全文获取类型
收费全文 | 967篇 |
免费 | 85篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 15篇 |
综合类 | 3篇 |
化学工业 | 245篇 |
金属工艺 | 21篇 |
机械仪表 | 48篇 |
建筑科学 | 24篇 |
矿业工程 | 3篇 |
能源动力 | 41篇 |
轻工业 | 129篇 |
水利工程 | 12篇 |
石油天然气 | 11篇 |
无线电 | 78篇 |
一般工业技术 | 181篇 |
冶金工业 | 119篇 |
原子能技术 | 8篇 |
自动化技术 | 118篇 |
出版年
2024年 | 5篇 |
2023年 | 5篇 |
2022年 | 19篇 |
2021年 | 46篇 |
2020年 | 38篇 |
2019年 | 42篇 |
2018年 | 43篇 |
2017年 | 46篇 |
2016年 | 49篇 |
2015年 | 30篇 |
2014年 | 42篇 |
2013年 | 83篇 |
2012年 | 45篇 |
2011年 | 65篇 |
2010年 | 53篇 |
2009年 | 45篇 |
2008年 | 45篇 |
2007年 | 32篇 |
2006年 | 26篇 |
2005年 | 25篇 |
2004年 | 24篇 |
2003年 | 22篇 |
2002年 | 13篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 16篇 |
1998年 | 30篇 |
1997年 | 16篇 |
1996年 | 19篇 |
1995年 | 21篇 |
1994年 | 17篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1976年 | 7篇 |
1975年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有1056条查询结果,搜索用时 20 毫秒
21.
22.
Effect of vitamin A supplementation on diarrhoea and acute lower-respiratory-tract infections in young children in Brazil 总被引:1,自引:0,他引:1
ML Barreto LM Santos AM Assis MP Araújo GG Farenzena PA Santos RL Fiaccone 《Canadian Metallurgical Quarterly》1994,344(8917):228-231
A beneficial effect of periodic vitamin A supplementation on childhood mortality has been demonstrated, but the effect on morbidity is less clear. We investigated the effect of vitamin A supplementation on diarrhoea and acute lower-respiratory-tract infections (ALRI) in children from northeastern Brazil in a randomised, double-blind, placebo-controlled community trial. 1240 children aged 6-48 months were assigned vitamin A or placebo every 4 months for 1 year. They were followed up at home three times a week, and data about the occurrence and severity of diarrhoea and ALRI were collected. Any child with cough and respiratory rate above 40 breaths per min was visited by a paediatrician. The overall incidence of diarrhoea episodes was significantly lower in the vitamin-A-supplemented group than in the placebo group (18.42 vs 19.58 x 10(-3) child-days; rate ratio 0.94 [95% Cl 0.90-0.98]). The benefit of supplementation was greater as regards severe episodes of diarrhoea; the incidence was 20% lower in the vitamin A group than in the placebo group (rate ratio 0.80 [0.65-0.98]). With the standard definition of diarrhoea (> or = 3 liquid or semi-liquid stools in 24 h) the effect of vitamin A on mean daily prevalence did not reach significance, but as the definition of diarrhoea was made more stringent (increasing number of stools per day), a significant benefit became apparent, reaching for diarrhoea with 6 or more liquid or semi-liquid stools in 24 h a 23% lower prevalence. We found no effect of vitamin A supplementation on the incidence of ALRI. The reduction in severity of diarrhoea may be the most important factor in the lowering of mortality by vitamin A supplementation. 相似文献
23.
