Phase transformations in nanocomposites based on fumed silica, alumina and rare earth metal oxides Ln2O3 (Ln = Nd, Gd)

Currently, fumed silica and alumina are generally known as adsorbents and supports for adsorbents and catalysts. At the same time, they, as nano-sized reagents, can be used to create modern materials containing silicates and aluminates of metals, in particular, rare-earth metals for the needs of optics, electronics and other applications. In this work, phase transformations and the structure of composites based on fumed silica, alumina and oxides of the lanthanum group Ln 2 O 3 (Ln=Nd, Gd) as dependent on the ratio of fumed oxides and nature of Ln 2 O 3 have been studied. X-ray diffraction and thermal analysis (DTA) methods were used. The samples were heated to 1000, 1200, 1400 °C corresponding to changes in the DTA curves on the derivatograms. It is shown that silicates of oxoapatite type Nd 9.33 Si 6 O 26 and Gd 9.34 Si 6 O 26 are formed in composites based on fumed silica and oxides of Ln 2 O 3 at a temperature below 1400 °C. The phase of Nd 9.33 Si 6 O 26 was found to be transitional during the formation of stable phases of Nd 2 Si 2 O 7 and cristobalite at 1400 °C. It is suggested that no observation of Gd 2 Si 2 O 7 is connected with lower reactivity of Gd 2 O 3 compared with Nd 2 O 3 . In the composites based on Ln 2 O 3 and both fumed oxides there are change in character of the interactions of the components near 1250 °C. First, formation of Nd 9.33 Si 6 O 26 , NdAlO 3 and polymorphic transformation of Gd 2 O 3 from cubic to hexagonal modification are observed. After 1250 °C interaction between fumed oxides occurs, which is accompanied by the formation of mullite and amorphous phase while maintaining various modifications of Gd 2 O 3 . In composites based on Ln 2 O 3 and fumed alumina the formation of aluminates NdAlO 3 and GdAlO 3 occurs at 1000 and 1200 °C accordingly. Simultaneously with the formation of GdAlO 3 , the polymorphic transformation of Gd 2 O 3 is observed up to 1400 °C. In the presence of both fumed oxides, it is limited to the formation of mullitic phase. Thus, apparently, Gd 2 O 3 exhibits less reactivity with respect to fumed alumina and silica compared with Nd 2 O 3 .