全文获取类型
收费全文 | 32608篇 |
免费 | 2358篇 |
国内免费 | 1818篇 |
专业分类
电工技术 | 1301篇 |
技术理论 | 1篇 |
综合类 | 1703篇 |
化学工业 | 12340篇 |
金属工艺 | 1731篇 |
机械仪表 | 942篇 |
建筑科学 | 1035篇 |
矿业工程 | 401篇 |
能源动力 | 690篇 |
轻工业 | 4685篇 |
水利工程 | 366篇 |
石油天然气 | 2036篇 |
武器工业 | 231篇 |
无线电 | 2583篇 |
一般工业技术 | 2434篇 |
冶金工业 | 888篇 |
原子能技术 | 493篇 |
自动化技术 | 2924篇 |
出版年
2024年 | 82篇 |
2023年 | 489篇 |
2022年 | 2205篇 |
2021年 | 2218篇 |
2020年 | 842篇 |
2019年 | 777篇 |
2018年 | 742篇 |
2017年 | 887篇 |
2016年 | 1109篇 |
2015年 | 1190篇 |
2014年 | 1599篇 |
2013年 | 1802篇 |
2012年 | 2008篇 |
2011年 | 2412篇 |
2010年 | 1676篇 |
2009年 | 1879篇 |
2008年 | 1744篇 |
2007年 | 1868篇 |
2006年 | 1745篇 |
2005年 | 1406篇 |
2004年 | 1189篇 |
2003年 | 1033篇 |
2002年 | 886篇 |
2001年 | 707篇 |
2000年 | 598篇 |
1999年 | 555篇 |
1998年 | 422篇 |
1997年 | 397篇 |
1996年 | 376篇 |
1995年 | 354篇 |
1994年 | 307篇 |
1993年 | 205篇 |
1992年 | 173篇 |
1991年 | 147篇 |
1990年 | 133篇 |
1989年 | 101篇 |
1988年 | 78篇 |
1987年 | 49篇 |
1986年 | 71篇 |
1985年 | 46篇 |
1984年 | 48篇 |
1983年 | 31篇 |
1982年 | 36篇 |
1981年 | 30篇 |
1980年 | 32篇 |
1979年 | 34篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1975年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
在cdma20001x前向信道中,要对一段信号进行传送,一般选用QPSK调制,因为QPSK调制比QAM更适合噪音环境。采用TI6000系列的TMS320C6711芯片处理前向信道信号,可以对复杂性和实时性较高的信号做采集、量化、编码、调制等实时处理,主要功能通过软件编程实现,从而使系统具有结构灵活、可靠性高、可扩展等优点。 相似文献
52.
A. I. D’Souza M. G. Stapelbroek P. N. Dolan P. S. Wijewarnasuriya R. E. DeWames D. S. Smith J. C. Ehlert 《Journal of Electronic Materials》2003,32(7):633-638
The 1/f noise in photovoltaic (PV) molecular-beam epitaxy (MBE)-grown Hg1−xCdxTe double-layer planar heterostructure (DLPH) large-area detectors is a critical noise component with the potential to limit
sensitivity of the cross-track infrared sounder (CrIS) instrument. Therefore, an understanding of the origins and mechanisms
of noise currents in these PV detectors is of great importance. Excess low-frequency noise has been measured on a number of
1000-μm-diameter active-area detectors of varying “quality̶1; (i.e., having a wide range of I-V characteristics at 78 K). The
1/f noise was measured as a function of cut-off wavelength under illuminated conditions. For short-wave infrared (SWIR) detectors
at 98 K, minimal 1/f noise was measured when the total current was dominated by diffusion with white noise spectral density
in the mid-10−15A/Hz1/2 range. For SWIR detectors dominated by other than diffusion current, the ratio, α, of the noise current in unit bandwidth
in(f = 1 Hz, Vd = −60 mV, and 16;f = 1 Hz) to dark current Id(Vd = −60 mV) was αSW-d = in/Id ∼ 1 15; 10−3. The SWIR detectors measured at 0 mV under illuminated conditions had median αSW-P = in/Iph ∼ 7 15; 10−6. For mid-wave infrared (MWIR) detectors, αMW-d = in/Id ∼ 2 15; 10−4, due to tunneling current contributions to the 1/f noise. Measurements on forty-nine 1000-μm-diameter MWIR detectors under
illuminated conditions at 98 K and −60 mV bias resulted in αMW-P = in/Iph = 4.16 177; 1.69 15; 10−6. A significant point to note is that the photo-induced noise spectra are nearly identical at 0 mV and 100 mV reverse bias,
with a noise-current-to-photocurrent ratio, αMW-P, in the mid 10−6 range. For long-wave infrared (LWIR) detectors measured at 78 K, the ratio, αLW-d = in/Id ∼ 6 15; 10−6, for the best performers. The majority of the LWIR detectors exhibited αLW-d on the order of 2 15; 10−5. The photo-induced 1/f noise had αLW-P = in/Iph ∼ 5 15; 10−6. The value of the noise-current-to-dark-current ratio, α appears to increase with increasing bandgap. It is not clear if
this is due to different current mechanisms impacting 1/f noise performance. Measurements on detectors of different bandgaps
are needed at temperatures where diffusion current is the dominant current. Excess low-frequency noise measurements made as
a function of detector reverse bias indicate 1/f noise may result primarily from the dominant current mechanism at each particular
bias. The 1/f noise was not a direct function of the applied bias. 相似文献
53.
