Thermal Stability Analysis of the Mg/TeO2 Ignition Composition after 180 °C Exposure

In order to satisfy the performance requirements of the pyrotechnic ignition composition of a space mission under an extreme thermal environment, it is necessary to analyze and verify the thermal stability of magnesium/tellurium dioxide (Mg/TeO2) ignition composition at a temperature of 180 °C. The thermal stability of the ignition composition of Mg/TeO2 and its components after exposure to 180 °C for 2–10 days was studied by means of apparent morphology analysis, differential scanning calorimetry (DSC), X-ray diffraction (XRD), content change analysis, and the P-t curve test. The results showed that after exposure to 180 °C for 2–10 days, no obvious changes, such as ruptures, expansion, or shrinkage, were found by optical microscope, and no changes in morphology and surface details were found by scanning electron microscope (SEM). XRD showed that no other new substance was found in the mixture except magnesium hydroxide (Mg (OH)2). DSC showed that the main reaction peak temperature of the ignition composition of Mg/TeO2 was after 500 °C and that no endothermic/exothermic reaction occurred before 380 °C. The exothermic pre-reaction took place at 381 °C to 470 °C, the weight loss ratio was within 0.71%, the content of the magnesium component varied from 0.49% to 0.90%, the peak pressure attenuation of the ignition composition of 360-mesh Mg/TeO2 was 8.07%, and the pressure rise time was basically unchanged. The results showed that the ignition composition of Mg/TeO2 had good thermal stability after exposure to 180 °C temperatures.


Introduction
Aerospace pyrotechnic devices usually consist of energetic materials and actuating devices, such as separating nuts, explosive bolts, and cutters. They are used in the connection, unlocking, and separation of parts of spacecraft, such as launch vehicles and satellites, and engine start-up, safetyself-destruction, payload dispersion, and other functional actions of spacecraft [1]. As the primary

Materials
The ignition composition of Mg/TeO 2 mainly contains magnesium powder and tellurium dioxide (TeO 2 ) at a mass ratio of 1:1, and fluororubber adhesive and boron nitride (BN) with mass percentages of 10% and 1%, respectively. TeO 2 has a melting point of 773 • C, a boiling point of 1260 • C, and a sublimation temperature of 450 • C. It is decomposed by heat to generate oxygen and tellurium oxide, which can be used as an oxidant for high-temperature-resistant pyrotechnics. Fluororubber can be used for a long time at 250 • C and a short time at 300 • C; it has excellent heat resistance, oxidation resistance, oil resistance, corrosion resistance, and atmospheric aging resistance.
The purity of magnesium powder was more than 98.5%, had a particle size of 40 µm and 50 µm, and it came from Taishan Weihao Magnesium Powder Co., Ltd. The purity of TeO 2 was more than 99%, the particle size was 5 µm, and came from Beijing Chemical Works. The tensile strength of fluororubber was 250 MPa, and the elongation was 320%, and came from Shanghai Sanaifu New material Co., Ltd. The purity of BN was more than 99%, and the particle size range was 10 µm-20 µm, and came from Tianyuan Chemical Co., Ltd.
The preparation protocol for the Mg/TeO 2 ignition composition was: weighing of raw materials, mixing of agent powder, pressing of agent powder, crushing and granulation, particle screening, drying of finished products. The ignition composition of Mg/TeO 2 can be divided into three types according to the conditions of granulation and mesh: the ignition composition of 360-mesh Mg/TeO 2 without granulation (sample A), the ignition composition of 300-mesh Mg/TeO 2 with granulation (sample B), and the ignition composition of 360-mesh Mg/TeO 2 with granulation (sample C). In order to better analyze the experimental phenomena, the tests were also carried out on magnesium powder, TeO 2 , and fluororubber.

