全文获取类型
收费全文 | 20447篇 |
免费 | 1728篇 |
国内免费 | 941篇 |
专业分类
化学 | 13903篇 |
晶体学 | 37篇 |
力学 | 1143篇 |
综合类 | 120篇 |
数学 | 4993篇 |
物理学 | 2920篇 |
出版年
2024年 | 21篇 |
2023年 | 266篇 |
2022年 | 579篇 |
2021年 | 518篇 |
2020年 | 660篇 |
2019年 | 572篇 |
2018年 | 541篇 |
2017年 | 647篇 |
2016年 | 927篇 |
2015年 | 873篇 |
2014年 | 968篇 |
2013年 | 1531篇 |
2012年 | 1337篇 |
2011年 | 1247篇 |
2010年 | 941篇 |
2009年 | 1289篇 |
2008年 | 1086篇 |
2007年 | 1251篇 |
2006年 | 1015篇 |
2005年 | 861篇 |
2004年 | 766篇 |
2003年 | 631篇 |
2002年 | 529篇 |
2001年 | 506篇 |
2000年 | 431篇 |
1999年 | 385篇 |
1998年 | 376篇 |
1997年 | 312篇 |
1996年 | 324篇 |
1995年 | 245篇 |
1994年 | 189篇 |
1993年 | 178篇 |
1992年 | 142篇 |
1991年 | 126篇 |
1990年 | 122篇 |
1989年 | 88篇 |
1988年 | 75篇 |
1987年 | 69篇 |
1986年 | 67篇 |
1985年 | 39篇 |
1984年 | 59篇 |
1983年 | 21篇 |
1982年 | 36篇 |
1981年 | 31篇 |
1980年 | 23篇 |
1979年 | 29篇 |
1978年 | 41篇 |
1977年 | 41篇 |
1976年 | 41篇 |
1974年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Kirtikumar D. Bharwad Priyanka A. Shah Pranav S. Shrivastav Vinay S. Sharma Puran Singhal 《Biomedical chromatography : BMC》2020,34(1):e4708
A rapid, simple and sensitive ultra-performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) method has been developed to quantify fenoprofen, a nonsteroidal anti-inflammatory drug in human plasma for a pharmacokinetic study in healthy subjects. Owing to high levels of protein binding, protein precipitation followed by solid-phase extraction was employed for the extraction of fenoprofen and fenoprofen-d3 (used as internal standard) from 200 μL human plasma. Separation was performed on a BEH C18 (50 × 2.1 mm, 1.7 μm) column using methanol−0.2% acetic acid in water (75:25, v/v) under isocratic elution. Electrospray ionization was operated in the negative mode for sample ionization. Ion transitions used for quantification in the selected reaction monitoring mode were m/z 241/197 and m/z 244/200 for fenoprofen and fenoprofen-d3, respectively. Under the optimized conditions, fenoprofen showed excellent linearity in the concentration range 0.02–20 μg/mL (r2 ≥ 0.9996), adequate sensitivity, favorable accuracy (96.4–103.7%) and precision (percentage coefficient of variation ≤4.3) with negligible matrix effect. The validated method was successfully applied to a pharmacokinetic study of fenoprofen in healthy subjects. The significant features of the method include higher sensitivity, small plasma volume for processing and a short analysis time. 相似文献
182.
Delfino Chamorro-Arenas Alejandro A. Nolasco-Hernández Dr. Lilia Fuentes Dr. Leticia Quintero Prof. Dr. Fernando Sartillo-Piscil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(21):4671-4676
Remote and multiple functionalization of piperidines without the use of transition-metal catalysts and elaborate directing groups is one of the major challenges in organic synthesis. Herein is reported an unprecedented two-step protocol that enables the multiple functionalization of piperidines to either 4-substituted or trans-3,4-disubstituted 2-piperidones. First, by exploiting the duality of TEMPO reactivity, which under oxidative and thermal conditions fluctuates between cationic and persistent-radical form, a novel multiple C(sp3)-H oxidation of piperidines to α,β-unsaturated 2-piperidones was developed. Second, the intrinsic low reactivity of the unsaturated piperidones toward conjugated Grignard additions was overcome by using trimethylsilyl chloride (TMSCl) as Lewis acid. Subsequently, conjugated Grignard addition/electrophilic trapping protocol provided substituted 2-piperidone intermediates, some of which were then transformed into pharmaceutical alkaloids. 相似文献
183.
