全文获取类型
收费全文 | 30855篇 |
免费 | 5782篇 |
国内免费 | 3265篇 |
专业分类
化学 | 21955篇 |
晶体学 | 349篇 |
力学 | 1803篇 |
综合类 | 136篇 |
数学 | 3015篇 |
物理学 | 12644篇 |
出版年
2024年 | 135篇 |
2023年 | 692篇 |
2022年 | 1207篇 |
2021年 | 1291篇 |
2020年 | 1394篇 |
2019年 | 1373篇 |
2018年 | 1167篇 |
2017年 | 1049篇 |
2016年 | 1598篇 |
2015年 | 1521篇 |
2014年 | 1950篇 |
2013年 | 2397篇 |
2012年 | 2864篇 |
2011年 | 2875篇 |
2010年 | 1873篇 |
2009年 | 1784篇 |
2008年 | 1999篇 |
2007年 | 1743篇 |
2006年 | 1629篇 |
2005年 | 1287篇 |
2004年 | 977篇 |
2003年 | 766篇 |
2002年 | 757篇 |
2001年 | 593篇 |
2000年 | 468篇 |
1999年 | 597篇 |
1998年 | 516篇 |
1997年 | 498篇 |
1996年 | 494篇 |
1995年 | 426篇 |
1994年 | 343篇 |
1993年 | 281篇 |
1992年 | 279篇 |
1991年 | 224篇 |
1990年 | 203篇 |
1989年 | 151篇 |
1988年 | 93篇 |
1987年 | 79篇 |
1986年 | 105篇 |
1985年 | 73篇 |
1984年 | 38篇 |
1983年 | 44篇 |
1982年 | 29篇 |
1981年 | 22篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1957年 | 4篇 |
1923年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
Intercalationchemistryoflayeredtransitionmetaldisulfideshasbeenextensivelystudiedinthepastyears[1].Ithasbeenestablishedthattheintercalationreactiondependsstronglyonelectronicstructureofthetransitionmetaldisulfides.Forinstance,variousguestmoleculeshavebe… 相似文献
852.
853.
镓在裸Si系和SiO2/Si系掺杂效应 总被引:3,自引:0,他引:3
Based on the diffusion action of gallium in silicon and SiO2 ,a diffusion model of gallium doping in bare silicon system and SiO_2/Si system is first presented in this paper ,the gallium doping effect in the two systems is analyzed theoretically. Experiments and applications have proved that the use of the open-tube gallium deffusion in SiO2/Si system can substantially improve diffusion quality and device characteristics . 相似文献
854.
X. Zhao Y. Itoh Y. Aoyagi T. Sugano K. Hirata Y. Kobayashi T. Ohdaira R. Suzuki T. Mikado 《Journal of Radioanalytical and Nuclear Chemistry》1996,211(1):31-38
Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials. 相似文献
855.
Xingang Kong Jiarui Zhang Jianfeng Huang Jiayin Li Yi Qin Ting Zhao Qi Feng 《中国化学快报》2019,30(3):771-774
SnNb2O6 and Sn2Nb2O7 nanosheets were synthetized via microwave assisted hydrothermal method, and innovatively employed as anode materials for lithium-ion battery. Compared with Sn2Nb2O7 and the previously reported pure Sn-based anode materials, the SnNb2O6 electrode exhibited outstanding cycling performance. 相似文献
856.
A new fluorinated polystyrene bearing a p-sulbstiuted perfluoro[1-(2-fluorosulfonylethoxy)]ethyl group was synthesized via one-electron oxidation of polystyrene by perfluoro[2-(2-fluorosulfonylethoxy)]propionyl peroxide at different peroxide to polystyrene molar ratios.The yield of perfluoroalkylation decreases with the increase of the reactant molar ratio.The modified polymer has been characterized by various techniques:the ring pefluoro[1-(2-fluorosulfonylethoxy)]ethylation has been proved by FT-IR and ^19FNMR;the X-ray photoelectron spectra(XPS) show the maximum binding energy of F18,O18,C18(two kinds of carbon atoms,namely C-H and C-F)and S2p,respectively; desulfonylation of the fluorinated polystyrene appearing at 217℃ has been found by its thermogravimetric analysis (TGA).The determinations of contact angle,refractive index and glass transition temperature of the modified polymer have disclosed that when the contact angle increases with the increase of the molar ratio,the refractive index and glass transition temperature decrease.The polydispersity values indicate that the degradation of the polymer chains did not occur during the reaction. 相似文献
857.