Methods
The maximum environmental temperature during the operation of a deep space exploration spacecraft is 130 • C. Considering a certain margin, we propose carrying out stability analysis according to the temperature condition of 150 • C. According to the relevant regulations of GJB1307A-2004 [18], the minimum temperature of decomposition, spontaneous combustion, and melting of explosives should be at least 30 • C higher than the predicted maximum use temperature, so the test temperature was set to 180 • C.
In some cases, pyrotechnics are exposed to extreme thermal temperatures for 2 days, so the designed test days were at normal conditions (60 • C, 4 h), rated working days (2 days), and extended working days (5 days, 10 days).
According to the relevant standards and theory for the stability analysis of pyrotechnics, the Mg/TeO 2 ignition composition and its components were stored at 180 • C for 2 days, 5 days, and 10 days. After that, apparent morphology, XRD, DSC, weight loss ratio, content analysis, and P-t curve tests were carried out to evaluate the thermal stability of the ignition composition of Mg/TeO 2 . The test methods are described as follows: Scanning electron microscopy (SEM) was used to scan and image the surface of the sample point by point with a focused electron beam. The samples were characterized by using S-4800 (Hitachi, Japan) at 15kV with a point resolution of 1.0 nm. X-ray diffraction (XRD): when the X-ray is incident on the crystal, the direction and intensity of the diffraction rays in space distribution are closely related to the crystal structure. The sample was measured with a Bruker D8 Advance diffractometer using 40 kV and 40 mA at Cu-K α monochromatized incident radiation.
Differential scanning calorimetry (DSC) is a technique to measure the change of heat flow rate of a sample with respect to the reference material under the control of the temperature program. The samples were tested by DSC-Q10 (TA Instruments, USA) with the heating rate of 5 • C·min −1 from 50 • C to 520 • C in a nitrogen atmosphere.
Weight loss ratio: the weight changes of the samples before and after storage at 180 • C were determined with an electronic balance (Mettler Toledo MX5, Switzerland), with an accuracy of 0.001 mg. The weight of the sample is about 2-10 mg.
Content analysis: during the test, a sample weight of 0.11-0.13 g and hydrochloric acid of 40 mL were used for the reaction. The content of active magnesium was determined according to the amount of hydrogen produced by the chemical reaction of magnesium and hydrochloric acid. Two parallel determination results are needed, and the difference should be less than 0.4%.
The P-t test system included a closed bomb, pressure sensor, power supply, signal acquisition, and processing equipment. The closed bomb's cavity was 10 mL, and the measurement range of the pressure sensor (Kistler 601A, Germany) was 0-25 MPa. Power supply: Agilent E3634A. Signal acquisition and processing equipment consisted of a signal conditioner (Dewetron DEWE-RACK-16) and a dynamic analyzer (Dewetron DEWE-5001-TR). Table 1 gives a summary of high-temperature storage conditions, test materials, and thermal stability analysis test items.

