Carbonated hydroxyapatite (CHA) represents a great potential for bone tissue engineering, owing to its solubility and biodegradation rate, both of which can be regulated by the quantity of carbonate substitution. Sintering of CHA must be carried out under a CO₂-rich atmosphere to prevent thermal decomposition and get carbonation. Carbonation of the hydroxyl structural site OH of the apatite result from solid-gas exchange reactions following reaction: CO₂(g) + 2 OH_OH = CO₃^-_OH + V⁺_OH + H₂O(g) (Eq. 1). Sintering studies performed under conventional resistive device revealed that carbonate ions in the OH sites delay the densification [1,2]. To reach sufficient consolidation, high sintering temperature or longer sintering dwell time are needed and may be accompanied by thermal decomposition, grain growth or uncontrolled amount of carbonate. The addition of small amount of water vapor in the atmosphere allow reversing Eq. 1 and slightly reducing grain growth but at the cost of thermal stability. The project SiBio (Sintering & Biodegradation of carbonated hydroxyapatite ceramics) aims to study the microwave sintering (MW) to overcome these drawbacks. As MW sintering is a volumetric heating process, heat treatment up to ten times faster than for classical resistive sintering can be reached. Thus, a wider range of chemistries as well as more biologically reactive microstructures can be considered.
Preliminary investigation of the MW sintering of CHA revealed the necessity to control as well the atmosphere of thermal treatment to reach pure parts [3]. This work present the ongoing development of a new MW equipment allowing to perform reactive thermal treatment. For this purpose, a single-mode cavity (SAIREM, France) working at a fixed frequency of 2.45 GHz and associated to a generator of controlled gas mixture (Serv’Instrumentation, France) is used. The presentation will be focused on the main challenges linked to the development of this new setup. First, the sample position and the length of the cavity must be adjusted to reach the resonance (stationary wave). Secondly, the cavity has to be instrumented to control the thermal treatment and follow the shrinkage. It implies the use of temperature contactless measurement with a bichromatic pyrometer (Lumasense Technology, Germany) connected to a specific homemade LabVIEW software. Then, the sintering cell made of aluminosilicate fibers (RATH®, Germany) has to be designed to carry out thermal treatments without hindering the solid-gas reactions. Finally, the MW cavity is connected to a gas mixture generator, calibrated to produce controlled atmospheres of Ar, CO₂ and H₂O gaseous mixture at a total pressure of 100 kPa.
 
This study is supported by the French “Agence Nationale de la Recherche” under grant number ANR-21-CE08-0004 and by the Federative Structure IngéLyse.

[1] Lafon, J. P., Champion, E., & Bernache-Assollant, D. (2008). J Eur Ceram Soc, 28(1), 139-147.
[2] Guillou, S., PhD Mines Saint-Etienne, 2020
[3] Petit, C., Le Tiec, A., Pancrazi, L., & Douard, N. (2022). Solid State Phenomena, 340, 119-130.