One-Pot Synthesis of Alumina-Titanium Diboride Composite Powder at Low Temperature

Abstract

Alumina-titanium diboride (Al₂O₃-TiB₂) composite powders were synthesised via aluminothermic reduction of TiO₂ and B₂O₃, mediated by a molten chloride salt (NaCl, KCl, or MgCl₂). The effects of salt type, initial batch composition, and firing temperature/time on the phase formation and overall reaction extent were examined. Based on the results and equilibrium thermodynamic calculations, the mechanisms underpinning the reaction/synthesis processes were clarified. Given their evaporation losses at test temperatures, appropriately excessive amounts of Al and B₂O₃ are needed to complete the synthesis reaction. Following this, phase-pure Al₂O₃-TiB₂ composite powders composed of 0.3–0.6 μm Al₂O₃ and 30–60 nm TiB₂ particles were successfully fabricated in NaCl after 5 h at 1050 °C. By increasing the firing temperature to 1150 °C, the time required to complete the synthesis reaction could be reduced to 4 h, although the sizes of Al₂O₃ and TiB₂ particles in the resultant phase pure composite powder increased slightly to 1–2 μm and 100–200 nm, respectively.

1. Introduction

Alumina (Al₂O₃) is a representative high performance ceramic material possessing numerous superior properties. Apart from its high melting point, it exhibits high hardness and mechanical strength, excellent wear and chemical resistances, as well as good electrical/thermal insulation [1]. These, along with its ready availability, make Al₂O₃ an attractive ceramic material applicable to a wide range of structural and functional applications, in, e.g., wear-resistant components, high-speed cutting tools, armors, refractory linings/crucibles, electrical and chemical insulators, and artificial hip joints [2,3,4]. Unfortunately, it suffers from low fracture toughness and poor thermal shock resistance [5,6], negatively affecting its service performance and life. One of the commonly adopted strategies for overcoming this is to make a composite [7], by combining it with another reinforcement phase, e.g., particles/platelets, or fibrous phases such as carbon fibres and carbon nanotubes. Titanium diboride (TiB₂) stood out as one of the reinforcement phases, owing to its good structural and thermodynamic compatibility with Al₂O₃ [8,9] and its excellent properties such as high melting point, high hardness/elastic modulus, relatively low density, and good thermal/electrical conductivities [10]. Previous studies found that incorporation of TiB₂ particles into Al₂O₃ conferred much improved hardness, strength, fracture toughness and electrical conductivity on the resultant Al₂O₃-TiB₂ composite materials [11,12,13,14,15,16], making them suitable for a variety of demanding applications, e.g., in electrodes, cutting tools, wear parts, lightweight armors, high-temperature/glow-plug heaters, and heat exchangers [14,17,18].

To fabricate high performance Al₂O₃-TiB₂ composites, it is key to prepare and use high quality counterpart powders. Conventionally, such composite powders were prepared by directly mixing pre-synthesised TiB₂ and Al₂O₃ powders in a ball mill [11,12,14,15,16,17,19,20]. Unfortunately, the final bulk composites exhibited unsatisfactory service performance arising mainly from poor dispersion and distribution of TiB₂ in Al₂O₃. To overcome this, in-situ formation of the two constituent phases was suggested [21]. For example, a self-propagating high-temperature synthesis (SHS) method was attempted to form in-situ Al₂O₃ and TiB₂ from powder mixtures of Al, TiO₂/Ti, and B₂O₃/B/H₃BO₃ [18,22,23,24,25,26,27,28,29,30,31,32]. However, this method suffered from several weaknesses; in particular, difficulties to control the synthesis process and synthesise phase pure materials. Apart from SHS, mechanochemical synthesis [8,9,33,34,35], and milling assisted sol-gel process [36] were investigated. With these two techniques, a long processing time was required, and the resultant powders were often heavily agglomerated, and contaminated during prolonged milling.

In recent years, our group has successfully synthesised several ultra-high temperature boride powders via magnesiothermic reduction of oxide precursors in a molten salt [37,38,39]. In this work, such a molten salt synthesis (MSS) technique was further developed to synthesise high-quality Al₂O₃-TiB₂ composite powders at relatively low temperatures. The effects of processing factors such as salt type, initial batch composition, and firing temperature/time on the synthesis process were examined, and the synthesis conditions optimised. Based on the results, and thermodynamic calculations, the mechanisms underpinning the reaction/synthesis process were proposed.

