全文获取类型
收费全文 | 9287篇 |
免费 | 411篇 |
国内免费 | 61篇 |
专业分类
化学 | 6390篇 |
晶体学 | 93篇 |
力学 | 279篇 |
数学 | 808篇 |
物理学 | 2189篇 |
出版年
2023年 | 54篇 |
2022年 | 190篇 |
2021年 | 226篇 |
2020年 | 178篇 |
2019年 | 172篇 |
2018年 | 172篇 |
2017年 | 144篇 |
2016年 | 296篇 |
2015年 | 269篇 |
2014年 | 354篇 |
2013年 | 596篇 |
2012年 | 743篇 |
2011年 | 823篇 |
2010年 | 469篇 |
2009年 | 385篇 |
2008年 | 616篇 |
2007年 | 580篇 |
2006年 | 512篇 |
2005年 | 473篇 |
2004年 | 408篇 |
2003年 | 325篇 |
2002年 | 326篇 |
2001年 | 216篇 |
2000年 | 177篇 |
1999年 | 129篇 |
1998年 | 69篇 |
1997年 | 79篇 |
1996年 | 95篇 |
1995年 | 80篇 |
1994年 | 75篇 |
1993年 | 57篇 |
1992年 | 60篇 |
1991年 | 47篇 |
1990年 | 51篇 |
1989年 | 34篇 |
1988年 | 26篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 26篇 |
1984年 | 18篇 |
1983年 | 17篇 |
1982年 | 18篇 |
1981年 | 16篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1975年 | 10篇 |
1972年 | 9篇 |
排序方式: 共有9759条查询结果,搜索用时 15 毫秒
171.
Two novel heptadentate ligands, pentaaza macrocycles with two pendant xpyridyl and phenol groups, were prepared and the crystal structure of the manganese(II) complex of N,N′-bis(2-pyridylmethyl)-pentaaza macrocycle revealed a pentagonal bipyramidal geometry. 相似文献
172.
Summary Clenbuterol has been determined in urine by solidphase extraction on a C18 cartridge, diazotization of the eluate with nitrite, coupling of the diazonium ion with 1-(naphthyl)ethylenediamine, and
separation of the azo dye formed by HPLC with a C18 column and a micellar mobile phase containing 0.1 M sodium dodecyl sulphate, 12%n-butanol and 0.05 M citrate buffer, pH 3. Recoveries higher than 90% were obtained by mixing the samples with a 20% 0.2 M
NaOH before extraction. Limits of detection of 51 and 6.7 ng L−1 were obtained with spectrophotometric and thermal lens spectrometric detection, respectively; respective repeatabilities
were 3.1% (5 μg mL−1) and 5.6% (0.16 μg mL−1). 相似文献
173.
The reaction of hydroxy peroxy radicals (RO(2)) with NO represents one of the most crucial tropospheric processes, leading to terrestrial ozone formation or NO(x)() removal and chain termination. We investigate the formation of hydroxy peroxy nitrites (ROONO) and nitrates (RONO(2)) from the OH-isoprene reactions using DFT and ab initio theories and variational RRKM/master equation (vRRKM/ME) formalism. The binding energies of ROONO from NO addition to RO(2) are determined to be in the range of 20-22 kcal mol(-)(1), and the bond dissociation energies of ROONO to form an alkoxy radical (RO) and NO(2) range from 6 to 9 kcal mol(-)(1). Isomerization of ROONO to RONO(2) is exothermic by 22-28 kcal mol(-)(1). The entrance and exit channels of the RO(2)-NO reaction are found to be barrierless, and the rate constants to form ROONO are calculated to be 3 x 10(-)(12) to 2 x 10(-)(11) cm(3) molecule(-)(1) s(-)(1) using the canonical variational transition state theory. The vRRKM/ME analysis reveals negligible stabilization of excited ROONO and provides an assessment of ROONO isomerization to RONO(2). 相似文献
174.
Brow ME Dai J Park G Wright MW Gillman IG Manderville RA 《Photochemistry and photobiology》2002,76(6):649-656
The photolysis (>300 nm) of ochratoxin A (OTA, N-[[(3R)-5-chloro-8-hydroxy-3-methyl-1-oxo-7-isochromanyl]carbonyl]-3-phenyl-L-alanine, 1) in the presence of excess (2 and 12 molar equiv) cysteine (CySH) has been investigated and found to yield sulfur adducts 5 and 6 that are characterized by liquid chromatography-mass spectrometry and 1H-NMR spectroscopy. The adduct 5 was ascribed to the Michael addition conjugate resulting from covalent attachment of CySH to the ochratoxin quinone (4) generated by photooxidation of OTA. This species was also formed by photolysis of a synthetic sample of the hydroquinone of OTA (ochratoxin hydroquinone, 3) in the presence of 12 equiv L-CySH. The conjugate 5 derived from photolysis of 3 with L-CySH was used for 1H-NMR analysis. The sulfur adduct 6 was the major species detected from covalent attachment of CySH to photoactivated OTA, and it resulted from direct displacement of the OTA Cl atom by CySH. The implications of the cysteinyl adducts to the in vivo toxicity of OTA are discussed, with particular emphasis given to conjugate 5, as products from the photooxidative pathway may be of relevance to the nephrotoxic properties of OTA. 相似文献
175.
