全文获取类型
收费全文 | 4005篇 |
免费 | 235篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 60篇 |
综合类 | 2篇 |
化学工业 | 935篇 |
金属工艺 | 63篇 |
机械仪表 | 100篇 |
建筑科学 | 99篇 |
矿业工程 | 7篇 |
能源动力 | 138篇 |
轻工业 | 696篇 |
水利工程 | 38篇 |
石油天然气 | 24篇 |
无线电 | 243篇 |
一般工业技术 | 525篇 |
冶金工业 | 754篇 |
原子能技术 | 25篇 |
自动化技术 | 534篇 |
出版年
2024年 | 17篇 |
2023年 | 27篇 |
2022年 | 93篇 |
2021年 | 155篇 |
2020年 | 94篇 |
2019年 | 119篇 |
2018年 | 131篇 |
2017年 | 103篇 |
2016年 | 138篇 |
2015年 | 112篇 |
2014年 | 158篇 |
2013年 | 240篇 |
2012年 | 193篇 |
2011年 | 229篇 |
2010年 | 184篇 |
2009年 | 214篇 |
2008年 | 149篇 |
2007年 | 156篇 |
2006年 | 110篇 |
2005年 | 75篇 |
2004年 | 84篇 |
2003年 | 93篇 |
2002年 | 76篇 |
2001年 | 58篇 |
2000年 | 69篇 |
1999年 | 65篇 |
1998年 | 264篇 |
1997年 | 191篇 |
1996年 | 132篇 |
1995年 | 89篇 |
1994年 | 67篇 |
1993年 | 68篇 |
1992年 | 22篇 |
1991年 | 24篇 |
1990年 | 15篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 14篇 |
1983年 | 17篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 10篇 |
1977年 | 19篇 |
1976年 | 28篇 |
1973年 | 7篇 |
排序方式: 共有4243条查询结果,搜索用时 15 毫秒
21.
We hypothesized that hydrocephalus can cause death of brain cells and that generation of new brain cells might compensate for the cell loss. Hydrocephalus was induced in 3-week-old rats by injection of kaolin into the cisterna magna. The brains were studied 1 to 4 weeks later by histochemical, immunochemical, and ultrastructural methods. The ventricles enlarged progressively. Some axons in the corpus callosum were injured as early as 1 week, but axonal damage was not prevalent until 4 weeks when ventriculomegaly became severe. Dying cells detected by DNA end labeling and often identified as oligodendrocytes by electron microscopy were evident in white matter. Late-stage hydrocephalus was associated with a significant increase in the quantity of dying cells. Hydrocephalus was associated with increased Ki67 labeling and bromodeoxyuridine incorporation in the subependymal zone. Reactive changes were identified among astrocytes, oligodendroglia, and microglia. We conclude that hydrocephalus causes, in addition to axonal injury, gradual cell death in the cerebrum, particularly the white matter. The brain response includes production of new glial cells, but whether the new cells play any beneficial role remains unknown. 相似文献
22.
SL Kamholz RJ Brewer G Grijalva J Burack MJ Del Rio M Vaynblatt N Lawson S Squinto WL Fodor AJ Norin 《Canadian Metallurgical Quarterly》1997,21(9):951-955
The lack of sufficient suitable human donor lungs for the many patients requiring pulmonary transplantation as life-saving therapy for end-stage lung diseases has generated extensive interest in cross-species lung transplantation. Ethical concerns and those of animal rights advocates have prompted studies of nonprimate species as potential solid organ donors for humans. This paper provides an overview of some of the laboratory studies of cross-species pulmonary transplantation performed over the past 20 years and focuses, in particular, on more recent work (from our laboratory and others) in the area of porcine-to-primate pulmonary xenotransplantation. 相似文献
23.
IEEE 802.11-saturation throughput analysis 总被引:1,自引:0,他引:1
To satisfy the emerging need of wireless data communications, the IEEE is currently standardizing the 802.11 protocol for wireless local area networks. This standard adopts a CSMA/CA medium access control protocol with exponential backoff. We present a simple analytical model to compute the saturation throughput performance in the presence of a finite number of terminals and in the assumption of ideal channel conditions. The model applies to both basic and request-to-send/clear-to-send (RTS/CTS) access mechanisms. Comparison with simulation results shows that the model is extremely accurate in predicting the system throughput 相似文献
24.
