Microstructural Engineering of Hydroxyapatite Membranes to Enhance Proton Conductivity

Abstract

A new approach to enhancing proton conductivity of ceramics is demonstrated by aligning proton conductive pathways and eliminating resistive grain boundaries. Hydroxyapatite (HAP) membranes are synthesized by multistage crystallization onto palladium. The synthesis involves three steps: electrochemical deposition of HAP seeds, secondary hydrothermal crystallization onto the seed layer to promote c‐axis growth normal to the substrate, and tertiary hydrothermal crystallization to promote a‐axis growth to fill the gaps between the aligned crystals. The c‐axis alignment with crystal domains spanning the membrane thickness significantly enhances proton conduction since protons are primarily transported along the c‐axes of HAP crystals. The novel HAP membranes display proton conductivity almost four orders of magnitude higher than traditional sintered HAP ceramics. The HAP membranes on palladium hydrogen membrane substrates hold promise for use in intermediate‐temperature fuel cells, chemical sensors, and other devices. The synthesis approach presented may also be applied to other ion‐conducting membrane materials to enhance transport properties.

Experimental Visualization of Chemical Bonding and Structural Disorder in Hydroxyapatite through Charge and Nuclear-Density Analysis

“…The nuclear density maps in Figures c, d, a, and d also show larger spatial distributions of hydrogen. These results strongly suggest the higher diffusivity of protons compared with other ionic species in HAp, which is consistent with the proton conduction in HAp reported in the literature. , …”

Control of hydroxyapatite coating by self‐assembled monolayers on titanium and improvement of osteoblast adhesion

“…EDX spectra (Figure ) of the three HA coating comprises O, P, and Ca peaks, and the Ca/P ratios are 1.38, 1.39, and 1.42, respectively, which are less than that of stoichiometric HA (that is, 1.67). The nonstoichiometry of HA is consistent with reports from literature on hydrothermal synthesis of HA crystals …”

Thermodynamics-Oriented Crystal Growth Enables Lignocellulose-Enticed Nanostructured Hydroxyapatite with Boosted Phosphorus Fertilizer Utility

“…Compared to the unmodified HAP, the lignocellulose-modified HAP showed inferior TG curves (left panel of Figure 2c) and prominently discerned peaks on DTG curves (middle panel of Figure 2c), indicating that the introduction of lignocellulose during synthesis decreased the thermal stability of the nanoenabled HAP. In the temperature domain below 800 °C, the weight losses (∼400 and ∼700 °C) on the lignocellulose-modified HAP could be ascribed to those deposited carbonaceous products arising from the hydrothermal carbonization of the added lignocellulose, 15,46 corresponding to the observed discrepancy of percent yield mentioned before, while in the domain above 800 °C, the weight loss arose from the thermolysis of those OH groups in the HAP nanocrystals, 47 where any marginal loss was observed on the unmodified HAP, except for mTCP-HAP and TCP-HAP that the decomposition of present CaCO 3 also contributed to the loss evidenced by the continuous loss signal up to 1000 °C. These statements were also supported by the changes of these DTA curves of the modified HAP (right panel of Figure 2c), in which deceased heat inputs were observed due to the decomposition of carbonaceous deposits below 800 °C, while increased heat was required above 800 °C when compared to the unmodified HAP.…”