Nanometer-scale phase separation and formation of delta ZrH₂ in Cu-Zr binary amorphous alloys

Fadonougbo, Julien O.,Suh, Jin-Yoo,Shim, Cheol-Hwee,Kim, Gyeung-Ho,Fleury, Eric,Cho, Young Whan ELSEVIER SCIENCE 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.721 No.-

<P><B>Abstract</B></P> <P>Different Cu-Zr alloys were hydrogenated under 100 bars of hydrogen pressure at different temperatures. The hydrogenation induced transformation of the initially amorphous phase into a polycrystalline structure characterized by its nanoscale (<5 nm), mostly composed of metallic Cu and ZrH<SUB>2</SUB>. Calorimetry measurements after hydrogenation showed a low temperature exothermic transformation occurring in the alloys hydrogenated at temperatures below 473 K, followed by multiple endothermic peaks at higher temperature attributed to dehydrogenation of different hydride phases. Activation barrier energies of the phase transformation were derived from Kissinger's method, and further characterization involving transmission electron microscopy revealed the existence of delta ZrH<SUB>2</SUB> with cubic structure contrasting with the typical epsilon ZrH<SUB>2</SUB> with tetragonal structure. This study evidences the effect of hydrogen pressure, temperature, and the alloy chemistry on the nature of the hydride formation in Cu-Zr binary amorphous alloys during the hydrogenation procedure.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Hydrogen was introduced at high pressure and low temperature into melt-spun amorphous alloys. </LI> <LI> Calorimetry investigations evidenced the formation of a hydride phase at low temperature. </LI> <LI> The formation of delta ZrH<SUB>2</SUB> was highlighted by means of combined XRD and TEM. </LI> <LI> The microstructural separation induced by hydrogen was in nanometer scale of 5 nm. </LI> <LI> Elevated pressure, together with low temperature were responsible for delta phase formation. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Low temperature formation of Mg₂FeH₆ by hydrogenation of ball-milled nano-crystalline powder mixture of Mg and Fe

Fadonougbo, Julien O.,Jung, Jee-Yun,Suh, Jin-Yoo,Lee, Young-Su,Shim, Jae-Hyeok,Cho, Young Whan Elsevier 2017 Materials & Design Vol.135 No.-

<P><B>Abstract</B></P> <P>Low temperature formation of Mg<SUB>2</SUB>FeH<SUB>6</SUB> is demonstrated by hydrogenation of Mg-Fe elemental powder mixture at a temperature as low as 350°C which is lower than the conventional process temperature, 500°C. To enable the low temperature synthesis, the powder mixture of Mg and Fe has been prepared by high energy ball milling using different process control agents (PCAs). A systematic study on the ball milling and hydrogenation conditions has been carried out to maximize the yield of the ternary line compound. The hydrogenation conditions together with the particle size of the starting materials turn out to play a significant role in the hydrogenation kinetics of the system. An optimized condition has demonstrated a significant hydrogenation as well as a robust cycling ability at low temperature which suggests the strong potential of the process for practical applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Mg<SUB>2</SUB>FeH<SUB>6</SUB> was synthesized by hydrogenation of various Mg-Fe powder mixtures. </LI> <LI> A high yield of 81wt% of Mg<SUB>2</SUB>FeH<SUB>6</SUB> was obtained at 450°C. </LI> <LI> A promising yield (>69wt%) at 350°C under 60bars of H<SUB>2</SUB> was achieved. </LI> <LI> Cyclic experiments demonstrated the potential of the process for practical applications. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Synthesis of Mg₂FeH₆ by hydrogenation of Mg/Fe powder mixture prepared by cold roll milling in air: Effects of microstructure and oxygen distribution

Jung, Jee Yun,Fadonougbo, Julien O.,Suh, Jin-Yoo,Lee, Young-Su,Huh, Joo-Youl,Cho, Young Whan Elsevier 2018 International journal of hydrogen energy Vol.43 No.34

