Amorphous carbon (AC) is present in the bulk and on the surface of nanostructured carbon materials (NCMs) and exerts a significant effect on the physical, chemical and mechanical properties of NCMs. ...Thus, the determination of AC in NCMs is extremely important for controlling the properties of a wide range of materials. In this work, a comparative study of the effect of heat treatment on the structure and content of amorphous carbon in deposited AC film, nanodiamonds, carbon black and multiwalled carbon nanotube samples was carried out by TEM, XPS, XRD and Raman spectroscopy. It has been established that the use of the 7-peak model for fitting the Raman spectra makes it possible not only to isolate the contribution of the modes of amorphous carbon but also to improve the accuracy of fitting the fundamental G and D
(D) modes and obtain a satisfactory convergence between XPS and Raman spectroscopy. The use of this model for fitting the Raman spectra of deposited AC film, ND, CB and MWCNT films demonstrated its validity and effectiveness for investigating the amorphous carbon in various carbon systems and its applicability in comparative studies of other NCMs.
Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order
. Unlike their nonmagnetic counterparts, magnetic topological ...insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics
, such as the quantum anomalous Hall effect
and chiral Majorana fermions
. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic
and electronic
properties of these materials, restricting the observation of important effects to very low temperatures
. An intrinsic magnetic topological insulator-a stoichiometric well ordered magnetic compound-could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi
Te
. The antiferromagnetic ordering that MnBi
Te
shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ
topological classification; ℤ
= 1 for MnBi
Te
, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi
Te
exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling
and axion electrodynamics
. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect
and chiral Majorana fermions
.
The U–Th–Pb age (SIMS) of magmatic (
T
= 730–744°C) zircon from intermediate and felsic metavolcanics (1923–1926 Ma) of the supracrustal complex in the Kaskama block of the Inari terrane ...(northwestern part of the Kola–Norwegian region of the Fennoscandian Shield) were obtained for the first time. This makes it possible to attribute these metavolcanics to the Paleoproterozoic Kalevian suprahorizon. The source of primary metarhyodacite and metabasalt in the Kaskama block was the Paleoproterozoic continental lithosphere not younger than 2390–2384 Ma with an essential contribution of juvenile material (εNd(
Т
) = +1.2…+2.8). The ages of the spatially related tonalites of the Kuroaivi massif (1936 ± 7 Ma, εNd (
T
) = +0.1) and of the granitoid massifs of the Southern Pechenga zone (1950–1940 Ma) are older, within the errors of the U–Th–Pb age determinations, than the volcanosedimentary deposits of the Kaskama unit and the Vepsian of the South Pechenga zone. This stage of granite formation separates two geodynamic stages in the Kola–Norwegian region of the Fennoscandian Shield: the early continental rifting in the Yatulian–Ludikovian time and a stage similar to the suprasubductional formation setting of the continental lithosphere of the Inari terrane (1926–1850 Ma).
We present a systematic analysis of the spatio-temporal evolution of sources of hard X-ray (HXR) pulsations in solar flares. We concentrate on disk flares whose impulsive phases are accompanied by a ...series of more than three successive peaks (pulsations) of HXR emission detected in the RHESSI 50 – 100 keV energy channel with a four-second time cadence. Twenty-nine such flares observed from February 2002 to June 2015 with characteristic time differences between successive peaks
P
≈
8
–
270
s
are studied. The main observational result of the analysis is that sources of HXR pulsations in all flares are not stationary, they demonstrate apparent movements or displacements in the parent active regions from pulsation to pulsation. The flares can be subdivided into two main groups depending on the character of the dynamics of the HXR sources. Group 1 consists of 16 flares (
55
%
) that show systematic dynamics of the HXR sources from pulsation to pulsation with respect to a magnetic polarity inversion line (MPIL), which has a simple extended trace on the photosphere. Group 2 consists of 13 flares (
45
%
) that show more chaotic displacements of the HXR sources with respect to an MPIL with a more complex structure, and sometimes several MPILs are present in the parent active regions of such flares. Based on the observations, we conclude that the mechanism of the flare HXR pulsations (at least with time differences of the considered range) is related to successive triggering of the flare energy release process in different magnetic loops (or bundles of loops) of the parent active regions. Group 1 flare regions consist of loops stacked into magnetic arcades that are extended along MPILs. Group 2 flare regions have more complex magnetic structures, and the loops are arranged more chaotically and randomly there. We also found that at least 14 (
88
%
) group 1 flares and 11 (
85
%
) group 2 flares are accompanied by coronal mass ejections (CMEs),
i.e.
the absolute majority of the flares we studied are eruptive events. This gives a strong indication that eruptive processes play an important role in the generation of HXR pulsations in flares. We suggest that an erupting flux rope can act as a trigger of the flare energy release. Its successive interaction with different loops of a parent active region can lead to apparent motion of HXR sources and to a series of HXR pulsations. However, the exact mechanism responsible for generating the pulsations remains unclear and requires a more detailed investigation.
