Thermoelectric materials synthesized by self-propagating high temperature synthesis process and methods thereof

FIELD

The present disclosure relates to thermoelectric materials prepared by self-propagating high temperature synthesis (SHS) process combining with plasma activated sintering (PAS) and a method for preparing the same. More specifically, the present disclosure relates to a new criterion for combustion synthesis and the method for preparing thermoelectric materials which can meet the new criterion.

BACKGROUND

In the heat flow of the energy consumption in the world, there is about 70% of the total energy wasted in the form of heat. If those large quantities of waste heat can be recycled effectively, it would relief the energy crisis in the world. Thermoelectric (TE) materials convert heat into electricity directly through the Seebeck effect. Thermoelectric materials offer many advantages including: no moving parts; small and lightweight; maintenance-free; no pollution; acoustically silent and electrically “quiet”. Thermoelectric energy conversion has drawn a great attention for applications in areas such as solar thermal conversion, industrial waste heat recovery. The efficiency of a TE material is strongly related to its dimensionless figure of merit ZT, defined as ZT=α²σT/κ, where α, σ, κ and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity, and the absolute temperature, respectively. To achieve high efficiency, a large ZT is required. High electrical conductivity, large Seebeck coefficient, and low thermal conductivity are necessary for a high efficient TE material. However those three parameters relate with each other. Hence decoupling the connection of those parameters is key issue to improve the thermoelectric performance. A lot of investigation shows that nanostructure engineering can weak the coupling to enhance the thermoelectric property.

Until now, most researchers have utilized top down approach to obtain nanostructure (mechanic alloy, melt spinning, etc). But all those processing is of high energy consumption. In addition, some investigator used bottom up fabrication to synthesize low dimensional material (Wet chemical method). Efficient synthesis and its adaptability to a large-scale industrial processing are important issues determining the economical viability of the fabrication process. So far, thermoelectric materials have been synthesized mostly by one of the following methods: melting followed by slow cooling; melting followed by long time annealing, multi-step solid state reactions, and mechanical alloying. Each such processing is time and energy consuming and not always easily scalable. Moreover, it is often very difficult to control the desired stoichiometry and microstructure. All those difficulty is of universality in all those thermoelectric material. Hence developing a technology which not only can synthesize the samples in large scale and short period but also can control the composition and microstructure precisely is of vital importance for the large scale application.

Self-propagating high-temperature synthesis (SHS) is a method for synthesizing compounds by exothermic reactions. The SHS method, often referred to also as the combustion synthesis, relies on the ability of highly exothermic reactions to be self-sustaining, i.e., once the reaction is initiated at one point of a mixture of reactants, it propagates through the rest of the mixture like a wave, leaving behind the reacted product. What drives this combustion wave is exothermic heat generated by an adjacent layer. In contrast with some other traditional method, the synthesis process is energy saving, exceptionally rapid and industrially scalable. Moreover, this method does not rely on any equipment. Base on the experiments, Merzhanov suggested an empirical criterion, T_ad>1800 K, as the necessary precondition for self-sustainability of the combustion wave, where T_adis the maximum temperature to which the reacting compact is raised as the combustion wave passes through. It restricts the scope of materials that can be successfully synthesized by SHS processing.

SUMMARY

In order to solve the problem of existing technology, the objects of the present disclosure is to provide an ultra-fast fabrication method for preparing high performance thermoelectric materials. By using this method, it can control the composition very precisely, shorts the synthesis period, and is easy to scale up to kilogram. High thermoelectric performance can be obtained. Moreover, we found that the criterion often quoted in the literature as the necessary precondition for self-sustainability of the combustion wave, T_ad≥1800 K, where T_adis the maximum temperature to which the reacting compact is raised as the combustion wave passes through, is not universal and certainly not applicable to thermoelectric compound semiconductors. Instead, we offer new empirically-based criterion, T_ad/T_mL>1, i.e., the adiabatic temperature must be high enough to melt the lower melting point component. This new criterion covers all materials synthesized by SHS, including the high temperature refractory compounds for which the T_ad≥1800 K criterion was originally developed. Our work opens a new avenue for ultra-fast, low cost, mass production fabrication of efficient thermoelectric materials and the new insight into the combustion process greatly broadens the scope of materials that can be successfully synthesized by SHS processing.

In accordance with the present disclosure, the above objects of the present disclosure can be achieved by the following steps.

1. The new criterion for the combustion synthesis of binary compounds is as following.

1) The adiabatic temperatures T_adof the binary compounds are calculated by thermodynamic data (enthalpy of formation and the molar specific heat of the product) and Eq. (1). Where Δ_fH_298Kis enthalpy of formation for the binary compounds, T is temperature, H_298K⁰is the enthalpy of the binary compounds at 298 K, and C is the molar specific heat of the product and the integral includes latent heats of melting, vaporization, and phase transitions, if any present. The reactants for the combustion reaction are pure elemental for the binary compounds.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adCdT (1)

When there is no phase transition and the adiabatic temperature is lower than the melting point of the binary compound, Equation (1) can be simplified into Equation (2) shown below, where C_pis the the molar specific heat of the product in solid state.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adC_pdT (2)

When there is no phase transition and the adiabatic temperature is higher than the melting point of the binary compound and lower than the boiling point of of the binary compound, Equation (1) can be simplified into Equation (3) shown below, where C_p, C″_pis the the molar specific heat of the product in solid state and liquid state respectively, T_mis the melting point of the binary compound, ΔH_mis the enthalpy change during fusion processing.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^mC_pdT+ΔH_m+∫_T_m^T^adC″_pdT (3)

When there is no phase transition and the adiabatic temperature is higher than the boiling point of of the binary compound, Equation (1) can be simplified into Equation (4) shown below, where C_p, C″_p, C′″_pis the the molar specific heat of the product in solid, liquid and gaseous state respectively, T_m, T_bis the melting point and boiling point of the binary compound, respectively. ΔH_m, ΔH_bis the enthalpy change during fusion and gasification processing repectively.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^mC_pdT+ΔH_m+∫_T_m^T^BC″_pdT+ΔH_B+∫_T_B^T^adC′″_pdT (4)

When phase transition exists during the heating processing and the adiabatic temperature is higher than the phase transition temperature of the binary compound, the Equation (1) can be simplified into Equation (5) as below, where C_p, C′_pis the the molar specific heat of the product in solid before or after phase transition respectively, T_tris the phase transition temperature of the binary compound, ΔH_tris the enthalpy change during phase transition processing.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^trC_pdT+ΔH_tr+∫_T_tr^T^adC′_pdT (5)

When phase transition exists during the heating processing and the adiabatic temperature is higher than the phase transition temperature and the melting point of the binary compound, the Equation (1) can be simplified into Equation (6) as below, where C_p, C′_p, C″_pis the molar specific heat of the product in solid before or after phase transition and the molar specific heat of the product in liquid state respectively, T_tr, T_mis the phase transition temperature and melting point of the binary compound respectively, ΔH_tr, ΔH_mis the enthalpy change during phase transition processing and fusion processing.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^trC_pdT+ΔH_tr+∫_T_tr^T^mC′_pdT+ΔH_m+∫_T_m^T^adC″_pdT (6)

When phase transition exists during the heating processing and the adiabatic temperature is higher than the phase transition temperature and the boiling point of the binary compound, the Equation (1) can be simplified into Equation (7) as below, where C_p, C′_p, C″_pis the molar specific heat of the product in solid before or after phase transition and the molar specific heat of the product in liquid state respectively, T_tr, T_mis the phase transition temperature and melting point of the binary compound respectively, ΔH_tr, ΔH_mis the enthalpy change during phase transition processing and fusion processing.

$\begin{matrix} - Δ_{f} H_{298 K} = H_{T}^{0} - H_{298 K}^{0} = \int_{298 K}^{T_{tr}} C_{p} dT + Δ H_{tr} + \int_{T_{tr}}^{T_{m}} C_{p}^{'} dT + Δ H_{m} + \int_{T_{m}}^{T_{B}} C_{p}^{″} dT + Δ H_{B} + \int_{T_{b}}^{T_{ad}} C_{p}^{′′′} dT & (7) \end{matrix}$

2. T_mLrepresents the melting point of the component with lower melting point. The SHS reaction to be self-sustaining, the value of T_ad/T_m,Lshould be more than 1, i.e., the heat released in the reaction must be high enough to melt the component with the lower melting point, or the combustion wave can not be self propagated.

