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   3356篇
  免费   414篇
  国内免费   331篇
化学   949篇
晶体学   7篇
力学   156篇
综合类   49篇
数学   1104篇
物理学   482篇
无线电   1354篇
  2024年   8篇
  2023年   27篇
  2022年   80篇
  2021年   104篇
  2020年   106篇
  2019年   91篇
  2018年   96篇
  2017年   119篇
  2016年   148篇
  2015年   161篇
  2014年   278篇
  2013年   243篇
  2012年   260篇
  2011年   264篇
  2010年   208篇
  2009年   239篇
  2008年   229篇
  2007年   217篇
  2006年   188篇
  2005年   186篇
  2004年   136篇
  2003年   127篇
  2002年   100篇
  2001年   62篇
  2000年   56篇
  1999年   63篇
  1998年   44篇
  1997年   52篇
  1996年   34篇
  1995年   27篇
  1994年   36篇
  1993年   14篇
  1992年   13篇
  1991年   10篇
  1990年   12篇
  1989年   5篇
  1988年   10篇
  1987年   5篇
  1986年   1篇
  1985年   9篇
  1984年   5篇
  1983年   6篇
  1982年   6篇
  1981年   6篇
  1979年   6篇
  1978年   2篇
  1977年   1篇
  1969年   1篇
排序方式: 共有4101条查询结果,搜索用时 953 毫秒
41.
1. INTRODUCTION Microbial pollution will bring about various problems in industry and other vital fields, such as causing decomposing of materials, harming people抯 health. In order to reduce these problems, new antibacterial materials have been demanded. Recently, much attention has been paid to inorganic materials including zinc oxide [1~4]. These inorganic antibacterial materials are now substituting for organic materials to avoid releasing noxious organic molecules harmful to humans;…  相似文献   
42.
以共沉淀法合成了硅铝系载体(AS和SA),并采用改性天然高铝石为原料合成了矿物质载体(MC-1和MC-2),采用浸渍法合成了镍负载型催化剂。用常压固定床管式反应器,考察了液化石油气水蒸气重整反应活性,通过XRD、物理吸附等手段对催化剂进行了表征。结果表明,以MC-1、MC-2和SA、AS为载体的催化剂的水蒸气重整活性明显高于目前普遍使用的Ni/α-Al2O3催化剂。以SA为载体,负载5%镍的催化剂具有最好的水蒸气重整活性。在750 ℃,水碳比为2.0的条件下转化率达100%,C1选择性达94.46%。XRD分析结果表明1 300 ℃焙烧后的SA载体中存在α-Al2O3晶型和莫来石结构。硅、铝添加次序对水蒸气重整活性影响较大,硅-铝添加次序的催化剂样品的活性高于铝-硅添加次序的样品,C1选择性提高12.7%。  相似文献   
43.
Dispersion of MoO3, NiO, ZnO on rutile TiO2 with low specific surface area was studied with Mercury Porosimeter, SEM, XPS and Ammonia Extraction method. The dispersion thresholds of MoO3, NiO, ZnO on three rutile TiO2 carriers were obtained with XPS, and com-pared with those on anatase TiO2 with high specific surf are area. Ammonia Extraction method was used to identify the surface oxide species interarting with support surface in different strength and it was found that the proportions of oxides that can not be extracted by ammonia extraction are different for MoO3, NiO and ZnO which are supported on rutile TiO2.  相似文献   
44.
在载体堇青石表面涂覆一层LaAlO3作为第二载体,用浸渍法制备负载型LaMnO3、LaCoO3等钙钛矿型燃烧催化剂时,发现能促进LaMnO3等活性相的生长,从而提高了二甲苯完全氧化的催化活性;对于负载型LaCo3O3催化剂,可避免因用γ-Al2O35作第二载体而生成的低活性CoAl2O4类尖晶石相,因而有利于提高催化剂的耐热性。  相似文献   
45.
