Crystal chemistry and luminescence of the Eu-activated alkaline earth aluminate phosphors

Alkaline earth monoaluminate phosphors activated with Eu²⁺ were synthesized by microwave processing and alkaline earth hexaaluminate phosphors activated by Eu²⁺ were synthesized by hydrothermal reactions. The emission spectra of the monoaluminates are similar to phosphors prepared by high-temperature firing except for BaAl₂O₄ which emits farther in the blue. The spectra of the hexaaluminate phosphors in general have their emission peak at different wavelengths than what are nominally the same compositions prepared by high-temperature firing.     

Luminescence excitation mechanisms of rare earth doped phosphors in the VUV range

Progress in the development of new luminescent materials is directly related to our understanding of physical processes of energy absorption and relaxation in solids. As far as the VUV energy is concerned, the most suitable materials are probably the large band gap inorganic lattices activated by rare earth ions. Optical excitation in these systems result either in a direct excitation of the luminescence center or an excitation of the host lattice which partly transfer the energy to the emitting levels of the activator. Every steps of the luminescence mechanism enter in competition with nonradiative losses or undesired luminescence which have to be minimized in order to get a high luminescence efficiency. Determination of the dominant transfer and energy loss mechanisms can be performed by time resolved luminescence spectroscopy using UV–VUV synchrotron radiation (SR) excitation. We report here, the result of studies of a large number of rare earth doped materials, performed in the frame of programmes for search of new efficient VUV phosphors and scintillators. The experiments were performed using XUV–VUV SR from SuperAco and DCI storage rings at LURE.     

A potential red-emitting phosphors scheelite-like triple molybdates LiKGd_2(MoO_4)_4:Eu~(3+) for white light emitting diode applications

A series of novel red-emitting phosphors scheelite-like triple molybdates LiKGd2-xEux(MoO4)4(0.1 ≤ x ≤ 1.9) were synthesized by solid state reaction method and their photoluminescence properties were investigated. The photoluminescence results show that all samples can be excited efficiently by UV (396 nm) light and blue (466 nm) light and emit red (615 nm) light with line spectra,which are coupled well with the characteristic emission from UV-LED and blue LED,respectively. The experimental results and thei...     

Pulsed laser deposition of Y2O3 : Eu thin film phosphors

Thin films of Y₂O₃ : Eu cathodoluminescent (CL) phosphors were deposited using pulsed laser deposition using deposition temperature between 250°C and 800°C, O₂ pressures between residual vacuum (2×10⁻⁵ Torr) and 6 Torr, and post annealing up to 1200° for 1 h in air. The CL efficiency of the best thin film was about one third that of the starting powder. The brightness and efficiency of the thin films improved as the deposition temperature, O₂ pressure and post annealing temperature were increased, except that O₂ pressures above 600 mTorr did not significantly improve the CL properties. At deposition temperatures >600°C, the surface morphology changed from a smooth film to a nodular deposit for O₂ pressures >200 mTorr, with nodule dimensions ≈100 nm. Simultaneously, the CL properties improved dramatically because of enhanced optical scattering out of the thin film. Optical scattering was discussed in terms of anomalous diffraction. The CL properties also improved dramatically with high temperature post annealing. This effect was interpreted in terms of improved crystallinity and activation of the Eu. The low brightness and efficiency of thin films versus powder was affected by depletion of the Eu in the thin films owing to the deposition process.     

Red emitting MTiO3 (M = Ca or Sr) phosphors doped with Eu3+ or Pr3+ with some cations as co-dopants

Ca (or Sr)TiO₃:Eu³⁺, M (Li⁺ or Na⁺ or K⁺) and CaTiO₃:Pr³⁺, M (Li⁺ or Na⁺ or Ag⁺ or K⁺ or Gd³⁺ or La³⁺) powders were prepared by combustion synthesis method and the samples were further heated to ～1000 °C to improve the crystallinity. The structure and morphology of materials were examined by X-ray diffraction (XRD) and a scanning electron microscopy (SEM). The morphologies of SrTiO₃:Eu³⁺, CaTiO₃:Eu³⁺ or CaTiO₃:Pr³⁺ powders co-doped with other metal ions were very similar. Small and coagulated particles of nearly cubical shapes with small size distribution having smooth and regular surface were formed. Photo-luminescence spectra of CaTiO₃:Pr³⁺ and co-doped either with Li⁺, Na⁺, K⁺, Ag⁺, La³⁺ or Gd³⁺ ions showed red emissions at 613 nm due to the ¹D₂ → ³H₄ transition of Pr³⁺. The variation of intensity of emission peak with different co-doping follows the order: K⁺ > Ag⁺ > Na⁺ > Li⁺ > La³⁺ > Gd³⁺. The characteristic emissions of CaTiO₃:Eu³⁺ lattices had strong emission at 614 and 620 nm for ⁵D₀ → ⁷F₂ with other weak transitions observed at 580, 592, 654, 705 nm for ⁵D₀ → ⁷F_n transitions where n = 0, 1, 3, 4 respectively in all host lattices. Photoluminescence intensity in SrTiO₃:Eu³⁺ is more than CaTiO₃:Eu³⁺ lattices. A remarkable increase of photoluminescence intensity (in ⁵D₀ → ⁷F₂ transition) was observed if co-doped with Li⁺ ions in CaTiO₃:Eu³⁺ and SrTiO3:Eu³⁺.     

