Rossby waves and α\alpha-effect

Rossby waves drifting in the azimuthal direction are a common feature at the onset of thermal convective instability in a rapidly rotating spherical shell. They can also result from the destabilization of a Stewartson shear layer produced by differential rotation as expected in the liquid sodium experiment (DTS) working in Grenoble, France. A usual way to explain why Rossby waves can participate to the dynamo process goes back to Busse (1975). In his picture, the flow geometry is a cylindrical array of parallel rolls aligned with the rotation axis. The axial flow component (the component parallel to the rotation axis) is (i) maximum in the middle of each roll and changes its sign from one roll to the next. It is produced by the Ekman pumping at the fluid containing shell boundary. The corresponding dynamo mechanism can be explained in terms of an $\alpha$-tensor with non-zero coefficients on the diagonal. In rapidly rotating objects like the Earth's core (or in a fast rotating experiment), Rossby waves occur in the limit of small Ekman number ($\approx 10^{-15}$). In that case, the main source of the axial flow component is not the Ekman pumping but rather the ``geometrical slope effect'' due to the spherical shape of the fluid containing shell. This implies that the axial flow component is (ii) maximum at the borders of the rolls and not at the centers. If assumed to be stationary, such rolls would lead to zero coefficients on the diagonal of the $\alpha$-tensor, making the dynamo probably less efficient if possible at all. Actually, the rolls are drifting as a wave, and we show that this drift implies non--zero coefficients on the diagonal of the $\alpha$-tensor. These new coefficients are in essence very different from the ones obtained in case (i) and cannot be interpreted in terms of the heuristic picture of Busse (1975)

Nevirapine- and efavirenz-associated hepatotoxicity under programmatic conditions in Kenya and Mozambique.

To describe the frequency, risk factors, and clinical signs and symptoms associated with hepatotoxicity (HT) in patients on nevirapine- or efavirenz-based antiretroviral therapy (ART), we conducted a retrospective cohort analysis of patients attending the ART clinic in Kibera, Kenya, from April 2003 to December 2006 and in Mavalane, Mozambique, from December 2002 to March 2007. Data were collected on 5832 HIV-positive individuals who had initiated nevirapine- or efavirenz-based ART. Median baseline CD4+ count was 125 cells/μL (interquartile range [IQR] 55-196). Over a median follow-up time of 426 (IQR 147-693) days, 124 (2.4%) patients developed HT. Forty-one (54.7%) of 75 patients with grade 3 HT compared with 21 (80.8%) of 26 with grade 4 had associated clinical signs or symptoms (P = 0.018). Four (5.7%) of 124 patients with HT died in the first six months compared with 271 (5.3%) of 5159 patients who did not develop HT (P = 0.315). The proportion of patients developing HT was low and HT was not associated with increased mortality. Clinical signs and symptoms identified 50% of grade 3 HT and most cases of grade 4 HT. This suggests that in settings where alanine aminotransferase measurement is not feasible, nevirapine- and efavirenz-based ART may be given safely without laboratory monitoring

Exploring the optical properties of La 2 Hf 2 O 7 :Pr 3+ nanoparticles under UV and X-ray excitation for potential lighting and scintillating applications

New optical materials with efficient luminescence and scintillation properties have drawn a great deal of attention due to the demand for optoelectronic devices and medical theranostics. Their nanomaterials are expected to reduce the cost while incrementing the efficiency for potential lighting and scintillator applications. In this study, we have developed praseodymium-doped lanthanum hafnate (La2Hf2O7:Pr3+) pyrochlore nanoparticles (NPs) using a combined co-precipitation and relatively low-temperature molten salt synthesis procedure. XRD and Raman investigations confirmed ordered pyrochlore phase for the as-synthesized undoped and Pr3+-doped La2Hf2O7 NPs. The emission profile displayed the involvement of both the 3P0 and 1D2 states in the photoluminescence process, however, the intensity of the emission from the 1D2 states was found to be higher than that from the 3P0 states. This can have a huge implication on the design of novel red phosphors for possible application in solid-state lighting. As a function of the Pr3+ concentration, we found that the 0.1%Pr3+ doped La2Hf2O7 NPs possessed the strongest emission intensity with a quantum yield of 20.54 ± 0.1%. The concentration quenching, in this case, is mainly induced by the cross-relaxation process 3P0 + 3H4 → 1D2 + 3H6. Emission kinetics studies showed that the fast decaying species arise because of the Pr3+ ions occupying the Hf4+ sites, whereas the slow decaying species can be attributed to the Pr3+ ions occupying the La3+ sites in the pyrochlore structure of La2Hf2O7. X-ray excited luminescence (XEL) showed a strong red-light emission, which showed that the material is a promising scintillator for radiation detection. In addition, the photon counts were found to be much higher when the NPs are exposed to X-rays when compared to ultraviolet light. Altogether, these La2Hf2O7:Pr3+ NPs have great potential as a good down-conversion phosphor as well as scintillator material

Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. Here, we introduce the molten-salt synthesis (MSS) method for making metal oxide nanomaterials. Advantages over other methods include its simplicity, greenness, reliability, scalability, and generalizability. Using pyrochlore lanthanum hafnium oxide (La2Hf2O7) as a representative, we describe the MSS protocol for the successful synthesis of complex metal oxide nanoparticles (NPs). Furthermore, this method has the unique ability to produce NPs with different material features by changing various synthesis parameters such as pH, temperature, duration, and post-annealing. By fine-tuning these parameters, we are able to synthesize highly uniform, non-agglomerated, and highly crystalline NPs. As a specific example, we vary the particle size of the La2Hf2O7 NPs by changing the concentration of the ammonium hydroxide solution used in the MSS process, which allows us to further explore the effect of particle size on various properties. It is expected that the MSS method will become a more popular synthesis method for nanomaterials and more widely employed in the nanoscience and nanotechnology community in the upcoming years

Samarium-Activated La2Hf2O7 Nanoparticles as Multifunctional Phosphors

Recent developments in the field of designing novel nanostructures with various functionalities have pushed the scientific world to design and develop high-quality nanomaterials with multifunctional applications. Here, we propose a new kind of doped metal oxide pyrochlore nanostructure for solid-state phosphor, X-ray scintillator, and optical thermometry. The developed samarium-activated La2Hf2O7 (LHOS) nanoparticles (NPs) emit a narrow and stable red emission with lower color temperature and adequate critical distance under near-UV and X-ray excitations. When the LHOS NPs are exposed to an energetic X-ray beam, the Sm3+ ions situated at the symmetric environment get excited along with those located at the asymmetric environment, which results in a low asymmetry ratio of Sm3+ under radioluminescence compared to photoluminescence. High activation energy and adequate thermal sensitivity of the LHOS NPs highlight their potential as a thermal sensor. Our results indicate that these Sm3+-activated La2Hf2O7 NPs can serve as a multifunctional UV, X-ray, and thermographic phosphor

Thermally Induced Disorder-Order Phase Transition of Gd 2 Hf 2 O 7 :Eu 3+ Nanoparticles and its Implication on Photo-and Radioluminescence

Crystal structure has a strong influence on the luminescence properties of lanthanide-doped materials. In this work, we have investigated the thermally induced structural transition in Gd 2 Hf 2 O 7 (GHO) using Eu 3+ ions as the spectroscopic probe. It was found that complete phase transition from the disordered fluorite phase (DFP) to the ordered pyrochlore phase (OPP) can be achieved in GHO with the increase of annealing temperature from 650 ? 1100 ? 1300 °C. OPP is the more stable structural form for the GHOE nanoparticles (NPs) annealed at a higher temperature based on the energy calculation by density functional theory (DFT). The asymmetry ratio of the GHOE-650 NPs was the highest, whereas the quantum yield, luminescence intensity, and lifetime values of the GHOE-1300 NPs were the highest. Emission intensity of Eu 3+ ions increases significantly with the phase transition from the DFP to OPP phase and is attributed to the higher radiative transition rate (281 s -1 ) of the 5 D 0 level of the Eu 3+ ion in the environment with relatively lower symmetry (C 2v ) because of the increase of crystal size. As the structure changes from DFP to OPP, radioluminescence showed tunable color change from red to orange. The Eu 3+ local structure obtained from DFT calculation confirmed the absence of inversion symmetry in the DFP structure, which is consistent with the experimental emission spectra and Stark components. We also elucidated the host to dopant optical energy transfer through density of states calculations. Overall, our current studies present important observations for the GHOE NPs: (i) thermally induced order-disorder phase transition, (ii) change of point group symmetry around Eu 3+ ions in the two phases, (iii) high thermal stability, and (iv) tunability of radioluminescent color. This work provides fundamental understanding of the relationship between the crystal structure and photophysical properties of lanthanide-doped materials and helps design a strategy for advanced optoelectronic materials

