全文获取类型
收费全文 | 17771篇 |
免费 | 3169篇 |
国内免费 | 2413篇 |
专业分类
化学 | 12819篇 |
晶体学 | 205篇 |
力学 | 1287篇 |
综合类 | 160篇 |
数学 | 1894篇 |
物理学 | 6988篇 |
出版年
2024年 | 33篇 |
2023年 | 364篇 |
2022年 | 636篇 |
2021年 | 602篇 |
2020年 | 748篇 |
2019年 | 655篇 |
2018年 | 579篇 |
2017年 | 507篇 |
2016年 | 812篇 |
2015年 | 792篇 |
2014年 | 993篇 |
2013年 | 1208篇 |
2012年 | 1604篇 |
2011年 | 1666篇 |
2010年 | 1113篇 |
2009年 | 1036篇 |
2008年 | 1193篇 |
2007年 | 1036篇 |
2006年 | 1059篇 |
2005年 | 872篇 |
2004年 | 689篇 |
2003年 | 573篇 |
2002年 | 569篇 |
2001年 | 477篇 |
2000年 | 397篇 |
1999年 | 453篇 |
1998年 | 318篇 |
1997年 | 298篇 |
1996年 | 337篇 |
1995年 | 287篇 |
1994年 | 272篇 |
1993年 | 201篇 |
1992年 | 191篇 |
1991年 | 155篇 |
1990年 | 133篇 |
1989年 | 112篇 |
1988年 | 108篇 |
1987年 | 66篇 |
1986年 | 60篇 |
1985年 | 43篇 |
1984年 | 31篇 |
1983年 | 26篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1966年 | 1篇 |
1957年 | 5篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
β‐Functionalization of Carboxylic Anhydrides with β‐Alkyl Substituents through Carbene Organocatalysis 下载免费PDF全文
Zhichao Jin Shaojin Chen Yuhuang Wang Pengcheng Zheng Prof. Dr. Song Yang Prof. Dr. Yonggui Robin Chi 《Angewandte Chemie (International ed. in English)》2014,53(49):13506-13509
The first NHC‐catalyzed functionalization of carboxylic anhydrides is described. In this reaction, the β carbon behaves as a nucleophilic carbon and undergoes asymmetric reactions with electrophiles. Anhydrides with challenging β‐alkyl substituents work effectively. 相似文献
942.
Two Eu(III) ternary luminescent complexes, Eu(tpb-H)3(Tppo)2 and Eu(tpb-H)3(Topo)2 (Tppo: triphenylphosphine oxide, Topo: trioctylphosphine oxide, tpb: 4,4,4-trifluoro-1-phenyl-1,3-butanedione) were synthesized using β-diketonates and phosphine oxides as ligands. Luminescent polymers were fabricated by incorporating the deuterated Eu(III) complexes in a PMMA matrix. Luminescent PMMA containing Eu(tpb-D)3(Tppo)2 exhibited relatively higher quantum yield, faster radiation rate, sharper red emission and larger stimulated emission cross-section and the results indicated prepared luminescent polymers including Eu(tpb-D)3(Tppo)2 showed promising results for applications in novel organic Eu(III) devices. Additionally, the Eu(III) complexes and luminescent PMMA showed good thermostabilization. 相似文献
943.
含Ti的HMS介孔材料催化剂及其对丙烯环氧化催化性能的研究 总被引:1,自引:1,他引:0
分别用水热合成法和气相四氯化钛(TiCl4)接枝法制备了Ti-HMS和Ti/HMS催化剂.表征结果表明,经过气相TiCl4接枝后的样品依然保持HMS(Hexagonal Mesoporous Silica,缩写为HMS)介孔材料特征,钛(Ti)物种主要以四配位的活性位形式存在.经过甲基接枝处理的催化材料,增加了表面的疏水性.丙烯环氧化反应结果表明,SN-Ti/HMS具有更高的催化性能.在2 500 h的稳定试验中,过氧化氢异丙苯(CHP)转化率大于99.0%,环氧丙烷(PO)选择性大于96.0%.研究和优化了环氧化反应工艺条件.采用浓度为30%的CHP为原料,CHP重量空速为1.0 h-1,床层温度为100℃,反应压力为3.0 MPa. 相似文献
944.
经过一系列化学反应,在吡啶环上引入特丁基及甲醛基,再与2,6-二异丙基苯胺反应生成2-(N-2,6-二异丙基苯亚氨基)亚甲基-6-特丁基吡啶(g)配体,并合成了相应的配合物[NiBr2(g)](h)。配合物h的单晶结构表明其为单配体单金属结构,属于正交晶系,a=1.527 76(14)nm,b=1.608 46(15)nm,c=1.837 77(17)nm,晶胞体积4.516 0(7)nm3,Z=8。在助催化剂MAO的作用下,考察了催化乙烯聚合特性,在0℃时,所得聚合产物中包括支化聚合物和齐聚物,其中支化聚合物的分子量Mw=1.09×104,其支链大部分为甲基、丁基和长支链,支链密度为58.5。 相似文献
945.
