Synthesis and Sintering Behavior of Lanthanum Chromite Doped with Strontium and Cobalt for SOFC Interconnect Applications

Lanthanum Chromite has been the main candidate as interconnect material for high temperature fuel cell applications, SOFC. So all researches on the direction to clarify some aspects of this material continue to be an interesting contribution to understand and modify the behavior of the Lanthanum Chromite in use. The effort of this work is attaining doped Lanthanum Chromite by combustion synthesis, optimizing the parameters of the synthesis process and studying the influence of the conditioning powder process as milling, atmosphere and temperature of the sintering and also additives concentration on the final density, crystalline structures and expansion thermal behavior. The main characterization techniques used were BET determinations, Archimedes density method, dilatometry, scanning electronic microscopy, X ray diffraction and Rietveld method for refinements of the phases quantification. The homogeneous Lanthanum Chromite doped with Strontium and Cobalt with sintered densities around 97th% and thermal expansion coefficient compatible for SOFC applications (12x10°C) were attained with control of the adequate concentrations additives, synthesis conditions, the control of the atmosphere, time and temperature of the sintering process. INTRODUCTION The rapid development of fuel cell technology in recent years, together with a global effort to reducing greenhouse gas emissions is focusing world opinion on options for future hydrogen economies. The solid oxide fuel cells (SOFC) are promising clean and efficient power sources, particularly for stationary applications [1,2]. Although many of the challenges in the development of high performance low-cost SOFCs can be solved with improving materials. Among one of the major challenges in the SOFC development is the interconnect materials whose the main attribute is to provide the conductive path for electrical current to pass between the electrodes and the external circuit [3, 4]. This material must have good electrical conductivity to minimize ohmic losses. The interconnect material is contacted to both cathode and the anode, that is required to be stable in both oxidizing and reducing conditions. SOFCs operate at high temperatures and the interconnect material must have a thermal expansion coefficient (TEC) close to those of other cell components to minimize thermal stresses. And also other requirements of the interconnect materials include adequate mechanical strength, low permeability to oxygen and hydrogen, reasonable thermal conductivity and easily to fabricate to contributing for the cost-effective manufacture of fuels cells. Several types of ABO3 Chromites, including La(M)CrO3, where M is Mg, Ca, Sr or Co have been developed that meet these requirements. It is also required a high density to provide gas tightness in the interconnection. It is known that most Cr-containing oxides are difficult to sinter in air to high density because of vapor-phase transport, which cause grain growth without densification [1, 5, 6]. Sintering of Chromites in air is relied on the formation of liquid phases by introducing sintering aids, such as, Co, Ni or Ca with formation of second perovskite phase, although some problems were reported resulting from such additives in SOFC applications [7,8,9]. Although Co additions promote the densification of LaCrO3 in air at low temperatures it also increase the expansion thermal coefficient (TEC) after sintering. The influence of the Srdoping, is verified to contributing by the reduction on the TEC in the same conditions. Besides of sintering processing, the precursor powder are primordial to obtain high density interconnects. The characteristics of the precursor powder depend on the method of the synthesis. There are a variety of methods of multi-cation oxide precursor powder synthesis. Conventional method for multi-cation oxide preparation is based on solid-state reaction technique. In this method, ball milling and grinding steps are time consuming and energy intensive. The calcinations and sintering with long holding times at high temperatures make the method expensive [2]. Citrate method (Pechini) [10,11] is another way for multication oxide synthesis. Popa [12] synthesized LaCoO3 powders by a polymerizable complex technique based on the Pechini-type reaction route. Synthesis by combustion reaction is been used to produce a lot of ceramic materials. This method’s main advantage is the possibility of obtaining homogenous composition nanometric particles ceramic powder. It is considered a fast, simple and economic process [13]. The process begins with a mixture of oxidant reagents (nitrates, sulfates, carbonates, among others) and an organic fuel (urea, carboidrazina, maleica hydrazine, etc.), which acts as a reducing reagent. The solution with the mixture is warmed up until auto-ignition, reaching high temperatures simultaneous with a fast and sustainable reaction [14]. The