Polymer-derived β-SiAlON:Eu2+ phosphors

A series of β-SiAlON:Eu²⁺ phosphors were synthesized from single-source precursors, perhydropolysilazane chemically modified with Al(OCH(CH₃)₂)₃, AlCl₃, and EuCl₂. The single-source precursors were converted to β-SiAlON:Eu²⁺ phosphors by pyrolysis under flowing N₂ or NH₃ at 1000°C, followed by heat treatment at 1800°C under an N₂ gas pressure at 980 kPa. By varying the molar ratio of the chemical modifiers, β-SiAlON:Eu²⁺ with the compositions close to the theoretical ones expressed as Si_6−zAl_zO_z−2yN_8−z+2y:yEu²⁺ were synthesized, where the z values and Eu²⁺ contents were controlled in the ranges of .44–.78 and .35–1.48 mol%, respectively. The polymer-derived β-SiAlON:Eu²⁺ phosphors exhibited green emission under excitation at 460 nm attributed to the 4f⁷–4f⁶(7f³)5d¹ transition of dopant Eu²⁺. High-angle annular dark-field-scanning transmission microscopy analysis confirmed that the doped-Eu²⁺ existed interstitially within the channels along the c axis of host β-SiAlON. Compared with the conventional powder metallurgy route, the polymer-derived ceramic route in this study offers some advantages in the grain growth of host β-SiAlON and photoluminescence properties in terms of green emission intensity under excitation at 460 nm, and the highest intensity was achieved for the polymer-derived β-SiAlON:Eu²⁺ with z = .64 and .37 mol% Eu²⁺.     

SHS of Eu2+-doped β-SiAlON phosphors: Impacts of N2 pressure and Si particle size

Self-propagation high-temperature synthesis (SHS) method was developed as a highly-efficient route for the preparation of Eu-doped β-SiAlON green phosphors. The as-synthesized products with composition Si_5.5Al_0.5O_0.5N_7.5:0.035Eu²⁺ were featured with high phase purity, good uniformity and equiaxed grain morphology. Strong green light emission spectra covering the range of 450–650 nm were obtained under the excitation of UV or blue light. The impacts of the nitrogen pressure in the reaction system and average particle size of Si reactant, which were two of the most vital parameters in the SHS reaction, were systematically investigated. Based on the experimental results, these two factors were confirmed to have striking influences on the phase composition, microstructure and photoluminescence properties of resultant phosphors. In the SHS for β-SiAlON:Eu²⁺ phosphors, there demonstrated that the selection of N₂ pressure of 4 MPa and the utilization of Si reactants with fine particle size would bring about remarkable enhancement in the photoluminescence properties.     

Significantly improved photoluminescence of the green-emitting β-sialon:Eu2+ phosphor via surface coating of TiO2

The β-sialon:Eu²⁺ phosphor particles were successfully coated by TiO₂ nanoparticles via the sol-gel method. The TiO₂-coated β-sialon:Eu²⁺ phosphor had a significantly improved photoluminescence (PL) performance under the 365 nm excitation, due to the localized surface plasmon resonance (LSPR) at the interface between the TiO₂ coating layer and phosphor surface. The emission intensity of the TiO₂-coated β-sialon:Eu²⁺ prepared with the titanium (IV) tetrabutoxide (Ti(OC₄H₉)₄, TTBO):H₂O = 1:0.5 volume ratio was dramatically increased by ~24%. When the preparation temperature was 500°C, it was responsible for superior PL intensity by considering the important domination factors of higher anatase content and spherical particle shape of the TiO₂ coating layer to the LSPR effect. The coating around the phosphor surface by the TiO₂ nanoparticles would be an effective technique to improve the PL efficiency of phosphor for the application in the white light-emitting diodes (LEDs), by utilizing the LSPR effect of the semiconductor coating layer, instead of conventional metal plasmonic materials.     

