全文获取类型
收费全文 | 30072篇 |
免费 | 2681篇 |
国内免费 | 882篇 |
专业分类
电工技术 | 1246篇 |
技术理论 | 5篇 |
综合类 | 1341篇 |
化学工业 | 5965篇 |
金属工艺 | 1791篇 |
机械仪表 | 1830篇 |
建筑科学 | 1861篇 |
矿业工程 | 681篇 |
能源动力 | 1126篇 |
轻工业 | 2259篇 |
水利工程 | 380篇 |
石油天然气 | 1633篇 |
武器工业 | 150篇 |
无线电 | 3456篇 |
一般工业技术 | 4515篇 |
冶金工业 | 1627篇 |
原子能技术 | 342篇 |
自动化技术 | 3427篇 |
出版年
2024年 | 81篇 |
2023年 | 533篇 |
2022年 | 679篇 |
2021年 | 1187篇 |
2020年 | 972篇 |
2019年 | 907篇 |
2018年 | 1027篇 |
2017年 | 1060篇 |
2016年 | 1028篇 |
2015年 | 1217篇 |
2014年 | 1514篇 |
2013年 | 2045篇 |
2012年 | 1972篇 |
2011年 | 2163篇 |
2010年 | 1780篇 |
2009年 | 1712篇 |
2008年 | 1584篇 |
2007年 | 1517篇 |
2006年 | 1526篇 |
2005年 | 1309篇 |
2004年 | 871篇 |
2003年 | 792篇 |
2002年 | 739篇 |
2001年 | 606篇 |
2000年 | 659篇 |
1999年 | 756篇 |
1998年 | 665篇 |
1997年 | 495篇 |
1996年 | 465篇 |
1995年 | 380篇 |
1994年 | 310篇 |
1993年 | 219篇 |
1992年 | 167篇 |
1991年 | 127篇 |
1990年 | 100篇 |
1989年 | 73篇 |
1988年 | 85篇 |
1987年 | 47篇 |
1986年 | 30篇 |
1985年 | 40篇 |
1984年 | 32篇 |
1983年 | 22篇 |
1982年 | 16篇 |
1981年 | 15篇 |
1980年 | 21篇 |
1979年 | 13篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1974年 | 7篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
71.
采用有限元素法 ,分别数值计算了平底孔道、台阶式孔道和尖劈状孔道等 3种孔眼模型中不同中心频率 (10 0 k Hz、5 0 0 k Hz、1MHz)的散射超声脉冲波。计算结果表明 ,无论选用哪一种孔底模型 ,均可以得到较为明显的对应于孔底的声波信号。但是 ,孔道越不规则 ,使用的超声信号的主频越高 ,则孔底信号越差。为了兼顾孔底信号质量和信号采集、传输的代价 ,在未来的检测仪器中应采用尽可能低的测量频率 (低于 5 0 0 k Hz) 相似文献
72.
73.
74.
万家寨引黄工程北干线系统输水损失,是关乎输水运行成本和水资源有效利用的重要问题之一。由于各类建筑物施工方法,材料及工艺的差异。其输水损失差异较大,计算方法和计算参数各不相同,文章在对各类建筑物进行输水损失计算的基础上,分析了不同类型建筑物的输水损失特点,提出了减少水量损失的建议和设想。 相似文献
75.
The low-frequency noise in the devices often appears to be a superposition of 1/f noise and white noise, which are induced by different physical mechanisms or microscopic defects. The separation of the two noises is necessary especially in the characterization of quality and reliability by means of noise measurement. The traditional separation method is usually used in frequency domain with limited frequency range, and the estimated value of exponent index γ for 1/fγ noise has considerable error when the ratio of 1/f noise and white noise is smaller than 20 dB. In this paper, based on wavelet denoising theory, a new method is proposed to separate 1/fγ noise from white noise in time domain and can be used to estimate γ values with much better accuracy even if the ratio of 1/f noise to white noise in low enough. 相似文献
76.
77.
本文提出一种与现有以太网兼容的综合话音/数据协议.描述了协议模型及实现方法.并用PASCAL语言在IBM-PC机上模拟研究了该协议的性能.模拟结果表明,该协议能满足话音的实时要求,性能良好,且数据传输性能没有明显下降.协议克服了以太网传输话音延迟不确定的缺点. 相似文献
78.
Aimin Xiang Zhongjie Du Qinghua Zeng Chen Zhang Hangquan Li 《Polymer International》2005,54(10):1366-1370
This paper presents recent efforts on the preparation of flexible polyhedral particles via concentrated emulsion templating polymerization in which the hydrophilic monomer (acrylamide) and hydrophobic monomer (butyl acrylate) are polymerized simultaneously in the continuous and dispersed phase, respectively. Such templating polymerization has been enhanced in our systems owing to the introduction of acrylamide monomer and their higher polymerization rate in continuous phase as compared with butyl acrylate in dispersed phase. Diffusion between the different phases was also inhibited. Furthermore, the stability of the concentrated emulsion and the molecular weight of the produced poly(butyl acrylate) were found to be significantly affected by the amount of redox initiator. The morphology of the particles could be controlled by varying the volume fraction of the dispersed phase and the polyhedral particles were achieved at higher volume fraction. Copyright © 2005 Society of Chemical Industry 相似文献
79.
Mg-Al hydrotalcites intercalated with five different interlayer anions—CO32−, SO42−, Cl−, HPO42− or terephthalate—were synthesized by either the coprecipitation or ion-exchange method. The structure of the as-synthesized samples and the presence of intended anions in the interlayer gallery of hydrotalcites were determined by X-ray diffraction and FTIR spectroscopy. On calcination at 600 °C the materials were transformed into mixed metal oxides. The kind of the counterbalancing anions present in the parent hydrotalcite influences strongly textural parameters of the obtained Mg-Al oxides. Both temperature-programmed desorption of NH3 and CO2, and test reaction of 2-methyl-3-butyn-2-ol (MBOH) conversion were used to determine the acidity and basicity of the samples. The hydrotalcite derived mixed Mg-Al oxides showed the presence of Brønsted and Lewis acid and base sites. However, the strong basic character of the solids caused that acetone and acetylene were observed as the major products of MBOH conversion. 相似文献
80.
Oh Kee Kwon Kang Ho Kim Eun Deok Sim Jong Hoi Kim Hyun Soo Kim Kwang Ryong Oh 《Photonics Technology Letters, IEEE》2005,17(3):537-539
Tunable external-cavity lasers with low power variation over a broad tuning range are demonstrated using asymmetric multiple quantum-wells with a wide and flat gain. For a 2.8-/spl mu/m-wide ridge waveguide laser of cavity length 380 /spl mu/m, when used in a grating external cavity and with an antireflection coating on one facet of laser diode chip, the power variation of less than -1 dB is obtained over a range of 80 nm. This extremely low power variation is a direct result of the spectrally flat gain. 相似文献