Cellular MgAl2O4 spinels prepared by reactive sintering of emulsified suspensions

Cellular MgAl2O4 spinels prepared by reactive sintering of emulsified suspensions

Materials Letters 164 (2016) 190–193 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet C...

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Materials Letters 164 (2016) 190–193

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Cellular MgAl2O4 spinels prepared by reactive sintering of emulsified suspensions N. Vitorino a,b,n, C. Freitas b, A.V. Kovalevsky a, J.C.C. Abrantes a,b, J.R. Frade a a b

CICECO-Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal UIDM, ESTG, Polytechnic Institute of Viana do Castelo, 4900 Viana do Castelo, Portugal

art ic l e i nf o

a b s t r a c t

Article history: Received 3 July 2015 Received in revised form 12 October 2015 Accepted 31 October 2015 Available online 2 November 2015

Emulsification of mixed precursor powder mixtures (Al2O3 þMgCO3) and subsequent reactive firing were, for the first time, combined to prepare single phase cellular MgAl2O4 and corresponding MgxAl3  xO4 spinels with deviations from ideal stoichiometry. Suspensions with 50% v/v solid load were emulsified in melted paraffin, with collagen additions to assist consolidation by gelcasting. Early burnout stages and firing conditions were adjusted to avoid collapse of green bodies on heating and to induce favorable microstructural changes, with emphasis on porosity and percolation. & 2015 Elsevier B.V. All rights reserved.

Keywords: Cellular monoliths MgAl2O4 spinels Reactive firing Emulsified suspensions

1. Introduction MgAl2O4 spinel is widely used in metallurgical, structural, chemical, optical, electrotechnical, catalysis and electronic industries [1–3] for a broad range of applications such as humidity sensors [3–5] catalyst supports [4–6], refractory materials for cement rotary kilns and steel ladles [1,3,4], etc. Many of these applications require specific microstructural features such as high surface area, small crystallite size and appropriate surface functionalization capabilities, especially targeting catalytic supports applications [6]. The MgAl2O4 spinel has been prepared by a variety of methods [2], namely conventional solid-state-reaction, self-heat-sustained [1], wet-chemical methods of co-precipitation [2,5,6], precipitation [6], sol–gel of metal alkoxides [2,5,6], spray-drying [1,5], hydrothermal technique [2,3], plasma spray decomposition [2,3], combustion, freeze drying [2,5], controlled hydrolysis of metal alkoxides decomposition of organometallic compounds in supercritical fluids, microwave assisted combustion [3,6,7], etc. Though solid-state-reaction is most often used to preparare MgAl2O4 spinels, this requires long processing time, including multiple calcining steps to obtain single phase material, with undue effects on microstructural features such as abnormal grain growth and remnant porosity [1,4,5]. Significant volume expansion occurs n Corresponding author at: CICECO-Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal. E-mail address: [email protected] (N. Vitorino).

http://dx.doi.org/10.1016/j.matlet.2015.10.169 0167-577X/& 2015 Elsevier B.V. All rights reserved.

during the synthesis of MgAl2O4 spinel from its oxide precursors (about 8%) with negative impact on the ability to process dense bodies in a single stage of reactive sintering [2,8]. To overcome this problem, alumina and magnesia raw materials are normally annealed at  1400 °C (depending on the MgO/Al2O3 ratio) to get an appreciable amount of spinel phase, followed by an additional high temperature treatment (4 1600 °C) to achieve desired densification [1,4,8]. The preparation methods listed above are broadly used to obtain powders and dense ceramics based on MgAl2O4 spinel. As an alternative, for those applications requiring high surface area to volume ratio (e.g., sensors, catalytic supports, etc.) this material can be processed as porous monolithic bodies with large connectivity between the pores, to allow percolation and enhanced gas–solid interactions, without undue pressure drop or high resistance to plugging. Thus, this work aims to demonstrate the possibility to process porous cellular MgAl2O4 monoliths by emulsification of Al2O3 þMgCO3 aqueous suspensions with melted paraffin and subsequent reactive firing.

