Preparation and characterization of macroporous γ-LiAlO2

Materials Letters 57 (2003) 3593 – 3597 www.elsevier.com/locate/matlet

Preparation and characterization of macroporous g-LiAlO2 Sergey Sokolov, Andreas Stein * Department of Chemistry, University of Minnesota, 207 Pleasant Street, S.E., Minneapolis, MN 55455, USA Received 6 January 2003; accepted 30 January 2003

Abstract Lithium aluminate powders with three-dimensional, periodic arrays of interconnected macropores (275 – 325 nm in diameter) were synthesized by poly(methyl methacrylate) latex colloidal crystal templating via wet-chemical process. Calcination removed the polymer template and converted the inorganic precursors into a high surface area (up to 56 m2/g), macroporous skeleton of g-LiAlO2. Under certain synthesis conditions, lithium-poor LiAl5O8 nanocrystals were also present. Optimized synthesis conditions and structural characterisics of the materials are presented. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Lithium aluminate; Porosity; Macroporous; Colloidal crystal; Composite; Templating

1. Introduction g-Lithium aluminate exhibits good thermochemical and irradiation stability, making it a potential candidate for a tritium-breeding blanket in fusion reactors as well as for ceramic matrixes in molten carbonate fuel cells [1,2]. Previously, g-LiAlO2 was prepared by conventional solid state methods [3,4], which required processing temperatures of 1000 jC and higher. Sol–gel methods allow a low temperature syntheses of g-LiAlO2. For example, hydrolysis of lithium alkoxides and aluminum alkoxides followed by calcination at 550 jC affords g-LiAlO2 [5]. Other examples include preparations of this material from aluminum sec-butoxide where the sources of lithium

* Corresponding author. Tel.: +1-612-624-1802; fax: +1-612626-7541. E-mail address: [email protected] (A. Stein).

can be hydroxide [6,7], acetate [8] or nitrate [6]. However, all of the above mentioned syntheses employ metal alkoxides, which are relatively expensive and moisture sensitive, thus requiring careful handling. In these regards, metal salts seem to be more practical precursors. In a recent study of precursor effects on the morphology of lithium aluminate by Kwon and Park [9], lithium and aluminum nitrates were used among other precursors in hydrothermal preparations. The authors observed the formation of boehmite, but no LiAlO2 phases were formed from lithium nitrate – aluminum nitrate system. Other researchers reported that spray drying of a pH-adjusted aqueous solution of lithium carbonate and aluminum nitrate yielded a mixture of a- and h-aluminates, which was converted to g-aluminate by annealing at 900 jC [10]. In this paper, we describe a templated wet-chemical process, which yields g-LiAlO2 with a high specific surface area and homogeneous porosity. This

0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00131-9

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process is based on colloidal crystal templating, which utilizes close-packed arrays of monodisperse spheres (e.g., polystyrene and poly(methyl methacrylate)) as molds [11]. Voids within such arrays are filled with inorganic precursors that may undergo further chemical transformations to the desired product phases. The templating spheres are removed by either calcination, extraction or etching. Pores in the resulting material are ordered, uniformly sized and typically a few hundred nanometers in diameter. The materials are denoted three-dimensionally ordered macroporous (3DOM) materials. Compositions of known up-todate 3DOM materials include silica, metal oxides, metals and alloys, metal chalcogenides, carbon and polymers [12].

2. Experimental 2.1. Sample preparation PMMA latex spheres were synthesized as described elsewhere [13]. These were close-packed by gravity sedimentation to form colloidal crystals. The following two procedures used to prepare composites of metal precursors in the PMMA templates differ in the manner in which the lithium precursor was introduced:

ml of a methanolic solution of aluminum nitrate nonahydrate (0.9 M). The excess of the solution was removed by suction and the impregnated colloidal crystals were allowed to dry in air at room temperature for 2 h. Then, the dried material was placed on a filter paper in a Bu¨chner funnel and 40 ml of 28 wt.% aqueous ammonia/methanol (a 1:1 mixture by volume) containing 0.4 M lithium nitrate was applied dropwise to the composite under suction. The materials were dried and calcined either as in method I or at lower temperatures for longer time periods. 2.2. Characterization The crystalline phases of the samples were identified by powder X-ray diffraction on a Siemens D500 diffractometer using CuKa radiation. SEM images were obtained on a Hitachi S800 instrument using an accelerating voltage of 10 kV. TGA and DSC analyses were performed in air, using a Netzsch Simultaneous Thermal Analyzer STA 409 PC with alumina crucibles and a heating rate of 10 jC/min. Nitrogen gas adsorption was carried out on a RXM100 Catalyst Characterization System (Advanced Scientific Design) and specific surface areas were calculated by the BET method.

