首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2271篇
  免费   120篇
  国内免费   386篇
化学   2594篇
晶体学   18篇
力学   12篇
综合类   11篇
数学   4篇
物理学   138篇
  2024年   2篇
  2023年   16篇
  2022年   19篇
  2021年   34篇
  2020年   62篇
  2019年   62篇
  2018年   53篇
  2017年   79篇
  2016年   68篇
  2015年   51篇
  2014年   88篇
  2013年   272篇
  2012年   114篇
  2011年   91篇
  2010年   94篇
  2009年   118篇
  2008年   149篇
  2007年   149篇
  2006年   143篇
  2005年   144篇
  2004年   128篇
  2003年   118篇
  2002年   148篇
  2001年   87篇
  2000年   63篇
  1999年   51篇
  1998年   51篇
  1997年   44篇
  1996年   65篇
  1995年   37篇
  1994年   42篇
  1993年   42篇
  1992年   27篇
  1991年   17篇
  1990年   11篇
  1989年   5篇
  1988年   7篇
  1987年   6篇
  1986年   3篇
  1985年   3篇
  1984年   4篇
  1983年   1篇
  1982年   2篇
  1981年   6篇
  1979年   1篇
排序方式: 共有2777条查询结果,搜索用时 15 毫秒
51.
本文介绍水杨配合与4-氨基-3,5-二乙基-1,2,4-三唑缩合而成对称三唑Schif碱(SAETZ)与氯化铜(CuCl2)形成一种新的配合物Cu(SAETZ)2(SAETZ=4-(邻羟苯基亚甲基)-亚胺-3,5-二乙基-1,2,4-三唑)。配合物的晶体结构表明,分子中两个偶氮甲碱的N原子及两个酚氧原子与中心Cu原子形成规则的平面配位结构。晶体属单斜晶系,空间群P21/n,a=8.688(2),b=9.314(1),c=16.515(4),β=94.34(2)。,V=1332.5(7)3,Z=2。  相似文献   
52.
以交联聚乙烯醇为载体的离子交换剂对蛋白质的分离性能   总被引:2,自引:0,他引:2  
本文对阴离子交换剂DEA—PVT常压液相离子交换色谱分离蛋白质的性能、分离条件进行了探讨。结果表明其对蛋白质的分离性能良好,容易洗脱。与载体交联聚乙烯醇相比,DEA—PVT对蛋白质的非特异性吸附明显降低。  相似文献   
53.
唐宏科  陈琦 《合成化学》2007,15(5):643-646
以过硫酸铵为引发剂,N,N-亚甲基双丙烯酰胺为交联剂,淀粉与丙烯酸/醋酸乙烯酯混合单体通过接枝共聚,制备了吸水及耐盐性能均较好的淀粉接枝丙烯酸/醋酸乙烯酯高吸水性树脂(CGAV)。最佳工艺条件为:淀粉10.0 g,m(混合单体)∶m(淀粉)=4∶1,w(引发剂)=0.3%,w(交联剂)=0.05%,于45℃反应2h~3 h。在最佳工艺条件下制得的CGAV吸去离子水率760 g.g-1,吸0.9%NaC l水溶液率68 g.g-1。  相似文献   
54.
钟平  郭圣荣 《中国化学》2004,22(10):1183-1186
2-Azido-3,5-dichloropyridine and 2-azido-5-chloro-3-fluoropyridine were given by reaction of sodium azide with 2,3,5-trichloropyridine, 3,5-dichloro-2-fluoropyridine or 5-chloro-2,3-difluoropyridine in ionic liquids. 1,3-Dipolar cycloaddition of 2-azido-3,5-dichloropyridine or 2-azido-5-chloro-3-fluoropyridine to alkynes in ionic liquids afforded the corresponding 1,4,5-trisubstituted [1,2,3]-triazoles in good yields and regioselectivities.  相似文献   
55.
The mer‐octahedral complexes(2‐carbonyl)(4‐Me)(6‐tBu)phenolato[C,O]hydridotris(trimethylphosphine)cobalt(III) ( 1 ) or ‐(1‐carbonyl)(2‐oxo)(1,2‐diphenylethene)[C,O]hydridotris(trimethylphosphine)cobalt(III) ( 2 ) via formal insertion of propynoic acid ethyl ester into Co‐H functions afford pentacoordinate vinylcobalt(III) 3 and 4 , respectively, that are diamagnetic and attain a square pyramidal structure as exemplified by an X‐ray diffraction analysis of 3 .  相似文献   
56.