Micropatterning of a semicrystalline poly(vinylidene fluoride) (PVDF) solution was performed by a temperature controlled capillary micromolding where the rate of solvent evaporation was controlled by substrate temperature. In order to choose proper solvents for micropatterning, we have investigated the solubility of PVDF in various organic solvents and crystal structures of the PVDF bulk films cast from the solvents. The films prepared from the polar solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO) dominantly showed γ type crystals regardless of preparation temperature, while the films from tetrahydrofuran (THF) exhibit α type crystals and the ones from acetone and methyl ethyl ketone (MEK) show the characteristics of both α- and γ-PVDF. The quality of micropatterns and shapes of the PVDF crystals in the patterns significantly depend on solvent evaporation rates. Micropatterns of PVDF formed in DMF at 120 °C showed the best uniformity in shape. Crystals of the PVDF nucleated at the center regions of microchannels tended to be elongated with the b-axis of γ-PVDF crystals along the channels as the concentration of the solution decreased. In contrast, crystals nucleated at the corner regions of the channels had their b-axis oriented perpendicular to the channels. In line patterns with the width of 2 μm, the corner nucleated crystals were dominant and a resulting bamboo-like crystalline microstructure was observed in which the b-axis of γ-PVDF crystals, fast growth direction, is oriented normal to the microchannels. The crystal structures of the bulk films and the micropatterns were characterized by X-ray diffractometer, Fourier transform infrared spectroscope in Attenuated Total Reflection mode, Polarized Optical and Scanning Electron Microscope. 相似文献
176.
The retention of 131I on ceramic rod coated with silver nitrate followed by coating with a polyurethane membrane to be applied in brachytherapy
was studied. The concentration of silver nitrate (20 g/l), the volume of 131I as a sodium iodide solution (100 μl), the pH of the reaction mixture (pH 9) etc., were optimized to get a maximum uptake
on the ceramic rod. The concentration of coating solution (5%, polyurethane in tetrahydrofuran) was also optimized to obtain
a minimum leaching of 131I activity in normal saline solution. After coating with a polyurethane membrane, the 131I absorbed on the ceramic rod exhibited low leachability (0.03%). This method can be applied for the preparation of 125I interstitial sources to be used in eye and prostate cancer therapy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
177.
Much improved catalytic carbon-carbon bond-forming reactions between aryl chlorides and Grignard reagents has been achieved using nickel(II) ion on nanoporous carbon. 相似文献
178.
Relationship between microhardness and fluorine contents on tooth enamel determined by PIGE analysis
D. S. Ma D. I. Paik D. Y. Park H. S. Moon Y. I. Chang J. B. Kim 《Journal of Radioanalytical and Nuclear Chemistry》1997,217(2):225-227
The remineralization effect of fluoride has been measured by surface microhardness on tooth enamel. The purpose of this study was to investigate the relationship between microhardness and fluorine concentration on tooth enamel. Twelve sound bovine enamel specimens were prepared and immersed in 0.05% NaF solution for 1, 3, 6, 24 and 36 hours, respectively. The concentration of fluorine in specimens were measured by PIGE analysis and surface microhardness of each specimen was measured by surface microhardness tester. Fluorine concentration was increased by immersing time. There was no change in microhardness of each specimen by fluorine content. The results of this study suggest that there was no relationship between the fluorine concentration and surface microhardness in sound tooth enamel. PIGE analysis can be used effectively to assess the remineralization effect of fluorine content in tooth enamel. 相似文献
179.
G.?A.?Mun I.?K.?Nam R.?R.?SarsengalievEmail author Z.?S.?Nurkeeva P.?I.?Urkimbaeva K.?Park 《Colloid and polymer science》2004,282(10):1111-1117
Amphiphilic hydrogels of copolymers of the vinyl ether of ethylene glycol and vinyl isobutyl ether were synthesized by -radiation-induced free radical polymerization. Hydrogels with certain copolymer compositions showed thermo-sensitive behavior in aqueous solutions. The swelling behavior of the hydrogels in cetylpyridinium bromide aqueous solution was studied. Increased swelling of the hydrogels was observed in the surfactant solutions. The increased swelling was more prominent for the hydrogels with a higher content of hydrophobic moiety in the copolymer composition, and with higher surfactant concentration. The reason for this phenomenon is discussed. Treatment of some hydrogels in the surfactant solutions resulted in higher swelling ability in distilled water with distinct thermo-induced contraction over a narrow temperature interval. 相似文献
180.
[reaction: see text] A new recyclable supported catalyst system for orthoalkylation was devised using a self-assembly consisting of the barbiturate and 2,4,6-triaminopyrimidine H-bonding motifs. At high temperature, the system is completely homogeneous so as to give an efficient catalytic activity, while it is heterogenized at room temperature to form an insoluble solid phase for the easy recovery of the catalyst after the reaction. 相似文献