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   22229篇
  免费   2522篇
  国内免费   1315篇
电工技术   1740篇
综合类   1560篇
化学工业   3453篇
金属工艺   603篇
机械仪表   677篇
建筑科学   753篇
矿业工程   1520篇
能源动力   1040篇
轻工业   851篇
水利工程   161篇
石油天然气   3644篇
武器工业   144篇
无线电   2752篇
一般工业技术   1011篇
冶金工业   2077篇
原子能技术   145篇
自动化技术   3935篇
  2024年   52篇
  2023年   275篇
  2022年   486篇
  2021年   672篇
  2020年   702篇
  2019年   596篇
  2018年   568篇
  2017年   728篇
  2016年   893篇
  2015年   880篇
  2014年   1453篇
  2013年   1587篇
  2012年   1852篇
  2011年   1981篇
  2010年   1360篇
  2009年   1293篇
  2008年   1217篇
  2007年   1429篇
  2006年   1312篇
  2005年   1112篇
  2004年   909篇
  2003年   843篇
  2002年   763篇
  2001年   601篇
  2000年   441篇
  1999年   388篇
  1998年   303篇
  1997年   245篇
  1996年   231篇
  1995年   224篇
  1994年   138篇
  1993年   110篇
  1992年   80篇
  1991年   61篇
  1990年   58篇
  1989年   43篇
  1988年   26篇
  1987年   26篇
  1986年   23篇
  1985年   18篇
  1984年   14篇
  1983年   9篇
  1982年   12篇
  1980年   7篇
  1979年   6篇
  1978年   6篇
  1977年   6篇
  1976年   7篇
  1974年   3篇
  1951年   5篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
The purpose of presented research is to find out the influence of air-jet yarn from the mixture of CO/PA (50/50) and CO/PES (67/33) fibres on functionality of woven fabric in the plain weave.The first part of the study is directed to the mechanical and physical properties of air-jet yarns (16.7 tex) from CO/PA and CO/PES fibres, while the second part of the study focuses on permeability properties of fabrics with air-jet yarns in the weft. The results show that air-jet yarn in the weft direction influences on the water vapor transmission and thermal conduction increase and air permeability decrease.Incorporation of air-jet yarn in the weft direction also improves breaking extension level.The results of presented research also show that the incorporation of air-jet yarn in the weft has statistical important influence on thermal conduction, water-vapor and air permeability changes.  相似文献   
72.
《Journal of dairy science》2019,102(12):11291-11297
Housing in hospital pens may be beneficial for lame cows due to soft flooring and less competition for resources. We compared recovery from lameness among dairy cows housed in designated hospital pens, with deep-litter straw, with recovery among cows housed together with the rest of the lactating cows in their home pens, with cubicles and slatted or solid concrete floors. Additionally, we compared lying behavior in the 2 groups of cows. A total of 168 lame dairy cows from 5 herds were included in the study. Each herd was visited once weekly, and lame cows (locomotion score 3 or 4 on a 5-point scale) were examined in a hoof-trimming chute, trimmed, and randomly assigned to 1 of 2 treatments: housing in a hospital pen (termed “treatment”; 72 cows) or housing under the herds' standard conditions together with the rest of the lactating cows in the herd (termed “control”; 96 cows). Cows were locomotion scored weekly until they were no longer lame or until they had been part of the trial for 3 weeks (i.e., 2, 3, or 4 locomotion scorings per cow). We categorized cows to describe the progression of lameness over time: recovery (divided into fast, medium, or slow), improvement without recovery, constant lameness, or worsening of lameness. Lying behavior was recorded in a sample of 60 of the 168 cows for a period of 5 d. Overall, recovery from lameness was significantly different between treatment and control cows. The proportion of cows included in the study with locomotion score 4 and a subsequent improvement was significantly higher among treatment cows than among control cows. Among cows included with locomotion score 4, 40% of treatment cows also had a locomotion score 4 at the fourth locomotion scoring, 46% had improved to a score 3, and 14% were no longer lame. In comparison, 73% of control cows had a locomotion score 4 at the fourth locomotion scoring, 16% had improved to a score 3, and 11% were no longer lame. We found no differences between treatment and control cows for mean daily lying time, number of steps per day, number of daily lying bouts, or mean duration of lying bouts. Housing of lame dairy cows in a hospital pen with a soft surface, easier access to feed and water, a smaller group size, and reduced waiting time for milking may have positive effects on recovery from lameness. Typically, farmers have housed only severely lame cows in hospital pens. However, our results indicate that less severely lame cows may also benefit from a stay in a hospital pen.  相似文献   
73.
