Comparison of energy transfer between Terbium and Ytterbium ions in glass and glass ceramic: Application in photovoltaic

The structural and optical properties of thin layers based on 70%SiO 2 –30%HfO 2 doped with different concentra- tion of rare earth ions (terbium and ytterbium) have been studied with a view to integrating them in a photovoltaic cell as a spectral conversion layer in order to improve its efficiency, by using down-conversion process. These thin films were synthesized by using sol gel technique and deposited on the pure silica substrate by dip-coating method. The DC layer can be placed on the front side of a solar cell and can enhance the current by converting ultraviolet (UV) photons into a large number of visible photons. In present study two series of samples are compared, the first series corresponds to samples treated at 900 °C (glass- S) while the second series concerns samples treated at 1000 °C (glass-ceramic- SC). These series are based on 70SiO 2 –30HfO 2 activated by different molar concentrations of rare earths [Tb + Yb]/[Si + Hf] = 7%, 9%, 12%, 15%, 17%, 19% and 21%. Photoluminescence results of reference samples (without Yb 3 + ) showed an emission from 5 D 4 to 7 F J ( J = 3, 4, 5, 6) level characteristic transitions of Tb 3 + , with a maximum peak in the green centered at 543.5 nm cor- responding to the 5 D 4 →7 F 5 transition. For the co-doped samples a clear NIR PL emission around 980 nm was detected, due to the 2 F 5/2 →2 F 7/2 transition of Yb 3 + ions. From luminescence decay curves of Tb 3 + maximum emission peak ( 7 F 5 →5 D 4 transition at 543.5 nm) we have identified the energy transfer efficiency. The quantum efficiency increases by increasing the total [Tb + Yb] concentration. The most significant yield was achieved with [Tb + Yb] = 19%, the maximum quantum transfer efficiency obtained was 190% for glass-ceramic samples and 161% for glassy one

Optical properties of Tb3mathplusmathplus/Yb3mathplusmathplus co-doped silica-hafnia glass and glass-ceramic thin film

In this paper, optical properties of Tb3+/Yb3+ co-doped 70SiO2 - 30HfO2 glass and glass-ceramic thin films prepared by dip coating on Si O2 substrate are investigated using transmittance measurements. Such systems are usually used for down-converting the high-frequency sunlight in order to tune the solar spectrum into the bandgap of the semi-conductor based photovoltaic system. The interference modulated transmission spectra tau(lambda) of 70SiO2 - 30HfO2- x Tb3+ - y Yb3+ (with molar ratio x/y = 1.8/0, 1.8/7.2, 3/12) at normal incidence were recorded in the spectral wavelength region from 250 nm up to 2500 nm. The corresponding optical constants and thicknesses were deduced by analyzing the transmittance spectra using the straightforward method proposed by Swanepoel. The dispersion of the real part of the refractive index and the absorption were then deduced. The results show thatthe increase in the concentration of rare earth ions leads to a slight increase in the refractive index, this is also observed in thickness variation.Furthermore, the energy bandgap has been determined from the absorption coefficient values using the direct transition model proposed by Tauc. The corresponding value is around 3 eV with a slightly higher bandgap for glass films as compared to glass-ceramic films

Risk factors of mild rectal bleeding in very low birth weight infants: a case control study

BACKGROUND: Mild rectal bleeding (MRB) is a particular clinical entity different from necrotizing enterocolitis, which significantly influences neonatal care in preterm infants. We aimed to determine the risk factors and to evaluate prospectively the clinical course of MRB. METHODS: We consecutively included in a case–control study all infants with birth weight ≤ 1500 g or gestational age ≤ 32 weeks admitted to our unit, and presenting MRB, defined as either isolated or associated with mild clinical or radiological signs. We matched each Case with two Controls. Clinical data before, after and at time of MRB were collected, together with stool cultures at time of MRB (or at similar postnatal age in Controls). Multiple logistic regression analysis was performed to determine independent risk factors for the development of MRB. RESULTS: During 4 years, among 823 very low birth weight (VLBW) infants admitted to our unit, 72 (8.8%) had MRB. The median duration of rectal bleeding was 1.1 [1–2] days and the fasting period lasted 2.9 [2–10] days. A relapse occurred in 24% of cases. In multivariate analysis, only hypertension during pregnancy (p = 0.019), growth restriction at onset of bleeding (p = 0.026), and exposure to ibuprofen (p = 0.003) were independent risk factors for MRB. In Cases there were more infants with Clostridium Difficile in stools than in Controls (p = 0.017). CONCLUSION: Hypertension during pregnancy, even without intrauterine growth restriction, appeared to carry the same risk for MRB as exposure to ibuprofen and extrauterine growth restriction