Advanced Materials Research Vols. 197-198

Abstract: An investigation was made of low voltage TiO₂ varistors doped with Ta₂O₅ and La₂O₃. TiO₂ ceramics doped with 0.7 mol% La₂O₃ and 0.1 mol% Ta₂O₅ were sintered at different temperature ranging from 1350 to 1450°С . The influence of sintering temperature on microstructure and nonlinear properties of the (La, Ta)-doped TiO₂ ceramics was studied. The varistor of 99.2 mol%-0.7 mol%La₂O₃-0.1 mol% Ta₂O₅ composite sintered at 1380°С has a maximal nonlinear coefficient of α =5.2 and a low breakdown voltage of 7.6 V/mm, which is consistent with its highest grain-boundary barriers. According to these results, it is suggested that the sample sintered at 1380°С forms the most efficient boundary barrier layer. Therefore, the sintering temperature is a very important varible which should not be despised in the project of TiO₂ based varistors production.

Abstract: SrAl₂O₄:Eu²⁺, Dy3⁺ long persistence phosphors are synthesized by microwave plasma method. The effects of molar ratio of Dy³⁺ and Eu²⁺ on the phase composition, the spectrum properties and long afterglow property of phosphors are investigated in the paper. The analysis results show that spectrum properties depressed clearly following the increase of Dy³⁺/ Eu²⁺. And the afterglow property of phosphors are improved by doping of the Dy³⁺, but suppressed by excessive doping. On the one hand, this is because that the number of electrons trapped by defect levels increased with the increase of Dy³⁺. But on the other hand, it’s difficult to occur electric transition if doping excessive Dy³⁺, so leads to performance depressed. Above-mentioned results, it’s confirm that the optimized Dy³⁺/Eu²⁺ molar ratio is 2:1 when SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors synthesized by microwave plasma method.

Abstract: The CaCu_3-xZn_xTi₄O₁₂ (CCZTO) (x=0, 0.06, 0.1, 0.2) ceramics were prepared by Sol–gel method. The XRD pattern shows that crystal structure of the sample with no Zn is basically single-phase. The SEM results indicate that the samples have porous structures and the gain size decreases with the content of Zn increasing. The dielectric properties of CCZTO were analyzed by Agilent 4294A. The experimental results show that the dielectric constants are less than that in other papers reported obviously because of their mesoporous structures. With the little doping amount(x=0.06, 0.1), a sharp decrease happened in the dielectric constant (ε_r). The higher ε_r was found when x=0.2, but its dielectric loss (D) had greatly increased and frequency stability of dielectric property became worse. The uniformity of gains was put forward to explain the results which showed great impact on the dielectric constant.

Abstract: DAST (4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate) crystal has been grown by slope nucleation method (SNM) at different cooling rates (0.5°С/d and 0.1°С/d). The morphology of crystal growing surfaces was observed by scanning electron microscope (SEM) and microscope. Different growth striations were found on the growth surfaces. The arrangements of crystal ions and cooling rate have a great influence on the crystal shape. The relationship between the DAST crystal molecular structure and the crystal shape was systematically studied by analyzing the growing surface morphology. Two-dimensional nucleation mechanism was found on growing faces with increasing supersaturation.

Abstract: It is important to develop a new special material to satisfy the requirement of producing capacitor with higher property. BZT(Ba(Zr_xTi_1-x)O₃) ceramics of La₂O₃ and Sm₂O₃ doped was fabricated by the conventional method of solid reaction. These ceramics were tested by X-ray diffraction (XRD), Impedance Tester, and LCR Tester with temperature controlling systems. The results indicate that dielectric property of BZT ceramics doped 0.05mol% La2O3, 0.2mol% Sm₂O₃ simultaneously and calcined in 1280°C is the best. Moreover, it was found that Sm₂O₃ had the functions of moving Tc of matrix material to a low temperature and making the peak wider.

