全文获取类型
收费全文 | 81318篇 |
免费 | 430篇 |
国内免费 | 401篇 |
专业分类
化学 | 26151篇 |
晶体学 | 811篇 |
力学 | 6798篇 |
数学 | 32499篇 |
物理学 | 15890篇 |
出版年
2020年 | 52篇 |
2019年 | 43篇 |
2018年 | 10461篇 |
2017年 | 10288篇 |
2016年 | 6124篇 |
2015年 | 909篇 |
2014年 | 361篇 |
2013年 | 466篇 |
2012年 | 3978篇 |
2011年 | 10678篇 |
2010年 | 5758篇 |
2009年 | 6154篇 |
2008年 | 6771篇 |
2007年 | 8931篇 |
2006年 | 434篇 |
2005年 | 1538篇 |
2004年 | 1702篇 |
2003年 | 2114篇 |
2002年 | 1147篇 |
2001年 | 295篇 |
2000年 | 343篇 |
1999年 | 183篇 |
1998年 | 215篇 |
1997年 | 175篇 |
1996年 | 241篇 |
1995年 | 146篇 |
1994年 | 111篇 |
1993年 | 131篇 |
1992年 | 97篇 |
1991年 | 91篇 |
1990年 | 84篇 |
1989年 | 90篇 |
1988年 | 86篇 |
1987年 | 78篇 |
1986年 | 83篇 |
1985年 | 85篇 |
1984年 | 77篇 |
1983年 | 68篇 |
1982年 | 81篇 |
1981年 | 70篇 |
1980年 | 71篇 |
1979年 | 75篇 |
1978年 | 59篇 |
1977年 | 40篇 |
1976年 | 43篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Neringa Kleizien? Virgin? Amankavi?ien? Ulf Berg Carsten Schicktanz Klaus Schlothauer Algirdas ?a?kus 《Monatshefte für Chemie / Chemical Monthly》2006,43(9):1109-1117
Condensation of 1-substituted 1,2,3,9a-tetrahydro-9H-imidazo[1,2-a]indol-2-ones with 5-nitrosalicylaldehyde afforded 1′-[(N-monosubstituted carbamoyl)methyl]indoline nitrospirobenzopyrans.
Treatment of the latter with strong base led to the formation of a mixture of cis/trans-5a,13-methano-1,3-benzoxazepino[3,2-a]indoles. Results of semiempirical calculations gave evidence that such a transformation of nitrospirobenzopyrans to bicyclic
indole derivatives could proceed via a single transition state, where the negatively charged carbon atom attacks the vinylic double bond of the spiropyran system. 相似文献
992.
Chris G. Vonk 《Colloid and polymer science》1979,257(10):1021-1032
Summary By Fourier transformation of the intensity distribution in the small angle X-ray scattering diagram of a polymer fibre one obtains the corresponding two-dimensional correlation function. This is compared with correlation functions calculated on the basis of different models of the semi-crystalline morphology. Here, two steps may be distinguished: in the first, the correctness of a model may be checked with the aid of qualitative features, such as the relative positions and sizes of positive and negative areas; in the second a quatitative agreement is pursued by fixing the various parameters involved. This method was applied to cold drawn low density polyethylene before and after annealing at different temperatures. In the case of the non-annealed sample, the microparacrystalline model, discussed byHosemann andLoboda-Cackovic (J. Appl. Cryst.11, 540 (1978)) was found to give the best fit. Annealing at 80 °C or higher temperatures seems to lead to what has been indicated as a three-dimensional chess-board structure; it consists of fibrils in which crystalline and amorphous regions alternate in such a way that the crystalline regions in a fibril are adjacent to amorphous regions in neighbouring fibrils.
