Preparation and Characterization of Undoped and Chromia-Doped Porous Alumina Using Different-Sized Particles

Highly porous alumina (Al 2 O 3 ) was prepared by sintering of Al 2 O 3 powder using ammonia bicarbonate (NH 4 HCO 3 ) as a pore-forming agent and chromia (Cr 2 O 3 ) as a sintering additive. In order to investigate the inuence of particle shape and size on the characterization of sintered porous Al 2 O 3 , the starting Al 2 O 3 powders included commercial disk micro-sized powder and synthesized spherical nanopowder. The nanoscale Al 2 O 3 powder was produced via combustion synthesis route. At the optimal pore-forming agent concentration, the porous Al 2 O 3 sintered by nanoparticles had a smaller pore size and a lower total porosity than the one prepared by microparticles. The differences of open porosity and closed porosity between porous Al 2 O 3 synthesized by micro and nano-scale powders with and without Cr 2 O 3 dopant were also discussed. In addition, the compressive strength of porous Al 2 O 3 achieved by nano-sized powders, especially with Cr 2 O 3 dopant, had a higher value in comparison with the one prepared by micro-sized powders.

The challenges in the fabrication of macroporous ceramics are to tailor pore structure and mechanical strength. There are two typical types of pores, i.e. open pores and closed pores. While the open pores connect to the outside of the materials, the closed pores are individual and inaccessible. Porous materials used as lters and carriers require a highly open pore content, whereas those materials employed in sonic and thermal insulators require a highly closed pore content [18]. The porous fraction must be controlled in relation to the mechanical strength, since these properties are generally inversely related. One of the common strategies enhancing the strength of the highly porous Al 2 O 3 is to add sintering additives including chromia (Cr 2 O 3 ), titania (TiO 2 ), magnesia (MgO), calcium oxide (CaO), etc. [19][20][21]. With a certain amount, these sintering additives reacts with Al 2 O 3 to create a solid solution or an intermediate phase on Al 2 O 3 particle surface, thus improves the sintering ability of Al 2 O 3 particles. Further, these additives can enhance the strength of bulk materials via dispersion strengthening or solid solution strengthening. Additionally, the mechanical strength of highly porous Al 2 O 3 has been tailored using ultra ne initial powders [22][23][24]. It is well documented that nanoparticles lead to the great densi cation of bulk materials due to their high speci c surface area and induce excellent strength owing to the Hall-Petch strengthening.
For processing highly macroporous ceramics, sintering is an effective method combining with replica, sacri cial template, or direct foaming techniques [6,25]. The sacri cial template method has been widely used due to its low-cost and simple process steps. This technique basically includes a dispersion of a sacri cial phase throughout the ceramic precursors, subsequently, compaction to form a green body, and nally decomposition of sacri cial agents to obtain the pores [25,26]. The pore formers are commonly organic agents or salts such as wax spheres, naphthalene particles, poly methyl methacrylate, starch, NaCl, and ammonia bicarbonate (NH 4 HCO 3 ) [27][28][29][30]. The decomposition of pore formers may leave the undesired residual impurities in the nal product, hence the selection of pore-forming agents and the decomposition process should be carefully controlled [26,31]. Additionally, most of the pores achieved in the sacri cial template are open pores. Mohanta et.al [32] stated that highly porous alumina could be produced by mixing Al 2 O 3 powder with rice husk as a sacri cial agent, followed by uniaxial pressing to obtain the green compacts, and nally sintering at 1700 o C for 2h. When the content of rice husk increased, the open porosity proportionally rose with the increase of total porosity. The total porosity could reach the maximum values of over 65% at 40% concentration of 75-180 µm-sized rice husk, however, the open porosity was approximately equal to total porosity, i.e. up to 65%. At the total porosity of 65%, the porous materials exhibited a compressive strength of 9.18 MPa. In order to attain the closed pores, it has been reported that a slurry or solution of ceramic precursors and sacri cial agents should be formed [26,[33][34][35][36]. Porous alumina with high closed porosity was fabricated by gel-casting of Al 2 O 3 powder and polyethylene, subsequently sintering at 1400 o C for 1h [37]. The closed porosity of bulk samples could reach a maximum of 43% when the total porosity was 62%. At the total porosity over 70%, the porous materials possessed a low compressive strength of 3.7 MPa.
In previous researches, porous Al 2 O 3 has been successfully prepared by sintering of micro-sized Al 2 O 3 powder using NH 4 HCO 3 as the pore former [38,39]. The porosity of porous Al 2 O 3 could be controlled by adjusting the concentration of pore-forming agents, and most of pore structure was the open pores. Sintering additives including Cr 2 O 3 and TiO 2 could signi cantly enhance the mechanical properties of porous Al 2 O 3 . However, the mechanical strength of highly porous Al 2 O 3 samples prepared with sintering additives was poor. Therefore, the objective of this study is to investigate the porous structure and mechanical strength of a highly porous Al 2 O 3 prepared from undoped and Cr 2 O 3 -doped Al 2 O 3 nanopowders. It was expected that nanoparticles could promote sinterability and enhance the mechanical strength of porous samples. Additionally, the content of open and closed porosities could be adjusted by initial powders and concentration of pore former without the usage of ceramic suspension.

