Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.794357
Title: Structure and properties of materials under extreme conditions
Author: Moody, Gregory Stephen
ISNI:       0000 0004 8499 5227
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2019
Availability of Full Text:
Access from EThOS:
Access from Institution:
Abstract:
The method of in situ neutron diffraction was used to investigate the pressure-induced structural transformations at ambient temperature, in the modified silicate network glasses MgSiO3 and CaSiO3, and the chalcogenide glass AsSe. Additionally, a series of calibration experiments were conducted to investigate the power-temperature relationships of a high pressure high temperature (HPHT) setup for neutron diffraction. The structure of magnesium silicate glass (MgSiO3) was investigated at pressures up to 17.5(5) GPa using in situ neutron diffraction. The densification process was found to be dominated by an increase in the Mg-O coordination number n̄ O Mg from 4.50(5) at ambient to 6.1(1) at 17.5(5) GPa. Additionally, the distribution of Mg-O bond lengths r MgO was found to be highly asymmetric at ambient conditions. This asymmetry is manifested by a high r shoulder originating from the Mg-O pair-distribution function gMgO(r ), which disappears at higher pressures. In contrast, no change was observed in the Si-O coordination number or bond length. The experimental work was complemented by molecular dynamics simulations. It is proposed that the pressure induced change to the Mg coordination environment is driven by an increase in the fraction of magnesium to bridging oxygen atom bonds in the network. The structure of calcium silicate glass (CaSiO3) was investigated at pressures up to 17.5(5) GPa using in situ neutron diffraction. A small increase in the Si-O coordination number n̄ O Si was observed, from 4.0 at ambient, to 4.12(10) at 17.5(5) GPa. This coordination change is intermediate between that measured for MgSiO3 and SiO2 glasses. It is proposed that the inclusion of a network modifying cation to silica glass delays the pressure-driven distortion of the SiO4 tetrahedra, and that Mg2+ delays this process to a greater extent than Ca2+. The experimental work was complemented by molecular dynamics simulations. The Ca-O correlations are not directly accessible using neutron diffraction, but the accompanying molecular dynamics simulations have predicted an increase in the Ca-O coordination number n̄ O Ca from 6.0 at ambient, to 7.4 at 17.5 GPa. It is proposed that, as in the case of MgSiO3, the increased Ca coordination number is driven by an increase in fraction of calcium to bridging oxygen atom bonds present in the network. The structure of arsenic selenide glass (AsSe) was investigated at pressures up to 14.4(5) GPa using in situ neutron diffraction. It was found that the effective coordination number n̄ 0 decreases from a value of 2.35(10) at ambient, to 2.1(1) at 14.4(5) GPa. This result contrasts with previous work on the lower As concentration glass As0.4Se0.6, where a lower ambient effective coordination number of 2.2(1) was measured, and this value remained constant across the pressure range. It is proposed that the higher ambient effective coordination of AsSe results from the higher concentration of As in the glass, and that As is more susceptible to a pressure driven coordination change. Across the measured pressure range, the pair-distribution functions show that the position of the peak associated with nearest neighbour distances remains constant, whilst the peaks associated with intra-molecular distances shift to lower values of r. It is therefore proposed that the densification process of AsSe glass is dominated by a shortening of intra-molecular distances, rather than nearest neighbour bonds. A series of calibration experiments were conducted to investigate the power-temperature relationship of a HPHT setup for neutron diffraction. Generally a linear relationship was observed, which was verified from fits, between power and/or gasket temperature, and sample temperature. Furthermore, an increase of sample pressure lowered the sample temperature for a given power. The results show that it is possible to operate the HPHT setup in an automated manner i.e without the need for a calibrant material, and that HPHT setups with altered components and/or dimensions could be calibrated in a similar way.
Supervisor: Zeidler, Anita ; Salmon, Philip Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.794357  DOI: Not available
Share: