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Conductivity measurements of β″-alumina type NH+4-gallate by dc four-terminal method
Solid State lonics 35 (1989) 217-222 North-Holland. Amsterdam
CONDUCTIVITY MEASUREMENTS BY dc F O U R - T E R M I N A L M E T H O D
OF [3"-ALUMINA TYPE NH2-GALLATE
H. IKAWA, K. S H I M A ¢~, T. T A N I G U C H I , K. U R A B E 2, O. F U K U N A G A Department of Inorganic Materials, Tokyo Institute of Technology, O-oka),ama, Meguro-ku, Tokyo 152, Japan
and J. M I Z U S A K I Institute o/Environmental Science and Technology, Yokohama National University, Hodogaya-ku, Yokohama 240, Japan
Received 17 October 1988
Electrical currents and potential differences in NH + -[3"-gallatesingle c~,stals doped with ZnO were measured in various atmospheres using sputtered terminal electrodes and Pt wire probes. The currents were functions of the atmosphere, applied voltage, electrode material, and time. Small currents experienced in all of the measurements were attributed to thc low activity of the electrodes. The ionic conductivity of the crystal, evaluated from the current and potential difference, was very.much smaller than that reported from complex impedance measurements. The inconsistency was explained by a depressed gradient of electrochemical potential of the conducting ion(s).
1. Introduction Protonic ion conductors based on [3- and [3"-alum i n a s wherein N a + ions are replaced by H 3 0 + a n d / or N H + ions have been extensively studied because of their potential applications as electrolytes in fuel cells, water dissociation cells, and sensors. Though m a n y studies [ 1-6 ] have been m a d e o f protonic [3and [3"-alumina, reported conductivities are inconsistent. This is due to the difficulties in p r e p a r i n g the specimens [7,8]. Since 13- and [3"-alumina crystals are cleaved during the ion exchange by the larger protonic ions. The gallium analogues o f J3- and 13"a l u m i n a ( a b b r e v i a t e d as [3- and [3"-gallate) have b r o a d e r c o n d u c t i o n planes accept protonic ions without serious d e l a m i n a t i o n . The ionic conductivities o f NH2-[3-gallate and NH2-13"-gallate, measured in high quality single crystals, have been reported to be 5 X 1 0 5 S / c m at 2 0 0 ° C [71 and ~¢ Present address: Mitsubishi Chemical Industry Ltd., Yokohama 227. Japan. Present address: Ryukoku University, Otsu-shi 520-21 Japan.
4.4X 10 -2 S / c m at 190°C [8,9], respectively. The electrochemical m e a s u r e m e n t s using protonic [3"-alumina and [3"-gallate electrolytes are limited in spite o f their high conductivities. The difficulty in fabrication o f electrolyte ceramics [ 10,1 1 ] may be the m a i n reason for this restriction. Nicholson et al. have succeeded in designing polycrystalline H 3 0 +[3/13"-alumina, and have p e r f o r m e d steam electrolysis [ 10] and p r o d u c e d a H 2 / 0 2 fuel cell [ 12]. Tsurumi et al. have fabricated NH+-[3"/13-gallate ceramics, and have measured E M F s o f hydrogen [ 11 ] and steam [ 13 ] concentration cells. The conductivities o f those polycrystalline electrolytes, especially those o f the latter type [ 11,13 ] were so small as to give some doubt about the high conductivity in the crystal. This p a p e r reports one o f two studies in which the ionic conductivity in N H +-[3''-gallate has been ree x a m i n e d using a complex i m p e d a n c e technique [14]. High conductivities o f the previous reports [8,9] have been reconfirmed, and low conductivities o f polycrystalline bodies have been attributed to the extremely large grain b o u n d a r y resistance. The
218
11. lkawa el al./fl"-alumina type N t t J -~allal('
present study reports on conductivity m e a s u r e m e n t s by a d c four-terminal method in various atmospheres.
2. Experimental
2. 1. Specimen co'stals The specimen crystals were that prepared by Ikawa et al. [14]: single crystals o f Na+-[3"-gallate d o p e d with ZnO were synthesized by the alkali evaporation m e t h o d [ 15] from reagents Ga~O3, Na2CO3, and Z n O mixed in a m o l a r ratio o f 5.00:2.80:0.70. Single crystals of the Na + type ca. 10 m m in the longest d i m e n s i o n were converted into Rb + type before the final exchange into N H ~ type in molten NH4NO3. The chemical c o m p o s i t i o n s o f crystals were analyzed by means of SEM installed with an EDX apparatus (Seiko E G & G , SED 8600). The estimated chemical formulae of N H 2 -13"-gallate was (NH4)~s,: ,(H~O),Rbo/liGalolZn<,~Oi7 [14], according to the reported expression form [ 16,17 ]. Single crystals o f high quality were picked up under a microscope and Laue photographs of them were taken to identify symmetry. Platinum was sputtered onto two opposite faces normal to the (001) plane o f rectangular crystals. The thickness of the sputtered Pt estimated from its a p p e a r a n c e was ca. 20 nm. The sizes of the crystals were 0.421 by 2.4 by 4.0 m m (sample no. 1), 0.873 by 2.3 by 3.0 m m (sample no. 2) and 0.115 by 1.6 by 2.0 m m (sample no. 3). The electrode material sputtered onto the no. 3 crystal was Au.
