The Effects of Magnesia Nano Particles and Magnesium Carbonate. on Sintering Behavior of Nano Alumina

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1 June 18-22, 2012, pp ISBN / ISBN CAN 2012 TENTH AES-ATEMA INTERNATIONAL CONFERENCE AES-ATEMA 2012 Tenth International Conference on Advances and Trends in Engineering Materials and their Applications (Montreal, CANADA: June 18 22, 2012) The Effects of Magnesia Nano Particles and Magnesium Carbonate on Sintering Behavior of Nano Alumina Mehran Dadkhah 1, Majid Jafari 2 *, Abdollah Saboori 3 1 Master of science, Dept. of Materials Engineering, Islamic Azad university of Najaf abad, Isfahan, Iran ( Dadkhah.Mehran@yahoo.com) 2 Assistant professor Dept. of Materials Engineering, Islamic Azad university of Najaf abad, Isfahan, Iran ( Mjafari27@yahoo.com) 3 Master of science, Dept. of Materials Engineering, Islamic Azad university of Najaf abad, Isfahan, Iran ( Saboori1985@yahoo.com) *Corresponding Author Abstract Sintering of nano-alumina powder with different constitutions of Magnesia has been performed and physical and morphology of sintered alumina were investigated. Nano-alumina with particle size of less than 50nm was synthesized by mechanical intensive milling, nano-mgo was prepared by chemical method and micro size of magnesium carbonate was used as a comparison. The ball shaped samples were formed by isostatic pressing and sintering was implemented at 1570 C for 4 hours in oxidant atmosphere. Microstructure studies were carried out by scanning electron microscopy and density of the samples has been measured by Archimedean method. Out coming results shown that with increase the amount of nano-mgo and MgCO 3 density of samples were decreased because of spinal formation of alumina-mgo however results also demonstrated that density of nano alumina-doped MgO is more than of nano alumina-doped MgCO 3. Keywords Sintering, Magnesia, Alumina, Magnesium carbonate, Nano. 1 Introduction Alumina is widely used as a high

2 Mehran Dadkhah, Majid Jafari, Abdollah Saboori temperature structural and wear-resistant material due to its high melting point, high hardness, high strength, good thermal conductivity, etc. Being an amphoteric material, alumina is used in acidic as well as in basic environments. It is compatible with other oxide and nonoxide ceramics (ZrO 2, mullite, cordierite, SiC, B 4 C, Si 3 N 4, etc.) to form particulate or whiskerreinforced composites, which possess good thermo-mechanical properties. Due to the above versatility nature, alumina and its composites account for more than 70% of structural ceramics [1]. Sintering is a processing technique used to produce density-controlled materials and components from metal or/and ceramic powders by applying thermal energy [2]. The driving force for sintering is the reduction of the total interfacial energy. The total interfacial energy of a powder compact is expressed as A, where is the specific surface energy and A the total surface area of the compact. The reduction of the total energy can be expressed as Δ( A) = AΔ + ΔA. Here, the change in interfacial energy (Δ ) is due to densification and the change in interfacial area (ΔA) is due to grain coarsening, also named grain growth. For solid-state sintering,(δ ) is related to the replacement of solid/vapor interfaces (surfaces) by solid/solid interfaces (grain boundaries). Then, during sintering, the reduction in total interfacial energy occurs via densification and grain growth, which are indeed two competing mechanisms. In the early stage of sintering of ceramics, bridges form between the elemental crystallites. In that case, samples are consolidating without volume change or shrinkage. This step, named consolidation, can be simply simulated by investigating matter transport between the surface of two tangent spheres, representing two elemental crystallites, and the surface of the forming bridge [3]. In that case, matter displacement proceeds either by surfacediffusion, gas phase transport or bulk diffusion in the grains [3]. When the samples begin to shrink, this signifies that the residual porosity is now removed. This subsequent step is named densification. A simple way to simulate densification consists in investigating matter transport between the center of the surface formed by two intersecting spheres (grain boundary), still representing the two elemental crystallites, and the surface of the forming bridge [3]. In that case, matter displacement proceeds either by grain boundary diffusion or bulk diffusion in the grains [3]. As stated earlier, during sintering, grain growth is also active and this reduces the total interfacial energy. When grain boundaries and pores are displaced simultaneously (a pore is always attached to a grain boundary) grain growth is controlled either by the grain boundaries or by the pores. If grain growth is controlled by grain boundaries, the pore dragging effect on the grain boundaries motion is negligible. If grain growth is controlled by pores, the displacement rate of a pore/grain boundary ensemble is fixed by the displacement rate of the pore. When grain growth is controlled by grain boundaries, the matter displacement invoked is the atomic migration from one grain towards another grain through the separating grain boundary [3]. In the case of grain growth controlled by pores, matter displacement proceeds either by surface-diffusion at the pore surface, gas phase transport in the pore or bulk diffusion in the grains [3]. In this study, we focused on the solid-state sintering behavior in oxidant environment of alumina doped pure/ MgO/ MgCO 3 powders shaped by isostatic press. The key to achieve a porefree micro structure (translucency) is the prevention of pore-grain boundary separation. In a single phase solid state 2012, Advanced Engineering Solutions, Ottawa, Canada.