Design of a superb Eu2+–Mn2+ co-doped narrow-band green phosphor via nearly 100% energy transfer efficiency

The excellent narrow-band emitters, especially the green ones, are regarded as a pivotal research direction for light-emitting diodes (LED) backlights in liquid-crystal displays (LCDs). A nearly single-peak green emission centered at 513 nm with a full width at half maximum of 28 nm is reached in KAl₁₁O₁₇:0.1Eu²⁺, 0.15Mn²⁺ phosphor via nearly 100% energy transfer (ET) efficiency, and the extended X-ray absorption fine structure analysis elucidates its mechanism, which is that Eu²⁺ and Mn²⁺ are constrained to form Eu²⁺–Mn²⁺ pairs with a small distance 3.7 Å caused by the local environment relaxation inducement. Meanwhile, by creating an unhindered energy flow between Eu²⁺, Mn²⁺ and K⁺/O²⁻ defect levels through ET and multilevel electron trapped and recombination process, the KAO:Eu²⁺, Mn²⁺ phosphors perform superb photoluminescence property with a high color purity of 83%, an excellent thermal stability (94%@200°C), and unexceptionable internal and external quantum efficiencies of 91.7% and 66.4%, which all are superior to characteristics of commercial β-SiAlON:Eu²⁺ phosphor. Moreover, the white LED fabricated using KAO:Eu²⁺, Mn²⁺ to provide green component shows a wide color gamut of 105% National Television System Committee. These results indicate a potential for an application of our material in LCD–LED backlights, and the design of such local relaxation-induced structure provides a significative reference to develop the new narrow-band emitters.     

Composition-dependent properties of Ca-α-SiAlON:Eu2+ phosphors prepared by combustion synthesis

Herein, we put forward a simple combustion synthesis strategy for the highly efficient preparation of Eu-doped Ca-α-SiAlON yellow phosphors with different composition of Ca_(m/2−x)Eu_xSi_12−m-nAl_m+nO_nN_16−n. The as-synthesized phosphors were endowed with outstanding photoluminescence behavior of yellow emission peaking at ~580 nm under excitation of near-ultraviolet (UV) or blue light. In the designed experiments, products with different compositions, which were determined by the varying m and n values, were obtained by adjusting the proportion of starting reactants. Further, the dependence of composition on the overall properties of products was systematically studied. It was found that the m and n values could have distinct impact on the phase composition, microstructure and photoluminescence properties of as-synthesized phosphors. The most prominent enhancement of spectral intensity was achieved in the sample with composition of m=1.5 and n=0.8 in this research system. The resultant Ca-α-SiAlON:Eu²⁺ phosphors were simultaneously featured with high purity, favorable uniformity and equiaxial morphology. A continuous red shift phenomenon in emission wavelength with the increase of m value and an inverse blue shift with the increase of n value were both observed and rationally uncovered.     

Aqueous processing of carbothermally prepared Ca-α-SiAlON and β-SiAlON Powders: powder and suspension characterisation

Properties of carbothermally prepared Ca-α-SiAlON and β-SiAlON powders and aqueous suspensions thereof were determined. The isoelectric points of Ca-α-SiAlON and β-SiAlON were 3·4 and 4·6. After addition of deflocculant, Dolapix CE64, the behaviour of both suspensions is nearly identical. The isoelectric points become 5·5 and 5·3, respectively. Despite differences in bulk composition, grain size distribution, grain size and shape, both SiAlON suspensions show a similar dependence of a zeta potential on pH. Optimum slip casting properties, i.e. lowest viscosity values (below 10 mPa s), the highest absolute zeta potential values, the smallest floc size and sediment volume were found between pH 10–11 for both powders. The potentials of the different suspension characterisation techniques were compared and zeta potential and viscosity measurements were found the most convenient.     

