全文获取类型
收费全文 | 4074篇 |
免费 | 113篇 |
国内免费 | 30篇 |
专业分类
化学 | 2587篇 |
晶体学 | 15篇 |
力学 | 118篇 |
数学 | 697篇 |
物理学 | 800篇 |
出版年
2023年 | 27篇 |
2022年 | 66篇 |
2021年 | 93篇 |
2020年 | 84篇 |
2019年 | 62篇 |
2018年 | 79篇 |
2017年 | 65篇 |
2016年 | 129篇 |
2015年 | 77篇 |
2014年 | 121篇 |
2013年 | 277篇 |
2012年 | 238篇 |
2011年 | 288篇 |
2010年 | 191篇 |
2009年 | 141篇 |
2008年 | 248篇 |
2007年 | 244篇 |
2006年 | 218篇 |
2005年 | 203篇 |
2004年 | 151篇 |
2003年 | 126篇 |
2002年 | 111篇 |
2001年 | 70篇 |
2000年 | 54篇 |
1999年 | 41篇 |
1998年 | 43篇 |
1997年 | 55篇 |
1996年 | 53篇 |
1995年 | 38篇 |
1994年 | 43篇 |
1993年 | 43篇 |
1992年 | 43篇 |
1991年 | 34篇 |
1990年 | 23篇 |
1989年 | 15篇 |
1988年 | 22篇 |
1987年 | 18篇 |
1986年 | 26篇 |
1985年 | 42篇 |
1984年 | 27篇 |
1983年 | 28篇 |
1982年 | 40篇 |
1981年 | 41篇 |
1980年 | 19篇 |
1979年 | 19篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1975年 | 17篇 |
1973年 | 15篇 |
排序方式: 共有4217条查询结果,搜索用时 109 毫秒
81.
Sandra Jiménez-Alonso Ana Estévez-Braun Ángel G. Ravelo Rafael Zárate Matías López 《Tetrahedron》2007,63(14):3066-3074
Several bis-pyrano-l,4-benzoquinones have been synthesized by a double domino Knoevenagel hetero Diels-Alder reaction. The synthetic approach is highly efficient allowing the construction of complex polycyclic scaffolds with six new σ-bonds. These reactions performed more efficiently and more rapidly using microwave irradiation. The resulting bis-pyrano-1,4-benzoquinones are the first examples of a double domino Knoevenagel hetero Diels-Alder reaction. Our approach represents a novel contribution to the chemistry of 2,5-dihydroxy-l,4-benzoquinones and the first general method for the synthesis of bis-pyranobenzoquinones. 相似文献
82.
P. Gronchi A. Kaddouri P. Centola R. Del Rosso 《Journal of Sol-Gel Science and Technology》2003,26(1-3):843-846
SiO2 and Al2O3 supported Ni catalysts were synthesized in the form of xerogels: the SiO2 based materials were prepared starting from Ni propionate or glycolate salts and reacting them with tetraethoxysilane (TEOS) in propionic acid, Si(ethylene glycolate) or sodium silicate. The Al2O3 supported catalysts were prepared similarly from Ni propionate salts with Al iso-propoxide salts. Narrow metal particles and strong metal support interactions are observed in the sol-gel catalysts. The metal dispersion was higher for Al2O3 based materials than the SiO2 ones and it deeply depends on the Ni precursor for the silica supported Ni. Wet impregnated oxides with similar Ni loading have higher metal surface area than those from sol-gel processing. The influence of surface differences on the catalytic activity of the materials was studied following the CH4 and CO2 reaction in dry reforming conditions by pulse reaction tests. 相似文献
83.
María Angeles Monge Enrique Gutirrez Puebla María Del Mar Ramos Gallego Carmen Pardo Jos Elguero 《Journal of heterocyclic chemistry》1992,29(2):499-502
The fluoroborate of 1H-2-methylbenzotriazolium monohydrate crystallizes in the Pbca space group [Z = 8, a = 16.734(3), b = 19.153(9), c = 6.937(1) Å], The hydrogen bond network between the three molecules, 2MeBzTr, HBF4 and H2O, corresponds to a situation intermediate between the fluoroborate of a protonated benzotriazole (BF4?.2MeBzTrH+) and a neutral benzotriazole hydrogen-bonded to fluoroboric acid (HBF4,2MeBzTr). The behaviour of the title compound in solution is also intermediate between these two extreme situations. Thus, the present compound is one of the rare examples of a proton solvated only by very weak hydrogen bond acceptors. 相似文献
84.