3 Mehran Dadkhah, Majid Jafari, Abdollah Saboori sintering, maximum densification occurs when pores located at grain boundaries are removed by lattice or grain boundary diffusion processes. In the absence of magnesia, pores become entrapped within the alumina grains as abnormal grain growth takes place during sintering [4]. Once residual pores become trapped within the grain, they are impossible to be removed in a reasonable firing time since the lattice transport required is extremely slow. This prohibits further densification [5]. 2 Experimental Commercial alumina (Alcan, France) and magnesium carbonate raw powders were selected as the starting materials for investigation. Table 1 shows chemical analysis of raw powders. MgO nano particles is synthesized by chemical method from Mg (NO 3 ) 2 and oxalic acid as precursors. The powders are very pure and exhibit a high specific surface area (SSA). An Isostatic press is used for shaping and PVA were incorporated as a binder. After mixing of alumina powders with various percent of magnesia (0, 0.1, 0.3, 0.5 weight percent) and magnesium carbonate with same percentage, they are formed by isostatic press as ball. The diameter of the samples for sintering runs was typically 10 mm in diameter. The average relative green density was 68% for alumina. When setting was completed, the green samples were left in an electrical furnace for a period of several hours. Therefore, it was decided to use 1570 C as the sintering temperatures for pure alumina, magnesia-doped alumina and magnesium carbonate doped alumina. The soak time at temperature was 4 hour, whatever the material investigated. The isothermal sintering runs constituting the experimental matrix were conducted in oxide, in a standard electrical furnace. For each run on each material, at least two samples were sintered at the same time. The sintering furnace cycles involved heating up at 10 C min -1, holding at the soak temperature and free cooling to room temperature. The apparent volume mass of the sintered samples was measured using the Archimedean method with deionized water. Then, the relative density, D, was obtained using a theoretical volume mass of g/cm 3 for alumina (calculated from the elemental lattice structure of the -alumina phase). As-sintered microstructures were characterized by observing fracture surfaces that were thermally etched for 30 minute by SEM for alumina (Etching was performed 300 C below the sintering temperature). The average grain size, G, was measured from SEM micrographs, using a lineintercept method and taking into account at least 200 grains. A three-dimensional correction factor of 1.2 was used, meaning individual grains were approximated by spheres [3]. Table 1. Chemical analysis of raw powders Powder Al 2 O 3 SiO 2 MgO LOI(%w/w) Al 2 O 3 (Alcan) MgCO Results and Discussion 3-1- physical Properties From the sintering studies, it is observed that the samples are sintered to 94 96% at 1570 C. Table2 shows different component of nano Alumina samples contain various percent of mineral magnesium carbonate and magnesia nano particles with measured density by Archimedean method. Table2- Density of nano alumina samples contain

4 Relative Density(%) Relative Density(%) Relative Density(%) Mehran Dadkhah, Majid Jafari, Abdollah Saboori different amount of magnesia nano particles and magnesium carbonate percent Sample code Relative Density(%) 0 N-Alumina N-Al 2 O 3-0.1%MgO N-Al 2 O 3-0.3%MgO N-Al 2 O 3-0.5%MgO N-Alumina N-Al 2 O 3-0.2%MgCO N-Al 2 O 3-0.6%MgCO N-Al 2 O 3-1.1%MgCO The curves in Figures 1 and 2, which were plotted for magnesia and magnesium carbonate, show percent of relative density changes with percent of MgO. The effect of MgO doping to densification can be easily understood. In the sintering process, both densification and grain growth are in a competition. The densification process is limited if mass transport occurs for grain growth. In the same way, the grain growth is limited if mass transport occurs for densification. Since the presence of MgO in Al 2 O 3 reduces the grain growth of alumina, the mass transport is mainly for densification Nano %MgO Fig. 1. Changes of relative density(%) with percent of MgO %MgCO3 Fig. 2. Changes of relative density(%) with percent of MgCO Nano Nano,MgO Nano,MgCO %MgO Fig. 3. Changes of relative density(%) with percent of MgO and MgCO 3. Therefore, to some extent, denser ceramics can be expected for higher MgO doping but in this research sintering temperature was not sufficient therefore densification of samples have been decreased with increase percent of MgO and MgCO 3. However, due to existence of impurities in mineral magnesium carbonate and formation of liquid phase, densification of these samples have been decreased more than aluminadoped MgO samples as shown in Fig Phase analysis XRD patterns of alumina-doped MgO samples and pure alumina are shown in Fig. 3. According to these patterns, it is clear that spinel (MgAl 2 O 4 ) phase has been formed and number of its peaks has been increased with percent of Magnesia. XRD patterns of alumina-doped MgCO 3 samples 2012, Advanced Engineering Solutions, Ottawa, Canada.

