全文获取类型
收费全文 | 110044篇 |
免费 | 1858篇 |
国内免费 | 615篇 |
专业分类
化学 | 46070篇 |
晶体学 | 949篇 |
力学 | 7375篇 |
综合类 | 1篇 |
数学 | 37196篇 |
物理学 | 20926篇 |
出版年
2023年 | 246篇 |
2022年 | 331篇 |
2021年 | 607篇 |
2020年 | 789篇 |
2019年 | 882篇 |
2018年 | 10852篇 |
2017年 | 10589篇 |
2016年 | 7095篇 |
2015年 | 1913篇 |
2014年 | 1437篇 |
2013年 | 2206篇 |
2012年 | 5847篇 |
2011年 | 12756篇 |
2010年 | 6949篇 |
2009年 | 7145篇 |
2008年 | 8367篇 |
2007年 | 10408篇 |
2006年 | 1951篇 |
2005年 | 2896篇 |
2004年 | 2837篇 |
2003年 | 3007篇 |
2002年 | 2016篇 |
2001年 | 775篇 |
2000年 | 735篇 |
1999年 | 599篇 |
1998年 | 579篇 |
1997年 | 533篇 |
1996年 | 617篇 |
1995年 | 498篇 |
1994年 | 401篇 |
1993年 | 400篇 |
1992年 | 335篇 |
1991年 | 312篇 |
1990年 | 277篇 |
1989年 | 272篇 |
1988年 | 251篇 |
1987年 | 227篇 |
1986年 | 214篇 |
1985年 | 281篇 |
1984年 | 260篇 |
1983年 | 196篇 |
1982年 | 252篇 |
1981年 | 244篇 |
1980年 | 221篇 |
1979年 | 224篇 |
1978年 | 235篇 |
1977年 | 174篇 |
1976年 | 162篇 |
1975年 | 160篇 |
1974年 | 135篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Daniel L. Feldheim Del R. Lawson Charles R. Martin 《Journal of Polymer Science.Polymer Physics》1993,31(8):953-957
We report a strong dependence of the thermal stability of Nafion® perfluorosulfonate ionomer on the nature of the counterion associated with the fixed sulfonate site. These results were obtained using thermal gravimetric analysis on a series of alkali metal and alkyl ammonium cation-exchanged Nafion films. We have found that the temperature of decomposition of Nafion is inversely dependent on the size of the exchanged cation; i.e., Nafion films show improved thermal stability as the size of the counter cation decreases. We attribute this inverse relationship of thermal stability with counterion size to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-coun-terion interaction. © 1993 John Wiley & Sons, Inc. 相似文献
42.
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
43.
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
44.
For z1,z2,z3∈Zn, the tristance d3(z1,z2,z3) is a generalization of the L1-distance on Zn to a quantity that reflects the relative dispersion of three points rather than two. A tristance anticodeAd of diameter d is a subset of Zn with the property that d3(z1,z2,z3)?d for all z1,z2,z3∈Ad. An anticode is optimal if it has the largest possible cardinality for its diameter d. We determine the cardinality and completely classify the optimal tristance anticodes in Z2 for all diameters d?1. We then generalize this result to two related distance models: a different distance structure on Z2 where d(z1,z2)=1 if z1,z2 are adjacent either horizontally, vertically, or diagonally, and the distance structure obtained when Z2 is replaced by the hexagonal lattice A2. We also investigate optimal tristance anticodes in Z3 and optimal quadristance anticodes in Z2, and provide bounds on their cardinality. We conclude with a brief discussion of the applications of our results to multi-dimensional interleaving schemes and to connectivity loci in the game of Go. 相似文献
45.
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
46.
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
47.
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
48.
49.
We consider a randomized version of the greedy algorithm for finding a large matching in a graph. We assume that the next edge is always randomly chosen from those remaining. We analyze the performance of this algorithm when the input graph is fixed. We show that there are graphs for which this Randomized Greedy Algorithm (RGA) usually only obtains a matching close in size to that guaranteed by worst-case analysis (i.e., half the size of the maximum). For some classes of sparse graphs (e.g., planar graphs and forests) we show that the RGA performs significantly better than the worst-case. Our main theorem concerns forests. We prove that the ratio to maximum here is at least 0.7690…, and that this bound is tight. 相似文献
50.
Graham Smith 《The Journal of the Operational Research Society》1989,40(9):761-770
Courses which teach discrete-event simulation are based on many different simulation languages. The requirements for a language to support teaching simulation are discussed. In particular, it is recommended that such languages separate into distinct modules those aspects of simulation which are taught as separate topics. Implementation of the separation is discussed. The SEESIM language, developed as a teaching aid, is described, and examples of its use are given. Straightforward use of SEESIM can be learned quickly, yet the language provides facilities for a staged introduction to advanced concepts of simulation. 相似文献