全文获取类型
收费全文 | 5127篇 |
免费 | 865篇 |
国内免费 | 741篇 |
专业分类
化学 | 3354篇 |
晶体学 | 164篇 |
力学 | 250篇 |
综合类 | 94篇 |
数学 | 828篇 |
物理学 | 2043篇 |
出版年
2024年 | 13篇 |
2023年 | 81篇 |
2022年 | 128篇 |
2021年 | 168篇 |
2020年 | 208篇 |
2019年 | 193篇 |
2018年 | 196篇 |
2017年 | 206篇 |
2016年 | 251篇 |
2015年 | 238篇 |
2014年 | 264篇 |
2013年 | 387篇 |
2012年 | 490篇 |
2011年 | 460篇 |
2010年 | 389篇 |
2009年 | 383篇 |
2008年 | 404篇 |
2007年 | 372篇 |
2006年 | 316篇 |
2005年 | 253篇 |
2004年 | 180篇 |
2003年 | 158篇 |
2002年 | 197篇 |
2001年 | 168篇 |
2000年 | 118篇 |
1999年 | 87篇 |
1998年 | 51篇 |
1997年 | 37篇 |
1996年 | 47篇 |
1995年 | 31篇 |
1994年 | 37篇 |
1993年 | 31篇 |
1992年 | 26篇 |
1991年 | 27篇 |
1990年 | 27篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1980年 | 2篇 |
1977年 | 5篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1971年 | 4篇 |
排序方式: 共有6733条查询结果,搜索用时 31 毫秒
881.
Xin Chen Mengke Li Zongxue Yu Qiang Ke 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2016,90(12):2413-2417
The oxygen reduction reaction (ORR) catalyzed by mononuclear and planar binuclear cobalt (CoPc) and iron phthalocyanine (FePc) catalysts is investigated in detail by density functional theory (DFT) methods. The calculation results indicate that the ORR activity of Fe-based Pcs is much higher than that of Co-based Pcs, which is due to the fact that the former could catalyze 4e- ORRs, while the latter could catalyze only 2e- ORRs from O2 to H2O2. The original high activities of Fe-based Pcs could be attributed to their high energy level of the highest occupied molecular orbital (HOMO), which could lead to the stronger adsorption energy between catalysts and ORR species. Nevertheless, the HOMO of Co-based Pcs is the ring orbital, not the 3d Co orbital, thereby inhibiting the electron transfer from metal to adsorbates. Furthermore, compared with mononuclear FePc, the planar binuclear FePc has more stable structure in acidic medium and more suitable adsorption energy of ORR species, making it a promising non-precious electrocatalyst for ORR. 相似文献
882.
An improvement on a generalized preconditioned Hermitian and skew-Hermitian splitting method (GPHSS), originally presented by Pan and Wang (J. Numer. Methods Comput. Appl. 32, 174–182, 2011), for saddle point problems, is proposed in this paper and referred to as IGPHSS for simplicity. After adding a matrix to the coefficient matrix on two sides of first equation of the GPHSS iterative scheme, both the number of required iterations for convergence and the computational time are significantly decreased. The convergence analysis is provided here. As saddle point problems are indefinite systems, the Conjugate Gradient method is unsuitable for them. The IGPHSS is compared with Gauss-Seidel, which requires partial pivoting due to some zero diagonal entries, Uzawa and GPHSS methods. The numerical experiments show that the IGPHSS method is better than the original GPHSS and the other two relevant methods. 相似文献
883.
The amide A band of protein is sensitive to the hydrogen bands of amide groups of proteins. However, it is hard to distinguish the amide A band of aqueous protein in situ directly, since it overlaps with O-H stretching vibration of water. In this work, we presented a new analytical method of Raman ratio spectrum, which can extract the amide A band of proteins in water. To obtain the Raman ratio spectrum, the Raman spectrum of aqueous protein was divided by that of pure water. A mathematical simulation was employed to examine whether Raman ratio spectrum is effective. Two kinds of protein, lysozyme and α-chymotrypsin were employed. The amide A bands of them in water were extracted from Raman ratio spectra. Additionally, the process of thermal denaturation of lysozyme was detected from Raman ratio spectrum. These results demonstrated the Raman ratio spectra could be employed to study the amide A modes of proteins in water. 相似文献
884.