Apparent Morphology
The analysis of the apparent morphology of pyrotechnic particles is the most intuitive research method for stability analysis. The use of digital cameras, optical microscopes, and SEM can improve the completeness and precision of the apparent topography analysis. Changes in the appearance of the agent before and after the 180 • C test were observed, and the change rule of the physical stability of the agent was analyzed.
3.1.1. The Ignition Composition of Mg/TeO 2 Figure 1 shows the particle appearance morphology photos of three kinds of Mg/TeO 2 ignition compositions, magnified by an optical microscope, before and after the high-temperature test at 180 • C for 2-10 days. The results showed that after exposure to 180 • C for 2 days, the color changed slightly, and the color changed less after 2-10 days. There were no appearance changes such as breakages, cracks, expansions and looseness, pits, shrinkage, melting, and surplus materials of the agent particles, and a little more metallic luster appeared.
It can be seen from Figure 2 that due to the granulation and the use of adhesive, the SEM images of sample B and sample C are different from those of sample A. Sample A presented irregular lumps, while sample B and sample C presented larger size agglomerates. Compared with the samples tested at normal temperature and high temperature, there was no change in the morphology and surface details of the composition. It can be seen from Figure 2 that due to the granulation and the use of adhesive, the SEM images of sample B and sample C are different from those of sample A. Sample A presented irregular lumps, while sample B and sample C presented larger size agglomerates. Compared with the samples tested at normal temperature and high temperature, there was no change in the morphology and surface details of the composition.     It can be seen from Figure 2 that due to the granulation and the use of adhesive, the SEM images of sample B and sample C are different from those of sample A. Sample A presented irregular lumps, while sample B and sample C presented larger size agglomerates. Compared with the samples tested at normal temperature and high temperature, there was no change in the morphology and surface details of the composition.  slightly darker silver-gray, and developed a more metallic luster. TeO 2 changed from white to light black, and fluororubber changed from white to brown-black.  5 show the changes of magnesium powder, TeO2, and fluororubber before and after exposure to 180 °C for 2 days. It can be seen that the magnesium powder changed from silver-gray to slightly darker silver-gray, and developed a more metallic luster. TeO2 changed from white to light black, and fluororubber changed from white to brown-black.    According to the above color change results, it was indicated that TeO2 and fluororubber were subject to high-temperature deterioration, and magnesium powder was also slightly deteriorated. The reason the color of the sample becomes darker after the ignition composition of Mg/TeO2 is exposed to 180°C is the darkening of the TeO2 and fluororubber, and the slightly more metallic luster is due to the oxidation of the trace magnesium on the surface.

XRD Analysis
An XRD test was carried out on sample A after it was stored at 60 °C for 4 h and 180 °C for 10 days, and the composition changes were compared and analyzed. The results are shown in Figure 6.   5 show the changes of magnesium powder, TeO2, and fluororubber before and after exposure to 180 °C for 2 days. It can be seen that the magnesium powder changed from silver-gray to slightly darker silver-gray, and developed a more metallic luster. TeO2 changed from white to light black, and fluororubber changed from white to brown-black.    According to the above color change results, it was indicated that TeO2 and fluororubber were subject to high-temperature deterioration, and magnesium powder was also slightly deteriorated. The reason the color of the sample becomes darker after the ignition composition of Mg/TeO2 is exposed to 180°C is the darkening of the TeO2 and fluororubber, and the slightly more metallic luster is due to the oxidation of the trace magnesium on the surface.

XRD Analysis
An XRD test was carried out on sample A after it was stored at 60 °C for 4 h and 180 °C for 10 days, and the composition changes were compared and analyzed. The results are shown in Figure 6. show the changes of magnesium powder, TeO2, and fluororubber before and after exposure to 180 °C for 2 days. It can be seen that the magnesium powder changed from silver-gray to slightly darker silver-gray, and developed a more metallic luster. TeO2 changed from white to light black, and fluororubber changed from white to brown-black.    According to the above color change results, it was indicated that TeO2 and fluororubber were subject to high-temperature deterioration, and magnesium powder was also slightly deteriorated. The reason the color of the sample becomes darker after the ignition composition of Mg/TeO2 is exposed to 180°C is the darkening of the TeO2 and fluororubber, and the slightly more metallic luster is due to the oxidation of the trace magnesium on the surface.

XRD Analysis
An XRD test was carried out on sample A after it was stored at 60 °C for 4 h and 180 °C for 10 days, and the composition changes were compared and analyzed. The results are shown in Figure 6. According to the above color change results, it was indicated that TeO 2 and fluororubber were subject to high-temperature deterioration, and magnesium powder was also slightly deteriorated. The reason the color of the sample becomes darker after the ignition composition of Mg/TeO 2 is exposed to 180 • C is the darkening of the TeO 2 and fluororubber, and the slightly more metallic luster is due to the oxidation of the trace magnesium on the surface.

