Precision Measurement of the 2

Abstract

The 4f¹³ 6s^{2 2}F_7/2–4f¹⁴ 5d ²D_5/2 3·43 μm infrared transition in ¹⁷²Yb⁺ has been driven for the first time and measured to be 87 360 087(4) MHz. The frequency was determined by probing a cloud of ¹⁷²Yb⁺ ions held in a radiofrequency trap in the presence of helium buffer gas. The infrared radiation was generated by difference frequency mixing in LiNbO₃. The frequency measurement is part of a programme to locate the 4f¹⁴ 6s ²S_1/2–4f¹³ 6s^{2 2}F_7/2 467 nm ultra-narrow transition in laser-cooled Yb⁺.     

Hyperfine Structure of the 6P1/2

Abstract

The hyperfine splitting of the 6P_1/2–6P_3/2 fine structure transition in ²⁰⁷Pb⁺ ions at 710 nm has been measured by c.w. laser excitation and subsequent observation of the weak fluorescence radiation at the same wavelength. The ions were confined in an rf quadrupole trap and buffer gas cooled by collisions with He atoms at 10^?5 mbar pressure. From the observed level splitting and the simultaneously recorded transmision of a Fabry-Perot etalon we determined the hyperfine splitting constants A(6P_1/2) = 13·001(48) GHz and A(6P_3/2) = 0·589(23) GHz. From a small impurity of ²⁰⁸Pb of our isotopically enriched sample we determined the isotope shift of the transitions to Δv(207–208) = 252(40) MHz.     

Synthesis and annealing effects on the optical spectroscopy properties of red-emitting Gd(P<Subscript>0.5</Subscript>V<Subscript>0.5</Subscript>)O<Subscript>4</Subscript>: x at.% Eu<Superscript>3+</Superscript>

In this work, Gd(P_0.5V_0.5)O₄: x at.% Eu³⁺ phosphors with different dopant concentrations (x?=?1, 3, 5, 6, 7, 9) were synthesized through chemical coprecipitation method. The phosphors were characterized by XRD, SEM, infrared spectroscopy, photoluminescence excitation, emission spectra and CIE. The results of XRD indicate that the obtained phosphors have the tetragonal phase structure. Eu³⁺ emission transitions arise mainly from the ⁵D₀ level to the ⁷F_J (J?=?0, 1, 2, 3, 4) manifolds. The emission intensity and crystalline of Gd(P_0.5V_0.5)O₄:x at% Eu³⁺ powders are increasing with annealing temperature at 600, 800, 1000, 1100, and 1200 °C, respectively. The introduction of VO₄^3? can broaden the range of UV excitation spectrum wavelength and enhance the transition between ⁵D₀ → ⁷F₁ to ⁵D₀ → ⁷F₂ for long wavelength emission. And the most dominant emission peak of Eu³⁺ for ⁵D₀ → ⁷F₂ transition is closer to pure red light at 622 nm. The maximum emission intensity of the phosphors is the concentration of 6 at.% Eu³⁺ because of the distance of the neighbor Eu³⁺ ions reaching a certain critical value and the influence of multipolar interaction. Compared to commercial phosphors Y₂O₃:Eu³⁺ and (Y,Gd)BO₃:Eu³⁺, our work yielded a longer wavelength red light emission intensity and a higher proportion of red light to orange light. All our results indicate that color purity of this phosphor turns it into a promising red phosphor in ultraviolet-pumped light-emitting diodes.     

Kinetics of grain growth in 0·1 at.-%B doped nickel aluminide containing 23·2 at.-%Al

Abstract

Nickel aluminide containing 23·2 at.-%Al and 0·1 at.-%B was cold rolled and fully recrystallised at 825, 850, 875, 900, 925, 975, and 1000°C. Increase in grain size as a function of isothermal annealing time at these temperatures was measured. All the data showed excellent fit with the developed equations. The value of m, the exponent of the grain growth equation (D^m - D^m₀ = Kt), was 2·5. The activation energy for grain growth was 263 kJ g-atom^-1. Small changes in the boron content had an appreciable effect on the kinetic parameters. This was attributed to the solute drag effect of boron.     

