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Syntheses of submicron magnesium oxide powders
MATERIALS CHEM#TR&UND Materials
Chemistry
Materials
Syntheses d’Zng&ierie des Mathiaux
Science
of submicron K. Chhor,
Lnboratoire
and Physics 40 (1995) 63-68
magnesium
J.F. Bocquet,
et des Hautes Pressions, Received
Communication
oxide powders
C. Pommier
CNRS LP 1311, Universite’ Paris XII& Avenue J.B. Clhnent, France
25 May 1994; accepted 17 October
93430 VWetaneuse,
1994
Abstract Submicron MgO powders have been synthesized from thermal decomposition of magnesium chelates or acetate at around 200 “C in liquid alcohol or supercritical alcohol-CO, mixtures. Because of a similar coordination of the magnesium atom in both types of precursors, a common amorphous alkoxy-hydroxy derivative is first obtained. When the reaction occurs in a supercritical medium with carbon dioxide as cosolvent, the microstructure of the solid is quite different and a CO, group is incorporated into the alkoxy bond. In all cases, the intermediate compound crystallizes into cubic magnesium oxide under thermal treatment between 400 and 600 “C; the elementary particle size in the powders obtained is less than 100 nm. Keywords: Magnesium oxide powders; Submicron MgO powders
1. Introduction
Most commercial MgO powders are synthesized by thermal decomposition of magnesium hydroxide or salts [1,2] and used as refractories. In such powders, the primary particles remain aggregated, usually in a shape similar to that of the precursor compound. For more specific applications, finer and less aggregated powders with higher purity are needed. Oxidation of pure metal magnesium in air has been realized [3,4]. When this reaction is performed in the vapor phase, nanometer particles with a controlled size (10-200 nm) have been obtained [4,5], and the optimum diameter for better forming and sintering seems to be around 100 nm [6,7]. The hydrolysis and polycondensation of magnesium alkoxides (sol-gel process) has also been performed under controlled pH conditions [S]. The amorphous solid material obtained at room temperature leads to crystalline submicron powdered MgO when thermally treated above 600-900 “C. An increasingly popular application of magnesium oxide is its use as a protective layer on plasma displays: it prevents degradation of the dielectric lead-based device substrate and contributes to enhancement of the discharge efficiency as well as to a lowering of the operating voltage. Thin overcoatings are usually deposited by physical vapor techniques such as electron beam evaporation [9]. Chemical routes starting with
0254-0584/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0254-0584(94)01452-M
metal organic precursors have also been proposed. For example, a thin coating of magnesium acetylacetonate dissolved in hexylcarbitol can be fixed into MgO above 430 “C [lo]. Many studies have been repbrted on the sol-gel processing of oxide films, starting with metal alkoxides and using dip coating or spin casting methods [11,12]. However, for coating plasma devices, damage to the glass substrate caused by thermal treatment above 500 “C has to be avoided. We have recently reported the synthesis of narrowsized submicron oxide powders from thermal decomposition of metal alkoxides in a supercritical fluid [13,14]. Such a reaction medium allows rapid nucleation and prevents hard agglomeration of primary particles. For example, decomposition of titanium alkoxides in supercritical ethanol leads to the formation of TiO, powders with a narrow size distribution (20-50 nm). These primary particles are softly associated into OS-2 /*rn spherical agglomerates easily destroyed under ultrasonic sound treatment [14]. We report below the results of similar syntheses of magnesium oxide, starting with metal chelates (acetylacetonate and the hexafluorinated derivative) or acetate as precursors. 2. Experimental Magnesium acetylacetonate acetonate (denoted Mg(acac),
and hexafluoroacetyland Mg(hfa),, respec-
K Chhor et al. I Materials
64
Chemistry
tively) were commercial products from Strem. These solids were hydrated with an undetermined number of water molecules (n G 2). Magnesium acetate (Strem) was hydrated with 6 water molecules and had a purity higher than 99%. These compounds were used as precursors in the following syntheses. In some experiments they were dehydrated under vacuum before study. The carbon dioxide used as cosolvent (purity N45) was provided by Air Liquide (France). Two laboratory-made high-pressure reactors were used for reactions in alcohol or in alcohol-CO, mixtures. The first reactor is a blind stainless-steel cylindrical tube with an internal volume of around 130 cm3 and can operate up to 400 “C and 400 bar. The second one is a sapphire cell with an internal volume of around 8 cm3, allowing visual observation of reactions occurring inside. However, because of the vespel joints needed for pressure tightness, the operating conditions are limited to below 300 “C and 250 bar. The synthesized powders were characterized by Xray diffraction (Siemens diffractometer with Cu Ka radiation), IR spectroscopy (Perkin-Elmer 580 instrument) and scanning electron microscopy (Cambridge 460 instrument).
and Physics 40 (1995) 63-68
1 . 1800
.