M Drlicek G Bianchi G Bogliun B Casati W Grisold C Kolig U Liszka-Setinek L Marzorati E Wondrusch G Cavaletti 《Canadian Metallurgical Quarterly》1997,244(2):85-89
PURPOSE: To determine the association between lipids, microalbuminuria and systemic blood pressure. Urinary albumin excretion rate (AER) was determined in timed overnight urine samples by radioimmunoassay. Microalbuminuria was defined when two out of three urine samples had AER ranging 20-200 micrograms/min. Lipids were determined by colorimetric methods (total cholesterol, HDL cholesterol and triglycerides). METHODS: Fifty patients with insulin dependent diabetes mellitus (28 females, 22 males) aged 21.9 +/- 7 years and with diabetes duration of 6.8 +/- 5.8 years attending the outpatients diabetes clinic were studied cross-sectionally. RESULTS: Microalbuminuria was present in 12% of our patients. A high systolic blood pressure (SBP) was found in microalbuminuric patients (p = 0.003). No difference concerning serum lipids were found in comparison between normo and microalbuminuric patients, although 20% of all patients had increased cholesterol and LDL cholesterol and 4% had high HDL cholesterol and triglycerides levels. Stepwise multiple regression analysis showed that SBP was the only significant independent variable to influence AER (r = 0.42 r2 = 0.18 p = 0.002). CONCLUSION: Although in our study, microalbuminuria was associated only with SBP, the independent alteration of lipids in young IDDM patients must be considered as a possible additional risk factor for cardiovascular disease. 相似文献
25.
26.
27.
Luiz F. Martha L. J. Gray A. R. Ingraffea 《International journal for numerical methods in engineering》1992,35(9):1907-1921
An efficient hypersingular boundary integral equation method for three-dimensional fracture mechanics was presented in a previous paper. The details of the numerical implementation of this method are further discussed herein. In particular, an algorithm for achieving the required differentiability of the crack surface displacement function is discussed. To illustrate the utility of the method, computational results for several strongly interacting multiple-crack geometries are presented. The calculated stress intensity factors are in excellent agreement with those obtained by an approximate analytical method due to Kachanov and Laures. 相似文献
28.
29.
Trends in Hydrological Parameters of a Southern Brazilian Watershed and its Relation to Human Induced Changes 总被引:5,自引:1,他引:4
Moraes Jorge M. Pellegrino Giampaolo Q. Ballester Maria V. Martinelli Luiz A. Victoria Reynaldo L. Krusche Alex V. 《Water Resources Management》1998,12(4):295-311
The Piracicaba river basin is a subtropical watershed located in the southeastern region of Brazil. With an area of 12 400 km2, the basin is a typical example of new landscape resulting from development in tropical and sub-tropical regions: establishment of intensive industrial and agricultural processes were followed by significant population growth and water management. This scenario has led to significant increase in water demand and decrease in water quality. The main objective of this study is the detection of changes in the patterns of flow and precipitation in the basin, and its possible relation to man-induced changes. Statistical analyses were performed on records of precipitation, evapotranspiration and streamflow, from 1947 to 1991. Precipitation and evapotranspiration totals showed significant increasing trends for the entire basin. From eight streamflow gauge stations, half showed significant decreasing trend. The most probable cause of such trends is the export of water from the basin to the metropolitan region of São Paulo city. 相似文献
30.
Morosi S. Marabissi D. Enrico Del Re Fantacci R. Del Santo N. 《Power Delivery, IEEE Transactions on》2006,21(4):1892-1897
This paper deals with a variable rate discrete multitone modulation system for broadband power-line communications, based on the bit-loading algorithm proposed by Leke and Cioffi. In the proposed system a suitable least mean square channel estimator is considered, which is based on the insertion of a training sequence (TS). The proposed approach will be compared with the ideal channel estimates, showing its effectiveness. Moreover, different TS lengths will be compared. The system performance, expressed in terms of bit rate and bit-error rate, is derived by simulation with and without estimation errors. The propagation environment has been assumed as a frequency-selective multipath fading channel with additive colored Gaussian noise, according to the in-building networks model. 相似文献