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   3524篇
  免费   165篇
  国内免费   7篇
电工技术   57篇
综合类   7篇
化学工业   870篇
金属工艺   54篇
机械仪表   66篇
建筑科学   98篇
矿业工程   21篇
能源动力   111篇
轻工业   285篇
水利工程   26篇
石油天然气   19篇
无线电   362篇
一般工业技术   671篇
冶金工业   332篇
原子能技术   35篇
自动化技术   682篇
  2023年   41篇
  2022年   120篇
  2021年   138篇
  2020年   81篇
  2019年   97篇
  2018年   102篇
  2017年   95篇
  2016年   130篇
  2015年   88篇
  2014年   139篇
  2013年   233篇
  2012年   192篇
  2011年   236篇
  2010年   163篇
  2009年   149篇
  2008年   149篇
  2007年   147篇
  2006年   138篇
  2005年   112篇
  2004年   104篇
  2003年   94篇
  2002年   90篇
  2001年   49篇
  2000年   47篇
  1999年   59篇
  1998年   56篇
  1997年   56篇
  1996年   48篇
  1995年   41篇
  1994年   28篇
  1993年   33篇
  1992年   26篇
  1991年   17篇
  1990年   17篇
  1989年   22篇
  1988年   28篇
  1987年   22篇
  1986年   15篇
  1985年   24篇
  1984年   17篇
  1983年   19篇
  1982年   20篇
  1981年   12篇
  1980年   19篇
  1979年   22篇
  1978年   18篇
  1977年   16篇
  1976年   20篇
  1975年   18篇
  1973年   12篇
排序方式: 共有3696条查询结果,搜索用时 15 毫秒
991.
This paper analyzes the socio-cultural, political and economic conditions prevalent during the inception of nuclear power programs in Japan and South Korea in order to identify commonalities which support nuclear power program expansion. The study identifies six factors as having a clear influence on supporting nuclear power development: (1) strong state involvement in guiding economic development; (2) centralization of national energy policymaking and planning; (3) campaigns to link technological progress with national revitalization; (4) influence of technocratic ideology on policy decisions; (5) subordination of challenges to political authority, and (6) low levels of civic activism. The paper postulates that insights from this study can be used to assess the propensity of nations which have the emergent capacity to support nuclear power development to actually embark on such programs.  相似文献   
992.
Objective: The Stroop (Stroop, 1935) is a frequently used neuropsychological test, with poor performance typically interpreted as indicative of disinhibition and frontal lobe damage. This study tested those interpretations by examining relationships between Stroop performance, behavioral disinhibition, and frontal lobe atrophy. Method: Participants were 112 patients with mild cognitive impairment or dementia, recruited through UCSF's Memory and Aging Center. Participants received comprehensive dementia evaluations including structural MRI, neuropsychological testing, and informant interviews. Freesurfer, a semiautomated parcellation program, was used to analyze 1.5T MRI scans. Behavioral disinhibition was measured using the Neuropsychiatric Inventory (Cummings, 1997; Cummings et al., 1994) Disinhibition Scale. The sample (n = 112) mean age was 65.40 (SD = 8.60) years, education was 16.64 (SD = 2.54) years, and Mini-Mental State Examination (MMSE; Folstein et al., 1975) was 26.63 (SD = 3.32). Hierarchical linear regressions were used for data analysis. Results: Controlling for age, MMSE, and color naming, Stroop performance was not significantly associated with disinhibition (β = 0.01, ΔR2 = 0.01, p = .29). Hierarchical regressions controlling for age, MMSE, color naming, intracranial volume, and temporal and parietal lobes, examined whether left or right hemisphere regions predict Stroop performance. Bilaterally, parietal lobe atrophy best predicted poorer Stroop (left: β = 0.0004, ΔR2 = 0.02, p = .002; right: β = 0.0004, ΔR2 = 0.02, p = .002). Of frontal regions, only dorsolateral prefrontal cortex atrophy predicted poorer Stroop (β = 0.001, ΔR2 = 0.01, p = .03); left and right anterior cingulate cortex atrophy predicted better Stroop (left: β = ?0.003, ΔR2 = 0.01, p = .02; right: β = ?0.004, ΔR2 = 0.01, p = .02). Conclusion: These findings suggest Stroop performance is a poor measure of behavioral disinhibition and frontal lobe atrophy even among a relatively high-risk population. (PsycINFO Database Record (c) 2011 APA, all rights reserved)  相似文献   
993.
994.
995.
Walter Short  Victor Diakov 《风能》2013,16(4):491-500
The variability of wind and solar is perceived as a major obstacle in employing otherwise abundant renewable energy resources. On the basis of the available geographically dispersed data for the Western USA, we analyze to what extent the geographic diversity of these resources can offset their variability. We determine the best match to loads in the western portion of the USA that can be achieved with wind power and photovoltaics (PV) with no transmission limitations. Without storage and with no curtailment, wind and PV can meet up to 50% of loads in Western USA. It is beneficial to build more wind than PV mostly because the wind contributes at night. When storage is available, the optimal mix has almost 75% as much nominal PV capacity as wind, with the PV energy contribution being 32% of the electricity produced from wind. With only 10 GW of storage (twice the pumped hydro storage capacity that already exists in the Western Electric Coordinating Council), up to 82% of the load can be matched with wind and PV, while in the same time curtailing just 10% of the renewable energy throughout the year. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
996.
