全文获取类型
收费全文 | 1558篇 |
免费 | 37篇 |
国内免费 | 46篇 |
专业分类
化学 | 196篇 |
晶体学 | 1篇 |
力学 | 17篇 |
综合类 | 7篇 |
数学 | 516篇 |
物理学 | 245篇 |
综合类 | 659篇 |
出版年
2023年 | 3篇 |
2022年 | 13篇 |
2021年 | 16篇 |
2020年 | 9篇 |
2019年 | 15篇 |
2018年 | 21篇 |
2017年 | 15篇 |
2016年 | 24篇 |
2015年 | 21篇 |
2014年 | 51篇 |
2013年 | 99篇 |
2012年 | 43篇 |
2011年 | 86篇 |
2010年 | 87篇 |
2009年 | 90篇 |
2008年 | 93篇 |
2007年 | 130篇 |
2006年 | 112篇 |
2005年 | 103篇 |
2004年 | 72篇 |
2003年 | 63篇 |
2002年 | 52篇 |
2001年 | 47篇 |
2000年 | 38篇 |
1999年 | 44篇 |
1998年 | 30篇 |
1997年 | 34篇 |
1996年 | 21篇 |
1995年 | 16篇 |
1994年 | 24篇 |
1993年 | 17篇 |
1992年 | 14篇 |
1991年 | 12篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 2篇 |
1982年 | 17篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有1641条查询结果,搜索用时 15 毫秒
91.
92.
A 'chaos expansion' of the intersection local time functional of two independent Brownian motions in R
d
is given. The expansion is in terms of normal products of white noise (corresponding to multiple Wiener integrals). As a consequence of the local structure of the normal products, the kernel functions in the expansion are explicitly given and exhibit clearly the dimension dependent singularities of the local time functional. Their L
p
-properties are discussed. An important tool for deriving the chaos expansion is a computation of the 'S-transform' of the corresponding regularized intersection local times and a control about their singular limit. 相似文献
93.
祝新艳 《科技情报开发与经济》2007,17(13):135-137
介绍了信息素养的概念及内容,分析了大学生信息素养的现状,指出了加强大学生信息素养教育的必要性,阐述了高校图书馆在大学生信息素养教育中应当采取的策略。 相似文献
94.
Smitha Cheruku Andrei P?un Francisco J. Romero-Campero Mario J. Pérez-Jiménez Oscar H. Ibarra 《自然科学进展》2007,17(4):424-431
In contrast to differential equations, P systems are an unconventional model of computation which takes into consideration the discrete character of the quantity of components and the inherent randomness that exists in biological phenomena. The key feature of P systems is their compartmentalised structure which represents the heterogeneity of the structural organisation of the cells, and where one can take into account the role played by membranes in the functioning of the system, for example signalling at the cell surface, selective uptake of substances from the media, diffusion across different compartments, etc. We show here that P systems can be a reliable tool for Systems Biology and could even outperform in some cases the current simulation techniques based on differential equations. We will also use a strategy based on the well known Gillespie algorithm but running on more than one compartment called Multi-compartmental Gillespie Algorithm. 相似文献
95.
进入20世纪,列宁领导的十月革命就突破了马克思、恩格斯一国无法获胜的推断。但无产阶级在一国获胜与在“主要文明国家”获胜所形成的环境条件大不相同,险恶的国际环境对社会主义国家民族问题产生了极为重大、极为复杂的影响。正因如此,如果套用无产阶级在“主要文明国家”同时胜利为前提条件的构想,去认识和处理一国和少数国家建立起社会主义制度条件下的国内民族问题,认为只要在社会主义条件下民族问题就可迎刃而解,那就要犯极大的错误。 相似文献
96.
The adsorption of NH3 molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH3 molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH3 incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH3 molecule an energy barrier of ∼0.3 eV was calculated, yielding NH2 on the adatom and H on the restatom. When two molecules are adsorbed, the NH3-NH3 interaction yields the weakening of a N-H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH3 molecule at the adatom-restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV). 相似文献
97.
The possibilities of graphic STM image simulation of a clean Si(1 1 1) 7 × 7 surface at atomic level are indicated. The presented procedure takes into account various types of deformation on the surface near the Fermi level in order to classify them and explain their origin. It also gives a clear hint to insert relevant physical phenomena in a suggested analysis. This goal is achieved exploiting the results of DAS (dimmer adatom stacing fault) model by means of standard mathematical programmes. A clean Si(1 1 1) 7 × 7 surface is considered as the representative example, but similar evaluation is possible for another non-metal and metal surfaces. 相似文献
98.
This paper presents a model of lead-time policies for a production system, such as an integrated steel mill, in which the bottleneck process requires a minimum batch size. An accurate understanding of internal lead-time quotations is necessary for making good customer delivery-date promises, which must take into account processing time, queueing time and time for arrival of the requisite volume of orders to complete the minimum batch size requirement. The problem is modeled as a stochastic dynamic program with a large state space. A computational study demonstrates that lead time for an arriving order should generally be a decreasing function of the amount of that product already on order (and waiting for minimum batch size to accumulate), which leads to a very fast and accurate heuristic. The computational study also provides insights into the relationship between lead-time quotation, arrival rate, and the sensitivity of customers to the length of delivery promises. 相似文献
99.
R. Mével S. Javoy F. Lafosse N. Chaumeix G. Dupré C.-E. Paillard 《Proceedings of the Combustion Institute》2009,32(1):359-366
Hydrogen-nitrous oxide mixtures have been studied for decades. In addition to their fundamental interest, they might play an important role in semi-conductor industry safety. Indeed, in silane-nitrous oxide mixtures, widely used in this industry, the silane molecule readily decomposes into molecular hydrogen which can react violently with nitrous oxide. Despite numerous shock tube studies on H2-N2O delay times, the pressure effect has never been addressed. The present work aims at studying this effect and at developing a detailed kinetic mechanism able to accurately reproduce experimental delay time data. Delay times of H2-N2O-Ar mixtures have been measured behind reflected shock waves in the 1300-2000 K temperature range and at a pressure around 300 kPa. Mixtures equivalence ratios ranged between 0.5 and 2, and the dilution was 98 and 99 mol% Ar. The present results and those from a previous study carried out in our institute show, first that, in the studied conditions, the equivalence ratio has no influence on delay times, and second, that the pressure increase drastically reduces the delay times. A kinetic model has been constructed from previously published mechanisms and tested against the present data with a mean error of 29%. Moreover, other delay time data from the literature for H2-O2-Ar, NH3-Ar and H2-N2O-Ar mixtures are also correctly reproduced as well as macroscopic parameters such as reduced activation energies. 相似文献
100.
Ethanol organosolv lignin extracted from Miscanthus × giganteus with differing levels of severity (1.75 < CS<2.8) were subjected to comprehensive structural characterization by 13C, 31P NMR, FTIR spectroscopy and gel permeation chromatography. The results were compared to those from milled wood lignin from the same feedstock. The results showed that an increase in the severity of the treatment enhanced the dehydration reactions on the side chain and the condensation of lignin, increased the concentration of phenol groups and decreased the molecular mass of lignin fragments. It appeared that for the experimental conditions generally employed the cleavage of α-aryl ether bonds is primarily reaction responsible for lignin depolymerization under the organosolv conditions examined. 相似文献