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Investigation of the bulk conductivity of BaZr0.95M0.05O3-α (M =Al, Er, Ho, Tm, Yb and Y)

under wet N2

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2002 J. Phys. D: Appl. Phys. 35 397

(http://iopscience.iop.org/0022-3727/35/4/316)

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 35 (2002) 397–401 PII: S0022-3727(02)24734-8

Investigation of the bulk conductivity ofBaZr0.95M0.05O3−α (M = Al, Er, Ho,Tm, Yb and Y) under wet N2

Mouloud Laidoudi1, Ibrahim Abu Talib and Ramli Omar

School of Applied Physics, Faculty of Science and Technology,Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia

E-mail: laidoudi@hotmail.com

Received 10 May 2001, in final form 13 September 2001Published 1 February 2002Online at stacks.iop.org/JPhysD/35/397

AbstractSamples of BaZr0.95M0.05O3−α (where M = Al, Er, Ho, Tm, Yb and Y)were prepared by solid state reaction method. The bulk conductivity of thespecimens under wet N2 exposure was studied as a function of temperature,dopant ionic radius and the electronegativity of dopants. The correlationbetween the conductivity and electronegativity of the dopant, which has notbeen reported in any other studies, will be discussed.

1. Introduction

The investigation of proton conduction at high temperaturesin compounds which have the composition AB1−xMxO3−α

(where A = Ba, Sr or Ca, B = Zr, Ce and M is a trivalentor divalent cation as a dopant, α = x/2, and x is theconcentration of dopants) is the current subject of research[1–6]. Due to the advantages of these ceramics we mayuse them as electrolytes for different electrochemical cells athigh temperatures such as high-temperature hydrogen sensors[7–9] and high-temperature fuel cells [10]. The protonsarise in the matrix of these ceramics by equilibrium with thewater vapour or hydrogen containing atmosphere at elevatedtemperatures.

The protonic conductors based on zirconates such asdoped CaZrO3, BaZrO3 and SrZrO3 in general exhibit amechanical strength and chemical stability greater than thosebased on cerates such as doped BaCeO3 and SrCeO3 [11]. Wetherefore chose the BaZrO3 ceramic, as our topic of study.In this paper, we report the bulk conductivity as a functionof temperature, ionic radii and the electronegativities ofdopants.

2. Experimental

The samples of BaZr0.95M0.05O3−α were prepared bythe conventional solid state reaction method of BaCO3

1 Author to whom correspondence should be addressed.

(99.997%, Aldrich), ZrO2 (99%, Fluka) and M2O3 accordingto the equation

BaCO3 + (1 − x)ZrO2 +x

2M2O3

→ BaZr1−xMxO3−α + CO2 (1)

where x = 0.00 and 0.05. The dopant purity values were:Al2O3 (99.99%, Johnson Matthey), Y2O3 (99.99%, Johnson),Tm2O3 (99.99%, Aldrich), Yb2O3 (99.99%, Johnson) Ho2O3

(99.9%, Fluka chemika) and Er2O3 (99.999%, Stremchemicals).

The appropriate amounts of raw materials were mixedwell, then calcined in air at 1300–1450˚C for 20–40 h in anelectrical tube furnace programmable down to 1˚C min−1. Thecalcined powders were ground manually for a period between30 min and 4 h. The x-ray technique was used for verificationof the formation the single phase in the samples (SeimensD2000, CuKα radiation, λ = 1.541 78 Å).

The ceramic powders were pressed hydrostatically(10–11 t for 5 min) into pellets (12.9 mm in diameter, 1–2 mmthickness) then sintered at 1350–1500˚C for 10–20 h. Theporosities were obtained from the ratio of the mass to thevolume of each sample. An electronic balance of 10−4 gresolution was used for the measurement of the mass of thesamples and the vernier gauge was used for the measurementof the dimensions of the samples. The porosity of theobtained samples is listed in table 1. Both sides of thesamples were painted by Pt paste (Engelhard A4338) and thenthe samples were fired to make a good contact between theelectrodes and the pellets. The coated samples were used

0022-3727/02/040397+05$30.00 © 2002 IOP Publishing Ltd Printed in the UK 397

M Laidoudi et al

Table 1. The porosity (%) of BaZr0.95M0.05O3−α samples (whereM = Y, Yb, Al, Er, Tm and Ho) and of the undoped BaZrO3.

