Effects of Cr content and morphology on the luminescence properties of the Cr-doped alpha-Al2O3 powders

Al₂O₃:Cr³⁺ samples were synthesized via hydrothermal and microwave solvothermal methods and thermal decomposition of Cr³⁺ doped precursors. The sample characterizations were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD results indicated that Cr³⁺ doped samples were pure α-Al₂O₃ phase after being calcined at 1573 K. SEM results showed that the length and diameter of these Cr³⁺ doped alumina microfibers by hydrothermal route were about 2–5 μm and 100–300 nm, respectively; the obtained α-Al₂O₃ based powders via the microwave solvothermal method were microspheres with an average diameter about 1–2 μm. PL spectra showed that the Al₂O₃:Cr³⁺ samples presented a broad R band at 696 nm. It is shown that the 0.3 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ is optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 24 Å. It is found that the curve followed the single-exponential decay. Furthermore, the luminescence properties of the samples are also dependent on the morphology.     

Energy transfer,tunable emission and optical thermometry in Tb3+/Eu3+ co-doped transparent NaCaPO4 glass ceramics

Tb³⁺/Eu³⁺ co-doped glass ceramics containing NaCaPO₄ nanocrystals were successfully synthesized via traditional melt-quenching route with further heat-treatment and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence spectroscopy. The energy transfer process of Tb³⁺→Eu³⁺ was confirmed by excitation and emission spectra and luminescence decay curves, and the energy transfer efficiency was also estimated. The results indicated that the efficient emission of Eu³⁺ was sensitized by Tb³⁺ under the excitation of 378 nm, realizing tunable emission in the transparent bulk glass ceramics containing NaCaPO₄ nanocrystals. Furthermore, optical thermometry was achieved by the fluorescence intensity ratio between Tb³⁺:⁵D₄→⁷F₅ (~542 nm) and Eu³⁺:⁵D₀→⁷F₂ (~612 nm). The maximum absolute sensitivity of 4.55% K⁻¹ at 293 K and the maximal relative sensitivity of 0.66% K⁻¹ at T=573 K for Tb³⁺/Eu³⁺ co-doped transparent NaCaPO₄ glass ceramic are obtained. It is expected that the investigated transparent NaCaPO₄ glass ceramics doped with Tb³⁺/Eu³⁺ have prospective applications in display technology and optical thermometry.     

Structure and optical properties of transparent Er3+-doped CaF2–silica glass ceramic prepared by controllable sol–gel method

Transparent Er³⁺-doped CaF₂–silica glass ceramics were prepared by the direct physical introduction of Er³⁺ doped CaF₂ nanocrystals into acid-catalyzed sol–gel silica glass. The physical methods of ball milling, ultrasonic baths, and stirring were investigated to disperse Er³⁺ doped CaF₂ nanocrystals in the silica sols. The CaF₂–silica sol mixture went through gelation and heat-treatment to form Er³⁺-doped CaF₂–silica glass ceramics. The morphology of Er³⁺ doped CaF₂ in silica glass did not change after heat-treatment at 600 °C for 10 h. The experimental results showed that Er³⁺ doped CaF₂ in the glass ceramic prepared with the assistance of ball milling possesses the best dispersity and homogeneity. The highest in-line transmittance of the glass ceramic reached up to 85% in visible region. Glass ceramic exhibits efficient up-conversion emissions corresponding to the Er³⁺:⁴F_9/2→⁴I_15/2 transition and long lifetime of ⁴F_9/2 level (1.73 ms) under 980 nm excitation.     

Luminescent properties of Li(Ga1-xCrx)5O8 (LGCO) phosphors

In this work, we investigate the synthesis of LiGa₅O₈ ceramic powders through a polyvinyl alcohol-based sol-gel technique and their optical properties when doped with high Cr³⁺ concentrations (5, 25 and 50 mol% with respect to the Ga³⁺ sites). The results indicate that the main crystalline phase, LiGa₅O₈, is obtained after calcining the samples at 1000 °C/2 h in a static air atmosphere. Via X-ray photoelectron spectroscopy, Cr is confirmed to exist in its trivalent oxidation state and the evaluation of the optical properties is performed via photoluminescence excited from visible to vacuum ultraviolet energy range and with X-ray excited optical luminescence, indicating the typical Cr³⁺ emission at the near-infrared energy range. The crystal field and Racah parameters are calculated and the influence of Cr³⁺ concentration in the host material indicates luminescence suppression/quenching and a redshift for a higher amount of these ions.     