The influence of processing parameters on luminescent oxides produced by combustion synthesis

Rare-earth activated oxide phosphors have application in high energy photoluminescent (plasma panels) and cathodoluminescent (field emission devices) flat panel displays. These phosphors are composed of a highly insulating host lattice with fluorescence arising from the 3d→3d, 5d→4f or 4f→4f transitions in transition metal or rare earth ions. Fabrication of complex host compositions Y₂SiO₅, Y₃Al₅O₁₂, Y₂O₃, and BaMgAl₁₀O₂₇ along with controlled amounts of the activators (Cr³⁺, Mn²⁺, Ce³⁺, Eu²⁺, Eu³⁺, Tb³⁺, Tm³⁺) represent a challenge to the materials synthesis community. High purity, compositionally uniform, single phase, small and uniform particle size powders are required for high resolution and high luminous efficiency in the new flat panel display developments. This paper will review the synthesis techniques and present physical and luminescent data on the resulting materials.

1. Introduction

The visible-light-generating components of emissive, full color, flat panel displays are called phosphors. Phosphors are composed of an inert host lattice and an optically excited activator, typically a 3d or 4f electron metal. For application in the emerging full color, flat panel display industry, thermally stable, high luminous efficiency, radiation resistant, fine particle size powders are required. The demands of these newer technologies have produced a search for new materials and synthesis techniques to improve the performance of phosphors.Oxide phosphors were found to be optimal for field emission display (FED) and plasma panel display (PDP) devices. Compared with a cathode ray tube, an FED operates with lower energy (3–10 keV) but higher current density (1 mA/cm²) beams impinging on the phosphors. This requires more luminous efficient and thermally stable materials. Luminous efficiency is defined as the ratio of the energy out (lumens) to the input energy. Outgassing from the highly efficient sulfide based phosphors has been shown to degrade the cathode tips of the field emitter array and cause irreversible damage [1]. For PDPs, high energy photons (147 nm, 8.5 eV) impinge on the phosphor powders and cause a reduction in luminous efficiency of the display over time because of radiation damage induced in the material [2].Another requirement is on the particle size distribution: there is a maximum and minimum particle size limitation to the powders. For FED applications, about five particle layers are required to achieve optimal light output [3]. Large particles (>8 μm) require thicker layers, increasing the phosphor cost and also producing more light scattering. Additionally the pixel pitch (250 μm) places a maximum on particle size [4]. Alternatively, it was found that small particles (<0.2 μm) do not have high luminous efficiency arising from grain boundary effects [5]. The activator ion in the crystal is most efficient when located in the bulk material in a regular crystal field. Activators located on the surface or on the grain boundaries are thought to be non-luminescent or even luminescence quenching regions.For full color displays, three phosphor compositions are necessary to emit in the red (611–650 nm), green (530–580 nm) and blue (420–450 nm) regions of the visible spectrum. Some oxide based phosphors used in FEDs are the red-emitting (Y_1−xEu_x)₂O₃, the green-emitting (Y_1−xTb_x)₃Al₅O₁₂ and the blue-emitting (Y_1−xCe_x)₂SiO₅ [6]. For some PDPs the red-emitting component is (Y_1−xEu_x)₂O₃, the green-emitting is Zn_1−xMn_xSi₂O₅ and the blue-emitting is (Ba_1−xEu_x)MgAl₁₀O₁₇ [7]. Sulfide phosphors are also used in FEDs but suffer from the aforementioned degradation problems.

