全文获取类型
收费全文 | 267篇 |
免费 | 19篇 |
国内免费 | 4篇 |
专业分类
化学 | 179篇 |
力学 | 2篇 |
综合类 | 1篇 |
数学 | 61篇 |
物理学 | 47篇 |
出版年
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 14篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 11篇 |
2017年 | 5篇 |
2016年 | 16篇 |
2015年 | 12篇 |
2014年 | 11篇 |
2013年 | 33篇 |
2012年 | 18篇 |
2011年 | 18篇 |
2010年 | 8篇 |
2009年 | 16篇 |
2008年 | 11篇 |
2007年 | 16篇 |
2006年 | 10篇 |
2005年 | 4篇 |
2004年 | 2篇 |
2003年 | 7篇 |
2002年 | 9篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1975年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有290条查询结果,搜索用时 15 毫秒
101.
Mohamed Lazoul Azzedine Boudrioua Lotfy Mokhtar Simohamed Alexis Fischer Lung Han Peng 《Applied physics. B, Lasers and optics》2013,110(4):459-464
We report a detailed study of multiple wavelengths optical parametric generation in a single grating one-dimensional periodically poled lithium tantalate crystal. Simultaneous collinear and non-collinear generations are observed around the pump collinear direction. Similar spectra are found in continuous spatial positions symmetrical to the collinear direction, with decreasing signal and increasing idler wavelengths, associated with decreasing powers when the far-field angle increases. A phase matching scheme is proposed to describe these interactions. Numerical simulations emphasize the agreement of our phase matching scheme with the experimental results. Single-pass gain and high pump power density are very likely at the origin of the simultaneous collinear and non-collinear QPM interactions. 相似文献
102.
Ehsan Mohammadpour Mokhtar Awang 《Applied Physics A: Materials Science & Processing》2012,106(3):581-588
An effective finite-element (FE) approach for modeling the structure and the deformation of single-walled carbon nanotubes
(SWCNTs) and multi-walled carbon nanotubes (MWCNTs) is presented. An individual tube was modeled using a frame-like structure
with beam elements. The effect of van der Waals forces, crucial in MWCNTs, was modeled by spring elements. The success of
this new carbon nanotube (CNT) modeling approach was verified by comparing the simulation results for single- and multi-walled
nanotubes and graphene with other experimental and computational results available in the literature. Simulations of final
deformed configurations were in excellent agreement with the atomistic models for various deformations. The proposed approach
successfully predicts the experimentally observed values for mechanical behavior of SWCNTs and MWCNTs. The results demonstrated
that the proposed FE technique could provide a valuable tool for studying the mechanical behavior of different types of nanotubes,
as well as their effectiveness as load-bearing entities in nanocomposite materials. 相似文献
103.
Chen T Wang Q Chen R Zhang B Jewart C Chen KP Maklad M Swinehart PR 《Optics letters》2012,37(6):1064-1066
We present spatially resolved Rayleigh scattering measurements in different polarization-maintaining (PM) fibers for high-temperature pressure sensing. The pressure-induced birefringence in the fiber cores is interrogated using polarization-resolved frequency-swept interferometry. The pressure responses of a PM photonic crystal fiber and a twin-air-hole PM fiber are investigated for a pressure range of 0 to 13.8 MPa (0-2000 psi) at room temperature and at temperatures as high as 800 °C. The proposed sensing system provides, for the first time to our knowledge, a truly distributed pressure-sensing solution for high-temperature applications. 相似文献
104.
Awad A. Ibraheem M. A. Hassanain S. R. Mokhtar M. A. Zaki Zakaria M. M. Mahmoud M. El-Azab Farid 《Physics of Atomic Nuclei》2012,75(8):969-972
Microscopic folding calculations based upon the effective M3Y nucleon-nucleon interaction and the nuclearmatter densities of the interacting nuclei have been carried out to explain recently measured experimental data of the 6He+120Sn elastic scattering cross section at four different laboratory energies near the Coulomb barrier. The extracted reaction cross sections are also considered. 相似文献
105.
Jean-Marc Richard Xavier Artru Mokhtar Elchikh Jacques Soffer Oleg Teryaev 《中国物理 C》2009,33(12):1153-1158
We discuss model-independent constraints on spin observables in exclusive and inclusive reactions, with special attention to the case of photoproduction. 相似文献
106.
