全文获取类型
收费全文 | 3541篇 |
免费 | 121篇 |
国内免费 | 22篇 |
专业分类
化学 | 2098篇 |
晶体学 | 11篇 |
力学 | 145篇 |
数学 | 499篇 |
物理学 | 699篇 |
无线电 | 232篇 |
出版年
2024年 | 5篇 |
2023年 | 44篇 |
2022年 | 81篇 |
2021年 | 122篇 |
2020年 | 107篇 |
2019年 | 96篇 |
2018年 | 65篇 |
2017年 | 54篇 |
2016年 | 142篇 |
2015年 | 98篇 |
2014年 | 143篇 |
2013年 | 186篇 |
2012年 | 241篇 |
2011年 | 272篇 |
2010年 | 153篇 |
2009年 | 135篇 |
2008年 | 218篇 |
2007年 | 210篇 |
2006年 | 216篇 |
2005年 | 197篇 |
2004年 | 153篇 |
2003年 | 133篇 |
2002年 | 132篇 |
2001年 | 57篇 |
2000年 | 28篇 |
1999年 | 29篇 |
1998年 | 33篇 |
1997年 | 41篇 |
1996年 | 37篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 19篇 |
1992年 | 9篇 |
1991年 | 18篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1975年 | 9篇 |
1973年 | 6篇 |
排序方式: 共有3684条查询结果,搜索用时 15 毫秒
51.
Michele Sacerdoti Maria Chiara Dalconi Barbara Cavicchi Stefano Colonna 《Journal of solid state chemistry》2004,177(6):1781-1788
Sol-gel routes were used to prepare Ta 10 at% and Nb 5 at% and 10 at% doped titania nanosized powders. When fired between 410°C and 850°C the doped titania powders are in the anatase phase; further heating up to 1050°C is required to obtain the rutile phase. The presence of dopant atoms delays the rate of transformation as compared with pure titania powders. Doping also affects the rate of grain growth and increases the conductance response to gas. To better understand the role played by dopant atoms in inhibiting both phase transformation to rutile and grain growth, X-ray Absorption Spectroscopy measurements were performed at the LIII-LI absorption edges of Ta and Nb K absorption edge. Analysis was restricted to the anatase phase because the transformation to rutile phase, obtained by firing at 1050°C, is accompanied by the formation of undesired Ta and Nb oxides (Ta2O5 and Nb2TiO7, respectively). Extended X-ray Absorption Fine Structure and X-ray Absorption Near-Edge Spectroscopy analysis results indicate that in nanostructured anatase both tantalum and niobium atoms substitute Ti cations with +5 valence state. 相似文献
52.
Franco Cecconi Carlo A. Ghilardi Stefano Midollini Annabella Orlandini Piero Zanello 《Polyhedron》1986,5(12):2021-2031
The reaction of hydrogen sulphide with [Co(H2O)6](BF4)2 and triethylphosphine in the presence of sodium tetraphenylborate or tetrabutylammonium hexafluorophosphate gave the paramagnetic clusters [Co6(μ3-S)8(PEt3)6](Y) (Y = BPh4, (1), PF6, (2)). These compounds can be easily reduced by sodium napthalenide to the diamagnetic species [Co6(μ3-S)8(PEt3)6] · 2C4H8O (3). The molecular structures of 1 and 3 have been established by single-crystal X-ray diffraction methods. Crystal data: (1) space group P
, a = 19.481(9), b = 15.562(7), c = 12.390(b) Å, α = 92.70(8), β = 94.50(7), γ = 94.10(9)°, Z = 2, (3) space group R
, a = 11.780(6) Å, α = 92.50(7)°, Z = 1. Both structures were solved by the heavy atom method and refined by full-matrix least-squares techniques to the conventional R factors values of 0.050 for 1 and 0.044 for 3 on the basis of 4251 and 1918 observed reflections, respectively. The two clusters [Co6(μ3-S)8)(PEt3)6]1+,0 are isostructural, the inner core consisting of an octahedron of cobalt atoms with all the faces symmetrically capped by triply bridging sulphur atoms. Each metal centre is additionally linked to a triethylphosphine group so that each cobalt atom is co-ordinated by four sulphur atoms and one phosphorus in a distorted square pyramidal environment. The addition of one electron whilst leaving unchanged the geometry of the inner framework, induces small changes in the structural parameters, the average Co---Co and Co---P distances being 2.794 (3) and 2.162 (2) Å for 1 and 2.817 (3) and 2.138 (2) Å for 3 respectively. Electrochemistry in non-aqueous solvents shows the electron-transfer sequence
The tricationic species is stable only in the short time of cyclic voltammetric tests. 相似文献
Full-size image (3K)
53.