XRD Analysis
An XRD test was carried out on sample A after it was stored at 60 • C for 4 h and 180 • C for 10 days, and the composition changes were compared and analyzed. The results are shown in Figure 6. It can be seen from the XRD pattern of Figure 6 that the position of the characteristic diffraction peak of sample A is mainly concentrated at 25°-70°, the agent stored at 180 °C for 10 days corresponds to the characteristic peak positions of samples stored at 60 °C for 4 h, and the diffraction peak shapes are similar. The order of the strongest peak and the second strongest peak changed, and the peak intensity changed, this phenomenon is related to the flatness of pressed sheet in XRD test, and there are defects in the crystal, but these are not the factors that affect the ignition ability of the composition. The characteristic peak diffraction angles of magnesium are mainly at 32°, 34°, 36°, 47°, 57°, 62°, and 68°, and the characteristic peak diffraction angles of TeO2 are mainly at 26°, 29°, 37°, 48°, and 55°. After storage at 180 °C for 10 days, Mg(OH)2 was found in the mixture (diffraction angles of about 18° and 38°); this was due to the reaction between magnesium and moisture in the air during the transfer process of the sample after the high-temperature test. Apart from this, no other new products were found.

DSC Analysis
Through the testing and analysis of the change rule of the characteristic parameters of the DSC curve before and after exposure to 180 °C, the thermal stability of the agent can be evaluated [19]. The characteristic parameters of the DSC curve include the initial reaction temperature, the peak temperature of the reaction, the heat of the reaction, and the change rate of the reaction heat. The calculation formula for the reaction heat change rate is shown in equation (1): where x is the number of days of storage at 180 °C, Q60°C, 4h is the reaction heat of the sample exposed to 60°C for 4 h, and Q180°C, x days is the reaction heat of the sample exposed at 180 °C for x days.  It can be seen from the XRD pattern of Figure 6 that the position of the characteristic diffraction peak of sample A is mainly concentrated at 25 • -70 • , the agent stored at 180 • C for 10 days corresponds to the characteristic peak positions of samples stored at 60 • C for 4 h, and the diffraction peak shapes are similar. The order of the strongest peak and the second strongest peak changed, and the peak intensity changed, this phenomenon is related to the flatness of pressed sheet in XRD test, and there are defects in the crystal, but these are not the factors that affect the ignition ability of the composition. The characteristic peak diffraction angles of magnesium are mainly at 32 • , 34 • , 36 • , 47 • , 57 • , 62 • , and 68 • , and the characteristic peak diffraction angles of TeO 2 are mainly at 26 • , 29 • , 37 • , 48 • , and 55 • . After storage at 180 • C for 10 days, Mg(OH) 2 was found in the mixture (diffraction angles of about 18 • and 38 • ); this was due to the reaction between magnesium and moisture in the air during the transfer process of the sample after the high-temperature test. Apart from this, no other new products were found.

