全文获取类型
收费全文 | 33458篇 |
免费 | 2121篇 |
国内免费 | 1465篇 |
专业分类
电工技术 | 1853篇 |
综合类 | 1862篇 |
化学工业 | 6489篇 |
金属工艺 | 3199篇 |
机械仪表 | 2314篇 |
建筑科学 | 2168篇 |
矿业工程 | 1392篇 |
能源动力 | 3125篇 |
轻工业 | 1720篇 |
水利工程 | 650篇 |
石油天然气 | 1072篇 |
武器工业 | 290篇 |
无线电 | 2116篇 |
一般工业技术 | 4922篇 |
冶金工业 | 1070篇 |
原子能技术 | 554篇 |
自动化技术 | 2248篇 |
出版年
2024年 | 99篇 |
2023年 | 594篇 |
2022年 | 956篇 |
2021年 | 1010篇 |
2020年 | 1067篇 |
2019年 | 945篇 |
2018年 | 963篇 |
2017年 | 1141篇 |
2016年 | 1201篇 |
2015年 | 1286篇 |
2014年 | 1970篇 |
2013年 | 2472篇 |
2012年 | 1960篇 |
2011年 | 2794篇 |
2010年 | 2029篇 |
2009年 | 2052篇 |
2008年 | 1891篇 |
2007年 | 2026篇 |
2006年 | 1667篇 |
2005年 | 1445篇 |
2004年 | 1235篇 |
2003年 | 1007篇 |
2002年 | 830篇 |
2001年 | 640篇 |
2000年 | 611篇 |
1999年 | 564篇 |
1998年 | 461篇 |
1997年 | 437篇 |
1996年 | 346篇 |
1995年 | 277篇 |
1994年 | 236篇 |
1993年 | 148篇 |
1992年 | 127篇 |
1991年 | 108篇 |
1990年 | 93篇 |
1989年 | 76篇 |
1988年 | 62篇 |
1987年 | 30篇 |
1986年 | 36篇 |
1985年 | 32篇 |
1984年 | 24篇 |
1983年 | 26篇 |
1982年 | 29篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1975年 | 2篇 |
1960年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Complex structural contribution of the morphotropic phase boundary in Na0.5Bi0.5TiO3 - CaTiO3 system
Roy Roukos Sara Abou Dargham Jimmy Romanos Fatima Barakat Denis Chaumont 《Ceramics International》2019,45(4):4467-4473
The correlation between structure and dielectric properties of lead-free (1-x)Na0.5Bi0.5TiO3 - xCaTiO3 ((1-x)NBT - xCT) polycrystalline ceramics was investigated systematically by X-ray diffraction, combined with impedance spectroscopy for dielectric characterizations. The system shows high miscibility in the entire composition range. A morphotropic phase boundary (MPB), at 0.09?≤?x?<?0.15 was identified where rhombohedral and orthorhombic symmetries coexist at room temperature. The fraction of orthorhombic phase increases gradually with x in the MPB region. Dielectric measurements reveal that the relative permittivity increase with addition of Ca2+. This behavior is unusual with this kind of doping. A thermal hysteresis occurred only in the MPB composition which varies in a non-monotonically manner with x, detected by dielectric properties. This phenomenon is related to the crystalline microstructure by a linear relationship between the fraction of each phase and dielectric properties, and, more precisely, to the strong interaction between rhombohedral and orthorhombic phases. 相似文献
993.
Yongjie Zhang Shaoxiang Liang Yi Zhang Rulin Li Zhidong Fang Shuai Wang Hui Deng 《Ceramics International》2021,47(2):1855-1864
Understanding surface kinetics of SiO2 growth on single crystal SiC at elevated temperatures is crucial to fabricate high-performance SiC-based devices. However, the role of oxygen in the evolution mechanism of SiC surface at atomic scale has not been comprehensively elaborated. Here, we reveal the manipulation effect of oxygen on the competitive growth of thermal oxidation SiO2 (TO-SiO2) and thermal chemical vapor deposition SiO2 (TCVD-SiO2) on the 4H-SiC substrate at 1500 °C. TO-SiO2 is formed by the thermal oxidation of SiC, in which the substrate undergoes layer-by-layer oxidation, resulting in an atomically flat SiC/TO-SiO2 interface. TCVD-SiO2 growth includes the sublimation of Si atoms, the reaction between sublimated Si atoms and reactive oxygen, and the adsorption of gaseous SixOy species. A relatively high sublimation rate of Si atoms at SiC atomic steps causes the transverse evolution of the nucleation sites, leading to the formation of nonuniform micron-sized pits at the SiC/TCVD-SiO2 interface. The low oxygen concentration favors TCVD-SiO2 growth, whose crystal quality is much better than that of TO-SiO2 due to the high surface mobility in the thermal CVD process. We further achieve the epitaxial growth of graphene on 4H-SiC in an almost oxygen-free reaction atmosphere. Additionally, ReaxFF reactive molecular dynamic simulation results illustrate that the decrease in oxygen concentration can promote the growth kinetics of SiO2 on single crystal SiC from being dominated by thermal oxidation to being dominated by thermal CVD. 相似文献
994.
