Improvement of mid-temperature ZT in a Bi-Se-Te via a two two-step sintering process

A wide range of thermoelectric materials are available for selection. The standard classification is based on the temperature range of the application. This study fabricated mid-temperature Bi-Se-Te using spark plasma sintering (SPS). This analysis focused on the impact of sintering conditions on thermoelectric properties. The structural analysis indicated that initial and secondary sintering processes effectively produced the low-temperature thermoelectric material Bi2Se0.45Te2.55. The sintering duration and temperature changes mainly influenced the grain boundary density, with elevated temperatures inducing defects that impacted the performance. Secondary sintering resulted in layered structures with elongated grains, which enhanced the phonon scattering effect. This configuration markedly decreased the thermal conductivity, increasing the ZT value by 60%.


Introduction
Energy demand has been increasing since the Industrial Revolution.Electricity is essential for various aspects of modern life, such as food production, clothing manufacturing, residential power, transportation, and entertainment.Renewable energy development is gaining attention due to environmental pollution, climate change, and the global fossil fuel crisis.Among the various renewable energy sources, solar power is considered among the most promising.The total amount of solar radiation reaching the Earth's surface is approximately 1.2 ×10 5 TW [1], far exceeding the energy consumed by humans.Simultaneously, all power is eventually converted into heat and dissipated.Therefore, proper utilization of thermal energy in the environment is crucial to ensure the future sustainability of Earth and the advancement of green energy.
The selection of thermoelectric materials includes various options, and the standard classification is based on the temperature range of the application.Thermoelectric materials can be divided into three categories based on their optimal application temperature [2,20].High-temperature type: With an optimal application temperature above 850 K, silicon germanium (SiGe) alloys and their related compounds belong to this category.High-temperature materials are primarily used for waste-heat recovery [3,4].Medium-high-temperature type: These have an optimal application temperature range of 450-850 to K; lead telluride (PbTe) and its related alloys belong to this category [5].Medium-high-temperature materials are primarily used for waste-heat recovery.Low-temperature type: These exhibit optimal application temperatures below 450 K; bismuth telluride (Bi 2 Te 3 ) and its related alloys are typical examples.Thermoelectric coolers primarily use low-temperature thermoelectric materials [6,7].
The strategies employed to enhance the thermoelectric performance of materials include the following.Adjusting the electronic structure of the material [2,8]], such as doping with hetero-elements [9], enhancement of crystal structure of phonon-glass-electronic-crystal [10,11], decreasing the potential barrier effect at grain boundaries [12,13], minimization of the phonon scattering effect [14,15], and fabricating low-dimensional nanostructures [16,17].
This study used BiTe, a low-temperature thermoelectric material, as the matrix, with Se introduced as the doping agent.The Spark Plasma Sintering (SPS) technique was used for sintering, known for its rapid, low-temperature capabilities and ability to produce high-density products through applied pressure.After an initial sintering, the material underwent a secondary sintering leveraging the pressure attributes of SPS.The hypothesis is that the additional pressure during the second step of sintering will induce a layered, directionally structured formation, enhancing photonic scattering and potentially elevating the value of the thermoelectric figure of merit (ZT).

