全文获取类型
收费全文 | 8153篇 |
免费 | 233篇 |
国内免费 | 61篇 |
专业分类
化学 | 5365篇 |
晶体学 | 42篇 |
力学 | 280篇 |
数学 | 1605篇 |
物理学 | 1155篇 |
出版年
2023年 | 52篇 |
2022年 | 172篇 |
2021年 | 195篇 |
2020年 | 166篇 |
2019年 | 169篇 |
2018年 | 141篇 |
2017年 | 114篇 |
2016年 | 279篇 |
2015年 | 239篇 |
2014年 | 241篇 |
2013年 | 465篇 |
2012年 | 534篇 |
2011年 | 600篇 |
2010年 | 364篇 |
2009年 | 358篇 |
2008年 | 521篇 |
2007年 | 487篇 |
2006年 | 488篇 |
2005年 | 413篇 |
2004年 | 354篇 |
2003年 | 281篇 |
2002年 | 293篇 |
2001年 | 100篇 |
2000年 | 73篇 |
1999年 | 67篇 |
1998年 | 79篇 |
1997年 | 87篇 |
1996年 | 98篇 |
1995年 | 58篇 |
1994年 | 79篇 |
1993年 | 55篇 |
1992年 | 57篇 |
1991年 | 44篇 |
1990年 | 54篇 |
1989年 | 48篇 |
1988年 | 41篇 |
1987年 | 45篇 |
1986年 | 31篇 |
1985年 | 47篇 |
1984年 | 63篇 |
1983年 | 41篇 |
1982年 | 45篇 |
1981年 | 31篇 |
1980年 | 40篇 |
1979年 | 29篇 |
1978年 | 30篇 |
1977年 | 27篇 |
1976年 | 14篇 |
1974年 | 15篇 |
1973年 | 14篇 |
排序方式: 共有8447条查询结果,搜索用时 0 毫秒
101.
Cestari AR Vieira EF Pinto AA Lopes EC 《Journal of colloid and interface science》2005,292(2):363-372
In this work, a hybrid silica/chitosan was synthesized and characterized by nitrogen elemental analysis and thermal analysis (TG, DTG, DTA, and DSC) and BET surface area. The hybrid was used in adsorption studies of two anionic dyes from aqueous solutions. A rise of temperature accelerates mass transfer of dyes into the hybrid. However, the maximum adsorption capacities reach similar values from 25 to 55 degrees C. The kinetic data were first evaluated in relation to the decrease of the time-related residual concentration of the dyes in solution, where the second-order model has presented the best fitting. The solid-phase interaction of dye data presents a rough fitting to the traditional first-order Lagergren kinetic model. However, a modified Avrami kinetic equation was successfully fitted to the kinetic quantities, where from five to seven kinetic regions were found. A pore-diffusion model has also demonstrated that the diffusion is the rate-controlling interaction mechanism. However, the experimental-calculated comparative values are the best way to evaluate a specific aqueous- or solid-phase kinetic model. 相似文献
102.
Fused pyrazolopyrimidines. I. Pyrazolo[4,3-e]-v-triazolo[1,5-a]pyrimidine. A new heterocyclic system
Misbahul Ain Khan Antonio Carlos Carreira Freitas 《Journal of heterocyclic chemistry》1980,17(7):1603-1604
5-Azido-4-cyano-1-phenylpyrazole reacts with benzylcyanide in the presence of sodium methoxide to give the title ring system. 相似文献
103.
Verdejo B Aguilar J Doménech A Miranda C Navarro P Jiménez HR Soriano C García-España E 《Chemical communications (Cambridge, England)》2005,(24):3086-3088
L-glutamate and L-aspartate selectivity is achieved by the action of two Cu2+ metal ions rightly disposed in a cyclophane-type macrocyclic framework; electrochemical sensing of glutamate has been achieved by adsorption of the copper complexes on graphite electrodes. 相似文献
104.
Copper bronze catalyzed Heck reaction in ionic liquids 总被引:1,自引:0,他引:1
Heck reaction of aryl iodides and activated aryl bromides catalyzed by copper bronze in tetrabutylammonium bromide as solvent and tetrabutylammonium acetate as base was developed. The effective catalysts are Cu nanoparticles deriving from the reaction of iodobenzene with copper bronze. These nanoparticles are very stable in tetraalkylammonium salts, are easily recycled, and can be stored for months without a loss of catalytic efficiency. [reaction: see text] 相似文献
105.
Edwin Weber Orm Hager Concepcion Foces-Foces Antonio L. Llamas-Saiz 《Journal of inclusion phenomena and macrocyclic chemistry》1999,34(2):199-220
New chiral host compounds based on mandelic acid derivatives having methyl (6a, b and 8a, b) or bromo substituents (7a, b) attached to the phenyl ring of mandelic acid and involving additional aromatic groups were synthesized. The inclusion properties of both the racemic and the optically resolved host species are reported, including solvent co-crystallization as well as chiroselective and vapour sorptive inclusion. The structures of the free racemic host compound 6b and of the DMSO inclusion compounds of optically resolved and racemic 8 (8a and 8b, respectively) have been determined by X-ray analysis. Enantiomeric pairs of molecules in 6b form centro-symmetric dimers by mutual hydrogen bonding of one hydroxyl group while the other is involved in O-–H ... interactions. The guest molecules in the DMSO complexes of 8a and 8b are bound via hydrogen bonds to two host molecules related by translation along crystallographic axes. Parallels to previous hosts of this type are drawn. 相似文献
106.
