Effect of amine-modified boron nitride (BN) on ammonium perchlorate decomposition

Abstract

Solid propellant is an important material in space exploration. It is also used in the automobile and demolition industries as well as in ejection seats and airbags where precise control of burning rate is crucial. In the present work, a novel multiphase boron nitride (BN) material containing NH₂ was synthesized and found to have a large impact on energetic materials. The reaction of the BN material with solid oxidizer ammonium perchlorate (AP) was characterized using DSC/TGA, SEM, XPS, and a high-pressure propellant strand burner. Primary amine groups on the BN material participates in a chemical reaction with the AP crystals causing a dramatic weight loss during low-temperature decomposition which is important for high-precision applications. While the burning rates of propellants containing 0.5% of the micron- and nano-sized BN materials increased by about 30% over the baseline formulation with no BN, similar propellants containing the synthesized BN material decreased the burning rate by as much 25%.

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Introduction

Solid propellant is an energetic material composed of metal fuel (usually magnesium or aluminum^1,2) and solid oxidizer bound by a polymer matrix which also serves as the fuel. Along with these basic components, other ingredients are often included into a propellant mixture such as plasticizer and burn-modifying additives.^3–10 Intense research in this area is warranted by the material's utility in a wide variety of areas such as demolition and space exploration.^11–13 Solid propellant has several advantages over its liquid counterpart, among which include tunability, shape controllability, storability and chemical and thermal stability. The most common oxidizer used in solid propellants is ammonium perchlorate (AP) salt due to its strong oxidizing property and its similar decomposition profile to other common components of solid propellant, such as hydroxyl-terminated polybutadiene (HTPB) polymer. Despite its widespread use and decades of research dedicated to the topic, there exists no consensus on the mechanism of its thermal decomposition or intermediate species within the crystalline salt.

Ammonium perchlorate (NH₄ClO₄) is an ionic salt consisting of ammonium and perchlorate ions. As seen in Fig. 1, NH₄ClO₄ is a white, solid crystalline material that is orthorhombic at room temperature and undergoes a phase transition to cubic at 240 °C. Thermal decomposition happens in two distinct stages: low-temperature decomposition (LTD) which happens between 300 and 380 °C and high-temperature decomposition (HTD), which occurs above 380 °C. The material experiences up to a 20% weight loss during LTD, in which the mechanism of decomposition has been a subject of debate. It is believed that either an electron is transferred from the perchlorate ion to the ammonium ion, or a proton is transferred from the ammonium ion to the perchlorate ion forming NH₃ and HClO₄. It is also believed the LTD phase is stunted by severe disorder in the crystal lattice.¹⁴ HTD is characterized by sublimation of the remaining solid material leading to 100% weight loss of the material. Other materials have been previously shown to increase the weight loss of ammonium perchlorate during different stages of its decomposition, including iron oxide, metallic copper, and titania.^1,15–17 The mechanism for these materials to produce the weight loss is that of a catalyst donating an electron to the ions of the AP crystal which is later returned to the catalyst as the AP decomposes into its most basic components.^17,18 The present study shows a progression of SEM images displaying the topography of the AP crystals at intervals of the decomposition with a novel amine modified boron nitride (BN) derivative material.

Fig. 1 SEM images of (top) hydrothermally synthesized BN showing thin ribbon-like structure and (bottom left) commercially obtained micron-sized BN and (bottom right) nano BN, both of which show plate-like structures. The 2D BN nanoribbon is approximately 1 micron wide and 5 to 10 microns long, while the commercial micron BN is approximately 1 micron in diameter, and the nano BN is approximately 200 nm in diameter.

The presence of amine groups during ammonium perchlorate is being investigated for its role in the initial stages of thermal decomposition. Since amine does not exist in a solid state on its own, it requires a host material to attach to in order to make thorough mixture in AP a possibility. Typical candidates for the delivery of amine into the mixture include urea, melamine and glycine. These organic materials often have high molecular weights therefore adding unnecessary “dead weight” to a propellant. On top of adding dead weight, the backbones of these materials serve no other purpose in a propellant after amine dissociates. Boron nitride is an ideal candidate to study the effects of amine functional groups on propellant and ammonium perchlorate decomposition.

