The Young's modulus, shear modulus and Poisson's ratio of monolayer arsenene with different sizes were calculated by finite element method, so as to explore the influence of dimension and orientation on the me...The Young's modulus, shear modulus and Poisson's ratio of monolayer arsenene with different sizes were calculated by finite element method, so as to explore the influence of dimension and orientation on the mechanical properties of monolayer arsenene. The calculation results show that the small size has a significant effect on the mechanical properties of the monolayer arsenene. The smaller the size, the larger the Young's modulus and Poisson's ratio of the monolayer arsenene. The size change has a great influence on the Young's modulus of the arsenene handrail direction, and the Young's modulus of the zigzag direction is not sensitive to the size change. Similarly, the size change has a significant effect on the shear modulus of arsenene in the handrail direction, while the shear modulus in the zigzag direction has no significant effect on its size change. For the Poisson's ratio, the situation is just the opposite, and the effect of the size change on the Poisson's ratio of the arsenene zigzag direction is greater than that of the handrail direction.展开更多
Catalytic epoxidation of alkenes is an important type of organic reaction in chemical industry,and the deep insight into catalyst deactivation will help to develop new epoxidation process.In this work,series of quater...Catalytic epoxidation of alkenes is an important type of organic reaction in chemical industry,and the deep insight into catalyst deactivation will help to develop new epoxidation process.In this work,series of quaternary ammoniums bearing different cationic sizes,i.e.MTOA+(methyltrioctylammonium,[(C_(8)H_(17))_(3)CH_(3)N]+),HTMA+(hexadecyltrimethylammonium,[(C_(16)H_(33))(CH_(3))_(3)N]+) and DMDOA+(dimethyldioctadecylammonium,[(C_(18)H_(37))_(2)(CH_(3))_(2)N]+) were incorporated with polyoxometalate (POM) anions to prepare phase transfer catalysts (PTCs),which were used in the styrene epoxidations.Among them,(MTOA)_(3)PW_(4)O_(24)exhibits the best catalytic performance judged from the highest styrene conversion rate(52%) and styrene oxide selectivity (93%),during which the styrene epoxidation conditions were optimized.Meanwhile,the deactivation mechanism of this kind of PTCs was proposed firstly,i.e.in the case of low H_(2)O_(2) content,the oxidant can only be used in the styrene epoxidation,in which the catalyst can transform into stable Keggin-type POM.But when the content of H_(2)O_(2) is higher,the excess H_(2)O_(2) can reactivate the Keggin-type POM into active (PW_(4)O_(24))_(3)-anions,which can trigger the ring-opening polymerization of styrene oxide.Consequently,the catalyst is deactivated by adhered poly(styrene oxide)irreversibly,which was determined by NMR spectra.In this situation,the active moiety{PO_(4)[WO(O_(2))_(2)]_(4)}_(3)-in phase-transfer catalytic system can break into some unidentified species with low W/P ratio with the presence of epoxides.This work will be beneficial for the design of new PTCs in alkene epoxidation in fine chemical industry.展开更多
A novel process for synthesizing nano-ceramics powders, named mechanical & therm al activation processing, is discussed in the present paper. It is a processing based on thermal activation in liquid phase (molten ...A novel process for synthesizing nano-ceramics powders, named mechanical & therm al activation processing, is discussed in the present paper. It is a processing based on thermal activation in liquid phase (molten salt) after mechanical activ ation. The nanometer-sized TiC particles (15-20nm) have been synthesized by the method, and analyzed by X-ray diffraction (XRD), transmission electron microscop e (TEM), scanning electron microscopy (SEM) and energy dispersion X-ray (EDX) sp ectroscopy. An interface interaction between liquid (molten salt) and solid (fin al product particles) phases plays a dominating role for the control of product particles size. The mechanism for the formation of nanometer-sized TiC particles has been discussed.展开更多
Low-basis-weight paper lacks normal strength and stiffness. Waterborne epoxy resin could be employed with oxidized starch to improve paper stiffness through surface sizing. In this study,the mechanism of enhancing sti...Low-basis-weight paper lacks normal strength and stiffness. Waterborne epoxy resin could be employed with oxidized starch to improve paper stiffness through surface sizing. In this study,the mechanism of enhancing stiffness by adding waterborne epoxy resin was fully investigated. The results indicated that through surface sizing with epoxy resin,the paper thickness was preserved,whereas the elastic modulus increased significantly and the epoxy resin had positive impact on single fiber strength. A rigid resin layer and interpenetrating polymer network formed on the surface and in the inner layer of the paper,respectively. The formed resin layer and interpenetrating polymer network strongly supported the paper,leading to the improvement of the elastic modulus and stiffness. The stiffness improvement through surface sizing was mainly due to the formation of a fibrous composite layer and penetration of the sizing agent into the inner layers of the paper. The better the combination between fiber and sizing agent,the higher were the elastic modulus and the stiffness of the whole paper.展开更多
