A Cs3Bi2I9 single crystal material with dual thermal transport behavior, its preparation method and application

Abstract

This invention relates to a Cs3Bi2I9 single-crystal material with dual thermal transport behavior, its preparation method, and its applications, belonging to the field of single-crystal material technology. The preparation method includes the following steps: CsI and BiI3 are mixed and ground uniformly according to a stoichiometric ratio, then transferred to a quartz tube, vacuum-sealed, and sequentially subjected to high-temperature melting, quenching, and annealing to obtain a Cs3Bi2I9 polycrystalline ingot with a density of not less than 97%; the Cs3Bi2I9 polycrystalline ingot is placed in a sealed quartz tube, and the sealed quartz tube containing the Cs3Bi2I9 polycrystalline ingot is placed in a vertical temperature gradient furnace, first melted at high temperature, and then cooled to room temperature to obtain the Cs3Bi2I9 single-crystal material with dual thermal transport behavior. Compared with the prior art, this invention innovatively produces a Cs3Bi2I9 single-crystal material with unique dual thermal transport behavior, providing a new design concept for controlling heat transfer through crystal chemistry, and also opening up a new path for developing anisotropic thermoelectric materials with thermal management functions.

Description

Technical Field

[0001] This invention relates to the field of single crystal material preparation technology, and in particular to a Cs3Bi2I9 single crystal material with dual thermal transport behavior, its preparation method and application. Background Technology

[0002] Low thermal conductivity halide perovskites are attracting increasing attention in thermal management and thermoelectric conversion due to their unique structural properties and lattice dynamics. By suppressing heat conduction, these materials have the potential to extend device lifespan and promote the development of high-efficiency energy conversion technologies. Lead-free metal halides have emerged as ideal candidates for thermal management, possessing tunable structures, excellent optoelectronic properties, good stability, and environmental friendliness. Among these properties, low thermal conductivity is a crucial indicator for achieving efficient thermal management.

[0003] The diverse geometries of polyhedra in Cs3Bi2I9 make it an ideal material for studying the structural chemical origins of particle-like and wave-like heat transport phenomena. Biswas et al. (Advanced Functional Materials 33, 2304607 (2023)) reported a fully inorganic halide perovskite bulk Cs3Bi2I9 polycrystalline material with a thermal conductivity as low as 0.2 W / mK. Furthermore, studies have shown that reducing the grain size can further decrease the thermal conductivity of Cs3Bi2I9 (Physical Chemistry ChemicalPhysics 26, 21801-21809 (2024)). When the grain size is reduced from 200 nm to 20 nm, the room-temperature thermal conductivity decreases from 0.25 W / mK to 0.18 W / mK (indicating particle-like transport). More importantly, enhanced grain boundary scattering weakens the temperature dependence of the thermal conductivity.

[0004] However, current research results are mostly based on the thermal conductivity of polycrystalline Cs3Bi2I9, and there is still a lack of research on the thermal conductivity of single-crystal Cs3Bi2I9 materials. Furthermore, single-crystal Cs3Bi2I9 materials are mainly used in the field of detectors. In addition, existing single-crystal Cs3Bi2I9 materials are mostly prepared using solution crystallization methods. For example, CN118147749A discloses a method for controlling the growth of bismuth-based perovskite Cs3Bi2I9 single crystals, which requires the addition of alcohol polymers as nucleation regulators. However, single crystals grown by solution methods generally suffer from excessive nucleation sites and excessively fast crystallization rates, resulting in high crystal defect densities and severely affecting the intrinsic properties of single-crystal Cs3Bi2I9 materials.

[0005] Therefore, a new method for preparing single-crystal Cs3Bi2I9 materials still needs to be developed in order to further study its thermal conductivity and applications. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects of the prior art and provide a Cs3Bi2I9 single crystal material with dual thermal transport behavior, its preparation method and application.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] This invention first provides a method for preparing a Cs3Bi2I9 single crystal material with dual thermal transport behavior, the preparation method comprising the following steps:

[0009] S1: CsI and BiI3 are mixed and ground evenly according to stoichiometric ratio, then transferred to a quartz tube, vacuum sealed, and subjected to high-temperature melting, quenching and annealing in sequence to obtain a Cs3Bi2I9 polycrystalline ingot with a density of not less than 97%.

