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   1703篇
  免费   340篇
  国内免费   219篇
化学   1151篇
晶体学   45篇
力学   118篇
综合类   16篇
数学   204篇
物理学   728篇
  2024年   3篇
  2023年   41篇
  2022年   90篇
  2021年   76篇
  2020年   82篇
  2019年   73篇
  2018年   68篇
  2017年   72篇
  2016年   82篇
  2015年   87篇
  2014年   114篇
  2013年   107篇
  2012年   159篇
  2011年   169篇
  2010年   116篇
  2009年   123篇
  2008年   119篇
  2007年   80篇
  2006年   100篇
  2005年   69篇
  2004年   51篇
  2003年   49篇
  2002年   55篇
  2001年   35篇
  2000年   16篇
  1999年   26篇
  1998年   26篇
  1997年   29篇
  1996年   21篇
  1995年   12篇
  1994年   17篇
  1993年   15篇
  1992年   5篇
  1991年   7篇
  1990年   4篇
  1989年   11篇
  1988年   9篇
  1987年   5篇
  1986年   7篇
  1985年   13篇
  1984年   4篇
  1983年   3篇
  1982年   2篇
  1976年   1篇
  1975年   1篇
  1974年   1篇
  1973年   4篇
  1972年   1篇
  1969年   1篇
  1961年   1篇
排序方式: 共有2262条查询结果,搜索用时 15 毫秒
71.
2, 5-呋喃二甲酸二甲酯(DMFDCA)这一生物质衍生的增值化学品是石油基聚合物单体对苯二甲酸(TPA)的理想替代品。本研究采用一步共热解法合成了两种廉价金属修饰的氮掺杂多孔碳催化剂CoMn@NC,并将其用于5-羟甲基糠醛(HMF)在温和条件下的需氧氧化。由Co3Mn2@NC-800催化HMF在50 ℃和常压氧气的条件下反应12 h后,得到产率为85%的DMFDCA。多孔催化剂的高比表面积提高了传质效率。Co纳米粒子(NPs)和呈原子级分散的Mn与掺杂在碳中的氮配位形成M―Nx。富含吡啶氮的碳基体中的缺电子金属位点有利于HMF和氧的活化。氧形成的超氧自由基阴离子的存在确保了半缩醛中间体和5-(羟基甲基)-2-糠酸甲酯(HMMF)的羟甲基的脱氢氧化,从而高选择性得到DMFDCA。该催化剂性能稳定,可适用于各种取代芳醇。该催化体系具有用于生产聚合物单体羧基酯的应用潜力。  相似文献   
72.
运用密度泛函理论(DFT)对可待因及福尔可定进行几何构型优化,从而对标准品粉末的拉曼光谱的振动模式进行指认和归属,并与低浓度水平标准溶液的表面增强拉曼谱图进行比较。进一步优化了表面增强拉曼光谱检测条件,并摸索可待因及福尔可定的测定下限及定量分析的可行性。结果表明,可待因及福尔可定大部分特征峰拉曼位移的理论计算值、拉曼光谱测定值、表面增强拉曼光谱测定值是一致的,但会有一定程度的蓝移和红移;可待因、福尔可定的测定下限均为10 mg·L^(-1)。可待因在631.29 cm^(-1)和1 595.26 cm^(-1)处、福尔可定在628.58 cm^(-1)和1 251.41 cm^(-1)处的特征峰强度比值,与其对应的质量浓度(40~100 mg·L^(-1))呈线性关系。对空白基质进行加标回收试验,可待因和福尔可定的回收率分别为99.0%~105%和102%~104%,测定值的相对标准偏差(n=5)分别为5.3%,5.9%。上述方法可为这两种管制药品提供拉曼光谱检测的理论依据和快检支持。  相似文献   
73.
The effect of a thermal annealing treatment on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and fullerene (C60) composites is investigated. Upon thermal annealing at 120 °C, short-circuit current and power conversion efficiency (η) are more than tripled, while a sharp rise by eight times in and η is found for the device annealed at 200 °C. It is concluded that the improved phase separation between MEH-PPV and C60 leads to the enhancement of and η at 120 °C, while thermodynamic molecule arrangement at the higher temperature of ∼200 °C induces a significant increase in all photovoltaic parameters of composite devices except the open-circuit voltage .  相似文献   
74.
According to the drug hybridization principle, a series of novel 1,5-diaryl substituted pyrazole secnidazole ester derivatives ( 6aa – 6gc ) have been synthesized by the combinations of various 1,5-diarylpyrazole-3-carboxylic acids with secnidazole. The in vitro antitumor/cytotoxicities activities against tumor and normal cell lines, including NCI-H460 (lung tumor cell), MCG-803 (gastric tumor cell), Skov-3 (ovarian tumor cell), BEL-7404 (liver tumor cell) and HL-7702 (normal liver cell), have been evaluated using MTT assay. All compounds showed promising inhibitory activities against four tumor cell lines. The IC50 of 6bc against the BEL-7404 cell was 2.03 μM, and those of 6fc against the NCI-H460, MCG-803 and Skov-3 were 1.34, 0.14, and 0.87 μM, respectively. All these values were much lower than those of the cisplatin. Furthermore, 6fc and 6bc were also verified to be considerably safe for normal human liver cell, since the lower IC50 values than cisplatin. Based on these results, the cell cycle analysis, apoptosis ratio detection and mitochondrial membrane potential assay of 6fc and 6bc were further performed aiming to investigate their inhibition mechanism of BEL-7404 cells. It is revealed that they have effectively inhibited the cell growth by arresting the BEL-7404 cells at S phase and induced apoptosis through the mitochondria-mediated pathway.  相似文献   
75.