2. Materials and Methods

The TiO₂ (≥99%, 100–300 nm), B₂O₃ (99.98%) and Al (99.5%, ~44 μm) powders were used as the main starting materials, along with KCl (>99%), NaCl (>99%) and anhydrous MgCl₂ (>98%). All of these materials except for Al (Alfa Aesar, Lancashire, UK) were from Sigma-Aldrich (Gillingham, UK). The three main starting materials were mixed in stoichiometric (indicated by the overall Reaction (1)) or nonstoichiometric proportions (with excess Al and B₂O₃), in a mortar and pestle, and further combined with 5 times (by weight) of each of the salts. The final powder mix was contained in an alumina crucible and heated in an argon protected tube furnace to a temperature between 850 and 1150 °C and held for 4–6 h. After cooling to room temperature, the reacted powder coexisting with the residual salt was subjected to repeated hot water washing. The remaining salt-free product powder was dried overnight at 80 °C before being subjected to detailed characterisations.

3TiO₂ + 3B₂O₃ + 10Al = 3TiB₂ + 5Al₂O₃

(1)

Phase formation and reaction extent in samples were evaluated based on their X-ray diffraction (XRD) spectra recorded by an X-ray diffractometer (D8 Advance, Bruker, Germany) operated at 40 mA/40 kV (using CuKα radiation), and at a scan rate of 2° (2θ) min⁻¹, and the ICDD cards used are: TiB₂ (35-741), Al₂O₃ (corundum) (10-173), Ti₂O₃ (43-1033), Al₃Ti (37-1449), Al₁₈B₄O₃₃ (32-3), and MgAl₂O₄ (21-1152). Samples were also gold- or carbon-coated for further microstructural examinations using a scanning electron microscope (SEM, Nova 600, FEI, Hillsboro, OR, USA) and a transmission electron microscope (TEM, JEM 2100, JEOL, Tokyo, Japan), linked with energy-dispersive X-ray spectroscopy (EDS, Oxford Instrument, Oxford, UK).

To assist in clarification of the mechanisms underpinning the whole reaction/synthesis process, thermodynamic calculations were carried out using the commercial FactSage package [40]. The Gibbs energy values for each of the intermediate reactions and the overall reaction (Reaction (1)), as a function of temperature, were calculated.

3. Results and Preliminary Discussion

3.1. Influence of Salt Type on Al₂O₃-TiB₂ Formation

Figure 1 shows XRD patterns of stoichiometric samples after 4 h firing in different salts at 850 °C. After firing in KCl, α-Al₂O₃ and TiB₂ were identified as the primary phases in the sample (Figure 1a). Some intermediate Al₁₈B₄O₃₃ and Al₃Ti were also detected, along with a trace of Ti₂O₃, indicating the low extents of Reaction (1) and Al₂O₃-TiB₂ formation. Replacing KCl with NaCl (Figure 1b) led to an evident increase in the relative peak heights of both Al₂O₃ and TiB₂, and a decrease in those of Al₃Ti, while those of intermediate Al₁₈B₄O₃₃ and Ti₂O₃ changed little, demonstrating the enhanced extents of Al₂O₃-TiB₂ formation, and the better effect of NaCl than KCl on accelerating the overall reaction. As discussed previously [37,38,39], several processing parameters—in particular, the solubility and mobility of reaction species in the molten salt—play crucial roles in MSS. Unfortunately, these key processing parameters are not available, so it is difficult to figure out the reasons behind the better accelerating effect of NaCl in the present case. When NaCl was replaced with MgCl₂, the peak heights of Al₂O₃ and TiB₂ further increased (Figure 1c), and no Al₁₈B₄O₃₃, Al₃Ti or Ti₂O₃ was detected, suggesting much enhanced extents of Reaction (1). Nevertheless, an undesirable by-product phase, magnesium aluminate spinel (MgAl₂O₄), was formed in this case, due to likely the reaction between the formed Al₂O₃ and the MgCl₂ (Reaction (2) [41]. In the three cases, except for α-Al₂O₃, no other transition aluminas were detected at such a low firing temperature. The reason for this was not clear, but it could be due to the locally enhanced salt bath temperature arising from the exothermic reactions, as suggested by the thermodynamic calculations presented later (see Section 4 below). Given the better accelerating effect of NaCl than KCl, and the formation of undesired by-product MgAl₂O₄ in the case of using MgCl₂, NaCl was chosen specifically to form a liquid reaction medium for the following MSS investigations.

