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Surface Passivation of MgAl2O4 Spinel Powder by Chemisorbing H3PO4 for Easy Aqueous Processing Susana M. Olhero,† Ibram Ganesh,†,‡ Paula M. C. Torres,† and Jose´ M. F. Ferreira*,† Department of Ceramics and Glass Engineering, CICECO, UniVersity of AVeiro, 3810193-AVeiro, Portugal, and Centre for AdVanced Ceramics, International AdVanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad-500 005, A.P., India ReceiVed April 26, 2008. ReVised Manuscript ReceiVed June 1, 2008 A stoichiometric MgAl2O4 spinel (MAS) powder was synthesized by heat treating at 1400 °C for 2 h a compacted mixture of R-Al2O3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H3PO4 and Al(H2PO4)3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.
Introduction Magnesium aluminate (MgAl2O4) spinel (MAS) possesses high melting point (2135 °C), high hardness (16.1 GPa), relatively low density (3.58 g cm-3), excellent transmittance between 0.25 and 5.0 µm wavelengths, high strength (180 MPa) at room and at elevated temperature, relatively low thermal expansion coefficient (∼9 × 10-6 °C1- between 30 and 1400 °C), and hence high thermal shock resistance and high chemical inertness.1–4 Further, it does not react with SiO2 until 1735 °C, with MgO or CaO until 2000 °C, and with Al2O3 until 1925 °C, and, except alkaline earth metals, it can be in contact with all other metals.5 Owing to these properties, it has been employed for various purposes such as optical systems for pressure vessels, bulletproof vehicles, IR window and dome applications,6–15 as * To whom correspondence should be addressed. Telephone: 351-234370242. Fax: 351-234-370204. E-mail:
[email protected]. † University of Aveiro. ‡ International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI). (1) Baudin, C.; Martinez, R.; Pena, P. J. Am. Ceram. Soc. 1995, 78(7), 857– 1862. (2) Peterson, R. C.; Lager, G. A.; Hitterman, R. L. Am. Mineral. 1991, 76(9-10), 1455. (3) Green, K. E.; Hastert, J. L.; Roy, D. W. Proc. SPIE-Int. Soc. Opt. Eng. 1989, A90-34551, 14–74. (4) Sainz, M. A.; Mazzoni, A.; Aglietti, E.; Caballero, A. Proc. UNITESR′95 1995, 387–392. (5) Belding, J. H., Letzgus, E. A. U.S. Patent No. 3 950 504, April 13, 1976. (6) Ji-Guang, L.; Ikegami, T.; Jong-Heum, L.; Mori, T. J. Am. Ceram. Soc. 2000, 83(11), 2866–2868. (7) Patel, P. J.; Gilde, G. A.; Dehmer, P. G.; McCauley, J. W. The AMTIAC Newsletter, Fall 2000. (8) Harris, D. C. Proc. SPIE-Int. Soc. Opt. Eng. 1992, 32. (9) Roy, D. W. Presented to DARPA/ARL, Transparent Armor Workshop, Annapolis, MD, Nov 16, 1998. (10) Wei, G. C. J. Phys. D: Appl. Phys. 2005, 38, 3057–3065. (11) Rozenburg, K.; Reimanis, I. E.; Kleebe, H.-J.; Cook, R. L. J. Am. Ceram. Soc. 2007, 90(7), 2038–2042. (12) Roy, D. W.; Evans, S. H. Proc. SPIE-Int. Soc. Opt. Eng. 1993, 2018. (13) Roy, D. W.; Martin, G. G. Proc. SPIE-Int. Soc. Opt. Eng. 1992, 1760. (14) Bratton, R. J. J. Am. Ceram. Soc. 1974, 57(7), 283–286. (15) Kumar, P.; Sandhage, K. H. Near Net-Shaped Magnesium Aluminate Spinel by the Oxidation of Solid Magnesium-Bearing Precursors. TMS Outstanding Student Paper Contest Winner-1998, Graduate Division, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH.
