首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5411篇
  免费   685篇
  国内免费   27篇
化学   1687篇
晶体学   11篇
力学   331篇
综合类   103篇
数学   2744篇
物理学   1247篇
  2023年   43篇
  2022年   99篇
  2021年   120篇
  2020年   124篇
  2019年   139篇
  2018年   100篇
  2017年   138篇
  2016年   163篇
  2015年   97篇
  2014年   199篇
  2013年   317篇
  2012年   268篇
  2011年   280篇
  2010年   215篇
  2009年   291篇
  2008年   337篇
  2007年   319篇
  2006年   308篇
  2005年   221篇
  2004年   170篇
  2003年   187篇
  2002年   196篇
  2001年   155篇
  2000年   126篇
  1999年   140篇
  1998年   109篇
  1997年   99篇
  1996年   84篇
  1995年   58篇
  1994年   62篇
  1993年   61篇
  1992年   63篇
  1991年   41篇
  1990年   35篇
  1989年   26篇
  1988年   36篇
  1987年   32篇
  1986年   18篇
  1985年   78篇
  1984年   94篇
  1983年   46篇
  1982年   80篇
  1981年   90篇
  1980年   53篇
  1979年   51篇
  1978年   50篇
  1977年   34篇
  1976年   20篇
  1974年   15篇
  1973年   18篇
排序方式: 共有6123条查询结果,搜索用时 46 毫秒
1.
In the past, the preferred strategy for the identification of unknown compounds was to search in an appropriate mass spectral database for spectra obtained using either electron ionisation (GC‐MS analyses) or collision‐induced dissociation (LC‐MS/MS analyses). Recently, an increase has been seen in the use of accurate mass instruments and spectra‐less databases, based on monoisotopic accurate mass alone. In this article, we describe a systematic workflow for the screening and identification of new flame retardants. This approach utilises LC‐quadrupole‐time‐of‐flight MS and spectra‐less databases based only on monoisotopic accurate mass for the identification of ‘unknowns’. An in‐house database was built, and the input parameters used in the data analysis process were optimised for flame retardant chemicals, so that it can be easily transferred to other laboratories. The procedure was successfully applied to dust, foam and textiles from car interiors and indoor consumer products. The developed method was demonstrated for the main new flame retardant present in Antiblaze V6 and for the three unreported reaction by‐products/impurities present in the same technical mixture. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
2.
3.
Mass-growth is usually perceived as a non-isochoric process, but classes of soft tissue that exhibit incompressible or nearly incompressible in vitro behaviour may have gone through growth stages which are isochoric or nearly isochoric. The present paper aims thus to complement and complete the non-isochoric mass-growth modelling framework presented in [1], [2] by presenting a relevant formulation for isochoric deformation processes that exhibit features of simultaneous elastic and plastic mass-growth. The refined modelling route that is followed is slightly different, and more general to that followed in [2], to which, however, is also applicable. Because mass density and stress levels are expected to increase faster than they would in analogous non-isochoric mass-growth situations, purely pseudo-elastic or purely pseudo-plastic stages of isochoric mass-growth are rather unlikely to alternate in the manner implied in [1] for their non-isochoric counterparts. Purely pseudo-elastic and purely pseudo-plastic isochoric mass-growth models can however still be obtained as particular cases of the present formulation. These issues as well as additional features that characterise the present model are detailed and clarified further through the complete, closed form solution of a particular, example problem application in which the mass density and the shape of the growing continuum are subjected to continuous time change.  相似文献   
4.
This note is to present some results on the group invertibility of linear combina- tions of idempotents when the difference of two idempotents is group invertible.  相似文献   
5.
A selective release system was demonstrated with a dual‐cargo loaded MSNs. When stimulated by different signals (UV or H+), this system could selectively release different kinds of cargoes individually. Furthermore, this system has been used to provide a combination of chemotherapy and biotherapy for cancer treatment. This controlled release system could be an important step in the development of more effective and sophisticated nanomedicine and nanodevices, due to the possibility of selective release of a complex multi‐drug.  相似文献   
6.