4Al₂O₃ + 3MgCl₂ = 3MgAl₂O₄ + 2AlCl₃

(2)

3.2. Influence of Firing Temperature on Al₂O₃-TiB₂ Formation

Figure 2 presents XRD patterns of stoichiometric samples after 4 h firing in NaCl at different temperatures. At 850 °C, as already described above, Al₂O₃ and TiB₂ were formed as the primary phases, but some intermediate Al₁₈B₄O₃₃, Al₃Ti and Ti₂O₃ were detected (Figure 2a/Figure 1b). Upon increasing the temperature to 950 (Figure 2b) and 1050 °C (Figure 2c), the relative peak heights of Al₂O₃ and TiB₂ increased significantly. Meanwhile, Al₃Ti disappeared, and Ti₂O₃ changed little, but the peak heights of Al₁₈B₄O₃₃ increased. These results implied that increasing the firing temperature favoured not only the Al₂O₃-TiB₂ formation but also the Al₁₈B₄O₃₃ formation. Further increasing the temperature to 1150 °C led to a further increase in the peak heights of Al₁₈B₄O₃₃ and Ti₂O₃ but decrease in those of Al₂O₃ and TiB₂ (Figure 2d), indicating the adverse effect of such further temperature increase on the Al₂O₃-TiB₂ formation. This, according to our previous studies on MSS of transition metal borides [37,38,39], was due to probably evaporation loss of Al at relatively high reaction temperatures [42]. To confirm this, the influence of using excess Al on the Al₂O₃-TiB₂ formation was further investigated, as described and discussed next.

3.3. Influence of Excess Al on Al₂O₃-TiB₂ Formation

Figure 3 illustrates the influence of excess Al on phase evolution in samples after 4 h firing in NaCl at 1150 °C. The use of 20 wt% more Al resulted in considerable increase in the peak heights of Al₂O₃ and TiB₂ and decrease in those of Al₁₈B₄O₃₃ and Ti₂O₃ (Figure 3a,b). By further increasing excess Al to 25 wt%, the peak heights of Al₂O₃ and TiB₂ were further increased. Meanwhile, Ti₂O₃ was completely eliminated, and only minor Al₁₈B₄O₃₃ remained (Figure 3c), illustrating positive effects from the Al compensation. Nevertheless, the formation of phase-pure Al₂O₃-TiB₂ was still not achieved.

To eliminate Al₁₈B₄O₃₃, the excess Al was further increased to 30 wt%. Unfortunately, some Al₃Ti was formed instead (Figure 3d), implying that there was no sufficient supply of B in the system, due to probably evaporation loss of B₂O₃ at such a relatively high temperature [43,44].

3.4. Influence of Excess B₂O₃ on Al₂O₃-TiB₂ Formation

To confirm and address the issue with B₂O₃ evaporation loss, the influence of using excess B₂O₃ (along with 30 wt% excess Al) on the Al₂O₃-B₂O₃ formation in the sample fired at 1150 °C for 4 h was further investigated as an example. When 10 wt% excess B₂O₃ was used, the peak heights of both Al₂O₃ and TiB₂ increased, whereas those of Al₃Ti decreased considerably (Figure 4b), compared with in the case of stoichiometric sample (Figure 4a). Further increasing the excess B₂O₃ to 20 wt% led to complete elimination of Al₃Ti, and the formation of phase pure Al₂O₃-TiB₂ (Figure 4c).

3.5. Influence of Firing Time on Al₂O₃-TiB₂ Formation and Further Optimisation of Synthesis Condition

To reveal the influence of firing time on the Al₂O₃-TiB₂ formation and further optimise the synthesis condition, samples containing 30 wt% excess Al and 20 wt% excess B₂O₃ were fired in NaCl at 1050 °C for different time periods, and then analysed by XRD (Figure 5). As shown in Figure 5a,b, with increasing the time at 1050 °C from 4 to 5 h, Al₁₈B₄O₃₃ disappeared, whereas the peak heights of Al₃Ti increased. Further extending the time to 6 h resulted in little change in the XRD pattern (Figure 5c), and Al₃Ti was still detected. This, as mentioned above (Section 3.4), indicated that 20 wt% excess B₂O₃ might not be sufficient to compensate for the evaporation loss of B₂O₃ under this condition, in other words, more excess B₂O₃ had to be used. As verified by Figure 6, increasing excess B₂O₃ from 20 wt% to 25 wt% led to significant decrease in the peak heights of Al₃Ti, and further increasing the excess amount to 30 wt%, all the intermediate phases including Al₃Ti disappeared, and phase pure Al₂O₃-TiB₂ powder was finally obtained.

3.6. Microstructure of Al₂O₃-TiB₂ Product Powder

As demonstrated in Figure 4c and Figure 6c and discussed above, phase pure Al₂O₃-TiB₂ powder could be formed in NaCl after 5 h firing at 1050 °C (using 30 wt% excess Al and 30 wt% excess B₂O₃), or 4 h firing at 1150 °C (using 30 wt% excess Al and 20 wt% excess B₂O₃), which was further confirmed by microstructural characterisation. Figure 7, as an example, gives TEM images of the product powder prepared under the former condition, along with the corresponding EDS, showing the coexistence of submicron-sized Al₂O₃ particles (0.3–0.6 μm) and nanosized TiB₂ particles (30–60 nm). Except these two phases, no other impurity phases were seen, as already revealed by XRD (Figure 6c). Figure 8 further displays SEM images of the product powder formed under the latter condition mentioned above. The formation of phase pure Al₂O₃-TiB₂ in this case was also confirmed by EDS, along with XRD (Figure 4c). However, due to the higher synthesis temperature, Al₂O₃ and TiB₂ particles in the composite powder became slightly bigger (1–2 μm Al₂O₃ and 100–200 nm TiB₂).

