Elsevier

Powder Technology

Volume 267, November 2014, Pages 339-345
Powder Technology

Synthesis of mullite–MoSi2 nano-composite through a mechanochemical processing

https://doi.org/10.1016/j.powtec.2014.07.037Get rights and content

Highlights

  • Mullite-MoSi2 nano-composite from the materials of MoO3, Si and Al was synthesized.

  • Mechanochemical processing was employed for the synthesis of the composite.

  • The mechanically induced self-propagating reaction (MSR) took place in 23–30 min.

  • Mullite-based composite consists of the stable α-MoSi2 with tetragonal crystals.

  • The crystallite size of 20 nm and the particle size of about 155 nm were obtained.

Abstract

Nano-composite mullite–MoSi2 with 40 wt.% MoSi2 was produced from the raw materials of MoO3, Al and Si by mechanochemical processing. Different Al amounts (stoichiometric amounts of 1, 1.5 and 2 mol) and milling times (15, 23, 30, 60, 180, 300 min) were applied to obtain this composite. A planetary ball mill with a rotation speed of 250 rpm and a ball-to-powder ratio (BPR) of 40:1 was employed. The composite was formed using 1.5 mol of Al and occurrence of mechanically induced self-propagating reaction (MSR) in 23–30 min. The evolutions of phases and their microstructure were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDX). By means of transmission electron microscopy (TEM) the particle size and morphology of the obtained powders were examined. The mean crystallite size of the composite changed from 60 nm in 30 min to 20 nm in 5 h; the average particle size was about 155 nm in 5 h.

Graphical abstract

MoO3, Al and Si powders were ball milled for different durations. XRD diffraction was used to detect the product phases. The mullite-α-MoSi2 composite was formed and its particles were analyzed by scanning electron microscopy.

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Introduction

Molybdenum disilicide (MoSi2) has attracted many researchers over the last three decades due to its unique properties such as high melting point (2030 °C), relatively low density (6.28 g/cm3), metallic-like thermal and electrical conductivities, very good oxidation and corrosion resistance at high temperatures [1], [2], [3], [4]. These properties make it ideally suited for high temperature structural applications. However, it has some intrinsic limitations like low ductility at low temperatures (roughly below 1000 °C) and poor high-temperature creep strength [4], [5], [6]. Thus, it is essential to increase room-temperature toughness and high-temperature strength for practical applications. Molybdenum disilicide is also widely used in the production of heating elements. A continuously adhering silica film that forms on a heater surface protects it from oxidation reliably up to 1700 °C. Above 1700 °C, the oxygen diffusivity in SiO2 is high because of the high equilibrium vacancy concentration that would result in an enhanced rate of oxidation. Below 700 °C, the SiO2 layer is not continuous and that leads to penetration of oxygen and pest disintegration in the porous MoSi2 product in the temperature range of 400–600 °C [2], [7], [8], [9], [10]. Therefore, another challenge lying in MoSi2 is to control the pest oxidation.

Fortunately, MoSi2 is thermodynamically stable with a wide variety of potential ceramic reinforcements, including SiC, Si3N4, ZrO2, Al2O3, mullite, TiB2, and TiC. Thus many material researchers focused on the field of synthesis and characterization of MoSi2-based composites and reached satisfactory results [4], [5], [6], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].

Mullite ((Al4SiO8)1.2) possesses many attractive properties as follows; high melting point (1830 °C), excellent high temperature strength and creep resistance, low density (3.2 g/cm3), good chemical stability, good thermal shock behavior, inherent oxidation stability, low thermal expansion coefficient, low thermal conductivity and dielectric properties.

Due to the aforementioned properties, it is a strong candidate material for structural applications under thermal and mechanical loads [20], [21], [22], [23], [24], [25], [26], [27].

A composite of mullite–MoSi2 can compensate for the oxidation issues and low creep resistance of MoSi2. As a heating element, MoSi2 could play the role of a passing current through the mullite matrix [28]. In the work of Kisly and Kodash[9], the mullite-based coating formed on the Al2O3–MoSi2 alloy surface during annealing reliably protected the alloy up to 1800 °C in the air. Fei et al.'s study [11] indicated that mullite formation in the Al2O3–MoSi2 coating improved its oxidation resistance behavior. Composites of molybdenum disilicide, reinforced with mullite whiskers and particles, were made with the aid of a powder processing method by McFayden et al. [29]. Zaki et al. [28] reached a mullite–MoSi2 composite using combustion synthesis, which showed a good oxidation resistance in the range of 1000–1300 °C. Furthermore, the heat produced by the MoSi2 formation reaction [30] can promote the endothermic reaction of mullite formation (ΔHf  120 kJ·mol 1 at 298 K for (Al4SiO8)1.2) [31].

In the present investigation, an attempt has been made to make a nano-structure mullite–MoSi2 composite from the raw materials of MoO3, Si and Al through mechanochemical reactions. Different Al amount and milling times were tested to obtain the composite. XRD, EDX and SEM were used to study the product composition and microstructure. Additionally, TEM was implemented to examine the particle size and morphology of the products. It is interesting to note that few researches have tried to make mullite–MoSi2 composite and so far ball milling has not been used as a method of synthesis.

Section snippets

Materials and methods

The raw materials used in this study were comprised of powders of MoSi2, Si and Al with purity of 95%, 99% and 99.5% and particle sizes of 250, 300 and 100 μm, respectively. Experiments of ball milling were carried out in a planetary ball mill with a steel vial of 39 cm3 volume and steel balls with diameters of 8, 10 and 12 mm. The ambient was under dry condition with air pressure of 1 atm. The rotation speed of 250 rpm and a ball-to-powder ratio (BPR) of 40:1 were used with the primary powder

Thermodynamic calculations

ΔH298, Tad and the amount of vaporized Al2O3 were calculated for reactions (1), (2), (3)by writing a thermodynamic model with Excel software. Table 1 shows that ΔH298 increases with the increment of Al amount. Tad is also higher for the systems with more Al amount, although it remains constant in the vaporizing temperature of Al2O3 (3253 K) for reactions (2), (3).

Results and discussion

Fig. 1 shows the comparison of XRD results of samples containing 1 mol Al with different ball milling durations. It can be seen that after 30 min, no reaction was detected. However, the reaction took place in sample X1-1 h so that the primary mixture was converted to Mo, α-MoSi2 and MoO2. This means that the MSR reaction occurs between 30 min and 1 h after starting the ball milling; therefore, activation time of at least 30 min is required. The peaks of the Al2O3 phase were not revealed by XRD. There

Conclusions

A mullite-α-MoSi2 nano-composite was successfully prepared by mechanochemical synthesis. The composite is mullite-based, containing around 60 wt.% mullite and its MoSi2 is a stable kind of α with tetragonal crystals. The crystallite size of around 18 nm and average particle size of about 155 nm were obtained for the composite after 5 h of ball milling. The temperature of the ball milling vial wall confirmed the XRD results, showing that the MSR reaction in X1.5 samples took place in a duration

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