全文获取类型
收费全文 | 21022篇 |
免费 | 1075篇 |
国内免费 | 844篇 |
专业分类
电工技术 | 574篇 |
技术理论 | 1篇 |
综合类 | 1585篇 |
化学工业 | 1648篇 |
金属工艺 | 2463篇 |
机械仪表 | 4212篇 |
建筑科学 | 893篇 |
矿业工程 | 1096篇 |
能源动力 | 403篇 |
轻工业 | 697篇 |
水利工程 | 167篇 |
石油天然气 | 1552篇 |
武器工业 | 166篇 |
无线电 | 918篇 |
一般工业技术 | 2423篇 |
冶金工业 | 1079篇 |
原子能技术 | 333篇 |
自动化技术 | 2731篇 |
出版年
2024年 | 47篇 |
2023年 | 206篇 |
2022年 | 439篇 |
2021年 | 426篇 |
2020年 | 466篇 |
2019年 | 369篇 |
2018年 | 390篇 |
2017年 | 552篇 |
2016年 | 638篇 |
2015年 | 713篇 |
2014年 | 1093篇 |
2013年 | 1116篇 |
2012年 | 1402篇 |
2011年 | 1479篇 |
2010年 | 1157篇 |
2009年 | 1154篇 |
2008年 | 1155篇 |
2007年 | 1418篇 |
2006年 | 1343篇 |
2005年 | 1186篇 |
2004年 | 1031篇 |
2003年 | 959篇 |
2002年 | 748篇 |
2001年 | 634篇 |
2000年 | 518篇 |
1999年 | 460篇 |
1998年 | 332篇 |
1997年 | 311篇 |
1996年 | 234篇 |
1995年 | 205篇 |
1994年 | 163篇 |
1993年 | 104篇 |
1992年 | 89篇 |
1991年 | 78篇 |
1990年 | 65篇 |
1989年 | 48篇 |
1988年 | 39篇 |
1987年 | 24篇 |
1986年 | 11篇 |
1985年 | 32篇 |
1984年 | 18篇 |
1983年 | 21篇 |
1982年 | 14篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1964年 | 3篇 |
1963年 | 4篇 |
1956年 | 3篇 |
1955年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
《International Journal of Hydrogen Energy》2022,47(12):8130-8144
Compared to liquid/gas hydrogen tank, the pipeline is an economical way for hydrogen transportation. With the quick development of utility tunnel in China, hydrogen pipeline enters the gas compartment can be expected soon. However, all the safety requirements of the gas compartment in the current standards are designed for natural gas, and the applicability for hydrogen is unknown. Therefore, a series of studies were started to investigate the safety of hydrogen in utility tunnel. In this work, a real utility tunnel locates at Shanghai was selected as the physical object. A 3D numerical model was built and successfully validated by a scaled tunnel test. The model has the maximum deviation of +9.5%. After that, a comparatively study of the dispersion behavior of CH4 and H2 was conducted. The assumed scenario was a 20 mm small-hole leaks with gauge pressure of 1.0 MPa in the middle of the tunnel. Numerical results shown that, H2 has a larger dispersion velocity and higher concentration, and is more dangerous compared to CH4. The current emergency ventilation strategy of air change rate of 12 times/h is not effective enough to dilute the H2 flammable cloud. The alarm time of the testing points shown strong linear law. There was a sharp variation in the range of 20%–100% LFL (Lower Flammable Limit), so the alarm strategy in the tunnel standards is too ideal for both CH4 and H2. The numerical results in the present study could provide a guidance for the design and safety management of the hydrogen tunnel. 相似文献
2.
The evaluation of the volumetric accuracy of a machine tool is an open challenge in the industry, and a wide variety of technical solutions are available in the market and at research level. All solutions have advantages and disadvantages concerning which errors can be measured, the achievable uncertainty, the ease of implementation, possibility of machine integration and automation, the equipment cost and the machine occupation time, and it is not always straightforward which option to choose for each application. The need to ensure accuracy during the whole lifetime of the machine and the availability of monitoring systems developed following the Industry 4.0 trend are pushing the development of measurement systems that can be integrated in the machine to perform semi-automatic verification procedures that can be performed frequently by the machine user to monitor the condition of the machine. Calibrated artefact based calibration and verification solutions have an advantage in this field over laser based solutions in terms of cost and feasibility of machine integration, but they need to be optimized for each machine and customer requirements to achieve the required calibration uncertainty and minimize machine occupation time.This paper introduces a digital twin-based methodology to simulate all relevant effects in an artefact-based machine tool calibration procedure, from the machine itself with its expected error ranges, to the artefact geometry and uncertainty, artefact positions in the workspace, probe uncertainty, compensation model, etc. By parameterizing all relevant variables in the design of the calibration procedure, this simulation methodology can be used to analyse the effect of each design variable on the error mapping uncertainty, which is of great help in adapting the procedure to each specific machine and user requirements. The simulation methodology and the analysis possibilities are illustrated by applying it on a 3-axis milling machine tool. 相似文献
3.
