Transparent Cr2O3-doped alumina ceramics were prepared by slip casting, followed by pre-sintering in ambient atmosphere and hot isostatic pressing. The effect of dopant concentration on material ...properties, including microstructure and optical properties was evaluated. Real in-line transmittance in the range of 20–44 % was measured for the ceramics with the mean grain size <520 nm: the transmittance decreased with increasing grain size and Cr content. The excitation spectra consisted of two broad bands with maxima at 404 nm and 558 nm, corresponding to 4A2g → 4T1g and 4A2g → 4T2g transitions of Cr3+ ions in octahedral sites of α-Al2O3. The intensive deep red narrow emissions under violet/green light excitation, R-lines (2Eg → 4A2g transition), were observed at 692.5 nm and 693.8 nm, that are very close to ruby single crystal. The highest emission was achieved at the Cr3+ concentration of 0.4 at.%. The luminescence decay curves exhibited single-exponential behaviour with decay times of ∼3.6 ms.
•The luminescent ZnO-doped Y2O3 ceramics was prepared by infiltration of Y2O3 with Zn2+ ions and pressureless sintering.•The presence of ZnO grains embedded into the Y2O3 matrix was proved using EBSD ...technique.•The whole visible spectral range of PL emission was achieved in the ZnO-doped Y2O3 ceramics.•Effect of Zn2+ dopant concentration and sintering temperature on luminescence.•Warm white light emission with maximum centered around 570–590 nm depending on ZnO.
Y2O3 ceramics containing up to 4 at.%Zn2+ were prepared by infiltrating a zinc acetate water solution into a pre-sintered matrix of Y2O3. After pressureless sintering for 2 h at 1400 °C, ZnO grains dispersed in an Y2O3 matrix were identified. The photoluminescence (PL) spectra of the Y2O3:ZnO ceramics exhibit a very broad emission band over the entire visible spectral range with an intensity maximum at about 570−590 nm. The intensity of the PL emission increases with an increasing concentration of additive Zn2+, reaching a maximum between 3 and 4 at.%. The annealing temperature also affects the emission intensity. The Y2O3-ZnO ceramic emits yellowish (warm) white light with the CIE 1931 colour coordinates (0.39,0.43) and a correlated colour temperature in the range of 4000−4600 K. The luminescence decay kinetics at RT exhibit multi-exponential behaviour with faster (∼180 ns) and slower (1.3μs and 8.0μs) decay components and the average lifetime spanning into the range of 5.3–6.5μs.
Rapid rotation is a fundamental characteristic of classical Be stars and a crucial property allowing for the formation of their circumstellar disks. Past evolution in a mass and angular momentum ...transferring binary system offers a plausible solution to how Be stars attained their fast rotation. Although the subdwarf remnants of mass donors in such systems should exist in abundance, only a few have been confirmed due to tight observational constraints. An indirect method of detecting otherwise hidden companions is offered by their effect on the outer parts of Be star disks, which are expected to be disrupted or truncated. In the context of the infrared and radio continuum excess radiation originating in the disk, the disk truncation can be revealed by a turndown in the spectral energy distribution due to reduced radio flux levels. In this work, we search for signs of spectral turndown in a sample of 57 classical Be stars with radio data, which include new data for 23 stars and the longest-wavelength detections so far (λ 10 cm) for two stars. We confidently detect the turndown for all 26 stars with sufficient data coverage (20 of which are not known to have close binary companions). For the remaining 31 stars, the data are inconclusive as to whether the turndown is present or not. The analysis suggests that many if not all Be stars have close companions influencing their outer disks. If confirmed to be subdwarf companions, the mass transfer spin-up scenario might explain the existence of the vast majority of classical Be stars.
Oxide ceramics with pyrochlore structure such as La2Zr2O7 are promising materials for advanced multifunctional applications including extreme environment, such as space applications. They combine low ...thermal conductivity, suitable coefficient of thermal expansion to join supporting materials and have optical response in the UV–VIS region with up-converting and down-converting properties after doping, required for coating, sensors, and electro-optic applications. The effect of doping elements such as Er3+ and Yb3+, replacing isomorphically lanthanum in the La2Zr2O7 with pyrochlore cubic structure, on material photophysical properties was investigated by selective doping, in the molar ratio of Er3+/Yb3+ ranging from 0.5 to 5. The reflectance spectra of materials prepared by sol-gel co-precipitation method after sintering at 1400 °C, investigated in the UV–VIS–NIR range showed as expected well resolved absorptions due to the 4f–4f intra-configurational transitions. At the same time, dielectric properties of sintered samples were investigated by terahertz time domain spectroscopy. The refractive indices observed in the range of ~4.46–5.15, indicate high dielectric permittivity at THz frequencies applicable in energy storage materials or THz communication components. For all samples a broad absorption band was found in the THz region and the band intensities were clearly associated with the doping amount of Er and Yb cations into the La2Zr2O7 structure, at frequencies 0.75–1.02 THz. Significant correlation at the level of R2 = 0.911 was found between the absorption band corresponding to the 4I15/2 → 2H11/2 transition at 520.5 nm and the integrated intensities of THz bands. Dielectric ceramic materials such as titanium-, tantalum- and niobium- oxide based ceramics have permittivity in the range 20–30. The permittivity of studied La2Zr2O7 was ~20 and increased after doping by Er3+/Yb3+ to ~21–26.
