245 research outputs found
Phase transition in Pr0.5Ca0.5CoO3 and related cobaltites
We present an extensive investigation (magnetic, electric and thermal
measurements and X-ray absorption spectroscopy) of the Pr0.5Ca0.5CoO3 and
(Pr1-yYy)0.7Ca0.3CoO3 (y=0.0625-0.15) perovskites, in which a peculiar
metal-insulator (M-I) transition, accompanied with pronounced structural and
magnetic anomalies, occurs at 76 K and 40-132 K, respectively. The inspection
of the M-I transition using the XANES data of Pr L3-edge and Co K-edge proofs
the presence of Pr4+ ions at low temperatures and indicates simultaneously the
intermediate spin to low spin crossover of Co species on lowering the
temperature. The study thus definitively confirms the synchronicity of the
electron transfer between Pr3+ ions and Co^(3+/4+)O3 subsystem and the
transition to the low-spin, less electrically conducting phase. The large
extent of the transfer is evidenced by the good quantitative agreement of the
determined amount of the Pr4+ species, obtained either from the temperature
dependence of the XANES spectra or via integration of the magnetic entropy
change over the Pr4+ related Schottky peak in the low-temperature specific
heat. These results show that the average valence of Pr3+/Pr4+ ions increases
(in concomitance with the decrease of the formal Co valence) below TMI for
(Pr0.925Y0.075)0.7Ca0.3CoO3 up to 3.16+ (the doping level of the CoO3 subsystem
decreases from 3.30+ to 3.20+), for (Pr0.85Y0.15)0.7Ca0.3CoO3 up to 3.28+ (the
decrease of doping level from 3.30+ to 3.13+) and for Pr0.5Ca0.5CoO3 up to
3.46+ (the decrease of doping level from 3.50+ to 3.27+).Comment: 19 pages, 11 figure
Adaptive Disturbance Tracking Theory with State Estimation and State Feedback for Region II Control of Large Wind Turbines
A theory called Adaptive Disturbance Tracking Control (ADTC) is introduced and used to track the Tip Speed Ratio (TSR) of 5 MW Horizontal Axis Wind Turbine (HAWT). Since ADTC theory requires wind speed information, a wind disturbance generator model is combined with lower order plant model to estimate the wind speed as well as partial states of the wind turbine. In this paper, we present a proof of stability and convergence of ADTC theory with lower order estimator and show that the state feedback can be adaptive
Initial conditions for turbulent mixing simulations
In the context of the classical Rayleigh-Taylor hydrodynamical instability, we examine the much debated question of models for initial conditions and the possible influence of unrecorded long wave length contributions to the instability growth rate α.У контексті класичної гідродинамічної нестійкості Релея - Тейлора вивчено дискусійне питання моделей для початкових умов і можливий вплив незафіксованих далекосяжних вкладів на швидкість росту нестійкості альфа
- …