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111.
112.
Detector systems with cooled non-thallium-activated NaI-crystals allow to improve the time resolution in -coincidence experiments. Such a system has been developed and yielded typical time resolutions a factor of 2.5 better than conventional NaI(Tl)-systems. Two examples for possible applications are given. 相似文献
113.
114.
R.R. Netz H. Orland 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):81-98
Variational methods are applied to a single polyelectrolyte chain. The polymer is modeled as a Gaussian chain with screened
electrostatic repulsion between all monomers. As a variational Hamiltonian, the most general Gaussian kernel, including the
possibility of a classical or mean polymer path, is employed. The resulting self-consistent equations are systematically solved
both for large and small monomer-monomer separations along the chain. In the absence of screening, the polymer is stretched
on average. It is described by a straight classical path with Gaussian fluctuations around it. If the electrostatic repulsion
is screened, the polymer is isotropically swollen for large separations, and for small separations the polymer correlation
function is calculated as an analytic expansion in terms of the monomer-monomer separation along the chain. The electrostatic
persistence length and the electrostatic blobsize are inferred from the crossover between distinct scaling ranges. We perform
a global analysis of the scaling behavior as a function of the screening length and electrostatic interaction strength , where is the Bjerrum length and A is the distance of charges along the polymer chain. We find three different scaling regimes. i) A Gaussian-persistent regime with Gaussian behavior at small, persistent behavior at intermediate, and isotropically swollen behavior at large length
scales. This regime occurs for weakly charged polymers and only for intermediate values of the screening length. The electrostatic
persistence length is defined as the crossover length between the persistent and the asymptotically swollen behavior and is given by and thus disagrees with previous (restricted) variational treatments which predict a linear dependence on the screening length
.ii) A Gaussian regime with Gaussian behavior at small and isotropically swollen behavior at large length scales. This regime occurs for weakly
charged polymers and/or strong screening, and the electrostatic repulsion between monomers only leads to subfluent corrections
to Gaussian scaling at small separations. The concept of a persistence length is without meaning in this regime. iii) A persistent regime , where the chain resembles a stretched rod on intermediate and small scales. Here the persistence length is given by the
original Odijk prediction, , if the overstretching of the chain is avoided. We also investigate the effects of a finite polymer length and of an additional
excluded-volume interaction, which modify the resultant scaling behavior. Applications to experiments and computer simulations
are discussed.
Received 24 December 1997 相似文献
115.
The effective charge of hydrophobic surfaces and in particular of the air–water interface is a crucial parameter for electrochemistry, colloidal chemistry and interfacial science, but different experiments give conflicting estimates. Zeta-potential and disjoining-pressure measurements point to a strongly negative surface charge, often interpreted as being due to adsorbing hydroxide ions. In contrast, surface tension measurements of acids and bases suggest the hydronium ion to be surface active, in agreement with some surface-specific non-linear spectroscopy results. The air–electrolyte interfacial tension exhibits a characteristic minimum at millimolar electrolyte concentration for all salts, the so-called Jones–Ray effect, which points to competitive adsorption mechanisms present in dilute electrolyte solutions. We show that all these puzzling experimental findings can be explained by the presence of trace amounts of surface-active charged impurities, most likely anionic surfactants. 相似文献
116.
117.
Praseodymium oxide is one of the group of binary oxides that show a wide range of stoichiometry, but in which the deviation
from the ideal stoichiometry is not continuous, as is expected from the classical model of randomly distributed isolated point
defects. Instead, there is a number of phases of unique structures and stoichiometries with no appreciable concentrations
of point defects, and thus essentially no compositional width. These structures are closely related to each other, and to
a simple parent structure. In the case of praseodymium oxide this is the fluorite structure.
Transitions between these phases upon heating and cooling, as well as upon changes in the oxygen activity of the environment,
occur very rapidly, indicating unusually high oxygen mobility. The equilibrium phase at ambient temperature in air is not
PrO2, but Pr6O11.
These phases have been shown to have cation sublattices essentially identical to that of fluorite-type PrO2. The stoichiometric changes are accommodated by the presence of topotactically ordered arrangements of large localized concentrations
of oxide ion vacancies which divide the structure up into microdomains.
Experiments have shown that Pr6O11 can absorb significant amounts of water, even at ambient temperature, apparently by the electrically neutral mechanism proposed
by Stotz and Wagner. This involves the absorption of oxide ions at sites where there were previously oxide ion vacancies.
Protons are also absorbed, and reside in interstitial sites. Consistent with the high mobility of oxygen in this structure,
this water is irreversibly desorbed at relatively low temperatures.
Electrochemical experiments will be reported that have been undertaken to investigate the insertion behavior of lithium into
these unusual structures, both with and without the initial presence of protons as the result of the absorption of water.
Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999. 相似文献
118.
<正> 7 解的唯一性 如果不考虑非平衡态和不可逆性,则等能密度理论中的基本命题与经典弹性理论中的在形式上完全相同。无论整个系统的动能是大还是小,图6中(sumform)_o上的面力T_i和在(sumform)_c和在的位移u_i将始终在空间坐标系x_i和时刻i下给出。与一般力学中相同,必须知道有关位移u_i及其时间导数的初始条件。它们的影响则可通过(38)和(39)向等能面转化,从而得到 相似文献
119.
A.G. Moreira R.R. Netz 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,21(1):83-96
Using a field-theoretic approach, we derive the first few coefficients of the exact low-density (“virial”) expansion of a
binary mixture of positively and negatively charged hard spheres (two-component hard-core plasma, TCPHC). Our calculations
are nonperturbative with respect to the diameters d+ and d- and charge valences q+ and q- of positive and negative ions. Consequently, our closed-form expressions for the coefficients of the free energy and activity
can be used to treat dilute salt solutions, where typically d
+∼d
- and q
+∼q
-, as well as colloidal suspensions, where the difference in size and valence between macroions and counterions can be very
large. We show how to map the TCPHC on a one-component hard-core plasma (OCPHC) in the colloidal limit of large size and valence
ratio, in which case the counterions effectively form a neutralizing background. A sizable discrepancy with the standard OCPHC
with uniform, rigid background is detected, which can be traced back to the fact that the counterions cannot penetrate the
colloids. For the case of electrolyte solutions, we show how to obtain the cationic and anionic radii as independent parameters
from experimental data for the activity coefficient.
Received 6 September 2001 / Received in final form 20 May 2002 Published online 24 September 2002 相似文献
120.
Umar Rashid Elarbi Chatir Leonardo Medrano Sandonas PA Sreelakshmi Arezoo Dianat Rafael Gutierrez Gianaurelio Cuniberti Saioa Cobo Veerabhadrarao Kaliginedi 《Angewandte Chemie (International ed. in English)》2023,62(16):e202218767
By employing a mechanically controllable break junction technique, we have realized an ideal single molecular linear actuator based on dithienylethene (DTE) based molecular architecture, which undergoes reversible photothermal isomerization when subjected to UV irradiation under ambient conditions. As a result, open form (compressed, UV OFF) and closed form (elongated, UV ON) of dithienylethene-based molecular junctions are achieved. Interestingly, the mechanical actuation is achieved without changing the conductance of the molecular junction around the Fermi level over several cycles, which is an essential property required for an ideal single molecular actuator. Our study demonstrates a unique example of achieving a perfect balance between tunneling width and barrier height change upon photothermal isomerization, resulting in no change in conductance but a change in the molecular length, which results in mechanical actuation at the single molecular level. 相似文献