1 Introduction

Thermoelectric generator (TEG) is a device used to convert thermal energy into electric energy by using the Seebeck effect [1,2,3]. Many different inorganic semiconductor materials such as Antimony telluride (Sb2Te3), Bismuth selenide (Bi2Se3), Bismuth telluride (Bi2Te3), and Cobalt antimonide (CoSb3) are extensively researched and currently employed as either n-type or p-type materials to form the pn junction in the TEG [4,5,6,7]. Furthermore, Si-based materials, such as magnesium silicide (Mg2Si) and silicon-germanium (SiGe), have become increasingly well known for their use in the production of thermoelectric electricity [8, 9]. While bulk silicon exhibits favorable electrical characteristics, its relatively high thermal conductivity of 140 W/m.K presents a problem even at lower temperatures (300 K) [10, 11]. By utilizing Ge or Mg with Si, it can take advantage of the special combination of advantageous electrical characteristics and decreased thermal conductivity which enables effective thermoelectric power generation in high-temperature conditions that exceed 700 K [12, 13]. On the other hand, for moderate and low-temperature applications, among the various thermoelectric material and besides its less toxicity, Bi2Te3 and its alloys are commercialized as TEG elements because of its various advantages and unique properties such as (1) versatility of production by different methods, (2) High power factor and figure of merit, (3) easily tune the electric conductivity between the p-type and n-type through additives and dopants, (4) acceptable mechanical properties and wide range of operating temperatures [4, 14,15,16,17].

The Bi2Te3-based alloys are manufactured with different methods such as solid-state reaction, hydrothermal synthesis, and mechanical alloying. Nano-sized powder metallurgy manufacturing methods is a promising approach for the synthesis of highly performance Bi2Te3-based alloys due to the possibility of fine tuning the material properties, controlled addition of the additives in addition to it can be processed on a large scale for commercial production [4, 18, 19]. In the mechanical alloying manufacturing method, the elements powders (Bi, Te) are firstly mixed with the proper concentration and the alloy is formed in nano-sized particles by ball milling [20]. The nano-alloy is then sintered to the final product by various sintering techniques such as spark plasma sintering (SPS) [14], Hot isostatic pressing (HIP) [14, 15, 21], Hot pressing [22], and microwave sintering [14]. Sintering is a process used to compact and fuse thermoelectric materials into a solid mass. The electric, thermal, and thermoelectric properties of the alloy can be manipulating by mixing the additives during the powder ball milling step or via changing the sintering method or even alteration of the sintering parameters.

HIP is one of the most effective sintering methods used with Bi2Te3-based alloys, which involves the application of heat and high-pressure inert gas to make the powder evenly pressurized during the heating process and promote the compactness of the material. It is widely used due to its simplicity and cost-effective equipment compared to other techniques such as SPS and microwave sintering. Also, it can be used in mass production and suitable for large-scale and commercial production. It gives homogeneous properties because of large number of samples can be performed at the same time. However, HIP may result in low carrier mobility due to induced grain boundaries and low relative density which results in poor overall thermoelectric performance in terms of ZT.

Bismuth telluride (Bi2Te3) and its alloys are among the promising thermoelectric materials due to their high thermoelectric efficiency [1, 5, 14, 18, 22,23,24]. Many experiments have been done to improve the TE performance of Bi2Te3-based alloys sintered using HIP as it is a V2VI3 compound semiconductor material with narrow bandgap [25]. Lead (Pb) was used with Bi2Te3 to form Bi1.99Pb0.01Te3, which improves Seebeck coefficient and decreases thermal conductivity which resulted in ZT ~ 0.42 [26]. Antimony (Sb) was utilized in 75%Sb2Te3–25%Bi2Te3 to enhance the p-type ZT up to ~ 1.15 owing to high electrical conductivity and low thermal conductivity [21]. Selenium (Se)-doped bismuth telluride (Bi2Te2.7Se0.3) is n-type alloy which achieved a high ZT reaching a value of ~ 0.54 by reducing thermal conductivity and increasing electrical conductivity [4]. However, Pb, Sb, and Se are considered as toxic and not environmentally friendly elements [27,28,29].

