全文获取类型
收费全文 | 7038篇 |
免费 | 352篇 |
国内免费 | 14篇 |
专业分类
化学 | 4042篇 |
晶体学 | 24篇 |
力学 | 163篇 |
数学 | 992篇 |
物理学 | 2183篇 |
出版年
2023年 | 75篇 |
2022年 | 65篇 |
2021年 | 91篇 |
2020年 | 165篇 |
2019年 | 172篇 |
2018年 | 104篇 |
2017年 | 87篇 |
2016年 | 257篇 |
2015年 | 210篇 |
2014年 | 224篇 |
2013年 | 354篇 |
2012年 | 427篇 |
2011年 | 480篇 |
2010年 | 238篇 |
2009年 | 177篇 |
2008年 | 346篇 |
2007年 | 304篇 |
2006年 | 252篇 |
2005年 | 289篇 |
2004年 | 188篇 |
2003年 | 170篇 |
2002年 | 109篇 |
2001年 | 109篇 |
2000年 | 95篇 |
1999年 | 87篇 |
1998年 | 61篇 |
1997年 | 71篇 |
1996年 | 79篇 |
1995年 | 82篇 |
1994年 | 63篇 |
1993年 | 74篇 |
1992年 | 77篇 |
1991年 | 58篇 |
1990年 | 52篇 |
1989年 | 52篇 |
1988年 | 53篇 |
1987年 | 51篇 |
1986年 | 49篇 |
1985年 | 52篇 |
1984年 | 58篇 |
1983年 | 48篇 |
1982年 | 60篇 |
1981年 | 60篇 |
1979年 | 54篇 |
1978年 | 53篇 |
1976年 | 50篇 |
1975年 | 61篇 |
1964年 | 45篇 |
1963年 | 45篇 |
1956年 | 48篇 |
排序方式: 共有7404条查询结果,搜索用时 31 毫秒
71.
Efficient fermentation control requires the measurement of biological parameters. Three techniques were tested for monitoring. In the first, the NADH-fluorescence of micro-organisms was measured in batch and in continuous cultures under aerobic and anaerobic conditions, providing information on the metabolic status of the cells. The effects of cell concentration and of different substrates (glucose, ethanol and oxygen) were studied. The second technique is the calorimetric determination of various substrates, such as penicillin or enzymes, by an enzyme/thermistor device. With immobilized penicillin acylase (E.C. 3.5.1.11) or penicillinase (E.C. 3.5.2.6), penicillin was determined selectively in a fermentation broth. The thermistor was also used to measure penicillin acylase activity. The third technique is laser flow cytometry. A commercial double-beam flow cytometry system was used to determine cell size, light scattering and the protein, DNA and RNA contents of single cells. Flow cytometry allows rapid and sensitive control of fermentation processes with genetically modified E. coli 5K (pHM12) cells. The results of monitoring the cell size, light scattering, and protein and DNA contents of different micro-organisms during fermentation are outlined. 相似文献
72.
Sandip Halder Theodor Schneller Rainer Waser Florian Thomas 《Journal of Sol-Gel Science and Technology》2007,41(3):203-207
Nickel thin films have been sputtered on standard Si/SiO2 substrates with TiO2 as an adhesive layer. The thermal stability of these substrates was analyzed. SEM images show an increase in grain size with
annealing temperature. They were found to be stable till 800°C, beyond which the nickel layer disintegrated. These substrates
were used for deposition of BaTiO3 and (Ba,Sr)TiO3 dielectric thin films under a reducing atmosphere. The dielectric thin films were processed with various pyrolysis and annealing
temperatures in order to optimize the dielectric properties. Increased pyrolysis temperatures showed an increase in the grain
size. Results on these nickelised substrates were finally compared with dielectric films deposited on platinized silicon substrates
under identical conditions but crystallized in an oxygen atmosphere. 相似文献
73.
