Surfactant-induced modification in photoluminescence properties of Ce0.9-xY0.1DyxO2-δ phosphors for single host white light emitting diodes

Dy³⁺/Y³⁺ codoped spherical ceria particles were synthesized via a sol-gel method using P123 as a surfactant. P123 amount was varied to control particle size around 70–200 nm. Photoluminescence excitation spectra of as-obtained ceria nano phosphors exhibited broadband around 280–305 nm due to O²⁻ to Ce⁴⁺ and Dy³⁺ charge transfer along with several sharp peaks around 325–365 nm due to f-f transitions. Strong UV excitation showed that the current phosphors were apposite for NUV-LEDs. The emission spectra of Ce_0.895Y_0.1 Dy_0.005O_2-δ phosphors comprised 489 (blue) and 579 nm (yellow) emissions corresponding to Dy³⁺ f-f transitions. The increase in emission intensities with P123 concentration was observed due to enhancement in crystallinity and particle size. Low Ω₂ (Judd Ofelt parameter) of 1.1 × 10⁻²⁰ cm² for Ce_0.895Y_0.1Dy_0.005O_2-δ phosphors synthesized without P123 indicated high symmetry neighboring Dy³⁺ ions. The increase in P123 concentration resulted in the elevation of Ω₂ value, leading to asymmetry surrounding Dy³⁺ ions. The local environment of Dy³⁺ was asymmetric, which caused the electric-dipole transition to upend the magnetic-dipole transition. Hence, the blue-to-yellow emission ratio (B/Y) was varied with P123, facilitating white light emission with color-coordinate (0.38,0.34). Photoluminescenc intensity of synthesized phosphors retained 86% of initial intensity at 500K (activation energy of 0.26 eV), indicating high stability at elevated temperature. The LEDs fabricated with present phosphors exhibited a color-rendering index and correlated color temperatures around 76.3 and 6065 K, respectively. The suitable Judd-Ofelt parameters and moderate activation energy suggested the feasibility of Ce_0.885Y_0.1Dy_0.015O_2-δ phosphors for white-light-emitting diodes.     

Structural characterization,energy transfer,and Judd–Ofelt analysis of pure and the Eu3+-doped Ca2LiMg2V3O12 phosphors

A novel vanadate host Ca₂LiMg₂V₃O₁₂ (CLMV) and the Eu³⁺-doped samples were synthesized via a solid-state reaction method. The phase formation and the morphological analysis were studied in detail. The Rietveld refinement result shows that the host belongs to cubic space group Ia-3d (230) with lattice parameter, a = 12.3948 Å, V = 1904.23 Å³, and Z = 8. The diffuse reflectance spectroscopy measurement estimated the bandgap of the host and the CLMV:0.05Eu³⁺ phosphors. The host exhibits a broad absorption band (peak at 345 nm) ranging from 240 to 380 nm, which is attributed to the charge transfer in the O²⁻–V⁵⁺ complex. Under near UV excitation (λ_exc = 345 nm), the host gives a broad emission band covering the visible region from 400 to 730 nm and the emission is in the bluish–green region of the CIE diagram. When the host is doped with the Eu³⁺ ions and excited at 345 nm, the emission spectrum depicts the superimposition of the characteristic emission bands (red emission) of the Eu³⁺ ions corresponding to the f–f transitions over the broad emission band of the host. The calculated color coordinates (9600 to 2280 K) demonstrated the color tuning ability of the phosphor as the dopant concentration is increased in the host. This is because the VO₄³⁻ group plays the sensitiser role and partially transfers energy with the Eu³⁺ ions. When the same set of phosphors were excited at the dominant characteristic excitation band (λ_exc = 394 nm) of the Eu³⁺, the characteristic emission bands of the Eu³⁺ in the orange–red region were observed. As the electric dipole transition of the Eu³⁺ was found to be dominant, the prepared phosphors possessed high color purity (CP). The energy transfer mechanism and the lifetime values were also presented. The temperature-dependent PL studies showed good thermal stability of the optimum sample. Various radiative transition properties were analyzed by the Judd–Ofelt theory. The photometric results reveal the color tuning ability and CP of the CLMV:xEu³⁺ phosphors.     

