全文获取类型
收费全文 | 4282篇 |
免费 | 214篇 |
国内免费 | 41篇 |
专业分类
化学 | 2702篇 |
晶体学 | 38篇 |
力学 | 158篇 |
数学 | 475篇 |
物理学 | 1164篇 |
出版年
2023年 | 38篇 |
2022年 | 63篇 |
2021年 | 223篇 |
2020年 | 179篇 |
2019年 | 181篇 |
2018年 | 181篇 |
2017年 | 139篇 |
2016年 | 205篇 |
2015年 | 151篇 |
2014年 | 214篇 |
2013年 | 383篇 |
2012年 | 312篇 |
2011年 | 317篇 |
2010年 | 246篇 |
2009年 | 195篇 |
2008年 | 217篇 |
2007年 | 185篇 |
2006年 | 141篇 |
2005年 | 80篇 |
2004年 | 104篇 |
2003年 | 57篇 |
2002年 | 60篇 |
2001年 | 36篇 |
2000年 | 32篇 |
1999年 | 36篇 |
1998年 | 20篇 |
1997年 | 35篇 |
1996年 | 30篇 |
1995年 | 13篇 |
1994年 | 30篇 |
1993年 | 19篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1990年 | 21篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1985年 | 24篇 |
1984年 | 27篇 |
1983年 | 17篇 |
1982年 | 23篇 |
1981年 | 16篇 |
1980年 | 38篇 |
1979年 | 15篇 |
1977年 | 17篇 |
1976年 | 13篇 |
1973年 | 13篇 |
1972年 | 11篇 |
1969年 | 14篇 |
1968年 | 12篇 |
排序方式: 共有4537条查询结果,搜索用时 15 毫秒
991.
A convenient and efficient method for the estimation of cobalt(II) ions in the presence of other metal ions is described. Interference of metal ions such as iron(II), iron(III), nickel(II), manganese(II), and copper(II) have been investigated. Only iron(III) ions seriously affect this determination. Copper(II) and nickel(II) ions do not interfere if present in a molar-ratio less than 1:2 in the cobalt(II) ion solution. Cobalt(II)-nickel(II) and cobalt(II)-copper(II) binary mixtures can be efficiently analyzed at selective wavelengths. 相似文献
992.
S. Ahmad A. Mannan I. H. Qureshi 《Journal of Radioanalytical and Nuclear Chemistry》1993,170(1):165-170
A routine procedure has been developed for the simultaneous determination of uranium and thorium in high concentration thorium ores. INAA is used to determine the uranium and thorium concentration. However, for very low concentrations of uranium a radiochemical procedure based on the use of NPy/benzene as an extractant has to be employed. The precision and accuracy of the method has been determined by analyzing IAEA and NBL standard thorium/uranium ores. 相似文献
993.
Shahida Waheed Asma Rahman Naila Siddique Shujaat Ahmad Matthias Rossbach 《Accreditation and quality assurance》2007,12(6):311-316
The results from a proficiency test exercise for the determination of natural levels of radionuclides in mushroom reference
material carried out by laboratories in six different countries are discussed. These laboratories submitted data on seven
radionuclides, namely 134Cs, 137Cs, 40K, 90Sr, 241Am, 226Ra, and 210Po. However, only the data received for three of these radionuclides could be statistically evaluated to obtain consensus
mean values and confidence intervals (at a significance level of 0.05). The contents of 134Cs, 137Cs and 40K were quantified as 4.4, 2,899 and 1,136 Bq/kg, respectively. 相似文献
994.
A. Mannan S. Waheed S. Ahmad I. H. Qureshi 《Journal of Radioanalytical and Nuclear Chemistry》1992,162(1):111-123
Several potentially toxic trace elements, namely, Hg, Pb, Cd, As, Sb, Br and Se have been measured by INAA in combination with AAS techniques in the integrated diet representative of the inhabitants of Gujranwala, a highly industrialized city of Pakistan. The dietary intake values for these elements have been estimated from the prevailing concentration level in the summer and winter season diets, which reveals that present intake data are well within the reported WHO values and can be considered to be safe. Posslble sources of food contamination by the toxic elements and their adverse impacts on human heaith are briefly discussed. 相似文献
995.