Materials And Method
The initial Al 2 O 3 powders without and with Cr 2 O 3 dopant with different particle shape and sizes were prepared in two following routes.
Route A: To attain the micro-Al 2 O 3 and micro-Al 2 O 3 /Cr powders, the starting materials were commercial Al 2 O 3 and Cr 2 O 3 powders (Sigma-Aldrich, Inc., Germany) with the purity of 99.5% and 99.9%, respectively.
The powder mixture of 99.5 wt.% Al 2 O 3 and 0.5 wt.% Cr 2 O 3 was ball-milled for 24h using Al 2 O 3 balls with ball powder mass ratios of 20/1 in a highly pure ethanol solution and then dried in a furnace at 120 ο C for 24h.
Route B: The nano-Al 2 O 3 and nano-Al 2 O 3 /Cr powders were synthesized via solution combustion method.
The starting materials were Al(NO 3 ) 3 .9H 2 O (99.99%, Sigma Aldrich, Germany) and Cr(NO 3 ) 3 .6H 2 O (99.99%, Sigma Aldrich, Germany) as oxidizers and urea (CH 4 N 2 O, 99%, Sigma Aldrich, Germany) as a fuel. Each precursor was stoichiometrically balanced and then dissolved in distilled water. The aqueous solution was preheated at 500 ο C in an electric resistance furnace (Linn HT1300, Germany). The combustion reaction occurred according to the reactions (1) and (2) to form voluminous products. The combustion-synthesized product was de-agglomerated for 24h in a highly pure ethanol solution using Al 2 O 3 balls with ball-powder mass ratios of 20/1. The milled powder was dried at 120 ο C for 24h and then calcined at 1100 ο C for 2h.
The powders obtained through two routes were individually mixed with NH 4 HCO 3 content of 30 to 90 vol% by drying ball-mixing for 3h. The green bodies were formed by uniaxial pressing from powder mixture under a pressure of 300 MPa. The green pellets were annealed at 200 ο C for 2h to eliminate the pore former according to the reaction (3) and then at 500 o C for 2h to remove the binders. Finally, the pellets were sintered in an electrical resistance furnace (HT1600, Linn, Germany) at 1550 ο C for 2h in an argon atmosphere.
The total porosity, open porosity and closed porosity of sintered pellets were measured using the Archimedes principle. The phase identi cation was implemented by X-ray diffraction (XRD, D5000 Siemens, Germany). The morphology of fractured surfaces was observed using the eld emission scanning electron microscopy (FESEM, Hitachi S4800, Japan). The compressive strength was tested 10 times by a compressive strength tester (MTS 809, USA) as same as the standard compressive testing method speci ed in JIS-R 1608-2003 standard. The tested sample size was φ5mm x 12.5mm, and the load was applied gradually at a rate of 0.5mm/min.

