全文获取类型
收费全文 | 4157篇 |
免费 | 78篇 |
国内免费 | 17篇 |
专业分类
化学 | 2476篇 |
晶体学 | 27篇 |
力学 | 121篇 |
数学 | 762篇 |
物理学 | 866篇 |
出版年
2021年 | 39篇 |
2020年 | 36篇 |
2019年 | 41篇 |
2017年 | 26篇 |
2016年 | 63篇 |
2015年 | 67篇 |
2014年 | 67篇 |
2013年 | 156篇 |
2012年 | 141篇 |
2011年 | 172篇 |
2010年 | 104篇 |
2009年 | 89篇 |
2008年 | 170篇 |
2007年 | 133篇 |
2006年 | 127篇 |
2005年 | 114篇 |
2004年 | 89篇 |
2003年 | 86篇 |
2002年 | 108篇 |
2001年 | 72篇 |
2000年 | 80篇 |
1999年 | 77篇 |
1998年 | 57篇 |
1997年 | 68篇 |
1996年 | 54篇 |
1995年 | 67篇 |
1994年 | 57篇 |
1993年 | 55篇 |
1992年 | 63篇 |
1991年 | 39篇 |
1990年 | 45篇 |
1989年 | 38篇 |
1988年 | 54篇 |
1987年 | 50篇 |
1986年 | 52篇 |
1985年 | 74篇 |
1984年 | 51篇 |
1983年 | 45篇 |
1982年 | 56篇 |
1981年 | 52篇 |
1980年 | 70篇 |
1979年 | 52篇 |
1978年 | 43篇 |
1977年 | 60篇 |
1976年 | 47篇 |
1975年 | 42篇 |
1974年 | 42篇 |
1973年 | 29篇 |
1971年 | 29篇 |
1933年 | 24篇 |
排序方式: 共有4252条查询结果,搜索用时 31 毫秒
61.
Urška L. Štangar Urh Černigoj Polonca Trebše Ksenija Maver Silvia Gross 《Monatshefte für Chemie / Chemical Monthly》2006,137(5):647-655
Summary. Transparent TiO2 films with a high photodegradation activity towards an azo dye in aqueous solution were prepared by sol–gel processing. Films
on soda–lime glass supports protected with a thin silica barrier layer exhibited better crystallization and monodisperse nanoparticles,
higher absorption of light below 370 nm, and higher photocatalytic activity than those films deposited on bare glass supports
proving the detrimental effect of interdiffused sodium ions on the development of the anatase nanostructure. The effect of
substrate was more pronounced in thinner films (300 nm) than in thicker ones (1200 nm), which were achieved by adding a template
(i.e. Pluronic F127) to the sol. 相似文献
62.
Friedrich Scholl 《Fresenius' Journal of Analytical Chemistry》1969,245(1-2):49-54
Zusammenfassung Für das zur Herstellung von Bleibatterien verwendete Grauoxid (bestehend aus einem feinteiligen Gemisch von ca. 60 bis 90% Bleioxid, PbO, und einem Rest metallischem Blei) wurde ein Verfahren zur kontinuierlichen bzw. quasikontinuierlichen, automatischen Bestimmung des PbO-Gehaltes an den Fertigungsanlagen benötigt. Eine schnelle Analyse mit vergleichsweise geringem Aufwand war durch Messung der Lösungswärme bei Behandlung einer abgewogenen Menge Grauoxid mit verd. Essigsäure möglich. Unter technisch bequem realisierbaren Bedingungen wurden Temperaturerhöhungen von einigen Grad erhalten. Der Einfluß verschiedener Arbeitsbedingungen, der verschiedenen PbO-Modifikationen und der Teilchengröße und -form wurde untersucht. Zur Zeit wird eine für die Fertigung bestimmte, automatische Apparatur aufgebaut, die bei einer Analysenfolge von 1 min die Bestimmung mit einer Standardabweichung unter ±1% durchzuführen gestattet.
Automatic procedure for the determination of the lead oxide content of gray oxides by measurement of the heat of solution
A continuous or semicontinuous automatic method for the determination of the lead oxide content was needed in the manufacturing process of gray oxides (a fine particle mixture of about 60 to 90% lead oxide, PbO, and metallic lead, used in manufacturing lead batteries).A rapid determination with comparatively small effort was possible by measuring the heat of solution of a weighed amount of gray oxides treated with acetic acid. Temperature rise of a few degrees could be reached under technically realisable conditions. The influence of different working conditions, of the particle size and shape and of the PbO modifications were investigated. An automatic apparatus for the determination is at present being built and will give one determination per minute with a standard deviation of less than ±1%.相似文献
63.
Viroids: a class of subviral pathogens 总被引:13,自引:0,他引:13
64.
Ronald L. Cerny Kenneth B. Tomer Michael L. Gross 《Journal of mass spectrometry : JMS》1986,21(10):655-660
Combining desorption ionization with tandem mass spectrometry overcomes the disadvantage of limited fragmentation accompanying desorption and permits mixtures of closely related substances to be investigated directly. These features of the combination are illustrated by completing the structure-proof of a minor component of an ornithine-containing lipid mixture isolated from Thiobacillus thiooxidans. The minor component is a homolog of the major constituent and differs from the principal component owing to the presence of a double bond in lieu of a cyclopropyl ring in one of the constituent fatty acids. Another feature of the combined method is the potentially complementary nature of collision-activated dissociation spectra of protonated and cationized biomolecules. This is illustrated by the differences in the collision-activated dissociations of the [M + Na]+ of sucrose, desorbed by field desorption, and [M + H]+, desorbed by fast atom bombardment. A third illustration is the application of field desorption and tandem mass spectrometry to an organometallic compound. The combined approach allows the ligands to be identified and the relative ligand binding energies to be approximated. 相似文献
65.
