
15
0.1 m NaOH
2 days
B-+A+&A
17
0.025 m HCl
2 days
A-+B+A-+B
22
0.1 m NaOH
5 days
B+A+B+A
30
0.025 m HCl
8 days
A-+B+A
0.025 m HCl
30
test cycles
-3 h
electrode
SP-4 glass SP-4 glass SP-4 glass SP-4 glass SP-4 glass SP-4 glass SP-4 glass SP-4 glass SP-4
B-A
678 676 672 680 670 677 670 674
67 2 681 669 683 665 674 665 670 639 676 664 669 674 676 655 67 1 67 5 676 628 667 683 677
662 671 653 67 2 627 672 683 681
SP-4 glass SP-4
A-+B-+A
A-B
A-B
659 672 650 673 6 64 669 670 671 659 670 672 672
B-A
64 8 67 1 66 5 671 669 673
a A = acid; B = base. After this set of measurements the system was rinsed and solution was replaced with fresh 0.025 m HCl; measurements were continued after temperature equilibrium had been reestablished.
-1
7
1
I
I
d I
I
I l l -&
------_A
'bl
3c
. -
DAY 3 0 - A F T E R fiEPLPClNG SOLJT O N
Figure 10. Comparison of responses of a zirconia sensor and a glass electrode to rapid changes in pH at 95 "C. TIME
Figure 12. Response of new sensor to smaller increments of 5 m HC1 and 5 m KOH; T = 95 OC.
4 i
IL '
*+iMKOH
OIMKOH
*bI MIN
Y
0 U
+ A
> 0 TIME
Figure 11. Effect of age on response rate of sensor to 25 mL increments of 5 m HC1 and 5 m KOH; T = 95 "C.
response of the glass electrode was again obtained. This is seen from the last set of data in the table. The transient responses of the glass electrode and the zirconia sensor on the first and thirtieth days are compared in Figure 10. Here it is seen that the time constant of the zirconia electrode is comparablewith that of the HA glass electrode. Over still longer periods of operation at 95 "C some loss of response rate has been encountered even with our best sensors. This is illustrated in Figure 11where the initial response to changes between 0.025 m HC1 and 0.1 m KOH is compared with that realized after about three months of use. The differences become more pronounced when smaller increments of acid and base are added as shown by the data in Figures 12 and 13. In contrast to the rapid response of the new sensor over the entire pH range, the aged sensor has become more sluggish. In the more acidic
t
Figure 13. Response of three month old sensor t o smaller increments of 5 m KOH; T = 95 "C.
and more basic regions, additions of acid or base result in responses that are only slightly slower than the original. In the mid-pH region, however, the response is considerably more sluggish and a sigmoidal break is seen in the potential-time plot. Similar behavior has been observed with sensors that have been operated at 285 "C for as long as two weeks. Upon cooling to 95 "C and testing a t this lower temperature, the theoretical voltage response to pH changes is still obtained, but even longer response times are observed. This behavior suggests that our solutions are slowly penetrating into grain boundaries or other discontinuities
Ind. Eng. Chem. Prod. Res. Dev.
in the structure and that functional groups are being titrated in the intermediate pH range. These functional groups may be derived from the slow hydrolysis of second phase materials resulting from impurities in the ceramic. For example, alumina could slowly hydrolyze to AlOOH (Kennedy, 1959) which would probably be titratable in the mid-pH region. Silica should also hydrolyze, but we would expect silicic acid to titrate in a more acidic region. Even in the absence of impurities, segregation of yttria in grain boundaries could have a similar effect since Y (0H)3is stable relative to Yz03in water under our conditions (Shafer and Roy, 1959). In contrast to the above, ZrOz is stable relative to the hydrolyzed forms (Nakamura et al., 1977; Tani et al., 1983). While we are apparently dealing with a real phenomenon here, it does not seem to be a critical one at present with our best ceramics. We have, however, seen similar behavior with other ceramics when the pause associated with the "titration" has been over an hour in duration. It is suggested that this type of behavior, rather than an "acid error" may have been the cause of the observations of Tsuruta and Macdonald (1982). Conclusions The use of zirconia membrane sensors for the determination of the pH of high-temperature solutions is a viable application. Such sensors may also be useful for lower temperature measurements in highly alkaline media because of the absence of the "alkaline error" associated with glass electrodes. While the characteristics of selected ceramics are adequate for present applications, slow degradation of response time is observed with even the best. It appears likely that further improvement will be possible with additional attention to purity of materials and detail of fabrication.
