全文获取类型
收费全文 | 2098篇 |
免费 | 64篇 |
国内免费 | 10篇 |
专业分类
电工技术 | 39篇 |
综合类 | 1篇 |
化学工业 | 497篇 |
金属工艺 | 68篇 |
机械仪表 | 38篇 |
建筑科学 | 24篇 |
矿业工程 | 3篇 |
能源动力 | 112篇 |
轻工业 | 82篇 |
水利工程 | 21篇 |
石油天然气 | 17篇 |
无线电 | 282篇 |
一般工业技术 | 466篇 |
冶金工业 | 224篇 |
原子能技术 | 6篇 |
自动化技术 | 292篇 |
出版年
2023年 | 15篇 |
2022年 | 51篇 |
2021年 | 56篇 |
2020年 | 40篇 |
2019年 | 47篇 |
2018年 | 69篇 |
2017年 | 61篇 |
2016年 | 49篇 |
2015年 | 39篇 |
2014年 | 74篇 |
2013年 | 172篇 |
2012年 | 95篇 |
2011年 | 112篇 |
2010年 | 84篇 |
2009年 | 81篇 |
2008年 | 80篇 |
2007年 | 98篇 |
2006年 | 55篇 |
2005年 | 58篇 |
2004年 | 44篇 |
2003年 | 58篇 |
2002年 | 45篇 |
2001年 | 33篇 |
2000年 | 32篇 |
1999年 | 27篇 |
1998年 | 74篇 |
1997年 | 55篇 |
1996年 | 32篇 |
1995年 | 31篇 |
1994年 | 28篇 |
1993年 | 36篇 |
1992年 | 25篇 |
1991年 | 31篇 |
1990年 | 35篇 |
1989年 | 18篇 |
1988年 | 14篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 17篇 |
1984年 | 19篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 19篇 |
1980年 | 18篇 |
1979年 | 15篇 |
1978年 | 9篇 |
1977年 | 9篇 |
1976年 | 11篇 |
1975年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有2172条查询结果,搜索用时 15 毫秒
11.
We consider a system comprising a finite number of nodes, with infinite packet buffers, that use unslotted ALOHA with Code
Division Multiple Access (CDMA) to share a channel for transmitting packetised data. We propose a simple model for packet
transmission and retransmission at each node, and show that saturation throughput in this model yields a sufficient condition
for the stability of the packet buffers; we interpret this as the capacity of the access method. We calculate and compare
the capacities of CDMA‐ALOHA (with and without code sharing) and TDMA‐ALOHA; we also consider carrier sensing and collision
detection versions of these protocols. In each case, saturation throughput can be obtained via analysis of a continuous time
Markov chain. Our results show how saturation throughput degrades with code‐sharing. Finally, we also present some simulation
results for mean packet delay. Our work is motivated by optical CDMA in which “chips” can be optically generated, and hence
the achievable chip rate can exceed the achievable TDMA bit rate which is limited by electronics. Code sharing may be useful
in the optical CDMA context as it reduces the number of optical correlators at the receivers. Our throughput results help
to quantify by how much the CDMA chip rate should exceed the TDMA bit rate so that CDMA‐ALOHA yields better capacity than
TDMA‐ALOHA.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
12.
The scalar transform is a new representation for signals, offering a perspective that is different from the Fourier transform. We introduce the notion of a scalar periodic function. These functions are then represented through the discrete scale series. We also define the notion of a strictly scale-limited signal. Analogous to the Shannon interpolation formula, we show that such signals can be exactly reconstructed from exponentially spaced samples of the signal in the time domain. As an interesting, practical application, we show how properties unique to the scale transform make it very useful in computing depth maps of a scene 相似文献
13.
14.
