EXPERIMENTAL INVESTIGATION OF THE TRANSIENT CHARACTERISTICS AND MECHANICAL LOSSES IN Ni-Mn-Ga ALLOYS

The reported study was funded by RFBR, project number 20-32-90195

OBTAINING INTERMETALLIC COMPOUNDS IN Al–Ti–Zn SYSTEM

Binary intermetallic compounds – titanium aluminides (TiAl, Ti3Al) – when added to the alloys, significantly increase their strength and special properties. The most promising direction to produce intermetallic compounds are mechanochemical technologies, including mechanical alloy building. Mechanical alloying makes it possible to introduce much smaller particles into the metal matrix than can be achieved using standard powder metallurgy technologies. In addition to mechanical synthesis, aluminum-based intermetallic compounds were produced by self-propagating high-temperature synthesis (SHS) of solid chemical compounds. The synthesis was carried out according to a multistage scheme: preparation of titanium and aluminum powder, mixing; synthesis of the Al3Ti intermetallic compound by the SHS method in vacuum followed by mechanical activation of stoichiometric charges. The aim of the research was to study the dynamics of the development of nanodispersed phases in the process of synthesis during mechanical alloying. The power absorbed by the unit mass of the material for different processing times of the charge was calculated. When the level of the specific power (dose) of mechanical treatment was 3.5 kJ/g, the maximum content of intermetallic compound in the resulting material was achieved. Based on calculations and the data obtained during X-ray phase analysis, the dependence of the change in the content of ternary intermetallic compounds in the final product on the absorbed power was determined. As a result of the studies using raster electron microscopy and X-ray analysis, it was found that mechanical alloying of nanostructured intermetallic compounds Ti4ZnAl11 and Ti25Zn9Al66 with the size of nanodisperse phases less than 12 nm in the Al–Ti–Zn system, the weight ratio of proportion of the latter reaches 74 %

Optical spectra, crystal-field parameters, and magnetic susceptibility of the new multiferroic NdFe3(BO3)4

We report high-resolution optical absorption spectra for NdFe3(BO3)4 trigonal single crystal which is known to exhibit a giant magnetoelectric effect below the temperature of magnetic ordering TN = 33 K. The analysis of the temperature-dependent polarized spectra reveals the energies and, in some cases, symmetries and exchange splittings of Nd3+ 84 Kramers doublets. We perform crystal-field calculations starting from the exchange-charge model, obtain a set of six real crystal-field parameters, and calculate wave functions and magnetic g-factors. In particular, the values g(perpendicular) = 2.385, g(parallel) = 1.376 were found for the Nd3+ ground-state doublet. We obtain Bloc=7.88 T and |JFN|= 0.48 K for the values of the local effective magnetic field at liquid helium temperatures at the Nd3+ site and the Nd - Fe exchange integral, respectively, using the experimentally measured Nd3+ ground-state splitting of 8.8 cm-1. To check reliability of our set of crystal field parameters we model the magnetic susceptibility data from literature. A dimer containing two nearest-neighbor iron ions in the spiral chain is considered to partly account for quasi-one-dimensional properties of iron borates, and then the mean-field approximation is used. The results of calculations with the exchange parameters for Fe3+ ions Jnn = -6.25 K (intra-chain interactions) and Jnnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with the experimental data.Comment: 13 pages, 8 figures, 2 table

Positive Magneto-Resistance in Quasi-1D Conductors

We present here a simple qualitative model that interpolates between the high and low temperature properties of quasi-1D conductors. At high temperatures we argue that transport is governed by inelastic scattering whereas at low temperatures the conductance decays exponentially with the electron dephasing length. The crossover between these regimes occurs at the temperature at which the elastic and inelastic scattering times become equal. This model is shown to be in quantitative agreement with the organic conductor $TTT_2I_{3-\delta}$. Within this model, we also show that on the insulating side, the positive magnetoresistance of the form $(H/T)^2$ observed in $TTT_2I_{3-\delta}$ and other quasi-1D conductors can be explained by the role spin-flip scattering plays in the electron dephasing rate.Comment: 4 pages, Latex, no figure

Negative magneto-resistance of electron gas in a quantum well with parabolic potential

We have studied the electrical conductivity of the electron gas in parallel electric and magnetic fields directed along the plane of a parabolic quantum well (across the profile of the potential). We found a general expression for the electrical conductivity applicable for any magnitudes of the magnetic field and the degree of degeneration of the electron gas. A new mechanism of generation of the negative magnetoresistance has been revealed. It has been shown that in a parabolic quantum well with a non-degenerated electron gas the negative magnetoresistance results from spin splitting of the levels of the size quantization.Comment: 15 pages, 3 figure

Nanocrystalline lanthanide-doped Lu3Ga5O12 garnets: interesting materials for light-emitting devices