2.2. Four-lerminal dc measurement Platinum wires o f 0.2 m m in d i a m e t e r were fixed by Pt paste (JMC, K T - 5 ) onto the sputtered terminal faces. The two probes to measure potential differences were Pt wires of 0.1 m m in diameter. They were fixed using the Pt paste onto the two faces o f the crystals with separations o f ca. 1.2 ram. The specimen was fixed in a Pyrex glass tube o f 20 m m in d i a m e t e r and various gases were introduced into the tube. The t e m p e r a t u r e o f the specimen was kept at 120°C. Gas a n d / o r m i x e d gas from containers were allowed to pass through flow meters and a gas mixer. The total flow rate was 60 m ~ / m i n in general,
and was 120 m ~ / m i n for the mixed gas c o m p o s e d of argon, water and a m m o n i a . The end o f the gas tube was open to the atmosphere. The partial water vapor pressure was controlled by bubbling the carrier gas - argon or a r g o n / h y d r o g e n - through gas syringes filled with water held at a given temperature. A constant dc potential was supplied to the specimen through a power source (YHP. 6114A). The current and potential difference between the two probes were m o n i t o r e d by an a m m e t e r (Toa Denpa, P M I 8 C ) and a voltmeter with an i m p e d a n c e larger than 1000 M ~ ( T a k e d a Riken, T R 6 8 4 1 ) . U n d e r fixed conditions, the current was a function o f time; it came to a steady value after a duration o f two to three hours. The potential difference was unstable and was affected by marginal changes in experimental conditions. The two values obtained by exchanging the polarities of dc supply were averaged. The surface currents measured using the probe wire were less than 1.5 nA.
2.3. Slep and pulse methods A rectangular pulse of a constant current (0.1-0.4 ~A, 0.1-1 s) was supplied using a galvanostat (Hokuto Denko, HA-301 ) and a pulse generator with a nominal rising edge less than 0.3 ~as ( H o k u t o Denko, HB-211), and the potential difference between the terminal electrodes of the specimen was m o n i t o r e d by a synchroscope with a rising edge less than 0.03 ps (Sony Tektronix, 485A). Pulses of various quantities o f electricity prepared by capacitors ( 10, 470, 570, 1000 p F ) and a dc generator ( 1-6 V ) were supplied to the specimen or the specimen and a capacitor (200 p F ) connected in series and the potential differences across the specimen and capacitor were m o n i t o r e d by the two synchroscopes. No discrete IR j u m p which might enable the resistance of the specimen to be evaluated was recorded by these measurements. Complex ion relaxation processes of the specimen [14] may be the reason for this, but further e x a m i n a t i o n is needed to explain these results.
tt. I k a w a el a l . / f l " - a l u m i n a type Y H + -q,allate
3. Results Fig. 1 shows part o f continuous records o f the current and potential difference before and after a stepwise change o f the applied voltages to the terminal electrodes from - 2 . 0 V to 1.7 V. The partial water pressure was 25.6 kPa, and it was balanced by argon with a m b i e n t pressure. A pulse-like increase in current is followed by a parabolic decrease, and the current comes to a constant value after two to three hours. A similar pulse-like increase in counter current was recorded when the terminal electrodes were short-circuited after a measurement. A polarization o f the specimen is evident from these results: i.e. a non-uniform distribution o f electric charge carriers occurred in the crystal. Thc steady state current and potential difference were functions o f e n v i r o n m e n t a l gas, applied voltage and the material of electrodes. Two o f the measurements are shown in fig. 2, and table 1 lists some sets o f those data together with apparent c o n d u c t i v i t y calculated from the current and potential difference. A n o n - o h m i c relation between the applied voltage and current is evident from fig. 2A. Similar nono h m i c relations were seen for all the m e a s u r e m e n t s in argon containing water and w a t e r / a m m o n i a . The applied voltage at the turning point o f the relation was ca. 1.5 V for the m e a s u r e m e n t s using Pt electrode (sample no. 1, 2), and was ca. 2.6 V for that using Au electrode (sample no. 3). The n o n - o h m i c relations were less evident for the m e a s u r e m e n t s in
219
argon and argon containing hydrogen or a m m o n i a (table 1 ). And an almost ohmic relation was seen for the m e a s u r e m e n t s in argon containing water and hydrogen (fig. 2B). The current was also affected with the kind o f gas and its partial pressure. The current increased nearly linearly with partial pressure o f water in argon containing water within the limit o f this experiment (table 1). The roughly estimated normalized current under high applied voltages ( 1.8-2.0 V) was largest in the a t m o s p h e r e of argon containing water and hydrogen, and smallest in argon (table l ). Conductivities are also listed in table 1. Like the current, the conductivity was also a function o f applied voltage. Further, it increased with the current. The largest conductivity recorded in this experiment was 2 . 4 × 10 s S / c m in argon containing water and hydrogen under an applied voltage 1.9 V (table 1 ). The value is significantly different from the reported value 3.5 × 10 3 S / c m [ 14] at 120°C. It is clear from these results that we have failed to evaluate conductivity o f this crystal by the m e t h o d employed. Incidentally, the conductivities calculated from the currents and applied voltages were far smaller than those listed in the table, which were calculated from the currents and potential differences, except the conductivities obtained in argon containing water and hydrogen.
4. Discussion
4.1. Activity of electrode SampLe No.1 500
1
v
:=L
I
0
< \3 <
-500 Ar+HaO/25.6kPa
~)
510 1130 Time /rnin
1;0
Fig. 1. Part of" c o n t i n u o u s records of the current and potential difference before and after a stepwise change of the applied voltages from - 2 . 0 to 1.7 V m e a s u r e d at 120=C in argon c o n t a i n i n g water with a partial pressure 25.6 kPa.
