全文获取类型
收费全文 | 4753篇 |
免费 | 308篇 |
国内免费 | 32篇 |
专业分类
化学 | 3286篇 |
晶体学 | 57篇 |
力学 | 169篇 |
数学 | 752篇 |
物理学 | 829篇 |
出版年
2023年 | 29篇 |
2022年 | 101篇 |
2021年 | 144篇 |
2020年 | 97篇 |
2019年 | 188篇 |
2018年 | 151篇 |
2017年 | 107篇 |
2016年 | 240篇 |
2015年 | 154篇 |
2014年 | 246篇 |
2013年 | 475篇 |
2012年 | 280篇 |
2011年 | 293篇 |
2010年 | 208篇 |
2009年 | 198篇 |
2008年 | 213篇 |
2007年 | 203篇 |
2006年 | 203篇 |
2005年 | 169篇 |
2004年 | 154篇 |
2003年 | 130篇 |
2002年 | 172篇 |
2001年 | 72篇 |
2000年 | 109篇 |
1999年 | 64篇 |
1998年 | 40篇 |
1997年 | 46篇 |
1996年 | 35篇 |
1995年 | 36篇 |
1994年 | 35篇 |
1993年 | 37篇 |
1992年 | 32篇 |
1991年 | 23篇 |
1990年 | 16篇 |
1989年 | 29篇 |
1988年 | 27篇 |
1987年 | 20篇 |
1986年 | 17篇 |
1985年 | 29篇 |
1984年 | 24篇 |
1983年 | 22篇 |
1982年 | 22篇 |
1981年 | 26篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 13篇 |
1975年 | 17篇 |
1959年 | 8篇 |
排序方式: 共有5093条查询结果,搜索用时 15 毫秒
991.
Khalil Al Mamari Hamid Ennajih Hafid Zouihri Rachid Bouhfid Seik Weng Ng El Mokhtar Essassi 《Tetrahedron letters》2012,53(18):2328-2331
The 1,3-dipolar cycloaddition of 1-allyl-5-haloisatin derivatives as dipolarophiles with the azomethine ylides generated in situ from N-allylisatin and l-proline to furnish novel dispiro-oxindoles has been investigated. The structures and relative stereochemistry of both types of cycloadducts were confirmed by single crystal X-ray diffraction, 1H and 13C NMR spectroscopy and mass spectrometry. 相似文献
992.
Aleksandra Tyl El?bieta Che?mecka Magdalena Jab?ońska Maria Nowak Joachim Kusz Karol Pasterny Roman Wrzalik 《Structural chemistry》2012,23(2):325-332
The X-ray structure, synthesis, theoretical calculation and IR spectra of 1-naphthaleneacrylic acid are reported. The titled
compound crystallizes in the monoclinic C 2/c space group with unit cell parameters: a = 14.556(3), b = 5.1332(10), c = 26.832(5) ?, β = 97.02(3)°, V = 1989.8(7) ?3, Z = 8 and form typical centrosymmetric hydrogen-bonded dimers. Theoretical calculations of 1-naphthylacrylic acid isolated
molecule and hydrogen-bonded dimer have been carried out using density functional theory at the B3LYP level. For optimized
structures the vibrational spectra have been then calculated and compared with experimental IR spectrum. The assignment and
characterization of theoretical vibrational spectra were based on the potential energy distribution analysis. This comparison
has shown that the theoretical spectrum for the dimer structure is in good agreement with the experimental one. Structural
comparisons with naphthalene, and with some substituted 2-propenoic acids have shown influence of the substituent on conformation
of the naphthalene ring or 2-propenoic moiety. 相似文献
993.
