Ion-kinetic D'Angelo mode

An extension of hydrodynamic D'Angelo mode of inhomogeneous sheared plasma flow along the magnetic field into the short-wavelength limit, where the hydrodynamic treatment is not valid, has been considered. We find that D'Angelo mode in this wavelength range is excited by inverse ion Landau damping and becomes the shear flow driven ion-kinetic mode.Comment: 9 pages, 1 figur

Influence of Gamma-X mixing on optical orientation and alignment of excitons in (In,Al)As/AlAs quantum dots

The effect of Gamma-X mixing on the energy levels fine structure of indirect in k-space excitons in an ensemble of (In,Al)As/AlAs quantum dots with type I band alignment was experimentally studied. Using the methods of optical spin orientation and optical alignment in a magnetic field, an increase in the anisotropic exchange splitting of excitonic levels (from approximately 0.6 to 5 ueV) due to the Gamma-X mixing was revealed. The extent of direct electronic states admixing to indirect ones depends on the size of the quantum dot. On the other hand, the optical and spin properties of excitons change radically with increasing of the Gamma states admixture to the X states: in the absence of a magnetic field, the optical orientation of excitons decreases from 18 to 3%, while the alignment of excitons is restored from 6 to 53%.Comment: 12 page

New transitions and energy levels of water vapor by high sensitivity CRDS near 1.73 and 1.54 µm

This contribution is part of a long term project aiming at improving the water absorption spectroscopy by high sensitivity cavity ring down spectroscopy (CRDS) in the near infrared. Two new sources of CRDS spectra are considered: (i) The room temperature absorption spectrum of water vapor in natural isotopic abundance is recorded near 1.73 µm. A series of recordings was performed from 5693 to 5991 cm−1 with a pressure value of about 6 Torr. The noise equivalent absorption (αmin) of the spectra is better than 10− 10 cm−1. A total of 1453 lines were assigned to 1573 transitions of four water isotopologues (H2 16O, H2 17O, H2 18O and HD16O). Their intensities span more than five orders of magnitude from 3.0 × 10−30 to 4.7 × 10−25 cm/molecule at 296 K. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Schwenke and Partridge. Two hundred fifty-one lines (assigned to 280 transitions) are observed for the first time and twelve energy levels are newly determined. The comparison of the obtained line parameters with those of the HITRAN database is discussed. Forty-six line positions are observed to significantly differ from their HITRAN values (δν = │νHITRAN – νCRDS│ > 0.02 cm−1). The derived set of energy levels is compared to those recommended by an IUPAC task group. (ii) The room temperature CRDS spectrum of water vapor highly enriched in 17O was recorded near 1.54 µm (6223–6672 cm−1) at a pressure of 12 Torr. Compared to a previous study, the higher pressure of the recordings allowed for extending the observations. Overall, twenty-six new levels were determined for both H2 17O and HD17O. All these observations together with other recent measurements will allow for an extension and an update of our empirical database in the 5693– 8340 cm−1 region. © 2019 Elsevier Lt

Swirling astrophysical flows - efficient amplifiers of Alfven waves

We show that a helical shear flow of a magnetized plasma may serve as an efficient amplifier of Alfven waves. We find that even when the flow is purely ejectional (i.e., when no rotation is present) Alfven waves are amplified through the transient, shear-induced, algebraic amplification process. Series of transient amplifications, taking place sequentially along the flow, may result in a cascade amplification of these waves. However, when a flow is swirling or helical (i.e., some rotation is imposed on the plasma motion), Alfven waves become subject to new, much more powerful shear instabilities. In this case, depending on the type of differential rotation, both usual and parametric instabilities may appear. We claim that these phenomena may lead to the generation of large amplitude Alfven waves and the mechanism may account for the appearance of such waves in the solar atmosphere, in accretion-ejecion flows and in accretion columns. These processes may also serve as an important initial (linear and nonmodal) phase in the ultimate subcritical transition to MHD Alfvenic turbulence in various kinds of astrophysical shear flows.Comment: 12 pages, 11 figures, accepted for publication (25-11-02) in Astronomy and Astrophysic

