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Synthesis and characterization of Al3+ and M (M = W6+, In3+, Nb5+, Mg2+) co-doped lanthanum silicate oxy-apatite electrolytes
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Synthesis and characterization of Al3þ and M (M ¼ W6þ, In3þ, Nb5þ, Mg2þ) co-doped lanthanum silicate oxy-apatite electrolytes Lei Dai a,b, Wen Han a, Yuehua Li a,**, Ling Wang a,b,* a
College of Chemical Engineering, North China University of Science and Technology, Tangshan 063009, PR China Hebei Province Key Laboratory of Photocatalytic and Electrocatalytic Materials for Environment, Tangshan 063009, PR China
b
article info
abstract
Article history:
Apatite-type La10Si5Al0.9M0.1O27d (M ¼ W6þ, In3þ, Nb5þ or Mg2þ) are successfully synthe-
Received 19 February 2016
sized by the high-temperature solid state reaction method. The composition, microstruc-
Received in revised form
ture and electrical conduction performance of the samples are characterized by X-ray
9 April 2016
diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance
Accepted 28 April 2016
spectroscopy (EIS), respectively. The dense co-doped lanthanum silicates electrolytes with
Available online 27 May 2016
pure hexagonal apatite-type structure are obtained after sintered at 1873 K for 6 h.
Keywords:
temperature range of 673e1073 K. The influences of the co-doped W6þ content on the
La10Si5Al0.9W0.1O26.65 has the highest total conductivity among the co-doped samples in the Oxy-apatite lanthanum silicate
properties of the La10Si5Al1xWxO27±d (0 x 1) are investigated. It is found that with
Co-doping
increasing the W6þ content, the equiaxed apatite-type grains are replaced gradually by the
Microstructure
rod-like grains, accompanied by the formation of impurity phases (La6W2O15, La2SiO5 and
Electrolyte
La2Si2O7) in the samples. The La10Si5Al0.9W0.1O26.65 exhibits the highest conductivity of
Electrical conductivity
3.03 102 S cm1 at 1073 K. The oxygen pressure independency of the total conductivity suggests that La10Si5Al0.9W0.1O26.65 remains an almost pure oxygen ionic conductor. La10Si5Al0.9W0.1O26.65 also shows good stability in 20% H2/Ar, wet Ar and pure CO2 atmospheres, respectively. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Lanthanum silicates with an apatite-type structure have attracted great attention as alternative electrolytes to YSZ due to their high oxygen ion conductivity at intermediate temperatures (773e1023 K) [1e4]. The oxy-apatite lanthanum
silicates also exhibit pure oxygen ion conduction over a wide oxygen partial pressure range and good stable performance under various gas atmospheres [5e7]. Meanwhile, there are other advantages of low electrical conduction activation energy, moderate thermal expansion coefficients, a wide range of materials selection, and relatively low cost of materials processing. Therefore, the excellent properties make the
* Corresponding author. College of Chemical Engineering, North China University of Science and Technology, Tangshan 063009, PR China. Tel./fax: þ86 315 2592170. ** Corresponding author. Tel./fax: þ86 315 2592170. E-mail addresses: [email protected] (Y. Li), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.04.210 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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apatite lanthanum silicates into the potential electrolytes for intermediate temperature operations, which have been widely used in solid oxide fuel cells and electrochemical gas sensors [8e13]. In addition, compared with most of other ionic conductors, the apatite-type structure is more tolerant to extensive aliovalent doping, which is an effective approach to optimize the electrical conduction of lanthanum silicates [14]. The improvement of oxygen ion conductivity of lanthanum silicates by aliovalent doping can be realized via partial substitution of La3þ or Si4þ site with cations. The cationic substitutions on the La site with other rare earth elements [15e17] or alkaline earth elements [18e23] have been investigated. The samples containing oxygen excess exhibit high oxygen ion conductivity, which indicates that oxygen excess is more important than cationic vacancies to achieve good oxide-ion conductivity of oxy-apatite lanthanum silicate [21].
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Compared with doping on the La3þ site, Si4þ site doping has a more extensive range of cations selection and shows to be more effective in increasing the total conductivity [24]. Up to now, many efforts have been made to further enhance the electrical conduction performance of apatite-type lanthanum silicates via partial substitution of Si4þ site with low valence state cations (such as Mg2þ [25e27], Al3þ [28e31], In3þ [32], Mn2þ [33], Fe3þ [34], Co2þ [35] or Sn4þ [36]) or high valence state cations (such as W6þ [37] or Nb5þ [38]). Furthermore, these various cations doping also can improve the sinterability of oxyapatite lanthanum silicates and restrain the formation of impurity phases (La2SiO5 and La2Si5O7). Among the cations doping on the Si4þ site, Al3þ doping has been investigated very systematically and provide a significant improvement of oxygen ion conductivity of oxyapatite lanthanum silicates [28e31]. Recently, some research also has been reported on the oxyapatite lanthanum silicates with the cations co-doping on
Fig. 1 e XRD patterns and SEM surface images of La10Si5Al0.9M0.1O27¡d (M ¼ W, In, Nb or Mg) sintered at 1873 K for 6 h.
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both La3þ and Si4þ site, such as Ba and Al co-doping [39], Sr and Al or Fe co-doping [40], Ca and Fe co-doping [41]. However, the research of the co-doping strategy on Si4þ site is limited. Although, the Si4þ site substituted by Zn2þ and Mg2þ in La10Si6O27 electrolyte has been reported, the single Zn2þsubstituted La10Zn0.2(SiO4)5.8O2.5 gives the highest oxygen ion conductivity [42]. Meanwhile, the influences of the co-doped cation types and contents on the structure and electrical conductivity of oxy-apatite lanthanum silicates are not well understood. In this paper, the oxy-apatite lanthanum silicates co-doped on the Si4þ site with Al3þ and different other cations (W6þ, In3þ, Nb5þ or Mg2þ) are successfully synthesized by the hightemperature solid state reaction method. The influences of the species and content of the doping cations on the composition, microstructure and electrical conductivity of apatitetype lanthanum silicates are investigated in detail.
The electrical conductivities of the co-doped apatite lanthanum silicates were measured by ac impedance spectroscopy using the Solartron Analytical SI 1260 impedance/ gain-phase analyzer with the Solartron Analytical SI 1287 electrochemical interface. Pt layers were used as the electrical collectors, which was prepared by painting Pt paste on the polished surfaces of the sample pellets and then treated in air at 1123 K for 1 h. The electrical conductivity was measured in air in the temperature range of 673e1073 K. The compleximpedance measurements of the samples were carried out in the frequency range of 1 MHz to 0.01 Hz under open circuit voltage using AC amplitude of 5 mV. The obtained spectra were analyzed using the Zview software associated with the Solartron instrument.
Results and discussion
Experimental
Characterization of co-doped La10Si5Al0.9M0.1O27d (M ¼ W, In, Nb or Mg)
The co-doped La10Si5Al1xMxO27d (M ¼ W, In, Nb or Mg, x ¼ 0e1) electrolytes were synthesized by the high temperature solid state reaction process. According to the stoichiometric ratios of La10Si5Al1xMxO27d with different composition, the appropriate amounts of raw materials (Analytically pure La2O3, SiO2, Al2O3, WO3, In2O3, Nb2O5 and MgO powders) were mixed and then ball-milled with anhydrous alcohol for 24 h. After dried, the mixed powders were calcined at 1573 K for 6 h in air. The calcined powders were finely ground and uniaxially pressed into a cylindrical flat mold utilizing fitted stainless steel disks with a pressure of 20 MPa to obtain the disk-shaped samples (13 mm in diameter and 1 mm in height). The disk-shaped samples were then sintered at 1873 K for 6 h to form the dense pellets. In order to examine the chemical stability of the sintered pellets in different atmospheres, the sample powders were exposed to flowing 20% H2/Ar, wet Ar or pure CO2 at 1073 K for 10 h, respectively. The bulk densities of all the sintered samples were measured by the Archimedes method with an immersion medium of deionized water. The theoretical bulk densities of the samples were calculated from the values of lattice parameters. The composition of the samples was identified by X-ray diffraction (XRD, Rigaku, D/MAX2500PC) analysis under Cu-Ka radiation with the incidence beam angle of 2 in the range of 10e90 . The microstructure of the samples was investigated by field emission scanning electron microscopy (SEM, Hitachi, S-4800).