国外某地含有丰富的烧绿石资源,属碱性碳酸盐矿体,原矿Nb2O5含量大于1%,在经历长流程胺法烧绿石浮选之后,尾矿含Nb2O5较高,铌资源未能得到综合回收,该烧绿石尾矿仍具有较高的回收价值。由于该尾矿长期在尾矿库堆存,药剂污染严重、含泥量较大,本工艺预先采用“磨矿擦洗-脱泥”的工艺改善其浮选性质,然后通过“磁选-脱钙”的工艺进行杂质矿物抛废,最终采用螯合类组合捕收剂实现烧绿石的选择性高效捕收。在原料Nb2O5品位0.71%的条件下,获得了Nb2O5品位55.07%、回收率38.23%的烧绿石精矿,尾矿中烧绿石资源得到回收。  相似文献   
74.
Stem cell therapy is one of the most promising candidate treatments for spinal cord injury. Research has shown optimistic results for this therapy, but clinical limitations remain, including poor viability, engraftment, and differentiation. Here, we isolated novel peripheral nerve-derived stem cells (PNSCs) from adult peripheral nerves with similar characteristics to neural-crest stem cells. These PNSCs expressed neural-crest specific markers and showed multilineage differentiation potential into Schwann cells, neuroglia, neurons, and mesodermal cells. In addition, PNSCs showed therapeutic potential by releasing the neurotrophic factors, including glial cell-line-derived neurotrophic factor, insulin-like growth factor, nerve growth factor, and neurotrophin-3. PNSC abilities were also enhanced by their development into spheroids which secreted neurotrophic factors several times more than non-spheroid PNSCs and expressed several types of extra cellular matrix. These features suggest that the potential for these PNSC spheroids can overcome their limitations. In an animal spinal cord injury (SCI) model, these PNSC spheroids induced functional recovery and neuronal regeneration. These PNSC spheroids also reduced the neuropathic pain which accompanies SCI after remyelination. These PNSC spheroids may represent a new therapeutic approach for patients suffering from SCI.  相似文献   
75.
聚合物驱油已成为老油田必要的提高采收率方法之一,但是其聚合物在地下驱油的流动机理难以明确。区别于传统实验,分子模拟技术从微观角度上在计算机上进行建模仿真-计算机实验,不仅弥补宏观实验和理论之间的空白,还具有研发周期短、低成本和低风险的优点。该研究方法具有较高灵活度,可满足复杂地层的极端条件,抵消实验和理论上的过度简化,以接近实际储层情况进行研究,甚至开展一些无法完成的实验。由于该技术的出现,探索机理的研究方法不仅多了一种,还增添了可以佐证实验与理论的工具。本文综述了分子模拟技术在油田聚合物驱油中的应用,对在聚合物驱油剂研发和黏弹驱油机理探讨两方面分别进行了阐述,揭示该技术提高聚合物驱油采收率的辅助作用。  相似文献   
76.
湖北某铜矿浮选在常规加药条件下,前两槽不能有效形成泡沫层,浮选液面经常出现“翻花”现象,以致选矿指标波动大,起泡剂用量大幅上升,因此开展了浮选试验研究。试验结果表明,在总药量不变的情况下,适当降低粗选捕收剂用量,使有用矿物有序上浮,有效改善了浮选泡沫层的稳定性。在生产现场应用后,浮选现场泡沫稳定,层次感分明,彻底解决了浮选前两槽不起泡、翻花等问题,选铜回收率提高0.69%,起泡剂单耗降低18g/t,取得较好的应用效果。  相似文献   
77.
