全文获取类型
收费全文 | 1561篇 |
免费 | 35篇 |
国内免费 | 7篇 |
专业分类
化学 | 1046篇 |
晶体学 | 4篇 |
力学 | 31篇 |
数学 | 223篇 |
物理学 | 299篇 |
出版年
2021年 | 8篇 |
2020年 | 11篇 |
2019年 | 17篇 |
2018年 | 17篇 |
2017年 | 14篇 |
2016年 | 33篇 |
2015年 | 19篇 |
2014年 | 18篇 |
2013年 | 81篇 |
2012年 | 79篇 |
2011年 | 77篇 |
2010年 | 43篇 |
2009年 | 63篇 |
2008年 | 65篇 |
2007年 | 63篇 |
2006年 | 80篇 |
2005年 | 61篇 |
2004年 | 67篇 |
2003年 | 63篇 |
2002年 | 51篇 |
2001年 | 33篇 |
2000年 | 42篇 |
1999年 | 17篇 |
1998年 | 14篇 |
1997年 | 17篇 |
1996年 | 29篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 26篇 |
1992年 | 22篇 |
1991年 | 20篇 |
1990年 | 18篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 20篇 |
1986年 | 10篇 |
1985年 | 23篇 |
1984年 | 28篇 |
1983年 | 22篇 |
1982年 | 20篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 22篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1976年 | 21篇 |
1975年 | 8篇 |
1974年 | 16篇 |
1973年 | 16篇 |
1968年 | 9篇 |
排序方式: 共有1603条查询结果,搜索用时 31 毫秒
131.
In this paper, the surface acoustic wave velocity results acquired from the inspection of specially manufactured and characterised alumina ceramic materials are presented. Ultrasonic velocity data of alumina-based ceramics in the range 60-100% theoretical density was generated utilising non-contacting laser-ultrasonic measurements based on laser generation and detection of surface acoustic waves with the objective of creating a routine technique for industrial advanced alumina inspection. With linear fitting the surface acoustic wave velocity data serves as a calibration graph for using laser ultrasonics for routine monitoring of alumina. A second laser ultrasonic technique based on the laser generation and foil transducer detection of surface acoustic waves was used to validate the surface acoustic wave velocities measured by the laser generation/detection technique. 相似文献
132.
Phospholipid lung surfactant and nanoparticle surface toxicity: Lessons from diesel soots and silicate dusts 总被引:3,自引:0,他引:3
William E. Wallace Michael J. Keane David K. Murray William P. Chisholm Andrew D. Maynard Tong-man Ong 《Journal of nanoparticle research》2007,9(1):23-38
Because of their small size, the specific surface areas of nanoparticulate materials (NP), described as particles having at
least one dimension smaller than 100 nm, can be large compared with micrometer-sized respirable particles. This high specific
surface area or nanostructural surface properties may affect NP toxicity in comparison with micrometer-sized respirable particles
of the same overall composition. Respirable particles depositing on the deep lung surfaces of the respiratory bronchioles
or alveoli will contact pulmonary surfactants in the surface hypophase. Diesel exhaust ultrafine particles and respirable
silicate micrometer-sized insoluble particles can adsorb components of that surfactant onto the particle surfaces, conditioning
the particles surfaces and affecting their in vitro expression of cytotoxicity or genotoxicity. Those effects can be particle surface composition-specific. Effects of particle
surface conditioning by a primary component of phospholipid pulmonary surfactant, diacyl phosphatidyl choline, are reviewed
for in vitro expression of genotoxicity by diesel exhaust particles and of cytotoxicity by respirable quartz and aluminosilicate kaolin
clay particles. Those effects suggest methods and cautions for assaying and interpreting NP properties and biological activities. 相似文献
133.
The location set covering problem continues to be an important and challenging spatial optimization problem. The range of practical planning applications underscores its importance, spanning fire station siting, warning siren positioning, security monitoring and nature reserve design, to name but a few. It is challenging on a number of fronts. First, it can be difficult to solve for medium to large size problem instances, which are often encountered in combination with geographic information systems (GIS) based analysis. Second, the need to cover a region efficiently often brings about complications associated with the abstraction of geographic space. Representation as points can lead to significant gaps in actual coverage, whereas representation as polygons can result in a substantial overestimate of facilities needed. Computational complexity along with spatial abstraction sensitivity combine to make advances in solving this problem much needed. To this end, a solution framework for ensuring complete coverage of a region with a minimum number of facilities is proposed that eliminates potential error. Applications to emergency warning siren and fire station siting are presented to demonstrate the effectiveness of the developed approach. The approach can be applied to convex, non-convex and non-contiguous regions and is unaffected by arbitrary initial spatial representations of space. 相似文献
134.