Oxy-apatite type La10Si5Al0.9M0.1O27d electrolytes co-doped on Si4þ site with Al3þ and another cation, such as W6þ, In3þ, Nb5þ or Mg2þ, were synthesized by the high-temperature solid state reaction method. The XRD patterns and SEM surface images of the samples sintered at 1873 K for 6 h are shown in Fig. 1. It can be seen from the XRD results that all the diffraction peaks of the samples correspond to the single hexagonal apatite-type structure of La10(SiO4)6O3 (JCPDS No.53-0291) without impurity phases, which indicates that Si4þ has been partially substituted by Al3þ and M ion successfully. The refined unit cell parameters of the samples are listed in Table 1. Clearly, the cell volume (V0) of the samples increases when Al3þ is partially substituted by W6þ, In3þ, Nb5þ or Mg2þ due to their larger effective ionic radii (RW ¼ 0.042 nm, RIn ¼ 0.062 nm, RNb ¼ 0.048 nm, RMg ¼ 0.057 nm) than that of Al3þ (RAl ¼ 0.039 nm), which is in agreement with the consequence of XRD and indicates that these cations have been introduced into the crystal lattice. Among the samples, La10Si5Al0.9W0.1O26.65 is of the highest relative bulk density. Fig. 1 also shows the typical SEM images of the surface of La10Si5Al0.9M0.1O27d (M ¼ W, In, Nb or Mg) pellets sintered at 1873 K for 6 h. It is clearly seen that the samples are quite dense without observable pores. La10Si5AlO26.5 is mainly composed of rod-like grains accompanied by some small equiaxed grains. For La10Si5Al0.9W0.1O26.65 and La10Si5Al0.9In0.1O26.5, the grains are equiaxed and relatively uniform, although a few of rod-like grains exist. In contrast, the rod-like grains become dominant in La10Si5Al0.9Nb0.1O26.6 and
Table 1 e Cell parameter values and bulk densities of La10Si5Al0.9M0.1O27¡d. Samples
a¼b (nm)
c (nm)
V0 (nm3)
Theoretical density (g cm3)
Measured density (g cm3)
Relative density (%)
La10Si5AlO26.5 La10Si5Al0.9W0.1O26.65 La10Si5Al0.9In0.1O26.5 La10Si5Al0.9Nb0.1O26.6 La10Si5Al0.9Mg0.1O26.45
0.9681 0.9719 0.9757 0.9729 0.9726
0.7205 0.7249 0.7254 0.7211 0.7222
0.5848 0.5930 0.5980 0.5911 0.5916
5.623 5.596 5.524 5.587 5.556
5.427 5.428 5.341 5.212 5.234
96.5 97.2 96.7 93.3 94.2
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Fig. 2 e Impedance spectra of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) measured in the temperature range of 673e1073 K in air. La10Si5Al0.9Mg0.1O26.45 samples with a few of excessively grown rods. The electrochemical impedance spectra of the co-doped apatite lanthanum silicate samples were measured and recorded respectively. Fig. 2 shows the typical impedance spectra measured at different temperatures for La10Si5Al0.9W0.1O26.65 sample sintered at 1873 K for 6 h. Based on the equivalent circuit model consisting of a serial association of (R//CPE) elements (R: resistance; CPE: constant phase element) attributed to electrolyte or electrode processes, the
high and medium frequency arcs can be identified with the grain bulk (Rg) and grain boundary (Rgb) resistances, respectively [32,41]. The electrical conduction of the co-doped apatite lanthanum silicates is controlled by the grain and grain boundary contributions together. In addition, the semicircle or tail at low frequencies represents the electrode interface diffusion, which is the representative of Wagner ionic diffusion effect [37,38]. At the temperature range of 673e923 K, the two arcs presented the grain bulk and boundary resistances can be observed. With the temperature
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5
Fig. 4 e XRD patterns (A) of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h and (B) a local magnification of diffraction peaks from (A).
increasing to 973 and 1073 K, the arcs contributed from both the grain bulk and boundary processes disappear due to the instrumental limitation. Only the semicircle attributed to the electrode interface process appears. Therefore, the intercept of the semicircle with the real axis at high frequency direction is designed to total resistance of the sample, which is similar with the results in literature [43].
Fig. 3 e Impedance spectra (A), a comparison of Rg and Rgb (B) measured at 923 K in air and Arrhenius plots of the total conductivity (C) of La10Si5Al0.9M0.1O27¡d (M ¼ W, In, Nb or Mg).
Table 2 e Total conductivity at typical temperatures and corresponding conduction activation energy of La10Si5Al0.9M0.1O27¡d. Conductivity (S cm1)
Samples 673 K La10Si5AlO26.5 La10Si5Al0.9W0.1O26.65 La10Si5Al0.9In0.1O26.5 La10Si5Al0.9Nb0.1O26.6 La10Si5Al0.9Mg0.1O26.45
1.11 2.03 1.81 8.23 1.35
104 104 104 105 104
923 K 4.91 7.36 6.79 4.37 3.78
103 103 103 103 103
Activation energy (eV) 1073 K 1.52 3.03 2.13 1.45 1.20
102 102 102 102 102
0.86 0.83 0.82 0.89 0.81
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Fig. 5 e SEM images of the surface (AeD) and the fractured cross-section (EeH) of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h.
Fig. 3A and B shows a comparison of Rg and Rgb for La10Si5Al0.9M0.1O27d samples based on the impedance data measured at 923 K. Compared with La10Si5AlO26.5, both Rg and Rgb of the W6þ or In3þ co-doped samples decrease obviously, while the two values increase with Nb5þ or Mg2þ co-doping. The
total conductivity (s) of the samples can be calculated from the total resistances (R ¼ Rg þ Rgb), through the following equation: s ¼ l=SR where l is the sample thickness and S is the pellet surface area.
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XRD and SEM analysis of La10Si5Al1xWxO27±d
Relative bulk density / %
98 97 96 95 94 93 92 91 90 0.0
0.2
0.4
X
0.6
0.8
1.0
Fig. 6 e The relationship between W6þ content (x) and the relative bulk densities of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h.
Fig. 3C shows the relationship between ln (sT) and 10,000 T1 for the co-doped oxy-apatite lanthanum silicates with different cations. It is clearly seen that the total conductivity of all the samples varies linearly with the measurement temperature in the range of 673e1073 K, which is well fitted to the Arrhenius equation. Compared with La10Si5AlO26.5, the total conductivity of the samples co-doped W6þ or In3þ is distinctly improved in all measurement temperature. On the contrary, the total conductivity of the sample becomes lower after co-doped by Nb5þ. The total conductivity of the samples at the typical temperatures and the activation energy for electrical conduction are shown in Table 2. The activation energy of the samples co-doped with W6þ, In3þ, Nb5þ and Mg2þ is 0.83, 0.82, 0.89 and 0.81 eV, respectively. The previous studies have identified that the ionic conductivity of silicate-based apatites containing oxygen excess is mainly mediated by oxygen interstitial migration [14,22]. Cation doping on Si site can effectively improve the formation of oxygen interstitial through the local distortions and structural relaxation, which consequently improves the total conductivity. The nature difference of the doped cations, especially the ionic radius, results in the difference of cell distortion and restraining the oxygen ion, and then the conductivity of silicate-based apatite is changed accordingly [44]. The influences of different cation doping on the total conductivity of La10Si5Al0.9M0.1O27d electrolytes indicate that a synergistic action between doped Al3þ and W6þ or In3þ promotes the oxygen ion conduction, which should be attributed to the improvement of the oxygen interstitial formation. Meanwhile, the In3þ doping can generate the oxygen vacancies, which is another positive factor for the improvement of the oxygen ion conduction [32]. Among the co-doped samples, La10Si5Al0.9W0.1O26.65 exhibits the highest total conductivity of 3.03 102 S cm1 at 1073 K, which is higher or comparable to previous reports [37,40e42]. Based on above results, the influences of the W6þ content on the properties of the La10Si5Al1xWxO27±d are investigated.
Fig. 4 shows the XRD patterns of La10Si5Al1xWxO27±d (x ¼ 0, 0.1, 0.3, 0.5, 1) pellets sintered at 1823 K for 6 h. The main diffraction peaks of all the samples are in agreement with the hexagonal apatite-type structure of La10(SiO4)6O3 (JCPDS No.53-0291), but phase purity is different for different codoped W6þ content. It is clearly seen from Fig. 4A that the single hexagonal apatite-type structure of the samples is obtained when x ¼ 0 or 0.1. With the W6þ content increases to 0.3, La6W2O15 phase (JCPDS No. 37-0124) with an orthorhombic structure appears, which is similar to the phenomenon in the previous literature [37]. When the W6þ content is more than 0.5, the diffraction peaks of La2SiO5 and La2Si5O7 appear. Fig. 4B shows the enlargement of Fig. 4A in the 2q range of 29.5e32.5 . The effective ionic radii of hexavalent W6þ (RW ¼ 0.042 nm), tetravalent Si4þ (RSi ¼ 0.026 nm) and trivalent Al3þ (RAl ¼ 0.039 nm) are different. Thus, the lattice distortion of La10Si5Al1xWxO27±d is relatively large, which makes the diffraction peaks shift to the low angle side. As shown in Fig. 4B, when the W6þ content increases from 0 to 0.1, the characteristic diffraction peaks of the samples shift observably to the low angle. However, the positions of characteristic diffraction peaks remain almost unchanged with further increasing the W6þ content, indicating a saturated solution of W6þ. Combined with the component analysis as mentioned above, the solid solubility limit of W6þ in the apatite-type lattice of La10Si5Al1xWxO27±d is in the range of 0.1e0.3. Fig. 5 shows the typical SEM images of the surface and cross-section of La10Si5Al1xWxO27±d (x ¼ 0.1, 0.3, 0.5, 1) pellets sintered at 1873 K for 6 h. It is clearly seen that La10Si5Al0.9W0.1O26.65 sample shows a relatively uniform structure. The grain boundary is clear and the equiaxed grain is full-grown with a grain size of 1e4 mm (Fig. 5A and E). When the W6þ content reaches 0.3, the grains size becomes uneven and some rod-like big grains dramatically grow (Fig. 5B) and a few closed pores appear (Fig. 5F). Although, WO3 is a high melting point compound, it can form the new phase with low melt point and promote the densification of sample through the liquid phase diffusion under capillary action along with grains rearrangement [45]. Therefore, the increasing of the W6þ content leads to the dramatic grain growth. With further increasing the W6þ doping content (x ¼ 0.5, 1.0), the equiaxed apatite-type grains are fully replaced by the rod-like grains, as shown in Fig. 5C and D. Meanwhile, compared with the sample with x ¼ 0.3, there are more observable closed pores in the samples, especially in the sample with x ¼ 1.0 (Fig. 5H). Although there are impurities (La6W2O15, La2SiO5 and La2Si2O7) in the samples when the W6þ doping content is more than 0.3 according to the XRD results (Fig. 4), the segregation or aggregation of impurities cannot be observed in the SEM images. Fig. 6 shows the influence of the W6þ doping content on the relative bulk densities of La10Si5Al1xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h. It can be seen that the relative bulk density of the La10Si5Al0.9W0.1O27±d sample is the highest. With further increasing the W6þ content, the relative bulk density decreases gradually.
