Investigation of sodium ion depletion layers in electrothermally poled bioglasses by combining impedance spectroscopy with ToF-SIMS depth profiling

The anodic process during electro-thermal poling of one bioactive and three bioabsorbable glasses was investigated by means of impedance spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The bioactive glass with the nominal composition 46.4% SiO2, 25.2% Na2O, 25.2% CaO, 3.2% P2O5 (46S4) and the three bioabsorbable glasses of the CaX series ((55 − X)% Na2O, X% CaO, 45% P2O5, with X = 20, 30, 40) were poled at temperatures between 473 K and 513 K using voltages between 50 V and 490 V. Both impedance spectroscopy studies during poling and subsequent ToF-SIMS profiling show that the thickness of the Na+ ion depletion layer under the anode increases in a linear fashion with increasing poling voltage. However, the same absolute values for the thickness were only obtained under the assumption that the dielectric permittivity of the depletion layer is about 2–3 times lower than the dielectric permittivity of the bulk glass.