Effects of oxidizer structure on thermal and combustion behavior of Fe2O3/Zr thermite

Performance of MOF-derived micrometer porous Fe2O3 as the oxidizer in Zr-fuelled thermite is compared with commercial nano-sized Fe2O3 by characterizing thermal and combustion behavior of Fe2O3/Zr mixture via differential scanning calorimetry, optical emission measurement as well as composition and morphology analysis on condensed combustion products. Results show that thermal behaviors of Fe2O3/Zr with a slow heating rate have little difference regardless of the kind of Fe2O3. However, MOF-derived micrometer porous Fe2O3 show an obvious superiority in enhancing combustion of Fe2O3/Zr heated by a high rate. Combustion reactions of Fe2O3/Zr under high heating rates are probably rate-controlled by condensed reaction. The better performance of MOF-derived Fe2O3 is attributed to its larger contact area with Zr particle in that micrometer porous Fe2O3 particles are easily broken into primitive nano-sized particles, which effectively avoid the agglomeration of oxidizer. The MOF-derived Fe2O3 particles obtained at calcination temperature of 550 °C enable the best combustion performance of Fe2O3/Zr thermite. This should be because the crystallinity and porous structure of 550 °C-Fe2O3 are more favorable for the mass transfer process during high-rate combustion.


Introduction
Zirconium (Zr) has good ignitibility and combustibility as well as high volumetric energy density of ∼1.86 kcal cm −3 , which creates a vast opportunity for its use as explosives and propellants [1,2]. Fe 2 O 3 is a widely used low cost oxidizer possessing good oxygen donation ability as well as catalytic effects on reactivity of energetic materials. Previous studies [3,4] showed that Fe 2 O 3 could be used to improve the combustion performance of Zr particles. Though these studies proposed that coating Zr particles with Fe 2 O 3 was a good way to exert the promoting effect of Fe 2 O 3 on combustion process, it is difficult to obtain such composite particles on a large scale. Directly mixing different components of energetic materials such as Fe 2 O 3 and Zr particles may still be the most practical way currently [5]. However, nanometer particles often have the difficulty in uniform dispersion, though they are expected to have better performances than the micrometer particles. In recent years, microsized metal-oxide particles having nanometer building blocks and porous structure are gradually perceived to be potentially more advantageous than corresponding nano-sized metal-oxides in many fields. Jian [6] demonstrated that hollow CuO spheres of large particle sizes had better performance than the commercial small-sized nanometer CuO particles in CuO/Al thermite. Chandru [7] prepared nano-structured Fe 2 O 3 catalysts with particle sizes of several hundred micrometers and showed that these Fe 2 O 3 had better catalytic performance in the combustion of AP-based composite propellant as compared with the small-sized nano Fe 2 O 3 rod. Bezmelnitsyn [8] showed that thermites composed of mesoporous Fe 2 O 3 and Al nano-particles exhibited suitable characteristics as an excellent propellant initiator. Chen [9] found that large-sized sea-urchin shape MnO 2 with hierarchical structure had better catalytic effects than the small-sized linear n-MnO 2 on thermal decomposition of AP. Hu [10] compared the catalytic performance of hollow mesoporous CuO microspheres with solid nano-CuO and micro-CuO particles on decomposition of AP, and confirmed the better performance of hollow mesoporous CuO microspheres than others. The superior performance of porous micro-particles composed of nano-sized assembly particles can be generally ascribed to the following two aspects. (1) Porous structure favors the large surface area and thus large interfacial contact areas between metal oxides and other components, and then shortens heat and mass transfer distance during reaction [11]; (2) Reasonable porous structure provides more pathways for the convective propagation [12,13] of the reaction energy. Therefore, micro-sized metal oxides having nano-sized assembly blocks and porous structure open up a new avenue to tailoring the performance of energetic materials.