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Fig. 7 e Impedance spectra of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) measured at 673 K in air and corresponding equivalent circuits.
Electrical conductivity of La10Si5Al1xWxO27±d Fig. 7 shows the typical impedance spectra measured at 673 K and corresponding equivalent circuit of La10Si5Al1xWxO27±d with different W6þ content. There are two visible semicircular arcs attributed to the grain and grain boundary process at high and intermediate frequency, respectively. Both the grain
resistance Rg and the grain boundary resistance Rgb of the samples are calculated according to the fitting results based on the corresponding equivalent circuit by Zview software. The influence of the W6þ content on the Rg and Rgb is shown in Fig. 8A. Compared with the La10Si5AlO26.5 sample without W6þ doping, both Rg and Rgb of the W6þ co-doped samples with x 0.5 decrease distinctly, indicating the improved
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Fig. 9 e The total conductivity of La10Si5Al0.9W0.1O26.65 as a function of the oxygen partial pressure measured at 673, 923 and 1073 K, respectively.
conductivity. However, when the cation doping content is excessive, the obvious differences of the ionic radius (Si4þ: 0.026 nm; Al3þ: 0.039 nm; W6þ: 0.042 nm) and electronegativity (Si4þ: 1.90; Al3þ:1.61; W6þ: 2.36) will inhibit the path for the oxygen interstitial migration [6,31e33,37,44]. Therefore, Rg and Rgb increase with the W6þ content over 0.3. Meanwhile, too much W6þ doping content results in the grain growth accompanied by the appearance of pores (as shown in Fig. 5) and formation of impurities (La6W2O15, La2SiO5 and La2Si2O7, as shown in Fig. 4) in the samples, which also have negative effect on the electrical conductivity of lanthanum silicate apatites. The impurities, La2SiO5 and La2Si2O7, are easily formed when the oxy-apatite phases are synthesized with the solid-state reaction method, and have been proved to tend to degrade the electrical performances of the electrolyte [3,4,14,46]. The conductivity of La6W2O15 phase is very low and has a negative influence on the conductivity of interstitial oxide-ion in apatite-type lanthanum silicates ceramics [37]. The Arrhenius plots of the total conductivities for La10Si5Al1xWxO27±d with different W6þ content calculated from the fitting results are shown in Fig. 8B. It can be seen that La10Si5Al0.9W0.1O26.65 possesses the highest conductivity, while the total conductivity of La10Si5WO28 is the lowest in the temperature range of 673e1073 K. Although the conductivity of the samples decreases when the W6þ content is more than 0.1, the total conductivity of the samples with the W6þ content of 0.3 and 0.5 is still higher than the W6þ undoped La10Si5AlO26.5. Table 3 shows the conduction activation energy calculated
Fig. 8 e The relationship between W6þ content (x) and bulk resistances or grain boundary resistances measured at 673 K in air (A) and Arrhenius plots (B) of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0).
conduction. Among the samples, the sample with x ¼ 0.1 possesses the minimum Rg and Rgb. With the W6þ content increasing from 0.3 to 1.0, both Rg and Rgb increase gradually. When the W6þ doping content reaches 1.0, both Rg (6.49 104 U cm2) and Rgb (4.86 104 U cm2) are much larger than those of the La10Si5AlO26.5 without W6þ doping. Cation doping can result in both positive and negative influences on the total conductivity of the samples [44]. When the W6þ doping content is small (x ¼ 0.1), the positive influence, that is, the improvement of the oxygen interstitial formation is dominant, resulting in the increase of the total
Table 3 e Total conductivity at typical temperatures and corresponding conduction activation energy of La10Si5Al1¡xWxO27±d. Conductivity (S cm1)
Samples 673 K La10Si5AlO26.5 La10Si5Al0.9W0.1O26.65 La10Si5Al0.7W0.3O26.95 La10Si5Al0.5W0.5O27.25 La10Si5WO28
1.11 2.03 1.59 1.35 1.02
923 K 4
10 104 104 104 106
4.91 7.36 5.95 5.19 1.04
Activation energy (eV) 1073 K
3
10 103 103 103 104
1.52 3.03 1.75 1.47 4.93
102 102 102 102 104
0.86 0.83 0.84 0.85 1.04
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lanthanum silicates is improved by W6þ or In3þ co-doping. Especially, with the W6þ content of 0.1, Al3þ and W6þ codoped lanthanum silicate apatite exhibits the highest total conductivity of 3.03 102 S cm1 at 1073 K. With further increasing the W6þ content, the equiaxed apatite-type grains are replaced gradually by the rod-like grains, and the impurity phases of La6W2O15, La2SiO5 and La2Si2O7 appear in the samples leading to lowering conductivity of the apatite lanthanum silicates. The total conductivity independence of the oxygen partial pressure suggests that La10Si5Al0.9W0.1O26.65 remains an almost pure ionic conductor. In addition, the stability test under different atmospheres at 1073 K for 10 h shows the excellent chemical stability of La10Si5Al0.9W0.1O26.65.
Fig. 10 e XRD patterns of La10Si5Al0.9W0.1O26.65 sintered at 1873 K for 6 h after exposed to different atmospheres at 1073 K for 10 h.
from the slope of the linear fitting according to the Arrhenius equation and the total conductivity of the samples at different temperatures. The electrical conduction activation energy of the samples with the doped W6þ content of 0.1e0.5 is almost the same as that of the undoped W6þ La10Si5AlO26.5, indicating that the enhancement in conductivity is due to an increase in charge carriers [6]. In order to verify the oxygen ion conduction of the samples, the conductivity of the samples at different oxygen partial pressure is determined. Fig. 9 shows the relationship between oxygen pressure and total conductivity of La10Si5Al0.9W0.1O26.65 at 673, 923 and 1073 K, respectively. As expected, the total conductivity of La10Si5Al0.9W0.1O26.65 is independence of the oxygen partial pressure, suggesting that the major charge carrier of the sample is oxide ion rather than electronic defects such as electrons and holes [5].
Stability of La10Si5Al0.9W0.1O26.65 To evaluate the chemical stability of the La10Si5Al0.9W0.1O26.65, the sample powders from pellet sintered at 1823 K for 6 h are exposed to 20% H2/Ar, wet Ar or pure CO2 at 1073 K for 10 h, respectively. The powder samples after stability test were analyzed by XRD and the results are shown in Fig. 10. No structural changes, impurity phases and segregation are observed, indicating the high stability of La10Si5Al0.9W0.1O26.65, though monitoring for much longer time measurements and further examination employing various analyses, such as the electrical properties and micro-morphology, are needed in the future work.
Conclusions Apatite lanthanum silicates co-doped on Si4þ site with Al3þ and another cation, such as W6þ, In3þ, Nb5þ or Mg2þ, have been successfully synthesized by the high-temperature solid state reaction method. The conductivity of the apatite
Acknowledgments The authors are grateful to financial support from Natural Science Foundation of China (51272067 and 51472073), Ironsteel United Foundation of Hebei Province of China (E2014209009 and E2016209359) and the Education Department of Hebei Province of China (Z2015140).
references
[1] Nakayama S, Kageyama T, Aono H, Sadaoka Y. Ionic conductivity of lanthanoid silicates Ln10(SiO4)6O3 (Ln ¼ La, Nd, Sm, Gd, Y, Ho, Er and Yb). J Mater Chem 1995;5:1801e5. [2] Nakayama S, Sakamoto M. Electrical properties of new type high oxide ionic conductor RE10Si6O27 (RE ¼ La, Pr, Nd, Sm, Gd, Dy). J Eur Ceram Soc 1998;18:1413e8. [3] Slater PR, Sansom JEH, Tolchard JR. Development of apatitetype oxide ion conductors. Chem Rec 2004;4:373e84. [4] Li B, Liu W, Pan W. Synthesis and electrical properties of apatite-type La10Si6O27. J Power Sources 2010;195:2196e201. [5] Liu W, Yamaguchi S, Tsuchiya T, Miyoshi S, Kobayashi K, Pan W. Sol-gel synthesis and ionic conductivity of oxyapatite-type La9.33þxSi6O26þ1.5x. J Power Sources 2013;235:62e6. [6] Mineshige A, Ohnishi Y, Sakamoto R, Daiko Y, Kobune M, Yazawa T, et al. Effect of cation doping on ionic and electronic properties for lanthanum silicate-based solid electrolytes. Solid State Ionics 2011;192:195e9. [7] Nakao T, Mineshige A, Kobune M, Yazawa T, Yoshioka H. Chemical stability of La10Si6O27 and its application to electrolytes for solid oxide fuel cells. Solid State Ionics 2008;179:1567e9. [8] Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935e9. [9] Nakayama S, Ikesue A, Higuchi Y, Sugawara M, Sakamoto M. Growth of single-crystals of apatite-type oxide ionic conductor from sintered ceramics by a seeding method. J Eur Ceram Soc 2013;33:207e10. [10] Argirusis C, Jothinathan E, Sourkouni G, Van der Biest O, Jomard F. Oxygen self-diffusion and conductivity measurements in apatite type electrolyte materials for SOFCs. Solid State Ionics 2014;257:53e9. [11] Takeda N, Itagaki Y, Aono H, Sadaoka Y. Preparation and characterization of Ln9.33þx/3Si6xAlxO26 (Ln ¼ La, Nd and Sm) with apatite-type structure and its application to a potentiometric O2 gas sensor. Sens Actuators B 2006;115:455e9.
