全文获取类型
收费全文 | 193943篇 |
免费 | 3521篇 |
国内免费 | 716篇 |
专业分类
电工技术 | 3612篇 |
综合类 | 156篇 |
化学工业 | 31970篇 |
金属工艺 | 8302篇 |
机械仪表 | 5540篇 |
建筑科学 | 5262篇 |
矿业工程 | 999篇 |
能源动力 | 4969篇 |
轻工业 | 18807篇 |
水利工程 | 1836篇 |
石油天然气 | 3153篇 |
武器工业 | 2篇 |
无线电 | 20919篇 |
一般工业技术 | 37900篇 |
冶金工业 | 33537篇 |
原子能技术 | 4423篇 |
自动化技术 | 16793篇 |
出版年
2021年 | 1750篇 |
2019年 | 1578篇 |
2018年 | 2730篇 |
2017年 | 2743篇 |
2016年 | 2908篇 |
2015年 | 2171篇 |
2014年 | 3463篇 |
2013年 | 9242篇 |
2012年 | 5422篇 |
2011年 | 7412篇 |
2010年 | 5953篇 |
2009年 | 6689篇 |
2008年 | 6801篇 |
2007年 | 6726篇 |
2006年 | 5663篇 |
2005年 | 5174篇 |
2004年 | 4877篇 |
2003年 | 4554篇 |
2002年 | 4531篇 |
2001年 | 4464篇 |
2000年 | 4197篇 |
1999年 | 4270篇 |
1998年 | 10546篇 |
1997年 | 7570篇 |
1996年 | 5807篇 |
1995年 | 4436篇 |
1994年 | 3742篇 |
1993年 | 3673篇 |
1992年 | 2705篇 |
1991年 | 2629篇 |
1990年 | 2492篇 |
1989年 | 2504篇 |
1988年 | 2436篇 |
1987年 | 2190篇 |
1986年 | 2114篇 |
1985年 | 2500篇 |
1984年 | 2325篇 |
1983年 | 2106篇 |
1982年 | 1971篇 |
1981年 | 2067篇 |
1980年 | 1885篇 |
1979年 | 1888篇 |
1978年 | 1851篇 |
1977年 | 2167篇 |
1976年 | 2750篇 |
1975年 | 1632篇 |
1974年 | 1604篇 |
1973年 | 1664篇 |
1972年 | 1379篇 |
1971年 | 1290篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Aline Kövilein Christin Kubisch Liyin Cai Katrin Ochsenreither 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2020,95(3):513-526
Malic acid derived from fossil resources is currently applied in the food and beverage industries with a medium global production capacity. However, in the transition from a fossil-based to a bio-based economy, biotechnologically produced l -malic acid may become an important platform chemical with many new applications, especially in the field of biopolymers. In this review, currently used petrochemical production routes to dl -malic acid are outlined and insights into possible bio-based alternatives for microbial l -malic acid production are provided. Besides ecological reasons, the possibility to produce enantiopure l -malic acid by microbial fermentation is the biggest advantage over chemical synthesis. State-of-the-art and open challenges concerning production host engineering, substrate choice and downstream processing are addressed. With regard to production hosts, a literature overview is given covering the leading natural production strains of Aspergillus, Ustilago and Aureobasidium, as well as Escherichia coli as the most important engineered recombinant host. The utilization of renewable substrates as an alternative to glucose is emphasized in particular as a key aspect for a competitive bio-based production. Out of the alternative substrates discussed in this review, the industrial side-streams crude glycerol and molasses seem to be most promising for large-scale l -malic acid production. © 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. 相似文献
102.
Climate change raises many concerns for urban water management because of the effects on all aspects of the hydrological cycle. Urban water infrastructure has traditionally been designed using historical observations and assuming stationary climatic conditions. The capability of this infrastructure, whether for storm-water drainage, or water supply, may be over- or under-designed for future climatic conditions. In particular, changes in the frequency and intensity of extreme rainfall events will have the most acute effect on storm-water drainage systems. Therefore, it is necessary to take future climatic conditions into consideration in engineering designs in order to enhance water infrastructure investment planning practices in a long time horizon. This paper provides the initial results of a study that is examining ways to enhance urban infrastructure investment planning practices against changes in hydrologic regimes for a changing climate. Design storms and intensity-duration-frequency curves that are used in the engineering design of storm-water drainage systems are developed under future climatic conditions by empirically adjusting the general circulation model output, and using the Gumbel distribution and the Chicago method. Simulations are then performed on an existing storm-water drainage system from NE Calgary to investigate the resiliency of the system under climate change. 相似文献
103.
