Introduction

To improve the energy density and reaction activity [1], micro-sized solid propellant ingredients such as metals [2,3,4], metal oxides [5], oxidants [6] and nitrocellulose fibers [7] have been substituted with nano-materials. In the past several decades, carbides have been used as additives for solid propellants [8,9,10]. However, related applications of nano-carbides are rarely reported.

Compared to other metal fuels, boron (B) has elevated gravimetric (58.86 kJ g−1) and volumetric calorific value (137.73 kJ cm−3) [11], which raised interests for studying the oxidation mechanisms and methods that will allow advancing the heat release of B [12, 13]. Huang et al. [14] and Xi et al. [15], respectively, investigated the catalytic action of metal oxides (micro-sized) on B oxidation and found that the initial oxidation temperature and ignition delay time of B might be reduced by addition of certain metal oxides such as bismuth oxide, iron oxide, and stannic oxide. However, these metal oxides exhibited little effect on heat release or oxidation ratio of B since their mass and chemical properties before and after the reaction remained unchanged.

In an earlier work published by our group [16], B was mixed with boron carbide (micro-sized) before ignition and enhanced heat release of B was noticed in addition to the easy ignition. Several approaches have been determined on how boron carbide could promote the ignition and combustion of B. The first consisted of boron carbide ignition at lower temperature than that of B, which can elevate the environment temperature and induce B ignition [17]. The second relied on gaseous oxidation products of boron carbide like carbon dioxide and carbon monoxide that are helpful in breaking down the liquid oxide film generated on B particle surface [18]. The major difference between metal oxides and boron carbide consisted of self-oxidation of the latter to produce new substances and release heat during combustion. In other words, metal oxides serve as catalysts and boron carbide as accelerant for B oxidation.

Based on the above findings, a novel approach combining nanotechnology with catalysis and auxoaction was developed using nano-carbides as accelerant for B oxidation. The alternative carbide (nano-sized) should easily ignite to release gaseous products and help break down the liquid oxide film on B formed during combustion, where oxidation products can serve as catalysts for B oxidation.

In this study, four nano-carbides (nano-boron carbide (nB4C), nano-titanium carbide (nTiC), nano-zirconium carbide (nZrC), and nano-silicon carbide (nSiC)) were evaluated. Each nano-carbide was mixed with amorphous B (micro-sized) at a mass ratio of 2:8 and then analyzed by thermogravimetric differential scanning calorimetry (TG-DSC)-combined thermal analysis system. The acceleration mechanisms of nano-carbides were explored by means of FactSage 6.2 software. The results showed suitable nano-carbides can accelerate oxidation of B by several approaches and improve energy release properties of B prominently.

Experimental

Materials

The nano-carbides used in this study were provided by Aladdin Industrial Co., China. The amorphous B reagent was purchased from Baoding Zhongpuruituo Technology Co., Ltd., China. The purity and median particle size of the specimens are listed in Table 1.

Table 1 Physical parameters of the used materials
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To prepare the material mixtures, 0.2 g nano-carbide and 0.8 g B were mechanically stirred in a planetary mixer for 20 min to ensure better uniformity.

Thermal analysis

The thermal analysis experiments were performed on a TA DSCQ1000 TG-DSC combined thermal analysis system. For each test, 5 mg of mixed powder was placed in an aluminum oxide crucible and then heated from room temperature to 1000 °C at the rate of 10 °C min−1. All tests were performed in an air atmosphere at constant gas flow of 120 mL min−1. To ensure better reproducibility, at least two tests were carried out for each sample.

Thermodynamic calculations

FactSage jointly developed by FACT–Win and ChemSage thermochemical is an extremely powerful tool for thermochemical calculations [19]. The FactSage package is comprised of database, calculation, and manipulation modules for accessing and manipulating pure substances and solution databases.