11350
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 1 3 4 0 e1 1 3 5 0
[12] Wang L, Han B, Dai L, Zhou HZ, Li YH, Wu YL, et al. An amperometric NO2 sensor based on La10Si5NbO27.5 electrolyte and nano-structured CuO sensing electrode. J Hazard Mater 2013;262:545e53. [13] Dai L, Meng WW, Meng W, Zhou HZ, Yang GX, Li YH, et al. An impedance metric NH3 sensor based on La10Si5MgO26 electrolyte and nano-structured CoWO4 sensing electrode. J Electrochem Soc 2016;163:B1e7. [14] Kendrick E, Islam MS, Slater PR. Developing apatites for solid oxide fuel cells: insight into structural, transport and doping properties. J Mater Chem 2007;17:3104e11. [15] Higuchi M, Kodaira K, Nakayama S. Nonstoichiometry in apatite-type neodymium silicate single crystals. J Cryst Growth 2000;216:317e21. [16] Arikawa H, Nishiguchi H, Ishihara T, Takita Y. Oxide ion conductivity in Sr-doped La10Ge6O27 apatite oxide. Solid State Ionics 2000;136e137:31e7. [17] Xiang J, Liu ZG, Ouyang JH, Yan FY. Synthesis, structure and electrical properties of rare-earth doped apatite-type lanthanum silicates. Electrochim Acta 2012;65:251e6. [18] Nojiri Y, Tanasea S, Iwasaa M, Yoshiokac H, Matsumuraa Y, Sakaia T. Ionic conductivity of apatite-type solid electrolyte material, La10XBaXSi6O27X/2(X ¼ 0e1), and its fuel cell performance. J Power Sources 2010;195:4059e64. [19] Zhang L, He HQ, Wu HW, Li CZ, Jiang SP. Synthesis and characterization of doped La9ASi6O26.5 (A ¼ Ca, Sr, Ba) oxyapatite electrolyte by a water-based gel-casting route. Int J Hydrogen Energy 2011;36:6862e74. [20] Vincent A, Savignat SB, Gervais F. Elaboration and ionic conduction of apatite-type lanthanum silicates doped with Ba, La10xBax(SiO4)6O3x/2 with x ¼ 0.25e2. J Eur Ceram Soc 2007;27:1187e92. [21] Beaudet-Savignat S, Vincent A, Lambert S, Gervais F. Oxide ion conduction in Ba, Ca and Sr doped apatite-type lanthanum silicates. J Mater Chem 2007;17:2078e87. [22] Tolchard JR, Islam MS, Slater PR. Defect chemistry and oxygen ion migration in the apatite-type materials La9.33Si6O26 and La8Sr2Si6O26. J Mater Chem 2003;13:1956e61. [23] Sansom JEH, Kendrick E, Tolchard JR, Islam MS, Slater PR. A comparison of the effect of rare earth vs Si site doping on the conductivities of apatite-type rare earth silicates. J Solid State Electrochem 2006;10:562e8. [24] Marrero-Lopez D, Martı´n-Sedeno MC, Pena-Martı´nez J, RuizMorales JC, Nu´nez P, Aranda MAG, et al. Evaluation of apatite silicates as solid oxide fuel cell electrolytes. J Power Sources 2010;195:2496e506. [25] Yoshioka H. High oxide ion conductivity in Mg-doped La10Si6O27 with apatite-type structure. Chem Lett 2004;33:392e3. [26] Yin GC, Yin H, Wang X, Sun ML, Zhong LH, Cong RD, et al. Mg doping effect on high-pressure behaviors of apatite-type lanthanum silicate. J Alloys Comp 2014;611:24e9. [27] Guillot S, Beaudet-Savignat S, Lambert S, Roussel P, Vannier R. Effect of the dopant nature on the conductivity of oxygen overstoichiometric oxyapatites with controlled microstructures. Solid State Ionics 2011;185:18e26. [28] Inoubli A, Kahlaoui M, Sobradosc I, Chefia S, Madani A, Sanz J, et al. Influence of anionic vacancies on the conductivity of La9.33Si6-xAlxO26-x/2 oxide conductors with an oxyapatite structure. J Power Sources 2014;271:203e12. [29] Shaula AL, Kharton VV, Marques FMB. Ionic and electronic conductivities, stability and thermal expansion of La10x(Si,Al)6O26±d solid electrolytes. Solid State Ionics 2006;177:1725e8.
[30] Shaula AL, Kharton VV, Marques FMB. Oxygen ionic and electronic transport in apatite-type La10x(Si, Al)6O26±d. J Solid State Chem 2005;178:2050e61. [31] Zhou J, Ye XF, Li JL, Wang SR, Wen TL. Synthesis and characterization of apatite-type La9.67Si6-xAlxO26.5-x/2 electrolyte materials and compatible cathode materials. Solid State Ionics 2011;201:81e6. [32] Xiang J, Liu ZG, Ouyang JH, Yan FY. Ionic conductivity of oxyapatite La10Si6-xInxO27þd solid electrolyte ceramics. J Power Sources 2014;251:305e10. [33] McFarlane J, Barth S, Swaffer M, Sansom JEH, Slater PR. Synthesis and conductivities of the apatite-type systems, La9.33þxSi6-yMyO26þz (M ¼ Co, Fe, Mn) and La8Mn2Si6O26. Ionics 2002;8:149e54. [34] Kharton VV, Shaula AL, Patrakeev MV, Waerenborgh JC, Rojas DP, Vyshatko NP, et al. Oxygen ionic and electronic transport in apatite-type solid electrolytes. J Electrochem Soc 2004;151:A1236e46. [35] Shi Q, Lu L, Jin H, Zhang H, Zeng Y. Electrical properties and thermal expansion of cobalt doped apatite-type lanthanum silicates based electrolytes for IT-SOFC. Mater Res Bull 2012;47:719e23. [36] Xiang J, Liu ZG, Ouyang JH, Zhou Y, Yan FY. Synthesis and electrical conductivity of La10Si5.5B0.5O27þd (B ¼ In, Si, Sn, Nb) ceramics. Solid State Ionics 2012;220:7e11. [37] Xiang J, Ouyang JH, Liu ZG. Microstructure and electrical conductivity of apatite-type La10Si6-xWxO27þd electrolytes. J Power Sources 2015;284:49e55. [38] Xiang J, Ouyang JH, Liu ZG, Qi GC. Influence of pentavalent niobium doping on microstructure and electrical conductivity of oxy-apatite La10Si6O27 electrolytes. Electrochim Acta 2015;153:287e94. [39] Cao XG, Jiang SP. Sinterability and conductivity of barium doped aluminium lanthanum oxyapatite La9.5Ba0.5Si5.5Al0.5O26.5 electrolyte of solid oxide fuel cells. J Alloys Comp 2012;523:127e33. [40] Cao XG, Jiang SP. Effect of Sr and Al or Fe co-doping on the sinterability and conductivity of lanthanum silicate oxyapatite electrolytes for solid oxide fuel cells. Int J Hydrogen Energy 2014;39:19093e101. [41] Cao XG, Jiang SP, Li YY. Synthesis and characterization of calcium and iron co-doped lanthanum silicate oxyapatites by sol-gel process for solid oxide fuel cells. J Power Sources 2015;293:806e14. [42] Setsoafia DDY, Hing P, Jung SC, Azad AK, Lim CM. Sol-gel synthesis and characterization of Zn2þ and Mg2þ doped La10Si6O27 electrolytes for solid oxide fuel cells. Solid State Sci 2015;48:163e70. [43] Yang TR, Zhao HL, Han J, Xu NS, Shen YN, Du ZH, et al. Synthesis and densification of lanthanum silicate apatite electrolyte for intermediate temperature solid oxide fuel cell via co-precipitation method. J Eur Ceram Soc 2014;34:1563e9. [44] Najib A, Sansom JEH, Tolchard JR, Slater PR, Islam MS. Doping strategies to optimise the oxide ion conductivity in apatite-type ionic conductors. Dalton Trans 2004:3106e9. [45] Xia YJ, Bai YJ, Wu XJ, Zhou DF, Wang ZC, Liu XJ, et al. Effect of sintering aids on the electrical properties of Ce0.9Nd0.1O2-d. Solid State Sci 2012;14:805e8. [46] Chesnaud A, Dezanneau G, Estournes C, Bogicevic C, Karolak F, Geiger S, et al. Influence of synthesis route and composition on electrical properties of La9.33þxSi6O26þ3x/2 oxy-apatite compounds. Solid State Ionics 2008;179:1929e39.
