全文获取类型
收费全文 | 1380篇 |
免费 | 24篇 |
国内免费 | 16篇 |
专业分类
化学 | 827篇 |
晶体学 | 44篇 |
力学 | 58篇 |
数学 | 128篇 |
物理学 | 363篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 64篇 |
2021年 | 55篇 |
2020年 | 32篇 |
2019年 | 31篇 |
2018年 | 35篇 |
2017年 | 66篇 |
2016年 | 61篇 |
2015年 | 55篇 |
2014年 | 96篇 |
2013年 | 127篇 |
2012年 | 148篇 |
2011年 | 95篇 |
2010年 | 76篇 |
2009年 | 51篇 |
2008年 | 40篇 |
2007年 | 32篇 |
2006年 | 41篇 |
2005年 | 28篇 |
2004年 | 14篇 |
2003年 | 27篇 |
2002年 | 16篇 |
2001年 | 27篇 |
2000年 | 17篇 |
1999年 | 24篇 |
1998年 | 14篇 |
1997年 | 7篇 |
1996年 | 12篇 |
1995年 | 5篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 32篇 |
1986年 | 26篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1976年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有1420条查询结果,搜索用时 312 毫秒
211.
This study attempts to determine the most desirable quantity of De-Link R suitable for recycling the rubber powder generated in large quantities as a waste product of manufacturing rubber balls and artificial eggs. Attempts were also made to assess the extent to which the devulcanization system scissions the crosslinks in the rubber vulcanizate by means of solubility test. Monsanto moving die rheometer (MDR 2000) was used to determine the rheological properties and cure characteristics of the recycled compound (DE-VULC). The tensile properties of the vulcanizates were correlated with the results from MDR 2000. The behaviour pattern of torque maximum, torque difference, cure time and tensile properties indicated that 6 phr is the optimum concentration of De-Link R. The resulting DE-VULC is not fully devulcanized as it dissolves only to a limited extent in a suitable solvent. 相似文献
212.
《Journal of Geometry and Physics》1999,32(2):189-210
We consider the general dimensional (complex) Minkowski spaces and the extended twistor spaces. We show that the fundamental solutions of the complex wave or Laplace equations are explicitly represented by the integrals of some closed forms on the twistor spaces. The closed form is defined from labeled trees explained in graphs theory, and is written, as the cohomology class, by the linear combination of the logrithmic forms on some hyperplane configuration complement in some complex affine space. 相似文献
213.
Enhanced Slime Mould Algorithm for Multilevel Thresholding Image Segmentation Using Entropy Measures
Image segmentation is a fundamental but essential step in image processing because it dramatically influences posterior image analysis. Multilevel thresholding image segmentation is one of the most popular image segmentation techniques, and many researchers have used meta-heuristic optimization algorithms (MAs) to determine the threshold values. However, MAs have some defects; for example, they are prone to stagnate in local optimal and slow convergence speed. This paper proposes an enhanced slime mould algorithm for global optimization and multilevel thresholding image segmentation, namely ESMA. First, the Levy flight method is used to improve the exploration ability of SMA. Second, quasi opposition-based learning is introduced to enhance the exploitation ability and balance the exploration and exploitation. Then, the superiority of the proposed work ESMA is confirmed concerning the 23 benchmark functions. Afterward, the ESMA is applied in multilevel thresholding image segmentation using minimum cross-entropy as the fitness function. We select eight greyscale images as the benchmark images for testing and compare them with the other classical and state-of-the-art algorithms. Meanwhile, the experimental metrics include the average fitness (mean), standard deviation (Std), peak signal to noise ratio (PSNR), structure similarity index (SSIM), feature similarity index (FSIM), and Wilcoxon rank-sum test, which is utilized to evaluate the quality of segmentation. Experimental results demonstrated that ESMA is superior to other algorithms and can provide higher segmentation accuracy. 相似文献
214.
Guan-Yeow Yeap Faridah Osman Keisuke Ito Daisuke Takeuchi Chien-Min Lin Hong Cheu Lin 《Liquid crystals》2017,44(5):809-821
A series of symmetrical S-shaped mesogens based on 4,4′-bis-(6-bromohexyloxy)biphenyl as a central unit containing two 2-{6-[4-(4-substitutedphenylazo)phenoxy]hexyloxy}phenol as side-chain groups has been successfully synthesised. The terminal substituent was varied from halogen (X = F, Cl, Br and I) to non-halogen (X = C2H5 and OC2H5). The oligomers with C2H5 and OC2H5 substituents exhibit predominantly the monotropic nematic (N) phase. The OC2H5-containing derivatives possess long-range stability of N phase than its C2H5-containing analogue in which it has small range of N phase stability. As for halogen-containing analogues, oligomer with F exhibits monotropic N phase whilst oligomers with Cl and Br exhibit monotropic N and smectic A (SmA) phases. In addition, homologue with Br shows additional phase which is smectic B (SmB) phase upon further cooling. However, the oligomers in which F, Cl and Br were substituted by I exhibits purely monotropic SmA and SmB phases. X-ray diffraction measurements reveal that the smectic phase is inclined to the monolayer structure. 相似文献
215.
