Roles of doping ions in persistent luminescence of SrAl<Subscript>2</Subscript>O<Subscript>4</Subscript>: Eu<Superscript>2+</Superscript>,<Emphasis Type="Italic">RE</Emphasis><Superscript>3+</Superscript> phosphors

The polycrystalline Eu²⁺ and Dy³⁺ codoped strontium aluminates SrAl₂O₄: Eu²⁺,Dy³⁺ were prepared by a solid-state reaction. The UV-excited photoluminescence, persistent luminescence, and thermoluminescence of the SrAl₂O₄: Eu²⁺,Dy³⁺ phosphors with different compositions and ion doping was studied and compared. The results showed that the Eu²⁺ ion doped in SrAl₂O₄: Eu²⁺,Dy³⁺ phosphors is not only the UV-excited luminescent center but also the persistent luminescent center. The Dy³⁺ ion introduced into SrAl₂O₄: Eu²⁺ crystal matrix can hardly yield any luminescence under UV excitation but acts as an electron trap with a suitable depth for persistent luminescence. The Dy³⁺ codoping would effectively enhance the persistent luminescence and thermoluminescence. Different codoping RE ³⁺ ions have a different effect on persistent luminescence. Only the RE ³⁺ ions (for example, Dy³⁺ and Nd³⁺), which have suitable optical electronegativity, can form suitable electron traps and effectively improve the persistent luminescence of SrAl₂O₄: Eu²⁺. Based on the above observations, a persistent luminescence mechanism, electron transfer model, was proposed and illustrated. The text was submitted by the authors in English.     

Characterization of luminescent SrAl2O4 films doped with terbium and europium ions deposited by ultrasonic spray pyrolysis technique

SrAl₂O₄, SrAl₂O₄:Tb³⁺ and SrAl₂O₄:Eu³⁺:Eu²⁺ films were synthesized by means of the ultrasonic spray pyrolysis technique. These samples, characterized by X-Ray Diffraction, showed the monoclinic phase of the strontium aluminate. Images of the surface morphology of these films were obtained by SEM and the chemical composition was measured by EDS and XPS. The photoluminescence and cathodoluminescence characteristics of the films were studied as a function of the terbium and europium concentrations. The optimal PL emission intensities were reached at 8?at% for terbium doped films and 6?at% for europium doped samples. The CL emission spectra for europium doped films showed the typical bands of Eu³⁺ ions and also a broadband centered at 525?nm which is attributed to Eu²⁺ ions. XPS measurements confirm the presence of Eu³⁺ and Eu²⁺ in europium doped SrAl₂O₄ films, without having been subjected to a reducing atmosphere. Chromatic diagrams exhibited green color for SrAl₂O₄:Tb³⁺ films, red and yellow colors for SrAl₂O₄:Eu³⁺:Eu²⁺ films. The PL decay curves were also obtained: the averaged decay time was 2.7?ms for SrAl₂O₄:Tb³⁺ films and 1.9?ms for SrAl₂O₄:Eu³⁺ films. Similar results were obtained by the stretched exponential model.     

Photoluminescence characteristics and mechanism of SrAl2O4 co-doped with Eu3+ and Cu2+

SrAl₂O₄ co-doped with Cu²⁺ and Eu³⁺ was prepared at high temperature in a weakly oxidizing atmosphere by solid states reaction. X-ray diffraction (XRD) pattern of the sample shows that the doped sample exhibits SrAl₂O₄ crystalline phase. No characteristic peaks of dopant have been observed in XRD pattern of doped sample. The excitation and emission spectra of CuEu:SrAl₂O₄, Eu:SrAl₂O₄, Cu:SrAl₂O₄ samples consist of many sharp peaks. The excitation and emission spectra of the SrAl₂O₄ sample co-doped with Cu²⁺ and Eu³⁺ are significantly different from those of Eu:SrAl₂O₄ and Cu:SrAl₂O₄ samples. The novel photoluminescence (PL) characteristic of the co-doped sample is attributed to the composite luminescence of Cu²⁺ and Eu³⁺ ions in SrAl₂O₄ matrix.     