E. M Andrade J. R Araújo Neto M. J. S Guerreiro J. C. N Santos H. A. Q Palácio 《Water Resources Management》2017,31(13):4103-4116
Dry tropical forests account for over 1,000,000 km2, and there is still lack of knowledge on their hydrologic processes. The curve number (CN) hydrologic model developed by the Natural Resources Conservation Service (NRCS) is widely applied for runoff determination in various parts of the world, but not so in tropical semiarid regions. This study analyzes the impact of land use changes on the CN model in a tropical semiarid environment, in two catchments of native dry tropical forest and thinned dry tropical forest land use from 2009 to 2012. The CN model was calibrated and validated for the NRCS recommended initial abstraction ratio λ = 0.2, and for λ evaluated from rainfall and runoff data. A reliability analysis was performed using Monte Carlo simulation. Model goodness-of-fit was assessed with statistical criteria. A total of 42 and 40 rainfall-runoff events were analyzed for the native and thinned dry tropical forest, respectively. Characteristic λ values of 0.15 and 0.11 were determined for the two respective catchments. Although CN values were similar for both land uses, CNλ=0.20 = 80 and CNmedian λ = 77, the thinned catchment showed a higher CN model parameters variability. The CN model was more sensitive to variations of CN values than to those of λ. This study showed that no matter the vegetation management in a dry tropical forest environment, modeled runoff is not affected by λ, but rather affected by CN, which represents soil, landuse and management. 相似文献
24.
The aim of this work is to analyse the possibility of using co-gasification technology to process coal mixed with wastes to take profit of its energy content and at the same time to minimize the environmental impact associated with the use of wastes and to diminish the costs of flue gas treatment. The addition to coal of different types of materials, like: pine based waste, petcoke and polyethylene (PE), was not found to give rise to any operational problems, regarding both the feeding system and gasification process and led to higher energy conversions, however, the gas presented higher tars and hydrocarbons content. Several catalysts were tested, such as, dolomite, olivine, nickel and magnesium oxides, zinc oxides and cobalt and molybdenum oxides. Catalyst action was analysed in tars release and also in ammonia compounds reduction. The presence of catalysts allowed increasing hydrogen release, whilst there was a decrease in hydrocarbons and tars contents. A nickel-magnesium oxide was the catalyst that led to the highest reduction in hydrocarbons and tars. This catalyst also led to the lowest NH3 content in the fuel gas produced, due to the catalyst efficiency in NH3 destruction. 相似文献
25.
D. S. Rosa I. Chiovatto Neto M. R. Calil A. G. Pedroso C. P. Fonseca S. Neves 《应用聚合物科学杂志》2004,91(6):3909-3914
The thermal and mechanical properties of low‐density polyethylene (LDPE), poly(ε‐caprolactone) (PCL), and their blends were evaluated. Differential scanning calorimetry showed that increasing the PCL content of the blend did not change the LDPE melting temperature, but reduced the crystallinity by up to 16.8%. This behavior was related to interactions between the PCL chains and the crystalline phase of LDPE. Tensile strength and elongation at break values for the blends were lower than those for the pure polymers, which suggested an incompatibility between the polymers. The values for Young's modulus under tensile increased when PCL was added to LDPE. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:3909–3914, 2004 相似文献
26.
Luciana Gampert Miranda Carolina Pereira Peglow Victória Cardoso Deorristt Geórgia Campos Sales Neto Ariela Milbrath Cardoso Suzana Frighetto Ferrarini Marçal José Rodrigues Pires 《应用聚合物科学杂志》2021,138(7):49842
The use of polymeric films incorporated with zeolite-TiO2 composites associated with UV radiation can be an alternative in the removal of volatile organic compounds (VOCs) through the adsorption and photodegradation processes. This study produced poly(lactic acid) (PLA) films incorporated with 13× zeolite, TiO2, and 13×-TiO2 zeolite composite to remove n-butanol and evaluate the by-products generated in the process. The results showed that 13× zeolite and TiO2 added individually or as a composite to PLA, gave the polymer matrix a significant increase in the removal capacity of n-butanol. The best performance was presented by the zeolite-TiO2, composite, confirming a synergistic effect. However, the formation of CO and CO2 exceeded the expected values, with the verification that the polymeric matrix underwent photodegradation action by TiO2. The polymeric film only containing zeolite is the most suitable for the removal of VOCs, as it did not present degradation of the PLA, generating a lower concentration of by-products. 相似文献
27.
