The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Juncheng Liu - One of the best experts on this subject based on the ideXlab platform.
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Microstructure analysis of Al2O3/Er3Al5O12/ZrO2 DSECs prepared using high-Frequency Zone melting method
Journal of Alloys and Compounds, 2020Co-Authors: Shunheng Wang, Juncheng LiuAbstract:Abstract The microstructure of directionally solidified eutectic ceramics (DSECs) is mainly determined by the eutectic growth mode, properties of each phase, and solidification processing parameters. The Al2O3/Er3Al5O12(EAG)/ZrO2 DSECs were prepared using high-Frequency induction melting, and their phase components, and microstructures were investigated. The results indicated that the samples were composed only of Al2O3, EAG, and ZrO2 phases. As an solidification rate increased from 5 to 500 mm/h, the microstructure was refined, and the distribution of the eutectic phases changed. This could be explained by the relationship between the undercooling, the growth rate and the melting entropy. The regular microstructure appeared in the samples prepared at low growth rate and it exhibited the form of triangles or squares. A polygonal colony structure was found in the DSECs obtained at 50 mm/h, which was related to the non-uniformed distribution of temperature fields near the solid-liquid interface. Furthermore, both of the internal microcrack and amorphous structure were identified in the rapidly solidified samples prepared at 500 mm/h.
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Microstructure and mechanical properties of Al2O3/Er3Al5O12/ZrO2 prepared by a high-Frequency Zone melting method
International Journal of Refractory Metals and Hard Materials, 2019Co-Authors: Shunheng Wang, Juncheng Liu, Shuoyan ZhaiAbstract:Abstract Oxide directionally solidified eutectic ceramics (DSECs) have excellent mechanical properties, oxidation resistance, and ablation resistance in an ultra-high temperature environment above 1600 °C. In this study, Al2O3/Er3Al5O12/ZrO2 ternary DSECs were prepared by high-Frequency induction Zone melting. Their diameters were considerably greater than those of samples prepared with other processing techniques under the conditions where the microstructures were uniform. The component, microstructure, and mechanical properties in these samples were investigated. The results indicate that these DSECs were composed of only Al2O3, Er3Al5O12, and ZrO2 phases. As the solidification rate increased, both the eutectic phase size and eutectic spacing decreased continuously, and the microstructure was refined. The relationship between the eutectic spacing and solidification rate satisfied the formula λ2ν ≈ 33.8 ± 0.49. A polygonal colony structure appeared at a growth rate of 6.7 μm/s, which was associated with the uneven distribution of the temperature field. The hardness and indentation fracture resistance increased marginally with an increase in solidification rate, achieving 17.6 ± 0.8 GPa and 5.0 ± 0.5 MPa·m1/2, respectively. The hardness of the Al2O3/Er3Al5O12/ZrO2 ternary DSEC was 18.3% less than that of the Al2O3/Er3Al5O12 binary DSEC owing to the reduced hardness of the ZrO2 phase added in the eutectic composition. The ternary DSEC's indentation fracture resistance was 88.4% greater than that of the binary eutectic ceramic owing to the phase transformation toughening of the ZrO2 phase.
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Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-Frequency Zone melting
Ceramics International, 2019Co-Authors: Shunheng Wang, Zongfu Chu, Juncheng LiuAbstract:Abstract Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600 °C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al2O3/GdAlO3 eutectic ceramic was prepared with horizontal high-Frequency induction Zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al2O3/GdAlO3 eutectic ceramic was composed only of the Al2O3 phase and GdAlO3 phase penetrating mutually, and the Al2O3 phase was the substrate in which the GdAlO3 phase was embedded. As the solidification rate increased from 1 to 5 mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5 μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36 GPa and 3.12 MPa•m1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-Frequency induction Zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO3 phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.
Shunheng Wang - One of the best experts on this subject based on the ideXlab platform.
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Microstructure analysis of Al2O3/Er3Al5O12/ZrO2 DSECs prepared using high-Frequency Zone melting method
Journal of Alloys and Compounds, 2020Co-Authors: Shunheng Wang, Juncheng LiuAbstract:Abstract The microstructure of directionally solidified eutectic ceramics (DSECs) is mainly determined by the eutectic growth mode, properties of each phase, and solidification processing parameters. The Al2O3/Er3Al5O12(EAG)/ZrO2 DSECs were prepared using high-Frequency induction melting, and their phase components, and microstructures were investigated. The results indicated that the samples were composed only of Al2O3, EAG, and ZrO2 phases. As an solidification rate increased from 5 to 500 mm/h, the microstructure was refined, and the distribution of the eutectic phases changed. This could be explained by the relationship between the undercooling, the growth rate and the melting entropy. The regular microstructure appeared in the samples prepared at low growth rate and it exhibited the form of triangles or squares. A polygonal colony structure was found in the DSECs obtained at 50 mm/h, which was related to the non-uniformed distribution of temperature fields near the solid-liquid interface. Furthermore, both of the internal microcrack and amorphous structure were identified in the rapidly solidified samples prepared at 500 mm/h.