Visible and ultraviolet upconversion and near infrared downconversion luminescence from lanthanide doped La2Zr2O7 nanoparticles

Single materials which have a multitude of photophysical processes by selective doping are in high demand in the community of material science due to their multifunctional applications. Upconversion (UC) and downconversion (DC) luminescence properties of lanthanide doped nanoparticles (NPs) are imperative for their broad application potentials. Here, we describe two series of either doubly or triply doped La2Zr2O7 NPs synthesized by a molten salt method. For the former, La2Zr2O7:Yb,Er NPs display bright red and moderate green UC (VUC) and NIR-B DC (NDC) at around 1550 nm, which are highly desirable for in-vivo bioimaging applications. For the latter, La2Zr2O7:Gd,Yb,Tm NPs demonstrate ultraviolet UC (UVUC) which can be exploited for water purification. In addition, we systematically investigated the effects of sensitizer doping level on the morphology, crystal structure and UC and DC emission intensity of these NPs. The processes involved in the VUC, NDC and UVUC emissions are evaluated in detail by pump power dependence studies which reveal that the VUC emissions are two-photon processes whereas the UVUC emission is a five-photon absorption process. The mechanisms of all these three luminescence processes have been extensively explained based on energy transfer and f-f transition processes. The idea provided in this work extends the knowledge on doping induced UC and DC luminescence in pyrochlore NPs which show multifunctionalities in light emission properties

THE EFFECT OF CARDIORESPIRATORY FITNESS ON THE ASSESSMENT OF THE PHYSICAL WORKING CAPACITY AT THE FATIGUE THRESHOLD

Purpose: The purpose of this study was to determine if different cardiorespiratory fitness levels (maximal oxygen uptake or VO2max) affect neuromuscular fatigue as measured by the physical working capacity at the fatigue threshold (PWCFT). Methods: Fourteen adults (14 men; mean ± SD; age = 20.79 ± 0.89 years; body weight = 80.7 ± 10.91 kg; height = 178.4 ± 5.29 cm) volunteered to participate in the investigation. Each participant performed an incremental cycle ergometry test to fatigue while electromyographic (EMG) signals were measured from the vastus lateralis (VL) muscle. Mean, standard deviation, and range values were calculated for the power outputs determined by the PWCFT. The relationships for EMG amplitude and power output for each participant were examined using linear regression (SPSS software program, Chicago, IL). An alpha level of p ≤ 0.05 was considered significant for all statistical analyses. Results: Participants were divided in a low and high fitness levels according to their VO2max values. A paired dependent t-test was used to determine if there were significant mean differences in power outputs associated to the PWCFT test for the low (Mean ± SD 162.5 ± 90.14 W) and high (173.21 ± 49.70 W) VO2max groups. The results of the dependent t-test indicated that there were no significant mean differences (p \u3e 0.05) between the high and low VO2max groups. The zero-order correlation for the power outputs between groups were not significantly correlated (r = 0.23). Conclusion: The results of the present investigation indicated t

Disorder driven asymmetry and singular red emission in doped Lu2Hf2O7 nanocrystals with no charge compensating defects

High performance luminescent materials possess low symmetry, high color purity, no charge compensating defects, and high quantum yield. In this work, we have synthesized Lu2Hf2O7 (LuHO) and Lu2Hf2O7:Eu3+ (LuHOE) nanocrystals (NCs) using a molten salt synthesis and confirmed that both are stabilized in defect fluorite structure with a high degree of structural disordering. The LuHO NCs depicted green emission under ultraviolet irradiation, which decreases and increases after being treated in oxidizing and reducing environments, respectively, confirming the role of oxygen vacancies in the emission process. The LuHOE NCs (i) show excitation wavelength-dependent host to dopant energy transfer efficiency, (ii) give a singular red emission with high color purity of ~95%, (iii) have a dominant occupation of the Lu3+ sites by Eu3+ ions without the formation of charge compensating defects (CCDs), and (iv) possess low non-radiative channels with a quantum yield of ~88%. We believe these LuHOE NCs with singular red emission, high quantum yield, and color purity acquired through structural disordering and the absence of CCDs warrant further investigation as efficient phosphors