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1.
The refined grains in both as-cast Mg-10Y-1Zn-2Zr (WZ101?K, wt.%) and Mg-10Y-1Al (WA101, wt.%) alloys have been found to have superior thermal stability after solution treatment. Plate-shaped, but different polytypes of long-period stacking ordered (LPSO) phases distributed along grain boundaries are considered as the predominant inhibitors of grain growth. Based on the plate-shaped features of the LPSO phases, a modified Zener’s model is established and the predicted limiting grain sizes are in a good agreement with the experimental results. Therefore, this model has strong potential for designing advanced Mg-RE-based alloys with high strength and high creep resistance at high temperatures. 相似文献
2.
碳化硅(SiC)是典型的层状结构化合物。到目前为止,已发现了150种以上的多型体。作者曾发展了一种特殊的劳厄法可有效地鉴定碳化硅多型体。这种方法,我们发现了85种碳化硅新多型体。为了测定其中一些多型体的结构,拍摄了回摆和魏森堡照相作结构分析,但没有成功。因为这些照相中只有一些基本类型6H,15R和8H的衍射斑点而没有高层多型体的斑点,这是由于新多型体在晶体中含量很少且这些薄晶体多型体处于基本类型6H,8H,15R中间的缘故。本文提出了一种测定碳化硅多型体晶体结构的劳厄法。提出了计算劳厄斑点衍射强度的方法。对结构系列[(33)m32]3,[(33)m34]3,[(22)m23]3和[(44)m43]3多型体的结构因子Fhkl的计算方法作了简化。利用这种方法对9种碳化硅新多型体的晶体结构作了测定,其结构用z字形堆垛(Жданов符号)表示时为231R:[(33)1232]3,249R:[(33)1332]3,321R:[(33)1732]3, 339R:[(33)1832]3,237R:[(33)1234]3,417R:[(33)2234]3, 453R:[(33)2434]3,93R:[(44)343]3,261R:[(44)1043]3。
关键词: 相似文献
3.
The electronic and structural properties of 9R diamond are studied by performing density functional (DFT) calculations, and compared with those of four other known polytypes (2H, 3C, 4H, and 6H). The calculated lattice constants, the unit cell energy, and the relative stability of these polytypes all agree well with the experimental sequence of diamond polytypes. It is found that the band gap of the cubic structure is considerably smaller than that of other polytypes. The trend of band gap increasing with the increase of hexagonality that has been found by others with other polytypes is not applicable to 9R polytype. 相似文献
4.
Kazuhisa Sato Shunya Tashiro Shuhei Matsunaga Yohei Yamaguchi Takanori Kiguchi Toyohiko J. Konno 《哲学杂志》2018,98(21):1945-1960
We have studied three-dimensional (3D) structures and growth processes of 14H-type long-period stacking order (LPSO) formed in Mg97Zn1Gd2 cast alloys by single tilt-axis electron tomography (ET) using high-angle annular dark-field scanning transmission electron microscopy. Evolution of the solute-enriched stacking faults (SFs) and the 14H LPSO by ageing were visualised in 3D with a high spatial resolution in multi-scale fields of views from a few nanometres to ~10 μm. Lateral growth of the solute-enriched SFs and the LPSO in the (0?0?0?1)Mg plane is notable compared to the out-of-plane growth in the [0?0?0?1]Mg direction. The 14H LPSO grows at the cost of decomposition of the (Mg, Zn)3Gd-type precipitates, and accompany a change of in-plane edge angles from 30 to 60°. We have updated the Time–Temperature–Transformation diagram for precipitation in Mg97Zn1Gd2 alloys: starting temperatures of both solute-enriched SFs and LPSO formation shifted to a shorter time side than those in the previous diagram. 相似文献
5.
