全文获取类型
收费全文 | 13476篇 |
免费 | 1232篇 |
国内免费 | 954篇 |
专业分类
电工技术 | 495篇 |
综合类 | 868篇 |
化学工业 | 3364篇 |
金属工艺 | 1168篇 |
机械仪表 | 436篇 |
建筑科学 | 402篇 |
矿业工程 | 208篇 |
能源动力 | 456篇 |
轻工业 | 1020篇 |
水利工程 | 97篇 |
石油天然气 | 675篇 |
武器工业 | 90篇 |
无线电 | 1975篇 |
一般工业技术 | 1102篇 |
冶金工业 | 690篇 |
原子能技术 | 285篇 |
自动化技术 | 2331篇 |
出版年
2024年 | 32篇 |
2023年 | 182篇 |
2022年 | 317篇 |
2021年 | 360篇 |
2020年 | 351篇 |
2019年 | 284篇 |
2018年 | 273篇 |
2017年 | 380篇 |
2016年 | 524篇 |
2015年 | 504篇 |
2014年 | 710篇 |
2013年 | 843篇 |
2012年 | 946篇 |
2011年 | 1181篇 |
2010年 | 941篇 |
2009年 | 1016篇 |
2008年 | 859篇 |
2007年 | 979篇 |
2006年 | 963篇 |
2005年 | 636篇 |
2004年 | 549篇 |
2003年 | 557篇 |
2002年 | 450篇 |
2001年 | 347篇 |
2000年 | 297篇 |
1999年 | 246篇 |
1998年 | 149篇 |
1997年 | 145篇 |
1996年 | 115篇 |
1995年 | 79篇 |
1994年 | 60篇 |
1993年 | 65篇 |
1992年 | 55篇 |
1991年 | 59篇 |
1990年 | 26篇 |
1989年 | 31篇 |
1988年 | 26篇 |
1987年 | 17篇 |
1986年 | 22篇 |
1985年 | 19篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1976年 | 2篇 |
1975年 | 5篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1951年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
For the first time, we present the unique features exhibited by power 4H–SiC UMOSFET in which N and P type columns (NPC) in the drift region are incorporated to improve the breakdown voltage, the specific on-resistance, and the total lateral cell pitch. The P-type column creates a potential barrier in the drift region of the proposed structure for increasing the breakdown voltage and the N-type column reduces the specific on-resistance. Also, the JFET effects reduce and so the total lateral cell pitch will decrease. In the NPC-UMOSFET, the electric field crowding reduces due to the created potential barrier by the NPC regions and causes more uniform electric field distribution in the structure. Using two dimensional simulations, the breakdown voltage and the specific on-resistance of the proposed structure are investigated for the columns parameters in comparison with a conventional UMOSFET (C-UMOSFET) and an accumulation layer UMOSFET (AL-UMOSFET) structures. For the NPC-UMOSFET with 10 µm drift region length the maximum breakdown voltage of 1274 V is obtained, while at the same drift region length, the maximum breakdown voltages of the C-UMOSFET and the AL-UMOSFET structures are 534 and 703 V, respectively. Moreover, the proposed structure exhibits a superior specific on-resistance (Ron,sp) of 2 mΩ cm2, which shows that the on-resistance of the optimized NPC-UMOSFET are decreased by 56% and 58% in comparison with the C-UMOSFET and the AL-UMOSFET, respectively. 相似文献
3.
Fei Xu Xiangli Shi Qingzhu Zhang Wenxing Wang 《International journal of molecular sciences》2015,16(8):18714-18731
The chlorothiophenoxy radicals (CTPRs) are key intermediate species in the formation of polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs). In this work, the formation of CTPRs from the complete series reactions of 19 chlorothiophenol (CTP) congeners with H and OH radicals were investigated theoretically by using the density functional theory (DFT) method. The profiles of the potential energy surface were constructed at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) contribution at 600–1200 K. The present study indicates that the structural parameters, thermal data, and rate constants as well as the formation potential of CTPRs from CTPs are strongly dominated by the chlorine substitution at the ortho-position of CTPs. Comparison with the study of formation of chlorophenoxy radicals (CPRs) from chlorophenols (CPs) clearly shows that the thiophenoxyl-hydrogen abstraction from CTPs by H is more efficient than the phenoxyl-hydrogen abstraction from CPs by H, whereas the thiophenoxyl-hydrogen abstraction from CTPs by OH is less impactful than the phenoxyl-hydrogen abstraction from CPs by OH. Reactions of CTPs with H can occur more readily than that of CTPs with OH, which is opposite to the reactivity comparison of CPs with H and OH. 相似文献
4.
《Ceramics International》2020,46(3):2868-2876
In order to improve the stability of PZT-based sensors, the mechanical, dielectric, ferroelectric and piezoelectric properties of PZT-5H under impact load were studied experimentally by using the separated Hopkinson pressure bar (SHPB) with an electrical output measurement device. At the same time, the experimental study on the material properties of PZT-5H before and after the impact was carried out. The effect of impact cracks on the output voltage of PZT-5H was also analyzed. The results show that the dynamic piezoelectric constants of PZT-5H under low stress impact (10–50 MPa) are different from those under quasi-static state, and the empirical relationship between them and the peak stress is obtained through experiments. The dielectric properties of PZT-5H did not change under low stress impact, but micro-cracks occurred in the material and dielectric loss increased at high frequencies. Under short circuit, the residual polarization intensity of PZT-5H decreases sharply due to impact load. While the impact load causes the secondary polarization and the increase of the residual polarization intensity of PZT under open circuit. When the stress is over 45 MPa, the PZT-5H breaks. The formation of cracks causes abnormal discharge voltage and gap discharge. 相似文献
5.
6.
氯甲烷在镁修饰的ZSM-5分子筛催化剂上催化转化研究 总被引:1,自引:1,他引:0
在镁修饰的ZSM-5分子筛催化剂上,氯甲烷可以被催化转化为烃类产品。分别采用了离子交换和浸渍的方法来修饰ZSM-5分子筛。离子交换的Mg-ZSM-5催化剂和浸渍的Mg/ZSM-5催化剂都可以提高反应产物中的低碳烯烃的选择性,降低烷烃的选择性,并且保持较高的反应活性,但高的浸渍量对反应活性有一定的影响。NH3-TPD的结果表明,浸渍镁的ZSM-5催化剂的强酸中心数目明显减少。镁的修饰对催化剂酸性的影响导致了产物中低碳烯烃的增加。浸渍Mg的ZSM-5催化剂是潜在氯甲烷转化生成低碳烯烃的催化剂。 相似文献
7.
8.
9.
张焱明 《石油化工腐蚀与防护》2006,23(5):55-57
2005年8月,中国石油天然气股份有限公司辽阳石化分公司炼油厂第二循环水场水质出现异常情况.经调查分析,发现是脱硫装置E3410A换热器介质NH3泄漏后进入循环冷却水系统所致.介绍了NH3泄漏后的水质变化趋势及危害,并提出了防治措施. 相似文献
10.