Green and red upconversion luminescence of Er-doped K0.5Na0.5NbO3 ceramics

Er³⁺ doped K_0.5Na_0.5NbO₃ (KNN) lead-free piezoelectric ceramics were synthesized by the solid-state reaction method. The upconversion emission properties of Er³⁺ doped KNN ceramics were investigated as a function of Er³⁺ concentration and incident pumping power intensity. Bright green (~555 nm) and red (670 nm) upconversion emission bands were obtained under 980 nm excitation at room temperature, which are attributed to (²H_11/2, ⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively. The upconversion emission intensity can be adjusted by changing Er³⁺ concentration, and the mechanism of upconversion processes involve not only a two-photon absorption but also a three-photon absorption. In addition to the admirable intrinsic piezoelectric properties of KNN, this kind of material may have potential application as a multifunctional device by integrating its upconversion and piezoelectric property.     

Microstructure, electrical properties, and aging behavior of ZnO-Pr6O11-CoO-Cr2O3-Y2O3-Er2O3 varistor ceramics

The microstructure, electrical properties, and aging behavior of the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃-Er₂O₃ varistor ceramics were investigated for different contents of Er₂O₃. The microstructure consisted of ZnO grain and an intergranular layer (Pr, Y, and Er-rich phases) as a secondary phase. The increase of Er₂O₃ content decreased the average grain size and increased the sintered density. As the Er₂O₃ content increased, the breakdown field increased from 4206 V/cm to 5857 V/cm and the nonlinear coefficient increased from 32.6 to 48.6. The varistor ceramics added with 1.0 mol% Er₂O₃ exhibited excellent stability by exhibiting −0.2% in the variation rate of the breakdown field and −2.7% in the variation rate of the nonlinear coefficient for aging stress of 0.95 E_1 mA/150 °C/24 h.     

Carbonate-precipitation synthesis of Yb:Y2O3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb³⁺:Y₂O₃) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb³⁺:Y₂O₃ nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ion. Homogeneous Yb³⁺:Y₂O₃ nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.     

An approach to Y2O3:Eu optical nanostructured ceramics

Y₂O₃:Eu³⁺ (1 at.%) translucent nanostructured ceramics with total forward transmission achieving ∼70% of the theoretical limit has been obtained by the transformation-assisted consolidation of custom-made cubic Y₂O₃:Eu³⁺ nanopowders under high pressure (HP). Sintering under the pressure of 7.7 GPa and temperatures in the 100-500 °C range leads to the partial cubic-to-monoclinic phase transition that results in two-phase Y₂O₃:Eu³⁺ nanoceramics. The average grain size of ceramics d ≤ 50 nm for both Y₂O₃:Eu³⁺ polymorph is comparable with crystallite size of initial nanopowders (d ∼ 40 nm), indicating that the grain growth factor is near unity. The phase compositions, morphology, densities, preliminary optical and luminescent properties of synthesized nanostructured ceramics have been studied.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Synthesis and electrical properties of Ce0.8Sm0.2O1.9 ceramics for IT-SOFC electrolytes by urea-combustion technique

Fine Ce_0.8Sm_0.2O_1.9 (SDC) powders with a fluorite cubic phase were prepared using a urea-combustion technique. The sinterability and microstructural evolution of the resulting ceramics were investigated. The results indicate that the ceramics sintered above 1350 °C display relative densities of about 98.5% along with significant grain growth. With respect to their electrical conduction properties, the specimens sintered above 1350 °C exhibit an excellent total ionic conductivity of 0.082 S cm⁻¹ at 800 °C in air. However, when measured in an N₂ + 33.3%H₂ atmosphere, a pronounced Warburg feature appears in the impedance plots of the SDC ceramics, together with a significant increase of the total conductivity at measuring temperatures above 550 °C, due to the introduction of a mixed Ce⁴⁺/Ce³⁺ valence state and the generation of the electronic conduction in the reducing atmosphere.     

Electrical properties of Li1.3Ge1.4Ti0.3Al0.3(PO4)3 superionic ceramics

Preparation and electrical characterization of NASICON-type compound, Li_1.3Ge_1.4Ti_0.3Al_0.3(PO₄)₃, are described. The solid solution is obtained with Ge⁴⁺ → Ti⁴⁺ and Ge⁴⁺ → Al³⁺ substitutions in LiGe₂(PO₄)₃. The powder has been fabricated by a solid state reaction and the structural characteristics of it have been studied by X-ray. Ceramic samples have been sintered by varying the sintering duration from 1 to 3 h. Samples were studied by complex impedance spectroscopy in the frequency range 1 MHz-1.2 GHz and temperature range 300-600 K. Two regions of relaxation dispersion were found. The dispersions were related to the fast Li⁺ ion transport in the grains and grain boundaries. Variation of the sintering duration has no considerable effect on electrical properties of the ceramics.     

