全文获取类型
收费全文 | 40742篇 |
免费 | 4104篇 |
国内免费 | 2807篇 |
专业分类
电工技术 | 2401篇 |
技术理论 | 1篇 |
综合类 | 3831篇 |
化学工业 | 6219篇 |
金属工艺 | 3106篇 |
机械仪表 | 1765篇 |
建筑科学 | 2812篇 |
矿业工程 | 1912篇 |
能源动力 | 1794篇 |
轻工业 | 3298篇 |
水利工程 | 1189篇 |
石油天然气 | 2476篇 |
武器工业 | 575篇 |
无线电 | 4151篇 |
一般工业技术 | 4413篇 |
冶金工业 | 2955篇 |
原子能技术 | 1015篇 |
自动化技术 | 3740篇 |
出版年
2024年 | 166篇 |
2023年 | 539篇 |
2022年 | 1083篇 |
2021年 | 1307篇 |
2020年 | 1488篇 |
2019年 | 1238篇 |
2018年 | 1248篇 |
2017年 | 1603篇 |
2016年 | 1726篇 |
2015年 | 1742篇 |
2014年 | 2607篇 |
2013年 | 2844篇 |
2012年 | 3162篇 |
2011年 | 3202篇 |
2010年 | 2375篇 |
2009年 | 2316篇 |
2008年 | 2091篇 |
2007年 | 2582篇 |
2006年 | 2163篇 |
2005年 | 1876篇 |
2004年 | 1618篇 |
2003年 | 1401篇 |
2002年 | 1180篇 |
2001年 | 1024篇 |
2000年 | 865篇 |
1999年 | 706篇 |
1998年 | 607篇 |
1997年 | 518篇 |
1996年 | 482篇 |
1995年 | 402篇 |
1994年 | 303篇 |
1993年 | 230篇 |
1992年 | 217篇 |
1991年 | 166篇 |
1990年 | 146篇 |
1989年 | 100篇 |
1988年 | 67篇 |
1987年 | 45篇 |
1986年 | 43篇 |
1985年 | 35篇 |
1984年 | 30篇 |
1983年 | 17篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 16篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1975年 | 9篇 |
1959年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
在Nakagami-m衰落信道下,分析了两跳固定增益放大转发中继通信系统的性能.首先采用基于概率密度函数(PDF)的性能分析法推导了无协作分集时系统的中断概率和平均误符号率(ASER)的闭合表达式,然后采用基于矩生成函数(MGF)的方法推导了有协作分集时系统的中断概率和ASER表达式以及接收信噪比的n阶矩的闭合表达式.仿真结果显示,所推导的闭合表达式与数值仿真结果吻合良好,协作分集和较大的m值可提升系统性能,中继前后两跳的信道质量对系统性能的影响并不相同. 相似文献
962.
依据公积金的性质,在其运作非营利的前提下,以收支平衡模型为衡量标准分析我国现行公积金增值收益分配的不公平性。提出公平改进后的住房公积金运作模型,并通过分析其内生变量之间的互动关系,得出公积金储户的公平性损失是由利率管制引起的。利率管制使公积金存贷款利率僵化,不能通过利率机制把增值收益返还给储户;在利率管制的背景下,激励机制的缺乏导致公积金管理的低效率对储户也造成了一定损失。 相似文献
963.
964.
Several countermeasures against the prevalence of infectious diseases have recently been issued, and one of them, the ultraviolet germicidal irradiation (UVGI) system, has been carefully considered for building environments especially. Besides experimental methods to evaluate the germicidal performance of upper air UVGI systems, this research introduces two numerical methods using the concept of ventilation performance, and illustrates the methods with a ward model. The first calculates the average residence time of air using the concept of local purging flow rate (L-PFR) which is multiplied by the average UV intensity of the upper area to obtain a UV dose. The other calculates the UV dose with the distributional UV intensity and deals with UV intensities as contaminant sources. The results of the illustrative cases with a ward model show that the method using the L-PFR concept could not clearly identify the difference in UV doses for each case with different exhaust opening setups, although the other could. The results from the method using the distributional UV intensity indicated the layout of ventilation openings and upper-room UVGI systems are important to optimize the germicidal performance. 相似文献
965.