Alessandra Gentili Fulvia Caretti Giuseppe D’Ascenzo Lucia Mainero Rocca Stefano Marchese Stefano Materazzi Daniela Perret 《Chromatographia》2007,66(9-10):669-676
Two new methods of analysis, based on liquid chromatography–tandem mass spectrometry, for simultaneous determination of trichothecenes A, B, and D in maize flour and oil have been developed and validated in accordance with European Commission decision 2002/657/EC (recovery, CCα, CCβ, and precision). The trichothecenes were extracted from maize flour by matrix solid-phase dispersion, with recoveries ≥79%, and from maize oil by liquid–liquid extraction, with recoveries ≥78%. Limits of quantitation ranged between 0.03 and 50 μg kg?1, depending on the electrospray response to each analyte and on the matrix. Monitoring of flour and oil samples with this HPLC–MS–MS method revealed the presence of deoxynivalenol, T-2 toxin, and diacetoxyscirpenol at very low concentrations. 相似文献
54.
Marchese S Curini R Gentili A Perret D Rocca LM 《Rapid communications in mass spectrometry : RCM》2004,18(3):265-272
A simple and rapid method using reversed-phase liquid chromatography/tandem mass spectrometry (LC/MS/MS) for the simultaneous determination of the urinary metabolites of benzene, toluene, xylene and styrene in human urine specimens and standard solutions is described. A hybrid quadrupole/time-of-flight (QqTOF) mass spectrometer was compared for the determination of metabolite of aromatic solvents in urine samples. The metabolites selected were: trans,trans-muconic acid, hippuric acid, o-, m- and p-methylhippuric acid and phenylglyoxylic acid. The compounds were well separated from each other on narrow-bore 1-mm i.d. reversed-phase LC C-18 columns. Average recoveries for loading 100 microL of urine samples varied from 88-110% and the quantification limits were less than 30 ng/mL for each analyte (3 ng/mL for trans,trans-muconic acid). The qualitative information obtained (mass accuracy, resolution and full-scan spectra) with the QqTOF mass spectrometer allows a secure identification of analytes in biological matrices. 相似文献
55.
The authors study an example, suggested by E. De Giorgi, of a second-order uniformly elliptic partial differential operator in divergence form with continuous coefficients on a smooth domain in the plane such that the associated harmonic measure on the boundary is not absolutely continuous with respect to the ordinary surface measure. 相似文献
56.
