Original ArticleInnovative strategy for designing proton conducting ceramic tapes and multilayers for energy applications
Introduction
Tape casting is the most widely used technique for large scale fabrication of green sheets to be used in multilayer ceramic technology [1,2]. Since the 50s, tape casting has been mainly exploited for the commercial production of electronic substrates and multilayer capacitors [3,4]. In this area, the research has been applied in multilayer structures of a large variety of dielectric, ferroelectric, insulator and piezoelectric materials for electronic components such as capacitors, actuators, transducers, varistors, resistors, energy harvesters, etc. [5]. More recently, tape cast multilayer structures have been developed for structural applications [[6], [7], [8], [9], [10], [11]], YAG-based lasers [[12], [13], [14]], Solid Oxide Fuel Cells [[15], [16], [17], [18]], Solid Oxide Electrolysers cells [19,20], Lithium / Sodium Ion Batteries [[21], [22], [23]] and Gas Separation Membranes [[24], [25], [26], [27]], confirming tape casting as powerful and flexible industrial technology.
The technique generally requires the following steps (Fig. 1): i) slurry preparation, ii) casting and drying, iii) punching and lamination and iv) thermal treatments. Each step of the process must be carefully optimized in order to obtain the final multilayer product with the desired properties. The green sheets, in particular, are the fundamental building blocks for the multilayers development and need to meet several requirements to enable high quality and reliable final products.
The green ceramic tapes are generally produced starting from well-optimized slurries. These are typically prepared by dispersing the ceramic powder in a solvent with the addition of significant amounts of organic compounds (binders, plasticizers, dispersing agents and eventually pore formers and surfactants) necessary to give, after casting and solvent evaporation, flexible and crack-free tapes [28,29]. In this respect, the green tape can be considered as a polymer-ceramic composite where the mutual physicochemical interaction of the components defines the mechanical characteristics of the material in terms of elasticity, strength and toughness. These important properties strongly affect the further processing steps: tapes handling, cutting and lamination and deformation during the thermal treatments.
The determination of the mechanical properties of the green tapes is therefore of paramount importance in order to optimize the whole ceramic process. Up to now, the mechanical properties of the green tapes have been assessed through stress-strain measurements in tensile mode to optimize tapes composition (i.e PVP-PVA-gelatin co-binder [30], binder and plasticizer content [[31], [32], [33], [34]]), adjust water based suspension [[35], [36], [37], [38], [39], [40]], asses the influence of leached cations (i.e. Ba2+ [41]), optimize the solid loading [42,43].
However, the complex structure and the viscoelastic behaviour of a green tape cannot be described only using static methods. The macromolecular mobility is in fact dependent upon many factors related either to the intrinsic material properties (i.e. chemical structure, molecular architecture, molecular weight and crosslinking, copolymers and blends, plasticizers, molecular orientation, fillers) and the operating conditions (i.e. time, temperature, frequency, stress or strain applied) [44].
To the authors knowledge the viscoelastic properties in terms of stiffness (modulus), ability to recover from deformation (elasticity), tendency to flow (viscosity) for green ceramic tapes have never been investigated so far. The lack of a deep understanding of the thermomechanical properties of the ceramic tapes has hindered the development of validated and standardized methods able to predict the lamination conditions necessary for the multilayers production and therefore simplify the optimization process.
In this paper we will present for the first time, the use of advanced Dynamical Mechanical Analysis (DMA), commonly used for polymer-based materials, to univocally define the lamination process conditions needed to obtain products with the suitable morphological and functional characteristics.
To firstly demonstrate the potential of this technique, the characterization of proton conductive ceramic tapes based on the solid solution of yttrium-doped barium cerate and barium zirconate (BCZY) will be presented as case study. The use of this material combined to the tape casting technology has received increasing interest in proton-conducting intermediate-temperature solid-oxide fuel cells and electrolysers (SOFC/SOECs) [[45], [46], [47]] as well as in hydrogen separation membranes [[48], [49], [50]]. These devices are constituted by a porous/dense laminated structure where the porous layer is obtained by mixing organic or inorganic pore-forming agents into the ceramic slurry before tape casting.
In this study, the influence of the slurry formulation, in particular of the pore former content, on the thermo-mechanical properties of the tapes, will be investigated using a dynamic mechanical analyser. This work will allow to develop a characterization protocol for green tapes able to optimize the lamination process through the identification of parameters such as lamination temperature and viscosity.
Section snippets
Experimental
For the production of the tapes, BCZY powder (BaCe0.65Zr0.20Y0.15O3-δ, Specific Surface Area (SSA) = 5.8 m2/g), supplied by Marion Technology was used as starting material. Rice Starch (RS, moisture content between 7.5–13 %, Aldrich, USA), with average particle size of 5−6 μm was used as sacrificial pore forming agent.
The slurries were prepared by adding to the starting ceramic powders the desired amounts of solvent (azeotropic mixture of ethyl alcohol and methyl ethyl ketone (EtOH-MEK),
Thermomechanical behaviour of the green tapes
The effect of the pore former (rice starch) load onto the mechanical properties of BCZY green tapes was firstly investigated. The pore former amount is in fact a key variable that governs the final porosity of the ceramic layers and, therefore, the gas permeation, an important parameter for the final performances of solid oxide fuel cells/electrolyzers [52] and gas separation membranes [53].
Dog-bone samples were obtained from the different tapes and stress-strain curves were collected using the
Conclusions
This work explored for the first time the possibility to use Dynamic Mechanical Analysis as a powerful tool to characterize and design green tapes and multilayers for energy applications.
DMA allowed to find helpful results correlating the formulation of the tapes with their thermomechanical behaviour and then with the lamination process.
In particular, it was found that the rice starch, used as pore former, has a ceramic particle-like blocking effect on the chain mobility at low temperature.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This work was funded by the agreement between the Italian Ministry of Economic Development and the Italian National Research Council “Ricerca di sistema elettrico”, under the frame of the Project: “Materiali di frontiera per usi energetici”.
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