Spontaneous transformations in the solid state: Towards porous and biphasic materials

This dissertation describes new routes to macroporous and hierarchically porous monoliths of oxides and metals. The routes are applicable to a broad range of materials, leading to pore walls with functions in energy transduction, catalysis, and photovoltaic materials. While the porous structures formed are random, the average architectural features are well-defined, and the porosity is highly interconnected.;We begin by developing metathetic solid state reactions which yield intimately mixed composites of K2SO4 and functional perovskites (piezoelectric PbTiO3, catalytic La1-xSrxMnO 3) as bulk monoliths. Dissolution of the soluble salt in water leaves behind macroporous monoliths of the desired perovskite phase. We then extend selective leaching to biphasic systems based on ZnO and a second, immiscible, phase (NiO, ZnFe2O4, ZnMn2O4). Leaching of the ZnO phase leads to robust oxide monoliths with highly interconnected pores. The porous structures that result are topologically identical to the sacrificial phase, and hence, microstructural control directly leads to control of the resulting pore structure. We also look at how functionality may be layered sequentially by exposing porous monoliths to reactive atmospheres or solutions.;We also develop routes where porosity arises from transformations with an intrinsic volume loss, such as the reduction of Mn3O4 to MnO. The volume loss inherent in this transformation is expressed as rectangular mesopores penetrating through the crystallites. Other systems are described which involve the removal of a sacrificial element from within a phase to induce a volume loss, such as the reduction and evaporation of Zn from Zn 2TiO4 to form porous TiO2. As these routes are shape-conserving, they may be applied to macroporous materials to form hierarchical macro/mesoporous architectures.;In some of these preparations, the resulting mesopores are aligned locally with certain crystallographic directions. This coupling between morphology and crystallography provides a macroscopic handle on nanoscale structure. Epitaxial thin films of precursor oxide phases were grown on lattice matched single crystal substrates and subsequently rendered porous through reduction. We find that the epitaxy of the pore-forming material results in pores that are crystallographically aligned with the substrate.