Post-Spinel Phases in the Earth’s Mantle Iskrina, A. V.; Bobrov, A. V.; Spivak, A. V.
Geochemistry international,
04/2022, Letnik:
60, Številka:
4
Journal Article
Recenzirano
Odprti dostop
—
The post-spinel phases include compounds with stoichiometry
and structures of the calcium ferrite CaFe
2
O
4
, calcium titanate CaTi
2
O
4
, and marokite CaMn
2
O
4
types. The structures with a ...centered
Cmcm
(
Bbmm
) and primitive
Pnma
(
Pmcn
) and
Pbcm
(
Pmab
) cells are distinguished in this family of topologically related compounds with a “marokite” channel formed by six octahedra. The sites A and B are occupied by various cations, in particular, Cr, Al, Mg, Fe, Ca, Ti, Fe, Na, and Si, which implies the formation of solid solutions of a wide compositional range. In nature, such high-pressure phases were found in meteorites, as inclusions in diamond crystals, and in rocks from some metamorphic complexes. This review provides a characterization of natural mineralogical finds, the results of an experimental study of post-spinel phases of various compositions and their solid solutions, as well as crystal chemical simulation and assessment of likely compositions and the areas of stability of compounds with a “marokite” channel. The discrepancy between the results of individual studies indicates the necessity to clarify the stability parameters and probable isostructural transitions, and, ultimately, to improve the classification of post-spinel phases.
This paper presents the results of studying the thermal stability of the grain structure, mechanical properties, and specific electrical resistivity of the Al-0.5 wt%Mg-xSc ultrafine-grained (UFG) ...alloys with varying scandium content (x = 0, 0.2, 0.3, 0.4, and 0.5 wt.%). The alloys were produced by induction casting and Equal-Channel Angular Pressing (ECAP). The alloys with Sc concentration above 0.3 wt% contained submicron Al3Sc primary particles, the amount of which increased with increasing Sc concentration. The decomposition kinetics of the Al–Sc solid solution in the UFG alloys was determined by the diffusion intensity along the dislocation cores in the lattice as well as at the grain boundaries. High thermal stability of the UFG structure in the alloys was ensured by ECAP at 225 оC whereas the recrystallization onset temperature was 375 оC. The UFG alloys exhibited high plasticity at the room and elevated temperatures and good combination of strength and electrical conductivity.
•Al-0.5%Mg-(0–0.5)%Sc ultrafine-grained (UFG) alloys were obtained by ECAP.•The recrystallization temperature in the UFG alloys was found to be 375 оC.•The Al3Sc particles nucleate via the grain boundaries at lower temperatures.•The Al3Sc particles nucleate via the lattice dislocation cores at higher temperatures.•The UFG alloys exhibited high strength, good plasticity and electrical conductivity.
A method for measuring cross sections at
colliders for the case where the center-of-mass frame of colliding particles moves in the laboratory frame is proposed. Within this method, the energy ...dependence of the cross section is extracted from the angular distribution of interaction products. The method applied to the
process is found to be sensitive. This method provides the possibility of studying the fine structure of the cross section near the threshold at scales much less than the energy spread of the beams used. Similar measurements may be implemented in experiments at the Super Charm-Tau Factory.
The effect of Sc on plasticity at elevated temperatures(from 300 to 500 °C) and strain rates(from 10−3 to 10−1 s−1)of ultrafine-grained (UFG) Al–0.5%Mg–Sc alloyswas investigated. In the UFG alloys ...with 0.4% and 0.5%Sc at 500 °C and strain rate 1·10−1 s−1, high elongation to failure (~1060%) was achieved. The fractographic analysis of the fracturesin the Al–0.5%Mg alloy demonstrated the crack growth zone to occupy almost whole fracture area.The self-destruction in the crack growth zone has a viscous character.In the UFG alloys with Sc, numerous pits formed as a result of micropore junction were observed. Dependencies of dynamic grain growth (DGG) rate on the temperature and strain rate were determined. The DGG in the Al–0.5%Mg–Sc alloys was found. The kinetics of the DGG in the Al–0.5%Mg–0.2%Sc alloy was shown to be limited by mobility of junction disclinations. In the Al–0.5%Mg–0.5%Sc alloy, it is limited by mobility of oriented misfit dislocations. Al3Sc particles were shown to be possible origin of the micropore formation and of accelerated destruction of the Al–0.5%Mg–Sc alloys at high temperature deformation.
