全文获取类型
收费全文 | 249641篇 |
免费 | 3402篇 |
国内免费 | 438篇 |
专业分类
电工技术 | 4399篇 |
综合类 | 295篇 |
化学工业 | 41579篇 |
金属工艺 | 9607篇 |
机械仪表 | 7579篇 |
建筑科学 | 6040篇 |
矿业工程 | 1411篇 |
能源动力 | 5441篇 |
轻工业 | 25721篇 |
水利工程 | 2558篇 |
石油天然气 | 5134篇 |
武器工业 | 33篇 |
无线电 | 25683篇 |
一般工业技术 | 46197篇 |
冶金工业 | 45892篇 |
原子能技术 | 6011篇 |
自动化技术 | 19901篇 |
出版年
2021年 | 2438篇 |
2019年 | 2211篇 |
2018年 | 3761篇 |
2017年 | 3659篇 |
2016年 | 4003篇 |
2015年 | 2469篇 |
2014年 | 4229篇 |
2013年 | 10817篇 |
2012年 | 6633篇 |
2011年 | 8912篇 |
2010年 | 6982篇 |
2009年 | 7718篇 |
2008年 | 8019篇 |
2007年 | 8108篇 |
2006年 | 7235篇 |
2005年 | 6337篇 |
2004年 | 6082篇 |
2003年 | 5845篇 |
2002年 | 5856篇 |
2001年 | 5694篇 |
2000年 | 5518篇 |
1999年 | 5414篇 |
1998年 | 12711篇 |
1997年 | 9034篇 |
1996年 | 6955篇 |
1995年 | 5408篇 |
1994年 | 4813篇 |
1993年 | 4709篇 |
1992年 | 3723篇 |
1991年 | 3446篇 |
1990年 | 3657篇 |
1989年 | 3539篇 |
1988年 | 3350篇 |
1987年 | 2941篇 |
1986年 | 2983篇 |
1985年 | 3428篇 |
1984年 | 3287篇 |
1983年 | 2937篇 |
1982年 | 2775篇 |
1981年 | 2856篇 |
1980年 | 2757篇 |
1979年 | 2655篇 |
1978年 | 2714篇 |
1977年 | 2980篇 |
1976年 | 3843篇 |
1975年 | 2405篇 |
1974年 | 2268篇 |
1973年 | 2409篇 |
1972年 | 1982篇 |
1971年 | 1873篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
Dr. Emilianne M. Limbrick Audrey E. Yñigez-Gutierrez Callie C. Dulin Dagmara K. Derewacz Dr. Jeffrey M. Spraggins Dr. Kathryn M. McCulloch Prof. T. M. Iverson Prof. Brian O. Bachmann 《Chembiochem : a European journal of chemical biology》2020,21(23):3349-3358
Everninomicins are orthoester oligosaccharide antibiotics with potent activity against multidrug-resistant bacterial pathogens. Everninomicins act by disrupting ribosomal assembly in a distinct region in comparison to clinically prescribed drugs. We employed microporous intergeneric conjugation with Escherichia coli to manipulate Micromonospora for targeted gene-replacement studies of multiple putative methyltransferases across the octasaccharide scaffold of everninomicin effecting the A1, C, F, and H rings. Analyses of gene-replacement and genetic complementation mutants established the mutability of the everninomicin scaffold through the generation of 12 previously unreported analogues and, together with previous results, permitted assignment of the ten methyltransferases required for everninomicin biosynthesis. The in vitro activity of A1- and H-ring-modifying methyltransferases demonstrated the ability to catalyze late-stage modification of the scaffold on an A1-ring phenol and H-ring C-4’ hydroxy moiety. Together these results establish the potential of the everninomicin scaffold for modification through mutagenesis and in vitro modification of advanced biosynthetic intermediates. 相似文献
93.
