全文获取类型
收费全文 | 333106篇 |
免费 | 2913篇 |
国内免费 | 941篇 |
专业分类
化学 | 173267篇 |
晶体学 | 5013篇 |
力学 | 15142篇 |
综合类 | 10篇 |
数学 | 36697篇 |
物理学 | 106831篇 |
出版年
2021年 | 2485篇 |
2020年 | 2748篇 |
2019年 | 2998篇 |
2018年 | 3923篇 |
2017年 | 3823篇 |
2016年 | 5816篇 |
2015年 | 3616篇 |
2014年 | 5669篇 |
2013年 | 14581篇 |
2012年 | 11317篇 |
2011年 | 13874篇 |
2010年 | 9559篇 |
2009年 | 9470篇 |
2008年 | 12592篇 |
2007年 | 12788篇 |
2006年 | 12157篇 |
2005年 | 11003篇 |
2004年 | 10005篇 |
2003年 | 8843篇 |
2002年 | 8721篇 |
2001年 | 9921篇 |
2000年 | 7642篇 |
1999年 | 5887篇 |
1998年 | 4934篇 |
1997年 | 4895篇 |
1996年 | 4486篇 |
1995年 | 4235篇 |
1994年 | 4054篇 |
1993年 | 3807篇 |
1992年 | 4353篇 |
1991年 | 4445篇 |
1990年 | 4208篇 |
1989年 | 4025篇 |
1988年 | 4007篇 |
1987年 | 3940篇 |
1986年 | 3698篇 |
1985年 | 4874篇 |
1984年 | 5181篇 |
1983年 | 4269篇 |
1982年 | 4585篇 |
1981年 | 4414篇 |
1980年 | 4245篇 |
1979年 | 4470篇 |
1978年 | 4721篇 |
1977年 | 4565篇 |
1976年 | 4664篇 |
1975年 | 4376篇 |
1974年 | 4405篇 |
1973年 | 4524篇 |
1972年 | 2959篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
301.
Three-dimensional laminar forced convection including steady-periodic transition is investigated up to periodic-chaotic transition
in the fully developed region of coolant passages with staggered arrays of pin fins. Comparative examples concern overall
pressure losses and heat transfer characteristics of circular, square and elliptical pins made of nickel and copper. In the
numerical model, transient conjugate heat transfer is assumed and space periodicities in pressure, velocity components and
temperatures are taken into account. In the range of operative conditions investigated, overall friction factors increase
almost linearly with the Reynolds number, while the increase of overall Nusselt numbers with the Reynolds number is characterized
by two slope changes connected with the onset of streamwise vortices, and the shedding of transverse vortices, respectively.
The use of copper, instead of nickel, increases the overall Nusselt number with all shapes, but is particularly beneficial
to the elliptical section. Square pins are characterized by the highest values of friction factors, but are also the best
performers as far as convection enhancing is concerned. The reverse is true for the elliptical pins which are characterized
by the lowest values of friction factors, but are the worst performers as far as convection enhancing is concerned. On the
basis of overall performances, the elliptical pins made of copper are the best choice, at least in the upper range of Reynolds
numbers investigated. 相似文献
302.
Andrew C. Brooks Peter Day John D. Wallis 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(5):o245-o247
The crystal structure of the title compound [systematic name: 2‐(1,3‐dithiolo[4,5‐b][1,4]dithiin‐2‐ylidene)‐6,6‐bis(methoxyethoxymethoxymethyl)‐1,3‐dithiolo[4,5‐b][1,4]dithiepine], C21H30O6S8, a spiro‐substituted BEDT–TTF analogue [BEDT–TTF is bis(ethylenedithio)tetrathiafulvalene], has a strongly bent heterocyclic framework. The seven‐membered ring adopts a pseudo‐chair conformation with notably widened ring bond angles, especially at the methylene C atoms [119.49 (11) and 117.60 (11)°]. The axial side chain adopts an extended conformation, but the equatorial side chain curls back on itself and the O atom nearest the ring system is involved in three short contacts to H atoms (2.45–2.53 Å). The molecules pack in centrosymmetrically related pairs, which are isolated from each other by columns of the polyether side chains. This study emphasizes the ease of distortion of the neutral bis(propylenedithio)tetrathiafulvalene ring structure, and how the need to accommodate side chains can easily override the tendency of these donor systems to form stacks in the crystalline state. 相似文献
303.
Our research group has been interested in synthesis of proton transfer compounds as new supramolecular synthons for the onepot preparation of self-assembled transition metal complex-organo-networks since 2000. We have focused on the proton delivery from acids, which are considered as suitable proton donors, to amines as proton acceptors. The results were production of several proton transfer ion pairs possessing some remaining donor sites applied for coordination to metallic centers in preparation of metal-organic structures. Some of the complexes showed contributions of both cationic and anionic fragments of the starting ion pair, while some others contained only one of these species as ligand. Much of the investigations on reviewed compounds focused on the concept of supramolecular systems, co-crystallization, stereochemically active lone pairs, coordination polyhedron and mainly on various interactions involve including van der Waals, ion pairing, hydrogen bondings, face to face π-π stackings and edge to face C-H...π, C-O...π, N-H...π, S-O...π, Ti...π and Hg-Cl...π interactions. The mentioned interactions are the most commonly used strategies in the extension of supramolecular structures. 相似文献
304.
