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

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This research aims to verify the 180 °C thermal stability of the ignition composition of Mg/TeO₂, to meet the charge requirements of separation devices such as the cutter for the Mars probe.

Abstract

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.

1. 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, safety- self-destruction, payload dispersion, and other functional actions of spacecraft [1]. As the primary energetic material in pyrotechnic devices, the ignition composition is ignited by external stimulation (electric energy, laser energy), and then detonates the secondary energetic material. The gas generated by the secondary energetic material drives the actuating devices to complete the predetermined task. The pyrotechnic devices used in deep space exploration missions are exposed to extreme thermal environments, and the maximum operating temperature can reach 130 °C [2,3], so the selection of an ignition composition that is insensitive and thermally resistant is crucial [4].

The energetic materials used for ignition can be classified by compounds and mixtures. The ignition performance of compounds is not easy to adjust, among which lead styphnate (LTNR) is most commonly used, and its use temperature should be limited to –40 °C to 110 °C [5]. The thermal weight loss ratio of potassium picrate (KP) after exposure to 100 °C for 48 h is 0.12% [6], and the initial decomposition temperature of potassium styphnate (K₂TNR) is about 310 °C [7]. The ignition ability of the mixed ignition composition is strong, which is usually composed of combustible agents (boron, magnesium, aluminum, zirconium, silicon, etc.) and oxidants (potassium nitrate, potassium perchlorate, potassium chlorate, sodium nitrate, polytetrafluoroethylene, etc.). Li et al. [8] believed that the main thermal weight loss temperature of boron/potassium nitrate (B/KNO₃) ignition composition was between 545 °C and 765 °C, and the thermal stability of the composition at 180 °C was verified. Babar and Malik [9] found that the critical ignition temperature of aluminum/barium nitrate (Al/Ba(NO₃)₂), magnesium/ammonium perchlorate (Mg/NH₄ClO₄), and magnesium/potassium permanganate (Mg/KMnO₄) were 599 °C, 338 °C, and 289 °C, respectively. Ji et al. [10] believed that the exothermal peak of aluminum/zirconium/potassium perchlorate (Al/Zr/KClO₄) ignition composition was around 530 °C, and the thermal weight loss was concentrated at 400 °C to 600 °C.

The ignition composition of Mg/TeO₂ has the advantages of simple preparation, heat stability, and reliable functioning. The formulation can be adjusted according to the performance requirements of the ignition object, and it meets the requirement of 1A/1W/5min no-fire (the composition will not be ignited if 1 ampere constant current or 1 watt of electric energy is applied to the composition for 5 min), so the ignition composition of Mg/TeO₂ can be used as primary energetic material of an insensitive igniter [11,12]. William and Wenonah studied the high-temperature resistance performance of the ignition composition of Mg/Te/TeO₂ (27/67/6), after it was stored at 204 °C to 240 °C for 24 h. The ignition current (the average current value that causes the composition to ignite) increased slightly, and storage at 72 °C and 105 °C for 2 months had little impact on the ignition time [13]. Yan et al. exposed the ignition composition of Mg/TeO₂ to a high temperature of 165 °C for 6 days and found that its output peak pressure decreased by less than 5%, the pressure rise time decreased by 8.5%, and the ignition current decreased by 12.8–15.7%. Therefore, it was considered that the ignition composition of Mg/TeO₂ could be used as an ignition composition that is resistant to 165 °C [14]. Pouretedal and Ravanbod believed that in the DSC test of the ignition composition of magnesium/oxidant in a nitrogen atmosphere, magnesium and nitrogen will react to form magnesium nitride, the decomposition of the oxidant, and the reaction of magnesium and oxide will occur [15,16]. Pourmortazavi et al. discovered that the melting point and ignition temperature of the ignition composition of magnesium/oxidant increases with the increase of the heating rate in the TGA-DSC test [17].

Space pyrotechnic devices require pyrotechnic composition to withstand a high temperature of 180 °C. At present, there is no research on the thermal stability of the ignition composition of Mg/TeO₂ after exposure to the high temperature of 180 °C for several days. Therefore, it was necessary to carry out comprehensive testing and verification of the physical, chemical, and explosion stability of the ignition composition of Mg/TeO₂ under extreme high temperatures, so as to provide a basis for the selection of a high-temperature-resistant ignition composition for space pyrotechnic devices.

