Open access peer-reviewed chapter

Combustion Catalyst: Nano‐Fe2O3 and Nano‐Thermite Al/ Fe2O3 with Different Shapes

By Ningning Zhao, Jiachen Li, Fengqi Zhao, Ting An, Rongzu Hu and Haixia Ma

Submitted: October 19th 2015Reviewed: June 29th 2016Published: October 5th 2016

DOI: 10.5772/64748

Downloaded: 1295

Abstract

In order to enable the energetic materials to possess a more powerful performance, adding combustion catalysts is a quite effective method. Granular, oval, and polyhedral Fe2O3 particles have been prepared by the hydrothermal method and used to fabricate Al/Fe2O3 thermites. All the Fe2O3 and Al/Fe2O3 thermite samples were characterized using a combination of experimental techniques including scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), and high‐resolution TEM (HRTEM). The non‐isothermal decomposition kinetics of the composites and nitrocellulose (NC) can be modeled by the Avrami‐Erofeev equation f(α)=3(1–α)[–ln(1–α)]1/3/2 in differential form. Through the thermogravimetric analysis infrared (TG‐IR) analysis of decomposition processes and products, it is speculated that Fe2O3 and Al/Fe2O3 can effectively accelerate the thermal decomposition reaction rate of NC by promoting the O‐NO2 bond cleavage. Adding oxides or thermites can distinctly increase the burning rate, decrease the burning rate pressure exponent, increase the flame temperature, and improve the combustion wave structures of the ammonium perchlorate/hydroxyl‐terminated polybutadiene (AP/HTPB) propellants. Among the three studied, different shapes of Fe2O3, the granular Fe2O3, and its corresponding thermites (Al/Fe2O3(H)) exhibit the highest burning rate due to larger surface area associated with smaller particle size. Moreover, Al/Fe2O3(H) thermites have more effective combustion‐supporting ability for AP/HTPB propellants than Fe2O3 structures and the other two as‐prepared Al/Fe2O3 thermites.

Keywords

  • combustion catalyst
  • thermal decomposition mechanism
  • combustion wave structure

1. Introduction

Energetic materials (explosives, propellants, and pyrotechnics) are necessary material bases of high‐performance weapons and ammunition, which are used extensively for both civil, military, and space applications. In order to enable the energetic materials to possess a more powerful performance, such as the high quantity of heat release, the high combustion temperature, the fast burning rate, and so on, adding combustion catalysts is a quite effective method.

In recent years, researchers pay much attention to the preparation and application of the combustion catalysis of nanoscale. Many studies reported that catalysts in nanoscale exhibit the absolute advantages both in accelerating the thermal decomposition process of the main energetic materials such as cyclotrimethylene trinitramine (RDX), nitrocellulose (NC), cyclotetramethylene tetranitramine (HMX), 2,4,6,8,10,12‐hexanitro‐2,4,6,8,10,12‐hexanitro hexaazaisowurtzitane (CL‐20), and 3‐nitro‐1,2,4‐triazol‐5‐one (NTO), and in enhancing the ignition and combustion performances of the solid. For instance, the nano‐sized Cr2O3 particles decrease the ignition delay time by a factor 3.5 (16 ± 2 vs 54 ± 4 ms) and accelerate the combustion rate (340 ± 10 mm s-1) of the Al/Cr2O3 thermite, which is fabricated by Cr2O3 micro‐ or NPs (Φ ≈20 nm) and Al NPs (Φ ≈50 nm) [1]. Pantoya [2] reported that nanocomposite thermites (Al/MoO3) can significantly reduce the ignition delay time compared with micron‐composite thermites. Nitrocellulose nanofiber‐based thermite textiles were studied and compared with the pure nitrocellulose and nano‐aluminum incorporated nanofiber; the result indicates that the burning rates were enhanced by adding the Al/CuO thermite [3].

The abovementioned nanothermite contains two parts: metal fuel (Al, used due to its low cost, high density, and the efficient catalytic property [4]) and metal oxides (Fe2O3, CuO, MnO2, MoO3, PbO [5], Bi2O3, etc.). The nanothermite system, as the metastable intermolecular composites (MICs) [6], can enhance the reactivity [79] through the oxidation‐reduction reactions, which lead to high burning rate [10], high heat production [11], and negligible gas generation. The traditional thermite, Al/Fe2O3, is prepared in various nanoparticle size, shape, and composition [12] in order to be better applied in free‐standing heat sources, airbag ignition materials, hardware destruction devices, welding torches [13], and energetic material field. Both Al and Fe2O3 particles have been used as catalysts not only in the thermal decomposition process of the main energetic components but also in composite solid propellants [1417]. However, the effects of Al/Fe2O3 nanoparticles on the thermal behavior and non‐isothermal decomposition kinetics of NC are barely investigated. And, to the best of our knowledge, there has been no report about the dependence of catalytic properties of Al/Fe2O3 thermites on the morphology of Fe2O3 particles in combustion reactions to date.

Nitrocellulose (NC) is extensively applied as a main component in gun, blasting gelatin, dynamites, and rocket propellants [1821] owing to its high flammability and explosiveness. In order to obtain more information about NC, the thermal decomposition mechanism of NC has been investigated. It is shown that the fission of oxygen‐nitrogen bond is the first and rate‐determining step during the decomposition process [2225]. Quantities of NO2 gases, derived from the O‐NO2 bond cleavage, could stagnate in the polymer skeleton and lead to promote the secondary autocatalytic reactions (i.e., the heterogeneous reactions in condensed phase) [26]. Furthermore, Mahajan et al. [27] reported that copper oxide influences the combustion/thermal decomposition of NC in a way so as to retard the breaking of O‐NO2 bonds in solid phase. With the excellent characteristics of nanomaterials, we study the influence of Fe2O3 particles and Al/Fe2O3 thermites on thermal behavior and non‐isothermal decomposition kinetics of NC in order to provide basic data for establishing the combustion model and studying the combustion process.

