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DETERMINTAION of SINTERING MECHANISM and GRAIN GROWTH KINETICS of Mgo-DOPED Al2o3

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DETERMINTAION of SINTERING MECHANISM and GRAIN GROWTH KINETICS of Mgo-DOPED Al2o3 JTM DETERMINTAION OF SINTERING MECHANISM AND GRAIN GROWTH KINETICS OF MgO-DOPED Al2O3 oleh : Muhammad Akbar Rhamdhani *, Syoni Soepriyanto**, Aditianto Ramelan *, A. Barliansyah *** Sari MgO dalam bentuk larutan padat dapat tersegregasi di sepanjang batas butir keramik Al2O3. Ini akan menurunkan mobilitas batas butir dan juga menghambat pertumbuhan butir. Artikel ini membahas tentang penentuan mekanisme sintering dan kinetika pertumbuhan butir Al2O3 yang didoping oleh MgO yang disinter pada 1773K. Model penyusutan linear digunakan untuk menentukan mekanisme sintering. Besar butir rata-rata dari sampel ditentukan dari data difraksi sinar-x dengan menggunakan metode fluktuasi statistic. Hasil menunjukkan bahwa mekanisme sintering untuk sampel dengan 0.1wt% dan 0.3 wt% MgO didominasi oleh difusi permukaan, dan untuk sampel dengan 0.5 wt% dan 1.0 wt% MgO didominasi oleh difusi batas butir. Besar butir rata-rata untuk sampel Al2O3 yang disinter selama 2 jam adalah 21.1μm. Untuk waktu sintering yang sama, doping MgO sebanyak 0.1 wt% sampai 1.0 wt% menurunkan besar butir rata-rata menjadi masing-masing 12 sampai 10μm. Kata kunci : MgO-doped Al2O3, model penyusutan linear, metode fluktuasi statistik, sintering, pertumbuhan butir Abstract MgO in the form of a solid solution can segregate along the Al2O3 ceramic grain boundary. This will lower the grain boundary mobility and also inhibit the grain growth. This article describes the determination of the sintering mechanism and grain growth kinetics of MgO-doped Al2O3 sintered at 1773K. A linear shrinkage model was used for determining the sintering mechanism. Average grain size of ceramic samples was determined from their x-ray diffraction data, by implementing the statistical fluctuation method (SFM). The results showed that the sintering mechanism for samples with 0.1 wt% and 0.3 wt% MgO was dominated by surface diffusion, while for 0.5 wt% and 1.0 wt% MgO dominated by grain boundary diffusion. The average grain size for pure Al2O3 sample, sintered for 2 hours, was 21.1μm. For the same sintering time, MgO doping of 0.1 wt% to 1.0 wt% reduced the average grain size down to 12 to 10μm, respectively. Keywords : MgO-doped Al2O3, linear shrinkage model, statistical fluctuation method, sintering, grain growth * Materials Engineering Study Program, Department of Mechanical Engineering, Institute of Technology Bandung. Ganesha 10, Bandung, Indonesia, Ph/Fax (62) 22 2508144 contact e-mail: [email protected] ** Department of Mining Engineering, Institute of Technology Bandung *** Department of Chemistry, Institute of Technology Bandung I. INTRODUCTION wt%) allows Al2O3 ceramics to be sintered to its theoretical density and its translucent state. MgO Since the work of Coble (1961), MgO-doped is also known to eliminate discontinuous grain Al2O3 has been the subject of numerous growth, suppress pore-grain boundary separation investigations. A small addition of MgO (∼0.25 and decrease the average grain growth rate 148 Journal JTM Vol. XII No. 3/2005 (Johnson and Coble, 1978; Franken and Gehring, growth and linear shrinkage can be obtained as 1981; Burke and Prochazka, 1981; Berry and follows: Harmer, 1986; Kingery et al., 1997). 2 ΔLx1 ⎛⎞ = ⎜⎟ .….……. (2) Using secondary ion mass spectrometry (SIMS) LAo 4 ⎝⎠ imaging, Soni et al. (1995) had shown that Mg is segregated along the Al2O3 grain boundaries thus By combining Equations (1) and (2), the suppressing grain growth. The presence of following is obtained, MgAl2O4 as a second phase also contributes to the p suppression of grain growth as it acts as a pinning ⎛⎞ΔL t 2 ⎜⎟= ZT() ………… (3) agent. L Aq ⎝⎠o The key to achieve a pore-free microstructure where p and q are exponent parameters, ΔL is the (translucency) is the prevention of pore-grain linear shrinkage, and Lo is the initial length. The boundary separation. In a single phase solid state values of p, q, and Z(T) for each transport sintering, maximum densification occurs when mechanism are also summarized in Table 1. pores located at grain boundaries are removed by lattice or grain boundary diffusion processes. In the absence of magnesia, pores become entrapped 1.2. Statistical Fluctuation Model (SFM) within the alumina grains as abnormal grain growth takes place during sintering (Handwerker X-ray diffraction technique can be used for et al., 1989). Once residual pores become trapped determining the average grain size of a material. within the grain, they are impossible to be Scherrer (Cullity, 1978) developed an expression removed in a reasonable firing time since the for measuring average grain size which is based lattice transport required is extremely slow. This on line broadening of x-ray diffraction pattern prohibits further densification. peaks. His equation, however, is accurate only for materials