全文获取类型
收费全文 | 4019篇 |
免费 | 111篇 |
国内免费 | 15篇 |
专业分类
化学 | 3211篇 |
晶体学 | 35篇 |
力学 | 50篇 |
数学 | 418篇 |
物理学 | 431篇 |
出版年
2022年 | 65篇 |
2021年 | 100篇 |
2020年 | 63篇 |
2019年 | 59篇 |
2018年 | 48篇 |
2017年 | 45篇 |
2016年 | 117篇 |
2015年 | 96篇 |
2014年 | 123篇 |
2013年 | 222篇 |
2012年 | 220篇 |
2011年 | 259篇 |
2010年 | 152篇 |
2009年 | 145篇 |
2008年 | 275篇 |
2007年 | 225篇 |
2006年 | 228篇 |
2005年 | 210篇 |
2004年 | 170篇 |
2003年 | 138篇 |
2002年 | 141篇 |
2001年 | 68篇 |
2000年 | 45篇 |
1999年 | 49篇 |
1998年 | 38篇 |
1997年 | 44篇 |
1996年 | 48篇 |
1995年 | 46篇 |
1994年 | 44篇 |
1993年 | 32篇 |
1992年 | 39篇 |
1991年 | 32篇 |
1989年 | 29篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 25篇 |
1984年 | 26篇 |
1983年 | 17篇 |
1982年 | 34篇 |
1981年 | 30篇 |
1980年 | 31篇 |
1979年 | 23篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1975年 | 15篇 |
1973年 | 14篇 |
1956年 | 19篇 |
1955年 | 37篇 |
1954年 | 57篇 |
排序方式: 共有4145条查询结果,搜索用时 461 毫秒
61.
McMahon WA Aleo VA Schultz AM Horter BL Lindberg KG 《Journal of AOAC International》2003,86(5):947-953
The 3M Petrifilm Staph Express Count plate method was compared with AOAC Official Method 975.55 for the enumeration of Staphylococcus aureus in selected foods. Four foods--cooked, diced chicken; cured ham; smoked salmon; and pepperoni--were analyzed for S. aureus by 12 collaborating laboratories. For each food tested, the collaborators received 8 blind test samples consisting of a control sample, a low inoculation level, a medium inoculation level, and a medium inoculation level with background flora, each in duplicate. The mean log10 counts for the methods were comparable for all 4 foods. The repeatability and reproducibility variances of the 24 h Petrifilm Staph Express Count plate method were similar to those of the 72 h standard method. 相似文献
62.
The halide anion effect on the adsorption of 1,2,3-triazole on Ag and Au colloidal nanoparticles has been investigated by means of surface enhanced Raman scattering (SERS), UV-visible absorption spectroscopy, and scanning electron microscopy. To interpret the SERS spectra, the vibrational spectra of 1,2,3-triazole were assigned with the help of density functional theoretical (DFT) calculations of the two tautomers of 1,2,3-triazole, both free and bound to Ag and Au adatoms. Upon addition of halide anions, both tautomers interact with the Ag surface through one nitrogen atom. Analogous behavior is observed in the case of basified Au colloids, whereas at the usual pH of these hydrosols (approximately 6) the adsorption of 1,2,3-triazole is the same of that observed in halide-free colloids. 相似文献
63.
Vijayasarathi Nagarajan Barbara Müller Oksana Storcheva Klaus Köhler Andreas Pöppl 《Research on Chemical Intermediates》2007,33(8):705-724
Interactions and binding sites of the solvent molecules chloroform and ethanol to bis(acetylacetonate)oxovanadium(IV) (VO(acac)2) complexes in (frozen) solutions have been investigated by pulsed electron nuclear double resonance, sum peak electron spin
echo envelope modulation and hyperfine sublevel correlation spectroscopy. The experimental proton hyperfine coupling data
of coordinating solvent molecules have been interpreted using quantum chemical calculations (density functional theory). Experimental
and computed hyperfine couplings indicate that ethanol coordinates to vanadium in the equatorial plane of VO(acac)2 and chloroform interacts via hydrogen bonding to oxygens of the acac ligands. 相似文献
64.
Mechanical properties decay and morphological behaviour of biodegradable films for agricultural mulching in real scale experiment 总被引:1,自引:0,他引:1
Giacomo Scarascia-Mugnozza Giuliano Vox Barbara Immirzi 《Polymer Degradation and Stability》2006,91(11):2801-2808
The use of plastic materials in agriculture causes the serious drawback of huge quantities of waste. The introduction of biodegradable materials, which can be disposed directly into the soil, can be one possible solution to this problem. Biodegradable materials are actually innovative materials; therefore, their physical properties must be evaluated in relation to their functionality during the use in field. In the present research results of experimental tests carried out on biodegradable films used in strawberries protected cultivation are presented. The decay of some relevant physical parameters of biodegradable films during the cultivation period was monitored by laboratory tests (SEM analysis, mechanical tensile tests and infrared reflectance spectroscopy). Infrared spectroscopy clearly indicated that the mechanical degradation starts from the starch component of the material. Tensile tests showed that the value of elongation at break of biodegradable materials decreased in some cases by 300% after 10 days of field application. 相似文献
65.
