全文获取类型
收费全文 | 2718篇 |
免费 | 102篇 |
国内免费 | 7篇 |
专业分类
化学 | 2125篇 |
晶体学 | 7篇 |
力学 | 43篇 |
数学 | 384篇 |
物理学 | 268篇 |
出版年
2024年 | 2篇 |
2023年 | 25篇 |
2022年 | 78篇 |
2021年 | 94篇 |
2020年 | 71篇 |
2019年 | 76篇 |
2018年 | 37篇 |
2017年 | 42篇 |
2016年 | 109篇 |
2015年 | 96篇 |
2014年 | 102篇 |
2013年 | 166篇 |
2012年 | 195篇 |
2011年 | 201篇 |
2010年 | 153篇 |
2009年 | 113篇 |
2008年 | 169篇 |
2007年 | 150篇 |
2006年 | 136篇 |
2005年 | 136篇 |
2004年 | 130篇 |
2003年 | 102篇 |
2002年 | 98篇 |
2001年 | 34篇 |
2000年 | 40篇 |
1999年 | 31篇 |
1998年 | 25篇 |
1997年 | 25篇 |
1996年 | 30篇 |
1995年 | 21篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 21篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1970年 | 3篇 |
排序方式: 共有2827条查询结果,搜索用时 15 毫秒
51.
Lavinia Corina Ciungu George Georgescu Claudia Mureşan 《Archive for Mathematical Logic》2013,52(3-4):335-376
States have been introduced on commutative and non-commutative algebras of fuzzy logics as functions defined on these algebras with values in [0,1]. Starting from the observation that in the definition of Bosbach states there intervenes the standard MV-algebra structure of [0,1], in this paper we introduce Bosbach states defined on residuated lattices with values in residuated lattices. We are led to two types of generalized Bosbach states, with distinct behaviours. Properties of generalized states are useful for the development of an algebraic theory of probabilistic models for non-commutative fuzzy logics. 相似文献
52.
In this paper we consider weakly hyperbolic equations of higher orders in arbitrary dimensions with time-dependent coefficients and lower order terms. We prove the Gevrey well-posedness of the Cauchy problem under $C^k$ -regularity of coefficients of the principal part and natural Levi conditions on lower order terms which may be only continuous. In the case of analytic coefficients in the principal part we establish the $C^\infty $ well-posedness. The proofs are based on using the quasi-symmetriser for the corresponding companion system and inductions on the order of equation and on the frequency regions. The main novelty compared to the existing literature is the possibility to include lower order terms to the equation (which have been untreatable until now in these problems) as well as considering any space dimensions. We also give results on the ultradistributional and distributional well-posedness of the problem, and we look at new effects for equations with discontinuous lower order terms. 相似文献
53.
We introduce a new method for solving box-constrained mixed-integer polynomial problems to global optimality. The approach, a specialized branch-and-bound algorithm, is based on the computation of lower bounds provided by the minimization of separable underestimators of the polynomial objective function. The underestimators are the novelty of the approach because the standard approaches in global optimization are based on convex relaxations. Thanks to the fact that only simple bound constraints are present, minimizing the separable underestimator can be done very quickly. The underestimators are computed monomial-wise after the original polynomial has been shifted. We show that such valid underestimators exist and their degree can be bounded when the degree of the polynomial objective function is bounded, too. For the quartic case, all optimal monomial underestimators are determined analytically. We implemented and tested the branch-and-bound algorithm where these underestimators are hardcoded. The comparison against standard global optimization and polynomial optimization solvers clearly shows that our method outperforms the others, the only exception being the binary case where, though, it is still competitive. Moreover, our branch-and-bound approach suffers less in case of dense polynomial objective function, i.e., in case of polynomials having a large number of monomials. This paper is an extended and revised version of the preliminary paper [4]. 相似文献
54.
Nitrogen‐Doped Ordered Mesoporous Carbon Supported Bimetallic PtCo Nanoparticles for Upgrading of Biophenolics 下载免费PDF全文
Dr. Guang‐Hui Wang Dr. Zhengwen Cao Dr. Dong Gu Norbert Pfänder Ann‐Christin Swertz Bernd Spliethoff Hans‐Josef Bongard Dr. Claudia Weidenthaler Dr. Wolfgang Schmidt Dr. Roberto Rinaldi Prof. Dr. Ferdi Schüth 《Angewandte Chemie (International ed. in English)》2016,55(31):8850-8855
Hydrodeoxygenation (HDO) is an attractive route for the upgrading of bio‐oils produced from lignocellulose. Current catalysts require harsh conditions to effect HDO, decreasing the process efficiency in terms of energy and carbon balance. Herein we report a novel and facile method for synthesizing bimetallic PtCo nanoparticle catalysts (ca. 1.5 nm) highly dispersed in the framework of nitrogen‐doped ordered mesoporous carbon (NOMC) for this reaction. We demonstrate that NOMC with either 2D hexagonal (p6m) or 3D cubic (Im m) structure can be easily synthesized by simply adjusting the polymerization temperature. We also demonstrate that PtCo/NOMC (metal loading: Pt 9.90 wt %; Co 3.31 wt %) is a highly effective catalyst for HDO of phenolic compounds and “real‐world” biomass‐derived phenolic streams. In the presence of PtCo/NOMC, full deoxygenation of phenolic compounds and a biomass‐derived phenolic stream is achieved under conditions of low severity. 相似文献
55.
