Research electrical properties of Cu doped FeTe2 prepared by solid state reaction

The present study successfully prepared CuxFeTe2 (x=0.01, 0.05, 0.1, 0.15, and 0.2) compounds using a high-temperature solid state reaction sintering technique under preparation conditions ranging from 623 K to 823 K. The phase structure and microstructure of CuxFeTe2 were determined using X-ray diffraction and scanning electron microscopy. Simultaneously, we studied the Seebeck coefficient and electrical resistivity of the sample. It was found that the samples prepared at 673 K had a purer phase, and the thermal power coefficient was better for samples with a Cu doping concentration below 0.1.


Introduction
With the rapid development of social productivity and changes in human lifestyles, environmental pollution, and energy scarcity have become increasingly serious issues.Thermoelectric conversion technology, as a new, efficient, and environmentally friendly energy conversion technology, has received wide attention domestically and abroad.Thermoelectric devices prepared using thermoelectric materials have significant advantages in energy conversion, such as being small in size, having no moving parts, no pollution, being lightweight, having long steady-state operating cycles, and being able to work under extreme conditions.This provides a feasible solution for direct energy conversion between heat and electricity [1].Thermoelectric materials can use natural temperature differences and industrial waste heat to generate thermoelectric power.They can also use non-polluting natural energy sources and have comprehensive social benefits.
Thermoelectric materials are functional materials that can directly convert thermal energy into electrical energy through the movement of internal carriers [2][3][4].However, different materials have different thermoelectric properties.We chose the pyrite-type compound FeTe 2 , which belongs to the orthorhombic crystal system with high symmetry, with lattice parameters a=5.26Å, b=6.27Å, and c=3.87Å.The crystal structure of FeTe 2 has a high degree of symmetry and special properties in electricity, optics, magnetism, and superconductivity, and some studies have shown that it is also a potential thermoelectric material [5,6].
As early as the 1980s, Alvi and Nazir [7] calculated a new type of band structure for ternary alloys such as NiMnSb and PtMnSb and called these compounds "half-metallic" magnetic materials.These materials are a new type of functional material.The traditional preparation methods for FeTe 2 are hightemperature and high-pressure, long-time high-temperature reactions, hydrothermal methods, and hightemperature solid state reaction methods.During the high-temperature solid state reaction sintering process, the low melting point and volatility of tellurium have an important impact on the preparation of samples, depending on the heating rate used [8].Copper is relatively cheap compared to rare earth elements, and copper metal has low electrical resistivity and excellent mechanical processing properties.Therefore, in this study, we use high-temperature solid state reaction sintering to prepare Cu-doped FeTe 2 alloys and analyze and study their preparation methods and electrical properties under different heating rate conditions.This study investigated the thermoelectric properties of the alloy compound Cu x FeTe 2 , the effect of Cu doping on its thermoelectric properties, and provided technical references for the research of this system.

Experiment
The purity of the experimental raw material is 4n, with a particle size of about 300 mesh, Fe powder, Te powder, and Cu powder as starting materials, accurately weighed according to the chemical formula Cu x FeTe 2 (x=0.01,0.05, 0.1, 0.15, and 0.2).After weighing, the mixture is evenly mixed under inert gas protection and then pressed into cylindrical samples.The samples are synthesized in a vacuum furnace at different temperatures and holding conditions and removed after cooling to room temperature.
The vacuum furnace model used for solid state reaction sintering is HMZ-1700-20.The phase structure analysis of the sample was performed using an X-ray diffractometer (Cu Kα radiation, diffraction angle from 20 o to 80 o ).At room temperature, the electrical resistivity of the samples was measured using an RTS-9 four-probe testing instrument.The Seebeck coefficient was measured using a self-made.It calibrated the Seebeck tester, maintaining a temperature difference of 10℃ at both ends of the sample to measure its thermoelectric potential, from which the Seebeck coefficient of the sample was calculated, with a measurement error of ±5%.The room temperature thermal conductivity of the samples was measured using an LFA475 laser thermal conductivity meter from NETZSCH in Germany.The ZT value of the samples was obtained through the formula ZT= S 2 σT/κ.