It is clear from these results that the current reflects the ability o f electrode to make the charge carrying ion (s). Within the limit o f this experiment, the activity increases with the applied voltage, and it is largest in argon containing water and hydrogen, and smallest in argon. The small current recorded in argon may be caused by a trace a m o u n t of water introduced into the system a n d / o r partial d e c o m p o sition of the crystal under the higher applied voltages. The applied voltages o f 1.5 V and 2.6 V at the respective turning points o f the V-I relations using the Pt and Au electrodes, suggest that the d e c o m p o s i t i o n o f water at the electrode is accelerated at higher applied voltages than the turning point. There m a y be
220
II. lkawa el al./fl" alumina O'pe N i l ) -gallale
4.0 Sampte No.2
( B } - 1000
Ar+He/18.2kPa +H20/lO'OkPa / I • ,
1.4
Sample No.2 Ar+H20/25.6kPa
900 800
(-~} 3.0
700
1.2
600
1.0
500 /
.~0.8
,"
:L _" 2.0
i
600 <
/
500 \ 3 < 4OO
• 400 <
ii •
0.6
5 0 0 ~<
5OO 1.0
0.4
2OO
0.2
tO0
0
0.5 1.0 1.5 2.0 Applied Vo[fege / V
0
2OO I00 i
0
F i g . 2 . Current and potential difference as functions of applied voltage measured v, atcr/h?drogen ( B ).
i
i
i
0.5 1.0 1.5 2.0 Apptied Vottage / V
at 120
C in
0
argon containing water (A), and containing
Table 1 Some selected relations among composition of gas, applied voltage to the terminal electrodes, current, potential difference between the probes and apparent conductivit.x. The electrode materials are PI for samples no. 1/no. 2, and Au for sample no. 3. The gas is balanced by' argon with the ambient pressure. Sample no.
Gas composition
Applied voltage ( V ) - current -potential diflk~rence ( mV ) - apparent conductivity (10 ~Scm i)
Ar+ H20/10.0kPa Ar+ H-O/25.6kPa
1.9-230nA-260-1.1, 1.7-92nA-230-0.5, 1.5-36nA-240-0.2, 1.0-9nA-153 1.95 590nA-370, 1.7-150nA-360, 1.3-48nA-290, 1.0-33nA-380, 0.5-10nA 220, 1.3-69nA-400, 1.7-190nA-360 1.9-680nA-390-2.1, 1.6-100nA-340-0.4, 1.3-30nA-240-0.1, 1.0 14nA-140-0.1 1.9-1.0/c~k-480 1.2, 1.6-350nA-420 0.5, 1.3-150nA-300-0.3, 1.0-77nA-200-0.2 1.0-1 hA, 1.5-2nA, 2.0-8nA, 2.2-13nA, 2.4-28nA, 2.8-110nA, 3.0-360nA 2.0-3.7#A-870-2.3, 1.9 3.5l~A-810-2.4, 1.6-2.71~A-670-2.2. 1.3-2.1/zA-580-2.0, 1.0 1.21~A-430-1.5,0.6 610nA-270-l.2 1.9-130nA 370-0.2,1.8-96nA 320-0.2. 1.6-50nA-250 0.1,1.2-16nA-110, 0.8-TnA 9 1.9-590nA-390-0.8, 1.6-400nA-310-0.7, 1.3-230nA-280-0.4, 1.0-130nA-220-0.3 1.9-33nA-380,1.6-22nA-300, 1.3 14nA-220, 1.0-9nA-150 1.9-43nA-410,1.6-21nA-320, 1.3-10nA-220, 1.0 4nA-150
Ar+ H20/35.5kPa Ar+ H,O/25.6kPa A r + H 2 0 10.0kPa A r + H 2 0 I 0.0kPa + He/18.2kPa Ar+ H_~O/5.0kPa +NH~/2.5kPa Ar+H,/20.3kPa Ar+NH~/5.1kPa Ar
no critical voltage for the decomposition of hydrogen. That may be the reason why the V-1 relations in gases containing hydrogen are different from those
in argon containing water. In any case, the polarizations detected show that the ability of the electrode to make conducting i o n ( s ) is insufficient.
H. Ikawa et al./I3"-alumina type NH~ -gallale
4.2. Reason for the failure of the conductivity measurement Yokota [ 18 ] has developed a theory of mixed conduction - electronic and ionic conductions - based on the Hebb-Wagner dc polarization theory [ 19,20]. He has derived expressions for the two potential distributions - electrochemical potentials of electrons and ions measured by the use of probes consisting of an electronic conductor and an ionic conductor, respectively. Couturier et al. [21] have proposed a model to explain the current variation versus time for an ionic conductor having the very low mobility of electrons and have derived a method of ionic conductivity measurement using Pt probes to detect the electrochemical potential of electrons. The method employed in this study is identical to the latter, and the electronic conductivity of NH~-[3"-gallate is also very low [2,5]. We have failed, however, in ionic conductivity measurement. The reason of the failure is discussed next. Fig. 3 shows potential distributions in three cells Pt/ionic c o n d u c t o r s / P t at a time zero when submitted to a voltage step and after a duration reaching
-J
Agl.93Te
.......
e
Ag*,t
. . . . . . .
.... e
"
I
_e[_~.
---.
.......
e
_e~..,,
PbF 2 --
--4e
+
_
+
NH~,-,B"-GaLLote ""---~2
p",
.....
go
.....
.
q ~ p".- ~le
Fig. 3. Distributions of chemical potentials of neutral species/~R and electrochemical potentials of electrons qe in the cells Pt/ Ag, ,,~Te/Pt (silver ion and electron conductor), PI/PbF_,/Pt (fluoride ion conductor) and Pt/NH~-13"-gallate/Pl (protonic ion (P+) conductor) at a time zero submitted to a voltage step (left) and at a quasi-equilibrium state (right). The vertical line represents the potential and electrode.
221
a quasi-equilibrated state. The decomposition of the specimen caused by a large applied voltage is neglected in this discussion. At a time when a voltage is applied to Ag~ ~,3Te, Ag + ions and electrons start to move owing to the gradients of electrochemical potentials o f A g + ion (~/Ag + ) and electron (qe), respectively. However, the chemical potential of neutral silver/tAg is uniform within the specimen. After a while the gradient of ~/Ag+ and consequently the flux of Ag + ions vanishes because of the blocking character of the Pt electrode. At a quasi-equilibrium state there is a next relation among the three potentials. /tAg = ~/Ag+ + tle.