a humidity sensor,16 as an alternative material to replace the conventional carbon anode in aluminum electrolytic cells,17 and as an effective refractory material for cement rotary kilns and steel ladles.18 Recently, it has also been considered as an attractive matrix material for ceramic-matrix composites because of its good chemical compatibility with alumina, zirconia, and mullite ceramics.10 In spite of these advantages, this material suffers from a problem of volume expansion (∼8%) associated with its phase formation from alumina and magnesia.19 Due to this problem, dense MAS ceramics cannot be prepared following a single-stage reaction sintering process.20 Alternatively, the compacted mixture of raw materials is initially calcined at about 1400 °C to obtain a powder with a spinel content >90%. This spinel powder is then fine ground, compacted, and sintered at >1650 °C to form dense ceramics.20 Due to the involvement of two firing cycles, the cost of production of these ceramics has been increased considerably. Furthermore, this powder also reacts with water because of its basic nature,16 causing the coagulation of its aqueous suspensions, limiting the practical solids loading to about 30 vol %, especially when the deagglomeration process by ball milling is conducted for more than 2 h. Due to these reasons, it is rather difficult to obtain stable and concentrated aqueous suspensions, which are required for near-net shape forming of ceramics following colloidal processing techniques. In fact, nearnet shaping is of great importance because expensive postsintering machining operations can be minimized or even eliminated, reducing the cost of production considerably.21 Besides these (16) Shimizu, Y.; Arai, H.; Seiyama, T. Sens. Actuators 1985, 7, 11–22. (17) Angappan, S.; Berchmans, L. J.; Augustin, C. O. Mater. Lett. 2004, 58, 2283–2289. (18) Ganesh, I.; Bhattacharjee, S.; Saha, B. P.; Johnson, R.; Rajeshwari, K.; Sengupta, R.; Ramanarao, M. V.; Mahajan, Y. R. Ceram. Int. 2002, 28, 245–253. (19) Nakagawa, Z. E.; Enomoto, N.; Yi, I. S.; Asano, K. Proc. UNITECER′95 1995, 379–386. (20) Ganesh, I.; Olhero, S. M.; Rebelo, A. H.; Ferreira, J. M. F. J. Am. Ceram. Soc. 2008, 91(16), 1905–1911. (21) Janney, M. A.; Nunn, S. D.; Walls, C. A.; Omatete, O. O.; Ogle, R. B.; Kirby, G. H.; McMillan, A. D. Gelcasting. In The Handbook of Ceramic Engineering; Rahman, M. N., Ed.; Marcel Dekker: New York, 1998; pp 115.
10.1021/la801300m CCC: $40.75 2008 American Chemical Society Published on Web 08/01/2008
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advantages, aqueous processing offers several environmental and economic benefits apart from the ability to manipulate interparticle forces in aqueous suspensions. Since the fracture in the ceramic materials originates from microstructural imperfections such as pores and inclusions causing poor mechanical reliability, an effective deagglomeration of the powder particles in the suspensions is very essential. However, for the reasons stated above, this target is difficult to achieve in the particular case of MAS powder.22 In view of the above reasons, at present, MAS components for infrared dome and radome applications are being fabricated by hot isostatic pressing (∼1500 °C and 200-400 MPa pressure) followed by extensive machining to obtain the desired final shape.8,9 However, this process is quite expensive due to the involvement of a large amount of postsintering machining operations. Alternatively, the cold isostatic pressing (CIPing) and freeze casting (FC) techniques were also employed for fabricating near-net shape components, which would minimize the cost of production.12–14 The advantages of these processes over the hot-pressing technique are low cost of production due to the involvement of ambient conditions, the use of inexpensive materials (aluminum or steel) for mold fabrication, and minimum postsintering machining operations. Furthermore, these ceramics were found to have minimal residual stresses. However, the product yield of these processes was found to be not encouraging, and the mechanical strength of the green consolidates was poor. Recently, a pressureless molten metal infiltration technique has also been employed for the fabrication of these components with near-net shape.15 In this process, initially, the molten Mg is infiltrated into porous (65-70% dense) Al2O3 preformed