TECHNICAL FIELD

[0001] The following relates to a nanocomposite thermoelectric material and to a method of manufacturing the nanocomposite thermoelectric material. It also relates to a thermoelectric system comprising the nanocomposite material and to methods of using the nanocomposite material.

BACKGROUND

[0002] Thermoelectric devices are based on a phenomenon known as the thermoelectric effect in which there is a direct conversion of a temperature gradient across two dissimilar materials into electricity. The materials which can be used to make thermoelectric devices are known as thermoelectric materials. The thermoelectric effect is reversible meaning that electricity can also be directly converted into a temperature gradient.

[0003] The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances. The Peltier Effect is a phenomenon in which heat is emitted or absorbed when an electric current passes across a junction between two materials. Thermoelectrics (TEs) are able to provide heating/cooling and yet be a source of electrical power; this is due to the Seebeck and Peltier effects.

[0004] Solid state TE modules can couple heat flow and electric current. This approach offers a viable route for power generation from high heat sources, such as nuclear energy. In addition, depending on the direction of the current flow, a TE can be used for heating and cooling applications. The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy production and utilization. However, in order to meet that role, more efficient thermoelectric materials are needed that are suitable for high-temperature applications. Based on the thermoelectric effects described above, one can build a thermoelectric module for power generation or cooling systems.

[0005] The efficiency of thermoelectric devices is characterized by the thermoelectric material’s figure of merit (ZT), which is a function of several transport coefficients; where S is the Seebeck coefficient, s is the electrical conductivity, T is the temperature, K _e is the thermal energy transported by charge carriers and K _L is the lattice thermal energy transported by phonons. The electrical transport term (S ² o) is called the power factor.

S ² o T ZT = -

K _L + Kg

[0006] The ZT of a material changes with temperature so thermoelectric materials in use today are optimized for specific temperature regions where their ZT values are the highest. The larger the figure of merit, the better the efficiency of the thermoelectric cooler or power generator. Therefore, there is significant interest in improving figure of merit in thermoelectric materials for many industrial and energy applications.

[0007] Metal selenides have attracted considerable attention due to their interesting properties and potential applications. A commonly used thermoelectric material is Tin Selenide (SnSe). The SnSe system has special properties related to the presence of a metal (Sn) and a chalcogen (Se), with different valences and ionicities that govern the structure and the properties. SnSe is a semiconductor compound that is widely used in holographic recording systems, optical and optoelectronic materials in infrared electronic, memory switching devices, thermoelectric generators and in photoelectrical cells. SnSe is a stable and simple compound consisting of earth-abundant elements and exhibits an intrinsically ultralow thermal conductivity.

[0008] SnSe-based thermoelectric materials have captured much attention since single crystal SnSe exhibited record high ZT. However, the practical application is limited due to the poor mechanical properties and harsh production conditions of single crystals. SnSe based materials have shown unexpectedly low thermal conductivities and high power factors; thus it has become a very promising thermoelectric material. Both the electrical and thermal transport properties of SnSe are found to be promising. The latter is enabled by the contribution of multiple electronic valence bands. SnSe provides a low enough electrical resistivity resulting in a moderate power factor. It has been observed that the thermal conductivity of SnSe is intrinsically ultralow, resulting in a high ZT value.

[0009] Other commonly used thermoelectric materials include Bismuth Telluride (Bi ₂ Te3), Antimony Telluride (Sb2Te ₃ ), polycrystalline Cuprous Selenide (Cu ₂ Se); Silicon-germanium (SiGe), etc.

[0010] However, current methods of manufacturing polycrystalline thermoelectric materials can exhibit unsatisfactory mechanical properties. Thermoelectric modules and devices may be subject to various mechanical loads while operating, and thus a mechanically poor material may not be suitable. Mechanical loads could cause physical deformation and damages due to high temperatures.

[0011] Thermoelectric materials can be doped by introducing impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties. Doping is an efficient way to improve thermoelectric performance. Typically, materials such as Iodine, Sodium, Bismuth and Bromine are used for doping.