Naturalfibre as a reinforcing agent has been widely used in many industrial applications.Nevertheless,several factors need to be considered,such as the size and weight percentage of thefibre used in binding.Using fused ...Naturalfibre as a reinforcing agent has been widely used in many industrial applications.Nevertheless,several factors need to be considered,such as the size and weight percentage of thefibre used in binding.Using fused deposition modelling(FDM),this factor was investigated by varying the size of naturalfibre as the responding variable with afixed weight percentage of kenaffibre.The process of modifying the naturalfibre in terms of size might increase the dispersion of kenaffibre in the polymer matrix and increase the adhesion bonding between thefibre and matrix of composites,subsequently improving the interfacial bonding between these two phases.In this paper,the effect offibre size was evaluated by performing the mechanical test,Scanning Electron Micrograph(SEM)to observe the morphology of the composites,and also by surface analysis.The surface roughness was visualised using a 3D profilometer and thefigure was illustrated as colour shading in the image.The composite withfibre size≤100μm displayed better tensile andflexural strength,compared to other sizes.In conclusion,by reducing the size of thefibre,the composites could develop high strength performance for industrial applications.展开更多
Experiments on thermal decomposition of nano-sized calcium carbonate were carried out in a thermo-gravimetric analyzer under non-isothermal condition of different heating rates (5 to 20K·min^-1). The Coats and Re...Experiments on thermal decomposition of nano-sized calcium carbonate were carried out in a thermo-gravimetric analyzer under non-isothermal condition of different heating rates (5 to 20K·min^-1). The Coats and Redfern''s equation was used to determine the apparent activation energy and the pre-exponential factors. The mechanism of thermal decomposition was evaluated using the master plots, Coats and Redfern's equation and the kinetic compensation law. It was found that the thermal decomposition property of nano-sized calcium carbonate was different from that of bulk calcite. Nano-sized calcium carbonate began to decompose at 640℃, which was 180℃ lower than the reported value for calcite. The experimental results of kinetics were compatible with the mechanism of one-dimensional phase boundary movement. The apparent activation energy of nano-sized calcium carbonate was estimated to be 151 kJ·mo1^-1 while the literature value for normal calcite was approximately 200 kJ ·mol^-1. The order of magnitude of Dre-exvonential factors was estimated to be 10^9 s^-1.展开更多
Experiments on maxillofacial bone tissue engineering showed the promising result;however, its healing mechanisms and effectiveness had not been fully understood. The aim of this study is to compare the bone healing me...Experiments on maxillofacial bone tissue engineering showed the promising result;however, its healing mechanisms and effectiveness had not been fully understood. The aim of this study is to compare the bone healing mechanism and osteogenic capacity between bovine bone mineral loaded with hAMSC and autogenous bone graft in the reconstruction of critical size mandibular bone defect. Critical size defects were made at the mandible of 45 New Zealand white rabbits reconstructed with BBM-hAMSC, BBM alone, and ABG, respectively. At the end of first, second, and twelfth weeks, five rabbits from each experimental week were sacrificed for histology and immunohistochemistry staining. Expressions of vascular endothelial growth factor (VEGF), bone mor-phogenic proteins-2 (BMP2), Runx2 and the amount of angiogenesis were analyzed in the first and second week groups, while expressions of Runx2, osteocalcin, collagen type-I fibres, trabecular area and bone incorporation were analyzed in the twelfth week groups. The result showed that expressions of VEGF, BMP2 and Runx2 as well as the amount of angiogenesis were higher in ABG compared with BBM-hAMSC group in the first and second weeks of healing. The result of twelfth week of healing showed that expressions of Runx2 and osteocalcin as well as the thickness of collagen type-I fibres were significantly higher in BBM-hAMSC compared to ABG group, while there was no statistically difference in trabecular area and bone incorporation between BBM-hAMSC and ABG group. This study concluded that early healing activities were higher in auto-genous bone graft than in BBM-hAMSC, while osteogenic activities in the late stage of healing were higher in BBM-hAMSC compared to autogenous bone graft. It was also concluded that the osteo-genic capacity of BBM-hAMSC was comparable to autogenous bone graft in the reconstruction of critical size defect in the mandible.展开更多
As a necessary factor for a successful business,entrepreneurial spirit is an important means for enterprises to survive in the unstable environment of fierce competition. Under the circumstance of mass entrepreneurshi...As a necessary factor for a successful business,entrepreneurial spirit is an important means for enterprises to survive in the unstable environment of fierce competition. Under the circumstance of mass entrepreneurship and innovation,a large number of small and medium-sized private enterprises have emerged in all walks of life in China. This paper examines the differences in the definition of entrepreneurial spirit by different scholars,the influence of Chinese entrepreneurship on the development of small and medium-sized private enterprises,the origin of the spirit of private entrepreneurs with Chinese characteristics,and the disadvantages of the spirit of small and medium-sized private entrepreneurs in China. It is necessary to further improve entrepreneurial spirit to contribute to the development of small and medium-sized private enterprises and help China to build the national economic system.展开更多
