Ammonium nitrate(AN)were purchased from Tianjin Guangfu Chemical Co., Ltd.(Tianjin, China).

Raw PZT is prepared with the method mentioned in reference [14]. The next step is to prepare “AN+PZT” composites. Herein, please note that grinding the mixture of AN and PZT without any liquid medium is forbidden because this would cause an exothermic solid reaction between AN and PZT, which produces a lot of gas and it smells like ammonia. In this work, we dissolve both AN and PZT into deionized water together. Then put the aqueous solution into a water bath oven at 70℃. Several days later, the water evaporates off and the mixture is obtained. Several mixtures are prepared, in which the mass fraction of PZT in the mixture are 3%, 5%, 10%, 15%, 20%, and 30%. The obtained samples are marked as “0.97AN+0.03PZT”, “0.95AN+0.05PZT”, “0.9AN+0.1PZT”, “0.85AN+0.15PZT”, “0.8AN+0.2PZT”, and “0.7AN+0.3PZT”, respectively.

Morphology was observed with a field-emission scanning electron microscope(SEM, JEOL JSM-7500). IR analysis was performed on American thermofisher scientific Nicolet 6700 infrared spectrometer(iodine bromide tablet). The phases of the samples were investigated with an X-ray powder diffractometer(XRD, Bruker Advance D8)using Cu K_α radiation at 40kV and 30mA. Thermal analyses were performed on a differential scanning calorimeter(TA Model Q600)at heating rates of 20℃/min(N₂ atmosphere, sample mass of approximately 5mg, and Al₂O₃ crucible). DSC-IR analysis was carried out at heating rates of 10℃/min by using a thermal analyzer system(DSC, Mettler Toledo)coupled with a Fourier transforms infrared spectrometer.

The IR spectrum of raw PZT is presented in Figure 1. The broad absorption peaks locating at 3378cm^-1 and 3269cm^-1 relate to the symmetric and anti-symmetric vibration of O—H bond in crystallized water. The peak locating at 1665cm^-1 corresponds to stretching vibration of N〖FY=,*2〗N bond and N—N in tetrazole ring. The strong peak locating at 1397cm^-1 is ascribed to stretching vibration of C—N bond in tetrazole ring. The peaks locating at 1195, 1148, 1063, and 1026cm^-1 are also attributed to stretching vibration of C—N bond in tetrazole ring. The peaks locating at 777cm^-1 and 732cm^-1 relate to the in-plane and out-plane vibrations of tetrazole ring. These results consist with the molecular structure of PZT hydrate(Figure 2).

Fig.1 IR spectrum of raw PZT

Fig.2 Molecular structure of PZT

SEM images of all “AN+PZT” composites are showed in Figure 3.Figure 3(a)presents the micron morphology of “0.97AN+ 0.03PZT”.Its particle size is about 300-500μm and its particle surface is very complicated. It seems that the particle surface of “0.97AN+0.03PZT” is etched, which may be resulted from the evaporation of the solvent. The micron morphologies of other composites are similar as that of “0.97AN+0.03PZT”. XRD patterns of raw PZT, pure AN, and all composites are illustrated in Figure 4. Raw PZT is a kind of crystal. Comparing patterns of composites with patterns of raw PZT and pure AN, we can find that some new phases form. The main diffraction peaks of raw PZT locate at 2θ of 32.6° and 11.1°. In all patterns of “AN+PZT”, the diffraction peak at 2θ of 32.6° still exists. The main diffraction peak of pure AN locate at 2θ of 29.1° and 40.2°. In all patterns of “AN+PZT”, the diffraction peaks at 2θ of 29.1° and 40.2° still exist. This means that all “AN+PZT” composites are polycrystalline of AN and PZT. However, the new peak locating at 2θ of 34.3° also exist in the patterns of all composites samples. This implies that eutectic of PZT and AN forms when the solvent is evaporated off. Each composite is the crystal mixture of raw PZT, pure AN, and the co-crystal.

Fig.3 SEM images of samples

Fig.4 XRD patterns of samples

Thermal decomposition of raw PZT is investigated by TG/DSC analysis and the result is showed in Figure 5. Figure 5(a)illustrates the DSC trace of raw PZT. A broad endothermic peak locates at 87.5-158.1℃, attributed to the sublimation of crystallized water in PZT molecules with heat absorption of +251.3J/g. A exothermic peak, relating to thermolysis of PZT, locates at 234.3-269.3℃ with heat release of -105.6J/g. Figure 5(b)and Figure 5(c)illustrate the TG/DTG curves of raw PZT. The TG curve indicates that the consumption of the decomposition contains two steps which correspond to the sublimation of crystallized water and decomposition of PZT. In the first consumption step, the mass loss is 15.3%. At elevated temperature, the consumption becomes very steep with mass loss of 48.5%. After that, as increasing of temperature, the mass loss does not change. This means there is still 36.2% material does not decompose and remains as residue. In DTG curve, it is revealed that the rate of the first consumption is very low and the rate of the second consumption is very high. This means the sublimation of crystallized water is a slow process and the decomposition of PZT is a fast process.

Fig.5 Thermal analysis of raw PZT

After understood thermolysis of raw PZT, we investigate the effect of raw PZT on phase transformation and thermal decomposition of ammonium nitrate, and the results are showed in Figure 6. The data extracted from DSC traces are listed in Table 1.

