全文获取类型
收费全文 | 4856篇 |
免费 | 315篇 |
国内免费 | 33篇 |
专业分类
化学 | 3350篇 |
晶体学 | 57篇 |
力学 | 179篇 |
数学 | 775篇 |
物理学 | 843篇 |
出版年
2023年 | 31篇 |
2022年 | 134篇 |
2021年 | 144篇 |
2020年 | 101篇 |
2019年 | 190篇 |
2018年 | 158篇 |
2017年 | 107篇 |
2016年 | 248篇 |
2015年 | 157篇 |
2014年 | 252篇 |
2013年 | 482篇 |
2012年 | 286篇 |
2011年 | 299篇 |
2010年 | 213篇 |
2009年 | 205篇 |
2008年 | 219篇 |
2007年 | 207篇 |
2006年 | 204篇 |
2005年 | 171篇 |
2004年 | 155篇 |
2003年 | 130篇 |
2002年 | 173篇 |
2001年 | 73篇 |
2000年 | 110篇 |
1999年 | 65篇 |
1998年 | 40篇 |
1997年 | 46篇 |
1996年 | 35篇 |
1995年 | 36篇 |
1994年 | 35篇 |
1993年 | 37篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 16篇 |
1989年 | 29篇 |
1988年 | 27篇 |
1987年 | 20篇 |
1986年 | 17篇 |
1985年 | 30篇 |
1984年 | 24篇 |
1983年 | 21篇 |
1982年 | 20篇 |
1981年 | 26篇 |
1980年 | 16篇 |
1979年 | 16篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 13篇 |
1975年 | 16篇 |
1959年 | 8篇 |
排序方式: 共有5204条查询结果,搜索用时 78 毫秒
921.
922.
Mohamed El Otmani 《Journal of Theoretical Probability》2009,22(3):601-619
In this paper, we study the reflected solution of one-dimensional backward stochastic differential equation driven by Teugels
martingales and an independent Brownian motion. We prove the existence and uniqueness of the solution using a penalization
method combined with Snell envelope theory.
相似文献
923.
This paper presents a variational multi-scale constitutive model in the finite deformation regime capable of capturing the mechanical behavior of nanocrystalline (nc) fcc metals. The nc-material is modeled as a two-phase material consisting of a grain interior (GI) phase and a grain boundary (GB) phase. A rate-independent isotropic porous plasticity model is employed to describe the GB phase, whereas a crystal-plasticity model which accounts for the transition from partial dislocation to full dislocation mediated plasticity is employed for the GI phase. Assuming the rule of mixtures, the overall behavior of a given grain is obtained via volume averaging. The scale transition from a single grain to a polycrystal is achieved by Taylor-type homogenization. It is shown that the proposed model is able to capture the inverse Hall-Petch effect. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
924.
Sawant S Youssef D Mayer A Sylvester P Wali V Arant M El Sayed K 《Chemical & pharmaceutical bulletin》2006,54(8):1119-1123
Sarcophine (1), a cembranoid diterpene is known to inhibit the process of tumorigenesis. Sarcophine can be isolated in large amounts from the Red Sea soft coral Sarcophyton glaucum and hence is an ideal target for semisynthetic or biocatalytic modifications. Hydroxylated derivatives of 1 were reported to improve its anticancer activity. Despite the promising results and ready availability, there are limited attempts towards further diversifying the library of sarcophine derivatives. Hence, the current study targets the epoxide ring to generate sulfur-containing derivatives of sarcophine by reacting it with ammonium thiocyanate and Lawesson's reagent. Structure elucidation of the products was based on extensive 1D and 2D NMR and high resolution mass spectrometry, in addition to mechanistic considerations. The effect of these derivatives on highly malignant +SA mammary epithialial cell proliferation is reported. Anti-inflammatory potential of sarcophine and its derivatives is also demonstrated. 相似文献
925.
Mrabet KE Poitevin M Vial J Pichon V Amarouche S Hervouet G Lalere B 《Journal of chromatography. A》2006,1134(1-2):151-161
Matrix reference materials (MRM) are essential tools for the validation of analytical protocols. Nowadays, there are no such materials for the determination of herbicides in water. Pesticides stored in acetonitrile and stored on solid phase extraction (SPE) cartridges previously percolated with a water sample spiked with triazines and phenylureas have proven to be good candidates for reference materials because of their satisfactory stability under appropriate temperature conditions. To evaluate the behaviors of these materials containing pesticides and to be analyzed by liquid chromatography, a collaborative study including 15 laboratories has been organized. Observed reproducibility on candidate materials after the removal of extreme results was 16.1% for the vials with pesticides in acetonitrile (at around 0.125 mg/L) directly analyzed, 29.2% for a water sample spiked with the pesticides (at around 0.5 microg/L) analyzed after preconcentration on a cartridge and 26.7% for the cartridges previously percolated with a water containing the pesticides (250 mL at around 0.5 microg/L for each pesticide) analyzed after elution. Such dispersion values are quite compatible with the requirement of a further certification for such materials. 相似文献
926.