3. Results and discussion 2.1.1. Method I Colloidal crystals composed of 400 nm diameter PMMA spheres (ca. 7 g) were soaked for 2 min in 20 ml of a methanolic solution containing lithium nitrate (0.9 – 3.2 M) and aluminum nitrate nonahydrate (0.8 – 0.9 M). The excess of the solution was removed by suction and the impregnated colloidal crystals were dried in air at room temperature for 2 h. Then, 20 ml of 28 wt.% aqueous ammonia/methanol (a 1:1 mixture by volume) was applied dropwise to the composite to form a precipitate within the colloidal crystals. After drying at room temperature for 12 h, materials were calcined in a flowing mixture of air/nitrogen (0.2 l min 1:1 l min 1), first at 300 jC for 3 h (heating rate: 2 jC min 1), then at 850 – 900 jC for 3 h. 2.1.2. Method II Colloidal crystals composed of 430 nm diameter PMMA spheres (ca. 7 g) were soaked for 2 min in 20

After calcination of the composite obtained by method I that contained lithium and aluminum precursors in stochiometric amounts (Li/Al mole ratio = 1.0), the XRD pattern of the product exhibited two phases, LiAl5O8 (PDF#38-1425) as the major phase and g-LiAlO2 (PDF#38-1464) as the minor phase (Fig. 1a). A SEM image showed that the material was macroporous, containing periodic voids with average diameters of 275 nm (Fig. 2a). The 31% linear shrinkage compared to the size of PMMA spheres was attributed to loss of solvent and nitrate ions, as well as sintering during the calcination process. The voids were interconnected through open windows, ca. 40 –80 nm in diameter. The walls were composed of fused LiAl5O8 and gLiAlO2 nanocrystals with average sizes of 11 and 19 nm, respectively, estimated from powder XRD data. A BET specific surface area of 48 m2/g was calcu-

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Fig. 1. XRD patterns of lithium aluminates. Samples (a) and (b) were obtained by method I from LiNO3 – Al(NO3)3 precursor solutions with Li/ Al mole ratios of 1.0 and 3.9, respectively; samples were calcined for 3 h at 900 jC (a) and 850 jC (b). Samples (c) and (d) were obtained by method II and calcined at 850 jC for 3 h and at 650 jC for 10 h, respectively.

lated from nitrogen adsorption measurements. The low Li/Al ratio in the product suggests that lithium was partially lost during the precipitation step with the alcoholic ammonia solution, since both lithium nitrate and lithium hydroxide are more soluble than aluminum hydroxide in an aqueous ammonia-methanol mixture. Preparations employing an excess of lithium nitrate resulted in 3DOM products with larger proportions of g-LiAlO2. Mixed LiAlO2/

LiAl5O8 phases were produced with Li/Al ratios up to 3.4, pure g-LiAlO2 was obtained only with a 290% stochiometric excess of lithium (Li/Al = 3.9; Fig. 1b). However, materials prepared with Li/Al ratios above 3 were less periodic than the other samples (Fig. 2b); they also contained significant amounts of bulk material along with porous regions and exhibited lower surface areas due to larger crystallite sizes (Table 1).

Fig. 2. SEM images of LiAlO2/LiAl5O8 mixed products prepared by method I with precursor solutions Li/Al mole ratios of 1.0 (a) and 3.9 (b). (c) SEM image of LiAlO2 prepared by method II and calcined at 850 jC for 3 h.

