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   100034篇
  免费   10692篇
  国内免费   3452篇
电工技术   6124篇
技术理论   1篇
综合类   6577篇
化学工业   19088篇
金属工艺   8160篇
机械仪表   4128篇
建筑科学   9193篇
矿业工程   2631篇
能源动力   9169篇
轻工业   9134篇
水利工程   1983篇
石油天然气   4113篇
武器工业   682篇
无线电   5972篇
一般工业技术   11888篇
冶金工业   6579篇
原子能技术   2023篇
自动化技术   6733篇
  2024年   105篇
  2023年   1640篇
  2022年   3367篇
  2021年   4594篇
  2020年   3557篇
  2019年   3115篇
  2018年   3085篇
  2017年   3088篇
  2016年   4016篇
  2015年   4579篇
  2014年   6282篇
  2013年   6946篇
  2012年   6644篇
  2011年   6196篇
  2010年   4685篇
  2009年   4868篇
  2008年   4285篇
  2007年   6422篇
  2006年   6419篇
  2005年   5494篇
  2004年   4174篇
  2003年   3864篇
  2002年   3105篇
  2001年   2248篇
  2000年   1760篇
  1999年   1494篇
  1998年   1105篇
  1997年   962篇
  1996年   889篇
  1995年   836篇
  1994年   730篇
  1993年   546篇
  1992年   494篇
  1991年   347篇
  1990年   332篇
  1989年   295篇
  1988年   193篇
  1987年   166篇
  1986年   142篇
  1985年   136篇
  1984年   156篇
  1983年   108篇
  1982年   114篇
  1981年   55篇
  1980年   65篇
  1979年   38篇
  1978年   27篇
  1964年   36篇
  1962年   65篇
  1959年   54篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
In the debate on the decarbonisation of heat, renewable electricity tends to play a much more dominant role than green gases, despite the potential advantages of gas in terms of utilising existing transportation networks and end-use appliances. Informed comparisons are hampered by information asymmetry; the renewable electricity has seen a huge grid level deployment whereas low-carbon hydrogen or bio-methane have been limited to some small, stand-alone trials. This paper explores the regulatory and commercial challenges of implementing the first UK neighbourhood level 100% low-carbon hydrogen demonstration project. We draw on existing literature and action research to identify the key practical barriers currently hindering the ability of strategically important actors to accelerate the substitution of natural gas with low carbon hydrogen in local gas networks. This paper adds much needed contextual depth to existing generic and theoretical understandings of low-carbon hydrogen for heat transition feasibility. The learnings from pilot projects, about the exclusion of hydrogen calorific value from the Local Distribution Zone calorific value calculation, Special Purpose Vehicle companies, holding of liability and future costs to consumers, need to be quickly transferred into resilient operational practice, or gas repurposing projects will continue to be less desirable than electrification using existing regulations, and with more rapid delivery.  相似文献   
2.
某铅锌矿原矿铅含量为1.26%,含锌6.53%,含硫30.38%。生产上采用“铅锌依次优先浮选-中矿顺序返回”工艺流程,生产指标为铅精矿铅品位50.69%,含锌12.61%,铅回收率75.53%,锌精矿锌品位48.77%,含铅1.59%,锌回收率73.91%。铅锌互含较高,锌精矿指标不理想。为了解决该问题,本文在了解现场生产工艺流程及矿石性质的基础上,针对该铅锌矿开展了详细的选矿工艺优化试验,通过对部分药剂制度进行优化,采用特效捕收剂BK-LY11,同时在锌浮选回路采用中矿再磨工艺,显著改善了铅锌互含情况,有效提高了铅锌选别指标,并成功应用于生产实践,优化后获得的铅、锌回收率分别提高了5.83、8.46个百分点。  相似文献   
3.
为提高跨临界CO2空气源热泵热水系统运行性能,对不同压缩机频率、电子膨胀阀开度和水流量下的系统及部件运行特性进行了实验研究,并对循环加热模式下的调节方式进行了优化。结果表明:当阀开度减小时,系统性能系数COP和制热量随过热度的增大而减小;压缩机存在最大电效率点,并且电效率极值点会随着运行频率的增大而向低过热度段发生移动;与电子膨胀阀调节方式相比,水流量调节方案下系统COP提升了13.22%;在循环加热模式下不同加热时期的最优阀开度并不相同,根据最优高压理论对其调节方式进行了优化,最大优化率为15.96%。  相似文献   
4.
为了降低热电联产系统能量调度算法计算的复杂度,减少计算时间,提出了一种基于If-Then-Else规则的热电联产系统能量调度方法。通过引入逻辑变量来描述设备的启停状态和工作模式,建立混合逻辑动态模型,在模型预测控制的框架下,根据实时电价以及本地能源、电力负荷和热力负荷的预测结果,为模型中的二元决策变量赋值,从而将模型中的混合整数线性规划问题转化为线性规划问题。最后,通过仿真将本文提出的方法和混合整数线性规划方法进行比较,结果表明,本文所采用的方法在性能上几乎没有损失,平均计算时间降低65%。  相似文献   
5.
