全文获取类型
收费全文 | 2551篇 |
免费 | 87篇 |
国内免费 | 16篇 |
专业分类
化学 | 1926篇 |
晶体学 | 49篇 |
力学 | 89篇 |
数学 | 139篇 |
物理学 | 451篇 |
出版年
2023年 | 15篇 |
2022年 | 42篇 |
2021年 | 60篇 |
2020年 | 53篇 |
2019年 | 77篇 |
2018年 | 68篇 |
2017年 | 57篇 |
2016年 | 106篇 |
2015年 | 82篇 |
2014年 | 143篇 |
2013年 | 224篇 |
2012年 | 191篇 |
2011年 | 201篇 |
2010年 | 118篇 |
2009年 | 87篇 |
2008年 | 149篇 |
2007年 | 136篇 |
2006年 | 124篇 |
2005年 | 113篇 |
2004年 | 79篇 |
2003年 | 62篇 |
2002年 | 62篇 |
2001年 | 16篇 |
2000年 | 27篇 |
1999年 | 13篇 |
1998年 | 20篇 |
1997年 | 17篇 |
1996年 | 21篇 |
1995年 | 16篇 |
1994年 | 19篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1991年 | 19篇 |
1990年 | 8篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 15篇 |
1984年 | 20篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 14篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1969年 | 5篇 |
1964年 | 5篇 |
排序方式: 共有2654条查询结果,搜索用时 15 毫秒
21.
22.
Chandra Raju 《Czechoslovak Journal of Physics》1986,36(12):1350-1357
In this paper, we show that 2m
e
m
/(m
e
2
+m
2
= (g
V/g
A)
e
2
. From this expression, the Weinberg mixing parameter is shown to be 0·2254 or 0·2746. Assuming that the electron and muon neutrino masses are degenerate, we find thatm
v = (g
V/g
A)
e
2
. (m
e
m
)/M
WL, where MWL is the mass of the standard W± boson. The neutrino mass turns out to be 6·5 eV. The -neutrino mass is found to be about 81 MeV. The masses of c, t, s and b quarks are found to be respectively 1·7 GeV, 21·2 GeV, 0·57 GeV and 2·18 GeV by assumingm
d=m
u= 0·3 GeV. All these masses agree with other estimates except the b quark which has about half of its expected value. 相似文献
23.
Suresh C. Shrivastawa Vikash C. Garg Arun K. Dey 《Fresenius' Journal of Analytical Chemistry》1969,248(5-6):305-306
Summary Eriochromecyanine RC has been used as chromogenic reagent for the photometric determination of rhodium(III) on a micro scale. A coloured Rh(III)-ECRC chelate with
max 530 nm at pH 4.0 (25°C) is formed. A detailed study has been made concerning to characteristics, composition and stability of the chelate. Standard deviation was ±1.17%.
Über die photometrische Bestimmung von dreiwertigem Rhodium mit Eriochromcyanin RC
Zusammenfassung Ergebnisse einer Untersuchung über die Zusammensetzung und Stabilität des Komplexes werden mitgeteilt. Das Absorptionsmaximum liegt bei 530 nm (pH 4,0; 25°C). Bestimmungen konnten mit einer Standardabweichung von ±1,17% durchgeführt werden.相似文献
24.
Basheer C Jayaraman A Kee MK Valiyaveettil S Lee HK 《Journal of chromatography. A》2005,1100(2):137-143
A novel sorbent, dihydroxylated polymethylmethacrylate (DHPMM), coated on hollow-fiber membrane, is used for the polymer-coated hollow-fiber microextraction of trace amounts of natural and synthetic estrogens, such as diethylstilbestrol, estrone, 17beta-estradiol and 17alpha-ethynylestradiol, in aqueous samples. In this procedure, estrogens were extracted using the functionalized polar DHPMM polymer with derivatization using N-methyl-N-(trimethylsilyl)trifluoroacetamide followed by gas chromatography-mass spectrometric analysis. The detection limits for estrogens in aqueous sample were between 0.03 and 0.8 ng l(-1) and the calibration curves were linear over the concentration range 0.05-10 microgl(-1) and had correlation coefficients of >0.994. The relative standard deviations (RSDs) were <15% (n = 3). This simple, accurate, sensitive and selective analytical method is applicable to the determination of trace amounts of estrogens in reservoir and potable water samples. 相似文献
25.
C. G. R. Nair Suresh Mathew K. N. Ninan 《Journal of Thermal Analysis and Calorimetry》1991,37(10):2325-2334
The thermal decomposition of tris(ethylenediamine)copper(II) sulphate has been studied using TG, DTG and DTA. The different stages of decomposition have been identified by these techniques in conjunction independent pyrolysis and X-ray diffraction. The kinetics and mechanism of the first two stages of deamination of the complex were evaluated. The activation parameters for the deamination reaction were computed from the TG and DTA curves using four integral methods. The two stages of deamination follow the mechanism of random nucleation with the formation of one nucleus on each particle (Mampel equation). The thermodynamic parameter namely heat of reaction (DH) for the two deamination processes was also evaluated.
