Thermal Expansion Of Rare-Earth Pyrosilicates

摘要

The use of RE_2Si_2O_7 materials as environmental barrier coatings (Ebcs) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE_2Si_2O_7. High-temperature X-ray diffraction (HTXRD) patterns were collected on different RE_2Si_2O_7 polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE_2Si_2O_7 compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE_2Si_2O_7 compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A-RE_2Si_2O_7 polymorph while the rest of polymorphs exhibited an aniso-tropic unit cell expansion with the biggest expansion directed along the REO_x polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE_2Si_2O_7 compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A-RE_2Si_2O_7 compounds, with an average of 12.1 × 10~(-6) K~(-1), whereas the lowest values are those of β-and γ-RE_2Si_2O_7, which showed average ABCTE values of ～4.0 × 10~(-6) K~(-1).