首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   298篇
  免费   3篇
电工技术   5篇
化学工业   43篇
金属工艺   4篇
机械仪表   14篇
建筑科学   21篇
能源动力   20篇
轻工业   25篇
水利工程   1篇
石油天然气   2篇
无线电   16篇
一般工业技术   57篇
冶金工业   37篇
原子能技术   3篇
自动化技术   53篇
  2021年   6篇
  2020年   1篇
  2019年   4篇
  2018年   4篇
  2017年   5篇
  2016年   5篇
  2015年   5篇
  2014年   9篇
  2013年   12篇
  2012年   12篇
  2011年   16篇
  2010年   12篇
  2009年   20篇
  2008年   14篇
  2007年   18篇
  2006年   14篇
  2005年   8篇
  2004年   9篇
  2003年   9篇
  2002年   13篇
  2001年   5篇
  2000年   4篇
  1999年   7篇
  1998年   9篇
  1997年   4篇
  1996年   4篇
  1995年   3篇
  1994年   4篇
  1993年   5篇
  1992年   3篇
  1991年   1篇
  1990年   4篇
  1989年   2篇
  1988年   2篇
  1987年   2篇
  1986年   3篇
  1985年   4篇
  1984年   7篇
  1983年   3篇
  1982年   3篇
  1981年   4篇
  1980年   5篇
  1979年   1篇
  1978年   2篇
  1977年   2篇
  1976年   3篇
  1975年   2篇
  1973年   2篇
  1972年   1篇
  1971年   2篇
排序方式: 共有301条查询结果,搜索用时 15 毫秒
1.
High-yielding dairy cows are often fed high proportions of cereal grain and pulses. For several reasons, it would be desirable to replace these feed sources with forage, which is not suitable for human consumption. Feeding large amounts of forage to dairy cows could also make dairy production more publicly acceptable in the future. In this study, we estimated genetic parameters for total dry matter intake (DMI), DMI from forage (DMIFor), energy-corrected milk (ECM), and ECM produced from forage (ECMFor). A total of 1,177 lactations from 575 cows of Swedish Red (SR) and Holstein (HOL) dairy breeds were included in the study. Mixed linear animal random regression models were used, with fixed effect of calving season and lactation week nested within parity 1 and 2+, fixed effect of calving year, and random regression coefficients for breeding value (up to linear) and permanent environmental effect (up to quadratic) of the cow. Heritability for DMI and DMIFor was generally higher for HOL than for SR in all-parity data and in later parities; however, the opposite was true for first parity. Heritability for DMI and DMIFor during the first 8 wk averaged 0.11 and 0.15, respectively, in all-parity data for the 2 breeds. Corresponding values for ECMFor and ECM were 0.21 and 0.29, respectively. In first parity, values were 0.32, 0.36, 0.28, and 0.51, respectively. The genetic correlation between DMI and DMIFor was high, above 0.83, and fairly constant across the lactation. The genetic correlation between ECMFor and ECM was close to unity in the later part of lactation for both breeds, but was around 0.8 in the early lactation for both breeds; it decreased for HOL to 0.54 in wk 17. The genetic correlations between DMI and ECMFor and between DMIFor and ECMFor were low and negative for HOL (absolute value ~0.2–0.3), but changed for SR from weakly positive in early lactation to negative values and back to positive toward the end of lactation. For most traits, the correlation between wk 1 and wk 8 into the lactation was very high; the lowest value was for DMI in HOL at 0.81. The genetic correlation between parities was rather high in the first part of the lactation. During the first 8 wk, the correlation was lower for HOL than for SR, except for ECM. We found that DMIFor and ECMFor showed reasonably large heritability, and future work should explore the possibility of genomic evaluations.  相似文献   
2.
Due to high figure of merit, Ca3Co4 ? xO9 + δ (CCO) has potential as p-type material for high-temperature thermoelectrics. Here, the influence of processing including solid state sintering, spark plasma sintering and post-calcination on stability, microstructure and thermoelectric properties is reported. By a new post-calcination approach, single-phase materials were obtained from precursors to final dense ceramics in one step. The highest zT of 0.11 was recorded at 800 °C for CCO with 98 and 72% relative densities. In situ high-temperature X-ray diffraction in air and oxygen revealed a higher stability of CCO in oxygen (~970 °C) than in air (~930 °C), with formation of Ca3Co2O6 which also showed high stability in oxygen, even at 1125 °C. Since achievement of phase pure high density CCO by post-calcination method in air is challenging, the phase stability of CCO in oxygen is important for understanding and further improvement of the method.  相似文献   
3.
