农业直接关系到粮食安全和农户收入,既高度依赖于资源环境又反作用于资源环境。农业可持续发展是实现可持续发展目标的关键。通过CiteSpace对Web of Science和中国知网在2000—2019年发表的关于农业可持续发展评价的1174篇英文和1043篇...农业直接关系到粮食安全和农户收入,既高度依赖于资源环境又反作用于资源环境。农业可持续发展是实现可持续发展目标的关键。通过CiteSpace对Web of Science和中国知网在2000—2019年发表的关于农业可持续发展评价的1174篇英文和1043篇中文文章进行文献计量分析,总结出农业可持续发展的研究前沿、热点和趋势,以期为农业可持续发展研究和决策管理提供参考。结果表明:农业可持续发展的研究前沿聚焦在食物系统弹性与粮食安全、农业生态集约化与可持续集约化、都市农业与农业多功能性、农业生态系统服务权衡与协同4个方面。研究热点主要关注气候变化对农业系统的影响及其适应性管理、农业生态系统服务管理、农业活动的环境影响和农业系统可持续性综合评价。建议在今后的研究中进一步解析“气候-土地-水-能源-粮食”耦合机制,系统开展农业生态系统的“影响因素-作用机理-调控方法”研究。展开更多
A compound,3-ethyl-3-hydroxymethyloxetane(EHO),was synthesized with diethyl carbonate and trihydroxypropane as raw materials,3-ethyl-3-allylmethoxy oxetane(EAMO)was synthesized with EHO and allyl bromide,and 1,1,3,3,5...A compound,3-ethyl-3-hydroxymethyloxetane(EHO),was synthesized with diethyl carbonate and trihydroxypropane as raw materials,3-ethyl-3-allylmethoxy oxetane(EAMO)was synthesized with EHO and allyl bromide,and 1,1,3,3,5,5-hexamethyl-1,5-bis[(3-ethyl-3-methoxyoxetane)propyl]trisiloxane(HMBEMOPTS)was synthesized with EAMO and 1,1,3,3,5,5-hexamethyltrisiloxane(HMTS).HMBEMOPTS is a novel UV-curable oligomer.The test of photo-DSC shows the photosensitivity of HMBEMOPTS is better than the ordinary oxetane,3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane.HMBEMOPTS was mixed with bisphenol A type epoxy resin E-51 to prepare a cationic UV-curable system,and triarylsulfonium hexafluoroantimonate(UV-6976)was used as a cationic photoinitiator.The mechanical tests of coating films prove that when the mass fraction of HMBEMOPTS is 50%,the mechanical properties of the curing system are the best.The impact strength of the UV-curable films is measured to be 40 kg·cm and the flexibility is 2 mm;the tensile strength and flexural strength of the prepared specimens are 20.74 MPa and 13.43 MPa,respectively.The experimental results show that HMBEMOPTS can effectively improve photosensitivity and flexibility of the photosensitive resin.展开更多
The properties of arctic underwater acoustic channel were researched using BurkeTwersky(BT) model and ray theory.In the BT model,sea ice ridge is assumed as a set of semi-elliptical cylinder randomly distributed on a ...The properties of arctic underwater acoustic channel were researched using BurkeTwersky(BT) model and ray theory.In the BT model,sea ice ridge is assumed as a set of semi-elliptical cylinder randomly distributed on a stress-free surface.By approximation of low frequency and high frequency of the BT model,the reflection coefficients were calculated at different frequencies.On the basis of ray theory,the acoustic field was calculated and the properties of underwater acoustic channel was analyzed.Compared to underwater acoustic channel properties when the boundary is air,the results show that under the ice surface some sound rays disappear during transmission,because the ice surface reflection coefficients is less than absolute soft surface.Especially,the higher the frequency,the higher the under-ice transmission loss.As a result,under-ice condition is unfavourable for acoustic signals to travel far.Besides,as to the channel structure,the reflection coefficient of ice layer is smaller,the channel multi-path structure is not significant compared to water-air interface without ice.The research results are valuable for our understanding of hydroacoustic channel and the prediction of the sonar system working in the polar region.展开更多
The Kirchhoff approximation with a modified reflection coefficient is used to calculate the three-dimensional acoustic scattering of a Gaussian rough under-ice surface.The concept of a local statistical average reflec...The Kirchhoff approximation with a modified reflection coefficient is used to calculate the three-dimensional acoustic scattering of a Gaussian rough under-ice surface.The concept of a local statistical average reflection coefficient of an under-ice surface is proposed in the calculation model.The scattered sound field of a two-dimensional Gaussian rough under-ice surface is divided into coherent scattering and incoherent scattering.A formula is derived for the scattering coefficient of each scattering component,and the three-dimensional scattering intensity is obtained.The relationships between the scattering intensity and(ⅰ)the root-meansquare height of the Gaussian rough under-ice surface,(ⅱ)the angle of incidence,and(ⅲ)the sound frequency are analyzed.The scattering intensity of a Gaussian rough under-ice surface is measured in a laboratory water tank,and the calculation results of the theoretical model are verified.The experimental results are compared with those of the theoretical model using(ⅰ)the present local statistical average reflection coefficient of an under-ice surface and(ⅱ)the mirror reflection coefficient of an under-ice surface from the literature.The calculation results of the model using the local statistical average reflection coefficient agree well with the experimental results.展开更多