[0012] Carbon nanoparticles, such as carbon nanotubes, carbon nanorods, nanoplatelets or graphene are known materials. Various applications for these materials have been considered. The incorporation of nanoparticles into composite materials has been studied over the last number of years. It is well known that nano-inclusions can scatter phonons effectively. It has also been reported that the nano-inclusions can enhanced the power factor.

[0013] Hence there is a need for materials that exhibit favorable thermoelectric and mechanical properties simultaneously.

SUMMARY

[0014] SnSe bulk materials with a layered structure and highly anharmonic lattice vibrations can feature an intrinsically low thermal conductivity and high thermoelectric efficiency. It has been found that incorporating carbon nanoparticles into such materials results in nanocomposite material that exhibits high electrical conductivity and improved power factor at elevated temperatures, and an increase in microhardness.

[0015] In one aspect there is provided a thermoelectric material of the formula A _y /B _x Ci- _x D

wherein:

A is a carbon nanoparticle selected from the groups consisting of carbon nanotubes (CNT), carbon nanorods, nanoplatelets and graphene;

B is an element selected from the group consisting of Na, Bi, Sb, As, P, N, B, Al, Ga, In and Tl;

C is an element selected from the group consisting of Sn, Ge, Pb;

D is an element selected from the group consisting of Si, Se, Te; x is 0 to 1 ; y is greater than 0 and y is less than or equal to 0.1

[0016] In a further aspect there is provided a thermoelectric system comprising a thermoelectric material of the formula A _y /B _x Ci- _x D as defined above.

[0017] In a further aspect there is provided a use of a thermoelectric material according of the formula A _y /B _x Ci- _x D as defined above in a thermoelectric device.

[0018] In a still a further aspect there is provided a process of preparing a thermoelectric material of the formula A _y /B _x Ci- _x D as defined above comprising: i) combining the components B, C and D of the formula A _y /B _x Ci- _x D in stoichiometric proportions; ii) annealing the combined components in a sealed vessel; iii) combining the annealed product of ii with component A and ball milling; iv) sintering the ball milled material of iii) to produce a product of the formula A _y /B _x Ci- _x D

[0019] In yet another aspect there is provided a thermoelectric system comprising a material adaptable for inclusion in a thermoelectric device, wherein said material has a formula A _y /B _x Ci- _X D wherein A is a molecule selected from the group consisting of single wall carbon nano tubes (SWCNT), multiwall carbon nanotubes (MWCNT), carbon nanorods, nanoplatelets and graphene; B is an element selected from the group consisting of Na, Bi, Sb, As, P, N, B, Al, Ga, In, Tl; C is an element selected from the group consisting of Sn, Ge, Pb; and D is an element selected from the group consisting of Si, Se, Te.; x is 0 to 1 and y is greater than 0 and y is less than or equal to 0.1 [0020] In still another aspect there is provided a thermoelectric device comprising: one or more elements adapted for inclusion in a thermoelectric device; wherein said device comprises: a material having the formula A _y /B _x Ci- _x D; a heatsink; an n-type material; a p-type material; and a heat source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments will now be described with reference to the appended drawings wherein:

[0022] FIG. 1 illustrates the crystal structure of A _y /B _x Ci- _x D;

[0023] FIG. 2 illustrates a B _x Ci- _x D unit cell;

[0024] FIG. 3 illustrates a process by which a powder mixture of elements B, C and D is processed to make an ingot of B _x Ci- _x D under Vacuum pressure of -0.0005 Pa and Temperature of -1223 K;

[0025] FIG. 4 illustrates the processed CNTs;

[0026] FIG. 5 illustrates the components of the composition A _y /B _x Ci- _x D being added to a grinding vial;

[0027] FIG. 6 illustrates the process of grinding and spark plasma sintering (SPS) to form the completed A _y /B _x Ci- _x D;

[0028] FIG. 7 illustrates an example of a typical thermoelectric device;

[0029] FIG. 8 shows photos of a sample of SnSe before heat treatment (left) and after heat treatment (right);

[0030] FIG. 9 Shows the powder diffraction pattern of the Na0 _. 03Sn0 _. 97Se sample after SPS; [0031] FIG 10A is a graph of electrical conductivity, FIG.10 B is a graph of the Seebeck coefficient and FIG. 10C is a graph of the power factor of the SnSe sample without carbon nanotubes (CNTs);