Carbon materials play a crucial role in the development and advancement of thermoelectric materials. There are various types of carbon materials that are commonly used in thermoelectric applications. Some of the key carbon materials are Graphite [1, 2], Carbon Nanotubes [30], and Graphene [31, 32]. Carbon materials offer unique advantages such as thermal conductivity suppression, enhanced electrical conductivity, band structure tailoring, scalability, and cost-effectiveness. Overall, carbon materials’ distinct characteristics make them critical for the development and enhancement of thermoelectric [33].

In this regard, the current work focuses on enhancing the properties of Bi2Te3 thermoelectric material using nano-graphite particles as an additive. Graphite Nanoparticles GTNPs is an excellent additive for thermoelectric applications as it provides phonons blocking and the interlayer weak van der Waals interaction, which results in low thermal conductivity and a step-like shape in the electronic transition with mini-gaps [5, 30, 32, 34, 35]. A mixed mechanical milling process along with the cost-effective hot isostatic sintering process are implemented to prepare the nanocomposites. The effects of adding different amounts of GTNPs (0.2, 0.35, and 0.5 wt%) to the Bi2Te3 on the structural and electronic properties are investigated and comprehensively discussed. The thermoelectric performances of the prepared BT-xGTNPs samples are evaluated by measuring the key parameters such as Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) in the temperature range (300–600 K) and systematically discussed. Overall enhancements of the thermoelectric performances of Bi2Te3 are exhibited with adding the GTNPs and maximum figure of merit (ZT) was achieved for the sample containing 0.35wt.% GTNPs that reached ~ 0.8 at 540 K, which represents 51% enhancement compared to that of the Bi2Te3 control sample.

The utilization of a cost-effective mass production sintering process, coupled with the incorporation of relatively inexpensive carbon material (Graphite), contributes to the cost-efficiency of the production process. This approach allows for the large-scale production of thermoelectric materials at a lower cost, making them more economically viable for a variety of applications. Furthermore, the mixing process used to combine Bi2Te3 with GTNPs incorporates magnetic stirring and ultrasonication. This methodology is useful in preventing GTNPs agglomeration and reducing their tendency to act as nanoballs if the mixing process was done in the ball milling machine. The mixing procedure uses magnetic stirring and ultrasonication to achieve a homogeneous dispersion of GTNPs within the Bi2Te3 alloy, improving the homogeneity and integrity of the final composite material.

2 Experimental work

2.1 Materials and samples preparation

The materials used in this work were Bismuth powder (Bi, 38 μm, 99.9% purity), Trillium powder (Te, 53 μm, 99.9% purity) (Source KOJUNDO KOREA CO., LTD.), and Graphite Nanoparticles (GTNPs, 30 nm), which were used as an additive for Bi2Te3. The Mechanical alloying (MA) process was performed using a ball milling machine (RETSCH, Planetary ball mill, PM400) with ZrO2 jars. Firstly, the Bi and Te powders were loaded and mixed in the jars with a Bi:Te weight ratio of 2:3 at% in a controlled atmosphere chamber using Argon gas to prevent oxidation. The ball-to-powder weight ratio was 10:1 and the milling speed was 380 rpm for different intervals of time till get the Bi2Te3 homogeneous alloy. After that, the process of size refining of the powder was carried out by adding Ethanol into the jars with a ratio of 1:1 wt% and milling was continued for 24 h. Finally, the Bi2Te3 samples were dried in a vacuum furnace at 60 °C overnight to evaporate all used fluids [13, 20, 36]. The nanocomposite (BT-xGTNPs) samples were prepared by mixing the Bi2Te3 nano-powder with different wt% of GTNPs, where (x = 0.2, 0.35, and 0.5 wt%) in ethanol and the mixing was conducted using magnetic stirring and ultrasonication with alternative period of 6 h each to ensure a complete and good pervasion of GTNPs between the Bi2Te3 particles. The nanocomposites were collected and dried in the vacuum furnace overnight [37]. All prepared Bi2Te3 samples were compressed into pellets (13 mm in diameter, 3 mm thick, 2.5 g each) in a stainless-steel die under a load of 3 tons using a hydraulic hand press (SPECAC, Atlas 15T). The nanocomposite pellets were sintered using Hot Isostatic Pressing (HIP, American Isostatic Presses, HP630) under controlled Argon atmosphere at 400 °C and pressure of 27 × 103 Psi for 2.5 h. The pressure and temperature were controlled during the HIP according to a typical sintering cycle for all samples. A summary for the entire experimental process is schematically given in Fig. 1.

