The electrical conductivity of sintered specimens of Ta2O5 with additions of foreign oxides

Solid State Ionics 12 (1984) 235-242 North-Holland, Amsterdam

THE ELECTRICAL CONDUCTIVITY OF SINTERED SPECIMENS OF TazOS WITH ADDITIONS OF FOREIGN OXIDES

Oivind JOHANNESEN* and Per KOFSTAD Institute of Chemistry, University of Oslo, Blindern, Oslo 3, Norway

The effect of CaO additions on the electrical conductivity of Ta,O, is discussed. The electrical conductivity and ionic transport numbers of Ta,Os containing TiO, have been measured at temperatures from 990 to 1300” C. At temperatures above 1200” C a substantial increase in ionic conductivity was observed. It is suggested that this increase is due to the substitutionally dissolved Ti cations in the high-temperature form of Ta,Os. L’influence d’addition de CaO sur la conductivite electrique de Ta,Os est discutee. La conductivite Clectrique et le nombre de transport ionique de Ta,O, contenant TiOz ont et6 mesures dans le domaine de temperature 990-1300” C. Aux temperatures superieures a 1200” C, une forte augmentation de la conductivite ionique a 6tC observee. Cette augmentation peut etre attribuee a la presence de cations titane en positions substitutionelles dans la structure haute temperature de Ta,O,.

1. Introduction

Ta205 goes through a phase transition from a lowtemperature (L-Ta@$ to a high-temperature (HTa,05) form at about 1360” C [l]. In a study of the electrical conductivity and ionic transport numbers of L-TaZ05 as a function of partial pressure of oxygen (l-10-” atm), the defect structure has been interpreted in terms of an oxygen vacancy model [2] (similar to calcia-stabilized zirconia). In order to test the validity of the model the electrical conductivity data have been separated into the ionic and the electronic contributions by subtracting the ionic conductivity from the total conductivity [2]. The results are shown in fig. 1. At near-atmospheric pressures (l10e4 atm 0,) the concentration of oxygen vacancies is fixed by the presence of lower-valent impurities and in this impurity controlled region (extrinsic region) the conductivity was found to be predominantly ionic [2]. Further, in the extrinsic region the n-type conductivity is given by U,CCPGt14 and the p-type by c,,a Pb/p (cf p. 239). At lower PO, values (lo-“* Present

address: State University,

Center for Solid State Science, Arizona Tempe, Arizona 85281, USA.

10-i’ atm) the conductivity is electronic and controlled by the defect formation and motion of electrons. In this region the conductivity is proportional t0 PG’,/6((ra P$) [2]. The above studies suggest that ionic conductivity of the oxide phase may be increased and extended by substitutional dissolution of lower-valent cations (doping). A program has therefore been initiated to study the effects of various oxide additions to Ta205 [3].

2. Experimental 2.1. Material and specimen preparation The specimens were prepared from TaZ05 and foreign oxide powders. The impurity contents have been examined by means of mass spectrographic analysis, and the results are to be published elsewhere [3]. The oxides were mixed in agate mortar, then cold pressed and subsequently presintered in an alumina boat for 24 h at 1100” C. The presintered samples were again ground to fine powders and pressed to

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/ Sintered

specimens csf Ta20,

the electrical conductivity was measured with a conductance bridge operating at 1592 Hz”. Ionic transport numbers were obtained by measuring the EMF’t across the specimen while maintaining different oxygen’pressures at opposite sides. Only small chemical potential (i.e. partial pressures of oxygen) differences were applied. Gases with different partial pressures of oxygen were obtained by using gas mixtures: Art 0, for the range l-lo-” atm O2 and CO+CO, for 10e4lO-2o atm Oz. The total pressure in the apparatus was always 1 atm, and the partial pressure of oxygen was determined by a calcia-stabilized zirconia cell situated near the specimen. The relative amounts of gases were controlled by flowmeters.

3. Results and diicussion 3.1. Ta205 withadditions of CaO

OXYQEN PRESSURE, atm. Fig. 1. Electrical conductivity of Ta20, as a function of Pa,. The total conductivity is subdivided into (if ionic cortductivity (horizontal iine at the region of minimum in the eiectrical conductivity), (ii) n-type conductivity, a,, (iii) p-type conductivity, up

tablets 2-4 mm thick and 20 mm in diameter. These were pressure-sintered in a graphite die where the sintering process was carried out at low partial pressures of oxygen f 1O-24atm 0,) in order to achieve high densi~cation. The reduced specimen turned gray (or black) after sintering. However, they became white (or pale yellow) when oxidized in air (24-170 h) at 1100” C. After this treatment the observed densities {measured pycnometrically with Kerosene) were not lower than 95% of the estimated theoretical ones.