Es-Safi NE Khlifi S Kollmann A Kerhoas L El Abbouyi A Ducrot PH 《Chemical & pharmaceutical bulletin》2006,54(1):85-88
From the hydromethanolic extract of the aerial parts of Globularia alypum grown in Morocco, a new chlorinated iridoid glucoside, globularioside has been isolated beside 5 known iridoid glycosides, globularin, globularicisin, globularidin, globularinin and globularimin. This is the first report of a chlorinated iridoid in G. alypum and in the Globulareaceae. Unlike all other known 7-chlorinated iridoid glucosides where the chlorine atom exhibits an alpha configuration, globularioside incorporate the chlorine atom as a 7beta substituent. The structures of the isolated compounds were established on the basis of ESI-MS, MS-MS, 1D and 2D NMR spectral analysis. 相似文献
927.
El Hadj Rhouma MB Ben Lakhdar Z Berriche H Spiegelman F 《The Journal of chemical physics》2006,125(8):084315
The 4s and 5s Rydberg excited states of NaAr(n)* clusters are investigated using a pseudopotential quantum-classical method. While NaAr(n) clusters in their ground state are known to be weakly bound van der Waals complexes with Na lying at the surface of the argon cluster, isomers in 4s or 5s electronically excited states of small NaAr(n)* clusters (n< or =10) are found to be stable versus dissociation. The relationship between electronic excitation and cluster geometry is analyzed as a function of cluster size. For both 4s and 5s states, the stable exciplex isomers essentially appear as sodium-centered structures with similar topologies, converging towards those of the related NaAr(n)+ positive ions when the excitation level is increased. This is consistent with a Rydberg-type picture for the electronically excited cluster, described by a central sodium ion solvated by an argon shell, and an outer diffuse electron orbiting around this NaAr(n)+ cluster core. 相似文献
928.
A study of the vibrational behavior of five saturated monoacid triacylglycerides is performed by Raman spectroscopy at various temperatures in two separate spectral ranges: 1780–1700 and 3100–2650 cm−1. The samples are studied in polycrystalline phase at room temperature, in isotropic liquid phase, and in polycrystalline phase after cooling from the isotropic liquid phase. The CO stretching mode of these triglycerides changes significantly according to the temperature: we observe three components, or an unresolved doublet, or a resolved doublet. The I(2845)/I(2880) ratios (in the C–H stretching spectral region) of the different saturated monoacid triglycerides vary also according to the temperature. The study of these two indicators (the CO stretching mode and the I(2845)/I(2880) ratio) has permitted us to determine the polymorphic forms of the studied triglycerides. 相似文献
929.
The following novel solvatochromic probes were synthesized: 2,6-dibromo-4-[(E)-2-(1-alkylpyridinium-4-yl)ethenyl] phenolate, where the alkyl groups are methyl, n-butyl, n-hexyl, and n-octyl, respectively. Solvatochromism of three of these probes (C(1), C(4), and C(8)) was studied in 36 protic and aprotic solvents. A modified linear solvation energy relationship has been applied to the data obtained at 25 degrees C. Correlation of (empirical) polarities with other solvent properties showed more dependence on lipophilicity than on basicity. A similar conclusion has been reached for a series of other solvatochromic indicators. Exceptions are those that carry acidic hydrogens, being biased toward solvent basicity. Thermosolvatochromism has been studied in mixtures of water with methanol, 1-propanol, acetonitrile, and DMSO. Thermosolvatochromic data have been treated according to a model that explicitly considers the presence in bulk solution of three "species": water, organic component, and solvent-water hydrogen-bonded aggregate. Solvation by the latter is favored over solvation by either of the two precursor solvents (aqueous DMSO is an exception). Temperature increase resulted in desolvation of the probes, due to concomitant decrease of the structures of the component solvents. The above-mentioned modified solvation equation has been successfully applied to solvatochromism in aqueous methanol and aqueous 1-propanol. 相似文献
930.
Eric Bonnefille Nancy El Hawi Heinz Gornitzka Claude Couret 《Journal of organometallic chemistry》2006,691(26):5619-5625
The synthesis of stable monomeric dimesityloxygermylene-amine (MesO)2Ge-NR3 [NR3 = Et2NH, (C6H11)2NH, Et3N, dabco, tmeda] and chloromesityloxygermylene-amine MesO(Cl)Ge-tmeda complexes is presented. All compounds have been fully characterized by spectroscopic methods. The insertion reaction of (MesO)2Ge-tmeda 1a with dimethyl disulfide, bis(trimethylsilyl)peroxide and iodomethane is reported. Cycloaddition reactions with dimethylbutadiene are especially effective without any activation and with dimethyl acetylenedicarboxylate led selectively to compounds 7 and 8. In the latter case, the transient formation of a germirene has been evidenced by a trapping reaction. The structures of 3, 7 and 8 were determined by single-crystal X-ray diffraction. 相似文献