全文获取类型
收费全文 | 37325篇 |
免费 | 1274篇 |
国内免费 | 9篇 |
专业分类
化学 | 21009篇 |
晶体学 | 166篇 |
力学 | 774篇 |
数学 | 6173篇 |
物理学 | 10486篇 |
出版年
2023年 | 201篇 |
2021年 | 301篇 |
2020年 | 481篇 |
2019年 | 322篇 |
2018年 | 434篇 |
2017年 | 415篇 |
2016年 | 962篇 |
2015年 | 828篇 |
2014年 | 933篇 |
2013年 | 2043篇 |
2012年 | 1569篇 |
2011年 | 1919篇 |
2010年 | 967篇 |
2009年 | 802篇 |
2008年 | 1842篇 |
2007年 | 1799篇 |
2006年 | 1595篇 |
2005年 | 1399篇 |
2004年 | 1107篇 |
2003年 | 949篇 |
2002年 | 897篇 |
2001年 | 737篇 |
2000年 | 660篇 |
1999年 | 449篇 |
1998年 | 408篇 |
1997年 | 407篇 |
1996年 | 509篇 |
1995年 | 455篇 |
1994年 | 474篇 |
1993年 | 555篇 |
1992年 | 454篇 |
1991年 | 346篇 |
1990年 | 383篇 |
1989年 | 308篇 |
1988年 | 324篇 |
1987年 | 325篇 |
1986年 | 294篇 |
1985年 | 471篇 |
1984年 | 459篇 |
1983年 | 422篇 |
1982年 | 434篇 |
1981年 | 438篇 |
1980年 | 401篇 |
1979年 | 361篇 |
1978年 | 404篇 |
1977年 | 386篇 |
1976年 | 333篇 |
1975年 | 320篇 |
1974年 | 304篇 |
1973年 | 311篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
[11C]Carbon monoxide at low concentrations, aryl halides and amines were used in the palladium-mediated synthesis of twenty 11C-amides. In the study several approaches to improve the radiochemical yield were explored. Eight of the selected amides were prepared by in situ activation of the amines using lithium bis(trimethylsilyl)amide and the radiochemical yields of these reactions were improved compared to utilising a previous reported method. In the synthesis of 1-[carbonyl-11C]benzoyl-3-methyl-1H-indole (11) from 3-methyl-1H-indole (25), the corresponding organotin-amine was prepared prior to the acylation reaction. In a typical experiment, N-(4-hydroxyphenyl)[carbonyl-11C]acetamide (5) was prepared in 15% radiochemical yield using 4-aminophenol (20) but the yield increased to 63% when the amine was activated by lithium bis(trimethylsilyl)amide. 相似文献
942.
Helvi Hödrejärv 《Analytical and bioanalytical chemistry》1997,357(2):191-196
The history of analytical chemistry, started in 1802, can be divided into different periods: 1802–1892 the Baltic German period in Universitas Dorpatensis, 1892–1917 the period of russification in the University, 1918–1939 the independent Estonian University of Tartu and opening of the chair of inorganic and analytical chemistry at Tallinn Technical University, 1940–1944 loss of independence and years of war, 1945–1991 the years in the Soviet Union, 1991-up to now the re-establishment of independence. The first chemical laboratory was founded at Universitas Dorpatensis by Prof. Ph. Arzt in 1802. The first practice in analytical chemistry was opened in 1847 by Prof. C. Göbel. The chair of chemistry was separated from pharmacy in 1850. A separate chair of analytical chemistry was opened at Tartu University in 1947. 相似文献
943.