Employing the method of zone- refining single crystals of PbI2 of different purities were grown. Using X-ray diffraction technique their structure was found to be of the type 12R and in some cases 12R+4H. The crystals were re-examined after prolonged storage of several months at room temperature. Neither the crystals of very high purity nor of relatively less purity transformed. But thecrystals having intermediate purities did transform from 12R to 2H. This observation establishes a clear link between impurities and phase transitions. The results have been explained on the basis of nucleation and growth mechanism of stacking faults. Also it has been concluded that the presence of impurities is a must for the formation of polytypes. 相似文献
6.
Yukitomo Komura 《Phase Transitions》2013,86(1-4):495-507
This paper summarizes polytypes in alloys having layer structures such as Laves phases and Sm-Ni intermetallics which were studied by X-ray diffraction and electron microscopy. In the Mg-base pseudo-binary Laves phases, many stacking variants were found as a function of electron concentration. They are 4H, 6H, 3C, 4H, 21R, 10H, 16H, 9R, 6H', 14H, 8H and 2H in order of increasing e/a from 0.67 to 2.0. Many polytypes were also observed in the specimens of Sm2Ni7 and Sm5Ni19 compounds. Polytypes found in Sm2Ni7 are 2H, 3R, 4H, 5T, 9R, 12R, and for Sm5Ni19 2H, 3R, 4H, 5T, 6H, 7T, 9R, 12R, 15R, 18R were recognized. Stacking faults, intergrowth and many defect structures were observed under the electron microscope. 相似文献
7.
Lattice imaging technique of high resolution electron microscopy has been employed to examine 4H, 6H and 9R ABO3 perovskite polytypes. The lattice images can be correlated with the lattice periodicity and the stacking sequence of AO3 layers and BO6 octahedra. The study shows the utility and validity of the lattice imaging technique for the study of relatively close-packed
systems.
Commonwealth Visiting Professor, University of Oxford (1974–75). 相似文献
8.
The crystal growth aspect of mica polytypism is reviewed with emphasis on the relationships between the growth mechanisms of the basal faces and the ordering of the unit modules of the mica structure. On synthetic micas, it is shown that the most common short-period polytypes (basic structures) and the disordered layer-stacking sequences as well originate during the early 3D-nucleation and 2D-layer growth mechanisms. During those stages, basic structures may be randomly faulted to various extents. Subsequently, spiral growth may take place and originate long-period (or complex) polytypes. Supported by such observations on synthetic as well as natural micas, the principles of the perfect-matrix and faulted-matrix models of the screw dislocation theory or polytypism are remembered and shown to be powerful in explaining most of the complex polytypes of the micas. On natural biotite, original data are given about the very nature of stacking disorder and also about weak but significant differences between the chemical compositions of coexisting 1M, 2M1 and 1Md polytypes. Lastly, the use of polytype inheritance to find out the phase transformation process between biotite and chlorite is illustrated. 相似文献
9.
本文提出了用X射线劳埃法鉴定SiC六方多型体类型的方法。在详细地研究了xH类型与基本类型6H,15R和4H的倒易阵点间相互配置关系的基础上,推导出了这些具体关系,这种关系对于xH—6H来说仅有十二种,对于xH—15R仅有三十种,对于xH—4H仅有八种。上述的点间关系表和推求xH类型单位晶胞密堆积层数的公式同样可以普遍适用于以其他X射线照相法鉴定SiC六方多型体类型的工作中。用本文所提出的方法研究了许多实验室升华法制备的SiC单晶体以及部分工业SiC晶体。发现了七种六方SiC新多型体141H,80H,58H,55H,15H,9H和7H。新类型的定间羣为C3v1(C3m),六方晶胞c轴参数分别为:355.26?,201.57?,146.14?,138.58?,37.794?,22.676?和17.637?。 相似文献
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11.
