Characterization of SrAl2O4:Eu2+, Dy3+ phosphor nano-powders produced by microwave synthesis route

SrAl₂O₄ phosphor nano-powders activated with heavy elements such as Eu and Dy were prepared by microwave synthesis method. Using this method led to the reduction of processing time. Various calcinations times have been employed to produce pure SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor materials. It was found out that microwave synthesis technique led to reduction of optimum calcinations time to 9 min. XRD analysis showed that the powders were nearly pure SrAl₂O₄ phase, in which the SrAl₂O₄ host phase has the maximum fraction of monoclinic SrAl₂O₄ phase. The critical pH to achieve pure SrAl₂O₄ phase determined to be equal to 4. For the synthesized SrAl₂O₄:Eu²⁺, Dy³⁺, the properties of photoluminescence such as emission, excitation and decay time were examined. Fluorescent spectrophotometer results revealed that two excitation peaks are appeared at 280 and 339 nm and an emission peak at 515 nm. The crystallite size of these pigments is about 58.22 nm after calcinations for 9 min in microwave as determined by Scherrer’s formula. SEM was used to study the morphology and shape of powders.     

Synthesis of Y2O2S:Eu3+, Mg2+, Ti4+ hollow microspheres via homogeneous precipitation route

Abstract

A phosphorescent material in the form of Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ hollow microspheres was prepared by homogeneous precipitation using monodispersed carbon spheres as hard templates. Y₂O₃:Eu³⁺ hollow microspheres were first synthesized to serve as the precursor. Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ powders were obtained by calcinating the precursor in a CS₂ atmosphere. The crystal structure, morphology and optical properties of the composites were characterized. X-ray diffraction measurements confirmed the purity of the Y₂O₂S phase. Electron microscopy observations revealed that the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ particles inherited the hollow spherical shape from the precursor after being calcined in a CS₂ atmosphere and that they had a diameter of 350–450 nm and a wall thickness of about 50–80 nm. After ultraviolet radiation at 265 or 325 nm for 5 min, the particles emitted strong red long-lifetime phosphorescence originating from Eu³⁺ ions. This phosphorescence is associated with the trapping of charge carriers by Ti⁴⁺ and Mg²⁺ ions.     

Synthesis of Y2O2S:Eu3+, Mg2+, Ti4+ hollow microspheres via homogeneous precipitation route

A phosphorescent material in the form of Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ hollow microspheres was prepared by homogeneous precipitation using monodispersed carbon spheres as hard templates. Y₂O₃:Eu³⁺ hollow microspheres were first synthesized to serve as the precursor. Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ powders were obtained by calcinating the precursor in a CS₂ atmosphere. The crystal structure, morphology and optical properties of the composites were characterized. X-ray diffraction measurements confirmed the purity of the Y₂O₂S phase. Electron microscopy observations revealed that the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ particles inherited the hollow spherical shape from the precursor after being calcined in a CS₂ atmosphere and that they had a diameter of 350–450 nm and a wall thickness of about 50–80 nm. After ultraviolet radiation at 265 or 325 nm for 5 min, the particles emitted strong red long-lifetime phosphorescence originating from Eu³⁺ ions. This phosphorescence is associated with the trapping of charge carriers by Ti⁴⁺ and Mg²⁺ ions.     

Synthesis and luminescence properties of red long-lasting phosphor Y2O2S:Eu3+, Mg2+, Ti4+ nanoparticles

We report an effective method to synthesize Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ nanoparticles. Tube-like Y(OH)₃ were firstly synthesized by hydrothermal method to serve as the precursor. Nanocrystalline long-lasting phosphor Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ was obtained by calcinating the precursor with co-activators and S powder. XRD investigation shows a pure phase of Y₂O₂S, indicating no other impurity phase appeared. SEM and TEM observation reveals that the precursor synthesized via a hydrothermal routine has tube-like structure and the final phosphor reveals a hexagonal shape. The fine nanoparticles which have the particle size ranging from 30 to 50 nm show uniform size and well-dispersed distribution. From the spectrum, the main emission peaks are ascribed to Eu³⁺ ions transition from ⁵D_J (J = 0, 1, 2) to ⁷F_J (J = 0, 1, 2, 3, 4). After irradiation by 325 nm for 10 min, the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ long-lasting phosphor shows very bright red afterglow and the longest could last for more than 1 h even after the irradiation source had been removed. It is considered that the long-lasting phosphorescence is due to the contribution from the electron traps with suitable trap depth.     

