全文获取类型
收费全文 | 87550篇 |
免费 | 2128篇 |
国内免费 | 1920篇 |
专业分类
化学 | 31761篇 |
晶体学 | 922篇 |
力学 | 7311篇 |
综合类 | 95篇 |
数学 | 33139篇 |
物理学 | 18370篇 |
出版年
2023年 | 166篇 |
2022年 | 315篇 |
2021年 | 360篇 |
2020年 | 379篇 |
2019年 | 447篇 |
2018年 | 10708篇 |
2017年 | 10511篇 |
2016年 | 6535篇 |
2015年 | 1431篇 |
2014年 | 910篇 |
2013年 | 1077篇 |
2012年 | 4639篇 |
2011年 | 11312篇 |
2010年 | 6306篇 |
2009年 | 6670篇 |
2008年 | 7253篇 |
2007年 | 9274篇 |
2006年 | 715篇 |
2005年 | 1766篇 |
2004年 | 1940篇 |
2003年 | 2351篇 |
2002年 | 1505篇 |
2001年 | 662篇 |
2000年 | 548篇 |
1999年 | 370篇 |
1998年 | 376篇 |
1997年 | 275篇 |
1996年 | 309篇 |
1995年 | 221篇 |
1994年 | 159篇 |
1993年 | 159篇 |
1992年 | 107篇 |
1991年 | 122篇 |
1990年 | 86篇 |
1989年 | 87篇 |
1988年 | 81篇 |
1987年 | 74篇 |
1986年 | 69篇 |
1985年 | 54篇 |
1984年 | 56篇 |
1983年 | 47篇 |
1982年 | 45篇 |
1981年 | 51篇 |
1980年 | 52篇 |
1979年 | 51篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
The enantioselective O-allylic alkylation of acetophenone oxime with various Morita-Baylis-Hillman (MBH) carbonates has been accomplished by the catalysis of a commercially available cinchona alkaloid (DHQD)2PHAL. The corresponding O-allylic products were obtained in moderate to excellent yields up to 96% ee. 相似文献
922.
Dynamics of ice nucleation on water repellent surfaces 总被引:3,自引:0,他引:3
Alizadeh A Yamada M Li R Shang W Otta S Zhong S Ge L Dhinojwala A Conway KR Bahadur V Vinciquerra AJ Stephens B Blohm ML 《Langmuir : the ACS journal of surfaces and colloids》2012,28(6):3180-3186
Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications. 相似文献
923.
Qiong Liu Peilong Yang Huiying Luo Pengjun Shi Huoqing Huang Kun Meng Bin Yao 《Applied biochemistry and biotechnology》2012,166(6):1442-1453
An endo-β-1,4-mannanase encoding gene, man5, was cloned from Bispora antennata CBS 126.38, which was isolated from a beech stump. The cDNA of man5 consists of 1,299 base pairs and encodes a 432-amino-acid protein with a theoretical molecular mass of 46.6 kDa. Deduced
MAN5 exhibited the highest amino acid sequence identity of 58% to a β-mannanase of glycoside hydrolase family 5 from Aspergillus aculeatus. Recombinant MAN5 was expressed in Pichia pastoris and purified to electrophoretic homogeneity. The specific activity of the final preparation towards locust bean gum was 289
U mg−1. MAN5 showed optimal activity at pH 6.0 and 70 °C and had good adaptation and stability over a broad range of pH values.
The enzyme showed more than 60% of peak activity at pH 3.0–8.0 and retained more than 80% of activity after incubation at
37 °C for 1 h in both acid and alkaline conditions (pH 4.0–11.0). The K
m and V
max values were 1.33 mg ml−1 and 444 μmol min−1 mg−1 and 1.17 mg ml−1 and 196 μmol min−1 mg−1 for locust bean gum and konjac flour, respectively. Of all tested metal ions and chemical reagents, Co2+, Ni2+, and β-mercaptoethanol enhanced the enzyme activity at 1 mM, whereas other chemicals had no effect on or partially inhibited
the enzyme activity. MAN5 was highly resistant to acidic and neutral proteases (trypsin, α-chymotrypsin, collagenase, subtilisin
A, and proteinase K). By virtue of the favorable properties of MAN5, it is possible to apply this enzyme in the paper and
food industries. 相似文献
924.
U. Ceren Başköse Sevgi Haman Bayarı Semran Sağlam Hacı Özışık 《Central European Journal of Chemistry》2012,10(2):395-406
The conformational analysis of rasagiline [N-propargyl-1(R)-aminoindan] was performed by the density functional theory (DFT) B3LYP method using the 6–31++G (d,p) basis set. A single
point energy calculations based on the B3LYP optimized geometries were also performed at MP2/6-31++G (d, p) level. The vibrational
frequencies of the most stable conformer of rasagiline was calculated at the B3LYP level and vibrational assignments were
made for normal modes on the basis of scaled quantum mechanical force field (SQM) method. The influence of mesylate and ethanedisulfonate
salts on the geometry of rasagiline free base and its normal modes are also discussed.
相似文献
925.
J. Steven Morris Vickie L. Spate Stacy B. Crane Alejandra Gudino 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(2):409-414
Scalp hair is routinely used to assess exposure to toxic trace elements and nutritional status of some required trace elements.