with average grain size smaller than The current study focuses on the determination of 1μm. For grain sizes or particles larger than 1 μm, the dominant sintering mechanism and the grain Warren (1960) developed the statistical growth kinetics of MgO-doped Al2O3. The linear fluctuation method. The principle of the model is shrinkage model is used to determine the sintering as follows: in ideal cases of a material with fine mechanism and the grain growth kinetics are grain sizes, the change of orientation and material investigated by measuring the average grain size position relative to the diffractometer does not from x-ray diffraction patterns and by change the total area under the peak in the implementing the statistical fluctuation method diffraction pattern. Conversely, for bigger grain (SFM) developed by Warren (1960). sizes where fewer grains contribute to the peak, the area under the peak will vary depending on the orientation and material position. The SFM 1.1. Two Dimensional Linear Shrinkage Model technique for measuring average grain size was developed based on this variation of the area From previous studies (Swandajani, 1994; under the peak of the pattern. Soepriyanto et al., 1995; Lumanauw, 1996), the neck growth between grains for each transport The mathematical formulation of the approach is mechanism during sintering can be generalized written as follows: 2 into the following mathematical equation: 3jAYYΩ−γ ⎡ ⎤ 3 o ⎣ ⎦ D = ………… (4) n 2 ⎛⎞x t 2πμ2 ⎡⎤Y ….……... (1) ⎣⎦ ⎜⎟= BT() m ⎝⎠AA where D is the average diameter of the grain (assuming a spherical shape), j is the multiplicity where n and m are the exponent parameters, t is factor for a particular plane, μ is the linear the time of sintering, x is the neck radius, and A is absorption coefficient, Y is the total counts, and the sphere radius. Each transport mechanism has Y is the average counts of a peak intensity for a different values of n, m, and B(T) as summarized particular plane. Ao is the beam slit area and Ω is in Table 1. By approximating the value of ρ with the space angle and can be calculated using the 2 x /4A and ΔL with ρ, a relationship between neck following equation: Journal JTM Vol. XII No. 3/2005 149 ωl a high-temperature horizontal-tube resistant .………… (5) =Ω 2 furnace in air atmosphere. Doping concentrations R sin4 θ of 0.1 wt%, 0.3 wt%, 0.5 wt% and 1.0 wt% of where ω and l are the width and the length of the MgO and sintering times of 1, 2, and 3 hours were receiving slit, R is the radius of the diffractometer, used as the experimental parameters for this study. and θ is the Bragg diffraction angle. All the geometrical parameters and the configuration of Linear shrinkage of the fired ceramics was the diffractometer are shown in Figure 1. The measured using vernier-calipers and micrometers. detailed derivation of the statistical fluctuation The density measurements were conducted using equations has been described elsewhere (Warren, a water-immersion technique. The linear 1960; Rhamdhani, 2000; Di Nunzio and shrinkage model was used to determine the Abbruzzese, 1992; Ginting et al., 1997). sintering mechanism. The investigation of the grain growth kinetics was carried out by The feasibility of this technique had been measuring the average grain size from x-ray analyzed by many investigators and acceptable diffraction data by implementing the SFM. error measurements were obtained, i.e. 10-30%. The technique was successfully implemented in measuring the average grain size of KCl (Warren, 1960), 0.01wt%C-steel (Rhamdhani, 2000) and III. RESULTS AND DISCUSSION Zircalloy (Ginting et al., 1997). However, to the authors’ knowledge there has not been any work 3.1. Sintering Mechanism to evaluate this method for measuring grain size of ceramic sample. The effect of MgO doping on the linear shrinkage and the bulk density is shown in Figure 3. To some extent, the greater the amount of MgO II. EXPERIMENTAL PROCEDURE added, the greater the linear shrinkage and as a result, the density is greater as well. In the range Alumina ceramic samples were prepared from of experimental parameters and conditions pure Al O (99.9%) and MgO (99.9%) powders 2 3 studied, the theoretical density of alumina ceramic obtained from Merck. Calcinations of Al O at 2 3 could not be achieved. Longer sintering time, 1373 K were conducted to transform any phases higher sintering temperature, as well as more present in the powders into the alpha phase, i.e. controlled atmosphere are required for complete γ ,,θκ→ α. X-ray diffraction analyses were densification. conducted to confirm this transformation. The effect of MgO doping to densification can be By sieving, the mean particle size of the starting easily understood. In the sintering process, both alumina powder was determined to be 55 μm. densification and grain growth are in a This value, however, represents a mean competition. The densification process is limited agglomerated-particle size. Figure 2(a) shows a if mass transport occurs for grain growth.
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