Sc3+–Lu3+-Doped α-SiAlONs

Sc³⁺ and dual Sc³⁺–Lu³⁺-doped α-SiAlON compositions were sintered by hot pressing, and the formation behavior, microstructure, and mechanical properties were assessed. It was found that the small cation Sc³⁺ could not be accommodated into the α-SiAlON structure alone. The addition of Lu₂O₃ in the composition induces the Sc³⁺ cation to enter the α-SiAlON structure, and leads to the production of α-SiAlON with an elongated-grain microstructure. Transmission electron microscopy analysis shows that α-SiAlON grains always contain an α-Si₃N₄ core, implicating heterogeneous nucleation to be in present in a mixed lutetium/scandium-doped α-SiAlON system.     

Fabrication and Photoluminescence Characteristics of Eu3+-Doped Lu2O3 Transparent Ceramics

A novel co-precipitation process was adopted for the preparation of highly sinterable europium-doped lutetia powders using ammonium hydroxide (NH₃·H₂O) and ammonium hydrogen carbonate (NH₄HCO₃) as the mixed precipitant. The resultant powders calcined at 1000°C for 2 h showed good dispersity and excellent sinterability. Highly transparent polycrystalline lutetia ceramics with a relative density of ∼99.9% were fabricated by pressureless sintering in flowing H₂ atmosphere at 1850°C for 6 h without any additives. The average grain sizes of the transparent material were estimated to be 50–60 μm. Optical in-line transmittance in the visible wavelength region for Lu₂O₃ ceramics (1 mm in thickness) reached 80%. The luminescence and decay behavior of the obtained transparent plate and the corresponding nanophosphors were also investigated.     

Eu2+-Doped Glass Ceramics Containing BaF2 Nanocrystals as a Potential Blue Phosphor for UV-LED

The Eu²⁺-doped glass ceramics containing BaF₂ nanocrystals were prepared and their luminescence properties were investigated. The excitation spectra of Eu²⁺-doped glass ceramics showed an excellent overlap with the main emission region of an ultraviolet light-emitting diode (UV-LED) centered at 380 nm. The 450 nm emission of Eu²⁺ in glass ceramics under the 385 nm excitation was much stronger than that in glass. The Eu²⁺-doped glass ceramics containing BaF₂ nanocrystals may be used as a potential blue-emitting phosphor for UV-LED.     

Fabrication and Luminescent Properties of Nd3+-Doped Lu2O3 Transparent Ceramics by Pressureless Sintering

The fabrication of transparent Nd³⁺ ion-doped Lu₂O₃ ceramics is investigated by pressureless sintering under a flowing H₂ atmosphere. The starting Nd-doped Lu₂O₃ nanocrystalline powder is synthesized by a modified coprecipitant processing using a NH₄OH+NH₄HCO₃ mixed solution as the precipitant. The thermal decomposition behavior of the precipitate precursor is studied by thermogravimetric analysis and differential thermal analysis. After calcination at 1000°C for 2 h, monodispersed Nd³⁺:Lu₂O₃ powder is obtained with a primary particle size of about 40 nm and a specific surface area of 13.7 m²/g. Green compacts, free of additives, are formed from the as-synthesized powder by dry pressing followed by cold isostatic pressing. Highly transparent Nd³⁺:Lu₂O₃ ceramics are obtained after being sintered under a dry H₂ atmosphere at 1880°C for 8 h. The linear optical transmittance of the polished transparent samples with a 1.4 mm thickness reaches 75.5% at the wavelength of 1080 nm. High-resolution transmission electron microscopy observations demonstrate a "clear" grain boundary between adjacent grains. The luminescent spectra showed that the absorption coefficient of the 3 at.% Nd-doped Lu₂O₃ ceramic at 807 nm reached 14 cm⁻¹, while the emission cross section at 1079 nm was 6.5 × 10⁻²⁰ cm².     

Strong Green and Red Upconversion Emission in Er3+-Doped Na1/2Bi1/2TiO3 Ceramics

Upconversion emission properties of Na_1/2Bi_1/2TiO₃:Er³⁺ ceramics prepared by the solid-state reaction method were analyzed as a function of Er³⁺ concentration and incident pump power. Strong green (550 nm) and red (670 nm) emission bands were observed with 980 nm excitation at room temperature. Experimental results showed that the emission bands can be tuned by changing Er³⁺ concentration. Upconversion processes in these samples result from not only a two-photon excited-state absorption process but also a nonradiative energy transfer and cross-relaxation process.     

