The influence of the overheating of the Ga–14.2 mol % In eutectic alloy melt on the presolidification cooling is studied on 2-g samples by cyclic thermal analysis. In the case of the overheating of ...the liquid eutectic above the eutectic temperature
T
e
= 288.3 by 28–30 K and subsequent cooling, the overheating dependence of the supercooling has a continuously increasing character. The limiting supercooling is ~25 K regardless of the preliminary overheating of the melt, from 30 to 100 K. The solidification from the supercooled state is found to have an “explosive” character at a rate of 40–45 K/s. The supercooling is not affected by long-term (for several hours) isothermal holding of the melt in overheated and supercooled states. The kinetic and thermodynamic parameters of the eutectic solidification are calculated using fusibility thermograms.
Abstract
Raman spectroscopy is one of the most suitable tools for studying few-layer graphene. The position of the G band and the defect-induced D and D' bands in the spectra of perfect single-layer ...graphene with sp
2
-hybridized carbon atoms and hydrogenated graphene with 27.7% sp
3
-hybridized carbon atoms are simulated using the Density Functional Theory (DFT) method with Perdew-Burke-Ernzerhof (PBE) functional. In the case of perfect graphene, the Raman G band is predicted at 1612 cm
-1
. In the case of the hydrogenated structure, a new feature appears. Namely, along with the G band, now shifted to 1591 cm
-1
, an additional feature, located at 1703 cm
-1
, is clearly seen. The latter is due to oscillations, involving six atomic benzene rings, containing two sp
3
-hybridized C atoms. According to our results, the presence of defects, related to sp
3
hybridized carbon, gives rise to the appearance of the defect D' band in the Raman spectrum of defective graphene. This study shows that it is possible to simulate Raman spectra using the DFT method, with the results qualitatively matching the experimental data.
A neutron collimator is developed to attenuate the neutron flux and reduce the residual induced activity in the interportal space of the diagnostic system of neutral particle analyzers of the ITER ...tokamak reactor. The collimator is installed in the port plug of the ITER vacuum vessel in front of an inlet to the vacuum pipeline of the diagnostic system. The collimator design has a cellular structure with 80% transparency for the neutral atom beam that goes out of the plasma and is recorded by the analyzers. However, because of increased scattering of neutrons in the collimator, their flux in the interportal room in the service zone of diagnostic systems of equatorial port no. 11 is significantly reduced and allows the equivalent dose rate in this zone to be decreased by several times. Thermal analysis showed that, during the reactor operation in modes with generation of the maximum power of 500 MW, the plasma radiation will cause the heating of the collimating grid to a temperature not exceeding 250°C, which makes it possible to select the stainless steel (316L(N)-ITER grade) as a material for manufacturing the collimator. In this case the cyclic strength of the collimator meets the ITER requirements, and it can be used without replacement during the entire deuterium-tritium experiment of the tokamak reactor.
The explosive solidification of supercooled liquids is analyzed using heating and cooling thermograms of various substances. As proof of the phenomenon of explosive solidification, thermograms in the ...temperature–time coordinates are presented for tellurium, bismuth, water, and acetic acid. A schematic cooling curve is used to demonstrate the structural changes occurring in a liquid phase and the stages of solidification, namely, the formation of crystal nuclei, their coalescence, and subsequent isothermal solidification. Explosive solidification at the second stage is characterized by a rapid temperature rise from a supercooling region to the melting temperature at the rates exceeding the cooling rate (i.e., heat removal) by 2–3 orders of magnitude. An attempt is made to explain the phenomenon of explosive solidification in terms of the well-known postulates of the cluster-coalescence model of solidification and the theory of chain reactions. The calculations demonstrate that the critical nucleus sizes are comparable with the unit cell sizes of the corresponding crystal lattices and the nucleus formation energy is comparable with the intermolecular bond energies. Based on the calculations of the heat released during nucleus formation and the interfacial surface energy released during the coalescence of crystal nuclei, we advance a hypothesis that crystalline clusters and stable crystal nuclei can serve as the “building blocks” of crystal growth along with molecules. The energy released during the formation of nuclei and their coalescence is shown to be equivalent to electromagnetic radiation quanta and to generate new centers of solidification, multiplication, and coalescence of nuclei. The calculations demonstrate that the energy released during the coalescence of many nuclei is high enough to quickly heat a substance from the supercooling region to the melting temperature. By analogy with the well-known thermal explosion diagram, we plotted and analyzed a similar diagram for the time dependence of the heat release and the heat removal during explosive solidification. The critical values of the beginning of the explosive process and the cooling rate of a liquid phase are found. The final stage of equilibrium solidification after explosive solidification is estimated.
