| dc.description.abstract | Solid-to-solid phase transitions induced by external stimuli underpin numerous technological applications and enable the stabilization of metastable polymorphs characterized by atomic connectivities and electronic structure distinct from that of the thermodynamic phase. Some strategies to stabilize metastable polymorphs include size reduction (such that surface energy preferences overcome bulk free energy differences), topochemical modification, application of high pressure, or interfacial strain. The primary focus of this work is on controlling solid-to-solid phase transitions to leverage the benefits afforded by stabilizing these metastable phases. Utilization of the insulator-to-metal transition in VOv2 for use in thermochromic fenestration will be examined in detail. Briefly, nanocomposite thin films have been prepared containing VOv2 nanocrystals embedded within methacrylic acid-ethyl acrylate copolymer matrices. Control over particle size, refractive index matching, and surface monolayer/surfactant mediated dispersion allows for mitigation of internal scattering mechanisms that negatively impact visible light transmittance and degrade the magnitude of near-infrared modulation. An unprecedented combination of visible light transmittance and solar heat gain modulation is demonstrated for hydrothermally prepared nanocrystals with a size of 44±30 nm encapsulated with a 5 nm thick SiOv2 shell and dispersed within a methacrylic acid-ethyl acrylate copolymer matrix at a loading of 0.6 mg/mL. Another noteworthy phase transition, cubic HfOv2, a material that is only stable at temperatures >2600°C, can be stabilized as a result of outwards oxygen diffusion and epitaxial matching within core—shell VOv2@HfOv2 structures when annealed at 650°C under an Ar ambient. The VOv2 is reduced to rhombohedral V2Ov3, which templates the nucleation of cubic HfOv2 as a result of a crystallographic epitaxial relationship that facilitates nucleation of this metastable polymorph over that of the thermodynamically stable monoclinic phase. Freestanding cubic HfOv2 has been stabilized by acid dissolution of the V2vO3 core. The process to embed these particles in nanocomposite thin films for gate dielectric applications will also be examined. The synthesis of the negative thermal expansion material, cubic HfV2Ov7, through an interdiffusion reaction between VOv2@HfOv2 precursors arranged in a core—shell configuration and ball-milling of HfOv2 and ultrasmall VOv2 in a 1:2 ratio will also be discussed as annealing the solid mixture in air at 600°C yields phase-pure, cubic HfV2Ov7. | |
