Effect of the composition and crystallite size on the thermoelectric properties of layered tungsten dichalcogenides

2020 ◽  
Vol 61 (11) ◽  
2020 ◽  
Vol 61 (11) ◽  
pp. 1721-1728
Author(s):  
G. E. Yakovleva ◽  
A. Yu. Ledneva ◽  
A. I. Romanenko ◽  
V. E. Fedorov ◽  
B. A. Kolesov ◽  
...  

A range of dense well oriented graphites of high chemical purity has been prepared by stress recrystallization of pyrolytic material. Systematic trends in the galvanomagnetic and thermoelectric properties have been investigated in relation to c-axis distribution function, crystallite size, and basal plane dislocation concentration, using temperatures from 300 to T2 °K and magnetic fields up to 6700 G. Basal plane properties of ideal graphite are evaluated in terms of the trends observed. Electronic properties have been measured parallel to the c axis for corresponding graphites with a range of defect concentrations. From the trends established, estimates are made of various properties of ideal defect-free graphite in the c -axis direction, and of anisotropy ratios. Effects of neutron irradiation on some of the electronic properties are also described. Results are discussed in terms of multi-carrier models.


2010 ◽  
Vol 667-669 ◽  
pp. 1089-1094 ◽  
Author(s):  
Gerda Rogl ◽  
Michael Zehetbauer ◽  
Michael Kerber ◽  
Peter Rogl ◽  
Ernst Bauer

For thermoelectrics it is important to produce thermodynamically stable bulk nanostructured materials. Ball milling/hot pressing was shown to reduce the crystallite size by a factor of 100 and to reach about 100 nm with dislocation densities of 1012 – 1013m-2. Thereby thermoelectric properties of single, double and multifilled Sb-based skutterudites were improved significantly leading to figures of merit ZT, which in some cases are twice as high as those of their microstructured counterparts. With HPT treatment the crystallite size can be decreased to even 50 nm with dislocation densities as high as 1015m-2. The small grains as well as the high dislocation density result in a further lowering of thermal conductivity holding a high potential for future enhancement of ZT.


2012 ◽  
Vol 1449 ◽  
Author(s):  
Marco Gálvez-Saldaña ◽  
Gina Montes-Albino ◽  
Oscar Perales-Perez

ABSTRACTA systematic study was carried out to determine the effect of the composition and annealing atmosphere (air and N2) on the structural, optical and magnetic properties of pure, doped and co-doped ZnO [Zn(1-y)(CoV)yO] nanocrystalline powders and films. The (Co+V) doping level, ‘y’, was fixed at 2 at% with variable individual concentrations of Co and V species. Powders and films were synthesized via a sol-gel approach where the films were grown on silicon (100) substrates. X-ray diffractometry verified the formation of the ZnO host structure after annealing of the precursor phases. The variation of the average crystallite size of Co-V (2 at.%) ZnO powders annealed in air at 500°C were negligible and averaged 33 nm. Photoluminescence (PL) measurements of powder corroborated the formation of high-quality ZnO host structure, as well as in films annealed in air. In turn, XRD and PL measurements confirmed an enhanced crystallinity of the ZnO host, with an average crystallite size of 41 nm, for films annealed at 500°C under a N2 atmosphere. M-H measurements evidenced a ferromagnetic behavior at room temperature in powders and films that was dependent on the type and amount of the dopant species.


Author(s):  
Thomas R. McKee ◽  
Peter R. Buseck

Sediments commonly contain organic material which appears as refractory carbonaceous material in metamorphosed sedimentary rocks. Grew and others have shown that relative carbon content, crystallite size, X-ray crystallinity and development of well-ordered graphite crystal structure of the carbonaceous material increases with increasing metamorphic grade. The graphitization process is irreversible and appears to be continous from the amorphous to the completely graphitized stage. The most dramatic chemical and crystallographic changes take place within the chlorite metamorphic zone.The detailed X-ray investigation of crystallite size and crystalline ordering is complex and can best be investigated by other means such as high resolution transmission electron microscopy (HRTEM). The natural graphitization series is similar to that for heat-treated commercial carbon blacks, which have been successfully studied by HRTEM (Ban and others).


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