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   119582篇
  免费   10946篇
  国内免费   5634篇
电工技术   5950篇
技术理论   8篇
综合类   10355篇
化学工业   21075篇
金属工艺   2989篇
机械仪表   5519篇
建筑科学   14889篇
矿业工程   5821篇
能源动力   6760篇
轻工业   6922篇
水利工程   19313篇
石油天然气   9275篇
武器工业   549篇
无线电   3435篇
一般工业技术   6873篇
冶金工业   4171篇
原子能技术   1306篇
自动化技术   10952篇
  2024年   394篇
  2023年   1444篇
  2022年   2804篇
  2021年   3415篇
  2020年   3648篇
  2019年   3078篇
  2018年   3031篇
  2017年   3671篇
  2016年   4176篇
  2015年   4280篇
  2014年   7385篇
  2013年   7444篇
  2012年   8712篇
  2011年   9266篇
  2010年   6730篇
  2009年   7166篇
  2008年   6466篇
  2007年   8251篇
  2006年   7448篇
  2005年   6763篇
  2004年   5373篇
  2003年   4858篇
  2002年   4118篇
  2001年   3327篇
  2000年   2772篇
  1999年   2096篇
  1998年   1549篇
  1997年   1261篇
  1996年   980篇
  1995年   890篇
  1994年   658篇
  1993年   530篇
  1992年   427篇
  1991年   340篇
  1990年   260篇
  1989年   260篇
  1988年   158篇
  1987年   163篇
  1986年   127篇
  1985年   119篇
  1984年   109篇
  1983年   60篇
  1982年   40篇
  1981年   18篇
  1980年   23篇
  1979年   22篇
  1977年   14篇
  1976年   6篇
  1975年   5篇
  1951年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In pot and field experiments conducted in 1990 and 1991 on an acid sandy, phosphorus (P) deficient soil in Niger, West Africa, the effect of seed coating on seedling emergence, early growth and grain yield of pearl millet (Pennisetum glaucum (L.) R. Br.) was studied. Seeds of pearl millet were coated with different rates (0; 0.5; 1.0; 2.0; 5.0; 10.0 mg P seed–1) and types of P fertilizers (single superphosphate, ammonium dihydrogen phosphate; monocalcium phosphate, sodium dihydrogen phosphate and sodium triphosphate). Seedling emergence was generally reduced at coating rates higher than 0.5 mg P seed–1 and prevented with single superphosphate and sodium triphosphate at rates higher than 5 mg P seed–1. No correlation was found between the pH and osmomolity of the coatings and final emergence of millet seedlings. The most favourable effect on plant growth and P content was achieved with ammonium dihydrogen phosphate (AHP) as seed coating. This was attributed to the enhancement effect of ammonium on P uptake. Compared to the untreated control dry matter production at 20 days after planting (DAP) was increased by 280%, P content per plant by 330%, total biomass at maturity by 30% and grain yield by 45%. Although seed coating with AHP may be harmful to seedlings emergence, it represents a suitable method to enhance early growth and increase yield of pearl millet.  相似文献   
992.
We discuss the use of low-dimensional physical models of the voice source for speech coding and processing applications. A class of waveform-adaptive dynamic glottal models and parameter identification procedures are illustrated. The model and the identification procedures are assessed by addressing signal transformations on recorded speech, achievable by fitting the model to the data, and then acting on the physically oriented parameters of the voice source. The class of models proposed provides in principle a tool for both the estimation of glottal source signals, and the encoding of the speech signal for transformation purposes. The application of this model to time stretching and to fundamental frequency control (pitch shifting) is also illustrated. The experiments show that copy synthesis is perceptually very similar to the target, and that time stretching and “pitch extrapolation” effects can be obtained by simple control strategies.  相似文献   
993.
本文采用氟离子选择电极法,该法具有测定简便、快速、灵敏、选择性好、可测定浑浊、有色样品等优点,最低检出浓度为0.05mg/L,适用浓度范围宽,在测定含陶瓷工业废水氟量中取得良好效果。  相似文献   
994.
Water hydraulic systems use water as a pressure medium and, thus, do not pose such adverse environmental impacts as oil hydraulics. Microbial deterioration of the pressure medium and biofouling of the surfaces restrict the applicability of the water‐based technology. The potential of microbial growth control by UV‐irradiation and filtration was studied in a pilot‐scale water hydraulic system. The UV‐irradiation (25 m Ws cm?2) of the pressure medium reduced the total viable counts of bacteria by 1–2 log10 cfu cm?3, whereas the total microbial cell numbers and the numbers of surface‐attached microorganisms remained unaffected. Prefiltration (1.2 µm, absolute) of the pressure medium decreased the total microbial cell number in the water phase and retarded the attachment of bacteria. The filtration during the operation (2 µm, absolute) decreased the total numbers of microbial cells and the total viable counts in the pressure medium, and microbial attachment on the surfaces. Microbial attachment was not prevented by filtration. The microbial water quality obtained by pre‐ and on‐line filtration of the pressure medium was sufficient to ensure the long‐term operation of the water hydraulic system assuming that clean work practices are complied with in assembly and during the operation. © 2002 Society of Chemical Industry  相似文献   
995.