Tuning the interfacial reaction between CaO–SrO–Al2O3–B2O3–SiO2 sealing glass–ceramics and Cr-containing interconnect: Crystalline structure vs. glass structure

In this paper, Al₂O₃ was added to CaO–SrO–B₂O₃–SiO₂ sealing system to tailor the structure of sealing glass–ceramics and glass–ceramics/metal interfacial reaction. The addition of alumina in glasses contributes to increasing fraction of bridging oxygen in silica tetrahedral as well as the change in boron environment from three-fold to four-fold (BO₄ → BO₃). The devitrification tendency of glasses also decreases with increasing Al₂O₃ content. The condensed glass structure and increasing residual glass content play opposite roles on the interfacial reaction, consequently resulting in a maximum fraction of Cr⁶⁺ in reaction couples between Cr₂O₃ and glass containing 6 mole% Al₂O₃ at 700 °C. In addition, the good bonding can be observed at the interface between Cr-containing interconnect (Crofer 22APU) and glass containing 4 mole% Al₂O₃, held at 700 °C for 100 h. The reported results support the suitability of the prepared glass–ceramics as sealing materials for SOFC applications.     

Structural-phase state and lasing of 5–15 at% Yb3+:Y3Al5O12 optical ceramics

Yttrium aluminum garnet (Yb³⁺:Y₃Al₅O₁₂) laser ceramics doped by 5, 10 and 15 at% of ytterbium ions were obtained by reactive sintering. Optimal sintering temperature range for the formation of highly-dense transparent Yb³⁺:Y₃Al₅O₁₂ ceramics under normal recrystallization conditions was found to be T = 1750–1800 °C. The influence of Yb³⁺ ions on structural-phase state, phase composition, microstructure, optical and luminescent properties of sintered samples was experimentally investigated. It was shown that lattice parameter a of Yb³⁺:Y₃Al₅O₁₂ ceramics decreases linearly with increasing of Yb³⁺ concentration in a good agreement with L. Vegard’s rule, that indicates to the formation of (Y_1−xYb_x)₃Al₅O₁₂ (х = 0.05–0.15) substitutional solid solutions. No concentration quenching of Yb³⁺ luminescence was observed in Yb³⁺:Y₃Al₅O₁₂ within the 5–15 at% doping range. Quasi-CW lasing of Yb³⁺:Y₃Al₅O₁₂ ceramics was studied under diode-pumping at 970 nm. A highest slope efficiency of about 50% was obtained for 15 at%-doped Yb³⁺:Y₃Al₅O₁₂ ceramics sintered at T = 1800 °C for 10 h.     

Exploring the luminescence of tin-incorporated nickel-sensitized lithium gallate short-wave-infrared phosphors

A series of Ni²⁺-sensitized LiGa₅O₈ nanocrystals doped with varying amounts of Sn⁴⁺ were synthesized via a high-temperature solid-state method. X-ray diffraction and scanning electron microscopy were employed to investigate the microstructure of the LiGa₅O₈ host, and optical spectroscopy was used to examine the effects of Sn⁴⁺ addition on the emission spectra and persistent luminescence (PersL) properties, which indicated a homogeneous distribution of the constituent elements in the phosphor. The Sn⁴⁺ incorporation led to an approximately sixfold enhancement of photoluminescence intensity at room temperature and decrease in the energy transition of Ni²⁺(³T₂(³F)→³A₂(³F)), which resulted in ∼65 nm bathochromic shift in the photoluminescence emission maxima. However, the addition of Sn⁴⁺ dopant led to a decrease in the quantum yield of luminescence compared to that of the original phosphor. Temperature-dependent thermoluminescence measurements revealed that doping of LiGa₅O₈ with Sn⁴⁺ interfered with the Ni²⁺ trap centers. Moreover, Ni²⁺-sensitized Sn⁴⁺-doped LiGa₅O₈ nanocrystals exhibited an afterglow effect that persisted for up to 300 s.     

