HPHT Synthesis And Performance Optimization Of Si-based And Ge-based Clathrate Thermoelectric Materials

Energy shortage and environmental pollution have emerged as some of the most critical issues,which barricade the development of the human society,they are not only independent,but also dependent each other.So far,fossil fuels are still the main energy source.The excessive consumption of energy causes the environmental pollution and ecological destruction,which is disastrous to human survival and life environment.Therefore,the development of new functional material,recycling of waste energy,changes the energy structure,save un-renewable resources and improve living environment become important,particularly.The scientist studied and developed new green energy conversion materials “Thermoelectric Materials” for effectively using the wasteful energy in the production and living.Thermoelectric?TE?material is one kind of function materials that can directly convert electricity and heat.Regarding TE materials,they have unique features,such as high reliability,quiet operation,long life,no moving parts,and low environmental impact.TE materials are going to be useful for all kinds of applications,for instance,recovering waste heat,environmentally friendly refrigeration,and harvesting solar energy.Therefore,the research and utilization of high-performance TE materials have great significance in optimizing the allocation of resources and improving the ecological environment.The performance of a TE material is generally characterized by the dimensionless figure of merit,ZT = S2?T/?,where S is the Seebeck coefficient,? is the electrical conductivity,T is the absolute temperature,and ? is the thermal conductivity.Thermoelectric materials with large values of ZT are necessary for efficient thermoelectric devices.Therefore,much effort has been devoted to the search for new materials and new fabrication techniques to enhance ZT.Si and Ge based clathrate TE materials which possess own “phonon glass electron crystal”?PGEC?characteristic are considered to be one of the most promising TE materials in high temperature region,and we can optimize and improve the thermoelectric performance from a variety of ways.For now,many methods have been used to synthesize clathrate TE materials,such as powder metallurgy,hydrothermal method,melting,and spark plasma sintering and so on.In this research,we adopt the high temperature and high pressure?HPHT?synthesis method for the preparation of TE materials.HPHT could synthesize the materials that cannot be achieved at atmospheric pressure,and it has the ability to intercept part of the excellent properties gained at high pressure.HPHT method possesses the unique advantages,such as fast reaction,simple operation,good repeatability,good mechanical properties and high purity of sample.In this paper,we successfully prepared a series of Si and Ge based clathrate TE materials by means of HPHT method,such as,Ba₈Si₄₆,Ba8 Cux Si46-x,Ba8Cu5 Gex Si41-x and Ba₈Ga₁₆Ge₃₀.The effects of synthetic pressure,doping elements,doping amount and microstructure on thermoelectric properties are systematically investigated.The main research results are as follows:?1?The silicon clathrate Ba₈Si₄₆ was successfully one-step synthesized using low-cost antioxidative azide Ba?N₃?₂ and Si powder as precursors by means of high pressure and high temperature?HPHT?under the condition of 3GPa,840?,0.5h,firstly.All the diffraction peak positions of the sample match very well with the standard diffraction data of Ba₈Si₄₆.We performed elemental mapping using TEM,shows obvious homogeneity in the distribution of the Ba and Si elements in the clathrate phase.The sample showed superconductivity with critical temperatures of 8.3K.In addition,the silicon clathrate Ba₈Si₄₆ samples were also synthesized using Ba Si2 and Si powder as raw materials in condition of 3GPa,840?,0.5h.As well,the distribution of two elements Ba and Si are homogeneity,the enrichment phenomenon is nonexistent.The electrical resistivity of bulk sample synthesized using Ba Si2 and Si powder is very low,the value is 0.6m?cm at room temperature,which is suitable to be used as thermoelectric materials to study.?2?We successfully prepared the initial clathrate Ba8 Cux Si46-x?x=0,1,2,3,4,5?samples using Ba Si2,Cu and Si powder as raw materials