All aerobic organisms have developed different mechanisms for neutralising the free radicals, mostly produced by the monoelectronic reduction of O(2), and preventing the severe damages these can provoke. The efficiency of these mechanisms can be assessed, in different matrices, by a simple and direct chemiluminescent assay (CL) based on luminol oxidation catalysed by horseradish peroxidase. Light emission is mediated by the production of free radicals and it is inhibited after a sample addition in a way that is directly proportional to the sample total content of molecules displaying antioxidant activity. The performances of this chemiluminescent assay were compared with those of two spectrophotometric methods already applied in clinical practice. First spectrophotometric method measures, like CL assay, the total antioxidant capacity, whereas the second one determines free thiol groups content. The chemiluminescent assay has a linearity interval between 0.60 and 9.46 mumol l(-1) of Trolox (y=34.91x+3.10; r=0.999; n=5) with an imprecision, expressed as CV, of 3.8% and an inaccuracy, expressed as percentage recovery, of 109%. The first spectrophotometric method, based on the same reference standard, the Trolox molecule, has a linearity interval between 0.2 and 2.5 mmol l(-1) of Trolox (y=-0.01x+4.54; r=0.95; n=5); the thiol groups assay has a linearity interval between 0.1 and 1 mmol l(-1) of l-cysteine (y=1.68x-47.09; r=0.998; n=5). Different clinical samples of plasma from healthy individuals, obese subjects and patients with liver diseases were tested. Interesting correlations were obtained among the three methods, but no significant correlations emerged between antioxidant capacity and clinical parameters. Significant differences were there only between men and women among obese subjects and between drinkers and non-drinkers among liver disease patients. 相似文献
57.
Giancarlo Cravotto Marina Beggiato Giovanni Palmisano Stefano Tollari Werner Bonrath 《Tetrahedron letters》2005,46(13):2267-2271
Pd-catalyzed homo- and cross-couplings of boronic acids and aryl halides were successfully carried out both in aqueous media under high-intensity ultrasound (US) and in DME under microwave (MW). Heterogeneous catalysis with Pd/C was employed, avoiding phosphine ligands and phase-transfer catalysts. In a trial series involving 15 different iodo- and bromoaryls and 7 boronic acids, both energy sources drastically reduced reaction times affording biaryls in acceptable to good yields. With palladium(II) acetate as catalyst, electron-deficient aryl chlorides also reacted, affording a few biaryls in acceptable yields. Ullmann-type zinc-mediated homocoupling of iodo- and bromoaryls in the presence of Pd/C under CO2 atmosphere was achieved in aqueous media under US, though not under MW. Suzuki homo- and cross-couplings were also carried out in a new reactor developed in our laboratory, featuring combined US and MW irradiation, further improving a green synthetic method. 相似文献
58.
This paper reports the results on a study of ion chromatography (IC) behaviour on strong cationic exchange column of CH3-(CH2)n−1-NH2 (n=1-6) type linear monoamines. The eluents were mixtures of HCl-NaCl, HNO3-NaNO3, HClO4-NaClO4, H2SO4-Na2SO4, HClO4-NaClO4-CH3CN. Amines were revealed with an amperometric detector. The influence of different counter-ions on retention factors, k′, is discussed. Relationships between k′ and pE (E=sum of Na+ and H+ concentrations), and length of alkylic chain, and sensitivity coefficient S, are reported and discussed. The addition of acetonitrile (ACN) in the eluent was also taken into account. Suitable considerations are derived from these data. An example of monoamines separation is given. 相似文献
59.
Concetta De Stefano Claudia Foti Antonio Gianguzza Frank J. Millero Silvio Sammartano 《Journal of solution chemistry》1999,28(7):959-972
The hydrolysis of trimethyltin(IV) has been studied by potentiometry (H+ -glass electrode) and calorimetry in various salt media (NaNO3, NaCl, KCl, Na2SO4, and NaNO3—NaCl mixtures). The effect of ionic strength on the hydrolysis constants is accounted for by a simple Debye–Hückel type equation and by Pitzer equations. The results allow us to obtain H for hydrolysis and the temperature dependence of the Pitzer parameters. The resulting coefficients can be used to examine the speciation of (CH3)3Sn+ in multicomponent electrolyte solutions, such as natural waters, over a wide range of temperature and ionic strength. 相似文献
60.
Piero Dalla Croce Paola Del Buttero Stefano Mayorana Raffaele Vistocco 《Journal of heterocyclic chemistry》1978,15(3):515-517
Thiete 1,1-dioxide reacts with 2 moles of α-chlorobenzalphenylhydrazine and methyl phenylhydrazonochloroacetate in the presence of triethylamine leading to the pyrazole derivatives 6 and 7 whose structure and mechanism of formation are discussed. 相似文献