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Intensity increase of absorption band in THz region after La2Zr2O7 ceramic doping with Er and Yb.
•La2Zr2O7 doped with Er3+/Yb3+ ratio ranging from 0.5 to 5 was synthesized.•Sintering at 1400 °C revealed cubic pyrochlore structure with luminescence properties.•Refractive indices of La2Zr2O7 materials in the THz region were found to be 4.46–5.15.•A broad absorption band was found in pure and doped La2Zr2O7 at 1.02–0.75 THz.•The integrated intensities of absorption band increased systematically with increasing Er/Yb ratio rising from 10 to 39 cm−1 THz.
Selected layered clay minerals of smectite group with different chemical composition (e.g. synthetic hectorite, synthetic smectite and natural smectite) were modified by rare earth cations such as ...Er3+ and a stochiometric mixture of Er3+/Yb3+. Raw and modified smectites were systematically heated up to temperatures ensuring their dehydration (400 °C), dehydroxylation (800 °C) and formation of new high temperature phases (1200 °C). The products of heating were investigated by terahertz-time domain spectroscopy to follow changes of their dielectric properties in the far-infrared region. The frequency-dependent refractive index of raw and modified clays was determined in the range of 0.2–3.8 THz (6.6–126.5 cm−1). The heating at 800 °C and above resulted in increase frequency dependent index of refraction and decrease absorption coefficient in comparison to the raw clays. The ion-exchanged smectites by rare earth cations further decreased the absorption coefficient of the samples heated at 1200 °C. Raw natural smectite sample revealed a low frequency band at 0.86 THz, already reported in the literature, which shifted to 0.67 THz after sample modification by rare earth cations. Heating of modified JP to 1200 °C resolved a new absorption band at ∼3.40 THz in the ceramic phase produced from raw natural smectite.
•Cation exchange reaction was used for modification of clays by selected rare earth elements.•Effect of sample heating up to 400, 800 and 1200 °C was studied.•Terahertz time domain spectroscopy was used to study the refractive index dispersion.•The frequency dependence of absorption in THz region resolved new typical absorption bands.•The high temperature treatment can be used for synthesis of new ceramic phosphor materials.
The Eu3+-doped transparent aluminas were prepared by wet shaping technique followed by pressure-less sintering and hot isostatic pressing. The effect of dopant amount on microstructure, real in-line ...transmission (RIT), photoluminescence (PL) properties, hardness and fracture behaviour was studied. The RIT decreased with increasing amount of the dopant. The PL emission spectra of Al2O3:Eu3+ ceramics exhibited predominant red light emission with the highest intensity (under 394nm excitation) for material containing 0.125at.% of Eu3+ and colour coordinates (0.645, 0.355) comparable with commercial red phosphors. The doping resulted in hardness increase from 26.1GPa for undoped alumina to 27.6GPa for Eu-doped sample. The study of fracture surfaces showed predominantly intergranular crack propagation micro-mechanism.
Abstract
Classical Be stars are possible products of close binary evolution, in which the mass donor becomes a hot, stripped O- or B-type subdwarf (sdO/sdB), and the mass gainer spins up and grows a ...disk to become a Be star. While several Be+sdO binaries have been identified, dynamical masses and other fundamental parameters are available only for a single Be+sdO system, limiting the confrontation with binary evolution models. In this work, we present direct interferometric detections of the sdO companions of three Be stars—28 Cyg, V2119 Cyg, and 60 Cyg—all of which were previously found in UV spectra. For two of the three Be+sdO systems, we present first orbits and preliminary dynamical masses of the components, revealing that one of them could be the first identified progenitor of a Be/X-ray binary with a neutron star companion. These results provide new sets of fundamental parameters that are crucially needed to establish the evolutionary status and origin of Be stars.
