全文获取类型
收费全文 | 31801篇 |
免费 | 5421篇 |
国内免费 | 3855篇 |
专业分类
化学 | 23152篇 |
晶体学 | 379篇 |
力学 | 2090篇 |
综合类 | 264篇 |
数学 | 3582篇 |
物理学 | 11610篇 |
出版年
2024年 | 82篇 |
2023年 | 628篇 |
2022年 | 1181篇 |
2021年 | 1108篇 |
2020年 | 1303篇 |
2019年 | 1343篇 |
2018年 | 1086篇 |
2017年 | 1095篇 |
2016年 | 1506篇 |
2015年 | 1534篇 |
2014年 | 1827篇 |
2013年 | 2300篇 |
2012年 | 2913篇 |
2011年 | 2934篇 |
2010年 | 1995篇 |
2009年 | 1956篇 |
2008年 | 2157篇 |
2007年 | 1866篇 |
2006年 | 1795篇 |
2005年 | 1486篇 |
2004年 | 1106篇 |
2003年 | 911篇 |
2002年 | 888篇 |
2001年 | 754篇 |
2000年 | 579篇 |
1999年 | 605篇 |
1998年 | 468篇 |
1997年 | 445篇 |
1996年 | 444篇 |
1995年 | 379篇 |
1994年 | 316篇 |
1993年 | 275篇 |
1992年 | 261篇 |
1991年 | 248篇 |
1990年 | 192篇 |
1989年 | 163篇 |
1988年 | 122篇 |
1987年 | 103篇 |
1986年 | 108篇 |
1985年 | 89篇 |
1984年 | 70篇 |
1983年 | 64篇 |
1982年 | 38篇 |
1981年 | 40篇 |
1980年 | 35篇 |
1979年 | 26篇 |
1978年 | 27篇 |
1977年 | 29篇 |
1976年 | 24篇 |
1973年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
952.
Toeplitz and Hankel type operators on the upper half-plane 总被引:3,自引:0,他引:3
An orthogonal decomposition of admissible wavelets is constructed via the Laguerre polynomials, it turns to give a complete decomposition of the space of square integrable functions on the upper half-plane with the measurey
dxdy. The first subspace is just the weighted Bergman (or Dzhrbashyan) space. Three types of Ha-plitz operators are defined, they are the generalization of classical Toeplitz, small and big Hankel operators respectively. Their boundedness, compactness and Schatten-von Neumann properties are studied.Research was supported by the National Natural Science Foundation of China. 相似文献
953.
The electrical behavior of the cationic, polyacetylene-based, conjugated ionomer, poly[(2-cyclooctatetraenylethyl)trimethylammonium trifluoromethanesulfonate], sandwiched between gold electrodes is reported. The steady-state current of this mixed ionically/electronically conducting system is assigned to be unipolar diffusive hole transport for voltages below approximately 1.4 V, giving way to bipolar migratory transport above approximately 1.4 V. In the low-voltage regime, a non-Faradaically controlled doping model is proposed where p-doping at the anode is balanced by the charging of an ionic double layer at the cathode. In the high-voltage regime, n- and p-type regions extend from the electrodes as the voltage becomes sufficient to drive disproportionation and the electric field required by the redistribution of ions begins to substantially influence carrier transport. The assignment of a transport mechanism is primarily based on analyzing the decay of the steady-state system under short-circuit and open-circuit conditions. First, it is shown that the power describing the power-law decay of the short-circuit current is characteristic of the steady-state carrier profile. Second, it is argued that a component of the time-dependent, open-circuit voltage decaying more rapidly than the time scale for ion motion is indicative of a substantial migratory component to steady-state transport, as observed in the high-voltage regime. The hole and electron mobilities are estimated to be on the order of 10(-7)-10(-6) cm(2) V(-1) s(-1). 相似文献
954.
955.