The advantages and disadvantages of hair as a biologic monitor have been comprehensively discussed in the literature for many
years. Among the concerns is distinguishing between exogenous and endogenous contributions. Nested in this issue is the longitudinal
distribution of a trace element along the hair strand. The typical observation for many elements of interest is that the element
concentration increases from the root end to the distal end; and this is attributed to continuing contamination from exogenous
sources. In this study we used neutron activation analysis to measure 14 trace elements in 6 mm segments of full-length scalp
hair from three healthy members of the same household having light-urban environmental exposure. To extend the data set for
selenium, we included three adult female subjects with longer than average scalp hair. From these trace-element concentrations
we calculated the root-to-distal end ratios as a profile diagnostic of trace-element distributions. Ratios fall into three
diagnostic categories, >1, ≈1, and <1 corresponding to profiles having decreasing root-to-distal concentrations, unchanging
concentrations, and increasing concentrations, respectively. Of the 14 elements measured, only Se has R > 1, Zn and S have R ≈ 1, and the remaining 11 elements all have R < 1 in the order: As > I > Hg ≈ Au ≈ Mg ≈ Mn ≈ Sb ≈ Ca > Cu > Al ≈ Ag. R
Se is greater than 1 and increases with hair length (P = 0.02) corresponding to a continuous longitudinal loss of Se in stark and puzzling contrast to the other elements measured.
An analogous loss of Se in the nail monitor was not observed leading us to conclude that the nail is less prone to misclassification
of selenium status in epidemiological studies. 相似文献
926.
青黛中微量元素含量测定分析 总被引:6,自引:0,他引:6
用原子吸收分光光度法测定了青黛中的7种微量元素。结果表明,青黛中含有丰富的人体必需的微量元素和极微量的有害元素镉和铅。 相似文献
927.
E. I. Klimova E. A. Vazquez Lopez T. Klimova M. Martinez Garcia Ortega S. Hernandez L. Ruiz Ramirez 《Russian Journal of General Chemistry》2004,74(11):1757-1762
Ferrocenyltetrahydropyrimidin-2-ones were prepared by reactions of linear and cyclic ,-unsaturated ketones of the ferrocene series with urea in i-PrOH in the presence of t-BuOK. The structures of the compounds prepared were studied by 1H and 13C NMR and IR spectroscopy, and also by single crystal X-ray diffraction.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 11, 2004, pp. 1887–1893.Original Russian Text Copyright © 2004 by E. Klimova, Lorez, T. Klimova, Garcia, Hernandez, Ramirez.This revised version was published online in April 2005 with a corrected cover date. 相似文献
928.
Ielpo P Cassano D Lopez A Pappagallo G Uricchio VF De Napoli PA 《Chemistry Central journal》2012,6(Z2):S5
Background
Ground waters are an important resource of water supply for human health and activities. Groundwater uses and applications are often related to its composition, which is increasingly influenced by human activities.In fact the water quality of groundwater is affected by many factors including precipitation, surface runoff, groundwater flow, and the characteristics of the catchment area. During the years 2004-2007 the Agricultural and Food Authority of Apulia Region has implemented the project “Expansion of regional agro-meteorological network” in order to assess, monitor and manage of regional groundwater quality. The total wells monitored during this activity amounted to 473, and the water samples analyzed were 1021. This resulted in a huge and complex data matrix comprised of a large number of physical-chemical parameters, which are often difficult to interpret and draw meaningful conclusions. The application of different multivariate statistical techniques such as Cluster Analysis (CA), Principal Component Analysis (PCA), Absolute Principal Component Scores (APCS) for interpretation of the complex databases offers a better understanding of water quality in the study region.Results
Form results obtained by Principal Component and Cluster Analysis applied to data set of Foggia province it’s evident that some sampling sites investigated show dissimilarities, mostly due to the location of the site, the land use and management techniques and groundwater overuse. By APCS method it’s been possible to identify three pollutant sources: Agricultural pollution 1 due to fertilizer applications, Agricultural pollution 2 due to microelements for agriculture and groundwater overuse and a third source that can be identified as soil run off and rock tracer mining.Conclusions
Multivariate statistical methods represent a valid tool to understand complex nature of groundwater quality issues, determine priorities in the use of ground waters as irrigation water and suggest interactions between land use and irrigation water quality.929.
Development of three end-capped para-benzoyl calix[4,6, or 8]arene bonded stationary phases for HPLC
Hu K Yu A Zhang J Wen F Liang S Zhao X Ye B Wu Y Zhang S 《Journal of chromatographic science》2012,50(2):123-130
Three end-capped para-benzoyl calixarene bonded silica gel stationary phases are prepared and characterized by elemental analysis, infrared spectroscopy, and thermal analysis. The comparison and selectivity of these phases are investigated by using PAHs, disubstituted benezene, and naphthalene positional isomers as probes. Possible separation mechanism based on the different interactions between calixarenes and analytes are discussed. The results indicate that the separation for those analytes are influenced by the supramolecular interaction including π-π interaction, π-electron transfer interactions, space steric hindrance, and hydrogen bonding interaction on the calixarene columns. Importantly, the aromatic probes with polar groups such as -OH, -NO(2), and -NH(2) could regulate the selectivity of calixarene-bonded stationary phases. 相似文献
930.
We herein report comparative analysis of two versions of the biosynthetic gene clusters of antimycins, a natural product family possessing up to 44 distinct entities. The biosynthetic pathway of antimycins is amenable to the high structural variation of the substrates, supported by successes in heterologous expression of the ant cluster and in fluorine incorporation. The latter facilitated the investigation of the structure-activity relationship into the usually invariable 3-formamidosalicylic acid moiety of the molecules. 相似文献