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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.

This paper describes a study of rapid microwave sintering of zinc-oxide based varistor ceramics undertaken with the goal to elucidate the mechanisms that underlie fast densification of these materials and compare them with those acting in other oxide ceramic materials in which ultra-rapid microwave sintering has been demonstrated previously. The samples used in the experiments were compacted from a commercial powder mixture with a nominal composition ZnO + 1.5 mol. % Bi2O3 + 1 mol. % Sb2O3. The microwave sintering was carried out on gyrotron systems for high-temperature processing of materials, operating at a frequency of 24 GHz with a maximum output power of 5 kW [1]. The samples were heated at a constant heating rate, selected in the range of 10 – 130 °C/min, up to a temperature of 1100 – 1300 °C with no isothermal hold. The temperature of the center and periphery of the samples, the automatically regulated microwave power and the shrinkage determined using an optical dilatometry systems were recorded throughout all experiments. To isolate the influence of the temperature non-uniformity arising during volumetric microwave heating, some heating runs were carried out with the sample enclosed in a SiC susceptor ring. In addition, dc electric conductivity of the samples was assessed in situ during microwave heating. In all processes carried out without the SiC susceptor ring a pronounced decrease in microwave power occurred at a temperature of about 600 °C. Simultaneously, an abrupt increase in the densification rate was observed. These are manifestations of the thermal instability associated with the liquid phase formation, which has been previously demonstrated to be a possible reason for ultra-rapid densification during microwave sintering of different ceramic materials [2–4]. In the presence of the SiC susceptor the thermal instability was not observed, and the maximum of the densification rate shifted towards higher temperatures. The temperature difference between the center and the periphery of the samples heated without the susceptor reached 200 °C due to a significant amount of microwave power absorbed in them volumetrically. However, the final sintered density of these samples, 95 – 96 % of the theoretical value, was not lower than the density of the samples heated uniformly using the susceptor to the same center temperature.