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   548篇
  免费   163篇
  国内免费   32篇
化学   71篇
晶体学   3篇
力学   138篇
综合类   9篇
数学   60篇
物理学   462篇
  2024年   3篇
  2023年   15篇
  2022年   15篇
  2021年   13篇
  2020年   10篇
  2019年   16篇
  2018年   7篇
  2017年   15篇
  2016年   28篇
  2015年   17篇
  2014年   47篇
  2013年   35篇
  2012年   35篇
  2011年   55篇
  2010年   38篇
  2009年   39篇
  2008年   35篇
  2007年   57篇
  2006年   28篇
  2005年   28篇
  2004年   23篇
  2003年   19篇
  2002年   13篇
  2001年   19篇
  2000年   17篇
  1999年   13篇
  1998年   13篇
  1997年   17篇
  1996年   16篇
  1995年   6篇
  1994年   5篇
  1993年   4篇
  1992年   5篇
  1991年   2篇
  1990年   5篇
  1989年   3篇
  1988年   6篇
  1987年   5篇
  1986年   1篇
  1984年   5篇
  1983年   1篇
  1982年   5篇
  1981年   2篇
  1978年   1篇
  1969年   1篇
排序方式: 共有743条查询结果,搜索用时 15 毫秒
1.
Application of rotary tillage has been increased due to less tillage passes required, reduced draft, and greater efficiency through reduction in wheel slippage. Early failure of the bearing of tractor power take-off (PTO) shaft was observed in tractors of power range 30–35 horsepower during rotary tillage. An instrumentation setup involving an extended octagonal ring transducer (EORT) was developed and installed at the bottom of the bearing to measure the axial load and the vertical component of the radial load. The horizontal component of radial load was measured by strain gauges. Based on measured loads, the bearing life was assessed. Independent variables were: operating depth, number of blades, gear setting, engine speed, and tyre size. The average axial and radial loads varied from 786–3869 N, and 134–430 N, respectively. However, bearing experienced very high peak loads during each trial. The peak axial and radial loads was recorded between 1081–7534 N and 566–1794 N, respectively. The estimated bearing life based on peak loads was 171.98–28341.39 h. Based on the findings, it may be concluded that the average loads were not sufficient to cause quick failure of PTO bearing, rather sudden peak loads might be the root cause of early failure.  相似文献   
2.
以有序介孔碳(OMC)球为离子-电子转换层,制备了固态氯离子选择性电极,构建了基于离子敏感的场效应晶体管(ISFET)的手持式传感系统,用于检测人体血清中的氯离子。优化了OMC前驱体的碳化温度,探究了OMC形貌结构对电极传感性能的影响;电极柔性化制备后考察了其在手持系统中对氯离子的检测效果。结果表明,最优条件下,电极在5.12×10^-4~1.02 mol/L的浓度范围呈现线性响应,响应斜率为60 mV/decade。该柔性电极在手持传感系统中展现出高灵敏度和重现性,可用于人体血清样品中氯离子的检测,其回收率为96.3%~104.9%。  相似文献   
3.
厚度模压电超声换能器作为超声波发射、接收以及电信号间转换的载体,是超声成像与检测系统的核心器件,一般由压电层、匹配层和背衬层3部分组成。超声换能器的性能一定程度上决定着整体超声设备的性能,影响了其在工业、医学、军事等领域的应用。该换能器的关键性能指标(带宽、灵敏度)除了受到压电层的影响,还与匹配层、背衬层等无源声学材料的设计密切相关。该文综述了近年来厚度模压电超声换能器无源声学材料(匹配层、背衬层和声透镜)的研究进展,提出了当前该类材料面临的难题和解决途径,并对其未来发展方向进行了展望。  相似文献   
4.
In this article, we introduce the algebra of block-symmetric cylinders and we show that symmetric cylindrical constructions on base-graphs admitting commutative decompositions behave as generalized tensor products. We compute the characteristic polynomial of such symmetric cylindrical constructions in terms of the spectra of the base-graph and the cylinders in a general setting. This gives rise to a simultaneous generalization of some well-known results on the spectra of a variety of graph amalgams, as various graph products, graph subdivisions and generalized Petersen graph constructions. While our main result introduces a connection between spectral graph theory and commutative decompositions of graphs, we focus on commutative cyclic decompositions of complete graphs and tree-cylinders along with a subtle group labeling of trees to introduce a class of highly symmetric graphs containing the Petersen and the Coxeter graphs. Also, using techniques based on recursive polynomials we compute the characteristic polynomials of these highly symmetric graphs as an application of our main result.  相似文献   
5.
针对未来空间天文学应用的超分辨率光谱成像仪器的需求,对低噪声柱面微通道板(MCP)的制备方法及其性能进行了研究. 提出了一种将光学抛光与热成型相结合的新的柱面MCP制备方法,利用不含放射性元素的低噪声MCP玻璃,制备出曲率半径为400mm、尺寸为30mm′46mm、长径比为80:1、通道直径12.5mm、通道间距15mm的柱面MCP,并将其与感应电荷楔条形阳极(WSA)组成光子计数探测器,对其暗计数率、分辨率进行了检测,暗计数率约为0.1counts/cm2×s.  相似文献   
6.
