76 research outputs found
Growth of twin-free b-oriented YBa2Cu3O7-x Films
No full textYBa2Cu3O7 - x films of b orientation have been grown on the surface of (100)SrLaGaO4 crystal. It is demonstrated that the films pass to the superconducting state at a temperature of 89 K. In the nonsuper-conducting region, the film resistance decreases linearly with a decrease in temperature at a rate of 30 Omega per 100A degrees C. According to the X-ray diffraction data on the crystallographic structure of the YBa2Cu3O7 - x films, they are single-crystal and b-oriented. It is shown that the films contain no domains of other crystallographic orientations, including a-oriented domains. Therefore, the films are free of structural twins
Growth of YBa2Cu3O7 films with [110] tilt of CuO planes to surface on SrTiO3 crystals
No full textYBa2Cu3O7 films with the CuO plane tilted to the surface have been grown on SrTiO3 crystalline substrates by pulsed laser deposition. This tilt was obtained by rotating the film lattice with respect to the substrate surface around its [110] axis oriented parallel to the surface. The zero tilt of the CuO plane was implemented at the orientation of the SrTiO3 crystal surface parallel to the (100) plane. The rotation angles were varied in the range from 0A degrees to 70A degrees. It is found that, being tilted at any angle, the CuO planes of the film remain parallel to one or several crystallographic planes of the (100)-type substrate. In the range of tilt angles from 0A degrees to 49A degrees, the film has a single-crystal structure. Above 49A degrees the film is transformed into a three-domain texture and its surface roughness sharply increases
Growth of YBa2Cu3O7 films with 100 tilt of CuO planes to the surface on SrTiO3 crystals
No full textYBa2Cu3O7 films with CuO planes tilted to the surface have been grown on SrTiO3 crystals by pulsed laser deposition. The tilt angle was varied in the range from 0A degrees (CuO planes are parallel to the surface) to 70A degrees. At this tilt, the crystalline lattice of the film was rotated by rotating the substrate crystalline lattice around its axis [100], which is oriented parallel to the surface. The zero tilt of CuO planes corresponds to the orientation of the SrTiO3 surface parallel to the (100) plane. It is established that the CuO planes of the film remain parallel to one or several (100)-type planes of the substrate at an arbitrary tilt angle. In the range of tilt angles from 0A degrees to 41A degrees, the film has a single-crystal structure. At tilt angles above 41A degrees, the film is transformed into a two-domain texture and its surface roughness sharply increases
Growth of Ultrathin Twin-Free b-Oriented YBa2Cu3O7-x Films
No full textTwin-free b-oriented YBa2Cu3O7 - x films with a thickness less than 40 nm have been epitaxially grown on (100)SrLaGaO4 crystals. Based on the temperature dependence of resistance, the onset temperature of the transition to the superconducting state is found to be 90 K; the transition width is 4 K. The film growth has been performed in two stages. A (100)PrBa2Cu3O7 - x buffer layer was previously grown on a (100)SrLaGaO4 substrate by rf magnetron sputtering in an Ar-O-2 gas mixture at a continuous and monotonic increase in temperature from 660 to 830A degrees C. The main YBa2Cu3O7 - x film was grown on the buffer layer surface by pulsed laser deposition in an oxygen medium at a fixed temperature (800A degrees C). The above processes were implemented in different chambers, which were connected by a vacuum channel for transporting samples. Both films were grown in situ, without contacting atmosphere in all growth stages. An X-ray diffraction study has shown that the YBa2Cu3O7 - x films are single-crystal and free of precipitates of other phases and domains of other orientations
Temperature sensitivity and noise of Josephson detector on bicrystal sapphire substrates at 77 K
No full textIn YBa 2 Cu 3 O 7 - x films grown on sapphire bicrystal substrates, the Josephson junctions are prepared based on artificial grain boundaries formed by the turn of the crystal lattices about the [100] axis. The films are deposited by the laser ablation method on the buffer CeO 2 layer. The critical film temperature reaches 88.5 K with a transition width of 1.5 K. Junctions from 2 to 3-μ m wide are integrated into the planar log-periodic antennas and their characteristics are measured at 77 K. The characteristic voltage I c R n reaches 570 μV. With exposure to external radiation at a frequency of 113 GHz, the Shapiro steps were observed on the current-voltage characteristic. The temperature sensitivity of this detector placed in a quasi-optical receiving unit is measured. At the modulation of the input radiation temperature 77 K/300 K, a response of more than 200 nV is observed at the detector output. At the modulation frequency, intrinsic noise is about 1 nV/Hz 1/2 , which corresponds to a temperature resolution of 1 K. \ua9 2007 Pleiades Publishing, Ltd
Rethinking the light water reactor fuel cycle