Pablo J. Blanco Pablo J. Sánchez Eduardo A. de Souza Neto Raúl A. Feijóo 《Archives of Computational Methods in Engineering》2016,23(2):191-253
A unified variational theory is proposed for a general class of multiscale models based on the concept of Representative Volume Element. The entire theory lies on three fundamental principles: (1) kinematical admissibility, whereby the macro- and micro-scale kinematics are defined and linked in a physically meaningful way; (2) duality, through which the natures of the force- and stress-like quantities are uniquely identified as the duals (power-conjugates) of the adopted kinematical variables; and (3) the Principle of Multiscale Virtual Power, a generalization of the well-known Hill-Mandel Principle of Macrohomogeneity, from which equilibrium equations and homogenization relations for the force- and stress-like quantities are unequivocally obtained by straightforward variational arguments. The proposed theory provides a clear, logically-structured framework within which existing formulations can be rationally justified and new, more general multiscale models can be rigorously derived in well-defined steps. Its generality allows the treatment of problems involving phenomena as diverse as dynamics, higher order strain effects, material failure with kinematical discontinuities, fluid mechanics and coupled multi-physics. This is illustrated in a number of examples where a range of models is systematically derived by following the same steps. Due to the variational basis of the theory, the format in which derived models are presented is naturally well suited for discretization by finite element-based or related methods of numerical approximation. Numerical examples illustrate the use of resulting models, including a non-conventional failure-oriented model with discontinuous kinematics, in practical computations. 相似文献
28.
29.
Daniel Dominguez Gouvêa Cyro de A. Assis D. Muniz Gilson A. Pinto Alberto Avritzer Rosa Maria Meri Leão Edmundo de Souza e Silva Morganna Carmem Diniz Vittorio Cortellessa Luca Berardinelli Julius C. B. Leite Daniel Mossé Yuanfang Cai Michael Dalton Lucia Happe Anne Koziolek 《Software and Systems Modeling》2013,12(4):765-787
In this paper, we report on our experience with the application of validated models to assess performance, reliability, and adaptability of a complex mission critical system that is being developed to dynamically monitor and control the position of an oil-drilling platform. We present real-time modeling results that show that all tasks are schedulable. We performed stochastic analysis of the distribution of task execution time as a function of the number of system interfaces. We report on the variability of task execution times for the expected system configurations. In addition, we have executed a system library for an important task inside the performance model simulator. We report on the measured algorithm convergence as a function of the number of vessel thrusters. We have also studied the system architecture adaptability by comparing the documented system architecture and the implemented source code. We report on the adaptability findings and the recommendations we were able to provide to the system’s architect. Finally, we have developed models of hardware and software reliability. We report on hardware and software reliability results based on the evaluation of the system architecture. 相似文献
30.
Guilherme T. de Assis Alberto H. F. Laender Marcos André Gonçalves Altigran S. da Silva 《World Wide Web》2009,12(3):285-319
Focused crawlers have as their main goal to crawl Web pages that are relevant to a specific topic or user interest, playing
an important role for a great variety of applications. In general, they work by trying to find and crawl all kinds of pages
deemed as related to an implicitly declared topic. However, users are often not simply interested in any document about a
topic, but instead they may want only documents of a given type or genre on that topic to be retrieved. In this article, we
describe an approach to focused crawling that exploits not only content-related information but also genre information present
in Web pages to guide the crawling process. This approach has been designed to address situations in which the specific topic
of interest can be expressed by specifying two sets of terms, the first describing genre aspects of the desired pages and
the second related to the subject or content of these pages, thus requiring no training or any kind of preprocessing. The
effectiveness, efficiency and scalability of the proposed approach are demonstrated by a set of experiments involving the
crawling of pages related to syllabi of computer science courses, job offers in the computer science field and sale offers
of computer equipments. These experiments show that focused crawlers constructed according to our genre-aware approach achieve
levels of F1 superior to 88%, requiring the analysis of no more than 65% of the visited pages in order to find 90% of the
relevant pages. In addition, we experimentally analyze the impact of term selection on our approach and evaluate a proposed
strategy for semi-automatic generation of such terms. This analysis shows that a small set of terms selected by an expert
or a set of terms specified by a typical user familiar with the topic is usually enough to produce good results and that such
a semi-automatic strategy is very effective in supporting the task of selecting the sets of terms required to guide a crawling
process. 相似文献