Change In Thermal Expansion Coefficient Research Articles

Synergistic effect of indium doping on thermoelectric performance of cubic GeTe-based thin films

Germanium Telluride (GeTe) has been widely explored as a promising lead-free thermoelectric material in its rhombohedral and cubic phases. However, the structural transition between these two phases at ∼700 K causes an abrupt change of thermal expansion coefficient, challenging its broader practical applications. Also, as characterized by multi-valence bands and strong anharmonic interaction, the high-temperature cubic phase exhibits a higher power factor, lower thermal conductivity, and ultimately superior thermoelectric performance than its rhombohedral counterpart. Prompted by these, in this work, the cubic phase of Ge0.9Sb0.1Te (presented as GeSbTe in the following content) nanocrystalline thin film is successfully realized by RF sputtering followed by post-annealing treatment. Additionally, indium, as an electron donor to the germanium site and an effective scattering center, further moderates carrier concentration, enhances the Seebeck coefficient and reduces thermal conductivity. The optimal composition achieves an estimated peak zT of ∼1.95 and an estimated average zT of ∼1.11 within the temperature range of 300 K–575 K, showcasing GeTe as a compelling candidate for applications close to room temperature.

Mechanism Study of Symmetric Mechanism-Rolling Bearing Dynamic System Considering the Influence of Temperature Factor

Due to the symmetry of their structure and the way they rotate, rolling bearings are often used at high temperatures and high speeds. When the temperature changes, the material properties of the rolling bearing change, which in turn causes the dynamic model equation of the rolling bearing to change. Given the problem in that the conventional dynamic model equation does not consider temperature changes, but because temperature changes cause changes in material performance parameters resulting in differences between the dynamic simulation signals and the actual signals, this paper fully considers three factors, namely, the thermal expansion coefficient, the hardness value, and the friction coefficient. With the influence of temperature changes, a study on the mechanism of the symmetric mechanism-rolling bearing dynamic system considering the influence of temperature factors is proposed. First, combined with the material properties of the rolling bearing, the changes in thermal expansion coefficient, hardness value, and friction coefficient caused by temperature changes were analyzed. Second, the functional relationship expressions obtained above were substituted into the existing conventional dynamic model. Finally, the comprehensive functional relationship formula was substituted into the conventional dynamic model to obtain the rolling bearing dynamic model under temperature difference changes. By studying the mechanism of the symmetric mechanism-rolling bearing dynamic system that considers the influence of temperature factors, this method can undertake a more comprehensive consideration of the dynamic analysis research into rolling bearing fault diagnosis, thereby verifying the effectiveness of the method.

Cubic phase stabilization and thermoelectric performance optimization in AgBiSe2–SnTe system

AgBiSe2 is a lead-free thermoelectric (TE) material that demonstrates excellent TE performance in its high-temperature cubic phase. However, the known order-disorder transition at around 580 K induces internal stress caused by the sudden change in thermal expansion coefficient, negatively impacting TE device operation. In this study, we heavily alloyed AgBiSe2 with SnTe to increase configurational entropy (ΔS) and maintain disordered cubic phase throughout the entire temperature range. This structure variation from hexagonal to cubic increased the density-of-state effective mass as demonstrated by experimental results and first-principles calculations. Additionally, the lattice thermal conductivity was reduced by multiple point defects. Overall, the average ZT value from 300 K to 773 K increased to 0.3 after SnTe alloying. Our study provides an additional perspective of entropy engineering on thermoelectric properties beyond the conventional phonon scattering, which helps to tailor properties in other TE materials with phase transition.

