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   27874篇
  免费   4551篇
  国内免费   4246篇
化学   21491篇
晶体学   440篇
力学   1352篇
综合类   300篇
数学   3273篇
物理学   9815篇
  2024年   58篇
  2023年   510篇
  2022年   662篇
  2021年   954篇
  2020年   1131篇
  2019年   1169篇
  2018年   970篇
  2017年   1027篇
  2016年   1337篇
  2015年   1344篇
  2014年   1698篇
  2013年   2150篇
  2012年   2574篇
  2011年   2695篇
  2010年   2034篇
  2009年   1944篇
  2008年   2070篇
  2007年   1917篇
  2006年   1771篇
  2005年   1431篇
  2004年   1038篇
  2003年   824篇
  2002年   776篇
  2001年   693篇
  2000年   616篇
  1999年   517篇
  1998年   377篇
  1997年   337篇
  1996年   307篇
  1995年   282篇
  1994年   265篇
  1993年   193篇
  1992年   163篇
  1991年   167篇
  1990年   146篇
  1989年   102篇
  1988年   86篇
  1987年   65篇
  1986年   71篇
  1985年   59篇
  1984年   29篇
  1983年   22篇
  1982年   26篇
  1981年   11篇
  1980年   10篇
  1979年   13篇
  1977年   4篇
  1971年   4篇
  1957年   4篇
  1936年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
 提出了以吸附和催化原理灭活病毒的设想,旨在开发出对病毒有过滤、吸附及灭活作用的高效非特异性催化材料,应用于各种防护设施,有效控制非典型肺炎(SARS)的传播.采用与SARS病毒相似的副流感病毒作为模拟对象,进行了吸附及灭活该病毒的催化材料研究,并考察了催化材料对哺乳动物细胞的毒性.结果表明,病毒气溶胶的阻留及吸附结果与基于DNA吸附的色谱分析结果相一致;部分材料可以强烈地吸附病毒(100%),甚至在强烈振荡下并洗脱至第3次,病毒也不能脱附;一些材料不仅可以吸附病毒,而且强烈振荡后的洗脱液虽然表现出一定的血凝效价,但接种鸡胚后,病毒并不增殖,说明材料具有明显的催化病毒灭活性能;对细胞毒性极低的材料可以用在与人体接触的防护材料和设施中.筛选出的性能优异的催化材料,拟进一步考察其对SARS病毒的灭活作用.  相似文献   
982.
皂甙的三维定量构效关系研究   总被引:7,自引:0,他引:7  
针对目标分子柔性大的特点,在比较分子场分析(CoMFA)方法中采用交叉验证相关系数平方R^2引导的构象选择法。对12个皂甙分子的生物活性进行了三维定量构效关系研究。探讨了几种探针对构效关系结果的影响,并选择了一种较合理的“复合”探针方案。应用该复合探针构建CoMFA模型,发现影响药效的立体场与静电场的贡献分别为40%和40%,其它能量项的贡献为20%。该模型交叉验证的相关系数平方R^2为0.653,非交叉验证的R^2为0.991,方差比F(4,7)值130.195(即置信度99%以上),活性预计的标准偏差与极差比(s/△γ)为4.2%,表明模型具有较好的预测能力。根据该模型,预计在指定位置添加位阻较大的基团活性值提高将会比较明显。  相似文献   
983.
CF3O2自由基和NO反应机理的理论研究   总被引:1,自引:0,他引:1  
用密度泛函理论(DFT)的B3LYP方法, 分别在6-31G、6-311G、6-311+G(d)基组水平上研究了CF3O2自由基和NO反应机理. 研究结果表明, CF3O2自由基和NO反应存在三条可行的反应通道, 优化得到了相应的中间体和过渡态. 从活化能看, 通道CH3O2+NO→IM1→TS1→IM2→TS2→CF3O+ONO的活化能最低, 仅为70.86 kJ•mol-1, 是主要反应通道, 主要产物是CF3O和NO2. 而通道CH3O2+NO→IM1→TS3→CF3ONO2和CH3O2+NO→TS4→IM3→TS5→IM4→TS6→CF3O+NOO的活化能较高, 故该反应难以进行.  相似文献   
984.