A novel method of membrane preparation for use in vanadium redox flow batteries by preswelling of poly(2,5‐benzimidazole) films in organic solvent followed by supercritical CO2 assisted solvent removal is proposed. Influence of the organic solvent type on the morphology, proton, and vanadyl ion transport properties is studied. The performance of the obtained membranes inside single cells of vanadium redox flow batteries is compared to pristine dense poly(2,5‐benzimidazole) and Nafion 115 membranes. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46262.  相似文献   
997.
Hensel’s symbolic lifting is a highly effective method for the solution of a general or structured (e.g. Toeplitz or Hankel) linear system of equations with integer or rational coefficients of bounded length. It can handle ill conditioned inputs, for which numerical methods become costly. Lifting amounts to recursive multiplications by vectors of the input coefficient matrices and its precomputed inverse modulo a fixed integer s. Such multiplications only involve small numbers of data movements and arithmetic operations with bounded precision. The known methods for precomputation of the inverse are more costly, however; in particular they involve more data movements. As our remedy for this bottleneck stage we create an auxiliary matrix sharing its inverse modulo s with the input matrix, and we readily compute this inverse by applying numerical iterative refinement, which is a numerical counterpart of lifting. In the case of general unstructured as well as Toeplitz, Hankel, and other popular structured inputs our hybrid algorithms involve a small number of data movements and optimal number of Boolean (that is bitwise) operations (up to a logarithmic factor). We extend the algorithms to nearly optimal computation of polynomial greatest common divisors (gcds), least common multiples (lcms) and Padé approximations, as well as the Berlekamp-Massey reconstruction of linear recurrences. We also cover Newton’s lifting for matrix inversion, specialize it to the case of structured input, and combine it with Hensel’s to enhance the overall efficiency. Our initialization techniques work for Newton’s lifting as well. Furthermore we extend all our lifting algorithms to allow their initialization modulo powers of two, thus implementing them in the binary base.  相似文献   
998.
Chemical and electronic interactions between CdSe nanocrystal quantum dots (NQDs) and Ru‐polypyridine complexes are studied in solution. It is shown that photoluminescence (PL) can be used to effectively monitor the formation of NQD‐complex assemblies in real time. It is also shown that with the aid of Langmuir isotherm modeling, the PL studies can be used to quantitatively characterize the composition of the assemblies and the strength of electronic interactions between their components. The approach demonstrated here is general and can be applied to other systems that combine semiconductor NQDs and appropriately functionalized organometallic or organic molecules interacting with NQDs via energy transfer, charge transfer, or other mechanisms leading to quenching of NQD emission.  相似文献   
999.
High mobility of nodes in vehicular ad hoc networks (VANETs) may lead to frequent breakdowns of established routes in conventional routing algorithms commonly used in mobile ad hoc networks. To satisfy the high reliability and low delivery‐latency requirements for safety applications in VANETs, broadcasting becomes an essential operation for route establishment and repair. However, high node mobility causes constantly changing traffic and topology, which creates great challenges for broadcasting. Therefore, there is much interest in better understanding the properties of broadcasting in VANETs. In this paper we perform stochastic analysis of broadcasting delays in VANETs under three typical scenarios: freeway, sparse traffic and dense traffic, and utilize them to analyze the broadcasting delays in these scenarios. In the freeway scenario, the analytical equation of the expected delay in one connected group is given based on statistical analysis of real traffic data collected on freeways. In the sparse traffic scenario, the broadcasting delay in an n‐vehicle network is calculated by a finite Markov chain. In the dense traffic scenario, the collision problem is analyzed by different radio propagation models. The correctness of these theoretical analyses is confirmed by simulations. These results are useful to provide theoretical insights into the broadcasting delays in VANETs. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
1000.
Relaxation behaviors of polyethylene, polypropylene, and polycarbonate have been studied by positron annihilation lifetime spectroscopy (PALS). In PALS positron sources made of radioisotopes are used to inject positrons into polymer as a micro probe. The injected positron probes can induce radiation effect, which plays an important role in detecting the polymer relaxation behavior through electrons trapped in shallow potentials at low temperature. Monitoring the intensity (I3) of ortho-positronium (o-Ps), transitions of γ and δ relaxation can be measured by PALS as a secondary effect. In this experiment, the change of I3 below Tg is connected with the number of the trapped electrons, which can be excited from the shallow potential by the thermal motion of polymer structures and visible light irradiation. In the PALS measurements of non-irradiated PP samples, relaxation of methyl groups was observed as low as 50 K, which can be assigned as the δ relaxation. Relaxations of β and γ were also observed for the non-irradiated PP samples between 100–370 K. However for the 3 MGy γ-ray irradiated PP samples, only β relaxation was observed because the large radiation dose caused a large number of scissions of –CH3 groups from main chains and the characteristics changed. For the irradiated samples, radiation hardening was observed. Electronic Publication  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