at 680-700 °C. After solidification, the Mg-Al2O3-bearing precursors are oxidized in flowing oxygen at 430 °C/40 h or 700 °C/6 h. The mixtures of MgO and Al2O3 thus obtained are further annealed in oxygen at 1200 °C for 15 h to obtain the MAS material, which is then further sintered for 10 h at 1700 °C in flowing Ar to achieve a maximum density of about 92.5% of the theoretical. The linear shrinkage upon firing was only 0.6%. However, these products are not suitable for many of the applications due to their low density. Considering the importance of stable and concentrated aqueous MAS suspensions for fabricating near-net shape ceramics for IR domes and windows for certain strategic applications, we have undertaken a systematic study and developed a surface treatment process to successfully passivate the MAS powder against water reaction. Orthophosphoric acid (H3PO4) and aluminum dihydrogen phosphate [Al(H2PO4)3] were utilized as coating agents.23 A stoichiometric MAS powder synthesized by heat treating at 1400 °C for 2 h a compacted mixture of R-Al2O3 and MgO, crushing, and milling was treated in ethanol solution containing H3PO4 and Al(H2PO4)3. The surface treated powder could be dispersed with the help of tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) to obtain stable aqueous-based suspensions with 41-45 vol % solids loading. All the suspensions were evaluated by rheometric studies, gelcast, dried (>90% relative humidity and >90 °C), and characterized for green strengths to evaluate the suitability of surface modified MAS powder for near-net shaping capability. (22) Kaiqi, L.; Wei, P.; Zhengkun, F.; Yongfeng, L.; Bingjun, W. Key Eng. Mater. 2008, 368-372, 1149–1151. (23) Ganesh, I.; Olhero, S. M.; Arau´jo, A. B.; Correia, M. R.; Sundararajan, G.; Ferreira, J. M. F. Langmuir 2008, 24(10), 5359–5365.
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Experimental Section Synthesis of MAS Powder. A stoichiometric mixture of alumina (CT-3000SG, Alcoa-Chemie GmbH, Ludwigshafen, Germany; average particle size, 1.84 µm; BET SSA, 4.06 m2 g-1) and calcined caustic magnesia (Jose´ M. Vaz Pereira, S.A., Porto, Portugal; average particle/agglomerate size, 5.63 µm; BET SSA, 15.15 m2 g-1) was dispersed in an azeotropic mixture of 60 vol % methyl ethyl ketone (MEK) (Honeywell, Riedel-de Haen, Hanover, Germany) and 40 vol % absolute ethanol (E) (Merck, Darmstadt, Germany) with the help of Hypermer KD1 (a polyester/polyamine copolymer having an estimated MW of about 10 000 g mol-1, Imperial Chemical Industries PLC, London, U.K.) to achieve about 40 vol % solids loading in a suspension. The resultant suspension was deagglomerated for 24 h in a polypropylene bottle using alumina balls by maintaining a 1:3 weight ratio between the powder and balls. The homogenized suspension was separated from the alumina balls and then transferred to a glass beaker, which was placed in a refrigerator (Whirlpool 310 Deluxe, Whirlpool, Madrid, Spain) at -5 °C. The consolidated mass was evacuated at a pressure of 1 × 10-1 Torr using a turbo pump (98.93 L per minute capacity, model 949-9315, Varian DS-102, Torino, Italy) at -5 °C and further dried at about 40 °C in an electric hot-air oven. The gelation mechanism involved in this process was mainly based on the cooling down of dissolved dispersant molecules, inducing in situ gelation (i.e., TIG) and the formation of a rigid network bridging the suspended particles.24 The dried mass was calcined in an electrically heated open-air muffle furnace for 2 h at 1400 °C to obtain a stoichiometric MAS powder.20 The calcined and planetary ball milled (3 h, weight ratio between powder and balls ) 1:3) MAS powder exhibited a BET SSA of 3.98 m2 g-1. Henceforth, this powder is termed as A-MAS powder. Surface Treatment of MAS Powder. In a typical experiment, 245 g of the MAS powder synthesized as described above was suspended in an absolute ethanol to obtain 250 mL of a 30 vol % suspension in a 500 mL volume three-neck round-bottom (RB) flask. The RB flask was fitted with an equalization funnel and valve to pass dry nitrogen gas, and was placed into a thermostatic oil bath (150 mm diameter and 75 mm height, Thermol-100, Biolabs, Hyderabad, India, -50° to +250 °C). In a separate experiment, 2 g of aluminum dihydrogen phosphate, Al(H2PO4)3 (assay g97.0%, Fluka, Seelze, Germany) was digested in 5 mL of hot orthophosphoric