Magnesium oxide nanopowder is synthesized using magnesium nitrate hexahydrate and oxalic acid as precursors via the sol-gel method. In order to investigate the effect of magnesia nanopowders on the physical properties of sintered alumina, 0.1, 0.3, and 0.5 wt% of MgO are added to alumina. The prepared specimens were sintered at 1570°C for 4 hours under an inert atmosphere. The morphology and size of nanopowders were characterized by transmission electron microscope (TEM) and scanning electron microscope (SEM). Structural analysis was investigated by means of Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). Outcomes show that by increasing the percentage of MgO, spinel phase (MgAl2O4) has been formed in the structure of alumina. During the sintering process, spinel phase diffused through the grain boundaries and pinned the grain boundaries which led to decrease in grain sizes. So, by decreasing the grain size, the physical properties of sintered alumina have improved.

1. Introduction

α-Al2O3 has the highest hardness and density among aluminum oxides that is formed by heating of other polymorphs of alumina or alumina hydrate at a high temperature. Additionally, α-alumina can be produced by melting process of alumina hydrate at up to more than 2000°C and subsequently its cooling and crushing [1]. Regarding formation of particulate or whisker-reinforced composites with good thermomechanical properties, alumina is compatible with other ceramics such as ZrO2, mullite, cordierite, SiC, B4C, and Si3N4. Due to the above versatility of its nature, alumina and its composites include more than 70% of structural ceramics [2]. Another reason alumina is significant compared to other engineering oxide ceramics is due to a lower sintering temperature which makes it economical. In recent years, nanomaterials attract more attention in comparison with micron size materials due to a high surface area which leads to its specific properties. For instance, particles with nanodimension can increase strength, ductility, and formation of ceramics. Moreover, nanoparticles are used for producing transparent coats. Nanoalumina powders are also applied to the production of ceramic parts with hardness and wear resistance as well as raw materials in the catalyst industry [3, 4]. Furthermore, alumina nanosize with regard to micron size alumina can reach a higher density at lower temperature; hence it is utilized as a raw material in the generation of alumina ceramic bodies. There are different methods to synthesize nanomaterials including mechanical alloying [5], vapor phase reaction route [6], precipitation [7], sol-gel [8], and combustion [9]. The mechanical alloying route takes too long and also during this process the final powders can be polluted. In vapor phase methods there are some disadvantages such as high cost equipment and low efficiency. Among these methods, sol-gel is a simple and economical route with high efficiency and specific properties at low temperature like high surface area in volume ratio, fine particle size distribution, and so on [10].

Among the oxide compounds, magnesium oxide (MgO) has interesting properties and is employed in industrial applications [1120]. In 1961, Coble investigated the effect of a small amount of magnesia on sintering of alumina for the first time and they reported that the adding of MgO less than 0.25 wt% led to obtaining a density near the theoretical value [21]. The present study has focused on the synthesis of nanopowder via sol-gel method by magnesium nitrate and oxalic acid as a precursor and investigating the effect of synthesized MgO powder on the wear property of alumina.

2. Materials and Methods

The magnesia (MgO) nanopowder used in the present study is prepared according to a sol-gel method by use of [(Mg(NO3)2·6H2O) (Merck, 99%)] and oxalic acid [(COOH)2·2H2O, (Merck, 99%)] as precursors in 1 : 1 molar ratio [22]. In the first step, the starting materials are dissolved in ethanol separately to acquire clear solutions. Subsequently, these two solutions are mixed together to achieve a thick white gel. The white gel is aged for 12 hours and then dried at 100°C for 24 hours. In the next step, the product is ground, sieved, and calcinated at 600°C for 2 hours and cooled at a rate of 10°C/min. To examine the effect of synthesized MgO nanopowder on the physical properties of alumina, 0.1, 0.3, and 0.5 wt% MgO are added to alumina powder (Commercial alumina, Alcan France, Micron size) and the samples are formed by cold isostatic press. The diameter of samples for sintering runs is typically 10 mm in diameter. The average relative green density is 68% for alumina. After completing the setting, the green samples are left in a standard electrical furnace at 1570°C with 4 hours soaking time for pure alumina and magnesia-doped alumina. For a more precise investigation, at least two samples are sintered at the same temperatures. In order to increase the sintering rate, inert atmosphere is applied. Examining the influence of MgO nanopowders on the physical properties of alumina has been done by measuring the wear and density of samples after sintering. The density of the samples is measured by Archimedes’ method by employing distilled water for the immersion media. Then, the relative density () is obtained by using a theoretical volume mass of 3.976 gcm−3 for alumina (calculated from the elemental lattice structure of the α-alumina phase). Apart from the study on the formation of MgO and its stability, thermal analysis of the dried product is performed with a computer-controlled thermogravimetric analysis (TGA) [22]. For the identification of phase(s) and determining the average crystallite size, X-ray powder diffractometer with Philips, λ = 1.5404 Å is used. Additionally, scanning electron microscopes (SEM Seron tech and Vega, TEM Philips CM10) are utilized for observing the morphology and microstructure development of the sintered specimens. Microstructures of sintered samples are characterized by observing fracture surfaces that are thermally etched for 30 minutes (etching is performed at 300°C below the sintering temperature). The average grain size, G, is determined from the SEM micrographs employing line intercept method [23]. A three-dimensional correction factor of 1.2 is used, meaning individual grains are approximated by spheres.

