Description:TECHNICAL FIELD
Present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to Bi0.5Sb1.5Te3 (BST). Further, embodiments of the disclosure disclose a method for improving figure of merit (zT) of Bi0.5Sb1.5Te3 (BST) by improving power factor and reducing thermal conductivity of Bi0.5Sb1.5Te3 (BST).
BACKGROUND OF THE DISCLOSURE
Thermoelectric materials (TE) are widely used in an array of industries. These materials are primarily used because of their characteristics to convert heat energy to electrical energy. Such materials works based on the Seebeck effect and Peltier effect. These materials are widely used to utilize waste heat energy, thus making a particular process more productive and energy efficient. The efficiency of energy conversion of such materials from heat energy to electrical energy is given by a quantity known as figure of merit (zT). This figure of merit (zT) is related to physical parameters such as Seebeck coefficient S, thermal conductivity k, electrical conductivity s and absolute temperature T and is given by the expression:
zT =S2?T/ k
where, k is sum of lattice thermal conductivity-?lat and electrical thermal conductivity-?el.
Large Seebeck co-efficient S, with high electrical conductivity ?el and reduced thermal conductivity ?lat are the ideal properties of good thermoelectric (TE) material for large scale power generation. The materials such as Bismuth telluride (Bi2Te3) alloy, Lead Telluride (PbTe) alloy, Silicon-Germanium alloy and Bi0.5Sb1.5 Te3 (BST) are widely chosen thermoelectric materials. Bi0.5Sb1.5Te3 (BST) is popular and widely used as thermoelectric materials (TE), due to its inherent characteristics such as high energy conversion efficiency at ambient temperature.
Further, interdependency of the aforementioned physical parameters has limited the thermoelectric performance (zT) in a single material. Hence, experimentally an effective way of improving figure of merit zT is only possible either by tuning the power factor (sS2) or by minimizing the thermal conductivity (?lat+?el). In this view, efforts have been employed to improve power factor (sS2) by modulating electronic structure by band convergence and creating resonant levels. The power factor sS2 can also be improved by tuning the charge carrier concentration (n) and effective mass of the dopant (m*). Further, the ?lat can be optimized by tunning intrinsic phonon transportation via alloying, nanostructuring, mesostructuring, all-length scale hierarchical structuring, lattice anharmonicity, and by inducing defects in materials.
Since discovery of tetradymites, a class of narrow band gap semiconductors with high thermoelectric performance, Bi2Te3, and Sb2Te3 has low ?L and feasible for the tunable electronic structure. For example, the power factor sS2 of ~58 ??W cm-1 K-2 was obtained at 300 K along a zone melting direction in Bi2Te3. Addition of selenium (Se) resulted in figure of merit zT ~1.1 at 350 K in the case of Bi2(Te0.95Se0.05)3 and figure of merit zT ~0.8 at 560 K for Bi2(Te0.5Se0.5)3.

Conventionally, such results have shown high figures of merit zT in Bi2Te3 -Sb2Te3 based alloys, and a decrease in figure of merit zT at high temperature. This was mainly due to thermally activated bipolar contribution (?bi). However, several breakthrough strategies, have been applied to surpass the thermal activated bipolar effect. Some of them include, improving charge carrier concentration thereby decreasing minor carrier density which can suppress the negative effect on thermoelectric properties of the Bi0.5Sb1.5Te3 (BST). Further strategies include introducing copper (Cu) to the antimony (Sb) sites of Bi0.5Sb1.5Te3 (BST) for improved carrier concentration and thereby a peak zT of ~1.4 at 430 K was achieved. The state-of-the-art average zT of ~1.21 is observed between 300-500 K upon Ca doped in Bi0.5Sb1.5Te3 (BST).
Several prior arts exist to improve figure of merit zT of thermoelectric materials (TE) as discussed below:
Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X., discloses aspects on High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science, 2008, 320, 634-638, observed a strong anisotropic thermoelectric property (figure of merit zT ~1.4 at 373 K) in ball-milled Bi0.5Sb1.5Te3 (BST) by hot pressing the nanostructured samples. Kim et al., showed peak zT ~1.86 at 320 K with ?lat of ~0.3 W.m-1K-1 in liquid phase compaction of Bi0.5Sb1.5Te3 (BST) owing to the induced dense dislocations which scatter phonons efficiently.

