全文获取类型
收费全文 | 1691篇 |
免费 | 58篇 |
国内免费 | 8篇 |
专业分类
化学 | 1102篇 |
晶体学 | 57篇 |
力学 | 30篇 |
数学 | 167篇 |
物理学 | 401篇 |
出版年
2023年 | 15篇 |
2022年 | 33篇 |
2021年 | 40篇 |
2020年 | 41篇 |
2019年 | 53篇 |
2018年 | 45篇 |
2017年 | 37篇 |
2016年 | 60篇 |
2015年 | 41篇 |
2014年 | 58篇 |
2013年 | 119篇 |
2012年 | 108篇 |
2011年 | 114篇 |
2010年 | 73篇 |
2009年 | 73篇 |
2008年 | 99篇 |
2007年 | 86篇 |
2006年 | 53篇 |
2005年 | 57篇 |
2004年 | 49篇 |
2003年 | 33篇 |
2002年 | 35篇 |
2001年 | 36篇 |
2000年 | 59篇 |
1999年 | 58篇 |
1998年 | 18篇 |
1997年 | 19篇 |
1996年 | 12篇 |
1995年 | 13篇 |
1994年 | 10篇 |
1993年 | 15篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 11篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1977年 | 12篇 |
1976年 | 7篇 |
1975年 | 6篇 |
1974年 | 8篇 |
1973年 | 5篇 |
排序方式: 共有1757条查询结果,搜索用时 46 毫秒
141.
Pradeep Mathur Raj Kumar Joshi Amrendra K. Singh 《Journal of organometallic chemistry》2010,695(24):2687-2694
A facile, one pot, high yield synthesis of α,β-vinylester (1-14) and alkoxy substituted γ-lactones (15-28) has been achieved by the photochemical reaction of terminal acetylene (ferrocenyl phenyl trimethylsillyl, hexyl and cyclohexyl) with alcohol (methanol, ethanol and isopropanol) and carbon monoxide in presence of iron pentacarbonyl as a catalyst. The selectivity of the compounds depends on the time of photolysis of the reaction as well as the solvent used. A stable reaction intermediate ferrole was isolated, and further photolysis with alcohols, resulted in the formation of α,β-vinylester. All the compounds were fully characterised by spectroscopic methods and the molecular structures of compounds 1, 16, 17 and 20 were established crystallographically. 相似文献
142.
The potentiometric response characteristics of zinc ion selective PVC-based membrane electrode employing 1,12,14-triaza-5,8-dioxo-3(4),9(10)-dibenzoyl-1,12,14-triene as an inophore was investigated. The proposed electrode exhibits a Nernstian behavior with a slope of 29.2 ± 0.4 mV per decade with a working concentration range of 1.3 × 10?7–1.0 × 10?1 mol L?1 and a detection limit of 1.0 × 10?8 mol L?1. The membrane having the composition as TDODBCPT:O-NPOE:PVC:OA; 7:57:30:6 wt.% exhibits the best results. It has a fast response time of 7 s and can be used for at least 100 days without any considerable divergence in potential. The proposed electrode show good discrimination of Zn2+ ion from diverse ions. The potential response remains constant over a pH range of 3.5–9.2. The electrode found well work under laboratory conditions. The proposed sensor directly used for determination of zinc ions in human hair sample, wastewater and an indicator electrode with EDTA titration. 相似文献
143.
Haladhara Naik Guinyun Kim Ashok Goswami Sarbjit Singh Vijay Kumar Manchanda Devesh Raj Srinivasan Ganesan Young Do Oh Hee-Seock Lee Kyung Sook Kim Man-Woo Lee Moo-Hyun Cho In Soo Ko Won Namkung 《Journal of Radioanalytical and Nuclear Chemistry》2010,283(2):439-445
The mass–yield distributions of various fission products have been determined in the 50-, 60- and 70-MeV end point bremsstrahlung induced fission of natPb using off-line γ-ray spectrometric technique in the electron linac at Pohang Accelerator Laboratory, Korea. The mass–yield distributions are symmetric with average mass of 102.34, 102.25 and 102.03 and FWHM of 21, 22 and 23 mass unit, respectively. From the present data and literature data in the 50–85 MeV bremsstrahlung induced fission of 209Bi the following observations were obtained: (i) The average masses of the yield distributions in the 50–85 MeV bremsstrahlung induced fission of natPb and 209Bi are around 102.25 ± 0.25 and 103 ± 0.5, respectively. (ii) The FWHM of the mass–yield distributions increases from about 21 mass units at 50 MeV to 23 mass units at 70–85 MeV, which is explained from the point of increase in multi-chance fission probability with increasing excitation energy. (iii) Within the bremsstrahlung energy range of 50–85 MeV, the role of nuclear structure effect in the mass–yield distribution was observed in the photo-fission of 209Bi, whereas it was not seen in case of natPb. This may be due to the presence of so many isotopes in natPb unlike mono-isotopic 209Bi. 相似文献
144.