Fig. 1
figure 1

A schematic diagram for the experimental workflow

Full size image

2.2 Material characterization

Crystallographic analysis of the samples was executed using X-ray powder diffractometer (XRD, Shimadzu, XRD-6100) equipped with CuKα radiation source with wavelength λ = 1.5418 Å. The surface morphology, grain sizes, and microstructure of the samples were measured using scanning electron microscopy (SEM, JEOL JSM-5900) and transmission electron microscopy (TEM, JEOL JEM-2100 F). The electrical conductivity measurements were measured at different temperature using a four-point probe head onto samples connected to a Keithley (SMU 2400). The thermal conductivity measurement was carried out into a temperature-controlled furnace using the thermal analyser (Hot Disk TPS 2500s) equipped with a 7577 Kapton-coated sensor that serves as both a heat source and a thermometer. Hall coefficient was measured at RT using (Ecopia, HMS3000) with a magnetic field of 0.51 T and 1 mA electric current with Au electrodes.

2.3 Thermoelectric measurements

The Seebeck coefficient of the samples was measured through a temperature range (300–570 K) on homebuilt calibrated apparatus fabricated upon the principal idea of Seebeck coefficient. Figure 2 shows the designed and a photograph for the final fabricated device.

Fig. 2
figure 2

a Designed and b fabricated Seebeck Coefficient test rig

Full size image

The design was inspired from some recent literature [31, 38, 39]. The main parts of the apparatus were fabricated from stainless steel for most parts, whereas copper was chosen for the heating element. A ceramic housing tube was used as enclosure for the apparatus to prevent heat leakage and maintain a stable temperature difference during the measurements. The Bi2Te3 sample was placed between the copper block with the heating coil inside (hot side) and a non-heated copper block on the other side (cold side). A piece of graphite tape was mounted on both sides of the sample to cover the whole circular surface and make sure that the measurements were done through the entire cross section. A nanovoltameter (Keithley, Model 2182 A), PID temperature controller (Autonics, TZN4S-14 S), and a 4-channel data logger (HUATO, HE804) with two type-K thermocouples were employed to measure the potential difference (ΔV), set the hot side temperature (T), and measure the temperature difference across the sample (ΔT), respectively. The Seebeck coefficient (S) was calculated according to Eq. (1):

$$S=\varDelta V/\varDelta T.$$
(1)

3 Results and discussions

3.1 Structural properties

3.1.1 As-prepared BT-xGTNPs samples

As mentioned in the experimental section, two steps of ball milling process were carried out: the first was a dry milling and the second step was a wet milling using Ethanol. The purpose of the dry milling was alloying Bi and Te to prepare Bi2Te3 as the control thermoelectric material for this work. A sample was taken every 15 min from the mixture and examined using XRD to confirm the synthesis of Bi2Te3. All examined samples after more than 60 min of milling revealed no trace of Bi or Te. However, the dry ball milling was continued for 2 h to ensure alloying homogeneity. Figure 3 shows the XRD patterns obtained for Bi, Te, and Bi2Te3 after 2 h of milling. All the diffraction peaks of the Bi2Te3 sample after 2 h milling match the diffraction planes of the hexagonal Bi2Te3 phase [23]. This reflects the successful synthesis of Bi2Te3 using the ball milling process form its constituents metal powders. Also, it can be noticed that the width of the Bi2Te3 peaks is larger than those for Bi or Te, which demonstrates that the dry milling process has an assisted role in reducing the Bi2Te3 particle size.

