Larger Pr Research Articles

Ultra-high resistive switching current ratio and improved ferroelectricity and dielectric tunability performance in a BaTiO3/La0.7Sr0.3MnO3 heterostructure by inserting a SrCoO2.5 layer.

A BaTiO3/SrCoO2.5 (BTO/SCO) bilayer and a BTO single film were prepared by radio frequency magnetron sputtering on La0.7Sr0.3MnO3 (LSMO) buffered SrTiO3 (001) substrates. Interestingly, compared with reported BTO-based films, the BTO/SCO/LSMO heterostructure has a maximum ON/OFF current ratio of ∼945. More interestingly, compared with the BTO single layer, a larger Pr (∼18.4 μC cm-2) and larger dielectric tunability (∼71.9%) were achieved in the BTO/SCO bilayer. The improved performance may be attributed to the large tetragonality and improved oxygen vacancy concentrations in the BTO/SCO/LSMO heterostructure. Furthermore, our BTO/SCO/LSMO stacks exhibit potential for flexible electronic informational devices.

Sensitivity of an Idealized Tropical Cyclone to the Configuration of the Global Forecast System–Eddy Diffusivity Mass Flux Planetary Boundary Layer Scheme

The intensity and structure of simulated tropical cyclones (TCs) are known to be sensitive to the planetary boundary layer (PBL) parameterization in numerical weather prediction models. In this paper, we use an idealized version of the Hurricane Weather Research and Forecast system (HWRF) with constant sea-surface temperature (SST) to examine how the configuration of the PBL scheme used in the operational HWRF affects TC intensity change (including rapid intensification) and structure. The configuration changes explored in this study include disabling non-local vertical mixing, changing the coefficients in the stability functions for momentum and heat, and directly modifying the Prandtl number (Pr), which controls the ratio of momentum to heat and moisture exchange in the PBL. Relative to the control simulation, disabling non-local mixing produced a ~15% larger storm that intensified more gradually, while changing the coefficient values used in the stability functions had little effect. Varying Pr within the PBL had the greatest impact, with the largest Pr (~1.6 versus ~0.8) associated with more rapid intensification (~38 versus 29 m s−1 per day) but a 5–10 m s−1 weaker intensity after the initial period of strengthening. This seemingly paradoxical result is likely due to a decrease in the radius of maximum wind (~15 versus 20 km), but smaller enthalpy fluxes, in simulated storms with larger Pr. These results underscore the importance of measuring the vertical eddy diffusivities of momentum, heat, and moisture under high-wind, open-ocean conditions to reduce uncertainty in Pr in the TC PBL.

Structural and Electric Properties of MnO2-Doped KNN-LT Lead-Free Piezoelectric Ceramics

Structural, ferroelectric, dielectric, and piezoelectric properties of K0.5Na0.5NbO3-LiTaO3-xmol%MnO2 lead-free piezoelectric ceramics with 0.0 ≤ x ≤ 0.3 were studied. The ceramic samples were synthesized through the conventional solid-state reaction method. The MnO2 addition can reduce the sintering temperature of KNLNT ceramics. Compared with undoped KNLNT ceramic, the piezoelectric measurements showed that piezoelectric properties of K0.5Na0.5NbO3-LiTaO3-xMnO2 were improved (d33 = 251 pC/N) when x = 0.1. In addition, KNLNT-xMnO2 ceramics have larger Pr(20.59~21.97 μC/cm2) and smaller Ec(10.77~6.95 kV/cm), which indicates MnO2 has excellent softening property, which improves the ferroelectric properties of KNLNT ceramics This work adds relevant information regarding of potassium sodium niobate K0.5Na0.5NbO3 (KNN) when doped Li, Ta, Mn at the B-site.