Fig.6 DSC curves of AN+PZT composites

It indicates that four endothermic peaks exist in DSC trace of pure AN. The peak locating at 57.1℃ is ascribed to the phase transformation at ambient temperature. This is the most headache problem in phase transition of AN, because phase transformation at this point is accompanied by a significant volume expansion which results in crack formation in the propellant grain ^[7]. Please see from the DSC trace of “0.97AN+0.03PZT”; by doped with 3% raw PZT, the phase transformation peak at ambient temperature disappears. The disappearance also occurs in other DSC traces of the composite samples. This means that the addition of raw PZT effectively inhibited the phase transition of AN at ambient temperature. The phase modification of AN by K⁺ and resulting changes in the transition temperature have been explained based on the replacement of some of the ammonium ions by potassium ions. The radius of potassium ion is 0.133nm compared with 0.148nm for the ammonium ion and hence, replacement of NH⁺₄ by K⁺ favors the AN(III)structure. Consequently, a steady fall in the IV-III transition temperature is observed as the replacement of ammonium ions by potassium ions increases. In DSC trace of pure AN, the peak locating at 137.4℃ corresponds to the phase transition of AN(II)to AN(I). Addition of raw PZT shows no inhibition effect on this phase transition. But it is clear that with increasing the content of raw PZT, the temperature of peak B decreases. And when the mass fraction of PZT accesses to 30%, this phase transition also disappears. In DSC trace of pure AN, the peak locating at 173.5℃ corresponds to the melting point of AN. Similar to peak B, the temperature of peak C(the melting point)also decreases as increasing of raw PZT content. In DSC trace of pure AN, the biggest endothermic peak locating at 313.5℃ is attributed to thermal decomposition of AN. Dissociation of NH₄NO₃(to NH₃ and HNO₃)is a strong endothermic process, and the heat release from redox reaction of NH₃ with HNO₃ can not offset the heat absorption of this dissociation. Thus, under the condition of 0.1MPa, the decomposition of AN presents an endothermic process in DSC trace. The addition of raw PZT does not change the endothermic feature of AN decomposition. However, as the content of PZT increasing, the area of the decomposition peak decreases and peak temperature also decreases. This means that addition of raw PZT promotes thermal decomposition temperature of AN.

Table 1 Characteristic parameters of phase transformation, melting,and decomposition of samples

Note: T_p-t is phase transformation temperature; T_m is the melting point; T_d is decomposition temperature.

For further investigating the effect of PZT on thermal decomposition of AN, DSC-IR analyses are performed and the results are illustrated in Figure 7. Figure 7(a)indicates that much gas produce in time lapse of 1182-1629s. The detected signal is very strong. We extract the IR spectra of the gas products at 1182, 1343, 1466, 1532, and 1629s, and show them in Figure 7(b). It indicates that the main decomposition products of pure AN are N₂O, HNO₃, and H₂O. This is accordance with the common mechanism of AN ^[15]. Firstly, AN dissociates to NH₃ and HNO₃; and the HNO₃ decomposes to ·OH and ·NO₂ radicals; and ·OH reacts with NH₃ to form ·NH₂ and H₂O; and then ·NH₂ reacts with ·NO₂ to produce N₂O and H₂O(see Eqs.1-4).

NH₄NO₃→NH₃+HNO₃+2.18kJ/g(1)

HNO₃→·OH+NO₂(2)

NH₃+·OH→·NH₂+H₂O(3)

NH₂+NO₂→[NH₂NO₂]→N₂O+H₂O(4)

This is the radical mechanism for thermal decomposition of AN. Please note that, herein, no NH₃ gas exists in gas products of pure AN, but many HNO₃ gas are detected. This means that the oxidization of NH₃ by decomposition products of HNO₃ is not the rate-limiting step for thermal decomposition of AN. Thermolysis of AN is dominated by decomposition of HNO₃, which has been confirmed in Izato's and Sinditskii's works ^[16-17]. In this work, the detection of HNO₃ also reveals that decomposition of HNO₃ is the rate-limiting step for thermolysis of AN. Now, please see Figure 7(c)and 7(d), they illustrate the result of AN doped with 20% raw PZT, i.e. the sample “0.8AN+0.2PZT”. The final decomposition products of “0.8AN+0.2PZT” are N₂O and H₂O. Comparing with the result of pure AN, we can find no HNO₃ exists in decomposition products of “0.8AN+0.2PZT”. This confirms that HNO₃ has reacted with something without decomposition. In Figure 7(d), please note that there are many NH₃ detected as intermediate product. The NH₃ may be produced from dissociation of AN, or from decomposition of raw PZT. In order to disclose this mechanism, the DSC-IR analysis of raw PZT is also performed and the result is showed in Figure 7(e)and 7(f). Figure 7(e)indicates that the decomposition of raw PZT is divided into two steps. Figure 7(f)reveals that the first step corresponds to sublimation of crystallized water and the second step relates to the decomposition of PZT. It is obvious that no NH₃ exists in decomposition products of raw PZT. Thus, in Figure 7(f), we speculate that the products(N₂O, NO₂, CO)are resulted from the reaction of N and C atoms(in PZT)with H₂O steam, since there are no oxidizing groups existing in molecules of raw PZT. Herein, the H₂O steam acts as the oxidizer. Now, let's go back to decomposition of “0.8AN+0.2PZT” in Figure 7(d). The produced NH₃ should be the reminder of dissociation of AN. As abovementioned, AN dissociates to NH₃ and HNO₃ firstly. It is speculated that the PZT skeletons directly react with HNO₃(prior to NH₃)to produce N₂O and H₂O. Hence, the NH₃ is detected as an intermediate product. Of course, all NH₃ gas eventually reacts with decomposition products of HNO₃, since no NH₃ is detected in final products of “0.8AN+0.2PZT”. We have known that the rate-limiting step for decomposition of AN is the decomposition of HNO₃. Here, it confirms that addition of raw PZT improves the consumption of HNO₃, i.e. thermal decomposition of AN is promoted in mechanism.

Fig.7 DSC-IR spectra of samples