全文获取类型
收费全文 | 21010篇 |
免费 | 2115篇 |
国内免费 | 1148篇 |
专业分类
电工技术 | 1419篇 |
技术理论 | 3篇 |
综合类 | 1559篇 |
化学工业 | 3209篇 |
金属工艺 | 1306篇 |
机械仪表 | 1420篇 |
建筑科学 | 1877篇 |
矿业工程 | 571篇 |
能源动力 | 564篇 |
轻工业 | 1348篇 |
水利工程 | 396篇 |
石油天然气 | 1053篇 |
武器工业 | 216篇 |
无线电 | 2490篇 |
一般工业技术 | 2480篇 |
冶金工业 | 1039篇 |
原子能技术 | 227篇 |
自动化技术 | 3096篇 |
出版年
2024年 | 78篇 |
2023年 | 376篇 |
2022年 | 658篇 |
2021年 | 973篇 |
2020年 | 735篇 |
2019年 | 554篇 |
2018年 | 590篇 |
2017年 | 656篇 |
2016年 | 620篇 |
2015年 | 906篇 |
2014年 | 1070篇 |
2013年 | 1180篇 |
2012年 | 1319篇 |
2011年 | 1366篇 |
2010年 | 1271篇 |
2009年 | 1150篇 |
2008年 | 1218篇 |
2007年 | 1107篇 |
2006年 | 1101篇 |
2005年 | 1021篇 |
2004年 | 802篇 |
2003年 | 618篇 |
2002年 | 626篇 |
2001年 | 556篇 |
2000年 | 530篇 |
1999年 | 535篇 |
1998年 | 433篇 |
1997年 | 395篇 |
1996年 | 362篇 |
1995年 | 301篇 |
1994年 | 246篇 |
1993年 | 210篇 |
1992年 | 145篇 |
1991年 | 105篇 |
1990年 | 103篇 |
1989年 | 84篇 |
1988年 | 54篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 15篇 |
1982年 | 17篇 |
1981年 | 7篇 |
1980年 | 21篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 13篇 |
1971年 | 4篇 |
1959年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 310 毫秒
1.
以智能反射面(intelligent reflecting surface,IRS)辅助的无线携能通信(simultaneous wireless information and power transfer,SWIPT)系统为背景,研究了该系统中基于能效优先的多天线发送端有源波束成形与IRS无源波束成形联合设计与优化方法。以最大化接收端的最小能效为优化目标,构造在发送端功率、接收端能量阈值、IRS相移等多约束下的非线性优化问题,用交替方向乘子法(alternating direction method of multipliers,ADMM)求解。采用Dinkelbach算法转化目标函数,通过奇异值分解(singular value decomposition,SVD)和半定松弛(semi-definite relaxation,SDR)得到发送端有源波束成形向量。采用SDR得到IRS相移矩阵与反射波束成形向量。结果表明,该系统显著降低了系统能量收集(energy harvesting,EH)接收端的能量阈值。当系统总电路功耗为?15 dBm时,所提方案的用户能效为300 KB/J。当IRS反射阵源数与发送天线数均为最大值时,系统可达最大能效。 相似文献
2.
Tao Feng Mingde Tong Shuotian Yao Hejun Li Shifeng Wen Hongjiao Lin 《Journal of the European Ceramic Society》2021,41(1):158-166
Composites based on hafnium carbide and reinforced with continuous naked carbon fiber with and without PyC interface were prepared at low temperature by precursor infiltration and pyrolysis and chemical vapor deposition method. The microstructure, mechanical property, cyclic ablation and fiber bundle push-in tests of the composites were investigated. The results show that after three times ablation cycles, the bending strength of samples without PyC interface decreased by 63.6 %; the bending strength of samples with PyC interface only decreased by 37.8 %. The force displacement curve of the samples with PyC interface presented a well pseudoplastic deformation state. The mechanical behavior difference of two kinds of composites was due to crucial function of PyC interface phase including protection of fiber and weakening of fiber/matrix interface. 相似文献
3.
4.
5.