Dopant element Y Yb Al Er Tm Ho UndopedThe porosity of 3 7 20 19 14 23 20

the samples (%)

as the elctrolyte of the electrochemical cell for the EMFand conductivity measurements. EMFs of the cell weremeasured by a Keithley multimeter and ac conductivities weremeasured by complex impedance (Solartron 1255). However,the impedance spectra were taken after pretreatment in wetN2 (with PH2O = 0.031 atm) for 30 min at each measurementtemperature, because this time was found to be more thanenough to reach full equilibration in each sample. The detailsof samples preparation and characterization are described inearlier reports [12, 13].

3. Results and discussion

3.1. X-ray diffraction and EMF measurements

X-ray diffraction (XRD) patterns of BaZr0.95M0.05O3−α

confirmed single-phase perovskite structure with the cubicunit cell. The 2θ values in this work were approximately inaccordance with those of ASTM standard data for the non-doped barium zirconate ceramics. Proton conduction of thespecimens at 400 � T � 900˚C was verified by means of EMFmeasurements of the hydrogen and water vapour concentrationcells. The results showed that the EMFs were stable. Thenegative terminal was in the higher hydrogen partial pressurecompartment of the hydrogen concentration cell and in thewet compartment of the water vapour concentration cell, whichmeans that ceramic BaZr0.95M0.05O3−α is a protonic conductor.Also the undoped BaZrO3 showed unstable EMFs, whichindicates the absence of the oxygen ion vacancies in thematrix. We can then say that the doping by trivalent cationsplays a major role in proton conduction. Some of the EMFresults of our work are mentioned in [12–16]. The protontransport number was also calculated. For example, the sampleof BaZr0.95Tm0.05O3−α showed a proton transport numbertH = 0.7 over the range of temperature (500–900˚C) [15].

3.2. Bulk conductivity studies

Figure 1 depicts the impedance spectra obtained for the sampleof BaZr0.95Y0.05O3−α at different temperatures, it is clear eachspectrum consist of two semicircles. Slade et al [17] andBohn et al [18] ascribed the low-frequency arc to the processesoccurring between the electrodes and the electrolyte, whereasthe high-frequency arc corresponds to the bulk response.The left-hand intercept of the high-frequency semicircle isdisplaced from the origin owing to the unavoidable impedancesrelated to the cell [17]. The bulk conductivity was then deducedfrom the high-frequency semicircle and, by using equation (2),the bulk conductivity, σ , was calculated:

σ = 1

R

L

A(2)

where L is the thickness of the ceramic disc (pellet) and A

is the area of cross section of the electrode and R is the bulkresistance of the electrolyte.

Studies on the bulk conductivity of BaZr0.95M0.05O3−α

were reported by Iwahara (M = Ga, In, Y, Nd and Dy inH2 atmosphere) [5] and by Slade et al (M = Y and Ybunder wet N2) [17]. Slade et al found that the conductiv-ity for to Yb-doped was higher than that of Y-doped com-pound. To our knowledge, studies on the bulk conductivityof BaZr0.95M0.05O3−α doped by Al, Er, Ho and Tm have notbeen carried out by any other workers. So far, only Yajimaet al [11] and Iwahara et al [5] have attempted to find a rela-tionship between the conductivity and ionic radius of dopantin SrZrO3 and in BaZrO3, respectively. In our work, we firststudied the bulk conductivity of BaZr0.95M0.05O3−α as a func-tion of temperature in the range 400 � T � 900˚C in wet N2

(PH2O = 0.031 atm). The results plotted in figure 2 showed theArrhenius-type of the conductivities. All the conductivity val-ues of BaZrO3 doped by Al, Er, Ho and Tm almost overlap eachother and are located between the conductivities of BaZrO3

doped by Yb and Y. We also note that the undoped sampleshowed the smallest conductivity within the temperature range400–900˚C. The activation energies and pre-exponential fac-tors are represented in the table 2. It is evident that the sampledoped by Y showed the smallest activation energy.

