Thermal Expansion Coefficient Ratio Research Articles

Anisotropic phonon properties in SiP2 monolayer: A first-principles study

Recently, the micromechanical exfoliation method has effectively separated the two-dimensional (2D) SiP2 flake, which exhibits superior performance in field-effect transistors and photodetectors. In this paper, first-principles calculations were utilized to investigate the phononic properties of a SiP2 monolayer, which would be significant to explain the thermal deformation of 2D SiP2-based devices. We found that the acoustic phonon branches of SiP2 monolayer demonstrate significant in-plane anisotropy. The anisotropic ratios for the group velocities are 2.41 and 1.66 for the longitudinal and transverse acoustic modes, respectively. Meanwhile, the anisotropic ratio of the negative thermal expansion coefficient only considering the contribution from acoustic phonons is 0.14, while this ratio is ∼0.279 occurs at 120 K when both acoustic and optical phonons are taken into account. For the Raman-active optical phonons, the largest Grüneisen constant of these Raman-active phonons reaches 22.76, while the largest anisotropic ratio of phonon frequency is up to 2.16. To interpret this significant in-plane anisotropy, we calculated the electron density distribution, crystal orbital Hamilton populations (COHP) and interatomic force constants (IFCs), and found the maximal IFC in the x-direction is 5.79 times greater than y-direction, suggesting it as a major origin of the phononic anisotropy.

A method for assessing non-uniformity of thermally induced stresses in the cross-section of energy piles

The non-uniform distribution of thermally induced stresses within energy piles presents a significant challenge when assessing their thermo-mechanical behavior. This study proposes an innovative approach to assess the non-uniformity of thermally induced stresses in energy piles by employing a novel parameter known as the concentration coefficient of thermally induced stress (Kth). Kth serves a dual role: firstly, it quantifies the non-uniformity of thermally induced stresses in the cross-section of energy piles, and secondly, it enhances the accuracy of determining the average thermally induced stress and axial force in energy piles when combined with field testing. Sensitivity analyses were performed using both the Taguchi method and the full factorial test method to investigate the influence of various factors on Kth. The descending order of impact weights for each influencing factor on the non-uniformity of thermally induced stress is as follows: pile-soil stiffness ratio, pile configuration, soil's Poisson's ratio, soil-pile thermal expansion coefficient ratio and slenderness ratio, with pile-soil stiffness ratio, pile configuration and soil Poisson's ratio combined effects take up over 90% of the observed variations in Kth. Notably, when the soil stiffness is low, Kth can exceed a value of thirteen, indicating an extreme non-uniformity in the thermally induced stress distribution across the cross-section. Additionally, a lower number of heat exchanger pipes is associated with a more non-uniform thermally induced stress distribution, but the non-uniformity tends to stabilize as the number of U-shaped heat exchanger pipes exceeds three.

Thermally induced mechanical interactions of energy pile groups subjected to cyclic nonsymmetrical thermal loading

Prior to this study, the long-term thermally-induced mechanical interactions of cyclic nonsymmetrical thermally loaded energy pile groups in soft clay have not been systematically elucidated, particularly with a limited understanding of the interactions among operating and non-operating energy piles, soil, and caps. Observing such a challenge, this paper presents a three-dimensional (3D) thermomechanical finite-element model (FEM), which considers the thermal-dependent behavior at the pile-soil interface and is validated using typical centrifuge tests reported in the literature. Special attention is given to the influence of soil-pile thermal expansion coefficient ratio, pile-soil stiffness ratio, and slab-soil stiffness ratio on the abovementioned interaction. The results indicate that a larger soil-pile thermal expansion coefficient ratio intensifies the interaction between energy piles and the surrounding soil, causing cyclic variations in thermally mobilized friction. With increasing pile-soil stiffness ratios, the differential thermomechanical responses among different piles within a group are enhanced because the toe and shaft resistances mobilized by the cap through nonoperating energy piles are weakened in the interaction process among energy piles, nonoperating energy piles, and the cap. Moreover, the increase in the slab-soil stiffness ratio exacerbates this interaction, strengthening the integrity of the pile groups.

