全文获取类型
收费全文 | 9106篇 |
免费 | 160篇 |
国内免费 | 55篇 |
专业分类
化学 | 3378篇 |
晶体学 | 231篇 |
力学 | 719篇 |
综合类 | 1篇 |
数学 | 999篇 |
物理学 | 3993篇 |
出版年
2023年 | 39篇 |
2022年 | 105篇 |
2021年 | 101篇 |
2020年 | 79篇 |
2019年 | 105篇 |
2018年 | 137篇 |
2017年 | 100篇 |
2016年 | 163篇 |
2015年 | 125篇 |
2014年 | 225篇 |
2013年 | 396篇 |
2012年 | 564篇 |
2011年 | 668篇 |
2010年 | 528篇 |
2009年 | 524篇 |
2008年 | 539篇 |
2007年 | 483篇 |
2006年 | 503篇 |
2005年 | 360篇 |
2004年 | 343篇 |
2003年 | 279篇 |
2002年 | 230篇 |
2001年 | 442篇 |
2000年 | 357篇 |
1999年 | 237篇 |
1998年 | 182篇 |
1997年 | 163篇 |
1996年 | 165篇 |
1995年 | 127篇 |
1994年 | 116篇 |
1993年 | 82篇 |
1992年 | 92篇 |
1991年 | 99篇 |
1990年 | 87篇 |
1989年 | 71篇 |
1988年 | 53篇 |
1987年 | 36篇 |
1986年 | 32篇 |
1985年 | 44篇 |
1984年 | 35篇 |
1983年 | 22篇 |
1982年 | 31篇 |
1981年 | 33篇 |
1980年 | 28篇 |
1979年 | 20篇 |
1978年 | 25篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1975年 | 19篇 |
1974年 | 16篇 |
排序方式: 共有9321条查询结果,搜索用时 15 毫秒
171.
Treatment of steroidal tertiary alcohols or olefins with N3H/BF3—éthérate/benzene gave 5-azido, 13-azido and 14-azido steroids. Reaction mechanism has been discussed. Chemical degradation and 13C NMR study confirmed structural assignments. 相似文献
172.
Quantitative structure-activity/property relationships (QSAR/QSPR) studies have been exploited extensively in the designs of drugs and pesticides, but few such studies have been applied to the design of colour reagents. In this work, the topological indices A(x1) - A(x3) suggested in this laboratory were applied to multivariate analysis in structure-property studies. The topological indices of 43 phosphone bisazo derivatives of chromotropic acid were calculated. The structure-property relationships between colour reagents and their colour reactions with cerium were studied using A (x1 a (x)3) indices with satisfactory results. The purpose of this work was to establish whether QSAR can be used to predict the contrasts of colour reactions and in the longer term to be a helpful tool in colour reagent design. 相似文献
173.
Zhen-Qiang Yu Xiaodong Li Wei Wan Xin-Shun Li Kuo Fu Yue Wu Alexander D. Q. Li 《Chemical science》2021,12(9):3146
Balancing the rigidity of a π-conjugated structure for strong emission and the flexibility of liquid crystals for self-assembly is the key to realizing highly emissive liquid crystals (HELCs). Here we show that (1) integrating organization-induced emission into dual molecular cooperatively-assembled liquid crystals, (2) amplifying mesogens, and (3) elongating the spacer linking the emitter and the mesogen create advanced materials with desired thermal–optical properties. Impressively, assembling the fluorescent acceptor Nile red into its host donor designed according to the aforementioned strategies results in a temperature-controlled Förster resonance energy transfer (FRET) system. Indeed, FRET exhibits strong S-curve dependence as temperature sweeps through the liquid crystal phase transformation. Such thermochromic materials, suitable for dynamic thermo-optical sensing and modulation, are anticipated to unlock new and smart approaches for controlling and directing light in stimuli-responsive devices.A temperature-sensitive Förster resonance energy transfer system was constructed using a highly emissive liquid crystal co-assembled with Nile red, enabling thermo-optical modulation for controlling and directing light in stimuli-responsive devices. 相似文献
174.
