Key Engineering Materials Vols. 602-603

Abstract: Fiber reinforced and aluminum phosphate-based composite was studied in this experiment. The composite was prepared through the process of lamination- hot press- sintering, binding material and fillers. The proper mixture ratio of Al₂O₃:AlPO₄:H₂O was 10:2.88:2.88.The slurry with this ratio was proper to prepare the composite. It indicated that the mechanical property of composite was excellent which was prepared by the slurry with this ratio. In order to improve the Mechanical property of silica glass fiber / aluminum phosphate composite, Al2O3 powders were graded in the slurry. Mechanical properties of composites were improved evidently.

Abstract: The fatigue crack initiation life at various stress ratios for GH4133B superalloy specimen with different circular notch radios are studied at ambient temperature and atmospheric pressure utilizing the fracture mechanics method. It is shown that a controlling parameter ΔK_I/ρ^0.5 can be adopted to characterize the fatigue crack initiation life, and the effect of notch radius on fatigue crack initiation life increases with increasing control parameter, while the fatigue crack initiation life firstly increases and then decreases with increasing stress ratio. The fatigue fracture surfaces of circular notched compact tension specimens of GH4133B superalloy for fatigue crack initiation tests at various stress ratios are investigated using a scanning electron microscopy, and the fracture surface morphologies in the fatigue source region are analyzed. It is found that the separation between carbide inclusion on the surface or subsurface and matrix, or the separation between nickel-rich second phase particle and matrix, or the interaction between persistent slip bands and inclusion, mainly results in the fatigue crack initiation.

Abstract: The short fatigue crack growth tests for circular notched compact tension specimens of GH4133B superalloy used in turbine disk of aero-engine are carried out at ambient temperature and atmospheric pressure. The stress intensity factor ranges and the fatigue crack growth rates at various stress ratios are measured, and the corresponding effective stress intensity factor ranges considering the crack closure effect are calculated. It is shown that the effective stress intensity factor range ΔK_eff, can be applied to describe the deceleration and acceleration of crack growth rate during the short crack propagation. The fatigue fracture surface morphologies in the short crack growth region are investigated using a scanning electron microscopy. It is found that there is a cleavage step between two adjacent radial striations, a series of early fatigue striations exist on the cleavage step, and some secondary cracks perpendicular to the direction of main crack propagation emerge on the fracture surface, the superalloy exhibits a mixed fracture mode in the short crack growth region, which reveals the microscopic mechanism of short crack propagation that the fatigue crack growth rate is primarily higher, and then gradually decreases with the propagation of short crack.

Abstract: The structural and thermal expansion properties of solid solutions In_2-xFe_xMo₃O₁₂ have been characterized using X-ray diffraction. The lattice parameters decrease linearly with increasing Fe content at room temperature. All compounds exhibit monoclinic structure at room temperature, and transform to orthorhombic phase upon heating. Monoclinic In_2-xFe_xMo₃O₁₂ (x = 0.3, 1.0 and 1.3) possess strong positive thermal expansion, while the linear thermal expansion coefficient of orthorhombic phase varies from zero/near-zero to positive with increasing Fe³⁺ content.

Abstract: Goal of the present research was to study dielectric properties of BiNbO₄ ceramics fabricated by solid state reaction route followed with pressureless sintering. The samples were fabricated from the mixture of oxides, viz. Nb₂O₅ and Bi₂O₃. Apart from stoichiometric amount an excess of 3%, 5% and 10% by mole of Bi₂O₃ was used. It was found that an excess amount of Bi₂O₃ oxide caused formation of Bi₅Nb₃O₁₅ minor phase in amount of 9wt%, 18wt% and 25wt%, respectively. The dielectric properties were studied by impedance spectroscopy within the frequency range ν=20Hz1MHz and temperature range T=RT550°C. It was found that the presence of the minor phase in amount of 18wt% caused a decrease in dielectric losses of the material under studies.

Abstract: The effects of rare-earth ions Sc³⁺ and Gd³⁺ on phase transition, microstructure and dielectric properties were investigated systematically. Gd-and Sc-co-doped nanopowders were synthesized via one-step sol-gel method. After xerogel was calcined at 800 °C for 2h, (1x)BaTiO₃-xGdScO₃ (0 x 0.06) nanopowders with narrow particle distribution was obtained. The nanopowders were then pressed into pellets and sintered at 1300~1450 °C respectively. The results indicated that the dielectric peaks of samples were broadened since double doping restrained the grain growth of BaTiO₃ ceramics. For x = 0.06, pellets sintered at different temperatures can obtain a low curie temperature of 25 °C. Meanwhile, accompanied by an increasing sintering temperature, the dielectric loss of samples decreased gradually, achieving a optimal value of 1.1%.