1600 1400 VI number (cm-‘)
Fig. 1. IR spectra of Mg(acac), dissolved in methanol and heated at (a) 60 “C, (b) 120 “C, (c) 150 “C, (d) 190 “C. (A) solvent and chelate bands; (AA) chelate carbonyl stretching band; (AAA) decomposition product carbonyl stretching band.
3. Results 3.1. Syntheses from magnesium chelates 3.1.1.
Thermal stability of magnesium chelates
In order to determine the thermal stability of the solid magnesium chelates, differential thermal analyses were performed. For both commercial compounds, two successive endothermic phenomena are evidenced, the first one, above around 110 “C, being associated with water loss. With magnesium acetylacetonate, the second one, associated with thermal decomposition, is observed in a narrow temperature range around 265 “C. The stability of the fluorinated derivative is lower and decomposition occurs simultaneously with departure of the last water molecules: a large peak is spread between about 155 and 200 “C. Thermograms of dehydrated Mg(acac), exhibit only a single narrow decomposition peak at 265 “C. The weight loss observed above 190 “C for Mg(hfa)z is probably associated with both decomposition and sublimation phenomena. These chelates dissolve in methanol and ethanol. No solubility measurements were made, but solutions containing 2 g of solute per 100 cm3 of solvent are currently used. However, it is noticed that the dissolution rates of the solids are lower in ethanol than in methanol. The thermal behaviour of the chelate molecules can be different in the solid state and when dissolved in alcohol. In order to check this assumption, solutions were heated at various temperatures in a closed reactor, and samples were recovered and subsequently analysed
Fig. 2. Scanning electron micrograph Mg(acac), in methanol at 300 “C.
of powder obtained from
by IR spectrometry. Fig. 1 shows the evolution of spectra in the range 1300-1800 cm-‘, recorded on Mg(acac), solutions in methanol, as a function of temperature. It can be seen that the intensities of two bands around 1540 and 1600 cm-l, associated with the C-O ligand vibration, begin to decrease at 150 “C and vanish at 190 “C. At this temperature, a new band appears in the range 1700-1750 cm-l and can be related with the stretching of a carbonyl group in a decomposition product, namely acetylacetone. 3.1.2. Powder formation in methanol or ethanol The critical temperatures of methanol and ethanol are 239 and 241 “C, respectively, i.e. higher than that of the thermal decomposition of the solid or dissolved magnesium chelates.
K. Chhor et al. I Materials
Chemists
and Physics 40 (1995) 63-68
65
In all cases, the dried solids are amorphous and look like a polymeric material such as a gel. Scanning electron microscopy shows that the particles are l-3 pm in size, with a lamellar microstructure. Figs. 2 and 3(a) are representative micrographs of the as-prepared powders and show that their microstructure is the same for both precursors, heated either in supercritical state or in the liquid phase only. As shown in Fig. 4, the IR spectra of the powders obtained exhibit bands associated with hydrogen-bonded and free OH group stretching modes (3300 and 3600 cm-‘, respectively), while those related to C-O vibrations are located around 1100 cm-‘. The presence of Mg-0 bonds is observed in the range 500-600 cm-’ and around 380 cm-l. The chemical analyses of these powders are also in accordance with a macromolecular structure in which Mg(OCH,), and Mg(OH), groups would be associated. The relative contents of these two species depend on the experimental conditions; for example, they are 55 and 45%, respectively, when starting with Mg(acac), in ethanol, heated at 320 “C. When these as-prepared powders are heated in air, they transform into magnesium oxide. X-ray diffraction studies of the treated samples show that partial crystallization occurs above about 400 “C. However, sharp diffraction peaks associated with complete transformation into the cubic MgO structure are observed at higher temperature, around 600 “C. This transformation can also be evidenced by electron microscopy. The micrograph of the treated sample (Fig. 3(b)), can be compared with that of the corresponding as-prepared powder (Fig. 3(a)): although the grain size and shape are similar, the polymer-amorphous aspect of the lamellar structure has disappeared and the diameter of elementary crystallized particles can be estimated at around 50 nm. 3.1.3. Powder formation in supercritical alcoholK0,
mix-
tures
Fig. 3. Scanning electron micrographs of powder obtained from Mg(hfa)z in ethanol: (a) as-prepared solid at 240 “C; (b) after thermal treatment at 600 “C.
