全文获取类型
收费全文 | 10530篇 |
免费 | 166篇 |
国内免费 | 38篇 |
专业分类
化学 | 6051篇 |
晶体学 | 100篇 |
力学 | 362篇 |
数学 | 1706篇 |
物理学 | 2515篇 |
出版年
2022年 | 105篇 |
2021年 | 130篇 |
2020年 | 138篇 |
2019年 | 138篇 |
2018年 | 124篇 |
2017年 | 111篇 |
2016年 | 193篇 |
2015年 | 166篇 |
2014年 | 183篇 |
2013年 | 592篇 |
2012年 | 375篇 |
2011年 | 475篇 |
2010年 | 341篇 |
2009年 | 281篇 |
2008年 | 429篇 |
2007年 | 386篇 |
2006年 | 374篇 |
2005年 | 333篇 |
2004年 | 342篇 |
2003年 | 277篇 |
2002年 | 280篇 |
2001年 | 192篇 |
2000年 | 182篇 |
1999年 | 148篇 |
1998年 | 118篇 |
1997年 | 135篇 |
1996年 | 169篇 |
1995年 | 121篇 |
1994年 | 123篇 |
1993年 | 155篇 |
1992年 | 156篇 |
1991年 | 160篇 |
1990年 | 117篇 |
1989年 | 119篇 |
1988年 | 131篇 |
1987年 | 182篇 |
1986年 | 148篇 |
1985年 | 205篇 |
1984年 | 188篇 |
1983年 | 149篇 |
1982年 | 171篇 |
1981年 | 181篇 |
1980年 | 171篇 |
1979年 | 171篇 |
1978年 | 167篇 |
1977年 | 163篇 |
1976年 | 145篇 |
1975年 | 161篇 |
1974年 | 128篇 |
1973年 | 125篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
D. Van Dyck 《Mikrochimica acta》2002,138(3-4):153-180
With the resolution becoming sufficient to reveal individual atoms, HREM is now entering the stage where it can compete with
X-ray methods to quantitatively determine atomic structures of materials without much prior knowledge, but with the advantage
of being applicable to aperiodic objects such as crystal defects. In our view the future electron microscope will be characterised
by a large versatility in experimental settings under computer control such as the illumination conditions (TEM-STEM), CBED,
detecting conditions (diffraction, image, ptychography) and many other tunable parameters such as focus (g), voltage, spherical aberration (C
s
), beam tilt, etc. Since modern detectors can detect single electrons, also the counting statistics is known. The only limiting
factor in the experiment will be the total number of electrons that interact with the object during the experiment due to
the limitations in the exposure time or in the object damage. However, instrumental potentialities will never be exploited
fully if not guided by an experimental strategy. Here intuitive guidelines can be very deceptive. For instance an image made
with the best electron microscope (C
s
= 0) at the best focus (g = 0) from the best object (phase object) would show no contrast at all. Hence, questions such as what is the best C
s
, focus, object thickness, etc. can only be answered properly if done using a method of experiment design. 相似文献
982.
Series of dipeptides, including homodipeptides and alanyl dipeptides, were separated using quadruplex (G-quartet) DNA stationary phases in open-tubular capillary electrochromatography (OTCEC). The stationary phases were constructed by covalently attaching the DNA oligonucleotides to the inner capillary surface. Three different G-quartet forming oligonucleotides were investigated: the two-plane G-quartet forming thrombin-binding aptamer, the four-plane analogue of the thrombin-binding aptamer, and a two-plane oligonucleotide identical to the thrombin-binding aptamer except for the replacement of the guanine by thymine in the central bridging loop of the G-quartet structure. Results were compared with results obtained using capillary electrophoresis on a bare capillary and OTCEC using an oligonucleotide with the same base composition as the thrombin-binding aptamer but in a different sequence that does not allow G-quartet formation as the stationary phase. 相似文献
983.
