Thermoelectric properties of Mg3SbxBi2−x thin films
Authors:   , Sadowski Grzegorz 
Published: August 2024

Thermoelectric materials produce an electrical potential when exposedto a temperature gradient. The lack of moving parts makes thermoelectric devices quiet and reliable, and therefore often used for off-gridenergy generation. When deposited in the form of thin films, thesurface to volume ratio is far larger than for bulk materials, grantingthem some useful attributes due to quantum electron confinement andphonon-scattering effects. Mg3SbxBi2−x is a thermoelectric materialwith good properties close to room temperature. This thesis exploresthe transport properties of Mg3SbxBi2−x thin films deposited using dcmagnetron sputtering.Mg3Bi2 films were synthesized between room temperature and400 ◦C – a relatively low deposition temperature of 200 ◦C proved tobe necessary for single crystal growth to avoid loss of Mg due to itsrelatively high vapor pressure. Results from energy-dispersive X-rayspectroscopy and X-ray diffraction confirm the loss of Mg at 300 ◦C andabove, while at room temperature the film was polycrystalline. Theepitaxy was confirmed using X-ray pole figures and computer simulation based on density functional theory. The thermoelectric propertieswere measured between room temperature and 200 ◦C. Mg3Bi2 is asemimetal with low electrical resistivity. The power factor S2ρ , where Sis the Seebeck coefficient and ρ is the electrical resistivity, had a peakvalue of 200 μWm−1K−2 at room temperature. Through measuringthe carrier concentration and mobility, it has been confirmed that thedecrease in power factor as the temperature increases is due to thebipolar effect – the small bandgap of the material being insufficient toprevent the minority carrier excitation. The electrons and holes neutralize, decreasing the net current, and therefore limiting the Seebeckcoefficient. Decreasing the bipolar effect may be possible by increasingthe majority carrier concentration and enlarging the band gap of thematerial.Mg3Sb2 is a semiconductor – it has significantly higher electrical resistivity and Seebeck coefficient as compared to Mg3Bi2. Alloying bothmaterials together can result in a semiconducting material with appreciably lower thermal conductivity. This is especially important forMg3Bi2 which as a semimetal has high thermal conductivity. Furthermore, the increase in Sb content increases the energy gap and shiftsthe bipolar effect, making the material more suitable for higher temper-atures. It must be noted that low band gap is more desirable for roomtemperature materials than for high temperature thermoelectrics. FiveMg3SbxBi2−x samples were synthesized at 200 ◦C with x ranging from0.00 to 1.19. Sample characterization included composition, crystalstructure and transport properties. An Sb peak has been observedin X-ray diffraction results for x ≥ 1. The precipitation of Sb due tominute deficiency of Mg in those samples can affect the properties – thefocus of the study became x < 1 instead. Higher Sb content results ina more polycrystalline structure, higher band gap, electrical resistivityand Seebeck coefficient. For room temperature implementations, highBi content is advantageous. This work has prospects for further study ofMg3SbxBi2−x based thin films and their synthesis on flexible substrates,which are often sensitive to higher temperatures.