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   28252篇
  免费   2882篇
  国内免费   1777篇
电工技术   3286篇
技术理论   3篇
综合类   3130篇
化学工业   3152篇
金属工艺   852篇
机械仪表   1271篇
建筑科学   3460篇
矿业工程   1169篇
能源动力   1110篇
轻工业   2013篇
水利工程   1197篇
石油天然气   2542篇
武器工业   237篇
无线电   2180篇
一般工业技术   2898篇
冶金工业   1321篇
原子能技术   654篇
自动化技术   2436篇
  2024年   27篇
  2023年   342篇
  2022年   705篇
  2021年   1023篇
  2020年   985篇
  2019年   716篇
  2018年   685篇
  2017年   780篇
  2016年   963篇
  2015年   1061篇
  2014年   1723篇
  2013年   1845篇
  2012年   2080篇
  2011年   2348篇
  2010年   1749篇
  2009年   1835篇
  2008年   1652篇
  2007年   1972篇
  2006年   1734篇
  2005年   1484篇
  2004年   1188篇
  2003年   1024篇
  2002年   905篇
  2001年   740篇
  2000年   580篇
  1999年   505篇
  1998年   391篇
  1997年   316篇
  1996年   291篇
  1995年   250篇
  1994年   201篇
  1993年   129篇
  1992年   109篇
  1991年   85篇
  1990年   91篇
  1989年   78篇
  1988年   83篇
  1987年   36篇
  1986年   21篇
  1985年   20篇
  1984年   26篇
  1982年   12篇
  1981年   11篇
  1980年   10篇
  1964年   10篇
  1962年   7篇
  1961年   8篇
  1960年   9篇
  1959年   8篇
  1955年   8篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Transportation of heavy crude oil via pipelines possesses many technological issues that are inherently flow related. Accurate prediction of flow characteristics is an essential step for a reliable piping design of transporting the crude oil. A rheology-based Computational Fluid Dynamics (CFD) model of the Iraqi heavy crude oil flow through a horizontal pipe (1 m length of 3/4 in. inside diameter) was developed using the commercial software Ansys 15 Fluent. By using power law rheological model, the Iraqi heavy crude oil exhibits a non-Newtonian dilatant behavior over the examined shear rate range of 1–40 s−1. The proposed axi-symmetric CFD model identifies velocity profile and generates values of friction factor, which are validated with experimental measurements. Additionally, wall shear stress and entrance length were numerically predicted and compared with well-established correlations from the literature for Non-Newtonian flow. Detailed results of the CFD model exhibited a reliable prediction of the characteristics of heavy crude oil flow.  相似文献   
3.
Background: Recently, it was reported that leucine-rich repeat-containing G-protein-coupled receptor 4 (LGR4, also called GPR48) is another receptor for RANKL and was shown to compete with RANK to bind RANKL and suppress canonical RANK signaling during osteoclast differentiation. The critical role of the protein triad RANK–RANKL in osteoclastogenesis has made their binding an important target for the development of drugs against osteoporosis. In this study, point-mutations were introduced in the RANKL protein based on the crystal structure of the RANKL complex and its counterpart receptor RANK, and we investigated whether LGR4 signaling in the absence of the RANK signal could lead to the inhibition of osteoclastogenesis.; Methods: The effects of point-mutated RANKL (mRANKL-MT) on osteoclastogenesis were assessed by tartrate-resistant acid phosphatase (TRAP), resorption pit formation, quantitative real-time polymerase chain reaction (qPCR), western blot, NFATc1 nuclear translocation, micro-CT and histomorphological assay in wild type RANKL (mRANKL-WT)-induced in vitro and in vivo experimental mice model. Results: As a proof of concept, treatment with the mutant RANKL led to the stimulation of GSK-3β phosphorylation, as well as the inhibition of NFATc1 translocation, mRNA expression of TRAP and OSCAR, TRAP activity, and bone resorption, in RANKL-induced mouse models; and Conclusions: The results of our study demonstrate that the mutant RANKL can be used as a therapeutic agent for osteoporosis by inhibiting RANKL-induced osteoclastogenesis via comparative inhibition of RANKL. Moreover, the mutant RANKL was found to lack the toxic side effects of most osteoporosis treatments.  相似文献   
4.
利用烧碱、保险粉、N-甲基吡咯烷酮制备NMP溶剂溶解靛蓝。以100%纯靛蓝颗粒作为基准物,通过分光光度计法,在最大吸收波长下测试低质量浓度靛蓝溶液的吸光度。用最小二乘法处理测试数据并制作校验曲线,最终确定靛蓝溶解液的质量浓度因子。此方法先溶解经纱靛蓝,再测试溶解液的吸光度,经验证可计算出不同深度经纱靛蓝的上染率。  相似文献   
5.
6.
