Use of geopolymers as innovative materials for wicking

Wicking is the transport of a fluid through a porous medium due to capillary suction at liquid-gas interfaces at the surface or within the porous medium. Porous wick structures are integral part of fluid capacitance and transport in many systems, such as heat pipes. A heat pipe is a heat transfer device widely used in space applications. It can exchange large quantities of heat by means of evaporation and condensation of a working fluid. The evaporator is the heart of the device and it contains capillary-porous structures generally based on metals or organic polymers. Both plastic and metals have some drawbacks such as low capillarity (organics) and high diffusivity, thermal expansion and high cost (metals). A possibility to increase the capacity of thermal exchange is through the replacement of conventional materials with ceramic like materials, such as geopolymers, thanks to their fine interconnected porosity, high thermal and chemical inertia and low thermal conductivity. Geopolymers are alkali bonded ceramics produced at low temperature (< 100 °C) by reacting an aluminosilicate powder with an aqueous alkali hydroxide and/or alkali silicate solution. The microstructure of the geopolymer matrix consists of nanoprecipitates and mesopores. Moreover, geopolymerization allows the tailoring of the porosity by direct and indirect foaming methods. It follows that geopolymers can be used to develop porous near-net-shape components in the pore size range from nanometres to millimetres and total porosity from 30% up to 90%. This study is based on the prototyping of the evaporator of a loop heat pipe device (LHP), where conventional wicks are replaced by innovative geopolymer wicks. In detail, two concentric wicks were realized by optimizing the compositions and the shaping process conditions. The external (primary) wick, with fine and uniform porosity for the development of an high capillary pressure, was achieved through the casting in situ, while the macroporous secondary wick was produced by direct foaming. Preliminary tests with acetone as working fluid, in the range -35°C ÷ +60°C, revealed as the tailored porosity of the wicks allowed the operation of the LHP, furthermore, the use of geopolymer materials combined with the production process at low temperature and with simple process techniques allowed to lower the production costs of the wicks.