In Situ Formation of Titanium Diboride/Magnesium Titanate Composites by Magnesiothermic-Based Combustion Synthesis

Abstract

In situ formation of TiB₂–Mg₂TiO₄ composites was investigated by combustion synthesis involving the solid-state reaction of Ti with boron and magnesiothermic reduction of B₂O₃. Certain amounts of MgO and TiO₂ were added to the reactant mixtures of Ti/B/Mg/B₂O₃ to act as the moderator of highly exothermic combustion and a portion of the precursors to form Mg₂TiO₄. Two combustion systems were designed to ensure that synthesis reactions were sufficiently energetic to carry on self-sustainably, that is, in the mode of self-propagating high-temperature synthesis (SHS). Consistent with thermodynamic analyses, experimental results indicated that the increase in pre-added MgO and TiO₂ decreased the combustion temperature and propagation velocity of the flame front. MgO was shown to have a stronger dilution effect on combustion exothermicity than TiO₂, because the extent of magnesiothermic reduction of B₂O₃ was reduced in the MgO-added samples. In situ formation of the TiB₂–Mg₂TiO₄ composite was achieved from both types of samples. It is believed that, in the course of the SHS progression, Mg₂TiO₄ was produced through a combination reaction between MgO and TiO₂, both of which were entirely or partially generated from the metallothermic reduction of B₂O₃. The microstructure of the products exhibited fine TiB₂ crystals in the shape of short rods and thin platelets that existed within the gaps of Mg₂AlO₄ grains. Both constituent phases were well distributed. A novel and efficient synthesis route, which is energy- and time-saving, for producing Mg₂TiO₄-containing composites was demonstrated.

1. Introduction

Titanium diboride (TiB₂), belonging to the family of ultra-high temperature ceramics (UHTCs) [1,2,3], possesses a high melting point, low density, metallic electrical conductivity, good thermal stability, excellent wear resistance, corrosion and oxidation resistance, and thermal shock resistance [3,4]. Such a unique combination of material properties renders TiB₂ suitable for applications in molten metal crucibles, cutting tools, wear resistance parts, high-temperature structural components, cathodes for alumina smelting, ballistic armors, rocket nozzles, etc. [3,4,5,6]. Moreover, TiB₂ has received increasing attention for use as a high-temperature microwave-absorbing material recently on account of its high electrical conductivity, great thermal stability, and outstanding resistance to harsh conditions [3]. Many TiB₂-based ceramics, such as TiB₂/MgO, TiB₂/Al₂O₃, TiB₂/SiC, and TiB₂/Al₂O₃/MgAl₂O₄ composites, were recognized for their excellence in high-temperature microwave absorption and dielectric properties [7,8,9,10,11]. Magnesium titanate spinel, Mg₂TiO₄, is particularly renowned for its microwave dielectric properties [12,13], which motivates the fabrication of an innovative composite composed of TiB₂ and Mg₂TiO₄. When compared with their constituent components, TiB₂–Mg₂TiO₄ composites enhance the microwave absorption and dielectric properties of TiB₂ and increase the corrosion, oxidation, and thermal shock resistances of Mg₂TiO₄. It is significant to develop the TiB₂–Mg₂TiO₄ composite as a new refractory material for microwave-tunable devices, voltage-controlled oscillators, dielectric substrates, phase shifters, filters, and antennas to be utilized in harsh environments [12,13,14,15].

There are three stable magnesium titanates (MgTiO₃, Mg₂TiO₄, and MgTi₂O₅) in the MgO–TiO₂ system. Two of them, MgTiO₃ and Mg₂TiO₄, exhibit excellent microwave dielectric properties [14], and especially, there is a growing interest in developing Mg₂TiO₄-based composites as a novel microwave dielectric material [14,15,16,17,18,19]. Conventionally, Mg₂TiO₄ has been fabricated by the solid-state reaction between TiO₂ and MgO under elevated temperatures of 1300–1500 °C for 4–6 h [15,16,17,18]. Cheng et al. [14] prepared Mg₂TiO₄ with MgO and TiO₂ by high-energy ball milling for 30 h, and nanosized Mg₂TiO₄ powders were synthesized at 1000 °C, about 300 °C lower than that by a conventional solid-state reaction process. Composite-type Mg₂TiO₄-MgTiO₃ ceramics were produced by the solid-phase sintering of TiO₂ and MgO powders at 1290–1410 °C for 2–8 h [15]. Solid solutions of MgAl₂O₄-Mg₂TiO₄ spinel ceramics were obtained by the solid-state reaction at 1350–1450 °C for 4 h [18]. Zhang et al. [19] prepared Mg₂TiO₄-based composite ceramics by the hydrothermal method. After removing the solvent and binder at 550 °C for 6 h, Mg₂TiO₄ was synthesized at sintering temperatures below 1300 °C.

