The study of thermoelectric energy and mechanical properties by modifying carbon fiber fabric

With the acceleration of global industrialisation, the scarcity and depletion of the world’s energy resources has become a problem that no country can ignore. It is a serious obstacle to the long-term stable development of society. Exploring and developing new energy sources has become the trend of global energy development. In this paper, hydrogen peroxide is used to treat carbon fibre materials. In addition, the thermoelectric reinforcing agent Bi2Te3 is doped into carbon fibre film materials by electrochemical deposition for the purpose of measuring the Seebeck coefficient and electrical conductivity. Technical abbreviations will be explained at the first use. By investigating how changes in the surface structure of carbon fibre fabrics and the addition of bismuth telluride affect their thermoelectric properties, this study establishes a framework for improving the thermoelectric capabilities of carbon fibre fabrics. Experimental results show that carbon fiber fabric treated with H2O2 have excellent thermoelectric and mechanical properties.


Introduction
There is a growing demand for energy.However, conventional fossil fuels are struggling to adapt to future energy needs due to resource constraints and regeneration challenges.Smart wearable materials are innovative and intelligent materials.They have the potential to support the development of strategic industries such as artificial intelligence, aerospace and neuroscience [1].A thermoelectric material is a new energy material that realises the interconversion between thermal and electrical energy through the movement of carriers within a solid.The temperature difference between the two ends of the material can generate a voltage and a temperature difference near the boundary of two different conductors (when a current flows).Flexible thermoelectric materials are able to convert thermal energy into electrical energy by exploiting the temperature difference between the human body and the environment [2].This efficient conversion is expected to meet the pressing need for constant power supply in smart wearable devices.However, it has proven difficult to create thermoelectric materials with optimal flexibility and thermoelectric properties at the same time.This has limited their practical application in the wearable industry [3].In the present work, carbon fibre materials are treated with H2O2 as a solvent and Bi2Te3 is electrochemically incorporated into the carbon fibre film as a thermoelectric enhancer, resulting in flexible thermoelectric materials that retain the exceptional mechanical properties of the CFF.Such materials have great potential to advance the fields of green energy and waste heat recovery applications.First, 0.3883g bismuth nitrate pentahydrate (Bi(NO3)3 ⋅ 5H2O, Aladdin, >99%), 0.1586g tellurium oxide (TeO2, Aladdin, >99%) and 7ml nitric acid (HNO3, Sinopharm, 65.0-68.0%)were dissolved in deionised water (H2O, Aladdin, ≤ 20ml) to prepare an aqueous electrolyte.The CFF was then impregnated with hydrogen peroxide (H2O2, Sinopharm, 20-45%) and covered with cling film for 12 h.The treated CFF was then ultrasonicated for 10 min with deionised water and dried.Following this, a platinum plate (2.5×2.5 cm2) served as the counter electrode, while the reference electrode was the Hg/Hg2Cl2/(saturated KCl) (SCE) electrode.The treated CFF served as the working electrode and was used to deposit Bi2Te3 by the constant potential deposition method (CHI 660E,Chenhua) with stirring.The material was purified by using deionised water to remove any residual reagents, and the purified film was then dried under vacuum at 60 °C for 9h.The experimental procedure consisted of four confident steps (Fig. 1).

Characterization of physical properties
The electrical conductivity (σ), carrier concentration (n) and mobility (μ) were measured by a Hall effect measurement system (CH-Magnetoelectricity Technology, CH-100) at room temperature and 500 mT magnetic field conditions (Table 1).The training scripts were implemented primarily in the cosmol framework.Create geometric models, add the CFF from the library, and electrodeposite bismuth telluride (Fig. 2).The mechanical properties of the model are tested after modelling (Fig. 3).

