首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   44661篇
  免费   2192篇
  国内免费   1634篇
电工技术   1626篇
技术理论   2篇
综合类   1416篇
化学工业   6161篇
金属工艺   3389篇
机械仪表   2300篇
建筑科学   2421篇
矿业工程   806篇
能源动力   1436篇
轻工业   2340篇
水利工程   499篇
石油天然气   1869篇
武器工业   126篇
无线电   4943篇
一般工业技术   9145篇
冶金工业   4870篇
原子能技术   538篇
自动化技术   4600篇
  2024年   110篇
  2023年   482篇
  2022年   762篇
  2021年   1077篇
  2020年   871篇
  2019年   868篇
  2018年   1064篇
  2017年   1125篇
  2016年   1065篇
  2015年   1167篇
  2014年   1665篇
  2013年   2726篇
  2012年   2290篇
  2011年   2820篇
  2010年   2369篇
  2009年   2415篇
  2008年   2415篇
  2007年   2388篇
  2006年   2289篇
  2005年   2126篇
  2004年   1499篇
  2003年   1352篇
  2002年   1259篇
  2001年   1209篇
  2000年   1142篇
  1999年   1308篇
  1998年   1914篇
  1997年   1434篇
  1996年   1262篇
  1995年   871篇
  1994年   695篇
  1993年   512篇
  1992年   368篇
  1991年   362篇
  1990年   243篇
  1989年   240篇
  1988年   172篇
  1987年   113篇
  1986年   87篇
  1985年   69篇
  1984年   48篇
  1983年   38篇
  1982年   37篇
  1981年   33篇
  1980年   18篇
  1979年   11篇
  1978年   10篇
  1976年   19篇
  1975年   11篇
  1973年   10篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
We report the first demonstration of proton-exchanged optical planar waveguides in x-cut and z-cut LiNbO3 doped with 6 mol% ZnO using adipic acid as proton source. These waveguides exhibit a graded index profile which can be modeled by a linear step function with a surface index increase of 0.135 and 0.14 for x-cut and z-cut waveguides, respectively. The diffusion constant Do and the activation energy Q are characterized optically to be 1.64×109 μm2/h and 88.8 KJ/mol for x-cut waveguides, and 1.478×109 μm2/h and 91.25 KJ/mol for z-cut waveguides, respectively. The diffusion rate along the z-axis is slower than that along the x-axis, whereas the surface index increase on z-cut waveguides is larger than that on x-cut waveguides.  相似文献   
72.
In this study, 16 cases of unilateral alveolar cleft with cleft lip and palate were repaired with autografts of cancellous bone (13 cases) or hydroxyapatite (3 cases). The grafts were covered by reflected mucoperiosteal flaps and a mucosal flap from the upper lip. Twelve of the thirteen cases were followed up for 1-5 years. Nine of whom using cancellous bone had bony continuity of the maxilla and 7 cases erupted permanent maxillary canines within the area of autografts. None of the 3 cases using hydroxyapatite erupted a canine tooth. The results showed that autograft was better than hydroxyapatite in terms of maxillary canine eruption.  相似文献   
73.
研究了Al2O3-TiB2陶瓷刀具材料在1000℃下的氧化行为,用XRD、SEM分析了氧化后的相组成及显微结构。结果表明:Al2O3-TiB2陶瓷材料在1000℃空气中氧化增重符合抛物线规律;随TiB2含量的增加,该材料的抗氧化能力下降。  相似文献   
74.
The represention of circuit variables in frequency-domain complex matrix and nonlinearities by power series gives rise to a nonlinear distortion prediction technique, the intermodulation-balance method. This operates entirely in the frequency domain. The technique is verified by a single-amplifier filter with a two-tone input signal  相似文献   
75.
Epidemiological studies have linked dietary fiber to the prevention of human colorectal cancer and suggest that short chain fatty acids such as butyric acid, which is produced by fermentation of dietary fiber in the large intestine, may be an important mediator of the protective effects of fiber. We investigated the role of Bcl-2 deregulation on the sensitivity of colorectal carcinoma cells to undergo butyrate-induced apoptosis. Here we report an inverse relationship between the levels of Bcl-2 and the sensitivity of colorectal carcinoma cell lines to undergo apoptosis in response to butyrate. Overexpression of Bcl-2 in colorectal carcinoma DiFi cells resulted in suppression of butyrate-induced apoptosis and enhanced cell survival in response to butyrate. Butyrate-induced apoptosis was accompanied by inhibition of expression of a 30 kDa protein (p30, immunorecognized by anti-Bcl-2 mAb) and this cellular effect of butyrate was inhibited by Bcl-2 overexpression. These findings suggest that deregulation of Bcl-2 in human colorectal carcinoma cells confers resistance to induction of apoptosis by butyrate, a dietary micronutrient.  相似文献   
76.
InAs channel field-effect transistors of 1-μm gate length were grown by molecular beam epitaxy and observed to operate at channel electric fields (20 kV/cm) higher than previously demonstrated and several times greater than the threshold for impact ionization in bulk InAs. Voltage gains on the order of 10 were observed with transconductances as high as 414 mS/mm and output conductances as low as 33 mS/mm. These voltage gains are comparable to those of GaAs-based devices and are the highest observed for InAs channel devices. The results demonstrate the potential for practical room-temperature operation of InAs FETs  相似文献   
77.
78.
79.
To investigate the prognostic factors of primary liver cancer (PLC) and improve the long-term results, 1,248 cases of PLC were analysed. Univariate analysis demonstrated that discovery approach, staging of PLC, original gamma-GPT, resection, radical resection, original AFP, tumor size, tumor number, and tumor capsule have very significant effects on prognosis of PLC (all P < 0.001); cirrhosis, HBsAg, local resection, and tumor embolus in portal vein were also significant difference (all P < 0.05); age, sex, original AFP, hepatitis, and differentiation of PLC cells were no significant difference (all P > 0.05). Multivariate analysis demonstrated that original gamma-GPT, radical resection, tumor size, and tumor number were the most significant prognostic factors (all P < 0.001). Some aspects improving long-term survival were discussed.  相似文献   
80.
The authors demonstrate, for the first time, a three node optically time division multiplexed network with a maximum capacity of 40 Gbit/s. Electro-optical switches were used for both `drop and insert' and demultiplexing, and electronic clock recovery provided synchronisation throughout the network  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号