Effects of adding B<Subscript>2</Subscript>O<Subscript>3</Subscript> as a flux on phosphorescent properties in synthesis of SrAl<Subscript>2</Subscript>O<Subscript>4</Subscript>: (Eu<Superscript>2+</Superscript>, DY<Superscript>3+</Superscript> phosphor

SrAl₂O₄: (Eu²⁺, Dy³⁺) phosphor was prepared by solid state reaction. B₂O₅ as a flux was added in SrAl₂O₄:(Eu ²⁺, Dy³⁺) in order to accelerate a solid state reaction. In this paper, the effects of B₂O₃ on the crystal structure and the phosphorescent properties of the material have been evaluated. The synthesized phosphor exhibited a broad band emission spectrum peaking at 520 nm, and the spectrum peak showed little effect by the B₂O₃ contents. The maximum afterglow intensity of the SrAl₂O₄: (Eu²⁺, Dy³⁺) phosphor was obtained at the B₂O₃ content of 5%. Adding the B₂O₃ caused uniform distortion to the crystal structure of the phosphor and resulted in reducing the lengths of a and c axes and Β angle of the SrAl₂O₄ crystal. The uniform distortion was accompanied with crystal defects which can trap the holes generated by the excitation of Eu²⁺ ions. The afterglow characteristic of the SrAl₂O₄: (Eu²⁺, Dy³⁺) phosphor was thus enhanced.     

Effect of Al/Sr ratio on the luminescence properties of SrAl2O4:Eu2+, Dy3+ phosphors

Recent studies have brought out many phosphors like Eu²⁺, Dy³⁺-doped alkaline earth aluminates. The trivalent Dy³⁺ ions as co-dopants greatly enhance the duration and intensity of persistent luminescence. These phosphors show excellent properties, such as high quantum efficiency, long persistence of phosphorescence, good stability and suitable color emission.In this work the effect of Al/Sr ratio on the afterglow and phosphorescence decay properties of Eu²⁺ and Dy³⁺ co-activated strontium aluminates synthesized by a solid-state process has been investigated. The luminescence properties of samples were investigated by means of excitation spectra, emission spectra and X-ray diffraction analysis.A variety of strontium aluminates, such as SrAl₂O₄, Sr₄Al₂O₇, Sr₃Al₂O₆, Sr₃Al₂(Eu, Dy, Y)O_7.5, Al₅(Eu, Dy, Y)O₁₂, Sr₄Al₁₄O₂₅, SrAl₁₂O₁₉ and (Eu, Dy, Y)AlO₃ have been identified in the samples prepared from starting precursors with Al/Sr mole ratios ranging from 0.44 to 5. The afterglow decay rate was found to be the fastest for sample with Al/Sr ratio of 4.18, in which SrAl₄O₇ phase was dominant. The afterglow decay rate for phosphor with Al/Sr ratio of 2, in which SrAl₂O₄ phase was dominant, was detected to be slow. Moreover, the emission spectra of the samples shift to yellow-green long wavelength from bluish-green-ultraviolet short wave with the increase of Al/Sr ratios resulting from the change in the composition.     

Long Afterglow SrAl2O4:Eu2+,Dy3+ Phosphors as Luminescent Down‐Shifting Layer for Crystalline Silicon Solar Cells

SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors can convert near ultraviolet light with lower sensitivity to the solar cell to yellow‐green light at which the solar cell has higher sensitivity and exhibit the excellent luminescent property of long persistence. Therefore, in this study, the authors firstly synthesized the fine SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors and then produced SrAl₂O₄:Eu²⁺,Dy³⁺/SiO₂ composite films as spectral shifters to understand the effects of SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor on photoelectric conversion efficiencies of a crystalline silicon photovoltaic module. Under one sun illumination, the composite film containing an appropriate amount of SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor enhances the photoelectric conversion efficiency of the cell through spectral down‐shifting as compared to the bare glass substrate, and the maximum achieves 11.12%. In contrast, the commercial SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor composite film is not effective for improving the photoelectric conversion efficiency because of the relatively lower visible light transmittance of film caused by the large aggregates. After one sun illumination for 1 min, the light source was turned off, and the cell containing the synthesized SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor still shows an efficiency of 1.16% in the dark due to the irradiation by the long persistent light from SrAl₂O₄:Eu²⁺,Dy³⁺, which provides a possibility to fulfill the operation of solar cells even in the dark.     