2. Materials and methods Cellular ceramic emulsions were prepared using a previously proposed methodology [9,10]. Precursors powders, (Al2O3 Alcoa CT3000, and MgCO3 Panreac) were used in a 1:1 M ratio (MgCO3: Al2O3) to prepare suspensions with 50 vol% of solids content, stabilized by adjusting viscosity at about with E0.6 Pa s by

N. Vitorino et al. / Materials Letters 164 (2016) 190–193


Table 1 Sintering conditions, total porosity (% P) and constriction factors (f) and average cell diameter (D) of prepared cellular MgAl2O4 monoliths. Denomination

Sintering conditions Segment 1

MgAl2O4 MgAl2O4 MgAl2O4 MgAl2O4

1550 1650 1550 3s 1650 3s

Segment 2


D (μm)

72.5 68.5 71.1 65.9

0.92 1.53 1.31 1.59

40.8 33.4 37.6 30.7

Segment 3

Ramp rate (°C/min)

T (°C)

t (h)

Ramp rate (°C/min)

T (°C)

t (h)

Ramp rate (°C/min)

T (°C)

t (h)




– – 2

– – 350

– – 3


1550 1650 1550 1650




-20 MgCO3

-40 Organics decomposition

-60 0


Expected MgCO3 decomposition from the dryed green body

dried green body

200 400 600 Temperature (ºC)







Intensity (a.u.)

weight losses (%)



s s




Fig. 1. Thermogravimetric curves for dried green body (thick line) and reference MgCO3 precursor material (thin line), obtained on heating at 5 K/min. The dotted line indicate the expected weight losses due to decomposition of MgCO3 from dried green body, the dashed line shows the contribution from burning the organic constituents.


50 Element content, % mol





10 0



1 1.5 Distance, mm



45 2θ /º



Fig. 3. XRD patterns of powdered cellular MgAl2O4 and Mg0xAl3  xO4 ceramics obtained by different heat treatments.





Fig. 2. Al and Mg concentration profiles across the 1650 3 s sample, measured by EDS.

addition of Dolapix PC-67. Suspensions added to melted paraffin (Merck, M.P. E58 °C) in a paraffin:suspension volume ratio of 1.5, followed by the addition of an aqueous solution (5 wt%) of the

anionic surfactant sodium lauryl sulfate (Sigma-Aldrich L-6026), with 0.06:1 volume ratio relative to the volume of emulsified suspension. Collagen (OXOID LP0008) was also added as a shape stabilizer (5 wt% relative to the water content). Emulsification was promoted by mechanical stirring at 1000 rpm for 10 min. After 72 h of drying at room temperature, samples were heattreated with several intermediate dwell stages, to allow controlled weight losses, and finally fired during 2 h at different temperatures (Table 1). The treatment conditions were selected based on the results, obtained in [9,10], in order to minimize the risks of disruption due to sudden release of gases and to ensure strong consolidation of the porous matrix. Thermogravimetry and differential thermal analysis (Netzsch STA409EP) were used to identify temperature ranges with relevant weight losses and to de-convolute losses ascribed to endothermic decomposition of MgCO3 and decomposition of the organic phases; this was used for adjusting the heat treatment conditions to avoid undue internal pressure variations and collapse of the green bodies.


N. Vitorino et al. / Materials Letters 164 (2016) 190–193

MgAl2O4 1550

MgAl 2O4 1550 3s

MgAl2O4 1650

MgAl 2O4 1650 3s

Mg0.9Al2.1O4 1550

Mg0.9Al2.1O4 1650

Fig. 4. SEM micrographs of fractured MgAl2O4 and Mg0.9Al2.1O4 cellular ceramics, processed under various conditions (Table 1).

Constriction to percolation between adjacent cells was estimated by impedance spectroscopy measurements, after impregnation of the cellular ceramics with a collagen solution (5:100 weight ratio). Differences in electrical resistivity between collagen solution and alumina allow one to trace current constrictions between adjacent cells, described by a constriction factor [11]

f = xo

A R L ρgel


where ρgel represents electrical resistivity of the collagen solution

(15.4 Ω m), x o is open porosity, A /L geometric area:thickness ratio

(m) and R the electrical resistance of the impregnated sample. Al foil was used to provide electrical contacts. The impedance spectra were taken in the frequency range 102–106 Hz, using an impedance bridge HP 4284A. Phase purity of fired monoliths was confirmed by XRD (Bruker D8 Advance DaVinci, with a step of 0.02° and a residence time of 0.5 s), after crushing the monoliths to powder. Diffraction patterns were analyzed using ICDD (International Center of Diffraction Data, PDF 4) in EVA (Bruker) software. The microstructure and elemental distribution in monoliths were assessed by scanning electron microscopy (SEM – Hitachi SU1510) and energy dispersive X-Ray spectroscopy (EDS-Bruker Quantax 200), performed on fracture surface.