acid, H3PO4 (85% assay, AR grade, Qualigens, Mumbai, India).23 This solution was then mixed with 50 mL of ethanol and added drop-by-drop to the above alcohol-based MAS suspension with the help of an equalization funnel. The RB flask was then continuously refluxed at 80 °C for 24 h while passing N2 at the rate of 100 mL min-1. The content of the RB flask was agitated with a magnetic stirrer (5MLHDX, Remi, Hyderabad, India).23 The treated MAS slurry was filtered off and washed with fresh ethanol several times in order to remove the excess of H3PO4 and Al(H2PO4)3. After distillation, the ethanol was reused for washing the treated powder. Henceforth, this treated powder is termed as T-MAS powder. Preparation and Rheological Characterization of High Concentrated Aqueous MAS Suspension. Aqueous suspensions with 41-45 vol % solids were prepared by dispersing the surface treated MAS powder in an aqueous organic premix solution obtained by dissolving 20 wt % methacrylamide (MAM), methylenebisacrylamide (MBAM), and n-vinylpyrrolidinone (NVP) in 3:1:3 ratio in deionized water with the help of 25 wt % aqueous TMAH and Duramax D-3005 employed at the ratios of 35 and 30 µL g-1 powder, respectively. The as-prepared suspensions were deagglomerated for 24 h in a polyethylene bottle containing Teflon balls. The viscosity of the suspension was measured using a rotational rheometer (Bohlin C-VOR Instruments, Worcestershire, U.K.). The measuring configuration adopted was a cone and plate (4°, 40 mm, and gap of 150 µm), and flow measurements were conducted between 0.1 and 800 s-1.23 Consolidation of Suspensions. The MAS suspensions containing 41-45 vol % solids were consolidated by adding the polymerization (24) Xu, X.; Ferreira, J. M. F. J. Am. Ceram. Soc. 2005, 88(3), 593–598.
Surface PassiVation of MgAl2O4 Spinel Powder
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Figure 1. XRD spectra of A-MAS and T-MAS powders.
initiator (10 wt % aqueous solution of ammonium persulfate, APS) and the catalyst (tetramethylethylenediamine, TEMED) at the ratios of 4 and 2 µL g-1 suspension, respectively.21 Afterward, the suspensions were cast into nonporous white petroleum jelly coated split-type aluminum molds (60 mm × 30 mm × 30 mm), which were then allowed to set under ambient conditions until the completion of the gelling process.21 The as-consolidated green bodies were demolded and dried under controlled humidity conditions to avoid cracking and nonuniform shrinkage due to rapid drying. The dried gelcast MAS samples were subjected to flexural strength analysis as per a procedure described elsewhere.25 X-Ray Diffraction (XRD) Analysis. XRD patterns of the asreceived and surface treated powders were recorded on a Bruker (Karlsruhe, Germany) D8 advanced system using a diffracted beam monochromated Cu KR (0.15418 nm) radiation source. The intensity data were collected over a 2θ range of 3-80° with a 0.02° step size and using a counting time of 1 s per point. Crystalline phases were identified by comparison with PDF-4 reference data from the International Centre for Diffraction Data (ICDD).26 Fourier Transform Infrared (FT-IR) Spectra. FT-IR spectra were recorded using a Nicolet 740 FTIR spectrometer at ambient conditions with a normal resolution of 4 cm-1 and averaging 100 spectra. Prior to analysis, thin cylinders (diameter ∼ 1 cm) were prepared by uniaxial pressing (9 ton) a powder mixture of 2 mg of sample powder with 200 mg of KBr.23 Brunauer-Emmett-Teller (BET) Surface Area Measurement. A Gemini Micromeritics BET surface area analyzer (model 2360, Micromeritics, Norcross, GA) was used for specific surface area measurements of the powders. The surface area was measured by nitrogen physisorption at liquid nitrogen temperature (-196 °C) by taking 0.162 nm2 as the area of the cross section of a N2 molecule.23 Zeta-Potential Measurements. The zeta-potentials of powders in 10-3 M KCl aqueous solutions were measured on a zeta meter (Delsa 440 Sx, Coulter, Buckinghamshire, U.K.). Dilute HNO3 and TMAH solutions were used for pH adjustment. Stock suspensions containing 5 wt % ball milled (24 h) powder were prepared without and with different added amounts of TMAH, Duramax D-3005, or their mixtures.25 Coarser particulates and supernatants containing finer particles were separated by centrifugation (model R23, Remi, Mumbai, India) at 3000 rpm for 30 min.
