The work performed within Pi4NoRM pursued the consecution of two scientific and technical objectives:
1-Optimizing the scale-up of ε-Fe2O3 production and sintering for integration on circulators
2-Designing and testing proof-of-concept thin film circulators
For achieving Objective 1, the three following tasks were implemented:
T1.1 Optimising the scale-up of ε-Fe2O3 synthesis
We have optimized the bulk sol-gel synthesis of large high aspect ratio ε-Fe2O3 in amorphous SiO2 matrices, considering the influence of thermal treatment and the addition of Y3+ and other rare earths (La3+, Dy3+) to obtain batches of above 5 g material, avoiding the crystallization of SiO2 and the formation of undesired polymorphs. We have obtained insights into the nanorod growth mechanism, and we have investigated modifications of the synthesis protocol to accelerate the gelation, which is the step that limits the production turnover.
T1.2 Preparation of sintered ε-Fe2O3 parts. The prepared materials were sintered into 8 mm diameter disks of thicknesses ranging between 1 and 2 mm by Spark Plasma Sintering (SPS) under different pressures ranging from 50 to 450 MPa after optimizing the sintering time and temperature. Tests of standard sintering at high temperature in a furnace after isostatic pressing of the powders revealed some degree of transformation to hematite. The sintering of nanocomposites before etching the silica was not successful (friable disks were obtained).
T1.3 Exploiting the piezomagnetic effect in ε-Fe2O3 circulators We further investigated the piezomagnetic effect in ε-Fe2O3 disks sintered by SPS, and it was not possible to systematically reproduce the responses of pressure-induced magnetization previously observed in disks prepared by SPS. Since for most samples a zero magnetization was observed in the previous measurement the magnetization was low, the task was abandoned. Moreover, within the materials preparation objective, we also prepared Al-substituted Sr, Ca hexaferrites of composition Sr0.75Ca0.25Al3Fe9O19 with natural resonances above 100 GHz within the WB8 band (90-140 GHz), which is a complementary material to ε-Fe2O3, with its natural resonance lying within the WB5 band (140-220 GHz).
Objective 2 was implemented through the following tasks:
T2.1 Design and simulation of Y-junction circulators. We simulated the response of the ε-Fe2O3 and hexaferrite materials for Y-junction circulators for the WR8 (90-140 GHz) and WR5 (140-220 GHz) bands. The optimized geometries of the sintered ferrites corresponded to disks of 1 mm diameter and 0.7 mm thick, and 0.7 mm diameter and 0.5 mm thick for the WB8 band.
T2.2 Implementation and testing of waveguide circulators Y-junction circulators were fabricated, based on the designs of T2.1 to operate within the range 140-220 GHz, containing ferrites in the junction. The testing of the circulators revealed moderate non-reciprocal response, without externally applied magnetic fields, for Sr0.75Ca0.25Al3Fe9O19 in the W8 band and ε-Fe2O3, in the WB5 band. Differences of less than 5 dB between S21 and S12 (transmissions in opposite directions), which are still far from the 15 dB required for a commercial product.
T2.3 Implementation and testing of thin-film circulators. We performed tests to implement strip circulators in the WB8 and WB5 bands by depositing gold coplanar waveguides on commercial Rogers substrates, which showed strong losses, which limited the progress in the implementation of thin-film circulators.
