Oxidation resistance of impurity doped Mg 2 Si grown from the melt

We have investigated oxidation resistance of melt grown Mg2Si crystals doped with Sb or Bi impurity by thermal annealing above 600°C, large difference of oxidation speed was observed in the Sb-doped, Bi-doped and non-doped Mg2Si crystals. The sample weight of Bi-doped and non-doped Mg2Si increased about 128% and 104%, respectively, while that of Sb-doped one unchanged after thermal annealing at 650°C for 6h. TG/DTA analysis also revealed the changing of oxidation speed and reaction temperature in the Sb-doped, Bi-doped and non-doped Mg2Si crystals. These results indicate that Sb-dopant improves the oxidation resistance whereas Bi-dopant deteriorates the oxidation resistance of Mg2Si.


Introduction
Magnesium half silicide, Mg2Si, is a promising n-type thermoelectric material that consists of abundant, low toxic and inexpensive elements [1].Hence, practical application of Mg2Si is expected in the intermediate temperature range because of its high thermoelectric properties [2].A number of donor impurities such as Bi, Sb, As, P and Al are investigated to optimize the electron concentration for obtaining the optimal ZT and thermoelectric properties in Mg2Si [3][4][5][6][7][8][9][10]. On the other hand, durability of the thermoelectric material is a major important issue as similar to the ZT for practical applications.In particular, thermal oxidation resistance of the material affects directly on the thermoelectric performance and mechanical strength of thermoelectric generators (TEGs).However, the reports on oxidation resistance of Mg2Si crystal are limited, so far [4,11,12].
Previously, we have reported the effect of Sb and Bi dopant on the lattice thermal conductivity in the melt grown Mg2Si crystals [13].In this report, we investigate the oxidation resistance of Sb and Bi doped Mg2Si crystals grown from the melt.

Experimental
Bulk crystals of Mg2Si having well-developed crystalline domains were grown by the vertical Bridgman method using a BN coated alumina crucible [7][8][9].Source materials of Si (99.999%) and Mg (99.98%) were weighed at stoichiometric composition and charged in the crucible with a certain amount of Bi (99.999%) or Sb (99.9999%) dopant element, where the amount of dopant ratio of X = [Sb or Bi]/[Mg2Si] was varied between X = 0 and 3 mol%.Bulk samples with a dimension of 3 x 3 x 2 mm were prepared from the melt grown crystals for the oxidation resistance experiments.All faces of the samples were polished by the #4000 SiC abrasive powder.The volume density of the samples was evaluated by the Archimedes method.The net dopant concentration in the measured bulk samples was determined by X-ray fluorescent (XRF) analysis using ZSX Primus II (Rigaku Co. Ltd).
Oxidation resistance of the Mg2Si crystals was studied on both of the bulk and powdered samples by annealing them in air atmosphere.The annealing temperature and time were varied between 550°C and 650°C and between 1 and 12 hours, respectively.In order to study the oxidation mechanism of Mg2Si in detail, thermogravimetric and differential thermal analysis (TG/DTA) of the Mg2Si powder was carried out at a heating rate of 10 °C /min in air flowing at 100 ml/min.