We present pseudo-potential calculations of geometrical structures of stable isomers of LiAr
n
clusters with both an electronic ground state and excited states of the lithium atom. The Li atom is perturbed by argon atoms
in LiAr
n
clusters. Its electronic structure obtained as the eigenfunctions of a single-electron operator describing the electron in
the field of a Li+Ar
n
core, the Li+ and Ar atoms are replaced by pseudo-potentials. These pseudo-potentials include core-polarization operators to account for
the polarization and correlation of the inert core with the valence Lithium electron [J Chem Phys 116, 1839 1]. The geometry optimization of the ground and excited states of LiAr
n
(n = 1–12) clusters is carried out via the Basin-Hopping method of Wales et al. [J Phys Chem 101, 5111 2; J Chem Phys 285, 1368 3]. The geometries of the ground and ionic states of LiAr
n
clusters were used to determine the energy of the high excited states of the neutral LiAr
n
clusters. The variation of the excited state energies of LiAr
n
clusters as a function of the number of argon atoms shows an approximate Rydberg character, corresponding to the picture
of an excited electron surrounding an ionic cluster core, is already reached for the 3s state. The result of optical transitions calculations shows that the absorption spectral features are sensitive to isomer
structure. It is clearly the case for transitions close to the 2p levels of Li which are distorted by the cluster environment. 相似文献
994.
Belkacem Belaabed Jean Luc Wojkiewicz Saad Lamouri Noureddine El Kamchi Nathalie Redon 《先进技术聚合物》2012,23(8):1194-1201
Composites based on conductive organic/inorganic fillers dispersed in insulating matrix have been widely investigated because of their widespread applications such as electromagnetic shielding, electrostatic discharge, and sensors. In this context, novel composite materials based on epoxy resin matrix charged with polyaniline (PANI)‐doped para‐toluene sulfonic acid were elaborated. Fourier transform infrared spectroscopy, X‐ray diffraction and scanning electron microscopy were used to check the structure and the morphology of the samples. Viscoelastic behavior and thermal stability of the composites were explored by dynamic mechanical thermal analysis and thermogravimetric analysis. It was shown that the PANI particles exhibited a partial crystalline structure and were homogeneously dispersed in epoxy matrix. Consequently, this structure affected the thermal stability and viscoelastic properties of the composites. Furthermore, the dielectric and electrical properties were investigated up to 1 MHz. Measurements of dielectric properties revealed that with loading fillers in matrix, the dielectric parameters increased to high values at low frequency then decreased at values around 40 and 32 of real and imaginary parts, respectively, at 1 MHz with 15% of PANI content. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
995.
E. V. Drobysheva A. A. Aleksandrov M. M. El’chaninov 《Russian Journal of Organic Chemistry》2012,48(3):423-425
Electrophilic substitution reactions (formylation and acylation) in the series of 2-[2-(2-furyl)vinyl]- and 2-[2-(2-thienyl)vinyl]benzothiazoles
leads to the corresponding derivatives at the α-position of the furan or thiophene ring. The presence of a vinylene bridge
weakens deshielding effect of the benzothiazole fragment on π-excessive heterocycles, so that such compounds react at a higher
rate and under milder conditions as compared to hetarylbenzothiazoles having no vinylene bridge. 相似文献
996.
E. Elshereafy Maysa A. Mohamed M. M. EL-Zayat A. A. El Miligy 《Journal of Radioanalytical and Nuclear Chemistry》2012,293(3):941-947
Nitrile butadiene rubber (NBR) was mixed with high density polyethylene (HDPE) thermoplastics with different ratio namely (100/20), (100/40), (100/60) and (100/80). The obtained blends were subjected to gamma irradiation with varying dose from 50 to 250 kGy. The induced crosslinking and hence the improvement in the different properties were followed up as a function of irradiation dose. Mechanical properties as tensile strength, tensile modulus at 50 % elongation, elongation at break percent, permanent set and hardness were carried out as a function of irradiation dose and blend ratio. Moreover, physical properties namely, gel fraction % and swelling number were found to improve with the increase of irradiation dose up to 250 kGy and with the increase of the content of HDPE in blend. Moreover, presence of NBR enhances the shrinking properties of the obtained blend which can be used as a good heat shrinkable material. 相似文献
997.