3. Based on the new criterion for combustion synthesis of thermoelectric compounds, the above and other objects can be accomplished by the provision of a method for preparing thermoelectric materials by SHS combining Plasma activated sintering which comprises following steps:

1) Choose two single elemental as the starting material for the reaction
2) The adiabatic temperatures T_adof the binary compounds are calculated by thermodynamic data (enthalpy of formation and the molar specific heat of the product) and Eq. (1). Where Δ_fH_298Kis enthalpy of formation for the binary compounds, T is temperature, H_298K⁰is the enthalpy of the binary compounds at 298 K, and C is the molar specific heat of the product and the integral includes latent heats of melting, vaporization, and phase transitions, if any present. The reactants for the combustion reaction are pure elemental for the binary compounds.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adCdT (1)

When there is no phase transition and the adiabatic temperature is lower than the melting point of the binary compound, the Equation (1) can be simplified into Equation (2) as below, where C_pis the the molar specific heat of the product in solid state.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adC_pdT (2)

When there is no phase transition and the adiabatic temperature is higher than the melting point of the binary compound and lower than the boiling point of of the binary compound, the Equation (1) can be simplified into Equation (3) as below, where C_p, C″_pis the the molar specific heat of the product in solid state and liquid state respectively, T_mis the melting point of the binary compound, ΔH_mis the enthalpy change during fusion processing.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^mC_pdT+ΔH_m+∫_T_m^T^adC″_pdT (3)

When there is no phase transition and the adiabatic temperature is higher than the boiling point of of the binary compound, the Equation (1) can be simplified into Equation (4) as below, where C_p, C″_p, C′″_pis the the molar specific heat of the product in solid, liquid and gaseous state respectively, T_m, T_bis the melting point and boiling point of the binary compound, respectively. ΔH_m, ΔH_bis the enthalpy change during fusion and gasification processing repectively.

$\begin{matrix} - Δ_{f} H_{298 K} = H_{T}^{0} - H_{298 K}^{0} = \int_{298 K}^{T_{m}} C_{p} dT + Δ H_{m} + \int_{T_{m}}^{T_{B}} C_{p}^{″} dT + Δ H_{B} + \int_{T_{B}}^{T_{ad}} C_{p}^{′′′} dT & (4) \end{matrix}$

3) T_mLrepresents the melting point of the component with lower melting point. The SHS reaction to be self-sustaining, the value of T_ad/T_m,Lshould be more than 1, i.e., the heat released in the reaction must be high enough to melt the component with the lower melting point, or the combustion wave can not be self propagated.

4) Self propagating high temperature synthesis: Stoichiometric amounts of single elemental powders with high purity were weighed and mixed in the agate mortar and then cold-pressed into a pellet. The pellet obtained was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. Single phase binary compounds are obtained after SHS.

According to the above step, the binary compounds are mostly thermoelectric material, high temperature ceramics and intermetallic.

According to the above step, the purity of the single elemental powder is better than 99.99%.

According to the above step, the pellet was sealed in a silica tube under the pressure of 10⁻³Pa or Ar atmosphere. The components react under the pressure of 10⁻³Pa or Ar atmosphere.

According to the above step, the pellet after SHS was crushed into powders and then sintered by spark plasma sintering to obtain the bulks.

Moreover, we found that the criterion suggested by Merzhanov as the necessary precondition for self-sustainability of the combustion wave, T_ad≥1800 K, where T_adis the maximum temperature to which the reacting compact is raised as the combustion wave passes through, is not universal and certainly not applicable to thermoelectric compound semiconductors. Instead, we offer new empirically-based criterion, T_ad/T_mL>1, i.e., the adiabatic temperature must be high enough to melt the lower melting point component. When this happens, the higher melting point component rapidly dissolves in the liquid phase of the first component and generates heat at a rate high enough to sustain propagation of the combustion wave. This new criterion covers all materials synthesized by SHS, including the high temperature refractory compounds for which the T_ad≥1800 K criterion was originally developed. Our work opens a new avenue for ultra-fast, low cost, mass production fabrication of efficient thermoelectric materials and the new insight into the combustion process greatly broadens the scope of materials that can be successfully synthesized by SHS processing.

It is another object for present disclosure to provide a method for preparing ternary or quarternary thermoelectric materials. Choose elemental powder with high purity as the starting material for the reaction. Stoichiometric amounts of single elemental powders with high purity were weighed and mixed in the agate mortar and then cold-pressed into a pellet. The pellet obtained was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. Single phase compounds are obtained after SHS. The pellet was crushed into powder and then sintered by spark plasma sintering to otain the bulk thermoelectric materials. The detailed synthesis procedure for ternary or quarternary thermoelectric materials is as following.

The ultra-fast synthesis method for preparing high performance Half-Heusler thermoelectric materials with low cost comprises the steps of

1) Stoichiometric amounts ABX of high purity single elemental A, B, X powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet.
2) The pellet was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water.
3) The obtained pellet in step 2) was crushed, hand ground into a fine powder, and then sintered by PAS. The densely bulks half heusler with excellent thermoelectric properties is obtained after PAS.

In step 1), what we choose for elemental A can be the elemental in IIIB, IVB, and VB column of periodic Table, such as one of or the mixture of the Ti, Zr, Hf, Sc, Y, La, V, Nb, Ta. What we choose for elemental B can be the elemental in VIIIB column of periodic Table, such as one of or the mixture of the Fe, Co, Ni, Ru, Rh, Pd, and Pt. What we choose for elemental B can be the elemental in IIIA, IVA, VA column of periodic Table, such as one of or the mixture of the Sn, Sb, and Bi. In step 3), the parameter for spark plasma sintering is with the temperature above 850° C. and the pressure around 30-50 MPa.

The detail of the ultra-fast preparation method of high performance BiCuSeO based thermoelectric material is as following.

1) Weigh Bi₂O₃, PbO, Bi, Cu, and Se according to the stoichiometric ratio (1-p):3p:(1-p):3:3 (p=0, 0.02, 0.04, 0.06, 0.08, 0.1) and mix them in the agate mortar and then cold-pressed into a pellet.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water.
3) The obtained pellet Bi_1-pPb_pCuSe in step 2) was crushed, hand ground into a fine powder, and then sintered by PAS. The densely bulks Bi_1-pPb_pCuSe with excellent thermoelectric properties is obtained after PAS.

In step 3), the parameter for spark plasma sintering is with the temperature above 670° C. and the pressure of 30 MPa holding for 5-7 min.

The detail of the ultra-fast preparation method of high performance Bi₂Te₃based thermoelectric material is as following.

1) Stoichiometric amounts Bi₂Te_3-xSe_xof high purity single elemental Bi, Te, Se powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water.
3) The obtained pellet Bi₂Te_3-xSe_xin step 2) was crushed, hand ground into a fine powder, and then sintered by PAS. The densely bulks Bi₂Te_3-xSe_xwith excellent thermoelectric properties is obtained after PAS.

In step 3), load the Bi₂Te_3-xSe_xpowder with single phase into the graph die. The parameter for spark plasma sintering is with the temperature around 420-480° C. and the pressure of 20 MPa holding for 5 min.

The detail of the ultra-fast preparation method of high performance PbS_1-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts PbS_1-xSe_xof high purity single elemental Pb, S, Se powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water.
3) The obtained pellet PbS_1-xSe_xin step 2) was crushed, hand ground into a fine powder, and then sintered by PAS. The densely bulks PbS_1-xSe_xwith excellent thermoelectric properties is obtained after PAS.

In step 3), load the PbS_1-xSe_xpowder with single phase into the graphite die. The parameter for spark plasma sintering is with the temperature of 550° C. and the pressure of 35 MPa holding for 7 min.

The detail of the ultra-fast preparation method of high performance Mg₂Si based thermoelectric material is as following.

1) Stoichiometric amounts Mg_2(1+0.02)Si_1-nSb_n(0≤n≤0.025) of high purity single elemental Mg, Si, Sb powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water.
3) The obtained pellet Mg_2(1+0.02)Si_1-nSb_n(0≤n≤0.025) in step 2) was crushed, hand ground into a fine powder, and then sintered by PAS. The densely bulks PbS_1-xSe_xwith excellent thermoelectric properties is obtained after PAS.

