INTERACTION BETWEEN FINE MGAL 2 O 4 SPINEL AND Α-AL 2 O 3 POWDERS AT HIGH TEMPERATURES

Transcription

1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 8, August 2017, pp , Article ID: IJMET_08_08_189 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed INTERACTION BETWEEN FINE MGAL 2 O 4 SPINEL AND Α-AL 2 O 3 POWDERS AT HIGH TEMPERATURES Soumen Pal Department of Manufacturing Engineering, School of Mechanical Engineering, VIT University, Vellore, Tamil Nadu, India ABSTRACT The co precipitated spinel forming precursor MAH and high purity α-al 2 O 3 have been used to prepare alumina-spinel admixtures accompanied by attrition milling to ascertain the spinel-alumina interactions at high temperatures. Pellets sintered at different temperatures have been characterized with regard to B.D., %A.P., %ρ th, %closed pores, etc. Microstructural study has been conducted by SEM. Positive effect of MAH addition ranges from to 1.0 wt% spinel in alumina, and > 99% ρ th could be attained at 1650 C, 2h. Solid solution interaction of submicron to nano size spinel-alumina powders occurs during sintering and some α-al 2 O 3 get accommodated in the spinel as solid solution. Keywords: MgAl 2 O 4 spinel, α-al 2 O 3, solid solution, sintering, interaction, grain growth Cite this Article: Soumen Pal, Interaction between fine MgAl2O4 spinel and α- Al2O3 powders at high temperatures, International Journal of Mechanical Engineering and Technology 8(8), 2017, pp INTRODUCTION Magnesium aluminate spinel (MgAl 2 O 4 ) is a unique ceramic material in the MgO Al 2 O 3 binary system. MgAl 2 O 4 can accommodate huge amount of α-al 2 O 3 at high temperatures in the form of MgAl 2 O 4 α-al 2 O 3 solid solution and this fact is revealed by the MgO Al 2 O 3 binary phase diagram. During cooling, α-al 2 O 3 separates out of spinel solid solution and distinct MgAl 2 O 4 and α-al 2 O 3 phases are confirmed by XRD studies at room temperature. The work of Coble on evidence of precipitation of α-al 2 O 3 (in the form of lath) from spinel solid solution during cooling was reported by Kingery in his book Introduction to Ceramics 1. This lath type α-al 2 O 3 precipitation has again been described by Kingery (with the help of figures from the work of Bansal and Heuer) in the later edition 2. Furthermore, it may be mentioned here that development of microstructure of alumina-spinel interaction at 1600 C was investigated by Pal 3. He used submicron ( µm) alumina powder and chemically pure coarser agglomerated spinel powder prepared by co precipitation technique 4

2 Interaction between fine MgAl2O4 spinel and α-al2o3 powders at high temperatures and reported solution of needle like α-al 2 O 3 during cooling the alumina-spinel composites containing 15 to 25 wt% spinel. In the previous subsection (i.e. subsection 3.2.1) it has been indicated that such type of precipitation of α-al 2 O 3 is not very prominent when aluminaspinel composites made of coarser alumina and coarser spinel powders were cooled from 1600 C. An important fact to note is that commercial alumina enriched dense spinel (wt% Al 2 O 3 ~90 and wt% MgO ~9-10) aggregates 5 has been available in the market for refractory applications during the past few decades. Characterization of this material by XRD and microstructural analysis reveals 5 that it contains spinel (major amount) and α-al 2 O 3 (minor amount), and both phases form coherent dense microstructure (angular spinel and alumina laths). This type of specific microstructure and phase composition has been largely due to very fast cooling during its production at high temperature. Shimada et al 6 reported that spinel and α-al 2 O 3 powders form spinel-alumina solid solution having compositions MgO-nAl 2 O 3 (1 n<3). They prepared different spinel-alumina composites with n = 1, 1.8, 2.0, 2.2, 2.5 and 3.0 respectively. Pellets were pressed isostatically at MPa and sintered at 1500 and 1550 C for 3 16 h. All the sintered pellets, except the one having n = 1, contained both spinel and α-al 2 O 3 phases. The pellet with n = 1 consisted only spinel phase. However, only MgAl 2 O 4 α-al 2 O 3 solid solution phase was obtained by post-hiping of these sintered pellets at 1750 C and 150 MPa for one hour. For 1 n 2.5, the pellets showed 90% in-line optical transmittance at µm wave length. In the present work, coprecipitated spinel forming precursor MAH and high purity α- Al 2 O 3 have been used to prepare alumina-spinel admixtures to a wide range of compositions accompanied by attrition milling to ascertain the spinel-alumina interactions at high temperatures. Pellets sintered at different temperatures have been characterized with regard to B.D., %A.P., %ρ th, %closed pores, etc. Microstructural study has been conducted by SEM. 2. EXPERIMENTAL 2.1. Materials Chemical analysis of the alumina powder reveals less impurities (Al 2 O wt%, Na 2 O wt%, Fe 2 O wt%, CaO+MgO wt%, SiO wt%). A total of 9 batch compositions were considered (Table 1) with 100 wt% α-al 2 O 3 as batch AS-1 and 90 wt% α- Al 2 O wt% spinel as batch AS-9. All the batches were attrition milled (500 RPM) in an attritor (Attritor Model 01 HD/ 01 HDDM, Union Process Ltd., USA) for 4h with isopropyl alcohol (liquid media) and 5mm diameter alumina balls (grinding media) in stabilized toughened cubic zirconia pot to avoid contamination. For the batches AS-2 to AS-9, the 4h attired and 110 C dried magnesium aluminate hydrate (MAH) powder was used for wt% spinel addition [the MAH cake at room temperature was dried at 110 C until constant weight and agated, and then it was attired for 4h]. Alumina powder was added in the batches considering the fact that it has 0.84 wt% L.O.I at 1200 C, 2h. In the batches AS-2 to AS-9, wt% spinel was added considering the fact that the 110 C dried attired MAH powder has L.O.I of wt% at 1200 C, 2h Particle size distribution and green body making Particle size distribution (Table 2) of 110 C dried batch AS-1 was determined with Nano- Sizer (ZetaSizer Nano-S, Model-Zen1600, Malvern, USA) by laser beam technique using sodium hexameta phosphate as dispersing agent. The dried (110 C) powders of all the batches were lightly a gated and then considered as starting materials for green body preparation. For all batch compositions (i.e. batches AS-1 to AS-9) thickness of the green pellets was maintained at ~6 mm. For each composition, 10 green pellets were prepared by a editor@iaeme.com

