Letter www.acsami.org
Selective Growth of Noble Gases at Metal/Oxide Interface Keisuke Takahashi,* Hiroshi Oka, and Somei Ohnuki Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8278, Japan S Supporting Information *
ABSTRACT: The locations and roles of noble gases at an oxide/metal interface in oxide dispersed metal are theoretically and experimentally investigated. Oxide dispersed metal consisting of FCC Fe and Y2Hf2O7 (Y2Ti2O7) is synthesized by mechanical alloying under a saturated Ar gas environment. Transmission electron microscopy and density functional theory observes the strain field at the interface of FCC Fe {111} and Y2Hf2O7 {111} whose physical origin emerges from surface reconstruction due to charge transfer. Noble gases are experimentally observed at the oxide (Y2Ti2O7) site and calculations reveal that the noble gases segregate the interface and grow toward the oxide site. In general, the interface is defined as the trapping site for noble gases; however, transmission electron microscopy and density functional theory found evidence which shows that noble gases grow toward the oxide, contrary to the generally held idea that the interface is the final trapping site for noble gases. Furthermore, calculations show that the inclusion of He/Ar hardens the oxide, suggesting that material fractures could begin from the noble gas bubble within the oxides. Thus, experimental and theoretical results demonstrate that noble gases grow from the interface toward the oxide and that oxides behave as a trapping site for noble gases. KEYWORDS: metal oxide interface, oxide particles, noble gas, transmission electron microscopy, density functional theory
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experimentally synthesized and analyzed by using transmission electron microscopy while density functional theory is implemented in order to evaluate the atomic scale behavior of the noble gases and metal/oxide interfaces.. Oxide dispersion-strengthened (ODS) steels are synthesized via mechanical alloying under a saturated Ar gas environment.15,16 Two common yttrium-based oxides, anion-deficient fluorite structure Y2Hf2O7 and the pyrochlore structure Y2Ti2O7, are selected as they have good thermal stability and mechanical strength. Each oxide is synthesized with austenitic stainless steel. Prealloyed stainless steel powder and 0.35 wt % Y2O3 powder with 0.1 wt % Ti and 0.6 wt % Hf are mechanically alloyed using a planetary ball mill. The material is then heat treated at 1150 °C for 2 h followed by hot extrusion. The expected final alloy composition is Fe(FCC)-16Cr-14Ni-(0.05−0.1)C-2.5Mo(0.06−0.18)Ti-0.35Y 2 O 3 -(0−0.6)Hf. JEM-2010 and H9000UHR microscopes are used for transmission electron microscopy (TEM) observation where a thin foil for observation is prepared using a standard twin-jet electropolishing technique. The synthesized ODS steel (Y2Hf2O7 and FCC Fe) are analyzed where the structure of Y2Hf2O7 is identified as an anion-deficient fluorite structure. The Y2Hf2O7 particle is
he metal/oxide interface in structural materials has been extensively investigated as it plays a key role for the thermal stability and mechanical strength of oxide dispersed metal.1−4 In addition, the presence of noble gases at such interfaces generates scientific and engineering interest as changes in the properties of these materials could be a result of noble gases at the interfaces. In particular, He and Ar gases are two common noble gases seen in metal/oxide interfaces where He atoms are induced by transmutation reactions and Ar atoms are introduced during material processing under fully saturated Ar gases in order to prevent oxidation.5,6 Such noble gases are considered to play a crucial role in material fracturing and embrittlement where they generally form few nanometers of noble gas bubbles during the annealing process.7,8 It is also reported that trapping He gases in Y−Ti based oxides is effective compared to trapping He in an Fe matrix in regards to preventing material fractures.9−11 In general, noble gases have low solubility, making the defect site behave as a trapping site.12 However, the location and the role of noble gases at the metal/ oxide interface are not well understood, thereby making it difficult to determine whether noble gases stay at the interface or grow toward the metal or oxide sites. Here, the location and effect of He and Ar atoms at the metal/oxide interface of oxide dispersed metal are experimentally and theoretically investigated. Two common oxides, Y2Hf2O7 and Y2Ti2O7, are examined as both oxides enhance the structural and thermal properties of Fe.10,13,14 The interface of Y2Hf2O7, Y2Ti2O7, and face centered cubic iron (FCC Fe) are © XXXX American Chemical Society
Received: December 4, 2015 Accepted: February 3, 2016
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DOI: 10.1021/acsami.5b11792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces embedded in a cube-on-cube orientation with the surrounding FCC matrix with no rotation.13 Transmission electron microscope analysis indicates surface orientations of Y2Hf2O7 (111) and FCC Fe (111) as shown in Figure 1a. Furthermore,
Figure 2. Atomic model of (a) the multilayer of FCC Fe {111} and Y2Hf2O7. Black circle indicates the oxygen-heavy area. (b) He atoms at FCC Fe {111} and Y2Hf2O7. (c) Ar atoms at FCC Fe {111} and Y2Hf2O7. Atomic color codes: Fe, purple; O, red; Y, light blue; Hf, dark blue; He, yellow; Ar, green. Figure 1. (a, b) Bright-field images of a single faceted oxide particle (Y2Hf2O7) showing the strain fields. Corresponding electron diffraction patterns and diffraction vectors g are also shown. The plane indexes of particle/matrix interface are indicated in a. (c) Ar bubble in oxide particle (Y2Ti2O7) at the interface of oxide particle (Y2Ti2O7) and Fe matrix.
