Preparation and luminescent properties of Cr3+:Al2O3 nano-powders by low-temperature combustion synthesis

Cr³⁺:Al₂O₃ nano-powders were prepared through low-temperature combustion synthesis (LCS) method by using glucose as a dispersion agent for the first time. The Cr³⁺:Al₂O₃ nano-powders samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and luminescence spectrometer. XRD results showed that pure α-Al₂O₃ phase was obtained for the sample fired at 1100 °C for 0.5 h. TEM results indicated that nano-powders were well dispersed. Luminescence spectrum analysis results indicated that the excitation spectrum of Cr³⁺:Al₂O₃ nano-powders consisted of two bands peaking at 462 nm and 579 nm, respectively, and the emission spectrum consisted of two bands peaking at 692 nm and 668 nm, respectively.     

Structural and photoluminescent properties of Al2O3: Cr3+ nanoparticles via solution combustion synthesis method

In the present work, we have discussed the structural and photoluminescent properties of Al₂O₃ nanoparticles doped with Cr³⁺ ion prepared through solution combustion synthesis (SCS) technique. SCS is a well-known method for the production of different metal oxides and composite materials such as metal matrix composites and for producing this need an extra reduction step. The set of samples differing in activator concentration were studied carefully by means of structural and optical characterization methods. In particular, the transmission electron microscopy (TEM) has been deployed together with X-ray diffraction (XRD) technique to determine fundamental structural properties of nanoparticles. XRD results showed that pure α-Al₂O₃ single phase was obtained and TEM result indicates that nanoparticles are spherical in shape. The selected area electron diffraction (SAED) and Energy dispersive analysis by X-rays (EDAX) analysis suggested the crystallinity and chemical composition of the Cr³⁺ doped Al₂O₃. The change in crystal structure parameters was obtained by Rietveld refinement method. The optical characterization focused mainly on the basic excitation and emission features and their sensitivity to the dopant concentrations. The excitation spectrum of Cr³⁺-doped Al₂O₃ nanopowders consist of two bands peaking at 406 nm and 570 nm and the emission spectrum consist of two bands peaking at 694 nm and 670 nm.     

Synthesis,structure and optical properties of the laser synthesized Al2O3 nanopowders depending on the crystallite size and vaporization atmosphere

A series of Al₂O₃ nanoparticles with the sizes ranging from ~ 2 to 21 nm, according to XRD, HRTEM and BET, was obtained by vaporization of α-Al₂O₃ in flowing helium and argon at different pressures under the action of a cw CO₂ laser. The particle size was changed by varying the composition of ambient gas (He, Ar) and its pressure in a vaporization chamber from 0.034 to 0.9 bar. The effect of the synthesis conditions on the properties of Al₂O₃ nanopowders is presented and discussed. Particles with the size smaller than 6–7 nm have a faceted shape, whereas the shape of larger particles is close to spherical. It was found that the resulting nanopowders consist of a mixture of transition aluminas, among which the γ-polymorph is dominant. Thermal analysis showed that for smaller nanoparticles the phase transition to α-Al₂O₃ occurs at a lower temperature. In nominally pure Al₂O₃ nanoparticles with different sizes, photoluminescence at RT and 80 K is determined by trace level concentrations (≤10^–3 wt%) of uncontrolled impurities of 3d elements (Cr³⁺(O_h) and Fe³⁺(T_d)), surface hydroxyl groups, and point defects represented mostly by F-type centers. The acquired experimental data indicate that manifestation of the size effect for Al₂O₃ nanoparticles has a boundary corresponding to 6–7 nm. Al₂O₃ nanoparticles with the dominant γ-Al₂O₃ structure obtained by the laser method can be promising in many chemical reactions, particularly as the supports of catalysts and sorbents, for which the high specific surface area is of special importance.     

Luminescent properties of Al2O3:Ce single crystalline films under synchrotron radiation excitation

The paper is dedicated to study the luminescent and scintillation properties of the Al₂O₃:Ce single crystalline films (SCF) grown by LPE method onto saphire substrates from PbO based flux. The structural quality of SCF samples was investigated by XRD method. For characterization of luminescent properties of Al₂O₃:Ce SCFs the cathodoluminescence spectra, scintillation light yield (LY) and decay kinetics under excitation by α-particles of Pu²³⁹ source were used. We have found that the scintillation LY of Al₂O₃:Ce SCF samples is relatively large and can reach up to 50% of the value realized in the reference YAG:Ce SCF. Using the synchrotron radiation excitation in the 3.7–25 eV range at 10 K we have also determined the basic parameters of the Ce³⁺ luminescence in Al₂O₃ host.     

