88 research outputs found
Investigation of the oxohalogenide Cu4Te5O12Cl4 with weakly coupled Cu(II) tetrahedra
The crystal structure of the copper(II) tellurium(IV) oxochloride
CuTeOCl (Cu-45124) is composed of weakly coupled
tetrahedral Cu clusters and shows crystallographic similarities with the
intensively investigated compound CuTeOX, with X~=~Cl,
Br (Cu-2252). It differs from the latter by a larger separation of the
tetrahedra within the crystallographic ab plane, that allows a more direct
assignment of important inter-tetrahedra exchange paths and the existence of an
inversion center. Magnetic susceptibility and specific heat evidence
antiferromagnetic, frustrated correlations of the Cu spin moments and long
range ordering with =13.6 K. The entropy related to the transition is
reduced due to quantum fluctuations. In Raman scattering a well structured low
energy magnetic excitation is observed at energies of 50K
(35cm. This energy scale is reduced as compared to Cu-2252.Comment: 11 pages, 9 figures, further information see
http://www.peter-lemmens.d
Band filling and interband scattering effects in MgB: C vs Al doping
We argue, based on band structure calculations and Eliashberg theory, that
the observed decrease of of Al and C doped MgB samples can be
understood mainly in terms of a band filling effect due to the electron doping
by Al and C. A simple scaling of the electron-phonon coupling constant
by the variation of the density of states as function of electron
doping is sufficient to capture the experimentally observed behavior. Further,
we also explain the long standing open question of the experimental observation
of a nearly constant gap as function of doping by a compensation of the
effect of band filling and interband scattering. Both effects together generate
a nearly constant gap and shift the merging point of both gaps to higher
doping concentrations, resolving the discrepancy between experiment and
theoretical predictions based on interband scattering only.Comment: accepted by PR
Mechanochemical Synthesis and Magnetic Characterization of Nanosized Cubic Spinel FeCr₂S₄ Particles
Nanosized samples of the cubic thiospinel FeCr2S4 were synthesized by ball milling of FeS and Cr2S3 precursors followed by a distinct temperature treatment between 500 and 800 °C. Depending on the applied temperature, volume weighted mean (Lvol) particle sizes of 56 nm (500 °C), 86 nm (600 °C), and 123 nm (800 °C) were obtained. All samples show a transition into the ferrimagnetic state at a Curie temperature TC of ∼ 167 K only slightly depending on the annealing temperature. Above TC, ferromagnetic spin clusters survive and Curie–Weiss behavior is observed only at T ≫ TC, with T depending on the heat treatments and the external magnetic field applied. Zero-field-cooled and field-cooled magnetic susceptibilities diverge significantly below TC in contrast to what is observed for conventionally solid-state-prepared polycrystalline samples. In the low-temperature region, all samples show a transition into the orbital ordered state at about 9 K, which is more pronounced for the samples heated to higher temperatures. This observation is a clear indication that the cation disorder is very low because a pronounced disorder would suppress this magnetic transition. The unusual magnetic properties of the samples at low temperatures and different external magnetic fields can be clearly related to different factors like structural microstrain and magnetocrystalline anisotropy
Multi‐Method Characterization of the High‐Entropy Spinel Oxide MnCoNiCuZnFeO: Entropy Evidence, Microstructure, and Magnetic Properties
The novel spinel Cu0.2Co0.2Mn0.2Ni0.2Zn0.2Fe2O4 comprising six transition metal cations was successfully prepared by a solution-combustion method followed by distinct thermal treatments. The entropic stabilization of this hexa-metallic material is demonstrated using in situ high temperature powder X-ray diffraction (PXRD) and directed removal of some of the constituting elements. Thorough evaluation of the PXRD data yields sizes of coherently scattering domains in the nanometre-range. Transmission electron microscopy based methods support this finding and indicate a homogeneous distribution of the elements in the samples. The combination of 57Fe Mössbauer spectroscopy with X-ray absorption near edge spectroscopy allowed determination of the cation occupancy on the tetrahedral and octahedral sites in the cubic spinel structure. Magnetic studies show long-range magnetic exchange interactions which are of ferri- or ferromagnetic nature with an exceptionally high saturation magnetization in the range of 92–108 emu g−1 at low temperature, but also an anomaly in the hysteresis of a sample calcined at 500 °C
Исследование процесса каталитической депарафинизации дизельных топлив
Цель работы – исследование процесса каталитической депарафинизации дизельных топлив с использованием метода математического моделирования для оптимизации технологических параметров работы установки. В процессе исследования проводилась оценка влияния температуры и расхода сырья на процесс каталитической депарафинизации, а также оптимизация технологического режима реактора депарафинизации. В результате исследования определены оптимальные технологические условия работы реактора депарафинизации.The object of the research is diesel fuel catalytic dewaxing unit L-24-10/2000 Ltd. “KINEF”. The aim of the work is to study diesel fuel catalytic dewaxing process using the method of mathematical modelling in order to optimize technological parameters of the unit. In this work the influences of temperature and feedstock flow rate were evaluated, as well as technological parameters of the dewaxing reactor operation were optimized. As a result the optimal technological conditions of the reactor operation were determined
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