The possibility to prepare hybrids made by poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA) and/or poly(ethyl acrylate) (PEtA) with TiO2 was studied. The processes of polymer formation-radical polymerization and sol-gel process for inorganic network —were achieved simultaneously. Due to a high reactivity of titanium isopropoxide (TIP) in the sol-gel process, a complexant comonomer, allyl acetoacetate (AlAcAc), was used. Covalent bonds between polymer and inorganic chains were obtained by addition of trialkoxysilane derivates with vinyl (VTES) or methacryloyl (MPTS) groups. The presence of TIP inhibits the radical polymerization of vinyl acetate (VAc). The PVAc-TiO2 hybrids were produced by the sol-gel process of TIP in the presence of pre-obtained PVAc. Except for VTES and MPTS, trialkoxysilane derivates with methyl (MeTES), octyl (OTES) and phenyl (PTES) groups were used. The thermal stability of hybrids is strongly affected by TiO2 presence and by the type of trialkoxysilane derivates. The thermal stability of PVAc hybrids decreases in the presence of TiO2 inorganic network. The glass transition temperature of polymers increases in the presence of the inorganic network.  相似文献   
57.
Reactions of tricarbonyl compounds with vinyl diazo compounds 2 were carried out. Reaction of 1,2,3-indanetrione with 2a,b,c gave the spiroindan-1,3-dione-2,2′-benzodihydrooxepin 7a,b,c, but not normal products oxirane and dihydrofuran derivatives expected from intermediate vinyl carbonyl ylides 4. Formation of 7 requires isomerization of vinyl carbonyl ylides 4 bearing a (Z)-cyanostyryl group to unstable (E)-form 5 and subsequent cyclization to oxepin 6 followed by a 1,5-hydrogen shift. However, reaction of 2 with six-membered cyclic tricarbonyl compounds 1,2,3-trioxo-2,3-dihydrophenalene 11 and dimethylalloxane 13 gave the dioxole 12 and the dihydrofuran 14, respectively, typical products expected from vinyl carbonyl ylides.  相似文献   
58.
Dense membranes were prepared from poly(vinyl alcohol)–poly(N-vinyl pyrrolidone) (PVA–PVP) blends of different compositions and studied in swelling and dehydration by pervaporation of three organic solvents contaminated by 5 wt% water. The swelling generally increases with the PVP content. No extraction occurs in water–tetrahydrofuran (THF) and water–methyl ethyl ketone (MEK) mixtures. In ethanol containing 10 wt% of water, there is no extraction for blends containing less than 40 wt% PVP and an increasing extraction beyond this PVP content. The pervaporation flux of the water–ethanol mixture increases drastically at the same threshold whereas the water permselectivity falls to a low level. The values of the diffusion and permeability coefficients determined from transient permeation of the test water–ethanol mixture exhibit a similar sudden increase at the same PVP content threshold. This singular behavior of the blend membranes is interpreted by a strong affinity of the PVP component to ethanol, combined with a disappearance of crystallites in the blend at this threshold. Consequently the amorphous membrane can swell freely according to the affinity of the PVP component, leading to the observed behavior.  相似文献   
59.
本文研究了马来酸酐(MAn)在过氧化苯甲酰(BP)引发作用下与醋酸乙烯酯(VAc)的交替共聚反应。红外光谱证明了交替共聚物的结构,分析结果表明共聚物是由反应单体技1:1摩尔比例组成。当c(BPO)=6.8×10~(-3)moi/L,c(VAc)=3.4×10~(-3)mol/L,p(MAn)=32.7g/L,63~65℃反应18小时,转化率可达92%以上。  相似文献   
60.
本文研究聚乙烯醇与丙基乙烯基亚砜经Michael加成反应合成含亚砜基的改性聚乙烯醇功能高分子.用IR、~1H-NMR、X-射线衍射等鉴定反应产物.结果表明,随着亚砜基含量的增加聚合物的结晶性降低,而在冷水,DMSO,CH_3OH,C_2H_5OH和CH_2Cl_2中的溶解性提高.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号