When alcoholic solutions of Mg(acac), or Mg(hfa), are heated in a closed reactor, white solid particles are formed above about 200 “C, from a reaction occurring in the liquid phase. This temperature is in accordance with that reported above for the decomposition of the dissolved chelate. In some experiments, the mixtures were heated up to 320 “C in the supercritical state (P = 200 bar). The powders recovered after cooling have been studied.
In order to test if the physical state of the reaction medium (liquid or supercritical phase) in which solid particles are formed has some influence in their chemical nature and microstructure, we used alcohol-CO, mixtures as solvents. These mixtures were about 10 mol.% in ethanol or methanol and their critical temperatures
I\
\ 4000
3000
Fig. 4. IR spectrum at 2.50 “C.
2000 1600 WAVE NUYBEA
of powder obtained
1200 (4)
800
400
from Mg(acac)z in methanol
66
K. Chhor et al. / Materials
Chemistry and Physics 40 (1995) 63-68
were around 110 “C. Solutions of chelates in alcohol (0.5-l g solute in 10 cm3 solvent) were first introduced into the high-pressure cell and CO, was added to fill half of the reactor volume with liquid phase. On heating, powder formation occurs above about 200 “C from Mg(hfa), or Mg(acac), solutions. Fig. 5(a) is an electron micrograph of a powder obtained from Mg(hfa), in an ethanol-CO, mixture at 210 “C. The spherical particle diameters are between 0.5 and 2 pm, but higher magnification reveals much
I
4000
all. 3000
1200 2000 WAVE IIIJKIHI (cm-‘)
Fig. 6. IR spectrum of powder obtained ethanol-CO, mixture at 210 “C.
400
!4
from Mg(hfa)* in supercritical
finer primary particles ( < 30 nm) hardly agglomerated. The IR spectrum (Fig. 6) shows important absorption in the range 1400-1700 cm-‘, which could be related to the presence of CO groups. When methanol-CO, mixtures are used as solvent, these primary particles appear to be less strongly associated and the grain substructure is clearly seen in Fig. 5(b). X-ray diffraction studies indicate that the solids obtained are amorphous. Upon heating, these powders partly crystallize at 500 “C into cubic magnesium oxide. The transformation is complete at 1000 “C and does not affect the microstructure: no significant change is evidenced on the electron micrographs. 3.2. Syntheses from magnesium acetate Anhydrous magnesium acetate is known to decompose on heating above 323 “C [15]. When heating a methanol (or ethanol) solution in a batch reactor, solid particle formation occurs above about 200 “C. Fig. 7(a) is a micrograph of the powder recovered after heating an ethanol solution of magnesium acetate to 260 “C. A gel lamellar structure, similar to that of Fig. 2 (obtained starting with magnesium chelate), is observed. However, X-ray diffraction analysis shows the presence of a low amount of crystalline magnesium carbonate in the formed powder. When this as-prepared solid is heated to 550 “C, the gel lamellar structure disappears (Fig. 7(b)) and 50-80 nm particles are observed (Fig. 7(c)). Crystallization into cubic magnesium oxide is then evidenced in the X-ray diffraction spectra.
4. Discussion
Fig. 5. Scanning electron micrographs of powders obtained from Mg(hfa)* (a) in a supercritical ethanol-CO, mixture at 210 “C, (b) in a methanol-CO, mixture at 200 “C.
The above results clearly show that the chemical composition and the microstructure of the powders obtained mostly depend on the nature of the reaction medium rather than on the starting magnesium precursor. It is known that metal chelates ML, react in methanol to methoxo derivatives polymeric form [ML(CH,O)(CH,OH)], [16]. It is likely that such an intermediate is formed in the above experiments. This compound can decompose further into magnesium alkoxide and partly hydrolyzed under the reaction con-
K. Chhor et al. I MateriaIs Chemistry and Physics 40 (1995) 63-68
67
Fig. 7. Scanning electron micrographs of powder obtained from magnesium acetate in ethanol: (a) as-prepared solid at 260 “C, (b), (c) after thermal treatment at 500 “C.
ditions, according to the following scheme: ML,
RoH) [ML(OR)(ROH)], + nLH ROH + H,O
I
[M(OW,W%xl,
+ nLH
where L is the chelating anion (acac) or (hfa).
