| With the miniaturization and integration of optoelectronic devices,the associated heating problems have become more and more serious,which will affect the working efficiency of optoelectronic devices,and even reduce the service life.Therefore,the heating problem of optoelectronic devices needs to be addressed urgently.In this case,Professor Maldovan of MIT proposed the concept of thermocrystal in 2013,providing a new way to solve the heat dissipation problem of optoelectronic devices.At present,the research on thermocrystals is still in its infancy and mainly focuses on improving the thermal insulation performance.Among them,the biggest challenge faced by thermocrystals is that the frequency range of thermal phonons in ordinary semiconductor materials is very wide,while the frequency range of thermal bandgap corresponding to thermocrystals is very narrow,which is the main reason that affects the thermal insulation performance of thermocrystals.In addition,the theoretical model for solving the related characteristics of thermocrystals also needs improvement.Therefore,in this paper,the theoretical model of heat transfer in thermocrystals is improved,the related factors affecting the thermal insulation performance of thermocrystals are studied,and the methods to improve the thermal insulation performance are proposed.It has been proved that improving the thermal insulation performance of thermocrystal will contribute to the control and management of heat,and the heat generated by optoelectronic devices can be directed by designing the structure of the thermocrystal.This paper was funded by the National Natural Science Foundation of China in the process of completion.The specific research work is as follows:a)The theoretical model of thermocrystals is summarized and improved.For the calculation of the thermal conductivity of the matrix material of thermocrystal,this paper presents calculation formulas for some of the parameters.In addition,in the process of solving the thermal spectrum of the matrix material of thermocrystal,the formula to calculate the thermal spectrum of the matrix material of thermocrystal is presented in this paper.In the process of obtaining the band structure of thermocrystal,this paper decoupled the transport of thermal phonons to XY mode and Z mode for the first time,and on this basis proposed the calculation formula of thermal transmittance.b)We explore the characteristics of the thermal spectrum of different semiconductor materials and the characteristics of thermal bandgap of thermocrystal thin films with different semiconductor materials as the matrix materials,mainly including C,Si,Ge and Sn.The results show that with the increase of atomic number,the thermal spectrum of semiconductor materials gradually moves to the low-frequency direction,and the bandgap frequency range of thermocrystal films also moves to the low-frequency direction.In addition,we also found that with the increase of the air hole radius and the decrease of the lattice constant,the thermal bandgap frequency range of thermocrystal film also increased gradually.Among them,with the decrease of lattice constant,the thermal bandgap frequency range of thermocrystal thin film will also move to the high frequency direction,showing an inverse relation between them.In addition,the frequency range of bandgap in Z mode of thermocrystal thin film is larger than that in XY mode.By comparison,when we select 10μm-thick carbon film or 1μm-thick silicon,germanium or tin film and choose lattice constant of 2nm and air hole radius of r=0.48 a,thermocrystal thin film could reduce the thermal conductivity of the matrix material by up to 19.6%.Finally,we investigate the heat flow transmission of line-defect waveguide of thermocrystal thin film with semiconductor material as the matrix material,and the results show that the heat flow transmission efficiency of line-defect waveguide of thermocrystal thin film with pure semiconductor material as the matrix material is very low.c)We explore different shapes of air hole in the structural unit on the influence of the thermal bandgap of thermocrystal films.In order to reduce the difficulty of the actual preparation,we will replace the matrix material from pure semiconductor to silicon germanium alloy with doping germanium nanoparticles and increases the lattice constant.The numerical results show that the thermal bandgaps of thermocrystal films with different elliptic air hole shapes have different sizes of broadening compared with that of circular air hole shapes.Among them,the thermal bandgap frequency range of thermocrystal film with the thickness of 1mm,the lattice constant of 5nm,the air hole semi-major-axis of 0.48 a,the eccentricity of 0.808 and cross-shaped elliptic air hole,is more than 2 times that of thermocrystal film with circular air hole,and it can increase the extent that the thermocrystal reduces the thermal conductivity of the matrix material up to 24.46% from 15.07%,which is equivalent of increasing the extent that the thermocrystal reduces the thermal conductivity of the matrix material by 62%.d)The structure of a one-dimensional super-thermocrystal is designed to enhance the extent of reducing the thermal conductivity of matrix material by thermocrystal thin film.The numerical results show that if monocrystalline silicon is used as the matrix material,the structure can double the thermal bandgap frequency range of thermocrystal thin film.However,if silicon germanium alloy with doping germanium nanoparticles is used as the matrix material,the structure can expand the thermal bandgap frequency range of thermocrystal thin film by three times,thus increasing the extent that the thermocrystal reduces the thermal conductivity of the matrix material up to 60%.Based on this structure,we design a super-thermocrystal line-defect waveguide.The results show that most of the heat flow is confined to the waveguide,which realizes the guide transmission of heat flow in the waveguide.e)A two-dimensional elliptic microcavity is designed to regulate spontaneous emission spectrum.The results show that the local state density of the two-dimensional elliptic microcavity is related to its aperture size and eccentricity.When the aperture size is0.56a(a is the length of the semi-major-axis of the elliptic microcavity)and the eccentricity is 0.95,the two-dimensional elliptic microcavity can extend the full width at half maximum of spontaneous emission spectrum to twice of the original,and the two-dimensional elliptic microcavity can be applied to the spontaneous emission spectrum of any Gaussian shape.In contrast,the regulation of thermal radiation by similar two-dimensional elliptic microcavity of thermocrystal is investigated.The results show that the elliptic microcavity of thermocrystal can suppress the transmission of thermal-phonons in a certain frequency range to a certain extent,thus enhancing the storage effect of heat radiation in the microcavity.Many novel applications can be realized when the extent of reducing the thermal conductivity of matrix material by thermocrystal thin film is greatly improved.For example,thermal imaging and thermal coating can be achieved by using the negative refraction effect of thermocrystals,or efficient photothermal coupling can be achieved by using the similarity between thermocrystals and photonic crystals.In the future,like photonic crystals or phononic crystals,thermocrystals will be able to control and manage heat flow,revolutionizing our lives. |
PDF Full Text Request