全文获取类型
收费全文 | 15490篇 |
免费 | 289篇 |
国内免费 | 36篇 |
专业分类
电工技术 | 191篇 |
综合类 | 17篇 |
化学工业 | 1887篇 |
金属工艺 | 272篇 |
机械仪表 | 308篇 |
建筑科学 | 476篇 |
矿业工程 | 23篇 |
能源动力 | 242篇 |
轻工业 | 1608篇 |
水利工程 | 108篇 |
石油天然气 | 29篇 |
无线电 | 1372篇 |
一般工业技术 | 1843篇 |
冶金工业 | 5698篇 |
原子能技术 | 166篇 |
自动化技术 | 1575篇 |
出版年
2022年 | 105篇 |
2021年 | 189篇 |
2020年 | 114篇 |
2019年 | 167篇 |
2018年 | 202篇 |
2017年 | 197篇 |
2016年 | 237篇 |
2015年 | 140篇 |
2014年 | 267篇 |
2013年 | 594篇 |
2012年 | 450篇 |
2011年 | 566篇 |
2010年 | 451篇 |
2009年 | 441篇 |
2008年 | 503篇 |
2007年 | 496篇 |
2006年 | 405篇 |
2005年 | 404篇 |
2004年 | 319篇 |
2003年 | 317篇 |
2002年 | 287篇 |
2001年 | 294篇 |
2000年 | 282篇 |
1999年 | 396篇 |
1998年 | 1869篇 |
1997年 | 1123篇 |
1996年 | 722篇 |
1995年 | 513篇 |
1994年 | 432篇 |
1993年 | 454篇 |
1992年 | 169篇 |
1991年 | 160篇 |
1990年 | 165篇 |
1989年 | 179篇 |
1988年 | 166篇 |
1987年 | 128篇 |
1986年 | 137篇 |
1985年 | 178篇 |
1984年 | 108篇 |
1983年 | 100篇 |
1982年 | 85篇 |
1981年 | 112篇 |
1980年 | 114篇 |
1979年 | 91篇 |
1978年 | 50篇 |
1977年 | 167篇 |
1976年 | 295篇 |
1975年 | 56篇 |
1974年 | 42篇 |
1973年 | 60篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Felix A. Miranda Carl H. Mueller Fred W. Van Keuls Guru Subramanyam Sivaruban Vignesparamoorthy 《Integrated ferroelectrics》2013,141(1):121-131
The inclusion of voltage-tunable barium strontium titanate (BSTO) thin films into planar band pass filters offers tremendous potential to increase their versatility. The ability to tune the passband so as to correct for minor deviations in manufacturing tolerances, or to completely reconfigure the operating frequencies of a microwave communication system, are highly sought-after goals. However, use of ferroelectric films in these devices results in higher dielectric losses, which in turn increase the insertion loss and decrease the quality factors of the filters. This study explores the use of patterned ferroelectric layers to minimize dielectric losses without degrading tunability. Patterning the ferroelectric layers enables us to constrict the width of the ferroelectric layers between the coupled microstrip lines, and minimize losses due to ferroelectric layers. Coupled one-pole microstrip bandpass filters with fundamental resonaces at ~7.2 GHz and well-defined harmonic resonances at ~14.4 and ~21.6 GHz, were designed, simulated and tested. For one of the filters, experimental results verified that its center frequency was tunable by 528 MHz at a center frequency of 21.957 GHz, with insertion losses varying from 4.3 to 2.5 dB, at 0 and 3.5 V/ w m, respectively. These data demonstrate that the tuning-to-loss figure of merit of tunable microstrip filters can be greatly improved using patterned ferroelectric thin films as the tuning element, and tuning can be controlled by engineering the ferroelectric constriction in the coupled sections. 相似文献
992.