In this contribution, granular, oval, and polyhedral Fe2O3 particles have been prepared by the hydrothermal method and used to fabricate Al/Fe2O3 thermites by integrating Al nanopowders with Fe2O3 at a stoichiometric ratio of Fe2O3:Al (71.1wt%:28.9wt%). All the Fe2O3 and Al/Fe2O3 thermite samples were characterized using a combination of experimental techniques including scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM) and high‐resolution TEM. The effects of Fe2O3 nanoparticles and Al/Fe2O3 on the thermal decomposition of NC have been investigated by the differential scanning calorimetry (DSC) method and the thermogravimetry with Fourier transform infrared analysis (TG‐IR). The influences of Fe2O3 and the corresponding thermite on the combustion properties of the ammonium perchlorate/hydroxyl‐terminated polybutadiene (AP/HTPB) composite propellant were investigated and compared. Moreover, the combustion wave structures and the flame temperatures of AP/HTPB composite propellants containing thermites Al/Fe2O3 are obtained at 4 MPa.

2. Experimental section

2.1. Synthesis of Fe2O3 particles and Al/Fe2O3 thermites

The granular, oval, and polyhedral Fe2O3 particles were prepared following the procedures developed from our reports [28, 29], and denoted as Fe2O3(H), Fe2O3(o), and Fe2O3(p), respectively. Three corresponding thermites Al/Fe2O3(H), Al/Fe2O3(o), and Al/Fe2O3(p) were prepared [29] with a stoichiometric ratio of Fe2O3:Al (71.1wt%:28.9wt%) based on the calculation [30].

2.2. Preparation of Fe2O3‐NC and Al/Fe2O3‐NC

The Fe2O3 particles or Al/Fe2O3 thermite was evenly mixed with NC via grinding to obtain the composite materials, respectively. For the Fe2O3‐NC composites, the Fe2O3:NC mass ratio was 1:1, while for the Al/Fe2O3‐NC composites it was 1:1, too. The grinding process was maintained for 30 min to obtain light red or dark gray composite materials. The products were used for differential scanning calorimetry experiment, in order to assess the thermal behavior and the effects of Fe2O3 particles or Al/Fe2O3 on NC.

2.3. Preparation of AP/HTPB propellant formulations

The as‐prepared Fe2O3(H), Fe2O3(o), Fe2O3(p), and their corresponding thermites are used as the burning rate modifiers in the preliminary AP/HTPB propellant formulation [29] as shown in Table 1.

No.HTPB system/%Al/%Coarse AP/%Superfine AP/%AdditivesAdditives/%
N014.315.318.452.0none0.0
F114.315.318.452.0Fe2O3(H)2.0
F214.315.318.452.0Fe2O3(o)2.0
F314.315.318.452.0Fe2O3(p)2.0
S114.314.518.452.0Al/Fe2O3(H)2.8
S214.314.518.452.0Al/Fe2O3(o)2.8
S314.314.518.452.0Al/Fe2O3(p)2.8

Table 1.

The composition and content of composite propellant.

2.4. Samples characterization

The physical phase, composition, morphology, and structure of materials were characterized by SEM‐EDS, TEM, XRD, and FT‐IR. X‐ray diffractograms were recorded on a D/MAX‐3C (Japan) instrument using Cu Kα1 radiation (λ = 0.15406 nm) at 40‐kV voltage and a 40‐mA current ranging from 10° to 80°. SEM observations were carried out on a Quanta 400 FE‐SEM (FEI Co., USA) at an acceleration voltage of 30 kV. EDS was measured using an INCAIE350 testing device from OXFORD Instruments INC (UK) with a discharge voltage of 4–10 kV and a distance of exactly 1 mm between the electrodes. The morphology and size of as‐obtained products were investigated with a transmission electron microscope and high‐resolution TEM on a Libra 200FE (Carl Zeiss SMT Pte Ltd., Germany). The sample structure and composition were characterized using Bruker Tensor 27 infrared spectrometer.

The specific surface area was determined with Brunauer‐Emmett‐Teller (BET) Procedure (Autosorb‐1C‐TCD, American Quantachrome Instruments).

The thermal behavior of the samples was investigated using DSC (Q2000, TA Co.) at a heating rate of 10°C min–1 from room temperature to 300°C in an N2 atmosphere at a flow rate of 50 mL min–1 under ambient atmospheric pressure. To explore the reaction mechanism of the intense exothermic decomposition processes of NC and Fe2O3‐NC and to obtain the corresponding kinetic parameters (apparent activation energy (Ea/kJ mol–1), pre‐exponential constant (A/s–1)) and the most probable kinetic model function, the DSC curves at the heating rates of 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0°C min–1 were dealt by mathematic means.

The thermal decomposition studies of NC and Fe2O3‐NC were also performed by the thermogravimetry (Netzsch STA409) with Fourier transform infrared (Brucker V70) analysis technique under nitrogen environment at the heating rate of 10°C min–1.

The thermal behavior of the prepared thermites was carried out on a TA Instrument (Q600) device with a 10°C min–1 heating rate, using N2 with a flow rate of 100 mL min–1.

Burning parameters of AP/HTPB propellant including the rate and the pressure exponent were obtained by acoustic emission method by the AE/BX‐2006 multifunction system [29].

3. Structure characterization

3.1. Morphological characterization

The microstructure characterizations of the Fe2O3 particles and Al/Fe2O3 thermites are determined by analytical SEM as well as TEM.

Figure 1 shows the SEM images of the granular, oval, and polyhedral Fe2O3 particles and the corresponding Al/Fe2O3 thermite. From Figure 1(a), it can be found that Fe2O3(H) particles are granular in shape with a relatively small size (average 200 nm) and seem somewhat aggregated. Fe2O3(o) particles (Figure 1(c)) have an oval shape and a rough surface morphology due to the adhesion of scrap irons. The shape of Fe2O3(p) (Figure 1(e)) particles is polyhedral, which is quite different from the other two samples. A closer examination of the SEM images indicates that Fe2O3(p) particles are not very uniform in size, agglomerated, and have larger surface‐area‐to‐volume (S/V) ratio than that of Fe2O3(o) particles. Figure 1(b), (d), and (f) show the SEM images of Al/Fe2O3(H), Al/Fe2O3(o), and Al/Fe2O3(p), respectively. Some degree of aggregation can be found in the three thermite systems. Also, there seems to be favorable interfacial between Al and Fe2O3 particles.