The authors are grateful to the authorities of Vikram Sarabhai Space Centre for providing the instrumental facilities. The help of Dr. K. Krishnan and Mr. Viswanathan Asari in the TG/DTA instrumental work is gratefully appreciated. One of the authors (S.M.) acknowledges with gratitude the aid given to him by the Council of Scientific and Industrial Research (India) in the form of a senior research fellowship for carrying out this work. 相似文献
Zusammenfassung Mittels TG, DTG und DTA wurde die thermische Zersetzung von Tris(ethylendiamin)-kupfer(II)-sulfat untersucht. Anhand dieser Methoden wurden in Verbindung mit einer gesonderten Pyrolyse und Röntgendiffraktion die einzelnen Schritte dieser Zersetzung identifiziert. Weiterhin wurde die Kinetik und der Mechanismus der beiden ersten Schritte der Desaminierung des Komplexes entwickelt. Unter Anwendung von vier Integralmethoden wurden aus den TG- und DTA-Kurven die Aktivierungsparameter der Desaminierungsreaktion berechnet. Beide Schritte der Desaminierung verlaufen nach dem Mechanismus der Random-Keimbildung mit der Bildung von einem Keim pro Partikel (Mampel-Gleichung). Die Reaktionswärme der zwei Desaminierungsprozesse wurde ebenfalls bestimmt.
The authors are grateful to the authorities of Vikram Sarabhai Space Centre for providing the instrumental facilities. The help of Dr. K. Krishnan and Mr. Viswanathan Asari in the TG/DTA instrumental work is gratefully appreciated. One of the authors (S.M.) acknowledges with gratitude the aid given to him by the Council of Scientific and Industrial Research (India) in the form of a senior research fellowship for carrying out this work. 相似文献
26.
C. Suresh Reddy Y. Hari Babu N. Ravi Sankar C. Devendranath Reddy 《Journal of heterocyclic chemistry》2004,41(6):1027-1031
Novel 3‐(substituted)‐2,4,8,15‐tetroxa‐3‐phosphadispiro[5.2.5]hexadecane‐3‐oxides (3‐12) have been synthesized by cyclization of 1,5‐dioxaspiro[5.5]undecande‐3,3‐dimethanol (1) with various substituted aryl phosphorus dichloridates (2) in dry toluene‐THF in the presence of triethylamine at 40‐60 oC. Their molecular structures were determined by ir, nmr and mass spectral studies and were screened for antifungal activity against Curvularia lunata and Aspergillus niger, and antibacterial activity on Staphylococcus aureus and Escherichia coli. Most of them possess significant activity. 相似文献
27.
Oxidative transformation of ciprofloxacin by alkaline permanganate – A kinetic and mechanistic study
Kiran A. Thabaj Suresh D. KulkarniShivamurti A. Chimatadar Sharanappa T. Nandibewoor 《Polyhedron》2007
This spectroscopic study presents the kinetics and degradation pathways of oxidation of ciprofloxacin by permanganate in alkaline medium at constant ionic strength of 0.04 mol−3. Orders with respect to substrate, oxidant and alkali concentrations were determined. Effect of ionic strength and solvent polarity of the medium on the rate of the reaction was studied. The oxidation products were identified by LC-ESI-MS technique. Product characterization of ciprofloxacin reaction mixtures indicates the formation of three major products corresponding to m/z 263, 306, and 348 (corresponding to full or partial dealkylation of the piperazine ring). The piperazine moiety of ciprofloxacin is the predominant oxidative site to KMnO4. Product analyses showed that oxidation by permanganate results in dealkylation at the piperazine moiety of ciprofloxacin, with the quinolone ring essentially intact. The reaction kinetics and product characterization point to a reaction mechanism that likely begins with formation of a complex between ciprofloxacin and the KMnO4, followed by oxidation at the aromatic N1 atom of piperazine moiety to generate an anilinyl radical intermediate. The radical intermediates subsequently undergo N-dealkylation. Investigations of the reaction at different temperatures allowed the determination of the activation parameters with respect to the slow step of proposed mechanism. The proposed mechanism and the derived rate laws are consistent with the observed kinetics. 相似文献
28.
The reaction of cis-[ClP(μ-NtBu)]2 with N,N′-dimethylurea leads to the formation of both mono- and di-substituted derivatives [ClP(μ-NtBu)2P(NMeCON(H)Me)] and [(μ-NtBu)P(NMeCON(H)Me)]2, instead of monomeric, dimeric or higher oligomeric macrocycles. The structure of [ClP(μ-NtBu)2P(NMeCON(H)Me)] shows rare non-bonded P?Cl and intermolecular hydrogen bonding interactions leading to a 2D-sheet like structure. 相似文献
29.
The problem of calculating the thermodynamic properties of two-dimensional semiclassical hard-body fluids is studied. Explicit expressions are given for the first-order quantum corrections to the free energy, equation of state, and virial coefficients. The numerical results are calculated for the planar hard dumbbell fluid. Significant features are the increase in quantum corrections with increasing eta and increasing L*=L/sigma(0). 相似文献
30.
An efficient and mild method for the reduction of aromatic nitro and azido groups on solid support using Al/NiCl2·6H2O and Al/NH4Cl is described. This solid phase reduction technique has been applied towards the synthesis of DNA binding pyrrolo[2,1-c][1,4]benzodiazepine antitumour antibiotics. 相似文献