For the measurements of the electrical conductivity the opposite sides of the tablets were polished and then platinized by means of ion sputtering. The specimens were placed in a resistance-heated furnace, and

Earlier investigations have shown that CaO has no doping effect on L-Taz05 [2]. The only difference between L-Ta205 and samples containing CaO is that the overall conductivity decreases with increasing amounts of CaO. The same relative decrease in electrical conductivity is observed both in the high and low PO, regions. This is illustrated in fig. 2 which compares the conductivity of L-Ta205 with that of samples containing CaO at 1 atm Oz. The decrease in conductivity has been interpreted in view of a classical dispersion theory derived by Maxwell and Rayleigh [4,5]. According to this model the decrease in total conductivity has been explained by the presence of a low or non-conducting calcium tantalate phase embedded in the L-Ta205 matrix

L&31. In order to test the validity of the model an attempt has been made to produce the pure calcium tantalate phase(s). In good agreement with the assumption of a low or non-conducting second phase, a pronounced decrease in the overall conductivity was observed (figs. 3 and 4). This yields a ratio of ~+,,,/~~~0.015 at lOOO”C,and 0.02 at 1200°C in the * Wayne Kerr Automatic Precision Bridge Model B-905. ‘The EMF has been measured with a digital electrometer Keithiey Model 616, autoranging voltmeter.

0. Johannesen,

237

P. Kofstad / Sintered specimens of Ta,O,

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Fig. 4. Electrical conductivity (VT) of the calcium tantalate rich specimen and TazOs at 1 atm 0, as a function of the reciprocal absolute temperature.

(UT) of Ta,Os,

(Ta,05)0.96(CaO)0.04 and (Taz05)o.&Ca%.0s at 1 atm 0~ as a function

of the reciprocal

absolute

temperature.

high-pressure range, where a,,, refers to the conductivity of the sample containing CaO and go to the LTa205. X-ray studies show that the sintered specimen in addition to the calcium tantalate phase(s) contains

x Calcium tantalete rich specimen l

*Wagner’sconsiderations

Ta205

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small amounts of the L-Ta205. It is assumed that the conductivity data in the high-pressure range reflect the ionic conductivity due to L-Ta205 embedded in the calcium tantalate matrix. However, at low PO, values ( 10-9-10-‘5 atm) the decrease in the conductivity was essentially smaller than at higher pressures. Thus, at PG:3.5 atm the ratio of u,,,/oo=0.2 at 1000” C. This behavior can be discussed in view of Wagner’s dispersion model [3,6]. According to this model [6] there can be an electric double layer at the interface between the dispersed phase and the matrix, which can markedly affect the electrical conductivity. If the electron concentration in the matrix is sufficiently low, the excess electrons in the dispersed phase may determine the overall electrical conductivity of the sample*. Assuming a constant volume

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Fig. 3. Electrical conductivity of the calcium specimen and TazOs as a function of PO,.

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[6] are analogous with the theory of the diffuse double layer introduced by Gouy [7] for the interface between liquid mercury and aqueous solutions. By solving numerically a modified Poisson-Boltzmann equation it follows that the electrical conductivity is estimated to be proportional to the volume fraction of the dispersed phase, and further inversely proportional to the square radius of the dispersed particle.

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fraction of the dispersed phase it can be shown [6] that the increase in the electrical conductivity is directly related to a decrease in the radius of the dispersed particle. From this point of view, even small amounts of L-Ta205 embedded in the tantalate matrix, might result in a marked enhancement of the electrical conductivity in the low-pressure ranges, where L-Ta20s is essentially a pure n-conductor (cf. p. 235 and fig. 3 at lower PO, (10-‘1-10-‘6 atm)). Considering the electrical properties of L-Ta205 showing both electronic and ionic conductor behavior, a further study of dispersion phenomena in the TazOS-CaO system would be of great interest. Careful examinations of the amounts of L-Taz05 and tantalates combined with a complete knowledge of sizes and shapes of the dispersed particles, can be an important contribution to clarify the electrical conductivity of two- (or multi-) phase mixtures. 3.2. Ta20s with additions of TiO,

