paper journal of the society of inorganic materials, japan

Paper Journal of the Society of Inorganic Materials, Japan 13, 336-344 (2006)

Formation Process of Magnesium Aluminate due to Solid-State

Reaction of Highly-Dispersed and Nanometer-Sized Particles

Kiyoshi ITATANI, Akio NAITO, Ian J. DAVIES*, Satoru SANO** and Seiichiro KODA (Department of Chemistry, Faculty of Science and Engineering, Sophia University, 7-1

Kioi-cho, Chiyoda-ku, Tokyo 102-8554; *Department of Mechanical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia; **Ube Material

Industries, Ltd., 1985 Kogushi, Ube-shi, Yamaguchi 755-8510)

The formation process of magnesium aluminate (MgAl2O4) due to the solid-state reac-

tion of highly-dispersed and nanometer-sized aluminum and magnesium compounds has

been examined by high-temperature X-ray diffractometry (HT-XRD) , synchrotron radia-

tion diffractometry (SRD) and X-ray photoelectron spectroscopy (XPS) . The starting

compounds were ƒ¿- and y-aluminum oxide (a- and y-Al2O3; primary particle sizes, 105

and 31.6 nm, respectively) as aluminum sources, and magnesium oxide (MgO; 41.3 nm)

and magnesium hydroxide (Mg (OH) 2; 61.1 nm) as magnesium sources. Through the com-

bination of these compounds, four powder mixtures were prepared, namely, (0 cy-Al203

and MgO, (ii) y-Al203 and MgO, (iii) ce--Al2O3 and Mg (OH)2, and (iv) ƒÁ-Al2O3 and

Mg (OH) 2. Phase change investigation during the heating of these mixtures indicated that

the formation of MgAl2O4 due to the reaction of y-Al2O3 with Mg (OH) 2 was faster when

compared to the other combinations; almost single phase of MgAl2O4 could be obtained

when this mixture was heated at 1200•Ž for 1 h. More detailed investigation on the forma-

tion process of MgAl2O4 was conducted using the precursor mixture of y-Al2O3 and

Mg (OH) 2 heat-treated at 800•Ž for 1 h. The data obtained from SRD and XPS suggested

that small amounts of MgAl2O4 and ce-Al2O3, together with y-Al2O3 and MgO , were present in this precursor. The formation of MgAl2O4 due to the reaction of y-Al2O3 with Mg (OH)2

was found to occur readily due to active mass transfer as a result of the very small primary

particle and agglomerate sizes.

(Received Apr. 17, 2006)

(Accepted Jun. 5, 2006)

Key words : Magnesium aluminate spinel, High-temperature X-ray diffractometry, Syn-

chrotron radiation diffractometry, X-ray photoelectron spectroscopy

1. Introduction

The magnesium aluminate spinel (MgAl2O4;

MA spinel) has excellent physical, mechanical

and chemical properties, e.g., melting point

(2105•Ž) , modulus of rupture (165 MPa at 1300

℃1)), fracture toughness (3.0 MPa・m1/2 at 1200

℃1)) and resistance against chemical attacks

from molten aluminum alloy2) , alkali vapors3)

and sodium-sulfate melts3). As such, it is current-

ly utilized as a refractory for furnace walls and

firebricks1) and also has the potential for applica-

tion as humidity sensors2),3)

High-purity MA spinel powder has been

prepared by numerous chemical synthesis

techniques, e.g., solid-state reaction4) , mecha-

nical alloying5) , co-precipitation6) , sol-gel7), alkoxide8), spray pyrolysis9), freeze-dryine , supercritical fluids" and vapor phase oxidation'2). The most practical technique for the preparation of MA spinel powder has been that of solid-state reaction due to its advantages of (i) precise control of chemical composition, (ii) reduced fabrication costs, and (iii) simpler production of complex-shaped ceramics. Many researchers, therefore, have paid attention to the types of starting compounds, in addition to the heating temperature, for the preparation of high-purity MA spinel through solid-state reaction. Starting powders for the preparation of MA spinel have included aluminum oxide (Al2O3; a and y forms) , aluminum hydroxide (Al (OH) 3) ,

336

Kiyoshi ITATANI, et al. Journal of the Society of Inorganic Materials, Japan 13, (2006) 337

and aluminum oxide hydroxide (AlO (OH) ) as

aluminum sources, and magnesium oxide

(MgO) and magnesium hydroxide (Mg (OH)2)

as magnesium sources.

