Key Engineering Materials Vol. 633

Abstract: The synthetic quartz crystal is widely utilized in electronic and optical components due to its high frequency and temperature stability, good dielectric properties, low thermal coefficient of linear expansion, and wide range of optical transparency. However, radiation effects due to cosmic rays are responsible for a frequency shift for quartz oscillators in space systems, which impair their performance. Sweeping quartz at high electric field is a well-established method for improving device performance in a radiation environment. The present paper focuses on the voltage current characteristic of swept quartz. First, the sweeping conduction mechanism is ionic conduction. Second, as the voltage increases, the current increases first, then decreases, and then increases. ICP-AES results indicated that the sweeping process make Na+ a oriented locomotion. Third, the etch channel tensity is obviously reduced to less than 3/cm3, which is a very promising result for space applications. Moreover, the radiation resistance properties are improved after electrical sweeping.

Abstract: Electroless Ni–P coatings with different phosphorus contents have been developed to deposit on phenolic plastic in plating bath with different pH values. The effects of pH value on the phosphorus content and the microstructure were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The hardness of the coating was measured using Vicker’s indenter and the wear resistance was evaluated by a block-on-ring wear tester under dry condition. The results showed that with the increase of the pH value ranging from 4.4 to 5.4, the phosphorus content exhibited a decreasing trend from 11.01 wt.% to 7.76 wt.% . The structure of the electroless Ni–P coating transformed from amorphous phase, to a mixture of amorphous phases and nanocrystalline, and finally to nanocrystalline phase with the decrease of phosphorus content. The wear resistance of phenolic plastic was obviously enhanced by plating Ni–P coating. The hardness increased with the decrease of phosphorus content. However, the wear resistance did not always follow the hardness, indicating that not only the hardness but also other factors, such as internal stress and surface morphology, had an effect on the wear resistance of Ni–P coating.

Abstract: A novel silica aerogel insulation composite was prepared by using TiO₂ nanowires as modifier via sol-gel processing and supercritical drying methods in order to address issues of the poor mechanical and the infrared shading performance of silica aerogel. The effects of TiO₂ nanowire content on the mechanical and adiabatic performance were investigated by X-Ray diffraction (XRD), nitrogen adsorption (BET), field emission scanning electron microscopy (FESEM), static compression test and thermal analysis. The results show that TiO₂ nanowire reinforced aerogels had great thermal insulation properties, while high surface area, low density and mechanical strength were retained.

Abstract: The effect of LnAlO₃ (Ln = Nd, Sm) additives on the densification, phase evolution, microstructure and dielectric properties of BaO-Nd₂O₃-5TiO₂ (BNT5) ceramics was investigated. The LnAlO₃ (Ln = Nd, Sm) had greatly influence on the densification behavior of the BNT5 ceramics and the grain sizes of BNT5 ceramics could be effectively refined by LnAlO₃ additive. The microstructure of BNT5 ceramics was transformed from quasi-rectangular grain to columnar grain while the content of LnAlO₃ was increasing. The dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) were reduced with the increase of LnAlO₃ content, whereas the Q×f values showed a non-linear behaviour with LnAlO₃ content increasing. The BNT5 ceramics doped with 12.5 wt% SmAlO₃, sintered at 1320°C for 4 h, had the excellent dielectric properties of ε_r = 62.78, Q×f = 11108 GHz and τ_f = -1.55 ppm/°.

Abstract: Glass-forming ability in the P₂O₅-ZnO-SrO-K₂O system was investigated using differential thermal analyzer (DTA). The results showed that adding appropriate amount of K₂O could improve the glass-forming ability of phosphate glasses. Meanwhile, the spectroscopic properties of Tm/Tb/Mn co-doped phosphate glasses were studied by photoluminescence (PL) measurements and CIE color coordinate. Blue, green, and reddish orange emission bands were observed in the PL spectra of the phosphate glasses. The combination of these emission bands gave white light by ultraviolet light excitation (360nm). As the Tb₄O₇ was increased,a gradual shift from white emission to yellow emission was observed in the PL spectra of the glasses. These three ions-doped phosphate glasses are potential candidates for white light-emitting diodes.

Abstract: Corundum-based castables were prepared by using tabular corundum, micropowder of MgAl₂O₄ spinel and calcium aluminate cement as raw materials, as well as the addition of dispersion agent and retarder. The effect of spinel contents on physical properties of the castables has been investigated. The results showed that as the samples were heat treated at 1100 °C and 1600 °C, the linear shrinkage rate decreased and the strength of rupture and crushing at room temperature increased with spinel content increasing, while apparent porosity and bulk density remained the same. The hot modulus of rupture increased from 12 MPa to 20MPa with 67% growth rate, and crushing strength after thermal shock increased from 80MPa to 105MPa with 32% growth rate. The increasing of high-temperature properties was attributed to the staggered distribution between MgAl₂O₄ spinel and CA₆.