《International Journal of Hydrogen Energy》2020,45(30):15281-15293
In this work, absorption/desorption isotherms of the LaNi3·6Mn0·3Al0·4Co0.7 alloy, have been determined from the experimental data at three temperatures (TFluid = 298 K, TFluid = 303 K, and TFluid = 313 K). However, the experimental isotherms are compared with a proposed theoretical model. The physicochemical parameters of the proposed model are determined from the experimental data. Using these parameters, the absorption and desorption processes of hydrogen by the LaNi3·6Mn0·3Al0·4Co0.7 alloy can be compared. During the absorption/desorption process, the behaviors of each parameter are studied under the effect of temperature and pressure. In addition, internal energy, entropy, and enthalpy are calculated by using the proposed model. On the other hand, the temperature and pressure effects on the variation of these functions have been studied. The calculated physicochemical parameters suggested that the hydrogen absorption/desorption process in the LaNi3·6Mn0·3Al0·4Co0.7 alloy was feasible, spontaneous and exothermic in nature. 相似文献
995.
Prakhar Jain Shubham Bauskar Manasi Gyanchandani 《International journal of imaging systems and technology》2020,30(1):112-125
Detection of anemia can be done by examining the hemoglobin concentration level in the blood using complete blood count, which is an invasive, time-consuming, and costly technique. Preliminary methods for detecting anemia include examining the color of the palpebral conjunctiva, which is a non-invasive method, but color perception may vary from person to person. This study aims to develop a computerized non-invasive technique for anemia detection. We propose a novel machine learning model using the artificial neural network to detect anemic patients from the images of eye conjunctiva. Since limited and small dataset has been used in the earlier approaches, this may cause over fitting of the model. We have improved the number of available training images using image augmentation techniques. To standardize a non-invasive method, we have used computer vision algorithms for preprocessing and feature extraction. This article derives the backpropagation rules mathematically for adjusting the weights for the proposed neural network model. After hyper parameter tuning and using the mathematically derived backpropagation rules, the model was able to achieve the best accuracy of 97.00% with sensitivity 99.21% and specificity 95.42% on the created dataset. 相似文献
996.
《Calphad》2020
The X-ray diffraction, Scanning Electron Microscopy, Differential Scanning Calorimetry, dilatometric and electrical conductivity measurements were used to study the structural and physicochemical properties of selected silver-rich alloys from Ag–Al system. All the studied alloys, containing from 10 to 37 at. % of Al (Ag90Al10, Ag85Al15, Ag77Al23, Ag75Al25, Ag72Al28, Ag70Al30, Ag63Al37), were prepared from high purity metals by melting in a glove-box filled with a high purity argon atmosphere. The obtained X-ray diffraction patterns and microstructure observation of alloys containing up to 15 at. % of Al suggested that in this range only solid solution of silver exists. The thermal analysis showed heat effects related to phase transitions in Ag–Al system. In addition, the thermal expansion studies revealed an anomalous behavior in expansion for some composition of alloys associated with the phase transition. The electrical conductivity values rapidly changed, which may be associated with the formation of different phase areas in the Ag–Al system.Based on the results obtained in this work and critically reviewed literature data a thermodynamic re-optimization of the binary Ag–Al system using CALPHAD method was proposed. A good agreement between calculation and experiment was found. 相似文献
997.
BackgroundThe consumers’ trend toward naturalness and “clean-label” products advocates the development of “bio-mediated” tools including new processes for the generation of flavors. Today, many fundamental studies demonstrate the feasibility of producing individual flavor compounds or more complex flavoring preparations by fermentation or by enzymatic reactions. However, to turn research into industrial applications, the processes have to be simplified and optimized by combining chemistry, biology and process engineering know-how.Scope and approachThis review summarises recent basic research and development on cell and enzyme based formation of volatile flavors with focus on smart combinations of biocatalytic and thermal steps to enrich the natural flavor profile of foods. Ideally, targeted bioconversion of specific raw materials and ingredients will release flavor precursors required to generate the desired flavor profile by appropriate thermal processing.Key findings and conclusionsThe combination of fermentation or enzymatic treatment of raw materials with heat-induced food processes (e.g. drying, extrusion, roasting) represent an elegant approach in industrial food processing to generate flavors under mild conditions. This requires a good control of fermentation or enzymatic reaction steps to produce suitable substrates at the optimal concentrations adapted to the thermal processes. Using traditional cooking and minimal processing conditions (nature-inspired strategies) has become an attractive approach to generate authentic flavor profiles resonating with consumers’ demand for more naturalness. 相似文献
998.