Fig. 3
figure 3

XRD results for Bi, Te, and dry ball-milled Bi2Te3

Full size image

Wet milling process was carried out using Ethanol as an organic wetting medium with the ratio of 1:1 wt% to further reduce the particle size of Bi2Te3 and reach to the nano-scale size. The main benefit of using ethanol is to prevent the powder from sticking to the walls of the jars and the grinding beads, which reduces materials wasting. Moreover, the grinding beads were replaced with smaller ones of 3 mm diameter with the same weight ratio to enhance the collisions between the powder particles and the grinding beads. The particle sizes were measured using (SEM) after 24 h of wet milling, and the results before and after the wet milling process are shown in Fig. 4. The dry milling yielded Bi2Te3 of about 1 µm particle size and with a poor size distribution as revealed in the histogram in Fig. 4a. On the other side, the wet milling process yielded as small as 200 nm Bi2Te3 particles and adequate size distribution.

Fig. 4
figure 4

SEM image for Bi2Te3 after a dry milling and b wet milling

Full size image

As mentioned previously, nanocomposites from Bi2Te3 and different ratios of graphite nanoparticles (GTNPs) were prepared by wet mixing in ethanol followed by collecting and drying. All the (BT-xGTNPs) nanocomposite samples were examined using TEM microscopy to evaluate the successful pervasion of GTNPs around and in between the Bi2Te3 particles and the acquired micrographs are shown in Fig. 5. As seen in Fig. 5a, the average particle size of GTNPs was measured to be ~ 30 nm. It is much smaller than the particle size of Bi2Te3 (~ 200 nm) and will easily give an assisted role in filling the pores in the pristine Bi2Te3 powder. The micrographs of the BT-xGTNPs nanocomposites in Fig. 5b–d show the successful incorporation of the GTNPs (light particles) into the pores of the Bi2Te3 (dark particles). They also show an increase in the shielding effect of the Bi2Te3 with the increase in the loading of the GTNPs. In high loaded BT-xGTNPs samples (x = 0.35 and 0.50 wt%), most of the base powder particles were completely shielded with GTNPs.

Fig. 5
figure 5

TEM images for a GTNPs and b–d BT-xGTNPs

Full size image

3.1.2 HIP-sintered BT-xGTNPs samples

As shown in Fig. 6, the addition of the GTNPs resulted in an increase by 9% in the composite relative density (RD) values from 83% for pristine Bi2Te3 powder to 90% for BT-0.50GTNPs before sintering, while GTNPs worked as a filler for the pores between the Bi2Te3 particles. After the HIP sintering process, these values were further increased to 90% and 93%, respectively. This is a direct result of removing the humidity from the samples and reducing the size of air gaps by the action of the high Argon pressure and temperature applied during the HIP sintering process.

Fig. 6
figure 6

Relative density of BT-xGTNPs and grain size

Full size image

XRD measurements were performed for all samples after sintering and the acquired patterns are represented in Fig. 7. All XRD patterns revealed no oxidation occurred during the sintering process, which verifies a fruitful HIP sintering process.

Fig. 7
figure 7

XRD for BT-xGTNPs for (x = 0.00, 0.20, 0.35, and 0.50 wt.%)

Full size image

Scherrer’s equation is extensively used in materials science as it offers a facile way to calculate the size of crystallites within a sample. The analysis is based on the broadening of X-ray diffraction peaks occurred upon the changes in the crystallite’s lateral dimensions [40]. Hence, the average crystallite size (D) of all sintered BT-xGTNPs samples was estimated using Scherrer’s equation expressed in Eq. (2) [41]

$$D=\frac{K\lambda }{\beta \text{cos}{\theta }_{B}},$$
(2)

where β is the Full Width Half Maximum (FWHM) [42] of the peak, the CuKαI wavelength λ = 1.5418 Å, K is the shape factor (= 0.9), and θB is the Bragg angle.

