全文获取类型
收费全文 | 2766篇 |
免费 | 79篇 |
国内免费 | 6篇 |
专业分类
化学 | 1248篇 |
晶体学 | 9篇 |
力学 | 45篇 |
数学 | 262篇 |
物理学 | 802篇 |
无线电 | 485篇 |
出版年
2022年 | 22篇 |
2021年 | 36篇 |
2020年 | 38篇 |
2019年 | 24篇 |
2018年 | 22篇 |
2017年 | 36篇 |
2016年 | 54篇 |
2015年 | 53篇 |
2014年 | 51篇 |
2013年 | 99篇 |
2012年 | 123篇 |
2011年 | 124篇 |
2010年 | 93篇 |
2009年 | 84篇 |
2008年 | 131篇 |
2007年 | 113篇 |
2006年 | 115篇 |
2005年 | 144篇 |
2004年 | 102篇 |
2003年 | 80篇 |
2002年 | 93篇 |
2001年 | 52篇 |
2000年 | 62篇 |
1999年 | 24篇 |
1998年 | 30篇 |
1997年 | 35篇 |
1996年 | 52篇 |
1995年 | 39篇 |
1994年 | 53篇 |
1993年 | 66篇 |
1992年 | 55篇 |
1991年 | 36篇 |
1990年 | 51篇 |
1989年 | 35篇 |
1988年 | 36篇 |
1987年 | 40篇 |
1986年 | 33篇 |
1985年 | 66篇 |
1984年 | 47篇 |
1983年 | 31篇 |
1982年 | 39篇 |
1981年 | 30篇 |
1980年 | 28篇 |
1979年 | 41篇 |
1978年 | 28篇 |
1977年 | 21篇 |
1976年 | 20篇 |
1975年 | 22篇 |
1974年 | 29篇 |
1973年 | 17篇 |
排序方式: 共有2851条查询结果,搜索用时 375 毫秒
1.
2.
João Marcos G. Barbosa Milenna K. Fernandes Rodrigues Lurian C. David Taynara C. e Silva Danielly A. Fortuna Lima Naiara Z. Pereira Emmanuel B. D'Alessandro Anselmo E. de Oliveira Paulo H. Jorge da Cunha Maria Clorinda S. Fioravanti Nelson R. Antoniosi Filho 《Biomedical chromatography : BMC》2020,34(11):e4935
An innovative volatolomic approach employs the detection of biomarkers present in cerumen (earwax) to identify cattle intoxication by Stryphnodendron rotundifolium Mart., Fabaceae (popularly known as barbatimão). S. rotundifolium is a poisonous plant with the toxic compound undefined and widely distributed throughout the Brazilian territory. Cerumen samples from cattle of two local Brazilian breeds (‘Curraleiro Pé-Duro’ and ‘Pantaneiro’) were collected during an experimental intoxication protocol and analyzed using headspace (HS)/GC–MS followed by multivariate analysis (genetic algorithm for a partial least squares, cluster analysis, and classification and regression trees). A total of 106 volatile organic metabolites were identified in the cerumen samples of bovines. The intoxication by S. rotundifolium influenced the cerumen volatolomic profile of the bovines throughout the intoxication protocol. In this way, it was possible to detect biomarkers for cattle intoxication. Among the biomarkers, 2-octyldecanol and 9-tetradecen-1-ol were able to discriminate all samples between intoxicated and nonintoxicated bovines. The cattle intoxication diagnosis by S. rotundifolium was accomplished by applying the cerumen analysis using HS/GC–MS, in an easy, accurate, and noninvasive way. Thus, the proposed bioanalytical chromatography protocol is a useful tool in veterinary applications to determine this kind of intoxication. 相似文献
3.
