Introduction

Aluminum (Al) powder, as a very common energetic material, is widely used in propellants [1] and nano-composite metal materials [2]. It increases the specific impulse of the propellant by increasing combustion temperature in the solid rocket motor and decreasing the molecular weight of products [3]. In addition, there is a high amount of Al on the earth, which results to a lower cost of Al mining. However, the formed solid oxide would adhere on the surface of metal particles and make the combustion process of metal more complex than that of hydrocarbon fuels.

Particle diameter is one of the factors that affect the combustion process of Al powder. In order to understand the combustion processes of nano- and micro-scale Al powders, several experiments were performed on the Al powder combustion. The results showed that nano-scale Al particle clouds burnt faster than micro-scale Al particle clouds in air [4]. Yan [5] showed that the melting enthalpy and melting temperature of Al powder decreased with the decreases in Al particles size. Moreover, nano-scale Al particle has more attractive oxidation energetics and fast reaction kinetics than micro-scale or larger Al particles [6]. All these results showed the different reaction characteristics between nano- and micro-scale Al powders.

In addition, there were some studies on the reaction characteristics of Al in dry air [7–9] and various mixed environments [10–13], but few studies carried out on thermal reaction characteristics of Al powder in carbon dioxide (CO2) atmosphere [14, 15]. The reaction of Al and CO2 releases large amounts of energy. On the other hand, Martian atmospheres consist of 95 % CO2, making the achievement of in situ resource utilization (ISRU) possible. Therefore, Al/CO2 is regarded as a promising fuel to realize an exploration mission to Mars in the future. However, there is little knowledge about the reaction of Al powder and CO2, so it would be conducive to the development of Mars exploration if the thermal reaction mechanism of Al powder and CO2 is clarified.

In this paper, thermal reaction characteristics of nano- and micro-scale Al powders were studied by a thermogravimetric analyzer coupled with a differential scanning calorimetry (TG-DSC) which were widely used and well-understood. Scanning electron microscopy (SEM) was also used to observe the morphology of thermal reaction products of Al powder. And the components of products were studied by X-ray diffraction (XRD). At the same time, thermal reaction mechanisms of nano- and micro-scale Al powders were inferred.

Experimental

Al powder used in the present work was provided by Nanometer Material Limited Company in Henan province. The average diameters of micro- and nano-scale Al powders were 1–2 µm and 50 nm, respectively. The corresponding active Al contents measured by chemical method [16] were 94.0 and 82.0 %, respectively.

Oxidation of Al powder was studied using a Netzsch STA 449C simultaneous TG-DSC thermal analyzer. Thermogravimetric analysis (TG) was used to measure the sample mass gain and the indicative of the oxidation when Al powder was heated in CO2 atmosphere. Heat flux of Al powder was studied by DSC. Al powder with a mass range of 4–6 mg was heated at the heating rate of 10 °C min−1 from 20 to 1400 °C under atmospheric pressure conditions, and CO2 (99.99 %) was used as the oxidant. Reaction gas (CO2) flow and protective gas (CO2) flow were controlled at 40 and 10 mL min−1, respectively. In addition, both TG and DSC baselines were corrected by subtraction of predetermined baselines run under identical conditions except for the absence of sample.

The thermal reaction products of Al powder quenched at different particular temperature were collected when the samples were cooled down to the room temperature. The morphology of product which was quenched at 1400 °C was observed by FEI scanning electron microscopy (SEM) (Model-NOVA NANO SEM430). And components of products quenched at different temperatures were analyzed by Bruker X-ray diffractometer (XRD) (Model-D8 ADVANCE, CuKa, λ = 1.54187).

Results and discussion

Thermal reaction characteristics of nano- and micro-scale Al powders

It can be seen from Fig. 1 that the oxidation of nano-scale Al powder and CO2 can be divided into two primary stages: induction stage and mass gain stage. There is no mass gain from 20 to 470 °C, called induction stage. After the induction stage, there is a stage of mass gain that can be further divided into three substages. The first mass gain is observed at 470–690 °C, which is called stage I. The second mass gain ranges from 690 to 875 °C which is called stage II. Stage III is found at 875–1136 °C. However, there are a few mass losses when the temperature is above 1136 °C. Actually, whether in wet or dry CO2, carbon can be produced in the reaction between aluminum and CO2 [17]. The reaction between C and CO2 (Boudouard reaction) takes place when the reaction temperature is above 1000 °C [18]. Therefore, the tail of TG curve of nano-scale Al powder drops down after 1136 °C due to the Boudouard reaction (C(s) + CO2(g) → 2CO(g)).

