Onset of selective laser flash sintering of AlN

Abstract

Flash sintering uses a combination of heating and electric fields to rapidly densify ceramics. Previously, it has been shown that a scanning laser can be used to initiate flash sintering in localized regions on an yttria-stabilized zirconia (YSZ) sample in a process known as selective laser flash sintering (SLFS). In this work, we show using a combination of measurements of electric current flowing through the sample and observations of necks formed between powder particles that aluminum nitride (AlN) can also undergo SLFS. Scan conditions required to initiate SLFS are characterized over a range of laser powers and laser scan speeds in a dry nitrogen environment. It is shown that initiation of SLFS in AlN is governed by both the local input energy density per scan and heat dissipation and a numerical model is developed to predict temperatures during SLFS. Assuming the minimum temperature along the conductive path determines the onset of SLFS, the minimum temperature and time required is 450-670 K in 2-0.25 s for the pressed AlN pellets used in this study for laser scan speeds of 33-300 m/s, laser powers of 10-30 W, and an applied electric field of 3000 V/cm.

Mechanisms responsible for the onset of selective laser flash sintering of 8‐YSZ

“…A finite element model, adapted from a similar one developed for aluminum nitride by Gouws, 14 was used to determine the temperature distribution in the pellet resulting from laser scanning. The purpose of this model was to determine the temperature within the pellet due only to the laser heating during SLFS, without the effects of Joule heating.…”

Effects of interparticle neck size on initiation of selective laser flash sintering of 8‐YSZ

“…The experimental measurement of the temperatures that arise during SLFS is very challenging because of the difficulty in measuring surface and subsurface temperatures at the required spatial and temporal scales that are relevant to the highly dynamic SLFS environment. Instead, an FEM model, adapted from a previously published model, 31 was used to determine the temperature distribution throughout the powder bed from heating caused by absorption of the laser. The FEM simulations were implemented in multi-physics software (COMSOL version 5.5 and 6.0, COMSOL, Inc., Burlington, MA, USA).…”

“…The model does account for heat transfer within the powder bed and radiative cooling to the atmosphere, assuming an emissivity = 0.9. 31 The model geometry, shown in Figure 3, is a solid block with effective material properties that correspond to a region of the pressed pellet that experiences heating from the laser that is scanned on the surface of the pressed pellet. The length of the block, 17 mm, is the length of The meshing that is used is shown in Figure 4, including the custom ultrafine free tetrahedral meshing with an element size of 0.01-0.1 mm that is used in the smaller rectangular prism volume, along with the locations of the negative and positive electrodes and the direction of the laser scan.…”

The onset of selective laser flash sintering in undoped and doped lanthanum chromite

“…18 Compared to furnace-based flash sintering, SLFS also offers advantages for studying the mechanisms for the onset of flash because temperature is controlled locally, which allows much faster heating and cooling rates than what is experienced in a furnace. Materials that have been studied with this technique include 8YSZ, 17,19,20 which is an extrinsic ionic conductor at high temperatures and AlN, 21 which is an intrinsic electronic conductor at high temperatures. Both materials are poor electrical conductors at room temperature and require high laser powers to reach the temperatures where electrical conduction is sufficient to initiate SLFS.…”