Kim, S. I.; Lee, K. H.; Mun, H. A.; Kim, H. S.; Hwang, S. W.; Roh, J. W.; Yang, D. J.; Shin, W. H.; Li, X. S.; Lee, Y. H., discloses aspects on Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics. Science 2015, 348, 109-114, reported an average figure of merit zT of 1.1 (300-573 K) by incorporating manganese (Mn) to the antimony (Sb) site of Bi0.5Sb1.5Te3 (BST).
Qin, H.; Liu, Y.; Zhang, Z.; Wang, Y.; Cao, J.; Cai, W.; Zhang, Q.; Sui, J., discloses aspects on improved thermoelectric performance of p-type Bi0.5Sb1.5Te3 through Mn doping at elevated temperature. Materials Today Physics, 2018, 6, 31-37, presented high zTave ~1.47 in p-type Bi0.396Sb1.525In0.075Cu0.004Te3 (300-500 K) by creating deep levels that regulated charge carriers.
Hu, L.; Meng, F.; Zhou, Y.; Li, J.; Benton, A.; Li, J.; Liu, F.; Zhang, C.; Xie, H.; He, J., discloses aspects on leveraging Deep Levels in Narrow Bandgap Bi0.5Sb1.5Te3 for Record-High zTave Near Room Temperature. Advanced Functional Materials, 2020, 30, 2005202, obtained zT ~1.20 at 303 and 1.38 at 383 K in 0.44 vol.%??-Si3N4 +BST composite.
Further, Dou, Y.; Qin, X.; Li, D.; Li, Y.; Xin, H.; Zhang, J.; Liu, Y.; Song, C.; Wang, L., discloses aspects on enhanced thermoelectric performance of BiSbTe-based composites incorporated with amorphous Si3N4 nanoparticles. RSC Advances, 2015, 5, 34251-34256, observed ~ zT of 1.32 at 320 K by adding 0.75 wt% ZnAlO secondary phase to the BST matrix.
Further, Zhang, T.; Zhang, Q.; Jiang, J.; Xiong, Z.; Chen, J.; Zhang, Y.; Li, W.; Xu, G., discloses aspects on enhanced thermoelectric performance in p-type BiSbTe bulk alloy with nanoinclusion of ZnAlO. Applied Physics Letters, 2011, 98, 022104, has reported a zT of ~1.56 at 400 K in 0.2 vol% PbSe alloyed Bi0.5Sb1.5Te3.
Jiang, Z.; Ming, H.; Qin, X.; Feng, D.; Zhang, J.; Song, C.; Li, D.; Xin, H.; Li, J.; He, J., discloses aspects on achieving High Thermoelectric Performance in p-Type BST/PbSe Nanocomposites through the Scattering Engineering Strategy. ACS Applied Materials & Interfaces, 2020, 12, 46181-46189, obtained ~1.35 zT at 350 K in 2 wt% Zn4Sb3 incorporated BST with a conversion efficiency of 8.74 % ?T 200 K.
However, most of the prior arts as described above either exhibits low figure of merit or require complex process to improve the figure of merit.
The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.

SUMMARY OF THE DISCLOSURE
One or more shortcomings of the prior art are overcome by method as claimed and additional advantages are provided through the method as described in the present disclosure.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

