Study on the influence of al content on elevated temperature oxidation of MgF2 in HFP/air atmosphere

The influence of Al content on the high-temperature oxidation of MgF2 in an air/HFP atmosphere at temperatures ranging from 700 to 1000 °C has been studied with XRD and EDS. The results reveal that the addition of Al accelerated the oxidation of MgF2. With increasing Al content in MgF2, the oxidation of MgF2 was increased. At the same Al content, the oxidation of MgF2 also increased as the temperature rose. These results suggest that the increase of the Al content of MgF2 and the temperature decreased the high-temperature stability of MgF2.

on molten magnesium but also offers a theoretical foundation for controlling and optimizing the protection of HFP gas on molten magnesium.

Experimental procedure
The raw materials in the experiment are 99.99% commercial MgF 2 powder and 99.8% Al powder.The protective gas is a blend of HFP and air, where HFP (HFP > 99.9%, O 2 < 20 ml m -3 , H 2 O <10 ml m -3 ) has not been treated before use.MgF 2 powder with an average particle dimension of 72 μm and Al powder with an average particle diameter of 45 μm were blended in a rotating vaporizer with Al/MgF 2 ratios of 5%, 9%, and 15% by weight, respectively.The mixed Al/MgF 2 powder was put in a stainless steel mould of Φ 10 mm×30 mm to press the sample.After holding the pressure at 100 MP for 15 minutes, a green body sample with the size of Φ10mm×1.0mm was obtained.
The pressed disk sample was placed into a quartz glass tube in an open vacuum/atmosphere tube furnace, and the experiment was conducted under an HFP/air-mixed atmosphere.Before the experiment, the air in the quartz glass tube was exhausted to ensure that the atmosphere was HFP/air, and the temperature parameters were set.The experimental procedure was as follows: (1) The green body samples containing 0%, 5%, 9%, and 15% Al were kept at an HFP concentration (volume fraction) of 1%, 0.5%, 0.1%, 0.05%, and 0.01% at 1000°C for 2 h, respectively.(2) The green body samples containing 0%, 5%, 9% and 15% Al were reacted at various temperatures in 0.01% HFP/air for 2 h.The composition of the oxidized sample was established with an APOLLOX X-ray fluorescence spectrometer.The X-ray source was a copper target with a voltage of 40 kV, a current of 40 mA, a scanning speed of 4°/min, and a step width of 0.02°.