Tuning energy storage and light transmittance properties in ZrO2-modified Na2O–K2O–Nb2O5–SiO2 glass-ceramics

Crystallization kinetics in ZrO₂-modified glass-ceramics, (35-x)Na₂O–5K₂O–40Nb₂O₅–20SiO₂-xZrO₂ (x = 0, 1, 3, 5, 10 mol%), is analyzed by crystallization activation energy E_c and kinetic parameter k. A large kinetic parameter k of 1.657 × 10²⁵ is found for the sample with x = 1, which indicates a high tendency toward crystallization. NaNbO₃ is formed as the major phase, and its crystallinity changes significantly by the addition of ZrO₂. It is found that the dielectric permittivity and maximum polarization are affected by crystallinity. Large domains are formed, and a relaxation process appears at high frequencies. In addition, the breakdown strength (BDS) is affected by the strong electric field distortion rather than the resistance in this system. Complex impedance analysis shows that resistance decreases with the addition of ZrO₂. The electric field distortion of the glass-ceramics is simulated by COMSOL. A low interfacial polarization activation energy E_a of 0.821 eV is achieved by the addition of ZrO₂. The maximum pulse power density reaches 90 MW/cc at 350 kV/cm for the sample with x = 10, and the pulse energy density attains 0.91 J/cm³ at 300 kV/cm for the sample with x = 1. Light transmittance is achieved because of the low crystallinity and the disappearance of cristobalite SiO₂ in the Na₂O–K₂O–Nb₂O₅–SiO₂ glass-ceramics.     

Sintering behavior,phase evolution and microwave dielectric properties of thermally stable Li2O-3MgO-mTiO2 ceramics (1≤m≤6)

Li₂O-3MgO-mTiO₂ (1≤m≤6) ceramics were prepared by the solid state reaction method. X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy techniques were used to investigate the phase composition and crystal structure. With increasing m values, the phase structures of ceramics changed as: (Li₂Mg₃TiO₆, m=1)→(Li₂Mg₃Ti₄O₁₂ and Mg₂TiO₄, m=2,3)→(Li₂Mg₃Ti₄O₁₂, m=4)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃ and Li₂MgTi₃O₈, m=5)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃, Li₂MgTi₃O₈ and MgTi₂O₅, m=6). The optimized sintering temperature was lowered from 1275 °C to 1050 °C. When m=5, Li₂O-3MgO-5TiO₂ ceramics showed good microwave dielectric properties at a wide sintering temperature range of 1000–1200 °C, and the best microwave dielectric properties of Q×f=71,726 GHz, ε_r=21.9 and τ_f=−20.9 ppm/°C were obtained at a relatively low sintering temperature of 1050 °C.     

Color tuning in (Ca1−xSrx)8MgLu(PO4)7:Eu2+,Mn2+ phosphor via host composition design and energy transfer

In this work, a series of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ and Eu²⁺/Mn²⁺-codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors were prepared via the combustion-assisted solid-state reaction process. XRD patterns and Rietveld refinements were used to verify the incorporations of Sr into Ca₈MgLu(PO₄)₇:Eu²⁺. Upon the same excitation wavelength of 380 nm, the emission peaks of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ (0≤x≤1) phosphors red-shifted from 453 to 519 nm with increasing Sr/Ca ratio. The red-shift of the Eu²⁺ emission with increasing Sr/Ca ratio was ascribed to the change of Eu²⁺ emission at different lattice sites. With variation of the Mn²⁺ content, the emission color of Eu²⁺/Mn²⁺ codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors exhibited the luminescence tunable from greenish blue to white and eventually to red. The energy transfer from Eu²⁺ to Mn²⁺ in Ca_6.5Sr_1.5MgLu(PO₄)₇ host matrix was demonstrated to be of a resonant type via a dipole- dipole mechanism with the critical distance of ∼16.7 Å. By the Sr substitution for Ca and properly tuning by the relative composition change of Eu²⁺/Mn²⁺, chromaticity coordinates of (0.329, 0.326) can be reached at near UV light excitation. The combination of host composition design and energy transfer may provide a novel strategy to obtain white light and tunable luminescence.     