Y Masuo H Wang D Pélaprat Z Q Chi W Rostène 《Chemical & pharmaceutical bulletin》1992,40(9):2520-2524
We have recently demonstrated that [3H]ohmefentanyl, a non-peptidergic opioid ligand which was suggested to cross the blood brain barrier in contrast to other peptidergic opioid ligands, bound not only to mu opioid receptor sites but also to sigma sites. In order to examine whether [3H]ohmefentanyl can be used as a marker for mu sites, we investigated the effects of brain lesions on [3H]ohmefentanyl binding site densities, as compared with [3H][D-Ala2, MePhe4, Gly-ol5]enkephalin ([3H]DAGO), a selective mu ligand. These binding site densities were measured by quantitative autoradiography in the rat striatum and substantia nigra, two brain structures known to contain a high density of mu receptors, following lesions of the nigro-striatal dopaminergic pathway and striatal intrinsic neurons. Following unilateral nigral lesion with 6-hydroxydopamine, [3H]ohmefentanyl binding site densities were decreased in the patches (-35%) and matrix (-20%) of the ipsilateral striatum and in the lesioned substantia nigra pars compacta (-49%). Unilateral striatal lesion with quinolinic acid induced 72%, 61% and 50% decreases in [3H]ohmefentanyl binding in the patches and matrix of the lesioned striatum and in the ipsilateral substantia nigra pars reticulata, respectively. Similar results were obtained in the binding of [3H]DAGO. Indeed, a significant linear correlation was observed between [3H]ohmefentanyl and [3H]DAGO binding site densities. Therefore, mu opioid receptors may be mainly located on intrinsic neurons in the striatum, dopaminergic cell bodies in the substantia nigra pars compacta and nerve terminals of striatal efferents in the substantia nigra pars reticulata.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
175.
A comprehensive study has been conducted to compare the adsorptions of alkali metals (including Li, Na, and K) on the basal plane of graphite by using molecular orbital theory calculations. All three metal atoms prefer to be adsorbed on the "middle hollow site" above a hexagonal aromatic ring. A novel phenomenon was observed, that is, Na, instead of Li or K, is the weakest among the three types of metal atoms in adsorption. The reason is that the SOMO (single occupied molecular orbital) of the Na atom is exactly at the middle point between the HOMO and the LUMO of the graphite layer in energy level. As a result, the SOMO of Na cannot form a stable interaction with either the HOMO or the LUMO of the graphite. On the other hand, the SOMO of Li and K can form a relatively stable interaction with either the HOMO or the LUMO of graphite. Why Li has a relatively stronger adsorption than K on graphite has also been interpreted on the basis of their molecular-orbital energy levels. 相似文献
176.
Measurements of dipole strengths of chlorophylls in solution are reviewed and correlated. The refractive index dependence is found to be expressible in a simple empirical fashion that does not rely on the concept of vacuum dipole strength. The index dependence in some respects contradicts the dependence expected on the basis of effective field theories. 相似文献
177.
178.
The deposition of PbO2 on platinum and glassy carbon electrodes was investigated experimentally. The comparison of the different behaviour on the two types of electrodes is presented. The effects of variation of temperature, potential, nucleation order and induction time-lags on the electrocrystallisation process are discussed.The present investigations have confirmed that electrocrystallisation of PbO2 occurs through 3-D nucleation and growth on glassy carbon and most probably also on platinum. The data on platinum contains considerable noise from simultaneous chemical processes and is not amenable for analysis in a simple way. The steady state behaviour is unusual because it has generally been assumed that charge transfer is rate determining under most conditions. However, as shown here, if lattice incorporation is slow even at the steady state, it provides a method for separating the activation energies associated with the nucleation and growth rate constants. Finally, the dissolution of PbO2 into the platinum substrate and its implication on transient rates merits more attention. In this context the Pt-PbO2 system is probably not a good choice because of the large contribution of substrate oxidation to the measured transient rates. 相似文献
179.
180.
超大型航天结构具有超大柔性、超低固有频率的特点,空间机器人在轨组装时应尽可能避免激起超大型结构的柔性振动.空间机器人组装超大型结构模块的过程分成抓捕阶段、位姿调整与稳定阶段、安装阶段和爬行阶段.通过对安装阶段的动力学与控制研究,提出共线安装的轨迹规划方法,有效避免了柔性结构振动.首先,采用自然坐标法和绝对节点坐标法建立主结构-空间机器人-待组装结构的在轨组装系统动力学模型.然后,将共线安装的要求转化为空间机器人的轨迹规划约束,要求空间机器人质心到主结构/待组装结构的距离保持不变,实现共线安装的轨迹规划.数值仿真表明:提出的组装方法在组装过程中可有效避免超大型结构的横向运动,降低夹持力矩.最后,分析了系统参数对组装过程动力学响应的影响,为超大型航天器的在轨组装提供了参考. 相似文献