Addition of α-arylmethylidene- or α-alkylidene-β-keto ester enolate to N-activated aldimines via the imino aldol pathway followed by intramolecular aza-Michael reaction in a domino fashion has been developed, and a highly diastereoselective route to substituted piperidines is reported. Enantiopure piperidines are synthesized from chiral sulfinyl imines. Formation and the observed stereoselectivity of the products have been rationalized by mechanistic and computational studies. 相似文献
145.
Gautam Prasanna Kar Anirban Karmakar Jubaraj B. Baruah 《Journal of chemical crystallography》2010,40(8):702-706
Abstract
Crystal structures of two bis-phenols namely bis-(3,5-dimethyl-2-hydroxyphenyl)(3-amino phenyl)methane 1 and bis-(3,5-dimethyl-4-hydroxyphenyl)(3-aminophenyl) methane 2 are determined. The compound 1 crystallises in monoclinic P21/c with a = 12.2579(16) ?, b = 16.0906(19) ?, c = 10.6664(13) ?, β = 115.417(7)°, V = 1900.2(4) ?3 whereas 2 crystallizes in monoclinic C2/c, a = 9.2538(2) ?, b = 18.6579(4) ?, c = 23.2725(5) ?, β = 98.796(2)°, V = 3970.89(15) ?3. The crystal lattice of both the compounds shows presence of N–H···π interactions but no O–H···π interactions. 相似文献146.
Cosmological models with variable G in C-field cosmology for barotropic fluid distribution in FRW space-time are investigated. To get the deterministic model of the
universe, we have assumed that G=R
n
where R is the scale factor and n the constant. To obtain the results in terms of cosmic time t, we have assumed n=−1. We find that for n=−1, Creation field (C) and spatial volume increase with time, G and ρ (matter density) decreases with time, the model represent accelerating universe. Thus inflationary scenario exists in the
model. The model is also free from horizon. The results so obtained match with the astronomical observations. 相似文献
147.
The radiative neutron capture on lithium-7 is calculated model independently using a low-energy halo effective field theory. The cross section is expressed in terms of scattering parameters directly related to the S-matrix elements. It depends on the poorly known p-wave effective range parameter r(1). This constitutes the largest uncertainty in traditional model calculations. It is explicitly demonstrated by comparing with potential model calculations. A single parameter fit describes the low-energy data extremely well and yields r(1)≈-1.47 fm(-1). 相似文献
148.
H. Naik P. M. Prajapati S. V. Suryanarayana P. N. Pathak D. R. Prabhu V. Chavan D. Raj P. C. Kalsi A. Goswami S. Ganesan V. K. Manchanda 《The European Physical Journal A - Hadrons and Nuclei》2011,47(8):1-5
The νp process is a primary nucleosynthesis process which occurs in core-collapse supernovae. An essential role in this process is being played by electron antineutrinos. They generate, by absorption on protons, a supply of neutrons which, by (n, p) reactions, allow to overcome waiting point nuclei with rather long beta-decay and proton-capture lifetimes. The synthesis of heavy elements by the νp process depends sensitively on the $\bar \nu _e$ luminosity and spectrum. As has been shown recently, the latter are affected by collective neutrino flavor oscillations which can swap the $\bar \nu _e$ and $\bar \nu _{\mu ,\tau }$ spectra above a certain split energy. Assuming such a swap scenario, we have studied the impact of collective neutrino flavor oscillations on the νp-process nucleosynthesis. Our results show that the production of light p-nuclei up to mass number A = 108 is very sensitive to collective neutrino oscillations. 相似文献
149.
In this paper, multilayer structures of porous silicon were fabricated by using electrochemical etching and characterized for its optical properties and surface morphology. Samples of monolayer of porous silicon were grown to study the characteristics of porous layer formation with respect to applied current density, etching time and hydrofluoric acid concentrations. Photoluminescence peaks of red emission at wavelength 695 and 650 nm were observed from multilayer porous silicon structures. By atomic force microscopy measurement, hillocks like surface were clearly observed within the host material, which confirmed the formation of pores. 相似文献
150.
D.M. Raj KumarM. Manivel Raja K. PrabaharV. Chandrasekaran Asok PoddarR. Ranganathan K.G. Suresh 《Journal of magnetism and magnetic materials》2011,323(13):1750-1754
The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd5SixGe4−x compounds with x=1.7-2.3. Microstructural studies reveal the presence of Gd5(Si,Ge)4-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x=1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x=2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100 K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of −8 μV/K was obtained at the magneto-structural transition for the x=2 compound. 相似文献