Enhanced photovoltaic effect in Ca and Mn co-doped BiFeO3 epitaxial thin films

In this work, we proposed modification of BiFeO3 (BFO) by Mn and Ca co-doping, and realized the epitaxial growth of Bi0.98Ca0.02Fe0.95Mn0.05O3 (BCFMO) thin films on (100)-oriented SrTiO3 (STO) substrates with a SrRuO3 (SRO) buffer layer via pulsed laser deposition (PLD). A decreased bandgap (Eg = 2.41 eV) and enhanced remanent polarization (Pr = 92.5 μC/cm2) were simultaneously achieved in the BCFMO thin film. A tunable photovoltaic (PV) effect by changing the direction of the poling electric field was observed in the BCFMO-based device. We investigated the PV mechanism for the device by analyzing its band structure and attribute it to the coupling effects of the ferroelectric depolarization field and the interface Schottky barriers. The BCFMO-based device under favorable upward poling shows the best PV performance with an open-circuit voltage (Voc) of −0.29 V and short-circuit current density (Jsc) of 0.26 mA/cm2, which are much higher than those for the BFO-based device (Voc = -0.11 V, Jsc = 0.08 mA/cm2), resulting from the narrower bandgap and larger Pr in the BCFMO film. The approach of simultaneously introducing Ca and Mn into the BFO, enabling both a higher polarization and narrower bandgap, opens a new way for improving the performance of BFO-based PV devices.

Effect of Prandtl Number on Mixed Convective Heat Transfer from a Porous Cylinder in the Steady Flow Regime.

The effect of the Prandtl number (Pr) on the flow and heat transfer from a porous circular cylinder with internal heat generation in the mixed convection regime is numerically investigated. The steady flow regime is considered over the ranges of the Reynolds number (Re), Darcy number (Da), and Richardson number (Ri), varying from 5 to 40, 10−6 to 10−2, and 0 to 2, respectively. The wake structure, the temperature distribution, and the heat transfer rate are discussed. Besides precipitating the growth of the recirculating wake, the Prandtl number is found to have a significant impact on the thermal characteristics. The concave isotherms, resembling a saddle-shaped structure, occur behind the cylinder at larger Pr, resulting in swells of the isotherms pairing off at the lateral sides. These swells are found to have a negative effect on heat transfer owing to a relatively smaller temperature gradient there. Then, the heat transfer rate in terms of the local Nusselt number (Nu) and enhancement ratio (Er) is calculated, which is closely related to Pr, Re, Da, and Ri. The local minimum heat transfer rate along the cylinder surface is found at the position where the swells of the isotherms form.

Flow organization and heat transfer in two-dimensional tilted convection with aspect ratio 0.5

Thermal convection in a two-dimensional tilted cell with aspect ratio (Γ = width/height) 0.5 is studied using direct numerical simulations. The considered tilt angle β ranges from 0° to 90°. The Prandtl number Pr dependence is first studied in the range of 0.01 ≤ Pr ≤ 100 for a fixed Rayleigh number Ra = 107. The Ra dependence is also investigated in the range of 106 ≤ Ra ≤ 109 for a fixed Pr = 0.71. Different flow states are identified over the β − Pr parameter space. It is found that the flow tends to organize in stable vertically-stacked double-roll state (DRS) for small Pr and small β, while this DRS becomes unstable and flow reversals happen with the increase of β. This finding complements our previous study of flow reversals in tilted cells with Γ = 1 and 2 [Wang et al., J. Fluid Mech. 849, 355–372 (2018)]. For relatively larger Pr, the flow gives way to a stable triple-roll state or an unstable triple-roll state for small β. Moreover, multiple states in the turbulent regime are found for Ra ≥ 108, between which the flow can or cannot switch. In the latter case, the Nu are different for the two states with the same number of convection rolls, but different orientations. It is found that the Nu(β)/Nu(0) and Re(β)/Re(0) dependence is strongly influenced by a combination of Ra and Pr. In the present system, we interestingly find that the earlier conclusion that Nu decreases with increasing β close to β = 90° for Γ = 1 does not hold for the present Γ = 0.5 case with small Pr.