通过实验研究ZF6铅玻璃在~(60)Co放射源相同剂量率不同累积剂量照射下透光率的变化规律。将辐照后的铅玻璃在室外阳光下、室内自然光下和不透光的抽屉中放置观察,发现阳光照射对铅玻璃辐照着色的恢复起着重要作用,在建立铅玻璃透光率随辐照时间变化的数学模型时,室内自然光对铅玻璃透光率的影响可忽略。实验数据验证了受辐照时铅玻璃透光率的变化与辐照时间符合指数函数关系。根据文献资料推导出铅玻璃透光率变化与光照时间为线性函数关系,但发现其与实验数据的符合程度较差,本文根据样条曲线插值法推导出铅玻璃透光率与光照时间为二次函数关系,与实验数据的符合效果较好。  相似文献   
78.
介绍了尼龙66连续聚合纺丝生产工艺路线,改造前反应器排放大量的过热蒸汽直接进入大气,造成能源浪费和环境污染。通过技术改造,新增饱和蒸汽塔、过滤器等废汽回收利用装置,根据高压蒸汽减温减压装置原理,将反应器排放的1.85 MPa,245℃过热蒸汽在饱和塔内进行喷淋,经过滤器除杂转换为0.8~1.0 MPa饱和蒸汽,作为浓缩槽热源;浓缩槽排放蒸汽的余热改造后作为第三温水系统和冬季取暖的热源,排汽余热得到充分利用。运行过程中,通过稳定控制反应器压力、保持反应器排汽阀的喷淋水流量和压力稳定等工艺条件,定期清理疏水阀等,可有效预防管路和设备堵塞。改造后,装置运行稳定,能耗降低,尼龙66聚合物相对黏度的均匀性和原丝一等品率均有所提高。  相似文献   
79.
A new air-gap membrane distillation (AGMD) module for desalination with internal latent-heat-recovery which consisted of parallel hollow fiber membranes and heat exchange hollow fibers was successfully developed. The influences of feed flow rate, feed temperature and feed initial concentration on AGMD process were investigated. The vapor pressure polarization coefficient (η) was introduced to measure the reduction in the effective driving force for mass transfer with regard to the driving force imposed. Among all AGMD experiments, the maximum water vapor permeate flux (JD) of 5.30 kg/m2 h and the gained output ratio (GOR) of 5.70 were obtained. A theoretical model based on the mass and energy balances of the hot feed side was established to calculate the temperature and the local water vapor permeate flux distributions along the hollow fiber membrane, which showed that the temperature drop and local water vapor permeate flux drop were much larger at the upper part than those at the lower part of the membrane module in the hot feed side.  相似文献   
80.
The environmental impact reduction and operating costs savings associated with the purification and recovery of solvent waste in the manufacture of active pharmaceutical ingredients (API's) were investigated. A software toolbox has been developed that combines Aspen Plus® process simulation with SimaPro® and Ecosolvent life cycle assessment (LCA) databases. A LCA approach was used in order to consider the environmental impact beyond pharmaceutical production plant boundaries. The feasibility of a relatively small flexible equipment-skid capable of recovering multiple solvent waste streams was evaluated. Distillation and pervaporation were considered to separate binary waste solvent mixtures. Optimum distillation reflux ratio and feed stage were determined to maximize the environmental impact reductions and operating cost savings. The optimum reflux ratio was significantly higher than 1.2 times the minimum reflux ratio suggested by traditional heuristics. The emissions and cost reductions obtained were as much as 49% and 56% higher, respectively, as compared to using the conventional optimum reflux ratio. A comprehensive cash flow analysis showed that the recovery of low volume solvent waste streams is economically feasible, despite traditional thinking. Three case studies from Pfizer are presented to show how our software tool can aid in green engineering decision making.  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