. (W5)0,9(Ti02)0,, 10-20

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OXYGEN

The electrical conductivity and ionic transport numbers of a sample containing 10 mol% TiOZ have been measured at temperatures from 990 to 1300” C [3] (figs. 5 and 6). The variation of the conductivity at 990 and 1100” C is closely related to the results reported above. However, with decreasing pressure the conductivity rapidly approaches that of L-Ta205 (fig. 5). It is difficult to give an unequivocal interpretation of the results and several possibilities can be considered [3]. This includes the structural explanation of the Ta205TiOz system given by Roth and co-workers [8,9], where oxygen deficiency has been connected to the presence of distortion planes. Thus, the results may reflect conductivity in a two- or multi-phase mixture of titanium tantalate phases embedded in a matrix of a phase which is directly related to the L-Ta205 form (named R-phase). Due to the structural similarity between R and L-Ta205 it has been suggested that the electrical properties of the R-phase are essentially the same as those of L-Ta205, and the data can be analyzed in the same way as for L-Ta20S (fig. 7). In a more detailed consideration, at 990 and 1100” C the decrease in ionic conductivity of (Ta2’05)0.9(Ti02)0.1 in the high-pressure region (fig. 8), and the increased electrical conductivity at low pressures (compared to that of L-Ta205, fig. 5) have

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Fig. 5. Electrical conductivity of (Taz0,),,,(Ti02),, as a function of Po, at temperatures from 990 to 1300” C compared with the electrical conductivity of Ta,O, at 1100” C.

been related to the possible decrease in the concentration of distortion planes as a result of structural changes in the R-phase [3]. As shown in fig. 5 a substantially increased electrical conductivity of (Ta205)0.9(Ti02)0.1 was observed in the temperature range of 1200 to 1300” C. It should be remarked that the conductivity at 1200” C after heating at 1300” C is qualitatively higher than the measured conductivity at the same temperature i.e. 1200” Ct before heating at 1300” C. At high PO, ( 1-10e4 atm) the conductivity is predominantly ionic and an increase in the ionic transport number from ti = 0.7 at 1100” C to ti = 0.95 at 1200” C was observed.

’ In the experiments the temperature was increased to about 1220” C and the electrical conductivity was subsequently measured on the equilibrated sample at 1200” C. Before heating at 1220” C the conductivity at 1200” C exhibits the same variation in Po, as at 990 and 1100” C [3]. However, after heating at 1220” C an overall enhanced conductivity was observed, which is suggested to be related to the presence of the H-Ta,O,, phase (fig. 6, ref. [3]).

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can be expressed by the quasichemical equation*: 4 12dO’C before at 12209

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where one assumes titanium with one negative effective charge. Provided that the concentration of doubly charged oxygen vacancies is fixed by the concentration of dissolved Ti cations, the electroneutrahty condition, can be approximated by:

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OXYGEN PRESSURE.

atm.

Fig. 6. Ionic transport number of (TazOs)O..+(TiO,)O,,as a function of Po, at temperatures from 1100 to 1300” C. Theoretical value of the ionic transport number ri is calculated from fi = q/o,, where a, is the estimated ionic conductivity in the extrinsic region and a, is the measured total conductivity.

It is suggested [3] that the increase in ionic conductivity is due to the presence of the H-Ta205 form and reflects an enhanced [VJ due to the increased solubility of Ti02 in H-Taz05 with increasing temperature. This is in agreement with Roth et al. [S], who found that the addition of approximately 10 mol% TiOz lowers the L-Ta205*H-Ta,05 transition temperature from about 1360”C to about 1200” C, and that TiO:! exhibits an increasing solubility in H-Ta205 with increasing temperature. Their results [8] seem to be in accordance with preliminary X-ray studies at this Laboratory [lo]. However, the H-Ta205 was not observed at 1200” C but appeared at 1220” C’. At all temperatures above 1200” C the powder patterns revealed a multi-phase mixture with the presence of R, H-Ta205 and an additional titanium tantalate phase. The amount of H-Ta205 gradually becomes larger with increasing temperature, whereas the amount of the L-TazOS related phase (R) showed the opposite trend. The results at high PO, (l-10W4 atm, fig. 7) have been interpreted in terms of an oxygen vacancy model where [VG] is fixed by the concentration of Ti cations substitutionally dissolved in the H-Taz05 phase. The substitutional dissolution of a Ti cation in H-Ta205 *This is in agreement with the electrical conductivity measurements indicating a phase transition from L-Ta20s to H-Ta,Os at 1220” C (cf. footnote on the previous page).