Prestress loss of twisted wire strands and parallel wire strands at elevated temperatures

High-strength prestressing cables exposed to high temperature result in the prestress loss, potentially leading to failure and even collapse of prestressing structures. This paper experimentally investigates thermal expansion of 1860 MPa steel wires and prestress loss of 1860 MPa twisted wire strands and parallel wire strands at elevated temperatures initially. Strands are localised heated in prestress loss tests. Different highest temperatures and initial prestress levels are considered. The test results show that EC2 underestimates the thermal strain of 1860 MPa steel wire at more than 450 °C. Prestress loss of the prestressing strand increases as the temperature increases. Cable types have little influence on the final prestress loss ratio. The strand with higher initial prestress level results in less final prestress loss ratio at less than 400 °C. A series fitting formulas of thermal strain, thermal expansion coefficient and final prestress loss ratio of prestressing strands are proposed. In addition, the finite element model of prestressing strand at elevated temperatures is established and validated. The numerical analysis shows that the temperature of the strand in non-heated region is significant, and the stress of the strand rises and then fall during heating stage, due to the balance between thermal expansion and degradation of mechanical properties. Finally, the analytical model of the prestress loss of the strands at elevated temperatures is proposed. The effect of thermal expansion, degradation of the mechanical properties and high-temperature creep of the strand in the heated region and non-heated region is considered respectively. Simplified method is obtained for more efficient computation. The analytical results have great agreement with the test results. The application of analytical method on practical engineering is discussed, and improved calculation equation is proposed for large span cables exposed to localised fire.

Cohesive and Thermal Properties of Sodium Cyanide-Halide Mixed Crystals

In order to analyse the cohesive and thermal properties of sodium cyanide-halide mixed crystals an Extended Three Body Force Shell Model (ETSM) has been applied by incorporating the effect of translational-rotational (TR) coupling. We have conducted theoretical research on cohesive and thermal properties, such as cohesive energy (, molecular force constant (f), compressibility (), Restrahlen frequency (, Debye temperature (D), Gruneisen parameter (), Moelwyn Hughes constants (F1) and the ratio of volume thermal expansion coefficient (v) to volume specific heat (Cv), as a function of temperature within the temperature range 50K T 300K at concentration x=0, 0.27, 0.58 and 1. The current model computations and the findings of the available experiments are in good agreement. The ETSM is a sufficiently realistic model and may be applied to a variety of other mixed crystals in this family.

Parametric study on interfacial crack propagation in solid oxide fuel cell based on electrode material

Interfacial delamination is one of the typical failure modes of solid oxide fuel cell, which is caused by interfacial crack propagation. In order to improve the stability, durability and mechanical integrity of the cell, the influence of electrode material properties on the interfacial crack propagation is studied. Based on a large number of experimental data, the material model of anode and cathode adopts approximate linear model and the different material optimization schemes are set by changing the material parameters, i.e., the ratio of elastic modulus and thermal expansion coefficient between electrode and electrolyte. The crack energy release rate and crack propagation length are taken as important objective functions to compare the extent of interfacial crack propagation under different material optimization schemes. The internal relationship between electrode material parameters and interfacial crack propagation behavior is analyzed, and the optimization scheme of electrode material is obtained to reduce the possibility of delamination in the cell. This research provides guidance for the improvement of stability and integrity of solid oxide fuel cell.

Thermal Buckling of Laminated Composite Plates Using a Simple Four Variable Plate Theory

In this study, the thermal buckling behavior of composite laminate plates cross-ply and angle-ply all edged simply supported subjected to a uniform temperature field is investigated, using a simple trigonometric shear deformation theory. Four unknown variables are involved in the theory, and satisfied the zero traction boundary condition on the surface without using shear correction factors, Hamilton's principle is used to derive equations of motion depending on a Simple Four Variable Plate Theory for cross-ply and angle-ply, and then solved through Navier's double trigonometric sequence, to obtain critical buckling temperature for laminated composite plates. Effect of changing some design parameters such as, orthotropy ratio (E1/E2), aspect ratio (a/b), thickness ratio (a/h), thermal expansion coefficient ratio (α2/α1), are investigated, which have the same behavior and good agreement when compared with previously published results with maximum discrepancy (0.5%).