[0032] FIG. 11 A is a graph of electrical conductivity, FIG. 11 B is a graph of the Seebeck coefficient and FIG. 11C is a graph of the power factor of the SnSe sample with 0.1 wt. %

CNTs;

[0033] FIG 12A is a graph of electrical conductivity, FIG. 12B is a graph of the Seebeck coefficient and FIG. 12C is a graph of the power factor of the Nao _. 03Sno _. 97Se sample without CNTs;

[0034] FIG. 13A is a graph of electrical conductivity, FIG. 13B is a graph of the Seebeck coefficient and FIG. 13C is a graph of the power factor of Na0 _. 03Sn0 _. 97Se with 0.1 wt. % CNTs.

DETAILED DESCRIPTION

[0035] A method of manufacturing a nanocomposite material which exhibits an enhanced thermoelectric effect is described herein. The thermoelectric material is composed of a nano inclusion material embedded into an p-type metal chalcogen nanocomposite.

[0036] Thermoelectric materials typically require a large Seebeck coefficient, low electrical resistivity and low thermal conductivity. However, as the Seebeck coefficient, the thermal conductivity, and the electrical resistivity are strongly dependent on each other, it can be challenging to improve them simultaneously in thermoelectric materials.

[0037] In an embodiment, the composition of the proposed thermoelectric material is as follows: A _y /B _x Ci- _x D where A is carbon nanoparticle and can be selected from the group consisting carbon nanotubes, such as single wall carbon nanotube (SWCNT) and multi-wall carbon nanotube (MWCNT), carbon nanoparticles and graphite nanoplatelets; B is a dopant and can be an element selected from the group consisting of Na, Bi, Sb, As, P, N, B, Al, Ga, In and Tl; C is an element selected from the group consisting of Sn, Ge and Pb; and D is an element selected from the group consisting of Si, Se and Te. In a further embodiment A is SWCNT or MWCNT. In a further embodiment B is Na or Bi. In still a further embodiment C is Sn and in still a further embodiment D is Se. In one embodiment x of the formula A _y /B _x Ci- _x D is 0 to 1. In a further embodiment x is about 0 to about 0.75, In still a further embodiment x is about 0 to about 0.5 in still a further embodiment x is about 0 to about 0.1. In still a further embodiment x is about 0 to about 0.05. In still a further embodiment x is about 0 to about 0.01. In still a further embodiment x is about 0.01 to about 0.5. In still a further embodiment x is about 0.01 to about 0.1. In still a further embodiment x is greater than 0 and less than or equal to 1. In one embodiment y of the formula A _y /B _x Ci- _x D, is greater than 0 and y is less than or equal to 0.1. In a further embodiment y is greater than 0 and less or equal to 0.05. In still a further embodiment y is greater than 0 and less or equal to 0.05. In still a further embodiment y is greater than 0 and less or equal to 0.01. In still a further embodiment y is about 0.01 to about 0.1. in still a further embodiment y is about 0.01 to about 0.05.

[0038] Doping is commonly used to introduce impurities into a material for the purpose of modulating its electrical and optical and structural properties. Thermoelectric materials are typically narrow gap semiconductors and the doping utilizes a process known as band gap engineering or tuning. Group V elements can be used as an n-type dopant by substitution of ‘C’ atoms and are shown as Ί3’ in the composition formula: A _y /B _x Ci- _x D. In addition, the matrix material can be doped with a dopant, such as an n-type or a p-type dopant. When a dopant is included in the matrix, its concentration with regards to the matrix is preferably less than 1% w/w. For example, Bismuth can be used as an n-type dopant and Boron as a p-type dopant. Doping enhances the thermoelectric properties of a material comprising ‘CD’ by increasing the ZT value at specific temperatures by providing the optimum carrier concentration for the power factor.

[0039] Carbon nanoparticles have extraordinary electrical, mechanical and thermal properties; thus, carbon nanoparticles may be embedded in thermoelectric matrix materials. Introducing nanoparticles into the matrix of formula B _x Ci- _x D enhances the thermoelectric performance of polycrystalline materials. Suitable nanoparticles can be single wall carbon nanotubes, multiwall carbon nanotubes, carbon nanorods, nanoplatelets or graphene.

