Properties of the Feynman-alpha method applied to accelerator-driven subcritical systems

A Monte Carlo study of the Feynman-method with a simple code simulating the multiplication chain, confined to pertinent time-dependent phenomena has been done. The significance of its key parameters (detector efficiency and dead time, k-source and spallation neutrons multiplicities, required number of fissions etc.) has been discussed. It has been demonstrated that this method can be insensitive to properties of the zones surrounding the core, whereas is strongly affected by the detector dead time. In turn, the influence of harmonics in the neutron field and of the dispersion of spallation neutrons has proven much less pronounced.     

Theoretical Investigations on Electrocaloric Properties of \mathrm{PbZr}_{0.95}\mathrm{Ti}_{0.05}\mathrm{O}_{3} Thin Film

Based on a phenomenological model, the electrocaloric effect (ECE) accompanied with the ferroelectric-to-paraelectric phase transition in a PbZr $_{0.95}$ 0.95 Ti $_{0.05}$ 0.05 O $_{3}$ 3 thin film was investigated. The extracted data reveal many features of the ECE such as electrocaloric entropy changes, heat capacity changes, and temperature changes as functions of temperature due to different electric fields shifts. From the behavior of the PbZr $_{0.95}$ 0.95 Ti $_{0.05}$ 0.05 O $_{3}$ 3 thin film in phase transitions, it leads to a large change of heat capacity of 105.94 J  ${\cdot }\,$ · kg ${^{-1}}\,{\cdot }\,{^{\circ }}$ ? 1 · ° C, a temperature change of 22.44 K, and a relative cooling power of 1469 J  ${\cdot }$ ·  kg $^{-1}$ ? 1 .     

$hbox{Mg}_{1-{rm x}}hbox{Co}_{rm x}hbox{Fe}_{1.98}hbox{O}_{4}$ Ceramics With Promising Magnetodielectric Properties for Antenna Miniaturization

We have studied the effects of concentration of cobalt and sintering temperature on the complex relative permittivity and permeability of MgFe_1.98O₄ ferrite ceramics. All samples were doped with 3 wt% Bi₂O₃ to improve the sintering properties of the Mg_1-xCo_xFe_1.98O₄ ceramics and thus attain a low dielectric loss tangent. X-ray diffraction data indicated that the ferrite samples all have a single-phase spinel structure and that there was no reaction between the ferrite and Bi₂O₃. In terms of microstructure development and grain growth, the effect of Bi₂O₃ is much more pronounced than that of Co. At a given concentration of Co, the magnetic properties can be fairly explained by a magnetic circuit model. The relationship between static permeability and the concentration of Co can be understood by using the single-ion model combined with the local uniaxial anisotropic model. Among the samples we prepared, Mg_0.96Co_0.04Fe_1.98O₄ sintered at 1000degC for 2 h has almost equal real permeability and permittivity-both approximately 10 - as well as sufficiently low dielectric and magnetic loss tangent over a range 3 to 30 MHz [the high-frequency (HF) band]. These properties that should be useful for miniaturization of HF antennas.     

Radiative Properties of Ceramic $$\hbox {Al}_{2}\hbox {O}_{3}$$, AlN,and $$\hbox {Si}_{3}\hbox {N}_{4}$$Si3N4: I. Experiments

The radiative properties of dense ceramic \(\hbox {Al}_{2}\hbox {O}_{3}\), AlN, and \(\hbox {Si}_{3}\hbox {N}_{4}\) plates are investigated from the visible to the mid-infrared region at room temperature. Each specimen has different surface finishings on different sides of the laminate. A monochromator was used with an integrating sphere to measure the directional-hemispherical reflectance and transmittance of these samples at wavelengths from 0.4 \(\upmu \hbox {m}\) to 1.8 \(\upmu \hbox {m}\). The specular reflectance was obtained by a subtraction technique. A Fourier-transform infrared spectrometer was used to measure the directional-hemispherical or specular reflectance and transmittance with appropriate accessories from about 1.6 \(\upmu \hbox {m}\) to 19 \(\upmu \hbox {m}\). All measurements were performed at near-normal incidence on either the smooth side or the rough side of the sample. The experimental observations are qualitatively interpreted considering the optical constants, surface roughness, and volume scattering and absorption.     

Prediction of Mass Evaporation of \mathrm{Fe}_{50}\mathrm{Co} _{50} During Measurements of Thermophysical Properties Using an Electrostatic Levitator

This paper describes the prediction of mass evaporation of \(\mathrm{Fe}_{50}\mathrm{Co} _{50}\) at% alloys during thermophysical property measurements using the electrostatic levitator at NASA Marshall Space Flight Center in Huntsville, AL. The final mass, final composition, and activity of individual component are considered in the calculation of mass evaporation. The predicted reduction in mass and variation in composition are validated with six ESL samples which underwent different thermal cycles. The predicted mass evaporation and composition shift show good agreement with experiments with the maximum relative errors of 4.8 % and 1.7 %, respectively.     

Application of Non-Isothermal Thermogravimetric Method to Interpret the Decomposition Kinetics of \hbox {NaNO}_{3}, \hbox {KNO}_{3}, and \hbox {KClO}_{4}

The non-isothermal thermogravimetric method was used to study the thermal decomposition of \(\hbox {KClO}_{4}, \hbox {KNO}_{3}\) , and \(\hbox {NaNO}_{3}\) at heating rates of (5, 10, 15, and 20)  \(\hbox {K}\cdot \hbox {min}^{-1}\) . The activation energy of thermal decomposition reactions was computed by isoconversional methods of Ozawa–Flynn–Wall, Kissinger–Akahiro–Sunose, and Friedman equations. Also, the kinetic triplet of the thermal decomposition of salts was determined by the model-fitting method of the modified Coats–Redfern equation. The activation energies of \(\hbox {KClO}_{4}, \hbox {KNO}_{3}\) , and \(\hbox {NaNO}_{3}\) of (293 to 307, 160 to 209, and 192 to 245)  \(\hbox {kJ}\cdot \hbox {mol}^{-1}\) , respectively, are obtained by non–isothermal isoconversional methods. The modified Coats and Redfern method showed that the most probable mechanism functions \(g(\alpha )\) of \([-\hbox {ln}(1 - \alpha )]^{1/3}\) (model A3: Arami–Erofeev equation) and \((1 - \alpha )^{-1}- 1\) (model F2: second order) can be used to predict the decomposition mechanisms of \(\hbox {KClO}_{4}\) , \(\hbox {KNO}_{3}\) , and \(\hbox {NaNO}_{3}\) , respectively.     

Elastic Properties of Monoclinic \hbox {ZrO}_{2} at Finite Temperatures Via First Principles

The structural and elastic properties of orthorhombic $\hbox {ZrO}_{2}\,(m\hbox {-ZrO}_{2})$ as a function of temperature are investigated by the generalized gradient approximation (GGA) correction scheme in the framework of density functional theory (DFT) and the quasi-harmonic Debye model. The thirteen independent elastic constants of $m\hbox {-ZrO}_{2}$ at temperatures to 3200 K are theoretically investigated for the first time. It is found that with increasing temperature, all elastic constants change, especially $C_{35}\hbox { and }C_{25}$ change rapidly in the temperature range of 1400 K to 1600 K and 2200 K to 2600 K, respectively. We also obtain the bulk modulus $B$ , shear modulus $G$ , Young’s moduli $E$ , as well as Poisson’s ratio $\sigma $ of $m\hbox {-ZrO}_{2}$ at high temperatures. Our work suggests that it is very important to predict the melting properties of materials via the elastic constants at temperatures.