Formation of TiB₂–MgAl₂O₄ Composites by SHS Metallurgy

Abstract

TiB₂–MgAl₂O₄ composites were fabricated by combustion synthesis involving metallothermic reduction reactions. Thermite reagents contained Al and Mg as dual reductants and TiO₂ or B₂O₃ as the oxidant. The reactant mixtures also comprised elemental Ti and boron, as well as a small amount of Al₂O₃ or MgO to serve as the combustion moderator. Four reaction systems were conducted and all of them were exothermic enough to proceed in the mode of self-propagating high-temperature synthesis (SHS). The reaction based on B₂O₃/Al/Mg thermite and diluted with MgO was the most exothermic, while that containing TiO₂/Al/Mg thermite and Al₂O₃ as the diluent was the least. Depending on different thermites and diluents, the combustion front temperatures in a range from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s were measured. The XRD spectra confirmed in situ formation of TiB₂ and MgAl₂O₄. It is believed that MgAl₂O₄ was synthesized through a combination reaction between Al₂O₃ and MgO, both of which can be totally or partially produced from the metallothermic reduction of B₂O₃ or TiO₂. The microstructure of the TiB₂–MgAl₂O₄ composite exhibited fine TiB₂ crystals surrounded by large densified MgAl₂O₄ grains. This study demonstrated an energy-saving and efficient route for fabricating MgAl₂O₄-containing composites.

1. Introduction

TiB₂ has been one of the most studied ultra-high temperature ceramics (UHTCs) due to its unique properties, including a high melting point (3225 °C), high hardness (33 MPa), high Young’s modulus (530 MPa), excellent wear and oxidation resistance, thermal shock resistance, chemical inertness, and good electric conductivity [1,2,3]. Combination of these properties makes TiB₂ an ideal candidate for use in ballistic armors, crucibles, metal evaporation boats, cutting tools, wear resistance parts, and cathodes for alumina smelting [4,5,6]. Many ceramic phases, such as Al₂O₃, SiC, B₄C, and MgAl₂O₄, have been considered as the reinforcement to improve fracture toughness, oxidation resistance, heat resistance, and mechanical strength of the TiB₂-based composites [7,8,9,10]. Moreover, a recent study showed that TiB₂–Al₂O₃–MgAl₂O₄ composite possesses temperature insensitive and enhanced microwave absorption properties [11]. Magnesium aluminate spinel, MgAl₂O₄, as an additive has been rarely studied, possibly because the fabrication of MgAl₂O₄ via either the direct solid-state reaction of oxides or wet chemical methods requires multiple steps that are complicated and time-consuming [12,13,14]. However, MgAl₂O₄ is an attractive component due to its high melting point, chemical inertness, high hardness, corrosion resistance, high mechanical strength, and low cost [12].

Among various fabrication routes for preparing multiphasic ceramics, metallothermic reduction reactions (i.e., thermite reactions) combined with combustion synthesis have been recognized as a promising technique for in situ formation of MgAl₂O₄-containing composites [15]. Combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS), which is based on strongly exothermic reactions, has merits of low energy consumption, short reaction time, simple equipment and operation, high-purity products, and in situ formation of composite components [16,17,18]. Moreover, aluminothermic and magnesiothermic reduction reactions have Al₂O₃ and MgO as their respective by-products, both of which are precursors for the formation of MgAl₂O₄. Consequently, Omran et al. [19] applied the reduction-based SHS technique to produce MgAl₂O₄–W–W₂B composites through magnesiothermic reduction of B₂O₃ and WO₃ in the presence of Al₂O₃. Zaki et al. [20] synthesized MoSi₂– and Mo₅Si₃–MgAl₂O₄ composites by SHS with a reducing stage from raw materials consisting of MoO₃, SiO₂, and Al as aluminothermic reagents and MgO as a precursor. Similarly, MgO was added into the reactive mixture of TiO₂, B₂O₃, and Al to fabricate TiB₂–MgAl₂O₄ composites by thermitic combustion synthesis [21]. Generally, most of the previous studies on the formation of MgAl₂O₄-containing composites had prior addition of one of two precursors (Al₂O₃ or MgO) in the green samples.