Abstract: Microstructure control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the challenges for increasing capacitive volumetric efficiency and high voltage dielectric properties. In this paper, the X5R-MLCCs with ultra-thin dielectric layers (~1.2 μm) owning uniform grain size distribution were prepared by wet casting process. The microstructures and dielectric properties of the MLCCs were investigated. The existence of core-shell structure was proved by transmission electron microscopy observation and energy dispersive spectroscopy analysis. The existence of core-shell structure makes the temperature coefficient of capacitance (TCC) performance meet X5R standard. Moreover, a highly accelerated lifetime test (HALT) result shows that MLCCs with ultra-thin layers under high electric field are more easily to fail with increasing test temperatures. And the results reveal that the activation energy is similar to the value reported for mid-dielectric constant dielectrics.

Abstract: Multilayer ceramic capacitors (MLCC) are important functional components of electronic information technology. The development of AC MLCC requires low dielectric loss and high AC breakdown voltage. In this paper, Y-Al-Ga-Si co-doped barium titanate ceramics were prepared by conventional solid state method. Microstructures, surface morphology and dielectric properties were investigated by X-ray diffraction, SEM, and LCR analyzer, respectively. Y³⁺ entered into the lattice of BaTiO₃, replaced A-sites and B-sites, suppressed grain growth effectively, and made crystal structure change from tetragonal to pseudo-cubic, which reduced dielectric loss and lowered the Curie peak. The sintering characteristic and permittivity can be improved by the incorporation of Al and Ga. BaTiO₃ -0.06Y₂O₃ - 0.02Ga₂O₃ -0.01Al₂O₃ -0.01SiO₂ ceramics sintered at 1380°C achieved good dielectric properties: ε_r= ~2223, tanδ =~1.1% (at 1kHz), ΔC/C₂₅ <~15.26% (from 55°C to 150°C).

Abstract: The lattice structure of a high-k BL3TC5 [(Ba_1-xLa_x)(Ti_1-x/4-yCe_y)O₃ (x = 0.03, y = 0.05)] ceramic with cubic structure at room temperature was investigated using temperature-dependent X-ray diffraction (XRD) and Raman spectroscopy. The XRD results indicated that with increasing temperature, the cubic lattice of BL3TC5 showed a lower coefficient of linear expansion (α = 7.7 × 10^-6/°C) relative to the cubic BaTiO₃ (9.3 × 10^-6/°C), but no abnormal change in cell volume occurred around the dielectric peak temperature T_m (= 38 °C). The peak position of the ~835 cm^-1 Raman band induced by the substitution of La³⁺ ions for Ba sites in BL3TC5 was nearly independent of temperature on each side of T_m, whereas a slight fall (3 cm^-1) in the peak position around T_m was detectable. The link between the dielectric peak with DPT and the lattice change was discussed.

Abstract: Ba_0.55Sr_0.4Ca_0.05TiO₃-MgO (BSCTM) with Mn²⁺ additive composite ceramics were prepared by the conventional solid state method with BaCO₃, SrCO₃, CaCO₃, MgO and MnCO₃ as raw materials and B₂O₃ and Li₂CO₃ as sintering agent. The influence of the MnCO₃ additive on the sinterability and dielectric properties of BSCTM composite ceramics are investigated. The morphologies were analysized by scanning electron microscopy (SEM). The dielectric constant and dielectric loss of ceramics were measured by LCR meter at 1 kHz and 1 MHz. The temperature dependences of dielectric constant were measured by high-low temperature incubator tank and LCR meter at 1 MHz and a temperature range from-55 to 125 °C. The dielectric tunabilities were tested by C-T-V converter and LCR meter at 1 MHz and room temperature (25 °C). SEM results showed that BSCTM ceramics became more compact with the increase of Mn²⁺ content, because Mn²⁺ helped to form more oxygen vacancies for accelerating densification process. The addition of Mn²⁺ largely reduced the dielectric loss while had little effect on dielectric constant. The dielectric constant and dielectric loss of 0.5mol% Mn²⁺ doped BSCTM ceramics were 222 and 0.001, respectively. The dielectric tunability increased at first and then decreased with the increase of Mn²⁺ content. The tunability of 0.5mol% Mn²⁺ doped BSCTM ceramics were 4.04% under a DC electric field of 8.2 kV/cm.