Reynolds numbers and the elliptic approximation near the ultimate state of turbulent Rayleigh–Bénard convection

We report results of Reynolds-number measurements, based on multi-point temperature measurements and the elliptic approximation (EA) of He and Zhang (2006 Phys. Rev. E 73 055303), Zhao and He (2009 Phys. Rev. E 79 046316) for turbulent Rayleigh–Bénard convection (RBC) over the Rayleigh-number range and for a Prandtl number Pr ≃ 0.8. The sample was a right-circular cylinder with the diameter D and the height L both equal to 112 cm. The Reynolds numbers ReU and ReV were obtained from the mean-flow velocity U and the root-mean-square fluctuation velocity V, respectively. Both were measured approximately at the mid-height of the sample and near (but not too near) the side wall close to a maximum of ReU. A detailed examination, based on several experimental tests, of the applicability of the EA to turbulent RBC in our parameter range is provided. The main contribution to ReU came from a large-scale circulation in the form of a single convection roll with the preferred azimuthal orientation of its down flow nearly coinciding with the location of the measurement probes. First we measured time sequences of ReU(t) and ReV(t) from short (10 s) segments which moved along much longer sequences of many hours. The corresponding probability distributions of ReU(t) and ReV(t) had single peaks and thus did not reveal significant flow reversals. The two averaged Reynolds numbers determined from the entire data sequences were of comparable size. For both ReU and ReV could be described by a power-law dependence on Ra with an exponent ζ close to 0.44. This exponent is consistent with several other measurements for the classical RBC state at smaller Ra and larger Pr and with the Grossmann–Lohse (GL) prediction for ReU (Grossmann and Lohse 2000 J. Fluid. Mech. 407 27; Grossmann and Lohse 2001 86 3316; Grossmann and Lohse 2002 66 016305) but disagrees with the prediction by GL (Grossmann and Lohse 2004 Phys. Fluids 16 4462) for ReV. At the dependence of ReV on Ra changed, and for larger Ra , consistent with the prediction for ReU (Grossmann and Lohse 2000 J. Fluid. Mech. 407 27; Grossmann and Lohse Phys. Rev. Lett. 2001 86 3316; Grossmann and Lohse Phys. Rev. E 2002 66 016305; Grossmann and Lohse 2012 Phys. Fluids 24 125103) in the ultimate state of RBC.

Unsteady mixed convection visco-elastic flow and heat transfer in a thin film flow over a porous stretching sheet with internal heat generation

In this study, a two-dimensional unsteady mixed convection flow of an incompressible visco-elastic fluid over a porous thermal forming thin film stretching sheet with internal heat generation has been studied. Two novel items x and are presented for free convection in porous space important parameters and an internal heat generation phenomena, respectively. The similarity transformation and an implicit finite-difference method have been used to analyze the present problem. The numerical solutions of the flow velocity distributions, temperature profiles, the wall unknown values of f''(0) and for calculating the heat transfer of the similar boundary-layer flow are carried out as functions of the unsteadiness parameter (S), the Prandtl number (Pr), the visco-elastic parameter (α), the porous parameter (), the space-dependent parameter (A) and temperature-dependent parameter (B) for heat source/sink and the free convection parameter (G). The effects of these parameters have also been discussed. The results show that it will produce greater heat transfer effect with a larger Pr, G and α, but S, A and B will reduce heat transfer effect. Key words: Visco-elastic, thin film flow, porous medium, finite-difference method, heat transfer, unsteady stretching sheet, mixed convection, internal heat generation.

Structure and piezoelectric properties of new (Bi0.5Na0.5)1−x−yBax(Yb0.5Na0.5)yTiO3 lead-free ceramics

New lead-free ceramics (Bi0.5Na0.5)1−x−yBax(Yb0.5Na0.5)yTiO3 (x = 0.02–0.10 and y = 0–0.04) have been prepared by an ordinary sintering technique and their structure and piezoelectric properties have been studied. X-ray diffraction shows that Ba2+ and Yb3+ diffuse into the Bi0.5Na0.5TiO3 lattices to form a solid solution with a pure perovskite structure and a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases is formed at 0.04 < x < 0.10. The partial substitutions of Ba2+ and Yb3+ for A-site ions of Bi0.5Na0.5TiO3 decrease effectively the coercive field Ec and improve significantly the remanent polarization Pr. The ceramics with x = 0.06 and y = 0–0.02 situate within the MPB and possess the lower Ec and larger Pr, and thus exhibit optimum piezoelectric properties: d33 = 155–171 pC/N and kp = 29.2–36.7%. The temperature dependences of the dielectric and ferroelectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above Td.