In one non-limiting embodiment of the present disclosure, a method for improving figure of merit (zT) of Bi0.5Sb1.5Te3 (BST) is disclosed. The method comprises synthesis of phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) and phase pure polycrystalline GeSe. Synthesized Bi0.5Sb1.5Te3 (BST) and GeSe are subjected crushing to form granules. These Bi0.5Sb1.5Te3 (BST) and GeSe granules are further subjected to grinding to obtain phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) fine powder and phase pure polycrystalline GeSe fine powder. These fine powders are then subjected to mixing to obtain the Bi0.5Sb1.5Te3 (BST)-GeSe fine powder mixture. This mixture is now subjected to further grinding in a ball mill for 12 mins at a grinding speed of 450 rpm, for 12 cycles with pauses for 10 mins followed by reverse on for the total number of 12 cycles, to obtain Bi0.5Sb1.5Te3(BST)-GeSe nanocomposite. The Bi 0.5 Sb1.5Te3(BST)-GeSe nanocomposite is further subjected to sintering at a temperature of 370°C for 15 mins. An axial pressure of 3.2 KN is applied under vacuum pressure of 10-5 torr during sintering, to obtain Bi0.5Sb1.5Te3 (BST)-GeSe composite with improved figure of merit (zT).
In an embodiment, the method of synthesis of phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) involves mixing stochiometric amount of high-quality Bismuth (Bi), Antimony (Sb), and Tellurium (Te) in a quartz tube to obtain a stochiometric mixture. The quartz tube with the stochiometric mixture is exposed to vacuum pressure of 10-5 torr in a sealed quartz ampule. The quartz ampule is then subjected to heating in a first instance, to a temperature of 723K for a period of 12 hrs further subjected to heating in a second instance, to a temperature of 1123 K for a period of 5 hrs. The quartz ampule is then held at the same temperature of 1123 K for 10 hrs. This is further followed by slow cooling of the quartz ampule to room temperature for a period of 15 hrs to obtain Bi0.5Sb1.5Te3 (BST) ingots.
In an embodiment, the method of synthesis of phase pure polycrystalline GeSe involves mixing stochiometric amount of high-quality Germanium (Ge) and Selenium (Se) in a quartz tube to obtain a stochiometric mixture. The quartz tube with the stochiometric mixture is exposed to vacuum pressure of 10-5 torr in a sealed quartz ampule. The quartz ampule is then subjected to heating in a first instance, to a temperature of 723K for a period of 12 hrs, and further subjected to heating in a second instance, to a temperature of 1123 K for a period of 7 hrs. The quartz ampule is then held at the same temperature of 1123 K for 10 hrs. This is further followed by slow cooling of the quartz ampule to room temperature for a period of 18 h to obtain GeSe ingots.
In an embodiment, the crushing and grinding of synthesized Bi0.5Sb1.5Te3 (BST) and GeSe is carried out in a mortar and pestle in an inert atmosphere.
In an embodiment, a predefined wt% in range of 0.25, 0.5 and 0.75 of crushed phase pure polycrystalline GeSe is mixed with pristine crushed phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) to obtain Bi0.5Sb1.5Te3 (BST)- GeSe mixture.
In an embodiment, the sintering is performed in a spark plasma sintering (SPS) system for a period of 30 mins.
In an embodiment, the sintering involves dwelling period of 3 to 5 min under constant pressure of 3.1KN under vacuum pressure of 10-5 torr.

In an embodiment, the relative density of the sintered Bi0.5Sb1.5Te3 (BST)-GeSe composite is about 96% to 99% of Bi0.5Sb1.5Te3.
In an embodiment, a Bi0.5Sb1.5Te3 (BST)-GeSe composite is manufactured.

In an embodiment, the Bi0.5Sb1.5Te3 (BST)-GeSe composite shows an increased p-Type carrier concentration as wt% of GeSe is increased.
In an embodiment, the figure of merit (zT) is at ~1.65 for the Bi0.5Sb1.5Te3 (BST)-GeSe composite at temperature of 323 K for 0.5 wt% of phase pure polycrystalline GeSe in the Bi0.5Sb1.5Te3 (BST).
In an embodiment, the Bi0.5Sb1.5Te3 (BST)-GeSe composite comprises composition of 0.005g of GeSe per 1g of Bi0.5Sb1.5Te3(BST).
In an embodiment, the Bi0.5Sb1.5Te3 (BST)-GeSe composite shows an increase in power factor (sS2) by 75.7 % from 26 ?W cm-1K-2 to 45.67 ?W cm-1K-2 at 323 K for 0.5 wt% GeSe alloyed with Bi0.5Sb1.5Te3 (BST) compared to that of pristine Bi0.5Sb1.5Te3 (BST).
It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Figure 1 is a flow chart disclosing method steps for increasing figure of merit (zT) of Bi0.5Sb1.5Te3 (BST), in accordance with an embodiment of the present disclosure.
Figure.2 illustrates graphical illustration of room temperature PXRD patterns of a)as prepared BST, as prepared GeSe,ball-milled BST and b) BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) nanocomposites., in accordance with an embodiment of the present disclosure.
Figures.3a and 3b illustrates graphical illustration of room temperature XRD patterns of BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites along (a) parallel and (b) perpendicular to the SPS pressing directions in accordance with an embodiment of the present disclosure.

Figure. 4 illustrates graphical illustration of temperature dependent electrical conductivity in heating-cooling cycles of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites along (a) parallel and (b) perpendicular to the SPS pressing directions in accordance with an embodiment of the present disclosure.
Figure. 5 illustrates graphical illustration of temperature dependent Seebeck coefficient in heating-cooling cycles of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites along (a) parallel and (b) perpendicular to the SPS pressing directions in accordance with an embodiment of the present disclosure.
Figure. 6 illustrates graphical illustration of temperature dependent power factor (sS2) of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites measured along (a) parallel (||) and (b) perpendicular (?) to the SPS pressing directions in accordance with an embodiment of the present disclosure.
Figure 7 illustrates graphical illustration of the temperature dependent total thermal conductivity (?) of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites measured along (a) parallel (||) and (b) perpendicular (?) to the SPS pressing directions in accordance with an embodiment of the present disclosure.

