全文获取类型
收费全文 | 20188篇 |
免费 | 3955篇 |
国内免费 | 2900篇 |
学科分类
数理化 | 27043篇 |
出版年
2024年 | 21篇 |
2023年 | 396篇 |
2022年 | 498篇 |
2021年 | 689篇 |
2020年 | 887篇 |
2019年 | 839篇 |
2018年 | 761篇 |
2017年 | 723篇 |
2016年 | 1055篇 |
2015年 | 1034篇 |
2014年 | 1192篇 |
2013年 | 1563篇 |
2012年 | 1949篇 |
2011年 | 2010篇 |
2010年 | 1443篇 |
2009年 | 1395篇 |
2008年 | 1570篇 |
2007年 | 1286篇 |
2006年 | 1194篇 |
2005年 | 940篇 |
2004年 | 755篇 |
2003年 | 597篇 |
2002年 | 575篇 |
2001年 | 487篇 |
2000年 | 430篇 |
1999年 | 398篇 |
1998年 | 320篇 |
1997年 | 287篇 |
1996年 | 276篇 |
1995年 | 221篇 |
1994年 | 205篇 |
1993年 | 175篇 |
1992年 | 154篇 |
1991年 | 126篇 |
1990年 | 131篇 |
1989年 | 99篇 |
1988年 | 72篇 |
1987年 | 51篇 |
1986年 | 55篇 |
1985年 | 46篇 |
1984年 | 38篇 |
1983年 | 29篇 |
1982年 | 25篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1974年 | 4篇 |
1972年 | 2篇 |
1966年 | 2篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
101.
In this paper, an interior point cutting plane method (IPCPM)is applied to solve optimal power flow (OPF) problems. Comparedwith the simplex cutting plane method (SCPM), the IPCPM is simpler,and efficient because of its polynomial-time characteristic.Issues in implementing IPCPM for OPF problems are addressed,including (1) how to generate cutting planes without using thesimplex tableau, (2) how to identify the basis variables inIPCPM, and (3) how to generate mixed integer cutting planes.The calculation speed of the proposed algorithm is further enhancedby utilizing the sparsity features of the OPF formulation. Numericalsimulations on IEEE 14-300-bus test systems have shown thatthe proposed method is effective. 相似文献
102.
103.
Based on the generalized Huygens-Fresnel diffraction integral, a recurrence propagation equation of Hermite-Gaussian (H-G) beams through a paraxial optical ABCD system with hard-edge aperture is derived, which permits us to obtain the analytical propagation expression for H-G beams of any order. The advantages of our analytical results are analyzed, and the application is illustrated with numerical examples. 相似文献
104.
105.
基于新型长周期光纤光栅的掺铒光纤放大器 总被引:10,自引:8,他引:2
报道了基于高频CO2激光脉冲写入的新型长周期光纤光栅的低噪音掺铒光纤放大器,这种新型长周期光纤光栅是用大约几千Hz的高频CO2激光脉冲对光纤玻璃热冲击作用而形成的.在铒纤中插入一个长周期光纤光栅,会明显减少掺铒光纤放大器的放大自发辐射(ASE)噪音.报道了两种低噪音掺铒光纤放大器,作为前置放大器和线路放大器,它们的ASE噪音指数分别从4.0 dB减少到3.5 dB和从4.8 dB减少到4.3 dB,并且在作为线放时,其小信号增益从30 dB提高到37 dB,降噪及提高增益效应十分显著. 相似文献
106.
用密度泛函B3LYP/ 6 3 1G(d)方法 ,对质子化丙酮分子团簇 (CH3COCH3) nH+ (n =1~ 7)弱相互作用体系进行了全自由度能量梯度优化 ,得到了该系列团簇的稳定结构及其对应的体系能量 .通过对构型的分析得出了质子化丙酮分子团簇 (CH3COCH3) nH+ (n =1~ 7)的生长规律 .计算了中性丙酮分子团簇体系的质子亲合能并总结出其变化趋势 .分析讨论了质子化团簇的红外振动光谱 ,发现质子化团簇的振动光谱普遍较中性环型团簇的振动光谱复杂 ,最强的振动峰来源于质子在溶剂壳中两个氧原子之间的振动 ,而且随着团簇尺寸的增加羰基的伸缩振动峰的数目也随之增多 相似文献
107.
Hong‐Qing Wang Zhao‐Jie Liu Li‐Min Yang Ming‐Wu Ding 《Journal of heterocyclic chemistry》2004,41(3):393-397
A series of new 1H‐pyrazolo[3,4‐d]pyrimidin‐4(5H)‐one Derivatives 5 has been designed and regio‐selectively synthesized via a tandem aza‐Wittig reaction. The structures of all compounds prepared have been confirmed by 1H NMR, IR, EI‐MS spectroscopy and elemental analyses. The results of preliminary bioassay indicate that most compounds 5 possess an inhibition effect against Botrytis cinereapers and Pyricularia oryzae at the concentration of 50 mg/L. 相似文献
108.
1689-nm diode lasers used in medical apparatus have been fabricated and characterized. The lasers had pnpn InP current confinement structure, and the active region consisted of 5 pairs of InGaAs quantum wells and InGaAsP barriers. 相似文献
109.
Numerical studies were conducted to investigate the natural convection heat transfer around a uniformly heated thin plate
with arbitrary inclination in an infinite space. The numerical approach was based on the finite volume technique with a nonstaggered
grid arrangement. For handling the pressure–velocity coupling the SIMPLE-algorithm was used. QUICK scheme and first order
upwind scheme were employed for discretization of the momentum and energy convective terms respectively. Plate width and heating
rate were used to vary the modified Rayleigh number over the range of 4.8×106 to 1.87×108. Local and average heat transfer characteristics were compared with regarding to the inclination angle. The empirical expressions
for local and average Nusselt number were correlated. It has been found that for inclination angle less than 10∘, the flow and heat transfer characteristics are complicated and the average Nusselt number can not be correlated by one equation
while for inclination angle larger than 10∘, the average Nusselt number can be correlated into an elegant correlation.
Received on 18 April 2001 / Published online: 29 November 2001 相似文献
110.