INFLUENCE OF SINTERING TEMPERATURE ON THE STRUCTURAL OF Mg 2 Si 0.3 Sn 0.7 ALLOY PREPARED BY POWDER METALLURGY

The efficiency of energy consumption can be improved by reducing the heat lost during the combustion process of automotive engines. By converting waste heat energy into other energy sources that can be used directly, efficiency can be achieved. Mg 2 Si 0.3 Sn 0.7 is a metal alloy that has the potential to convert waste heat into power. In this research, we used the powder metallurgy method to synthe-size the Mg 2 Si 0.3 Sn 0.7 -based alloy. A mixture of Mg, Si, and Sn powders that had been milled for 2 hours was insulated in the stainless-steel tube to avoid oxidation and combustion. Heat treatment was conducted with temperature variations of 700, 750, and 800°C for 4 hours to investigate the influence of sintering temperature on the crystal structure of the Mg 2 Si 0.3 Sn 0.7 -based alloy. The microstructure and formation of the material were examined using an X-ray diffractometer (XRD) and a scanning electron microscope (SEM). Based on XRD analysis, it was found that the Mg 2 Si 0.3 Sn 0.7 phase and a small amount of magnesium oxide (MgO) phase have formed. Due to the phase transition from Mg 2 Si 0.3 Sn 0.7 to Mg 2 Sn at a sintering temperature of 800°C, the cubic lattice constant -a changes from 0.6631 nm to 0.6765 nm.


INTRODUCTION
Fossil fuels as a fuel in various fields such as transportation, electricity generation and industry are still an option today.In general, the majority of processes generate a substantial amount of waste heat.The use of wasted thermal energy can lead to an increase in fuel consumption.Through the Seebeck and Peltier phenomena, this heat can be converted directly into other energy sources.The Seebeck phenomenon states that if two materials have a temperature difference between their surfaces, a potential difference will result in both materials.Furthermore, this material will be known as a thermoelectric material.When a material has a potential difference, the Peltier phenomenon states that applying heat to one surface will result in a cool temperature on the opposite surface.This material is referred to as a thermocooler material.Magnesium based metal alloys, such as Mg2Si, Mg2Ge, and Mg2Sn, have the potential to be used as thermoelectric materials.The ZT value of a material can be used to determine its thermoelectricity (ZT>1) The ZT values for the three magnesiumbased alloys are still lower than 1.Enhancing thermoelectric properties, particularly the ZT value, has been achieved through doping in materials.When Sn and Ge elements are doped on the Si site, the ZT value of Mg2Si-based alloys increases [1].The Mg2Si0.3Sn0.7 alloy has thermoelectric properties that are similar to those of p-type thermoelectric materials [1]- [4].The ZT value of the Mg2Si0.3Sn0.7-basedalloy was increased up to 1.4 by doping several elements [1].The Mg2Si0.3Sn0.7 alloy generation process is generally based on the phase diagram of the Mg-Si-Sn system or pseudo-binary Mg2Si-Mg2Sn [5]- [7].When manufacturing the Mg2Si0.3Sn0.7-basedalloy, the heating method is challenging.Kun Cheng et al synthesized the Mg2Si0.3Sn0.7-basedalloy through the melting technique [2].Magnesium, silicon, and tin were weighed in accordance with their respective stoichiometric compositions of Mg2.03Si0.3Sn0.7 and then all of the components were sealed in a tantalum tube.The melting occurred at 1057°C for 7 hours, was quenched in water, and then annealed for 72 hours at 700°C.Huang et al employed the solid-state reaction approach followed by hot pressing to synthesize the Mg2(Si0.3Sn0.7)0.99Sb0.01alloy [8].After the raw materials Mg, Si, Sn, and Sb have been weighed in accordance with the formula, they are mixed and put on a Mo crucible.To prevent the formation of magnesium oxide, the sample was carefully sealed in a silica tube before the heating procedure.At 580°C, the sample was heated for 15 hours at a relatively low heat rate.The heat-treated products are crushed and then heated to 750°, 800°, and a combination of the two temperatures during a hotpressing process.The solid-state reaction method led to the formation of Mg2Si0.3Sn0.7,Mg2Si, and Mg2Sn phases based on their X-ray diffraction investigation.When it was combined with hot pressing, the method was found to be effective in producing Mg2Si0.3Sn0.7-basedalloy.In this study, we report that the sintering temperature influences the evolution from the Mg2Si0.3Sn0.7 phase to the Mg2Sn phase.Through X-ray diffraction and scanning electron microscope investigations, it was discovered that the growth of grains occurred with an increase in sintering temperatures.

