全文获取类型
收费全文 | 496721篇 |
免费 | 44087篇 |
国内免费 | 24248篇 |
专业分类
电工技术 | 33460篇 |
技术理论 | 49篇 |
综合类 | 38270篇 |
化学工业 | 76879篇 |
金属工艺 | 29537篇 |
机械仪表 | 32642篇 |
建筑科学 | 40245篇 |
矿业工程 | 16405篇 |
能源动力 | 13995篇 |
轻工业 | 34720篇 |
水利工程 | 10608篇 |
石油天然气 | 29837篇 |
武器工业 | 4649篇 |
无线电 | 55886篇 |
一般工业技术 | 52276篇 |
冶金工业 | 23482篇 |
原子能技术 | 5265篇 |
自动化技术 | 66851篇 |
出版年
2024年 | 949篇 |
2023年 | 7179篇 |
2022年 | 12649篇 |
2021年 | 19246篇 |
2020年 | 14860篇 |
2019年 | 11770篇 |
2018年 | 13595篇 |
2017年 | 15497篇 |
2016年 | 13999篇 |
2015年 | 20143篇 |
2014年 | 25560篇 |
2013年 | 30659篇 |
2012年 | 34408篇 |
2011年 | 37303篇 |
2010年 | 33688篇 |
2009年 | 31812篇 |
2008年 | 31378篇 |
2007年 | 29766篇 |
2006年 | 29201篇 |
2005年 | 24805篇 |
2004年 | 16979篇 |
2003年 | 14884篇 |
2002年 | 13957篇 |
2001年 | 12356篇 |
2000年 | 11468篇 |
1999年 | 11370篇 |
1998年 | 8366篇 |
1997年 | 7125篇 |
1996年 | 6713篇 |
1995年 | 5622篇 |
1994年 | 4556篇 |
1993年 | 3042篇 |
1992年 | 2417篇 |
1991年 | 1802篇 |
1990年 | 1429篇 |
1989年 | 1165篇 |
1988年 | 932篇 |
1987年 | 571篇 |
1986年 | 458篇 |
1985年 | 268篇 |
1984年 | 219篇 |
1983年 | 158篇 |
1982年 | 150篇 |
1981年 | 110篇 |
1980年 | 133篇 |
1979年 | 67篇 |
1978年 | 26篇 |
1977年 | 32篇 |
1976年 | 38篇 |
1951年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Yunyi Wu Xiaohui Wang Caifu Zhong Longtu Li 《Journal of the European Ceramic Society》2018,38(4):1434-1441
In this work, in order to optimize the electrical performance of (Na0.85K0.15)0.5Bi0.5TiO3 (NKBT) thin films, 20 nm-thick BaTiO3 (BTO) layer was utilized by deliberately coating in the NKBT film-substrate interface or in the NKBT film, i.e., BTO layers coated in sequence with NBKT layers. The BTO layer, especial coated in the NKBT film, was beneficial for crystallization process and more preferable to form a denser film morphology. The BTO-coated NKBT composite films exhibited much enhancement in electrical properties compared to the films without BTO layer. Accordingly, a high effective piezoelectric coefficient d33* of 75 pm/V and remnant polarization Pr of 22.1 μC/cm2, as well as a low leakage current density of 1.2 × 10?5 A/cm2 were obtained in the 460 nm-thick composite film with BTO layers coated in the NBKT film. It meant that this kind of BTO-coated NKBT composite film could perform as a potential candidate for the lead-free piezoelectric applications. The observed enhancement in the electrical properties with the introduction of BTO layer could be mainly explained by the weakened influence of domain pinning in the film-electrode interface and grain boundaries due to the decreased strain in the film-electrode interface and better crystallinity in the highly (110)-oriented NKBT films, thereby enhancing motion of domain-walls. 相似文献
992.
Yiguang Jiang Benxue Jiang Qihua Zhu Nan Jiang Pande Zhang Shuilin Chen Xu Hu Ge Zhang Jintai Fan Liangbi Su Jiang Li Long Zhang 《Journal of the European Ceramic Society》2018,38(5):2404-2409
Different deformation rates of Nd,Y-codoped CaF2 transparent ceramics were prepared by ceramization of single crystals. The deformation rate effects on the crystallization behaviors, microstructures, mechanical properties, and optical performances were investigated for the first time. The results indicate that the comprehensive performances of Nd,Y-codoped CaF2 ceramic (△a?=?62%) are the most optimal compared with other ceramics having different deformation rates (△a?=?34%, 40%, 50%, and 75%). In further investigations of the optical properties, the Nd,Y-codoped CaF2 ceramic (△a?=?62%) sample exhibited a high transparency (Ta?>?91%, 3-mm thick,250?~?1200?nm), low light scattering, superior fracture toughness (K1c?~?0.71?MPa·m1/2), strong fluorescence emission, long lifetime (τ?=?348.72?μs), and broad FWHM (29.2?nm), promising a good candidate for high-power laser material. 相似文献
993.