The occurrence of similar reactions, when starting with magnesium acetate, can be understood by noting the similar molecular structures of both types of precursor (Fig. 8(a), (b)). The coordination number of magnesium is retained through the above sequence, and the chemical composition of the solids obtained can be described by the overall formula shown in Fig. 8(c), without any assumptions about the relative positions of binding OR and OH groups along the macromolecular chain. As
68
K. Chhor et al. / MatetiaLr Chemistry
Fig. 8. Similarity of the magnesium atom coordination in (a) magnesium chelates, (b) magnesium acetate and (c) intermediate compound obtained in MgO synthesis.
noted previously, the OH content of the compounds obtained depends on the experimental conditions and particularly on the amount of water in the system. Because of the presence of numerous residual alkoxy groups, the solids have a great tendency to be further hydrolyzed, even by atmospheric water vapor. Such compounds can be thermally decomposed into magnesium oxide at higher temperature (above about 450 “C); this behavior is quite similar to that of amorphous gels obtained from the hydrolysis and polycondensation alkoxides [8]. The decomposition of magnesium chelates in supercritical alcohol-CO, mixtures occurs in a temperature range similar to that in liquid alcohols. However, the chemical nature and microstructure of the recovered powders are very different. It is likely that the reaction between magnesium chelate and alcohol, leading to the intermediate polymer [ML(OR)(ROH)], mentioned above, should occur via a monomeric species ML(OR), ROH which, in the present case, can react with CO, used as cosolvent. It is effectively known that CO, can add to an alkoxide group to form carbonate ester anion [18]. On the IR spectrum in Fig. 6, strong absorption between 1200 and 1750 cm-’ can be related to the presence of such a group. The C= 0 and C-O stretching modes appear around 1700 and 1200 cm-‘, respectively, and the O-C-O deformation is seen in the range 1400-1600 cm-l. Absorptions observed at lower frequencies (1150-300 cm-l) are attributed the ethoxy vibrational modes, and the large band observed at around 3450 cm-’ can be attributed to adsorbed water molecules. 5. Conclusions
Alkoxides are usually used as precursors in magnesium oxide synthesis by a sol-gel route involving hydrolysis
and Physics 40 (1995) 63-68
and polycondensation. In magnesium chelates or acetate, the metal atom has the same coordination as in alkoxides. We have shown that their thermal decomposition in alcohol solutions containing little water (from the hydrated starting compounds) leads to amorphous solids with a lamellar structure which can be described as alkoxy-hydroxy derivatives [Mg(OR),(OH),_,],. Under thermal treatment above 400 “C, this intermediate compound transforms into crystallized magnesium oxide, with a primary particle size less than 100 nm. The use of a supercritical reaction medium does not appear to bring any major changes, as in previous studies of TiO, and MgAl,O, powder synthesis [13,14]. Although the chemical nature and microstructure of the intermediate solid obtained around 200 “C are different, the transformation into crystallized MgO occurs in the same temperature range as above and the final powder characteristics are similar. From a practical point of view, the synthesis of submicron MgO powders from magnesium acetate (a less expensive precursor than metal organic derivatives) and using the proposed process can be of interest.
References T.J. Gardner and G.L. Messing, Am. Ceram. Sot. Bull., 63 (1984) 1498. 121 J. Green, .I. Mater. Sci., 18 (1983) 637. [31 J.L. Boldu, L.A. Boatner and M.M. Abraham, J. Am. Gram. Sot., 73 (1990) 2345. [41 A. Nishida, A. Ueki and Y. Yoshida, Adv. Ceram., 21 (1987) 265. PI T. Watari, K. Nakayoshi and A. Kato, J. Chem. Sot. Jpn., (1984) 1075. WI A. Nishida, K Yoshida, H. Igarashi and W. Kobayashi, Adv. Gram., 21 (1987) 271. [71 A. Kato and Y. Toda, Am. Ceram. Sot. Bull., 64 (1985) 440. I81 T. Lopez, I. Garcia-Cruz and R. Gomez, J. Catal., 127 (1991) 75. K.C. Park and E.J. Weitzmann, IBM J. Res. Dev., 22 (1978) R 607. E. Vogel, N.C. Andreakis, W.E. Quinn and T.J. Nelson, Adv. WI Ceram., 21 (1987) 131. H. Dislich, in L.C. Klein (ed.), Sol-gel Technologyfor Thin Films, WI Fibers, Preforms, Electronics and Speciality Shapes, Noyes Press, Park Ridge, NJ, 1988, p. 50. 1121 G. Yi and M. Sayer, Ceram. Bull., 70 (1991) 1173. P31 M. Barj, J.F. Bocquet, K. Chhor and C. Pommier, J. Mater. Sci., 27 (1992) 2187. iI41 K. Chhor, J.F. Bocquet and C. Pommier, Mater. Chem. Phys., 32 (1992) 249. WI Handbcwk of Chemirtry and Physics, CRC Press, Boca Raton, FL, 1994. J.A. Bertrand and D. Caine, J. Am. Chem. Sot., 86 (1964) PI 2298. P71 P.G. Debenedetti and SK. Kumar, AIChE J., 34 (1988) 645. P31 J. March, Advanced Organic Chemistry, Wiley, New York, 1992, p. 893.