[0010] S2: Place the Cs3Bi2I9 polycrystalline ingot obtained in S1 into a sealed quartz tube, place the sealed quartz tube containing the Cs3Bi2I9 polycrystalline ingot into a vertical temperature gradient furnace, melt it at high temperature first, and then cool it to room temperature to obtain Cs3Bi2I9 single crystal material with dual thermal transport behavior.

[0011] The axial temperature gradient in the vertical temperature gradient furnace is 6-10 ℃ / cm;

[0012] The cooling process employs a segmented cooling method, first cooling to 450-470 ℃ at a rate of 1.5-2.5 ℃ / h, and then cooling to room temperature at a rate of 8-12 ℃ / h.

[0013] The Cs3Bi2I9 single crystal material exhibits anisotropic dual thermal transport behavior, specifically as follows:

[0014] Along the direction parallel to the crystallographic c-axis, the thermal conductivity of the Cs3Bi2I9 single crystal material is mainly particle-like, with a thermal conductivity of no more than 0.30 W / mK at room temperature, and gradually decreasing in the temperature range of 300-450 K.

[0015] Along the direction perpendicular to the crystallographic c-axis, the thermal conductivity of the Cs3Bi2I9 single crystal material is mainly wavy, with a thermal conductivity of no more than 0.18 W / mK at room temperature, gradually decreasing in the temperature range of 300-400 K and gradually increasing in the temperature range of 400-500 K.

[0016] Furthermore, in step S1, the molar ratio between CsI and BiI3 is 3:2.

[0017] Furthermore, in step S1, the purity of both CsI and BiI3 is not less than 98%.

[0018] Furthermore, in step S1, the vacuum is evacuated to a vacuum level of less than 6 Pa.

[0019] Furthermore, in step S1, the high-temperature melting temperature is 650-750 ℃, preferably 700 ℃; the high-temperature melting time is 5-7 h, preferably 6 h.

[0020] Furthermore, in step S1, the quenching medium is water.

[0021] Further, in step S1, the annealing temperature is 250-300 °C, preferably 277 °C; the annealing time is 36-60 h, preferably 48 h. Annealing, by heating the material to a certain temperature and holding it for a period of time, allows the atoms within the material to rearrange, thereby releasing residual stress. This not only helps improve the stability of the material, but the diffusion and recrystallization processes of atoms also help homogenize the composition and structure of the material, thereby improving the consistency of material performance and reducing local performance differences. In addition, annealing can also promote grain growth and eliminate stress and impurity phases.

[0022] Furthermore, in step S2, during the high-temperature melting process, the temperature of the tip of the quartz tube is heated to 650-750℃, preferably 700℃, and maintained for 8-12 h, preferably 10 h.

[0023] Furthermore, in step S2, the axial temperature gradient in the vertical temperature gradient furnace is preferably 8 °C / cm. By precisely controlling the axial temperature gradient of the vertical temperature gradient furnace to achieve directional solidification growth, lattice stress concentration can be effectively suppressed, resulting in high-quality single crystals with significantly reduced dislocation density. The closed-system design effectively suppresses component volatilization, minimizing stoichiometric deviation.

[0024] The segmented cooling method of this invention has significant advantages in the single crystal growth process, including reducing thermal stress, optimizing crystal quality, improving crystal stability, reducing the risk of crystal cracking, and improving crystal uniformity. By reasonably controlling the rate and temperature of segmented cooling, the quality and performance of single crystals can be significantly improved.

[0025] The present invention also provides an application of Cs3Bi2I9 single crystal material with dual thermal transport behavior in the fabrication of anisotropic thermal management devices.

[0026] The Cs3Bi2I9 single-crystal material with unique bidirectional thermal transport behavior obtained by this invention provides a new approach for the design of advanced thermal management devices: along the c-axis, the thermal conductivity decreases with increasing temperature, making it suitable for self-regulating thermal insulation systems that automatically suppress heat conduction at high temperatures (such as overheat protection layers in electronic devices); perpendicular to the c-axis, the thermal conductivity increases with increasing temperature, enabling the construction of directional enhanced heat dissipation channels and achieving intelligent heat diffusion under high-temperature conditions (such as active heat dissipation interfaces in spacecraft). By synergistically utilizing the above-mentioned bidirectional thermal transport behavior, a single piece of Cs3Bi2I9 single-crystal material can simultaneously achieve thermal shielding and heat diffusion functions, breaking through the inherent limitations of traditional isotropic single-crystal materials in thermal management design.