Dietrich Dpp Mohsen Abdel-Motaal Gomaa Gerald Henkel Ahmed Mokhtar Nour El-Din 《Journal of heterocyclic chemistry》1995,32(2):603-610
Nucleophilic attack by N2 of N1 N2-diarylformamides 1a-c on C-2 of chloranil (2) and subsequently by N1 on C-1 of 2 initiates the formation of benzimidazolinones 8a-c. In contrast, when 1b-e is reacted with 2,3-dichloro-1,4-naphthoquinone (9) , both chlorine atoms are successively substituted by the two nitrogen atoms and 2-(arylamino)-3-(N-formylarylamino)-1,4-naphthoquinones 13b-e result, which (probably via their cyclic tautomers 12b-e ) may be cyclodehydrogenated to form N1,N3-diarylnaphtho[2,3-d] imidazoline-2,4,9-triones (as 14b,c ). On the other hand, N1,N2diarylacetamidines 15a-d attack 2 and 9 at C-2 with N2 but subsequently exert nucleophilic character at the acetamidine α-carbon attacking C-1 of 2 and 9 , respectively, thus forming 1-aryl-2-(arylimino)-3a-hydroxy-2,3,3a,6-tetrahydro-1N-indol-6-ones 18a-d and 3-aryl-2-(arylimino)-9b-hydroxy-2,3,5,9b-tetrahydro-1-H-benz[e]indol-5-ones 19b,c , respectively. The latter may be thermally dehydrated to the fully conjugated 2,5-dihydro-3H-benz[e]indol-5-ones 20b,c. Unambiguous structural assignments for 18b and 20c are made on the basis of X-ray crystal structure analyses. 相似文献
107.
Taibi Z Saoudi B Boudelaa M Trigui H Belghith H Gargouri A Ladjama A 《Applied biochemistry and biotechnology》2012,166(3):663-679
An extracellular thermostable xylanase from a newly isolated thermophilic Actinomadura sp. strain Cpt20 was purified and characterized. Based on matrix-assisted laser desorption–ionization time-of-flight mass
spectrometry analysis, the purified enzyme is a monomer with a molecular mass of 20,110.13 Da. The 19 residue N-terminal sequence
of the enzyme showed 84% homology with those of actinomycete endoxylanases. The optimum pH and temperature values for xylanase
activity were pH 10 and 80 °C, respectively. This xylanase was stable within a pH range of 5–10 and up to a temperature of
90 °C. It showed high thermostability at 60 °C for 5 days and half-life times at 90 °C and 100 °C were 2 and 1 h, respectively.
The xylanase was specific for xylans, showing higher specific activity on soluble oat-spelt xylan followed by beechwood xylan.
This enzyme obeyed the Michaelis–Menten kinetics, with the K
m and k
cat values being 1.55 mg soluble oat-spelt xylan/ml and 388 min−1, respectively. While the xylanase from Actinomadura sp. Cpt20 was activated by Mn2+, Ca2+, and Cu2+, it was, strongly inhibited by Hg2+, Zn2+, and Ba2+. These properties make this enzyme a potential candidate for future use in biotechnological applications particularly in
the pulp and paper industry. 相似文献
108.
Qingyun Qian Dr. Javier Ruiz‐Martínez Prof. Dr. Mohamed Mokhtar Prof. Dr. Abdullah M. Asiri Prof. Dr. Shaeel A. Al‐Thabaiti Prof. Dr. Suliman N. Basahel Hendrik E. van der Bij Dr. Jan Kornatowski Prof. Dr. Bert M. Weckhuysen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(34):11204-11215
The formation of hydrocarbon pool (HCP) species during methanol‐to‐olefin (MTO) and ethanol‐to‐olefin (ETO) processes have been studied on individual micron‐sized SAPO‐34 crystals with a combination of in situ UV/Vis, confocal fluorescence, and synchrotron‐based IR microspectroscopic techniques. With in situ UV/Vis microspectroscopy, the intensity changes of the λ=400 nm absorption band, ascribed to polyalkylated benzene (PAB) carbocations, have been monitored and fitted with a first‐order kinetics at low reaction temperatures. The calculated activation energy (Ea) for MTO, approximately 98 kJ mol?1, shows a strong correlation with the theoretical values for the methylation of aromatics. This provides evidence that methylation reactions are the rate‐determining steps for the formation of PAB. In contrast for ETO, the Ea value is approximately 60 kJ mol?1, which is comparable to the Ea values for the condensation of light olefins into aromatics. Confocal fluorescence microscopy demonstrates that during MTO the formation of the initial HCP species are concentrated in the outer rim of the SAPO‐34 crystal when the reaction temperature is at 600 K or lower, whereas larger HCP species are gradually formed inwards the crystal at higher temperatures. In the case of ETO, the observed egg‐white distribution of HCP at 509 K suggests that the ETO process is kinetically controlled, whereas the square‐shaped HCP distribution at 650 K is indicative of a diffusion‐controlled process. Finally, synchrotron‐based IR microspectroscopy revealed a higher degree of alkylation for aromatics for MTO as compared to ETO, whereas high reaction temperatures favor dealkylation processes for both the MTO and ETO processes. 相似文献
109.
110.
New selective and sensitive Spectrophotometric methods for quinone determination are described. Quinone samples are treated with either methanolic solution of rhodanine, fluorene in ammoniacal media or aqueous semicarbazide hydrochloride solution. Semicarbazide is the most sensitive reagent for the determination of 1,4-benzoquinone, 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone. Quinones substituted with electron withdrawing groups are better determined using rhodanine. The average recovery is 99.1%, 98.7% and 99.2% using rhodanine, fluorene and semicarbazide as reagents, respectively. 相似文献