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Influence of preparation procedure on some physical properties of BICUVOX
70
Materials Science a n d Engineering, t~2/
! 9~ ~ 7~-~: ~
Influence of preparation procedure on some physical properties of BICUVOX F. Krok, W. Bogusz, P. Kurek, M. Wasiucionek, W. Jakubowski and J. Dygas Institute of Physics, Warsaw University of Technology, Ul. Chodkiewicza 8, 02-525 Warszawa (Poland) (Received February 2, 1993)
Abstract The influence of preparation conditions on some physical properties of BICUVOX. 10 (Bi2V0.90Cu0.10Os.35) is reported. Differential thermal and X-ray analyses indicate that synthesis of the compound can be accomplished at 610 °C. The densification characteristics show that high density BICUVOX ceramic can be obtained after several minutes of sintering at 800 °C. The electrical conductivity was studied by a.c. impedance methods. Separation of the contributions of grain interiors and grain boundaries to the total resistance was possible at temperatures below around 350 °C. The Arrhenius plots of the total conductivity consist of two straight line sections with a change in slope at around 500 °C. The activation energy was about 0.48 eV at high temperatures for all the samples. Below around 500 °C the activation energy was in the range from 0.62 to 0.72 eV depending on the preparation procedure. Lower values of activation energy and higher values of conductivity were obtained for well-sintered samples, which is related to the smaller contribution of the grain boundaries to the total electrical resistance of a sample.
1. Introduction Oxygen ion conductors can be applied in many electrochemical devices, e.g. as solid electrolytes in sensors of the partial pressure of oxygen or in fuel cells. However, the low conductivity of the currently used electrolytes (e.g. yttria- or calcia-stabilized zirconia) causes serious problems--the devices based on them have to be heated to high temperatures in order to achieve high enough conductivity. This creates some complications in terms of the construction of various devices and decreases the area of their application. Therefore particular attention is now paid to the search for new materials which exhibit high conductivity at lower temperatures. The new family of compounds characterized by the formula Bi2V l_xMexOS.5_ 3x/2 (0 ~ X ~ 1, Me is a metal such as Cu, Ni etc.), called BIMEVOX, and exhibiting high ionic conductivity was described recently [1-3]. The highest conductivity was found in BICUVOX characterized by the value of x equal to 0.1 (BICUVOX.10); at around 250 °C the conductivity was nearly two orders of magnitude higher than that for zirconia-based electrolytes. The interest in these materials arises not only from their potential applications. The mechanism of such high conduction of oxygen ions is also interesting. However, the experimental data for BIMEVOX compounds are very scanty to date. First of all, investiga0921-5107/93/$6.00
tions in the field of technology are necessary to establish the relation between conditions of preparation and the physical properties of the obtained material. In this work the results of such an investigation in the case of BICUVOX.10 are reported.
2. Processing of ceramic samples of BICUVOX Polycrystalline BICUVOX was prepared from Bi203, V205 and CuO (all analytical grade). The preparation consists of the following steps: (1) the wet ball milling (in toluene with zirconia balls) of the mixture of raw materials; (2) the synthesis of the compound carried out for 20 h at a chosen temperature; (3) the postsynthesis milling (as in the starting step) with the addition of a binder (polyethylene glycol); (4) the isostatic pressing of the pellet-shaped samples (10 mm in diameter, 2-4 mm in thickness); and (5) the sintering of the samples. All these steps in the preparation of BICUVOX can affect the properties of the resulting material, e.g. through the influence on the morphology of the powder or on the degree of completion of the synthesis reaction. The powders were milled (steps 1 and 3) for about 20 h. The milling was carried out for such a long time before synthesis in order to get fully homogeneous © 1993 - Elsevier Sequoia. All rights reserved
F. Krok et al.
/
Preparation of B1CUVOX
powder to facilitate the reaction, and after synthesis in order to crush the obtained agglomerates of grains. The proper choice of the experimental conditions applied in the other steps of BICUVOX preparation (synthesis temperature, value of isostatic pressure and sintering temperature) was the result of investigations.
2.1. The synthesis of BICUVOX To choose the proper temperature for synthesis by reaction in the solid state two criteria should be taken into account: (1) the powders should completely react; (2) the process of the synthesis reaction should be separated from the process of sintering. Because of the first criterion the synthesis reaction should be carried out at the highest possible temperature. However, such a choice would be in disagreement with the second criterion, since heating powders at high temperatures causes simultaneous sintering of the powder. Sintering disturbs synthesis because mass streams in both processes are perpendicular each to other. It is thus concluded that the synthesis temperature should be sufficiently high to allow the reaction to achieve completion but low enough to avoid the process of sintering at this stage. The investigation of the synthesis reaction of BICUVOX was carried out by means of differential thermal analysis (DTA) measurements (Derivatograph Q-1500D, MOM, Budapest). The DTA curve for the mixture of powders of the chemical composition corresponding to BICUVOX. 10 which was wet milled and dried in air is shown in Fig. 1 (curve a). Curve b in the same figure represents the measurements made on a mixture of powders of the same composition which was not wet milled but only ground in a mortar for 0.5 h. Two thermal events can be distinguished in these runs. The first is the exothermic peak in curve a with the centre at about 200 °C. Since this peak is absent in the curve for dry-milled powder we attribute it to the residual organics in the wet-milled powder. The second peak, also exothermic, was found for a temperature
c
J t-
~6 × t~
6 160 260 360 460 s60 660 760 s60 t [°C]
Fig. 1. DTA curves of mixed powders of Bi20~ , V205 and CuO obtained in air at l0 °C m i n - l : curve a, first heating of wetmilled (20 h) material; curve b, first heating of dry-milled (0.5 h) material; curve c, second heating.
