This thesis addresses the energetics of nanostructured, amorphous, and molten materials in several systems of potential practical importance in technology. The two themes which unify the separate projects on different materials involve (1) nanodomains and nanoclusters, as applied to amorphous polymer-derived ceramics (PDCs), and chalcogenide-containing borosilicate glasses, and (2) complex amorphous structures as seen in molten and glassy phases in Fe 2 O 3 , Rb 3 H(SO 4 ) 2 , and La 2 O 3 -SiO 2 -HfO 2 based systems. Obtaining data on thermodynamics of materials is essential to better understand, control, and predict their stability, structure, and properties in a desired application or process.
PDCs are silicon oxycarbides (SiCO), silicon carbonitrides (SiCN), and silicon oxycarbonitrides (SiCNO), applicable as high temperature materials and in protective coatings for electronic packaging. They can incorporate large amounts of carbon and can create unusual nanostructures. The calorimetric measurements of heats of oxidative dissolution in a molten oxide solvent show that SiCO amorphous ceramics possess a negative enthalpy of formation from crystalline constituents (silicon carbide, cristobalite, and graphite), providing a thermodynamic hindrance to crystallization, while SiCNO ceramics possess a slightly negative (or near zero) enthalpy, relative to their crystalline constituents.
The measured interfacial energy of CdS 0.1 Se 0.9 nanoparticles (1-40 nm) embedded in borosilicate glass is 0.56 J/m 2 suggests that the glass matrix can accommodate and relax the strain in the nanoparticles, increasing their stability, which is important for optical device performance.
Near-zero enthalpies of mixing were measured for a series of La 2 O 3 -HfO 2 -SiO 2 glasses (prepared using containerless processing techniques), approximately along the join [0.73SiO 2 - 0.27(xHfO 2 - (1-x)La 2 O 3 ), 0≤ x≤0.3]. These glasses may be useful as laser and gate dielectric materials.
Drop solution calorimetry in molten sodium molybdate revealed the enthalpy of the solid state disporportionation Rb 3 H(SO 4 ) 2 (s) [arrow right] Rb 2 SO 4 (s) + RbHSO 4 (s) to be essentially zero (0.85 ± 2.73 kJ/mol), supporting the assertion that the observed transformation at 202°C involves melting rather than just solid-solid reaction to a superionic state. The standard enthalpy of formation of Rb 3 H(SO 4 ) 2 from the elements at 25°C is -2602 ± 10 kJ/mol. Acid sulfates are proposed as alternatives to current fuel cell electrolytes.
High temperature reaction calorimetry at both 700°C and 1353°C on several iron ores has been used to examine and predict the thermodynamics of iron oxide uptake during sintering processes, important to steel making.
Questions concerning stability and structure of nanocrystalline, amorphous, glassy, and molten materials have been addressed using high temperature calorimetry and several spectroscopic techniques (EMPA, (S)TEM, high energy synchrotron XRD, XRF, NMR, photoluminescence, neutron scattering, and Ramen spectroscopy).
The thesis also contains archived contributions to collaborative in-progress projects on the energetics of La 2 Si 2 O 7 and triple-chain silicates, and neutron scattering and Raman spectroscopy measurements of negative thermal expansion [A 2 M 3 O 12 family (A= Al, Sc, Cr, Fe, In, and M = W, Mo )] materials.
My Research