全文获取类型
收费全文 | 1605103篇 |
免费 | 32451篇 |
国内免费 | 8445篇 |
专业分类
化学 | 712115篇 |
晶体学 | 20803篇 |
力学 | 76507篇 |
综合类 | 112篇 |
数学 | 247012篇 |
物理学 | 389590篇 |
无线电 | 199860篇 |
出版年
2021年 | 14873篇 |
2020年 | 17494篇 |
2019年 | 17622篇 |
2018年 | 16725篇 |
2017年 | 15001篇 |
2016年 | 31211篇 |
2015年 | 22227篇 |
2014年 | 33025篇 |
2013年 | 79011篇 |
2012年 | 44319篇 |
2011年 | 44944篇 |
2010年 | 43103篇 |
2009年 | 47277篇 |
2008年 | 46876篇 |
2007年 | 45002篇 |
2006年 | 47870篇 |
2005年 | 42144篇 |
2004年 | 41819篇 |
2003年 | 38511篇 |
2002年 | 38745篇 |
2001年 | 38903篇 |
2000年 | 33583篇 |
1999年 | 29938篇 |
1998年 | 27821篇 |
1997年 | 27606篇 |
1996年 | 27094篇 |
1995年 | 24840篇 |
1994年 | 24351篇 |
1993年 | 23753篇 |
1992年 | 23928篇 |
1991年 | 24016篇 |
1990年 | 22850篇 |
1989年 | 22448篇 |
1988年 | 21658篇 |
1987年 | 20360篇 |
1986年 | 19194篇 |
1985年 | 25655篇 |
1984年 | 26661篇 |
1983年 | 22463篇 |
1982年 | 23815篇 |
1981年 | 22987篇 |
1980年 | 22245篇 |
1979年 | 22314篇 |
1978年 | 23450篇 |
1977年 | 23051篇 |
1976年 | 22659篇 |
1975年 | 21342篇 |
1974年 | 20994篇 |
1973年 | 21442篇 |
1972年 | 15617篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
941.
In view of immense importance of silylenes and the fact that their properties undergo significant changes on substitution with halogens, here, we have used B3LYP/6-311++G** level of theory to access the effects of 1–4 halogens (X = F, Cl, Br, and I) on four unprecedented sets of cyclopentasilylene-2,4-dienes; with the following formulas: SiC4H3X ( 1 X ), SiC4H2X2 ( 2 X ), SiC4HX3 ( 3 X ), and SiC4X4 ( 4 X ). In going down from F to I, the singlet (s)-triplet (t) energy gap (ΔEs-t, a possible indication of stability), and band gap (ΔEH-L) decrease while nucleophilicity (N), chemical potential (μ), and proton affinity (PA) increase. The overall order of N, μ, and PA for each X is 2 X > 1 X > 3 X > 4 X . Precedence of 2 X over 1 X is attributed to the symmetric cross conjugation in the former. The highest and lowest N are shown by 2 I and 4 F . The trend of divalent angle () for each X is 4 X > 1 X > 3 X > 2 X . The results show that in going from electron withdrawing groups (EWGs) to electron donating groups (EDGs), the ΔEs-t and ΔEH-L decrease while N, μ, and PA increase. Also, rather high N of our scrutinized silylenes may suggest new promising ligands in organometallic chemistry. 相似文献
942.
943.
Vanessa R. Machado Louis P. Sandjo Giovanni L. Pinheiro Milene H. Moraes Mario Steindel Moacir G. Pizzolatti 《Natural product research》2018,32(3):275-281
The natural product lupeol 1 was isolated from aerial parts of Vernonia scorpioides with satisfactory yield, which made it viable to be used as starting material in semisynthetic approach. Ten lupeol derivatives 2–11 were prepared by classical procedures. Including, five new esters derivatives 7–11, which were obtained by structural modifications in the isopropylidene fragment. All semisynthetic compounds and lupeol 1–11 were confirmed by 1H NMR, 13C NMR and HRMS. Their antiprotozoal activity was evaluated in vitro against L. amazonensis and T. cruzi. Derivative 6 showed the best antitrypanosomal activity (IC50 = 12.48 μg/mL) and the lowest cytotoxic derivative (CC50 = 161.50 μg/mL). The mechanism of action of the most active derivatives (4, 6 and 11) is not dependent from the enzyme trypanothione reductase. 相似文献
944.