71
close to 600 °C. We ascribe this peak to the synthesis reaction of BICUVOX. This is in agreement with the suggestion given by Abraham et al. [1] who carried out synthesis of BICUVOX at 600 °C. The shape and the position of the second peak depend on the history of the powders. The fine-grained homogeneous powder (wet milled for 20 h) exhibits a broad peak with the centre at 595 °C, whereas less homogeneous powder (dry milled for 0.5 h) exhibits a much narrower peak shifted to higher temperatures (with the centre at about 610 °C). On the basis of DTA measurements the temperature of 610 °C was chosen as the temperature at which the process of synthesis was carried out in this preparation of BICUVOX. 10. Finally it was found that the repeated-DTA run (curve c) did not show any thermal events in the studied materials in the temperature range from 20 to 700 °C. This was also so for the materials which were allowed to stay in air for many days.
2.2. The isostatic pressing Measurements of the green density of the samples as a function of applied pressure were carried out in order to choose the proper value of isostatic pressure. The applied pressure was in the range from 50 to 400 MPa. The results--logarithm of density against pressure-plotted in logarithmic coordinates are presented in Fig. 2. The change in the slope of the curve plotted in such coordinates indicates the change in susceptibility of a studied material to pressure. This susceptibility is related to the forces of internal friction counteracting the densification of the grainy material [4]. At the value of pressure corresponding to the change in slope a loss of cohesion of agglomerates of crystallites occurs. This loss of cohesion is related to breaking of the bridges which bind crystallites into agglomerates. The value of isostatic pressure applied during the pressing of pellets should be higher than this critical value in order to form compacts of high enough density. Preparation of samples with such a value of pressure creates advantageous conditions for sintering. On the basis of this study, an isostatic pressure of 350 MPa was used in the preparation of BICUVOX pellets. It turned out that the final density of the sintered material did not depend significantly on the pressure applied in preparation of the green material. The criterion concerning a high enough value of isostatic pressure is mainly important in the case when sintering is controlled by the solid state diffusion process. However, in a situation when the liquid phase is present during sintering, the density of the sintered material should not be strongly affected by the degree to which the powder was compacted during pressing. This was the case encountered in our study.
72
t;~ Krok et al.
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I'reparation ~f [ I( (., I~OX
2.3. The density o f B I C U V O X . I O material
The sintering of the isostatically pressed discs of BICUVOX.10 was carried out in air on an alumina plate. The samples were isolated from the plate by a layer of BICUVOX powder. The temperature and time of sintering were varied in the range from 740 to 870 °C and from 0.25 to 40 h respectively. The density of the sintered samples was measured by immersion in isobutyl alcohol. The densification characteristics for two sintering temperatures are represented in Fig. 3. The influence of the temperature of sintering on the density of samples sintered for a fixed time (20 h) is shown in Fig. 4. It can be seen that the proper sintering of BICUVOX.10 requires a temperature close to 800 °C. The high density can be obtained at this temperature even after several minutes of sintering. As can be seen in Fig. 4, sintering carried out at temperatures lower than
_~ 0.26
!
(125
3. Properties of the sintered BICUVOX 3.1. The crystal structure
The crystal structure of the obtained material was examined by X-ray diffraction analysis. The powder pattern obtained for a well-sintered sample is presented in Fig. 5--it reveals only tetragonal symmetry in the investigated material. The calculated lattice parameters listed in Table 1 are in good agreement with those obtained by Abraham et al. [1 ], also submitted in the table. The crystallographic data obtained for our samples were found to be practically independent of the sintering conditions; the lattice
;!
oo/ 21o
780 °C did not allow samples of high density to bc obtained. Overheating also led to a decrease of the density of the material which is related to the formation of internal pores visible by microscope.
o~ ~~ 7"0I
g 6.5t
~oL
215
760
8oo
9o0
sintering temp. [°C ]
tog P [ MPQ ]
Fig. 2. The logarithmic dependence of the logarithm of green density on the value of the isostatic pressure.
Fig. 4. Effect of sintering temperature oil the density of the BICUVOX.10 samples sintered for 20 h.
r %
0 03
Q - 793 °C
7.0 b - 813 °C
C "0 6.5
6.0
I 10
I 20 sintering time [ h ]
Fig. 3. The densification characteristics for BICUVOX samples sintered at 793 °C (curve a) and 813 °C (curve b).
30
40
50
60
70
28--~
Fig. 5. X-ray diffraction pattern (Co Ka radiation) of the obtained BICUVOX.10 material.
F. Krok et al. /
Preparationof BICUVOX
73
TABLE t. Unitcell constantsderived from X-ray diffractionof BICUVOX.10 material
TABLE 2. The densityand microhardnessof BICUVOX.10 samples sintered under various conditions
Reference
a (A)
c (A)
Sample
This work 1
3.92 3.907
15.44 15.41
parameters for the samples sintered at various temperatures and durations were essentially the same. 3.2. Microhardness The microhardness of the BICUVOX.10 samples was measured by the Vickers method. The results are presented in Table 2. The temperature and time of sintering as well as the density of the measured samples are submitted in the same table. A correlation between density and microhardness is observed. The higher the density of the samples, the higher the microhardness, whereas the samples of lower density, regardless of the cause of low density (sintered at too low a temperature, overheated etc.), are not so hard.