粒子群优化算法在自适应偏振模色散补偿中的性能研究 总被引:1,自引:0,他引:1
反馈控制算法是偏振模色散的自适应补偿器的关键组成部分,将粒子群优化算法(PSO)引入到偏振模色散自适应补偿系统中。该算法的优点是具有快速收敛到全局最佳值的能力、避免搜索陷入局部极值的能力、抗噪声能力和多自由度控制能力。理论上分析了粒子群优化算法的两个分类———全局邻居结构粒子群优化(GPSO)和局部邻居结构粒子群优化(LPSO)在搜索全局最佳值方面的能力优劣,给出了局部邻居结构粒子群优化算法成功率达100%的三种邻居拓扑结构。实验表明:在补偿一阶偏振模色散时,全局邻居结构和局部邻居结构搜索全局最佳的成功率都能满足要求,全局邻居结构算法收敛速度快。而在补偿二阶偏振模色散时,全局邻居结构成功率降低,而局部邻居结构仍可以满足要求。 相似文献
12.
Extreme room temperature conductivity enhancements have been reported for nanocrystalline AgI of up to ×?10(4) relative to bulk β-AgI (Guo et?al 2005 Adv.?Mater. 17 2815-9). These samples were identified as possessing 7H and 9R polytype structures, which can be considered as heterostructures composed of thin, commensurate layers in the β (wurtzite) and γ (zincblende) phases. It has been proposed that space-charge layer formation at β|γ-interfaces causes near complete disordering of the Ag(+)?sublattice in these polytypes, resulting in a massive intrinsic enhancement of ionic conductivity. We have performed molecular dynamics simulations of β- and γ-AgI and mixed β|γ?superlattices, to study the effect of heterostructuring on intrinsic defect populations and Ag(+) transport. The ionic conductivities and Ag(+)?diffusion coefficients vary as β?>?7H?≈?9R?≈?10L?>?γ. The β|γ-heterostructured polytypes show no enhancement in defect populations or Ag(+)?mobilities relative to the β-AgI phase, and instead behave as simple composites of β- and γ-AgI. This contradicts the proposal that the extreme conductivity enhancement observed for 7H and 9R polytypes is explained by extensive space-charge formation. 相似文献
13.
Richard Nader Elie Moussaed Michel Kazan Joerg Pezoldt Pierre Masri 《Superlattices and Microstructures》2008,44(2):191-196
Structural and optical measurements were performed on silicon carbide (SiC) samples containing several polytypes. The SiC samples investigated were grown on (111) Si substrates by solid source molecular beam epitaxy (SSMBE). Several quantities of Ge were predeposited before the growth procedure. The influence of Ge on the SiC polytypes formation was studied by X-Ray, FTIR and μ-Raman characterizations methods. The spectra of the samples with less than one Ge monolayer exhibit a mixture of 2H, 15R and 3C–SiC polytypes. This mixture is due to the mismatch between the heterostructure layers. We propose that the Ge predeposition in the heterostructure can be used to stabilize and unify the polytypes formation. 相似文献
14.
Koji Hagihara Michiaki Yamasaki Masahito Honnami Hitoshi Izuno Masakazu Tane Takayoshi Nakano 《哲学杂志》2015,95(2):132-157
Formation of curious deformation bands has been reported as one of the deformation mechanisms occurring in an Mg-based long-period stacking ordered (LPSO) phase. The origin of the deformation band is still unknown, and the possibility of the deformation kink band and/or the deformation twin has been discussed. To clarify this, the crystallographic nature of deformation bands formed in the LPSO phase was examined by scanning electron microscope–electron backscatter diffraction (SEM-EBSD) pattern analysis. The results were compared to those of the deformation kink bands formed in hcp-Zn and deformation twins formed in hcp-Mg polycrystals. The deformation bands in the LPSO phase was confirmed not to exhibit a fixed crystal orientation relationship with respect to the matrix, different from the case shown in the deformation twin. Instead, the deformation band in the LPSO phase showed three arbitrariness on its crystallographic nature: an ambiguous crystal rotation axis that varied on the [0 0 0 1] zone axis from band to band; an arbitral crystal rotation angle that was not fixed and showed relatively wide distributions; and a variation in crystal rotation angle depending on the position even within a deformation band boundary itself. These features were coincident with those observed in the deformation bands formed in Zn polycrystals, suggesting that the formed deformation bands in LPSO phase crystals are predominantly deformation kink bands. 相似文献
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16.