BN is an attractive material for many applications due to its high thermal and chemical stability.^19–22 The recent rise in interest in boron nitride is a result of the enormous study on graphene as BN can form identical structures with very similar properties. The two most common structures are cubic, which tends to be more densely packed and resembling bulk material and hexagonal, which is more common when synthesizing nanomaterials. The transition temperature between cubic and hexagonal BN has long been debated and is very sensitive to defects, purity and grain size.²² Boron nitride was recently studied as a propellant additive to prevent corrosion in the combustion chamber,²³ an important application as the space industry moves toward reusable rockets. It was discovered that a small amount of powder made by the procedure described by Li et al.,²⁴ mixed with ammonium perchlorate has a drastic impact on the decomposition of the AP powder due to the presence of amine functional groups from the precursor, melamine. This makes it a novel, multifunctional material combining the anti-corrosive properties of BN with the burn rate tailorability associated with the presence of amine. Besides being a host for amine functional groups, it is important to study the effects of pure BN on propellant mixtures due to the high thermal diffusivity and low density of the material.

In this study, we demonstrate that boron nitride synthesized from a melamine precursor will have the same effect on the burn profile and regression rate of the propellant as the melamine. This similar effect is due to amine groups imparted to the BN compound from melamine, which is not commonly used due to its explosive nature. The strong nucleophilic nature of primary amine groups allows protons transferred from the ammonium anion to the perchlorate cation to be intercepted by NH₂. This proposed mechanism of decomposition of ammonium perchlorate then follows the equation below.

NH₄ClO₄ + NH₂ → ClO⁻₄ + 2NH₃ (1)

The commercial BN, with no functional groups, increases the burning rate by increasing the thermal conductivity of the composite. This effect is more prevalent in the micron-sized BN compared to the nano BN. Provided as follows is a description of the materials and methods utilized in this paper, including the materials synthesis and characterization. The results of the characterization and burning rate tests are then described, followed by a discussion of the results.

Materials and methods

Materials synthesis

Commercial BN samples were obtained from Sigma Aldrich and used without further processing. Melamine (C₃N₆H₆) and boric acid (H₃BO₃) precursors used to synthesize BN hydrothermally were both obtained from Fisher Scientific and used without further processing. For this study, boron nitride ribbons were synthesized via a hydrothermal method as described by Li et al.²⁴ Typically, H₃BO₃ was mixed with C₃N₆H₆, and a small amount of 0.1 M HNO₃ was introduced to adjust the pH to 6.5. Then a triblock copolymer was added as a template. The mixture was stirred vigorously and heated to 85 °C for 6 hours and then cooled to room temperature. A white precipitate formed and the mixture was filtered and dried in a vacuum oven to remove any residual water. The dry powder was then heated to a temperature of 400 °C, which was chosen because it is above the vaporization point of both precursors. Li, et al.,²⁴ describes a final step of heating the material up to 1300 °C to crystalize the material. This step was forgone for this study. The physical and chemical properties of the material as well as its performance in propellant systems were characterized using SEM, XPS, TGA/DSC, and propellant strand burning.

Characterization

For the SEM study, the synthesized BN material was mixed with AP at 1 wt%, and three samples were made by heating the mixture using the TA instruments SDT Q600 to 300 °C, 330 °C, and 360 °C, respectively and allowed to cool in air before examination. A sample of pure AP was also heated to these temperatures. These three temperature were chosen because they represent the approximate beginning, middle and end of the low-temperature decomposition of the ammonium perchlorate. SEM images were taken of the heated samples for comparison of pore formation and growth when AP is in the presence of our BN material.

