全文获取类型
收费全文 | 539篇 |
免费 | 17篇 |
国内免费 | 2篇 |
专业分类
化学 | 427篇 |
晶体学 | 3篇 |
力学 | 11篇 |
数学 | 65篇 |
物理学 | 52篇 |
出版年
2023年 | 3篇 |
2022年 | 13篇 |
2021年 | 28篇 |
2020年 | 13篇 |
2019年 | 14篇 |
2018年 | 11篇 |
2017年 | 11篇 |
2016年 | 15篇 |
2015年 | 27篇 |
2014年 | 13篇 |
2013年 | 44篇 |
2012年 | 31篇 |
2011年 | 45篇 |
2010年 | 30篇 |
2009年 | 20篇 |
2008年 | 32篇 |
2007年 | 26篇 |
2006年 | 28篇 |
2005年 | 24篇 |
2004年 | 21篇 |
2003年 | 24篇 |
2002年 | 13篇 |
2001年 | 4篇 |
2000年 | 6篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1995年 | 5篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 6篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1974年 | 3篇 |
1973年 | 1篇 |
1941年 | 1篇 |
1933年 | 1篇 |
排序方式: 共有558条查询结果,搜索用时 31 毫秒
151.
The asymptotic performance ratio of an on-line algorithm for uniform parallel machine scheduling with release dates 总被引:1,自引:0,他引:1
Jobs arriving over time must be non-preemptively processed on one of m parallel machines, each running at its own speed, so as to minimize a weighted sum of the job completion times. In this on-line
environment, the processing requirement and weight of a job are not known before the job arrives. The Weighted Shortest Processing
Requirement (WSPR) heuristic is a simple extension of the well known WSPT heuristic, which is optimal for the single machine
problem without release dates. According to WSPR, whenever a machine completes a job, the next job assigned to it is the one
with the least ratio of processing requirement to weight among all jobs available for processing at this point in time. We
analyze the performance of this heuristic and prove that its asymptotic competitive ratio is one for all instances with bounded job processing requirements and weights. This implies that the WSPR algorithm generates a
solution whose relative error approaches zero as the number of jobs increases. Our proof does not require any probabilistic
assumption on the job parameters and relies extensively on properties of optimal solutions to a single machine relaxation
of the problem.
Research supported in part by ONR Contracts N00014-90-J-1649 and N00014-95-1-0232, NSF Contracts DDM-9322828, DMI-9732795,
DMI-0085683 and DMI-0245352, NUS Academic Research Grant R314-000-046-112, and a research grant from the Natural Sciences
and Research Council of Canada (NSERC). 相似文献
152.
Liliana V. Lukashuk Andrey B. Lysenko Eduard B. Rusanov Alexander N. Chernega Konstantin V. Domasevitch 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(4):m140-m143
In the isostructural title complexes, [M(C9H6N3O2)2(H2O)2] [M = CoII, (I), CdII, (II), and CuII, (III); the metal centres reside on a twofold axis in the space group C2/c for (I) and (II)], the metal centres are surrounded by four O atoms from two O,O′‐bidentate carboxylate groups and by two trans‐coordinated aqua ligands, forming a distorted octahedral environment. The molecules possess four hydrogen‐bond donor (two aqua ligands) and four hydrogen‐bond acceptor sites (two triazole groups), and aggregate by self‐association, forming two‐dimensional hydrogen‐bonded frameworks [via O—H⋯N interactions; O⋯N = 2.749 (3)–2.872 (3) Å]. The layers are parallel and are tightly packed with short interlayer distances of 4.93, 4.95 and 5.01 Å for (I), (II) and (III), respectively. 相似文献
153.
The adsorption process of 3-chloro phenol from aqueous solution on a activated carbon prepared from African palm stone and
which presents a specific surface area of 685 m2 g−1, a greater quantity of total acid groups and a pHPZC of 6.8 is studied. The adsorption isotherms are determined at pH values of 3, 5, 7, 9 and 11. The adsorption isotherms are
fitted to the Langmuir model and the values of the maximum quantity adsorbed that are between 96.2 and 46.4 mg g−1 are obtained along with the constant KL with values between 0.422 and 0.965 L mg−1. The maximum quantity adsorbed diminishes with the pH and the maximum value for this is a pH of 5. The immersion enthalpies
of the activated carbon in a 3-chloro phenol solution of constant concentration, of 100 mg L−1, are determined for the different pH levels, with results between 37.6 and 21.2 J g−1. Immersion enthalpies of the activated carbon in function of 3-chloro phenol solution concentration are determined to pH
5, of maximum adsorption, with values between 28.3 and 38.4 J g−1, and by means of linearization, the maximum immersion enthalpy is calculated, with a value of 41.67 J g−1. With the results of the immersion enthalpy, maximum quantity adsorbed and the constant KL, establish relations that describe the adsorption process of 3-chloro phenol from aqueous solution on activated carbon. 相似文献
154.