Graded‐index plastic optical fibers, composed of doped polymers, have advantages over conventional glass optical fibers, but need to be developed further for practical application. Here, a variety of aromatic sulfide dopants were synthesized, and their effects on the refractive indexes and glass transition temperatures (Tg) of poly(methyl methacrylate) and methyl 2‐chloroacrylate/2,2,2‐trichloroethyl methacrylate copolymers were studied. While polymers containing large dopants exhibited relatively high refractive indices, their Tg values were low, making these materials unsuitable for graded‐index plastic optical fiber applications. Six dopants yielded polymers that exhibited higher Tg values than the conventionally used (diphenyl sulfide)‐doped polymer. The dopant dibenzothiophene, in particular, yielded polymers with the highest refractive indexes and Tg values, and polymers containing (phenylthio)benzene dopants also performed well. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
76.
Porous organic frameworks perform a variety of functions, owing to their extremely large surface areas, but the dynamics of the structural elements have never been explored. Our discovery of ultra‐fast molecular rotors (106 Hz at 225 K) in their architectures allows us to look at them from a new perspective. The constructive elements are robust struts and rapid rotors, resulting in a dynamic material whose motion can be frozen or released at will. The rotational motion can be actively regulated in response to guests. As the temperature is increased, the rotors spin ever faster, approaching free‐rotational diffusion at 550 K. The unusual combination of remarkable nanoporosity with fast dynamics is intriguing for engineering oscillating dipoles and producing responsive materials with switchable ferroelectricity, and for applications spanning from sensors to actuators, which capture and release chemicals on command.  相似文献   
77.
固相萃取-高效液相色谱法测定果蔬中异菌脲残留量   总被引:1,自引:0,他引:1  
建立了固相萃取-高效液相色谱法测定果蔬中异菌脲残留量的方法。样品用乙腈提取,C18固相萃取柱净化,甲醇-水(70:30,V/V)为流动相,经C18液相色谱柱分离,DAD紫外检测器(218 nm)检测。结果表明:异菌脲在0.1~2.0 mg/L范围内线性关系良好(R2=0.9998),方法定量限(以S/N=10计)为0.05 mg/kg,在0.05,0.1,0.5 mg/kg添加水平下的加标回收率范围为81.3%~98.3%,相对标准偏差(n=6)为3.9%~8.9%。方法适用于大多数水果、蔬菜中异菌脲残留量的测定。  相似文献   
78.
A convenient reproducible technique is reported for the fabrication of large‐area gold semishell arrays by mechanically pressing porous anodic alumina (PAA) stamps into gold/polymer bilayer structures that serve as robust and cost‐efficient surface‐enhanced Raman‐scattering (SERS) substrates. The surface structure can be tuned further to optimize the enhancement factor according to optional PAA fabrication parameters and imprinting pressures. Finite‐difference time‐domain calculations indicate that the structure may possess excellent SERS characteristics due to the high density and abundance of hot spots.  相似文献   
79.
纳米结构Au/Fe_2O_3的制备、表征及催化氧化性能   总被引:1,自引:0,他引:1  
分别以纳米和块状氧化铁为载体,通过沉积沉淀法制备了Au/Nano-Fe2O3和Au/Bulk-Fe2O3,并对其进行了表征和催化氧化性能测试。结果表明:对于Au/Nano-Fe2O3,5 nm Au颗粒被尺寸相当的Nano-Fe2O3所包覆,形成新颖的类似核壳结构;对于Au/Bulk-Fe2O3,3 nm Au颗粒高度分散于Bulk-Fe2O3的表面。在1-苯乙醇的氧化反应中,Au/Nano-Fe2O3显示出比Au/Bulk-Fe2O3更好的催化活性。活性的增强主要与小尺寸的Nano-Fe2O3以及Au和Nano-Fe2O3更大的接触界面有关。相比于广泛受到重视的Au的尺寸效应来说,对于Au/Nano-Fe2O3而言,Fe2O3尺寸的影响更大。  相似文献   
80.
滕腾  陈进  陈旭林  余荣民  卢灿忠 《结构化学》2014,33(11):1661-1665
A three-coordinate Ag(I) complex, [Ag(Mepzpy)(PAr3)]BF4·H2O(1, Mepzpy = 3-methyl-1-(2-pyridyl)pyrazole, PAr3 = tri(o-tolyl)phosphine), was synthesized from the reaction of Ag(CH3CN)4BF4, PAr3 and Mepzpy in CH3 CN at room temperature. The compound was characterized by UV-vis, NMR and X-ray single-crystal structure analysis. It crystallizes in triclinic space group P1 with a = 10.2251(5), b = 10.6014(5), c = 15.7012(5) , α = 92.963(3), β = 92.641(3), γ = 114.647(4)o, V = 1540.6(1) 3, Z = 2, Mr = 676.24, Dc = 1.458 g/cm3, F(000) = 688, μ = 6.187 mm-1, GOOF = 1.038, the final R = 0.0530 and w R = 0.1493 for 6371 observed reflections with I 2σ(I). Compound 1 is ionic. It is composed of a BF4- anion and a [Ag(Mepzpy)(PAr3)]+ cation. The Ag(I) ion adopts a distorted trigonal pyramidal coordination geometry defined by two nitrogen atoms and a phosphorous atom. The complex emits blue luminescence with maximum peaks at 470 nm in solid state at room temperature.  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