Synthesis,characterization and luminescence of Sr3Al2O6 phosphor with trivalent rare earth dopant

The luminescent properties of Sr₃Al₂O₆ doped and co-doped with the rare earths (Ln³⁺ = Eu³⁺, Dy³⁺, Eu³⁺ and Dy³⁺) have been studied. The material was synthesized by reflux method and fired up to 900 °C for 16 h. The X-ray diffraction pattern confirms that the synthesized material consists of Sr₃Al₂O₆ as main phase. The photoluminescence study gives a clear evidence of europium stabilizing in trivalent form and surprisingly with no presence of europium in the divalent state. The addition of Dy³⁺ as co-dopant in the Sr₃Al₂O₆:Eu³⁺ matrix shows the quenching effect in the photoluminescence (PL) spectra. The photoluminescence intensity of Eu³⁺ falls gradually on increasing the concentration of the co-dopant in the range from 0.1 mole% to 2.0 mole%. The significantly intense thermoluminescence (TL) glow peak was obtained for Sr₃Al₂O₆:Eu³⁺, Dy³⁺ (1% and 0.1%) at around 194 °C when irradiated with 10 kGy dose from Sr-90 β source.     

Host-sensitized phosphorescence of Mn4+, Pr3+,4+ and Nd3+ in MgAl2Si2O8

Mn⁴⁺ doped and Pr^3+,4+, Nd³⁺ co-doped MgAl₂Si₂O₈-based phosphors were first of all synthesized about 1300 °C. They were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X-ray powder diffraction (XRD), photoluminescence (PL) and scanning electron microscopy (SEM). The luminescence mechanism of the phosphors, which showed broad red emission bands in the range of 610–715 nm and had a different maximum intensity when activated by UV illumination, was discussed. Such a red emission can be attributed to the intrinsic d–d transitions of Mn⁴⁺.     

Physical and spectroscopic properties of multi-component Na2O–PbO–Bi2O3–SiO2 glass ceramics with Cr2O3 as nucleating agent

Transparent glass ceramics, synthesized from melt quenching followed by heat treatment, of the composition 10Na₂O–30PbO–10Bi₂O₃–(50 − x)SiO₂:xCr₂O₃ (mol%), where 0 ⩽ x ⩽ 0.5, were characterized with XRD, DTA, SEM and EDS. Physical and spectroscopic studies, viz., optical absorption, electron paramagnetic resonance (EPR), FTIR and Raman were investigated. The characterization of the host glass ceramic has revealed that the formation of a major phase of sodium silicate along with two minor phases such as lead silicate and bismuth oxide. By integrating Cr₂O₃ to the host glass additional crystal phases viz., NaCrO₂, Na₂Cr₂O₇ and Pb(CrO₄) which are the complexes of Cr³⁺ and Cr⁶⁺ ions were also developed. As the concentration of nucleating agent is increased, a part of the Cr⁶⁺ ions is found to reduce in to Cr³⁺ ions. Spectroscopic studies have revealed that with an increase in the concentration of Cr₂O₃ from 0.1 to 0.5 mol%, there is a gradual increase in the intensity of vibrational modes of various asymmetric structural units of silicate, bismuthate and chromate in the glass ceramic network at the expense of symmetrical structural units. The analysis of the results of these studies has indicated that in the samples containing higher concentration of Cr₂O₃, chromium ions exists predominantly in Cr³⁺ state and occupy the octahedral positions in glass ceramic matrix and such glass ceramic samples are suitable for lasing action.     

Thermal stability and annealing behavior of photoluminescence from Eu doped YAG

Eu₂O₃ doped Y₃Al₅O₁₂ (YAG) crystals have been grown using a floating zone technique and evaluated thermal stability and annealing behavior of PL for a fluorescence thermo-sensor application. Color of the crystals grown varies from deep purple to colorless with O₂ concentration of the growth atmosphere and annealing in air. Photoluminescence (PL) peaking at λ = 590, 607, 624, 647 and 709 nm due to Eu³⁺ ions are observed from the crystals under UV excitation. Anomalous temperature dependence of PL intensity, which is observed in as-grown crystals, is improved greatly by annealing through the heat cycle. From annealing behavior of optical absorption spectra, residual Eu²⁺ ions are suggested to be responsible for the de-coloration and the improvement of anomalous temperature dependence of Eu doped YAG crystals.     