Jane S. Murray Pat Lane Anian Nieder Thomas M. Klapötke Peter Politzer 《Theoretical chemistry accounts》2010,127(4):345-354
Si-pentaerythritol tetranitrate (PETN), Si[CH2ONO2]4, is a silicon analog of the widely used explosive PETN, C[CH2ONO2]4. Si-PETN is extremely sensitive to impact, much more so than PETN. This was attributed by Liu et al. to Si-PETN having a
much lower activation barrier to decomposition, via a facile rearrangement that is not as readily available to PETN, and which
releases considerable energy that can promote further steps. We have investigated computationally why the barrier to the rearrangement
is so much lower for Si-PETN than for PETN, using 5, (H3C)3C–CH2ONO2, and 6, (H3C)3Si–CH2ONO2, as models for PETN and Si-PETN. Reaction force analysis shows that most of the difference between the rearrangement barriers
for 5 and 6 comes about in the initial (reactant) stages of the processes, in which 6 benefits from a 1,3 electrostatic interaction involving a positive σ–hole on the silicon and the negative linking oxygen.
The analogous interaction is weaker in 5, since the central carbon does not have positive σ–holes; furthermore, this carbon is less able than silicon to temporarily
expand its coordination sphere. A similar explanation involving a positive silicon σ–hole and a linking oxygen is proposed
for Si-PETN. The greater exothermicity of the rearrangement of 6 (and also Si-PETN) can be rationalized, following Liu et al., in terms of the formation of the strong Si–O bond. 相似文献
135.
Jeffrey M. Spraggins Julie A. Lloyd Murray V. Johnston Julia Laskin Douglas P. Ridge 《Journal of the American Society for Mass Spectrometry》2009,20(9):1579-1592
The gas-phase fragmentation reactions of singly charged angiotensin II (AngII, DR+VYIHPF) and the ozonolysis products AngII+O (DR+VY*IHPF), AngII+3O (DR+VYIH*PF), and AngII+4O (DR+VY*IH*PF) were studied using SID FT-ICR mass spectrometry, RRKM modeling, and molecular dynamics. Oxidation of Tyr (AngII+O)
leads to a low-energy charge-remote selective fragmentation channel resulting in the b
4
+O fragment ion. Modification of His (AngII+3O and AngII+4O) leads to a series of new selective dissociation channels. For
AngII+3O and AngII+4O, the formation of [MH+3O]
+
−45 and [MH+3O]
+
−71 are driven by charge-remote processes while it is suggested that b
5
and [MH+3O]
+
−88 fragments are a result of charge-directed reactions. Energy-resolved SID experiments and RRKM modeling provide threshold
energies and activation entropies for the lowest energy fragmentation channel for each of the parent ions. Fragmentation of
the ozonolysis products was found to be controlled by entropic effects. Mechanisms are proposed for each of the new dissociation
pathways based on the energies and entropies of activation and parent ion conformations sampled using molecular dynamics. 相似文献
136.
An alternative approach to synthesize pedamide,a key building block of pederin was described.Iodine-induced asymmetric heterocyclization was used as the key step to construct the skeleton,a tetrahydropyran ring with three chiral centers.Brown's asymmetric allylation and Lewis acid-mediated allylation were investigated to introduce chains and chiral alcohols.Sharpless dihydroxylation decorated the side chain.And high optically pure target was obtained by removing the epimers formed in these reactions on c... 相似文献
137.
138.
J. A. Sanchez H. D. Murray W. Minajew K. Swetljakow S. Frolow P. P. Ssasanow W. A. Blinow H. H. Evers N. Strafford D. O. Jones H. R. Lee Erich Müller E. Dachselt L. Desvergnes S. Palkin K. Lehmstedt O. Zumstein K. Heller und Z. Fleischhaus 《Fresenius' Journal of Analytical Chemistry》1931,86(5-6):262-267
Ohne Zusammenfassung 相似文献
139.
S Murray G O'Malley I K Taylor A I Mallet G W Taylor 《Journal of chromatography. A》1989,491(1):15-25
A gas chromatographic-mass spectrometric assay has been developed for the measurement of N tau-methylimidazoleacetic acid in urine and plasma. The method uses the isopropyul ester 3,5-bistrifluoromethylbenzoyl derivative of N tau-methylimidazoleacetic acid and electron capture negative ion chemical ionisation mass spectrometry. The derivative has very good chromatographic properties and a negative ion mass spectrum which contains only a molecular ion at m/z 422. When this ion is specifically monitored, an amount of derivative equivalent to 1 pg of parent compound can be detected. A deuterated analogue of N tau-methylimidazoleacetic acid was synthesised for use as an internal standard and this allowed the development of an assay for N tau-methylimidazoleacetic acid, in urine with a precision of 2.9% and in plasma with a precision of 1.5%. 相似文献
140.