A thermodynamic version of FactSage 6.2 was used to evaluate the equilibrium species produced in systems containing B/carbide/oxygen (O2), B/carbide, and B/oxide at temperatures below 1000 °C. Equilib module of FactSage was employed to treat the above systems at heterogeneous equilibrium states. The calculation temperature range varied from 450 to 1000 °C at step length of 50 °C. For B/carbide/oxygen (O2), the initial input chemical compositions were set to 8 g B, 2 g carbide, and 50 g O2 (O2 in excess). For B/carbide, O2 was eliminated and chemical compositions were 8 g B and 2 g carbide (B in excess). For B/oxygen, carbide was replaced by the corresponding oxide, and oxide mass was set to the calculated value found in B/carbide/oxygen (O2) system at equilibrium (x g). Therefore, the chemical composition in B/carbide could be expressed as 8 g B and x g oxide (B in excess).

Results and discussion

nB4C

Figure 1 shows the TG and DSC curves of the original B and (B + nB4C) systems. A slight decrease in mass was noticed when heated up to 100 °C. This can be attributed to the hygroscopic nature of the initial oxide layer present on B. During storage of B, particle surface will slowly oxidize to form a thin oxide layer of several to hundreds of nanometers [20,21,22]. The oxide layer will then react with water vapor in the presence of air to form boric acid (H3BO3) or metaboric acid (HBO2). After heating, H3BO3 and HBO2 decomposed again to create boron oxide (B2O3) and release water vapor, leading to mass loss and heat absorption. Moreover, the heat capacities of the samples changed after the decomposition reactions, which caused a slight slide down in heat flow since ~ 450 °C [13, 23].

Fig. 1
figure 1

TG and DSC curves of original B and (B + nB4C) systems

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For original B system, the mass gain of B started from 800 °C and was caused by the oxidation of B. This was accompanied by significant heat release. For (B + nB4C) system, the mass gain started at low temperature (~ 600 °C), suggesting that addition of nB4C facilitated the oxidation. Two exothermic peaks were observed in the DSC curve of (B + nB4C) system. The first one occurred between 600 and 700 °C and the second between 700 and 1000 °C, showing how nB4C accelerated the B oxidation reaction. The nB4C oxidation was initiated at around 600 °C [16, 24] followed by oxidation of B. The generated gaseous carbon dioxide (CO2) during nB4C oxidation induced a final mass gain of (B + nB4C), which was less than that of original B.

The equilibrium species temperature profiles between 450 and 1000 °C for (8B + 2B4C + 50O2), (8B + 2B4C), and (8B + 1.59CO2) are presented in Fig. 2. The equilibrium mixture of (8B + 2B4C + 50O2) such as O2(g), CO2(g), and B2O3(l) is depicted in Fig. 2a. CO2 was produced by oxidation of B4C, while B2O3 originated from oxidation of both B and B4C. The oxidation reactions can be summarized by Eqs. (1) and (2). As shown in Fig. 2b, B was unable to react with B4C in the absence of O2. However, B reacted with CO2 to create B4C and B2O3 according to Eq. (3) (Fig. 2c). The cyclic reduction of B4C was formed by a combination of Eqs. (2) and (3). Theoretically, B4C may act as catalyst for B oxidation under O2 atmosphere. However, B4C hardly achieved any catalysis during the actual oxidation process since the generated gaseous CO2 could easily diffuse before reacting with B.

Fig. 2
figure 2

Equilibrium species for (B + B4C) oxidation system: (a) (8B + 2B4C + 50O2), (b) (8B + 2B4C), and (c) (8B + 1.59CO2)