This study represents the first attempt to prepare TiB₂–MgAl₂O₄ composites from the SHS powder metallurgy simultaneously involving aluminothermic and magnesiothermic reduction of TiO₂ or B₂O₃. Only a small amount of Al₂O₃ or MgO was included in the reactive mixture to serve as the combustion moderator and part of the precursors for the formation of MgAl₂O₄. Four SHS reaction systems formulated with different metallothermic reagents and combustion diluents were investigated. In this work, combustion exothermicity and kinetics of the combustion wave of the SHS process, as well as compositions and microstructures of the final products were explored.

2. Materials and Methods

The starting materials adopted by this study included TiO₂ (Acros Organics, Geel, Belgium, 99.5%), B₂O₃ (Acros Organics, 99%), Al₂O₃ (Alfa Aesar, Haverhill, MA, USA, 99%), MgO (Acros Organics, 99.5%), Al (Showa Chemical Co., Tokyo, Japan, <45 μm, 99.9%), Mg (Alfa Aesar, <45 μm, 99.8%), Ti (Alfa Aesar, <45 μm, 99.8%), and amorphous boron (Noah Technologies, San Antonio, TX, USA, <1 μm, 93.5%). Four SHS reactions were formulated for the synthesis of 3TiB₂–MgAl₂O₄ composites. Two metallothermic reagents (i.e., thermites) were considered; one is composed of TiO₂, Al, and Mg, as shown in Equations (1) and (2), and the other comprises B₂O₃, Al, and Mg, as in Equations (3) and (4). Due to strong exothermicity of combustion, Al₂O₃ with an amount of 0.3 mol. was included in Equations (1) and (3) as the combustion moderator (or combustion diluent) in order to attain stable propagation of the combustion wave. The pre-added Al₂O₃ also acted as part of the precursor for the synthesis of MgAl₂O₃. Likewise, an equal amount of MgO was adopted by Equations (2) and (4) and MgO played the same role as Al₂O₃ in Equations (1) and (3).

$$\left(1.55Ti{O}_2+1.4Al+Mg\right)+0.3A{l}_2{O}_3+1.45Ti+6B\to 3Ti{B}_2+MgA{l}_2{O}_4$$

(1)

$$\left(1.85Ti{O}_2+2Al+0.7Mg\right)+0.3MgO+1.15Ti+6B\to 3Ti{B}_2+MgA{l}_2{O}_4$$

(2)

$$\left(1.033B_2{O}_3+1.4Al+Mg\right)+0.3A{l}_2{O}_3+3Ti+3.934B\to 3Ti{B}_2+MgA{l}_2{O}_4$$

(3)

$$\left(1.233B_2{O}_3+2Al+0.7Mg\right)+0.3MgO+3Ti+3.534B\to 3Ti{B}_2+MgA{l}_2{O}_4$$

(4)

Combustion exothermicity of the above four reactions, Equations (1)–(4), was evaluated by calculating their adiabatic combustion temperatures (T_ad) from the following energy balance equation [17,22] with thermochemical data taken from [23].

$$\Delta H_r+\underset{298}{\overset{T_{ad}}{{\displaystyle \int }}}{\displaystyle \sum }{n}_j{c}_p\left(P_j\right)dT+{\displaystyle \sum }_{298-T_{ad}}{n}_{j}L\left(P_j\right)=0$$

where ∆H_r is the reaction enthalpy at 298 K, c_p and L are the heat capacity and latent heat, n_j is the stoichiometric coefficient, and P_j refers to the product component.