In progressive particle or layered composites based on a combination of BaTiO3 and Al2O3, serving as e.g. ceramic harvesters, new phases are formed during heat treatment. The dominant one is ...BaTiAl6O12. This study provides information about the microstructural, mechanical and optical properties of the BaTiAl6O12 ceramics. The evolution of the phases during the solid-state reaction synthesis of the BaTiAl6O12 was monitored. The fully dense samples prepared by spark plasma sintering had indentation Vickers hardness and indentation elastic modulus within ranges of 10.1–13.7GPa and 132.0–187.0GPa, depending on loading force. The three-point bending tests of the BaTiAl6O12 samples resulted in flexural strength of 129.9MPa and fracture toughness of 1.8MPam1/2. The sample showed blue broad-band emission under UV excitation due to the charge-transfer transition of the Ti4+ and defect sites. The BaTiAl6O12 evinced low permittivity (ɛ′)=16 and dielectric loss (tanδ) <0.0003 at a frequency 1kHz.
En los compuestos de partículas o capas progresivas basados en una combinación de BaTiO3 y Al2O3, que sirven, por ejemplo, como capacitadores cerámicos, se forman fases nuevas durante el tratamiento térmico. La dominante es BaTiAl6O12. Este estudio proporciona información sobre las propiedades microestructurales, mecánicas y ópticas del cerámico BaTiAl6O12. Se monitorizó la evolución de las fases durante la síntesis por reacción en estado sólido de BaTiAl6O12. Las muestras totalmente densas preparadas mediante spark plasma sintering exhibieron una dureza Vickers por indentación y un módulo elástico por indentación entre 10,1-13,7GPa y 132,0-187,0GPa, respectivamente, dependiendo de la fuerza de carga aplicada. Los ensayos de flexión en tres puntos de las muestras de BaTiAl6O12 dieron como resultado una resistencia a la flexión de 129,9MPa y una tenacidad a la fractura de 1,8MPam1/2. La muestra presentó una emisión azul de banda ancha bajo excitación UV debido a la transición de transferencia de carga del Ti4+ y sitios defectuosos. El BaTiAl6O12 mostró una baja permitividad (ɛ′)=16 y una pérdida dieléctrica (tanδ) <0,0003 a una frecuencia de 1kHz.
Green light emitting Zn2SiO4:Mn2+ phosphors have been synthetised by the solid-state reaction in ambient atmosphere at 1300°C for 2h, with ZnO, SiO2 and MnO2 as the reagents. The ZnO/SiO2 molar ratio ...varied from 2 to 0.5. The doping level was in a lower concentration range (0.01≤x≤0.05). The effect of both the Mn2+ concentration and ZnO/SiO2 molar ratio on luminescence intensity and decay was investigated in detail. The microstructure and phase composition of prepared phosphors were characterised by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). XRD results indicate that the pure α-Zn2SiO4 phase with rhombohedral structure was obtained after heat treatment. The prepared phosphors exhibit a strong green emission centred at 525nm from the 4T1→6A1 forbidden transition. The highest emission intensity was observed for phosphors with ZnO/SiO2 molar ratio equal to 1.0, and the Mn2+ concentration x=0.03 (ZSMn3). The emission intensity of the ZSMn3 phosphor is comparable with the commercial Zn2SiO4:Mn2+ phosphor. The decay curves can be characterised by double exponential function. After fitting a fast component τ1∼2ms and a slow component τ2∼10ms were obtained. The decay times decrease significantly with increasing Mn2+ concentration. The decay time and luminescence mechanism depend on the excitation light wavelength. Temperature dependent luminescence of the ZSMn3 phosphor in the temperature range of 25–200°C was studied.
•GeSbNa:B novel glass system was developed by the conventional melt quenching method.•Energy band gap (Eg) values increased with the content of B2O3 (mol%).•Ed, n and So values enhanced with the ...increase of boron ions in the glass network.•Enhanced K value or optical conductivity of the glasses were observed.•Obtained low phonon energy (0.073–0.102 eV) in comparison to tellurite glasses.
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Borate incorporated novel germanium based glass system was fabricated by traditional melt-quenching technique and is used as a probe for the generation of efficient lasers. In this context, we have explored in detail the impact of B2O3 addition on optical properties of GeSbNa (germanate-antimono-sodium) glass composition. The optical absorption spectra reveal that the cut-off edge of the glasses is shifted to lower wavelengths, due to smaller number of (NBO’s) non-bridging oxygen atoms. Using optical energy band gap values, the phonon energy (EΩ) of the glass system was calculated. Extinction coefficient (k) values were found to decrease in the wavelength range of 290–335 nm which is used for low loss optical devices. The refractive index (RI) of the glasses decreased, indicating that the entire B2O3 doped GeSbNa glass system obeys the normal dispersion mechanism. In addition, the boron mixed GeSbNa glass system shows low phonon energy, making it ideal for rare earth ion hosting glass systems for laser rods and promising candidate for manufacturing optical fibers.
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