Zusammenfassung Durch Fourier-Transformation der Intensitätsverteilung im Kleinwinkelstreudiagramm von Polymerfasern bekommt man die zugehörige zweidimensionale Korrelationsfunktion. Diese wird verglichen mit Korrelationsfunktionen, die für verschiedene Modelle der teilkristallinen Morphologie berechnet wurden.Hier werden zwei Schritte unterschieden. Im ersten Schritt wird die Richtigkeit eines Modells überprüft mit Hilfe von qualitativen Merkmalen, wie z. B. die relativen Lagen und Größen der positiven und negativen Bereiche. Im zweiten Schritt wird eine quantitative Übereinstimmung dadurch angestrebt, daß die unterschiedlich bezüglichen Parameter festgestellt werden. Diese Methode wurde angewandt bei kaltverstrecktem Polyäthylen niedriger Dichte, sowohl vor wie nach Temperung bei verschiedenen Temperaturen. Im Falle einer nicht getemperten Probe zeigte sich, daß das mikrokristalline Modell, vorgeschlagen von Hosemann und Loboda-Cackovic (J. Appl. Cryst.11, 540 (1978)), am besten zutrifft. Temperung bei 80°C oder bei höheren Temperaturen scheint zu einer Struktur zu führen, die als dreidimensionale Schachbrett-Struktur bezeichnet wurde. Sie besteht aus Fibrillen, in denen kristalline und amorphe Bereiche abwechseln, und zwar so, daß die kristallinen Bereiche in einer Fibrille nur von amorphen Bereichen in nächstliegenden Fibrillen umgeben sind.相似文献
993.
The oxidation of polyunsaturated fatty acids such as arachidonic and linoleic acid initiates a plethora of cell signaling pathways in animals and plants. The chemistry of the enzymatic oxidation has been investigated for several enzymes, most notably prostaglandin synthase and the lipoxygenases, revealing many surprises and impressive examples of enzymatic control of hydrogen atom abstraction and subsequent oxygenation. 相似文献
994.
Yin G Buchalova M Danby AM Perkins CM Kitko D Carter JD Scheper WM Busch DH 《Journal of the American Chemical Society》2005,127(49):17170-17171
The reactive intermediates and mechanisms of oxygenation of olefins by manganese complexes were investigated by treating olefins with newly synthesized [MnIV(Me2EBC)(OH)2](PF6)2 in the presence and absence of peroxide and by studying its catalytic epoxidation reaction in normal aqueous solution and, individually, with isotopically labeled H218O, 18O2, and H218O2. The manganese oxo species is not the reactive intermediate for the oxygen transfer process mediated by this manganese complex. A novel manganese(IV) peroxide intermediate, MnIV(Me2EBC)(O)(OOH)+, was captured by mass spectrometry and is proposed as the intermediate that oxygenates olefins in this catalytic system. 相似文献
995.
The use of reduced overlap population (ROP) analysis from EHMO calculations as a means to gauge the presence of metal–metal
bonds in triosmium clusters is examined. A number of triosmium clusters containing bridging ligands have been used as test
cases, including the bis-ethoxy bridged cluster Os3(μ-OEt)2(CO)10, 1b, and some of its group 15 derivatives Os3(μ-OEt)2(CO)9(EPh3), 2. These latter clusters are newly synthesized and have been characterized completely, including by single-crystal X-ray crystallographic
studies.
In honour of Professor Dieter Fenske on his 65th birthday. 相似文献
996.
Chris Mussell Céline S. J. Wolff Briche Chris Hopley Gavin O’Connor 《Accreditation and quality assurance》2007,12(9):469-474
The method used at LGC for analysis of “total” 19-norandrosterone (19-norandrosterone glucuronide plus “free” 19-norandrosterone)
in urine for the Comité Consultatif pour la Quantité de Matière Pilot Study (CCQM-P68) is described. The analytical method
used was a modified version of the method developed at the National Measurement Institute of Australia, which used a hydrolysis
and derivatisation procedure first described by the German Sports University. This method is routinely used by World Anti-Doping
Agency-accredited laboratories for sports drug testing. The main modifications made to the method were the use of 19-norandrosterone
glucuronide as a calibration standard and 19-norandrosterone glucuronide-d4 as an isotopically labelled internal standard,
and the use of a bench-top quadrupole gas chromatograph–mass spectrometer. The results produced by LGC (2.14 ± 0.15 ng g−1 expanded uncertainty, coverage factor k = 2) were in excellent agreement with those from other participating national metrology institutes and thus further validates
the exact-matching isotope-dilution mass spectrometric procedures used at LGC for a wide range of reference measurement applications,
including measurement of ng g−1 levels of steroids in a biological matrix. 相似文献
997.