Residual α–Si3N4 in O' Crystals in CeO2-Doped O'+β' SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si₃N₄ inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO₂-doping and its high doping level as well as the high Al₂O₃ concentration were thought to be the main reasons for accelerating the reaction between the α-Si₃N₄ and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β-Si₃N₄ transformation. The large O' grains and the α-Si₃N₄ inclusions have a pronounced effect on the strength degradation of O'+β' ceramics.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Formation Mechanisms of La3+-Doped CaO·6Fe2O3 with the Magnetoplumbite Structue

Several experiments were conducted to investigate the formation mechanisms of the magnetoplumbite phase in La³⁺-doped CaO 6Fe₂O₃. It is shown that the CaO·2Fe₂O₃ phase plays a crucial role in forming the magnetoplumbite phase. The formation mechanisms are proposed and verified.     

Domain Structure and Fatigue Behavior of La3+-Doped SrBi2Ta2O9 Thin Films

Ferroelectric SrBi_1.4La_0.6Ta₂O₉ (SBLT) thin films were grown onto Pt/Ti/SiO₂/Si substrates by pulsed-laser deposition. With the aid of X-ray diffractometry, piezoresponse scanning probe microscopy, and ferroelectric-property measurements, a correlation between microstructure, as well as domain structure and ferroelectric properties, was established. Excluding the effect of preferential orientation on ferroelectric properties, the increase in remanent polarization was attributed to distortion of the perovskite-like sublattice and atom displacement. Despite the co-instantaneous observation of a 90° domain and slight fatigue behavior in the SBLT films, the 90° domain-wall clamping did not seem to account for the fatigue in the SBLT films. Instead, strain-stress aggravation of the SBLT sublattice, due to the substitution of La³⁺ into Bi³⁺ sites, decreased the self-regulated flexibility of the (Bi₂O₂)²⁺ layers and caused fatigue in the SBLT.     

Reassessment of the Electronic-Conduction Behavior above the Verwey-Like Transition of Ni2+- and Al3+-Doped LiMn2O4

The electronic conduction behavior above the Verwey-like transition temperature ( T _v) of Ni²⁺- and Al³⁺-doped LiMn₂O₄ has been reassessed. The nonadiabatic small-polaron model is adopted to assess the conduction mechanism. The Jahn–Teller distortion and short-range ordering are found to play a role in the variations of the lattice parameter and conduction behavior above T _v. It is also found that the effect of the dopant content on the activation energy is minor but is significant on the preexponential factor during the hopping process above T _v. In simultaneously comparing the effects of the dopants with quite different chemistry on the electronic-conduction behavior in Ni²⁺- and Al³⁺-doped LiMn₂O₄, it indeed provides a deep sight into understanding the fundamental conduction mechanism above T _v.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

La2O3-Al2O3-SiO2 Glasses for High-Power, Yb3+-Doped, 980-nm Fiber Lasers

Single-mode semiconductor pumps have failed to keep pace with the increasing power requirements of Er-doped fiber amplifiers (EDFAs), so there is a need for high-powered 980-nm sources. Yb³⁺-doped tapered fiber lasers can provide high-power output by conversion of a low-brightness, high-powered, 920-nm, multimode broad stripe laser diode to a high-brightness, 980-nm, single-mode output. The tapered fiber laser requires a fiber with high numerical aperture (NA) (>0.4), a rectangular core, and good Yb³⁺ spectroscopy for efficient operation. CVD-based fiber fabrication methods are incapable of delivering fibers with an NA > ∼0.3 or with good efficiency at 980 nm so a new method of high-NA fiber fabrication was developed. The core glass composition is critical for maintaining a high-NA fiber with good power extraction while avoiding phase separation, loss, and clustering. The SiO₂ level controlled the NA and interdiffusion between core and clad, while the Al₂O₃/La₂O₃ ratio controlled phase separation. A La₂O₃-Al₂O₃-SiO₂ glass was developed that is compatible with a pure SiO₂ cladding glass and has a laser slope efficiency of 70% at 980 nm. The optimized fiber composition yielded 450 mW of 980-nm power in a single-mode fiber.     

Properties of Dy3+-Doped Ge–As–Ga–Se Chalcogenide Glasses

In the present work, properties of Dy³⁺-doped chalcogenide glasses as a stoichiometric formula of (0.9− x )GeSe₂– x AsSe_3/2–0.1GaSe_3/2 ( x =0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were investigated as a comparison with the previously studied Ge–Ga–Se system. Property measurements include differential thermal analysis, density, Vis-NIR and IR transmission spectra, and fluorescent emission spectra. Judd–Ofelt analysis was performed using the calculated oscillator strengths of glasses. All properties of glasses show obvious dependence on compositions. Especially, the improved devitrification resistance and the longer fluorescence life at 1.34 μm emission were observed with the increasing As content. Noticeably, abnormal changes in some properties occurred to the glass whose average coordination number corresponds to the threshold of two-dimensional–three-dimensional topological phase transition.     