Amperometric method for determining the degree of complexation of polyelectrolytes with cationic surfactants 总被引:1,自引:0,他引:1
Hakkarainen S Gilbert SL Kontturi AK Kontturi K 《Journal of colloid and interface science》2004,272(2):404-410
The complexation of sodium polystyrene sulfonate with monovalent cationic surfactants at a microsized liquid/liquid interface has been studied using electrochemistry. The method is based on measurement of surfactant ion transfer across the interface between two immiscible electrolyte solutions (ITIES). The complexation of various cationic surfactants (alkylpyridinium- and trimethylammonium-) with oligosized polystyrene sulfonate was measured. Binding isotherms were used to determine the degree of binding as a function of the surfactant chain length and type of head group. It was found that the hydrophobicity of the surfactant was the predominant factor. The effect of the polyelectrolyte chain length on the binding mechanism was studied using cetylpyridinium chloride as a complexing agent. It was found that binding affinity, as well as cooperativity of the binding process, decreases with decreasing polyelectrolyte chain length. Thermodynamics of surfactant binding was measured using titration microcalorimetry. The thermodynamic data obtained show that the enthalpy of surfactant binding is not dependent on polymer chain length, but an increase in chain length makes the binding process entropically more favorable. 相似文献
944.
S. Stichleutner E. Kuzmann Z. Homonnay Z. Klencsár A. Vértes K. Havancsák W. T. König O. Heiml G. Gritzner 《Journal of Radioanalytical and Nuclear Chemistry》2004,261(1):133-135
57Fe Mössbauer spectroscopy was used to study the effects of heavy ion irradiation on (Bi0.93Pb0.17)Sr1.9Ca2.05(Cu1.02Fe0.01)3Oy superconductors. The Mössbauer spectra of the irradiated superconductor showed significant changes in the subspectra belonging to different Cu microenvironments into which Fe ions were substituted. The relative occurrence of Fe occupying the square pyramidal fivefold oxygen coordinated Cu sites decreased while Fe3 + substituting the square planar fourfold oxygen coordinated Cu sites increased upon bombardment by 246 MeV 86Kr8+ ions. This observation was interpreted on the basis of partial replacement of the apical oxygen atoms in the square pyramids surrounding one of the Cu sites. Such oxygen vacancies create additional square planar coordination sites for Cu. These results also support the covalent character of Cu-O bonds in the superconducting layer as well as the ionic character of bond of apical oxygen to Cu. 相似文献
945.
R. Riemschneider B. Böttcher S. Georgi 《Monatshefte für Chemie / Chemical Monthly》1960,91(4):630-638
Zusammenfassung Benzthiazoliumsalze (I) reagieren, wie an 20 Beispielen gezeigt wird, mit nucleophilen Partnern unter gleichzeitiger Labilisierung des Oniumsystems zu 2-substituierten 2,3-Dihydro-benzthiazolen (dequartärisierende nucleophile Substitution). Der Reaktionsverlauf wird von der Art der nucleophilen Komponente bestimmt: Freie Amino-(Hydrazino)-Verbindungen und I lassen sich im Molverhältnis 21, die Alkalisalze von Aminocarbonsäuren und-sulfosäuren im Molverhältnis 11 umsetzen. Verbindungen mit reaktiver Methylengruppierung reagieren glatt mit der äquivalenten Menge I, wenn in Gegenwart einer Base gearbeitet wird. Die verwendeten I-Verbindungen sind: 2-Äthoxy-3-methyl-benzthiazolium-tetrafluoroborat (VII) und 2-Methylmercapto-3-methylbenzthiazolium-methylsulfat (VIII). 相似文献
946.
A. Uhrig L. Banyai S. Gaponenko A. Wörner N. Neuroth C. Klingshirn 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,20(1):345-348
In this contribution we present and discuss our measurements on CdS1?x Se x quantum dots in a glass matrix. In linear absorption measurements we find the typical blue shift of the transitions with decreasing crystallite radius due to quantization. The luminescence shows a significant Stokes shift with respect to absorption, which is interpreted in terms of strong exciton-phonon coupling and allows to deduce the Huang-Rhys factor S. Under high excitation we find an additional emission band on the high energy side, which can be attributed to the recombination of an excited two electron-hole pair state to a one electron-hole pair state in agreement with theory. Pump and probe beam experiments give a bleaching but no hole burning. Finally we discuss some open questions especially concerning the high energy structures in the absorption spectrum. 相似文献
947.
