Title

Etude des propriétés physiques et électriques de matériaux céramiques utilisés en application spatiale

Date of Award

2015

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Sophie Guillemet

Second Advisor

P Thierry

Abstract

Dielectric materials used on satellites are subject to radiative and thermal extreme stresses which may lead to disturbances on board instrumentation. The application efficiency can then decrease significantly due to charging and aging effects of used ceramics. With the aim to understand and predict these phenomena, the mechanisms investigation of charges transport and electrical aging on these ceramics is of high importance. The scientific approach of this study was to define a protocol and an experimental method which allows characterising the electrical and physico-chemical behaviours of raw boron nitride and coated with a thin coating of alumina. For this purpose, a parametric study was performed in the irradiation chamber, named CEDRE (at ONERA Toulouse) in order to assess the influence of some parameters such as, incident energy, primary electron flux, temperature, ionising dose, on charging, relaxation and electrical aging kinetics of these industrial ceramics. This study demonstrated that it is possible to greatly limit the dielectrics charging thanks to the use of a ceramic coating and suitable annealing thermal treatment. Indeed, the high secondary electron emission of alumina and the increase of surface conductivity generated by the annealing thermal treatment partly govern the low surface potential of coated boron nitride. Some alumina coating were subsequently elaborated through PVD-RF and then characterised in the irradiation chamber in order to identify the preparation parameters which allow optimising the electrical properties of system. It was shown that the optimisation of the roughness and the coating thickness limits the surface potential of ceramics. An experimental study was conducted in the frame of an international collaboration with the Materials Physics Group of the Utah State University (Logan, USA), in order to investigate the influence of nature and densities of electron defects on the electrical properties of different ceramics. The cathodoluminescence method was used and brought to light the origin of total conductivity difference between materials, raw, coated and annealed. A new method to measure the surface potential under continuous electron irradiation was developed and then validated. A partial discharges mechanism was identified on surface of annealed samples with this optimised device. Ageing processes of the irradiated materials was also studied in the irradiation chamber to reproduce the observed degradation in orbit over the long time. It was demonstrated that the charging of annealed coated materials is noticeable when the sample receive a critical ionising dose. Several physico-chemical characterisations were thus performed at CIRIMAT in order to study the evolution of structural and chemical properties of ceramics. This evolution was correlated with that of electrical properties after deterioration under critical electron irradiation. The contamination and deterioration mechanisms of coated ceramics are responsible of the electrical aging observed experimentally. Finally, these thorough experimental characterisations allowed the development of physical model for the description of the different mechanisms involved on irradiated ceramics and coating.

Comments

Visiting Scholar from Office National d’Etudes et Recherches Aérospatiales (ONERA), Département Environnement Spatial (DESP) Toulouse, France, PhD Université Paul Sabatier, Toulouse, France, November 2015.

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