The effect of Sintering condition on the microstructure and Electrical Conductivity of Apatite- type La
9.33(SiO
4)
6O
2ceramic
A. Saoudel
1, N. Azzouz
1, S. Boulfrad
2and J. T. Sirr Irvine
21
LIME, Faculty of Sciences and Technology, Jijel Universty, P.O.Box 98 Jijel 18000, Algeria.
2
School of chemistry,University of St Andrews, Fife KY 16 9ST Scotland, UK
*Corresponding author. Tel: +213 790 85 40 41.
E-mail address:a_saoudel@yahoo.com
Abstract
Rare-earth apatite-type lanthanum silicates, La9.33(SiO4)6O2 ,is prepared in air by the conventional solid state reaction for solid oxide electrolyte. The microstructure and electrical properties of La9.33(SiO4)6O2 ceramic are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS).The principal objective is the study of the effect of the sintering condition(time, temperature), on the morphology and the electrochemical properties of this phase. La9.33(SiO4)6O2ceramic consist of a hexagonal apatite type structure and a small amount of a second phase of La2SiO5due to the law temperature of sintering.
Electrical properties of the sample have been studied between 302 and 802°C by the complex impedance method. The results of the conductivity measurements obtained between 302 and 605°C are treated first, in the total form of the sample, and then, by separating the grain from the grain boundary. Electrical conductivity of the La9,33(SiO4)6O2apatite-type was found of a value of 1.04 10-3S.cm-1 at 605°C. This value is higher than that obtained with yttria stabilized zirconia( YSZ) at the same intermediate temperatures.
Keywords Electrolyte; Apatite; Lanthanum silicate; Ionic conductivity I. Introduction
The solid state electrolyte used in SOFC are required for high ionic conductivity, better stability, chemical and thermal compatibility, impermeability by the reacting gases, high strength and toughness, easy fabrication and low cost[1]. Yttria-stabilized zirconia (YSZ) has been accepted as the most promising electrolyte material due to its good physical and chemical properties such as high chemical, thermal, and mechanical stability in addition to high ionic conductivity [2].
The La9,33(SiO4)6O2 apatite structure materials as a new class of oxide-ion conductors for SOFCs, first found by Nakayama , Aono, Sadaoka and Sakamoto [3, 4], have high ionic conductivity at intermediate temperatures (700–800 °C), suitable thermal expansion matching with the electrode material, and other benefits [5-7]. The ionic conductivities of La9.33(SiO4)6O2 are found to be 7.2×10-5Scm-1at 500oC and 1.4×10-3S cm-1at 800
oC, respectively [8]. Furthermore, compared with most of other ionic conductors, one can find that the apatite-type structure is more tolerant to
extensive aliovalent doping, which is an approach to improve oxide ionic conductivity of lanthanum silicates [9]. However, by far the applicability of lanthanum silicates as solid electrolytes is limited due to the high sintering temperature (1700-1800
oC) and extensive holding time for the traditional solid-state reaction [10]. However, the influence of sintering parameters on structure and electrical conductivity of oxy-apatite lanthanum silicates remains unclear in the open literature. This paper deals with the study of sintering parameters on structure and electrical conductivity of oxy-apatite lanthanum silicates. Rare earth silicate oxyapatites are generally prepared by solid-state synthesis.
II. Experimental procedure II.1. Powder preparation
The apatite-type lanthanum silicates La9.33(SiO4)6O2
was prepared by solid state reaction using high purity La2O3 (Aldrich, 99.9%) and SiO2 (Prolabo, 99%) powders. The mixture with stoichiomitry of La2O3and SiO2are mixed thoroughly and calcined in air at 1200 °C for 3h. The obtained powders are
hand milled in an agate mortar and pressed under a pressure of 5 tons at room temperature into pellets of about 1-2 mm thickness and 13 mm of diameter.
It was then submitted to tow successive cycles of sintering in air at 1400 °C for 3h and 1550 °C for 4h.
2.2. Characterizations
Powder X-ray diffraction (XRD) patterns were collected on a Siemens D8-advance powder diffractometer by use of CuKα radiation (radiation of λ=1.5405A°) in the angular range of 20 ≤ 2θ ≤ 80º 0.02°/step, 2s/step. Rietveld refinement of the DRX pattern was performed using janna 2006 softrware.The crystalline phase was identified from a comparison of the registered patterns with the international center of diffraction data (ICDD) powder diffraction files [11]. The micro structural characterization of the sample was done on a FE- SEM (ZEISS ultra 55) scanning electron microscopy (SEM).