DSC/TGA and propellant strand burning were done to determine the performance of the material in a propellant mixture. One-half wt% BN material additive in AP mixture was used in a baseline propellant mixture of 80% oxidizer/20% HTPB binder. The propellant strands were extruded and cured for one week and burned in a Crawford-style burner and compared to similar samples without BN (i.e., the baseline propellant). DSC/TGA was done using TA Instruments SDT Q600 under argon flow using a heating rate of 20°C min⁻¹ up to 800 °C. For the DSC/TGA and propellant strand burner studies, the performance of the amine modified BN material was compared against micron-sized and nano-sized BN commercially obtained from Sigma Aldrich.

X-ray photoelectron spectrometer (XPS) measurements were accomplished in an ion-pumped chamber (evacuated to 2 × 10⁻⁹ Torr) of an PE-PHI5400 spectrometer, employing Mg-Kα radiation (BE = 1253.6 eV). The binding energy (BE) for the samples was calibrated by setting the measured BE of C1s to 284.6 eV. Peakfit software was further used to identify the chemical state of boron and nitrogen spectra according to the previous reports. XPS was done on the synthesized BN material as well as the residue of the 1 wt% BN in AP mixture after heating up to 600 °C, which is well above the temperature at which the AP fully decomposes, so it is assumed the remaining residue is entirely the BN material.

Results and discussion

The presented work is a step toward more controllable propellants for use in many industries and a better understanding of the decomposition mechanism of ammonium perchlorate. Solid propellant plays a crucial role in rocketry, demolition, airbag deployment and ejection seat deployment, the application of which is very sensitive to the burning rate of the propellant. In this work, amine functional groups attached to synthesized boron nitride were studied to find their effect on the burn characteristics of solid propellant.

The hydrothermal synthesis process yielded a white powder which was imaged using SEM, shown in Fig. 1. The boron nitride structures are all ribbons roughly 3 μm wide and 10 to 15 μm long. Fig. 1 (top right) shows the transparency of the BN ribbons as the ribbon laying underneath is clearly visible. For this reason, the thickness was assumed to be roughly on the order of 100 nm. Similar ribbon-like structures have been observed on activated boron nitride prepared by hydrothermal process, and the BN material showed high crystallinity.²⁴ This synthesized BN material was seen to be amorphous by XRD because the material was synthesized at lower temperatures (see ESI Fig. S3†). SEM images of both commercial samples showed a similar morphology with the only difference between the two being size (Fig. 1). Both samples had consistent plate-like structures with the micron sample being approximately 1 μm in diameter, while the nano sample was the same structure with a diameter of 200 nm. Particle thickness is approximately one tenth of the diameter in each case.

The DSC/TGA graphs, Fig. 2c, show the thermal decomposition profile of the synthesized BN to be unlike that of the commercial BN, or either reactants used (boric acid and melamine) suggesting that what was synthesized was a multiphase material consisting of h-BN, B₂O₃, B₄C as well as NH₂ amine groups as reported by Singh²⁵ to be a possible product when reacting boric acid and melamine. FTIR on the sample (ESI†) shows infrared absorption in our material from amine groups similar to that of melamine, but without the heterocyclic nitrogen absorption, making it clear that a reaction took place and the primary amine groups are present in our new material. Similarly, the FTIR from our material after being heated to 600 °C shows no absorption from amine groups, meaning all amine groups dissociate from the BN material before reaching 600 °C.

Fig. 2 (a and b) TGA/DSC plot of 1 wt% BN additive in AP. Advanced decomposition during LTD phase with amine/BN material due to amine interaction with AP salt during initial steps of decomposition, leading to large exothermic peak and advanced weight loss in this temperature region. Materials lacking NH₂ groups had minimal effect on AP decomposition. (c) TGA analysis of reactants and product of hydrothermal synthesis procedure from Li, et al. (without final heating to 1300 °C as described) and commercial BN. Synthesized material contains neither pure melamine or boric acid, nor pure h-BN but rather a mixture of materials from a reaction between boric acid and melamine (d) TGA of 10 mol% NH₂ in AP from melamine and glycine, respectively. Both additives show a large impact on the low temperature decomposition of AP due to the presence of primary amine groups.