2 4 7 10 11 14
Sinteringconditions Temperature (°C)
Time (h)
764 740 847 813 793 793
20 20 20 0.25 0.25 5
Microhardness (kGmm -2)
6.31 6.10 6.52 7.10 6.86 7.00
340 75 270 500 200 400
T=94':'C
100 go
G
o •
•
\
1'00
2'00
3'00 T= 202.5°C
1,0 OoooO •
•
• Ooqbqdpo
20'
3.3. The electrical conductivity of BICUVOX.IO Electrical properties of ceramic BICUVOX.10 samples were investigated here by means of the impedance spectroscopy method in the frequency range (10-5)x105 Hz using a Tesla B M 5 0 7 impedance meter. The samples had the shape of a long rectangular bar (approximately 3 x 3 x 6 mm 3) or a pellet (10 mm in diameter and 2-3 mm in thickness) and were covered with Pt electrodes sputtered in a cathode discharge in air. The electrodes possessed some degree of porosity and allowed penetration of atmospheric gases to the surface of the electrolyte. The impedance of the samples was measured in the temperature range from about 100 to 650 °C (a small number of samples was investigated up to 750 °C). The thermal evolution of the impedance spectra of the samples is represented by three selected plots shown in Fig. 6. The plots represent partial impedance spectra because of the limited frequency range used in our measurements. Below 200 °C the plots consist of two semicircles (Fig. 6(a)). The left-hand high frequency semicircle beginning at the origin of coordinates expresses the impedance dispersion caused by the geometrical capacitance of the sample. The right-hand semicircle is related to grain boundaries. For the flat thin samples two such semicircle plots can be obtained for lower temperatures (as in Fig. 6(a)) than for the long samples with small cross-section area.
Density (gcm -~)
'
/
/
£0 T= 354 °C
0.05
0.10
~
015'
02'0
0.25' R/k~
Fig. 6. Measured impedance spectra for (a) sample 1, (b) sample
5 and (c) sample 10.
From around 200 to 350 °C elements of the lefthand semicircle disappear (they are displaced towards high frequencies out of the frequency range used) and on the low frequency side (right) part of a third semicircular arc, attributed to electrodes, appears (Fig. 6(b)). In both temperature ranges the separate determination of the bulk electrical resistance R b and the grain boundary resistance Rgb of the sample is possible. Above 350 °C elements of the second semicircle gradually disappear from the frequency range used; the third semicircle passes into a straight line with a slope close to 45 ° (Fig. 6(c)) and, at still higher temperatures, decreases and disappears leaving only the 45 ° straight line. Therefore only the total resistance R t = R b + Rgb of the sample can be determined in this range of temperatures. It will be shown, however, that at high
74
t~ Krok et al.
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Preparation ~f fllCU VOX
temperatures the difference in values between the total and the bulk conductivities becomes negligible (this corresponds to disappearance of the third semicircle). The "partial" impedance spectra presented in Fig. 6 constitute for each case some part of the entire impedance spectrum shown schematically in Fig. 7. A whole spectrum of the entire shape of Fig. 7 was never found for the range of frequencies used for any investigated sample--this range of frequencies was too narrow to allow determination of all dispersions for one given temperature. Electrical parameters of the samples were derived from experimental data by means of fitting the impedance function of a chosen electrical equivalent circuit to the measured data. The function was fitted using the least-squares method. It was found that the electrical equivalent circuit shown in Fig. 8 can successfully model the entire spectrum of Fig. 7: the partial impedance spectra can be modelled by the corresponding circuit being a "part" of the circuit of Fig. 8. Such partial equivalent circuits were applied in the fitting procedure. The symbols used to describe the elements of the equivalent circuit in Fig. 8 are the same as those in the description of Fig. 7. In this paper only the electrical conductivity and the influence of technology on it will be discussed. The
other parameters derived in the course ot fitting the impedance of the equivalent circuit to the measured data will be the subject of another paper. The dependence of the electrical conductivities of the samples on the temperature was as usual plotted in ln(oT) vs. 1/T coordinates (Arrhenius plots). For the samples under investigation, this dependence plotted for the total conductivity o t is composed of two straight line segments with a smooth change in slope at a temperature close to 500 °C i486-508 °C depending on the sample). The high temperature segment of the plot was essentially the same for all the samples (in the limits of measurement errors), despite the differences in density, porosity etc. The Arrhenius dependence of the total conductivity for three samples is shown in Fig. 9. The presented data were chosen purposely to illustrate the influence of sintering on electrical conductivity (sample 3 was sintered at too low a temperature, sample 10 was well sintered and sample 1 was overheated). The plots correspond to the impedance spectra taken after the samples were first heated over 500 °C; otherwise during the first heating cycle the obtained values of the conductivity are slightly lower than those for cooling-the first heating over 500 °C "stabilizes" the electrical conductivity and no hysteresis was found for subsequent cycles of heating and cooling. The difference in conductivity between the first heating and subsequent cooling is shown in Fig. 10.
-X
8.°I v" 4.0
R~
R2 R3
R
Fig. 7. Whole impedance spectrum of a BICUVOX.10 sample (schematic).
"7 I -I-- / 0.
"~No.1) 0.631eV ~o.10) 0.616eV
-,.oF
I (N°3)0"730ieV ~1 xN~° Fig. 8. The proposed electrical equivalent circuit of a BICUVOX.10 sample corresponding to the whole impedance spectrum of Fig. 7 (Rl = Rb, R2= Ru + Rgb, R3= Rb + Rgb + Rc; W denotes Warburg impedance; Cg is the geometrical capacitance; Pd~ and Pgu are the constant phase angle (cpa) elements for the double layer and grain boundaries respectively).
110
1.5
2.0 2.5 IO00/T [ K -1]
Fig. 9. Temperature dependence of the total electrical conductivity for three samples of BICUVOX.10. Sintering conditions for the samples are given in Table 2.
F. Krok et al.
/
4.0[
8.0
v. 2. 0
o
//
4.o
(3
75
Preparation of BICUVOX
o
0
o.o '
I-.10
"~'~
~. ,~_(b/ 0.77_~2e V
I--'
q -2.0
0.0 (Q)
(c) 0.713 oV
-4.0
-4.0
__
_.,
_0
"~
0.656 eV
--<.q.-...
eV
-6.0
I
I
1.0
1.5
[
[
2.0 2.5 IO00/T [ K4 ]
I
1.8
I
I
2.0
2.2
I
2.4
IO00/T [ K -1]
Fig. 10. Temperature dependence of the total electrical conductivity for sample 3: curve a, first heating; curve b, cooling.
