全文获取类型
收费全文 | 5092篇 |
免费 | 27篇 |
国内免费 | 6篇 |
专业分类
化学 | 2834篇 |
晶体学 | 65篇 |
力学 | 124篇 |
数学 | 572篇 |
物理学 | 1530篇 |
出版年
2016年 | 36篇 |
2013年 | 173篇 |
2012年 | 97篇 |
2011年 | 96篇 |
2010年 | 70篇 |
2009年 | 84篇 |
2008年 | 115篇 |
2007年 | 133篇 |
2006年 | 128篇 |
2005年 | 157篇 |
2004年 | 127篇 |
2003年 | 103篇 |
2002年 | 105篇 |
2001年 | 68篇 |
2000年 | 64篇 |
1999年 | 77篇 |
1998年 | 43篇 |
1997年 | 73篇 |
1996年 | 99篇 |
1995年 | 94篇 |
1994年 | 60篇 |
1993年 | 63篇 |
1992年 | 58篇 |
1991年 | 62篇 |
1990年 | 64篇 |
1989年 | 59篇 |
1988年 | 70篇 |
1987年 | 79篇 |
1986年 | 53篇 |
1985年 | 62篇 |
1984年 | 72篇 |
1983年 | 63篇 |
1982年 | 75篇 |
1981年 | 61篇 |
1980年 | 82篇 |
1979年 | 59篇 |
1978年 | 78篇 |
1977年 | 68篇 |
1976年 | 64篇 |
1975年 | 70篇 |
1974年 | 48篇 |
1973年 | 45篇 |
1972年 | 36篇 |
1969年 | 36篇 |
1968年 | 43篇 |
1967年 | 40篇 |
1955年 | 59篇 |
1934年 | 43篇 |
1933年 | 36篇 |
1928年 | 38篇 |
排序方式: 共有5125条查询结果,搜索用时 15 毫秒
991.
The hydrogen-bonded complexes between methylacetate (MeAc) and water or hydrochloric acid have been studied by infrared spectrometry in a low temperature Argon matrix. The νCO, νCC and νCC ional modes of MeAc show a splitting to the low and to the high frequency side of the free molecule. This suggests that water and HCl interact with the keto and ether oxygens. This conclusions is supported by the appearance of two main absorptions in the νHCl region. These results are discussed as a function of the gas-phase basicity of the two oxygen atoms, derived from the O(1s) core electron binding energies and from the ionization potentials. 相似文献
992.
993.
Chih-Hao Lee Hsin-Ho Chang Hui-Chia Su Yu-Han Wu Chih-Wei Hu L.J. Chang A. Ioffe K. Bussmann Th. Brückel 《Physica B: Condensed Matter》2009,404(17):2565-2567
The magnetic correlation length of a mixed nickel ferrite powder was studied by a newly commissioned depolarized neutron beamline at the W3 port of Tsing Hua Open Pool Reactor (THOR). In this work, Ni ferrite powder samples with different packing densities were studied. The magnetic correlation lengths of the sample were observed to be 2 μm at virgin state and about 3.1 μm at remanent state from the packing density of 20–60%. This magnetic domain size is smaller than particle size. No significant change of domain size at this packing density implies the domain wall motion is hindered by the porosity effectively up to at least 60% of packing density. 相似文献
994.
A laser frequency comb allows the conversion of the very rapid oscillations of visible light of some 100’s of THz down to
frequencies that can be handled with conventional electronics. This capability has enabled the most precise laser spectroscopy
experiments yet that allowed to test quantum electrodynamics, to determine fundamental constants and to search for possible
slow changes of these constants. Using an optical frequency reference in combination with a laser frequency comb has made
it possible to construct all optical atomic clocks, that are now outperforming even the best cesium atomic clocks. In future
direct frequency comb spectroscopy might enable high resolution laser spectroscopy in the extreme ultraviolet for the first
time. Frequency combs are also used to calibrate astronomical spectrographs and might reach an accuracy that is sufficient
to observe the expansion of the universe in real time. 相似文献
995.
Complementary results from 13C intermolecular nuclear Overhauser effects (NOE), 1H-13C heteronuclear Overhauser spectroscopy (HOSEY) and 1H-NMR diffusion measurements were used for probing the structure of the first solvation shell of uridine in water. It is demonstrated that a cyclic dihydrate is formed. The two water molecules produce two hydrogen bonds with the two oxygen atoms from the pyrimidine ring and accept only one hydrogen bond from the amide proton. The dihydrate has only a short lifetime as compared with the rotational correlation time of the free nucleoside. The chemical exchange constant of the amide proton with water is then estimated by diffusion experiments. The results are consistent with previous data obtained for uracil in water and provide interesting information about water accessibility in nucleic acid bases. 相似文献
996.
