全文获取类型
收费全文 | 6950篇 |
免费 | 118篇 |
国内免费 | 10篇 |
专业分类
电工技术 | 73篇 |
综合类 | 3篇 |
化学工业 | 449篇 |
金属工艺 | 31篇 |
机械仪表 | 48篇 |
建筑科学 | 111篇 |
矿业工程 | 5篇 |
能源动力 | 82篇 |
轻工业 | 294篇 |
水利工程 | 23篇 |
石油天然气 | 14篇 |
无线电 | 409篇 |
一般工业技术 | 440篇 |
冶金工业 | 4684篇 |
原子能技术 | 55篇 |
自动化技术 | 357篇 |
出版年
2022年 | 24篇 |
2021年 | 47篇 |
2020年 | 22篇 |
2019年 | 53篇 |
2018年 | 50篇 |
2017年 | 54篇 |
2016年 | 70篇 |
2015年 | 35篇 |
2014年 | 61篇 |
2013年 | 163篇 |
2012年 | 85篇 |
2011年 | 113篇 |
2010年 | 101篇 |
2009年 | 92篇 |
2008年 | 133篇 |
2007年 | 112篇 |
2006年 | 86篇 |
2005年 | 77篇 |
2004年 | 64篇 |
2003年 | 74篇 |
2002年 | 64篇 |
2001年 | 83篇 |
2000年 | 61篇 |
1999年 | 207篇 |
1998年 | 1393篇 |
1997年 | 796篇 |
1996年 | 538篇 |
1995年 | 286篇 |
1994年 | 261篇 |
1993年 | 288篇 |
1992年 | 72篇 |
1991年 | 89篇 |
1990年 | 110篇 |
1989年 | 97篇 |
1988年 | 107篇 |
1987年 | 97篇 |
1986年 | 64篇 |
1985年 | 92篇 |
1984年 | 33篇 |
1983年 | 33篇 |
1982年 | 45篇 |
1981年 | 51篇 |
1980年 | 62篇 |
1979年 | 30篇 |
1978年 | 42篇 |
1977年 | 116篇 |
1976年 | 253篇 |
1975年 | 39篇 |
1974年 | 14篇 |
1973年 | 16篇 |
排序方式: 共有7078条查询结果,搜索用时 15 毫秒
1.
Activity-directed synthesis (ADS) is a structure-blind, functional-driven molecular discovery approach. In this Concept, four case studies highlight the general applicability of ADS and showcase its flexibility to support different medicinal chemistry strategies. ADS deliberately harnesses reactions with multiple possible outcomes, and allows many chemotypes to be evaluated in parallel. Resources are focused on bioactive molecules, which emerge in tandem with associated synthetic routes. Some of the future challenges for ADS are highlighted, including the realisation of an autonomous molecular discovery platform. The prospects for ADS to become a mainstream lead generation approach are discussed. 相似文献
2.
We have improved the electronic properties of narrow-bandgap (Tauc gap below 1.5 eV) amorphous-silicon germanium alloys (a-SiGe:H) grown by hot-wire chemical vapor deposition (HWCVD) by lowering the substrate temperature and deposition rate. Prior to this work, we were unable to grow a-SiGe:H alloys with bandgaps below 1.5 eV that had photo-to-dark conductivity ratios comparable with our plasma-enhanced CVD (PECVD) grown materials [B.P. Nelson et al., Mater. Res. Soc. Symp. 507 (1998) 447]. Decreasing the filament diameter from our standard configuration of 0.5 mm to 0.38 or 0.25 mm provides first big improvements in the photoresponse of these alloys. Lowering the substrate temperature from our previous optimal temperatures (Tsub starting at 435 °C) to at 250 °C provides additional photo-to-dark conductivity ratio increasing by two orders of magnitude for growth conditions containing 20–30% GeH4 in the gas phase (relative to the total GeH4+SiH4 flow). 相似文献
3.
Predicting Forest Age Classes from High Spatial Resolution Remotely Sensed Imagery Using Voronoi Polygon Aggregation 总被引:1,自引:0,他引:1
Efficient identification of forest age is useful for forest management and ecological applications. Here we propose a user-assisted method for determining forest age using high spatial resolution remotely sensed imagery. This method requires individual trees to be extracted from imagery and represented as points. We use a local maximum filter to generate points that are converted to Voronoi polygons. Properties of the Voronoi polygons are correlated with forest age and used to aggregate points (trees) into areas (stands) based on three forest age classes. Accuracy of the aggregation ranges from approximately 68% to 78% and identification of the mature class is more consistent and accurate than the younger classes. 相似文献
4.
