全文获取类型
收费全文 | 877篇 |
免费 | 63篇 |
国内免费 | 2篇 |
专业分类
化学 | 666篇 |
晶体学 | 4篇 |
力学 | 11篇 |
数学 | 156篇 |
物理学 | 105篇 |
出版年
2023年 | 5篇 |
2022年 | 17篇 |
2021年 | 28篇 |
2020年 | 27篇 |
2019年 | 29篇 |
2018年 | 20篇 |
2017年 | 14篇 |
2016年 | 36篇 |
2015年 | 47篇 |
2014年 | 56篇 |
2013年 | 55篇 |
2012年 | 62篇 |
2011年 | 68篇 |
2010年 | 41篇 |
2009年 | 29篇 |
2008年 | 49篇 |
2007年 | 62篇 |
2006年 | 49篇 |
2005年 | 46篇 |
2004年 | 30篇 |
2003年 | 27篇 |
2002年 | 21篇 |
2001年 | 8篇 |
2000年 | 15篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1969年 | 3篇 |
1958年 | 2篇 |
1954年 | 1篇 |
1909年 | 2篇 |
排序方式: 共有942条查询结果,搜索用时 187 毫秒
71.
The fluid of two-dimensional hard disks is investigated over a range of densities by Monte Carlo simulations in order to detect and characterize structural changes which take place when the condition of freezing and melting is approached. A novel method is proposed based on the use of the Voronoi tessellation and a certain shape factor which turns out to be a clear indicator of the presence of different underlying substructures (domains). Close to the freezing condition the probability distribution of the shape factor develops a second distinct maximum corresponding to a predominant presence of near-regular hexagons, whereas the original peak, having its origin primarily in pentagons and distorted hexagons, diminishes and disappears at melting density. 相似文献
72.
73.
De Santis F Zona D Bellagotti R Vichi F Allegrini I 《Analytical and bioanalytical chemistry》2004,380(5-6):818-823
Ambient ozone was measured in a forest in Castelporziano (Italy) characterised by the prevailing presence of Holm-oak trees (Quercus ilex L.) from June to November 2003. Two methods for measuring ozone were used: long-term monitoring using diffusive samplers at three heights within the canopy, and continuous monitoring at two heights using the UV method. Results for one week mean ozone levels above and below the canopy from the diffusive samplers were compared to those obtained using the automatic analyser at the same levels. A good correlation between the two sampling techniques was found. Continuous monitoring showed a daily cycle with a midday maximum and a nocturnal minimum. While the forest floor consistently had the lowest ozone concentration, there were no differences during most daytime hours. The midday maximum is clearly due to downward mixing with O3-rich air from above. The night-time ozone decay within the canopy is the result of dry deposition of O3 and most likely due to reaction with biogenically produced NO. AOT40 within and above the canopy mostly exceeded the critical levels. 相似文献
74.
Left Cotorsion Rings 总被引:1,自引:0,他引:1
It is proved that if R is an associative ring that is cotorsionas a left module over itself, and J is the Jacobson radicalof R, then the quotient ring R/J is a left self-injective vonNeumann regular ring and idempotents lift modulo J. In particular,if R is indecomposable, then it is a local ring. 2000 MathematicsSubject Classification 16D50, 16D90, 16L30. 相似文献
75.
Effects of uncertainties in the domain on the solution of Dirichlet boundary value problems 总被引:1,自引:0,他引:1
Summary. A domain with possibly non-Lipschitz boundary is defined as a limit of monotonically expanding or shrinking domains with
Lipschitz boundary. A uniquely solvable Dirichlet boundary value problem (DBVP) is defined on each of the Lipschitz domains
and the limit of these solutions is investigated. The limit function also solves a DBVP on the limit domain but the problem
can depend on the sequences of domains if the limit domain is unstable with respect to the DBVP. The core of the paper consists
in estimates of the difference between the respective solutions of the DBVP on two close domains, one of which is Lipschitz
and the other can be unstable. Estimates for starshaped as well as rather general domains are derived. Their numerical evaluation
is possible and can be done in different ways.
Received October 16, 2001 / Revised version received January 16, 2002 / Published online: April 17, 2002
RID="*"
ID="*" The research was funded partially by the National Science Foundation under the grants NSF–Czech Rep. INT-9724783 and
NSF DMS-9802367
RID="**"
ID="**" Support for Jan Chleboun coming from the Grant Agency of the Czech Republic through grant 201/98/0528 is appreciated 相似文献
76.
In this paper we construct elliptic boundary value problems whose standard finite element approximations converge arbitrarily slowly in the energy norm, and show that adaptive procedures cannot improve this slow convergence. We also show that the -norm and the nodal point errors converge arbitrarily slowly. With the -norm two cases need to be distinguished, and the usual duality principle does not characterize the error completely. The constructed elliptic problems are one dimensional.
77.
Ivo Vrkoč 《Applications of Mathematics》2000,45(4):318-319
78.
Ivo Petráš 《Nonlinear dynamics》2009,57(1-2):157-170
This paper deals with a new fractional-order chaotic system. It is based on the concept of Volta’s system, where the mathematical model of Volta’s system contains fractional-order derivatives. This system has simple structure and can display a double-scroll attractor. The behavior of the integer-order and the fractional-order Volta’s system with total order less than three which exhibits chaos is presented as well. Computer simulations are cross-verified by the numerical calculation and the Matlab/Simulink models. 相似文献
79.
80.
Anneli Kruve Koit Herodes Ivo Leito 《Rapid communications in mass spectrometry : RCM》2010,24(7):919-926
Optimization of both the ionization process and ion transportation in the mass spectrometer is of crucial importance in order to achieve high sensitivity and low detection limits and acceptable accuracy in liquid chromatography/electrospray ionization mass spectrometry (LC/ESI‐MS) analysis. In this paper four optimization procedures of electrospray interface and quadrupole ion‐trap mass spectrometer parameters (ESI‐MS) (nebulizer gas and drying gas flow rate, end plate voltage, capillary voltage, skimmer voltage, octopoles direct current and radio frequency, trap drive and lens voltages) were studied on three pesticides – thiabendazole, aldicarb and imazalil. The results demonstrate that the methodology of optimization strongly influences the effectiveness of finding true optima of the operating parameters. Both eluent flow rate and composition during optimization have to mimic the situation during real analysis as closely as possible in order to achieve parameters giving the highest sensitivity. Therefore, post‐column addition of analyte to the mobile phase identical in composition to the one in which analyte elutes during real analysis combined with software‐based optimization was found to be the most effective and fastest method for achieving intensity maxima. The parameters most strongly affecting ion formation and transportation, hence sensitivity, were capillary voltage, direct current of the first octopole, trap drive and the second lens for all pesticides under study. In addition to sensitivity and detection limit matrix effect was considered in the optimization process. It was found that the matrix effect can be reduced but not eliminated by adjusting the ESI and MS parameters. The optimal parameters from the point of view of the matrix effect can only be found with factorial design. Parameters giving higher sensitivity tended to be more affected by matrix effect causing higher ionization suppression by co‐eluting compounds. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献