Synthesis, phases and microstructures
Carbon fibre is a high-strength, high-modulus fibre with a carbon content of more than 90%, which is produced from acrylic and viscose fibres by means of oxidative carbonisation at high temperatures.
Carbon fibre is much lighter than steel and other metallic materials, with a density of 1.5-2.0g/cm3.The tensile strength of carbon fibre cloth can reach more than 3000MPa, which is 5-10 times higher than the strength of steel and other metallic materials.Its Young's modulus is large and high stiffness, which can be used to produce high-strength and high-stiffness structural parts.Carbon fibre is the highest among existing structural materials when comparing two comprehensive indices of specific strength and specific modulus.The thermal expansion coefficient of carbon fibre is different from that of other fibres.It has anisotropic characteristics, parallel to the fibre direction is negative (-0.72×10-6 ~-0.90×10-6K-1), while perpendicular to the fibre direction is positive (32×10-6 ~22×10-6 K-1).In addition, carbon fibres have significant shape anisotropy.They are soft and can be processed into a wide variety of fabrics.At the same time, carbon fiber materials also have defects.Therefore, the production of carbon fiber fabrics generally needs to go through PAN fiber carbonization.The structure of PAN carbon fiber thus produced is a highly ordered woven structure consisting of highly stretched, axially oriented fibers and voids, so there are anomalies, uneven diameter size, surface contamination, internal impurities, foreign impurities, various cracks, voids, bubbles, etc. on PAN carbon fiber.At present, people often use the nitric oxidation to treat carbon fibre, in order to achieve the purpose of improving its surface activity, further increase the specific surface area of carbon fabric, rich in its surface pore structure, to achieve the technical effect of improving the electrochemical properties of carbon fabric, but the process.In addition, the Seebeck coefficient and carrier mobility of carbon fibre materials are very low.This leads to an extremely low power factor of the material itself.The adopted CFF had a thickness of approximately 310 μm and a low surface density of around 120 g m−2, yet exhibited excellent strength.These results demonstrate the impressive capabilities of the CFF and its potential for various applications.The original σ and S values were as low as 36 S cm−1 and −6 μV K−1.The surface impurities of the CFF were removed through treatment with H2O2, and hydrophilicity was improved to enhance the post-depositional bonding of bismuth telluride to the CFF.Bismuth telluride is one of the earliest and best-developed thermoelectric materials.It is a powder with good electrical conductivity but low thermal conductivity, and the most optimal material for the current temperature difference.The results of its tests on the resistive layer show that: The material has topological insulating properties, which allows the free flow of electrons on its surface and does not cause any energy consumption.In practice, bismuth telluride is very easy to produce and use, while the hexahedral layered structure of bismuth telluride compounds makes them easy to dope.This makes it relatively easy to tune the properties of the composites and improve the thermoelectric properties of the materials.It is well known that doping can be carried out in a cheap, fast and simple way using electrochemical deposition techniques.Ions in solution during deposition can form a film on the cathode by a reduction reaction.Microscopic electrodeposition behaviour occurs when the cathode and anode are externally powered.An aqueous or organic solvent containing both positively charged cations and negatively charged anions is usually the ideal solvent for the electrolyte.In cathodic deposition, diffusive and convective transport from other parts of the solution first adsorbs hydrated ions near the cathode.The hydrated skin is then stripped off at the solution/cathode interface and the ions, after charge transfer, are converted to adsorbed particles.They move across the surface to form granular adsorbed particles, which eventually form a film.Bi2Te3 was electrodeposited in the three-electrode system using the constant potential mode.Prior to deposition, a linear scanning voltammogram (LSV) at a scan rate of 0.01 V s-1 was used to determine the optimum deposition parameters.To avoid the negative effect of concentration polarisation, the electrolyte was agitated during the LSV and deposition.The results showed that under the influence of the electric field force, the electrochemical reaction started at about 0.1 V (initial potential) and the Bi3+ and HTeO2+ cations migrated towards the interfacial layer of the working electrode.After acquiring electrons on their surfaces, the Bi and Te atoms arrange themselves perpendicular to the substrate to form the Bi2Te3 layer in the sequence Te1-Bi-Te2-Bi-Te1(Fig.4).The electrochemical reaction can be represented as [4]: Bi 3+ + 3 HTeO 2+ + 18 e -+ 9H + → Bi2Te3 + 6 H2O (1) The preferential reduction of HTeO2+ to Te0 is due to factors such as ionic radius, ionic charge, and electronegativity.Te0 then reacts with Bi3+ to form Bi2Te3 [5][6][7].Therefore, the electrochemical reaction of Bi2Te3 can be divided into two steps [8]: As the deposition potential moves towards a negative value, bubbles form on the CE due to the reaction of oxygen precipitation.This disturbs the electric field between the CE and the WE, resulting in a jittery LSV curve.Since the tellurium content in BixTey varies inversely with the overpotential, the sample composition can be controlled by adjusting the deposition potential according to the difference in reduction capacity of different ions.To prevent the precipitation reaction of oxygen, the deposition potential should be in the range of 0.00 to 0.08 V at an ion concentration ratio of CBi3+ /CHteO2+=8 mM:10 mM and a total charge of 180C.At deposition potentials of 0.00 ~0.02 V, the surface of the CFF is completely covered by Bi2Te3 in the form of fine needles.As a result, the fibre diameter increases.Since higher overpotential implies higher current density, It not only leads to smaller particle size, but also to the formation of dendritic structures, which increases particle boundary density and decreases μ.In addition, too high of a deposition rate increases the flaw density of Bi2Te3, and the intensity of Bi2Te3 peaks in XRD spectra is lower at 0.00 V and 0.02 V potentials.The Bi2Te3 grains continue to grow and the grain boundary density decreases as the deposition potential increases from 0.04 V to 0.06 V.In particular, at 0.06 V, the Bi2Te3 uniformly fills the interior of the CFF and the morphology of the grains changes from The morphology changes from fine needle-like to dense rice-like, resulting in higher XRD peak intensity and reduced defects.Bi2Te3 grows wrapped around several carbon nanotubes at this potential, likely due to the high charge density at the fibre bundles.The deposited fibres grow to approximately ~30 μm and ~20 μm in diameter, respectively.Bi2Te3 can only be deposited at the warp and weft interwoven carbon nanotube nodes due to the low overpotential at a deposition potential of 0.08 V.This leads to the exposure of some of the carbon nanotubes.Nevertheless, the growth rate of the Bi2Te3 grains surpasses the nucleation rate, resulting in the formation of larger rugby-like grains.The film's surface roughens and numerous micropores form within it as a result of the large grain size.