The Mg-Gd-Y-Zn-Zr alloys are representational and potential age-hardening systems as reported in the past ten years,but their mechanical properties are still dependent on the grain size and its distribution.The effect...The Mg-Gd-Y-Zn-Zr alloys are representational and potential age-hardening systems as reported in the past ten years,but their mechanical properties are still dependent on the grain size and its distribution.The effect of bimodal structure on mechanical properties of Mg-8Gd-3Y-0.5Zr alloy with bimodal grain size distributions was investigated.The results suggested that the volume fraction of fine grain(FG)and coarse grain(CG)could be controlled by combined processes of hot forging,extrusion and annealing.And for the present alloys with bimodal grain size distribution,the improvement of strength is still attributed to the grain refinement.The morphology of bimodal grain size distribution has a marked impact on the ductility of the alloy,i.e.with the increase of coarse grain volume fraction,the elongation to failure increases at the beginning and then decreases.The mechanism of the toughening effect of bimodal grain size distribution on the Mg-Gd-Y-Zn-Zr alloys with bimodal grain size structure has been discussed.展开更多
Six kinds of micro bridge-beam specimens with different sizes are fabricated using photolithography technology for bending test. Beam specimens with trapezoidal section could be representatives of those with rectangle...Six kinds of micro bridge-beam specimens with different sizes are fabricated using photolithography technology for bending test. Beam specimens with trapezoidal section could be representatives of those with rectangle and square section, which are usually applied in MEMS. Nano indentation method used in bending test can be applied to both elastic and plastic materials. Also, some mechanical properties parameters such as the modulus of elasticity, hardness and the bending strength are obtained. The average modulus of elasticity of SCS is 170.295 0±2.485 0 GPa, showing no size effects, but the bending strength ranges from 3.24 GPa to 10.15 GPa, displaying strong size effects, and the average hardness is 9.496 7±1.753 3 GPa,in which no obvious size effects are observed.展开更多
A high-Ti 6061 alloy was rolled with strains up to 0. 8 - 2. 0 and at 350 - 550 ℃ . Microstructures that developed during deformation and subsequent solution heat treatment (SHT) were observed by using optical and tr...A high-Ti 6061 alloy was rolled with strains up to 0. 8 - 2. 0 and at 350 - 550 ℃ . Microstructures that developed during deformation and subsequent solution heat treatment (SHT) were observed by using optical and transmission electron microscopy. Microstructure evolution during SHT depends mainly on the initial rolling temperature,and it was found that the higher this temperature is,the coarser the grains are. After rolling at 400 ℃ ,well-defined cells and subgrains were formed, which induced further sites for recrystallization nucleation during subsequent SHT. The recrystallization mechanism was found to be subgrain rotation,with a final grain size smaller than 200 μm. Increasing the rolling temperature to 500 ℃ results in a low density of dislocations distributed uniformly in the deformed matrix and fewer nucleation sites during subsequent SHT. The recrystallization mechanism is grain boundary bulging,while the final grain size approaches several millimeters. Finally,a hot forming process of high-Ti 6061 alloy for inhibiting grain coarsening was proposed,and verified by experiments.展开更多
The thermoplasticity of duplex stainless steel 2205(DSS2205) is better than that of lean duplex steel 2101(LDX2101), which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase rat...The thermoplasticity of duplex stainless steel 2205(DSS2205) is better than that of lean duplex steel 2101(LDX2101), which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy(OM), electron backscatter diffraction(EBSD), Thermo-Calc software, and transmission electron microscopy(TEM). The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.展开更多
Six kinds of beryllium powders with different particle sizes (4~15 μm) and low oxygen prepared by impact grinding were compacted and consolidated by cold hot isostatic pressing (CIP HIP). The tensile strength, yield...Six kinds of beryllium powders with different particle sizes (4~15 μm) and low oxygen prepared by impact grinding were compacted and consolidated by cold hot isostatic pressing (CIP HIP). The tensile strength, yield strength, elongation and micro yield strength(MYS) of the materials were tested and it showed that the strength of the materials, especially the yield strength and micro yield strength(MYS) increase obviously with the refinement of grain size. From the XRD and TEM, the second phase is BeO which is finely dispersed in matrix. This is considered to be the main strengthening mechanism for CIP HIPed beryllium materials with higher purity.展开更多
Functions of the polyamines in neutral rosin sizing were investigated using X-ray photoelectron spectroscopy (XPS) and laser diffraction particle analysis. The polyamine with a higher charge density and a smaller unit...Functions of the polyamines in neutral rosin sizing were investigated using X-ray photoelectron spectroscopy (XPS) and laser diffraction particle analysis. The polyamine with a higher charge density and a smaller unite size could retain more rosin and cover larger fiber surfaces. The XPS spectra demonstrated that polyallylamine (PAAm) and polyvinylamine (PVAm) could react with a rosin size to form a -OC-N-CO- structure, but polydimethylamino ethyl methacrylate (PDMAEMA) could not. The formation of this structure may be a key step for effective sizing.展开更多