Electrical properties have been measured using a SOLARTRON 1260 Impedance/Gain Phase Analyzer. The electrical measurements were taken under air by interval of 50°C in an active temperature range of 302°C until 802°C. Before any measurement, the sample was maintained for 30 minutes at the temperature of measurement in order to reach thermodynamic balance. The Zview 3.2c software was used to analyze the impedance data.
III. Results and discussion
III.1 Phase and microstructural studies
The XRD patterns of La9.33(SiO4)6O2 powder obtained after calcinations and the two successive cycles of sintering are shown in Fig.1. As can be seen from Fig. 1, powder calcined at 1200°C for 3h presented the formation of the apatite phase La9.33(SiO4)6O2 with a small amount, tow second phases La2SiO5, La2Si2O7and the trace of La2O3. After the first sintering at 1400°C/3h Fig.1, the most intense peaks belong to La9.33(SiO4)6O2phase, but the impurity of La2SiO5 and La2Si2O7 phases were also observed with a small amount. As seen in Fig.1, after the second sintering at 1550°C/4h, only pure apatite structure was found; however, a trace amount of La2SiO5was observed. According to the La2O3/SiO2 phase diagram[12] La2SiO5 phase is more stable than apatite phase below1600°C. This compound was refined in the hexagonal system of the space group P63 /m. The lattice parameters of La9.33(SiO4)6O2 ceramic are calculated by use of JANA 2006 software [13]. Table 1 shows the lattice parameters of La9.33(SiO4)6O2ceramic. The
Samples Lattice
parameters( ˚ A) V(Å3) Reference a (Å) c (Å)
La9.33(SiO4)6O2 9.708Å 7. 193 587.227 This work
9.721 7.187 588.20 [8]
Table. 1. Lattice parameters of La9.33(SiO4)6O2sintered at 1550
°C/4 h.
similar results are reported in previous studies [14], and compared to 01-074-9552 ICSD file card (the international centre for diffraction data) [11].
Fig. 1. XRD paterns of powders calcined at 1200°C for 3h; first sintering at 1400°C for 3h and second sintering at 1500°Cfor 4h.
After elaboration, the SEM photographs taken from the surface of pellets are displayed in Fig. 2. We have to note that the used magnification for the sample is very important to give idea on grain shape and know that the grain size range between 1- 10 µm, which is larger than 150 nm reported by Celerier et al. [14]. As is it well shown in fig. 2(a), the sintered powder has a flat hexagonal shape. The hexagonal habit can be related to the apatite structure with a hexagonal crystal lattice [15]. Then starting from the fig. 2(b), we can say that the grains are well attached the one to the others with the existence of the few closed pores. from fig. 2(c), we observe two colors white and black apartiennent to the grains and pores, respectively.
Some pores existed in the sintered body, and all of which were located at the grain boundaries or at the triple points.
hand milled in an agate mortar and pressed under a pressure of 5 tons at room temperature into pellets of about 1-2 mm thickness and 13 mm of diameter.
It was then submitted to tow successive cycles of sintering in air at 1400 °C for 3h and 1550 °C for 4h.
2.2. Characterizations
Powder X-ray diffraction (XRD) patterns were collected on a Siemens D8-advance powder diffractometer by use of CuKα radiation (radiation of λ=1.5405A°) in the angular range of 20 ≤ 2θ ≤ 80º 0.02°/step, 2s/step. Rietveld refinement of the DRX pattern was performed using janna 2006 softrware.The crystalline phase was identified from a comparison of the registered patterns with the international center of diffraction data (ICDD) powder diffraction files [11]. The micro structural characterization of the sample was done on a FE- SEM (ZEISS ultra 55) scanning electron microscopy (SEM).
Electrical properties have been measured using a SOLARTRON 1260 Impedance/Gain Phase Analyzer. The electrical measurements were taken under air by interval of 50°C in an active temperature range of 302°C until 802°C. Before any measurement, the sample was maintained for 30 minutes at the temperature of measurement in order to reach thermodynamic balance. The Zview 3.2c software was used to analyze the impedance data.