Thermoelectric properties
The material's thermoelectric conversion efficiency primarily relies on the eigenvalue coefficient ZT, where ZT = S2κT/λ.S denotes the Seebeck coefficient, κ represents conductivity, T is the absolute temperature, λ signifies thermal conductivity, and S2κ stands for the power factor [10] (Fig. 5).The power factor The carrier of CFF is 7.79 × 1020 cm-3, which significantly surpasses that of Bi2Te3.When Bi2Te3 is deposited on CFF, the n of the heterofilm decreases by a factor of three compared to that of CFF due to various factors.Consequently, the carriers in the sample are dependent on the amount of Bi2Te3 present, so that the Te content increases from 56.7 to 61.8 at.% when the deposition potential is increased from 0.00 V to 0.08 V.The optimum stoichiometric ratio for Bi2Te3 can be achieved at a deposition potential of 0.04 V.In certain Te-rich samples, the Te content exceeds that of the CFF due to complex influencing factors.In particular, in Te-rich samples, some Te atoms occupy the Bi vacancy, generating electrons and forming the para-defect Te • Bi , thus reducing n.The atomic ratio of Bi is increased by reducing the deposition potential from 0.08 V to 0.06 V.However, excessive overpotential can lead to the formation of a significant number of point defects.Among these defects of Bi2Te3, V • • Te has the lowest formation energy (1.287 eV), resulting in predominantly V • • Te point defects.The simultaneous ionisation of electrons leads to a slight increase in n in the sample and an increase in the deposition potential from 0.06 to 0.00 V.In addition, the deposition potential of Bi2Te3 is increased by the simultaneous electron ionisation.The carbon nanotubes within the CFF do not produce an electrically interconnected structure, so there is a spatial separation between most of the adjoining fibres.However, this isolation confines electrons to specific carbon nanotubes.The low mobility of CFF is mainly due to its poor crystallinity.In order to improve its properties, Bi2Te3 is deposited on CFF, resulting in a core-shell structure of the fibres with the outer layer of Bi2Te3 wrapped around the core.The core-shell structures provide electrical connectivity, while the interfaces between the fibres increase electron transport speed.The zigzag electron transport further increases the value of μ, resulting in a significant improvement in the μ of Bi2Te3/CFF.Deposition of Bi2Te3/CFF at 0.08 V yields Bi2Te3 with the largest grain size, indicating a robust surface for the Bi2Te3 phase with most of the voids inside.Moreover, the insufficient coverage of Bi2Te3 on the fibre leads to exposed areas on the fibre surface.Enhancing the fibre coverage can significantly increase the value of μ to 4.6 cm2V-1s-1.Lowering the deposition potential from 0.08 V to 0.06 V reduces the grain size of Bi2Te3, decreases pore density, and results in a more compact structure.Bi2Te3 has the capability to interconnect optical fibres, significantly improving its mobility (μ).However, reducing the potential to between 0.06 and 0.00 V results in a decrease in grain size and an increase in point defect density, leading to a decrease in μ.The maximum value of μ for the sample is 7.4 cm2V-1s-1, occurring at 0.06 V.The conductivity (σ) is determined by the product of n and μ.After Bi2Te3 deposition, n decreases by three orders of magnitude, while μ increases by one to two orders of magnitude.To demonstrate the flexibility of the material, it is applied to a series of cylindrical surfaces of decreasing radius and the rate of change of σ with respect to the radius is measured [11].It is important to note that even rigid materials can become flexible when made thin enough.For example, crystalline silicon-based microelectronics can be made flexible when the thickness is approximately 0.1 mm.This is used in many portable electronics applications [12].As a result, it is important to take thickness into account when comparing the flexibility of materials with different thicknesses.In order to address this issue, Snyder et al. [13] have recently proposed an optimum value for the flexibility, fFOM = h/2r, where h is the thickness and r is the radius of curvature prior to fracture.The higher the value of fFOM, the better the flexibility.At a thickness of 1-2 μm, the fFOM of Bi2Te3 films deposited on polyimide by RF magnetron co-sputtering is only 1.5×10-4, resulting in a low critical thermoelectric load.The researchers assert that the flexibility of Bi2Te3 can be enhanced by incorporating conducting polymers using various methods, such as spin coating [14], drop coating [15], vapour phase polymerisation [16], vacuum filtration [17], etc [18][19].According to their findings, the resulting films have an fFOM in the range of 1.0 × 10-4 ~5.0 × 10-3, owing to the excellent intrinsic flexibility of conducting polymers due to the bendability and rotatability of the molecular chains.Wearable electronics are often subjected to tensile forces during use.If the materials used have low tensile strength, cracks can easily form during the stretching process, which can affect the function of the device.For this reason, high tensile strength is required for flexible thermoelectric materials.CFF fabrics feature a plain weave structure, with carbon fibre fabrics of 10 μm radius woven alternately up and down in both the warp and weft directions.The points of the weave are not tightly knotted, The weave points are not tightly knotted and the interaction between the warp and weft carbon fibres is solely by friction.When subjected to tensile stress in the direction of the weft, the carbon fibre cloth perpendicular to the direction of the stress remains almost unaffected, but the carbon fibre cloth parallel to the direction of the tensile stress experiences the greatest stress and breaks.However, it is easy for the carbon fibre cloth in the weft direction to break independently at different lengths, resulting in an irregular fracture, because it is not cross-linked.In contrast, the power factor of the composites obtained from the hydrogen peroxide experiments clearly exceeds that obtained from the mixed acid media treatment typically used.Comparing the reported data on the flexibility of TE films based on bismuth telluride, the fFOM value of the material obtained in this experiment is similar to that of the Bi2Te3/CF shown in the graph (Fig. 6).In addition, the 200 test was performed with a bending radius of 10 mm and the decrease in σ is less than 10%.H2O2 solution has oxidising properties and, when heated to a certain temperature, can cause oxidation of the weak boundary layer or graphite microcrystals present on the surface of the CFF, resulting in the formation of grooves on the surface.At the same time, the hydrogen peroxide solution is weakly acidic compared to the mixed acid, so the etching degree of the CFF is weaker, and the excellent mechanical properties of the CFF are well preserved.

