全文获取类型
收费全文 | 4677篇 |
免费 | 100篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 38篇 |
综合类 | 1篇 |
化学工业 | 1011篇 |
金属工艺 | 56篇 |
机械仪表 | 81篇 |
建筑科学 | 242篇 |
矿业工程 | 21篇 |
能源动力 | 75篇 |
轻工业 | 438篇 |
水利工程 | 35篇 |
石油天然气 | 20篇 |
无线电 | 221篇 |
一般工业技术 | 855篇 |
冶金工业 | 1092篇 |
原子能技术 | 33篇 |
自动化技术 | 562篇 |
出版年
2022年 | 31篇 |
2021年 | 43篇 |
2020年 | 46篇 |
2019年 | 46篇 |
2018年 | 58篇 |
2017年 | 42篇 |
2016年 | 72篇 |
2015年 | 67篇 |
2014年 | 127篇 |
2013年 | 274篇 |
2012年 | 188篇 |
2011年 | 250篇 |
2010年 | 169篇 |
2009年 | 183篇 |
2008年 | 236篇 |
2007年 | 200篇 |
2006年 | 212篇 |
2005年 | 189篇 |
2004年 | 147篇 |
2003年 | 138篇 |
2002年 | 127篇 |
2001年 | 84篇 |
2000年 | 100篇 |
1999年 | 74篇 |
1998年 | 78篇 |
1997年 | 96篇 |
1996年 | 88篇 |
1995年 | 69篇 |
1994年 | 77篇 |
1993年 | 83篇 |
1992年 | 81篇 |
1991年 | 53篇 |
1990年 | 78篇 |
1989年 | 73篇 |
1988年 | 55篇 |
1987年 | 76篇 |
1986年 | 70篇 |
1985年 | 78篇 |
1984年 | 76篇 |
1983年 | 55篇 |
1982年 | 58篇 |
1981年 | 61篇 |
1980年 | 35篇 |
1979年 | 51篇 |
1978年 | 48篇 |
1977年 | 37篇 |
1976年 | 32篇 |
1975年 | 32篇 |
1974年 | 28篇 |
1973年 | 27篇 |
排序方式: 共有4781条查询结果,搜索用时 15 毫秒
61.
Gary E. Bulman Ed Siivola Ryan Wiitala Rama Venkatasubramanian Michael Acree Nathan Ritz 《Journal of Electronic Materials》2009,38(7):1510-1515
Thin-film Bi2Te3- and Sb2Te3-based superlattice (SL) thermoelectric (TE) devices are an enabling technology for high-power and low-temperature applications,
which include low-noise amplifier cooling, electronics hot-spot cooling, radio frequency (RF) amplifier thermal management,
and direct sensor cooling. Bulk TE devices, which can pump heat loads on the order of 10 W/cm2, are not suitable in these applications due to their large size and low heat pumping capacity. Recently, we have demonstrated
an external maximum temperature difference, ΔT
max, as high as 58 K in an SL thin-film p–n couple. This state-of-the-art couple exhibited a cold-side minimum temperature, T
cmin, of −30.9°C. We regularly attain ΔT
max values in excess of 53 K, in spite of the many significant electrical and thermal parasitics that are unique to thin-film
devices. These measurements do not use any complex thermal management at the heat sink to remove the heat flux from the TE
device’s hot side. We describe here multistage SL cooling technologies currently being developed at RTI that can provide useful
microcooling cold-side temperatures of 200 K. This effort includes a three-stage module employing independently powered stages
which produced a ΔT
max of 101.6 K with a T
cmin of −75°C, as well as a novel two-wire three-stage SL cascade which demonstrated a T
cmin of −46°C and a ΔT
max of nearly 74 K. These RTI modules are only 2.5 mm thick, significantly thinner than a similar commercial three-stage module
(5.3 mm thick) that produces a ΔT
max of 96 K. In addition, TE coolers fabricated from these thin-film SL materials perform significantly better than the extrapolated
performance of similar thickness bulk alloy materials. 相似文献
62.