III. Results and discussion
III.1 Phase and microstructural studies
The XRD patterns of La9.33(SiO4)6O2 powder obtained after calcinations and the two successive cycles of sintering are shown in Fig.1. As can be seen from Fig. 1, powder calcined at 1200°C for 3h presented the formation of the apatite phase La9.33(SiO4)6O2 with a small amount, tow second phases La2SiO5 , La2Si2O7and the trace of La2O3. After the first sintering at 1400°C/3h Fig.1, the most intense peaks belong to La9.33(SiO4)6O2phase, but the impurity of La2SiO5 and La2Si2O7 phases were also observed with a small amount. As seen in Fig.1, after the second sintering at 1550°C/4h, only pure apatite structure was found; however, a trace amount of La2SiO5was observed. According to the La2O3/SiO2 phase diagram[12] La2SiO5 phase is more stable than apatite phase below1600°C. This compound was refined in the hexagonal system of the space group P63 /m. The lattice parameters of La9.33(SiO4)6O2 ceramic are calculated by use of JANA 2006 software [13]. Table 1 shows the lattice parameters of La9.33(SiO4)6O2ceramic. The
Samples Lattice
parameters( ˚ A) V(Å3) Reference a (Å) c (Å)
La9.33(SiO4)6O2 9.708Å 7. 193 587.227 This work
9.721 7.187 588.20 [8]
Table. 1. Lattice parameters of La9.33(SiO4)6O2sintered at 1550
°C/4 h.
similar results are reported in previous studies [14], and compared to 01-074-9552 ICSD file card (the international centre for diffraction data) [11].
Fig. 1. XRD paterns of powders calcined at 1200°C for 3h; first sintering at 1400°C for 3h and second sintering at 1500°Cfor 4h.
After elaboration, the SEM photographs taken from the surface of pellets are displayed in Fig. 2. We have to note that the used magnification for the sample is very important to give idea on grain shape and know that the grain size range between 1- 10 µm, which is larger than 150 nm reported by Celerier et al. [14]. As is it well shown in fig. 2(a), the sintered powder has a flat hexagonal shape. The hexagonal habit can be related to the apatite structure with a hexagonal crystal lattice [15]. Then starting from the fig. 2(b), we can say that the grains are well attached the one to the others with the existence of the few closed pores. from fig. 2(c), we observe two colors white and black apartiennent to the grains and pores, respectively.
Some pores existed in the sintered body, and all of which were located at the grain boundaries or at the triple points.
20 25 30 35 40 45 50 55 60 65 70 75 80
AA A A
A
*
*
A
AA A
A
A A
A
* *
*
*
* *
* *
+ +
+ +
x x
x x x
x x x x
A
A A
A A A
A A
A
A La9.33Si6O26
* La2SiO5
Intensity / a.u.
1550/4h
1400/3h
1200/3h
2-theta/degree
+ La2Si2O7 x La2O3
hand milled in an agate mortar and pressed under a pressure of 5 tons at room temperature into pellets of about 1-2 mm thickness and 13 mm of diameter.
It was then submitted to tow successive cycles of sintering in air at 1400 °C for 3h and 1550 °C for 4h.
2.2. Characterizations
Powder X-ray diffraction (XRD) patterns were collected on a Siemens D8-advance powder diffractometer by use of CuKα radiation (radiation of λ=1.5405A°) in the angular range of 20 ≤ 2θ ≤ 80º 0.02°/step, 2s/step. Rietveld refinement of the DRX pattern was performed using janna 2006 softrware.The crystalline phase was identified from a comparison of the registered patterns with the international center of diffraction data (ICDD) powder diffraction files [11]. The micro structural characterization of the sample was done on a FE- SEM (ZEISS ultra 55) scanning electron microscopy (SEM).
Electrical properties have been measured using a SOLARTRON 1260 Impedance/Gain Phase Analyzer. The electrical measurements were taken under air by interval of 50°C in an active temperature range of 302°C until 802°C. Before any measurement, the sample was maintained for 30 minutes at the temperature of measurement in order to reach thermodynamic balance. The Zview 3.2c software was used to analyze the impedance data.