(2)

Following a classical oxygen model the formation of oxygen vacancies can be given by: 00=V~+2e+j02(g)

(3)

and the corresponding defect equilibrium is written: [VJn’Pb/,” = Kv;;,

(4)

where n designates the concentration of electrons and Kv;; the equilibrium constant. By combining eqs. (2), (4) and the expression for the intrinsic electronic equilibrium (np = KJ the electron concentration n and the electron hole concentration p become: n = Kg/p = (2Kv;; /[Ti&,])“2F~~‘4,

(9

where & and Kv;; are the equilibrium constants. Thus, it is concluded that in the extrinsic region the electron hole and the electron conductivities are proportional to u, 0~Pri,” and a,~ P;;t’4, respectively. Following the above model, the ionic conductivity represents transport of doubty charged oxygen vacancies and, accordingly, the ionic conductivity tr,, is given by

=const T-’ exp (-Al&,/RT),

(6)

where vv;; is the mobility of oxygen vacancies and AH, is the activation energy for the mobility of oxygen vacancies. Then, with sufficiently large [T&j the H-Ta205 may become essentially a pure ionic conductor over a large range of oxygen pressures. To test the validity of the above model, the total electrical conductivity was separated into ionic and electronic conductivities by subtracting the ionic conductivity from the total conductivity. The results of such an analysis are illustrated in fig. 7. (The ionic * The

symbols refer to the Kriiger-Vink

notation.

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P. Kofstad

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Following the oxygen vacancy model, a plot of log 5iT versus l/T (after heating at 1300” C) yields the activation energy of 70 kJ/mol for the mobility of oxygen vacancies (fig. 8). Fig. 9 shows the electrical conductivity at 868 and 750” C of (Ta205)0.9(Ti02)0.1 quenched from 1300” C. It should be emphasized that the conductivity of the quenched sample at 868” C was measured after waiting 4 d. The quenched sample is almost a pure ionic conductor at 868” C in the oxygen partial pressure range of l-10-* atm. In the low-pressure range (lo+- lo-l9 atm 02) the variation in conductivity is almost the same as at 1300” C, and the sample exhibits pure n-conductor behavior. The deviation from the oxygen vacancy model, similar to the one observed at higher temperatures, is illustrated by the fact that the ionic transport numbers estimated from the EMF measurements at low pressures are lower (essentially close to zero) than the ionic transport numbers calculated in accordance with the model. This is shown in fig. 10, where ti”O estimated from the EMF measurements at PO, = 1O-‘2.4atm has

PRESSURE, atm.

Fig. 7. Electrical conductivity of (Taz05)o.9(Ti0,),,1 as a function of Po,. The total conductivity is subdivided into (i) ionic conductivity (horizontal line at the region of minimum in the electrical conductivity), (ii) n-type conductivity, o,, (iii) p-type conductivity, ur,

conductivity is given by ~iion= Uttti,where a, denotes the total electrical conductivity and ti the ionic transport number.) According to the proposed model one should await that the total electrical conductivity in the extrinsic region approaches the dependence of the electron conductivity i.e. U”a P;:‘4 at lower PO,. However, such a dependence was never observed. On the contrary, the electrical conductivity at 1200 and 1300” C exhibited a considerably smaller variation with PO,, and a general feature at low PO, values (
Taz% Ua205)ogUi0~)ol bh

122O'C

(TazOs)c9(TrOr)aI ah 122O'C 0

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90

13OOY

90

Fig. 8. Ionic conductivity (UT) of Ta,O, and (Taz05)o,9(TiOZ),,, as a function of the reciprocal absolute temperature; b.h. = before heating, a.h. = after heating, a.q. = after quenching.

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OXYGEN PRESSURE, Mm. Fig. 9. Electrical conductivity of (Ta205)o.9(Ti02),,1 (quenched from 1300” C) as a function of PO, at temperatures of 868 and 750” C. The symbol a.q. (4 d) refers to the electrical conductivity of a quenched sample measured after waiting 4 d; b.q. = before quenching, a.q. = after quenching, H = H-Ta,O, phase, K = additional second phase (or phases).

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atm

PRESSURE,

Fig. 10. Ionic transport number of (Taz05)o.s(TiOz)o.1 as a function of P,,. Measurements have been performed on a sample quenched from 1300” C after waiting 4 d. The theoretical value of t, is calculated from ri = aJo, (cf. this page and fig. 6).

