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排序方式: 共有225条查询结果,搜索用时 11 毫秒
91.
92.
Szmigielski R Surratt JD Vermeylen R Szmigielska K Kroll JH Ng NL Murphy SM Sorooshian A Seinfeld JH Claeys M 《Journal of mass spectrometry : JMS》2007,42(1):101-116
In the present work, we have characterized in detail the chemical structures of secondary organic aerosol (SOA) components that were generated in a smog chamber and result from the photooxidation of isoprene under high-NO(x) conditions typical for a polluted atmosphere. Isoprene high-NO(x) SOA contains 2-methylglyceric acid (2-MG) and oligoester derivatives thereof. Trimethylsilylation, in combination with capillary gas chromatography (GC)/ion trap mass spectrometry (MS) and detailed interpretation of the MS data, allowed structural characterization the polar oxygenated compounds present in isoprene SOA up to 2-MG trimers. GC separation was achieved between 2-MG linear and branched dimers or trimers, as well as between the 2-MG linear dimer and isomeric mono-acetate derivatives thereof. The electron ionization (EI) spectra of the trimethylsilyl derivatives contain a wealth of structural information, including information about the molecular weight (MW), oligoester linkages, terminal carboxylic and hydroxymethyl groups, and esterification sites. Only part of this information can be achieved with a soft ionization technique such as electrospray (ESI) in combination with collision-induced dissociation (CID). The methane chemical ionization (CI) data were used to obtain supporting MW information. Interesting EI spectral differences were observed between the trimethylsilyl derivatives of 2-MG linear and branched dimers or trimers and between 2-MG linear dimer mono-acetate isomers. 相似文献
93.
Two series of ordered mesoporous organosilica (OMO) SBA-15 materials with surface and bridging groups were fabricated by varying
the organic precursor addition at different synthesis stages. The consequence of the delayed introduction of organic precursor
on the structural and adsorption properties of the resulting OMOs was investigated. The OMOs studied were synthesized via
co-condensation of tetraethyl orthosilicate (TEOS) and ureidopropyltrimethoxysilane (UPS) as well as TEOS and bis(triethoxysilylpropyl)
disulfide (BTDS) in the presence of poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide) triblock copolymer Pluronic
P123 (EO20PO70EO20). The aforementioned OMOs were characterized by nitrogen adsorption-desorption isotherms at −196 °C and powder X-ray diffraction
(XRD). Nitrogen adsorption isotherms were used to estimate the pore volume, mesopore diameter and the BET specific surface
area, whereas the XRD data provided information about structural ordering and unit cell of the samples studied.
Ryan Felix, undergraduate student from Oberlin College (Oberlin, OH, USA) participating in the NSF-REU program during Summer
2006. 相似文献
94.
A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes 下载免费PDF全文
Dr. Laurence J. Kershaw Cook Dr. Rafal Kulmaczewski Dr. Rufeida Mohammed Stephen Dudley Simon A. Barrett Dr. Marc A. Little Prof. Robert J. Deeth Prof. Malcolm A. Halcrow 《Angewandte Chemie (International ed. in English)》2016,55(13):4327-4331
The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base‐metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2]2+ (bpp=2,6‐di{pyrazol‐1‐yl}pyridine) can be high‐spin, low‐spin, or spin‐crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T ) in solution vs. the relevant Hammett parameter show that the low‐spin state of the complex is stabilized by electron‐withdrawing pyridyl (“X”) substituents, but also by electron‐donating pyrazolyl (“Y”) substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe‐L σ and π bonding. 相似文献
95.
Rafal Kruszynski 《Central European Journal of Chemistry》2008,6(4):542-548
2,4-dinitrophenylhydrazine hydrochloride hydrate (I) was determined by X-ray crystallography, and the intermolecular interaction
energies were calculated in terms of Natural Bond Orbital analysis. The asymmetric unit of (I) consists of a dinitrophenylhydrazinium
cation, a chloride anion and a water molecule. The interatomic distances and angles in (I) show no unusual values. In the
structure there are intermolecular N—H⊎⊎⊎O, N—H⊎⊎⊎Cl, O—H⊎⊎⊎Cl, C—H⊎⊎⊎O hydrogen bonds with bonding energy ranging form 16.03
to 0.76 kcal mol−1. These hydrogen bonds create the following N1 motifs: 6D, S(5), S(6), C(6), C(9). N1D motifs become infinite at the third level and are 2C32(6), C32(7).