本文以工业纯铝和混合稀土RE(Ce和La)为原料,制备出Al-8RE、Al-10RE和Al-15RE中间合金,并探讨了Al-RE中间合金对变形铝合金3102的细化机理。结果表明:Al-RE中间合金由α-Al、Al11RE3(Al11Ce3和Al11La3)、Ce和La组成,不同稀土含量Al-RE中间合金微观组织形貌不同,Al-8RE、Al-10RE和Al-15RE中间合金组织分别为亚共晶组织,伪共晶组织和过共晶组织,随稀土含量增加,Al-RE中间合金中Al11RE3相数量增多,尺寸增大;Al-RE中间合金对3102合金具有显著的细化效果,其中Al-10RE中间合金的细化效果最好;添加Al-RE中间合金可以提高产品性能,当添加10%RE时,性能提升最大。  相似文献   
6.
Hybrid nanofluid boundary layer flow past a stretching surface with zero mass flux boundary condition is explored in this article. The main aim of this article is to analyze the electromagnetohydrodynamic role in a hybrid nanofluid containing silver and molybdenum disulfide nanoparticles. The self-similar solution is embedded to reduce the governing partial differential equation into algebraic equations and a shooting algorithm is applied to obtain the solution of the resultant boundary value problem. Variation in momentum, energy, and nanoparticle concentration is explained through graphical profiles. Nusselt number and drag force coefficients are computed for various flow parameters and their impact on the nanofluid and hybrid nanofluid is computed and presented and explained in a comparative fashion. It is observed that the velocity profile shows the opposite nature with respect to the electric field and magnetic field. For electric field parameter velocity accelerates whereas for magnetic parameter velocity diminishes. Nusselt number increases with electric field parameter and nanoparticle volume fraction.  相似文献   
7.
有机朗肯循环(Organic Rankine Cycle,ORC)可实现中低温热能的有效利用,但其热利用效率仍可进一步提高,而内置换热器的使用可有效提高循环的热利用效率。因此,文章构建了内置换热器ORC系统的热力学模型。研究了当蒸发温度变化时,R600,R601a,R236ea,R245fa,R245ca,R123,R600a,R114和R142b对内置换热器ORC系统性能的影响,并比较了内置换热器ORC系统与传统ORC系统的净输出功率和热效率。结果表明:在最优蒸发温度下,采用R236ea的内置换热器ORC系统净输出功率大于其余工质,为32.40 kW,其比第二最大净输出功率R600a系统相对增大1.16%;采用R601a的内置换热器ORC系统的热效率最大。内置换热器ORC系统的最大净输出功率及热效率均较传统ORC系统显著增大。  相似文献   
8.
The development of efficient filters is an essential part of industrial machinery design, specifically to increase the lifespan of a machine. In the filter chamber design considered in this study, the magnetic material is placed along the horizontal surface of the filter chamber. The inside of the filter chamber is layered with a porous material to restrict the outflow of unwanted particles. This study aims to investigate the flow, pressure, and heat distribution in a dilating or contracting filter chamber with two outlets driven by injection through a permeable surface. The proposed model of the fluid dynamics within the filter chamber follows the conservation equations in the form of partial differential equations. The model equations are further reduced to a steady case through Lie's symmetry group of transformation. They are then solved using a multivariate spectral-based quasilinearization method on the Chebyshev–Gauss–Lobatto nodes. Insights and analyses of the thermophysical parameters that drive optimal outflow during the filtration process are provided through the graphs of the numerical solutions of the differential equations. We find, among other results, that expansion of the filter chamber leads to an overall decrease in internal pressure and an increase in heat distribution inside the filter chamber. The results also show that shrinking the filter chamber increases the internal momentum inside the filter, which leads to more outflow of filtrates.  相似文献   
9.
In the present paper, therapeutic treatment of infected tumorous cells has been studied through mathematical modeling and simulation of heat transfer in tissues by using a nonlinear dual-phase lag bioheat transfer model with Dirichlet boundary condition. The components of volumetric heat source in this model such as blood perfusion and metabolism are assumed experimentally validated temperature-dependent function, which gives more accurate temperature distribution in tissues through this model. We have used the finite difference and RK (4, 5) techniques of numerical methods to solve the proposed problem and obtained the exact solution in a particular case. After comparison, we got a good agreement between them. We have used dimensionless quantities throughout this paper. The effect of relaxation and thermalization time with respect to dimensionless temperature distribution has been analyzed in the treatment process.  相似文献   
10.
We considered the magnetohydrodynamic (MHD) free convective flow of an incompressible electrically conducting viscous fluid past an infinite vertical permeable porous plate with a uniform transverse magnetic field, heat source and chemical reaction in a rotating frame taking Hall current effects into account. The momentum equations for the fluid flow during absorbent medium are controlled by the Brinkman model. Through the undisturbed state, both the plate and fluid are in a rigid body rotation by the uniform angular velocity perpendicular to an infinite vertical plate. The perpendicular surface is subject to the homogeneous invariable suction at a right angle to it and the heat on the surface varies about a non-zero unvarying average whereas the warmth of complimentary flow is invariable. The systematic solutions of the velocity, temperature, and concentration distributions are acquired systematically by utilizing the perturbation method. The velocity expressions consist of steady-state and fluctuating situations. It is revealed that the steady part of the velocity field has a three-layer characteristic while the oscillatory part of the fluid field exhibits a multi-layer characteristic. The influence of various governing flow parameters on the velocity, temperature, and concentration are analyzed graphically. We also discuss computational results for the skin friction, Nusselt number, and Sherwood number in the tabular forms.  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