Barbara Szafran Jan Pawlaczyk 《Journal of inclusion phenomena and macrocyclic chemistry》1995,23(4):277-288
The 1 : 1 inclusion complex involving sulfafurazole (SF) and-cyclodextrin (-CD) is prepared by the freeze-drying method and characterized on the basis of its chemical analysis, thermal behavior, infrared spectrum, X-ray powder pattern and13C NMR spectrum in DMSO-d6 solution. The stability constant of the inclusion complex was determined by the solubility method. The effect of cyclodextrin on the UV absorption spectrum of sulfafurazole was also observed. 相似文献
66.
Barbara Priwitzer 《Geometriae Dedicata》1995,58(3):245-258
LetP=(P, L) be a compact projective plane with 0P< and let be a compact connected subgroup of Aut(P). If dim dimE – dimP, whereE is the elliptic motion group of the corresponding classical plane, then E or is isomorphic to a point stabilizerE
0 inE, cf. [31]. Here we consider the case E
0. It is shown that the action of on the point spaceP is equivalent to the classical action ofE
0. For dimP {8, 16} the planeP is uniquely determined by a 2-dimensional subplane with SO2 Aut().Für H. Reiner Salzmann zum 65. Geburtstag 相似文献
67.
68.
Bazzicalupi C Bencini A Berni E Bianchi A Ciattini S Giorgi C Maoggi S Paoletti P Valtancoli B 《The Journal of organic chemistry》2002,67(25):9107-9110
The synthesis of three new tris-macrocycles, containing three [12]aneN(4) (L1), [12]aneN(3)O (L2), or [14]aneN(4) (L3) moieties appended to a tren unit, is reported. The crystal structure of the [(Na(ClO(4))(6)) subset L1(2)H(13)]Na(6)Cl(2)(ClO(4))(12) compound shows the anionic cluster [Na(ClO(4))(6)](5)(-) assembled inside the cavity defined by two bowl-shaped polyammonium receptors, held by multiple charge-charge and hydrogen bond interactions. 相似文献
69.
Baciocchi E Bietti M Chiavarino B Crestoni ME Fornarini S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(2):532-537
The gas-phase acidity of ionized benzyl alcohol and of some of its derivatives with selected reference bases has been studied by Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. The aim was to relate the gas-phase reactivity to the behavior in aqueous solution of the radical cations of benzyl alcohols bearing methoxy substituent(s) on the phenyl ring which are known to undergo deprotonation at both the CH2 and OH groups. The dual reactivity behavior is confirmed in the gas phase, in which the prototypical ion, C6H5CH2OH*+, is deprotonated at both the CH2 and OH groups, whereas the ring hydrogens are not involved. An increasing extent of O-deprotonation is shown as the strength of the base increases. Appropriate methyl substitution, as in the radical cations of C6H5C(Me)2OH and C6H5CH2OMe, allows only O- or C-acidity. The two processes are characterized by comparable thermodynamic features with a Gas-phase Basicity (GB) value of 852 kJ mol(-1) for the cumyloxyl radical and 850 kJ mol(-1) for the alpha-methoxybenzyl radical. The possible origin of the observed mechanistic dichotomy is discussed. 相似文献
70.
Dzygiel A Rzeszotarska B Masiukiewicz E Cmoch P Kamieński B 《Chemical & pharmaceutical bulletin》2004,52(2):192-198
Methyl 5-amino-1H-[1,2,4]triazole-3-carboxylate hydrochloride (1). and free ester (2). were obtained and 2 was reacted with Ac(2)O to give the acetylated products 3-6. Compounds 1-6 were studied using HPLC, GC-MS, FTIR and multinuclear NMR spectroscopy, including the cross-polarisation magic angle spinning (CPMAS) technique. The results of the acetylation of 2 were compared to those of the acetylation of 5-amino-1H-[1,2,4]triazole, and for 2 a significant decrease in the susceptibility to acetylation was found. The reaction of 2 with Ac(2)O at 20 degrees C, regardless of the amount and the concentration of the latter, including neat Ac(2)O, proceeds fully regioselectively and leads to one product: methyl 1-acetyl-5-amino-1H-[1,2,4]triazole-3-carboxylate (3). In sharp contrast to 5-amino-1H-[1,2,4]triazole, neither an additional monoacetylated isomer, whether annular or exocyclic, nor any diacetylated derivative could be detected. The diacetylation of 2 requires the process to be carried out in neat boiling Ac(2)O and, as in the case of 5-amino-1H-[1,2,4]triazole, gives two diacetylated isomers. These are methyl 1-acetyl-3-(acetylamino)-1H-[1,2,4]triazole-5-carboxylate (4) and 1-acetyl-5-(acetylamino)-1H-[1,2,4]triazole-3-carboxylate (5). Hypothetical pathways of their formation have been suggested. A mixture of 4 and 5 upon hydrolysis of the ring acetyl group gives the monoacetylated derivative methyl 5-(acetylamino)-1H-[1,2,4]triazole-3-carboxylate (6). The spectroscopic, structural and conformational characteristics of compounds 1-6 have been given and methods for their preparation have been provided. 相似文献