In step 3), load the Mg_2(1+0.02)Si_1-nSb_n(0≤n≤0.025) powder with single phase into the graphite die. The parameter for spark plasma sintering is with the temperature of 800° C. with the heating rate 100° C./min and the pressure of 33 MPa holding for 7 min. Since the content of Sb in Mg_2(1+0.02)Si_1-nSb_n(0≤n≤0.025) is very low, the impact of Sb on the SHS processing can be ignored.

The detail of the ultra-fast preparation method of high performance Cu_aMSn_bSe₄thermoelectric material is as following.

1) Stoichiometric amounts Cu_aMSn_bSe₄(M=Sb, Zn, or Cd; a=2 or 3; b=1 or 0) of high purity single elemental Cu, M, Sn, Se powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet. For Cu₃SbSe₄, Weigh the elemental Cu, Sb Se powder according to the ratio of Cu:Sb:Se=3:(1.01˜1.02):4, and mixed in the agate mortar and then cold-pressed into a pellet. For Cu₂ZnSnSe₄, Weigh the elemental Cu, Zn, Sn, Se powder according to the ratio of Cu:Zn:Sn:Se=2:1:1:4, and mixed in the agate mortar and then cold-pressed into a pellet. For Cu₂CdSnSe₄, Weigh the elemental Cu, Cd, Sn, Se powder according to the ratio of Cu:Cd:Sn:Se=2:1:1:4, and mixed in the agate mortar and then cold-pressed into a pellet.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 1^0-3Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water. The obtained pellet Cu_aMSn_bSe₄in step 2) was crushed, hand ground into a fine powder.

The detail of the ultra-fast preparation method of high performance Cu₂SnSe₃thermoelectric material is as following.

1) Weigh high purity single elemental Cu, Sn, Se powders according to the ratio of Cu:Se:Sn=2.02:3.03:1 and mixed in the agate mortar and then cold-pressed into a pellet.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water.
3) The obtained pellet Cu₂SnSe₃in step 2) was crushed, hand ground into a fine powder, and then sintered by PAS. The densely bulks Cu₂SnSe₃with excellent thermoelectric properties is obtained after PAS.

In step 3), load the Cu₂SnSe₃powder with single phase into the graphite die. The parameter for spark plasma sintering is with the temperature around 500-550° C. with the heating rate 50-100° C./min and the pressure around 30-35 MPa holding for 5-7 min.

The detail of the ultra-fast preparation method of high performance CoSb₃based thermoelectric material is as following.

1) Stoichiometric amounts Co_4-eM_eSb_12-fTe_f(0≤e≤1.0, 0≤f≤1.0, M=Fe or Ni) of high purity single elemental Co, M, Sb, Te powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air or quenched in the salt water.
3) The obtained pellet Co_4-eM_eSb_12-fTe_f(0≤e≤1.0, 0≤f≤1.0, M=Fe or Ni) in step 2) was crushed, hand ground into a fine powder, and then sintered by PAS. The densely bulks Co_4-eM_eSb_12-fTe_f(0≤e≤1.0, 0≤f≤1.0, M=Fe or Ni) with excellent thermoelectric properties is obtained after PAS.

In step 3), load the Co_4-eM_eSb_12-fTe_f(0≤e≤1.0, 0≤f≤1.0, M=Fe or Ni) powder with single phase into the graphite die. The parameter for spark plasma sintering is with the temperature of 650° C. with the heating rate 100° C./min and the pressure of 40 MPa holding for 8 min.

Compared with the conventional synthesis technique, the advantage of the disclosure is as below.

- 1. SHS method is very convenient and does not rely on any equipment. But for some other methods such as Mechanic alloy, Melt spinning, etc all those processing demand complicated equipments. For chemical method, the yield is very low and it is very difficult to condense the sample. Moreover all those processing except SHS processing is energy consuming. Self-propagating high-temperature synthesis (SHS) is a method for synthesizing compounds by exothermic reactions. The SHS method, often referred to also as the combustion synthesis, relies on the ability of highly exothermic reactions to be self-sustaining, i.e., once the reaction is initiated at one point of a mixture of reactants, it propagates through the rest of the mixture like a wave, leaving behind the reacted product. What drives this combustion wave is exothermic heat generated by an adjacent layer. In contrast with some other traditional method, the synthesis process is energy saving, exceptionally rapid and industrially scalable.
- 2. Since Self-propagating high-temperature synthesis (SHS) can be finished in a very short time. It can control the composition very precisely. Moreover, the Non-equibrium microstructure can be obtained since large temperature gradient exists during the SHS processing.
- 3. It shortens the synthesis periods very significantly by about 90% in comparson with conventional method.

Based on the above content, without departing from the basic technical concept of the present disclosure, under the premise of ordinary skill in the art based on the knowledge and means of its contents can also have various forms of modification, substitution or changes, such as T_ad>T_mL, or T_mL<T_ad.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows Powder XRD pattern of compounds thermoelectric after SHS for embodiment example 1.

FIG. 2 shows Powder XRD pattern of Sb₂Te₃and MnSi_1.70pellets after SHS in different region for embodiment example 2.

FIG. 3 shows the ratio of between T_adand T_mLfor compounds thermoelectrics PbS, PbSe, Mg₂Si, Mg₂Sn, Cu₂Se, Bi₂Se₃, PbTe, Bi₂Te₃in embodiment example 1 and high temperature intermetallic and refractory in embodiment example 3.

FIG. 4 shows XRD pattern of Cu₂Se after SHS (in step 2) and after SHS-PAS (in step 3) of embodiment example 4

FIG. 5 shows FESEM image of Cu₂Se after SHS (in step 2) of embodiment example 4

FIGS. 6(a) and (b) show FESEM images of Cu₂Se after SHS-PAS (in step 3) of embodiment example 4.

FIG. 7 shows the temperature dependence of ZT (in step 3) of embodiment example 4.

FIG. 8 shows XRD pattern of the powder in step 2 of embodiment example 5.1 and bulk in step 3 of embodiment example 5.1

FIG. 9 shows the microstructure of the powder in step 2 of embodiment example 5.1.

FIG. 10 shows XRD pattern of the powder in step 2 of embodiment example 5.2

FIG. 11 shows the XRD pattern of the powder in step 2 of embodiment example 5.3 and bulk in step 3 of embodiment example 5.3

FIG. 12 shows the temperature dependence of power factor and ZT of bulks obtained in step 3 of embodiment example 5.3

FIG. 13 shows the XRD pattern of the powder obtained in step 2 of embodiment example 6

FIG. 14 shows the XRD pattern of the Bi₂Te_2.7Se_0.3compound in step 2 of embodiment example 7.1 and Bi₂Te_2.7Se_0.3bulk in step 3 of embodiment example 7.1

FIG. 15(a) shows FESEM image of Bi₂Te_2.7Se_0.3after SHS-PAS (in step 3) of embodiment example 7.1. FIG. 15(b) shows enlarged FESEM image of Bi₂Te_2.7Se_0.3after SHS-PAS.

FIG. 16 shows temperature dependence of ZT for Bi₂Te_2.7Se_0.3compound (in step 3) of embodiment example 7.1 and the data from the reference.

FIG. 17 shows the XRD pattern of the Bi₂Te_2.7Se_0.3compound in step 2 of embodiment example 7.2

FIG. 18 shows the XRD pattern of the Bi₂Te₂Se compound in step 2 of embodiment example 7.3

FIG. 19 shows the XRD pattern of powder after SHS in embodiment example 8.1

FIG. 20 shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 8.2

FIG. 21 shows the XRD pattern of powder after SHS in embodiment example 8.3

FIG. 22 shows the XRD pattern of powder after SHS in embodiment example 8.4

FIG. 23(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 8.5. FIG. 23(b) shows SEM image of the powder after SHS (with the magnification 5000 and 8000) in embodiment example 8.4. FIG. 23(c) shows the temperature dependence of ZT in compareson with the sample synthesized by melting method in embodiment example 8.4.

FIG. 24(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 9.1. FIG. 24(b) shows SEM image of the powder after SHS (with the magnification 5000 and 10000) in embodiment example 9.1. FIG. 24(c) shows SEM image of the bulks after SHS-PAS (with the magnification 2000 and 10000) in embodiment example 9.1.

