全文获取类型
收费全文 | 10912篇 |
免费 | 626篇 |
国内免费 | 187篇 |
专业分类
化学 | 7345篇 |
晶体学 | 51篇 |
力学 | 523篇 |
数学 | 1119篇 |
物理学 | 1442篇 |
无线电 | 1245篇 |
出版年
2024年 | 23篇 |
2023年 | 90篇 |
2022年 | 345篇 |
2021年 | 488篇 |
2020年 | 535篇 |
2019年 | 565篇 |
2018年 | 608篇 |
2017年 | 522篇 |
2016年 | 765篇 |
2015年 | 505篇 |
2014年 | 745篇 |
2013年 | 1272篇 |
2012年 | 918篇 |
2011年 | 800篇 |
2010年 | 559篇 |
2009年 | 520篇 |
2008年 | 509篇 |
2007年 | 424篇 |
2006年 | 278篇 |
2005年 | 243篇 |
2004年 | 185篇 |
2003年 | 159篇 |
2002年 | 148篇 |
2001年 | 51篇 |
2000年 | 42篇 |
1999年 | 44篇 |
1998年 | 32篇 |
1997年 | 25篇 |
1996年 | 32篇 |
1995年 | 22篇 |
1994年 | 15篇 |
1993年 | 17篇 |
1992年 | 26篇 |
1991年 | 14篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 9篇 |
1987年 | 17篇 |
1986年 | 6篇 |
1985年 | 17篇 |
1984年 | 18篇 |
1983年 | 8篇 |
1982年 | 16篇 |
1981年 | 8篇 |
1980年 | 16篇 |
1979年 | 9篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Mohammad Rahimizadeh Tahmineh Bazazan Ali Shiri Mehdi Bakavoli Hassan Hassani 《中国化学快报》2011,22(4):435-438
Preyssler-type heteropoly acid is introduced as a new,mild and efficient catalyst for protection of a variety of carbonyl compounds with 1,3-propane dithiol. 相似文献
992.
Abbas Pirmohammadi Mohammad Hossein Zarifi 《Analog Integrated Circuits and Signal Processing》2012,71(3):473-479
In this article, an inverter based transconductor using double CMOS pair is proposed for implementation of a second order lowpass Gm?CC Filter. The proposed operational transconductance amplifier (OTA) and biquad filter are designed using standard 0.35???m CMOS technology. Simulation results demonstrate the central frequency tunability from 10?kHz to 2.8?MHz which is suitable for the wireless specifications of Bluetooth (650?kHz), CDMA 2000 (700?kHz) and Wideband CDMA (2.2?MHz) applications. The power consumption of the filter is 445?nW and 178???W at 10?kHz and 2.8?MHz from 3.3?V supply voltage, respectively. The active area occupied by the designed filter on the silicon is 215?×?720???m2. The proposed approach guarantees the upper bound on THD to be ?40?dB for 300?mVpp signal swing. Employing the double CMOS pair in the inverters causes PSRR to reach 68.6?dB which is higher than similar works. 相似文献
993.
Mahdi Khosravy Mohammad Reza Asharif Katsumi Yamashita 《Signal, Image and Video Processing》2011,5(3):379-388
This paper discusses the theoretical foundation of Stone’s BSS (Stone in Neural Comput 13:1559–1574, 2001; Stone in Independent
Component Analysis: A Tutorial Introduction, A Bradford Book, London, 2004), and it proposes a novel BSS approach based on
second-order statistics of the responses of two different linear filters to source signals. The proposed approach which includes
Stone’s BSS as a special case helps us to understand how generalized eigenvalue decomposition (GEVD) concludes separating
vectors in Stone’s BSS. It obtains the separating vectors by simultaneous diagonalization of covariance matrices of two different
linear filters responses to the mixtures. The two employed linear filters are selected dependent on source signals structures
under the assumption that they have different responses to source signals. Here, two FIR filters with coefficients selected
in an opposite probabilistic way have been suggested for the proposed BSS. The proposed BSS method has been compared with
Stone’s BSS, SOBI and AMUSE over speech and image mixtures in different noise levels. 相似文献
994.