Highly efficient photostimulated luminescence of Pb2+ doped SrAl2O4:Eu2+, Dy3+ borate glass for long-term stable optical information storage

Despite the transformative role in society, information storage materials remain vulnerable to the corrosion by water, oxygen and heat, while topological engineering of glass provides an attractive solution to this tricky problem. Here, a considerable discovery is reported that the doping of Pb²⁺ ions could greatly affect the luminescence behavior of SrAl₂O₄:Eu²⁺, Dy³⁺ borate glass, resulting in a controllable property between long persistent luminescence and photostimulated luminescence. Specifically, high concentration Pb doped samples featuring the deeper continuously distributed trap levels with 0.97–1.47 eV performed highly efficient photostimulated luminescence. In other words, the ultraviolet-visible photons could be “written” in the deeper traps and then “read out” under the stimulation of a 980 nm near-infrared laser. From the combined structural and luminescence characterizations, it was speculated that the deeper trap originated from the increase of oxygen vacancies at defect levels. The practical anti-counterfeiting application was successfully realized based on this material with superior photostimulated luminescence phenomenon, which rendered the SrAl₂O₄:Eu²⁺, Dy³⁺ borate glass shine in a new field such as anti-counterfeiting, yet as a promising candidate for information storage application.     

Laser synthesis and luminescence properties of SrAl2O4:Eu2+, Dy3+ phosphors

A laser melting method has been developed for the synthesis of highly luminescent, long-lasting SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors. The high temperature achieved in high-power density CO₂ laser irradiation of mixtures of SrCO₃, Al₂O₃, Eu₂O₃, and Dy₂O₃ enabled the one-step, fast synthesis of these phosphors in air at atmospheric pressure. X-ray diffraction, Raman spectroscopy, and scanning electron microscopy characterization studies reveal that the produced materials consist of monoclinic SrAl₂O₄ grains extensively surrounded by rare-earth ion-enriched grain boundaries. The photoluminescence properties of laser-produced SrAl₂O₄:Eu²⁺, Dy³⁺ materials are discussed. The results reported here suggest that this laser melting method is a promising route for the synthesis of ceramic phosphors. It is presented as an alternative to the conventional sol–gel and solid-state methods, which require the use of high-temperature furnaces, flux additives, and reducing atmospheres.     

Sunlight‐activated long persistent luminescent polyurethane incorporated with amino‐functionalized SrAl2O4:Eu2+,Dy3+ phosphor

An amino‐terminated long persistent luminescent phosphor (Amino‐SrAl₂O₄:Eu²⁺,Dy³⁺) was prepared based on inorganic SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor, chemically modified with 3‐aminopropyltriethoxysilane (KH550). Fourier transform infrared and X‐ray photoelectron spectral, thermogravimetric and scanning electron microscopic measurements confirmed the successful synthesis of Amino‐SrAl₂O₄:Eu²⁺,Dy³⁺. Then this amino‐functionalized phosphor was introduced into polyurethane (PU) through urea linkages, and the effects of the chemical combination of Amino‐SrAl₂O₄:Eu²⁺,Dy³⁺ and PU on the morphology, structure, storage stability, and mechanical, thermal and luminescent properties of the resultant long persistent luminescent polyurethane (LPLPU) were investigated. Compared with SrAl₂O₄:Eu²⁺,Dy³⁺/PU composites prepared by physical blending, the LPLPU shows better mechanical properties and storage stability due to the good compatibility of Amino‐SrAl₂O₄:Eu²⁺,Dy³⁺ with PU. More residues and higher initial decomposition temperature are observed because the interaction of the amino‐phosphor and PU delays the degradation. Study of the luminescent effect reveals that the LPLPU shows more than 10 h afterglow after cessation of the excitation light, and the brightness of green light in darkness is basically the same as that of LPLPU and SrAl₂O₄:Eu²⁺,Dy³⁺/PU. © 2016 Society of Chemical Industry     