III. Results and discussion
III.1 Phase and microstructural studies
The XRD patterns of La9.33(SiO4)6O2 powder obtained after calcinations and the two successive cycles of sintering are shown in Fig.1. As can be seen from Fig. 1, powder calcined at 1200°C for 3h presented the formation of the apatite phase La9.33(SiO4)6O2 with a small amount, tow second phases La2SiO5 , La2Si2O7and the trace of La2O3. After the first sintering at 1400°C/3h Fig.1, the most intense peaks belong to La9.33(SiO4)6O2phase, but the impurity of La2SiO5 and La2Si2O7 phases were also observed with a small amount. As seen in Fig.1, after the second sintering at 1550°C/4h, only pure apatite structure was found; however, a trace amount of La2SiO5was observed. According to the La2O3/SiO2 phase diagram[12] La2SiO5 phase is more stable than apatite phase below1600°C. This compound was refined in the hexagonal system of the space group P63 /m. The lattice parameters of La9.33(SiO4)6O2 ceramic are calculated by use of JANA 2006 software [13]. Table 1 shows the lattice parameters of La9.33(SiO4)6O2ceramic. The
Samples Lattice
parameters( ˚ A) V(Å3) Reference a (Å) c (Å)
La9.33(SiO4)6O2 9.708Å 7. 193 587.227 This work
9.721 7.187 588.20 [8]
Table. 1. Lattice parameters of La9.33(SiO4)6O2sintered at 1550
°C/4 h.
similar results are reported in previous studies [14], and compared to 01-074-9552 ICSD file card (the international centre for diffraction data) [11].
Fig. 1. XRD paterns of powders calcined at 1200°C for 3h; first sintering at 1400°C for 3h and second sintering at 1500°Cfor 4h.
After elaboration, the SEM photographs taken from the surface of pellets are displayed in Fig. 2. We have to note that the used magnification for the sample is very important to give idea on grain shape and know that the grain size range between 1- 10 µm, which is larger than 150 nm reported by Celerier et al. [14]. As is it well shown in fig. 2(a), the sintered powder has a flat hexagonal shape. The hexagonal habit can be related to the apatite structure with a hexagonal crystal lattice [15]. Then starting from the fig. 2(b), we can say that the grains are well attached the one to the others with the existence of the few closed pores. from fig. 2(c), we observe two colors white and black apartiennent to the grains and pores, respectively.
Some pores existed in the sintered body, and all of which were located at the grain boundaries or at the triple points.
20 25 30 35 40 45 50 55 60 65 70 75 80
AA A A
A
*
*
A
AA A
A
A A
A
* *
*
*
* *
* *
+ +
+ +
x x
x x x
x x x x
A
A A
A A A
A A
A
A La9.33Si6O26
* La2SiO5
Intensity / a.u.
1550/4h
1400/3h
1200/3h
2-theta/degree
+ La2Si2O7 x La2O3
Fig.2. SEM images showing the microstructure of La9.33(SiO4)6O2: sintered at 1550°C for 4h
III.2 Impedance and electrical conductivity AC impedance spectroscopy (-Z’’ vs. Z’) of La9.33(SiO4)6O2 ceramic is measured as a function of frequency in the temperature range of 302 to 802°C in air. Typical Impedance data are displayed in the form of Nyquist plots (Fig. 3) for La9.33Si6O26
sintered at 1550 °C from 302 °C and 605 °C and corresponding equivalent circuit. These impedance data are representative of sample investigated in the course of this study. At low temperature, electrical response of sample essentially consists in one depressed arc at high frequency (in the 102-105Hz frequency range) and the electrode contribution, at low frequency (<102 Hz) (Fig. 3 a–f). Ceramic electrical response might be constituted of two processes (‘g’: grain ‘gb’: grain boundaries) that are difficult to separate when strongly overlapped.
Some attempts have been made to distinguish both contributions. With the increasing of the temperature, the grain and grain boundary semicircles rapidly decrease.
b
d
a
c
e
f
Fig.2. SEM images showing the microstructure of La9.33(SiO4)6O2: sintered at 1550°C for 4h
III.2 Impedance and electrical conductivity AC impedance spectroscopy (-Z’’ vs. Z’) of La9.33(SiO4)6O2 ceramic is measured as a function of frequency in the temperature range of 302 to 802°C in air. Typical Impedance data are displayed in the form of Nyquist plots (Fig. 3) for La9.33Si6O26
sintered at 1550 °C from 302 °C and 605 °C and corresponding equivalent circuit. These impedance data are representative of sample investigated in the course of this study. At low temperature, electrical response of sample essentially consists in one depressed arc at high frequency (in the 102-105Hz frequency range) and the electrode contribution, at low frequency (<102 Hz) (Fig. 3 a–f). Ceramic electrical response might be constituted of two processes (‘g’: grain ‘gb’: grain boundaries) that are difficult to separate when strongly overlapped.