Phase transition of Er3+-doped Al2O3 powders prepared by the non-aqueous sol–gel method

The Er³⁺-doped Al₂O₃ powders have been prepared by the non-aqueous sol–gel method using the aluminum isopropoxide as precursor, acetylacetone as chelating agent, nitric acid as catalyzer, and hydrated erbium nitrate, as dopant under isopropanol environment. The phase structure and phase transition of the Er³⁺-doped Al₂O₃ powders were investigated by using thermogravimetry/differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The phase contents diagram for the Er-doped Al–O system with the doping concentration up to 5 mol% was described at the sintering temperature from 550 to 1250 °C. There were the three crystalline types of Er³⁺-doped Al₂O₃ phases, γ-, θ- and α-(Al, Er)₂O₃, and the two relative stoichiometric compounds composed of Al, Er, and O, ErAlO₃ and Al₁₀Er₆O₂₄ phases in the Er–Al–O phase contents diagram. The Er³⁺ doping suppressed crystallization of the γ and θ phases and delayed phase transition of the γ → θ and θ → α. The increased Er³⁺ doping concentration and the elevated sintering temperature enhanced the precipitation of the ErAlO₃ and Al₁₀Er₆O₂₄ phases. The preparation procedure for the Er³⁺-doped Al₂O₃ powders in the non-aqueous sol–gel process, including chelating, hydrolysis, peptization, doping and gelation, has a significant effect on the phase formation and its transition for the Er³⁺-doped Al₂O₃ powders.     

Polyvinylpyrrolidone (PVP)-assisted hydrothermal synthesis of luminescent YVO4:Eu3+ microspheres

Spherical YVO₄:Eu³⁺ microstructures were hydrothermally synthesized by the reaction of NH₄VO₃, Y₂O₃, and Eu₂O₃ at 180 °C for 24 h with the assistance of polyvinylpyrrolidone (PVP) as a surfactant. The resulting products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The experimental results showed that ball-like YVO₄:Eu³⁺ microspheres with a diameter of about 4–5 μm, corresponding to the SEM observations, formed at 180 °C for 24 h using 0.2 g PVP with the molecular weight of 20,000 g mol^?1. The amount of PVP and the reaction time of hydrothermal processing were found to play a key role in the formation of YVO₄:Eu³⁺ microspheres. It has been observed that the relative luminescence intensities of the as-synthesized samples increased with increasing hydrothermal reaction times due mainly to the increase of crystallinity.     

Crystal growth and photoluminescence characteristics of Ca2MgSi2O7:Eu3+ thin films grown by pulsed laser deposition

Ca₂MgSi₂O₇:Eu³⁺ films were deposited on Al₂O₃ (0 0 0 1) substrates by pulsed laser deposition. The films were grown at various oxygen pressures ranging from 100 to 400 mTorr. The crystallinity and surface morphology of the films were examined by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD and AFM respectively showed that the Ca₂MgSi₂O₇:Eu³⁺ films had a zircon structure and consisted of homogeneous grains ranging from 100 to 400 nm depending on the deposition conditions. The radiation emitted was dominated by a red emission peak at 620 nm. The maximum PL intensity of the Ca₂MgSi₂O₇:Eu³⁺ films grown at 300 mTorr was increased by a factor of 1.3 compared to that of Ca₂MgSi₂O₇:Eu³⁺ films grown at 100 mTorr. The crystallinity, surface roughness and photoluminescence of the thin-film phosphors were strongly dependent on the deposition conditions, in particular, the oxygen partial pressure.     

Effect of surfactants on preparation of nanoscale α-Al2O3 powders by oil-in-water microemulsion

The nanoscale α-Al₂O₃ powders have been synthesized by the O/W microemulsion system using cyclohexane as the oil phase, ultrapure water as the water phase, OP-10 and alcohol as the surfactant and co-surfactant. It has been found that the nature of surfactants played an important role to regulate the size and morphologies of the α-Al₂O₃ nanoparticles. Three different nonionic surfactants (OP-10, Triton X-100 and Tween-80) have been used for the preparation of microemulsions. The microemulsions were characterized by Zeta Potential Analyzer. The results showed that the OP-10 system possesses wide and stable microemulsion phase regions. The synthesized α- Al₂O₃ powders have been comprehensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD). The powders calcined at 900–1150 °C showed the presence of alumina phase with crystal structure, The SEM images showed that the powders was an average diameter of 30–100 nm.     