PI
Chemistry
Materials
Syntheses d’Zng&ierie des Mathiaux
Science
of submicron K. Chhor,
Lnboratoire
and Physics 40 (1995) 63-68
magnesium
J.F. Bocquet,
et des Hautes Pressions, Received
Communication
oxide powders
C. Pommier
CNRS LP 1311, Universite’ Paris XII& Avenue J.B. Clhnent, France
25 May 1994; accepted 17 October
93430 VWetaneuse,
1994
Abstract Submicron MgO powders have been synthesized from thermal decomposition of magnesium chelates or acetate at around 200 “C in liquid alcohol or supercritical alcohol-CO, mixtures. Because of a similar coordination of the magnesium atom in both types of precursors, a common amorphous alkoxy-hydroxy derivative is first obtained. When the reaction occurs in a supercritical medium with carbon dioxide as cosolvent, the microstructure of the solid is quite different and a CO, group is incorporated into the alkoxy bond. In all cases, the intermediate compound crystallizes into cubic magnesium oxide under thermal treatment between 400 and 600 “C; the elementary particle size in the powders obtained is less than 100 nm. Keywords: Magnesium oxide powders; Submicron MgO powders
1. Introduction
Most commercial MgO powders are synthesized by thermal decomposition of magnesium hydroxide or salts [1,2] and used as refractories. In such powders, the primary particles remain aggregated, usually in a shape similar to that of the precursor compound. For more specific applications, finer and less aggregated powders with higher purity are needed. Oxidation of pure metal magnesium in air has been realized [3,4]. When this reaction is performed in the vapor phase, nanometer particles with a controlled size (10-200 nm) have been obtained [4,5], and the optimum diameter for better forming and sintering seems to be around 100 nm [6,7]. The hydrolysis and polycondensation of magnesium alkoxides (sol-gel process) has also been performed under controlled pH conditions [S]. The amorphous solid material obtained at room temperature leads to crystalline submicron powdered MgO when thermally treated above 600-900 “C. An increasingly popular application of magnesium oxide is its use as a protective layer on plasma displays: it prevents degradation of the dielectric lead-based device substrate and contributes to enhancement of the discharge efficiency as well as to a lowering of the operating voltage. Thin overcoatings are usually deposited by physical vapor techniques such as electron beam evaporation [9]. Chemical routes starting with
0254-0584/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0254-0584(94)01452-M
metal organic precursors have also been proposed. For example, a thin coating of magnesium acetylacetonate dissolved in hexylcarbitol can be fixed into MgO above 430 “C [lo]. Many studies have been repbrted on the sol-gel processing of oxide films, starting with metal alkoxides and using dip coating or spin casting methods [11,12]. However, for coating plasma devices, damage to the glass substrate caused by thermal treatment above 500 “C has to be avoided. We have recently reported the synthesis of narrowsized submicron oxide powders from thermal decomposition of metal alkoxides in a supercritical fluid [13,14]. Such a reaction medium allows rapid nucleation and prevents hard agglomeration of primary particles. For example, decomposition of titanium alkoxides in supercritical ethanol leads to the formation of TiO, powders with a narrow size distribution (20-50 nm). These primary particles are softly associated into OS-2 /*rn spherical agglomerates easily destroyed under ultrasonic sound treatment [14]. We report below the results of similar syntheses of magnesium oxide, starting with metal chelates (acetylacetonate and the hexafluorinated derivative) or acetate as precursors. 2. Experimental Magnesium acetylacetonate acetonate (denoted Mg(acac),
and hexafluoroacetyland Mg(hfa),, respec-
K Chhor et al. I Materials
64
Chemistry
tively) were commercial products from Strem. These solids were hydrated with an undetermined number of water molecules (n G 2). Magnesium acetate (Strem) was hydrated with 6 water molecules and had a purity higher than 99%. These compounds were used as precursors in the following syntheses. In some experiments they were dehydrated under vacuum before study. The carbon dioxide used as cosolvent (purity N45) was provided by Air Liquide (France). Two laboratory-made high-pressure reactors were used for reactions in alcohol or in alcohol-CO, mixtures. The first reactor is a blind stainless-steel cylindrical tube with an internal volume of around 130 cm3 and can operate up to 400 “C and 400 bar. The second one is a sapphire cell with an internal volume of around 8 cm3, allowing visual observation of reactions occurring inside. However, because of the vespel joints needed for pressure tightness, the operating conditions are limited to below 300 “C and 250 bar. The synthesized powders were characterized by Xray diffraction (Siemens diffractometer with Cu Ka radiation), IR spectroscopy (Perkin-Elmer 580 instrument) and scanning electron microscopy (Cambridge 460 instrument).
and Physics 40 (1995) 63-68
1 . 1800
.