Second, we tried to find a relationship between theisotherm conductivities as a function of ionic radius of dopantin BaZr0.95M0.05O3−α as plotted in figure 3 (the ionic radii weretaken from [19]). The same shape of isotherm conductivitieswas also reported by Yajima et al [11] and by Iwahara et al [5].

In the third step, we tried to find a relationship between theconductivity of BaZr0.95M0.05O3−α and the electronegativityof the dopants M = Al, Er, Ho, Tm, Yb and Y, andthe electronegativity of Zr in the case of undoped BaZrO3.The study was carried out based on the assumption that theconductivity is dependent on electronegativity, l, of the dopantelements for the case of doped BaZrO3 and is dependenton electronegativity of B-cation, which is Zr, for the caseof undoped BaZrO3. Figure 4 depicts the logarithm ofisotherm conductivities of BaZr0.95M0.05O3−α as a functionof the electronegativity of dopants and of zirconium forthe case of non-doped BaZrO3 (the electronegativities ofdopants were taken from [19]). The following remarkscan be made: the smallest conductivity is that of undopedBaZrO3. As the electronegativity increased in the range ofelectronegativity l � 1.4, the isotherm conductivity decreasesuntil the electronegativity l = 1.4 (which corresponds to theelectronegativity of Zr); after that, the conductivity started toincrease with increasing electronegativity in the range l � 1.4.

Figure 5 shows plots of the logarithm of the bulkconductivity as a function of the electronegativity for l � 1.4in wet N2. The plots are straight lines, which are parallel toeach other. It is interesting to note that the conductivities ofEr, Ho and Tm are almost the same. These results lead usto suggest that the logarithm of the isotherm conductivity ofBaZr0.95M0.05O3−α in the range l � 1.4 decreases linearlywith increasing electronegativity. In the case of compoundsdoped with elements with l � 1.4 the conductivity seems toincrease with increasing electronegativity. However, since weonly have the conductivity data for the undoped and Al-dopedBaZrO3 in this range, we could not presently specify the typeof the relationship, which may need further study.

398

Investigation of the bulk conductivity of BaZr0.95M0.05O3−α under wet N2

Real part of the impedance Z' [Ohm]

0.0e+0 5.0e+4 1.0e+5 1.5e+5 2.0e+5 2.5e+5 3.0e+5 3.5e+5

Imag

inar

y pa

rt o

f th

e im

peda

nce

Z"

[Ohm

]

0.0e+0

T = 800°C

T = 900°C

1 Hz

25 Hz

1.24 103 Hz

107 Hz

5.80103 Hz

1 Hz

41.5 Hz

T = 600°C

T = 700°C

T = 500°C

4.0e+4

8.0e+4

1.2e+5

1.6e+5

2.0e+5

2.4e+5

2.8e+5

3.2e+5

Figure 1. Impedance spectra of the sample of BaZr0.95Y0.05O3−α (of thickness 1.08 mm and diameter 1.158 cm) as a function of temperaturein wet N2 (PH2O = 0.031 atm).

103/T [K–1]

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Log

σT

[S

cm–1

K]

AlErYbTmHoY

Conductivity of undoped BaZrO3

–6

–5

–4

–3

–2

–1

0

Figure 2. Bulk conductivity of sintered oxides ofBaZr0.95M0.05O3−α (where M = Al, Er, Ho, Tm, Y and Yb) in wetN2 (PH2O = 0.031 atm) atmosphere.

Table 2. The activation energies Ea and pre-exponential factorslog A of the conductivity of BaZr0.95M0.05O3−α (where M = Al, Er,Yb, Tm, Ho and Y) and of the undoped BaZrO3.

Dopant element Al Er Yb Tm Ho Y UndopedActivation 0.51 0.46 0.46 0.51 0.47 0.45 0.46

energy Ea (eV)Pre-exponential 4.30 3.85 4.20 4.46 3.87 3.23 2.85

factorlog(A/K S cm−1)

Ionic radius of dopant [Å]

0.92

Log

σ[ S

cm

-1]

–7

–6

–5

–4

–3

–2

500°C600°C700°C800°C900°C

0.93Y

0.94Yb

0.95Tm

0.96Er

0.97Ho

Figure 3. Bulk conductivity as a function of ionic radius of dopantin BaZr0.95M0.05O3−α (where M = Er, Ho, Tm, Yb and Y) in wet N2

(PH2O = 0.031 atm).