Numerical Investigation of Residual Stress Formation Mechanisms in Flash-Butt Welded Rail

For the construction of long and continuous railway lines as well as the replacement of defected rails, rails are joined using flash-butt welding. Under various localized temperatures and thermo-mechanical stresses, a residual stress can develop in the flash-butt welded joint. The residual stress can affect the performance and reliability of the welded rail, particularly in terms of progressive structural damage caused by repeated wheel load. In the present work, the mechanisms of residual stress formation in a flash-butt welded rail and the influence of upsetting force (including its temperature range and magnitude) were investigated using the thermal elastic–plastic finite element analysis. The formation mechanisms of residual stress involved the changes in thermal expansion coefficient, strain, and elastic modulus of the welded joint with respect to temperature. The calculated cooling temperatures and residual stresses in the flash-butt welded joint were in good agreement with the measured results. Compressive residual stresses were observed around the rail head and the rail foot (i.e., approximately −648 MPa at the rail head and −495 MPa at the rail foot), while tensile residual stresses were observed at the rail web (i.e., approximately 165 MPa). It was observed that the investigated compressive upsetting force predominantly induced plastic deformation within the welded joint, resulting in minimal alteration of stress. Consequently, the investigated ranges of upsetting temperature and upsetting forces had an insignificant impact on the formation of residual stress.

Magnetic-Field-Induced Sign Changes of Thermal Expansion in DyCrO4

The anharmonicity of lattice vibration is mainly responsible for the coefficient of thermal expansion (CTE) of materials. External stimuli, such as magnetic and electric fields, thus cannot effectively change the CTE, much less the sign variation from positive to negative or vice versa. In this study, we report significant magnetic field effects on the CTE of zircon- and scheelite-type DyCrO4 prepared at ambient and high pressures, respectively. At zero field, the zircon-type DyCrO4 exhibits a negative CTE below the ferromagnetic-order temperature of 23 K. With increasing field up to ≥1.0 T, however, the sign of the CTE changes from negative to positive. In the scheelite phase, magnetic field can change the initially positive CTE to be negative with a field up to 2.0 T, and then a reentrant positive CTE is induced by enhanced fields ≥3.5 T. Both zircon and scheelite phases exhibit considerable magnetostrictive effects with the absolute values as high as ∼ 800 ppm at 2 K and 10 T. The strong spin–lattice coupling is discussed to understand the unprecedented sign changes of the CTE caused by applying magnetic fields. The current DyCrO4 provides the first example of field-induced sign change of thermal expansion, opening up a way to readily control the thermal expansion beyond the conventional chemical substitution.

Electrodeposition of Functionally Graded Brazing Interlayers for Enhanced Joint Strength between Fusion Plasma-Facing Materials and Heat Sinks

In response to the significant plasma science and engineering breakthroughs witnessed over the past decade, the fusion research community has developed an ambitious roadmap to achieve large-scale, energy-positive fusion reactors that will enable economically competitive fusion electricity. Critical to the success of commercial fusion energy is the development of materials able to serve as plasma facing components (PFCs) that can withstand the unprecedented high heat, particle, and neutron fluxes within the fusion reactor. Tungsten is a leading candidate for the plasma-facing portion of these PFCs because of its high melting point, high sputtering resistance, and low tritium retention. Copper or copper-based alloys (e.g., copper-chromium-zirconium alloy C18150) and reduced activation ferritic/martensitic (RAFM) steels have been proposed for the heat sink materials behind the tungsten armor due to their high thermal conductivity. Brazing is considered a promising method for joining the tungsten armor and heat sink layer. However, the extreme mismatch in the coefficient of thermal expansion (CTE) between tungsten and these potential heat sink materials makes directly joining these two layers challenging. This thermal stress can be reduced by incorporating a joining layer of functionally graded materials between the tungsten and heat sink that imparts gradual changes in CTE. Copper/tungsten or copper/tungsten carbide composites are leading candidates for these functionally graded materials. For these composites, gradual changes in the CTE are achieved by varying the volume fraction of tungsten/tungsten carbide in the composite, from tungsten-rich at the tungsten armor to tungsten-poor at the heat sink.This talk will discuss the development of a scalable, electrochemical approach for fabricating interlayers with functionally graded CTE that can decrease the thermal mismatch between tungsten PFCs and copper or RAFM-based heat sinks. In this approach, the copper/tungsten or copper/tungsten carbide composite are electrodeposited onto the tungsten PFC. This component is then bonded to the heat sink via brazing. A key objective of the work is to achieve compositional control during electrodeposition through the use of highly tunable pulse and pulse-reverse electric fields. Engineering of the electric field enables control of the mass transport and crystallization phenomena in the deposition process and thus enables control of composite properties such as composition, porosity, and surface roughness. Results from electrodeposition trials and mechanical testing of brazed joints under ambient conditions as an initial screen for interlayer performance will be discussed, in anticipation of high-heat flux testing of joint performance under fusion-relevant conditions.