Summary
The paper presents a cross-linked core-shell structure created by electrodepositing Bi2Te3 on a carbon fibre fabric.The cross-linked core-shell structure in the Bi2Te3/carbon fibre fabric significantly enhances the carrier mobility and greatly improves the final power factor.This is due to the lower carrier concentration compared to carbon fibre fabric and the higher Seebeck coefficient.Overall, the results demonstrate the effectiveness of the cross-linked core-shell structure in improving the performance of the Bi2Te3/carbon fibre fabric.Since the H2O2 treatment is less destructive to the braided carbon fibre structure, while the braided fibre structure can promote the stress transfer, the superior value of flexibility of the material itself remains stable.The core-shell structure and the induced enhanced knotting effect also ensured that the tensile strength of the material was not weakened.Compared with the widely used strong acid treatment of CFF, this paper proposes a new method: by treating raw CFF with a H2O2(20% -45%) and electrochemically depositing bismuth telluride, it is possible to enhance the thermoelectric and mechanical properties of the material.This approach provides a novel research idea for reducing the thermal conductivity of thermoelectric materials while increasing the Seebeck coefficient and electrical conductivity.It is expected that with the advancement of science and economic development, research on low thermal conductivity thermoelectric materials will emerge as a new research focus in the field of thermoelectric materials.However, there are certain aspects that require further study and investigation： (1) A systematic study of the structure of the CFF and bismuth telluride thermoelectric materials at different temperatures has been carried out with the aim of establishing a relationship between the