Results and Discussions
Fig. 1 gives the XRD results of MgF 2 powder after heating in air with various contents of HFP at 1000°C.It is evident that as HFP content decreased, the strength of MgO peaks was enhanced.In the air with 1%, 0.5%, 0.1%, and 0.05% HFP, MgF 2 peaks were only found, and no peaks of MgO were present.In air with 0.01% and 0.005% HFP, apart from MgF 2 peaks, there were also peaks of MgO, and the strength of MgO peaks was enhanced as HFP content decreased.This suggests that at 1000°C, MgF 2 was not oxidized in the air with HFP content over 0.05%.But MgF 2 was oxidized in the air with HFP content less than 0.01% and the oxidation of MgF 2 was intensified as the HFP content decreased.Therefore, the following studies were mainly carried out in 0.01% HFP/air.Fig. 2 presents the XRD results of MgF 2 after heating in 0.01% HFP/air at various temperatures.As seen in Fig. 2, when the temperature was below 850°C, the peaks of MgF 2 were only observed, indicating that MgF 2 was not oxidized at this temperature.At temperatures over 900°C, the peaks of MgO were present, and with temperature increasing, the strength of MgO peaks gradually increased and that of MgF 2 peaks tended to decrease.This indicates that the oxidation of MgF 2 occurred above 900°C, and the oxidation was intensified with temperature rising.The EDS results of MgF 2 after heating in 0.01% HFP/air at various temperatures are displayed in Table 1.It is observed that the O element content was 0 at 750°C; therefore, MgF 2 did not undergo oxidation at this temperature.At 800 and 850°C, the O element content was not high, indicating a weak oxidation of MgF 2 .At temperatures above 900°C, the O element content significantly increased with temperature, indicating that MgF 2 undergoes significant oxidation within this temperature range.The XRD results of 5% Al-MgF 2 after heating in 0.01% HFP/air at various temperatures are presented in Fig. 3.As illustrated in Fig. 3, at temperatures below 800°C, only the peak of MgF 2 was present.At temperatures above 850°C, MgO peaks were observed, and the strength of the peaks was gradually enhanced with temperature increasing, especially at 950 and 1000°C, the majority of MgF 2 peaks were transformed into the peaks of MgO.This indicates that in 0.01% HFP/air mixtures at lower temperatures, MgF 2 was almost not oxidized, but at higher temperatures, MgF 2 was significantly oxidized and the oxidation degree of MgF 2 increased with increasing temperature, which was higher than that without the addition of Al.The EDS results of 5% Al-MgF 2 after heating in 0.01% HFP/air at various temperatures are displayed in Table 2.It is noted from Table 2 that at 650800°C, the level of O element was not high and increased weakly.Above 850°C, the O element level increased with increasing temperature, and the increase was very obvious.This result is almost consistent with the XRD results.The XRD results of 9% Al-MgF 2 after heating in 0.01% HFP/air at various temperatures are presented in Fig. 4. It is found that at temperatures below 750°C, the spectrum was almost the peak of MgF 2 .When the temperature was above 800°C, MgO peaks were observed, the intensity of the peaks gradually increased with the temperature increasing, and the degree of enhancement was very obvious.Especially at 950 and 1000°C, almost all the peaks of MgF 2 were transformed into MgO peaks.This indicates that at lower temperatures in the HFP/air atmosphere, little oxidation of MgF 2 occurred.However, at higher temperatures, MgF 2 underwent significant oxidation and as the temperature increased, the degree of MgF 2 oxidation increased significantly, which was higher than the case with the addition of 5% Al.The EDS results of 9% Al-MgF 2 heated in 0.01% HFP/air at various temperatures are given in Table 3.It is shown that below 750°C, the O element content was not high and increased weakly.Above 800°C, the O element level was increased with increasing temperature, especially up to 950 and 1000°C, where the O element level maintained a high-quality fraction.This result is almost consistent with the correspondent XRD results.The XRD results of 15% Al-MgF 2 after heating in 0.01% HFP/air at various temperatures are depicted in Fig. 5. Fig. 5 shows that at temperatures below 700°C, the spectrum was almost the peak of MgF 2 .Above 750°C, the peak of MgO appeared, and with temperature increasing, the strength of MgO peak was increased, especially at 950 and 1000°C, where almost all the peaks of MgF 2 were converted into MgO peaks.This indicates that in 0.01% mixtures below 700°C, MgF 2 oxidation was weak, but at higher temperatures, MgF 2 experienced significant oxidation, and the oxidation was intensified with increasing temperature, which was higher than the case with the addition of 9% Al.The EDS results of 15% Al-MgF 2 after heating in 0.01% HFP/air at various temperatures are represented in Table 4.It shows that below 700°C, the O element content was not high.Above 750°C, the O element content was increased with increasing temperature, especially up to 950 and 1000°C, where the O element content was higher and the F element content was lower.This result is almost equal to the corresponding XRD result.

Conclusion
The influence of Al content on the oxidation of MgF 2 in an air/HFP atmosphere from 700 to 1000°C has been studied by XRD and EDS.The results reveal that the addition of Al accelerated the oxidation of MgF 2 .With increasing Al content, the oxidation of MgF 2 was increased.At the same Al content, with temperature increasing, the oxidation of MgF 2 also increased.This result indicates that increasing the Al content in MgF 2 and the temperature reduced the thermal stability of MgF 2 , which helps to better understand the mechanism of HFP gas protection of magnesium liquid, and offers a theory base for controlling and optimizing HFP gas protection of magnesium liquid.

Figure 1 .
Figure 1.XRD results of MgF 2 heated in air with various contents of HFP at 1000 °C [10].