Figure 8 illustrates graphical illustration of the temperature dependent combination of lattice thermal conductivity (?lat) and bipolar (?bi) thermal conductivity of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites measured along (a) parallel (||) and (b) perpendicular (?) to the SPS pressing directions in accordance with an embodiment of the present disclosure.
Figure 9 illustrates graphical illustration of the temperature dependent thermoelectric figure of merit (zT) of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites measured along (a) parallel (||) and (b) perpendicular (?) to the SPS pressing directions in accordance with an embodiment of the present disclosure.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristics of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusions, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.
Embodiments of the present disclosure discloses a method for improving figure of merit (zT) of Bi0.5Sb1.5Te3 (BST). Thermoelectric (TE) materials are the materials that shows the capability to convert heat energy into electric energy. Thus, these materials can be utilised to improve the fuel efficiency and used as an alternative source of energy in many applications by collecting waste heat, and therefore finding alternate energy solutions. For constructing high-performance thermoelectric (TE) devices for an array of applications, certain TE materials have been targeted such as Bismuth telluride (Bi2Te3) alloy, Lead Telluride (PbTe) alloy, Silicon-Germanium alloy and Bi0.5Sb1.5Te3 (BST) etc. The performance of these TE materials is measured by a physical quantity called as figure of merit zT, which relates to the physical parameters by zT =S2?T/(?lat+?el), S-thermopower, T-the absolute temperature, ?-electrical conductivity, ?lat-lattice thermal conductivity and ?el is the electrical thermal conductivity. Development of highly efficient thermoelectric materials (TE) have gained significant attention in the conversion of waste heat to electricity. Therefore, obtaining high-performance TE materials involves maximizing its figure of merit (zT). The figure of merit (zT) may be maximized by improving power factor and reducing thermal conductivity. Based on the above, Bi0.5Sb1.5Te3 (BST) may be considered as the best suitable material or choice for near room temperature thermoelectric (TE) power generation due to its intrinsic bonding and favourable electronic structure.
Referring now to figure 1, which illustrates a method for improvement in zT of BST by alloying x wt% (x=0, 0.25, 0.5, and 0.75) GeSe in BST matrix via ball-milling followed by a spark plasma sintering (SPS) [herein referred to as SPS]. In an embodiment, a p-type carrier concentration is increased with an increase in x wt% GeSe (x=0, 0.25, 0.5, and 0.75) in BST and power factor (sS2) has remarkably enhanced to 75.7 % from 26 to 45.67 ?W cm-1K-2 at 323 K along a spark plasma sintering (SPS) in perpendicular direction for 0.5 wt% GeSe alloyed BST compared to that of the pristine BST. Room temperature (RT) Rietveld refinement of XRD data reveals an increased c-cell parameter with the increased incorporation of GeSe, implying the existence of GeSe in the van der Waals layers of BST. Therefore, incorporation of GeSe in BST matrix could act as an additional phonon scatterer to achieve low lattice thermal conductivity (?lat) of 0.5 Wm-1K-1 in BST-0.5 wt% GeSe along SPS?.
