Synthesis and Characterization of Fine Grained High Density La 2 Mo 2 O 9 -based Oxide-ion Conductors

J. Mater. Sci. Technol., Vol.24 No.5, 2008 761 Synthesis and Characterization of Fine Grained High Density La 2 Mo 2 O 9 -based Oxide-ion Conductors Jianxin WANG 1,2), Qin WANG 1,2), Xianping WANG 1), Chun LI 1) and Qianfeng FANG 1,2) 1) Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China 2) Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315040, China [Manuscript received April 30, 2007, in revised form June 27, 2007] A cost-effective technique, including nanocrystalline powder preparation using a modified Pechini method and a two-step low-temperature sintering route, was developed for the synthesis of high performance La 2 Mo 2 O 9 - based oxide-ion conductors. The optimum parameters of the compaction pressure, the first step and the second step sintering temperatures for the synthesis of fine grained, high density and uniform La 2 Mo 2 O 9 - based oxide-ion conductors were determined by a series of sintering experiments. High density and uniform sintered La 2 Mo 2 O 9 samples with average grain size from 0.8 to 5 µm and La 1.96 K 0.04 Mo 2 O 8.96 sample with average grain size as small as 500 nm were synthesized by using this cost-effective method. The impedance measurement results show that the as-fabricated La 2 Mo 2 O 9 -based ceramics possess much higher ionic conductivity than that obtained by solid state reaction method. It is found that in the range of 0.8 5 µm the grain size of dense La 2 Mo 2 O 9 samples prepared from the nanocrystalline powders has little influence on their conductivities. KEY WORDS: Oxide ion conductor; La 2Mo 2O 9; Nanocrystalline; Two-stage sintering 1. Introduction The oxide-ion conductors based on La 2 Mo 2 O 9 compound have attracted special attention owing to their relatively high ionic conductivity at medium temperatures (600 800 C) [1 17]. The ionic conductivity of La 2 Mo 2 O 9 as high as 0.06 s/cm at 800 C is a little higher than that of stabilized zirconia, the most widely used oxide electrolyte. In most of these works, La 2 Mo 2 O 9 -based ceramics are usually fabricated via conventional solid-state reaction method [1-3,5-11]. In order to obtain high density La 2 Mo 2 O 9 samples, ballmilling and high sintering temperatures (950 1100 C) are necessary, which frequently introduces impurities such as zirconia into the final samples [8,12]. To obtain high purity and high density La 2 Mo 2 O 9 samples, the preparation of nanocrystalline powders is proven to be a better choice [12], since nanocrystalline powders provide faster densification kinetics, lower sintering temperatures, better mechanical and electrical properties of the electrolytes [12 17]. Different synthetic techniques such as solgel [14 21] and freeze-dried precursor method [12,13] have been used to prepare nanocrystalline powders of La 2 Mo 2 O 9. These synthesis processes are suitable to obtain dense samples of La 2 Mo 2 O 9, but to achieve high conductivity the sintering temperature has to be increased to above 950 C when common isothermal pressure-less sintering processes were used [12 14]. Other sintering modes were also used to prepare La 2 Mo 2 O 9 ceramics from nanocrystalline powders [15 19], but the ionic conductivity of these samples is not as high as expected owing to the low density or low purity. Dense and finegrained La 2 Mo 2 O 9 ceramics with much high conductivity were prepared from ultrafine powders by Prof., Ph.D., to whom correspondence should be addressed, E-mail: qffang@issp.ac.cn. spark-plasma sintering (SPS) method [16]. However, the conductivity perpendicular to the direction of rodtype grains is lower than that parallel to the grains. Therefore, it is necessary to develop other new synthesis process suitable to obtain high purity and high density La 2 Mo 2 O 9 -based ceramics with homogeneous fine grains. In our previous work [17], La 2 Mo 2 O 9 samples with a grain size of 1 3 µm and relative density of 94% 96% were prepared by the novel three-stage sintering method from nanocrystalline powders, and an remarkable enhancement in conductivity was observed in comparison with the samples fabricated via conventional solid-state reaction method. In this paper it is reported that after improvement of this synthesis technique, the density of La 2 Mo 2 O 9 -based samples can reach 97% 99% of the theoretical density while the average grain size was kept below 1 µm. An average grain size of 500 nm is obtained for a K doped La 1.96 K 0.04 Mo 2 O 8.96 sample with relative density of 97%. An alternative procedure is established for the preparation of high dense nanocrystalline La 2 Mo 2 O 9 - based ceramics. 2. Experimental 2.1 Sample preparation The integral fabrication process of La 2 Mo 2 O 9 - based ceramics can be divided into three subsequent steps: powder preparation and treatments, compaction into a green shape, and final two-step sintering. Since each step specifically influences the microstructure, it is important and necessary to systematically control and optimize each parameter. Firstly, nanocrystalline La 2 Mo 2 O 9 -based powders were prepared by using a modified Pechini method as reported in literature [15 21]. Briefly, La 2 O 3 (0.005 mol AR) dissolved in a dilute nitric acid and (NH 4 ) 6 Mo 7 O 24 4H 2 O (0.000714 mol AR) dissolved