( 1)
The two Pt probes are in equilibrium with the respective q values of the specimen at points x~ and x> The potential difference between the two probes gives the value of the electrochemical potentials difference between x~ and x_,. We can evaluate the conductivity of electrons by using the potential difference. For the case of PbF> the electronic conduction is negligible and the gradient of composition is also negligible except for at interfaces because of the low applied field and stoichiometric character of PbFe [21 ]. After the voltage step is applied, ions move to create space charges and to cancel the electrostatic field. Finally the current at the interfaces is due to electron tunneling between the electrode and localized states in the gap, through the narrow potential barrier at each interface. The current in the bulk is purely ionic and cancels the charge of tunneling electrons [21]. In this state it has been said that /tF in the bulk is uniform and as a result, the difference between ~IF-(,v~ ) and qF (.v~) is equal to the difference between qe(x~) and ~le(x_,). Accordingly, the ionic conductivity can be calculated by using the potential difference of electrons. The charge carriers of protonic 13- and /3"-aluminas/gallates have been provided to be protons, H~O + a n d / o r NH~- ions, but no definite determination of the carriers and their transference number have been made [ 11 ]. In this discussion, accordingly, the protonic charge carrier(s) in NH+-6"-gallate is expressed tentatively by the P+ ion. At the beginning /tP of the specimen is uniform and in equilibrium with the gas phase. When the voltage step was applied, the large current recorded was due to the
222
1t. lkawa el a/./fl' alumina type N l t j -ga//ate
m o v e m e n t o f P + ions. The current was pulse-like because the ability of the electrode to make P* ions was very low. The transient current shown in fig. 1 is similar to that reported for the partially ion blocking P t / A g B r / A g cell [22 ], and it may be also similar in having composition gradients. At a quasi-equilibrated state, the gradient of r/P + in NH2-13"-gallate is depressed and the gradient of #P is close to that of ~le (fig. 3) depending on the equation. ~LP= qP + + ~le -
(2 )
Unlike the case fo PbF> the difference between 71e(.v~ ) and ~le(xe ) of this case is much larger than the difference between J/P+ (.v~) and ~IP ~ (.v,). This is the reason of the failure in conductivity measurements. The two reasons are listed for the generation of the composition gradient. One is the higher applied voltage, and the other is a character of NH4+ -13"-gallate. The number of cations in the conduction plane of 13and ~"-alumina structures are sensitive to the experimental conditions. Especially, protonic 13- and 13"-alumina c o m p o u n d s change their compositions around the conduction planes during the ion exchange processes at a low temperature (ca. 2 0 0 : C )
[17,23]. The time spent amounting two to three hours before the current settles (fig. t ), may be the time spent in changing the structure to a c c o m m o d a t e the chemical potential(s) of protonic species. The conductivit}' of the four terminal method in argon containing water and hydrogen almost coincided with that of the two terminal method because a rather high activity of the electrode may not generate a localized state of potential at the interface.
Acknowledgement The authors are grateful to Prof. T. Maruyama of this Institute for his nice suggestions. Part of this work is supported by the Japan Securities Scholarship Foundation.
References
[ 1 ] N. Baffler, J.C. Badot and Ph. Colombam Solid State lonics 13(1984) 233. [ 2 ] Ph. Colomban. J.P. Boilot, A. Kahn and G. Lucazeau. Nouv. J. Chim. 2 (1978) 21. [3 ] N. Baffler, J.C. Badot and Ph. Colomban, Solid State lonics 2 (1980) 107. [4] G.C. Farringlon and J.k. Briant, Mat. Rex. Bull. 13 (1978) 763. [5] G.C. Farrington and J.L. Briant, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy ( North-Holland, Amsterdam. 1979 ) p. 395. [6] A. Teitsma, M. Sayer. S.L Segel and P.S. Nicholson, Mal. Res. Bull. 15 (1980) 1611. [ 7 ] H. Ikawa. T. Tsurumi, K. Urabe and S. Udagawa, Solid State tonics 20 (1986) 1. [8] T. Tsurumi, H. Ikawa, K. Urabe, T. Nishimura, T. Oohashi and S. Udagawa, Solid State lonics 25 (1987) 143. [ 9 ] H . Ikawa, T. Tsurumi, T. Oohashi, K. Urabc and S. Udagawa, J. Ceram. Soc. Japan. 92 ( 1984 ) 473. [10] P.S. Nicholson, M.Z. Munshi, G. Singh, M. Sayer and M.F. Bell, Solid State lonics 18/19 (1986) 699. [ I I ] T . Tsurumi, H. Ikawa, M. lshimori, K. Urabc and S. Udagawa, Solid State lonics 21 (1986) 31. [12]M.Z. Munshi and P.S. Nicholson. Solid State lonics 23 (1987) 203. [13] H. Ikawa, T. Oohashi, M. lshimori. T. Tsurumi. K. Urabe and S. Udagawa, in: High tech ceramics, ed. P. Vincenzini ( Elsevier Science Publishers, Amsterdam, 1987 ) p. 2137. [14] H. Ikawa, K. Shima, T. Tsurumi and O. Fukunaga, Proc. Intern. Seminar on Solid State Ionic Devices. eds. B.V.R Chowdari and S. Radhakirshna (World Scientific. Singapore. 1988) p. 497. [15] I_.M. Foster and J.E. Scardefield, J. Electrochem. Soc. 124 (1977) 434. [ 16] J.O. Thomas and G.('. Farrington, Acta Cryst. B39 (1983) 227. [17]T. Tsurumi, H. lkawa, T. Nishimura, K. Urabe and S. Udagawa, ,I. Solid State Chem. 71 ( 1987 ) 154. [ 18] 1. Yokota, J. Phys. Soc. Japan. 16 ( 1961 ) 2213. [ 19] M. Hebb, J. Chem. Phys. 20 (1952) 185. [20] C. Wagner, Proc. V.I.T.('.E. 7 (1955) 361. [21 ] G. ('outurier, J. Salardenne. ('. Sribi and M. Rosso, Solid State lonics 9/10 ( 1983 ) 699. [22 ] ,1. Mizusaki, J. Sasaki. S. Yamauchi and K. Fueki, Solid State lonics 7 (1982) 323. [23]H. Ikawa, T. Tsurumi, M. lshimori, K. Urabe and S. Udagawa, J. Solid State Chem. 60 (1985) 51.
