全文获取类型
收费全文 | 79689篇 |
免费 | 6337篇 |
国内免费 | 4903篇 |
专业分类
电工技术 | 3018篇 |
技术理论 | 1篇 |
综合类 | 4329篇 |
化学工业 | 26178篇 |
金属工艺 | 7706篇 |
机械仪表 | 2570篇 |
建筑科学 | 4022篇 |
矿业工程 | 1097篇 |
能源动力 | 2182篇 |
轻工业 | 6611篇 |
水利工程 | 543篇 |
石油天然气 | 4273篇 |
武器工业 | 609篇 |
无线电 | 7420篇 |
一般工业技术 | 8992篇 |
冶金工业 | 3251篇 |
原子能技术 | 1564篇 |
自动化技术 | 6563篇 |
出版年
2024年 | 221篇 |
2023年 | 1210篇 |
2022年 | 2554篇 |
2021年 | 4163篇 |
2020年 | 2468篇 |
2019年 | 2093篇 |
2018年 | 1830篇 |
2017年 | 2341篇 |
2016年 | 2883篇 |
2015年 | 2914篇 |
2014年 | 3953篇 |
2013年 | 4474篇 |
2012年 | 4864篇 |
2011年 | 6049篇 |
2010年 | 4390篇 |
2009年 | 5116篇 |
2008年 | 4356篇 |
2007年 | 4877篇 |
2006年 | 4560篇 |
2005年 | 3732篇 |
2004年 | 3085篇 |
2003年 | 2934篇 |
2002年 | 2465篇 |
2001年 | 2047篇 |
2000年 | 1809篇 |
1999年 | 1523篇 |
1998年 | 1144篇 |
1997年 | 928篇 |
1996年 | 840篇 |
1995年 | 749篇 |
1994年 | 668篇 |
1993年 | 512篇 |
1992年 | 458篇 |
1991年 | 359篇 |
1990年 | 338篇 |
1989年 | 232篇 |
1988年 | 185篇 |
1987年 | 162篇 |
1986年 | 185篇 |
1985年 | 155篇 |
1984年 | 153篇 |
1983年 | 110篇 |
1982年 | 111篇 |
1981年 | 124篇 |
1980年 | 116篇 |
1979年 | 99篇 |
1977年 | 66篇 |
1976年 | 69篇 |
1975年 | 81篇 |
1974年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
71.
为了系统地评价胸腺五肽作为辅助药物治疗各种肺癌的疗效及其对机体免疫功能的影响,利用电子检索收集有关胸腺五肽联合放疗或化疗方案治疗肺癌的临床随机对照试验文献,对符合纳入标准的文献,采用RevMan5.3 软件进行系统评价。最终共纳入文献 9 篇,总样本量 784 例。Meta 分析结果表明,胸腺五肽作为辅助药物治疗各种肺癌提高总有效率的差异无统计学意义[OR = 1.44, 95%CI(0.99, 2.10), P =0.06 > 0.05]。在对免疫功能的影响方面,胸腺五肽的使用显著增高外周血中的 CD3+ 细胞水平[OR = 5.88, 95% CI(2.34, 9.42), P =0.001],CD4+ 细胞水平也显著上升[OR =8.32, 95%CI(5.22, 11.42), P < 0.00001] , CD4+ /CD8+比值也有明显的提高[OR = 0.38, 95% CI(0.18, 0.59), P=0.0002],但 CD8+ 细胞水平的差异无统计学意义[OR =-3.12, 95% CI ( -9.02, 2.79), P >0.05]。总的来说,本研究在一定程度上反映了在辅助治疗肺癌方面,胸腺五肽能显著提高外周血中的 CD3+ 细胞水平、CD4+ 细胞水平、CD4+/CD8+ 比值。而对于治疗的有效率、CD8+ 细胞水平,差异无统计学意义。 相似文献
72.
The need for feature selection and dimension reduction is felt as a fundamental step in security assessment of large power systems in which the number of features representing the state of power grids dramatically increases. These large amounts of attributes are not proper to be used for computational intelligence (CI) techniques as inputs, because it may lead to a time consuming procedure with insufficient results and they are not suitable for on-line purposes and updates.This paper proposes a combined method for an online voltage security assessment in which the dimension of the token data from phasor measurement units (PMUs) is reduced by principal component analysis (PCA). Then, the features with different stability indices are put into several categories and feature selection is done by correlation analysis in each category. These selected features are then given to decision trees (DTs) for classification and security assessment of power systems.The method is applied to 39-bus test system and a part of Iran power grid. It is seen from the results that the DTs with reduced data have simpler splitting rules, better performance in saving time, reasonable DT error and they are more suitable for constant updates. 相似文献
73.
74.