N. Vitorino et al. / Materials Letters 164 (2016) 190–193

3. Results and discussion Fig. 1 shows the thermogravimetry (TG) curves for green ceramic body obtained by casting the emulsified suspension and subsequent drying and MgCO3 reference. The initial losses at up to about 100 °C correspond to endothermic evaporation of residual humidity, and TG results obtained for MgCO3 were used to deconvolute the corresponding CO2 contribution to losses of the green body (dotted line) and remaining losses ascribed to elimination of paraffin and other additives (collagen and surfactant). These results were used as guidelines for selecting the firing conditions, preventing detrimental increase in internal pressure. Thus, the firing cycle includes an initial ramp at low rate and isothermal plateau at 200 °C (Table 1), to ensure slow water losses. Some regimes also comprised a second plateau at 350 °C to ensure that the initial losses are self controlled by slow supply of oxygen, which is required to sustain oxidative decomposition of organics; this is expected to originate porosity for subsequent decomposition of MgCO3, without undue internal pressure. Processing methods based on ceramic suspensions of powder mixtures and their emulsification may be affected by segregation, depending on relevant parameters such as solid load, viscosity, stirring rate, setting and conditions of collagen gelcasting. Thus, EDS profiles were performed to assess the uniformity of chemical composition through the monolith (Fig. 2). The estimates are close to the stoichiometric atomic ratio in the MgAl2O4 spinel (Mg: Al E1:2), confirming efficient dispersion of precursors during the steps of preparation of ceramic suspensions, emulsification, consolidation and drying of the green body. Actually, one cannot exclude slight deviation from stoichiometry, mainly because MgCO3 may include slight hydration, as suggested by the complex features of its thermogravimetry [12]. XRD confirmed formation of a single phase spinel after firing (Fig. 3), without traces of precursors or other secondary phases, probably because the composition range of the MgxAl3  xO4 spinel phase is relatively wide at high temperatures [13]. The latter is additionally confirmed by processing cellular ceramics with non-stoichiometric compositions, as shown also in Fig. 3 (for x ¼0.9). Porosity, cell size and percolation are the key parameters for the applicability of porous cellular ceramics. A summary of results is shown in Table 1, emphasizing major effects of firing conditions on cell diameter, percolation, characterized by the constriction factor, and slight changes in porosity. These factors are related as discussed in previous works [10,11], percolation increase with increasing porosity, and also increase with cell size. Percolation drops with increase in peak firing temperature. High resistivity and porosity were observed for samples sintered at 1650 °C with 3 sintering steps (Table 1); this means that the percolation between cells is lower in this case. Grain growth is also observed on increasing the firing temperature from 1550 to 1650 °C (Fig. 4). On the other hand, introduction of isothermal plateau at 350 °C into processing route yields cellular ceramics with improved connectivity between MgAl2O4 grains, probably assisted by slower MgCO3 decomposition and corresponding reorganization of the green skeleton of Al2O3 þ MgO (formed after MgCO3 decomposition).

4. Conclusions One demonstrated that cellular MgAl2O4 ceramics can be


processed by emulsification of mixed Al2O3 þ MgCO3 precursor suspensions and reactive firing. By adjusting the initial stages of heat treatment, one controlled the evolution of gases resulting from residual humidity, elimination of pore former and decomposition of MgCO3, to ensure that the resulting porosity prevents undue internal pressure and collapse of the green samples. Reactive firing allows one to change porosity and percolation. Earlier burnout stages also contribute to microstructural changes in the resulting cellular ceramics, probably by facilitating the reorganization of the resulting Al2O3 þMgO green packing, before reactive firing at higher temperatures. The proposed method is also suitable to process single phase cellular ceramics with adjusted MgxAl3  xO4 compositions, based on frozen-in conditions achieved at high firing temperatures.

Acknowledgments This work was developed in the scope of the project CICECOAveiro Institute of Materials (Ref. FCT UID /CTM /50011/2013, PTDC/CTM-ENE/2073/2012 and PEst-C/CTM/LA0011/2013, and Grants SFRH/BPD/99367/2013 and IF/00302/2012), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. SEM and XRD facilities were funded by FEDER Funds through QREN – Aviso SAIECT-IEC/2/2010, Operação NORTE-07-0162-FEDER-000050.

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