Comparison of dispersants performance on the suspension Lu2O3:Eu stability and high-density compacts on their basis

The conditions for obtaining a stable Lu₂O₃:Eu³⁺ suspension of spherical particles with a diameter of 100 nm using three dispersants possessing an electrosteric stabilizing effect (Dolapix CE 64, Darvan 821 A, Darvan C-N) have been studied. It has been shown that in colloidal processing of ceramics the packing density and microstructure of green bodies can be controlled by regulating the interactions between ceramic particles in the suspension. The influence of the molecular weight and concentration of the dispersant on the stability of Lu₂O₃:Eu³⁺ suspensions containing 5-10 vol.% of the solid loading has been considered. It has been determined that use of Dolapix CE 64 with a concentration of 1 mass.% in the alkaline pH range allows to obtain suspensions with high stability and low viscosity (∼1.7 сP). Such suspensions were used to produce compacts with a maximum relative density of ∼52% and uniform density distribution by the pressure slip casting method. The obtained compacts were densified into translucent Lu₂O₃:Eu³⁺ ceramics by the vacuum sintering method.     

Optical properties of Er,Yb co-doped YAG transparent ceramics

The transparent polycrystalline erbium and ytterbium co-doped yttrium aluminum garnet (Er,Yb:YAG) ceramics with various Yb contents from 5% to 25% were prepared by the solid-state reaction and the vacuum-sintering technique. The in-line transmittances of the mirror-polished ceramics exceed 80% from the visible band to the infrared band. The samples are very compact with few pores. The average grain size of the Er,Yb:YAG ceramic is about 15 μm. The upconversion luminescence spectra, infrared luminescence spectra and luminescence decay curves of the ceramics were observed and discussed. For 1%Er doped YAG ceramic, the best ion ratio of Yb³⁺ and Er³⁺ is around 15:1.     

Synthesis and emission analysis of RE (Eu or Dy):Li2TiO3 ceramics

The development and photoluminescence analysis of Eu³⁺or Dy³⁺ ions in the matrix of lithium titanate (Li₂TiO₃) ceramics by using a solid state reaction method are reported. Emission spectra of Eu³⁺:Li₂TiO₃ ceramics have shown strong red emission at 611 nm (⁵D₀ → ⁷F₂) with λ_exci = 392 nm (⁷F₀ → ⁵L₆) and from the Dy³⁺:Li₂TiO₃, a blue emission at 493 nm (⁴F_9/2 → ⁶H_15/2) and also an yellow emission at 582 nm (⁴F_9/2 → ⁶H_13/2) have been observed with λ_exci = 366 nm (⁶H_15/2 → ⁶P_5/2). Both the rare-earth ions containing ceramics have displayed their brighter emission performance from their measured spectral results. In addition, X-ray diffraction (XRD), Fourier transform infra red (FTIR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) have been used to characterize the structural properties of (Eu³⁺ or Dy³⁺):Li₂TiO₃ ceramics.     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

Preparation and mid-infrared 2.7 µm luminescence property of high content Er3+-doped (Y0.9La0.1)2O3 transparent ceramics pumped at 980 nm

High content Er³⁺ doped (Y_0.9La_0.1)₂O₃ transparent ceramics have been prepared by conventional ceramic process. Absorption spectra, mid-infrared, up-conversion and near-infrared emission spectra of Er³⁺ pumped at 980 nm have been investigated. The mechanisms of energy transfer processes have been discussed. Large values of Judd–Ofelt parameter Ω₂ (5.73 × 10^–20 cm²) and spectral quality factor X (3.71) have been obtained. The greatly enhanced green up-conversion emission in the high Er³⁺ doped sample is considered important for the applications in up-converters. The much enhanced mid-infrared 2.7 µm and up-conversion emissions, as well as the depressed near-infrared 1.5 µm emission demonstrate the efficient population inversion of Er³⁺:⁴I_11/2 → ⁴I_13/2 in high Er³⁺-doped ceramics for the 2.7 µm emission. These results suggest that high Er³⁺-doped (Y_0.9La_0.1)₂O₃ transparent ceramics are promising host materials for the applications of mid-infrared lasers and infrared-to-visible up-converters.     

Structural and luminescence properties of RE (RE = Eu, Tb):(MgCa)2Bi4Ti5O20 ceramics

Rare-earth ions (Eu³⁺, Tb³⁺) activated magnesium calcium bismuth titanate [(MgCa)₂Bi₄Ti₅O₂₀] ceramics were prepared by conventional solid state reaction method for their structural and luminescence properties. By using XRD patterns, the structural properties of ceramic powders have been analyzed. Emission spectrum of Eu³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown strong red emission at 615 nm (⁵D₀ → ⁷F₂) with an excitation wavelength λ_exci = 393 nm and Tb³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown green emission at 542 nm (⁵D₄ → ⁷F₅) with an excitation wavelength λ_exci = 376 nm. In addition, from the measurements of scanning electron microscopy (SEM), Fourier transform-infrared (FTIR) and energy dispersive X-ray analysis (EDAX) results the morphology, structure and elemental analysis of these powder ceramics have been studied.     