of Ar gas at the interface would depend on factors such as pressure of Ar gas during mechanical alloying and the number of defects at the interface. Thus, the metal/oxide interface has known to acts as a trapping site for inert gases; however, Figure 1c implies that Ar gases grow toward the oxide. The grid-based projector-augmented wave (GPAW) method within the density functional theory is implemented for first principle calculation.17 The generalized gradient approximation of Perdew−Burke−Ernzerhof (PBE) is implemented for exchange correlation.18 The grid spacing is set to 0.22 Å and 0.1 eV of smearing was applied. The spin polarized approximation was performed. The Brillouin zone sampling coupled with the Monkhorst−Pack scheme and 4 × 4 × 1 k points were applied.19 The wave function is expanded to linear combination of numerical atomic orbitals. Bader charge analysis was implemented for analyzing the charge transfers.20,21 The dissolution energy per atom of He and Ar atom (Ed) was calculated based on eq 1:
TEM observation reveals strain fields at the (111) particle/ matrix interface, seen in Figure 1b, where the black contrast is confirmed to be strain fields in the bright-field images. Thus, the interface consists of Y2Hf2O7 (111) and FCC Fe (111) where strain fields are observed at the interface. The synthesis of ODS steel is executed under a fully saturated Ar gas environment. Hence, inclusion of Ar gas within the material must be considered. TEM analysis shows that an Ar bubble is formed at the interface of Y2Ti2O7 and FCC Fe shown in Figure 1c where the bubble is located on the oxide side rather than the Fe matrix side. Note that the quantitative chemical analysis detected 0.0031 wt % of Ar in synthesized ODS steel while Ar is not detected in plain steel. Please see the Supporting Information for further details of quantitative chemical analysis and additional TEM images of the Ar bubble on the interface of the oxide particle (Y2Ti2O7) and Fe matrix. The size of the oxide particle in Figure 1c is slightly larger than the typical oxide particle observed in common ODS steels. However, the size effect toward the behavior of Ar gas is generally minimum as Ar bubbles are observed in 15 nm of oxide particle shown in Supporting Information. One can consider that Ar bubbles can be observed in few to ten nm of oxide particles unless the size of the oxide particle is few to 100 atoms, a size which generally has nonconstant structural and physical properties. In addition, one can note that the quantity
ΔEd = E(Fe + Y2Hf2O7 + He/Ar) − E(Fe + Y2Hf2O7) − E(He/Ar)
(1)
The multilayers of FCC Fe {111} and Y2Hf2O7 {111} are modeled as the experiment shows the interface consists of FCC Fe {111} and Y2Hf2O7 {111} as shown in Figure 1a. The structure of Y2Hf2O7 is constructed based on the experimental coordinates.22 The termination of the nonoxide layer in the top layer of Y2Hf2O7 {111} was considered as the experiment uses the mechanical milling process and results in oxidation on the surface. The interface of 3 × 3 supercell of FCC Fe {111} and 2 × 2 supercell of Y2Hf2O7 {111} is created where the lattice mismatch is 4% FCC Fe {111} has four atomic layers where the bottom two layers are fixed. Y2Hf2O7 {111} has six atomic B
DOI: 10.1021/acsami.5b11792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 3. (a) Dissolution energy of He and Ar atom in Fe, Hf, and Y defects at the interfaces. (b) Change of the bulk modulus of Y2Hf2O7 with He and Ar atoms.
