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   53042篇
  免费   3964篇
  国内免费   2079篇
电工技术   1801篇
技术理论   1篇
综合类   2553篇
化学工业   12816篇
金属工艺   982篇
机械仪表   2614篇
建筑科学   766篇
矿业工程   881篇
能源动力   2147篇
轻工业   11271篇
水利工程   266篇
石油天然气   19809篇
武器工业   65篇
无线电   311篇
一般工业技术   1262篇
冶金工业   840篇
原子能技术   52篇
自动化技术   648篇
  2024年   281篇
  2023年   688篇
  2022年   1302篇
  2021年   1605篇
  2020年   1773篇
  2019年   1774篇
  2018年   1492篇
  2017年   1883篇
  2016年   2030篇
  2015年   1848篇
  2014年   2929篇
  2013年   3278篇
  2012年   3502篇
  2011年   3635篇
  2010年   2596篇
  2009年   2413篇
  2008年   2148篇
  2007年   2788篇
  2006年   3030篇
  2005年   2667篇
  2004年   2370篇
  2003年   2208篇
  2002年   1943篇
  2001年   1622篇
  2000年   1451篇
  1999年   1203篇
  1998年   976篇
  1997年   835篇
  1996年   663篇
  1995年   510篇
  1994年   397篇
  1993年   258篇
  1992年   237篇
  1991年   189篇
  1990年   131篇
  1989年   90篇
  1988年   49篇
  1987年   46篇
  1986年   23篇
  1985年   61篇
  1984年   55篇
  1983年   39篇
  1982年   26篇
  1981年   8篇
  1980年   14篇
  1979年   3篇
  1978年   3篇
  1976年   4篇
  1951年   5篇
  1944年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
A brown coal, peat and oil shale were subjected to a rapid pyrolysis process and medium-heat-value gases together with tar were collected. The char residue was of high activity and suitable for gasification to create a two-stage gasification system. The coal-tar is used for manufacturing liquid fuels and chemicals.  相似文献   
32.
A monitoring program was performed to investigate heavy metal content alteration due to exploratory drilling for oil using non-aqueous fluids (NAFs) in Brazilian offshore, 900 m deep. Fourteen elements were monitored in 54 sites and it was verified that after drilling activities the average Ba concentration was remarkably increased with respect to background level, even 1 year after the activity. A minor increase in Mn and a moderate increase in Al concentrations were verified. The Cd, Cr, Ni, As, Co, Cu, Pb, V, and Zn concentrations were at the background levels, ca. 1 year after the NAFs drilling materials deposition on the seafloor. The Al, Ba, Cd, Cr, Cu, Ni and Zn mean concentrations were significantly different (P<0.05) between the three sampling operations (cruises) performed, while As, Cd, Fe and Pb presented different mean values according to the distance of the oil well, independent of the sampling operation. Interaction between sampling operation and distance was observed for Mn. In all sediment samples the Hg concentration was below the detection limit (0.07 μg g−1).  相似文献   
33.
丁锦萍 《山西化工》2002,22(1):31-33
介绍了国外利用天然气生产油口的生产工艺,探讨了山西省利用丰富的煤层气资源发展合成油的可能性和优势,并提出了相应的发展建议。  相似文献   
34.
Adsorption isotherms of sesame oil in a concentrated miscella system   总被引:2,自引:0,他引:2  
The adsorption of peroxides, unsaturated carbonyls, free fatty acids and carotenoids from unrefined sesame oil on vegetable carbon (5%, w/w) in a miscella system was studied. Three different solvent conditions (hexane/ethanol, 100:0, 95:5 and 75:25, vol/vol), combined in a factorial design with five levels of solvent (0, 10, 20, 30 and 40%, w/w), were used to develop the miscella. Equilibrium adsorption was not achieved during the 100 min of adsorption, mainly because the oil components were involved in oxidation reactions during the adsorption process. However, for a given solvent concentration, adsorption of the oil components showed a significant linear regression on their respective initial concentration in the miscella (Ci). Peroxides and carbonyls showed, at all solvent levels investigated, an affinity for the carbon more independent of their Ci than free fatty acids and carotenoids. In general, at the same Ci, a higher adsorption was achieved as solvent concentration increased. The results indicated that free fatty acid adsorption may depend on competitive adsorption based on molecule hydrophobicity. However, in spite of the higher hydrophobicity of carotenoids, compared with free fatty acids, they might not be competing for the same adsorbing sites. Ethanol showed a prooxidant effect that increased peroxide production during adsorption but did not affect the reaction involved in carbonyl production.  相似文献   
35.