Selection of reversed-phase liquid chromatographic columns with diverse selectivity towards the potential separation of impurities in drugs 总被引:1,自引:0,他引:1
Van Gyseghem E Jimidar M Sneyers R Redlich D Verhoeven E Massart DL Vander Heyden Y 《Journal of chromatography. A》2004,1042(1-2):69-80
To select appropriate stationary phases from the continuously expanding supply of potentially suitable HPLC columns, the properties of 28 frequently applied stationary phases were determined by measuring several chromatographic parameters. From these results, based on chromatographic expertise, eight stationary phases with different properties and selectivities were selected. The aim of this study is to apply chemometric tools to evaluate the initially selected set of columns, i.e. a more systematic approach for making such a selection is examined. Starting from the information obtained on the 28 stationary phases, the re-evaluation was performed independently based on the chemometric techniques Pareto-optimality, principal component analysis (PCA), and Derringer's desirability functions. The aim was to select a set of efficient columns exhibiting large selectivity differences. The chemometrically selected stationary phases were divided in groups based on hydrophobicity, a critical retention-determining property in reversed-phase chromatography. This allowed to further reducing the selection to three columns. It is demonstrated that the selection by the chemometric approaches in general is fairly comparable with the initial selection. 相似文献
984.
Ping Li Fei Rong Yibing Xie Van Hu Chunwei Yuan 《Journal of Analytical Chemistry》2004,59(10):939-944
S-naproxen imprinted polymer was prepared with acrylamide as a functional monomer, 1,4-butanediyl diacrylate as a new kind of crossliner, and 2,4,6-trimethylbenzoylphenyl-phosphinic acid ethyl ester as photoinitiator. Their UV and IR spectra were applied to study the interactions between the template and the monomer. Binding experiments showed that the S-naproxen imprinted polymer had better recognition capability for the template than that of a blank polymer. The adsorption distribution coefficient KD of S-naproxen on molecularly-imprinted polymer was 153.2 g/mL, and the separation factor was 1.95 when the initial concentration of substrate was 2.0 mmol/L. Scatchard curves suggested that there were two classes of binding sites in the imprinted polymer and only one class in the blank polymer. Computer simulation using Hyperchem showed the existence of the interactions between the template and the functional monomer as well as the models of the complexes formed by the template and the monomer. 相似文献
985.
Carbonnelle S Loco JV Overmeire IV Windal I Wouwe NV Leeuwen SV Goeyens L 《Talanta》2004,63(5):1255-1259
Differences between chemical activated luciferase gene expression (CALUX) bioassay and chemoanalyses results are observed.This paper shows that calculations of the TEQ values using REP values instead of WHO TEF values give different results. The REP values do affect the results obtained by the CALUX technique. These differences are more marked for the dioxin like PCB compounds (CALUX TEQ values are lower than WHO TEQ values) than for the dioxin compounds (CALUX TEQ values are higher than WHO TEQ values).The CALUX results were compared with the concentrations of the congeners’ spiked into the oil. 相似文献
986.
Synthesis of enantiomeric-pure cyclohexenyl nucleoside building blocks for oligonucleotide synthesis
Lipases were used for the resolution of (±) (4aR, 7R, 8aS)-2-phenyl-4a,7,8,8a-tetrahydro-4H-1,3-benzodioxine. This separation was carried out on preparative scale and used for the synthesis of eight phosphoramidites of cyclohexenyl nucleosides (d- and l-series). 相似文献
987.
