Elsevier

Solid State Ionics

Volume 354, 15 October 2020, 115414
Solid State Ionics

Quantitative analysis of proton solubility in YSZ using SIMS

https://doi.org/10.1016/j.ssi.2020.115414Get rights and content

Highlights

  • Deuteron solubilities in 8YSZ were quantitatively determined using SIMS as a measure of proton solubilities.

  • Deuterons incorporate mainly into the grain boundaries in 8YSZ.

  • Deuteron solubilities per unit grain boundary area were determined to be 3.2 × 1013 cm−2 at 600 °C.

Abstract

The solubilities of protons in 8YSZ single crystal and polycrystals at 600 °C were investigated using secondary ion mass spectrometry (SIMS). As a measure of proton solubility, deuteron solubility in YSZ was measured because of its lower background level in SIMS measurements. It became possible to quantitatively determine the solubility using a new analytical method proposed here, which includes a procedure to remove the small background signal included in the deuterium signal. The new method was employed for determining the solubilities in YSZ single crystal and polycrystals. Although the solubility in the single crystal was below the detection limit of SIMS (~1016 cm−3), the solubilities in the polycrystals were significantly larger than that in the single crystal. The solubility shows a proportional relationship with the grain boundary density, which strongly suggests that deuterons incorporate mainly into the grain boundaries in YSZ. The deuteron solubility per unit grain boundary area was determined to be 3.2 × 1013 cm−2.

Introduction

Yttria-stabilized zirconia (YSZ) is one of the most well-known fluorite-type oxide ion conductors, which is widely used as electrolyte materials for various electrochemical devices, such as solid oxide fuel cells (SOFCs). For that reason, the transport behavior of oxide ions in YSZ has been broadly studied and well understood in detail. From the results based on several experiments such as proton solubility [[1], [2], [3], [4], [5], [6]] and hydrogen permeability measurements [7], it was suggested that not only oxide ions but also protons exist in YSZ under wet conditions; however, its transport behavior is still unclear in contrast to the oxide ions because of the difficulty in measuring small conductivities and solubilities of protons. Recently, it has been pointed out that protons in electrolyte materials play an important role in electrode reactions and degradations in SOFCs [8]. Therefore, understanding of the proton transport behavior is beneficial to improve the efficiency and the durability of SOFCs. As the conductivity depends on the carrier density, i.e., the solubility of protons, detailed information about the solubility is crucial for understanding the transport behavior of protons.

There are several studies on the proton/deuteron solubilities in YSZ as summarized in Table 1 [[1], [2], [3], [4], [5], [6]]. The corresponding Arrhenius plot of the reported solubilities is also shown in Fig. 1. The reported solubilities are widely distributed in the range of 1016–1018 cm−3, even at similar temperatures. This discrepancy should originate from its slight solubility in YSZ, which causes large errors in the experimentally determined solubilities. In addition, it was suggested that YSZ polycrystals have higher proton solubility as compared with YSZ single crystal [2,6], so the differences among the grain boundary densities of polycrystalline samples may also be reflected in the distribution of the reported solubilities. In order to confirm these speculations, an improved quantitative method to determine the proton solubility is required. Furthermore, such method will enable one to obtain the precise relationship between parameters, for instance temperature or water vapor pressure and solubility, which will help us to understand the mechanism for proton incorporation into YSZ.

Secondary ion mass spectrometry (SIMS) has a high sensitivity to detect hydrogen and its isotopes such as deuterium, so it is a powerful technique to quantify their concentrations. However, because the background level for hydrogen signal is relatively high in typical SIMS measurements, solubilities of deuterons are usually measured instead of protons when analyzing low concentrations (<1020 cm−3) [[2], [3], [4], [5], [6],9]. SIMS analysis of the deuteron solubility in YSZ was previously performed by several research groups [[2], [3], [4], [5], [6]]. The solubility was determined by measuring the deuteron concentration in YSZ annealed in D2O-containing atmosphere. However, as shown in Table 1 and Fig. 1, the obtained solubilities exhibit a large scatter in data. This discrepancy arises from inaccuracies in the analytical process of SIMS data (this will be explained further in Section 2.2). In the present study, we have corrected the inaccuracies and proposed a new analytical method for quantitative determination of deuteron solubility in YSZ. Using the proposed method, deuteron solubilities in 8 mol% Y2O3-stabilized ZrO2 (8YSZ) single crystal and polycrystals at 600 °C were determined. From their solubility difference and the proportional relationship between the solubility and the grain boundary density, it was inferred that deuterons incorporate mainly into the grain boundaries.

Section snippets

Preparation of YSZ samples and deuteron incorporation process

YSZ polycrystalline samples were prepared from commercially available 8 mol% yttria-stabilized zirconia (Zr0.85Y0.15O1.93: 8YSZ) powders, TZ-8Y, purchased from Tosoh Co., Japan. After the powders were pelletized by cold isostatic pressing at 390 MPa for 5 min, they were sintered at several temperatures, 1700, 1400, and 1300 °C for 5 h, to obtain polycrystals with several grain boundary densities. Surfaces of prepared samples were polished using emery paper and diamond paste. Grain boundary

Results and discussion

Fig. 4 shows the back-scattered electron images of YSZ polycrystalline samples prepared at various sintering temperatures. All samples were well-sintered and there seems to be no open pores. In addition, the grains have several different contrasts, which usually indicate that there are variations of composition or crystal orientation of the grains. In this case, because the grains have the same composition, different contrast corresponds to variation of crystal orientation, which means

Conclusions

In the present study, the deuteron solubilities in 8YSZ single crystal and polycrystals at 600 °C were determined by SIMS, using the new analytical method we proposed. In the analysis of SIMS data, specifically we proposed a procedure to remove the background included in the deuterium signal, which enabled improved accuracy of the determined solubility. As a result of the solubility measurements of deuterons in YSZ, although the solubility in the single crystal was below the detection limit of

CRediT authorship contribution statement

Takuya Yamaguchi: Methodology, Formal analysis, Investigation, Writing-original draft.

Tomohiro Ishiyama: Formal analysis, Writing - review & editing.

Haruo Kishimoto: Conceptualization, Writing - review & editing.

Katherine Develos-Bagarinao: Formal analysis, Writing - review & editing.

Katsuhiko Yamaji: Methodology, Conceptualization, Investigation, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) project, “Basic study on rapid evaluation method of SOFC durability”.

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