全文获取类型
收费全文 | 1080篇 |
免费 | 29篇 |
国内免费 | 4篇 |
专业分类
化学 | 733篇 |
晶体学 | 20篇 |
力学 | 17篇 |
数学 | 114篇 |
物理学 | 229篇 |
出版年
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 13篇 |
2019年 | 13篇 |
2018年 | 10篇 |
2017年 | 10篇 |
2016年 | 26篇 |
2015年 | 23篇 |
2014年 | 26篇 |
2013年 | 41篇 |
2012年 | 51篇 |
2011年 | 47篇 |
2010年 | 23篇 |
2009年 | 30篇 |
2008年 | 20篇 |
2007年 | 42篇 |
2006年 | 37篇 |
2005年 | 44篇 |
2004年 | 36篇 |
2003年 | 44篇 |
2002年 | 22篇 |
2001年 | 18篇 |
2000年 | 26篇 |
1999年 | 21篇 |
1998年 | 13篇 |
1997年 | 16篇 |
1996年 | 18篇 |
1995年 | 11篇 |
1994年 | 13篇 |
1993年 | 12篇 |
1992年 | 12篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1986年 | 12篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1981年 | 9篇 |
1979年 | 9篇 |
1978年 | 13篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1972年 | 9篇 |
1968年 | 13篇 |
1966年 | 10篇 |
1965年 | 17篇 |
1964年 | 11篇 |
1962年 | 8篇 |
排序方式: 共有1113条查询结果,搜索用时 312 毫秒
21.
22.
We report a high-power source of coherent picosecond light pulses based on optical parametric generation and amplification in LiB3O5 and AgGaS2 crystals. The spectral range of this continuously tunable source covers the visible, near-infrared and medium-infrared spectrum from 0.41 to 12.9 m. An optical parametric generator and amplifier, consisting of two type-I phase-matched LiB3O5 crystals and a diffraction grating, is pumped by the third harmonic of a picosecond Nd:YAG laser and provides spectrally narrow, high-power pulses from 0.41 to 2.4 m. Energy conversion efficiencies up to 16 percent are achieved. The pulse duration is about 14 ps, the bandwidth between 10 and 30 cm–1. The tuning range is extended to 12.9 m by mixing the infrared output between 1.16 and 2.13 m with the fundamental of the Nd:YAG laser in type-I-phase-matched AgGaS2 crystals. Up to 25 percent of the pulse energy at 1.064 m is converted into parametric infrared pulses. Bandwidths between 3 and 8 cm–1 and a pulse duration of approximately 19 ps are measured for these pulses. We also observe a retracing behaviour in the tuning curve of AgGaS2 not reported before. 相似文献
23.
One-Bond P? P Coupling Constant of Diphosphate Anions The coupling constant 1JPP of diphosphate anions depends upon the electronegativities of the substituents at phosphorus. The simple linear relationship which has been established also holds for other P? P compounds involving tetra-coordinate phosphorus, with the exception of compounds with bulky substituents. 相似文献
24.
Krause A. A. Rumler A. Hagen F. Jansch H. J. Shturm I. G. Dubur G. Ya. 《Chemistry of Heterocyclic Compounds》1992,28(1):67-70
The condensation of salts of amidinium vinylogs with N-R-thiocarbamoylacetamides gave 3-(N-R)carbamoyl-5-(pyrid-4-yl)pyridine-2(1H)-thiones, which undergo oxidative cyclization in concentrated sulfuric acid with the formation of 3-oxo-5-(pyrid-4-yl)isothiazolo[5,4-b]pyridines. The oxidation and alkylation of the compounds referred to were investigated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 75–79, January, 1992. 相似文献
25.
Kevin F. Morris Eugene J. Billiot Fereshteh H. Billiot Jordan A. Ingle Kevin B. Krause Corbin R. Lewis 《Journal of Dispersion Science and Technology》2019,40(5):716-727
Molecular dynamics (MD) simulations were used to investigate the binding of six chiral compounds to the amino acid-based molecular micelle (MM) poly-(sodium undecyl-(L)-leucine-leucine) or poly(SULL). The MM investigated is used as a chiral selector in capillary electrophoresis. The project goal was to characterize the chiral recognition mechanism in these separations and to move toward predictive models to identify the best amino acid-based MM for a given separation. Poly(SULL) was found to contain six binding sites into which chiral compounds could insert. Four sites had similar sizes, shapes, and electrostatic properties. Enantiomers of alprenolol, propranolol, 1,1′-bi-2-naphthyl-2,2′-diyl hydrogen phosphate, 1,1′-bi-2-naphthol, chlorthalidone, or lorazepam were separately docked into each binding pocket and MD simulations with the resulting intermolecular complexes were performed. Solvent-accessible surface area calculations showed the compounds preferentially associated with binding sites where they penetrated into the MM core and shielded their non-polar atoms from solvent. Furthermore, with five of the six compounds the enantiomer with the most favorable free energy of MM association also experienced the most favorable intermolecular hydrogen bonding interactions with the MM. This result suggests that stereoselective intermolecular hydrogen bonds play an important role in chiral discrimination in separations using amino acid-based MMs.GRAPHICAL ABSTRACT 相似文献
26.
