全文获取类型
收费全文 | 23416篇 |
免费 | 849篇 |
国内免费 | 1257篇 |
专业分类
系统科学 | 734篇 |
丛书文集 | 635篇 |
教育与普及 | 198篇 |
理论与方法论 | 33篇 |
现状及发展 | 179篇 |
综合类 | 23739篇 |
自然研究 | 4篇 |
出版年
2024年 | 87篇 |
2023年 | 193篇 |
2022年 | 378篇 |
2021年 | 381篇 |
2020年 | 388篇 |
2019年 | 357篇 |
2018年 | 334篇 |
2017年 | 434篇 |
2016年 | 426篇 |
2015年 | 650篇 |
2014年 | 979篇 |
2013年 | 786篇 |
2012年 | 1218篇 |
2011年 | 1200篇 |
2010年 | 881篇 |
2009年 | 1061篇 |
2008年 | 1014篇 |
2007年 | 1472篇 |
2006年 | 1395篇 |
2005年 | 1161篇 |
2004年 | 1083篇 |
2003年 | 988篇 |
2002年 | 934篇 |
2001年 | 899篇 |
2000年 | 765篇 |
1999年 | 701篇 |
1998年 | 649篇 |
1997年 | 670篇 |
1996年 | 564篇 |
1995年 | 489篇 |
1994年 | 509篇 |
1993年 | 427篇 |
1992年 | 421篇 |
1991年 | 383篇 |
1990年 | 306篇 |
1989年 | 330篇 |
1988年 | 279篇 |
1987年 | 180篇 |
1986年 | 93篇 |
1985年 | 39篇 |
1984年 | 11篇 |
1983年 | 1篇 |
1955年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
张良忠 《北京理工大学学报》1998,18(3):331-335
探讨在电子光学成像系统中像评定的一种途径。方法,从静电这像差均方根理论出发,追迹计算大量空间轨迹的落点及系统均根半径,并由此衍算成弦财制传递函数。 相似文献
152.
研究了甲酸和乙酸于水中和不同有机介质中的分配系数,重点研究了含21、24和27碳原子的三烷基叔胺对甲酸和乙酸的成盐萃取作用。同时,在不同稀释剂及其用量、温度以及甲酸和乙酸浓度的条件下,确定了小分子羧酸的分配系数及其变化规律,最后,根据有机羧酸叔胺盐的热分解行为获得了纯度达90%的甲酸和乙酸,从而为重有机工业的酸回收提供了工艺技术设计的依据。 相似文献
153.
RbCl在混合溶剂中的介质效应 总被引:1,自引:0,他引:1
应用一价阳离子选择电极和氯离子选择电极组成及逆电池。通过测定电池的电动势,根据溶液热力学原理,引用扩展的Debye-Hueckel公式,计算求得了RbCl在(H2O ̄DMF)混合溶剂中的活度系数,一级,二级和总介质效应,并对RbCl的介质效应进行了讨论。 相似文献
154.
李健 《山东师范大学学报(自然科学版)》1998,13(3):275-278
利用变换矩阵和ABCD定律讨论了激光谐振空对高我束的变换,推导出高斯光束参数通过光腔输出镜的变换公式。并对热透镜腔,光束远场发散角等问题进行了讨论。 相似文献
155.
Four basic explicit analytical solutions of non-Fourier heat conduction equation simulating IC chip thermal condition are
derived. Some other analytical series solutions can also be derived with the above-mentioned basic solutions. Analytical solution
has its own irreplaceable important meaning in theory. In addition, as the standard solution to check the accuracy, convergence
and effectiveness of various numerical computational methods and their differencing schemes, grid generation ways and so on,
the analytical solution is very useful also for the newly rapidly developing computational heat transfer. 相似文献
156.
