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⁺.     

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.     

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.     

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.     

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.     

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%.     

ARGET-ATRP using β-CD as reducing agent for the synthesis of PMMA-b-PS-b-PMMA triblock copolymers

We studied the role of β-cyclodextrin (CD), a polyhydroxy cyclic sugar as reducing agent (RA) for the copper mediated activators regenerated by electron transfer atom transfer radical polymerization (ATRP) with the successful chain extension of methyl methacrylate blocks on the bifunctional polystyrene macroinitiator (PS-MI) using nonaqueous solvent system for the first time. The PS-MI was prepared by ATRP, had a polydispersity of 1.05 and gave triblock copolymers in the presence of β-CD. We found that alcoholic groups of β-CD show provisional reducing character and in situ convert Cu(II) to Cu(I) during polymerization. A well-defined triblock copolymer with narrow molecular weight distribution (polydispersity index < 1.5) was obtained at 100 °C. The molecular weight of the block copolymers increased linearly with monomer conversion and the reaction showed good control over the molecular weights when compared to the theoretical values. We show that the mild reducing power of β-CD is retained in a nonaqueous solvent also and is efficient with catalyst/RA ratio of 1:0.5. The effect of temperature on the reducing competency of β-CD was evaluated and found to be active at around 80 °C even though its melting temperature is 290 °C. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47117.     

The influence of precursors and additives on the hydrothermal synthesis of VO2: A route for tuning the metal–insulator transition temperature

Thermochromic materials have attracted the attention of scientific and technological researchers due to their ability to change color depending on the temperature. Vanadium dioxide (VO₂) is capable of considerable polymorphs and has aroused interest mainly because its metal–insulator transition (MIT) presents a thermochromic characteristic at a relatively low temperature. This work aimed to obtain vanadium oxide nanostructures using hydrothermal synthesis to tune the MIT temperature. Ammonium metavanadate or vanadium pentoxide was used as a precursor of vanadium, oxalic acid as a reducing agent, and sodium molybdate as an additive. The starting materials were homogenized and inserted in a hydrothermal reactor at 180 °C. After 24 h of synthesis, part of the resulting product was heat-treated at 400 °C for 3 h. The powders obtained were characterized by their structure, morphology, and thermal properties. The results showed a fiber/rod-shaped VO₂ (M) morphology. Distinct strategies were used to obtain the crystalline phase of interest (VO₂(M)), and the presence of a reversible change occurring at ~68 °C was evaluated according to the parameters from the VO₂ phase transition. The addition of sodium molybdate favored a 22% reduction in the MIT temperature when the precursor used was vanadium pentoxide, indicating possible doping in the structure increased the effects of smaller crystallite size and the presence of crystalline phases. This work opens new perspectives for applications of the vanadium oxides obtained, such as in thermal sensors and/or intelligent materials.     

Partial wet route for YAG powders synthesis leading to transparent ceramic: A core–shell solid-state reaction process

Synthesis of Y₃Al₅O₁₂ (YAG) powders respectively presents morphology control and chemical stoichiometry problems when employing the solid-state reaction or the wet-chemical route. YAG powder retaining the morphology of Al₂O₃ powder was designed and synthesized via a partial wet-chemical process with yttrium ions precipitating on the Al₂O₃ particles. The formation process of the Y-compound/Al₂O₃ core–shell structure is discussed on the basis of zeta-potential measurements and HRTEM results. Two stages, including direct precipitation at the surface of the Al₂O₃ particles and the assembly of the yttrium precipitate from explosive nucleation onto the yttrium compound-coated Al₂O₃ particles, are proposed. A spherical surface reaction process is illustrated. A pure YAG phase can be realized at a temperature about 300 °C lower than that of the traditional solid-state reaction process.     

Influence of Bi2O3 flux in the structural and photoluminescence properties of SrAl2O4:Eu2+ phosphors

SrAl₂O₄:Eu²⁺ phosphors with various content of Bi₂O₃ flux were synthesized and analyzed. It was observed that the crystallinity and the particle size of the phosphors were increased with the addition of Bi₂O₃ flux. These phenomena are considered to be caused via the melting of the Bi₂O₃ flux particles during the synthesis of the phosphors. The melted Bi₂O₃ flux increased the mobility and homogeneity of solid reactants, thereby enhancing the photoluminescence intensity of the phosphors. SrAl₂O₄:Eu²⁺ phosphors with Bi₂O₃ as the flux exhibited a broad green emission with a peak at 520 nm. The highest photoluminescence emission intensity was observed when 5 mol% Bi₂O₃ flux was added into the phosphors. The emission is due to 4f⁶5d→4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ions. Moreover, Bi₂O₃ flux extended the application of the ultraviolet excited phosphors toward the blue-light excited phosphors. Nevertheless, the influence of Bi₂O₃ on the afterglow and the emission color of SrAl₂O₄:Eu²⁺ phosphors were not significant. This research indicated that Bi₂O₃ flux is effective flux for synthesizing SrAl₂O₄:Eu²⁺ phosphors.     