Scandium was readily detected by paper chromatography using methyl acetate as the solvent and quinalizarin for the developer. The reddish-violet color formed with scandium resembled the colers obtained with thorium, titanium, zirconium and the rare earths, but the difference in RF values gave definite separation of these elements. Positive chromatographs were obtained by using 0.01 ml of solution 0.005M respect to scandium. 相似文献
27.
H. W. Krause 《Reaction Kinetics and Catalysis Letters》1979,10(3):243-246
New chiral rhodium complexes of a set of dimenthylphosphine-substituted, crosslinked styrene-divinylbenzene copolymers have been tested for the enantioselective hydrogenation of Z--acetamidocinnamic acid. The supported catalysts have been found to give optical yields up to 60% but suffer with respect to the catalytic activity.
, . (Z)-- . N--(R)- 60%.相似文献
28.
A. Krause S. Zieliski und R. Fiedorow 《Fresenius' Journal of Analytical Chemistry》1962,187(5):350-353
Zusammenfassung Es wird eine Methode beschrieben, nach der, gegebenenfalls unter Zuhilfenahme von Testlösungen, Spurenmengen von Co2+ in einem Konzentrationsbereich von 10 mg bis zu 0,001 mg herab in 1 ml sich recht genau auf Grund der Katalyse der H2O2-Indigocarminreaktion in Gegenwart von Calciumcarbonat als Träger bestimmen lassen. Da das Volumen der Versuchslösung stets 60 ml beträgt (siehe oben), beläuft sich die kleinste noch bestimmbare Co2+-Konzentration auf 10–6 g Co2+ in 60 ml was einer Verdünnung von 16·107 entspricht. In bezug auf diese Empfindlichkeit steht die Methode den colorimetrischen bzw. spektrophotometrischen1,6 und auch den komplexchemischen7 Verfahren nicht nach. Sie bietet aber im Vergleich mit diesen insofern gewisse Vorteile, als man die störenden Fremdionen meist nicht zu entfernen braucht, was auf eine beträchtliche Zeitersparnis hinauskommt. Schließlich ist die katalytische Methode, für die mit entsprechenden Abänderungen auch eine Temperatur von 30° C genügen würde, in ihrer Handhabung und Ausführung sehr einfach und bedarf, abgesehen vom Thermostaten, keiner kostspieligen Apparatur. 相似文献
29.
Rodella Cristiane B. Franco Roberto W.A. Magon Claudio J. Donoso Jose P. Nunes Luis A.O. Saeki Margarida J. Aegerter Michel A. Sargentelli Vagner Florentino Ariovaldo O. 《Journal of Sol-Gel Science and Technology》2002,25(1):83-88
Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V2O5/TiO2 catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm–1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm–1 and 770 to 850 cm–1. The Raman spectra do not exhibit characteristic peaks of crystalline V2O5. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V4+ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V4+ ions are located in sites with octahedral symmetry substituting for Ti4+ ions in the rutile structure. Magnetically interacting V4+ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V2O5, a partial oxidation of V4+ to V5+ is apparent from the EPR results. 相似文献
30.
Contributions to the Chemistry of Phosphorus. 167. Constitutional and Configurational Isomers of Pentaphosphane(7), P5H7 Phosphane mixtures containing 10—15 P-% of pentaphosphane(7), P5H7, are obtained by thermolysis of diphosphane, P2H4, or as residue from distillation of crude diphosphane [3]. According to the complete analysis of the 31P{1H}-NMR spectrum on the basis of selective population transfer experiments, P5H7 exists as a mixture of three diastereomers of n-P5H7 — 1a (erythro, erythro), 1b (erythro, threo), 1c (threo, threo) — and of the constitutional isomer 2-phosphinotetraphosphane 2 (iso-P5H7, largest relative isomeric abundance). The correlation between the diastereomers and the observed spin systems results from the preferred gauche orientation of neighboring free electron pairs, the dependence of 1J(PP) on dihedral angles, and the 3J(PP) long range couplings. From the 31P-NMR data of the phosphane molecules PnHn+2 with n = 1—5 general relationships for the δ(31P) values and the 1J(PP) coupling constants of chain-type phosphorus hydrides as a function of their structural parameters are derived. 相似文献