Novel Ti–Si–C composites for SOFC interconnect materials: Production optimization

Abstract

This paper presents a first part of research of designing a new Ti–Si–C-based composite for potential application as a solid oxide fuel cells interconnect material. Spark plasma sintering technique was used for the production and the effects of sintering parameters on microstructure, phase composition, as well as mechanical and electrical properties were investigated. Optimization of the sintering conditions then allowed to obtain a fine-grained material with properties (bending strength 433 MPa, hardness of 9.6 GPa, electrical resistance 26 mΩ cm) surpassing those of Crofer steels, materials typically used in the SOFC interconnect applications nowadays. This is a very promising result, yielding our Ti–Si–C composite a serious candidate for new-generation lightweight interconnect materials.

Introduction

Solid oxide fuel cells (SOFC) are electrochemical energy conversion devices that directly convert chemical energy of fuel and oxygen into electricity and heat. In comparison to other energy generator devices, SOFC possess relatively high energy conversion efficiency (up to 90%), high power density, a low level of generated pollutants, and relatively flexible choice of fuel (various hydrocarbons C_nH_m, hydrogen H₂, carbon dioxide CO₂, etc.) [1]. The harsh operating conditions of SOFC (oxidizing/reducing environment, high temperature, long operation time, etc.) require a careful choice of materials for the main components of the cell, such as the solid electrolyte, the anode, the cathode, and the interconnect [2]. For a broader commercialization of SOFC technology, the following promising directions were prioritized in the industry during last years: reduction of the operating temperature of a fuel cell system below 700 °C, improvement of long-term phase, structural and corrosion stability of the components, and development of more light-weight and compact SOFC systems suitable for mobile or airborne applications [[3], [4], [5]]. In these, the improvement of the interconnect component is considered as one of the most important routes in SOFC structural optimization [5], aiming at increased electrical performance, thermal stability, and operational life-time [6]. The interconnect is therefore one of the key parts of the SOFC energy system. Its main roles are the electrical connection of adjacent cells in a multiple cell-stack, multiplication of the SOFC electrical power, and physical separation of the fuel and the oxidant (oxygen, air) located in the anode and cathode parts of a cell, respectively. At present, high chromium ferritic steels (20–25 vol% Cr, so-called Crofer steels) are widely used as interconnect materials for intermediate temperature SOFC (operation temperatures of 700–800 °C) owing to their suitable thermal expansion, good oxidation resistance, conductive oxide scale, and relatively low cost. Despite the advantages, there are several major issues connected to their application, such as long-term oxidation at the SOFC operating conditions followed by a rapid growth of a chromia layer, and poisoning of the porous cathode by volatile Cr migration [[7], [8], [9], [10]]. These issues lead to a significant upsurge in the cell's resistivity and, as a result, SOFC performance degradation. Needless to say, development of a new thermally-stable chromium-free light-weight interconnect material could contribute to a significant improvement and commercialization of the SOFC system.

As alternative materials for SOFC interconnects, conductive nano-laminated ceramic of ternary compounds, so-called MAX phases, have been recently proposed [11,12]. In general, MAX phases have a formula M_n+1AX_n (n = 1, 2, or 3), where M is a transition metal, A is an A-group element (mostly IIIA and IVA), and X is C or N [13,14]. These materials combine the advantages of both ceramics and metals, such as low density, high chemical stability, high Young's modulus and fracture toughness, high electrical and thermal conductivity, and good machinability. In particular, the Ti₃SiC₂ phase is the most studied compound, having a unique combination of low specific weight (D = 4.52 g cm⁻³), high Young's modulus (∼330–340 GPa), low electrical resistivity (ρ = 0.22 μΩ m at 27 °C) and high thermal conductivity (33–40 W m⁻¹ K⁻¹ in the 27–1027 °C temperature range), excellent resistance to thermal shock, easy machinability, high fracture toughness ranging from 10 to 15 MPa m^0.5, and the average coefficient of thermal expansion (CTE, 9.1 × 10⁻⁶ K⁻¹) close to that of other SOFC components (12 × 10⁻⁶ K⁻¹ for ZrO₂-30 vol% Ni, 13.3 × 10⁻⁶ K⁻¹ for Ni, 11 × 10⁻⁶ K⁻¹ for 8YSZ) [[15], [16], [17]]. Such combination of properties makes the Ti₃SiC₂ compound more suitable for SOFC interconnect application than traditional carbide (e.g., TiC, SiC) or silicide (MoSi₂) ceramics, which have excellent high-temperature corrosion resistance, but unsuitably low electrical conductivity, thermal conductivity, and fracture toughness [[18], [19], [20]]. Nevertheless, there are concerns about long-term corrosion stability of pure Ti₃SiC₂ compound in SOFC operating conditions and its insufficient mechanical properties (low hardness and strength). As a remedy, substitutional doping of Ti₃SiC₂ compound with Ta and Nb was therefore employed in recent studies [11,12] to improve its long-term oxidation resistance and electrical conductivity to be used as an interconnect for intermediate-temperature SOFC, however, no information on the effect of this doping on the mechanical behavior and physical properties of Ti₃SiC₂ was reported.