DSC Analysis
Through the testing and analysis of the change rule of the characteristic parameters of the DSC curve before and after exposure to 180 • C, the thermal stability of the agent can be evaluated [19]. The characteristic parameters of the DSC curve include the initial reaction temperature, the peak temperature of the reaction, the heat of the reaction, and the change rate of the reaction heat. The calculation formula for the reaction heat change rate is shown in equation (1): where x is the number of days of storage at 180 • C, Q 60 • C, 4h is the reaction heat of the sample exposed to 60 • C for 4 h, and Q 180 • C, x days is the reaction heat of the sample exposed at 180 • C for x days.    From the DSC curve of the ignition composition of Mg/TeO2, it can be seen that there were two reaction peaks in the samples: the pre-reaction peak and the main reaction peak. Before 380 °C, there was no endothermic/exothermic reaction; the exothermic pre-reaction took place at 381 °C to 470 °C, and the initial reaction temperature was 381 °C to 439 °C. The main reaction peak temperature was more than 500 °C, and the initial reaction temperature of the pre-reaction was more than 200 °C    From the DSC curve of the ignition composition of Mg/TeO2, it can be seen that there were two reaction peaks in the samples: the pre-reaction peak and the main reaction peak. Before 380 °C, there was no endothermic/exothermic reaction; the exothermic pre-reaction took place at 381 °C to 470 °C, and the initial reaction temperature was 381 °C to 439 °C. The main reaction peak temperature was more than 500 °C, and the initial reaction temperature of the pre-reaction was more than 200 °C  (a) overall view (b) partial view     From the DSC curve of the ignition composition of Mg/TeO2, it can be seen that there were two reaction peaks in the samples: the pre-reaction peak and the main reaction peak. Before 380 °C, there was no endothermic/exothermic reaction; the exothermic pre-reaction took place at 381 °C to 470 °C, and the initial reaction temperature was 381 °C to 439 °C. The main reaction peak temperature was more than 500 °C, and the initial reaction temperature of the pre-reaction was more than 200 °C From the DSC curve of the ignition composition of Mg/TeO 2 , it can be seen that there were two reaction peaks in the samples: the pre-reaction peak and the main reaction peak. Before 380 • C, there was no endothermic/exothermic reaction; the exothermic pre-reaction took place at 381 • C to 470 • C, and the initial reaction temperature was 381 • C to 439 • C. The main reaction peak temperature was more than 500 • C, and the initial reaction temperature of the pre-reaction was more than 200 • C higher than the working environment temperature of the agent. Therefore, it can be judged that the ignition composition of Mg/TeO 2 had good thermal stability after being exposed to 180 • C.

T/℃
It can be seen from Figure 10a that the pre-reaction peak temperature change of sample A was 36.4 • C, the peak temperature change within 2 days was relatively large, and the change within 2-10 days was significantly reduced (≤2 • C). The pre-reaction peak temperature change of sample B and sample C were 14.3 • C and 7.9 • C, respectively. days was significantly reduced (≤2 °C). The pre-reaction peak temperature change of sample B and sample C were 14.3 °C and 7.9 °C, respectively.
It is indicated from Figure 10b that the reaction heat curves of three kinds of Mg/TeO2 ignition compositions were almost parallel, and there were inflection points around 2 days, the reaction mechanism was different before and after 2 days. The pre-reaction heat of sample B and sample C was more than one time higher than that of sample A; the higher reaction heat of sample B and sample C waws related to the thermal decomposition reaction of fluororubber start from 412 °C to 438 °C [20].
In Figure 10c, the pre-reaction heat change rate of the ignition composition of Mg/TeO2 changed significantly with the increase of the exposure time at 180 °C. The pre-reaction heat change rate of sample A first increased and then decreased, and the pre-reaction heat change rate (fitting value) after exposure at 180 °C for 2 to 5 days reached 80 to 122%. The pre-reaction heat change rates of sample B and sample C both increased with the increase of exposure time at 180 °C, and the pre-reaction heat change rates after exposure for 10 days were 17.19 and 13.88%, respectively. The fluororubber adhesive itself is a high temperature resistant material, and its thermal decomposition starts from 412 °C to 438 °C. It can be seen from Figure 10b that the thermal decomposition of fluororubber adhesive increased the pre-reaction heat of the composition, but the temperature at this time was much higher than the working temperature of the composition. It can be seen from Figure 10a and Figure 10c that the characteristic temperature change and the change It is indicated from Figure 10b that the reaction heat curves of three kinds of Mg/TeO 2 ignition compositions were almost parallel, and there were inflection points around 2 days, the reaction mechanism was different before and after 2 days. The pre-reaction heat of sample B and sample C was more than one time higher than that of sample A; the higher reaction heat of sample B and sample C waws related to the thermal decomposition reaction of fluororubber start from 412 • C to 438 • C [20].
In Figure 10c, the pre-reaction heat change rate of the ignition composition of Mg/TeO 2 changed significantly with the increase of the exposure time at 180 • C. The pre-reaction heat change rate of sample A first increased and then decreased, and the pre-reaction heat change rate (fitting value) after exposure at 180 • C for 2 to 5 days reached 80 to 122%. The pre-reaction heat change rates of sample B and sample C both increased with the increase of exposure time at 180 • C, and the pre-reaction heat change rates after exposure for 10 days were 17.19 and 13.88%, respectively.
The fluororubber adhesive itself is a high temperature resistant material, and its thermal decomposition starts from 412 • C to 438 • C. It can be seen from Figure 10b that the thermal decomposition of fluororubber adhesive increased the pre-reaction heat of the composition, but the temperature at this time was much higher than the working temperature of the composition. It can be seen from Figures 10a and 10c that the characteristic temperature change and the change rate of pre-reaction heat of sample B and sample C were smaller, and the use of fluororubber adhesive increasesd the thermal stability of the composition. Since the peak temperature of the main reaction of the three Mg/TeO 2 ignition compositions was greater than 500 • C, the occurrence and variation of the pre-reaction were not expected to have a significant impact on functionality.