Enhanced narrow green emission and thermal stability in γ-AlON: Mn2+, Mg2+ phosphor via charge compensation

A series of novel green emitting γ-AlON: Mn²⁺, Mg²⁺ and γ-AlON: Mn²⁺, Mg²⁺, M (M?=?Li⁺, Na⁺, K⁺, Si⁴⁺) phosphors were fabricated with the gas-pressure sintering reaction process. The phase structure, morphology and photoluminescence properties of the phosphors were characterized via X-ray powder diffraction, scanning electron microscopy, and photoluminescence spectroscopy. Meanwhile, the charge compensation methods were utilized to eliminate the disadvantage of charge imbalance between Al³⁺ and Mn²⁺. The results show that the luminescence intensity of Mn²⁺ was maximally enhanced by the introduction of Si⁴⁺ ions, which was 2.36 times that of the sample without charge compensator. Moreover, as the temperature reached at 150?°C, thermal stability of the samples contained charge compensator Li⁺ and Si⁴⁺ were improved to 93% and 90% of that the room temperature, respectively, while the original sample was 85%. These luminescence properties were enhanced due to the introduction of charge compensators which reduce defects caused by charge imbalance. In addition, the specific mechanisms were discussed in detail. In general, the charge compensation could be used as an effective strategy to strengthen thermal stability and luminescence performances of phosphors.     

Tunable photoluminescence of apatite phosphor Ca5.95−xSrxLa4(SiO4)2(PO4)4O2:0.05Eu2+ and its application in light-emitting diodes

A series of new apatite phosphors Ca_5.95−xSr_xLa₄(SiO₄)₂(PO₄)₄O₂:0.05Eu²⁺ (x = 0-5.95) were prepared with the solid-state method. The variations of the occupation rate and cell parameters were investigated in detail, demonstrating that the phosphors are pure phases and that the different occupation rates of La³⁺, Ca²⁺, and Sr²⁺ ions are due to the different electrostatic bond strengths. The reflectance and photoluminescence excitation spectra prove that the phosphors can be efficiently excited with near-ultraviolet (n-UV) light. The broad redshift (50 nm) in the photoluminescence spectra is attributed to the increase in the crystal field splitting when the Ca²⁺ ion is replaced by the larger Sr²⁺ ion. At 150°C, the obtained phosphors maintain an emission intensity of ~67%-77% of that at room temperature (25°C), which indicates relatively the high performance of apatite phosphors in the temperature-dependence experiment. Because of the substitution of the small Ca²⁺ ion by the large Sr²⁺ ion, the emission color changes from green to yellow. Finally, a series of self-made light emitting diodes lamps were fabricated by coating the Ca_5.95−xSr_xLa₄(SiO₄)₂(PO₄)₄O₂:0.05Eu²⁺ phosphors with commercial blue and red phosphors on an n-UV chip (λ_ex = 370 nm). The self-made white-emitting lamps display a continuous changing correlated color temperature (4053-9353 K) or commission international de L'eclairgae (from [0.29, 0.28] to [0.38, 0.37]), implying that the series apatite phosphors have great potential to meet the different requirements of applications.     

A single-phase white-emitting La10(SiO4)6O3：Eu2+/Eu3+ phosphor for near-UV LED-based application

A novel single-phase white-emitting phosphor La₁₀(SiO₄)₆O₃ (LSO): xEu has been synthesized by high-temperature solid-state reaction. Its crystal structure, luminescence properties, fluorescence decay time and oxygen vacancies have been characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectra. XRD result shows a typical oxyapatite structure with the space group of P6₃/m. Characteristic excitation and emission peaks of Eu²⁺ and Eu³⁺ were observed from PL studies. The optimum doping concentration of Eu was found to be 7.5 mol% (x = 0.075). In this work, the lifetimes of Eu³⁺ and Eu²⁺ were considerably longer than those from some references. Under the excitation of different near ultraviolet (n-UV) longer wavelengths (λex = 360, 370, and 380 nm), the white light emission can be realized with the CIE chromaticity coordinates (0.3907, 0.3595), (0.3472, 0.3282), and (0.3504, 0.3062) for the phosphor LSO: 0.075Eu. The chromaticity coordinates of the phosphor were all located in the white region. Therefore, it is suggested that the explored LSO: 0.075Eu phosphor can be a good candidate for white light-emitting diodes (W-LEDs) application.     

Characteristics of α′- and β-Sr2SiO4:Eu phosphor powders prepared by spray pyrolysis

α′- and β-Sr₂SiO₄:Eu²⁺ phosphor powders were prepared by spray pyrolysis from the spray solutions with and without NH₄Cl flux. The phosphor powders prepared from the spray solutions with the addition amount of NH₄Cl flux between 2 and 5 wt% of phosphor had regular polyhedron structures and main crystal structure of β-Sr₂SiO₄. On the other hand, the phosphor powders prepared from the spray solution with high addition amount of NH₄Cl flux as 6 wt% of phosphor had irregular morphology and crystal structure of α′-Sr₂SiO₄. The mean size of the β-Sr₂SiO₄ phosphor powders with regular polyhedron structure was 5.2 μm. The β-Sr₂SiO₄:Eu²⁺ phosphor powders had higher photoluminescence intensities than the α′-Sr₂SiO₄:Eu²⁺ phosphor powders. The β-Sr₂SiO₄:Eu²⁺ phosphor powders prepared from the spray solution with 5 wt% NH₄Cl flux of phosphor had the maximum photoluminescence intensity. The wavelengths of the emission spectra showing the maximum peak intensities changed from 543.2 to 561.8 nm when the addition amount of NH₄Cl flux was increased from 2 to 6 wt% of phosphor.     

Sol–gel-deposition of thin TiO2:Eu thermographic phosphor films

A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this application, the temperature-dependent luminescence properties of europium (III)-doped anatase (TiO₂:Eu³⁺) thin films were studied. The films were prepared by the sol–gel method using dip coating. The structures and the morphology of the films were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electron dispersive X-ray spectroscopy (EDX) was used to verify the europium concentration within the films. For using the films as temperature sensors the optical properties are the main concern. Therefore, the emission spectra of the films were measured after ultraviolet laser excitation (355 nm). They indicate that the red characteristic emission (617 nm) of TiO₂:Eu³⁺ due to the ⁵D₀ → ⁷F₂ electric dipole transition is the strongest. The decay time constant of the exponential emission decay under UV excitation with a Nd:YAG laser (355 nm, f = 10 Hz) is strongly temperature dependent in the range from 200 °C up to 400 °C; making it useful for temperature evaluation. The temperature dependence was measured for the emission line at 617 nm; the results demonstrate that anatase doped europium (III) can be used as a thermographic phosphor.     

In-air self-reduction synthesis and photoluminescent properties of Eu2+–Eu3+ activated CaAl2SixO2x+4 phosphors

In this paper, CaAl₂Si_xO_2x+4:Eu²⁺/Eu³⁺ (2%, molar ratio) phosphors with different SiO₂ concentrations (x=0, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 1.5, and 2) were prepared by a high-temperature solid-state reaction under air condition with the absence of the reduction atmosphere. The crystalline phase, photoluminescent properties and luminescent lifetimes of the prepared samples were systematically studied. The results indicated that Eu²⁺ was reduced and obtained in air through self-reduction. The doping of SiO₂ in calcium aluminates strongly affects the crystalline phase, excitation and emission spectra, CIE (Commission Intemationale de I'Eclairage 1931 chromaticity) (x, y) position and lifetimes of europium. It is interesting and important that different luminescent colors can be obtained and modulated by changing the content of SiO₂.     

