Dielectric performance of pyrochlore-type Bi2MgNb2-xTaxO9 ceramics: The effects of tantalum doping

The solid solutions based on the pyrochlore-type system Bi₂MgNb_2-xTa_xO₉ were formed in the compositional range х = 0–2.0 (Bi_1·6Mg_0·8Nb_1.6-tTa_tO_7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi₂MgNb_2-xTa_xO₉ (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi_1·6Mg_0·8Ta_1·6O_7.2). The increase in the Ta⁵⁺ concentration led to the decrease in the dielectric permeability from 104 (Bi_1·6Mg_0·8Nb_1·6O_7.2) to 20 (Bi_1·6Mg_0·8Ta_1·6O_7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme.     

A Digital Deformation Vacuum Gauge with an LC-Generator

The design of a sylphon bellows sensor and the basic circuits of an LC-generator and of a microprocessor unit are presented. An analytical pressure–frequency conversion function and a special method of adjusting the sensor ensure an error of less than 0.05%. The dynamic range is up to 10⁵. The instruments developed cover the ranges 10³, 10⁴, and 10⁵ Pa.     

NANODEV: A Nanoelectronic-Device Simulation Software System

The concept and structure of the NANODEV simulation software are described. NANODEV deals with nanoelectronic devices that exploit single-electron tunneling, resonant tunneling, or quantum interference. It can use both simplified and sophisticated models and enables one to evaluate a wide variety of devices and configurations. The capabilities of NANODEV are illustrated by examples.     

Model of the magmatic thermolysis of coal matter deep in the earth (short communication)

A model of contact thermolysis was constructed based on a combined set of equations for heat transfer from a magmatic intrusion to a coal bed and the kinetics of thermal coal conversion. This model was illustrated by the generation of petroleum hydrocarbons deep in the earth by the thermolysis of the sapropelic matter of boghead.     

Relative Yield and Period of Individual Groups of Delayed Neutrons in 233U, 235U, and 239Pu Fission by Epithermal Neutrons

The relative yield of delayed neutrons and the half-life of their precursor nuclei in fissioning of ²³³U, ²³⁵U, and ²³⁹Pu by epithermal neutrons are measured on a setup which is based on a KG-2.5 electrostatic accelerator. The experimental samples are irradiated in a polyethylene cube, one face of which is irradiated by a neutron beam consisting of T(p, n)³He neutrons. A procedure is developed for averaging the group parameters of delayed neutrons from different series of measurements, taking account of their correlation. A comparative analysis of the data of this work and in ENDF/BVI in terms of the average half-life of the precursor nuclei of the delayed neutrons and using the dependence of the reactivity on the asymptotic period of the reactor is performed.It is shown that the data of this work agree, to within the uncertainty limits, with the recommended experimental data and differ substantially from the ENDF/BVI data.     

Selection of Protector Parameters for Right Bidirectional Transducers

The paper compares experimental and calculated pulse shapes and spectra of signals for bidirectional right transducers with radically different protectors, namely, those from epoxy glue with a filler and ceramic. The new data support the previously made conclusion (from calculated parameters of emitted pulses) that in protectors from composite materials (with low propagation velocities of longitudinal elastic waves) signal spectra can have several pronounced peaks, and their waveforms can exhibit beats (oscillating amplitude modulation).     

Relative yield of delayed neutrons and half-life of their precursor nuclei with fissioning of 239Pu by 14.2–17.9 MeV neutrons

The energy dependence of the relative yield of delayed neutrons and of half-lives of their precursor nuclei with fissioning ²³⁹Pu by 14.2–17.9 MeV neutrons is measured. The data obtained are analyzed in terms of the average half-life 〈T〉 of the precursor nuclei of the delayed neutrons. It is shown that the observed growth of 〈T〉 with increasing primary neutron energy in the energy range studied is due predominately to opening of the channels for the emission fission of nuclei. Comparing the data on the average half-life of the precursor nuclei of delayed neutrons obtained in the present work with those obtained by other authors showed that the anomalously high values of 〈T〉 are due to methodological features of the experiment which are related with the properties of the reaction T(d, n)⁴He used as a neutron source on electrostatic charged-particle accelerators. The data obtained are of great value for developing instrumentational nondestructive means for determining the nuclide composition of nuclear fuel based on the detection of delayed neutrons. __________ Translated from Atomnaya énergiya, Vol. 101, No. 6, pp. 441–444, December, 2006.