再生骨料混凝土抗拉强度和抗剪强度试验研究 总被引:1,自引:1,他引:0
考虑混凝土两个强度等级(C20、C30)和再生骨料取代率,进行再生混凝土劈裂抗拉试验、非标准试件的抗折试验和等高变宽梁的抗剪试验,研究再生混凝土抗拉、抗剪性能和变形,获得了以下成果:1)再生混凝土的劈裂强度稍低于普通混凝土。2)再生混凝土抗折强度离散范围较大,与立方体抗压强度间没有显著的回归关系。3)再生混凝土的抗剪能力随取代率增加没有明显的变化,极限切应力与混凝土的强度回归相关性较差。4)抗剪试验梁的主拉应变曲线在大约70%最大值以下时呈直线,超过此值以后逐渐弯曲。试验实测的名义切应力、剪切弹性模量较低。 相似文献
966.
967.
968.
Biooxidation of sulphide under denitrifying conditions is a key process in control of souring in oil reservoirs and in treatment of gas and liquids contaminated with sulphide and nitrate. In this work, biooxidation of sulphide was studied using a representative culture originated from an oil reservoir. Effects of sulphide concentration, sulphide to nitrate molar ratio, and loading rates of sulphide and nitrate on their removal rates and composition of the end products were investigated. In the batch system sulphide removal rate passed through a maximum as sulphide concentration was increased from 2.1 to 16.3 mM, with the highest rate (2.06 mM h−1) observed with 10.7 mM sulphide. Nitrate removal was coupled to sulphide oxidation and the highest removal rate was 1.05 mM h−1. In the continuous bioreactors fed with 10 and 5, 15 and 7.5, and 20 and 10 mM sulphide and nitrate, cell wash-out occurred as dilution rate was increased above 0.15, 0.13 and 0.08 h−1, respectively. Prior to cell wash-out linear increases in sulphide and nitrate removal rates were observed as loading rate was increased. The highest sulphide and nitrate removal rates of 2.0 and 0.92 mM h−1 were obtained in the bioreactor fed with 15 mM sulphide and 7.5 mM nitrate at loading rates of 2.1 and 0.93 mM h−1, respectively. Short residence times and high sulphide to nitrate ratios promoted the formation of sulphur, a desired end product for ex situ treatment of contaminated streams. Combination of long residence times and low sulphide to nitrate ratios, which favours formation of sulphate, is the suitable strategy for in situ removal of H2S from oil reservoirs. 相似文献
969.
Der-Fong Juang Chao-Hsien Lee Chung-Shin Yuan 《The Science of the total environment》2010,408(20):4524-4531
To understand the potential threat of volatile organic compounds (VOCs) to the health of residents living close to a heavily polluted river, this study investigated the species and the concentration of VOCs evaporating from a river and surveyed the health condition of the nearby residents. Air samples were taken seasonally at the upstream, midstream, and downstream water surfaces of the river, and at different locations at certain distances from the river. These samples were analyzed qualitatively and quantitatively through gas chromatography and electron capture detector (GC/ECD) for chlorinated organic compounds, and through gas chromatography and flame ionization detector (GC/FID) for ordinary hydrocarbons. The health data obtained from valid health questionnaires of 908 residents were analyzed through Statistical Package for Social Science (SPSS) software. Twenty-six species of VOCs were identified in the environment adjacent the river, many of which are carcinogenic or believed to be carcinogenic to humans. However, results of this study shows that the VOCs evaporating from the polluted river have not been definitively identified as a major factor of cancer in the residents. However, the risk of suffering from certain chronic diseases may increase in residents living less than 225 m away from the river due to the high levels of evaporated VOCs. Residents living less than 225 m away from the river and with nearby specific industries are 3.130 times more at risk of suffering from chronic diseases than those with no nearby specific industries. 相似文献
970.
南方山谷型填埋场渗滤液产量及水位控制措施 总被引:2,自引:0,他引:2
在我国南方建成的第一批山谷型填埋场大多存在渗滤液产量大、堆体中渗滤液水位高的问题。基于对南方几个典型填埋场的现场调查和相关测试结果,从水量平衡的角度分析了影响山谷型填埋场渗滤液产量的关键因素,探讨了我国现有山谷型垃圾填埋场在渗滤液产量及水位控制方面存在的问题,并借鉴国内外相关工程经验教训提出了一些改进措施(包括采用带有垂直防渗体的截洪沟来控制场外汇水区域地下径流补给量、采用新型临时覆盖材料及工艺避免在堆体中形成渗滤液阻滞层、渗滤液收集与导排系统的防淤堵设计及淤堵反冲洗维护措施等)。 相似文献