Synthesis and Na + conduction properties of the ceramics of Na 5 YSi 4 O 12 -type phosphate-substituted solid solutions
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Synthesis and Na1 conduction properties of the ceramics of Na5 YSi4 O12 -type phosphate-substituted solid solutions Kimihiro Yamashita Division of Inorganic Materials, Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda, Tokyo 101, Japan
Motohide Matsuda Department of Environmental Chemistry and Materials, Faculty of Environmental Science and Technology, Okayama University, 2-1-1 Tsushima-Naka, Okayama 700, Japan
Takao Umegaki Department of Industrial Chemistry, Faculty of Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-03, Japan (Received 28 October 1997; accepted 14 February 1998)
The Na1 -superionic conducting ceramics with Na5 YSi4 O12 -type structure were synthesized using the composition formula of Na313x2y Y12x Py Si32y O9 , in which some portions of Na1 ions are considered to replace Y31 as Nas1318x24yd/3 [Nas4x21d/3 Ys424xd/3 ] [P4y/3 Sis1224yd/3 ]O12 according to Na5 YSi4 O12 -type composition formula. The average ionic radius of the 6-coordinated Y31 sites [ s4x 2 1dy3 3 rNa 1 s4 2 4xdy3 3 rY ] approximately covered the range of rGd and to rSm . The ionic conduction properties were strongly dependent upon the combination of composition parameters, x and y. In this report, the dependence of conduction properties on the composition is discussed on the basis of structural considerations. The use of glass-making processing is favorable for the fabrication of Na1 conducting electrolyte tubes, which has been the key to the technological development of 1 MW NayS battery plants. However, the processing technique cannot be applied to well-known byb 00 – aluminas (e.g., NaAl11 O17 and NaAl5 O8 ) and Nasicons (Na11x Zr2 P32x Six O12 ) because their high inclusion of Al2 O3 or ZrO2 brings about the inhomogeneous melting or crystallization from glasses. Recently, we have successfully prepared Na1 -superionic conducting glassceramics using phosphate-substituted Na5 YSi4 O12 -type compound.1,2 Only a limited number of works have been carried out for this type of Na1 conductors, although its excellent ionic conduction property of the Na5 YSi4 O12 has long been recognized for these two decades.3 –16 Recently, we have prepared glass-ceramics of Na5 YSi4 O12 -type solid solutions with the formula Na313x2y Y12x Py Si32y O9 . The glass-ceramics thus prepared have exhibited high Na1 conductivities of ca. 0.1 Sycm at 300 ±C, which are comparable to those of byb 00 –aluminas and Nasicons. To date, we have also synthesized more than 70 specimens with various sets of the parameters x and y according to the composition formula. A phase diagram has very recently been established for the formation of the Na5 YSi4 O12 -type solid solutions, which indicates that with PO32 but also not only the substitution of SiO42 4 4 31 the reduction of Y was indispensable for the formation of Na5 YSi4 O12 -type solid solutions. We have also found that the composition parameters affect the conduction J. Ma
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