As a potential alternative, self-propagating high-temperature synthesis (SHS) or combustion synthesis capitalizes on highly exothermic reactions and is efficient and economical in time, energy, and costs [20,21]. When a traditional SHS process combines with metallothermic reduction of oxide compounds, this composite type of synthesis scheme symbolizes an in situ production route to prepare composite materials containing Al₂O₃ and magnesium aluminate spinel (MgAl₂O₄) [22,23,24,25,26,27]. For example, the aluminothermic reduction of WO₃ and B₂O₃ was incorporated with SHS to prepare Al₂O₃-WSi₂-WB₂-WB composites [22]. TiB₂-Al₂O₃ and NbB₂-Al₂O₃ composites were produced via an aluminothermic-based SHS technique with reactant mixtures containing Al-TiO₂-B₂O₃ and Al-Nb₂O₅-B₂O₃, respectively [23]. MgAl₂O₄-based composites with MoSi₂ and Mo₅Si₃ were produced by the SHS process involving co-reduction of SiO₂ and MoO₃ by Al in the presence of MgO [24]. Composite materials of MoSi₂ and MgAl₂O₄ were also synthesized by incorporating the metallothermic reduction of MoO₃ by Al and Mg as dual reducing agents into the Mo–Si combustion system [25]. The addition of MgO into a reactant mixture consisting of TiO₂, B₂O₃, and Al was applied to prepare TiB₂–MgAl₂O₄ composites through a reduction-based combustion process [26]. A recent study on the synthesis of TiB₂–MgAl₂O₄ composites via SHS indicated that MgAl₂O₄ was produced by a combination reaction between MgO and Al₂O₃, both of which were partially or entirely generated from co-reduction of TiO₂ and B₂O₃ by Al and Mg [27]. Magnesium titanate Mg₂TiO₄ refers to an inverse spinel compound and has the same spinel structure and space group as MgAl₂O₄ [18]. Nonetheless, there are no studies on the combustion synthesis of Mg₂TiO₄-based composites in the available literature.

This study made the first effort to investigate the fabrication of TiB₂–Mg₂TiO₄ composites by metallothermic and self-sustaining combustion synthesis, within which magnesiothermic reduction of B₂O₃ was integrated with solid-state combustion between Ti and boron in the presence of MgO and TiO₂. The role of pre-added MgO and TiO₂ was explored as a combustion moderator and precursor of Mg₂TiO₄. The exothermicity of the SHS reaction, referring to the enthalpy of the reaction and adiabatic combustion temperature, was evaluated. The combustion wave kinetics of the SHS reaction were examined by measuring the combustion front velocity and temperature, and the activation energy of the reaction was deduced. The composition and microstructure of the synthesized products were analyzed.

2. Materials and Methods

This study utilized the following oxide and elemental compounds as the reactants, including MgO (Acros Organics, Mount Olive, NJ, USA, 99.5%), TiO₂ (Acros Organics, Mount Olive, NJ, USA, 99.5%), Mg (Alfa Aesar, Ward Hill, MA, USA, <45 μm, 99.8%), B₂O₃ (Acros Organics, Mount Olive, NJ, USA, 99%), amorphous boron (B) (Noah Technologies, San Antonio, TX, USA, <1 μm, 93.5%), and Ti (Alfa Aesar, Ward Hill, MA, USA, <45 μm, 99.8%). Two reaction systems were designed and formulated as Equations (1) and (2) for the synthesis of 1.5TiB₂ + Mg₂TiO₄ composites.

$$\left(\frac{4-\mathrm{x}}{3}\right){\mathrm{B}}_2{\mathrm{O}}_3+\left(2-\mathrm{x}\right)\mathrm{M}\mathrm{g}+\mathrm{x}\mathrm{M}\mathrm{g}\mathrm{O}+\frac{5}{2}\mathrm{T}\mathrm{i}+\left(\frac{1+2\mathrm{x}}{3}\right)\mathrm{B}\to 1.5\mathrm{T}\mathrm{i}{\mathrm{B}}_2+{\mathrm{M}\mathrm{g}}_2{\mathrm{T}\mathrm{i}\mathrm{O}}_4$$

