Combustion Synthesis of NbB₂–Spinel MgAl₂O₄ Composites from MgO-Added Thermite-Based Reactants with Excess Boron

Abstract

The formation of NbB₂–MgAl₂O₄ composites from the MgO-added thermite-based reaction systems was investigated by self-propagating high-temperature synthesis (SHS). Two thermite mixtures, Nb₂O₅/B₂O₃/Al and Nb₂O₅/Al, were, respectively, adopted in Reactions (1) and (2). The XRD analysis confirmed the combination of Al₂O₃ with MgO to form MgAl₂O₄ during the SHS process and that excess boron of 30 atom.% was required to yield NbB₂–MgAl₂O₄ composites with negligible NbB and Nb₃B₄. The microstructure of the composite reveals that rod-shaped MgAl₂O₄ crystals are closely interlocked and granular NbB₂ are embedded in or scattered over MgAl₂O₄. With the addition of MgAl₂O₄, the fracture toughness (K_IC) of 4.37–4.82 MPa m^1/2 was obtained for the composites. The activation energies E_a = 219.5 ± 16 and 167.9 ± 13 kJ/mol for Reactions (1) and (2) were determined from combustion wave kinetics.

1. Introduction

Transition metal (IVB and VB) diborides, such as TiB₂, ZrB₂, NbB₂, and TaB₂, have been referred to as ultra-high-temperature ceramics (UHTCs). Transition metal diborides crystallize in the hexagonal A1B₂ type structure (space group P6/mmm) with c/a ratio close to unity. In this arrangement, the hexagonal nets of metal atoms and triangle nets of pure boron atoms are alternately stacked along the c axis [1]. Besides their melting points exceeding 3000 °C, they possess a unique combination of high hardness, thermal conductivity, electrical conductivity, excellent chemical stability, corrosion resistance, and thermal shock resistance [2,3,4,5]. They have found a broad range of applications in the mechanical, automobile, aerospace industries, etc. [5]. To improve the refractory properties of metal borides and carbides, Al₂O₃ or MgAl₂O₄ (magnesium aluminate spinel) has been considered as an additive. Extensive studies have been conducted on understanding the production and characteristics of different metal borides reinforced by Al₂O₃ [6,7,8,9,10], but relatively few studies have focused on the boride–MgAl₂O₄ composite [11].

MgAl₂O₄ is of particular interest due to its exceptional mechanical, thermal, and optical properties, such as high melting point (2135 °C), high hardness (16 GPa), relatively low density (3.58 g/cm³), high mechanical strength (135–216 MPa), good transmittance in the wavelength range of 0.25 to 5.0 μm, high thermal shock resistance, high chemical inertness, low dielectric constant, and low thermal expansion coefficient [12,13]. MgAl₂O₄ spinel has a close-packed face-centered cubic (fcc) structure of space group Fd3m (number 227). There are eight MgAl₂O₄ units per cubic cell. Mg and Al cations occupy 1/8 of the tetrahedral sites and 1/2 of the octahedral sites and there are 32 oxygen ions in the unit cell. In the normal spinel, all Al³⁺ ions are in octahedral coordination with local symmetry D_3d and all Mg²⁺ ions are in tetrahedral coordination with point group symmetry T_d. The anion sublattice is arranged in a pseudo-cubic close-packed (ccp) spatial configuration. There are four layers of AlO₆ octahedral chains along the c axis in one unit cell [12]. Fabrication of MgAl₂O₄ and other spinel compounds like NiFe₂O₄ and CoFe₂O₄ by either solid-state or wet-chemistry reaction methods usually requires long processing time and complicated steps [14,15,16,17,18].

With the merits of energy and time savings, simple operation, and high-purity products, combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) has been recognized as an alternative route to material synthesis and processing [19,20]. When the SHS process is combined with the aluminothermic or magnesiothermic reduction of metal oxides to generate Al₂O₃ or MgO, the multistage combustion reaction represents an in situ approach to producing MgAl₂O₄-containing composites [21,22,23,24,25]. Horvitz and Gotman [21] obtained TiAl–Ti₃Al–MgAl₂O₄ composites from the powder compacts composed of 2TiO₂–Mg–4Al via the SHS reaction and thermal explosion. Magnesiothermic reduction of WO₃ and B₂O₃ in the presence of Al₂O₃ under the SHS manner was conducted by Omran et al. [22] to prepare MgAl₂O₄–W and MgAl₂O₄–W–W₂B composites. With the use of MoO₃, SiO₂, and Al as the thermite reagents mixing with Mo, Si, and MgO powders for the SHS reaction, the formation of various molybdenum silicides (MoSi₂, Mo₅Si₃, and Mo₃Si) and MgAl₂O₄ composites was achieved [23,24,25].

