全文获取类型
收费全文 | 1443篇 |
免费 | 13篇 |
专业分类
电工技术 | 4篇 |
综合类 | 1篇 |
化学工业 | 90篇 |
金属工艺 | 65篇 |
机械仪表 | 8篇 |
建筑科学 | 40篇 |
能源动力 | 32篇 |
轻工业 | 67篇 |
水利工程 | 5篇 |
无线电 | 122篇 |
一般工业技术 | 104篇 |
冶金工业 | 871篇 |
原子能技术 | 5篇 |
自动化技术 | 42篇 |
出版年
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 4篇 |
2019年 | 11篇 |
2018年 | 10篇 |
2017年 | 11篇 |
2016年 | 12篇 |
2015年 | 8篇 |
2014年 | 13篇 |
2013年 | 23篇 |
2012年 | 20篇 |
2011年 | 30篇 |
2010年 | 32篇 |
2009年 | 28篇 |
2008年 | 30篇 |
2007年 | 31篇 |
2006年 | 36篇 |
2005年 | 22篇 |
2004年 | 11篇 |
2003年 | 23篇 |
2002年 | 21篇 |
2001年 | 15篇 |
2000年 | 21篇 |
1999年 | 48篇 |
1998年 | 273篇 |
1997年 | 156篇 |
1996年 | 116篇 |
1995年 | 60篇 |
1994年 | 51篇 |
1993年 | 64篇 |
1992年 | 10篇 |
1991年 | 16篇 |
1990年 | 9篇 |
1989年 | 19篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 23篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1979年 | 2篇 |
1978年 | 7篇 |
1977年 | 25篇 |
1976年 | 60篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有1456条查询结果,搜索用时 15 毫秒
1.
2.
3.
AJ Dunipace EJ Brizendine ME Wilson W Zhang BP Katz GK Stookey 《Canadian Metallurgical Quarterly》1998,128(8):1392-1400
On the basis of observations that endemic fluorosis occurs more often in malnourished populations, a series of studies tested the hypothesis that deficient dietary intake of calcium, protein or energy affects fluoride metabolism so that the margin of safe fluoride exposure may be reduced. The objective of the investigation was to determine whether changes in fluoride metabolism in nutritionally deficient rats resulted in manifestation of any extraskeletal toxic fluoride effects not observed in healthy animals. This investigation included two studies, one that monitored the effect of calcium deficiency on the effects of chronic fluoride exposure, and a second study that observed fluoride effects in rats that were deficient either in protein or in energy and total nutrient intake. Control and experimental rats received drinking water containing 0, 0.26 (5), 0.79 (15) or 2.63 (50) mmol fluoride/L (mg/L) for 16 or 48 wk. Control rats were fed optimal diets and experimental rats were fed diets deficient in calcium (Study 1) or protein (Study 2). An additional group of experimental rats (Study 2) was provided with a restricted amount of diet; thus these rats were deficient in energy and total nutrient intake. The intake, excretion and retention of fluoride were monitored; after the rats were killed, tissue fluoride levels and biochemical markers of tissue function were analyzed. Bone marrow cells were harvested from some of the rats, after 48 wk of treatment, for determining the frequency of sister chromatid exchange, a marker of genetic damage. Although there were significant differences among fluoride treatment groups in fluoride excretion and retention that resulted in significantly greater fluoride levels in tissues of the experimental rats, we were unable to detect any harmful, extraskeletal biochemical, physiologic or genetic effects of fluoride in the nutritionally deficient rats. 相似文献
4.
The performance of sequential decoding of long constraint length convolutional codes is evaluated for Rayleigh fading channels. Sequential decoding is not practical below a certain theoretical signal-to-noise ratio, and these theoretical limits are calculated for a number of modulation methods and code rates. As an example, with BPSK modulation, soft decisions and code rate 1/2, the theoretical signal-to-noise ratio per information bit is 5.7 dB. Above this limit the bit error rate can be made arbitrarily small by increasing the constraint length at no significant complexity cost. Furthermore, it is shown that with carefully chosen quantization steps, 8 level uniform quantization gives a negligible loss also for sequential decoding on a Rayleigh fading channel. Simulation results using 8 level quantization correspond well with the theoretical performance bounds. Also, the performance on a correlated channel with finite interleaving has been obtained. With an interleaver depth of 50×50 and a normalized doppler frequency equal to 0.01 we are only 0.5 dB away from the performance with perfect interleaving. Finally, bit error rate results show this scheme to compete well with Turbo codes. 相似文献
5.
6.
7.
8.
9.
10.
L Fr?yland DK Asiedu H Vaagenes A Garras O Lie GK Totland RK Berge 《Canadian Metallurgical Quarterly》1995,36(12):2529-2540
The mechanism behind the hypolipidemic effect of tetradecylthioacetic acid (CMTTD, a non-beta-oxidizable 3-thia fatty acid) was studied in hamsters fed a high cholesterol diet (2%), which resulted in hyperlipidemia. Treating hyperlipidemic hamsters with CMTTD resulted in a progressive hypocholesterolemic and hypotriacylglycerolemic effect. Decreased plasma cholesterol was followed by a 39% and 30% reduction in VLDL-cholesterol and LDL-cholesterol, respectively. In contrast, the HDL-cholesterol content was not affected, thus decreasing the VLDL-cholesterol/HDL-cholesterol and LDL-cholesterol/HDL-cholesterol ratios. 3-Hydroxy-3-methylglutaryl- (HMG) CoA reductase activity and its mRNA level were unchanged after CMTTD administration. Also, the LDL receptor and LDL receptor-related protein (LRP-4) mRNAs were unchanged. The decrease in plasma triacylglycerol was accompanied by a 45% and 56% reduction in VLDL-triacylglycerol and LDL-triacylglycerol, respectively. The hypolipidemic effect of CMTTD was followed by a 1.4-fold increase in mitochondrial fatty acid oxidation and a 2.3-fold increase in peroxisomal fatty acid oxidation. CMTTD treatment led to an accumulation of dihomo-gamma-linolenic acid (20:3n-6) in liver, plasma, very low density lipoprotein, and heart. Noteworthy, CMTTD accumulated more in the heart, plasma, and VLDL particles compared to the liver, and in the VLDL particle alpha-linolenic acid (18:3n-3) decreased whereas eicosatetraenoic acid (20:4n-3) increased. In addition, linoleic acid (18:2n-6) and the total amount of polyunsaturated fatty acids decreased, the latter mainly due to a decrease in n-6 fatty acids. The present data show that CMTTD was detected in plasma and incorporated into VLDL, liver, and heart. The relative incorporation (mol%) of CMTTD was heart > VLDL > liver. In conclusion, CMTTD causes both a hypocholesterolemic and hypotriacylglycerolemic effect in hyperlipidemic hamsters. 相似文献