CONDUCTIVITY MEASUREMENTS BY dc F O U R - T E R M I N A L M E T H O D
OF [3"-ALUMINA TYPE NH2-GALLATE
H. IKAWA, K. S H I M A ¢~, T. T A N I G U C H I , K. U R A B E 2, O. F U K U N A G A Department of Inorganic Materials, Tokyo Institute of Technology, O-oka),ama, Meguro-ku, Tokyo 152, Japan
and J. M I Z U S A K I Institute o/Environmental Science and Technology, Yokohama National University, Hodogaya-ku, Yokohama 240, Japan
Received 17 October 1988
Electrical currents and potential differences in NH + -[3"-gallatesingle c~,stals doped with ZnO were measured in various atmospheres using sputtered terminal electrodes and Pt wire probes. The currents were functions of the atmosphere, applied voltage, electrode material, and time. Small currents experienced in all of the measurements were attributed to thc low activity of the electrodes. The ionic conductivity of the crystal, evaluated from the current and potential difference, was very.much smaller than that reported from complex impedance measurements. The inconsistency was explained by a depressed gradient of electrochemical potential of the conducting ion(s).
1. Introduction Protonic ion conductors based on [3- and [3"-alum i n a s wherein N a + ions are replaced by H 3 0 + a n d / or N H + ions have been extensively studied because of their potential applications as electrolytes in fuel cells, water dissociation cells, and sensors. Though m a n y studies [ 1-6 ] have been m a d e o f protonic [3and [3"-alumina, reported conductivities are inconsistent. This is due to the difficulties in p r e p a r i n g the specimens [7,8]. Since 13- and [3"-alumina crystals are cleaved during the ion exchange by the larger protonic ions. The gallium analogues o f J3- and 13"a l u m i n a ( a b b r e v i a t e d as [3- and [3"-gallate) have b r o a d e r c o n d u c t i o n planes accept protonic ions without serious d e l a m i n a t i o n . The ionic conductivities o f NH2-[3-gallate and NH2-13"-gallate, measured in high quality single crystals, have been reported to be 5 X 1 0 5 S / c m at 2 0 0 ° C [71 and ~¢ Present address: Mitsubishi Chemical Industry Ltd., Yokohama 227. Japan. Present address: Ryukoku University, Otsu-shi 520-21 Japan.
4.4X 10 -2 S / c m at 190°C [8,9], respectively. The electrochemical m e a s u r e m e n t s using protonic [3"-alumina and [3"-gallate electrolytes are limited in spite o f their high conductivities. The difficulty in fabrication o f electrolyte ceramics [ 10,1 1 ] may be the m a i n reason for this restriction. Nicholson et al. have succeeded in designing polycrystalline H 3 0 +[3/13"-alumina, and have p e r f o r m e d steam electrolysis [ 10] and p r o d u c e d a H 2 / 0 2 fuel cell [ 12]. Tsurumi et al. have fabricated NH+-[3"/13-gallate ceramics, and have measured E M F s o f hydrogen [ 11 ] and steam [ 13 ] concentration cells. The conductivities o f those polycrystalline electrolytes, especially those o f the latter type [ 11,13 ] were so small as to give some doubt about the high conductivity in the crystal. This p a p e r reports one o f two studies in which the ionic conductivity in N H +-[3''-gallate has been ree x a m i n e d using a complex i m p e d a n c e technique [14]. High conductivities o f the previous reports [8,9] have been reconfirmed, and low conductivities o f polycrystalline bodies have been attributed to the extremely large grain b o u n d a r y resistance. The
218
11. lkawa el al./fl"-alumina type N t t J -~allal('
present study reports on conductivity m e a s u r e m e n t s by a d c four-terminal method in various atmospheres.
2. Experimental
2. 1. Specimen co'stals The specimen crystals were that prepared by Ikawa et al. [14]: single crystals o f Na+-[3"-gallate d o p e d with ZnO were synthesized by the alkali evaporation m e t h o d [ 15] from reagents Ga~O3, Na2CO3, and Z n O mixed in a m o l a r ratio o f 5.00:2.80:0.70. Single crystals of the Na + type ca. 10 m m in the longest d i m e n s i o n were converted into Rb + type before the final exchange into N H ~ type in molten NH4NO3. The chemical c o m p o s i t i o n s o f crystals were analyzed by means of SEM installed with an EDX apparatus (Seiko E G & G , SED 8600). The estimated chemical formulae of N H 2 -13"-gallate was (NH4)~s,: ,(H~O),Rbo/liGalolZn<,~Oi7 [14], according to the reported expression form [ 16,17 ]. Single crystals o f high quality were picked up under a microscope and Laue photographs of them were taken to identify symmetry. Platinum was sputtered onto two opposite faces normal to the (001) plane o f rectangular crystals. The thickness of the sputtered Pt estimated from its a p p e a r a n c e was ca. 20 nm. The sizes of the crystals were 0.421 by 2.4 by 4.0 m m (sample no. 1), 0.873 by 2.3 by 3.0 m m (sample no. 2) and 0.115 by 1.6 by 2.0 m m (sample no. 3). The electrode material sputtered onto the no. 3 crystal was Au.