2. Phosphor synthesis techniques

Synthesis of oxide phosphors has been achieved by a variety of routes: solid-state reactions [8 and 9], sol–gel techniques [10], hydroxide precipitation [11], hydrothermal synthesis [12 and 13] and combustion synthesis [14, 15, 16 and 17]. Solid-state reactions are performed at high temperatures, typically around 1600°C, because of the refractory nature of the oxide precursors. For multielement compositions, an incomplete reaction is often obtained with undesirable precursor products present in the final product. This technique requires several heating and grinding steps in order to achieve well-reacted, small particle size phosphors. For sol–gel and hydroxide precipitation methods, dilute solutions of metallorganics or metal salts are reacted and condensed into an amorphous or weakly crystalline mass. The advantage of these methods is that atomically mixed powders are obtained in the as-synthesized condition and problems associated with incomplete reactions are avoided. However, these as-synthesized materials must also be heat treated to high temperatures to crystallize the desired phase and to achieve particle sizes greater than 0.2 μm. Hydrothermal synthesis is a low temperature and high pressure decomposition technique that produces fine, well-crystallized powders [13]. These powders must also be heat treated to high temperature to extract the maximum luminous efficiency. Combustion synthesis is a novel technique that has been applied to phosphor synthesis in the past few years. This technique produces highly crystalline powders in the as-synthesized state and will be described in more detail in Section 3.

3. Combustion synthesis of oxide phosphors

Combustion synthesis involves the exothermic reaction between metal nitrates and a fuel. Combustion synthesis is an important powder processing technique generally used to produce complex oxide ceramics such as aluminates [18, 19, 20 and 21], ferrites [22, 23, 24 and 25], and chromites [26 and 27]. The process involves the exothermic reaction of an oxidizer such as metal nitrates, ammonium nitrate, and ammonium perchlorate [28], and an organic fuel, typically urea (CH₄N₂O), carbohydrazide (CH₆N₄O), or glycine (C₂H₅NO₂).The combustion reaction is initiated in a muffle furnace or on a hot plate at temperatures of 500°C or less; much lower than the phase transition of the target material. In a typical reaction, the precursor mixture of water, metal nitrates, and fuel decomposes, dehydrates, and ruptures into a flame after about 3–5 min. The resultant product is a voluminous, foamy powder which occupies the entire volume of the reaction vessel. The chemical energy released from the exothermic reaction between the metal nitrates and fuel can rapidly heat the system to high temperatures (>1600°C) without an external heat source. Combustion synthesized powders are generally more homogeneous, have fewer impurities, and have higher surface areas than powders prepared by conventional solid-state methods [28].The mechanism of the combustion reaction is quite complex. The parameters that influence the reaction include: type of fuel, fuel to oxidizer ratio, use of excess oxidizer, ignition temperature, and water content of the precursor mixture. In general, a good fuel should react non-violently, produce non-toxic gases, and act as a complexant for metal cations [28]. Complexes increase the solubility of metal cations, thereby preventing preferential crystallization as the water in the precursor solution evaporates [29]. The adiabatic flame temperature, T_f, of the reaction is influenced by the type of fuel, fuel to oxidizer ratio, and the amount of water remaining in the precursor solution at the ignition temperature [27]. The flame temperature can be increased with the addition of excess oxidizer such as ammonium nitrate [28], or by increasing the fuel/oxidizer molar ratio. The following equation can be used to approximate the adiabatic flame temperature for a combustion reaction:

(1)

where ΔH_r and ΔH_p are the enthalpies of formation of the reactants and products, respectively, c_p is the heat capacity of products at constant pressure, and T₀ is 298 K. Measured flame temperatures are typically lower than calculated values of flame temperature as a result of heat loss. Table 1 lists the various phosphor compositions that were synthesized by combustion synthesis.

Table 1. Phosphor compositions obtained by combustion synthesis

An example of a stochiometric combustion reaction of yttrium, aluminum and terbium nitrate with carbohydrazide to form (Y_1−xTb_x)₃Al₅O₁₂ is:

(2)

(1−x)Y(NO₃)₃+5Al(NO₃)₃+3xTb(NO₃)₃+15CH₆N₄O→ (Y_1−xTb_x)₃Al₅O₁₂+42N₂+15CO₂+45H₂O.

When complete combustion occurs, the only gaseous products obtained are N₂, CO₂, and H₂O, making this an environmentally clean processing technique. The generation of gaseous products increases the surface area of the powders by creating micro- and nanoporous regions. For the earlier reaction, for every mole of solid produced, 102 mol of gas are produced.The difference in particle size with the use of different fuels depends upon the number of moles of gaseous products released during combustion. As more gases are liberated, the agglomerates are disintegrated and more heat is carried from the system thereby hindering particle growth. A greater number of moles of gas are produced in combustion reactions with carbohydrazide. If complete combustion is assumed, the gaseous product amounts liberated in combustion reactions with glycine, urea and carbohydrazide, are shown in Table 2. The reactions shown are for 2 mol of nitrate producing 1 mol of metal sesquioxide. If Y₃Al₅O₁₂ (YAG) is produced, the reactions must be multiplied by four, as 8 mol of nitrate are used in the reaction.