The relaxation of the highly vibrationally excited CO (v = 1–8) by CO2 is studied by timeresolved Fourier transform infrared emission spectroscopy (TR FTIR). 193 nm laser photolysis of the mixture
of CHBr3 with O2 generates the highly vibrationally excited CO(v) molecules. TR FTIR records the intense infrared emission of CO(v→v-1). The
vibrational populations of each level of CO(v) have been determined by the method of spectral simulation. Based on the evolution
of the time resolved populations and the differential method, 8 energy transfer rate constants of CO(v = 1–8) to CO2 molecules areobtained: (5.7±0.1), (5.9±0.1), (5.2±0.2), (3.4±0.2), (2.4±0.3), (2.2±0.4), (2.0±0.4) and (1.8±0.6) (1014 cm3 · molecule−1 · s−1), respectively. A two-channel energy transfer model can explain the feature of the quenching of CO(v) by CO2. For the lower vibrational states of CO, the vibrational energy transfers preferentially to the u3 mode of CO2 For the higher levels, the major quenching channel changes to the vibrational energy exchange between CO(v→v-1) and the u1 mode of CO2. 相似文献
157.
Jianguo Ren Xianbin Wang Jianfa Chen Chunyuan Li Hui Yang Ziyuan Ouyang 《科学通报(英文版)》1998,43(9):761-761
The isotopic cumpositions of helium have been investigated in hot spring gases sampled from the Liaodong Peninsula. China.
The3He/4He ratios range from 0.14 Ra (0.14 times the atmospheric3He/4He ratio of 1.40 × 10-6 to 0.72 Ra.3He/4He versus4He/20Ne ratios and He contents suggest that a small amount of mantle-derived helium reaches the earth’s surface. Helium iwtope
ratios of the hot spring gases show a distribution pattern similar to the regional heat flow values, indicating that the positive
thermal anomaly occurring in the rtrea is closely associated with the presence of mantle-derived helium. The observations
described above provide some significant evidence that the molten magma originating from partial melting of the upper mantle
may have intruded into the continental crust along the activity fault. The coupling of the presence of mantle He and heat
flow anomalies to seismic activity in studied region seem to imply that there is an internal relationship between mantle degassing
and seismic activity. 相似文献
158.
The vibrational energy transfer from highly vibrationally excited CO to H2O molecules is studied by time-resolved Fourier transform infrared emission spectroscopy (TR FTIR). Following the 193 nm laser
photolysis of CHBr3 and O2 the secondary reactions generate CO(v). The infrared emission of CO(v → v−1) is detected by TR FTIR. The excitation of H2O molecules is not observed. By the method of the spectral simulation and the differential technique, 8 rate constants for
CO(v)/H2O system are obtained: (1.7 ±0.1), (3.4 ±0.2), (6.2 ±0.4), (8.0 ±1.0), (9.0 ±2.0), (12 ±3), (16 ±4) and (18 ±7) (1013cm3 · molecule-1· s-1). At least two reasons lead to the efficient energy transfer. One is the contributions of the rotational energy to the vibational
energy defect and the other is the result of the complex collision. With the SSH andab initio calculations, the quenching mechanism of CO(v) by H2O is suggested. 相似文献
159.
The lattice distortion of nanoscale silver particles prepared is studied by using the chemical method. The particles lattice
contract was found, and the lattice contraction become greater while the particle size decreases. The results were explained
with surface energy and surface tension. 相似文献
160.
The effect of lanthanum ions on the structural and conformational change of yeast tRNA Phe was studied by 1H NMR. The results suggest that the tertiary base pair (G-15)(C-48), which was located in the terminal in the augmented dihydrouridine helix (D-helix), was markedly affected by adding La 3+ and shifted 0.33 downfield. Based pair (U-8)(A-14), which is associated with a tertiary interaction, links the base of the acceptor stem to the D-stem and anchors the elbow of the L structure, shifted 0.20 upfield. Another imino proton that may be affected by La 3+ in tRNA Phe is the tertiary base pair (G-19)(C-56). The assignment of this resonance is tentative since it is located in the region of highly overlapping resonances between 12.6 and 12.2. This base pair helps to anchor the D-loop to the TΨC loop. 相似文献