随着社会经济的发展和城镇化进程的加速,生态环境日益恶化,人类逐渐认识到不断出现的环境问题带来的严重后果,生态修复作为改善生态环境、保障人类福祉的重要手段,其科学研究和实践探索迅速发展。针对当前我国国土综合整治和生态修复的...随着社会经济的发展和城镇化进程的加速,生态环境日益恶化,人类逐渐认识到不断出现的环境问题带来的严重后果,生态修复作为改善生态环境、保障人类福祉的重要手段,其科学研究和实践探索迅速发展。针对当前我国国土综合整治和生态修复的理念和方法侧重于非生物环境和小范围特定环境的局限,在明确国际生态修复基本概念和内涵的基础上,基于CiteSpace对1988—2020年Web of Science期刊引文数据库中的4220篇生态修复文献,分析发文数量、关键词、前沿热点等,梳理和总结国际生态修复理念演变、不同目标和尺度下的修复方法与措施,提出我国新时期国土综合整治理念应建立可持续发展的社会—生态耦合系统,整治方法应加强生物多样性和生态系统保护、关注景观修复,整治管理应建立社会参与、绩效监督、多元化投入机制,以及整治实施应因地制宜,循序渐进。展开更多
Differential capacitive detection has been widely used in the displacement measurement of the proof mass of vibratory gyroscopes, but it did not achieve high resolutions in angle detection of rotational gyroscopes due...Differential capacitive detection has been widely used in the displacement measurement of the proof mass of vibratory gyroscopes, but it did not achieve high resolutions in angle detection of rotational gyroscopes due to restrictions in structure, theory, and interface circuitry. In this paper, a differential capacitive detection structure is presented to measure the tilt angle of the rotor of a novel rotational gyroscope. A mathematical model is built to study how the structure's capacitance changes with the rotor tilt angles. The relationship between differential capacitance and structural parameters is analyzed, and preliminarily optimized size parameters are adopted. A lownoise readout interface circuit is designed to convert differential capacitance changes to voltage signals. Rate table test results of the gyroscope show that the smallest resolvable tilt angle of the rotor is less than 0.47(0.00013?),and the nonlinearity of the angle detection structure is 0.33%, which can be further improved. The results indicate that the proposed detection structure and the circuitry are helpful for a high accuracy of the gyroscope.展开更多
The Norrish type II reaction is an interesting photochemical process through Csp3-H functionalization of N,N-disubstituted oxo-amine or oxo-amide provides access to diverse synthetic building blocks by different pathw...The Norrish type II reaction is an interesting photochemical process through Csp3-H functionalization of N,N-disubstituted oxo-amine or oxo-amide provides access to diverse synthetic building blocks by different pathways.Due to the finite lifetime of the excited state and rapid changes in the intermediates,there are significant challenges in controlling the stereochemical outcomes of this reaction over the strong racemic background reaction.The oxazolidin-4-one skeleton is a useful structural motif found in bioactive molecules,and it is the one that is produced by a not well-studied route among the three routes in Norrish type II cyclization.Herein we report the asymmetric version of this process,and a variety of oxazolidin-4-ones is achieved from arylα-oxoamides via triplet excimers with three H-transfer steps interspersed by a cyclization step under mild,visible light.In the presence of chiral N,N-dioxide/metal complex catalysts,diastereo-and enantiocontrol in this photochemical reaction happens with enantioselective protonation of enol intermediates as the crucial stereo-determining step,getting the better of the thermodynamical control pathway.Trans-oxazolidine-4-ones can be obtained in high enantioselectivity,which can be transferred into optically active cis-oxazolidine-4-ones after the treatment with a base.展开更多
Rotating speed is a critical parameter affecting the performance of rotor gyroscopes. Rotor gyroscopes must operate at the rated rotating speed. To shorten the start time of the ball-disk rotor gyroscope, this paper p...Rotating speed is a critical parameter affecting the performance of rotor gyroscopes. Rotor gyroscopes must operate at the rated rotating speed. To shorten the start time of the ball-disk rotor gyroscope, this paper presents a new design of the drive system for a ball-disk rotor gyroscope. The drive system is monitored by a microeontroUer. First, the microcontroller generates a sine pulse width modulation signal to drive the permanent magnet rotor. Second, the position of the rotor is detected according to the back electromotive force in the non-energized coil. Third, a piecewise closed-loop control algorithm is implemented to keep the angular acceleration of the rotor within the safe range automatically during the acceleration process and when running at a constant speed. This control algorithm can avoid rotor stalling due to loss of steps. Experimental result shows that with the help of adaptive quick-start technique, the start time of the device can be shortened by up to 36.6%.展开更多
