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Experimental data on the diffusion combustion of a round hydrogen microjet in a concurrent coaxial flow are presented. The effects on the combustion of a concurrent air flow and an air flow premixed with nanopowder of TiO2 are of interest. The hydrogen microjet emanates from a round micronozzle, which is surrounded by a coaxial slit to produce the concurrent flow. Combustion events found in these conditions are similar to those observed in the previous studies on the diffusion combustion of hydrogen microjets at subsonic and supersonic velocities. In a subsonic range, the so-called “bottleneck-flame region” is generated close to the nozzle exit, while in high-speed conditions, the flame separates from the nozzle. At increasing velocity of both the hydrogen microjet and the concurrent flow, the “bottleneck-flame region” is still found and the combustion becomes more intense. The “bottleneck-flame region” is suppressed at the microjet velocity approaching transonic values.

This paper presents experimental data on the effect of inert (He, N2, Ar, CO2) and reactive (СН4, O2, CF3Br, (CH3O)3PO) gas additives to hydrogen and ambient air on flame blow-off during diffusion combustion of a hydrogen microjet emanating from a micronozzle into air. It is found that the addition of the above gases to hydrogen narrows the microjet velocity range of stable combustion of hydrogen. For hydrogen with the additives studied, the critical velocity of flame blow-off depends primarily on the mean molecular weight of the combustible mixture. When the gases are added to the air surrounding the diffusion flame, the critical velocity of flame blow-off is determined by their inhibiting effect on the chemical reactions of hydrogen oxidation. The results can be used to determine the minimum concentrations of flame-suppressing additives required to prevent the ignition and combustion of hydrogen in its accidental releases.

Experimental data on the phenomenon of nozzle choking at diffusion combustion of a high-speed hydrogen microjet at its ignition close to the nozzle are presented. As is found, such a phenomenon is due to the nozzle heating by the «bottleneck flame region» which is generated at the origin of microjet. This flow region persists up to transonic velocities of the microjet preventing from cooling of the nozzle and the transition to supersonic speed. In the case of hydrogen ignition far from the nozzle exit in supersonic conditions, the «bottleneck flame region» is suppressed, the flame becomes detached from the nozzle which is no longer heated so that the supersonic range is attained. The subsonic combustion of hydrogen microjet is stabilized by the «bottleneck flame region» while the supersonic one becomes more stable at the generation of shock cells. The results of the present study provide new details on the combustion of hydrogen microjets and could by useful for the operation of different burners.

The objective of this work is an experimental study of the influence of initial and boundary conditions at the nozzle exit upon diffusion combustion of a hydrogen microjet. It is found that the initial mean velocity profile and the presence (or absence) of a material with large heat capacity surrounding the nozzle exit may have a pronounced effect on the flame structure and combustion of the round hydrogen microjet. The rates of fuel consumption (i.e., the efflux velocities of hydrogen) providing diffusion combustion of the round hydrogen microjet, the flame detachment, and the origination of the «bottleneck flame» in the cases of a top-hat and a parabolic mean velocity profiles at the nozzle exit are determined. The variations of the extent of «bottleneck flame» with the hydrogen flow rate are made clear. Also we examine the diminution of the extent of «bottleneck flame» with the growth of hydrogen flow rate in three cases of initial conditions at the nozzle exit.

In the second part of the present study, experimental results on the round and plane hydrogen microjets diffusion combustion at various nozzle diameter depending on microjet velocity are shown. Of special interest is the contribution to the combustion process of the so-called «bottleneck flame» region which is observed at diffusion combustion of hydrogen both in the round and plane microjets. The «bottleneck flame» appears as a spherical closed volume of laminar combustion at mixing of the hydrogen microjet with ambient air. This spherical volume is surrounded by a narrow layer of density gradient overcoming which the laminar microjet and its flame become turbulent. A diminution of the «bottleneck flame» with growth of the microjet velocity is revealed.

In the first part of this study, experimental (hot–wire and visualization) results on the effect of transverse acoustic oscillations on characteristics of round and plane air microjets at low Reynolds numbers and on their propane and hydrogen combustion are presented. In both cases, the transverse acoustic excitation results in the flame bifurcation. A phenomenon observed at the nozzle exit is the so-called «bottleneck flame». This is found both at acoustic excitation and without it at combustion of hydrogen. More details on this phenomenon are given in the second part of the study titled as: “Features of diffusion combustion of hydrogen in round and plane high-speed microjets (Part II)”.