Fig. 11. Temperature dependence of the bulk (curve a), grain boundary (curve b) and total (curve c) electrical conductivity of sample 3 for cooling from 750 °C.
The electrical conductivity at low temperatures (less than 500 °C) is dependent on the technology conditions of the samples. The activation energy for the samples sintered at too low a temperature (0.72 eV)is higher than that for overheated (0.631 eV) or optimally sintered (0.62 eV) samples. The conductivity at 300 °C for the first set of samples is about 1 x 1 0 - 3 S cm- 1 whereas for the other two it is close to 2 x 10 -3 S
carried out by means of DTA leads to the following conclusions. ( 1 ) The starting powders have completely reacted at a temperature around 600 °C (peaks related to the starting powders of Bi203, V205 and CuO are not present in the DTA curve). (2) The proper temperature of synthesis should be chosen taking into account the degree of homogeneity of the powder related to the quality of milling. (3) The synthesized material is thermally stable. The densification characteristics show that the BICUVOX.10 samples can be sintered at 800 °C in a very short time (a few minutes only). The fast sintering kinetic suggests the presence of some amount of liquid phase during sintering. Additional confrmation of this is the fact that the density of sintered samples did not depend significantly on the value of the pressure applied in isostatic pressing of green samples, which has already been discussed in Section 2.2. The microhardness, which may be considered as a measure of the mechanical properties of a material, is quite low for all BICUVOX samples (about 500 kG mm -2 for properly sintered material compared with about 1000 kG mm 2 for fl-A1203). Its proportionality to the density of samples indicates that the mechanical properties of BICUVOX are mostly controlled by intergrain or grain boundary phases. The value of the total electrical conductivity at 300 °C for low density samples (equal to about 1 × 10-3 S cm-1) is not much lower than that for high
cm-
1.
The separation of the bulk and the grain boundary conductivities was possible only at temperatures lower than around 350 °C. The temperature dependence of both conductivities for one of the samples is shown in Fig. 11. The activation energies for both do not differ much from one another (0.656 _ 0.003 eV for the bulk against 0.713 _+0.008 eV for the total conductivity) and the values of the conductivity at 300 °C amount to 1 . 4 3 x 1 0 -3 S cm-~ and 0 . 9 5 x 1 0 3 S c m i respectively. At temperatures below 100°C a deviation of In(aT) vs. 1 / T from the straight line is observed. This seems to be related to the increase in conductivity of BICUVOX samples caused by atmospheric moisture.
4. Discussion The investigations carried out in the field of technology allow the best conditions for preparation of BICUVOX. 10 material to be established. The analysis
76
F. Krok et al.
/
Preparation o['BI('UVOX
density samples (about 2 x 10 .3 S cm-~). It may be concluded therefore that, for BICUVOX, processing conditions play a much less important role than for other solid state electrolytes. The sintering conditions (temperature and time) affect the ionic conductivity of BICUVOX at temperatures below 500 °C while the values of the conductivity at high temperatures are practically independent of the preparation procedure. The values of the activation energy of the conductivity at temperatures below 500 °C for well-sintered samples are lower than for samples not fully sintered owing to the use of too low a sintering temperature. Lower values of conductivity and higher values of activation energy were observed at temperatures below 500 °C during the first heating cycle than during cooling and subsequent heating-cooling cycles. Much larger differences between the conductivity values measured during the cooling and heating cycle were reported by Reiselhuber et al. [5] for BICUVOX samples prepared from coarse-grained materials. The results obtained by them for samples prepared from finely milled powders are similar to ours. The change of slope in the Arrhenius plots of conductivity at temperatures of about 500 °C seems to be related to a certain order-disorder transition in the arrangement of oxygen ions in the BICUVOX structure. Our DTA measurements did not reveal any thermal effects at temperatures around 500 °C in sintered material. However, a small endothermic peak in the DTA curve at a temperature around 480 °C was reported for BICUVOX. 10 by Reiselhuber et al. [5]. The value of activation energy of the total conductivity is significantly higher (Ea> 0.71 eV) for low density porous samples than for high quality dense samples (E a ~ 0.65 eV). When we refer those values to
the activation energies for bulk conduction ( E~ ~ 0.64 eV on average) and grain boundary conduction (0.78 eV), it may be concluded that the activation energy of the total conductivity for high density samples is close to the activation energy for bulk intragrain conduction. The value of the activation energy of the total conduction for low density samples (which is close to 0.75 eV ) is much more grain boundary like. Such behaviour is caused by larger values of grain boundary resistance in low density samples, which dominates the total resistance of the sample at low temperatures.
Acknowledgments The authors express sincere thanks to Dr. M. Psoda (Warsaw University of Technology) for performing X-ray analysis and Professor M. W. Breiter and Dr. K. Reiselhuber (Technische Universitat Wien) for helpful discussions and communicating their results on BICUVOX before publication. This work has been supported by the Komitet Badafi Naukowych under project PB/1179/2/91.
References 1 F. Abraham, J. C. Bovin, G. Mairesse and G. Nowogrocki, Solid&ate lonics, 40-41 (1990) 934. 2 T. Iharada, A. Hammouehe,J. Fouletier,M. Kleitz, J. C. Bovin and G. Mairesse, Solid State lonics, 48 ( 1991 ) 257. 3 R.N. Vannier, G. Mairesse, G. Nowogrocki,F. Abraham and J. C. Bovin, Solid State Ionics, 53-56 (1992) 713. 4 K. Haberko, Scientific Bulletins of the University of Mining and Metallurgy (Cracow), Ceramics, No. 47, University of Mining and Metallurgy,Cracow, 1983. 5 K. Reiselhuber, G. Dorner and M. W. Breiter, Electrochim. Acta, in press.
