全文获取类型
收费全文 | 3184篇 |
免费 | 58篇 |
国内免费 | 7篇 |
专业分类
化学 | 2085篇 |
晶体学 | 17篇 |
力学 | 37篇 |
数学 | 538篇 |
物理学 | 572篇 |
出版年
2016年 | 56篇 |
2015年 | 46篇 |
2014年 | 48篇 |
2013年 | 76篇 |
2012年 | 87篇 |
2011年 | 118篇 |
2010年 | 63篇 |
2009年 | 77篇 |
2008年 | 107篇 |
2007年 | 106篇 |
2006年 | 101篇 |
2005年 | 92篇 |
2004年 | 86篇 |
2003年 | 84篇 |
2002年 | 78篇 |
2001年 | 54篇 |
2000年 | 51篇 |
1999年 | 27篇 |
1998年 | 37篇 |
1997年 | 24篇 |
1996年 | 37篇 |
1995年 | 47篇 |
1994年 | 41篇 |
1993年 | 45篇 |
1992年 | 37篇 |
1991年 | 29篇 |
1990年 | 38篇 |
1989年 | 47篇 |
1988年 | 57篇 |
1987年 | 44篇 |
1986年 | 39篇 |
1985年 | 51篇 |
1984年 | 31篇 |
1983年 | 26篇 |
1982年 | 24篇 |
1981年 | 47篇 |
1980年 | 42篇 |
1979年 | 50篇 |
1978年 | 47篇 |
1977年 | 32篇 |
1976年 | 29篇 |
1975年 | 41篇 |
1974年 | 27篇 |
1973年 | 37篇 |
1972年 | 31篇 |
1971年 | 38篇 |
1934年 | 27篇 |
1933年 | 24篇 |
1931年 | 19篇 |
1912年 | 25篇 |
排序方式: 共有3249条查询结果,搜索用时 15 毫秒
191.
192.
The optimisation of ultrasonic cleaning procedures for dairy fouled ultrafiltration membranes 总被引:3,自引:0,他引:3
Muthukumaran S Kentish S Lalchandani S Ashokkumar M Mawson R Stevens GW Grieser F 《Ultrasonics sonochemistry》2005,12(1-2):29-35
Ultrafiltration (UF) of whey is a major membrane based process in the dairy industry. However, commercialization of this application has been limited by membrane fouling, which has a detrimental influence on the permeation rate. There are a number of different chemical and physical cleaning methods currently used for cleaning a fouled membrane. It has been suggested that the cleaning frequency and the severity of such cleaning procedures control the membrane lifetime. The development of an optimal cleaning strategy should therefore have a direct implication on the process economics. Recently, the use of ultrasound has attracted considerable interest as an alternative approach to the conventional methods. In the present study, we have studied the ultrasonic cleaning of polysulfone ultrafiltration membranes fouled with dairy whey solutions. The effects of a number of cleaning process parameters have been examined in the presence of ultrasound and results compared with the conventional operation. Experiments were conducted using a small single sheet membrane unit that was immersed totally within an ultrasonic bath. Results show that ultrasonic cleaning improves the cleaning efficiency under all experimental conditions. The ultrasonic effect is more significant in the absence of surfactant, but is less influenced by temperature and transmembrane pressure. Our results suggest that the ultrasonic energy acts primarily by increasing the turbulence within the cleaning solution. 相似文献
193.
The multibubble sonoluminescence (MBSL) emission intensity from aqueous solutions containing simple aliphatic organic acids (RCOOH) and bases (RNH2) and mixtures of the two types of solutes has been examined as a function of pH. In solutions containing either an organic acid or base, under pH conditions where the solutes are predominately in their ionized form (i.e., RCOO- and RNH3+), the MBSL intensity is identical with that obtained in pure water. Alternatively, under pH conditions where the solutes are in their un-ionized form the MBSL intensity is suppressed. However, in solute mixtures of RCOO- and RNH3+ in the pH range of 7 to 9, the MBSL intensity was significantly suppressed relative to that from water. To explain the results of the mixed solute system it has been postulated that when the bubble/solution interface experiences the extreme temperature conditions that accompany bubble collapse, proton transfer occurs between acid-base ion-pair complexes, [RCOO-...RNH3+], adsorbed at the bubble/solution interface. The neutral forms of the solutes then evaporate into the bubble during its expansion phase and through a complex series of events, over a number of bubble oscillations, reduce the core temperature of the collapsing bubble and hence the SL intensity. 相似文献
194.
