全文获取类型
收费全文 | 89207篇 |
免费 | 2060篇 |
国内免费 | 1762篇 |
专业分类
化学 | 31848篇 |
晶体学 | 923篇 |
力学 | 7539篇 |
综合类 | 95篇 |
数学 | 33215篇 |
物理学 | 19409篇 |
出版年
2023年 | 174篇 |
2022年 | 299篇 |
2021年 | 318篇 |
2020年 | 393篇 |
2019年 | 357篇 |
2018年 | 10676篇 |
2017年 | 10488篇 |
2016年 | 6466篇 |
2015年 | 1285篇 |
2014年 | 868篇 |
2013年 | 1108篇 |
2012年 | 4629篇 |
2011年 | 11353篇 |
2010年 | 6222篇 |
2009年 | 6678篇 |
2008年 | 7271篇 |
2007年 | 9393篇 |
2006年 | 878篇 |
2005年 | 1902篇 |
2004年 | 1989篇 |
2003年 | 2335篇 |
2002年 | 1341篇 |
2001年 | 563篇 |
2000年 | 571篇 |
1999年 | 466篇 |
1998年 | 515篇 |
1997年 | 409篇 |
1996年 | 460篇 |
1995年 | 338篇 |
1994年 | 318篇 |
1993年 | 253篇 |
1992年 | 202篇 |
1991年 | 193篇 |
1990年 | 188篇 |
1989年 | 142篇 |
1988年 | 138篇 |
1987年 | 144篇 |
1986年 | 139篇 |
1985年 | 117篇 |
1984年 | 90篇 |
1983年 | 69篇 |
1982年 | 76篇 |
1981年 | 65篇 |
1980年 | 66篇 |
1979年 | 59篇 |
1978年 | 48篇 |
1914年 | 45篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 3 毫秒
31.
超临界流体色谱流程设计及其应用 总被引:1,自引:1,他引:0
本文设计了多功能超临界流体色谱流程,流程中包括毛细管/微填充柱SFC,GC,计算机控制温度、压力、密度及信号采集、处理,配置有超临界流体萃取池,解决了超临界流体色谱分流口易堵问题。利用该流程,将石腊、DC-200气相色谱固定相、黄油、蜂蜡、救心油、红花油等样品进行超临界流体色谱分离。 相似文献
32.
A new secoiridoid, multiflorin (1) has been isolated from the EtOAc-soluble fraction of Jasminum multiflorum (Oleaceace). The structure of I was determined by application of spectral methods. 相似文献
33.
A forecasting model is developed for the number of daily applications for loans at a financial services telephone call centre. The purpose of the forecasts and the associated prediction intervals is to provide effective staffing policies within the call centre. The model building process is constrained by the availability of only 2 years and 7 months of data. The distinctive feature of the data is that demand is driven in the main by advertising. The analysis given focuses on applications stimulated by press advertising. Unlike previous analyses of broadly similar data, where ARIMA models were used, a model with a dynamic level, multiplicative calendar effects and a multiplicative advertising response is developed and shown to be effective. 相似文献
34.
Chee Sing Yap 《The Journal of the Operational Research Society》1989,40(7):649-658
This paper describes three case studies which investigated issues relating to the implementation and management of advanced information technology. The benefits derived from information technology were found to vary from one organization to another. These studies provide further evidence to support earlier research findings that senior management has an important role to play in the effective exploitation of information technology, and that the investment approach taken by an organization has a significant effect on the development of computer-based information systems. The effectiveness of a system depends not only on applying the appropriate technology, but also on how successfully technical and behavioural issues are resolved. 相似文献
35.
A profit-maximizing supply chain network design model with demand choice flexibility 总被引:1,自引:0,他引:1
Zuo-Jun Max Shen 《Operations Research Letters》2006,34(6):673-682
We present a profit-maximizing supply chain design model in which a company has flexibility in determining which customers to serve. The company may lose a customer to competition if the price it charges is too high. We show the problem formulation and solution algorithm, and discuss computational results. 相似文献
36.
M I Salema A P B Póvoa A Q Novais 《The Journal of the Operational Research Society》2006,57(6):615-629
In this paper, an MILP formulation is proposed for the design of a reverse logistics network based on a warehouse location–allocation model, which optimizes, simultaneously, the forward and reverse networks. A single product model with unlimited capacity is first defined. Subsequently, the model is extended to a multi-product capacitated recovery network model, where capacity limitations and a multi-product system can be considered. The proposed model is compared to published work in the field, where different model assumptions have been proposed. Two cases are described so as to gain a better insight into the model and allow a comparative analysis. 相似文献
37.
Wei Gengping~ Shen Jianhua~ 《高校应用数学学报(英文版)》2006,21(3):320-326
This paper studies the nonautonomous nonlinear system of difference equationsΔx(n)=A(n)x(n)+f(n,x(n)),n∈Z,(*) where x(n)∈R~N,A(n)=(a_(ij)(n))N×N is an N×N matrix,with a-(ij)∈C(R,R) for i,j= 1,2,3,...,N,and f=(f_1,f_2,...,f_N)~T∈C(R×R~N,R~N),satisfying A(t+ω)=A(t),f(t+ω,z)=f(t,z) for any t∈R,(t,z)∈R×R~N andωis a positive integer.Sufficient conditions for the existence ofω-periodic solutions to equations (*) are obtained. 相似文献
38.
Data Envelopment Analysis (DEA) offers a piece-wise linear approximation of the production frontier. The approximation tends to be poor if the true frontier is not concave, eg in case of economies of scale or of specialisation. To improve the flexibility of the DEA frontier and to gain in empirical fit, we propose to extend DEA towards a more general piece-wise quadratic approximation, called Quadratic Data Envelopment Analysis (QDEA). We show that QDEA gives statistically consistent estimates for all production frontiers with bounded Hessian eigenvalues. Our Monte-Carlo simulations suggest that QDEA can substantially improve efficiency estimation in finite samples relative to standard DEA models. 相似文献
39.
Zygmunt Bak 《Czechoslovak Journal of Physics》2002,52(1):A25-A28
We consider RKKY interaction in a quasi 2D system with nonparabolic dispersion. In our paper we calculate the RKKY range function assuming the in-layer confinement via effective dimensionality approach. We show, that indirect magnetic exchange in our system can be modelled by the effective spectral dimension which equals one. 相似文献
40.