全文获取类型
收费全文 | 17211篇 |
免费 | 1955篇 |
国内免费 | 1555篇 |
学科分类
工业技术 | 20721篇 |
出版年
2024年 | 109篇 |
2023年 | 467篇 |
2022年 | 724篇 |
2021年 | 871篇 |
2020年 | 776篇 |
2019年 | 713篇 |
2018年 | 695篇 |
2017年 | 763篇 |
2016年 | 750篇 |
2015年 | 719篇 |
2014年 | 1095篇 |
2013年 | 1216篇 |
2012年 | 1139篇 |
2011年 | 1204篇 |
2010年 | 843篇 |
2009年 | 901篇 |
2008年 | 945篇 |
2007年 | 956篇 |
2006年 | 793篇 |
2005年 | 747篇 |
2004年 | 635篇 |
2003年 | 562篇 |
2002年 | 479篇 |
2001年 | 401篇 |
2000年 | 321篇 |
1999年 | 277篇 |
1998年 | 222篇 |
1997年 | 209篇 |
1996年 | 155篇 |
1995年 | 185篇 |
1994年 | 123篇 |
1993年 | 108篇 |
1992年 | 89篇 |
1991年 | 78篇 |
1990年 | 79篇 |
1989年 | 69篇 |
1988年 | 60篇 |
1987年 | 29篇 |
1986年 | 17篇 |
1985年 | 31篇 |
1984年 | 20篇 |
1983年 | 28篇 |
1982年 | 21篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1966年 | 6篇 |
1964年 | 6篇 |
1959年 | 7篇 |
1955年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
G. Ranzi M. A. Bradford B. Uy 《International journal for numerical methods in engineering》2004,61(5):657-672
The use of the conventional semi-analytical stiffness method in finite element analysis, in which interpolation polynomials are used to develop the stiffness relationships, leads to problems of curvature locking when beam-type elements are developed for composite members with partial interaction between the materials of which it is comprised. The curvature locking phenomenon that occurs for composite steel–concrete members is quite well reported, and the general approach to minimizing the undesirable ramifications of curvature locking has been to use higher-order polynomials with increasing numbers of internal nodes. This paper presents an alternate formulation based on a direct stiffness approach rather than starting from pre-defined interpolation polynomials, and which does not possess the undesirable locking characteristics. The formulation is based on a more general approach for a bi-material composite flexural member, whose constituent materials are joined by elastic shear connection so as to provide partial interaction. The stiffness relationships are derived, and these are applied to a simply supported and a continuous steel–concrete composite beam to demonstrate the efficacy of the method, and in particular its ability to model accurately both very flexible and very stiff shear connection that causes difficulties when implemented in competitive semi-analytical algorithms. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
42.
43.
Not all Group Support Systems are identical, as is demonstrated by their software implementations. We discuss two existing implementations of group support tools and the process models underlying them. We demonstrate that fundamental to both processes is the merging or integration of individual data. Based on this and other empirical research, the Shared Context Model (SCM) of cooperative work is adopted and we show that it supports existing processes and others. We expect that groups will find merging their work easier with the SCM. This model is presented and embedded in the architecture and implementation of four group tools. Because these tools are destined to be used by dispersed groups, synchronously or asynchronously, an object-based communication and control mechanism is incorporated. Finally, as graphics and multi-tasking have been shown to be increasingly important, the tools are implemented in Microsoft Windows for personal computers attached to local area networks. 相似文献
44.
四种别藻蓝蛋白三聚体的时间分辨荧光光谱研究 总被引:6,自引:0,他引:6
研究了从Anabanavaribilis中提取的4种别藻蓝蛋白APCI,APCII,APCII及APCB三聚体的稳态光谱和皮秒荧光光谱。采用Monte-Carlo方法对瞬态荧光光谱进行拟合,实验结果表明:APCI荧光有两个带,其中第一个带位于662nm,有两个时间组分:35.8ps和1.67ns;第二个带位于680nm,有两个时间组分:34.2ps和1.64ns;APCII瞬态荧光位于660nm,有两个时间组分:20.4ps和1.64ns;APCII瞬态荧光位于660nm,有两个时间组分:23.8ps和1.76ns;APCB瞬态荧光有两个带,其中第一个带位于662nm,有两个时间组分:36.6ps和1.45ns;第二个带位于680nm,有两个时间组分:25.8ps和1.62ns。实验结果一方面说明了藻胆体核内4种别藻蓝蛋白形成能量传递的两条途径;另一方面瞬态荧光解叠结果揭示了APCI和APCB三聚体内能量传递的超快过程。 相似文献
45.