3 Soumen Pal Batch Sl. No. Wt. % Spinel unidirectional hydraulic press at a forming pressure 1550 Kgs/cm 2. Average green B.D. of the pellets for each composition was measured from mass and dimension of pellets [mass was determined in an analytical balance of accuracy gm and dimension by digital slide calipers of accuracy cm]. Further, average green B.D. on loss free basis for each composition was determined, and the values of green densities (normal and loss free basis) have been incorporated in Table 1. Wt. % α- Al 2O 3 Table 1 Batch composition details along with green B.D. (loss free basis). Tr. Sp. Gr (TSG) Gr. BD (gm/cm 3 ) from Weight & dimen. wt. (gm) of spinel in batch wt. (gm) of alumina in batch at 110 C Total wt. (gm) of the batch at 110C Wt.% Loss Spinel Alumina Total loss Gr. BD (gm/c m 3 ) (loss free basis) %ρ th for Gr. B.D. (loss free basis) AS AS AS AS AS AS AS AS AS Note: The word spinel in columns 6 and 9 means spinel forming precursor and due to spinel forming precursor respectively. The Word Alumina In Column 10 Means Alumina Dried At 110 C Table 2 Particle size distribution of 110C dried AS-1 batch powder Size (dia. in nm): Mean Volume (%): Measurement of physical properties of sintered bodies Two pellets of each composition were fired at each of the sintering temperatures 1450, 1500, 1550, 1600 and 1650 C with same heating schedule [from room temperature (30 C) to 1200 C at 7 C/min, and from 1200 C to the respective sintering temperature at 5 C/min]. A dwell time of 2h was kept at each of the sintering temperatures. Furnace cooling was followed up to 800 C from the sintering temperatures, and after 800 C the furnace was turned off [since after ~800 C, cooling rate will have negligible effect on the development microstructure of sintered pellets]. Electrical muffle furnace [Naskar & Company, India] with MoSi 2 heating element was used for these sintering studies. Bulk density (B.D.) and % apparent porosity (%A.P.) of the sintered pellets were measured by boiling method using deionized water. For measurement of true specific gravity (TSG), pellets of the batch AS-1 and 1200 C calcined MAH 4 hr attrited powder were fired at 1600 C, 2h and agated (-300 B.S. sieve). Then TSG was measured by pycnometric method. TSG of the rest of the compositions were calculated using the additive principle. Due to experimental constraints, the TSG of the batches AS-2 to AS-9 were not determined by pyconometric method. It is assumed that the difference in TSG values between additive principle and actual determination by pyconometric method would be less.

