全文获取类型
收费全文 | 1417篇 |
免费 | 122篇 |
国内免费 | 103篇 |
专业分类
化学 | 773篇 |
晶体学 | 10篇 |
力学 | 16篇 |
综合类 | 14篇 |
数学 | 579篇 |
物理学 | 250篇 |
出版年
2023年 | 7篇 |
2022年 | 43篇 |
2021年 | 33篇 |
2020年 | 52篇 |
2019年 | 35篇 |
2018年 | 39篇 |
2017年 | 42篇 |
2016年 | 49篇 |
2015年 | 49篇 |
2014年 | 65篇 |
2013年 | 139篇 |
2012年 | 68篇 |
2011年 | 57篇 |
2010年 | 61篇 |
2009年 | 88篇 |
2008年 | 76篇 |
2007年 | 90篇 |
2006年 | 68篇 |
2005年 | 54篇 |
2004年 | 70篇 |
2003年 | 67篇 |
2002年 | 51篇 |
2001年 | 43篇 |
2000年 | 28篇 |
1999年 | 42篇 |
1998年 | 25篇 |
1997年 | 34篇 |
1996年 | 24篇 |
1995年 | 20篇 |
1994年 | 16篇 |
1993年 | 11篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1990年 | 2篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有1642条查询结果,搜索用时 15 毫秒
911.
912.
913.
Magnus Fröhling Frank Schwaderer Hauke Bartusch Otto Rentz 《European Journal of Operational Research》2010
By-products accrue in all stages of industrial production networks. Legal requirements, shortening of primary resources and their increasing prices make their recycling more and more important. For the re-integration into the economic cycle the scope of common supply chain management is enlarged and so-called closed-loop supply chains with adapted and new planning tasks are developed. In process industries this requires a detailed modelling of the recycling processes. This is of special relevance for operational planning tasks in which an optimal usage of a given production system is envisaged. This contribution presents an integrated planning approach for a real-world case study from the zinc industry to achieve such an adequate process modelling. We consider the planning problem of a company that operates four metallurgical recycling plants and has to allocate residues from different sources to these recycling sites. The allocation determines the raw material mix used in the plants. This blending has an effect on the transportation costs and the costs and revenues of the individual technical processes in the recycling plants. Therefore in this problem transportation and recycling planning for multiple sites have to be regarded in an integrated way. The necessary detailed process modelling is achieved by the use of a flowsheet process simulation system to model each recycling plant individually. The models are used to derive linear input–output functions by multiple linear regression analyses. These are used in an integrated planning model to calculate the decision-relevant input and output flows that are dependent upon the allocation of the residues to the recycling sites. The model is embedded in a decision support system for the operational use. An example application and sensitivity analyses demonstrate and validate the approach and its potentials. The approach is transferable to other recycling processes as well as to other processes in process industries. 相似文献
914.
There are various importance sampling schemes to estimate rare event probabilities in Markovian systems such as Markovian reliability models and Jackson networks. In this work, we present a general state-dependent importance sampling method which partitions the state space and applies the cross-entropy method to each partition. We investigate two versions of our algorithm and apply them to several examples of reliability and queueing models. In all these examples we compare our method with other importance sampling schemes. The performance of the importance sampling schemes is measured by the relative error of the estimator and by the efficiency of the algorithm. The results from experiments show considerable improvements both in running time of the algorithm and the variance of the estimator. 相似文献
915.
利用随机环境中马氏链的Hofp极大遍历引理和Brunel极大遍历引理,给出了随机环境中马氏链的Chacon-Ornstein定理和Chacon认证定理. 相似文献
916.
The development of sensors that recognize Lewis oligosaccharides can help the diagnosis and early detection of cancer. Herein, we reported the design and synthesis of a series of anthracene-based bisboronic acids (9a-9e) with different N-substituents near the boronic acid unit. Among them, compound 9a could recognize sialyl Lewis X(sLex) with selectivity over other Lewis sugars, and could significantly stain sLex-expressing HEPG2 cells with selectivity over the range of 0.1-10 μmol/L. Compound 9a possibly has two properly positioned boronic acids caused by the steric hindrance by the near N-benzyl substituent group, which empower its sLex selectivity and higher binding affinity. 相似文献
917.
Geng‐Min Lin Ming‐Chuan Cheng Shun‐Jie Liou Ho‐Sung Tsao Chih‐Hsun Lin Yi R. Lin Gene‐Hsiang Lee Chun‐hsien Chen Shie‐Ming Peng 《中国化学会会志》2019,66(9):1157-1164
Metal string complexes contain a linear metal‐atom chain in which the metal centers are coordinated by four equatorial and two axial ligands. With a variety of transition‐metal elements and ligands, the structural framework drives the flourishing of molecular design and properties. The one‐dimensional configuration makes the compounds suitable for the studies of quantum transport across molecular junctions. In this study, we report the conductance measurements and transmission spectra of three trinickel metal strings, [Ni3(dpa)4(NCS)2] ( 1 ), [Ni3(dzp)4(NCS)2] ( 2 ), and [Ni3(dpa)4(CN)2] ( 3 ) (Hdpa = dipyridylamine, Hdzp, diazaphenoxazine) in which 1 is a prototypical compound, dzp of 2 represents an equatorial ligand more rigid than dpa of 1 , and ─CN is an axial ligand with a ligand‐field effect stronger than ─NCS of 1 . Measurement results of molecular junctions for 1 , 2 , and 3 are 2.69, 3.24, and 17.4 MΩ, respectively. The highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) gaps calculated by density functional theory in the gas phase for 1 , 2 , and 3 are about 2.65, 2.34, and 3.85 eV, respectively. Zero‐bias transmission spectra of 1 – 3 show that transmission peaks lie just above EFermi (the Fermi energy of the gold electrode), suggesting LUMO‐dominant transport pathways. The transmission peaks at EFermi are associated with LUMO+2 found in the gas phase. LUMOs in the free space are located at nearly 1 eV below EFermi. The shift of molecular orbitals from their isolated form and the alignment of LUMO+2 with the electrode Fermi level manifest the importance and significant of the electrodes' self‐energy on electron transport. 相似文献
918.
Guilherme M.A. Almeida Francisco A.B.F. de Moura Marcelo L. Lyra 《Physics letters. A》2018,382(20):1335-1340
We study quantum-state transfer in XX spin-1/2 chains where both communicating spins are weakly coupled to a channel featuring disordered on-site magnetic fields. Fluctuations are modeled by long-range correlated sequences with self-similar profile obeying a power-law spectrum. We show that the channel is able to perform almost perfect quantum-state transmissions even in the presence of significant amounts of disorder provided the degree of those correlations is strong enough, with the cost of having long transfer times and unavoidable timing errors. Still, we show that the lack of mirror symmetry in the channel does not affect much the likelihood of having high-quality outcomes. Our results suggest that coexistence between localized and delocalized states can diminish effects of static perturbations in solid-state devices for quantum communication. 相似文献
919.
We obtained polyvinyl alcohol films dyed with Brilliant Yellow (BY) and have determined their spectral and polarizing characteristics
in the short-wavelength region of the visible spectrum (400–430 nm). We have shown that BY imparts a polarizing power of ≥99%
to the film. We measured the orientational parameters characterizing the degree of ordering of the polymer chains and the
dye molecules for different BY content in the film. An increase in BY concentration above 0.3 wt.% is accompanied by a decrease
in its orientational parameter, which is connected with aggregation of dye molecules and a change in the distribution of the
molecular axes relative to the film orientation axis.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 466–472, May–June, 2009. 相似文献
920.