The kinetic influence of particle sizes of 1 to

100 ,um on the reaction between Al2O3 and MgO

powders has been examined by Beretka and

Brown13), who reported that the reaction temper-

ature decreased with decreasing particle size,

and that the powder properties, e.g., smaller par-

ticle size, narrow particle size distribution, and

uniform particle geometry, enhanced the reac-

tion rate of solids. Recently, nanometer-sized

aluminum and magnesium compounds with rela-

tively little agglomeration have been commer-

cially available. Typical starting powders that

fulfill such requirements would be Al2O3 and

MgO powders prepared by vapor-phase reaction.

Moreover, highly-dispersed and nanometer-sized

magnesium hydroxide powder has started to

become commercially available; the high reactiv-

ity of such Mg (OH) 2 powder has allowed the

preparation of transparent forsterite (Mg2SiO4)

powder through the solid-state reaction of MgO

(derived from Mg (OH) 2) and SiO2 at compara-

tively low temperatures14). Therefore, these

compounds may also be potential starting pow-

ders for the preparation of MA spinel at reduced

temperatures. On the basis of such information,

the present authors have investigated the forma-

tion process of MA spinel starting from

nanometer-sized and highly-dispersed aluminum

and magnesium compounds; some advanced X-

ray techniques for characterization of the result-

ing powders, i.e., high-temperature X-ray

diffractometry (HT-XRD) , synchrotron radia-

tion diffractometry (SRD) and X-ray photoelec-

tron spectroscopy (XPS) , were applied in order

to investigate the formation process of MA

spinel.

2 Experimental procedures

2. 1 Preparation of MA spinel

The starting compounds were a-aluminum

oxide (a-Al2O3; TM-DAR; Taimei Chemicals,

Co. Ltd., Nagano, Japan) and y-aluminum oxide

(ƒÁ-Al203; C. I. Kasei, Co. Ltd., Tokyo) as alumi-

num sources, and magnesium oxide (MgO; C. I.

Kasei, Co. Ltd., Tokyo) and magnesium

hydroxide (Mg (OH) 2; MH-VO5P; Ube Materi-

als Industries, Ube) as magnesium sources.

Among these compounds, a-Al203 was produced

by the pyrolysis of ammonium aluminum car-

bonate hydroxide ( NH4A1CO3 ( OH )2 ) 15) /

whereasγ-Al2O3 and MgO were produced by

vapor-phase oxidation (VPO). On the other

hand, Mg(OH)2was produced by the reaction of

VPO-derived MgO with water vapor. Through

the combination of these compounds, four types

of powder mixtures with a stoichiometric compo-

sition of MA spine1(A1/Mg=2.0)were pre-

pared,i.e.,(i)α-Al2O3 and MgO,(ii)γ-A12O3

and MgO,(iii)α-A12O3 and Mg(OH)2, and(iv)

γ-Al2O3 and Mg(OH)2. Following ball-milling at

room temperature for 24 h, the powders were

heated at a temDerature between 700℃and 1200

℃for l h;the heating rate from room tempera-

ture to the desired temperature was fixed at 10℃

・min-1 . The resulting heat-treated mixtures

were then pulverized using an alumina mortar

and pestle.

2.2 Evaluation

Crystalline phases of the heat-treated powders

were characterized using an X-ray diffractome-

ter (XRD; Model RINT2000V/P, Rigaku,

Tokyo) and monochromatic CuKa radiation at

40 kV and 40 mA. In addition, changes in crys-

talline phase during heating from room tempera-

ture to 1200•Ž were examined utilizing the X-ray

diffractometer equipped with a furnace (heating

elements: platinum) . The specific surface area of

the starting powders was measured using the

Brunauer-Emett-Teller (BET) method: nitrogen

gas was used as an adsorption gas. The primary

particle size was calculated assuming the particle

shapes to be either cubic or spherical. Agglomer-

ate sizes of the primary particles were deter-

mined using a laser diffraction particle size

analyzer (Model Microtrac HRA, Nikkiso,

Tokyo; dispersion medium, methanol) .

