全文获取类型
收费全文 | 87737篇 |
免费 | 810篇 |
国内免费 | 445篇 |
专业分类
化学 | 31043篇 |
晶体学 | 893篇 |
力学 | 7004篇 |
数学 | 32777篇 |
物理学 | 17275篇 |
出版年
2023年 | 56篇 |
2022年 | 148篇 |
2021年 | 253篇 |
2020年 | 193篇 |
2019年 | 214篇 |
2018年 | 10568篇 |
2017年 | 10352篇 |
2016年 | 6349篇 |
2015年 | 1149篇 |
2014年 | 684篇 |
2013年 | 977篇 |
2012年 | 4575篇 |
2011年 | 11288篇 |
2010年 | 6118篇 |
2009年 | 6430篇 |
2008年 | 7243篇 |
2007年 | 9360篇 |
2006年 | 778篇 |
2005年 | 1810篇 |
2004年 | 1973篇 |
2003年 | 2331篇 |
2002年 | 1372篇 |
2001年 | 473篇 |
2000年 | 474篇 |
1999年 | 297篇 |
1998年 | 275篇 |
1997年 | 233篇 |
1996年 | 303篇 |
1995年 | 209篇 |
1994年 | 161篇 |
1993年 | 158篇 |
1992年 | 121篇 |
1991年 | 116篇 |
1990年 | 105篇 |
1989年 | 106篇 |
1988年 | 87篇 |
1987年 | 71篇 |
1986年 | 68篇 |
1985年 | 77篇 |
1984年 | 65篇 |
1983年 | 59篇 |
1982年 | 65篇 |
1981年 | 60篇 |
1980年 | 63篇 |
1979年 | 62篇 |
1978年 | 56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 235 毫秒
21.
22.
Elsye Agustina Jeungchoon Goak Suntae Lee Youngho Seo Jun-Young Park Naesung Lee 《ChemistryOpen》2015,4(5):613-619
Iron catalysts have been used widely for the mass production of carbon nanotubes (CNTs) with high yield. In this study, UV/visible spectroscopy was used to determine the Fe catalyst content in CNTs using a colorimetric technique. Fe ions in solution form red–orange complexes with 1,10-phenanthroline, producing an absorption peak at λ=510 nm, the intensity of which is proportional to the solution Fe concentration. A series of standard Fe solutions were formulated to establish the relationship between optical absorbance and Fe concentration. Many Fe catalysts were microscopically observed to be encased by graphitic layers, thus preventing their extraction. Fe catalyst dissolution from CNTs was investigated with various single and mixed acids, and Fe concentration was found to be highest with CNTs being held at reflux in HClO4/HNO3 and H2SO4/HNO3 mixtures. This novel colorimetric method to measure Fe concentrations by UV/Vis spectroscopy was validated by inductively coupled plasma optical emission spectroscopy, indicating its reliability and applicability to asses Fe content in CNTs. 相似文献
23.
24.
Guangrui Chen Yuxing Yan Dr. Jie Wang Prof. Yong Sik Ok Guiyuan Zhong Prof. Bu Yuan Guan Prof. Yusuke Yamauchi 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(44):19831-19836
Mesoporous core–shell nanostructures with controllable ultra-large open channels in their nanoshells are of great interest. However, soft template-directed cooperative assembly to mesoporous nanoshells with highly accessible pores larger than 30 nm, or even above 50 nm into macroporous range, remains a significant challenge. Herein we report a general approach for precisely tailored coating of hierarchically macro-/mesoporous polymer and carbon shells, possessing highly accessible radial channels with extremely wide pore size distribution from ca. 10 nm to ca. 200 nm, on diverse functional materials. This strategy creates opportunities to tailor the interfacial assembly of irregular mesostructured nanounits on core materials and generate various core–shell nanomaterials with controllable pore architectures. The obtained Fe,N-doped macro-/mesoporous carbon nanoshells show enhanced electrochemical performance for the oxygen reduction reaction in alkaline condition. 相似文献
25.
26.
The structure, chemical composition, and magnetic properties of electrochemically deposited nanocrystalline Co-Ni-Fe films
were investigated using a number of techniques. A high saturation magnetic induction up to B
s
= 21 kG was attained. An enhancement of the saturation magnetization compared to the ideal anticipated one was revealed,
which correlated with the nonlinear behavior of the structural phase composition and lattice parameters with the change of
the composition.
The text was submitted by the authors in English. 相似文献
27.
Laser welding is one of the most precise welding processes in joining sheet metals. In laser welding, performing real time evaluation of the welding quality is very important to enhance the efficiency of the welding process. In this study, the plasma and spatter, which are generated during laser welding, are measured using UV and IR photodiodes. The factors that influence weld quality are classified into five categories; optimal heat input, slightly low heat input, low heat input, partial joining due to gap mismatch, and nozzle deviation. The data number deviated from reference signals and their standard deviations were also considered to evaluate the qualities. A system was also formulated to perform real time evaluations of the weld quality using a fuzzy multi-feature pattern recognition with the measured signals. 相似文献
28.
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
29.
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
30.
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