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   3927篇
  免费   97篇
  国内免费   348篇
化学   3556篇
晶体学   56篇
力学   16篇
综合类   10篇
数学   2篇
物理学   732篇
  2024年   28篇
  2023年   243篇
  2022年   53篇
  2021年   51篇
  2020年   86篇
  2019年   75篇
  2018年   60篇
  2017年   85篇
  2016年   111篇
  2015年   103篇
  2014年   139篇
  2013年   217篇
  2012年   333篇
  2011年   290篇
  2010年   217篇
  2009年   251篇
  2008年   247篇
  2007年   269篇
  2006年   215篇
  2005年   205篇
  2004年   172篇
  2003年   103篇
  2002年   98篇
  2001年   78篇
  2000年   71篇
  1999年   81篇
  1998年   86篇
  1997年   67篇
  1996年   42篇
  1995年   65篇
  1994年   39篇
  1993年   30篇
  1992年   40篇
  1991年   28篇
  1990年   21篇
  1989年   9篇
  1988年   8篇
  1987年   6篇
  1986年   10篇
  1985年   4篇
  1984年   8篇
  1982年   6篇
  1981年   4篇
  1980年   3篇
  1978年   3篇
  1976年   6篇
  1975年   1篇
  1974年   2篇
  1973年   1篇
  1972年   1篇
排序方式: 共有4372条查询结果,搜索用时 15 毫秒
81.
K. Surendra 《Tetrahedron letters》2006,47(13):2133-2136
A new and convenient procedure for the synthesis of homoallylic alcohols directly from aromatic acetals and dioxolanes has been developed with very good yields under biomimetic conditions using a Zn-mediated Barbier-type allylation in the presence of β-cyclodextrin in water.  相似文献   
82.
报道了35例食道癌患者血清铜/锌比值及血清硒水平,结果发现食道癌患者血清铜/锌比值增加,血清硒水平下降,具有显著性差异(P〈0.001);同时提示随着食道癌分期增加,铜/锌比值进一步增加,血硒水平进一步下降,但无统计学差异。上述结果对临床诊断及观察病情变化有一定意义。  相似文献   
83.
The effects of M3O3 (M = Al, Sc etc.) in Cu-ZnO-M2O3 catalysts on methanol synthesis at low pressure were studied with ESR, XPS and TPR spectroscopy. The results of ESR showed that the generation of monovalent cationic defects was because the valence state and electronic charge on the ZnO lattice lost their balance as M3+ doped into ZnO. The induced effect by Sc3+ is stronger than that by Al3+. The results of XPS and TPR indicated that the amount and stabilization of Cu+ on the surface of reduced copper-based catalyst and its catalytic activity were affected by the monovalent cationic defects on the surface of ZnO.  相似文献   
84.
采用偏振光反光度、紫外及可见光谱、电子显微镜形貌观察、粒子大小测定及电子衍射等方法加以系统研究硫化锌纳米级粒子化膜的制备与气相性质的影响.  相似文献   
85.
The aqueous dispersion of polyurethaneurea-acrylate (PUA) including small amount of fluorinated block copolymers containing carboxyl groups (PATF), which can be dissolved in water, was used to make films and the surface properties of these films were examined. The experimental data show that the modified PUA film exhibits a hydrophobic surface property, although the original surface of PUA film is hydrophilic. The surface composition of the modified PUA film was measured by ATR and XPS. The results indicate that there is a concentration gradient of the fluorine groups along the lines of thickness of the modified film and towards the outmost surface layer, resulting from the migration of fluorinated blocks to the air side surface of the modified PUA film during the film formation process. However, the PUA film can not be modified effectively by adding the sodium salt of PATF, since the urethane groups in the system are easy to occupy on the surface of the film.  相似文献   
86.
Due to the toxic and carcinogenic properties of hexavalent chromium ion, the corrosion protection with chromating technique needs replacement. Several environmentally friendly alternative metal pretreatments have already been proposed. One of these methods is the application of self-assembling molecules to form mono- or multilayers on the metal surfaces. These layers can prevent metal dissolution due to their dense and stable structure. The objective of our studies was to protect zinc surface against corrosion, with a thin phosphonate layer. Aqueous solutions of diphosphonic acid with different alkyl chain lengths were applied with different treatment times. The layer formation, stability, and corrosion protection of these films were monitored by electrochemical impedance spectroscopy and the effect of 1,5-diphosphono-pentane (DPP) on zinc was studied by polarization curves. The wetting properties were determined by static contact angle measurement. 1,5-Diphosphono-pentane forms a thin layer, with a pronounced protective ability in neutral aqueous solutions. The application of self-assembling molecules can be a promising method to replace the chromating technique on zinc surface.  相似文献   
87.
用健儿三宝冲剂治疗的儿童钙,铁,锌缺乏症,经临床观察,取得了较好的疗效。  相似文献   
88.
The water-structural contributions to the entropies and heat capacities of hydration of over 120 ions and the viscosity B-coefficients of nearly 80 aqueous ions are tabulated and correlated. B-coefficients for many more ions are predicted from this relationship and from their dependence on ionic size and charge. The structural entropies determine a unique scale of water structure making and breaking by the ions.  相似文献   
89.
乙烯砜型染料与丙烯腈的水相沉淀聚合吴致宁,张雪松,杨锦宗大连理工大学精细化工系,大连,116012)关键词高分子染料、沉淀共聚合、乙烯砜型染料、丙烯腈近年来,随着人们对功能材料具有高附加值的认识的不断深人,特别是伴随各类功能性染料结构的开发,高分子染...  相似文献   
90.
Pentaazadienido Complexes of Zinc, Cadmium, and Mercury. The Crystal Structure of [Cd(EtOC6H4-N5-C6H4OEt)2(py)2] and [Hg(tol-N5-tol)2(py)] The pentaazadienido complexes [M(EtOC6H4N5C6H4OEt)2] (M = Zn ( 1 ), Cd ( 2 )) are formed by the reaction of [M(NH3)4]2+ with [EtOC6H4N5C6H4OEt]? in aqueous ammonia. 2 crystallizes from pyridine as [Cd(EtOC6H4N5C6H4OEt)2py2] ( 3 ) with the triclinic space group P1 and a = 937.2(2); b = 1422.7(2); c = 2085.5(2) pm; α = 75.28(1)°; β = 94.74(1)°; γ = 99.75(1)°; Z = 2. The central Cd2+ ion of 3 exhibits an octahedral coordination by two pyridine ligands in cis arrangement and two (N1, N3)-2+ chelating pentaazadienide ions. The reaction of [HgI4]2 with the 1,5-di(tolyl)pentaazadienide anion in aqueous ammonia affords [Hg(p-tol-N5-tol)2] ( 4 ), which crystallizes from pyridine in form of [Hg(tol-N5-tol)2py] ( 5 ) with the space group P1 and a = 1176.2(4); b = 1203.1(3); c = 1295.6(5) pm; α = 100.77(3)°; β = 110.08(3)°; γ = 94.29(2)°; Z = 2. In 5 the Hg2+ cation is threefold coordinated by two monodentate (N3)-η1 pentaazadienid anions and one pyridine ligand. Within the N5 chains of the pentaazadienid anions of 3 and 5 localized N? N double bonds are found in the positions N1? N2 and N4? N5 with distances between 125 and 129 pm.  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