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   200405篇
  免费   2327篇
  国内免费   612篇
电工技术   3479篇
综合类   149篇
化学工业   29079篇
金属工艺   8064篇
机械仪表   6083篇
建筑科学   4849篇
矿业工程   1304篇
能源动力   5134篇
轻工业   17210篇
水利工程   2308篇
石油天然气   4040篇
武器工业   8篇
无线电   23100篇
一般工业技术   40106篇
冶金工业   37470篇
原子能技术   5089篇
自动化技术   15872篇
  2021年   1621篇
  2019年   1579篇
  2018年   2584篇
  2017年   2650篇
  2016年   2794篇
  2015年   1802篇
  2014年   3084篇
  2013年   8970篇
  2012年   5082篇
  2011年   7069篇
  2010年   5614篇
  2009年   6383篇
  2008年   6468篇
  2007年   6547篇
  2006年   5601篇
  2005年   5253篇
  2004年   5020篇
  2003年   4906篇
  2002年   4540篇
  2001年   4666篇
  2000年   4588篇
  1999年   4749篇
  1998年   11429篇
  1997年   8151篇
  1996年   6426篇
  1995年   4682篇
  1994年   4288篇
  1993年   4087篇
  1992年   3232篇
  1991年   3078篇
  1990年   2885篇
  1989年   2958篇
  1988年   2833篇
  1987年   2406篇
  1986年   2311篇
  1985年   2724篇
  1984年   2512篇
  1983年   2349篇
  1982年   2097篇
  1981年   2207篇
  1980年   2035篇
  1979年   2134篇
  1978年   2122篇
  1977年   2369篇
  1976年   3198篇
  1975年   1837篇
  1974年   1761篇
  1973年   1778篇
  1972年   1444篇
  1971年   1340篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Optical channel waveguides in silicon diffused from GeSi alloy   总被引:1,自引:0,他引:1  
A technique for fabricating low-loss and polarisation-independent channel waveguides in silicon is reported. The waveguides are obtained by Ge-indiffusion using either a GeSi alloy or a system of alternating Ge and Si layers. Typical fabrication parameters for single-mode waveguides are given.<>  相似文献   
992.
During the last decade, a substantial amount of progress has been made in the understanding of the relationship between the dietary constituents and the development of colon cancer in man. The information base is sufficiently convincing with respect to an enhancing effect as a function of total fat intake and a protective effect of certain dietary fibers in colon cancer. The populations with high incidence of colon cancer are characterized by consumption of high-dietary fat which may be a risk factor in the absence of factors that are protective, such as use of whole-grain cereals, high fibrous foods and vegetables mainly of cruciferous type. Application of the findings made thus far in colon cancer research for the general public is, therefore, to have a far-reaching impact on the major premature, killing diseases in the western world.  相似文献   
993.
Fermented salami was prepared containing 0, 9, 18 and 27% defatted sesame flour (DSF). Chemical and sensory analyses were performed on the products. Results indicated that DSF could be used at either the 9 or 18% level without detrimental effect upon sensory attributes but salami containing 27% DSF will have less desirable flavor.  相似文献   
994.
A dozen CuO samples prepared under various conditions and from different starting materials were evaluated as cathode materials for a primary Li/CuO cell. The “thin electrode” method was used for rapid evaluation of the samples. Both coulombic efficiency and discharge voltage depend considerably on the method of synthesis. No correlation was found between the specific surface area and the resistivity of the samples on the one hand and the cathode performance on the other. Best results were obtained from CuO prepared by the oxidation of Cu2O under controlled temperature and time of oxidation.  相似文献   
995.
996.
This newsletter provides information on past and upcoming events related to the IEEE Computer Society's Test Technology Technical Council and the test community.  相似文献   
997.
