全文获取类型
收费全文 | 48551篇 |
免费 | 8841篇 |
国内免费 | 11502篇 |
专业分类
化学 | 36321篇 |
晶体学 | 1512篇 |
力学 | 3056篇 |
综合类 | 1055篇 |
数学 | 6044篇 |
物理学 | 20906篇 |
出版年
2024年 | 44篇 |
2023年 | 578篇 |
2022年 | 1266篇 |
2021年 | 1454篇 |
2020年 | 1698篇 |
2019年 | 1852篇 |
2018年 | 1552篇 |
2017年 | 1933篇 |
2016年 | 2100篇 |
2015年 | 2512篇 |
2014年 | 2900篇 |
2013年 | 3807篇 |
2012年 | 4304篇 |
2011年 | 4509篇 |
2010年 | 3884篇 |
2009年 | 3977篇 |
2008年 | 4388篇 |
2007年 | 3777篇 |
2006年 | 3809篇 |
2005年 | 3213篇 |
2004年 | 2587篇 |
2003年 | 2003篇 |
2002年 | 2070篇 |
2001年 | 1852篇 |
2000年 | 1568篇 |
1999年 | 1053篇 |
1998年 | 651篇 |
1997年 | 468篇 |
1996年 | 440篇 |
1995年 | 381篇 |
1994年 | 382篇 |
1993年 | 336篇 |
1992年 | 285篇 |
1991年 | 194篇 |
1990年 | 176篇 |
1989年 | 145篇 |
1988年 | 145篇 |
1987年 | 96篇 |
1986年 | 89篇 |
1985年 | 84篇 |
1984年 | 61篇 |
1983年 | 61篇 |
1982年 | 31篇 |
1981年 | 39篇 |
1980年 | 20篇 |
1979年 | 45篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1959年 | 9篇 |
1936年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
51.
An incremental bundle method for portfolio selection problem under second-order stochastic dominance
Numerical Algorithms - We propose a preconditioner to accelerate the convergence of the GMRES iterative method for solving the system of linear equations obtained from discretize-then-optimize... 相似文献
52.
景俊 《原子与分子物理学报》2020,37(6):935-939
原子分子系统与量子化的电磁场或光子模式耦合的系统是非相对论量子力学理论研究和实验研究的主要对象和模型. 现实系统必然与外界环境耦合,且即便原子隔绝较好、光学腔壁品质因子足够高,原子系统也不等价于少数几个能级构成的简单模型:它仍然有不为零的几率跃迁到不可控的能级空间、与原子相互作用的自由空间真空场的量子效应也必须考虑. 本文将结合开放量子系统理论的基本要素与原子光子的基本模型,对原子分子系统在电磁场中发生的耗散以及量子退相干过程做简单综述,并重点介绍描述量子系统退相干过程的主流理论工具——主方程. 相似文献
53.
加速器中子活化实验是核专业本科实验教学中的重要实验之一.本文在利用14 MeV快中子辐照238U诱发裂变截面实验过程中引导学生挖掘实验数据,测量了裂变核素(89Rb)的重要核参数—半衰期.本工作运用低本底HPGe-γ谱仪测定裂变产物的一系列γ射线能谱,解谱得到89Rb的特征γ峰计数,从而确定89Rb的半衰期.数据分析中应用89Rb核素两条γ射线分别独立得到89 Rb的半衰期为15.30±0.09 min和15.25±0.07 min.测量结果在与文献数据符合较好的基础上,提高了测量精度. 相似文献
54.
55.
56.
You Qin Rongrong Zhou Jian Jin Jing Xie Hao Liu Ping Cai Jun Shu Yahui Zhao Luqi Huang Shuihan Zhang 《Biomedical chromatography : BMC》2020,34(8):e4841
Ophiocordyceps xuefengensis (O. xuefengensis), a new species of caterpillar fungus, has been identified as the sister taxon of Ophiocordyceps sinensis (O. sinensis). The aims of the present study are to evaluate the anticancer activity and to qualitatively analyze the potential bioactive chemical constituents of O. xuefengensis and O. sinensis, comparatively. An MTT assay was used to evaluate the in vitro anticancer activities of different fractions from O. xuefengensis and O. sinensis. The results show that ethyl acetate fractions of O. xuefengensis and O. sinensis have significant in vitro anticancer activity. These two bioactive fractions were analyzed by ultra-performance liquid chromatography–electrospray ionization with quadrupole–time of flight tandem mass spectrometry technology. A total of 82 compounds and 101 compounds were identified or tentatively characterized in the bioactive fractions of O. xuefengensis and O. sinensis, respectively. Among these compounds, 68 existed in both O. xuefengensis and O. sinensis. A total of 67 compounds were reported in O. xuefengensis and 8 compounds were reported in caterpillar fungus for the first time. This is the first detailed comparative analysis of the in vitro anticancer activity and chemical ingredients between O. xuefengensis and O. sinensis. The application of this work will provide reliable fundamental pharmacological substances for the use of O. xuefengensis by Yao people. 相似文献
57.
