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Cation Influence on Negative Thermal Expansion in the A2M3O12 Family

Gates, Stacy D.
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1216698704

Abstract Details

2008, Doctor of Philosophy, University of Toledo, Chemistry.
Negative thermal expansion (NTE) describes materials that shrink when they are heated. Most materials expand when heated displaying positive thermal expansion coefficient. Several oxide families display negative thermal expansion behavior. Recently, it has been shown that NTE materials can be used to reduce thermal expansion,and to achieve a more desirable expansion coefficient α, when incorporated into composites. In general, thermal expansion varies for every material. Therefore, problems may arise when attempting to bond multiple materials for applications if the thermal expansion coefficients differ significantly. Such a mismatch in thermal expansion may result in stresses, cracks, or separation at the interface. This can cause inefficiencies, and even device failures. Preparation of composite materials containing NTE oxides should allow increased control over thermal expansion to produce better matches while preserving the properties of the original matrix. NTE is commonly found in the A2M3O12 family (A=trivalent metal; M=W, Mo). These materials crystallize in an orthorhombic unit cell, and their α-value is highly dependent on the identity of the cations (A3+ and M6+). Some compositions show a phase transition to a monoclinic structure with α>0 at low temperatures. Literature suggests that the magnitudes of NTE, as well as the crystal structure and phase transition temperatures, are affected by cation substitution. However, current theories only apply to a small number of these compounds. This research explores the various factors that influence the behavior of this oxide family of materials. When examining the influence of cation substitution on thermal expansion and phase transition behavior, synthesis of compounds using traditional solid-state methods limits the number of cations that can be incorporated into this structure. To overcome these limitations, a non-hydrolytic sol-gel (NHSG) method was used during preparation of the mixed metal compounds. The NHSG process provides a low temperature route to compounds that cannot be prepared by ceramic methods, and enables cation substitution without sacrificing the homogeneity of the mixed metal oxide. The compounds were characterized by powder X-ray diffraction in combination with Rietveld refinement, thermal analysis, and scanning electron microscopy. Additionally, some compounds were examined by diffraction studies under pressure.
Cora Lind, PhD (Advisor)
Amanda Bryant-Friedrich, PhD (Committee Member)
Jon Kirchhoff, PhD (Committee Member)
Mark Mason, PhD (Committee Member)
192 p.
Chemistry
negative thermal expansion; X-ray diffraction; scandium tungstate; metal oxides; non-hydrolytic sol-gel

Recommended Citations

Citations

  • Gates, S. D. (2008). Cation Influence on Negative Thermal Expansion in the A2M3O12 Family [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1216698704

    APA Style (7th edition)

  • Gates, Stacy. Cation Influence on Negative Thermal Expansion in the A2M3O12 Family. 2008. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1216698704.

    MLA Style (8th edition)

  • Gates, Stacy. "Cation Influence on Negative Thermal Expansion in the A2M3O12 Family." Doctoral dissertation, University of Toledo, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1216698704

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Toledo and OhioLINK.