全文获取类型
收费全文 | 30679篇 |
免费 | 3305篇 |
国内免费 | 1498篇 |
专业分类
电工技术 | 3907篇 |
技术理论 | 1篇 |
综合类 | 2261篇 |
化学工业 | 4369篇 |
金属工艺 | 4211篇 |
机械仪表 | 1254篇 |
建筑科学 | 1556篇 |
矿业工程 | 1947篇 |
能源动力 | 1216篇 |
轻工业 | 1488篇 |
水利工程 | 750篇 |
石油天然气 | 1139篇 |
武器工业 | 127篇 |
无线电 | 2271篇 |
一般工业技术 | 2675篇 |
冶金工业 | 4238篇 |
原子能技术 | 545篇 |
自动化技术 | 1527篇 |
出版年
2024年 | 66篇 |
2023年 | 424篇 |
2022年 | 858篇 |
2021年 | 1121篇 |
2020年 | 1027篇 |
2019年 | 854篇 |
2018年 | 796篇 |
2017年 | 1003篇 |
2016年 | 1209篇 |
2015年 | 1152篇 |
2014年 | 1877篇 |
2013年 | 2091篇 |
2012年 | 2120篇 |
2011年 | 2343篇 |
2010年 | 1749篇 |
2009年 | 1633篇 |
2008年 | 1401篇 |
2007年 | 1889篇 |
2006年 | 1751篇 |
2005年 | 1507篇 |
2004年 | 1301篇 |
2003年 | 1089篇 |
2002年 | 1085篇 |
2001年 | 890篇 |
2000年 | 723篇 |
1999年 | 576篇 |
1998年 | 464篇 |
1997年 | 415篇 |
1996年 | 376篇 |
1995年 | 343篇 |
1994年 | 301篇 |
1993年 | 198篇 |
1992年 | 179篇 |
1991年 | 128篇 |
1990年 | 113篇 |
1989年 | 107篇 |
1988年 | 85篇 |
1987年 | 42篇 |
1986年 | 37篇 |
1985年 | 34篇 |
1984年 | 30篇 |
1983年 | 11篇 |
1982年 | 29篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1974年 | 3篇 |
1959年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
In the recent advancements in image and video analysis, the detection of salient regions in the image becomes the initial step. This plays a crucial role in deciding the performance of such algorithms. In this work, a Multi-Resolution Feature Extraction (MRFE) technique that makes use of Discrete Wavelet Convolutional Neural Network (DWCNN) for generating features is employed. An Enhanced Feature Extraction (EFE) module extracts additional features from the high level features of the DWCNN, which are used to frame both channel as well as spatial attention models for yielding contextual attention maps. A new hybrid loss function is also proposed, which is a combination of Balanced Cross Entropy (BCE) loss and Edge based Structural Similarity (ESSIM) loss that effectively identifies and segments the salient regions with clear boundaries. The method is tested exhaustively with five different benchmark datasets and is proved superior to the existing state-of-the-art methods with a minimum Mean Absolute error (MAE) of 0.03 and F-measure of 0.956. 相似文献
2.
Jieyi Yu Xianguo Liu Rui Su Xuefeng Zhang Xinglong Dong 《Ceramics International》2021,47(17):23950-23957
Increasing the dielectric loss capacity plays an important role in enhancing the electromagnetic absorption performance of materials. It remains a challenge to simultaneously introduce multiple types of dielectric losses in the material. In this work, we show that the atomic and interfacial dipole polarizations can be simultaneously enhanced by substituting N species into both carbon coating layers and bulk TiC lattices of a core-shell TiC@C material. Additionally, substitution of N species results more exposed TiC(111) facets and refines the TiC grain sizes in the bulk material, which is beneficial for enhancing the scattering of the external electromagnetic waves. The maximum reflection loss of the N substituted TiC@C material is measured as ?47.1 dB with an effective absorbing bandwidth of 4.83 GHz at 1.9 mm, which illustrates a valuable way to further tuning the electromagnetic absorption performance of this type of materials. 相似文献
3.
Tomoki Murata Toru Asaka Sakyo Hirose 《Journal of the American Ceramic Society》2021,104(11):5794-5802
Dielectric tunability has been extensively investigated in ferroelectric materials, which exhibit a negative tunability of dielectric permittivity in an external electric field. In contrast, positive tunability is rare and has been reported only in a few antiferroelectric materials. We present positive (and negative) tunability in the titanite, CaTiSiO5. The dielectric property of CaTiSiO5 was measured up to an extraordinarily high electric field of 40 MV/m. A nonlinear polarization field loop with no hysteresis was obtained. The dielectric permittivity of εr ~ 25 increases up to εr ~ 40 at 20 MV/m and room temperature. Although titanite has an antipolar structure and is expected to be “antiferroelectric,” its dielectric response in high electric fields up to ~40 MV/m differs from that of conventional antiferroelectrics. We demonstrate that the phase-transition temperature and dielectric tunability could be modulated through the chemical substitution of Ca1−xLaxTiSi1−xAlxO5, in which the destabilization of the long-range antipolar order is revealed by transmission electron microscopy analysis. These results indicate that the observed dielectric response to an electric field may originate from the unique features of the antipolar and domain structures in CaTiSiO5. 相似文献
4.
5.
