Direct Cold Sintering of α-Al₂O₃ Ceramics in a Pure Water Medium

1. Introduction

Cold sintering process (CSP) introduced by Randall et al. in 2016 [1] is known as a rapidly developing technique for an express and energy-saving production of ceramic and composite materials, the range of which is expanding annually. Providing materials consolidation and sintering at a tremendously lowered temperature, the CSP was applied to the ceramics and composites manufacturing as an alternative to the high-temperature as well as multistage techniques. The processes accompanying the CSP are commonly interpreted by the dissolution-precipitation mechanism [2]. At the initial stage, the transient liquid acts similar to the temporary technological binder in the uniaxial compaction of powders facilitating the particles rearrangement. Next, on heating, it might partially dissolve the solid particles and transport the substance from the high- to low-stress surface regions. For the moment, materials with more than a hundred compositions have been densified using the CSP including those which are unavailable for the traditional routes because of their thermodynamic incompatibility [3,4,5,6,7].

An upgrade in an energy budget embedded in CSP is highly desirable to produce materials with extremely high sintering temperature, for example, refractories. Several authors reported a success in manufacturing of electrolyte materials such as those based on yttria-stabilized zirconia [8,9,10] and sodium-β-alumina [11], also magnesia ceramics [12] via single CSP or combined with post heat treatment (PHT) 1200-1300 °C. Conventionally sintered above 1400 °C, these oxides were densified up to 83-93% of their theoretical density and preserved a fine-grained microstructure due to CSP at temperatures of about 30% of their melting points. However, such examples remain far from numerous against the background of achievements in CSP.

The most stable α-modification of alumina (α-Al₂O₃) is a well-known refractory material (sintered above 1500 °C) with a wide application range from abrasion to microelectronic and biomedical materials. α-Al₂O₃ is highly appreciated due to a combination of chemical stability, mechanical strength, dielectric properties, biological compatibility, and availability [13,14,15]. In particular, porous α-Al₂O₃ ceramics are of interest for liquid and gas filters, supports for catalysts and metal-organic frameworks (MOFs) [16,17,18,19]. Ohji et al. [20] in their review classified the approaches for preparation of a macroporous ceramic structures in four categories: partial sintering, the use of a sacrificial templates, replica technique, and direct foaming. In recent years, the alumina porous ceramics was reported to be manufactured following the methods mentioned above [21,22,23,24] as well as via a rapidly developing 3D printing techniques [25,26] with a precise control of its pore structure. However, these approaches always conceal a compromise of the achieved properties on one hand and a multi-stage, energy consumption, high-cost auxiliary compounds and equipment on another.

For the moment, several works demonstrated an application of the CSP to manufacture alumina ceramics. Herisson de Beauvoir et al. [27] prepared semi-transparent material consisting of boehmite and amorphous alumina by processing of aluminum hydroxide hydrate at temperatures up to 400 °C and a pressure of 500 MPa for 30-180 min. PHT at 500 °C in air led to a formation of γ-Al₂O₃. Kang et al. [28] performed CSP of mixed γ- and α-Al₂O₃ in a medium of glacial acetic acid at 300 °C and 300 MPa for 1 h. The PHT above 1250 °C led to highly dense α-Al₂O₃ ceramics. Suleiman et al. [29] applied CSP for the compaction of Al₂O₃-NaCl composites which transformed into porous alumina ceramics on PHT at 1200-1500 °C for 30 min. Later, Gao et al. [30] demonstrated a direct preparation of γ-Al₂O₃ translucent ceramics by the CSP in 10 M NaOH at 350 °C and 500 MPa without a need of PHT.

Our previous work [31] showed a capability of CSP combined with spark plasma sintering (SPS) to obtain porous boehmite material which transformed in α-Al₂O₃ ceramics with a porosity of about 60 % on the PHT. In this case, γ-Al(OH)₃ was used as a raw powder. Besides, a formation of α-Al₂O₃ in CSP-SPS conditions was observed at 450 °C and 70 MPa in a presence of 5 wt.% of water, but the material demonstrated a poor transport strength. In the current research, we develop the CSP for a direct obtaining of a porous α-Al₂O₃ ceramics starting from γ-Al(OH)₃ powder following the previous results. The work reveals the effects of the processing temperature and pressure on the phase and microstructural transformations during the CSP of alumina in a presence of pure water.