H Daims F Maixner S Lücker K Stoecker K Hace M Wagner 《Water science and technology》2006,54(1):21-27
Nitrite-oxidizing bacteria of the genus Nitrospira are key nitrifiers in wastewater treatment plants. Pure cultures of these organisms are unavailable, but cultivation-independent molecular methods make it possible to detect Nitrospira-like bacteria in environmental samples and to investigate their ecophysiology. Comprehensive screening of natural and engineered habitats and of public databases for 16S rRNA sequences of Nitrospira-like bacteria revealed a surprisingly high biodiversity in the genus Nitrospira, which comprises at least four phylogenetic sublineages. All Nitrospira-like bacteria detected in wastewater treatment plants belonged to the sublineages I and II. Subsequently, the population dynamics of different Nitrospira-like bacteria were monitored, by quantitative fluorescence in situ hybridization with rRNA-targeted probes, confocal laser scanning microscopy and digital image analysis, during incubation of nitrifying activated sludge in media containing different nitrite concentrations. These experiments showed that Nitrospira-like bacteria, which were affiliated with the phylogenetic sublineages I or II of the genus Nitrospira, responded differently to nitrite concentration shifts. Previously unknown properties of Nitrospira-like bacteria were discovered in the course of an environmental genomics project. Implications of the obtained results for fundamental understanding of the microbial ecology of nitrite oxidizers as well as for future improvement of nutrient removal in wastewater treatment plants are discussed. 相似文献
104.
The main factors that determine the uniformity with which smelting products are tapped from a blast furnace are the rate of accumulation of the products in the hearth and the tapping rate. The first parameter is in turn determined by the stability of the furnace’s operation and can vary significantly even over the course of 24 h. The second parameter is determined mainly by the condition of the iron notch, the maintenance it receives, any changes that occur in the diameter of the channel of the notch, and the quality of the materials used to form the notch. Recommendations are presented to optimize tapping of the smelting products. 相似文献
105.
The Chelyabinsk Metallurgical Combine has periodically used Kachkanar agglomerate to make conversion pig iron since 1972.
Use of this material reached a peak in 1998, when it comprised an average of 27.1% of the charge. In some months, the amount
of this material charged into the furnace reached highs of 45.6 and 74.1% on certain furnaces. The coolers on the iron notches
of furnaces 4 and 5 burned during that year, the quality of the smelting products and the condition of the ladles rapidly
deteriorated, and the smelting rate declined. No Kachkanar agglomerate was used in 2000–2001, but it was again added to the
charge in February 2002. The amount used was smaller than previously, averaging 10.7% for the shop as a whole. It was determined
that using a charge with 15% Kachkanar agglomerate has both positive and negative consequences.
N. N. Gorshkov, A. V. Denisov, and V. Kh. Barinov also participated in this study.
__________
Translated from Metallurg, No. 3, pp. 43–45, March, 2006. 相似文献
106.
B. Schönfelder G. Gottstein L. S. Shvindlerman 《Metallurgical and Materials Transactions A》2006,37(6):1757-1771
While the motion of twist boundaries can be readily studied by atomistic simulations with molecular dynamics (MD) under the
action of an elastic driving force, the approach fails for tilt boundaries. This is due to the interaction of the elastic
stress with the grain boundary (GB) structure, which causes plastic strain by GB sliding. A novel concept, the orientation
correlated driving force, is introduced to circumvent this problem. It is shown that this concept can be successfully applied
to the study of the migration of tilt boundaries. The migration behavior of several twist and tilt GBs was investigated. The
transition from low-to high-angle boundaries can be captured, and a structural transition of tilt boundaries was found at
high temperatures, which also affected the migration behavior. The results compare well with experimental results of the motion
high-angle boundaries, but for low-angle boundaries, the agreement is poor.
This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces
in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden. 相似文献
107.
Stievano I.S. Maio I.A. Canavero F.G. Siviero C. 《Advanced Packaging, IEEE Transactions on》2006,29(1):31-38
This paper addresses the impact of device macromodels on the accuracy of signal integrity and performance predictions for critical digital interconnecting systems. It exploits nonlinear parametric models for both single-ended and differential devices, including the effects of power supply fluctuations and receiver bit detection. The analysis demonstrates that the use of well-designed macromodels dramatically speeds up the simulation as well it preserves timing accuracy even for long bit sequences. 相似文献
108.
109.
It was found that vanadyl porphyrin complexes synthesized from petroleum metal porphyrin concentrates stimulated epoxidation during the olefin oxygenation process. The yields of obtained oxiranes turned out to be 38–75%, depending on the olefin structure. An epoxidation mechanism that suggests the formation of a protonated dioxygen adduct as an intermediate during oxygenation of olefins in the presence of vanadyl porphyrin complexes was proposed. An analogy is drawn between the epoxide formation reaction upon the catalytic oxygenation of olefins and the Prilezhaev reaction. 相似文献
110.
The Magnitogorsk Metallurgical Combine has conducted a study of the effect of technological factors on the hydrogen content
of chromium-nickel-molybdenum steel after vacuum degassing. It was established that the most important factor is the hydrogen
content of the steel before the degassing operation. The study also determined the effects of the circulation coefficient,
the duration of the degassing operation, and the vacuum used in the treatment.
__________
Translated from Metallurg, No. 7, pp. 68–69, July, 2006. 相似文献