1600 1400 VI number (cm-‘)
Fig. 1. IR spectra of Mg(acac), dissolved in methanol and heated at (a) 60 “C, (b) 120 “C, (c) 150 “C, (d) 190 “C. (A) solvent and chelate bands; (AA) chelate carbonyl stretching band; (AAA) decomposition product carbonyl stretching band.
3. Results 3.1. Syntheses from magnesium chelates 3.1.1.
Thermal stability of magnesium chelates
In order to determine the thermal stability of the solid magnesium chelates, differential thermal analyses were performed. For both commercial compounds, two successive endothermic phenomena are evidenced, the first one, above around 110 “C, being associated with water loss. With magnesium acetylacetonate, the second one, associated with thermal decomposition, is observed in a narrow temperature range around 265 “C. The stability of the fluorinated derivative is lower and decomposition occurs simultaneously with departure of the last water molecules: a large peak is spread between about 155 and 200 “C. Thermograms of dehydrated Mg(acac), exhibit only a single narrow decomposition peak at 265 “C. The weight loss observed above 190 “C for Mg(hfa)z is probably associated with both decomposition and sublimation phenomena. These chelates dissolve in methanol and ethanol. No solubility measurements were made, but solutions containing 2 g of solute per 100 cm3 of solvent are currently used. However, it is noticed that the dissolution rates of the solids are lower in ethanol than in methanol. The thermal behaviour of the chelate molecules can be different in the solid state and when dissolved in alcohol. In order to check this assumption, solutions were heated at various temperatures in a closed reactor, and samples were recovered and subsequently analysed
Fig. 2. Scanning electron micrograph Mg(acac), in methanol at 300 “C.
of powder obtained from
by IR spectrometry. Fig. 1 shows the evolution of spectra in the range 1300-1800 cm-‘, recorded on Mg(acac), solutions in methanol, as a function of temperature. It can be seen that the intensities of two bands around 1540 and 1600 cm-l, associated with the C-O ligand vibration, begin to decrease at 150 “C and vanish at 190 “C. At this temperature, a new band appears in the range 1700-1750 cm-l and can be related with the stretching of a carbonyl group in a decomposition product, namely acetylacetone. 3.1.2. Powder formation in methanol or ethanol The critical temperatures of methanol and ethanol are 239 and 241 “C, respectively, i.e. higher than that of the thermal decomposition of the solid or dissolved magnesium chelates.
K. Chhor et al. I Materials
Chemists
and Physics 40 (1995) 63-68
65
In all cases, the dried solids are amorphous and look like a polymeric material such as a gel. Scanning electron microscopy shows that the particles are l-3 pm in size, with a lamellar microstructure. Figs. 2 and 3(a) are representative micrographs of the as-prepared powders and show that their microstructure is the same for both precursors, heated either in supercritical state or in the liquid phase only. As shown in Fig. 4, the IR spectra of the powders obtained exhibit bands associated with hydrogen-bonded and free OH group stretching modes (3300 and 3600 cm-‘, respectively), while those related to C-O vibrations are located around 1100 cm-‘. The presence of Mg-0 bonds is observed in the range 500-600 cm-’ and around 380 cm-l. The chemical analyses of these powders are also in accordance with a macromolecular structure in which Mg(OCH,), and Mg(OH), groups would be associated. The relative contents of these two species depend on the experimental conditions; for example, they are 55 and 45%, respectively, when starting with Mg(acac), in ethanol, heated at 320 “C. When these as-prepared powders are heated in air, they transform into magnesium oxide. X-ray diffraction studies of the treated samples show that partial crystallization occurs above about 400 “C. However, sharp diffraction peaks associated with complete transformation into the cubic MgO structure are observed at higher temperature, around 600 “C. This transformation can also be evidenced by electron microscopy. The micrograph of the treated sample (Fig. 3(b)), can be compared with that of the corresponding as-prepared powder (Fig. 3(a)): although the grain size and shape are similar, the polymer-amorphous aspect of the lamellar structure has disappeared and the diameter of elementary crystallized particles can be estimated at around 50 nm. 3.1.3. Powder formation in supercritical alcoholK0,
mix-
tures
Fig. 3. Scanning electron micrographs of powder obtained from Mg(hfa)z in ethanol: (a) as-prepared solid at 240 “C; (b) after thermal treatment at 600 “C.
When alcoholic solutions of Mg(acac), or Mg(hfa), are heated in a closed reactor, white solid particles are formed above about 200 “C, from a reaction occurring in the liquid phase. This temperature is in accordance with that reported above for the decomposition of the dissolved chelate. In some experiments, the mixtures were heated up to 320 “C in the supercritical state (P = 200 bar). The powders recovered after cooling have been studied.