Roger M Barros N Arliguie T Thuéry P Maron L Ephritikhine M 《Journal of the American Chemical Society》2006,128(27):8790-8802
Reaction of U(NEt(2))(4) with HS-2,4,6-(t)Bu(3)C(6)H(2) (HSMes) gave U(SMes)(3)(NEt(2))(py) (1), whereas similar treatment of U[N(SiMe(3))SiMe(2)CH(2)][N(SiMe(3))(2)](2) afforded U(SMes)[N(SiMe(3))(2)](3) (2) and U(SMes)(3)[N(SiMe(3))(2)]. The first neutral homoleptic uranium(IV) thiolate to have been crystallographically characterized, U(SMes)(4) (4), was isolated from the reaction of U(BH(4))(4) and KSMes. The first homoleptic thiolate complex of uranium(III), U(SMes)(3) (5), was synthesized by protonolysis of U[N(SiMe(3))(2)](3) with HSMes in cyclohexane. The crystal structure of 5 exhibits the novel eta(3) ligation mode for the arylthiolate ligand. Comparison of the crystal structure of 5 with those of the isomorphous lanthanide congeners Ln(SMes)(3) (Ln = La, Ce, Pr, and Nd) indicates that the U-S, U-C(ipso)(), and U-C(ortho)() bond lengths are shorter than the corresponding ones in the 4f-element analogues, when taking into account the variation in the ionic radii of the metals. The distance between the uranium and the carbon atoms involved in the U...H-C epsilon agostic interaction of each thiolate ligand is shorter, by approximately 0.05 A, than that expected from a purely ionic bonding model. The lanthanide(III)/actinide(III) differentiation was analyzed by density functional theory (DFT). The nature of the M-S bond is shown to be ionic strongly polarized at the sulfur for M = U and iono-covalent (i.e. strongly ionic with low orbital interaction), for M = Ln. The strength of the U...H-C epsilon agostic interaction is proposed to be controlled by the maximization of the interaction between U(+) and S(-) under steric constraints. The eta(3) ligation mode of the arylthiolate ligand is also obtained from DFT. 相似文献
997.
Kera S Casu MB Schöll A Schmidt T Batchelor D Rühl E Umbach E 《The Journal of chemical physics》2006,125(1):014705
We report on a combined experimental and theoretical carbon and nitrogen K-edge near-edge x-ray absorption fine structure investigation on condensed metal-free phthalocyanine (H2Pc). Based on the results from improved virtual orbital calculations, all resonances in the experimental high-resolution data can be assigned to various electronic transitions. The comparison between experiments and calculations further shows that a significant influence of the core hole, which affects both the transition energies and the cross sections, is present and must be considered in theoretical approaches. Moreover, additional fine structure is clearly resolved for the first N 1s-->pi* transition, which can be interpreted as vibronic coupling to the electronic core excitation. 相似文献
998.
Th. K. Mavrogordatos 《JETP Letters》2017,106(12):815-820
We highlight the importance of quantum fluctuations in organizing a dissipative quantum phase transition for the driven Jaynes–Cummings interaction with variable qubit-cavity detuning. The system response presents a substantial difference from the predictions of the semiclassical theory, the extent of which is revealed in the properties of quantum bistability, and visualized with the help of quasi-distribution functions for the cavity field, subject to an appropriate scale parameter. States anticipated by the neoclassical theory of radiation coexist in the quantum picture, following the occurrence of spontaneous dressed-state polarization and phase bistability at resonance. 相似文献
999.
M. Burianek M. Mühlberg M. Woll M. Schmücker Th. M. Gesing H. Schneider 《Crystal Research and Technology》2009,44(10):1156-1162
Pure and homogeneous single crystals of orthorhombic mullite‐type Bi2M4O9 (M = Al3+, Ga3+, Fe3+), and a mixed Bi2Fe1.7Ga2.3O9 crystal from an equimolar Ga/Fe composition were grown by the top seeded solution growth (TSSG) method. All these compounds melt incongruently in the range of about 800 and 1100 °C. In case of bismuth gallate and ferrate inclusion‐free crystals with dimensions up to several cubic centimeters can be grown. Limited solubility in Bi2O3 and the high steepness of the liquidus curve are the reasons for getting only small imperfect bismuth aluminate crystals. In contrast to ceramic materials preparation reported in literature, divalent calcium and strontium could not be incorporated into the mullite‐type structure during the melt growth process. Several fundamental physical properties like heat capacity, thermal expansion, heat conductivity, elastic constants, high‐pressure behavior and oxygen diffusivity were determined by different research groups using single‐crystalline samples from the as‐grown materials. Furthermore, the refractive indices of Bi2Ga4O9 were measured in the range of 0.430 and 0.700 μm. Such as many other bismuth containing compounds the refractive indices of Bi2Ga4O9 are larger than 2, and Bi2Ga4O9 is an optic biaxial positive crystal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
1000.
Sp.G. Pantelakis Th.B. Kermanidis G.N. Haidemenopoulos 《Theoretical and Applied Fracture Mechanics》1996,25(2):139
Paint removal is required in a series of aeronautical procedures such as maintenance and repair. Today's paint stripping processes which are based on application of chemicals and abrasion are inadequate for modern aircraft structures in addition to environmental contamination. several alternative techniques are in progress. However, the aspect of material property degradation when developing novel, alternative paint stripping techniques has not been properly faced up to present. The influence of two novel paint stripping processes on the mechanical properties of the substrate 2024 7351 aluminium alloy has been investigated. The paint stripping processes included laser radiation with excimer, CO2, TEA-C02 and YAG laser sources as well as plasma etching. These processes have been applied for the removal of polyurethane coating which is a typical aeronautical paint system. The results indicated no significant degradation in yield strength and ultimate tensile strength. However a significant degradation in tensile ductility and toughness is observed with the application of all paint stripping processes, the highest degradation being associated with the ultraviolet excimer laser and plasma etching. On the other hand there is a considerable extension in fatigue life, which depends on the paint stripping process and the applied stress amplitude. At high stress there is no appreciable effect while at low stress there is an order of magnitude life extension associated with CO2 laser paint stripping. At moderate stresses, there is an up to sixfold life extension associated with the excimer laser processing. 相似文献