There is huge scientific interest in the neuropeptide oxytocin (OXT) due to its putative capacity to modulate a wide spectrum of physiological and cognitive processes including motivation, learning, emotion, and the stress response. The present review seeks to increase the understanding of the role of OXT in an individual’s vulnerability or resilience with regard to developing a substance use disorder. It places specific attention on the role of social stress as a risk factor of addiction, and explores the hypothesis that OXT constitutes a homeostatic response to stress that buffers against its negative impact. For this purpose, the review summarizes preclinical and clinical literature regarding the effects of OXT in different stages of the addiction cycle. The current literature affirms that a well-functioning oxytocinergic system has protective effects such as the modulation of the initial response to drugs of abuse, the attenuation of the development of dependence, the blunting of drug reinstatement and a general anti-stress effect. However, this system is dysregulated if there is continuous drug use or chronic exposure to stress. In this context, OXT is emerging as a promising pharmacotherapy to restore its natural beneficial effects in the organism and to help rebalance the functions of the addicted brain.  相似文献   
7.
The realization of liquid metal-based wearable systems will be a milestone toward high-performance, integrated electronic skin. However, despite the revolutionary progress achieved in many other components of electronic skin, liquid metal-based flexible sensors still suffer from poor sensitivity due to the insufficient resistance change of liquid metal to deformation. Herein, a nacre-inspired architecture composed of a biphasic pattern (liquid metal with Cr/Cu underlayer) as “bricks” and strain-sensitive Ag film as “mortar” is developed, which breaks the long-standing sensitivity bottleneck of liquid metal-based electronic skin. With 2 orders of magnitude of sensitivity amplification while maintaining wide (>85%) working range, for the first time, liquid metal-based strain sensors rival the state-of-art counterparts. This liquid metal composite features spatially regulated cracking behavior. On the one hand, hard Cr cells locally modulate the strain distribution, which avoids premature cut-through cracks and prolongs the defect propagation in the adjacent Ag film. On the other hand, the separated liquid metal cells prevent unfavorable continuous liquid-metal paths and create crack-free regions during strain. Demonstrated in diverse scenarios, the proposed design concept may spark more applications of ultrasensitive liquid metal-based electronic skins, and reveals a pathway for sensor development via crack engineering.  相似文献   
8.
共享控制策略作为基于临场感的遥操作机器人的主要控制模式,能够充分利用操作者的感知、判断和决策能力,也能发挥出机器人自身的优势. 阐述遥操作机器人临场感技术;综述遥操作共享控制策略的发展现状,主要基于触觉反馈引导、运动学限制规避以及共享因子分配等,对各控制策略的原理进行介绍,并梳理和分析遥操作共享控制策略发展中的瓶颈和不足,如共享因素的单一化或僵硬化、时延问题和机器人自主判断能力有限等问题. 针对研究存在的局限性,从3个方面对未来的发展提出展望,分别为提升干预水平、加强机器人意图预测、结合机器学习, 具有一定的指导意义.  相似文献   
9.
胡训健  卞康  刘建  谢正勇  陈明  李冰洋  岑越  刘振平 《岩土工程学报》2021,(4):725-733,I0003,I0004
基于颗粒离散元方法,通过构建损伤因子,提出了平行黏结水弱化模型,建立了考虑胶结物力学参数的非均质性的颗粒流模型。通过室内试验和数值模拟计算结果的对比分析,验证了所提模型的正确性和适用性。主要结论如下:(1)岩石胶结物的非均质性对岩石宏观力学性质存在一定影响。随着均质性因子m的增加,岩石均质性增加,单轴抗压强度和弹性模量也随之增加,符合指数函数关系;(2)随着黏结面积系数的增加,岩石所储存的总应变能的总量和增速逐渐降低;(3)岩石在干燥状态下,微裂纹倾角集中于80°~100°,随着黏结面积系数的增加,微裂纹倾角的分布范围逐渐增加;(4)随着黏结面积系数的增加,岩石破裂面更为密集,且贯通性增强。研究结果可为深埋隧道遇水产生围岩大变形、库岸涉水边坡变形等问题的细观机制研究提供了一定的依据和理论指导。  相似文献   
10.
公园绿地是城市中重要的公共活动场所,其使用会受到公园自身及其所处城市空间等多种因素的影响。利用手机大数据,通过对城市个体活动轨迹的识别,提取人们到访城市公园的交通出行方式,从而探寻公园出行的时空差异性。同时,通过对网络开源数据的获取,将公园的城市区位、周边环境、业态布局、自身条件和交通可达性等特征进行量化处理,并构建一组线性回归模型来确定影响公园使用的相关因素。结果表明:时间上,工作日和休息日的公园使用特征有较大差异;空间上,城市建成环境较完善地区的城市公园使用频率较高。通过回归模型,发现城市区位对公园使用的影响最大,而公园周边的交通设施并不会对公园使用产生显著影响。研究结果将有助于政策制定者优化城市公园的规划设计与维护管理。  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