(1)

$$\left(\frac{4-2\mathrm{y}}{3}\right){\mathrm{B}}_2{\mathrm{O}}_3+2\mathrm{M}\mathrm{g}+\mathrm{y}{\mathrm{T}\mathrm{i}\mathrm{O}}_2+\left(\frac{5-2\mathrm{y}}{2}\right)\mathrm{T}\mathrm{i}+\left(\frac{1+4\mathrm{y}}{3}\right)\mathrm{B}\to 1.5\mathrm{T}\mathrm{i}{\mathrm{B}}_2+{\mathrm{M}\mathrm{g}}_2{\mathrm{T}\mathrm{i}\mathrm{O}}_4$$

(2)

where x and y are stoichiometric coefficients indicating the number of moles of MgO and TiO₂ in the green mixtures of the reaction system (1) and (2), respectively. To ensure a self-sustaining process, the reaction system (1) was conducted with 0.1 ≤ x ≤ 1.0 and (2) with 0.1 ≤ y ≤ 1.0 in this study.

The metallothermic reagents of both combustion systems comprised B₂O₃ as the oxidant and Mg as the reducing agent. Ti and boron powders were adopted for the production of TiB₂. Pre-added MgO and TiO₂ not only acted as a combustion diluent to moderate the highly exothermic reactions, but they also constituted a part of the precursors for the formation of Mg₂TiO₄. The other part of MgO and TiO₂ required for the synthesis of Mg₂TiO₄ was supplied from metallothermic reduction reactions.

The combustion exothermicity of the reaction systems (1) and (2) was investigated by first calculating the enthalpy of reaction (ΔH_r) at 298 K and, second, by evaluating the adiabatic combustion temperature (T_ad) under different stoichiometric coefficients of x and y. Equation (3) was the energy balance equation [26,27] for computing T_ad, and the required thermochemical data were taken from [28].

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

(3)

where n_j is the stoichiometric coefficient of the product (i.e., n_j = 1.5 for TiB₂ and n_j = 1.0 for Mg₂TiO₄ in this study), c_p(P_j) is the specific heat of the product, and L(P_j) is the latent heat of the product.

The well-mixing of dry reactant powders was accomplished by a tumbler ball mill. Reactant powders and alumina grinding balls of 2.5 mm in diameter were contained in a glass cylindrical bottle that rotated about its longitudinal axis. The tumbler ball mill operated at 90 rpm, and the milling time was 4 h. Then, the blended powders were uniaxially compressed in a stainless-steel mold under a packing pressure of 70–80 MPa to prepare test specimens with a height of 12 mm, a diameter of 7 mm, and a relative density of 55%. The SHS experiments were performed in a windowed combustion chamber. The chamber was first purged with high-purity (99.99%) argon for 3 min and then filled with argon at 0.25 MPa. A schematic diagram of the experimental setup [29] is shown in Figure 1. The ignition of the powder compact was achieved by a heated tungsten coil with a voltage of 120 V and a current of 5 A. Based on the time series of recorded combustion images, the propagation velocity of the self-sustaining combustion wave (V_f) was determined from the time derivative of the combustion front trajectory. The exposure time of each recorded image was set at 0.1 ms. A beam splitter with a mirror characteristic of 75% transmission and 25% reflection was used to optically superimpose a scale onto the image of the sample in order to accurately measure the instantaneous locations of the combustion front. The location of the combustion front in each recorded photo was determined by the variation in the gray-level number of pixels. The IMAQ Vision for LabVIEW software (version 6.1) was used to identify the position with the largest directional derivative, which is considered the location of the combustion front. The trajectory of the combustion front was then constructed by plotting the position versus time, and the derivative of position with respect to time was the combustion front velocity.

