Additive fabrication of hydroxyapatite ceramics using millimeter-wave and sub-terahertz radiation

This paper reports recent results on the use of millimeter-wave and sub-terahertz radiation for additive manufacturing of hydroxyapatite ceramic parts for prospective biomedical applications. Layer-by-layer sintering has been implemented using 24 GHz / 5 kW gyrotron systems for high-temperature processing of materials. Layers of a hydroxyapatite suspension prepared by colloidal processing were deposited by doctor blade on a 70 % dense pre-sintered hydroxyapatite substrate. The layer thickness was 0.1 – 0.15 mm. Each layer was sintered in a separate millimeter-wave heating run with a heating rate of 10 – 20 °C/min to a maximum temperature of 1150 °C without an isothermal hold. After cooling, the next layer was deposited and the sintering process was repeated. The obtained multi-layer samples had uniform structure with no visible boundaries between the layers in the SEM images. Another approach was based on localized heating by a focused beam of sub-terahertz radiation. The radiation source was a 263 GHz / 1 kW cw gyrotron which is capable of delivering an intensity of up to 25 kW/cm2 when full power is focused into a beam with a transverse size on the order of the wavelength. The hydroxyapatite powder was contained in a crucible made of porous alumina. The dimensions of the powder layer were 57 × 12 × 2.5 mm. The powder layer was illuminated by the focused radiation beam so that the focal point was at its surface. By moving the crucible relative to the beam at a velocity of about 0.5 mm/s, the scanning of the wave beam over the powder surface was implemented. Upon exposure to the focused sub-terahertz radiation with a power of about 100 W, localized consolidation of the powder was achieved in a 3–5 mm wide zone. As demonstrated by numerical modeling, focused radiation of the millimeter-wave band (≥24 GHz) can also be used for localized sintering with a corresponding increase in the focal spot size. The rising dependences of the material’s millimeter-wave absorptivity on temperature and density result in narrowing the heated area in the process of sintering. This approach can have prospective applications in orthopedic medicine due to a possibility to develop porosity gradients. On the whole, the reported results indicate that the use of millimeter-wave and sub-terahertz radiation has an application potential in developing additive methods of controlled localized consolidation.