全文获取类型
收费全文 | 3724篇 |
免费 | 91篇 |
国内免费 | 15篇 |
专业分类
化学 | 2324篇 |
晶体学 | 15篇 |
力学 | 196篇 |
数学 | 754篇 |
物理学 | 541篇 |
出版年
2023年 | 19篇 |
2022年 | 53篇 |
2021年 | 99篇 |
2020年 | 77篇 |
2019年 | 54篇 |
2018年 | 66篇 |
2017年 | 59篇 |
2016年 | 121篇 |
2015年 | 107篇 |
2014年 | 88篇 |
2013年 | 195篇 |
2012年 | 210篇 |
2011年 | 255篇 |
2010年 | 148篇 |
2009年 | 125篇 |
2008年 | 213篇 |
2007年 | 229篇 |
2006年 | 171篇 |
2005年 | 162篇 |
2004年 | 150篇 |
2003年 | 123篇 |
2002年 | 107篇 |
2001年 | 45篇 |
2000年 | 30篇 |
1999年 | 41篇 |
1998年 | 35篇 |
1997年 | 35篇 |
1996年 | 41篇 |
1995年 | 29篇 |
1994年 | 30篇 |
1993年 | 52篇 |
1992年 | 19篇 |
1991年 | 29篇 |
1990年 | 30篇 |
1989年 | 20篇 |
1988年 | 26篇 |
1987年 | 24篇 |
1986年 | 28篇 |
1985年 | 46篇 |
1984年 | 44篇 |
1983年 | 29篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 30篇 |
1979年 | 25篇 |
1978年 | 29篇 |
1977年 | 25篇 |
1976年 | 22篇 |
1975年 | 13篇 |
1974年 | 22篇 |
排序方式: 共有3830条查询结果,搜索用时 15 毫秒
1.
Sébastien Lemaire Giuliano Giambastiani Barbara Pacini Giovanni Poli 《Journal of organometallic chemistry》2003,687(2):291-300
A new type I palladium-catalyzed pseudo-domino process is developed, in which a single Pd-based catalytic system promotes two mechanistically unrelated, sequential catalytic cycles in chronologically distinct order. Suitable precursors undergo an allylic alkylation and a Heck coupling in sequence, affording polycondensed pyrrolidone derivatives. Depending on the starting precursors, intra/inter or doubly intramolecular processes can be obtained. The allylic alkylation process takes place always very smoothly. On the other hand, the Heck coupling turns out to be rather difficult either when the process is intermolecular, or when an intramolecular process generates polycondensed structures featuring three fused bonds connected to a common carbon atom. In such difficult cases, use of the Herrmann-Beller phosphapalladacycle allowed to catalyze the coupling. This study demonstrates also that allylic alkylations can be catalyzed by the Herrmann-Beller phosphapalladacycle. 相似文献
2.
3.
M. De Falco F. De Giovanni C. Musella R. Schmidt 《Rendiconti del Circolo Matematico di Palermo》2003,52(1):70-76
A subgroupH of a groupG is said to bepermutable ifHX=XH for every subgroupX ofG. In this paper the structure of groups in which every subgroup either is abelian or permutable is investigated.
This work was done while the last author was visiting the University of Napoli Federico II. He thanks the “Dipartimento di
Matematica e Applicazioni” for its financial support. 相似文献
4.
5.
Giovanni Tartarini Alessandra Lena Davide Passaro Lorenzo Rosa Stefano Selleri Pier Faccin Enrico Maria Fabbri 《Optical and Quantum Electronics》2006,38(9-11):869-876
A comprehensive numerical tool has been developed for the evaluation of the performances of Radio over Fiber (RoF) links intended for wireless signal distribution.At the transmitter end an appropriate set of rate equations allows to model the optical source as a solitary laser or as an appropriately injection locked laser. The optical channel is modeled putting into account the combined effect of fiber dispersion, laser source non ideal performances (e.g. non-linear effects, frequency chirp), and quadratic detection of the receiving photodiode. The simulation model developed can be a useful tool at the design stage allowing a preliminary evaluation of the characteristics of real RoF links. 相似文献
6.