Zhuang Tang Yuwei Huang Kangning Wu Jianying Li 《Journal of the European Ceramic Society》2018,38(4):1569-1575
Significantly enhanced breakdown field of 24.52 kV cm?1 as well as noteworthy nonlinear coefficient of 8.11 and low dielectric loss of 0.077 were obtained in Ca0.6Sr0.4Cu3Ti4O12 ceramic. It was proved from impedance spectra that improved breakdown field was attributed to enhanced grain boundary resistance and elevated Schottky barrier height, which was further found resulting from reduced donor densities in C-V measurements. In addition, it was found that the activation energy originated from oxygen vacancies was increased, indicating the generation of oxygen vacancies was suppressed. Since oxygen vacancies acted as donors in depletion layers, it is reasonable to deduce that the reduced donor density was mainly ascribed to the decreased oxygen vacancies. In conclusion, maximum integrated action of strong solid solution effect and weak Sr-stretching effect was achieved when Sr/Ca ratio is 40/60, leading to greatly elevated potential barrier height and enhanced breakdown field consequently. 相似文献
994.
Tao Li Zehui Du Nobumichi Tamura Mao Ye Saikumar Inguva Wei Lu Xierong Zeng Shanming Ke Haitao Huang 《Journal of the European Ceramic Society》2018,38(4):1488-1497
(1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 ((1-x)PZN-xPT in short) is one of the most important piezoelectric materials. In this work, we extensively investigated (1-x)PZN-xPT (x = 0.07–0.11) ferroelectric single crystals using in-situ synchrotron μXRD, complemented by TEM and PFM, to correlate microstructures with phase transitions. The results reveal that (i) at 25 °C, the equilibrium state of (1-x)PZN-xPT is a metastable orthorhombic phase for x = 0.07 and 0.08, while it shows coexistence of orthorhombic and tetragonal phases for x = 0.09 and x = 0.11, with all ferroelectric phases accompanied by ferroelastic domains; (ii) upon heating, the phase transformation in x = 0.07 is Orthorhombic → Monoclinic → Tetragonal → Cubic. The coexistence of ferroelectric tetragonal and paraelectric cubic phases was in-situ observed in x = 0.08 above Curie temperature (TC), and (iii) phase transition can be explained by the evolution of the ferroelectric and ferroelastic domains. These results disclose that (1-x)PZN-xPT are in an unstable regime, which is possible factor for its anomalous dielectric response and high piezoelectric coefficient. 相似文献
995.
Chunchun Li Huaicheng Xiang Minyu Xu Ying Tang Liang Fang 《Journal of the European Ceramic Society》2018,38(4):1524-1528
Two low-permittivity dielectric materials Li2AGeO4 (A?=?Zn, Mg) were prepared via the solid-state reaction method. X-ray diffraction analysis and Rietveld refinement indicated that both ceramics crystallize in an orthorhombic olivine structure with a space group Pmn21. Dense ceramics with high relative density and homogeneous microstructure were obtained. Li2ZnGeO4 densified at 1200?°C possessed a relative permittivity εr?=?6.5, a quality factor Q?×?f?=?35,400?GHz, and a temperature coefficient of resonant frequency. Li2MgGeO4 exhibited εr?=?6.1, Q?×?f?=?28,500?GHz, and τf?=?–74.7?ppm/°C when sintered at 1220?°C. Additionally, the large negative τf values of Li2AGeO4 (A?=?Zn, Mg) ceramics were successfully adjusted compensated by forming composite ceramics with CaTiO3 and near-zero τf values of +2.9?ppm/°C and +5.8?ppm/°C were achieved in 0.92Li2ZnGeO4-0.08CaTiO3 and 0.90Li2MgGeO4-0.10CaTiO3, respectively. 相似文献
996.
Core–shell type multi‐arm azide polymers based on hyperbranched copolyether as potential energetic materials in solid propellants 下载免费PDF全文
A series of novel multi‐arm azide copolymers (POGs) with the same hyperbranched poly[3‐ethyl‐3‐(hydroxymethyl)oxetane] core (PEHO‐c) and different content of linear glycidyl azide polymer shell (GAP‐s) have been synthesized by sequential cationic ring‐opening polymerization and azidation. Detailed structural information of these copolyethers was deduced from Fourier transform infrared, 1H NMR and inverse gated decoupled 13C NMR spectroscopies, matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry, gel permeation chromatography and elemental analysis. The molecular weight of POG having GAP‐s and PEHO‐c with a molar ratio 14.95:1 (Rs/c) was around 31 000 g mol?1, far above that of linear GAP (around 4000 g mol?1). The apparent viscosity and glass transition temperature (?51 to ?23 °C) decreased first and then slightly increased with increasing molecular weight. Thermal analysis revealed that all the obtained POGs exhibited excellent resistance to thermal decomposition up to 220 °C. Moreover, the energetic properties, investigated using oxygen bomb calorimetric measurements, indicated that the enthalpy of formation of the POGs was higher than that of general linear GAP, but similar to that of branched GAP under reasonable Rs/c. The compatibilities of the POGs with common materials used in solid propellants were studied using differential scanning calorimetry and the results indicated that the POGs had good compatibility with these materials. © 2017 Society of Chemical Industry 相似文献
997.
