全文获取类型
收费全文 | 22870篇 |
免费 | 794篇 |
国内免费 | 84篇 |
专业分类
电工技术 | 293篇 |
综合类 | 44篇 |
化学工业 | 5147篇 |
金属工艺 | 479篇 |
机械仪表 | 460篇 |
建筑科学 | 1719篇 |
矿业工程 | 160篇 |
能源动力 | 686篇 |
轻工业 | 2135篇 |
水利工程 | 182篇 |
石油天然气 | 56篇 |
武器工业 | 1篇 |
无线电 | 1483篇 |
一般工业技术 | 4163篇 |
冶金工业 | 2769篇 |
原子能技术 | 137篇 |
自动化技术 | 3834篇 |
出版年
2023年 | 152篇 |
2022年 | 308篇 |
2021年 | 534篇 |
2020年 | 312篇 |
2019年 | 327篇 |
2018年 | 493篇 |
2017年 | 426篇 |
2016年 | 530篇 |
2015年 | 479篇 |
2014年 | 689篇 |
2013年 | 1506篇 |
2012年 | 1081篇 |
2011年 | 1518篇 |
2010年 | 1092篇 |
2009年 | 1044篇 |
2008年 | 1265篇 |
2007年 | 1099篇 |
2006年 | 939篇 |
2005年 | 865篇 |
2004年 | 774篇 |
2003年 | 688篇 |
2002年 | 660篇 |
2001年 | 392篇 |
2000年 | 376篇 |
1999年 | 412篇 |
1998年 | 409篇 |
1997年 | 371篇 |
1996年 | 340篇 |
1995年 | 295篇 |
1994年 | 301篇 |
1993年 | 297篇 |
1992年 | 250篇 |
1991年 | 214篇 |
1990年 | 201篇 |
1989年 | 230篇 |
1988年 | 186篇 |
1987年 | 177篇 |
1986年 | 174篇 |
1985年 | 239篇 |
1984年 | 214篇 |
1983年 | 184篇 |
1982年 | 168篇 |
1981年 | 195篇 |
1980年 | 130篇 |
1979年 | 147篇 |
1978年 | 143篇 |
1977年 | 127篇 |
1976年 | 138篇 |
1975年 | 120篇 |
1974年 | 105篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
“Energy Security” has typically, to those involved in making energy policy, meant mostly securing access to oil and other fossil fuels. With increasingly global, diverse energy markets, however, and increasingly transnational problems resulting from energy transformation and use, old energy security rationales are less salient, and other issues, including climate change and other environmental, economic, and international considerations are becoming increasingly important. As a consequence, a more comprehensive operating definition of “Energy Security” is needed, along with a workable framework for analysis of which future energy paths or scenarios are likely to yield greater Energy Security in a broader, more comprehensive sense. Work done as a part of the Nautilus Institute's “Pacific Asia Regional Energy Security” (PARES) project developed a broader definition of Energy Security, and described an analytical framework designed to help to compare the energy security characteristics – both positive and negative – of different quantitative energy paths as developed using software tools such as the LEAP (Long-range Energy Alternatives Planning) system. 相似文献
992.
Accelerating oil import dependence in energy consuming nations highlights the importance of having energy supplies at sufficient levels and at stable and reasonable prices. Consequently, it is crucial that oil exporters realize their full production potential. Current debates on energy security are often focused on short-term risks e.g. sudden disruptions due to wars, domestic instability, etc. However, when it comes to assessing oil supplier reliability it is equally important to assess their longer term ability and willingness to deliver oil to the global market. This study analyzes the effects of petroleum investment policies on crude oil production trends in 14 major oil producing countries (2000–2010) by focusing on the political–institutional frameworks that shape the investment conditions for the upstream oil sector. Our findings indicate that countries with less favorable oil sector frameworks systematically performed worse than countries with investor friendly and privatized sectors. The findings indicate that assessments based on remaining reserves and planned production capacities alone could inflate expectations about future oil supplies in a world where remaining crude reserves are located in countries with unfavorable investment frameworks. 相似文献
993.
The spectacular recent economic growth in the Asia-Pacific region in general, and in many of the economies of Northeast Asia in particular, has spurred a vast expansion in the need for energy services, and an expansion in the demand for the fuels that help to supply these services. Future projections suggest that the growth of fossil fuel use in Northeast Asia, especially in China, will have major consequences for financial and fuel markets and pollution both regionally and globally. Before the project described in this paper was initiated, there was no ongoing forum for energy experts from all of the countries of the region to meet, informally and in an unofficial capacity, to discuss openly and in a targeted fashion the energy situations in their countries, and to work together to evaluate the energy efficiency costs and benefits of different ways of meeting regional demand. The Asian Energy Security (AES) project provides such a forum, and as such constitutes a unique resource in the engagement of the countries of Northeast Asia on the topic of energy security. 相似文献
994.