To find the induced strain (ε) in the grain lattice, the Williamson–Hall (W–H) method [43] was employed. The FWHM of diffraction peaks and the grain size (D) are utilized in the commonly used W–H method in materials research to determine the strain value [27, 44, 45] according to Eq. (3).

$$\beta \text{cos}\theta =4\varepsilon \text{sin}\theta + \frac{K\lambda }{D}.$$
(3)

Table 1 summarizes the calculated crystallite size and lattice-induced strain obtained from the XRD analysis for all BT-xGTNPs. As seen from the table, D is decreasing with increasing the ratio of the GTNPs, which is suggested to improve the thermoelectric properties due to lower thermal conductivity via phonon scattering mechanism. Conversely, the induced strain (ε) is nearly equal for all samples as they are subjected to the same sintering process.

Table 1 Crystallite size and lattice strain obtained from the XRD analysis
Full size table

Figure 8 shows the SEM micrographs acquired for the four prepared samples after HIP Sintering process. It is clear that the alternative stirring and sonication processes played an essential part in GTNPs uniform distribution and preventing particles agglomeration within the nanocomposite, which is supposed to tune the microstructure of the samples after HIP sintering by producing denser and homogenate structures. As seen from Fig. 8a, the HIP sintering for the pristine Bi2Te3 powder resulted in large and non-homogeneous grains. On the other hand, the micrographs of GTNPs containing samples in Fig. 8b–d reveal finer and more homogeneous grains. This ensures the success of our proposed approach of using GTNPs to produce Bi2Te3 with fine and close packed grains for thermoelectric material applications using ball milling and HIP techniques.

Also, the amount of the GTNPs had a significant impact on grain refinement of the nanocomposite. Notably, the sample with the highest GTNPs content exhibited the finest structure. In contrast, the pristine Bi2Te3 displayed a mixture of coarse and fine grains as indicated by the yellow arrows in Fig. 8a. However, with the introduction of even a small amount of graphite, the Bi2Te3 grain size noticeably decreased, and the homogeneity of the structure is furtherly improved, correlating with an increase in graphite content. The presence of GTNPs additionally promotes the formation of nucleation sites, leading to the creation of numerous small Bi2Te3 grains within the alloy. The presence of graphite also acts as a filler material, effectively filling gaps and pores in the pristine Bi2Te3 structure which is highly recommended to improve the electrical conductivity of the Bi2Te3-based nanocomposite and enhance its overall thermoelectric properties.

Fig. 8
figure 8

SEM images of sintered samples a pristine Bi2Te3, b x = 0.20, c x = 0.35, and d x = 0.50

Full size image

3.2 Thermoelectric properties

The measurements of Seebeck coefficient (S), electrical, and thermal conductivity were done in temperature range between 300 and 570 K with incremental step of 30 K and 5 K/min as a heating rate.

The Seebeck coefficients (S) were measured using the fabricated setup described before. As can be seen from Fig. 9a, all measured S are negative values for all the BT-xGTNPs samples throughout the tested temperature range. These firmly indicate an n-type thermoelectric semiconductors with electrons as majority charge carriers, which was also confirmed by the Hall measurements in Fig. 9c. These results also prove that GTNPs did not change the n-type nature of the pristine Bi2Te3. All BT-xGTNPs samples show an identical behavior of S values, which increase with temperature until reaching its peak value, which is almost in the range of 510–540 K. However, the pristine Bi2Te3 showed the lowest S among all the tested samples and an overall S enhancement was observed with the increase in the GTNPs content in the samples. For the pristine Bi2Te3 sample, the recorded S value at RT was 90 µV/K and reached 125 µV/K at 540 K with an increase of about 39%. The maximum recorded S values were for samples with x = 0.50wt.% (BT-0.5GTNPs), which were 120 µV/K at RT and 154 µV/K at 540 K. Hence, an improvement of 33% in the S value at the RT and 23.2% at 540 K of the BT-0.5GTNPs compared to the pristine Bi2Te3.

