Thermochemistry of Metal Nitrides in the Ca/Zn/N System

Structure and Thermodynamics of Silicon Oxycarbide Polymer-Derived Ceramics with and without Mixed-Bonding

Silicon oxycarbides synthesized through a conventional polymeric route show characteristic nanodomains that consist of sp² hybridized carbon, tetrahedrally coordinated SiO_4, and tetrahedrally coordinated silicon with carbon substitution for oxygen, called “mixed bonds.” Here we synthesize two preceramic polymers possessing both phenyl substituents as unique organic groups. In one precursor, the phenyl group is directly bonded to silicon, resulting in a SiOC polymer-derived ceramic (PDC) with mixed bonding. In the other precursor, the phenyl group is bonded to the silicon through Si-O-C bridges, which results in a SiOC PDC without mixed bonding. Radial breathing-like mode bands in the Raman spectra reveal that SiOC PDCs contain carbon nanoscrolls with spiral-like rolled-up geometry and open edges at the ends of their structure. Calorimetric measurements of the heat of dissolution in a molten salt solvent show that the SiOC PDCs with mixed bonding have negative enthalpies of formation with respect to crystalline components (silicon carbide, cristobalite, and graphite) and are more thermodynamically stable than those without. The heats of formation from crystalline SiO₂, SiC, and C of SiOC PDCs without mixed bonding are close to zero and depend on the pyrolysis temperature. Solid state MAS NMR confirms the presence or absence of mixed bonding and further shows that, without mixed bonding, terminal hydroxyls are bound to some of the Si-O tetrahedra. This study indicates that mixed bonding, along with additional factors, such as the presence of terminal hydroxyl groups, contributes to the thermodynamic stability of SiOC PDCs.

Sample seal-and-drop device and methodology for high temperature oxide melt solution calorimetric measurements of PuO2

Thermodynamic properties of refractory materials, such as standard enthalpy of formation, heat content, and enthalpy of reaction, can be measured by high temperature calorimetry. In such experiments, a small sample pellet is dropped from room temperature into a calorimeter operating at high temperature (often 700 °C) with or without a molten salt solvent present in an inert crucible in the calorimeter chamber. However, for hazardous (radioactive, toxic, etc.) and/or air-sensitive (hygroscopic, sensitive to oxygen, pyrophoric, etc.) samples, it is necessary to utilize a sealed device to encapsulate and isolate the samples, crucibles, and solvent under a controlled atmosphere in order to prevent the materials from reactions and/or protect the personnel from hazardous exposure during the calorimetric experiments. We have developed a sample seal-and-drop device (calorimetric dropper) that can be readily installed onto the dropping tube of a calorimeter such as the Setaram AlexSYS Calvet-type high temperature calorimeter to fulfill two functions: (i) load hazardous or air-sensitive samples in an air-tight, sealed container and (ii) drop the samples into the calorimeter chamber using an "off-then-on" mechanism. As a case study, we used the calorimetric dropper for measurements of the enthalpy of drop solution of PuO₂ in molten sodium molybdate (3Na₂O·4MoO₃) solvent at 700 °C. The obtained enthalpy of -52.21 ± 3.68 kJ/mol is consistent with the energetic systematics of other actinide oxides (UO₂, ThO₂, and NpO₂). This capability has thus laid the foundation for thermodynamic studies of other Pu-bearing phases in the future.

A study on the thermal conversion of scheelite-type ABO4 into perovskite-type AB(O,N)3

Scheelite-type SrMO₄ oxides (M = Mo, W) convert upon thermal treatment under an ammonia atmosphere into an intermediate scheelite-type oxynitride phase SrMO_4−yN_y at 600 °C, and subsequently rearrange into perovskite-type oxynitrides SrMO_3−xN_x at higher temperatures.

The carotenoids as anti-oxidants--a review

Carotenoids are abundant in many fruits and vegetables and they play diverse roles in photobiology, photochemistry and medicine. This review concerns the reactivity of carotenoids with singlet oxygen and the interaction of carotenoids with a range of free radicals. Mechanisms associated with the anti- and pro-oxidant behaviour of carotenoids are discussed including carotenoid interactions with other anti-oxidants.