Nanocrystalline Lu3Ga5O12, with average particle sizes of 40 nm, doped with a wide variety of luminescent trivalent lanthanide ions have been prepared using a sol\u2013gel technique. The structural and morphological properties of the powders have been investigated by x-ray powder diffraction, high resolution transmission electron microscopy and Raman spectroscopy. Structural data have been refined and are presented for Pr3+, Eu3+, Gd3+, Ho3+, Er3+ and Tm3+ dopants, while room temperature excited luminescence spectra and emission decay curves of Eu3+-, Tm3+- and Ho3+-doped Lu3Ga5O12 nanocrystals have been measured and are discussed. The Eu3+ emission spectrum shows typical bands due to 5D0 \u21927FJ (J = 0, 1, 2, 3, 4) transitions and the broadening of these emission bands with the non-exponential behaviour of the decay curves indicates the presence of structural disorder around the lanthanide ions. Lanthanide-doped nanocrystalline Lu3Ga5O12 materials show better luminescence intensities compared to Y2O3, Gd3Ga5O12 and Y3Al5O12 nanocrystalline hosts. Moreover, the upconversion emission intensity in the blue-green region for the Tm3+- and Ho3+-doped samples shows a significant increase upon 647.5 nm excitation with respect to other common oxide hosts doped with the same lanthanide ions

Comparative characterization of different kinds of chromatographic quantification using the double standard addition method

Различные варианты обработки результатов количественного газохроматографического анализа способом двойной стандартной добавки сопоставлены по точности. Три основных из них: I – простое сравнение данных, получаемых с использованием однократной и двойной добавок, II – аппроксимация зависимости m(S) в координатах «площадь пика определяемого компонента» (S) – «масса добавки» (mдоб) методом наименьших квадратов по уравнению линейной регрессии и III–вычисление количеств определяемых компонентов (mx) по каждой из стандартных добавок с последующей линейной экстраполяцией их значений на «нулевую» стандартную добавку, mx(mдоб® 0). Показано, что результаты определений в различных вариантах стандартных добавок сопоставимы по точности, но несколько занижены относительно заданных количеств аналитов. Главной причиной таких систематических погрешностей является испарение растворителя при последовательном дозировании проб одних и тех же образцов в хроматограф. В результате площади пиков, определяемые после ввода стандартных добавок в образцы, оказываются несколько завышенными, что и приводит к занижению результатов. Второй (менее значимый) фактор – незначительное увеличение объема образцов за счет добавок определяемых компонентов. Отмечено, что погрешности определений различными вариантами способа стандартной добавки не превышают случайных составляющих погрешностей. Лучшие результаты (с учетом знаков отклонений) обеспечивает вычисление содержания определяемого аналита методом двойной стандартной добавки с экстраполяцией результатов на «нулевую» величину добавки. Для исключения влияния «человеческого фактора» (увеличение точности результатов в ходе анализа серий однотипных образцов за счет опыта аналитиков) все параллельные определения были проведены студентами бакалавриата Института химии Санкт-Петербургского государственного университета в ходе выполнения ими лабораторных работ. Такая орга­низация экспериментов повышает их достоверность, поскольку исключает зависимость результатов от различий в квалификации аналитиков.Different algorithms for processing the quantitative gas chromatographic analysis data using the double standard addition method are compared for their accuracy. Three principal approaches are possible for such processing: I – simple comparison of values determined by single and double standard additions, II – approximation of «peak area of analyte» (S) – «mass of standard addition» (madd) dependence by the least squares method [linear regression, m(S)], and III – independent quantification of analyte with both standard additions followed by the linear extrapolation of two subresults on the socalled «zero standard addition», mx(madd ® 0). It is concluded that the quantitation results obtained using the various modes of the method are comparable in accuracy, but somewhat underestimated relative to the specified amounts of analytes. The principal reason of such systematic errors is the evaporation of the solvent during the successive injecting of the same samples into the gas chromatograph. Due to this reason the peak areas, measured after the standard addition, appear to be slightly increased and this leads to the systematic underestimation of the results. The second (less important) factor is the small increase of the sample volumes due to the addition of the components to be determined. It is confirmed that the systematic errors of different modes of standard addition are not exceeding the values of their random uncertainties. The optimal results (considering their signs of deviations) are provided using the double standard addition method with extrapolation of subresults on «zero standard addition». In order to exclude the possible influence of «human factor» (increasing the re­sults precision during the series of analyses of similar samples due to the rising experience of analytical chemists) all parallel measurements have been per­for­med by bachelor students of the Chemistry Institute of the St. Petersburg State University in the course of their laboratory practical works in chromatography. Such organization of experiments increases their credibility as it excluded the dependence of the results on the qualification of chemists.Студенческая лабораторная работа, результаты которой составили предмет настоящего сообщения, выполнена с использованием оборудования Ресурсного Центра «Методы анализа состава вещества» Санкт-Петербургского государственного университета. Авторы благодарят сотрудников Центра за содействие.The students’ work, results of which are discussed in the current paper, was carried out using the equipment of the “Methods of analysis of substance’s composition” Resource Centre at the St. Petersburg State University. The authors are grateful to the staff of this Center for assistance