《Journal of organometallic chemistry》1993,456(2):C27-C28
The preparation and molecular structures, as determined by single crystal X-ray diffraction, of the complexes Ag2(O2CCCl3)2 (HO2CCCl3)(IAr), where IAr = iodobenzene and 1,2-diiodobenzene, are reported. In the former the iodobenzene functions as a μ2-bridge. 相似文献
216.
《Superlattices and Microstructures》1994,15(3):345
The engineering of the valence subbands for device applications has concentrated on the energy separation between heavy- and light-hole states. We show that the degree of overlap between the envelope functions of heavy- and light-hole states can affect the in-plane dispersion of the highest hole subband. We consider ways to reduce this overlap by spatially separating the heavy- and light-hole states to different layers, while maximizing their energy separation. Strain-compensated superlattices where opposite strains are introduced in the well and barrier regions offer such possibilities and lead to a significant increase of the optical gain in semiconductor lasers. We consider the InxGa1-xAsyP1-y /Inx'Ga1-x'Asy'P1-y' system grown on an InP substrate where the wells are under biaxial compression while the barriers are under tension. In this type of structures, the electron and heavy-hole states are confined to the compressive layers whereas the light-holes are confined to the tensile layers. We also discuss the possibility of confining light-hole and electron states to wells under tension, of potential benefit for lasers operating in the transverse magnetic (TM) mode. 相似文献
217.
《Tetrahedron letters》1997,38(48):8341-8344
Reaction of methyl 2,3-anhydro-5,6-O-cyclohexylidene-β-D-allofuranoside with 2-methyl-2-propenylmagnesium chloride selectively gave methyl 5,6-O-cyclohexylidene-3-deoxy-3-C-(2-methyl-2-propenyl)-β-D-glucofuranoside, which was converted into the C1∼C10 fragment of carbonolide B by a sequence of reactions involving hydroboration of the prochiral double bond, oxidative cleavage between C5C6, and subsequent stereoselective three-carbon elongation at the C5 position. 相似文献
218.
《European Journal of Operational Research》1997,97(1):213-214
In a recent volume of European Journal of Operational Research a Case Study concerning fund allocation using goal programming was reported. However, the goal program model given was incomplete and the solution provided incorrect. In this short communication we propose a more correct formulation. The correctly formulated problem has multiple solutions. It is demonstrated that at least one solution is better than the solution reported in the original paper. 相似文献
219.
《European Journal of Operational Research》1996,91(1):38-55
A dynamic programming model is developed for the optimization of statistical process control for finite production runs. The case of one-sided control charts for variables is examined first. A dynamic chart allows all three parameters, namely the sampling interval, sample size and control limit location, to change during production, as sample information becomes available and the state of the process is updated using Bayesian methods. The economic performance of the optimal dynamic chart is compared to that of the optimal static control chart through a number of numerical examples. It is concluded that substantial improvements may result from the adoption of dynamic charts. The benefits of direct observability of assignable causes are also estimated. Then, a generalized theoretical formulation is provided for two-sided dynamic control charts. The increased size of the state and decision spaces leads to a discussion of practical implementation issues. An example of computing transition probabilities between states of different stages is given. The major conclusion of this research is that significant cost savings may be realized through the application of dynamic control charts and even greater benefits may be reaped by investing in process understanding and improvements. 相似文献
220.
《Microporous Materials》1995,3(4-5):497-510
23Na Magic-angle spinning (MAS), double rotation (DOR) and two-dimensional nutation nuclear magnetic resonance (NMR) and static 139La NMR spectroscopy were applied to study the location and migration of sodium and lanthanum cations in faujasites. Generally, 23Na MAS NMR spectroscopy of as-exchanged and hydrated zeolites LaNaY was used for the quantitative determination of non-localized Na+ in the large cavities at a 23Na NMR shift of −9 ppm and of sodium cations observed at −13 ppm. The latter originate from Na+ ions located on position SII in the large cavities, on position SI in the hexagonal prisms and on positions SII′ and/or SI′ in the sodalite cages. The 23Na MAS NMR signal at about −13 ppm was found to be caused by two coonents. The component that is characterized by a quadrupolar interaction causing a field-dependent shift and a signal at v1 = 2vrf in the two-dimensional quadrupolar nutation spectra is attributed to Na+ enclosed in the sodalite cages. The 23Na MAS NMR spectra of dehydrated lanthanum-exchanged faujasites are characterized by a low-field Gaussian line of Na+ located on SI positions in the hexagonal prisms and a high-field quadrupole pattern of Na+ located on positions SII and SI′. The migration of lanthanum cations from the large cavities to position SI′ in the sodalite cages was monitored by 139La NMR spectroscopy and verified by a theoretical estimation of the electric field gradient. The lanthanum migration was found to be coupled with a strain of SiOT and AlOT angles observed by 29Si and 27Al MAS NMR high-field shifts, respectively. 相似文献