Small amounts of reaction products below the

detection limit of HT-XRD were examined by

SRD. Diffraction patterns were obtained using

the Australian National Beamline Facility

(ANBF) at the 2.5 GeV Photon Factory (High

Energy Accelerator Research Organization

(KEK) , Tsukuba) . The principle of the meas-

urement is shown in Fig. 1, together with a pho-

tograph of the apparatus. The ANBF beamline

was able to produce monochromatic (using a

water cooled Si (111) monochromator) synchro-

tron X-ray radiation with an energy range of 4-

21 keV. The monochromator was positioned 3 m

from the specimen goniometer and the beamline

contained a "de-tune" facility for the rejection of

harmonic energies. Crystalline phases of the

heat-treated powder were examined using a

beam energy of 12.398 keV (equivalent to 0.1

338 Formation Process of Magnesium Aluminate due to Solid-State Reaction of Highly-Dispersed and Nanometer-Sized Particles

nm wavelength) with a beam width of 2 mm and a beam height of 0.2 mm. The synchrotron X-ray beam was incident at an angle of 10 degrees to the sample and this angle was fixed during the experiment. The sample was irradiated by X-rays for 12 min under vacuum. The diffracted radiation was collected using imaging plates with the intensity data information being read by an imaging plate scanner. In order to aid analysis, SRD angle data obtained in this work was con-verted to values corresponding to the CuKo/ wavelength.

Examination of the reaction products formed on the surfaces of the heat-treated powder was conducted through analysis of the binding ener-

gies of the Al (2p) and Mg (2s) transitions using an X-ray photoelectron spectroscope with AlKa radiation ( XPS; ULVAC - PHI 5800ci, Chigasaki) .

3 Results and discussion

3.1 Properties of starting powders As mentioned earlier, the solid-state reaction

is known to be promoted when the powder pos-sesses the following properties: (i) sub-micrometer-sized particles, (ii) narrow particle size distribution, (iii) uniform particle geometry, and (iv) little agglomeration. Prior to checking

the formation process of MA spinel, therefore,

we examined the properties of the starting pow-

ders in order to determine whether the present

powders met these requirements. Properties of

the starting powders have been listed in Table 1

with the purities of all starting powders exceed-

ing 99.9%. The specific surface areas of these

powders were ordered as follows: ƒÁ-Al2O3 (54.3

m2 •E g-1) > Mg (OH) 2 (41.6 m2•Eg-1) > MgO

(40.6 m2. g -1) > ƒ¿-Al2O3 (14.3 m2. g-1). On the

other hand, the primary particle sizes calculated

on the basis of powder density and specific sur-

face areas were: ƒ¿-Al2O3 (105 nm) > Mg (OH) 2

(61.1 nm) > MgO (41.3 nm) > ƒÁ-Al2O3 (31.6

nm) .

The solid-state reaction would presumably be

affected not only by the specific surface area and

primary particle size but also by the degree of ag-

glomeration of the primary particles in the start-

ing powder, as strong agglomeration of the pri-

mary particles and larger agglomerate sizes res-

trict the solid-state reaction. In order to evaluate

the degree of agglomeration, therefore, the ag-

glomerate sizes were determined using a laser

diffraction particle size analyzer with results

being shown in Fig. 2. The agglomerate sizes of

the ce-Al2O3 powder were distributed in the

range of 0.06 to 3 ,um (Fig. 2 (a) ) . Similar

results were obtained in the case of y-Al2O3 pow-

der (Fig. 2 (b)) ; however, the distribution curve

of this powder was slightly shifted toward lower

agglomerate sizes.

The agglomerate sizes of the MgO powder

were distributed in the range of 0.1 to 3 ,um (Fig.

2 (c)) , whereas those of the Mg (OH)2 powder

were distributed in the range of 0.01 to 1,um

(Fig. 2 (d)) . The median agglomerate size of the

MgO powder was 0.44 ,um (440 nm) , whereas

that of the Mg (OH)2 powder was 0.07 ,um (70

nm) . The average agglomerate size of the

Mg (OH)2 particles was significantly smaller

than that of the MgO particles. These Mg (OH) 2

particles were found to be highly dispersible.

Fig. 1 Photograph (above) and principle (below) of the SRD.

Table 1 Properties of the starting powders.


3.2 Formation process of MA spinel at

elevated temperatures

As previously mentioned, four kinds of mix-

tures were prepared for this work, i.e., (i) ƒ¿-

Al2O3 and MgO, (ii) ƒÁ-Al2O3 and MgO, (iii) ƒ¿-

Al2O3 and Mg (OH) 2, and (iv) ƒÁ-Al2O3 and

Mg (OH) 2. Investigation of phase changes dur-

ing heating of these mixtures from room temper-

ature up to 1200°C was then examined by HT-

XRD. First of all, phase changes that occurred

during heating of the mixture of ƒ¿-Al2O3 and

MgO and that of ƒÁ-Al2O3 and MgO have been

shown in Fig. 3. The characteristic Miller in-

dices employed in this experiment were as fol-

lows: (100) for ƒ¿-Al2O3, (400) for ƒÁ-Al2O3,

(400) for MgO and (422) for MA spinel. It

should also be stressed that the lack of preferred

orientation of these compounds enabled direct

comparison of the X-ray diffraction intensities.

