DOI
https://doi.org/10.25772/PK6G-YR03
Author ORCID Identifier
0000-0002-6053-6865
Defense Date
2021
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Dr. Everett Carpenter
Abstract
Refrigeration systems for the cooling of commercial and residential buildings are a major drain on energy resources and source of greenhouse gas emissions. Magnetic refrigeration, which employs the magnetocaloric effect, has the potential to mitigate the energy drain and greenhouse gas emissions for air conditioning. In order to commercialize magnetic refrigerators, a material with a high magnetic entropy change, ΔS, near room temperature which is simple and inexpensive to create is needed.1 Current options for this solid refrigerant are either expensive, difficult to synthesize, or have a ΔS or Tc (the Curie Temperature, where ΔS is maximized) below room temperature. Lanthanum manganite nanomaterials (LaxA1-xMnO3, where A = Ca2+, Sr2+, Na+, Ba2+, etc) exhibit promise as efficient, inexpensive solid refrigerants.2
In lanthanum manganites the magnetocaloric properties arise from the Zener double exchange interactions between the Mn ions across the Mn-O-Mn bond. The magnetic properties, namely Tc and ΔS, can therefore be tuned by altering the ratio of Mn3+ to Mn4+ and the Mn-O-Mn bond distances and angles through the introduction of various metals at the a-site.3 By entering the nanoscale, crystallite size effects also become a point of control for both Tc and ΔS. The purpose of this work was to examine each of these three areas as independently as possible in the La0.60Ca0.40MnO3 material, to optimize the Pechini sol-gel synthesis of these materials, and to apply those findings to La0.75Ca0.25MO3 materials.
The Mn-O-Mn bond distances and angles were altered by introducing a-site cation size disorder through substitution of Sr2+ in place of Ca2+ in the La0.60Ca0.40-xSrxMnO3 crystal lattice. As Sr2+ was introduced the lattice parameters increased and crystallite size decreased. The lattice distortion created a 73% decrease in ΔS – from 5.6 J/kgK to 1.5 J/kgK.4 Any Tc affects were offset by the decreasing crystallite size. By contrast, introducing Na+ into the lattice in place of Ca2+ did not alter the lattice parameters of La0.60Ca0.4-xNaxMnO3 materials or the resultant crystallite sizes, but did increase the amount of Mn4+ and decrease the amount of Mn3+. The Tc values increased dramatically, however, from 265K to 333K, and ΔS saw a 33% decrease from 4.1 J/kgK to 2.8 J/kgK.5 This implies that Mn-O-Mn bond distances and angles are primarily responsible for ΔS, while Mn oxidation states are primarily responsible for Tc. Work done to optimize the synthesis parameters revealed that larger crystallites are ideal as well as the optimal synthesis parameters to achieve the largest crystallite sizes.6 These findings were successfully applied to La0.75Ca0.25MnO3 materials and the Tc value was increased from 200K, shown in the literature, to 280K.7
Through this work, multiple materials with viable magnetic properties were created. These include La0.60Ca0.28Na0.12MnO3 and both the La0.75Ca0.25MnO3 and La0.60Ca0.40MnO3 materials created with the optimized synthesis method. In addition, the understanding of how affecting the Zener double exchange interactions through both Mn-O-Mn bond distances and angles and the Mn oxidation states changes the magnetic properties of lanthanum manganites was improved. By creating nanoscale LCMO nanomaterials with this new understanding, materials ready for practical testing in prototype magnetic refrigerators can be inexpensively and reliably developed.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
4-25-2021