Variation of the photoluminescence and vacuum ultraviolet excitation characteristics of BaZr(BO3)2:Eu3+ by the incorporation of Al3+, La3+, or Y3+ into the lattice

The photoluminescence (PL) and vacuum ultraviolet (VUV) excitation properties are studied for the BaZr(BO₃)₂:Eu³⁺ phosphor with incorporating the Al³⁺, La³⁺, or Y³⁺ ion into the lattice. The excitation spectrum shows an absorption band in the VUV region with the band-edge at 200 nm and a very weak charge transfer band of Eu³⁺ at about 226 nm. The luminescence spectrum shows a strong emission at 615 nm (⁵D₀ → ⁷F₂ transition) and weak emission at 594 nm (⁵D₀ → ⁷F₁ transition) in BaZr(BO₃)₂:Eu³⁺, with a good red color purity. The PL intensity is increased by incorporating Al³⁺ into the BaZr(BO₃)₂ lattice. The PL intensity has also increased by incorporating La³⁺ into the lattice, however, the red color purity has deteriorated because of the increased centrosymmetric nature of the site. With the incorporation of Y³⁺ into the BaZr(BO₃)₂ lattice, the PL characteristics of the Eu³⁺ activator resembles that in the YBO₃ lattices. The intensity of the red PL for the Eu³⁺ activator is the highest with good color purity for BaZr(BO₃)₂:Eu³⁺ incorporated with both Al³⁺ (10%) and La³⁺ (0.5%).     

Al2O3–FeCrAl composites and functionally graded materials fabricated by reactive hot pressing

Al₂O₃–FeCrAl composites were fabricated by mixing Fe₂O₃, Al and Cr powders and then reactive hot pressing. The high temperature alloy FeCrAl was formed by the reaction of extra Al, Cr and the Fe reduced from Fe₂O₃. The Al₂O₃–FeCrAl composites with various Al₂O₃ fractions were successfully fabricated by the proper addition of extra Fe, Cr, Al or Al₂O₃ powders. A five-layer functionally graded material of YSZ–FeCrAl was fabricated using the Al₂O₃–FeCrAl composites with compositions of 25, 53.2 and 75 vol.% Al₂O₃ as interlayer. The results from XRD analysis, optical microscope observation and thermal cycling test show that the composites fabricated by this method consist of α-Al₂O₃ phase and (Fe, Cr, Al) solid solution. The α-Al₂O₃ grain formed by this in-situ reaction between Fe₂O₃ and Fe is ultrafine and uniform distribution. The three-point bending strength is 305.0 MPa for the composite with 53.2 vol.% Al₂O₃ prepared by the reactive hot pressing, about 20% higher than that of the composite with same composition prepared by ex situ hot pressing method (252.0 MPa). No cracking was found in the functionally graded materials after 10 thermal cycles up to 1000 °C due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM.     

Effects of ultraviolet excitation on the spectroscopic properties of Sm3+ and Tb3+ doped aluminophosphate glasses

Li₂O–BaO–Al₂O₃–La₂O₃–P₂O₅ glasses optically activated with rare earth ions with the 4f⁵, and 4f⁸ electronic configuration (Sm³⁺ and Tb³⁺, respectively) were analyzed by Raman spectroscopy, absorption, excitation photoluminescence, decay curves and temperature dependent photoluminescence. The spectroscopic characteristics of the as-prepared and heat treated samples at temperatures below and above T_g were studied as well as their room temperature photometric properties under ultraviolet excitation. All the doped glasses exhibit typical signatures of the lanthanides in their trivalent charge state. For the samarium doped glass heat treated at 250 °C (<T_g) the Sm²⁺ luminescence was also observed. The analysis of the luminescence efficiency was performed in the interval range of 14 K to room temperature, where the integrated intensity of the luminescence was found to decrease for the Sm³⁺ and Tb³⁺ ions in the studied temperature range. Luminescence decay curves were found to be non-exponential for the ⁴G_5/2 → ⁶H_7/2 and ⁵D₃ → ⁷F₄ transitions of the Sm³⁺ and Tb³⁺ ions, respectively. The results strongly suggest the occurrence of energy transfer processes through cross relaxation phenomena, mediated by dipole–dipole interaction in all the studied samples. The decay of the ⁵D₄ → ⁷F₅ emission of the Tb³⁺ ions was found to be single exponential with a time constant of ∼3.1 ms. Based on the spectroscopic characteristics, models for recombination processes are proposed. The room temperature luminance photometric properties with ultraviolet excitation show that the samarium doped glasses have much lower luminance intensity (around 0.3 Cd/m²) when compared with the 6–7 Cd/m² observed for the terbium doped ones.     