First-principles prediction of thermodynamic properties and mechanical properties of Ti2AX (A=Al, Ga; X=C, N) M2AX phase at different pressures and temperatures

First-principles calculations were used to examine the mechanical and thermodynamic properties of Ti2AX (A = Al,Ga; X = C,N) M2AX phase at various pressures and temperatures. The calculated values of elastic constants show that Ti2AlC, Ti2AlN, Ti2GaC and Ti2GaN are mechanically stable under pressures of 0–50 GPa. The changes of thermal expansion coefficient, Debye temperature, heat capacity and Grünesen constant were investigated at 1200 K temperature and 50 GPa pressure. The results of temperature and pressure on constant heat capacity (Cv) and elastic constant are exactly the opposite, which indicates that Ti2GaN has the largest heat release or heat storage capacity, and the order of the final Debye temperature are: Ti2AlN > Ti2AlC > Ti2GaN > Ti2GaC.

Suppressing lone-pair expression endows room-temperature cubic structure and high thermoelectric performance in GeTe-based materials

The stereochemical expression of ns2 lone-pair electrons in specific Ⅳ-Ⅵ group semiconductors is pivotal to lattice instability, which induces ferroelectric phase transition. In the thermoelectric device working temperature range, the distinct change of thermal expansion coefficient, however, is introduced by phase transition and then severely restrains material's critical applications. Herein, taking the example of thermoelectric GeTe, the hidden role of manipulating lone pair expression to control the crystal structure is theoretically unraveled. Moreover, it is experimentally demonstrated that LiBiTe2-alloyed GeTe possess a room-temperature cubic structure and competitive thermoelectric performance. Density functional theory calculations reveal that reducing the charge orientation caused by the stereochemical activity of cation lone-pair electrons in GeTe is an effective strategy to achieve room-temperature cubic structure. LiBiTe2 is found to be a novel alloying agent to realize crystal structure manipulating, carrier concentration optimization, multi-band convergence, and lattice softening. Benefiting from the optimized electrical and thermal transport properties, an ultrahigh ZTave of 1.22 within 300–773 K in cubic (GeTe)0.80(LiBiTe2)0.20 is achieved. This work provides a new insight into manipulating lone pair expression to control crystal structure, and promotes the development and application of cubic GeTe-based materials.

High-performance lead-free cubic GeTe-based thermoelectric alloy

The ferroelectric phase transition of GeTe thermoelectric materials caused by the 4s2 lone-pair electrons of Ge2+ leads to the sudden change of thermal expansion coefficient, which severely restrains its practical applications. Herein, we demonstrate a successful way to suppress the ferroelectric phase-transition temperature down to below room temperature in GeTe. Our approach involves LiSbTe2 alloying, where the charge distribution is manipulated to reduce the influence of the stereochemical activity of the cation lone-pair electrons on the crystal structure. LiSbTe2 alloying can also promote multi-band convergence and enhance phonon scattering. Consequently, a superior ZTave of 1.38 within 300—773 K is achieved in cubic (GeTe)0.70(LiSbTe2)0.30 alloys, which is significantly higher than the reported results for room temperature or near-room temperature cubic GeTe systems so far. Moreover, the calculated conversion efficiency of 17% is also the highest among all nontoxic p-type polycrystalline thermoelectric materials. This work takes a solid step forward in the application of GeTe.

