全文获取类型
收费全文 | 4418篇 |
免费 | 246篇 |
国内免费 | 20篇 |
专业分类
化学 | 2967篇 |
晶体学 | 24篇 |
力学 | 123篇 |
综合类 | 3篇 |
数学 | 765篇 |
物理学 | 802篇 |
出版年
2023年 | 53篇 |
2022年 | 43篇 |
2021年 | 74篇 |
2020年 | 108篇 |
2019年 | 100篇 |
2018年 | 84篇 |
2017年 | 76篇 |
2016年 | 176篇 |
2015年 | 159篇 |
2014年 | 144篇 |
2013年 | 264篇 |
2012年 | 205篇 |
2011年 | 198篇 |
2010年 | 166篇 |
2009年 | 139篇 |
2008年 | 197篇 |
2007年 | 213篇 |
2006年 | 177篇 |
2005年 | 187篇 |
2004年 | 133篇 |
2003年 | 122篇 |
2002年 | 158篇 |
2001年 | 81篇 |
2000年 | 59篇 |
1999年 | 74篇 |
1998年 | 74篇 |
1997年 | 77篇 |
1996年 | 71篇 |
1995年 | 67篇 |
1994年 | 57篇 |
1993年 | 44篇 |
1992年 | 31篇 |
1991年 | 37篇 |
1990年 | 35篇 |
1989年 | 23篇 |
1988年 | 21篇 |
1987年 | 31篇 |
1986年 | 33篇 |
1985年 | 29篇 |
1984年 | 31篇 |
1983年 | 22篇 |
1982年 | 24篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 25篇 |
1978年 | 20篇 |
1977年 | 39篇 |
1976年 | 18篇 |
1974年 | 19篇 |
1973年 | 21篇 |
排序方式: 共有4684条查询结果,搜索用时 203 毫秒
1.
2.
Designs, Codes and Cryptography - We study security functions which can serve to establish semantic security for the two central problems of information-theoretic security: the wiretap channel, and... 相似文献
3.
The synthesis of oligonucleotides containing 7-(2-deoxy-β -D -erythro-pentofuranosyl)adenine (N7Ad; 1 ) is described. Compound 1 was obtained from the precursor 4-amino-1H -imidazole-5-carbonitrile 2-deoxyribonucleoside 6 and was found to be much more labile than Ad. The N6-benzoyl protecting group (see 8 ) destabilized the N-glycosylic bond further and was difficult to remove by NH3-catalyzed hydrolysis. Therefore, a (dimethyl-amino)methylidene residue was introduced (→ 9 ). Amidine 9 was blocked at OH? C(5′) with the dimethoxytrityl residue ((MeO)2Tr), and phosphonate 4 as well as phosphoramidite 5 were prepared under standard conditions. Phosphonate 4 was employed in solid-phase oligonucleotide synthesis. Homooligonucleotides as well as self-complementary oligonucleotides were prepared. The oligomer d[(N7A)11-A] ( 11 ) formed a duplex with d(T12) ( 13 ). Antiparallel chain polarity and reverse Watson-Crick base pairing was deduced from duplex formation of the self-complementary d[(N7A)8-T8] ( 14 ). 相似文献
4.
Werner Leißner 《Results in Mathematics》1991,20(1-2):530-537
5.
6.
Hans Maaß 《Mathematische Annalen》1941,118(1):312-318
Ohne Zusammenfassung 相似文献
7.
8.
Starch belongs to the polyglucan group. This type of polysaccharide shows a broad β-relaxation process in dielectric spectra at low temperatures, which has its molecular origin in orientational motions of sugar rings via glucosidic linkages. This chain dynamic was investigated for α(1,4)-linked starch oligomers with well-defined chain lengths of 2, 3, 4, 6, and 7 anhydroglucose units (AGUs) and for α(1,4)-polyglucans with average degrees of polymerization of 5, 10, 56, 70, and so forth (up to 3000; calculated from the mean molecular weight). The activation energy (Ea) of the segmental chain motion was lowest for dimeric maltose (Ea = 49.4 ± 1.3 kJ/mol), and this was followed by passage through a maximum at a degree of polymerization of 6 (Ea = 60.8 ± 1.8 kJ/mol). Subsequently, Ea leveled off at a value of about 52 ± 1.5 kJ/mol for chains containing more than 100 repeating units. The results were compared with the values of cellulose-like oligomers and polymers bearing a β(1,4)-linkage. Interestingly, the shape of the Ea dependency on the chain length of the molecules was qualitatively the same for both systems, whereas quantitatively the starch-like substances generally showed higher Ea values. Additionally, and for comparison, three cyclodextrins were measured by dielectric relaxation spectroscopy. The ringlike molecules, with 6, 7, and 8 α(1,4)-linked AGUs, showed moderately different types of dielectric spectra. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 188–197, 2004 相似文献
9.
Holger Poths Erik Wischerhoff Rudolf Zentel Axel Sch nfeld Guido Henn Friedrich Kremer 《Liquid crystals》1995,18(5):811-818
Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo- and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs. It is found that polymerization increases the spontaneous polarization Ps. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respect to their molecular structures. Each oligomer is already a good model for its corresponding polymer concerning the soft mode dynamics. For the local β-relaxation a similar temperature dependence of the relaxation times τ for the cyclic tetramer and for the side chain polysiloxanes is observed. The long axial rotation of the twin, having a very efficient decoupling, is significantly faster, thus resembling the monomer. 相似文献
10.
Mustafa Çelebier Ertan Şahin Nilgün Ancın Nurşen Altuntaş Öztaş Selma Gül Öztaş 《应用有机金属化学》2007,21(10):viii-viii
The article referenced above was first published online on 30 August 2007 with incorrect pagination; the pagination has now been corrected online and in print. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献