In an embodiment, the Bi0.5Sb1.5Te3 (BST)- 0.5 wt% GeSe composite, comprises composition of 0.005g of GeSe per 1g of Bi0.5Sb1.5Te3(BST). As a result of this, a figure of merit zTmax of ~1.6 may be achieved in 0.5 wt% GeSe along SPS? at 323 K.
According to various embodiment of the present disclosure, a method for improving figure of merit (zT) of Bi0.5Sb1.5Te3 (BST) is disclosed. The method includes a first step of synthesizing phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) [block 101a] and phase pure polycrystalline GeSe [block 101b]. The synthesis of phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) includes mixing of high-quality Bismuth (Bi), Antimony (Sb), and Tellurium (Te) in a quartz tube to obtain a stochiometric mixture. This mixture mixed in the quartz tube is exposed to vacuum pressure of 10-5 torr in a sealed quartz ampule. Further, heating the quartz ampule for a first instance to a temperature of 723K for a period of 12 hrs followed by heating the quartz ampule in a second instance to a temperature of 1123 K for a time period of 5 hrs. The quartz ampule is then held at 1123 K for 10 hrs, followed by slow cooling the quartz ampule to room temperature for a period of 15 hrs. Once this process is complete, the Bi0.5Sb1.5Te3 (BST) are formed into ingots [block 102]. Similarly, the synthesis of phase pure polycrystalline GeSe comprises the mixing of high-quality Germanium (Ge) and Selenium (Se) in a quartz tube to obtain a stochiometric mixture. This mixture mixed in the quartz tube is exposed to vacuum pressure of 10-5 torr in a sealed quartz ampule. Further, heating the quartz ampule for a first instance, to a temperature of 723K for a period of 12 hrs followed by heating, the quartz ampule for a second instance to a temperature of 1123 K for a period of 7 hrs. The quartz ampule is held at 1123 K for 10 hrs followed by slow cooling the quartz ampule to the room temperature for a period of 18 hrs. Similarly, like the above process, the GeSe are formed into ingots [block 102].
The synthesized Bi0.5Sb1.5Te3 (BST) ingots and GeSe ingots are subjected to crushing and grinding to obtain phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) fine powder and phase pure polycrystalline GeSe to fine powder, respectively. In an embodiment, this step of grinding and crushing is carried out using a mortar and pestle in an inert atmosphere (also termed as a glove box). The powders thus obtained are mixed in mass fraction ranging from x= 0 wt% to x= 0.75 wt% [block 103]. Further, the mixture is grinded [block 104] in a ball mill for 12 mins at a grinding speed of 450 rpm, for 12 cycles with pauses for 10 mins. Once the grinding is complete a reverse grinding is carried out for a total number of 12 cycles, to obtain Bi0.5Sb1.5Te3 (BST)-GeSe nanocomposite.
A final step involves sintering [block 105] the Bi0.5Sb1.5Te3 (BST)-GeSe nanocomposite in a spark plasma sintering unit at a temperature of 370°C for 15 mins. In an embodiment, during sintering, an axial pressure of 3.2 KN may be applied under vacuum condition, to obtain Bi0.5Sb1.5Te3 (BST)-GeSe composite which shows improved figure of merit (zT).
Experiment and Test study:

