全文获取类型
收费全文 | 1374篇 |
免费 | 54篇 |
国内免费 | 5篇 |
专业分类
化学 | 981篇 |
晶体学 | 10篇 |
力学 | 37篇 |
数学 | 243篇 |
物理学 | 162篇 |
出版年
2023年 | 17篇 |
2022年 | 22篇 |
2021年 | 38篇 |
2020年 | 27篇 |
2019年 | 24篇 |
2018年 | 16篇 |
2017年 | 19篇 |
2016年 | 44篇 |
2015年 | 48篇 |
2014年 | 39篇 |
2013年 | 67篇 |
2012年 | 77篇 |
2011年 | 107篇 |
2010年 | 75篇 |
2009年 | 54篇 |
2008年 | 76篇 |
2007年 | 85篇 |
2006年 | 78篇 |
2005年 | 67篇 |
2004年 | 63篇 |
2003年 | 55篇 |
2002年 | 51篇 |
2001年 | 23篇 |
2000年 | 12篇 |
1999年 | 20篇 |
1998年 | 13篇 |
1997年 | 22篇 |
1996年 | 13篇 |
1995年 | 11篇 |
1994年 | 9篇 |
1993年 | 12篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1976年 | 8篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1970年 | 3篇 |
排序方式: 共有1433条查询结果,搜索用时 218 毫秒
101.
102.
Giancarlo Verardo Irene Duse Alessandro Callea 《Rapid communications in mass spectrometry : RCM》2009,23(11):1607-1618
Underivatized oligosaccharides were analyzed by electrospray ionization (ESI) using a linear ion trap mass spectrometer in the negative ion mode with post‐column addition of an aqueous solution of formic acid. Under these conditions all oligosaccharides showed the presence of the corresponding formate adduct [M + HCOO]? with high intensity and easy subsequent low‐energy collision‐induced dissociation (CID) fragmentation using successive MSn experiments. A careful examination of the mass spectra obtained from these MSn experiments pointed out some significant differences useful to identify and quantify the single components in mixtures of coeluted disaccharides. This new sensitive and rapid method was successfully applied to the quantification of oligosaccharides in some juices minimizing sample handling. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
103.
The deployment of machine learning models is expected to bring several benefits. Nevertheless, as a result of the complexity of the ecosystem in which models are generally trained and deployed, this technology also raises concerns regarding its (1) interpretability, (2) fairness, (3) safety, and (4) privacy. These issues can have substantial economic implications because they may hinder the development and mass adoption of machine learning. In light of this, the purpose of this paper was to determine, from a positive economics point of view, whether the free use of machine learning models maximizes aggregate social welfare or, alternatively, regulations are required. In cases in which restrictions should be enacted, policies are proposed. The adaptation of current tort and anti-discrimination laws is found to guarantee an optimal level of interpretability and fairness. Additionally, existing market solutions appear to incentivize machine learning operators to equip models with a degree of security and privacy that maximizes aggregate social welfare. These findings are expected to be valuable to inform the design of efficient public policies. 相似文献
104.
Irene Gouvinhas Nelson Machado Amadeo Girons‐Vilaplana Snia Gomes Teresa Carvalho Raúl Domínguez‐Perles Ana I. R. N. A. Barros 《Journal of Chemometrics》2016,30(9):548-558
The olive tree (Olea europaea L.) can be affected by Colletotrichum acutatum, causing a loss of yield and quality of the final products, whilst the incidence of this fungal infection depends on several factors, including cultivar susceptibility. Thus, the effect of C. acutatum infection in cultivars displaying different susceptibilities to this fungal disease (‘Galega Vulgar’ ‐ susceptible, ‘Cobrançosa’ ‐ moderately susceptible, ‘Picual’ ‐ tolerant) has been assessed through spectrophotometric methods and HPLC, while the FTIR spectra of the cuticles have been concomitantly registered, resorting to the ATR accessory. With the support of multivariate analysis, these spectra allowed to discriminate olives with distinct infection times, besides retrieving evidences concerning the different susceptibility of each cultivar, while these observations were reinforced by the spectrophotometric and chromatographic methods. Furthermore, the assessment of the phenolic profile evidenced individual compounds in the distinct cultivars, so as their variations in response to the fungal infection. 相似文献
105.
Dr. Irene Vassalini Dr. Laura Borgese Michele Mariz Prof. Stefano Polizzi Dr. Giuliana Aquilanti Prof. Paolo Ghigna Prof. Andrea Sartorel Prof. Vincenzo Amendola Prof. Ivano Alessandri 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(23):6689-6693
Oxygen evolution reaction (OER) is the most critical step in water splitting, still limiting the development of efficient alkaline water electrolyzers. Here we investigate the OER activity of Au–Fe nanoalloys obtained by laser-ablation synthesis in solution. This method allows a high amount of iron (up to 11 at %) to be incorporated into the gold lattice, which is not possible in Au–Fe alloys synthesized by other routes, due to thermodynamic constraints. The Au0.89Fe0.11 nanoalloys exhibit strongly enhanced OER in comparison to the individual pure metal nanoparticles, lowering the onset of OER and increasing up to 20 times the current density in alkaline aqueous solutions. Such a remarkable electrocatalytic activity is associated to nanoalloying, as demonstrated by comparative examples with physical mixtures of gold and iron nanoparticles. These results open attractive scenarios to the use of kinetically stable nanoalloys for catalysis and energy conversion. 相似文献
106.