Among various methods to prepare porous structured micro-sized metal oxides, metal-organic frameworks (MOF)-templating is an effective approach. MOF-derived metal oxides easily have large surface area, high porosity and adjustable morphology [14][15][16], providing diverse means for their performance tailoring. However, MOF-derived metal oxides have rarely been studied as oxidizers in energetic materials [17,18]. In

Experimental
Fe 2 O 3 was synthesized by using MIL-53(Fe) as the template and a calcination treatment. MIL-53(Fe)-derived Fe 2 O 3 was prepared by referencing previous work by Banerjee [19]. In detail, 1.35 g FeCl 3 ·6H 2 O and 0.83 g H 2 BDC were added into 25 ml DMF and magnetically stirred for 30 min at room temperature. Then the solution was transferred into an autoclave of 100 ml, and kept at 150°C for 2 h. After cooling down to room temperature a rust-colored precipitate was obtained, which was then separated by centrifugation. As-obtained product was washed with deionized water, followed by centrifugation to separate the solid phase. After repeating the washing and centrifugation processes for four times, the color of the solid phase turned into light-yellow. The resulted light yellow solid phase was dried at vacuum at 80°C for 18 h to remove the residual solvent and water. Finally, the precursor was calcined at different temperatures to obtain Fe 2 O 3 with different microstructures. The phase structure and the morphology of the precursor before and after calcination were characterized by XRD and SEM, respectively. TG test was conducted to help determine the proper calcination temperature of the precursor.
As-obtained Fe 2 O 3 were evenly mixed with Zr powders (particle sizes of mesh 400) to obtain Fe 2 O 3 /Zr mixture (weight ratio of 45:55) with a mortar and pestle. Thermal behavior of Fe 2 O 3 /Zr mixture in air atmosphere was examined by differential scanning calorimetry (DSC) technique at a heating rate of 10 K min −1 . Combustion characteristics of Fe 2 O 3 /Zr mixtures were evaluated by measuring the optical emission signal of the sample with a fiber optic spectrometer (Ocean Optics HR4000 CG-UV-NIR) and a Si detector (Thorlabs, DET 10 A) combined with an oscilloscope (Tektronix 4054B). The schematic experimental setup was shown in figure 1, in which the loose powder sample was placed in the groove inside the sample holder, and ignited by a NiCr hotwire with diameter of 0.5 mm. For each test, the resistance of the NiCr hotwire was kept to be ∼0.6 Ω by fixing the length of the hotwire, and the current employed was 10 A. The heating rate was ∼660 K s −1 . The morphology and composition of Fe 2 O 3 /Zr mixtures as well as their combustion products were characterized by SEM-EDS and XRD. For reference, commercial nano-Fe 2 O 3 having an averaged particle size of 30 nm was used to compare with MOF-derived Fe 2 O 3 . n-Fe 2 O 3 powders were first ultrasonically dispersed in ethanol, and then wetly mixed with Zr powders until ethanol was evaporated.   figure S1(b). It is seen that most of the MIL-53(Fe) particles have trigonal column-like morphologies, and the averaged particle size is ∼100 μm. To determine the transformation temperature from MIL-53(Fe) to Fe 2 O 3 , TG test was conducted. The TG result (figure S2) shows that the pyrolysis temperature of MIL-53(Fe) should be higher than 420°C. Therefore, two calcination temperatures, i.e. 450°C and 550°C were selected to tune the microstructure of as-obtained Fe 2 O 3 . Corresponding products are named as 450°C-Fe 2 O 3 and 550°C-Fe 2 O 3 , respectively. Figure 2 shows comparison of XRD patterns of MIL-53(Fe)-derived Fe 2 O 3 with commercial n-Fe 2 O 3 . It is seen that 450°C-Fe 2 O 3 and 550°C-Fe 2 O 3 show sharp diffraction peaks within 30°∼70°, which are identified to the phase of α-Fe 2 O 3 . In addition, an amorphous halo peak presents at around 20°for each sample, which is more obvious for 450°C-Fe 2 O 3 . By referencing the XRD pattern of MIL-53(Fe), this amorphous peak is attributed to the incomplete pyrolysis of MIL-53(Fe). Diffraction peaks of n-Fe 2 O 3 are wider than that of MIL-53(Fe)-derived Fe 2 O 3 , indicating that the crystallinity of the former is poorer. Figure 3 shows SEM images of n-Fe 2 O 3 , 450°C-Fe 2 O 3 and 550°C-Fe 2 O 3 . It is observed that n-Fe 2 O 3 powders agglomerate seriously, though the nominal particle size is ∼30 nm. The sizes of n-Fe 2 O 3 agglomerations range from several μm to ∼15 μm. The particle sizes and morphologies of MOF-derived Fe 2 O 3 change with the calcination temperature of MIL-53(Fe). 450°C-Fe 2 O 3 particles mainly show trigonal prism-like porous morphologies (high-magnification SEM images can be seen in figure S3), and the averaged particle size is above 20 μm. Some irregular-shaped small particles of ∼5 μm can also be seen to scatter around or on the surfaces of those regular-shaped large particles. Those large regular particles are composed of a huge number of nanometer particles, and many pores and cracks can be seen on the surfaces of particles. 550°C-Fe 2 O 3 particles also present trigonal prism-like outer shapes, but the averaged particle size increases to ∼50 μm. What's more, the surfaces of particles seem less porous, which should be due to sintering of the assembly particles. (highmagnification SEM images can be seen in figure S4).

