全文获取类型
收费全文 | 2287篇 |
免费 | 182篇 |
国内免费 | 8篇 |
专业分类
化学 | 1743篇 |
晶体学 | 21篇 |
力学 | 15篇 |
数学 | 77篇 |
物理学 | 429篇 |
无线电 | 192篇 |
出版年
2024年 | 5篇 |
2023年 | 29篇 |
2022年 | 28篇 |
2021年 | 39篇 |
2020年 | 68篇 |
2019年 | 84篇 |
2018年 | 48篇 |
2017年 | 33篇 |
2016年 | 112篇 |
2015年 | 87篇 |
2014年 | 99篇 |
2013年 | 115篇 |
2012年 | 193篇 |
2011年 | 229篇 |
2010年 | 100篇 |
2009年 | 83篇 |
2008年 | 164篇 |
2007年 | 126篇 |
2006年 | 112篇 |
2005年 | 118篇 |
2004年 | 96篇 |
2003年 | 59篇 |
2002年 | 54篇 |
2001年 | 34篇 |
2000年 | 32篇 |
1999年 | 22篇 |
1998年 | 12篇 |
1997年 | 22篇 |
1996年 | 19篇 |
1995年 | 14篇 |
1994年 | 19篇 |
1993年 | 23篇 |
1992年 | 28篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 6篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1968年 | 5篇 |
排序方式: 共有2477条查询结果,搜索用时 0 毫秒
1.
Watanabe Sou Senzaki Tatsuya Shibata Atsuhiro Nomura Kazunori Takeuchi Masayuki Nakatani Kiyoharu Matsuura Haruaki Horiuchi Yusuke Arai Tsuyoshi 《Journal of Radioanalytical and Nuclear Chemistry》2019,322(3):1273-1277
Journal of Radioanalytical and Nuclear Chemistry - Extraction chromatography flow-sheet employing octyl(phenyl)-N,N-diisobutylcarbonoylmethylphosphine oxide and bis(2-ethylhexyl) hydrogen phosphate... 相似文献
2.
Direct Addition of Amides to Glycals Enabled by Solvation‐Insusceptible 2‐Haloazolium Salt Catalysis
Yuya Nakatsuji Yusuke Kobayashi Yoshiji Takemoto 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(40):14253-14257
The direct 2‐deoxyglycosylation of nucleophiles with glycals leads to biologically and pharmacologically important 2‐deoxysugar compounds. Although the direct addition of hydroxyl and sulfonamide groups have been well developed, the direct 2‐deoxyglycosylation of amide groups has not been reported to date. Herein, we show the first direct 2‐deoxyglycosylation of amide groups using a newly designed Brønsted acid catalyst under mild conditions. Through mechanistic investigations, we discovered that the amide group can inhibit acid catalysts, and the inhibition has made the 2‐deoxyglycosylation reaction difficult. Diffusion‐ordered two‐dimensional NMR spectroscopy analysis implied that the 2‐chloroazolium salt catalyst was less likely to form aggregates with amides in comparison to other acid catalysts. The chlorine atom and the extended π‐scaffold of the catalyst played a crucial role for this phenomenon. This relative insusceptibility to inhibition by amides is more responsible for the catalytic activity than the strength of the acidity. 相似文献
3.
4.
Hiroshi Yamauchi Keiko Takahashi Mari Mashiko Juichiro Saitoh Yukio Yamamura 《应用有机金属化学》1992,6(4):383-388
We calculated the intake of each chemical species of dietary arsenic by typical Japanese, and determined urinary and blood levels of each chemical species of arsenic. The mean total arsenic intake by 35 volunteers was 195±235 (15.8-1039) μg As day?1, composed of 76% trimethylated arsenic (TMA), 17.3% inorganic arsenic (Asi), 5.8% dimethylated arsenic (DMA), and 0.8% monomethylated arsenic (MA): the intake of TMA was the largest of all the measured species. Intake of Asi characteristically and invariably occurred in each meal. Of the intake of Asi, 45-75% was methylated in vivo to form MA and DMA, and excreted in these forms into urine. The mean measured urinary total arsenic level in 56 healthy volunteers was 129±92.0 μg As dm?3, composed of 64.6% TMA, 26.7% DMA, 6.7% Asi and 2.2% MA. The mean blood total arsenic level in the 56 volunteers was 0.73±0.57 μg dl?1, composed of 73% TMA, 14% DMA and 9.6% Asi. The urinary TMA levels proved to be significantly correlated with the whole-blood TMA levels (r = 0.376; P<0.01). 相似文献
5.