This study made an attempt to investigate the preparation of NbB₂–MgAl₂O₄ in situ composites via a reduction-based SHS process. The multistage combustion process involves the aluminothermic reduction of Nb₂O₅ and B₂O₃ in the presence of MgO, elemental interaction of Nb with B, and the combination of MgO with Al₂O₃. To resolve the loss of boron in the form of gas-phase B₂O₂ and BO during the SHS process [8,9], this study examined test specimens with excess boron. The effects of thermite reagents and excess boron on the reaction exothermicity and combustion wave kinetics were explored. Moreover, the phase composition, microstructure, and mechanical properties of the final products were characterized.

2. Materials and Methods

The starting materials utilized by this study included niobium (V) oxide (Nb₂O₅) (Strem Chemicals, <45 μm, 99.9%, Newburyport, MA, US), B₂O₃ (Strem Chemicals, 99.9%), Al (Showa Chemical Co., <45 μm, 99.9%, Tokyo, Japan), Nb (Strem Chemicals, <45 μm, 99.8%), amorphous boron (Noah Technologies Corp., <1 μm, 92%, San Antonio, TX, US), and MgO (Alfa Aesar, 99%, Ward Hill, MA, US). Two MgO-added combustion systems with different thermite reagents were studied. As expressed below, Reaction (1) comprises Nb₂O₅ and B₂O₃ as thermite oxidants, while Reaction (2) has Nb₂O₅ only. For both reaction systems, Al acts as the thermite reductant.

$$\left[\frac{x}{2}{\mathrm{Nb}}_2{\mathrm{O}}_5+\left(1-\frac{5}{6}x\right){\mathrm{B}}_2{\mathrm{O}}_3+2\mathrm{Al}\right]+\left(\frac{11}{3}x-2\right)\mathrm{B}+\mathrm{MgO}\to x{\mathrm{NbB}}_2+{\mathrm{MgAl}}_2{\mathrm{O}}_4$$

R(1)

$$\left(\frac{3}{5}{\mathrm{Nb}}_2{\mathrm{O}}_5+2\mathrm{Al}\right)+\left(y-\frac{6}{5}\right)\mathrm{Nb}+2y\mathrm{B}+\mathrm{MgO}\to y{\mathrm{NbB}}_2+{\mathrm{MgAl}}_2{\mathrm{O}}_4$$

R(2)

where x and y are stoichiometric coefficients signifying the mole number of NbB₂ formed per unit mole of MgAl₂O₄ in Reactions (1) and (2), respectively.

The experiments of Reaction (1) were performed with x = 0.6–1.0. The increase of x raises the amount of Nb₂O₅ but reduces that of B₂O₃ in the reactant mixture. Under this condition, more amorphous boron is added to make up for the decrease of boron provided from B₂O₃. Samples of Reaction (2) were formulated with y = 1.2–1.8. The increase of y augments elemental Nb and B for the production of a larger amount of NbB₂, but has no change in the molar quantity of thermite reagents, Nb₂O₅ and Al, in Reaction (2). A combination of Reactions (1) and (2) renders this study feasible to obtain products with a molar proportion of NbB₂/MgAl₂O₄ from 0.6 to 1.8. In addition, the SHS reactions with test specimens containing excess boron of 20 and 30 atom.% were conducted to examine the extent of boron loss during combustion and to compensate for the relatively low purity (92%) of amorphous boron used in this study. It should be noted that boron has high hardness, great stability in extreme environments, and good resistance to heat. It has several forms, the most common of which is amorphous boron, and is unreactive to oxygen, water, acids, and alkalis. While barely reactive at room temperature, boron reacts strongly at high temperature with metals to form borides. Moreover, its reducing properties allow it to react with numerous compounds and, as in the case of oxygenated or halogenated compounds, in a violent manner [26].

