全文获取类型
收费全文 | 74636篇 |
免费 | 8238篇 |
国内免费 | 5440篇 |
专业分类
电工技术 | 6765篇 |
技术理论 | 4篇 |
综合类 | 6516篇 |
化学工业 | 9252篇 |
金属工艺 | 4733篇 |
机械仪表 | 5464篇 |
建筑科学 | 5785篇 |
矿业工程 | 2462篇 |
能源动力 | 2224篇 |
轻工业 | 6238篇 |
水利工程 | 2468篇 |
石油天然气 | 3437篇 |
武器工业 | 990篇 |
无线电 | 9029篇 |
一般工业技术 | 6921篇 |
冶金工业 | 2890篇 |
原子能技术 | 1086篇 |
自动化技术 | 12050篇 |
出版年
2024年 | 391篇 |
2023年 | 1190篇 |
2022年 | 2568篇 |
2021年 | 3432篇 |
2020年 | 2562篇 |
2019年 | 1941篇 |
2018年 | 2231篇 |
2017年 | 2594篇 |
2016年 | 2318篇 |
2015年 | 3579篇 |
2014年 | 4341篇 |
2013年 | 5101篇 |
2012年 | 6056篇 |
2011年 | 6520篇 |
2010年 | 5924篇 |
2009年 | 5505篇 |
2008年 | 5853篇 |
2007年 | 5559篇 |
2006年 | 4769篇 |
2005年 | 3846篇 |
2004年 | 2781篇 |
2003年 | 1888篇 |
2002年 | 1771篇 |
2001年 | 1493篇 |
2000年 | 1257篇 |
1999年 | 785篇 |
1998年 | 441篇 |
1997年 | 332篇 |
1996年 | 285篇 |
1995年 | 254篇 |
1994年 | 192篇 |
1993年 | 131篇 |
1992年 | 104篇 |
1991年 | 62篇 |
1990年 | 53篇 |
1989年 | 39篇 |
1988年 | 36篇 |
1987年 | 23篇 |
1986年 | 16篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 11篇 |
1980年 | 21篇 |
1979年 | 12篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1959年 | 14篇 |
1951年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
A first‐principles‐based effective Hamiltonian is developed and employed to investigate finite‐temperature structural properties of a prototype of perovskite halides, that is CsPbI3. Such simulations, when using first‐principles‐extracted coefficients, successfully reproduce the existence of an orthorhombic Pnma state and its iodine octahedral tilting angles around room temperature. However, they also yield a direct transformation from Pnma to cubic upon heating, unlike measurements that reported the occurrence of an intermediate long‐range‐tilted tetragonal P4/mbm phase in‐between the orthorhombic and cubic phases. Such disagreement, which may cast some doubts about the extent to which first‐principle methods can be trusted to mimic hybrid perovskites, can be resolved by “only” changing one short‐range tilting parameter in the whole set of effective Hamiltonian coefficients. In such a case, some reasonable values of this specific parameter result in the predictions that i) the intermediate P4/mbm state originates from fluctuations over many different tilted states; and ii) the cubic phase is highly locally distorted and develops strong transverse antiphase correlation between first‐nearest neighbor iodine octahedral tiltings, before undergoing a phase transition to P4/mbm under cooling. 相似文献
3.
4.
Yuanyuan Zhang Haiyan Lu Bing Wang Zhuo Zhang Xiaorong Lin Zhongzheng Chen Bin Li 《International Journal of Food Science & Technology》2015,50(6):1397-1404
Removal of imidacloprid and acetamiprid in tea infusions by microfiltration membrane using dead‐end model was investigated in the present study. The results showed that microfiltration significantly promoted the removal of both pesticides (P < 0.05) in tea infusions. Furthermore, the extent of removal was strongly influenced by the pore size of membrane, operational pressure and the concentrations of tea infusions. The initial concentration of imidacloprid and acetamiprid showed no significant effect on their removal rates. The maximum removal rates were 79.7% for imidacloprid and 81.9% acetamiprid. The changes in major chemical components of tea infusions after microfiltration were evaluated. The results indicated that microfiltration caused no considerable changes in total polyphenols and total free amino acids, and small but statistically significant losses (6.3–18.0%) of eight catechins and three methylxanthines when filtration volume reached to 200 mL. The present study validated the application of microfiltration as a potentially feasible and promising method for the removal of imidacloprid and acetamiprid residues from tea infusions. 相似文献
5.
双检合并技术在海底电缆地震资料处理中被广泛应用,可衰减电缆鬼波的影响。传统方式以水、陆检记录的波场记录相同为假设,电缆鬼波压制效果差,合并效果并不理想。在分析交叉鬼波化双检合并技术基本原理的基础上,综合利用水检资料与陆检鬼波褶积的结果和陆检资料与水检鬼波褶积的结果求取刻度算子,同时在地震资料处理中利用陆检资料标定水检资料,从而优化了交叉鬼波化双检合并技术。实际资料处理结果表明,改进后的技术可有效衰减电缆鬼波,分离出上行波场,大幅度拓宽地震资料频带,提高地震资料分辨率,从而提供更丰富的构造细节信息。 相似文献
6.
为了研究荷电细水雾对瓦斯爆炸超压的影响规律和机理,采用小尺寸管道模拟瓦斯爆炸,研究不同荷电电压作用下的瓦斯爆炸超压和平均压升速率,以及不同雾通量作用下的瓦斯爆炸超压.结果表明:随着荷电电压的升高,瓦斯爆炸超压和平均压升速率受到明显的抑制;随着雾通量的增加,瓦斯爆炸超压明显降低.在实验条件下,和普通细水雾相比,当雾通量为4L、荷电电压为8kV时,瓦斯爆炸超压峰值降低10.798kPa,降幅达49.78%;平均压升速率峰值降低180.468kPa/s,降幅达49.90%. 相似文献
7.
针对现有海量点云可视化方法存在索引构建时间长、内存占用大等问题,研究一种八叉树索引结合OSG分页结点的快速可视化方法,可在占用较小内存的基础上快速建立点云索引并实时调度。采用八叉树索引结构对海量点云进行数据组织,建立各层级的八叉树结点并以文件映射的方式分块保存,对结点文件重组织转换为支持OSG渲染引擎的多分辨率点云数据。采用基于OSG分页结点的实时调度技术,对海量点云进行高质量可视化。与目前两款主流的点云数据处理商业软件进行实验对比分析,结果表明所提方法具有索引建立速度快、内存占用小等优点,同时可视化交互更加流畅,适用于各种配置计算机下海量点云数据的调度管理与实时可视化。 相似文献
8.
9.
10.
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