The SHS experiments were conducted in a windowed combustion chamber filled with Ar at 0.25 MPa. Reactant powders were well mixed in a tubular ball mill and then uniaxially compressed into cylindrical test specimens with a diameter of 7 mm, a height of 12 mm, and a relative density of 55%. The sample compact was ignited by an electrically heated tungsten coil. An R-type bare-wire thermocouple (Pt/Pt-13%Rh) with a bead size of 125 μm was used to measure the combustion temperature. The propagation velocity of combustion wave (V_f) was determined by the time derivative of the flame-front trajectory constructed from the recorded series of combustion images. Phase compositions of the products were identified by an X-ray diffractometer (XRD, Bruker D2 Phaser, Karlsruhe, Germany). Microstructures and constituent elements of the products were examined by the scanning electron microscopy (SEM, Hitachi, Tokyo, Japan, S3000H) and energy dispersive spectroscopy (EDS). Details of the experimental methods were reported elsewhere [24].

3. Results and Discussion

3.1. Combustion Exothermicity of Reduction-Based SHS Reactions

Figure 1 presents the calculated ∆H_r and T_ad of reactions Equations (1)–(4) and shows that Equation (4) has the highest values while Equation (1) has the lowest ones. Both ∆H_r and T_ad increase from Equations (1)–(4). Specifically, the values of T_ad are 2530 K, 2595 K, 2783 K, 2897 K for Equations (1)–(4), respectively. A comparison between Equations (1) and (2) revealed that the combustion moderator Al₂O₃ appeared to impose a stronger dilution effect on combustion than MgO, which led to a lower T_ad for Equation (1) than Equation (2). Similar results were observed in Equations (3) and (4). These findings could also be explained by the fact that metallothemic reduction of TiO₂ or B₂O₃ by Al is more exothermic than that by Mg [15,25].

On the other hand, because the B₂O₃-based thermite is more energetic than the one using TiO₂ [25], Equation (3) has a higher T_ad than Equation (1). Similarly, Equation (4) has a higher T_ad than Equation (2). According to the calculated T_ad, it is realized that the thermite oxidants (i.e., B₂O₃ versus TiO₂) have a more pronounced influence on combustion exothermicity than the diluent oxides (i.e., Al₂O₃ versus MgO).

3.2. Combustion Temperature and Self-Propagating Velocity

Two series of the SHS processes recorded from reactions Equations (1) and (3) are illustrated in Figure 2a,b, respectively. It is apparent that upon ignition, the reaction was initiated and characterized by a self-sustaining combustion wave. More intense combustion accompanied with a faster combustion wave was observed in Figure 2b, when compared with that in Figure 2a. Combustion luminosity and flame spreading speed reflected the degree of reaction exothermicity. As mentioned above, B₂O₃/Al/Mg-based Equation (3) is more energetic than TiO₂/Al/Mg-based Equation (1). Similar combustion behavior was also noticed in Equations (2) and (4).

Figure 3 depicts typical combustion temperature profiles measured from four different reactions. All profiles exhibit a steep temperature rise followed by a rapid descent, which is characteristic of the SHS reaction that features a fast combustion wave and a thin reaction zone. The peak value is considered as the combustion front temperature (T_c). When compared with pinnacles in the contours of Equations (1) and (2), sharper peaks were detected in the profiles of Equations (3) and (4). This implied a faster combustion wave in Equations (3) and (4). As shown in Figure 3, the values of T_c from Equation (1)–(4) in ascending order are 1348 °C, 1445 °C, 1660 °C, and 1736 °C. It should be noted that the measured combustion front temperatures are in agreement with the calculated reaction exothermicity.

It is useful to note in Figure 3 that the curves of Equations (3) and (4) have a shape peak with a pronounced shoulder. The shape peak was a result of the fast combustion wave. The pronounced shoulder could be caused by the occurrence of volumetric synthesis reactions after the passage of the rapid combustion wave.

Figure 4 plots the measured combustion wave propagation velocities (V_f) and temperatures (T_c) of four reactions. The rising trend of V_f from Equations (1)–(4) is consistent with that of T_c. This can be understood by the fact that the propagation of combustion wave is essentially governed by layer-by-layer heat transfer from the reaction zone to unreacted region, and therefore, is subject to the combustion front temperature. As presented in Figure 4, the average combustion velocities are 3.9, 4.7, 5.1, and 5.7 mm/s for Equation (1)–(4), respectively. It is worth noting that the measured combustion temperature not only justified the reaction exothermic analysis, but confirmed the temperature dependence of combustion wave velocity.