FIG. 25(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 9.2. FIG. 25(b) shows SEM image of the powder after SHS (with the magnification 5000 and 10000) in embodiment example 9.2. FIG. 25(c) shows SEM image of the bulks after SHS-PAS (with the magnification 2000 and 10000) in embodiment example 9.2.

FIG. 26(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 9.3. FIG. 26(b) shows SEM image of the powder after SHS (with the magnification 5000 and 10000) in embodiment example 9.3. FIG. 26(c) shows SEM image of the bulks after SHS-PAS (with the magnification 2000 and 10000) in embodiment example 9.3.

FIG. 27(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 9.4. FIG. 27(b) shows SEM image of the powder after SHS (with the magnification 5000 and 10000) in embodiment example 9.4. FIG. 27(c) shows SEM image of the bulks after SHS-PAS (with the magnification 2000 and 10000) in embodiment example 9.4.

FIG. 28(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 9.5. FIG. 28(b) shows SEM image of the powder after SHS (with the magnification 5000 and 10000) in embodiment example 9.5. FIG. 28(c) shows SEM image of the bulks after SHS-PAS (with the magnification 2000 and 10000) in embodiment example 9.5. FIG. 28(d) shows the temperature dependence of ZT in compareson with the sample synthesized by other method in embodiment example 9.5.

FIG. 29 shows the XRD pattern of Cu₃SbSe₄powder after SHS in step 3 of embodiment example 10.1.

FIG. 30 shows the XRD pattern of Cu₃SbSe₄powder after SHS in step 3 of embodiment example 10.2.

FIG. 31 shows the XRD pattern of Cu₂ZnSnSe₄powder after SHS in step 3 of embodiment example 10.3.

FIG. 32 shows the XRD pattern of Cu₂ZnSnSe₄powder after SHS in step 3 of embodiment example 10.4.

FIG. 33 shows the XRD pattern of Cu₂CdSnSe₄powder after SHS in step 3 of embodiment example 10.5.

FIG. 34 shows the XRD pattern of Cu₃SbSe₄powder after SHS in step 3 of embodiment example 10.6.

FIG. 35 shows the XRD pattern of Cu₂SnSe₃powder after SHS in step 2 of embodiment example 11.1

FIG. 36 shows the XRD pattern of Cu₂SnSe₃powder after SHS in step 2 of embodiment example 11.2

FIG. 37 shows the XRD pattern of Cu₂SnSe₃powder after SHS-PAS of embodiment example 11.2

FIG. 38 shows the temperature dependence of ZT for Cu₂SnSe₃in embodiment example 11.2

FIG. 39 shows the XRD pattern of Cu₂SnSe₃powder after SHS in embodiment example 11.3

FIG. 40(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 12.1. FIG. 40(b) shows SEM image of the powder after SHS (with the magnification 5000 and 20000) in step 2 of embodiment example 12.1. FIG. 40(c) shows SEM image of the bulks after SHS-PAS (with the magnification 5000 and 20000) in step 3 of embodiment example 12.1.

FIG. 41(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 12.2. FIG. 41(b) shows SEM image of the powder after SHS (with the magnification 5000 and 20000) in step 2 of embodiment example 12.2. FIG. 41(c) shows SEM image of the bulks after SHS-PAS (with the magnification 5000 and 20000) in step 3 of embodiment example 12.2.

FIG. 42(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 12.3. FIG. 42(b) shows SEM image of the powder after SHS (with the magnification 5000 and 20000) in step 2 of embodiment example 12.3. FIG. 42(c) shows SEM image of the bulks after SHS-PAS (with the magnification 5000 and 20000) in step 3 of embodiment example 12.3.

FIG. 43(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 12.4. FIG. 43(b) shows SEM image of the powder after SHS (with the magnification 5000 and 20000) in step 2 of embodiment example 12.4. FIG. 43(c) shows SEM image of the bulks after SHS-PAS (with the magnification 5000 and 20000) in step 3 of embodiment example 12.4.

FIG. 44(a) shows the XRD pattern of powder after SHS and after SHS-PAS of embodiment example 12.5. FIG. 44(b) shows SEM image of the powder after SHS (with the magnification 5000 and 20000) in step 2 of embodiment example 12.5. FIG. 44(c) shows SEM image of the bulks after SHS-PAS (with the magnification 5000 and 20000) in step 3 of embodiment example 12.5.

FIG. 45(a) shows the temperature dependence of ZT for Co_3.5Ni_0.5Sb₁₂in step 3 of embodiment example 12.1 compared with the data from reference.(in the reference, the sample synthesized by Melt-annealing and PAS. It takes about 240 h)

FIG. 45(b) shows the temperature dependence of ZT for Co₄Sb_11.4Te_0.6in step 3 of embodiment example 12.5 compared with the data from reference. (In the reference, the sample is synthesized by Melt-annealing and PAS. It takes about 168 h)

DETAILED DESCRIPTION

For a better understanding of the present disclosure, several embodiments are given to further illustrate the disclosure, but the present disclosure is not limited to the following embodiments

EMBODIMENT EXAMPLE 1
Embodiment Example 1.1

Based on the new criterion, the detailed synthesis procedure of Bi₂Te₃is as following.

(1) Elemental Bi, Te powder with high purity were Chosen as starting material.

(2) The adiabatic temperature can be calculated by using molar enthalpy of forming Bi₂Te₃and the molar heat capacity according to the following formula. The molar enthalpy of forming Bi₂Te₃at 298K Δ_fH_298Kis −78.659 kJ·mol⁻¹
−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adCdT

Assuming the adiabatic temperature is lower than the melting point of Bi₂Te₃, there is no phase transition during the combustion processing. The above formula can be simplified as below.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adC_pdT

The molar heat capacity of Bi₂Te₃in solid state is 107.989+55.229×10⁻³T JK⁻¹mol⁻¹, solve the equation and then the adiabatic temperature can be obtained as 860 K. Since the calculated adiabatic temperature is 860 K, which is lower than the melting point of Bi₂Te₃. The result obtained is consistent with the assumpation. Hence the adiabatic temperature is 860 K.

$\begin{matrix} Δ_{f} H_{298 K}^{0} = - 78.659 k J {mol}^{- 1} \\ = - \int_{298 K}^{T_{ad}} (107.989 + 55.229 \times 10^{- 3} T) dT \\ = - [107.989 \times (T_{ad} - 298) + 0.5 \times 55.229 \times 10^{- 3} \times (T_{ad}^{2} - 298^{2})] \end{matrix}$

(3) Since the molten point of Te and Bi is 722.5 K, 544.44 K respectively. The component with lower melting point is Bi. The ratio between the adiabatic temperature and the melting point of the component with lower melting point is 1.58. According to the new criterion for combustion synthesis, self propogating high temperature reaction between Bi and Te can be self sustained.

(4) The SHS synthesis of Bi₂Te₃can be achieved by the following steps.

a) Stoichiometric amounts of high purity Bi(4N), and Te(4N) powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet with the dimension of ϕ15×18 mm under the pressure 8 MPa holding for 10 min.

b) The pellet obtained in the step a) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample. Once started, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air.

c) The obtained pellet in the step b) was crushed, hand ground into a fine powder, Single phase Bi₂Te₃compounds is obtained.

Embodiment Example 1.2

Based on the new criterion, the detailed synthesis procedure of Cu₂Se is as following.

(1) Elemental Cu, Se powder with high purity were Chosen as starting material.

(2) The adiabatic temperature can be calculated by using molar enthalpy of forming Cu₂Se and the molar heat capacity according to the following formula. The molar enthalpy of forming Cu₂Se at 298K Δ_fH_298Kis −66.107 kJmol⁻¹.

−×_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adCdT

Assuming the adiabatic temperature is lower than the temperature of α-β phase transition of Cu₂Se, there is no phase transition during the combustion processing. The above formula can be simplified as below.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adC_pdT

The molar specific heat capacity in solid state of α phase Cu₂Se is 58.576+0.077404T Jmol⁻¹K⁻¹. Substitute the equitation with the heat capacity and molar enthalpy of forming Cu₂Se. And solve the equation. The calculated adiabatic temperature can be obtained as 922.7 K, which is much higher than the temperature of α-β phase transition of Cu₂Se corresponding to 395 K. it is inconsistent with the hypothesis.