Rafael Lucena 《Analytica chimica acta》2004,509(1):47-54
A multipurpose chamber capable of implementing a variety of continuous separation techniques is reported. The proposed module can be used in conjunction with flow manifolds coupled on-line to optical detectors in order to develop simple autoanalyzers for full sample pretreatment. The analytical applicability of such a device is discussed with specific regard to gas diffusion, dialysis, solid-phase extraction (SPE), and precipitation/dissolution techniques. Its versatility and simplicity are among the main advantages. Four different automated analyzers based on the use of the multipurpose chamber have been developed for monitoring of different quality control parameters of liquid food samples, namely, carbon dioxide in beer, ascorbic acid in fruit juice, caffeine in soft drinks and lactose, fat and protein in milk. The results obtained were compared with those provided by the existing alternatives for the same applications in terms of sensitivity and precision. 相似文献
85.
Rafael Sorkin 《International Journal of Theoretical Physics》1978,17(5):369-376
It is proved without resort to calculus methods that every continuous group multiplier for R can be reduced to the identity by a continuous remultiplication. The method introduced may generalize to infinitedimensional Abelian groups such as occur in analyzing the projective representations of the Bondi-Metzner-Sachs (BMS) group. 相似文献
86.
Maria Rosaria Del Giudice Anna Borioni Carlo Mustazza Franco Gatta 《Journal of heterocyclic chemistry》1994,31(6):1503-1507
A number of 2-aryl-substituted pyrido[3,2-e] and [4,3-e][1,2,4]triazolo[1,5-c]pyrimidines and [1,2,4]triazolo[1,5-c]pteridines 11,12a,b,e , their corresponding 5-carbonyl derivatives 7,8a,b,e and some pyrimido[5,4-e][1,2,4]triazolo[1,5-c]pyrimidin-5-ones 7,8c,d have been synthesized, according to different pathways. The new tricyclic heterocycles were prepared with the aim of studying their possible benzodiazepine receptors affinity. 相似文献
87.
Alcala R Shabaker JW Huber GW Sanchez-Castillo MA Dumesic JA 《The journal of physical chemistry. B》2005,109(6):2074-2085
Reaction kinetics studies were conducted for the conversions of ethanol and acetic acid over silica-supported Pt and Pt/Sn catalysts at temperatures from 500 to 600 K. Addition of Sn to Pt catalysts inhibits the decomposition of ethanol to CO, CH4, and C2H6, such that PtSn-based catalysts are active for dehydrogenation of ethanol to acetaldehyde. Furthermore, PtSn-based catalysts are selective for the conversion of acetic acid to ethanol, acetaldehyde, and ethyl acetate, whereas Pt catalysts lead mainly to decomposition products such as CH4 and CO. These results are interpreted using density functional theory (DFT) calculations for various adsorbed species and transition states on Pt(111) and Pt3Sn(111) surfaces. The Pt3Sn alloy slab was selected for DFT studies because results from in situ (119)Sn M?ssbauer spectroscopy and CO adsorption microcalorimetry of silica-supported Pt/Sn catalysts indicate that Pt-Sn alloy is the major phase present. Accordingly, results from DFT calculations show that transition-state energies for C-O and C-C bond cleavage in ethanol-derived species increase by 25-60 kJ/mol on Pt3Sn(111) compared to Pt(111), whereas energies of transition states for dehydrogenation reactions increase by only 5-10 kJ/mol. Results from DFT calculations show that transition-state energies for CH3CO-OH bond cleavage increase by only 12 kJ/mol on Pt3Sn(111) compared to Pt(111). The suppression of C-C bond cleavage in ethanol and acetic acid upon addition of Sn to Pt is also confirmed by microcalorimetric and infrared spectroscopic measurements at 300 K of the interactions of ethanol and acetic acid with Pt and PtSn on a silica support that had been silylated to remove silanol groups. 相似文献
88.
Optimisation of stir bar sorptive extraction for the analysis of volatile phenols in wines 总被引:2,自引:0,他引:2
An easy, fast and reliable analytical method is proposed for the determination of the concentration of volatile phenols (ethyl- and vinylphenols) in wines. The novel stir bar sorptive extraction (SBSE) technique is employed, following a simple and fast procedure that allows 15 samples to be extracted simultaneously using very small sample volume. Extracts are desorbed in a thermodesorption system (TDS) coupled on-line to a gas chromatograph-mass spectrometry system. The SBSE offers better recovery and linear regression coefficient (r2) for the four volatile phenols than solid-phase extraction (SPE). The mass spectrometric detection in selected ion monitoring mode contributes to the lower detection limit and good sensibility obtained with this method. 相似文献
89.
90.
Facile synthesis of derivatives of 2,4-diphenyl-3-azabicyclo[3.3.1]nonane and 7,9-diphenyl-8-azabicyclo[4.3.1]decane The facile synthesis of hydantoins, cyanhydrins and aminonitriles derived from 2,4-diphenyl-3-azabicyclo[3.3.1]nonanone and 7,9-diphenyl-8-azabicyclo[4.3.1]decanone is described. Configurations at C(9) or C(10) of the new compounds wth pharmaceutical and synthetical utility is deduced from their spectral properties. 相似文献