Sintering and compensation effect of donor and acceptor codoped 3 mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350°C. The enhancement is accounted for by a liquid phase sintering mechanism. While the addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol% at 1300–1600°C results in the decrease of final density and in the destabilization of the tetragonal (t) phase of the 3 mol% Y₂O₃-t-ZrO₂ (TZP). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m) ZrO₂ phase and a second Ta₂Zr₆O₁₇ phase. The decrease is attributed to the increase of m-ZrO₂ content in these samples. Complete phase transformation from t-ZrO₂ to m-ZrO₂ observed in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor and acceptor codoping defect chemistry.     

Polymorphism of Bismuth Sesquioxide. II. Effect of Oxide Additions on the Polymorphism of Bi2O3

The effect of small oxide additions on the polymorphism of Bi₂O₃ was studied by means of high-temperature x-ray diffractometry. Solidus and occasional liquidus temperatures were approximated, so that tentative partial phase diagrams for 33 oxide additions were constructed. Only the monoclinic and the cubic forms of Bi₂O3 were found to be stable. Other phases, frequently reported by previous investigators, such as tetragonal and body-centered cubic (b.c.c.), were shown to form metastably from cooled liquid or cubic. An impure b.c.c. phase of distinct but variable composition appeared in systems of ZnO, PbO, B₂O₃, Al₂O₃, Ga₂O₃, Fe₂O₃, SiO₂, GeO₂, TiO₂, and P₂O₅. The impure b.c.c. phase in the systems with SiO₂, GeO₂, and TiO₂ melted congruently about 100 °C above the m.p. of Bi₂O₃. The impure b.c.c. phase was formed metastably in systems with Rb₂O, NiO, MnO, CdO, V₂O₅, and Nb₂O₅; the conditions of formation were dependent on composition, preparation, and heating schedules. The impure b.c.c. phases, both stable and metastable, had smaller unit cell dimensions than that of pure Bi₂O₃.     

Chemical and structural properties of the system Fe2O3-In2O3

The system Fe₂O₃-In₂O₃ was studied using X-ray diffraction,⁵⁷Fe Mössbauer spectroscopy and infrared spectroscopy. The samples were prepared by chemical coprecipitation and thermal treatment of the hydroxide coprecipitates. For samples heated at 600 °C, a phase, α- (Fe_1?x In _x )₂O₃, isostructural with α-Fe₂O₃, exists for 0?x?0.8, and a phase C-(Fe_1?x In _x )₂O₃, isostructural with cubic In₂O₃, exists for 0.3?x?/1. In the two-phase region these two phases are poorly crystallized. An amorphous phase is also observed for 0.3?x?0.7. For samples heated at 900 °C the two-phase region is wider and exists for 0.1?x?0.8 with the two phases well crystallized. In these samples an amorphous phase is not observed.⁵⁷Fe Mössbauer spectroscopy of samples prepared at 600 °C indicated a general tendency of the broadening of spectral lines and the decrease of numerical values of the hyperfine magnetic field (HMF) with increasing molar fraction In₂O₃ in the system Fe₂O₃-In₂O₃. The samples prepared at 900 °C, in the two-phase region, are characterized by a constant HMF value of 510 kOe at room temperature. Infrared spectroscopy was also used to follow the changes in the infrared spectra of the system Fe₂O₃-In₂O₃ with gradual increase of molar fraction of In₂O₃. A correlation between X-ray diffraction, Mössbauer spectroscopic and infrared spectroscopic results was obtained.     

Preparation and Magnetic Properties of Nd3+, Al3+, Ca2+ Substituted M-Type Strontium Hexaferrites

Strontium ferrite with Nd³⁺, Al³⁺ and Ca²⁺ substitution of Fe³⁺ and Sr²⁺ ions were prepared by the conventional solid phase reaction process. The Nd³⁺ substitution shows 10 %–20 % improvement in coercivity for the substitution content less than 10 %. The Ca²⁺ substitution is favorable to the enhancement of saturation magnetization due to the accelerated reaction of Fe₂O₃ and SrCO₃. The samples with Al³⁺ substitution of Fe³⁺ show the lowest saturation magnetization, although the highest coercivity was achieved for a homogeneous grain size less than 1 μm. The combinatory substitution Nd³⁺, Ca²⁺ and Al³⁺ leads to the optimum magnetic properties with σ _s=52 A?m²/kg and H _cj=412 kA/m.     