599
1983,22, 599-603
It would also be desirable to achieve higher conductivities. This might be achieved through the substitution of alternative stabilizers for the present yttria, perhaps scandia. It appears less likely that alternative systems based on ceria or bismuth oxide will prove attractive because of the aggressiveness of high-temperature aqueous environments. Registry No. Zirconia, 1314-23-4; yttria, 1314-36-9.
Literature Cited Danielson, M. J. Corrosion 1979, 35, 201. Dell, R. M.; Hooper, A. I n "Solid Electrolytes", Hagenmuller, P.; Vangool, W., Ed.; Academic Press: New York, 1978; p 291. Etsell, T. H.; Flengas, S. N. Chem. Rev. 1970, 70, 339. Indig, M. E.; McIlree, A. R. Corrosion 1979, 35, 268. Kennedy, G. C. Am. J . Sci. 1959, 257, 568. Nakamura, K.; Hirano, S . ; Somiya, S. Am. Ceram. SOC. Bull. 1977, 56, 513. Nledrach, L. W. Science 198Oa, 207, 1200. Niedrach, L. W. J . Nectrochem. SOC. 1980b, 127, 2122. Niedrach, L. W.;Stoddard, W. H. "The Development of a High Temperature pH Electrode for Geothermal Fluids. Final Report-Task I " , prepared for the Pacific Northwest Laboratory operated by Battelle Memorial Institute under prime contract No. DE-Ac-06-76-RLO-1830 for the United States Department of Energy, Division of Geothermal Energy, Report No. PNL 3857 UC-66, Mar 1981. Nledrach, L. W.; Stoddard, W. H. "The Development of a High Temperature pH Electrode for Geothermal Fluids. Final Report-Task I11 and Year-End Summary" prepared for the Pacific Northwest Laboratory operated by Battelle Memorial Institute under prime contract No. DE-AC-06-76-RLO1630 for the United States Department of Energy, Division of Geothermal Energy, Report No. PNL 4651, Feb 1983. Shafer, M. W.; Roy, R. J . Am. Ceram. SOC. 1959, 42, 563. Tani, E.; Yoshimura, M.; Somiya, S. J . Am. Ceram. SOC. 1983, 66, 11. Tsuruta, T.; Macdonald, D. D. J . Nectrochem. SOC. 1982, 729, 1221. Vetter, K. J. "Electrochemical Kinetics", Translated by S. Bruckenstein and B. Howard, Academic Press: New York, 1967; p 17 ff.
Receiued for review May 10, 1983 Accepted May 28, 1983 Presented a t the 85th Annual Meeting of the American Ceramic Society, Chicago, IL, Apr 26, 1983.
High-Temperature Electrical Conductivity of Rigid Polyurethane Foam Ralph T. Johnson, Jr. Sandia National Laboratories, Albuquerque, New Mexico 87 185
The electrical conductivity of a rigid polyurethane foam, used for electronic encapsulation, has been measured (ohm-cm)-l but it increases during thermal decom osition to 340 O C . The conductivity below -150 O C is dramatically to IO- (ohm-cm)-' as the temperature is increased to 260-270 OC. At higher temperatures the conductance continues to increase, but some of this increase is due to changes in sample geometry resulting from sample softening. Experiments in air and nitrogen environments showed no significant dependence of the conductivity on the atmosphere. The insulatingcharacteristics are compared to those of phenolic- and silicone-based materials (glass-fabric and glass microsphere reinforced) which are used for electronic case housings and component encapsulation. The insulating characteristics to -200 O C for the polyurethane foam are similar to those of the silicones and are better than that of the phenolics. At higher temperatures (2270 "C)the phenolics and particularly the silicones appear to be better insulators.
-
?!
Introduction Rigid polyurethane foams have a number of important applications including encapsulants for electronics components. For situations where there may be an accidental exposure to a high-temperature environment, it is im-
portant to know how the electrical insulating characteristics of the foam degrade with temperature. This paper examines the change in the electrical conductivity of a rigid polyurethane foam, commonly used as an encapsulant, over the temperature range from 24 to 340 O C in air and ni-
0196-4321/83/1222-0599$01.50/0@ 1983 American Chemical Society
