全文获取类型
收费全文 | 30670篇 |
免费 | 1538篇 |
国内免费 | 137篇 |
专业分类
化学 | 15611篇 |
晶体学 | 243篇 |
力学 | 696篇 |
数学 | 2182篇 |
物理学 | 5755篇 |
无线电 | 7858篇 |
出版年
2024年 | 49篇 |
2023年 | 331篇 |
2022年 | 426篇 |
2021年 | 788篇 |
2020年 | 606篇 |
2019年 | 648篇 |
2018年 | 574篇 |
2017年 | 550篇 |
2016年 | 974篇 |
2015年 | 758篇 |
2014年 | 1132篇 |
2013年 | 1765篇 |
2012年 | 1986篇 |
2011年 | 2176篇 |
2010年 | 1454篇 |
2009年 | 1455篇 |
2008年 | 2041篇 |
2007年 | 1790篇 |
2006年 | 1809篇 |
2005年 | 1598篇 |
2004年 | 1434篇 |
2003年 | 1258篇 |
2002年 | 1127篇 |
2001年 | 755篇 |
2000年 | 689篇 |
1999年 | 541篇 |
1998年 | 396篇 |
1997年 | 359篇 |
1996年 | 386篇 |
1995年 | 265篇 |
1994年 | 257篇 |
1993年 | 266篇 |
1992年 | 211篇 |
1991年 | 179篇 |
1990年 | 168篇 |
1989年 | 131篇 |
1988年 | 104篇 |
1987年 | 90篇 |
1986年 | 57篇 |
1985年 | 89篇 |
1984年 | 62篇 |
1983年 | 56篇 |
1982年 | 70篇 |
1981年 | 71篇 |
1980年 | 39篇 |
1979年 | 49篇 |
1978年 | 49篇 |
1976年 | 41篇 |
1975年 | 38篇 |
1974年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
A new hybrid active power filter (APF) topology 总被引:12,自引:0,他引:12
In this paper, a new hybrid active power filter topology is presented. A higher-voltage, low-switching frequency insulated gate bipolar transistor (IGBT) inverter and a lower-voltage high-switching frequency metal oxide semiconductor field effect transistor (MOSFET) inverter are used in combination to achieve harmonic current compensation. The function of the IGBT inverter is to support utility fundamental voltage and to compensate for the fundamental reactive power. The MOSFET inverter fulfills the function of harmonic current compensation. To further reduce cost and to simplify control, the IGBT and MOSFET inverters share the same DC-link via a split capacitor bank. With this approach harmonics can be cancelled over a wide frequency range. Compared to the conventional APF topology, the proposed approach employs lower dc-link voltage and generates less noise. Simulation and experimental results show that the proposed active power filter topology is capable of compensating for the load harmonics 相似文献
52.
Jong Uhn Kim 《Transactions of the American Mathematical Society》2002,354(3):1117-1135
In this paper we discuss an initial-boundary value problem for a stochastic nonlinear equation arising in one-dimensional viscoelasticity. We propose to use a new direct method to obtain a solution. This method is expected to be applicable to a broad class of nonlinear stochastic partial differential equations.
53.
Gauss periods give an exponentiation algorithm that is fast for many finite fields but slow for many other fields. The current paper presents a different method for construction of elements that yield a fast exponentiation algorithm for finite fields where the Gauss period method is slow or does not work. The basic idea is to use elements of low multiplicative order and search for primitive elements that are binomial or trinomial of these elements. Computational experiments indicate that such primitive elements exist, and it is shown that they can be exponentiated fast. 相似文献
54.
Neodymium-substituted Bi4Ti3O12 (BNdT) thin films were prepared by a chemical solution deposition technique on platinum- coated silicon substrates. All of
the samples were annealed at the relatively low temperature of 600 °C by a rapid thermal annealing process in different atmospheres,
such as O2, air, and nitrogen, and vacuum. Irrespective of different annealing atmospheres, all of the BNdT thin films exhibit good
ferroelectric properties, such as a saturated hysteresis loop, good fatigue endurance, and low leakage current density. A
large remanent polarization (Pr) of ∼48 μC/cm2 with an electric field of 240 kV/cm was observed from the BNdT thin film annealed in O2 atmosphere. The BNdT thin films annealed in nitrogen and vacuum, at reduced oxygen partial pressures, exhibit smaller Pr than that annealed in oxygen. The difference of Pr of the BNdT thin films annealed in different atmospheres may originate from differences in the grain sizes and the number
of oxygen vacancies.
PACS 77.55.+f; 77.80.-e; 77.80.Fm; 81.15.-z 相似文献
55.
56.
Taekyun Kim 《Journal of Mathematical Analysis and Applications》2002,273(1):236-242
The purpose of this paper is to give a proof of Kummer type congruence for the q-Bernoulli numbers of higher order, which is an answer to a part of the problem in a previous publication (see Indian J. Pure Appl. Math. 32 (2001) 1565-1570). 相似文献
57.
Homo-apioneplanocin A was efficiently synthesized via stereoselective hydroxymethylation, regio- and chemoselective hydroboration, and chemoselective oxidation as key steps from d-ribose. 相似文献
58.
Vinyl and diene derivatives of thiolactomycin have been prepared via Horner-Wadsworth-Emmons olefination from protected 5-formyl-3,5-dimethylthiotetronic acid. Several 4-position protecting groups and a variety of phosphonates were evaluated, with MOM protection and β-ketophosphonates yielding the highest ratio of the desired product to deformylated product. 相似文献
59.
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
60.
Smart TDI readout circuit for long-wavelength IR detector 总被引:3,自引:0,他引:3
Byunghyuk Kim Hee Chul Lee 《Electronics letters》2002,38(16):854-855
A smart time delay and integration (TDI) readout circuit is suggested which performs background suppression, cell-to-cell non-uniformity compensation, and dead pixel correction. Using the smart TDI readout circuit, the integration capacitor area occupying almost the whole area of a unit-cell can be reduced to one-fifth and transimpedance gain can increase by five times. From measurement results, it is found that the skimming current error for a few hundred nA background current is < 1.25 nA corresponding to LSB/2 of ADC and the non-uniformity introduced by cell-to-cell background current variation is reduced to 1.02 nA 相似文献