Mechanochemical synthesis of Al2O3/Co nanocomposite by aluminothermic reaction

In this work, Al₂O₃/Co nanocomposite was successfully prepared by mechanochemical reaction between Co₃O₄ and Al powders in a planetary high energy ball mill. The mechanism of the reaction was dealt using X-ray diffraction (XRD), differential thermal analysis (DTA), and thermodynamics calculations. It was found that Co₃O₄ reacts with Al through a self-sustaining combustion reaction after an incubation period of 50 min and the reaction between Co₃O₄ and Al involves two steps. First, Co₃O₄ reacts with Al to form CoO and Al₂O₃ at the temperature around melting point of Al, and at higher temperature, CoO reacts with remaining Al to form Co and Al₂O₃. Mechanical activation process decreases the reaction temperature from 1041 °C for as-received Co₃O₄ and Al powder mixture to 869 °C for 45 min milled powders. After annealing of powder milled for 12 h, no phase transformation has been detected. The crystallite sizes of both α-Al₂O₃ and Co remained in nanometeric scale after annealing at 1000 °C for 1 h.     

Concentration quenching,surface and spectral analyses of SrF2:Pr3+ prepared by different synthesis techniques

Pr³⁺ doped strontium fluoride (SrF₂) was prepared by hydrothermal and combustion methods. The phosphors were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) spectroscopy. XRD patterns indicated that the samples were completely crystallized with a pure face-centred cubic (space group: Fm3m) structure. SEM images showed different morphologies which is an indication that the morphology of the SrF₂:Pr³⁺ phosphor strongly depends on the synthesis procedure. Both the SrF₂:Pr³⁺ samples exhibit blue–red emission centred at 488 nm under a 439 nm excitation wavelength (λ_exc) at room temperature. The emission intensity of Pr³⁺ was also found to be dependent on the synthesis procedure. The blue–red emission has decreased with an increase in the Pr³⁺ concentration. The optimum Pr³⁺ doping level for maximum emission intensity was 0.4 and 0.2 mol% for the hydrothermal and combustion samples, respectively. The reduction in the intensity for higher concentrations was found to be due to dipole–dipole interaction induced concentration quenching effects.     

Compositional variation of photoluminescence from Mn doped MgAl2O4 spinel

Spinel (MgAl₂O₄) crystals doped with 1.0% Mn have been grown by floating zone (FZ) technique with various Mg compositions, x = MgO/Al₂O₃, from 0.2 to 1.0. Compositional variations of photoluminescence are evaluated for a fluorescence thermometer application using crystals grown. Strong photoluminescence (PL) peak is observed at λ from 512 to 520 nm from the crystals grown from compositions, x, from 0.3 to 1.0. Peak wavelength of PL increases linearly from 512 to 520 nm with x. Weak PL peaking at λ = 750 nm is also observed from the specimens. Compositional variations of PL are considered to be due to the variation of crystal field surrounding the Mn²⁺ ions. The variation of crystal field strength agrees with the compositional variation of lattice constant.     

Synthesis and spectroscopic characterization of the K4BaSi3O9:Eu3+

K₄BaSi₃O₉:Eu³⁺ polycrystals were synthesized by solid state method. X-ray powder diffraction measurements confirmed structure of the samples. The excitation and the emission spectra of orthorhombic K₄BaSi₃O₉ doped with Eu³⁺ were investigated. The excitation spectrum exhibits a broad band with maximum at 220 nm corresponding to the charge transfer (CT) transition between O²⁻ and Eu³⁺ ions and smaller 4f–4f transitions. The emission of investigated phosphor was excited at 395 nm and has quantum efficiency (QE) equal 27%. The emission maximum at 616.5 nm was assigned to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The luminescence decay profiles as well as the thermal quenching were measured and analyzed. K₄BaSi₃O₉:Eu³⁺has high temperature quenching of the emission T_0.5 = 335 °C.     

Nature of intrinsic and impure luminescence of MAlP2O7 crystals

The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP₂O₇ (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP₂O₇ crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the ⁴Т₂ → ⁴А₂ radiation transitions in the impurity Cr³⁺ ions located in the intermediate crystal field.