In Reaction (1), the aluminothermic reduction of Nb₂O₅ (the reaction enthalpy, ΔH_r = −536 kJ/mol of Al₂O₃ and the adiabatic temperature, T_ad = 2756 K) is more energetic than that of B₂O₃ (ΔH_r = −403.8 kJ/mol of Al₂O₃ and T_ad = 2315 K) [27,28]. The intermetallic reaction of Nb + 2B to yield NbB₂ with formation enthalpy ΔH_f = −175.3 kJ/mol and T_ad = 2315 K is exothermically comparable to the B₂O₃ + 2Al reaction [29]. The combination reaction between MgO and Al₂O₃ to form MgAl₂O₄ is weakly exothermic with ΔH_r = −35.6 kJ [23]. This means that with the increase of NbB₂ content (the x value), two opposing effects govern the combustion exothermicity of Reaction (1). As far as Reaction (2) is concerned, the increase of y for the formation of a larger NbB₂ content has a dilution effect on combustion. To elucidate the combustion exothermicity, calculation of T_ad of Reactions (1) and (2) under different stoichiometric coefficients was performed according to the following equation [23,30] with thermochemical data taken from [29].

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

where ΔH_r is the reaction enthalpy at 298 K, n_j is the stoichiometric constant, C_p and L are the heat capacity and latent heat, and P_j refers to the product. Calculations of the adiabatic temperature were based upon the final products of stoichiometric reactions described in R(1) and R(2).

Reactant powders were mixed in a ball mill and then uniaxially compressed in a stainless-steel mold at a pressure of 70–80 MPa to form cylindrical test samples with 7 mm in diameter, 12 mm in height, and a relative density of 60%. The relative density of the test specimen is related to the initial components. The theoretical density (ρ_TD) of the test specimen is calculated from the mass fraction (Y) and density (ρ) of each component through the following equation.

$$\frac{1}{{\rho }_{TD}}=\frac{Y_{Nb2O5}}{{\rho }_{Nb2O5}}+\frac{Y_{B2O3}}{{\rho }_{B2O3}}+\frac{Y_{Al}}{{\rho }_{Al}}+\frac{Y_B}{{\rho }_B}+\frac{Y_{MgO}}{{\rho }_{MgO}}$$

The SHS experiments were conducted in a windowed combustion chamber filled with high-purity (99.99%) argon. The propagation velocity of the combustion wave (V_f) was determined from the time sequence of recorded pictures. The reaction temperature was measured by the Pt/Pt-13%Rh bare wire thermocouple with a bead diameter of 125 μm. A thin ceramic (SiO₂) coating is usually to prevent the catalytic effect on the thermocouple in the measurement of gas-phase flame temperature of a combustion mixture involving hydrogen, methane, or propane as the fuel. Because solid-state combustion with argon as the surrounding gas is under investigation, the thermocouple without an inert coating was used by this work. Details of the experimental setup and scheme were described elsewhere [25,31]. After the SHS process, phase constituents of the products were analyzed by an X-ray diffractometer (Bruker D2, Billerica, MA, US) using CuK_α radiation. The microstructure of the final product was examined by a scanning electron microscope (Hitachi S3000H, Tokyo, Japan) and elemental proportion was deduced from the energy dispersive spectroscopy (EDS). For Vickers hardness (H_v) and fracture toughness (K_IC) measurement, selected experiments with the reactant compact placing in a steel mold were conducted. Upon the completion of the self-sustaining combustion reaction, densification of the product was carried out by a hydraulic press machine [25].

3. Results and Discussion

3.1. Combustion Wave Kinetics and Reaction Temperature

Figure 1 illustrates a typical series of combustion images obtained by this study, which was recorded from the powder compact of Reaction (2) with y = 1.4. As can be seen in Figure 1, a well-defined combustion wave is established upon ignition and propagates throughout the entire sample in a self-sustaining manner. This demonstrates sufficient reaction exothermicity of the reactant mixture. After combustion, the burned sample essentially retained its original shape.