The combustion temperature was measured by a bare Pt/Pt-13%Rh (R-type) thermocouple with a bead diameter of 125 μm, and the thermocouple was mounted at a position about 6–7 mm from the ignited top plane of the sample. For the synthesized products, the phase composition was identified by a Bruker D2 X-ray diffractometer (XRD Phaser, Karlsruhe, Germany). The diffractometer features a high-precision theta–theta goniometer and uses CuK_α radiation with wavelength λ = 1.5406 Å. The range of the scan was 20°–80° with a scanning rate of 0.05°/s. The microstructure and elemental analysis of the final products were examined by scanning electron microscopy (Hitachi S3000H, Tokyo, Japan) and energy-dispersive X-ray spectroscopy. An acceleration voltage of 15 kV was used in SEM. The number of EDS scans was 4, and the scan time was 60 s.

3. Results and Discussion

3.1. Combustion Exothermicity Analysis

Variations in ΔH_r and T_ad were calculated for Equations (1) and (2) as a function of their stoichiometric coefficients (x and y) in the range between 0.1 and 1.0 and are presented in Figure 2a,b, respectively. Both reactions were highly energetic with T_ad above 2600 K, which justifies self-sustaining combustion. However, due to the dilution effect of pre-added MgO and TiO₂, combustion exothermicity was lowered by increasing the amounts of MgO and TiO₂. There are two major heat-releasing reactions involved in the reaction systems (1) and (2). One is the solid-state reaction of Ti with boron to form TiB₂, with ΔH_r = −315.9 kJ/mol of TiB₂. The other is the magnesiothermic reduction of B₂O₃ to generate boron and MgO, which liberates ΔH_r of 177.2 kJ/mol of MgO [28]. Because Mg is a stronger reducing agent than Ti, the reduction of B₂O₃ was initiated by Mg. Once combustion started off, a portion of Ti could participate in the reaction with B₂O₃ to produce TiO₂. The reaction of B₂O₃ with Ti is less exothermic than that with Mg and has a lower reaction enthalpy of ΔH_r = −108.8 kJ/mol of TiO₂ [28]. For the synthesis of Mg₂TiO₄ from Reaction (1) under 0.1 ≤ x ≤ 1.0, MgO was partly produced from the magnesiothermic reduction of B₂O₃ and partly supplied from pre-added MgO, but TiO₂ was totally generated from the reaction of Ti with B₂O₃. Because Reaction (2) adopts TiO₂-added samples, the sources of MgO and TiO₂ for the formation of Mg₂TiO₄ are opposite to those of Reaction (1). It should be noted that the formation of Mg₂TiO₄ from a solid-state reaction between TiO₂ and MgO is an endothermic process with ΔH_r = 302.8 kJ/mol of Mg₂TiO₄ [28].

Figure 2a,b shows the decrease in ΔH_r from −925 to −765 kJ for Reaction (1) with increasing MgO content from x = 0.1 to 1.0, within which T_ad declines from 3055 to 2660 K. Similarly, as revealed in Figure 2a,b, the increase in TiO₂ from y = 0.1 to 1.0 in Reaction (2) leads to a decrease in both ΔH_r (from −932 to −845 kJ) and T_ad (from 3075 to 2860 K). A more pronounced decrease in both ΔH_r and T_ad was observed for Reaction (1), suggesting a stronger dilution effect on combustion imposed by MgO than TiO₂ addition. This was attributed to the fact that the increase in MgO in Reaction (1) decreased the magnitude of the magnesiothermic reduction of B₂O₃. However, the amount of MgO produced from the reduction of B₂O₃ was not affected by the addition of TiO₂ in Reaction (2). That is, the extent of magnesiothermic reduction of B₂O₃ remained unchanged in Reaction (2). As mentioned above, the reduction of B₂O₃ by Mg is more exothermic than the reaction of B₂O₃ with Ti. Moreover, it is noteworthy that the number of moles of MgO required to form Mg₂TiO₄ is twice as many as that of TiO₂.

3.2. Self-Propagating Combustion Wave Kinetics

Two typical time sequences of recorded combustion images are illustrated in Figure 3a,b, which shows the SHS processes of powder compacts of the reaction systems (1) with x = 0.5 and (2) with y = 0.5, respectively. As shown in Figure 3, a scale image was recorded and displayed on the left-hand side of each recorded photo; the scale bar has a unit of 1 mm. It is evident that both combustion processes became self-sustaining upon initiation and featured a distinct combustion front propagating along the powder compact. The combustion of the MgO-added sample shown in Figure 3a appeared to be less violent and took a longer time to complete the flame propagation when compared with that of the TiO₂-containing sample in Figure 3b. This implies a higher exothermicity of Reaction (2) and agrees with thermodynamic calculations. Moreover, the visual observation of recorded films clearly revealed that the degree of combustion strength and the intensity of burning glow were gradually alleviated as the amount of MgO and TiO₂ increased in the samples.

