(718e) Giant Electret Polarization in Electrochemically Deposited Hydroxyapatite Ceramic Coatings

Calcium phosphate coatings synthesized electrochemically were discovered to retain extremely large quasi-permanent electrical polarization. The coatings are synthesized onto a cathode surface from an aqueous electrolyte solution under a low temperature galvanostatic process. Calcium cations are concentrated near the cathode surface due to Coulombic attraction, while hydroxide ions are generated at cathode surface due to water splitting reactions. Changes in pH and ionic strength near the cathode surface result in the nucleation and growth of a thin calcium phosphate coating. X-ray diffraction spectra show that the only crystal phase observed in the coating is hydroxyapatite, although other amorphous phases are likely present. The coating grows within the electrical double layer at the cathode/electrolyte interface, where ions experience extreme electric fields and ion concentration gradients. As the coating grows on the cathode surface, ions become trapped in the dielectric calcium phosphate layer. These trapped charges charges result in an internal electric field in the coating oriented in the opposite direction as the applied field during synthesis. Since the trapped charges are immobilized at room temperature in the solid coating, the calcium phosphate is an electret with stored charge among the highest measured for any material. The unique polarization mechanism will be discussed, and biomedical applications of the hydroxyapatite coatings in implants will be demonstrated. In addition, synthesis of metal and semiconductor nanoparticles on the coatings will be described. When acting as a support for metal and semiconductor particles, the coatings may impact catalytic processes due to the strong internal electric field.