4 Interaction between fine MgAl2O4 spinel and α-al2o3 powders at high temperatures 2.4. SEM studies Majority of the selected sintered pellets were viewed in scanning electron microscope (QUANTA 200 MK2 with EDS attachment, FEI, Netherlands). Only a few of the pellets were observed in another electron microscope (JEOL, Model JXA-6400, Japan), depending on availability. Grain/pore morphology, grain size and phase distribution (spinel-alumina) were looked into in the pellets. With the help of EDS (attached with SEM) studies, compositional analysis of a few grains, grain boundaries and pore regions was determined. 3. RESULTS AND DISCUSSION 3.1. Physical Properties Sintering behaviour of attrited alumina powders in presence of spinel forming precursor (MAH) has been studied in terms of B.D., %A.P., %ρ th and %C.P. of the sintered pellets. Results have been represented by two types of co-relation physical properties versus composition at each sintering temperature, and physical properties versus sintering temperature at each composition. Fig. 1(a, b, c, d) depicts the effect of addition of spinel on sintered properties of all batches. Incorporation of spinel has impact on densification and other related properties. It is to be noted that addition of dried powder up to 10 wt% spinel compositions causes considerable amount of weight loss and affects the sintered properties (for instance lower %ρth and higher %A.P. can be seen for 5 and 10 wt% spinel addition compositions). From Fig. 1(a, b, c, d), it may be interpreted that effect of spinel addition has marginal positive effect on sintering at 1450 C compared to other sintering temperatures. But 1500 C onwards, significant positive effect in B.D. and %ρ th (Figs. 1a and 1c) of spinel addition can be seen in the composition range to 2.0 wt% spinel compositions. We observe that there is a considerable reduction of values of sintered properties (B.D. and %ρ th ) and increase in %A.P. (Fig. 1b) values from 2 to 5 wt% spinel addition. However, from 5 to 10 wt% spinel addition, this change does not occur appreciably. Figure 1 (a): Effect of MgAl 2 O 4 addition on B.D. of all batches editor@iaeme.com

5 Soumen Pal Figure 1 (b) Effect of MgAl 2 O 4 addition on %A.P. of all batches. Figure 1 (c) Effect of MgAl 2 O 4 addition on %ρ th of all batches. Figure 1 (d): Effect of MgAl 2 O 4 addition on %C.P. of all batches

6 Interaction between fine MgAl2O4 spinel and α-al2o3 powders at high temperatures Fig. 1d represents the effect of spinel addition on %C.P. change at different sintering temperatures. Obviously, at 1450 C, extent of sintering is less compared to other sintering temperatures. Therefore at this temperature, variation of %C.P. is not that significant (values range from to 1.630). Another important observation is that, sintered pellets contain higher %C.P. at 1550 C. Existence of more closed pores is probably detrimental for achieving higher densification and fine grain microstructure. Fig. 2(a, b, c, d) incorporates the effect of heat treatment temperatures on sintering behaviour of all batches. As expected, increase in sintering temperature has positive effect of B.D. and %ρ th. Beyond 2 wt% spinel addition, the sintered properties get affected and overall the results are not positive. Unexpected %C.P. values in the C range and also higher values beyond 1 2 wt% spinel addition have been observed. Figure 2 (a) Effect of sintering temperatures on B.D. for different MgAl 2 O 4 additions. Figure 2 (b) Effect of sintering temperatures on %A.P. for different MgAl 2 O 4 additions editor@iaeme.com

7 Soumen Pal Figure 2 (c) Effect of sintering temperatures on %ρ th for different MgAl 2 O 4 additions. Figure 2 (d) Effect of sintering temperatures on %C.P. for different MgAl 2 O 4 additions SEM Investigation Existence of spinel phase (secondary phase) probably as alumina enriched spinel has not been prominent in the SEM images of the batches AS-2 to AS-4. Microstructures for the pellets AS-2 to AS-4 are more or less similar at 1650 C (2h) and the same has been shown in Fig. 3(a). Representative closed pores in the pellets can be seen in Fig. 3(b) which shows the closed pores in AS-3 pellet at 1550 C, 2h. In Fig. 3(b), the black arrows depict intra-granular closed pores and the white arrow indicates inter-granular pore. SEM analysis along with sintered properties suggests that addition up to 1 wt% spinel has positive effect in sintering of α-al 2 O 3. Microstructures of samples containing 2 to 10 wt% spinel have been depicted in Fig. 4(a, b, c, d), and presence of some independent spinel phases in them can be observed. Also photographs from 3 wt% spinel addition batch compositions in Fig. 4 [i.e. Figs. 4b, 4c and 4d] reveal slightly less dense microstructure. In Fig. 4, the white arrows indicate some of the spinel phases (black arrows show the alumina phase). Spectrum of EDS analysis of a few