H. Sunaoshi Y. Fukashiro M. Furukawa M. Yamauchi S. Hayashibe T. Shinozuka M. Fujioka I. Satoh M. Wada S. Matsuki 《Hyperfine Interactions》1993,78(1-4):241-245
The ground state hyperfine splitting of87Sr+ was measured with a precision of 1×10–8. The experiments were performed with an RF ion trap connected to an ISOL (isotope separator on-line), where all the possible transitions between Zeeman sublevels were observed by a laser-microwave double resonance method. The magnetic dipole hyperfine constant was determined to beA=–1 000 473.673 (11) kHz. 相似文献
6.
The relationship between the four components, (1) fluorescence intensity, (2) arsenic concentration, (3) pH and (4) total dissolved solids, (TDS) measured in well waters from areas in Taiwan where blackfoot disease (BFD) is endemic was studied, as well as the relationships between the four degrees of BFD and each of the above four symptomatic components, in order to evaluate the etiological factors of BFD more progressively. The following 95% confidence intervals were obtained in well water samples (n = 1189): fluorescence intensity, 26.837–32.570; arsenic concentration, 0.103–0.127 mg dm?3; pH, 7.466–7.519; and TDS 733.063–801.647 mg dm?3. Fluorescence intensities of the four degrees of BFD were not all the same (F = 64.54, P < 0.001), and nor were arsenic concentrations (F = 72.03, P < 0.001), pH values (F = 7.30, P < 0.001), nor TDS values (F = 10.76, P < 0.001). In addition, multiple comparisons indicate that the higher the epidemical degree, the higher the fluorescence intensities, arsenic concentrations and pH values become; however, such a relationship is not found for TDS values. Moreover, the fluorescence intensities have positive linear correlations with arsenic concentrations (r = 0.49, P < 0.001), pH (r = 0.25, P < 0.001), and TDS (r = 0.18, P < 0.001), as do the arsenic concentrations with pH (r = 0.22, P < 0.001). Of the four epidemical degree groups, pairs are not significantly different from one another in correlation coefficients between fluorescence intensity and arsenic concentration, which implies a steady relationship between fluorescent compounds and arsenic. We conclude that fluorescent compounds in well water, as possible etiological factors of BFD, are closely related to arsenic along with pH and TDS values in the areas where BFD is endemic. In addition, we infer that a complex is formed by fluorescent compounds, arsenic and other metals. 相似文献
7.
H. Ohmori S. Tomiyoshi H. Yamauchi H. Yamamoto 《Journal of magnetism and magnetic materials》1987,70(1-3):249-251
The compound Mn3Sn shows an interesting magnetic behavior. A triangular screw spin structure has been observed below 250 K, whose period is independent of temperature. The screw structure is sensitive to heat treatment, alloy composition and impurity content. 相似文献
8.
9.
Double-band operation (85 and 900 MHz) of a collinear monopole antenna is numerically demonstrated. The antenna consists of three linear wires connected with two coils, and has a height of 67 cm. One coil acts as a phase compensation element to increase the gain and the other as a decoupling element for use in the mobile telephone band.<> 相似文献
10.
Nakano H. Iwatsuki M. Sakurai M. Yamauchi J. 《Antennas and Propagation, IEEE Transactions on》2003,51(4):712-718
A rectangular aperture of A/sub x//spl times/A/sub y/, cut in the top conducting plate of a triplate transmission line and backed by a cavity, radiates a tilted beam off the direction normal to the aperture. The mechanism of the radiation is explained using the Poynting vector distribution above the aperture and the phase distribution of the electric field over the aperture. The tilt angle is calculated as a function of side length A/sub x/ for a representative value of A/sub y/=18 mm=0.747/spl lambda//sub 12.45/, where /spl lambda//sub 12.45/ is the wavelength at a test frequency of 12.45 GHz. A tilted beam of approximately 27/spl deg/ is realized at A/sub x//A/sub y/=8/9 with a gain of approximately 8 dB. Using this value of A/sub x//A/sub y/, an array antenna composed of rectangular cavity-backed aperture elements is investigated. The array forms a tilted fan beam without phase shifters. The frequency responses of the gain and input impedance are discussed. 相似文献