In order to test if the physical state of the reaction medium (liquid or supercritical phase) in which solid particles are formed has some influence in their chemical nature and microstructure, we used alcohol-CO, mixtures as solvents. These mixtures were about 10 mol.% in ethanol or methanol and their critical temperatures
I\
\ 4000
3000
Fig. 4. IR spectrum at 2.50 “C.
2000 1600 WAVE NUYBEA
of powder obtained
1200 (4)
800
400
from Mg(acac)z in methanol
66
K. Chhor et al. / Materials
Chemistry and Physics 40 (1995) 63-68
were around 110 “C. Solutions of chelates in alcohol (0.5-l g solute in 10 cm3 solvent) were first introduced into the high-pressure cell and CO, was added to fill half of the reactor volume with liquid phase. On heating, powder formation occurs above about 200 “C from Mg(hfa), or Mg(acac), solutions. Fig. 5(a) is an electron micrograph of a powder obtained from Mg(hfa), in an ethanol-CO, mixture at 210 “C. The spherical particle diameters are between 0.5 and 2 pm, but higher magnification reveals much
I
4000
all. 3000
1200 2000 WAVE IIIJKIHI (cm-‘)
Fig. 6. IR spectrum of powder obtained ethanol-CO, mixture at 210 “C.
400
!4
from Mg(hfa)* in supercritical
finer primary particles ( < 30 nm) hardly agglomerated. The IR spectrum (Fig. 6) shows important absorption in the range 1400-1700 cm-‘, which could be related to the presence of CO groups. When methanol-CO, mixtures are used as solvent, these primary particles appear to be less strongly associated and the grain substructure is clearly seen in Fig. 5(b). X-ray diffraction studies indicate that the solids obtained are amorphous. Upon heating, these powders partly crystallize at 500 “C into cubic magnesium oxide. The transformation is complete at 1000 “C and does not affect the microstructure: no significant change is evidenced on the electron micrographs. 3.2. Syntheses from magnesium acetate Anhydrous magnesium acetate is known to decompose on heating above 323 “C [15]. When heating a methanol (or ethanol) solution in a batch reactor, solid particle formation occurs above about 200 “C. Fig. 7(a) is a micrograph of the powder recovered after heating an ethanol solution of magnesium acetate to 260 “C. A gel lamellar structure, similar to that of Fig. 2 (obtained starting with magnesium chelate), is observed. However, X-ray diffraction analysis shows the presence of a low amount of crystalline magnesium carbonate in the formed powder. When this as-prepared solid is heated to 550 “C, the gel lamellar structure disappears (Fig. 7(b)) and 50-80 nm particles are observed (Fig. 7(c)). Crystallization into cubic magnesium oxide is then evidenced in the X-ray diffraction spectra.
4. Discussion
Fig. 5. Scanning electron micrographs of powders obtained from Mg(hfa)* (a) in a supercritical ethanol-CO, mixture at 210 “C, (b) in a methanol-CO, mixture at 200 “C.
The above results clearly show that the chemical composition and the microstructure of the powders obtained mostly depend on the nature of the reaction medium rather than on the starting magnesium precursor. It is known that metal chelates ML, react in methanol to methoxo derivatives polymeric form [ML(CH,O)(CH,OH)], [16]. It is likely that such an intermediate is formed in the above experiments. This compound can decompose further into magnesium alkoxide and partly hydrolyzed under the reaction con-
K. Chhor et al. I MateriaIs Chemistry and Physics 40 (1995) 63-68
67
Fig. 7. Scanning electron micrographs of powder obtained from magnesium acetate in ethanol: (a) as-prepared solid at 260 “C, (b), (c) after thermal treatment at 500 “C.
ditions, according to the following scheme: ML,
RoH) [ML(OR)(ROH)], + nLH ROH + H,O
I
[M(OW,W%xl,
+ nLH
where L is the chelating anion (acac) or (hfa).
