全文获取类型
收费全文 | 5946篇 |
免费 | 153篇 |
国内免费 | 46篇 |
专业分类
化学 | 3715篇 |
晶体学 | 35篇 |
力学 | 208篇 |
数学 | 1274篇 |
物理学 | 913篇 |
出版年
2022年 | 46篇 |
2021年 | 57篇 |
2020年 | 65篇 |
2019年 | 67篇 |
2018年 | 58篇 |
2017年 | 49篇 |
2016年 | 96篇 |
2015年 | 116篇 |
2014年 | 135篇 |
2013年 | 312篇 |
2012年 | 294篇 |
2011年 | 326篇 |
2010年 | 222篇 |
2009年 | 207篇 |
2008年 | 306篇 |
2007年 | 315篇 |
2006年 | 305篇 |
2005年 | 272篇 |
2004年 | 267篇 |
2003年 | 216篇 |
2002年 | 208篇 |
2001年 | 98篇 |
2000年 | 91篇 |
1999年 | 98篇 |
1998年 | 73篇 |
1997年 | 66篇 |
1996年 | 90篇 |
1995年 | 66篇 |
1994年 | 73篇 |
1993年 | 81篇 |
1992年 | 65篇 |
1991年 | 43篇 |
1990年 | 75篇 |
1989年 | 59篇 |
1988年 | 47篇 |
1987年 | 53篇 |
1986年 | 47篇 |
1985年 | 68篇 |
1984年 | 81篇 |
1983年 | 51篇 |
1982年 | 78篇 |
1981年 | 79篇 |
1980年 | 94篇 |
1979年 | 75篇 |
1978年 | 81篇 |
1977年 | 76篇 |
1976年 | 71篇 |
1975年 | 52篇 |
1974年 | 38篇 |
1973年 | 59篇 |
排序方式: 共有6145条查询结果,搜索用时 372 毫秒
181.
Marguerite Dols Magali Remaud-Simeon René-Marc Willemot Michel Vignon Pierre F. Monsan 《Applied biochemistry and biotechnology》1997,62(1):47-59
Leuconostoc mesenteroides NRRL B-1299 dextransucrase was fractionated into soluble (SGT) and insoluble (IGT) enzyme preparations differing by their dextran content. In spite of this, they displayed the same Km for sucrose (10 g/L) and the same activation energy (35 kJ/mol). But the presence of cells and insoluble dextran led to the IGT behaving like an immobilized enzyme: stabilization against thermal denaturation and diffusional limitations at low substrate concentrations were observed. On the other hand, the behavior of SGT was influenced by the presence, in the preparation, of soluble dextran that reduced enzyme inhibition by excess substrate. SGT and IGT present very different pH profiles. In the presence of 4 g/L of soluble dextran, IGT was activated and displayed the same susceptibility to pH as SGT. The activation of IGT was highly dependent on the nature of the acceptor added but also on the pH of the reaction medium. IGT and SGT synthesize both soluble and insoluble polymer containing α(l → 2), α(l → 3), and α(l → 6) linkages. A larger amount of insoluble dextran is elaborated by SGT. The polymer structures, examined by13C NMR spectrometry, revealed that they differ mainly by their α(l → 3) linkage content (from 0 to 11%). This linkage seems to be partly responsible for the dextran insolubility and can be completely eliminated by carrying out the synthesis of soluble polymer at pH 7.4 with SGT. 相似文献
182.
The structures of Beccapoline and Beccapolinium, two new isoquinoline alkaloids from , Annonaceae, have been elucidated by spectroscopic analysis. They are the first examples of natural bisaporphines. 相似文献
183.
Bruno Hasiak Guy Ricart Didier Barbry Daniel Couturier Michel Hardy 《Journal of mass spectrometry : JMS》1981,16(1):17-20
The ammonia chemical ionization desorption spectra of N,N-dimethyl quaternary ammonium iodides in addition to high protonated molecular ion [M + H]+ intensity, show signals for an ion radical composed of N-methyl abstracted salt cation and ammonia [C + NH3? CH3]+˙. These ions corresponding to the cation +2 show increased importance in the chemical ionization mode, using the same reagent gas. The technique of chemical ionization desorption appears suitable for the analysis of salts, and thus for the determination of the molecular weight of both anion and cation. 相似文献
184.
Négrier F Marceau E Che M Giraudon JM Gengembre L Löfberg A 《The journal of physical chemistry. B》2005,109(7):2836-2845
1.5 Ni wt %/Al2O3 catalysts have been prepared by incipient wetness impregnation using [Ni(diamine)x(H2O)(6-2x)]Y2 precursors (diamine = 1,2-ethanediamine (en) and trans-1,2-cyclohexanediamine (tc); x = 0, 1, and 2; Y = NO3- and Cl-), to avoid the formation, during calcination, of difficult-to-reduce nickel aluminate. N2 was chosen for thermal treatment to help reveal and take advantage of the reactions occurring between Ni2+, ligands, counterions, and support. In the case of [Ni(en)2(H2O)2]Y2 salts used as precursors, in situ UV-vis and DRIFT spectroscopies show that after treatment at 230 degrees C Ni(II) ions are grafted to alumina via two OAl bonds and that the diamine ligands still remain coordinated to grafted nickel ions but in a monodentate way, bridging the cation with the alumina surface. With Y = Cl-, the chloride counterions desorb as hydrogen chloride, and hydrogen released upon decomposition of the en ligands is able to reduce a fraction of nickel ions into metal as evidenced by XPS. In contrast, with Y = NO3-, compounds such as CO or NO are formed during thermal treatment, indicating that nitrate ions burn the en ligands. After thermal treatment at 500 degrees C, a surface phase containing Ni(II) ions forms, characterized by XPS and UV-vis spectroscopy. Temperature-programmed reduction shows that these ions can be quantitatively reduced to the metallic state at 500 degrees C, in contrast with the aluminate obtained when the preparation is carried out from [Ni(H2O)6]2+, which is reduced only partly at 950 degrees C. On the other hand, a total self-reduction of nickel complexes leading to 2-5-nm metal particles is obtained upon thermal treatment via the hydrogen released by a hydrogen-rich ligand such as tc, whatever the Y counterion. An appropriate choice of the ligand and the counterion allows then to obtain selectively Ni(II) ions or a dispersed reduced nickel phase after treatment in N2, as a result of the reactions occurring between the chemical partners present on alumina. 相似文献
185.
