Thermal conduction in a mirror-unstable plasma Komarov, S. V; Churazov, E. M; Kunz, M. W ...
Monthly notices of the Royal Astronomical Society,
07/2016, Letnik:
460, Številka:
1
Journal Article
Recenzirano
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The plasma of galaxy clusters is subject to firehose and mirror instabilities at scales of order the ion Larmor radius. The mirror instability generates fluctuations of magnetic-field strength ...δB/B ∼ 1. These fluctuations act as magnetic traps for the heat-conducting electrons, suppressing their transport. We calculate the effective parallel thermal conductivity in the ICM in the presence of the mirror fluctuations for different stages of the evolution of the instability. The mirror fluctuations are limited in amplitude by the maximum and minimum values of the field strength, with no large deviations from the mean value. This key property leads to a finite suppression of thermal conduction at large scales. We find suppression down to ≈0.2 of the Spitzer value for the secular phase of the perturbations’ growth, and ≈0.3 for their saturated phase. The effect operates in addition to other suppression mechanisms and independently of them. Globally, fluctuations δB/B ∼ 1 can be present on much larger scales, of the order of the scale of turbulent motions. However, we do not expect large suppression of thermal conduction by these, because their scale is considerably larger than the collisional mean free path of the ICM electrons. The obtained suppression of thermal conduction by a factor of ∼5 appears to be characteristic and potentially universal for a weakly collisional mirror-unstable plasma.
The effect of equal-channel angular pressing on structural phase transformations and mechanical properties of the as-cast hypoeutectic Al–4Ca–1.3Fe–0.8Mn–0.1Zr–0.1Sc alloy has been studied. ECAP was ...carried out under isothermal conditions at a temperature of 200 °C, with a channel intersection angle of 110º, using the BC route and the number of passes N = 6. After ECAP, a substructure with an increased dislocation density has developed in the alloy. ECAP led, firstly, to the evolution of the dendritic structure (dendrites elongate and merge with each other), and secondly, to the refinement of the Al4Ca, Al4Ca(Fe) and Al3−x(Fe, Mn) eutectic phase particles to nano- and submicron scale, their mixing and partial segregation of calcium. The tensile tests of the ECAP processed alloy showed that its strength increased, while its ductility decreased. The yield point increased from 122 to 257 MPa, the ultimate tensile strength increased from 174 to 281–314 MPa, and the relative elongation decreased from 6 to 1–3%. After ECAP, the strength is higher, and the relative elongation is lower for longitudinal specimens compared to transverse specimens.
•The as-cast Al-4Ca-1.3Fe-0.8Mn-0.1Zr-0.1Sc hypoeutectic alloy was processed by ECAP at 200 °C.•The elongation of the (Al)-dendrites along the pressing direction and their merging occurs.•The refinement of the eutectic phase particles to nano- and submicron scale occurs.•The eutectic phase particles mixing and partial Ca segregation occurs as a result of mass transfer.•The 1.7–2.2-fold increase in the alloy’s strength while maintaining moderate ductility occurs.
The effect of pre-hardening of steel substrate on mechanical and tribological properties of TiN and TiAlN coating–steel substrate composites was studied for various treatment conditions. We have ...found that prior nitrocarburization of the steel substrate increases the microhardness by 7 times, the wear resistance of the working surface of the composite up to 2.3 times, the resilience of the composite by 4.5 times. It is thus feasible to produce thinner coatings in composite layered systems with no drawbacks regarding performance. We also suggest a method to determine wear resistance of the deposited coatings, as well as to estimate the influence of the hardness gradient and contribution of all layers to overall durability of such layered systems.
•The nitrocarburization enables to increase the microhardness by 7 times.•The thermochemical treatment enables to increase the wear resistance up to 2.3 times.•A new method for determining wear resistance of the deposited coatings is proposed.•The non-additive effect of nitrocarburization and PVD coating deposition was discovered.
The features of the magnetic state of an array of parallel oriented permalloy ribbons are discussed. The arrays are made by explosive lithography. The ribbons have a thickness of 180 nm, a width of ...2.8 µm, and a length of about 4 mm. The distance between ribbons in different samples varies in the range from 300 nm to 4 μm. It is found that the ribbons in the regions far from the end faces are in a single-domain state with small-angle ripples, the magnetization distribution of which correlates with inhomogeneities of the ribbon side surfaces. Moreover, there is a distinct relationship in the spatial distribution of the ripples between adjacent ribbons with a relatively small distance between them. This makes it possible to evaluate the role of the magnetostatic coupling of magnetic subsystems of array elements and to estimate the characteristic value of the random stray field that pins the magnetization.
Al/CNT coatings were deposited on Al substrates by a ball impact treatment method using balls of 3, 4 and 8 mm in diameter and treatment time of 1–4 h under ambient pressure and air atmosphere ...conditions. The morphology and structure of fabricated coatings were investigated by scanning and transmission electron microscopy. Also, the obtained coatings were characterized for microhardness and wear resistance. The results revealed that smaller balls yield better characteristics of coatings as compared with larger balls. The best condition was found to be 3-mm grinding ball and 3-h treatment. It was found that the thickness of hardened layer is about 150 μm for all treatment conditions examined. The Al/CNT coated samples demonstrated a better tribological behavior in comparison with the uncoated Al substrate.