M. De Keijser G. J. M. Dormans P. J. Van Veldhoven P. K. Larsen 《Integrated ferroelectrics》2013,141(2):131-137
Abstract Organometallic chemical vapour deposition is a suitable technique for the deposition of thin films of oxidic compounds such as lead zirconate titanate, PbZrxTi1?xO3. Above a deposition temperature of about 600°C stoichiometric PbZrxTi1?xO3 films can be grown on platinized silicon wafers within a large process window, independent of the precursor partial pressures and the deposition temperature. This is the result of a self-regulating mechanism. The PbZrxTi1?xO3 films have excellent ferroelectric properties exhibiting high values, up to 60μC/cm2, for the remanent polarisation. The value of the coercive field strength varies between 50 and 180 kV/cm, dependent on the composition. Layers with comparable properties can also be grown at lower temperatures, down to 500°C. In this case careful control of the gas-phase composition is required to obtain films with the correct stoichiometry. 相似文献
993.
994.
CHENG Jipeng ZHANG Xiaobin YE Ying TAO Xinyong LIU Fu LI Yu G Van Tendeloo 《武汉理工大学学报(材料科学英文版)》2006,21(1):29-31
1 IntroductionCarbon nanotubes (CNTs) are globally in the lime-light as a newfancy material of the 21st century andtheyare broadening their applications to almost all scientificareas . Several methods , such as arc-discharge va-porization[1],laser vaporization[2],chemical vapor depo-sition (CVD)[3-4]and others[5], have been developed tofabricate CNTs (including single and multi wall) up tonow.It is very well possible that the CVD method,be-cause of its simplicity andlowcost ,will provide … 相似文献
995.
Animal studies suggest that neuroactive steroids, in particular progesterone and its metabolites, have stress-dampening effects. However, few studies have explored these effects in humans. In this study, we investigated the effects of acute progesterone administration on responses to the Trier Social Stress Test (TSST). Healthy men participated in the TSST 3.5 hrs after intramuscular injection of 0, 50, or 100 mg progesterone (N = 16, 14, and 14). We measured cardiovascular (heart rate, blood pressure), hormonal (plasma adrenocorticotrophic hormone, cortisol, and noradrenaline), and subjective (e.g., anxiety, arousal) responses to stress in the three groups. Before the TSST, progesterone injections increased plasma levels without altering physiological or subjective states. Stress produced its expected physiological and subjective effects among placebo-treated individuals. Progesterone 50 mg attenuated peak increases in plasma cortisol and reduced changes in negative mood and alertness after stress, yet it increased plasma noradrenaline and systolic blood pressure. Progesterone 100 mg also attenuated stress-induced increases in alertness and arousal, yet it potentiated stress-induced increases in diastolic pressure. Thus, progesterone dampened some of the psychological effects of stress but produced inconsistent effects on physiological stress responses. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
996.
Claire Duménil David Woud Francesco Pinto Jeroen T. Alkema Ilse Jansen Anne M. Van Der Geest Sanne Roessingh Jean-Christophe Billeter 《Journal of chemical ecology》2016,42(3):259-269
Individuals can make choices based on information learned from others, a phenomenon called social learning. How observers differentiate between which individual they should or should not learn from is, however, poorly understood. Here, we showed that Drosophila melanogaster females can influence the choice of egg-laying site of other females through pheromonal marking. Mated females mark territories of high quality food by ejecting surplus male sperm containing the aggregation pheromone cis-11-vaccenyl acetate (cVA) and, in addition, deposit several sex- and species-specific cuticular hydrocarbon (CHC) pheromones. These pheromonal cues affect the choices of other females, which respond by preferentially laying eggs on the marked food. This system benefits both senders and responders, as communal egg laying increases offspring survival. Virgin females, however, do not elicit a change in the egg-laying decision of mated females, even when food has been supplemented with ejected sperm from mated females, thus indicating the necessity for additional cues. Genetic ablation of either a female’s CHC pheromones or those of their mate results in loss of ability of mated females to attract other females. We conclude that mated females use a pheromonal marking system, comprising cVA acquired from male ejaculate with sex- and species-specific CHCs produced by both mates, to indicate egg-laying sites. This system ensures information reliability because mated, but not virgin, females have both the ability to generate the pheromone blend that attracts other flies to those sites and a direct interest in egg-laying site quality. 相似文献
997.
