全文获取类型
收费全文 | 5696篇 |
免费 | 107篇 |
国内免费 | 29篇 |
专业分类
化学 | 3499篇 |
晶体学 | 15篇 |
力学 | 169篇 |
数学 | 1293篇 |
物理学 | 856篇 |
出版年
2020年 | 58篇 |
2019年 | 40篇 |
2016年 | 82篇 |
2015年 | 70篇 |
2014年 | 71篇 |
2013年 | 203篇 |
2012年 | 176篇 |
2011年 | 191篇 |
2010年 | 131篇 |
2009年 | 131篇 |
2008年 | 184篇 |
2007年 | 210篇 |
2006年 | 172篇 |
2005年 | 190篇 |
2004年 | 154篇 |
2003年 | 127篇 |
2002年 | 112篇 |
2001年 | 45篇 |
2000年 | 55篇 |
1999年 | 51篇 |
1998年 | 45篇 |
1997年 | 66篇 |
1996年 | 65篇 |
1995年 | 78篇 |
1994年 | 80篇 |
1993年 | 73篇 |
1992年 | 86篇 |
1991年 | 57篇 |
1990年 | 63篇 |
1989年 | 73篇 |
1988年 | 82篇 |
1987年 | 71篇 |
1986年 | 84篇 |
1985年 | 120篇 |
1984年 | 116篇 |
1983年 | 84篇 |
1982年 | 120篇 |
1981年 | 121篇 |
1980年 | 110篇 |
1979年 | 120篇 |
1978年 | 106篇 |
1977年 | 107篇 |
1976年 | 96篇 |
1975年 | 112篇 |
1974年 | 81篇 |
1973年 | 90篇 |
1972年 | 55篇 |
1971年 | 63篇 |
1970年 | 65篇 |
1966年 | 47篇 |
排序方式: 共有5832条查询结果,搜索用时 31 毫秒
1.
Andreas Hegetschweiler Aljosha-Rakim Jochem Anna Zimmermann Johannes Walter Thorsten Staudt Tobias Kraus 《Particle & Particle Systems Characterization》2021,38(7):2000236
Different colloidal particle characterization methods are examined for their suitability to determine the particle size distribution of particles extracted from steels. Microalloyed steels are dissolved to extract niobium and titanium carbonitride particles that are important for the mechanical properties of these steels. Such particles have sizes ranging from several nanometers to hundreds of nanometers depending on the precipitation stage during the thermomechanically controlled rolling process. The size distribution of the particles is analyzed by dynamic light scattering (DLS), analytical ultracentrifugation (AUC), and hollow fiber flow field-flow fractionation (HF5) and compared to data obtained for reference particles as well as data from electron microscopy, the standard sizing technique used in metallurgy today. AUC and HF5 provide high-quality size distributions, average over large particle numbers that enables statistical analysis, and yield useful insights for alloy design; however, DLS fails due to a lack of resolution. Important aspects in the conversion and comparison of size distributions obtained for broadly distributed particle systems with different measurement principles and the role of surfactants used in sample preparation are discussed. 相似文献
2.
Thomas H. Walter Bonnie A. Alden Kenneth Berthelette Jessica A. Field Nicole L. Lawrence Justin McLaughlin Amit V. Patel 《Journal of separation science》2022,45(8):1389-1399
We have characterized a sulfobetaine stationary phase based on 1.7 μm ethylene-bridged hybrid organic–inorganic particles, which is intended for use in hydrophilic interaction chromatography. The efficiency of a column packed with this material was determined as a function of flow rate, demonstrating a minimum reduced plate height of 2.4. The batch-to-batch reproducibility was assessed using the separation of a mixture of acids, bases, and neutrals. We compared the retention and selectivity of the hybrid sulfobetaine stationary phase to that of several benchmark materials. The hybrid sulfobetaine material gave strong retention for polar neutrals and high selectivity for methyl groups, hydroxy groups, and configurational isomers. Large differences in cation and anion retention were observed among the columns. We characterized the acid and base stability of the hybrid sulfobetaine stationary phase, using accelerated tests at pH 1.3 and 11.0, both at 70°C. The results support a recommended pH range of 2–10. We also investigated the performance of columns packed with this material for metal-sensitive analytes, comparing conventional stainless steel column hardware to hardware that incorporates hybrid surface technology to mitigate interactions with metal surfaces. Compared to the conventional columns, the hybrid surface technology columns showed a greatly improved peak shape. 相似文献
3.
Fortunati Alessandro Bacigalupo Andrea Lepidi Marco Arena Andrea Lacarbonara Walter 《Nonlinear dynamics》2022,108(2):765-787
Nonlinear Dynamics - The cellular microstructure of periodic architected materials can be enriched by local intracellular mechanisms providing innovative distributed functionalities. Specifically,... 相似文献
4.
Nathalie Guarnizo Diego Oliveros Walter Murillo-Arango María Bianney Bermúdez-Cardona 《Molecules (Basel, Switzerland)》2020,25(24)
Plants have innate immune systems or defense mechanisms that respond to the attack of pathogenic microorganisms. Unlike mammals, they lack mobile defense cells, so defense processes depend on autonomous cellular events with a broad repertoire of recognition to detect pathogens, which compensates for the lack of an adaptive immune system. These defense mechanisms remain inactive or latent until they are activated after exposure or contact with inducing agents, or after the application of the inductor; they remain inactive only until they are affected by a pathogen or challenged by an elicitor from the same. Resistance induction represents a focus of interest, as it promotes the activation of plant defense mechanisms, reducing the use of chemical synthesis pesticides, an alternative that has even led to the generation of new commercial products with high efficiency, stability and lower environmental impact, which increase productivity by reducing not only losses but also increasing plant growth. Considering the above, the objective of this review is to address the issue of resistance induction with a focus on the potential of the use of oligosaccharides in agriculture, how they are recognized by plants, how they can be used for commercial products and perspectives. 相似文献
5.