When the mixture of ƒ¿-Al2O3 and MgO was

heated above 1000°C, X-ray diffraction intensi-

ties of ƒ¿-Al2O316) and MgO17) started to decrease

(Fig. 3 (a) ) , whilst the X-ray diffraction intensi-

ty of MgAl2O418) started to increase. Although

the X-ray diffraction intensity of MgAl2O4 in-

creased with a further increase in temperature,

(a)

(b)

significant amounts ofα-A12O3 and MgO still

remained even at 1200℃(Fig.3(a)).

Similar results were obtained when the mix-

ture ofγ-A12O3 and MgO was heated above 1000

℃.The X-ray diffraction intensity of MgA12O4

increased, whilst those of ƒÁ-Al2O3 and MgO

decreased (Fig. 3 (b)) , with increasing tempera-

ture up to 1200•Ž. However, the X-ray diffrac-

tion intensities of ƒÁ-Al2O3 and MgO at 1200•Ž

were lower compared to the case of ƒ¿-Al2O3 and

MgO.

Next, phase changes during heating of the

mixture of ƒ¿-Al2O3 and Mg (OH) 2 and that of ƒÁ-

Al2O3 and Mg (OH) 2 have been shown in Fig. 4.

The characteristic Miller index for Mg (OH)2

was (101) ; Miller indices of the other com-

pounds have been described previously. When

the mixture of ƒ¿-Al2O3 and Mg (OH) 2 was heat-

ed to 350•Ž, Mg (OH) 2 disappeared to form MgO

(Fig. 4 (a)) . X-ray diffraction intensities of the

Fig. 2 Agglomerate-size distributions of the start-

ing powders.

(a) : ƒ¿-Al2O3 powder, (b) : ƒÁ-Al2O3 powder

(c) : MgO powder, (d) : Mg (OH) 2 powder

Fig. 3 Phase changes during the heating of (a) the

mixture of ƒ¿-Al2O3 and MgO and (b) that

of y-Al2O3 and MgO. Note that the results

were obtained by HT-XRD.

■ : γ-Al2O3 (2θ=45.80 ; (400))

□ : α-Al2O3 (2θ=41.7° ; (100))

△ : MgO (2θ=42.9° ; (400))

○ : MAspinel (2θ=38.5° ; (422))


(a)

(b)

α-Al203 and MgO started to decrease at approxi-

mately 950•Ž, whereas the X-ray diffraction in-

tensity of MgAl2O4 increased with increasing

temperature up to 1200•Ž. Similar phase

changes occurred during heating of the mixture

of y-Al2O319) and Mg (OH) 220); the X-ray diffrac-

tion intensity of MgAl2O4 increased with increas-

ing temperature, whilst the X-ray diffraction in-

tensities of ce-Al2O3 and MgO decreased (Fig. 4

(b) ) .

On the basis of the results shown in Figs. 3

and 4, the reaction process of oe-Al2O3 or y-Al2O3

with MgO or Mg (OH) 2 may be expressed as fol-

lows:

(1)

(2)

X-ray diffraction intensities of a-Al203 and

MgO relative to those of the MA spinel at 1200•Ž

may be classified according to the combination of

the starting powders: y-Al2O3 and Mg (OH)2 <

α-A12O3 and Mg(OH)2<γ-A12O3 and MgO<α-

Al2O3 and MgO. This arrangement indicates that

the MgO powder which formed immediately fol-

lowing the pyrolysis of Mg (OH) 2 appears to be

active in the promotion of mass transfer for the

formation of MA spinel. In particular, the solid-

state reaction of y-Al2O3 with Mg (OH) 2 for the

formation of MA spinel proceeds faster when

compared to the utilization of other combinations

of compounds. Previously, the present authors

found that the morphology of MgO particles

formed immediately following the pyrolysis of

Mg (OH) 2 powder maintains the external frame-

work of the original Mg (OH) 2 particles21),22).