Results And Discussion
The combustion-synthesized and commercial alumina powders without and with dopants were denoted as nano-Al 2 O 3 , nano-Al 2 O 3 /Cr, micro-Al 2 O 3 , and micro-Al 2 O 3 /Cr, respectively in the following part. XRD patterns of nano-Al 2 O 3 and nano-Al 2 O 3 /Cr powders before and after annealing at 1100 ο C for 2h are depicted in Fig. 1. Most re ections of α-Al 2 O 3 phase as being given in the ICDD 00-046-1212 le were identi ed in combustion-synthesized powders. Although the phase transformation from γ-Al 2 O 3 to α-Al 2 O 3 occurs at above 1100 ο C [40,41], the combustion reaction could produce the α-Al 2 O 3 powder at the low pre-heat temperature of 500 ο C. The exothermicity of the reactions from the gaseous decomposition of reactants generated a large heat amount, thus a high temperature was achieved within a short duration. The ame temperature of the combustion reaction between Al(NO 3 ) 3 and CH 4 N 2 O was observed and calculated at 1550 ο C and 1427 ο C, respectively [42]. Both observed and calculated ame temperatures were much higher than the temperature of α-Al 2   samples revealed a higher grain growth than that of undoped ones, as the presence of solid solution (Al,Cr) 2 O 3 on Al 2 O 3 particle surfaces enhanced the bonding ability at the grain boundary. samples as a function of pore former concentrations. The total porosity of samples sintered from doped and/or nano-sized powders without pore former and with a xed pore former concentration was much lower than that of undoped and/or micro-sized candidates (Fig. 4(a)). The micro-Al 2 O 3 and micro- When the pore former amount increased, the closed porosity tended to increase, then reach the maxima at the pore former concentration of 30% for micro-Al 2 O 3 /Cr sample, and 50% for nano-Al 2 O 3 ones, and subsequently decrease ( Fig. 4(b)). The highest closed porosity in micro-Al 2 O 3 /Cr was much lower than both nano-Al 2 O 3 and nano-Al 2 O 3 /Cr, and the maximum closed porosity in nano-Al 2 O 3 /Cr was higher than nano-Al 2 O 3 . The closed porosity as a percentage of total porosity was 24%, 48%, and 77% corresponding  used. This unusual phenomenon may be caused by the NH 4 HCO 3 particle shape, the agglomeration of NH 4 HCO 3 particles during the powder mixing processes, the pressure increasing during pore former removal, or less shrinkage during sintering processes of the micro-sized powders. However, the pore size was changed with different size of Al 2 O 3 powders. In the case of 50% pore former, the pore size relating to nano-Al 2 O 3 and nano-Al 2 O 3 /Cr was much smaller than that of micro-Al 2 O 3 /Cr due to the high densi cation of nanoparticles. Pore quantity in nano-Al 2 O 3 and nano-Al 2 O 3 /Cr was mainly closed pores, while most of the pores in micro-Al 2 O 3 /Cr were open pores. This agreed with the porosity data as mentioned before (Fig. 4b). When the amount of pore formers increased up to the maximum values, i.e. 70% for micro-Al 2 O 3 /Cr, 90% for nano-Al 2 O 3 and nano-Al 2 O 3 /Cr, the large volume of NH 4 HCO 3 particles caused that pores were easier to connect to each other. Hence, the pore shape could not be observed in micro-Al 2 O 3 /Cr and nano-Al 2 O 3 cases at the maxima of pore forming agents, while pores in nano-Al 2 O 3 /Cr could maintain a certain shape.
The microstructure of NH 4 HCO 3 pore-forming agents was observed by SEM with different magni cation ( Fig. 6(a),(b)). The particles of NH 4 HCO 3 had an irregular shape, hence the pore shape of porous samples was not also apparent. In high magni cation, it is observed that the NH 4 HCO 3 particle was porous, this facilitated the complete decomposition process. The particle size of NH 4    Both nanoparticles and Cr 2 O 3 dopant promote the densi cation of pore walls, hence aid in closing the pores in porous alumina. Therefore, porous ceramics prepared by nanoparticles possessed the smaller pore size and the lower total porosity; and could further exhibit the higher maximum closed porosity at the higher pore former concentration than ones sintered by the microparticles. The identical pore sizes and shapes between nano-Al 2 O 3 and nano-Al 2 O 3 /Cr were attributed to the good consolidation of pore walls; thus, the pore size represented the particle size of pore-forming agents. However, the existence of Cr 2 O 3 caused the increase in maximum closed porosity and maintained a certain pore shape in porous alumina having over 80% total porosity. Additionally, the nanoparticles induce the grain boundary strengthening in sintered bulk materials, while the Cr 2 O 3 dopant enhances the strength of materials via the solid solution strengthening. As a result, the alumina samples prepared by Cr 2 O 3 -doped nanoparticles had the lower total porosity and the higher compressive strength than the other ones at a low pore former concentration. Nevertheless, Cr 2 O 3 dopant did not have a signi cant effect on compressive strength at high total porosity, i.e. 80%.

Conclusion
The initial particle shape and size had an in uence on the sintering ability, pore structures, and mechanical properties of the porous alumina fabricated through sintering combined with the sacri cial template method. Nanoparticles and Cr 2 O 3 dopant lead to a greater sintering rate and densi cation.
Consequently, the porous alumina prepared from undoped and/or micro-sized powders possessed higher porosity than ones achieved from Cr 2 O 3 -doped and/or nano-sized powders at a xed pore former concentration. The closed porosity varied with different starting Al 2 O 3 powders and reached the maximum up to 30% for nano-Al 2 O 3 /Cr at 50% pore former. The porous alumina prepared from the nanoparticles had pore sizes smaller than those obtained from the micro-Al 2 O 3 powders. In addition, nanoparticles and Cr 2 O 3 dopant enhanced the compressive strength of porous Al 2 O 3 samples. When the total porosity was over 80%, the compressive strength of highly porous alumina achieved by Cr 2 O 3 -doped ne initial powders could reach 15.2 MPa. The obtained results suggested that the proposed process could be e ciently used to tailor the pore structure and mechanical properties in porous Al 2 O 3 ceramics by changing the initial powders as well as the pore former quantity.