全文获取类型
收费全文 | 1583篇 |
免费 | 30篇 |
国内免费 | 17篇 |
专业分类
化学 | 1061篇 |
晶体学 | 11篇 |
力学 | 23篇 |
数学 | 318篇 |
物理学 | 217篇 |
出版年
2023年 | 5篇 |
2022年 | 27篇 |
2021年 | 37篇 |
2020年 | 21篇 |
2019年 | 29篇 |
2018年 | 29篇 |
2017年 | 23篇 |
2016年 | 58篇 |
2015年 | 33篇 |
2014年 | 50篇 |
2013年 | 104篇 |
2012年 | 92篇 |
2011年 | 102篇 |
2010年 | 72篇 |
2009年 | 90篇 |
2008年 | 94篇 |
2007年 | 94篇 |
2006年 | 96篇 |
2005年 | 72篇 |
2004年 | 74篇 |
2003年 | 41篇 |
2002年 | 55篇 |
2001年 | 20篇 |
2000年 | 28篇 |
1999年 | 16篇 |
1998年 | 20篇 |
1997年 | 16篇 |
1996年 | 20篇 |
1995年 | 18篇 |
1994年 | 8篇 |
1993年 | 15篇 |
1992年 | 13篇 |
1991年 | 7篇 |
1990年 | 16篇 |
1989年 | 10篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1973年 | 6篇 |
1966年 | 2篇 |
排序方式: 共有1630条查询结果,搜索用时 489 毫秒
161.
The self-consistent model of heat and mass transfer during convective drying of capillary porous media describing both the
first and the second periods of drying is presented in Musielak (Wydawnictwo Politechniki Poznańskiej, seria Rozprawy, nr
386 (2004a); Chem. Process Eng. 25, 393–409 (2004b)). The results of simulations of processes in steady conditions are shown (Musielak Wydawnictwo Politechniki
Poznańskiej, seria Rozprawy, nr 386 (2004a); Chem. Process Eng. 25, 393–409 (2004b)). The main aim of the present work is to compare experimental results with those from numerical simulations.
Three convective drying processes have been performed experimentally. The first and the second periods of drying are considered,
during which the humidity of air in the dryer changes due to evaporation. The first process is used to establish drying parameters,
whereafter the two remaining processes are simulated. Good agreement between experimental and simulation results is found,
both qualitative and quantitative. 相似文献
162.
Dorota Prukala Igor Khmelinskii Jacek Koput Mateusz Gierszewski Tomasz Pędziński Marek Sikorski 《Photochemistry and photobiology》2014,90(5):972-988
The photophysical properties of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S0–Si transitions were investigated using the ab initio methods [MP2, CIS(D), EOM‐CCSD] with the correlation‐consistent basis sets. Also the time‐dependent density functional theory (TD‐DFT) has been employed. The lowest singlet excited states of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close‐lying π, π* and n, π* transitions. Experimental steady‐state and time‐resolved spectral studies indicate formation of an isoalloxazinic excited state via excited‐state double‐proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5‐deazaalloxazine molecule. Solvatochromism of both 5‐deazaalloxazine and its 1,3‐dimethyl substituted derivative was analyzed using the Kamlet–Taft scale and four‐parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute–solvent hydrogen‐bonding interactions in their excited state. 相似文献
163.
Sergey N. Tverdomed Jacek KolanowskiEnno Lork Gerd-Volker Röschenthaler 《Tetrahedron》2011,67(21):3887-3903
Herein, we report a new and convenient methodology for the synthesis of ortho-XCF2 arylphosphonates via Diels-Alder reaction of selected 1,3-butadienes with XCF2-≡-P(O)(OEt)2, followed by the aromatization of the cyclic vinylphosphonates obtained using the KMnO4/Al2O3 system. The reactivity of ortho-XCF2 arylphosphonates was then examined to give the respective dichlorides that were converted to the corresponding phosphonic acids, phosphine oxides or a carboxylic acid (upon hydrolysis of the CF2Br group). When ortho-XCF2 arylphosphonates (X=Br) were treated with Zn/CuBr and an electrophile, the dimeric product ArCFCFAr was isolated only. The lithiation of the CF2H group (X=H) allowed however to obtain products of nucleophilic substitution with various electrophiles. 相似文献
164.
Jacek PecynaRichard P. Denicola Bryan RingstrandAleksandra Jankowiak Piotr Kaszynski 《Polyhedron》2011,30(15):2505-2513
The preparation of iodo acid [closo-1-CB9H8-1-COOH-10-I]− (1) is optimized and scaled from 1 to 40 g of B10H14. The improved preparation of the [arachno-6-CB9H13-6-COOH]− (5) uses four times smaller volume and can be run conveniently in up to 40 g scale in a 3-L vessel. The optimized oxidation of 5 to [closo-2-CB9H9-2-COOH]− (4) requires less oxidant, 12 times smaller volume, and significantly shorter reaction time. The overall yields of the iodo acid 1 as the [NMe4]+ salt are typically 8-10% (10-12 g) for 40 g of B10H14. The iodo acid 1 was transformed to amino acid 8, then to dinitrogen acid 10, and finally to sulfonium acid 2[3] in overall yield of about 13%. The search for a more efficient phosphine ligand for the Pd-catalyzed amination process was not fruitful. Three routes to the sulfonium acid 2[n] were investigated, and the best yield of about 47% was obtained for Cs2CO3-assisted cycloalkylation. Liquid crystalline ester of acid 2[3] and 4-butoxyphenol was prepared and investigated. 相似文献
165.
