全文获取类型
收费全文 | 62207篇 |
免费 | 14239篇 |
国内免费 | 3288篇 |
专业分类
化学 | 52969篇 |
晶体学 | 526篇 |
力学 | 1352篇 |
综合类 | 67篇 |
数学 | 4213篇 |
物理学 | 7826篇 |
无线电 | 12781篇 |
出版年
2024年 | 93篇 |
2023年 | 586篇 |
2022年 | 731篇 |
2021年 | 1114篇 |
2020年 | 2580篇 |
2019年 | 3820篇 |
2018年 | 2095篇 |
2017年 | 1609篇 |
2016年 | 4770篇 |
2015年 | 4938篇 |
2014年 | 4995篇 |
2013年 | 6037篇 |
2012年 | 5061篇 |
2011年 | 4298篇 |
2010年 | 4313篇 |
2009年 | 4301篇 |
2008年 | 4077篇 |
2007年 | 3233篇 |
2006年 | 2861篇 |
2005年 | 2892篇 |
2004年 | 2369篇 |
2003年 | 2134篇 |
2002年 | 2713篇 |
2001年 | 1922篇 |
2000年 | 1772篇 |
1999年 | 844篇 |
1998年 | 382篇 |
1997年 | 327篇 |
1996年 | 344篇 |
1995年 | 281篇 |
1994年 | 248篇 |
1993年 | 208篇 |
1992年 | 190篇 |
1991年 | 162篇 |
1990年 | 174篇 |
1989年 | 143篇 |
1988年 | 114篇 |
1987年 | 80篇 |
1986年 | 65篇 |
1985年 | 102篇 |
1984年 | 80篇 |
1983年 | 52篇 |
1982年 | 48篇 |
1981年 | 52篇 |
1980年 | 40篇 |
1979年 | 49篇 |
1977年 | 41篇 |
1976年 | 37篇 |
1975年 | 36篇 |
1973年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
5‐Amino‐4‐methyl‐2‐phenyl‐6‐substitutedfuro[2,3‐d]pyrimidines ( 2a‐c ) were reacted with 2,5‐dimethoxytetrahydrfuran to afford the pyrrolyl derivatives 3a‐c . Compound 3a was chosen as intermediate for the synthesis of poly fused heterocycles incorporated furopyrimidines moiety 4–11 . Some of the synthesized compounds were screened for their antibacterial and antifungal activities. 相似文献
104.
This paper reports that the growth of RuOx(110) thin layer growth on Ru(0001)
has been investigated by means of scanning tunnelling microscope (STM). The STM
images showed a domain structure with three rotational domains of RuOx(110)
rotated by an angle of 120℃.
The as-grown RuOx(110) thin layer is expanded from the bulk-truncated
RuOx(110) due to the large mismatch between RuOx(110) and the
Ru(0001) substrate. The results also indicate that growth of RuOx(110)
thin layer on the Ru(0001) substrate by oxidation tends first to formation
of the Ru-O (oxygen) chains in the [001] direction of RuOx(110). 相似文献
105.
就一般非完整约束系统,从约束方程满足的变分恒等式出发,利用增广位形流形上的向量场定义三类非自由变分,即非完整变分:vakonomic变分、Hlder变分、Suslov变分,并讨论它们之间的关系以及它们成为自由变分的充要条件.利用非完整变分以及相应的积分变分原理建立两类动力学方程:vakonomic方程和Routh方程或Chaplygin方程.通过vakonomic方程分别与Routh方程和Chaplygin方程比较,得到它们具有共同解的两类充分必要条件.这些条件并不是约束的可积性条件.
关键词:
非完整约束
非完整变分
Chetaev条件
vakonomic动力学 相似文献
106.
Chang-Ho Lee Sutono A. Sangwoo Han Kyutae Lim Pinel S. Tentzeris E.M. Laskar J. 《Advanced Packaging, IEEE Transactions on》2002,25(3):374-384
Presents design, implementation, and measurement of a three-dimensional (3-D)-deployed RF front-end system-on-package (SOP) in a standard multi-layer low temperature co-fired ceramic (LTCC) technology. A compact 14 GHz GaAs MESFET-based transmitter module integrated with an embedded bandpass filter was built on LTCC 951AT tapes. The up-converter MMIC integrated with a voltage controlled oscillator (VCO) exhibits a measured up-conversion gain of 15 dB and an IIP3 of 15 dBm, while the power amplifier (PA) MMIC shows a measured gain of 31 dB and a 1-dB compression output power of 26 dBm at 14 GHz. Both MMICs were integrated on a compact LTCC module where an embedded front-end band pass filter (BPF) with a measured insertion loss of 3 dB at 14.25 GHz was integrated. The transmitter module is compact in size (400 /spl times/ 310 /spl times/ 35.2 mil/sup 3/), however it demonstrated an overall up-conversion gain of 41 dB, and available data rate of 32 Mbps with adjacent channel power ratio (ACPR) of 42 dB. These results suggest the feasibility of building highly SOP integrated RF front ends for microwave and millimeter wave applications. 相似文献
107.
Wen Zhang Hui Huang Yu Ding Margherita Gavagnin Ernesto Mollo Guido Cimino Yue‐Wei Guo 《Helvetica chimica acta》2006,89(4):813-820
Three new polyoxygenated steroids, muricesteroid ( 1 ), and menellsteroids A ( 2 ) and B ( 3 ), were isolated from two species of the South China Sea gorgonian Muricella flexuosa and Menella verrucosa Brundin , respectively. The structures of these new compounds were elucidated on the basis of extensive spectroscopic analysis, chemical methods and comparison with known related compounds. 相似文献
108.
This paper reports that the m-plane GaN layer is grown on (200)-plane LiAlO2 substrate by metal-organic chemical wpour deposition (MOCVD) method. Tetragonal-shaped crystallites appear at the smooth surface. Raman measurement illuminates the compressive stress in the layer which is released with increasing the layer's thickness. The high transmittance (80%), sharp band edge and excitonic absorption peak show that the GaN layer has good optical quality. The donor acceptor pair emission peak located at -3.41 eV with full-width at half maximum of 120 meV and no yellow peaks in the photoluminescence spectra partially show that no Li incorporated into GaN layer from the LiAlO2 substrate. 相似文献
109.
It is proved that the variety of relevant disjunction lattices has the finite embeddability property. It follows that Avron's relevance logic RMI min has a strong form of the finite model property, so it has a solvable deducibility problem. This strengthens Avron's result that RMI min is decidable. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
110.
Woo-Jin Han Eun-Kyoung Kim Yung-Hwan Oh 《Electronics letters》2002,38(6):292-294
A novel frame interpolation technique for two-band linear predictive coding (LPC) vocoders is proposed for maintaining natural speech quality at bit rates below 1 kbit/s. Experimental results show that the speech quality of the proposed vocoder is quite natural at bit rates 880 bit/s and comparable to that of 4.8 kbit/s CELP 相似文献