Diego A. Blanco-Martínez Liliana Giraldo Juan C. Moreno-Piraján 《Journal of Thermal Analysis and Calorimetry》2009,96(3):853-857
The immersion enthalpies of modified activated carbons were determined, with commercial CarbochemTM–PS230 (CAG) as the initial activated carbon, which was modified by: chemical treatment with HNO3 7 mol L−1 (CAO) and thermal treatment under flow of H2 (CAR) in function of the adsorbed quantity of monohydroxilated phenols, catechol, resorcinol and hydroquinone at a pH of
7 in aqueous dissolutions in order to characterize the solid–solution interaction and evaluate the influence of the chemical
characteristics of the activated carbon in the phenol adsorption. The results show a variation in the immersion enthalpy in
function of the adsorbed quantity of phenol and the initial dissolution concentration; which shows that the intensity of the
interaction changes in function of the composition of the liquid phase. The immersion enthalpies present the following arrangement:
catechol > resorcinol > hydroquinone, with a −ΔHinm of 35.7; 30.8 and 24.6 Jg−1, respectively, at a pH of 7 for a 100 mg L−1 phenol monohydroxilated solution. 相似文献
155.
Mabel Cuesta Humberto Ramos Quoirin 《NoDEA : Nonlinear Differential Equations and Applications》2009,16(4):469-491
Let Δ
p
denote the p-Laplacian operator and Ω be a bounded domain in . We consider the eigenvalue problem
for a potential V and a weight function m that may change sign and be unbounded. Therefore the functional to be minimized is indefinite and may be unbounded from below.
The main feature here is the introduction of a value α(V, m) that guarantees the boundedness of the energy over the weighted sphere . We show that the above equation has a principal eigenvalue if and only if either m ≥ 0 and α(V, m) > 0 or m changes sign and α(V, m) ≥ 0. The existence of further eigenvalues is also treated here, mainly a second eigenvalue (to the right) and their dependence
with respect to V and m.
相似文献
156.
Yaghmur A de Campo L Salentinig S Sagalowicz L Leser ME Glatter O 《Langmuir : the ACS journal of surfaces and colloids》2006,22(2):517-521
In our recent work, we reported on the effect of varying temperature and solubilizing tetradecane (TC) on the structural transitions observed in dispersed particles based on the monolinolein (MLO)-water-TC system. At a given temperature, the addition of TC induces a transition of the internal structure from the bicontinuous cubic phase, Pn3m, to the reversed hexagonal, H2, and to the isotropic liquid phase (water-in-oil (W/O) microemulsions). Our present study focuses on the discovery of a Fd3m phase (reversed discontinuous micellar cubic), which is formed in the MLO-water-TC system at a specific TC/MLO weight ratio. It is situated between the H2 and the isotropic liquid phase (W/O microemulsion). Remarkably, it is not found in the absence of TC by increasing the temperature. The Fd3m structure was investigated in detail by means of small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). The present work proves that the structural transformation in the dispersed particles from H2 (hexosomes) to the W/O microemulsion system (emulsified microemulsion (EME)) is indirect and it occurs gradually via an emulsified intermediate phase. Specifically, in addition to the nanostructured aqueous dispersions described above, we present new TC-loaded aqueous dispersions with a confined intermediate phase, which is a discontinuous micellar cubic phase of the symmetry Fd3m. We denoted this type of emulsified particles as "micellar cubosomes". 相似文献
157.
Ma. Beatriz García Liliana R. Orelli Isabel A. Perillo 《Journal of heterocyclic chemistry》2006,43(6):1703-1707
The reactivity of some representative 5‐arylpyrimidoquinoxaline 6‐oxides 1 was investigated. Reduction with sodium borohydride afforded the corresponding N‐deoxy compounds 2. Reaction with methyl iodide in excess led selectively to N‐methyl derivatives 3 . The preference for N vs. O‐alkylation is analyzed considering electronic effects. Alkaline hydrolysis of compounds 1 involved initial nucleophilic attack to the amidine carbon (C4a), and subsequent regioselective ring opening leading to 4‐(3‐aminopropyl)‐2‐aryl‐quinoxaline‐3‐one 1‐oxides 4. The regioselectivity observed in alkaline hydrolysis is also discussed. 相似文献
158.
A pressurised liquid extraction (PLE) method for the analysis of ochratoxin A (OTA) in bread samples is given. Parameters such as solvent, temperature, pressure and time were investigated thoroughly. The optimized PLE conditions were: methanol as extraction solvent, 80 degrees C, 2000 psi and a 5-min cycle. OTA was determined by liquid chromatography coupled with fluorescence detection and confirmed by methyl ester derivatization. Under these conditions OTA recovery is 92.3% with a RSD of 5%. Limits of detection and quantification were 0.02 and 0.06 microg/kg, respectively. The proposed method was applied to 20 bread samples, finding two positive samples with OTA levels below the maximum permitted levels by the European Union. 相似文献
159.
Dobrzańska L Lloyd GO Raubenheimer HG Barbour LJ 《Journal of the American Chemical Society》2006,128(3):698-699
We describe the structure and permeability of a crystalline material that appears to be nonporous in a conventional sense. The material is initially formed as a solvate, and removal of the solvent molecules under relatively mild conditions proceeds via a single-crystal to single-crystal transformation, leaving the host structure intact. Although discrete unoccupied voids of 108 A3 are present in the structure, it is not possible to map open channels that represent an intuitive pathway for guest diffusion. Despite the apparent absence of pores, the material is permeable to a variety of gases including H2, O2, N2, CO, CH4, CO2, and I2. These findings show that porosity in crystalline systems cannot always be rationalized by considering the static structures and that as-yet unknown dynamic and cooperative mechanisms prevail by which porosity can be induced. 相似文献
160.