Sample preparation
Powder metallurgy was employed to synthesize the Mg2Si0.3Sn0.7-basedalloy using commercially available magnesium powders (Merck Millipore), silicon powders (Sigma Aldrich), and tin powders (Sigma Aldrich) as initial materials.To prepare the Mg2Si0.3Sn0.7-basedalloy, Mg, Si and Sn powders were weighed at the atomic ratio of Mg:Si:Sn=2:0.3:0.7.The initial powders were then ground for 2 hours in a shaker mill to further homogenize the mixture.In the milling process, steel balls are employed at a ratio of ball to powders weight (BPR) of 2:1.The milled powder is sealed and compacted in a stainlesssteel tube.This technique has been demonstrated to be effective in minimizing magnesium oxidation while sintering process at high temperatures (above the melting point of Mg) [9], [10].The samples were then sintered in a muffle furnace with variations in temperature of 700, 750 and 800°C for 4 hours.After the holding time was reached, all samples were permitted to naturally cool to room temperature in the furnace.The samples in this research were coded and presented in Table 1.To characterize the sintered samples, it was divided into two sections.For X-ray diffraction (XRD) testing, a piece was mechanically removed from the stainless-steel tube and ground using agate mortar.To observe the morphology of the surface samples, the other pieces are inserted into the resin to be grinded and polished.