[0040] Nanocomposites with constituent sizes of <50nm may be used to enhance the figure of merit (ZT) of bulk thermoelectric materials.

[0041] The term “nano-sized inclusion”, as used herein, generally refers to inclusions, into the host material, such as nanoparticles, whose dimensions are equal or preferably less than about 100 nm. For example, they can refer to nanoparticles having an average cross-sectional diameter in a range of about 1 nm to about 100 nm. Particles for inclusion may be considered nano-sized if they are between about 1 nm and about 100nm in at least one dimension. In another embodiment the nano-sized particles are between about 1 nm and about 100nm in two dimensions and in still another embodiment the nano-sized particles are between about 1 nm and about 100nm in three dimensions.

[0042] Nanostructured compounding can also be used to improve the thermoelectric properties of a material comprising ‘CD’. The arrangement of atoms is often described as a zigzag double layer of C and D atoms, such as Se ^2- and Sn ²⁺ , with strong intralayer bonds and weak van der Waals interactions between the layers. SnSe nanostructure exhibits orthorhombic symmetry of either Pnma or Cmcm space group depending on temperature. Nanostructured compounding of bulk materials can be achieved by hot pressing of mechanically alloyed nanopowders. This technique has shown the ability to increase figure of merit (ZT) of thermoelectric materials. The enhancement of ZT is believed to be due to a large reduction of thermal conductivity caused by the increased phonon scattering at the grain boundaries of the nanostructures combined with an increased power factor at high temperatures.

[0043] Nanostructured materials can enhance thermoelectric performance by lowering the thermal conductivity of the material. Stronger phonon scattering can be induced by increasing the nano-surfaces and interfaces in nano-scale samples. The effect of nanostructures on reducing thermal conductivity can be significant at low temperatures. Therefore, at high- temperature ranges, the contribution of short wavelength phonons to the lattice thermal conductivity dominates and point defects can have much stronger effects on the lattice thermal conductivity than nanostructures. One solution may include the use of a wide spectrum of phonon scattering centers to scatter a range of phonon wavelengths to scatter the low energy acoustic phonons.

[0044] Thermoelectric properties which have high directional dependency can be seen as important obstacles which must be overcome to use thermoelectric devices in current applications. An improvement in the manufacturing method of thermoelectric materials and development of a suitable nanocomposite will enable the widespread use of TE devices.

[0045] FIG. 1 illustrates an embodiment of the crystal structure of A _y /B _x Ci- _x D. Element ‘A’ (104), can be embedded into the matrix comprised of B _x Ci- _x D (102, 103, 101). Element ‘A’ can be dispersed into the matrix randomly or homogeneously. Element ‘B’ 102 is a dopant that replaces element C 103. In one embodiment A is a carbon nanotube (CNT), B is Se, C is Bi and D is Sn. FIG. 2 illustrates an embodiment of one unit cell of B _x Ci- _x D wherein B is Se, C is Bi and D is Sn.

[0046] FIG. 3 illustrates an embodiment of forming a B _x Ci- _x D ingot. A high purity powder mixture of B _x , Ci- _X and D (301) was weighed in stoichiometric proportions and loaded into an Argon-filled quartz ampoule. The ampoule 302 was sealed under vacuum to a pressure of approximately ~ 0.1 Pa. The ampoule was then heated to 1223 K over 10 h, then soaking at 1223 K for 24 h, followed by furnace-cooling to room temperature. Element B (Bi) diffused into the lattice of C and D (SnSe) and substituted for C (Sn ²⁺ ) to obtain n-type doped lattice (Bi- doped SnSe) as depicted in FIG. 2. The obtained ingot, B _x Ci- _x D, was ground into a powder.

[0047] In one embodiment the carbon nanotubes (CNTs) to be used in this nanocomposite can have single-wall structure and can be synthesized using thermal chemical vapor deposition (CVD). For purification, separation of CNT layers, the CNTs can be stirred in strong acid (such as HCI) under a weak ultrasonic treatment in an ice bath for about 30 min at about 25°C, followed by drying in a vacuum. The ice bath can be used to counteract a rising temperature during the mixing process.

