Thermal Stability of Nanocrystalline Ag–Cu–Al Alloy Films with Densely Dispersed Alumina Particles Prepared via Reactive Sputtering Using Ar–O₂ Mixed Gas

Abstract

To address the challenges associated with the fabrication of highly O₂-permeable Ag membranes suitable for on-site and downsized O₂ separators for large-scale industrial applications, we investigated nanocrystalline heat-resistant Ag–Cu–Al alloy thin films. Ag–1 at. pct. Al and Ag–2 at. pct. Cu–1 at. pct. Al alloy thin films with nanocrystalline structures were prepared through a reactive sputtering method employing Ar–O₂ mixed gas under the condition of supplying excess O/Al flux ratio above the stoichiometric ratio of Al₂O₃ (> 3/2). To examine the thermal stability of the nanocrystalline Ag alloy films, heating tests were conducted at 350 °C for 1 hour and 300 °C for 1 to 1000 hours in air, and the Ag crystal grain size, nanoindentation hardness, Young’s modulus, and electrical resistivity were measured. The results show that the Ag–2 at. pct. Cu–1 at. pct. Al thin film has excellent thermal stability, maintaining its nanostructure with an average grain size of 30 nm after heating at 300 °C for 1000 hours in air. The effects of the addition of 2 at. pct. Cu and the O₂ partial pressures during sputtering on the thermal stability of the Ag nanocrystalline structures were discussed using model calculations. It was found that supplying excess O/Al flux ratio above the stoichiometric ratio of Al₂O₃ (> 3/2) during sputtering reduced the dissolved Al concentration at the Ag grain boundary to substantially zero, almost completely suppressed the Ostwald ripening of nano-sized Al₂O₃ particles during sputtering and long-term heating, and allowed the Al₂O₃ particles to maintain high pinning force. Even under the excess oxygen supply conditions of O/Al > 3/2, the 2 at. pct. Cu addition was preferred for further Ag grain refinement in terms of promoting Al₂O₃ nucleation at the early stage of the film deposition on the substrate due to the effect of suppressing recrystallization of the Ag matrix.

Appendices

Appendix 1

O₂ flux through an Ag film (J_O2) is shown as follows:

$$ J_{{{\text{O}}_{2} }} = D_{{\text{O}}} \cdot S_{{\text{O}}} \cdot {{\left( {\sqrt {P_{{{\text{O}}_{2} }} \left( {\text{I}} \right)} - \sqrt {P_{{{\text{O}}_{2} }} \left( {{\text{II}}} \right)} } \right)} \mathord{\left/ {\vphantom {{\left( {\sqrt {P_{{{\text{O}}_{2} }} \left( {\text{I}} \right)} - \sqrt {P_{{{\text{O}}_{2} }} \left( {{\text{II}}} \right)} } \right)} d}} \right. \kern-0pt} d} $$

(A1)

where D_O is the diffusion coefficient of O in Ag, S_O is solubility of O in Ag under P_O2 = 1 atm, and symbols of (I) and (II) are inlet and outlet sides of O for Ag films, respectively. We estimated that J_O2 > 0.1 Ncc/cm²/min would be required in a large-scale steelmaking plant. To achieve the value of J_O2 for an Ag single-crystal film under the conditions of d = 1 μm, P_O2(I) = 2.1 atm (pressurized air at 10 atm) and P_O2(II) = 1 atm (pure O₂ at 1 atm), using Eqs. [A2] and [A3], the film temperature of > 700 °C, i.e., oxygen permeability P ($={D}_{{\text{O}}}\cdot {S}_{{\text{O}}}$) of > 3×10^-5 Ncc/cm/min/atm^0.5 is required.

$$ D_{{\text{O}}} = D_{{\text{O}}}^{{{\text{lat}}}} $$

(A2)

$$ S_{{\text{O}}} = S_{{\text{O}}}^{{{\text{lat}}}} $$

(A3)

where the superscript of lat represents lattice of Ag. Values of ${D}_{{\text{O}}}^{{\text{lat}}}$ and ${S}_{{\text{O}}}^{{\text{lat}}}$ are listed in Table AI.

Table AI Values of D_O, S_O, and δ to Estimate P for Ag Thin Films

When the Ag film has nanocrystalline structure, the values of D and S can be evaluated as follows:

$$ D_{{\text{O}}} = D_{{\text{O}}}^{{{\text{lat}}}} \cdot \left( {1 - f^{{{\text{gb}}}} } \right) + D_{{\text{O}}}^{{{\text{gb}}}} \cdot f^{{{\text{gb}}}} $$

(A4)

$$ S_{{\text{O}}} = S_{{\text{O}}}^{{{\text{lat}}}} \cdot \left( {1 - f^{{{\text{gb}}}} } \right) + S_{{\text{O}}}^{{{\text{gb}}}} \cdot f^{{{\text{gb}}}} $$

(A5)

$$ D_{{\text{O}}}^{{{\text{gb}}}} = D_{{\text{O}}}^{{\text{L}}} $$

(A6)

$$ S_{{\text{O}}}^{{{\text{gb}}}} = S_{{\text{O}}}^{{\text{L}}} $$

(A7)

$$ f^{{{\text{gb}}}} = \delta /\left( {d_{{\text{g}}} /2 + \delta } \right) $$

(A8)

where f^gb is an area fraction of Ag grain boundaries in the cross-section parallel to the Ag film surface, δ is a grain boundary width of Ag, and the superscripts of gb and L represent the grain boundary and the liquid state of Ag, respectively. Here, the grain boundary is assumed to be in the liquid state with a width of three atoms. Values of ${D}_{{\text{O}}}^{{\text{gb}}}$, ${S}_{{\text{O}}}^{{\text{gb}}}$, and δ are listed in Table AI.

To obtain the high value of P > 3×10^-5 Ncc/cm/min/atm^0.5 corresponding to two thousand times larger than that of single-crystal Ag at a waste gas temperature of 300 °C in a steelmaking plant, fast grain boundary diffusion of Ag films with d_g < 50 nm corresponding to f^gb > 4 pct is required.

Appendix 2

See Figures A1 and A2.

Fig. A1

BF-STEM observations and particle analyses of the cross-section perpendicular to the substrate of the Ag–2 at. pct. Cu–1 at. pct. Al sputtered film before heating (Sample No. 4); full cross-section of the thin film (a), enlarged sub-surface region (b), and EDX analysis (c); fine particles at positions 1 and 2 were identified as those of Al₂O₃ containing a small amount of CuO; Ga peaks are due to contamination during the FIB sectioning process, and Mo and Si peaks originate from the Mo mesh support and silica substrate, respectively

Fig. A2

BF-TEM observations and particle analysis of the cross-section perpendicular to the substrate of the Ag–2 at. pct. Cu–1 at. pct. Al film (Sample No. 4) after heating at 300 °C for 1000 h in air; full cross-section of the thin film (a), enlarged sub-surface region (b), nano-beam electron diffraction pattern with Miller indices at the position of the white circle (c), and EDX analysis (d); the coarse particle indicated by the white arrow was identified as CuO (tenorite) with monoclinic crystal structure