全文获取类型
收费全文 | 47356篇 |
免费 | 4951篇 |
国内免费 | 1977篇 |
专业分类
电工技术 | 1262篇 |
综合类 | 2754篇 |
化学工业 | 12333篇 |
金属工艺 | 7537篇 |
机械仪表 | 2707篇 |
建筑科学 | 1987篇 |
矿业工程 | 1747篇 |
能源动力 | 1174篇 |
轻工业 | 3945篇 |
水利工程 | 397篇 |
石油天然气 | 992篇 |
武器工业 | 465篇 |
无线电 | 2750篇 |
一般工业技术 | 7832篇 |
冶金工业 | 5401篇 |
原子能技术 | 372篇 |
自动化技术 | 629篇 |
出版年
2024年 | 229篇 |
2023年 | 1160篇 |
2022年 | 1314篇 |
2021年 | 1745篇 |
2020年 | 1882篇 |
2019年 | 1634篇 |
2018年 | 1411篇 |
2017年 | 1696篇 |
2016年 | 1610篇 |
2015年 | 1634篇 |
2014年 | 2355篇 |
2013年 | 2667篇 |
2012年 | 2948篇 |
2011年 | 3011篇 |
2010年 | 2334篇 |
2009年 | 2374篇 |
2008年 | 2087篇 |
2007年 | 2891篇 |
2006年 | 2906篇 |
2005年 | 2609篇 |
2004年 | 2158篇 |
2003年 | 2107篇 |
2002年 | 1726篇 |
2001年 | 1435篇 |
2000年 | 1273篇 |
1999年 | 1024篇 |
1998年 | 855篇 |
1997年 | 654篇 |
1996年 | 562篇 |
1995年 | 450篇 |
1994年 | 411篇 |
1993年 | 293篇 |
1992年 | 222篇 |
1991年 | 147篇 |
1990年 | 125篇 |
1989年 | 109篇 |
1988年 | 55篇 |
1987年 | 32篇 |
1986年 | 24篇 |
1985年 | 26篇 |
1984年 | 20篇 |
1983年 | 19篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1975年 | 4篇 |
1951年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
In this work, the longitudinal permeability of squarely packed dual-scale fiber preforms is studied theoretically. These fiber preforms are composed of aligned porous tows and the tows are tightly packed. The effective permeability is calculated as a parallel-like network of intra-tow permeability and inter-tow permeability, which are quantified by Darcy’s law and the inscribed radius between tows, respectively. The jump velocity at the interface between inter-tow fluids and porous tows is considered, as derived by substituting Beavers and Joseph’s correlation into Brinkman’s equation. We further examine the effects of intra-tow permeability on the effective permeability of the fibrous system with three interface conditions: (1) interface velocity = 0, (2) interface velocity = mean intra-tow velocity, and (3) interface velocity = jump velocity. The jump-velocity-based model is found to be closest to numerical data. The influence of the fiber volume fraction of tows on the effective permeability is also analyzed. 相似文献
2.
We propose all printed and highly stable organic resistive switching device (ORSD) based on graphene quantum dots (G-QDs) and polyvinylpyrrolidone (PVP) composite for non-volatile memory applications. It is fabricated by sandwiching G-QDs/PVP composite between top and bottom silver (Ag) electrodes on a flexible substrate polyethylene terephthalate (PET) at ambient conditions through a cost effective and eco-friendly electro-hydrodynamic (EHD) technique. Thickness of the active layer is measured around 97 nm. The proposed ORSD is fabricated in a 3 × 3 crossbar array. It operates switching between high resistance state (HRS) and low resistance state (LRS) with OFF/ON ratio ∼14 for more than 500 endurance cycles, and retention time for more than 30 days. The switching voltage for set/reset of the devices is ±1.8 V and the bendability down to 8 mm diameter for 1000 cycles are tested. The elemental composition and surface morphology are characterized by XPS, FE-SEM, and microscope. 相似文献
3.
4.
Thin Film Thermoelectric Metal–Organic Framework with High Seebeck Coefficient and Low Thermal Conductivity 下载免费PDF全文
5.
Upconversion phosphors are known as a material system that can convert near-infrared light into visible/ultraviolet emissions by sequentially absorbing multiple photons. The studies on upconversion materials often use two rare earth (RE) ions as a sensitizer-activator pair. We investigated the influences on luminescence intensity depending on Cr-doping content (x) of hexagonal NaLu0.98–xCrxF4Er0.02 (x = 0–0.9) upconversion material by substituting Lu3+ ions with Cr3+in the absence of Gd3+. The change in upconversion luminescence intensity appears with saddle-like shape. We suggest that Cr3+ ions play the dual role as a constituent in host lattice and a sensitizer in the upconversion process. Optimal conditions for gaining the strongest upconversion emission correspond to x = 0.3–0.5, where there are effective energy transfers between Cr3+ and Er3+ ions and CrEr dimers. Apart from these values, the emission intensity decreases rapidly which can be ascribed to the absence of multiple-photon absorption for the case of low Cr3+ contents, and to the coupling between Cr3+ and/or Er3+ ions for the case of high Cr3+ contents. Magnetization and electron-spin-resonant measurements were performed to understand the correlation between the optical and magnetic properties. 相似文献
6.
Tatsuro Tanaka Yoshifumi Maeda Shinji Yamamoto Toru Iwao 《Electrical Engineering in Japan》2019,207(1):15-23
The arc welding has been used in various welding methods because it is inexpensive and high in strength after welding. However, it is a problem that accidents such as collapse of the bridge occur because of the welding defects. The welding of low cost and high productivity is required without the welding defects. The pulsed TIG welding is inexpensive and capable of high‐quality welding. The electromagnetic force contributing to penetration changes because the transient response of arc temperature and iron vapor generated from anode occurs. However, the analysis of pulsed TIG welding with metal vapor has been elucidated only metal vapor concentration near anode with transient phenomenon and heat flux. Thus, the theoretical elucidation of penetration depth with control factor has not been researched. In this paper, the contribution of metal vapor mass at the periphery part of pulsed arc to the electromagnetic force in the weld pool is elucidated. As a result, the iron vapor mass at periphery part decreased with increasing the frequency. The iron vapor was stagnated at axial center within one cycle. The electromagnetic force to the penetration depth direction in weld pool increased at axial center. Therefore, the metal vapor mass at periphery part plays an important role for the electromagnetic force increment at axial center. 相似文献
7.
8.
9.
10.
The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase diagram of the Ti-C system at 0 K was obtained by elucidating formation enthalpies as a function of compositions, and their mechanical and metallic properties of titanium carbides were investigated systematically. We also discussed the relation of titanium concentration to the both mechanical and metallic properties of titanium carbides. It has been found that the average valence electron density and tractility improved at higher concentrations of titanium, while the degree of covalent bonding directionality decreased. To this effect, the hardness of titanium carbide decreases as the content of titanium increases. Our results indicated that the titanium content significantly affected the metallic properties of the Ti-C system. 相似文献