Figure 4 presents the measured combustion wave velocities (V_f) of the reaction systems (1) and (2) containing different molar contents of pre-added MgO and TiO₂. Figure 3 indicates that for the reaction system (1), as the amount of MgO increases from 0.1 to 1.0 mol, the combustion wave velocity decreases significantly from 8.6 to 2.1 mm/s. The deceleration of the combustion wave resulted largely from the lessening of combustion exothermicity. Because heat transfer by conduction from the thin reaction zone to its adjacent unburned region plays an important role in establishing a self-sustaining combustion wave, the propagation velocity, in large part, is governed by the reaction zone temperature. For the reaction system (2), the decline in combustion wave spreading speed was relatively moderate, and the flame velocity was lowered from 11 to 7.2 mm/s when the number of moles of TiO₂ increased from 0.1 to 1.0. Not only was the combustion wave velocity of the reaction system (2) higher than that of Reaction (1), but also the discrepancy between them became larger as the amounts of MgO and TiO₂ augmented. The velocity difference between the two reaction systems expanded from about 2.4 to 5.1 mm/s. This indicates that MgO imposes a stronger attenuation effect on combustion, and its influence is more intense as MgO increases.

Three typical combustion temperature profiles are depicted in Figure 5a,b, and they were respectively measured from the powder compacts of the reaction systems (1) and (2) with different MgO and TiO₂ contents. All profiles exhibit a sharp positive gradient, signifying the speedy arrival of the combustion front. Because of a thin reaction zone, the peak value is considered the combustion front temperature (T_c). After the rapid progression of the combustion wave, the profile showed a dramatic temperature decrease, indicating the burned sample was experiencing a rapid cooling rate. The temperature profile is characteristic of the SHS reaction, which features a thin reaction zone and a speedy combustion wave. Three temperature profiles depicted in Figure 5a show T_cs of 1607 °C, 1466 °C, and 1312 °C for the samples of Reaction (1) at x = 0.25, 0.5, and 1.0, respectively. Due to higher exothermicity for Reaction (2), as revealed in Figure 5b, higher T_cs reaching 1658 °C, 1588 °C, and 1467 °C were detected correspondingly for the powder compacts of y = 0.25, 0.5, and 1.0. As expected, the highest combustion front temperatures for the reaction systems (1) and (2) were measured at x = 0.1 and y = 0.1, respectively. The highest T_c of Reaction (1) approached 1670 °C, which is slightly lower than that of Reaction (2) at 1705 °C. It is worth mentioning that the dependence of T_c on the content of MgO and TiO₂ not only validated the thermal analysis calculations but also justified the variation in combustion wave velocity with the stoichiometric coefficient. When compared with the calculated adiabatic temperature T_ad, however, the measured combustion front temperature T_c appeared to be much lower. This suggests that combustion of the powder compact was subject to extensive heat losses to the surrounding argon gas by conduction and convection, to the steel sample holder by conduction, and to the chamber inner wall by radiation.

According to combustion wave kinetics [30], the activation energy (E_a) of a solid-state combustion reaction can be determined from a modified Arrhenius rate equation, as can be seen in Equation (4).

$${V_f}^2=\frac{2\lambda }{\rho Q}\frac{R{T_c}^2}{E_a}{k}_o\exp (-E_a/R{T}_c)$$

(4)

where λ is the thermal conductivity, R is the universal gas constant, Q is the heat of the reaction, and k_o is a constant. The use of experimental data for V_f and T_c to construct the variation in ln(V_f/T_c)² with 1/T_c has been widely employed to deduce E_a [27,31]. That is, the slope of a linear line correlating ln(V_f/T_c)² and 1/T_c signifies E_a/R. Figure 6 plots two sets of experimental data for the reaction systems (1) and (2) and their respective best-fitted linear lines and slopes. From the slopes of two straight lines, the activation energies E_a of 183 and 91 kJ/mol were obtained for (1) and (2), respectively. A larger E_a for (1) means a higher kinetic barrier for the synthesis reaction to occur. Because the magnesiothermic reduction of B₂O₃ is considered the initiation step of the SHS process in (1) and (2), the extent of magnesiothermic reduction of (1) is less than that of (2). In addition, the TiO₂ required in (1) must be all produced from the reduction reaction of B₂O₃ by Ti, which is more difficult to achieve than the magnesiothermic reduction of B₂O₃. On the contrary, (2) contains pre-added TiO₂ and fulfills a complete magnesiothermic reduction. From the thermodynamic point of view, the combustion exothermicity of (2) is greater than that of (1), which also facilitates the reaction to be initiated and carried out.