Properties of SPEEK based PEMs for fuel cell application

Peer reviewed: YesNRC publication: Ye

Выбор материала чувствительных элементов акселерометров на основе ПАВ

Introduction. At the present, sensors based on surface acoustic waves (SAW) is a rapidly developing direction and a promising replacement for classical sensors, especially in those areas where long-term performance of latter is questionable. The principle of operation of SAW sensors is based on acoustic vibrations, therefore, the choice of piezoelectric material of а console, considering external influences on a future device and its operating conditions, is the most important task. Currently, many monocrystalline structures and their sections have been synthesized and created for the devices using SAW. The main materials used for the manufacture of substrates are crystals of quartz (SiO2), lithium niobate (LiNbO3), lithium tantalate (LiTaO3) and film aluminum nitride (AlN). Also, new crystal structures: langasite (La3Ga5SiO14), langatate (La3Ga5.5Ta0.5O14), langanite and others were produced. The problem of using such materials for the manufacture of consoles is the lack of systematized data on important characteristics for the propagation of surfactants, for example, the elasticity tensor of the 4th rank. One of the key problems for the further development of SAW-based sensors is the one-way fastening of rectangular and triangular sensitive elements (SE) in sensor housing. In order to overcome the above drawback an MMA surfactant thing based on a membrane SE for a more uniform distribution of a load over the surface of the SE was proposed.Aim. To show the advantages of using AlN as the SE material of a ring wave resonator on SAW.Materials and methods. The theoretical part of the research was carried out using the finite element method. Mathematical processing was implemented in AutoCAD 2019 and in COMSOL Multiphysics 5.4.Results. The use of AlN, which acts as the SE material for measuring an acceleration based on SAW was proposed. The proposed solution was compared with existing prototypes based on the use of SiO2 / LiNbO3 membranes, which were characterized by strong anisotropic properties. A 3D model of the SE of a ring wave resonator on surface waves was created. Using computer simulations and COMSOL Multiphysics software, it was shown that the thing was capable to withstand exposures in excess of 10 000 g, and an isotropic AlN sensor overcomed the limitations of both the low sensitivity of SiO2 and the low temperature stability of LiNbO3. AlN demonstrated almost double resistance to irreversible mechanical deformations as compared to SiO2, which, in turn, allows an additional 1.5-fold increase in sensitivity compared to quartz – based sensors.Conclusion. Based on the data obtained by the modeling, it can be concluded that the use of AIN as SE material is promising, especially for measuring high acceleration values, but with restrictions on temperature sensitivity of the material.Введение. Датчики на основе поверхностных акустических волн (ПАВ) являются стремительно развивающимся направлением и перспективной заменой классических датчиков, особенно в тех сферах, где длительная работоспособность последних под вопросом. Принцип работы датчиков на ПАВ основан на акустических колебаниях, поэтому выбор пьезоэлектрического материала консоли с учетом внешних влияний на будущее устройство и его условий работы является важнейшей задачей. Синтезировано и создано множество монокристаллических структур и их срезов для устройств на поверхностных акустических волнах. Основными материалами, применяемыми для изготовления подложек, являются кристаллы кварца (SiO2), ниобата лития (LiNbO3), танталата лития (LiTaO3) и пленочный нитрид алюминия (AlN). Производятся новые кристаллические структуры: лангасит (La3Ga5SiO14), лангатат (La3Ga5.5Ta0.5O14), ланганит и др. Проблема применения подобных материалов для изготовления консолей – отсутствие систематизированных данных о важных характеристиках для распространения ПАВ, к примеру тензора упругости 4-го ранга. Чтобы преодолеть указанный недостаток, предложена конструкция микромеханического акселерометра на основе ПАВ, основанного на мембранном ЧЭ для более равномерного распределения нагрузки по поверхности ЧЭ. Одна из ключевых проблем для дальнейшего развития датчиков на основе ПАВ - одностороннее закрепление прямоугольных и треугольных чувствительных элементов (ЧЭ) в корпусе датчика.Цель работы. Показать преимущества использования AlN как материала чувствительного элемента кольцевого волнового резонатора на поверхностных акустических волнах.Материалы и методы. Применение метода конечных элементов и математическая обработка в AutoCAD 2019 и COMSOL Multiphysics 5.4.Результаты. Предложено использовать AlN в качестве материала чувствительного элемента для измерения ускорения на основе ПАВ. Предлагаемое решение сравнивалось с существующими прототипами, основанными на использовании мембран SiO2/LiNbO3, которые характеризуются сильными анизотропными свойствами. Создана 3D-модель ЧЭ кольцевого волнового резонатора на поверхностных волнах. Используя компьютерное моделирование и программное обеспечение COMSOL Multiphysics доказано, что конструкция способна выдерживать воздействия свыше 10 000 g и чувствительный элемент на основе изотропного AlN преодолевает ограничения как низкой чувствительности SiO2, так и малой температурной стабильности LiNbO3. AlN демонстрирует почти двойную устойчивость к необратимым механическим деформациям по сравнению с SiO2, что, в свою очередь, позволяет дополнительно повысить чувствительность в 1.5 раза по сравнению с датчиками на основе кварца.Заключение. Исходя из созданной модели, можно сделать вывод о перспективности использования нитрида алюминия как материала для чувствительного элемента, особенно для измерения больших значений ускорения, но c ограничениями по температурной чувствительности материала