The transformation from graphite to turbostratic graphite by means of the treatment with high energy ball milling was investigated by X ray powder diffraction method. It is believed that the size effect of nano cry...The transformation from graphite to turbostratic graphite by means of the treatment with high energy ball milling was investigated by X ray powder diffraction method. It is believed that the size effect of nano crystal leads to this transformation. A possible transformation mechanism is proposed from the change of the eletronic structure of the hexagonal plane of the carbon atoms.展开更多
There have been various theoretical attempts by researchers worldwide to link up different scales of plasticity studies from the nano-, micro- and macro-scale of observation, based on molecular dynamics, crystal plast...There have been various theoretical attempts by researchers worldwide to link up different scales of plasticity studies from the nano-, micro- and macro-scale of observation, based on molecular dynamics, crystal plasticity and continuum mechanics. Very few attempts, however, have been reported in ultra-precision machining studies. A mesoplasticity approach advocated by Lee and Yang is adopted by the authors and is successfully applied to studies of the micro-cutting mechanisms in ultra-precision machining. Traditionally, the shear angle in metal cutting, as well as the cutting force variation, can only be determined from cutting tests. In the pioneering work of the authors, the use of mesoplasticity theory enables prediction of the fluctuation of the shear angle and micro-cutting force, shear band formation, chip morphology in diamond turning and size effect in nano-indentation. These findings are verified by experiments. The mesoplasticity formulation opens up a new direction of studies to enable how the plastic behaviour of materials and their constitutive representations in deformation processing, such as machining can be predicted, assessed and deduced from the basic properties of the materials measurable at the microscale.展开更多
Metallic thin films have attracted much attention owing to their unique mechanical properties,which are widely used in micro-/nano-devices.In this review,several key topics about the thin films in the micron to nano-s...Metallic thin films have attracted much attention owing to their unique mechanical properties,which are widely used in micro-/nano-devices.In this review,several key topics about the thin films in the micron to nano-scales are covered.First,the plastic deformation mechanisms in face-centered-cubic(FCC)metals,in particular the sizedependent deformation twinning at small scales,are discussed based on a deformation-mechanism map.Microstructural evolution is then briefly discussed from the perspective of the ratio of effective-to-internal stresses,while the stress-driven grain growth is discussed based on a twinning-mediated mechanism.The last section elucidates the size-dependent mechanical properties of metallic thin films,such as yield strength,ductility and mechanical fatigue behavior.展开更多
Projection micro stereolithography(PμSL)has emerged as a powerful three-dimensional(3D)printing technique for manufacturing polymer structures with micron-scale high resolution at high printing speed,which enables th...Projection micro stereolithography(PμSL)has emerged as a powerful three-dimensional(3D)printing technique for manufacturing polymer structures with micron-scale high resolution at high printing speed,which enables the production of customized 3D microlattices with feature sizes down to several microns.However,the mechanical properties of as-printed polymers were not systemically studied at the relevant length scales,especially when the feature sizes step into micron/sub-micron level,limiting its reliable performance prediction in micro/nanolattice and other metamaterial applications.In this work,we demonstrate that PμSL-printed microfibers could become stronger and significantly more ductile with reduced size ranging from 20μm to 60μm,showing an obvious size-dependent mechanical behavior,in which the size decreases to 20μm with a fracture strain up to~100%and fracture strength up to~100 MPa.Such size effect enables the tailoring of the material strength and stiffness of PμSL-printed microlattices over a broad range,allowing to fabricate the microlattice metamaterials with desired/tunable mechanical properties for various structural and functional applications.展开更多
基金Funded by Project of Liaoning Provincial Department of Education (No.LZGD2019003)。
文摘The Young's modulus, shear modulus and Poisson's ratio of monolayer arsenene with different sizes were calculated by finite element method, so as to explore the influence of dimension and orientation on the mechanical properties of monolayer arsenene. The calculation results show that the small size has a significant effect on the mechanical properties of the monolayer arsenene. The smaller the size, the larger the Young's modulus and Poisson's ratio of the monolayer arsenene. The size change has a great influence on the Young's modulus of the arsenene handrail direction, and the Young's modulus of the zigzag direction is not sensitive to the size change. Similarly, the size change has a significant effect on the shear modulus of arsenene in the handrail direction, while the shear modulus in the zigzag direction has no significant effect on its size change. For the Poisson's ratio, the situation is just the opposite, and the effect of the size change on the Poisson's ratio of the arsenene zigzag direction is greater than that of the handrail direction.