Speed S Vicente R Aravena D Ruiz E Roubeau O Teat SJ El Fallah MS 《Inorganic chemistry》2012,51(12):6842-6850
The syntheses, structural characterization, and magnetic behavior of two new hexanuclear copper(II) complexes derived from R-phosphonic acids and 1,3-bis(dimethylamino)-2-propanol (Hbdmap) with formulas [Cu(6)(μ-bdmap)(3)(μ(3)-Ph-PO(3))(2)(μ(3)-O···H···μ(3)-O)(ClO(4))(2)(H(2)O)]·5H(2)O (1) and [Cu(6)(μ-bdmap)(3)(μ(3)-t-Bu-PO(3))(2)(μ(3)-O···H···μ(3)-O)(μ(1,3)-dca)(dca)(H(2)O)]·6H(2)O (2) (Ph-H(2)PO(3) = phenylphosphonic acid, t-Bu-H(2)PO(3) = tert-butylphosphonic acid, dca = dicyanamide) are reported. Compounds 1 and 2 are hexanuclear 3.111 R-phosphonate(2-)/1,3-bis(dimethylamino)-2-propanolato(1-) cages including in the center the [μ(3)-O···H···μ(3)-O](3-) unit. The temperature dependence of the magnetic properties of 1 and 2 clearly indicates an overall strong antiferromagnetic coupling confirmed by DFT calculations. 相似文献
998.
Houcine Ammar Fatma Abdelkafi Souhir Abid Bouchra Benjelloum-Mlayah Rachid El Gharbi Michel Delmas 《Chemistry of Natural Compounds》2012,48(1):99-102
Alfa grass pulping was successfully performed in hydro-organic acid medium under mild conditions (107°C, atmospheric pressure, cooking
time: 3 h). Use of an acetic acid/formic acid/water mixture as pulping liquor was perfectly suitable for selective isolation
of pulp, lignin, and hemicelluloses. The unbleached pulp obtained in good yield was first delignified by peroxyacids in organic
acid medium and then bleached with hydrogen peroxide in a basic medium to give pulp offering good physico-chemical and mechanical
characteristics. 相似文献
999.
The heterogeneous interaction of H(2)O(2) with TiO(2) surface was investigated under dark conditions and in the presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H(2)O(2) ([H(2)O(2)](0) = (0.17-120) × 10(12) molecules cm(-3)), irradiance intensity (J(NO(2)) = 0.002-0.012 s(-1)), relative humidity (RH = 0.003-82%), and temperature (T = 275-320 K). Under dark conditions, a deactivation of TiO(2) surface upon exposure to H(2)O(2) was observed, and only initial uptake coefficient of H(2)O(2) was measured, given by the following expression: γ(0)(dark) = 4.1 × 10(-3)/(1 + RH(0.65)) (calculated using BET surface area, estimated conservative uncertainty of 30%) at T = 300 K. The steady-state uptake coefficient measured on UV irradiated TiO(2) surface, γ(ss)(UV), was found to be independent of RH and showed a strong inverse dependence on [H(2)O(2)] and linear dependence on photon flux. In addition, slight negative temperature dependence, γ(ss)(UV) = 7.2 × 10(-4) exp[(460 ± 80)/T], was observed in the temperature range (275-320) K (with [H(2)O(2)] ≈ 5 × 10(11) molecules cm(-3) and J(NO(2)) = 0.012 s(-1)). Experiments with NO addition into the reactive system provided indirect evidence for HO(2) radical formation upon H(2)O(2) uptake, and the possible reaction mechanism is proposed. Finally, the atmospheric lifetime of H(2)O(2) with respect to the heterogeneous loss on mineral dust was estimated (using the uptake data for TiO(2)) to be in the range of hours during daytime, i.e., comparable to H(2)O(2) photolysis lifetime (~1 day), which is the major removal process of hydrogen peroxide in the atmosphere. These data indicate a strong potential impact of H(2)O(2) uptake on mineral aerosol on the HO(x) chemistry in the troposphere. 相似文献
1000.
Finite nonassociative division algebras (i.e., finite semifields) with 243 elements are completely classified. Nine Knuth orbits were found, two of which are new. All are primitive, and all but the twisted field planes are fractional dimensional. 相似文献