[0027] Therefore, the successful preparation of Cs3Bi2I9 single crystal material with dual thermal transport behavior based on the present invention can provide a design basis for longitudinal thermal transport through crystal chemical engineering, open up a path for the development of anisotropic thermoelectric materials with thermal management capabilities, and can be widely used in thermoelectric materials, thermal insulation materials and other fields that require efficient thermal management.

[0028] Compared with the prior art, the present invention has the following technical advantages:

[0029] (1) The present invention uses a precisely controlled vertical gradient freezing method to achieve directional solidification growth of single crystals, obtains high-quality Cs3Bi2I9 single crystal material with significantly reduced dislocation density, and unexpectedly discovers that it has a unique dual heat transport coexistence behavior, providing a new design idea for regulating heat transport through crystal chemistry, and also opening up a new path for developing anisotropic thermoelectric materials with thermal management function.

[0030] (2) Compared with the traditional solution crystallization method for preparing single-crystal Cs3Bi2I9 materials, this invention innovatively adopts a precisely controlled vertical gradient freezing method. Through a segmented cooling strategy, it can effectively solve the problems of excessive nucleation sites and excessively fast crystallization rate in solution-grown single crystals. This method can significantly reduce the crystal defect density, improve the quality, stability and uniformity of the crystal, and reduce production costs, demonstrating significant economic practicality and innovation. In addition, this invention also found that the cooling rate has a significant impact on the crystal growth quality. Therefore, by further optimizing the cooling rate control strategy and adopting a segmented cooling method, and finally cooling to room temperature at a slower rate, this precise improvement in cooling method not only effectively avoids crystal cracking but also significantly improves crystal stability.

[0031] (3) The present invention discovers that the prepared Cs3Bi2I9 single crystal material exhibits significant thermal transport anisotropy, with its thermal conductivity value not exceeding 0.30 W / mK parallel to the c-axis (particle behavior dominant) and not exceeding 0.18 W / mK perpendicular to the c-axis (wave behavior dominant). This unique bidirectional thermal transport behavior provides a new approach for the design of advanced thermal management devices. Specifically, along the c-axis, the thermal conductivity decreases with increasing temperature, making it suitable for self-regulating thermal insulation systems that automatically suppress heat conduction at high temperatures (such as overheat protection layers for electronic devices); perpendicular to the c-axis, the thermal conductivity increases with increasing temperature, enabling the construction of directional enhanced heat dissipation channels to achieve intelligent heat diffusion under high-temperature conditions (such as active heat dissipation interfaces for spacecraft). By synergistically utilizing the bidirectional thermal transport behavior, a single piece of material can simultaneously achieve thermal shielding and heat diffusion functions, breaking through the inherent limitations of isotropic materials in thermal management design.

[0032] (4) The thermal conductivity of the Cs3Bi2I9 single crystal material obtained by the present invention is no higher than 0.18 W / mK in the direction perpendicular to c. This value is much lower than that of most traditional low thermal conductivity materials (such as glass, ceramics, etc.), indicating that it has excellent performance in thermal insulation. Especially in applications that require ultra-low thermal conductivity (such as thermoelectric materials, heat insulation coatings, etc.), Cs3Bi2I9 single crystal material shows great potential.

[0033] (5) The Cs3Bi2I9 single crystal material prepared by this invention does not undergo phase transformation or phase decomposition over a wide temperature range (up to 500 K). Compared with many existing materials, this material can still maintain its structure and thermal properties at higher temperatures, making it suitable for use in high-temperature environments without the material properties deteriorating due to thermal expansion or phase transformation.

