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Ca0.98Eu0.02Al1−4δ/3Si1+δN3 (δ = 0–0.36) red-emitting phosphors were prepared by carbothermal reduction and nitridation method with stable and inexpensive CaCO3 as Ca source. Optimal nominal composition was obtained at δ = 0.18, showing intense emission peaked at 625 nm and high external quantum efficiency of 71%. The emission wave length could be successfully tuned from 630 to 606 nm with increasing δ value. Ca0.98Eu0.02Al1−4δ/3Si1+δN3 phosphors provided two coordinated environments for Eu2+ ions, resulting in two fitted Gaussian peaks. Energy transfer from Eu2+ sites in Si-rich environments to those in Si/Al-equivalent modes has been confirmed by analysis of the decay curve of each peak. The decay behaviors suggested that energy transfer effect slowed with higher δ value. Finally, warm white light was created by combining as-prepared red-emitting Ca0.98Eu0.02Al0.76Si1.18N3 and yellow-emitting YAG:Ce3+ phosphors with a blue-emitting chip, exhibiting a color rendering index Ra of 91 at a low correlated color temperature of 3500 K with a luminous efficiency of 79 lm/W.
Ce3+-activated lanthanide silicon oxynitride (La5Si3O12N, La4SiO7N2, LaSiO2N, and La3Si8O4N11) phosphors were prepared by firing the mixture of La2O3, Si3N4, SiO2, and CeO2 at 1500–1600 °C under a 0.5 MPa nitrogen atmosphere. Diffuse reflection spectrum, photoluminescence spectra, and temperature-dependent luminescence of these phosphors are presented in this work. A blue emission of Ce3+ in all lanthanide silicon oxynitrides was observed under ultraviolet irradiation, which is strongest in La3Si8O4N11:Ce3+. The concentration quenching and thermal quenching of the samples were discussed.
Nano-sized cerium-doped lutetium aluminum garnet (LuAG:Ce) powders were prepared via a sol-gel combustion process from a mixed solution of metal nitrates, using organic glycine as a fuel. The purified crystalline phase of LuAG:Ce was obtained after calcination at 1000 °C for 2 h. The obtained phosphors were agglomerated and had a foamy-like morphology, consisting of pointed crystallites with uniform size of about 40 nm. Both the photoluminescence and the radioluminescence of the calcined powders showed the same two emission bands, corresponding to transitions from the lowest 5d excited state (2D) to the 4f ground state of Ce3+ (2F5/2, 2F7/2). Using the prepared powders, polycrystalline LuAG:Ce optically transparent ceramics were successfully fabricated at 1850 °C for 10 h under vacuum without sintering aids and annealed at 1450 °C for 20 h in air. The sintered ceramics are transparent with an in-line light transmittance in the visible wavelength range of about 50% and have a uniform microstructure with an average grain size of about 8 μm. The radioluminescence of the transparent ceramics is similar to that for calcined powders, except higher in intensity.
The ductile-to-brittle transition was observed in a superplastic silicon nitride nanoceramic. This transition depends on strain rates and deformation temperatures. Generally, the material exhibits ductility at low strain rates and high deformation temperatures. At 1600 °C, the material is brittle when the strain rates are higher than 10−3/s. At a fixed strain rate of 10−3/s, the material exhibits brittleness when the temperatures are lower than 1550 °C. Moreover, critical strain rate for the brittle to ductile transition depends on deformation temperature. The critical strain rates increase with increases in the deformation temperature. When the deformation temperature is 1700 °C, the critical strain rates reach a maximum at 10−2/s. The extent of superplastic deformation in the present material was found to be limited not by intergranular cavitation but by the initiation and growth of surface cracks.
Using a pure α–SiC starting powder and an oxynitride glass composition from the Y–Mg–Si–Al–O–N system as a sintering additive, a powder mixture was hot-pressed at 1850 °C for 1 h under a pressure of 20 MPa and further annealed at 2000 °C for 4 h in a nitrogen atmosphere of 0.1 MPa. High-resolution electron microscopy and x-ray diffraction studies confirmed that a small amount of β–SiC was observed in the liquid-phase-sintered α–SiC with this oxynitride glass, indicating stability of β–SiC even at high annealing temperature, due to the nitrogen-containing liquid phase.
A quantitative texture analysis, including calculations of the orientation distribution function, is applied to investigate the preferred orientation of β–Si3N4 in a fine-grained material containing almost equiaxed grains that has been hot-pressed, annealed, and plane-strain compressed. The results show that (i) plane strain compression can produce relatively strong textures that were dependent on the compressive strain; (ii) the basal plane of hexagonal β–Si3N4 was normal to the hot-pressing direction for the hot-pressed and annealed samples, whereas it was parallel to the stress axis for deformed samples; and (iii) the mechanisms for texture development were preferred grain growth for the annealed sample and grain rotation for the hot-pressed and deformed samples, respectively.
A refractory silicon nitride joint, which contains β–Si3N4 grains and grain boundary amorphous phase in the joined layer, was developed with the aid of a ceramic adhesive based on the system Si3N4–Y2O3–SiO2–Al2O3. The similarity in chemistry and microstructure between the parent ceramic and the joint zone indicates that the joining mechanism is the same as that involved in the sintering of Si3N4. The resultant joint exhibits a high bond strength of 550 MPa at 25 °C and retains a strength of 332 MPa at 1000 °C. Post-joining hot-isostatic pressing was applied to strengthen the joint, resulting in increased strengths of 668 MPa at room temperature and 464 MPa at 1000 °C.
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