Papers by Author: Ping Hu

Abstract: With honeycomb lattice of sp² hybridized carbon atoms, graphene has demonstrated excellent electrical and mechanical properties. One of its promising applications is to fabricate graphene-ceramic composite to improve the mechanical properties. In order to quantify the strength between graphene-ZrB₂ interactions, molecular dynamic method was utilized to simulate typical interface of graphene/ZrB₂ ceramic structure. Berendsen method was used to control the temperature and pressure during the whole simulation process. Universal potential function was employed to simulate the force filed between graphene and ZrB₂ structure. The binding structures of graphene/ZrB₂ (0001) interface were analyzed in detail and the bonding energy of the interface was calculated. The influence of numbers of graphene layer and sandwich structures on the bonding energy of interface is discussed. The study helped to understand the influence of graphene on mechanical properties of ZrB₂ ceramic.

Abstract: Laminated ZrB₂-SiC-C_g ceramics were successfully prepared by tape casting - dipping and tape casting - stacking respectively. The effect of different molding process on the mechanical properties of materials was investigated. The microstructure and fracture behavior were characterized as well. The flexural strength, fracture toughness and fracture work of laminated ZrB₂-SiC-C_g ceramics prepared by tape casting-stacking were 427 MPa, 11.3 MPa·m^1/2, 415 J/m², respectively. The thickness of ZrB₂-SiC layers was about 250 μm, and the thickness of graphite layers was about 25 μm. As a comparison, Laminated ZrB₂-SiC-C_g ceramics prepared by tape casting - dipping showed non-uniform thickness of the ZrB₂-SiC layers and graphite layers. And the flexural strength, fracture toughness and fracture work of laminated ZrB₂-SiC-C_g ceramics prepared by tape casting-dipping were 252 MPa, 5.7 MPa·m^1/2, 104 J/m², respectively. The improvement on the mechanical properties of laminated ZrB₂-SiC-C_g ceramics prepared by tape casting - stacking was attributed to the uniform graphite layer and ZrB₂-SiC layer and the reducing amount of graphite infiltrated into ZrB₂-SiC matrix layer.

Abstract: In this paper, the dynamical behaviors of the centrifugal flywheel governor with external disturbance is studied and it has abundant nonlinear behavior.The influence of system parameter is discussed by Lyapunov exponents spectrum and global bifurcation diagram, which accurately portray the partial dynamic behavior of the centrifugal flywheel governor. The routes to chaos are analyzed using Poincaré sections, which are found to be more complex . Periodic and chaotic motions can be clearly distinguished by Poincaré sections, bifurcation diagrams and Lyapunov exponents. Then, the paper proposes coupledfeedback control and adaptive control methods to achieve the chaotic the centrifugal flywheel governor system synchronization, the numerical simulation was provided in order to show the effectiveness of coupled feedback control and adaptive control methods for the synchronization of the chaotic nonautonomous centrifugal flywheel governor system.

Abstract: This paper presented the application of microwave-discharge plasma apparatus which was used to evaluate the catalytic properties of ZrB₂-based ultra-high temperature ceramics in the simulated real service environment by the wall temperature response method based on the heat balance principle. The results showed that the material composition had a significant influence on the catalytic properties of ZrB₂-based ultra-high temperature ceramics, and the catalytic activity of ZrB₂-SiC composites with Cr addition had been increased significantly. The relationship between catalytic properties of ZrB₂-based ultra-high temperature ceramics and surface composition was discussed in detail. The composition optimization was considered to be a very effective way to inhibit the recombination reactions of dissociated atoms on the surface of ultra-high temperature ceramics.

Abstract: The dispersion of SiC nanopowders in ethanol solution was studied by sedimentation test, particle size measurement and TEM analysis. The dispersion behavior of SiC nanopowders in ethanol solution was strongly dependent on the pH values, types and amounts of dispersant. PEI was found to be effective for the dispersion of SiC nanopowders in ethanol solution. With the addition of PEI, the isoelectric points of SiC nanopowders in ethanol solution were at pH 9.5, and shift to pH 12.3. The stability of SiC suspension increased with the dispersant content increasing until reached 2.5 wt% PEI. The suitable pH value for the dispersion of SiC nanopowders should lower than 10.