Abraham G. Alvarado Rosaura Hernández-Montelongo Martin Rabelero Lourdes A. Pérez-Carrillo Jorge E. Puig Francisco López-Serrano 《Polymer Engineering and Science》2020,60(2):223-232
A simple but comprehensive model considering homogeneous and micellar nucleation, coagulation, entry of radicals to particles and to micelles and radicals' exit from particles, is presented. The model is validated, in a starved semicontinuous heterophase polymerization of ethyl methacrylate, at three monomer addition rates. The model accurately describes the overall and instantaneous conversion, the average particle density and diameter, and the number and weight average molar masses evolutions over time. It is found that even though the average number of radicals is much smaller than 0.5, the system is not 0-1. An empirical function was used to describe the gel effect. The homogeneous nucleation was the prevailing mechanism for particle formation and large exit rates of radicals were observed. POLYM. ENG. SCI., 60: 223–232, 2019. © 2019 Society of Plastics Engineers 相似文献
94.
Kovchavtsev A. P. Aksenov M. S. Nastov’yak A. E. Valisheva N. A. Gorshkov D. V. Sidorov G. Yu. Dmitriev D. V. 《Technical Physics Letters》2020,46(5):469-472
Technical Physics Letters - The C–V characteristics of Au/Al2O3/In0.52Al0.48As and Au/SiO2/In0.52Al0.48As metal–insulator–semiconductor structures have been studied. It has been... 相似文献
95.
96.
Daniel Biedermann Jan Schneider Hendrik Drachsler 《Journal of Computer Assisted Learning》2021,37(5):1217-1231
Digital distractions can interfere with goal attainment and lead to undesirable habits that are hard to get red rid of. Various digital self-control interventions promise support to alleviate the negative impact of digital distractions. These interventions use different approaches, such as the blocking of apps and websites, goal setting, or visualizations of device usage statistics. While many apps and browser extensions make use of these features, little is known about their effectiveness. This systematic review synthesizes the current research to provide insights into the effectiveness of the different kinds of interventions. From a search of the ‘ACM’, ‘Springer Link’, ‘Web of Science’, ’IEEE Xplore’ and ‘Pubmed’ databases, we identified 28 digital self-control interventions. We categorized these interventions according to their features and their outcomes. The interventions showed varying degrees of effectiveness, and especially interventions that relied purely on increasing the participants' awareness were barely effective. For those interventions that sanctioned the use of distractions, the current literature indicates that the sanctions have to be sufficiently difficult to overcome, as they will otherwise be quickly dismissed. The overall confidence in the results is low, with small sample sizes, short study duration, and unclear study contexts. From these insights, we highlight research gaps and close with suggestions for future research. 相似文献
97.
Ugur Gezer Ebru E. Y?rüker Metin Keskin Cemil Burak Kulle Yoganiranjana Dharuman Stefan Holdenrieder 《International journal of molecular sciences》2015,16(12):29654-29662
Circulating nucleic acids (CNAs) are under investigation as a liquid biopsy in cancer as potential non-invasive biomarkers, as stable structure in circulation nucleosomes could be valuable sources for detection of cancer-specific alterations in histone modifications. Our interest is in histone methylation marks with a focus on colorectal cancer, one of the leading cancers respective the incidence and mortality. Our previous work included the analysis of trimethylations of lysine 9 on histone 3 (H3K9me3) and of lysine 20 on histone 4 (H4K20me3) by chromatin immuno- precipitation-related PCR in circulating nucleosomes. Here we asked whether global immunologic measurement of histone marks in circulation could be a suitable approach to show their potential as biomarkers. In addition to H3K9me3 and H4K20me3 we also measured H3K27me3 in plasma samples from CRC patients (n = 63) and cancer free individuals (n = 40) by ELISA-based methylation assays. Our results show that of three marks, the amounts of H3K27me3 (p = 0.04) and H4K20me3 (p < 0.001) were significantly lower in CRC patients than in healthy controls. For H3K9me3 similar amounts were measured in both groups. Areas under the curve (AUC) in receiver operating characteristic (ROC) curves indicating the power of CRC detection were 0.620 for H3K27me3, 0.715 for H4K20me3 and 0.769 for the combination of both markers. In conclusion, findings of this preliminary study reveal the potential of blood-based detection of CRC by quantification of histone methylation marks and the additive effect of the marker combination. 相似文献
98.
99.
Journal of Computer and Systems Sciences International - A problem of the synthesis of programmed motions of mechanical systems based on the use of time polynomials is considered. The programmed... 相似文献
100.