Preparation and photoluminescence properties of MgNb2O6:Eu3+, Bi3+ red-emitting phosphor

Rare earth Eu³⁺-doped MgNb₂O₆ red-emitting phosphor was prepared by solid-state reaction. Structure and photoluminescence properties of the samples were characterized by X-ray diffraction (XRD), scan electron microscopy (SEM) and fluorescence spectrophotometer. Meanwhile, the effect of the co-activator Bi³⁺ on the PL of the MgNb₂O₆:Eu³⁺ phosphor was studied. The results showed that the pure phase of MgNb₂O₆ could be available after firing at 1200 °C. The Mg_1?x Nb₂O₆:Eu _x ³⁺ phosphors could be effectively excited by the UV irradiation (273 nm) and emit red light at 615 nm due to the forced electric dipole ⁵ D ₀ → ⁷ F ₂ transitions on Eu³⁺, which indicated that Eu³⁺ occupied the non-inversion symmetry sites in the MgNb₂O₆ host lattice. So, the addition of the co-activator Bi³⁺ not only increased the excitation band of the MgNb₂O₆:Eu³⁺ phosphor at about 330 nm, but also strengthened the PL intensity at 615 nm. Therefore, MgNb₂O₆:Eu³⁺, Bi³⁺ might find application to InGaN chip-based white light emitting diodes.     

Synthesis of Long Afterglow SrAl2O4: Eu2+, Dy3+ Phosphors Through Microwave Route

A novel and fast microwave route is used for the synthesis of SrAl₂O₄: Eu²⁺, Dy³⁺ powder phosphors. Based on the XRD peaks, the powder phosphors were identified as SrAl₂O₄ phase, which is monoclinic (a = 8.4424Å, b = 8.822 Å, c = 5.1607Å, = 93.415°). Compared with those synthesized by solid-state reaction process, the phosphors show a smaller grain size (about 4.8 m). It exhibited broadband peaks in both the excitation and emission spectra. A clear blue shift occurs in the excitation and emission spectra of these phosphors compared to those synthesized by solid-state reaction process. The excitation peaks lied between 300 nm and 450 nm, and the main emission peaks lied around 507 nm. The afterglow curve shows that the initial luminescent intensity of the phosphors synthesized through microwave route decreases greatly.     

Phases ε obtenues par decomposition des spinelles non-stoechiometriques dans les systemes Ga2O3-MgO,Al2O3-NiO,Al2O3-AlN et Al2O3-Li2O

In the systems Al₂O₃-AlN, Al₂O₃-NiO, Al₂O₃-Li₂O and Ga₂O₃-MgO, non-stoechiometric spinels, when decomposed at high temperatures, form an intermediate metastable phase ε. This phase has a one-dimensional periodic antiphase-domain structure. The antiphase boundaries are parallel to the (310) plane and the displacement vector is $\text{1}\text{4}$ [110] when referred to the spinel structure. Across each antiphase boundary, some octahedral and tetrahedral sites share faces instead of corners. In the case of ε_AlN, cationic vacancies occupy these sites; in the other cases, this is likely to go along with a segregation of the more charged ions.     

Sintering and compensation effect of donor- and acceptor-codoped 3mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350 °C. The enhancement is accounted for by a liquid phase sintering mechanism. The addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol % at 1300–1600 °C results in the destabilization of tetragonal (t-) phase and the decrease of final density in 3 mol% Y₂O₃-TZP (tetragonal ZrO₂ polycrystals). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m-) ZrO₂ and a second phase of Ta₂Zr₆O₁₇. The decreasing in final density is attributed to the increase of m-ZrO₂ content. Complete destabilization of t-ZrO₂ to m-ZrO₂ in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor- and acceptor-codoping defect chemistry.     