Michael Khl Gary Jorgensen Stefan Brunold Bo Carlsson Markus Heck Kenneth Mller 《Solar Energy》2005,79(6):618-623
The economic viability of solar collector systems for domestic hot water (DHW) generation is strongly linked to the cost of such systems. Installation and hardware costs must be reduced by 50% to allow significant market penetration. An attractive approach to cost reduction is to replace glass and metal parts with less expensive, lighter weight polymeric components. Weight reduction decreases the cost of shipping, handling, and installation. The use of polymeric materials also allows the benefits and cost savings associated with well established manufacturing processes, along with savings associated with improved fastening, reduced part count, and overall assembly refinements. A key challenge is to maintain adequate system performance and assure requisite durability for extended lifetimes. Results of preliminary and ongoing screening tests for a large number of candidate polymeric glazing materials are presented. Based on these results, two specific glazings with moderate and poor weathering stability are selected to demonstrate how a service lifetime methodology can be applied to accurately predict the optical performance of these materials during in-service use. A summary is given for data obtained by outdoor exposure and indoor testing of polyvinyl chloride (PVC) and high temperature modified polycarbonate copolymer (coPC) materials, and an initial risk analysis is given for the two materials. Screening tests and analyses for service lifetime prediction are discussed. A methodology that provides a way to derive correlations between degradation experienced by materials exposed to controlled accelerated laboratory exposure conditions and materials exposed to in-service conditions is given, and a validation is presented for the methodology based upon durability test results for PVC and coPC. 相似文献
63.
Many energy-related investments are made without a clear financial understanding of their values, risks, and volatilities. In the face of this uncertainty, the investor—such as a building owner or an energy service company—will often choose to implement only the most certain and thus limited energy-efficiency measures. Conversely, commodities traders and other sophisticated investors accustomed to evaluating investments on a value, risk, and volatility basis often overlook energy-efficiency investments because risk and volatility information are not provided. Fortunately, energy-efficiency investments easily lend themselves to such analysis using tools similar to those applied to supply side risk management. Accurate and robust analysis demands a high level of understanding of the physical aspects of energy-efficiency, which enables the translation of physical performance data into the language of investment. With a risk management analysis framework in place, the two groups—energy-efficiency experts and investment decision-makers—can exchange the information they need to expand investment in demand-side energy projects. In this article, we first present the case for financial risk analysis in energy efficiency in the buildings sector. We then describe techniques and examples of how to identify, quantify, and manage risk. Finally, we describe emerging market-based opportunities in risk management for energy efficiency. 相似文献
64.
The Los Angeles Department of Water and Power (LADWP) operates a 600 mgd (2,270 ML/day) direct filtration water treatment plant which includes a 7,900 lb/day (149 kg/hr) ozonation system. Ozone is applied as a preoxidant for the purposes of disinfection and microflocculation. The ozonation system is unique in that high purity oxygen is generated on–site and is used as the feed–gas in a once–through system. The process was selected through competitive bids and evaluated for total present worth from 20–year life cycle costs which included capital plus energy expenses. Power consumption (and penalty) was valued at $6,500/kW. System power demand was measured at nine ozone production rates. A minimum specific energy of 6.5 kWh/lb (14.3 kWh/kg) of ozone was observed when generating ozone at a concentration between 5 and 6 % (wt) [65 and 80 g/m3; where the standard temperature and pressure are 70F (21.11 C) and 1 atm, respectively]. The test methodology and data assessment considerations were developed jointly by the owner and manufacturer, and produced results with practical significance beyond the performance testing objective. 相似文献
65.
Densification controlled by solution-precipitation during liquid-phase sintering was analyzed for the aluminamagnesium aluminosilicate glass system. As a model system for liquid-phase sintering, narrowly sized alumina powders and up to 20 vol% magnesium aluminosilicate glass samples were isothermally sintered at 1550° to 1650°C. Densification rate increases with increasing liquid content and sintering temperature but decreases with increasing density. For samples with >15% grain growth, the densification rate during the solution-precipitation stage of sintering was proportional to (particle size)−2 and thus interface reaction-controlled. Activation energies ranged from 270 to 500 kJ/mol over the relative density range of 66% to 96%, respectively. The low activation energy is attributed to densification by particle rearrangement, whereas the higher activation energy is due to densification controlled by interface-reaction-controlled solution-precipitation. Intermediate activation energies are attributed to simultaneous densification by the two mechanisms. 相似文献
66.
