Photoinduced ordering and anchoring properties of azo-dye films

We study both theoretically and experimentally anchoring properties of photoaligning azo-dye films in contact with a nematic liquid crystal depending on photoinduced ordering of azo-dye molecules. In the mean field approximation, we found that the bare surface anchoring energy linearly depends on the azo-dye order parameter and the azimuthal anchoring strength decays to zero in the limit of vanishing photoinduced ordering. From the absorption dichroism spectra measured in the azo-dye films that are prepared from the azo-dye derivative with polymerizable terminal groups (SDA-2) we obtain dependence of the dichroic ratio on the irradiation dose. We also measure the polar and azimuthal anchoring strengths in nematic liquid crystal (NLC) cells aligned by the azo-dye films and derive the anchoring strengths as functions of the dichroic ratio. Though linear fitting of the experimental data for both anchoring strengths gives reasonably well results, it, in contradiction with the theory, predicts vanishing of the azimuthal anchoring strength at certain nonzero value of the azo-dye order parameter. By using a simple phenomenological model we show that this discrepancy can be attributed to the difference between the surface and bulk order parameters in the films.Comment: revtex4, 25 pages, 9 figure

Orientational instabilities in nematics with weak anchoring under combined action of steady flow and external fields

We study the homogeneous and the spatially periodic instabilities in a nematic liquid crystal layer subjected to steady plane {\em Couette} or {\em Poiseuille} flow. The initial director orientation is perpendicular to the flow plane. Weak anchoring at the confining plates and the influence of the external {\em electric} and/or {\em magnetic} field are taken into account. Approximate expressions for the critical shear rate are presented and compared with semi-analytical solutions in case of Couette flow and numerical solutions of the full set of nematodynamic equations for Poiseuille flow. In particular the dependence of the type of instability and the threshold on the azimuthal and the polar anchoring strength and external fields is analysed.Comment: 12 pages, 6 figure

КАК «НАУЧИТЬ» СПЛАВЫ МАГНИЯ ПРОТИВОСТОЯТЬ ЭКСПЛУАТАЦИОННЫМ РИСКАМ

The paper studies regularities and mechanisms of structure and phase formation in the surface layers of magnesium alloys when they are processed by method of micro-arc oxidation [MAO]. It has been determined that the same specific features of structure formation, namely: existence of a thin dense inner sublayer and a thicker outer sublayer with developed porosity are common for all types of coatings on the surface of magnesium and aluminum alloys. Such structural state of a protective coating can not be considered as a guaranteed protection against operational impacts, taking into account the fields of their primary application that is aviation construction, automotive construction, instrumentation, building construction, etc. The paper has analyzed the effect of alkaline electrolytes with varying chemical composition due to additions of sodium fluoride or potassium on the structure and properties of these alloys as well as on the level of basic performance characteristics of the layers formed in such electrolytes. On the basis of the analysis a conclusion has been made that it is possible to extend their life-span under operational conditions. It has been revealed that the existing techniques and methods for process control of MAO aluminum and magnesium alloys, particularly processing modes and technological equipment capacity determine a nature of structure formation and changes in a phase composition of the formed coatings.Изучены закономерности и механизмы структурои фазообразования в поверхностных слоях магниевых сплавов при их обработке методом микродугового оксидирования. Определено, что для всех видов покрытий на поверхности магниевых и алюминиевых сплавов характерны одинаковые особенности структурообразования: наличие тонкого плотного внутреннего подслоя и более толстого с развитой пористостью наружного. Такое структурное состояние защитного покрытия не является гарантированной защитой объектов от эксплуатационных воздействий, учитывая области их преимущественного использования – авиа-, автомобиле-, приборостроение, строительство и т. п. Проанализировано влияние щелочных электролитов с изменяющимся химическим составом за счет добавок фторидов натрия или калия на структуру и свойства указанных сплавов, а также на уровень основных эксплуатационных характеристик слоев, формируемых в таких электролитах. На основании этого сделан вывод о возможности продления их ресурса в эксплуатационных условиях. Выявлено, что существующие приемы и способы управления процессом микродугового оксидирования алюминиевых и магниевых сплавов, в частности режимы обработки и мощность технологического оборудования, определяют характер структурообразования и изменение фазового состава формируемых покрытий