Properties of polyamide 6,10/poly(vinyl alcohol) blends and impact on oxygen barrier performance 下载免费PDF全文
Jia‐wei Wu Ya‐qiong Huang Hong‐bo Li James Runt Jen‐taut Yeh 《Polymer International》2018,67(4):453-462
The oxygen transmission rate, average volume of free‐volume cavities (Vf) and fractional free volume (Fv) of polyamide 6,10 (PA610)/poly(vinyl alcohol) (PVA) (i.e. PA610xPVA05y, PA610xPVA08y and PA610xPVA14y) blend films reduced to minimum values when their PVA contents reached corresponding optimal values. Oxygen transmission rate, Vf and Fv values obtained for optimal PA610xPVAzy blown films were reduced considerably with decreasing PVA degrees of polymerization. The oxygen transmission rate of the optimal bio‐based PA61080PVA0520 blown film was only 2.4 cm3 (m2·day·atm)?1, which is about the same as that of the most often used high‐barrier polymer, ethylene–vinyl alcohol copolymer. Experimental findings from dynamic mechanical analysis, differential scanning calorimetry, wide‐angle X‐ray diffraction and Fourier transform infrared spectroscopy of the PA610xPVAzy blends indicate that PA610 and PVA in the blends are miscible to some extent at the molecular level when the PVA contents are less than or equal to the corresponding optimal values. The considerably enhanced oxygen barrier properties of the PA610xPVAzy blend films with optimized compositions are attributed to the significantly reduced local free‐volume characteristics. © 2017 Society of Chemical Industry 相似文献
998.
High‐performance thermosets with tailored properties derived from methacrylated eugenol and epoxy‐based vinyl ester 下载免费PDF全文
Yuehong Zhang Yuzhan Li Vijay Kumar Thakur Zhenhua Gao Jiyou Gu Michael R Kessler 《Polymer International》2018,67(5):544-549
A renewable chemical, eugenol, is methacrylated to produce methacrylated eugenol (ME) employing the Steglich esterification reaction without any solvent. The resulting ME is used as a low‐viscosity co‐monomer to replace styrene in a commercial epoxy‐based vinyl ester resin (VE). The volatility and viscosity of ME and styrene are compared. The effect of ME loading and temperature on the viscosity of the VE–ME resin is investigated. Moreover, the thermomechanical properties, curing extent and thermal stability of the fully cured VE–ME thermosets are systematically examined. The results indicate that ME is a monomer with low volatility and low viscosity, and therefore the incorporation of ME monomer in VE resins allows significant reduction of viscosity. Moreover, the viscosity of the VE–ME resin can be tailored by adjusting the ME loadings and processing temperature to meet commercial liquid molding technology requirements. The glass transition temperatures of VE–ME thermosets range from 139 to 199 °C. In addition, more than 95% of the monomer is incorporated and fixed in the crosslinked network structure of VE–ME thermosets. Overall, the developed ME monomer exhibits promising potential for replacing styrene as an effective low‐viscosity co‐monomer. The VE–ME resins show great advantages for use in polymer matrices for high‐performance fiber‐reinforced composites. This work is of great significance to the vinyl ester industry by providing detailed experimental support. © 2018 Society of Chemical Industry 相似文献
999.
1000.
目的: 建立快速灵敏的LC-MS/MS法测定人血浆中阿莫西林浓度,并用于两种阿莫西林胶囊的一致性评价。方法: 采用岛津公司LCMS-8060型LC-MS/MS仪,以MRM模式测定阿莫西林(m/z 366.00/114.00)的浓度,d4-阿莫西林作内标(m/z 370.10/114.05),离子源为ESI源。色谱柱选用Waters ACQUITY BEH C18(2.1×50 mm,1.7 μm),梯度洗脱。血浆样本加入内标,经甲醇沉淀蛋白后取上清液进样检测。结果: 所建方法经验证,其线性、准确度、精密度、最低定量限、提取回收率、特异性、基质效应、稳定性等各项指标均符合CFDA的指导原则及最新核查标准要求,并较文献报道中的方法有处理简单、灵敏度高、色谱峰形好的优点。结论: 所建方法快速、灵敏,适用于人血浆中阿莫西林浓度的检测。用于一致性评价的样本实测,两种制剂生物等效。 相似文献