Table 1 illustrates room temperature carrier concentration (p) and carrier mobility (?) of BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75).
Table 1
BST-x wt% GeSe p.1019 (cm-3) µ (cm2 V-1S-1)
x=0 1.05 195
x=0.25 1.16 248
x=0.5 1.25 345
x=0.75 1.41 317

In an embodiment, reagents have been used in order to conduct the experiment and test studies of some of these reagents are of high-quality elements of analytical grade without further purification for the synthesis of Bi0.5Sb1.5Te3 and GeSe. High-purity bismuth [Bi, Alfa Aesar (99.999%)], antimony [Sb, Alfa Aesar (99.999%)], tellurium [Te, Alfa Aesar (99.999%)], selenium [Se, Alfa Aesar (99.999%)], and germanium [Ge, Sigma Aldrich (99.999%)] are also used.

In an embodiment, powder X-ray diffraction (PXRD) is carried out. Room temperature PXRD of as prepared Bi0.5Sb1.5Te3, GeSe, and the ball-milled x wt% GeSe-BST nanocomposites are recorded on Rigaku diffractometer using CuKa source of wavelength (? = 1.54059 Å). Rietveld refinement is performed using GSAS & EXPGUI software to understand the influence of alloying BST with GeSe on the structural parameters via ball-milling.
In another embodiment, thermal conductivity measurements are also carried out on the obtained BST-GeSe nanocomposite. Thermal diffusivity (D) of all the SPS processed samples are measured under N2 atmosphere by using Netzsch LFA-457 (Laser Flash Analysis (LFA) technique) in the temperature range 296 – 523 K. In an embodiment, disc-shaped samples (10 mm x 2 mm) and square-shaped sample (8 mm x 8 mm x 2.5 mm) were obtained, and their properties are measured along parallel (||) and perpendicular (?) to the SPS pressing directions, respectively. Total thermal conductivity (?) is estimated using ? = D x Cp x ?, where Cp and ? are the heat capacity and the density of a sample. The experimental Cp of pristine BST (x=0) to estimate the total thermal conductivity of all other BST- x wt% GeSe (x= 0.25, 0.5, and 0.75) samples. Later, an electronic thermal conductivity (?el) is estimated using the Wiedemann-Franz law, ?el = L??T, where L=Lorentz number, ??= electrical conductivity, and T=temperature. Lorentz number is calculated by considering the single parabolic band (SPB) model and dominant acoustic phonon scattering by fitting the temperature dependent experimental Seebeck values [reduced chemical potential as obtained from the S(T)]. Further ?el was subtracted from the total thermal conductivity (?) to achieve lattice thermal conductivity (?lat).
In another embodiment, electrical conductivity (??) and seebeck coefficients (S) of all parallelepiped shape samples are measured on ZEM-3 (ULVAC-RIKO) instrument in the temperature range of 296 – 523 K under helium (He) inert atmosphere using four-probe geometry along the different SPS pressing directions.
In another embodiment, the incorporation of GeSe in BST matrix acts as an additional phonon scatterer to achieve low lattice thermal conductivity. Out of the three samples i.e. BST- x wt% GeSe (x= 0.25, 0.5, and 0.75), the Bi0.5Sb1.5Te3 (BST)- 0.5 wt% GeSe shows maximum figure of merit, zTmax of ~1.6. The Bi0.5Sb1.5Te3 (BST)- 0.5 wt% GeSe sample comprises a composition of 0.005g of GeSe per 1g of Bi0.5Sb1.5Te3(BST).
Now referring to figure.2a relates to graphs of powder X-ray diffraction (PXRD) patterns of synthesized pristine Bi0.5Sb1.5Te3 (BST) and GeSe prepared via vacuum-sealed tube melting along with the ball-milled Bi0.5Sb1.5Te3 (BST), according to an exemplary embodiment of the present disclosure. The Bi0.5Sb1.5Te3 (BST) patterns are indexed and all the reflections are in agreement with the rhombohedral layered structure (R-3m). Similarly, as synthesized GeSe is indexed to a stable orthorhombic system of space group (Pnma).

Referring to figure.2b, which illustrates the powder X-ray diffraction (PXRD) patterns of ball-milled nanocomposites of Bi0.5Sb1.5Te3 (BST)-x wt% GeSe (x=0, 0.25, 0.5 and 0.75), according to an exemplary embodiment of the present disclosure. The broadening of diffraction peaks is attributed to the formation of smaller crystallites.

Referring to figure.3 which illustrates powder X-ray diffraction (PXRD) patterns of all SPS processed composites, according to an exemplary embodiment of the present disclosure. Figure 3a and figure 3b represents powder X-ray diffraction (PXRD) pattern along parallel and perpendicular to the SPS pressing directions, respectively. No GeSe reflections are noticed in composites within the detection limit as the minimum amount of GeSe (x=0.25, 0.5, and 0.75 wt%) is allowed to embed in the BST matrix. On the other hand, when powder nanocomposites are SPS sintered, point (009) reflections may be noticed in parallel directions to the SPS pressing. Considerable variation in the relative intensities of points (1010) and (110) reflections in SPS? and SPS? indicates an anisotropic nature of BST. GeSe alloyed BST shows a systematic peak shift toward lower 2?, supporting information (SI) suggesting cell expansion upon alloying. The a-cell parameter of (x=0.25, 0.5, and 0.75 wt%) obtained by the Rietveld refinement shows negligible variation, whereas the c-parameter has been increased upon GeSe alloying, indicating the incorporation of GeSe in the van der Waal layers.

Referring now to figure. 4, which illustrates a graph of temperature-dependent electrical conductivity (s) of BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites along with different pressing directions, according to an exemplary embodiment of the present disclosure. All the samples exhibit decreased (s) as a function of an increase in temperature reflecting the degenerate semiconductor behaviour. However, an increase in the wt% addition of GeSe from 0 to 0.75, at room temperature, s increases from 313 to 669 Scm-1 along the SPS? and 295 to 706 Scm-1 respectively along SPS?. Moreover, s of all the samples are higher along SPS? than that of SPS? which could be the intrinsic anisotropic character where layers are stacked along the parallel direction. Further, a good reversibility in electrical conductivity is observed along the SPS pressing directions.