4. Theoretical model of the correlation ofthe conductivity and electronegativity

From figure 5 we calculated the slopes and the pre-exponentialconstants of the plots of log σ vs electronegativity in therange l � 1.4, at different temperatures. The results areshown in table 3, which indicate that the slopes, β, areindependent of temperature. The average value of these isβ = −4.5267, whereas the pre-exponential constants aretemperature dependent.

399

M Laidoudi et al

Electronegativity [Pauling]

1.0 1.1Yb

1.2Er, Tm

Ho

1.3Y

1.4Zr

1.5Al

1.6

Log

arith

m o

f co

nduc

tivity

Log

σ [

S cm

–1]

–8

–7

–6

–5

–4

–3

–2500°C600°C700°C800°C900°C

Figure 4. Bulk conductivity of sintered BaZr0.95M0.05O3−α (whereM = Al, Er, Ho, Tm, Y, Yb and Zr) as a function ofelectronegativity in wet N2 (PH2O = 0.031 atm).

Electronegativity [Pauling]

1.05 1.10Yb

1.15 1.20Er, Tm

Ho

1.25 1.30Y

1.35 1.40Zr

1.45

Log

arith

m o

f th

e co

nduc

tivity

Log

σ [

S cm

–1]

–8

–7

–6

–5

–4

–3

–2

500° C600° C700° C800° C900° C

Figure 5. Bulk conductivity of sintered BaZr0.95M0.05O3−α (whereM = Er, Ho, Tm, Y, Yb and Zr) as a function of electronegativity inthe range of electronegativity (l � 1.4) in wet N2

(PH2O = 0.031 atm).

From figure 5 and table 3 we may deduce the relationshipbetween the bulk conductivity, σ , of BaZr0.95M0.05O3−α andthe electronegativity, l, of the dopant for l � 1.4 as follows:

log σ∝(−l) (3)

orσ = σ0 exp(−βl) = σ0 exp(−4.5267l) (4)

Table 3. The slopes and pre-exponential constants of theconductivities of BaZr0.95M0.05O3−α vs the electronegativity ofdopants M (where M = Er, Ho, Tm, Yb, Y and Zr) at differenttemperatures.

T (˚C) 500 600 700 800 900Slopes (P −1) −4.6171 −4.4014 −4.4781 −4.5838 −4.5535Pre-exponential −0.6269 0.0976 0.5734 1.2169 1.6612

log(σ0/S cm−1)

where σ0 is the pre-exponential constant and β is the slope ofthe log σ vs l. The value of β is taken from table 3. Thus, wemay write the conductivity as a function of temperature andelectronegativity as

σ(T , l) = σ ′0

Texp −(βl) exp −

(Ea

KbT

)(5)

where Ea and Kb have their usual meanings and σ ′0 is the

pre-exponential factor.The model has been examined qualitatively to see whether

it can be applied to the results obtained by Iwahara et al(1993) on BaZr0.95M0.05O3−α and by Yajima et al (1992) onSrZr0.95M0.05O3−α in hydrogen atmosphere. The model seemsto work qualitatively for SrZr0.95M0.05O3−α with M = Yb, Y,In, Al and Zr [11] and in BaZr0.95M0.05O3−α with M = Y,In, Ga and Zr [5]. However, the model does not workwith the results obtained from Ga-doped SrZrO3 [11] andDy- and Nd-doped BaZrO3 [5]. We may attribute this tothe different atmospheres the samples were exposed to. Inour work the conductivities were measured in wet N2 whilethose of Iwahara [5] and Yajima [11] were obtained in H2

atmosphere.

5. Conclusion

We have demonstrated the relationship between theconductivity of BaZr0.95M0.05O3−α and the electronegativity oftrivalent dopant cations for l � 1.4. For l � 1.4 we have dataonly for the compound doped by Al. For l � 1.4 it seems thatthe conductivity increases with increasing electronegativity,but we do not know exactly how they are correlated and thusfurther investigations are needed.

Acknowledgment

The authors thank the Ministry of Science, Technology andEnvironment, Malaysia for the IRPA grant (09-02-02-0006) tocarry out this project.

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