The multicomponent reactions of aldehydes, electron deficient alkynes and amines have been successfully performed to yield a number of symmetrical 2,6‐unsubstituted 1,4‐dihydropyridines (1,4‐DHPs). This method has been found generally applicable for the synthesis of both N‐substituted and N‐unsubstituted 1,4‐DHPs by employing secondary amine to activate the alkyne component via enaminoester intermediates. The present method runs through an enamine type activation, which is different from the known approach employing AcOH as solvent. 相似文献
946.
Zhuo Li Yang Zhang Fang-Hong Tong Ting-Ting Jiang Hui-Ping Zheng Jian-Nong Ye Qing-Cui Chu 《中国化学快报》2014,25(4):640-644
A hollow-fiber liquid-phase microextraction (HF-LPME) method has been developed for the purification and preconcentration of biogenic polyamines and their precursor amino acids in human saliva. Putrescine (Put), cadaverine (Cad), spermidine (Spe), ornithine (Orn), lysine (Lys), and arginine (Arg) were determined by the CE-LIF detection after microextraction. Several factors that affect extraction efficiency, separation, and detection were investigated. Under the optimum conditions, six analytes could achieve baseline separation within 30 min, exhibiting a linear calibration at three orders of magnitude (r2 〉 0.998); the obtained enrichment factors of HF-LPME were between 19 (for Orn) and 2] 8 (for Cad), and the LODs were in the range of 0.0072-0.26 nmol/L. The proposed HF-LPME/CE-LIF method has been successfully applied for the sensitive analyses of the real-world saliva samples collected from healthy volunteers and different patients with oral diseases, providing a potential method for primary non-invasive diagnosis of some oral diseases. 相似文献
947.
The asymmetric desymmetrization of cyclic anhydrides via the addition of carbon-based nucleophiles has been the focus of considerable levels of interest because it leads to optically active products. Over the past 20 years, a variety of different catalytic asymmetric alkylation reactions have been developed for the desymmetrization of cyclic anhydrides using different metal reagents as nucleophiles and using chiral ligands. The purpose of this review is to provide an overview of significant developments in this field. ~ 2013 Fen-Er Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. 相似文献
948.
以聚二烯丙基二甲基胺盐酸盐(PDDA)为连接剂,采用原位自组装方式将MoO3和WO3负载到碳纳米管(CNTs)上,然后通过乙二醇还原法负载Pt纳米颗粒,得到Pt纳米颗粒均匀分布的Pt/MoO3-WO3/CNTs催化剂.当氧化物总量控制在10 wt%,MoO3与WO3摩尔比为1:0.5时,Pt/MoO3-WO3/CNTs催化剂催化甲醇氧化活性最高,甲醇氧化峰电流If高达835 A/gPt.WO3和MoO3的加入提高了催化剂的甲醇氧化活性、抗CO中毒能力和稳定性,使得Pt/MoO3-WO3/CNTs催化剂表现出优异的甲醇电催化氧化性能. 相似文献
949.
纤维素直接催化转化制乙二醇是一条极具吸引力的生物质转化途径,有助于减轻化石能源资源的消耗。综述了从该反应途径的发现到获得高效、高稳定性催化剂的快速发展过程。基于对钨基催化剂的大量研究结果,本文讨论了反应机制,明确了反应路径、催化剂状态、钨物种及加氢催化活性中心各自在串联反应中的作用。围绕该反应过程的工业化应用需要,讨论了有关原生木质纤维素生物质催化转化以及高效反应过程的发展策略。在此基础上,将纤维素催化转化制乙二醇过程与生物质发酵制丙酮-丁醇-乙醇的生物炼制路线进行整合,构建出一个理想的反应过程潜在应用范例。最后,对纤维素催化转化制乙二醇反应过程进行了总结和前景展望. 相似文献
950.
Self-assembly behavior of copolymers with super segregated structure containing fluorinated segments
Copolymers with super segregated structure of hydrophilic methoxy poly(ethylene glycol) (mPEG) and fluorophilic poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PFA) were prepared. And just because of this super segregated structure which was resulted from the extremely strong incompatibility between the two blocks, several interesting self- assembly behaviors of the copolymers were displayed and studied under different conditions. Transmission electron microscope (TEM) showed that with the increase of PFA in the polymerization system, the incompatibility in this super segregated structure became stronger, and the self-assembly behavior changed from ball-like or rod-like to vesicles, and finally collapsed to sheet-like. The self-assembly behavior changed likewise when the initial concentration increased. And the interesting formation of these barrel-like and spindle-like vesicles was finally studied with different cooling speeds. It's finally found that with this super segregation structure, these new self-assembly morphology might be formed due to the extremely strong incompatibility between mPEG and PFA segments. 相似文献