3. Results and Discussion

3.1. Magnesia Nanopowder
3.1.1. Thermal Analysis of Synthesized Nano-MgO

The thermal behavior of the dried product was investigated by raising its temperature up to 800°C at the rate of 4°C/min. According to differential thermal analysis (DTA), two main levels of weight loss are seen in Figure 1 [22]. In the first stage, at a temperature in the range of 201–282°C, a weight loss of about 24% is observed which is attributed to the removal of crystal water of MgC2O4·2H2O. In the second stage, another weight loss of about 47.5% has been seen between 416–564°C which is ascribed to the decomposition of MgC2O4 and the formation of magnesia nanopowders. Residual materials are indicated as weight loss 1% at the temperature of up to 564°C [22]. With attention to the results, the decomposition temperature of dried gel powder and formation of MgO are arranged in the range of 500–1000°C.

3.1.2. Phase Analysis and Microstructure Evolution

Particle size and phase analysis of nanomaterials are characterized by X-ray diffraction. The X-ray diffraction (XRD) pattern of a calcined specimen at 600°C is shown in Figure 2. According to this pattern, it is clear that there are some peaks in the range of 10°–80° (to 2) that are in good agreement with the standard pattern of periclase. Generally, in the XRD pattern, particle size vigorously depends on the intensity and angle of the maximum peak. In the Scherrer equation , nanocrystallite size is calculated by full-width at half-maximum of peak (β) and XRD radiation of wavelength λ nm. However, if the value of is the same for all of the peaks of a pattern, should be equal. Therefore, for a typical 5 nm crystallite size and = 0.15405 nm the peak at 2θ = 170° must be more than ten times wider with respect to the peak at 2θ = 10°, which is never observed. Thus, the present study has used modified Scherrer equation to estimate more accurately nanocrystallite size using XRD which was presented by Monshi et al. [24]. The modified Scherrer equation plots against and obtains the intercept of a least squares line regression, ln = , from which a single value of is obtained through all of the available peaks.

As can be viewed in Figure 2, most of the peaks are matched with a mineral material that is called magnesium oxide with the chemical formula MgO, a cubic crystallography structure, and a space group F23 with group space number 196, the lattice parameters = 4.22 Å, and angles α = β = γ = 90°. The XRD pattern agrees fairly well with the TG diagram. In the TG diagram, the decomposition of magnesium oxalate is at 526°C, following which MgO is formed. In order to calculate the nanocrystallite size of MgO, FWHM for three main peaks should be determined and according to the modified Scherrer equation [24] the plot of against must be drawn. The calculations for measuring the nanocrystallite size of synthesized MgO are shown in Table 1. The crystal size of synthesized MgO according to this plot is calculated at about 15 nm. The plot of against is illustrated in Figure 3.

The crystal size of synthesized MgO was also assessed by Transmission electron microscope along with the modified Scherer method and the TEM image (Figure 4) verify the calculations.

Figure 4 shows the image of transmission electron microscopy of calcined magnesia powder at 600°C. It is noticeable that magnesia nanopowders with a size of less than 70 nm have been formed as agglomerates.

3.2. Investigating the Physical Properties of Alumina Samples after Sintering

As can be observed in Figure 5, the average percentage of relative density for samples in the range of 1500–1650°C is fluctuational. With the increase in temperature from 1500°C to 1550°C, the average percentage of relative density has declined from 94.7% to 93.2%. On the contrary, by raising the temperature from 1550°C to 1650°C it has resulted in decreasing the amount of porosity and obtaining a denser body. Although these changes in the percentage of relative density can be attributed to percentage of porosity, it is clear that there is a relation between the temperature and relative density in such a way that at the temperatures below 1550°C the diffusion process cannot be completed, so after 1550°C by increasing the temperature the density will increase while below this temperature the story is different.

To survey the effect of amount of nanomagnesia on the physical properties of alumina, the specimens with various percentages of MgO ( = 0, 0.1, 0.3, 0.5 wt%) were sintered at 1570°C for 4 hours which is close to the industrial condition for sintering of alumina. Figure 6 indicates the changes in percentage of relative density with particle size of samples with different MgO. It is evident that these two parameters have a converse correlation with each other. However, the comparison between diagrams 5 and 6 finds that, by raising the percentage of magnesia, the particles sizes decreased which leads to an increase in the density of specimens.

