Low-Temperature Synthesis of Ultrafine La0.7Sr0.3MnO3 Powder via Chelate Homogenization

Ultrafine La_0.7Sr_0.3MnO₃ powders were prepared via homogenization in chelate solutions, followed by the calcination of solid precursors at 700°C in air or oxygen, and their phase composition and average particle size were determined. The solid precursors were obtained from a solution of polynuclear La, Sr, and Mn chelates (diethylenetriaminepentaacetates) by three procedures: (1) gelation of the solution, followed by air drying of the resultant gel; (2) gelation followed by microwave dehydration; and (3) microwave dehydration of the solution. The results demonstrate that the way in which the chelate solution is converted into solid foams has little effect on the phase composition and particle size of the powders. At the same time, the phase composition and particle size of the reaction products depend on the calcination atmosphere (air or oxygen). Calcination in oxygen (700°C, 10 h) ensures the preparation of phase-pure La_0.7Sr_0.3MnO₃ powders, with an average particle size of 30 nm, from the three precursors.     

International collaboration on salvaging nuclear-powered submarines

The problem of salvaging multipurpose nuclear-powered submarines, Russias capabilities for solving this problem without any assistance, and the need for international assistance to hasten the solution are examined. Multi- and bilateral mechanisms for international collaboration in this field are presented. The status of the collaboration with all countries participating in the global partnership is reviewed.Translated from Atomnaya Énergiya, Vol. 97, No. 5, pp. 387–395, November, 2004.     

Anomalous Reentrant Resistance Phenomena in Superconducting Sr1 ? xKxBiO3: Recovery of Superconductivity with Electric or Magnetic Field

The temperature-dependent resistivity of Sr_{1 – x}K_xBiO₃, with x = 0.4–0.6, has been measured as a function of the magnetic field (or electrical current). Although X-ray diffraction results are more or less identical to single phase for the measured 10 samples, the EDS results indicate that the potassium content, x, varies from 0.4 to 0.6 and the electrical resistance varies quite sensitively from sample to sample. For the samples of resistivity less than 4 m cm at room temperature with (15 K)/(273 K) < 0.9, superconductivity is observed with T_c 12 K. Other samples exhibit a reentrant resistance below the superconducting transition temperature. The reentrant resistance, however, decreases as the external magnetic field (or electrical current) is applied, and some samples show the recovery of superconductivity upon the application of a magnetic field (or electrical current). Disorderness in the junction area between superconducting grains seems to be vital for the observed anomalous reentrant resistance. Further investigations are on to understand this intriguing phenomena.     

Complete salvaging of decommissioned objects of the nuclear-powered fleet in the northwest and methodological approaches

The basic problems of complete salvaging of the decommissioned Russian nuclear-powered fleet in the northwest are examined: salvaging of nuclear-powered submarines, nuclear-powered surface ships, nuclear tenders, and ecological remediation of the objects of the shored infrastructure supporting the operation of the ships in the nuclear-powered nanval fleet of Russia. The basic methodological aproaches whose implementation will make it possible to determine rational ways to decrease the ecological threat and to increase national safety are formulated on the basis of an analysis of the scales and complexity of the problems which have accumulated concerning the complete salvaging and remediation of the objects of the nuclear-powered fleet. __________ Translated from Atomnaya énergiya, Vol. 101, No. 1, pp. 4–11, July, 2006.     

Sorption of Water by Aromatic Polyamides

Protection of Metals and Physical Chemistry of Surfaces - Isotherms of water-vapor sorption by aramid fibers and films of various nature were determined by static sorption methods. Structural and...     

Development of antimony-based impregnating alloys to produce electrographite frame composite materials

The properties of antimony-based alloys used as an impregnating metal in electrographite composites for creating antifriction composite materials are studied. The linear shrinkage, penetrability, corrosion resistance, and mechanical strength of the alloys are experimentally determined.     

Study of the characteristics of ultraviolet light-emitting diodes based on GaN/AlGaN heterostructures grown by chloride-hydride vapor-phase epitaxy

The results of work on developing and studying ultraviolet (UV) light-emitting diodes (LEDs) based on GaN/AlGaN heterostructures fabricated on Al₂O₃(0001) substrates by the chloride-hydride vaporphase epitaxy are presented. The maximum in the electroluminescence spectrum is located in the wavelength range of 360–365 nm, and its full width at half maximum is 10–13 nm. At a working current of 20 mA, the optical density and efficiency of the UV LED are 1.14 mW and 1.46%, respectively.     

Quantum Dot Infrared Photodetectors: Photoresponse Enhancement Due to Potential Barriers

Potential barriers around quantum dots (QDs) play a key role in kinetics of photoelectrons. These barriers are always created, when electrons from dopants outside QDs fill the dots. Potential barriers suppress the capture processes of photoelectrons and increase the photoresponse. To directly investigate the effect of potential barriers on photoelectron kinetics, we fabricated several QD structures with different positions of dopants and various levels of doping. The potential barriers as a function of doping and dopant positions have been determined using nextnano³ software. We experimentally investigated the photoresponse to IR radiation as a function of the radiation frequency and voltage bias. We also measured the dark current in these QD structures. Our investigations show that the photoresponse increases ~30 times as the height of potential barriers changes from 30 to 130 meV.