全文获取类型
收费全文 | 1152篇 |
免费 | 34篇 |
国内免费 | 143篇 |
专业分类
化学 | 583篇 |
晶体学 | 13篇 |
力学 | 56篇 |
综合类 | 5篇 |
数学 | 473篇 |
物理学 | 199篇 |
出版年
2023年 | 21篇 |
2022年 | 17篇 |
2021年 | 11篇 |
2020年 | 24篇 |
2019年 | 14篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 25篇 |
2015年 | 16篇 |
2014年 | 39篇 |
2013年 | 86篇 |
2012年 | 70篇 |
2011年 | 63篇 |
2010年 | 56篇 |
2009年 | 58篇 |
2008年 | 68篇 |
2007年 | 59篇 |
2006年 | 62篇 |
2005年 | 53篇 |
2004年 | 51篇 |
2003年 | 35篇 |
2002年 | 34篇 |
2001年 | 43篇 |
2000年 | 39篇 |
1999年 | 43篇 |
1998年 | 25篇 |
1997年 | 28篇 |
1996年 | 35篇 |
1995年 | 38篇 |
1994年 | 27篇 |
1993年 | 27篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1972年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有1329条查询结果,搜索用时 78 毫秒
991.
An original focus on univariate calibration as an experimental process of quantitative analysis is presented. A novel classification system is introduced against the background of the present situation concerning nomenclature of calibration methods. Namely, it has been revealed that four methods well-known in analytical chemistry: the conventional method, the internal standard method, the indirect method and the dilution method, can be split into those carried out in both the interpolative and the extrapolative mode. It is then shown that the basic procedures of all these methods can be modified including different approaches, such as matrix-matched technique, spiking the sample with a reactant, bracketing calibration, and others. For the first time (as compared to monographies dealing with univariate calibration) it is reviewed how the methods are mixed and integrated with one another thereby creating new calibration strategies of extended capabilities in terms of enhanced resistance to the interference and non-linear effects – as the main sources of systematic calibration errors. As additional novelty, rationally possible combinations of the calibration methods – not met hitherto in the literature – have been predicted. Finally, some general rules relating to calibration are formulated and the main calibration problems that still need to be solved are displayed. 相似文献
992.
993.
Avi Sigler Moshe Stupel 《International Journal of Mathematical Education in Science & Technology》2016,47(8):1226-1243
Solution of problems in mathematics, and in particular in the field of Euclidean geometry is in many senses a form of artisanship that can be developed so that in certain cases brief and unexpected solutions may be obtained, which would bring out aesthetically pleasing mathematical traits. We present four geometric tasks for which different proofs are given under the headings: standard proof, elegant proof, and the proof without words. The solutions were obtained through a combination of mathematical tools and by dynamic investigation of the geometrical properties. 相似文献
994.
51.Intr0ductionUsuallyasolitonequationcanbeexpressedasthecompatibleconditionofanover-determinedsystem,theso-calledLaxpair-Wehavea1readyknownthatthetwol1nearequationsintheLaxpaircanben0nlinearizedtobetw0c0mpatible,comp1etelyintegrableHamilt0niansystemsintheLioll"i1lesenseundersomecertainc0nstraintconditionsbetweentheeigenfunctionsandthepotentials(seeL1J).Am0ngtheseconstraintconditionsaretheretheBargmannconstraint{see[1,2,4j),theC.Neumannconstraint(see[l,4J),andthesymmetryconstra1nt(see[5,6j… 相似文献
995.
张无畏 《高校应用数学学报(A辑)》1999,14(1):2
本文研究了两类分段对称系统周期解的存在性,得到了系统相应的周期解存在的充分条件.作为应用,证明了一类具有分段对称性的时滞微分方程的周期解的存在性,推广了现有的相关结果. 相似文献
996.
The widespread distribution of plastics, their persistence and ability to act as a vector of toxic chemicals has rendered them concerning emergent pollutants. The quantification of these contaminants is highly relevant for the evaluation of anthropogenic impacts on aquatic and terrestrial ecosystems and dependent of the efficacy of methods to separate microplastics from environmental matrices. Little information is available about the microplastic extraction methods on complex samples – i.e. samples with multiple types of matrices. Herein, methods for the separation of microplastics from complex samples are summarized and discussed based on their advantages and drawbacks focused on a comparative analysis of their efficiency on organic matter removal, polymer recovery and preservation of plastic integrity. The efficiency on microplastic recovery and organic matter reduction, as well as the examination of the effects of treatments on plastics are closely linked to the density and digestion approaches selected, the polymer features and the environmental matrix analyzed. High-density salt solutions are more effective for density separation, while oxidative methods have recurrently shown better rates of organic matter reduction (particularly in vegetal-rich samples) and plastic recovery, with little impact on plastics, while 10 % KOH has been described as highly efficient in samples containing animal organic matter. This comparative analysis highlights the benefits and limitations of different approaches for the analysis of microplastics in complex samples which may be helpful for the optimization and harmonization of the methods. 相似文献
997.
