J O U R N A L O F C H E M I S T R Y Materials Microwave synthesis of LaCrO3 Masato Iwasaki,a Hirotsugu Takizawa,*a Kyota Uheda,a Tadashi Endoa and Masahiko Shimadab aDepartment of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980–8579, Japan. E-mail: takizawa@aim.che.tohoku.ac.jp bInstitute for Advanced Materials Processing, Tohoku University, Katahira, Sendai 980–8577, Japan Received 2nd June 1998, Accepted 14th September 1998 The synthesis of LaCrO3 from a stoichiometric mixture of Cr2O3 and La2O3 powder by microwave irradiation was examined using a multi-mode 28 GHz microwave heating system.In the La2O3–Cr2O3 system, Cr2O3 strongly absorbs microwaves while La2O3 is transparent to microwaves. The reaction proceeded rapidly and LaCrO3 could be synthesised within 15 min of irradiation.Surprisingly, the temperature required for the microwave synthesis (440 °C) is much lower than that for conventional synthesis (>1200 °C) using an electric furnace. The eVect of the partial pressure of oxygen in the microwave chamber on the formation of LnCrO3 was investigated. The formation of LnCrO3 is enhanced with increase in the partial pressure of oxygen for Ln=La and Nd, while opposite behavior is seen for Ln=Gd, Tb, Dy and Ho. 1 Introduction 2 Experimental The increase in temperature of Cr2O3 and La2O3, and the Lanthanum chromite, LaCrO3, has received much interest as an electrode or interconnector for solid oxide fuel cells (SOFC) possible synthesis of LaCrO3 by microwave irradiation were determined. Multi-mode microwave heating equipment (Fuji and a heating element for high-temperature electric furnaces.1 Generally, LaCrO3 is synthesised by solid state reaction of the Dempa Kogyo Co., LTD, Japan; FMS-10–28) was used.Appropriate amounts of Cr2O3 and La2O3 powder were mixed component oxides at high temperatures (T>1200 °C) in air. Since the diVusion of reactants is very slow in the solid state and pressed into a pellet (10 mm in diameter and 5 mm in thickness).A hole (1.6 mm in diameter and 3 mm in depth) reaction, the reaction requires a long heating period and intermediate grinding to achieve good homogeneity. Attempts was drilled at the centre of pellet to insert a thermocouple. In the microwave chamber (multi-mode cavity; 0.8 m in diameter to synthesise LaCrO3 at lower temperature have been made by hydrothermal reaction2 and thermal decomposition of a and 1.2 m in length), a platinum sheathed Pt–Pt/Rh10% thermocouple (1.5 mm in diameter) was inserted into the mixed organometallic complex of La and Cr.3 sample pellet and then the sample was surrounded by quartz Microwave heating has been used for the syntheses of wool as a heat insulating material which is transparent to various inorganic materials, e.g., YBa2Cu3O7-d,4,5 La2CuO4,5 microwaves.A platinum sheath was used to achieve an eVective b-SiC,6 BaWO4,7 Bi4V2O11, PbV2O6.8 If some constituent shielding from the microwaves. A schematic illustration of the materials of a chemical reaction system strongly absorb microsample set up is shown in Fig. 1. The temperature was moni- waves, the resulting heat generated can be used to drive a tored during microwave irradiation. We considered that moni- solid state reaction with another component. This is especially tored temperature was not aVected by microwave dielectric applicable for solid state reaction systems containing C, V2O5, fields as suggested by Rowly et al.13 After irradiation, the Cr2O3, CuO, MnO2, PbO2, WO3 or Fe3O4.9 Detailed aspects of microwave processing have been reviewed by Sutton.10 The above mentioned examples were investigated by using microwaves with a frequency of 2.45 GHz which is widely applied for heating and cooking foodstuVs.It is known by experiment that the required size of the microwave chamber to achieve a uniform electric field distribution is directly proportional to the wavelength of the microwaves.11 The temperature rise of a material due to coupling with the microwaves is theoretically proportional to the dielectric loss factor of the material.9 However, because the