Monodisperse nanospheres of yttrium oxysulfide: Synthesis,characterization, and luminescent properties

A red long-lasting phosphorescent material, monodisperse Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ nanospheres have been prepared successfully. Y(OH)(CO₃): Eu³⁺ nanospheres were firstly synthesized via an urea-based homogeneous precipitation technique to serve as the precursor. Nanospheres long-lasting phosphors Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ were obtained by calcinating the precursor in CS₂ atmosphere. XRD investigation shows a pure phase of Y₂O₂S, indicating no other impurity phase appeared. SEM observation reveals that the structures are nanosphere. The Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ nanospheres with particle size about 100–150 nm show uniform size and well-dispersed distribution. After irradiation by ultraviolet radiation with 325 nm for 5 min, the phosphor emitted red color long-lasting phosphorescence corresponding to typical emission of Eu³⁺ ion. The main emission peaks are ascribed to Eu³⁺ ions transition from ⁵D_J (J = 0, 1, 2) to ⁷F_J (J = 0, 1, 2, 3, 4). Both the PL spectra and luminance decay revealed that this phosphor had efficient luminescent and long-lasting properties. It was considered that the red-emitting long-lasting phosphorescence was due to the persistent energy transfer from the traps to the Ti⁴⁺ and Mg²⁺ ions.     

Spectrum regulation of YAG:Ce transparent ceramics with Pr,Cr doping for white light emitting diodes application

YAG:Ce transparent ceramics with high luminous efficiency and color render index were prepared via a solid state reaction-vacuum sintering method. Cr³⁺and Pr³⁺ were applied to expand the spectrum of YAG:Ce transparent ceramics. As prepared ceramics exhibit luminescence spectrum ranging from 500 nm to 750 nm, which almost covers full range of visible light. After the concentration optimization of Ce³⁺, Pr³⁺ and Cr³⁺, high quality white light was obtained by coupling the YAG:Ce,Pr,Cr ceramics with commercial blue LED chips. Color coordinates of the YAG:Ce,Pr,Cr ceramics under 450 nm LED excitation vary from cold white light to warm white light region. The highest luminous efficiency of WLEDs encapsulated by transparent YAG:Ce,Pr,Cr ceramic was 89.3 lm/W, while its color render index can reach nearly 80. Energy transfers between Ce³⁺ → Pr³⁺ and Ce³⁺ → Cr³⁺ were proved in co-doped ceramic system. Transparent luminescence ceramics accomplished in this work can be quite prospective for high power WLEDs application.     

Annealing induced discoloration of transparent YAG ceramics using divalent additives in solid-state reaction sintering

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y₃Al₅O₁₂ (YAG) ceramics. However, Si⁴⁺ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr³⁺ → Cr⁴⁺). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T = 84.6%, at 1064 nm) were obtained after vacuum sintering at 1840 °C for 8 h and without subsequent annealing in air. An absorption peak centered at ∼320 nm was observed before annealing, and it extended to ∼550 nm after annealing at 1450 °C for 10 h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca²⁺F⁺] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.     

Doping behaviors of NiO and Nb2O5 in BaTiO3 and dielectric properties of BaTiO3-based X7R ceramics

Doping behaviors of NiO and Nb₂O₅ in BaTiO₃ in two doping ways and dielectric properties of BaTiO₃-based X7R ceramics were investigated. When doped in composite form, the additions rendered higher solubility than that doped separately due to the identical valence between the complex (Ni_1/3²⁺Nb_2/3⁵⁺)⁴⁺ and Ti⁴⁺. NiO–Nb₂O₅ composite oxide was more effective in broadening dielectric constant peaks which was responsible for the temperature-stability of BaTiO₃ ceramics. A reduction in grain size was observed in the specimens with 0.5–0.8 mol% NiO–Nb₂O₅ composite oxide, whereas the abnormal growth of individual grains took place in the 1.0 mol% NiO–Nb₂O₅ composite oxide-doped specimen. When the specimen of BaTiO₃ doped with 0.8 mol% NiO–Nb₂O₅ composite oxide was sintered at 1300 °C for 1.5 h in air, good dielectric properties were obtained and the requirement of (EIA) X7R specification with a dielectric constant of 4706 and dielectric loss lower than 1.5% were satisfied.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Fabrication and properties of transparent Tb:YAG fluorescent ceramics with different doping concentrations

