| dc.description.abstract | The present work has highlighted the deuterium behaviour during implantation in Cu, Ti and Pd, particularly relating to its migration, trapping and deuteride formation. The technique of neutron?yield measurement during implantation along with the theoretical analysis of the neutron?yield behaviour provided information on the dynamics of defect interactions. SIMS, glancing?incidence XRD and SEM studies on deuterium?implanted samples were employed to study the evolution of implanted deuterium profiles, structure of deuterides and surface topography. The main conclusions are:
i) During implantation, the deuteride phase in Ti and possibly in Pd forms at the surface and then grows inwards rather than forming at the peak of the implantation profile.
ii) The structure of the titanium deuteride was found to be ??TiD? which is stabilised by irradiation.
iii) The diffusion of deuterium in Cu, during implantation at room temperature, is via a vacancy?mediated migration of deuterium trapped in vacancies. A diffusion constant around 5 × 10?¹? cm²/sec is estimated.
iv) The concentration profile of deuterium implanted in copper obtained by SIMS matched with the damage profile, further supporting deuterium trapping by defects.
v) The trapped?deuterium profile in Cu shows interesting features. The deuterium profile splits into two peaks at a dose of 1 × 10¹? D? ions/cm², which is much higher than the minimum dose for blister formation. This behaviour is attributed to the control of nucleation and growth of bubbles by the lateral compressive stress generated in the implanted layer of Cu.
vi) The time dependence of the neutron yield shows a power?law behaviour whose exponent is correlated to various solid?state processes occurring during implantation.
vii) The neutron?yield rate showed peaks with a time constant of ~10 seconds. These peaks are attributed to the pressure?driven growth and relaxation kinetics of bubbles.
Scope for further experimental and theoretical studies in this area are indicated below:
— Deuterium atoms at room temperature are trapped in vacancies, from which they diffuse by a substitutional (vacancy or divacancy) or dissociative mechanism. In order to obtain the dominant mode of diffusion among these, knowledge about the diffusion rates due to these processes is essential for comparing these with the experimental values. At present, there are no such studies reported in the literature. A possible method for obtaining these is to calculate the activation energies using the Embedded Atom Method [31,32].
— The peaks in the neutron yield are related to the pressure?driven growth and relaxation kinetics of bubbles. A detailed stability analysis of the coupled rate equations is necessary to understand these processes in detail.
— The development of the bubble structure as a function of implantation dose by transmission electron microscopy would provide important microstructural information on deuterium?bubble structure, bubble size and density which are essential input parameters for understanding the oscillation in neutron?yield behaviour and the splitting of the deuterium concentration profile.
— Measurement of deuterium pressure inside the bubbles is required. At present, no such experimental data exist.
— The deuteriding behaviour in palladium needs further experimental investigation at room temperature and at 134 K by in?situ measurement of deuterium depth profiles during implantation.
— The structure of deuterides formed during implantation is likely to be different from that formed under thermal conditions. More experimental investigation on deuteride formation during implantation is needed. It will be interesting to extend these studies to Hf and Zr systems which are expected to behave similar to Ti.
— Ion implantation can provide a powerful tool to study the behaviour of hydrides at high deuterium concentrations where preparation of hydrides by other methods is difficult.
— The deuterium diffusion in deuterides having high [D]/[Ti] ratio is going to be very different as most of the low?lying solution sites are occupied. More experimental and theoretical investigations in this area are required.
— We have observed that the presence of deuterium enhances the SIMS signal from matrix atoms of Ti. This requires further investigation by SIMS at high mass resolution. The SIMS study at high mass resolution will also enable direct quantification of deuterium signals with the use of D? signal from different isotopes of Ti. | |
