全文获取类型
收费全文 | 676篇 |
免费 | 49篇 |
国内免费 | 29篇 |
专业分类
化学 | 502篇 |
晶体学 | 3篇 |
力学 | 22篇 |
数学 | 82篇 |
物理学 | 145篇 |
出版年
2023年 | 11篇 |
2022年 | 13篇 |
2021年 | 14篇 |
2020年 | 25篇 |
2019年 | 32篇 |
2018年 | 22篇 |
2017年 | 15篇 |
2016年 | 40篇 |
2015年 | 25篇 |
2014年 | 28篇 |
2013年 | 39篇 |
2012年 | 41篇 |
2011年 | 41篇 |
2010年 | 29篇 |
2009年 | 26篇 |
2008年 | 41篇 |
2007年 | 41篇 |
2006年 | 28篇 |
2005年 | 21篇 |
2004年 | 19篇 |
2003年 | 8篇 |
2002年 | 21篇 |
2001年 | 6篇 |
2000年 | 18篇 |
1999年 | 10篇 |
1998年 | 11篇 |
1997年 | 9篇 |
1996年 | 5篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1969年 | 2篇 |
排序方式: 共有754条查询结果,搜索用时 15 毫秒
131.
Strain-stiffening response in transient networks formed by reverse wormlike micelles 总被引:2,自引:0,他引:2
Strain-stiffening, that is, an increase in material stiffness at large deformations, is a property of many biological materials. Currently, model systems for the study of this phenomenon are elastic networks (gels) of semiflexible filamentous biopolymers such as actin, keratin, or fibrin. Here, we demonstrate strain-stiffening in a class of viscoelastic solutions, comprising reverse wormlike micelles. These structures are formed by the coassembly of the physiological surfactants, lecithin and bile salt, in an organic solvent, cyclohexane. In contrast to the biopolymer gels, the networks here are transient and are formed by the physical entanglement of relatively flexible worms. Our results suggest that neither a permanent network nor a high filament rigidity is required for strain-stiffening. We suggest a different origin, based on a temporary strain-induced increase in the volume fraction of entangled worms. Our system can also serve as a convenient synthetic model for future studies into this phenomenon. 相似文献
132.
Reverse vesicles are spherical containers in organic liquids (oils) consisting of an oily core surrounded by a reverse bilayer. They are the organic counterparts to vesicles in aqueous solution and could potentially find analogous uses in encapsulation and controlled release. However, few examples of robust reverse vesicles have been reported, and general guidelines for their formation do not exist. We present a new route for forming stable unilamellar reverse vesicles in nonpolar organic liquids, such as cyclohexane and n-hexane. The recipe involves mixing short- and long-chain lipids (lecithins) with a trace of a salt such as sodium chloride. The ratio of short- to long-chain lecithin controls the type and size of self-assembled structure. As this ratio is increased, a spontaneous transition from reverse micelles to reverse vesicles occurs. Small-angle neutron scattering (SANS) and transmission electron microscopy (TEM) confirm the presence of unilamellar vesicles in the corresponding solutions. Average vesicle diameters can be tuned from 60 to 250 nm depending on the sample composition. 相似文献
133.
134.
Kiem PV Tri MD Tuong le VD Tung NH Hanh NN Quang TH Cuong NX Minh CV Choi EM Kim YH 《Chemical & pharmaceutical bulletin》2008,56(9):1270-1275
Two new phenyl glycosides, mangliesides A and B (1, 2), a new ionol glycoside, manglieside C (3), two new lignan glycosides, mangliesides D and E (4, 5), were isolated from the leaves of Manglietia phuthoensis, along with two known lignans, 3-methoxymagnolol (6) and obovatol (7). Their structures were established by means of 1D and 2D NMR, electrospray ionization (ESI)-MS and HR-ESI-MS experiments. Among them, compounds 2 and 5 significantly (p<0.05) increased the growth and differentiation of osteoblastic MC3T3-E1 cells in vitro. 相似文献
135.
A series of calix[4]arene-based binuclear platinum(II) complexes, Pt2LCl2 (1, L = 5,11,17,23-tetra- tert-butyl-25,27-di[methoxy(4-phenyl)-(C;N;N)]-26,28-dihydroxycalix[4]arene, HC;N;N = 6-phenyl-2,2'-bipyridine), [Pt2L(mu-dppCn)](ClO4)2 (dppCn = bis(diphenylphosphino)-methane (2, n = 1), -ethane (3, n = 2), -propane (4, n = 3), and [Pt2L(PPh3)2](ClO4)2 (5), have been designed and synthesized in this work. Spectroscopic investigation demonstrates that p- tert-calix[4]arene is capable of assembling the two square-planar [(C;N;N)Pt(II)] units in a face-to-face manner, simultaneously suppressing intermolecular aggregation and increasing the solubility of the studied complexes. Facile replacement of the chloride ligand in 1 by the strongly sigma-donating ancillary phosphine ligands affords binuclear platinum(II) complexes with improved photophysical properties. All of the complexes are emissive both in the fluid/glassy solution and in the solid state, except for 1 in the solid state at room temperature. Moreover, the absorption and emission energies of the complexes are sensitive to the ancillary ligands. Varying the tethered phosphine auxiliaries from dppm (2) and dppe (3) to dppp (4) and PPh3 (5) modulates the intramolecular metal-metal (Pt...Pt) and ligand-ligand (pi-pi) distances, thereby leading to a switch of 3MMLCT and excimeric 3(pipi*) excited states to a common 3MLCT excited state. 相似文献
136.