762 J. Mater. Sci. Technol., Vol.24 No.5, 2008 in an appropriate amount of water were mixed with gentle stirring. For synthesis of La 1.96 K 0.04 Mo 2 O 8.96 powders solution of potassium citrate was used. Citric acid (0.03 mol AR) dissolved in water was added slowly to the mixed solution. To avoid precipitation, the ph value of the solution was maintained below 2 by adding drops of concentrated nitric acid. After the mixed solution has been kept stirring in a water bath at 40 C for about 1 h, an appropriate amount of polyethylene glycol 20000 was added as a disperser. The solutions were then filtrated and kept in a water bath at 80 C with constant stirring until gelation was completed. At last the as-prepared gels were dried at 120 C for 24 h. The ultrafine La 2 Mo 2 O 9 -based powders were obtained by calcining the dried gels at 550 600 C for 3 h. After addition of some polyvinyl alcohol (PVA) solution as binders, the obtained powders were ground by hand instead of the common ballmilling, to avoid contamination by silicon or zirconia. In the compaction process the powders are embedded in a mould and pressed in one direction by a piston. Since the homogeneous filling of powders in the mould is necessary to reach high density, the rheology of powders must be improved to get a homogeneous filling. The addition of PVA is one method to improve the powders rheology. Another improvement is the application of the graphite as lubricant in the pressing process. Briefly, after the graphite powders were put into the mould and pressed under a pressure of 600 MPa, the mould was wiped carefully by tampons and used for the La 2 Mo 2 O 9 nanocrystalline powders. The remainder graphite on the inner wall of the mould can markedly decrease the friction between the inner wall of the mould and the powders. The remainder graphite on the brink of green pellets will completely disappear after sintering. In this study, green pellets of about 12 mm in diameter and 1 2 mm in thickness were shaped by uniaxial pressing at a pressure range of 200 800 MPa. It is worth to point out that isostatic pressing may be a better choice to obtain homogeneous filling of powders. Finally, the green pellets were held at 450 C for 3 h to eliminate the PVA and then sintered with normal isothermal or a novel Chen-type sintering mode [22] in air. For the former, the green pellets were heated at a rate of 3 C/min and then kept at a given temperature until densification is completed. In the socalled Chen-type sintering process, the heating schedule comprises of two stages, that is, the green pellet is first heated to a higher temperature to achieve an intermediate density and then immediately cooled down and held at a lower temperature until it becomes fully dense. In order to tune the microstructure of the resultant ceramics and to research and take advantage of the grain-size-dependent physical properties of La 2 Mo 2 O 9 -based ceramics, a great many of sintering experiments were employed on the special green pellets with different sintering schedules. 2.2 Characterization methods Phase formation of the powders and the sintered samples were characterized by X-ray diffraction (XRD) in a Philips X pert PRO X-ray diffractometer with CuKα radiation. The Rietveld refinement method [23] was used to determine lattice parameters from the resultant spectra collected at room temperature. The microstructure of the sintered samples was examined by field emission scanning electron microscopy (FESEM, Sirion 200 FEG). The surfaces of the sintered samples were polished, thermally etched, and then coated with a thin gold layer for better image definition. Grain size measurements were carried out on the surface SEM micrographs of the etched samples by linear intercept technique as described by Mendelson [24]. Density of the samples was determined by weight/geometric measurements and theoretical density is calculated according to the lattice constant deduced from the XRD data. The electrical properties of the sintered samples in air are determined by impedance spectroscopy using a frequency response analyzer (Hioki 3531 Z Hi-Tester, 42 Hz to 5 MHz). Before measurements, thin silver films are sputtered onto both sides of the samples as electrodes, which are connected to the experimental setup through platinum wires. The data points of impedance were collected with a temperature interval of 50 C over the temperature range of 300 650 C. After each change in temperature the specimen was allowed to equilibrate for at least 20 min before the measurement was taken. 