The Development of New Multivalent Cation Conducting Solid Electrolyte and Their Application for Gas Sensors

到目前为止，三价阳离子由于其高价态而被认为是固体中迁移性差的离子种类，而单价或二价离子导电固体被深入研究。最近，我们成功地研制了NASICON型结构的三价阳离子导电（M_<0.2>Zr_<0.8>）_&lt;20/19&gt; Nb（PO_4）_3固体电解质（M =三价阳离子）。（M_<0.2>Zr_<0.8>）_&lt;20/19&gt;（PO_4）_3固体的三价离子电导率与典型的二价O_2离子导电固体的三价离子电导率一样高<2->。然而，必须提高离子电导率以用于各种电化学装置，例如气体传感器。此外，由于（M_<0.2>Zr_<0.8>）_&lt;20/19&gt;（PO_4）_3的机械强度不够，我们试图在提高（M_<0.2>Zr_<0.8>）_&lt;20/19&gt;（PO_4）_3固体的机械强度的同时，提高其离子电导率。本文选择（Al_<0.2>Zr_<0.8>）_&lt;20/19&gt; Nb（PO_4）_3作为靶材，因为它是（M_<0.2>Zr_<0.8>）_&lt;20/19&gt;（PO_4）_3系列中三价离子导电率最高的一种，并<0.2><0.8>采用B_2O_3作为烧结助剂合成了（M_ Zr_）_&lt;20/19&gt;（PO_4）_3<0.2><0.8>固体。此外，还将上述B_2O_3掺杂的（Al_<0.2>Zr_<0.8>）_（20/19）Nb（PO_4）_3固体与具有氧离子<2->导电性的氧化钇稳定的氧化锆（YSZ）和稀土硫酸盐基敏感辅助电极一起制成了CO_2气敏元件。在550℃时，传感器输出电动势与CO_2浓度的对数之间符合能斯特理论关系，能准确地检测CO_2浓度。

UP to now, trivalent cation has been believed as a poor migrating ion species in solids because of the high valence state, whereas monovalent or divalent ion conducting solids were intensively studied. Recently, we have been succeeded in developing the trivalent cation conducting (M_<0.2>Zr_<0.8>)_<20/19> Nb(PO_4)_3 solid electrolytes (M = trivalent cations) with NASICON type structure. The trivalent ion conductivity of the (M_<0.2>Zr_<0.8>)_<20/19>(PO_4)_3 solid is as high as those for representative divalent O^<2-> ion conducting solid. However, it is necessary to enhance the ion conductivity to utilize for various electrochemical devises such as gas sensors. Furthermore, since the mechanical strength of (M_<0.2>Zr_<0.8>)_<20/19>(PO_4)_3 is not enough, we try to improve bath the ion conductivity aril the mechanical strength of the (M_<0.2>Zr_<0.8>)_<20/19>(PO_4)_3 solid. Here, we selected (Al_<0.2>Zr_<0.8>)_<20/19> Nb(PO_4)_3 as a target material due to it highest trivalent ion conductivity among the (M_<0.2>Zr_<0.8>)_<20/19>(PO_4)_3 series, and we synthesized the (M_<0.2>Zr_<0.8>)_<20/19>(PO_4)_3 solid with a sintering aid of B_2O_3.By adding 6-8 Wt% B_2O_3 at the sintering step of the (Al_<0.2>Zr_<0.8>)_<20/19> Nb(PO_4)_3 solid, B_2O_3 vaporized With high aggregation between sample particles resulting in high mechanical strength and high density of the sintered sample. Furthermore, the trivalent Al^<3+> ion conductivity was also enhanced two times compared with that for B_2O_3 non-doped sample.We have also fabricated the CO_2 gas sensor with above mentioned B_2O_3 added (Al_<0.2>Zr_<0.8>)_<20/19> Nb(PO_4)_3 solid together with O^<2-> ion conducting yttria stabilized zirconia (YSZ) and rare earth oxysulfate based sensing auxiliary electrode. Tile present sensor can detect the CO_2 concentration exactly with obeying theoretical Nernstian relationship between sensor output EMF and the logarithm of the CO_2 concentration at 550℃.