全文获取类型
收费全文 | 6821篇 |
免费 | 253篇 |
国内免费 | 27篇 |
专业分类
化学 | 4749篇 |
晶体学 | 66篇 |
力学 | 186篇 |
数学 | 1020篇 |
物理学 | 1080篇 |
出版年
2023年 | 51篇 |
2022年 | 91篇 |
2021年 | 203篇 |
2020年 | 167篇 |
2019年 | 165篇 |
2018年 | 118篇 |
2017年 | 121篇 |
2016年 | 266篇 |
2015年 | 226篇 |
2014年 | 236篇 |
2013年 | 365篇 |
2012年 | 422篇 |
2011年 | 515篇 |
2010年 | 297篇 |
2009年 | 283篇 |
2008年 | 406篇 |
2007年 | 397篇 |
2006年 | 384篇 |
2005年 | 356篇 |
2004年 | 318篇 |
2003年 | 256篇 |
2002年 | 228篇 |
2001年 | 92篇 |
2000年 | 80篇 |
1999年 | 85篇 |
1998年 | 59篇 |
1997年 | 63篇 |
1996年 | 90篇 |
1995年 | 60篇 |
1994年 | 56篇 |
1993年 | 49篇 |
1992年 | 55篇 |
1991年 | 49篇 |
1990年 | 32篇 |
1989年 | 29篇 |
1988年 | 19篇 |
1987年 | 25篇 |
1986年 | 35篇 |
1985年 | 37篇 |
1984年 | 25篇 |
1983年 | 38篇 |
1982年 | 24篇 |
1981年 | 16篇 |
1980年 | 20篇 |
1979年 | 21篇 |
1978年 | 19篇 |
1977年 | 28篇 |
1976年 | 11篇 |
1973年 | 9篇 |
1971年 | 13篇 |
排序方式: 共有7101条查询结果,搜索用时 31 毫秒
21.
Andrea Santagati Agostino Marrazzo Giuseppe Granata 《Journal of heterocyclic chemistry》2003,40(5):869-873
Derivatives containing the thieno[2,3‐d]pyrimidin‐4‐one system, potential selective COX‐2 inhibitors, were prepared starting from ethyl ester of 2‐isothiocyanato‐5‐phenyl‐3‐thiophenecarboxylic acid ( 2 ); their structural elucidation is also reported. 相似文献
22.
Andrea Vietri 《Graphs and Combinatorics》2007,23(1):111-121
We analyse 3-subset difference families of Z2d+1⊕Z2d+1 arising as reductions (mod 2d+1) of particular families of 3-subsets of Z⊕Z. The latter structures, namely perfect d-families, can be viewed as 2-dimensional analogues of difference triangle sets having the least scope. Indeed, every perfect d-family is a set of base blocks which, under the natural action of the translation group Z⊕Z, cover all edges {(x,y),(x′,y′)} such that |x−x′|, |y−y′|≤d. In particular, such a family realises a translation invariant (G,K3)-design, where V(G)=Z⊕Z and the edges satisfy the above constraint. For that reason, we regard perfect families as part of the hereby defined translation designs, which comprise and slightly generalise many structures already existing in the literature. The geometric context allows
some suggestive additional definitions. The main result of the paper is the construction of two infinite classes of d-families. Furthermore, we provide two sporadic examples and show that a d-family may exist only if d≡0,3,8,11 (mod 12). 相似文献
23.
24.
25.
Alberto Caprara Andrea Lodi Silvano Martello Michele Monaci 《Discrete Optimization》2006,3(4):317-326
We address the problem of packing a given set of rectangles into the minimum size square. We consider three versions of the problem, arising when the rectangles (i) are squares; (ii) have a fixed orientation; (iii) can be rotated by 90. For each case we study lower bounds, and analyze their worst-case performance ratio. In addition, we evaluate through computational experiments their average performance on instances from the literature. 相似文献
26.
Andrea Loi 《Differential Geometry and its Applications》2006,24(3):311-319
In this paper we study the Calabi diastasis function of Hermitian symmetric spaces. This allows us to prove that if a complete Hermitian locally symmetric space (M,g) admits a Kähler immersion into a globally symmetric space (S,G) then it is globally symmetric and the immersion is injective. Moreover, if (S,G) is symmetric of a specified type (Euclidean, noncompact, compact), then (M,g) is of the same type. We also give a characterization of Hermitian globally symmetric spaces in terms of their diastasis function. Finally, we apply our analysis to study the balanced metrics, introduced by Donaldson, in the case of locally Hermitian symmetric spaces. 相似文献
27.
The ternary system CsCl?NaCl?LaCl3 was investigated by means of differential thermal analysis and X-ray powder diffraction analysis. There exists one congruently melting compound, Cs2NaLaCl6, crystallizing with the cubic elpasolite structure. No quasi-binary section exists for the whole system, however three binaries range from the ternary compound Cs2NaLaCl6 to NaCl, CsLa2Cl7 and Cs3LaCl6 resp., dividing the system in three areas of composition: one triangle, Cs3LaCl6?Cs2NaLaCl6?CsLa2Cl7, containing additionally a compound Cs2LaCl5 below 510°C, and the two areas CsCl?NaCl?Cs2NaLaCl6?Cs3LaCl6 and Cs2NaLaCl6?NaCl?LaCl3?CsLa2Cl7, containing a mixed crystal range between LaCl3 and Na3La5Cl18. These areas could be further divided in five triangles, so that the whole system contains six Alkemade triangles. 相似文献
28.
29.
Michelina Soccio Lara Finelli Nadia Lotti Valentina Siracusa Tiberio A. Ezquerra Andrea Munari 《Journal of Polymer Science.Polymer Physics》2007,45(13):1694-1703
Poly(butylene naphthalate) (PBN), poly(diethylene naphthalate) (PDEN), and poly(thiodiethylene naphthalate) (PTDEN) were synthesized and characterized in terms of chemical structure and molecular weight. The polyesters were examined by TGA, DSC, and DMTA. All the polymers showed a good thermal stability, even though depending on chemical structure. At room temperature they appeared as semicrystalline materials; the effect of the introduction along the PBN polymer chain of ether oxygen atoms or sulfur ones was a lowering in the Tg value, a decrement of Tm, and a decrease of the crystallization rate. Changing in chemical structure also affects the main α absorption associated with the glass transition which moves to lower temperature and whose energetic requirements decrease. The results were explained as due to the presence of highly flexible C? S? C or C? O? C bonds in the polymeric chain. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1694–1703, 2007 相似文献
30.