Inerfield M. Skones W. Nelson S. Ching D. Cheng P. Wong C. 《Solid-State Circuits, IEEE Journal of》2003,38(9):1524-1532
To date, the development of multifunction multicarrier digital receivers for cellular base station and military communications applications has been limited by the demanding dynamic range requirements for the analog-to-digital converter (ADC). The use of oversampling delta-sigma modulators provides a promising approach to overcoming the dynamic range barriers Nyquist-rate converters face in the same applications. This paper discusses issues involved in the design of high-speed high dynamic range wide-band delta-sigma ADCs for such communications applications. Test results of prototype designs are also presented. The delta-sigma modulators described in this paper operate at sampling frequencies ranging from 1 to 2.5 GHz with center frequencies ranging from dc to 100 MHz, providing between 74 and 84.2 dB signal-to-noise ratio (12 and 13.7 bits) for bandwidths of 25 and 12.5 MHz, respectively. The loop filters are continuous-time low-pass and bandpass implementations of order 6 and 10, and were fabricated in an InP heterojunction bipolar (HBT) technology. A typical tenth-order design consumes 6 W of power and occupies a die area of 23.5 mm/sup 2/. 相似文献
4.
A new architecture for implementing finite-impulse response (FIR) filters using the residue number system (RNS) is detailed. The design is based on using a restricted modulus set, with moduli of the form 2/sup n/,2/sup n/-1, and 2/sup n/+1. This does not restrict the modulus set to the common 3 modulus set {2/sup n/-1,2/sup n/,2/sup n/+1}, but any number of pairwise relatively prime moduli of this form, for example, {5,7,17,31,32,33}. Based on a comparison with a 2's complement design, the new RNS design can offer a significant speed improvement. The gain is obtained by using a set of small moduli, selected so as to minimize critical path delay and area. An algorithmic approach is used to obtain full adder based architectures that are optimized for area and delay. The modulus set is optimum based on cost parameters for each modulus. This new architecture presents a practical approach to implementing a fast RNS FIR filter. 相似文献
5.
Pentacene-based organic thin-film transistors 总被引:7,自引:0,他引:7
Yen-Yi Lin Gundlach D.I. Nelson S.F. Jackson T.N. 《Electron Devices, IEEE Transactions on》1997,44(8):1325-1331
Organic thin-film transistors using the fused-ring polycyclic aromatic hydrocarbon pentacene as the active electronic material have shown mobility as large as 0.7 cm2/V-s and on/off current ratio larger than 108; both values are comparable to hydrogenated amorphous silicon devices. On the other hand, these and most other organic TFT's have an undesirably large subthreshold slope. We show here that the large subthreshold slope typically observed is not an intrinsic property of the organic semiconducting material and that devices with subthreshold slope similar to amorphous silicon devices are possible 相似文献
6.
7.
We study the nonlinear propagation of light in optical fibers in the strong nonlinearity limit. In this limit, the transverse guiding properties are altered and cannot be decoupled from the longitudinal part of the wave equation. We consider a refractive index profile with no bound modes at low intensities and show that stable self-guiding occurs at a critical intensity. This critical intensity is a property of the guide and can be made much lower than the threshold for unstable self-focusing. We consider a particular design for which self-guiding occurs at three orders of magnitude lower than the self-focusing threshold. 相似文献
8.
Using lower-cost lasers, 30% savings are possible for 10-Gb/s uncompensated metro transmission over NZDF, compared to G.652 fiber. We present WDM transmission results for a mixed plant of NZDF rings interconnected to G.652.C-fiber access laterals. 相似文献
9.
Jacobson M. A. Nelson D. K. Konstantinov O. V. Matveentsev A. V. 《Semiconductors》2005,39(12):1410-1414
Semiconductors - It is established experimentally that the peak in the photoluminescence spectrum of the In0.2Ga0.8N/GaN heterostructure with a quantum well shifts by ~150 meV as the power... 相似文献
10.
We discuss the concept of coarse wavelength-division multiplexing (CWDM) for metro networks. After reviewing the requirements on components such as lasers and fiber, we propose different architectures for a flexible upgrade of existing CWDM systems. 相似文献