In this paper we investigated the BER performance of DS-CDMA using various chip-waveforms, which include three time-limited chip-waveforms and two band-limited chip-waveforms. Closed-form formulae were derived for evaluating the achievable bit-error rate performance with the aid of the standard Gaussian approximation, when communicating over a Nakagami-m channel. The time-limited waveforms impose a low implementational complexity, since they maybe over sampled and read from a look-up table. However, they are outperformed by the frequency-domain raised-cosine waveform as well as the optimum waveform specifically designed by Cho and Lehnert for achieving the lowest possible bit error rate 相似文献
945.
V. A. Babenko V. V. Grigor’yants I. P. Shilov 《Journal of Communications Technology and Electronics》2006,51(6):618-630
Basic definitions related to the main electrodynamic characteristics of surface space-charge waves are formulated. These characteristics and their relationships with the geometric dimensions of guiding structures (a dielectric cylinder and a planar metal-dielectric waveguide) are determined. 相似文献
946.
George K. Karagiannidis 《Communications Letters, IEEE》2006,10(12):828-830
The distribution of the modulus of the sum of random phase vectors, is of great practical importance in several applications which deal with sums of sinusoidal signals (wireless multipath transmissions, radars, optical communications, etc). In this letter, simple closed-form expressions are presented for the probability density function (PDF), the cumulative distribution function (CDF), the moment generating function (MGF) and the moments of the envelope distribution of the sum of L non-identical random Nakagami-m phase vectors with integer fading parameters. Moreover, the average over this distribution of the Gaussian Q-function and of the squared Q-function, are also presented in closed-form 相似文献
947.
In this paper, we propose a new peak-to-average power ratio (PAPR) reduction algorithm for OFDM transmission. The algorithm is based on simple amplitude predistortion of a subset of the input symbol blocks, the predistorted symbols being selected according to a metric which measures their contribution to the output signal samples of large. magnitude. This metric-based algorithm is simple, highly flexible, and can be implemented as a one-shot process, although its performance can be further improved by iterating the process one or more times. 相似文献
948.
Molino A. Vacca F. Masera G. Nguyen T.Q. 《Vision, Image and Signal Processing, IEE Proceedings -》2006,153(6):860-868
To efficiently compute the phase difference (PD) between two complex numbers, two novel approaches are described. The problem of fast PD computation is central in many applications. As a case study, the main focus is on the phase correlation technique that is used for motion estimation. Starting from the problem statement, the system requirements are dealt with showing how PD requires a remarkable amount of computational resources. Reduced complexity techniques are then proposed and specifically tailored to suit the application needs. Each solution is completely implemented both in 0.25 mum as well as 0.13 mum CMOS. The so-called LUT-ROT exhibits noteworthy figures in terms of area occupation, delay and power dissipation, saving nearly 50% in terms of area and power when compared to recent work on this subject 相似文献
949.
Stievano I.S. Maio I.A. Canavero F.G. Siviero C. 《Advanced Packaging, IEEE Transactions on》2006,29(1):31-38
This paper addresses the impact of device macromodels on the accuracy of signal integrity and performance predictions for critical digital interconnecting systems. It exploits nonlinear parametric models for both single-ended and differential devices, including the effects of power supply fluctuations and receiver bit detection. The analysis demonstrates that the use of well-designed macromodels dramatically speeds up the simulation as well it preserves timing accuracy even for long bit sequences. 相似文献
950.
A. Yu. Grinev E. N. Voronin 《Journal of Communications Technology and Electronics》2006,51(3):255-262
It is shown that two approaches can efficiently be applied to calculate impulse responses of antennas radiating superwideband signals. The simultaneous use of these approaches makes it possible to lucidly describe the operation of certain antennas and substantially simplify the calculations. The impulse responses of some short-video-pulse scanning array antennas and the shapes of radiated signals are calculated. 相似文献