全文获取类型
收费全文 | 1546篇 |
免费 | 272篇 |
国内免费 | 135篇 |
专业分类
化学 | 448篇 |
晶体学 | 3篇 |
力学 | 145篇 |
综合类 | 41篇 |
数学 | 1011篇 |
物理学 | 305篇 |
出版年
2024年 | 2篇 |
2023年 | 23篇 |
2022年 | 39篇 |
2021年 | 54篇 |
2020年 | 86篇 |
2019年 | 57篇 |
2018年 | 59篇 |
2017年 | 73篇 |
2016年 | 86篇 |
2015年 | 62篇 |
2014年 | 78篇 |
2013年 | 140篇 |
2012年 | 81篇 |
2011年 | 81篇 |
2010年 | 67篇 |
2009年 | 59篇 |
2008年 | 82篇 |
2007年 | 80篇 |
2006年 | 79篇 |
2005年 | 85篇 |
2004年 | 67篇 |
2003年 | 50篇 |
2002年 | 67篇 |
2001年 | 43篇 |
2000年 | 60篇 |
1999年 | 47篇 |
1998年 | 30篇 |
1997年 | 36篇 |
1996年 | 27篇 |
1995年 | 19篇 |
1994年 | 17篇 |
1993年 | 20篇 |
1992年 | 8篇 |
1991年 | 8篇 |
1990年 | 6篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有1953条查询结果,搜索用时 234 毫秒
51.
This work concentrates on extending the utilization of multiple dual mode (MDM) counter‐current chromatography in chiral separations. Two aromatic acids, 2‐(6‐methoxy‐2‐naphthyl)propionic acid (NAP) and 2‐phenylpropionic acid (2‐PPA), were enantioseparated by MDM counter‐current chromatography using hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD) as chiral selector. The two‐phase solvent systems consisting of n‐hexane/ethyl acetate 0.1 mol/L phosphate buffer pH 2.67 containing 0.1 mol/L HP‐β‐CD (7.5:2.5:10 for NAP and 7:3:10 for 2‐PPA, v/v/v) were used. Conventional MDM and modified MDM were compared according to peak resolution under current separation mechanism. The influence of elution time after the first‐phase inversion and number of cycles for MDM were investigated. Peak resolution of NAP and 2‐PPA increased from 0.62 to 1.05 and 0.72 to 0.84, respectively, using optimized MDM conditions. Being an alternative elution method for counter‐current chromatography, MDM elution greatly improved peak resolution in chiral separations. 相似文献
52.
Papori Goswami 《合成通讯》2013,43(13):2271-2278
A wide range of substituted coumarin derivatives were synthesized by refluxing in acetonitrile, ethyl acetoacetate, and ethyl benzoyl acetate with a wide range of structurally diverse phenol derivatives within a short reaction time with a catalytic combination of pyridine dicarboxylic acid as organocatalyst and nanocrystalline ZnO. 相似文献
53.
Prof. Dr. A. Stephen K. Hashmi Tobias Lauterbach Dipl.‐Chem. Pascal Nösel Dr. Mie Højer Vilhelmsen Dr. Matthias Rudolph Dr. Frank Rominger 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(3):1058-1065
A series of dinuclear gold σ,π‐propyne acetylide complexes were prepared and tested for their catalytic ability in dual gold catalysis that was based on the reaction of an electrophilic π‐complex of gold with a gold acetylide. The air‐stable and storable catalysts can be isolated as silver‐free catalysts in their activated form. These dual catalysts allow a fast initiation phase for the dual catalytic cycles without the need for additional additives for acetylide formation. Because propyne serves as a throw‐away ligand, no traces of the precatalyst are generated. Based on the fast initiation process, side products are minimized and reaction rates are higher for these catalysts. A series of test reactions were used to demonstrate the general applicability of these catalysts. Lower catalyst loadings, faster reaction rates, and better selectivity, combined with the practicability of these catalysts, make them ideal catalysts for dual gold catalysis. 相似文献
54.
We present here a fine singularity analysis of solutions to the Laplace equation in special polygonal domains in the plane. We assume piecewise constant Neumann data on one component of the boundary. Our motivation is to study the so‐called Berg effect, which is explained in the introduction. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
55.