裂缝-溶洞型碳酸盐岩的油水两相流动形态复杂,本文针对此类油藏水驱油机理研究做了大量的文献调研,阐述了此类油藏的水驱油机理研究现状,并提出新的试验研究方法和建议,以进一步加深对这类油藏驱油理论的认识。  相似文献   
996.
In the context of a European union‐supported network on “Reactive Surfactants for Heterophase Polymerization,” different polymerizable surfactants (surfmers) have been synthesized and engaged in the emulsion polymerization of styrene, butyl acrylate, and acrylic acid. The thermomechanical properties of films cast from these different latices are reported in this article. The evolution of the mechanical properties with temperature and the effect of water molecules on these properties are studied. We observed that the studied surfactants do not influence the properties of the dry films. However, some differences due to grafting of reactive surfactants appeared when the films were wet. The amount of water uptake is drastically decreased when only reactive surfactants are present in the film. Concerning the mechanical behavior of the wet films, a decrease of the plastic flow stress is observed for all the samples whatever the nature of the surfactant (reactive or conventional). Hence, calorimetric measurements and dynamic mechanical analysis are used to identify the possible mechanisms that induce the change in the mechanical behavior of the latex films. In the case of reactive surfactant grafted to the polymer, the very low value of water uptake is accompanied by a plasticization of the polymer. In contrast, no plasticizing effect is observed in the case of nonreactive surfactant, even if the amount of water is very large. Finally, the tensile behavior of the styrene–butyl acrylate copolymer versus temperature is analyzed in the frame of the quasi point defects (qpd) model. Both rubber elasticity and chain orientation effects are taken into account to describe the behavior laws at large extensions (i.e., ? ≈ 1.2). © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1686–1700, 2002; DOI 10.1002/app.10548  相似文献   
997.
根据2003-2013年淮河流域五省的降水量、径流系数、人均水资源量、人均GDP、人口密度等相关数据,构建水资源短缺风险评价体系,用熵权法对指标赋值,运用可变模糊模型对淮河流域及各省的水资源短缺风险进行评估和时空差异分析。结果表明:2003-2013年间淮河流域水资源短缺风险值总体较高,且呈缓慢的增长趋势,2004年降水较少,风险值达到最高;十年间河南省风险增加最为明显,而山东省较为稳定,风险增加也最低;在淮河流域五个区域中,河南省的风险程度最高,达到3.52;江苏省、山东省次之;安徽省和湖北省相对较低,达到2.86和2.51。水资源短缺风险二级指标分析发现,危险性最强的是河南省,安徽省最小;水资源短缺易损性最强的是山东省,湖北省最小;水资源短缺暴露性最强的是江苏省,山东省最小;水资源短缺可恢复性最好的是湖北省,河南省最差。同时,所有评价指标中人口密度、人均GDP、降水量对水资源短缺风险的影响较大。  相似文献   
998.
针对新疆自治区阜康市水资源日益短缺及生态环境逐渐恶化的现实情况,根据DPSIR框架展开水资源承载力因子分析,构建了水资源承载力多因素基础指标体系;应用主成分分析法筛选出主要指标,采用熵权法进行赋权;构建综合评价模型对水资源承载力进行计算。研究表明:筛选出的4个主成分累计贡献率为92.016%,能充分代表多因素基础指标体系;阜康市九年间(2006年-2014年)水资源承载力综合评分值在0.498~0.531之间波动,下降速率缓慢且波动幅度减小,整体上处于开发饱和阶段。社会和经济发展的压力带来的农业用水的增加是导致水资源承载力下降的主要指标,扩大地下水开采和推行的节水灌溉等压采措施则在很大程度上缓解了下降的趋势。  相似文献   
999.
通过对济宁市防治水污染技术研究, 提出水污染防治在政策措施上实施治污抓源头, 杜绝不达标排放; 在城市 建设规划上, 把河湖生态湿地建设、 改善居住生活环境与城市建设有机结合; 在河湖治理上, 坚持工程、 生物措施并 举; 在工程管护上, 坚持依法治水。利用南水北调东线工程建设, 实施水污染防治建设, 通过对济宁市水污染防治成 果的阐释, 为今后水污染治理提供成功的经验和做法。  相似文献   
1000.
水资源短缺是制约城市经济社会发展的重要因素,合理配置和有效利用有限的水资源是解决水资源供需矛盾的有效途径。以许昌市国家水生态文明试点城市建设为例,在分析水资源开发利用现状的基础上,运用定额法对需水量进行预测。以多年平均水资源量为基础,2011年作为研究基准年,根据水资源取用方式、工程方案及各种水量需求等,利用MIKE BASIN建立水资源配置模型,对许昌市水资源供需平衡进行模拟,通过三次供需平衡分析,得到不同规划水平年水资源供需配置方案,最终达到水资源供需平衡,使有限的水资源发挥最大效益。研究结果表明,本地水资源结合南水北调、引黄调蓄、中水回用等多模式联合调度,可以满足许昌市中长期经济社会可持续发展用水需求,对未来许昌市水资源可持续利用具有重要的指导意义。  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