全文获取类型
收费全文 | 601篇 |
免费 | 45篇 |
国内免费 | 103篇 |
专业分类
化学 | 237篇 |
晶体学 | 1篇 |
力学 | 157篇 |
综合类 | 2篇 |
数学 | 97篇 |
物理学 | 255篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2021年 | 15篇 |
2020年 | 17篇 |
2019年 | 20篇 |
2018年 | 11篇 |
2017年 | 31篇 |
2016年 | 25篇 |
2015年 | 15篇 |
2014年 | 35篇 |
2013年 | 43篇 |
2012年 | 19篇 |
2011年 | 48篇 |
2010年 | 33篇 |
2009年 | 39篇 |
2008年 | 50篇 |
2007年 | 47篇 |
2006年 | 42篇 |
2005年 | 42篇 |
2004年 | 33篇 |
2003年 | 16篇 |
2002年 | 18篇 |
2001年 | 10篇 |
2000年 | 23篇 |
1999年 | 16篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 12篇 |
1995年 | 9篇 |
1994年 | 4篇 |
1993年 | 10篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有749条查询结果,搜索用时 15 毫秒
21.
The fast transient fluorescence (FTRF) technique was used to study critical exponents at the glass transition in free-radical crosslinking copolymerization (FCC) for two different monomeric systems, methyl methacrylate (MMA) and styrene (S). Pyrene (Py ) was used as a fluorescence probe. The fluorescence lifetimes of Py from its decay traces were measured and used to monitor the gelation process. Changes in the viscosity of the pregel solutions due to glass formation dramatically enhance the fluorescent yield of aromatic molecules. This effect is used to study the glass transition upon gelation of MMA and S monomeric systems as a function of time, at various temperatures and crosslinker concentrations. The results are interpreted in the view of percolation theory. The gel fraction and weight average degree of polymerization exponents β and γ are found to be 0.37 ± 0.02 and 1.66 ± 0.07 in agreement with percolation results. 相似文献
22.
《Current Applied Physics》2018,18(3):340-344
In this paper, we investigate the performance of ring oscillators composed of gate-all-around (GAA) silicon nanowire (NW) field-effect transistors (FETs) with four different numbers of NW channels, for sub-10-nm logic applications. Our simulations reveal that ring oscillators with double, triple, and quadruple NW channels exhibit improvements of up to 50%, 85%, and 97%, respectively, in the oscillation frequencies (fosc), compared to a ring oscillator with a single NW channel, due to the large drive current, in spite of the increased intrinsic capacitance of a given device. Moreover, our work shows that the fosc improvement ratio of the ring oscillators becomes saturated with triple NW channels with additional load capacitances of 0.1 fF and 0.01 fF, which are similar to, or less than the intrinsic device capacitance (∼0.1 fF). Thus, our study provides an insight for determining the capacitive load and optimal number of NW channels, for device development and circuit design of GAA NW FETs. 相似文献
23.
J. Swiatkiewicz 《Phase Transitions》2013,86(9-10):891-896
Transient behaviour of the excited states originated from the neutral and the charged species is discussed using data from time-resolved experiments. Complex transient characteristics obtained for the slightly charged poly-(bithiophene) film during electrochemical doping were explained in terms of sequential molar fraction changes of the neutral species, the radical cation and the π-dimerized radical cations. Femtosecond transient absorption measurement carried out in time range of 0-4 ns for a two-photon excited dye reveals a multi-exponent decay with well separated relaxation times and the distinct relaxation mechanisms. At high irradiance and a sufficient fluence, the multi-photon excitation processes affect the subpicosecond response and lead to formation of the photo-products. 相似文献
24.
《Current Applied Physics》2020,20(5):648-652
The transient photocurrent (TPC) technique was performed to explore the dynamics of excitons and carriers at organic active layer/buffer layer interfaces. A special device with ITO/PEIE/NPB/C60/Al structure was designed to study the interfacial processes at the NPB/C60 interface. An external electrical field was provided to neutralize the built-in electrical field of the device. Interestingly, a new phenomenon was observed, wherein the polarity of the TPC changed from negative to positive under an external electrical field. The initial negative signal was ascribed to exciton separation by the built-in field in C60, and the subsequent positive signal can be attributed to the diffusion of electrons that accumulate at the NPB/C60 interface. TPC measurements shown that further increasing the external electrical field causes polarity to change twice. Analyzing the two changes in polarity revealed that the NPB did not only extract holes from C60 but also provided an effective interface for exciton dissociation. 相似文献
25.
《Applied Mathematical Modelling》2014,38(9-10):2673-2688
The in-bore process that occurs when a pistol is fired involves multiple physical models. This process is brief and typically measured in microseconds. Furthermore, propellants produce high temperatures and pressure gases during the burning process. These factors have made experimentation and simulation of the in-bore behavior of bullets difficult. This study uses a nonlinear transient finite element method (FEM) to simulate the in-bore behavior of a 9 mm bullet after being fired, where the chamber pressure is calculated by Vallier–Heydenreich formula and is used as the input loading. A gunshot experiment is conducted to verify the accuracy of computational results. The maximum difference between the numerical results and real experimental data is only 2.56% (including muzzle velocity and width and depth of engraved bullet vestiges), indicating that the simulation is credible.The discussed simulation is capable of obtaining the plastic deformation and kinematic status of the bullet and the stress history and distribution of the gun barrel. The numerical results can provide complete data of the entire in-bore process, improve the drawbacks during real in-bore ballistic research experiments, and assist engineers in designing and developing other novel systems. The simulation can save considerable time when designing small arms barrels. 相似文献
26.