Inerfield M. Skones W. Nelson S. Ching D. Cheng P. Wong C. 《Solid-State Circuits, IEEE Journal of》2003,38(9):1524-1532
To date, the development of multifunction multicarrier digital receivers for cellular base station and military communications applications has been limited by the demanding dynamic range requirements for the analog-to-digital converter (ADC). The use of oversampling delta-sigma modulators provides a promising approach to overcoming the dynamic range barriers Nyquist-rate converters face in the same applications. This paper discusses issues involved in the design of high-speed high dynamic range wide-band delta-sigma ADCs for such communications applications. Test results of prototype designs are also presented. The delta-sigma modulators described in this paper operate at sampling frequencies ranging from 1 to 2.5 GHz with center frequencies ranging from dc to 100 MHz, providing between 74 and 84.2 dB signal-to-noise ratio (12 and 13.7 bits) for bandwidths of 25 and 12.5 MHz, respectively. The loop filters are continuous-time low-pass and bandpass implementations of order 6 and 10, and were fabricated in an InP heterojunction bipolar (HBT) technology. A typical tenth-order design consumes 6 W of power and occupies a die area of 23.5 mm/sup 2/. 相似文献
5.
A new architecture for implementing finite-impulse response (FIR) filters using the residue number system (RNS) is detailed. The design is based on using a restricted modulus set, with moduli of the form 2/sup n/,2/sup n/-1, and 2/sup n/+1. This does not restrict the modulus set to the common 3 modulus set {2/sup n/-1,2/sup n/,2/sup n/+1}, but any number of pairwise relatively prime moduli of this form, for example, {5,7,17,31,32,33}. Based on a comparison with a 2's complement design, the new RNS design can offer a significant speed improvement. The gain is obtained by using a set of small moduli, selected so as to minimize critical path delay and area. An algorithmic approach is used to obtain full adder based architectures that are optimized for area and delay. The modulus set is optimum based on cost parameters for each modulus. This new architecture presents a practical approach to implementing a fast RNS FIR filter. 相似文献
6.
A revised methodology is described for research on metacognitive monitoring, especially judgments of learning (JOLs), to investigate psychological processing that previously has been only hypothetical and unobservable. During data collection a new stage of recall occurs just prior to the JOL, so that during data analysis the items can be partitioned into subcategories to measure the degree of JOL accuracy in ways that are more analytic than was previously possible. A weighted-average combinatorial rule allows the component measures of JOL accuracy to be combined into the usual overall measure of metacognitive accuracy. An example using the revised methodology offers a new explanation for the delayed-JOL effect, in which delayed JOLs are more accurate than immediate JOLs for predicting recall. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
7.
Specific brain receptors for oxytocin have been described in several mammalian species. The distribution of these receptors differs greatly across species and in the rat, receptor binding in specific brain regions appears to depend upon gonadal steroids. This study used in vitro receptor autoradiography to examine the effects of testosterone on oxytocin receptor binding in the mouse forebrain. Three groups of male mice were compared: castrates treated with blank capsules, castrates treated with testosterone filled capsules, and intact males. Irrespective of steroid treatment, the distribution of oxytocin receptors in mouse forebrain differed markedly from patterns previously described in the rat. In addition to these species differences in receptor distribution, testosterone had effects in the mouse which differed from the induction of receptors previously reported in the rat. In the mouse ventromedial nucleus of the hypothalamus, binding in the untreated castrate males was approximately double that observed in either the intact or the testosterone-treated castrates. In other regions of the mouse brain, such as the intermediate zone of the lateral septum, binding to oxytocin receptors was increased with testosterone treatment. These results suggest that the brain oxytocin receptor varies across species not only in its distribution but also in its regional regulation by gonadal steroids. These apparently paradoxical changes in oxytocin receptor binding may result from either direct or indirect effects of gonadal steroids in mouse brain. 相似文献
8.
9.
10.
The Ca2+ sensitivity of cardiac myofibrillar force production can be decreased by acidosis or inorganic phosphate (P(i)) and increased by caffeine. To investigate whether the source of tissue influences the potency of these agents, we compared the actions of acidosis (change of pH from 7.0 to 6.2), P(i) and caffeine (both 20 mM) on force production of skinned cardiac muscles from adult ventricle, adult atrium and neonate ventricle of the rat. Maximum Ca(2+)-activated force was reduced by all three interventions and the responses of the different muscle types to a given intervention were similar. Acidosis reduced myofibrillar Ca2+ sensitivity by 1.09 and 1.04 pCa units in adult ventricle and atrium, respectively, and P(i) reduced it by 0.19 and 0.22 pCa units. However, each effect was only one-third as great in the neonate ventricle, which showed falls of 0.33 pCa units for acidosis and 0.06 for P(i). In contrast, caffeine raised the Ca2+ sensitivity by the same amount (approximately 0.4 pCa units) in all three muscle types. The differential effect between adult and neonate seen with both acidosis and P(i) suggests some similarity in the mechanisms by which these factors decrease Ca2+ sensitivity. In contrast, the equal effects of caffeine on neonate and adult suggests that caffeine acts by a completely different mechanism. The lower pH- and P(i)-sensitivity of the neonatal ventricle can help to explain why neonatal and adult myocardium exhibit differential force responses to ischaemia (or hypoxia alone). 相似文献