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821.
We present a nonlocal density-functional theory of polymeric fluids consisting of freely jointed Lennard-Jones chains with explicit consideration of the segment size, van der Waals attraction, and structural correlations due to chain connectivity. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the short-ranged repulsion and the first-order thermodynamic perturbation theory for chain connectivity. The contribution of the long-ranged attraction to the Helmholtz energy functional is taken into account using a quadratic density expansion with the direct correlation function obtained from the first-order mean-spherical approximation. The numerical performance of the density-functional theory is compared well with the simulation results from this work as well as those from the literature for the segment-level density profiles and correlation functions of Lennard-Jones chains in slit pores, near isolated nanoparticles, or in bulk. 相似文献
822.
A simplified enzyme-based fiber optic sensor system has been developed for selective determination of hydrogen peroxide. Horseradish peroxidase (HRP) is immobilized on bovine albumin matrix with glutaraldehyde. A new fluorimetric substrate, thiamine is used to indicate the sensing process. Under optimized condition the measuring range of sensor is up to 1 x 10(-4)M hydrogen peroxide with a limit of detection of 5 x 10(-7)M in a 5 min response period. It can be easily incorporated in multienzyme sensors for biochemical substances which produce hydrogen peroxide under catalytic oxidation by their oxidase. This possibility has been tested for the determination of uric acid, D-amino acid, L-amino acid, glucose cholesterol, choline and acetylcholine, respectively, using a membrane with co-immobilized oxidase and horseradish peroxidase. 相似文献
823.
In this study, three kinds of L ‐lactide‐based copolymers, poly(lactide‐co‐glycolide) (PLGA), poly(lactide‐co‐p‐dioxanone) (PLDON) and poly(lactide‐co‐caprolactone) (PLC), were synthesized by the copolymerization of L ‐lactide (L) with glycolide (G), or p‐dioxanone (DON) or ε‐caprolactone (CL), respectively. The copolymers were easily soluble in common organic solvents. The compositions of the copolymers were determined by 1H‐NMR. Thermal/mechanical and shape‐memory properties of the copolymers with different comonomers were compared. Moreover, the effect of the chain flexibility of the comonomers on thermal/mechanical and shape‐memory properties of the copolymers were investigated. The copolymers with appropriate lactyl content showed good shape‐memory properties where both the shape fixity rate (Rf)and the shape recovery rate (Rr) could exceed 95%. It was found that the comonomers with different flexible molecular chain have different effects on their thermal/mechanical and shape‐memory properties. Among them, PLGA has the highest mechanical strength and recovery rate while PLC copolymer has high recovery rate when the lactyl content exceeded 85% and the lowest transition temperature (Ttrans). Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
824.
CeO2‐promoted Na‐Mn‐W/SiO2 catalyst has been studied for catalytic oxidation of methane in a micro‐stainless‐steel reactor at elevated pressure. The effect of operating conditions, such as GHSV, pressure and CH4/O2 ratio, has been investigated. 22.0% CH4 conversion with 73.8% C2‐C4 selectivity (C2/C3/C4 = 3.8/1.0/3.6) was obtained at 1003 K, 1.5 × 105 h?;1 GHSV and 1.0 MPa. The results show: Elevated pressure disadvantages the catalytic oxidation of methane to C2‐C4 hydrocarbons. Large amounts of C3 and C4 hydrocarbons are observed. The unfavorable effects of elevated pressure can be overcome by increasing GHSV; the reaction is strongly dependent on the operating conditions at elevated pressure, particularly dependent on GHSV and ratio of CH4/O2. Analyses by means of XRD, XPS and CO2‐TPD show that CO2 produced from the reaction makes a weakly poisoning capacity of the catalyst; information of changeful valence on Ce and Mn was detected over the near‐surface of the Ce‐Na‐W‐Mn/SiO2 catalyst; the existence of Ce3+/Ce4+ and Mn2+/Mn3+ ion couple supported that the reaction over the catalyst followed the Redeal‐Redox mechanism. Oxidative re‐coupling of C2H6 and CH4 in gas phase or over surface of catalyst produces C3 or C4 hydrocarbons. 相似文献
825.
Lingjun Chou Yingchun Cai Bing Zhang Jianzhong Niu Shengfu Ji Shuben LiState Key Laboratory for Oxo Synthesis Selective Oxidation Lanzhou Institute of Chemical Physics Chinese Academy ofSciences Lanzhou P.R.China 《天然气化学杂志》2002,(Z2)
Na-Mn-W/SiO2 catalysts were studied for the oxidative coupling of methane (OCM) in a micro fixed bed reactor made of stainless steel reactor at elevated pressures. The effect of operating conditions, such as GHSV, pressure, temperature and CH4/O2 ratio on the catalytic performance of OCM was investigated. The C2+ selectivity of 80.3% was obtained at a CH4 conversion of 16.1% at 750℃, 1.5×105h-1 GHSV, and 0.6 MPa. Also, there is a small output of C3 and C4 hydrocarbons in the tail gas. The results show that unfavorable effects due to elevated pressure can be overcome by increasing GHSV, and the OCM reaction is strongly dependent on the operating conditions at elevated pressures, particularly GHSV and the CH4/O2 ratio. 相似文献
826.
