全文获取类型
收费全文 | 268902篇 |
免费 | 3588篇 |
国内免费 | 1277篇 |
专业分类
电工技术 | 5136篇 |
综合类 | 278篇 |
化学工业 | 40869篇 |
金属工艺 | 11358篇 |
机械仪表 | 7508篇 |
建筑科学 | 7374篇 |
矿业工程 | 804篇 |
能源动力 | 6892篇 |
轻工业 | 25751篇 |
水利工程 | 2051篇 |
石油天然气 | 1567篇 |
武器工业 | 3篇 |
无线电 | 32676篇 |
一般工业技术 | 48666篇 |
冶金工业 | 56128篇 |
原子能技术 | 4101篇 |
自动化技术 | 22605篇 |
出版年
2021年 | 1953篇 |
2019年 | 1756篇 |
2018年 | 2728篇 |
2017年 | 2583篇 |
2016年 | 2903篇 |
2015年 | 2278篇 |
2014年 | 3727篇 |
2013年 | 12365篇 |
2012年 | 6328篇 |
2011年 | 8909篇 |
2010年 | 7140篇 |
2009年 | 7891篇 |
2008年 | 8631篇 |
2007年 | 8676篇 |
2006年 | 7938篇 |
2005年 | 7347篇 |
2004年 | 6943篇 |
2003年 | 6922篇 |
2002年 | 6895篇 |
2001年 | 6843篇 |
2000年 | 6179篇 |
1999年 | 6850篇 |
1998年 | 17041篇 |
1997年 | 12041篇 |
1996年 | 9420篇 |
1995年 | 7104篇 |
1994年 | 6338篇 |
1993年 | 6158篇 |
1992年 | 4470篇 |
1991年 | 4251篇 |
1990年 | 3958篇 |
1989年 | 3877篇 |
1988年 | 3769篇 |
1987年 | 3278篇 |
1986年 | 3153篇 |
1985年 | 3833篇 |
1984年 | 3473篇 |
1983年 | 3112篇 |
1982年 | 2890篇 |
1981年 | 2847篇 |
1980年 | 2718篇 |
1979年 | 2601篇 |
1978年 | 2500篇 |
1977年 | 3042篇 |
1976年 | 4386篇 |
1975年 | 2135篇 |
1974年 | 1994篇 |
1973年 | 1986篇 |
1972年 | 1618篇 |
1971年 | 1422篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
In this paper we investigate the impact of finite laser linewidths (i.e. source partial coherence) on the crosstalk performance of optical frequency division multiplexing (OFDM) networks employing single-cavity Fabry-Perot demultiplexers. Results are presented that show the important limitation imposed by the finite laser linewidths on the attainable bit error rate (BER), maximum number of users, and the required power penalties to overcome this impairment 相似文献
942.
Ferdinand P. Ferragu O. Lechien J.L. Lescop B. Magne S. Marty V. Rougeault S. Kotrotsios G. Neuman V. Depeursinge Y. Michel J.B. Van Uffelen M. Varelas D. Berthou H. Pierre G. Renouf C. Jarret B. Verbandt Y. Stevens W. Voet M.R.H. Toscano D. 《Lightwave Technology, Journal of》1995,13(7):1303-1313
Recent developments of stability control in mines, essentially based on Ge-doped fiber Bragg gratings (FBG) are reported including results about the different aspects of the system: accurate characterizations of FBG, sensor network topology and multiplexing method, user interface design and sensor packaging 相似文献
943.
MC Ocké D Kromhout A Menotti C Aravanis H Blackburn R Buzina F Fidanza A Jansen S Nedeljkovic A Nissienen 《Canadian Metallurgical Quarterly》1995,61(4):480-484
This ecologic study aimed to investigate whether differences in population mortality from lung, stomach and colorectal cancer among the 16 cohorts of the Seven Countries Study could be explained by differences in the average intake of anti-oxidant (pro)vitamins. In the 1960s, detailed dietary information was collected in small sub-samples of the cohorts by the dietary record method. In 1987, food-equivalent composites representing the average food intake of each cohort at baseline were collected locally and analyzed in a central laboratory. The vital status of all participants was verified after 25 years of follow-up. The average intake of vitamin C was strongly inversely related to the 25-year stomach-cancer mortality (r = -0.66, p = 0.01), also after adjustment for smoking and intake of salt or nitrate. The average intake of alpha-carotene, beta-carotene, and alpha-tocopherol were not independently related to mortality from lung, stomach or colorectal cancer, nor was vitamin C related to lung and colorectal cancer. 相似文献
944.
Mineralization occurred both in fetal rat calvarial cells and UMR 106 osteoblastic cells when they were cultured in medium containing L-ascorbate and beta-glycerophosphate as evidenced by von Kóssa staining as well as deposition of calcium ions and inorganic phosphate in the cells. When compared with corresponding non-mineralized cell cultures, both the mineralized cultures of calvarial cells and UMR 106 cells did not exhibit any change in intracellular bone-specific alkaline phosphatase activities which were measured by wheatgerm lectin precipitation method. Our results support the hypothesis that mineralization may not exert any direct negative feedback on matrix protein synthesis in osteoblasts during bone formation. 相似文献
945.
946.
947.
948.
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
949.
950.
Hillmer H. Grabmaier A. Zhu H.-L. Hansmann S. Burkhard A. 《Lightwave Technology, Journal of》1995,13(9):1905-1912
We have implemented and studied a new type of tunable multiple-section semiconductor distributed feedback (DFB) laser using tailored chirped DFB gratings. Arbitrarily and continuously chirped DFB gratings are defined by bent waveguides on homogeneous grating fields with ultrahigh spatial precision. The mathematical bending functions are optimized in this case to provide enlarged wavelength tuning ranges. We present the results of model calculations, the technological device realization and experimental results of the DFB laser characterization e.g. a tuning range of 5.5 nm without wavelength gaps and high side mode suppression ratio 相似文献