The effect of the Thanksgiving Holiday on weight gain

Background  

More people than ever are considered obese and the resulting health problems are evident. These facts highlight the need for identification of critical time periods for weight gain. Therefore the purpose was to assess potential changes that occur in body weight during the Thanksgiving holiday break in college students.     

Surface Interaction of PuO2, UO2+x and UO3 with Water Ice

The interaction of PuO₂, UO_2+x and UO₃ with water ice was studied using ultraviolet photoelectron spectroscopy (UPS). Water was adsorbed at 80–120 K as thick ice multilayers. Surface modification after desorption of the ice around 200 K was investigated. Main information on the surface oxidation state was obtained by highly surface sensitive UPS-HeII spectra, probing primarily the first monolayer. The oxidation state was directly deduced from the intensity of the actinide 5f levels. The surface character of the phenomenon was further confirmed by comparing HeII spectra with the more bulk sensitive HeI spectra. Spectral interpretation was done using the cross-section variations in HeI and HeII spectra and by comparing the spectra with theoretical density of states curves, obtained by LSDA + U calculations. It was shown previously that reduction to Pu₂O₃ takes place, when the ice covered PuO₂ films are warmed up under UV light and ice is desorbed. In this paper, this effect was investigated in further detail. It was shown that only the top surface layer is reduced. Reduction is inhibited by surface diffusion of oxygen trapped in the films during sputter deposition and not incorporated in the lattice. UO_2+x and UO₃ also undergo reduction, but to a significant lesser extent than PuO₂. Reoxidation of surface U by bulk oxygen was much slower than that of surface Pu. It was shown that for all oxides, reduction needs the illumination of an ice overlayer by UV light. Surface reduction by atomic hydrogen was investigated to check for possible influence of ice photolysis products. UO₃ was shown to be reduced to UO₂ while PuO₂ is not further reduced. The observations are explained by photochemical decomposition of water by the UV light (used for UPS) at the oxide-ice interface. It is thought that the oxide acts as photocatalyst, absorbing light and splitting adsorbed water. The thick ice layer traps the reaction products on the surface, thereby enabling them to react and reduce the surface. Why only the reductants (probably H) and not the concomitant oxidants react with the surface is still unknown.     

Densification kinetics and sintering behavior of UO2 and 0.5 wt.%MnO-doped UO2

In this work, the sintering kinetics of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by a high-temperature dilatometer heated up to 1500°C. In addition, the sintering behavior of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by pressureless sintering technique. The results showed that MnO doping enhanced the grain boundary diffusion of UO₂, which can effectively decrease the densification temperature and promote grain growth. The sintering temperature of UO₂ was significantly reduced by about 200°C with the addition of 0.5 wt.%MnO. The microscopic morphology studies showed that there were still fine particles agglomerated, forming sintered spheres in the matrix even if no severe agglomeration and bimodal size distribution were observed in raw UO₂ powder. The microstructure evolution of the sintered sphere and UO₂ matrix during the densification process was studied by isothermal sintering. Finally, the present analyses indicated that the densification of UO₂ matrix can be accelerated by adding MnO or increasing the sintering temperature, thus improving the densification inhomogeneity of UO₂ matrix.     

Helium behaviour in stoichiometric and hyper-stoichiometric UO2

Understanding the long-term behaviour of the UO₂ spent fuel in terms of helium build-up and oxidation is a very important issue for safety aspects of storage and disposal. Although helium behaviour in stoichiometric UO₂ has been studied by many authors, it has not been well established and there is a lack of experimental studies in non-stoichiometric UO₂. In this study, an infusion technique was chosen to introduce helium in stoichiometric and hyperstoichiometric single and polycrystalline UO₂ and polycrystalline U₃O₈ samples. Characterization of the samples before and after infusion and after thermal desorption measurements were performed by thermogravimetry, X-ray diffraction, laser flash technique and scanning electron microscopy. It was observed that the increase of stoichiometry in UO₂ affects barely the dissolved helium quantity (under the same infusion conditions as in stoichiometric), while it has a more pronounced influence on the helium diffusion. These two effects are much more pronounced for U₃O₈.     

UO2 2+ Sorption in Natural Mexican Erionite and Y Zeolite

Abstract

The use of Y zeolite and erionite to remove UO₂ ²⁺ ions from aqueous solutions has been investigated. The effect of temperature, the concentration of UO₂ ²⁺ uptake, the diffusion coefficients, and the ion-exchange isotherms were also studied. X-ray diffraction, thermal analyses, and transmission electron microsocopy were used to characterize the solids. The UO₂ ²⁺ content was determined by neutron activation analysis. The diffusion coefficients are not clearly affected by temperature or concentration. They definitely depend on the structure of each zeolite. The UO₂ ²⁺ exchange capacities were 0.2037 and 0.7345 meq UO₂ ²⁺/g for erionite and Y zeolite, respectively. It was found that UO₂ ²⁺ is exchanged with the cations of the large cavity in Y zeolite. In the case of erionite, UO₂ ²⁺ is exchanged with the cations localized in sites where the charge densities are very high.     

