Application of the cold sintering process to the electrolyte material BaCe0.8Zr0.1Y0.1O3-δ

doi:10.1016/j.jeurceramsoc.2020.03.043

Journal of the European Ceramic Society

Volume 40, Issue 9, August 2020, Pages 3445-3452

https://doi.org/10.1016/j.jeurceramsoc.2020.03.043 Get rights and content

Highlights

•
The cold sintering process (CSP) decreases the densification temperature of electrolyte BCZY.
•
CSP enhances the densification and the conductivity of BCZY.
•
Post annealing at 1200 °C increases the grain boundary conductivity.

Abstract

This paper describes and discusses the application of the original sintering process named cold sintering to the electrolyte material BaCe_0.8Zr_0.1Y_0.1O_3-δ to enhance its densification at a temperature below that needed in a conventional sintering. This new technique enables the acceleration of the densification resulting in a more compacted microstructure with an unexpected high relative density of 83 % at only 180 °C. A subsequent annealing at 1200 °C further enhances the densification which reaches 94 %. The electrochemical performance of CSP sintered ceramics was investigated and optimized by varying different process parameters. The comparison with the conventional sintered material reveals an increase of the total conductivity by mostly increasing the grain boundary one. This result emphasizes the benefits of CSP to not only reduce the sintering temperature but also to enhance the electrochemical properties.

Introduction

BCZY type proton conducting electrolyte such as BaCe_0.8Zr_0.1Y_0.1O_3-δ has gained a great attention as one of the most promising candidate for intermediate temperature solid oxide fuel cells, due to the combination of a high bulk conductivity associated to a chemical stability [1]. However, some of challenges to manufacture a dense BCZY ceramic are the high sintering temperature typically 1500–1600 °C [[2], [3], [4], [5], [6]], and the long processing duration in conventional method which can damage the material and cause barium evaporation resulting in a decrease of electrochemical properties [7,8]. Numerous studies have been carried out to improve the sinterability of BCZY electrolyte material by varying its composition. Results have shown that BCZY performance is strongly dependent on Zr/Ce ratio, and a formula of BaCe_0.8Zr_0.1Y_0.1O_3-δ can be a good compromise with a sintering temperature of 1400 °C and a high conductivity level in dry air above 10⁻² S. cm⁻¹ at 700 °C, for a sample of 92 % relative density [1]. In the same context, researches have shown that introducing sintering aids such as ZnO, NiO, CuO, or Al₂O₃ [[9], [10], [11], [12], [13]], can often decrease the sintering temperature without lowering the electrochemical properties. Always with the aim of saving energy, many innovative processes, assisted by modern techniques such as spark plasma sintering (SPS) or Flash sintering have been developed and successfully demonstrated to be effective in accelerating the diffusion process and increasing the driving force for densification. However, temperature required to obtain a dense ceramic with standard densification processes is generally higher than 1100 °C. More recently, a novel sintering process inspired from hydrothermal method has been developed in Randall group from Pennsylvania State University [[14], [15], [16], [17], [18]]. This original technique called cold sintering has been reported to have densified a large range of ceramic materials at a very low temperature <200 °C [14] compared to the usual sintering temperatures, using an appropriate amount of liquid phase (3–30 wt%) leading to dense ceramic materials of 80–99 % relative density [15], under pressure going from 50 to 500 MPa during a short time period of 1−60 min [16,17].

In a previous paper [19], we demonstrated the feasibility of applying the cold sintering process to a refractory ceramic material with an incongruent dissolution. One of the main achievement was the reduction of the sintering temperature in comparison with the conventional method. The use of water as liquid transient resulted in higher densification, however, it also generates the formation of a secondary phase. This phenomenon has been reported in literature with different incongruent material such as BaTiO₃ and PZT [16,18], [20,21]. The impurity phase can be removed by optimizing the amount of solvent and proceeding with a post heat treatment. This paper aimed essentially to study the electrochemical properties of the BCZY electrolyte obtained by cold sintering process. The evolution of the electrochemical behavior of bulk and grain boundaries as function of the different process parameters is investigated.

Section snippets

Powder synthesis

BCZY powder of composition BaCe_0.8Zr_0.1Y_0.1O_3-δ, was synthesized by nitrate-glycine method as described in previous work [1]. Stoichiometric amounts of metal nitrate salts precursors: Ba(NO₃)₂, Ce(NO₃)₃, ZrO(NO₃)₂, and Y(NO)₃ were dissolved separately in deionized water under stirring on a hot-plate then mixed together. Glycine, used as the fuel of the autocombustion reaction, is added to the mixture with a subsequent use of an ammonia solution to neutralize the pH. The final solution is heated

Effect of the CSP parameters

The electrolyte material performance depends on the sintering process parameters such as quantity of solvent, cold sintering temperature or pressure. The optimization of these parameters and the study of their effect on the electrolyte microstructure and electrochemical behaviour have been first investigated.

Discussion

Previous results for samples prepared by conventional sintering in our group are recalled on Fig. 13 [1]. For these samples, we obtained an evolution trend similar to Fig. 2, i.e. the total conductivity of the sample limited at low temperatures by the grain boundary, and at high temperatures by the bulk, but the intersection of the two curves was observed at higher temperature, near 530 °C, instead of 200 °C, as it was shown on Fig. 2.

As previously demonstrated, the main benefit of the cold

Conclusion

In summary, this work has shown the possibility to use the cold sintering process (CSP) to densify a proton refractory electrolyte material with incongruent dissolution issue (BCZY). Different parameters have been optimized and the electrolyte material was successfully cold sintered leading to a high relative density of 83 %, at low temperature equal to 180 °C under 375 MPa using 5 % wt of water. Compared to the 63 % relative density of the "dry" pressed specimen, this result shows the power of

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of Competing Interest

None.

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Citation Excerpt :
While CSP was successfully applied to a wide range of materials [1], leading to dense samples using a single step sintering process, it remains challenging and sometimes leads to incomplete densification [3]. In many cases, CSP is used as a pre-sintering step, leading to better grain to grain contacts, followed by a high temperature treatment to complete material’s sintering [4–6]. This is thus a two-step sintering process.
The development of in situ characterization techniques is mandatory for the development of the Cold Sintering Process (CSP). Here we show the first development of in situ impedance analyzes to follow materials evolution during sintering. This characterization brings key information about grain boundary formation at the solid/liquid interface, both in terms of thermodynamic and kinetics. Through the example of ZnO sintering in the presence of acetic acid aqueous solution, known for being easily Cold Sintered, we demonstrate the role of the liquid phase, notably the acetic acid role, regarding chemical composition, structure and microstructure of the obtained densified ceramics. It turns out this is a promising in-situ characterization technique that should be applied to a wide range of materials.

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Original ArticleApplication of the cold sintering process to the electrolyte material BaCe0.8Zr0.1Y0.1O3-δ

Highlights

Abstract

Introduction

Section snippets

Powder synthesis

Effect of the CSP parameters

Discussion

Conclusion

Funding

Declaration of Competing Interest

Solid State Ion.

Solid State Ion.

J. Power Sources

Int. J. Hydrogen Energy

Mater. Chem. Phys.

Solid State Ion.

J. Alloys Compd.

Solid State Ion.

Prog. Mater. Sci.

Solid State Ion.

Electrochim. Acta

Electrochim. Acta

Solid State Ion.

Original Article
Application of the cold sintering process to the electrolyte material BaCe_0.8Zr_0.1Y_0.1O_3-δ