[0048] FIG. 4 and 5 show an embodiment of the nanotubes 402 being added to the ground B _x Ci- _x D powder 401, to form a combined mixture 501. The synthesized B _x Ci- _x D powder and the purified carbon nanotubes (CNT) powders can be ground together using an agate mortar and pestle. A range of about 0.01% to about 0.5% weight fraction of CNT to B _x Ci- _x D is suitable.

FIG. 6 shows an embodiment of the ground composite powders being loaded into a closed grinding vial 502. The composite powders were ground via grinding machine 602 at room temperature for 20 minutes followed by a rest period of 15 minutes, with 12 cycles. The grinding can also be performed at cryogenic temperatures. The resulting composite powders were loaded into a graphite (or carbide) die 603 and can be densified by spark-plasma sintering method 604 (or by hot-pressing) at pressure of 40-60 MPa for 5 to 10 minutes (or 60 min for hot-pressing). The particles within the nanocomposites can be in the nanometer size range although grain growth can take place during sintering.

[0049] FIG. 7 illustrates an example of a thermoelectric device. The thermoelectric device consists of two dissimilar thermoelectric materials 701, 702 joined at their ends: an n-type 703 and a p-type 704 semiconductor. A direct electric current 705 will flow in the circuit when there is a temperature difference between the ends of the materials. The elements 703, 704 are electrically connected in series with current flowing alternatively through p-type and n-type thermoelectric materials. The thermoelectric materials are formed of nanocomposites of the invention. The thermoelectric materials n-type 703 and p-type 704 of the devices are connected through electrically conductive bridges 701,702. Application of a current causes transfer of heat from one side of the thermoelectric cooler to the other, thereby lowering the temperature at one side while increasing the temperature at the opposite side

[0050] The CNT nanoparticles can be used to reduce the lattice thermal conductivity due to the developed phonon scattering by newly-formed interfaces between the matrix and CNTs. The embedded CNTs work as effective reinforcing agents, leading to the improvement of mechanical behavior of overall nanocomposite. Additionally, the uniform size nanoparticles scatter electrons less than their equivalent ionized impurities and therefore may improve the carrier mobility significantly. On the other hand, the hot pressing method promises an opportunity to produce thermoelectric modules in high volumes.

[0051] The embedded CNTs within the B _x Ci- _x D matrix promise a much larger scale in the production of thermoelectric modules. Most of the thermoelectric materials used currently require a very high cost per unit to produce and also require significant time to produce. Additionally, the low efficiency at elevated temperatures makes a bottleneck of using thermoelectric modules as an energy generator. The newly developed and proposed novel thermoelectric nanocomposite made by embedding CNTs within the B _x Ci- _x D matrix was demonstrated to outperform all reported bulk thermoelectric materials, such as low thermal conductivity over 1000 K, thereby earmarking it as a material system for future use in efficient thermoelectric power generation from high heat sources.

[0052] Thermoelectric devices fabricated from the nanocomposite materials described herein have the potential to be widely used in daily life, in large scale and high-power commercial generators, to establish a reliable, efficient, environmentally friendly and clean energy supply in the global marketplace and to protect the surrounding environment.

[0053] Examples:

[0054] 1. General Methods

[0055] 7.1 Starting materials [0056] The starting materials were Sn (99.999 wt.%, Alfa Aesar), Se (99.999 wt.%, Alfa Aesar) and Na (99.8 wt.% Thermofisher Scientific, store under oil and in the glove box), multiwall carbon nanotubes (99 wt.%, Nanolntegris Technologies Inc.).

[0057] 1.2 Handling of the starting materials and samples.

[0058] All the starting materials, SnSe and Nao _. 3Sno _. 97Se samples with and without multiwall carbon nanotubes (CNTs thereafter) were handled in the Ar-filled filled glove box with the partial pressure of O2 less than 0. 1 ppm (parts per million).

[0059] 1.3 Preparation of the polycrystalline samples.

[0060] For the synthesis SnSe and Nao _. 3Sno _. 97Se with and without CNTs or Sn purification, the elements with appropriate ratios were loaded into the silica tubes (fused quartz, ID 10mm, OD 12mm) with one end sealed and one end open. The silica tubes were closed with the valves inside the glove box and taken out of the box. The tubes were transferred and attached to a vacuum line, where the valves were open, and tubes were evacuated. The evacuated tubes were sealed off with a methane-oxygen torch.

