全文获取类型
收费全文 | 16474篇 |
免费 | 163篇 |
国内免费 | 15篇 |
专业分类
教育 | 11613篇 |
科学研究 | 1635篇 |
各国文化 | 217篇 |
体育 | 1174篇 |
综合类 | 9篇 |
文化理论 | 143篇 |
信息传播 | 1861篇 |
出版年
2021年 | 159篇 |
2020年 | 227篇 |
2019年 | 339篇 |
2018年 | 518篇 |
2017年 | 520篇 |
2016年 | 490篇 |
2015年 | 328篇 |
2014年 | 455篇 |
2013年 | 3280篇 |
2012年 | 404篇 |
2011年 | 415篇 |
2010年 | 364篇 |
2009年 | 357篇 |
2008年 | 414篇 |
2007年 | 365篇 |
2006年 | 322篇 |
2005年 | 308篇 |
2004年 | 327篇 |
2003年 | 271篇 |
2002年 | 264篇 |
2001年 | 279篇 |
2000年 | 310篇 |
1999年 | 260篇 |
1998年 | 181篇 |
1997年 | 224篇 |
1996年 | 204篇 |
1995年 | 176篇 |
1994年 | 190篇 |
1993年 | 169篇 |
1992年 | 247篇 |
1991年 | 209篇 |
1990年 | 216篇 |
1989年 | 234篇 |
1988年 | 207篇 |
1987年 | 179篇 |
1986年 | 186篇 |
1985年 | 182篇 |
1984年 | 161篇 |
1983年 | 190篇 |
1982年 | 132篇 |
1981年 | 148篇 |
1980年 | 141篇 |
1979年 | 172篇 |
1978年 | 152篇 |
1977年 | 134篇 |
1976年 | 111篇 |
1975年 | 105篇 |
1974年 | 103篇 |
1973年 | 109篇 |
1972年 | 88篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Mary A. Anderson Frank Bowles C. O. Taiwo Jan D. Beckmann L. J. Lewis James R. Liesch Godfrey N. Brown H. L. Elvin Shib K. Mitra Richard F. Behrendt Graeme Kemelfield Hugh W. R. Hawes W. A. Dodd A. L. Tibawi 《International Review of Education/Internationale Zeitschrift für Erziehungswissenschaft/Revue internationale l'éducation》1971,17(2):220-251
73.
74.
75.
This paper summarizes some of the experimental evidence of the apparent biological effects produced by atmospheric ionization. Thermionic ion sources are discussed and the results of experimental determinations of their characteristics are presented. 相似文献
76.
77.
Martin A. Pomerantz 《Journal of The Franklin Institute》1946,241(6):415-433
The secondary electron emission from alkaline-earth oxide-coated cathodes has been investigated under both continuous and pulsed bombardment. Various factors affecting the yield, such as dependence upon primary voltage, collecting voltage, temperature, time, and angle of incidence, are noted, and the present state of the theory is discussed.Experiments have been performed with three types of apparatus. Yield vs. Energy data reveal values of δ of 4–7 at room temperature, with a more or less flat maximum at approximately 1,000 volts primary energy.The yield increases with temperature in an exponential manner, and plots of log Δδ (i.e. δK° ? δ300°K) vs. 1/T give straight lines. Values of Q1 between 0.9 – 1.5 eV. are generally indicated, and from extrapolation of these curves, yields exceeding 100 at 850° C. are deduced. The secondary emission depends upon the degree of activation, and increases with enhancement of the thermionic emission characteristics. Short-time effects such as growth or decay of secondary current after the onset of primary bombardment or persistence after the cessation of bombardment have not been observed, and values of yield obtained by pulsed methods are in accord with those obtained under D.C. conditions. Tail phenomena reported by J. B. Johnson and interpreted as “enhanced thermionic emission” from oxidecoated cathodes become manifest only under experimental conditions characterized by certain space-charge effects, and have been effectively simulated by bombarding a tantalum target adjacent to an electron-emitting tungsten filament. Various measurements of the energy distribution of secondary electrons as a function of primary voltage and temperature have been obtained. It was observed that the average energy of the secondary electrons decreases with temperature at a rate which more than compensates for the increase in the number of secondaries emitted per incident primary. The mechanism of the observed dependence of yield upon temperature is not well understood. Various alternative explanations are discussed and, in the light of the present state of our knowledge, regarded as untenable. 相似文献
78.
79.
80.
A report is given of the investigation of the iodine content of oyster shells taken from the reefs found in the Gulf of Mexico. The pulverized shells as sold on the market contain 200 to 300 parts per billion of iodine. Water soluble and alcohol soluble iodides appear to be absent. The granulated undried shells contain 500 parts per billion of iodine. Apparently there is a loss of iodine due to the temperature at which the pulverized shells are dried. 相似文献