Results and discussion
Figs. 1 (a) and 1 (b) show a typical Mg2Si bulk crystal and a prepared bulk sample for the oxidation resistance test, respectively.All tested bulk samples (Sb-, Bi-and non-doped samples) had no significant voids and cracks on the surface and, their volume densities were agreed with those of theoretical density.Table 1 lists the dopant ratio XSb and XBi in raw materials and net dopant concentration determined by XRF analysis.The net dopant concentration in both Sb-doped and Bi-doped samples were increased depending on XSb and XBi, respectively.Fig. 2 plots the weight change of annealed bulk samples (#1_non-dope, #3_Sb0.5 and #8_Bi2.0)against the annealing time up to 12 hours under the annealing temperature at 550°C and 650°C.In the annealing experiments at 550°C, the weight of #1_non-dope and #2_Sb0.5 samples unchanged after the annealing for 12 hours, whereas the weight of #8_Bi2.0increased slightly.In the annealing experiments at 650°C, the weight of #1_non-dope and #8_Bi2.0samples increased remarkably with increasing the annealing time, whereas that of #3_Sb0.5 was almost the same after 12 hours annealing.Figs. 3 and 4 show the photographs of the samples annealed at 550°C and 650°C, respectively.The samples annealed at 550°C kept initial shape after 12 hours annealing although their surface became dark gray color due to oxidation.After 12 hours annealing at 650°C , the #1_non-dope and #8_Bi2.0samples were broken to pieces while the Sb doped sample kept initial shape.Clearly, oxidation resistance of bulk Mg2Si was varied by affecting by the dopant impurity.Scanning electron microscopic (SEM) images on those oxidized surface are shown in Fig 5 .The thick oxide layer on Bi-doped sample is easy to peel off from the sample as compared to the Sb-doped sample.
Further, in order to investigate the influence of dopant concentration on the oxidation resistance, TG/DTA measurement was carried out for the Sb and Bi-doped powdered samples with various dopant concentrations.oxidation from 550°C, which was consistent with oxidation temperatures reported [4,14].In the Bi-doped samples, increase of oxidation rate was observed with increasing the Bi concentration.On the other hand, in the Sb doped samples, the oxidation rate was unchanged depended on the Sb concentration.Similar trend was also observed in DTA measurement as shown in Fig 6 (b).Sharp and high DTA peaks around 562 °C and 622 °C were found in the #8_Bi2.0sample, while in the #3_Sb0.5 sample, the DTA peaks were relatively low and the peak temperatures were shifted to around 583 °C and 680 °C.
For the investigation of the oxidation mechanism, powdered samples that were grounded from the bulk samples with a mortar were annealed at 650 °C in atmosphere.Fig. 7 (a) shows the XRD patterns of the powdered Mg2Si samples (#1_non-dope, #3_Sb0.5 and #8_Bi2.0)annealed at 650 °C for 1 hour.The diffraction peaks corresponding to Si and MgO were newly observed in the XRD patterns measured after the annealing, even though the both phases were not observed in the pre-annealed samples.On the other hand, diffraction peaks related to bismuth oxide or antimony oxide were not found in the XRD measurement.
During the annealing of Mg2Si in atmosphere, following reaction is occurred between Mg2Si and O2 [11,15,16]: Therefore, the X-ray peak intensities both of MgO and Si would represent the progress of oxidation.The X-ray peak intensity ratios of Si 111/Mg2Si 111 and MgO 200/Mg2Si 111 of the measured samples were plotted in Fig. 7 (b).The intensity ratios both of Si and MgO in Bi doped sample is about 10 times larger than those of non-doped one, whereas those in Sb doped sample is about 20% less than those of non-doped one.This result is consistent with our thermal annealing test and TG/DTA measurement.

Conclusion
The oxidation resistance of Sb or Bi-doped Mg2Si crystals was investigated by thermal annealing and TG-DTA analysis.We found that the changing of oxidation speed and reaction temperature depend on dopant element and concentration.We also found that Bi dopant enhances the oxidation of Mg2Si, whereas Sb dopant suppresses the oxidation of Mg2Si.

Fig. 1 .
Fig.2plots the weight change of annealed bulk samples (#1_non-dope, #3_Sb0.5 and #8_Bi2.0)against the annealing time up to 12 hours under the annealing temperature at 550°C and 650°C.In the annealing experiments at 550°C, the weight of #1_non-dope and #2_Sb0.5 samples unchanged after the annealing for 12 hours, whereas the weight of #8_Bi2.0increased slightly.In the annealing experiments at 650°C, the weight of #1_non-dope and #8_Bi2.0samples increased remarkably with increasing the annealing time, whereas that of #3_Sb0.5 was almost the same after 12 hours annealing.Figs.3 and 4show the photographs of the samples annealed at 550°C and 650°C, respectively.The samples annealed at 550°C kept initial shape after 12 hours annealing although their surface became dark gray color due to oxidation.After 12 hours annealing at 650°C , the #1_non-dope and #8_Bi2.0samples were broken to pieces while the Sb doped sample kept initial shape.Clearly, oxidation resistance of bulk Mg2Si was varied by affecting by the dopant impurity.Scanning electron microscopic (SEM) images on those oxidized surface are shown in Fig 5.The thick oxide layer on Bi-doped sample is easy to peel off from the sample as compared to the Sb-doped sample.Further, in order to investigate the influence of dopant concentration on the oxidation resistance, TG/DTA measurement was carried out for the Sb and Bi-doped powdered samples with various dopant concentrations.Fig.6(a) shows the results of TG measurement.In all samples, it had begun

Fig. 2 .
Fig. 2. Relationship between the annealing time and weight change of Mg2Si samples annealed at (a) 550°C and (b) 650°C.