基金financial supported by the National Natural Science Foundation of China (22078065)Key Program of Qingyuan Innovation Laboratory (00221001)Quanzhou City Science & Technology Program of China (2020C008R)。
文摘Catalytic epoxidation of alkenes is an important type of organic reaction in chemical industry,and the deep insight into catalyst deactivation will help to develop new epoxidation process.In this work,series of quaternary ammoniums bearing different cationic sizes,i.e.MTOA+(methyltrioctylammonium,[(C_(8)H_(17))_(3)CH_(3)N]+),HTMA+(hexadecyltrimethylammonium,[(C_(16)H_(33))(CH_(3))_(3)N]+) and DMDOA+(dimethyldioctadecylammonium,[(C_(18)H_(37))_(2)(CH_(3))_(2)N]+) were incorporated with polyoxometalate (POM) anions to prepare phase transfer catalysts (PTCs),which were used in the styrene epoxidations.Among them,(MTOA)_(3)PW_(4)O_(24)exhibits the best catalytic performance judged from the highest styrene conversion rate(52%) and styrene oxide selectivity (93%),during which the styrene epoxidation conditions were optimized.Meanwhile,the deactivation mechanism of this kind of PTCs was proposed firstly,i.e.in the case of low H_(2)O_(2) content,the oxidant can only be used in the styrene epoxidation,in which the catalyst can transform into stable Keggin-type POM.But when the content of H_(2)O_(2) is higher,the excess H_(2)O_(2) can reactivate the Keggin-type POM into active (PW_(4)O_(24))_(3)-anions,which can trigger the ring-opening polymerization of styrene oxide.Consequently,the catalyst is deactivated by adhered poly(styrene oxide)irreversibly,which was determined by NMR spectra.In this situation,the active moiety{PO_(4)[WO(O_(2))_(2)]_(4)}_(3)-in phase-transfer catalytic system can break into some unidentified species with low W/P ratio with the presence of epoxides.This work will be beneficial for the design of new PTCs in alkene epoxidation in fine chemical industry.
基金The project was supported by China Postdoctoral Science Foundation(No.2003034452)National Natural Science Foundation of China(No.50371027).
文摘A novel process for synthesizing nano-ceramics powders, named mechanical & therm al activation processing, is discussed in the present paper. It is a processing based on thermal activation in liquid phase (molten salt) after mechanical activ ation. The nanometer-sized TiC particles (15-20nm) have been synthesized by the method, and analyzed by X-ray diffraction (XRD), transmission electron microscop e (TEM), scanning electron microscopy (SEM) and energy dispersion X-ray (EDX) sp ectroscopy. An interface interaction between liquid (molten salt) and solid (fin al product particles) phases plays a dominating role for the control of product particles size. The mechanism for the formation of nanometer-sized TiC particles has been discussed.
