Papers by Author: Yu Zhou

Abstract: Bi4-xLaxTi3O12 (BLT, x=0, 0.75, 1, 2) ceramic powders were prepared with sol-gel and solid phase reaction processes, respectively. BLT powders were synthesized after decarbonization at 300°C for 1h followed by calcination at 800°C for 2h. The ceramics synthesized with cold isostatic pressing and sintering at 1150°C exhibit a perovskite-like structure. The influence of sintering parameters and La doping content on density, microstructure and dielectric properties of the BLT were discussed. With the increase of La doping content in Bi4-xLaxTi3O12, dielectric constant increases and dielectric loss gradually decreases. The dielectric constant and dielectric loss of Bi4-xLaxTi3O12 (x=2) are 132.7 and 0.012 at 100 KHz, respectively.

Abstract: The microstructure and ferroelectric domains of SrBi2Ta1.6Nb0.4O9 ceramics were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). The X-ray diffraction patterns show that the lattice constants a and b decrease, and c increases by doping with Nb into SBT sample. Accordingly, it has large strain and lattice distortion in the lattice This suggests that the Nb atoms partially occupy the location of the Ta atoms in the lattice. From TEM observations, the grains show (008) preferred orientations in the sample, which agrees well with the XRD results. The 90° domain walls are identified by the 90° rotation relationship of the electron diffraction pattern about the [001] zone axis. The 180° domain walls and anti-phase boundaries (APBs) in Nb-doped SBT ceramics are also observed, which are irregularly shaped and highly curved. The traditional α-fringes can be found in the Nb-doped SBT ceramics, which are the evidence of large strains in the lattice.

Abstract: Hydroxyapatite coating was prepared by alternate soaking strategy in short period time and ambient condition. According to the pH near equality between chitosan acidic acid solution and H2PO4 - aqueous solution, we proposed to choose H2PO4 - as one of hydroxyapatite precursor instead of traditional candidate (HPO4 2-). The properties of chitosan hydrogel containing plenty of water provide enough spaces for hydroxyapatite precursor to diffuse into framework spontaneously. XRD, FTIR and SEM were used to characterize the component and microstructure of mineralized chitosan. The ageing process helps to transfer amorphous calcium phosphate in chitosan framework into hydroxyapatite. The hydroxyapatite coating is gradient structure according to the result of SEM.

Abstract: Calcium phosphate phases with laminar-plate structure were converted from calcite powders after soaking in phosphate buffer solutions of pH’s 6.0-8.0 at 37 °C for 9 days. The effect of pH values on the conversion of calcite crystals was investigated by X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy. If the pH value of a buffer solution is kept at 6.0, calcite powders are converted mainly to dicalcium phosphate dehydrate (DCPD) or octacalcium phosphate (OCP). If the pH value is kept at 6.4 or 7.0, calcite powders are converted mainly to OCP. Hydroxyapatite (HAP) with poorly crystalline can be obtained from calcite powders both by treatment of a basic buffer solution, and by treatment of an acid buffer solution without regulating its pH value during the reaction. The conversion mechanism of calcite crystals is a dissolution-precipitation reaction.

Abstract: In this research, SrBi2+xTa2O9 (SBT) powders with different Bi content were prepared using conventional solid-state reaction method. The SrBi2+xTa2O9 ceramics (x=0 ,0.2 ,0.4 ,0.6) were fabricated by the cold isostatic compaction and sintering in the air. Microstructure and dielectric, ferroelectric properties of SBT ceramics were investigated by XRD, SEM, EDAX and dielectric tester. The effects of Bi content on dielectric, ferroelectrc properties in SBT ceramics were discussed.

Abstract: ZrO2(3Y) ceramics were sintered by vacuum hot-pressing and impacted by split Hopkinson method. The microstructure of the materials was studied by X-ray diffraction and scanning electron microscopy. The results show that the phase transformation from tetragonal phase to monocline phase occurred in the ceramics during dynamic loading process, and the transformation rate is about 30%. From the results of scanning electron microscopy, it can be found that the microcracks formed in the materials under the dynamic loading, and there is a kind of liked-amorphous layer appeared on the fragment surface.

Abstract: Ytterbia and Lutecia were adopted to stabilize α-sialon ceramics during a two-step hot press sintering, respectively. Although Yb3+ and Lu3+ possess the similar small ion size (0.86 Å and 0.85 Å) as two adjacent rare-earth elements, Yb- and Lu-α-sialon ceramics present very different microstructure and properties. Yb-α-sialon ceramic is fully composed of α-sialon phase without any secondary phase. The ceramic incorporated with Lu2O3 also consists of α-sialon phase, but a trace amount of intergranular phase J′ (Lu4Si2-xAlxO7+xN2-x) still remains at the boundary. Microstructure observation indicates the fully equiaxial grain morphology of Yb-α-sialon ceramic. However, elongated grains appear in the Lu-α-sialon ceramic, coexisting with small equiaxed grains. Owing to the almost full α-sialon phase assemblage, both ceramics present high hardness with the value of over 21 GPa. Lu-α-sialon ceramic possesses good toughness, which is attributed to the elongated grains assisting the facilitation of toughening mechanism. The equiaxied grain morphology results in a comparatively lower toughness for Yb-α-sialon ceramic.

Abstract: Amorphous Si-B-C-N ceramics obtained by high energy ball milling and hot pressing using hexagonal boron nitride (h-BN), graphite (C) and amorphous Si as starting materials have been studied. The mechanical milling with high energy resulted in the generation of large amounts of amorphous composites only milled for 5 h. Si-B-C-N powders were consolidation by hot pressing at 1850 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that small amount of BN and SiC crystal lies in the amorphous matrix. The flexural strength reached the maximal value of 137.2 MPa at a mole ratio of BN/(Si+C) being 0.6.

Abstract: The Al2O3/LiTaO3 (ALT) composite ceramics were fabricated by hot pressing method and polarization treated at different temperatures along height and width directions. Effects of polarization treatment on mechanical properties of the ALT composite ceramic were investigated. Flexural strength decreased with the increase of polarization temperature. Meanwhile, the flexural strength of samples polarized in height direction is higher than that polarized in width direction. The composite ceramic fractures intragranularly, and many rupture steps in polarization direction were observed on fractographs of the composite ceramics. Domain switching in LiTaO3 particles increased the resistance of crack propagation and improved the mechanical properties of the polarized ALT composite ceramics.

Abstract: For the purpose of studying material cluster design and shape design of a certain arc-shaped thermal-protection component rationally, the ablation behavior and thermal stress distribution are studied by using the method of finite element numerical simulation. The study includes ablation tests, numerical simulation of temperature field, calculation of ablation thickness and numerical simulation of unsteady thermal stress field of the component. The simulation results are consistent with the results of ablation tests, which shows that the shape design of the arc-shaped thermal-protection component is rational and the dangerous periods of the component ablation are the time of initial heating and initial ablation boundary retreat.