Thermal behavior of Fe 2 O 3 /Zr
To examine effects of the type of Fe 2 O 3 on thermal behavior of Fe 2 O 3 /Zr mixture heated in air, DSC curves of three mixtures are compared, and TG-DSC curves of pure Zr powders are given for reference. It is seen from figure 5(a) that an obvious weight gain of pure Zr powders begins from ∼250°C and ends at around 800°C, accompanied by a broad exothermic peak centering at 613.5°C. This corresponds to the oxidization of Zr powders by air. Figure 5(b) shows that each Fe 2 O 3 /Zr sample has one broad exothermic peak. The exothermic onset temperatures of three Fe 2 O 3 /Zr mixtures resemble that of pure Zr, which are significantly lower than the thermal decomposition temperature of pure Fe 2 O 3 (∼1100°C). This indicates that thermal reaction of   [20]. However, the differences in temperatures and areas of exothermic peaks for different Fe 2 O 3 /Zr mixtures are very small, suggesting no obvious dependences on the particle size and morphology of Fe 2 O 3 . One possible reason is that the test is conducted in air atmosphere and with a low heating rate, in which case the dominated reaction is the oxidation of Zr powders by air due to the high reactivity of Zr.

Combustion characteristics of Fe 2 O 3 /Zr thermite
Combustion tests under high heating rates were conducted to further examine the influence of different Fe 2 O 3 on reaction characteristics of Fe 2 O 3 /Zr mixtures. Figure 6 shows optical emission spectra and temporal evolutions of optical emission intensity during combustion of different Fe 2 O 3 /Zr mixtures. It is necessary to point out that collection of the optical emission spectrum of n-Fe 2 O 3 /Zr mixture failed under the same testing distance, which was because the optical emission intensity was so weak that the signal could not be detected by the spectrometer. Optical emission spectra ( figure 6(a)) of MOF-derived Fe 2 O 3 /Zr are continuum spectra on  which two impurity peaks (Na atoms at ∼589 nm and K atoms at 766 nm) are superimposed. Therefore, the combusting sample can be approximately viewed as a greybody, and the intensity of the optical emission in figure 6(a) can be interpreted as an indicative of the flame temperature. It is concluded that 550°C-Fe 2 O 3 /Zr mixture has a higher flame temperature than that of the 450°C-Fe 2 O 3 /Zr mixture. Temporal evolutions of optical emission intensities in figure 6(b) further reveal that n-Fe 2 O 3 /Zr mixture has the lowest flame temperature among the three samples, though their combustion times are similar.
These results confirm that the size and microstructure of Fe 2 O 3 indeed have an obvious influence on combustion characteristics of Fe 2 O 3 /Zr mixtures under high heating rates. This is different from the case of heating Fe 2 O 3 /Zr at low heating rates like DSC tests. Figure 7 shows XRD patterns of condensed combustion products of different Fe 2 O 3 /Zr mixtures. The major composition of n-Fe 2 O 3 /Zr after combustion is ZrO 2 , Fe and some unreacted Zr, suggesting that the combustion is not complete. The combustion products of Fe 2 O 3 /Zr containing MOF-derived Fe 2 O 3 are mainly composed of ZrO 2 and Fe, which is corresponding to the complete redox reaction between Fe 2 O 3 and Zr. Besides, there is a small impurity peak presented at ∼30°for each sample. This is probably resulted from the oxidation products (NiCr 2 O 4 , PDF#23-0432) of NiCr hotwire used for ignition. Figure 8 shows SEM images of combustion products of different Fe 2 O 3 /Zr mixtures. It is clearly observed that morphologies of three samples differ significantly. Combustion products of n-Fe 2 O 3 /Zr (figures 8(a)-(b)) show two distinct sizes and morphologies. One is the very large spherical particles of ∼30 μm, which is far larger than the scale of both n-Fe 2 O 3 agglomerations and Zr particles. The other is similar with the n-Fe 2 O 3 /Zr mixture before combustion, indicating that combustion reaction is not complete, which is in accordance with XRD results in figure 7. On contrary, combustion products of MOF-derived Fe 2 O 3 /Zr (figures 8(c)-(f)) are  nearly only composed of large scale spherical particles. Compared with 450°C-Fe 2 O 3 , 550°C-Fe 2 O 3 results in larger sized spherical combustion products. The enlarged images show that these spherical particles are hollow, implying that interior expansion or explosion may have occurred [21,22].