Figure 1.

SEM images of Fe2O3 (×100,000 magnification) and thermites Al/Fe2O3 (×60,000 magnification). (a) Fe2O3(H), (b) Al/Fe2O3(H), (c) Fe2O3(o), (d) Al/Fe2O3(o), (e) Fe2O3(p), and (f) Al/Fe2O3(p).

Investigations of the low‐magnification TEM image (Figure 2(b)) of Fe2O3(H) nanoparticles indicate that most of the particles have an irregular sphere geometrical structure, and usually possess rough surfaces. Typical HRTEM images of the small part of Fe2O3(H) nanoparticles were obtained and are shown in Figure 2(b), (c), and (d). Just one set of clear lattice fringes with the interplanar distance of 0.25 nm could be seen in Figure 2(b), (d), inset (i), and (v), which can be indexed to the (110) plane of rhombohedral Fe2O3(H) structure. Excellent crystallinity is also confirmed by corresponding fast Fourier‐transform (FFT) transformation (inset in Figure 2(iii)).

Figure 2.

(a) and (e) show TEM images of Fe2O3(H) and Al/Fe2O3(H), respectively; (b, c, d) HRTEM images of Fe2O3(H) nanoparticles. Insets i and v are the high‐resolution images of Fe2O3(H) nanoparticles, insets ii and iv show a high‐resolution image of Fe2O3(H) nanoparticles containing stacking faults and dislocation tangles/networks on the surface, respectively, and inset iii shows the corresponding fast Fourier‐transform (FFT) pattern of Fe2O3(H) nanoparticles.

Particles containing a certain extent of lattice defects such as dislocation and stacking fault caused by the high pressure, temperature, and concentration through the hydrothermal treatment have also been found. Figure 2(c) gives an example of a series of diagonal and straight‐stacking faults within a particle throughout most of the surface. It can be seen more clearly in an enlargement of a local region (inset ii). Figure 2(d), the area “D” marked black pane and the corresponding inset (iv), shows a high‐resolution image of the Fe2O3(H) nanoparticles containing dislocation tangles/networks on the surface. These linear and plane defects mentioned above have profound effects on the growth and property of the Fe2O3(H) nanoparticles [31, 32].

Figure 3 shows the oval Fe2O3(o) particles with the length‐to‐diameter ratio (L/D ratio) of 1.47–1.59. From Figure 4(b), it is really easy to find out the rough surface of Fe2O3(o) particle, which is consistent with the SEM measurement. In the TEM image of Al/Fe2O3(o) thermites, the small spherical Al nanoparticles stick together, and also with the Fe2O3(o) particles.

Figure 3.

(a) and (c) show TEM images of Fe2O3(o) and Al/Fe2O3(o), respectively; (b) HRTEM image of Fe2O3(o) and the corresponding fast FFT pattern (inset).

Figure 4(a) and (c) show the TEM images of the Fe2O3(p) and Al/Fe2O3(p), respectively. It is obvious to see that almost all of the Fe2O3(p) particles are polyhedral in shape, which adhere to the Al particles as seen in Figure 4(c). The corresponding selected area electron diffraction (SAED) pattern shown in Figure 4(b) indicates that the Fe2O3(p) particles are single crystals. Figure 4(d) shows the fringes with the interplanar distance of 0.25 nm in a typical HRTEM image of a Fe2O3(p) particle, which agree well with the (110) lattice spacing of the rhombohedral hematite.

Figure 4.

(a) and (c) show TEM images of Fe2O3(p) and Al/Fe2O3(p), respectively, (b) Selected area electron diffraction (SAED) pattern of Fe2O3(p) and (d) HRTEM image of Fe2O3(p) and the corresponding fast FFT pattern (inset).

Figures 57 show the SEM images of Fe2O3(H), Fe2O3(p), Fe2O3(o), and the corresponding Al/Fe2O3, respectively. Take Figure 5, for instance. The SEM observation of Fe2O3(H)‐NC and Al/Fe2O3(H)‐NC in Figure 5 shows that the two composites have rough, irregular surface morphology under low magnification, probably due to the agglomeration of Fe2O3(H) nanoparticles or Al/Fe2O3(H). From Figure 5(a) and (c), it can be found that the vast majority of Fe2O3(H) particles or Al/Fe2O3(H) adhered on the surfaces of NC short fibers. Also, some small Fe2O3(H) (or Al/Fe2O3(H)) agglomeration and NC fragments can be observed. The enlargement of a local region on the surface of Fe2O3‐NC and Al/Fe2O3(H)‐NC in Figure 2(b) and (d) indicates that the mechanical‐grinding treatment has not changed the basic shape and particle size of Fe2O3 and Al.

Figure 5.

SEM images of Fe2O3(H)‐NC and Al/Fe2O3(H)‐NC. (a) Fe2O3(H)‐NC (×400 magnification), (b) Fe2O3(H)‐NC (×50,000 magnification), (c) Al/Fe2O3(H)‐NC (×200 magnification), and (d) Al/Fe2O3(H)‐NC (×50,000 magnification).

Figure 6.

SEM images of Fe2O3(p)‐NC and Al/Fe2O3(p)‐NC. (a) Fe2O3(p)‐NC (×1000 magnification), (b) Fe2O3(p)‐NC (×50,000 magnification), (c) Al/Fe2O3(p)‐NC (×200 magnification), and (d) Al/Fe2O3(p)‐NC (×25,000 magnification).

Figure 7.

SEM images of Fe2O3(o)‐NC and Al/Fe2O3(o)‐NC. (a) Fe2O3(o)‐NC (×1000 magnification), (b) Fe2O3(o)‐NC (×50,000 magnification), (c) Al/Fe2O3(o)‐NC (×200 magnification), and (d) Al/Fe2O3(o)‐NC (×25,000 magnification).