S.M. Afzal S.G. Nakhate S.A. Ahmad 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1996,37(3):249-253
Isotope shift (IS) Δσ (142–150) have been measured in 92 spectral lines of Nd+ in the region 3290–3955 Å, employing recording Fabry-Perot spectrometer and highly enriched isotopic samples excited in a liquid-nitrogen cooled hollow cathode. Earlier IS measurement in UV region of Nd II spectrum is available for only a few lines, New classification has been made for several of the spectral lines studied presently. We have also revised the earlier suggested classification for some of the Nd II lines on the basis of observed isotope shifts. The transitions presently studied mostly involve high lying odd parity energy levels above 30,000 cm?1 having neither electronic configuration assignment nor term isotope shifts (ΔT) evaluated. The present study would thus enable to evaluate ΔT values of some of these high lying odd levels, which should be helpful in suggesting possible electronic configurations. 相似文献
996.
Epoxidations of alkyl-substituted alkenes, with hydrogen peroxide as the oxygen source, are catalyzed by CH(3)ReO(3) (MTO). The kinetics of 28 such reactions were studied in 1:1 CH(3)CN-H(2)O at pH 1 and in methanol. To accommodate the different requirements of these reactions, (1)H-NMR, spectrophotometric, and thermometric techniques were used to acquire kinetic data. High concentrations of hydrogen peroxide were used, so that diperoxorhenium complex CH(3)Re(O)(eta(2)-O(2))(2)(H(2)O), B, was the only predominant and reactive form of the catalyst. The reactions between B and the alkenes are about 1 order of magnitude more rapid in the semiaqueous solvent than in methanol. The various trends in reactivity are medium-independent. The rate constants for B with the aliphatic alkenes correlate closely with the number of alkyl groups on the olefinic carbons. The reactions become markedly slower when electron-attracting groups, such as halo, hydroxy, cyano, and carbonyl, are present. The rate constants for catalytic epoxidations with B and those reported for the stoichiometric reactions of dimethyldioxirane show very similar trends in reactivity. These findings suggest a concerted mechanism in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of B. These data, combined with those reported for the epoxidation of styrene (a term intended to include related molecules with ring and/or aliphatic substituents) by B and by the monoperoxo derivative of MTO, suggest that all of the rhenium-catalyzed epoxidations occur by a common mechanism. The geometry of the system at the transition state can be inferred from these data, which suggest a spiro arrangement. 相似文献
997.
Zahid Hassan Hidayat Hussain Viqar Uddin Ahmad Shazia Anjum Gennaro Pescitelli Tibor Kurtn Karsten Krohn 《Tetrahedron: Asymmetry》2007,18(24):2905-2909
The absolute configuration of 1β,10β-epoxydesacetoxymatricarin 1, a sesquiterpenoid isolated from Carthamus oxycantha, was established by TDDFT CD calculations in combination with single crystal X-ray analysis. Since the solid-state CD spectrum shows contributions from intermolecular interactions in the crystal, 1 represents a test case for our solid-state CD/TDDFT approach. 相似文献
998.
A novel one-pot, three-component condensation reaction of an aldehyde, 2-aminobenzothiazole and 2-naphthol or 6-hydroxyquinoline in water to give 2′-aminobenzothiazolomethyl naphthols or 5-(2′-aminobenzothiazolomethyl)-6-hydroxyquinolines in high yields at 90 °C without using any catalyst, is described. 相似文献
999.
Ibrahim A. Ahmad 《Probability Theory and Related Fields》1980,54(2):119-124
Summary By representing the strong law of large numbers for partial sums in its equivalent form that an integer-valued random variable N
is finite with probability one it is possible to obtain resutls on the rate of convergence as 0.In this note a rate of convergence is obtained when the index of the sum itself is a random variable (not necessarily independent of the observations).Research supported in part by the National Research Council of Canada while the author was with McMaster University. The author is indebted to the referees for several helpful comments and criticisms 相似文献
1000.
Sami H. Mahmood Maen A. Gharaibeh Hassan Abu-Safia Ahmad S. Saleh 《Hyperfine Interactions》1993,77(1):255-264
We report the results of a Mössbauer study of the alloy sytem FeAl1?x Co x forx ≥ 0.3 at temperatures down to 83 K. Magnetic splitting is observed forx ≥ 0.35 at all temperatures. However, forx=0.3, no splitting is observed at room temperature, and superparamagnetic behavior occurs at LN2 temperature. The magnetically split spectra are fitted each with a distribution of hyperfine fields and the average hyperfine field \(\bar B_{hf} \) as a function of temperature is obtained. The variation of \(\bar B_{hf} \) withT is explained using the model of magnetic clusters with collective magnetic excitations from which the saturation hyperfine field and the magnetic anisotropy energy for these clusters are obtained. Also, the results are discussed using the model of random atomic distributions, and the agreement between the calculated and the experimentally obtained distributions of hyperfine fields is found improve asx increases. 相似文献