全文获取类型
收费全文 | 39912篇 |
免费 | 4936篇 |
国内免费 | 2716篇 |
专业分类
电工技术 | 5136篇 |
综合类 | 4555篇 |
化学工业 | 2803篇 |
金属工艺 | 1377篇 |
机械仪表 | 2534篇 |
建筑科学 | 3750篇 |
矿业工程 | 1121篇 |
能源动力 | 1632篇 |
轻工业 | 1365篇 |
水利工程 | 1383篇 |
石油天然气 | 3673篇 |
武器工业 | 467篇 |
无线电 | 4851篇 |
一般工业技术 | 4076篇 |
冶金工业 | 1001篇 |
原子能技术 | 1184篇 |
自动化技术 | 6656篇 |
出版年
2024年 | 70篇 |
2023年 | 482篇 |
2022年 | 1056篇 |
2021年 | 1256篇 |
2020年 | 1352篇 |
2019年 | 1079篇 |
2018年 | 955篇 |
2017年 | 1302篇 |
2016年 | 1391篇 |
2015年 | 1602篇 |
2014年 | 2591篇 |
2013年 | 2526篇 |
2012年 | 3175篇 |
2011年 | 3401篇 |
2010年 | 2504篇 |
2009年 | 2552篇 |
2008年 | 2393篇 |
2007年 | 2884篇 |
2006年 | 2493篇 |
2005年 | 2095篇 |
2004年 | 1706篇 |
2003年 | 1515篇 |
2002年 | 1299篇 |
2001年 | 1064篇 |
2000年 | 846篇 |
1999年 | 664篇 |
1998年 | 510篇 |
1997年 | 491篇 |
1996年 | 416篇 |
1995年 | 360篇 |
1994年 | 339篇 |
1993年 | 252篇 |
1992年 | 209篇 |
1991年 | 154篇 |
1990年 | 116篇 |
1989年 | 128篇 |
1988年 | 90篇 |
1987年 | 41篇 |
1986年 | 40篇 |
1985年 | 34篇 |
1984年 | 20篇 |
1983年 | 20篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 12篇 |
1979年 | 6篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1959年 | 13篇 |
1955年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
The present research work concentrates on viscous dissipation, Dufour, and heat source on an unsteady magnetohydrodynamics natural convective flow of a viscous, incompressible, and electrically conducting fluid past an exponentially accelerated infinite vertical plate in the existence of a strong magnetic field. The presence of the Hall current induces a secondary flow in the problem. The distinguishing features of viscous dissipation and heat flux produced due to gradient of concentration included in the model along with heat source as they are known to arise in thermal-magnetic polymeric processing. The flow equations are discretized implicitly using the finite difference method and solved using MATLAB fsolve routine. Numerical values of the primary and secondary velocities, temperature, concentration, skin friction, Nusselt number, and Sherwood number are illustrated and presented via graphs and tables for various pertinent parametric values. The Dufour effect was observed to strengthen the velocity and temperature profile in the flow domain. In contrast, due to the impact of viscous dissipation, the local Nusselt number reduces. The study also reveals that the inclusion of the chemical reaction term augments the mass transfer rate and diminishes the heat transfer rate at the plate. 相似文献
2.
针对现有基于视频监控的人流量统计方案成本高、算法复杂且不利于个人隐私保护的局限性,利用毫米波雷达体积小、成本低、分辨率高的特点,提出了一种基于双时间点检测的人流量监测方法。该方法先获取人体目标散射点位置和多普勒频移信息来构成点云数据,然后根据多普勒频移正负来判断人体的运动方向,并筛选具有高多普勒频移值的点云数据以降低干扰点对聚类结果的影响;在双时间点对特定区域内人员数量进行统计,并根据双时间点之间所获取的点云数据聚类结果对所统计人员数据进行修正。实验结果表明,该方法能够用匿名的方式以较高的正确率统计人员进出。 相似文献
3.
《Journal of the European Ceramic Society》2021,41(14):6983-6990
Ternary 0.552Pb(Ni1/3Nb2/3)O3-xPbZrO3-(0.448-x)PbTiO3 (PNN-PZ-PT) ceramics near the triple point compositions were fabricated by an improved two-step sintering method. The triple point composition 0.552PNN-0.135PZ-0.313PT ceramic has outstanding piezoelectric performance with piezoelectric coefficient d33 = 1200 pC/N. Its easy fabrication and low cost make this piezoelectric material an excellent candidate for high sensitivity sensors and ultrasonic transducers. The evolution of domain structures for ceramics with composition near the triple point provides deeper insight into the mechanism of ultrahigh piezoelectric properties of PNN-PZ-PT ceramics. 相似文献
4.