Abstract: Based on the theoretical model, magnetic field distribution of rectangular teeth, two-sides dilated shape and one-side dilated shape teeth structure with common other conditions were calculated using finite element method when the sealing gap was 0.1mm and 0.12mm. The comparison of their results with the same sealing gap showed that rectangular teeth structure had the highest magnetic leakage. Moreover, the magnetic field distribution of sealing structures with rectangular stages on both the shaft and pole pieces under the same design and sealing gap were also calculated using the same method, whose result was compared with rectangular stages on pole pieces only. The comparison showed that the former did not have higher pressure capability obviously but led to higher magnetic leakage.

Abstract: MAl₂O₄:Eu²⁺0.01, Dy³⁺0.02 (M=Ca, Sr and Ba) materials were synthesized by combustion method. X-ray powder diffraction (XRD) patterns indicate that all the samples are of high purity and crystallinity. The grain sizes of MAl₂O₄:Eu²⁺, Dy³⁺ are similar. The luminescent properties of phosphors were studied systematically. The main peaks of emission spectra of the luminescent phosphors changes from blue-purple to yellow-green and then to blue-green due to the different distortion existed in different host lattices. The decay time of phosphors varies a lot, and SrAl₂O₄:Eu²⁺, Dy³⁺ shows the best persistence characteristic. Thermoluminescence analysis indicates that the trap levels in MAl₂O₄:Eu²⁺, Dy³⁺ (M=Ca, Sr and Ba) are different, and the trap level in SrAl₂O₄ host is the most moderate to exhibit long afterglow at room temperature.

Abstract: BaO-CeO₂-TiO₂ microwave dielectric ceramics with the different content of TiO₂ were fabricated by a solid-state reaction method using BaCO₃, CeO₂, TiO₂ powders as starting materials in view of the dispute. The sintering characteristics, phase composition, micro-structures and microwave dielectric properties of BaO-CeO₂-nTiO₂ microwave dielectric ceramics with different n values sintered at different temperatures were investigated. The results showed that the sintering temperature of BaO-CeO₂-nTiO₂ microwave dielectric ceramics was lower (< 1300°C), and CeO₂ didn’t tend to form polynary compounds. When n = 3, the sintered ceramics had excellent dielectric properties: ε_r ≈40.3, tanδ ≈1.02×10^-3 (1 MHz); ε_r ≈ 41.2, Q•f =19922GHz (6.865 GHz), τ_f = +22 ppm/°C .

Abstract: MoSi₂ and SiC ceramics were prepared by traditional powder metallurgy method. A novel ceramic thermocouple (CTC) used in the high temperature and high corrosion environment was assembled by SiC as the cathode and MoSi₂ as the anode. The thermo-emf of CTC was tested from 25 °C to 1600 °C in air. The results show that there is a simple relationship between the thermo-emf of CTC and the temperature. By holding temperature for 70 h at 1500 °C, the deviation of thermo-emf is ±0.37%, and better than 0.75% which is the criteria of industry thermocouple. And also the response characteristics of CTC were analyzed.

Abstract: Barium strontium titanate (Ba_0.6Sr_0.4TiO₃: BST) powders were prepared by the oxalate co-precipitation method. The ceramics from thus obtained BST powders with B₂O₃-Li₂CO₃ addition were prepared by conventional oxide mixing method. The powders and the ceramics were observed and analyzed by SEM and X-ray diffraction. The effects of B₂O₃-Li₂CO₃ addition and powder properties on the sintering behavior and the dielectric properties of BST-based ceramics were investigated. SEM results revealed that the BST powders are micron-sized with cauliflower-shaped rough surface leading the specific surface area reaches 18.52m²/g. In dilatometric studies, the ceramics without B₂O₃-Li₂CO₃ addition didn’t produce the desired shrinkage and dense microstructure at relative low temperatures (<1000°C). However, the sintering temperature is decreased to 850°C by addition of small amount (≤3wt %) of B₂O₃ and Li₂CO₃. This was also verified in sintered microstructures. The XRD results showed that the main phase of the ceramics was BST without any other crystalline phases of remarkable extent. With increasing of B₂O₃-Li₂CO₃ content, the permittivity and the tunability of the ceramics were decreased.