全文获取类型
收费全文 | 1074篇 |
免费 | 43篇 |
国内免费 | 2篇 |
专业分类
化学 | 838篇 |
晶体学 | 4篇 |
力学 | 18篇 |
数学 | 125篇 |
物理学 | 134篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 59篇 |
2021年 | 53篇 |
2020年 | 36篇 |
2019年 | 38篇 |
2018年 | 28篇 |
2017年 | 18篇 |
2016年 | 48篇 |
2015年 | 67篇 |
2014年 | 52篇 |
2013年 | 77篇 |
2012年 | 77篇 |
2011年 | 75篇 |
2010年 | 58篇 |
2009年 | 64篇 |
2008年 | 68篇 |
2007年 | 65篇 |
2006年 | 54篇 |
2005年 | 38篇 |
2004年 | 30篇 |
2003年 | 19篇 |
2002年 | 17篇 |
2001年 | 8篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1965年 | 1篇 |
1963年 | 2篇 |
排序方式: 共有1119条查询结果,搜索用时 15 毫秒
161.
A new on-line Gd preconcentration and determination system associated to flow injection (FI) method was developed. 2,2′-(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo) bisbenzenearsonic acid (Arsenazo III) was used as a complexing agent at pH 2.5. A reactor containing the polyamide membrane was used for the retention of the Gd complex by chemofiltration. The complex was then removed from the reactor with buffer solution pH 9. The variables affecting the combined on-line preconcentration-absorptiometric method have been evaluated and optimised. The coupling of the on-line preconcentration and spectrophotometric flow through detection led to a detection limit of 15 μg l−1 for a preconcentration time of 5 min at 2 ml/min. The method was successfully applied to biological samples. 相似文献
162.
Adriana Vlez Feij Maria Alice Parente Mara Behlau Srgio Haussen Maria Cecília De Veccino Beatriz Castellar de Faria Martignago 《Journal of voice》2004,18(3):341-347
The objective is to investigate the presence of dysphonic symptoms in multiple sclerosis (MS) patients and to compare quantitative acoustic parameters in multiple sclerosis patients and normal individuals. The method of study was an 8-month controlled cross-sectional that was carried out with 106 individuals (30 MS, 76 controls). Both groups included males and females from 20 to 55 years. Exclusion criteria were prior vocal disorder, laryngeal microsurgery, recent endotracheal intubation, tumors, laryngeal, lung or mediastinal metastases, respiratory disease, and other associated neurological diagnoses. For dysphonic symptoms (qualitative variables), associations were assessed using Mantel-Haenszel's chi2 test, with Yates correction or the Fisher exact test when necessary. Statistical significance was set at p< or =0.05. Dysphonia was observed in 70% of MS individuals versus 33% of controls (p=0.01). Association was found between MS and dysphonia (OR: 2.2, CI 95%: 1.13-4.25). Fundamental frequency was higher among MS patients (p=0.01). Fundamental frequency deviation was significantly higher in MS women (but not men) than controls (p=0.00). Jitter was higher in MS men than in all other groups (p=0.00). Results suggest that evaluation and treatment of MS patients should be revised, evaluating voice alterations in relation to other signs. MS seems to intensify gender effect on fundamental frequency deviation, noise, and jitter, with MS women presenting fewer voice variations than men. 相似文献
163.
Kensuke Naka Adriana Gelover‐Santiago Takashi Uemura Yoshiki Chujo 《Journal of polymer science. Part A, Polymer chemistry》2003,41(5):708-715
Dithiafulvene oligomers ( 3 ) were prepared by cycloaddition polymerization of aldothioketenes with their alkynethiol tautomers derived from 1,4‐diethynylbenzene ( 2 ) with the addition of 1‐ethynyl‐4‐methylbenzene ( 1 ) as a monofunctionalized compound. Different feed ratios of 2 / 1 were used to control the molecular weights of 3 . The structures of 3 were confirmed by IR and 1H NMR spectroscopies in comparison with those of 2‐(4‐tolylidene)‐4‐tolyl‐1,3‐dithiol ( 4 ) as a model compound, which was obtained by the treatment of lithium 2‐tolylethynethiolate with water in Et2O. The number‐average degree of polymerization (DP) and the number‐average molecular weight were measured by gel permeation chromatographic and 1H NMR analysis. DP increased with an increasing feed ratio of 2 / 1 . The ultraviolet–visible spectra of 3 in diluted acetonitrile showed that the absorption maxima of 3 increased with an increasing DP of 3 . These redshifts are ascribed to an effective expansion of the π‐conjugation system in 3 . The oligomers exhibited a maximum conjugation length of seven repeating units. The redox properties of 3 were examined by cyclic voltammetry. The oxidation half‐peak potentials (Ep/2) of 3 were slightly cathodically shifted with increasing DP. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 708–715, 2003 相似文献
164.
