全文获取类型
收费全文 | 1103篇 |
免费 | 80篇 |
国内免费 | 18篇 |
专业分类
化学 | 880篇 |
晶体学 | 3篇 |
力学 | 40篇 |
数学 | 127篇 |
物理学 | 151篇 |
出版年
2023年 | 6篇 |
2022年 | 13篇 |
2021年 | 17篇 |
2020年 | 32篇 |
2019年 | 20篇 |
2018年 | 21篇 |
2017年 | 17篇 |
2016年 | 35篇 |
2015年 | 31篇 |
2014年 | 54篇 |
2013年 | 77篇 |
2012年 | 94篇 |
2011年 | 96篇 |
2010年 | 56篇 |
2009年 | 57篇 |
2008年 | 81篇 |
2007年 | 60篇 |
2006年 | 68篇 |
2005年 | 74篇 |
2004年 | 58篇 |
2003年 | 54篇 |
2002年 | 27篇 |
2001年 | 16篇 |
2000年 | 14篇 |
1999年 | 15篇 |
1998年 | 10篇 |
1997年 | 8篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 10篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有1201条查询结果,搜索用时 9 毫秒
61.
S. G. Hutchison C. M. Wai J. Dong R. J. Kearney 《Plasma Chemistry and Plasma Processing》1995,15(2):353-367
The feasibility of producing titanium metal from titanium tetrachloride in a thermal plasma under equilibrium and adiabatic expansion conditions has been theoretically investigated. Free energy minimization and adiabatic expansion calculations to simulate a nozzle expansion were used to study the practicality of production. The crucial requirements for the production of titanium powder from TiCl4 and H2 appear to be rapid quenching of the plasma gas at high temperature (e.g., 3700 K) and appropriate reactant concentrations. Quenching of tire plasma gas and production of titanium powder can be achieved by adiabatic expansion through a nozzle. Preliminary experimental data indicate that titanium powder of approximately 5 nm in size can be produced in an argon plasma rising a nozzle expansion approach. 相似文献
62.
63.
Manual shaking-enhanced, ultrasound-assisted emulsification microextraction (MS-USAEME) combined with ultraperformance liquid chromatography (UPLC) with UV detection has been developed for the determination of five endocrine-disrupting phenols (EDPs) in seawater samples and detergent samples: 4-tert-butylphenol (4-t-BP), 4-cumylphenol (4-CP), 4-tert-octylphenol (4-t-OP), 2,4-di-tert-butylphenol (2,4-di-t-BP) and 4-nonylphenol (4-NP). Optimum conditions were found to be: 25 μL 1-bromohexadecane as extraction solvent, 5 mL of aqueous sample and 1 g of NaCl to control the ionic strength; manual shaking for 10 s; ultrasonication for 1 min; centrifugation for 3 min at 5000 rpm (speed). For MS-USAEME, manual shaking for 10 s is essential for effective extraction when the ultrasonic extraction time is as brief as 1 min. The small volume of aqueous sample enhances the effect of manual shaking significantly. For seawater samples, the limit of detection (LOD) was 0.5-2.8 ng mL(-1), the limit of quantification (LOQ) was 1.8-9.3 ng mL(-1) with the relative standard deviation (RSD) in the range 4.2-10.3%. For detergent samples, the LOD was 0.4-2.4 ng mL(-1), LOQ was 1.6-8.2 ng mL(-1) and RSD 4.7-10.0%. The relative recovery was 96-109% for seawater samples and 81-106% for the detergent samples. 相似文献
64.
65.
TM Ton F Himawan JW Chang PW Chan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(38):12020-12027
A method to prepare α,α-acyl amino acid derivatives efficiently by Cu(OTf)(2) +1,10-phenanthroline (1,10-phen)-catalyzed amination of 1,3-dicarbonyl compounds with PhI?NSO(2) Ar is described. The mechanism is thought to initially involve aziridination of the enolic form of the substrate, formed in situ through coordination to the Lewis acidic metal catalyst, by the putative copper-nitrene/imido species generated from the reaction of the metal catalyst with the iminoiodane source. Subsequent ring opening of the resultant aziridinol adduct under the Lewis acidic conditions then provided the α-aminated product. The utility of this method was exemplified by the enantioselective synthesis of a precursor of 3-styryl-2-benzoyl-L-alanine. 相似文献
66.
