The Pore Microstructure Evolution and Porous Properties of Large Capillary Pressure Wicks Sintered with Carbonyl Nickel Powder

Abstract

We investigated the effect of different sintering temperatures ranging from 200 °C to 600 °C on the porous properties and pore microstructure of large capillary pressure wicks made of carbonyl nickel powder. The evolution model of hydraulic diameter was established and verified by the maximum pore diameter. Hydraulic diameter changed as the roughness of particle surfaces decreased and sintering necks grew large during sintering. In the contact-formation stage and the initial sintering stage (200–500 °C), the decrease in the roughness of particle surfaces played a decisive role, contributing to an increase in hydraulic diameter. In the intermediate sintering stage (600 °C), the growth of sintering necks dominated the process, however the hydraulic diameter was reduced. These results show that the maximum pore diameter first increased and then decreased in the same way as our evolution model. Permeability and capillary performance of the wicks first increased and then declined with increasing sintering temperature. We found the optimal sintering temperature to be 400 °C, at which point the wicks achieved the maximum pore diameter of 1.21 μm, a permeability of 1.77 × 10−14 m2, and their highest capillary performance of 1.46 × 10−8 m.

Effect of low thermal conductivity wick on the performance of compact loop heat pipe

“…This is credited to the fact, higher sintering temperatures led to smoother surfaces of particles and larger sintering necks between the particles. In addition, a large sintering shrinkage ratio of a wick caused a decrease in its porosity 30 . Hence at 700°C the porosity is low for sample S1.…”

“…In the sintering process, atoms on rough surfaces move from a convex point to a concave point, leading to a decrease in roughness of particle surfaces. Moreover, the final shape of the particles depends on the surface diffusion coefficient, which increases with the sintering temperature 30 . These circumstances encouraged the increase of capillary pressure, conversely, it also lead to decline of permeability.…”

“…In addition, a large sintering shrinkage ratio of a wick caused a decrease in its porosity. 30 Hence at 700°C the porosity is low for sample S1. Further, because of the incomplete evaporation of moisture content at lower temperatures pore formation is low in the samples.…”

“…Moreover, the final shape of the particles depends on the surface diffusion coefficient, which increases with the sintering temperature. 30 These circumstances encouraged the increase of capillary pressure, conversely, it also lead to decline of permeability. Hence, as the sintering temperature further increased to 700°C, the sintering necks grew rapidly and greater reduction was found in the area of pores compared to the perimeter, leading to the decline in the mean pore diameter.…”