Epoxidation of natural rubber studied by nmr spectroscopy

Epoxidation of natural rubber with peroxyacids in homogeneous solution (CHCl₃, benzene) and in latex occurs without main chain fission or isomerization of the cis configuration. There are two possible modes of addition of oxygen to the double bond, which gives rise to diastereoisomers in dyad structures, as shown by the occurrence of splitting of ¹³C NMR resonances in 100% epoxidized rubber. In partially epoxidized rubber the ¹³C NMR resonances of the methylene carbon atoms have been fully assigned using pulse sequences and lanthanide shift reagents, in terms of triad sequences. Resonances in the olefinic region of the spectrum require assignment in terms of pentad sequences whereas those from the oxirane region are much more closely spaced and are assigned using triads. Intensity measurements on the methylene carbon NMR resonances show that the epoxidation reaction is a random process in both homogeneous solution and in latex particles.     

Lateral space-charge effects on ballistic electron transport across graded heterojunctions

Hot electron transport across graded compound semiconductor heterojunctions has been explored using a two-dimensional formulation of the self-consistent ensemble Monte Carlo method. The Al_xGa_1-xAs/GaAs heterojunction imbedded into a vertical field effect transistor with two ohmic contacts (source, drain) and two lateral Schottky gates has been used as an example. Lateral space charges modulated by the gates are shown to control ballistic injection of electrons over the heterojunction under steady state conditions. The transient response to a gate pulse is found to be determined by carrier transit from the heavily doped source contact region into the channel. A conceptual one-dimensional section model is used to explain the Monte Carlo results.     

Simultaneous a posteriori detection and identification of a predetermined number of quasi-periodic fragments in a sequence based on their segments

An a posteriori (off-line) approach to the simultaneous detection and identification of quasi-periodic fragments in a numerical sequence using their segments is analyzed. The solution of the problem is proposed for the case when the number of the desired fragments is known. The following assumptions are made. (i) Each desired fragment of the numerical sequence coincides with an element of the given alphabet of the reference sequences that have equal lengths (i.e., number of elements). (ii) Only a segment (part) of the desired fragment can be processed; unprocessable parts of the fragment are interpreted as lost data. (iii) The numbers of the elements in the sequence that correspond to the beginning of the desired fragment and the boundaries of the segment in the fragment represent deterministic (rather than random) quantities; the boundaries of the segments are different for different fragments; and the desired fragments are quasi-periodic in the sequence. (iv) Gaussian uncorrelated noise impedes the observation of the sequence that contains quasi-periodic segments of the reference sequences. It is demonstrated that the essence of the problem under study lies in the verification of a set of hypotheses on the mean value of a random Gaussian vector (the power of this set exponentially increases with an increase in the dimension of the vector—the sequence length). An effective a posteriori algorithm that provides for maximum-likelihood detection and identification is validated. The estimates of the time and space complexity are related to the parameters of the problem. The results of the numerical simulation are presented. This work was supported by the Russian Foundation for Basic Research, project no. 03-01-00036 and 06-01-00058. Aleksandr V. Kel’manov. Born April 25, 1952. Graduated from Izhevsk State Technical University in 1974. Received candidate’s degree in 1980 and doctoral degree in 1994. Leading researcher of the Sobolev Institute of Mathematics, Siberian Division, Russian Academy of Sciences. Scientific interests: mathematical methods for pattern recognition; discrete optimization; effective algorithms for the analysis and recognition of random sequences; algorithms for the solution of applied problems; and methods and algorithms for the processing, recognition, and synthesis of voice signals. Author of more than 120 papers. Sergei A. Khamidullin. Born March 28, 1952. Graduated from Novosibirsk State University in 1974. Received candidate’s degree in 1997. Senior researcher of the Sobolev Institute of Mathematics, Siberian Division, Russian Academy of Sciences. Scientific interests: mathematical methods for pattern recognition; discrete optimization; effective algorithms for the analysis and recognition of random sequences; algorithms for the solution of applied problems; and methods and algorithms for the processing, recognition, and synthesis of voice signals. Author of more than 80 papers.     

Simplified steady-state model of an SCR-inverter-basedload-commutated commutatorless series motor

This paper presents a simplified analytical model of a silicon-controlled-rectifier based, load-commutated inverter driven, commutatorless series motor to predict its speed-current, torque-current and torque-speed characteristics. The model has been formulated, keeping in view the steady state equivalent circuit of a conventional DC series motor. The proposed analytical expressions relating torque and current with speed are found to follow an inverse law and that relating torque and current is found to be parabolic, as is expected in a "series" machine. The drive has been implemented, loaded with a DC generator and experimentally similar characteristics have been obtained, as predicted by the analytical model. The characteristics obtained experimentally are seen to follow the ones, predicted by the analytical model closely     

A correlation between the microhardness and the mobility of twinning dislocations in bismuth crystals exposed to constant magnetic field and pulsed electric field

It is established that bismuth crystals under the simultaneous action of a constant magnetic field and current pulses exhibit a correlation between the microhardness and the mobility of twinning dislocations. It is shown that application of the external fields favors translation of the twinning dislocations along the twin-matrix boundaries.     

TEM study of PLZT ceramics

The microscopic structures of PLZT(7.9/70/30 and x/65/35, x = 7 or 8) ceramics were studied by means of transmission electron microscopy. The presence of micro-regions in PLZTs was first verfied.     

Operation of an experimental facility for fabrication of fuel elements and fuel assemblies of the bor-60 containing vibrocompacted fuel

Translated from Atomnaya Énergiya, Vol. 67, No. 5, pp. 320–323, November, 1989.     

Distribution of one of the ascitic fluid antigens of ovarian cancer patients in malignant tumors of the stomach and intestines

The indirect immunofluorescent technique was used to study the distribution of the ascitic fluid antigen (AgD) on histological sections from surgical specimens of gastric and colon tumours from proximal to distal lines of resection. AgD was found in gastric tumours exceptionally in the tumour area and in colon cancer--both in the tumour stroma and in surrounding histologically normal mucosa.     

A randomized voltage vector switching scheme for three-level powerinverters

In this paper, a feasibility study of a randomized space vector switching scheme for three-level inverters is reported. Based on the traditional space vector concept for pulse width modulation (PWM) inverters, the sequence of the space vector is randomized in this scheme. The overall vector, resulting from the randomization, is mathematically identical to that of the traditional space vector. It has been found that the randomized scheme can effectively spread the switching noise over a wide frequency range and at the same time keep the steady-state link capacitor voltage balanced. The proposed scheme has been implemented in real time in a digital controller and its features have been experimentally confirmed