全文获取类型
收费全文 | 1961篇 |
免费 | 225篇 |
国内免费 | 63篇 |
专业分类
化学 | 712篇 |
晶体学 | 1篇 |
力学 | 536篇 |
综合类 | 8篇 |
数学 | 479篇 |
物理学 | 513篇 |
出版年
2023年 | 17篇 |
2022年 | 38篇 |
2021年 | 47篇 |
2020年 | 46篇 |
2019年 | 49篇 |
2018年 | 45篇 |
2017年 | 65篇 |
2016年 | 103篇 |
2015年 | 81篇 |
2014年 | 130篇 |
2013年 | 178篇 |
2012年 | 118篇 |
2011年 | 133篇 |
2010年 | 111篇 |
2009年 | 130篇 |
2008年 | 101篇 |
2007年 | 102篇 |
2006年 | 82篇 |
2005年 | 84篇 |
2004年 | 68篇 |
2003年 | 71篇 |
2002年 | 52篇 |
2001年 | 45篇 |
2000年 | 26篇 |
1999年 | 53篇 |
1998年 | 37篇 |
1997年 | 28篇 |
1996年 | 22篇 |
1995年 | 18篇 |
1994年 | 23篇 |
1993年 | 15篇 |
1992年 | 16篇 |
1991年 | 19篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1972年 | 2篇 |
排序方式: 共有2249条查询结果,搜索用时 46 毫秒
51.
Abstract A rapid and sensitive spectrophotometric method is developed for the quantitative determination of piperazine, piperazine salts and diethylcarbamazine citrate. The method is based on the formation of donor-acceptor complexes between the compound and chloranilic acid. The interaction product has a characteristic absorption maximum at about 345 nm. The system obeys Beer's law in the concentration range of 1–10 ugml?1 of piperazine hexahydrate, adipate, phosphate, citrate and diethylcarbamazine citrate. The effects of several experimantal variables have been studied and the stoichiometric composition of the complex has been determined. The developed procedure has been found to be simple, rapid and precise and its application to various dosage forms was demonstrated and yields results comparable with the standard method. 相似文献
52.
Versatile chromatographic method for catechin determination in development of topical formulations containing natural extracts
下载免费PDF全文
![点击此处可从《Biomedical chromatography : BMC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
《Biomedical chromatography : BMC》2018,32(2)
Catechin is found in several natural sources, as Eugenia dysenterica and Syzygium cumini extracts. Its antioxidant and UV‐protective properties suggest a potential use in cosmetic and dermatological formulations. A simple analytical method capable of giving support to experiments performed along the development of topical formulations containing this natural substance (i.e. drug assay, skin permeation and stability studies), however, is still needed. Thus, this work aimed to develop and validate a selective HPLC method for catechin determination during the development of topical formulations. Separation was achieved using an RP‐C18 column (300 × 3.9 mm; 10 μm), with a mobile phase of methanol–phosphoric acid 0.01 m (15: 85, v/v), a flow rate of 0.8 mL/min, temperature set at 40°C and UV detection at 230 nm. The method was linear in a range from 0.5 to 10.0 μg/mL (r = 0.9998), precise with an overall variation coefficient of 5.5% and accurate with catechin recovery from the skin layers >85%. Additionally, the method was sensitive (limit of detection, 0.109 μg/mL; limit of quantification, 0.342 μg/mL) and selective against plant extracts, skin matrices and formulation interferents, as well as catechin degradation products. It was also robust regarding both methodology parameters and analytical stability. 相似文献
53.
In this article, an ALE finite element method to simulate the partial melting of a workpiece of metal is presented. The model includes the heat transport in both the solid and liquid part, fluid flow in the liquid phase by the Navier–Stokes equations, tracking of the melt interface solid/liquid by the Stefan condition, treatment of the capillary boundary accounting for surface tension effects and a radiative boundary condition. We show that an accurate treatment of the moving boundaries is crucial to resolve their respective influences on the flow field and thus on the overall energy transport correctly. This is achieved by a mesh‐moving method, which explicitly tracks the phase boundary and makes it possible to use a sharp interface model without singularities in the boundary conditions at the triple junction. A numerical example describing the welding of a thin‐steel wire end by a laser, where all aforementioned effects have to be taken into account, proves the effectiveness of the approach.Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
54.
55.