Novel prostaglandin F(2)(alpha) derivatives, functionalized at C13 and C14, have been prepared. 17-Phenyl-18,19,20-trinorprostaglandin F(2)(alpha) isopropyl ester [(15S)-1] and its epimer [(15R)-1] were stereoselectively epoxidized, using Sharpless conditions, to produce each of the four diastereomeric epoxides (15S)-2, (15S)-3, (15R)-2, and (15R)-3. Treatment of the four epoxides with LiOH stereospecifically-produced the pentahydroxy substituted analogues 12 and 13. Alternatively, epoxides 2 and 3were allowed to react with thiophenolate ion. The attack of the sulfur nucleophile on the epoxide occurred at either C13 or C14 depending on the stereochemistry of the epoxide and of C15. 相似文献
948.
Jenö Kóbor Ferenc Fülöp M. Sami El-Gharib Gábor Bernáth 《Journal of heterocyclic chemistry》1984,21(1):149-153
A large number of 4-substituted-9,10-dialkoxy-1,6,7,11b-tetrahydropyrimido[6,1-a]isoquinolin-2-ones were prepared by the reaction of 1-(ethoxycarbonylmethyl)-6,7-dialkoxy-1,2,3,4-tetrahydroisoquinolines with iminoethers. Reaction of the corresponding isoquinoline-1-acetic acid derivatives with iminoethers led to the formation of N-acyl-1,2,3,4-tetrahydroisoquinolin-1-acetamides. In the hydrolysis of the prepared 4-substituted-pyrimido[6,1-a]isoquinolin-2-ones, the corresponding N-acyl-1,2,3,4-tetrahydroisoquinolin-1-acetamides were obtained. While reduction of the 4-phenyl derivative resulted in the corresponding 1,3,4,6,7,11b-hexahydropyrimidinone. The steric structures of the tetrahydro- and hexahydropyrimido[6,1-a]isoquinolines were determined by nmr spectroscopy. 相似文献
949.
von Euler H Olsson JM Hultenby K Thörne A Lagerstedt AS 《Bioelectrochemistry (Amsterdam, Netherlands)》2003,59(1-2):89-98
INTRODUCTION: Electrochemical treatment (EChT) has been taken under serious consideration as being one of several techniques for local treatment of malignancies. The advantage of EChT is the minimal invasive approach and the absence of serious side effects. Macroscopic, histopathological and ultra-structural findings in liver following a four-electrode configuration (dog) and a two-electrode EChT design (dog and rat) were studied. MATERIALS AND METHODS: 30 female Sprague-Dawley rats and four female beagle dogs were studied with EChT using Platinum:Iridium electrodes and the delivered dose was 5, 10 or 90 C (As). After EChT, the animals were euthanized. RESULTS: The distribution of the lesions was predictable, irrespective of dose and electrode configuration. Destruction volumes were found to fit into a logarithmic curve (dose-response). Histopathological examination confirmed a spherical (rat) and cylindrical/ellipsoidal (dog) lesion. The type of necrosis differed due to electrode polarity. Ultra-structural analysis showed distinct features of cell damage depending on the distance from the electrode. Histopathological and ultra-structural examination demonstrated that the liver tissue close to the border of the lesion displayed a normal morphology. CONCLUSIONS: The in vivo dose-planning model is reliable, even in species with larger tissue mass such as dogs. A multi-electrode EChT-design could obtain predictable lesions. The cellular toxicity following EChT is clearly identified and varies with the distance from the electrode and polarity. The distinct border between the lesion and normal tissue suggests that EChT in a clinical setting for the treatment of liver tumours can give a reliable destruction margin. 相似文献
950.
Ernst M Detken A Böckmann A Meier BH 《Journal of the American Chemical Society》2003,125(51):15807-15810
Proteins are not always available in amounts desirable for solid-state magic-angle spinning (MAS) nuclear-magnetic resonance (NMR) spectroscopy. To maximize the signal-to-noise ratio achievable with small samples, the filling factor must be optimized by using small-diameter MAS rotors. These rotors have the added benefit of allowing higher radio frequency field amplitudes during polarization transfer steps and during decoupling periods as well as allowing higher spinning frequencies. We demonstrate the advantages of relatively fast MAS (30 kHz using a 2.5 mm rotor) compared to MAS at 12 kHz for the 10.4 kDa model protein Crh with 93 residues and show that the signal-to-noise ratio in two-dimensional correlation spectra can be significantly improved by taking advantage of optimized pulse sequences available with rapid MAS. 相似文献