The mean agglomerate size of Mg (OH) 2 may be

44 nm, which is much smaller than that of the

original MgO powder (70 nm) . The faster solid-

state reaction of ƒÁ-Al2O3 with Mg (OH) 2 for the

formation of MA spinel can, therefore, be related

to the smaller agglomerate sizes of the resulting

MgO particles, as well as those of the original

Mg (OH) 2 particles, when compared to the case

of nanometer-sized MgO powder.

3.3 Detailed examination of the formation

process of MA spinel

Since the solid-state reaction of y-Al2O3 with

Mg (OH) 2 was faster than for the case of other

combinations of compounds, we further exa-

mined the formation process of MA spinel

through some advanced X-ray analyses. First of

all, typical conventional XRD patterns of the

mixtures of ƒÁ-Al2O3 and Mg (OH) 2 heated at 800

℃,1000℃and 1200℃for 1 h have been shown in

Fig . 5. Crystalline phases within the mixture

heated at 800•Ž for 1 h were found to be y-Al2O3

and MgO, whereas the mixture heated at 1000•Ž

for 1 h contained cr-Al2O3, together with y-Al2O3

and MgO. Furthermore, an almost single phase

of MA spinel (trace of ƒ¿-Al2O3) could be ob-

tained when this mixture was heated at 1200•Ž

for 1 h. These results agree well with those ob-

tained by HT-XRD (see Fig. 4 (b) ) . The forma-

tion of MA spinel described above may be con-

trolled by the initial stage of a reaction between

aluminum and magnesium compounds.

Following this, the properties of the mixtures

heated at 800•Ž for 1 h, which corresponds to the

initial stage of the reaction (or the stage immedi-

ately prior to initiation of the solid-state reac-

tion) , were examined by SRD. The SRD tech-

Fig. 4 Phase changes during the heating of (a) the

mixture of a—Al2O3 and Mg (OH)2 and (b)

that of ƒÁ-Al2O3 and Mg (OH) 2. Note that the

results were obtained by T-TT—XRD_


nique has the characteristics of (i) a highly

monochromatic, extremely high intensity (108 •`

1012 photons •E s-1) light source when compared

to the case of a laboratory X-ray generator, (ii)

an energy resolution (E/AE) of 2.4 x 103, (iii)

high accuracy of measured angles (5 •~ 10 -4

degrees) , (iv) direction of high intensity light

onto small specimen areas due to low diver-

gence, and (v) possible selection of optimum X-

ray energy for each experiment. Even if the

amount of reaction product is below the detec-

tion limit of the conventional XRD technique, it

may still be detected using this SRD technique.

Thus, phase identification within multiphase

powder, including the investigation of phase

transformation (e.g., ƒÁ- to ƒ¿-Al2O3) , is more

efficiently performed using SRD when compare

to the standard XRD technique.

Firstly, SRD patterns of the mixture of ƒ¿-

Al2O3 and MgO and that of ƒÁ-Al2O3 and MgO,

both heated at 800•Ž for 1 h, have been shown in

Fig. 6. The SRD pattern of the heat-treated mix-

ture of ƒ¿-Al2O3 and MgO contained ƒ¿-AlO3,

MgO and MA spinel (Fig. 6 (a) ) , whereas that

of the heat-treated mixture of ƒÁ-Al2O3 and MgO

included not only ƒÁ-Al2O3 and MgO but also a-

Al2O3 and MA spinel (trace) (Fig. 6 (b)) . Se-

condly, SRD patterns of the mixture of ƒ¿-Al2O3

and Mg (OH) 2 and that of ƒÁ-Al2O3 and Mg

(OH) 2, both heated at 800•Ž for 1 h, are shown

(a)

(b)

in Fig. 7. The SRD pattern of the heat-treated

mixture of ƒ¿-Al2O3 and Mg (OH) 2 contained ƒ¿-

Al2O3, MgO and MA spinel (Fig. 7 (a) ) ,

whereas that of heat-treated mixture of ƒÁ-Al2O3

and Mg (OH) 2 included not only ƒÁ-Al2O3 and

MgO but also ƒ¿-Al2O3 (trace) and MA spinel

(trace) (Fig. 7 (b)) .

The results in Figs. 6 and 7 indicate that a por-

tion of the ƒÁ-Al2O3, utilized as an aluminum

source, transforms into ƒ¿-Al2O3 that has not

been detected by conventional XRD. Since the

transformation of ƒÁ- to a-Al2O3 generally occurs

at 1100-1200•Ž, the lowering of the transforma-

tion temperature may be associated with the

presence of magnesium ions (Mg2 +) in the ƒÁ-

Al2O3 and the mechanical grinding effect.