Dr. Colin R. Groom Dr. Frank H. Allen 《Angewandte Chemie (International ed. in English)》2014,53(3):662-671
The Cambridge Crystallographic Data Centre (CCDC) was established in 1965 to record numerical, chemical and bibliographic data relating to published organic and metal–organic crystal structures. The Cambridge Structural Database (CSD) now stores data for nearly 700 000 structures and is a comprehensive and fully retrospective historical archive of small‐molecule crystallography. Nearly 40 000 new structures are added each year. As X‐ray crystallography celebrates its centenary as a subject, and the CCDC approaches its own 50th year, this article traces the origins of the CCDC as a publicly funded organization and its onward development into a self‐financing charitable institution. Principally, however, we describe the growth of the CSD and its extensive associated software system, and summarize its impact and value as a basis for research in structural chemistry, materials science and the life sciences, including drug discovery and drug development. Finally, the article considers the CCDC’s funding model in relation to open access and open data paradigms. 相似文献
56.
Applied Mathematics and Mechanics - The recently developed hard-magnetic soft (HMS) materials can play a significant role in the actuation and control of medical devices, soft robots, flexible... 相似文献
57.
A new numerical model for simulations of wave transformation,breaking and long‐shore currents in complex coastal regions
下载免费PDF全文
![点击此处可从《国际流体数值方法杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In this paper, we propose a model based on a new contravariant integral form of the fully nonlinear Boussinesq equations in order to simulate wave transformation phenomena, wave breaking, and nearshore currents in computational domains representing the complex morphology of real coastal regions. The aforementioned contravariant integral form, in which Christoffel symbols are absent, is characterized by the fact that the continuity equation does not include any dispersive term. A procedure developed in order to correct errors related to the difficulties of numerically satisfying the metric identities in the numerical integration of fully nonlinear Boussinesq equation on generalized boundary‐conforming grids is presented. The Boussinesq equation system is numerically solved by a hybrid finite volume–finite difference scheme. The proposed high‐order upwind weighted essentially non‐oscillatory finite volume scheme involves an exact Riemann solver and is based on a genuinely two‐dimensional reconstruction procedure, which uses a convex combination of biquadratic polynomials. The wave breaking is represented by discontinuities of the weak solution of the integral form of the nonlinear shallow water equations. The capacity of the proposed model to correctly represent wave propagation, wave breaking, and wave‐induced currents is verified against test cases present in the literature. The results obtained are compared with experimental measures, analytical solutions, or alternative numerical solutions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
58.
This paper is concerned with nonisentropic hydrodynamic models for two-carrier plasmas, which take the form of Euler equations for conservation laws of mass density, current density and energy density for two-carrier plasmas, coupled to Poisson’s equation for self-consistent electronic field. Due to the nonlinear coupling and cancellation between electrons and ions, the expected dissipation rates of densities for two carriers are no longer available in comparison with the one-carrier case, which leads to the lack of exponential stability near constant equilibrium in the whole space. In order to capture the weaker dissipation and obtain global solutions in spatially critical Besov spaces, calculus techniques which have been recently developed in Chemin–Lerner spaces, will be further applied. 相似文献
59.
Agnieszka Lewiska Marta Doma-Kdzia Kinga Kierul Micha Bochynek Dominika Pannert Piotr Nowaczyk Marcin ukaszewicz 《Molecules (Basel, Switzerland)》2021,26(4)
The skin is constantly exposed to external and internal factors that disturb its function. In this work, two nanosystems-levan nanoparticles and a surfactin-stabilized nanoemulsion were preserved (tested for microbial growth) and characterized (size, polydispersity, Zeta potential, and stability). The nanosystems were introduced in the model formulations-cream, tonic, and gel, and confirmed by TEM. The analysis showed that nanoemulsion has a spherical morphology and size 220–300 nm, while levan nanoparticles had irregular shapes independently of the use of matrix and with particle size (130–260 nm). Additionally, we examined the antiradical effect of levan nanoparticles and nanoemulsion in the prototype of formulations by scavenging DPPH (2,2-diphenyl-1-picrylhydrazyl; EPR spectroscopy). The model cream with both nanosystems and the whole range of products with nanosystems were evaluated in vivo for hydration, elasticity, smoothness, wrinkles and vascular lesions, discoloration, respectively. The cream improved skin condition in all tested parameters in at least 50% of volunteers. The use of more comprehensive care, additionally consisting of a tonic and gel, reduced the previously existing skin discoloration to 10.42 ± 0.58%. The presented prototype formulations are promising in improving skin conditions. 相似文献
60.
R. Feola F. Iandoli 《Annales de l'Institut Henri Poincaré (C) Analyse Non Linéaire》2019,36(1):119-164
We prove local in time well-posedness for a large class of quasilinear Hamiltonian, or parity preserving, Schrödinger equations on the circle. After a paralinearization of the equation, we perform several paradifferential changes of coordinates in order to transform the system into a paradifferential one with symbols which, at the positive order, are constant and purely imaginary. This allows to obtain a priori energy estimates on the Sobolev norms of the solutions. 相似文献