全文获取类型
收费全文 | 220852篇 |
免费 | 4300篇 |
国内免费 | 1671篇 |
专业分类
电工技术 | 5326篇 |
技术理论 | 3篇 |
综合类 | 1573篇 |
化学工业 | 30687篇 |
金属工艺 | 8841篇 |
机械仪表 | 6994篇 |
建筑科学 | 5752篇 |
矿业工程 | 1674篇 |
能源动力 | 5426篇 |
轻工业 | 16019篇 |
水利工程 | 2155篇 |
石油天然气 | 4299篇 |
武器工业 | 165篇 |
无线电 | 29739篇 |
一般工业技术 | 40332篇 |
冶金工业 | 46681篇 |
原子能技术 | 3789篇 |
自动化技术 | 17368篇 |
出版年
2022年 | 1266篇 |
2021年 | 2219篇 |
2020年 | 1654篇 |
2019年 | 1925篇 |
2018年 | 2879篇 |
2017年 | 2897篇 |
2016年 | 2903篇 |
2015年 | 2398篇 |
2014年 | 3794篇 |
2013年 | 9405篇 |
2012年 | 5961篇 |
2011年 | 7694篇 |
2010年 | 6360篇 |
2009年 | 7064篇 |
2008年 | 7443篇 |
2007年 | 7406篇 |
2006年 | 6819篇 |
2005年 | 6362篇 |
2004年 | 5983篇 |
2003年 | 5760篇 |
2002年 | 5589篇 |
2001年 | 5882篇 |
2000年 | 5446篇 |
1999年 | 6021篇 |
1998年 | 16090篇 |
1997年 | 10713篇 |
1996年 | 8299篇 |
1995年 | 6094篇 |
1994年 | 5304篇 |
1993年 | 5240篇 |
1992年 | 3550篇 |
1991年 | 3372篇 |
1990年 | 3279篇 |
1989年 | 3104篇 |
1988年 | 2852篇 |
1987年 | 2279篇 |
1986年 | 2333篇 |
1985年 | 2628篇 |
1984年 | 2326篇 |
1983年 | 2069篇 |
1982年 | 1915篇 |
1981年 | 2053篇 |
1980年 | 1797篇 |
1979年 | 1670篇 |
1978年 | 1660篇 |
1977年 | 2026篇 |
1976年 | 2706篇 |
1975年 | 1427篇 |
1974年 | 1371篇 |
1973年 | 1319篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
111.
Temperature history can have a significant effect on the strength of water-saturated chalk.In this study,hydrostatic stress cycles are applied to understand the mechanical response of chalk samples exposed to temperature cycling between each stress cycle,compared to the samples tested at a constant temperature.The total accumulated strain during a stress cycle and the irreversible strain are reported.Chalk samples from Kansas(USA)and Mons(Belgium),with different degrees of induration(i.e.amount of contact cementation),were used.The samples were saturated with equilibrated water(polar)and nonpolar Isopar H oil to quantify water weakening.All samples tested during 10 stress cycles with varying temperature(i.e.temperature cycled in between each stress cycle)accumulated more strain than those tested at constant temperatures.All the stress cycles were performed at 30℃.The two chalk types behaved similarly when saturated with Isopar H oil,but differently when saturated with water.When saturated with water,the stronger Kansas chalk accumulated more total strain and more irreversible strain within each stress cycle than the weaker Mons chalk. 相似文献
112.
113.
Jianmou Lu Yanli Huang Shunyan Ren 《International Journal of Control, Automation and Systems》2020,18(5):1250-1263
This paper investigates the general decay synchronization and general decay H∞ synchronization problems of multi-weighted coupled reaction-diffusion 相似文献
114.
Y.K. Yang F.L. Liu Y.W. Zhang M.F. Li F. Ling H.T. Wu 《Ceramics International》2018,44(11):12238-12244
In this work, ultra-low loss Li2MgTi0.7(Mg1/3Nb2/3)0.3O4 ceramics were successfully prepared via the conventional solid-state method. X-ray photoelectron spectroscopy (XPS), thermally stimulated depolarization current (TSDC) and bond energy were used to determine the distinction between intrinsic and extrinsic dielectric loss in (Mg1/3Nb2/3)4+ ions substituted ceramics. The addition of (Mg1/3Nb2/3)4+ ions enhances the bond energy in unit cell without changing the crystal structure of Li2MgTiO4, which results in high Q·f value as an intrinsic factor. The extrinsic factors such as porosity and grain size influence the dielectric loss at lower sintering temperature, while the oxygen vacancies play dominant role when the ceramics densified at 1400?°C. The Li2MgTi0.7(Mg1/3Nb2/3)0.3O4 ceramics sintered at 1400?°C can achieve an excellent combination of microwave dielectric properties: εr =?16.19, Q·f?=?160,000?GHz and τf =??3.14?ppm/°C. In addition, a certain amount of LiF can effectively lower the sintering temperature of the matrix, and the Li2MgTi0.7(Mg1/3Nb2/3)0.3O4-3?wt% LiF ceramics sintered at 1100?°C possess balanced properties with εr?=?16.32, Q·f?=?145,384?GHz and τf =??16.33?ppm/°C. 相似文献
115.