Assuming the adiabatic temperature is higher than the phase transition temperature but is lower than the molten point of Cu₂Se, the formula can be simplified as below.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^trC_pdT+ΔH_tr+∫_T_tr^T^adC′_pdT

The molar specific heat capacity in solid state of α phase and β phase Cu₂Se are 58.576+0.077404T Jmol⁻¹K⁻¹, 84.098 Jmol⁻¹K⁻¹, respectively. The molar enthalpy of α-β phase transition of Cu₂Se is 6.820 KJ·mol⁻¹. We substitute the equation with the specific heat capacity and molar enthalpy, and solve the equation. The adiabatic temperature can be obtained as 1001.5 K, which is higher than the α-β phase transition temperature and lower than the molten point of Cu₂Se. It is consistent with the hypothesis. Hence the adiabatic temperature is 1001.5 K.

66107=∫_2298K^395K(58.576+0.077404T)dT+6820+∫_395K^T^ad84.098dT

(3) Since the molten point of Cu and Se is 1357 K, 494 K respectively. The component with lower melting point is Se. The ratio between the adiabatic temperature and the melting point of the component with lower melting point is 2.03. According to the new criterion for combustion synthesis, self propogating high temperature reaction between Cu and Se can be self sustained.

Embodiment Example 1.3

Based on the new criterion, the detailed synthesis procedure of PbS is as following.

(1) Elemental Pb, S powder with high purity were Chosen as starting material.

(2) The adiabatic temperature can be calculated by using molar enthalpy of forming PbS and the molar heat capacity according to the following formula. The molar enthalpy of forming PbS at 298K Δ_fH_298Kis −98.324 kJmol⁻¹.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adCdT

Assuming the adiabatic temperature is lower than the molten temperature of PbS, there is no phase transition during the combustion processing. The above formula can be simplified as below.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adC_pdT

The molar specific heat capacity of PbS in solid state is 46.735+0.009205T Jmol⁻¹K⁻¹. Substitute the equitation with the heat capacity and molar enthalpy of forming PbS. And solve the equation.

98324=∫_298K^T^ad(46.435+0.009205T)dT

The calculated adiabatic temperature can be obtained as 2023 K, which is much higher than the molten point of PbS corresponding to 1392 K. it is inconsistent with the hypothesis.

Assuming the adiabatic temperature is higher than the molten point but is lower than the boiling point of PbS, the formula can be simplified as below.

−Δ_fH_298K=H_298K⁰−H_T⁰=∫_298K^T^mC_pdT+ΔH_m+∫_T_m^T^adC″_pdT

The molar specific heat capacity of PbS in solid state is 46.735+0.009205T Jmol⁻¹K⁻¹. The molar specific heat capacity of PbS in liquid state is 61.923 Jmol⁻¹K⁻¹. The molar enthalpy between solid state and liquid state is 36.401 KJmol⁻¹. We substitute the equation with the specific heat capacity and molar enthalpy, and solve the equation. The adiabatic temperature can be obtained as 1427 K, which is higher than the molten point (1392 K) and lower than the boiling point (1609 K) of PbS. it is consistent with the hypothesis. Hence the adiabatic temperature is 1427 K.

98324=∫_298K^{1392 K}(46.435+0.009205T)dT+36401+∫_1392K^T^ad61.923dT

(3) Since the molten point of Pb and S is 600 K, 388 K respectively. The component with lower melting point is S. The ratio between the adiabatic temperature and the melting point of the component with lower melting point is 3.68. According to the new criterion for combustion synthesis, self propogating high temperature reaction between Pb and S can be self sustained.

By using the method above, the ratio between adiabatic temperature and the molten point of lower molten point component of Bi₂Se₃, PbSe, Mg₂Sn and Mg₂Si are calculated as shown in table 1. The ratio between adiabatic temperature and the molten point of lower molten point component of those compounds thermoelectric is larger than unit. Hence, all those compounds thermoelectric can be synthesized by SHS by choosing single elemental as starting materials. However, the adiabatic temperature of all those compounds is dramatically lower than 1800 K. As an example, the well-known and important thermoelectric compounds Bi₂Te₃and Bi₂Se₃have their adiabatic temperature well below 1000 K. According to the criterion T_ad≥1800 K suggested by Merzhanov, the reaction leading to their formation should not have been self-sustaining. Obviously, the criterion fails in the case of compound semiconductors.

TABLE 1

Parameters of SHS for thermoelectric materials.

Adiabatic

temper-

Material

Molar enthalpy
Specific Heat capacity
ature

system
Reaction
(kJmol⁻¹)
(JK⁻¹mol⁻¹)
(T_ad/K)
T_ad/T_m,L

Bi₂Te₃
2Bi + 3Te→Bi₂Te₃
Δ_fH⁰_298K: −78.659
107.989 + 55.229 × 10⁻³ T
860
1.58

Bi₂Se₃
2Bi + 3Se→Bi₂Se₃
Δ_fH⁰_298K: −139.955
86.818 + 48.953 × 10⁻³ T
995
2.01

ΔmH⁰_995K: 85.772

Cu₂Se
2Cu + Se→Cu₂Se
Δ_fH⁰_298K: −66.107
58.576 + 77.404 × 10⁻³ T
1001
2.03

ΔtH⁰_395K: 6.820
84.098

PbS
Pb + S→PbS
Δ_fH⁰_298K: −98.324
46.735 + 9.205 × 10⁻³ T
1427
3.68

ΔmH⁰_1392K: 36.401
61.923

PbSe
Pb + Se→PbSe
Δ_fH⁰_298K: −99.998
47.237 + 10.000 × 10⁻³ T
1350
2.73

ΔmH⁰_1350K: 49.371

Mg₂Sn
2Mg + Sn→Mg₂Sn
Δ_fH⁰_298K: −80.000
68.331 + 35.797 × 10⁻³ T +
1053
2.01

1.919 × 10⁵T⁻²

Mg₂Si
2Mg + Si→Mg₂Si
Δ_fH⁰_298K: −79.496
107.989 + 55.229 × 10⁻³T
1282
1.39

Based on the success with the combustion synthesis of Cu₂Se, we apply the SHS technique to Bi₂Te₃, Bi₂Se₃, Cu₂Se, PbTe, PbS, PbSe, SnTe, Mg₂Sn and Mg₂Si compounds thermoelectric. In each case, high purity powders are used as a starting material and weighed according to the desired stoichiometry above. The powders are mixed in an agate mortar and are pressed into pellets. Each respective pellet is sealed in a silica tube under the pressure of 10⁻³Pa. The pellets are locally ignited at the bottom by the flame of a torch.

FIG. 1 shows XRD pattern of the powder after SHS in embodiment example 1, which indicate that single phase Bi₂Te₃, Bi₂Se₃, Cu₂Se, PbS, PbSe, Mg₂Sn and Mg₂Si can be obtained after SHS directly. Hence, all compounds which can meet the new criterion specifying that the SHS process will proceed whenever the adiabatic temperature exceeds the melting point of the lower melting point component of the compact can be synthesized by SHS.

EMBODIMENT EXAMPLE 2
Embodiment Example 2.1

Based on the new criterion, the detailed synthesis procedure of MnSi 1.70 is as following.

(1) Elemental Mn, Si powder with high purity were Chosen as starting material.

(2) The adiabatic temperature can be calculated by using molar enthalpy of forming MnSi_1.70and the molar heat capacity according to the following formula. The molar enthalpy of forming MnSi_1.70at 298K Δ_fH_298Kis 75.60 kJmol⁻¹.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adCdT

Assuming the adiabatic temperature is lower than the molten point of MnSi_1.70corresponding to 1425 K, there is no phase transition during the combustion processing. The above formula can be simplified as below.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adC_pdT

The molar specific heat capacity of MnS_1.70in solid state is 71.927+4.615×10⁻³T−13.067×10⁵T⁻²JK⁻¹mol^{31 1}. Substitute the equitation with the heat capacity and molar enthalpy of forming MnSi_1.70. And solve the equation. The calculated adiabatic temperature can be obtained as 1314 K, which is lower than the molten point of MnSi_1.70corresponding to 1425 K. it is consistent with the hypothesis. Hence the adiabatic temperature is 1314 K.