Preparation of Silicon Carbide/Aluminum Nitride Ceramics Using Organometallic Precursors 总被引:1,自引:0,他引:1
Corinna L. Czekaj Michael L. J. Hackney William J. Hurley Jr. Leonard V. Interrante Gary A. Sigel Paul J. Schields Glen A. Slack 《Journal of the American Ceramic Society》1990,73(2):352-357
Solid solutions of 2H -SiC/AlN can be prepared at temperatures less than 1600°C by rapid pyrolysis ("hot drop") of mixtures of [(Me3 Si)0.80 ((CH2 =CH)MeSi)1.0 (MeHSi)0.35 ] n (VPS) or [MeHSiCH2 ] n (MPCS) with [R2 AlNH2 ]3 , where R=Et, i -Bu or simply by slow pyrolysis of the precursor mixture in the case of [Et2 AlNH2 ]3 . In contrast, slow pyrolysis of mixtures of VPS or MPCS with [ i -Bu2 AlNH2 ]3 yields a composite of 2 H -AlN and 3 C -SiC at 1600°C, which transforms into a single 2 H -SiC/AlN solid solution on heating to 2000°C. The influences of the nature of the precursor and processing conditions on the structure, composition, and purity of the SiC/AlN materials are discussed. 相似文献
67.
Silicon Nitride Derived from an Organometallic Polymeric Precursor: Preparation and Characterization 总被引:1,自引:0,他引:1
Wayde R. Schmidt Vijay Sukumar William J. Hurley Jr. Roberto Garcia Robert H. Doremus Leonard V. Interrante Gary M. Renlund 《Journal of the American Ceramic Society》1990,73(8):2412-2418
Partially crystalline Si3 N4 , with nanosized crystals and a specific surface area greater than 200 m2 /g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH3 to 1000°C. This polymer does not contain N initially. Crystallization to high-purity α-Si3 N4 proceeds with additional heating above 1400°C under N2 . The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000° to 1600°C under N2 , are consistent with an amorphous-to-crystalline transformation. Although macroscopic consolidation and local densification occur at 1400°C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering process. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by polymeric precursor routes. 相似文献
68.
Two experiments compared automatic semantic and episodic priming effects in adult aging. In the 1st experiment, target words were semantically primed; in the 2nd experiment, targets were primed by repetition of semantically unrelated words. Both experiments involved a pronunciation task with response signals at fixed times following target onset. Consequently, priming was measured as improvement in the percentage of correct responses. Priming was also calculated with speed–accuracy measures of intercept and slope. Both types of priming effect were significant in the percentage correct and slope measures, but no age group differences were found. Furthermore, the magnitudes of the priming effects were equivalent. The age-resistant nature of semantic and episodic priming, as well as evidence for a common theoretical mechanism, is discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
69.
70.
Cohen Andrew R. Bjornsson Christopher S. Temple Sally Banker Gary Roysam Badrinath 《IEEE transactions on pattern analysis and machine intelligence》2009,31(8):1386-1403
An algorithmic information-theoretic method is presented for object-level summarization of meaningful changes in image sequences. Object extraction and tracking data are represented as an attributed tracking graph (ATG). Time courses of object states are compared using an adaptive information distance measure, aided by a closed-form multidimensional quantization. The notion of meaningful summarization is captured by using the gap statistic to estimate the randomness deficiency from algorithmic statistics. The summary is the clustering result and feature subset that maximize the gap statistic. This approach was validated on four bioimaging applications: 1) It was applied to a synthetic data set containing two populations of cells differing in the rate of growth, for which it correctly identified the two populations and the single feature out of 23 that separated them; 2) it was applied to 59 movies of three types of neuroprosthetic devices being inserted in the brain tissue at three speeds each, for which it correctly identified insertion speed as the primary factor affecting tissue strain; 3) when applied to movies of cultured neural progenitor cells, it correctly distinguished neurons from progenitors without requiring the use of a fixative stain; and 4) when analyzing intracellular molecular transport in cultured neurons undergoing axon specification, it automatically confirmed the role of kinesins in axon specification. 相似文献