By employing extremely short laser pulses, researchers have synthesized porous zinc oxide pellets that can be used to fabricate a humidity sensor, which can measure atmospheric humidity levels1.
Conventional surface engineering techniques employ toxic, highly reactive chemicals. To devise a safe technique, the researchers used a focused laser beam to create pores on the surfaces of pellets made from zinc oxide powder. The pellets were placed in a vacuum chamber and irradiated by typically 600 laser pulses over a period of 60 seconds. The pellets were then left to cool for 30 minutes in the vacuum chamber.
The laser pulses melted the pellets, creating a melt pool. When trapped gases in the pellets exceeded their solubility limit in the melt pool, they formed bubbles. Thermocapillary forces then moved these bubbles from low-temperature to high-temperature regions in the melt pool. When laser-induced heating was stopped, the pellets cooled and solidified, either trapping or releasing the gas bubbles and creating pores on the pellet surfaces in the process.
The researchers found that increasing the laser pulse energy significantly increased the surface porosity of the pellets. A higher laser pulse energy created a lower temperature gradient that reduced the bubbles’ velocity and prolonged the time for them to reach the surface. This increased their chances of becoming trapped during solidification and resulted in larger mean pore sizes.
The researchers say that it is possible to reproducibly fabricate such laser-treated porous surfaces of zinc oxide pellets. They say that these porous pellets could potentially be used to fabricate fast-acting humidity sensors.
