# Caractérisation diélectrique des hydroxyapatites carbonatées à hautes températures

Romain Journiac, Giuseppe Annino, Roberto d'Ambrosio, Andrea Cintio, Clémence Petit, Christophe Meunier, Nathalie Douard, David Eglin

► To cite this version:

Romain Journiac, Giuseppe Annino, Roberto d'Ambrosio, Andrea Cintio, Clémence Petit, et al. Caractérisation diélectrique des hydroxyapatites carbonatées à hautes températures. Journées Annuelles du Groupe Français de la Céramique, Mar 2025, Orléans, France. hal-05003840

HAL Id: hal-05003840

https://hal.science/hal-05003840v1

Submitted on 25 Mar 2025

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
## CONTEXT AND OBJECTIVES

* Carbonated hydroxyapatite (CHA) is a biodegradable ceramic suitable for bone substitution. Its biodegradation can be tailored by its microstructure and the location and amount of carbonate ions substitution in HA's lattice [1]. Carbonate ions can be located at both the hydroxyl (A-site) and phosphate (B-site) sites of HA ($Ca_{10}(PO_{4})_{6}(OH)_{2}$), giving respectively A-type carbonated HA ($C_{A}HA$) or B-type carbonated HA ($C_{B}HA$).

* $C_{A}HA$ can be obtained by sintering of HA under $CO_{2}$ rich atmosphere. However, due its inertia, conventional (CV) sintering does not allow for precise control of the final microstructure and carbonate substitution [2].

* Microwave (MW) sintering presents an interesting alternative for producing $C_{A}HA$ with versatile physico-chemical features allowing better modulation of its biological behavior. Preliminary results [3] highlighted the need to optimize the MW heating setup to minimize, among other factors, thermal gradients. Additionally, a thorough understanding of the dielectric properties would enable better control of MW heating.

The dielectric properties are defined by: $ε^* = ε' + jε''$

$ε^*$: complex dielectric permittivity with $ε'$: real part and $ε''$: imaginary part (dielectric loss)

The absorbed power during MW heating is: $P = 2πfε''E^2$

$E$: norm of the electric field, $f$: frequency of electromagnetic wave

* To produce CHA with controlled physico-chemical properties through MW sintering, this study aims at investigating the evolution of $ε'$ and $ε''$ of HA with temperature under different atmospheres using both CV and MW heating setups.

## EXPERIMENTAL SETUPS AND PRELIMINARY RESULTS

All measurements are made on HA green pellets (diameter: 6 mm) obtained by uniaxial compression of a powder synthesized by co-precipitation and calcined at 950°C. Its specific surface area is $5.9 ± 0.2 m^2.g^{-1}$ (preparation of powder: CERAMATHS, Univ. Polytechnique Hauts-de-France).

### Principle of the measurement

The dielectric properties are measured by using the cavity perturbation method. In this method, the presence of a material in a resonant cavity changes the resonance frequency and the quality factor of the cavity, compared to the empty cavity. $ε'$ and $ε''$ are calculated from the resonance frequencies and the quality factors of the empty and the filled cavity [4]. Two setups are used in this study to heat the sample: one based on a CV heating and one based on a MW heating.

### 1 - Setup based on MW heating

**Setup presentation:**

The heating and measurement steps are carried out in the same resonant cavity. This latter is connected to a gas circulation system (to work **under controlled atmospheres**). A thermal camera measures the temperature. A tunable MW source (2.4-2.5 GHz) and the Vector Network Analyzer (VNA) are connected to enable heating of the sample and acquisition of the resonance curves.

**Improving sintering homogeneity:**

MW heating must be homogeneous. For this purpose, an optimization of the sample's thermal insulation is necessary. Pellets were heated in the MW setup with different sample's surroundings. On the picture: heating was more and more homogeneous from left to right.

<figure><img src="image_2.png" /><figcaption>Picture of the heating tests results</figcaption></figure>

After numerous heating tests (from 20°C to 885°C at 20°C/min in air), a suitable configuration is achieved. It is composed by a refractory shielding and 2 SiC rings used as susceptors. The refractory upper part is pierced for temperature measurement. In addition, 2 disks of sintered HA are used to insulate the upper and bottom faces of the sample.

<figure><img src="image_3.png" /><figcaption>(a) Picture of the cylindrical box surrounding the sample and (b) scheme of the cross section of this box</figcaption></figure>

<figure><img src="image_4.png" /></figure>

### 2 - Setup based on CV heating

<figure><img src="image_5.png" /><figcaption>Picture of the CV setup in Pisa</figcaption></figure>

**Setup presentation:**

The sample is located in a quartz tube and held between two quartz rods. This sample holder is placed inside the furnace, at a given temperature during a holding time of 20 min for thermal equilibrium. Then, the set is quickly moved down to a resonant cavity during 0.5 sec. A VNA records the resonance curves. An offline treatment on COMSOL is performed to determine $ε'$ and $ε''$ by fitting experimental resonance curves. With this setup, the measurements can be carried out up to 1100°C **under air or under vacuum**.

### Temperature measurement of the dielectric properties of HA using the CV setup:

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>Dielectric properties measurements of HA under heating using the CV setup: (a) $ε'$ and (b) $ε''$</figcaption></figure>

* $ε'$ and $ε''$ steeply increase from 800°C.

* The increase of $ε''$ is related to a better absorption of MW energy by the material.

* From 500°C, HA undergoes the dehydration reaction: $2OH_{aH} \rightleftharpoons V_{OH}^{0} + O'_{OH} + H_{2}O(g)$

* At 900°C, HA's densification starts, the sample shrinks.

* Possible link between the composition of the A site in the HA lattice and the evolution of dielectric properties.

* These results show that even a slight local overheating can induce an increase of dielectric losses and subsequently an uncontrolled heating.

## CONCLUSIONS AND PROSPECTS

* A setup, dedicated to the dielectric characterization, was optimized to heat homogeneously the HA samples and perform the measurements in the same cavity.

* First measurements were performed via CV heating and showed an increase of dielectric loss for the HA pellets from 800°C. A link can be done with the composition of the A sites. To confirm this hypothesis, similar measurements will be further performed under vacuum and $CO_{2}$/water vapor mixed atmospheres.

* These first results highlight the importance of the A site composition in the MW/material interactions (and thus, the link with the carbonatation of the A site).

## References

[1] Spence G. et al., 2006, Key Eng. Mater. 309-311, 133-136.

[2] Guillou S. et al., 2022, J. Therm. Anal. Calorim. 147, 13135-13150.

[3] Petit C. et al., 2022, Solid State Phenom. 340, 119-130.

[4] Dube D.C. et al., 1988, J. App. Phys. 63, 2466.

## Acknowledgements

This study was funded by the JECS Trust (Contract: 2024 404) and the French Agence Nationale de la Recherche (Project SiBio, grant number: ANR-21-CE08-0004).