Doped Strontium Aluminate Research Articles

Mechanoluminescence glow curves of rare-earth doped strontium aluminate phosphors

The present paper reports the mechanoluminescence (ML) glow curves of rare-earth doped strontium aluminate phosphors. When Sr3Al2O6:Eu, Dy phosphor mixed in epoxy resin is compressed at a fixed pressing rate or fixed strain rate, its elastico ML (EML) intensity increases linearly with deformation time or pressure and attains a maximum value Im at the time tm, at which the deformation is stopped. Under the pressed condition, the decay time for fast decrease of EML after tm, gives the time-constant for stopping the cross-head of the testing machine used to deform the sample, and decay time for slow decrease of EML gives the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. When SrAl2O4:Eu phosphor mixed in resin is compressed at a fixed rate, then the EML intensity increases linearly with pressure and when the pressure is decreased at a fixed rate, then the EML intensity decreases exponentially with time, in which the decay time of EML is equal to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. The EML intensity of SrAl2O4:Eu film excited by the impact stress, initially increases with time, attains a peak value and later on it decreases exponentially with time, in which the fast decay of EML intensity gives the decay time of impact stress and the decay time of the slow decrease of the EML intensity gives the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. The piezoelectrically-induced detrapping model is found to be suitable for the EML of rare-earth doped strontium aluminate phosphors. Expressions derived on the basis of the piezoelectrically-induced detrapping model are able to explain satisfactorily the characteristics of the EML of the phosphors. It is shown that several parameters of the phosphors can be determined from the ML glow curves.

One-step synthesis and morphology evolution of luminescent Eu2+ doped strontium aluminate nanostructures

Several shapes of Eu2+ doped strontium aluminate have been simply obtained by hydrolysis of Sr2+ and Al3+ in hydrothermal and solvothermal systems under mild conditions. The influence of reaction conditions on the formation of the samples has been considered. Flower-like nanoparticles are synthesized in glycol, when the solvent is substituted by H2O, and/or adding NaOH or stirring for 30 min before injecting the prepared Eu(NO3)3, needle cluster-like, sheet-like and lantern-like structures are synthesized. At the same time, the doped Eu3+ is reduced to Eu2+ at much lower temperature, and the emission peaks result from the transition of 4f65d–4f7 of Eu2+ around 405 nm, 435 nm and 530 nm are also observed, no typical peak of Eu3+ is found, which indicates the complete reduction from Eu3+ to Eu2+. The results of this article may give a guide to synthesize the similar Eu2+ doped luminescent materials.

Structure and luminescence properties of Sr4Al14O25:Eu2+, Dy3+ by Ca2+ doping

The rare-earth doped strontium aluminate phosphors SrxCa4-xAl14O25:Eu2+,Dy3+（x＝0，0.8，1.6，2.4，3.2，4） were synthesized by a high temperature solid-state reaction method. The X-ray diffraction studies show that phase of the phosphors is orthorhombic Sr4Al14O25 for x4O7 for x > 2.4. The results indicate that the phase structure of the samples changes from Sr4Al14O25 to CaAl4O7 when the value of x changes from 0 to 4. In addition, a blue shift was found in the emission spectra due to the change in nature of the Eu2+ surroundings. The decay characteristics show that the phosphor samples with different Ca2+ contents have different afterglow time, and the afterglow time also changes with the value of x. The measurement of thermoluminescence reveals that the trap depth of the phosphor samples with different Ca2+ contents is different. The samples with deeper traps have longer afterglow time.

Synthesis and optical properties of ultra-fine Sr 5Al 2O 8:Eu 3+ nanorod phosphor from a low-heating-temperature solid-state precursor method

A novel Eu 3+ doped strontium aluminate (Sr 5Al 2O 8) nanorod phosphors with high brightness and long afterglow were prepared by sintering the precursor (produced by a low-heating-temperature solid-state method) at 1100 °C in a reducing atmosphere. Sr 5Al 2O 8:Eu 3+ phosphor nanorods with diameters of 20–50 nm and lengths of several microns were obtained, and their optical properties were systematically studied. The main peaks in the excitation and emission spectra can be viewed as the typical emission of Eu 3+. The ultraviolet–visible (UV–vis) absorption spectrum revealed that Eu 3+ was effectively excited by the ultraviolet light, which caused the transition 5D 0 → 7F J ( J = 1, 2).