Ferroelectric and piezoelectric properties of [(Bi0.98La0.02Na1−x Li x )0.5]0.94Ba0.06TiO3 lead-free ceramics

Lead-free ceramics [(Bi0.98La0.02Na1−x Li x )0.5]0.94Ba0.06TiO3 have been prepared by an ordinary sintering technique and their ferroelectric and piezoelectric properties have been studied. The results of X-ray diffraction reveal that Li+, Ba2+, and La3+ diffuse into the Bi0.5Na0.5TiO3 lattices to form a new solid solution with a pure perovskite structure. The partial substitution of Li+ lowers the coercive field E c and improves the remanent polarization P r. Because of the larger P r and lower E c, the ceramic with x = 0.10 exhibits optimum piezoelectric properties: d 33 = 212 pC/N and k P = 36.1%. The partial substitution of Li+ for Na+ shifts the depolarization temperature T d toward low temperature. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics contain both the polar and non-polar regions near/above T d, which cause the polarization hysteresis loop become deformed and the ceramics become depolarized.

Natural convection flows in porous trapezoidal enclosures with various inclination angles

Simulations were carried out using penalty finite element analysis with bi-quadratic elements to investigate the influence of uniform and non-uniform heating of bottom wall within a trapezoidal enclosure of various inclination angles (φ). Parametric study has been carried out for a wide range of Rayleigh number (Ra)(103⩽Ra⩽106), Prandtl number (Pr)(0.026⩽Pr⩽988.24) and Darcy number (Da)(10-3⩽Da⩽10-5). Numerical results are presented in terms of stream functions, isotherm contours and Nusselt numbers. The heat transfer is primarily due to conduction at lower values of Darcy number (Da) and convection dominant heat transfer is observed at higher Da values. The intensity of circulation increases with increase in Darcy number. Increase in the intensity of circulations and larger temperature gradient are also observed with increase in φ from 0° to 45° especially at larger Pr and Ra. Non-uniform heating of the bottom wall produces greater heat transfer rate at the center of the bottom wall than uniform heating at all Rayleigh and Darcy numbers, but average Nusselt number is lower for non-uniform heating. Local heat transfer rates are found to be relatively greater for φ=0°. It is observed that the local heat transfer rate at the central portion of bottom wall is larger for non-uniform heating case. Average Nusselt number plots show higher heat transfer rates at the bottom wall for φ=0° as compared to φ=45° and φ=30°. It is observed that the average heat transfer rate at the bottom wall is found to be invariant with respect to φ at higher Ra for non-uniform heating. Critical Rayleigh numbers for conduction dominant heat transfer cases have been obtained and the power law correlations between average Nusselt number and Rayleigh numbers are presented for convection dominated regimes.

Crystal Structure and Ferroelectric Properties of Bi4Si3O12-Added Sr-Ce-Bi-Ta-O System

We investigated crystal structure and ferroelectric properties of bismuth layered-structure compounds prepared by alloying Ce substituted Sr 1− x Ce x Bi 2 Ta 2 O 9 (SCBT) with Bi 3 Si 4 O 12 (BSO) using a solid state reaction method. From the P-E hysteriesis measurement, the SCBT has larger Pr and Ec compared with SBT. Pr increased and Ec decreased by adding BSO to SCBT. The crystal structure was determined by the Rietveld analysis using neutron powder diffraction data. The results of analysis indicated that the SCBT and that added with BSO had a larger spontaneous polarization, and a smaller tilting angle (α a ) in comparison with those of SBT. Moreover, the distortion of TaO 6 octahedron was changed in the SCBT and that added with BSO.

Photophysical properties of Pr(iii) and Er(iii) complexes of poly(pyrazolyl)borates