3.2. Structure and composition

Structure and composition of Fe2O3 and thermite were characterized using EDS, XRD, and FTIR techniques. The results [29] show that the three prepared iron oxides are Fe2O3 with a stoichiometric ratio of O:Fe (3:2), because their typical XRD patterns coincided with JCPDS: 33‐0664, and the hematite lattice vibration is identified at 480 and 571 cm-l [33]. The EDS data show that the thermite samples contain Al element. XRD patterns of thermites reveal no reaction between Al (JCPDS: 65‐2869) and Fe2O3. It can be found that the presence of water peaks in FTIR spectra of thermites, which is a common phenomenon in the nanomaterials [3438] especially with the presence of Al particles.

4. Thermal analysis

To explore the reaction mechanism of the intense exothermic decomposition process of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC and to obtain the corresponding kinetic parameters (apparent activation energy (Ea/kJ mol-1), pre‐exponential constant (A/s-1)) and the most probable kinetic model functions, the DSC curves at six heating rates of 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0°C min-1 were dealt by mathematic means, and the temperature data corresponding to the conversion degrees (α) were found. The values of were obtained by Ozawa’s method from the iso‐conversional DSC curves at the heating rates of 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0°C min-1, and the Eαα relation is shown in Figure 8. From Figure 8, one can see that the activation energy slightly changes in the section of 0.10–0.80 (α), and the ranges were selected to calculate the non‐isothermal reaction kinetics parameters.

Figure 8.

Eα versus α curve of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC by Flynn‐Wall‐Ozawa’s method.

Six integral methods (MacCallum‐Tanner, Šatava‐Šesták, Agrawal, general integral, universal integral, and Flynn‐Wall‐Ozawa) and one differential method (Kissinger) were employed [3943]. Forty‐one types of kinetic model functions and the basic data were put into the integral and differential equations for calculation. The kinetic parameters and the probable kinetic model function were selected by the logical choice method and satisfying the ordinary range of the thermal decomposition kinetic parameters for energetic materials (Ea = 80–250 kJ mol-1, logA = 7–30 s-1). These data together with their appropriate values of linear correlation coefficient (r), standard mean square deviation (Q), and believable factor (d, where d = (1–r)Q) are presented in Tables 24. The values of Ea and logA obtained from each single non‐isothermal DSC curve are in good agreement with the calculated values obtained by Kissinger’s method and Ozawa’s method. We consider the Fe2O3‐NC composites as an example, and conclude that the reaction mechanism of the intense exothermic decomposition process of Fe2O3‐NC is classified as Avrami‐Erofeev equation G(α)=[–ln(1–α)]2/3. Substituting f(α) with 3(1–α)[–ln(1–α)]1/3/2, Ea with 192.11 kJ mol-1, and A with 1018.54 s-1 in Eq. (1),

dαdT=Aβf(α)eE/RTE1

where f(α) and dα/dT are the differential model function and the rate of conversion, respectively.

Methodβ/°C min-1Ea/kJ mol-1log (A/s-1)rSd
MacCallum‐Tanner5207.9820.550.99834.45 × 10-47.50 × 10-7
10205.4020.250.99844.11 × 10-46.38 × 10-7
15209.3320.680.99883.25 × 10-43.98 × 10-7
20209.3420.670.99873.49 × 10-44.61 × 10-7
25211.7520.910.99902.56 × 10-42.48 × 10-7
30210.1020.760.99824.69 × 10-48.30 × 10-7
Šatava‐Šesták5204.5520.230.99834.45 × 10-47.50 × 10-7
10202.1219.940.99844.11 × 10-46.38 × 10-7
15205.8320.350.99883.25 × 10-43.98 × 10-7
20205.8320.330.99873.49 × 10-44.61 × 10-7
25208.1120.560.99902.56 × 10-42.48 × 10-7
30206.5620.420.99824.69 × 10-48.30 × 10-7
Agrawal5207.2020.490.99822.37 × 10-34.30 × 10-6
10204.5420.180.99832.19 × 10-33.67 × 10-6
15208.3720.600.99871.73 × 10-32.29 × 10-6
20208.3320.580.99861.86 × 10-32.65 × 10-6
25210.6820.810.99901.36 × 10-34.43 × 10-6
30209.0320.660.99812.49 × 10-34.77 × 10-6
General integral5205.8118.980.99852.36 × 10-34.33 × 10-6
10203.3018.690.99832.18 × 10-33.69 × 10-6
15207.2219.100.99871.72 × 10-32.30 × 10-6
20207.2419.090.99861.85 × 10-32.66 × 10-6
25209.6419.330.99891.36 × 10-34.43 × 10-6
30208.0219.180.99812.48 × 10-34.79 × 10-6
Universal integral5207.2020.490.99822.37 × 10-34.30 × 10-6
10204.5420.190.99832.19 × 10-33.67 × 10-6
15208.3720.600.99871.73 × 10-32.29 × 10-6
20208.3320.580.99861.86 × 10-32.65 × 10-6
25210.6820.820.99901.36 × 10-34.43 × 10-6
30209.0320.660.99812.49 × 10-34.77 × 10-6
Mean207.4820.22
Flynn‐Wall‐Ozawa185.68 (Eeo)0.9998
197.56 (Epo)0.9979
Kissinger199.68 (EK)19.820.9977
Mean (EeO, EpO, EK)194.31

Table 2.

Calculated values of kinetic parameters of decomposition reaction for NC.

Note: E with the subscript of eo and po is the apparent activation energy obtained from the onset temperature (Te) and the peak temperature (Tp) by Ozawa’s method, E with the subscript of K is the apparent activation energy obtained from the peak temperature (Tp) by Kissinger’s method.


