Nanostructured AlN/YAlO3 Ceramics via Electroconsolidation (SPS): Comparative Synthesis Routes, Grain-Boundary Engineering, and a Predictive Acoustic Model for Acoustoelectronic Devices Operating at High Temperatures

We present a predictive materials-science study of nanostructured AlN ceramics incorporating a controlled grain-boundary phase, intended for acoustic signal-processing components (correlators, filters, delay lines) capable of operating in extreme thermal and radiation environments where conventional piezoelectric single crystals are impractical. We compare three electroconsolidation (spark plasma sintering, SPS) routes for introducing a controlled intercrystalline phase into an AlN ceramic matrix: (1) single-stage SPS of unmodified AlN nanopowder, in which adsorbed surface oxygen forms a thin oxynitride (Al-O-N) interlayer; (2) a three-stage route in which an open-porous AlN framework (~90% density) is impregnated with a yttrium salt solution, thermally converted to oxide, and finally consolidated to full density, forming a YAlO3 (YAP) perovskite intercrystalline phase; and (3) single-stage SPS of AlN with a CeO2 + Li2CO3 additive, forming a CeAlO3 grain-boundary phase via a low-temperature Li-Ce melt. We adopt Route 2 as the baseline for acoustic applications, since the YAlO3 phase chemically binds residual oxygen, densifies grain boundaries, and is expected to minimize acoustic attenuation and maximize signal repeatability; Route 3 is retained as an auxiliary route for tuning acoustic impedance. We derive a structural and acoustic model of the resulting AlN/YAlO3 interphase, calibrate baseline elastic and acoustic-mode parameters against real literature measurements, and explicitly flag which parameters remain predictions pending direct measurement — including two disagreeing estimates for interphase thickness. We summarize AlN's relevant property envelope (radiation tolerance mechanisms, thermal/electrical/mechanical/chemical properties) and its known technological limitations (oxygen sensitivity, cost, hydrolysis, machining and metallization difficulty), and lay out a comparative characterization programme across all three routes to be carried out at a partner materials-science laboratory.

This is a theoretical/predictive proposal preceding synthesis: all structural and acoustic predictions require experimental confirmation via the comparative characterization programme (Section 6) before the model can be considered validated.