[0061] For the Nao _.3 Sno _.97 Se samples with and without CNTs, the silica tubes coated with the carbon on the inside walls were used.

[0062] The sealed-off silica tubes were placed into larger silica tubes (ID 14mm, OD 16mm), which were evacuated and sealed off in the same manner. The second silica tubes were used to prevent oxidation of the samples, in case the inner silica tubes crack during cooling.

[0063] All the samples were heated in a box furnace with the programable temperature controller.

[0064] After the synthesis the tubes were transferred into the glove box where they were opened, and the samples were retrieved.

[0065] 1.4 Purification of tin metal.

[0066] Before use, the tin metal purchased from the supplier was purified.

[0067] Using the method describe above, the tin metal was sealed inside the silica tubes. The tin sample was heated according to the following profile:

+ 100° / -50° / ice quenched

RT - 5 950°C (dwell 24h) - 5 350°C (dwell 4h) - > RT [0068] After cooled to room temperature (RT), the tin sample was transferred into the glove box, open, and the top layer containing oxide impurities was scrapped off.

[0069] 1.5 Ball milling.

[0070] The SnSe and Nao _. 3Sno _. 97Se with and without CNTs were loaded into a 65 ml stainless steel milling jar with 10mm stainless steel balls; the weight ratio between the balls and samples was around 6:1. The jar was closed with a lid with a rubber gasket, which was then taped with parafilm. This was done inside the glove box.

[0071] The milling jar was taken out of the glove box and transferred to the Fritsch Pulverisette 6 planetary ball mill. The samples were milled at 200 rpm for 20 minutes followed by the rest period of 15 minutes. This cycle was repeated 12 times.

[0072] After ball milling, the jar was transferred into the glove box, where it was opened. [0073] 1.6 Spark plasm sintering (SPS).

[0074] The ball-milled SnSe and Nao _. 3Sno _. 97Se with and without CNTs were loaded into the graphite dies (15 mm ID) with graphite punches at the bottom and top. The dies were sealed in a plastic bag inside the glove box. The jar was taken from the glove box and transferred to the Fuji Dr. Sintere SPS instrument. A graphite die with the sample was taken out of the jar and transferred quickly to the SPS chamber and evacuated.

[0075] The sample would be heated under an axial pressure of 40MPa in dynamic vacuum according to the following profile:

+ 100° *-/ current is switched off

RT - 5 500°C (dwell 5min) - > RT

[0076] After the SPS, the samples were transferred into the glove box until further use.

[0077] 1.7 Sample cutting.

[0078] The spark plasma sintered (SPS) samples were cut into 2mm ^c 3mm ^c 10mm rectangular bars for thermoelectric measurements.

[0079] Cutting was done with a low-speed diamond saw, and kerosene was used as a lubricant to prevent sample oxidation during cutting. [0080] 1.8 X-ray powder diffraction.

[0081] The samples were analyzed by X-ray powder diffraction (PXRD) on a PANalytical X’Pert Pro diffractometer with the CUKCH radiation and an X’Celerator detector. The samples were finely ground using an agate mortar and pestle and deposited on zero-background silicon discs. The diffraction data were collected in the 2Q range of 20° to 90°. Rietveld refinement (Rietica program) was used to determine the sample purity and lattice parameters.

[0082] 1.9 Thermoelectric measurements.

[0083] Electrical conductivity and Seebeck coefficient for the samples were measured on a ULVAC-RIKO ZEM-3 instrument from room temperature (RT) up to 500°C at 50°C increments.

[0084] 1.10 Microhardness measurements.

[0085] Vickers microhardness was measured using a diamond indenter on a Zwick Roell ZHU 2.5 instrument, where the force applied was 2 N and the indent was kept for 10 s. The equation Hv = 1.854 ^c L/(2d) ² was used to determine the Vickers hardness values (kgf mm ^-2 ), with L being the indentation load and 2d the diagonal length of the indentation.

[0086] 2. Sample Preparation and Characterization.

[0087] 2.1. Synthesis of the SnSe samples without and with 0.1 wt.% carbon nanotubes.

