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   810768篇
  免费   9662篇
  国内免费   3203篇
电工技术   13927篇
综合类   3224篇
化学工业   118200篇
金属工艺   34871篇
机械仪表   25913篇
建筑科学   18183篇
矿业工程   5218篇
能源动力   20449篇
轻工业   57146篇
水利工程   9500篇
石油天然气   16688篇
武器工业   73篇
无线电   94283篇
一般工业技术   163705篇
冶金工业   139310篇
原子能技术   17714篇
自动化技术   85229篇
  2021年   6744篇
  2020年   5255篇
  2019年   6299篇
  2018年   24003篇
  2017年   23047篇
  2016年   20755篇
  2015年   7934篇
  2014年   12384篇
  2013年   33927篇
  2012年   22162篇
  2011年   34710篇
  2010年   28236篇
  2009年   28689篇
  2008年   30323篇
  2007年   31012篇
  2006年   21289篇
  2005年   20229篇
  2004年   19468篇
  2003年   19190篇
  2002年   17919篇
  2001年   17434篇
  2000年   16456篇
  1999年   17110篇
  1998年   43372篇
  1997年   30266篇
  1996年   23279篇
  1995年   17468篇
  1994年   15364篇
  1993年   15616篇
  1992年   11253篇
  1991年   10755篇
  1990年   10428篇
  1989年   10196篇
  1988年   9728篇
  1987年   8576篇
  1986年   8539篇
  1985年   9516篇
  1984年   8786篇
  1983年   8081篇
  1982年   7386篇
  1981年   7523篇
  1980年   7290篇
  1979年   7015篇
  1978年   7015篇
  1977年   7858篇
  1976年   10144篇
  1975年   6065篇
  1974年   5853篇
  1973年   5986篇
  1972年   5080篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Trace element analysis of Cretan wines and wine products   总被引:2,自引:0,他引:2  
The object of this research is to investigate the ways and the degree of contamination of Cretan grapes from the area of Chania and their alcoholic products, with the elements aluminium, arsenic, cadmium, copper, chromium, iron, lead, manganese, nickel and zinc. Fifteen samples of grapes were collected and used for the production of experimental wines from rinsed and unrinsed grapes. A microwave furnace was used for the digestion and dissolution of the experimental wines, the precipitates that originated in these wines, as well as the wines of the corresponding producers. The analyses of all mentioned samples as well as 34 local alcoholic distillates were performed using total reflection X-ray fluorescence and graphite furnace atomic absorption spectrometry. The concentrations for all the elements that were determined were almost in all cases, well below the maximum permissible levels by the Greek and the European Union legislation.  相似文献   
102.
A study has been made of the coagulation of a 50 mg l−1 aqueous suspension of kaolinite for a range of aluminium sulphate and hydrogen ion concentrations. The rate of flocculation was determined from jar tests after stirring for 1 h. The data permitted a stability limit diagram to be drawn from which some indication of the coagulation mechanisms of the system could be ascertained.  相似文献   
103.
With increasing use of saline water for irrigating urban landscapes, soil salinization is becoming a concern. This study examined spatial variation in soil salinity over the length of selected fairways at five golf courses, and of large turf areas at two public parks located in west Texas and southern New Mexico. Salinity of water used for irrigation ranged from 680 to 2700 mg L−1, and the sites consisted mostly of Aridisols (upland soils) or Entisols (alluvial soils). Soil salinity distribution at sites consisting of deep Aridisols was spatially independent with the coefficient of variability (CV) ranging from 24 to 42%. The sites consisting of shallow Aridisols over a calcic horizon had erratic and random soil salinity distributions with an average CV of 37%. Since soil salinity distributions in Aridisols and Alfisols appeared to be spatially independent within the distance of fairways, soil sampling adequacy can be determined by the conventional probability statistics. However, salinity distributions at Entisols sites were spatially dependant to a length of 100 m or more. In addition, the clayey Entisols used for public parks had high levels of salt accumulation (>10 dS m−1 in the saturation extract) with the CV exceeding 60%. When salinity readings were stratified by soil type distribution, the CV was reduced to 28% on the average. The number of samples required to obtain the mean salinity over a typical length of fairways (250–300 m) within a deviate range of 20% averaged 6 in deep sandy Aridisols and Alfisols, and 13 each in other cases, provided that sampling of Entisols is made based on soil type distributions. These sampling requirements are greater than those for field soil moisture or saturation water contents. The spacing to collect the required number of samples over the prevailing length of fairways was between 23 and 38 m. Soil sampling for salinity appraisal is most problematic in Entisols, but can be made simple if a detailed soil map is available. In Aridisols containing a calcic horizon, it is probable to have saline spots over poorly permeable caliche.  相似文献   
104.
