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   90089篇
  免费   1134篇
  国内免费   433篇
电工技术   865篇
综合类   2318篇
化学工业   12755篇
金属工艺   5035篇
机械仪表   3194篇
建筑科学   2254篇
矿业工程   569篇
能源动力   1406篇
轻工业   3837篇
水利工程   1312篇
石油天然气   356篇
无线电   10147篇
一般工业技术   17642篇
冶金工业   3461篇
原子能技术   325篇
自动化技术   26180篇
  2023年   69篇
  2022年   150篇
  2021年   174篇
  2020年   157篇
  2019年   147篇
  2018年   14587篇
  2017年   13497篇
  2016年   10118篇
  2015年   703篇
  2014年   422篇
  2013年   583篇
  2012年   3343篇
  2011年   9666篇
  2010年   8474篇
  2009年   5790篇
  2008年   6979篇
  2007年   7936篇
  2006年   314篇
  2005年   1351篇
  2004年   1244篇
  2003年   1295篇
  2002年   661篇
  2001年   221篇
  2000年   303篇
  1999年   167篇
  1998年   313篇
  1997年   208篇
  1996年   173篇
  1995年   138篇
  1994年   116篇
  1993年   141篇
  1992年   119篇
  1991年   120篇
  1990年   84篇
  1989年   75篇
  1988年   81篇
  1987年   75篇
  1986年   88篇
  1985年   106篇
  1984年   99篇
  1983年   95篇
  1982年   78篇
  1981年   63篇
  1978年   49篇
  1977年   62篇
  1976年   82篇
  1968年   60篇
  1966年   49篇
  1955年   64篇
  1954年   69篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
111.
112.
We recently diagnosed subcutaneous fat necrosis in a newborn female infant using fine needle aspiration cytology. The neonate had perinatal asphyxia and her mother a difficult labor, but otherwise the neonate was well developed and healthy and within a few days developed well-defined areas of subcutaneous induration all over. On cytologic examination of the aspirated material, many necrotizing fat cells were found with refractile, needle-shaped crystals arranged in a sheaflike or starburst pattern. Subcutaneous fat necrosis of the newborn is uncommon today, and although the histologic findings from excised tissue are known, this appears to be the first case in which the diagnosis was possible from a sample of fine needle aspirate.  相似文献   
113.
A reverse phase high-performance liquid chromatography method for the quantitation of sulfacetamide, sulfadiazine, sulfamerazine, and sulfamethazine in various combinations has been developed. The method is simple, accurate, precise and reproducible. The percent relative standard deviations based on 6 injections were 2.1, 0.6, 1.9, and 1.6 for sulfacetamide, sulfadiazine, sulfamerazine, and sulfamethazine, respectively. The ratio of peak heights (drug/internal standard) wer closely related (r value 0.99 or better) to concentrations (± 20% of the standard solution concentrations). The results of synthetic mixtures showed quantitative recovery and method was successfully applied to commercial dosage forms (tablets and suspension). Extraction of sulfa drugs from the dosage forms required a very simple procedure.  相似文献   
114.
115.
116.
Nickel and chromium are essential ingredients in alloys increasingly important for energy-efficient, environmentally friendly modern technology. Quantitative assessment of the flows of these metals through the world economy from resource extraction to final disposal informs resource policy, energy planning, environmental science, and waste management. This article summarizes the worldwide technological cycles of nickel and chromium in 2000. Stainless steel is the major use of these metals, but they serve numerous other special needs, as in superalloys for high-temperature service, as plating materials, and in coinage. Because they are used primarily in alloys, novel recycling issues arise as their use becomes more widespread. “... the great New York and St. Louis double track, nickel plated railroad...” — Norwalk, Ohio, Chronicle 10 March 1881 announces arrival of surveyors for the future Nickel Plate Railway “Later [1911] I formed an alloy of Iron and Chromium, which showed remarkable resistance to rust and tarnish ... [It was] rediscovered by an Englishman named Brearley, in 1914.” —Elwood Haynes to Stephen F. Roberts, 17 January 1925  相似文献   
117.
A comparative study for the nucleation of diamond was carried out using surface treatment like (i) surface scratching with 1 μm diamond paste and (ii) surface etching using chlorine plasma at different RF powers (50, 100 and 150 W). Atomic force microscopic study shows variation in roughness from 31 nm to 110 nm. Scratching results in random scratches, whereas plasma etches a surface uniformly. Scanning electron microscopic observations show well faceted crystallites with a predominance of angular shaped grains corresponding to 〈100〉 and 〈110〉 crystallite surfaces for the scratched as well as plasma etched substrate. Surface etching at 150 W plasma power results in a better growth in comparison with 50 and 100 W plasma powers. Chlorine-radical is found responsible for the changes in the growth morphology. Raman spectroscopy shows a sharp peak at 1,332 cm−1 and a peak at ∼1,580 cm−1 for both samples.  相似文献   
118.
To achieve higher fatigue resistance against subsurface crack generation, both the refinement of grain structure and the introduction of mobile dislocations on various slip systems have been shown to be effective in the 32Mn-7Cr austenitic steel. A novel treatment which consisted of cold grooved rolling and partial recrystallization was introduced to modify the microstructure. High-cycle fatigue properties and fatigue-crack generation were investigated for both the solution-treated (ST) and the partially recrystallized (PR) materials at 77 K. The PR material displayed higher fatigue strength than the ST material, especially in the high-cycle regime. No subsurface crack generation was detected for the PR material; however, it appeared in the lower peak stress and/or in the longer-life range for the ST material. Intergranular facets formed a subsurface crack initiation site in the ST material. Since the dislocation structure that developed in the fatigued PR material assisted homogeneous and multidirectional plastic deformation, the localized deformation and/or the stress concentration at the grain boundaries by coplanar arrays were believed to be relieved. Therefore, intergranular cracking due to incompatibility at a grain boundary may disappear.  相似文献   
119.
Concurrent and reactive programs are specified by their behaviours in the presence of a nondeterministic environment. In a natural way, this gives a specification (ARW) of an atomic variable in the style of Abadi and Lamport. Several implementations of atomic variables by lower level primitives are known. A few years ago, we formulated a criterion to prove the correctness of such implementations. The proof of correctness of the criterion itself was based on Lynch’s definition of atomicity by serialization points. Here, this criterion is reformulated as a specification HRW in the formal sense. Simulations from HRW to ARW and vice versa are constructed. These now serve as a constructive proof of correctness of the criterion. Eternity variables are used in the simulation from HRW to ARW. We propose so-called gliding simulations to deal with the problems that appear when occasionally the concrete implementation needs fewer steps than the abstract specification.  相似文献   
120.
The paper describes some geotechnical properties of the soft soils in Guangzhou College City, China, which are difficult to both sample and test. Laboratory and in situ tests were carried out to assess the physical and mechanical indices of the soils. The data were statistically analyzed and linear regression undertaken such that equations could be developed by which the geotechnical properties of the soft soils can be predicted. The statistical validity of the degree of correlation confirmed that using these equations, the mechanical indices can be estimated from physical indices determined by routine testing.   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