Figure 4 .
Figure 4. Crystal structure of Bi2Te3 and preferential orientation of the electrodeposited polycrystalline Bi2Te3 film on CF substrate [9]

Figure 5 .
Figure 5.The power factorThe carrier of CFF is 7.79 × 1020 cm-3, which significantly surpasses that of Bi2Te3.When Bi2Te3 is deposited on CFF, the n of the heterofilm decreases by a factor of three compared to that of CFF due to various factors.Consequently, the carriers in the sample are dependent on the amount of Bi2Te3 present, so that the Te content increases from 56.7 to 61.8 at.% when the deposition potential is increased from 0.00 V to 0.08 V.The optimum stoichiometric ratio for Bi2Te3 can be achieved at a

Figure 6 .
Figure 6.The figure of merit for flexibility reported[9] To demonstrate the flexibility of the material, it is applied to a series of cylindrical surfaces of decreasing radius and the rate of change of σ with respect to the radius is measured[11].It is important to note that even rigid materials can become flexible when made thin enough.For example, crystalline silicon-based microelectronics can be made flexible when the thickness is approximately 0.1 mm.This is used in many portable electronics applications[12].As a result, it is important to take thickness into account when comparing the flexibility of materials with different thicknesses.In order to address this issue, Snyder et al.[13] have recently proposed an optimum value for the flexibility,

Table 1 .
Seebeck and electrical conductivity of the CFF after treatment with different liquid medium