Characterization of strontium aluminate phosphors prepared from milled SrCO3

SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors were prepared by solid-state reaction from milled SrCO₃. The effect of milling treatment of SrCO₃ on the formation and physical properties of SrAl₂O₄ phosphors was investigated by DTA, XRD, BET, SEM and PL. The results indicate that small crystallite size and large specific surface area of the milled SrCO₃ were able to increase the contact points between the reactants and to reduce the average transport distance for materials diffusion. Therefore, the solid-state reaction can be accelerated and the formation of SrAl₂O₄ was facilitated. On the other hand, the number of nucleation sites was also suggested to be increased that leads to a decrease in SrAl₂O₄ crystallite size and an increase in specific surface area. The increased specific surface area was proposed to increase the emission intensity and afterglow decay.     

Structural-chemical mechanism of formation of solid solutions in the La2SrAl2O2-Ho2SrAl2O7 system

The processes of phase formation in the La₂O₃-Ho₂O₃-SrO-Al₂O₃ system are investigated in the temperature range 1200–1500°C. The structural characteristics of the compounds described in the studied system are presented. It is established that the formation of (La_1?x Ho_x)₂SrAl₂O₇ solid solutions proceeds through the formation of LaAlO₃, LaSrAlO₄, SrAl₂O₄, and SrHo₂O₄ compounds. An increase in the holmium content and the temperature leads to a crossover from the mechanism in which the interaction of LaAlO₃ and LaSrAlO₄ is a limiting stage to the mechanism in which the decisive role is played by the interactions of SrAl₂O₄ with Ho₂O₃ and SrHo₂O₄ with Al₂O₃.     

Enhanced Light Storage of SrAl2O4 Glass‐Ceramics Controlled by Selective Europium Reduction

A luminescent Eu, Dy: SrAl₂O₄ glass‐ceramics with high transparency in the visible region was successfully synthesized using the frozen sorbet technique with the control of O₂ partial pressure () for the oxidation of Eu²⁺ ions. The glass‐ceramics include Eu²⁺, Eu³⁺, and Dy³⁺ ions, and thus exhibits three characteristic types of emission bands, 4f–5d at around 520 nm (Eu²⁺ ions), 4f–4f at 610 nm (Eu³⁺ ions), and 480 nm (Dy³⁺ ions). The Eu, Dy: SrAl₂O₄ glass‐ceramics provide remarkable long‐persistent luminescence under dark condition. The glass‐ceramics also exhibits color‐changing luminescence in the visible region based on their remarkable light storage properties. The luminescent Eu, Dy: SrAl₂O₄ glass‐ceramics using the frozen sorbet technique with control of are promising materials for application in novel photonic and light storage materials.     

The influence of SrSO4 on the tribological properties of NiCr–Al2O3 cermet at elevated temperatures

The tribological properties of NiCr–40 wt%Al₂O₃ (NC40A) cermet-based self-lubricating composites containing 10 wt%–30 wt% SrSO₄ against alumina ball were investigated at elevated temperatures. The results indicated that the friction coefficients and wear rates were significantly reduced by adding different amounts of SrSO₄ above 200 °C. NC40A–10SrSO₄ composite exhibited satisfactory tribological properties above 200 °C due to the formation of synergistic lubricating films SrAl₄O₇ and NiCr₂O₄ on the contact surface, while low friction coefficient and wear rate of NC40A–30SrSO₄ composite at 400 °C were attributed to the synergistic lubricating effect of Sr₄Al₂O₇, SrAl₂O₄ and NiCr₂O₄.     

