全文获取类型
收费全文 | 6731篇 |
免费 | 1618篇 |
国内免费 | 361篇 |
专业分类
化学 | 1318篇 |
晶体学 | 25篇 |
力学 | 1466篇 |
综合类 | 83篇 |
数学 | 648篇 |
物理学 | 5170篇 |
出版年
2024年 | 21篇 |
2023年 | 64篇 |
2022年 | 196篇 |
2021年 | 209篇 |
2020年 | 205篇 |
2019年 | 204篇 |
2018年 | 203篇 |
2017年 | 271篇 |
2016年 | 328篇 |
2015年 | 275篇 |
2014年 | 484篇 |
2013年 | 470篇 |
2012年 | 415篇 |
2011年 | 500篇 |
2010年 | 349篇 |
2009年 | 407篇 |
2008年 | 428篇 |
2007年 | 434篇 |
2006年 | 353篇 |
2005年 | 371篇 |
2004年 | 264篇 |
2003年 | 302篇 |
2002年 | 282篇 |
2001年 | 216篇 |
2000年 | 204篇 |
1999年 | 189篇 |
1998年 | 145篇 |
1997年 | 129篇 |
1996年 | 133篇 |
1995年 | 102篇 |
1994年 | 75篇 |
1993年 | 65篇 |
1992年 | 62篇 |
1991年 | 51篇 |
1990年 | 37篇 |
1989年 | 49篇 |
1988年 | 25篇 |
1987年 | 34篇 |
1986年 | 25篇 |
1985年 | 22篇 |
1984年 | 22篇 |
1983年 | 13篇 |
1982年 | 25篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 16篇 |
1978年 | 5篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1957年 | 3篇 |
排序方式: 共有8710条查询结果,搜索用时 9 毫秒
41.
A method is described for the rapid and automatic analysis of flexible molecular alignments using multidimensional scaling and a normalized scoring scheme. A projection scheme was devised to separate orientational and conformational effects. It is shown that the approach can be utilized for the identification of common binding orientations or to the study of differences in partioning behavior. It is suggested that the method can be employed as a novel approach exploring molecular similarity as a dynamic property, so that it includes aspects of motion (by way of mutual orientations), conformations and molecular properties. 相似文献
42.
Delayed feedbacks are quite common in many physical and biological systems and in particular many physiological systems. Delay
can cause a stable system to become unstable and vice versa. One of the well-studied non-biological chemical oscillators is
the Belousov-Zhabotinskii (BZ) reaction. This gives relaxation oscillations for a considerable period of time under batch
conditions. This paper deals with the effect of perturbing the limit cycle oscillation of BZ reaction by employing a delayed
electrical feedback to the system under batch conditions. The parameters chosen to study are external resistance and delay.
For various resistances and delays the system was electrically perturbed and found to exhibit various complex mixed mode oscillations.
The dynamic features are accounted for by the Oregonator model, with time delay incorporated in one of the variables.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
43.
C. Joly S. Goizet J. C. Schrotter J. Sanchez M. Escoubes 《Journal of membrane science》1997,130(1-2):63-74
The effect of introduction of silica particles prepared by the sol-gel technique on the gas transport properties of a polyimide film was studied. The sorption and permeation of N2, O2, CO2, H2 and CH4 were studied and correlated with morphological changes in the polymer structure. From sorption isotherms, we observed that the composite membrane presents higher solubility coefficients than the polyimide one. The solubility coefficient ratio between the composite and the polyimide is about 1.5–2.0. The isotherms were analyzed in terms of the dual mode sorption. The Henry's coefficient and the Langmuir's affinity and saturation constants were obtained allowing to calculate the Langmuir to Henry concentration ratios as function of the gas pressure. These ratios decrease until zero within a certain pressure range as long as the Langmuir's mode is acting and they are higher for the polyimide membrane when compared with the composite one. This study was completed with calorimetric measurements during the sorption. The gas interaction energy in kJ/mol decreases within the same pressure range as previously described. The measured energies are higher for the polyimide film when compared with the composite one because the polyimide membrane presents a stronger energetic effect caused by a higher Langmuir's contribution. From permeation studies at 3.155 Pa, the composite membrane showed higher permeability coefficients and permselectivities than the polyimide one. All these results were explained, taking into account the difference on the imidization degree of both membranes and the morphological changes which may be induced by the silica nodules in the organic/inorganic interphases. 相似文献
44.