Evaluation of steady-state flow curves from model molecular weight distributions

Graessley's theory of entanglement was applied to several model distributions. The distribution functions chosen were such that the differential weight distributions were triangular with respect to a log molecular weight axis. The molecular weight level, breadth of the molecular weight distribution, skewness, and blending of simple distributions were studied in their effect on the steady-state flow curve. The governing factor for the shape of the reduced flow curve was shown to be M?_z+1·M?_z/M?_w². Other general features of the flow curve predicted by Graessley's theory were discussed.     

An Evaluation of the Thermophysical Properties of Stoichiometric CeO2 in Comparison to UO2 and PuO2

The thermal conductivity of stoichiometric CeO₂ was determined through measurement of thermal expansion from 313 to 1723 K, thermal diffusivity from 298 to 1473 K, and specific heat capacity from 313 to 1373 K. The thermal conductivity was then calculated as the product of the density, thermal diffusivity, and specific heat capacity. The thermal conductivity was found to obey an (A + BT)^?1 relationship with A = 6.776×10^?2 m·K·W^?1 and B = 2.793 × 10^?4 m·W^?1. Extrapolations of applied models were made to provide suggested data for the specific heat capacity, thermal diffusivity, and thermal conductivity data up to 1723 K. Results of thermal expansion and heat capacity measurements agreed well with the limited low‐temperature data available in the literature. The thermal conductivity values provided in the current study are significantly higher than the only high‐temperature data located for CeO₂. This is attributed to the tendency of CeO₂ to rapidly reduce at elevated temperatures given the available partial pressure of O₂ in air at ambient pressure. The CeO₂ data are compared to literature values for UO₂ and PuO₂ to evaluate its suitability as a surrogate in nuclear fuel systems where thermal transport is a primary criterion for performance     

Effect of molecular weight on shape memory behavior in polyurethane films

Understanding the relationship between the number‐average molecular weight (M_n) and the shape memory behavior of polymers is crucial for a complete picture of their thermomechanical properties, and hence for the development of smart materials, and, in particular, in textile technology. We report here on the study of shape memory properties as a function of M_n of polymers. Shape memory polyurethanes (SMPUs) of different M_n were synthesized, with various catalyst contents or molar ratio(r = NCO/OH) in the composition. In particular, two types of SMPU, namely T_m and T_g types according to their switch temperature type, were synthesized to compare the influence of M_n on their shape memory behavior. X‐ray diffraction, differential scanning calorimetry, dynamic mechanical analysis, and shape memory behavior results for the SMPUs are presented. The results indicate that the melting temperature (T_m), the glass transition temperature (T_g), the crystallinity, and the crystallizability of the soft segment in SMPUs are influenced significantly by M_n, before reaching a critical limit around 200 000 g mol^?1. Characterization of the shape memory effect in PU films suggests that the T_m‐type films generally show higher shape fixities than the T_g‐type films. In addition, this shape fixity decreases with increasing M_n in the T_g‐type SMPU, but the shape recovery increases with M_n in both types of SMPU. The shape recovery temperature, in contrast, decreases with M_n as suggested by the result of their thermal strain recovery. It is concluded that a higher molecular weight (M_n > 200 000 g mol^?1) is a prerequisite for SMPUs to exhibit higher shape recovery at a particular temperature. Copyright © 2007 Society of Chemical Industry     

On the origin of order-disorder in drawn polyacrylonitrile

Drawn polyacrylonitrile can be characterized only insufficiently with regard to its molecular and supermolecular structures which substantially determine the physical properties. This paper points out that the so-called “superstructure” of polyacrylonitrile fibers can be cleared up after suitable thermooxidative treatment using x-ray small-angle scattering technique. The chemical reactions during the thermo-oxidative treatment lead to a “self-contrasting” effect so that the fibers as such are severely damaged yet information about the supermolecular structure can be obtained. The investigations show that changes in the course of fiber production (spinning, drawing, and annealing processes) affect alterations of the superstructure. The described method of investigation represents a so far unknown possibility for the purpose of detecting the supermolecular structure of drawn polyacrylonitrile.     

On the origin of bubbles in gas-fluidized beds

In gas-fluidized beds bubbling starts when the voids formed by the oscillating bottom layer of the bed exceed a minimal size. The so-called vibration criterion derived predicts the porosity at the bubble point in close agreement with the experimental data. Comparison of the other criteria proposed in the literature on the basis of the transition number

shows that the voids formed under the critical conditions will grow and that stable bubbles can exist.     

On the role of acidity in catalytic oxidation

The role of the catalyst surface acid-base properties on the heterogeneously-catalyzed oxidation reaction mechanisms is discussed. Acid-base properties depend on the covalent/ionic character of the metal-oxygen bonds and are involved in some steps of the oxidation reactions, such as the activation of the C---H hydrocarbon bonds, the step associated with the evolution of alkoxide species and the desorption/overoxidation of the partial oxidation products. Thus they participate with the cation redox properties in determining the selective/unselective catalyst behavior.     

