Transparent Cr-Doped Al2O3 Made by Pulsed Electric Current Sintering Process

Khanh Quoc Dang and Makoto Nanko

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AZojomo (ISSN 1833-122X) Volume 6 November 2010

Topics Covered

Abstract

Al₂O₃ doped with a small amount of chromium is called ruby and is widely used in various applications. Al₂O₃-Cr₂O₃ powder mixture was prepared by drying Al₂O₃ aqueous slurry with Cr(NO₃)₃ and consolidated by pulsed electric current sintering (PECS) processing into Cr-doped Al₂O₃ polycrystals. The PECS process was performed in vacuum at sintering temperature of 1200°C with the heating rate ranging from 2 to 100 K/min under different applied uniaxial pressures of 40, 80 and 100 MPa. PECS with slow heating rate and high pressure leads to highly densified bodies and transparent Cr-doped Al₂O₃ polycrystals.

Keywords

Cr-doped Al₂O₃, Ruby, Pulsed Electric Current Sintering (PECS), Density, Transparent, Microstructure

Introduction

Pure corundum (Al₂O₃) is a rare mineral, perfectly colourless and transparent. Small amounts of metallic elements such as chromium, iron, titanium or vanadium can substitute for aluminium in the corundum crystal structure, giving rise to many colour variations. Ruby, this magnificent red variety from the multi-coloured corundum family, consists of aluminium oxide and chromium as well as very fine traces of other elements. The advantages of ruby are high heat-resistance, high melting point, high mechanical strength, high hardness and excellent chemical stability. Ruby is widely used for hydraulics, chemicals, airflow filters, solid-state lasers, solar energy, decorative products, optical components like prisms and wedges, etc. [1, 2].

Pulsed electric current sintering (referred to as PECS), also known as spark plasma sintering (SPS) or plasma activated sintering (PAS), is the latest pressure-sintering process to consolidate advanced materials such as ceramics, metallic materials and their composites. The sintering process consists of placing raw powder inside an electrically-conductive die, closing the unit with two punches pressed uniaxially, and applying a pulsed electric current momentarily through an electrically-conductive die, in some cases, through the sample to heat up the specimen [3-24]. PECS for producing transparent ceramics has also been reported [17-25]. Some reports described the successful sintering of transparent alumina polycrystals by PECS [17-22]. In particular, Kim et al. demonstrated that, for PECS processing of Al₂O₃, a slow heating rate is efficient for densification and transparency [17-19].

However, there are no reports on sintering of coloured Al₂O₃ with any doping by the PECS process. In the present study, Al₂O₃-Cr₂O₃ powder mixture was prepared by drying Al₂O₃ powder slurry with Cr(NO₃)₃. The powder mixture was reacted and sintered into transparent Cr-doped Al₂O₃ polycrystals by the PECS process with various conditions, particularly, different heating rates and applied pressures.

Experimental Procedure

A commercial á-Al₂O₃ powder (99.99% purity, TM-DAR, Taimei Chemicals Co. Ltd., Japan) was mixed with Cr(NO₃)₃ (Nacalai Tesque Inc., Japan) as a source material of Cr₂O₃ in distilled water. The aqueous slurry was dried by dropping it into a glassware pot heated at 350°C. The mass of Cr₂O₃ concentration in the final samples was equalled to be 70 ppm. The Al₂O₃-Cr₂O₃ powder mixture was ground by a conventional dry milling process using an alumina mortar for 30 min. The sintering experiments were conducted by using Dr. Sinter Model SPS-1050 (Sumitomo Coal Mining Co.) with 12/2 in ON/OFF pulse pattern, which is recommended by the PECS supplier. The Al₂O₃-Cr₂O₃ powder mixture was put into the graphite die (ϕ30 in outer diameter, ϕ15.4 in inner diameter and 30 mm in height) with a ϕ1.8 x 3 mm hole for temperature measurement by a pyrometer. Sintering temperature was controlled by a preset heating program and it was measured by an optical pyrometer focused on the surface hole of the die during heating. The PECS process was conducted at sintering temperature of 1200°C for 20 min under various uniaxial pressures between 40 and 100 MPa in vacuum. Heating rate of 2 and 100 K/min was selected.