Shingo Urata Hiroyuki Hijiya Kazuhiko Niwano Jun Matsui 《Journal of the American Ceramic Society》2022,105(6):4200-4207
To theoretically explore amorphous materials with a sufficiently low dielectric loss, which are essential for next-generation communication devices, the applicability of a nonequilibrium molecular dynamics simulation employing an external alternating electric field was examined using alkaline silicate glass models. In this method, the dielectric loss is directly evaluated as the phase shift of the dipole moment from the applied electric field. This method enabled us to evaluate the dielectric loss in a wide frequency range from 1 GHz to 10 THz. It was observed that the dielectric loss reaches its maximum at a few THz. The simulation method was found to qualitatively reproduce the effects of alkaline content and alkaline type on the dielectric loss. Furthermore, it reasonably reproduced the effect of mixed alkalines on the dielectric loss, which was observed in our experiments on sodium and/or potassium silicate glasses. Alkaline mixing was thus found to reduce the dielectric loss. 相似文献
6.
7.
《Ceramics International》2022,48(3):3059-3069
In this work, an ultralight nanocomposite of graphene oxide aerogels as a matrix and nickel-zinc ferrite (Ni0.7Zn0.3Fe2O4) nanoparticles as a second phase for the absorption of electromagnetic waves in the frequency of 1–18 GHz were fabricated by the hydrothermal - freeze-drying method. α-Al2O3 nanoparticles were used for further impedance matching for applications in electromagnetic wave absorption. XRD, SEM, EDS, and VNA analyses were used to characterize the sample. The effects of the amount of Ni0.7Zn0.3Fe2O4 (NZF) nanoparticles (GO: NZF volume percent ratio = 5:1 and 2:1) on the absorption of electromagnetic waves were investigated. 相似文献
8.
Effect of heat loss on the syngas production by fuel-rich combustion in a divergent two-layer burner
《International Journal of Hydrogen Energy》2022,47(34):15270-15281
The effect of heat loss on the syngas production from partial combustion of fuel-rich in a divergent two-layer burner is numerically studied using two-dimensional model with detailed kinetics GRI-Mech 1.2. Both the radiation and wall heat losses to the surrounding are considered in the computations. It is shown that two types heat losses have different effects on the syngas production. The radiation heat loss has significant effect on the syngas temperature and the syngas temperature is dropped as radiation heat loss is increased, but it has neglected effect on the reforming efficiency and methane conversion efficiency. The wall heat loss has a comprehensive effect on the syngas production. The wall heat loss not only reduces the conversion efficiency, but also significantly decreases the syngas temperature. The effect of wall heat loss becomes weak as the equivalence is increased. The reforming efficiency drops from 0.440 to 0.424 for equivalence ratio of 2 and mixture velocity of 0.17 m/s for the predictions between adiabatic wall and non-adiabatic conditions. 相似文献
9.
Wonje Jang Junhyuk Hyun Jhonghyun An Minho Cho Euntai Kim 《IEEE/CAA Journal of Automatica Sinica》2022,9(1):187-204
The essential requirement for precise localization of a self-driving car is a lane-level map which includes road markings (RMs). Obviously, we can build the lane-level map by running a mobile mapping system (MMS) which is equipped with a high-end 3D LiDAR and a number of high-cost sensors. This approach, however, is highly expensive and ineffective since a single high-end MMS must visit every place for mapping. In this paper, a lane-level RM mapping system using a monocular camera is developed. The developed system can be considered as an alternative to expensive high-end MMS. The developed RM map includes the information of road lanes (RLs) and symbolic road markings (SRMs). First, to build a lane-level RM map, the RMs are segmented at pixel level through the deep learning network. The network is named RMNet. The segmented RMs are then gathered to build a lane-level RM map. Second, the lane-level map is improved through loop-closure detection and graph optimization. To train the RMNet and build a lane-level RM map, a new dataset named SeRM set is developed. The set is a large dataset for lane-level RM mapping and it includes a total of 25157 pixel-wise annotated images and 21000 position labeled images. Finally, the proposed lane-level map building method is applied to SeRM set and its validity is demonstrated through experimentation. 相似文献
10.
为明确大豆蛋白纳米纤维的结构形成和扩宽铁强化剂的食品工业应用,以大豆分离蛋白(soy protein isolate,SPI)为原料,通过5 h的酸热处理制备纳米纤维(soy protein isolate fibrils,Fib SPI),系统研究纤维形成前后蛋白结构的变化,并进一步制备铁纳米颗粒(iron nanoparticles,Fe NPs),探究Fib SPI对铁的稳态化作用。研究结果表明:在酸热处理过程中,SPI产生大量的β-折叠结构,其与硫磺素T结合,显示出增强的荧光强度;此外,7S组分先发生降解,利于纤维成核形成,随后11S逐渐被水解,促进纤维生长;同时水解产生大量的小肽组分,提高了产物的还原力。研究进一步利用Fib SPI递送铁纳米颗粒(Fe NPs),发现与原始SPI相比,铁纳米颗粒可在Fib SPI原位形成胶体稳定的铁-大豆蛋白纳米纤维复合物(Fe FibSPI),并以Fe(II)形式存在,其对乳液体系色泽及稳定性的影响较硫酸亚铁或氯化铁小。该研究可为构建新型植物基铁强化剂递送体系提供理论和方法指导。 相似文献