汽相生长的ZnS·Cu单晶交流电致发光线的疏密与生长层线的疏密相对应。用X射线周转晶体法对1050℃汽相生长的ZnS晶体结构进行了分析。结果表明,生长层线疏密不同,结构差别很大,层线区的结构特点是层错的无规分布。ZnS·Cu单晶交流电致发光线的产生与结构中层错的无规分布有关。 相似文献
17.
A. A. Zhokhov V. M. Masalov D. V. Matveev M. Yu. Maksimuk I. I. Zver’kova S. S. Khasanov S. Z. Shmurak A. P. Kiselev A. V. Bazhenov G. A. Emel’chenko 《Physics of the Solid State》2009,51(8):1723-1729
The method for carbothermal reduction of spherical particles of amorphous silicon dioxide is developed, and hexagonal α-SiC polytype nanocrystals are synthesized. The prepared samples are characterized by X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, and electron microscopy. The silicon carbide nanocrystals prepared have sizes in the range 5–50 nm depending on the diameter of initial silicon dioxide particles. A detailed analysis of the positions of the lines in the Raman spectra, their broadening, and shift makes it possible to reliably establish that the samples under investigation predominantly contain the 6H and 4H silicon carbide polytypes and insignificant amounts of the 2H and 3C phases. The 15R and 21R polytypes in the samples are absent. It is noted that the samples are characterized by a substantial size effect: the luminescence intensity of small silicon carbide nanocrystals is more than three times higher than that of large SiC nanocrystals. 相似文献
18.
Shang-Peng Gao 《Solid State Communications》2012,152(19):1817-1820
Geometry optimizations are performed for three polytypes of h-BN using density functional theory with dispersion correction for the van der Walls interaction. Quasiparticle band structure calculations are carried out to solve the controversy on band gap type of h-BN. Band energies are corrected by GW method. The h-BN with Bk structure has an indirect band gap of 5.840 eV. Two kinds of h-BN polytypes are shown to be mechanically stable and have quasi-direct band gap type. 相似文献
19.
采用平面波超软赝势法和范数不变赝势法对几种SiC多型体的几何结构、能带结构等进行了系统的研究.结果表明:6HSiC导带最低点在ML线上U点,用平面波超软赝势法计算时U点在(0000,0500,0176)点附近;而用范数不变赝势法计算时在导带最低点附近能带呈现不连续点,不连续点出现在(0000,0500,0178)点附近.两种赝势法计算结果相比,用平面波超软赝势法得到的导带最低点位置更靠近布里渊区M(0,05,0)点.在平面波超软赝势下,随着六角度的增加,cp,cpa增大的趋势较为明显,能隙和价带宽度变宽的趋势也较为明显.在计算极限内,绝对零度下4HSiC系统能量最低、最稳定,而Ewald能量显示3CSiC最稳定.
关键词:
密度泛函理论
电子结构
SiC 相似文献
20.
The formation of ⊥c texture of WS2 thin films by solid state reaction between the spray deposited WO3 and gaseous sulfur vapours with Ni interfacial layer has been reported. X-ray diffraction technique has been used to measure the degree of preferred orientation and texture of WS2 films. Scanning electron microscopy, transmission electron microscopy and atomic force microscopy have been used to characterize the microstructure and morphology. The electronic structure and chemical composition was studied using X-ray photoelectron spectroscopy. The WS2 films comprise single crystalline quality hexagonal crystallites of 15 μm × 15 μm size with their basal planes parallel to the substrate. The film consists of turbostratic stacking sequence of 2H and 3R polytypes of WS2. The tungsten-to-sulfur ratio was estimated to be 1:1.8. The various qualitative models used to explain promotional effects are briefly outlined and the plausible underlying mechanism of formation of ⊥c texture with nickel, in this study, is given. 相似文献