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   3104篇
  免费   18篇
  国内免费   2篇
电工技术   5篇
综合类   3篇
化学工业   252篇
金属工艺   23篇
机械仪表   26篇
建筑科学   26篇
矿业工程   1篇
能源动力   17篇
轻工业   151篇
水利工程   8篇
无线电   67篇
一般工业技术   138篇
冶金工业   2292篇
原子能技术   13篇
自动化技术   102篇
  2022年   8篇
  2021年   18篇
  2020年   11篇
  2019年   9篇
  2018年   13篇
  2017年   12篇
  2016年   19篇
  2015年   8篇
  2014年   20篇
  2013年   30篇
  2012年   33篇
  2011年   52篇
  2010年   29篇
  2009年   36篇
  2008年   49篇
  2007年   35篇
  2006年   46篇
  2005年   35篇
  2004年   20篇
  2003年   29篇
  2002年   19篇
  2001年   21篇
  2000年   15篇
  1999年   97篇
  1998年   805篇
  1997年   433篇
  1996年   272篇
  1995年   142篇
  1994年   135篇
  1993年   130篇
  1992年   33篇
  1991年   39篇
  1990年   26篇
  1989年   35篇
  1988年   27篇
  1987年   38篇
  1986年   30篇
  1985年   39篇
  1984年   15篇
  1983年   10篇
  1982年   8篇
  1981年   15篇
  1980年   23篇
  1978年   8篇
  1977年   41篇
  1976年   87篇
  1975年   7篇
  1974年   7篇
  1973年   7篇
  1968年   8篇
排序方式: 共有3124条查询结果,搜索用时 15 毫秒
51.
The catalytic activity and selectivity of heterodonor phosphine rhodium catalysts prepared in situ were tested in the hydroformylation of functional alkenes (ethyl acrylate, methyl methacrylate, styrene, 4-vinyl-1-cyclohexene, dicyclopentadiene and cis-1,2,3,6-tetrahydrophthalic anhydride). Systematic variation of the heterodonor atom in the ortho position of the ligand showed that the heterodonor atom has a significant influence on the activities and selectivities of the reaction. However, the activity seems to depend mainly on the modifying ligand, and the regioselectivity mainly on the substrate (i.e., the structure and functionality of the alkene). Nevertheless, regioselective control is only obtained through synergy between the substrate and the catalyst. Clear regiocontrol was observed in the hydroformylation of ,-unsaturated esters and styrene with an in situ formed o-(thiomethylphenyl)diphenylphosphine rhodium catalyst.  相似文献   
52.
In the present work, the process of biodiesel production in a pilot plant has been studied using beef tallow as raw materials with methanol and potassium hydroxide as catalyst. The biodiesel quality is regulated by Brazilian specifications (Resolution 42) by the National Agency of Petroleum (ANP). The alkaline transesterification of animal fat with methanol produces a biodiesel with high quality and also with a good conversion rate. The process is possible but the economical viability must be improved by recovering methanol and glycerin. The obtained results have been used for industrial scale up of the process.  相似文献   
53.
The esterification of benzoic acid with n‐hexanol, n‐octanol, 2‐ethyl hexanol and n‐decanol was investigated in detail. An analysis of the reaction kinetics of esterification in the presence of different commercially available chemical catalysts was carried out. The effects of catalyst type and loading on the reaction rate were studied. Although the considered reaction is bimolecular, it showed a first‐order behavior, and a linear dependence with respect to the catalyst concentration was observed. Hence, a new approach is presented to describe the reaction kinetics accurately over a wide range. The application of biotechnological synthesis applying different enzymes as catalysts offers an interesting alternative besides chemical synthesis. Especially an esterase from Bacillus subtilis immobilized on Sepabeads EC‐EP showed high stability and was applied for 2 days in the synthesis of hexyl benzoate. Nevertheless, the chemical reaction route remains superior with respect to the catalyst activities under the applied conditions, which were 25 kU/g for the chemical reaction and 0.7 kU/g for the best enzymatic conversion.  相似文献   
54.