A highly transparent and thermally stable copolymer of 1‐adamantyl methacrylate and styrene
下载免费PDF全文
![点击此处可从《Polymer International》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Thermal and optical properties of copolymers of 1‐adamantyl methacrylate (AdMA) and styrene (St) prepared by free radical polymerization in the bulk are investigated. The copolymer forms an azeotrope when the composition is AdMA/St = 55/45 mol%. The glass transition temperature and decomposition temperature of the azeotropic copolymer are 170 and ca 340 °C, respectively. The refractive index increases nonlinearly with St content from 1.522 to 1.591. The light scattering loss at 633 nm is 28.1 dB km?1, which is less than half of that of polystyrene. The total optical loss including molecular vibrational absorption, which is evaluated using a copolymer‐based optical fiber, is 292–645 dB km?1 at 500–700 nm. These values correspond to transmittances of 86–93% for a 1 m optical path length. © 2014 Society of Chemical Industry 相似文献
75.
76.
Peng Jiang Jiafeng Lu Kun Li Xiquan Chen Ruiqi Dan 《Fullerenes, Nanotubes and Carbon Nanostructures》2020,28(5):381-386
AbstractPreparation condition can affect the structure and the properties of nanofiber membrane. In order to explore suitable conditions to prepare the Fe3O4/PVDF nanofiber membrane with good hydrophobicity, the hydrophobicity of Fe3O4/PVDF nanofiber membranes obtained by electrospinning was investigated by changing preparation conditions like weight percentage of Fe3O4 nanoparticles, blending quality concentration of poly (vinylidene fluoride) (PVDF) and Fe3O4 nanoparticles, and positive voltage. And the variations of hydrophobicity of Fe3O4/PVDF nanofiber membranes modified by 1H, 1H, 2H, 2H-perfluorodecyl trimethoxysilane were studied. The results show that the hydrophobicity of Fe3O4/PVDF nanofiber membranes has changed under different preparation conditions. The contact angles of samples increased after a modification by 1H, 1H, 2H, 2H-perfluorodecyl trimethoxysilane, which indicates that the hydrophobicity of Fe3O4/PVDF nanofiber membranes has been enhanced. 相似文献
77.
Han Yan Gu Yi Chen Lei Li Bing Liu Xiang Ming Chen 《International Journal of Applied Ceramic Technology》2020,17(2):790-796
SrLa[Ga1−x(R0.5Ti0.5)x]O4 (R = Mg, Zn) ceramics were prepared by a standard solid state sintering method. The single-phase ceramics with K2NiF4-type layered perovskite structure and I4/mmm space group were obtained, indicating that SrLa(R0.5Ti0.5) and SrLaGaO4 can form the unlimited solid solutions. With increasing x for R = Mg and Zn, εr increases monotonously, the Qf value first increases and then decreases, while τf increases from a negative to a positive value. The optimized microwave dielectric properties were obtained as following: εr = 23.3, Qf = 89 400 GHz, τf = −0.8 ppm/°C for SrLa[Ga0.6(Mg0.5Ti0.5)0.4]O4 and εr = 23.3, Qf = 76 200 GHz, τf = 0.2 ppm/°C for SrLa[Ga0.7(Zn0.5Ti0.5)0.3]O4, indicating that the present solid solution ceramics are the promising candidates as microwave resonator materials for the telecommunication applications. 相似文献
78.
F. Schröder R. M. Ward A. R. Walpole R. P. Turner M. M. Attallah J.-C. Gebelin 《Materials Science & Technology》2015,31(3):372-384
Linear friction welding of the Ti6Al4V alloy is studied. A new definition of the energy input rate is proposed, based on an integration over time of the in-plane force and velocity; a strong correlation with the upset rate is then found. The effective friction coefficient is estimated to be 0·5±0·1 for varying frequencies and amplitudes, with only a weak dependence on the processing conditions displayed. A model is proposed that accounts for both the conditioning and equilibrium stages of the process, which is shown to be in good agreement with the experimental data. The model is used to study the mechanism by which the flash is formed. A criterion is proposed by which the rippled nature of its morphology can be predicted. 相似文献
79.
《Ceramics International》2020,46(3):2868-2876
In order to improve the stability of PZT-based sensors, the mechanical, dielectric, ferroelectric and piezoelectric properties of PZT-5H under impact load were studied experimentally by using the separated Hopkinson pressure bar (SHPB) with an electrical output measurement device. At the same time, the experimental study on the material properties of PZT-5H before and after the impact was carried out. The effect of impact cracks on the output voltage of PZT-5H was also analyzed. The results show that the dynamic piezoelectric constants of PZT-5H under low stress impact (10–50 MPa) are different from those under quasi-static state, and the empirical relationship between them and the peak stress is obtained through experiments. The dielectric properties of PZT-5H did not change under low stress impact, but micro-cracks occurred in the material and dielectric loss increased at high frequencies. Under short circuit, the residual polarization intensity of PZT-5H decreases sharply due to impact load. While the impact load causes the secondary polarization and the increase of the residual polarization intensity of PZT under open circuit. When the stress is over 45 MPa, the PZT-5H breaks. The formation of cracks causes abnormal discharge voltage and gap discharge. 相似文献
80.