Luigi Ambrosio Irene Fonseca Paolo Marcellini Luc Tartar 《Archive for Rational Mechanics and Analysis》1999,149(1):23-47
. Existence of minimizers for a volume-constrained energy $ E(u) := \int_{\Omega} W(\nabla u)\, dx Existence of minimizers for a volume-constrained energy E(u) : = òW W(?u) dx E(u) := \int_{\Omega} W(\nabla u)\, dx where LN({u = zi}) = ai, i = 1, ?, P, {\cal L}^N(\{u = z_i\}) = \alpha_i, i = 1, \ldots, P, is proved for the case in which ziz_i are extremal points of a compact, convex set in \Bbb Rd\Bbb R^d and under suitable assumptions on a class of quasiconvex energy densities W. Optimality properties are studied in the scalar-valued problem where d=1d=1, P=2P=2, W(x)=|x|2W(\xi)=|\xi|^2, and the &-limit as the sum of the measures of the 2 phases tends to \L(W)\L(\Omega) is identified. Minimizers are fully characterized when N=1N=1, and candidates for solutions are studied for the circle and the square in the plane. 相似文献
107.
The enantioselective synthesis of 3(S)-hydroxy polygodial and its acetyl derivative is described. The construction of the 3-hydroxy drimane skeleton was based on the titanium-catalyzed radical cyclization of (10S)-10, 11-epoxy-farnesyl acetate. Only underivatized 3(S)-hydroxy polygodial showed activity in assays on VR1 vanilloid receptor in HEK cells transfected with the human VR1. 相似文献
108.
Three new N2S2 donor ligands 1,1′-((2-(2-(phenylthio)phenylthio)phenyl)methylene)bis(3,5-R-1H-pyrazole), R = H (LH), R = Me (LMe), R = i-Pr (Li-Pr) have been prepared and characterized. These bifunctional ligands incorporate two distinct chelate donor systems, by virtue of the presence of bispyrazole and bisthioether functions. The preferred conformation of these ligands is such that the N2 and S2 donor moieties may be oriented in opposite directions, thus favoring the formation of molecular chains when treated with AgBF4. The X-ray structures of Ag(I) complexes show that, depending on the steric hindrance present on the pyrazole rings, these ligands behave as κ4-SSNN-μ bridging tetradentate (when R = H), or κ3-SNN-μ bridging tridentate (when R = Me, i-Pr). Interestingly, [Ag(LH)]BF4 crystallizes in the chiral space group P41, with the molecular chain that is folded around the 41 screw axis. 相似文献
109.
Stefano Gialanella Fabrizio Girardi Gloria Ischia Ivan Lonardelli Maurizio Mattarelli Maurizio Montagna 《Journal of Thermal Analysis and Calorimetry》2010,102(3):867-873
This study deals with some microstructural and crystallographic aspects of the thermally induced transformation of goethite
(α-FeOOH) into hematite (α-Fe2O3), occurring at about 300 °C. Powder specimens of goethite have been annealed in air at different temperatures, ranging from
200 °C up to 1,000 °C. The resulting products have been analyzed for a complete characterization of the changes brought about
by the thermal treatments, using a multianalytical approach, based on: thermogravimetry, differential thermal analysis, transmission
electron microscopy, Raman spectroscopy, and X-ray diffraction. At lower temperatures, the transition to hematite produces
no important changes in size and shape of the original goethite grains. Recrystallization, and partial sintering, occurs only
at temperatures in excess of 800 °C. The relevant evolution of pores present in both phases has been also considered, as it
may provide important indications on the actual formation mechanism of hematite. 相似文献
110.
Sofia Georgakakou Michael Kazanis Irene Panderi 《Analytical and bioanalytical chemistry》2010,397(6):2161-2170
A validated method based on liquid chromatography/positive ion electrospray–mass spectrometry (LC-ESI/MS) is described for the quantification of perindopril and its active metabolite, perindoprilat, in human plasma. The assay was based on 500-μL plasma samples, following solid-phase extraction using Oasis HLB cartridges. All analytes and the internal standard (trandolapril) were separated by hydrophilic interaction liquid chromatography using a SeQuant Zic-HILIC analytical column (150.0?×?2.1 mm i.d., particle size 3.5 μm, 200 Å) with isocratic elution. The mobile phase consisted of 10% 5.0 mM ammonium acetate water solution in a binary mixture of acetonitrile/methanol (60:40, v/v) and pumped at a flow rate of 0.10 mL min?1. Quantitation of the analytes was performed with selected ion monitoring (SIM) in positive ionization mode using electrospray ionization interface. The assay was found to be linear in the concentration range of 5.0–500.0 ng mL?1 for perindopril and perindoprilat. Intermediate precision were found less than 3.5% over the tested concentration ranges. A run time of less than 6.0 min for each sample made it possible to analyze a large number of human plasma samples per day. The method is the first reported application of HILIC in the analysis of angiotensin-converting enzyme inhibitors and can be used to quantify perindopril and perindoprilat in human plasma covering a variety of pharmacokinetic or bioequivalence studies. 相似文献