Ningning Fu Jiaxing Li Ming Wang Lili Ren Youqing Luo 《International journal of molecular sciences》2020,21(22)
An obligate mutualistic relationship exists between the fungus Amylostereum areolatum and woodwasp Sirex noctilio. The fungus digests lignin in the host pine, providing essential nutrients for the growing woodwasp larvae. However, the functional properties of this symbiosis are poorly described. In this study, we identified, cloned, and characterized 14 laccase genes from A. areolatum. These genes encoded proteins of 508 to 529 amino acids and contained three typical copper-oxidase domains, necessary to confer laccase activity. Besides, we performed molecular docking and dynamics simulation of the laccase proteins in complex with lignin compounds (monomers, dimers, trimers, and tetramers). AaLac2, AaLac3, AaLac6, AaLac8, and AaLac10 were found that had low binding energies with all lignin model compounds tested and three of them could maintain stability when binding to these compounds. Among these complexes, amino acid residues ALA, GLN, LEU, PHE, PRO, and SER were commonly present. Our study reveals the molecular basis of A. areolatum laccases interacting with lignin, which is essential for understanding how the fungus provides nutrients to S. noctilio. These findings might also provide guidance for the control of S. noctilio by informing the design of enzyme mutants that could reduce the efficiency of lignin degradation. 相似文献
116.
Shuncheng Liu Kejian Wang Zhongfeng Zhang Yueqing Ren Lanlan Chen Xiaojie Sun Wenbin Liang 《Polymer Engineering and Science》2020,60(10):2640-2652
Three kinds of ethylene-octene copolymers (POE) were melt-blended with high-density polyethylene (PE-HD) in different proportions. Detailed characterizations were conducted to analyze their structural differences of POE and its effects in toughening PE-HD. The higher molecular weight POE can improve the toughness of PE-HD. 60:40 PE-HD/POE is elongated to break up to 700% while impact strength is 84.7 kJ/m2 at −30°C, which is 21-fold of PE-HD. In the brittle to ductile transition (BDT) during impact, the fracture mechanism changes from the crazing mode to the shear yield-plastic deformation mode. The BDT temperature decreases as the POE molecular weight and its content increase. The interface strength in tension is estimated to access their effects. The Boltzmann-type models were successfully extended to describe the typical S-shaped curves in BDT of notched impact strength vs POE content or temperature. The supplementary decay model is suggested for the attenuation in toughening. Transition map in impact is proposed to select the use range of composition (c ) and temperature (T ) for high toughness. The curves are converted into 3D graph of T -c -impact strength for illustrating their coupling-separate effects, and further into the contour map of impact strength in T -c space for finding their partial equivalence. 相似文献
117.
Effect of Supports and Promoters on the Performance of Ni-Based Catalysts in Ethanol Steam Reforming
Thanh Khoa Phung Thong Le Minh Pham Anh-Nga T. Nguyen Khanh B. Vu Ha Ngoc Giang Tuan-Anh Nguyen Thanh Cong Huynh Hong Duc Pham 《化学工程与技术》2020,43(4):672-688
Ethanol steam reforming (ESR) is one of the potential processes to convert ethanol into valuable products. Hydrogen produced from ESR is considered as green energy for the future and can be an excellent alternative to fossil fuels with the aim of mitigating the greenhouse gas effect. The ESR process has been well studied, using transition metals as catalysts coupled with both acidic and basic oxides as supports. Among various reported transition metals, Ni is an inexpensive material with activity comparable to that of noble metals, showing promising ethanol conversion and hydrogen yields. Additionally, different promoters and supports were utilized to enhance the hydrogen yield and the catalyst stability. This review summarizes and discusses the influences of the supports and promoters of Ni-based catalysts on the ESR process. 相似文献
118.
119.
Robert Groth Luke T. Cravigan Sadegh Niazi Zoran Ristovski Graham R. Johnson 《Journal of the Royal Society Interface》2021,18(178)
The airborne dynamics of respiratory droplets, and the transmission routes of pathogens embedded within them, are governed primarily by the diameter of the particles. These particles are composed of the fluid which lines the respiratory tract, and is primarily mucins and salts, which will interact with the atmosphere and evaporate to reach an equilibrium diameter. Measuring organic volume fraction (OVF) of cough aerosol has proved challenging due to large variability and low material volume produced after coughing. Here, the diametric hygroscopic growth factors (GF) of the cough aerosol produced by healthy participants were measured in situ using a rotating aerosol suspension chamber and a humidification tandem differential mobility analyser. Using hygroscopicity models, it was estimated that the average OVF in the evaporated cough aerosol was 0.88 ± 0.07 and the average GF at 90% relative humidity (RH) was 1.31 ± 0.03. To reach equilibrium in dry air the droplets will reduce in diameter by a factor of approximately 2.8 with an evaporation factor of 0.36 ± 0.05. Hysteresis was observed in cough aerosol at RH = ∼35% and RH = ∼65% for efflorescence and deliquescence, respectively, and may depend on the OVF. The same behaviour and GF were observed in nebulized bovine bronchoalveolar lavage fluid. 相似文献
120.
Li Wei Ren Yinghui Li Chenfang Li Zhipeng Li Maojun 《The International Journal of Advanced Manufacturing Technology》2020,106(3):921-935
The International Journal of Advanced Manufacturing Technology - The wear of micro-grinding tools less than 1 mm in diameter is crucial for achieving acceptable surface quality and accurate... 相似文献