Surface energy plays an important role in the mechanical performance of nanomaterials; however, determining the surface energy density of curved surfaces remains a challenge. In this paper, we conduct atomic simulatio...Surface energy plays an important role in the mechanical performance of nanomaterials; however, determining the surface energy density of curved surfaces remains a challenge. In this paper, we conduct atomic simulations to calculate the surface energy density of spherical surfaces in various crystalline metals. It is found that the average surface energy density of spherical surfaces remains almost constant once its radius exceeds 5 nm. Then, using a geometrical analysis and the scaling law, we develop an analytical approach to estimate the surface energy density of spherical surfaces through that of planar surfaces. The theoretical prediction agrees well with the direct atomic simulations, and thus provides a simple and general method to calculate the surface energy density in crystals.展开更多
Wavelet theory is efficient as an adequate tool for analyzing single epoch GPS deformation signal. Wavelet analysis technique on gross error detection and recovery is advanced. Criteria of wavelet function choosing an...Wavelet theory is efficient as an adequate tool for analyzing single epoch GPS deformation signal. Wavelet analysis technique on gross error detection and recovery is advanced. Criteria of wavelet function choosing and Mallat decomposition levels decision are discussed. An effective deformation signal extracting method is proposed, that is wavelet noise reduction technique considering gross error recovery, which combines wavelet multi-resolution gross error detection results. Time position recognizing of gross errors and their repairing performance are realized. In the experiment, compactly supported orthogonal wavelet with short support block is more efficient than the longer one when discerning gross errors, which can obtain more finely analyses. And the shape of discerned gross error of short support wavelet is simpler than that of the longer one. Meanwhile, the time scale is easier to identify.展开更多
Nanoparticles in air are of particular concern for public health and employee exposure in work-places. Therefore, it is very important to prepare effective filters for their removal. In this work filters were prepared...Nanoparticles in air are of particular concern for public health and employee exposure in work-places. Therefore, it is very important to prepare effective filters for their removal. In this work filters were prepared from nanocellulose, i.e. cellulose nanofibrils (CNF). CNF was produced using two methods giving two different qualities of CNF. One quality had negative charges on the fibril surfaces while the other was neutral, and had in addition thinner fibrils compared to the other qualities. Filter samples were produced from water dispersions of CNF, by removal of the water by freeze drying. The performance of the CNF based filters was assessed and compared with filters based on synthetic polymer fibres. The ability to collect NaCl particles with a broad size distribution, ranging from nanometer to micrometer scale, was determined. CNF filters showed quality values comparable with the synthetic polymer based filters. Filters based on both the two CNF qualities had very good filtration efficiency for a given pressure drop across the filter.展开更多
Surface mechanical attrition treatment（SMAT） has been recently applied to bulk polycrystalline magnesium（Mg） alloys with gradient grain size distribution from the impact surface to inside matrix, hence effectively...Surface mechanical attrition treatment（SMAT） has been recently applied to bulk polycrystalline magnesium（Mg） alloys with gradient grain size distribution from the impact surface to inside matrix, hence effectively improving the alloys＇ mechanical performances. However, in-depth understanding of their mechanical property enhancement and grain size-dependent fracture mechanism remains unclear. Here,we demonstrated the use of in situ micro-tensile testing inside a high resolution scanning electron microscope（SEM） to characterize the microstructure evolution, in real time, of SMATed Mg alloy AZ31 samples with different grain sizes of ~10 μm（＇coarse-grain sample＇） and ~5 μm（＇fine-grain sample＇）, respectively, and compared the results with those of a raw Mg alloy AZ31. The quantitative tensile tests with in situ SEM imaging clearly showed that fracture of ‘fine-grain sample＇ was dominated by intergranular cracks,while both trans-granular and intergranular cracks led to the final failure of the ‘coarse-grain samples＇.It is expected that this in situ SEM characterization technique, coupled with quantitative tensile testing method, could be applicable for studying other grain-refined metals/alloys, allowing to optimize their mechanical performances by controlling the grain sizes and their gradient distribution.展开更多
As a wide-bandgap semiconductor, 4H-SiC is an ideal material for high-power and high-frequency devices, and plays an increasingly important role in developing our country’s future electric vehicles and 5G techniques....As a wide-bandgap semiconductor, 4H-SiC is an ideal material for high-power and high-frequency devices, and plays an increasingly important role in developing our country’s future electric vehicles and 5G techniques. Practical applications of SiCbased devices largely depend on their mechanical performance and reliability at the micro-and nanoscales. In this paper, singlecrystal [0001]-oriented 4H-SiC nanopillars with the diameter ranging from ~200 to 700 nm were microfabricated and then characterized by in situ nanomechanical testing under SEM/TEM at room temperature. Loading-unloading compression tests were performed, and large, fully reversible elastic strain up to ~6.2% was found in nanosized pillars. Brittle fracture still occurred when the max strain reached ~7%, with corresponding compressive strength above 30 GPa, while in situ TEM observation showed few dislocations activated during compression along the [0001] direction. Besides robust microelectromechanical system(MEMS), flexible device and nanocomposite applications, the obtained large elasticity in [0001]-oriented 4H-SiC nanopillars can offer a fertile opportunity to modulate their electron mobility and bandgap structure by nanomechanical straining,the so called "elastic strain engineering", for novel electronic and optoelectronic applications.展开更多
基金supported by the National Natural Science Foundation of China (Grants 11272249 and 11525209)
文摘Surface energy plays an important role in the mechanical performance of nanomaterials; however, determining the surface energy density of curved surfaces remains a challenge. In this paper, we conduct atomic simulations to calculate the surface energy density of spherical surfaces in various crystalline metals. It is found that the average surface energy density of spherical surfaces remains almost constant once its radius exceeds 5 nm. Then, using a geometrical analysis and the scaling law, we develop an analytical approach to estimate the surface energy density of spherical surfaces through that of planar surfaces. The theoretical prediction agrees well with the direct atomic simulations, and thus provides a simple and general method to calculate the surface energy density in crystals.