Fan HH Guo L Li KF Wong MS Cheah KW 《Journal of the American Chemical Society》2012,134(17):7297-7300
We report the synthesis and investigation of multiphoton absorption properties of a novel series of diphenylamino-end-capped ladder-type oligo(p-phenylene)s which exhibit greatly enhanced and efficient multiphoton (from two- to five-photon) upconverted blue photoluminescence with which the record-high intrinsic three-photon absorption cross-section of 4.56 × 10(-76) cm(6) s(2) in the femtosecond regime has been obtained. Exceptionally efficient two- to five-photon-excited lasing in the blue region has also been demonstrated in which the highest two-photon-excited lasing efficiency of 0.34% has been achieved. 相似文献
67.
Dehestani A Lam WH Hrovat DA Davidson ER Borden WT Mayer JM 《Journal of the American Chemical Society》2005,127(10):3423-3432
Osmium tetroxide is reduced by molecular hydrogen in the presence of ligands in both polar and nonpolar solvents. In CHCl3 containing pyridine (py) or 1,10-phenanthroline (phen), OsO4 is reduced by H2 to the known Os(VI) dimers L2Os(O)2(mu-O)2Os(O)2L2 (L2 = py2, phen). However, in the absence of ligands in CHCl3 and other nonpolar solvents, OsO4 is unreactive toward H2 over a week at ambient temperatures. In basic aqueous media, H2 reduces OsO4(OH)n(n-) (n = 0, 1, 2) to the isolable Os(VI) complex, OsO2(OH)4(2-), at rates close to that found in py/CHCl3. Depending on the pH, the aqueous reactions are exergonic by deltaG = -20 to -27 kcal mol(-1), based on electrochemical data. The second-order rate constants for the aqueous reactions are larger as the number of coordinated hydroxide ligands increases, k(OsO4) = 1.6(2) x 10(-2) M(-1) s(-1) < k(OsO4(OH)-) = 3.8(4) x 10(-2) M(-1) s(-1) < k(OsO4(OH)2(2-)) = 3.8(4) x 10(-1) M(-1) s(-1). The observation of primary deuterium kinetic isotope effects, k(H2)/k(D2) = 3.1(3) for OsO4 and 3.6(4) for OsO4(OH)-, indicates that the rate-determining step in each case involves H-H bond cleavage. Density functional calculations and thermochemical arguments favor a concerted [3+2] addition of H2 across two oxo groups of OsO4(L)n and argue against H* or H- abstraction from H2 or [2+2] addition of H2 across one Os=O bond. The [3+2] mechanism is analogous to that of alkene addition to OsO4(L)n to form diolates, for which acceleration by added ligands has been extensively documented. The observation that ligands also accelerate H2 addition to OsO4(L)n highlights the analogy between these two reactions. 相似文献
68.
69.
Carbon‐doped titania (C‐TiO2) nanoparticles were synthesized by the sol–gel method at different calcination temperatures (300–600°C) employing titanium tetraisopropoxide (TTIP) as the titanium source and polyoxyethylene sorbitan monooleate (Tween 80) as the carbon source. The physical properties of C‐TiO2 samples were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The photocatalytic activities were checked through the photodegradation of phenolphthalein (PHP) under ultraviolet irradiation. The UV spectrum showed that the carbon doping extends the absorption range of TiO2 to the visible region. However, the photocatalytic activity is affected by the electron–hole recombination phenomenon, as revealed by the photoluminescence (PL) study. According to the PL spectra, carbon doping reduces the edge‐to‐edge electron–hole recombination. Nevertheless, the number of defect sites is greatly influenced by the calcination temperature of C‐TiO2. C‐TiO2 that was calcined at 400°C showed the highest photodegradation percentage of PHP, which was mainly attributed to the synergic effect of the low direct edge‐to‐edge electron–hole recombination, high content of defect sites, and retention of active electrons on the surface hydroxyl group. 相似文献
70.
Quan‐Bin Han Wai‐Lun Tang Cai‐Xia Dong Hong‐Xi Xu Zhi‐Hong Jiang 《Journal of separation science》2013,36(7):1304-1310
Two‐phase solvent system plays crucial role in successful separation of organic compounds using counter‐current chromatography (CCC). An interesting two‐phase solvent system, composed of chloroform/ethyl acetate/methanol/water, is reported here, in which both phases contain sufficient organic solvents to balance their dissolving capacities. Adjusting the solvent system to get satisfactory partition coefficients (K values) for target compounds becomes relatively simple. This solvent system succeeded in sample preparation of aconitine (8.07 mg, 93.69%), hypaconitine (7.74 mg, 93.17%), mesaconitine (1.95 mg, 94.52%) from raw aconite roots (102.24 mg, crude extract), benzoylmesaconine (34.79 mg, 98.67%) from processed aconite roots (400.01 mg, crude extract), and yunaconitine (253.59 mg, 98.65%) from a crude extract of Aconitum forrestii (326.69 mg, crude extract). 相似文献