Millimeter wave absorption in hydroxyapatite and 3YSZ ceramics in wide temperature range

Abstract

[EN] In the field of ceramic-based materials processing the last three decades has been marked by significant academic and industry interest in Additive Manufacturing (AM) technology due to its capability to produce ceramic parts with complex geometry and customizable materials properties. Conceptually, AM technology is a layer-by-layer fabrication of three dimensional physical parts directly from computer-aided design [1]. Solidification of the parts prepared from substances containing ceramic powder may be performed either by conventional heat treatment of a part as whole or by directed energy deposition. Both these strategies can be implemented using gyrotron-based millimeter-wave facilities allowing alternatively both the uniform heating of large-size parts in multi-mode cavities and local heating by focused wave-beams [2]. Hydroxyapatite- and yttria-stabilized zirconia-based ceramics are widely used in biomedical applications due to their high biocompatibility. The knowledge of their microwave absorption variation with temperature and porosity as the materials are densified, is necessary to optimize the scheme of microwave heating. 8 mm diameter disks for the measurements were prepared by uniaxial compacting from commercially available hydroxyapatite (HA) powder and yttria-stabilized zirconia (3YSZ) powder (Tosoh corp.). The measurements were performed at 24 GHz 3 kW gyrotron system. Samples for measurements were placed into the gyrotron system applicator and surrounded with porous alumina based thermal insulation. The design of the applicator and insulation allowed performing optical measurements of both the sample size and temperature distribution over the surface of the sample using a digital monochrome CCD camera. Measurements were made by the calorimetric method, when the microwave power absorbed in the sample is determined basing on the difference of the heating/cooling rates at the moments of intentional abrupt change of the microwave power at different sample temperatures. Absorption coefficient was determined as a division of the absorbed power to the incident microwave power. Special calibration experiments were made for determining microwave power density in the applicator and inside the thermal insulation. The method allows to measure absorption coefficients in situ during the sintering process. Absorption coefficients of HA were obtained in the range of 200 C - 1200 C, and for 3YSZ - in the range of 400 C - 1400 C both in situ during sintering and for as sintered samples. Dependencies of the absorption coefficients on the temperature and porosity are discussed.