Fibre-optic thermometer using sensor materials with long fluorescence lifetime

A fibre-optic thermometer using newly developed phosphor sensor materials is described. A highly sensitive fibre-optic fluorescence thermometer is developed using chromium ion (Cr3+) doped phosphor crystals with a longer lifetime than that of conventional ruby sensors. Long fluorescent europium doped strontium aluminate phosphors co-doped with various auxiliary activators (SrAl2O4:Eu2+, Ln3+, Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), were also grown and evaluated. In the fibre-optic thermometer using Cr3+ doped phosphor, PL lifetime decreased at temperatures ranging from 270 K to 750 K. The sensitivity of the fibre-optic thermometer varies with the host materials of Cr3+ ions. In the fibre-optic thermometer using SrAl2O4:Eu2+, Ln3+ doped phosphor, the value and lifetime of afterglow phosphorescence from the SrAl2O4:Eu2+, Ln3+ phosphors varied greatly in the narrow temperature region because the afterglow phosphorescence was dominated by thermal excitation of trapped carriers. Since the trap depth and the trap density depend on auxiliary activator elements, the sensitivity and temperature range of the fibre-optic thermometer using SrAl2O4:Eu2+, Ln3+ phosphors varied with auxiliary activator elements of SrAl2O4:Eu2+, Ln3+ phosphors.

Effect of temperature on the luminescence processes of SrAl2O4:Eu2+

The UV excited and persistent luminescence properties as well as thermoluminescence (TL) of Eu2+ doped strontium aluminates, SrAl2O4:Eu2+ were studied at different temperatures. Two luminescence bands peaking at 445 and 520nm were observed at 20K but only the latter at 295K. Both Sr-sites in the lattice are thus occupied by Eu2+ but at room temperature efficient energy transfer occurs between the two sites. The UV excited and persistent luminescence spectra were similar at 295K but the excitation spectra were different. Thus the luminescent centre is the same in both phenomena but excitation processes are different. Two TL peaks were observed between 50 and 250°C in the glow curve. Multiple traps were, however, observed by preheating and initial rise methods. With longer delay times only the high temperature TL peak was observed. The persistent luminescence is mainly due to slow fading of the low temperature TL peak but the step-wise feeding process from high temperature traps is also probable.

The mechanism of long phosphorescence of SrAl 2− xB xO 4 (0< x

The role of B 2O 3 in realizing the long phosphorescence of Eu(II)+Dy(III) doped strontium aluminates has been investigated. IR and solid state 27 Al MAS NMR spectra show the incorporation of boron as BO 4 in the AlO 4 framework of SrAl 2O 4 and Sr 4Al 14O 25. Phosphor, made free of glassy phases by washing with hot acetic acid+glycerol, did not show any photoconductivity under UV irradiation, indicating that the mechanism involving hole conduction in valence band is untenable for long phosphorescence. EPR studies confirm the presence of both electron and hole trap centers. Dy 3+ forms substitutional defect complex with borate; [Dy–BO 4–V Sr] 2−, and acts as a hole trap center. The electron centers are formed by the oxygen vacancies associated with BO 3 3−, i.e. [BO 3–V O] 3−. Under indigo light or near UV irradiation, the photoinduced electron centers are formed as [BO 3–V O(e′)] 4−. The holes are released from [Dy–BO 4–V Sr(h )] 1− under thermal excitation at room temperature. The recombination of electrons with holes releases energy which is expended to excite Eu 2+ to induce long phosphorescence.

Effect of boron substitution on the preparation and luminescence of Eu 2+ doped strontium aluminates

A series of monoclinic strontium aluminates, SrAl 2− x B x O 4:Eu 2+ (0≤ x≤0.3), were prepared by a solid state reaction. The effect of boron on the preparation, structure and luminescence was studied by thermoanalytical, X-ray powder diffraction (XPD), UV excited as well as thermoluminescence (TL) methods. Boron decreased clearly the reaction threshold temperature but concentration was not found important. Boron decreased also the amount of impurities. The monoclinic structure remained the same irrespective of the boron content. No Al–B solid solution was formed. The luminescence band position, shape, and width were not changed as a function of the boron content. The UV excited luminescence intensity was enhanced but persistent luminescence suppressed when the boron concentration increased. The thermoluminescence peak shifted slightly towards lower temperatures and the TL intensity was suppressed by increasing boron concentration maybe due to the modification of trap concentration and depth.