Govindan Suresh Kumar Gopalu Karunakaran Easwaradas Kreedapathy Girija Evgeny Kolesnikov Nguyen Van Minh Mikhail V. Gorshenkov Denis Kuznetsov 《Ceramics International》2018,44(10):11257-11264
We report the rapid microwave-assisted hydrothermal synthesis of mesoporous hydroxyapatite (HAp) nanocrystals with controlled size, morphology, and surface area using various organic modifiers as regulators. The products were analyzed for their crystalline nature, phase purity, morphology, particle size and pore size distribution. Results indicated that ascorbic acid, cetyltrimethyl ammonium bromide (CTAB) and polyvinylpyrrolidone (PVP) play an important role to obtain needle like, rod like and fiber like mesoporous HAp nanocrystals with different specific surface area by controlling growth habit of HAp along c-axis. In addition, the prepared samples were B-type carbonated HAp similar to bone minerals. Therefore, the present approach can be a promising way to obtain precursor for making tissue engineering scaffolds, drug/protein delivery carriers and bone fillers with tunable characteristics. 相似文献
998.
The objective of this work was to promote the formation of the aragonite polymorph of calcium carbonate, which has some valuable applications in industry, via the mineral carbonation route. The combination of ultrasound with magnesium ions promoted the formation of pure aragonite crystals at optimum conditions. It was possible to synthesize high purity aragonite precipitates at temperatures ranging from 24 °C to 70 °C, with the resulting powders possessing varying particle size distributions (from sub-micron up to 20 μm) and crystal morphologies (from acicular needles to novel hubbard squash-like particles). Several process parameters were found to influence the produced calcium carbonate polymorph ratios (aragonite over calcite). Higher values of magnesium-to-calcium ratio, intermediate ultrasound amplitude (60%), continuous ultrasound application (100% cycle), introduction of ultrasound pre-breakage, lowering of the CO2 flow rate, and increase in the relative concentration (g/L Ca(OH)2), all promoted aragonite formation. A potential route for industrial production of this material has been identified via a fed-batch process, which effectively reutilizes magnesium chloride while maintaining high aragonite yield. The results presented herein are significantly superior to aragonite formation using only single promoting techniques, typically found in literature, and go beyond by focusing on pure (>99%) aragonite formation. 相似文献
999.
Matthew D. Miller A. Willem deGroot John W. Lyons Freddy A. Van Damme Bill L. Winniford 《应用聚合物科学杂志》2012,123(2):1238-1244
This report describes the application of a recently developed polyolefin characterization tool based upon gradient adsorption high‐temperature liquid chromatography (HT‐LC) using a graphitic carbon stationary phase to polyolefin homopolymer and previously unreported copolymer systems. Polyolefin‐based materials find utility in a broad range of applications and are differentiated by parameters such as molecular weight and comonomer content. Polymer comonomer distribution is commonly determined by crystallinity‐based separations (ATREF, CRYSTAF). These techniques, however, are time consuming. In addition, some semicrystalline polymers undergo cocrystallization, impacting the techniques' universal utility. Adsorption‐based HT‐LC can ideally overcome the limitations of crystallinity‐based separations, shedding new light on the composition of randomly‐polymerized polyolefins. In this report the basic separation capability of the adsorption HT‐LC technique, using a graphitic carbon column, is demonstrated for poly (ethylene‐co‐octene) and poly(ethylene‐co‐propylene) systems and compared with select precipitation/redissolution HT‐LC and ATREF results. Select results in this paper are also compared and contrasted to other recent publications on similar separations of polyolefins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011 相似文献
1000.
通过每个工序的多次试验,得出适用于轿车门型材的最佳铝合金成分,铸造和挤压工艺参数,保证了轿车门铝合金型材的组织、抗拉强度和弯曲成形等使用性能。 相似文献