Hydrothermal phase equilibria in Ln2O3-H2O-CO2 systems for Tm,Yb and Lu

Phase diagrams for Tm₂O₃-H₂O-CO₂. Yb₂O₃-H₂O-CO₂ and Lu₂O₃-H₂O-CO₂ systems at 650 and 1300 bars have been investigated in the temperature range of 100–800°C. The phase diagrams are far more complex than those for the lighter lanthanides. The stable phases are Ln(OH)₃, Ln₂(CO₃)₃.3H₂O (tengerite phase), orthorhombic-LnOHCO₃, hexagonal-Ln₂O₂CO₃. LnOOH and cubic-Ln₂O₃. Ln(OH)₃ is stable only at very low partial pressures of CO₂. Additional phases stabilised are Ln₂O(OH)₂CO₃and Ln₆(OH)₄(CO₃)₇ which are absent in lighter lanthanide systems. Other phases, isolated in the presence of minor alkali impurities, are Ln₆O₂(OH)₈(CO₃)₃. Ln₄(OH)₆(CO₃)₃ and Ln₁₂O₇(OH)₁₀,(CO₃)₆. The chemical equilibria prevailing in these hydrothermal systems may be best explained on the basis of the four-fold classification of lanthanides.     

Liquidus surface in the HfO2-Y2O3-La2O3 system

The liquidus surface in the HfO₂-Y₂O₃-La₂O₃ system was studied by differential thermal analysis in helium at temperatures of up to 2500°C, derivative thermal analysis in air at temperatures of up to 3000°C, x-ray diffraction, optical microscopy, and electron microscopy. The liquidus surface was found to comprise five primary crystallization fields-those of theH-Y₂O_3-,C-Y₂O_3-,F-HfO_2-, andX-La₂O₃-based solid solutions and the pyrochlore phase La₂Hf₂O₇. Three invariant equilibria were identified in the system studied-two peritectics and one eutectic.     

Joint mechanical activation of MnO2, Fe2O3 and graphite: Mutual influence on the structure

The structural changes of MnO₂, Fe₂O₃ and graphite under separate and joint mechanical activation in high-energy planetary ball mill were studied by X-ray diffraction analysis, Raman spectroscopy and chemical analysis. Separate mechanical processing resulted in nanostructured states of MnO₂, Fe₂O₃ and graphite with the size of coherent scattering regions of 25, 12 and 6 nm, respectively, and the average particle size of 15–20 nm. Along with nanoparticles of globular shape, Fe₂O₃ nanorods were found to be formed during separate milling. No mechanochemical effect was found after separate milling. Under joint mechanical activation of nanostructured manganese and iron oxides with graphite, phase transformations toward less stable forms of oxides (Mn₂O₃, Mn₃O₄, Fe₃O₄) were found. When co-milled with α-Fe₂O₃, graphite was found to exfoliate to graphene layers. The graphite phase remained under the combined mechanical activation with MnO₂. Dynamic recrystallization of α-Fe₂O₃ phase also proceeded during joint mechanical activation of nanostructured Fe₂O₃ and graphite.     

Phase separation in tellurite glass-forming systems containing B2O3,GeO2,Fe2O3,MnO,CoO,NiO and CdO

The boundaries between the regions of single-phase and two-phase glasses were established in tellurite glass-forming systems containing B₂O₃ and one of the following oxides: GeO₂, Fe₂O₃,CoO, NiO, MnO and CdO. The character of the microstructures inside and outside the regions of stable phase separation were determined by electron microscopy. It was shown that the existing microheterogeneities may either result from incomplete liquid immiscibility during melting and supercooling or be due to typical metastable separation.     