CEM03 and LAQGSM03 - new modeling tools for nuclear applications

An improved version of the Cascade-Exciton Model (CEM) of nuclear reactions realized in the code CEM2k and the Los Alamos version of the Quark-Gluon String Model (LAQGSM) have been developed recently at LANL to describe reactions induced by particles and nuclei for a number of applications. Our CEM2k and LAQGSM merged with the GEM2 evaporation/fission code by Furihata have predictive powers comparable to other modern codes and describe many reactions better than other codes; therefore both our codes can be used as reliable event generators in transport codes for applications. During the last year, we have made a significant improvements to the intranuclear cascade parts of CEM2k and LAQGSM, and have extended LAQGSM to describe photonuclear reactions at energies to 10 GeV and higher. We have produced in this way improved versions of our codes, CEM03.01 and LAQGSM03.01. We present a brief description of our codes and show illustrative results obtained with CEM03.01 and LAQGSM03.01 for different reactions compared with predictions by other models, as well as examples of using our codes as modeling tools for nuclear applications.Comment: 12 pages, 10 figures, to be published in Journal of Physics: Conference Series: Proc. Europhysics Conf. on New Trends in Nuclear Physics Applications and Technologies (NPDC19), Pavia, Italy, September 5-9, 200

РОЛЬ УЛЬТРАЗВУКА В МЕХАНИЗМАХ АНОДНО-КАТОДНЫХ ВЗАИМОДЕЙСТВИЙ ПРИ ЭЛЕКТРОИСКРОВОМ ЛЕГИРОВАНИИ

The paper reveals results of investigations on mass transfer kinetics and dynamics of coating formation while using integral electrospark alloying method with additional ultrasonic exposure at different stages of formation. Nowadays, a classical method for electrospark alloying with hard-alloy anodes and impulse AC voltage frequency on the vibration exciter coil from 20 to 1600 Hz has been mainly used for application of protective and strengthening coatings within permissible thickness and characteristics. The key aspect of ultrasonic exposure application (frequency 22–44 kHz) during electrospark alloying is the possibility to increase further thickness of coatings to be formed even after reaching a brittle fracture threshold of the coating material. Methodology of the executed research activity has been based on integrated studies (gravimetric, metallographic, X-ray diffraction and electron microscopic) of coatings which are to be formed through compositions produced while using method of high-energy hot compaction and a “refractory carbide (WC) and a binding material“ system in the form of alloy based on nickel from the series of “colmonoy” Ni – Ni3B system which is alloyed with additions of copper and silicon. The initial surface treatment within ultrasonic frequency range (22–44 kHz) contributes to a noticeable increase in the mass transfer rate, which is primarily determined by chemical composition and thermodynamic stability of anodes. It is due to surface activation in the process of its preliminary deformation at ultrasonic frequency which creates additional conditions for striking of a spark.The final ultrasonic treatment improves coating quality due to its additional forging that leads to an increase of its structure homogeneity and density.Приведены результаты исследований кинетики массопереноса и динамика формирования покрытий интегральным способом электроискрового легирования с дополнительным ультразвуковым воздействием на разных стадиях их формирования. В настоящее время для нанесения защитно-упрочняющих покрытий в пределах допустимых толщин и характеристик в основном применяется классический метод электроискрового легирования с использованием твердосплавных анодов и частотой импульсного переменного напряжения на катушке вибровозбудителя от 20 до 1600 Гц. Главной особенностью применения ультразвукового воздействия (частота 22–44 кГц) в процессе электроискрового легирования является возможность дальнейшего наращивания толщины формируемых покрытий даже после достижения порога хрупкого разрушения материала покрытия. Методика проводимой работы базировалась на комплексных гравиметрических, металлографических, рентгеноструктурных и электронно-микроскопических исследованиях формируемых покрытий на основе композиций, изготовленных методом высокоэнергетического горячего прессования, и системы «тугоплавкий карбид (WC) – связка» в виде сплава на основе никеля из серии «колмоной» системы Ni – Ni3B, легированной добавками меди и кремния. Первоначальная обработка поверхности в диапазоне ультразвуковых частот 22–44 кГц способствует заметному возрастанию скорости массопереноса, которая определяется прежде всего химическим составом и термодинамической устойчивостью анодов. Это объясняется активацией поверхности в процессе ее предварительного деформирования с ультразвуковой частотой с созданием дополнительных условий для возникновения искрового разряда. Завершающая ультразвуковая обработка улучшает качество покрытия вследствие его дополнительной проковки, приводя к увеличению однородности его структуры и повышению ее плотности

