全文获取类型
收费全文 | 26013篇 |
免费 | 57篇 |
国内免费 | 87篇 |
专业分类
系统科学 | 287篇 |
丛书文集 | 611篇 |
教育与普及 | 70篇 |
理论与方法论 | 97篇 |
现状及发展 | 12230篇 |
研究方法 | 1013篇 |
综合类 | 11579篇 |
自然研究 | 270篇 |
出版年
2013年 | 137篇 |
2012年 | 320篇 |
2011年 | 650篇 |
2010年 | 151篇 |
2008年 | 411篇 |
2007年 | 492篇 |
2006年 | 434篇 |
2005年 | 466篇 |
2004年 | 412篇 |
2003年 | 464篇 |
2002年 | 417篇 |
2001年 | 795篇 |
2000年 | 794篇 |
1999年 | 462篇 |
1992年 | 452篇 |
1991年 | 363篇 |
1990年 | 405篇 |
1989年 | 387篇 |
1988年 | 385篇 |
1987年 | 388篇 |
1986年 | 409篇 |
1985年 | 517篇 |
1984年 | 351篇 |
1983年 | 331篇 |
1982年 | 276篇 |
1981年 | 320篇 |
1980年 | 375篇 |
1979年 | 870篇 |
1978年 | 684篇 |
1977年 | 711篇 |
1976年 | 530篇 |
1975年 | 544篇 |
1974年 | 851篇 |
1973年 | 693篇 |
1972年 | 669篇 |
1971年 | 786篇 |
1970年 | 1017篇 |
1969年 | 880篇 |
1968年 | 804篇 |
1967年 | 813篇 |
1966年 | 683篇 |
1965年 | 508篇 |
1964年 | 147篇 |
1959年 | 300篇 |
1958年 | 480篇 |
1957年 | 347篇 |
1956年 | 318篇 |
1955年 | 278篇 |
1954年 | 275篇 |
1948年 | 183篇 |
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
61.
A diffusion chamber technique based on time-lag analysis for the estimation of effective diffusion coefficients of radiolabelled macromolecules of varying molecular weights through native mucus gel is reported. For all solutes studied, a reduction in effective diffusion coefficients was observed with a retardation of solute flux in both aqueous and mucus layers. Over the molecular weight range of solutes investigated (126-186,000 Daltons), a consistent effect of molecular weight was evident with regard to the retarding effect of mucus. No apparent or absolute molecular weight cut-off for macromolecular transfer was exhibited. However, at high molecular weights (greater than 30,000 Daltons) the retardation was greatly enhanced. The results confirm that mucus can be regarded as a gel with finite pores, but that it does not constitute an absolute barrier to even high molecular weight solutes. 相似文献
62.
F. Acerbi 《Archive for History of Exact Sciences》2003,57(6):465-502
63.
Contemporary developments in economicmethodology have produced a vibrant agenda ofcompeting positions. These include, amongothers, constructivism, critical realism andrhetoric, with each contributing to the Realistvs. Pragmatism debate in the philosophies of thesocial sciences. A major development in theneo-pragmatist contribution to economicmethodology has been Quine's pragmatic assaulton the dogmas of empiricism, which are nowclearly acknowledged within contemporaryeconomic methodology. This assault isencapsulated in the celebrated Duhem-Quinethesis, which according to a number ofcontemporary leading philosophers of economics,poses a particularly serious methodologicalproblem for economics. This problem, asreflected in Hausman's analysis, consists ofthe inability of economics to learn fromexperience, thereby subverting the capacity totest economic theories. In this paper wedispute this position. Our argument is basedon a combination of Quine's holism with VanFraassen's constructive empiricism, especiallythe latter's analysis of empirical adequacy andhis pragmatic approach to explanation. Theresulting reorientation of economic methodologyrestores the capacity of economics to learnfrom experience and reinstates the imperativeof developing alternatives to orthodoxtheorizing in economics. 相似文献
64.
Coalignment of vimentin intermediate filaments with microtubules depends on kinesin. 总被引:19,自引:0,他引:19
Intermediate filaments in most types of cultured cells coalign with microtubules. Depolymerization of microtubules results in collapse of vimentin and desmin intermediate filaments to the nucleus where they form a perinuclear cap. Collapse can also be induced by microinjection of antibodies against intermediate filament or microtubule proteins. Thus, two filament systems interact with each other. But the molecules mediating this interaction are unknown. One of the candidates for this role is a microtubule motor kinesin. Recent data showed that kinesin is involved in the plus end-directed movement of the membranous organelles along microtubules such as radial extension of lysosomes in macrophages and centrifugal movement of pigment in melanophores. Here we report that injection of the anti-kinesin antibody into human fibroblasts results in the redistribution of intermediate filaments to a tight perinuclear aggregate but had no effect on the distribution of microtubules. Thus, kinesin is involved not only in organelle movement but also in interaction of the two major cytoskeletal systems, intermediate filaments and microtubules. 相似文献
65.
66.
卢武星 《北京师范大学学报(自然科学版)》1994,(4)
1.0MeV208Pb离子在非晶Si中的投影射程RP和射程偏差ΔRP作为注量和温度二者的函数用背散射法进行测定.注量的变化范围为5×1013~7×1014cm-2.注入是在室温和t=-120℃下完成的.由由实验所确定的投影射程,射程偏差与注量或温度无关,并且分别等于295和72.2nm.与TRIM86的计算结果相比较,发现RP的偏离为18%,而ΔRP的偏离为36%.RP和ΔRP二者与注量及温度的无关性,排除了所观察到的与TRIM的矛盾是由于注入期间辐射增强扩散或离子束混合效应而引起的解释。 相似文献
67.
68.
Non-Minimum Phase Nonlinear System Predictive Control Based on Local Recurrent Neural Networks 总被引:3,自引:0,他引:3
Abstract: After a recursive multi-step-ahead predictor for nonlinear systems based on local recurrent neural networks isintroduced, an intelligent PID controller is adopted to correct the errors including identified model errors and accumulatederrors produced in the recursive process. Characterized by predictive control, this method can achieve a good controlaccuracy and has good robustness. A simulation study shows that this control algorithm is very effective. 相似文献
69.
Dynamic Characteristics of Inter-Satellite Links in LEO Networks 总被引:1,自引:0,他引:1
To establish an efficient inter-satellite link (ISL) in an LEO network, the effect of geometric characteristics of ISL on the ISLs and the devices on the LEO satellite should be examined. Because of the continuous movement of the LEO satellite, the time-varying behaviours of the ISL's geometric charactersistics continuously change with the changes of the satellite's position on the orbit. These dynamic geometric characteristics of the ISLs are important for ISL's performance analyzing and the design of the devices on the LEO satellite. This paper describes dynamic geometric characteristics of ISL, analyzes the impact of these regulations on the tracking system of the satellite's antenna and the power adjusting system of the satellite's transmitter, with the Iridium system as an example. 相似文献
70.
Qiangfu Zhao Zhong Wang Youan KeDept. of Electronic Eng. Beijing Institute of Technology P.O.Box Beijing China 《系统工程与电子技术(英文版)》1991,(2)
It has long been realized that the problem of radar imaging is a special case of image reconstruction in which the data are incomplete and noisy. In other fields, iterative reconstruction algorithms have been used successfully to improve the image quality. This paper studies the application of iterative algorithms in radar imaging. A discrete model is first derived, and the iterative algorithms are then adapted to radar imaging. Although such algorithms are usually time consuming, this paper shows that, if the algorithms are appropriately simplified, it is possible to realize them even in real time. The efficiency of iterative algorithms is shown through computer simulations. 相似文献