layers where the top two layers are fixed. The constructed FCC Fe {111}/Y2Hf2O7 {111} is fully relaxed. The calculated atomic model of the interface of FCC Fe {111} and Y2Hf2O7 {111} is shown in Figure 2. Major structural relaxation is observed at the interface upon relaxation. The O atoms are transferred from Y2Hf2O7 {111} to the surface of FCC Fe {111}, resulting in oxygen dispersion over FCC Fe. The structure of FCC Fe {111} is elongated by 0.62 Å toward Y2Hf2O7 {111} in areas where the displaced oxygen atoms are concentrated. Thus, surface reconstruction is induced due to the reduction of Y2Hf2O7 {111}. Surface reconstruction shown in Figure 2a supports the experimental result shown in Figure 1b where the strain field is observed at the interface of oxide and iron. The physical origin of the strain field at the interface is considered due to the charge transfer at the interface. Bader charge analysis indicates that elongated Fe atoms are positively charged by 0.9. This suggests that chemical bonding is formed
between the O atoms from Y2Hf2O7 {111} and elongated Fe atoms. In general, the charge state of the interface is described as the Fe side is positively charged, whereas the Y2Hf2O7 is negatively charged. He and Ar atoms up to six atoms are placed at the interface of FCC Fe {111} and Y2Hf2O7 {111} in order to evaluate the location of noble gases. Both Ar and He atom segregate the interface of FCC Fe {111} and Y2Hf2O7 {111}. The segregation can be explained by the closed-shell structure of He and Ar atoms, resulting in low solubility. When the interface is saturated with He atoms, He atoms start to move toward oxides as seen in Figure 2b. One could consider that the average interatomic distance of Y2Hf2O7 is approximately twice larger than FCC Fe; hence, He atoms move toward oxides when He atoms are saturated. In the case of Ar, Ar atoms create an even larger segregation than He atoms do at the interface where Ar atoms completely push the oxide from the interface. This can be contributed to the fact that Ar has a large atomic C
DOI: 10.1021/acsami.5b11792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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ACKNOWLEDGMENTS CPU time is funded by the Japan Society for the Promotion of Science. Computational work is supported in part by Hokkaido university academic cloud,information initiative center, Hokkaido University, Sapporo, Japan.
radius in comparison to He atoms. This supports the relatively large Ar bubble in oxide shown in Figure 1c. Thus, theoretical investigation reveals that He and Ar atoms segregate the oxide and metal interface, eventually leading the He and Ar atoms to grow toward the oxides. Metal and oxide interfaces often have point defects although the heat treating process eliminates major defects. Here, the point defects (Fe,Hf,Y) are considered at the interface where the dissolution energy of He and Ar atoms with each of point defects (Fe,Hf,Y) is evaluated at the interface. Figure 3a shows the dissolution energy of He and Ar atoms when the different defects (Fe,Hf,Y) are placed at the interface. Note that positive energy indicates exothermic matter, as high energy is unstable. It is clearly seen that He and Ar atoms are stable in Y/Hf defects compared to those in Fe defects. In particular, when placing the He and Ar atoms in Fe defects they push themselves from the defect toward the interface. Thus, He and Ar atoms are found to grow toward oxides, showing good agreement with theoretical work which demonstrates that He is energetically stable in oxides site at the interface of Fe and oxides.11 In particular, defects in the oxide promotes growth of He and Ar atoms toward oxides. In other words, the oxides behave as a trapping site of He and Ar atoms. He and Ar atoms are experimentally and theoretically proven to grow toward oxides while the oxides become the trapping sites for He and Ar atoms. Here, the physical effect of He and Ar atoms within oxides are evaluated. He and Ar atoms are placed into bulk Y2Hf2O7 and the bulk modulus is calculated with the increase of He and Ar atoms. Figure 3b shows that the bulk modulus of Y2Hf2O7 dramatically increases as the number of He and Ar atoms increases. This indicates that Y2Hf2O7 increasingly hardens against compression as the number of He and Ar atoms increase. These results potentially suggest that material fractures could originate from the oxides starting from the location of noble gas bubbles that form as the result of annealing the material. In conclusion, the characterization of noble gases at the interface of metal and oxides are theoretically and experimentally investigated. Transmission electron microscopy and density functional theory calculations observed strain fields at the interface of Y2Hf2O7 {111} and FCC Fe {111} where the physical origin rests on surface reconstruction due to charge transfer. Furthermore, both experiment and theory found evidence that He and Ar atoms segregate the interface and grow toward the oxide, which then hardens upon the inclusion of He and Ar atoms. This implies that material fractures could originate from the oxide containing noble gas. These results should help better understand the behavior of noble gases at the metal/oxide interface and help prevent material failure when designing materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11792. Table S1 and Figures S1 and S2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acsami.5b11792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
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DOI: 10.1021/acsami.5b11792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
