M.Sc Thesis


M.Sc StudentLea Halimi
SubjectIntegration of a Stress-Adaptive Material and Evaluation
of the Chemical Expansion for the Mechanical
Properties Improvement of Solid Oxide
Fuel Cells
DepartmentDepartment of Chemical Engineering
Supervisor Professor Tsur Yoed
Full Thesis textFull thesis text - English Version


Abstract

A new approach for calculating the non-isothermal chemical expansion coefficient (CTEC) of gadolinia doped ceria (CGO) is presented.  Chemical expansion occurs in materials such as oxides while changing the environment (PO2, temperature, etc.). Increasing or decreasing the temperature causes changes in the oxygen chemical potential and hence chemical induced effects in the material.

 As a result, the thermal expansion coefficient (CTE) measured in the usual way while heating, contains contributions of both thermal and chemical expansion.

Modulated temperature dilatometry is used in order to sort out chemical expansion coefficient (CTEC) from total expansion.

In this work the material of choice is CGO with several doping concentrations: Ce1-xGdxO2-0.5-x-δ where 0.1≤x≤0.2. CGO is of a great interest since it is used as an electrolyte for intermediate temperatures solid oxide fuel cells (SOFC).

In addition, CGO is believed to be a “smart ceramic” that can adjust to the stress developed in the material through interaction between defects. A "smart ceramic" adjusts to the stress developed by a mechanism called "the chemical strain effect".

The chemical strain effect is a transformation of strain related energy into chemical energy of interaction between point defects. In other words, interaction between point defects in the solid can release some of the developing stress. Therefore, CTEC and CTE measurements could serve as a step forward for deeper understanding of the role of defects in mechanical response to changes in the environment.

The CTEC’s of CGO at 500, 600 and 700°C, at both low and high oxygen partial pressure (PO2) were calculated via modulated temperature dilatometery and are presented in this work.

In addition, the CGO samples at different doping concentration were prepared by two different synthesis methods: co-precipitation and glycine nitrate process (CGO). Those two methods are compared in attempt to reach dense ceramic as required from an electrolyte.