Organometallic precursors,an alternative route for the doping of silicon nitride powders?

Two silicon nitride powders prepared by diimide precipitation and direct nitridation were doped with 5 vol.% yttria using (i) a commercial Y₂O₃ powder and (ii) a soluble Y-organometallic compound as additive. The aim of this work was a direct comparison between those two doping methods. Homogeneity of additive distribution was characterized by Fourier transform infra-red spectroscopy (FT-IR), photo electron spectroscopy (XPS) and transmission electron microscopy (TEM). Depending on the doping technique used, no major difference in sintering behavior and microstructure evolution were monitored. However, chemical doping results in (i) an enhanced densification rate in the early stage of sintering, (ii) a slight shift of the densification rate maximum (ds/dt) to higher temperatures, and (iii) in a slightly finer final microstructure. Note that doping via organometallic precursors can only be successfully applied, when the powder particle surface reveals the presence of silanol groups, as in case of diimide starting powders. Electron microscopy, Organometallic dopants, Sintering, Si₃N₄     

Photoluminescence and LED application of β-SiAlON:Eu2+ green phosphor

We successfully prepared β-SiAlON:Eu²⁺ phosphors with composition of Eu_xSi_6?zAl_zO_yN_8?y (y = z ? 2x, x = 0.018, z = 0.23) by gas-pressured synthesis for application to LED. The crystal phase, microstructure, PL emission and thermal quenching properties were investigated in detail. The β-SiAlON:Eu²⁺ phosphors absorbed broad UV–vis spectral region, and showed a single intense broadband emission near 538 nm. The Stokes shift and zero-phonon line were calculated mathematically, and also estimated from the spectral data. The β-SiAlON:Eu²⁺ green phosphor showed superior thermal quenching properties compared to commercial silicate (SrBaSiO₄:Eu²⁺) green phosphor. The white light-emitting diode (LED) using the prepared β-SiAlON:Eu²⁺ green phosphor exhibited high color gamut, and good optical stability in high working temperature.     

Citrate-based sol–gel synthesis and luminescent properties of Y6WO12:Eu3+, Dy3+ phosphors for solid-state lighting applications

The rare-earth ions (Eu³⁺, Dy³⁺) doped Y₆WO₁₂ phosphors were prepared by a citrate-based sol–gel method. The morphologies and structural properties of the as-prepared and doped samples were analyzed by scanning electron microscope images and X-ray diffraction patterns. The luminescent properties were studied by examining the excitation and emission spectra of the samples. The Eu³⁺ and Dy³⁺ ions doped samples exhibited their characteristic emission bands in the visible region under ultraviolet light excitation. The temperature-dependent photoluminescence (PL) properties of the samples were also investigated. The PL spectra of the synthesized samples by the sol–gel method were compared with those of the bulk sample prepared by a solid-state reaction. Similarly, the Commission International de I’Eclairage chromaticity coordinates and the decay times of Y₆WO₁₂:Eu³⁺ (3 mol%) and Y₆WO₁₂:Dy³⁺ (2 mol%) phosphors were studied.     

Sintering of powders in the CrSi2–Ti(Ta)Si2 systems depending on the methods for synthesis

Composite materials based on refractory silicides, in particular their solid solutions, are widely used in many areas of engineering for parts of heating elements and heat resistance protective coatings. This paper presents the findings obtained in the study of peculiarities of solid-phase interaction during synthesis of Cr_0.9Ta_0.1Si₂ and Cr_0.5Ti_0.5Si₂ solid solutions depending on the synthesis conditions. The effect of the powder dispersity on compaction of targets from Cr_0.9Ta_0.1Si₂ and Cr_0.5Ti_0.5Si₂ powders has been studied, and it has been established that the use of nanosized powders complicates the process of pressing. Also, sintering of targets from nanosized Cr_0.9Ta_0.1Si₂ and Cr_0.5Ti_0.5Si₂ powders was studied. The sintered targets were established to have a small grain size and uniform porosity all over the volume. Thanks to small closed porosity, the targets exhibit high heat resistance under thermal shock.     

Epichlorohydrin cross-linked β-cyclodextrin: an environmental method for the synthesis of 2-arylbenzothiazoles derivatives in water

In the present study, we report an environmentally benign synthesis of 2-arylbenzothiazoles derivatives from o-aminothiophenol and aldehydes in aqueous medium using β-cyclodextrin polymer as a catalyst and air as an oxidant. The polymer showed excellent catalytic activity, recovered, reused six times, and the catalyst efficiency remained unchanged. This suggests that the catalyst is an efficient and green catalyst for the synthesis of 2-arylbenzothiazoles derivatives and the obtained results could be promising for industrial synthesis of 2-arylbenzothiazoles derivatives.     