Materials Science a n d Engineering, t~2/
! 9~ ~ 7~-~: ~
Influence of preparation procedure on some physical properties of BICUVOX F. Krok, W. Bogusz, P. Kurek, M. Wasiucionek, W. Jakubowski and J. Dygas Institute of Physics, Warsaw University of Technology, Ul. Chodkiewicza 8, 02-525 Warszawa (Poland) (Received February 2, 1993)
Abstract The influence of preparation conditions on some physical properties of BICUVOX. 10 (Bi2V0.90Cu0.10Os.35) is reported. Differential thermal and X-ray analyses indicate that synthesis of the compound can be accomplished at 610 °C. The densification characteristics show that high density BICUVOX ceramic can be obtained after several minutes of sintering at 800 °C. The electrical conductivity was studied by a.c. impedance methods. Separation of the contributions of grain interiors and grain boundaries to the total resistance was possible at temperatures below around 350 °C. The Arrhenius plots of the total conductivity consist of two straight line sections with a change in slope at around 500 °C. The activation energy was about 0.48 eV at high temperatures for all the samples. Below around 500 °C the activation energy was in the range from 0.62 to 0.72 eV depending on the preparation procedure. Lower values of activation energy and higher values of conductivity were obtained for well-sintered samples, which is related to the smaller contribution of the grain boundaries to the total electrical resistance of a sample.
1. Introduction Oxygen ion conductors can be applied in many electrochemical devices, e.g. as solid electrolytes in sensors of the partial pressure of oxygen or in fuel cells. However, the low conductivity of the currently used electrolytes (e.g. yttria- or calcia-stabilized zirconia) causes serious problems--the devices based on them have to be heated to high temperatures in order to achieve high enough conductivity. This creates some complications in terms of the construction of various devices and decreases the area of their application. Therefore particular attention is now paid to the search for new materials which exhibit high conductivity at lower temperatures. The new family of compounds characterized by the formula Bi2V l_xMexOS.5_ 3x/2 (0 ~ X ~ 1, Me is a metal such as Cu, Ni etc.), called BIMEVOX, and exhibiting high ionic conductivity was described recently [1-3]. The highest conductivity was found in BICUVOX characterized by the value of x equal to 0.1 (BICUVOX.10); at around 250 °C the conductivity was nearly two orders of magnitude higher than that for zirconia-based electrolytes. The interest in these materials arises not only from their potential applications. The mechanism of such high conduction of oxygen ions is also interesting. However, the experimental data for BIMEVOX compounds are very scanty to date. First of all, investiga0921-5107/93/$6.00
tions in the field of technology are necessary to establish the relation between conditions of preparation and the physical properties of the obtained material. In this work the results of such an investigation in the case of BICUVOX.10 are reported.
2. Processing of ceramic samples of BICUVOX Polycrystalline BICUVOX was prepared from Bi203, V205 and CuO (all analytical grade). The preparation consists of the following steps: (1) the wet ball milling (in toluene with zirconia balls) of the mixture of raw materials; (2) the synthesis of the compound carried out for 20 h at a chosen temperature; (3) the postsynthesis milling (as in the starting step) with the addition of a binder (polyethylene glycol); (4) the isostatic pressing of the pellet-shaped samples (10 mm in diameter, 2-4 mm in thickness); and (5) the sintering of the samples. All these steps in the preparation of BICUVOX can affect the properties of the resulting material, e.g. through the influence on the morphology of the powder or on the degree of completion of the synthesis reaction. The powders were milled (steps 1 and 3) for about 20 h. The milling was carried out for such a long time before synthesis in order to get fully homogeneous © 1993 - Elsevier Sequoia. All rights reserved
F. Krok et al.
/
Preparation of B1CUVOX
powder to facilitate the reaction, and after synthesis in order to crush the obtained agglomerates of grains. The proper choice of the experimental conditions applied in the other steps of BICUVOX preparation (synthesis temperature, value of isostatic pressure and sintering temperature) was the result of investigations.
2.1. The synthesis of BICUVOX To choose the proper temperature for synthesis by reaction in the solid state two criteria should be taken into account: (1) the powders should completely react; (2) the process of the synthesis reaction should be separated from the process of sintering. Because of the first criterion the synthesis reaction should be carried out at the highest possible temperature. However, such a choice would be in disagreement with the second criterion, since heating powders at high temperatures causes simultaneous sintering of the powder. Sintering disturbs synthesis because mass streams in both processes are perpendicular each to other. It is thus concluded that the synthesis temperature should be sufficiently high to allow the reaction to achieve completion but low enough to avoid the process of sintering at this stage. The investigation of the synthesis reaction of BICUVOX was carried out by means of differential thermal analysis (DTA) measurements (Derivatograph Q-1500D, MOM, Budapest). The DTA curve for the mixture of powders of the chemical composition corresponding to BICUVOX. 10 which was wet milled and dried in air is shown in Fig. 1 (curve a). Curve b in the same figure represents the measurements made on a mixture of powders of the same composition which was not wet milled but only ground in a mortar for 0.5 h. Two thermal events can be distinguished in these runs. The first is the exothermic peak in curve a with the centre at about 200 °C. Since this peak is absent in the curve for dry-milled powder we attribute it to the residual organics in the wet-milled powder. The second peak, also exothermic, was found for a temperature
c
J t-
~6 × t~
6 160 260 360 460 s60 660 760 s60 t [°C]
Fig. 1. DTA curves of mixed powders of Bi20~ , V205 and CuO obtained in air at l0 °C m i n - l : curve a, first heating of wetmilled (20 h) material; curve b, first heating of dry-milled (0.5 h) material; curve c, second heating.
