全文获取类型
收费全文 | 35847篇 |
免费 | 4431篇 |
国内免费 | 2476篇 |
专业分类
电工技术 | 2408篇 |
技术理论 | 4篇 |
综合类 | 6245篇 |
化学工业 | 4094篇 |
金属工艺 | 4748篇 |
机械仪表 | 2766篇 |
建筑科学 | 2490篇 |
矿业工程 | 365篇 |
能源动力 | 2325篇 |
轻工业 | 784篇 |
水利工程 | 166篇 |
石油天然气 | 126篇 |
武器工业 | 253篇 |
无线电 | 3266篇 |
一般工业技术 | 6164篇 |
冶金工业 | 545篇 |
原子能技术 | 328篇 |
自动化技术 | 5677篇 |
出版年
2024年 | 123篇 |
2023年 | 822篇 |
2022年 | 972篇 |
2021年 | 1067篇 |
2020年 | 1277篇 |
2019年 | 1146篇 |
2018年 | 1082篇 |
2017年 | 1381篇 |
2016年 | 1402篇 |
2015年 | 1586篇 |
2014年 | 2254篇 |
2013年 | 2367篇 |
2012年 | 3271篇 |
2011年 | 3388篇 |
2010年 | 2888篇 |
2009年 | 3452篇 |
2008年 | 2117篇 |
2007年 | 2899篇 |
2006年 | 2499篇 |
2005年 | 942篇 |
2004年 | 597篇 |
2003年 | 567篇 |
2002年 | 503篇 |
2001年 | 419篇 |
2000年 | 485篇 |
1999年 | 449篇 |
1998年 | 298篇 |
1997年 | 255篇 |
1996年 | 292篇 |
1995年 | 245篇 |
1994年 | 225篇 |
1993年 | 215篇 |
1992年 | 199篇 |
1991年 | 178篇 |
1990年 | 148篇 |
1989年 | 100篇 |
1988年 | 152篇 |
1987年 | 203篇 |
1986年 | 186篇 |
1985年 | 50篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 14篇 |
1979年 | 2篇 |
1965年 | 1篇 |
1959年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
《International Journal of Heat and Mass Transfer》2006,49(23-24):4305-4324
This study employs a direct numerical simulation (DNS) technique to study the flow, turbulence structure, and passive scalar plume transport behind line sources in an unstably stratified open channel flow. The scalar transport behaviors for five emission heights (zs = 0, 0.25H, 0.5H, 0.75H, and H, where H is the channel height) at a Reynolds number of 3000, a Prandtl number and a Schmidt number of 0.72, and a Richardson number of −0.2 are investigated. The vertically meandering mean plume heights and dispersion coefficients calculated by the current DNS model agree well with laboratory results and field measurements in literature. It is found that the plume meandering is due to the movement of the positive and negative vertical turbulent scalar fluxes above and below the mean plume heights, respectively. These findings help explaining the plume meandering mechanism in the unstably stratified atmospheric boundary layer. 相似文献
102.
103.
《Journal of power sources》2006,159(2):1042-1047
The direct methanol fuel cell (DMFC) is regarded as a promising candidate in portable electronic power applications. Bipolar plate stacks were systematically studied by controlling the operating conditions, and by adjusting the stack structure design parameters, to develop more commercial DMFCs. The findings indicate that the peak power of the stack is influenced more strongly by the flow rate of air than by that of the methanol solution. Notably, the stack performance remains constant even as the channel depth is decreased from 1.0 to 0.6 mm, without loss of the performance in each cell. Furthermore, the specific power density of the stack was increased greatly from ∼60 to ∼100 W l−1 for stacks of 10 and 18 cells, respectively. The current status of the work indicates that the power output of an 18-cell short stack reaches 33 W in air at 70 °C. The outer dimensions of this 18-cell short stack are only 80 mm × 80 mm × 51 mm, which are suitable for practical applications in 10–20 W DMFC portable systems. 相似文献
104.
B. Wilthan G. Pottlacher 《稀有金属(英文版)》2006,25(5):592-596
1. Introduction Metal working industry needs accurate thermo- physical properties of liquid metals and alloys as input data for different simulation routines to im- prove the performance of their products. Within this paper we continue the systematic investigation of the dependence of emissivity of binary alloys on the relative concentration of the constituent elements (for a previous paper see [1]). 2. Experimental method A fast ohmic pulse heating technique is applied to heat metallic wire-s… 相似文献
105.