Abstract: The purpose of this study was to study the surface characteristics of the dental 3Y-TZP (3 mol% yttria stabilized tetragonal zirconia) ceramic treated by machining process and analysis what impact on the mechanical properties of the 3Y-TZP dental restorations. Dense core ceramic discs were made of 3Y-TZP. The samples were partially sintered to get part strength after uniaxial pressure moulding. The partially sintering green body could be treated by grinding, incising and milling and incising to simulate the CAD-CAM (computer aided design/computer aidedmanufacturing) process in dentistry. The partially sintering samples were then sintered to achieve full density. The dense sintered ceramics were also treated by grinding and incising to simulate the dental restorations adjusting process in dental clinic. The both surface of thespecimens were studied with an optical microscope and field emission SEM. Bulges were observed on the surface of the sample after uniaxial pressure moulding process. The defects due to partially sintered green body milling process were different from incising process in size and shape. Groves are observed on the surfaces of the samples after grinding process. The conclusions were that (i) machining of the final sintered dental 3Y-TZP ceramic brought more critical damage than the machining of partly-sintered green-bodies; (ii) the milling process ofthe partially sintered green-body brought less damage to the 3Y-TZP samples than the incising process.

Abstract: Laminated composite of TiB₂ based ceramic to Ti-6Al-V alloy in graded microstructure were achieved by combustion synthesis in ultra-high gravity field. XRD and FESEM results showed the presence of the graded microstructures at the joint which was characterized by multilevel (TiC_1-x-TiB-TiB₂ + TiB₂-Ti-TiC_1-x-TiB + TiB₂-TiC_1-x-TiB-Ti + TiB-TiC_1-x-Ti + TiC_1-x-Ti) and multiscale (micro-submicro-nano) of the size and the distribution of TiB₂ phases and TiB phases. It was considered that liquid fusion, atomic interdiffusion and a series of metallurgical reaction during solidification were reasoned for the continuously-graded microstructure from the ceramic to Ti alloy. Ballistic mass effectiveness coefficients of TiB₂ based ceramic and the laminated composite with TiB₂ based ceramic to Ti alloy were calculated to be 3.25 and 7.35 respectively by DOP test. The continuously-graded microstructures at the joint from the ceramic to Ti ally was considered to effectively reduce acoustic impedance mismatch between the ceramic and the metal, resulting in the enhanced ballistic performance of the target against the projectile by initiating intensive shear coupling effect.

Abstract: Bi_3.25Ho_0.75Ti_2.97V_0.03O₁₂ (BHTV) ceramics was prepared by solid state reaction. This sample had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well developed plate-like grains with random orientation. BHTV caused a large shift of the Curie temperature ( T_C ) of Bi₄Ti₃O₁₂ (BIT) from 675 °C to 398 °C. The remanent polarization and the coercive field of the BHTV ceramics were 28μC/cm² and 56kV/cm at an electric field of 90kV/cm, respectively. Furthermore, the dielectric permittivity and dissipation factor were 298 and 2.8×10^-3 at 1MHz, respectively. These ferroelectric properties of BHTV ceramics are superior to V-doped Bi₄Ti₃O₁₂ (~20μC/cm² and 80kV/cm) and (Sr, Ta)-doped Bi₄Ti₃O₁₂ (~12μC/cm² and 71kV/cm) ceramics. In addition, the dense ceramics of BHTV could be obtained by sintering at temperatures 100─200 °C lower than those of the SrBi₂Ta₂O₉ system.

Abstract: The electrical properties of Dy-doped bismuth titanate,Bi_4-xDy_xTi₃O₁₂ (BDT) ceramics prepared by a conventional electroceramic technique were investigated. XRD analyses revealed Bi-layered perovskite structure in all samples. SEM micrographs showed randomly oriented and plate-like morphology. For the samples with x=0.25 and 1.0 the current-voltage characteristics exhibited negative differential resistance behaviors and their P-V hysteresis loops were characterized by large leakage current, whereas for the samples with x=0.5 and 0.75 the current-voltage characteristics showed simple ohmic behaviors and their P-E hysteresis loops were the saturated and undistorted hysteresis loops. The remanent polarization ( P_r ) and coercive field (E_c) of the BDT ceramic with x=0.75 were above 16μC/cm² and 75KV/cm , respectively.