71
close to 600 °C. We ascribe this peak to the synthesis reaction of BICUVOX. This is in agreement with the suggestion given by Abraham et al. [1] who carried out synthesis of BICUVOX at 600 °C. The shape and the position of the second peak depend on the history of the powders. The fine-grained homogeneous powder (wet milled for 20 h) exhibits a broad peak with the centre at 595 °C, whereas less homogeneous powder (dry milled for 0.5 h) exhibits a much narrower peak shifted to higher temperatures (with the centre at about 610 °C). On the basis of DTA measurements the temperature of 610 °C was chosen as the temperature at which the process of synthesis was carried out in this preparation of BICUVOX. 10. Finally it was found that the repeated-DTA run (curve c) did not show any thermal events in the studied materials in the temperature range from 20 to 700 °C. This was also so for the materials which were allowed to stay in air for many days.
2.2. The isostatic pressing Measurements of the green density of the samples as a function of applied pressure were carried out in order to choose the proper value of isostatic pressure. The applied pressure was in the range from 50 to 400 MPa. The results--logarithm of density against pressure-plotted in logarithmic coordinates are presented in Fig. 2. The change in the slope of the curve plotted in such coordinates indicates the change in susceptibility of a studied material to pressure. This susceptibility is related to the forces of internal friction counteracting the densification of the grainy material [4]. At the value of pressure corresponding to the change in slope a loss of cohesion of agglomerates of crystallites occurs. This loss of cohesion is related to breaking of the bridges which bind crystallites into agglomerates. The value of isostatic pressure applied during the pressing of pellets should be higher than this critical value in order to form compacts of high enough density. Preparation of samples with such a value of pressure creates advantageous conditions for sintering. On the basis of this study, an isostatic pressure of 350 MPa was used in the preparation of BICUVOX pellets. It turned out that the final density of the sintered material did not depend significantly on the pressure applied in preparation of the green material. The criterion concerning a high enough value of isostatic pressure is mainly important in the case when sintering is controlled by the solid state diffusion process. However, in a situation when the liquid phase is present during sintering, the density of the sintered material should not be strongly affected by the degree to which the powder was compacted during pressing. This was the case encountered in our study.
72
t;~ Krok et al.
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I'reparation ~f [ I( (., I~OX
2.3. The density o f B I C U V O X . I O material
The sintering of the isostatically pressed discs of BICUVOX.10 was carried out in air on an alumina plate. The samples were isolated from the plate by a layer of BICUVOX powder. The temperature and time of sintering were varied in the range from 740 to 870 °C and from 0.25 to 40 h respectively. The density of the sintered samples was measured by immersion in isobutyl alcohol. The densification characteristics for two sintering temperatures are represented in Fig. 3. The influence of the temperature of sintering on the density of samples sintered for a fixed time (20 h) is shown in Fig. 4. It can be seen that the proper sintering of BICUVOX.10 requires a temperature close to 800 °C. The high density can be obtained at this temperature even after several minutes of sintering. As can be seen in Fig. 4, sintering carried out at temperatures lower than
_~ 0.26
!
(125
3. Properties of the sintered BICUVOX 3.1. The crystal structure
The crystal structure of the obtained material was examined by X-ray diffraction analysis. The powder pattern obtained for a well-sintered sample is presented in Fig. 5--it reveals only tetragonal symmetry in the investigated material. The calculated lattice parameters listed in Table 1 are in good agreement with those obtained by Abraham et al. [1 ], also submitted in the table. The crystallographic data obtained for our samples were found to be practically independent of the sintering conditions; the lattice
;!
oo/ 21o
780 °C did not allow samples of high density to bc obtained. Overheating also led to a decrease of the density of the material which is related to the formation of internal pores visible by microscope.
o~ ~~ 7"0I
g 6.5t
~oL
215
760
8oo
9o0
sintering temp. [°C ]
tog P [ MPQ ]
Fig. 2. The logarithmic dependence of the logarithm of green density on the value of the isostatic pressure.
Fig. 4. Effect of sintering temperature oil the density of the BICUVOX.10 samples sintered for 20 h.
r %
0 03
Q - 793 °C
7.0 b - 813 °C
C "0 6.5
6.0
I 10
I 20 sintering time [ h ]
Fig. 3. The densification characteristics for BICUVOX samples sintered at 793 °C (curve a) and 813 °C (curve b).
30
40
50
60
70
28--~
Fig. 5. X-ray diffraction pattern (Co Ka radiation) of the obtained BICUVOX.10 material.
F. Krok et al. /
Preparationof BICUVOX
73
TABLE t. Unitcell constantsderived from X-ray diffractionof BICUVOX.10 material
TABLE 2. The densityand microhardnessof BICUVOX.10 samples sintered under various conditions
Reference
a (A)
c (A)
Sample
This work 1
3.92 3.907
15.44 15.41
parameters for the samples sintered at various temperatures and durations were essentially the same. 3.2. Microhardness The microhardness of the BICUVOX.10 samples was measured by the Vickers method. The results are presented in Table 2. The temperature and time of sintering as well as the density of the measured samples are submitted in the same table. A correlation between density and microhardness is observed. The higher the density of the samples, the higher the microhardness, whereas the samples of lower density, regardless of the cause of low density (sintered at too low a temperature, overheated etc.), are not so hard.