Small volume low mechanical stress cytometry using computer-controlled Braille display microfluidics
This paper describes a micro flow cytometer system designed for efficient and non-damaging analysis of samples with small numbers of precious cells. The system utilizes actuation of Braille-display pins for micro-scale fluid manipulation and a fluorescence microscope with a CCD camera for optical detection. The microfluidic chip is fully disposable and is composed of a polydimethylsiloxane (PDMS) slab with microchannel features sealed against a thin deformable PDMS membrane. The channels are designed with diffusers to alleviate pulsatile flow behaviors inherent in pin actuator-based peristaltic pumping schemes to maximize hydrodynamic focusing of samples with minimal disturbances in the laminar streams within the channel. A funnel connected to the microfluidic channel is designed for efficient loading of samples with small number of cells and is also positioned on the chip to prevent physical damages of the samples by the squeezing actions of Braille pins during actuation. The sample loading scheme was characterized by both computational fluidic dynamics (CFD) simulation and experimental observation. A fluorescein solution was first used for flow field investigation, followed by use of fluorescence beads with known relative intensities for optical detection performance calibration. Murine myoblast cells (C2C12) were exploited to investigate cell viability for the sample loading scheme of the device. Furthermore, human promyelocytic leukemia (HL60) cells stained by hypotonic DNA staining buffer were also tested in the system for cell cycle analysis. The ability to efficiently analyze cellular samples where the number of cells is small was demonstrated by analyzing cells from a single embryoid body derived from mouse embryonic stem cells. Consequently, the designed microfluidic device reported in this paper is promising for easy-to-use, small sample size flow cytometric analysis, and has potential to be further integrated with other Braille display-based microfluidic devices to facilitate a multi-functional lab-on-a-chip for mammalian cell manipulations. 相似文献
137.
Xiao-Ling Wu Gui-Hong Liao Lin Luo Xu-Feng Shan Li-Ping Zhang Chen-Ho Tung 《Tetrahedron》2007,63(15):3133-3137
Two novel hydroxyl- and carboxyl-functionalized cubane-like photodimers from methyl 2-naphthoate have been successfully achieved under mild conditions. X-ray crystal structures confirm that the cubane-like structure is well retained in these two derivatives and the intermolecular OH?O hydrogen bonding plays an important role in crystal packing. It is of significance that the isolated racemic mixture can be subsequently resolved into two optically pure enantiomers by high-performance liquid chromatography. 相似文献
138.
Danoff EJ Wang X Tung SH Sinkov NA Kemme AM Raghavan SR English DS 《Langmuir : the ACS journal of surfaces and colloids》2007,23(17):8965-8971
We demonstrate the unique ability of catanionic vesicles, formed by mixing single-tailed cationic and anionic surfactants, to capture ionic solutes with remarkable efficiency. In an initial study (Wang, X.; Danoff, E. J.; Sinkov, N. A.; Lee, J.-H.; Raghavan, S. R.; English, D. S. Langmuir 2006, 22, 6461) with vesicles formed from cetyl trimethylammonium tosylate (CTAT) and sodium dodecylbenzenesulfonate (SDBS), we showed that CTAT-rich (cationic) vesicles could capture the anionic solute carboxyfluorescein with high efficiency (22%) and that the solute was retained by the vesicles for very long times (t1/2 = 84 days). Here we expand on these findings by investigating the interactions of both anionic and cationic solutes, including the chemotherapeutic agent doxorubicin, with both CTAT-rich and SDBS-rich vesicles. The ability of these vesicles to capture and hold dyes is extremely efficient (>20%) when the excess charge of the vesicle bilayer is opposite that of the solute (i.e., for anionic solutes in CTAT-rich vesicles and for cationic solutes in SDBS-rich vesicles). This charge-dependent effect is strong enough to enable the use of vesicles to selectively capture and separate an oppositely charged solute from a mixture of solutes. Our results suggest that catanionic surfactant vesicles could be useful for a variety of separation and drug delivery applications because of their unique properties and long-term stability. 相似文献
139.
Huang X Zhao F Li Z Tang Y Zhang F Tung CH 《Langmuir : the ACS journal of surfaces and colloids》2007,23(9):5167-5172
Tetrakis(aryloxy)phthalocyanine (4c) and its Zn congeners (4a and 4b) and Ni congener (4d) were synthesized, and their self-assembling properties in coordinating and non-coordinating solvents were investigated by absorption and fluorescence spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), and transmission electron microscopy (TEM). Compounds 4a and 4b in non-coordinating solvents exhibit red-shifted and split Q-bands in absorption spectra even at very low concentrations, suggesting J-aggregate formation. The MALDI-TOF MS for the samples of 4a and 4b prepared from chloroform solutions gives the monomer and aggregate signals. The TEM images of such samples display an indefinite two-dimensional network structure. The aggregates break up into monomers when a coordinating solvent is added to the solution. The driving force for the aggregate formation is proposed to be the complementary coordination of the ether oxygen in the aryloxy groups of one molecule to the core Zn of another molecule of phthalocyanine. 相似文献
140.
O'Donnell P.J. Xu Q.P. Tung H.K.K. 《The European Physical Journal C - Particles and Fields》1999,10(2):313-317
Corrections to the exact heavy–quark symmetry results are expected to come from the inverse powers of the heavy-quark mass.
We show, by an explicit calculation using the quark model, that the breaking of the spin symmetry is suppressed by other kinematic
effects even when the quark masses are not heavy.
Received: 21 January 1999 / Published online: 20 May 1999 相似文献