The use of marble in Architecture began on the 7th century B.C. and continued to be used without stop untill today. Greece and especially the East Macedonia region is an extremely privileged area on marble deposits both in quantities and qualities and colours. In the present work the main physical and engineering properties of some varieties of greek marbles are given and a comparison with the international standards is made. The experimental material (marbles) was from East Macedonia, Argolida-Arcadia and Magnessia, three of the biggest marble deposit areas in Greece. From the results obtained, it is concluded that the marbles examined satisfy all the international standard codes and thus they can be used as building stones and decoration materials.  相似文献   
105.
The problem … maintain fuselage integrity and a survivable cabin environment during a post-crash fire. Its high melting point made titanium an attractive candidate for the large-scale test discussed here. Note: This paper is an adaptation of Report No. FAA-RD-71-3, “Titanium Fuselage Environmental Conditions in Post-Crash Fires,” prepared by the author for the Federal Aviation Administration. The report was also the basis for Mr. Sarkos’ presentation at the 75th Annual Meeting of the National Fire Protection Association in San Francisco on May 18, 1971.  相似文献   
106.
Results are presented for laboratory and field tests of weak saturated soils serving as beds for embankments in Ho Chi Min City and various regions of the Mekong River Delta (Vietnam). A method is outlined for quantitative evaluation of the stress-strain state (SSS) of the weak saturated clayey beds (plane problem). A method is described for quantitative evaluation of the deformation and stability of weak beds in the initial, intermediate, and stabilized stages of the formation and transformation of the SSS. __________ Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 2–6, September–October, 2005.  相似文献   
107.
Lead has numerous acute and chronic adverse effects on human beings. This is especially true for infants and children. The main path of lead ingestion in children can be different according to housing and living situation. The intake of lead through drinking water is commonly due to metal corrosion. The users plumbing can be an important factor. In recent years, many lead pipes in Germany have been replaced by pipes made of an alternative material. The aim of this study is to assess the present state of drinking water contamination and the resulting exposure of infants to lead. For this purpose mothers of new-born babies were offered a free examination of their drinking water. After a written declaration of consent had been obtained and after the infant in question had reached an age of 3 months, a stagnation sample of cold tap-water after overnight stagnation together with a random daytime sample was obtained from the family. The collected samples were analysed by atomic absorption spectrometry for their lead concentration. In total, 1485 samples from households were collected. Of the 1434 stagnation samples, 3.1% had lead concentrations greater than 0.01 mg/l (recommended limit of the WHO) and 0.6% had concentrations above the limit of the German drinking water regulation (0.04 mg/l). The values for the 1474 random daytime samples were 2.1% above 0.01 mg/l and 0.2% greater than 0.04 mg/l, respectively. By region, the areas Bovenden, Friedland, Duderstadt, Northeim and Rosdorf were particularly affected. The highest measured concentrations of lead in the stagnation samples were 0.11 mg/l and 0.15 mg/l in the random daytime samples, respectively.  相似文献   
108.
The aim of this study is to analyse and compare three variants of a family house in order to evaluate the total environmental impacts produced during the whole building life cycle. The first variant corresponds to the standard in force in Switzerland, the second alternative to the requirements of a quality control label for houses with low energy consumption and the third case is a very low energy consumption building. The three variants have the same architectural aspect but different insulation thicknesses and types, different energy production systems and the use of different renewable energies. The calculation of the environmental impacts is carried out by means of a life cycle analysis, which includes not only the impact related to the energy consumption during the occupancy stage, but also the materials manufacture, transport, replacement and elimination at the end of the building lifetime. The results are expressed with the Swiss and the UCTE (Union for the Co-ordination of Transmission of Electricity) electricity mix to analyse how they influence the building impact.  相似文献   
109.
110.
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