metallic oxide production by simple calcinations process, where the metallic nitrates are decomposed into oxides, needs a constant supply of energy. On the other hand, the combustion reaction, usually has its ignition at temperatures lower than the phase formation ones. The energy released during the exothermic reaction between the nitrates and the fuel in the combustion reaction can heat fast and intensely the system, maintaining the heat for a long period, even without an external heat source to guarantee the desired phase formation [14, 15]. That is the reason why the combustion synthesis is considered more worth-while than simple thermic calcinations. The heat releases gas species before the nitrate decomposition, when the water is released, the temperature increases staring the ignition of the fuel (oxidation). The reaction’s outset is observed by flames that achieve high temperatures, around 1000°C [14], they guarantee ceramic formation and crystallization in a short period of time. This is a method that is considered simple because it does not need further treatment steps. Oxi-reduction reactions as well combustion reactions are usually exothermic and if they are not properly controlled, an explosion can happen. Nevertheless, the reactions from nitrates and urea are self propagating they are exothermic but not explosive. Urea is commercially available, low price and produces higher temperatures what makes it interesting to the synthesis process [14, 15, 16](Segadães et al., 1998). The base of combustion reaction for powder synthesis comes from explosives and propellants’ thermodynamics concepts. Jain [17] proposed a simple calculation method of stochiometric coefficient establishing a valence balance of elements on reducing and oxidant reactants form the mixture. Stoichiometric composition calculation of the nitrate/urea mixture is based on the total valences from the fuel and oxidant, and it is valid when the concerning valence values are: [(Σ oxidant elements coefficients (O) /n(-1)Σ reducing element coefficient (F)) = 1] and it corresponds to the maximum energy released to the reaction. The considered valences are the ones form the product elements of the combustion reactions that are: CO2, H2O and N2. In this calculation, the carbon and the hydrogen are considered reducing elements, presenting the valences +4 and +1 respectively. The oxygen is considered an oxidant element with a valence of -2, and the nitrogen is considered valence zero. When extrapolate this concept to oxides that means metals like La and Cr (or other) can be considered reducing elements. Taking into account the structure Lanthanum Chromite (LaCrO3), it is considered the valences corresponding to the coordination of the desired components, that is +3 for both. In stoichiometric mixtures, there are a relation between oxidant and reducing total power and the reaction heat calculated from the formation heat of the reactants and products from the reaction [18]. The temperature of the flame produced by the combustion depends on the reducing power available and the quantity of gases that are formed. This conditions influence the characteristics of the produced powder, like particle size and agglomeration state. In their studies, Manoharan and Patil [19], observed the urea influence in some characteristics of the produced powder. After the reaction its use provided a better porosity because this fuel produces less gas per mol of fuel (1 mol of urea = 1 mol of gas) [13, 19]. The metallic nitrate are the most used salts to the synthesis on combustion reactions since they are soluble in water and in general they have low decomposing temperatures what guarantees a good homogeneity in the solution. Biamino and Badini [20] studied the behavior involving nitrates and the urea during the combustion reaction to obtain LaCrO3 and they observed that besides the apparent simplicity of the method, a series of intermediate complex compounds are formed and they are difficult to present in only one chemical reaction. In the last years many studies are been made to obtain LaCrO3 from combustion reaction technique using several types of fuels. Morelli [11, 21, 22], as well as Biamino and Badini [20] used urea as fuel to the synthesis of LaCrO3. Comparing this route of synthesis to the oxide mixture technique and Pechini, Morelli [11], they observed that the obtained powder from combustion presented better sinterability than ones obtained by other techniques. The present work reports synthesis of Cobalt and Strontium doped Lanthanum Chromite by combustion reaction synthesis. This method was chosen because it is a relatively simple and fast method, and it uses for the reactions as start materials or reagents easily found in the market [13]. However this simple method is strongly influenced by the kind and the concentration of the fuel and the kind and content of the additive used. It will influence the structures and the morphology of the powder resultant. EXPERIMENTAL The aqueous nitric solutions of the metals were