When this amine-modified BN material is added to AP, the low-temperature decomposition phase is dramatically accelerated due to the amine interaction with the ionic components during the initial step in decomposition. Fig. 2a and b show the LTD of AP when in the presence of the BN material modified with NH₂ groups. It can be seen from the DSC graphs that each sample shows endothermic peaks around 240 °C and 430 °C which correspond to the characteristic phase transformation of AP and the final sublimation of the AP crystal, respectively. The DSC graphs also show an exothermic peak around 325 °C which corresponds to the breaking of the ionic bonds in the crystal. This exothermic peak is much higher in the modified BN sample and is unaffected in the samples with the commercial BN compared to the pure AP sample.

The TGA graphs also show a drastic change in the sample with the amine modified BN. The pure AP graph has a two-stage decomposition profile in which the decomposition begins around 300 °C and is finalized after 450 °C with an inflection point in the weight percentage curve around 360 °C that distinguishes the two phases as being low-temperature decomposition and high-temperature decomposition. The pure AP loses anywhere between 20% and 30% of its total weight during the LTD phase and the remainder during HTD. This weight loss trend remains unaffected by the commercial BN additive, but the modified BN causes the AP sample to lose over 60 wt% during the LTD, which corresponds to the large exothermic peak seen in the DSC plot. This drastic weight loss is due to the strong nucleophilic nature of primary amine groups. Assuming the mechanism of AP decomposition is a proton transfer between ammonium and perchlorate ions, the amine groups intercept the proton via a lower energy reaction than the one between the ionic components of the crystal, making decomposition occur faster and at a lower temperature. Similar LTD weight-loss effects are seen when using iron oxide and copper chromite additives, however the mechanism in those reactions are different than the proposed mechanism for amine groups.²⁶ To ensure this effect is a result of the presence of amine groups on the BN, a sample was made by heating the NH₂/BN material up to 800 °C to dissociate the primary amines, and the remaining material was added to AP. The FTIR spectrum of the preheated BN material shows no absorption from NH₂. The TGA of the preheated BN/AP mixture shows no effect on the LTD of the AP, showing the effect seen was a result of amine groups reacting with AP and not the presence of any other compounds.

The proton transfer within an ionic salt is the first step in thermal decomposition and necessarily occurs first. The change in the decomposition caused by the presence of amine is not seen with the mixture of AP and the micron- or nano-sized BN, which only show minor changes in the decomposition onset temperature and exothermic peak location, which could be due to the higher thermal diffusivity of pristine BN. The amine-modified BN material and the commercially obtained micron- and nano-sized BN alter the AP decomposition via different mechanisms. The amine/BN material alters the decomposition chemically by providing an alternate route to the final decomposition products, while the micron- and nano-sized BN materials alter the decomposition by simply increasing the thermal diffusivity of the mixture which allows the mixture to reach the decomposition energy faster during the heating process as identified by TGA analysis. The HTD stage is unaffected by all BN additives. The DSC plots of the AP/BN mixtures show a large exothermic peak during the decomposition of the AP with the amine modified BN material. The large exothermic peak is not seen in the decomposition of the AP mixture with micron- and nano-sized BN, which is more evidence that the reaction between the amine BN derivative material and the AP is chemical in nature and the effect of micron- and nano-sized BN on the AP is simply thermal.

To further investigate the role of primary amine groups in the decomposition, TGA analysis was done on mixtures of melamine in AP and glycine in AP, both of which contain NH₂ functional groups. The mixtures were mixed in a way to make a 10 mol% NH₂ in AP, which corresponds to a weight mixture of 0.04 wt% melamine in AP and 6.156 wt% glycine in AP. Fig. 2d shows this TGA analysis. Both additives show a similar effect on the LTD of AP as was seen with the NH₂-modified BN.