The evidence for the stabilizing nature of the H–H bonding in planar biphenyl is succinctly reviewed. The stabilizing nature
of the H–H bonding is revealed through a comparison of the atomic energy of every atom in planar biphenyl with the same atom
in the twisted equilibrium structure. It is shown that the barrier to rotation via the planar transition state is the net
resultant of a stabilisation of the four ortho-hydrogen atoms (by 8 kcal/mol each), a stabilisation of the two para-carbon atoms (by 3 kcal/mol each) and by the dominant destabilisation of the two carbon atoms joining the two rings—the two
junction carbon atoms—(by 22 kcal/mol each). The energetic stabilisation of the four ortho-hydrogen atoms is further shown to be in large proportion due to the formation of the hydrogen–hydrogen interatomic surface.
Furthermore, neither the “bond order” between the two junction carbon atoms nor the total electron delocalisation between
the two rings exhibit a significant change in going from the planar to the twisted equilibrium geometry. These findings are
in contrast with the classical view of a balance between “steric non-bonded repulsion” and better electron delocalisation
as a function of the twist dihedral angle. Similar conclusions have been recently reached by Pacios and Gómez through a study
of the electrostatic potential at the position of the hydrogen nuclei.
We dedicate this article to Professor TM Krygowski on the occasion of his 70th birthday wishing him a long and productive
life. 相似文献
998.
Bjrn E. Berg Ellen M. Hansen Synnve Gjrven Tyge Greibrokk 《Journal of separation science》1993,16(6):358-363
Supercritical carbon dioxide can be utilized both as an introduction solvent in capillary SFC and as a reaction medium; both possibilities are illustrated in this study. An off-line SFE unit was modified for on-line SFE-SFC. To facilitate rapid depressurization of the extraction cell and to prevent memory effects, a 6-port valve was installed at the outlet line of the extraction unit. An increased background signal was obtained when PEEK polymer was used in the construction of the extraction vessel; when stainless steel was used, the blanks improved. The synthesis of methyl esters and butyl esters of fatty acids from triglycerides in edible fat was examined using an immobilized lipase as a catalyst in on-line SFE-SFC. As a result of 30 minutes reaction – extraction time, high yields of fatty acid esters were obtained at a pressure of 150 bar and a temperature of 50°C. 相似文献
999.
A multi-wall carbon nanotube (MWNT)/cetyl pyridine bromine (CPB) composite film modified glassy carbon electrode (GCE) was
developed for the electrochemical determination of hymecromone in phosphonate buffer. Electrochemical behaviour of hymecromone
at the composite film electrode was investigated with voltammetry. Compared with an irreversible oxidation of hymecromone
at the bare GCE, the oxidation peak current was enhanced greatly at the film electrode. Some parameters such as pH, scan rate,
accumulation potential and accumulation time were optimized. Under optimal conditions, an oxidation peak at 0.82 V was employed
to determine hymecromone electrochemically. A linearity between the oxidation peak current and the hymecromone concentration
was obtained in the range of 3.0 × 10−7 − 2.0 × 10−5 mol 1−1 with a detection limit of 8.0 × 10−8 mol 1−1. The proposed procedure was successfully applied to assay hymecromone in pharmaceutical formulation with satisfactory results.
The text was submitted by the authors in English. 相似文献
1000.
Zhang De-Qing Wang Shao-Jun Sun Hong-Shan Wang Xiu-Li Cao Mao-Sheng 《Journal of Sol-Gel Science and Technology》2007,41(2):157-161
Lead zirconate titanate nanopowders Pb(Zr0.52Ti0.48)O3 (PZT) were prepared by modified sol-gel process in ethylene glycol system with zirconium nitrate as the zirconium source.
The research showed that it was critical to add lead acetate after the reaction of zirconium nitrate and tetrabutyl titanate
in ethylene glycol system for preparing PZT of exact titanium content. The reaction mechanisms of the sol synthesis, preparation
of xerogel and agglutinating process were characterized through using FT-IR, NMR, TG-FTIR, and GC-MS. The experiment proved
that ethylene glycol system did not rely on hydrolysis and condensation reactions in the process of the sol formation, but
on the formation of chain or network large molecules from complexation of ethylene glycol and all Ti and minor Pd, Zr. In
the preparation of xerogel, the complexation reaction was so completed that it formed large molecules network composed of
metal and dioxyethyl. Bulk weight loss happened before 350°C in the process of sintering xerogel to prepare PZT nanopowders.
Volatile matters and vapor phase decomposition resultants were primarily oxy-compounds including ethylene glycol, aldehyde-ketone
compounds, carbon dioxide and nitrate radical conversion matters. After 350°C, primary vapor phase decomposition resultants
were carbon dioxide and minor carbonyl compound. 相似文献