Low-Fire Processing of ZrO2–SnO2–TiO2 Ceramics

Effects of a liquid-phase-sintering aid, BaCuO₂+ CuO (BCC), on densification and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics have been investigated. The densification kinetics of ZST are greatly enhanced with the presence of 2.5–5 wt% BCC, but become retarded when the amount of BCC increases further. At a given BCC content, moreover, slower densification kinetics are observed with a larger particle size of ZST. The above results are attributed to a chemical reaction taking place at the interface of BCC/ZST during firing. The ZST dissolves into BCC, forming crystalline phases of ZrO₂, SnO₂, CuO, and BaTi₈O₁₆ which reduce the amount of BCC flux available for liquid-phase sintering. The crystallization kinetics become more significant, compared with densification kinetics, with increasing the amount of BCC and the particle sizes of ZST. For samples with 2.5–5 wt% BCC, a high relative sintered density is obtained at 1000°C and the resulting microwave ceramics have a dielectric constant and a value of Q at 7 GHz in the ranges of 35–38 and 2800–5000, respectively.     

Characterization and Microwave Dielectric Properties of M2+Nb2O6 Ceramics

This paper details the investigation of the quality factor ( Q ), dielectric permittivity (ɛ_r) and temperature coefficient of resonant frequency (τ_f) of the TE_01δ mode of the columbite binary niobate ceramics, with the formula MNb₂O₆ where M=²⁺ cation, in relation to their degree of sintering, microstructure and phase composition. The ceramics were made from a mixed oxide preparative route and fired over a range of temperatures from 800° to 1400°C, and most formed the columbite structure. A comprehensive study was made of the niobates containing the transition metal cations M=Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, and the group II metal cations M=Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. All columbite niobates were found to have ɛ_r between 17 and 22 and negative τ_f values between –45 and –76 ppm/°C, and ZnNb₂O₆, MgNb₂O₆, CaNb₂O₆, and CoNb₂O₆ had high Q f values of 84 500, 79 600, 49 600, and 41 700 GHz, respectively. The Q f of MgNb₂O₆ was found to rise to over 95 000 GHz when heated at 1300°C for 50 h.     

La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Synthesis and Photoluminescence Properties of Eu3+-Doped Calcium Phosphates

Eu³⁺-doped (1–12 mol%) calcium hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) powders were synthesized by a precipitation method and their photoluminescence (PL) properties were investigated. Eu has a limited solubility in HA and Eu₂O₃ was detected by X-ray diffraction above 3% doping, whereas a nearly single β-phase was obtained up to 6% doping in TCP but EuPO₄ appeared in the 12% Eu-doped specimen. Electron spin resonance measurement confirmed that europium ions exist as Eu³⁺ in both samples. Eu³⁺-doped HA exhibited a strong emission at 575 nm with several minor peaks at 610–640 nm and Eu³⁺-doped TCP had an intense emission at 613 nm with secondary peaks at 590–600 nm, which were consistent with the earlier reports determined at low temperatures. In Eu-doped TCP, the PL intensity increased with increasing calcination temperature without phase transformation. The more the Eu added, the higher the PL intensity obtained in both cases.     

Structure of Na2O–CaO–P2O5–SiO2 Glass–Ceramics with Multimodal Porosity

We have investigated the evolution of the structure of nano–macro porous CaO–Na₂O–P₂O₅–SiO₂ bioactive glass–ceramics by Fourier transform infrared (FTIR) and Raman spectroscopies, and X-ray diffraction (XRD). A controlled devitrification, followed by a chemical leaching treatment is used to produce a multimodal distribution of nano/macro pores that are expected to improve cell attachment. Data show that the leaching process removes the sodium- and calcium-containing crystalline phases that are formed during the ceramming heat treatment. The primary Si–O peaks in the infrared spectra blue shift with leaching, indicating that the sample becomes SiO₂ rich. In parallel, the fraction of nonbridging oxygen decreases. These results suggest a restructuring of the glass network far below the glass transition temperature. The stresses from leaching, capillary forces, and subsequent restructuring develop and grow, eventually producing cracks in the sample.     

Kinetic Study of Cu+−Cu2+ Equilibrium in Sodium Na2O−Al2O3−B2O3 Glass

The Cu⁺−Cu²⁺ equilibrium in sodium aluminoborate glass with an air atmosphere was studied at 950°, 1050°, and 1150°C. Equations suggested to represent the equilibrium reaction between oxidized and reduced species and oxygen were critically examined. An equation to represent the Cu⁺−Cu²⁺ equilibrium reaction was suggested. The reaction was observed to follow first-order kinetics. Enthalpy of the reaction was—12.5 kcal/mol.