Tailorable thermal expansion in leucite‐pollucite materials derived from geopolymers for environmental barrier coatings

Abstract

The K[AlSi 2 O 6 ]-Cs[AlSi 2 O 6 ] pseudo-binary system was synthesized by geopolymer crystallization. The thermal expansion properties of these materials were studied by in situ high-temperature X-ray diffraction to characterize thermal expansion behavior for potential application as environmental barrier coatings. Tailorable thermal expansion through changing cation stoichiometry allowed reduced thermal expansion mismatch with SiC f /SiC composites compared to rare-earth-based coatings. K E Y W O R D Senvironmental barrier coatings (EBC), geopolymers, synthesis, thermal expansion 3398 | STEVESON aNd KRIVEN application for the derivative materials as one-pot coatings that could be applied without requiring the suspension processing of typical ceramic coatings and for near net shape, bulk ceramic parts. 14,15 Geopolymers are X-ray amorphous, nanoporous framework aluminosilicate glass structures. 16 Charge balancing Group 1 cations and water molecules are incorporated throughout the structure in molecular pores in the framework. 17 Geopolymer materials have chemical similarity and stoichiometric equivalence to members of the feldspar and feldspathoid mineral families. These minerals consist of a framework of mostly Q 4 -bonded aluminate and silicate tetrahedra which form cages that enclose alkali cations and water molecules. The cages are aligned along certain crystallographic directions to form "pore channels." The local structure of geopolymers, within 1-2 nearest neighbor atoms, is identical to that of framework zeolites of equivalent compositions. [18][19][20][21][22] In this work, materials of the analcime family, having the formulawere studied. The exact temperature at which crystallization occurs increases with the ionic radius of the interframework cation due to the dependence of diffusive mobility on cation size. [23][24][25] Thermal stability of the amorphous geopolymer phase is also dependent upon stoichiometry. In geopolymers in general, the formation of crystalline phases is promoted by high Al 2 O 3 content, large interframework cation size, and low SiO 2 content. 16,[26][27][28][29][30] On heat treatment to an appropriate temperature, the long-range structure of potassium and cesium geopolymers undergo reorganization to that of crystalline leucite, K[AlSi 2 O 6 ] or K 2 O•Al 2 O 3 •4SiO 2 , and pollucite, Cs[AlSi 2 O 6 ] or Cs 2 O•Al 2 O 3 •4SiO 2 , respectively. How to cite this article: Steveson AJ, Kriven WM. Tailorable thermal expansion in leucite-pollucite materials derived from geopolymers for environmental barrier coatings.