Figure 2. FT-IR spectra of A-MAS and T-MAS powders.
The X-ray diffraction patterns of MAS (A-MAS) obtained by the calcination of a compact stoichiometric mixture of R-Al2O3 and calcined caustic MgO at 1400 °C for 2 h and the A-MAS powder after treating in a solution mixture of ethanol, H3PO4, and Al(H2PO4)3 for 24 h at 80 °C (T-MAS) are presented in Figure 1. It can be seen that both of the powders exhibit XRD peaks only due to stoichiometric MgAl2O4 spinel (ICDD file no.
00-21-1152). No peaks due to the precursor raw materials corundum (ICDD file no. 00-46-1212) and periclase (ICDD file no. 00-45-946) are seen. Interestingly, the phosphoric acid treated powder also did not show any extra XRD peaks other than the those for MAS. These results show that, after calcination at 1400 °C for 2 h, the compact mixture of alumina and magnesia reacted completely, leading to the formation of a single phase MAS powder. Formation of MgAl2O4 by the solid-state reaction of corundum and periclase is explained by the Wagner mechanism.21 The reaction proceeds by counter diffusion of the cations through the product layer, with oxygen ions remaining at the initial sites. To keep the electroneutrality, 3Mg2+ diffuse toward the alumina side and 2Al3+ diffuse toward the magnesia side to form 3 mol of MgAl2O4. Therefore, shorter diffusion paths make the reaction between alumina and magnesia occur faster. Powders with fine median particles size (i.e., 2 µm) can be mixed very intimately to accelerate the reaction. In the present case, although the magnesia raw material has an apparently larger D50 ) 5.63 µm, its specific surface area of 15.15 m2 g-1 indicates that the primary particles are finer and the measured average size is for the agglomerates, which tend to be destroyed during ball milling. These results are in good agreement with the literature reports.18,20 Further, it can also be noted that no crystalline phases are formed during the surface treatment of MAS powder in a reaction mixture of ethanol, H3PO4, and Al(H2PO4)3. This observation is also concurrent with earlier studies.23 Figure 2 shows the FT-IR spectra of A-MAS and T-MAS powders recorded between 400 and 4000 cm-1. Both powders exhibit two major transmittance bands at 509 and 698.85 cm-1, a medium size transmittance band at about 3450.6 cm-1, and a minor transmittance band at 1625.2 cm-1. The A-MAS powder exhibits two additional minor transmittance bands at 2334.6 and 2893.2 cm-1, whereas the T-MAS exhibits a new band at about 1092.5 cm-1. The common transmittance bands shown by both powders at around 3450 cm-1 and at around 1625 cm-1 correspond to broad -OH stretching and H-O-H bending vibrations, respectively.27,28 Water of hydration can be easily distinguished from hydroxyl groups by the presence of the H-O-H bending motion, which also produces a medium band
(25) Olhero, S. M.; Tari, G.; Coimbra, M. A.; Ferreira, J. M. F. J. Eur. Ceram. Soc. 2000, 20, 423–429. (26) Cullity, B. D. Elements of XRD, 2nd ed.; Addison-Wesley: Reading, MA, 1978.
