Crystallization behaviors of nanosized MgO particles from magnesium alkoxides

doi:10.1016/S0021-9797(03)00034-1

Journal of Colloid and Interface Science

Volume 259, Issue 1, 1 March 2003, Pages 127-132

https://doi.org/10.1016/S0021-9797(03)00034-1 Get rights and content

Abstract

The effects of the size of the alkoxy group on the thermal decomposition behavior of magnesium alkoxides (magnesium methoxide and ethoxide) and the crystallization behavior of MgO was investigated using thermogravimetry, Fourier-transformed infrared spectroscopy, X-ray powder diffraction, and transmission electron microscopy. As the size of the alkyl group increased, the decomposition temperature decreased and resultant MgO crystallization of the alkoxide precursor was enhanced. In an inert N₂ atmosphere, the decomposition temperature of magnesium ethoxide was about 260 °C, which was lower than that of magnesium methoxide by approximately 70 °C. The degree of the crystallization of MgO particles from the ethoxide was also significantly higher than that of the methoxide. This result is explained in terms of the OR bonding strength of the alkoxide. With use of the Kissinger method, the activation energy for the thermal decomposition of magnesium alkoxide was found to be dependent on the size of the alkyl group. The activation energies were 161±23 and 130±24 kJ/mol for the magnesium methoxide and the magnesium ethoxide, respectively.

Introduction

MgO has received a great deal of attention for its applications in the chemical and electronic industries. MgO powder has been utilized as a catalyst [1], [2], [3], [4], [5], [6], [7], [8] and MgO thin films have been used as buffer layers for superconducting and ferroelectric thin films [9], [10], [11], [12], [13], [14], [15]. Furthermore, MgO thin films have been investigated for use as a protective layer in AC-plasma display panels (AC-PDP) [16], [17], [18], [19], [20]. The substrate used in these panels is made of a soda-lime silicate with a low melting temperature; one of the major requirements of the MgO thin film is a low crystallization temperature. Since the sol–gel process has the advantages of low crystallization temperature and low cost, sol–gel-derived MgO thin films have been studied intensively in recent years [12], [13], [14], [15].

To enhance the crystallization behavior of sol–gel-derived films or powders, the hydrolysis reaction of the component alkoxide was investigated as a function of the alkoxy radical size [21], amount of water [22], and acid and base catalysts [7], [22]. In our preliminary study, however, the MgO film deposited from a hydrolyzed sol was found to be unsuitable for the protective layer application in AC-PDP [23]. The film produced through the hydrolysis reaction demonstrated unstable secondary electron emission properties because of nanopores present in the film. The nanopores were formed during the topotactic reaction of Mg(OH)₂ to MgO. This suggested that a new manufacturing approach was needed to prepare MgO film for application as protective layers. In that study, MgO films were prepared using a pyrolysis process, without an associated hydrolysis reaction. In the pyrolysis process, the selection of raw alkoxide is critical to the subsequent decomposition and the crystallization process of the films. Desu found that the deposition rate from a TEOS (tetraethoxysilane) source was higher than that from a TMOS (tetramethoxysilane) source in the CVD process [24]. The difference in the decomposition behavior was attributed to the size of the alkyl group. In the MgO system, the effect of the size of the alkoxy group was also studied. Thoms et al. investigated the decomposition mechanism of Mg–alkoxide and showed that MgO powder morphology depended on the Mg–alkoxy group size [25]. Considering that the decomposition of the alkoxy group is closely related to crystallization behavior, the difference in the size of the alkoxy group is expected to change the crystallization of the alkoxide. However, the decomposition and crystallization behavior of MgO has not been studied systematically as a function of the alkoxy group.

In this study, MgO powders were prepared from two types of Mg–alkoxides (magnesium methoxide and magnesium ethoxide). The thermal decomposition/crystallization behavior of the MgO was investigated from the viewpoint of the alkoxy group size to find the effect of the alkoxy group size.

Section snippets

Experimental

In the present study, magnesium ethoxide [Mg(OC₂H₅)₂] in powder form (designated as ME) and a magnesium methoxide [Mg(OCH₃)₂] solution in methanol were selected. To prepare the magnesium methoxide powder (designated as MM) without the undesired hydrolysis reaction, the magnesium methoxide solution in methanol was dried in a vacuum chamber. The ME and MM powders were heat-treated under different thermal conditions; i.e., atmosphere and temperature. Heat-treated alkoxide powders were quenched at

Results

The FT-IR spectra of MM and ME raw precursors and powders quenched at 300, 350, 400, and 600 °C in N₂ are shown in Fig. 1. The absorption bands in Fig. 1 were indexed according to Table 1, which summarizes the organic modes reported in the literature [13], [25], [29], [30], [31], [32]. The CO stretching mode observed at 1015–1130 cm⁻¹ and the CH stretching mode observed at 2750–2970 cm⁻¹ correspond to the alkoxy group. The absorption band (MgO stretching mode at 500–600 cm⁻¹) in raw powders

Discussion

Using FT-IR and TGA data, we observed that the ME powder started decomposition at a lower temperature than the MM powder. These results revealed that the decomposition behavior of the alkoxide was influenced by the size of the alkoxy group. The alkoxide with a larger alkyl group began decomposition at a lower temperature. The difference in the decomposition temperature, in turn, influenced the crystallization of the resultant alkoxide powders. The XRD and TEM results revealed that the MgO from

Conclusions

(1) XRD and TEM results revealed a different in crystallization behavior in MgO powders. The magnesium oxide phase was not observed in the methoxide powder, but in the magnesium ethoxide one, processed at the same temperature. This suggests that the crystallization behavior of MgO powders is influenced by the type of alkoxide used in processing.

(2) TG analysis in N₂ showed that the initial decomposition temperature of the alkoxide was approximately 330 °C for magnesium methoxide and

Acknowledgements

This work was funded by the Center for Iron & Steel Research. The authors would also like to thank Ki Ho Maeng for kindly supporting TEM measurement.

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¹: Current address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

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Crystallization behaviors of nanosized MgO particles from magnesium alkoxides

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Acknowledgements

Mater. Sci. Eng. B

Solid State Ionics

Chem. Mater.

Chem. Mater.

Chem. Mater.

Chem. Mater.

Kinet. Catal.

Langmuir

Solid State Chem.

Chem. Mater.

Appl. Phys. Lett.

J. Cryst. Growth

Appl. Phys. Lett.

J. Mater. Res.

Chem. Mater.

Appl. Phys. Lett.

J. Sol–Gel Sci. Technol.