Effect of M3+ (M = Bi,Al) co-doping on the luminescence enhancement of Ca2ZnSi2O7:Sm3+ orange-red−emitting phosphors

In this paper, the luminescence properties of Ca₂ZnSi₂O₇:Sm³⁺ phosphors were improved by co-doping with M³⁺ (M = Bi, Al) via the sol-combustion method. The structure and luminescence properties of the Ca₂ZnSi₂O₇:Sm³⁺, M³⁺ samples were investigated in detail, especially the luminescence enhancement effect of Bi³⁺/Al³⁺ co-doping. XRD results indicated that moderate Bi³⁺/Al³⁺ co-doping in the host structure did not change the tetragonal structure of Ca₂ZnSi₂O₇. The series of Ca₂ZnSi₂O₇:Sm³⁺, M³⁺ phosphors could be excited by 402 nm near-ultraviolet light and several significant emission peaks were obtained at 567, 604 and 650 nm, which originated from the electron transitions of Sm³⁺ from ⁴G_5/2 to ⁶H_5/2, ⁶H_7/2 and ⁶H_9/2 levels, respectively. The luminescence intensity of Ca₂ZnSi₂O₇:Sm³⁺ was markedly enhanced through Bi³⁺/Al³⁺ co-doping, which could be explained by Al³⁺ decreasing the crystal field symmetry and greatly increasing the red luminescence intensity, and Bi³⁺ functioning as a sensitizer to increasing the luminescence intensity through energy transfer from Bi³⁺ to Sm³⁺ ions. In conclusion, the excellent Ca₂ZnSi₂O₇:Sm³⁺, M³⁺ phosphors have potential application as red phosphors in white light emitting diodes.     

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.     

Structure,Judd-Ofelt analysis and spectra studies of Sm3+-doped MO-ZnO-B2O3–P2O5 (M = Mg,Ca, Sr,Ba) glasses for reddish-orange light application

In the present work, a series of Sm³⁺-doped MO-ZnO-B₂O₃–P₂O₅ (M = Mg, Ca, Sr, Ba) glasses were prepared. The glass structure and luminescence properties were investigated by XRD, DSC, IR, absorption spectroscopy, Judd-Ofelt theory and photoluminescence spectra. The J-O parameters of Sm³⁺-doped glasses follow the trend of Ω₄＞Ω₆＞Ω₂. Under the excitation of 401 nm Xenon lamp, Sm³⁺-doped glasses exhibited four emissions from the transitions of ⁴G_5/2→⁶H_J/2 (J = 5, 7, 9, 11) in the visible spectra. The luminous intensity of Sm³⁺ increases with the asymmetry in local environments and decreases with the increasing radius of the alkaline-earth cation. Among the as-prepared glass, the Sm³⁺-doped glass containing magnesium oxide exhibits higher values of stimulated emission cross-section (2.18 × 10⁻²¹ cm²), gain bandwidth (1.40 × 10⁻²⁷ cm³), and optical gain (3.83 × 10⁻²⁴ cm²). All the Sm³⁺-doped glasses show intense orange light in the CIE 1931 chromaticity diagram with a high color purity exceeding 99%. In addition, the time-resolved emission spectra reveal the decay process of the Sm³⁺ ions for the transitions ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 in the glass containing magnesium oxide. It suggests that Sm³⁺-doped alkaline-earth zinc borophosphate glasses could be a potential candidate for reddish-orange light-conversion fluorescent materials based on the ultraviolet light-emitting diode.     

Fabrication,tunable fluorescence emission and energy transfer of Tm3+-Dy3+ co-activated P2O5–B2O3–SrO–K2O glasses

A growing demand for white light-emitting diodes (W-LEDs) gives rise to continuous exploration of functional fluorescence glasses. In this paper, Tm³⁺/Dy³⁺ single- and co-doped glasses with composition (in mol%) of 30P₂O₅–10B₂O₃–23SrO–37K₂O were synthesized using the melt-quenching method in air. The physical properties, glass structure, luminescence characteristics and energy transfer mechanism of the glasses were systematically studied. As glass network modifiers, Tm³⁺ and Dy³⁺ ions can densify the glass structure. Excitation wavelength and doping concentration of Tm³⁺/Dy³⁺ ions have a direct impact on the emission intensities of blue and orange light as well as the color coordinate of the as-prepared glasses. A white light very close to standard white light can be obtained under 354 nm excitation when the content of Tm³⁺ and Dy³⁺ is 0.2 mol% and 1.0 mol%, respectively. The results of the emission spectra and decay curves reveal the existence of energy transfer from Tm³⁺ to Dy³⁺. The analytic results based on the Inokuti-Hirayama model indicate that the electrical dipole-dipole interaction may be the main mechanism of energy transfer. Moreover, Tm³⁺/Dy³⁺ co-activated glass phosphor has good thermal stability and chrominance stability and it is a promising candidate for white LEDs and display device.     

