全文获取类型
收费全文 | 3326篇 |
免费 | 113篇 |
国内免费 | 7篇 |
专业分类
化学 | 1731篇 |
晶体学 | 125篇 |
力学 | 91篇 |
数学 | 262篇 |
物理学 | 667篇 |
无线电 | 570篇 |
出版年
2023年 | 25篇 |
2022年 | 56篇 |
2021年 | 92篇 |
2020年 | 72篇 |
2019年 | 70篇 |
2018年 | 86篇 |
2017年 | 53篇 |
2016年 | 113篇 |
2015年 | 85篇 |
2014年 | 103篇 |
2013年 | 271篇 |
2012年 | 168篇 |
2011年 | 163篇 |
2010年 | 126篇 |
2009年 | 118篇 |
2008年 | 149篇 |
2007年 | 128篇 |
2006年 | 116篇 |
2005年 | 115篇 |
2004年 | 99篇 |
2003年 | 81篇 |
2002年 | 83篇 |
2001年 | 46篇 |
2000年 | 34篇 |
1999年 | 35篇 |
1998年 | 30篇 |
1997年 | 30篇 |
1996年 | 38篇 |
1995年 | 39篇 |
1994年 | 30篇 |
1993年 | 45篇 |
1992年 | 47篇 |
1991年 | 41篇 |
1990年 | 38篇 |
1989年 | 41篇 |
1988年 | 35篇 |
1987年 | 29篇 |
1986年 | 33篇 |
1985年 | 33篇 |
1984年 | 37篇 |
1983年 | 31篇 |
1982年 | 30篇 |
1981年 | 35篇 |
1980年 | 30篇 |
1979年 | 28篇 |
1978年 | 22篇 |
1977年 | 35篇 |
1976年 | 18篇 |
1975年 | 23篇 |
1974年 | 24篇 |
排序方式: 共有3446条查询结果,搜索用时 812 毫秒
1.
Prasad Ravi Kumar Madhu Santanoo Ramotra Prashant Edla Damodar Reddy 《Wireless Networks》2021,27(6):4111-4122
Wireless Networks - In Wireless Sensor Networks (WSNs), where power consumption is a huge concern, the improvement of the network’s lifetime is an area of constant study and innovation. The... 相似文献
2.
Gladson S. Chrisben Prasad P. Siva Manickam Bhaskar 《Analog Integrated Circuits and Signal Processing》2021,109(3):571-583
Analog Integrated Circuits and Signal Processing - As the 5G era beckons in the world of communication and information technology, there is a surge in demand for battery-operated, mobile wireless... 相似文献
3.
Self‐Assembly and Gelation of Poly(aryl ether) Dendrons Containing Hydrazide Units: Factors Controlling the Formation of Helical Structures
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Partha Malakar Prof. Dr. Edamana Prasad 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(13):5093-5100
Self‐assembly of AB2 and AB3 type low molecular weight poly(aryl ether) dendrons that contain hydrazide units were used to investigate mechanistic aspects of helical structure formation during self‐assembly. The results suggest that there are three important aspects that control helical structure formation in such systems with acyl hydrazide/hydrazone linkage: i) J‐type aggregation, ii) the hydrogen‐bond donor/acceptor ability of the solvent, and iii) the dielectric constant of the solvent. The monomer units self‐assemble to form dimer structures through hydrogen‐bonding and further assembly of the hydrogen‐bonded dimers leads to macroscopic chirality in the present case. Dimer formation was confirmed by NMR spectroscopy and by mass spectrometry. The self‐assembly in the system was driven by hydrogen‐bonding and π–π stacking interactions. The morphology of the aggregates formed was examined by scanning electron microscopy, and the analysis suggests that aprotic solvent systems facilitate helical fibre formation, whereas introduction of protic solvents results in the formation of flat ribbons. This detailed mechanistic study suggests that the self‐assembly follows a nucleation–elongation model to form helical structures, rather than the isodesmic model. 相似文献
4.