$\begin{matrix} Δ_{f} H_{298 K}^{0} = - 75.601 k J {mol}^{- 1} \\ = - \int_{298 K}^{T_{ad}} (71.927 + 4.615 \times 10^{- 3} T - 13.067 \times 10^{5} T^{- 2}) dT \\ = - [71.927 \times (T_{ad} - 298) + 0.5 \times 4.615 \times (T_{ad}^{2} - 298^{2}) + \\ 13.067 \times 10^{5} (T_{ad}^{- 1} - 1 / 298)] \end{matrix}$

(3) Since the molten point of Mn and Si is 1519 K, 1687 K respectively. The component with lower melting point is Mn. The ratio between the adiabatic temperature and the molten point of the component with lower molten point is 0.88. According to the new criterion for combustion synthesis, self propagating high temperature reaction between Mn and Si to form MnSi_1.70cannot be self sustained.

Embodiment Example 2.2

Based on the new criterion, the detailed synthesis procedure of Sb₂Te₃is as following.

(1) Elemental Sb, Te powder with high purity were Chosen as starting material.

(2) The adiabatic temperature can be calculated by using molar enthalpy of forming Sb₂Te₃and the molar heat capacity according to the following formula. The molar enthalpy of forming Sb₂Te₃at 298K Δ_fH_298Kis −56.484 kJmol⁻¹.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adCdT

Assuming the adiabatic temperature is lower than the molten point of Sb₂Te₃corresponding to 890.7 K, there is no phase transition during the combustion processing. The above formula can be simplified as below.

−Δ_fH_298K=H_T⁰−H_298K⁰=∫_298K^T^adC_pdT

The molar specific heat capacity of Sb₂Te₃in solid state is 112.884+53.137×10⁻³T JK⁻¹mol⁻¹. Substitute the equitation with the heat capacity and molar enthalpy of forming Sb₂Te₃. And solve the equation. The calculated adiabatic temperature can be obtained as 702 K, which is lower than the molten point of Sb₂Te₃corresponding to 890.7 K. it is consistent with the hypothesis. Hence the adiabatic temperature is 702 K.

$\begin{matrix} Δ_{f} H_{298 K}^{0} = - 56.484 k J {mol}^{- 1} \\ = - \int_{298 K}^{T_{ad}} (112.884 + 53.137 \times 10^{- 3} T) dT \\ = - [112.884 \times (T_{ad} - 298) + 0.5 \times 53.137 \times 10^{- 3} \times (T_{ad}^{- 2} - 298^{2})] \end{matrix}$

(3) Since the molten point of Sb and Te is 903.755 K, 722.5 K respectively. The component with lower molten point is Te. The ratio between the adiabatic temperature and the molten point of the component with lower molten point is 0.98. According to the new criterion for combustion synthesis, self propagating high temperature reaction between Sb and Te to form Sb₂Te₃cannot be self sustained.

Table 2 shows the molar enthalpy of forming Sb₂Te₃and MnSi_1.70at 298 K, specific heat capacity of Sb₂Te₃and MnSi_1.70, adiabatic temperature T_adand the ratio between the adiabatic temperature and the molten point of the component with lower molten point. Since the calculated ratio T_ad/T_m,Lfor both materials is less than the unity, i.e., the heat of reaction is too low to melt the lower melting point component. This impedes the reaction speed and prevents the reaction front to self-propagate.

TABLE 2

Thermodynamic parameters for Sb₂Te₃and MnSi_1.70.

Adiabatic

temper-

Material

Molar enthalpy
Specific Heat capacity
ature

system
Reaction
(kJmol⁻¹)
(JK⁻¹mol⁻¹)
(T_ad/K)
T_ad/T_m,L

Sb₂Te₃
2Sb + 3Te→Sb₂Te₃
Δ_fH⁰_298K: −56.484
112.884 + 53.137 × 10⁻³ T
702
0.98

MnSi_1.70
Mn + 1.70Si→MnSi_1.70
Δ_fH⁰_298K: −75.601
71.927 + 4.615 × 10⁻³ T −
1314
0.88

13.067 × 10⁵T⁻²

In order to prove that Sb₂Te₃cannot be synthesized by SHS, The experimental as below has been done.The detailed synthesis procedure is as below.

(1) Stoichiometric amounts Sb₂Te₃of high purity single elemental Sb, Te powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet (ϕ15×18 mm) with the pressure of 8 MPa holding for 10 min.
(2) The pellet obtained in step (1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample with hand torch. Although the reaction between Sb and Te was ignited at the bottom, the combustion wave cannot be self-propagated and go through the whole pellet.
(3) The different parts of the pellet (specifically the bottom and the top of the pellet) in step (2) were characterized by XRD.

The proof for MnSi_1.70that cannot be synthesized by SHS is the same as that of Sb₂Te₃. The detailed synthesis procedure is as below.

(1) Stoichiometric amounts MnSi_1.70of high purity single elemental Mn, Si powders were weighed and mixed in the agate mortar and then cold-pressed into a pellet.
(2) The pellet was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by point-heating a small part (usually the bottom) of the sample with hand torch. Although the reaction between Mn and Si was ignited at the bottom, the combustion wave cannot be self-propagated and go through the whole pellet.
(3) The different parts of the pellet (specifically the bottom and the top of the pellet) in step (2) were characterized by XRD.

FIG. 2 shows the XRD pattern of bottom part of the top part of the MnSi_1.70and Sb₂Te₃pellet. MnSi and Sb₂Te₃compounds are observed after ignition by the torch indicating the reaction started. However at the top the pellets of the mixture none of compounds except single elemental Mn, Si, Sb, Te, is observed indicating that the reaction cannot be self-sustained after ignition.

EMBODIMENT EXAMPLE 3

Assessing available experimental data for high temperature ceramics and intermetallics, such as TiB, ZrB₂, TiB₂, TiSi, ZrSi₂, NiAl, CoAl, ZrC, TiC and MoSi₂, which can be synthesized by SHS and meet the criterion suggested by Merzhanov that the system will not be self-sustaining unless T_adreaches at least 1800 K. the adiabatic temperature and the ratio between adiabatic temperature and the molten point of the component with lower molten point are calculated as shown in table 3. The data indicate that the adiabatic temperature of all high temperature intermetallics (borides, carbides, silicates) is, indeed, more than 1800 K. Moreover, the ratio between adiabatic temperature and the molten point of the component with lower molten point of those high temperature intermetallics (borides, carbides, silicates) is larger than unit, which can meet the new criterion.

TABLE 3

Thermodynamic parameter for high

temperature ceramics and intermetallics

High temperature

Adiabatic

ceramics and

temperature

intermetallics
Reaction
(T_ad/K)
T_ad/T_mL

TiB
Ti + B→TiB
3350
2.00599

TiB₂
Ti + 2B→TiB₂
3190
1.91018

ZrB2
Zr + 2B→ZrB₂
3310
1.78437

TiC
Ti + C→TiC
3210
1.92216

ZrC
Zr + C→ZrC
3400
1.83288

TiSi
Ti + Si→TiSi
2000
1.1976

NiAl
Ni + Al→NiAl
1910
2.04497

CoAl
Co + Al→CoAl
1900
2.03426

MoSi2
Mo + 2Si→MoSi₂
1900
1.12626

ZrSi2
Zr + 2Si→ZrSi₂
2063
1.22288

FIG. 3 shows the the ratio between adiabatic temperature and the molten point of the component with lower molten point of the compounds in embodiment example 1 and the high temperature ceramics and intermetallics in embodiment example 3. It is very clear that the ratio between adiabatic temperature and the molten point of the component with lower molten point of those high temperature intermetallics (borides, carbides, silicates) is larger than unit, which can meet the new criterion.

Merzhanov suggested an empirical criterion that the system will not be self-sustaining unless T_adreaches at least 1800 K based on high temperature ceramics and intermetallics. However, the empirical criterion restricted the scope of the material can be synthesized by SHS. In contrast, the adiabatic temperature of thermoelectric semiconductors is dramatically lower than 1800 K. According to the criterion T_ad≥1800 K, the reaction leading to their formation should not have been self-sustaining. Moreover, at that high temperature above 1800 K most thermoelectric compounds would decompose due to high volatility of their constituent elements. It seems hopeless for thermoelectric materials to be synthesized by SHS. In this disclosure, SHS was applied to synthesize Bi₂Te₃, Bi₂Se₃, Bi₂S₃, Cu₂Se, PbS, PbSe, SnTe, Mg₂Sn and Mg₂Si compounds thermoelectric for the first time. However, we failed to synthesize Sb₂Te₃and MnSi_1.70by SHS. In order to find the new thermodynamics criterion, we examined the ratio formed by the relevant thermodynamic parameters: the adiabatic temperature, T_ad, divided by the melting temperature of the lower melting point component, T_m,L. For the SHS reaction to be self-sustaining, the value of T_ad/T_m,Lshould be more than 1.

