全文获取类型
收费全文 | 6168篇 |
免费 | 222篇 |
国内免费 | 253篇 |
专业分类
电工技术 | 106篇 |
综合类 | 200篇 |
化学工业 | 404篇 |
金属工艺 | 182篇 |
机械仪表 | 266篇 |
建筑科学 | 272篇 |
矿业工程 | 43篇 |
能源动力 | 237篇 |
轻工业 | 168篇 |
水利工程 | 8篇 |
石油天然气 | 74篇 |
武器工业 | 24篇 |
无线电 | 653篇 |
一般工业技术 | 429篇 |
冶金工业 | 41篇 |
原子能技术 | 1466篇 |
自动化技术 | 2070篇 |
出版年
2023年 | 26篇 |
2022年 | 63篇 |
2021年 | 61篇 |
2020年 | 88篇 |
2019年 | 81篇 |
2018年 | 67篇 |
2017年 | 118篇 |
2016年 | 148篇 |
2015年 | 121篇 |
2014年 | 230篇 |
2013年 | 236篇 |
2012年 | 265篇 |
2011年 | 432篇 |
2010年 | 314篇 |
2009年 | 303篇 |
2008年 | 281篇 |
2007年 | 354篇 |
2006年 | 345篇 |
2005年 | 309篇 |
2004年 | 266篇 |
2003年 | 287篇 |
2002年 | 226篇 |
2001年 | 167篇 |
2000年 | 188篇 |
1999年 | 155篇 |
1998年 | 151篇 |
1997年 | 140篇 |
1996年 | 149篇 |
1995年 | 133篇 |
1994年 | 104篇 |
1993年 | 96篇 |
1992年 | 91篇 |
1991年 | 81篇 |
1990年 | 66篇 |
1989年 | 60篇 |
1988年 | 55篇 |
1987年 | 41篇 |
1986年 | 53篇 |
1985年 | 41篇 |
1984年 | 45篇 |
1983年 | 35篇 |
1982年 | 30篇 |
1981年 | 25篇 |
1980年 | 20篇 |
1979年 | 17篇 |
1978年 | 18篇 |
1977年 | 15篇 |
1976年 | 14篇 |
1974年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有6643条查询结果,搜索用时 15 毫秒
991.
Electricity load forecasting plays an important role in the strategy management of electricity power system. Learning methods such as artificial neural networks, and more recently, support vector regression machines (SVR) have been introduced to this field. In practices we often expect a fast forecasting, while standard algorithms based on the whole data set are time consuming. To this end, in this paper we introduce local learning strategy considering only a subset of candidates in the neighborhood of the test point and present a general form for it. Concretely, we consider the combination of KNN and ε-SVR for its powerful generalization ability and simplicity. As for model optimizing, Pattern Search method is used for model selection and multiple kernels are developed to improve the performance. Intensive experiments on a real world electricity load forecasting have been carried out and the results show that our methods can improve the performance at a reduced computation cost. Consequently local learning strategy provides a promising alternative for fast electricity load forecasting. 相似文献
992.
本文从中国木雕工艺发展历史入手,逐一解析了中国木雕主要流派的造型特点、雕刻技艺、雕刻题材,从历史和文化的角度阐述了江西余江宗教造像工艺木雕的形成与发展,并揭示了其深层次的特征及内涵. 相似文献
993.
《Ceramics International》2022,48(1):754-759
Thermal control coatings (TCCs) are an essential part of the thermal control systems in the spacecraft. Solar absorptance and emittance are the key performance parameters of TCCs. To develop an ultra-low solar absorption and stable inorganic TCCs for surface radiator, different TCCs were prepared by co-sintering ZnO and SiO2 nanoparticles to form Zn2SiO4/SiO2 pigment in this work, and the optical properties and radiation stability were systematically studied. It is found that the coating based on composite pigment has high reflectivity in the ultraviolet band and excellent optical performance possessing the low solar absorption of 0.06. In addition, the Zn2SiO4/SiO2 coating demonstrates the highest proton and electron radiation stability because that SiO2 between Zn2SiO4 particles acts as the relaxation center of the defects caused by radiation. 相似文献
994.