This high-resolution study of the latest Holocene dinoflagellate cyst record from Gullmar Fjord, on the west coast of Sweden, provides evidence for the recognition of two major dinoflagellate communities within the fjord over the last 85 years. These communities may have their origins with the history of cultural eutrophication within the region, but are more likely to be associated with the wider phenomenon of the North Atlantic Oscillation and/or the complex hydrographical response of the fjord to various changing climatic environments between 1915 and 1999. The changing dinoflagellate cyst populations are compared in detail with the many hydrographical parameters available from this well studied fjord with its long instrumental records. Indeed the dinoflagellate cysts fail to demonstrate a convincing ongoing eutrophication record for the fjord but do show a major change in the cyst assemblages at about 1969/1970 at a time when the NAO was changing from a negative phase to the present-day positive phase. Gullmar Fjord is important in the history of dinoflagellate cyst studies, being the site of the 1954 study by Erdtman in which viable cysts, produced within the phytoplankton, were first documented within the water column.  相似文献   
4.
This paper endogenizes the value of an information set which has to be produced and protected. The profit is inverse U shaped in security investment and production effort. The breach probability is commonly assumed to decrease convexly in security investment, which means that modest security investment is sufficient to deter most perpetrators. We allow the breach probability to be not only convex, but concave, which means that substantial security investment is needed to deter most perpetrators. Convexity versus concavity depends on the security environment, perpetrators, technology, and law enforcement. A firm strikes a balance between producing and protecting an information set dependent on seven model parameters for production, protection, convexity, concavity, vulnerability, and resource strength.  相似文献   
5.
A game theoretic model of six kinds of players is analyzed, i.e. countries, central banks, banks, firms, households, and financial inter-governmental organizations. Each player has a strategy set, with strategies such as setting interest rates, lending, borrowing, producing, consuming, investing, importing, exporting, defaulting, and penalizing default. Markets for goods, debt, and capital are modeled endogenously. This conceptualization of strategic opportunities for as many as six types of players is richer than anything that has been attempted earlier. 2005–2011 empirical data for Greece is used to analyze how utility is impacted by public consumption and lump sum transfers, and negative productivity shocks, and to analyze equilibrium over several time periods with and without the possibility of default. 2007–2008 empirical data for Greece and Germany is used to determine how the two countries’ utilities depend on Greece’s public 2007 consumption, with and without negative productivity shocks. Greece’s high debt burden is shown to make default optimal when productivity shocks are large and the default penalty is small. We find that Germany has limited ability, through its available strategies, to prevent a Greek default, and may need to resort to unconventional tools such as debt forgiveness and changing the default penalty.  相似文献   
6.
Coarse grained superalloys are of large interest in high temperature applications, and can be found in e.g. gas turbine components, where great care must be given with respect to high temperature fatigue. Due to the large grain size, the material behaviour at e.g. sharp notches cannot be considered homogeneous. As a consequence, the fatigue behaviour is likely to expose a large variation. In order to numerically investigate this variation, a Monte Carlo analysis has been carried out by 100 FE-simulations of notched specimens, where placements and orientations of the grains were randomised. Furthermore, each grain was modelled as a unique single-crystal, displaying both anisotropic elastic and plastic behaviour and tension/compression asymmetry. The effect of randomness was investigated by the obtained dispersion in fatigue crack initiation life. It was concluded that the fatigue life behaviour of coarse grained nickel-base superalloys may show a considerable variation, which cannot be captured by one single deterministic analysis based on data for a homogenised material. Furthermore, the dispersion is of such a magnitude that it needs to be taken into account in industrial applications where highly stressed coarse grained materials are used.  相似文献   
7.