High dielectric and microwave absorption properties of ultra-thin 1-xSrTiO3-δ − xSrAl12O19 films

To reduce the thickness of the microwave absorbing materials, we have prepared 1-xSrTiO_3-δ?xSrAl₁₂O₁₉ ceramics by hot?pressing sintering in the vacuum. The microstructure, dielectric, thermogravimetric analysis and microwave absorbing properties of 1-xSrTiO_3-δ?xSrAl₁₂O₁₉ were systematically investigated and discussed. The 0.95SrTiO_3-δ??0.05SrAl₁₂O₁₉ has high permittivity, the real part is from 1662.2 to 704.9 and the imaginary part is from 208.6 to 12. The absorption bandwidth (reflection loss ≤?5?dB) of 0.95SrTiO_3-δ??0.05SrAl₁₂O₁₉ can cover 8.6???12.4?GHz and its thickness is only 0.232?mm which is much thinner than these recently reported by other researchers. For 0.942SrTiO_3-δ??0.058SrAl₁₂O₁₉, the peak value of reflection loss is up to ??58.5?dB with a thickness of 0.75?mm. The 1-xSrTiO_3-δ?xSrAl₁₂O₁₉ films could be excellent candidates for highly efficient and ultra?thin microwave absorbing materials.     

SrAl12O19: Fe3+ @3-aminopropyl triethoxysilane: Ambient aqueous stable near-infrared persistent luminescent nanocomposites

Near-infrared (NIR) persistent luminescent nanoparticles (N-PLNPs) endow a long-term in vivo imaging with deep tissue penetration and high signal to noise ratio. However, synthesis route and applicable afterglow center for N-PLNPs are still limited. Here, we report on a new synthesis by employing chemical precipitation as the central pivot, which is simpler and has controllable and reproducible routes than the existing technique. We also introduce Fe³⁺ ion as a new member to join the group of afterglow emitters. Although its NIR luminescence is ubiquitous, its NIR afterglow is still almost never reported. In this paper, SrAl₁₂O₁₉: Fe³⁺ N-PLNPs display a NIR persistent luminescence from 750 to 1000 nm, which is assigned to ⁴T₁(⁴G)→⁶A₁(⁶S) transition of Fe³⁺. Furthermore, a surface-amination technique is proposed to improve the stability of SrAl₁₂O₁₉: Fe³⁺ N-PLNPs in aqueous solution. After encapsulating the N-PLNPs with 3-aminopropyl triethoxysilane (APTES), SrAl₁₂O₁₉: Fe³⁺ @APTES nanocomposites exhibit a hydrophilic stability beyond 20 days in aqueous solution. The results make them valuable in studying the biological functions of biomolecules and monitoring cellular networks in their native contexts.     

Atomic layer deposition (ALD) of nanoscale coatings on SrAl2O4-based phosphor powders to prevent aqueous degradation

The aqueous degradation of Eu²⁺-activated and Dy³⁺-codoped strontium aluminate (SrAl₂O₄:Eu²⁺, Dy³⁺, SA2-Green) long afterglow phosphors synthesized from solid-state reaction and coated with nanoscale metal oxide protective layers (≤12 nm) via atomic layer deposition (ALD) is investigated. Uncoated phosphor powders degrade rapidly upon water immersion and lose their green phosphorescence within 48 hours of water exposure. Postmortem investigations reveal hydration and decomposition of the SrAl₂O₄ phase. ALD of ~10 nm Al₂O₃ or ~12 nm TiO₂ is found to significantly improve the powder's resistance to aqueous degradation. All ALD-coated powders show minimal structural and chemical degradation and retain phosphoresence after 48 hours of water immersion. This enhanced durability offers a new pathway for applying long afterglow phosphors to outdoor applications like roadway markings or safety signage and for their incorporation into more eco-friendly waterborne coatings.     

