A First-principles Study On Material Thermal Expansion And Design Of Novel Negative Thermal Expansion Materials

Thermal expansion is a responsive behavior of material macroscopic dimensions to temperature changes.As temperature rises,most materials exhibit positive expansion behavior.The essence of positive expansion behavior is the increase of interatomic distance caused by anharmonic effects.However,some materials exhibit anomalous negative thermal expansion behavior,such as water near its freezing point,showing‘heat-shrink and cold-stretch’.Thermal expansion phenomenon is ubiquitous,and the large-scale dimensional changes of materials caused by temperature fluctuations can affect the performance and precision of devices.Therefore,fine-tuning the thermal expansion behavior of materials has become particularly important,especially in the fields of aerospace and precision devices.The conventional method of tuning involves compositing positive and negative thermal expansion materials to achieve low or even zero expansion coefficients at a certain temperature.However,finding negative expansion materials that match positive expansion materials is the key to this method.Currently,research on the physical mechanism of anomalous thermal expansion of materials mainly focuses on the intrinsic properties of materials,involving electronic mechanisms and structural mechanisms.In terms of electronic mechanisms,charge transfer,ferroelectric phase transition,magnetic phase transition,etc.,can cause significant negative expansion behavior due to the resulting redistribution of charges.In terms of structural mechanisms,materials with open framework structures,including metal oxides,metal fluorides,and cyanides,etc.,are mainly composed of rigid rotating units and bridging atoms that vibrate laterally.The high degree of freedom in the structure can result in significant volume contraction.A clear understanding of the thermal expansion mechanism of materials is beneficial to provide theoretical guidance for the thermal management of materials and corresponding devices.However,there are still many problems waiting to be solved in the current research on the thermal expansion mechanism of materials.On the one hand,the thermal expansion behavior and mechanism explanation of non-intrinsic structured materials are still lacking;on the other hand,we still lack a unified understanding of the thermal expansion mechanism,especially for anomalous negative expansion behavior.Therefore,the relationship between structure-composition and thermal expansion properties needs to be discovered and elucidated.In this paper,based on density functional theory and first-principles calculations,we explore the complex thermal expansion mechanism of materials and conduct the following research work:1.Revealing the influence of twin grain boundary on the thermal expansion of materials with different bonding types.Based on first-principles phonon calculations within the framework of quasi-harmonic approximation,we study the effect of twin grain boundary（TGB）on the thermal expansion of insulating materials.We select diamond structure solids（C,Si,and Ge）and rutile（Sn O₂）as representatives of the covalent and ionic bonding materials that are important systems commonly accompanied with TGB.We found distinct effects of TGB on thermal expansion in two types of materials.For diamond-structure solids,the thermal expansion of（111）-oriented twinning structure varies subtly from that of pristine material within the temperature range studied,which is primarily induced by the modification of vibrational phonon mode by the TGB effect.Distinctly,the thermal expansion of Sn O₂twinning increase substantially by comparison with the pristine case and depend strongly on the twin orientation.The（101）twinning shows much larger thermal expansion than the（301）twinning.Further analysis indicates the physical mechanism can be mainly attributed to the effect of rotational degree of freedom of the Sn O₆octahedron motif and the stress induced by TGB.Our work reveals the physical mechanism underlying the distinct effect of TGB on thermal expansion caused by different chemical bonding,and meanwhile provides an insightful understanding of the relationship between specific structural features and thermal expansion in solid-phase materials.2.Proposing effective descriptors to evaluate the in-plane thermal expansion behavior and driving mechanisms of layered materials.Two-dimensional layered materials show promising applications in miniaturized devices,such as transistors,spintronics,and field emitters.However,substantial thermal management issues,including thermal mismatch and thermal stress,may degrade device performance.To address such challenges,the thermal expansion（TE）anisotropy determined by the structure feature of layered material needs to be well understood.Here,we propose two new descriptors to evaluate the TE behavior of layered materials,namely the axial elastic deviation factor₄4))and axial net thermal stress1)1)₄4))along i-th direction.The former,defined as the normalized elastic element difference of material elastic tensor C and compliance tensor S,can distinguish whether the thermal expansion of a material is driven by phonons(with small₄4)))or elastic property(with large₄4)))with few computational costs.The latter,axial stress（in GPa/K）induced by temperature,shows an accurate determination of the positive or negative thermal expansion along different in-plane directions of layered materials.Based on the analysis of descriptors,we found that Pt S₂ and Pt Se₂ featured with larger axial elastic deviation factor（>23%）.Considering the elastic property,we for the first time report the in-plane negative thermal expansion in Pt S₂（-1.2 ppm/K）and Pt Se₂（-0.8 ppm/K）.Our work provides a unified understanding of TE causes of layered materials via effective descriptors,which can serve as a guideline for high-throughput screening of thermal expansion materials and subsequent device design3.Developing an efficient algorithm to characterize the connectivity of framework structures for prediction of new negative thermal expansion materials.Open framework structures(e.g.,Sc F₃,Sc₂W₃O₁₂,etc.)exhibit significant potential for thermal expansion tailoring owing to their high atomic vibrational degrees of freedom and diverse connectivity between polyhedral units,displaying positive/negative thermal expansion（PTE/NTE）coefficients at a certain temperature.Despite the proposal of several physical mechanisms to explain the origin of NTE,an accurate mapping relationship between the structural-compositional properties and thermal expansion behavior is still lacking.This deficiency impedes the rapid evaluation of thermal expansion properties and hinders the design and development of such materials.We developed an algorithm for identifying and characterizing the connection patterns of structural units in open-framework structures and constructed a descriptor set for the thermal expansion properties of this system,which is composed of connectivity and elemental information.Our developed descriptor,aided by machine learning（ML）algorithms,can effectively learn the thermal expansion behavior in small sample datasets collected from literature-reported experimental data（246 samples）.The trained model can accurately distinguish the thermal expansion behavior（PTE/NTE）,achieving an accuracy of 92%.Additionally,our model predicted six new thermodynamically stable NTE materials,which were validated through first-principles calculations.Our results demonstrate that developing effective descriptors closely related to thermal expansion properties enables ML models to make accurate predictions even on small sample datasets,providing a new perspective for understanding the relationship between connectivity and thermal expansion properties in the open framework structure.