PLUREL是一个探讨城市边缘区土地利用战略与可持续影响评估工具的综合项目,选取7个案例以探索城市土地之间利用的关系.自从二战以后,城市化是土地用途变化最为显著的过程.不同空间模式、文化、规划政策和迥异的城市化或城市衰退的驱动力等因素导致城乡之间出现土地利用联系和功能关联.研究阐述了城市扩张的后果及正面影响,并从7个方面提出了城市边缘区的土地利用战略及可持续影响评估工具:交通、土地利用和露天空间规划之间的更好协同作用,优良治理和政策集成方法,通过保护和密实化实现城市控制,发展拥有美丽内城区的紧凑型花园城市,确保生物多样性、创建促进健康的交通系统,保护和开发蓝色和绿色基础设施,促进本地生产和短循环和生态服务系统.以期通过已有的城市发展和城乡关系建设知识和经验,努力向发展中国家提供支持.  相似文献   
8.
A defender allocates its resource between defending an object passively and striking preventively against an attacker seeking to destroy the object. With no preventive strike the defender distributes its entire resource between deploying false targets, which the attacker cannot distinguish from the genuine object, and protecting the object. If the defender strikes preventively, the attacker's vulnerability depends on its protection and on the defender's resource allocated to the strike. If the attacker survives, the object's vulnerability depends on the attacker's revenge attack resource allocated to the attacked object. The optimal defense resource distribution between striking preventively, deploying the false targets and protecting the object is analyzed. Two cases of the attacker strategy are considered: when the attacker attacks all of the targets and when it chooses a number of targets to attack. An optimization model is presented for making a decision about the efficiency of the preventive strike based on the estimated attack probability, dependent on a variety of model parameters.  相似文献   
9.
Two full size strip detectors were investigated in this study: one with p+ strips (p+/n/n+) and another with n+ strips (n+/p/p+). Both detectors, are made of magnetic Czochralski silicon (MCz-Si) and irradiated to S-LHC fluencies, were tested with 225 GeV muon beam in the CERN H2 area. The Current Injected Detector (CID) sensors were operated in a cooling box capable of providing a −53 °C temperature. Results indicate a relative charge collection efficiency (CCE) at 5×1015 neq/cm2 above 30% in irradiated p+/n/n+ CID detector at 600 V bias voltage. The signal to noise ratio of this CID module was about eight and a forward current of 30 μA was needed for detector biasing. In standard reverse bias, the same detector could not provide a sufficiently large signal for particle tracking purposes. A p-type (n+/p/p+) sensor was irradiated to a fluence of 2×1015 neq/cm2 and measured under the same test beam conditions. According to the theory of CIDs developed by the CERN RD39 Collaboration, this detector module could be biased up to only 230 V due to the low irradiation fluence. The CCE at 230 V was 35% in CID operation and 20% when reverse biased.  相似文献   
10.
In the process of service life prediction or estimation of building and construction components and materials, data of the prevailing exposure environment, (the conditions at and around a building or construction) is required. However, most environmental data is measured by and available from meteorological and air quality research communities. This data is collected at macro and meso levels, some distance from the object studied, and raises the need to transform data in order to describe the specific, local conditions adjacent to that object. To estimate levels of degradation agents in the exposure environment, especially those close to the building or construction at local and micro levels have to be considered. This paper will show and discuss useful environmental data sources, and how to transform such data by means of available distribution models.

Lorsqu'il s'agit de faire des prévisions ou des estimations de durée de vie de bâtiments ainsi que de composants et matériaux de construction, il faut disposer des données sur les conditions prévalantes d'exposition et d'environnement (conditions qui règnent dans un bâtiment ou à proximité). Or, la plupart des données environnementales sont mesurées et fournies par des instituts de recherche en météorologie et qualité de l'air. Ces données sont collectées à grande et à moyenne échelle, à une certaine distance de l'objet étudié, et elles doivent donc être transformées si l'on veut décrire les conditions spécifiques locales à proximité dudit objet. Pour estimer le niveau de dégradation dans l'environnement d'exposition, notamment à proximité des bâtiments ou constructions, il faut considérer les agents au micro-niveau et au niveau local. Cet article présente et examine des sources de données environnementales utiles et montre comment transformer ces données en utilisant les modèles de distribution existants.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号