Methodβ/°C min-1Ea/kJ mol-1log(A/s-1)rQd
MacCallum‐Tanner5.0182.7617.710.98884.32 × 10-24.82 × 10-4
10.0190.6718.630.99133.36 × 10-22.92 × 10-4
15.0178.1717.250.99491.97 × 10-21.00 × 10-4
20.0196.9319.310.99342.58 × 10-21.72 × 10-4
25.0203.1119.970.99402.32 × 10-21.38 × 10-4
30.0208.5920.600.99462.08 × 10-21.11 × 10-4
Šatava‐Šesták5.0180.7417.530.98884.32 × 10-24.82 × 10-4
10.0188.2118.400.99133.36 × 10-22.92 × 10-4
15.0176.4117.100.99491.97 × 10-21.00 × 10-4
20.0194.1219.040.99342.58 × 10-21.72 × 10-4
25.0199.9519.670.99402.32 × 10-21.38 × 10-4
30.0205.3820.270.99462.08 × 10-21.11 × 10-4
Agrawal5.0182.1617.690.98782.30 × 10-12.80 × 10-3
10.0189.9218.580.99061.79 × 10-11.69 × 10-3
15.0177.4517.220.99441.05 × 10-25.88 × 10-4
20.0196.0219.240.99281.37 × 10-19.95 × 10-4
25.0202.1119.890.99351.23 × 10-17.90 × 10-4
30.0206.7920.510.99421.11 × 10-16.44 × 10-4
General integral5.0180.7716.220.98772.29 × 10-12.81 × 10-3
10.0188.6717.110.99051.78 × 10-11.70 × 10-3
15.0176.2715.790.99441.05 × 10-15.87 × 10-4
20.0194.9117.780.99271.37 × 10-19.95 × 10-4
25.0201.0618.420.99351.23 × 10-17.98 × 10-4
30206.7819.030.99421.10 × 10-16.43 × 10-4
Universal integral5.0182.1617.690.98782.30 × 10-12.80 × 10-3
10.0189.9218.580.99061.79 × 10-11.69 × 10-3
15.0177.4517.220.99441.53 × 10-14.88 × 10-4
20.0196.0219.240.99281.37 × 10-19.95 × 10-4
25.0202.1119.890.99351.23 × 10-17.99 × 10-4
30.0207.7920.510.99421.11 × 10-16.44 × 10-4
Mean192.1118.54
Flynn‐Wall‐Ozawa188.33 (Epo)0.9993
Kissinger189.98 (EK)18.760.9992
Mean (EeO,EpO, EK)189.16

Table 3.

Calculated values of kinetic parameters of decomposition reaction for Fe2O3(H)‐NC.

Note: E with the subscript of eo and po is the apparent activation energy obtained from the onset temperature (Te) and the peak temperature (Tp) by Ozawa’s method, E with the subscript of K is the apparent activation energy obtained from the peak temperature (Tp) by Kissinger’s method.


Methodβ/°C min-1Ea/kJ mol-1log(A/s-1)rSd
MacCallum‐Tanner5174.5616.810.99582.84 × 10-31.21 × 10-5
10178.3017.270.99672.22 × 10-37.36 × 10-6
15185.2618.040.99711.94 × 10-35.61 × 10-6
20183.7917.870.99692.10 × 10-36.59 × 10-6
25207.7220.500.99652.33 × 10-38.11 × 10-6
30195.5019.150.99622.53 × 10-39.56 × 10-6
Šatava‐Šesták5173.0016.670.99582.84 × 10-31.21 × 10-5
10176.5317.110.99672.22 × 10-37.36 × 10-6
15183.1017.840.99711.94 × 10-35.61 × 10-6
20181.7117.680.99692.10 × 10-36.59 × 10-6
25204.3120.180.99652.33 × 10-38.11 × 10-6
30192.7618.890.99622.53 × 10-39.56 × 10-6
Agrawal5174.0016.790.99531.51 × 10-27.04 × 10-5
10177.6117.240.99641.18 × 10-24.29 × 10-5
15184.4517.990.99681.03 × 10-23.26 × 10-5
20182.9417.820.99661.12 × 10-23.84 × 10-5
25206.6720.420.99621.24 × 10-24.67 × 10-5
30194.4919.070.99591.34 × 10-25.53 × 10-5
General integral5174.0016.790.99531.51 × 10-27.04 × 10-5
10177.6117.240.99641.18 × 10-24.29 × 10-5
15184.4517.990.99681.03 × 10-23.26 × 10-5
20182.9417.820.99661.12 × 10-23.84 × 10-5
25206.6720.420.99621.24 × 10-24.67 × 10-5
30194.4919.070.99591.34 × 10-25.53 × 10-5
Universal integral5172.6415.350.99531.51 × 10-27.09 × 10-5
10176.3915.800.99631.17 × 10-24.31 × 10-5
15183.3216.550.99681.03 × 10-23.27 × 10-5
20181.8816.390.99651.11 × 10-23.85 × 10-5
25205.6518.940.99621.23 × 10-24.68 × 10-5
30193.5217.630.99591.34 × 10-25.53 × 10-5
Mean186.3417.91
Flynn‐Wall‐Ozawa172.66(Eeo)0.99414.87×10-3
194.23(Epo)0.99908.60×10-4
Kissinger196.18(EK)19.440.99894.56×10-3
Mean(EeO,EpO, EK)187.69

Table 4.

Calculated values of kinetic parameters of decomposition reaction for Al/Fe2O3(H)‐NC.

Note: E with the subscript of eo and po is the apparent activation energy obtained from the onset temperature (Te) and the peak temperature (Tp) by Ozawa’s method, E with the subscript of K is the apparent activation energy obtained from the peak temperature (Tp) by Kissinger’s method.


The kinetic equation of the exothermic decomposition reaction may be described as

dαdT=1018.71β(1α)[ln(1α)]1/3exp(2.31×104/T)E2

The values (Te0 and Tp0) of the onset temperature (Te) and peak temperature (Tp) corresponding to β→0 were obtained by Eq. (3), and the self‐accelerating decomposition temperature (TSADT) was obtained by Eq. (5) [3943]. The values (TSADT and Tp0) are 182.03 and 194.10°C, respectively.

Te(or p)=Te0(or p)+aβi+bβi2+cβi3 i = 14E3

where a, b, and c are coefficients.