Results of experimental studies on round and plane, macro- and microjets subjected to transverse acoustic field at low Reynolds numbers are presented. A new phenomenon associated with transformation of the round microjet to the plane one under acoustic forcing is revealed. Also, bifurcation of the round microjet is observed. It is shown that both plane macro- and microjets are prone to a sinusoidal instability. As is found, the plane microjet becomes twisted at its periphery in the direction of the oscillatory flow velocity induced by acoustic waves. Acoustic influence both upon the pseudo-plane and the planemicrojets results in their sinusoidal oscillations and bifurcation. New phenomena observed in the present experiments on the round and plane microjets are caused by a proportionality of the acoustic energy with that of the microjets.

Charged particle motion in reconnecting current sheets (CS) can be both regular and chaotic, depending on the values of transverse (perpendicular to the CS plane) and longitudinal (parallel to the electric field inside the CS) components of the magnetic field. The non-zero transverse field gives rise to chaos, whereas a sufficiently large longitudinal field tends to stabilize the motion. The longitudinal field change in time may be the cause of different regimes of electron acceleration in solar flares and magnetospheric substorms.

An improved method for hot pressing of high temperature superconducting (HTSC) powders prepared by conventional solid phase synthesis of the initial BaCO3, CuO and Ln2O3 allows one to obtain HTSC targets and magnetic shields possessing a high degree of homogeneity. By sputtering such targets HTSC films with critical current density of 3.3·106 A/cm2 have been deposited. Shields, prepared according to this method, show a shielding coefficient of 105 andHc of 79 Oe in the constant magnetic field and in the alternating magnetic field the amplitude is 90 Oe in the frequency range of 70–3000 Hz.

This chapter is concerned with the application of the spectroscopy of magnetic crystals. The value of the spin wave energy is estimated, and particular crystal magnetic structures are studied on the basis of the crystals light absorption spectra.

The reduction of the space-temporal symmetry of the medium during magnetic ordering leads to qualitative changes in its physical properties. For example, there are well-known effects such as piezomagnetism and the magnetoelectric effect [1] and some galvano- and thermomagnetic effects [2], forbidden in magnetically nonordered crystals and allowed in ordered crystals with a definite magnetic symmetry. The majority of optical properties of magnetically ordered media, which appear during magnetic ordering, are similar to those of magnetically nonordered media magnetized by an external magnetic field. For example, the spontaneous magnetic rotation of the plane of polarization in a ferromagnet is proportional to the magnetic moment, in the same way as the Faraday rotation in a paramagnet, induced by a field, is proportional to its magnetization. Similar correspondence can be found between the spontaneous and field-induced quadratic effects of magnetic linear birefringence. Spontaneous magnetic gyrotropic birefringence is possible in antiferromagnetic crystals of some classes, i.e., the birefringence of linearly polarized light, which changes sign when the direction of light propagation or the directions of elementary magnetic moments of the crystal are reversed [3–9]. Similar to this is the effect of induced gyrotropic birefringence in non-magnetic crystals found in non-centrosymmetric crystals in the presence of a magnetic field [9–11]. The sublattice structure of ferri- and antiferromagnetsis not an obstacle to this analogy. The optical properties of a multisublattice magnet can often be represented as a superposition of the properties of mutually penetrating magnetic subsystems.

The present chapter is concerned with crystals characterized by a small amount of a substitutional impurity of concentration c << 1. These crystals are of great interest both theoretically and experimentally. In particular, they are a suitable subject for the analysis of localized and delocalized impurity states which are fundamental for a number of physical effects. For the most part, we shall concentrate on antiferromagnets (AFM), as the major body of theoretical and experimental work has been done for this type of crystal.

The spontaneous Faraday rotation and the Cotton-Mouton magneto-optical effect discovered in relatively transparent iron garnets, and then in other ferro and ferrimagnets, have opened a remarkable possibility to observe magnetic domains directly by human eye, thus studying the variety of domain structures and analyzing structural changes induced by a magnetic field. Visual magneto-optical methods have played a highly essential part in the marking and refinement of data recording, data processing, and data transmission techniques using the properties of cylindrical magnetic domains. The new magneto-optical effects, for the first time, provide means to distinguish visually between collinear, 180-degree, or time-reversed antiferromagnetic (AFM) domains. The magneto-optical methods for analyzing AFM domain structures and the AFM domain wall are at their earliest stage of development, with the possibilities far from being exhausted. The special sensitivity of the new effects to the symmetry of the crystal magnetic structure can be used in test experiments aimed at specifying the magnetic symmetry of an antiferromagnet. For example, the linear magneto-optic effect (LMOE) or the quadratic magnetic rotation (QMR) can often provide an unambiguous answer to such questions as to whether the chemical and magnetic crystal unit cells coincide, or whether the crystal possesses an anti-inversion center. The new magneto-optical effects also prevent new possobilities for investigating multiphase magnetic states at phase transitions.

