Joseph J. Beaman

Methods for fabrication of multi-material or functionally graded ceramic composite architectures are of interest for numerous applications. However, conventional co-sintering of multi-material ceramic parts is a challenge because differences in the sintering behavior of the two materials leads to interfacial strain and, ultimately, component failure. Direct ink writing (DIW) is an extrusion-based additive manufacturing process that excels at multi-material printing because multiple extrusion nozzles can be installed on the same gantry system. Furthermore, the use of DIW as a method to fabricate multi-material ceramic green bodies offers an additional variable for controlling and potentially matching sintering kinetics in the slurry formulation used for two dissimilar feedstocks. In the work documented in this manuscript, we explored two strategies to successfully co-sinter multi-material ceramic oxides: slurry optimization to match sintering kinetics and material gradients to step from one material to another. This manuscript also quantifies the allowable mismatch that avoids part cracking in solid solution forming multi-material systems and discusses best strategies to reduce mismatch during co-sintering. Inks composed of gadolinium oxide (Gd 2 O 3 ) and zirconium oxide (ZrO 2 ), a surrogate for uranium oxide (UO 2 ), were thermally matched, which resulted in a sintering mismatch reduction of over 10%. It was found that ~1% mismatch is tolerable during debind cycles and that ~5% mismatch is manageable during sintering cycles after slurry formulations are optimized to match the sintering behavior. Use of continuous gradients is shown to reduce sintering mismatch, although geometric resolution may be lost due to solid solution formation.

Radio Frequency Additive Manufacturing (RFAM) is an additive manufacturing process that utilizes Radio Frequency (RF) radiation as the sole heat source to heat and sinter an entire object simultaneously. Parts are fabricated selectively from powders, similarly to powder bed fusion but with RF radiation replacing laser or electron beams as the energy source. Typical polymer powders, such as nylon 11 or 12, are relatively transparent to RF energy sources, but polymer powders that are doped with conductive additives selectively absorb RF energy. By depositing electrically conductive dopants into selective regions of an insulating polymer powder bed, those regions of the powder bed can be sintered quickly and volumetrically via RF radiation into engineered parts. Previous work demonstrated that heating uniformity is a challenge related to the dopant density and the geometry of the part, but simulations suggested that it can be addressed by functionally (spatially) grading the dopant density. In this work, those simulation-based, functionally graded designs are fabricated for the first time via a combination of binder jetting additive manufacturing and sintering in an RF heating apparatus. The heating uniformity and geometric accuracy of the functionally graded samples are evaluated and compared to that of uniformly doped samples. The results show that functionally graded samples exhibit enhanced heating uniformity and improved geometric accuracy.

Abstract A noncontact laser ultrasonics method for determining the onset temperature and the early stage sintering state is studied. Because this technique measures properties near the surface in selected regions on the sample, it is particularly well‐suited to parts produced by additive manufacturing routes such as selective laser flash sintering, where local variations in sintering state along one dimension result from the laser scan pattern. We demonstrate the ability to measure very small changes in interparticle neck size by measuring Rayleigh wave speeds. The changes in wave speed result initially from rapid changes in Young's modulus that occur at the onset and in the early stages of sintering, before significant densification is observed. This measurement method is demonstrated using alumina pellets that were partially sintered at different temperatures to produce parts with a range of neck sizes and relative densities. Using the laser ultrasonics technique, the onset of sintering is detected at temperatures between 520°C and 650°C, which is significantly below the onset sintering temperatures detectable using traditional methods.

Abstract Selective laser flash sintering utilizes a scanning laser as a heat source to locally initiate flash sintering in the regions scanned by the laser. A key challenge towards utilizing this process for additive manufacturing (AM) is the induced electrical current that arises during flash sintering. With traditional scan patterns conducted with static electric fields, electrical and thermal runaway and associated thermal shock cracking occur. In this study, several open‐loop control strategies with static and dynamic applied electric fields were utilized to control peak currents. These strategies were shown to be effective in reducing peak currents to below 1.0 µA and reducing the severity of, but not completely eliminating cracking. Alternative strategies are suggested that could lead to complete elimination of cracks.

Abstract Previous studies have shown that selective laser flash sintering (SLFS) can be initiated in dielectrics that exhibit ionic or electronic conduction at high temperature. These materials required high laser powers to reach the temperatures where electrical conduction is sufficient to initiate SLFS. In this study, SLFS in lanthanum chromite (LC), an intrinsic electronic conductor with high conductivity, and lanthanum strontium chromite (LSC), which is doped to further increase electronic conductivity, were investigated with a focus on understanding the initiation mechanisms. Results show that the initiation of SLFS in LC and LSC occurs when electronic charge carriers are activated and flow to the electrode where the current is measured. A combination of carriers produced at the electrode, temperature‐activated intrinsic charge carriers, and extrinsic charge carriers present in LSC due to doping are responsible for the facile initiation of SLFS.

Abstract Selective laser flash sintering (SLFS) is a variation of flash sintering where the only external heat source is a scanning laser. The scanning laser locally heats a region of the sample between two electrodes, initiating measured current flow at a threshold electric field and laser energy density. This study focuses on understanding how charge transport occurs during stage I SLFS in 8 mol% yttria stabilized zirconia. Two potential charge transport mechanisms are considered: (1) a continuous current moving along a hot line between electrodes and (2) charge transported in a discrete bundle within a hot spot, localized to the area under the laser beam. Two laser scan patterns are employed to experimentally determine how charge is transported during stage I SLFS. Numerical modeling is used to estimate the temperatures at which SLFS initiates, which allows the calculation of charge carrier densities and mobilities relevant for the onset of SLFS. Results demonstrate that both a discrete bundle of charges and continuous flow of charge carriers contribute to the current at the onset of SLFS.