dielectric loss factor correlates with temperature and microwave frequency,9 diVerent temperature profiles may be obtained even for the same material if the frequencies of microwaves used are diVerent.Chromium sesquioxide (Cr2O3) absorbs 2.45 GHz microwave radiation very strongly.12 If Cr2O3 absorbs 28 GHz microwave radiation in a similar manner, such microwaves can be utilised for the syntheses of various chromium double oxides. This paper reports the rapid and simple procedure for the synthesis of LaCrO3 by 28 GHz microwave irradiation.The oxygen pressure dependence on the formation of LnCrO3 Fig. 1 Schematic illustration of the cross section of the microwave chamber. (Ln=La, Nd, Gd, Tb, Dy and Ho) is also investigated. J. Mater. Chem., 1998, 8, 2765–2768 2765sample was cooled to room temperature in the microwave chamber and the obtained sample was characterised by X-ray diVraction analysis and scanning electron microscopy. 3 Results and discussion Fig. 2 shows the temperature–time profiles of (a)Cr2O3 and (b)La2O3 during 28 GHz microwave irradiation (0.3 kW output) in air. It is apparent that La2O3 is transparent to 28 GHz microwave radiation. In contrast to La2O3, the temperature of Cr2O3 rose rapidly. It was reported that the temperature of Cr2O3 rose rapidly above 1200 °C on exposure to a few minutes (referred to ‘thermal runaway’) of 2.45 GHz microwave irradiation.12 However, the drastic temperature rise was completed within 2 min and the temperature stabilized at ca. 500 °C over longer periods of irradiation when 28 GHz microwaves was irradiated on Cr2O3. The diVerence is thought to be due to the diVerence in the dielectric properties of Cr2O3 at 28 GHz and 2.45 GHz.X-Ray diVraction analysis of the irradiated sample did not reveal any phase changes. Fig. 3 shows the temperature–time profiles of a Fig. 4 X-Ray diVraction patterns of Cr2O3+La2O3 stoichiometric mixture after microwave irradiation of (a) 0.3 kW, (b) 0.5 kW and (c) 1 kW in air. Cr2O3+La2O3 stoichiometric mixture during 28 GHz microwave irradiation of (a) 0.3 kW, (b) 0.5 kW and (c) 1 kW in air.The profile of the Cr2O3+La2O3 mixture is similar to that of Cr2O3 with the maximum temperature reached for the Cr2O3+La2O3 mixture being slightly lower than that of Cr2O3. The profiles at various microwave power are quite similar except for diVerences in saturation temperature. X-Ray diVraction patterns of Cr2O3+La2O3 mixtures after microwave irradiation of various power outputs (a) 0.3 kW, (b) 0.5 kW and (c) 1 kW in air for 15 min are shown in Fig. 4. As seen, LaCrO3 was readily formed from a mixture of Cr2O3+La2O3 within a short time (15 min) at each microwave power, but the amount of LaCrO3 formed at 0.3 and 0.5 kW was slightly larger than that at 1 kW. This result is due to the Fig. 2 Temperature–time profiles of (a) Cr2O3 and (b) La2O3 during 0.3 kW microwave irradiation in air.Fig. 5 X-Ray diVraction patterns of Cr2O3+La2O3 stoichiometric Fig. 3 Temperature–time profiles of Cr2O3+La2O3 stoichiometric mixture during microwave irradiation of (a) 0.3 kW, (b) 0.5 kW and mixture after 0.3 kW microwave irradiation (a) in O2(3.7 atm), (b) in air and (c) in vacuo. (c) 1 kW in air. 2766 J. Mater. Chem., 1998, 8, 2765–2768Scanning electron microscopy showed that the particle size of the raw material powders, Cr2O3 and La2O3, were 0.39 and 0.79 mm, respectively. Fig. 6 shows a scanning electron micrograph of LaCrO3 obtained by microwave heating. In this case, microwave heating was carried out using a 0.3 kW output in 3.7 atm oxygen pressure for 15 min. After microwave irradiation, the average particle size of the resultant LaCrO3 was about 1.6 mm, with a size distribution of 0.4–3.2 mm.The synthesis of other lanthanide chromites, LnCrO3(Ln= Nd, Gd, Tb, Dy, Ho), from stoichiometric mixtures of Cr2O3 and Nd2O3, Gd2O3, Tb4O7, Dy2O3, or Ho2O3 have also been carried out. Fig. 7 shows the X-ray diVraction patterns of Cr2O3+Gd2O3 mixtures after 0.3 kW microwave irradiation for 30 min in various atmospheres.It is clear that GdCrO3 was formed in each