Fig. 9
figure 9

a Seebeck coefficient, b Electrical conductivity, and c carrier concentration and mobility of all BT-xGTNPs samples

Full size image

Figure 9b shows the measured values for electrical conductivity (σ) in the same temperature range for all BT-xGTNPs samples. The decline in σ values for all samples with rising measurement temperature indicates a metal-like conduction behavior of the Bi2Te3 semiconductor and matches well with the previous reports [1, 44, 46]. The addition of GTNPs to the Bi2Te3 has a significant effect on electrical conductivity due to the step-like shape in the electronic transition with mini-gaps [35]. The presence of 0.2wt.% and 0.35wt.% GTNPs improved the measured σ at RT from 7.16 × 104 S·m−1 of the pristine Bi2Te3 to 7.46 × 104 and 8.2 × 104 S·m−1, respectively. The enhancements in σ are mainly attributed to reducing the charge carriers scattering process at the Bi2Te3 grain boundaries and improving the charge mobility because of the GTNPs as discussed earlier in Fig. 8. Nevertheless, increasing the GTNPs to 0.5wt.% decreased the σ of the Bi2Te3 to 6.48 × 104 S·m−1 at RT and to a very low value of 5 × 104 S·m−1 at 570 K, which is not beneficial for thermoelectric applications.

To get a deeper understanding behind the reasons these observations, Hall effect measurements are performed for all BT-xGTNPs samples and the results are summarized in Fig. 9c and Table 2. An increase in the carrier concentration is obviously seen with the increase in the GTNPs ratio in the samples. The pristine Bi2Te3 yielded electron concentration of 9.62 × 1017 cm−3 and increased to a maximum value of 3.98 × 1018 cm−3 for the BT-0.35GTNPs sample. Furthermore, the mobility at RT increased from 1122 cm2·(V·s)−1 for base pristine sample to 1472 cm2·(V·s)−1 for 0.35wt.% GTNPs before decreasing again to 61.5 cm2·(V·s)−1 for 0.50wt.% GTNPs, which could explain the enhancements in the S and σ values obtained with increasing GTNPs ratio. Increasing temperature of the sample during Seebeck measurements will promote more free charge carriers, which are electrons in this case, by the thermal excitation process especially is that Bi2Te3 semiconductor is a narrow bandgap with Eg ≈ 0.2 eV. However, the further increase in the electron concentration for the BT-0.5GTNPs sample along with the increase in the electron thermalization and the increase in the crystal vibrations by the high temperature will increase the collision probability between the fast moving electrons and the vibrating lattice points [19, 47]. This will cause a reduction in the electrons’ mobility and the electrical conductivity of the sample BT-0.5GTNPs as observed in Fig. 9b and c. Also, the HIP sintering process with low GTNPs concentrations has promoted the grain growth until 0.35wt.% GTNPs. However, the grain growth was inhibited with extra loaded samples (0.50wt.% GTNPs) as shown in Fig. 8d.

Table 2 Hall transport coefficient
Full size table

The thermal conductivity of the materials consists of two components (k = ke + kl), the electronic part (ke) and the lattice part (kl) caused by the charge carriers and by the phonons vibrations, respectively. The electronic part of thermal conductivity was estimated in accordance with Wiedemann–Franz formula, ke = LTσ, where L is the Lorentz number (L = 2.45 × 10−8 W·Ω·K−2). Then, by subtracting, the lattice thermal conductivity can be determined and the results are plotted in Fig. 10.

Fig. 10
figure 10

Thermal conductivity a electronic, b lattice, and c total

Full size image

All GTNPs containing samples almost have the same values of k at RT and attained total k values below that of the base pristine Bi2Te3 sample within the full measurement range. At the RT, the k sharply decreased from 1.18 W·m−1·K−1 for the pristine Bi2Te3 to 0.98, 1, and 1.09 W·m−1·K−1 for x = 0.20, 0.35, and 0.50 wt%, respectively. All these k values were increased with temperature rising to reach the highest values of 1.4, 1.15, 1.22, and 1.28 W·m−1·K−1 for x = 0.00, 0.20, 0.35, and 0.50 wt%, respectively, at 570 K. The low k for samples containing GTNPs is basically because of the increase in phonon scattering due to the presence of nano-sized GTNPs at the Bi2Te3 grain boundaries as well as the smaller Bi2Te3 grains grown during the sintering process as discussed before.