[0088] Step 1. Purified tin metal and selenium were combined in the 1:1 molar ratio and total mass of 6 grams, sealed in the silica tubes, and annealed according to the following profile:

+ 100° ^c / _h -100° ^c / _h

RT - 5 950°C (dwell 6h) - 5 RT

[0089] FIG. 8 shows the sample before heat treatment and after heat treatment. SnSe sample before heat treatment is shown on the left and the SnSe sample after heat treatment is shown on the right.

[0090] The sample was transferred into the glove box, the silica tube was opened, and sample was retrieved.

[0091] Step 2. The powder X-ray diffraction data were collected on a small piece of the sample. The derive lattice constants are given in Table 1. The sample is labeled as “Melted”. [0092] Step 3. The sample was loaded into the stainless-steel milling jar in the glove box.

[0093] To prepare the SnSe sample with 0.1 wt.% CNTs. For each 0.999 grams of the sample, 0.001 gram of CNTs was added to the sample in the jar.

[0094] Step 4. The jar was transferred to the ball mill and the sample was milled.

[0095] Step 5. The jar was transferred into the glove box and opened.

[0096] Step 6. The powder X-ray diffraction was collected on a small piece of the ball milled sample. The lattice constants are given in Table 1. The sample is labeled as “Ball milled”.

[0097] Step 7. The sample was loaded into the graphite die, sealed in a plastic bag and transferred from the glove box to the SPS instrument.

[0098] Step 8. The sample was SPS sintered and moved back to the glove box.

[0099] Step 9. The powder X-ray diffraction was collected on a small piece of the SPS sample. The lattice constants are given in Table 1. The sample is labeled as “SPS”.

[00100] Step 10. The sample was taken out of the glove box and cut into a 2mm ^c 3mm ^c 10mm bar.

[00101] Step 11. Thermoelectric properties were measured on the bar. The properties are shown in Figure 3-4.

[00102] Step 12. Another sample prepared in an identical way (Steps 1, 3-5, 7-8) was mounted into the epoxy and polished. Microhardness was measured.

[00103] Table 1. Lattice constants of the SnSe samples with and without 0.1 wt. % CNTs.

[00104]

[00105] 2.2. Synthesis of the Nao. ₀₃ Sno. ₉₇ Se samples without and with 0.1 wt.% carbon nanotubes.

[00106] Step 1. Sodium, purified tin metal and selenium were combined in the 0.03:0.097:1 molar ratio and total mass of 1.5 grams (2.5 grams for the sample with CNTs), sealed in the silica tubes with carbon coating on the inside walls. The samples were annealed according to the following profile:

+ 100° ^c / _h -100° ^c / _h

RT - 5 950°C (dwell 6h) - 5 RT

[00107] The samples were transferred into the glove box, the silica tubes were opened, and samples were retrieved.

[00108] Step 2. The powder X-ray diffraction data were collected on a small piece of the samples. The derive lattice constants are given in Table 1. The samples are labeled as “Melted”.

[00109] Step 3. The samples were loaded into the stainless-steel milling jar in the glove box.

[00110] Note for the Nao _.03 Sno _.97 Se sample with 0.1 wt.% CNTs. For each 0.999 grams of the sample, 0.001 gram of CNTs was added to the sample in the jar. [00111] Step 4-12 were performed in a similar way to those described in section 2.1 above.

[00112] The lattice constants are shown in Table 2 and properties are shown in FIG. 12-13.

[00113] The powder diffraction patterns of the Na0 _. 03Sn0 _. 97Se samples with and without CNTs contained a graphite impurity peak are shown in FIG.9. This impurity originates from the carbon coating of the tube walls. Such peak was absent in the powder diffraction patterns of the SnSe samples.

[00114] FIG. 9 shows the powder diffraction pattern of the Na0 _. 03Sn0 _. 97Se sample after SPS.

[00115] Table 2. Lattice constants of the Nao _. 03Sno _. 97Se samples with and without 0.1 wt. % CNTs.

* Due to the small amount of the sample prepared, all the sample was used for the SPS.

[00116] 3. Thermoelectric properties

[00117] 3.1. Thermoelectric properties of the SnSe samples without CNTs

[00118] Electrical conductivity, Seebeck coefficient and power factor of the SnSe sample without CNTs are shown in FIG. 10A, FIG. 10B and FIG. 10C respectively.