45.
论述了激光拉曼光谱对高分子结构、结晶形态和表征,反应动力学过程和取向的研究,还介绍了纵向声学模、共振、高温高压、光波导和付里叶拉曼光谱在高分子研究中的最新进展。 相似文献
46.
基于Lagrange原理和假设模态法建立了旋转输液管的动力学模型.通过降阶升维的方法求解系统的特征值问题,并分析了旋转输液管自由振动特性.得到了不同端部集中质量和转速下,系统特征值随流速升高的演变轨迹.揭示了临界流速随系统参数的变化规律.研究发现,内部流体的流动对旋转输液管动力学特性存在显著影响.在某些参数组合下,系统低阶模态能够形成不同形式的内共振关系.预示了旋转输液管模型蕴含丰富的动力学现象. 相似文献
47.
本文首先运用考虑非期望产出的SBM-SupSBM模型估计我国各省市绿色技术创新效率并进行分类:稳定上升类、平稳类、大幅度波动类以及下降类。之后从企业内部和外部两方面对绿色技术创新效率提升的影响因素提出假设,着重考虑大幅度波动类别省份的效率提升。运用面板数据模型各影响因素进行验证,结论显示:企业规模对绿色技术创新效率波动较大的省份起到显著的抑制作用,环境规制和技术进步对绿色技术创新效率波动较大的省份作用不明显。针对分析结果,提出结论:在提升绿色技术创新效率的过程中,应注重企业规模和技术进步的协同作用,以及技术的开放程度。 相似文献
48.
This paper focus on the event-triggered sliding mode controller design for discrete-time switched genetic regulatory networks (GRNs) with persistent dwell time (PDT) switching. Firstly, the observation error dynamics of switched GRNs with PDT is constructed in the light of event-triggered sliding mode control (SMC) scheme. Next, sufficient conditions are derived to ensure the exponential stability of the augmented plant. Moreover, an event-triggered SMC law is synthesized to impel the system trajectories onto the sliding surface in a finite time. Finally, a verification example is provided to illustrate the effectiveness and potential of the proposed method. 相似文献
49.
Raman phonon spectroscopy has been used to study photodimerization reaction inp-chloro cinnamic acid (pCCA) crystal. The β-form of the crystal yields the 4,4′-dichloro-β-truxinic acid dimer. Six distinct low frequency phonon
bands are observed in thepCCA monomer crystal. On reaction progress, these bands show a monotonic shift to lower frequencies and broaden out. Finally,
in the dimer crystal the phonon spectrum shows two weak broad bands. These results suggest that the reaction is homogeneous
in the initial stages and, as the product concentration increases, the lattice becomes highly disordered. The reactant and
the product were characterised by infrared and Raman spectroscopy. The disappearance of aliphatic C=C bond stretching vibration
and appearance of cyclobutane ring deformation and cyclobutane ring-breathing vibrations on reaction confirm photodimerization
by cyclobutane ring formation. The large Stoke's shift between the absorption and emission band suggest strong exciton-phonon
coupling in the monomer lattice. This reaction seems to be phonon-mediated. 相似文献
50.
Circular high-performance thin-layer chromatography (HPTLC) has been used to differentiate a series of twelve tricyclic neuroleptics, using both normal phase and reverse phase procedures. The use of normal phase systems also allows the resolution of geometric isomers of chlorprothixene, clopenthixol and flupenthixol. Thirteen sulphonamides and Trirnethoprim may also be distinguished using HPTLC. 相似文献