Bulk density of the sintered bodies was determined by the liquid-replacement technique with toluene. Phase identification of Al₂O₃-Cr₂O₃ powder mixture was conducted by using X-ray diffraction (XRD). The distribution of Cr, Al, and O was observed by the energy dispersive X-ray spectroscopy (EDX). Microstructure of as-sintered samples was observed by scanning electron microscopy (SEM) with EDX for element mapping. The average grain size was determined by using the linear intercept method with the compensation coefficient of 1.126 [26].

Results And Discussion

Figure 1 shows XRD pattern of the Al₂O₃-Cr₂O₃ powder mixture. Samples with the single corundum phase were obtained. No contamination or other chemical reaction has occurred. Figure 2 shows the microstructure of the Al₂O₃-Cr₂O₃ powder mixture after grinding with an alumina mortar. Strong agglomeration of the Al₂O₃ particles was observed.

Figure 1. XRD pattern of Al₂O₃-Cr₂O₃ powder mixture.

Figure 2. Microstructure of the Al₂O₃-Cr₂O₃ powder mixture after milling process.

Figure 3 shows EDX elemental maps of Al₂O₃-Cr₂O₃ powder mixture after the milling process. The element maps showed that the Cr and Al exist homogeneously throughout the powder mixture.

Figure 3. SEM and EDX images of Al₂O₃-Cr₂O₃ powder mixture after milling process.

The relationship between the relative density of sintered body and an applied uniaxial pressure is shown in Figure 4. The relative density is increased with increasing applied uniaxial pressure. The relative density of ruby polycrystals sintered by PECS with slow heating rate was higher than that in the rapid heating rate under same applied uniaxial pressure. All the samples sintered under 100 MPa attained nearly the theoretical density (>99%). With an applied uniaxial pressure of 80 MPa and higher, fully densified samples were obtained already at the slowest heating rate. An applied uniaxial pressure and the heating rate play important roles in the densification during the PECS process of Al₂O₃-Cr₂O₃ powder mixture.

Figure 4. Relationship between relative density, D, and an applied uniaxial pressure, P, in the PECS of Al₂O₃-Cr₂O₃ powder mixture.

Figure 5 represents SEM images of fractured surface of ruby polycrystals sintered by the PECS process under various conditions. With an applied uniaxial pressure of 40 MPa, the average grain size of all sintered samples exhibits smaller grain size and number of pores exists in samples. Increasing an applied uniaxial pressure to 100 MPa, the ruby polycrystals with a heating rate of 2 K/min was fully densified. Less pores at the grain boundary junction and the fine grains formed after sintering under higher pressure in slower heating rate. The relationship between average grain size and an applied uniaxial pressure with various heating rates is shown in Figure 6. The results show that an applied uniaxial pressure had little effect on average grain size but the heating rate significantly affected on average grain size. The grain size of ruby polycrystals with slower heating rate was smaller than ones formed by rapid heating rate. It appears that the heating rate plays a greater role in grain growth than an applied uniaxial pressure.

Figure 5. SEM images of the fractured surface of ruby polycrystals sintered at 1200°C: a) and b) the heating rate of 2 K/min and an applied uniaxial pressure were 40 and 100 MPa; c) and d) the heating rate of 100 K/min and an applied uniaxial pressures were 40, and 100 MPa, respectively. Arrows in a), c) and d) are represent the pores.

Figure 6. Relationship between average grain size, D, and an applied uniaxial pressure, P, in PECS of Al₂O₃-Cr₂O₃ powder mixture. The error bars show maximum and minimum values.

