The kinetics of the PuO2 to Pu2O3 conversion.

In an oxidizing environment, the oxide formed on plutonium (Pu) metal is composed of a plutonium dioxide (PuO2) top layer and a thin cubic plutonium sesquioxide (Pu2O3) middle layer. In a reducing environment, the PuO2 layer auto-reduces to cubic Pu2O3. The speed and extent of this conversion depend on the combination of temperature and time. While PuO2 provides a strong diffusion barrier against unwanted Pu corrosion by gaseous species (like hydrogen), Pu2O3 does not, since its crystal structure has chains of oxygen vacancies. The kinetics of the PuO2 reduction are, therefore, of fundamental interest and enable researchers to better protect Pu from corrosion. In this report, the oxygen-diffusion-limited kinetics of the dioxide to sesquioxide conversion were obtained by dynamically heating a PuO2-covered Pu sample from 294 to 418 K in a high-vacuum vessel equipped with an in situ spectroscopic ellipsometer. The physical/chemical constraints in the conversion process were combined with the ellipsometry method of multi-sample analysis to track the percentage of PuO2 and to compute the extent of Pu2O3 formation. The resulting diffusion coefficients were compared against and then combined with complementary literature data to produce a comprehensive set of kinetic parameters for reliably modeling oxide conversion over a larger temperature range than spanned by prior studies. The extracted thermal activation energy barrier (43.7 kJ/mol) and pre-exponential factor (5.0 × 10-10 cm2/s) for the oxygen-diffusion-limited process can be used to accurately model the PuO2 to Pu2O3 transformation in vacuum and/or inert gas applications.

Temperature-Dependent Kinetic Prediction for Reactions Described by Isothermal Mathematics.

Most kinetic models are expressed in isothermal mathematics. This may lead unaware scientists either to the misconception that classical isothermal kinetic models cannot be used for any chemical process in an environment with a time-dependent temperature profile or, even worse, to a misuse of them. In reality, classical isothermal models can be employed to make kinetic predictions for reactions in environments with time-dependent temperature profiles, provided that there is a continuity/conservation in the reaction extent at every temperature-time step. In this article, fundamental analyses, illustrations, guiding tables, and examples are given to help the interested readers using either conventional isothermal reacted fraction curves or rate equations to make proper kinetic predictions for chemical reactions in environments with temperature profiles that vary, even arbitrarily, with time simply by the requirement of continuity/conservation of reaction extent whenever there is an external temperature change.

Mechanism and kinetic modeling of hydrogenation in the organic getter/palladium catalyst/activated carbon systems.

Experiments to measure the hydrogen uptake kinetics of DEB getter/Pd catalyst/activated carbon pellets have been performed under isothermal isobaric conditions. The extracted kinetics were then used to predict the performance of the getter pellets under different temperatures and pressures, including nonisobaric situations. For isothermal isobaric uptake at higher H2 pressure (666.6-2666.5 Pa), H2 solubility in the getter matrix is responsible for the uptake observed up to a 40-60% reacted fraction. Once the hydrogenated product becomes thicker, the diffusions of the reactants (atomic hydrogen and getter molecules) toward the reaction front become the rate limiting step. However, in a dynamic but very low H2 pressure, encountered in many vacuum electronic applications, the hydrogen spillover effect, over micrometer scale, becomes the dominant reaction mechanism. Despite such a complex dependence of the rate limiting mechanisms on the experimental environment, there is good agreement between kinetic prediction models and experiments. The investigation also reveals that the ultimate uptake capacity in the getter pellets scales inversely with the free volume of the vacuum vessel in which the DEB getter pellets are used, and that DEB getter pellets' performance greatly deteriorates during the final 10-15% capacity (as evidenced by the sharp bend in the slopes of the reacted fraction vs time curves at 85-90% reacted fraction).

Controlled manipulation of elastomers with radiation: Insights from multiquantum nuclear-magnetic-resonance data and mechanical measurements.

Filled and cross-linked elastomeric rubbers are versatile network materials with a multitude of applications ranging from artificial organs and biomedical devices to cushions, coatings, adhesives, interconnects, and seismic-isolation, thermal, and electrical barriers. External factors such as mechanical stress, temperature fluctuations, or radiation are known to create chemical changes in such materials that can directly affect the molecular weight distribution (MWD) of the polymer between cross-links and alter the structural and mechanical properties. From a materials science point of view it is highly desirable to understand, affect, and manipulate such property changes in a controlled manner. Unfortunately, that has not yet been possible due to the lack of experimental characterization of such networks under controlled environments. In this work we expose a known rubber material to controlled dosages of γ radiation and utilize a newly developed multiquantum nuclear-magnetic-resonance technique to characterize the MWD as a function of radiation. We show that such data along with mechanical stress-strain measurements are amenable to accurate analysis by simple network models and yield important insights into radiation-induced molecular-level processes.

H2O outgassing from silica-filled polysiloxane TR55.

Temperature-programmed desorption/decomposition (TPD) was employed to obtain the moisture content and outgassing kinetics of TR55, a silica-filled cross-linked polysiloxane. The total moisture content of TR55 in the as-received state and after 20-30 min of vacuum pumping in the load-lock prior to TPD was measured to be on the order of 0.35 wt%. Physisorbed H(2)O and chemisorbed H(2)O account for about 13.2 and 86.8%, respectively, of the 0.35 wt% measured moisture content. H(2)O outgassing models based on the kinetics measured from TPD experiments suggest that loosely bound chemisorbed water outgasses in a dry environment slowly but continuously over many decades at or a little above room temperature. However, physisorbed water can be easily pumped out in a matter of hours at around 400 K.