In the present study, we synthesized well-defined tadpole-shaped polystyrene (PS) via the combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted azide-alkyne cycloaddtion (SPAAC) reaction. A di-bromo ATRP initiator (Br-ini-Br) containing cyclopropenone-masked dibenzocyclooctyne group was used to prepare the linear PS with a cyclopropenone-masked dibenzocyclooctyne in the middle of the chain and bromo groups at both ends (Br-PS-Br). Then we used the single electron transfer-nitroxide radical coupling (SET-NRC) reaction to transfer the bromo end groups to azide groups (N3-PS-N3). After UV irradiation, the dibenzocyclooctyne group was quantitatively released, and intramolecularly reacted with alternative azide end group to produce the tadpole-shaped PS based on SPAAC reaction. 相似文献
885.
886.
给出广义e-ω-凹算子的定义,在假设算子A不是锥映射的前提下,得到了广义e-ω-凹算子的不动点的存在唯一性以及单调迭代列.最后,将主要结果应用到一类Hammerstein型积分方程中去. 相似文献
887.
Pictet–Spengler Synthesis of Quinoline‐Fused Porphyrins and Phenanthroline‐Fused Diporphyrins
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Ke Gao Norihito Fukui Seok II Jung Prof. Dr. Hideki Yorimitsu Prof. Dr. Dongho Kim Prof. Dr. Atsuhiro Osuka 《Angewandte Chemie (International ed. in English)》2016,55(42):13038-13042
Doubly and quadruply quinoline‐fused porphyrins were effectively synthesized through a reaction sequence consisting of Suzuki–Miyaura coupling of β‐borylated porphyrins with 2‐iodoaniline and subsequent Pictet–Spengler cyclization. These quinoline‐fused porphyrins display red‐shifted absorption bands and higher electron‐accepting abilities. This synthetic protocol also allowed the synthesis of phenanthroline‐fused porphyrin dimers, which bound either a NiII or ZnII cation. The resultant metal complexes displayed further red shifted absorption spectra and molecular twists to effect an almost perpendicular arrangement of the two porphyrins. 相似文献
888.
A Variety of Phase‐Transition Behaviors in a Niccolite Series of [NH3(CH2)4NH3][M(HCOO)3]2
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ran Shang Sa Chen Ke‐Li Hu Prof. Bing‐Wu Wang Prof. Zhe‐Ming Wang Prof. Song Gao 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(18):6199-6203
A niccolite series of [bnH22+][M(HCOO)3]2 (bnH22+=1,4‐butyldiammonium) shows four kinds of metal‐dependent phase transitions, from high temperature para‐electric phases to low‐temperature ferro‐, antiferro‐, glass‐like, and para‐electric phases. The conformational flexibility of bnH22+ and the different size, mass, and bonding character of the metal ion lead to various disorder‐order transitions of bnH22+ in the lattice and relevant framework modulations, thus different phase transitions and dielectric responses. The magnetic members display a coexistence or combination of electric and magnetic orderings in the low‐temperature region. 相似文献
889.
Existence and global exponential stability of almost periodic solution for delayed competitive neural networks with discontinuous activations
下载免费PDF全文
![点击此处可从《Mathematical Methods in the Applied Sciences》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In this paper, we study a class of delayed competitive neural networks with discontinuous activations. Without assuming the boundedness and local Lipschizian on the activation functions, some new criteria ensuring the existence and global exponential stability of almost periodic solutions for the neural network model considered in this work are established by constructing some suitable Lyapunov functionals and employing the theory of nonsmooth analysis. Finally, we present some applications and numerical examples with simulations to show the effectiveness of our main results. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
890.
Synthesis and evaluation of a maltose‐bonded silica gel stationary phase for hydrophilic interaction chromatography and its application in Ginkgo Biloba extract separation in two‐dimensional systems
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Qianying Sheng Kaiya Yang Yanxiong Ke Xinmiao Liang Minbo Lan 《Journal of separation science》2016,39(17):3339-3347
Maltose covalently bonded to silica was prepared by using carbonyl diimidazole as a cross‐linker and employed as a stationary phase for hydrophilic interaction liquid chromatography. The column efficiency and the effect of water content, buffer concentration, and pH value influenced on retention were investigated. The separation or enrichment selectivity was also studied with nucleosides, saccharides, amino acids, peptides, and glycopeptides. The results indicated that the stationary phase processed good separation efficiency and separation selectivity in hydrophilic interaction liquid chromatography mode. Moreover, a two‐dimensional hydrophilic interaction liquid chromatography× reversed‐phase liquid chromatography method with high orthogonality was developed to analyze the Ginkgo Biloba extract fractions. The development of this two‐dimensional chromatographic system would be an effective tool for the separation of complex samples of different polarities and contents. 相似文献