KnE Materials Science | Sino-Russian ASRTU Conference Alternative Energy: Materials, Technologies, and Devices | pages: 8–13

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1. Introduction

A widely developing field of alternative energetics is the creation of solid oxide fuel cells (SOFCs) working at intermediate temperatures (500–700 C) [1]. Today, yttria stabilized zirconia (YSZ) demonstrates the highest efficiency as electrolytic membrane of SOFC. However, electrolytes based on YSZ undergo a severe degradation of properties during the operation and have a high operating temperature 900 C.

In 2000, a novel family of oxygen ionic conductors was reported by Lacorre et al. [2], namely LAMOX. The ionic conductivity of lanthanum molybdate La 2 Mo 2 O 9 is comparable with that of YSZ reaching 0.06 S/cm at 800 C. La 2 Mo 2 O 9 undergoes a reversible phase transition around 580 C [3] from a high-temperature β-La 2 Mo 2 O 9 (cubic syngony, space group P2 1 3) to a low-temperature α-La 2 Mo 2 O 9 (monoclinic syngony, space group P2 1 ), resulting in a decrease in conductivity by 2–3 orders of magnitude at low temperatures. The crystallographic sites in β-La 2 Mo 2 O 9 for the occupation of oxygen ions are partially available, resulting in a high concentration of structural oxygen vacancies, which provides enough paths for the migration of oxygen ions [2,4]. A decrease in conductivity resulting from the α-β phase transition and low chemical stability in the reducing atmosphere limits practical applications of this material. Thereby, suppressing phase transition and maintaining high conductivity values at intermediate temperatures are of great importance.

In this article, heterogeneous doping technique was used to optimize the functional properties of lanthanum molybdate La 2 Mo 2 O 9 for the first time. It should be noted that conductivity of an inert additional phase La 2 Mo 3 O 12 is 2–3 orders of magnitude lower than conductivity of the matrix phase. Achievement of high values of conductivity in (1–x)La 2 Mo 2 O 9 xLa 2 Mo 3 O 12 (x = 0.15) composite system is associated with the so-called composite effect, which was first detected by Liang [5] in { (100%–x)LiI–xAl 2 O 3} composites. This phenomenon is manifested in increase of ionic conductivity of salt or oxide upon heterogeneous doping with an inert dispersed oxide.

2. Methods

The polycrystalline samples of La 2 Mo 2 O 9 and La 2 Mo 3 O 12 , as well as (1–x)La 2 Mo 2 O 9 xLa 2 Mo 3 O 12 (x = 0.15) composite system were obtained by means of conventional solid-state reaction method from La 2 O 3 and MoO 3 powders of the highly pure grade. The powders with ethanol were thoroughly mixed in an agate mortar. Then, three stages of annealing in air were carried out with a stepwise increase in the temperature from 450 to 950 C for La 2 Mo 2 O 9 and to 900 C for La 2 Mo 3 O 12 . Composites were sintered for 24 h at 900 C. The phase composition of obtained powder samples was established by means of X-ray phase analysis (XRD) at room temperature using a Bruker D8 Advance powder diffractometer (CuK α radiation, angle range of 2θ = 10–80 with a scan step of 0.02 ).

Pellets for conductivity measurements represented disks with thickness of 2–3 mm and diameter of 15 mm obtained by uniaxial compaction at 8 MPa in a textolite mold and sintered for 24 h at 900–950 C in air. Powder of the platinum with an alcohol solution of colophony (1 wt %) was used to apply electrodes on polished face surfaces of sintered pellets. Electrodes were sintered with slow heating at 1–2 C/min and conditioning for 6 h at 900 C.

Conductivity was measured using a two-electrode ac scheme with an Elins Impedancemeter Z-1000P in the frequency range of 500 Hz to 3 MHz on the basis of the complex impedance technique. Measurements were carried out in the cooling mode at the rate of 1 C/min, the impedance spectra were recorded on a PC every 25 min. The bulk and grain boundary conductivities were estimated using the equivalent circuit method in the ZView program.

The oxygen partial pressure Pо 2 was controlled and maintained using an oxygen electrochemical pump governed by a Zirconia-M automatic adjuster [6].

3. Results

According to the XRD data, all the XRD reflexes of La 2 Mo 2 O 9 were indexed by monoclinic symmetry with space group P2 1 , which agreed with that described in [2]. La 2 Mo 3 O 12 crystallized in monoclinic symmetry (space group C2/c), which agrees with the data of [7].

