Parametric study of the gel-combustion synthesis of nanocrystalline ZrO2-based powders
Graphical abstract
Highlights
► We exhaustively study gel-combustion routes for the synthesis of nanocrystalline oxides. ► Gel-combustion routes yield highly homogeneous nanocrystalline materials. ► The temperature of the material during the combustion reaction controls powder morphology. ► A novel gel-combustion route assisted with oxygen peroxide is reported.
Introduction
Gel-combustion synthesis, and, in general, soft chemical methods, is very popular nowadays as it is a simple and fast synthesis route to produce compositionally homogenous, crystalline, deagglomerated ceramic powders at much lower temperatures than classic ceramic synthesis methods. It allows the facile generation of relatively large quantities of simple [1], [2], [3], [4], [5], [6] or complex [7], [8], [9], [10], [11] ceramics, which present a nanometric size and/or metastable phases [1], [4], [9], [12], [13], [14].
Gel-combustion is based on the formation of an initial gel by thermal concentration of an aqueous mixture of the desired metal nitrates and organic fuels, followed by a combustion process. The reaction between the nitrate ion and the organic fuel is strongly exothermic and the great volume of produced gasses during the combustion promotes the violent disintegration of the precursor gel. A calcination process at moderate temperature eliminates the remaining organic residues. For ZrO2-based powders the synthesis yields a low density powder composed of highly crystalline nanometric grains [12], [13] with very small pores between them [15].
Zirconia-based ceramics have been intensely investigated because of their excellent electric and mechanical properties. In particular, TZP (‘tetragonal zirconia polycrystals’) ceramics exhibit high ionic conductivity at intermediate temperatures and high fracture toughness [16], [17]. For example, ZrO2–CeO2 substitutional solid solutions are extensively used as redox or oxygen storage promoters in three-way catalysts [18] and ZrO2–Y2O3 ceramics are used for several electrochemical devices [19], and biomedical applications [20].
Several authors have studied the influence of certain synthesis parameters on the properties and morphology of the final ceramic powder. Particular attention was paid to the nature and/or mixture of fuels [1], [5], [6], [7], [8], [9], [21] and the role of the oxidizer/fuel ratio [1], [2], [3], [6], [7], [9], [10], [11], [22], [23]. Few works reported studies on the influence of the metal/fuel ratio [24], [25], the final pH [26], [27] or the combustion temperature [1], [6], [10], [28]. As we will demonstrate, this temperature is a very important parameter to tailor properties as the crystallite size or BET surface area. Unfortunately, this temperature was measured on the flame [6], [10], [28], which may not be representative of the temperature of the solid, or with a thermocouple [1], which may not have reached thermal equilibrium due to the fast propagation of the reaction front. In this paper we report the temperature of the material during combustion.
Additionally, the influence of the mentioned parameters on the final product seems to depend on the material. CeO2[2], (ZrO2)0.92(Y2O3)0.08[23], SrCeO3[4] or Gd2O3[6], show a bigger crystallite size as the fuel/oxidizer ratio increases, while the opposite happens for Ce0.8Y0.2O1.9[22], Ba0.5Sr0.5Co0.8Fe0.2O3[7], Y2O3[3] or La0.7Ca0.3CrO3[11]. Al2O3–tZrO2[25] composite maintains approximately the same average crystallite size with an increase of the metal/fuel ratio, while for CeO2[24] the average crystallite size diminishes. Nevertheless, all papers report an optimum value depending on the property to be tailored: degree of agglomeration [3], [5], [22], [24], average crystallite size [1], [2], [3], [4], [5], [7], [8], [9], [12], [13], [22], [24], [25], crystal phase [6], [9], [10], [12], [13], average pore size and distribution [5], [15], BET specific surface area [2], [3], [5], [22], etc.
Currently, nanomaterials have a tremendous technological and basic importance. They present new phenomena and new or enhanced properties as a consequence of their small dimensions [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. In some cases, they are related to quantum effects that arise due to the small number of atoms. For example, CdSe quantum dots have a tunable emission spectrum and higher emission efficiency [39], [40]. CeO2-based nanoceramics present an enhanced ionic conductivity of one order of magnitude higher than their microceramic counterparts [38].