6.
Unusual Bonding and Properties in Main Group Element Chemistry: Rational Synthesis,Characterization, and Experimental Electron Density Determination of Mixed‐Valent Tetraphosphetes 下载免费PDF全文
Verena Breuers Prof. Dr. Christian W. Lehmann Prof. Dr. Walter Frank 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4596-4606
Five dispirocyclic λ3,λ5‐tetraphosphetes [{R2Si(NR1)(NR2)P2}2] (R1 = R2 and R1 ≠ R2) are easily prepared in almost quantitative yields via photolysis of the respective bis(trimethylsilyl)phosphanyldiazaphosphasiletidines with intense visible light. These deep‐yellow low‐coordinate phosphorus compounds can be considered as the first higher congeners of the well‐known cyclodiphosphazenes. The tetraphosphetes are remarkably stable in air and show unexpected molecular properties related to the unique bonding situation of the central four‐π‐electron four‐membered phosphorus ring. The extent of rhombic distortion of the central P4 ring is remarkable due to an unusually acute angle at the σ2‐phosphorus atoms. All of the P?P bonds are approximately equal in length. The distances are in the middle of the range given by phosphorus single and double bonds. The anisotropic absorption of visible light that can easily be observed in the case of the yellow/colorless dichroic crystals of [{Me2Si(NtBu)(NtBu)P2}2] and the exceptional 31P NMR chemical shift of the σ2‐phosphorus atoms are the most remarkable features of the λ3,λ5‐tetraphosphetes. In the case of [{Me2Si(NtBu)(NtBu)P2}2], the Hansen–Coppens multipole model is applied to extract the electron density from high‐resolution X‐ray diffraction data obtained at 100 K. Static deformation density and topological analysis reveal a unique bonding situation in the central unsaturated P4 fragment characterized by polar σ‐bonding, pronounced out‐of‐ring non‐bonding lone pair density on the σ2‐phosphorus atoms, and an additional non‐classical three‐center back‐bonding contribution. 相似文献
7.
Dr. Christian Schmitz Prof. Walter Leitner Dr. Giancarlo Franciò 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(30):10696-10702
A library of α,α,α,α‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL)‐based phosphoramidites has been synthesized and applied in the Ni‐catalyzed cycloisomerization of different dienes. Through the systematic variation of the three structural motifs of the lead structure, that is, the amine moiety, the protecting group, and the aryl substituents, the ligand features could be optimized for the asymmetric cycloisomerization of the model substrate diethyl diallylmalonate. The substrate scope of the new catalytic system was extended to other diallylic substrates, including unsymmetrical dienes. Overall remarkably high activities of up to approximately 13 500 h?1, very high selectivities toward five‐membered exo‐methylenecyclopentanes, and enantioselectivities of up to 92 % ee have been achieved. 相似文献
8.
Torands Revisited: Metal Sequestration and Self‐Assembly of Cyclo‐2,9‐tris‐1,10‐phenanthroline Hexaaza Macrocycles 下载免费PDF全文
Dr. Matthias Georg Schwab Dr. Masayoshi Takase Dr. Alexey Mavrinsky Dr. Wojciech Pisula Prof. Dr. Xinliang Feng Dr. José A. Gámez Prof. Dr. Walter Thiel Dr. Kunal S. Mali Prof. Dr. Steven de Feyter Prof. Dr. Klaus Müllen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(23):8426-8434
A series of novel toroidal cyclo‐2,9‐tris‐1,10‐phenanthroline macrocycles with an unusual hexaaza cavity are reported. Nickel‐mediated Yamamoto aryl–aryl coupling was found to be a versatile tool for the cyclotrimerization of functionalized 1,10‐phenathroline precursors. Due to the now improved processability, both liquid‐crystalline behavior in the bulk phase and two‐dimensional self‐assembly at the molecular level could be studied, for the first time, for a torand system. The macrocycles exhibited a strong affinity for the complexation of different metal cations, as evidenced by MALDI‐TOF analysis and spectroscopic methods. Experimental results were correlated to an extensive computational study of the cyclo‐2,9‐tris‐1,10‐phenanthroline cavity and its binding mode for metal cations. Due to the combination of several interesting features, toroidal macrocycles may find future applications in the field of ion and charge transport through molecular channels, as well as for chemical sensing and molecular writing in surface‐confined monolayers under STM conditions. 相似文献
9.
Heidi-Kristin Walter Peggy R. Bohländer Prof. Dr. Hans-Achim Wagenknecht 《ChemistryOpen》2015,4(2):92-96
DNA-based aptamers are commonly used recognition elements in biosensors for a range of target molecules. Here, the development of a wavelength-shifting optical module for a DNA-based adenosine-binding aptamer is described. It applies the combination of two photostable cyanine-styryl dyes as covalent modifications. This energy-transfer pair is postsynthetically attached to oligonucleotides via a copper(I)-catalyzed azide–alkyne cycloaddition by two structurally different approaches: 1) as nucleotide modifications at the 2′-position of uridines and 2) as nucleotide substitutions using (S)-amino-1,2-propanediol as acyclic linker between the phosphodiester bridges. Both dyes exhibit a remarkable photostability. A library of DNA aptamers consisting of different combinations of the two dyes in diagonal orientations were evaluated by their emission color contrast as readout. Further optimization led to aptasensors with improved fluorescent readout as compared with previously reported aptasensors. This approach described is synthetically facile using simple propargylated phosphoramidites as DNA building blocks. As such, this approach could be applied for other dyes and other chemical/biological applications. 相似文献
10.