2 4 7 10 11 14
Sinteringconditions Temperature (°C)
Time (h)
764 740 847 813 793 793
20 20 20 0.25 0.25 5
Microhardness (kGmm -2)
6.31 6.10 6.52 7.10 6.86 7.00
340 75 270 500 200 400
T=94':'C
100 go
G
o •
•
\
1'00
2'00
3'00 T= 202.5°C
1,0 OoooO •
•
• Ooqbqdpo
20'
3.3. The electrical conductivity of BICUVOX.IO Electrical properties of ceramic BICUVOX.10 samples were investigated here by means of the impedance spectroscopy method in the frequency range (10-5)x105 Hz using a Tesla B M 5 0 7 impedance meter. The samples had the shape of a long rectangular bar (approximately 3 x 3 x 6 mm 3) or a pellet (10 mm in diameter and 2-3 mm in thickness) and were covered with Pt electrodes sputtered in a cathode discharge in air. The electrodes possessed some degree of porosity and allowed penetration of atmospheric gases to the surface of the electrolyte. The impedance of the samples was measured in the temperature range from about 100 to 650 °C (a small number of samples was investigated up to 750 °C). The thermal evolution of the impedance spectra of the samples is represented by three selected plots shown in Fig. 6. The plots represent partial impedance spectra because of the limited frequency range used in our measurements. Below 200 °C the plots consist of two semicircles (Fig. 6(a)). The left-hand high frequency semicircle beginning at the origin of coordinates expresses the impedance dispersion caused by the geometrical capacitance of the sample. The right-hand semicircle is related to grain boundaries. For the flat thin samples two such semicircle plots can be obtained for lower temperatures (as in Fig. 6(a)) than for the long samples with small cross-section area.
Density (gcm -~)
'
/
/
£0 T= 354 °C
0.05
0.10
~
015'
02'0
0.25' R/k~
Fig. 6. Measured impedance spectra for (a) sample 1, (b) sample
5 and (c) sample 10.
From around 200 to 350 °C elements of the lefthand semicircle disappear (they are displaced towards high frequencies out of the frequency range used) and on the low frequency side (right) part of a third semicircular arc, attributed to electrodes, appears (Fig. 6(b)). In both temperature ranges the separate determination of the bulk electrical resistance R b and the grain boundary resistance Rgb of the sample is possible. Above 350 °C elements of the second semicircle gradually disappear from the frequency range used; the third semicircle passes into a straight line with a slope close to 45 ° (Fig. 6(c)) and, at still higher temperatures, decreases and disappears leaving only the 45 ° straight line. Therefore only the total resistance R t = R b + Rgb of the sample can be determined in this range of temperatures. It will be shown, however, that at high
74
t~ Krok et al.
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Preparation ~f fllCU VOX
temperatures the difference in values between the total and the bulk conductivities becomes negligible (this corresponds to disappearance of the third semicircle). The "partial" impedance spectra presented in Fig. 6 constitute for each case some part of the entire impedance spectrum shown schematically in Fig. 7. A whole spectrum of the entire shape of Fig. 7 was never found for the range of frequencies used for any investigated sample--this range of frequencies was too narrow to allow determination of all dispersions for one given temperature. Electrical parameters of the samples were derived from experimental data by means of fitting the impedance function of a chosen electrical equivalent circuit to the measured data. The function was fitted using the least-squares method. It was found that the electrical equivalent circuit shown in Fig. 8 can successfully model the entire spectrum of Fig. 7: the partial impedance spectra can be modelled by the corresponding circuit being a "part" of the circuit of Fig. 8. Such partial equivalent circuits were applied in the fitting procedure. The symbols used to describe the elements of the equivalent circuit in Fig. 8 are the same as those in the description of Fig. 7. In this paper only the electrical conductivity and the influence of technology on it will be discussed. The
other parameters derived in the course ot fitting the impedance of the equivalent circuit to the measured data will be the subject of another paper. The dependence of the electrical conductivities of the samples on the temperature was as usual plotted in ln(oT) vs. 1/T coordinates (Arrhenius plots). For the samples under investigation, this dependence plotted for the total conductivity o t is composed of two straight line segments with a smooth change in slope at a temperature close to 500 °C i486-508 °C depending on the sample). The high temperature segment of the plot was essentially the same for all the samples (in the limits of measurement errors), despite the differences in density, porosity etc. The Arrhenius dependence of the total conductivity for three samples is shown in Fig. 9. The presented data were chosen purposely to illustrate the influence of sintering on electrical conductivity (sample 3 was sintered at too low a temperature, sample 10 was well sintered and sample 1 was overheated). The plots correspond to the impedance spectra taken after the samples were first heated over 500 °C; otherwise during the first heating cycle the obtained values of the conductivity are slightly lower than those for cooling-the first heating over 500 °C "stabilizes" the electrical conductivity and no hysteresis was found for subsequent cycles of heating and cooling. The difference in conductivity between the first heating and subsequent cooling is shown in Fig. 10.
-X
8.°I v" 4.0
R~
R2 R3
R
Fig. 7. Whole impedance spectrum of a BICUVOX.10 sample (schematic).
"7 I -I-- / 0.
"~No.1) 0.631eV ~o.10) 0.616eV
-,.oF
I (N°3)0"730ieV ~1 xN~° Fig. 8. The proposed electrical equivalent circuit of a BICUVOX.10 sample corresponding to the whole impedance spectrum of Fig. 7 (Rl = Rb, R2= Ru + Rgb, R3= Rb + Rgb + Rc; W denotes Warburg impedance; Cg is the geometrical capacitance; Pd~ and Pgu are the constant phase angle (cpa) elements for the double layer and grain boundaries respectively).
110
1.5
2.0 2.5 IO00/T [ K -1]
Fig. 9. Temperature dependence of the total electrical conductivity for three samples of BICUVOX.10. Sintering conditions for the samples are given in Table 2.
F. Krok et al.
/
4.0[
8.0
v. 2. 0
o
//
4.o
(3
75
Preparation of BICUVOX
o
0
o.o '
I-.10
"~'~
~. ,~_(b/ 0.77_~2e V
I--'
q -2.0
0.0 (Q)
(c) 0.713 oV
-4.0
-4.0
__
_.,
_0
"~
0.656 eV
--<.q.-...
eV
-6.0
I
I
1.0
1.5
[
[
2.0 2.5 IO00/T [ K4 ]
I
1.8
I
I
2.0
2.2
I
2.4
IO00/T [ K -1]
Fig. 10. Temperature dependence of the total electrical conductivity for sample 3: curve a, first heating; curve b, cooling.