TSADT=Te0E4

The thermal ignition temperature (Tbe0 or TTIT) was obtained by substituting Eeo and Te0 into the equation of Zhang et al. (Eq. (5)) [44], and the critical temperatures of thermal explosion (Tbp0 or Tb) were obtained by substituting Epo and Tp0 in Eq. (5). The values (TTIT and Tb) are 191.44 and 204.16°C, respectively,

Tbe0(or bp0)=EoEo24EoRTe0(or p0)2RE5

The thermal behaviors of NC are also analyzed with the same method using the data in Figure 8. The results show that the reaction mechanism of the intense exothermic decomposition process of them is classified as reaction order f(α) = 3(1–α)[–ln(1–α)]1/3/2, G(a) = [–ln(1–α)]2/3. The DSC curves of Fe2O3(p)‐NC, Al/Fe2O3(p)‐NC, Fe2O3(o)‐NC, and Al/Fe2O3(o)‐NC at a heating rate of 10°C min-1 are listed in Figures S1 and S3 (Supplementary data), respectively. The Eαα relations of Fe2O3(p)‐NC, Al/Fe2O3(p)‐NC, Fe2O3(o)‐NC, and Al/Fe2O3(o)‐NC are shown in Figures S2 and S4 (Supplementary data). Table 5 and the supplementary data (Tables S1–S4) show the calculated values of kinetic parameters of decomposition reaction for NC, Fe2O3(H)‐NC, Al/Fe2O3(H)‐NC, Fe2O3(p)‐NC, Al/Fe2O3(p)‐NC, Fe2O3(o)‐NC, and Al/Fe2O3(o)‐NC. From Table 5, it can be found that (1) the Ea values of NC‐based composites containing Fe2O3 and Al/Fe2O3 are less than that of NC; (2) the Ea value of NC‐based composites containing Al/Fe2O3 is less than that of NC‐based composites containing the corresponding Fe2O3; (3) among the three Fe2O3 particles, Fe2O3(H) is the best catalyst because Ea of Fe2O3(H)‐NC is the lowest; (4) the Ea value of Al/Fe2O3(o)‐NC is 0.16 and 5.57 kJ mol-1 lower than that of Al/Fe2O3(H)‐NC and Al/Fe2O3(p)‐NC, respectively, but the thermal ignition temperature and the critical temperature of thermal explosion of Al/Fe2O3(o)‐NC are so high. Therefore, the prepared Fe2O3(H) and Al/Fe2O3(H) are the two kinds of promising catalysts developed in accelerating the decomposition rate or the process of NC.

SampleEa/kJ mol-1log(A/s-1)Te0/°CTp0/°CTbe0/°CTbp0/°CΔS/J·mol-1·K-1ΔH/kJ·mol-1ΔG/kJ·mol-1
NC207.4820.22181.76197.00191.42206.69138.40199.68134.61
Fe2O3(H)‐NC192.1118.54182.03194.10191.44204.16106.21189.98140.35
Al/Fe2O3(H)‐NC186.3417.91175.57190.02185.71199.5994.31196.18152.46
Fe2O3(p)‐NC200.6719.45176.38196.42185.53206.82123.72185.65127.56
Al/Fe2O3(p)‐NC191.7518.51180.29195.55190.25205.52106.31185.39135.56
Fe2O3(o)‐NC202.6919.68178.87187.64187.93197.14128.29195.50136.39
Al/Fe2O3(o)‐NC186.1817.87179.82191.87189.39201.9793.50187.43143.95

Table 5.

Calculated values of kinetic parameters of decomposition reaction for NC, Fe2O3(H)‐NC, Al/Fe2O3(H)‐NC, Fe2O3(p)‐NC, Al/Fe2O3(p)‐NC, Fe2O3(o)‐NC, and Al/Fe2O3(o)‐NC.

By thermal analysis, the addition of Fe2O3(H) and Al/Fe2O3(H) did not change the kinetic model function of NC, reduced the value of Ea, and the critical temperature of thermal explosion, thus Fe2O3(H) nanoparticles and Al/Fe2O3(H) thermites could accelerate the decomposition rate or process of NC. Furthermore, the effects of Fe2O3(H) nanoparticles and Al/Fe2O3(H) on the thermal decomposition of NC have been investigated by the thermogravimetry with Fourier transform infrared analysis (TG‐IR).

The TG‐IR‐hyphenated technique is a highly preferred approach for investigating the thermal degradation of energetic materials. The TG‐thermogravimetric derivative (TG‐DTG) curves of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC at a heating rate of 10°C/min are presented in Figure 9. Just one stage of the total mass loss can be found from the decomposition processes of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC. The total weight loss of NC is 68.41%, while that of Fe2O3(H)‐NC and Al/Fe2O3(H)‐NC are about 34.77 and 31.12%, respectively, which is lower than that of NC due to the remaining Fe2O3 and some residues.

A series of temperature values of typical points including the initial decomposition temperature (Ti), the extrapolated onset temperature (Te), the peak temperature (TL), the extrapolated end temperature (Tc), and the final temperature (Tf) deserve special attention. Compared with the degradation process of NC, these typical temperature values of Fe2O3(H)‐NC and Al/Fe2O3(H)‐NC obviously are reduced under the influence of Fe2O3(H) nanoparticles and Al/Fe2O3(H) as seen in Figure 9(b) and (c). The peak temperatures of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC are 209.43, 208.59, and 210.24°C, respectively.

The apparent variation of IR characteristic absorption peaks of the gaseous decomposition products of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC corresponding to the thermal decomposition process at the typical temperature points (Ti, Te, TL, Tc, and Tf) is shown in Figure 10 and Table 6. It can be found that the gaseous products detected include CO, NO2, NO, N2O, HCHO, and HCOOH during the decomposition process of NC with and without the Fe2O3(H) nanoparticles or Al/Fe2O3(H).

Figure 9.

TG‐DTG curves of NC (a), Fe2O3(H)‐NC (b), and Al/Fe2O3(H)‐NC (c).

Figure 10.

IR spectra of the gases evolved from the degradation of NC (a), Fe2O3(H)‐NC (b), and Al/Fe2O3(H)‐NC (c).