Sample characterization
In this study, we investigate the phase formed after the sintering process using an XRD.The testing was conducted at room temperature with a step size of 0.01° and an angle range of 2θ = 20-80°.The Rigaku Smart Lab X-ray diffractometer, which has a 240 mm goniometer radius, is operated by a copper radiation source (Cu kα = 0.15406 nm).Qualitative and quantitative characterization was carried out to investigate the effect of sintering temperature on the phase and its crystal structure.The Rietveld method is employed to determine the lattice parameters of the phase that was obtained [11].The surface morphology of the samples was observed using a JEOL JSM6390A scanning electron microscope (SEM).The sample surface that observed was a sintered solid surface that had been grinded and polished.The distribution of elements on the sample surface was determined through element mapping -energy dispersive X-ray spectroscopy (Mapping-EDAXS).220), (311), (400), (311), (422), and (511), which are the planes of the cubic phase Mg2Si0.3Sn0.7 [4], [12]- [15].There is a peak that matches the MgO phase peak at angles 2θ = 42.90 and 62.30° [14].These findings demonstrate that, despite sintering temperatures over the Mg melting point, the method employed can maintain the Mg2Si0.3Sn0.7 phase formation with less MgO.The SiO2 phase was detected at an angle of 2θ = 20.89°.The presence of the SiO2 is a follow-up impurity phase from the silicon powder raw material used [16].In comparison to M700 and M750, the sample sintered at 800°C (M800) has a significant difference.The diffraction peaks have been shifted to the left.Based on the investigation results, the peaks were detected at angles 2θ = 22.[8], [17].On sintering at a temperature of 800°C it turns out to give rise to a cubic phase of Mg2Si which is detected at an angle of 2θ = 23.95°.The occurrence of the Mg2Si phase is most likely due to sintering temperatures exceeding the melting point of the Mg2Si0.3Sn0.7 alloy, causing the Mg2Si0.3Sn0.7 phase to decompose into Mg2Sn and Mg2Si [18].The Mg2Sn alloy melts at a temperature of 774 °C [5], [6].As a result, phase formation now occurs through a solidification mechanism, instead of continuing as a liquid solid process.The Monshi-Scherrer method was utilized to determine the size of the crystallite to determine the impact of sintering temperature on the microstructure [19], [20].A linear line equation can be created using the ln (1/cos θ) vs ln FWHM plot, as displayed in Fig. 2b.The full width at half maximum (FWHM) of the three samples in the indexed planes ( 111), ( 220), (311), and (331), expressed in degrees, are shown in Table 2.After that, the crystal size is determined using this FWHM value.Line equations M700, M750, and M800 samples have constant c values of -5.5640, -5.8441, and -6.6683, respectively.As the sintering temperature rises, crystals of the Mg2Si0.3Sn0.7-basedalloy grow, as presented in Table 2.  Fig. 3 shows the refinement of the diffraction pattern of the Mg2Si0.3Sn0.7-basedalloy sample sintered at 800°C.Mg2Sn, Mg2Si, MgO, and SiO2 were the 4 phases used in the refinement process that were detected qualitatively.The Mg2Sn phase input for the calculation process is modified by using the Mg2Si0.3Sn0.7 formula and a cubic structure that is part of the space group fm-3m [7].The lattice constant -a at 0.6765 nm, as calculated, is similar to the lattice constant value of the Mg2Sn phase reported by Boudemagh et al [17].The lattice constant -a for all Mg2Si0.3Sn0.7-basedalloy samples are tabulated in Table 2. Despite calculations using the modified Mg2Sn phase, the lattice constants values of the M800 sample remained close to the Mg2Sn lattice constants value.This is in contrast to the M700 and M750 samples, which have the lattice constants -a value of 0.6631 nm and 0.6670 nm, respectively.According to Goyal et al, the Mg2Si0.3Sn0.7-basedalloy synthesized by the melting technique has the lattice constants -a of 0.6655 nm [3].Mg2Si0.3Sn0.7basedalloy was synthesized by Assahsahi et al through melting and spark plasma sintering (SPS) [4].They provided information on the lattice constants -a value of the Mg2Si0.3Sn0.7 phase at 0.6660 nm [4].These findings indicate that the phase transition of Mg2Si0.3Sn0.7 phase is influenced by the sintering temperature.The Mg2Si0.3Sn0.7 alloy will tend to turn into Mg2Sn alloy when heated above its melting point of Mg2Sn.4a shows the polishing surface of the M700 sample with irregular shapes and random directions.There are no significant metallic grain connections evident in the surface morphology of the M700 sample.

RESULTS AND DISCUSSION
As the sintering temperature increases, the surface begins to have a direction marked by polishing traces.The sample was sintered at 750°C and its surface microstructure is shown in Fig. 4b.It appears that a metallic morphology has begun to form with a regular shape compared to the sample sintered at 700°C.Fig. 4c demonstrates the polished surface with an enlarging grain surface.This is supported by the XRD analysis in Table 2 which shows that crystal growth occurs with increasing sintering temperature.In addition, the metal surface shows the majority of Mg2Sn phase present in the M800 sample.

CONCLUSIONS
Mg2Si0.3Sn0.7-basedalloy has successfully been synthesized using the powder metallurgy method.An X-ray diffractometer (XRD) and a scanning electron microscope (SEM) were used to investigate synthesized samples with variations in sintering temperature.Following is a list of some findings from the present research: 1.The alloying process is maintained successfully with minor magnesium oxide by heating in the stainless-steel tube.2. The cubic Mg2Si0.3Sn0.7 phase is formed when sintering at temperatures below 800°C.3. The cubic Mg2Si0.3Sn0.7 phase is turned into the cubic Mg2Sn phase by the sintering process at 800°C.Evidence for this phenomenon can be found in the lattice constantsa, which have a value change from 0.6631 nm to 0.6765 nm. 4. As the sintering temperature increases, the surface of the Mg2Si0.3Sn0.7-basedalloy becomes clearer, which leads to grain growth.