EMBODIMENT EXAMPLE 4

The detailed procedure of the ultra-fast preparation method of high performance Cu₂Se thermoelectric material with nano pores is as following.

1) Stoichiometric amounts Cu₂Se of high purity single elemental Cu, Se powders were weighed and mixed in the agate mortar. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ12 mm under the pressure of 10 MPa.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by the hot plate with the temperature of 573 K at the bottom of the sample. Once started, turn off the hot plate, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. Single phase Cu₂Se with nanostructures is obtained.
3) The obtained pellet Cu₂Se in step 2) was crushed, hand ground into a fine powder, and then the fine powder was loaded into a graphite die with size of ϕ15 mm and was vacuum sintered by PAS. The parameter for spark plasma sintering is with the temperature of 973 K with the heating rate 80 K/min and the pressure of 30 MPa holding for 3 min. The densely bulks Cu₂Se with nanostructure is obtained after PAS with the size of ϕ15×3 mm. the sample was cut into the right size for measurement and microstructure characterization by diamond saw.

FIG. 4 shows the powder XRD pattern of Cu₂Se after SHS and after SHS-PAS. Single phase Cu₂Se is obtained after SHS and after SHS-PAS.

Table 4 shows the actual composition of the powder in step 2) of embodiment example 4 and the bulks in step 3 of embodiment example 4 characterized by EPMA. The molar ratio between Cu and Se is ranged from 2.004:1 to 2.05:1. The actual composition is almost the same as the stoichiometric. This indicates that SHS-PAS technique can control the composition very precisely.

FIG. 5 shows the FESEM image of the fracture surface of the sample after SHS. Nano grains with the size of 20-50 nm distributes homogeneously on the grains in the micro-scale. FIG. 6 shows the FESEM image of the fracture surface of the sample after SHS-PAS. Lots of Nano pore with the size of 10-300 nm is observed.

FIG. 7 show the temperature dependence of ZT for Cu₂Se sample synthesized by SHS-PAS. The maximum ZT about 1.9 is attained at 1000 K, which is much higher than that reported in the reference.

TABLE 4

Nominal composition and actual composition for the powder after

SHS and the bulk after SHS-PAS in the embodiment example 4.

Nominal
Actual composition

Sample
composition
characterized by EPMA

Powder after SHS
Cu₂Se
Cu_2.004Se

Bulks after SHS-PAS
Cu₂Se
Cu_2.05Se

EMBODIMENT EXAMPLE 5
A Method for Ultra-Fast Synthesis of High Thermoelectric Performance Half-Heusler
Embodiment Example 5.1

The detailed procedure of the ultra-fast preparation method of high performance ZrNiSn thermoelectric material is as following.

1) Stoichiometric amounts ZrNiSn of high purity single elemental Zr (2.5N), Ni (2.5N), Sn (2.8N) powders were weighed and mixed in the agate mortar with the weight about 5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ12 mm under the pressure of 6 MPa holding for 5 min.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by the hand torch at the bottom of the sample. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. The whole SHS process takes 2 seconds.
3) The obtained pellet ZrNiSn in step 2) was crushed, hand ground into a fine powder, and then the fine powder was loaded into a graphite die with size of ϕ15 mm and was vacuum sintered by PAS. The parameter for plasma activated sintering is with the temperature of 1163-1173 K with the heating rate 80-100 K/min and the pressure of 30 MPa holding for 5-7 min. The densely bulks ZrNiSn is obtained after PAS with the size of ϕ15×3 mm. the sample was cut into the right size for measurement and microstructure characterization by diamond saw.

The phase composition of above samples were characterized by XRD. FIG. 8 shows XRD pattern for the samples obtained in step 2) and in step 3) of embodiment example 5.1. Single phase ZrNiSn is obtained in seconds after SHS. After PAS, XRD pattern does not change. FIG. 9 shows the microstructure of the sample in step 2) of embodiment example 5.1. FESEM image shows that the sample is well crystallized with some nanostructures.

Embodiment Example 5.2

The detailed procedure of the ultra-fast preparation method of high performance Ti_0.5Zr_0.5NiSn thermoelectric material is as following.

1) Stoichiometric amounts Ti_0.5Zr_0.5NiSn of high purity single elemental Ti (4N), Zr (2.5N), Ni (2.5N), Sn (2.8N) powders were weighed and mixed in the agate mortar with the weight about 5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ12 mm under the pressure of 6 MPa holding for 5 min.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by the hand torch at the bottom of the sample. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. The whole SHS process takes 2 seconds.

The phase compositions of above samples were characterized by XRD. FIG. 10 shows XRD pattern for the samples obtained in step 2) of embodiment example 5.2. Single phase Ti_0.5Zr_0.5NiSn solid solution is obtained in seconds after SHS.

Embodiment Example 5.3

The detailed procedure of the ultra-fast preparation method of high performance ZrNiSn_0.98Sb_0.02thermoelectric material is as following.

1) Stoichiometric amounts ZrNiSn_0.98Sb_0.02of high purity single elemental Zr (2.5N), Ni (2.5N), Sn (2.8N), Sb (5N) powders were weighed and mixed in the agate mortar with the weight about 5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ12 mm under the pressure of 6 MPa holding for 5 min.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by the hand torch at the bottom of the sample. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. The whole SHS process takes 2 seconds.
3) The obtained pellet ZrNiSn_0.98Sb_0.02in step 2) was crushed, hand ground into a fine powder, and then the fine powder was loaded into a graphite die with size of ϕ15 mm and was vacuum sintered by PAS. The parameter for plasma activated sintering is with the temperature of 1163-1173 K with the heating rate 80-100 K/min and the pressure of 30 MPa holding for 5-7 min. The densely bulks ZrNiSn_0.98Sb_0.02is obtained after PAS with the size of ϕ15×3 mm. The sample was cut into the right size for measurement and microstructure characterization by diamond saw.

The phase, microstructure and thermoelectric properties of above samples were characterized. FIG. 11 shows XRD pattern for the samples obtained in step 2) and in step 3) of embodiment example 5.3. Single phase ZrNiSn is obtained in seconds after SHS. After PAS, XRD pattern does not change. FIG. 12 shows the temperature dependence of power factor and ZT for sample in step 3) of embodiment example 5.3, which is comparable with the sample synthesized by induction melting with the same composition. At 873 K, the maximum ZT is 0.42.

EMBODIMENT EXAMPLE 6

The detailed procedure of the ultra-fast preparation method of high performance BiCuSeO thermoelectric material by SHS is as following.

1) Stoichiometric amounts BiCuSeO of high purity Bi₂O₃(4N), Bi (2.5N), Cu (2.5N), Se (2.8N) powders were weighed and mixed in the agate mortar with the weight about 10 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ12 mm under the pressure of 6 MPa holding for 5 min.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by the hand torch at the bottom of the sample. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. The whole SHS process takes 2 seconds.

The phase compositions of above samples were characterized by XRD. FIG. 13 shows XRD pattern for the samples obtained in step 2) of embodiment example 6. Almost Single phase BiCuSeO with trace of tiny amount Cu_1.75Se is obtained after SHS.

EMBODIMENT EXAMPLE 7
A Method for Ultra-Fast Synthesis of n Type Bi₂Te_3-xSe_xwith High Thermoelectric Performance
Embodiment Example 7.1

The detailed procedure of the ultra-fast preparation method of high performance n type Bi₂Te_3-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts Bi₂Te_2.7Se_0.3of high purity single elemental Bi (4N), Te (4N), Se (4N) powders were weighed and mixed in the agate mortar with the weight about 25 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ16 mm under the pressure of 10 MPa holding for 5 min.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by hot plate with the temperature of 773 K at the bottom of the sample. Once started, turn off the hot plate, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. Single phase Bi₂Te_2.7Se_0.3compounds is obtained after SHS.
3) The obtained pellet Bi₂Te_2.7Se_0.3in step 2) was crushed, hand ground into a fine powder, and then the fine powder was loaded into a graphite die with size of ϕ15 mm and was vacuum sintered by PAS. The parameter for plasma activated sintering is with the temperature of 753 K with the heating rate 100 K/min and the pressure of 20 MPa holding for 5 min. The densely bulks Bi₂Te_2.7Se_0.3is obtained after PAS with the size of ϕ15×2.5 mm. The sample was cut into the right size for measurement and microstructure characterization by diamond saw.

