Electrical properties and stability of Dy2O3-doped ZnO–Pr6O11-based varistors

The electrical properties and DC accelerated aging characteristics of ZnO–Pr₆O₁₁-based varistors were investigated with various Dy₂O₃ contents and sintering times. It was found that the Dy₂O₃ content and sintering time significantly affected electrical properties and their stability. The nonlinear exponent with increasing Dy₂O₃ content varied with ∨-shaped, reaching maximum of 58.6 at 1.0 mol% Dy₂O₃ for varistors sintered for 1 h and with ∧-shaped, reaching maximum of 46.3 at 0.5 mol% Dy₂O₃ for 2 h. The leakage current was in the range of 0.1–1.1 μA for varistors sintered for 1 h and 0.3–2.4 μA for 2 h. The increase of sintering time led to lowering of nonlinearity. The varistors with 0.5 mol% Dy₂O₃ sintered for 2 h exhibited the good stability, under DC stress of 0.95 V _{1 mA}/150 °C/24 h.     

Effect of oxygen flow rate on the structural and electrochemical properties of lithium nickel oxides synthesized by the sol–gel method

The structural and electrochemical properties of LiNiO₂ powders were investigated as a function of the oxygen flow rate employed in the preparation of lithium nickel oxide. It was found that oxygen played an important role in the synthesis of highly crystallized LiNiO₂(R3¯m). In the crystallization process of LiNiO₂, a deficiency of oxygen in the calcination reactor induced the formation of impurities and cubic rock-salt structure (Fm3m) in LiNiO₂ powders. For LiNiO₂ prepared at higher oxygen flow rates, the electrode delivered high discharge capacities with relatively good retention rates. But very low electrode capacity was obtained from LiNiO₂ prepared at lower oxygen flow rates.     

The effects of Ni and Li doping on the performance of lithium manganese oxide material for lithium secondary batteries

Layered Li_0.7[M_1/6Mn_5/6]O₂ (M=Li, Ni) was synthesized using a sol-gel method. P2-Na_0.7[M_1/6Mn_5/6]O₂ precursor was first synthesized by a sol-gel method, and then O2-Li_0.7[M_1/6Mn_5/6]O₂ was prepared by an ion exchange of Li for Na in P2-Na_0.7[M_1/6Mn_5/6]O₂ precursor. From charge/discharge curves, it was seen that Li_0.7[Li_1/6Mn_5/6]O₂ has two plateaus similar to those observed from a spinel structure, but Li_0.7[Ni_1/6Mn_5/6]O₂ holds a single plateau as observed from a typical layered structure. It was considered that Li_0.7[Li_1/6Mn_5/6]O₂ undergoes a phase transformation from layered to spinel structure during the charge/discharge cycle, but Li_0.7[Ni_1/6Mn_5/6]O₂ maintains O2-layered structure after the cycles. Li_0.7[Ni_1/6Mn_5/6]O₂ was higher in discharge capacity and retention rate than Li_0.7[Li_1/6Mn_5/6]O₂.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.     

Suppression of superconductivity in the Er1− x Pr x Ba2Cu3O7−δ

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er_1-x Pr _x Ba₂Cu₃O_7- system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.     

Electrical properties and stability against DC accelerated aging stress of lanthania doped praseodymia-based zinc oxide varistor ceramics

The microstructure, electrical properties, and stability against DC accelerated aging stress of the varistor ceramics, which are composed of ZnO–Pr₆O₁₁–CoO–Cr₂O₃–La₂O₃-based ceramics, were investigated for various La₂O₃ contents. The increase of La₂O₃ content led to more densified ceramics, whereas abruptly decreased the nonlinear properties by incorporating beyond 1.0 mol%. The highest nonlinearity was obtained from 0.5 mol% La₂O₃, in which the nonlinear exponent is 81.6 and the leakage current is 0.1 μA. As the La₂O₃ content increased, the donor concentration increased in from 0.64 × 10¹⁸ to 16.89 × 10¹⁸/cm³ and the barrier height greatly decreased with increasing La₂O₃ content, reaching a maximum (1.47 eV) in 0.5 mol% La₂O₃. The varistors doped with 0.5 mol% La₂O₃ exhibited high stability, in which the variation rates of varistor voltage, nonlinear exponent, and leakage current were −1.14%, −3.7%, and +100%, respectively, for stressing state of 0.95 V _{1 mA}/150 °C/24 h.     

Evaluation of fracture toughness degradation of CrMoV rotor steels based on ultrasonic nonlinearity measurements

The objective of this paper is to develop a nondestructive method for estimating the fracture toughness (K _IC) of CrMoV steels used as the rotor material of steam turbines in power plants. To achieve this objective, a number of CrMoV steel samples were heat-treated, and the fracture appearance transition temperature (FATT) was determined as a function of aging time. Nonlinear ultrasonics was employed as the theoretical basis to explain the harmonic generation in a damaged material, and the nonlinearity parameter of the second harmonic wave was the experimental measure used to be correlated to the fracture toughness of the rotor steel. The nondestructive procedure for estimating theK _IC consists of two steps. First, the correlations between the nonlinearity parameter and the FATT are sought. The FATT values are then used to estimateK _IC, using theK _IC versus excess temperature (i.e.,T-FATT) correlation that is available in the literature for CrMoV rotor steel.     

Nonlinear behavior of Tb4O7-modified ZnO-Pr6O11-based ceramics with high breakdown field

The effect of Tb4O7 on electrical behavior of the ZnO-Pr6O11-based varistor ceramics was investigated.Microstructural analysis indicated that the addition of Tb4O7 decreased average grain size from 3.6 to 3.2 μm and increased the sintered density from 5.58 to 5.68 g/cm3.As the amount of Tb4O7 increased,the breakdown field increased from 9393 to 12437 V/cm and the nonlinear coefficient increased from 50 to 65.The varistor ceramics added with 0.5 mol.% in the amount of Tb4O7 exhibited an excellent stability by exhibiting-0.1% in the variation rate of the breakdown field,0% in the variation rate of the nonlinear coefficient,and 8.8% in the variation rate of the leakage current density for DC-accelerated aging stress of 0.85 E1 mA/115℃/24 h.     

An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2

The effects of surface passivation on the photoluminescence (PL) properties of CdS nanoparticles oxidized by straightforward H₂O₂ injection were examined. Compared to pristine cadmium sulfide nanocrystals (quantum efficiency ≅ 0.1%), the surface-passivated CdS nanoparticles showed significantly enhanced luminescence properties (quantum efficiency ≅ 20%). The surface passivation by H₂O₂ injection was characterized using X-ray photoelectron spectroscopy, X-ray diffraction, and time-resolved PL. The photoluminescence enhancement is due to the two-order increase in the radiative recombination rate by the sulfate passivation layer.