Nonlinear varicose instability of a streaky structure is experimentally investigated in a wing boundary layer. Spatial evolution of the streaky structure perturbed by high-frequency traveling waves is compared at zero and adverse streamwise pressure gradients. The process of streak multiplication and generation of ?-shaped vortices is under examination. Results obtained testify to the strong influence of pressure gradient upon the breakdown of the streaky structure due to its varicose instability.

The action of volume magnetic forces markedly surpassing volume gravitational forces on a disperse ferromagnetic system strongly intensifies the densification of the powder during sintering. The substantial improvement in physical properties exhibited by porous specimens after magnetic sintering indicates a higher degree of perfection of interparticle contacts, resulting from the intensification of densification achieved during sintering in a magnetic field, and, possibly, an anisotropic arrangement of powder in a magnetic field.

A material with a well-developed surface is characterized by a narrowing of its AAPD curve and by the appearance of a narrow component in the small-angle region. The positron annihilation parameters of nickel powder compacts are greater than those of cast and worked nickel and grow with increasing degree of porosity, which is evidence for preferential positron localization on the pore surfaces. Using experimental data and a model of positron trapping by defects, estimates were made of the effective pore size¯Rv and pore concentration Cv as functions of the degree of porosity.

1. X-ray diffraction and calorimetric data show that the recrystallization of nickel sintered at 1200°C and deformed is no different from that of cast and deformed nickel of the same purity. 2. The thermal effects observed during the heating of compacts from carbonyl and reduced nickel powders are much more pronounced than those characterizing the heating of deformed nickel. Apart from this, powder compacts exhibit high-temperature regions of heat evolution which are totally absent in the case of deformed nickel. 3. From x-ray diffraction data, it follows that, with compacts, slower completion of recrystallization processes is observed in the temperature range 600–1100°C, i.e., after primary recrystallization, as revealed by calorimetric data.

1. A temperature of 1250°C and a deformation rate of 6. 10−5 sec−1 constitute conditions under which superductility manifests itself in sintered nickel containing dispersed ZrO2 inclusions. The factor m characterizing the sensitivity of yield stress to deformation rate then assumes the value 0.34. 2. Thermomechanical treatment of sintered carbonyl nickel and extruded dispersion-hardened nickel leads to loss of the superplastic deformability exhibited under certain conditions by the same materials in the undeformed sintered condition.

The optical “double” transition spectrum of crystalline oxygen is studied in the frequency range of 15000 to 31000 cm−1 at different temperatures (5 to 27 °K) and nitrogen concentrations (0 to 40%). A sharp integral intensity decrease of the absorption bands, their shift, and broadening are found in the temperature range investigated. With the insertion of nitrogen into oxygen new absorption bands with spectral positions close to the pure oxygen band frequencies appeared in the oxygen spectrum. The results show that the “double” transitions in oxygen are considerably induced by exchange interaction. It is also shown that the O−0 band intensity of the 23 Σ 21Δg and 23Σ 21 Σ transition is determined by the interaction not with acoustic phonons, as it was considered before, but with magnons corresponding to the Brillouin zone boundary and having an energy equal to the exchange one gμBHE. The O−0 band of the 23Σ 1Δg + 1Σ transition is interpreted as a pure electronic band. A consistent scheme, based on considering the exchange splitting of the 3Σ ground state of the oxygen molecules and the magnon excitation at light absorption is suggested to explain the “double” transition structures and their band behaviours at magnetic ordering. Das optische „Doppel” ubergangsspektrum von kristallinem Sauerstoff wird bei verschiedenen Temperaturen (5 bis 27°K) und Stickstoffkonzentrationen (0 bis 40%) im Frequenzbereich 15000 bis 31000 cm−1 untersucht. Ein starker Gesamtintensitatsabfall der Absorptionsbanden und deren Verschiebung und Verbreiterung wurden im untersuchten Temperaturbereich gefunden. Mit dem Einbau von Stickstoff in Sauerstoff treten im Sauerstoffspektrumm neue Absorptionsbanden in der Nahe der reinen Sauerstoffbanden auf. Die Ergebnisse zeigen, das die „Doppelubergange” im Sauerstoff durch Austauschwechselwirkung merklich induziert werden. Es wird ebenfalls gezeigt, das die Intensitat der O−0-Bande des (23Σ 21Δg)- und (23Σ 21Σ)-Ubergangs nicht durch Wechselwirkung mit akustischen Phononen, wie bisher angenommen, sondern durch Wechselwirkung mit Magnonen bestimmt wird, die den Grenzen der Brillouinzone entsprechen und eine der Austauschwechselwirkung gleiche Energie g μBHE besitzen. Die O−0-Bande des (23Σ 1Δg + 1Σ)-Ubergangs wird als reine Elektronenbande interpretiert. Ein konsistentes Niveaudiagramm, das auf der Berucksichtigung der Austauschaufspaltung des 3Σ-Grundzustands der Sauerstoffmolekule und der Magnonenanregung bei Lichtabsorption basiert, wird zur Erklarung der Strukturen der „Doppelubergange” und ihres Bandenverhaltens bei magnetischen Ordnungsprozessen vorgeschlagen.