3.3. Phase Composition and Microstructure of Synthesized Products

Figure 7a,b presents typical XRD spectra of TiB₂–Mg₂TiO₄ composites synthesized from the reaction systems (1) with x = 0.5 and (2) with y = 0.5, respectively. In situ formation of TiB₂ and Mg₂TiO₄ from self-sustaining combustion of (1) and (2) under these two conditions was confirmed, and almost no other phases were identified from the XRD analysis. It is believed that the formation of Mg₂TiO₄ was completed by a combination reaction between MgO and TiO₂ during the SHS process. For Reaction (1), MgO was partly from the pre-added precursor and partly from the product of the Mg–B₂O₃ thermite, while TiO₂ was totally generated from the reaction of Ti with B₂O₃. In contrast, the pre-added precursor in Reaction (2) was TiO₂, which reacted with thermite-produced MgO and TiO₂ to form Mg₂TiO₄. Similar to the synthesis of MgAl₂O₄ from SHS involving reducing stages [25], the evolution of Mg₂TiO₄ could be governed by a dissolution–precipitation mechanism. Namely, in situ-formed MgO and TiO₂ dissolved into pre-added TiO₂ or MgO particles, and Mg₂TiO₄ grains then precipitated as saturation was achieved.

It should be noted that for the reaction system (1), the XRD pattern of the final products synthesized from the samples of x = 0.9 and 1.0 indicated the presence of a trivial amount of another magnesium titanate, MgTiO₃, which was considered an intermediate phase for the formation of Mg₂TiO₄. During the SHS process, it is believed that the reaction of TiO₂ with MgO forms MgTiO₃ first, and MgTiO₃ subsequently combines with another MgO to become Mg₂TiO₄. Low reaction temperatures of about 1310–1340 °C for (1) with x = 0.9 and 1.0 could be responsible for this incomplete conversion. In contrast, the synthesized products from (2) with y = 0.1–1.0 contained no MgTiO₃ and other unwanted phases. This could be attributed to its high reaction temperatures of 1467–1705 °C.

In summary, the formation mechanism of TiB₂–Mg₂TiO₄ composites involves an elemental reaction, a metallothermic reduction, and a combination reaction. For both reaction systems, the initiation step was the magnesiothermic reduction of B₂O₃ to form MgO and boron, as in Reaction (5). Subsequently, the elemental reaction of Ti with boron took place to produce TiB₂, as in Reaction (6). Both Reactions (5) and (6) were highly exothermic, so the reduction of B₂O₃ by Ti, as in Reaction (7), was triggered to form TiO₂ and boron. Next, a series of combination reactions between MgO and TiO₂ occurred as in Reactions (8) and (9), which yielded MgTiO₃ and then Mg₂TiO₄.

$$\mathrm{M}\mathrm{g}+\frac{1}{3}{\mathrm{B}}_2{\mathrm{O}}_3\to \mathrm{M}\mathrm{g}\mathrm{O}+\frac{2}{3}B$$

(5)

$$\mathrm{T}\mathrm{i}+2\mathrm{B}\to \mathrm{T}\mathrm{i}{\mathrm{B}}_2$$

(6)

$$\mathrm{T}\mathrm{i}+\frac{2}{3}{\mathrm{B}}_2{\mathrm{O}}_3\to \mathrm{T}\mathrm{i}{\mathrm{O}}_2+\frac{4}{3}B$$

(7)

$$\mathrm{M}\mathrm{g}\mathrm{O}+\mathrm{T}\mathrm{i}{\mathrm{O}}_2\to \mathrm{M}\mathrm{g}\mathrm{T}\mathrm{i}{\mathrm{O}}_3$$

(8)

$$\mathrm{M}\mathrm{g}\mathrm{T}\mathrm{i}{\mathrm{O}}_3+\mathrm{M}\mathrm{g}\mathrm{O}\to {\mathrm{M}\mathrm{g}}_2\mathrm{T}\mathrm{i}{\mathrm{O}}_4$$