The influence of the stoichiometric coefficient and excess boron on the flame-front velocity of Reactions (1) and (2) is presented in Figure 2. It was found that for the samples without excess boron, the combustion wave velocity of Reaction (1) increased from 2.5 to 6.6 mm/s with x increasing from 0.6 to 1.0, while that of Reaction (2) decreased from 8.4 to 4.8 mm/s with y in the range from 1.2 to 1.8. To be presented lately, it is believed that the variation of combustion front velocity with the reaction stoichiometry depends mainly on the exothermicity of the SHS process. As also indicated in Figure 2, samples with excess boron of 30 atom.% exhibit lower combustion wave speeds when compared to those without additional boron. This could be attributable to a prolonged sequence of phase evolution of borides in response to the increase of boron.

Figure 3 plots several measured sample temperature profiles, which depict a sharp rise signifying the rapid arrival of the combustion wave and a peak value corresponding to the combustion front temperature (T_c). After the progression of the combustion wave, a substantial temperature decline is a consequence of heat loss to the surroundings. As revealed in Figure 3, the peak temperature (T_c = 1516 °C) of Reaction (1) with x = 0.9 is higher than that of x = 0.6 (T_c = 1317 °C) and the combustion front temperature of Reaction (2) decreases from 1606 °C to 1416 °C with an increase of y from 1.2 to 1.8. Based on the experimental measurement, the stoichiometric dependence of the combustion wave temperature is in agreement with that of flame-front velocity.

A comparison between the calculated adiabatic temperature and measured combustion front temperature of Reactions (1) and (2) is presented in Figure 4, indicative of a consistent variation of T_ad and T_c with the reaction stoichiometry. The increase of the combustion temperature of Reaction (1) with increasing NbB₂/MgAl₂O₃ ratio is ascribed to a larger proportion of Nb₂O₅ to B₂O₃ in the thermite mixture, since the aluminothermic reduction of Nb₂O₅ is more exothermic. However, a decline of the combustion temperature of Reaction (2) with NbB₂/MgAl₂O₃ molar ratio confirms the cooling effect on combustion by increasing Nb and B, because the elemental reaction of Nb with B is less energetic than the thermite reaction of Nb₂O₅ and Al. As shown in Figure 4, the values of T_ad are higher by about 350–400 °C than those of T_c. The discrepancy between T_c and T_ad might result from considerable heat loss mostly by radiation, substantial boron elimination from the reaction zone, and formation of boride phases different from the stoichiometric composition.

The activation energy (E_a) of solid-state combustion was deduced from combustion wave kinetics by constructing a correlation between ln(V_f/T_c)² and 1/T_c in a form of linear relationship [32,33]. Figure 5 depicts two sets of experimental data with best-fitted straight lines. From the slopes of straight lines, E_a = 219.5 ± 16 and 167.9 ± 13 kJ/mol were deduced for Reactions (1) and (2), respectively. A larger E_a for Reaction (1) means a higher kinetic barrier in comparison to Reaction (2). This could be caused most likely by the fact that the co-reduction of Nb₂O₅ and B₂O₃ by Al is required in Reaction (1) for the synthesis sequence to proceed, but Reaction (2) has only Nb₂O₅ to be reduced. According to Arrhenius kinetics, the activation energy of the solid-state reaction is governed by the reaction mechanism. In this study, aluminothermic reduction of metal oxides is considered as a first step of the SHS process, which is followed by a combination of Al₂O₃ and MgO to form MgAl₂O₄ and elemental interactions between Nb and B to produce NbB₂.

3.2. Phase Composition and Microstructure of As-Synthesized Products

Figure 6a–c displays the XRD patterns of SHS-derived products from Reaction (1) of x = 1.0 without and with excess boron. As shown in Figure 6, MgAl₂O₄ is identified and niobium borides exist in three phases including NbB₂, Nb₃B₄, and NbB. The formation of MgAl₂O₄ confirms a combination reaction between pre-added MgO and thermite-produced Al₂O₃. The presence of NbB and Nb₃B₄ denotes that the amount of boron is inadequate to transform all the borides into NbB₂. For the sample without extra boron, as shown in Figure 6a, NbB is the dominant boride phase. Besides, there is NbO₂ detected in the final product, indicative of an incomplete reduction of Nb₂O₅. For the sample with excess boron of 20 atom.%, Figure 6b indicates that the yield of Nb₃B₄ and NbB₂ is enhanced and NbO₂ is no longer detectable. Furthermore, Figure 6c reveals that more NbB₂ is formed in the final product obtained from the sample with extra boron of 30 atom.%. Although NbB₂ is the dominant boride in Figure 6c, the other two borides, NbB and Nb₃B₄, are not trivial. This means a substantial boron loss from the samples of Reaction (1) possibly through two different paths. One is the reduction of B₂O₃ and the other is the borothermal reaction between Nb₂O₅ and boron. Both reactions could generate gaseous B₂O₂ and BO, and they might expel from the sample compact.