文摘农业直接关系到粮食安全和农户收入,既高度依赖于资源环境又反作用于资源环境。农业可持续发展是实现可持续发展目标的关键。通过CiteSpace对Web of Science和中国知网在2000—2019年发表的关于农业可持续发展评价的1174篇英文和1043篇中文文章进行文献计量分析,总结出农业可持续发展的研究前沿、热点和趋势,以期为农业可持续发展研究和决策管理提供参考。结果表明:农业可持续发展的研究前沿聚焦在食物系统弹性与粮食安全、农业生态集约化与可持续集约化、都市农业与农业多功能性、农业生态系统服务权衡与协同4个方面。研究热点主要关注气候变化对农业系统的影响及其适应性管理、农业生态系统服务管理、农业活动的环境影响和农业系统可持续性综合评价。建议在今后的研究中进一步解析“气候-土地-水-能源-粮食”耦合机制,系统开展农业生态系统的“影响因素-作用机理-调控方法”研究。
基金Funded by the National Natural Science Foundation of China(No.21865017)。
文摘A compound,3-ethyl-3-hydroxymethyloxetane(EHO),was synthesized with diethyl carbonate and trihydroxypropane as raw materials,3-ethyl-3-allylmethoxy oxetane(EAMO)was synthesized with EHO and allyl bromide,and 1,1,3,3,5,5-hexamethyl-1,5-bis[(3-ethyl-3-methoxyoxetane)propyl]trisiloxane(HMBEMOPTS)was synthesized with EAMO and 1,1,3,3,5,5-hexamethyltrisiloxane(HMTS).HMBEMOPTS is a novel UV-curable oligomer.The test of photo-DSC shows the photosensitivity of HMBEMOPTS is better than the ordinary oxetane,3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane.HMBEMOPTS was mixed with bisphenol A type epoxy resin E-51 to prepare a cationic UV-curable system,and triarylsulfonium hexafluoroantimonate(UV-6976)was used as a cationic photoinitiator.The mechanical tests of coating films prove that when the mass fraction of HMBEMOPTS is 50%,the mechanical properties of the curing system are the best.The impact strength of the UV-curable films is measured to be 40 kg·cm and the flexibility is 2 mm;the tensile strength and flexural strength of the prepared specimens are 20.74 MPa and 13.43 MPa,respectively.The experimental results show that HMBEMOPTS can effectively improve photosensitivity and flexibility of the photosensitive resin.
基金supported by the National Natural Science Foundation of China(61471137)Huo Yingdong Education Foundation of China(151007)Fundamental Research Funds for the Central Universities(heucfd2050260166,heu10500170006)
文摘The properties of arctic underwater acoustic channel were researched using BurkeTwersky(BT) model and ray theory.In the BT model,sea ice ridge is assumed as a set of semi-elliptical cylinder randomly distributed on a stress-free surface.By approximation of low frequency and high frequency of the BT model,the reflection coefficients were calculated at different frequencies.On the basis of ray theory,the acoustic field was calculated and the properties of underwater acoustic channel was analyzed.Compared to underwater acoustic channel properties when the boundary is air,the results show that under the ice surface some sound rays disappear during transmission,because the ice surface reflection coefficients is less than absolute soft surface.Especially,the higher the frequency,the higher the under-ice transmission loss.As a result,under-ice condition is unfavourable for acoustic signals to travel far.Besides,as to the channel structure,the reflection coefficient of ice layer is smaller,the channel multi-path structure is not significant compared to water-air interface without ice.The research results are valuable for our understanding of hydroacoustic channel and the prediction of the sonar system working in the polar region.
基金supported by the National Key R&D Program of China(2018YFC1405900)National Natural Science Foundation of China(61631008)。
文摘The Kirchhoff approximation with a modified reflection coefficient is used to calculate the three-dimensional acoustic scattering of a Gaussian rough under-ice surface.The concept of a local statistical average reflection coefficient of an under-ice surface is proposed in the calculation model.The scattered sound field of a two-dimensional Gaussian rough under-ice surface is divided into coherent scattering and incoherent scattering.A formula is derived for the scattering coefficient of each scattering component,and the three-dimensional scattering intensity is obtained.The relationships between the scattering intensity and(ⅰ)the root-meansquare height of the Gaussian rough under-ice surface,(ⅱ)the angle of incidence,and(ⅲ)the sound frequency are analyzed.The scattering intensity of a Gaussian rough under-ice surface is measured in a laboratory water tank,and the calculation results of the theoretical model are verified.The experimental results are compared with those of the theoretical model using(ⅰ)the present local statistical average reflection coefficient of an under-ice surface and(ⅱ)the mirror reflection coefficient of an under-ice surface from the literature.The calculation results of the model using the local statistical average reflection coefficient agree well with the experimental results.