The complexes [M(L(1))(2)(NO(3))] and [M(L(2))(NO(3))(2)](M = Pr, Er; L(1)= the tetradentate ligand dihydrobis-[3-(2-pyridyl)pyrazolyl]borate; L(2)= the hexadentate ligand hydrotris-[3-(2-pyridyl)pyrazolyl]borate) were prepared and their structural and photophysical properties studied. All complexes are 10-coordinate. Crystallographic analysis of [M(L(1))(2)(NO(3))](M = Pr, Er) showed that for the smaller Er(iii) ions steric congestion at the metal centre results in two of the Er-N(pyridyl) distances being particularly long, which does not occur with the larger Pr(iii) ion that is better able to accommodate 10-fold coordination. On UV irradiation, both Pr(iii) complexes show, in the visible region of their luminescence spectra, transitions originating from both the (3)P(0) level (at ca. 21,000 cm(-1)) and the (1)D(2) level (at ca. 17,000 cm(-1)), a consequence of the fact that the lowest triplet state of the coordinated pyrazolylborate ligands lies at ca. 24,000 cm(-1) in each case so is high enough in energy to populate both levels. This contrasts with Pr(iii) complexes based on diketonate ligands in which the lower triplet energies of the ligands result in emission from the (1)D(2) level only. At longer wavelengths, near-infrared luminescence arising from the (1)D(2) emissive level is observed with lifetimes (in both the solid state and solution) being in the range 50-110 ns. For both Er(iii) complexes, luminescence at 1530 nm occurs following UV excitation of ligand-centred transitions. In CH(2)Cl(2) both complexes gave dual-exponential luminescence, with the major component having a lifetime characteristic of an intact Er(iii) complex (approximately 1.5 micros) and the minor component being much shorter lived (0.2-0.5 micros), suggestive of a species in which a ligand is partially detached and the metal is solvated, with the two forms interconverting slowly. This behaviour is consistent with the steric congestion and long M-N(pyridyl) bonds that were observed in [Er(L(1))(2)(NO(3))]. In the solid state both Er(iii) complexes gave very weak luminescence, which could be fitted to a single exponential decay with a lifetime similar to the longer-lived of the solution components.

1H-NMR Relaxation of Water Molecules Contained in the Cellulose Membranes and Their Biocompatibility.

Spin-lattice relaxation time, T1 of water molecules (H2O/Cellulose=0.08, 1.1, 2.0 (g/g)) was estimated by 1H-NMR technique for the five regenerated cellulose membranes with different water contents, and related to a biocompatibility and a structure of the membrane. In all water contents investigated here, two T1 components (T1, l>T1, s) were observed. A degree of complement activation, Ac which was estimated as an index of biocompatibility showed good correlation with porosity, Pr of the dried sample, not with an aggregation structure of cellulose chains (e. g. crystallinity), suggesting that the Ac was mainly influenced by a surface area of the membrane. In addition, it was found that the sample having the larger Pr or the higher Ac tends to have the shorter T1, l value, especially in the swollen state. These results lead to a conclusion that T1, l value estimated for the swollen sample can be regarded as an index parameter for a biocompatibility, because this value reflects a surface area of the swollen cellulose membrane.

Superconductivity in (Gd 1.85 − xPr xCe 0.15)CuO 4

We have carried out a systematic study of structural and superconducting characteristics of the system (Gd 1.85 − x Pr x Ce 0.15)CuO 4 over the entire composition range. Superconductivity was observed for Gd 0.75Pr 1.10Ce 0.15 CuO 4 at 6.5 K. Superconducting transition temperature ( T c) in this compound was found to increase to 18 K by subjecting it to high Ar pressure annealing. However, attempts to induce superconductivity in the undoped Gd 1.85Ce 0.15CuO 4 did not succeed. The results are explained as due to the increased lattice parameters of the doped compound, consequent upon the replacement of Gd 3+-ion by larger Pr 3+-ion, and the increase in the charge carrier concentration as a result of high pressure argon annealing.

Praseodymium-small rare earth gallium garnets with rare earths on two crystallographic sites

Garnets of the type {Pr 3− y R y }[R x Ga 2− x ](Ga 3)O 12, in which R is Lu, Yb, Tm, or Er, have been synthesized and studied. They are analogous to materials reported earlier with Nd in place of Pr, but the minimum y values which result in compliance with Vegard's law are higher. This is the case because the larger Pr 3+ ion requires a greater amount than does Nd 3+ of a given small rare earth ion, R 3+, to reduce the average dodecahedral site radius to a value which satisfies the R 3+ on the octahedral site. Magnetic susceptibility measurements over the temperature range 78–300°K show the materials to be paramagnetic with Curie-Weiss behavior but with Curie constants somewhat lower than expected.