Relating to the effect of Mg2 in the ƒÁ-Al2O3,

Okada et al.23) did not find any distinct effect on

the transformation of ƒÁ- to ƒ¿-Al2O3 but con-

firmed the formation of ƒ¿-Al2O3 before the ap-

pearance of MA spinel. On the other hand,

Hayashi et al. 24) pointed out that the transforma-

tion of ƒÁ- to ƒ¿-Al2O3 generally occurs along with

the formation of O-Al2O3, and that the nucleation

of ƒ¿-Al2O3 may preferentially occur at the sites

of structural defects, e.g., surfaces and interfaces

of particles. Overall, the present ƒ¿-Al2O3 may be

Fig. 5 Typical conventional XRD patterns of the

mixtures of ƒÁ-Al2O3 and Mg (OH)2 heated

at 800•Ž, 1000•Ž and 1200•Ž for 1 h.

Fig. 6 SRD patterns of (a) the mixture of ƒ¿-Al2O3

and MgO and (b) that of ƒÁ-Al2O3 and MgO,

both heated at 800•Ž for 1 h.


(a)

(b)

formed not only by the incorporation of MgO

into ƒÁ-Al2O3 but also by nucleation and growth

due to an increase in the density of defects

resulting from the grinding operation.

As the above results indicate, a very small

amount of ƒ¿-Al2O3 was detected from the heat-

treated mixture of ƒÁ-Al2O3 and Mg (OH) 2 at tem-

peratures as low as 800•Ž. Such ƒ¿-Al2O3 must

also react with MgO in order to form MA spinel.

Nevertheless, this formation process may vir-

tually be ignored during the heating of ƒÁ-Al2O3

and Mg (OH) 2, due to the formation of such a

small amount of ƒ¿-Al2O3.

The formation of MA spinel is thus confirmed

not by conventional XRD but by SRD. We fur-

ther examined the phases present in the heat-

treated mixture of ƒÁ-Al2O3 and Mg (OH) 2

through XPS with typical results being shown in

Fig. 8. The binding energies of Al (2p) was 74.5

eV, whereas the binding energy of Mg (2s) was

89.3 eV. The binding energies of Al (2p) in a-

and ƒÁ-Al2O3 is reported to be 74.325) and 74.6

eV26), respectively, whereas the binding energy

of MA spinel is 74.7 eV27). The present binding

energy of Al (2p) (74.5 eV) is positioned be-

tween the energies of Al (2p) in ƒ¿- and ƒÁ-Al2O3

and MA spinel. On the other hand, the binding

energy of Mg (2s), 89.3 eV, is in accordance with

(a) (b)

that in MA spinel27), and is clearly different from that in MgO (88.1 eV) 28). The XPS data, there-fore, also demonstrates the presence of MA spinel.

It is known that MA spinel may be formed by the inter-diffusion of Al3+ and Mg2+

(3)

The solid solubility of Al2O3 into MgO and that of MgO into Al2O3 have been reported by many researchers. For example, the solid solubility of Al2O3 into MgO is 0.04 mol% (1200-1600℃)29), whereas that of MgO intoα-A12O3 is 175

ppm by mass at 1880•Ž30). Also, the solid solubil-

ity of ƒÁ-Al2O3 seems to be very low31), although

no quantitative data are available. Generally,

MA spinel is present in the original MgO-side,

whereas the defective spinel, i.e., MgO .nAl2O3

(n > 1) , is formed in the original Al2O3-side32).

On the basis of the conventional XRD, SRD and

XPS data, we shall discuss the mass transfer be-

tween the Al2O3 and MgO sides at the initial

stage of reaction. The schematic diagram of the

mass transfer has been illustrated in Fig. 9. The

inter-diffusion of Al3+ and Mg2 + results in the

formation of MA spinel in the original MgO

grains and that of defective spinel (MgO .nAl2O3

(n > 1) ) and ƒ¿-Al2O3 in the original ƒÁ-Al2O3

grains. Here, ƒ¿-Al2O3 may be formed as a result

of the grinding operation and/or the solid solu-

tion. Li et al.33) also reported that the high reac-

tivity of ƒÁ-Al2O3 and MgO makes the formation

of MA spinel at 800•Ž possible. Their informa-

tion also anticipates the possibility of the prepa-

ration of MA spinel through the solid-state reac-

tion of the present ƒÁ-Al2O3 and Mg (OH) 2 mix-

ture at temperatures as low as 800•Ž, due to the

Fig. 7 SRD patterns of (a) the mixture of ƒ¿-Al2O3

and Mg (OH)2 and (b) that of ƒÁ-Al2O3 and

Mg (OH) 2, both heated at 800•Ž for 1 h.