2. Materials and Methods

2.1. Materials

The ignition composition of Mg/TeO₂ mainly contains magnesium powder and tellurium dioxide (TeO₂) at a mass ratio of 1:1, and fluororubber adhesive and boron nitride (BN) with mass percentages of 10% and 1%, respectively. TeO₂ 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 TeO₂ 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/TeO₂ 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/TeO₂ can be divided into three types according to the conditions of granulation and mesh: the ignition composition of 360-mesh Mg/TeO₂ without granulation (sample A), the ignition composition of 300-mesh Mg/TeO₂ with granulation (sample B), and the ignition composition of 360-mesh Mg/TeO₂ with granulation (sample C). In order to better analyze the experimental phenomena, the tests were also carried out on magnesium powder, TeO₂, and fluororubber.

2.2. 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/TeO₂ 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/TeO₂. 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. Thermal stability analysis test items and storage conditions corresponding to different test materials.

Test Materials	Storage Conditions	Test Items
Magnesium powder; TeO2; viton	60 °C, 4 h; 180 °C, 2 days	Apparent morphology; weight loss ratio
Sample A	60 °C, 4 h; 180 °C, 2 days, 5 days, 10 days	Apparent morphology; XRD; DSC; weight loss ratio; content analysis
Sample B	60 °C, 4 h; 180 °C, 2 days, 5 days, 10 days	Apparent morphology; DSC; weight loss ratio; content analysis
Sample C	60 °C, 4 h; 180 °C, 2 days, 5 days, 10 days	Apparent morphology; DSC; weight loss ratio; content analysis; P-t test

gives a summary of high-temperature storage conditions, test materials, and thermal stability analysis test items.

3. Test Results and Analysis

3.1. 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/TeO₂

Figure 1 shows the particle appearance morphology photos of three kinds of Mg/TeO₂ 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.

3.1.2. Magnesium Powder, TeO₂, Fluororubber

Figure 3, Figure 4 and Figure 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.

3.2. 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.

3.3. 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):

ΔQ/Q_60℃,4h = |(Q_60℃,4h − Q_{180℃,x days})/Q_60℃,4h| × 100%

(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. Figure 7, Figure 8 and Figure 9 are the DSC curves of the ignition composition of Mg/TeO₂ before and after exposure to 180 °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 higher than the working environment temperature of the agent. Therefore, it can be judged that the ignition composition of Mg/TeO₂ had good thermal stability after being exposed to 180 °C.

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.

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 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.

3.4. 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]:

W = (m₁ − m₂)/m × 100%

(2)

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. Weight loss ratio of the components of the Mg/TeO₂ ignition composition.

Type	Weight Loss Ratio after 60 °C, 4 h/%	Weight Loss Ratio after 180 °C, 2 Days/%	Weight Loss Ratio after 180 °C, 2 Days/% (with Cover)
Magnesium powder	0.0478	0.0325	0.0334
TeO2	0.8479	0.9407	0.7524
BN	0.1647	-0.0859	0.0000
fluororubber	0.0518	0.0848	0.1197
empty bottle	0.0009	0.0027

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.

3.5. 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):

X = {[(P₁ − P₂) × V × 0.002927]/[(273.2 + t) × m]} × 100

(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. Change data of the active magnesium content in the ignition composition of Mg/TeO₂ according to storage test days at 180 °C.

Type	Content of Magnesium after 60 °C, 4 h/%	Content of Magnesium after 180 °C, 2 Days/%	Content of Magnesium after 180 °C, 5 Days/%	Content of Magnesium after 180 °C, 10 Days/%
sample A	43.69	43.76	44.77	44.59
sample B	43.96	44.15	44.04	44.48
sample C	42.83	42.73	42.60	42.34

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.

3.6. 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. Mean values of characteristic parameters of the P-t curve of sample C after the 180 °C test.

Type	Storage Condition	Number of Sample	Average Peak Pressure p/MPa	Average Rise Tine t/ms	Change Rate of Peak Pressure/%
sample C	60 °C, 4 h	3	1.61	0.34	0
	180 °C, 5 days	3	1.54	0.39	–4.35
	180 °C, 10 days	3	1.48	0.38	–8.07

.

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.

4. 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.