As an alternative to substitutional doping, the development of Ti₃SiC₂-based composites reinforced with different carbide or silicide phases seems to be a prospective approach to improve mechanical and thermal properties, as shown in several reports [21,22]. Among the other reinforcements, TiC is one of the most suitable candidates for a Ti₃SiC₂ matrix due to their good thermal expansion match (7.4 × 10⁻⁶ K⁻¹ for TiC and 8.6 × 10⁻⁶ K⁻¹ for a-direction of Ti₃SiC₂ and 9.7 × 10⁻⁶ K⁻¹ for c-direction of Ti₃SiC₂) [23,24] and thermodynamic stability with Ti₃SiC₂ [25]. The additions of TiC carbide were shown to improve hardness, flexural strength, fracture toughness, wear resistance, and oxidation resistance of the composite as compared to bulk Ti₃SiC₂ [21,26]. Contrary to TiC, the effects of the presence of silicide compounds Ti₅Si₃ and TiSi₂ on the strength, corrosion resistance, or electrical properties of Ti₃SiC₂-based composites have not been studied in detail yet. In the production of the pure Ti₃SiC₂ phase, these compounds are usually considered as undesirable impurities. At the same time, the addition of Ti₅Si₃ and TiSi₂ phases can be beneficial for the improvement of the oxidation resistance, since both silicides have excellent oxidation and corrosion resistance at high temperatures due to the formation of protective SiO₂ layers [[27], [28], [29]]. Also, these silicides have low density, high melting temperatures, and good thermal expansion match with the Ti₃SiC₂ (7.1 × 10⁻⁶ K⁻¹ for Ti₅Si₃ and 9.9 × 10⁻⁶ K⁻¹ for TiSi₂) [27]. For the Ti₃SiC₂-based composites with the silicide reinforcements, only one work is devoted to the sintering of Ti₃SiC₂–Ti₅Si₃–TiC composites and their microstructure, hardness and tribological properties [30].

For all the reinforcement types (carbide, silicide or both types), the most important condition for obtaining good mechanical properties of the composite is the preservation of the fine grain size, as large grains of carbides or silicides are detrimental to the mechanical properties [22]. In this context, the use of advanced sintering techniques, such as spark plasma sintering, is an advantageous approach to fabricate Ti₃SiC₂-based composites with finer carbide/silicide reinforcements in contrast to the conventionally used methods, such as hot pressing [[31], [32], [33], [34], [35], [36], [37]], reactive melt infiltration [[38], [39], [40]], mechanical alloying (MA) [41], in situ reactive synthesis [42,43], self-propagating high-temperature synthesis (SHS) [32,44], three-dimensional printing [45], or pulse discharge sintering [26,35,46]. The employment of spark plasma sintering (SPS) allows to process MAX phases and related composites under relatively lower temperatures and significantly shorter times, thereby retaining a fine-grained microstructure, and consequently advanced mechanical properties [21,[47], [48], [49], [50]]. The previous results showed that the Ti₃SiC₂ obtained by SPS in the temperature range of 1200–1250 °C and a dwell time of 5–10 min under a constant pressure of 20–30 MPa reached the hardness values in the range of 4–5.5 GPa [[51], [52], [53]]. An additional improvement of the mechanical properties of pure Ti₃SiC₂ can be reached via the development of Ti₃SiC₂–TiC composites [21]. Nevertheless, the corrosion and electrical performance of these composites should be carefully evaluated in relation to potential SOFC interconnect application.

This work represents the first part of an experimental study focused on the development of alternative materials for fuel cell interconnects that would increase the SOFC system performance, while significantly reducing its weight. The aim was a development of new, high-quality Ti₃SiC₂-based composite strengthened by complex carbide-titanium reinforcement phases. To do that, a correlation between the SPS sintering conditions (sintering temperature, pressure, duration, and atmosphere) and the resulting microstructure, phase composition, mechanical properties was studied. Since these materials could potentially be used for solid oxide fuel cells as interconnects, special attention was paid to assessing the porosity, expansion behavior and electrical characteristics of the Ti–Si–C-based compacts.