H. Münzel F. Michel P. P. Coetzee V. Krivan 《Journal of Radioanalytical and Nuclear Chemistry》1977,37(1):267-273
The possible application of cyclotron-produced fast neutrons to activation analysis for oxygen based on the16O(n, p)16N reaction has been investigated. Neutrons were produced by bombarding a thick beryllium target with 22 to 45 MeV deuterons.
It was found that the sensitivity increases rapidly with the energy of the deuterons. Using 45 MeV deuterons and a 10 μA beam
current a sensitivity of about 20 counts per 1 μg oxigen could be achieved, enabling the determination of less than 1 μg oxigen.
In a direct comparison it was experimentally established that the sensitivity for cyclotron-produced neutrons assuming a deuteron
beam of about 10 μA, is up to two orders of magnitude higher than that achievable for 14 MeV neutrons with a flux of about
1010 n/s. The interference of fluorine is at about the same level for both the cyclotron-produced and 14 MeV neutrons. Using cyclotron-produced
fast neutrons in the investigated energy range, sodium and magnesium can also interfere, but only to a very much lower extent. 相似文献
186.
Karen Wright Fernando FormaggioClaudio Toniolo Roland TörökAntal Péter Michel WakselmanJean-Paul Mazaleyrat 《Tetrahedron letters》2003,44(22):4183-4186
Resolution of trans 3-(9-fluorenylmethyloxycarbonylamino)-1-oxyl-2,2,5,5-tetramethylpyrrolidine-4-carboxylic acid (Fmoc-POAC-OH) was quickly achieved upon esterification with (aR)-1,1′-binaphthyl-2,2′-diol, chromatographic separation of the obtained diastereomers, and facile saponification of the aryl ester function with removal of the chiral auxiliary. 相似文献
187.
Kanibolotsky A Roquet S Cariou M Leriche P Turrin CO de Bettignies R Caminade AM Majoral JP Khodorkovsky V Gorgues A 《Organic letters》2004,6(13):2109-2112
[structure: see text] In four new dendrimers terminated by 12 electroactive tetrathiafulvalenyl substituents, the tridimensional character of the inter- and intradendrimeric charge and electron transfer, and hence of the electroconductivity, is evidenced by examination of the electronic spectra of their corresponding neutral state and cation radical, dication, and mixed-valence salts, including a closed-shell anion. 相似文献
188.
In this paper, the transport of Cu(II) in the presence of lipophilic Cu(II) organic complexes through permeation liquid membranes (PLMs) have been investigated. In natural waters, small organic compounds, which form liposoluble neutral complexes with Cu(II), are potentially toxic and bioavailable. Hence, to understand the role of liposoluble Cu(II) complexes in natural waters, four organic ligands: phthalic acid, bipyridyl, pyrocatechol and hydroxyquinoline, which form uncharged or lipophilic Cu(II) complexes, were tested. The results showed that the transport of lipophilic Cu(II) complexes through PLM depends on the lipophilicity of the complex. Applications of PLMs in natural waters are presented. 相似文献
189.
Max Robba Michel Cugnon De Sevricourt Anne-Marie Godard 《Journal of heterocyclic chemistry》1979,16(6):1175-1180
The cleavage of the pyridazine ring of the acyltetrahydro [1] benzofuro [2,3-d] pyridazones was carried out by hydrolysis, alcoholysis or aminolysis reactions and they affect the lactam 3,4-bond. They lead chiefly to benzofuran derivatives with acid, ester, amide or hydrazide groups in the 2-position and eventually an acylated methylhydrazine group in the 3-position. The cyclization reactions of 3-hydrazinomethylbenzofuran-2-carboxylic acid and its derivatives or ethyl 3-bromome thylbenzofuran-2-carboxylate affords tetrahydrobenzofuropyridazones. The nmr spectra were studied. 相似文献
190.
Caroline LapeyreFlorence Bedos-Belval Hubert Duran Michel BaltasLiliane Gorrichon 《Tetrahedron letters》2003,44(12):2445-2447
The one-pot transesterification of diethylarylvinylphosphonates with N-acetylcysteamine has been achieved using phosphonochloridates as intermediates. Reaction of phosphonodiesters with (COCl)2 gave the corresponding chlorinated compounds, which were coupled with N-acetylcysteamine in presence of Et3N. 相似文献