Changes in the content of aliphatic waxes during industrial deodorization and/or physical refining of bleached olive oil were studied in an experimental discontinuous pilot plant of 250 kg deodorizer using nitrogen as stripping gas in place of steam. The kinetic constants for the decomposition of waxes during the deodorization process were determined. The reaction orders studied are zero (or can be considered zero) within the working interval. The values of rate constants, activation energy, frequency factor, increment of activation Gibbs free energy, activation enthalpy, and activation entropy are established.  相似文献   
36.
Phase transitions of canola oil sediment   总被引:3,自引:0,他引:3  
Canola sediment was obtained from an industrial filter cake by solvent extraction. When heated in the differential scanning calorimeter (DSC) (5–100°C), the sediment exhibited a single narrow melting peak at around 74.8°C. No solid-state polymorphic transformation of the material could be detected over this temperature range. The X-ray powder diffraction pattern of canola sediment resembled waxes from other sources with an orthorhombic unit cell. The phase transition behavior of canola sediment in oil was studied by both DSC and polarizing microscopy. With increasing ratio of oil/sediment, a reduction in both melting temperature and transition enthalpy was observed. The shape of the supercooling curve resembled that of the melting curve. The induction time was determined by spectrophotometry and was used to calculate the interfacial free energyσ between sediment and oil; σ=4.71 erg/cm2. The effect of temperature and sediment concentration on the clouding time of canola oil was studied; the clouding time was the shortest at 5°C.  相似文献   
37.
杜昭辉 《辽宁化工》2004,33(10):559-561
对不饱和聚酯树脂 (UPR)废料裂解制原料油技术进行了研究 ,系统地考察了催化裂解反应工艺条件。试验结果表明 ,催化裂解所需反应温度为 40 0~ 45 0℃ ,反应时间为 70~ 80min ;剂油比 1∶5 ;试验对 4种催化剂的催化性能进行了评价 ,研究表明单独使用YB -2催化剂 ,液相产品收率为 5 8.2 2%~ 5 9.1 4% ;YB -1和YB -2混合使用 ,最高液相产品收率为 68.76%。  相似文献   
38.
曹勇  龙小柱  李长久  毕颖 《辽宁化工》2004,33(11):627-628,664
对焦化柴油的不安定因素进行了研究.通过研究发现,焦化柴油中的酸性物质、碱性物质、微量金属及可见光均是影响焦化柴油安定性的因素,空气对焦化柴油的安定性影响较小.  相似文献   
39.
Effects of microwave heating on the oxidative stability ofd-tocopherols were studied in relation to the production of fatty acids in oils. During microwave heating, the stability of tocopherols decreased in the orderδ>β>γ>α. This order did not depend on the types of ethyl esters of fatty acids or oils present. But, the shorter the chainlength and the lower the degree of unsaturation of the fatty acid ethyl esters, the greater was the reduction in amount of individual tocopherols. A similar tendency was observed when tocopherol-stripped vegetable oils, with equimolar mixtures of tocopherols added, were treated under the same conditions. The reduction in tocopherols became greater with increasing levels of free fatty acids.  相似文献   
40.
棉籽油间歇式酯交换反应动力学的研究   总被引:45,自引:0,他引:45  
生物柴油(棉籽油甲酯)可以由棉籽油与甲醇在催化剂KOH存在下通过酯交换反应制得。故对棉籽油间歇式酯交换反应动力学进行了研究,并考察了反应温度和催化剂浓度对产物棉籽油甲酯浓度的影响。用液相色谱法分析棉籽油的组成;用气相色谱法分析产物中棉籽油甲酯的含量。由实验数据绘制的动力学曲线得到酯交换反应在开始阶段为准二级反应,以后紧接转为一级反应和零级反应,与文献报导的棕榈油酯交换反应动力学结果一致。由实验数据求出酯交换反应在开始阶段的动力学参数,35℃、45℃时的反应速率常数分别为0.9179Lmol-1min-1和1.049Lmol-1min-1,酯交换反应的活化能为10.88kJmol-1。根据实验结果得到棉籽油酯交换反应的最佳反应温度为45℃,最佳催化剂为1.1%KOH。  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