Terbium doped yttrium aluminum garnet (Tb:YAG) transparent ceramics with different doping concentrations were fabricated by the solid-state reaction method using commercial Y₂O₃, α-Al₂O₃ and Tb₄O₇ powders as raw materials. Samples sintered at 1750 °C for 20 h were utilized to observe the optical transmittance, microstructure and fluorescence characteristics. It is found that all the Tb: YAG ceramics with different doping concentrations exhibit homogeneous structures with grain size distributions around 22–29 µm. For the 5 at% Tb:YAG transparent ceramics, the grain boundaries are clean with no secondary phases. The photoluminescence spectra show that Tb:YAG ceramics emit predominantly at 544 nm originated from the energy levels transition of ⁵D₄→⁷F₅ of Tb³⁺ ions, and the intensity of the emission peak reaches a maximum value when the Tb³⁺ concentration is 5 at%. The in-line transmittance of the 5 at% Tb:YAG ceramics is 73.4% at the wavelength of 544 nm, which needs to be further enhanced by optimizing the fabrication process. We think that Tb:YAG transparent ceramics may have potential applications in the high-power white LEDs.     

Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Deep-red emission of Mn4+ and Cr3+ in (Li1−xAx)2MgTiO4 (A=Na and K) phosphor: Potential application as W-LED and compact spectrometer

Mn⁴⁺ doped and Mn⁴⁺/Cr³⁺ co-doped alkali metal titanate phosphors have been prepared by solid state reaction method. A part of Li⁺ ions in the Li₂MgTiO₄: Mn⁴⁺ are substituted with Na⁺ and K⁺ ions and consequently the intensity of Mn⁴⁺ emission at 678 nm is enhanced by 1.7 and 2.5 times, respectively. In the Mn⁴⁺/Cr³⁺ co-doped (Li_0.95K_0.05)₂MgTi_0.999O₄, both emission of Cr³⁺at 726 nm and emission of Mn⁴⁺ at 678 nm of Mn⁴⁺ are observed. It is interesting to find that the intensity ratio of 726–678 nm emissions in the Mn⁴⁺/Cr³⁺ phosphor continually increases with excitation wavelength increasing from 290 nm to 455 nm, which means that the intensity ratio in turn can be used to identify the excitation light wavelength. This refers a possible approach to design novel compact light-wavelength detector or spectrometer based on the phosphor. The mechanism of Na⁺ or K⁺ substitution induced luminescence enhancement in the Mn⁴⁺ phosphor and the competition between the Cr³⁺ and Mn⁴⁺ emissions in the Mn⁴⁺/Cr³⁺ co-doped has been discussed.     

Enhanced non-linear current-voltage response of Te-doped calcium copper titanate ceramics

The influence of partial replacement of Ti⁴⁺ ions by Te⁴⁺ in calcium copper titanate lattice on dielectric and non-linear current- voltage (I–V) characteristics was systematically studied. There was a remarkable increase in the values of the nonlinear coefficient (α) with Te⁴⁺ doping concentration in CaCu₃Ti_4-xTe_xO₁₂ (where, x=0, 0.1, 0.2).For instance, the α values increase from 2.9 (x=0) to 22.7 (x=0.2) for ceramics sintered at 1323 K/8 h. The room temperature value of current density (J) at the electrical field of 250 V/cm for CaCu₃Ti_3.8Te_0.2O₁₂ ceramics is almost 400 times higher than that of the pure CaCu₃Ti₄O₁₂ ceramics sintered at 1323 K. A systematic investigation into I–V behaviour as a function of temperature gave an insight into the conduction mechanisms of undoped and doped ceramics of calcium copper titanate (CCTO). The calculated potential barrier value for doped ceramics (~ 0.21 eV) dropped down to almost one third that of the undoped ceramics (~ 0.63 eV).     

Structure and luminescence properties of Eu3+ doped TiO2 nanocrystals and prolate nanospheroids synthesized by the hydrothermal processing