《Ceramics International》2022,48(8):10817-10820
Single-crystal grains of TaN were synthesized by heating Ta2O5, FeTa2O6, or FeTaO4 in a BN crucible together with Na metal in an Ar atmosphere at 1100 °C. The BN crucible acted a solid source of nitrogen. Aggregates of columnar ε-TaN single crystals 10–150 μm in size were formed on the inner wall of the BN crucible when either Ta2O5 or FeTa2O6 was used. On the other hand, platelet-like single crystals of θ-TaN 1–50 μm in size were obtained from FeTaO4. The results of wavelength-dispersive X-ray spectrometry indicated that the compositions of the ε-TaN and θ-TaN crystals were close to the stoichiometric ideal. 相似文献
5.
以实际案例和文献资料收集为基础,主要围绕食用油检测从采样到结果判定一系列过程中可能涉及的操作风险点进行分析,并对部分典型检验指标(苯并(a)芘、塑化剂、抗氧化剂、残留溶剂、过氧化值、酸价、辣椒素、黄曲霉毒素B1)进行分析,介绍了分析过程中实验室环境、接触器具、试剂、样本本底、特殊基质干扰、指标稳定性等对实验结果有关键影响的因素,并针对具体问题提出解决措施,旨在为食用油检验风险防控提供技术支撑。 相似文献
6.
7.
8.
整个精确放疗流程最关键、最基础的一个环节就是定位CT扫描精确程度,做好定位CT的质控关乎整个放疗流程的成败。介绍了JJG(闽)1086-2018放射治疗CT模拟定位机X射线辐射源在检定和质控中的流程和方法,从剂量性能要求、图像成像质量和机器机械精度方面对CT模拟定位机的质量控制做了定量的要求。连续8个月的测试结果表明:CT测量值误差在30HU以内,图像均匀性≤5.0HU,空间分辨力7Lp/cm,低对比度分辨力在对比度为1.0%,0.5%,0.3%时分别≤3.0mm,≤5.0mm,≤6.0mm,达到了规程的要求。CT模拟定位机的图像质量和机械精确程度得到了很好的保障,定位CT的剂量准确,影像参数和位置精确度有规可循。 相似文献
9.
The thermoeconomic behaviour of a nanoparticle seeded single effect LiBr‐H2O absorption refrigeration system (ARS) is investigated for a small scale application. In the proposed method, alumina nanoparticles with volume concentrations of 3%, 5%, and 7% are dispersed into an aqua lithium bromide solution. The multiobjective heat transfer search algorithm is employed to examine the design trade‐off between the coefficient of performance (COP) and total annualized cost (TAC). To analyze the overall performance of the system, the influence of five design parameters, namely the temperatures of the generator, absorber, evaporator, condenser and heat exchanger pipe diameter, are studied. It is found that with an increase in the COP, the TAC of the system is initially raised marginally, and after that, raised rigorously with further increment. The comparative results indicate that the COP and TAC of the nanofluid based ARS system are increased by about 7% and decreased by about 3.2%, respectively, corresponding to the Pareto points of the base ARS system. A lower break‐even point of about 2.6 years is achieved for the ARS system containing nanoparticles compared to the base ARS system. Overall, the ARS system containing 5% nanoparticles is the best solution from a thermodynamic and economic point of view. 相似文献
10.
A Monte Carlo source model using PENELOPE was developed to investigate different tritiated metals in order to design a better radioisotope source for betavoltaic batteries. The source model takes into account the self‐absorption of beta particles in the source which is a major factor for an efficient source design. The average beta energy, beta flux, source power output, and source efficiency were estimated for various source thicknesses. The simulated results for titanium tritide with 0° and 90° angular distributions of beta particles were validated with experimental results. The importance of the backscattering effect due to isotropic particle emission was analyzed. The results showed that the normalized average beta energy increases with the source thickness, and it reaches peak energy depending on the density and the specific activity of the source. The beta flux and power output also increase with increasing source thickness. However, the incremental increase in beta flux and power output becomes minimal for higher thicknesses, as the source efficiency decreases significantly at higher thicknesses due to the self‐absorption effect. Thus, a saturation threshold is reached. A low‐density source material such as beryllium tritide provided a higher power output with higher efficiency. A maximum power output of approximately 4 mW/cm3 was obtained for beryllium tritide with SiC. A form factor approach was used to estimate the optimum source thickness. The optimum source thickness was found near the thickness where the peak beta particle average energy occurs. 相似文献