Norman L. Allinger Lothar Schäfer K. Siam V. J. Klimkowski C. Van Alsenoy 《Journal of computational chemistry》1985,6(5):331-342
A series of ab initio calculations have been carried out, using the 4-21G basis set. Ethane and propane were first studied to obtain reference points. The effect of adding an electronegative atom (fluorine, or carbonyl oxygen) onto the framework was then studied as a function of the torsional angle about the single bond. Some pronounced trends in structural changes were observed, and these can in part be correlated with hyperconjugative effects. For example, fluoroethane has bond lengths which are shorter than those in ethane itself, by 0.024 Åin the C C bond, and 0.003 Åin the α C H bonds. These changes are essentially torsionally independent. On the other hand, in propionaldehyde, the C C bond length of the methyl group and the C H bond lengths of the hydrogens attached to the alpha carbon vary as a function of the torsion angle. If the methyl C C bond in the carbonyl plane is taken as a reference, the bond stretches .016 Åwhen the torsion angle is increased to 90°, an α C H bond similarly stretches up to .007 Å. Many of these geometric changes are large, well beyond the experimental errors in modern measurements. 相似文献
74.
4,4-Disubstituted Imidazole Derivatives from the Reaction of 3-Amino-2H-azirines with Salicylamide Reaction of 3-amino-2H-azirines 1a–c with salicylamide ( 7 ) in MeCN leads to imidazoles 10 and 11 in different rates, depending on the conditions. In the case of 1a and 1b, 11a and 11b , respectively, have been obtained as the main product at 50°; in reactions at 80°, 10a and 10b are the favored products (Tables 1 and 2). 2,2-Dimethyl-3-(N-methyl-N-phenylamino)-2H-azirine ( 1c ) reacts with 7 in MeCN mainly to 2-(2-hydroxyphenyl)-5,5-dimethyl-3,5-dihydroimidazol-4-one ( 10a ); in boiling toluene, 11c is formed with low preference (Table 3). The structure of the products has been established by spectroscopic means, and in the case of 10b and 11c , by X-ray crystallography. Two different reaction mechanisms for the formation of the products are discussed (Scheme 2). 相似文献
75.
Zusammenfassung Durch Hinzunahme von Magnesiumoxid zur Verbrennungsrohrfüllung wird der Anwendungsbereich des CHN-Automaten auf fluor- und phosphorhaltige Proben erweitert. Diese beiden Elemente werden im wesentlichen in der Magnesiumoxidzone der Füllung gebunden. Bei manchen phosphorhaltigen Verbindungen macht die geringe Pyrolysegeschwindigkeit einen längeren Analysezyklus notwendig. Die längere Verbrennungszeit wird durch eine Reduzierung des Gasstromes ermöglicht. Bei metallhaltigen Proben geben alle Verbindungen, deren Metalle unter den herrschenden Bedingungen definierte Rückstände bilden, ohne besondere Maßnahmen gute Werte. Die Bildung carbonathaltiger Rückstände bei Alkali und Erdalkali wird meistens von der Apparatur als solche registriert.
Die Firma Technicon hat uns für diese Untersuchungen dankenswerterweise einen CHN-Analyzer zur Verfügung gestellt. 相似文献
Summary The field of application of the CHN-automatic combustion devices is extended to samples containing fluorine and phosphorus by including magnesium oxide in the filling of the combustion tube. These two elements are essentially bound in the magnesium oxide zone. Many compounds containing phosphorus make it necessary to reduce the speed of the gas stream because of the low pyrolysis velocity. When samples containing metals are being analyzed, excellent results are obtained with all compounds whose metals yield definite residues under the prevailing conditions. The formation of residues containing carbonate in the case of alkali and alkaline earths is usually registered as such by the apparatus.
Die Firma Technicon hat uns für diese Untersuchungen dankenswerterweise einen CHN-Analyzer zur Verfügung gestellt. 相似文献
76.