Оптимизация конструкции встречно-штыревого преобразователя кольцевого резонатора на поверхностных акустических волнах

Introduction. Previous works considered the frequency characteristics and methods for fixing sensitive elements in the form of a wave ring resonator on surface acoustic waves in a housing made of various materials, as well as the influence of external factors on sensitive elements. It was found that the passband in such a case is sufficiently wide, which can affect adversely signal detection when measuring acceleration using the sensitive element under development. Therefore, it has become relevant to reduce the sensitive element’s bandwidth by changing the design of the interdigitated transducer (IDT).Aim. To demonstrate an optimal topology for an IDT with a low bandwidth, leading to improved signal detection when acceleration affects the sensitive element.Materials and methods. The finite element method and mathematical processing in AutoCAD and in COMSOL Multiphysics.Results. Nine topologies of IDT are proposed. All these types were investigated using the COMSOL Multiphysics software on lithium niobate substrates, which material acts as a sensitive element. The frequency characteristics are presented. The data obtained allowed an optimal design of the ring resonator to be proposed: an IDT with rectangular pins without selective withdrawal.Conclusion. Self-generation in a ring resonator can be performed by withdrawing no more than one pair of IDTs for 10 or more periods. In this case, the withdrawal of IDTs should be uniform. With an increase in the number of IDT withdrawals, the geometry of the ring resonator is violated, and the wave leaves the structure. The presence of a shared bus keeps the surface acoustic wave inside the IDT structure, and the narrowing of the periods towards the inner part of the structure makes it possible to improve the frequency characteristics of the ring resonator on surface acoustic waves.Введение. В предыдущих публикациях авторами рассматривались частотные характеристики и способы закрепления чувствительных элементов в виде волнового кольцевого резонатора на поверхностных акустических волнах в корпусе из различных материалов, а также влияние внешних факторов на чувствительные элементы. При анализе полученных результатов было замечено, что полоса пропускания достаточно широкая, что может отрицательно сказаться на детектировании сигнала при измерении ускорения с помощью разрабатываемого чувствительного элемента. Возникла необходимость уменьшить полосу пропускания чувствительного элемента за счет изменения конструкции встречноштыревого преобразователя (ВШП). Используя компьютерное моделирование COMSOL Multiphysics, было продемонстрировано несколько вариантов ВШП и показано, что эта цель может быть достигнута изменением геометрии штыря ВШП.Цель работы. Продемонстрировать оптимальную топологию ВШП с малой полосой пропускания, что влечет за собой улучшение детектирования сигнала при влиянии ускорения на чувствительный элемент.Материалы и методы. Применение метода конечных элементов и математическая обработка в AutoCAD и COMSOL Multiphysics.Результаты. Предложено 9 типов топологий встречно-штыревых преобразователей. Проведены исследования при помощи программного обеспечения COMSOL Multiphysics вышеуказанных типов ВШП на подложках из ниобата лития, выступающего в качестве материала чувствительного элемента, и представлены частотные характеристики. Проведена оценка полученных данных и сделаны выводы об оптимальной конструкции кольцевого резонатора: наиболее эффективной структурой является ВШП с прямоугольными штырями без селективного изъятия.Заключение. Генерация волны в кольцевом резонаторе может быть выполнена при изъятии не более одной пары ВШП на 10 и более периодов. При этом изъятие ВШП должно быть равномерным. При увеличении количества изъятых ВШП нарушается геометрия кольцевого резонатора и волна покидает конструкцию. Наличие общей шины позволяет удерживать поверхностную акустическую волну внутри конструкции ВШП, а сужение периодов к внутренней части конструкции позволяет улучшить частотные характеристики кольцевого резонатора на поверхностных акустических волнах

New palynological data for Toarcian (Lower Jurassic) deep-marine sandstones of the Western Caucasus, southwestern Russia

© 2018 Anna A. Goryacheva et al., published by Sciendo 2018. Information on Jurassic palynomorphs from the Greater Caucasus is potentially of great importance, but its availability to the international research community is severely limited. New palynological data for Toarcian deposits of the Western Caucasus are recorded in the present paper. Particularly, dinoflagellate cysts are described for the first time from the Bagovskaja Formation; palynomorphs are found in sandstone levels within this unit. The most representative assemblage includes pollen (with predominant bisaccate pollen), spores (Cyathidites being commonest), and dinoflagellate cysts amongst which the predominant taxon is Nannoceratopsis spiculata. The dinocyst assemblage implies a late Toarcian age for the upper part of the Bagovskaja Formation. On the basis of these new palynostratigraphical results, the range of the formation is extended; previously, only the lower part had been dated on ammonite evidence