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Microstructure and mechanical properties of Al2O3/Er3Al5O12/ZrO2 prepared by a high-Frequency Zone melting method
International Journal of Refractory Metals and Hard Materials, 2019Co-Authors: Shunheng Wang, Juncheng Liu, Shuoyan ZhaiAbstract:Abstract Oxide directionally solidified eutectic ceramics (DSECs) have excellent mechanical properties, oxidation resistance, and ablation resistance in an ultra-high temperature environment above 1600 °C. In this study, Al2O3/Er3Al5O12/ZrO2 ternary DSECs were prepared by high-Frequency induction Zone melting. Their diameters were considerably greater than those of samples prepared with other processing techniques under the conditions where the microstructures were uniform. The component, microstructure, and mechanical properties in these samples were investigated. The results indicate that these DSECs were composed of only Al2O3, Er3Al5O12, and ZrO2 phases. As the solidification rate increased, both the eutectic phase size and eutectic spacing decreased continuously, and the microstructure was refined. The relationship between the eutectic spacing and solidification rate satisfied the formula λ2ν ≈ 33.8 ± 0.49. A polygonal colony structure appeared at a growth rate of 6.7 μm/s, which was associated with the uneven distribution of the temperature field. The hardness and indentation fracture resistance increased marginally with an increase in solidification rate, achieving 17.6 ± 0.8 GPa and 5.0 ± 0.5 MPa·m1/2, respectively. The hardness of the Al2O3/Er3Al5O12/ZrO2 ternary DSEC was 18.3% less than that of the Al2O3/Er3Al5O12 binary DSEC owing to the reduced hardness of the ZrO2 phase added in the eutectic composition. The ternary DSEC's indentation fracture resistance was 88.4% greater than that of the binary eutectic ceramic owing to the phase transformation toughening of the ZrO2 phase.
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Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-Frequency Zone melting
Ceramics International, 2019Co-Authors: Shunheng Wang, Zongfu Chu, Juncheng LiuAbstract:Abstract Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600 °C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al2O3/GdAlO3 eutectic ceramic was prepared with horizontal high-Frequency induction Zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al2O3/GdAlO3 eutectic ceramic was composed only of the Al2O3 phase and GdAlO3 phase penetrating mutually, and the Al2O3 phase was the substrate in which the GdAlO3 phase was embedded. As the solidification rate increased from 1 to 5 mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5 μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36 GPa and 3.12 MPa•m1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-Frequency induction Zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO3 phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.
Davood Toghraie - One of the best experts on this subject based on the ideXlab platform.
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a comparison of the bolotin and incremental harmonic balance methods in the dynamic stability analysis of an euler bernoulli nanobeam based on the nonlocal strain gradient theory and surface effects
Mechanics of Materials, 2020Co-Authors: Pezhman Sourani, Mohammad Hashemian, Mostafa Pirmoradian, Davood ToghraieAbstract:Abstract This study addresses the dynamic stability of an Euler–Bernoulli nanobeam under time-dependent axial loading based on the nonlocal strain gradient theory (NSGT) and considering the surface stress effects. The studied nanobeam cross-section was rectangular, and simply-supported boundary conditions were assumed. Moreover, a uniform thermal gradient was applied to the nanobeam. The elastic medium was modeled based on the Pasternak theory. The strain–displacement relations were derived using the Von Karman equations. The governing equations were obtained by the energy method and applying the Hamilton's principle. Furthermore, the Bolotin and Incremental Harmonic Balance (IHB) methods were used to solve the differential equations. This study investigates the impact of such parameters as the small-scale parameter, the material length scale, surface effects, elastic medium parameters, temperature variations, geometry, and the static loading factor on the Dynamic Instability Region (DIR). The results are suggestive of the shift of the DIR to lower Frequency Zone by increasing the small-scale Eringen's nonlocal theory parameter, whereas an increase in the material length scale from the strain gradient theory moves the region to higher frequencies. In case the said parameters are equal, the result conforms to the classical beam theory. In addition, assuming a Pasternak medium and taking into account the effects of surface stress (Young's modulus and the residual stress of the surface) shifts the DIR to higher frequencies, whereas applying a compressive static load moves the region to lower frequencies. Moreover, depending on the thermal expansion coefficient of the medium, temperature variations can also displace the DIR.