Comparison of the effects of irradiation on iso-molded, fine grain nuclear graphites: ETU-10, IG-110 and NBG-25

Selecting graphite grades with superior irradiation characteristics is important task for designers of graphite moderation reactors. To provide reference information and data for graphite selection, the effects of irradiation on three fine-grained, iso-molded nuclear grade graphites, ETU-10, IG-110, and NBG-25, were compared based on irradiation-induced changes in volume, thermal conductivity, dynamic Young's modulus, and coefficient of thermal expansion. Data employed in this study were obtained from reported irradiation test results in the high flux isotope reactor (HFIR)(ORNL) (ETU-10, IG-110) and high flux reactor (HFR)(NRL) (IG-110, NBG-25). Comparisons were made based on the irradiation dose and irradiation temperature. Overall, the three grades showed similar irradiation-induced property change behaviors, which followed the historic data. More or less grade-sensitive behaviors were observed for the changes in volume and thermal conductivity, and, in contrast, grade-insensitive behaviors were observed for dynamic Young's modulus and coefficient of thermal expansion changes. The ETU-10 of the smallest grain size appeared to show a relatively smaller VC to IG-110 and NBG-25. Drastic decrease in the difference in thermal conductivity was observed for ETU-10 and IG-110 after irradiation. The similar irradiation-induced properties changing behaviors observed in this study especially in the DYM and CTE may be attributed to the assumed similar microstructures that evolved from the similar size coke particles and the same forming method.

Effect of rare-earth oxides on thermal behavior of alumina-silica glass sealants

Thermal behavior of (62.92SiO2–4.71Al2O3–14.83Na2O–5ZrO2–12.54SrO) – 5Re2O3 (Re = Y, La, Nd, Sm) glasses with superstoichiometric additives of rare-earth oxides has been investigated by scanning differential calorimetry, high temperature optical microscopy, and dilatometry. It is found out that an increase in characteristic temperatures (glass transition temperature and temperature of half-sphere formation) is caused by decreasing the ionic radius. The value of thermal expansion coefficient changes insignificantly.

Insights into thermodynamic properties of CsCl-type HfTM (TM = Fe, Ru, Os) compounds from first-principles calculations

In this work, we used the first-principles calculations based on density functional theory and quasi-harmonic Debye to predict the electronic structures, phonon dispersions and thermodynamic properties of HfTM (TM = Fe, Ru, Os) compounds. The obtained phonon dispersions of HfTM indicate that they are thermodynamically stable. Moreover, HfFe, HfRu and HfOs show metallic properties from the electronic structures. The bulk moduli and thermal expansion coefficients of HfTM compounds under temperature of 0 ~ 1500 K and pressure of 0 ~ 20 GPa were considered, and the results indicate that the bulk modulus reduces as the temperature increases, and thermal expansion coefficient changes quickly to temperature at low temperatures. The order of thermal expansion coefficient is HfFe > HfRu > HfOs and that of thermal stability is HfOs > HfRu > HfFe. Besides, Debye temperature is opposite to the trend of Gruneisen parameter under the same temperature and pressure.

Structural Characteristics of Thin Sr0.5Ba0.5Nb2O6 Films in the Temperature Range 20–500°C

Barium strontium niobate films ~300 nm in thickness have been grown on MgO(001) substrates by rf sputtering of a ceramic target with the stoichiometric composition Sr0.5Ba0.5Nb2O6 (SBN50) in an oxygen atmosphere. Composition–depth profiles across the films have been measured using secondary ion mass spectrometry, and we have determined their phase composition, structure, and c cell parameter in the temperature range 20–500°C. The results demonstrate that the SBN50 films are heteroepitaxial and have two crystallographic orientations with an angle of ~36.8° between them and identical tetragonal cell parameters (а = 12.438 A and с = 3.955 A). Their chemical composition does not vary in the thickness direction and corresponds to the composition of the ceramic target. The SBN50 films have a higher temperature of the ferroelectric-to-paraelectric phase transition than do SBN50 single crystals and polycrystalline films, and their thermal expansion coefficient changes sign near 100°C.