Weight Loss Ratio
The weight changes of samples before and after storage at 180 • C were determined. The thermal stability of samples was evaluated by thermogravimetric analysis. The calculation formula of the the weight loss ratio is shown in equation (2) [21]: where m 1 is the weight of the sample and tube before heating, m 2 is the weight of the sample and tube after heating, and m is the weight of the sample before heating. A weight loss ratio of ≤1% is the evaluation standard of good thermal stability [22]. The weight loss test for each component of the Mg/TeO 2 ignition composition in Table 2 shows that the weight loss ratio of magnesium powder was less than 0.05%, that of TeO 2 is less than 1%, that of BN was less than 0.2%, and that of fluororubber was less than 0.15%. Each component underwent only slight changes.  Figure 11 shows the trend of the weight loss ratio of the ignition composition of Mg/TeO 2 over time. It can be seen that the thermal weight loss ratio of sample A increased linearly with time. The weight loss ratio of sample B and sample C was greater than that of sample A and that there was a significant difference in the weight loss ratio before and after 2 days, the adhesive solvent volatilized rapidly at the beginning of the high-temperature test, and then the slope of weight loss rate became smaller due to the decrease in the total amount of solvent. The results showed that the weight loss ratio of the three kinds of Mg/TeO 2 ignition compositions was from 0.314 to 0.707% after being exposed to 180 • C for 2-10 days. Therefore, it can be judged that the Mg/TeO 2 ignition composition underwent a slight change, but that the thermal stability at 180 • C was good.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 14 rate of pre-reaction heat of sample B and sample C were smaller, and the use of fluororubber adhesive increasesd the thermal stability of the composition. Since the peak temperature of the main reaction of the three Mg/TeO2 ignition compositions was greater than 500 °C, the occurrence and variation of the pre-reaction were not expected to have a significant impact on functionality.

Weight Loss Ratio
The weight changes of samples before and after storage at 180 °C were determined. The thermal stability of samples was evaluated by thermogravimetric analysis. The calculation formula of the the weight loss ratio is shown in equation (2) [21]: where m1 is the weight of the sample and tube before heating, m2 is the weight of the sample and tube after heating, and m is the weight of the sample before heating. A weight loss ratio of ≤1% is the evaluation standard of good thermal stability [22]. The weight loss test for each component of the Mg/TeO2 ignition composition in Table 2 shows that the weight loss ratio of magnesium powder was less than 0.05%, that of TeO2 is less than 1%, that of BN was less than 0.2%, and that of fluororubber was less than 0.15%. Each component underwent only slight changes. Figure 11 shows the trend of the weight loss ratio of the ignition composition of Mg/TeO2 over time. It can be seen that the thermal weight loss ratio of sample A increased linearly with time. The weight loss ratio of sample B and sample C was greater than that of sample A and that there was a significant difference in the weight loss ratio before and after 2 days, the adhesive solvent volatilized rapidly at the beginning of the high-temperature test, and then the slope of weight loss rate became smaller due to the decrease in the total amount of solvent. The results showed that the weight loss ratio of the three kinds of Mg/TeO2 ignition compositions was from 0.314 to 0.707% after being exposed to 180 °C for 2-10 days. Therefore, it can be judged that the Mg/TeO2 ignition composition underwent a slight change, but that the thermal stability at 180 °C was good.

