全文获取类型
收费全文 | 52233篇 |
免费 | 6516篇 |
国内免费 | 3496篇 |
专业分类
电工技术 | 4985篇 |
技术理论 | 1篇 |
综合类 | 4226篇 |
化学工业 | 7787篇 |
金属工艺 | 3040篇 |
机械仪表 | 3453篇 |
建筑科学 | 4244篇 |
矿业工程 | 1770篇 |
能源动力 | 1567篇 |
轻工业 | 3937篇 |
水利工程 | 1357篇 |
石油天然气 | 2493篇 |
武器工业 | 669篇 |
无线电 | 6440篇 |
一般工业技术 | 5848篇 |
冶金工业 | 1953篇 |
原子能技术 | 512篇 |
自动化技术 | 7963篇 |
出版年
2024年 | 287篇 |
2023年 | 1314篇 |
2022年 | 2498篇 |
2021年 | 3524篇 |
2020年 | 2428篇 |
2019年 | 1872篇 |
2018年 | 1883篇 |
2017年 | 2056篇 |
2016年 | 1829篇 |
2015年 | 2536篇 |
2014年 | 3005篇 |
2013年 | 3473篇 |
2012年 | 4019篇 |
2011年 | 4118篇 |
2010年 | 3548篇 |
2009年 | 3325篇 |
2008年 | 3248篇 |
2007年 | 2937篇 |
2006年 | 2591篇 |
2005年 | 2297篇 |
2004年 | 1561篇 |
2003年 | 1313篇 |
2002年 | 1365篇 |
2001年 | 1131篇 |
2000年 | 877篇 |
1999年 | 733篇 |
1998年 | 467篇 |
1997年 | 418篇 |
1996年 | 372篇 |
1995年 | 303篇 |
1994年 | 250篇 |
1993年 | 137篇 |
1992年 | 123篇 |
1991年 | 96篇 |
1990年 | 62篇 |
1989年 | 56篇 |
1988年 | 38篇 |
1987年 | 26篇 |
1986年 | 36篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1982年 | 14篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1975年 | 5篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1959年 | 7篇 |
1951年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
31.
Breast cancer is the most frequently diagnosed cancer in women worldwide. The disease and its treatments exert profound effects on an individual’s physical and mental health. There are many factors that impact an individual’s risk of developing breast cancer, their response to treatments, and their risk of recurrence. The community of microorganisms inhabiting the gastrointestinal tract, the gut microbiota, affects human health through metabolic, neural, and endocrine signaling, and immune activity. It is through these mechanisms that the gut microbiota appears to influence breast cancer risk, response to treatment, and recurrence. A disrupted gut microbiota or state of ‘dysbiosis’ can contribute to a biological environment associated with higher risk for cancer development as well as contribute to negative treatment side-effects. Many cancer treatments have been shown to shift the gut microbiota toward dysbiosis; however, the microbiota can also be positively manipulated through diet, prebiotic and probiotic supplementation, and exercise. The objective of this review is to provide an overview of the current understanding of the relationship between the gut microbiota and breast cancer and to highlight potential strategies for modulation of the gut microbiota that could lead to improved clinical outcomes and overall health in this population. 相似文献
32.
We propose an analytical model to evaluate the lightpath blocking performance for a single ROADM node with intra-node add/drop contention,in which the number of lightpaths that can be added/dropped with the same wavelength is limited by the add/drop contention factor.Different models of traffic load per nodal degree are considered to validate the effectiveness of the analytical model.The simulation results show that the proposed analytical model is effective in predicting the performance for different values of add/drop contention factor C and for variable offered loads at the node.The add/drop contention factor shows an important impact on the lightpath blocking performance and properly raising the contention factor can significantly improve the lightpath blocking performance.When the add/drop contention factor C exceeds a certain level,the performance of a ROADM with intra-node contention is close to that of a contentionless ROADM. 相似文献
33.
34.
35.
