ompression followed by annealing at different temperatures. The functional properties such as porosity, percentage thermal solume and percentage densification of the sintered pellets are compiled. Bismuth oxide is found to be an effective sintering aid inhe effect of bismuth oxide addition on YZ was investigated through sintering studies, XRD, TGA/DTA, SEM and conductivity measuhe results obtained on YZ with and without bismuth oxide addition are discussed with respect to the requirement of an electrolyte fopplication.2004 Published by Elsevier B.V.
eywords:Electrolyte; Intermediate temperature solid oxide fuel cells; Electrical and sintering study
. Introduction
Fuel cells are electrochemical devices, which offer directonversion of a variety of fuels into electrical power. Sev-ral types of fuel cells are under development owing to theirnvironmental compatibility, potential for use in stationarynd distributed electric power stations as well as in trans-ortation applications. They are solid polymer electrolyte fuelells (SPEFC), alkaline fuel cell (AFC), direct methanol fuelell (DMFC), phosphoric acid fuel cell (PAFC), molten car-onate fuel cells (MCFC) and solid oxide fuel cell (SOFC).mong these systems SOFC has striking features, namely,igh energy conversion efficiency, use of non-precious ma-
erials, no liquids are involved and adaptation to variety of
fuels and invariant electrolyte[1–3]. The high temperatuoperation of SOFC causes many serious problems susevere restriction on the choice of materials, electrodetering programme, interfacial diffusion across the electroand electrodes and mechanical stresses due to differetheir thermal expansion/shrinkage value. The thermal cpatibility of the LSM cathode–YSZ interface is not adequenough on prolonged operation in SOFC resulting in themation of insulating phases across the interface leadiperformance degradation due to rise in the cell resistwith time [4]. In order to overcome these problems, thesearchers have introduced the reduced/intermediate temture solid oxide fuel cell (ITSOFC) operating at 600–800◦Care developed. Two approaches are considered in ITSresearch.
1. Use of this electrolyte membrane to make the ohmicdue to electrolyte as minimum as possible.
304 M. Kumar et al. / Materials Chemistry and Physics 92 (2005) 303–309
2. To identify new electrolyte materials, which exhibit highoxide ion conductivity at reduced temperature.
Attempts to replace YSZ by rare earth pyrochlores ex-hibiting high ion conductivity have been reported[5–12]. Inthis paper, we have discussed the synthesis, characterisationand sintering of the yttrium zirconate (YZ) parent pyrochlore.The effect of bismuth on the physical properties and sinter-ing behaviour of YZ pyrochlore based materials is broughtout.
2. Experimental
2.1. Powder preparation
High purity yttrium oxide, zirconyl nitrate, and glycinewere used as the starting materials. The stoichiometric com-positions of mixtures for the combustion were calculated us-ing the total oxidising (O) and reducing (F) valencies of thecomponents which serve as a numerical coefficient for thestoichiometric balance, so that the oxidant to the fuel ratiois 1:2.78 for the yttrium zirconate synthesis. The yttrium zir-conate (YZ) was obtained by glycine–nitrate synthesis routewhich involved rapid heating of an aqueous concentrated so-lution containing respective starting materials at 550◦C [13].T tiona asess itht eo eu xidep
Fig. 2. Thermal analysis and differential thermal analysis of yttrium zir-conate with bismuth oxide.
7.5% bismuth oxide addition and the YZ pellets containing5, 10, 15 and 20% bismuth oxide were subjected to sinter-ing at constant temperature, 1200◦C for studying the effectof bismuth oxide addition at 1200◦C, 3 h dwell time. Fromthe data obtained the sintering behaviour and the porosity ofthese pellets were measured by liquid adsorption techniqueusing dioxan as a medium. The conductivity measurementson YZ with and without bismuth oxide pellets were carriedout in air using ac impedance spectroscopy in the frequencyranging from 100 Hz to 100 kHz. The microstructure anal-ysis of these pellets were studied using scanning electronmicroscopy (SEM).
he solution initially boiled, underwent rapid degradand foaming followed by decomposition, generating guch as CO2, N2, H2O. The gases ignited and burnt wemperature above 1000◦C, yielding voluminous oxide. Thbtained oxide was calcined at 800◦C for 3 h to remove thnburnt materials and to obtain the most stable mixed ohases.
.2. Sample preparation and characterization
The combustion-derived pyrochlore powder was chaerised by powder X-ray diffraction (XRD), density, partiize, FTIR and TGA/DTA. The XRD patterns were obtaiith a diffractometer (JEOL-8030 X-ray diffractometer)
ng Cu-K� radiation. The density of the calcined pyrochlowders was measured using pycnometer with xylen
he liquid medium. Horiba Laser Particle Size AnalyLA-910) was used to determine the particle size distribuf calcined yttrium zirconate pyrochlores. Fourier transf
nfrared spectra of yttrium zirconate pyrochloresecorded using a Perkin-Elmer, paragon 500 model Fpectrometer as KBr pellets. The thermal characterisf the precalcined powder was carried out PL Therciences unit at a heating rate of 10◦C min−1 in staticir.
