Key Engineering Materials Vols. 336-338

Abstract: We have prepared the gallium oxide (Ga2O3) films on sapphire substrates by a thermal evaporation of GaN powders. We have characterized the films by using the x-ray diffraction (XRD), scanning electron microscopy (SEM), and the photoluminescence (PL). SEM and XRD revealed that the deposits were Ga2O3 thin films with monoclinic structure. PL spectrum of Ga2O3 films under excitation at 325 nm showed a blue emission.

Abstract: Ultrafine terbium-doped Lu2Hf2O7/Tb phosphor powders are prepared by a solution combustion process using glycine and urea as fuel. Phase evolution of the synthesized powder is determined by X-ray diffraction (XRD) techniques. Single-phase cubic Lu2Hf2O7/Tb crystalline powder is obtained by calcining the amorphous materials at 800oC and no intermediate phase is observed. Transmission electronic microscope (TEM) morphology shows that the resultant Lu2Hf2O7/Tb powders have uniform size and good homogeneity. The photoluminescence (PL) spectra of Tb3+ substituted for Lu3+ in Lu2Hf2O7 with different content has been measured on samples calcined at 1000oC.

Abstract: White anodic coatings are gained by growth in situ on the aluminum alloy. The coatings possess stable optical properties, excellent adhesive power, low cost, stability in the space environment, and so on, which can be widely used in the thermal control system of spacecrafts. In this article, the developed Al2O3 thermal control coatings that had a low solar absorptance and a high infrared emittance were gained in the electrolyte of sulfuric acid. The influences of various process parameters, such as electrolyte temperature, current density, anodizing time, on the optical properties of the coatings were investigated. AFM and XRD were used to characterize the microstructure of the coatings. Experimental results show the anodic coatings could reach a solar absorptance value lower than 0.20 and an infrared emittance higher than 0.80. Therefore, the coatings can be applied as important thermal control coatings for space applications.

Abstract: Near-infrared emission properties of Dy3+-Tm3+ co-doped Ge-Ga-Sb-Se glasses were investigated. Possible energy transfer routes and the regime of donor decay were discussed. Tm3+ is found to be an efficient sensitizer on the Dy3+:1.34μm luminescence. Probable energy transfer schemes include Tm3+:3H4 → Dy3+: 6F5/2 and Tm3+:3H5 → Dy3+:6F11/2· 6H9/2. The donor (Tm3+) excitation is transferred to an acceptor (Dy3+) by direct interaction or migrates among donors by diffusion-limited regime until it comes into the vicinity of an energy acceptor where direct interaction and transfer occur.

Abstract: Sr4Al14O25: Eu2+, Dy3+ luminescent ceramics with high brightness and long afterglow were prepared using glass frits as binders by sintering at lower temperature and air atmosphere. The effects of glass frits on the properties of luminescent ceramics were discussed. The analytical results indicate that the surface structure and adhesion with phosphor are influenced by glass frits and sintering temperature. When sintered at 850°C, the samples with frit1 or frit2 obtained good surface morphology and good adhesion. The sample with frit2 sintered at 850°C yielded the better surface morphology and good adhesion. The sample with frit1 sintering at 850°C possessed the better luminescent properties.

Abstract: In this paper, we have investigated the near-infrared luminescence emitting from NiSi2 passivated silicon nanocrystals (NCs) embedded in SiOx films. For comparison, we also prepared the regular specimen without NiSi2 passivation. In the both systems, the intensity of photoluminescence emission from NC-Si increased with the increase of annealing temperature, which was explained by the crystallization of amorphous silicon in SiOx films. The maximum intensity of near-infrared emission from NiSi2-passivated NC-Si was stronger by factor 5 than that of regular specimen without NiSi2 passivation. The model of NiSi2 passivation was employed to explain this phenomenon.

Abstract: We describe the fabrication of chip-type PTC elements (BaTiO3-based) by roll-forming process. Crack-free green tapes with good malleability are successfully obtained with the thickness typically in the range of 0.20~1.0mm. We also focus on reducing the room-temperature resistance of the chip-type PTC element without deteriorating its PTC effect, both by controlling the firing schedule and by introducing multilayer structure with electrode-bonding method. The typical size of the 5-layered chip-type PTC sample obtained is 8.0mm×5.0mm×1.95mm with R25=1.97- and Rmax/Rmin=4.8×105.

Abstract: In this work, aqueous gel-casting (AGC) method was used for preparing PTC (positive temperature coefficient) ceramic green body based on BaTiO3. We studied on modeling of liquid desiccant drying method for gelcasting ceramic bodies, and compared with the conventional drying methods.

Abstract: BaTiO3 powder was synthesized from BaCO3 and TiO2 using a domestic microwave oven. The samples were synthesized under different temperatures with various holding times. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the phase compositions and morphologies of the result samples. The main phase obtained at 950°C was BaTiO3, and the intermediate phases Ba2TiO4 and Ba4TiO9 were also detected. The pure, well-crystallized BaTiO3 powder could be obtained at 1050°C within 10 minutes and the particle size ranged from 300~500nm. In comparison with conventional synthesis, faster speed and finer grains could be achieved through microwave heating.