1. Tensile tests were carried out on fine- and coarse-grained sintered nickel specimens. The value of yield stressσy for the fine-grained nickel was an order of magnitude less than that for the coarse-grained material. 2. With the fine-grained sintered nickel, superplastic deformation occurred at a temperature of 0.6 Tm and a relative rate of tensile deformation of 6·10−5 sec−1. Under these conditions, the factor of yield stress sensitivity to deformation rate was m=0.88. 3. The phenomena observed in this work can provide a basis for a judicious selection of conditions for the hot plastic working of such materials.

1. The chemical composition of steel 4Kh2GSVMF ensures good workability in all stages of metallurgical preparation and mechanical and heat treatment. 2. As compared with standard steel 3Kh2V8F, steel 4Kh2GSVMF has higher heat resistance, hardability, plasticity, ductility, thermal stability at working temperatures, and has a lower quenching temperature. 3. This steel is recommended for severely heated extrusion tools (punches, dies, etc.). 4. The heat treatment of the steel consists of quenching from 970–1000°C in oil and tempering at 580–600°C to a hardness HRC 46-50.

The optical Faraday effect, magnetization, and light absorption spectrum in antiferromagnetic ferrous carbonate have been studied in detail in pulsed magnetic fields up to 300 kOe at 4.2–40°K.

1. A reducing atmosphere appreciably lowers, compared with a neutral atmosphere, the threshold of primary recrystallization in carbonyl and reduced nickel specimens. 2. The presence of stable foreign particles in dispersion-strengthened nickel inhibits primary recrystallization irrespective of the type of atmosphere in which sintering is performed.

1. At any given degree of plastic deformation the amount of energy stored is much greater in nickel containing 2 wt.% of dispersed ZrO2 inclusions that in pure nickel. 2. Dispersed zirconia inclusions generally delay the completion of recrystallization processes, as revealed by calorimetric measurements, by 150°C compared with recrystallization in pure nickel. 3. The positions and widths of the recovery and recrystallization temperature ranges for nickel containing dispersed ZrO2 inclusions and the amounts of heat evolved in these temperature ranges are strongly affected by the final size of the strengthening particles.

Optimum conditions for the synthesis of LnBa2Cu3O7−x and LnBa7−xSrxCuxO7−x ceramics, where Ln=La, Y, Yb, Tm, Gd have been found. The synthesis and decomposition processes are multistageous. The superconducting Y-Ba-Cu-O ceramics with density 6 g/cm3 has been prepared by hot pressing of powders.

A complex study of magnetization change, optical Faraday effect, and light absorption spectrum has been carried out on uniaxial antiferromagnetic crystals of MnF 2 of FeCO 3 under the phase transitions induced by strong external magnetic field. The measurements were made at 4.2 and 20.4 K, which is lower than Neel temperature (T n =68 and 38 K for MnF 2 and FeCO 3 , respectively). The field used was pulsed up to 400 kOe. MnF 2 : The magnetic properties have been studied in detail employing a pickup coil method in a narrow range of magnetic fields near the spin-flopping field (H c =92 kOe) with a strict field orientation along the tetragonal axis of the crystal symmetry. The angular field deviation ψ from the c 4 axis is controlled to within 1.5'. In the narrow range of angles ψ c depends on the sample form and the magnetization process is of a jump-like character. The measurements of the longitudinal (along the magnetic field) and transversal susceptibility components made using three pickup coils of mutually perpendicular axes and three coaxial coils shifted along the sample axis show that at ψ c ± 200 Oe the sample magnetically cleaves into interchanging layers of 10-5cm 3 volume which stretch along the symmetry axis of the sample and differ from each other by a longitudinal component of the magnetization vector.