Table 2. Number of moles of gas produced for different fuels per mole of metal sesquioxide formed

The BET surface area for the as-synthesized YAG phosphors made with glycine, urea and carbohydrazide was measured to be 19, 22 and 25 m²/g, respectively [16], which is consistent with the increase in number of moles of gas produced.Fig. 1 shows the efficiency in lumens per watt (lm/W) as a function of electron accelerating voltage for (Y_1−xTb_x)Al₅O₁₂ made by solid-state, hydrothermal synthesis and combustion synthesis. The combustion synthesized YAG produced in this work has low-voltage cathodoluminescence efficiencies that are comparable to powders produced by other techniques. The efficiencies for all three phosphors were essentially the same at voltages below 600 V. At these voltages, the penetration depth of the incident electron beam is low, (0.004 nm at 600 V) exciting the surface layer of the phosphor particles. In the higher voltage regime, (>600 V), the penetration depth of the electron beam is greater (0.03 nm at 1 kV). The efficiencies of solid-state and hydrothermal synthesized (Y_1−xTb_x)Al₅O₁₂ at these voltages were approximately 1.0 lm/W greater than combustion synthesized (Y_1−xTb_x)Al₅O₁₂. This is because of the smaller crystallite size of these powders (60 nm) compared with hydrothermal and solid-state synthesized powders (100 nm).

Fig. 1. Effect of synthetic route on the low-voltage cathodoluminescence efficiency of (Y_1−xTb_x)3Al₅O₁₂.     

Red shift of the PSL spectra by Al3+ doping in BaFBr:Eu2+

Abstract— Photostimulated luminescent (PSL) materials are currently used for digital storage and display in radiography. The phosphor BaFBr:Eu²⁺ doped with Al³⁺ is shown to have a very high PSL intensity, and the peak of the PSL spectrum shifts to a longer wavelength. Compared with BaFBr:Eu²⁺ doped with Ca²⁺, Sr²⁺, or Mg²⁺, BaAlFBr:Eu²⁺ is better suited to the stimulated light wavelength emitted by a semiconductor laser. The red‐shift mechanism is considered to be a FAcenter for BaFBr:Eu²⁺ doped with Ca²⁺, Sr²⁺, or Mg²⁺ and a F^Z1 center for BaFBr:Eu²⁺ doped with Al³⁺.     

Direct-driven electrochromic displays based on nanocrystalline electrodes

Electrochromic reflective displays based on nanocrystalline metal-oxide working electrodes with chemisorbed organic electrochromic molecules have been developed. We have combined such electrodes with nanocrystalline capacitive counter electrodes, a white reflector layer, and an electrochemically inert electrolyte. Device components and process technology have been developed, resulting in direct-driven displays with promising stability and attractive paper-like visual qualities. This paper describes the components, the design, the process methods, the performance, and the driving routines of the direct-driven devices.     

稀土荧光粉用作强度调制型光纤温度传感器温敏材料的研究

强度调制型光纤温度传感器,可以根据具体情况,选择不同的敏感材料作为传感材料,本文在实验研究和分析的基础上提出了稀土荧光粉作为温敏材料的可行性。     

A phosphor screen for high-resolution CRTs

Abstract— The resolution and image quality of small-screen CRTs have been analyzed. The resolution is primarily determined by the size of the luminous spot, which is determined both by the electron gun and by the size of phosphor particles. Image quality on phosphor screens is controlled by the packing density of the phosphor particles and by the number of layers of phosphor particles in the screen. We have developed a phosphor screen with 1.5 layers of phosphor particles in which packing voids have been minimized by the use of dusting techniques. The new phosphor screen used in a 0.5-in. CRT is capable of displaying sharp SVGA images (1000 × 800 pixels) that are comparable in quality with the images on 13-in. CRTs.     

Thermal stability of nanocrystalline W-Ti diffusion barrier thin films

Nanocrystalline W-Ti diffusion barrier thin films with different phase structures and Cu/barrier/Si multilayer structures were deposited on p-type Si(100) substrates by DC magnetron sputtering.These films were annealed at different temperatures for 1 h.The diffusion barrier properties and thermal stability were studied using four-probe tester(FPP),XRD,AFM,XPS,FESEM,and HRTEM.The experimental results showed that the films were stable up to 700℃.When the annealing temperature was increased,the Cu and Ti atoms...