Zeinab Noroozi TissehFereshteh Ahmadi Minoo DabiriHamid Reza Khavasi Ayoob Bazgir 《Tetrahedron letters》2012,53(28):3603-3606
A practical, simple, and efficient method for the synthesis of pyrano-fused spirooxindoles via an organocatalytic three-component reaction of isatins, malononitrile, and dialkyl acetylenedicarboxylate in the presence of 3,4-dimethylaniline as an organocatalyst in ethanol is reported. The structures of these products are confirmed by IR, 1H NMR and 13C NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. 相似文献
995.
996.
A dispersive liquid‐liquid microextraction method based on the dispersion of 1,2‐dichlorobenzene as an extraction solvent into an aqueous phase in the presence of ethanol as a dispersive solvent for the preconcentration of Co2+ and Ni2+ ions is discussed. 1‐Nitroso 2‐naphtol was used as a chelating agent prior to the extraction and the preconcentrated analyte was determined by flame atomic absorption spectrometry. The effect of various experimental parameters including the extraction and dispersive solvent type and volume, pH, amount of the chelating agent, etc. on the microextraction and complex formation was investigated for finding the optimum conditions. The enhancement factors were about 61.9 and 51.8, the calibration graphs were linear in the range of 10‐150 μgL?1 and 10‐250 μgL?1 with detection limits of 2.42 μgL?1 and 1.59 μgL?1, and RSD (n = 5) of 3.08% and 2.17% for cobalt and nickel, respectively. The method was successfully applied to the determination of Co and Ni in water and vitamin B12. 相似文献
997.
Mohammad Bayat Nader Zabarjad Shiraz Seyyed Hajar Hosseini 《Helvetica chimica acta》2010,93(11):2189-2193
A novel, efficient one‐pot approach for the synthesis of furopyrimidine and oxobenzofuran derivatives 4 by a multicomponent reaction of an isocyanide, an aldehyde, and a CH‐acid compound in CH2Cl2 is reported (Scheme 1 and Table). The reactions were completed after 20 h at room temperature. This method has the advantages of high yields, simple methodology, and easy workup. 相似文献
998.
Dispersive liquid-liquid microextraction (DLLME) combined with spectrofluorimetry was applied to the extraction, pre-concentration and analysis of thiamine (vitamin B1). The procedure is based on (a) the oxidation of thiamine with ferricyanide to form fluorescent thiochrome (TC), (b) the trapping of TC into a microextraction solvent, and (c) spectrofluorometric determination. Microextraction solvent and disperser solvent are directly injected into an aqueous solution containing TC. After centrifuging, phase separation is performed by sedimenting the fine droplets of the microextraction solvent on the bottom of a test tube. The settled phase is transferred into a fluorometer for the determination of thiamine at excitation/emission wavelengths of 375/438 nm. Under the optimized experimental conditions, the method provides a linear dynamic range of 0.2–100 ng mL?1, a detection limit of 0.06 ng mL?1, and a relative standard deviation of 3.0%. The method was successfully applied to pharmaceutical formulations and human urine. The results were validated by recovery test and by comparison with other methods, and were found to be highly satisfactory. 相似文献
999.
Thermal reduction has been applied to the preparation of copper nanoparticles (Cu-NPs) using three kinds of nonionic surfactants (Triton X-100, Tween-80, and dodecylamine). The Cu-NPs were formed by decomposition of copper(II) oxalate in presence of triphenylphosphine. The effect of the surfactants on the formation of the Cu-NPs was studied via X-ray diffraction, scanning electron microscopy, energy dispersive analysis of X-rays, transmission electron microscopy, thermogravimetric differential thermal analyses, and Fourier transform infra-red spectroscopy. It is shown that the Cu-NPs have an fcc crystal structure. Depending on the surfactant used, Cu-NPs with diameters between 8 and 20 nm can be prepared. The smallest Cu-NPs (8 nm) were formed in the presence of micelles of dodecylamine (yield 49%), while the largest particles (20 nm) were obtained with Triton X-100 (yield 99%). The use of Triton X-100 results in the highest yield and most uniform Cu-NPs. 相似文献
1000.