4. Further Discussion and Reaction/Synthesis Mechanism

Based on the results presented in Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8 and the preliminary discussion above, as well as on the thermodynamic calculations (Figure 9), the reaction/synthesis mechanisms (taking the case of using NaCl as an example) could be proposed; they are discussed as follows.

The firing temperatures (850–1150 °C) were above the melting points of NaCl, B₂O₃ and Al. So, at these temperatures, they all melted, forming a liquid NaCl pool, and B₂O₃ and Al liquid droplets. Then, the latter two would start to react with each other in the molten NaCl medium, forming B and Al₂O₃, according to Reaction (3).

On the other hand, as discussed in our previous work on MSS of TiB₂ [39], TiO₂ also would be reduced by the Al droplets in the molten NaCl medium, forming Ti and Al₂O₃ (Reaction (4)). The Al₂O₃ formed from Reactions (3) and (4) would partially react with the unreduced B₂O₃, in the NaCl medium, forming the intermediate Al₁₈B₄O₃₃ (Reaction (5)), as detected by XRD (Figure 1, Figure 2 and Figure 3 and Figure 5).

B₂O₃ + 2Al = 2B + Al₂O₃

(3)

3TiO₂ + 4Al = 3Ti + 2Al₂O₃

(4)

9Al₂O₃ + 2B₂O₃ = Al₁₈B₄O₃₃

(5)

With the optimisation of synthesis condition, the intermediate Al₁₈B₄O₃₃ could be further reduced by Al in the molten NaCl, forming additional B and Al₂O₃ according to Reaction (6), as verified by XRD (Figure 3 and Figure 5).

Apart from Al₁₈B₄O₃₃, another main intermediate phase, Al₃Ti, was formed from the reaction between the Ti formed from Reaction (4), and Al (Reaction (7)), as detected by XRD (Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6).

As discussed, and confirmed in our previous studies [37,38,39], the formed B and Ti (Reactions (3), (4) and (6)) were slightly dissoluble in the molten chloride salts. So, the dissolved B and Ti would diffuse through the molten salt medium and reacted with each other, forming the other desirable phase of TiB₂ (Reaction (8)).

Al₁₈B₄O₃₃ + 4Al = 4B + 11Al₂O₃

(6)

Ti + 3Al = Al₃Ti

(7)

Ti + 2B = TiB₂

(8)

B dissolved in the molten salt would also diffuse through the salt medium and react respectively with the Al₃Ti produced from Reaction (7), and the unreacted raw materialTiO₂, forming additional TiB₂ (Reactions (9) and (10)). Reaction (9) was mainly responsible for the complete elimination of this intermediate phase under the optimal conditions (Figure 4 and Figure 6).

Al₃Ti + 2B = TiB₂ + 3Al

(9)

10B + 3TiO₂ = 3TiB₂ + 2B₂O₃

(10)

As shown in Figure 9, the Gibbs energy values corresponding to Reactions (3)–(9) and the overall Reaction (1) are also negative, at test temperatures (and even at lower temperatures), suggesting that all of these exothermic Reactions are thermodynamically favourable, which further supports the plausibility of the reaction/synthesis mechanisms proposed above.

5. Conclusions

Low temperature synthesis of Al₂O₃-TiB₂ composite powders via aluminothermic reduction of TiO₂ and B₂O₃ in a molten chloride salt (KCl, NaCl or MgCl₂) was investigated. Processing parameters including salt type, initial batch composition, and firing temperature/time played important roles in the synthesis process. Phase pure Al₂O₃-TiB₂ nanocomposite powder could be prepared in NaCl after 5 h firing at 1050 °C, when 30 wt% excess Al and 30 wt% excess B₂O₃ were used. The particle sizes of Al₂O₃ and TiB₂ coexisting in the as-prepared composite powder were estimated to be 0.3–0.6 μm and 30–60 nm, respectively. By increasing the temperature to 1150 °C and using 30 wt% excess Al and 20 wt% excess B₂O₃, phase pure Al₂O₃-TiB₂ composite powder also could be prepared in NaCl after only 4 h, although sizes of Al₂O₃ and TiB₂ particles coexisting in the resultant composite power increased slightly to 1–2 μm and 100–200 nm, respectively.