Damping capacity of Al–12Si composites effected by negative thermal expansion of Y2W3O12 particle inclusions

Effects of Y2W3O12 particle inclusions and content on damping capacity of Al–12Si composites have been investigated. The Y2W3O12, which is a kind of negative thermal expansion material, can generate high local strain in the composites. Thermal expansion testing shows that the coefficient of thermal expansion (CTE) of Y2W3O12 has obvious changes between 100 and 200 °C The Y2W3O12/Al–12Si composites show damping peaks and display higher damping capacities than the pure Al–12Si alloy. The damping capacities of composites strongly depends on the content of Y2W3O12 inclusions. A thermal strain damping model is also established. Based on the model analysis results, there is a significant contribution of thermal strain to the damping capacity in the composites; the change in CTE of Y2W3O12 could cause the generation of damping peaks. Moreover, dislocation damping and interfacial damping are also main damping mechanisms in the investigated composites.

Investigation of thermal behavior of mixed-valent iron borates vonsenite and hulsite containing [OM4]n+and [OM5]n+oxocentred polyhedra byin situhigh-temperature Mössbauer spectroscopy, X-ray diffraction and thermal analysis

The investigation of elemental composition, crystal structure and thermal behavior of vonsenite and hulsite from the Titovskoe boron deposit in Russia is reported. The structures of the borates are described in terms of cation-centered and oxocentred polyhedra. There are different sequences of double chains and layers consisting of oxocentred [OM4]n+ tetrahedra and [OM5]n+ tetragonal pyramids forming a framework. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). Oxidation states and coordination sites of iron and tin in the oxoborates are determined using Mössbauer spectroscopy and compared with EDX and X-ray diffraction data (XRD). According to results obtained from high-temperature Mössbauer spectroscopy, the Fe2+ to Fe3+ oxidation in vonsenite and hulsite occurs at approximately 500 and 600 K, respectively. According to the high-temperature XRD data, this process is accompanied by an assumed deformation of crystal structures and subsequent solid-phase decomposition to hematite and warwickite. It is seen as a monotonic decrease of volume thermal expansion coefficients with an increase in temperature. A partial magnetic ordering in hulsite is observed for the first time with Tc ≃ 383 K. Near this temperature, an unusual change of thermal expansion coefficients is revealed. Vonsenite starts to melt at 1571 K and hulsite melts at 1504 K. Eigenvalues of thermal expansion tensor are calculated for the oxoborates as well as anisotropy of the expansion is described in comparison with their crystal structures.

Phase-Transition Dynamics of Forsterite from Glass to Liquid States

To investigate the transition mechanisms of forsterite from a glassy to molten state, we performed molecular dynamics calculations in a temperature range of 10–3000 K. The results show that the thermal expansion coefficient changes remarkably at around 1567 K during heating from 10 K. This temperature is the transition point from glassy to liquid states. To investigate the mechanisms of the phase transition, shear viscosity and self-diffusion processes of atoms were analyzed. The result shows that the increasing rates of diffusion coefficients with heating change at around 1300 and 1800 K. This suggests that a diffusion mechanism changes in the supercooled liquid state. From the observation of the trajectories of atoms, it was found that atoms migrate through discontinuous hopping in the supercooled liquid matrices. The hopping probability of atoms, which is located in positions with higher Si and lower Mg densities, is higher than the average probability at low temperatures. The result indicates a dynami...