文摘随着社会经济的发展和城镇化进程的加速,生态环境日益恶化,人类逐渐认识到不断出现的环境问题带来的严重后果,生态修复作为改善生态环境、保障人类福祉的重要手段,其科学研究和实践探索迅速发展。针对当前我国国土综合整治和生态修复的理念和方法侧重于非生物环境和小范围特定环境的局限,在明确国际生态修复基本概念和内涵的基础上,基于CiteSpace对1988—2020年Web of Science期刊引文数据库中的4220篇生态修复文献,分析发文数量、关键词、前沿热点等,梳理和总结国际生态修复理念演变、不同目标和尺度下的修复方法与措施,提出我国新时期国土综合整治理念应建立可持续发展的社会—生态耦合系统,整治方法应加强生物多样性和生态系统保护、关注景观修复,整治管理应建立社会参与、绩效监督、多元化投入机制,以及整治实施应因地制宜,循序渐进。
基金Project supported by the National Natural Basic Research Program(973)of China(No.2012CB934104)the National Natural Science Foundation of China(No.61071037)the Natural Science Foundation of Heilongjiang Province,China(No.F201418)
文摘Differential capacitive detection has been widely used in the displacement measurement of the proof mass of vibratory gyroscopes, but it did not achieve high resolutions in angle detection of rotational gyroscopes due to restrictions in structure, theory, and interface circuitry. In this paper, a differential capacitive detection structure is presented to measure the tilt angle of the rotor of a novel rotational gyroscope. A mathematical model is built to study how the structure's capacitance changes with the rotor tilt angles. The relationship between differential capacitance and structural parameters is analyzed, and preliminarily optimized size parameters are adopted. A lownoise readout interface circuit is designed to convert differential capacitance changes to voltage signals. Rate table test results of the gyroscope show that the smallest resolvable tilt angle of the rotor is less than 0.47(0.00013?),and the nonlinearity of the angle detection structure is 0.33%, which can be further improved. The results indicate that the proposed detection structure and the circuitry are helpful for a high accuracy of the gyroscope.
基金the financial support of the National Natural Science Foundation(grant no.22188101)the Science and Technology Department of Sichuan Province(grant no.2021YJ0561)Sichuan University(grant no.2020SCUNL204)for financial support.
文摘The Norrish type II reaction is an interesting photochemical process through Csp3-H functionalization of N,N-disubstituted oxo-amine or oxo-amide provides access to diverse synthetic building blocks by different pathways.Due to the finite lifetime of the excited state and rapid changes in the intermediates,there are significant challenges in controlling the stereochemical outcomes of this reaction over the strong racemic background reaction.The oxazolidin-4-one skeleton is a useful structural motif found in bioactive molecules,and it is the one that is produced by a not well-studied route among the three routes in Norrish type II cyclization.Herein we report the asymmetric version of this process,and a variety of oxazolidin-4-ones is achieved from arylα-oxoamides via triplet excimers with three H-transfer steps interspersed by a cyclization step under mild,visible light.In the presence of chiral N,N-dioxide/metal complex catalysts,diastereo-and enantiocontrol in this photochemical reaction happens with enantioselective protonation of enol intermediates as the crucial stereo-determining step,getting the better of the thermodynamical control pathway.Trans-oxazolidine-4-ones can be obtained in high enantioselectivity,which can be transferred into optically active cis-oxazolidine-4-ones after the treatment with a base.
基金Project supported by the National Basic Research Program of China （No. 2012CB934104）, the National Natural Science Foundation of China （No. 61474034）, the Natural Science Foundation of Heilongjiang Province of China （No. F201418）, and the Fundamental Research Funds for the Central Universities, China （Nos. HIT.NSRIF. 2014040 and HIT.NSRIF.2013040）
文摘Rotating speed is a critical parameter affecting the performance of rotor gyroscopes. Rotor gyroscopes must operate at the rated rotating speed. To shorten the start time of the ball-disk rotor gyroscope, this paper presents a new design of the drive system for a ball-disk rotor gyroscope. The drive system is monitored by a microeontroUer. First, the microcontroller generates a sine pulse width modulation signal to drive the permanent magnet rotor. Second, the position of the rotor is detected according to the back electromotive force in the non-energized coil. Third, a piecewise closed-loop control algorithm is implemented to keep the angular acceleration of the rotor within the safe range automatically during the acceleration process and when running at a constant speed. This control algorithm can avoid rotor stalling due to loss of steps. Experimental result shows that with the help of adaptive quick-start technique, the start time of the device can be shortened by up to 36.6%.