The phase behavior of short-chain fluids in slit pores is investigated by using a nonlocal-density-functional theory that takes into account the effects of segment size, chain connectivity, and van der Waals attractions explicitly. The layering and capillary condensation/evaporation transitions are examined at different chain length, temperature, pore width, and surface energy. It is found that longer chains are more likely to show hysteresis loops and multilayer adsorptions along with the capillary condensation and evaporation. Decreasing temperature favors the inclusion of layering transitions into the condensation/evaporation hysteresis loops. For large pores, the surface energy has relatively small effect on the pressures of the capillary condensation and evaporation but affects significantly on the layering pressures. It is also observed that all phase transitions within the pore take place at pressures lower than the corresponding bulk saturation pressure. The critical temperature of condensation/evaporation is always smaller than that of the bulk fluid. All coexistence curves for confined phase transitions are contained within the corresponding bulk vapor-liquid coexistence curve. As in the bulk phase, the longer the chain length, the higher are the critical temperatures of phase transitions in the pore. 相似文献
827.
C60衍生物与C60混合物中富勒烯的生长研究 总被引:1,自引:0,他引:1
富勒烯分子生长方面的研究,对于富勒烯的基础研究以及潜在应用有着重要的意义,因此引起了科学界的广泛兴趣和研究,Yeretzian[1]、Campbell[2],Rao[3]等人相继用不同的实验方法对C60的融合作了详细研究,但C60的生长动力学过程依然是个悬而未决的问题.我们实验室已对C60的正负离子解离以及C60加成衍生物中C60的解离和生长作了较深入的研究:发现C60正负离子解离时电荷具有不同的转移通道,即C朋解离时电荷留在碳笼上,而C60解离时电荷易转移到小碳簇上,且C60解离以较缓慢的笼子收缩过程进行间.衍生物基团(如节基、薛基)对C。。… 相似文献
828.
A substantial amount of experimental and numerical evidence has shown that the Derjaguin-Landau-Verwey-Overbeek theory is not suitable for describing those colloidal solutions that contain multivalent counterions. Toward improved understanding of such solutions, the authors report Monte Carlo calculations wherein, following Rouzina and Bloomfield, they postulate that, in the absence of van der Waals forces, the overall force between two isolated charged colloidal particles in electrolyte solutions is determined by a dimensionless parameter Gamma=z(2)l(B)/a, which measures the electrostatic repulsion between counterions adsorbed on the macroion surface, where z = counterion valence, l(B)=Bjerrum length, and a = average separation between counterions on the macroion surface calculated as if the macroion were fully neutralized. The authors find, first, that the maximum repulsion between like-charged macroions occurs at Gamma approximately 0.5 and, second, that onset of attraction occurs at Gamma approximately 1.8, essentially independent of the valence and concentration of the surrounding electrolyte. These observations might provide new understanding of interactions between electrostatic double layers and perhaps offer explanations for some electrostatic phenomena related to interactions between DNA molecules or proteins. 相似文献
829.
Shiming Liu Xiaoliang Qian Jianzhong Xiao 《Journal of Sol-Gel Science and Technology》2007,44(3):187-193
Nano-crystalline La0.8Sr0.2Co0.5Fe0.5O3±δ powder has been successfully synthesized by microwave assisted sol–gel (MWSG) method. The decomposition and crystallization
behavior of the gel-precursor was studied by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) analysis.
From the result of FT-IR and X-ray diffraction patterns, it is found that a perovskite La0.8Sr0.2Co0.5Fe0.5O3±δ was formed by irradiating the precursor at 700 W for 3 min, but the well-crystalline perovskite La0.8Sr0.2Co0.5Fe0.5O3±δ was obtained at 700 W for 35 min. Morphological and specific area analysis of the powder were done by transmission electron
microscopy (TEM), scanning electron microscope (SEM) and Brunauer–Emmett–Teller (BET). The surface areas measured was 38.9 m2/g and the grain size was ∼23 nm. Electrochemical properties of pure LSCF cathode on YSZ electrolyte at intermediate temperatures
were investigated by using AC impedance analyzer, which shows a low area specific resistance (0.077 Ω cm2 at 1073 K and 0.672 Ω cm2 at 953 K). Moreover, the synthesis period of 20 h usually observed for conventional heating mode is reduced to a few minutes.
Thus, the MWSG method is proved to be a novel, extremely facile, time-saving and energy-efficient route to synthesize LSCF
powders. 相似文献
830.
聚丙交酯/聚乙二醇多嵌段共聚物的合成及其性能 总被引:18,自引:0,他引:18
聚丙交酯 (PLLA)由于具有良好的生物降解性和生物相容性 ,在医学领域已经得到了广泛的临床应用 ,近来又被制备成细胞支架大量应用于组织工程中[1,2 ] ,但由于其疏水性而造成细胞亲和性不好 .聚乙二醇 (PEG)具有良好的亲水性 ,良好的生物相容性 ,但是PEG是非降解性的 ,只有低分子量的PEG可以被吞噬细胞所吞噬或透过肾滤膜而排出体外 ,因此 ,低分子量的PEG常被用来与丙交酯 (L LA)共聚以改善PLLA支架的亲水性 .聚丙交酯 聚乙二醇共聚物 (PLE)的三嵌段及两嵌段共聚物的合成及其性能的研究已被广泛报道[3~ 5] .研究… 相似文献