Propargylic alcohols are versatile precursors to many organic molecules including natural products and pharmaceutical compounds.[1] In our continuing interest in the development of practical methods for synthesis of ferrocene derivatives, we decided to investigate the efficiencies of ferrocenylacetylene addition to aromatic aldehydes under t-BuOKcatalyzed condition.[2] Interestingly, we found that the usually reliable coupling reaction between terminal alkyne of type 1 and aromatic aldehydes of type 2 does not furnish the expected propargylic alcohols, but that the isomeric aryl enones 3are found in high yield and very short reaction time (10~20 min). This provided a promising protocol for preparation of ferrocenyl chalcones in a practical, economical and mild sense compared with the traditional method. To the best of our knowledge, this unusual reaction was first observed for the addition of ferrocenylacetylene to aldehyde in a base-catalyzed manner (Eq. 1). 相似文献
956.
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
957.
Jian-Tao?Zhang Si-Xue?Cheng Ren-Xi?ZhuoEmail author 《Colloid and polymer science》2003,281(6):580-583
A series of thermosensitive and fast-response poly(vinyl alcohol) (PVA)/poly(N-isopropylacrylamide) (PNIPA) hydrogels were prepared by incorporating PVA into cross-linked PNIPA to form a semi-interpenetrating polymeric network (semi-IPN). Compared to the conventional PNIPA hydrogel, the semi-IPN hydrogels thus prepared exhibit significantly faster response rates and undergo full deswelling in 1 min (lose about 95% water within 1 min) when the temperature is raised above their lower critical solution temperature, and have larger equilibrium swelling ratios at room temperature. These improved properties are attributed to the incorporation of PVA, which forms water-releasing channels and results in increased hydrophilicity, into the PNIPA hydrogel networks. 相似文献
958.
A method for the determination of methylmercury (MeHg) fluxes across the air–water and air–soil interfaces was developed using an in situ chamber. The MeHg in the air coming out of the chamber was captured by a column containing sulfhydryl cotton fiber adsorbent. MeHg was then desorbed from the column by using 2 mol L–1 HCl. The MeHg in the effluent was extracted with benzene, and determined by gas chromatography with electron capture detection. Finally, the MeHg flux was calculated using the chamber. The method was applied to simulated experiments, and the results showed that the MeHg fluxes in the air–water system were higher than those in the air–soil–water system. The method was also successfully applied to the field measurements of an environment polluted by a chemical factory, and the results showed that the MeHg fluxes across the air–soil and air–water interfaces were 0.21–3.09 and 0.14–0.79 ng m–2 h–1, respectively. The method will be a useful tool in the environmental study of MeHg. 相似文献
959.
High-performance liquid chromatographic (HPLC) methods were developed for the analysis of two compounds in a series of new antiallergenic agents, 1-[4-[3-[4-[bis(4-fluorophenyl)hydroxymethyl]-1-piperidinyl] propoxy]-3-methoxyphenyl]ethanone and its active acidic metabolite in plasma. The methods utilize ultraviolet or fluorescence detection, liquid-liquid extraction or solid-phase extraction and reversed-phase HPLC. The drugs were quantitated in samples from bioavailability studies performed in dogs. Calibrations were in the ng/ml concentration range for both compounds in plasma. 相似文献
960.
The shear viscosity of liquid copper is studied using nonequilibrium molecular-dynamics simulations under planar shear flow conditions. We examined variation of viscosity as function of shear rate at a range of pressures (ca. 0 - 40 GPa). We analyzed these results using eight different phenomenological models and find that the observed non-Newtonian behavior is best described by the Powell-Eyring (PE) model: eta(gamma) = (eta(0)-eta(infinity))sinh(-1)(taugamma)(taugamma) + eta(infinity), where gamma is the shear rate. Here eta(0) (the zero-shear-rate viscosity) extracted from the PE fit is in excellent agreement with available experimental data. The relaxation time tau from the PE fit describes the shear response to an applied stress. This provides the framework for interpreting the shear flow phenomena in complex systems, such as liquid metal and amorphous metal alloys. 相似文献