Results and discussions
The XRD patterns of samples Cu x FeTe 2 with different Cu contents at a synthesis temperature of 400°C are shown in Figure 1.The diffraction peaks of the test spectrum match well with the FeTe 2 -PDF#14-0419 card in the PDF database, indicating that the phase of synthesized samples is mainly composed of the marcasite-type compound FeTe 2 .As the Cu content gradually increases, impurity peaks in the diffraction peaks also increase, and a certain amount of tellurium elemental is precipitated inside the sample.The melting points of copper, tellurium, and iron are quite different, and under prolonged heating and insulation conditions, tellurium is easily volatile.The sample is cooled and reprecipitated as an element when the temperature decreases.Generally, for preparing Cu x FeTe 2 (x=0.01,0.05, 0.1, 0.15, and 0.2) by solid-phase reaction, Cu doping should not be too high at 400°C, and less than 0.1 is better.Samples with Cu content above 0.1 will have pores and cavities inside after sintering and changing their properties.2, with a magnification of 5000 times.The synthesis temperature of the sample was 400°C.It can be seen from the SEM images that there are many pores inside the sample, which is caused by the solid state reaction.This phenomenon may be because the sample volatilizes and forms during the high-temperature synthesis process due to the low melting point of the Te element.It can be seen from Figure 2(a) that the crystallinity of the sample is poor, and the grain boundaries are blurred, which is consistent with the XRD measurement results.The SEM energy spectra in (b), (c), and (d) show that there is a certain segregation of Te, Fe, and Cu in the sample at this time, and there are many pores present.
The Cu-doped FeTe 2 sample prepared by the solid state reaction was tested for resistivity, and the resistivity-temperature curve is shown in Figure 3(a).The resistivity-temperature curves of Cu x FeTe 2 with x values of 0.01, 0.05, 0.1, 0.15, and 0.2 were tested from 350℃ to 550℃.As can be seen from Figure 3(a), the resistivity of the sample decreases with increasing synthesis temperature, and the decrease becomes gentle when the synthesis temperature is greater than 723 K.The sample with a Cu content of 0.05 has the lowest resistivity; its minimum resistivity is 1.74 mΩꞏcm.The change is more obvious at temperatures below 723 K.The sample of Cu-doped FeTe 2 prepared by the solid state reaction was tested for the Seebeck coefficient, and the Seebeck coefficient-temperature curve was obtained.The functional relationship between the Seebeck coefficient and the temperature of sample Cu x FeTe 2 is shown in Figure 3(b).It can be observed that the Seebeck values are all positive, indicating that the prepared thermoelectric material under these preparation conditions is a p-type semiconductor material.With increasing Cu doping, the curve becomes smoother as temperature increases.However, at higher temperatures, the sample undergoes greater changes, and the absolute value of Seebeck decreases with increasing temperature.The maximum Seebeck value occurs at Cu doping levels between 0.01 and 0.1, with the highest Seebeck coefficient of 75.66 μVK -1 obtained.
According to the Seebeck coefficient and resistivity measured for Cu x FeTe 2 (x=0.01,0.05, 0.1, 0.15, 0.2), the power factor curve versus temperature was calculated PF=S 2 σ using the formula.Figure 4(a) shows the curve of power factor versus temperature for Cu x FeTe 2 (x=0.01,0.05, 0.1, 0.15, 0.2).From the figure, it can be observed that the power factor is the best at 400℃, and the largest power factor is obtained when Cu content is 0.05.However, the power factor decreases with the preparation temperature.The maximum power factor of 95.24 μW/mK 2 was obtained when the sample synthesis temperature was 823 K.Although the largest power factor was obtained at 823 K, according to the phase analysis of the sample, the optimum temperature for FeTe 2 is still 723 K. Figure 4(b) shows the calculated ZT values near room temperature for the samples.The figure shows that the maximum ZT value of 0.0275 is obtained when the doping amount of copper is 0.05 and the synthesis temperature of the sample is 673 K.In future research, optimizing its electrical properties can be achieved through element doping and changing the heating and holding time.

Conclusion
The effects of preparation and electrical properties of Cu x FeTe 2 (x=0.01,0.05, 0.1, 0.15, 0.2) were investigated by solid state reaction at different sintering temperatures and with different Cu contents.The following conclusions were obtained: The temperature range for preparing Cu x FeTe 2 by solid state reaction was 350~550℃, with a heating time of 40 minutes and holding time within 60 minutes; the optimum temperature and Cu content were 400℃ and 0.05, respectively.The minimum electrical resistivity of the sample was obtained at 1.74 mΩꞏcm at a temperature of 400℃ and Cu content of 0.05.The maximum Seebeck coefficient of 75.66 μVK -1 was obtained at a temperature of 350℃ and Cu content of 0.01.The experiment obtained the maximum power factor at 95.23 μW/mK 2 .universities in Coal Industry (Qian Jiao Ji

Figure 1 .Figure 2 .
Figure 1.The XRD spectra of Cu x FeTe 2 with different Cu contents.

Figure 3 .
Figure 3. Resistivity and seebeck coefficient versus temperature of Cu x FeTe 2.

Figure 4 .
Figure 4.The variation of power factor and ZT of Cu x FeTe 2 with synthesis temperature.