Referring to figure. 5 which illustrates temperature dependent Seebeck coefficient in heating-cooling cycles of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites along (a) parallel and (b) perpendicular to the SPS pressing directions, according to an exemplary embodiment of the present disclosure.
Figure. 6 illustrates graphical representation of temperature dependent power factor (sS2) of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites measured along (a) parallel (||) and (b) perpendicular (?) to the SPS pressing directions. A typical ball-milled pristine BST exhibits sS2 of ~ 30 µWcm-1K-2 at ~ 300 K and then decreases to ~ 8 µWcm-1K-2 at ~ 523 K. Also, a significant improvement in sS2 from 25 to 45.7 µWcm-1K-2 is observed along the SPS? at ~ 295 K as a function of an increase in x wt% of GeSe in BST. However, sS2 is in a range of 22-25 µWcm-1K-2 near room temperature along SPS? for all composites. The dramatic improvement in sS2 along SPS? is attributes to the optimization of p-type carrier concentration.
Referring to figure. 7 which illustrates graphs of temperature dependent total thermal conductivity of (?) along different SPS pressing directions in the range 295-523 K, according to an exemplary embodiment of the present disclosure. ? gradually raises as a function of temperature from 0.76 Wm-1K-1 at ~300 K to 1.19 Wm-1K-1 at ~523 K along SPS? and from 0.88 Wm-1K-1 at ~300 K to 1.53 Wm-1K-1 at ~523 K along with SPS?. Increased ? with temperature is owing to the thermally active bipolar contribution. However, obtained ? along SPS? is slightly lesser than in SPS?. Nevertheless, wt% GeSe addition further increased ? from 0.76 W/mK for x=0 to 0.81 Wm-1K-1 for x=0.25 to 0.95 Wm-1K-1 for x=0.5 along the SPS?. Further, increase in x=0.75 results in ? decreased to 0.71 Wm-1K-1 along the SPS?. However, ? effectively increases with an increase in x wt% GeSe incorporation along the SPS? from 0.88 W/mK to 1.53 Wm-1K-1 at 300 K which could be attributed to the increased s.

Now Referring to figure. 8, which illustrates graphical representation of temperature dependent combination of lattice thermal conductivity (?lat) and bipolar (?bi) thermal conductivity of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites measured along (a) parallel (||) and (b) perpendicular (?) to the SPS pressing directions, according to an exemplary embodiment of the present disclosure. ? is the combination of ?=? ??_el+ ? ??_lat+ ? ??_bi components where, ?el is the electronic thermal conductivity, ?lat is the lattice thermal conductivity and ?bi is the bipolar thermal conductivity. Bipolar conduction not only decreases the Seebeck coefficient but also increases the total thermal conductivity due to the thermally activated bipolar diffusion. Minimum ?lat +?bi ~0.41 Wm-1K-1 along SPS? for the x=0.75 wt% and ~0.57 Wm-1K-1 along with SPS? for x=0.5 wt% has been obtained at 323 K.
Referring to figure. 9 which illustrates temperature dependent thermoelectric figure of merit (zT) of the BST-x wt% GeSe (x=0, 0.25, 0.5 and 0.75) composites measured along (a) parallel (||) and (b) perpendicular (?) to the SPS pressing directions, according to an exemplary embodiment of the present disclosure. As shown in the graphs, all samples exhibit higher zT along SPS? than BST (~0.98), and a peak zT of ~1.62 at 323 K has been obtained for x=0.5 wt% GeSe which is ~ 63% higher than the pristine BST. Therefore, significant improvement of zT along with SPS? is mainly due to the enhanced power factor and the obtained zT is highly reversible. The primary origin of improved zT in BST-0.5 wt% GeSe is the synergy between interfacial defects, enhanced power factor, and the reduced lattice thermal conductivity.
In the method of the present disclsoure, BST-x wt% GeSe composites (x=0, 0.25, 0.5, and 0.75) are synthesized by vacuum seal tube method followed by the ball-milling and SPS. In an embodiment, a high thermoelectric performance in p-type Bi0.5Sb1.5Te3 material by optimizing its thermoelectric properties by alloying with GeSe is obtained. Further, the p-type Bi0.5Sb1.5Te3 material has a peak zT ~1.62 at 323 K along SPS? for 0.5 wt% GeSe incorporations in BST. The origin of high zT in this composite is from the improved power factor, optimized p-type carrier concentration, improved phonon scattering at the interfaces, and nano-grain boundaries.
Equivalents:

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims:We Claim:
1. A method for improving figure of merit (zT) of Bi0.5Sb1.5Te3 (BST), the method comprising:
synthesizing, phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) and phase pure polycrystalline GeSe;
crushing and grinding the synthesized phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) and the synthesized phase pure polycrystalline GeSe to fine powders;
mixing the crushed and synthesized phase pure polycrystalline GeSe with the crushed and synthesized phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) to obtain a mixture;
grinding the mixture in a ball mill for 12 mins at a grinding speed of 450 rpm, for 12 cycles with pauses for 10 mins followed by reverse grinding for a total number of 12 cycles, to obtain Bi0.5Sb1.5Te3 (BST)-GeSe nanocomposite;

sintering the Bi0.5Sb1.5Te3 (BST)-GeSe nanocomposite at a temperature of 370 °C for 15 mins at an applied pressure of 3.2 KN under vacuum, to obtain Bi0.5Sb1.5Te3 (BST)-GeSe composite with improved figure of merit (zT).