Content Analysis
The content of active magnesium before and after the 180 • C storage test was determined by complexometric titration to analyze changes in the thermal stability of the agent [23,24]. The formula for calculating the percentage of active magnesium content is shown in equation (3): where P 1 is the atmospheric pressure, kPa; P 2 is the saturated vapor pressure of water, kPa; V is the gas volume, ml; t is the trachea temperature, • C; and m is the sample weight, mg. Figure 12 and Table 3 show the change data of the active magnesium content of the ignition composition of Mg/TeO 2 after exposure to 180 • C.

Content Analysis
The content of active magnesium before and after the 180 °C storage test was determined by complexometric titration to analyze changes in the thermal stability of the agent [23,24]. The formula for calculating the percentage of active magnesium content is shown in equation (3): where P1 is the atmospheric pressure, kPa; P2 is the saturated vapor pressure of water, kPa; V is the gas volume, ml; t is the trachea temperature, °C; and m is the sample weight, mg. Figure 12 and Table 3 show the change data of the active magnesium content of the ignition composition of Mg/TeO2 after exposure to 180 °C.  The variation of the active magnesium content of the three kinds of Mg/TeO2 ignition compositions was basically linear, and the overall variation was not large. The variation and dispersion of the magnesium content of sample A were higher than those of samples B and C, and the change of magnesium content of sample A after exposure to 180 °C for 5 days reached 1.08%. It can be seen from the fitted line that this change was caused by the dispersion of the sample. The magnesium content changes of the three agents after exposure to 180 °C for 10 days were 0.90%, 0.52% and 0.49%, respectively. In this test, the magnesium content was stable after a long period of high-temperature storage.

P-t Test
The characteristic parameter peak pressures and pressure rise times in the P-t curve of the pyrotechnic agent are important basis for evaluating its functionality. The thermal stability of the agent can be evaluated by the change of the characteristic parameters of the P-t curve before and after the 180 °C test. In the P-t test, Mg/TeO2 ignition composition needed 325 mesh screening [25], and sample C met the requirements of granulation and mesh number. The P-t test curve is shown in Figure 13, and the mean values of the characteristic parameters are shown in Table 4.  The variation of the active magnesium content of the three kinds of Mg/TeO 2 ignition compositions was basically linear, and the overall variation was not large. The variation and dispersion of the magnesium content of sample A were higher than those of samples B and C, and the change of magnesium content of sample A after exposure to 180 • C for 5 days reached 1.08%. It can be seen from the fitted line that this change was caused by the dispersion of the sample. The magnesium content changes of the three agents after exposure to 180 • C for 10 days were 0.90%, 0.52% and 0.49%, respectively. In this test, the magnesium content was stable after a long period of high-temperature storage.

P-t Test
The characteristic parameter peak pressures and pressure rise times in the P-t curve of the pyrotechnic agent are important basis for evaluating its functionality. The thermal stability of the agent can be evaluated by the change of the characteristic parameters of the P-t curve before and after the 180 • C test. In the P-t test, Mg/TeO 2 ignition composition needed 325 mesh screening [25], and sample C met the requirements of granulation and mesh number. The P-t test curve is shown in Figure 13, and the mean values of the characteristic parameters are shown in Table 4.  Figure 13. P-t curves of sample C before and after storage at 180 °C. With the increase of exposure days at 180 °C, the shape of the P-t curve of sample C did not change significantly; the peak pressure decreased, and the pressure rise time changed slightly. After being stored at 180 °C for 10 days, the peak pressure of sample C decreased by 0.13 Mpa, the proportion of decrease was 8.07%, and the pressure rise time changed by 0.04 ms. The results of the P-t test showed that the ignition ability of the Mg/TeO2 ignition composition decreased only slightly after exposure to 180 °C.