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Synthesis and characterization of Al3þ and M (M ¼ W6þ, In3þ, Nb5þ, Mg2þ) co-doped lanthanum silicate oxy-apatite electrolytes Lei Dai a,b, Wen Han a, Yuehua Li a,**, Ling Wang a,b,* a
College of Chemical Engineering, North China University of Science and Technology, Tangshan 063009, PR China Hebei Province Key Laboratory of Photocatalytic and Electrocatalytic Materials for Environment, Tangshan 063009, PR China
b
article info
abstract
Article history:
Apatite-type La10Si5Al0.9M0.1O27d (M ¼ W6þ, In3þ, Nb5þ or Mg2þ) are successfully synthe-
Received 19 February 2016
sized by the high-temperature solid state reaction method. The composition, microstruc-
Received in revised form
ture and electrical conduction performance of the samples are characterized by X-ray
9 April 2016
diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance
Accepted 28 April 2016
spectroscopy (EIS), respectively. The dense co-doped lanthanum silicates electrolytes with
Available online 27 May 2016
pure hexagonal apatite-type structure are obtained after sintered at 1873 K for 6 h.
Keywords:
temperature range of 673e1073 K. The influences of the co-doped W6þ content on the
La10Si5Al0.9W0.1O26.65 has the highest total conductivity among the co-doped samples in the Oxy-apatite lanthanum silicate
properties of the La10Si5Al1xWxO27±d (0 x 1) are investigated. It is found that with
Co-doping
increasing the W6þ content, the equiaxed apatite-type grains are replaced gradually by the
Microstructure
rod-like grains, accompanied by the formation of impurity phases (La6W2O15, La2SiO5 and
Electrolyte
La2Si2O7) in the samples. The La10Si5Al0.9W0.1O26.65 exhibits the highest conductivity of
Electrical conductivity
3.03 102 S cm1 at 1073 K. The oxygen pressure independency of the total conductivity suggests that La10Si5Al0.9W0.1O26.65 remains an almost pure oxygen ionic conductor. La10Si5Al0.9W0.1O26.65 also shows good stability in 20% H2/Ar, wet Ar and pure CO2 atmospheres, respectively. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Lanthanum silicates with an apatite-type structure have attracted great attention as alternative electrolytes to YSZ due to their high oxygen ion conductivity at intermediate temperatures (773e1023 K) [1e4]. The oxy-apatite lanthanum
silicates also exhibit pure oxygen ion conduction over a wide oxygen partial pressure range and good stable performance under various gas atmospheres [5e7]. Meanwhile, there are other advantages of low electrical conduction activation energy, moderate thermal expansion coefficients, a wide range of materials selection, and relatively low cost of materials processing. Therefore, the excellent properties make the
* Corresponding author. College of Chemical Engineering, North China University of Science and Technology, Tangshan 063009, PR China. Tel./fax: þ86 315 2592170. ** Corresponding author. Tel./fax: þ86 315 2592170. E-mail addresses: [email protected] (Y. Li), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.04.210 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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apatite lanthanum silicates into the potential electrolytes for intermediate temperature operations, which have been widely used in solid oxide fuel cells and electrochemical gas sensors [8e13]. In addition, compared with most of other ionic conductors, the apatite-type structure is more tolerant to extensive aliovalent doping, which is an effective approach to optimize the electrical conduction of lanthanum silicates [14]. The improvement of oxygen ion conductivity of lanthanum silicates by aliovalent doping can be realized via partial substitution of La3þ or Si4þ site with cations. The cationic substitutions on the La site with other rare earth elements [15e17] or alkaline earth elements [18e23] have been investigated. The samples containing oxygen excess exhibit high oxygen ion conductivity, which indicates that oxygen excess is more important than cationic vacancies to achieve good oxide-ion conductivity of oxy-apatite lanthanum silicate [21].
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Compared with doping on the La3þ site, Si4þ site doping has a more extensive range of cations selection and shows to be more effective in increasing the total conductivity [24]. Up to now, many efforts have been made to further enhance the electrical conduction performance of apatite-type lanthanum silicates via partial substitution of Si4þ site with low valence state cations (such as Mg2þ [25e27], Al3þ [28e31], In3þ [32], Mn2þ [33], Fe3þ [34], Co2þ [35] or Sn4þ [36]) or high valence state cations (such as W6þ [37] or Nb5þ [38]). Furthermore, these various cations doping also can improve the sinterability of oxyapatite lanthanum silicates and restrain the formation of impurity phases (La2SiO5 and La2Si5O7). Among the cations doping on the Si4þ site, Al3þ doping has been investigated very systematically and provide a significant improvement of oxygen ion conductivity of oxyapatite lanthanum silicates [28e31]. Recently, some research also has been reported on the oxyapatite lanthanum silicates with the cations co-doping on
Fig. 1 e XRD patterns and SEM surface images of La10Si5Al0.9M0.1O27¡d (M ¼ W, In, Nb or Mg) sintered at 1873 K for 6 h.
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both La3þ and Si4þ site, such as Ba and Al co-doping [39], Sr and Al or Fe co-doping [40], Ca and Fe co-doping [41]. However, the research of the co-doping strategy on Si4þ site is limited. Although, the Si4þ site substituted by Zn2þ and Mg2þ in La10Si6O27 electrolyte has been reported, the single Zn2þsubstituted La10Zn0.2(SiO4)5.8O2.5 gives the highest oxygen ion conductivity [42]. Meanwhile, the influences of the co-doped cation types and contents on the structure and electrical conductivity of oxy-apatite lanthanum silicates are not well understood. In this paper, the oxy-apatite lanthanum silicates co-doped on the Si4þ site with Al3þ and different other cations (W6þ, In3þ, Nb5þ or Mg2þ) are successfully synthesized by the hightemperature solid state reaction method. The influences of the species and content of the doping cations on the composition, microstructure and electrical conductivity of apatitetype lanthanum silicates are investigated in detail.
The electrical conductivities of the co-doped apatite lanthanum silicates were measured by ac impedance spectroscopy using the Solartron Analytical SI 1260 impedance/ gain-phase analyzer with the Solartron Analytical SI 1287 electrochemical interface. Pt layers were used as the electrical collectors, which was prepared by painting Pt paste on the polished surfaces of the sample pellets and then treated in air at 1123 K for 1 h. The electrical conductivity was measured in air in the temperature range of 673e1073 K. The compleximpedance measurements of the samples were carried out in the frequency range of 1 MHz to 0.01 Hz under open circuit voltage using AC amplitude of 5 mV. The obtained spectra were analyzed using the Zview software associated with the Solartron instrument.
Results and discussion
Experimental
Characterization of co-doped La10Si5Al0.9M0.1O27d (M ¼ W, In, Nb or Mg)
The co-doped La10Si5Al1xMxO27d (M ¼ W, In, Nb or Mg, x ¼ 0e1) electrolytes were synthesized by the high temperature solid state reaction process. According to the stoichiometric ratios of La10Si5Al1xMxO27d with different composition, the appropriate amounts of raw materials (Analytically pure La2O3, SiO2, Al2O3, WO3, In2O3, Nb2O5 and MgO powders) were mixed and then ball-milled with anhydrous alcohol for 24 h. After dried, the mixed powders were calcined at 1573 K for 6 h in air. The calcined powders were finely ground and uniaxially pressed into a cylindrical flat mold utilizing fitted stainless steel disks with a pressure of 20 MPa to obtain the disk-shaped samples (13 mm in diameter and 1 mm in height). The disk-shaped samples were then sintered at 1873 K for 6 h to form the dense pellets. In order to examine the chemical stability of the sintered pellets in different atmospheres, the sample powders were exposed to flowing 20% H2/Ar, wet Ar or pure CO2 at 1073 K for 10 h, respectively. The bulk densities of all the sintered samples were measured by the Archimedes method with an immersion medium of deionized water. The theoretical bulk densities of the samples were calculated from the values of lattice parameters. The composition of the samples was identified by X-ray diffraction (XRD, Rigaku, D/MAX2500PC) analysis under Cu-Ka radiation with the incidence beam angle of 2 in the range of 10e90 . The microstructure of the samples was investigated by field emission scanning electron microscopy (SEM, Hitachi, S-4800).