<P><B>Abstract</B></P> <P>Herein, we describe the synthesis of Mg<SUB>2</SUB>FeH<SUB>6</SUB> by hydrogenation of a 2.1 Mg:Fe (mol/mol) powder mixture prepared by cold roll milling (CRM) in air. The thickness of Fe layers and the amount and distribution of oxygen with number of CRM passes were systematically analyzed. CRM-induced microstructural changes were shown to play an important role in Mg<SUB>2</SUB>FeH<SUB>6</SUB> formation. Although repeated CRM effectively decreased the Fe layer thickness to values sufficient for the fast formation of Mg<SUB>2</SUB>FeH<SUB>6</SUB>, too much CRM passes decreased the total degree of hydrogenation due to inevitable oxidation of Mg in air. Both microstructure refinement and minimal oxidation are the prerequisites for efficient Mg<SUB>2</SUB>FeH<SUB>6</SUB> synthesis, with the former condition being achievable by optimizing the number of milling passes, and the latter one requiring CRM under an inert atmosphere.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Fast hydrogenation of Mg and Fe powder mixture obtained by cold roll milling (CRM). </LI> <LI> Development of extra fine layered structure with oxide interface by repeated CRM in air. </LI> <LI> Correlation between the amount and distribution of MgO and hydrogen storage capacity. </LI> </UL> </P>

Insulation Coating of Fe–Si–Cr Soft Magnetic Powder by Selective Oxidation

Jae‑Young Park,Kwangsuk Park,Bosung Seo,Julien O. Fadonougbo,Tae‑Wook Na,Ki Beom Park,Hyeon‑Tae Im,Nong‑Moon Hwang,Hyung‑Ki Park 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.7

This study examines the insulation coating technology of Fe–Si–Cr powder via selective oxidation annealing, which oxidizeselements selectively by controlling the oxidation potential. The study calculated the oxidation driving force of Fe, Si, andCr, and conducted a thermodynamic analysis of oxidation and reduction conditions according to temperature and oxidationpotential. Based on the results, a selective oxidation annealing was performed in an atmosphere in which Fe is reduced andonly Si and Cr are selectively oxidized. The oxidation potential was controlled through the partial pressure ratio of hydrogenand water vapor. The XPS analysis results confirmed that a Si and Cr complex oxide layer formed on the powder surfaceafter the selective oxidation annealing. Afterward, withstanding voltages were analyzed to evaluate the insulation property. Then, the withstanding voltage of the powder applying the selective oxidation annealing increased significantly comparedto that of the initial powder. Further analysis showed that the powder annealed in an air atmosphere had a significantly lowersaturation magnetic flux density than the initial powder, while the powder applying the selective oxidation annealing hadonly a slightly reduced saturation magnetic flux density.

Characterizations of Hydrogen Absorption and Surface Properties of Ti0.2Zr0.2Nb0.2V0.2Cr0.17Fe0.03 High Entropy Alloy with Dual Phases

Ki Beom Park,Jae‑Young Park,Young Do Kim,Julien O. Fadonougbo,Seongtak Kim,Hyo‑Kyu Kim,Jang‑Won Kang,Hyun‑Su Kang,Hyung‑Ki Park 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.2

In this study, we investigated the microstructures, hydrogen absorption kinetics, and surface oxides of aTi0.2Zr0.2Nb0.2V0.2Cr0.17Fe0.03high entropy alloy (HEA). The prepared HEA had a dual phase microstructure consistingof body-centered cubic (BCC) phase (32.2Ti-13.1Zr-30.3Nb-18.6 V-5.8Cr) and face-centered cubic (FCC) phase (11.3Ti-19.6Zr-13.2Nb-24.7 V-25.7Cr-5.5Fe). The HEA ingot absorbed hydrogen under a hydrogen pressure of 5 bar at roomtemperature without any thermal activation process. After hydrogenation, the FCC and BCC phases were transformed to amonohydride and a dihydride phase, respectively. To examine the hydrogen absorption behavior of each phase, two ingotshaving the same compositions as the BCC and the FCC phases were separately prepared. Though the BCC phase ingot didnot react with hydrogen, the FCC phase ingot absorbed hydrogen, which could result from the formation of a highly reactiveoxide layer on the FCC phase ingot. From the X-ray photoelectron spectroscopy results of the two ingots, although theBCC phase ingot contained Cr, no Cr was detected in the oxide layer. In contrast, the oxide layer on the FCC phase ingotdisplayed a high Cr concentration, and it seems that the reactivity of the oxide layer with hydrogen could be improved bythe presence of Cr in the surface oxides.