NCFe2O3(H)‐NCAl/Fe2O3(H)‐NC
T/°CGaseous productsT/°CGaseous productsT/°CGaseous products
164.92 (Tx) H2O, CO2160.47 (Tx1) H2O, CO2, NO2164.40 (Tx2) H2O, CO2, NO2, NO 
178.81 (Ti)H2O, CO2, NO2177.74 (Ti1)H2O, CO2, NO2180.19 (Ti2)H2O, CO2, NO2, NO
200.16 (Te)H2O, CO2, NO2, NO195.74 (Te)H2O, CO2, NO2, NO196.76 (Te)H2O, CO2, NO2, NO
209.43 (TL)H2O, CO2, NO2, NO, N2O, HCHO, HCOOH208.59(TL)H2O, CO2, CO, NO2, NO, N2O, HCHO, HCOOH210.24 (TL)H2O, CO2, CO, NO2, NO, N2O, HCHO, HCOOH
221.70 (Tc)H2O, CO2, CO, NO2, NO, HCHO, HCOOH223.31 (Tc)H2O, CO2, CO, NO2, NO, N2O, HCHO, HCOOH225.47 (Tc)H2O, CO2, CO, NO2, NO, N2O, HCOOH
247.45 (Tf)H2O, CO2, NO259.63 (Tf)H2O, CO2, CO, NO, HCOOH259.72 (Tf)H2O, CO2, NO2, NO

Table 6.

Gaseous products generated during the decomposition processes of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC at different temperatures.

Note: Tx, some temperature below the initial decomposition temperature; Tx1(Tx2), some temperature below Ti1(Ti2); Ti, the initial decomposition temperature; Ti1, the initial decomposition temperature of the obvious exothermic peak of Fe2O3(H)‐NC; Ti2, the initial decomposition temperature of the obvious exothermic peak of AL/Fe2O3(H)‐NC; Te, the extrapolated onset temperature; TL, the peak temperature; Tc, the extrapolated end temperature; Tf, the final temperature.


Table 6 lists the gaseous products generated during the decomposition processes of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC at the typical temperature points. At the initial decomposition temperature (Ti), the IR absorption peaks of H2O (3600–3740 cm-1), CO2 (2360, 670 cm-1), and NO2 (1593–1635 cm-1) [45, 46] are easily identifiable (Figure 10(a, Ii) and (b, Ii)). During the whole testing process, the existence of H2O and CO2 from the ambience is the outside disruptive factor, which cannot be ignored. In order to prove that H2O and CO2 gas detected at 178.81°C are not the products of NC decomposition, the variation of IR absorption peaks at 164.92°C is obtained, which is shown in Figure 10(a, Ix). The TG‐DTG curves of NC in Figure 9(a) show that the decomposition of NC has not occurred at 164.92°C and begins from 178.81°C. By comparing the curves Ix (164.92°C) and Ii (178.81°C) in Figure 10(a), the bands at 3600–3740 cm-1 are assigned to O‐H‐bonding‐stretching vibrational modes for water as an impurity because of the extremely similar absorptive intensities. Apart from the impurity peaks of H2O, it can be concluded that CO2 gas is not the initial degradation product because the intensities of the detected CO2 gas are basically unchanged at both 164.92 and 178.81°C shown in Figure 10(a, Ix, and Ii). Furthermore, the noticeable IR bands of NO2 are found in the region of 1593–1635 cm-1 at the beginning of the decomposition of NC shown in the curve Ii (Figure 10(a)) [46], which means that the NO2 gas is an initial degradation product. The above results concur with those of several previous studies [22, 23, 47, 48] in which the O‐NO2 bond is deemed to be the first step leading to the release of NO2

RONO2RO+NO2E6

The NO2 stagnates in the polymer skeleton and then reacts with the RO• radical or its degradation products. It is particularly necessary to point out that the signal of NO2 was present as shown in the IR spectrum in Figure 10(b, Ix) at 160.47°C (Tx1). The intensities of NO2 peaks increase with temperature, which is very different from the decomposition of NC. It is a fact that NC could be slowly decomposed as the temperature increases further, which has been enhanced by Fe2O3(H) nanoparticles due to their catalysis. That is, the Fe2O3(H) nanoparticles could accelerate the O‐NO2 bond cleavage and the release of NO2. For this reason, the absorption peaks of H2O and CO2 exist not only from the environment but also from the degradation of Fe2O3(H)‐NC at 160.47°C (Tx1). Moreover, both NO2 and NO are detected besides the H2O and CO2 gases, which indicates that Al/Fe2O3(H) can make NC decompose faster than Fe2O3(H) does.

Figure 11 shows the 3D‐IR spectra of gas products of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC at a heating rate of 10°C min-1. At the initial decomposition temperature, the IR absorption peaks of H2O (3600–3740 cm-1), CO2 (2360, 670 cm-1), and NO2 (1593–1635 cm-1) [46, 47] are easily identifiable (Figure 11). With the progress of the thermal decomposition process, the gas productions such as NO (1762–1965 cm-1), CO (2194 cm-1), N2O (2241 cm-1), HCHO (2814 and 1746 cm-1), and HCOOH (1080–1128 cm-1) are detected [26, 47]. By comparing the gaseous products generated during the decomposition processes of NC, Fe2O3(H)‐NC, and Al/Fe2O3(H)‐NC at different temperatures, it can be concluded that Fe2O3(H) and Al/Fe2O3(H) can accelerate the O‐NO2 bond cleavage and the release of NO2 gas. The evolution of HCOOH gases is the products of the secondary autocatalytic reactions of NC [24, 26]. From Figure 11, the intensities of all these gas products are very strong, which can be taken as an important signal of identifying the faster decomposition rate of Fe2O3(H)‐NC and Al/Fe2O3(H)‐NC composites than that of NC. One of the reasons for generating quantities of gases is that the condensed phases break down fast with the presence of Fe2O3(H) nanoparticles and Al/Fe2O3(H). It has to be pointed out that Al/Fe2O3(H) thermite is a better catalyst than Fe2O3(H) nanoparticles for reducing the activation energy and accelerating to break the O‐NO2 bond during the thermal decomposition process of NC.