Aisha Khatoon Muhammad Zargham Khan Zain ul Abidin Sheraz Ahmed Bhatti 《Food additives & contaminants. Part A, Chemistry, analysis, control, exposure & risk assessment》2018,35(3):538-545
A presence of mycotoxins in feed is one of the most alarming issues in the poultry feed industry. Ochratoxins, produced by several Aspergillus and Penicillium species, are important mycotoxin regarding the health status of poultry birds. Ochratoxins are further classified into to several subtypes (A, B, C, etc) depending on their chemical structures, but ochratoxin A (OTA) is considered the most important and toxic. Bentonite clay, belonging to phyllosilicates and formed from weathering of volcanic ashes, has adsorbent ability for several mycotoxins. The present study was designed to study the effects of bentonite clay upon OTA-induced immunosuppression in broiler chicks. For this, 480 day-old broiler chicks were procured from a local hatchery and then different combinations of OTA (0.15, 0.3, or 1.0 mg/kg) and bentonite clay (5, 10, and 20 g/kg) were incorporated into their feed. At 13, 30, and 42 days of age, parameters such as antibody responses to sheep red blood cells, in situ lymphoproliferative responses to mitogen (PHA-P), and in situ phagocytic activity (i.e., via carbon clearance) were determined respectively. The results indicated there was a significant reduction of total antibody and immunoglobulin titres, lymphoproliferative responses, and phagocytic potential in OTA-treated birds, suggesting clear immunosuppression by OTA in birds in a dose-dependent manner. These results were also significantly lower in all combination groups (OTA with bentonite clay), suggesting few to no effects of feeding bentonite clay upon OTA- induced alterations in different immune parameters. 相似文献
999.
《中国稀土学报(英文版)》2022,40(9):1480-1487
Low cost and high abundance rare earth elements Y and Ce have attracted tremendous interests of the industrial and scientific societies for fabricating the highly cost-performance efficient rare earth permanent magnets. However, the effect of separate replacement of Nd by Y or Ce on the performances of NdFeB-type magnet under the same atomic ratio and preparation conditions is still unclear. In this work, we systematically investigated the magnetic properties, thermal stabilities and service performances of (Nd0.8Y0.2)13.80FebalAl0.24Cu0.1B6.04 (atomic fraction, denoted as 20Y) and (Nd0.8Ce0.2)13.80FebalAl0.24Cu0.1B6.04 (atomic fraction, denoted as 20Ce) magnets. The results demonstrate that the μ0Mr, μ0Hc and (BH)max of 20Y magnet are respectively 1.325 T, 1.173 T and 343.467 kJ/m3, which are comprehensively higher than those of 20Ce magnet (μ0Mr = 1.310 T, μ0Hc = 0.948 T, (BH)max = 321.105 kJ/m3). Moreover, the 20Y magnet has higher thermal stability compared with 20Ce magnet which is favorable for the magnetic performances at elevated temperatures. The investigation of microstructure and elemental distribution indicates that the excellent magnetic performances of NdY-Fe-B magnet can be attributed not only to the preferable intrinsic properties 4πMs, Ha and Tc of Y2Fe14B, but also to the in-situ core–shell structure of the 2:14:1 matrix phase grain with Y-rich core and Nd-rich shell, along with the thicker grain boundary layer between the adjacent matrix phase grains in NdY-Fe-B magnet. Furthermore, the 20Y magnet exhibits better mechanical property and higher corrosion resistance than 20Ce magnet, which are helpful for prolonging the service life of the magnet in practical application. 相似文献
1000.
Research on the hot surface ignition of hydrogen‐air mixture under different influencing factors
下载免费PDF全文
![点击此处可从《国际能源研究杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
To further reveal the pre‐ignition characteristics of hydrogen internal combustion engine, the effect of hot surface characteristic parameters on the ignition characteristics of hydrogen‐air mixture was investigated in this research. Based on the prototype of the constant volume combustion bomb with an overhead glow plug, the duration from the heating of the hot surface to the combustion of hydrogen‐air mixture, the so‐called heating duration, was firstly researched under different fuel‐air equivalence ratio, initial temperature, initial pressure, hot surface temperature, and hot surface area, and the influence of each factor on the heating duration was analyzed. The results show that the order of the effect of each factor on the hot surface ignition is as follows: hot surface temperature > initial pressure of hydrogen‐air mixture > equivalent ratio > initial temperature of hydrogen‐air mixture > hot surface area. The influence of the hot surface characteristic parameters on the heating duration was further analyzed in detail. On this basis, the relationship among the critical ignition temperature, the heating duration and the hot surface area was researched and established. The results show that the heating duration is the only major factor affecting the critical ignition temperature. Finally, the research results were applied to analysis the pre‐ignition in hydrogen internal combustion engine. 相似文献