3.3. Composition and Microstructure Analyses of Synthesized Products

The XRD spectra of the final products synthesized from Equations (1) and (2) are shown in Figure 5a,b, respectively. Both indicated the formation of TiB₂ and MgAl₂O₄ along with two minor phases, magnesium titanate (MgTiO₃) and MgO. It is believed that MgAl₂O₄ was synthesized through a combination reaction between Al₂O₃ and MgO. Equation (1) and (2) were formulated with the same thermite reagents of TiO₂, Al, and Mg, but diluted by different metal oxides. That is, Al₂O₃ was partly pre-added and partly thermite-produced, while MgO was completely generated from the reduction of TiO₂ by Mg in Equation (1). In contrast, the required Al₂O₃ in Equation (2) was entirely produced from the reduction of TiO₂ by Al, but MgO was supplied in part from prior addition and in part from the reduction of TiO₂ by Mg. For both Equations (1) and (2), TiB₂ was synthesized from the reaction of elemental boron with reduced and metallic Ti.

Traces of MgO suggested an incomplete reaction due probably to the relatively low reaction temperatures of Equations (1) and (2). The presence of MgTiO₃ in the final products of Equations (1) and (2) could be attributed to the reaction of MgO with the thermite oxidant TiO₂ [26,27]. The formation of MgTiO₃ in the SHS-produced TiB₂–MgAl₂O₄ composites was also observed by Radishevskaya et al. [10] using Ti, boron, and MgAl₂O₄ as their starting materials and a partial decomposition of MgAl₂O₄ during combustion synthesis was considered as a possible route resulting in the formation of MgTiO₃.

The presence of MgO along with no detection of Al₂O₃ in the final products of Equations (1) and (2) suggested that the as-synthesized MgAl₂O₄ is an Al₂O₃-rich spinel. The formation of MgTiO₃ could also result in the production of Al₂O₃-rich spinel. According to Naghizadeh et al. [28], magnesium titanate compounds (MgTiO₃ and Mg₂TiO₄) were identified in the phase evolution of MgAl₂O₄ produced from TiO₂-containing samples and stoichiometric MgAl₂O₄ spinel shifted toward the Al₂O₃-rich type. Due to the formation of MgTiO₃, the amount of TiB₂ formed in the composite should be less than the stoichiometric amount.

Figure 6a,b exhibits the XRD patterns of the synthesized composites from Equations (3) and (4), respectively. In addition to TiB₂ and MgAl₂O₄, a small amount of MgTiO₃ was identified. The formation of MgAl₂O₄ from a combination reaction between Al₂O₃ and MgO was proved. Both Al₂O₃ and MgO can be totally or partially produced from the reduction of B₂O₃ by Al and Mg. For Equations (3) and (4) containing B₂O₃/Al/Mg-based thermite, TiB₂ was produced from the reaction of metallic Ti with reduced and elemental boron. Moreover, the formation of MgTiO₃ in Equations (3) and (4) might involve some interaction of Ti with B₂O₃ to form TiO₂ which further reacted with MgO. Unlike that in Equations (1) and (2), MgO was no longer detected in the final products of Equations (3) and (4).

MgTiO₃ ceramic has been proved to be an excellent dielectric material, owing to its high dielectric constant, low dielectric loss, high value of quality factors, and good temperature stability [29,30]. It is believed that a trace amount of MgTiO₃ as a minor phase existed in the as-synthesized TiB₂-MgAl₂O₄ products has no effect on the refractory properties of the composites. However, removal of MgTiO₃ from the TiB₂-MgAl₂O₄ composite would be difficult, since it could combine with MgAl₂O₄ in a solid solution form [28].