Fig. 1
figure 1

Thermal analysis patterns of nano- and micro-scale Al powders. a TG curves; b DTG curves

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Thermal reaction of micro-scale Al powder and CO2 can be also divided into two major stages: induction stage and mass gain stage. Mass gains from 20 to 579 °C are not detected at the induction stage. As for mass gain stage of micro-scale Al powder, compared with three substages of nano-scale Al powder, it can be divided into two substages. The first mass gain is observed at 579–697 °C, which is called stage I. The second mass gain ranges from 697 to 1188 °C, which is called stage II.

In order to compare the differences of mass gain between nano- and micro-scale Al powders, the mass gains of each stage are shown in Table 1. The mass gains of stage I, stage II, and stage III are represented by Δm I, Δm II, and Δm II, respectively. It can be obtained by the Eq. (1), where m is the mass at the end of each stage and m o is the initial mass.

$$ \Delta m = \frac{{m - m_{\text{o}} }}{{m_{\text{o}} }} $$
(1)
Table 1 Mass gains of each stage for nano- and micro-scale Al powders
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As shown in Table 1, the mass gain of nano-scale Al powder is 44.64 % at stage I, which means that the key reaction takes place at a low temperature for nano-scale Al powder. This result is consistent with the Ref. [19]. And the mass gains of stages II and III reduce gradually. Especially for stage III, there is only 4.90 % mass gain. For micro-scale Al powder, there is only 4.40 % mass gain at stage I. However, mass gain at stage II is 74.47 %, which indicates that the key reaction of micro-scale Al powder takes place at stage II.

DSC curves of nano- and micro-scale Al powders are shown in Fig. 2, and an endothermic peak around 660 °C is detected, which corresponds to the melting point of Al. Generally, onset temperature of extrapolated peak is used to define the melting point, causing the less dependence of extrapolated onset temperature on heating rate, thermal conductivity of sample, thickness of sample, and mass of sample compared with the peak temperature [20]. Thus, according to the extrapolation method, the melting points of nano- and micro-scale Al powders in this study are 652.6 and 654.0 °C, respectively. The melting point of nano-scale Al powder is slightly lower than that of micro-scale Al powder, which is consistent with the results that the melting temperature decreases with the decrease in Al particle size [5, 21]. In addition, the melting enthalpies of nano- and micro-scale Al powders are 33.42 and 250.6 J g−1, respectively, by calculating the melting peak area. This result indicates that the melting enthalpy also decreases with the decrease in Al particle size. It is worth noting that the melting enthalpy (melting endothermic peak) of nano-scale Al powder is smaller than that of micro-scale Al powder due to the exothermic oxidation of nano-scale Al powder at a low temperature, so there is a deep impact of particle size on the melting behavior of Al. In order to further understand the influence of particle size on melting behavior of Al powder, the mass gains of Al powder before the melting point and during the whole reaction are calculated, represented by Δm′ and Δm″, respectively. The results are shown in Table 2.

Fig. 2
figure 2

TG–DSC curves of nano- and micro-scale Al powders at a heating rate of 10 °C min−1

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Table 2 Mass gains of nano- and micro-scale Al powders before the melting point and during the whole reaction
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It can be noted from Table 2 that the mass gains of nano-scale Al powder before the melting temperature (Δm′) are far higher than those of micro-scale Al powder. Furthermore, mass gains of nano-scale Al powder before the melting point are relatively higher than those of the whole reaction. The results show that the key reaction of nano-scale Al powder takes place before the melting point, which means the solid form of Al powder that involves in oxidation. However, for micro-scale Al powder, the mass gains before the melting point are only 4.38 %, which is far lower than 78.87 % mass gains of the whole reaction. This shows that the key reaction of micro-scale Al powder takes place between molten Al and CO2.

Morphology of residuals

Figure 3a indicates the morphology of the residuals of nano-scale Al powder. It can be seen from Fig. 3a that the residuals are like corals which consist of irregular particles. And the main size of the residuals of Al particle is greater than 50 nm, the mean size of initial particles. According to the diffusion mechanisms, the volume increases when Al is oxidized to alumina [19], so the oxidation of nano-scale Al powder is consistent with a shrink core-type model [3].