Fig. 11. Temperature dependence of the bulk (curve a), grain boundary (curve b) and total (curve c) electrical conductivity of sample 3 for cooling from 750 °C.
The electrical conductivity at low temperatures (less than 500 °C) is dependent on the technology conditions of the samples. The activation energy for the samples sintered at too low a temperature (0.72 eV)is higher than that for overheated (0.631 eV) or optimally sintered (0.62 eV) samples. The conductivity at 300 °C for the first set of samples is about 1 x 1 0 - 3 S cm- 1 whereas for the other two it is close to 2 x 10 -3 S
carried out by means of DTA leads to the following conclusions. ( 1 ) The starting powders have completely reacted at a temperature around 600 °C (peaks related to the starting powders of Bi203, V205 and CuO are not present in the DTA curve). (2) The proper temperature of synthesis should be chosen taking into account the degree of homogeneity of the powder related to the quality of milling. (3) The synthesized material is thermally stable. The densification characteristics show that the BICUVOX.10 samples can be sintered at 800 °C in a very short time (a few minutes only). The fast sintering kinetic suggests the presence of some amount of liquid phase during sintering. Additional confrmation of this is the fact that the density of sintered samples did not depend significantly on the value of the pressure applied in isostatic pressing of green samples, which has already been discussed in Section 2.2. The microhardness, which may be considered as a measure of the mechanical properties of a material, is quite low for all BICUVOX samples (about 500 kG mm -2 for properly sintered material compared with about 1000 kG mm 2 for fl-A1203). Its proportionality to the density of samples indicates that the mechanical properties of BICUVOX are mostly controlled by intergrain or grain boundary phases. The value of the total electrical conductivity at 300 °C for low density samples (equal to about 1 × 10-3 S cm-1) is not much lower than that for high
cm-
1.
The separation of the bulk and the grain boundary conductivities was possible only at temperatures lower than around 350 °C. The temperature dependence of both conductivities for one of the samples is shown in Fig. 11. The activation energies for both do not differ much from one another (0.656 _ 0.003 eV for the bulk against 0.713 _+0.008 eV for the total conductivity) and the values of the conductivity at 300 °C amount to 1 . 4 3 x 1 0 -3 S cm-~ and 0 . 9 5 x 1 0 3 S c m i respectively. At temperatures below 100°C a deviation of In(aT) vs. 1 / T from the straight line is observed. This seems to be related to the increase in conductivity of BICUVOX samples caused by atmospheric moisture.
4. Discussion The investigations carried out in the field of technology allow the best conditions for preparation of BICUVOX. 10 material to be established. The analysis
76
F. Krok et al.
/
Preparation o['BI('UVOX
density samples (about 2 x 10 .3 S cm-~). It may be concluded therefore that, for BICUVOX, processing conditions play a much less important role than for other solid state electrolytes. The sintering conditions (temperature and time) affect the ionic conductivity of BICUVOX at temperatures below 500 °C while the values of the conductivity at high temperatures are practically independent of the preparation procedure. The values of the activation energy of the conductivity at temperatures below 500 °C for well-sintered samples are lower than for samples not fully sintered owing to the use of too low a sintering temperature. Lower values of conductivity and higher values of activation energy were observed at temperatures below 500 °C during the first heating cycle than during cooling and subsequent heating-cooling cycles. Much larger differences between the conductivity values measured during the cooling and heating cycle were reported by Reiselhuber et al. [5] for BICUVOX samples prepared from coarse-grained materials. The results obtained by them for samples prepared from finely milled powders are similar to ours. The change of slope in the Arrhenius plots of conductivity at temperatures of about 500 °C seems to be related to a certain order-disorder transition in the arrangement of oxygen ions in the BICUVOX structure. Our DTA measurements did not reveal any thermal effects at temperatures around 500 °C in sintered material. However, a small endothermic peak in the DTA curve at a temperature around 480 °C was reported for BICUVOX. 10 by Reiselhuber et al. [5]. The value of activation energy of the total conductivity is significantly higher (Ea> 0.71 eV) for low density porous samples than for high quality dense samples (E a ~ 0.65 eV). When we refer those values to
the activation energies for bulk conduction ( E~ ~ 0.64 eV on average) and grain boundary conduction (0.78 eV), it may be concluded that the activation energy of the total conductivity for high density samples is close to the activation energy for bulk intragrain conduction. The value of the activation energy of the total conduction for low density samples (which is close to 0.75 eV ) is much more grain boundary like. Such behaviour is caused by larger values of grain boundary resistance in low density samples, which dominates the total resistance of the sample at low temperatures.
Acknowledgments The authors express sincere thanks to Dr. M. Psoda (Warsaw University of Technology) for performing X-ray analysis and Professor M. W. Breiter and Dr. K. Reiselhuber (Technische Universitat Wien) for helpful discussions and communicating their results on BICUVOX before publication. This work has been supported by the Komitet Badafi Naukowych under project PB/1179/2/91.
References 1 F. Abraham, J. C. Bovin, G. Mairesse and G. Nowogrocki, Solid&ate lonics, 40-41 (1990) 934. 2 T. Iharada, A. Hammouehe,J. Fouletier,M. Kleitz, J. C. Bovin and G. Mairesse, Solid State lonics, 48 ( 1991 ) 257. 3 R.N. Vannier, G. Mairesse, G. Nowogrocki,F. Abraham and J. C. Bovin, Solid State Ionics, 53-56 (1992) 713. 4 K. Haberko, Scientific Bulletins of the University of Mining and Metallurgy (Cracow), Ceramics, No. 47, University of Mining and Metallurgy,Cracow, 1983. 5 K. Reiselhuber, G. Dorner and M. W. Breiter, Electrochim. Acta, in press.