Taking both the heterogeneous catalytic processes, including the surface formation of particles with excited internal degrees of freedom, and the processes of multicomponent diffusion and heat transfer in the MESOX apparatus fully into account makes it possible to obtain a recombination coefficient and an accommodation coefficient of the oxygen-atoms-on-quartz recombination energy which are in good agreement with the experimental data. The heterogeneous catalysis model constructed can be used effectively for predicting the heat fluxes to the surface of reentry vehicles on their entry into the Earth’s atmosphere. 相似文献
305.
306.
Simulation of evaporation in operation area of an evaporator is considered. We obtained a formalized model of this process and optimum conditions of an engineering system on the basis on the analysis. 相似文献
307.
Most adhesives and binders, including bitumen for asphalt mixture production, are presently produced from petrochemicals after
the refining of crude oil. The fact that crude oil reserves are a finite resource means that in the future, it may become
necessary to produce these materials from alternative and probably renewable sources. Suitable resources of this kind may
include polysaccharides, plant oils and proteins. This paper deals with the synthesis of polymer binders from monomers that
could, in future, be derived from renewable resources. These binders consist of polyethyl acrylate (PEA) of different molecular
weight, polymethyl acrylate (PMA) and polybutyl acrylate (PBA), which were synthesised from ethyl acrylate, methyl acrylate
and butyl acrylate, respectively, by atom transfer radical polymerisation. The rheological properties of these binders were
determined by means of oscillatory testing using a dynamic shear rheometer and combinations of stress/strain, temperature
and frequency sweeps. The results indicate that PEA can be produced to have rheological properties similar to that of ‘soft’
100/150 penetration grade bitumen, PMA with similar rheological properties to that of ‘hard’ 10/20 penetration grade bitumen,
while PBA, due to its highly viscous nature and low dynamic moduli, cannot be used on its own as a binder. The synthetic polymers
were found to be thermo-rheologically simple, and the shift factors, used to produce the dynamic moduli master curves, were
found to fit an Arrhenius function. 相似文献
308.
T. M. Prokop’eva V. A. Mikhailov M. K. Turovskaya E. A. Karpichev N. I. Burakov V. A. Savelova I. V. Kapitanov A. F. Popov 《Russian Journal of Organic Chemistry》2008,44(5):637-646
Bis(dialkylamide)hydrogen dibromobromates were synthesized and their reactivity was investigated in decomposition of diethylphosphonic, diethylphosphoric, and 4-toluenesulfonic acids 4-nitrophenyl esters. The nucleophilic reactivity of a typical α-nucleophile, hypobromite ion, is independent of the source of the active bromine. The cetyltrimethylammonium dibromobromate is a unique reagent for destruction of ecotoxicants. In weakly alkaline media the half-life of 4-nitrophenyl diethylphosphonate did not exceed 6 s at [BrO?] 0.02 mol l?1, and the apparent reaction rate compared to water increased ~40-fold. The main factor governing the micellar effects of surfactants is concentrating the substrate in the micellar pseudophase. 相似文献
309.
E. V. Sennikova A. S. Antsyshkina G. G. Sadikov A. V. Bicherov O. Yu. Korshunov G. S. Borodkin M. S. Korobov V. S. Sergienko N. N. Kharabaev A. D. Garnovskii 《Russian Journal of Coordination Chemistry》2008,34(5):315-321
The chelate and molecular complexes of 2-(2-benzazolyl, perimidyl)-indandiones-1,3 were synthesized for the first time. The structures of the ligands (tautomeric forms) and complex compounds were determined from the X-ray diffraction, 1H, 13C NMR, and IR data and the quantum-chemical calculations (RHF SCF in 6-31 G** basis). 相似文献
310.
F. Silva X. Bonnin J. Achard O. Brinza A. Michau A. Gicquel 《Journal of Crystal Growth》2008,310(1):187-203
Plasma-assisted CVD homoepitaxial diamond growth is a process that must satisfy many stringent requirements to meet industrial applications, particularly in high-power electronics. Purity control and crystalline quality of the obtained samples are of paramount importance and their optimization is a subject of active research. In the process of such studies, we have obtained high purity CVD diamond monocrystals with unusual morphologies, namely with apparent {1 1 3} stable faces. This phenomenon has led us to examine the process of CVD diamond growth and build up a 3D geometrical model, presented here, describing the film growth as a function of time. The model has been able to successfully describe the morphology of our obtained crystals and can be used as a predictive tool to predetermine the shape and size of a diamond crystal grown in a given process configuration. This renders accessible control of desirable properties such as largest usable diamond surface area and/or film thickness, before the cutting and polishing manufacture steps take place. The two latter steps are more sensitive to the geometry of the growth sectors, which will be addressed in a companion paper.Our model, applicable to the growth of any cubic lattice material, establishes a complete mapping of the final morphology state of growing diamond, as a function of the growth rates of the crystalline planes considered, namely {1 0 0}, {1 1 1}, {1 1 0}, and {1 1 3} planes, all of which have been observed experimentally in diamond films. The model makes no claim as to the stability of the obtained faces, such as the occurrence of non-epitaxial crystallites or twinning. It is also possible to deduce transient behavior of the crystal morphology as growth time is increased. The model conclusions are presented in the form of a series of diagrams, which trace the existence (and dominance) boundaries of each face type, in presence of the others, and where each boundary crossing represent a topology change in terms of number of faces, edges and vertices. We validate the model by matching it against crystals published in the literature and illustrate its predictive value by suggesting ways to increase usable surface area of the diamond film. 相似文献