On the Evaporation of C60 in Vacuum and Inert Gases at Temperatures Between 830 K and 1050 K

Abstract

The binary diffusion coefficient D_AB of subliming C₆₀ in He, N₂ and Ar gas has been determined at a gas pressure between 5 and 10 kPa. It resulted D_AB/(cm^2/s)=D_o(P_o/P_t(T/T_o)ⁿ as a function of the total pressure P_t of the vapor phase and temperature T. At T_o=273 K and P_o=1.0133×10⁵ P_a, the values D_o = (0.059±0.004) cm²/s, (0.011±0.003) cm²/s, (0.012±0.007) cm²/s, n = 1.77±0.06, 2.02±0.18, 1.77±0.37 were obtained for He, N₂, Ar and (830-1000) K, (800-1020) K, (875-1095) K, respectively. Only 40 wt% of the initial C₆₀ material yielded reliable D_AB where the vapor pressure of C₆₀ followed log₁₀(P/Pa) = -(8976±60)/T/K+(11.05±0.07) for T between 640 and 1055 K.     

Hydrogen permeation through mild steel in temperature range 20–500°C measured by hydrogen collection method

Abstract

Hydrogen permeation through steel plate maintained at various constant temperatures between 20 and 500°C was investigated using the hydrogen collection method. Fused salts were employed at elevated temperatures. Hydrogen entry was induced by step changes in cathodic charging current density. Emanating flux transients obtained at the hydrogen exit face corresponded closely to a model for permeation determined exclusively by bulk hydrogen diffusion through steel, induced by stepped changes in hydrogen concentration at the steel entry face subsurface, and zero hydogen at the exit face subsurface. An Arrhenius plot of log (D) v. 1/T, using diffusion coefficients D derived from permeation transients was approximately linear in the range 20–310°C. The values derived for activation energy E _a of 17 kJ mol ^-1 and for the pre-exponential factor D ₀ of 2·6 × 10^-3 cm2 s^-1, according to D = D ₀ exp (-E _a/RT) were similar to literature values. Between 310 and 500°C, stable permeation conditions were difficult to obtain, but flux measurements were repeatable and unaffected by moisture in air.     

Photoluminescence properties of Y3Al5O12:Eu nanocrystallites prepared by co-precipitation method using a mixed precipitator of NH4HCO3 and NH3·H2O

Europium-doped yttrium aluminum garnet (Y₃Al₅O₁₂:Eu, YAG:Eu) nanocrystallites were prepared by calcining the precursors obtained via a co-precipitation method using a mixed solution of NH₄HCO₃ and NH₃·H₂O as the precipitator. The results of XRD, FTIR and thermal analysis showed that phase-pure YAG:Eu without any other phases was obtained at 900 °C. TEM results indicated that the particle sizes are 50–100 nm. YAG:Eu nanocrystallites showed four emission bands ascribed to ⁵D₀ → ⁷F₁ transition (592 and 597 nm) and ⁵D₀ → ⁷F₂ transition (611 and 633 nm) of Eu³⁺, respectively. The intensity of the magnetic dipole transition (⁵D₀ → ⁷F₁) is stronger than that of the electric dipole transition (⁵D₀ → ⁷F₂). The influence of the precipitators with different molar ratios of NH₄HCO₃ to NH₃·H₂O on the thermal properties of the as-prepared precursors and luminescent properties of the resulting YAG:Eu nanocrystallites was also investigated.     

Effect of Laser Intensity Variation and Coherence on the Resonant Excitation of the 61P1–61D2 Transition of Atomic Mercury

Abstract

The effects of resonant laser excitation of an atomic transition whose lower state is not a ground state are reported. The system studied is the optical excitation to the 6¹D₂ state from the electron-excited 6¹P₁ state of Hg. Both the total fluorescence intensity and the time-resolved electron-photon coincidence fluorescence signal from the 6¹D₂–6³P₁ relaxation channel are observed as the laser intensity is varied. In the coincidence signals, a manifestation of Rabi cycling in the optically excited transition is evident. The experimental data are in generally good agreement with predictions from a quantum-electrodynamic model.     