2. Materials and Methods

Gibbsite γ-Al(OH)₃ (> 99.6 % purity, Pikalevo Alumina Refinery LLC, Pikalevo, Russia) and α-Al₂O₃ (Treibacher Industrie AG, Althofen, Austria) powders were used as starting materials in the CSP. The characterization of the initial powders by X-ray diffraction (XRD) and scanning electron microscopy (SEM) is presented in Appendix A (Figures A1, A2). A mixture of 95 wt. % γ-Al(OH)₃ and 5 wt. % α-Al₂O₃ was prepared by a quintuple joint sieving through a sieve with a 300 μm cell. The role of α-Al₂O₃ additive was to promote alumina nucleation during the cold sintering. Earlier, such effect of α-Al₂O₃ seeding was reported for the synthesis of its fine crystals in supercritical water [32] as well as for the conventional sintering from aluminum hydroxide at a lowered temperature [33]. The prepared mixture (1.000 g) was placed inside a mold equipped with a circular heater and a thermal insulation cover. Then, 0.2 ml of distilled water were added to the powder inside of the mold. Graphite sheets separated the inner wall of the mold and the punches from the prepared suspension. The filled mold was placed between the platforms of a uniaxial press, and a pressure of 90-350 MPa was applied to it. Then, the mold was heated up to a temperature of 380-450 °C with a rate of 860 °C h^-1 and held isothermally under the applied pressure for 30 min. After that, the pressure was relieved, and the mold cooled down to the room temperature. The obtained ceramic sample was removed from the mold and peeled from the graphite sheets.

The phase compositions of the initial powders and the obtained ceramic samples were studied by XRD method using Rigaku D/Max-2500 diffractometer (Rigaku Corp., Tokyo, Japan). Identification of the phases was provided with the use of PDF2 database [34]. Diffraction pattern profile fitting was carried out following the Le Bail method by the means of FullProf software [35]. The initial structural models were taken from the Crystallography Open Database [36]. The Rietveld method [37] was applied for the quantitative phase analysis of the ceramic samples. Microstructural study of the powders and ceramics was provided at Jeol JSM 6380 scanning electron microscope (Jeol Ltd., Tokyo, Japan). Particle and grain size distributions based on the measurements in the SEM images carried out using ImageJ software [38]. The density and porosity measurements were carried out following the Archimedes method with the use of kerosene as a saturating liquid.

3. Results

Cold sintering of the gibbsite powder mixed with 5 wt. % of α-alumina in a presence of 20 wt. % of distilled water as a transient liquid resulted in a formation of solid and transportable ceramic samples, an example of which is shown in Figure 1.

A mechanical pressure initially applied to the mold containing the starting powder mixture and an amount of distilled water significantly affected the phase contents of the final ceramic samples. Figure 2a shows the XRD patterns of the samples obtained by CSP at 450 °C under different values of the mechanical pressure. At the lowest pressure of 90 MPa, γ-Al(OH)₃ powder transformed into a mixture of α-Al₂O₃ (51.6 wt. %), χ-Al₂O₃, and a small amount of boehmite (γ-AlOOH) (Figure 2b). The increase in the pressure up to 220 MPa resulted in a complete transformation of γ-Al(OH)₃ into α-alumina. However, the further rise of the mechanical pressure to 350 MPa led to a formation of ceramics which consisted of 81.6 wt.% γ-AlOOH and only 18.4 wt.% of α-Al₂O₃ (Figure 2b). Carbon and silica impurities detected in XRD pattern got into the sample prepared at 220 MPa from the graphite paper, which contacted with the sample during the CSP, and a mortar used for the ceramics grinding before the XRD analysis.

The effect of CSP temperature on the phase contents of the alumina ceramics at a constant mechanical pressure is shown in Figure 3. A major phase of α-Al₂O₃ was found in the samples obtained at 380-450 °C and a pressure of 220 MPa. However, the ceramics manufactured at 410 °C contained a minor phase of boehmite (γ-AlOOH) in an amount of 5.1 wt.%. A metal (Fe) admixture is supposed to be brought into the sample from the inner surfaces of the mold.

At a mechanical pressure of 350 MPa, the samples formed in a temperature range of 380-450 °C contain a major part of boehmite (Figure 4a). The rise of the CSP temperature resulted in an increase in α-Al₂O₃ fraction in these samples from 4.9 to 18.4 wt.% (Figure 4b).

Crystallinity of boehmite and α-Al₂O₃ phases formed in CSP conditions was estimated by their mean crystallite sized calculated from XRD data (Table 1). After CSP at 220 MPa, boehmite performs slightly higher mean crystallite size (6.48 nm) than it results from the processing at 350 MPa (4.94 nm) at an equal temperature of 410 °C. γ-AlOOH to α-Al₂O₃ transformation was accompanied by a decrease in the crystallite size during the CSP at 220 MPa. On the opposite, when sintered at a pressure of 350 MPa, α-Al₂O₃ demonstrated an increase in its crystallite size compared to the neighboring boehmite phase. Crystallite sizes observed in α-Al₂O₃ were mainly higher in the case of CSP at 350 MPa than at 220 MPa. Low peak intensities in the XRD pattern of the sample obtained at 90 MPa impeded an accurate estimation of crystallite sizes of the phases it contained.