Fig. 2a illustrates
Fig.2aillustrates the shift in the diffraction peak of the Mg2Si0.3Sn0.7-basedalloy sample when the diffraction pattern is enlarged in the angle range 2θ = 22.2-23.8°.The pattern indicates that the highest peak of the Mg2Si0.3Sn0.7 sample has been moved by 0.48°.A peak shift and a phase transition from Mg2Si0.3Sn0.7 to Mg2Sn occur when the sintering temperature increases.Between the Mg2Si0.3Sn0.7 and the Mg2Sn phase, an intermediate arises.Additionally, Mejri et al reported the existence of this intermediary phase[6].According to Fig.2a, a higher sintering temperature results in a more crystallin phase of Mg2Si0.3Sn0.7-basedalloy.This indicates that the phase peak intensity value has increased by up to three times.According to the phase diagram of the Mg-Si-Sn system, the sintering process has reached the melting point of the Mg2Si0.3Sn0.7 phase, which has led to a rise in crystallinity.As a result, phase formation now occurs through a solidification mechanism, instead of continuing as a liquid solid process.The Monshi-Scherrer method was utilized to determine the size of the crystallite to determine the impact of sintering temperature on the microstructure[19],[20].A linear line equation can be created using the ln (1/cos θ) vs ln FWHM plot, as displayed

Fig. 3
Fig. 3 Refined pattern of the sample sintered at 800°C.

Fig. 4a -
Fig.4a-4c are surface images of Mg2Si0.3Sn0.7-basedalloy samples after grinding and polishing processes.Fig.4ashows the polishing surface of the M700 sample with irregular shapes and random directions.There are no significant metallic grain connections evident in the surface morphology of the M700 sample.As the sintering temperature increases, the surface begins to have a direction marked by polishing traces.The sample was sintered at 750°C and its surface microstructure is shown in Fig.4b.It appears that a metallic morphology has begun to form with a regular shape compared to the sample sintered at 700°C.Fig.4cdemonstrates the polished surface with an enlarging grain surface.This is supported by the XRD analysis in Table2which shows that crystal growth occurs with increasing sintering temperature.In addition, the metal surface shows the majority of Mg2Sn phase present in the M800 sample.Fig.5a and 5bshow the distribution of the elements contained on the surface of the Mg2Si0.3Sn0.7-basedalloy sample sintered at 700 and 750°C.The elements mapping image indicates that magnesium and tin elements are the dominant elements throughout the observation area.The alloy formula was created based on the content of these two elements, which is the reason for this.Magnesium oxide (MgO) is the most likely substance in Fig.5b, which is marked with a red line.It can be seen that the area with

Fig. 5a and
Fig.4a-4c are surface images of Mg2Si0.3Sn0.7-basedalloy samples after grinding and polishing processes.Fig.4ashows the polishing surface of the M700 sample with irregular shapes and random directions.There are no significant metallic grain connections evident in the surface morphology of the M700 sample.As the sintering temperature increases, the surface begins to have a direction marked by polishing traces.The sample was sintered at 750°C and its surface microstructure is shown in Fig.4b.It appears that a metallic morphology has begun to form with a regular shape compared to the sample sintered at 700°C.Fig.4cdemonstrates the polished surface with an enlarging grain surface.This is supported by the XRD analysis in Table2which shows that crystal growth occurs with increasing sintering temperature.In addition, the metal surface shows the majority of Mg2Sn phase present in the M800 sample.Fig.5a and 5bshow the distribution of the elements contained on the surface of the Mg2Si0.3Sn0.7-basedalloy sample sintered at 700 and 750°C.The elements mapping image indicates that magnesium and tin elements are the dominant elements throughout the observation area.The alloy formula was created based on the content of these two elements, which is the reason for this.Magnesium oxide (MgO) is the most likely substance in Fig.5b, which is marked with a red line.It can be seen that the area with