Alewives (Alosa pseudoharengus), the major prey fish for Lake Ontario, contain thiaminase. They are associated with development of a thiamine deficiency in salmonines which greatly increases the potential for developing an early mortality syndrome (EMS). To assess the possible effects of thiamine deficiency on salmonine reproduction we measured egg thiamine concentrations for five species of Lake Ontario salmonines. From this we estimated the proportion of families susceptible to EMS based on whether they were below the ED20, the egg thiamine concentration associated with 20% mortality due to EMS. The ED20s were 1.52, 2.63, and 2.99 nmol/g egg for Chinook salmon (Oncorhynchus tshawytscha), lake trout (Salvelinus namaycush), and coho salmon (Oncorhynchus kisutch), respectively. Based on the proportion of fish having egg thiamine concentrations falling below the ED20, the risk of developing EMS in Lake Ontario was highest for lake trout, followed by coho (O. kisutch), and Chinook salmon, with the least risk for rainbow trout (O. mykiss). For lake trout from western Lake Ontario, mean egg thiamine concentration showed significant annual variability during 1994 to 2003, when the proportion of lake trout at risk of developing EMS based on ED20 ranged between 77 and 100%. Variation in the annual mean egg thiamine concentration for western Lake Ontario lake trout was positively related (p < 0.001, r2 = 0.94) with indices of annual adult alewife biomass. While suggesting the possible involvement of density-dependent changes in alewives, the changes are small relative to egg thiamine concentrations when alewife are not part of the diet and are of insufficient magnitude to allow for natural reproduction by lake trout.  相似文献   
998.
New climate change scenarios for the Netherlands.   总被引:4,自引:0,他引:4  
A new set of climate change scenarios for 2050 for the Netherlands was produced recently. The scenarios span a wide range of possible future climate conditions, and include climate variables that are of interest to a broad user community. The scenario values are constructed by combining output from an ensemble of recent General Climate Model (GCM) simulations, Regional Climate Model (RCM) output, meteorological observations and a touch of expert judgment. For temperature, precipitation, potential evaporation and wind four scenarios are constructed, encompassing ranges of both global mean temperature rise in 2050 and the strength of the response of the dominant atmospheric circulation in the area of interest to global warming. For this particular area, wintertime precipitation is seen to increase between 3.5 and 7% per degree global warming, but mean summertime precipitation shows opposite signs depending on the assumed response of the circulation regime. Annual maximum daily mean wind speed shows small changes compared to the observed (natural) variability of this variable. Sea level rise in the North Sea in 2100 ranges between 35 and 85 cm. Preliminary assessment of the impact of the new scenarios on water management and coastal defence policies indicate that particularly dry summer scenarios and increased intensity of extreme daily precipitation deserves additional attention in the near future.  相似文献   
999.
1000.
Experiment 1 was to test effect of three ratios of energy to protein in complete mixed diets for 36 lactating cows in three, 28-d periods. Energy was varied with cottonseed hulls, pelleted ground corrugated boxes, and a mixture of the two. Crude protein was varied with soybean meal to give energy:crude protein of 5.7, 5.0, and 4.6 for each energy amount. Cottonseed meal was compared with soybean meal in corrugated box diets. Feed intake was much higher with cottonseed hulls, and appreciable feedlot bloat resulted from pelleted ground corrugated box diets. Data adjusted to equal feed intake showed significant effect of energy to crude protein ratio on milk yield and improved digestion of organic matter with soybean meal vs. cottonseed meal. Experiment 2 tested the hypothesis that lactating cows consuming high-protein alfalfa may benefit from supplemental protein. Diets were 50% forage. Six diets were 14 or 18% crude protein in three ratios of alfalfa hay to corn silage (0:100, 50:50, 100:0). Additional corn silage diets were to compare: 14 versus 18% protein from distiller's dried grains with solubles only and with .5 or .9% urea (four diets); two 14% protein diets compared .6% added potassium chloride with or without .5% urea. Thirty-six Holstein cows in early lactation received one of the 12 diets in each of three 28-d periods. Distiller's grains with solubles markedly depressed milk yield (2.2 kg/d) and milk protein (.22%); heat damage of distiller's grains was evident. Protein interacted with alfalfa so gain in milk from 18 versus 14% increased from .55 to 1.36 to 2.66 kg/d as alfalfa changed from 0 to 50 to 100%. Thus, crude protein of alfalfa was not as effective as that from soybean meal in supporting milk yield.  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