全文获取类型
收费全文 | 14942篇 |
免费 | 2651篇 |
国内免费 | 2332篇 |
专业分类
化学 | 11481篇 |
晶体学 | 216篇 |
力学 | 848篇 |
综合类 | 216篇 |
数学 | 1672篇 |
物理学 | 5492篇 |
出版年
2024年 | 52篇 |
2023年 | 345篇 |
2022年 | 561篇 |
2021年 | 579篇 |
2020年 | 736篇 |
2019年 | 689篇 |
2018年 | 633篇 |
2017年 | 551篇 |
2016年 | 776篇 |
2015年 | 815篇 |
2014年 | 958篇 |
2013年 | 1178篇 |
2012年 | 1308篇 |
2011年 | 1333篇 |
2010年 | 1047篇 |
2009年 | 971篇 |
2008年 | 980篇 |
2007年 | 843篇 |
2006年 | 760篇 |
2005年 | 696篇 |
2004年 | 638篇 |
2003年 | 518篇 |
2002年 | 561篇 |
2001年 | 461篇 |
2000年 | 361篇 |
1999年 | 313篇 |
1998年 | 213篇 |
1997年 | 157篇 |
1996年 | 168篇 |
1995年 | 142篇 |
1994年 | 101篇 |
1993年 | 83篇 |
1992年 | 67篇 |
1991年 | 75篇 |
1990年 | 66篇 |
1989年 | 38篇 |
1988年 | 37篇 |
1987年 | 30篇 |
1986年 | 21篇 |
1985年 | 17篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1978年 | 2篇 |
1971年 | 2篇 |
1957年 | 3篇 |
1936年 | 2篇 |
1916年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
J. Xu Y. Z. Meng S. J. Wang A. S. Hay 《Journal of polymer science. Part A, Polymer chemistry》2006,44(10):3328-3335
A series of novel soluble pyridazinone‐ or pyridazine‐containing poly(arylene ether)s were prepared by a polycondensation reaction. The pyridazinone monomer, 6‐(4‐hydroxyphenyl)pyridazin‐3(2H)‐one ( 1 ), was synthesized from the corresponding acetophenone and glyoxylic acid in a simple one‐pot reaction. The pyridazinone monomer was successfully copolymerized with bisphenol A (BPA) or 1,2‐dihydro‐4‐(4‐hydroxyphenyl)phthalazin‐1(2H)‐one (DHPZ) and bis(4‐fluorophenyl)sulfone to form high‐molecular‐weight polymers. The copolymers had inherent viscosities of 0.5–0.9 dL/g. The glass‐transition temperatures (Tg's) of the copolymers synthesized with BPA increased with increasing content of the pyridazinone monomer. The Tg's of the copolymers synthesized from DHPZ with different pyridazinone contents were similar to those of the two homopolymers. The homopolymers showed Tg's from 202 to 291 °C by differential scanning calorimetry. The 5% weight loss temperatures in nitrogen measured by thermogravimetric analysis were in the range of 411–500 °C. 4‐(6‐Chloropyridazin‐3‐yl)phenol ( 2 ) was synthesized from 1 via a simple one‐pot reaction. 2 was copolymerized with 4,4′‐isopropylidenediphenol and bis(4‐fluorophenyl)sulfone to form high‐Tg polymers. The copolymers with less than 80 mol % pyridazinone or chloropyridazine monomers were soluble in chlorinated solvents such as chloroform. The copolymers with higher pyridazinone contents and homopolymers were not soluble in chlorinated solvents but were still soluble in dipolar aprotic solvents such as N‐methylpyrrolidinone. The soluble polymers could be cast into flexible films from solution. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3328–3335, 2006 相似文献
52.
Ye Lin Feng Zeng‐Guo Zhao Yu‐Mei Wu Feng Chen Shi Wang Guo‐Qing 《Journal of polymer science. Part A, Polymer chemistry》2006,44(11):3650-3665
A novel cyclic ether monomer 3‐{2‐[2‐(2‐hydroxyethoxy)ethoxy]ethoxy‐methyl}‐3′‐methyloxetane (HEMO) was prepared from the reaction of 3‐hydroxymethyl‐3′‐methyloxetane tosylate with triethylene glycol. The corresponding hyperbranched polyether (PHEMO) was synthesized using BF3·Et2O as initiator through cationic ring‐opening polymerization. The evidence from 1H and 13C NMR analyses revealed that the hyperbranched structure is constructed by the competition between two chain propagation mechanisms, i.e. active chain end and activated monomer mechanism. The terminal structure of PHEMO with a cyclic fragment was definitely detected by MALDI‐TOF measurement. A DSC test implied that the resulting polyether has excellent segment motion performance potentially beneficial for the ion transport of polymer electrolytes. Moreover, a TGA assay showed that this hyperbranched polymer possesses high thermostability as compared to its liquid counterpart. The ion conductivity was measured to reach 5.6 × 10?5 S/cm at room temperature and 6.3 × 10?4 S/cm at 80 °C after doped with LiTFSI at a ratio of Li:O = 0.05, presenting the promise to meet the practical requirement of lithium ion batteries for polymer electrolytes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3650–3665, 2006 相似文献
53.
54.
55.
56.
微波常压法合成水杨酸酯 总被引:15,自引:0,他引:15
在浓硫酸催化下,采用微波常压法由水杨酸分别与正丁醇、异丁醇、正戊醇和异戊醇反应合成相应的水杨酸酯。结果表明:当水杨酸:醇:H2SO4=1:5.5:0.3(摩尔比)时,采用560W微波辐射22min,水杨酸酯的产率可达88.7% ̄96.4%,反应速度至少是常规反应的14倍。 相似文献
57.
用裂解气相色谱法研究PMR型聚酰亚胺前体在反应中的化学变化过程,以裂解产物醇和环戊二烯的生成率表示酰胺化或酰亚胺化及交联的程度,结果表明,酰胺化或酰亚胺化在50℃以下不发生,在180~220℃完成,降冰片烯端基在150℃以下不发生交联反应,在280℃,10~18小时可完成交联反应。 相似文献
58.
59.
60.
几种常见水果的微量元素分析 总被引:1,自引:0,他引:1
运用原子吸收分光光度法测定了几种常见水果中部分微量元素含量。其中榴莲的Mg、Zn、Cu、K含量位于四种水果之首。美国提子中Ca含量最高,芒果中Fe含量最高。以上差别具有统计学意义。六种元素的回收率为90%-106.5%。连续6次测定样品,变异系数(CV)为0.96%。 相似文献