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, February 2004.Includes bibliographical references (p. 249-262).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.The once through nuclear fuel cycle adopted by the majority of countries with operating commercial power reactors imposes a number of concerns. The radioactive waste created in the once through nuclear fuel cycle has to be isolated from the environment for thousands of years. In addition, plutonium and other actinides, after the decay of fission products, could become targets for weapon proliferators. Furthermore, only a small fraction of the energy potential in the fuel is being used. All these concerns can be addressed if a closed fuel cycle strategy is considered offering the possibility for partitioning and transmutation of long lived radioactive waste, enhanced proliferation resistance, and improved utilization of natural resources. It is generally believed that dedicated advanced reactor systems have to be designed in order to perform the task of nuclear waste transmutation effectively. The development and deployment of such innovative systems is technically and economically challenging. In this thesis, a possibility of constraining the generation of long lived radioactive waste through multi-recycling of Trans-uranic actinides (TRU) in existing Light Water Reactors (LWR has been studied. Thorium based and fertile free fuels (FFF) were analyzed as the most attractive candidates for TRU burning in LWRs. Although both fuel types can destroy TRU at comparable rates (about 1150 kg/GWe-Year in FFF and up to 900 kg/GWe-Year in Th) and achieve comparable fractional TRU burnup (close to 50a/o), the Th fuel requires significantly higher neutron moderation than practically feasible in a typical LWR lattice to achieve such performance.(cont.) On the other hand, the FFF exhibits nearly optimal TRU destruction performance in a typical LWR fuel lattice geometry. Increased TRU presence in LWR core leads to neutron spectrum hardening, which results in reduced control materials reactivity worth. The magnitude of this reduction is directly related to the amount of TRU in the core. A potential for positive void reactivity feedback limits the maximum TRU loading. Th and conventional mixed oxide (MOX) fuels require higher than FFF TRU loading to sustain a standard 18 fuel cycle length due to neutron captures in Th232 and U238 respectively. Therefore, TRU containing Th and U cores have lower control materials worth and greater potential for a positive void coefficient than FFF core. However, the significantly reduced fuel Doppler coefficient of the fully FFF loaded core and the lower delayed neutron fraction lead to questions about the FFF performance in reactivity initiated accidents. The Combined Non-Fertile and UO2 (CONFU) assembly concept is proposed for multi- recycling of TRU in existing PWRs. The assembly assumes a heterogeneous structure where about 20% of the UO2 fuel pins on the assembly periphery are replaced with FFF pins hosting TRU generated in the previous cycle. The possibility of achieving zero TRU net is demonstrated. The concept takes advantage of superior TRU destruction performance in FFF allowing minimization of TRU inventory. At the same time, the core physics is still dominated by UO2 fuel allowing maintenance of core safety and control characteristics comparable to all-UO2.by Evgeni Shwageraus.Ph.D
THz Josephson properties of grain boundary YBaCuO junctions on symmetric, tilted bicrystal sapphire substrates [Elektronisk resurs]
No full textSuperconducting Josephson junctions with high characteristic voltages (IcRn larger than 4 mV at 4.2 K) are fabricated by depositing YBa2Cu3O7-x on miscut sapphire bi-crystal substrates, where the tilting axis is along the grain boundary. The good junction quality and low microwave losses in sapphire gave high frequency response well into the THz region. High quality YBa2Cu3O7-x epitaxial films were deposited on tilted (vicinal) sapphire substrates with CeO2 buffer layers by pulsed laser deposition. YBaCuO films have smaller tilt angles, from 1.0o up to 10.3o, compared to inclination angles of the substrates from 1.5o to 13.6o. X-ray diffraction shows only a single orientation of the films in the a-b plane, as well as an absence of a-axis particles and outgrowths. Critical temperatures as high as Tc=88.589.0 K and Tc1.5 K were obtained in all films. The grain boundary in a common high-Tc superconducting junction is usually much less straight than in the bi-crystal substrate. The meandering of the artificial grain boundary in a tilted bi-crystal film is three times less than in an in-plane (un-tilted) bi-crystal. Tilted Josephson junctions of widths from 1.5 to 6 m were tested at temperatures from 0.26 K to 77 K. IcRn products as high as 4.5 mV were observed at T=4.2 K. Shapiro steps were observed at voltages over 3 mV under 300 GHz irradiation. Josephson radiation from the tilted junction was measured at frequencies up to 1.7 THz by a cryogenic bolometer. Suppressing the critical current with a magnetic field can separate Josephson radiation and thermal radiation. A parabolic dependence of the response on bias voltage for thermal radiation corresponds to an increase of junction temperature from 260 mK at zero bias to 3 K at 1 mV bias
Corporate Governance Transformation in Transition Economies: Evidence from Bulgaria
No full textThe paper presents study of corporate governance evolution in Bulgaria and its convergence to EU structures. The author discusses the traditional approach to separation of ownership and control in transition economies and modern ideas about the need for changing corporate governance paradigm. The study analyses main ownership and control structures in the Bulgarian industry and specific property-rights structures in corporatized state-owned firms, privatized firms and established de novo private enterprises. The research reveals the main constraints on corporate governance convergence to European structures in Bulgaria as the emerging enterprise sector duality and inefficient property-rights structures based on the state-owned firms transformation. The study analyses different paths for corporate governance and country development in the short run.
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