ABSTRACT
Producing soft magnetic alloys by additive manufacturing has the potential to overcome cracking and brittle fracture issues associated with conventional thermomechanical processing. Fe-Co alloys exhibit high magnetic saturation but low ductility that makes them difficult to process by commercial methods. Ni-Fe alloys have good ductility and high permeability in comparison to Fe-Co, but they suffer from low magnetic saturation. Functional grading between Fe-Co and Ni-Fe alloys through blown powder directed energy deposition can produce soft magnetic materials that combine and enhance properties beyond the strengths of the individual magnetic materials. This work focuses on the microstructure, crystal structure, and magnetic properties of functionally graded Fe49Co49V2/Ni80Fe16Mo4 coupons. The grading between the two materials is found to refine the microstructure, thereby improving the mechanical hardness without the use of a nonmagnetic element. Postbuild thermal treatments are found to recrystallize the microstructure and increase the grain size, leading to improved magnetic properties. Analysis of crystal structures provides an understanding of the solubility limits and phase equilibria between the BCC (Fe-Co) and FCC (Ni-Fe) structures. Success in functional grading of soft magnets may provide a pathway toward improving energy conversion efficiency through strategic combinations of high saturation and high strength materials.
ABSTRACT
AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M-site in the planar MX-layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu1-x Zn0.5x Sb solid solution. The transition from a fully-occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn0.5 Sb) leads to non-linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb-sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5â W mK-1 at 300â K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn0.5 Sb.
ABSTRACT
Thermoelectric materials convert heat energy into electricity, hold promising capabilities for energy waste harvesting, and may be the future of sustainable energy utilization. In this work, we successfully synthesized core-shell Bi2Te3/Sb2Te3 (BTST) nanostructured heterojunctions via a two-step solution route. Samples with different Bi2Te3 core to Sb2Te3 shell ratios could be synthesized by controlling the reaction precursors. Scanning electron microscopy images show well-defined hexagonal nanoplates and the distinct interfaces between Bi2Te3 and Sb2Te3. The similarity of the area ratios with the precursor ratios indicates that the growth of the Sb2Te3 shell mostly took place on the lateral direction rather than the vertical. Transmission electron microscopy revealed the crystalline nature of the as-synthesized Bi2Te3 core and Sb2Te3 shell. Energy-dispersive X-ray spectroscopy verified the lateral growth of a Sb2Te3 shell on the Bi2Te3 core. Thermoelectric properties were measured on pellets obtained from powders via spark plasma sintering with two different directions, in-plane and out-of-plane, showing anisotropic properties due to the nanostructure alignment in the pellets. All samples showed a degenerate semiconducting character with the electrical resistivity increasing with the temperature. Starting from Sb2Te3, the electrical resistivity increases with the increase in amounts of Bi2Te3. Thermal conductivity is lowered due to the increase in interfaces and additional phonon scattering. We show that the out-of-plane direction of the BTST 1-3 sample (where 1-3 indicates the ratio of BT to ST) demonstrates a high Seebeck value of 145 µV/K at 500 K which may be attributed to an energy filtering effect across the heterojunction interfaces. The highest overall zT is observed for the BTST 1-3 sample in the out-of-plane direction at 500 K. The zT values increase continuously over the measured temperature range, indicating a probable higher value at increased temperatures.
ABSTRACT
The discovery of new thermoelectric materials has the potential to benefit from advances in high-throughput methodologies. Traditional synthesis and characterization routes for thermoelectrics are time-consuming serial processes. In contrast, high-throughput materials discovery is commonly done by thin film growth, which may produce microstructures that are metastable or compositionally graded and, therefore, are challenging to characterize. As a middle ground between bulk synthesis and thin film deposition, we find that the aerosol deposition process can rapidly produce samples that exhibit electronic property trends consistent with those produced by traditional bulk means. We demonstrate rapid growth of discrete thermoelectric thick films of varying chemical compositions (Pb1-xSnxTe) from PbTe and SnTe polydisperse micrometer sized powder feedstocks. The high deposition rate (near 1 µm min-1) and resultant microstructures are advantageous as the diffusion length scales promote rapid thermal treatment and equilibrium phase formation. Room-temperature high-throughput measurements of the Seebeck coefficient and resistivity are compared to traditionally produced bulk materials. The Seebeck coefficient of the films follows the trends of traditional samples, but the resistivity is found to be more sensitive to microstructural effects. Ultimately, we demonstrate a framework for exploratory materials science using aerosol deposition and high-throughput characterization instrumentation.