•Al–0.5Mg–Sc UFG alloys have high superplastic characteristics.•A dynamic grain growth (DGG) with the exponent k=∂lg(ḋ)/∂lg(ε̇) = 0.8–1.3.•The DGG rate in the alloys with low Sc content is governed by the mobility of the joint disclinations.•The DGG rate in the alloys with high Sc content is governed by the mobility of the orientation mismatch dislocations.•A formation of pores takes place on the Al3Sc particles in the tensile state.
Mixing properties of solid solutions of the post-spinel phases with a composition of CaCr
2
O
4
–CaAl
2
O
4
, CaCr
2
O
4
–CaFe
2
O
4
, MgCr
2
O
4
–MgAl
2
O
4
, and MgCr
2
O
4
–MgFe
2
O
4
in the ...temperature range of 1873–2223 K and at a pressure of 18–25 GPa were studied by the semi-empirical modeling. These
PT
parameters were used to estimate a formation energy of impurity defects of trivalent metal ions (aluminum and iron) in isomorphic sites. The study results: (1) the studied binary solid solutions are characterized by a complete miscibility; (2) the incorporation of Fe
3+
impurity ion into the post-spinel phases of MgCr
2
O
4
and CaCr
2
O
4
is less favorable in terms of energy than that of Al
3+
ions over the entire pressure and temperature range under study. The results obtained were used to interpret compositions of the post-spinel phases forming inclusions in lower mantle diamonds.
The processes of precipitation of particles of Al
3
X (X = Zr, Yb, Er, Hf) in cast conductive aluminum alloys have been studied, including those alloyed with Mg and Si. The alloys have been produced ...using the induction casting technique. To study the precipitation kinetics of particles, methods for measuring specific electrical resistivity (SER) and microhardness have been used. It has been shown that the studied alloys can be divided into three groups. Group I contains the alloys in which an increase in the annealing temperature results in a decreases in SER due to the precipitation of particles. Group II contains the alloys in which the precipitation of particles occurs during the crystallization of an ingot. The SER value of these alloys is close to that of aluminum. The SER value of the alloys of Group III remains almost constant upon annealing and is 3.0–3.4 μΩ cm, which indicates the high stability of the solid solution. The Johnson–Mehl–Avrami–Kolmogorov equation is used to study the precipitation kinetics of particles in alloys of Group I. It has been established that the activation energy of the precipitation of particles in alloys of Group I is close to the activation energy of bulk diffusion, while the values of the decomposition rate coefficient (
n
= 0.5–0.8) in the Johnson–Mehl–Avrami–Kolmogorov equation are lower than the theoretical value
n
= 1.5 typical of the precipitation of particles in the bulk of grains. The observed contradiction is associated with the presence of large primary or eutectic Al
3
X particles in the structure of alloys. It has been shown that the Al–0.25% Zr–0.25% Er–0.15% Si alloy exhibits an appropriate set of properties; after annealing their characteristics meet the requirements for the designed alloys: SER is less than 2.95 μΩ cm and the microhardness is ~550 MPa.
Experiments at the Super Charm-Tau factory Achasov, Mikhail N.; Blinov, Vladimir E.; Bobrov, Alexander V. ...
Physics Uspekhi,
01/2024, Letnik:
67, Številka:
1
Journal Article
In this Letter we discuss our approach that makes possible creation of the steady-state ultracold plasma having various densities and temperatures by means of continuous two-step optical excitation ...of calcium atoms in the magneto-optical trap. A strongly coupled ultracold plasma can be used as an excellent test platform for studying many-body interactions associated with various plasma phenomena. The parameters of the plasma are studied using laser-induced fluorescence of calcium ions. The experimental results are well described by a simple theoretical model involving equilibration of the continuous source of charged particles by the hydrodynamical ion outflux and three-body recombination. The ultracold plasma with the peak ion density of 2.7×10^{6} cm^{-3} and the minimum electron temperature near 2 K has been prepared. Our steady-state approach in combination with the strong magnetic confinement of the plasma will make it possible to reach extremely strong coupling in such system.
This paper highlights advances made using the 4-bora-3a,4a-diaza-s-indacene (BODIPY) as a fluorophore in design and application of fluorescent sensors for microenvironment polarity. Sections of the ...paper cover broad analysis of a range of fluorescent indicators immobilized in ethyl- and methyl cellulose matrices. The present study demonstrates that BODIPY-based fluorescent materials could be successfully utilized for ratiometric detection of ethanol and acetone in gas phase. The achieved limit of detection value equals 0.02 mg/ml for acetone and 0.08 mg/ml for ethanol, whereas obtained sensoric materials are reusable without regeneration required.