Oxy-apatite type La10Si5Al0.9M0.1O27d electrolytes co-doped on Si4þ site with Al3þ and another cation, such as W6þ, In3þ, Nb5þ or Mg2þ, were synthesized by the high-temperature solid state reaction method. The XRD patterns and SEM surface images of the samples sintered at 1873 K for 6 h are shown in Fig. 1. It can be seen from the XRD results that all the diffraction peaks of the samples correspond to the single hexagonal apatite-type structure of La10(SiO4)6O3 (JCPDS No.53-0291) without impurity phases, which indicates that Si4þ has been partially substituted by Al3þ and M ion successfully. The refined unit cell parameters of the samples are listed in Table 1. Clearly, the cell volume (V0) of the samples increases when Al3þ is partially substituted by W6þ, In3þ, Nb5þ or Mg2þ due to their larger effective ionic radii (RW ¼ 0.042 nm, RIn ¼ 0.062 nm, RNb ¼ 0.048 nm, RMg ¼ 0.057 nm) than that of Al3þ (RAl ¼ 0.039 nm), which is in agreement with the consequence of XRD and indicates that these cations have been introduced into the crystal lattice. Among the samples, La10Si5Al0.9W0.1O26.65 is of the highest relative bulk density. Fig. 1 also shows the typical SEM images of the surface of La10Si5Al0.9M0.1O27d (M ¼ W, In, Nb or Mg) pellets sintered at 1873 K for 6 h. It is clearly seen that the samples are quite dense without observable pores. La10Si5AlO26.5 is mainly composed of rod-like grains accompanied by some small equiaxed grains. For La10Si5Al0.9W0.1O26.65 and La10Si5Al0.9In0.1O26.5, the grains are equiaxed and relatively uniform, although a few of rod-like grains exist. In contrast, the rod-like grains become dominant in La10Si5Al0.9Nb0.1O26.6 and
Table 1 e Cell parameter values and bulk densities of La10Si5Al0.9M0.1O27¡d. Samples
a¼b (nm)
c (nm)
V0 (nm3)
Theoretical density (g cm3)
Measured density (g cm3)
Relative density (%)
La10Si5AlO26.5 La10Si5Al0.9W0.1O26.65 La10Si5Al0.9In0.1O26.5 La10Si5Al0.9Nb0.1O26.6 La10Si5Al0.9Mg0.1O26.45
0.9681 0.9719 0.9757 0.9729 0.9726
0.7205 0.7249 0.7254 0.7211 0.7222
0.5848 0.5930 0.5980 0.5911 0.5916
5.623 5.596 5.524 5.587 5.556
5.427 5.428 5.341 5.212 5.234
96.5 97.2 96.7 93.3 94.2
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Fig. 2 e Impedance spectra of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) measured in the temperature range of 673e1073 K in air. La10Si5Al0.9Mg0.1O26.45 samples with a few of excessively grown rods. The electrochemical impedance spectra of the co-doped apatite lanthanum silicate samples were measured and recorded respectively. Fig. 2 shows the typical impedance spectra measured at different temperatures for La10Si5Al0.9W0.1O26.65 sample sintered at 1873 K for 6 h. Based on the equivalent circuit model consisting of a serial association of (R//CPE) elements (R: resistance; CPE: constant phase element) attributed to electrolyte or electrode processes, the
high and medium frequency arcs can be identified with the grain bulk (Rg) and grain boundary (Rgb) resistances, respectively [32,41]. The electrical conduction of the co-doped apatite lanthanum silicates is controlled by the grain and grain boundary contributions together. In addition, the semicircle or tail at low frequencies represents the electrode interface diffusion, which is the representative of Wagner ionic diffusion effect [37,38]. At the temperature range of 673e923 K, the two arcs presented the grain bulk and boundary resistances can be observed. With the temperature
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5
Fig. 4 e XRD patterns (A) of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h and (B) a local magnification of diffraction peaks from (A).
increasing to 973 and 1073 K, the arcs contributed from both the grain bulk and boundary processes disappear due to the instrumental limitation. Only the semicircle attributed to the electrode interface process appears. Therefore, the intercept of the semicircle with the real axis at high frequency direction is designed to total resistance of the sample, which is similar with the results in literature [43].
Fig. 3 e Impedance spectra (A), a comparison of Rg and Rgb (B) measured at 923 K in air and Arrhenius plots of the total conductivity (C) of La10Si5Al0.9M0.1O27¡d (M ¼ W, In, Nb or Mg).
Table 2 e Total conductivity at typical temperatures and corresponding conduction activation energy of La10Si5Al0.9M0.1O27¡d. Conductivity (S cm1)
Samples 673 K La10Si5AlO26.5 La10Si5Al0.9W0.1O26.65 La10Si5Al0.9In0.1O26.5 La10Si5Al0.9Nb0.1O26.6 La10Si5Al0.9Mg0.1O26.45
1.11 2.03 1.81 8.23 1.35
104 104 104 105 104
923 K 4.91 7.36 6.79 4.37 3.78
103 103 103 103 103
Activation energy (eV) 1073 K 1.52 3.03 2.13 1.45 1.20
102 102 102 102 102
0.86 0.83 0.82 0.89 0.81
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Fig. 5 e SEM images of the surface (AeD) and the fractured cross-section (EeH) of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h.
Fig. 3A and B shows a comparison of Rg and Rgb for La10Si5Al0.9M0.1O27d samples based on the impedance data measured at 923 K. Compared with La10Si5AlO26.5, both Rg and Rgb of the W6þ or In3þ co-doped samples decrease obviously, while the two values increase with Nb5þ or Mg2þ co-doping. The
total conductivity (s) of the samples can be calculated from the total resistances (R ¼ Rg þ Rgb), through the following equation: s ¼ l=SR where l is the sample thickness and S is the pellet surface area.
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XRD and SEM analysis of La10Si5Al1xWxO27±d
Relative bulk density / %
98 97 96 95 94 93 92 91 90 0.0
0.2
0.4
X
0.6
0.8
1.0
Fig. 6 e The relationship between W6þ content (x) and the relative bulk densities of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h.
Fig. 3C shows the relationship between ln (sT) and 10,000 T1 for the co-doped oxy-apatite lanthanum silicates with different cations. It is clearly seen that the total conductivity of all the samples varies linearly with the measurement temperature in the range of 673e1073 K, which is well fitted to the Arrhenius equation. Compared with La10Si5AlO26.5, the total conductivity of the samples co-doped W6þ or In3þ is distinctly improved in all measurement temperature. On the contrary, the total conductivity of the sample becomes lower after co-doped by Nb5þ. The total conductivity of the samples at the typical temperatures and the activation energy for electrical conduction are shown in Table 2. The activation energy of the samples co-doped with W6þ, In3þ, Nb5þ and Mg2þ is 0.83, 0.82, 0.89 and 0.81 eV, respectively. The previous studies have identified that the ionic conductivity of silicate-based apatites containing oxygen excess is mainly mediated by oxygen interstitial migration [14,22]. Cation doping on Si site can effectively improve the formation of oxygen interstitial through the local distortions and structural relaxation, which consequently improves the total conductivity. The nature difference of the doped cations, especially the ionic radius, results in the difference of cell distortion and restraining the oxygen ion, and then the conductivity of silicate-based apatite is changed accordingly [44]. The influences of different cation doping on the total conductivity of La10Si5Al0.9M0.1O27d electrolytes indicate that a synergistic action between doped Al3þ and W6þ or In3þ promotes the oxygen ion conduction, which should be attributed to the improvement of the oxygen interstitial formation. Meanwhile, the In3þ doping can generate the oxygen vacancies, which is another positive factor for the improvement of the oxygen ion conduction [32]. Among the co-doped samples, La10Si5Al0.9W0.1O26.65 exhibits the highest total conductivity of 3.03 102 S cm1 at 1073 K, which is higher or comparable to previous reports [37,40e42]. Based on above results, the influences of the W6þ content on the properties of the La10Si5Al1xWxO27±d are investigated.
Fig. 4 shows the XRD patterns of La10Si5Al1xWxO27±d (x ¼ 0, 0.1, 0.3, 0.5, 1) pellets sintered at 1823 K for 6 h. The main diffraction peaks of all the samples are in agreement with the hexagonal apatite-type structure of La10(SiO4)6O3 (JCPDS No.53-0291), but phase purity is different for different codoped W6þ content. It is clearly seen from Fig. 4A that the single hexagonal apatite-type structure of the samples is obtained when x ¼ 0 or 0.1. With the W6þ content increases to 0.3, La6W2O15 phase (JCPDS No. 37-0124) with an orthorhombic structure appears, which is similar to the phenomenon in the previous literature [37]. When the W6þ content is more than 0.5, the diffraction peaks of La2SiO5 and La2Si5O7 appear. Fig. 4B shows the enlargement of Fig. 4A in the 2q range of 29.5e32.5 . The effective ionic radii of hexavalent W6þ (RW ¼ 0.042 nm), tetravalent Si4þ (RSi ¼ 0.026 nm) and trivalent Al3þ (RAl ¼ 0.039 nm) are different. Thus, the lattice distortion of La10Si5Al1xWxO27±d is relatively large, which makes the diffraction peaks shift to the low angle side. As shown in Fig. 4B, when the W6þ content increases from 0 to 0.1, the characteristic diffraction peaks of the samples shift observably to the low angle. However, the positions of characteristic diffraction peaks remain almost unchanged with further increasing the W6þ content, indicating a saturated solution of W6þ. Combined with the component analysis as mentioned above, the solid solubility limit of W6þ in the apatite-type lattice of La10Si5Al1xWxO27±d is in the range of 0.1e0.3. Fig. 5 shows the typical SEM images of the surface and cross-section of La10Si5Al1xWxO27±d (x ¼ 0.1, 0.3, 0.5, 1) pellets sintered at 1873 K for 6 h. It is clearly seen that La10Si5Al0.9W0.1O26.65 sample shows a relatively uniform structure. The grain boundary is clear and the equiaxed grain is full-grown with a grain size of 1e4 mm (Fig. 5A and E). When the W6þ content reaches 0.3, the grains size becomes uneven and some rod-like big grains dramatically grow (Fig. 5B) and a few closed pores appear (Fig. 5F). Although, WO3 is a high melting point compound, it can form the new phase with low melt point and promote the densification of sample through the liquid phase diffusion under capillary action along with grains rearrangement [45]. Therefore, the increasing of the W6þ content leads to the dramatic grain growth. With further increasing the W6þ doping content (x ¼ 0.5, 1.0), the equiaxed apatite-type grains are fully replaced by the rod-like grains, as shown in Fig. 5C and D. Meanwhile, compared with the sample with x ¼ 0.3, there are more observable closed pores in the samples, especially in the sample with x ¼ 1.0 (Fig. 5H). Although there are impurities (La6W2O15, La2SiO5 and La2Si2O7) in the samples when the W6þ doping content is more than 0.3 according to the XRD results (Fig. 4), the segregation or aggregation of impurities cannot be observed in the SEM images. Fig. 6 shows the influence of the W6þ doping content on the relative bulk densities of La10Si5Al1xWxO27±d (x ¼ 0e1.0) sintered at 1873 K for 6 h. It can be seen that the relative bulk density of the La10Si5Al0.9W0.1O27±d sample is the highest. With further increasing the W6þ content, the relative bulk density decreases gradually.