For the effect of the heating rate on the grain size, there have been conflicting results. Some researchers reported that the grain size of alumina decreased with increasing the heating rate [9, 10, 28]. Those reports mentioned a small grain size at rapid heating for the alumina, which was opposite to the results of the present study and other reports [18, 29]. Their mechanism is explained by using the conventional empirical equation of grain growth. The cause of their conflicting results has not been clarified. However, their respective PECS experiments were conducted under different sintering conditions with different Al₂O₃ powders. The different sintering conditions may provide a clue for understanding the conflicting results. To explain the reason for larger grain size at higher heating rate, Murayama and Shin mentioned the possibility that a high defect concentration was produced by rapid heating and associated rapid plastic deformation during densification [29]. The defect produced by both heating and plastic deformation may therefore accelerate the grain growth during PECS processing. The initial defect concentration in the powder particles, which is different in different powder, should be also affected to sintering behaviour.

In order to achieve complete densification and fine grain size in pressure-sintering of alumina, Krell et al. emphasized the importance of homogeneous particle dispersion in the raw powder before sintering [30, 31]. They improved the final density and transparency by homogenizing the powder using such techniques as stirring, ultrasonification and milling. On the other hand, Kim et al. [17-19] considered that in addition to the low defect concentration, an effect similar to the precoarsening may act on the powder during slow heating rate at low temperature and the as-received Al₂O₃ powder was homogenized itself during PECS. Furthermore, some researchers reported that the Al₂O₃ powder with small amount doping element such as MgO or triple dopant combination of the 3 dopants: Mg, Y and La gave the fine grain size and the best transparency of samples [20, 21, 32]. Due to very limited solubility of elements in Al₂O₃, the grain size can be affected by segregation of dopants or impurities at grain boundary. As well as densification during the PECS process, the quality of the starting powder from powder preparation played a key role for grain growth during PECS processing.

As mentioned above, higher applied uniaxial pressure and slow heating rate resulted in complete densification and small grain size. The transparency of ruby polycrystals could be affected by both relative density and grain size. Transparency is inversely proportional to the grain size and the density [20, 27]. The difference in transparency of the ruby polycrystals sintered under high applied uniaxial pressure with various heating rates is shown in Figure 7. The ruby polycrystals with more porosity has lower apparent transparency than the dense ones. In addition to the effects of porosity, the grain size also influences transparency. Therefore, the ruby polycrystals changed gradually, the appearance from opaque to transparent with increasing applied uniaxial pressure and simultaneously decreasing heating rate. The transparency of ruby polycrystals is easily observed by the naked eyes.

Figure 7. Images of ruby polycrystals sintered by PECS at 1200°C under an applied uniaxial pressure of 100 MPa: a), b) the heating rates were 2 and 100 K/min, respectively. The thickness of these samples was approximately 2.5 mm.

However, some dots appeared inside the ruby polycrystals, as seen in Figure 7. These dots have high Cr concentration as shown by the EDX results. Figure 8 shows the SEM image and EDX elemental of fractured surface of sintered ruby polycrystals. Porous area, which is probably a black dot, shows high Cr concentration. Preparation of the homogeneous Al₂O₃-Cr₂O₃ powder mixture would be important for obtaining sintered ruby polycrystals with higher transparency without black dots. Powerful milling process such as high-speed milling may be effective in improving the homogeneity of the powder mixture.

Figure 8. SEM and EDX images of fractured surface of ruby polycrystals sintered at 1200°C with heating rate of 100 K/min under applied uniaxial pressure of 100 MPa.

Conclusions

Highly densified ruby polycrystals were successfully obtained with the PECS processing of Al₂O₃-Cr₂O₃ powder mixture prepared by drying aqueous Al₂O₃ slurry containing Cr(NO₃)₃. In present study, the PECS process with slow heating rate resulted in complete densification and small grain size. An applied uniaxial pressure had a little influence on microstructure, but had a significant effect on densification. The sintered ruby polycrystals with slow heating rate, e.g. 2 K/min, and high applied uniaxial pressure, e.g. 100 MPa, shows excellent transparency which is easily visible by the naked eyes. However, some dots appeared inside of ruby polycrystals. These dots are probably caused by heterogeneity in the Al₂O₃-Cr₂O₃ powder mixture.

References

Contact Details

Khanh Quoc Dang and Makoto Nanko
Department of Mechanical Engineering, Nagaoka University of Technology,
Nagaoka, Niigata 940-2188, Japan

E-mail : [email protected]

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 12[1] (2010) 19-24.