The fin surfaces of the condensing heat exchangers onboard manned spacecraft require a hydrophilic surface for effective gravity independent water separation, as well as an antimicrobial function to prevent bacterial growth. We have previously shown that silver exchanged zeolite A coating is highly hydrophilic and antimicrobial [1], and easily formed by a low temperature in-situ crystallization process. We have also shown that the coating remains highly hydrophilic and antimicrobial after long term leaching in water [2], and after many repeated bacterial inoculations. However, the silver exchanged zeolite A coating showed considerable degradation in mildly acidic and basic solutions. In addition, the zeolite A synthesis solution is highly alkaline, preventing the use of aluminum as a substrate. Aluminum alloys are much lighter and have a thermal conductivity that is roughly 15 times that of stainless steel, which would provide better performance and lower launching cost for the heat exchangers onboard manned spacecraft.

In order to remedy the issues associated with zeolite A, we propose the use of zeolite A incorporated in ZSM-5 to form a hybrid coating. ZSM-5 is a high silica zeolite which can be produced by an in-situ crystallization process using TPAOH as a template. ZSM-5 shows superior acid/base resistance, as well as corrosion resistance [3], and its synthesis solution is far less alkaline then that of zeolite A, allowing it to coat aluminum substrates. In order to achieve all desired properties, we prepared a coating that consisted of zeolite A seeds imbedded within a ZSM-5 matrix. The zeolite A seeds provided the hydrophilic and antimicrobial function, and the ZSM-5 matrix protected the seeds and allowed the coating to be produced on aluminum substrates. Data was obtained for hybrid coatings formed on top of a ZSM-5 pure phase membrane, as well as for hybrid coatings formed directly on the substrate using a template free ZSM-5 synthesis solution. All coatings were shown to be highly hydrophilic and antimicrobial after silver exchange.

References:

1. McDonnel AMP, Beving D, Wang A, Chen W, Yan YS. Hydrophilic and antimicrobial zeolite coatings for gravity-independent water separation. Advanced Functional Materials. 2005;15:336-340.

2. O'Neill C, Beving D, Chen W, Yan YS. Durability of hydrophilic and antimicrobial zeolite coatings under water immersion. AIChe Journal. 2006;52:1157-1161.

3. Cheng XL, Wang ZB, Yan YS. Corrosion-resistant zeolite coatings by in situ crystallization. Electrochemical And Solid State Letters. 2001;4:B23-26.