相似文献
96.
Miikka Visanko Juho Antti Sirviö Petteri Piltonen Rafal Sliz Henrikki Liimatainen Mirja Illikainen 《Cellulose (London, England)》2017,24(10):4173-4187
In the past, the direct production of lignin-containing nanofibers from wood materials has been very limited, and nanoscale fibers (nanocelluloses) have been mainly isolated from chemically delignified, bleached cellulose pulp. In this study, we have introduced a newly adapted, heat-intensified disc nanogrinding process for the enhanced nanofibrillation of wood nanofibers (WNF) with a high lignin content (27.4 wt%). The WNF produced this way have many unique and intriguing properties in their naturally occurring form, for example, being able to be dispersed in ethanol and having ethanol solution viscosities higher than water solution viscosities. When WNF nanopapers were formed with ethanol, the properties of the nanofibers were recoverable without a notable decrease in the viscosity or mechanical strength after redispersing them in water. The preservation of lignin in the WNF was noticed as an increase in the water contact angles (89°), the rapid removal of water in the fabrication of the nanopapers, and the enhanced strength of the nanopapers when subjected to high pressure and heat. The nanopapers fabricated from the WNF were mechanically stable, having an elastic modulus of 6.2 GPa, a maximum stress of 103.4 MPa, and a maximum strain of 3.5%. Throughout the study, characteristics of the WNF were compared to those of the delignified and bleached reference cellulose nanofibers. We envision that the exciting characteristics of the WNF and their lower cost of production compared to that of bleached cellulose nanofibers may offer new opportunities for nanocellulose and biocomposite research. 相似文献
97.
Campbell CJ Klajn R Fialkowski M Grzybowski BA 《Langmuir : the ACS journal of surfaces and colloids》2005,21(1):418-423
A new experimental technique is described that uses reaction--diffusion phenomena as a means of one-step microfabrication of complex, multilevel surface reliefs. Thin films of dry gelatin doped with potassium hexacyanoferrate are chemically micropatterned with a solution of silver nitrate delivered from an agarose stamp. Precipitation reaction between the two salts causes the surface to deform. The mechanism of surface deformation is shown to involve a sequence of reactions, diffusion, and gel swelling/contraction. This mechanism is established experimentally and provides a basis of a theoretical lattice-gas model that allows prediction surface topographies emerging from arbitrary geometries of the stamped features. The usefulness of the technique is demonstrated by using it to rapidly prepare two types of mold for passive microfluidic mixers. 相似文献
98.
Harrison RJ Kasama T White TA Simpson ET Dunin-Borkowski RE 《Physical review letters》2005,95(26):268501
The ability of certain magnetic minerals to acquire a remanent magnetization that opposes the direction of the Earth's magnetic field has fascinated rock magnetists since its discovery in 1951. Here, we determine the origin of this phenomenon, which is termed self-reversed thermoremanent magnetization (SR-TRM). We present direct transmission electron microscopy observations of negative exchange coupling across antiphase domain boundaries (APBs) in ilmenite-hematite. This coupling is linked intrinsically to the origin of SR-TRM and is responsible for the formation of two new classes of magnetic domain wall at APBs. We present simulations of the chemical and magnetic structure of the APBs and show that SR-TRM is generated by coupling between strongly ferrimagnetic Ti-rich domains and weakly ferrimagnetic Fe-rich domains, which form during the transition from short- to long-range cation order. 相似文献
99.
Rodrigo Bañ uelos Rafal Latala Pedro J. Mé ndez-Herná ndez 《Proceedings of the American Mathematical Society》2001,129(10):2997-3008
We derive an inequality for multiple integrals from which we conclude various generalized isoperimetric inequalities for Brownian motion and symmetric stable processes in convex domains of fixed inradius. Our multiple integral inequality is a replacement for the classical inequality of H. J. Brascamp, E. H. Lieb and J. M. Luttinger, where instead of fixing the volume of the domain one fixes its inradius.
100.
Rafal Klajn 《中国科学:化学(英文版)》2016,59(4):420-421
正When exposed to light,titanium dioxide(titania,TiO_2)can act as a powerful catalyst for degrading a variety of organic compounds.Anatase—one of the polymorphs of TiO_2—has a band gap of 3.2 e V;exposing it to ultraviolet light causes the formation of electron-hole pairs,which,in the presence of water,can generate hydroxyl radicals—strong oxidizing agents.The photocatalytic behavior of titania has enabled 相似文献