[0034] (6) The Cs3Bi2I9 single crystal material prepared by this invention is composed of relatively non-toxic and sustainably obtainable elements (Cs, Bi and I). Compared with some existing low thermal conductivity materials (such as compounds containing heavy metals or rare elements), it is more environmentally friendly and in line with the development trend of green materials. Attached Figure Description

[0035] Figure 1 The image shows the crystal structure of Cs3Bi2I9 prepared in Example 1.

[0036] Figure 2 The images shown are: A) X-ray diffraction pattern, B) scanning electron microscope image and elemental distribution map, C) temperature-dependent and optical crystal images, and D) optical crystal images of Cs3Bi2I9 polycrystalline and single crystal prepared in Example 1.

[0037] Figure 3 The optical bandgap diagram and X-ray photoelectron spectrum of Cs3Bi2I9 prepared in Example 1 are shown.

[0038] Figure 4 The temperature-dependent thermal conductivity diagram of Cs3Bi2I9 prepared in Example 1.

[0039] Figure 5 The image shows the sound velocity of Cs3Bi2I9 prepared in Example 1.

[0040] Figure 6 The room temperature Raman spectrum of Cs3Bi2I9 prepared in Example 1 is shown.

[0041] Figure 7 The image shows the room-temperature Raman amplification spectrum of Cs3Bi2I9 prepared in Example 1.

[0042] Figure 8 Images of Cs3Bi2I9 single crystals prepared in Comparative Examples 1 and 2, respectively. Detailed Implementation

[0043] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0044] Unless otherwise specified, the reagents, methods, instruments, and equipment used in this invention are conventional in the art. Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0045] Example 1:

[0046] This embodiment provides a Cs3Bi2I9 single crystal material with dual thermal transport behavior, and its preparation method is as follows:

[0047] S1: Weigh CsI and BiI3 with a purity ≥98% at a molar ratio of 3:2, place them in a quartz tube, evacuate to less than 6 Pa, and then seal. First, melt at 700 ℃ for 6 h, then quench in water, followed by annealing at 227 ℃ for 48 h, and cool to room temperature to obtain Cs3Bi2I9 ingot.

[0048] S2: Single-crystal Cs3Bi2I9 was grown using a vertical gradient freezing method. A sealed quartz tube containing a Cs3Bi2I9 ingot was placed in a vertical temperature gradient furnace with an axial temperature gradient of 8 °C / cm. The temperature (T0) at the tip of the quartz tube was heated to 700 °C and held for 10 h. Then, it was slowly cooled from 700 °C to 460 °C at a cooling rate of 2 °C / h, and then slowly cooled to room temperature at a cooling rate of 10 °C / h to obtain the Cs3Bi2I9 single-crystal material.

[0049] In this embodiment, a rapid hot-pressing method was used to prepare dense flakes of Cs3Bi2I9 polycrystalline material for subsequent testing. Specifically, the Cs3Bi2I9 ingot was ground into Cs3Bi2I9 powder. The Cs3Bi2I9 powder was placed into a graphite mold, an axial pressure of 60 MPa was applied, and the temperature was raised to 227 °C and held for 40 min. Subsequently, the pressure was released and the material was cooled to obtain dense flakes. The density of the Cs3Bi2I9 polycrystalline material was measured to be over 97% of the theoretical density.

[0050] Example 2:

[0051] This embodiment provides a Cs3Bi2I9 single crystal material with dual thermal transport behavior. The difference from Embodiment 1 is that the axial temperature gradient of the vertical temperature gradient furnace in this embodiment is 6 °C / cm. During cooling, it is first slowly cooled from 650 °C to 450 °C at a cooling rate of 1.5 °C / h, and then slowly cooled to room temperature at a cooling rate of 8 °C / h, thus obtaining the Cs3Bi2I9 single crystal material.

[0052] Example 3:

[0053] This embodiment provides a Cs3Bi2I9 single crystal material with dual thermal transport behavior. The difference from Embodiment 1 is that the axial temperature gradient of the vertical temperature gradient furnace in this embodiment is 10 °C / cm. During cooling, it is first slowly cooled from 750 °C to 470 °C at a cooling rate of 2.5 °C / h, and then slowly cooled to room temperature at a cooling rate of 10 °C / h, thus obtaining the Cs3Bi2I9 single crystal material.