The power factor (PF) of all tested BT-xGTNPs samples was calculated according to Eq. (4) within the temperature range and the results are graphically depicted in Fig. 11a.

$${PF=S}^{2}\sigma$$
(4)
Fig. 11
figure 11

Calculated values of a PF and b ZT for all BT-xGTNPs samples

Full size image

Interestingly, the sample with x = 0.35wt.% GTNPs retained the maximum PF value within the full tested temperature range despite it does not show the highest Seebeck coefficient values. This is mainly attributed to its obtained highest electric conductivity. Although, the BT-0.5GTNPs sample succeeded in achieving the highest S value, but herein σ dominates in the PF value as the difference in S between the BT-0.5GTNPs and the BT-0.35GTNPs is quite small whereas the σ is largely differ.

The overall efficiency of the thermoelectric material is expressed in terms of the dimensionless figure of merit (ZT) which is simply calculated according to Eq. (5):

$$ZT=\left(\sigma {S}^{2}/k\right) T=(PF/k) T.$$
(5)

The ZT for all tested BT-xGTNPs samples as a function of temperature were calculated and plotted in Fig. 11b. From the figure and in accordance with the previously discussed S, σ, k measurements, it was found that the BT-0.35GTNPs sample achieved highest ZT at all temperatures and reached its maximum ZT value of about 0.8 at 540 K. It is much higher than the 0.36 of the pristine Bi2Te3 and also higher than the 0.57 obtained for BT-0.5GTNPs at the same temperature. It is important to note that the ZT values shown here are some of the highest ZT values for n-type Bi2Te3-based alloys that have been recorded. The TE characteristics, including those of various n/p-type Bi2Te3-based materials, have been profiled in Table 3 for comparison purposes and plotted in Figs. 12 and 13.

Fig. 12
figure 12

Comparison between this study and other published studies for a Seebeck coefficient, b Electrical conductivity, and c PF

Full size image
Fig. 13
figure 13

Comparison between this study and other published studies for a Total thermal conductivity and b ZT

Full size image
Table 3 Seebeck coefficient, electrical, thermal conductivities, and figure of merit for some reported Bi2Te3-based alloys
Full size table

Figures 12 and 13 present a comparative analysis between the thermoelectric properties obtained in our study and those reported in other published studies. Figure 12 specifically illustrates the variations in the Seebeck coefficient, electrical conductivity, and power factor as a function of temperature. Notably, a consistent trend is observed across the majority of the reported results, indicating a reduction in these properties at temperatures exceeding 450 K. In contrast, our experimental data exhibit a distinctive characteristic, with the peak values recorded at a higher temperature of 550 K. Furthermore, the power factor, which combines the Seebeck coefficient and electrical conductivity, serves as a metric for evaluating the overall thermoelectric performance. The majority of the cited studies experience a decline in the power factor beyond 450 K and some are even at lower temperatures, indicating a diminishing thermoelectric efficiency. In contrast, our experimental findings showcase a unique peak in the power factor at 550 K, suggesting an improved performance compared to the literature results. In conjunction with these analysis, Fig. 13b presents a comparison of the overall figure of merit (ZT) across the various studies, further confirming the observed reduction in thermoelectric performance at the higher temperature of 550 K.