Materials and method
Bi 2 Te 3 (99.98%)and Bi 2 Se 3 (99.98%)were purchased from Alfa Aesar GmbH.Bi 2 Te 3 (52.5 g) and Bi 2 Se 3 (7.56g) were thoroughly mixed, and ball milled using a planetary ball mill (P6 Classic).Because Bi-Se-Te powder is prone to oxidation, all procedures were conducted in a nitrogen atmosphere.The ball mill was operated at 350 rpm for three hours.The alloy was finely ground and sieved before spark plasma sintering was used (SPS, Dr Sinter SPSS-515).The sintering parameters were set as follows: temperature range of 300-500 °C, time of 5-10 min, and pressure range of 40-60 MPa.The size of the die used in the SPS is Ø20 mm in diameter with a depth of 30 mm.The two-stage sintering parameters are shown in figure 1.
The specific heat capacities of the thermoelectric materials were measured using differential scanning calorimetry (DSC).Detailed heat analysis was conducted using the TA Instrument Explorer software.Each set was measured five times, and outliers were removed to obtain the average data.The thermal diffusivity values were measured using a physical property measurement system to assess the thermal conductivity coefficient of the material within the temperature range of 300 K-400 K.Each sample set was measured at least five times.The Seebeck coefficients and electrical properties of the thermoelectric materials were determined using a thermoelectric parameter measurement instrument (ZEM-3, ULVAC RIKO ZA07-9201).The measurement temperature range was 300 K-400 K, and the Seebeck coefficient of the material was measured at different temperature intervals.Each set was measured five times.
The crystal structure of the thermoelectric materials was analyzed using an X-ray diffractometer (XRD D2 Phaser / Bruker AXS (Gmbh, Germany)), with scanning angles between 10°and 60°.The microstructure was observed using scanning electron microscopy (SEM; AURIGA, Carl Zeiss, Oberkochen, Germany) and transmission electron microscopy (TEM; JEM-2100F Cs STEM (JEOL, Japan)) to examine the crystal aspect ratio and orientation of the internal grains.

Crystal and microstructure analysis
The XRD pattern of Bi 2 Se 0.45 Te 2.55 , referenced from the ICDD database (PDF#00-067-0431), is indicated using red lines in figure 2. The XRD patterns of the bulk material after the first step of sintering process were similar for both horizontal and vertical pressing.Thus, the structure of the Bi 2 Se 0.45 Te 2.55 bulk material obtained from a single sintering process was uniform, with no grains arranged in a specific direction.However, for the Bi 2 Se 0.45 Te 2.55 bulk material after the second step of sintering process, the XRD pattern showed peaks at 38 and 44°, which were noticeably higher for vertical pressing than for horizontal pressing.After the second step of sintering, the diffraction intensities of the (1,0,10) and (0,0,15) crystal planes inside the bulk material increased, indicating an increase in the number of these two sets of crystal planes within the material.
Moreover, the grains were arranged in a particular direction.The average grain size was calculated using the Williamson-Hall equation [18].After the first and second step of sintering, the grain sizes were 36.0 ± 0.5 and 20.0 ± 0.3 μm, respectively.The grain size after the second step of sintering showed a significant reduction compared to that after the first step of sintering.

SEM observation
Figure 3 depicts the fracture microstructures of the Bi2Se0.45Te2.55bulk material following both the first and secondary sintering processes.The Bi 2 Se 0.45 Te 2.55 bulk material from the first step of sintering was filled with tiny pores generated during the sintering process, leading to significant losses in the electrical and thermal conductivities of the material.Such a limitation hindered its ability to enhance the ZT.Compared to the material from the f first step of sintering, the second step of sintering material displayed a significantly reduced density of tiny pores.In the two-stage sintering process, the first step of sintering at lower temperatures and pressures promoted the expulsion of pores and degassing between powder particles, thus reducing pore formation.During the second step of sintering process, higher temperatures and pressures were employed to fill and repair the residual pores and defects, thereby improving the density and integrity of the material.Besides the pore distribution within the material, the grain aspect ratio from the second step of sintering exhibited greater magnitude and enhanced directional alignment.The Bi 2 Se 0.45 Te 2.55 bulk material showed a layered structure wherein the thermal and electrical conductivities were along the cross-section and parallell directions, respectively.The direction of thermal conductivity, which has more grain boundaries, enhances grain boundary scattering, complicates phonon propagation, and decreases thermal conductivity.Conversely, the direction of electrical conductivity, with fewer grain boundaries and larger grains, offers lower resistance and diminishes grain boundary scattering, thereby aiding electron transmission with minimal impact on electrical conductivity.

