Negative and Near-Zero Thermal Expansion in A2M3O12 and Related Ceramic Families: A Review

Abstract

This review presents the history of materials in the A2M3O12 and related ceramic families, including their unusual thermal expansion and the present understanding of its mechanism, and related factors such as hydroscopicity and the monoclinic to orthorhombic phase transition. Other properties, including thermomechanical, thermal and ionic conduction and optical properties, are presented in terms of current knowledge, challenges and opportunities for applications. One of the largest challenges is the production of monoliths, and various methods for consolidation and sintering are summarized. These ceramics have considerable promise when combined with other materials, and recent advances in such composites are presented. These matters are placed in the context of the potential applications of negative and near-zero thermal expansion ceramics, which still present challenges for future materials researchers.

Extrinsic Point Defects in Low-Positive Thermal Expansion Al2W3O12 and Their Effects on Thermal and Optical Properties

“…Some previous authors , based their explanation of this feature on a more flexible crystal structure since some of the A–O–M linkages are broken in oxygen-deficient A 2 M 3 O 12 materials, permitting stronger transverse acoustic vibrations of the structure-forming polyhedra. Fine-tuning of lattice flexibility is the core of the other two mechanisms for tuning the CTE in A 2 M 3 O 12 , namely, octahedral distortion and structural voids populated by small molecules, as detailed elsewhere. ,, Therefore, increased lattice flexibility would be a reasonable and acceptable model for the oxygen vacancy mechanism. However, the low-energy part of the phonon spectrum and heat capacity should be studied in more detail to put this model on a solid basis.…”

“…Negative-, zero-, and low-positive thermal expansion ceramics, such as the materials from the A 2 M 3 O 12 family, present great potential for a vast range of applications, from thermal shock resistance components and IR-transparent windows to white-light-emitting diodes and optical thermometry devices. − Although the physical properties required for these applications heavily depend on point defects, studies on point defects in the A 2 M 3 O 12 family are scant, except for a study on oxygen vacancy influence on the coefficient of thermal expansion (CTE) in In 0.6 (HfMg) 0.7 Mo 3 O 12 and another on point defects in HfMnMo 2 PO 12−δ . Both studies indicated that oxygen vacancies are able to modify the CTE in this ceramic family.…”

“…The literature on A 2 M 3 O 12 materials , presents two main mechanisms for tuning the CTE, which are (a) partial, or complete, substitution by another type of A 3+ cation, which causes changes in inherent distortion of the constituting octahedra, changing the flexibility of the lattice and (b) insertion of small molecules into empty dodecahedral, eightfold sites, present in the garnet crystal structure to which A 2 M 3 O 12 ceramics belong to. More recently, Cheng et al proposed that introducing extrinsic oxygen vacancies could provide another mechanism to tune the CTE since In 0.6 (HfMg) 0.7 Mo 3 O 12 with extrinsic oxygen vacancies, produced by heat treatment in an inert He atmosphere, presented a more negative CTE than its extrinsic-point-defect-free counterpart calcined in air.…”

Features of the Phase Preferences, Long- and Short-Range Order in Ln2(WO4)3 (Ln = Gd, Dy, Ho, Yb) with Their Relation to Hydration Behavior

“…The most typical Ln tritungstates Ln 2 (WO 4 ) 3 with the 2:3 lanthanoid:tungsten atomic ratio have been known for many decades [5][6][7]. In particular, a considerable interest in the Ln 2 (WO 4 ) 3 tungstates is observed in the past few decades because these compounds exhibit negative thermal expansion (NTE) [8][9][10][11].…”

Compósitos De Matriz Metálica À Base Da Alumix-231 Com Adição De Sílica Fundida

“…As Para entender melhor o efeito da tensão residual no CET, devido à incompatibilidade entre Alumix-231 e sílica fundida, a tensão residual gerada dentro de um compósito, foi calculada a partir da equação 1 [63]:…”