The interaction between fullerene C60 and heparin was studied using a fullerene C60‐coated piezoelectric quartz crystal sensor. The irreversible response of the piezoelectric quartz crystal was found which could be attributed to the quite strong adsorption of heparin onto the C60 molecule. Immobilized fullerene C60‐Heparin was prepared and successfully applied as a good inhibitor for blood clotting. Like solvated heparin, both wet and dry C60‐heparin solid all demonstrated excellent ability of anticoagulation of blood. The blood clotting time with C60‐heparin solid was found to be > 7 days, while only 17.9 min required for blood clotting time in the absence of C60‐heparin solid. Furthermore, the C60‐heparin coated artificial PVC blood vessels were prepared by coating fullerene C60 onto the surface of artificial PVC blood vessels, followed by the adsorption of water solvated heparin onto the fullerene C60 molecule to form C60‐heparin coating. The blood clotting time of blood in artificial PVC blood vessels with C60‐heparin coating was found to be > 30 days, while only ≤ 30 min. of blood clotting time without the C60‐Heparin coating was observed. The C60‐heparin coated artificial PVC blood vessels can be expected to be employed in human body for the anticoagulation of blood.  相似文献   
7.
We demonstrate that one can exhaustively determine the n‐bound eigenstates of a Hamiltonian H by constructing a sequence of supersymmetric (SUSY) partner Hamiltonians and invoking a time‐dependent quantum adiabatic switching algorithm for passage from the ground state of one to the other. The ground states of the initial pair H(0) and H(1) are constructed by solving the Riccati equation for the superpotential ?(0) for H(0) and adiabatically switching from the ground state Ψ of H(0) to the ground state Ψ of H(1). The charge operator Q is then used to recover the first excited state Ψ of H(0). The procedure is repeated for the ground states of SUSY pairs H(n + 1) and H(n + 2), and appropriate charge operators lead to the excited states Ψ of H(0) with , thereby exhausting the full eigenspectrum of H(0). The workability of the proposed method is shown with several well‐known examples. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011  相似文献   
8.
In this paper interfacial edge crack problems are considered by the application of the finite element method. The stress intensity factors are accurately determined from the ratio of crack-tip-stress value between the target given unknown and reference problems. The reference problem is chosen to produce the singular stress fields proportional to those of the given unknown problem. Here the original proportional method is improved through utilizing very refined meshes and post-processing technique of linear extrapolation. The results for a double-edge interface crack in a bonded strip are newly obtained and compared with those of a single-edge interface crack for different forms of combination of material. It is found that the stress intensity factors should be compared in the three different zones of relative crack lengths. Different from the case of a cracked homogeneous strip, the results for the double edge interface cracks are found to possibly be bigger than those for a single edge interface crack under the same relative crack length.  相似文献   
9.
10.
Consider K ≥ 2 independent copies of the random walk on the symmetric group SN starting from the identity and generated by the products of either independent uniform transpositions or independent uniform neighbor transpositions. At any time $n\in \mathbb{N}$, let Gn be the subgroup of SN generated by the K positions of the chains. In the uniform transposition model, we prove that there is a cut‐off phenomenon at time N ln(N)/(2K) for the non‐existence of fixed point of Gn and for the transitivity of Gn, thus showing that these properties occur before the chains have reached equilibrium. In the uniform neighbor transposition model, a transition for the non‐existence of a fixed point of Gn appears at time of order $N^{1+\frac{2}{K}}$ (at least for K ≥ 3), but there is no cut‐off phenomenon. In the latter model, we recover a cut‐off phenomenon for the non‐existence of a fixed point at a time proportional to N by allowing the number K to be proportional to ln(N). The main tools of the proofs are spectral analysis and coupling techniques. © 2011 Wiley Periodicals, Inc. Random Struct. Alg., 2012  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号