[0054] Taking the Cs3Bi2I9 single crystal material obtained in Example 1 as an example, the present invention performs the following tests and characterization:

[0055] (1) Crystal structure characterization

[0056] Figure 1 The crystal structure of Cs3Bi2I9 was refined by single-crystal X-ray diffraction (XRD) in the study, revealing lattice parameters of a=b=8.38 Å and c=21.11 Å, which are in good agreement with previously reported data.

[0057] Figure 1 In section B, a dodecahedral polyhedron with eight shared corners is shown within the unit cell, each polyhedron consisting of a Cs 2b Six I 6h and six I 12k Atomic composition. The bonding environment consists of six Cs. 2b -I 6h Bond (4.19 Å) and six Cs 2b -I 12k It consists of bonds (4.29 Å).

[0058] Figure 1 The C in the diagram illustrates four similar twelve-coordinate cubic octahedrons, whose bonding environment consists of three Cs. 4f -I 6h Bond (4.25 Å) and nine Cs 4f -I 12k Bond (4.19 / 4.21 Å). This series of faces shares a cubic octahedron parallel to the c-direction.

[0059] like Figure 1 As shown in Figure D, the unit cell contains two six-coordinated octahedrons [BiI6], which are formed by a Bi 4f Three I's 6h and three I 12k Composed of atoms, with bond lengths of three Bi atoms. 4f -I 6h Bond (3.23 Å) and three Bi bonds 4f -I 12k Bond (2.93 Å). Two octahedrons share [Bi₂I₉] via faces parallel to the c-direction. 3- Dimer.

[0060] (2) Structure and composition

[0061] like Figure 2 As shown in Figure A, the orientation of the single-crystal Cs3Bi2I9 was characterized by XRD (DX2000, Cu-Kα), and all diffraction peaks were indexed to the hexagonal crystal structure (ICSD#410726). No impurity peaks were observed.

[0062] like Figure 2 As shown in Figure B, the composition of single-crystal Cs3Bi2I9 was characterized using a scanning electron microscope (SEM, PhenomPro) equipped with an energy-dispersive spectroscopy (EDS) instrument, confirming its uniform composition.

[0063] like Figure 2 As shown in Figure C, temperature-dependent X-ray diffraction (XRD, Rigaku SmartLab, Cu-Kα) of Cs3Bi2I9 powder shows that no phase transition or phase decomposition occurs in the range up to 500 K.

[0064] Figure 2 Figure D shows a typical optical photograph of single-crystal Cs3Bi2I9. As can be seen from the figure, Example 1 ultimately yielded a single crystal with a diameter of 16 mm and a length of approximately 5 cm.

[0065] (3) Optical bandgap diagram and X-ray photoelectron spectroscopy test

[0066] The room-temperature optical band gap of Cs3Bi2I9 was determined by UV-Vis diffuse reflectance spectroscopy. The absorption spectrum showed an absorption edge at ~2.05 eV, with significant exciton transitions near 2.6 eV. Figure 3 (A)

[0067] The surface valence states of the sample were further investigated using X-ray photoelectron spectroscopy (XPS, Al-Kα radiation) with an ESCALAB 250Xi (ThermoFisher Scientific). Cs 3d 5 / 2 and Cs 3d 3 / 2 Peak values ​​were reached at 724.86 eV and 738.78 eV, validating Cs. + The existence of ( Figure 3 (B). The +1 oxidation state of cesium reflects its strong metallic properties, indicating that Cs tends to form ionic bonds. Figure 3 The middle C shows Bi 4f 7 / 2 and Bi 4f 5 / 2 Peak, confirming Bi 3+ state. Figure 3 The deconvolution of the bimodal I 3d peaks in the middle D reveals the I 3d 5 / 2 (619.34 eV) and I 3d 3 / 2 The (630.81 eV) component proves that I - The existence of.

[0068] (4) Thermal conductivity test

[0069] The thermal conductivity of single-crystal and polycrystalline Cs3Bi2I9 as a function of temperature is as follows: Figure 4 As shown in the figure, the Cs3Bi2I9 single crystal material prepared in Example 1 exhibits the coexistence of dual thermal transport, which occurs through particle-like collisions parallel to the c-axis and wave-like coherence perpendicular to the c-axis. The anisotropy in thermal transport results in a thermal conductivity of 0.29 W / mK parallel to the c-axis and 0.17 W / mK perpendicular to the c-axis.