Research findings about the effect of additives to alumina indicate that a rise in the amount of MgO can lead to increased density of sintered samples and control of grain growth. Magnesium ions diffuse in grain boundaries and pin them. MgO with creation vacancies increase and (lattice diffusion coefficient and grain boundary diffusion coefficient) and subsequently raise the rate of grain boundary diffusion which leads to an increased densification. Additionally, due to the relation between porosity and grain boundary, abnormal grain growth is prevented [25]. Furthermore, an increase in density is attributed to the formation of a second phase which is called spinel with chemical formula (MgAl2O4). By diffusion of the spinel phase in grain boundaries, the grain growth has been suppressed, so by raising the MgO percentage, the grain growth is prevented and finally the grain size will be decreased. X-ray diffraction and FT-IR of sintered samples verified the formation of spinel phase in samples. The XRD pattern of samples after sintering at 1570°C for 4 hours is shown in Figure 7. As can be observed, by increasing the nano-MgO percentage the intensity of peaks declines and the peaks which are related to formation of spinel are clearer. In the alumina specimens with 0.1 wt% and 0.5 wt% nano-MgO, the numbers of peaks which are related to spinel are two and three, respectively.

Figure 8 shows the FT-IR of sintered samples with 0.5 wt% MgO. As can be seen, the peak that appeared in 1097 cm−1 is related to Al–O–Mg bond. In accordance with the standard, peaks of Al–O bond in 520, 690, 914, 1379, 1633 cm−1 and peaks of Mg–O bond in 435, 541, 851, 1090, 1400, and 1500 cm−1 appear.

The wear changes of sintered alumina specimens against nano-MgO% are illustrated in Figure 9. As can be detected, by increasing the percentage of magnesia up to 0.3 wt% the wear of alumina samples increased; then by raising the MgO percentage to 0.5 wt%, this property is decreased up to five fold.

SEM images of sintered alumina samples with different MgO wt% at 1570°C for 4 hours under an inert atmosphere are shown in Figure 10. As can be observed, Al2O3-0.1 wt% MgO specimens with particle size of ~2.6 μm in comparison with other percentages of MgO indicate bigger particle size (Figure 10(a)). While, by raising MgO to 0.3 and 0.5 wt%, the particle size of specimens is decreased to 2.4 μm (Figure 10(b)) and 2 μm (Figure 10(c)), respectively.

By investigating sintering mechanism in alumina samples and the effect of additives on this process, it is found that pinning the grain boundaries is the simplest way for control of grain growth. In total, kinetic grain growth is controlled by the decrease in grain boundary motion which is due to grains pinning power. Fine particles and precipitations that do not dissolve into structure can affect stability of structure up to a high temperature. Due to that, the dissolving of alloying elements is used in order to delay grain growth. These elements with aggregation in grain boundaries or formation of second phase lead to a decrease in grain boundary motion. In this study, the presence of MgO in the alumina structure at 700°C causes the formation of spinel phase which remains in alumina body as precipitation. In the following sintering process at elevated temperatures, the presence of spinel in grain boundaries resulted in pinning the grain boundaries. This pining process is a reason for suppressesing the grain growth and also decreasing the grain size in alumina sintered bodies which finally lead to improve the physical properties of sintered Alumina. Figure 11 by applying EDAX analysis approves that the presence of magnesium, aluminum, and oxygen is in good agreement with the formation of spinel phase (MgAl2O4) at grain boundary.

4. Conclusion

In the present research, magnesia nanopowders were synthesized via sol-gel method. In order to investigate the effect of MgO nanopowders on the physical properties of sintered alumina, the alumina specimens with 0.1, 0.3, and 0.5 wt% of MgO were prepared by isostatic press as ball and were sintered at 1570°C for 4 hours under an inert atmosphere. Effects of MgO on properties of alumina include the following.(1)MgO nanopowders with an average crystal size less than 50 nm were synthesized via a simple and economical sol-gel method by Mg (NO3)2·6H2O and (COOH)2·2H2O as starting materials and ethanol as solvent.(2)By using the modified Scherrer equation, MgO with a particle size of 20 nm was synthesized and it was approved by electric microscope images (SEM, TEM).(3)In a certain amount of MgO, change in percentage of relative density by increasing the temperature from 1500°C to 1650°C was not lineal.(4)At a constant temperature, by raising MgO wt%, the average particle size of alumina has declined.(5)By decreasing the grain size of alumina samples by MgO addition, the percentage of relative density of samples has climbed.(6)At elevated temperatures the magnesia and alumina have formed spinel phase. Spinel phase with diffusion through the grain boundaries leads to the formation of a structure with fine grains.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.