Optical properties of divalent samarium-doped fluorochlorozirconate glasses and glass ceramics

Glass ceramics comprising Sm²⁺-doped barium chloride in both hexagonal and orthorhombic phases embedded in a fluorochlorozirconate glass matrix have been prepared. Divalent samarium doping was effected with the aid of NaBH₄ to partly reduce Sm³⁺ in the starting materials to Sm²⁺. The optical absorption spectrum in unannealed samples shows a strong, broad 4f → 5d band centred at around 340 nm and attributed to Sm²⁺, but no photoluminescence is observed. Following annealing to induce crystallisation of Sm²⁺-doped barium chloride crystals in the glass, photoluminescence is observed in the form of sharp line 4f → 4f transitions, and two broad 5d → 4f bands, the latter centred at around 680 and 900 nm. The 680 nm band disappears below about 250 K whilst the 900 nm band intensity increases with decreasing temperature. The 4f → 4f transitions for Sm²⁺ ions in the orthorhombic phase of barium chloride show an abrupt change in the relative intensity of transitions originating from the ⁵D₁ and ⁵D₀ levels at around 90 K. The effect is attributed to the role of the 5d level in thermally assisted indirect transitions between the ⁵D₁ and ⁵D₀ levels. In contrast, transitions from only the ⁵D₀ level are observed for the hexagonal phase. The temperature dependence of the photoluminescence intensity, and the observation of a different crystal field splitting pattern for the ⁷F₁ level, can be used to distinguish between the two phases. The splitting pattern for the ⁷F₁ level for the hexagonal phase of barium chloride suggests that both possible crystallographic barium sites are occupied by Sm²⁺ ions.     

Synthesis and characterization of Eu3+ activated hexagonal ABaGd(PO4)2 (A+=Li, Na, or K) phosphates

BaGd_0·5(PO₄)₂: Eu _0·5 ³⁺ and ABaGd_0·5(PO₄)₂: Eu _0·5 ³⁺ (where A⁺=Li, Na or K) double phosphates were synthesized by solid state diffusion method. These powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD results showed many distinct lines in the XRD spectra. SEM measurement showed grains of different sizes in double phosphates. The Eu³⁺-ion emission lines for⁵ D ₀→⁷ F _j (j=1, 2, 3, and 4) transitions showed a splitting into 3, 2, 1, and 2 components, respectively. Strength of these lines was enhanced by the addition of alkali ion to BaGd_0·5(PO₄)₂: Eu _0·5 ³⁺ double phosphate. However, as the alkali ion varied from Li⁺ to K⁺, the intensity of⁵ D ₀→⁷ F ₂ line (617 nm) in the red region increased at the cost of⁵ D ₀→⁷ F ₁ line (599 nm) in the orange region. Suitable explanation has been proposed for this phenomena.     

Structural and luminescent properties of Eu3+-doped GdSrAl3O7 nanophosphor

Eu³⁺-doped GdSrAl₃O₇ nanophosphor with promising luminescent properties has been synthesized by low-temperature solution combustion synthesis. The structural properties examined by X-ray diffractometer, scanning electron microscopy, and transmission electron microscopy showed that pure tetragonal GdSrAl₃O₇: Eu³⁺ red nanophosphor having narrow size distribution in 50–55 nm range could be readily obtained at low temperature 550 °C. The photoluminescent excitation and emission spectra, life time, and concentration effect were studied in detail. Under excitation at 266 nm, Eu³⁺-doped GdSrAl₃O₇ nanophosphor revealed weak green emission and strong red emission attributed to ⁵D₁ → ⁷F_1–2 and ⁵D₀ → ⁷F_0–3 transitions of Eu³⁺ ion, respectively in the region of 525–700 nm. The red emission from ⁵D₀ → ⁷F₂ transition at 616 nm exhibits the highest intensity under the optimized concentration of 10 mol% after which the quenching mechanism became relevant. Quenching behavior of the europium in the GdSrAl₃O₇ host was explained by nonradiative cross-relaxation phenomenon. Moreover, Eu³⁺-doped GdSrAl₃O₇ nanophosphor can generate light from orange to deeper red by properly tuning the concentration of europium ions based on the energy transfer principle.     