Coleman RS Perez RJ Burk CH Navarro A 《Journal of the American Chemical Society》2002,124(44):13008-13017
Evaluation of the sequence selectivity, noncovalent association, and orientation of the DNA cross-linking agent azinomycin B on its duplex DNA receptor is described. A strong correlation between sequence nucleophilicity and cross-linking yield was observed, and steric effects due to the thymine C5-methyl group were identified. Detailed studies on the role of the azinomycin naphthoate using viscometry, fluorescence contact energy transfer, and DNA unwinding assays point to a nonintercalative binding mode for this group. A kinetic assay for agent regioselectivity was used to determine the orientation of binding and covalent cross-link formation. 相似文献
107.
Ruiz-Medina A Ortega-Barrales P Fernández-de Córdova ML Molina-Díaz A 《Journal of AOAC International》2002,85(2):369-374
A simple, rapid, inexpensive, and automated flow-through solid-phase spectroscopic sensing device is proposed for the sequential determination of 2 vitamins: thiamine and ascorbic acid. The vitamins are concentrated on ion-exchange gels, thiamine on Sephadex SP C-25, and ascorbic acid on Sephadex QAE A-25; both solid supports are packed in 2 different flow cells. The absorbance is monitored directly on the solid phase with a double-beam spectrophotometer at 250 nm, without derivatization or additional elution. With the use of 2 carrier/self-eluting solutions (0.1 5M sodium acetate/acetic acid and 0.18M citric acid/K2HPO4) and a sample volume of 1000 microL, the sensor responds linearly in the range of 0.5-15 and 3-50 microg/mL with detection limits of 0.14 and 0.36 microg/mL for thiamine and ascorbic acid, respectively. When the method was applied to synthetic samples and pharmaceutical preparations, precise and accurate values were obtained. 相似文献
108.
Antonio Doménech María Teresa Doménech-Carbó María Luisa Vázquez de Agredos Pascual 《Journal of Solid State Electrochemistry》2007,11(9):1335-1346
The preparation of indigo from Indigofera suffruticosa following the procedures attributed to ancient Mayas was electrochemically monitored using the voltammetry-of-microparticles
approach. The mechanism formation of indigotin and indirubin from its precursors, indican and isatan, is discussed. Comparison
of voltammetric profiles for differently prepared and commercial indigos and genuine Maya Blue samples suggests that the preparation
procedure of indigo changed during the Late Classical Maya period.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
109.
Cimas A Rayón VM Aschi M Barrientos C Sordo JA Largo A 《The Journal of chemical physics》2005,123(11):114312
The singlet potential-energy surface for the N(2D)+CH2F(2A') reaction has been studied employing both second-order M?ller-Plesset and density-functional theories. The energies of the involved species have been refined using the Gaussian-2, complete basis set, and coupled-cluster singles and doubles (triples) methods. The reaction proceeds through the formation of an initial intermediate, which does not involve any activation barrier. Based on the energy profile for the singlet potential-energy surface, the preferred product should be the most exothermic one, namely, HCN+HF, followed by HNC+HF and FCN+H2. This result seems in contradiction with a computational study of the kinetics of the title reaction in terms of the statistical theories, which leads to the prediction that the production of HNC+HF should be the dominant channel. Consequently, a limited molecular-dynamics study has been carried out, concluding that in fact the system behaves in a nonstatistical way. According to the molecular-dynamics study, the most exothermic channel, HCN+HF, should be the dominant one. An analysis of the possible role of the singlet surface in the reaction of N(4S) with CH2F(2A') has also been carried out. The computational study shows that the microcanonical coefficients for the nonadiabatic channels are much smaller than the competing adiabatic ones. Therefore, the reaction of N(4S) with CH2F(2A') should proceed on the triplet surface without spin change. 相似文献
110.
The oxidation of 2-phenyl-3-arylaminoindoles (1a–e) has been studied in acetonitrile, dimethylformamide and propylene carbonate at a new type of platinum electrode with periodical renewal of the diffusion layer and at a rotating platinum electrode by cyclic voltammetry, d.c. and a.c. voltammetry, controlled potential coulometry and spectrometry (e.s.r. and u.v.). In the examined aprotic solvents and in the presence of a base (like water or diphenylguanidine) amines 1a–e undergo a two-electron oxidation to the corresponding imines. In a protophobic medium (acetonitrile or propylene carbonate) 1a–e are oxidized in two one-electron steps, the first of which leads to the formation of a cation radical (identified by e.s.r. spectrum), oxidizable at more positive potentials (second step). The second electron transfer, however, must be simultaneous with, or be immediately followed by, a very fast deprotonation reaction. The same e.e.c. sequence explains the observed bielectronic step of 1a–e in a protophilic aprotic solvent, like dimethylformamide. 相似文献