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Fig. 7 e Impedance spectra of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0) measured at 673 K in air and corresponding equivalent circuits.
Electrical conductivity of La10Si5Al1xWxO27±d Fig. 7 shows the typical impedance spectra measured at 673 K and corresponding equivalent circuit of La10Si5Al1xWxO27±d with different W6þ content. There are two visible semicircular arcs attributed to the grain and grain boundary process at high and intermediate frequency, respectively. Both the grain
resistance Rg and the grain boundary resistance Rgb of the samples are calculated according to the fitting results based on the corresponding equivalent circuit by Zview software. The influence of the W6þ content on the Rg and Rgb is shown in Fig. 8A. Compared with the La10Si5AlO26.5 sample without W6þ doping, both Rg and Rgb of the W6þ co-doped samples with x 0.5 decrease distinctly, indicating the improved
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 1 3 4 0 e1 1 3 5 0
Fig. 9 e The total conductivity of La10Si5Al0.9W0.1O26.65 as a function of the oxygen partial pressure measured at 673, 923 and 1073 K, respectively.
conductivity. However, when the cation doping content is excessive, the obvious differences of the ionic radius (Si4þ: 0.026 nm; Al3þ: 0.039 nm; W6þ: 0.042 nm) and electronegativity (Si4þ: 1.90; Al3þ:1.61; W6þ: 2.36) will inhibit the path for the oxygen interstitial migration [6,31e33,37,44]. Therefore, Rg and Rgb increase with the W6þ content over 0.3. Meanwhile, too much W6þ doping content results in the grain growth accompanied by the appearance of pores (as shown in Fig. 5) and formation of impurities (La6W2O15, La2SiO5 and La2Si2O7, as shown in Fig. 4) in the samples, which also have negative effect on the electrical conductivity of lanthanum silicate apatites. The impurities, La2SiO5 and La2Si2O7, are easily formed when the oxy-apatite phases are synthesized with the solid-state reaction method, and have been proved to tend to degrade the electrical performances of the electrolyte [3,4,14,46]. The conductivity of La6W2O15 phase is very low and has a negative influence on the conductivity of interstitial oxide-ion in apatite-type lanthanum silicates ceramics [37]. The Arrhenius plots of the total conductivities for La10Si5Al1xWxO27±d with different W6þ content calculated from the fitting results are shown in Fig. 8B. It can be seen that La10Si5Al0.9W0.1O26.65 possesses the highest conductivity, while the total conductivity of La10Si5WO28 is the lowest in the temperature range of 673e1073 K. Although the conductivity of the samples decreases when the W6þ content is more than 0.1, the total conductivity of the samples with the W6þ content of 0.3 and 0.5 is still higher than the W6þ undoped La10Si5AlO26.5. Table 3 shows the conduction activation energy calculated
Fig. 8 e The relationship between W6þ content (x) and bulk resistances or grain boundary resistances measured at 673 K in air (A) and Arrhenius plots (B) of La10Si5Al1¡xWxO27±d (x ¼ 0e1.0).
conduction. Among the samples, the sample with x ¼ 0.1 possesses the minimum Rg and Rgb. With the W6þ content increasing from 0.3 to 1.0, both Rg and Rgb increase gradually. When the W6þ doping content reaches 1.0, both Rg (6.49 104 U cm2) and Rgb (4.86 104 U cm2) are much larger than those of the La10Si5AlO26.5 without W6þ doping. Cation doping can result in both positive and negative influences on the total conductivity of the samples [44]. When the W6þ doping content is small (x ¼ 0.1), the positive influence, that is, the improvement of the oxygen interstitial formation is dominant, resulting in the increase of the total
Table 3 e Total conductivity at typical temperatures and corresponding conduction activation energy of La10Si5Al1¡xWxO27±d. Conductivity (S cm1)
Samples 673 K La10Si5AlO26.5 La10Si5Al0.9W0.1O26.65 La10Si5Al0.7W0.3O26.95 La10Si5Al0.5W0.5O27.25 La10Si5WO28
1.11 2.03 1.59 1.35 1.02
923 K 4
10 104 104 104 106
4.91 7.36 5.95 5.19 1.04
Activation energy (eV) 1073 K
3
10 103 103 103 104
1.52 3.03 1.75 1.47 4.93
102 102 102 102 104
0.86 0.83 0.84 0.85 1.04
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lanthanum silicates is improved by W6þ or In3þ co-doping. Especially, with the W6þ content of 0.1, Al3þ and W6þ codoped lanthanum silicate apatite exhibits the highest total conductivity of 3.03 102 S cm1 at 1073 K. With further increasing the W6þ content, the equiaxed apatite-type grains are replaced gradually by the rod-like grains, and the impurity phases of La6W2O15, La2SiO5 and La2Si2O7 appear in the samples leading to lowering conductivity of the apatite lanthanum silicates. The total conductivity independence of the oxygen partial pressure suggests that La10Si5Al0.9W0.1O26.65 remains an almost pure ionic conductor. In addition, the stability test under different atmospheres at 1073 K for 10 h shows the excellent chemical stability of La10Si5Al0.9W0.1O26.65.
Fig. 10 e XRD patterns of La10Si5Al0.9W0.1O26.65 sintered at 1873 K for 6 h after exposed to different atmospheres at 1073 K for 10 h.
from the slope of the linear fitting according to the Arrhenius equation and the total conductivity of the samples at different temperatures. The electrical conduction activation energy of the samples with the doped W6þ content of 0.1e0.5 is almost the same as that of the undoped W6þ La10Si5AlO26.5, indicating that the enhancement in conductivity is due to an increase in charge carriers [6]. In order to verify the oxygen ion conduction of the samples, the conductivity of the samples at different oxygen partial pressure is determined. Fig. 9 shows the relationship between oxygen pressure and total conductivity of La10Si5Al0.9W0.1O26.65 at 673, 923 and 1073 K, respectively. As expected, the total conductivity of La10Si5Al0.9W0.1O26.65 is independence of the oxygen partial pressure, suggesting that the major charge carrier of the sample is oxide ion rather than electronic defects such as electrons and holes [5].
Stability of La10Si5Al0.9W0.1O26.65 To evaluate the chemical stability of the La10Si5Al0.9W0.1O26.65, the sample powders from pellet sintered at 1823 K for 6 h are exposed to 20% H2/Ar, wet Ar or pure CO2 at 1073 K for 10 h, respectively. The powder samples after stability test were analyzed by XRD and the results are shown in Fig. 10. No structural changes, impurity phases and segregation are observed, indicating the high stability of La10Si5Al0.9W0.1O26.65, though monitoring for much longer time measurements and further examination employing various analyses, such as the electrical properties and micro-morphology, are needed in the future work.
Conclusions Apatite lanthanum silicates co-doped on Si4þ site with Al3þ and another cation, such as W6þ, In3þ, Nb5þ or Mg2þ, have been successfully synthesized by the high-temperature solid state reaction method. The conductivity of the apatite
Acknowledgments The authors are grateful to financial support from Natural Science Foundation of China (51272067 and 51472073), Ironsteel United Foundation of Hebei Province of China (E2014209009 and E2016209359) and the Education Department of Hebei Province of China (Z2015140).
references
[1] Nakayama S, Kageyama T, Aono H, Sadaoka Y. Ionic conductivity of lanthanoid silicates Ln10(SiO4)6O3 (Ln ¼ La, Nd, Sm, Gd, Y, Ho, Er and Yb). J Mater Chem 1995;5:1801e5. [2] Nakayama S, Sakamoto M. Electrical properties of new type high oxide ionic conductor RE10Si6O27 (RE ¼ La, Pr, Nd, Sm, Gd, Dy). J Eur Ceram Soc 1998;18:1413e8. [3] Slater PR, Sansom JEH, Tolchard JR. Development of apatitetype oxide ion conductors. Chem Rec 2004;4:373e84. [4] Li B, Liu W, Pan W. Synthesis and electrical properties of apatite-type La10Si6O27. J Power Sources 2010;195:2196e201. [5] Liu W, Yamaguchi S, Tsuchiya T, Miyoshi S, Kobayashi K, Pan W. Sol-gel synthesis and ionic conductivity of oxyapatite-type La9.33þxSi6O26þ1.5x. J Power Sources 2013;235:62e6. [6] Mineshige A, Ohnishi Y, Sakamoto R, Daiko Y, Kobune M, Yazawa T, et al. Effect of cation doping on ionic and electronic properties for lanthanum silicate-based solid electrolytes. Solid State Ionics 2011;192:195e9. [7] Nakao T, Mineshige A, Kobune M, Yazawa T, Yoshioka H. Chemical stability of La10Si6O27 and its application to electrolytes for solid oxide fuel cells. Solid State Ionics 2008;179:1567e9. [8] Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935e9. [9] Nakayama S, Ikesue A, Higuchi Y, Sugawara M, Sakamoto M. Growth of single-crystals of apatite-type oxide ionic conductor from sintered ceramics by a seeding method. J Eur Ceram Soc 2013;33:207e10. [10] Argirusis C, Jothinathan E, Sourkouni G, Van der Biest O, Jomard F. Oxygen self-diffusion and conductivity measurements in apatite type electrolyte materials for SOFCs. Solid State Ionics 2014;257:53e9. [11] Takeda N, Itagaki Y, Aono H, Sadaoka Y. Preparation and characterization of Ln9.33þx/3Si6xAlxO26 (Ln ¼ La, Nd and Sm) with apatite-type structure and its application to a potentiometric O2 gas sensor. Sens Actuators B 2006;115:455e9.