Figure 11.

3D‐IR spectra of gas products of NC (a), Fe2O3(H)‐NC (b), and Al/Fe2O3(H)‐NC (c) at a heating rate of 10°C min-1.

5. Combustion catalysts used in AP/HTPB propellants

Figure 12 shows the burning rate (u) versus pressure (p) curves of AP/HTPB propellants. Based on the experimental data, the pressure exponent (n) can be calculated [29, 49]. The catalytic efficiency is another evaluation index of the combustion catalyst [29], which is shown in Figure 13.

Figure 12.

Burning rate (u) versus pressure (p) curves of AP/HTPB composite propellants.

Figure 13.

Efficiency of AP/HTPB composite propellants.

The burning rate of all the formulations increases as the pressure rises within 4–15 MPa. For the formulations containing Fe2O3, the burning rate is in the order: F2<F3<F1. For the formulations with thermite, that order is S1>S2>S3. The BET‐specific surface areas of Fe2O3(H), Fe2O3(o), and Fe2O3(p) are 5.3, 3.1, and 6.9 m2/g, respectively. Possibly due to the aggregation of Fe2O3(H), its specific surface area is smaller than that of Fe2O3(p), which is opposite to what we expected simply based on the particle size. However, when the particles are dispersed to the formulations, Fe2O3(H) particles are believed to have a larger surface area than Fe2O3(p) based on the particle size. Therefore, the superior combustion performance can be ascribed to the dominant role of the surface area of the Fe2O3 particles. From Figures 12 and 13, the formulations containing thermites all have better burning rate and combustion‐supporting ability than the formulations containing the corresponding Fe2O3. And thermite with a smaller size of Fe2O3 (H) particles has a higher burning rate.

A good formulation is considered to have a high burning rate with a low pressure exponent. From Table 7, it can be found that Fe2O3 can take effect on decreasing the pressure exponent of the propellant. Considering the role of Al nanopowders and the thermite reaction in enhancing the burning rate, thermites Al/Fe2O3 are the better burning rate modifiers in improving the combustion performances of AP/HTPB composite propellants. Overall, the Al/Fe2O3(H) is the best combustion catalyst.

p/MPanN0nF1nF2nF3nS1nS2nS3
4–70.350.370.310.370.360.340.32
7–100.420.360.260.270.310.290.36
10–130.630.450.370.480.480.500.39
13–150.650.450.430.350.510.490.39

Table 7.

The burning rate pressure exponent of formulations under different pressure.

Moreover, the combustion wave structures of AP/HTPB composite propellants containing Al/Fe2O3 thermite obtained at 4 MPa are shown in Figure 14. The flames of the three composites (Figure 14) are entirely yellowish that is caused by the fuel‐rich diffusion flame generated by the decomposed gases of the binder and the AP particles [50]. There is no obvious dark zone, which is different from the double‐base propellants. Above the burning surface, both the AP flame and the diffusion flame produced by the decomposed gases exist simultaneously, leading to major heat release of the combustion process. High flame temperature up to 2700 K was generated at the center of the luminous flame as in the case of Al/Fe2O3(H) catalyst. Thick gray smoke was formed surrounding the yellowish flame of the formulation S2, due to the incomplete burning. When adding Al/Fe2O3(p) thermite to the AP/HTPB propellants, the front luminous flame of formulation S3 was blown down to the downstream and numerous Fe2O3 and Al particles were ejected from the burning surface. The results of the combustion wave structures and flame temperature characteristics indicate that the Al/Fe2O3(H)‐containing propellant formulation is the best formulation among the tested samples.

Figure 14.

Combustion wave structures of AP/HTPB composite propellants contain thermites Al/Fe2O3: (S1) Al/Fe2O3(H), (S2) Al/Fe2O3(o), and (S3) Al/Fe2O3(p).

6. Conclusion

In summary, the structural effect of different sizes and shaped Fe2O3 particles on the performance as enhancers, with or without Al, in the thermal decomposition process of NC and the combustion of AP/HTPB has been studied and compared. The as‐prepared Fe2O3 particles and Al/Fe2O3 have good compatibility with NC from DSC thermal analysis, suggesting the safely use of Fe2O3‐NC and Al/Fe2O3‐NC composites. The non‐isothermal decomposition kinetics of the composites and NC can be modeled by the Avrami‐Erofeev equation f(α)=3(1–α)[–ln(1–α)]1/3/2 in differential form. Through the TG‐IR analysis of decomposition processes and products of the composites and NC, it is speculated that the as‐prepared Fe2O3 particles and Al/Fe2O3 can effectively accelerate the thermal decomposition reaction rate of NC by promoting the O‐NO2 bond cleavage. Among the three studied different shapes of Fe2O3, the granular Fe2O3 and its corresponding thermite (Al/Fe2O3(H)) exhibit the highest burning rate due to the larger surface area associated with a smaller particle size. Moreover, the Al/Fe2O3(H) thermites have more effective combustion‐supporting ability for AP/HTPB composite propellants than Fe2O3 and the other two as‐prepared Al/Fe2O3 thermites. Moreover, adding the thermites to the composite propellants could contribute to increasing the flame temperature and improving the combustion wave structures of the formulations. In all, the addition of the prepared oxides or thermites can distinctly increase the burning rate, enhance the flame temperature, and decrease the burning rate pressure exponent of the AP/HTPB composite propellants.

Acknowledgments

This work was supported by the Program for the National Natural Science Foundation of China (No. 21373161), Research Fund for the Doctoral Program of Higher Education of China (RFDP, No. 20126101110009), and the New Century Excellent Talents in the University of Ministry of Education of China (NCET‐12‐1047).

© 2016 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Ningning Zhao, Jiachen Li, Fengqi Zhao, Ting An, Rongzu Hu and Haixia Ma (October 5th 2016). Combustion Catalyst: Nano‐Fe2O3 and Nano‐Thermite Al/ Fe2O3 with Different Shapes, Developments in Combustion Technology, Konstantinos G. Kyprianidis and Jan Skvaril, IntechOpen, DOI: 10.5772/64748. Available from:

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