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Dynamic stability of functionally graded nanobeam based on nonlocal Timoshenko theory considering surface effects
Physica B: Condensed Matter, 2017Co-Authors: Shahab Saffari, Mohammad Hashemian, Davood ToghraieAbstract:Abstract Based on nonlocal Timoshenko beam theory, dynamic stability of functionally graded (FG) nanobeam under axial and thermal loading was investigated. Surface stress effects were implemented according to Gurtin-Murdoch continuum theory. Using power law distribution for FGM and von Karman geometric nonlinearity, governing equations were derived based on Hamilton's principle. The developed nonlocal models have the capability of interpreting small scale effects. Pasternak elastic medium was employed to represent the interaction of the FG nanobeam and the surrounding elastic medium. A parametric study was conducted to focus influences of the static load factor, temperature change, gradient index, nonlocal parameter, slenderness ratio, surface effect and springs constants of the elastic medium on the dynamic instability region (DIR) of the FG beam with simply-supported boundary conditions. It was found that differences between DIRs predicted by local and nonlocal beam theories are significant for beams with lower aspect ratio. Moreover, it was observed that in contrast to high temperature environments, at low temperatures, increasing the temperature change moves the origin of the DIR to higher excitation Frequency Zone and leads to further stability. Considering surface stress effects shifts the DIR of FG beam to higher Frequency Zone, also increasing the gradient index enhances the Frequency of DIR.
Shuoyan Zhai - One of the best experts on this subject based on the ideXlab platform.
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Microstructure and mechanical properties of Al2O3/Er3Al5O12/ZrO2 prepared by a high-Frequency Zone melting method
International Journal of Refractory Metals and Hard Materials, 2019Co-Authors: Shunheng Wang, Juncheng Liu, Shuoyan ZhaiAbstract:Abstract Oxide directionally solidified eutectic ceramics (DSECs) have excellent mechanical properties, oxidation resistance, and ablation resistance in an ultra-high temperature environment above 1600 °C. In this study, Al2O3/Er3Al5O12/ZrO2 ternary DSECs were prepared by high-Frequency induction Zone melting. Their diameters were considerably greater than those of samples prepared with other processing techniques under the conditions where the microstructures were uniform. The component, microstructure, and mechanical properties in these samples were investigated. The results indicate that these DSECs were composed of only Al2O3, Er3Al5O12, and ZrO2 phases. As the solidification rate increased, both the eutectic phase size and eutectic spacing decreased continuously, and the microstructure was refined. The relationship between the eutectic spacing and solidification rate satisfied the formula λ2ν ≈ 33.8 ± 0.49. A polygonal colony structure appeared at a growth rate of 6.7 μm/s, which was associated with the uneven distribution of the temperature field. The hardness and indentation fracture resistance increased marginally with an increase in solidification rate, achieving 17.6 ± 0.8 GPa and 5.0 ± 0.5 MPa·m1/2, respectively. The hardness of the Al2O3/Er3Al5O12/ZrO2 ternary DSEC was 18.3% less than that of the Al2O3/Er3Al5O12 binary DSEC owing to the reduced hardness of the ZrO2 phase added in the eutectic composition. The ternary DSEC's indentation fracture resistance was 88.4% greater than that of the binary eutectic ceramic owing to the phase transformation toughening of the ZrO2 phase.
G. P. Srivastava - One of the best experts on this subject based on the ideXlab platform.
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The intrinsic lifetime of low-Frequency Zone-centre phonon modes in silicon nanowires and carbon nanotubes
Applied Surface Science, 2006Co-Authors: S. P. Hepplestone, G. P. SrivastavaAbstract:Abstract Estimates of the intrinsic lifetime of low-Frequency Zone-centre phonon modes in silicon nanowires and carbon nanotubes have been presented from the application of Fermi’s golden rule formula based upon an elastic continuum model for cubic anharmonicity. In particular, results have been presented for the lowest non-zero mode in both nanostructures, and also the breathing mode in the nanotube. Except for the ultrathin nanowire, the lifetime increases with size and decreases with an increase in temperature. Typically, these modes have a lifetime of the order of nanoseconds, almost a thousand times larger than the lifetimes of optical phonon modes in the corresponding bulk materials. Also, at room temperature the lifetime of the lowest non-zero mode is nearly an order of magnitude larger in the (20,20) nanotube than in the nanowire of similar thickness (width 2.2 nm).