2.2. Four-lerminal dc measurement Platinum wires o f 0.2 m m in d i a m e t e r were fixed by Pt paste (JMC, K T - 5 ) onto the sputtered terminal faces. The two probes to measure potential differences were Pt wires of 0.1 m m in diameter. They were fixed using the Pt paste onto the two faces o f the crystals with separations o f ca. 1.2 ram. The specimen was fixed in a Pyrex glass tube o f 20 m m in d i a m e t e r and various gases were introduced into the tube. The t e m p e r a t u r e o f the specimen was kept at 120°C. Gas a n d / o r m i x e d gas from containers were allowed to pass through flow meters and a gas mixer. The total flow rate was 60 m ~ / m i n in general,
and was 120 m ~ / m i n for the mixed gas c o m p o s e d of argon, water and a m m o n i a . The end o f the gas tube was open to the atmosphere. The partial water vapor pressure was controlled by bubbling the carrier gas - argon or a r g o n / h y d r o g e n - through gas syringes filled with water held at a given temperature. A constant dc potential was supplied to the specimen through a power source (YHP. 6114A). The current and potential difference between the two probes were m o n i t o r e d by an a m m e t e r (Toa Denpa, P M I 8 C ) and a voltmeter with an i m p e d a n c e larger than 1000 M ~ ( T a k e d a Riken, T R 6 8 4 1 ) . U n d e r fixed conditions, the current was a function o f time; it came to a steady value after a duration o f two to three hours. The potential difference was unstable and was affected by marginal changes in experimental conditions. The two values obtained by exchanging the polarities of dc supply were averaged. The surface currents measured using the probe wire were less than 1.5 nA.
2.3. Slep and pulse methods A rectangular pulse of a constant current (0.1-0.4 ~A, 0.1-1 s) was supplied using a galvanostat (Hokuto Denko, HA-301 ) and a pulse generator with a nominal rising edge less than 0.3 ~as ( H o k u t o Denko, HB-211), and the potential difference between the terminal electrodes of the specimen was m o n i t o r e d by a synchroscope with a rising edge less than 0.03 ps (Sony Tektronix, 485A). Pulses of various quantities o f electricity prepared by capacitors ( 10, 470, 570, 1000 p F ) and a dc generator ( 1-6 V ) were supplied to the specimen or the specimen and a capacitor (200 p F ) connected in series and the potential differences across the specimen and capacitor were m o n i t o r e d by the two synchroscopes. No discrete IR j u m p which might enable the resistance of the specimen to be evaluated was recorded by these measurements. Complex ion relaxation processes of the specimen [14] may be the reason for this, but further e x a m i n a t i o n is needed to explain these results.
tt. I k a w a el a l . / f l " - a l u m i n a type Y H + -q,allate
3. Results Fig. 1 shows part o f continuous records o f the current and potential difference before and after a stepwise change o f the applied voltages to the terminal electrodes from - 2 . 0 V to 1.7 V. The partial water pressure was 25.6 kPa, and it was balanced by argon with a m b i e n t pressure. A pulse-like increase in current is followed by a parabolic decrease, and the current comes to a constant value after two to three hours. A similar pulse-like increase in counter current was recorded when the terminal electrodes were short-circuited after a measurement. A polarization o f the specimen is evident from these results: i.e. a non-uniform distribution o f electric charge carriers occurred in the crystal. Thc steady state current and potential difference were functions o f e n v i r o n m e n t a l gas, applied voltage and the material of electrodes. Two o f the measurements are shown in fig. 2, and table 1 lists some sets o f those data together with apparent c o n d u c t i v i t y calculated from the current and potential difference. A n o n - o h m i c relation between the applied voltage and current is evident from fig. 2A. Similar nono h m i c relations were seen for all the m e a s u r e m e n t s in argon containing water and w a t e r / a m m o n i a . The applied voltage at the turning point o f the relation was ca. 1.5 V for the m e a s u r e m e n t s using Pt electrode (sample no. 1, 2), and was ca. 2.6 V for that using Au electrode (sample no. 3). The n o n - o h m i c relations were less evident for the m e a s u r e m e n t s in
219
argon and argon containing hydrogen or a m m o n i a (table 1 ). And an almost ohmic relation was seen for the m e a s u r e m e n t s in argon containing water and hydrogen (fig. 2B). The current was also affected with the kind o f gas and its partial pressure. The current increased nearly linearly with partial pressure o f water in argon containing water within the limit o f this experiment (table 1). The roughly estimated normalized current under high applied voltages ( 1.8-2.0 V) was largest in the a t m o s p h e r e of argon containing water and hydrogen, and smallest in argon (table l ). Conductivities are also listed in table 1. Like the current, the conductivity was also a function o f applied voltage. Further, it increased with the current. The largest conductivity recorded in this experiment was 2 . 4 × 10 s S / c m in argon containing water and hydrogen under an applied voltage 1.9 V (table 1 ). The value is significantly different from the reported value 3.5 × 10 3 S / c m [ 14] at 120°C. It is clear from these results that we have failed to evaluate conductivity o f this crystal by the m e t h o d employed. Incidentally, the conductivities calculated from the currents and applied voltages were far smaller than those listed in the table, which were calculated from the currents and potential differences, except the conductivities obtained in argon containing water and hydrogen.
4. Discussion
4.1. Activity of electrode SampLe No.1 500
1
v
:=L
I
0
< \3 <
-500 Ar+HaO/25.6kPa
~)
510 1130 Time /rnin
1;0
Fig. 1. Part of" c o n t i n u o u s records of the current and potential difference before and after a stepwise change of the applied voltages from - 2 . 0 to 1.7 V m e a s u r e d at 120=C in argon c o n t a i n i n g water with a partial pressure 25.6 kPa.
It is clear from these results that the current reflects the ability o f electrode to make the charge carrying ion (s). Within the limit o f this experiment, the activity increases with the applied voltage, and it is largest in argon containing water and hydrogen, and smallest in argon. The small current recorded in argon may be caused by a trace a m o u n t of water introduced into the system a n d / o r partial d e c o m p o sition of the crystal under the higher applied voltages. The applied voltages o f 1.5 V and 2.6 V at the respective turning points o f the V-I relations using the Pt and Au electrodes, suggest that the d e c o m p o s i t i o n o f water at the electrode is accelerated at higher applied voltages than the turning point. There m a y be
220
II. lkawa el al./fl" alumina O'pe N i l ) -gallale
4.0 Sampte No.2
( B } - 1000
Ar+He/18.2kPa +H20/lO'OkPa / I • ,
1.4
Sample No.2 Ar+H20/25.6kPa
900 800
(-~} 3.0
700
1.2
600
1.0
500 /
.~0.8
,"
:L _" 2.0
i
600 <
/
500 \ 3 < 4OO
• 400 <
ii •
0.6
5 0 0 ~<
5OO 1.0
0.4
2OO
0.2
tO0
0
0.5 1.0 1.5 2.0 Applied Vo[fege / V
0
2OO I00 i
0
F i g . 2 . Current and potential difference as functions of applied voltage measured v, atcr/h?drogen ( B ).
i
i
i
0.5 1.0 1.5 2.0 Apptied Vottage / V
at 120
C in
0
argon containing water (A), and containing
Table 1 Some selected relations among composition of gas, applied voltage to the terminal electrodes, current, potential difference between the probes and apparent conductivit.x. The electrode materials are PI for samples no. 1/no. 2, and Au for sample no. 3. The gas is balanced by' argon with the ambient pressure. Sample no.