•A novel method of plating of Al/CNT composite coatings on Al substrates is proposed.•Obtained coatings have microhardness ranging from 100 to 125 HV.•The maximum thickness of Al/CNT coatings is 150 μm.•Coatings deposition by ball impact treatment technique is simple and does not require any special preparation of samples.
Abstract
Using a constant volume piezometer and ultrasonic interferometer methods, the density and speed of sound in gaseous mixture R-125 (31 wt. %) + R-134a (69 wt. %) were measured within the ...temperature range from 293 to 393 K and at pressure from 0.18…0.47 to 2.5 MPa. The errors in the measuring temperature, pressure, density and speed of sound were ±20 mK, ±4 kPa, ± (0.15–0.3) %, ± (0.1–0.2) %, respectively. It was shown that the speed of sound values increase with temperature and decrease with pressure. The obtained results were compared with the calculations using the REFPROP software.
X-ray observations of hot gas in galaxy clusters often show steeper temperature gradients across cold fronts - contact discontinuities, driven by the differential gas motions. These sharp (a few kpc ...wide) surface brightness/temperature discontinuities would be quickly smeared out by the electron thermal conduction in unmagnetized plasma, suggesting significant suppression of the heat flow across the discontinuities. In fact, the character of the gas flow near cold fronts is favourable for suppression of conduction by aligning magnetic field lines along the discontinuities. We argue that a similar mechanism is operating in the bulk of the gas. Generic 3D random isotropic and incompressible motions increase the temperature gradients (in some places) and at the same time suppress the local conduction by aligning the magnetic field lines perpendicular to the local
temperature gradient. We show that the suppression of the effective conductivity in the bulk of the gas can be linked to the increase of the frozen magnetic field energy density. On average the rate of decay of the temperature fluctuations d〈δT
2〉/dt decreases as 〈B
2〉−1/5.
Comparative studies of the functional and mechanical characteristics of an alloy Ti−50.8 at % Ni with various initial structural states, as well as after aging at 430°C, 10 h have been carried out; ...the specific features of evolution of studied parameters using tensile tests in the temperature range of −196 ≤
Т
≤ +100°С were analyzed. The highest value of the strength characteristics (up to 1500 MPa), as well as the maximum difference between the dislocation yield stress and transformation yield stress (up to 1200 MPa) occurs after aging of an alloy with the initial developed dislocation substructure and amorphized volume of ~30%. The efficiency of aging of a material of the initially recrystallized structure depends on the grain size: the functional and mechanical characteristics of the fine-grain material almost do not change; in the initially coarse-grained material the studied characteristics increase significantly. The obtained regularities can be used for prediction of the set of functional and mechanical characteristics of titanium nickelide as a result of aging with various initial structures.
The galvanomagnetic characteristics of SmB
6
single crystals are studied within the temperature range of 1.9‒3.6 K at different orientations of the crystal faces. As a result, the electrical ...resistivities of the surfaces corresponding to the (100), (110), (111), and (211) crystallographic planes are determined. It is shown that the effective parameters of charge carriers, which determine the surface conductivity in SmB
6
, depend both on the orientation of the surface and on the method of its processing. It is found that the etching of polished polar surfaces formed by (100) planes leads at 1.9 K to a decrease in the density and to an increase in the mobility of surface
n
-type charge carriers from 113/
a
2
and 1.12 cm
2
/(V s) to 0.76/
a
2
and 18 cm
2
/(V s), respectively (lattice parameter
a
≈ 4.134 Å). For etched nonpolar surfaces corresponding to the (110) and (111) planes, the maximum density of surface charge carriers (per unit area of the surface Brillouin zone) is found to increase by factors of 2.3 and 3.9, respectively. It is proposed to use this parameter as a simple criterion to identify the features of electron transport due to the nontrivial topology of the band structure of SmB
6
.
The microstructure, morphology, micro hardness and electrochemical properties of the deposited Ni-Al coating on the surface of hypoeutectoid steel, Nickle, Aluminum and Titanium substrates were ...analyzed by scanning electron microscopy, X-ray diffraction, micro hardness and electrochemical analysis. In this study the Ni-Al coating was produced by mechanical alloying method using a self-constructed vibratory ball mill with subsequent laser treatment. The approximate thickness of the coating was about 100 μm. The conditions of laser treatment, allowing to obtain Ni-Al intermetallic coating on the pre-alloyed by mechanical cladding substrates, were determined.
•Ni-Al intermetallic coatings were obtained by mechanical alloying (MA) and laser treatment.•The optimal regime of laser treatment of the obtained MA coating was determinated.•Process conditions of the MA method and laser treatment not involve the overheating of the substrate.•Synthesized NiAl intermetallic coating has relatively good electrochemical properties.
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