[00119] 3.2. Thermoelectric properties of the SnSe samples with 0.1 wt.% CNTs

[00120] Electrical conductivity, Seebeck coefficient and power factor of the SnSe sample with 0.1 wt.% CNTs are shown in FIG. 11 A, FIG. 11B and FIG. 11 C, respectively.

[00121] 3.3. Thermoelectric properties of the Nao. ₀₃ Sno. ₉₇ Se sample without CNTs. [00122] Electrical conductivity, Seebeck coefficient and power factor of the Na0 _. 03Sn0 _. 97Se sample without CNTs are shown in FIG. 12A, FIG. 12B and FIG. 12C respectively.

[00123] 3.4. Thermoelectric properties of the Nao. ₀₃ Sno. ₉₇ Se sample with 0.1 wt.% CNTs.

[00124] Electrical conductivity, Seebeck coefficient and power factor of the Na0 _. 03Sn0 _. 97Se sample with 0.1 wt. % CNTs are shown in FIG. 13A, FIG. 13B and FIG.13C respectively.

[00125] It can be observed from FIG.12A and FIG. 13A that the the material comprising Na0 _. 03Sn0 _. 97Se with 0.1 wt.% CNTs has a lower electrical conductivity in the lower temperature range. While not wishing to be bound by theory, this may be due to the CNTs supressing the electrical conductivity in this range. The power factor is also observed to be lower in the 200- 400°C range for the material including CNTs, while the Seebeck coefficient appears substantially unchanged between the two materials. At 500°C, the electrical conductivity and power factor of Na0 _. 03Sn0 _. 97Se with 0.1 wt.% CNTs are higher than for those of Na0 _. 03Sn0 _. 97Se without CNTs (FIG.12A, 12C and FIG. 13A, 13C). While not wishing to be bound by theory, such changes may stem form a semiconducting-semimetals -semiconductor transition that may be occuring around 200 °C. These results will need to be explored further and the CNTs could be acting as a hopping center for the electrons in the host materials. It is unlikely CNTs affected the band gap of the host materials since the Seebeck coefficient is unchanged. Nevertheless, these results indicate that the addition of CNTs can be advantageuously used to manitpulate the electrical transport properties of a thermoelectric material, to modify or tune the material to have preffered properties for a particular application.

[00126] 4. Microhardness

[00127] 4.1. Microhardness of SnSe without CNTs.

[00128] According to Li, J. et al. Substantial Enhancement of Mechanical Properties for SnSe Based Composites with Potassium Titanate Whiskers. J Mater Sci: Mater Electron 2019, 30 (9), 8502-8507, the microhardness of SnSe without CNTs is 660 MPa.

[00129] 4.2. Microhardness of Sno. ₉₇ Nao. ₀₃ Se without CNTs.

[00130] The microhardness of the Na _0.03 Sn _0.97 Se without CNTs is 493 MPa.

[00131] 4.3. Microhardness of Sno. ₉₇ Nao. ₀₃ Se with 0.1 wt.% CNTs.

[00132] The microhardness of the Na _0.03 Sn _0.97 Se with 0.1 wt.% CNTs is 942 MPa. [00133] By comparing the results of 4.2 and 4.3, it can be observed that addition of CNTs significantly improves the mechanical properties of Na0 _. 03Sn0 _. 97Se; the microhardness increased by 91%. These results demonstrate that nanocomposite thermoelectric materials having improved microhardness properties have been prepared.

[00134] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skill in the art without departing from the purpose and scope of the invention as outlined in the claims. It will be appreciated that the examples provided herein are for illustrative purposes and are not intended to limit the invention in any way. Any figures provided herein are for the purpose of illustrating various aspects of the invention and not intended to limit the invention in any way. Different configurations and elements of the material can be used without departing from the principles expressed herein. Various modifications of the materials and methods described herein will be apparent to those skilled in the art as outlined in the appended claims. The disclosures of all prior art cited herein are incorporated by reference in their entirety. Should any reference incorporated herein include a definition that contradicts a definition in the present application the definitions provided herein will be supersede the definition in the incorporated reference.