The effect of excess boron on the formation of borides for Reaction (2) is presented in Figure 7a–c. Likewise, excess boron enhanced the production of NbB₂. It was found that the improvement is more effective for Reaction (2) than Reaction (1). Figure 7b unveils that NbB₂ prevails over NbB and Nb₃B₄ for the sample with excess boron of 20 atom.%. Moreover, as shown in Figure 7c, NbB and Nb₃B₄ become negligible in the resulting product from the sample with excess boron of 30 atom.%. This is because Reaction (2) contains no B₂O₃, only borothermal reduction of Nb₂O₅ could result in the loss of boron.

Typical microstructures of the fracture surface of the NbB₂–MgAl₂O₄ composites synthesized from Reactions (1) and (2) with 30 atom.% extra boron are illustrated in Figure 8a,b, respectively. As displayed in the micrographs of Figure 8a,b; the long rod-shaped MgAl₂O₄ crystals form a dense matrix with an interlocking structure and small NbB₂ grains are embedded in MgAl₂O₄ or distributed over the surface. Furthermore, MgAl₂O₄ and NbB₂ phases in Figure 8a are confirmed by the atomic ratios of Mg:Al:O = 14.1:28.9:57.0 and Nb:B = 32.7:67.3 deduced from the EDS spectrum. Similarly, in Figure 8b, MgAl₂O₄ and NbB₂ are determined to have Mg:Al:O = 13.7:29.8:56.5 and Nb:B = 33.1:66.9, both of which match well with their exact stoichiometries.

Densified products have a relative density of about 92%–95%. For the NbB₂–MgAl₂O₄ composite synthesized from Reaction (1) of x = 1.0, H_v = 15.2 GPa and K_IC = 4.82 MPa m^1/2 were determined. The composite obtained from Reaction (2) of y = 1.8 exhibits H_v = 16.9 GPa and K_IC = 4.37 MPa m^1/2. When compared with pure NbB₂ (K_IC = 3.76 MPa m^1/2) [34], the NbB₂–MgAl₂O₄ composite is toughened. Similar materials like TiB₂–Al₂O₃ composites fabricated by hot pressing at a sintering temperature of 1700 °C showed a relative density of about 96.2%, Vickers hardness of 24.8 GPa, and fracture toughness of 4.56 MPa m^1/2 [35]. This confirms that satisfactory toughness is obtained for the NbB₂–MgAl₂O₄ composite synthesized by this study.

4. Conclusions

This study prepared NbB₂–MgAl₂O₄ in situ composites with a molar ratio of NbB₂/MgAl₂O₄ from 0.6 to 1.8 by the SHS process with reducing stages. Within the scope of experimental variables, combustion front velocity and temperature increased with NbB₂ content for Reaction (1), because the proportion of Nb₂O₅ to B₂O₃ in the thermite mixture increased. On the other hand, Reaction (2) showed a decrease in combustion velocity and temperature as NbB₂ content increased, because of the dilution effect of additional Nb and B on combustion. The activation energies, E_a = 219.5 ± 16 and 167.9 ± 13 kJ/mol, were, respectively, deduced for Reactions (1) and (2), suggesting a higher kinetic barrier for Reaction (1).

The XRD analysis of the final product confirms the formation of MgAl₂O₄ from thermite-produced Al₂O₃ and pre-added MgO. For the samples without extra boron, three boride phases, NbB, Nb₃B₄, and NbB₂, were formed and dominated by NbB. Excess boron up to 30 atom.% effectively compensated for the loss of boron during combustion and promoted the formation of NbB₂ as the major boride compound. The microstructure of as-synthesized NbB₂–MgAl₂O₄ composites is characterized by interlocking rod-shaped MgAl₂O₄ crystals and small NbB₂ grains embedded in MgAl₂O₄ or distributed over the surface. Fracture toughness of the NbB₂–MgAl₂O₄ composite was improved up to K_IC = 4.37–4.82 MPa m^1/2.