Uwe‐Christoph Knig Michael Berkei Claudia Hirsch Hans Preut Terence Nigel Mitchell 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(12):e550-e551
The preparation and X‐ray analysis of the title compound, [Sn2Br4(CH3)4(C5H9NO)], are described. The compound contains two Sn atoms in the asymmetric unit, that complexed by N‐methylpyrrolidin‐2‐one being hexacoordinated (a), the other exhibiting pentacoordination (b). The most important features are three different Sn—Br bond lengths at both Sn atoms with the following values: (a) 2.5060 (9), 2.7152 (10) and 3.7118 (10) Å; (b) 2.5084 (10), 2.5279 (9) and 3.5841 (10) Å. 相似文献
56.
Concetta De Stefano Claudia Foti Antonio Gianguzza Frank J. Millero Silvio Sammartano 《Journal of solution chemistry》1999,28(7):959-972
The hydrolysis of trimethyltin(IV) has been studied by potentiometry (H+ -glass electrode) and calorimetry in various salt media (NaNO3, NaCl, KCl, Na2SO4, and NaNO3—NaCl mixtures). The effect of ionic strength on the hydrolysis constants is accounted for by a simple Debye–Hückel type equation and by Pitzer equations. The results allow us to obtain H for hydrolysis and the temperature dependence of the Pitzer parameters. The resulting coefficients can be used to examine the speciation of (CH3)3Sn+ in multicomponent electrolyte solutions, such as natural waters, over a wide range of temperature and ionic strength. 相似文献
57.
Self-assembled nanocages for hydrophilic guest molecules 总被引:1,自引:0,他引:1
Jones MC Tewari P Blei C Hales K Pochan DJ Leroux JC 《Journal of the American Chemical Society》2006,128(45):14599-14605
Reverse polymeric micelles are obtained following the association of polymeric amphiphiles in apolar media. To this date, reports of pharmaceutical applications for such micelles have been scarce, mainly because these systems have been studied in solvents that are not suitable for medical use. Here, alkylated star-shaped poly(glycerol methacrylate) polymers have been proposed in the design of oil-soluble reverse polymeric micelles. Micellar behavior was studied in various apolar solvents, including ethyl oleate, a pharmaceutically acceptable vehicle. The polymers were shown to assemble into spherical nanostructures (<40 nm) as determined by cryogenic transmission electron microscopy and atomic force microscopy studies. Interestingly, the reverse micelles were able to encapsulate various peptides/proteins (vasopressin, myoglobin, and albumin) in substantial amounts and facilitate their solubilization in oil. The nature of both the polymer used in micelle formation and the guest molecules was found to influence the ability of the micelle to interact with hydrophilic compounds. 相似文献
58.
Stefan Bode Claudia C. Quentmeier Pen-Nan Liao Tiago Barros Peter J. Walla 《Chemical physics letters》2008,450(4-6):379-385
The Car S1 → Chl energy transfer efficiency, ΦTransfer, in xanthophyll-cycle mutants of living plants and LHC II was investigated by selective Car S1 two-photon excitation. Before high-light illumination ΦTransfer, of the violaxanthin deficient mutant npq2 is 30% smaller than the corresponding value for wild type plants. For the zeaxanthin deficient mutant, npq1, ΦTransfer is 30% larger. Wild type Arabidopsis thaliana is the only variant which is capable of a light-dependent decrease of up to 40% and complete recovery to the original ΦTransfer values. In contrast, ΦTransfer remains constant during dark adaptation in both mutants. Surprisingly, changes in ΦTransfer of LHC II preparations were less than 5% only, when substituting violaxanthin by zeaxanthin. 相似文献
59.
Coexisting lamellar phases in water-oil-surfactant systems induced by the addition of an amphiphilic block copolymer 总被引:1,自引:0,他引:1
We report here a pioneering study using quadrupolar splitting NMR to detect new phases and phase compositions in the quasi-ternary microemulsion system water-decane-C(10)E(4)/PEP5-PEO5. The striking observation is that at certain compositions the polymer is apparently no longer incorporated into the membranes of the lamellar phase due to space restrictions. The polymer therefore induces a phase separation into two different lamellar phases L(alpha)(1) and L(alpha)(2) such that it fits into L(alpha)(1) while the excess surfactant forms a polymer-free L(alpha)(2) phase. 相似文献
60.