Diffusion of oxygen into polycrystalline Li-Ti ferrites under heavy electron exposure

The diffusion of polycrystalline ferrites under heavy electron exposure was studied in this work. The effects of radiation were evaluated by comparing the diffusivities of oxygen in the irradiated samples with those in the samples that received the heat treatments at the same temperatures and for the same time (873-1173 K and 240 min respectively). The diffusion coefficients of oxygen ions was determined by the new method based on measuring the activation energy of conductivity in the surface layers of ferrites. The basis for this method is the model of grains and interlayers, according to which high-conducting grains are separated by low-conducting inter-grain boundaries. Chemisorption of additional quantities of oxygen causes bending of the energy zones with the resulting formation of the inter-grain potential barrier. Thus, the depth profile of the oxygen accumulated in interlayers can be obtained from measuring the activation energy of conductivity of thin layers of the ferrite ceramics at different distances from the original surface. The diffusion coefficients of oxygen in the material under study can then be determined by fitting the corresponding solution of diffusion equation to this profile. Experimental evaluation of the above method was performed using Li-Ti ferrites synthesized by the ceramic technique. Samples were formed in tablets of 18 mm diameter and 3 mm thickness using unilateral moulding. Then they were sintered in air at 1100 0C during 240 minutes. The resistive furnace and the ELV-6 electron accelerator (manufactured by the Institute for Nuclear Research, Novosibirsk, Russia) were used to perform the thermal and thermo radiation anneals of ferrites. The accelerator provided a continuos 1.4 MeV electron beam. Surface conductivity was measured using two nichrome probes adjusted to the surface of the sample. The grain D and grain boundary Db diffusion coefficients of oxygen for the thermal anneal of ferrites and for the thermo radiation anneal were determined by approximating the experimental profiles with the corresponding solutions of diffusion equation. Approximation of profiles with the solution of Fick equation describing the diffusion from an unlimited source into semi-infinite crystal was used to evaluate Db for below mean anneal temperatures and D at warm temperatures. At mean anneal temperatures we used the solution of Fisher and Wipply for approximation. The diffusion coefficients determining with the solutions of diffusion equation at anneal temperatures of 600-900 degrees centigrade were equivalent. It can be seen that thermo radiation annealing results in increased values of the activation energy of diffusion and the preexponential factor D0