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$$4{\text{B}}\left( {\text{s}} \right) \, + 3{\text{O}}_{2} \left( {\text{g}} \right) \, \to \, 2{\text{B}}_{2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(1)
$${\text{B}}_{ 4} {\text{C}}\left( {\text{s}} \right) \, + {\text{ 4O}}_{ 2} \left( {\text{g}} \right) \, \to {\text{ 2B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right) \, + {\text{ CO}}_{ 2} \left( {\text{g}} \right)$$
(2)
$$1 6 {\text{B}}\left( {\text{s}} \right) \, + {\text{ 3CO}}_{ 2} \left( {\text{g}} \right) \, \to {\text{ 2B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right) \, + {\text{ 3B}}_{ 4} {\text{C}}\left( {\text{s}} \right)$$
(3)

nTiC

The TG and DSC curves of original B and (B + nTiC) systems are presented in Fig. 3. The mass gain and heat release of (B + nTiC) occurred much earlier than in B. Hence, nTiC significantly decreased the oxidation temperature of B. Moreover, the final mass gain of (B + nTiC) remained almost the same as in B though the oxidation of nTiC produced gaseous CO2. Hence, nTiC might also increase the oxidation ratio of B, suggesting the relevance of nTiC as accelerant for B oxidation reaction.

Fig. 3
figure 3

TG and DSC curves of original B and (B + nTiC) systems

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The mass gain of (B + nTiC) system can be divided into two continuous stages. The first started at approximately 500 °C and accompanied by slight heat release, and the second started around 650 °C and accompanied by severe heat release. The difference in heat release between the two stages was indicative of oxidation of B mainly during the second stage. Thus, TiO2 was already produced before oxidation of B. In addition, the continuous two stages revealed the presence of leftovers of unoxidized nTiC in the system. Therefore, nTiC and TiO2 could both contribute to oxidation of B.

Figure 4 gathers the equilibrium species temperature profiles between 450 and 1000 °C for (8B + 2TiC + 50O2), (8B + 2TiC), and (8B + 2.67TiO2). In excess O2 atmosphere (Fig. 4a), both B and TiC were completely oxidized to produce heterogeneous oxidation products, such as CO2(g), B2O3(l), and TiO2 rutile(s). The oxidation reaction of TiC is provided in Eq. (4). The gaseous product CO2 helped to break down the liquid oxide film on B particle surface formed by liquid product B2O3. In Fig. 4b, B reacted with TiC to create TiB2(s) and B4C(s) according to Eq. (5). The generated B4C can easily be oxidized (Eq. (2)). However, the oxidation of TiB2 was relatively slow below 1000 °C [25]. The elevated melting point of TiB2 (3225 °C [11]) may reduce the accumulated products of liquid oxide film present on B particle surface at low temperatures. As combustion temperature increased, TiB2 became oxidized according to Eq. (6), and the generated B2O3 was easily evaporated.

Fig. 4
figure 4

Equilibrium species for (B + TiC) oxidation system: (a) (8B + 2TiC + 50O2), (b) (8B + 2TiC), and (c) (8B + 2.67TiO2)

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As presented in Fig. 4c, B can also react with TiO2 to yield TiB2(s) and B2O3(l) following Eq. (7). The cyclic reduction of TiO2 will then be formed by combining Eq. (6) with Eq. (7). Since TiB2 and TiO2 were both present at the solid state, the catalysis can be achieved during oxidation, especially at high temperatures.

$${\text{TiC}}\left( {\text{s}} \right) \, + {\text{ 2O}}_{ 2} \left( {\text{g}} \right) \, \to {\text{ TiO}}_{ 2} \left( {\text{s}} \right) \, + {\text{ CO}}_{ 2} \left( {\text{g}} \right)$$
(4)
$$6 {\text{B}}\left( {\text{s}} \right) \, + {\text{ TiC}}\left( {\text{s}} \right) \, \to {\text{ TiB}}_{ 2} \left( {\text{s}} \right) \, + {\text{ B}}_{ 4} {\text{C}}\left( {\text{s}} \right)$$
(5)
$$2 {\text{TiB}}_{ 2} \left( {\text{s}} \right) \, + {\text{ 5O}}_{ 2} \left( {\text{g}} \right) \, \to {\text{ 2TiO}}_{ 2} \left( {\text{s}} \right) \, + {\text{ 2B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(6)
$$1 0 {\text{B}}\left( {\text{s}} \right) \, + {\text{ 3TiO}}_{ 2} \left( {\text{s}} \right) \, \to {\text{ 3TiB}}_{ 2} \left( {\text{s}} \right) \, + \, 2{\text{B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(7)