Fig. 8 Typical XPS spectra of (a) Al (2p) and (b)

Mg (2s) of the mixture of ƒÁ-Al2O3 and

Mg (OH) 2 heated at 800•Ž for 1 h.


increased heating time.

4 Conclusion

The formation process of magnesium

aluminate (MgAl2O4; MA spinel) due to the

solid-state reaction of nanometer-sized alumi-

num and magnesium compounds has been exa-

mined through high-temperature diffractometry

(HT-XRD), synchrotron radiation diffraction

(SRD) and X-ray photoelectron spectroscopy

(XPS) . The starting compounds were ƒ¿- and y-

aluminum oxide (ƒ¿- and ƒÁ-Al2O3; specific sur-

face area, 105 and 31.6 nm, respectively) as alu-

minum sources, and magnesium oxide (MgO;

41.3 nm) and magnesium hydroxide (Mg (OH) 2;

61.1 nm) as magnesium sources. Through the

combination of these compounds, four kinds of

mixtures were prepared, i.e., (i) ƒ¿-Al2O3 and

MgO, (ii) ƒÁ-Al2O3 and MgO, (iii) ƒ¿-Al2O3 and

Mg (OH) 2, and (iv) ƒÁ-Al2O3 and Mg (OH) 2. The

formation process of MA spinel examined in this

research is summarized as follows:

1) Phase change investigation during the

heating of these mixtures showed that the forma-

tion of MgAl2O4 due to the solid-state reaction of

γ-Al2O3 with Mg(OH)2was faster than the case

of other combinations. An almost single-phase of

MgAl2O4 was obtained when the mixture was

heated at 1200•Ž for 1 h.

2) More detailed investigation on the forma-

tion process of MgAl2O4 was conducted through

SRD and XPS, after the mixture of ƒÁ-Al2O3 and

Mg (OH) 2 was heat-treated at 800•Ž for 1 h.

Although the conventional XRD of this powder

indicated the presence of ƒÁ-Al2O3 and MgO, a

small amount of ƒ¿-Al2O3 and MgAl2O4 was addi-

tionally detected by SRD. Moreover, the XPS

results showed that the MgAl2O4 was chiefly de-

tected in the original MgO-side, whereas the ƒ¿-

Al2O3 was found in the original Al2O3-side.

3) The solid-state reaction of nanometer-

sized ƒÁ-Al2O3 and Mg (OH) 2 promoted the for-

mation of MgAl2O4 due to the active mass trans-

fer through very small primary particle and ag-

glomerate sizes.

References

1) C. Baudin, R. Martinez, P. Pena, J. Am. Cer-

am. Soc., 78, 1857 (1995) .

2) M. Sindel, N. A. Travitzky, N. Claussen, J.

Am. Ceram. Soc., 73, 2615 (1990) .

3) R. D. Maschio, B. Fabbri, C. Fiori, Ind. Cer-

am., 8, 121 (1988).

4) I. Ganesh, B. Srinivas, R. Johnson, B. P.

Saha, Y. R. Mahajan, Brit. Ceram. Trans.,

101, 247 (2002) .

5) D. Domanski, G. Urretavizcaya, F. J. Castro,

F. G. Gennari, J. Am. Ceram. Soc., 87, 2020

(2004) .

6) R. J. Bratton, Am. Ceram. Soc. Bull., 48, 759

(1969) .

7) D. Lepkova, A. Bartarjav, B. Samuneva, Y.

Ivanova, L. Georgieva, J. Mater. Sci., 26,

4861 (1991) .

8) M. Sugiura, O. Kamigaito, Yogyo-Kyokai-

Shi, 92, 605 (1984) .

9) S. Kanzaki, T. Nishida, N. Otsu, K. Saito,

Yogyo-Kyokai-Shi, 91, 164 (1983) .

10) C.-T. Wang, L.-S. Lin, S.-J. Yang, J. Am.

Ceram. Soc., 75, 2240 (1992) .

11) M. Barj, J. F. Bocquet, K. Chhor, C. Porn-

mier, J. Mater. Sci., 27, 2187 (1992) .

12) K. Itatani, H. Sakai, F. S. Howell, A.

Kishioka, M. Kinoshita, Br. Ceram. Trans.