Fabrication and characterizations of Eu2+-Dy3+ co-doped SrAl2O4 ceramics with persistent luminescence

(Sr_0.97Eu_0.01Dy_0.02)Al₂O₄ persistent luminescence (PersL) ceramics were fabricated by solid-state reactive sintering in vacuum combined with hot isostatic pressing (HIP) using H₃BO₃ as a sintering additive. The phase composition, microstructure, luminescence properties, trap state, and PersL performance of HIP post-treated (Sr_0.97Eu_0.01Dy_0.02)Al₂O₄ PersL ceramics were discussed. For the (Sr_0.97Eu_0.01Dy_0.02)Al₂O₄ PersL ceramics after HIP post-treatment, the initial luminescence intensity of the ceramics reached over 6400 mcd/m² with simulated daylight irradiation of 1000 lx for 5 min, and the persistent emission decay time > 17 h. This is much better than the SrAl₂O₄:Eu²⁺,Dy³⁺ PersL powders and the other luminescent ceramics. In addition, this method is a solid-state reactive sintering method for synthesizing ceramics, which has the advantages of low cost and simple operation, and is suitable for large-scale, high-volume industrial production.     

The dispersants effect on physical properties of long‐lasting luminescent polypropylene

Two series of blends, O‐PP₁₅ and O‐PP₃₅, were prepared by mixing polypropylene (PP), luminescent powders (SrAl₂O₄: Eu²⁺, Dy³⁺) of 15 and 35 μm average particle diameter, and hydrophobic dispersant at about 190°C in the Brabender mixer. The effect of amounts and diameter of luminescent powders on the physical properties of PP material were discussed herein. The luminescence and afterglow time tests indicated that the initial luminescence of all blends increased with the luminescent powders amounts. O‐PP₃₅ blends showed lower afterglow luminance than O‐PP₁₅ blends at low luminescent powder amounts. The melting and crystallization temperatures of the blends appeared at 152–168°C and 87–103°C, respectively. The blends displayed peaks attributable to a α crystal structure at 2θ = 18°–19°. The β crystal structure was only evident from its characteristic 2θ peak at 15°–16° in the WAXD pattern of the O‐PP₃₅ blends with high luminescent powder amounts. All of the blends had lower tensile strengths. However, the improvement in the luminescent powder distribution was evident from the SEM images after adding hydrophobic dispersant. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Phase evolution and thermal behaviors of the solid-state reaction between SrCO3 and Al2O3 to form SrAl2O4 under air and CO2-air atmospheres

The solid-state reactions between SrCO₃ and Al₂O₃ forming SrAl₂O₄ under air and CO₂-air atmospheres were investigated. The solid-state reaction between SrCO₃ and Al₂O₃ under a CO₂ atmosphere can be separated into multiple reaction stages. The first stage is attributed to the formation of SrAl₂O₄ resulting from the reaction between SrCO₃ and Al₂O₃. The diffusion of Al₂O₃ through the product layer then takes place to continue the reaction. At a higher temperature, the Sr₃Al₂O₆ formation reaction occurs due to the chemical reaction between SrCO₃ and SrAl₂O₄. The SrCO₃ is thermally finally decomposed. The resulting SrO may diffuse rapidly through the product layer, producing pure SrAl₂O₄ formation via a complicated diffusion process at a much higher temperature. These reaction stages occur at very close temperatures under an air atmosphere, leading to a complex reaction between the solids in air.     

Luminescent properties of SrAl2O4: Eu,Dy material prepared by the gel method

SrAl₂O₄:Eu,Dy materials were first prepared by the gel method. Compared with samples prepared by solid state reactions, the grain size of the gel method is greatly reduced to nanometer grade. A clear blue shift occurs in the excitation and emission spectra of nano SrAl₂O₄:Eu,Dy, of which the peak of the excitation and emission spectra are at 323 and 500 nm respectively. The brightness of nano SrAl₂O₄:Eu,Dy is greatly reduced. The blue shift and the change of luminescent intensity in nano SrAl₂O₄:Eu,Dy materials can be attributed to the effect of surface energy.