2. The method as claimed in claim 1, wherein the synthesis of phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) comprises:
mixing stochiometric amount of high-quality Bismuth (Bi), Antimony (Sb), and Tellurium (Te) in a quartz tube to obtain a stochiometric mixture;
exposing the quartz tube to vacuum pressure of 10-5 torr in a sealed quartz ampule;
heating in a first instance, the quartz ampule to a temperature of 723K for a time period of 12 hrs;
heating in a second instance, the quartz ampule to a temperature of 1123 K for a time period of 5 hrs;
holding the quartz ampule at a fixed temperature of 1123 K for 10 hrs;
slow cooling the quartz ampule to room temperature for a time period of 15 hrs to obtain Bi0.5Sb1.5Te3 (BST) ingots.

3. The method as claimed in claim 1, wherein the synthesis of phase pure polycrystalline GeSe comprises:
mixing stochiometric amount of high-quality Germanium (Ge) and Selenium (Se) in a quartz tube to obtain a stochiometric mixture;
exposing the quartz tube to vacuum pressure of 10-5 torr in a sealed quartz ampule;
heating in a first instance, the quartz ampule to a temperature of 723K for a time period of 12 hrs;
heating in a second instance, the quartz ampule to a temperature of 1123 K for a time period of 7 hrs;
holding the quartz ampule at a fixed temperature of 1123 K for 10 hrs;
slow cooling the quartz ampule to the room temperature for a period of 18 h to obtain GeSe ingots.

4. The method as claimed in claim 1, wherein the crushing and grinding of synthesized Bi0.5Sb1.5Te3 (BST) and GeSe is carried out in a mortar and pestle in an inert atmosphere.

5. The method as claimed in claim 1, wherein a predefined wt% in range of 0.25, 0.5 and 0.75 of crushed phase pure polycrystalline GeSe is mixed with pristine crushed phase pure polycrystalline Bi0.5Sb1.5Te3 (BST) to obtain Bi0.5Sb1.5Te3 (BST)- GeSe mixture.

6. The method as claimed in claim 1, wherein the sintering is performed in a spark plasma sintering (SPS) system for a period of 30 mins.

7. The method as claimed in claim 1, wherein sintering involves dwelling period of 3 to 5 min under constant pressure of 3.1KN under vacuum pressure of 10-5 torr.

8. The method as claimed in claim 1, wherein relative density of the sintered Bi0.5Sb1.5Te3 (BST)-GeSe composite was about 96% to 99% of Bi0.5Sb1.5Te3.

9. A Bi0.5Sb1.5Te3 (BST)-GeSe composite manufactured by the method of claim 1.

10. The Bi0.5Sb1.5Te3 (BST)-GeSe composite as claimed in claim 9, wherein the Bi0.5Sb1.5Te3 (BST)-GeSe composite shows increased p-Type carrier concentration as wt% of GeSe is increased.

11. The Bi0.5Sb1.5Te3 (BST)-GeSe composite as claimed in claim 9, wherein figure of merit (zT) is at ~1.62 for the Bi0.5Sb1.5Te3 (BST)-GeSe composite at temperature of 323 K for 0.5 wt% of phase pure polycrystalline GeSe in the Bi0.5Sb1.5Te3 (BST).

12. The Bi0.5Sb1.5Te3 (BST)-GeSe composite as claimed in claim 11, comprises composition of 0.005g of GeSe per 1g of Bi0.5Sb1.5Te3(BST).

13. The Bi0.5Sb1.5Te3 (BST)-GeSe composite as claimed in claim 9, wherein, power factor (sS2) is increased by 75.7 % from 26 ?W cm-1K-2 to 45.67 ?W cm-1K-2 at 323 K for 0.5 wt% GeSe alloyed with Bi0.5Sb1.5Te3 (BST) compared to that of pristine Bi0.5Sb1.5Te3 (BST).