Importance of Fourth-Order Zero-Field Splitting Terms in Random-Orientation EPR Spectra of Eu(II)-Doped Strontium Aluminate

This work discusses an issue caused by omitting the fourth-order zero-field splitting (ZFS) terms in the analysis of random-orientation EPR spectra. Eu(II)-doped strontium aluminates were employed as a model system of theS-state lanthanoid ions, which have relatively large second-rank ZFS components, comparable to the conventional X-band microwave energy. The initial estimation of second- and fourth-rank ZFS components was acquired uniquely from the W-band spectrum based on a perturbation treatment of the spin Hamiltonian, and they were refined by a hybrid-eigenfield simulation approach. A parallel simulation of the X- and W-band spectra with the refined parameters reasonably reproduced both the observed spectra. The reliability of the parameters was also verified by reproducing the off-principal-axis extra lines arising from the Ms = ½ ↔ -½ transition observed in the W-band spectrum. By comparison with an analysis based on the spin Hamiltonian omitting the fourth-order terms, it was concluded that omitting these terms results in an erroneous determination of the rhombic parameter (E) of the second-rank ZFS tensor. The obtained reliable second-rank ZFS tensors enabled us to invoke the superposition model, determining their principal-axis orientation. This is the first report of such relative orientation determined for Eu(II)-doped strontium aluminates.

Tuning of emission colours in strontium aluminate long persisting phosphor

Solid-state preparation, structure, and multi-colour luminescence of Eu2+ doped strontium aluminate (SRA) phosphors have been reported in this paper. It has been observed that a slight modification in the processing parameters such as type of reducing atmosphere, stoichiometric excess of one or more constituents, nature of fluxes, and intentional addition of carbon or rare-earth halides can drastically shift the emission colours of the SRA phosphor in the visible spectrum. X-ray diffraction analysis and scanning electron microscopy observations of the samples revealed the formation of different crystalline phases related to the SRA system. The luminescence measurements showed that the decay times varied with the emission colours of the samples.

High frequency EPR investigations of gadolinium(III)-doped strontium aluminates

The high frequency EPR spectra of SrAl2O4 doped with gadolinium(III) as a probe are discussed, focusing on the sites occupied by the lanthanides. This is because the emission due to europium(II) of SrAl2O4:Eu,Ln (Ln: lanthanides) is significantly affected by such substitution. The EPR spectrum of SrAl2O4:Dy,Gd showed a pattern typical for two overlapping resonances of 8S7/2 state ions with a λ = ∣D/E∣ of ⅓, suggesting that the gadolinium(III) sites have very low symmetry. From a comparison of the experimental and simulated EPR spectra it was found that both gadolinium(III) and europium(II) ions occupy strontium sites.

High frequency EPR of europium(ii)-doped strontium aluminate phosphors

The europium(II) sites in Sr4Al 14O25 ∶ Eu phosphor with blue emission were examined by means of high frequency (90 and 180 GHz) EPR spectroscopy. The ions occupy two low symmetry sites in the Sr4Al 14O25 lattice with |D| = 0.100 and 0.0907 cm−1 respectively; the ratio |E/D | is estimated to be 1/3 regardless of the substitution site. This implies that the crystal field surrounding europium(II) ions has non-axial symmetry. EPR spectral simulation suggests that the europium( II) ions preferentially occupy the site with the larger |D | when they substitute for strontium ions. Comparison of the EPR spectra of Sr4Al14O25 ∶ Eu and SrAl2O4 ∶ Eu,Dy phosphors shows that the Sr4Al14O25 ∶ Eu is present as an impurity in the non-stoichiometric SrAl2O 4 ∶ Eu,Dy when prepared with excess amounts of Al 2O3. Such materials have been found to have long phosphorescent characteristics.