An alternative approach to calculating the critical nucleus size and the nucleus formation energy has been developed. A significant difference between this method and the generally accepted so-called ...classical method, according to which a nucleation center begins to form at the melting temperature, is noted. The results are illustrated by the dependences of the nucleus size and the nucleus formation energy on the supercooling and by the Gibbs energy plotted as a function of the nucleus size in both the classical and the proposed versions.
Compression of a fiber array with a deuterated target mounted on its axis is studied at the Angara-5-1 facility (3.5 MA, 100 ns). Cylindrical arrays with an initial diameter of 12 mm made from ...polypropylene fibers with a diameter of 13.4 µm are used. The number of fibers varied from 30 to 120. The target with the density of 0.08–0.2 g/cm
3
and the diameter of 1 mm was made on the basis of deuterated polyethylene. A 10‑frame ultra-high-speed X-ray camera, optical slit scans, an integral X-ray pinhole camera, vacuum X-ray diodes, a crystal spectrograph, and neutron detectors are used to measure the plasma parameters in the Z‑pinch. It is found that the dynamics of plasma compression and evolution of local plasma formations, which are sources of neutrons and soft X-ray emission in the energy range of
E
> 150 eV, depend on the load configuration: the number of fibers, diameter, and density of the deuterated target. Efficient compression of the liner plasma, high concentration and temperature of the compressed target state, as well as the highest neutron yield (8 × 10
9
neutron/pulse) are observed in experiments with arrays with the fiber number of 60, inside which the target with the diameter of 1 mm and density of 0.2 g/cm
3
was placed. The electron density and temperature of the hot plasma in local formations are estimated as
n
e
≈ 10
21
cm
–3
,
T
e
≈ 1 keV, respectively. The average neutron energy was 2.6 ± 0.2 MeV. The intensity of the plasma formation
in µg/(cm
2
ns) of polypropylene fibers under the action of the discharge current of the facility is determined in experiments with fiber arrays.
A Passive Shield for the RED-100 Neutrino Detector Akimov, D. Yu; Aleksandrov, I. S.; Belov, V. A. ...
Instruments and experimental techniques (New York),
03/2021, Volume:
64, Issue:
2
Journal Article
Peer reviewed
Open access
—
A combined passive shield of the RED-100 two-phase emission neutrino detector has been developed and built for suppressing the background of external γ rays and neutrons. The shield is composed of ...a 5‑cm-thick copper layer (the inner layer is adjacent to the detector) and a water layer with a total thickness of approximately 70 cm (including the water inside the copper shield). The Monte Carlo simulation of the shielding efficiency has been performed. The obtained attenuation factor of the copper shield for the γ-ray background has been experimentally verified in a laboratory test using a NaI(Tl) scintillator detector. The γ‑ray background rejection factor of the full shield has also been calculated.
An extraordinary phenomenon of generation of coupled types of oscillations with a transverse field distribution corresponding to the ТЕМ
pq
eigenmodes with different pairs of indices
p
and
q
in the ...main laser cavity and in an external cavity has been experimentally detected. The excitation of coupled modes depends on the configuration and tuning of partial cavities. To solve the system of integral equations for a three-mirror cavity with mismatched spherical mirrors in the quasioptical approximation, it has been proposed to use modified boundary conditions including the coupling coefficients for eigenmodes in partial cavities.
—Two methods are considered for the production of wear-resistant coatings for cutting and stamping tools: plasma nitriding with subsequent oxidation; and plasma chemical vapor deposition of metals ...with preliminary nitriding. The microstructure of the modified layers, their microhardness, and the tool life are discussed.
We have analyzed the available data on the transition from equilibrium crystallization of melts of various substances to nonequilibrium explosive crystallization depending on preliminary heating of ...the liquid phase to temperature
. Analogously to the liquid-crystal state, superheating interval Δ
=
–
T
L
relative to melting temperature
T
L
is ascribed to the region of existence of a mesophase described by order parameter
Q
. A comparative analysis has been performed for the dependence of the free energy variation on the order parameter for clusters in the mesophase and on the sizes of crystalline nuclei in the metastable region below the melting point.
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