基金financially supported by the National Natural Science Funds of China(grant number31470599)the Priority Academic Program Development of Jiangsu Higher Education Institutions,China
文摘Low-basis-weight paper lacks normal strength and stiffness. Waterborne epoxy resin could be employed with oxidized starch to improve paper stiffness through surface sizing. In this study,the mechanism of enhancing stiffness by adding waterborne epoxy resin was fully investigated. The results indicated that through surface sizing with epoxy resin,the paper thickness was preserved,whereas the elastic modulus increased significantly and the epoxy resin had positive impact on single fiber strength. A rigid resin layer and interpenetrating polymer network formed on the surface and in the inner layer of the paper,respectively. The formed resin layer and interpenetrating polymer network strongly supported the paper,leading to the improvement of the elastic modulus and stiffness. The stiffness improvement through surface sizing was mainly due to the formation of a fibrous composite layer and penetration of the sizing agent into the inner layers of the paper. The better the combination between fiber and sizing agent,the higher were the elastic modulus and the stiffness of the whole paper.
基金The result was obtained through the financial support of the Ministry of Education Malaysia under Grant Number RACER/2019/FKM-CARE/F00408This paper’s publication also funded by UTeM Incentive Jurnal Q00049/JURNAL/2020/FKM.
文摘Naturalfibre as a reinforcing agent has been widely used in many industrial applications.Nevertheless,several factors need to be considered,such as the size and weight percentage of thefibre used in binding.Using fused deposition modelling(FDM),this factor was investigated by varying the size of naturalfibre as the responding variable with afixed weight percentage of kenaffibre.The process of modifying the naturalfibre in terms of size might increase the dispersion of kenaffibre in the polymer matrix and increase the adhesion bonding between thefibre and matrix of composites,subsequently improving the interfacial bonding between these two phases.In this paper,the effect offibre size was evaluated by performing the mechanical test,Scanning Electron Micrograph(SEM)to observe the morphology of the composites,and also by surface analysis.The surface roughness was visualised using a 3D profilometer and thefigure was illustrated as colour shading in the image.The composite withfibre size≤100μm displayed better tensile andflexural strength,compared to other sizes.In conclusion,by reducing the size of thefibre,the composites could develop high strength performance for industrial applications.
基金Supported by the Key Research of Science & Technology of Education(No.0202)and the Fundamental Research Plan of HuoYingdong(No.81063).
文摘Experiments on thermal decomposition of nano-sized calcium carbonate were carried out in a thermo-gravimetric analyzer under non-isothermal condition of different heating rates (5 to 20K·min^-1). The Coats and Redfern''s equation was used to determine the apparent activation energy and the pre-exponential factors. The mechanism of thermal decomposition was evaluated using the master plots, Coats and Redfern's equation and the kinetic compensation law. It was found that the thermal decomposition property of nano-sized calcium carbonate was different from that of bulk calcite. Nano-sized calcium carbonate began to decompose at 640℃, which was 180℃ lower than the reported value for calcite. The experimental results of kinetics were compatible with the mechanism of one-dimensional phase boundary movement. The apparent activation energy of nano-sized calcium carbonate was estimated to be 151 kJ·mo1^-1 while the literature value for normal calcite was approximately 200 kJ ·mol^-1. The order of magnitude of Dre-exvonential factors was estimated to be 10^9 s^-1.
文摘Experiments on maxillofacial bone tissue engineering showed the promising result;however, its healing mechanisms and effectiveness had not been fully understood. The aim of this study is to compare the bone healing mechanism and osteogenic capacity between bovine bone mineral loaded with hAMSC and autogenous bone graft in the reconstruction of critical size mandibular bone defect. Critical size defects were made at the mandible of 45 New Zealand white rabbits reconstructed with BBM-hAMSC, BBM alone, and ABG, respectively. At the end of first, second, and twelfth weeks, five rabbits from each experimental week were sacrificed for histology and immunohistochemistry staining. Expressions of vascular endothelial growth factor (VEGF), bone mor-phogenic proteins-2 (BMP2), Runx2 and the amount of angiogenesis were analyzed in the first and second week groups, while expressions of Runx2, osteocalcin, collagen type-I fibres, trabecular area and bone incorporation were analyzed in the twelfth week groups. The result showed that expressions of VEGF, BMP2 and Runx2 as well as the amount of angiogenesis were higher in ABG compared with BBM-hAMSC group in the first and second weeks of healing. The result of twelfth week of healing showed that expressions of Runx2 and osteocalcin as well as the thickness of collagen type-I fibres were significantly higher in BBM-hAMSC compared to ABG group, while there was no statistically difference in trabecular area and bone incorporation between BBM-hAMSC and ABG group. This study concluded that early healing activities were higher in auto-genous bone graft than in BBM-hAMSC, while osteogenic activities in the late stage of healing were higher in BBM-hAMSC compared to autogenous bone graft. It was also concluded that the osteo-genic capacity of BBM-hAMSC was comparable to autogenous bone graft in the reconstruction of critical size defect in the mandible.