Fig. 3
figure 3

SEM images of residuals after heated to 1400 °C. a Nano-scale Al powder; b micro-scale Al powder

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Figure 3b shows the morphology of the residuals of micro-scale Al powder. As can be seen from Fig. 3b, a number of larger globular particles are formed due to the aggregation of Al particles. It can be also easily found that some fragments with a rough outer surface appear. In addition, the inner surface of the fragment is also rough, which is marked with a red circle shown in Fig. 4. It can be found that some holes are formed on the spherical particles. The results show that the formation of fragments and holes can be explained by changes in pressure in Al core, between Al core and alumina. The melting of Al is accompanied by a 6 % volume increase due to the decreases in density from 2.7 to 2.4 g cm−3, which causes a compressive pressure within the Al core [22]. This high tensile stress leads to the shell’s dynamic fracture and spallation. Mechanical stress at the interface between the metal and oxide results in an increase in the reaction rate followed by the disruption of the film. Thus, the differences in the thermal expansion coefficients and densities of the metal and its oxide, as well as in the volume change in metal during phase or polymorphic transformations, are responsible for these fragments. In addition, the produced CO gases disrupt the oxide shell caused by the expansion of them and the holes are formed. If the amount of produced CO gas is very large and the space of the oxide coatings is small, hollow oxide shells of particles form as fragments from brittle fracturing of the thin walls due to the expansion of CO gas and aluminum core.

Fig. 4
figure 4

SEM image of micro-scale Al powder after heated to 1400 °C

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Components of residuals

The phase transformation of alumina film on Al particle surface by thermal oxidation was established by Levin [23], and the sequence of alumina phase transformations shown in Ref. [23] is amorphous → γ→δ → θ→α-Al2O3. Amorphous alumina is thermodynamically more stable than crystalline alumina [24]. The thickness of amorphous alumina layer increases with temperature increasing until it reaches a critical value [5, 24]. And then the transformation of amorphous alumina to a crystalline alumina polymorph takes place.

In order to understand the thermal reaction mechanisms of nano- and micro-scale Al powders with CO2 and to identify intermediate products, several Al samples were heated in CO2 at 10 °C min−1 to selected temperatures in this paper. And then the products quenched at selected temperature were analyzed by X-ray diffraction. The results are shown in Fig. 5.

Fig. 5
figure 5

XRD patterns of Al particles at different quenched temperatures. a 50 nm Al; b 1–2 μm Al

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The initial nano- and micro-scale Al powders (at 25 °C) were also analyzed by XRD as shown in Fig. 5. Although only diffraction peaks of Al for initial Al powder are detected, there may be some amorphous Al2O3, because amorphous Al2O3 could be hardly detected by XRD. After being heated to 600 °C, the formed alumina phases are referred to as γ-Al2O3 for nano-scale Al powder, which corresponds to stage I. This result shows that the polymorph of alumina phase presents as γ-Al2O3 in the reaction between nano-scale Al powder and CO2 before the Al melting point. When the temperature increases to 800 °C, peaks of γ-Al2O3 become stronger. This means the growth of γ-Al2O3 with temperature increasing. The γ-Al2O3 layer eventually transforms into α-Al2O3 at 1200 °C. Note that no θ-Al2O3 or δ-Al2O3 is detected at the changing of alumina crystal. It is consistent with the reports [23, 25] that the transformation of γ-Al2O3 to α-Al2O3 may proceed directly or via a number of intermediate phases.

For micro-scale Al powder, there are also some diffraction peaks of Al at 25 °C. At 600 °C, no new alumina crystal is found, and only diffraction peaks of Al are present. Weak diffraction peaks of γ-Al2O3 can be detected after being heated to 800 °C. When the temperature increases to 1000 °C, θ-Al2O3 is found and diffraction peaks of γ-Al2O3 become stronger. As the reaction temperature increases, strong diffraction peaks of α-Al2O3 are observed at 1400 °C, which indicates the formation of stable Al2O3 crystal. Thus, the transformation sequence of alumina film is amorphous → γ→θ → α-Al2O3.

As discussed above, it can be concluded that the main alumina crystal in products of the first, second, and third mass gains for nano-scale Al powder is γ-Al2O3, γ-Al2O3, and α-Al2O3, respectively. And for micro-scale Al powder, the main crystal of alumina phase at stage I may be amorphous Al2O3. However, at stage II, it has changed greatly, such as amorphous oxide → γ-Al2O3 or θ-Al2O3.