The dramatic change in LTD with the BN with amine groups is seen visually in the SEM images of the AP crystals. Fig. 3a–c show the AP crystals heated up to 300 °C, 330 °C and 360 °C. Small pores can be seen to form on the surface of the crystal and grow very slowly in this temperature range. The decomposed material in the crystal forms the gaseous products of the decomposition and accounts for the material's weight loss during this stage of decomposition. Fig. 3d–f shows the AP heated to 300, 330, and 360 °C while in the presence of the amine-BN and it shows the formation and rapid growth of surface pores. The dramatic pore growth is responsible for the increase in weight loss in this temperature range, which exposes the surrounding material to any of the reaction products formed during the decomposition. This proposed mechanism is in agreement with XPS results as is discussed below. The amine-modified BN is seen to undergo a chemical change during the decomposition of the AP/BN mixture. This reaction allows the pores to grow in diameter as well as depth into the crystal. The gaseous products of the decomposition diffuse away from the crystal and account for the drastic weight loss experienced during the LTD. The TGA results that showed no excess decomposition during LTD for micron and nano BN material which may produce very few holes in the AP crystals. This result is due to the lack of functional groups (see ESI-FTIR-Fig. 1†) attached to the micron and nano BN material which are solely responsible for the chemical changes in the AP decomposition process.

Fig. 3 SEM images of AP crystals after heated to 330 °C (a) and 360 °C (b). The pores shown on the surface of the crystal are the result of thermal decomposition and are the means by which gaseous material escapes the crystal to participate in the combustion reaction. (c) SEM image of AP particles heated to 300 °C showing very little decomposition. (d–f) AP crystals heated to 300 °C, 330 °C and 360 °C, respectively, in the presence of amine modified BN material. The decomposition of the AP crystals is far more advanced at these temperatures due to the presence of the functional groups derived from the melamine precursor.

To determine any chemical changes in the amine-modified BN materials, XPS was performed on the modified BN before and after undergoing the reaction with AP crystals. The experimental data were then fitted using peak fit software to deconvolute the peaks for boron (B1s) and nitrogen (N1s) components to identify surface compositional information and bonding. Fig. 4 shows the deconvoluted B1s and N1s for the BN material before and after heating with AP crystals. According to the analysis of the B1s spectra (Fig. 4a and b), the peaks are observed at around 190.6, 191.8 and 192.9 eV. Based on the available literature, the peak situated at 190.6 and 192.9 eV are assigned to B–C and B–O bonding, respectively.²⁷ The peak appearing at 191.8 eV is higher than that of B–C bonding and is lower than that of B–O bonding. Hence, the peak appearing at 191.6 eV might be related to B–N bonding,²⁸ although it can be B–N–B or B–N–C moiety in hexagonal (h)-BN, for which the peak was reported at 190.2–191.6 eV.^21,28–31 However, the amine-modified BN material was not heated at high temperature (1300 °C), so the formation of B–N–B/B–N–C or B–C–N may not be possible. Therefore, it has been assigned to B–N bonding. On the other hand, the N1s spectra are also analyzed (Fig. 4a and b), where the peaks are deconvoluted and the peak centers are observed to exist at around 397.9, 398.9 and 400.1 eV, which are assigned to N–O, B–N and N–H bonding, respectively.^32–35 The calculated concentration of N–H bond is decreased after reacting the BN material with AP (from 23% to 13%), which suggests that the amine group detaches from the BN material during AP decomposition and undergoes a chemical change which also agrees with FTIR results (see ESI-FTIR Fig. 1†). This chemical change is the result of the highly nucleophilic nature of the amine species that exist on the BN material and their interaction with AP via the proposed mechanism. The chemical change seen in the BN is further proof of the hypothesis that the material is participating in a chemical reaction with AP by cleaving the ionic bonds in the crystal.