Yu-Fen Zhao De-Qing Zhang Chu-Biao Xue Jia-Ning Zeng Gai-Jiao Ji 《Journal of mass spectrometry : JMS》1991,26(5):510-513
Positive ion fast atom bombardment mass spectrometry of N-diisopropyloxyphosphoryl dipeptides and tripeptides showed a novel cleavage pattern in that only the N-phosphoryl fragment ions gave intense peaks while the C-terminal series of ions was suppressed. The base peak was the N-phosphoryl imino ion responding to the N-terminal residue. These advantages are superior to those of other types of N-protecting groups. 相似文献
858.
Li ZT Zhang XL Lian XD Yu YH Xia Y Zhao CX Chen Z Lin ZP Chen H 《The Journal of organic chemistry》2000,65(17):5136-5142
A variety of novel calix[4]arene-incorporating crown ethers with or without intramolecular hydrogen bonding have been prepared by two efficient methods and utilized as donor rings to assemble calix[4]arene [2]catenanes based on pi-stacking interaction between hydroquinone and bipyridinium units. Treatment of calix[4]arene crown ethers 4, 10a, or 10b, whose cone conformation was fixed by intramolecular hydrogen bonding within the calix[4]arene moiety, with dicationic salt 15 x 2PF6 and dibromide 16 afforded the corresponding [2]catenanes 17a x 4PF6, 17b x 4PF6, and 17c x 4PF6 in 20%, 53%, and 55% yields, respectively, whereas from the reactions of 15 x 2PF6 and dibromide 16 in the presence of conformationally flexible 11 or 12 with a cone conformation kept by two propyl groups, [2]catenanes 18 x 4PF6 and 19 x 4PF6 were obtained in 12% and 6% yields. [2]Catenanes 21a x 4Cl, 21b x 4Cl, and 21c x 4Cl, incorporating calix[4]arene in both the donor and acceptor rings, were also successfully assembled from 10a or 10b, 16, and dicationic salts 20a x 2PF6 or 20b x 2PF6. The dynamic 1H NMR and absorption spectra of the [2]catenanes have been investigated, which revealed a strongest donor-acceptor interaction in 17a x 4PF6 and that the cone [2]catenanes 17a-c x 4PF6 can isomerize to the partial cone isomer at high temperature. The difference of the dynamic properties of these catenanes was discussed. The results demonstrate that catenation is one new general method to change the conformational distributions of calix[4]arenes. 相似文献
859.
A dicyano-containing [Fe(bpb)(CN)2]- building block has been employed for the synthesis of cyano-bridged heterometallic Ni(II)-Fe(III) complexes. The presence of steric bpb(2-) ligand around the iron ion results in the formation of low-dimensional species: five are neutral NiFe2 trimers and three are one-dimensional (1D). The structure of the 1D complexes consists of alternating [NiL]2+ and [Fe(bpb)(CN)2]- generating a cyano-bridged cationic polymeric chain and the perchlorate as the counteranion. In all complexes, the coordination geometry of the nickel ions is approximately octahedral with the cyano nitrogen atoms at the trans positions. Magnetic studies of seven complexes show the presence of ferromagnetic interaction between the metal ions through the cyano bridges. Variable temperature magnetic susceptibility investigations of the trimeric complexes yield the following J(NiFe) values (based on the spin exchange Hamiltonian H = -2J(NiFe) S(Ni) (S(Fe(1)) + S(Fe(2))): J(NiFe) = 6.40(5), 7.8(1), 8.9(2), and 6.03(4) cm(-1), respectively. The study of the magneto-structural correlation reveals that the cyanide-bridging bond angle is related to the strength of magnetic exchange coupling: the larger the Ni-N[triple bond]C bond angle, the stronger the Ni- - -Fe magnetic interaction. One 1D complex exhibits long-range antiferromagnetic ordering with T(N) = 3.5 K. Below T(N) (1.82 K), a metamagnetic behavior was observed with the critical field of approximately 6 kOe. The present research shows that the [Fe(bpb)(CN)2]- building block is a good candidate for the construction of low-dimensional magnetic materials. 相似文献
860.
For a system of flexible polymer molecules, the concepts of two concentrations, namely the segmental and the molecular concentrations, have been proposed in this paper. The former is equivalent to the volume fraction. The latter can be defined as the number of the gravity centers of macromolecules in a unit volume. The two concentrations should be correlated with each other by the conformational function of the polymer chain and should be discussed in different thermodynamic equations. On the basis of these concepts it has been proved that the Flory-Huggins entropy of mixing should be the result of the mixing “ideal gases of the gravity centers of macromolecules“. The general correlation between the free energy of mixing and the scattering function (structural factor) of polymer blends has been studied based on the general fluctuation theory. When the Flory-Huggins free energy of mixing is adopted, the de Gennes scattering function of a polymer blend can be derived. 相似文献