Thermal reaction mechanisms

Thermal reaction mechanisms of nano-scale Al powder

During induction stage (20–470 °C), there are no obviously mass gains of Al powder. This shows the existence of an integrated protective Al2O3 layer that adheres to Al particles, which is known as amorphous alumina [23]. As temperature increases, the thickness of amorphous alumina layer increases. The rate of this process is controlled by the outward diffusion of Al cations [26]. TG curve is the evidence of a slow oxidation of Al powder at induction stage. For the fast oxidation, oxidation process is illustrated in Fig. 6. As shown in Fig. 6, a large number of CO2 molecules penetrate the Al2O3 shell and the particle’s diameter becomes larger with the oxidation of nano-scale Al particle. When thickness of the oxide layer exceeds the critical thickness of amorphous alumina, the oxide transforms into γ-Al2O3 whose density is about 20 % higher than that of amorphous Al2O3. At the beginning of transformation, the smallest crystallite size of γ-Al2O3 has been found to be in the range of a few nanometers comparable with the thickness of oxide layer existing at this point on the Al surfaces [23]. Thus, it is suggested that the newly formed γ-Al2O3 partially covers the Al particle which is previously isolated by a continuous amorphous Al2O3 shell. Then, bare Al spots will be exposed to the environment and oxidized immediately by CO2. The above reaction procedure corresponds with the sharp mass increases in Al powder that is shown on TG curve during stage I, and the reaction between nano-scale Al and CO2 is dominated by chemical kinetics.

Fig. 6
figure 6

The reaction of nano-scale Al powder and CO2 at a low heating rate

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At stage II, solid Al transforms into the liquid phase with a little mechanical stress due to a low content of active Al. At the same time, the γ-Al2O3 layer that coats on the surface of Al particles grows. This stage is controlled by chemical reaction and diffusion of oxidant. As alumina layer becomes thicker, diffusion of oxidant becomes more difficult.

At stage III, stable α-Al2O3 is formed, which results in an abrupt reduction in oxidation rate shown on DTG curve. In this stage, the diffusion from the oxidant to Al core and the oxidation of active Al occur simultaneously. However, what differs from stage II is that the oxide layer on the surface of Al particle is made up of denser α-Al2O3, which leads to a more difficult diffusion of oxidant. On the other hand, the alumina layer is much thicker than before, so the reaction of stage III is slower than that of stage II. Thus, the whole reaction of nano-scale Al powder and CO2 is controlled by the diffusion process.

Thermal reaction mechanisms of micro-scale Al powder

Oxidation of micro-scale Al powder is shown in Fig. 7. During induction stage (20–579 °C), there are no mass gains. With temperature increasing, the thickness of amorphous alumina layer increases. When the temperature gets to the melting point of Al powder, solid Al starts to melt. And the density of active Al which is protected by Al2O3 layer decreases from solid Al (2.7 g cm−3) to liquid Al (2.4 g cm−3), and causes a 6 % volume increase in Al core. Volume expansion of Al core in the oxide layer leads to compress pressure within the Al core and the tensile stress on the Al2O3 layer. As a result, a pressure gradient generates within the layer and leads to the cracking of the alumina layer, followed by the reaction between Al and CO2 taking place. The produced CO gas disrupts the oxide shell, leading to formation of some holes. It is the disruption of Al2O3 shell and formation of holes that resulting in an increment of reaction rate, and the mass increases by 74.47 % at stage II.

Fig. 7
figure 7

The reaction of micro-scale Al powder and CO2 at a low heating rate

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Conclusions

Thermal reaction mechanisms of nano-scale Al powder are found to differ significantly from micro-scale Al powder by a thermogravimetric analyzer coupled with a differential scanning calorimetry, scanning electron microscopy, and X-ray diffraction.

  1. 1.

    An obvious difference of thermal reaction performance is the oxidation temperature between nano- and micro-scale Al powders. Low-temperature oxidation is prominent for nano-scale Al powder, while high-temperature reaction is the key reaction for micro-scale Al powder. Furthermore, mass gains of micro-scale Al powder are more than those of nano-scale Al powder due to the higher content of active Al. In addition, the oxidation of nano- and micro-scale Al powders in CO2 atmosphere can be divided into two stages, the induction stage and the mass gains stage. However, there are three mass gains stages for nano-scale Al powder and two mass gains stages for micro-scale Al powder at 20–1400 °C.

  2. 2.

    Some broken alumina fragments with a rough outer surface and holes appear in the thermal reaction products of micro-scale Al powder. However, fragments of oxide shell and the holes are not observed in the thermal reaction products of nano-scale Al powder. And thermal reaction products of nano- and micro-scale Al powders change differently with the temperature increasing. For nano-scale Al powder, γ-Al2O3 appears at 600 °C, which is before the melting point of Al. When the temperature reaches 1200 °C, stable α-Al2O3 is detected. However, for micro-scale Al powder, the temperature that γ-Al2O3 appears is 800 °C, which is higher than the Al melting point. When temperature rises to 1000 °C, γ-Al2O3 and θ-Al2O3 are found.

  3. 3.

    The thermal reaction process of nano-scale Al powder and CO2 is controlled by chemical kinetics and diffusion of oxidant at low heating rate. As for micro-scale Al powder, diffusion mechanism is also considered to be the thermal reaction mechanism except the disruption of the oxide shell.