全文获取类型
收费全文 | 131256篇 |
免费 | 17457篇 |
国内免费 | 5957篇 |
专业分类
电工技术 | 3755篇 |
技术理论 | 2篇 |
综合类 | 6960篇 |
化学工业 | 38439篇 |
金属工艺 | 12513篇 |
机械仪表 | 4419篇 |
建筑科学 | 10247篇 |
矿业工程 | 3173篇 |
能源动力 | 2686篇 |
轻工业 | 16036篇 |
水利工程 | 1965篇 |
石油天然气 | 4130篇 |
武器工业 | 764篇 |
无线电 | 7890篇 |
一般工业技术 | 23571篇 |
冶金工业 | 6212篇 |
原子能技术 | 886篇 |
自动化技术 | 11022篇 |
出版年
2024年 | 428篇 |
2023年 | 2834篇 |
2022年 | 4663篇 |
2021年 | 7117篇 |
2020年 | 5374篇 |
2019年 | 4834篇 |
2018年 | 5253篇 |
2017年 | 6183篇 |
2016年 | 7422篇 |
2015年 | 7968篇 |
2014年 | 9073篇 |
2013年 | 10139篇 |
2012年 | 8636篇 |
2011年 | 9001篇 |
2010年 | 6701篇 |
2009年 | 6939篇 |
2008年 | 6016篇 |
2007年 | 7604篇 |
2006年 | 7396篇 |
2005年 | 5996篇 |
2004年 | 4863篇 |
2003年 | 4240篇 |
2002年 | 3141篇 |
2001年 | 2130篇 |
2000年 | 1715篇 |
1999年 | 1347篇 |
1998年 | 1048篇 |
1997年 | 950篇 |
1996年 | 839篇 |
1995年 | 750篇 |
1994年 | 588篇 |
1993年 | 446篇 |
1992年 | 420篇 |
1991年 | 341篇 |
1990年 | 411篇 |
1989年 | 379篇 |
1988年 | 160篇 |
1987年 | 140篇 |
1986年 | 123篇 |
1985年 | 152篇 |
1984年 | 148篇 |
1983年 | 91篇 |
1982年 | 101篇 |
1981年 | 47篇 |
1980年 | 97篇 |
1966年 | 26篇 |
1964年 | 41篇 |
1962年 | 67篇 |
1959年 | 29篇 |
1955年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
61.
62.
Vitória Mussi Toschi Christine Bogicevic Houssny Bouyanfif Henri Laville Pierre-Eymeric Janolin 《Journal of the American Ceramic Society》2021,104(10):5257-5269
The morphotropic composition of the lead-free solid solution between Na0.5Bi0.5TiO3 and BaTiO3 (0.94 Na0.5Bi0.5TiO3-0.06 BaTiO3 or NBT-6BT) is of particular interest for the next generation of high-temperature capacitors but remains plagued by the diversity of dielectric properties reported in the literature. In order to explain the apparent inconsistencies among the reported dielectric properties of NBT-6BT, we examine the influence of stoichiometry, phase separation, and metallization method. We show that the nominal stoichiometry has a crucial effect, since increasing the nominal Na/Bi ratio increases conductivity and dielectric losses (tan δ). It also increases the real part of the permittivity (ε’) and the frequency dispersion of both ε’ and tan δ, thereby altering the shape of the evolution with temperature of the dielectric properties. Moreover it increases the depolarization temperature (Td) and decreases the temperature of maximum permittivity (Tm). Phase separation also occurs during the synthesis of NBT-6BT as Na evaporation leads to the formation of secondary Ba-containing phases. We report that these phases can have a positive impact on the dielectric properties: a moderate volume fraction (2.5 to 3.0%) and average grain surface (0.9 to 3.0 µm2) of these secondary Ba-containing phases increase the relative permittivity, decrease the dielectric losses, and increase the insulation resistance. We also show that the metallization method impacts the dielectric properties and therefore may contribute to the differences between various reports. The dielectric properties of NBT-6BT samples are measured during successive heating/cooling cycles and reveal that the permittivity value is lower during the first heating when silver paste, even cured, is used. These three components contribute to explaining the diversity of the reported dielectric properties of NBT-6BT. 相似文献
63.
64.