Liquid-liquid cloud point diagrams of solutions of nearly monodisperse samples of polystyrene (PS), and binary mixtures of nearly monodisperse PS’s, both in methylcyclohexane (MCH), were determined for several polymer molecular weights (Mw) at 0.1 MPa. The bimodal mixtures (PS[Mw(1),ρ(1)] + PS[Mw(2),ρ(2)], Mw(1)=90×103 g/mol, Mw(2)=13×103 g/mol, 5.78 × 103 g/mol, and 2.2 × 103 g/mol, ρ=1.06) were prepared constraining 〈Mw〉=38.6×103 g/mol, ρ=Mw/Mn is the polydispersity index. In each case the cloud point curves (CPC’s) for the bimodal mixtures are strongly skewed, lying well above CPC for 〈Mw〉 when φ<φCRITICAL, and below CPC for 〈Mw〉 when φ>φCRITICAL; φ is volume fraction polymer in the polymer/solvent mixture. The experimental results are discussed in the context of empirical and mean-field representations. 相似文献
988.
This review covers our extensive research activities in the area of fluorine containing phospha- and arsaalkenes as well as selenocarbonyls, which differ considerably in their properties and reactivities from their alkyl and aryl counterparts and thus contribute in a gratifying manner to the still growing field of unsaturated element-carbon compounds of 3rd and 4th row main group elements E. Of particular interest is the influence of the fluorine substituents and other small groups (OR, NR2) with either inductive and/or mesomeric effects on the polarity and reactivity of the EC bond. Addition reactions of proton acidic and hydridic polar HX reactants as well as [2+2], [3+2] and [4+2] cycloadditions have been thoroughly studied. The results obtained allow a classification of the EC systems within five different types, A to E, and prove a change from “normal” to “inverse” heteroalkenes in this sequence. The ligand properties of some derivatives have also been investigated in some detail. 相似文献
989.
Ricardo H. M. Godoi Ana F. L. Godoi Anna Worobiec Sandro J. Andrade Johan de Hoog Mary R. Santiago-Silva Rene Van Grieken 《Mikrochimica acta》2004,145(1-4):53-56
Biomass burning is an important primary and secondary source of aerosol particles. The presence of carbonaceous particles in the respirable size range makes the study of this fraction important in view of possible health and climatic effects. The annual burning of sugar cane plantations causes emission of huge amounts of pyrogenic particles. Aerosol samples were collected in Araraquara city, São Paulo state, Brazil, during the harvest season for fine and coarse particles and bulk; they were analysed by electron-probe microanalysis, including facilities for low-Z element determination (low-Z EPMA) and by energy-dispersive X-ray fluorescence (EDXRF), in order to investigate the elemental composition of individual particles and bulk samples, respectively. Numerical analysis of the EPMA results by hierarchical clustering shows high contributions of carbonaceous particles that can be distinguished mainly in two different types: biogenic and carbon-rich. Additionally, two significant contributions of aluminosilicate particles were identified: as rather pure aluminosilicates or mixed with carbonaceous species. The EDXRF results are compatible with those of aerosol particles in Amazon, which is nowadays one of the main sources of biogenic particles in the world. 相似文献
990.
Stefan?DiethelmEmail author Joseph?Sfeir Frank?Clemens Jan?Van herle Daniel?Favrat 《Journal of Solid State Electrochemistry》2004,8(9):611-617
Dense planar and tubular oxygen separation membranes of La0.6Ca0.4Fe0.75Co0.25O3– were investigated as reactors for the partial oxidation (POX) of methane to syngas. Their permeation properties were measured in an air/argon
pO2 gradient as a function of temperature. At 900 °C, the oxygen flux through a 1.26-mm-thick membrane was 0.075 mol/cm2·s and through a 0.25-mm-thick tube, 0.24 mol/cm2·s.For the POX measurements, a catalyst was added to the membrane and methane was introduced on the argon side. This resulted in a gradual increase of the oxygen flux with increasing concentration of methane, reaching 2 mol/cm2·s at 900 °C with pure methane. For the planar reactor, the CO selectivity reached 99% and the CH4 conversion 75% at 918 °C with pure methane. For the tubular reactor, the CO selectivity and CH4 conversion were 83 and 99%, respectively, under the same conditions. After 1,400 h of operation in a tubular POX reactor, the membrane was examined revealing phase demixing and local decomposition.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003 相似文献