The methodical basis, development, and current state of a new method of chronostratigraphic studies, i.e., strontium isotope stratigraphy (SIS), are considered. This method makes it possible to date ...and correlate geographically distant sedimentary sequences without involving the biostratigraphic and isotope geochronological data. SIS is based on secular variations in
87
Sr/
86
Sr in the paleocean, resulting from the redistribution of the roles of two global strontium flows formed in the mantle and continental reservoirs of the Earth. Isotopic homogeneity of Sr in the paleoceans and in the linked seas leads to the fact that the
87
Sr/
86
Sr ratio in the sea basins is individual for each geological time point and is inherited in marine chemogenic sediments under deposition of dissolved Sr as an isomorphic impurity. Low-Mg calcite and also fragments of fossilized paleontological remains buried in situ are the best minerals that are capable of retaining the Sr isotopic signature of the sedimentation environment. SIS is carried out with geochemical diagnostics of secondary alteration of the studied material and selective dissolution of the samples to produce a carbonate material that adequately reflects isotopic signature of the sedimentary basin. Interregional correlations of the Proterozoic and Cenozoic sea sediments and their relation to the SIS-based stratigraphic scale are given as an example.
The evolution of opinions about the isotope age of the general stratigraphic subdivisions of the Upper Proterozoic in Russia (Lower, Middle, Upper Riphean and Vendian) during the second half of the ...20th century and the beginning of the 21st century is considered, and the current data on the age of these subdivisions are analyzed in detail. It is shown that the isotope age of the lower boundaries of the Lower, Middle, and Upper Riphean and Vendian should be estimated at 1750 ± 50, 1400, 1030, and 640 ± 5 Ma, respectively. The role of the historical–geological criteria in the substantiation of these boundaries is studied.
The magnetocaloric effect in nanosystems based on exchange-coupled ferromagnets with different Curie temperatures is calculated within the mean-field theory. Good agreement between the results of the ...mean-field theory and the Landau theory, valid near the critical phase transition temperature, is demonstrated for a flat-layered Fe/Gd/Fe structure. We show that a high magnetic cooling efficiency in this system is attainable in principle and prove the validity of the Maxwell relation, enabling an experimental verification of the predictions made. The theory developed for flat-layered structures is generalized to a granular medium.
Published and original data on the Sr isotopic characterization of carbonates from the Riphean and Vendian key sections of the Southern Urals, Siberia, Asia, Africa, Australia, and North America are ...considered in compliance with the suggested principles of reconstructing the Sr isotopic composition of the Proterozoic seawater. The suggested methodic approach is used to plot the reference curve of the
87
Sr/
86
Sr variations in the Riphean and Vendian oceans. During the time span of 1600–1250 Ma, the
87
Sr/
86
Sr variations were in a narrow range corresponding to 0.70456–0.70494, but approaching the date of about 1030 Ma, the
87
Sr/
86
Sr ratio rose to 0.70601–0.70611 and then quickly declined to 0.70519–0.70523 near the date of 1000 Ma. In the second half of the late Riphean and in the Vendian, the ratio grew almost steadily from 0.70521–0.70535 to values of 0.70874–0.70885 characteristic of the Late Vendian time. The subsequent regular growth of that ratio in seawater lasted from 840 to 550 Ma, though there were short-term epochs when the ratio noticeably dropped to 0.70561–0.70575 at approximately 760 Ma and to 0.70533–0.70538 at 670–660 Ma. After the mid-Late Vendian maximum, it declined to 0.70812–0.70823 at the end of the Nemakit-Daldynian Age and decreased to 0.70806–0.70812 during the Tommotian Age of the Early Cambrian. As is shown, the Sr isotopic variations in the Riphean and Vendian oceans were interrelated with global tectonic events in geospheres and formation stages of the Rodinia and Gondwana supercontinents. The Baikalian Complex of Siberia is considered in the work as a case in point illustrating advantages of the expounded approach with respect to age substantiation of particular stratigraphic subdivisions.
This review highlights recent research on the application of core–shell structured materials as catalysts in the oxidation of alcohols to value-added products, such as benzaldehyde, acetophenone, ...benzophenone, cinnamaldehyde, and vanillin, among others. While the application of various unconventional energy inputs (such as microwave and ultrasound irradiation) was reported, this paper focuses on conventional heating. The oxidation of homocyclic aromatic, heterocyclic aromatic, aliphatic, and alicyclic alcohols catalyzed by core–shell composite catalysts is addressed. This work also highlights some unique advantages of core–shell nanomaterial catalysis, namely the flexibility of combining individual functions for specific purposes as well as the effect of various parameters on the catalytic performance of these materials.
PDF
