Theoretical Study On Thermal And Thermoelectric Properties Of Novel Metastable Silicon Crystal Structures

As the main carrier of thermal transport in materials,especially in semiconductors（insulators）,phonons（quantum description of lattice vibrations）are the basis for understanding the thermal transport properties of materials from a microscopic perspective and exploiting and regulating their physical properties.The investigation of phonon thermal transport properties can help us to explore materials with excellent heat dissipation properties,which can fundamentally solve the thermal dissipation problems faced by integrated circuits and high energy density batteries;help guide us to discover high-performance thermoelectric materials,improve existing thermoelectric materials,and alleviate the current energy and environmental crises;and also help discover excellent thermal management materials and promote the further development of"phononics"devices.Related research has become one of the hot spots and frontiers in condensed matter physics and materials science.Silicon crystals are the cornerstone of the modern semiconductor information industry and photovoltaic technology,due to their abundant reserves and mature preparation processes.Since diamond silicon possesses an indirect band gap and high lattice thermal conductivity,which greatly limits its photoelectric and thermoelectric conversion efficiency,it is particularly important to explore and discover new silicon metastable crystal structures with excellent physical properties.The previous work on the thermoelectric properties of new metastable silicon crystals has been initially explored,focusing mainly on predicting the thermoelectric properties of the materials from their thermal transport properties.However,the electron transport properties will also play an important role in the conversion efficiency of the material,and a more comprehensive and in-depth study of the thermoelectric transport properties of the new silicon structures is needed in order to find thermoelectric materials with excellent performance.In this paper,we systematically explore the thermal transport properties and thermoelectric conversion performance of several novel metastable silicon structures based on first-principles calculations combined with Boltzmann transport theory and find that Si₂₄,Si（o P32）,and o P16-Si show more excellent thermoelectric conversion efficiency than diamond silicon,reveal their microscopic transport images and potential physical mechanisms,and foreshadow their application in thermal management and thermoelectricity filed.The main research of this paper is as follows:1.Open framework structured Si₂₄is a recently experimentally prepared cage with a quasi-direct band gap and excellent light absorption efficiency.We have systematically investigated its thermal and electronic transport properties,and the results show that Si₂₄possesses a high Seebeck coefficient and a low lattice thermal conductivity.The analysis of phonon modes clarifies that its low thermal conductivity originates from the lower phonon group velocity and relaxation time.Combined with the electron relaxation time approximation,we further predict its thermoelectric figure of merit（ZT）and find that the ZT of Si₂₄along the xx lattice direction is 0.69（n-type doping）and 0.51（p-type doping）at 700 K,which is an order of magnitude higher than the thermoelectric conversion efficiency of diamond silicon.The results reveal the thermoelectric properties of Si₂₄and provide a theoretical basis for the design and construction of photovoltaic and thermoelectric composite devices based on this new metastable silicon crystal structure.2.Strain engineering is one of the most effective means to modulate the thermal transport properties of materials due to its flexibility and ease of implementation experimentally.We have systematically investigated the effect of strain on the thermal transport properties of the open framework structure Si₂₄.It is widely believed that the phonon thermal conductivity of the bulk material shows a positive correlation with compressive strain and a negative correlation with tensile strain.However,our calculations show that the lattice thermal conductivity of Si₂₄decreases abnormally under compressive strain and increases abnormally under a tensile strain up to 5%.By analyzing the phonon mode information which affects the thermal conductivity,we find that the anomalous strain behavior of Si₂₄is mainly attributed to the competition between phonon relaxation time and other phonons thermal physical quantities（specific heat capacity and phonon group velocity）.These findings further deepen our understanding of the phonon thermal transport properties of Si₂₄and elucidate the underlying physical mechanism of the anomalous strain dependence of the thermal conductivity of Si₂₄.3.Si（o P32）is a novel metastable silicon allotrope with an orthorhombic lattice,and its energy is close to that of diamond silicon.Based on first-principles calculations and Boltzmann transport theory,we explored its thermoelectric transport properties,and the calculation results showed that Si（o P32）exhibits a lower phonon thermal conductivity and a higher power factor compared with diamond silicon.The synergistic effect of reduced phonon group velocity and enhanced three-phonon scattering（phase space and intensity）results in the low thermal conductivity of this allotrope.Based on deformation potential theory calculations,we predicted that the ZT of Si（o P32）is up to 2.45 along the zz lattice direction at 700 K,which is superior to diamond silicon and Si₂₄and comparable to the conversion efficiency of current mainstream thermoelectric materials.These findings suggest that Si（o P32）is a highly competitive candidate in the field of thermoelectrics.4.Based on the RG2 structure search software developed by our group,we predicted a novel metastable silicon crystal structure,o P16-Si,which possesses lower energy than Si₂₄.Its dynamics and mechanical stability are revealed by calculating phonon dispersion relations and elastic constants.We also verified its thermodynamic stability using first-principles molecular dynamics,which fully demonstrates the possibility of this new silicon structure in terms of experimental preparation.We also investigated the Raman spectrum and the eigenvibration vector to provide a theoretical basis for the experimental characterization of the structure.In addition,we explored its physical properties in-depth and found that it has large strength and stiffness as well as high light absorption efficiency and thermoelectric conversion efficiency,which predicts that o P16-Si is a multifunctional material that can realize both photovoltaic and thermoelectric technologies.