全文获取类型
收费全文 | 588篇 |
免费 | 3篇 |
国内免费 | 3篇 |
专业分类
化学 | 432篇 |
晶体学 | 3篇 |
力学 | 30篇 |
数学 | 44篇 |
物理学 | 85篇 |
出版年
2022年 | 28篇 |
2021年 | 37篇 |
2020年 | 16篇 |
2019年 | 14篇 |
2018年 | 9篇 |
2017年 | 14篇 |
2016年 | 11篇 |
2015年 | 7篇 |
2014年 | 18篇 |
2013年 | 51篇 |
2012年 | 22篇 |
2011年 | 24篇 |
2010年 | 25篇 |
2009年 | 14篇 |
2008年 | 21篇 |
2007年 | 29篇 |
2006年 | 35篇 |
2005年 | 23篇 |
2004年 | 13篇 |
2003年 | 14篇 |
2002年 | 16篇 |
2001年 | 12篇 |
2000年 | 5篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 5篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 3篇 |
排序方式: 共有594条查询结果,搜索用时 234 毫秒
71.
The addition of 10?5 — 2 Torr of NO, a radical scavenger, is found to significantly quench the rate of photonucleation of nonane by NO2 or CH3I in a diffusion cloud chamber. This confirms a recently proposed radical mechanism for the photoinduced nucleation of these systems. The photonucleation rate of nonane induced by o-tolualdehyde (a system whose mechanism is not known) is similarly quenched by the addition of small amounts of NO, suggesting a radical mechanism. A mechanism for this system, based upon the formation of nonane radicals (resulting from hydrogen abstraction from nonane by the carbonyl molecules in the n,π* singlet or triplet state) followed by further reaction of the radicals to form low vapor pressure species, is discussed. Acetone, a system known to dissociate into radicals, is found to photoinduce nucleation of nonane when excited to the lowest singlet or triplet excited states. This adds further support to the proposed radical mechanism and suggests that acetone dissociates in its lowest singlet as well as its lowest triplet state. A theoretical model is outlined in which the production of large involatile alkanes (dimers and higher polymers) are formed from an initially produced nonane radical. These results are combined with binary nucleation theory in order to calculate the effect of these species on the rate of nucleation. These calculations indicate that low concentrations of these involatile species can indeed induce nucleation. The ability of small, photochemically produced polymers to induce nucleation is also examined and the time dependent space distribution of polymers (e.g., vinyl polymers) generated by chain transfer from a single free radical is derived. The small polymers formed in this process are analogous to the species formed in the photoinduced nucleation of alkane vapors. 相似文献
72.
A simple multivariate calibration method for analysis of two types of hypotensive mixture is described. The mixtures are composed of chlorthalidone with atenolol or chlorthalidone with both amiloride hydrochloride and atenolol. The components of the mixtures result in substantial spectral overlap—between 87.5 and 91.0%. Resolution of the mixtures under investigation has been accomplished mainly by using CLS and PCR methods. The components in each mixture have been simultaneously determined in three commercial dosage forms with high accuracy and without interference from commonly encountered excipients and additives. Good recoveries were obtained with both synthetic mixtures and commercial tablets. The results obtained were compared with those from pharmacopeial methods and found to be in good agreement. The results obtained from CLS and PCR were also compared with those obtained from a 1D spectrophotometric method. 相似文献
73.
Two new flavonoids, 8-hydroxyisoscoparin (1) and luteolin 7-O-glucoside 4'-sulfate (2), along with eight known flavonoids, including luteolin 7-O-glucoside 2'-sulfate, were isolated and identified from Washingtonia filifera. All compounds were characterized by (1)H-NMR, (13)C-NMR, CI-MS, FABMS and UV. The antioxidant activities of various W. filifera extracts were determined. 相似文献
74.
In this paper, numerical solutions of a reaction-diffusion chemotactic model of fractional orders for bacterial growth will
be present. A new solution is constructed in power series. The fractional derivatives are described in the Caputo sense. We
compare the experimental result obtained with those obtained by simulation of the chemotactic model without fractional derivatives.
The results show that the solution continuously depends on the time-fractional derivative. The resulting solutions spread
faster than the classical solutions and may exhibit asymmetry, depending on the fractional derivative used. We present results
of numerical simulations to illustrate the method, and investigate properties of numerical solutions. The Adomian’s decomposition
method (ADM) is used to find the approximate solution of fractional ‘reaction-diffusion chemotactic model. Numerical results
show that the approach is easy to implement and accurate when applied to partial differential equations of fractional order. 相似文献
75.
The eigenvalue problem for elliptic partial differential equation with multi-point nonlocal conditions
下载免费PDF全文
![点击此处可从《Journal of Applied Analysis & Computation》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ahmed Elsai Shaimaa Helal Ahmed El-Sayed 《Journal of Applied Analysis & Computation》2015,5(1):146-158
We study the spectral problem for the system of difference equations of a two-dimensional elliptic partial differential equation with nonlocal conditions. A new form of two-point nonlocal conditions that involve interior points is proposed. The matrix of the difference system is nonsymmetric thus different types of eigenvalues occur. The conditions for the existence of the eigenvalues and their corresponding eigenvectors are presented for the one dimensional problem. Then, these relations are generalized to the two-dimensional problem by the separation of variables technique. 相似文献
76.
Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition 总被引:1,自引:0,他引:1
Plasmonic metal nanoparticles have great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticle surface and ease of monitoring the light signal due to their strong scattering or absorption. In this work, we investigated the dependence of the sensitivity of the surface plasmon resonance (frequency and bandwidth) response to changes in their surrounding environment and the relative contribution of optical scattering to the total extinction, on the size and shape of nanorods and the type of metal, that is, Au vs Ag. Theoretical consideration on the surface plasmon resonance condition revealed that the spectral sensitivity, defined as the relative shift in resonance wavelength with respect to the refractive index change of surrounding materials, has two controlling factors: first the bulk plasma wavelength, a property dependent on the metal type, and second on the aspect ratio of the nanorods which is a geometrical parameter. It is found that the sensitivity is linearly proportional to both these factors. To quantitatively examine the dependence of the spectral sensitivity on the nanorod metal composition and the aspect ratio, the discrete dipole approximation method was used for the calculation of optical spectra of Ag-Au alloy metal nanorods as a function of Ag concentration. It is observed that the sensitivity does not depend on the type of the metal but depends largely on the aspect ratio of nanorods. The direct dependence of the sensitivity on the aspect ratio becomes more prominent as the size of nanorods becomes larger. However, the use of larger nanoparticles may induce an excessive broadening of the resonance spectrum due to an increase in the contribution of multipolar excitations. This restricts the sensing resolution. The insensitivity of the plasmon response to the metal composition is attributable to the fact that the bulk plasma frequency of the metal, which determines the spectral dispersion of the real dielectric function of metals and the surface plasmon resonance condition, has a similar value for the noble metals. On the other hand, nanorods with higher Ag concentration show a great enhancement in magnitude and sharpness of the plasmon resonance band, which gives better sensing resolution despite similar plasmon response. Furthermore, Ag nanorods have an additional advantage as better scatterers compared with Au nanorods of the same size. 相似文献
77.
In this paper, the Adomian’s decomposition method has been developed to yield approximate solution of the reaction-diffusion
model of fractional order which describe the evolution of the bacterium Bacillus subtilis, which grows on the surface of thin
agar plates. The fractional derivatives are described in the Caputo sense. The method introduces a promising tool for solving
many linear and nonlinear fractional differential equations. In these schemes, the solution takes the form of a convergent
series with easily computable components. Numerical results show that the approach is easy to implement and accurate when
applied to partial differential equations of fractional order. 相似文献
78.
Abstract The shielding effect of an iron sphere assembly has been tested for a Pu-α-Be neutron source placed in the center of the shield assembly. Emergent neutron and gamma spectra were measured with a stilbene scintillation counter. Discrimination between neutrons and gammas was achieved by the pulse shape discrimination technique based on the zero crossing method. Calculations have been made using the one-dimensional transport code ANISN-Westinghouse version (ANISN-W) and the EUR LiB 15/5 cross section data set. The agreement between measurements and calculations indicates that the cross section set and the calculation model are suitable for studying the iron shielding experiments over the neutron energy range 1.35–10 MeV and the gamma energy range 0.3–6 MeV. Total macroscopic cross sections for fast neutrons, linear attenuation coefficients for gamma rays and half-value thicknesses for neutrons and gammas for the whole energy range and at different energies have been obtained. 相似文献
79.
Mohamed G. Assy Hassan A. El‐Sayed Nabil H. Ouf Ahmed Hamza Hesham A. Morsy 《Journal of heterocyclic chemistry》2019,56(10):2954-2959
A simple and direct synthetic methodology for a novel series of azines and their annulated systems was performed. Heterocyclization of acyl isothiocyanate 2 with urea or malononitrile gave s‐triazine 4 and 1,3‐oxazine 7 derivatives, respectively. The reaction of heteroallene 1 with acetylacetone tolerated 2‐thioxopyridine derivative 9 . The latter compound underwent heterocyclization with urea, hydrazine hydrate, or phenyl hydrazine to give the annulated pyridines 10 – 12 . Pyrimidinethione 14 was resulted from reaction of acylisothiocyanate with enamine 13 . Condensation of compound 14 with hydrazine hydrate, phenyl hydrazine, urea, and 3‐nitrobenzaldehyde in the presence of ethyl cyanoacetate or sodium hydroxide afforded 15 – 20 , respectively. 相似文献
80.
Four new groups of 4-((2?-substitutedphenylimino)methyl)phenyl-4”-alkoxy benzoates, Ina-d, of Schiff base ester liquid crystals, were prepared and investigated for their mesophase formation and stability. Each group constitutes four homologous series that differ from each other by the lateral attached polar group X in the ortho position for the imine mesogen at terminal benzene ring that alternatively changed from F, Br, NO2 and lateral benzene ring. Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 6, 8, 10 and 12. Molecular structures of the prepared compounds were confirmed via elemental analysis, FT-IR, and 1H NMR spectroscopy. Mesomorphic properties were investigated by differential scanning calorimetry (DSC) and the phase identified by polarised light microscopy (PLM). A comparative study was made between the investigated compounds and their previously prepared laterally neat, 4-((4?-phenylimino)methyl)phenyl-4”-alkoxy benzoates (IIn); the result revealed that all lateral substituents not only decrease the melting temperature but also the mesophase stability and shown only nematic phase. Density functional theory (DFT) calculations for new lateral derivatives were discussed. 相似文献