研究制备了海绵负载纳米Al2O3微球的复合吸附剂(NAS),并用于对Se(Ⅳ)和Se(Ⅵ)的吸附。结果表明,合成的纳米Al2O3微球(NAO)的平均尺寸为200~400 nm,在海绵上负载NAO会使其分散性更好。当NAO负载量分别为80 mg/g和60 mg/g时,NAS对Se(Ⅳ)和Se(Ⅵ)的吸附性能为佳,分别需要60、120 min达到平衡,适应pH为2~5;两者均符合准2级动力学模型;NAS对Se(Ⅳ)、Se(Ⅵ)的最大吸附容量分别为137.2、143.9 mg/g,能很好地与Freundlich模型拟合,说明NAS表面不均匀,且属于多层吸附。经过2次的循环,对Se(Ⅳ)和Se(Ⅵ)的去除率有所降低,但均仍保持在一定的水平,说明NAS可再生循环利用。NAS作为一种新型吸附剂去除水中Se具有较好的应用前景。 相似文献
6.
The last decade has witnessed the convergence of three giant worlds:electronics,computer science and telecommunications.The next decade should follow this convergence in most of our activities with the generalization of sensor networks.In particular with the progress in medicine,people live longer and the aging of population will push the development of wireless person- 相似文献
7.
总结“三头小直径深层搅拌二次成墙法建造水泥土防渗墙施工技术”在多个堤身防渗小直径深层搅拌水泥土防渗墙工程中的应用情况,对搅拌头的钻头与喷浆方式进行了改进,经工程实践验证,改进后的搅拌头,有效降低了搅拌头的钻进扭力,使被搅动的成墙土料与喷入的水泥浆搅拌更均匀、充分,有较好的推广应用前景. 相似文献
8.
Do T. Hue Rebel Skirving Tong Chen John L. Williams Cynthia D.K. Bottema Kiro Petrovski 《Journal of dairy science》2021,104(7):8164-8176
Colostrum is essential for good neonate health; however, it is not known whether different calves absorb the nutrients from colostrum equally well. In this study, the absorption of protein, IgG, and γ-glutamyl transferase was compared in newborn dairy bull calves for 1 wk after feeding colostrum from different sources. Thirty-five Holstein-Friesian bull calves were randomly allocated into 3 groups and fed colostrum within 4 h after birth. Group A calves (n = 12) were bottle fed colostrum from their own dam for 3 d. Colostrum from these group A cows was also used as foster cow colostrum for the group B calves (n = 12), such that each group A and B calf pair received identical colostrum from each milking of the respective group A dam (10% of birth weight per day). The group C calves (n = 11) were fed 1 bottle (2 L) of pooled colostrum and transition milk (referred to as pooled colostrum), as was the standard practice on the dairy farm. The pooled colostrum was collected from the other dairy cows on the farm 0 to 4 d postpartum and stored at 4°C for less than 12 h. Blood was sampled from calves before the first feeding and at 1, 2, 3, and 7 d after birth. Levels of total solids, total protein, and IgG were higher in the dam colostrum than in the pooled colostrum. At birth, there were no differences between the calf groups for any measurements, and all calves had very low IgG levels. After receiving colostrum, the glucose, plasma γ-glutamyl transferase, serum total protein, and IgG concentrations increased significantly in all calves. There were no differences in any blood measurements at any time point between the pairs of group A and group B calves that received colostrum from the same cow except for the IgG concentration 2 d after birth. However, the group A calves had a higher total serum protein level and IgG concentration than the group C calves for all the time points after the first feeding. The group B calves had a higher IgG concentration than the group C calves on d 1, 2, and 7 after birth. Compared with groups A and B, there was no difference in the proportion of calves in group C that failed to have passive immunity transferred adequately based on the IgG threshold (<10 g/L). Thus, the calves receiving identical colostrum from the same cow had the same levels of IgG, and even the pooled colostrum provided sufficient transfer of IgG as the calves were fed within 4 h after birth. 相似文献
9.
Tong Le Fan Ronggui Yang Shuangchun Zhang Qiushi Pan Yi 《Chemistry and Technology of Fuels and Oils》2022,57(6):991-999
Chemistry and Technology of Fuels and Oils - Application of water flooding technologies in the oilfield causes intensive heterogeneity of the oil reservoir and improper circulation of the injected... 相似文献
10.