Samples of La 2 Mo 2 O 9 and La 2 Mo 3 O 12 can be described as single-phase, the impurity content does not exceed 1%. The XRD pattern of (1–x)La 2 Mo 2 O 9 xLa 2 Mo 3 O 12 (x = 0.15) composite contains only reflexes of two initial compounds; no chemical interaction is observed between the components.

The temperature dependences of bulk conductivity of La 2 Mo 2 O 9 and La 2 Mo 3 O 12 phases, as well as composite based on it are given in Figure 1. The bulk component of conductivity was calculated from the impedance spectra. An increase in conductivity observed in Figure 1 for composite with x = 0.15 by approximately one order of magnitude is associated with appearance of a composite effect in the studied system. The influence of La 2 Mo 3 O 12 heterogeneous dopant on ionic transport is more pronounced at temperatures lower than temperature of the α-β phase transition ( 580 C). It should also be noted that addition of inert phase does not suppress the phase transition and does not stabilize high-conductivity β-La 2 Mo 2 O 9 at room temperature, as occurs in some cases of homogeneous doping, for example, when Mo is replaced by W [8].

Figure 1

Temperature dependences of the bulk conductivity for (1–x)La 2 Mo 2 O 9 xLa 2 Mo 3 O 12 composites in air: – x = 0; – x = 0.15; – x = 1. The dotted lines indicate the temperature range corresponding to the α-β phase transition.

fig-1.jpg

It was reported in the early papers [2,4] that oxygen ions O 2- in La 2 Mo 2 O 9 acted as the main charge carriers. In this study, the nature of carriers was determined by conductivity measurements with a variation of the oxygen partial pressure Pо 2 in gas phase. Experimental isotherms of conductivity (Figure 2) are close to linear form and correspond to the electrolytic region of Ро 2 . According to the theoretical notions concerning the relationship between Ро 2 and the concentration of defects in the crystal lattice [9], σ ion is independent of Ро 2 in the medium electrolytic region, σ е,h Ро 2±1/m , where σ ion , σ h , σ е – ionic, hole, and electronic conductivity, respectively; the sign and the value of 1/m depend on the nature of carriers and the disordering type of crystal lattice. As follows from Figures 2(a) and (b), the conductivity values are almost unchanged with a variation of Pо 2 . The value of 1/m is close to zero; therefore, it can be concluded that dominant ionic conductivity is maintained both for La 2 Mo 2 O 9 matrix phase and for composites in the wide range of oxygen partial pressures 0.21–3.2 × 10 -5 atm. The ion transport numbers t ion calculated according to Equation (1) from the conductivity-Ро 2 dependences are 0.93–0.98.

t ion =σ ion /σ ion +σh
Figure 2

Oxygen partial pressure dependences of the conductivity for (a) La 2 Mo 2 O 9 at temperatures: – 830 C; – 780 C; – 730 C; – 630 C; – 530 C; and (b) (1–x)La 2 Mo 2 O 9 xLa 2 Mo 3 O 12 (open signs – x = 0; closed signs – x = 0.15) at temperatures: – 800 C; – 500 C.

fig-2.jpg

4. Conclusion

The phases of La 2 Mo 2 O 9 and La 2 Mo 3 O 12 , as well as (1–x)La 2 Mo 2 O 9 xLa 2 Mo 3 O 12 (x = 0.15) composite system were obtained by means of conventional solid-state reaction method. No chemical interaction is observed between the components; the impurity content does not exceed 1%.

It was established that introduction of 15 molar % La 2 Mo 3 O 12 resulted in an increase in conductivity of composite by approximately one order of magnitude. An increase in conductivity is manifestation of composite effect in the composite system. The influence of La 2 Mo 3 O 12 on electrical properties of composite is more pronounced at temperatures lower than temperature of the α-β phase transition ( 580 C). However, addition of inert phase does not suppress the phase transition.

The dominant ionic conductivity is maintained both for La 2 Mo 2 O 9 matrix phase and for composites in the range of oxygen partial pressures 0.21–3.2 × 10 -5 atm.

Funding

The work was supported by the Ministry of Education and Science of Russian Federation for financial support via the State Task No. 4.2288.2017/4.6.

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