通过grafting-onto方法合成高侧链接枝密度的环形侧链聚合物刷并进行结构表征.先通过开环易位聚合(ROMP)制备带有炔基侧基的线形聚降冰片烯衍生物主链(l-PND),通过原子转移自由基聚合(ATRP)和铜催化的叠氮-炔环加成(Cu AAC)反应制备带有一个叠氮基的环形聚丙烯酸叔丁酯侧链(c-Pt BA),再利用Cu AAC反应将环形侧链高密度地接入主链,得到环形侧链聚合物刷.通过核磁氢谱(1H-NMR)、凝胶渗透色谱(GPC)和红外光谱(IR)对聚合物主/侧链及聚合物刷的结构和组成进行了表征.该聚合物刷主链聚合度为400,高接枝密度的环形侧链使主链构象伸展.聚合物刷在原子力显微镜(AFM)下呈单分子蠕虫形貌(柱状形貌),分子长度为200 nm,直径为30 nm.  相似文献   
7.
气体的导热系数和黏度是重要的热物性参数,其数值大小取决于所处的热力学状态。在目前的导热系数和黏度主要测量方法中,待测工质在测量时需经历非定常的过程或处于具有物性梯度的非平衡态之下,使得待测工质的物性在时间或者空间上不处于一个确定的热力学状态。本文利用圆柱定程干涉法,通过分析气体导热系数和黏度导致的声波能量耗散,结合气体输运理论中对稀疏气体的描述,探索了在确定的热力学状态下同时测量气体导热系数和黏度的方法,并以氩(Ar)为例进行了实验验证。测量结果与已有文献一致性较好,初步证实了方法的可行性。  相似文献   
8.
Recently, we reported molecular dynamics simulations of stable cyclotron motions of ions and water molecules in a carbon nanotorus, induced by different rotating electric fields (EFs). This study is devoted to the calculation and characterisation of the magnetic field (MF) induced by these cyclotron motions. Results show that carbon nanotorus containing ions or water molecules acts as an EF-to-MF transducer. Components of the instantaneous induced MF show large-scale oscillations superimposed by strong fluctuations arising respectively from overall circular motion and random collisions of moving species. Analysis of the space-dependencies of the induced MF components shows that the induced MF is maximum at the centre of the nanotorus. The MF induced by cyclotron motion of ions follows the orders B(Ca2+)?>?B(Na+)?≈?B(K+) at E?=?1.0?V/nm and B(E?=?1.0?V/nm)?>?B(E?=?0.5?V/nm)?>?B(E?=?0.1?V/nm). The time-averaged induced MF of the cyclotron motion of 81 water molecules is almost 102 times stronger than that of ions. The induced MF strength is decreased with increasing distance from nanotorus and decays effectively at about 17.3–18.1 and 15.9–18.2?nm along the z-axis of the nanotorus for ions and water molecules, respectively. The magnitude of the MF induced by cyclotron motions of water molecules and ions, respectively, decreases and increases in the nanotorus with freed carbon atoms.  相似文献   
9.
压电驻极体(也称为铁电驻极体)是一类具有强压电效应的微孔结构驻极体材料,具有柔韧、低密度、低特性声阻抗等特征,是制备柔性空气耦合声电换能器的理想材料.针对器件对高灵敏度和高温工作环境的应用需求,本文报道高性能氟化乙丙烯/聚四氟乙烯(FEP/PTFE)复合膜压电驻极体的制备和性能表征.研究结果表明, FEP/PTFE膜的特性声阻抗为0.02 MRayl (1 Rayl=10 Pa·s/m);在小压强范围内的准静态压电电荷系数d33可高达800 pC/N,且具有良好的压强特性.基于FEP/PTFE复合膜压电驻极体的麦克风的灵敏度最高可达6.4 mV/Pa@1 kHz,远高于文献报道的相同结构的压电驻极体麦克风的灵敏度,且具有平坦的频响曲线.对于直径为20 mm的超声波发射器,当驱动电压Vp为600 V时,样品中轴线上距离器件表面100 mm处, 40—80 kHz频率范围内产生的超声波的声压级为80—90 dB (参考声压为20μPa).基于FEP/PTFE复合膜压电驻极体的声电换能器的热稳定性显著优于聚丙烯(PP)压电驻极体声电换能器:在125...  相似文献   
10.
The present work analyzes cylindrical diffusion flames (Tsuji burner) under low stretch condition, considering fuel injection also from the backward region of the burner. To highlight the fundamental aspects of this flame, some assumptions are imposed, like constant thermodynamic and transport coefficients, unitary Lewis number and no radiative heat loss. It is also considered potential flow model and incompressible Navier–Stokes model. Despite the simplicity of the former model, results from both models show good agreement. Also, an asymptotic analysis describing the problem far from the burner is able to capture the most important mechanisms controlling the flame, then the flame shape is determined and the dependence of the characteristic length scales on Peclet number (based on the burner properties), free stream velocity and stoichiometry is revealed. The results show that the flame width is proportional to the mass stoichiometric coefficient and reciprocal to the Peclet number the 1/4 power and free stream velocity the 3/4 power, and that the flame height is proportional to the square of the mass stoichiometric coefficient and to the square root of the ratio of Peclet number to free stream velocity. In addition, an asymptotic stability analysis reveals low-stretch flame extinction to be caused by reduction in fuel and oxidizer concentrations, which provides the range of the stoichiometric coefficient for stable regime, and at the same time the range of heat released.  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