Effects of different sintering additives on the synthesis of γ-Y2Si2O7 powders

Pure γ-Y₂Si₂O₇ powders were synthesized by the solid-liquid reaction method using Y₂O₃ and SiO₂ powders with Li₂O, MgO or Al₂O₃ additives. The effects of the metallic ions Li⁺, Mg²⁺ and Al³⁺ on the synthesis process were systematically investigated by X-ray diffraction and differential scanning calorimetry. The chemical kinetics of the Y₂Si₂O₇ synthetic process was calculated to illuminate the influences of the different metallic ions on the formation of silicate. The results indicate that the additives could effectively reduce the synthesis temperature by 100–300 °C. The apparent activation energy of the synthetic reaction was reduced by 79.75%, 65.16%, or 56.77% when 6 mol.% of Li₂O, MgO, or Al₂O₃ was added, respectively, and the reaction rate was also significantly increased.     

Influence of the synthesis route on sol–gel SiO2–TiO2 (1:1) xerogels and powders

Five different sol–gel routes are used in order to synthesize mixed SiO₂–TiO₂ materials. Simple mixing of the Ti and Si precursors, pre-hydrolyzing of TEOS, modification of the Ti alkoxide with acetic acid, isoamyl alcohol and acetylacetone lead to translucent gels with different time of gelation. Different techniques such as TGA, DTA, XRD and IR spectroscopy are used to characterize each material. IR spectroscopy revealed the presence of Si–O–Ti and Si–O–Si bonds for all the xerogels letting suppose a composite microstructure of the gels. Pre-hydrolyzing of TEOS and modification of Ti alkoxide with isoamyl alcohol are the most appropriate routes to retain the anatase phase up to 1100 °C.     

A simple solid–liquid grinding/templating route for the synthesis of magnetic iron/graphitic mesoporous carbon composites

Magnetic iron/graphitic mesoporous carbon composites were prepared by a simple one-step solid–liquid grinding/templating route. X-ray diffraction, nitrogen adsorption–desorption, transmission electron microscopy, vibrating-sample magnetometry and thermogravimetric analysis were used to characterize the samples. It was observed that a high content of magnetic iron nanoparticles could be embedded in the walls of graphitic mesoporous carbon matrix, and the resulting materials have a hierarchical mesostructure with a high specific surface area, large pore volume, and high saturation magnetization, giving the materials wide potential applications as catalyst supports and adsorbents.     

A survey of platinum hydrous oxide electrochemistry at elevated temperature: evidence for a new component in the β-deposit

Hydrous (or ) oxide. films produced on platinum in acid at room temperature usually give rise to two reduction peaks, in addition to the monolayer oxide response, on a negative sweep extending to about 0V. This has been attributed to the presence of two hydrous oxide components (designated as HO1 and HO2) in the film. It was confirmed in the present work that under more severe conditions, 3.0 m H₂SO₄ at 60°C, an additional, quite significant, peak — apparently due to the presence of a further hydrous oxide component (HO3) — may be observed. The approximate peak maximum potential values (RHE scale) were 0.4 V (HO1), 0.2 V (HO2) and 0.0 V (HO3); the E _p values are given here only as a guide: the processes involved occur under very irreversible conditions and the values vary with factors such as sweep rate, film thickness, temperature, etc. The differences in behaviour between these components is assumed to be due to factors such as the degree of aggregation and compactness in different regions of the gel-type, amorphous deposit. A brief account is given of the increasing evidence from different laboratories of unusual responses for platinum electrodes in aqueous acid solution.     

A new procedure for the synthesis of α-acyloxy aldehydes from ketones via α,β-epoxy sulfides

α-Acyloxy aldehydes, as O-protected α-hydroxy aldehydes, have been prepared in good yields via a new procedure involving the sequence: (1) reaction of ketones with sulfur ylides to give α,β-epoxy sulfides, (2) ring opening with carboxylic acids, (3) oxidation of the α-acyloxy-β-hydroxy sulfide products by mCPBA to the corresponding sulfones; and (4) acyl migration followed by elimination of the sulfinate group. The last transformation was readily accomplished, in very good yields, by equivalent amounts of triethylamine in dichloromethane at room temperature (method A); or by acyl migration assisted by complexation of the liberated alkanthiol accomplished in the α-acyloxy-β-hydroxy sulfides by Ag₂O in acetonitrile to give the same α-acyloxy aldehydes (method B). The presented procedure, compared to alternative procedures, has advantages including easily available key intermediates (α,β-epoxy sulfides), mildness of the reactions conditions, more general procedure (applied to different types of α-hydroxy aldehydes).