nZrC

The TG and DSC curves of original B and (B + nZrC) systems are illustrated in Fig. 5. nZrC was able to advance the oxidation of B but with inferior effects to those of nTiC and nB4C. The oxidation products of nZrC containing gaseous CO2 induced lower final mass gain of (B + nZrC) than that of original B.

Fig. 5
figure 5

TG and DSC curves of original B and (B + nZrC) systems

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The mass gain in (B + nZrC) system might be divided into two separated stages. The first occurred between 350 and 500 °C and the second between 750 and 1000 °C. The first stage was accompanied by slight heat release, while the second was accompanied by severe heat release. The first stage consisted of oxidation of nZrC, and the second dealt with oxidation of B. Since nZrC could easily be oxidized and both stages were separated by large temperature interval, most nZrC was oxidized before starting of the second stage [26].

The equilibrium species temperature profiles between 450 and 1000 °C for (8B + 2ZrC + 50O2), (8B + 2ZrC), and (8B + 2.39ZrO2) are provided in Fig. 6. Three final products [CO2(g), B2O3(l), and ZrO2 monoclinic(s)] were generated under O2 excess atmosphere (Fig. 6a). Thus, the oxidation reaction of ZrC can be expressed according to Eq. (8). In the absence of O2, B can still react with ZrC to generate ZrB2(s) and ZrC4(s) according to Eq. (9) (Fig. 6b). Similar to TiB2, the oxidation rate of ZrB2 is also slow below 1000 °C [27, 28]. Since all B transformed into ZrB2 in Eq. (9) and no B4C was generated, the reaction looked theoretically negative for oxidation and heat release in (B + nZrC) at low temperatures. Fortunately, due to the low initial oxidation temperature of ZrC, Eq. (9) would unlikely be the dominant reaction during combustion.

Fig. 6
figure 6

Equilibrium species for (B + ZrC) oxidation system: (a) (8B + 2ZrC + 50O2), (b) (8B + 2ZrC), and (c) (8B + 2.39ZrO2)

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In Fig. 6c, B also reacted with ZrO2 to generate ZrB2(s) and B2O3(l) according to Eq. (10). In this reaction, part of elemental B was directly oxidized to form B2O3. The other part remained in ZrB2 and can further be oxidized at higher temperatures (Eq. 11)). Thus, Eq. (10) would be the more suitable reaction during combustion of (B + nZrC) when compared to Eq. (9). Equations (10) and (11) formed the cyclic reduction of ZrO2, which meant that ZrO2 can also act as catalyst for B oxidation.

$${\text{ZrC}}\left( {\text{s}} \right) \, + {\text{ 2O}}_{ 2} \left( {\text{g}} \right) \, \to {\text{ ZrO}}_{ 2} \left( {\text{s}} \right) \, + {\text{ CO}}_{ 2} \left( {\text{g}} \right)$$
(8)
$$6 {\text{B}}\left( {\text{s}} \right) \, + {\text{ 4ZrC}}\left( {\text{s}} \right) \, \to {\text{ 3ZrB}}_{ 2} \left( {\text{s}} \right) \, + {\text{ ZrC}}_{ 4} \left( {\text{s}} \right)$$
(9)
$$1 0 {\text{B}}\left( {\text{s}} \right) \, + {\text{ 3ZrO}}_{ 2} \left( {\text{s}} \right) \, \to {\text{ 3ZrB}}_{ 2} \left( {\text{s}} \right) \, + {\text{ 2B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(10)
$$2 {\text{ZrB}}_{ 2} \left( {\text{s}} \right) \, + {\text{ 5O}}_{ 2} \left( {\text{g}} \right) \, \to {\text{ 2ZrO}}_{ 2} \left( {\text{s}} \right) \, + {\text{ 2B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(11)

nSiC

Figure 7 presents the TG and DSC curves of original B and (B + nSiC) systems. Unlike other mixtures, only one exothermic peak was observed in DSC curve of (B + nSiC). Earlier studies showed the initial oxidation temperature of nSiC powder (average particle size 200 nm) approached 783 °C [29], which was very close to that of original B (779 °C). Hence, mass gain and heat release processes of nSiC and B occurred almost in the same temperature range.