Relative density of the manufactured alumina ceramics calculated with respect to its phase contents did not exceed 65 % (Table 1). The values of the measured open porosity pointed to the domination of this type of the pores in the samples obtained at 90 and 220 MPa as well as in ceramics sintered at 380 °C at a pressure of 350 MPa. Lowering of the open porosity in a couple of samples processed at 410 and 450 °C with an applied pressure of 350 MPa indicates the pores’ closure in these conditions. The proportion of the closed pores reached 8.0 and 12.8 % in the mentioned samples, respectively.

A large intergranular pore space in α-Al₂O₃ ceramics obtained at a pressure of 220 MPa and temperatures of 380 and 450 °C could be observed in their fractured surfaces (Figure 5), which is in accordance with the measurements of the porosity. The grains possess mostly irregular shapes and broad size distributions from submicron up to ten microns. The mean values of the grain size are rather close in these ceramic samples and reach about 3 μm. Locally, the grains demonstrate intergrowth and coalescence, but their major part is at the stage of neck formation between the neighboring ones.

Formed at 350 MPa and 380 °C, the boehmite ceramics consist of submicron-sized grains with a smoothed slightly elongated shape (Figure 6a). Fine-grained regions are separated by a large pore space. Noteworthy that the neighboring grains support unidirectional orientation. An increase of the CSP temperature to 410 °C led to the appearance of single plate-like grains of 2-4 μm in diameter among the fine matrix (Figure 6b). After CSP at 450 °C, the trend to the mutual orientation of the neighboring grains continues among submicron rounded as well as micron-sized plate-like ones leading to their coalescence and the formation of faceted grains (Figure 6c, d). The latter are apparently attributed to α-Al₂O₃ which phase fraction significantly increased after the CSP at 450 °C compared to the processing at lower temperatures. The size of the mentioned alumina grains was comparable to that determined for α-Al₂O₃ formed at 220 MPa (Figure 5).

4. Discussion

Being heated in air at about 300 °C gibbsite γ-Al(OH)₃ is commonly known to decompose into boehmite γ-AlOOH, which eliminates water at about 500-550 °C with the formation of γ-Al₂O₃. The latter undergoes a sequence of transformations to δ- and θ-alumina modifications and finally forms α-Al₂O₃ above 1050 °C [39,40]. In this way, the transitional phases perform face-centered cubic packing of oxygen ions. Another way from γ-Al(OH)₃ to α-Al₂O₃ in an air atmosphere includes the formation of χ- and κ-alumina phases with hexagonal close-packed oxygen ions above 300 °C [39]. In a water medium (below as well as above the critical point of 374 °C, 22.1 MPa), the route from gibbsite to α-Al₂O₃ was reported to pass only through the formation of boehmite, which transforms directly into α-alumina [32,41,42]. In the current work, the heating of γ-Al(OH)₃ powder occurred in a presence of supercritical water as the applied mechanical pressure (> 90 MPa) and the temperature of isothermal dwell (> 380 °C) exceeded the critical parameters. Under these conditions, the formation of three new phases was observed, i.e. γ-AlOOH, χ- and α-Al₂O₃. All these phases were detected in the ceramics processed at 450 °C and 90 MPa. Worth noting that the starting gibbsite placed inside the mold for the CSP, experienced a heating with a relatively high rate. Earlier, Ingram-Jones et al. [43] [https://doi.org/10.1039/JM9960600073] reported that fast heating of fine γ-Al(OH)₃ (0.5 μm) powder mostly resulted in a sequence of phase transitions, first of which was its transformation to χ-Al₂O₃, while the coarse gibbsite particles (14 μm) prone to form boehmite as well as χ-Al₂O₃ at the initial step of their thermal decomposition. Gibbsite powder used in the current work could be classified as a fine-grained (Figure A1), and the formation of χ-Al₂O₃ during its relatively fast heating appears expectable. Along with χ-Al₂O₃, γ-AlOOH is supposed to form from gibbsite at 90 MPa and then almost completely decompose into α-Al₂O₃. In the samples prepared at a higher mechanical pressure, χ-Al₂O₃ was not found. At 220 MPa and 450 °C, a single-phase α-alumina apparently formed from an intermediate boehmite phase. However, after the CSP at 350 MPa and 450 °C, α-Al₂O₃ became a minor phase in the ceramic sample mostly containing of boehmite. The mentioned observations indicated that the initial step in the CSP of gibbsite was its dehydroxylation, the route of which depended on the mechanical pressure. The rise of the applied pressure hindered the elimination of OH-groups from the solid because of the related increase in the supercritical water density inside of the mold. Almost regardless of the temperature, α-Al₂O₃ dominated in the ceramics obtained at 220 MPa, while the samples prepared at 350 MPa contained boehmite as the major phase. Similar effect of the supercritical water density on the dehydroxylation processes in γ-AlOOH as well as in BaTiO₃ were reviewed by Hayashi et al. [44].