In this work,the ionospheric variability is analyzed by applying the wavelet decomposition technique to the noontime fo F2,F10.7,interplanetary magnetic field(IMF)Bz,Ap,and lower thermospheric temperature at pressure of 10?4 h Pa in 2002.Results show that the variance of periodic oscillations in the ionosphere is largest in the 2–4-day period and declines with the increase of the period.The maximum variance of the periodic oscillations in solar irradiation is in the 16–32-day period.For geomagnetic activities,most of the variance is about equally distributed on intervals of periods shorter than 32 days.Variance distributions of IMF Bz and lower thermospheric temperature are similar to those of the ionosphere.They show the maximum in the 2–4-day period and decline with the increase of the period.By analyzing the distributions of the variances,the potential connections between the ionosphere and the external sources are discussed. 相似文献
36.
37.
Hao Li Pengcheng Zhang Shengquan Yu Hongyu Yang Bin Tang Fuhai Li Shuren Zhang 《Ceramics International》2019,45(9):11639-11647
Mg2(Ti1-xSnx)O4 (x?=?0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg2(Ti1-xSnx)O4 average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg2(Ti1-xSnx)O4 compounds were obtained (= 12.18, ?=?170,130?GHz, ?=??53.1?ppm/°C, x?=?0.2). 相似文献
38.
Qianglong He Aiyang Wang Chun Liu Weimin Wang Hao Wang Zhengyi Fu 《Journal of the European Ceramic Society》2018,38(7):2832-2840
B4C-TiB2-SiC composites were fabricated via hot pressing using ball milled B4C, TiB2, and SiC powder mixtures as the starting materials. The impact of ball milling on the densification behaviors, mechanical properties, and microstructures of the ceramic composites were investigated. The results showed that the refinement of the powder mixtures and the removal of the oxide impurities played an important role in the improvement of densification and properties. Moreover, the formation of the liquid phases during the sintering was deemed beneficial for densification. The typical values of relative density, hardness, bending strength, and fracture toughness of the composites reached 99.20%, 32.84?GPa, 858?MPa and 8.21?MPa?m1/2, respectively. Crack deflection, crack bridging, crack branching, and microcracking were considered to be the potential toughening mechanisms in the composites. Furthermore, numerous nano-sized intergranular/intragranular phases and twin structures were observed in the B4C-TiB2-SiC composite. 相似文献
39.
We investigate the effects of doping and annealing on the dielectric properties of metal ions doped TiO2 ceramics. Colossal permittivity (CP) above 104 was observed in single Nb ion doped TiO2, which was dominated by electron transport related interfacial polarization. Moreover, the CP can be dropped to 120 when simultaneously introducing Mn ion into the sample. The disappearance of CP behaviors maybe due to the multivalence of Mn which would inhibit the reduction of Ti4+ to Ti3+, and thus reduce delocalized electrons. Interestingly, the CP was recovered for the (Nb+Mn) co-doped TiO2 after post-sintering heat treatment in N2 atmosphere. The recovery of CP in the sample after annealing can be ascribed to the semiconducting grain and the insulating grain boundary, according to impedance spectroscopy. We therefore believe that this work can help us understand the mechanism of CP from a new perspective. 相似文献
40.
With increasing consumption of natural gas (NG), small NG reservoirs, such as coalbed methane and oil field associated gas, have recently drawn significant attention. Owing to their special characteristics (e.g., scattered distribution and small output), small-scale NG liquefiers are highly required. Similarly, the mixed refrigerant cycle (MRC) is suitable for small-scale liquefaction systems due to its moderate complexity and power consumption. In consideration of the above, this paper reviews the development of mobile miniature NG liquefiers in Technical Institute of Physics and Chemistry (TIPC), China. To effectively liquefy the scattered NG and overcome the drawbacks of existing technologies, three main improvements, i.e., low-pressure MRC process driven by oil-lubricated screw compressor, compact cold box with the new designed heat exchangers, and standardized equipment manufacturing and integrated process technology have been made. The development pattern of “rapid cluster application and flexible liquefaction center” has been eventually proposed. The small-scale NG liquefier developed by TIPC has reached a minimum liquefaction power consumption of about 0.35 kW·h/Nm3. It is suitable to exploit small remote gas reserves which can also be used in boil-off gas reliquefaction and distributed peak-shaving of pipe networks. 相似文献