(9)

The samples for the SEM examination were taken from the cross-section surface of the synthesized sample. Figure 8a is the SEM image illustrating the microstructure of the fracture surface of the synthesized product of the reaction system (1) with x = 0.5. The morphology displays uniformly distributed Mg₂TiO₄ agglomerates (the irregular gray chunks) with a size of 3–5 μm. EDS analysis was conducted to identify Mg₂TiO₄ and TiB₂ from the SEM images. The locations of point analysis on the SEM images were indicated with an x sign; that is, P1 and P2 are pointed out in Figure 8a and P3 and P4 in Figure 8b. An atomic ratio of Mg:Ti:O = 26.7:15.2:58.1, which is close to the stoichiometry of Mg₂TiO₄, was detected for the agglomerates based on the EDS analysis at P1. TiB₂ crystals (the small white grains), which were confirmed by an atomic ratio of Ti:B = 32.4:67.6 at P2, existed in the interstices among Mg₂TiO₄ grains. TiB₂ crystals were formed as either short rods with a length of 1–2 μm or flattened platelets with a thickness of 0.5–1.0 μm. The final products of (1) all exhibited a similar microstructure to Figure 8a. For the TiB₂–Mg₂TiO₄ composite obtained from the reaction system (2), Figure 8b also unveils a similar microstructure to that shown in Figure 8a. Small TiB₂ crystals in the shape of short rods and thin platelets were present within the gaps of Mg₂AlO₄ grains. Both phases were well distributed and closely packed. For the Mg₂TiO₄ and TiB₂ grains in Figure 8b, elemental ratios of Mg:Ti:O = 29.1:13.8:57.1 and Ti:B = 34.6:65.4 were obtained by the EDS analysis at P3 and P4, respectively.

4. Conclusions

In situ formation of TiB₂–Mg₂TiO₄ composites with a molar ratio of TiB₂:Mg₂TiO₄ = 1.5:1 was completed by combustion synthesis integrating magnesiothermic reduction of B₂O₃ with a solid-state reaction between Ti and boron. Two types of green mixtures were considered: one adopted MgO as the combustion moderator and a part of the precursor of Mg₂TiO₄; the other employed TiO₂. The amounts of pre-added MgO and TiO₂ were varied within 0.1 to 1.0 mol to ensure combustion of both types of sample compacts was stable and sufficiently exothermic to take place in the SHS mode. A novel and rapid synthesis route for producing Mg₂TiO₄-containing composites was demonstrated.

According to the analysis of reaction exothermicity, combustion of the MgO-added sample is less energetic than that of the TiO₂-added sample. Experimental results were consistent with the calculations and indicated that the increase in pre-added MgO decreased the combustion front temperature from 1670 to 1312 °C, as well as the combustion wave velocity from 8.6 to 2.1 mm/s. Likewise, a decrease in combustion temperature from 1705 to 1467 °C and flame-front velocity from 11 to 7.2 mm/s was observed for the TiO₂-added samples. MgO was shown to have a greater dilution effect on combustion than TiO₂, because pre-added MgO reduced the extent of magnesiothermic reduction of B₂O₃ in the SHS process. Activation energies E_a of 183 and 91 kJ/mol were deduced for MgO- and TiO₂-added reaction systems, respectively, based on a modified Arrhenius rate equation with measured combustion wave velocities and temperatures. A larger E_a implies a higher kinetic barrier for MgO-added samples to be initiated and proceed because of the reduction of B₂O₃ by both Mg and Ti and low combustion exothermicity. The XRD analysis confirmed in situ formation of the TiB₂–Mg₂TiO₄ composite with almost no other minor phases. It is considered that in the course of the SHS process, Mg₂TiO₄ was produced from the interaction between MgO and TiO₂, both of which were totally or partially generated from the metallothermic reduction of B₂O₃. The dissolution of in situ-formed MgO and TiO₂ into pre-added TiO₂ or MgO particles led to the precipitation of Mg₂TiO₄ grains. The microstructure of the synthesized products based on SEM observations exhibited that two constituent components, TiB₂ and Mg₂TiO₄, were well distributed and closely engaged. Mg₂TiO₄ agglomerates have a size of 3–5 μm. TiB₂ crystals are in the form of short rods of 1–2 μm in length and thin platelets of 0.5–1.0 μm in thickness.