166.
Asymmetric aldol-Tishchenko reaction catalyzed by Yb-complexes with basic amino acid-derived ligands
Simple amino acid-derived esters have been identified as promising chiral sources for the ytterbium-catalyzed aldol-Tishchenko reaction of aromatic aldehydes with aliphatic ketones. The 1,3-anti-diols with three stereogenic centers were isolated in excellent yields, complete anti-diastereocontrol and enantioselectivities of up to 50% ee. 相似文献
167.
We demonstrate the power of high resolution, two dimensional laser induced fluorescence (2D-LIF) spectroscopy for observing rovibronic transitions of polyatomic molecules. The technique involves scanning a tunable laser over absorption features in the electronic spectrum while monitoring a segment, in our case 100 cm(-1) wide, of the dispersed fluorescence spectrum. 2D-LIF images separate features that overlap in the usual laser induced fluorescence spectrum. The technique is illustrated by application to the S(1)-S(0) transition in fluorobenzene. Images of room temperature samples show that overlap of rotational contours by sequence band structure is minimized with 2D-LIF allowing a much larger range of rotational transitions to be observed and high precision rotational constants to be extracted. A significant advantage of 2D-LIF imaging is that the rotational contours separate into their constituent branches and these can be targeted to determine the three rotational constants individually. The rotational constants determined are an order of magnitude more precise than those extracted from the analysis of the rotational contour and we find the previously determined values to be in error by as much as 5% [G. H. Kirby, Mol. Phys. 19, 289 (1970)]. Comparison with earlier ab initio calculations of the S(0) and S(1) geometries [I. Pugliesi, N. M. Tonge, and M. C. R. Cockett, J. Chem. Phys. 129, 104303 (2008)] reveals that the CCSD∕6-311G?? and RI-CC2∕def2-TZVPP levels of theory predict the rotational constants, and hence geometries, with comparable accuracy. Two ground state Fermi resonances were identified by the distinctive patterns that such resonances produce in the images. 2D-LIF imaging is demonstrated to be a sensitive method capable of detecting weak spectral features, particularly those that are otherwise hidden beneath stronger bands. The sensitivity is demonstrated by observation of the three isotopomers of fluorobenzene-d(1) in natural abundance in an image taken for a supersonically cooled sample. The ability to separate some of the (13)C isotopomers in natural abundance is also demonstrated. The equipment required to perform 2D-LIF imaging with sufficient resolution to resolve the rotational features of large polyatomics is available from commercial suppliers. 相似文献
168.
Free quadratic harness is a Markov process from the class of quadratic harnesses, i.e. processes with linear regressions and quadratic conditional variances. The process has recently been constructed for a restricted range of parameters in Bryc et al. (2010) [7] using Askey-Wilson polynomials. Here we provide a self-contained construction of the free quadratic harness for all values of the parameters. 相似文献
169.
An efficient computational method for a stochastic dynamic lot-sizing problem under service-level constraints 总被引:1,自引:0,他引:1
S. Armagan Tarim Mustafa K. Dogˇru Ula? Özen Roberto Rossi 《European Journal of Operational Research》2011,215(3):563-571
We provide an efficient computational approach to solve the mixed integer programming (MIP) model developed by Tarim and Kingsman [8] for solving a stochastic lot-sizing problem with service level constraints under the static-dynamic uncertainty strategy. The effectiveness of the proposed method hinges on three novelties: (i) the proposed relaxation is computationally efficient and provides an optimal solution most of the time, (ii) if the relaxation produces an infeasible solution, then this solution yields a tight lower bound for the optimal cost, and (iii) it can be modified easily to obtain a feasible solution, which yields an upper bound. In case of infeasibility, the relaxation approach is implemented at each node of the search tree in a branch-and-bound procedure to efficiently search for an optimal solution. Extensive numerical tests show that our method dominates the MIP solution approach and can handle real-life size problems in trivial time. 相似文献
170.
Let \(\mathcal Lf(x)=-\Delta f (x)+V(x)f(x)\), V?≥?0, \(V\in L^1_{loc}(\mathbb R^d)\), be a non-negative self-adjoint Schrödinger operator on \(\mathbb R^d\). We say that an L 1-function f is an element of the Hardy space \(H^1_{\mathcal L}\) if the maximal functionbelongs to \(L^1(\mathbb R^d)\). We prove that under certain assumptions on V the space \(H^1_{\mathcal L}\) is also characterized by the Riesz transforms \(R_j=\frac{\partial}{\partial x_j}\mathcal L^{-1\slash 2}\), j?=?1,...,d, associated with \(\mathcal L\). As an example of such a potential V one can take any V?≥?0, \(V\in L^1_{loc}\), in one dimension.
相似文献
$ \mathcal M_{\mathcal L} f(x)=\sup\limits_{t>0}|e^{-t\mathcal L} f(x)| $