Conclusions
In view of the lack of previous studies on the thermal stability of the Mg/TeO2 ignition composition after exposure to 180 °C temperatures, this article has conducted a series of tests to comprehensively analyze the physical, chemical, and explosive properties of the Mg/TeO2 ignition composition after exposure to 180 °C. The comprehensive evaluation showed that the Mg/TeO2 ignition composition had good thermal stability at 180 °C. The main conclusions are as follows: (1) Apparent morphology: after being exposed to 180 °C, the overall color of the Mg/TeO2 ignition composition became darker and showed a little more metallic luster. No breakages, cracks, expansion and looseness, pits, shrinkage, melting, or surplus materials were found by an optical microscope, and no changes of morphology and surface details were found by SEM. (2) XRD: X-ray diffraction patterns showed that the positions of the characteristic diffraction peaks of the main components did not change, and Mg(OH)2 was formed by the reaction of magnesium with moisture in the air. (3) DSC: before 380 °C, there was no endothermic/exothermic reaction of the Mg/TeO2 ignition composition, and there was a pre-reaction exothermic peak between 388 °C and 470 °C. The main exothermic peak was greater than 500 °C, and the initial reaction temperature was more than 200 °C higher than the working temperature. (4) Weight loss ratio: the weight loss ratio of the Mg/TeO2 ignition composition and its components after exposure to 180 °C were within 0.94% and 0.71%, respectively. (5) Content change: the active magnesium content of the three kinds of Mg/TeO2 ignition compositions varied from 0.49 to 0.90% after being exposed to 180 °C for 10 days. (6) P-t test: after being exposed to 180 °C for 10 days, the peak pressure of simple C only decreased by 8.07% (0.13 Mpa), and the change of the pressure rise time was 0.04 ms.  With the increase of exposure days at 180 • C, the shape of the P-t curve of sample C did not change significantly; the peak pressure decreased, and the pressure rise time changed slightly. After being stored at 180 • C for 10 days, the peak pressure of sample C decreased by 0.13 Mpa, the proportion of decrease was 8.07%, and the pressure rise time changed by 0.04 ms. The results of the P-t test showed that the ignition ability of the Mg/TeO 2 ignition composition decreased only slightly after exposure to 180 • C.

Conclusions
In view of the lack of previous studies on the thermal stability of the Mg/TeO 2 ignition composition after exposure to 180 • C temperatures, this article has conducted a series of tests to comprehensively analyze the physical, chemical, and explosive properties of the Mg/TeO 2 ignition composition after exposure to 180 • C. The comprehensive evaluation showed that the Mg/TeO 2 ignition composition had good thermal stability at 180 • C. The main conclusions are as follows: (1) Apparent morphology: after being exposed to 180 • C, the overall color of the Mg/TeO 2 ignition composition became darker and showed a little more metallic luster. No breakages, cracks, expansion and looseness, pits, shrinkage, melting, or surplus materials were found by an optical microscope, and no changes of morphology and surface details were found by SEM. (2) XRD: X-ray diffraction patterns showed that the positions of the characteristic diffraction peaks of the main components did not change, and Mg(OH) 2 was formed by the reaction of magnesium with moisture in the air. (3) DSC: before 380 • C, there was no endothermic/exothermic reaction of the Mg/TeO 2 ignition composition, and there was a pre-reaction exothermic peak between 388 • C and 470 • C. The main exothermic peak was greater than 500 • C, and the initial reaction temperature was more than 200 • C higher than the working temperature. (4) Weight loss ratio: the weight loss ratio of the Mg/TeO 2 ignition composition and its components after exposure to 180 • C were within 0.94% and 0.71%, respectively.