Nonhydrolytic sol-gel synthesis and characterization of YAG

Yttrium–aluminum oxides are interesting compounds, which are widely used as hosts for lasers and phosphors due to their stable physical and chemical properties. The manufacture of YAG has been investigated thoroughly. YAG powders are traditionally synthesized through the reaction of aluminum and yttrium powders at high temperatures. The work reported here involved an investigation into the preparation of YAG by a nonhydrolytic sol-gel route and the influence of heating time at low temperatures to obtain YAG from inorganic precursors (yttrium and aluminum chloride). AlCl₃, YCl₃ and ethanol were reflux reacted under an argon atmosphere. Europium III chloride was added as a structural probe. The powder was treated at 800 °C for 1, 2, 4, 8 and 16 h. The YAG structure was analyzed by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal analysis (TA) and photoluminescence (PL). The XRD revealed only peaks corresponding to the YAG phase. PL data showed that the YAG phase was formed in 2 min with samples pretreated at 50 °C. For the samples pretreated at 800 °C, the YAG phase appeared in 30 s. The excitation spectra presented a maximum of 394 nm corresponding to the ⁵L₆ level, while the emission spectra of Eu III ions showed characteristic transition bands arising from the ⁵D₀ → ⁷F _J (J = 1, 2, 3, 4) manifolds at maximum excitation. The magnetic dipole ⁵D₀ → ⁷F₁ transition exhibited greater intensity than the electric dipole ⁵D₀ → ⁷F₂ transition. This methodology proved efficient for obtaining YAG phase.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Structural relaxation and electrical conductivity of molybdovanadate glass

⁵⁷Fe Mössbauer spectrum of conductive barium iron vanadate glass with a composition of 20BaO·10Fe₂O₃·70V₂O₅ (in mol%) showed paramagnetic doublet peak due to distorted Fe^IIIO₄ tetrahedra with isomer shift (δ) value of 0.37 (±?0.01) mm s^?1. Mössbauer spectra of 20BaO·10Fe₂O₃·xMoO₃·(70???x)V₂O₅ glasses (x?=?20–50) showed paramagnetic doublet peaks due to distorted Fe^IIIO₆ octahedra with δ’s of 0.40–0.41 (±?0.01) mm s^?1. These results evidently show a composition-dependent change of the 3D-skeleton structure from “vanadate glass” phase, composed of distorted VO₄ tetrahedra and VO₅ pyramids, to “molybdate glass” composed of distorted MoO₆ octahedra. After isothermal annealing at 500 °C for 60 min, Mössbauer spectra also showed a marked decrease in the quadrupole splitting (Δ) of Fe^III from 0.70 to 0.77 to 0.58–0.62 (±?0.02) mm s^?1, which proved “structural relaxation” of distorted VO₄ tetrahedra which were randomly connected to FeO₄, VO₅, MoO₆, FeO₆ and MoO₄ units by sharing corner oxygen atoms or edges. DC-conductivity (σ) of barium iron vanadate glass (x?=?0) measured at room temperature was 3.2?×?10^?6 S cm^?1, which increased to 3.4?×?10^?1 S cm^?1 after the annealing at 500 °C for 60 min. The σ’s of as-cast molybdovanadate glasses with x’s of 20–50 were ca. 1.1?×?10^?7 or 1.2?×?10^?7S cm^?1, which increased to 2.1?×?10^?2 (x?=?20), 6.7?×?10^?3 (x?=?35) and 1.9?×?10^?4 S cm^?1 (x?=?50) after the annealing at 500 °C for 60 min. It was concluded that the structural relaxation of distorted VO₄ tetrahedra was directly related to the marked increase in the σ, as generally observed in several vanadate glasses.     

Creep of Pb–2·5Sb–0·2Sn alloy at low stresses

Abstract

The creep of a Pb–2·5Sb–0·2Sn alloy has been studied at stresses up to 6·5 MN m^?2 in the temperature range 318–348 K (0·53–0·58T_m) using helical specimens. At 333 K, a transition in the stress exponent from ~1 to 3 occurred at ~3 MN m^?2. The observed good agreements below the transition stress, both for experimental dE/do and predictions for Coble diffusional creep of lead, and for measured activation energy for creep and the activation energy for grain boundary self-diffusion in lead, suggest that grain boundary diffusional creep is the dominant mechanism. at low stresses. The presence of antimony does not seem to affect the magnitude of dE/do appreciably, and the results suggest that the grain boundary self-diffusivity of lead is not influenced by the presence of segregated antimony on the grain boundaries. The diffusional creep occurred above a threshold stress of magnitude ~0·5 MN m^?2, and its temperature dependence was characterised by an activation energy of ~20 kJ mol^?1, similar to the value of 23 ± 7 kJ mol^?1 typical of pure metals in the temperature range investigated. The stress exponent of ~3 observed for the power law regime suggests control by viscous glide of dislocations constrained by dragging of solute atmospheres. Preliminary tests on sagging beam specimens of as-worked material at an applied stress of 2·5 MN m^?2 and a test temperature of 333 K has provided the first direct evidence that anisotropic grain shape affects Coble creep. The specimen with the longest grain dimension along the stress axis underwent slower creep than the specimen with the longest grain dimension perpendicular to the stress axis. This observation is in qualitative agreement with theoretical predictions.