Thermal behavior,structure, crystallization and solubility of the melt-derived SiO2–P2O5–Na2O–F-MO (M = Ca,Sr, Zn) glasses

In this work, phospho-silicate glasses with SiO₂–P₂O₅–Na₂O–F-MO (M = Ca, Sr, Zn) composition were prepared by using the conventional melt quenching technology. Structural, physical, and chemical property tests were used to analyze the effects of different SrO and ZnO content on the structure and properties of the glasses. The results showed that the glass stability varied nonlinearly as CaO was replaced by SrO, which was mainly related to the different positions of Sr²⁺ and Ca²⁺ ions breaking the network connection in the network structure, and the substitution of ZnO for CaO led to a continuous decrease in the stability of the glasses. The immersion experiment showed that SrO doping was more feasible than ZnO doping to improve the biological activity of the glasses, and the doping of ZnO promoted the dissolution of ions in the glasses. The obtained results indicated that the glass samples prepared in this paper have potential biological activity, which has potential applications in dental treatment.     

Charge states of atoms in ceramic superconductors HgBa2Ca n − 1Cu n O2n + 2, Tl2Ba2Ca n − 1Cu n O2n + 4, and Bi2Sr2Ca n − 1Cu n O2n + 4 (n = 1–3)

The parameters of the electric-field-gradient tensor for copper sites in the HgBa₂Ca _{n ? 1}Cu _n O_{2n + 2}, Tl₂Ba₂Ca _{n ? 1}Cu _n O_{2n + 4}, and Bi₂Sr₂Ca _{n ? 1}Cu _n O_{2n + 4} (n = 1–3) lattices have been determined using ⁶⁷Cu(⁶⁷Zn) Mössbauer emission spectroscopy and calculated in the framework of the point-charge approximation. The agreement between the experimental and calculated parameters has been achieved under the assumption that the holes formed as a result of the decrease in the oxidation state of a part of the mercury, thallium, or bismuth atoms are distributed over the oxygen sites in the Cu-O or adjacent planes. It has been demonstrated that the oxidation state of cations can be controlled in high-temperature superconducting ceramic materials.     

The Impact of Cr2O3 on the Mechanical,Physical, and Radiation Shielding Characteristics of Na2B4O7–CaO–SiO2 Glasses

Silicon - The rapid-quenching route was used to prepare Na2B4O7–CaO–SiO2 glass formulations with various Cr2O3 concentrations. XRD analyses were used to determine the amorphous states...     

Synthesis and thermal stability of Re2Zr2O7, (Re = La,Gd) and La2(Zr1−xCex)2O7−δ compounds under reducing and oxidant atmospheres for thermal barrier coatings

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La₂Zr₂O₇, Gd₂Zr₂O₇, La₂Ce₂O_7?γ and La₂(Zr_0.7Ce_0.3)O_7?γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400 °C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar_(g)/3%H_2(g) atmosphere.     

Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part II, β-Yb2Si2O7 and β-Sc2Si2O7

The high-temperature (1500?°C) interactions of two promising dense, polycrystalline EBC ceramics, β-Yb₂Si₂O₇ and β-Sc₂Si₂O₇, with a calcia-magnesia-aluminosilicate (CMAS) glass have been explored as part of a model study. Unlike YAlO₃ and γ-Y₂Si₂O₇ in the accompanying Part I paper, little or no reaction is found between the Y-free EBC ceramics and the CMAS. In the case of β-Yb₂Si₂O₇, a small amount of reaction-crystallization product Yb-Ca-Si apatite solid solution (ss) forms, whereas none is detected in the case of β-Sc₂Si₂O₇. The CMAS glass penetrates into the grain boundaries of both EBC ceramics, and they both suffer from a new type of ‘blister’ cracking damage. This is attributed to the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the CMAS glass. The success of a ‘blister’-damage-mitigation approach is demonstrated, where 1?vol% CMAS glass is mixed into the β-Yb₂Si₂O₇ powder prior to sintering. The CMAS-glass phase at the grain boundaries promotes rapid CMAS-glass penetration, thereby avoiding the dilatation-gradient.     

