全文获取类型
收费全文 | 106388篇 |
免费 | 4576篇 |
国内免费 | 3102篇 |
专业分类
化学 | 43966篇 |
晶体学 | 1046篇 |
力学 | 8495篇 |
综合类 | 275篇 |
数学 | 34937篇 |
物理学 | 25347篇 |
出版年
2024年 | 106篇 |
2023年 | 594篇 |
2022年 | 922篇 |
2021年 | 969篇 |
2020年 | 1126篇 |
2019年 | 1096篇 |
2018年 | 11102篇 |
2017年 | 10867篇 |
2016年 | 7226篇 |
2015年 | 2112篇 |
2014年 | 1839篇 |
2013年 | 2369篇 |
2012年 | 6322篇 |
2011年 | 12951篇 |
2010年 | 7193篇 |
2009年 | 7559篇 |
2008年 | 8229篇 |
2007年 | 10257篇 |
2006年 | 1688篇 |
2005年 | 2503篇 |
2004年 | 2428篇 |
2003年 | 2719篇 |
2002年 | 1728篇 |
2001年 | 800篇 |
2000年 | 843篇 |
1999年 | 800篇 |
1998年 | 766篇 |
1997年 | 715篇 |
1996年 | 853篇 |
1995年 | 619篇 |
1994年 | 539篇 |
1993年 | 480篇 |
1992年 | 423篇 |
1991年 | 383篇 |
1990年 | 304篇 |
1989年 | 260篇 |
1988年 | 215篇 |
1987年 | 193篇 |
1986年 | 184篇 |
1985年 | 164篇 |
1984年 | 131篇 |
1983年 | 97篇 |
1982年 | 82篇 |
1981年 | 74篇 |
1980年 | 64篇 |
1979年 | 61篇 |
1978年 | 53篇 |
1975年 | 44篇 |
1914年 | 45篇 |
1909年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
51.
52.
玻璃陶瓷材料中Tm3+离子红外到蓝色上转换发光 总被引:4,自引:0,他引:4
系统研究了PbF2+GeO2+WO3ⅩⅣTmF3玻璃陶瓷材料中,在近红外光(1.06μm)激发下,Tm3+离子的发光特性.实验中观测到Tm3+离子的两组峰值位置分别在20920cm-1和22173cm-1的蓝色上转换发光,并证实这两组上转换发光分别与吸收三个和四个光子有关,同时建立了上转换发光的模型.为了选择最佳掺杂浓度,详细地测量了Tm3+离子峰值为20920cm-1的蓝色上转换发光强度与TmF3浓度的关系. 相似文献
53.
对由一类非线性抛物型变分不等方程所描述的无穷维动力系统,给出了存在全局吸引子及弱近似惯性流形的充分条件. 相似文献
54.
L-Quebrachitol (1) isolated from the root of Elaeagnus formosana Nakai (Elaeagnaceae) has not been reported before for any species of the genus of Elaeasnus. Its structure was established from spectral data and was completely characterized by single-crystal X-ray analysis. L-Quebrachitol crystallizes in the monoclinic system, space group P21, with cell parameters are a = 6.702(4), b = 7.207(4), c = 8.758(5) Å, β = 90.24(5)° and Z = 2. 相似文献
55.
56.
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
57.
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
58.
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
59.
Jian Shen Huizhong Zeng Zhihong Wang Shengbo Lu Huidong Huang Jingsong Liu 《Applied Surface Science》2006,252(22):8018-8021
Polycrystalline Pb(Zr0.55Ti0.45)O3 thin film was deposited on Pt/Ti/SiO2/Si(1 0 0) by radio-frequency-magnetron sputtering method, and the writing of charge bits on the surface of PZT thin film was studied by Kelvin probe force microscopy. It is found that the surface potential of the negative charge bits are higher than those of the corresponding positive ones. When ferroelectric polarization switching occurs, the potential difference becomes even more remarkable. A qualitative model was proposed to explain the origin of the asymmetric charge writing. It is demonstrated that the internal field in the interface layer, which is near the ferroelectric/electrode interface in ferroelectric film, is likely to be the cause for the occurrence of this phenomenon. 相似文献
60.
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