We report on a new approach to the synthesis of Eu³⁺ doped TiO₂ nanocrystals and prolate nanospheroids. They were synthesized by shape transformation of hydrothermally treated titania nanotubes at different pH and in the presence of Eu³⁺ ions. The use of nanotubes as a precursor to the synthesis of Eu³⁺ doped TiO₂ nanocrystals and prolate nanospheroids opens the possibility of overcoming the problems related to molecular precursors. The shapes and sizes of the nanotubes, Eu³⁺ doped TiO₂ nanocrystals and prolate nanospheroids were characterized by transmission electron microscopy (TEM) technique. Crystal structures of the resultant powders were investigated by X-ray diffraction (XRD) analysis. The percentage ratio of Eu³⁺ to Ti⁴⁺ ions in doped nanocrystals was determined using inductively coupled plasma atomic emission spectroscopy. The optical characterization was done by using fluorescence and ultraviolet-visible reflection spectroscopies. An average size of faceted Eu³⁺ doped TiO₂ nanocrystals was 13 nm. The lateral dimensions of Eu³⁺ doped TiO₂ prolate nanospheroids varied from 14 to 20 nm, while the length varied from 40 to 80 nm, depending on precursor concentrations. The XRD patterns revealed the homogeneous anatase crystal phase of Eu³⁺ doped TiO₂ nanocrystals and prolate nanospheroids independently of the amount of dopant. A postsynthetic treatment (filtration or dialysis) was applied on the dispersions of the doped nanoparticles in order to study the influence of the dopant position on photoluminescence (PL) spectra. In the red spectral region, room temperature PL signals associated with ⁵D₀ → ⁷F_J (J = 1–4) transitions of Eu³⁺ were observed in all samples. The increased contribution of dopants from the interior region of dialyzed nanocrystals to photoluminescence was confirmed by the increase of R value.     

Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics

New high temperature negative temperature coefficient (NTC) thermistor ceramics based on a xMgAl₂O₄–(1 − x)YCr_0.5Mn_0.5O₃ (x = 0.1, 0.4, 0.6) composite system have been successfully fabricated through spark plasma sintering (SPS) with a low sintering temperature and a short sintering period. The X-ray diffraction analysis indicates that the SPS-sintered composite ceramics consist of a cubic spinel MgAl₂O₄ phase and an orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase isomorphic to YCrO₃. The SPS-sintered composite ceramics have high relative density ranging from 94.1 to 97.4% of the theoretical density. X-ray photoelectron spectroscopy analysis corroborates the presence of Cr³⁺, Cr⁴⁺, Mn³⁺, and Mn⁴⁺ ions on lattice sites, which may result in the hopping conduction. The obtained ρ₂₅, B_25–150, and B_700–1000 of the SPS-sintered composite NTC thermistors are in the range of 1.53 × 10⁶–9.92 × 10⁹ Ω cm, 3380–5172 K, and 7239–9543 K, respectively. These values can be tuned by adjusting the MgAl₂O₄ concentration.     

“Dark hole” cure in Ba4.2Nd9.2Ti18O54 microwave dielectric ceramics

Large size Ba_4.2Nd_9.2Ti₁₈O₅₄ (BNT) ceramics doped with MnCO₃, CuO and CoO were prepared by the conventional solid-state method. Only a single BaNd₂Ti₄O₁₂ phase was formed in all samples. No second phase was found in the XRD patterns. The bulk density increases slightly because of the dopants. The SEM results showed that the grain size of Mn²⁺and Cu²⁺-doped BNT ceramics became larger with the increasing amount of dopants. The permittivity of all samples stays the same. However, the Q×f value of BNT ceramics increases by doping, especially with Mn²⁺ ions. The conductivity of BNT ceramic doped with Mn²⁺(0.5 mol‰) under high temperature is lower than that without doping. There are fewer defects in Mn²⁺-doped BNT ceramics. The XPS results indicated that Ti reduction was suppressed in BNT ceramics doped with 0.5 mol‰ Mn²⁺. BNT ceramics doped with 0.5 mol‰ Mn²⁺ ions sintered at 1320 °C for 2 h exhibited good microwave dielectric properties, with ε_r=88.67, Q×f=7408 GHz and τf = 82.98 ppm/°C.     

Infrared luminescence in Er3+:Yb3Al5O12 bulk ceramics prepared by sol–gel method

Erbium doped and pure ytterbium aluminium garnet (YbAG, Yb₃Al₅O₁₂) bulk ceramics were successfully prepared by a chelating sol–gel route based on the polyesterification of ethylenediaminetetraacetic acid (EDTA) with triethanolamine (TEA). The gel decomposition and phase formation in precursor powders were studied using XRD and TG/DTA. Amorphous precursor was directly converted to YbAG phase after calcinations at 800 °C. The influence of intermediate grinding on microstructure and luminescent properties was investigated. The discrete luminescence bands of the ⁴I_13/2 → ⁴I_15/2 transition were observed in the infrared emission spectra of erbium doped samples. The lifetime of luminescence at 1530 nm was 2.82 ms and 1.82 ms for the doped samples. This may be attributed to the different efficiency of surface recombination channel, caused by different grain size distribution. Prepared samples are suitable as a standard for photoluminescence measuring of Er-doped YbAG thin films.