3. Results and Discussion The sintering schedule, relative density, lattice constant, and average grain size of the sintered La 2 Mo 2 O 9 and La 1.96 K 0.04 Mo 2 O 8.96 specimens are summarized in Table 1. The sample P570 is the nanocrystalline powders prepared by sol-gel method. The B900, B800, B750 and K720 are the samples sintered by Chen-type sintering mode. K720 is the La 1.96 K 0.04 Mo 2 O 8.96 sample and the others are La 2 Mo 2 O 9 samples. Figure 1 shows the XRD patterns of the nanocrystalline powders and three sintered samples (P570, B800, B750, and K720). In the range of experimental errors, only the La 2 Mo 2 O 9 phase was detected. The room temperature phase can be indexed as a pseudocubic structure with space group P2 1 3, and the lattice constant of nanocrystalline powder was calculated as 0.7156 nm. The crystallite size of the powders prepared in this study was about 30 nm as deduced from the width of XRD peaks by Debye-Scherer equation after subtraction of the equipment widening. The lattice constant of the sintered samples is in the range of 0.7152 0.7161 nm, as listed in Table 1 and in accordance with the results reported by Basu et al. [14]. Fig.1 XRD patterns of samples P570, B800, B750, K720

J. Mater. Sci. Technol., Vol.24 No.5, 2008 763 Table 1 Characteristic parameters of the typical sintered La 2Mo 2O 9-based specimens Sample number Sintering schedule Relative density/% Lattice constant/nm Average grain size/nm P570 570 C 3 h 7.156 0.05±0.01 B900 900 C+650 C 20 h 99±1 7.152 4.5±0.2 B800 800 C+650 C 30 h 99±1 7.153 1.5±0.2 B750 750 C+650 C 50 h 97±1 7.153 0.8±0.1 K720 720 C+600 C 80 h 97±1 7.161 0.5±0.1 Fig.2 Ultimate relative densities of the first step sintered sample under different compaction pressures vs the sintering temperature The relative density of the sample after the first step sintering is analyzed under different compaction pressures and sintering temperature of the first step and delineated in Fig.2. For these sintered samples, the relative density increases monotonously with both the sintering temperature of the first step and the applied pressure and there is an obvious transition between 720 750 C in each curve. It is clear that the compaction pressure and sintering temperature are both important factors that influence the density of the samples. In order to reach a given value of the relative density after the first step sintering the sample can be compacted at higher pressure and then sintered at lower temperature or compacted at lower pressure and then sintered at higher temperature. Since inferior facies of the green pellets were observed under the applied pressure of 800 MPa, the pressure of 600 MPa was determined as the usual compaction pressure for this special La 2 Mo 2 O 9 nanocrystalline powders in this study. At the compaction pressure of 600 MPa, the first step sintering temperature should be above 720 C to obtain the dense sample in the second step sintering. As suggested by Chen and Wang [22], the normal densification process of ceramic bodies in the normal isothermal sintering processes is always accompanied by rapid grain growth, because the capillary driving forces for sintering (involving surfaces) and grain growth (involving grain boundaries) are comparable in magnitude, both being proportional to the reciprocal of grain size. Therefore, it is difficult to obtain high density samples with fine grains by using the normal isothermal sintering mode. For the two-stage sintering process it is believed that at the first stage the densification is controlled by grain-boundary migration, while at the second stage it is controlled by active grain boundary diffusion and the grain boundary migration is suppressed. To get high density samples with ultrafine grains in the two-step sintering process, a sufficiently high starting density should be obtained during the first step, because only when the density is above 70% the pores in green pellets can become subcritical and unstable against shrinkage under capillary action. These pores can be filled as long as grain-boundary diffusion allows it, even if the particle network is frozen at the second step. In our case, the density of La 2 Mo 2 O 9 -based samples after the first stage is higher than 75% of the theoretical density and therefore La 2 Mo 2 O 9 -based samples with relative density as high as 95% can be obtained at the second stage without additional grain growth. It was also suggested by Chen et al. [22] that the feasibility of densification without grain growth relies on the suppression of grain-boundary migration while keeping grain boundary diffusion active. A kinetic window for reaching full density without grain growth for pure Y 2 O 3 can be demarcated by an upperbound and a lower-bound temperature in the coordinate of the second stage sintering temperature plotted against the grain-size. Above the upper-bound temperature grain growth will be observed. This suggests that grain-boundary