Jeffrey J. Sweterlitsch Perminus Mungara Roger W. Jones Jay-lin Jane 《International Journal of Polymer Analysis and Characterization》2013,18(6):464-468
Soy protein–based polymers offer promising performance properties, but their characteristics are sensitively dependent on production conditions, so on-line monitoring could help provide the needed control during production. Mid-infrared spectroscopy combined with partial least squares offer the needed analysis, but the opacity of many materials in the mid-infrared range limits its conventional application. Transient infrared spectroscopy is a method of acquiring mid-infrared spectra from moving streams in real time that avoids the opacity problem. We apply transient infrared spectroscopy to a polymer of soy protein and polyisoprene-graft-maleic anhydride–modified natural rubber during its compounding extrusion to measure tensile strength and Young's modulus. 相似文献
27.
T.L.-G. Carlson K. Larsson Y. Miezis 《Journal of Dispersion Science and Technology》2013,34(2):197-208
In connection with studies of technical functionality of lipids in cereals, the aqueous systems of lipids from rye and tri-ticale flour are described and compared to different wheat culti-vars. Rye lipids give a lamellar liquid-crystalline phase with very small ordered regions, and this phase is continuously changed into an L2-phase with increasing water content. Triticale lipids exhibit phase equilibria in between lipids of wheat and of rye. Evidence is given for a significant role of the lamellar liquid--crystalline phase with regard to technical functionality when the cereal flour is worked in water to give a dough. 相似文献
28.
Dr. Agnese Marcelli Dr. Barbara Patrizi Prof. Alessandra Bonamore Prof. Alberto Boffi Prof. Maurizio Becucci Prof. Paolo Foggi 《Chemphyschem》2021,22(11):1134-1140
Flavohemoglobins have the particular capability of binding unsaturated and cyclopropanated fatty acids as free acids or phospholipids. Fatty acid binding to the ferric heme results in a weak but direct bonding interaction. Ferrous and ferric protein, in presence or absence of a bound lipid molecule, have been characterized by transient absorption spectroscopy. Measurements have been also carried out both on the ferrous deoxygenated and on the CO bound protein to investigate possible long-range interaction between the lipid acyl chain moiety and the ferrous heme. After excitation of the deoxygenated derivatives the relaxation process reveals a slow dynamics (350 ps) in lipid-bound protein but is not observed in the lipid-free protein. The latter feature and the presence of an extra contribution in the absorption spectrum, indicates that the interaction of iron heme with the acyl chain moiety occurs only in the excited electronic state and not in the ground electronic state. Data analysis highlights the formation of a charge-transfer complex in which the iron ion of the lipid-bound protein in the expanded electronic excited state, possibly represented by a high spin Fe III intermediate, is able to bind to the sixth coordination ligand placed at a distance of at 3.5 Å from the iron. A very small nanosecond geminate rebinding is observed for CO adduct in lipid-free but not in the lipid-bound protein. The presence of the lipid thus appears to inhibit the mobility of CO in the heme pocket. 相似文献
29.
Alkaptonuria (AKU) is an ultra-rare disease caused by mutations in homogentisate 1,2-dioxygenase (HGD) enzyme, characterized by the loss of enzymatic activity and the accumulation of its substrate, homogentisic acid (HGA) in different tissues, leading to ochronosis and organ degeneration. Although the pathological effects of HGD mutations are largely studied, less is known about the structure of the enzyme, in particular the pathways for dioxygen diffusion to the active site, required for the enzymatic reaction, are still uninvestigated. In the present project, the combination of two in silico techniques, Molecular Dynamics (MD) simulation and Implicit Ligand Sampling (ILS), was used to delineate gas diffusion routes in HGD enzyme. A route from the central opening of the hexameric structure of the enzyme to the back of the active site trough the protein moiety was identified as the path for dioxygen diffusion, also overlapping with a transient pocket, which then assumes an important role in dioxygen diffusion. Along the route the sequence location of the missense variant E401Q, responsible for AKU development, was also found, suggesting such mutation to be conducive of enzymatic activity loss by altering the flow dynamics of dioxygen. Our in silico approach allowed also to delineate the route of HGA substrate to the active site, until now only supposed. 相似文献
30.
利用激光闪光光解-瞬态吸收光谱技术研究了355 nm 光作用下六氟苯(C6F6)-HNO2水溶液的反应机理, 探讨了中间产物及其动力学行为, 并对终产物进行了分析. 实验表明, C6F6可与HNO2光解产生的OH自由基反应生成加合物C6F6…OH, 二级反应速率常数为1.8×109 L·mol -1·s-1, 加合物吸收峰位置在250、270和400 nm处; C6F6…OH 加合物通过消除反应生成C6F5O·, 其表观生成常数为6.1×105 s-1. C6F6…OH与O2复合转化为C6F6OHO2, 二级反应速率常数为2.8×106 L·mol-1·s-1, C6F6OHO2峰位置与C6F6…OH 加合物相似. 终产物分析表明, OH自由基与六氟苯发生消除HF的反应而生成C6F5OH, 有O2时, 还产生四氟醌C6F4O2, 但无论有氧还是无氧体系, 均不发生硝基化反应. 相似文献