Infinitely many homoclinic orbits for a class of second‐order damped differential equations 下载免费PDF全文
We investigate the existence and multiplicity of homoclinic orbits for the second‐order damped differential equations For Equation 1 where L(t) and W(t,u) are neither autonomous nor periodic in t. Under certain assumptions on g, L, and W, we get infinitely many homoclinic orbits for superquadratic, subquadratic and concave–convex nonlinearities cases by using fountain theorem and dual fountain theorem in critical point theory. These results generalize and improve some existing results in the literature. Copyright © 2015 JohnWiley & Sons, Ltd. 相似文献
56.
57.
Zhihui Pan Yingru Huang Haoyong Guo Tingjian Huang Gang Wen Huarong Yu Junguo He 《先进技术聚合物》2022,33(1):235-245
An advanced dual pH- and temperature-sensitive hydrogel (NASH2.5) was optimally synthesized through modification of N-isopropylacrylamide (NIPAM) hydrogel with introducing 5 mol% acrylic acid (AA) and 2.5 wt% sewage sludge ash (SSA). The swelling kinetic results showed that NASH2.5 exhibited both high equilibrium swelling ratio and swelling rate, which was attributed to the higher porous structure as shown in scanning electron microscope, and the more hydrogen bonding formed inside of the hydrogel as investigated in Fourier transform infrared spectrometer. In addition, its curve was better fitted to the pseudo-second-order model, indicating that the water absorption process was dominated by chemisorption through forming the hydrogen bonding among the water molecules and carboxyl/silanol groups of the hydrogel. Compared with the pure NIPAM hydrogel, the water transport mechanism switched from Case I diffusion to Case II diffusion by introduction of AA and further SSA. Furthermore, through the results of the deswelling kinetics in pH value change (from 9 to 4 and 2, respectively), temperature value change (from 25 to 40, 50, and 60°C, respectively), and dual pH and temperature values changes, NASH2.5 not only presented a high pH sensitivity, but also showed high sensitive to temperature by achieving high water recovery ratio in rapid dehydrated rate. Therefore, the dual stimuli-sensitive hydrogel with the simultaneously high performance of swelling and deswelling would provide a suitable alternative for specific applications such as pollutant adsorption. 相似文献
58.
Iodoarene catalysis is a powerful methodology that usually requires an excess of oxidant, or of redox mediator if the terminal oxidant is dioxygen, to generate the key hypervalent iodine intermediate to proceed efficiently. We report that, using the spiro‐cyclization of amides as a benchmark reaction, aerobic iodoarene catalysis can be enabled by relying on a pyrylium photocatalyst under blue light irradiation. This unprecedented dual organocatalytic system allows the use of low catalytic loading of both catalysts under very mild operating conditions. 相似文献
59.
Jose M. Cornejo-Bravo Claudia D. Becerra Kenia Palomino Héctor Magaña Ignacio Rivero Eduardo López-Maldonado 《Soft Materials》2021,19(1):89-99
ABSTRACT We prepared nano/microgels by precipitation copolymerization of N-isopropylacrylamide (NIPAAm), and one of three different carboxyalkyl methacrylamides [methacryloylamido hexanoic acid (M5), 8-methacryloylamido octanoic acid (M7), and 11-methacryloylamido undecanoic acid (M10)], either in the acid forms or as carboxylates (potassium salts). The hydrodynamic diameter (Dh) of the nano/microgels prepared with the carboxylates was smaller (≈100 nm for M10 copolymers), compared to the size of homopolymeric NIPAAm microgels prepared by dispersion polymerization (around 600 nm), indicating that the carboxylates act as surfactants reducing the size of the seeds during the polymerization process. These materials presented a swollen-shrunken transition temperature (T tr) similar to the T tr of the homopolymeric NIPAAm microgels, without pH sensitivity. On the other hand, the copolymeric microgels prepared from the acid form of the comonomers have a similar or bigger size than NIPAAm microgels. For these copolymers, the T tr can be tuned by the type and proportion of acid comonomer used and present pH sensitivity. This is important for biomedical applications such as positive temperature control release. Polyelectrolyte titration demonstrates that the nano/microgels prepared with the carboxylates behave as hard spheres, while the microgels prepared with the weak acid behave as porous materials. 相似文献
60.
We introduce a set of multi-way dual Cheeger constants and prove universal higher-order dual Cheeger inequalities for eigenvalues of normalized Laplace operators on weighted finite graphs. Our proof proposes a new spectral clustering phenomenon deduced from metrics on real projective spaces. We further extend those results to a general reversible Markov operator and find applications in characterizing its essential spectrum. 相似文献