全文获取类型
收费全文 | 309467篇 |
免费 | 3997篇 |
国内免费 | 713篇 |
专业分类
电工技术 | 6193篇 |
综合类 | 560篇 |
化学工业 | 46738篇 |
金属工艺 | 10324篇 |
机械仪表 | 8820篇 |
建筑科学 | 7898篇 |
矿业工程 | 862篇 |
能源动力 | 8568篇 |
轻工业 | 31080篇 |
水利工程 | 2532篇 |
石油天然气 | 3669篇 |
武器工业 | 8篇 |
无线电 | 39694篇 |
一般工业技术 | 58443篇 |
冶金工业 | 56632篇 |
原子能技术 | 5458篇 |
自动化技术 | 26698篇 |
出版年
2021年 | 2275篇 |
2019年 | 2141篇 |
2018年 | 3607篇 |
2017年 | 3449篇 |
2016年 | 3538篇 |
2015年 | 2541篇 |
2014年 | 4439篇 |
2013年 | 14271篇 |
2012年 | 7362篇 |
2011年 | 10112篇 |
2010年 | 7899篇 |
2009年 | 9131篇 |
2008年 | 9599篇 |
2007年 | 9577篇 |
2006年 | 8504篇 |
2005年 | 7887篇 |
2004年 | 7759篇 |
2003年 | 7496篇 |
2002年 | 7333篇 |
2001年 | 7543篇 |
2000年 | 7205篇 |
1999年 | 7545篇 |
1998年 | 17655篇 |
1997年 | 12831篇 |
1996年 | 10097篇 |
1995年 | 7872篇 |
1994年 | 7203篇 |
1993年 | 6903篇 |
1992年 | 5286篇 |
1991年 | 5066篇 |
1990年 | 4933篇 |
1989年 | 4781篇 |
1988年 | 4673篇 |
1987年 | 3886篇 |
1986年 | 4007篇 |
1985年 | 4717篇 |
1984年 | 4275篇 |
1983年 | 4021篇 |
1982年 | 3601篇 |
1981年 | 3761篇 |
1980年 | 3456篇 |
1979年 | 3418篇 |
1978年 | 3225篇 |
1977年 | 3814篇 |
1976年 | 4847篇 |
1975年 | 2780篇 |
1974年 | 2660篇 |
1973年 | 2675篇 |
1972年 | 2215篇 |
1971年 | 1963篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
941.
942.
Two cases of congenital absence of a cervical vertebral pedicle are reported. This condition includes hypoplasia of the ipsilateral posterior arch and may predispose to spinal cord injury. The radiographic and computed tomography (CT) findings are reviewed, and the importance of the ipsilateral oblique radiographic view and of CT in diagnosis is stressed. 相似文献
943.
Also with the Department of Agronomy. 相似文献
944.
In vitro transdifferentiation of retinal pigmented epithelial cells of the chick embryo into lens cells can be markedly enhanced by culture in the presence of testicular hyaluronidase and phenylthiourea. Since the commercial preparations of hyaluronidase that had previously been used were very crude, a search for the actual effective molecule(s) enhancing lens transdifferentiation was conducted. First, we purified the enzyme and tested the effect of the purified hyaluronidase. Highly purified hyaluronidase itself did not enhance lens transdifferentiation. The crude hyaluronidase was then separated according to affinity with heparin, considering the possibility that the fibroblast growth factor (FGF) is contained in the crude hyaluronidase. Transdifferentiation-enhancing activity was detected in the fraction which was bound to heparin and eluted with 2 M NaCl, where no hyaluronate-degrading activity existed. Analysis of the fraction by SDS-PAGE revealed the existence of an 18 kDa protein whose NH2-terminal sequence was identical to that of basic FGF. The basic FGF derived from bovine brain also enhanced lens transdifferentiation of pigmented epithelial cells. These findings suggest that basic FGF must play a major role in enhancing transdifferentiation of pigmented epithelial cells to lens cells. 相似文献
945.
946.
947.
948.
OBJECTIVE: To describe pediatric housestaff resuscitation experience and their ability to perform key resuscitation skills. DESIGN: Cohort study of 63 pediatric residents in a university-based training program. PARTICIPANTS AND METHODS: Investigators observed, scored, and timed resident performance on 4 key resuscitation skills. Cognitive ability was tested with 4 written scenarios. Housestaff provided self-reports of the number of months since their last American Heart Association Pediatric Advanced Life Support course, number of mock and actual codes attended, number of times skills were performed, and self-confidence with respect to resuscitation. RESULTS: A total of 45 pediatric residents (71%) participated. Median cognitive score was 5 (range, 1-5). Of all residents, 44 (97%) successfully bag mask-ventilated the mannequin; 24 (53%) and 36 (80%) used the correct bag and mask size, respectively. Thirty-nine residents (87%) placed a tube in the mannequin trachea, 12 (27%) checked that suction was working prior to intubation, and 30 (67%) chose the correct endotracheal tube size. Forty residents (89%) discharged the defibrillator, and 25 (56%) and 32 (71%) correctly chose asynchronous mode and infant paddles, respectively. Thirty-eight residents (84%) inserted an intraosseous line; 35 (78%) had correct placement. Median times for successful skill completion were 83 seconds for bag mask ventilation, 136 seconds for intubation, 149 seconds for defibrillation, and 68 seconds for intraosseous line placement. CONCLUSION: Pediatric housestaff previously trained in pediatric advanced life support were generally able to reach the end point of 4 key resuscitation skills but less frequently performed the specific subcomponents of each skill. This poor performance and the prolonged time to skill completion suggest the need for greater attention to detail during training. 相似文献
949.
MA Lindorfer CS Myung Y Savino H Yasuda R Khazan JC Garrison 《Canadian Metallurgical Quarterly》1998,273(51):34429-34436
The G protein beta5 subunit differs substantially in amino acid sequence from the other known beta subunits suggesting that beta gamma dimers containing this protein may play specialized roles in cell signaling. To examine the functional properties of the beta5 subunit, recombinant beta5 gamma2 dimers were purified from baculovirus-infected Sf9 insect cells using a strategy based on two affinity tags (hexahistidine and FLAG) engineered into the N terminus of the gamma2 subunit (gamma2HF). The function of the pure beta5 gamma2HF dimers was examined in three assays: activation of pure phospholipase C-beta in lipid vesicles; activation of recombinant, type II adenylyl cyclase expressed in Sf9 cell membranes; and coupling of alpha subunits to the endothelin B (ETB) and M1 muscarinic receptors. In each case, the efficacy of the beta5 gamma2HF dimer was compared with that of the beta1 gamma2HF dimer, which has demonstrated activity in these assays. The beta5 gamma2HF dimer activated phospholipase C-beta with a potency and efficacy similar to that of beta1 gamma2 or beta1 gamma2HF; however, it was markedly less effective than the beta1 gamma2HF or beta1 gamma2 dimer in its ability to activate type II adenylyl cyclase (EC50 of approximately 700 nM versus 25 nM). Both the beta5 gamma2HF and the beta1 gamma2HF dimers supported coupling of M1 muscarinic receptors to the Gq alpha subunit. The ETB receptor coupled effectively to both the Gi and Gq alpha subunits in the presence of the beta1 gamma2HF dimer. In contrast, the beta5 gamma2HF dimer only supported coupling of the Gq alpha subunits to the ETB receptor and did not support coupling of the Gi alpha subunit. These results suggest that the beta5 gamma2HF dimer binds selectively to Gq alpha subunits and does not activate the same set of effectors as dimers containing the beta1 subunit. Overall, the data support a specialized role for the beta5 subunit in cell signaling. 相似文献
950.