Séby F Castetbon A Ortega R Guimon C Niveau F Barrois-Oudin N Garraud H Donard OF 《Analytical and bioanalytical chemistry》2008,391(2):587-597
The European directive 2000/53/EC limits the use of Cr(VI) in vehicle manufacturing. Although a maximum of 2 g of Cr(VI) was
authorised per vehicle for corrosion prevention coatings of key components, since July 2007 its use has been prohibited except
for some particular applications. Therefore, the objective of this work was to develop direct analytical procedures for Cr(VI)
determination in the different steel coatings used for screws. Instead of working directly with screws, the optimisation of
the procedures was carried out with metallic plates homogeneously coated to improve the data comparability. Extraction of
Cr(VI) from the metallic parts was performed by sonication. Two extraction solutions were tested: a direct water extraction
solution used in standard protocols and an ammonium/ammonia buffer solution at pH 8.9. The extracts were further analysed
for Cr speciation by high-performance liquid chromatography (HPLC) inductively coupled plasma (ICP) atomic emission spectrometry
or HPLC ICP mass spectrometry depending on the concentration level. When possible, the coatings were also directly analysed
by solid speciation techniques (X-ray photoelectron spectroscopy, XPS, and X-ray absorption near-edge structure, XANES) for
validation of the results. Very good results between the different analytical approaches were obtained for the sample of coating
made up of a heated paint containing Zn, Al and Cr when using the extracting buffer solution at pH 8.9. After a repeated four-step
extraction procedure on the same portion test, taking into account the depth of the surface layer reached, good agreement
with XPS and XANES results was obtained. In contrast, for the coatings composed of an alkaline Zn layer where Cr(VI) and Cr(III)
are deposited, only the extraction procedure using water allowed the detection of Cr(VI). To elucidate the Cr(VI) reduction
during extraction at pH 8.9, the reactivity of Cr(VI) towards different species of Zn generally present in the coatings (metallic
Zn and zinc oxide) was studied. The results showed that metallic Zn rapidly reduces Cr(VI), whereas this reaction is less
evident in the presence of zinc oxide. Water was then retained for coatings containing metallic Zn. 相似文献
998.
This study investigated the transport of bacteria through goethite-coated sand, focusing on the effects of solution pH and coated sand content on the transport of Escherichia coli ATCC 11105. The first set of column experiments was performed in columns (length 30 cm, diameter 5 cm) packed with quartz sand coated with goethite in solution having a pH in the range of 6–9. The second was carried out in columns (length 30 cm, diameter 2.5 cm) with varying coated sand contents ranging from 0 to 100%. Results indicate that the bacteria transport in the coated sand was influenced by solution pH. Around pH 6 and 7, bacterial mass recoveries were low at 2.4–6.7% while they were high at 76.3–81.6% around pH 8 and 9. Around pH 8, the positively charged coated sand may convert to being negatively charged, causing an electrostatically repulsive interaction between the coated sand and bacteria, thus effecting a sharp change in the mass recovery. Results also reveal that the mass recovery decreased from 76.7 to 2.7% as the coated sand content increased from 0 to 100%, showing the nonlinear dependency of mass recovery on the content of coated sand. This study demonstrates the importance of the solution pH and coated sand content in the adhesion of bacteria to goethite-coated sand and furthermore contributes to the knowledge of bacterial removal in positively charged porous media. 相似文献
999.
A new Pb(II) one-dimensional coordination polymer {[Pb(PAA)2]n (1), PAA− = phenylacetate} was synthesized by the reaction of Pb(CH3COO)2 · 3H2O and ligand phenyl acetic acid. Compound 1 was structurally characterized by single-crystal X-ray diffraction. The crystal structure of this compound consists of one-dimensional polymeric units of [Pb(PAA)2] and the coordination number of PbII ions is six. The lead atoms have irregular coordination sphere containing stereo-chemically active lone pair and tetra-hapto (η4) interactions, thus attaining a total hapticity of 10 with environment C4O6Pb. The thermal stability of compound 1 was studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The results of studies of the stoichiometry and formation of complex 1 in methanol, ethanol and acetonitrile solutions were found to be in support of their solid state stoichiometry. 相似文献
1000.
The protonation equilibria for some phenolic acids in nonaqueous solutions have been studied by pH-potentiometry. The dissociation
constants, pK
a, of these phenolic acids and the thermodynamic functions, ΔG
o,ΔH
o and ΔS
o, for the successive and overall protonation processes of these phenolic acids have been derived at different temperatures
in three different mixtures of water and dioxane (mole fractions of dioxane were 0.083, 0.174 and 0.33). Titrations were also
carried out in (water + dioxane) with ionic strengths of 0.15, 0.20 and 0.25 mol⋅dm−3 NaNO3, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent,
dioxane, temperature and ionic strength on the protonation processes of phenolic acids is presented and discussed to determine
the factors which control these processes.
Ahmed E. Fazary; previous address: Egyptian Organization for Biological Products and Vaccines, 51 Wezaret El-Zeraa Street,
Agouza, Giza, Egypt. Tel. +2010-3017357. 相似文献