Abstract: Ablation behavior of ZrB₂-SiC sharp leading edges with five different curvature radiuses was investigated using an oxy-acetylene torch. During the test, the curvature radiuses were 0.15 mm, 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm, respectively. Under the same ablation condition, the smaller was the radius, the severer ablation underwent. The sharp leading edge with a curvature radius of 0.15 mm had the highest surface temperature and maximum surface temperature rising rate, exceeded 2100°C in less than 30 s. However, the surface of sharp leading edge with a curvature radius of 2.0 mm achieved only 1900°C in more than 60 s. After 5 min ablation, the mass and linear ablation rates were measured. All the five sharp leading edges evolved to nearly a same radius after ablation. The microstructure of the oxidation layers was also investigated. A ZrO₂-SiO₂ layer generated from oxidation of ZrB₂-SiC acts as a thermal barrier and reduces the diffusion of oxygen.

Abstract: The influence of hot pressing temperature and SiC content on the microstructure and mechanical properties of ZrB2-SiC ceramics was investigated. ZrB2 containing 20 volume percent SiC were prepared by hot pressing at 1850, 1900 and 1950°C for 60 min. Fully dense ceramic was obtained after hot pressing at temperature of 1950°C. In addition, the materials with SiC content of 0, 10vol.%, 15 vol.%, 20 vol.% and 30 vol.% hot pressed at 1950°C were also investigated. Results showed that the grain size of the ZrB2 significantly reduced on adding 10vol.% SiC and then decreased slightly with further increasing SiC content, whereas the grain size of SiC exhibited a opposite trend. The flexural strength of ZrB2-SiC ceramics remarkedly increased on adding 10vol.% SiC due to the significant decrease of ZrB2 particle size and then slightly increased with increasing SiC content up to 20vol.%. However, further increasing SiC content led to a reduction of the flexural strength.

Abstract: SiC whisker-reinforced ZrB2 matrix ultra-high temperature ceramic were prepared at 2000°C for 1 h under 30MPa by hot pressing and the effects of whisker on flexural strength and fracture toughness of the composites was examined. The flexural strength and fracture toughness are 510±25MPa and 4.05±0.20MPa⋅m1/2 at room temperature, respectively. Comparing with the SiC particles-reinforced ZrB2 ceramic, no significant increase in both strength and toughness was observed. The microstructure of the composite showed that the SiC whisker was destroyed because the SiC whisker degraded due to rapid atom diffusivity at high temperature. The results suggested that some related parameters such as the lower hot-pressing temperature, a short sintering time should be controlled in order to obtain SiC whiskerreinforced ZrB2 composite with high properties.

Abstract: The present study focuses on the dynamic oxidation resistance of five representative ZrB2-SiC based ultra-high temperature ceramics (UHTCs): ZrB2-SiC, ZrB2-SiC-Si3N4, ZrB2-SiC-TiB2, ZrB2-SiCHfB2 and ZrB2-SiC-ZrC using oxyacetylene torch and arc jet testing. The effects of second phase incorporation (Si3N4, TiB2, HfB2, ZrC) on oxidation resistance were compared and analyzed. The mechanism of oxidation based on experimental results and thermodynamic calculations were explored. Some approaches to improvement of oxidation resistance and future directions of UHTC are also presented.

Abstract: Two ZrB2-based composites were fabricated by hot-pressing with vacuum. ZrB2+20vol%SiC and ZrB2+20vol%SiC+8vol%TiB2 were selected as the starting compositions. The microstructures and phase constitutions of the composites were investigated and compared to these of monolithic ZrB2 material. For the ZrB2-based composites, with the addition of SiC particles, the excessive growth of ZrB2 materials is restricted and grain structure is refined. Meanwhile the fracture modes are changed, namely, from transgranular to mixed inter/transgranular. Strengthening mechanism is grain refining and the toughening mechanism is crack deflection, crack branching and grain refining. The role of TiB2 as an addition to the ZrB2 matrix was also examined and discussed.