Hydrogen Permeation and Crystallization Kinetics of Ni-based Amorphous Alloy Membranes
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Authors
Kim, Sang-Mun
Issue Date
2011
Type
Dissertation
Language
Keywords
Activation Energy , Amorphous Alloy , Crystallization , Hydrogen Permeation
Alternative Title
Abstract
Amorphous Ni-based alloy membranes show great promise as inexpensive, hydrogen-selective membrane materials. One of the main barriers to the commercial uptake of these materials is their tendency to crystallize during long-term operation at elevated temperatures, a process which decreases strength, durability and permeability. In this study, a series of Ni<sub>100-x</sub>Zr<sub>x</sub> (where x = 36.3 or 70 at.%), (Ni<sub>0.6</sub>Nb<sub>0.4</sub>)<sub>100-x</sub>Zr<sub>x</sub> and (Ni<sub>0.6</sub>Nb<sub>0.3</sub>Ta<sub>0.1</sub>)<sub>100-x</sub>Zr<sub>x</sub> (where x = 0, 10, 20 or 30 at.%) amorphous alloy membranes were prepared by melt spinning. The glass transition and crystallization kinetics of the amorphous alloy membranes have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The activation energies (E<sub>x</sub>) of crystallization were determined using Kissinger and Ozawa equations. A Johnson-Mehl-Avrami equation has also been applied for isothermal kinetics and corresponding time-temperature-transformation (TTT) diagrams are plotted. Intermetallic phases and surface morphology after the thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallization temperature, T<sub>x</sub>, for the eutectic Ni<sub>63.7</sub>Zr<sub>36.3</sub> and the near-eutectic Ni<sub>30</sub>Zr<sub>70</sub> alloys for a 10 K/min DSC heating rate were determined as 841.2 K and 668.9 K, respectively. The Arrhenius plot obtained for the Ni<sub>63.7</sub>Zr<sub>36.3</sub> alloy yielded E<sub>x</sub>=374.1 kJ/mol which is higher than the value for the Ni<sub>30</sub>Zr<sub>70</sub> alloy (E<sub>x</sub>=353.3 kJ/mol). The activation energy of the ternary Ni<sub>60</sub>Nb<sub>20</sub>Zr<sub>20</sub> alloy using Kissinger and Ozawa equations (517 kJ/mol and 505 kJ/mol) are higher than those of the binary Ni-Zr alloys (374.1 kJ/mol and 353.3 kJ/mol), and also the crystallization temperature (871.6 K) of the Ni<sub>60</sub>Nb<sub>20</sub>Zr<sub>20</sub> alloy is higher than those of binary Ni-Zr alloys at the heating rate of 10 K/min. In (Ni<sub>0.6</sub>Nb<sub>0.4</sub>)<sub>100-x</sub>Zr<sub>x</sub> (where x = 0, 10, 20 or 30 at.%) alloys, T<sub>x</sub> continuously decreased with increasing Zr concentration. The value of E<sub>x</sub> increased up to 10 at.% Zr addition, then decreased, which is consistent with the decrease in T<sub>x</sub>. In addition, the values of T<sub>x</sub> and E<sub>x</sub> increased with increasing Ta content in the (Ni<sub>0.6</sub>Nb<sub>0.3</sub>Ta<sub>0.1</sub>)<sub>100-x</sub>Zr<sub>x</sub> alloys.Hydrogen permeation properties of Ni-based amorphous alloy membranes were investigated over the 573-673 K temperature range at different hydrogen partial pressures. The linear relationship between the hydrogen permeation flux and the hydrogen partial pressure difference across the membrane indicates that permeation was controlled by diffusion through the bulk membrane. The effect of Zr concentration on the hydrogen permeability, the thermal stability and hydrogen embrittlement was also investigated. The hydrogen permeability of the (Ni<sub>0.6</sub>Nb<sub>0.4</sub>)<sub>70</sub>Zr<sub>30 </sub>alloy was higher (7.0 x 10<super>-9</super> mol m<super>-1</super> s<super>-1</super> Pa<super>-0.5</super>) than those of the (Ni<sub>0.6</sub>Nb<sub>0.4</sub>)<sub>90</sub>Zr<sub>10</sub> and Ni<sub>60</sub>Nb<sub>40</sub> alloys at 623 K (1.2 x 10<super>-9</super> and 4.9 x 10<super>-11</super>, respectively), but decreased with decreasing Zr concentration. The permeability of the alloy with Ta, (Ni<sub>0.6</sub>Nb<sub>0.3</sub>Ta<sub>0.1</sub>)<sub>70</sub>Zr<sub>30</sub> at 623 K was slightly lower (3.9 x 10<super>-9</super> mol m<super>-1</super> s<super>-1</super> Pa<super>-0.5</super>) than (Ni<sub>0.6</sub>Nb<sub>0.4</sub>)<sub>70</sub>Zr<sub>30</sub> alloy, but resulted in improved thermal stability and hydrogen embrittlement.
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