Effect of La-doping on the properties of CaCu3Ti4O12 dielectric ceramics

Nano-size Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ (χ = 0.00, 0.05, 0.10, 0.15 and 0.20) precursor powders were prepared via the sol–gel method and the citrate auto-ignition route and then processed into micro-crystal Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics under heat treatment. Characterization of the as-obtained ceramics with XRD and SEM showed an average grain sizes of ∼1–2 μm, indicating La³⁺ amount to have little impact on grain size. The room-temperature dielectric constant of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics sintered at 1000 °C was of the order of 10³–10⁴ despite the variation of χ values. Compared with CaCu₃Ti₄O₁₂, La³⁺-doped CaCu₃Ti₄O₁₂ showed a flatter dielectric constant curve related to frequency. It was found that the loss tangent of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics was less than 0.20 in ∼600–10⁵ Hz region, which rapidly decreased to a minimum value of 0.03 by La³⁺doping with χ = 0.05. Our measurement of the ceramics conductivities (σ) also indicated that the appropriate introduction of La³⁺ into CaCu₃Ti₄O₁₂ would distinctly result in its dielectric properties.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Structural,morphological and up-converting luminescence characteristics of nanocrystalline Y2O3:Yb/Er powders obtained via spray pyrolysis

Sub-micronic, spherical Y₂O₃:Yb/Er particles comprising clustered nano-units (70 nm) were prepared via ultrasonic spray pyrolysis from pure nitrate precursor solutions with different Yb/Er dopant ratios. The particles were additionally thermally treated at 1100 °C for 12, 24 and 48 h. The structural and morphological characteristics of particles were studied by X-ray powder diffraction, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, energy dispersive X-ray analysis and specific surface area (BET) and were further correlated with their advanced optical properties. For the recorded up-conversion emissions, originating from the following Er³⁺ transitions: [²H_9/2→⁴I_15/2] in blue (407–420 nm); [²H_11/2, ⁴S_3/2→⁴I_15/2] green: 510–590 nm; and [⁴F_9/2→⁴I_15/2] in red (640–720 nm) spectral region, the corresponding lifetimes were acquired in the wide temperature range (10–300 K). The most intense green up-conversion emission with the long decay of 550 ms is recorded for Y_1.97Yb_0.02Er_0.01O₃ particles thermally treated at 1100 °C for 24 h.     

Fabrication and spectroscopic investigations on Er3+, Ho3+: SrF2 transparent ceramics for 2.7 μm emission

3 at.% Er³⁺, x at.% Ho³⁺: SrF₂ (x = 0, 0.05, 0.1, 0.5, 1, 2) transparent ceramics, as the potential material for the 2.7 μm solid-state laser, were fabricated by hot-pressed sintering. XRD, TEM, SEM, and EDS measurements were used to investigate the phase composition, morphology, microstructure, and distribution of the elements of the nanoparticles and transparent ceramics. Results showed that the Er³⁺ ions and Ho³⁺ ions do not alter the SrF₂ crystal structure, and they are distributed uniformly in the sample. With the increase of the Ho³⁺ doping concentration, the lattice parameter decreased from 5.799 Å to 5.784 Å, and the average grain size decreased gradually. The maximum transmittance of as-obtained ceramics is approximately 93 % which is close to the theoretical transmittance of SrF₂. Moreover, the absorption spectra, emission spectra, and the lifetime of Er³⁺ and Ho³⁺ were investigated. The energy transfer processes between Er³⁺ and Ho³⁺ were discussed. After co-doping Ho³⁺, the lifetime difference between Er³⁺:⁴I_11/2 and Er³⁺:⁴I_13/2 levels was shortened from 8.50 ms to 1.12 ms. All the results show that the incorporation of Ho³⁺ with proper doping concentration is beneficial for achieving 2.7 μm laser output in Er³⁺: SrF₂ transparent ceramics.     

Effect of Ta2O5 in (Ca,Si, Ta)-doped TiO2 ceramic varistors

TiO₂ varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO₂ ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁴. Experimental evidence shows that small quantities of Ta₂O₅ improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10⁴ and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂–0.8 mol% Ta₂O₅ ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta⁵⁺ for Ti⁴⁺.     

Microstructural variation and dielectric properties of KTiNbO5 and K3Ti5NbO14 ceramics

KTiNbO₅ (KTN) and K₃Ti₅NbO₁₄ (3K5TN) ceramics sintered at 1150 °C and 1125 °C, respectively, exhibited a dense, homogeneous microstructure with a high relative density (≥96% of the theoretical density). Abnormal grain growth occurred in both specimens during sintering, and large (002) and (001) grains developed in KTN and 3K5TN ceramics, respectively. A dielectric constant (ε_r) of 13 and a dielectric loss of 2.9% at 10 MHz were obtained from KTN ceramics sintered at 1150 °C. The 3K5TN ceramics sintered at 1125 °C showed an ε_r of 15 and a dielectric loss of 12% at 10 MHz. The resistivity of KTN and 3K5TN ceramics was low and their ε_r and dielectric loss values displayed low-frequency dispersion (LFD); the presence of K⁺ ions between the layers could be responsible for their low resistivity and LFD.