J., 88, 13 (1989) . 13) J. Beretka, T. Brown, J. Am. Ceram. Soc., 66,

383 (1983) .

14) S. Sano, N. Saito, M. Takemoto, S. Matsu-

da, Y. Arita, N. Ohashi and H. Haneda, J.

Am. Ceram. Soc., 89, 568 (2006) .

15) S. Kato, T. Iga, S. Hatano, Y. Isawa, Yogyo-

Kyokai-Shi, 84, 255 (1976) .

16) Powder Diffraction File Card No. 10-173,

Fig. 9 Schematic diagram of the mass transfer

through Al2O3- and MgO-sides at the initial

stage of reaction.


JCPDS International Center for Diffraction

Data, Newton Square, PA (1960) .


JCPDS International Center for Diffraction Data, Newton Square, PA (1949) .







21) K. Itatani, K. Koizumi, F. S. Howell, A.

Kishioka, M. Kinoshita, J. Mater. Sci., 23,

2603 (1989) .

22) K. Itatani, K. Koizumi, F. S. Howell, A.

Kishioka, M. Kinoshita, J. Mater. Sci., 23,

3405 (1988) .

23) K. Okada, A. Hattori, T. Taniguchi, A.

Nukui, R. N. Das, J. Am. Ceram. Soc., 83,

928 (2000) .

24) K. Hayashi, S. Toyoda, H. Takebe, K.

Morinaga, J. Ceram. Soc. Japan, 99, 550

(1991) . 25) E. Paparazzo, Appl. Surf Sci., 25, 1 (1986) .

26) K. Arata, M. Hino, Appl. Catalysis, 59, 197

(1990) . 27) D. E. Haycock, C. J. Nicholls, D. S. Urch, M.

J. Webber, G. Wiech, J. Chem. Soc. Dalton Trans., 1785 (1978) .

28) V. I. Nefedov, D. Gati, B. F. Dzhurinskii, N.

P. Sergushin, Y. V. Salyn, Zh. Neorg.

Khimii, 20, 2307 (1975) .

29) A. F. Henriksen, W. D. Kingery, Ceramurgia

Int., 5, 11 (1979) .

30) C. Greskovich, J. A. Brewer, J. Am. Ceram.

Soc., 84, 420 (2001) .

31) K. Shirasuka, G. Yamaguchi, K. Takagasi,


32) G. Yamaguchi, M. Nakano, R. Uchimura,


33) J.-G. Li, T. Ikegami, J.-H. Lee, T. Mori, J.

Am. Ceram. Soc., 83, 2866 (2000) .

高分散ナノ粒子の固相反応によるアルミン酸マグネシウムの生成過程

板谷清司・内藤暁雄・Ian J. Davies*・佐野聡**・幸田清一郎

(上智大学理工学部,*Curtin University of Technology,**宇部マテリアルズ(株))

高分散性ナノ粒子の固相反応によるアルミン酸マグネシウム(MgA1204)の生成過程を高温X線回折法

(HT-XRD),シンクロトロン放射光分析法(SRD)およびX線光電子分光法(XPS)を用いて検討した.出発

粉体はアルミニウム源として α-および γ―酸化アルミニウム(α-および γ-A12O3;一次粒子径,各105および

31.6nm)を,またマグネシウム源として酸化マグネシウム(MgO;41 .3nm)および水酸化マグネシウム

(Mg(OH)2;61.1nm)を使用した.これらの化合物を組み合わせて,(i)α-A12O3およびMgO,(ii)γ 一A1203

およびMgO,(iii)crAl2O3およびMg(OH)2,および(iv)γ-A12O3およびMg(OH)2の四種類の混合粉体を調

製した.各混合粉体の相変化をHT-XRDによって調べた結果,γ-Al203とMg(OH)2を加熱した時に

MgA1204の生成が最も速かった.さらに,この混合物を1200℃,1h加熱するとほぼMgAl2O4の単一相が

得られた.SRDとXPSを用いて γ-A1203およびMg(OH)2を800℃ で1h加熱して得た混合粉体(前駆物

質)に含まれる相を調べたところ,この粉体からは γ-Al2O3とMgOの他に少量のMgAl204と α-Al2O3が検

出された.γ-A12O3およびMg(OH)2粉体はそれぞれ α-A12O3やMgO粉体と比較して一次粒子径および凝

集粒子径ともに微細であったことから,加熱中に両化合物の固相反応が急速に起こり,それに伴って

MgA12O4が効率良く生成したものと判断された.

paper journal of the society of inorganic materials, japan

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