全文获取类型
收费全文 | 6142篇 |
免费 | 813篇 |
国内免费 | 1406篇 |
专业分类
化学 | 3478篇 |
晶体学 | 133篇 |
力学 | 1825篇 |
综合类 | 82篇 |
数学 | 579篇 |
物理学 | 2264篇 |
出版年
2024年 | 24篇 |
2023年 | 80篇 |
2022年 | 210篇 |
2021年 | 190篇 |
2020年 | 264篇 |
2019年 | 216篇 |
2018年 | 211篇 |
2017年 | 284篇 |
2016年 | 300篇 |
2015年 | 232篇 |
2014年 | 351篇 |
2013年 | 552篇 |
2012年 | 357篇 |
2011年 | 433篇 |
2010年 | 296篇 |
2009年 | 352篇 |
2008年 | 374篇 |
2007年 | 402篇 |
2006年 | 395篇 |
2005年 | 408篇 |
2004年 | 337篇 |
2003年 | 299篇 |
2002年 | 233篇 |
2001年 | 242篇 |
2000年 | 211篇 |
1999年 | 175篇 |
1998年 | 141篇 |
1997年 | 141篇 |
1996年 | 110篇 |
1995年 | 98篇 |
1994年 | 89篇 |
1993年 | 78篇 |
1992年 | 54篇 |
1991年 | 50篇 |
1990年 | 36篇 |
1989年 | 25篇 |
1988年 | 23篇 |
1987年 | 17篇 |
1986年 | 18篇 |
1985年 | 11篇 |
1984年 | 14篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1971年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有8361条查询结果,搜索用时 15 毫秒
901.
902.
An experimental study of evaporation heat transfer coefficients for single circular small tubes was conducted for the flow of C3H8, NH3, and CO2 under various flow conditions. The test matrix encompasses the entire quality range from 0.0 to 1.0, mass fluxes from 50 to 600 kg m−2 s−1, heat fluxes from 5 to 70 kW m−2, and saturation temperatures from 0 to 10 °C. The test section was made of circular stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm in a horizontal orientation. The test section was uniformly heated by applying electric power directly to the tubes. The effects of mass flux, heat flux, saturation temperature, and inner tube diameter on the heat transfer coefficient are reported. Among the working refrigerants considered in this study, CO2 has the highest heat transfer coefficient. Laminar flow was observed in the evaporative small tubes, and was considered in the modification of boiling heat transfer coefficients and pressure drop correlations. 相似文献
903.
Imperfect bonding between the constitutive components can greatly affect the properties of the composite structures.An asymptotic analysis of different types of imperfect interfaces arising in the problem of 2D fibrereinforced composite materials are proposed.The performed study is based on the asymptotic reduction of the governing biharmonic problem into two harmonic problems.All solutions are obtained in a closed analytical form.The obtained results can be used for the calculation of pull-out and pushout tests,as well as for the investigation of the fracture of composite materials. 相似文献
904.
S. Suresh M. ChandrasekarS. Chandra Sekhar 《Experimental Thermal and Fluid Science》2011,35(3):542-549
An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6 mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2-10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%. 相似文献
905.
Air injection as a stabilization method is evaluated for flow boiling in a micro tube. Pyrex glass tube coated by ITO film is employed as a test tube for flow visualization with water as a working fluid. Air bubble and liquid slug lengths are controlled by changing air and liquid mass velocities. Wall temperatures and inlet/outlet pressures show very large fluctuations during flow boiling without air injection. Severe reverse flow is also observed from flow visualization. On the other hand, wall temperature and inlet/outlet pressures as well as visualized flow patterns become very stable with air injection. In addition, much higher heat transfer coefficients are obtained for air injected cases. It is observed from the flow visualization that the flow becomes much stable and shows regular patterns. 相似文献
906.
907.
在密闭容器中,用爆轰分解掺杂含有镍离子的混合炸药前驱体合成了核壳结构石墨包覆镍纳米颗
粒。调整混合炸药前驱体中碳源材料和金属源材料的有效摩尔比合成了球形、不同尺寸、核壳结构的磁性石
墨包覆纳米镍颗粒。采用X 射线衍射仪(XRD)、透射电镜(TEM)、能谱分析仪(EDX)和振动样品磁强计
(VSM)表征化学构成、结构形貌及磁性能。结果表明:颗粒大小主要分布在10~55nm 之间,复合纳米颗粒
主要由面心立方镍纳米晶体和石墨碳构成,常温下这些复合纳米颗粒主要表现出超顺磁性和铁磁性能。 相似文献
908.
909.
为研究大气腐蚀对Q420钢管构件服役期内耐撞性的影响,提出了计及腐蚀损伤的材料模型。引入损伤因子(ω)修正Voce模型,推导随腐蚀程度变化的低合金钢材本构方程,并通过加速腐蚀试验结果回归相应参数。利用ABAQUS软件定义构件材料特性,建立起受腐蚀的Q420钢管仿真模型,采用显式动力算法分析多种初始状态下,撞击体与不同腐蚀程度钢管的冲击响应规律。开展预腐蚀Q420钢管的落锤试验,将试验结果与数值计算结果进行对比,验证所建模型的合理性。结果表明:大气腐蚀导致材料名义强度降低,对Q420钢管抗撞击能力影响显著;随着腐蚀程度增加,冲击力峰值减小,撞击时间和深度增加;Q420钢管受腐蚀后抗冲击刚度减小,构件整体变形耗能增加,表明大气腐蚀使其抗冲击性能下降;同等动能增量下,增大撞击体初速度比增加初始质量获得的冲击力峰值增幅更大,而所得到的接触时间增幅更小。 相似文献
910.
对常温常压下柴油舱组集中透气管中柴油蒸汽的燃爆危险性以及阻隔防爆技术的抑爆效能开展了实验研究。利用nac HX-3高速相机和CY-YD-205压力传感器记录燃爆传播过程和爆炸超压,阻隔防爆装置分别采用新型抑爆小球和普通波纹型阻火器。结果表明:常温常压下,一旦柴油油舱发生爆炸,爆炸火球可以通过透气管传播到相邻油舱,并引起二次爆炸,具有较大的危险性;普通波纹型阻火器在爆炸过程中阻火失效,而新型抑爆小球具有较好的抑爆效果;相对于光滑透气管工况,在点火舱上方安装抑爆小球后,被点火舱内的最大爆炸超压可以显著地从552.5 kPa降低到35.0 kPa;抑爆小球的中空多孔结构是其阻隔防爆的关键,多孔结构不仅可以增大比表面积、扩大热损失,而且还可以有效地分割削弱反应面。 相似文献