Reaction of the triolide 1 from (R)-3-hydroxybutanoic acid with Lawesson's reagent 5 leads to the mono-, di-, and trithio derivatives 6–8 which can be isolated in pure form (20–40% yields), and which have crystal structures very similar to the parent triolide 1 (Fig. 1). Similarly, pentolide 3 is converted to mixtures of various thio derivatives, three of which are separated ( 10–12 ) by HPLC and fully characterized. The X-ray structures of the mono- and of one of the dithiopentolides ( 10, 12 ) differ remarkably from each other (Fig. 3). Reduction of the thiotriolides 6–8 (NaBH4, R3SnH, Cl3SiH, Raney-Ni) gives 12-membered rings containing up to three ether groups (chiral crown ethers, 15, 17–19 ) in poor yields. The thiotriolides react spontaneously and in yields of up to 96% with ammonia, certain primary amines, and hydroxylamine to give imine and oxime derivatives with intact 12-membered-ring backbones ( 20, 22–24, 30 , see crystal structures in Figs. 4–7). The rigid structure of all the derivatives of triolide 1 puts the C?O, C?S, and C?NR O-, S-, and N-atoms in juxtaposition (a feature reminiscent of the side chains in the iron-binder enterobactin, Fig. 6). Imines containing PPh2 groups are prepared ( 30, 33, 35 ) from the thiotriolides and tested as chiral ligands for PdII-catalyzed 1,3-diphenyallylations (→ 37 , enantiomer ratio up to 77:23). The reactions described demonstrate that multiple reactions of the triolide 1 from (R)-3-hydroxybutanoic acid which proceed through tetrahedral intermediates are possible without ring opening – the skeleton is remarkably stable, and this might be exploited as a template for bringing up to three pendent substituents into close proximity to allow a study of their interactions and cooperative properties. Also, the di- and trithio derivatives 7 and 8 could be used for cross-linking in molecules containing primary NH2 groups. 相似文献
77.
Mechanism of Thermal Dehydration of Sodium Dihydrogenphosphate The thermal dehydration of sodium dihydrogenphosphate (P1) to sodium cyclotriphosphate (CP3) and sodium polyphosphate, the so-called Maddrell's salt (MS), is controlled by nucleation and crystallization of CP3 and MS, respectively. The wellknown influence of various formation conditions on the CP3:MS product ratio is caused by amorphous and crystalline intermediate phases and their ability to form CP3 or MS nuclei. On seeding P1 with CP3 or MS the dehydration proceeds structure-controlled via dihydrogendiphosphate to pure CP3 or pure MS. The particularities of thermal dehydration of crystalline sodium dihydrogendiphosphate, and also the dehydration and reorganization of sodium phosphate glasses (Na:P = 1:1) with different mean chain lengths, under formation of CP3 or MS are described. 相似文献
78.
Summary Diphenyl(2-pyridyl)phosphine (PPh2pyl), phenylbis(2-pyridyl)-phosphine (PPhpyl2) and tris(2-pyridyl)-phosphine (Ppyl3) react with [Rh(acac)(CO)2] (acac=acetylacetonate) and Rh(8-oxy)(CO)2(8-oxy=8-hydroxyquinolinate) yielding [Rh(chel)(CO)(PPhxpyl3–x)]. The properties of these complexes were examined by spectral (i.r.,u.v.-vis,31P n.m.r.) and chemical methods. 相似文献
79.
The varying -bonding contributions in the title compounds caused by the different electronic and molecular structure of the chelate rings are used for explaining the large band splittings in the absorption spectra by trigonal symmetry. It is shown that usual ligand field theory and the angular overlap model are not able to account for the trigonal level splitting of Cr(acac)3 for which the coordination sphere of oxygen atoms is nearly octahedrally arranged. The experimental finding can, however, be rationalized by an extended angular overlap model which considers the phase coupling of -orbitals in the ligands leading to non-additive contributions to the metal-ligand bond energy.On leave of absence from the Bulgarian Academy of Sciences, Sofia, Bulgaria 相似文献
80.
Florian Bentivegna Michael Canva Alain Brun Frédéric Chaput Jean-Pierre Boilot 《Journal of Sol-Gel Science and Technology》1997,9(1):33-39
We study herein the rotational mobility of organic dye molecules and their ability to align on a strong optical electric field when they are encaged in the pores of an inorganic silica xerogel matrix. We compare the case of dye molecules simply dispersed in the pores of the gel—and possibly held by hydrogen bonds—to the case of molecules chemically grafted on the inner surface of these pores through covalent bonds. The study is led on hybrid silicon-zirconium based inorganic matrices doped with organic rhodamine B molecules. The stronger holding of the dopants when these are grafted to the matrix enhances the molecular alignment—and thus the induced anisotropy—as well as the remanence of this alignment. Furthermore, we show that submitting the samples to a supplementary drying at higher temperature tends to increase both the alignment anisotropy and its stability. We explain these results in terms of mobility of the molecules, in relation to their immediate environment. 相似文献