11350
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 1 3 4 0 e1 1 3 5 0
[12] Wang L, Han B, Dai L, Zhou HZ, Li YH, Wu YL, et al. An amperometric NO2 sensor based on La10Si5NbO27.5 electrolyte and nano-structured CuO sensing electrode. J Hazard Mater 2013;262:545e53. [13] Dai L, Meng WW, Meng W, Zhou HZ, Yang GX, Li YH, et al. An impedance metric NH3 sensor based on La10Si5MgO26 electrolyte and nano-structured CoWO4 sensing electrode. J Electrochem Soc 2016;163:B1e7. [14] Kendrick E, Islam MS, Slater PR. Developing apatites for solid oxide fuel cells: insight into structural, transport and doping properties. J Mater Chem 2007;17:3104e11. [15] Higuchi M, Kodaira K, Nakayama S. Nonstoichiometry in apatite-type neodymium silicate single crystals. J Cryst Growth 2000;216:317e21. [16] Arikawa H, Nishiguchi H, Ishihara T, Takita Y. Oxide ion conductivity in Sr-doped La10Ge6O27 apatite oxide. Solid State Ionics 2000;136e137:31e7. [17] Xiang J, Liu ZG, Ouyang JH, Yan FY. Synthesis, structure and electrical properties of rare-earth doped apatite-type lanthanum silicates. Electrochim Acta 2012;65:251e6. [18] Nojiri Y, Tanasea S, Iwasaa M, Yoshiokac H, Matsumuraa Y, Sakaia T. Ionic conductivity of apatite-type solid electrolyte material, La10XBaXSi6O27X/2(X ¼ 0e1), and its fuel cell performance. J Power Sources 2010;195:4059e64. [19] Zhang L, He HQ, Wu HW, Li CZ, Jiang SP. Synthesis and characterization of doped La9ASi6O26.5 (A ¼ Ca, Sr, Ba) oxyapatite electrolyte by a water-based gel-casting route. Int J Hydrogen Energy 2011;36:6862e74. [20] Vincent A, Savignat SB, Gervais F. Elaboration and ionic conduction of apatite-type lanthanum silicates doped with Ba, La10xBax(SiO4)6O3x/2 with x ¼ 0.25e2. J Eur Ceram Soc 2007;27:1187e92. [21] Beaudet-Savignat S, Vincent A, Lambert S, Gervais F. Oxide ion conduction in Ba, Ca and Sr doped apatite-type lanthanum silicates. J Mater Chem 2007;17:2078e87. [22] Tolchard JR, Islam MS, Slater PR. Defect chemistry and oxygen ion migration in the apatite-type materials La9.33Si6O26 and La8Sr2Si6O26. J Mater Chem 2003;13:1956e61. [23] Sansom JEH, Kendrick E, Tolchard JR, Islam MS, Slater PR. A comparison of the effect of rare earth vs Si site doping on the conductivities of apatite-type rare earth silicates. J Solid State Electrochem 2006;10:562e8. [24] Marrero-Lopez D, Martı´n-Sedeno MC, Pena-Martı´nez J, RuizMorales JC, Nu´nez P, Aranda MAG, et al. Evaluation of apatite silicates as solid oxide fuel cell electrolytes. J Power Sources 2010;195:2496e506. [25] Yoshioka H. High oxide ion conductivity in Mg-doped La10Si6O27 with apatite-type structure. Chem Lett 2004;33:392e3. [26] Yin GC, Yin H, Wang X, Sun ML, Zhong LH, Cong RD, et al. Mg doping effect on high-pressure behaviors of apatite-type lanthanum silicate. J Alloys Comp 2014;611:24e9. [27] Guillot S, Beaudet-Savignat S, Lambert S, Roussel P, Vannier R. Effect of the dopant nature on the conductivity of oxygen overstoichiometric oxyapatites with controlled microstructures. Solid State Ionics 2011;185:18e26. [28] Inoubli A, Kahlaoui M, Sobradosc I, Chefia S, Madani A, Sanz J, et al. Influence of anionic vacancies on the conductivity of La9.33Si6-xAlxO26-x/2 oxide conductors with an oxyapatite structure. J Power Sources 2014;271:203e12. [29] Shaula AL, Kharton VV, Marques FMB. Ionic and electronic conductivities, stability and thermal expansion of La10x(Si,Al)6O26±d solid electrolytes. Solid State Ionics 2006;177:1725e8.
[30] Shaula AL, Kharton VV, Marques FMB. Oxygen ionic and electronic transport in apatite-type La10x(Si, Al)6O26±d. J Solid State Chem 2005;178:2050e61. [31] Zhou J, Ye XF, Li JL, Wang SR, Wen TL. Synthesis and characterization of apatite-type La9.67Si6-xAlxO26.5-x/2 electrolyte materials and compatible cathode materials. Solid State Ionics 2011;201:81e6. [32] Xiang J, Liu ZG, Ouyang JH, Yan FY. Ionic conductivity of oxyapatite La10Si6-xInxO27þd solid electrolyte ceramics. J Power Sources 2014;251:305e10. [33] McFarlane J, Barth S, Swaffer M, Sansom JEH, Slater PR. Synthesis and conductivities of the apatite-type systems, La9.33þxSi6-yMyO26þz (M ¼ Co, Fe, Mn) and La8Mn2Si6O26. Ionics 2002;8:149e54. [34] Kharton VV, Shaula AL, Patrakeev MV, Waerenborgh JC, Rojas DP, Vyshatko NP, et al. Oxygen ionic and electronic transport in apatite-type solid electrolytes. J Electrochem Soc 2004;151:A1236e46. [35] Shi Q, Lu L, Jin H, Zhang H, Zeng Y. Electrical properties and thermal expansion of cobalt doped apatite-type lanthanum silicates based electrolytes for IT-SOFC. Mater Res Bull 2012;47:719e23. [36] Xiang J, Liu ZG, Ouyang JH, Zhou Y, Yan FY. Synthesis and electrical conductivity of La10Si5.5B0.5O27þd (B ¼ In, Si, Sn, Nb) ceramics. Solid State Ionics 2012;220:7e11. [37] Xiang J, Ouyang JH, Liu ZG. Microstructure and electrical conductivity of apatite-type La10Si6-xWxO27þd electrolytes. J Power Sources 2015;284:49e55. [38] Xiang J, Ouyang JH, Liu ZG, Qi GC. Influence of pentavalent niobium doping on microstructure and electrical conductivity of oxy-apatite La10Si6O27 electrolytes. Electrochim Acta 2015;153:287e94. [39] Cao XG, Jiang SP. Sinterability and conductivity of barium doped aluminium lanthanum oxyapatite La9.5Ba0.5Si5.5Al0.5O26.5 electrolyte of solid oxide fuel cells. J Alloys Comp 2012;523:127e33. [40] Cao XG, Jiang SP. Effect of Sr and Al or Fe co-doping on the sinterability and conductivity of lanthanum silicate oxyapatite electrolytes for solid oxide fuel cells. Int J Hydrogen Energy 2014;39:19093e101. [41] Cao XG, Jiang SP, Li YY. Synthesis and characterization of calcium and iron co-doped lanthanum silicate oxyapatites by sol-gel process for solid oxide fuel cells. J Power Sources 2015;293:806e14. [42] Setsoafia DDY, Hing P, Jung SC, Azad AK, Lim CM. Sol-gel synthesis and characterization of Zn2þ and Mg2þ doped La10Si6O27 electrolytes for solid oxide fuel cells. Solid State Sci 2015;48:163e70. [43] Yang TR, Zhao HL, Han J, Xu NS, Shen YN, Du ZH, et al. Synthesis and densification of lanthanum silicate apatite electrolyte for intermediate temperature solid oxide fuel cell via co-precipitation method. J Eur Ceram Soc 2014;34:1563e9. [44] Najib A, Sansom JEH, Tolchard JR, Slater PR, Islam MS. Doping strategies to optimise the oxide ion conductivity in apatite-type ionic conductors. Dalton Trans 2004:3106e9. [45] Xia YJ, Bai YJ, Wu XJ, Zhou DF, Wang ZC, Liu XJ, et al. Effect of sintering aids on the electrical properties of Ce0.9Nd0.1O2-d. Solid State Sci 2012;14:805e8. [46] Chesnaud A, Dezanneau G, Estournes C, Bogicevic C, Karolak F, Geiger S, et al. Influence of synthesis route and composition on electrical properties of La9.33þxSi6O26þ3x/2 oxy-apatite compounds. Solid State Ionics 2008;179:1929e39.