Microwave dielectric properties of novel temperature stable high Q Li2Mg1−xZnxTi3O8 and Li2A1−xCaxTi3O8 (A = Mg,Zn) ceramics

The Li₂Mg_1?xZn_xTi₃O₈ (x = 0–1) and Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li₂MgTi₃O₈ ceramic shows ?_r = 27.2, Q_u × f = 42,000 GHz, and τ_f = (+)3.2 ppm/°C and Li₂ZnTi₃O₈ has ?_r = 25.6, Q_u × f = 72,000 GHz, and τ_f = (?)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li₂Mg_0.9Zn_0.1Ti₃O₈ dielectric ceramic composition shows the best dielectric properties with ?_r = 27, Q_u × f = 62,000 GHz, and τ_f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.     

Phase transition and energy storage properties of Bi0.5Na0.5TiO3–Bi(Mg2/3Nb1/3)O3 lead-free ceramics

Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics have been extensively studied due to their excellent dielectric, piezoelectric and ferroelectric properties. The phase structure and functionalities of BNT can be feasibly adjusted by doping/forming solid solutions with other elements/components. In this work, Bi(Mg_2/3Nb_1/3)O₃ (BMN) was introduced into BNT by a conventional solid-state reaction to form a homogeneous solid solution of (1-x)(Bi_0.5Na_0.5)TiO₃-xBi(Mg_2/3Nb_1/3)O₃ (BNT-xBMN) with a perovskite structure. With the increase of BMN content, a phase transition from rhombohedral R3c to tetragonal P4bm has been confirmed by XRD, along with shifting the ferroelectric-paraelectric phase transition temperature to lower temperatures with broadening dielectric peaks. Furthermore, an optimized recoverable energy density of 1.405 J/cm³ was achieved for BNT-0.10BMN ceramics under a low applied electric field of 140 kV/cm, which is mainly attributed to the transformation from ferroelectric to ergodic relaxor phase.     

Dielectric and piezoelectric properties of (1−x)(Bi,Na)TiO3–x(Bi,K)TiO3 lead-free ceramics for piezoelectric energy harvesters

The piezoelectric and dielectric properties of the (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ (x=0.12, 0.14, 0.18, 0.20 and 0.30) lead-free ceramics were investigated. Specimens were prepared by the conventional mixed oxide method and sintered at 1170 °C in air. Scanning electron microscopy indicated that increasing x from 0.12 to 0.30 causes a decrease in the grain size. The (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ ceramics shows a homogeneous microstructure and excellent dielectric and piezoelectric properties. Specimens with optimum composition showed a piezoelectric charge constant d₃₃ of 166 pC/N, an electromechanical coupling factor k_p of 0.5, a dielectric constant ε_r of 1591.32 at 1 kHz and generated power output of 37.49 nW/cm².     

A novel low-fired,temperature-stable,and low-cost (1−x)BaCu(B2O5)−xTiO2 microwave dielectric ceramic

BaCu(B₂O₅) is a typical microwave dielectric ceramic (MDC) with a low sintering temperature, but it exhibits a large negative temperature coefficient of resonant-frequency (τ_f) which makes it difficult to use in wireless communications. We employ TiO₂ to improve its temperature-stability of resonant-frequency, and reveal the effects of TiO₂ on the densification and the microwave dielectric properties of BaCu(B₂O₅). Here we show that BaCu(B₂O₅) can be well-sintered at 825 °C with proper TiO₂ additions; we find that the TiO₂ grains homogeneously distribute in the boundaries of BaCu(B₂O₅) grains, resulting in the τ_f compensation of BaCu(B₂O₅). Enhanced temperature-stability of resonant-frequency can be achieved by increasing the content of TiO₂ properly. A novel temperature-stable (1-x)BaCu(B₂O₅)–xTiO₂ (x = 0.20) MDC (τ_f =?0.8 ± 3.0 ppm/°C, ε_r = 8.8 ± 0.36, Q×f = 28,612 ± 1170 GHz) is obtained using some low-cost raw materials. Our results provide the underlying insights needed to guide the design of temperature-stable MDCs for wireless communication applications.