In this paper initial value problems and nonlinear mixed boundary value problems for the quasilinear parabolic systems below $\[\frac{{\partial {u_k}}}{{\partial t}} - \sum\limits_{i,j = 1}^n {a_{ij}^{(k)}} (x,t)\frac{{{\partial ^2}{u_k}}}{{\partial {x_i}\partial {x_j}}} = {f_k}(x,t,u,{u_x}),k = 1, \cdots ,N\]$ are discussed.The boundary value conditions are $\[{u_k}{|_{\partial \Omega }} = {g_k}(x,t),k = 1, \cdots ,s,\]$ $\[\sum\limits_{i = 1}^n {b_i^{(k)}} (x,t)\frac{{\partial {u_k}}}{{\partial {x_i}}}{|_{\partial \Omega }} = {h_k}(x,t,u),k = s + 1, \cdots N.\]$ Under some "basically natural" assumptions it is shown by means of the Schauder type estimates of the linear parabolic equations and the embedding inequalities in Nikol'skii spaces,these problems have solutions in the spaces $\[{H^{2 + \alpha ,1 + \frac{\alpha }{2}}}(0 < \alpha < 1)\]$.For the boundary value problem with $\[b_i^{(k)}(x,t) = \sum\limits_{j = 1}^n {a_{ij}^{(k)}} (x,t)\cos (n,{x_j})\]$ uniqueness theorem is proved.  相似文献   
985.
With a hydrothermal technique, a layered titanium phosphate with the formula Ti2(H2PO4)(HPO4)(PO4)2 · 0.5C6N2H16 (denoted TP-J2) has been prepared by treating the Ti/H3PO4/H2O/1-methylpiperazine system directly. The as-synthesized products were characterized by powder X-ray diffraction, CP-MAS solid-state 31P NMR spectroscopy, thermogravimetric and differential thermal analyses (TG-DTA). The structure was solved by single-crystal X-ray diffraction analysis and it presents an extended γ-phase intercalated with organic amine. Crystal data: triclinic, , a = 8.106 (2) Å, b = 8.197 (2) Å, c = 11.658 (2) Å.  = 78.32 (3)°, β = 80.85 (3)°, γ = 77.90 (3)°, Z = 2. Additionally, the intercalation behavior of TP-J2 with n-alkyl monoamine (n = 2, 3, 4, 6, 8, 10 and 12) was investigated. Owing to the strong brønsted base, N,N′-dimethylpiperazine, resides in the interlayer, it presented unusual features of TP-J2 in contrast with that of γ-Tip.  相似文献   
986.
DNA tile based self-assembly provides an attractive route to create nanoarchitectures of programmable patterns. It also offers excellent scaffolds for directed self-assembly of nanometer-scale materials, ranging from nanoparticles to proteins, with potential applications in constructing nanoelectronic/nanophotonic devices and protein/ligand nanoarrays. This Review first summarizes the currently available DNA tile toolboxes and further emphasizes recent developments toward self-assembling DNA nanostructures with increasing complexity. Exciting progress using DNA tiles for directed self-assembly of other nanometer scale components is also discussed.  相似文献   
987.
A series of aryl and ferrocenyl pyrogallol[4]arenes have been synthesized by the HCl-catalyzed condensation reactions of pyrogallol with aromatic aldehydes and ferrocenecarbaldehyde. The fully acetyl and ethoxycarbonylmethoxy derivatives were also prepared and fully characterized. The crystal structures show that acylated phenyl pyrogallol[4]arene exists in rctt (cis-trans-trans) configuration, while the corresponding ferrocenyl pyrogallol[4]arene in rccc (all cis) configuration.  相似文献   
988.
SYNTHESIS OF ASILICA-SUPPORTED CARBOXYMETHYL CELLULOSE PLATINUM COMPLEX AND ITS CATALYTIC BEHAVIORS FOR HYDROGENATION OF ETHY...  相似文献   
989.
A novel lanthana-promoted nickel catalyst supported on silica for the liquid phase hydrogenation of m-dinitrobenzene to m-phenylenediamine was prepared by an incipient wetness sequential impregnation method. It was found that Ni-La/SiO2 catalyst exhibited high activity and stability for m-dinitrobenzene hydrogenation. Over this catalyst, the conversion of m-dinitrobenzene and the yield of m-phenylenediamine were up to 97.1% and 93.5%, respectively,at 373 K and 2.6 MPa hydrogen pressure after reaction for 1 h.  相似文献   
990.
Au colloids were used to fabricate nanoscale-tunable Au nanofilms on silicon for surface-enhanced IR absorption bases in both ambient and electrochemical environments. This wet process incorporates the self-assembly of colloidal Au monolayer using 3-aminopropyl trimethoxysilane as the organic coupler with subsequent chemical plating in an Au(III)/hydroxylamine solution. FTIR spectroscopy in transmission mode of the probe species SCN- was used to evaluate the apparent surface enhancement in IR absorption of 2D Au colloid arrays and chemically plated Au particles. The nanostructure of Au films was examined by atomic force microscopy. The IR and AFM results show that the apparent surface enhancement factor (1-2 orders of magnitude) increases with increasing sizes and/or contact, and the severe aggregation of Au nanoparticles may cause the bipolar band shape. Cyclic voltammetry on the Au nanofilm obtained by the above nucleation and growth strategy exhibits a feasible electrochemical stability and behavior. In situ ATR-FTIR measurement of p-nitrobenzoic acid adsorption demonstrates that the as-grown Au film yields rather promising surface enhancement as well.  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