TEM evidence
TEM was used to analyze the microstructure and interfacial conditions inside the materials.Figures 4(a) and 5(a) show cross-sectional images of the Bi 2 Se 0.45 Te 2.55 material after the first and second step of sintering, respectively.As evident, the material was predominantly single-crystalline with a crystal structure of R 3m.The atomic arrangement was viewed using high-resolution TEM.Interatomic distance calculations revealed the region's orientation, as figures 4(b) and 5(b) highlighted.Most areas exhibited a particular orientation.A uniform diffraction pattern was observed in the crystalline regions in selected-area electron diffraction (SAED) analysis, as depicted in figures 4(c) and 5(c), indicating a primarily single crystalline nature of the bulk material.The images obtained from SAED are in reciprocal space.Subsequent calculations of these diffraction spots and reference to the ICDD database revealed that their corresponding diffraction planes aligned with the XRD results.Further, TEM analysis confirmed that the bulk material was predominantly single-crystalline, and the arrangement of the material was a hexagonal close-packed structure.

Analysis of thermoelectric property
Figures 6-10 display the electrical conductivity, Seebeck coefficient, power factor, thermal conductivity, and ZT results for the first and second step of sintering processes.The grain size refinement decreased the electrical conductivity, and the Seebeck coefficient increased, enhancing the power factor.The thermal conductivity decreased significantly, resulting in a noticeable increase in the ZT value.Compared to the maximum ZT value of 0.726 obtained from a single sintering process, there was an increase of approximately 50% and reach 1.3 (at 375 K) [19,20].This increase can be attributed to a reduction in thermal conductivity.In the second step of sintering, the aspect ratio and internal defect morphology of the bulk material were adjusted, enhancing the layered structure inside the Bi 2 Se 0.45 Te 2.55 material.By increasing the grain aspect ratio, the probability of phonon scattering was further elevated, achieving lower thermal and lattice thermal conductivities and increasing its electrical conductivity, thus enhancing the ZT value.Therefore, the experiments indicate that

Conclusion
This study employed a two-step sintering process using a SPS furnace to synthesize the n-type ternary thermoelectric material, Bi 2 Se 0.45 Te 2.55 .The two-step sintering process enhanced the thermoelectric properties of a material.Grain refinement was achieved through this two-stage sintering process, which reduced the pores and defects within the crystal.Simultaneously, the grains inside the material were directionally aligned, leading to an overall improvement in the thermoelectric performance.Consequently, the ZT value increased from 0.826 in the single-step sintering method to 1.224 in the two-step sintering method, indicating an enhancement of approximately 60%.
This study explored the structural impacts of secondary sintering using the pressure features of SPS.Results highlighted that a layered structure aligned with the thermal flow direction significantly mitigated thermal conduction effects.This emphasizes the potential of refining later processing stages to enhance the material's thermoelectric application traits.However, the research didn't fully optimize aspects like doping amounts for

Figure 1 .
Figure 1.Relationship diagram of temperature, pressure, and time in the secondary sintering process.

Figure 2 .
Figure 2. XRD analysis results for the cross-section area after the sintering process.

Figure 3 .
Figure 3.The SEM images of Bi 2 Se 0.45 Te 2.55 by (a) first step of sintering (red dot was the pores position) and (b) second step of sintering (red dot was the layered structure).

Figure 4 .
Figure 4. First step of sintering (a) cross-sectional image, (b) HT-TEM image, and (c) SAED of the yellow box.

Figure 5 .
Figure 5. Second step of sintering (a) cross-sectional image, (b) HT-TEM image, and (c) SAED of the yellow box.

Figure 6 .
Figure 6.Comparison of the electrical conductivity between the first and second step of sintering.

Bi 2
Se 0.45 Te 2.55 can undergo pre-sintering second step of sintering to optimize the ZT value and be used in environments below 200 °C.

Figure 7 .
Figure 7.Comparison of the Seebeck coefficient between the first and second step of sintering.

Figure 8 .
Figure 8.Comparison of the power factor between the first and second step of sintering.