195.
Vanja Margetic Ticijana Ban Ota Samek Franz Leis Kay Niemax Roland Hergenröder 《Czechoslovak Journal of Physics》2004,54(4):423-429
Fluorescence measurements have been used to characterize the velocity of atoms in a femtosecond-laser-produced plasma. Nanogram
amounts of a copper sample were ablated by the focused radiation (λ=775 nm) of an all-solid-state laser. The laser was operated
at a pulse rate of 10 Hz with an energy of 200μJ per pulse. The microplasma expanded into a defined argon atmosphere of pressures between 0.02 and 850 mbar. Atomic fluorescence
was excited in the laser plume by a dye-laser pulse with the wavelength set to the line Cu I 282.4 nm. The narrowed beam of
the dye-laser was directed into the plasma at different heights above the sample surface. The fluorescence radiation was measured
with an échelle-spectrometer, equipped with an intensified-charge-coupled device as the detector. The velocity depends strongly
on the pressure of the ambient atmosphere and the distance from the sample surface. The highest velocity found at an argon
pressure of 0.02 mbar was 1.0×106 cm s−1. 相似文献
196.
A new continuous-flow system for the analysis of the complete stable isotopic composition of water vapor has been developed. The sample size is reduced to only 120 microg (identical with 120 nL of liquid substance) of water, yielding precisions of about 0.7, 1.3 and 7 per thousand for delta17O, delta18O and delta2H, respectively. The total time for the analysis of a sample is about 150 min including purging times. Oxidized steel surfaces can be a source of memory effects which can be corrected for. The system is predestined for atmospheric applications in the tropopause region, as the sample can be directly introduced into the system from a cryogenic trap. 相似文献
197.
The first and second cinchona rearrangement. Two fundamental transformations of alkaloid chemistry 总被引:1,自引:0,他引:1
Stereochemistry, products, and driving forces of the "first and second Cinchona rearrangement" have been investigated and a unified theory is presented. The first cage expansion affords [3.2.2]azabicyclic alpha-amino ether and is formulated via a configurationally stable bridgehead iminium ion and quasiequatorial nucleophilic attack. The second cage expansion affords beta-functionalized [3.2.2]azabicycles. In this case a nonclassical nitrogen-bridged cation is postulated to account for retention of configuration and potential reversibility of the cage expansion. The second rearrangement is favored for the so-called cinch bases (6'-R = H) in trifluoroethanol. Stereoelectronic factors, electron demand at C9, ground state conformation, and solvent type are crucial in all cases. A two-step protocol for preparing 9-epi-configured Cinchona alkaloids from 9-nat precursors is described. 相似文献
198.
Lobo J Michel EG Bachmann AR Speller S Kuntze J Ortega JE 《Physical review letters》2004,93(13):137602
Stepped Cu nanostripes with varying terrace widths are self-assembled during Ag-induced periodic faceting of vicinal Cu(111). By changing Ag coverage the average terrace size within individual Cu stripes is readily tuned, making it possible to select the one-dimensional or two-dimensional character of surface states. Furthermore, the average terrace size can be smoothly switched from 10 to 30 A, thereby tracking the transition from step-lattice, quasi-two-dimensional umklapp bands to terrace-confined one-dimensional quantum well states. 相似文献
199.
We consider a finite-range spin glass model in arbitrary dimension, where the strength of the two-body coupling decays to zero over some distance gamma(-1). We show that, under mild assumptions on the interaction potential, the infinite-volume free energy of the system converges to that of the Sherrington-Kirkpatrick one, in the Kac limit gamma-->0. This could be a first step toward an expansion around mean-field theory, for spin glass systems. 相似文献
200.
We present a temperature-independent Monte Carlo method for the determination of the density of states of lattice proteins that combines the fast ground-state search strategy of the new pruned-enriched Rosenbluth chain-growth method and multicanonical reweighting for sampling the complete energy space. Since the density of states contains all energetic information of a statistical system, we can directly calculate the mean energy, specific heat, Helmholtz free energy, and entropy for all temperatures. We apply this method to lattice proteins consisting of hydrophobic and polar monomers, and for the examples of sequences considered, we identify the transitions between native, globule, and random coil states. Since no special properties of heteropolymers are involved in this algorithm, the method applies to polymer models as well. 相似文献