Al-Li alloys being developed as lighter, substitutes for conventional high strength Al alloys are to be processed by routine
methods. During extrusion of a 8090 Al-Li alloy, the extrusion die container failed causing some alarm. This failed die container
was analysed to examine if the failure was caused by interaction of Li diffusing out of Al-Li alloy with the carbides of die
steel. The evidence, although not conclusive, is sufficient to exercise caution during such processing. 相似文献
46.
Vladimir Kulyukin Chaitanya Gharpure John Nicholson Grayson Osborne 《Autonomous Robots》2006,21(1):29-41
We present a robot-assisted wayfinding system for the visually impaired in structured indoor environments. The system consists
of a mobile robotic guide and small passive RFID sensors embedded in the environment. The system is intended for use in indoor
environments, such as office buildings, supermarkets and airports. We describe how the system was deployed in two indoor environments
and evaluated by visually impaired participants in a series of pilot experiments. We analyze the system’s successes and failures
and outline our plans for future research and development. 相似文献
47.
The paper presents experimental results of secondary ion energy distributions obtained for Ti and Si targets bombarded by 20-30 keV monoisotope Ar+ ion beam. The influence of the extraction voltages between target and a slit of the electrostatic energy analyzer entrance on the energy distributions of secondary ions was investigated. After optimization of the secondary ion extraction system, the mass spectra of secondary ions were also measured. The investigations were done using recently built experimental system. Experimental data are compared with the computer simulation results obtained using TRQR and SATVAL codes. 相似文献
48.
R. Kaji S. Adachi H. Sasakura S. Muto H. Kumano I. Suemune 《Journal of Superconductivity and Novel Magnetism》2007,20(6):447-451
We investigated optical pumping of nuclear spin polarizations in a single self-assembled In0.75Al0.25As/Al0.3Ga0.7As quantum dot. The nuclear spin polarization exhibits the abrupt jump and hysteresis in the excitation power dependence at
a particular excitation polarization. Measurement of circular polarization rate of the photoluminescence reveals that the
abrupt change of the nuclear spin polarization is created mainly by the spin flip-flop process between nuclei and an electron
of a positive charged exciton in this single quantum dot. Model calculation explains well the experimentally observed bistable
behavior in InAlAs quantum dot. By using this abrupt change, the sign and magnitude of electron and hole g-factors in z-direction are verified.
相似文献
49.
Jan‐Chan Huang 《应用聚合物科学杂志》2003,90(3):671-680
A modified method is discussed that is based on Farooque and Deshpande's method to obtain polymer–polymer interaction parameters using inverse gas chromatography (IGC) data. In the Farooque and Deshpande method, the ratio of the difference of probe–polymer interaction parameters between two polymers and the probe volume [(χ12 ? χ13)/V1] is used as the abscissa. In the modified method, the ratio [(?2χ12 + ?3χ13)/V1] is used as the abscissa. Experimental data previously reported for a poly(?‐caprolactone)‐polyepichlorohydrin (PCL/PECH) blend and a poly(ethyl acrylate)‐poly(vinyl propionate) (PEA/PVPr) blend are analyzed. It is found that the slopes obtained by the new method had smaller deviations from the theoretical values than the Farooque and Deshpande method. The standard deviations of both slopes and intercepts obtained from the new method are also smaller. Using the new method, the polymer–polymer interaction parameters obtained from the intercept are negative numbers for the PCL/PECH system and very small positive numbers for PEA/PVPr. Explanations are given for the probe and concentration dependency of the polymer–polymer interaction parameters that are generally observed in IGC studies. A new method for selecting the best probe for calculating the interaction parameter is discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 671–680, 2003 相似文献
50.
结构-波浪-海床耦合系统中大圆筒结构的波压力响应 总被引:5,自引:2,他引:3
以沉入式大圆筒结构为对象,通过设计随机波浪水槽实验,测取了在单筒和连续筒两种结构型式、两种沉入深度和两种波谱输入下结构表面的动波压力分布。波面高度以及波压力历程均在领域进行分析。通过计算结构表面沿高程和沿环向的波压力谱及波压力传递函数,将单筒实测结果与基于线性绕射理论的解析解进行了比较,同时也详细对比了两种结构型式下的实测结果。文中汇报了在研究中获得的新发现及进行的相应分析,特别是在结构——波浪——海床动力相互作用下对波压力响应机理以及针对连续筒的研究,可为工程应用提供有价值的参考。 相似文献