Phase equilibria in the CaF2-Al2O3-CaO system

Computations of phase equilibria in the CaF₂-Al₂O₃-CaO system have been carried out on the basis of experimentally found thermodynamic properties of all intermediate phases and melts. Coordinates of the phase equilibrium boundaries were determined by solving a system of equations expressing equality of chemical potentials of the components in coexisting phases. The nature and quantity of the coexisting phases were established by a search for the Gibbs energy minimum of the system. All the phases of the CaF₂-Al₂O₃-CaO system were taken into consideration. Calculated phase diagrams of the CaO-CaF₂, CaO-Al₂O₃ and CaF₂-Al₂O₃ binary subsystems are in good agreement with the data available in the literature. Isotherms of the CaF₂-Al₂O₃-CaO system were calculated at 1600, 1650, 1723 and 1773 K. A wide region of liquid separation into two phases is observed in the system. One phase is composed of practically pure CaF₂ with additions of several mol% of CaO and Al₂O₃, and the other consists of 50 to 65 mol% of CaF₂ only. Eleven invariant points of the CaF₂-Al₂O₃-CaO system include seven ternary eutectics, two ternary peritectics and two points of four-phase monotectic transition. The primary fields of crystallization of all the phases are alongated toward the CaF₂ apex, the CaO field being the widest and the 3CaO·Al₂O₃ field the narrowest. Seven junctions of the CaF₂-Al₂O₃-CaO phase diagram were represented. Computed saturation lines of CaF₂-Al₂O₃-CaO melt with CaO, Al₂O₃, CaO·6Al₂O₃ and CaO·2Al₂O₃, and also the positions of a number of characteristic points, agree well with the experimental data available. The present calculations reveal a number of details and peculiarities of the constitution of the CaF₂-Al₂O₃-CaO phase diagram.     

Ternary eutectics in the systems FeO-UO2-ZrO2 and Fe2O3-U3O8-ZrO2

The systems FeO-UO₂-ZrO₂ (in inert atmosphere) and Fe₂O₃-U₃O₈-ZrO₂ (in air) were studied. For the FeO-UO₂-ZrO₂ system, the eutectic temperature was found to be 1310°C, with the following component concentrations (mol %): 91.8 FeO, 3.8 UO₂, and 4.4 ZrO₂. For the Fe₂O₃-U₃O₈-ZrO₂ system, the eutectic temperature was found to be 1323°C, with the following component concentrations (mol %): 67.4 FeO_1.5, 30.5 UO_2.67, and 2.1 ZrO₂. The solubility limits of iron oxides in the phases based on UO₂(ZrO₂,FeO) and UO_2.67(ZrO₂,FeO_1.5) were determined.     

New understanding on formation mechanism of CaFe2O4 in Fe2O3 -Fe3O4-CaO-SiO2 system during sintering Process: Phase transformation and morphologies evolution

The complex calcium ferrite is the main binder phase of high basicity, high iron and low silicon sinter used in blast furnace (BF), and its formation amount and structure are important factors affecting sinter quality. This work research on the phase transformation and morphologies evolution of Fe₂O₃ -Fe₃O₄-CaO-SiO₂ systems roasted 900 °C ? 1200 °C in air atmosphere to understand the formation process of calcium ferrite. In CaO-Fe₃O₄ system, the prolonged roasting time and higher temperature promotes that CaFe₅O₇, CaFe₃O₅, and Ca₂Fe₂O₅ gradually transformed into the stable existence of CaFe₂O₄. In CaO-Fe₃O₄ system, the higher temperature promotes that the combination of Fe₃O₄ and CaO formed the stable CaFe₃O₅ with orthorhombic structure. The replacement reaction between the newly formed CaFe₃O₅ phase and the unreacted CaO phase occur to form Ca₂Fe₂O₅. In CaO-Fe₃O₄-SiO₂ system, FeSiO₃ can be combined with Fe₃O₄ to form Fe₂SiO₄ and Fe₂O₃. Under the action of high temperature, FeSiO₃ and CaO undergo displacement reaction to form the unstable CaSiO₃, the new formation of CaSiO₃ can be easily combined with CaO to form the stable Ca₂SiO₄. With the further increase of temperature, the complex calcium ferrite is formed in calcium silicate layer. The final product complex calcium ferrite and calcium silicate exist at the same time. The formation of calcium silicate has an unfavorable effect on formation of complex calcium ferrite in the sintering process.     

Some thermodynamic aspects of the high-temperature transition in doped V2O3

Measurements of the entropy change are reported for the high-temperature metal-insulator (MI) transitions in the (V_1–xCr_x)₂O₃ and (V_1–xAl_x)₂O₃ systems. It is emphasized that the entropy of the I phase exceeds that of the M phase. Evidence is presented to show that the M and I phases coexist over a narrow temperature range. The transformation is attended by enormous hysteresis effects; these indicate that the lattice plays an important role in the transition. The probable role of Cr³⁺ and Al³⁺ as a dopant in the V₂O₃ lattice is briefly discussed. A phase diagram for the dilute V₂O₃-Al₂O₃ alloy system is presented.