Shigehito Isobe Somei OhnukiWilliam I.F. David Matthias GutmannMartin O. Jones Peter P. EdwardsTakayuki Ichikawa Yoshitsugu Kojima 《International Journal of Hydrogen Energy》2011,36(13):7909-7913
We have synthesized a single crystal of lithium amide (LiNH2, LiND2) by melting method, and performed neutron diffraction of the single crystal at variable temperature. LiND2 is tetragonal structure and I-4 space group. Lattice parameters and unit cell volume of LiND2 at room temperature, 50 °C, 100 °C, 150 °C and 200 °C were determined. Both of the lattice parameters and the unit cell volume increase with increase of temperature. From these results, we have estimated coefficient of volumetric thermal expansion αV of LiND2 to be 222 × 10−6/K. With increase of temperature, all thermal ellipsoids gradually expand because of thermal vibration. 相似文献
995.
In spite of the statutory meat inspection at abattoirs, Taenia saginata cysticercus infection in cattle remains an economically important parasitic disease for the livestock industry by affecting food safety. The routinely performed standard meat inspection protocol has a low diagnostic sensitivity for the detection of T. saginata cysticerci infections. Therefore, an abattoir trial aiming to increase the detection level was undertaken. In three EU-approved abattoirs, several additional heart incisions were performed in a total of 1088 slaughtered cattle originating from 832 farms throughout Switzerland. Cysticerci as putative parasitic lesions were classified by visual examination during meat inspection and confirmed microscopically and/or by molecular analyses. With the EU-approved routine meat inspection, bovine cysticercosis was diagnosed in 1.8% (20/1088) of the slaughtered animals. Additional incisions into the heart muscle revealed a further 29 cases, indicating that the prevalence was at least 4.5%. All infected animals originated from individual farms. This straightforward technique had a significantly higher sensitivity and is feasible for routine practice. It also confirms that the prevalence of this zoonotic parasite in the cattle population is underestimated based on the routine abattoir reports. 相似文献
996.
The standard Codex HACCP approach was modified to allow a hazard analysis and critical control point determination to be conducted at an industry level and then used to determine the appropriate on-farm food safety control measures for pig production in Australia. A detailed risk-based profile with hazard identification, hazard characterisation and levels of microbial contamination for production and primary processing was used as a major technical resource to inform HACCP determinations. The process resulted in the identification of Critical Control Points for control of a specific physical hazard (non-recovered broken needles) and prevention of violations of Maximum Residue Limits with agricultural and veterinary chemicals. In relation to the identified microbiological hazards, it was noted that there are numerous Good Agricultural Practices (GAP) to prevent and/or reduce their risk and these controls would also need to be a feature of any on-farm food safety program. By applying a HACCP-based approach it was determined that the application of a set of Good Agricultural Practices on-farm would be effective in ensuring low risk. It was, therefore, concluded that on-farm food safety programs may not warrant full (i.e. Codex compliant) HACCP plans at the individual enterprise level provided appropriate GAP is in place. The results provide pig producers and the Australian pig industry with the elements of a HACCP-based food safety system that are scientifically justifiable, understandable and realistic to apply. These features are essential elements that underpin successful implementation and compliance by industry. 相似文献
997.
Jonathan A. Czuba James L. Best Kevin A. Oberg Daniel R. Parsons P. Ryan Jackson Marcelo H. Garcia Peter Ashmore 《Journal of Great Lakes research》2011,37(3):480-493
An integrated multibeam echo sounder and acoustic Doppler current profiler field survey was conducted in July 2008 to investigate the morphodynamics of the St. Clair River at the outlet of Lake Huron. The principal morphological features of the upper St. Clair River included flow-transverse bedforms that appear weakly mobile, erosive bedforms in cohesive muds, thin non-cohesive veneers of weakly mobile sediment that cover an underlying cohesive (till or glacio-lacustrine) surface, and vegetation that covers the bed. The flow was characterized by acceleration as the banks constrict from Lake Huron into the St. Clair River, an approximately 1500-m long region of flow separation downstream from the Blue Water Bridge, and secondary flow connected to: i) channel curvature; ii) forcing of the flow by local bed topography, and iii) flow wakes in the lee side of ship wrecks. Nearshore, sand-sized, sediment from Lake Huron was capable of being transported into, and principally along, the banks of the upper St. Clair River by the measured flow. A comparison of bathymetric surveys conducted in 2007 and 2008 identifies that the gravel bed does undergo slow downstream movement, but that this movement does not appear to be generated by the mean flow, and could possibly be caused by ship-propeller-induced turbulence. The study results suggest that the measured mean flow and dredging within the channel have not produced major scour of the upper St. Clair River and that the recent fall in the level of Lake Huron is unlikely to have been caused by these mechanisms. 相似文献
998.