Reflection characteristics of plane-S-wave at the free boundary of unsaturated porothermoelastic media

In the present work, the reflection of plane-S-wave at the free boundary of an unsaturated porothermoelastic medium is studied taking into consideration of the theory of porous media and the generalized thermoelasticity model. Based on the wave equations for an unsaturated porous medium considering thermo-fluid-solid coupling and the boundary conditions, the theoretical expressions of the reflection coefficient of five kinds of reflected waves at the free boundary of the unsaturated porothermoelastic medium, i.e., reflected P1 wave, reflected P2 wave, reflected P3 wave, reflected S wave and reflected thermal wave, are derived by using the classical Helmholtz potential function decomposition method. Then the numerical calculations are used to discuss the variation of the reflection coefficients of each reflected wave with the incident angle of plane-S-wave and the thermophysical parameters such as saturation, thermal expansion coefficient, thermal conductivity, medium temperature and phase-lag of heat flux and gradient of temperature. The results show that the reflection coefficient of each reflected wave is affected by not only the incident angle but also the saturation. The change of thermal expansion coefficient has a significant influence on the reflection coefficient of each reflected wave. The change of medium temperature only affects the reflection coefficient of the reflected P2 wave, reflected P3 wave and reflected thermal wave. The change of thermal conductivity, phase-lag of heat flux and phase-lag of gradient of temperature only has a large effect on the reflection coefficient of reflected thermal wave. These conclusions are helpful in the theoretical research and seismic exploration projects.

Effect of Sc substitution on thermophysical properties of tetragonal ScYSZ: Molecular dynamics simulation

This paper aims to provide an insight into thermophysical properties of series tetragonal Sc2O3-Y2O3 co-stabilized ZrO2 (ScYSZ) materials. Phonon density of states and point-defect scattering model were employed to explain the variation of thermal conductivity of ScYSZ materials. The results suggested that the smaller interionic distance of Sc3+–O2− than Y3+–O2− provided larger atomic attraction, which created an effective migration barrier to ensure the better structural stability of ScYSZ. Compared with Y2O3 stabilized ZrO2 (YSZ), ScYSZ had lower phonon group velocity and thermal conductivity, which gradually decreased with the increase of Sc3+ content. Thermal expansion coefficient of ScYSZ was comparable to that of YSZ due to the combined effect of temperature, structural density, lattice defect and chemical composition. The substitution of Sc3+ for Y3+ played a minor role in the change of thermal expansion coefficient of YSZ. ScYSZ with good thermophysical properties is a prospective candidate material for high-temperature TBCs application.

Microstructure and thermal properties of Cr7C3 coated graphite flakes/Al composites

Graphite flakes (Gf) were coated with Cr7C3 by salt bathing coating, and the Cr7C3 coated Gf reinforced Al matrix composites were prepared by gas pressure infiltration. Surface morphology of uncoated and Cr7C3 coated Gf, the interfacial characteristics of Cr7C3 coated Gf/Al composites were observed by SEM. Surface of Cr7C3 coated Gf is smooth without defects, the Cr7C3 coating effectively improved the wettability. When volume fraction of Gf is below 50%, the experimental value of thermal conductivity (TC) is basically consistent with the calculated value of the EMA model. When volume fraction of Gf is over 50%, the growth of deviates from the EMA model curve. Coefficient of thermal expansion (CTE) of composite was predicted by Kerner model and Orientation Disc model, because Kerner model does not consider the anisotropy of Gf and inaccurately predict the change of CTE. However, the Orientation Disc model can predict the change of CTE better, but there is certainly deviation.

Environment and strain energy related micromechanics analysis for properties of carbon nanotubes

AbstractIn the present study, a theoretical prediction of the effect of thermal environment on elastic properties of single‐walled carbon nanotubes (SWCNTs) is reported, and a molecular structural mechanics model is proposed in which the covalent bonds are traded as a truss. The influences of the temperature and the coefficient of thermal expansion (CTE) on Young's modulus and shear modulus of both armchair and zigzag SWCNTs are investigated. When CTE is and remains constant, the surface Young's modulus and the normal Young's modulus are reduced, respectively, as compared with the normal temperature. With the change of CTE, its effect on the elastic constant is far greater than the temperature effect itself. According to the principle of elastic theory, a temperature‐dependent continuum shell model of strain energy is also established. The strain energy of single‐walled carbon nanotubes decreases with the increase of the temperature.