MST/1139     

On Oxygen Diffusion in tetragonal zirconia

Oxygen diffusion in dense scales of tetragonal zirconia at 1100–1300°C has been studied by the so-called interruption kinetic method (or the Rosenberg method). Assuming that oxygen vacancies are the predominant defects in zirconia (ZrO₂), studies by this method provide values of the oxygen self diffusion coefficient in this oxide at its lower limit of stability, D, and the value in tetragonal zirconia can be expressed as D =2.2 · 10^?3 exp (?140(kJ/mole)/RT). The studies furthermore show that the maximum non-stoichiometry in tetragonal zirconia, X in ZrO_2?x, is small and has a value about x* ～ 0.03 at 1000–1300°C.     

Domaine vitreux,structure et conductivite electrique des verres du systeme LiCl /1b Li2O /1b P2O5

The glass forming region in the LiCl /1b Li₂O /1b P₂O₅ system was determined. Glass transition temperature was obtained from differential thermal analysis studies. The determination of the electrical conductivity as a function of the temperature and the composition in Li₂O and LiCl was carried out. The conductivity reaches a maximum value of the order of 5·10^?7 (ohm^?1·cm^?1) at 25° C. Raman spectra are examined in order to show the influence of the composition on the vitreous structure.     

Enhanced optoelectronics properties of europium(III) complexes with β-diketone and nitrogen heterocyclic ligands

Preparation and photoluminescence behavior of six new europium complexes with β-diketone 1-(2-hydroxy phenyl)-3-phenylpropane-1,3-dione (HPPP) and bathophenanthroline (batho), 2,2′-bipyridyl (bipy), 2,2′-biquinoline (biq), neocuproin (neo) and 1,10-phenanthroline (phen) are reported in solid state. The ligand (HPPP) and complexes Eu(HPPP)₃·H₂O (1), Eu(HPPP)₃·phen (2), Eu(HPPP)₃·batho (3), Eu(HPPP)₃·bipy (4), Eu(HPPP)₃·biq (5) and Eu(HPPP)₃·neo (6) were characterized by means of elemental analysis, infrared spectroscopy, proton nuclear magnetic resonance (¹H-NMR). The optical properties, thermal stability and crystalline nature were investigated by photoluminescence spectroscopy, thermogravimetric analysis and XRD respectively. The emission spectra show narrow intense emission band of central europium (III) metal ion that arise from the high intense ⁵D₀ → ⁷F₀ transition. The introduction of ancillary ligands like batho, bipy, biq, neo, phen enlarged the π-conjugation system in complexes as a result higher luminescence intensity, longer life time (τ) and higher quantum efficiency (η) observed in europium ternary complexes in comparison to Eu(HPPP)₃·H₂O (1). Based on the emission spectra, the luminescence decay curve was measured which indicated that the transfer of energy from HPPP ligand to the europium metal is more efficient in complexes 2–6 than complex 1.     

Simplified method of porosity prediction in directionally solidified Al–4·5 wt-%Cualloy

Abstract

An experiment was carried out to verify a simplified theory of porosity formation during the unidirectional solidification of Al–4·5 wt-%Cu alloy in cylindrical moulds 1·5 and 2·4 cm in diameter. For specimens 1·5 cm in diameter, a higher thermal gradient G and a lower solidus velocity V_s at low drawing speeds are measured compared with the specimens 2·4 cm in diameter, while this difference tends to be reversed at high drawing speeds. The experimental results also conjirm the roles of interdendritic fluid flow and surface tension effects in the formation of porosity. The porosity content tends to increase with increasing local solidification time or a decrease in G^0·4/V_s^1·6when this parameter is less than about 1·0 K^0·4min^1·6cm^?2.

MST/3122