Biological,physical, and chemical properties of wallostonite-added β-SiAlON ceramics

The different content of wollastonite was added to raw materials to prepare β-SiAlON ceramics; furthermore, in this study, the effect of wollastonite on the physical, chemical, and biological properties of β-SiAlON was explored. X-ray diffraction and scanning electron microscopy (SEM) demonstrated that Ca in β-SiAlON was uniformly distributed; however, the addition of wollastonite did not lead to the formation of the Ca-α-SiAlON phase. Wollastonite addition improved the sintering performance and increased the compressive strength of β-SiAlON ceramics. Moreover, SEM demonstrated that by adding wollastonite, the bonding of the sample became tighter. Ion release results demonstrated that the material had low ion release and stable chemical properties. Moreover, in cellular experiments, when the content of wollastonite is <12 wt% in β-SiAlON ceramics, β-SiAlON ceramics with or without Ca demonstrated satisfactory biocompatibility. The presence of some amount of Ca did not affect the adhesion, spreading, and proliferation of the MC3T3-E1 cells. Moreover, when wollastonite is <12 wt%, culturing MC3T3-E1 cells in the leaching solution of β-SiAlON ceramics confirmed that bone formation is unaffected. Thus, β-SiAlON ceramics containing Ca (wollastonite <12 wt%) demonstrated good physical and chemical properties; however, the biological properties remain unaffected. Therefore, the ceramics that were prepared are bioactive materials having excellent properties.     

A new route for the synthesis of β-sialon: Eu2+ phosphors using pyrophyllite powders

Eu²⁺ ion doped β-sialon (Si_6?zAl_zO_zN_8?z:xEu²⁺) phosphors were prepared from pyrophyllite-based mixtures by a carbothermal reduction and nitridation method first. Then, the effects of z values and the Eu concentration on the structure and the photoluminescence (PL) properties were investigated. β-Sialon was successfully synthesized as a dominant phase even at the low temperature of 1500 °C, while Si₃N₄ and AlN weakly coexisted as secondary phases. The PL excitation spectra exhibited the broad spectra covering up to 500 nm, and the greenish-yellow emission was observed at around 580 nm. At z=1 and x=0.012, excellent emission intensity could be attained.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Synthesis of β-SiAlON whiskers: dependence of uniform morphology upon preparation conditions

Abstract

Abstract

β-SiAlON whiskers with uniform morphology were prepared using reaction sintering method under different conditions. The effect of preparing conditions on the morphology of β-SiAlON whiskers was systematically studied by SEM, XRD, TEM and HRTEM. The results showed that single crystalline β-SiAlON whiskers with uniform morphology were successfully fabricated at 1773 K for 6 h under flowing nitrogen atmosphere. The well synthesised whiskers were of several hundreds of nanometres in diameter and a few hundreds of micrometres in length. Although the morphology and its size distribution are mainly determined by the reaction temperature and holding time, they can also be tailored by controlling the reaction atmosphere. The ratio of staring materials has no significant influence on the morphology of β-SiAlON whiskers. The growth of β-SiAlON whiskers follows a vapour–solid mechanism, and the formation of the belt-like whiskers is attributed to an anisotropic growth at the early nucleation/growth stage.     

Effect of Sr/Ca substitution on phase structure and photoluminescence properties of micro-SrxCa1−xAlSiN3: Eu2+ phosphor for high CRI white LEDs

Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors were prepared by using the high temperature solid state reaction in a 1.1 Mpa N₂ atmosphere. The phase structures, photoluminescence (PL) properties, and chromaticity properties of the phosphors affected by Sr/Ca Substitution have been investigated in detail. With increasing Sr content (x value), the crystal grain became bigger and the average grain size increased from 5 µm to 10 µm. PL emission bands of Sr_xCa_1−xAlSiN₃: Eu²⁺ showed a blue-shift from 660 (x=0) to 617 nm (x=0.8), the shoulder of the excitation spectra around 550 nm showed a slightly blue-shift and decay lifetime shortened from 776.96 (x=0.2) to 642.35 ns (x=0.8). Both the emission and excitation intensity of peak position increased with Sr content increased. The ideal white light with high CRI (Ra>88) can be obtained by mixing the Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors and commercial green phosphors with appropriate proportion of the components.     