65.
66.
67.
研究辊速差对连铸连轧7075铝板显微组织、织构及力学性能的影响。采用3种不同上辊/下辊转速比(ω/ω0,ω为上辊转速,ω0为下辊转速)1:1、1:1.2及1:1.4进行多次试验。结果显示,在最大辊速差条件下(ω/ω0=1:1.4),7075铝板在轧制方向的屈服强度和极限抗拉强度分别提高41.5%和21.9%。此外,当辊速比ω/ω0为1:1.4时,成品轧制板的平均晶粒尺寸减小36%,横剖面平均硬度增加约9.2%。织构研究结果显示,辊速差越大,成品各向同性及硬度越大。然而,采用不同辊速度的连铸连轧会导致变形板伸长率降低约6%。 相似文献
68.
Effect of Bimodal Grain Size Distribution on the Strain Hardening Behavior of a Medium-Entropy Alloy
The evolution of strain hardening behavior of the Fe_(50)(CoCrMnNi)_(50) medium-entropy alloy as a function of the fraction of recrystallized microstructure and the grain size was studied using the Hollomon and Ludwigson equations.The specimens under study were partially recrystallized,fully recrystallized with ultrafine-grained microstructure,and fully recrystallized with coarse grains.The yield strength decreases steadily as the fraction of recry stallized micro structure and grain size increases due to the recovery process and the Hall-Petch effect.Interestingly,the bimodal grain distribution was found to have a significant impact on strain hardening during plastic deformation.For instance,the highest ultimate tensile strength was exhibited by a 0.97 μm specimen,which was observed to contain a bimodal grain distribution.Furthermore,using the Ludwigson equation,the effect of the bimodal grain distribution was established from the behavior of K_2 and n1 curves.These curves tend to show very high values in the specimens with a bimodal grain distribution compared to those that show a homogenous grain distribution.Additionally,the bimodal grain distribution contributes to the extensive L(u|")ders strain observed in the 0.97 μm specimen,which induces a significant deviation of the Hollomon equation at lower strains. 相似文献
69.
Lun Feng William G. Fahrenholtz Gregory E. Hilmas Frédéric Monteverde 《Journal of the European Ceramic Society》2021,41(1):92-100
Dense high-entropy (Hf,Zr,Ti,Ta,Nb)B2 ceramics with Nb contents ranging from 0 to 20 at% were produced by a two-step spark plasma sintering process. X-ray diffraction indicated that a single-phase with hexagonal structure was detected in the composition without Nb. In contrast, two phases with the same hexagonal structure, but slightly different lattice parameters were present in compositions containing Nb. The addition of Nb resulted in the presence of a Nb-rich second phase and the amount of the second phase increased as the Nb content increased. The relative densities were all >99.5 %, but decreased from ~100 % to ~99.5 % as the Nb content increased from 0 to 20 at%. The average grain size decreased from 13.9 ± 5.5 μm for the composition without Nb additions to 5.2 ± 2.0 μm for the composition containing 20 at% Nb. The reduction of grain size with increasing Nb content was due to the suppression of grain growth by the Nb-rich second phase. The addition of Nb increased Young’s modulus and Vickers hardness, but decreased shear modulus. While some Nb dissolved into the main phase, a Nb-rich second phase was formed in all Nb-containing compositions. 相似文献
70.
The influences of atmosphere during processes of melting and heat treatment, heat treatment temperature, Fe3O4 content and basicity on the magnetic properties of magnetite-based glass ceramics were investigated. For sample containing 20 % Fe3O4 melted in different atmospheres, the highest saturation magnetisation was realized in 20vol% air + 80 vol% Ar, due to the fact that ratio of Fe3+ to Fe2+ in melt obtained in this atmosphere was close to 2. However, it was found that the coercivity of glass ceramics was not affected by the melting atmosphere. A high sintering temperature led to the decrease of saturation magnetisation and the increase of coercivity. As increasing Fe3O4 content, the main crystal phase transformed from CaSiO3 to CaFe0.6Al1.3Si1.08O6 and finally to magnetite phase, accompanied by the increase of saturation magnetisation and coercivity. In addition, the increase of basicity caused the decrease of saturation magnetisation and the increase of coercivity. 相似文献