[0070] In the temperature range of 300 K to 500 K, the thermal conductivity parallel to the c direction gradually decreases with increasing temperature, showing a weak temperature dependence; in contrast, the heat transfer perpendicular to the c direction mainly exhibits wave-like behavior and increases with increasing temperature.

[0071] The polycrystalline material prepared in this invention has a thermal conductivity of 0.20 W / mK at room temperature, which is very close to the reported thermal conductivity value of 0.19 W / mK in the literature (Advanced Functional Materials 33, 2304607 (2023)). The thermal conductivity gradually decreases with increasing temperature, exhibiting a weak temperature dependence (0.20~0.17 W / mK), and this trend is consistent with previous research results.

[0072] (5) Verification of relevant mechanisms

[0073] Essentially, temperature-dependent changes are described by a complex interplay between particle-like and wave-like transport mechanisms. In crystalline materials, heat carriers exhibit particle-like collisions governed by the Boltzmann transport equations. Conversely, in glasses, hot carriers exhibit wave-like coherence through Zener tunneling between secondary coupled vibrational eigenstates, as formalized in the Allen-Feldman equations. The temperature dependence of thermal conductivity is a key indicator of these microscopic mechanisms. The dominance of particle-like or wave-like transport fundamentally depends on the material's structural chemistry, which governs phonon scattering pathways and the degree of wave localization.

[0074] This invention measures the velocity of sound at room temperature using an ultrasonic pulse receiver (Olympus-NDT) equipped with an oscilloscope (Keysight). The velocity of sound in Cs3Bi2I9 single crystal and polycrystalline forms is as follows: Figure 5 As shown. The relatively low sound velocity indicates weak overall chemical bonding, which inhibits heat propagation and ultimately leads to ultra-low thermal conductivity. Along the c-direction, [Bi₂I₉] 3- The distortion of the dimer increases atomic disorder and reduces the transverse sound velocity. However, in polycrystalline Cs3Bi2I9, this effect is averaged due to the random orientation of individual grains. Therefore, the transverse sound velocity in polycrystalline Cs3Bi2I9 lies between the values ​​observed in single-crystal sound velocities parallel and perpendicular to the c-direction. In single-crystal Cs3Bi2I9, the planes share [CsI 12 The strong bonding force between the cubic octahedron and the [BiI6] octahedron induces high sound speeds parallel to the c-direction, while the co-angular [CsI] 12 The weak ringing vibration effect in a cubic octahedron and Cs 2b Localized ringing vibrations of atoms result in low sound speeds perpendicular to the c-direction.

[0075] The room-temperature Raman spectrum of Cs3Bi2I9 was measured using a Jobin-Yvon HR800 Raman system in a backscatter configuration. Figure 6 The wavenumber range is shown to be 25 cm. -1 Up to 150 cm -1The characteristic Raman activity pattern. Notably, we experimentally observed previously unreported Raman activity below 25 cm⁻¹. -1 This may correspond to a low-frequency vibration mode. For example... Figure 7 As shown, 5-25 cm -1 Magnified Raman spectra within the range show 9 cm⁻¹ -1 There is a prominent peak at this point, accompanied by a height of 15-25 cm. -1 Several weaker signals were observed between these. These observed vibrational features confirm the previously reported diagonal sharing of [CsI]. 12 Cubic octahedral dynamics and Cs 2b Prediction of localized ringing vibrations in atoms.

[0076] Comparative Example 1:

[0077] This comparative example provides a Cs3Bi2I9 single crystal material. The difference between this comparative example and Example 1 is that this comparative example does not use segmented cooling, but instead directly and slowly cools it from 700 ℃ to room temperature at a cooling rate of 10 ℃ / h.

[0078] Compared to Example 1, this comparative example directly employs a faster cooling rate, resulting in greater undercooling and causing the melt to crystallize at temperatures far from its melting point. This further destabilizes the crystal growth interface, producing an uneven growth interface. Due to the non-uniform temperature gradient, convection occurs in the melt, further disrupting the stability of the growth interface and leading to numerous defects such as dislocations, twins, microcracks, and voids in the crystal. Furthermore, due to the unstable growth interface, the crystal growth rate varies in different directions, resulting in irregular crystal shapes and poor-quality single crystals (e.g., ...). Figure 8 (As shown in A).