Irradiation-induced microstructural transformations in UO2 accelerated upon electronic energy deposition

The combination of electronic and nuclear energy deposition can have an effect on the defect production and evolution that is not simply the sum of both independent contributions. In nuclear reactors, the fuel is exposed to the irradiation of several particles such as the fission fragments (FF) whose electronic to nuclear energy loss ratio evolves along their path. To understand the impact of this energy-loss coupling on the fuel microstructure, single and dual-beam ion irradiations of uranium dioxide (UO₂) were carried out. The damage evolution was investigated by Raman spectroscopy analysis correlated with Transmission Electronic Microscopy observations. It was found a significant acceleration effect of the electronic energy dissipation on the formation and evolution of defects induced by the nuclear energy deposition. The magnitude of this acceleration was shown to depend on both the electronic energy loss level and the microstructure at the moment when the coupling occurs.     

A photo-induced dissolution of UO2 sintered pellets in a simulated solution

The objective of this study is to improve the established dissolution technique of UO₂ target by using a photochemical reaction. Photo-dissolution tests of UO₂ sintered powder and pellets were carried out in a simulated nitric acid solution at about 50 °C under UV irradiation. The simulated solution consists of 2 M nitric acid containing elements such as Cs, Sr, Zr, Ru, Mo and Nd. The light source is a Hg-lamp emitting 254 nm wavelength. As results, in the dark reaction, UO₂ sintered pellets were hardly dissolved, whereas UO₂ was rapidly dissolved after 7 hours of dissolution time in the UV irradiation. The very low dissolution rate in the dark reaction was due to surface characteristics of sintered pellets: UO₂ sintered pellet is very dense and has extremely low specific surface area. However, the dissolution rate of UO₂ sintered pellet was considerably increased in the simulated solution under UV irradiation. This was attributed to the fact that ruthenium and molybdenum ions in the simulated solution could accelerate the dissolution of UO₂ under UV irradiation. Additionally, when the pulverized sintered UO₂ powder was used, the dissolution rate of UO₂ increased more rapidly than that of UO₂ sintered pellet.     

Densification mechanisms of UO2 consolidated by spark plasma sintering

Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, Q_act, and the stress exponent, n, for SPS of nearly stoichiometric UO₂. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate Q_act and n, respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare Q_act. The average values were Q_act =96 kJ/mol (CHR), Q_act = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250?450 kJ/mol), creep (350?550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol).This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS.     

Effect of molecular weight on craze shape and fracture toughness in polycarbonate

The shape of the craze at the tip of a crack has been studied using optical microscopy on polycarbonates of various molecular weights at ?30°C. For all molecular weights studied the craze shape was well approximated by the Dugdale plastic zone model and this model was used to calculate the craze stress and the release rate in plane strain. It was found that the craze dimensions, the craze stress and the strain energy release rate in plane strain all increased with increasing molecular weight. Fracture of macroscopic specimens showed a mixed mode fracture in all molecular weights. By studying the effect of thickness the strain energy release rate in plane strain was calculated for various molecular weights. Agreement was found between these values and those determined from the craze shape measurements. The overall strain energy release rate, the strain energy release rate in plane strain and the contributions from the plane stress mode increased with increasing molecular weight.     

行业信息

<正>石墨烯和人脑工程入选欧盟旗舰技术项目新华网布鲁塞尔1月28日电欧盟委员会28日宣布,石墨烯和人脑工程两大科技入选"未来新兴旗舰技术项目",并分别设立专项研发计划。每项计划将在未来10年内分别获得10亿欧元的经费。石墨烯又称单层石墨,它仅由一层碳原子组成,是科学家于2004年发现的一种新材料。欧盟委员会认为,从长期看,这种新材料可能同钢铁、塑料一样重要,有可能代替硅成为信息技术的基础材料,还可能在能源、交通和医疗保健领域发挥重要作用。     

On the origin of strain hardening in glassy polymers

The influence of network density on the strain hardening behaviour of amorphous polymers is studied. The network density of polystyrene is altered by blending with poly(2,6-dimethyl-1,4-phenylene-oxide) and by cross-linking during polymerisation. The network density is derived from the rubber-plateau modulus determined by dynamic mechanical thermal analysis. Subsequently uniaxial compression tests are performed to obtain the intrinsic deformation behaviour and, in particular, the strain hardening modulus. At room temperature, the strain hardening modulus proves to be proportional to the network density, irrespective of the nature of the network, i.e. physical entanglements or chemical cross-links. With increasing temperature, the strain hardening modulus is observed to decrease. This decrease appears to be related to the influence of thermal mobility of the chains, determined by the distance to the glass-transition temperature (T−T_g).