The combustion-derived YZ powder and powder YZ warious weight percent of bismuth oxide addition were mnd crushed in an agate mortar and pelletized by appT load. These pellets were subjected to sintering at va
emperatures ranging from 900 to 1200◦C for 0, 2.5, 5.0 an
M. Kumar et al. / Materials Chemistry and Physics 92 (2005) 303–309 305
Fig. 3. Fourier transform infrared spectroscopy for yttrium zirconate.
Fig. 4. Particle size distribution of yttrium zirconate powder.
3. Results and discussions
The XRD pattern of the parent yttrium zirconate is shownin Fig. 1. Thed-values obtained are in agreement with theformation of pyrochlore phase as reported[13] in literature.No new phase is found even for the sample with 5% bismuthoxide addition. However, the JCPDS data files correspondingto the pyrochlore oxides are not available for comparison.
Table 1Physical properties of yttrium zirconate with and without bismuth oxide
Bi2O3
(wt.%)Powder density (g cm−3) % Theoretical density
Fig. 5. The effect of temperature on percentage of theoretical density foryttrium zirconate pellet with and without bismuth oxide.
Fig. 6. The effect of bismuth oxide content on percentage theoretical densityand on percentage of porosity factor for yttrium zirconate pellet with bismuthoxide.
306 M. Kumar et al. / Materials Chemistry and Physics 92 (2005) 303–309
Fig. 7. Microstructures of yttrium zirconate pellet with and without bismuth oxide.
By using Scherrer equation the average crystallite size ofyttrium zirconate pyrochlore materials are calculated fromthe broadening of a specific diffraction peak. The equationused is:
D = 0.9λ
β cosθ
whereD is the average size of the crystallites, 0.9 is the Scher-rer constant,X the wave length of radiation,β the peak widthat half-height andθ corresponds to the peak position. Thecrystallite size of YZ is 3.782 nm. It reveals that the densifi-cation of the YZ pyrochlore compound at high temperatureis less. Because of smaller crystallite size, the material has
larger surface free energy[14]. Generally, the reduction ofsurface free energy is the driving force for sintering of thematerial[15], which is the reason for the low densificationof YZ at high temperature. Hence, in order to sinter the YZmaterials well even at low temperature, it was felt necessaryto add bismuth oxide with various weight percent as sinteringaid.
The TG and the DTA patterns obtained on YZ with bis-muth oxide addition are shown inFig. 2. A gain in the weightoccurred initially and then a gradual loss occurred up to400◦C which was followed by weight gain up to 900◦C. TheDTA curve shows broad exothermic peak at 320◦C, whichis due to evolution of gases as observed in the TG curve.
M. Kumar et al. / Materials Chemistry and Physics 92 (2005) 303–309 307
Fig. 8. An idealized equivalent circuit (b) and its corresponding impedance plot (a).Cb,Rb andCgb,Rgb andCel,Rel represents resistance and capacitance forbulk, grain boundary and electrode process, respectively.
The DTA curve for YZ with bismuth oxide did not showany sharp endothermic peak which from due to melting ofbismuth oxide, at 824◦C, the melting point of bismuth ox-ide. The TG and DTA analysis did not show a weight loss inTG and endothermic peak in DTA in the temperature rangebetween 600 and 900◦C. It reveals that there is no indica-tion for removal of leaving any extra oxygen from the lattice[16].
The FTIR spectra obtained on pyrochlore materials areshown inFig. 3. It shows that the peak observed at 500 cm−1
corresponds to the pyrochlore phase, which is in good agree-ment with the literature data[13].
The particle size distribution of YZ is shown inFig. 4. It isevident that 60% of the particles in the bulk are below 7.3�min size. The bulk, tap and absolute densities of YZ powderwith and without bismuth oxide addition are presented inTable 1.
The effect of bismuth oxide addition on the densifica-tion behaviour of the representative compositions Y2Zr207with respect to temperature is clearly evident fromFig. 5,in which percent theoretical density is plotted against thesintering temperature. For all of the samples the extent ofsintering is negligible below 800◦C. For YZ without bis-muth oxide, the densification increases very slowly withincreasing temperature. A maximum of around 40% den-sification occurs at a sintering temperature of 1200◦C.Bt nd7
ture,t ddi-t inedp rcento iti to
40% at 1200◦C, but the percentage of densification increaseswith increase in the bismuth oxide content in YZ. This resultclearly shows the effectiveness of bismuth oxide as sinteringaid for the material, YZ.
The porosity values of the yttrium zirconate with and with-out bismuth oxide were measured by liquid adsorption tech-nique using dioxan as a medium. The effect of weight percentof bismuth oxide content on porosity factor is shown inFig. 6.It is evident that the percentage porosity factor decreases asfunction of bismuth oxide content in sintered YZ at 1200◦C,3 h.
The effectiveness of bismuth oxide addition as sinteringaid is also evident from the SEM fractograph of the sinteredspecimens as shown inFig. 7(a–e). A progressive change inmicrostructure with bismuth oxide is clearly visible.Fig. 7(a)shows no grains in the structure but small grains and smallpores are formed with addition of bismuth oxide.Fig. 7(c)shows dentritic structure. Even 20% bismuth oxide addi-tion is not sufficient enough for complete densification ofYZ.