《Thin solid films》2006,515(2):430-433
In this paper, we present an experimental study on the structural and crystalline properties of electron beam evaporated Mo/Si multilayers. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) experiments were used to characterize the crystalline nature of Mo and Si. TEM and XRD show that Si grows amorphous, while Mo clearly crystallizes. The Mo crystallites show no preferred growth orientations and a columnar growth mode, with crystallite sizes limited by the Mo layer thickness. From samples with low Mo content, we show that crystallization of Mo starts directly at the Mo/Si interfaces, with the formation of a Mo3Si compound. The crystallite lattice strains that develop during Mo-on-Si growth due to lattice mismatches, will add to the macroscopic multilayer stress. By comparing Mo lattice strains derived from XRD with changes in substrate curvatures, as determined by interferometry, we show that compressive and tensile substrate deformations are predominantly caused by the internal crystallite strains. 相似文献
106.
《Journal of the European Ceramic Society》2005,25(2-3):211-220
The inherent high temperature durability as well as the hardness of random inorganic networks composed of Si, B, N and C, can be raised through the incorporation of rigid structural elements (e.g. borazine rings) as well as through an increase of the carbon content. This has been shown by comparison of the high temperature durabilities and micromechanical properties of newly synthesized Si/B/N/C ceramics derived from different acyclic and cyclic single source precursors. Crosslinking of these specially designed monomers with methylamine provides highly homogeneous preceramic polymers, in which the predetermined structural features of the molecular precursors are embedded. Subsequent pyrolysis and calcination up to 1500 °C converts the polymers into all-inorganic amorphous silicon boron carbonitrides. Depending on the constitution of the precursors, the as-obtained materials exhibit a combination of a high carbon content together with borazine rings embedded into the covalent network. Comparing the high temperature stability, hardness and stiffness of the synthesized ceramics, the best performance is achieved when carbon rich borazine derivatives are employed as single source precursors. Thus, the high temperature durability can be raised up to at least 2000 °C, and, at the same time, the microhardness and elastic modulus is maximized up to 14.5 and 127 GPa, respectively. The new materials have been characterized by IR spectroscopy, elemental analysis, XRD, SEM, DTA/TG, and by nanoindentation. 相似文献
107.
《Control Engineering Practice》2006,14(8):897-908
Previous work has considered the use of sliding mode observers for fault detection and isolation (FDI) in uncertain linear systems whereby the unknown faults are reconstructed by appropriate processing of the so-called equivalent output error injection. The paper builds on this work and considers such a scheme within the broader context of fault tolerant control. Specifically, by correcting the faulty measurement by an estimate of the fault obtained from the sliding mode FDI scheme, good closed-loop performance is still maintained. An example of such a scheme, which has been implemented in real-time on a laboratory dc motor rig, is described. 相似文献
108.
109.
We performed the first-principles calculation to investigate the electronic structure and polarization behaviors in PbTiO3/SrTiO3 (PST) superlattices. The DOS (density of state) profiles show that there are strong hybridizations of atom Ti–O and Pb–O which play very important roles on ferroelectricity of the PbTiO3/SrTiO3 superlattices. Comparing to the corresponding paraelectric phase, we find the electrons of the PT (PbTiO3) layers occupy lower energy states and electrons of the ST (SrTiO3) layer occupy higher energy states. It is shown that the polarizations of the superlattices decrease with proportion of SrTiO3 increasing. The constant polarization of local layer indicates that PST superlattices with small modulation lengthen can be approximately considered as a single ferroelectric material. Furthermore, according to electrostatic model, we find that directions of internal electric fields in PT and ST layers are opposite. In PST superlattices, internal electric field in PT layer leads to the loss of polarization of this layer, but the polarization of ST layer is induced by internal electric field of this layer. Compared to the value of the polarization in bulk PbTiO3, polarization of PST is smaller. 相似文献
110.
The mechanisms of speciation of aluminate and silicate phases during dissolution and condensation stages of alumino-silicate
geopolymer reactions characterised by Si/Al ≥ 3, have been investigated and the results compared to predictions of the partial
charge model. Solid-state nuclear magnetic resonance (NMR) traces indicate that free [Al(OH)4]− species, present in lower silicate formulations such as Si/Al ≤ 1, do not occur in the present systems, suggesting that the condensation reaction between [Al(OH)4]− and silicate species is fairly quick and is consumed as soon as it is formed. This observation is also consistent with both
calorimetric measurements and model predictions, as the condensation time increased exponentially with increased Si/Al ratio
in the geopolymeric phase, indicating again that the high content of Al species in the gel phase greatly enhanced the condensation
rate. The experimental observations suggest that the condensation process in these systems occurs in two stages: (a) quick
condensation between aluminate and silicate species; followed by (b) a slow condensation stage solely involving silicate species. 相似文献