[0079] Comparative Example 2:

[0080] This comparative example provides a Cs3Bi2I9 single crystal material. The difference between this and Example 1 is that the axial temperature gradient of the vertical temperature gradient furnace in this comparative example is 30 °C / cm.

[0081] During crystal growth, the melt requires a certain degree of supercooling to begin crystallization. Supercooling is typically proportional to the crystal growth rate and inversely proportional to the temperature gradient. Compared to Example 1, when the axial temperature gradient of the vertical temperature gradient furnace increased to 30 °C / cm, poorer crystal quality was observed (e.g., Figure 8(As shown in Figure B). This larger temperature gradient reduces the supercooling of the melt. This is because the temperature near the growth interface is closer to the melting point, resulting in a significant decrease in the crystal growth rate. Furthermore, due to the reduced growth rate, a longer growth time is required to obtain crystals of the same size. A more precise temperature control system is also needed to maintain a stable growth environment. Any temperature fluctuations can lead to instability at the growth interface, thus affecting crystal quality.

[0082] Furthermore, the vertical temperature gradient furnace used in this invention differs from the dual-temperature zone Bridgman furnace in the prior art. Firstly, the vertical temperature gradient furnace of this invention, through its integrated furnace design, ensures a uniform and stable temperature gradient throughout the entire furnace chamber. Crystal growth can begin simply by placing a quartz tube containing polycrystalline material into the furnace and setting the appropriate program. This design reduces the impact of temperature fluctuations on crystal growth, improving the uniformity and stability of crystal quality. In contrast, when growing single crystals using a dual-temperature zone Bridgman furnace, the crystal growth quartz tube containing polycrystalline material needs to be placed in a high-temperature zone and heated to fully melt the polycrystalline material. Subsequently, the quartz tube is lowered and moved from the high-temperature zone to the low-temperature zone to achieve crystal growth. However, the movement of the quartz tube increases physical disturbance, significantly affecting crystal growth. Moreover, this invention, through a uniform temperature gradient and a stable growth environment, can grow high-quality single crystals (such as…). Figure 2 In a medium-temperature (D) furnace, the defect density in the crystal is low, and the physical properties are more uniform. While a dual-temperature Bridgman furnace can optimize crystal growth by adjusting the temperature of each zone, the independence of the temperature zones may lead to non-uniformity of the temperature gradient, thus affecting the crystal quality.

[0083] In summary, the Cs3Bi2I9 single-crystal material grown using the method of this invention exhibits excellent thermal insulation properties. Furthermore, it has been discovered that it displays different thermal conductivities along different crystal axes, not exceeding 0.30 W / mK parallel to the c-axis and not exceeding 0.18 W / mK perpendicular to the c-axis. By synergistically utilizing bidirectional heat transport behavior, the Cs3Bi2I9 single-crystal material can simultaneously achieve thermal shielding and thermal diffusion functions, overcoming the inherent limitations of isotropic materials in thermal management design.

[0084] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. The application of a Cs3Bi2I9 single crystal material with dual thermal transport behavior in the fabrication of anisotropic thermal management devices, characterized in that, The unit cell parameters of the Cs3Bi2I9 single crystal material are: a=b=8.38 Å, c=21.11 Å; The Cs3Bi2I9 single crystal material exhibits anisotropic dual thermal transport behavior, specifically as follows: Along the direction parallel to the crystallographic c-axis, the thermal conductivity of the Cs3Bi2I9 single crystal material is mainly particle-like, with a thermal conductivity of no more than 0.30 W / mK at room temperature, and gradually decreasing in the temperature range of 300-450 K. Along the direction perpendicular to the crystallographic c-axis, the thermal conductivity of the Cs3Bi2I9 single crystal material is mainly wavy, with a thermal conductivity of no more than 0.18 W / mK at room temperature, gradually decreasing in the temperature range of 300-400 K and gradually increasing in the temperature range of 400-500 K.

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