全文获取类型
收费全文 | 198261篇 |
免费 | 2126篇 |
国内免费 | 633篇 |
专业分类
电工技术 | 4063篇 |
综合类 | 123篇 |
化学工业 | 26762篇 |
金属工艺 | 7508篇 |
机械仪表 | 5468篇 |
建筑科学 | 4081篇 |
矿业工程 | 752篇 |
能源动力 | 4665篇 |
轻工业 | 14415篇 |
水利工程 | 1702篇 |
石油天然气 | 2842篇 |
武器工业 | 7篇 |
无线电 | 27305篇 |
一般工业技术 | 37455篇 |
冶金工业 | 45523篇 |
原子能技术 | 3562篇 |
自动化技术 | 14787篇 |
出版年
2021年 | 1248篇 |
2019年 | 1287篇 |
2018年 | 2129篇 |
2017年 | 2112篇 |
2016年 | 2177篇 |
2015年 | 1493篇 |
2014年 | 2632篇 |
2013年 | 7963篇 |
2012年 | 4477篇 |
2011年 | 6128篇 |
2010年 | 4942篇 |
2009年 | 5790篇 |
2008年 | 6159篇 |
2007年 | 6212篇 |
2006年 | 5650篇 |
2005年 | 5318篇 |
2004年 | 5269篇 |
2003年 | 5100篇 |
2002年 | 4914篇 |
2001年 | 5321篇 |
2000年 | 4924篇 |
1999年 | 5446篇 |
1998年 | 15672篇 |
1997年 | 10341篇 |
1996年 | 7948篇 |
1995年 | 5799篇 |
1994年 | 5019篇 |
1993年 | 5033篇 |
1992年 | 3389篇 |
1991年 | 3264篇 |
1990年 | 3220篇 |
1989年 | 3044篇 |
1988年 | 2787篇 |
1987年 | 2237篇 |
1986年 | 2304篇 |
1985年 | 2611篇 |
1984年 | 2313篇 |
1983年 | 2066篇 |
1982年 | 1901篇 |
1981年 | 2047篇 |
1980年 | 1794篇 |
1979年 | 1663篇 |
1978年 | 1657篇 |
1977年 | 2026篇 |
1976年 | 2708篇 |
1975年 | 1424篇 |
1974年 | 1370篇 |
1973年 | 1318篇 |
1972年 | 1124篇 |
1971年 | 957篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
51.
Closed-loop expressions for the minimum energy control of a single-input linear digital system are described. They are obtained by two different methods. The optimal inputs are given by state feedback with the bounded variable gains. The gains are independent of the initial state of the system 相似文献
52.
53.
T Takikawa H Hayashi N Nishimura M Yano T Isomura N Sakamoto 《Canadian Metallurgical Quarterly》1994,29(5):593-597
Chronic hepatitis C has been demonstrated to be associated with hepatic iron overload, and the hypothesis that the disease activity of hepatitis C is associated with iron cytotoxicity was tested in male volunteer blood donors. Sera with either antibody to hepatitis C virus or hepatitis B surface antigen were selected for determination of ferritin concentration and alanine aminotransferase activity. A correlation between serum ferritin concentration (Y; microgram/l) and alanine aminotransferase activity (X; IU/l) was found in donors with antibody to hepatitis C (log Y = 0.65 x log X + 0.98, r = 0.53, and P < 0.01). The correlation was lower in donors with hepatitis B surface antigen (r = 0.37; P < 0.01). Hepatitis C virus infection probably induces time-dependent iron accumulation associated with the progression of disease activity, while hepatitis B virus infection results in a variety of iron loads with different clinical features. The high disease activity related to hyperferritinemia suggests the presence of iron-induced liver damage in donors with hepatitis C. 相似文献
54.
Ipratropium bromide is a synthetic derivative of atropine with little absorption when used in inhalation, and therefore little secondary effects. The authors review its pharmacological properties and therapeutic efficacy in the treatment of asthma in children. Combined nebulized inhalation of ipratropium bromide and beta 2 sympathomimetic results in a more efficient and more sustained bronchodilatation than beta 2 sympathicomimetic alone in the treatment of acute asthma in children. Ipratropium bromide should be usefully introduced in the therapeutic scheme of acute asthma in children. Further studies will be necessary in order to determine its efficacy and tolerance in infants. 相似文献
55.
56.
The expression patterns of laminin A, B1, B2, and nidogen genes were identified by in situ hybridization in postimplantation mouse extraembryonic tissues and maternal decidua during the period when the chorioallantoic placenta is established. Laminin and nidogen genes were not coordinately expressed either in the decidua or in trophoblast cells, indicating that these genes are regulated independently in these cell types during the establishment of the placenta. Laminin A mRNA was absent from the decidua except in the outer layer of cells adjacent to the myometrium and in the central decidual zone adjacent to the remnant of the uterine epithelium on Day 9. At this stage laminin B1, B2, and nidogen genes were strongly expressed in these cells and also in other regions of the decidua. Laminin B1 mRNA was present at higher levels in the decidua capsularis than in the decidua basalis, while nidogen mRNA showed highest expression in the decidua basalis. Laminin B2 mRNA was produced uniformly throughout the decidua at very high levels, suggesting that laminin B2 chains may be an important component of the decidual matrix. By Day 11, the nidogen gene was expressed only in endothelial cells lining the maternal blood spaces within the decidua. Laminin B1 and nidogen mRNAs were found at high levels within trophoblast giant cells at all stages, while laminin A mRNA was detected in trophoblast giant cells at later stages and laminin B2 mRNA was not produced in high levels by these cells. The patterns of gene expression show a very high degree of regional specialization, suggesting that the extracellular matrices in different regions of the decidua and extraembryonic membranes are likely to be composed of quite different ratios of laminin and nidogen polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
57.
58.
A. A. Konstantinov N. V. Kurenkov A. B. Malinin T. E. Sazonova S. V. Sepman 《Atomic Energy》1989,67(3):696-698
Translated from Atomnaya Énergiya, Vol. 67, No. 3, pp. 215–216, September, 1989. 相似文献
59.
60.
Hayashi M. Tanaka H. Ohara K. Otani T. Suzuki M. 《Lightwave Technology, Journal of》2002,20(2):236-242
An all-optical multiplexing technique using wavelength division multiplexing (WDM)-time division multiplexing (TDM) conversion with an electroabsorption wavelength converter has been proposed and demonstrated. The effectiveness of this WDM-TDM conversion technique for various pulsewidth settings was experimentally investigated. The fluctuation of the signal performance, which was inevitably caused by the coherent crosstalk between adjacent pulses in the conventional optical time division multiplexing (OTDM) technique, were successfully suppressed, even in the case of wide pulse duration. High Q-factor performance has been maintained for a wide range of duty ration from 36% to 74%. By introducing this technique to the optical time division multiplexer, a highly stable and high-quality 40-Gb/s optical signal can be effectively produced without generating the short pulse or setting two tributaries at orthogonal polarization states, and without introducing high-speed electronics for signal multiplexing. The WDM-TDM conversion with an electroabsorption wavelength converter was extended to 60-Gb/s operation by using three 20-Gb/s tributaries. A clear eye opening was confirmed for a waveform after the WDM-TDM conversion of the 60-Gb/s signal 相似文献