《Arabian Journal of Chemistry》2020,13(2):4246-4253
This study reports a promising method of solid-phase extraction for determining the toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, 1,3,5-trimethylbenzene, and 1,2,4-trimethylbenzene in water samples by gas chromatography–mass spectrometry (GC–MS). Prior to this procedure, the magnesium–aluminum bimetallic hydroxides modified with sodium dodecylbenzenesulfonate (Mg/Al-SDBS-LDH) were prepared and served as the novel solid-phase extractant. The Mg/Al-SDBS-LDH has advantage of good hydrophobicity and larger spacing which facilitates the monoaromatic hydrocarbons (MAHCs) into the interlayer for adsorption. As a result, the seven MAHCs in 500 mL water samples were enriched greatly, and the theoretical enrichment factor reached to 125 times. Under the optimized conditions of solid-phase extraction (SPE) and GC–MS, the mass concentration of each MAHC (0.005–10, 0.01–10, or 0.05–10 ng/mL) had a fine linear relationship with peak area. The correlation coefficients were more than 0.995. The detection limits were between 0.001 and 0.01 ng/mL, and the RSD were between 3.1% and 6.6%. The method had been applied to determine the seven MAHCs in the Dongfengqu river water and laboratory wastewater of Chengdu University of Technology successfully. 相似文献
184.
Jie‐Lian Han Ying Qin Cheng‐Wei Ju Dongbing Zhao 《Angewandte Chemie (International ed. in English)》2020,59(16):6555-6560
Organosilicon compounds have been extensively utilized both in industry and academia. Studies on the syntheses of diverse organosilanes is highly appealing. Through‐space metal/hydrogen shifts allow functionalization of C?H bonds at a remote site, which are otherwise difficult to achieve. However, until now, an aryl to alkyl 1,5‐palladium migration process seems to have not been presented. Reported herein is the remote olefination, arylation, and borylation of a methyl group on silicon to access diverse vinyl‐, benzyl‐, and borylsilanes, constituting a unique C(sp3)?H transformation based on a 1,5‐palladium migration process. 相似文献
185.
The oxygen isotopic exchange technique is a powerful tool to investigate the oxygen transport kinetics in an oxide solid. In a solid oxide fuel cell, isotopic surface exchange and diffusion coefficients are classically determined by using the Isotopic Exchange Depth Profiling method followed by ex situ SIMS characterizations. Despite its relevance, the utilization of in situ or operando techniques to measure the isotopic exchange under an electrical bias remains marginal. We developed here a set-up which enables operando monitoring of oxygen exchange in SOFC type cells under polarization. The system has been used for studying the oxygen mobility dependency upon polarization on a symmetrical Pt/YSZ/Pt cell (YSZ: yttria-stabilized zirconia). Homomolecular and heterolytic exchange reactions were undertaken to investigate the oxygen activation step and discriminate the limiting step among the sequence of elementary steps which constitute the oxygen transport process in the SOFC system. Oxygen ions incorporation into the dense ionic conductor was identified to be the rate determining step, and its first order rate constant dependency on applied potential was established. 相似文献
186.
Sequential Semidefinite Program for Maximum Robustness Design of Structures under Load Uncertainty 总被引:1,自引:0,他引:1
A robust structural optimization scheme as well as an optimization algorithm are presented based on the robustness function. Under the uncertainties of the external forces based on the info-gap model, the maximization of the robustness function is formulated as an optimization problem with infinitely many constraints. By using the quadratic embedding technique of uncertainty and the S-procedure, we reformulate the problem into a nonlinear semidefinite programming problem. A sequential semidefinite programming method is proposed which has a global convergent property. It is shown through numerical examples that optimum designs of various linear elastic structures can be found without difficulty.The authors are grateful to the Associate Editor and two anonymous referees for handling the paper efficiently as well as for helpful comments and suggestions. 相似文献
187.
188.
Wiesława T. Obuchowska 《Computational Optimization and Applications》2004,29(1):69-90
In this paper we analyze properties of the analytic centers for perturbed convex problems. In particular we study the continuity of convex regions with respect to such characteristics as boundedness, full dimensionality and the existence of an analytic centre. Furthermore, we state the necessary and sufficient conditions for the existence of the analytic centre for possibly unbounded regions and we show that the analytic centre is identical for any minimal representation of the convex set. We are also concerned with the limiting behaviour of the central path as it approaches the set of optimal solutions of the problem. 相似文献
189.
In this note new Rosenbrock methods for ODEs, DAEs, PDEs and PDAEs of index 1 are presented. These solvers are of order 3,
have 3 or 4 internal stages, and fulfil certain order conditions to obtain a better convergence if inexact Jacobians and approximations
of
are used. A comparison with other Rosenbrock solvers shows the advantages of the new methods.
AMS subject classification (2000) 34A09, 65L80 相似文献
190.
Kouji Hashimoto Kenta Kobayashi Mitsuhiro T. Nakao 《Numerical Functional Analysis & Optimization》2013,34(4-5):523-542
We propose two methods to enclose the solution of an ordinary free boundary problem. The problem is reformulated as a nonlinear boundary value problem on a fixed interval including an unknown parameter. By appropriately setting a functional space that depends on the finite element approximation, the solution is represented as a fixed point of a compact map. Then, by using the finite element projection with constructive error estimates, a Newton-type verification procedure is derived. In addition, numerical examples confirming the effectiveness of current methods are given. 相似文献