atmosphere. However, in contrast to LaCrO3, the amount of GdCrO3 increased with decreasing the partial pressure of oxygen and single phase GdCrO3 was obtained in vacuo. During our extensive study, it was seen that the eVect of the atmosphere on the formation of the LnCrO3 Fig. 6 Scanning electron micrograph of LaCrO3 obtained by 0.3 kW phase by microwave irradiation can be divided into two groups microwave irradiation in O2(3.7 atm) atmosphere. i.e., enhancement or inhibition of reactivity upon increasing the partial pressure of O2. The former behavior is seen for LaCrO3 and NdCrO3, and the latter for GdCrO3, TbCrO3, diVerence in the maximum temperature at each microwave DyCrO3 and HoCrO3.Considering the lanthanide contraction, power and high microwave power is thus not required for the we can suppose that it is easy to synthesise LnCrO3 under synthesis of LaCrO3. It is notable that it takes only 15 min to high partial pressure of O2 if the ionic radius of Ln is relatively synthesise LaCrO3 by microwave irradiation, which is a very large (Ln=La, Nd), while the use of a low partial pressure short time as compared with conventional heating methods.14 of O2 is favoured if the ionic radius of Ln is relatively small Fig. 5 shows the X-ray diVraction patterns of a (Ln=Gd, Tb, Dy, Ho). This diVerence must be due to Cr2O3+La2O3 mixture after 0.3 kW microwave irradiation diVerences in the diVusion mechanism for the formation of (a) in O2(3.7 atm), (b) in air and (c) in vacuo for 15 min.It LnCrO3 from a Ln2O3 and Cr2O3 mixture for diVerent Ln. can be seen that LaCrO3 was formed in each atmosphere and Although the detailed diVusion mechanism under microwave the higher the partial pressure of O2, the smaller was the irradiation has, as yet, not been clarified, it should be pointed amount of the unreacted La2O3 and Cr2O3.When the out that the diVerence in crystal structure of Ln2O3, i.e., irradiation was carried out at a pressure of 3.7 atm oxygen, hexagonal A-type for Ln=La, Nd and cubic C-type for Ln= single phase LaCrO3 was obtained. Generally, conventional Gd, Tb, Dy and Ho, would aVect the diVusion mechanism for synthesis of LaCrO3 does not require a high oxygen pressure.the formation of LnCrO3 via solid state reaction with Cr2O3. Synthesis under high oxygen pressure leads to the formation In conclusion, a new simple procedure for the synthesis of of volatile chromium(VI) oxide.15 However, in the microwave LaCrO3 has been developed in the present study. Microwave synthesis, chromium(VI) oxides was not formed even at 3.7 atm heating is eVective for the synthesis of complex oxides contain- oxygen.This is due to considerably lower reaction temperature ing a component oxide strongly coupled with microwaves. for the microwave synthesis as compared with the conventional There is still an unsolved problem as to why the reaction synthesis. proceeded rapidly and completely within a short time at low temperature (440 °C).This observation suggests an existence of a ‘non-thermal eVect’ of the microwave electric field on diVusion of chemical species. Acknowledgement This work was supported in part by the new Energy and Industrial Technology Development Organization (NEDO) for the R&D Program (No. A-061) and the National Industrial Research Institute of Nagoya (NIRIN), Japan. Thanks are due to Mr.T. Saji and Mr. T. Kuge, Research and Development Department, Fuji Dempa Kogyo, Co., LTD, Tsukuba, Japan, for helpful discussions on experimental set up. References 1 A. Furusaki, H. Konno and R. Furuichi, J. Mater. Sci., 1995, 30, 2829. 2 M. Yoshimura, S. T. Song and S. Somiya, J. Ceram. Soc. Jpn., 1982, 90, 91. 3 S. Nakayama and M. Sakamoto, J. Ceram. Soc. Jpn., 1992, 100, 342. 4 K. G. K. Warrier, H. K. Varma, T. V. Mani and A. D. Damodaran, J. Am. Ceram. Soc., 1992, 75, 1990. 5 D. R. Baghurst, A. M. Chippindale and D.M. P.Mingos, Nature, Fig. 7 X-Ray diVraction patterns of Cr2O3+Gd2O3 stoichiometric 1988, 332, 311. 6 P. D. Ramesh, B. Vaidhyanathan, M. Ganguli and K. J. 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