基金Supported by Special Youth Project"A Comparative Study on the Cultivation Mode of Innovative Entrepreneurship Talents in China and South Korea"(120-1452226)
文摘As a necessary factor for a successful business,entrepreneurial spirit is an important means for enterprises to survive in the unstable environment of fierce competition. Under the circumstance of mass entrepreneurship and innovation,a large number of small and medium-sized private enterprises have emerged in all walks of life in China. This paper examines the differences in the definition of entrepreneurial spirit by different scholars,the influence of Chinese entrepreneurship on the development of small and medium-sized private enterprises,the origin of the spirit of private entrepreneurs with Chinese characteristics,and the disadvantages of the spirit of small and medium-sized private entrepreneurs in China. It is necessary to further improve entrepreneurial spirit to contribute to the development of small and medium-sized private enterprises and help China to build the national economic system.
基金supports of the National Key Research and Development Plan(Grant Nos.2016YFB0701201 and 2016YFB0301103)the National Natural Science Foundation of China(Grant Nos.51771109 and 51631006).
文摘The Mg-Gd-Y-Zn-Zr alloys are representational and potential age-hardening systems as reported in the past ten years,but their mechanical properties are still dependent on the grain size and its distribution.The effect of bimodal structure on mechanical properties of Mg-8Gd-3Y-0.5Zr alloy with bimodal grain size distributions was investigated.The results suggested that the volume fraction of fine grain(FG)and coarse grain(CG)could be controlled by combined processes of hot forging,extrusion and annealing.And for the present alloys with bimodal grain size distribution,the improvement of strength is still attributed to the grain refinement.The morphology of bimodal grain size distribution has a marked impact on the ductility of the alloy,i.e.with the increase of coarse grain volume fraction,the elongation to failure increases at the beginning and then decreases.The mechanism of the toughening effect of bimodal grain size distribution on the Mg-Gd-Y-Zn-Zr alloys with bimodal grain size structure has been discussed.
文摘Six kinds of micro bridge-beam specimens with different sizes are fabricated using photolithography technology for bending test. Beam specimens with trapezoidal section could be representatives of those with rectangle and square section, which are usually applied in MEMS. Nano indentation method used in bending test can be applied to both elastic and plastic materials. Also, some mechanical properties parameters such as the modulus of elasticity, hardness and the bending strength are obtained. The average modulus of elasticity of SCS is 170.295 0±2.485 0 GPa, showing no size effects, but the bending strength ranges from 3.24 GPa to 10.15 GPa, displaying strong size effects, and the average hardness is 9.496 7±1.753 3 GPa,in which no obvious size effects are observed.
基金Sponsored by the Project of High-level Talent of Hebei Province (Post-Doctoral Research Project of Hebei Province)
文摘A high-Ti 6061 alloy was rolled with strains up to 0. 8 - 2. 0 and at 350 - 550 ℃ . Microstructures that developed during deformation and subsequent solution heat treatment (SHT) were observed by using optical and transmission electron microscopy. Microstructure evolution during SHT depends mainly on the initial rolling temperature,and it was found that the higher this temperature is,the coarser the grains are. After rolling at 400 ℃ ,well-defined cells and subgrains were formed, which induced further sites for recrystallization nucleation during subsequent SHT. The recrystallization mechanism was found to be subgrain rotation,with a final grain size smaller than 200 μm. Increasing the rolling temperature to 500 ℃ results in a low density of dislocations distributed uniformly in the deformed matrix and fewer nucleation sites during subsequent SHT. The recrystallization mechanism is grain boundary bulging,while the final grain size approaches several millimeters. Finally,a hot forming process of high-Ti 6061 alloy for inhibiting grain coarsening was proposed,and verified by experiments.
基金financially supported by the National Natural Science Foundation of China (No. 51174026)the National Science and Technology Pillar Program during the Twelfth Five-Year Plan Period (No. 2012BAE04B02)
文摘The thermoplasticity of duplex stainless steel 2205(DSS2205) is better than that of lean duplex steel 2101(LDX2101), which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy(OM), electron backscatter diffraction(EBSD), Thermo-Calc software, and transmission electron microscopy(TEM). The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.
文摘Six kinds of beryllium powders with different particle sizes (4~15 μm) and low oxygen prepared by impact grinding were compacted and consolidated by cold hot isostatic pressing (CIP HIP). The tensile strength, yield strength, elongation and micro yield strength(MYS) of the materials were tested and it showed that the strength of the materials, especially the yield strength and micro yield strength(MYS) increase obviously with the refinement of grain size. From the XRD and TEM, the second phase is BeO which is finely dispersed in matrix. This is considered to be the main strengthening mechanism for CIP HIPed beryllium materials with higher purity.