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Table 1 Compositions and structural data for samples prepared by methods I and II Li/Al mole ratio in synthesis mixture

LiAlO2/ LiAl5O8 ratioa

Li/Al mole ratio in product

BET surface area (m2/g)

Crystallite sizeb of LiAlO2 (nm)

Method I 1.0 3.4 3.9

0.75 12.35 no LiAl5O8

0.30 0.77 1

48 35 14

19 22 32

Method II 0.9 (650 jC) 0.9 (850 jC)

no LiAl5O8 no LiAl5O8

1 1

56 35

10 16

a

LiAlO2/LiAl5O8 was estimated from the ratio of the intensities ˚) of the strongest XRD reflections: (101) for LiAlO2 (d = 3.995 A ˚ ). and (311) for LiAl5O8 (d = 2.387 A b Crystallite sizes were calculated by the Scherrer equation for line broadening, using the following reflections: (101) for LiAlO2 and (311) for LiAl5O8. A lanthanum hexaboride standard was used to account for instrumental line broadening.

Another approach involved introduction of lithium ions along with ammonium hydroxide in a second step, as described in method II of Section 2. This procedure avoided preferential leaching of lithium ions from the template. The XRD pattern of the product calcined at a final temperature of 850 jC for 3 h (Fig. 1c) corresponded to g-LiAlO2 (PDF#381464) with a crystallite size of 16 nm. Only traces of LiAl5O8 phase were observed. An extensively or-

dered porous structure (Fig. 2c) together with a relatively small crystallite size rendered the material with a BET surface area of 35 m 2/g. A linear shrinkage of 24% during processing resulted in an average pore size of 325 nm. The wall skeleton was more ‘‘strut-like’’ than in products formed by method I, so that windows between pores were correspondingly larger. In order to increase the surface area of the products further by minimizing grain growth through sintering, the minimum formation temperature for g-LiAlO2 from the given precursors was determined. A composite prepared via the second route was pre-calcined at 350 jC for 5 h in an oxygen-poor atmosphere (0.2 l min 1 air:1 l min 1 N2) to remove most of the PMMA template. The XRD pattern of the product could not be indexed due to the low crystallinity of the material. The DSC curve (Fig. 3) indicated an endothermic event around 100 jC (weight loss: 2.7%), which was attributed to the loss of moisture. A second event between 100 and 540 jC with a maximum at 318 jC was exothermic due to pyrolysis of residual polymer decomposition products and was accompanied by a major weight loss of 31.4%. Another exothermic event starting at 540 jC with a maximum at 679 jC corresponded to the formation of g-LiAlO2, as confirmed in bulk samples by XRD (see below). A coinciding weight loss of 4.2% was attributed to lattice dehydration.

Fig. 3. TGA and DSC curves of a PMMA-precursor composite prepared by method II and pre-calcined at 350 jC for 8 h.

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Based on this thermal analysis, two samples were prepared by method II and calcined at 550 and at 650 jC for 10 h. The sample prepared at 550 jC possessed a 3DOM structure and produced an XRD pattern with broad reflections from a h-LiAlO2 phase (PDF# 330785). The material was brown, indicative of residual carbon. The sample calcined at 650 jC was white and gave a powder pattern of the g-LiAlO2 phase (Fig. 1d). Due to the small crystallite size in the wall skeleton (10 nm), the sample exhibited a high surface area of 56 m2/g, which is higher than that reported for g-LiAlO2 obtained by sol – gel syntheses [14,15]. Thus, it was established that 3DOM g-LiAlO2 free of residual carbon could be formed at 650 jC.

4. Conclusion Three-dimensionally ordered macroporous gLiAlO2 was prepared via colloidal crystal templating from lithium and aluminum nitrates. It was demonstrated that the lithium precursor could be introduced either along with aluminum during the first step or dissolved in the precipitating solution during the second step. After the organic template was removed by calcination, g-LiAlO2 could be produced by both methods. However, the first method required a large excess of the lithium precursor, whereas only stochiometric amounts were needed in the second method. The material prepared via the second method possessed an extensively ordered macroporous structure with average pore diameters of 325 nm. Such morphology provided small crystallite sizes and a notably high surface area, which was further increased by lowering the processing temperature to the minimum adequate for this method of synthesis (650 jC). The open, interconnected pore structure of these ceramics is a desirable feature in host matrices for fluids, such as those used in molten carbonate fuel cells [2].

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Acknowledgements Parts of this research were funded by the David and Lucile Packard Foundation, the NSF (DMR9701507), the MRSEC program of the NSF (DMR9809364), the U.S. Army Research Laboratory and the U.S. Army Research Office (DAAD 19-01-10512), and the Office of Naval Research (grant number N00014-01-1-0810, subcontracted from NWU).

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