A capillary gas chromatographic method is described for the quantitative determination of liquid paraffin in blood. Paraffin is extracted from blood into n-heptane. After solvent evaporation and dissolution of the residue in 100–200 μl n-heptane one μl is injected into a gas chromatograph fitted with a fused silica capillary column (Permabond® OV-1-CB-0.1, 10 m × 0.32 mm i.d.) and flame ionization detector. Analysis is performed by using an oven program [50°C (3 min)?285°C (5 min), rise 10%min]. The sensitivity (1.5 ng hexadecane) and the reproducibility prove the applicability of the method for the determination of liquid paraffin in blood and for the study of the stability of the liquid paraffin hollow fiber membranes used in an extracorporeal liver support system.  相似文献   
46.
 Any analytical data is used to provide information about a sample. The "possible error" of the measurement can be of extreme importance in order to have complete information. The measurement uncertainty concept is a way to achieve quantitative information about this "possible error" using an estimation procedure. On the basis of the analytical result, the chemist makes a decision on the next step of the development process. If the uncertainty is unknown, the information is not complete; therefore this decision might be impossible. The major problem for the in-process control (IPC) procedure is that not only the repeatability but also the intermediate precision (which expresses the variations within laboratories related to different days, different analysts, different equipment, etc.) has to be good enough to make a decision. Unfortunately, the statistical information achieved from one single analytical run only gives information about the repeatability. This paper shows that the estimation of the measurement uncertainty for IPC is a way to solve the problem and gives the necessary information about the quality of the procedure. An example demonstrates that an estimate of uncertainty based on the standard deviations of an analytical method gives a value similar to one based on the standard deviations obtained from a control chart. Therefore, the estimation is both a very useful and also a very cost-effective tool. Though measurement uncertainty cannot replace validation in general, it is a viable alternative to validation for all methods that will never be used routinely. Received: 24 May 1996 Accepted: 10 August 1996  相似文献   
47.
玻璃载体表面脱氧核糖核酸的固定及其化学发光检测   总被引:2,自引:0,他引:2  
张国军  周宜开 《分析化学》1998,26(10):1205-1208
用硅烷化偶联剂把DNA直接共价固定在载玻片表面,将辣根过氧化物酶标记的探针与之进行核酸杂交,杂交后用增强的化学发光检测。方法的检出限为75pg。研究了DNA分子固定在玻璃载体表面的各种条件,并建立了在玻璃载体表面进行核酸杂交的体系,为研究光纤DNA生物传感器打下了基础。  相似文献   
48.
A series of Ti/Mg supported catalysts are prepared by using ball-milled mixtures of MgCl2-ethanol adducts and NaCl as supports, and 1-hexene polymerizations catalyzed by the novel catalysts are studied. It is found that the molecular weight distribution of poly(1-hexene) becomes apparently narrower when catalysts with doped supports are used, indicating that changing the structure of the support is an effective way to regulate the active center distribution of heterogeneous Ziegler-Natta catalyst.  相似文献   
49.
Yoichi M.A Yamada 《Tetrahedron》2004,60(18):4097-4105
The efficient heterogeneous Heck reaction was achieved by a new networked and supramolecular catalyst PdAS-V (1b). Employing of PdAS-V in 5.0×10−5 mol equiv. efficiently progressed the heterogeneous Heck reaction of a series of aryl iodides with acrylates, styrenes and acrylic acid. PdAS-V was successfully recycled five times without any decrease in its activity, and showed good stability in toluene and water, and hence the Heck reaction was efficiently performed in both reaction media. The use of 8.0×10−7 mol equiv. of PdAS-V resulted in the coupling product in 92% yield with the turnover number (TON) and the turnover frequency (TOF) of PdAS-V reached up to 1,150,000 and 12,000, respectively. The efficient synthesis of resveratrol was achieved via the PdAS-V-promoted Heck reaction.  相似文献   
50.
1,4-二乙氧基苯的相转移催化合成   总被引:1,自引:0,他引:1  
采用大孔型交联聚苯乙烯为载体研究制得新型季Lin盐型相转移催化剂Y98B,并将其于1,4-二乙氧基苯的催化合成反应,进行了产品的红外光谱分析和熔点测定,探讨了催化剂用量、对苯二酚与溴乙烷物的质量比、反应温度和反应时间等动力学条件对产品收率的影响。确定了在反应温度为70℃,mol对苯二酚:mol溴乙烷=1:3.5,反应时间4h,催化剂用量3.0g的条件下产品收率可达84.3%。  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