Fig. 7
figure 7

TG and DSC curves of original B and (B + nSiC) systems

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The initial oxidation temperature of (B + nSiC) system was slightly lower than that of original B, showing the modest acceleration effect of nSiC toward the B oxidation reaction. In the explored temperature range, the amount of unreacted nSiC was superior to that of oxidation product (SiO2) in (B + nSiC) since oxidation of nSiC mainly took place above 1100 °C [29]. Hence, the acceleration should mainly be attributed to nSiC rather than SiO2.

Figure 8 presents the equilibrium species temperature profiles between 450 and 1000 °C for (8B + 2SiC + 100O2), (8B + 2SiC), and (8B + 3SiO2). Under O2 excess atmosphere (Fig. 8a), CO2(g), B2O3(l), and SiO2(s) with three different crystal structures were generated. Therefore, the oxidation reaction of SiC can be expressed according to Eq. (12). Between 450 and 550 °C, the crystal structure of SiO2 existed as quartz (l), which will turn into quartz (h) at temperatures from 600 to 850 °C. From 900 to 1000 °C, the crystal structure will transform into tridymite (h), and crystal transition of SO2 would theoretically be accompanied by heat absorption. However, no endothermic peak was noticed from 550 and 900 °C in DSC curve of (B + nSiC) (Fig. 7), indicating the absence of generated SO2 in (B + nSiC) system at temperatures below 900 °C.

Fig. 8
figure 8

Equilibrium species for (B + SiC) oxidation system: (a) (8B + 2SiC + 100O2), (b) (8B + 2SiC), and (c) (8B + 3SiO2)

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In the absence of O2 (Fig. 8b), B reacted with SiC to produce SiB14(s), B4C(s), and SiB6(s) following Eqs. (13) and (14). These three products could easily be oxidized at low temperature following Eqs. (2), (15), and (16), respectively [16, 30]. Hence, SiC can hardly inhibit the accumulation of liquid oxide film present on surface of B particles.

In Fig. 8c, B reacted with SiO2 as according to Eq. (17) at temperatures higher than 500 °C but not easily since residues of B and SO2 were still present in the equilibrium species temperature profile. This indicated the weak catalysis of SiO2 following Eqs. (16) and (17) to form the cyclic reduction of SiO2.

$${\text{SiC}}\left( {\text{s}} \right) \, + {\text{ O}}_{ 2} \left( {\text{g}} \right) \, \to {\text{ SiO}}_{ 2} \left( {\text{s}} \right) \, + {\text{ CO}}_{ 2} \left( {\text{g}} \right)$$
(12)
$$1 8 {\text{B}}\left( {\text{s}} \right) \, + {\text{ SiC}}\left( {\text{s}} \right) \to {\text{ SiB}}_{ 1 4} \left( {\text{s}} \right) \, + {\text{ B}}_{ 4} {\text{C}}\left( {\text{s}} \right)$$
(13)
$$1 0 {\text{B}}\left( {\text{s}} \right) \, + {\text{ SiC}}\left( {\text{s}} \right) \to {\text{ B}}_{ 4} {\text{C}}\left( {\text{s}} \right) \, + {\text{ SiB}}_{ 6} \left( {\text{s}} \right)$$
(14)
$$2 {\text{SiB}}_{ 1 4} \left( {\text{s}} \right) \, + {\text{ 23O}}_{ 2} \left( {\text{g}} \right) \to {\text{ 2SiO}}_{ 2} \left( {\text{s}} \right) \, + \, 14{\text{B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(15)
$$2{\text{SiB}}_{ 6} \left( {\text{s}} \right) \, + \, 1 1 {\text{O}}_{ 2} \left( {\text{g}} \right) \to {\text{ 2SiO}}_{ 2} \left( {\text{s}} \right) \, + {\text{ 6B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(16)
$${\text{B}}\left( {\text{s}} \right) \, + {\text{ SiO}}_{ 2} \left( {\text{s}} \right) \, \to {\text{ SiB}}_{ 1 4} \left( {\text{s}} \right) \, + {\text{ B}}_{ 2} {\text{O}}_{ 3} \left( {\text{l}} \right)$$
(17)