Some attempts have been made to distinguish both contributions. With the increasing of the temperature, the grain and grain boundary semicircles rapidly decrease.
b
d
a
c
e
f
Fig.2. SEM images showing the microstructure of La9.33(SiO4)6O2: sintered at 1550°C for 4h
III.2 Impedance and electrical conductivity AC impedance spectroscopy (-Z’’ vs. Z’) of La9.33(SiO4)6O2 ceramic is measured as a function of frequency in the temperature range of 302 to 802°C in air. Typical Impedance data are displayed in the form of Nyquist plots (Fig. 3) for La9.33Si6O26
sintered at 1550 °C from 302 °C and 605 °C and corresponding equivalent circuit. These impedance data are representative of sample investigated in the course of this study. At low temperature, electrical response of sample essentially consists in one depressed arc at high frequency (in the 102-105Hz frequency range) and the electrode contribution, at low frequency (<102 Hz) (Fig. 3 a–f). Ceramic electrical response might be constituted of two processes (‘g’: grain ‘gb’: grain boundaries) that are difficult to separate when strongly overlapped.
Some attempts have been made to distinguish both contributions. With the increasing of the temperature, the grain and grain boundary semicircles rapidly decrease.
b
d
a
c
e
f
Fig. 3. AC impedance spectra of densified La9.33(SiO4)6O2
ceramic at 302◦C (a), 358◦C (b),408°C(c),459°C(d),508°C(e) and 605◦C (c); and corresponding equivalent circuits used in fitting impedance spectra data (g) of impedance spectra (a),(b) and (c) and (h) of impedance spectra (d),(e) and (f)
The fitting parameters of grain resistance (Rg), grain boundary resistance (Rgb), grain capacitance (CPEg) and grain boundary capacitance (CPEgb) for La9.33(SiO4)6O2 ceramic under different sintering conditions are all acquired by Zview software (Table 2).
Table. 2. Fitting parameters of the La9.33(SiO4)6O2 ceramic at different temperature of measure.
Total conductivity of La9.33(SiO4)6O2 ceramic at different temperatures is obtained from the following equation
RS
e
(1)
Whereeis the thickness of specimen, is the electrode area of the specimen surface and Ris the total resistance, which includes Rgand Rgb.
In our case, the best total ionic conduction is observed at 605 °C temperature (1.04 10-3S.cm -
1).We can thus say that our result is acceptable comparatively with values 1,1.10-4S.cm-1[ 3 ] 500
°C, 1,0.10-3S.cm-1[ 16 ] 800°C.
The temperature dependence of total conductivity for each composition is analyzed from the following Arrhenius equation:
KT
E
a0
exp
(2)Where σ is the total conductivity, σ0 is the pre- exponential factor related to the effective number of mobile oxide-ions, Ea is the activation energy for the electrical conduction process, k is the Boltzmann constant, and T is the absolute temperature.
Fig. 4 present the Arrhenius plot of the total conductivity of La9.33(SiO4)6O2 ceramic. The diffusion process of oxide-ions is thermally activated because the straight line is well fitted to the Arrhenius relation. Eaand σ0can be calculated from the slope and the intercept of the linear fits in the Arrhenius plots for this composition, respectively.
The conductivities of the La9.33Si6O26 at 800 oC from Refs. 10 and 14 are 2.0×10-3 Scm-1 and 7.3×10-4Scm-1. Therefore, the solid state synthesis in our study yields high performance of the La9.33Si6O26 electrolyte.
Fig .4. Arrhenius plots of the total conductivity for sintered sample.
4. Conclusions
(1) Oxy-apatite La9.33(SiO4)6O2 ceramic was prepared via high temperature solid state reaction.
The sintering parameters are optimized to be 1550°C and 4h to get a high purity of La9.33(SiO4)6O2 ceramic.
(2) Sintering parameters have a distinct influence on both the content of second phase La2SiO5 and the bulk density of La9.33(SiO4)6O2ceramic.
(3)The content of second phase La2SiO5 has a certain influence on the grain size of La9.33(SiO4)6O2. The second phase distributes mainly at the grain boundaries, which blocks the grain growth of La9.33(SiO4)6O2. However, the grain size and grain boundary amount have a distinct Températures
°C
grain grain boundary
Rg(Ω) CPEg.10-
12(F) Rgb(Ω) CPEgb.10-
9(F)
302 75681,3 0.687 110987 5.19
358 18774 0.8186 12719 8.28
408 5470.8 0.8111 5508.9 9.05
459 1579.6 0.727 1944.4 8.37
508 420.2 0.01119 761.5 6.96
605 67.69 0.02822 112.37 9.29
h g
influence on total conductivity of La9.33(SiO4)6O2
ceramic.
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