全文获取类型
收费全文 | 6176篇 |
免费 | 118篇 |
国内免费 | 32篇 |
专业分类
化学 | 3834篇 |
晶体学 | 16篇 |
力学 | 175篇 |
数学 | 1333篇 |
物理学 | 968篇 |
出版年
2020年 | 65篇 |
2019年 | 44篇 |
2016年 | 93篇 |
2015年 | 80篇 |
2014年 | 89篇 |
2013年 | 227篇 |
2012年 | 192篇 |
2011年 | 215篇 |
2010年 | 151篇 |
2009年 | 141篇 |
2008年 | 204篇 |
2007年 | 230篇 |
2006年 | 186篇 |
2005年 | 213篇 |
2004年 | 169篇 |
2003年 | 142篇 |
2002年 | 148篇 |
2001年 | 52篇 |
2000年 | 68篇 |
1999年 | 55篇 |
1998年 | 49篇 |
1997年 | 76篇 |
1996年 | 70篇 |
1995年 | 83篇 |
1994年 | 83篇 |
1993年 | 83篇 |
1992年 | 86篇 |
1991年 | 66篇 |
1990年 | 71篇 |
1989年 | 78篇 |
1988年 | 83篇 |
1987年 | 76篇 |
1986年 | 89篇 |
1985年 | 124篇 |
1984年 | 121篇 |
1983年 | 88篇 |
1982年 | 124篇 |
1981年 | 127篇 |
1980年 | 111篇 |
1979年 | 123篇 |
1978年 | 109篇 |
1977年 | 107篇 |
1976年 | 96篇 |
1975年 | 110篇 |
1974年 | 84篇 |
1973年 | 90篇 |
1972年 | 55篇 |
1971年 | 61篇 |
1970年 | 63篇 |
1966年 | 47篇 |
排序方式: 共有6326条查询结果,搜索用时 171 毫秒
51.
Unusual Bonding and Properties in Main Group Element Chemistry: Rational Synthesis,Characterization, and Experimental Electron Density Determination of Mixed‐Valent Tetraphosphetes 下载免费PDF全文
Verena Breuers Prof. Dr. Christian W. Lehmann Prof. Dr. Walter Frank 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4596-4606
Five dispirocyclic λ3,λ5‐tetraphosphetes [{R2Si(NR1)(NR2)P2}2] (R1 = R2 and R1 ≠ R2) are easily prepared in almost quantitative yields via photolysis of the respective bis(trimethylsilyl)phosphanyldiazaphosphasiletidines with intense visible light. These deep‐yellow low‐coordinate phosphorus compounds can be considered as the first higher congeners of the well‐known cyclodiphosphazenes. The tetraphosphetes are remarkably stable in air and show unexpected molecular properties related to the unique bonding situation of the central four‐π‐electron four‐membered phosphorus ring. The extent of rhombic distortion of the central P4 ring is remarkable due to an unusually acute angle at the σ2‐phosphorus atoms. All of the P?P bonds are approximately equal in length. The distances are in the middle of the range given by phosphorus single and double bonds. The anisotropic absorption of visible light that can easily be observed in the case of the yellow/colorless dichroic crystals of [{Me2Si(NtBu)(NtBu)P2}2] and the exceptional 31P NMR chemical shift of the σ2‐phosphorus atoms are the most remarkable features of the λ3,λ5‐tetraphosphetes. In the case of [{Me2Si(NtBu)(NtBu)P2}2], the Hansen–Coppens multipole model is applied to extract the electron density from high‐resolution X‐ray diffraction data obtained at 100 K. Static deformation density and topological analysis reveal a unique bonding situation in the central unsaturated P4 fragment characterized by polar σ‐bonding, pronounced out‐of‐ring non‐bonding lone pair density on the σ2‐phosphorus atoms, and an additional non‐classical three‐center back‐bonding contribution. 相似文献
52.
Solvent Influence on Cellulose 1,4‐β‐Glycosidic Bond Cleavage: A Molecular Dynamics and Metadynamics Study 下载免费PDF全文
Claudia Loerbroks Eliot Boulanger Prof. Dr. Walter Thiel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(14):5477-5487
We explore the influence of two solvents, namely water and the ionic liquid 1‐ethyl‐3‐methylimidazolium acetate (EmimAc), on the conformations of two cellulose models (cellobiose and a chain of 40 glucose units) and the solvent impact on glycosidic bond cleavage by acid hydrolysis by using molecular dynamics and metadynamics simulations. We investigate the rotation around the glycosidic bond and ring puckering, as well as the anomeric effect and hydrogen bonds, in order to gauge the effect on the hydrolysis mechanism. We find that EmimAc eases hydrolysis through stronger solvent–cellulose interactions, which break structural and electronic barriers to hydrolysis. Our results indicate that hydrolysis in cellulose chains should start from the ends and not in the centre of the chain, which is less accessible to solvent. 相似文献
53.
Frontispiece: Unusual Bonding and Properties in Main Group Element Chemistry: Rational Synthesis,Characterization, and Experimental Electron Density Determination of Mixed‐Valent Tetraphosphetes 下载免费PDF全文
54.
Control of reaction mechanisms in cationically polymerized epoxy resins facilitates the adjustment of morphology and mechanical properties 下载免费PDF全文
André Arnebold Florian Plander Karsten Thiel Stefanie Wellmann Andreas Hartwig 《Journal of Polymer Science.Polymer Physics》2016,54(21):2188-2199
Structure–property relations of cationically polymerized epoxy thermosets with different morphologies are examined. The morphology adjustment of amorphous epoxy based copolymers and partially crystalline polymer alloys is carried out with star‐shaped poly(ε‐caprolactone) (SPCL) bearing various numbers of hydroxyl end groups. These hydroxyl groups are known for their reactivity toward epoxides following the activated monomer (AM) mechanism. For this reason, four‐armed SPCL was synthesized with four hydroxyl end groups (SPCL‐tetraol) and, in addition, with successively esterified ones down to a SPCL with four ester end groups (SPCL‐tetraester). SPCL species bearing fewer or no hydroxyl end groups segregate into needle‐like nanodomains within the epoxy networks and, if the concentration is high enough, also into crystalline domains. The stronger phase separation of SPCL‐tetraester within the epoxy network compared with SPCL‐tetraol is due to a reduction of the AM mechanism. The mechanical properties resulting from different morphologies lead to a trade‐off between higher storage moduli and Tg values in the case of the more phase separated (and partially crystalline) polymer alloys and higher strain at break in the case of the amorphous copolymers. Nevertheless, in both cases toughness is improved or at least kept on the same level as for the pure epoxy resin. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2188–2199 相似文献
55.
Ludwin A.BASILIO-HERNáNDEZ Walter CARBALLOSA Jesús LEA?OS José M.SIGARRETA 《数学学报(英文版)》2019,35(3):338-354
We introduce the differential polynomial of a graph. The differential polynomial of a graph G of order n is the polynomial B(G; x) :=∑?(G)k=-nB_k(G) x~(n+k), where B_k(G) denotes the number of vertex subsets of G with differential equal to k. We state some properties of B(G;x) and its coefficients.In particular, we compute the differential polynomial for complete, empty, path, cycle, wheel and double star graphs. We also establish some relationships between B(G; x) and the differential polynomials of graphs which result by removing, adding, and subdividing an edge from G. 相似文献
56.
57.
58.
59.
60.