M. Friedrich K. Hiller M. Wiemer T. Geßner D. R. T. Zahn 《Analytical and bioanalytical chemistry》1998,361(6-7):558-559
Basic investigations have been carried out on the characterization of different processing steps in bond preparation using
etched <111> faces of silicon wafers for the incidence of the infrared beam to a multiple internal reflection geometry. The
method is very sensitive to the surface coverage and interface. Surface activation by RCA cleaning yields an increase of water
coverage and a decrease of SiH and CH groups. The detection limit for an oxide layer between silicon wafers has been found
to be about 3 nm.
Received: 18 July 1997 / Revised: 15 August 1997 / Accepted: 19 August 1997 相似文献
66.
Schultz György Hargittai István Rot Nicolette Bickelhaupt Friedrich 《Structural chemistry》1998,9(3):209-214
The molecular structure of 1,3,5-tris (trimethylstannyl) benzene has been determined by gas-phase electron diffraction. The C — C bond length is in good agreement with that in benzene. In agreement with the somewhat electron-releasing character of the substituents, the endocyclic bond angles at the substituents are somewhat smaller than 120°. The mean value of Sn — C bond lengths is greater than that in tetraphenyltin and tetramethyltin. The SnMe3 groups appear freely rotating around the Caryl — Sn bonds. The following bond lengths (r
g) and bond angles were determined: (Sn — C)mean 2.150 ± 0.007 Å, C — C 1.399 ± 0.005 Å, (C — H)mean 1.105 ± 0.006 Å, < C — C(Sn) — C 117.7 ± 1.7º, < Caryl — Sn — Cmethyl 106.7 ± 0.7º < Sn — C — H 111.5 ± 0.9º. 相似文献
67.
Friedrich J Dolg M Gansäuer A Geich-Gimbel D Lauterbach T 《Journal of the American Chemical Society》2005,127(19):7071-7077
The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive. 相似文献
68.
In this article, we describe the unique fragmentations of oligodeoxynucleotides (ODNs) whose phosphate groups are completely depleted of protons and replaced with metal ions. The production of the ubiquitous [a(n) - base] ions still occurs, but no longer by transfer of an acidic phosphate proton to an adjoining 3' base. Nor is the extent of the reaction determined by the proton affinity of that base. Rather, the reaction now occurs via a cleavage 3' to both pyrimidines and purines; cleavage 3' to pyrimidine is more favorable than that 3' to purine. We also demonstrate that an ODN is more stable in the gas phase when its phosphate groups are bound to metal ions than when its phosphate groups are attached to hydrogens. This study also provides further evidence for the ODN fragmentation mechanism that involves H transfer to a nucleobase. To establish the structural utility of this new fragmentation, we applied it to distinguishing small ODNs containing a photomodified cis,syn-cyclobutane pyrimidine dimer from the parent ODNs, a system that cannot be distinguished by collisional activation of precursor species that do not contain metal ions. 相似文献
69.
Hubertus Appler Lutz W. Gross Bernd Mayer Wilhelm P. Neumann 《Journal of organometallic chemistry》1985,291(1):9-23
Factors governing the ease and mechanism of 7-silabicyclo[2.2.1]heptadienes thermolysis in order to generate free silylenes and the corresponding benzene derivatives are investigated. For this purpose, 29 new compounds of the types VII–X have been prepared. No indications for a polar mechanism or an intermediate biradical could be found. The degradation is exactly of first order in all cases investigated sofar, and is enhanced by phenyl groups at the bridgehead C atoms, if a conformation coplanar with the basic ring is allowed by the neighbouring substituents, but is not enhanced by phenyl groups at the Si. The X-ray structure of two typical derivatives is discussed with this respect. A special mechanism is operating in the easy thermolysis of carbomethoxy-substituted compounds leading to cyclic sila enolether intermediates. 相似文献
70.
The collision-induced dissociation (CID) spectra of five alkylmethyleneimmonium ions (H2C-N+R1R2, (a) R1 = R2 = C2H5, (b) R1 = n-C3H7, R2 = H, (c) R1 = n-C3H7, R2 = CH3, (d) R1 = n-C3H7, R2 = C2H5, (e) R1 = R2 = n-C3H7) are reported and discussed in terms of the mechanism of alkane loss. The most abundant alkane losses result from 2-azaallylic bond cleavages within R1 and R2 leading to daughter ions of m/z 84. Ion d (R1 = n-C3H7, R2 = C2H5) was chosen for a deuterium-labelling study because it exhibited methane loss nearly free from interferences with other fragmentations. The methane lost consists to a great extent (95%) of the methyl moiety of R2. Whereas the methyl moiety obviously stays intact during the fragmentation process, the hydrogen additionally needed originates from all positions of R1 and the double-bonded methylene in an approximately random distribution, suggesting extensive hydrogen migrations preceding the transfer step. 相似文献