全文获取类型
收费全文 | 6829篇 |
免费 | 138篇 |
国内免费 | 18篇 |
专业分类
化学 | 4102篇 |
晶体学 | 89篇 |
力学 | 152篇 |
数学 | 777篇 |
物理学 | 1865篇 |
出版年
2021年 | 59篇 |
2020年 | 74篇 |
2019年 | 62篇 |
2016年 | 134篇 |
2015年 | 117篇 |
2014年 | 123篇 |
2013年 | 271篇 |
2012年 | 260篇 |
2011年 | 253篇 |
2010年 | 147篇 |
2009年 | 161篇 |
2008年 | 227篇 |
2007年 | 226篇 |
2006年 | 226篇 |
2005年 | 214篇 |
2004年 | 168篇 |
2003年 | 175篇 |
2002年 | 163篇 |
2001年 | 154篇 |
2000年 | 142篇 |
1999年 | 99篇 |
1998年 | 94篇 |
1997年 | 102篇 |
1996年 | 116篇 |
1995年 | 109篇 |
1994年 | 87篇 |
1993年 | 111篇 |
1992年 | 90篇 |
1991年 | 85篇 |
1990年 | 63篇 |
1989年 | 85篇 |
1988年 | 88篇 |
1987年 | 80篇 |
1986年 | 72篇 |
1985年 | 121篇 |
1984年 | 106篇 |
1983年 | 82篇 |
1982年 | 98篇 |
1981年 | 104篇 |
1980年 | 60篇 |
1979年 | 88篇 |
1978年 | 83篇 |
1977年 | 86篇 |
1976年 | 77篇 |
1975年 | 82篇 |
1974年 | 59篇 |
1973年 | 78篇 |
1968年 | 90篇 |
1967年 | 100篇 |
1966年 | 98篇 |
排序方式: 共有6985条查询结果,搜索用时 15 毫秒
41.
Schmidt B 《The Journal of organic chemistry》2004,69(22):7672-7687
A novel ruthenium-catalyzed tandem ring-closing metathesis (RCM) double-bond isomerization reaction is described in this paper. The utility of this method for the efficient syntheses of five-, six-, and seven-membered cyclic enol ethers is demonstrated. It relies on the conversion of a metathesis-active ruthenium carbene species to an isomerization-active ruthenium-hydride species in situ. This conversion is achieved by using various additives. Scope and limitations of the different protocols are discussed, and some mechanistic considerations based on (31)P and (1)H NMR spectroscopic studies are presented. 相似文献
42.
121Sb Mössbauer Spectra of Antimony (V) Compounds. III The Mössbauer resonance effect of 121Sb has been studied in the antimony(V) compounds I–VI. Isomer shift and quadrupole splitting variations are considered in the light of results of vibrational spectroscopy and in part of X-ray data. The antimony iodine azides II and IV , respectively, were obtained by reactions of R3Sb (R = CH3, C6H5) with iodine azide. 相似文献
43.
This paper provides answers to several questions raised by V. Klee regarding the efficacy of Mattheiss' algorithm for finding all vertices of convex polytopes. Several results relating to the expected properties of polytopes are given which indicate thatn-polytopes defined by large numbers of constraints are difficult to obtain by random processes, the expected value of the number of vertices of polytope is considerably less than Klee's least upper bound the expected performance of Mattheiss' algorithm is far better than Klee's upper bound would suggest. 相似文献
44.
A series of sialyloligosaccharides was synthesized using the transglycolytic activity of the sialidases from Vibrio cholerae, Clostridium perfringens, Salmonella typhimurium, and Newcastle disease virus. According to their hydrolytic activities the sialidases from V. cholerae and C. perfringens catalyze preferentially the formation of sialyl alpha(2-6)-linkages whereas the sialidases from S.typhimurium and Newcastle disease virus show a distinct preference for alpha(2-3) directed sialylations. Using combined chemical and enzymatic methodologies structures such as T-(Thomsen-Friedenreich) antigen [beta-D-Gal-(1-3)-alpha-D-GalNAc-OThr], Tn-(Thomsen nouveau) antigen (alpha-D-GalNAc-OThr) and beta-D-Gal-(1-4)-alpha-D-2-deoxy-Gal-OMe were sialylated in alpha(2-3)- and alpha(2-6)-positions regioselectively or in high regioisomeric excess and purified by simple isolation procedures. Depending on the enzyme source and acceptor structure yields for transsialylation varied between 10 and 30%. 相似文献
45.
New Compounds with Garnet Structure. VI. Vanadates The preparation of vanadate-garnets of the following three types is reported: (I) {Na3}[B2III](V3)O12 (BIII = Cr, Sc), (II) {LiCa2}[B2II](V3)O12 (BII = Mg), (III) {Ca2AIII}[Li2] (V3)O12 (AIII = In, Sc). The Cr-compound of type (I) decomposes above 690°C into a mixture of Cr2O3 and NaVO3. The analogous Fe-compound decomposes in a similar way already at 400°C; therefore the preparation by solid state reaction is not possible. Employing larger BIII-ions (Y, Yb, Lu) no garnets of type (I), but mixtures of BIIIVO4 (zircon structure) and Na3BIIIV2O8 are formed. Garnets of type (II) do not exist, when BII are Co and Ni. Mixtures of {Ca3}[LiBII](V3)O12 (garnet structure), LiBIIVO4 (spinel structure) and B3II(VO4)2 are formed. With type (III) for AIII = Y reaction occurs forming a mixture of YVO4, Ca3(VO4)2 and Li3VO4. 相似文献
46.
