STABILIZATION OF SNO2 ULTRAFINE PARTICLES BY ADDITIVES

In order to stabilize ultrafine particles of SnO2 which is essential to obtain high gas sensitivity, a systematic investigation was undertaken regarding the stabilizing effects of 5 at% impregnated foreign additives, consisting of oxides or polyoxy compounds of 31 metals and 3 non-metals. The data of specific surface area, SA, as well as SnO2 crystallite size, D, evaluated from X-ray diffraction showed that the additives could be classified into several groups according to the effectiveness. The most effective group, consisting of P Ba, Sm, Ba, P, Mo, W, Ca, Sr, Cr and In, could keep D less than 10 nm even after calcination at 900-degrees-C, whereas pure SnO2 underwent grain growth to have D of 13 and 27 nm at 600 and 900-degrees-C, respectively. Electron microscopy revealed that neck sizes, X, between crystallites were fairly proportional to D(X/D = 0.80). A simple analysis of SA and D data based on a monosized sphere model suggested that each crystallite was coordinated with 3-4.5 neighbours through the necks. The existing state and stabilizing mechanism of additives are discussed in conjunction with the electrical resistance of porously sintered elements.