Nathan S. Bosch J. David Allan David M. Dolan Haejin Han R. Peter Richards 《Journal of Great Lakes research》2011,37(2):263-271
The Soil and Water Assessment Tool (SWAT), a physically-based watershed-scale model, holds promise as a means to predict tributary sediment and nutrient loads to the Laurentian Great Lakes. In the present study, model performance is compared across six watersheds draining into Lake Erie to determine the applicability of SWAT to watersheds of differing characteristics. After initial model parameterization, the Huron, Raisin, Maumee, Sandusky, Cuyahoga, and Grand SWAT models were calibrated (1998-2001) and confirmed, or validated (2002-2005), individually for stream water discharge, sediment loads, and nutrient loads (total P, soluble reactive P, total N, and nitrate) based on available datasets. SWAT effectively predicted hydrology and sediments across a range of watershed characteristics. SWAT estimation of nutrient loads was weaker although still satisfactory at least two-thirds of the time across all nutrient parameters and watersheds. SWAT model performance was most satisfactory in agricultural and forested watersheds, and was less so in urbanized settings. Model performance was influenced by the availability of observational data with high sampling frequency and long duration for calibration and confirmation evaluation. In some instances, it appeared that parameter adjustments that improved calibration of hydrology negatively affected subsequent sediment and nutrient calibration, suggesting trade-offs in calibrating for hydrologic vs. water quality model performance. Despite these considerations, SWAT accurately predicted average stream discharge, sediment loads, and nutrient loads for the Raisin, Maumee, Sandusky, and Grand watersheds such that future use of these SWAT models for various scenario testing is reasonable and warranted. 相似文献
999.
Peter D. Blanken Christopher Spence Newell Hedstrom John D. Lenters 《Journal of Great Lakes research》2011,37(4):707-716
The surface energy balance of Lake Superior was measured using the eddy covariance method at a remote, offshore site at 0.5-h intervals from June 2008 through November 2010. Pronounced seasonal patterns in the surface energy balance were observed, with a five-month delay between maximum summer net radiation and maximum winter latent and sensible heat fluxes. Late season (winter) evaporation and sensible heat losses from the lake typically occurred in two- to three-day-long events, and were associated with significant release of stored heat from the lake. The majority of the evaporative heat loss (70-88%) and sensible heat loss (97-99%) occurred between October and March, with 464 mm (2008-2009) and 645 mm (2009-2010) of evaporative water loss occurring over the water year starting October 1. Evaporation was proportional to the horizontal wind speed, inversely proportional to the ambient vapor pressure, and was well described by the ratio of wind speed to vapor pressure. This ratio remained relatively constant between the two water years, so the differences in evaporative water loss between years were largely associated with differences in lake surface conditions (e.g. water temperature, ice cover, and ice duration). Since late-season water temperature decline is driven by evaporative and sensible heat loss, the potential for a negative feedback mechanism between evaporation and ice cover is discussed. 相似文献
1000.
Food safety demands calls for new approaches to better understand the safe and hygienic practices of specific sectors in the food supply chain. In the present study, sauerkraut growers were investigated to explore their understanding of food safety and hygiene practices. A combination of quantitative (questionnaire prepared for the study) and qualitative (ten in-depth interviews) methods was used in the study so that topics could be discussed more completely and in greater depth. The results indicated that sauerkraut growers possess a rather good level of knowledge concerning food safety, although some gaps were discovered (e.g., record keeping and the feeling that recording data was an unnecessary burden, inadequate acquaintance with health status requirements, insufficient knowledge of the cabbage production process of food safety professionals, etc.). Some differences were found when comparing the results of the qualitative and quantitative studies. The qualitative analysis highlighted some contradictions (e.g., an attitude toward the production process: “We do not need the HACCP system”; a feeling concerning product safety: “Why the need for data recording?”) and differences between the sauerkraut growers and a professional understanding of good practice guidelines for sauerkraut production. All other determinations concerning food safety knowledge and practices are presented and discussed. The findings can be used as a guide for reorganizing current education programs aimed to improve safe and hygiene practices at the farm level. 相似文献