Jun Xu Victor V. Terskikh Yueying Chu Anmin Zheng Yining Huang 《Magnetic resonance in chemistry : MRC》2020,58(11):1082-1090
Metal–organic frameworks (MOFs) are a class of important porous materials with many current and potential applications. Their applications almost always involve the interaction between host framework and guest species. Therefore, understanding of host–guest interaction in MOF systems is fundamentally important. Solid-state NMR spectroscopy is an excellent technique for investigating host–guest interaction as it provides information complementary to that obtained from X-ray diffraction. In this work, using MOF α-Mg3(HCOO)6 as an example, we demonstrated that 13C chemical shift tensor of organic linker can be utilized to probe the host–guest interaction in MOFs. Obtaining 13C chemical shift tensor components (δ11, δ22, and δ33, where δ11 ≥ δ22 ≥ δ33) in this MOF is particularly challenging as there are six coordinatively equivalent but crystallographically non-equivalent carbons in the unit cell with very similar local coordination environment. Two-dimensional magic-angle-turning experiments were employed to measure the 13C chemical shift tensors of each individual crystallographically non-equivalent carbon in three microporous α-Mg3(HCOO)6 samples with different guest species. The results indicate that the δ22 component (with its direction approximately being co-planar with the formate anion and perpendicular to the C−H bond) is more sensitive to the adsorbate molecules inside the MOF channel due to the weak C−H···O hydrogen bonding or the ring current effect of benzene. The 13C isotropic chemical shift, on the other hand, seems much less sensitive to the subtle changes in the local environment around formate linker induced by adsorption. The approach described in this study may be used in future studies on host–guest interaction within MOFs. 相似文献
58.
59.
Shantaram Kothavale Kyung Hyung Lee Prof. Jun Yeob Lee 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(4):845-852
Two efficient thermally activated delayed fluorescent (TADF) emitters were developed by utilizing CN-modified imidazopyridine as an acceptor unit. The CN-modified imidazopyridine acceptor was combined with either an acridine donor or a phenoxazine donor through a phenyl linker to produce two TADF emitters, Ac-CNImPy and PXZ-CNImPy. The acridine-based Ac-CNImPy emitter exhibited sky-blue emission with a CIE coordinate of (0.18, 0.38), whereas the phenoxazine-donor-based PXZ-CNImPy showed greenish-yellow emission with a CIE coordinate of (0.32, 0.58). A high photoluminescence quantum yield of 80 % was observed for the PXZ-CNImPy emitter compared with 40 % for the Ac-CNImPy emitter. Organic light-emitting diodes based on the PXZ-CNImPy emitter demonstrated high external quantum efficiency of 17.0 %. Hence, the CN-modified imidazopyridine unit can be considered as a useful electron acceptor for the future design of highly efficient TADF emitters. 相似文献
60.
Dongping Wang Jie Dong Wenjing Fan Prof. Dr. Xiang-Ai Yuan Prof. Dr. Jian Han Prof. Dr. Jin Xie 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(22):8508-8512
An unprecedented Mn(I)-catalyzed selective hydroarylation and hydroalkenylation of unsaturated amides with commercially available organic boronic acids is reported. Alkenyl boronic acids have been successfully employed for the first time in Mn(I)-catalyzed carbon–carbon bond formation. A wide array of β-alkenylated amide products can be obtained in moderate to good yields, which offers practical access to five- and six-membered lactams. This protocol has predictable regio- and chemoselectivity, excellent functional group compatibility and ease of operation in air, representing a significant step-forward towards manganese-catalyzed C−C coupling. 相似文献