Cu-Mg/Al_2O_3催化乙醇和乙二胺合成N-乙基乙二胺 总被引:1,自引:0,他引:1
在固定床反应器内,乙醇和乙二胺经分子间催化胺化反应合成了N-乙基乙二胺(NEED),考察了一系列铜系催化剂的活性,并筛选出催化性能较好的Cu-Mg/A l2O3催化剂;通过X射线衍射和透射扫描电子显微镜方法表征了助剂Mg对Cu/A l2O3催化剂的改性作用;考察了助催化剂Mg含量、反应温度、反应压力、进料量和乙二胺含量等对Cu-Mg/A l2O3催化剂性能的影响。实验结果表明,Cu-Mg/A l2O3催化剂具有很好的活性和选择性;在200℃、3.0MPa、质量分数20%的乙二胺乙醇溶液、进料量1.0mL/m in的条件下,乙二胺的转化率为71.56%,产物NEED的选择性和收率分别为92.62%和69.86%;经过20 d的连续使用表明,Cu-Mg/A l2O3催化剂具有良好的稳定性。 相似文献
54.
Frank te Beest Ad Peeters Kees van Berkel Hans Kerkhoff 《Journal of Electronic Testing》2003,19(4):397-406
Handshake circuits form a special class of asynchronous circuits that has enabled the industrial exploitation of the asynchronous potential such as low power, low electromagnetic emission, and increased cryptographic security. In this paper we present a test solution for handshake circuits that brings synchronous test-quality to asynchronous circuits. We add a synchronous mode of operation to handshake circuits that allows full controllability and observability during test. This technique is demonstrated on some industrial examples and gives over 99% stuck-at fault coverage, using test-pattern generators developed for synchronous circuits. The paper describes how such a full-scan mode can be achieved, including an approach to minimize the number of dummy latches in case latches are used in the data path of the handshake circuit. 相似文献
55.
56.
通过多年在蓬莱19-3油田FPSO和渤中25-1油田FPSO设备维护和管理的经历,从船体、上部模块、单点系泊等方面,对这两艘FPSO上的一些系统和设备在设计方面各自的特点进行对比和分析 相似文献
57.
用溶胶-凝胶技术在Bi(100)衬底上制备了单层和渐变型多层的BaxSr(1-X)TiO3薄膜,其膜层组分分别为:Ba0.7Sr0.3TiO3,Ba0.8Sr0.2TiO,Ba0.9Sr0.1TiO3,BaTiO3,对生长制备出的多层BaxSr(1-X)TiO3薄膜进行了变角度椭偏光谱测量,通过椭偏光谱解谱分析研究,首次得到了BaxSr(1-X)TiO3多层膜结构不同膜层的膜厚和光学常数,其结果显示:椭偏光谱分析得到的不同膜层的膜厚与卢瑟福背向散射测量得到的结果基本相符;渐变型多层膜中BaTiO3薄膜的折射率比单层BaTiO3薄膜折射率大许多,与体BaTiO3的折射率相接近,这说明渐变型多层膜中BaTiO3薄膜的光学性质与体材料的光学性质接近。 相似文献
58.
校园网宿舍网络管理模式的探讨 总被引:10,自引:1,他引:9
讨论了高校学生宿舍区网络的特点,结合天津商学院校园网的实践,讨论了校园网宿舍网络管理的模式。 相似文献
59.
Copper films with (1 1 1) texture are of crucial importance in integrated circuit interconnects. We have deposited strongly (1 1 1)-textured thin films of copper by atomic layer deposition (ALD) using [2,2,6,6-tetramethyl-3,5-heptadionato] Cu(II), Cu(thd)2, as the precursor. The dependence of the microstructure of the films on ALD conditions, such as the number of ALD cycles and the deposition temperature was studied by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy. Analysis of (1 1 1)-textured films shows the presence of twin planes in the copper grains throughout the films. SEM shows a labyrinthine structure of highly connected, large grains developing as film thickness increases. This leads to low resistivity and suggests high resistance to electromigration. 相似文献
60.
Dissolved organic carbon (DOC) dynamics were investigated over a two year period in the Upper Rhǒne River to examine the role of side-arms in providing DOC to the main channel in relation to discharge fluctuations, especially floods. Concentrations of DOC are shown to remain low in space and in time (average 1.5 mg ?1) and to be more related to global hydrological events (precipitation) than to any local flushing for backwaters during floods. The results do not support the general assumption that side-arms are the providers of DOC to the main channel. 相似文献