Hence, relying on the large driving force provided by the nano-scale grain size, a dense, coherent, and homogeneous BT-xGTNPs composite was successfully obtained via an effective milling and consolidation approach. The effect of milling parameters and graphite nanoparticles content on the microstructure and TE properties of the synthesized BT-xGTNPs n-type composite are investigated and presented in this report. Upon the investigated structures, the properties depend on the concentration of GTNPs. The low GTNPs concentrations (0.20, 0.35 wt%) can promote restricted grain growth by acting as a nucleation site for the growth of Bi2Te3 grains, resulting in the formation of small gaps and well-distributed GTNPs in the matrix, as schematically demonstrated in Fig. 14. The obtained composites have a relative density greater than 90%, but the milling process generates a high number of defects and/or voids, resulting in an excess of free volume and non-equilibrium grain boundaries, which are substituted by the GTNPs content. The resultant structure enhances the electrical conductivity of the composite by limiting the number of grain boundaries that can facilitate the flow of electrons and build conductive networks through the GTNPs points. In addition, the tailored structure reduces thermal conductivity through the high frequency and short-wavelength phonons dispersion. However, with the high GTNPs concentrations inhibited the grain growth by blocking the movement of Bi2Te3 atoms, leading to the formation of fine and more uniform grains with GTNPs inclusions as proved by the SEM micrograph in Fig. 8d. Unfortunately, the formed structure impedes the electrons paths and increases the electrical resistance without any sacrifice of thermal reduction. Hence, 0.35 wt% GTNPs is decided to be the optimal content in the BT-xGTNPs composite for comprising the electronic properties of Bi2Te3 to obtain the highest ZT (~ 0.8 at 540 K). These findings provide a guideline for the design of consolidation techniques that reduce grain growth and generate more stable conductive materials. It would also be important in practical applications, as thermoelectric generators based on ball-milled n-type Bi2Te3 will be subjected to severe and prolonged thermal excursions.

Fig. 14
figure 14

Schematic diagrams depict the mechanisms of electron and phonon movement through samples with different GTNPs content

Full size image

To provide a more comprehensive understanding of the impact of GTNPs on the physical properties, we can elucidate the electron and phonon dynamics, as depicted in Fig. 14. As can be seen by looking at the length of the blue arrow, the pristine sample has grain boundaries that prevent electrons from moving freely through the material. Furthermore, scattering occurs for phonons, as indicated by the green arrow’s orientation.

However, with the incorporation of GTNPs, notable changes occur. Firstly, the electron motion is enhanced when passing over the graphite particle due to its superior electrical conductivity. Consequently, the electron speed increases, facilitating improved charge transport throughout the material. This enhanced electron mobility can lead to favorable electrical properties, such as lower resistivity and increased conductivity. On the other hand, the addition of GTNPs also introduces increased scattering for phonons. Phonons, which represent lattice vibrations and heat transfer carriers, experience enhanced scattering due to the presence of GTNPs. This scattering effect can impede the phonon motion, leading to altered thermal conductivity properties. As a result, the thermal conductivity of the material may be reduced due to the increased scattering of phonons by the GTNPs.

4 Conclusions

Bi2Te3-based alloys are widely used TE materials which easily tune its properties with a tiny amount with one of many different additives. This study explored how adding GTNPs to Bi2Te3 nano-powder promotes its thermoelectric properties. Bi2Te3 nano-powder was ball-milled, and was given in three ratios of GTNPs (x = 0.2, 0.35, and 0.5 wt%). A relatively high 93% RD was achieved using HIP sintering method with 0.5 wt% GTNPs compared to 83% for unsintered pristine Bi2Te3 sample, which represent 12% increase in RD. The electron microscopy studies revealed lower grain sizes, whereas the crystallographic measurements revealed a minor decrease in the crystallinity of the BT-xGTNPs samples which was accredited to the presence of the GTNPs. Extensive detailed study for electrical, thermal, and thermoelectric measurements for all samples was carried out in the temperature range from RT to 570 K. The measurements showed improvements in the power factor and ZT for x = 0.35 wt% GTNPs at 540 K compared to pure Bi2Te3, despite the fact that the sample with 0.5 wt% has the highest Seebeck coefficient of 154 V/T at 540 K. Also, the Bi2Te3 with 0.35 wt% GTNPs showed the best electrical conductivity of 8.2 × 104 S/m and the lowest thermal conductivity of 1 W/m.K, These remarkable thermoelectric results achieved for the BT-xGTNPs were attributed to the better grain size and the improved electrical and thermal conductivities generated by the precise addition of GTNPs.