Some Irradiation Experiments with 2, 1-Benzisothiazoles 2, 1-Benzisothiazole ( 1 ) on irradiation with a mercury high-pressure lamp in benzene/diethylamine yields, after acetylation, 2-acetylamino-benzaldehyde ( 3 ; Scheme 1). Similarly, irradiation of 3-chloro-2, 1-benzisothiazole ( 2 ) in benzene/diethylamine leads to a mixture of 3-dimethylamino-2, 1-benzisothiazole ( 6a ) and N, N-diethyl-thioanthranilamide ( 7a ; Scheme 2). Benzisothiazole 6a , on irradiation, is not transformed into 7a . On the other hand, when 2 is irradiated in methanol a mixture of 3-methoxy-2, 1-benzisothiazole ( 4a ) and methyl anthranilate ( 5a ; Scheme 2) is obtained. In this case, 4a on irradiation in methanol or ethanol also yields 5a . No exchange of the methoxy group in 4a is observed when the irradiation is performed in ethanolic solution. Thus, 2, 1-benzisothiazoles 1 , 2 and 4a react photochemically by N,S-bond cleavage and hydrogen-atom abstraction from the solvent (Scheme 3). 3-Chloro-2, 1-benzisothiazole ( 2 ) shows a second photoreaction, i.e. nucleophilic exchange of the chloro substituent by methanol or diethyl amine. The latter reaction can also be observed thermally, e.g. in boiling methanol in the presence of methoxide ions. 相似文献
47.
The incongruent vaporization reactions of Ta2S and Ta6S have been investigated by mass-loss effusion in the temperature range 1576 to 1902 K. By extrapolation of PS(obs) to equilibrium the enthalpies of the reactions and Ta6S = 6 Ta(s) + S(g) were found to be and , respectively. Comparison between the above values, determined by a 2nd law treatment, and 3rd law values was used to derive fef (“free energy function”) values for Ta and S in the compounds. These postulated fef's, which apply only to the elements as present in the compounds measured, are compared to tabulated quantities for the pure solid elements to provide a criterion for 2nd and 3rd law evaluation. 相似文献
48.
Torsten Barfels Bernd Schmidt Andreas von Czarnowski Hans-Joachim Fitting 《Mikrochimica acta》2002,139(1-4):11-16
For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary
electron energy E
0 = 2…30 keV. Applying a constant incident power regime (E
0·I
0 = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx.
Thermally grown SiO2 layers of thickness d = 500 nm have been implanted by Ge+-ions of energy 350 keV and doses (0.5–5)1016 ions/cm2. Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of dm≅240 nm are expected. Afterwards the layers have been partially annealed up to T
a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of
the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth
towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster
formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm. 相似文献
49.
M. I. Krivopustov J. Adam V. Bradnova R. Brandt V. S. Butsev P. I. Golubev V. G. Kalinnikov J. Karachuk B. A. Kulakov E.-J. Langrock G. Modolo M. Ochs R. Odoj A. N. Premyshev V. S. Pronskich Th. Schmidt V. I. Stegailov J. S. Wan V. M. Zupko-Sitnikov 《Journal of Radioanalytical and Nuclear Chemistry》1997,222(1-2):267-270
First experiments on the transmutation of long-lived129I and237Np using relativistic protons of 3.7 GeV are described. Relativistic protons generate in extended Pb-targets substancial neutron fluences. These neutrons get moderated in paraffin and are used for transmutation as follows:129I(n,)130I and237Np(n,)238Np. The isotopes130I (T
1/2-12.36 h) and238Np (T
1/2=2.117 d) were identified radiochemically. One can estimate the transmutation cross-section (n,) in the given neutron field as (129I(n,))=(10±2)b and (237Np(n,))=(140±30)b The experiments were carried out in November 1996 at the Synchrophasotron, LHE, Dubna, Russia. The investigation has been performed at the Laboratory of High Energies, JINR, Dubna. 相似文献
50.
H. Schmidt 《Fresenius' Journal of Analytical Chemistry》1960,178(3):173-184
Zusammenfassung Es wird eine einfache Methode zur schnellen Bestimmung (1 Std) von Sorbinsäure in Lebensmitteln beschrieben. Die Sorbinsäure wird durch Wasserdampfdestillation verlustlos vom Lebensmittel abgetrennt und im Destillat mit saurer Kaliumdichromatlösung oxydiert. Dabei entsteht unter anderem Malondialdehyd, der mit Thiobarbitursäure einen beständigen roten Farbstoff bildet, dessen Absorptionsmaximum bei 532 nm liegt. Die rote Lösung wird colorimetriert, und über eine Eichgerade, die mit 2–10 g Sorbinsäure aufgestellt worden ist, wird der Gehalt an Sorbinsäure bestimmt. Die Methode wird durch keinen der üblichen Konservierungsstoffe gestört. Verschiedenen Lebensmitteln zugesetzte Sorbinsäuremengen wurden zu 100±2% wiedergefunden. Der qualitative Nachweis kann in vielen Lebensmitteln ohne Destillation innerhalb weniger Minuten visuell durchgeführt werden.Vorgetragen auf der 18. Arbeitstagung des Arbeitskreises Südwestdeutschland der Fachgruppe Lebensmittelchemie und gerichtliche Chemie in der Gesellschaft deutscher Chemiker in Weinheim (Bergstra\e) am 19./20. 5. 1960.Meinem Assistenten, Herrn F. Nagel, sei für die gewissenhafte Durchführung einer gro\en Zahl von Versuchen aufrichtig gedankt.Wir danken ferner der Fa. Rostock Gebrüder Friedrich Söhne, Elmshorn/Holstein, fÜr die Überlassung von Margarineproben mit Sorbinsäure und den Hoechster Farbwerken fÜr die Bereitstellung verschiedener sorbinsäurehaltiger Lebensmittel. 相似文献