Three-dimensional pattern formation, multiple homogeneous soft modes, and nonlinear dielectric electroconvection

Patterns forming spontaneously in extended, three-dimensional, dissipative systems are likely to excite several homogeneous soft modes ($\approx$ hydrodynamic modes) of the underlying physical system, much more than quasi one- and two-dimensional patterns are. The reason is the lack of damping boundaries. This paper compares two analytic techniques to derive the patten dynamics from hydrodynamics, which are usually equivalent but lead to different results when applied to multiple homogeneous soft modes. Dielectric electroconvection in nematic liquid crystals is introduced as a model for three-dimensional pattern formation. The 3D pattern dynamics including soft modes are derived. For slabs of large but finite thickness the description is reduced further to a two-dimensional one. It is argued that the range of validity of 2D descriptions is limited to a very small region above threshold. The transition from 2D to 3D pattern dynamics is discussed. Experimentally testable predictions for the stable range of ideal patterns and the electric Nusselt numbers are made. For most results analytic approximations in terms of material parameters are given.Comment: 29 pages, 2 figure

HOW TO MAKE MAGNESIUM ALLOYS BE RESISTANT TO OPERATIONAL RISKS

The paper studies regularities and mechanisms of structure and phase formation in the surface layers of magnesium alloys when they are processed by method of micro-arc oxidation [MAO]. It has been determined that the same specific features of structure formation, namely: existence of a thin dense inner sublayer and a thicker outer sublayer with developed porosity are common for all types of coatings on the surface of magnesium and aluminum alloys. Such structural state of a protective coating can not be considered as a guaranteed protection against operational impacts, taking into account the fields of their primary application that is aviation construction, automotive construction, instrumentation, building construction, etc. The paper has analyzed the effect of alkaline electrolytes with varying chemical composition due to additions of sodium fluoride or potassium on the structure and properties of these alloys as well as on the level of basic performance characteristics of the layers formed in such electrolytes. On the basis of the analysis a conclusion has been made that it is possible to extend their life-span under operational conditions. It has been revealed that the existing techniques and methods for process control of MAO aluminum and magnesium alloys, particularly processing modes and technological equipment capacity determine a nature of structure formation and changes in a phase composition of the formed coatings

ROLE OF ULTRASOUND IN MECHANISMS OF ANODE-CATHODE INTERACTIONS DURING ELECTROSPARK ALLOYING

The paper reveals results of investigations on mass transfer kinetics and dynamics of coating formation while using integral electrospark alloying method with additional ultrasonic exposure at different stages of formation. Nowadays, a classical method for electrospark alloying with hard-alloy anodes and impulse AC voltage frequency on the vibration exciter coil from 20 to 1600 Hz has been mainly used for application of protective and strengthening coatings within permissible thickness and characteristics. The key aspect of ultrasonic exposure application (frequency 22–44 kHz) during electrospark alloying is the possibility to increase further thickness of coatings to be formed even after reaching a brittle fracture threshold of the coating material. Methodology of the executed research activity has been based on integrated studies (gravimetric, metallographic, X-ray diffraction and electron microscopic) of coatings which are to be formed through compositions produced while using method of high-energy hot compaction and a “refractory carbide (WC) and a binding material“ system in the form of alloy based on nickel from the series of “colmonoy” Ni – Ni3B system which is alloyed with additions of copper and silicon. The initial surface treatment within ultrasonic frequency range (22–44 kHz) contributes to a noticeable increase in the mass transfer rate, which is primarily determined by chemical composition and thermodynamic stability of anodes. It is due to surface activation in the process of its preliminary deformation at ultrasonic frequency which creates additional conditions for striking of a spark.The final ultrasonic treatment improves coating quality due to its additional forging that leads to an increase of its structure homogeneity and density