The ionic transport number of the quenched sample was found to be essentially independent of the temperature for PO, = 1 atm. Thus, the activation energy of 73 kJ/mol for PO, = 1 atm (fig. 11) is qualitatively the same as the activation energy for the ionic TEMPERATURE,%

to be compared with the calculated value of ti f 0.4*. Further, the ionic conductivity of the quenched sample at 868” C is essentially higher than the ionic conductivity of the sample before heating at elevated temperatures. It is demonstrated by comparing the ionic conductivity of the quenched sample at 868” C ai=7.94x 10e3 R-l cm-’ with the ionic conductivity pi= 3.2X lop4 W’ cm-’ at 1100” C (fig. 8). At high pressures ( l-lop4 atm 0,) a slow decrease in the electrical conductivity was observed with time at 868 and 750” C, indicating a metastable system; no such decrease was observed in the low-pressure region (1O-8-1O-‘9 atm 0,) (fig. 9). Although the decrease in the electrical conductivity was most pronounced at 1 atm 02, it only amounted to 7.2 X 10d4 f12-’cm-’ measured after 20 h at 750” C. * The theoretical value of the ionic transport number r, calculated in accordance with the oxygen vacancy model is given by ri = CT,/U,, where ot is the total electrical conductivity and oi is the ionic conductivity. The total conductivity of the quenched sample at 868” C and PO, = 10-i2~“ atm is o, = 1.91 X lo-’ 0-i cm-‘, and the ionic conductivity of the quenched sample at the same temperature is ai = 7.94 X 10e3 a-’ cm-i. Thus, ti becomes: ri = 0.4.

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10-5

OXYGEN 76o’C

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16’5

g(Ti02)u.l

70

80

90

10

104 T,“K Fig. 11. Electrical

conductivity

(UT) of (Ta,O&

,.

JTiO,),,

(Ta2%)o.dHfWo.,, (TaZ0s)o.9,(CrzO~)o.03 and Ta,O, 1 atm O2 as a function of the reciprocal absolute temperature. The symbol a.q. (14 d) refers to the electrical ductivity of a quenched sample measured after waiting a.q. = after quenching.

at con14 d;

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P. Kofstad

conductivity. The activation energy of 73 kJ/mole has to be compared with the estimated activation energy for the mobility of oxygen vacancies, 70 kJ/mol, in the H-Ta205 (fig. 8). On this basis it is concluded that the pronounced ionic conductivity of the quenched sample at high pressures is due to the presence of the titanium-doped H-Ta205 phase. The metastability is primarily a consequence of quenching a multi-phase mixture where the R-phase is embedded in the matrix at 868” C. It is suggested that the slow decrease in the conductivity with time is due to the gradual conversion of the stabilized H-Ta205 into the R-phase. (The situation is illustrated in fig. 9.) A high ionic conductivity combined with a low activation energy of a completely stabilized H-Ta205 quenched from high temperatures (= 1600” C [8]) may reveal that the stabilized H-Ta205 is an attractive candidate for fuel cells below temperatures at which calcia-stabilized ZrO, can be used.

Acknowledgement The authors gratefully acknowledge a grant from the Norwegian Oil and Energy Department through the Norwegian Council for General Sciences and Humanities.

/ Sintered specimens of Ta,O,

References [ll

PI

[31 [41

[51 [61 [71 [81 [91

[JOI

A. Reisman, F. Holtzberg, M. Berkenblit and M. Berry, J. Am. Chem. Sot. 78 (1956) 4514. 0. Johannesen and P. Kofstad, in: Transport in nonstoichiometric compounds, Proc. 1st Intern. %onf., Krakow, Poland, 1980, ed. J. Nowotny (Elsevier, Amsterdam, 1982) p. 100. 0. Johannesen and P. Kofstad, unpublished results. J.C. Maxwell, A treatise on electricity and magnetism, 2nd Ed., Vol. 1 (Clarendon Press, Oxford, 1881) p. 435. Lord Rayleigh, Phil. Mag. 34 (1892) 481. C. Wagner, J. Phys. Chem. Solids 33 (1972) 1051. G. Gouy, CR Acad. Sci. 149 (1909) 654. J.L. Waring and R.S. Roth, J. Res. Nat. Bur. Standards, Phys. Chem. 72A (1968) 175. R.S. Roth and N.C. Stephenson, in: The chemistry of extended defects in non-metallic solids, eds. L. Eyring and M. O’Keeffe (North-Holland, Amsterdam, 1970) p. 167. S. Furuseth and K. Selte, private communication.