5 Intensity Mehran Dadkhah, Majid Jafari, Abdollah Saboori and pure alumina are shown that peaks of spinel have been formed too. a Alumina Spinel Al 2 O 3-0.5%MgO Al 2 O 3-0.3%MgO b Al 2 O 3-0.1%MgO Al 2 O 3 (Nano) theta Fig. 3. XRD patterns of MgO-doped alumina samples and pure alumina after sintering Microstructural Analysis The morphology of the fracture of the pure alumina, alumina-doped MgO and alumina - doped MgCO 3 samples after sintering at 1570 C for 4hr was observed using scanning electron microscopy. Figure 4 shows the SEM image of fracture morphology of the pure alumina, Al 2 O 3-0.1%MgO and Al 2 O 3-0.5%MgO samples that sintered for 4 hour. The images revealed that the average grain size of the alumina doped magnesia is smaller than the value of the pure alumina. c Fig. 4. SEM micrographs from fracture surface of sintered a) pure nano alumina, b) Al 2 O 3-0.1%MgO, c) Al 2 O 3-0.5%MgO, and thermal etch at 1200 C for15 min. The mechanism by which MgO reduces the boundary mobility in Al 2 O 3 is still the subject of some discussions. Two major

6 Average Grain Size(mm) Mehran Dadkhah, Majid Jafari, Abdollah Saboori mechanisms had been proposed: first, solute drag due to Mg segregation at the grain boundaries and second, pinning of the grain boundaries by fine second-particles of spinel MgAl 2 O 4. The solute drag mechanism is operating below the solubility limit. Grain growth can be also inhibited by the presence of pores, inclusions and solute impurities [6]. These inhibitors act as pinning agents that constrain the movement of the grain boundary. The shape of curvature also plays a role in this grain boundary behavior. A sharp curvature can drag these inhibitors while a shallow curvature cannot. In the ceramic samples studied, at least two inhibitors acted as pinning agents, i.e. pore and second phase particles [7]. Since we were dealing with initial and intermediate stages of sintering, the presence of large amount of pores was expected. The second phase particles, in this case MgAl 2 O 4, were also likely to form and be present during sintering and grain growth since the amount of MgO added was above its solubility limit. Solubility limit of MgO in Al 2 O 3 was reported to be a few hundred ppm, i.e. ~500 ppm [8]. The grain sizes of samples were calculated by Intercept method and SEM images. Average grain size versus relative density (%) is plotted in figure 5. The average grain size of MgO-doped alumina has been decreased with amount of MgO, as shown in figure 5. However, the average grain size of alumina-doped MgCO 3 is increased with percent of MgCO 3, it s because the presents of impurity in MgCO 3 mineral and formation of liquid phase that has been formed during sintering. Microstructure of Al 2 O 3-0.5% MgCO 3 is shown in figure Al 2 O 3 (Nano) 2-Al 2 O 3 (Nano)- 0.1%MgO 3-Al 2 O 3 (Nano)-0.5%MgO Relative Density(%) Fig. 5.Average grain size versus relative density. Fig. 6. SEM micrographs from fracture surface of sintered Al 2 O 3-0.5%MgCO 3, thermal etch at 1200 C for15 min. 4 Conclusion Alumina doped MgO/MgCO 3 in different proportions have been sintered at 1570 C for 4hrs in oxidant atmosphere in an electric furnace. Density and microstructure of , Advanced Engineering Solutions, Ottawa, Canada.

7 Mehran Dadkhah, Majid Jafari, Abdollah Saboori sintered alumina doped MgO/MgCO 3 were studied. Outcome results shown that the density of alumina doped nano-mgo samples is more than alumina doped MgCO 3 samples but the density of all samples has been decrease with increase the additives. On the other hand, with increasing the percentage of nano-mgo, the average grain size of alumina has been decreased but in comparison by increasing MgCO 3 because the present of impurity and formation of liquid phase the grain growth of alumina cannot be suppressed and abnormal grain growth was occurred. References [1] Panda P.K., V.A. Jaleel, G. Lefebvre, Thermal shock study of α-alumina doped with 0.2% MgO, Materials Science and Engineering A 485 (2008) [2] Kang J.L., Sintering: Densification, Grain Growth & Microstructure, Elsevier Butterworth- Heinemann, urlington, MA, [3] Bernache-Assollant D., Chimie-Physique du frittage, Editions Herme`s, Paris, [4] Handwerker, C.A., Morris, P.A. and Coble, R.L., 1989, J. Am. Ceram. Soc., vol. 72 (1), pp [5] Rhamdhani M. A., Syoni Soepriyanto, Determination of sintering mechanism and grain growth kinetics of MgO-doped Al 2 O 3, Journal JTM Vol. XII No. 3/2005. [6] Barsoum, M., 1997, Fundamentals of Ceramics, McGraw Hill, New York. [7] Baik, S. and Ik Bae, S., 1994, J. Am. Ceram. Soc., vol. 77 (10), pp [8] Chiang, Y-M., Birnie III, D.P. and Kingery, W.D., 1997, Physical Ceramics, Principles for Ceramic Science and Engineering, John Wiley and Sons, New York..