Analysis of thermal oxidation parameters

The thermal oxidation characteristic parameters are compiled in Table 2. The original B showed the highest values of initial oxidation temperature, temperature of maximum mass gain rate, and temperature of maximum heat flow. Thus, addition of all four nano-carbides can reduce the oxidation temperature of B at different degrees. The maximum mass gain rate and final mass gain of the four mixtures were all lower than those of original B, attributed to production of gaseous CO2 during oxidation. Although the maximum heat flow of B was the highest, it fell behind in the aspect of heat release (except B + nSiC). Among the four nano-carbides, nTiC showed the most benefit for oxidation of B as it reduced the initial oxidation temperature by 10.7%. Moreover, (B + nTiC) system revealed the largest maximum heat flow and heat release, suggesting the relevance of nTiC in increasing energy release of B. In the experimental temperature range, (B + nTiC) released 16.0% more heat than original B. By contrast, addition of nSiC showed negative effect on energy release, which decreased heat release by 4.8%. The latter might be attributed to lower calorific value of nSiC when compared to that of B, as well as the rare promotion of heat release of B by nSiC.

Table 2 Thermal oxidation parameters of the systems
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Conclusions

The accelerating effect of B oxidation using four nano-carbides (nB4C, nTiC, nZrC, and nSiC) and their action mechanisms were evaluated. The results showed that all four nano-carbides could reduce the initial oxidation temperature of B and can increase the heat release, except nSiC. However, the promotion effects and corresponding action mechanisms varied among nano-carbides. nTiC displayed the best acceleration among all four nano-carbides. The addition of nTiC reduced the initial oxidation temperature of B by approximately 10.7% and increased the heat release by 16.0%. TiB2 as reaction product of B with TiC reduced the accumulation of liquid oxide film on B particle surface at low temperatures. Meanwhile, the oxidation product of TiC (TiO2) acted as catalyst for B oxidation. nB4C was ranked as the second-best accelerant for B oxidation. The oxidation of nB4C occurred before that of B and might induce B oxidation. Moreover, the generated gaseous CO2 was found beneficial for breaking down the liquid oxide film present on B particle surface. The third best accelerant was nZrC since its oxidation product (ZrO2) acted as catalyst for B oxidation. By contrast, nSiC showed no value as nano-additive for acceleration of oxidation reaction of B. The addition of nSiC reduced the initial oxidation temperature of B by only 1.5% but decreased the heat release by 4.8%. This can be explained by the reaction products of B and SiC, which can easily be oxidized at low temperatures and hardly reduce the accumulation of liquid oxide film. As a summary, different acceleration mechanisms of the four nano-carbides are presented in Table 3.

Table 3 Acceleration mechanisms of the nano-carbides
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The thermal analyses and thermodynamic calculations provided the thermal oxidation characteristics of mixtures and acceleration mechanisms of the nano-carbides. However, the temperature range and heating rate of TG-DSC were both lower than those of actual ignition combustion processes. Relevant acceleration mechanisms above 1000 °C require experimental evidence, which will be the focus of future work related to effect of nano-carbides on ignition and combustion characteristics of B.