基金Supported by Specialized Research Fundfor the Doctoral Programof Higher Educationin China(No.20040290503) Science and Technology Fundationof CUMT(No.2005B020) .
文摘Wavelet theory is efficient as an adequate tool for analyzing single epoch GPS deformation signal. Wavelet analysis technique on gross error detection and recovery is advanced. Criteria of wavelet function choosing and Mallat decomposition levels decision are discussed. An effective deformation signal extracting method is proposed, that is wavelet noise reduction technique considering gross error recovery, which combines wavelet multi-resolution gross error detection results. Time position recognizing of gross errors and their repairing performance are realized. In the experiment, compactly supported orthogonal wavelet with short support block is more efficient than the longer one when discerning gross errors, which can obtain more finely analyses. And the shape of discerned gross error of short support wavelet is simpler than that of the longer one. Meanwhile, the time scale is easier to identify.
文摘Nanoparticles in air are of particular concern for public health and employee exposure in work-places. Therefore, it is very important to prepare effective filters for their removal. In this work filters were prepared from nanocellulose, i.e. cellulose nanofibrils (CNF). CNF was produced using two methods giving two different qualities of CNF. One quality had negative charges on the fibril surfaces while the other was neutral, and had in addition thinner fibrils compared to the other qualities. Filter samples were produced from water dispersions of CNF, by removal of the water by freeze drying. The performance of the CNF based filters was assessed and compared with filters based on synthetic polymer fibres. The ability to collect NaCl particles with a broad size distribution, ranging from nanometer to micrometer scale, was determined. CNF filters showed quality values comparable with the synthetic polymer based filters. Filters based on both the two CNF qualities had very good filtration efficiency for a given pressure drop across the filter.
基金supported by the National Key Basic Research Program (Grant No. 2012CB932203)the National Natural Science Foundation of China (Grant No. 51301147)+1 种基金the funding support from City University of Hong Kong (Grant Nos. 9610288 and 9680108)the funding support from the National Natural Science Foundation of China (Grant No. 51464234)
文摘Surface mechanical attrition treatment（SMAT） has been recently applied to bulk polycrystalline magnesium（Mg） alloys with gradient grain size distribution from the impact surface to inside matrix, hence effectively improving the alloys＇ mechanical performances. However, in-depth understanding of their mechanical property enhancement and grain size-dependent fracture mechanism remains unclear. Here,we demonstrated the use of in situ micro-tensile testing inside a high resolution scanning electron microscope（SEM） to characterize the microstructure evolution, in real time, of SMATed Mg alloy AZ31 samples with different grain sizes of ~10 μm（＇coarse-grain sample＇） and ~5 μm（＇fine-grain sample＇）, respectively, and compared the results with those of a raw Mg alloy AZ31. The quantitative tensile tests with in situ SEM imaging clearly showed that fracture of ‘fine-grain sample＇ was dominated by intergranular cracks,while both trans-granular and intergranular cracks led to the final failure of the ‘coarse-grain samples＇.It is expected that this in situ SEM characterization technique, coupled with quantitative tensile testing method, could be applicable for studying other grain-refined metals/alloys, allowing to optimize their mechanical performances by controlling the grain sizes and their gradient distribution.
基金supported by Hong Kong Research Grant Council (RGC)(Grant No. U11207416)City University of Hong Kong (Grant No.7005234)National Natural Science Foundation of China under the Excellent Young Scientists Fund (Grant No. 11922215)。
文摘As a wide-bandgap semiconductor, 4H-SiC is an ideal material for high-power and high-frequency devices, and plays an increasingly important role in developing our country’s future electric vehicles and 5G techniques. Practical applications of SiCbased devices largely depend on their mechanical performance and reliability at the micro-and nanoscales. In this paper, singlecrystal [0001]-oriented 4H-SiC nanopillars with the diameter ranging from ~200 to 700 nm were microfabricated and then characterized by in situ nanomechanical testing under SEM/TEM at room temperature. Loading-unloading compression tests were performed, and large, fully reversible elastic strain up to ~6.2% was found in nanosized pillars. Brittle fracture still occurred when the max strain reached ~7%, with corresponding compressive strength above 30 GPa, while in situ TEM observation showed few dislocations activated during compression along the [0001] direction. Besides robust microelectromechanical system(MEMS), flexible device and nanocomposite applications, the obtained large elasticity in [0001]-oriented 4H-SiC nanopillars can offer a fertile opportunity to modulate their electron mobility and bandgap structure by nanomechanical straining,the so called "elastic strain engineering", for novel electronic and optoelectronic applications.
