全文获取类型
收费全文 | 49394篇 |
免费 | 5652篇 |
国内免费 | 3911篇 |
专业分类
电工技术 | 4569篇 |
技术理论 | 3篇 |
综合类 | 4877篇 |
化学工业 | 6702篇 |
金属工艺 | 3190篇 |
机械仪表 | 3604篇 |
建筑科学 | 4002篇 |
矿业工程 | 1798篇 |
能源动力 | 1432篇 |
轻工业 | 4621篇 |
水利工程 | 1486篇 |
石油天然气 | 1880篇 |
武器工业 | 673篇 |
无线电 | 5475篇 |
一般工业技术 | 4832篇 |
冶金工业 | 1861篇 |
原子能技术 | 805篇 |
自动化技术 | 7147篇 |
出版年
2024年 | 279篇 |
2023年 | 823篇 |
2022年 | 1894篇 |
2021年 | 2453篇 |
2020年 | 1717篇 |
2019年 | 1300篇 |
2018年 | 1372篇 |
2017年 | 1425篇 |
2016年 | 1330篇 |
2015年 | 2070篇 |
2014年 | 2594篇 |
2013年 | 2969篇 |
2012年 | 3711篇 |
2011年 | 3947篇 |
2010年 | 3600篇 |
2009年 | 3521篇 |
2008年 | 3627篇 |
2007年 | 3670篇 |
2006年 | 3282篇 |
2005年 | 2775篇 |
2004年 | 2119篇 |
2003年 | 1436篇 |
2002年 | 1427篇 |
2001年 | 1216篇 |
2000年 | 1106篇 |
1999年 | 655篇 |
1998年 | 417篇 |
1997年 | 378篇 |
1996年 | 338篇 |
1995年 | 268篇 |
1994年 | 246篇 |
1993年 | 206篇 |
1992年 | 147篇 |
1991年 | 104篇 |
1990年 | 108篇 |
1989年 | 99篇 |
1988年 | 57篇 |
1987年 | 45篇 |
1986年 | 42篇 |
1985年 | 18篇 |
1984年 | 22篇 |
1983年 | 21篇 |
1982年 | 19篇 |
1981年 | 10篇 |
1980年 | 23篇 |
1979年 | 14篇 |
1978年 | 5篇 |
1977年 | 13篇 |
1959年 | 5篇 |
1951年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 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.
以蚕丝蛋白为模板,在相对温和的条件下通过生物矿化的手段形成具有特殊形貌的α-GaOOH颗粒,并通过在不同温度下煅烧α-GaOOH得到α-Ga2O3和β-Ga2O3.采用扫描电镜(SEM)、透射电镜(TEM)、X射线衍射仪(XRD)和荧光分光光度计(PL)等手段研究了丝素蛋白多肽和矿化时间对颗粒的影响,对其生物矿化机理进行了初步探讨.结果表明,所制备的β-Ga2O3具有优良的发光特性,丝素蛋白多肽模板以无定形的结构与产物结合在一起,并且经过高温烧结后仍以碳膜的形式包覆在材料的表面.这种碳膜结构对于提高材料的生物学性能起着重要的作用. 相似文献
4.
5.
7.
8.
9.
10.
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