We study the regularity of the stochastic representation of the solution of a class of initial–boundary value problems related to a regime-switching diffusion. This representation is related to the value function of a finite-horizon optimal stopping problem such as the price of an American-style option in finance. We show continuity and smoothness of the value function using coupling and time-change techniques. As an application, we find the minimal payoff scenario for the holder of an American-style option in the presence of regime-switching uncertainty under the assumption that the transition rates are known to lie within level-dependent compact sets. 相似文献
165.
Adriana F. Gabor 《Operations Research Letters》2010,38(6):582-584
We prove the NP-hardness of a consistency checking problem that arises in certain elimination strategies for solving Sudoku-type problems. 相似文献
166.
For several honeys from different countries, we study the dependence of the Logarithmic Shift Factor (LSF) with temperature that obeys the Williams–Landel–Ferry equation. We find that the LSF may be expressed in terms of a linear equation. On the other hand, the viscosities of different honeys present a corresponding state behavior through a master plot in terms of an adimensional temperature. This kind of behavior has been reported for other glass formers. 相似文献
167.
Adriana Balan 《代数通讯》2013,41(4):1491-1525
In this article, we consider categories of all semimodules over semirings which are p-Schreier varieties, i.e., varieties whose projective algebras are all free. Among other results, we show that over a division semiring R all semimodules are projective iff R is a division ring, prove that categories of all semimodules over proper additively π-regular semirings are not p-Schreier varieties (in particular, this result solves Problem 1 of Katsov [8]), as well as prove that categories of all semimodules over cancellative division semirings are, in contrast, p-Schreier varieties. 相似文献
168.
On the hydroperoxyl radical scavenging activity of two Edaravone derivatives: mechanism and kinetics
The free radical scavenging activity of two Edaravone (EDA) derivatives (C1 and D1) has been studied in aqueous and lipid solutions using the Density Functional Theory. Different mechanisms of reaction have been considered, as well as the acid/base equilibrium in aqueous solution. The studied compounds were found to be rather poor ?OOH scavengers in non‐polar environments, but better than the parent molecule, EDA, which suggests that the peroxyl scavenging activity of this kind of compounds in lipid media could be increased via structural modifications. In aqueous solution, at physiological pH, D1 is predicted to be a better peroxyl scavenger than C1, and slightly better than its parent molecule, EDA. Their excellent activity, under such conditions, is attributed to the electron transfer from their anionic forms. D1 was found to be among the best peroxyl scavengers known so far, with a rate constant for its reaction with ?OOH near the diffusion limit regime (2.3 × 108 M?1s?1). Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
169.
Let ${\mathcal P}$ be a partial order and ${\mathcal A}$ an arboreal extension of it (i.e. the Hasse diagram of ${\mathcal A}$ is a rooted tree with a unique minimal element). A jump of ${\mathcal A}$ is a relation contained in the Hasse diagram of ${\mathcal A}$ , but not in the order ${\mathcal P}$ . The arboreal jump number of ${\mathcal A}$ is the number of jumps contained in it. We study the problem of finding the arboreal extension of ${\mathcal P}$ having minimum arboreal jump number—a problem related to the well-known (linear) jump number problem. We describe several results for this problem, including NP-completeness, polynomial time solvable cases and bounds. We also discuss the concept of a minimal arboreal extension, namely an arboreal extension whose removal of one jump makes it no longer arboreal. 相似文献
170.
Trojanowska A Lewicka-Szczebak D Jedrysek MO Kurasiewicz M Wassenaar LI Izydorczyk K 《Isotopes in environmental and health studies》2008,44(2):163-175
The stable isotopic analyses of molecular oxygen dissolved in water (delta18O(DO)) and dissolved inorganic carbon (delta13C(DIC)), supplemented with basic chemical measurements, have been carried out on a diurnal basis to better understand the dynamics of photosynthesis and respiration in freshwater systems. Our observations have been carried out in a lowland dam reservoir, the Sulejow Lake (central Poland), during the summer cyanobacterial bloom. All data obtained, isotopic, hydrochemical, and biological, show a high mutual consistency. Namely, the lowest delta18O(DO) values, obtained at 10:00 and 14:00 (16.0 and 15.5 per thousand, respectively), correspond to the highest amount of cyanobacterial cells observed (66 and 63 mg dm(-3), respectively), whereas the minimum delta13C(DIC) (-10.6 per thousand) obtained at 22:00 corresponds to the maximum content of organic matter (110 mg dm(-3)). This evidence suggests that isotopic assays of delta18O(DO) and delta13C(DIC) are a reliable tool for the quantitative study of biochemical processes in freshwater systems. 相似文献