(27) Nyquist, R. A.; Kagel, R. O. The Handbook of Infra-Red (3800-45 cm-1) and Raman Spectra of Inorganic Compounds and Organic Salts; Academic Press Limited: London, U.K., 1996 (a 4 volume set). (28) Phambu, N. Mater. Lett. 2003, 57, 2907.
Results and Discussion
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Figure 4. Schematic representation of the phosphate layer chemisorbed onto the surface of a T-MAS powder particle.
Figure 3. (A) Zeta potential versus pH of the as-synthesized (A-MAS) and the surface treated (T-MAS) MAS powders. (B) Zeta potential of T-MAS powder as a function of added amounts of dispersants.
in the region 1600-1650 cm-1. Free water has a strong and broad absorption band centered in the region 3200-3400 cm-1.27,28 The high surface area of these materials results in rapid adsorption of water from the atmosphere because the FTIR samples were kept and ground in air. The transmittance bands noted at around 698.85 and at 509 cm-1 correspond to the AlO6 groups, which built up the MgAl2O4 spinel and indicate the formation of MgAl2O4 spinels for all the as-synthesized samples.29,30 Pure H3PO4 normally reveals a small transmittance band at 500-550 cm-1, a large transmittance band at 1500-1800 cm-1, and a low intensity band at 2000-3200 cm-1 due to different vibrations of phosphate molecule.27,28 Oliveira et al.31 observed an absorption band at 2366 cm-1 due to the formation of [Al(PO3)3]x upon the reaction between hydroxyl groups of AlN and H3PO4. The absence of two transmittance bands around 2334.6 and 2893.2 cm-1 and the presence of a new transmittance band at 1092.5 cm-1 for the T-MAS powder indirectly suggest that the phosphate coating on the surface of the treated powder is restricting the vibration of some of the functional groups, which might have been blocked with H2PO4- groups. The presence of a band at 1092.5 cm-1 for the T-MAS powder can be attributed to the phosphate coating formed on the surface of the powder. Since the above two characterization techniques (XRD and FT-IR) did not enable us to quantitatively estimate the amount of phosphorus adsorbed onto the surface of MAS particles, both A-MAS and T-MAS powders were also analyzed by energy dispersive X-ray (EDAX) measurements. The observed results (not shown here) showed that both powders contained negligible amounts of P. Interestingly, even the A-MAS powder contained phosphorus as an impurity (0.61%), while the amount of (29) Parmentier, J.; Richard-Plouet, M.; Vilminot, S. Mater. Res. Bull. 1998, 33, 1717. (30) Adak, A. K.; Saha, S. K.; Pramanik, P. J. Mater. Sci. Lett. 1997, 16, 234. (31) Oliveira, M.; Olhero, S. M.; Rocha, J.; Ferreira, J. M. F. J. Colloid Interface Sci. 2003, 261, 456–463.
phosphorus detected in the T-MAS powder was ∼1.21%. The change of zeta (ζ) potential of the A-MAS and T-MAS powders as a function of pH is presented in Figure 3A. It can be seen that the as-synthesized powder exhibits a pHiep in the range of 9-10 (between those of alumina and magnesia),32 whereas the treated one exhibits a pHiep at about 3. This shift in the pHiep confirms the anionic nature of the adsorbed phosphate species, conferring to the T-MAS powder completely different surface chemistry properties. It is generally accepted that the oxide film on the surface of the MAS powder exists in the form of AlOH and MgOH groups. These hydroxyl groups dissociate into water and confer to the particles a surface charge according to reactions 1 and 2:
(MgAl2O4)-M-OH + H+ T M-OH2+ (where M ) Al and Mg) (1) (MgAl2O4)-M-OH + -OH- T M-O- + H2O
(2)
The surface of MAS powder is usually covered with a processdependent alumina (i.e., θ- and γ-Al2O3), a hydrated alumina layer (γ-AlOOH and Al(OH)3), or both and brucite (Mg(OH)2) layers. Such layers are further developed during storage in the presence of humidity because of its basic nature.16 The surface hydroxyl groups play an important role in the formation of a protective layer against water reaction when the MAS powder is treated with H3PO4 and Al(H2PO4)3. The reactions of MAS powder surface with H3PO4 could be expressed as reaction 3:
Al(OH)3 + Mg(OH)2 + H3PO4 + nAl(H2PO4)3 f (n + 1)Al(H2PO4)3 + Mg(H2PO4)2 + 5H2O (3) The results presented in Figure 3A suggest that the A-MAS powder reacts with H+ ions in the acidic medium (reaction 1) and acquires a high density positive charge on its surface responsible for the measured high positive ζ values 30 000 Pa are suitable for casting nearnet-shape components that can be easily removed from the molds.25 Table 1 lists the suspension viscosity values (mPa s) at 230 s-1, gelation time, green density, percentage linear shrinkage upon drying, and green strength of T-MAS bodies consolidated by aqueous gelcasting. For simplicity purposes, different codes are given to samples, with GC standing for gelcasting and numbers 41-45 denoting the solids vol % in the suspensions. A significant increase of the suspension viscosity with increasing solids loading can be observed, reaching a highest value of 1291.5 mPa s at 230 s-1 for the suspension containing 45 vol % solids (GC-45). According to Janney et al.,21 45 vol % solids loading is high for certain powders, such as Ube E10 silicon nitride Starck, B10 silicon carbide, TOSOH TZ zirconias, and Baikowski high purity aluminas. In all these cases, the powders are exceedingly difficult to disperse because of their high specific surface areas. From Table 1, it can also be seen that the green density of consolidates increases with increasing solids loading, which is consistent with a nonliquid removal consolidation route in nonporous molds such as gelcasting.21,24 GC-45 exhibits a green density of 1.599 ( 0.012 g cm-3, which is a bit lower than the value of 1.665 ( 0.043 g cm-3 measured for samples consolidated by dry pressing of freeze-dried granules at 200 MPa. However, compacts with green density values similar to those obtained in the present
work by gelcasting exhibited good properties upon sintering at about 1650 °C.18,20 Table 1 also reveals that the percentage of linear shrinkage associated with consolidation and drying processes decreases with increasing solids loading, as expected.21 Nevertheless, all consolidates formed from suspensions having g43 vol % solids loading exhibit linear drying shrinkage of 14 MPa, with the highest value (15.638 ( 2.564 MPa) corresponding to the GC-45 sample. Such strength values enable green machining to obtain desired dimensions so that expensive postsintering machining operations can be minimized. These results indicate that aqueous gelcasting is a suitable process for fabricating high green strength MAS ceramics. Table 1 further indicates that a 45 vol % solids loading suspension can be transformed into a consolidated part by aqueous gelcasting in less than 5 min. The results presented in Table 1 indicate that stable and concentrated suspensions of stoichiometric MAS suitable for neat-shaping of ceramics by aqueous gelcasting could be prepared by treating the powder in an ethanol solution containing H3PO4 and Al(H2PO4)3 at 80 °C for 24 h.21
Conclusions Stoichiometric MgAl2O4 spinel (MAS) powder could be prepared from a mixture of R-Al2O3 and calcined caustic MgO, compacted by temperature-induced gelcasting (TIG) using an azeotropic mixture of methyl ethyl ketone and ethanol in a 60:40 volume ratio followed by calcination at 1400 °C for 2 h. Orthophosphoric acid and aluminum dihydrogen phosphate were revealed to be effective in passivating the surface of MAS against hydrolysis reactions. The as-treated powder could be used to prepare stable and concentrated (up to 45 vol % solids loading) aqueous suspensions suitable for casting operations using tetramethylammonium hydroxide (TMAH) and commercial Duramax D-3005 as dispersing agents. These suspensions were successfully used for near-net shaping of ceramic parts by gelcasting, a process that enables minimization or even elimination of the expensive postsintering machining operations, therefore decreasing the production costs of ceramic components. Acknowledgment. S.M.O. wishes to thank the Foundation for Science and Technology (FCT) of Portugal for the financial support under Grant SFRH/BPD/27013/2006. I.G. thanks SERCDST (Government of India) for the awarded BOYSCAST fellowship (SR/BY/E-04/06). The financial support of CICECO is also acknowledged. LA801300M