The aim of this work was to optimize the synthesis of ZrO2-based nanomaterials by gel-combustion. In the particular case of nanoceramics, the advantages are lost if the nano dimensions cannot be retained. Therefore, we have also looked for conditions that increase the sinterability, such as a large BET area.
For this purpose, we studied the influence of the different synthesis parameters (final pH of the gel, fuel type, fuel/metal ratio, final ceramic composition, combustion atmosphere, etc.) on the final ceramic powder. We undertook this study since no article combines all of them in a systematic way for the same material and, in particular, the role of the real combustion temperature, a key factor, is hardly discussed.
Section snippets
Gel-combustion synthesis routes
For the non-stoichiometric synthesis routes, 2.25 g of ZrOCl2·8H2O (Riedel-de Haën, Germany, 99.5%) and Ce(NO3)3·6H2O (Alpha Aesar, EE.UU, 99%) or Y2O3 (BDH, England, 99.99%), in adequate proportions, were dissolved in water. Then, 50 ml of nitric acid (Merck, Germany, 65%) was added. Since the chloride anions degrade the electrical properties, they were removed by thermal evaporation. This step was carefully controlled to ensure the same content of nitrate in all the samples (see Results and
Precursor gels
Fig. 1 displays different SAXS patterns for the Z3Y-5 synthesis as a function of the remaining time to the moment of combustion. A quick fall at low q and a subsequently uniform value in the rest of the studied q range is observed. Similar patterns were obtained for all the other routes. The shape of these curves indicates, according to X-ray scattering theory, that the gel was homogeneous up to the moment of the combustion reaction. This is an important fact, because the presence of
Conclusions and final remarks
We presented a detailed study of the effect on the powder morphology of several synthesis parameters of the gel-combustion routes.
Multiple techniques (FTIR, EXAFS) showed that the cations are complexed in the gel and thus maintained in solution ensuring a homogeneous mixture. Homogeneity also demonstrated by the SAXS analysis, reported here for the first time for the whole gelling process. This homogeneity is the origin of the compositional homogeneity of the ceramic powders produced by this
Acknowledgments
The authors wish to thank Lic. Raul Tarula (CITEFA) for his assistance with the FTIR measurements and Mr. Alejandro Fernandez (CITEFA) for his help with the syntheses. This work was supported by the Brazilian Synchrotron Light Laboratory (LNLS, Brazil, proposals D11A-SAXS1-3314 and D04B-XAFS1-6711), the scientific collaboration projects CNPq–CONICET and CAPES–MinCyT (Brazil–Argentina), CNPq (Brazil, PROSUL programs 490289/2005-3 and 490580/2008-4), Agencia Nacional de Promoción Científica y
Dr. Ismael Fábregas studied chemistry at the Universidad de Buenos Aires, Buenos Aires, Argentina. For his PhD, at CINSO (Solid State Research Center), CONICET, Argentina, he carried out research on the crystal structure and local atomic order of zirconia-based nanocrystalline solid solutions. Presently he is a research assistant at CINSO and works in refining of crystal structure, local order and electronic conductivity mainly in materials for solid oxide fuel cells.
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Dr. Ismael Fábregas studied chemistry at the Universidad de Buenos Aires, Buenos Aires, Argentina. For his PhD, at CINSO (Solid State Research Center), CONICET, Argentina, he carried out research on the crystal structure and local atomic order of zirconia-based nanocrystalline solid solutions. Presently he is a research assistant at CINSO and works in refining of crystal structure, local order and electronic conductivity mainly in materials for solid oxide fuel cells.
Dr. Diego G. Lamas is Associate Professor at the National University of Comahue (Neuquén, Argentina) and Independent Researcher of the National Scientific and Technical Research Council of Argentina. He received his PhD degree in Physics from the University of Buenos Aires, Argentina, in 1999. His investigations are mainly focused on synthesis, characterization, crystallographic and thermodynamic properties and applications of nanostructured materials, mainly nanoceramics for intermediate-temperature solid-oxide fuel cells (IT-SOFCs). During the last decade, he has been the head of the SOFC group at CINSO (Solid State Research Centre), CONICET-CITEFA, Argentina. He is the present vice president of the Argentine Association of Crystallography (AACr).