The effect of CaO on the physicochemical and biological properties of β-SiAlON ceramics

To investigate the effects of CaO on the physicochemical and biological properties of β-SiAlON, β-SiAlON ceramics containing CaO were prepared using the direct nitriding method. The results of X-ray powder diffraction and scanning electron microscopy demonstrated that β-SiAlON remained in the material phase because the addition of Ca did not result in the formation of Ca-α-SiAlON. However, CaO promoted the sintering of β-SiAlON grains and significantly decreased porosity and increased bulk density and compressive strength. According to a chemical stability study, when β-SiAlON ceramics powder was soaked in deionized water and a cell culture medium, it was noted to have negative electricity. A reaction occurred with the H⁺ and OH⁻ ions in the deionized water, leading to the formation of surface structures, such as Si-OH, Al-OH, and N-H. Moreover, the addition of CaO caused a different chemical reaction among ions or ion groups in the culture medium, and new chemical groups formed on the material surface that interacted with the culture medium, which resulted in an alteration of the zeta potential and surface chemical properties of β-SiAlON. MC3T3-E1 cells cultured on the surface of ceramics proved that the cells cannot adhere well to the surface of β-SiAlON ceramics with CaO, although they could proliferate well around those with only β-SiAlON. Therefore, the change in the surface chemical properties provides good anticell adhesion ability, which makes β-SiAlON with CaO a biocompatible material that can be used to prevent contamination caused by cell adhesion, with possible applications in biosensors or biomedical equipment that must be used in sterile environments.     

Large-scale Synthesis of β-SiC Nanochains and Their Raman/Photoluminescence Properties

Although the SiC/SiO₂ nanochain heterojunction has been synthesized, the chained homogeneous nanostructure of SiC has not been reported before. Herein, the novel β-SiC nanochains are synthesized assisted by the AAO template. The characterized results demonstrate that the nanostructures are constructed by spheres of 25–30 nm and conjoint wires of 15–20 nm in diameters. Raman and photoluminescence measurements are used to explore the unique optical properties. A speed-alternating vapor–solid (SA-VS) growth mechanism is proposed to interpret the formation of this typical nanochains. The achieved nanochains enrich the species of one-dimensional (1D) nanostructures and may hold great potential applications in nanotechnology.     

Green emitting β$ubeta$-SiAlON:Eu2+ phosphors derived from chemically modified perhydropolysilazanes

We report the first synthesis of β-SiAlON:Eu²⁺ phosphors from single-source precursors, perhydropolysilazane (PHPS), chemically modified with Al(OCH(CH₃)₂)₃, and EuCl₂. The reactions occurring during the precursor synthesis and the subsequent thermal conversion of polymeric precursors into β-SiAlON:Eu²⁺ phosphors have been studied by a complementary set of analytical techniques, including infrared spectroscopy, gas chromatography–mass spectrometry, thermogravimetry–mass spectrometry, X-ray diffraction (XRD), photoluminescence spectroscopy, and scanning electron microscopy. It has been clearly established that Al(OCH(CH₃)₂)₃ immediately reacted with PHPS to afford N–Al bonds at room temperature, whereas N–Eu bond formation was suggested to proceed above 600°C accompanied by the elimination of HCl up to 1000°C in flowing N₂. The subsequent 1800°C-heat treatment for 1 h under an N₂ gas pressure at 980 kPa allowed converting the single-source precursors into fine-grained β-SiAlON:Eu²⁺ phosphors. XRD analysis revealed that the Al/Si of .09 was the critical atomic ratio in the precursor synthesis to afford single-phase β-SiAlON (z = .55). Moreover, Eu²⁺-doping was found to efficiently reduce the carbon impurity in the host β-SiAlON. The polymer-derived β-SiAlON:Eu²⁺ phosphors exhibited green emission under excitation at 460 nm and achieved the highest green emission intensity at the critical dopant Eu²⁺ concentration at 1.48 at%.