We study a variation of the knapsack problem in which each item has a profit, a weight and a penalty; the sum of profits of the selected items minus the largest penalty associated with the selected items must be maximized. We present an ILP formulation and an exact optimization algorithm. 相似文献
7.
In mountain-rescue operations injured people are generally exposed to vibrations and shocks that can be potential causes of physical conditions worsening. Such vibrations can derive both from patient's body manipulations (e.g. when it is being loaded and immobilized on a stretcher) and from forces coming from the transport devices and vehicles. Despite the general feeling that during this kind of operations the levels of transmitted vibrations to the injured can be quite large and potentially dangerous, there is practically no study in literature providing reliable parameters (i.e. measurements) to support or dismiss these beliefs. This paper reports the results of a measurement campaign carried-out in order to outline, identify and quantify the excitations a human body is exposed to, during typical transportation phases related to mountain-rescue operations. The work mainly presents and discusses the experimental setup with the aim of focusing on the problems related to this kind of measurements; the results of the experimental campaign carried-out for the measurement of the vibrations undergone by a human body during a simulated rescue operation are presented and discussed as well. Such simulation includes three phases of transportation: on a hand-held stretcher, on an ambulance and on a helicopter. The work is not intended to supply a complete characterization and analysis of vibrations transmission during any rescue operation but just to provide a preliminary overview and to define a measurement method that can be applied for a more comprehensive characterization. With such aims measurements were carried out in on-field situations stated as “typical” by rescue experts and data then analyzed both with standard procedures and algorithms (e.g. ISO 2631s weighting curves) and with the commonly used statistical indexes; in the analysis it is important to be aware that standardized measurement procedures and indexes, created to verify comfort or health-risks of workers, might not fit the case of a generic patient who experienced a serious mountain accident. The work includes also a laboratory activity mainly related to mechanical characterization of the stretcher used in the field tests. The most interesting result of the study is the comparison of the vibration levels in the various rescue phases that, even when using different indicators, shows that the most critical issue is due to hand transportation despite the bad judgment usually expressed for helicopter flight. 相似文献
8.
Nonequilibrium statistical mechanics close to equilibrium is studied using SRB states and a formula [10] for their derivatives
with respect to parameters. We write general expressions for the thermodynamic fluxes (or currents) and the transport coefficients,
generalizing the results of [4, 5]. In this framework we give a general proof of the Onsager reciprocity relations.
Received: 2 December 1996 / Accepted: 13 March 1997 相似文献
9.
This paper develops a mathematical model of the ring-spinning process that takes into account its non-stationary nature. A complex system of differential equations is obtained, which from a mathematical point of view constitutes a ‘free-boundary’ problem. Its solution involves definition of suitable boundary conditions related to the mechanical characteristics of the process and of the spinning machine itself. The boundary conditions which determine the solution are pointed out. A numerical solution of the system of differential equations can be obtained by the Finite-Segments method, as shown in an example. 相似文献
10.
Marialuigia Macchione Giovanni De Filpo Fiore P. Nicoletta Giuseppe Chidichimo 《Liquid crystals》2005,32(3):315-319
Control of light intensity and colour are two of the major features required in the realization of smart windows. We designed a bi-functional polymer dispersed liquid crystal (PDLC) film in order to satisfy such requirements, i.e. it is able both to modulate the optical transmission, if an external electric field is applied, and to change colour if exposed to sunlight. A monomer/liquid crystal mixture was doped with a small amount of photochromic material and homeotropically aligned by means of rough surfaces. A transparent and pale pink coloured film was achieved after photopolymerization. Such a film changes colour upon exposure for some seconds to sunlight or ultraviolet radiation in a persistent but reversible manner. In addition, the film appears transparent without the application of an electric field (OFF state) and becomes opaque on application of a driving voltage of about 75 V (ON state), and thus the film operates in reverse mode with respect to conventional PDLCs. 相似文献