《International Journal of Hydrogen Energy》2019,44(58):30732-30742
A highly durable proton exchange membranes (PEM)s based on covalently supported ionic liquid (IL) bearing sulfonic acid imidazolium groups were successfully fabricated. The membrane preparation involved radiation induced grafting of 1-vinyl imidazole (1-VIm) onto poly(ethylene-co-tetraflouroethene) (ETFE) film, followed by covalent immobilization of 3-sulfopropyl and subsequent treatment with trifluoromethanesulfonic acid. The ionic conductivity of the supported IL membranes was increased with the increase in the concentration of IL and reached a maximum value of 138 mS cm−1 in a fully hydrated state with an ion exchange capacity of 4.82 mmol g−1 that is higher than Nafion with a similar thickness. The membranes displayed excellent chemical and mechanical stability. In addition, the dimensional and thermal stability of supported IL-membranes were significantly higher than commercial Nafion membranes. 相似文献
995.
This paper proposes a recovery plan for managing disruptions in a three-stage production-inventory system under a mixed production environment. First, a mathematical model is developed to deal with a disruption at any stage while maximizing total profit during the recovery-time window. The model is solved after the occurrence of a disruption event, with changed data used to generate a revised plan. We also propose a new and efficient heuristic for solving the developed mathematical model. Second, multiple disruptions are considered, where a new disruption may or may not affect the recovery plans of earlier disruptions. The heuristic, developed for a single disruption, is extended to deal with a series of disruptions so that it can be implemented for disruption recovery on a real-time basis. We compare the heuristic solutions with those obtained by a standard search algorithm for a set of randomly generated disruption test problems, and that show the consistent performance of our developed heuristic with lower computational times. Finally, some numerical examples and a real-world case study are presented to demonstrate the benefits and usefulness of our proposed approach. 相似文献
996.
M.S. AlBuriahi H.H. Hegazy Faisal Alresheedi I.O. Olarinoye H. Algarni H.O. Tekin H.A. Saudi 《Ceramics International》2021,47(5):5951-5958
This research article aims to study the effect of CdO addition on the radiation shielding characteristics of boro-tellurite glasses in the composition of 50B2O3 - (50-x) TeO2- xCdO, where x = 0, 10, 20, 30, 40 and 50 mol%. These glasses were exposed to gamma radiation and the transmitted gamma photons were evaluated for energies varying from 15 keV to 15 MeV using Geant4 simulation toolkit. The number of transmitted photons was then used to characterize the gamma shielding for the studied glasses in terms of linear/mass attenuation coefficients, MFP, Zeff, and HVL. The simulation outcomes were theoretically confirmed by using Phy-X software. The beta (electron) shielding characterization of the involved glasses was also investigated by determining the projectile range and stopping power using ESTAR software. Additionally, the fast neutron shielding characterization of the glasses was achieved by evaluating removal cross-section (ΣR). The results reveal that the CdO has a small influence on the shielding performance of the boro-tellurite glasses against gamma, beta, and neutron radiations. The shielding performance of the boro-tellurite glasses was compared with that of common shielding materials in terms of MFP. It can be concluded that the boro-tellurite glasses regardless of the concentration of CdO content have promising shielding performance to be used for radiation applications. 相似文献
997.
《Ceramics International》2022,48(14):20041-20052
The growing demand for radiation-resistant optical glasses for space and nuclear radiation applications has attracted significant research interest. However, radiation-resistant fluorophosphate glasses have been poorly studied. In this work, we report on the tailoring and performance of radiation-resistant fluorophosphate glasses that contained cerium through codoping with Sb2O3 and Bi2O3. The physical properties, optical properties, microstructure, and defects of fluorophosphate glasses were investigated using transmittance measurements, absorption measurements, as well as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. The results showed that the radiation resistance of all codoped fluorophosphate glasses was better than the undoped cerium-containing fluorophosphate glasses after 10–250 krad(Si) irradiation. Especially in glasses doped with Bi2O3, the optical density increment at 385 nm was only 0.1482 after 250 krad(Si) irradiation. The CeO2 prevented the development of phosphate-related oxygen hole center (POHC) defects, whereas further codoping with Bi2O3 suppressed the formation of oxygen hole center (OHC) and POEC defects, reducing the breaking of phosphate chains caused by CeO2. Bi3+ is more likely than Sb3+ to change the valence, affecting the transition equilibrium of intrinsic defects and reducing the concentration of defects produced by irradiation. When codoping with Sb2O3 and Bi2O3, Bi2O3 does not enhance radiation resistance owing to the scission effect of Sb2O3 on the phosphate chain, which is not conducive to the radiation resistance of glasses. This indicates that the cerium-containing fluorophosphate glasses doped with Bi2O3 can effectively suppress the defects caused by irradiation and improve the radiation resistance of the glasses. 相似文献
998.