FIG. 14 shows XRD pattern for the samples obtained in step 2) and in step 3) of embodiment example 7.1. Single phase Bi₂Te_2.7Se_0.3is obtained in seconds after SHS. After PAS, XRD pattern does not change.

FIG. 15 shows the FESEM image of the sample in step 3) of embodiment example 7.1. FESEM image shows typical layer structure is obtained with random distributed grains, indicating no preferential orientation.

FIG. 16 shows the temperature dependence of ZT for Bi₂Te_2.7Se_0.3. In comparison with the sample with the composition of Bi_1.9Sb_0.1Te_2.55Se_0.45in the reference (Shanyu Wang, J. Phys. D: Appl. Phys, 2010, 43, 335404) synthesized by Melting spinning combined with Spark plasma sintering. At 426 K, the maximum ZT of sample in step 3 of embodiment 7.1 is 0.95. At the temperature ranged from 300 K to 520 K, the average ZT value is larger than 0.7.

Embodiment Example 7.2

The detailed procedure of the ultra-fast preparation method of high performance n type Bi₂Te_3-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts Bi₂Te_2.7Se_0.3of high purity single elemental Bi (4N), Te (4N), Se (4N) powders were weighed and mixed in the agate mortar with the weight about 25 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ16 mm under the pressure of 10 MPa holding for 5 min.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by global explosion at 773 K in the furnace for 3 min. And then the pellet was cool down to room temperature in the air. Single phase Bi₂Te_2.7Se_0.3compounds is obtained after SHS.

FIG. 17 shows XRD pattern for the samples obtained in step 2) of embodiment example 7.2. Single phase Bi₂Te_2.7Se_0.3is obtained in seconds after global ignition.

Embodiment Example 7.3

The detailed procedure of the ultra-fast preparation method of high performance n type Bi₂Te_3-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts Bi₂Te₂Se of high purity single elemental Bi (4N), Te (4N), Se (4N) powders were weighed and mixed in the agate mortar with the weight about 25 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ16 mm under the pressure of 10 MPa holding for 5 min.
2) The pellet obtained in step 1) was sealed in a silica tube under the pressure of 10⁻³Pa and was initiated by hot plate with the temperature of 773 K at the bottom of the sample. Once started, turn off the hot plate, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air. Single phase Bi₂Te₂Se compounds is obtained after SHS.

FIG. 18 shows the XRD pattern for the samples obtained in step 2) of embodiment example 7.3. Single phase Bi₂Te₂Se is obtained in seconds after SHS.

EMBODIMENT EXAMPLE 8
A New Methods for Ultra-Tast Synthesis of PbS_1-xSe_xwith High Thermoelectric Performance
Embodiment Example 8.1

The detailed procedure of the ultra-fast preparation method of high performance n type PbS_1-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts PbS_0.22Se_0.8of high purity single elemental Pb (4N), S (4N), Se (4N) powders were weighed and mixed in the agate mortar with the weight about 4.5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ10 mm under the pressure of 5 MPa holding for 5 min, and then increase the pressure to 8 MPa holding for 10 min.
2) The pellet obtained in step 1) was initiated by hand torch at the bottom of the sample. Once started, move away the hand torches, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air.
3) The obtained pellet in step 2) was crushed, hand ground into a fine powder for XRD characterization.

FIG. 19 shows XRD pattern for the samples obtained in step 3) of embodiment example 8.1. Single phase PbS_0.2Se_0.8solid solution is obtained in seconds after SHS.

Embodiment Example 8.2

The detailed procedure of the ultra-fast preparation method of high performance n type PbS_1-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts PbS_0.42Se_0.6of high purity single elemental Pb (4N), S (4N), Se (4N) powders were weighed and mixed in the agate mortar with the weight about 4.5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ10 mm under the pressure of 5 MPa holding for 5 min, and then increase the pressure to 8 MPa holding for 10 min.
2) The pellet obtained in step 1) was initiated by hand torch at the bottom of the sample in the air. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air.
3) The obtained pellet in step 2) was crushed, hand ground into a fine powder for XRD characterization.

FIG. 20 shows XRD pattern for the samples obtained in step 2) and in step 3) of embodiment example 8.2. Single phase PbS_0.4Se_0.6is obtained in seconds after SHS. After PAS, XRD pattern does not change.

Embodiment Example 8.3

The detailed procedure of the ultra-fast preparation method of high performance n type PbS_1-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts PbS_0.62Se_0.4of high purity single elemental Pb (4N), S (4N), Se (4N) powders were weighed and mixed in the agate mortar with the weight about 4.5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ10 mm under the pressure of 5 MPa holding for 5 min, and then increase the pressure to 8 MPa holding for 10 min.
2) The pellet obtained in step 1) was initiated by hand torch at the bottom of the sample in the air. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air.
3) The obtained pellet in step 2) was crushed, hand ground into a fine powder for XRD measurement.

FIG. 21 shows XRD pattern for the samples obtained in step 3) of embodiment example 8.3. Single phase PbS_0.6Se_0.4is obtained in seconds after SHS.

Embodiment Example 8.4

The detailed procedure of the ultra-fast preparation method of high performance n type PbS_1-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts PbS_0.82Se_0.2of high purity single elemental Pb (4N), S (4N), Se (4N) powders were weighed and mixed in the agate mortar with the weight about 4.5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ10 mm under the pressure of 5 MPa holding for 5 min, and then increase the pressure to 8 MPa holding for 10 min.
2) The pellet obtained in step 1) was initiated by hand torch at the bottom of the sample in the air. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air.
3) The obtained pellet in step 2) was crushed, hand ground into a fine powder for XRD measurement.

FIG. 22 shows XRD pattern for the samples obtained in step 3) of embodiment example 8.4. Single phase PbS_0.8Se_0.2solid solution is obtained in seconds after SHS.

Embodiment Example 8.5

The detailed procedure of the ultra-fast preparation method of high performance n type PbS_1-xSe_xthermoelectric material is as following.

1) Stoichiometric amounts PbS_1.02of high purity single elemental Pb (4N), S (4N) powders were weighed and mixed in the agate mortar with the weight about 4.5 gram. And then the mixed powder was loaded into a stainless steel die and cold-pressed into a pellet with the size of ϕ10 mm under the pressure of 5 MPa holding for 5 min, and then increase the pressure to 8 MPa holding for 10 min.
2) The pellet obtained in step 1) was initiated by hand torch at the bottom of the sample in the air. Once started, move away from the hand torch, a wave of exothermic reactions (combustion wave) passes through the remaining material as the liberated heat of fusion in one section is sufficient to maintain the reaction in the neighboring section of the compact. And then the pellet was cool down to room temperature in the air.
3) The obtained pellet in step 2) was crushed, hand ground into a fine powder, and then the fine powder was loaded into a graphite die with size of ϕ15 mm and was vacuum sintered by PAS. The parameter for spark plasma sintering is with the temperature of 823 K with the heating rate 100 K/min and the pressure of 35 MPa holding for 7 min. The densely bulks PbS is obtained after PAS with the size of ϕ15×2.5 mm. The sample was cut into the right size for measurement and microstructure characterization by diamond saw.

FIG. 23(a) shows XRD pattern for the samples obtained in step 2) and in step 3) of embodiment example 8.5. FIG. 23(b) shows FESEM image of the sample in step 2) of embodiment example 8.5. FIG. 23(c) shows temperature dependence of ZT for the sample synthesized by SHS-PAS and traditional melting method.

As shown in FIG. 23, Single phase PbS is obtained in seconds after SHS. The grain size distributes in very large scales. After PAS, Single phase PbS can be maintained. In comparison with the sample synthesized by traditional method, the average ZT above 600 K is much higher for the sample synthesized by SHS-PAS. At 875 K, the maximum ZT is 0.57, which is one time higher than the sample synthesized by traditional method.

EMBODIMENT EXAMPLE 9
A New Methods for Ultra-Fast Synthesis of Mg₂Si with High Thermoelectric Performance
Embodiment Example 9.1