全文获取类型
收费全文 | 3454篇 |
免费 | 211篇 |
国内免费 | 123篇 |
专业分类
电工技术 | 149篇 |
综合类 | 681篇 |
化学工业 | 255篇 |
金属工艺 | 83篇 |
机械仪表 | 132篇 |
建筑科学 | 518篇 |
矿业工程 | 57篇 |
能源动力 | 42篇 |
轻工业 | 188篇 |
水利工程 | 41篇 |
石油天然气 | 51篇 |
武器工业 | 11篇 |
无线电 | 212篇 |
一般工业技术 | 174篇 |
冶金工业 | 650篇 |
原子能技术 | 4篇 |
自动化技术 | 540篇 |
出版年
2024年 | 5篇 |
2023年 | 22篇 |
2022年 | 52篇 |
2021年 | 71篇 |
2020年 | 79篇 |
2019年 | 65篇 |
2018年 | 57篇 |
2017年 | 61篇 |
2016年 | 80篇 |
2015年 | 80篇 |
2014年 | 173篇 |
2013年 | 190篇 |
2012年 | 151篇 |
2011年 | 231篇 |
2010年 | 213篇 |
2009年 | 229篇 |
2008年 | 243篇 |
2007年 | 258篇 |
2006年 | 253篇 |
2005年 | 195篇 |
2004年 | 196篇 |
2003年 | 232篇 |
2002年 | 164篇 |
2001年 | 110篇 |
2000年 | 74篇 |
1999年 | 47篇 |
1998年 | 31篇 |
1997年 | 36篇 |
1996年 | 20篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1984年 | 5篇 |
1982年 | 4篇 |
1970年 | 4篇 |
1965年 | 10篇 |
1964年 | 5篇 |
1963年 | 6篇 |
1962年 | 8篇 |
1961年 | 5篇 |
1960年 | 5篇 |
1957年 | 3篇 |
1956年 | 5篇 |
1955年 | 9篇 |
1954年 | 7篇 |
排序方式: 共有3788条查询结果,搜索用时 234 毫秒
1.
2.
《Journal of dairy science》2022,105(2):1661-1673
The objective of this study was to evaluate how meal patterns of recently weaned and group-housed dairy calves are subject to effects of previous dietary experience and associated with competition for feed. During the preweaning period, Holstein heifer and bull calves were provided diets differing in provision and presentation of forage as follows: (1) pelleted starter only (n = 12), (2) starter and chopped coastal bermudagrass provided in separate buckets (n = 13), (3) starter and chopped coastal bermudagrass mixed together in the same bucket (n = 15), or (4) starter, chopped coastal bermudagrass, and liquid molasses mixed together in 1 bucket (n = 13). At 58 ± 2 (mean ± standard deviation) days of age, following weaning, calves were mingled between treatments and moved into group housing in weekly age-based cohorts (7 ± 2 calves/group; 8 groups total), and all were provided the mixed diet (without molasses). Within group pens, calves were fed individually using the Calan Broadbent feeding system (American Calan Inc.). Calves were monitored for 1 wk following introduction to the group pen. Feed intake was measured daily. Behavioral data, describing feeding times and competition for feed, were recorded continuously for 48 h beginning after a 5-d adaptation to the group pen. Calves previously provided starter only had longer, less frequent meals than calves previously provided forage in any form, and they tended to consume less feed compared with those previously provided starter and hay separately, with calves previously provided mixed diets having intermediate intakes. We observed occurrences of displacements at bins, which were followed by replacements and feed stealing on some occasions, where 34% of calves consumed feed from a bin assigned to another calf on at least 1 occurrence, and 64% of calves were stolen from at least once. Competition at feed bins was not affected by previous dietary treatments, but was associated with meal patterns. Actor displacement rate was negatively associated with meal frequency and duration, but calves that were displaced more often were those that spent more time feeding. These results suggested a possible carryover effect of previous exposure to forage on postweaning meal patterns following a dietary change. Further, we found that competitive behavior varied considerably between individuals, occurring frequently despite feeding calves using individually-assigned feeding bins, and was associated with meal characteristics. These results highlighted the importance of considering both previous dietary experience and social factors when evaluating feeding behavior. 相似文献
3.
4.
《Contact lens & anterior eye》2021,44(2):192-219
This paper outlines changes to the ocular surface caused by contact lenses and their degree of clinical significance. Substantial research and development to improve oxygen permeability of rigid and soft contact lenses has meant that in many countries the issues caused by hypoxia to the ocular surface have largely been negated. The ability of contact lenses to change the axial growth characteristics of the globe is being utilised to help reduce the myopia pandemic and several studies and meta-analyses have shown that wearing orthokeratology lenses or soft multifocal contact lenses can reduce axial length growth (and hence myopia).However, effects on blinking, ptosis, the function of Meibomian glands, fluorescein and lissamine green staining of the conjunctiva and cornea, production of lid-parallel conjunctival folds and lid wiper epitheliopathy have received less research attention. Contact lens wear produces a subclinical inflammatory response manifested by increases in the number of dendritiform cells in the conjunctiva, cornea and limbus. Papillary conjunctivitis is also a complication of all types of contact lenses. Changes to wear schedule (daily disposable from overnight wear) or lens materials (hydrogel from SiHy) can reduce papillary conjunctivitis, but the effect of such changes on dendritic cell migration needs further study. These changes may be associated with decreased comfort but confirmatory studies are needed. Contact lenses can affect the sensitivity of the ocular surface to mechanical stimulation, but whether these changes affect comfort requires further investigation.In conclusion, there have been changes to lens materials, design and wear schedules over the past 20+ years that have improved their safety and seen the development of lenses that can reduce the myopia development. However, several changes to the ocular surface still occur and warrant further research effort in order to optimise the lens wearing experience. 相似文献
5.
6.
7.
8.
9.
This paper uses a mixed-methods approach to examine the relation between online academic disclosure and academic performance. A multi-ethnic sample of college students (N = 261; male = 66; female = 195; M age ≈ 22 years) responded to open-ended questions about their Facebook use. Thematic analysis revealed that over 14% of the Facebook wall posts/status updates (N = 714) contained academic themes; positive states were more frequent than negative and neutral states and students with lower GPAs expressed negative states more often. A path analysis suggested that academic performance may determine college students’ Facebook use, rather than the reverse. Implications for student support services are discussed. 相似文献
10.
In cooperative communications, employing the idle relay node for forwarding data can effectively improve the transmission rate and performance of the wireless networks. Therefore, various relay node selection algorithms are proposed. However, most of the existing algorithms just focus on the benefit caused by relay nodes, while ignoring the negative effect of the backoff duration time which does not exist in direct transmission, when more than one flow compete for the same relay node. To this end, considering both advantage and adverse impact caused by the relay node, we adopt the game-theoretic approach and propose a cooperative transmission strategy based on the Nash equilibrium, named the Nash Equilibrium Cooperative Transmission Strategy (NECTS). The NECTS can not only guarantee the gain of each flow, but also maximize the system performance, and improve the system throughput effectively. Simulation results show that the NECTS can take the advantage of cooperative communications. Meanwhile, it can avoid the unnecessary competition, and its performance outperforms that of the existing methods. 相似文献