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Reversible phase transformations in SeTe(As/Sb) combining ultrafast DSC and electron microscopy

Calon, J. E. M (2016) Reversible phase transformations in SeTe(As/Sb) combining ultrafast DSC and electron microscopy. Master's Thesis / Essay, Applied Physics.

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The reversible crystalline-amorphous transition of SeTeAs/SeTeSb has been investigated by the use of ultrafast DSC by using heating rates of 1 to 104 K/s. In the SeTeSb sample, Se3Sb2 crystals where formed which limited the influence of the antimony compared to SeTe. Furthermore, it was possible to switch SeTeAs reversibly between the crystalline and amorphous phase. SeTeAs was found to be a better glass former than SeTe, the critical quench rate of SeTeAs was found to be a factor 100 times lower than SeTe. At low temperatures, new transitions started showing up below 220 °C which decreased the heat capacity during crystallization and increased the heat capacity in another transition occurring at 250 – 300 °C. Isothermal measurements at 140 – 160 °C also showed a transition around 200 – 220 °C when the sample was reheated. Because ultrafast DSC did not provide a full explanation for the transitions occurring, a new method has been successfully developed to transfer samples treated in a Flash DSC to a scanning (SEM) or transmission electron microscope (TEM). By the use of this method the structure of SeTe was investigated with a TEM which was fully crystalline with crystals in the size order of 0.1 to 1 µm. Furthermore, heat treated samples of Se45Te45As10 that were crystallized at 5, 100 and -100 K/s were investigated with a TEM. These samples had a spherulitic structure which contained crystal lamella in the size order of 100 nm which was surrounded by amorphous material. Both SeTe and SeTeAs were measured to have a crystal structure belonging to the trigonal lattice system and had a P 31 2 1 symmetry. By the use of electron diffraction on SeTeAs, the lamella are found to grow in the {-1 2 0} or {-1 3 0} direction. Furthermore, Energy-Dispersive X-ray Spectroscopy (EDS) and Scanning Transmission Electron Microscopy (STEM) were used to investigate the composition of SeTeAs. It was found that the arsenic segregated into the amorphous phase (15 - 20 at%). Also, the amorphous phase contained 10 to 50 at% more selenium and 20 – 50 at% less tellurium than the crystal phase. The heating rates of 5 and 100 K/s did not seem to have a large influence on the composition and structure.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Applied Physics
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 08:30
Last Modified: 15 Feb 2018 08:30

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