Fig. 4 XPS data of hydrothermally synthesized amine modified BN material (a) and the same material after participating in thermal decomposition of AP (b).

This novel BN derivative material containing primary amine groups also showed a large difference in performance from the commercially available BN when in a full propellant. Fig. 5 shows the burning rate profile of the 80/20 propellant mixture with 0.5 wt% BN. The commercially bought micron and nano BN materials showed a slight increase in the burning rate, while the amine-modified BN ribbons showed a decrease in burning rate compared to the control sample. This decrease in burning rate is likely due to the increased presence of ammonia gas in the mixture from the reaction between amine and the ammonia salt, which hinders the combustion process. The increase in burning rate from the commercial material is expected due to the high thermal conductivity of BN which allows the burn to propagate through a propellant grain faster. It is also a very modest increase in burning rate compared to the metal-oxide additives that have recently been studied in the authors' laboratories and by others.^6,36,37 Since the commercial BN shows no catalytic activity on the decomposition of the AP, there is no reason to believe the increase in burning rate is due to anything other than the high thermal properties of the boron nitride. The decrease in burning rate seen with the amine-modified BN is due to the chemical reaction of the material with the AP crystals. The increased production of ammonium gas decreases the relative abundance of oxygen during the LTD, making the propellant mixture oxygen deficient. The overall impact of each additive on the burn profile of ammonium perchlorate and propellant mixtures is summarized in Table 1. The ability to combine these functional groups to BN, which has already been shown to benefit propellant technologies, adds a higher degree of control to the effect of boron nitride on combustion characteristics and makes the additive a single, multi-functional material. The variable mixture of “modified BN” with “clean” BN can allow for precise adjustments to be made on solid propellants, which are often used for applications in which burning rate requirements fall within a tight range.

Fig. 5 Burning rate plot of solid propellant strands of 80% AP/20% binder. The AP mixture has BN additive totalling 0.5 wt% of the entire propellant for each mixture. The additives were mixed into the AP powder prior to mixing the AP into the propellant mixture to ensure intimate contact between the BN and AP particles. The commercially obtained BN material increased the burning rate of the propellant, while the synthesized material hindered the burn rate.

Table 1 Chart comparing characteristics of studied additives and their respective effects on the burn profile of an 80/20 propellant mixture

Morphology of additive	N/A	Amine-modified ribbon-like structures	Micron platelets	Nano platelets
Effect on burn rate on propellant	N/A	Decrease (↓)	Increase (↑)	Increase (↑)
LTD weight loss	16.9% weight loss	53.1% weight loss	17.3% weight loss	18.9% weight loss
Exothermic peak max	0.032 W g^−1	2.51 W g^−1	−0.48 W g^−1	−1.49 W g^−1

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Conclusion

In summary, a new material containing boron nitride modified with amine groups was synthesized using a facile, low-cost hydrothermal synthesis process and was compared to commercially available micron- and nano-sized BN materials. It was identified that the modified BN material contained primary amine groups which caused weight loss of up to 60% in the ammonium perchlorate during the low-temperature decomposition phase through a chemical reaction with the ionic salt. When used in an 80/20 propellant mixture with 0.5 wt% additive, the modified BN material caused a decrease in burning rate by as much as 25%, while the micron- and nano-sized BN increased the burning rate by about 30% compared to the baseline, which makes the BN containing amine preferable for applications in which a slower burn is needed. A slight increase in burning rate was observed from larger additive particles due to the surface area-to-volume ratio difference combined with the mechanics of heat flow through a propellant grain. This tailorability of burning rate from the amine/BN material is important for high-precision applications such as air bag and ejection seat deployment as well as rocketry.

Acknowledgements

This work was supported in part by the TEES Turbomachinery Laboratory (TAMU authors). Dr Seal is thankful to partial support of the work through his nanotechnology laboratory.

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Footnote

† Electronic supplementary information (ESI) available: Schematic of BN synthesis process, FTIR and XRD analysis. See DOI: 10.1039/c6ra21300f

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