Seema Prasad 《Molecular Crystals and Liquid Crystals》2018,665(1):52-63
Molecular structure and vibrational spectroscopic studies of higher homologous series nematogenic p-n-alkylbenzoic acids (nBAC) that have 6 (6BAC) and 7 (7BAC) carbon atoms in the alkyl chain have been investigated using the Density Functional Becke3-Lee-Yang-Parr (B3LYP) level with the basis set 6-31++G (d.p) and Hartree Fock (HF) with the same basis set. The observed vibrational spectra has been resolved and assigned in detail for comparision with both the molecules. These results indicate that DFT and HF values are slightly different at both the levels. A comparision of chemical reactivity such as HOMO (EH), LUMO (EL) energies, energy gap (Eg), ionization energy (I), electron affinity (A), electro negativity (χ), chemical hardness (η), electronic chemical potential (μ), electrophilicity index (ω), and softness (S) has been made. It has been observed that the decrement has occurred in the energy band gap value of isolated molecule with increment in alkyl chain length. This provides valuable information regarding enhancing the stability of liquid crystal materials by maintaining the conductivity. 相似文献
5.
Wireless Personal Communications - The ongoing technological developments have totally transformed the lives of human beings. With the emergence of the smart era and the smart city concept, a few... 相似文献
6.
7.
GaAs MESFETs were fabricated using a spin-on platinum source as the gate material. They were subsequently aged at 200°C for up to 1000 h. DC electrical characterisation of the MESFETs was carried out during various stages of annealing. The aging behaviour of these MESFETs was compared with those fabricated using conventionally evaporated platinum sources. The results show that the performance of the MESFETs fabricated using a spin-on platinum source is comparable to those of MESFETs fabricated using a conventionally evaporated platinum source 相似文献
8.
K. Batra V. Prasad M. Mohan 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(2):191-198
The scattering of heavy ion with a multilevel Rydberg atom in the presence of an electromagnetic field is studied. The interaction
of Rydberg atom and the e.m field is explored using non-perturbative quasi-energy technique. Although the results are presented
for selected excitations but in actual calculations we have included many levels of the atom. The effect of various parameters
are shown on collisional excitation process. As an illustration detailed calculations are performed for the inelastic proton-Na
Rydberg atom collision accompanied by the transfer of photons and the effects of dressing due to the field are considered.
The emphasis of the present work is on collision induced transitions especially the case that involves change of orbital as
well as principal quantum number.
Received 26 December 2001 / Received in final form 8 April 2002 Published online 19 July 2002 相似文献
9.
Beeranahally H Doreswamy Madegowda Mahendra Hirihally C Devarajegowda Venkatesh B Devaru Sridhar M Anandalwar Javaregowda S Prasad 《Analytical sciences》2004,20(2):407-408
The title compound was extracted from a natural product and its structure was characterized by an X-ray diffraction method. It crystallizes in the tetragonal space group P41 with cell parameters a = 15.832(10)A, c = 11.622(10)A, Z = 4; the final residual factor is R1 = 0.0769. The structure has both intra and intermolecular hydrogen bonds. 相似文献
10.
Punt J.B. Sparreboom D. Brouwer F. Prasad R. 《Vehicular Technology, IEEE Transactions on》1998,47(4):1302-1313
Dynamic channel selection (DCS) is an algorithm for flexible resource sharing in mobile radio systems. The digital enhanced cordless telecommunications (DECT) standard implements a version of DCS based on time as well as frequency multiplexing. In this paper, mathematical models are developed to evaluate the probabilities of channel availability, desired carrier power, and the carrier-to-interference ratio (CIR) for a constant traffic load. These results can be used to compute the call setup blocking probability. The models reported in this study are based on a decentralized DCS according to the DECT standard specifications. The results show that blocking due to availability of a channel (resource blocking), not interference blocking, generally is the most important factor in overall call blocking. Furthermore, it is seen that resource blocking is sensitive to the implementation of portable sets. Also, with homogeneous static traffic, this type of DCS ran be as good as fixed channel allocation (FCA) with respect to resource blocking 相似文献