全文获取类型
收费全文 | 26301篇 |
免费 | 2961篇 |
国内免费 | 2691篇 |
专业分类
化学 | 3621篇 |
晶体学 | 124篇 |
力学 | 2346篇 |
综合类 | 622篇 |
数学 | 17861篇 |
物理学 | 7379篇 |
出版年
2024年 | 28篇 |
2023年 | 219篇 |
2022年 | 339篇 |
2021年 | 471篇 |
2020年 | 569篇 |
2019年 | 630篇 |
2018年 | 615篇 |
2017年 | 784篇 |
2016年 | 898篇 |
2015年 | 687篇 |
2014年 | 1343篇 |
2013年 | 1866篇 |
2012年 | 1360篇 |
2011年 | 1654篇 |
2010年 | 1442篇 |
2009年 | 1755篇 |
2008年 | 1746篇 |
2007年 | 1857篇 |
2006年 | 1644篇 |
2005年 | 1543篇 |
2004年 | 1288篇 |
2003年 | 1172篇 |
2002年 | 1076篇 |
2001年 | 956篇 |
2000年 | 892篇 |
1999年 | 774篇 |
1998年 | 679篇 |
1997年 | 651篇 |
1996年 | 464篇 |
1995年 | 360篇 |
1994年 | 302篇 |
1993年 | 230篇 |
1992年 | 186篇 |
1991年 | 171篇 |
1990年 | 160篇 |
1989年 | 121篇 |
1988年 | 110篇 |
1987年 | 104篇 |
1986年 | 82篇 |
1985年 | 110篇 |
1984年 | 96篇 |
1983年 | 62篇 |
1982年 | 80篇 |
1981年 | 73篇 |
1980年 | 50篇 |
1979年 | 51篇 |
1978年 | 42篇 |
1977年 | 31篇 |
1976年 | 34篇 |
1973年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
2.
3.
5.
Chemical bonding in representative astrophysically relevant neutral,cation, and anion HCnH chains 下载免费PDF全文
Ioan Baldea 《中国物理 B》2022,31(12):123101-123101
Most existing studies assign a polyynic and cumulenic character of chemical bonding in carbon-based chains relying on values of the bond lengths. Building on our recent work, in this paper we add further evidence on the limitations of such an analysis and demonstrate the significant insight gained via natural bond analysis. Presently reported results include atomic charges, natural bond order and valence indices obtained from ab initio computations for representative members of the astrophysically relevant neutral and charged HC2k/2k+1H chain family. They unravel a series of counter-intuitive aspects and/or help naive intuition in properly understanding microscopic processes, e.g., electron removal from or electron attachment to a neutral chain. Demonstrating that the Wiberg indices adequately quantify the chemical bonding structure of the HC2k/2k+1H chains—while the often heavily advertised Mayer indices do not—represents an important message conveyed by the present study. 相似文献
6.
7.
《Wave Motion》2018
Large cable net structures have been widely applied in aerospace engineering due to the feature of light-weight, high packaging efficiency, and high thermal stability. Structural vibrations induced by a variety of disturbances are inevitable in the space environment, resulting in the requirement of effective vibration control strategies for large cable net structures. Since the large cable net structures have many closely spaced vibrational modes in the range of low frequencies, traditional modal based control may cause modal truncation and spillover problems. In this paper, a wave-based boundary control strategy is adopted and its effectiveness to control the vibration of cable net structures is investigated, by transfer function analysis and numerical methods. It is found that the structural vibration can be absolutely resisted by applying the wave-based boundary controllers onto all the exterior nodes, when disturbances come from the external boundaries of the cable net. Our results in this paper can provide a theoretical basis for the vibration control of large cable net structures. 相似文献
8.
Multi-sensor data fusion is an evolving technology whereby data from multiple sensor inputs are processed and combined. The data derived from multiple sensors can, however, be uncertain, imperfect, and conflicting. The present study is undertaken to help contribute to the continuous search for viable approaches to overcome the problems associated with data conflict and imperfection. Sensor readings, represented by belief functions, have to be fused according to their corresponding weights. Previous studies have often estimated the weights of sensor readings based on a single criterion. Mono-criteria approaches for the assessment of sensor reading weights are, however, often unreliable and inadequate for the reflection of reality. Accordingly, this work opts for the use of a multi-criteria decision aid. A modified Analytical Hierarchy Process (AHP) that incorporates several criteria is proposed to determine the weights of a sensor reading set. The approach relies on the automation of pairwise comparisons to eliminate subjectivity and reduce inconsistency. It assesses the weight of each sensor reading, and fuses the weighed readings obtained using a modified average combination rule. The efficiency of this approach is evaluated in a target recognition context. Several tests, sensitivity analysis, and comparisons with other approaches available in the literature are described. 相似文献
9.
10.
Nitin P. Lobo K.V. Ramanathan T. Narasimhaswamy 《Magnetic resonance in chemistry : MRC》2020,58(11):988-1009
In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one- and two-dimensional (2D) solid-state 13C nuclear magnetic resonance (NMR) spectroscopy are described. Besides 13C chemical shifts, the 13C─1H dipolar couplings measured from 2D-separated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the 13C─1H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B1 and B2 mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state 13C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed. 相似文献