Ridwan Sakidja

Coating designs for oxidation control of Mo–Si–B alloys

Abstract Oxidation of a three phase (Mo+Mo 5 SiB 2 +Mo 3 Si) Mo–Si–B alloy in air at 1000 and 1200 °C yields borosilicate and MoO 2 product layers. The formation of MoO 2 indicates that the borosilicate layer restricts the oxygen activity. A coating strategy based upon a kinetic biasing to limit the oxygen flux and enhance oxidation resistance is presented.

Multiferroicity of Carbon‐Based Charge‐Transfer Magnets

A new type of carbon charge-transfer magnet, consisting of a fullerene acceptor and single-walled carbon nanotube donor, is demonstrated, which exhibits room temperature ferromagnetism and magnetoelectric (ME) coupling. In addition, external stimuli (electric/magnetic/elastic field) and the concentration of a nanocarbon complex enable the tunabilities of the magnetization and ME coupling due to the control of the charge transfer.

Aluminum Pack Cementation on Mo–Si–B Alloys Kinetics and Lifetime Prediction

Following aluminum pack cementation on Mo-Si-B alloys, the oxidation response has been examined in the range 700-1400°C in 0.20 X 10 5 Pa of oxygen under isothermal and cyclic conditions. The initial coating is mainly composed of Mo 3 Al 8 , which reacts with the oxygen to produce a surface layer of alumina. The alumina slows the rate of oxidation considerably. The coating also reacts with the alloy substrate to develop an intermediate layer mainly composed of Mo 3 (Al,Si) A15 phase. The kinetics of the oxidation and intermediate A15 phase formation reactions have been evaluated in order to model the alloy substrate recession and the overall coating lifetime.

Synergistic Strain Engineering Effect of Hybrid Plasmonic, Catalytic, and Magnetic Core–Shell Nanocrystals

Hybrid core-shell nanocrystals, consisting of distinct components, represent an emerging functional material system, which could facilitate synergistic coupling effects via integrating drastically different functionalities. Here we report a unique strain engineering effect induced by phase transformation between plasmonic core and magnetic shell materials, which leads to a facile surface reconstruction of bimetallic core-shell nanocrystals to enhance their synergistic magnetic and catalytic properties. This advancement dramatically results in two orders of magnitude enhancement in magnetic coercivity and significant improvement in catalytic activity. Mechanistic studies involving the kinetic measurement and theoretical modeling uncover a structural distortion and surface rearrangement mechanism during the core-shell phase transformation pathway. This facile methodology could potentially open up the new design of multifunctional artificial hybrid nanostructures by the combination of phase transformation and surface engineering for emerging technological applications.

Atomically Thin Al2O3 Films for Tunnel Junctions

Metal-Insulator-Metal tunnel junctions (MIMTJ) are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically-thin barriers demanded for enhanced quantum coherence. In this work, we employed in situ scanning tunneling spectroscopy (STS) along with molecular dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic layer deposition (ALD). We found that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically-thin Al2O3 tunnel barrier with a high quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson Junctions, show that ALD Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next-generation of MIMTJs.

Oxidation Performance of High Temperature Mo-Si-B Alloys and Coatings

Mo-Si-B alloys are attractive due to their high temperature mechanical properties and high melting temperature. The oxidation of multiphase alloys develops in two distinct stages. First, there is a transient stage that corresponds to the evaporation of the volatile MoO3 and to an initial high recession rate. The steady state stage of the oxidation begins when the slower forming borosilicate layer becomes continuous and inhibits further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to inhibit the transient stage, a coating strategy has been developed to capitalize on the interdiffusion reactions and to employ a kinetic bias to modify interface reaction products in order to maximize the high temperature stability and performance. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to synthesize metal-rich silicide and borosilicide surface layers. The analysis of the enhanced oxidation performance indicates that a strategy based upon the operating principles of interface reactions in multicomponent systems is effective for developing stable and robust coating systems.

Microstructural development of Mo(ss) + T2 two-phase alloys

The microstructure evolution involving Mo-B-Si as-solidified alloys with compositions in the Mo solid-solution(ss) + T2 two-phase field has been examined following arc-melting and rapid solidification processing (RSP). Several solidification pathways in the arc-melted alloys have been identified. Compositional segregation during conventional solidification results in the formation of additional phases such as borides in the arc-melted alloys which require a prolonged solid-state annealing to obtain equilibrated two-phase microstructures. The RSP methods employed, splat-quenching (SQ) and powder drop tube processing (DTP), allow for significant microstructural modifications that facilitate the attainment of uniform dispersions of Mo(ss) phase in a T2 phase matrix.

Microstructure Development in High-Temperature Mo-Si-B Alloys

Mo-Si-B alloys are considered as potential high temperature structural materials due to their high melting points (above 2000°C) and excellent oxidation resistance attributed to their self-healing characteristics over an extended temperature range. In the current study, the effect of alloying additions to achieve lower weight density and microstructure stability has been examined. The critical factor to the alloying additions appears to be the stability of the high melting ternary-based T 2 borosilicide phase.

The Ti3AlC2 MAX-phase as an efficient catalyst for oxidative dehydrogenation of n-butane

Abstract Dehydrogenation or oxidative dehydrogenation (ODH) of alkanes to produce alkenes directly from natural gas/shale gas is gaining in importance. Ti3AlC2, a MAX phase, which hitherto had not been used in catalysis, efficiently catalyzes the ODH of n‐butane to butenes and butadiene, which are important intermediates for the synthesis of polymers and other compounds. The catalyst, which combines both metallic and ceramic properties, is stable for at least 30 h on stream, even at low O2:butane ratios, without suffering from coking. This material has neither lattice oxygens nor noble metals, yet a unique combination of numerous defects and a thin surface Ti1−yAlyO2−y/2 layer that is rich in oxygen vacancies makes it an active catalyst. Given the large number of compositions available, MAX phases may find applications in several heterogeneously catalyzed reactions.

Composition- and oxidation-controlled magnetism in ternary FeCoNi nanocrystals

Ternary FeCoNi metallic nanostructures have attracted significant attention due to their high saturation magnetization, unique mechanical properties, and large corrosion resistance. In this study, we report a controlled synthesis of ternary FeCoNi nanocrystals using solution-based epitaxial core–shell nanotechnology. The thickness and stoichiometry of the FeCoNi nanocrystals affect their magnetic characteristics, which can be controlled by a phase transformation-induced tetragonal distortion. Furthermore, surface oxidation of the stoichiometry-controlled FeCoNi nanostructures can drastically enhance their magnetic coercivity (up to 8,881.6 Oe for AuCu–FeCo), and optimize the AuCu–FeCo0.8Ni0.2 performance corresponding to the saturated magnetization of 134.4 emu·g−1 and coercivity of 4,036.7 Oe, which opens the possibility of developing rare-earth free high energy nanomagnets.