Meijie Zhang Miaolin He Huazhi Gu Ao Huang Wuguo Xiang 《Ceramics International》2018,44(16):19319-19329
The microstructures of porous alumina materials with different porosities were established by introducing the departure factor of pore position and acentric factor of pore diameter to describe the distribution of pores in space and in size, respectively. The contribution of radiation and influence of pore distribution on the equivalent thermal conductivity were discussed based on numerical simulations by the finite volume method (FVM) considering both thermal conduction and radiation. When the pore diameter was less than 10?µm, the radiation component was less than 2%, and radiation could be neglected. Radiative heat transfer played a dominant role for materials with high porosity and large pore size at high temperatures. For micro pore materials (<?100?µm), broad pore size and non-uniform pore space distribution decreased the thermal conductivity across the entire temperature range. For materials with macro pores (>1?mm), broad pore distribution decreased the thermal conductivity at low temperatures and increased it at high temperatures. The basic prediction model of effective thermal conductivity for a two-component material, the Maxwell–Eucken model (ME1) and its modified model were corrected by introducing the pore structure factor. The results from experiments prove that the numerical values were satisfactory. 相似文献
999.
针对人脸识别因光照、姿态、表情、遮挡及噪声等多种因素的影响而导致的识别率不高的问题,提出一种加权信息熵(IEw)与自适应阈值环形局部二值模式(ATRLBP)算子相结合的人脸识别方法(IE (w) ATR-LBP)。首先,从原始人脸图像分块提取信息熵,得到每个子块的IEw;然后,利用ATRLBP算子分别对每个人脸子块提取特征从而得到概率直方图;最后,将各个块的IEw与概率直方图相乘,再串联成为原始人脸图像最后的特征直方图,并利用支持向量机(SVM)对人脸进行识别。在AR人脸库的表情、光照、遮挡A和遮挡B四个数据集上,IE (w) ATR-LBP方法分别取得了98.37%、94.17%、98.20%和99.34%的识别率。在ORL人脸库上,IE (w) ATR-LBP方法的最大识别率为99.85%;而且在ORL人脸库5次不同训练样本的实验中,与无噪声时相比,加入高斯和椒盐噪声后的平均识别率分别下降了14.04和2.95个百分点。实验结果表明,IE (w) ATR-LBP方法能够有效提高人脸在受光照、姿态、遮挡等影响时的识别率,尤其是存在表情变化及脉冲类噪声干扰时的识别率。 相似文献
1000.
《Digital Communications & Networks》2019,5(1):34-39
This paper proposes a novel Multiple-Input Multiple-Output (MIMO) transmission scheme based on Pattern Recognition (PR), which is termed as the PR aided Transmission Antenna Selection MIMO (PR-TAS aided MIMO). As the conventional TAS algorithms need to search all possible legitimate antenna subsets, they may impose some redundant calculations. In order to avoid this problem, we employ some pattern recognition methods to carry out the TAS algorithm in this paper. To be specific, two PR algorithms, namely the K-Nearest Neighbor (KNN) algorithm and the Support Vector Machine (SVM) algorithm, are introduced and redesigned to obtain a TAS with lower complexity but higher efficiency. Moreover, in order to improve the performance of the SVM, we propose a new feature extraction of channel matrix for the TAS. Our simulation results show that the proposed KNN and SVM based PR-TAS algorithms are capable of striking a flexible tradeoff between the complexity and the Bit Error Rate (BER), and the new feature can effectively improve the BER performance compared with the conventional feature extraction method. 相似文献