The occurrence of similar reactions, when starting with magnesium acetate, can be understood by noting the similar molecular structures of both types of precursor (Fig. 8(a), (b)). The coordination number of magnesium is retained through the above sequence, and the chemical composition of the solids obtained can be described by the overall formula shown in Fig. 8(c), without any assumptions about the relative positions of binding OR and OH groups along the macromolecular chain. As
68
K. Chhor et al. / MatetiaLr Chemistry
Fig. 8. Similarity of the magnesium atom coordination in (a) magnesium chelates, (b) magnesium acetate and (c) intermediate compound obtained in MgO synthesis.
noted previously, the OH content of the compounds obtained depends on the experimental conditions and particularly on the amount of water in the system. Because of the presence of numerous residual alkoxy groups, the solids have a great tendency to be further hydrolyzed, even by atmospheric water vapor. Such compounds can be thermally decomposed into magnesium oxide at higher temperature (above about 450 “C); this behavior is quite similar to that of amorphous gels obtained from the hydrolysis and polycondensation alkoxides [8]. The decomposition of magnesium chelates in supercritical alcohol-CO, mixtures occurs in a temperature range similar to that in liquid alcohols. However, the chemical nature and microstructure of the recovered powders are very different. It is likely that the reaction between magnesium chelate and alcohol, leading to the intermediate polymer [ML(OR)(ROH)], mentioned above, should occur via a monomeric species ML(OR), ROH which, in the present case, can react with CO, used as cosolvent. It is effectively known that CO, can add to an alkoxide group to form carbonate ester anion [18]. On the IR spectrum in Fig. 6, strong absorption between 1200 and 1750 cm-’ can be related to the presence of such a group. The C= 0 and C-O stretching modes appear around 1700 and 1200 cm-‘, respectively, and the O-C-O deformation is seen in the range 1400-1600 cm-l. Absorptions observed at lower frequencies (1150-300 cm-l) are attributed the ethoxy vibrational modes, and the large band observed at around 3450 cm-’ can be attributed to adsorbed water molecules. 5. Conclusions
Alkoxides are usually used as precursors in magnesium oxide synthesis by a sol-gel route involving hydrolysis
and Physics 40 (1995) 63-68
and polycondensation. In magnesium chelates or acetate, the metal atom has the same coordination as in alkoxides. We have shown that their thermal decomposition in alcohol solutions containing little water (from the hydrated starting compounds) leads to amorphous solids with a lamellar structure which can be described as alkoxy-hydroxy derivatives [Mg(OR),(OH),_,],. Under thermal treatment above 400 “C, this intermediate compound transforms into crystallized magnesium oxide, with a primary particle size less than 100 nm. The use of a supercritical reaction medium does not appear to bring any major changes, as in previous studies of TiO, and MgAl,O, powder synthesis [13,14]. Although the chemical nature and microstructure of the intermediate solid obtained around 200 “C are different, the transformation into crystallized MgO occurs in the same temperature range as above and the final powder characteristics are similar. From a practical point of view, the synthesis of submicron MgO powders from magnesium acetate (a less expensive precursor than metal organic derivatives) and using the proposed process can be of interest.
References T.J. Gardner and G.L. Messing, Am. Ceram. Sot. Bull., 63 (1984) 1498. 121 J. Green, .I. Mater. Sci., 18 (1983) 637. [31 J.L. Boldu, L.A. Boatner and M.M. Abraham, J. Am. Gram. Sot., 73 (1990) 2345. [41 A. Nishida, A. Ueki and Y. Yoshida, Adv. Ceram., 21 (1987) 265. PI T. Watari, K. Nakayoshi and A. Kato, J. Chem. Sot. Jpn., (1984) 1075. WI A. Nishida, K Yoshida, H. Igarashi and W. Kobayashi, Adv. Gram., 21 (1987) 271. [71 A. Kato and Y. Toda, Am. Ceram. Sot. Bull., 64 (1985) 440. I81 T. Lopez, I. Garcia-Cruz and R. Gomez, J. Catal., 127 (1991) 75. K.C. Park and E.J. Weitzmann, IBM J. Res. Dev., 22 (1978) R 607. E. Vogel, N.C. Andreakis, W.E. Quinn and T.J. Nelson, Adv. WI Ceram., 21 (1987) 131. H. Dislich, in L.C. Klein (ed.), Sol-gel Technologyfor Thin Films, WI Fibers, Preforms, Electronics and Speciality Shapes, Noyes Press, Park Ridge, NJ, 1988, p. 50. 1121 G. Yi and M. Sayer, Ceram. Bull., 70 (1991) 1173. P31 M. Barj, J.F. Bocquet, K. Chhor and C. Pommier, J. Mater. Sci., 27 (1992) 2187. iI41 K. Chhor, J.F. Bocquet and C. Pommier, Mater. Chem. Phys., 32 (1992) 249. WI Handbcwk of Chemirtry and Physics, CRC Press, Boca Raton, FL, 1994. J.A. Bertrand and D. Caine, J. Am. Chem. Sot., 86 (1964) PI 2298. P71 P.G. Debenedetti and SK. Kumar, AIChE J., 34 (1988) 645. P31 J. March, Advanced Organic Chemistry, Wiley, New York, 1992, p. 893.
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