The SEM image shown in Figure 7 illustrates the microstructure of fracture surface of the product synthesized from Equation (1) which contains a TiO₂/Al/Mg-based thermite. The morphology displays several large and solidified MgAl₂O₄ aggregates of 5–15 μm surrounded by fine-grain TiB₂ crystals with a particle size of about 1–2 μm. Moreover, EDS analysis of two characteristic regions in the product surface indicates that the atomic ratios of Ti:B = 35.2:64.8 and Mg:Al:O = 13.1:28.1:58.8 match well with the stoichiometries of TiB₂ and MgAl₂O₄, respectively.

For the final product of B₂O₃/Al/Mg-based Equation (4), the microstructure and elemental ratios of the components are presented in Figure 8. As can be seen, MgAl₂O₄ was formed as large densified aggregates of around 20 μm and TiB₂ crystals were in a short-rod form with a length of 2–4 μm or in a shape of fine grains of 1–2 μm. Based on the EDS analysis, the atomic ratio of the selected area in an aggregate is Mg:Al:O = 15.1:30.6:54.3 that is reasonably close to MgAl₂O₄. Short-rod crystals have a composition of Ti:B = 34.3:65.7, which certainly is TiB₂.

In summary, the addition of MgAl₂O₄ into TiB₂ enhanced the refractory properties, such as the high-temperature oxidation and corrosion resistance and thermal shock resistance [10,31,32]. Like many sintering aids, MgAl₂O₄ as an additive could improve densification of TiB₂ ceramics and reduce sintering temperatures [33,34]. The abnormal grain growth could be efficiently prevented during the sintering process. As a result, it is more likely to obtain a uniform grain distribution.

Moreover, the TiB₂-MgAl₂O₄ composite is a promising high-temperature microwave absorption material with a reflection loss less than –5 dB at 8.2–18.0 GHz in the temperature range of 25 °C to 1100 °C [11]. The composite also exhibits an extremely high tolerance against intense irradiation in harsh environments [35,36]. Therefore, the potential uses of the TiB₂-MgAl₂O₄ composite might include heat-resistant coatings, nozzles and nose cones of supersonic jets, microwave absorption components, diagnostic or detector windows in fusion devices, target materials in the nuclear applications, etc., [11,35,36].

4. Conclusions

In situ formation of 3TiB₂–MgAl₂O₄ composites was conducted by combustion synthesis combined with metallothermic reduction reactions involving Al and Mg as dual reductants. Thermite reagents with different oxidants were considered; one utilized TiO₂ and the other B₂O₃. The reactant mixtures also contained elemental Ti and boron. This study in total completed four SHS reactions, within which a small amount of Al₂O₃ or MgO was included in the reactive mixture to serve as the combustion moderator and part of the precursors for the formation of MgAl₂O₄. The overall synthesis reaction was exothermic enough to proceed in the SHS mode. An energy-saving and efficient fabrication route for the formation MgAl₂O₄-containing composites was demonstrated.

The analysis of combustion exothermicity indicated that the SHS reaction containing B₂O₃/Al/Mg-based thermites was more energetic than that adopting TiO₂ as the oxidant. Prior addition of Al₂O₃ had a greater cooling effect on combustion than that of MgO. Depending on different thermites and diluents, the measured combustion front temperatures ranged from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s. The temperature dependence of combustion wave velocity was justified. The XRD analysis confirmed in situ formation of TiB₂ and MgAl₂O₄. A small amount of MgTiO₃ was found as the impurity. It is believed that MgAl₂O₄ was synthesized through a combination reaction between Al₂O₃ and MgO, both of which can be totally or partially produced from the metallothermic reduction of B₂O₃ or TiO₂. The microstructure of the synthesized composite exhibited that MgAl₂O₄ was surrounded by closely packed TiB₂ grains. MgAl₂O₄ was formed as densified aggregates with a size of 5–20 μm. TiB₂ crystals were produced in a shape of short rods of 2–4 μm and fine grains of 1–2 μm.