Timothy P. Hogan

Here, porous nickel interlayers or porous platinum interlayers were shown to promote the wetting, spreading, and adhesion of silver on alumina-forming ferritic stainless steel (AFFSS) and chromia-forming ferritic stainless steel (CFFSS). These interlayers resulted in dense, crack-free AFFSS|Ag-Ni|CFFSS and AFFSS|Ag-Pt|CFFSS braze joints that, after 300 h in 650 °C air, displayed shear strengths up to 70 MPa similar to, or larger than, those of AFFSS|Ag-CuO|CFFSS or AFFSS|Ag|CFFSS joints subjected to identical treatment. Similarly, after exposure to 25 cycles of (50 switches between) 12 h of 650 °C air and 12 h of 650 °C 4%H₂–96%N₂, AFFSS|Ag-Ni|CFFSS and AFFSS|Ag-Pt|CFFSS braze joints displayed shear strengths significantly larger than those of AFFSS|Ag-CuO|CFFSS or AFFSS|Ag|CFFSS joints subjected to identical treatment. In addition, nickel and platinum were found to chemically getter surface-segregating steel constituents, particularly Al from the AFFSS. As a result, Ag-Ni and Ag-Pt electrical contact resistances on AFFSS and CFFSS substrates were several orders of magnitude lower than those of conventional Ag-CuO reactive air brazes or Heraeus C8710 Ag contact pastes. Together, these results suggest that Ag-Pt and especially Ag-Ni may be useful for the fabrication of durable joints, seals, and/or electrical contacts in electrical/electrochemical devices exposed to high temperatures and/or variable oxygen partial pressure environments.

The metal circuits and brazes used in electronic, energy conversion, and/or energy storage devices often have difficulty wetting and adhering to ceramic and/or ceramic-passivated metal substrates. Here, a novel Particle Interlayer Directed Wetting and Spreading (PIDWAS) technique is demonstrated that utilizes screen printing, the low wetting angle of silver on nickel, and the high work of adhesion between nickel and various ceramics to produce well-adhered, self-assembled silver patterns on ceramic and/or stainless-steel substrates not normally wet by silver. The resulting Ag-Ni circuits have higher sapphire adhesion strengths (up to 30 MPa on sapphire), higher densities (>97% on sapphire), and similar high-temperature electronic resistivities to those made using commercially-available Heraeus C8710 or DAD-87 silver circuit pastes. Similarly, Ag-Ni brazes between a variety of ceramic and/or stainless-steel substrates have better microstructural stability with rapid thermal cycling, reduction-oxidation cycling, and dual atmosphere isothermal aging than conventional Ag-CuO brazes. In addition, the residual Ni in these Ag-Ni circuits and brazes can be used to chemically getter surface segregated Al, ensuring low contact resistances on a variety of chromia- and/or alumina-passivated stainless steels. Reference: [1] Hu G, Zhou Q, Bhatlawande A, Park J, Termuhlen R, Ma Y, Bieler TR, Yu HC, Qi Y, Hogan T & Nicholas JD. Patterned Nickel Interlayers for Enhanced Silver Wetting, Spreading and Adhesion on Ceramic Substrates. Scripta Materialia, 2021; 196, 113767. 10.1016/j.scriptamat.2021.113767 [2] Park J, Phongpreecha T, Nicholas JD & Qi Y. Enhanced Liquid Metal Wetting on Oxide Surfaces via Patterned Particles. Acta Materialia, 2020; 199, 551-560. 10.1016/j.actamat.2020.08.037

Here, a van der Pauw method was used to compare the 25-450^oC sheet resistivity of Ag-Ni electrical contacts to those made from commercially-available Heraeus C8710 and DAD-87 silver circuit pastes. In addition, a circular transfer length method was used to compare the 25-600^oC contact resistivity of Ag-Ni, Heraeus C8710, and DAD-87 on lanthanum strontium manganate (LSM). After 10 hours at 750^oC in air, the Ag-Ni sheet resistivity was nearly double that of the commercial Ag pastes. However, after 10 hours at 750^oC in air, the Ag-Ni to LSM contact resistivity was between that of Heraeus C8710 and DAD-87. The low Ag-Ni sheet resistivity of ~10^-6 ohm-cm, the low Ag-Ni to LSM contact resistivity of ~10^-4 ohm-cm², and the previously-observed >10 times higher adhesion strength of Ag-Ni (compared to Heraeus C8710 or DAD-87) on ceramic substrates, suggests that Ag-Ni may be useful for a variety of high-temperature electronic circuit/current collector applications.

Zinc oxide nanowires with two distinct morphologies were synthesized on silicon substrates using a simple thermal evaporation and vapor transport method in an oxidizing environment. The as‐synthesized nanowires were coated with gold to allow excitation of surface plasmons over a broad frequency range. SERS studies with near‐IR excitation at 785 nm showed significant enhancement (average enhancement > 10⁶) with excellent reproducibility to detect monolayer concentrations of 4‐methylbenzenethiol (4‐MBT) and 1,2‐benzendithiol (1,2‐BDT) probe molecules. The Raman enhancement showed a strong dependence on the gold film thickness, and the peak enhancement was observed for a ∼40‐nm‐thick film. The Raman enhancement was stronger for randomly oriented nanowires compared to aligned ones suggesting the importance of contributions from the junctions of nanowires. Copyright © 2009 John Wiley & Sons, Ltd.

For the material (Pb_0.95Sn_0.05Te)_1-x(PbS)_x nanostructuring from nucleation and growth and spinodal decomposition were reported to enhance the thermoelectric figure of merit over bulk PbTe, producing ZT of 1.1 - 1.4 at 650 K for x = 0.08[1]. Thermoelectric modules made from (Pb_0.95Sn_0.05Te)_1-x(PbS)_x materials with various hot-side metal electrodes were fabricated and tested. Short circuit current was measured on unicouples of Pb_0.95Sn_0.05Te – PbS 8% (n-type) legs and Ag_0.9Pb₉Sn₉Sb_0.6Te₂₀ (p-type) legs over 10 (A) for a hot side temperature of 870K, and a cold side of 300K. Hot pressed (Pb_0.95Sn_0.05Te)_1-x(PbS)_x materials were also investigated for module fabrication. Investigations of the electrical properties of hot-pressed (Pb_0.95Sn_0.05Te)_1-x(PbS)_x materials are presented along with the latest advancements in the fabrication and characteristics of modules based on the processing of these materials.