全文获取类型
收费全文 | 49558篇 |
免费 | 4906篇 |
国内免费 | 2416篇 |
专业分类
电工技术 | 3378篇 |
技术理论 | 1篇 |
综合类 | 3945篇 |
化学工业 | 6704篇 |
金属工艺 | 1966篇 |
机械仪表 | 3401篇 |
建筑科学 | 7477篇 |
矿业工程 | 1013篇 |
能源动力 | 3400篇 |
轻工业 | 5267篇 |
水利工程 | 674篇 |
石油天然气 | 2359篇 |
武器工业 | 692篇 |
无线电 | 3082篇 |
一般工业技术 | 4746篇 |
冶金工业 | 2283篇 |
原子能技术 | 329篇 |
自动化技术 | 6163篇 |
出版年
2024年 | 277篇 |
2023年 | 925篇 |
2022年 | 1777篇 |
2021年 | 2113篇 |
2020年 | 2155篇 |
2019年 | 1819篇 |
2018年 | 1734篇 |
2017年 | 1988篇 |
2016年 | 2098篇 |
2015年 | 2135篇 |
2014年 | 3292篇 |
2013年 | 3155篇 |
2012年 | 3759篇 |
2011年 | 3806篇 |
2010年 | 2840篇 |
2009年 | 2861篇 |
2008年 | 2494篇 |
2007年 | 3200篇 |
2006年 | 2672篇 |
2005年 | 2274篇 |
2004年 | 1829篇 |
2003年 | 1517篇 |
2002年 | 1220篇 |
2001年 | 1017篇 |
2000年 | 766篇 |
1999年 | 603篇 |
1998年 | 412篇 |
1997年 | 347篇 |
1996年 | 335篇 |
1995年 | 227篇 |
1994年 | 179篇 |
1993年 | 134篇 |
1992年 | 107篇 |
1991年 | 96篇 |
1990年 | 76篇 |
1989年 | 59篇 |
1988年 | 44篇 |
1987年 | 37篇 |
1986年 | 20篇 |
1985年 | 32篇 |
1984年 | 43篇 |
1983年 | 28篇 |
1982年 | 26篇 |
1980年 | 47篇 |
1965年 | 19篇 |
1964年 | 28篇 |
1963年 | 26篇 |
1961年 | 22篇 |
1956年 | 18篇 |
1955年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Since electronic switching systems usually require very strict reliability requirements as well as good performance objectives, we need to jointly analyse the performance and reliability of switching systems. In this paper, we compare conventional time–space–time switches with single space switches with those with multiple separated space switches, from the viewpoints of reliability and performance. We consider time–space–time switching networks which consist of N incoming time switches, i.e. one NxN space switch, two (N/2)x(N/2) space switches, and four (N/4)x(N/4) space switches. We introduce a Markov reliability model to study the effect of failures and analyse the reliability and performance of three different types of switching networks in terms of average blocking probability and the mean time to unreliable operation, as we vary the offered traffic. As a result, T–S–T switching networks with multiple separated space switches exhibit better performance and reliability than those with single space switches. 相似文献
102.
The bond strength and slip of epoxy-coated reinforcing bars in concrete have been evaluated by carrying out single pullout and double pullout tests. In extended single pullout tests, slip measurements were made while tensile force was applied to reinforcing bars embedded in concrete. In double pullout tests, 20 cycles of load were applied at levels of steel stress between zero and 0·5 times characteristic steel strength. Strains were measured by electrical resistance strain gauges glued inside the bars. Both epoxy-coated and uncoated bars were used in the investigation, to obtain comparative results. The strain gradient along the bar was found to be less for the coated reinforcement. In general, the epoxy coating was found to increase slip in bond and thereby reduce the bond performance of coated bars. 相似文献
103.
104.
本文提出了一种在广播信道上基于倾听机制的独立网络性能管理系统。这种系统适用于用物理广播信道连接,但存在多种协议体系结构的网络系统,例如CIMS网络。 相似文献
105.
106.
107.
本法采用同一色谱条件反相高效液相色谱法测定棉铃宝中甲基对硫磷和辛硫磷的含量,方法简单易行,具有较高的精密度和准确度,其回收率分析为甲基对硫磷99.59%,辛硫磷101.0%。 相似文献
108.
Second-law-based thermodynamic analysis of two-stage and mechanical-subcooling refrigeration cycles 总被引:1,自引:0,他引:1
Thermodynamic analysis of HFC-134a vapor-compression refrigeration cycles is investigated by both the first and second laws of thermodynamics. Second-law analysis is carried out for both two-stage and mechanical-subcooling refrigeration cycles. The analysis is performed on each of the system components to determine their individual contribution to the overall system irreversible losses. It is found that most of the losses are due to a low compressor efficiency. Irreversibilities of expansion valves and condenser are also significant. In addition, it is shown that the optimum inter-stage pressure for two-stage and mechanical-subcooling refrigeration systems is very close to the saturation pressure corresponding to the arithmetic mean of the refrigerant condensation and evaporation temperatures. These results are compared with the existing practice in the industry. Furthermore, theoretical results of a two-stage refrigeration system performance are also compared with experimental values for a CFC-22 system. 相似文献
109.
Sensitivity analysis is used to show how uncertainties in basic factors, such as heat transfer, pressure drop, compressor efficiency, and thermophysical properties, influence the result of heat pump cycle performance estimations. By comparing the results of this analysis with uncertainty levels of estimations and experimental determinations, it is established to what extent improvements in accuracy are needed. The investigation was carried out for several different applications and, as working fluids, both pure substances and non-azeotropic mixtures were examined. It was found that evaporation heat transfer and compressor efficiency are two areas where improvements are needed. It is also shown that estimates using very accurate, substance-specific, equations of state differ from those using more general types by up to 1 % for COP and 2% for capacity. 相似文献
110.
Downhole pumps are being used increasingly more often in low-enthalpy geothermal wells. The depth at which these pumps are installed depends on the physical and chemical characteristics of the geothermal fluid, the production flow rate, and the reservoir pressure and permeability. In this study we have investigated the factors affecting pump setting depths in low-temperature, liquid-dominated geothermal systems and defined the relationship between flow rate and pressure drawdown based on multi-rate test results. The methodology proposed was applied to the Balcova-Narlidere geothermal field, Turkey. It was found that the most important parameters for determining the capacity and setting depth of a downhole pump were flow performance, non-condensable gas concentration, and temperature. The implementation of the methodology is illustrated. 相似文献