by HPHT method.All XRD patterns of the samples could be indexed to type-I clathrate phases.With the Cu element increasing,the Seebeck coefficient and the power factor improved,significantly.The Seebeck coefficient of Ba8Cu5Si41 reached-40?V/K,and the power factor reached 1.84?W/?cm K2?at room temperature.Doping of Cu and high pressure can significantly change microstructure of reaction products,carrier concentration band structure and phonon scattering to optimize the electrical property and improve the thermoelectric properties of type-I silicon based clathrate thermoelectric materials.?3?We successfully prepared Cu and Ge double-doped Si based clathrate bulk thermoelectric materials Ba8Cu5Ge8 x Si_41-8x?x=0,1,2,3?by HPHT method.The synthesized samples were pure phase,the diffraction peaks are in agreement with the standard atlas.The actual chemical composition of all samples basically is consistent with the nominal composition,and the clathrate samples exhibit no element enrichment and uniform distribution of the Ba,Cu,Si,Ge elements.Ge doping significantly improves the Seebeck coefficient of Si based clathrate thermoelectric materials.The Seebeck coefficient of Ba₈Cu₅Ge₂₄Si₁₇ sample reached the maximum value of-141.2V/K@673K.The HRTEM images shows that Ge doping and high pressure intervention has changed the microstructure,a large number of nanocrystalline,non crystalline region,the lattice orientation confusion,?dislocations,lattice curvatures,and fringes?are ubiquitous,and these microstructure and the Si/Ge alloy can effectively enhance the phonon scattering,reduce the lattice thermal conductivity.The thermal conductivity of Ba₈Cu₅Ge₂₄Si₁₇ obtained a minimum value of 0.91Wm^-1K^-1@573K,maximum ZT value of 0.42@673K.Under the conditions of different pressure,Cu and Ge double-doped Si based clathrate bulk thermoelectric materials Ba₈Cu₅Ge₂₄Si₁₇ was successfully prepared by the HPHT method.The electrical resistivity obviously decreased as pressure rises.The sample synthesized at 5GPa obtains the minimum electrical resistivity of 1.45m?cm@300K.The microstructures of nanocrystalline,dislocations,lattice curvatures and fringes increase as pressure rise,and the thermal conductivity significantly declines.The sample prepared at 5GPa gains the minimum thermal conductivity of 0.76Wm^-1K^-1@573K.The ZT value of sample synthesized at 4GPa obtains maximum value 0.55@673K.?4?We successfully prepared high performance n-type polycrystalline Ba₈Ga₁₆Ge₃₀ samples at different pressure?2GPa,3GPa,4GPa,5GPa?by HPHT method.All the diffraction peaks of the samples match very well with the standard atlas.A variety of different scales of micro/nano structure,such as nano ball,amorphous area,lattice disorder?dislocation,lattice bending,etc.?,generally exist in the sample,which plays a very important role to enhance the phonon scattering,optimize the electrical properties and reduce the thermal conductivity.The sample synthesized at 4GPa obtains the lowest thermal conductivity of 0.71Wm^-1K^-1@773K.The electrical resistivity of the samples decreased with the increase of the pressure,the electrical resistivity of the sample prepared at 5GPa gets the maximum value of 1.15m?cm@300K,the power factor inceased with the increase of the pressure,the power factor of the sample prepared at 5GPa gains the maximum value of 11.62?Wcm-1K-2@773K,and the sample synthesized at 5GPa gets the maximum ZT value of 1.14@773K,a 30% rise from the same sample synthesized at the ordinary conditions.In conclusion,these results show that the HPHT method can successfully prepared high performance Si and Ge based clathrate thermoelectric materials in a short time.We systematically researched the clathrate TE material in the threedimensional space of pressure?P?,temperature?T?and the chemical composition?X?.Revealed the impact mechanism on the electronic and phonon transport characteristics of crystal structure?structure transformation,the lattice distortion?,special microstructure?defects,density,etc.?,doping and composite of the clathrate TE material that prepared by HPHT method.Finally,we optimize the electronic and phonon transport properties through the pressure regulation,the composition optimization,the structure optimization and the optimization of microstructure for realizing the breakthrough of the thermoelectric properties.