Phase transformation of boehmite into α-alumina in a water medium under supercritical conditions has been extensively studied previously [32,42,45]. The observed evolution of the powders’ morphology derived two separate viewpoints on the alumina formation and growth. On one hand, Suchanek [42] found no evident orientation of boehmite particles on the surfaces of alumina crystals to prove the solid-state nature of the alumina nucleation and growth and supposed dissolution-precipitation or surface diffusion processes to govern the reaction. Plyasunov [46] reported that the dissolved form of alumina in the supercritical water is Al(OH)₃·H₂O. On another hand, Ivakin et al. [32] revealed homogenous nucleation of α-Al₂O₃ in boehmite particles. Currently obtained ceramic samples contained phases different from Al(OH)₃, the presence of which might point to the dissolution-precipitation mechanism. However, the microstructure observed in the samples obtained at 380 °C and 350 MPa (Figure 6a) resembled that demonstrated by Yamaguchi et al. [7] for porous boehmite material CSPed from γ-Al(OH)₃ at milder conditions (250 °C, 270 MPa). The authors assumed that the large pores in the material were relics of the completely dissolved gibbsite particles, while the boehmite grains nucleated and grew from the solution separating them. Important to note that the pore space in boehmite could result from an intensive elimination of water during the gibbsite decomposition.

A study of the microstructural changes during the CSP at different temperatures and pressure values shows that dehydroxylation of the initial gibbsite on heating first results in the development of a fine-grained boehmite structure (Figure 7a). Further processing leads to a coalescence of these submicron grains and a formation of larger plate-like grains of boehmite (Figure 7b). Several authors [12,30] reported that at this stage of CSP, an interaction of the solid with the water medium leads to a dissociative adsorption of H₂O as well as to condensation of H⁺ and OH^- with a formation of bridging oxygen between the metal ions. These processes have a decisive role in the oxygen ions diffusion, which facilitates and the mass transport between the grains and lowers the activation energy for the phase transitions. The boehmite grains of both rounded and plate-like morphologies under the CSP conditions tend to perform coalescence and finally transform homogenously into α-alumina modification (Figure 7c). Earlier, the coalescence of the particles was also demonstrated for the CSP of ZnO [47], CaCO₃ [3], and Na₂Mo₂O₇ [48].

Regardless of the phase composition, the obtained ceramics possessed a relative density in a range of 58-65 %, which is close to the density of the conventionally pressed compacts. It is likely that the porosity of the samples containing boehmite was supported by water elimination accompanying the ongoing transformation into alumina. The shape of the formed α-Al₂O₃ grains appeared similar to that observed in its crystals synthesized from the boehmite powder in a medium of supercritical water [32,42]. Compared to the parent boehmite grains, the grains of α-alumina increased in the size and approached to their Wulffs shape [49], which indicates the fall of the driving force for the sintering and corresponds to the remaining porosity.

Interestingly, that α-Al₂O₃ ceramics currently produced by CSP demonstrated a porosity which is almost totally of the open type and was comparable to the recent results obtained by a traditional route [22,50,51,52]. This would make the obtained material highly attractive as a filter, thermal insulator, a component of polymer- and metal-ceramic composite, etc., when combined with sufficient mechanical strength and chemical resistivity. Energy- and time-saving CSP technique also would be a reason of competitiveness for these alumina ceramics compared to the related alumina materials conventionally sintered at a high temperature.

5. Conclusions

(1)

For the first time, porous α-Al₂O₃ ceramics were processed directly by CSP at a temperature of 380-450 °C, a pressure of 220 MPa during 30 min. in a presence of pure water. The materials were characterized by predominately open-type porosity reaching 36 %.

(2)

Under the studied CSP conditions, the initial γ-Al(OH)₃ powder undergoes dehydration leading to γ-AlOOH and possible minor χ-Al₂O₃ phases. Further processed, γ-AlOOH forms elongated grains, which coalesce into plate-like ones and transform into the grains of α-Al₂O₃.

(3)

An increase in the applied pressure (up to 350 MPa) prevents the dehydration of γ-AlOOH and impedes the formation of the final α-Al₂O₃.