migration may involve an activation process that has higher activation energy than grain-boundary diffusion; thus, it is active at higher temperatures but is suppressed at lower temperatures. It is well known that the driving force for grain growth decreases with grain size, so the temperature required to activate the higher-energy process increases with grain size and the upper temperature in the window shifts to a higher value at a larger grain size. Below the lower-bound temperature the sintering will be exhausted before full density is achieved. This suggests that grain boundary diffusion itself can be suppressed at lower temperatures. Interface kinetics in very fine grain polycrystals is sometimes limited due to difficulties in maintaining sources and sinks to accommodate point defects. This effect should diminish at larger grain sizes, allowing the kinetic window to extend to lower temperatures. According to this suggestion and colligating our sintering experiments on the La 2 Mo 2 O 9 -based ceramics, an approximate kinetic window in which dense La 2 Mo 2 O 9 ceramic can be obtained without possible grain growth in the second sintering stage is delineated in Fig.3. It is clear that to obtain dense samples with grain size less than 1 µm the sintering temperature of the first step should be limited between 720 750 C and the sintering temperature of the second step should be limited between 600 700 C. Figure 4(a) (d) show the FESEM photographs of the sintered La 2 Mo 2 O 9 samples K720, B750, B800, B900 by the two step sintering mode as described in Table 1. It is clear that the microstructure of the sintered specimens is significantly dependent on the sintering temperature of the first step. The average grain size varies from 500 nm (K720) to 5 µm (B900) with increasing sintering temperature of the first step. So, with this sintering mode, one can tune the

764 J. Mater. Sci. Technol., Vol.24 No.5, 2008 Fig.3 Kinetic window for sintering dense La2 Mo2 O9 without grain growth at the second sintering stage. Solid symbols are conditions of successful second-step sintering runs. Open symbols above the upper boundary are conditions that showed grain growth in second-step sintering, and open symbols below the lower boundary are ones that did not reach full density Fig.5 Conductivity of samples K720, B750, B800, B900, and S970 vs temperature microstructure of the resultant ceramics by varying the sintering temperature of the first step, and an average grain size as small as 500 nm can be obtained in sintered La1.96 K0.04 Mo2 O8.96 specimen with a relative density of about 98%. The bulk conductivity of these sintered dense samples is characterized by impedance spectroscopy and presented as Arrhenius plots in Fig.5. The conductivity of all the La2 Mo2 O9 samples prepared from the nanocrystalline powders is almost the same and reaches 7.8 10 4 S/cm at 550 C and 0.018 S/cm at 600 C, which is 2 5 times that of macrocrystalline La2 Mo2 O9 samples (S970 which is fabricated via solid-state reaction method and sintered at 970 C) in the whole temperature range measured and close to the results obtained in samples sintered at 1100 C for 5 h from the nanocrystalline powders prepared by freeze-dried precursor method[13]. The conductivity of sample K720 is similar with that of the pure La2 Mo2 O9 samples prepared from the nanocrystalline powders except in the temperature range of 400 570 C where the conductivity is much higher due to the suppression of phase transition and reaches a value of 5 10 3 S/cm at 550 C. The enhancement of conductivity in the La2 Mo2 O9 -based samples can be attributed to the co-operation of the excellent performance of nanocrystalline powders and the advantage of the strategic two-stage low-temperature thermal processing[17]. This result also suggests that in the range of 0.8 5 µm the grain size of dense La2 Mo2 O9 samples prepared from the nanocrystalline powders has little influence on their conductivities. 4. Conclusion Fig.4 SEM micrographs of two step sintered La2 Mo2 O9 based samples: (a) K720, (b) B750, (c) B800, (d) B900 (thermally etched surfaces) In conclusion, high density and single-phase La2 Mo2 O9 -based ceramics with grain size of 500 nm 5 µm were successfully fabricated by using a novel cost-effective method. The grain size of the dense La2 Mo2 O9 -based ceramics can be controlled by the sintering temperature of the first step in the twostep sintering mode, and gain size as small as 500 nm of La1.96 K0.04 Mo2 O8.96 samples with a relative density of about 98% is obtained. This would facilitate the cost-effective preparation of dense nanostructured La2 Mo2 O9 -based materials starting from nanocrystalline powders. The as-fabricated dense samples possess higher ionic conductibility than the samples fabricated via conventional solid-state reaction method, which can be attributed to the co-operation of the ex-

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