Based on previous studies on micro-explosion phenomenon of Zr particles [21][22][23] and the condensed state reaction mechanisms of thermites at high heating rates [24], formation process of the observed large spherical products is explained as follows. For each Fe 2 O 3 /Zr mixture, ignition is initiated by the air-oxidization of Zr component, and the combustion reactions become self-sustaining when local solid-solid reaction between Zr and Fe 2 O 3 occurs at the contact interface. Oxidation of Zr particles proceeds inward from the surface due to the diffusion of oxygen across the Zr/ZrO 2 interface, producing core-shell structure particles with ZrO 2 as the shell and Zr as the core. At the same time, the Zr core and the ZrO 2 shell are heated rapidly by the released reaction heat. By referencing the maximum combustion temperature of Fe 2 O 3 /Zr (∼2380°C) [4] and Zr particles (∼2827°C) [25], it is inferred that neither evaporation of Zr (boiling point: ∼4377°C) nor evaporation of ZrO 2 (boiling point: ∼4300°C) shell can occur, but melting of Zr (melting temperature: ∼1852°C) is possible. Due to the difference in thermal expansion coefficients of ZrO 2 shell and Zr core, the ZrO 2 shell will burst, resulting in the outward splashing of melted Zr from the cracks on particle surface. These splashed liquid Zr drops are violently oxidized when exposure to oxygen (either from the oxygen in air or from the oxygen provided by Fe 2 O 3 ), forming large spherical particles with new ZrO 2 shell.
According to the above analysis, the difference in combustion performance of different Fe 2 O 3 /Zr mixtures should be resulted from the difference in the degree of solid-solid reaction at contact interfaces of Fe 2 O 3 with Zr, as is schematically illustrated in figure 9. The serious agglomeration of n-Fe 2 O 3 particles impedes the intimate contact of n-Fe 2 O 3 with Zr particle and thus oxygen transport from Fe 2 O 3 to Zr particles [26], so solid-solid reaction of them occurs locally, resulting in low heat release and low flame temperature. Therefore, the melting and splashing of Zr particle are difficult ( figure 9(a)). The whole combustion process of n-Fe 2 O 3 /Zr is probably dominated by the interface diffusion-controlled solid-solid reaction. However, MOF-derived Fe 2 O 3 particles have larger contact areas with Zr particles because most of them are broken into dispersed primitive nanometer particles during mixing with Zr particles, which enables the multi-point solid-solid reactions at Fe 2 O 3 /Zr interfaces, resulting in a large amount of heat release and ultra-high temperature rise of reactants ( figure 9(b)). The ultra-high temperature rise not only accelerates the thermal decomposition of Fe 2 O 3 to release gas oxygen, but also leads to the melting and splashing of most Zr particles. Consequently, the reaction mechanism of subsequent combustion of Zr has become beyond the solid-solid state reaction between Fe 2 O 3 and Zr. Compared with 450°C-Fe 2 O 3 /Zr, the combustion performance of 550°C-Fe 2 O 3 /Zr is better. This is probably due to the superior crystallinity and porous structure of 550 o C-Fe 2 O 3 , which favor the oxygen transport along grain boundaries [5].

Conclusions
MOF-derived micrometer porous Fe 2 O 3 particles are demonstrated to be a more efficient oxidizer than commercial nanometer Fe 2 O 3 for Fe 2 O 3 /Zr system. Thermal behaviors of Fe 2 O 3 /Zr mixture containing MOFderived Fe 2 O 3 and nano-Fe 2 O 3 have little difference when heated in air atmosphere with the low rate of 10 K min −1 . However, particle size and microstructure of Fe 2 O 3 have an obvious influence on combustion characteristics of Fe 2 O 3 /Zr mixture at the high heating rate of ∼660 K s −1 . MOF-derived Fe 2 O 3 enables Fe 2 O 3 /Zr mixture to have more complete reactions and higher flame temperatures than commercial nanometer Fe 2 O 3. This is ascribed to the larger contact areas between fragments of MOF-derived Fe 2 O 3 particles and Zr, which significantly increases the degree of solid-solid exothermic reaction at Fe 2 O 3 /Zr interfaces. MOF-derived Fe 2 O 3 particles obtained at calcination temperature of 550°C display the best performance due to their superior crystallinity and porous structure to other Fe 2 O 3 . Micrometer-sized porous metal oxide particles having nanosized building blocks may be a better choice than those nanometer counterparts for applications as the oxidizer in thermites.