Effect of processing parameters on the properties of freeze-cast Ni wick with gradient porosity

TL;DR

This study demonstrates a novel freeze-casting method to produce gradient porous Ni wicks in a single step, optimizing pore structure and capillarity for heat pipe applications.

Contribution

It introduces a single-step freeze-casting process for gradient porous Ni wicks, eliminating the need for joining separate wicks and enhancing liquid transfer efficiency.

Findings

Stearic acid improves pore stabilization and enlarges pores.

Different dispersants and thermal gradients produce distinct pore morphologies.

Gradient porosity can be controlled via thermal gradient and solidification velocity.

Abstract

Wicks are the main component of Loop Heat Pipe systems, whereby coolant liquid flow through their porous structure. They are usually formed by a primary wick to produce liquid transportation by capillary force, and a secondary wick that is continuously wetted by the liquid coolant. Traditionally, the two wicks are manufactured separately and subsequently joined, thereby creating an interface that reduces the liquid transfer efficiency. In order to overcome this situation, a gradient porous wick is proposed and successfully manufactured through the freeze-casting method in a single operation. The influence of two different dispersant agents, KD4 and stearic acid was studied on the processing parameters, final pore size and morphology, and capillarity performances. A variate of gradient porosity was obtained by applying a diverse thermal gradient and solidification front velocity during directional solidification. The rheological characterisation of the camphene-based NiO suspensions was performed using a rotational viscometer. The final pore size and morphology were characterised by Optical Microscopy, Field Emission Scanning Electron Microscopy, and X-ray computed tomography. The use of stearic acid improves the particle stabilisation and generates pore enlargement with an equiaxed pore structure, while commercial dispersant KD4 shows a dendritic pore morphology at lower thermal gradient.