Gas composition
Applied voltage ( V ) - current -potential diflk~rence ( mV ) - apparent conductivity (10 ~Scm i)
Ar+ H20/10.0kPa Ar+ H-O/25.6kPa
1.9-230nA-260-1.1, 1.7-92nA-230-0.5, 1.5-36nA-240-0.2, 1.0-9nA-153 1.95 590nA-370, 1.7-150nA-360, 1.3-48nA-290, 1.0-33nA-380, 0.5-10nA 220, 1.3-69nA-400, 1.7-190nA-360 1.9-680nA-390-2.1, 1.6-100nA-340-0.4, 1.3-30nA-240-0.1, 1.0 14nA-140-0.1 1.9-1.0/c~k-480 1.2, 1.6-350nA-420 0.5, 1.3-150nA-300-0.3, 1.0-77nA-200-0.2 1.0-1 hA, 1.5-2nA, 2.0-8nA, 2.2-13nA, 2.4-28nA, 2.8-110nA, 3.0-360nA 2.0-3.7#A-870-2.3, 1.9 3.5l~A-810-2.4, 1.6-2.71~A-670-2.2. 1.3-2.1/zA-580-2.0, 1.0 1.21~A-430-1.5,0.6 610nA-270-l.2 1.9-130nA 370-0.2,1.8-96nA 320-0.2. 1.6-50nA-250 0.1,1.2-16nA-110, 0.8-TnA 9 1.9-590nA-390-0.8, 1.6-400nA-310-0.7, 1.3-230nA-280-0.4, 1.0-130nA-220-0.3 1.9-33nA-380,1.6-22nA-300, 1.3 14nA-220, 1.0-9nA-150 1.9-43nA-410,1.6-21nA-320, 1.3-10nA-220, 1.0 4nA-150
Ar+ H20/35.5kPa Ar+ H,O/25.6kPa A r + H 2 0 10.0kPa A r + H 2 0 I 0.0kPa + He/18.2kPa Ar+ H_~O/5.0kPa +NH~/2.5kPa Ar+H,/20.3kPa Ar+NH~/5.1kPa Ar
no critical voltage for the decomposition of hydrogen. That may be the reason why the V-1 relations in gases containing hydrogen are different from those
in argon containing water. In any case, the polarizations detected show that the ability of the electrode to make conducting i o n ( s ) is insufficient.
H. Ikawa et al./I3"-alumina type NH~ -gallale
4.2. Reason for the failure of the conductivity measurement Yokota [ 18 ] has developed a theory of mixed conduction - electronic and ionic conductions - based on the Hebb-Wagner dc polarization theory [ 19,20]. He has derived expressions for the two potential distributions - electrochemical potentials of electrons and ions measured by the use of probes consisting of an electronic conductor and an ionic conductor, respectively. Couturier et al. [21] have proposed a model to explain the current variation versus time for an ionic conductor having the very low mobility of electrons and have derived a method of ionic conductivity measurement using Pt probes to detect the electrochemical potential of electrons. The method employed in this study is identical to the latter, and the electronic conductivity of NH~-[3"-gallate is also very low [2,5]. We have failed, however, in ionic conductivity measurement. The reason of the failure is discussed next. Fig. 3 shows potential distributions in three cells Pt/ionic c o n d u c t o r s / P t at a time zero when submitted to a voltage step and after a duration reaching
-J
Agl.93Te
.......
e
Ag*,t
. . . . . . .
.... e
"
I
_e[_~.
---.
.......
e
_e~..,,
PbF 2 --
--4e
+
_
+
NH~,-,B"-GaLLote ""---~2
p",
.....
go
.....
.
q ~ p".- ~le
Fig. 3. Distributions of chemical potentials of neutral species/~R and electrochemical potentials of electrons qe in the cells Pt/ Ag, ,,~Te/Pt (silver ion and electron conductor), PI/PbF_,/Pt (fluoride ion conductor) and Pt/NH~-13"-gallate/Pl (protonic ion (P+) conductor) at a time zero submitted to a voltage step (left) and at a quasi-equilibrium state (right). The vertical line represents the potential and electrode.
221
a quasi-equilibrated state. The decomposition of the specimen caused by a large applied voltage is neglected in this discussion. At a time when a voltage is applied to Ag~ ~,3Te, Ag + ions and electrons start to move owing to the gradients of electrochemical potentials o f A g + ion (~/Ag + ) and electron (qe), respectively. However, the chemical potential of neutral silver/tAg is uniform within the specimen. After a while the gradient of ~/Ag+ and consequently the flux of Ag + ions vanishes because of the blocking character of the Pt electrode. At a quasi-equilibrium state there is a next relation among the three potentials. /tAg = ~/Ag+ + tle.