8 Interaction between fine MgAl2O4 spinel and α-al2o3 powders at high temperatures representative spinel and alumina phases have been given to suggest their presence. Figs. 4(b) and 4(d) were recorded in the JEOL machine. Figure 3 (a) Microstructure of batch AS-2 pellet (1650 C, 2h) showing similar nature in the batches AS-1 to AS-4. Figure 3 (b) SEM image showing closed pores in AS-3 pellet at 1550 C, 2h. Figure 4 (a)sem photograph (1650 C, 2h) depicting the presence of spinel phase in pellet containing wt% spinel editor@iaeme.com

9 Soumen Pal Figure 4 (b) SEM image (1650 C, 2h) showing presence of spinel phase in pellet containing 3 wt% spinel. Figure 4 (c) SEM image (1650 C, 2h) showing presence of spinel phase in pellet containing 5 wt% spinel. Figure 4 (d) SEM image (1650 C, 2h) showing significant presence of spinel phase in pellet containing 10 wt% spinel Another observation is that inside the pores present on sample surface, tiny spinel grains could be seen on the surface of the alumina grains. Presence of this type of spinel on the internal surface of the pores was also reported by Sung et al 7. Available microstructures [Figs. 5a and 5b] have been shown to suggest this viewpoint. In Fig. 5, the white arrows indicate some of the spinel grains and spectrum of EDS analysis of representative spinel grain has

10 Interaction between fine MgAl2O4 spinel and α-al2o3 powders at high temperatures been provided. With the present experimental conditions, a maximum of % ρ th could be achieved in the alumina side samples. Figure 5 (a) SEM image showing presence of spinel inside the surface of pores in pellet containing 3 wt% spinel at 1550 C, 2h. Figure 5 (b) SEM image showing presence of spinel inside the surface of pores in pellet containing 4 wt% spinel at 1550 C, 2h. 4. CONCLUSIONS a) Co-grinding of the batch compositions by attrition milling containing alumina and spinel causes intimate mixing between them, and very fine powders are obtained. The green pellets prepared contain soft agglomerates. Further, it is expected that the green bodies contain some intergranular pores of dimensions larger than that of individual particles. These larger pores contribute to some abnormal grain growth during sintering. b) Uniform dispersion of the spinel forming precursor MAH in alumina contributes to the densification of alumina. Positive effect of addition ranges from to 1.0 wt% spinel in alumina, and > 99% ρ th could be attained at 1650 C, 2h. c) Solid solution interaction of submicron to nano size spinel-alumina powders occurs during sintering and some α-al 2 O 3 get accommodated in the spinel as solid solution. However, precipitation of α-al 2 O 3 in the form of laths from spinel solid solution has not been observed in the pellets (where wt% spinel is high). Fineness of the powders is editor@iaeme.com

11 Soumen Pal probably the main cause for absence of lath type precipitation of α-al 2 O 3, and thereby resulting in dense microstructure. REFERENCES [1] W. D. Kingery, Introduction to Ceramics, pp 265, John Wiley and Sons, New York, [2] W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2 nd Edition, pp 436, John Wiley and Sons, New York, [3] P. G. Pal, Ph. D Thesis: Study of the Sintering Kinetics of Alumina in Presence of Magnesio-Aluminate Spinel, Dept. of Chemical Technology, University of Calcutta, [4] S. G. Mukherjee and B. N. Samaddar, Spinel Formation from Coprecipitated Hydroxides of Aluminium and Magnesium, Trans. Ind. Ceram. Soc., 25[1] pp 33-34, [5] S. Zhang and W. E. Lee, Spinel Containing Refractories, pp , Editor Schacht, C., Refractories Handbook, Marcel-Dekker, [6] M. Shimada, T. Endo, T. Saito, and T. Sato, Fabrication of Transparent Spinel Polycrystalline Materials, Mater. Lett., 28, pp , [7] C. Sung, G. C. Wei, K. J. Ostreicher and W. H. Rhodes, Segregation of Magnesium to the Internal Surface of Residual Pores in Translucent Polycrystalline Alumina, J. Am. Ceram. Soc., 75[7], pp , [8] Faisal Naseer and Dr. Ajeet Kumar Rai, Study of Heat Transfer In A Corrugated Plate Heat Exchanger Using Al2o3 Microparticles. International Journal of Mechanical Engineering and Technology, 7(4), 2016, pp [9] T.Teressa, A.ChanduSagar, B.SaiNithin, K.SharanTeja, G.Suresh Reddy, P.Akhil Experimental Investigation on Heat Transfer Enhancement on Plate Heat Exchanger Using Al 2 O 3 International Journal of Mechanical Engineering and Technology,8(5),2017 pp