Rare pediatric non-compaction and restrictive cardiomyopathy are usually associated with a rapid and severe disease progression. While the non-compaction phenotype is characterized by structural defects and is correlated with systolic dysfunction, the restrictive phenotype exhibits diastolic dysfunction. The molecular mechanisms are poorly understood. Target genes encode among others, the cardiac troponin subunits forming the main regulatory protein complex of the thin filament for muscle contraction. Here, we compare the molecular effects of two infantile de novo point mutations in TNNC1 (p.cTnC-G34S) and TNNI3 (p.cTnI-D127Y) leading to severe non-compaction and restrictive phenotypes, respectively. We used skinned cardiomyocytes, skinned fibers, and reconstituted thin filaments to measure the impact of the mutations on contractile function. We investigated the interaction of these troponin variants with actin and their inter-subunit interactions, as well as the structural integrity of reconstituted thin filaments. Both mutations exhibited similar functional and structural impairments, though the patients developed different phenotypes. Furthermore, the protein quality control system was affected, as shown for TnC-G34S using patient’s myocardial tissue samples. The two troponin targeting agents levosimendan and green tea extract (-)-epigallocatechin-3-gallate (EGCg) stabilized the structural integrity of reconstituted thin filaments and ameliorated contractile function in vitro in some, but not all, aspects to a similar degree for both mutations.  相似文献   
55.
A ternary system consisting of cylcotetramethylene tetranitramine (octogen, HMX) as the solute, acetone respectively γ-butyrolactone as solvents and carbon dioxide as the anti-solvent was used to investigate the formation of fine particles with the GAS-process as well as the influence of the solvents upon the particle morphology under the same operating conditions. The precipitated particles were examined by infrared radiation spectroscopy (IR), laser light diffraction and light-optical microscopy. The results show that octogen with a narrow particle size distribution was obtained. The crystals are of the same modification as the raw material and of hight purity. The modification and the particle size are influenced by the solvent used for the recrystallization.  相似文献   
56.
57.
A hydroxymethylated resorcinol (HMR) was used as a coupling agent to improve glueability of silicone (aminofunctional polydimethylsiloxane) modified pine sapwood (Pinus sylvestris) with polyvinyl acetate (PVAc). Priming with HMR did not significantly increase tensile shear strength of unmodified and silicone treated wood under dry conditions. Under wet conditions, tensile shear strength of the control specimens was increased by 37% and of the silicone modified specimens by 13%. Despite of higher shear strength under wet conditions, the primed silicone treated specimens displayed lower degree of wood failure than the unprimed treated specimens.  相似文献   
58.
Vertically integrated amorphous silicon color sensor arrays   总被引:3,自引:0,他引:3  
Large-area color sensor arrays based on vertically integrated thin-film sensors were realized. The complete color information of each color pixel is detected at the same position of the sensor array without using optical filters. Sensor arrays consist of amorphous silicon thin-film color sensors integrated on top of amorphous silicon readout transistors. The spectral sensitivity of the sensors is controlled by the applied bias voltage. The operating principle of the color sensor arrays and the influence of device design on spectral sensitivity are described. Furthermore, the image quality of the sensor arrays is analyzed by measurements of the line spread function and the modulation transfer function.  相似文献   
59.
Objective: Evaluate the utility of the current 7-scale structure of the Life Situation Questionnaire—Revised (LSQ–R) using confirmatory factor analysis (CFA) and explore the factor structure of each set of items. Design: Adults (N = 1,543) with traumatic spinal cord injury (SCI) were administered the 20 satisfaction and 30 problems items from the LSQ–R. Results: CFA suggests that the existing 7-scale structure across the 50 items was within the acceptable range (root-mean-square error of approximation [RMSEA] = 0.078), although it fell just outside of this range for women. Factor analysis revealed 3 satisfaction factors and 6 problems factors. The overall fit of the problems items (RMSEA = 0.070) was superior to that of the satisfaction items (RMSEA = 0.80). RMSEA fell just outside of the acceptable range for Whites and men on the satisfaction scales. All scales had acceptable internal consistency. Conclusion: Results suggest the original scoring of the LSQ–R remains viable, although individual results should be reviewed for special population. Factor analysis of subsets of items allows satisfaction and problems items to be used independently, depending on the study purpose. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
60.
The purpose of this study was to see whether the social environment of the church is associated with changes in feelings of gratitude toward God over time. The theoretical thrust of the authors’ analyses is captured in the following conceptual linkages: (a) People who attend church frequently are more likely to believe that their congregation is highly cohesive; (b) individuals who worship in more cohesive congregations will receive more emotional support from fellow church members; (c) people who receive more emotional support from coreligionists will be more likely to feel they have a close spiritual connection with others; and (d) people who feel they are closely connected with other individuals will feel especially grateful to God. Data from a nationwide longitudinal survey of older adults provide support for each of these hypothesized relationships. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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