2',4'-Dihydroxyacetophenone, also known as resoacetophenone, is a commercially important intermediate which is generally
prepared by the acylation of resorcinol with acetic acid in the presence of a molar excess of zinc chloride, which leads to
waste disposal problems. The most frequently used acylating agents such as acetic anhydride and acetyl chloride have several
disadvantages and need to be replaced by cheap and benign agents. In this connection, acetic acid is a better choice but with
a non-polluting and reusable catalyst. The synthesis of 2',4'-dihydroxyacetophenone from resorcinol and acetic acid was carried
out in the presence of a variety of solid acid catalysts such as montmorillonite clay (K-10), dodecatungstophosphoric acid
(DTP) supported on K-10, sulfated zirconia and ion exchange resins. Amongst these catalysts, Amberlyst-36, an ion exchange
resin, was found to be the most effective. The effects of various parameters on the rate of reaction and selectivity were
investigated to establish the intrinsic kinetics of the reaction. It was possible to deduce the adsorption equilibrium constant
and rate constant simultaneously for the reaction including the corresponding energies of activation. The catalyst is reusable.
The process is in consonance with the principles of green chemistry.
Electronic Publication 相似文献
15.
In this paper we report the impact of hot-carrier stress on analog performance of n- and p-MOSFET's with conventional oxide, NH3-nitrided oxide (RTN) and reoxidized nitrided oxide (RTN/RTO) as gate dielectrics. Changes due to hot-carrier stress in crucial analog parameters viz., drain output resistance, voltage gain, and input offset voltage of a source coupled differential MOSFET pair are investigated. Results show that RTN/RTO gate dielectrics suppress degradation of analog parameters in n-MOSFET's but increase it slightly in p-MOSFET's, as compared to conventional oxide MOSFET's 相似文献
16.
Of all the methods customarily used to transform E. coli we found only electroporation to be effective for transformation of the Gram-negative bacterium Vitreoscilla, yielding 5.10(5) transformants/microgram of plasmid DNA. The conditions used were close to those described for E. coli E. coli plasmids are stably maintained in Vitreoscilla. This is the first report of exogenous DNA transfer in Vitreoscilla which opens the way for the application of recombinant-DNA techniques to study this unique group of organisms. 相似文献
17.
18.
OBJECTIVES: Injury is the leading cause of death in the male working population of Brazil. An important fraction of these deaths are work related. Very few cohort studies of steel workers, and none from developing countries, have reported on mortality from injuries. This paper analyses mortality from work and non-work related injuries among Brazilian steel workers. METHODS: Deaths during employment from 1 January 1977 to 30 November 1992 were analysed in a cohort of 21,816 male steel workers. Mortality rates specific for age and calendar year among the workers were compared with those of the male population of the state where the plant is located. Work related injuries were analysed by comparing the mortality rates for different subgroups of the cohort. RESULTS: The number of deaths (391) was less than half that expected based on death rates of the general population. Over 60% (242) of deaths were due to injuries. Mortality from most causes was substantially below that in the general population, but that from unintentional injury, was 50% above that of the general population. Standardised mortality ratios (SMRs) were highest for the youngest and the oldest employees and for labourers and clerical workers. Mortality from motor vehicle injury was twice that expected from population rates (SMR = 209, 95% confidence interval (95% CI) 176-244). There was a 67% fall in the age adjusted mortality from occupational injuries in the study period. CONCLUSION: The healthy worker effect in this cohort was greater than that commonly found in studies of occupational groups in developed countries, probably because of a greater socioeconomic gap between employed and unemployed populations in Brazil, and unequal distribution of health care resources. Mortality was especially high for motor vehicle injuries. The fall in mortality from occupational injuries during the study period was probably due to improvement in safety standards, increased automation, and better medical care. There is a need to investigate risk factors for unintentional injuries among steel workers, especially those due to motor vehicle injuries. Prevention of occupational and nonoccupational injuries should be a main priority in Brazil. 相似文献
19.
HB Joshi 《Canadian Metallurgical Quarterly》1997,88(9):197-198
20.