基金This work was supported by International Co-operation Project of Fujian Province(99-I-5).
文摘Functions of the polyamines in neutral rosin sizing were investigated using X-ray photoelectron spectroscopy (XPS) and laser diffraction particle analysis. The polyamine with a higher charge density and a smaller unite size could retain more rosin and cover larger fiber surfaces. The XPS spectra demonstrated that polyallylamine (PAAm) and polyvinylamine (PVAm) could react with a rosin size to form a -OC-N-CO- structure, but polydimethylamino ethyl methacrylate (PDMAEMA) could not. The formation of this structure may be a key step for effective sizing.
文摘The transformation from graphite to turbostratic graphite by means of the treatment with high energy ball milling was investigated by X ray powder diffraction method. It is believed that the size effect of nano crystal leads to this transformation. A possible transformation mechanism is proposed from the change of the eletronic structure of the hexagonal plane of the carbon atoms.
基金the Research Committee of The Hong Kong Polytechnic University and the Innovation Technology Commission of The Hong Kong SAR Government for their financial support of the Hong Kong Partner State Key Laboratory of Ultra-Precision Machining Technology
文摘There have been various theoretical attempts by researchers worldwide to link up different scales of plasticity studies from the nano-, micro- and macro-scale of observation, based on molecular dynamics, crystal plasticity and continuum mechanics. Very few attempts, however, have been reported in ultra-precision machining studies. A mesoplasticity approach advocated by Lee and Yang is adopted by the authors and is successfully applied to studies of the micro-cutting mechanisms in ultra-precision machining. Traditionally, the shear angle in metal cutting, as well as the cutting force variation, can only be determined from cutting tests. In the pioneering work of the authors, the use of mesoplasticity theory enables prediction of the fluctuation of the shear angle and micro-cutting force, shear band formation, chip morphology in diamond turning and size effect in nano-indentation. These findings are verified by experiments. The mesoplasticity formulation opens up a new direction of studies to enable how the plastic behaviour of materials and their constitutive representations in deformation processing, such as machining can be predicted, assessed and deduced from the basic properties of the materials measurable at the microscale.
基金supported by the National Key Research and Development Program of China(2017YFA0700701,2017YFB0702301)the National Natural Science Foundation of China(Grant Nos.51621063,51722104,51625103,51790482,51761135031 and 51571157)+2 种基金the 111 Project 2.0 of China(BP2018008),the Fok Ying Tong Education Foundation(161096)the Shaanxi Province innovative talents promotion Projects(2018KJXX-004)the Fundamental Research Funds for the Central Universities.
文摘Metallic thin films have attracted much attention owing to their unique mechanical properties,which are widely used in micro-/nano-devices.In this review,several key topics about the thin films in the micron to nano-scales are covered.First,the plastic deformation mechanisms in face-centered-cubic(FCC)metals,in particular the sizedependent deformation twinning at small scales,are discussed based on a deformation-mechanism map.Microstructural evolution is then briefly discussed from the perspective of the ratio of effective-to-internal stresses,while the stress-driven grain growth is discussed based on a twinning-mediated mechanism.The last section elucidates the size-dependent mechanical properties of metallic thin films,such as yield strength,ductility and mechanical fatigue behavior.
基金the financial support from Shenzhen Science and Technology Innovation Committee under the Grant Nos. JCYJ20170818103206501, Type C 202011033000145Changsha Municipal Science and Technology Bureau Project kh2201035supported by the City University of Hong Kong under the Grant No. 9667226
文摘Projection micro stereolithography(PμSL)has emerged as a powerful three-dimensional(3D)printing technique for manufacturing polymer structures with micron-scale high resolution at high printing speed,which enables the production of customized 3D microlattices with feature sizes down to several microns.However,the mechanical properties of as-printed polymers were not systemically studied at the relevant length scales,especially when the feature sizes step into micron/sub-micron level,limiting its reliable performance prediction in micro/nanolattice and other metamaterial applications.In this work,we demonstrate that PμSL-printed microfibers could become stronger and significantly more ductile with reduced size ranging from 20μm to 60μm,showing an obvious size-dependent mechanical behavior,in which the size decreases to 20μm with a fracture strain up to~100%and fracture strength up to~100 MPa.Such size effect enables the tailoring of the material strength and stiffness of PμSL-printed microlattices over a broad range,allowing to fabricate the microlattice metamaterials with desired/tunable mechanical properties for various structural and functional applications.