Synthesis and deformation behavior of nano-diphasic materials from natural dolomite

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Abstract

Nano-grained CaO/MgO and related composites have been synthesized from the mixed carbonate mineral, dolomite (CaMg(CO3)2), by pyrolytic reactive sintering. In CaO/MgO composites, CaO and MgO grains mutually inhibited the other’s grain growth. Effects of LiF doping on decomposition and sintering of dolomite were studied. The CaO/MgO nanocomposite with a small amount of LiF showed superplastic-like deformation at high temperatures. A preliminary machining test revealed that the CaZrO3/MgO in situ nanocomposite, which was synthesized from dolomite/zirconia mixed powder, was machinable at room temperature accompanying quasiplastic deformation.

Introduction

Dolomite (CaMg(CO3)2) has been studied widely and is well known because of its usefulness in the steel industry. Dolomite is also recognized as an important raw material because it can be used for producing fine CaO/MgO composites [1], [2], [3], [4], [5], [6], [7]. To illuminate the pyrolysis behavior of dolomite, Suzuki et al. [8] have recently studied the decomposition of dolomite and the formation of CaO/MgO nanopolycrystals via in situ scanning electron microscopy (SEM) and in situ X-ray diffraction (XRD) at high temperatures. Furthermore, dense bulk-form CaO/MgO and CaZrO3/MgO nanocomposites have been successfully fabricated using reactive hot-pressing from dolomite and dolomite/zirconia mixed powders, respectively [8].

In the present study, first, we demonstrate the direct observation of the CaO/MgO nano-polycrystals formation from a pure dolomite powder using in situ SEM-video system. Secondly, the effects of LiF doping on decomposition and sintering of dolomite are described. Finally, some characteristic behavior of the CaZrO3/MgO in situ nanocomposite, such as machinability and quasiplasticity are reported.

Section snippets

Dolomite source

The dolomite source was the same as that in the previous paper [8]. Natural Thailand dolomite ore, which was colorless (white), was crushed and ground in a WC–Co mortar, and screened to produce <200 mesh (<75 μm) powder for hot-pressing. A part of the powder was pestled into a particle size less than about 1–2 μm for the high-temperature SEM observation. This natural material is of high purity, and the Ca/Mg ratio (=1.017 in mole fraction) is almost exactly the “ideal” stoichiometry. An excess of

Nanophase formation during the decomposition of dolomite

In the previous work [8], we have reported that dolomite decomposed into CaO and MgO nanoparticles, and they sintered into submicron-sized CaO/MgO composite. In the present study, CaO and MgO mixed nanoparticles have been successfully photographed via SEM-video system (Fig. 1). The formation of very fine nanoparticles about 5 nm in size was clearly observed in situ during the pyrolysis (estimated at ∼600°C). Since the SEM chamber had a high-vacuum level, the full decomposition, i.e. CaMg(CO3)2

Conclusions

In this work, decomposition, sintering and deformation of dolomite-related materials are studied. Conclusions are the following:

  • 1.

    CaO and MgO mixed nanoparticles have been directly observed via SEM-video system. The formation of very fine nanoparticles about 5 nm in size was confirmed during the pyrolysis.

  • 2.

    Dense CaO/MgO in situ nanocomposite was produced by hot-pressing the pure dolomite powder. CaO and MgO mutually inhibited grain growth.

  • 3.

    The LiF-doped dolomite decomposed in two steps, while the

Acknowledgements

The authors gratefully acknowledge Kawatetsu Mining Co. Ltd. for the kind provision of pure dolomite ore. We gratefully acknowledge Prof. Niihara and Prof. Sekino at Osaka University for the use of the high-temperature SEM apparatus. We thank Dr. T. Kasuga at Chubu Electric Power Co. for chemical analyses. A part of this research was supported by STA, Japan, under the Fluidity Promoting Research System and the Encouragement of Basic Research at National Research Institutes (the Special

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