The ionic conductivity of Y2Zr2O7 with and without bis-muth oxide addition was measured by a two probe compleximpedance analysis. The ionic conductivity (σ) has been mea-sured in the range of temperature 500–800◦C. For all the sam-ples of YZ mixed with 5–20% bismuth oxide, all of whichwere sintered at 1200◦C for 3 h. In general, the ac impedanceo con-t lec-t om-pT s thep rep-r pec-t lowf n ide-
ut there is sudden increase in density at 1200◦C for yt-rium zirconate with bismuth oxide addition of 2.5, 5.0, a.5 wt.%.
For enhancing the sintering rate, at a fixed temperahe effect of various weight percent of bismuth oxide aion (i.e., 5, 10, 15 and 20%) has been studied. The obtaercent theoretical density is plotted against various pef bismuth oxide and shown inFig. 6. From the graph,
s found that the YZ without bismuth oxide densify only
f an ionic conductor measured by a two probe methodains contributions from the bulk, grain boundaries and erode/electrolyte interface, which can be reflected in a clex plane by three successive arc, as shown inFig. 8(a).he frequency increases from the right to the left acroslot. The arc at the high frequency end of the spectrumesents the bulk resistivity; the arc at the middle of the srum is a consequence of the grain boundary effect; therequency arc is assigned to the electrode response. A
308 M. Kumar et al. / Materials Chemistry and Physics 92 (2005) 303–309
Fig. 9. The impedance plots of yttrium zirconate samples with differentbismuth contents measured at 500◦C in air.
Fig. 10. Arrhenius plot for bulk conductivity of yttrium zirconate withoutbismuth oxide.
alized equivalent circuit for ceramic oxides corresponding tothe impedance plot is shown inFig. 8(b). In a practical case,however, not all these arcs can be observed, depending on thenature of the samples and testing conditions.Fig. 9shows theimpedance plots of samples with different bismuth contentsmeasured at 500◦C in air. This figure clearly shows that theaddition of bismuth oxide has a detrimental effect on grainboundary behaviour.
Fig. 10represents the plot of logσ and 1/T for the samplewithout bismuth oxide. It was found that a straight line andthe conductivity increases with increasing temperature. Theactivation energy of the plot is 7.841 kJ.
The effect of temperature on ionic conductivity for thesamples with various percent of bismuth oxide is shown in
Fig. 11. Arrhenius plots for bulk conductivity of yttrium zirconate withbismuth oxide.
Fig. 11. It was found that straight lines are obtained for thesamples with 5, 10, 15 and 20%. The conductivity values andenergy of activation are given inTable 1. From the graph,it clear that at 500◦C, the conductivity value increases withincreasing bismuth oxide content and the conductivity valuedecreases with increasing bismuth oxide content at 800◦C[17]. The reason for the conductivity change was also identi-fied from microstructures of those pellets. As expected, moreand more dense microstructure is formed with increasing bis-muth oxide content. Instead, the particles are grained withsmall pores, i.e., small pores increases with increasing bis-muth oxide content, however it has 83% of theoretical densityat 20% bismuth oxide added YZ which sintered at 1200◦C.As a result of experiment, the reduction of conductivity asincreasing the bismuth oxide may be due to the formation ofpores during sintering as shown in microstructure and com-pared with the unmixed YZ, bismuth oxide mixed samplesexhibit lower conductivity. This results mainly from an en-larged grain boundary effect[18] in bismuth oxide mixedsamples.
The effect of bismuth oxide on energy of activation andconductivity of pellets measured at 800◦C with and withoutbismuth oxide content are shown inFig. 12. It reveals thatthe energy of activation decreases with increasing bismuthoxide content, but the conductivity of pellets measured at800◦C initially decrease and the conductivity neither increasen As ar le fort th.C s nom ion.T nda
or decrease with increasing bismuth oxide content.esult, higher energy of activation seems to be responsibhe higher conductivity in yttrium zirconate without bismuompared to yttrium zirconate with bismuth oxide, there iarked difference in conductivity and energy of activathe energy of activation is not only for ion migration also responsible for defect formation[16].
M. Kumar et al. / Materials Chemistry and Physics 92 (2005) 303–309 309
Fig. 12. The effect of bismuth oxide content on bulk conductivity measuredat 800◦C and on energy of activation for yttrium zirconate pellets.
4. Conclusion
The glycine–nitrate combustion synthesis is a simple andconvenient method to prepare yttrium zirconate powders. Thethermal behaviour of YZ pellets is brought out from the steadystate sintering experiments to draw useful information on theinter-dependence of percentage shrinkage in volume and thepercentage densification factor with respect to the maximumattainable apparent percentage porosity for the first time. Thebulk conductivity value of YZ is in the range of 103 S cm−1
at temperature range between 500 and 800◦C bismuth oxidefound to be an effective sintering aid. Even though it has in-creased the densification, it decreases the conductivity valueof yttrium zirconate.
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