全文获取类型
收费全文 | 21441篇 |
免费 | 2458篇 |
国内免费 | 1484篇 |
学科分类
工业技术 | 25383篇 |
出版年
2024年 | 60篇 |
2023年 | 284篇 |
2022年 | 473篇 |
2021年 | 495篇 |
2020年 | 647篇 |
2019年 | 536篇 |
2018年 | 531篇 |
2017年 | 754篇 |
2016年 | 864篇 |
2015年 | 961篇 |
2014年 | 1281篇 |
2013年 | 1305篇 |
2012年 | 1609篇 |
2011年 | 1639篇 |
2010年 | 1223篇 |
2009年 | 1296篇 |
2008年 | 1286篇 |
2007年 | 1589篇 |
2006年 | 1399篇 |
2005年 | 1130篇 |
2004年 | 915篇 |
2003年 | 846篇 |
2002年 | 693篇 |
2001年 | 605篇 |
2000年 | 531篇 |
1999年 | 408篇 |
1998年 | 311篇 |
1997年 | 288篇 |
1996年 | 267篇 |
1995年 | 213篇 |
1994年 | 191篇 |
1993年 | 136篇 |
1992年 | 99篇 |
1991年 | 77篇 |
1990年 | 67篇 |
1989年 | 71篇 |
1988年 | 56篇 |
1987年 | 41篇 |
1986年 | 22篇 |
1985年 | 32篇 |
1984年 | 30篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 12篇 |
1959年 | 4篇 |
1956年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
71.
阐述静态电子轨道衡诸多优点;工作原理、允许误差及误差分配;轨道衡的结构组成部分、解析各部分的工作特性及技术指标要求;合理选配及推荐实例. 相似文献
72.
73.
One of the most frequently cited reasons for conducting a meta-analysis is the increase in statistical power that it affords a reviewer. This article demonstrates that fixed-effects meta-analysis increases statistical power by reducing the standard error of the weighted average effect size (T?.) and, in so doing, shrinks the confidence interval around T?.. Small confidence intervals make it more likely for reviewers to detect nonzero population effects, thereby increasing statistical power. Smaller confidence intervals also represent increased precision of the estimated population effect size. Computational examples are provided for 3 effect-size indices: d (standardized mean difference), Pearson's r, and odds ratios. Random-effects meta-analyses also may show increased statistical power and a smaller standard error of the weighted average effect size. However, the authors demonstrate that increasing the number of studies in a random-effects meta-analysis does not always increase statistical power. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
74.
Withdrawal from an escalating-dose, bingelike regimen of cocaine administration in rats produced significantly depressed levels of locomotor activity during the nocturnal portion of the day-night cycle. This effect was observed during the first 48 hrs of testing. Extracellular single-unit recordings of ventral tegmental area (VTA) dopamine (DA) neurons revealed no differences between saline- and cocaine-treated rats with respect to basal firing rates. However, significantly fewer spontaneously active VTA DA neurons were encountered in rats withdrawn from binge cocaine. As with the nocturnal hypoactivity, this effect was observed only during the first 48 hrs of withdrawal. These findings suggest that short-term DA neuron dysfunction during cocaine withdrawal temporally corresponds to behavioral disruptions that are similar to those described in human addicts. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
75.
The authors explore the extent to which eating disorders, specifically anorexia nervosa (AN) and bulimia nervosa (BN), represent culture-bound syndromes and discuss implications for conceptualizing the role genes play in their etiology. The examination is divided into 3 sections: a quantitative meta-analysis of changes in incidence rates since the formal recognition of AN and BN, a qualitative summary of historical evidence of eating disorders before their formal recognition, and an evaluation of the presence of these disorders in non-Western cultures. Findings suggest that BN is a culture-bound syndrome and AN is not. Thus, heritability estimates for BN may show greater variability cross-culturally than heritability estimates for AN, and the genetic bases of these disorders may be associated with differential pathoplasticity. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
76.
Turbo码采用修正的BAHL et al.算法实现解码.这是一种基于软值的概率迭代解码算法.本文在保持Turbo码迭代软解码算法优点的基础上,充分利用Turbo码编码器结构这一确知条件,结合代数解码原理,提出了一种Turbo码概率-代数联合解码算法.该算法结合了概率解码和代数解码的优点,又有效避免了误差传播的发生,使Turbo码的纠错性能在原经典算法的基础上得到进一步的提高.该算法不仅为降低Turbo码的比特误码率和误差地板值提供了一种新的研究途径,而且因其更好的纠错性能而具有十分明显的实用价值.仿真实验结果显示,在比特误码率(BER)为10-3~10-4时,与经典Turbo码解码算法相比,采用该算法能获得0.1dB左右的编码增益. 相似文献
77.
78.
Sanghoon Lee Chris Podilchuk Vidhya Krishnan Alan C. Bovik 《The Journal of VLSI Signal Processing》2003,34(1-2):149-166
By exploiting new human-machine interface techniques, such as visual eyetrackers, it should be possible to develop more efficient visual multimedia services associated with low bandwidth, dynamic channel adaptation and robust visual data transmission. In this paper, we introduce foveation-based error resilience and unequal error protection techniques over highly error-prone mobile networks. Each frame is spatially divided into foveated and background layers according to perceptual importance. Perceptual importance is determined either through an eye tracker or by manually selecting a region of interest. We attempt to improve reconstructed visual quality by maintaining the high visual source throughput of the foveated layer using foveation-based error resilience and error correction using a combination of turbo codes and ARQ (automatic reQuest). In order to alleviate the degradation of visual quality, a foveation based bitstream partitioning is developed. In an effort to increase the source throughput of the foveated layer, we develop unequal delay-constrained ARQ (automatic reQuest) and rate compatible punctured turbo codes where the punctual pattern of RCPC (rate compatible punctured convolutional) codes in H.223 Annex C is used. In the simulation, the visual quality is significantly increased in the area of interest using foveation-based error resilience and unequal error protection; (as much as 3 dB FPSNR (foveal peak signal to noise ratio) improvement) at 40% packet error rate. Over real-fading statistics measured in the downtown area of Austin, Texas, the visual quality is increased up to 1.5 dB in PSNR and 1.8 dB in FPSNR at a channel SNR of 5 dB. 相似文献
79.
Chuxiang Li Jianhua Lu Jun Gu Ming L. Liou 《The Journal of VLSI Signal Processing》2003,33(3):277-285
In this paper, error-resilient schemes are proposed to support robust video transmission for digital terrestrial TV broadcasting (DTTB). In particular, a temporal error concealment incorporated with a low-complexity block-matching is developed, achieving an effective reception of predictive pictures in harsh terrestrial environment. Special algorithms are also designed for isolated I-pictures. Moreover, combined with an intra/inter case prediction, an adaptive error concealment scheme is further contrived to fit for different error conditions. Extensive simulations have been conducted under various DTTB channel conditions, even with a very high packet error rate, to verify the effectiveness of the proposed schemes. 相似文献
80.
N. V. Khovanov 《Measurement Techniques》2007,50(3):255-258
A Bayesian model is proposed based on randomizing the systematic errors of the instruments. Conditions are identified under
which the randomization reduces the expected bias in estimating a measured quantity.
__________
Translated from Izmeritel’naya Tekhnika, No. 3, pp. 22–25, March, 2007. 相似文献