( 1)
The two Pt probes are in equilibrium with the respective q values of the specimen at points x~ and x> The potential difference between the two probes gives the value of the electrochemical potentials difference between x~ and x_,. We can evaluate the conductivity of electrons by using the potential difference. For the case of PbF> the electronic conduction is negligible and the gradient of composition is also negligible except for at interfaces because of the low applied field and stoichiometric character of PbFe [21 ]. After the voltage step is applied, ions move to create space charges and to cancel the electrostatic field. Finally the current at the interfaces is due to electron tunneling between the electrode and localized states in the gap, through the narrow potential barrier at each interface. The current in the bulk is purely ionic and cancels the charge of tunneling electrons [21]. In this state it has been said that /tF in the bulk is uniform and as a result, the difference between ~IF-(,v~ ) and qF (.v~) is equal to the difference between qe(x~) and ~le(x_,). Accordingly, the ionic conductivity can be calculated by using the potential difference of electrons. The charge carriers of protonic 13- and /3"-aluminas/gallates have been provided to be protons, H~O + a n d / o r NH~- ions, but no definite determination of the carriers and their transference number have been made [ 11 ]. In this discussion, accordingly, the protonic charge carrier(s) in NH+-6"-gallate is expressed tentatively by the P+ ion. At the beginning /tP of the specimen is uniform and in equilibrium with the gas phase. When the voltage step was applied, the large current recorded was due to the
222
1t. lkawa el a/./fl' alumina type N l t j -ga//ate
m o v e m e n t o f P + ions. The current was pulse-like because the ability of the electrode to make P* ions was very low. The transient current shown in fig. 1 is similar to that reported for the partially ion blocking P t / A g B r / A g cell [22 ], and it may be also similar in having composition gradients. At a quasi-equilibrated state, the gradient of r/P + in NH2-13"-gallate is depressed and the gradient of #P is close to that of ~le (fig. 3) depending on the equation. ~LP= qP + + ~le -
(2 )
Unlike the case fo PbF> the difference between 71e(.v~ ) and ~le(xe ) of this case is much larger than the difference between J/P+ (.v~) and ~IP ~ (.v,). This is the reason of the failure in conductivity measurements. The two reasons are listed for the generation of the composition gradient. One is the higher applied voltage, and the other is a character of NH4+ -13"-gallate. The number of cations in the conduction plane of 13and ~"-alumina structures are sensitive to the experimental conditions. Especially, protonic 13- and 13"-alumina c o m p o u n d s change their compositions around the conduction planes during the ion exchange processes at a low temperature (ca. 2 0 0 : C )
[17,23]. The time spent amounting two to three hours before the current settles (fig. t ), may be the time spent in changing the structure to a c c o m m o d a t e the chemical potential(s) of protonic species. The conductivit}' of the four terminal method in argon containing water and hydrogen almost coincided with that of the two terminal method because a rather high activity of the electrode may not generate a localized state of potential at the interface.
Acknowledgement The authors are grateful to Prof. T. Maruyama of this Institute for his nice suggestions. Part of this work is supported by the Japan Securities Scholarship Foundation.
References
[ 1 ] N. Baffler, J.C. Badot and Ph. Colombam Solid State lonics 13(1984) 233. [ 2 ] Ph. Colomban. J.P. Boilot, A. Kahn and G. Lucazeau. Nouv. J. Chim. 2 (1978) 21. [3 ] N. Baffler, J.C. Badot and Ph. Colomban, Solid State lonics 2 (1980) 107. [4] G.C. Farringlon and J.k. Briant, Mat. Rex. Bull. 13 (1978) 763. [5] G.C. Farrington and J.L. Briant, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy ( North-Holland, Amsterdam. 1979 ) p. 395. [6] A. Teitsma, M. Sayer. S.L Segel and P.S. Nicholson, Mal. Res. Bull. 15 (1980) 1611. [ 7 ] H. Ikawa. T. Tsurumi, K. Urabe and S. Udagawa, Solid State tonics 20 (1986) 1. [8] T. Tsurumi, H. Ikawa, K. Urabe, T. Nishimura, T. Oohashi and S. Udagawa, Solid State lonics 25 (1987) 143. [ 9 ] H . Ikawa, T. Tsurumi, T. Oohashi, K. Urabc and S. Udagawa, J. Ceram. Soc. Japan. 92 ( 1984 ) 473. [10] P.S. Nicholson, M.Z. Munshi, G. Singh, M. Sayer and M.F. Bell, Solid State lonics 18/19 (1986) 699. [ I I ] T . Tsurumi, H. Ikawa, M. lshimori, K. Urabc and S. Udagawa, Solid State lonics 21 (1986) 31. [12]M.Z. Munshi and P.S. Nicholson. Solid State lonics 23 (1987) 203. [13] H. Ikawa, T. Oohashi, M. lshimori. T. Tsurumi. K. Urabe and S. Udagawa, in: High tech ceramics, ed. P. Vincenzini ( Elsevier Science Publishers, Amsterdam, 1987 ) p. 2137. [14] H. Ikawa, K. Shima, T. Tsurumi and O. Fukunaga, Proc. Intern. Seminar on Solid State Ionic Devices. eds. B.V.R Chowdari and S. Radhakirshna (World Scientific. Singapore. 1988) p. 497. [15] I_.M. Foster and J.E. Scardefield, J. Electrochem. Soc. 124 (1977) 434. [ 16] J.O. Thomas and G.('. Farrington, Acta Cryst. B39 (1983) 227. [17]T. Tsurumi, H. lkawa, T. Nishimura, K. Urabe and S. Udagawa, ,I. Solid State Chem. 71 ( 1987 ) 154. [ 18] 1. Yokota, J. Phys. Soc. Japan. 16 ( 1961 ) 2213. [ 19] M. Hebb, J. Chem. Phys. 20 (1952) 185. [20] C. Wagner, Proc. V.I.T.('.E. 7 (1955) 361. [21 ] G. ('outurier, J. Salardenne. ('. Sribi and M. Rosso, Solid State lonics 9/10 ( 1983 ) 699. [22 ] ,1. Mizusaki, J. Sasaki. S. Yamauchi and K. Fueki, Solid State lonics 7 (1982) 323. [23]H. Ikawa, T. Tsurumi, M. lshimori, K. Urabe and S. Udagawa, J. Solid State Chem. 60 (1985) 51.