4.
5.
6.
《International Journal of Hydrogen Energy》2022,47(57):24242-24253
As hydrogen refueling stations become increasingly common, it is clear that a high level of economic efficiency and safety is crucial to promoting their use. One way to reduce costs is to use a simple orifice instead of an excess flow valve, which Japanese safety regulations have identified as a safety device. However, there is concern about its effect on refueling time and on risk due to hydrogen leakage. To clarify the effect, we did a study of model-based refueling time evaluation and quantitative risk assessment for a typical refueling station. This study showed that an orifice is an effective alternative safety device. The increase in refueling time was less than 10%, based on simulations using a dynamic physical model of the station. Neither was there a significant difference in the risk between a configuration with excess flow valves and one with an orifice. 相似文献
7.
Wassim Bahr 《国际自然能源杂志》2018,39(8):802-812
This paper assesses building integrated photovoltaic (BIPV) installation parameters based on the profit generated by a photovoltaic system. It takes into consideration a home building case study and it investigates its monthly energy demand based on a specific location and a typical occupancy. The capability of a photovoltaic (PV) system to generate more profit occurs when solar intensity is maximum while the electric energy price is at its highest rate. The paper traces a framework that encompasses different aspects such as energy demand, energy price, and solar intensity. This framework identifies profit alternatives according to different installation parameters. A tool that predicts a PV installation hourly electric energy production is developed. The profit generated is simulated for home buildings located in Beirut (Lebanon) and Xihua (China), both at 33.8° latitude north. The paper highlights a new approach for BIPV installations, taking into account weather conditions, energy demand, and electric energy utility rates. 相似文献
8.
常规的冲击钻井工具不能满足井眼轨迹控制要求、使用寿命有限、缓解脱压不显著等问题,影响了在定向井钻井中的推广应用。建立248;215.9 mm PDC钻头和岩石的有限元模型,分析钻井参数对破岩效率的影响规律,结果表明:冲击频率为17.5 Hz和35 Hz时,破岩效率最优; 破岩效率随钻压增大而增加,而后变化逐渐平缓; 破岩效率随钻头转速增大而增加; 破岩效率随冲击载荷的增加变化平缓。据此研制了新型水力旋冲钻井工具。现场试验表明,该工具符合定向水平井“一趟钻”工艺要求,使机械钻速提高56%,使用寿命达150 h,MWD信号正常。 相似文献
9.
In different computer models, shape is represented using different methodologies, to varying degrees of precision. This paper examines two approaches to shape representation, and their effects on accuracy in the context of cylindrical particle packing. Two discrete element method (DEM) based software packages are used. A X-ray CT scan of a packed bed provides the experimental measurements for comparison. Eight sphere-composite representations of the same cylindrical pellet were tested. Two of these gave results that quantitatively follow experimental measurements. A range of factors that in theory could affect accuracy of the simulation results are examined, including edge roundedness, surface roughness and restitutional behaviour as a function of sphere-composite representations. The conclusion is that, for packing at least, matching the object's overall shape and dimensions is not enough. Only when a high enough resolution is applied to corners and edges, could the sphere-composite approach possibly match the experimental data quantitatively. 相似文献
10.
When five axis CNC machine tools follow series linear toolpath segments, the drives experience velocity, acceleration and jerk discontinuities at the block transition points. The discontinuities result in fluctuations on machine tool motions which lead to poor surface quality. This paper proposes to insert quintic and septic micro-splines for the tool tip and tool-orientation, respectively, at the adjacent linear toolpath segments. Optimal control points are calculated for position and orientation splines to achieve C3 continuity at the junctions while respecting user-defined tolerance limits. The geometrically smoothed corners are traveled at a smoothly varying feed with cubic acceleration trajectory profile. The proposed method is experimentally demonstrated to show improvements in motion smoothness and tracking accuracy in five-axis machining of free-form surfaces found in dies, molds and aerospace parts. 相似文献