Stresses caused by Cd and Te inclusions in CdMnTe crystals and their impact on charge carrier transport

We present a defect analysis in Cd and Te inclusion regions within Bridgman-grown Cd0.95Mn0.05Te crystals for nuclear detectors. Crystal growth was performed under Cd- or Te-rich conditions so that Cd and Te inclusions could be investigated, respectively. Cathodoluminescence (CL) maps confirm that a Te inclusion does not generate defects in its vicinity, in contrary to a Cd inclusion, which induces microcracks in its neighborhood in the CdMnTe matrix. This originates from the higher thermal expansion coefficient ratio of Cd to the CdTe than Te to CdTe. Next, the relative stress analysis was carried out using the electron backscatter diffraction (EBSD) technique, which shows that a Te inclusion causes higher stresses in its environment than a Cd inclusion does. The reason for this is that the Cd inclusion releases the stresses due to a creation of the tensile cracks. Despite the fact that a Te inclusion stresses the crystal lattice more, it can have a smaller impact on charge carrier transport in nuclear detectors than a Cd inclusion. This effect we confirm by μτ product measurements, which determine charge carrier transport properties.

Magnetoelastic coupling and Grüneisen scaling in NdB4

We report high-resolution capacitance dilatometry studies on the uniaxial length changes in a NdB$_4$ single crystal. The evolution of magnetically ordered phases below $T_{\rm N}$= 17.2~K (commensurate antiferromagnetic phase, cAFM), $T_{\rm IT}$= 6.8~K (intermediate incommensurate phase, IT), and $T_{\rm LT}$= 4.8~K (low-temperature phase, LT) is associated with pronounced anomalies in the thermal expansion coefficients. The data imply significant magneto-elastic coupling and evidence of a structural phase transition at $T_{\rm LT}$ . While both cAFM and LT favor structural anisotropy $\delta$ between in-plane and out-of-plane length changes, it competes with the IT-type of order, i.e., $\delta$ is suppressed in that phase. Notably, finite anisotropy well above $T_{\rm N}$ indicates short-range correlations which are, however, of neither cAFM, IT, nor LT-type. Gr\"uneisen analysis of the ratio of thermal expansion coefficient and specific heat enables the derivation of uniaxial as well as hydrostatic pressure dependencies. While $\alpha$/$c_{\rm p}$ evidences a single dominant energy scale in LT, our data imply precursory fluctuations of a competing phase in IT and cAFM, respectively. Our results suggest the presence of orbital degrees of freedom competing with cAFM and successive evolution of a magnetically and orbitally ordered ground state.

Photo-Actuation Stress of Ultra-Drawn, Chain-Extended Polyethylene

The influence of solid-state drawing on the photo-actuation stress of oriented semi-crystalline polymers is explored experimentally and theoretically. Photo-responsive and ultra-drawn linear polyethylenes (PEs) containing a commercial benzotriazole dye (BZT) are studied as a model material. In accordance with previous studies, it is found that the actuation stress as a function of draw ratio exhibits an optimum and that the stress first increases and then decreases again slightly. A semi-empirical model is presented that describes the photo-actuation stress as a function of the draw ratio, Young's modulus, thermal expansion coefficient, and temperature difference between the illuminated and non-illuminated state. The Young's modulus and thermal expansion coefficient versus draw ratio curves are described using models based on the pseudo-affine deformation scheme in combination with a two-phase structural model in which highly oriented elements are connected in series with a disordered phase. Rather surprisingly, it is found that the model predicts that the photo-actuation stress is virtually independent of the draw ratio and the degree of chain orientation and extension. An excellent agreement is observed between model predictions and experimental results if the actuation stress is corrected for temperature effects related to the conversion of light into heat. The structure-property relationships derived here provide guidelines for the design of light-driven artificial muscle systems based on ultra-drawn polymers. For instance, it is predicted that ultra-drawn polyamides and polyesters are potentially excellent candidates for producing photo-actuators provided that suitable additives are used and this despite their low maximum attainable draw ratio, Young's modulus and strength.

Thermo-mechanical buckling of stepped circular bi-laminates

Thermo-mechanical buckling of stepped circular bi-laminates subjected to a uniform temperature field is investigated analytically. The governing equations are derived via the Principal of Stationary Potential Energy, resulting in self-consistent constitutive equations of the composite structure, as well as the equations of motion and the associated matching and boundary conditions for the composite structure. A loading parameter is identified, and closed form analytical solutions to the non-linear problem are obtained, yielding exact results within the context of the formulation. A stability criterion is established based on the second variation of the total potential energy of the system in order to identify regions of stable and unstable equilibrium configurations, and a critical temperature and a critical membrane force are determined. Simulation results based on the analytical solutions are generated and a critical and characteristic behavior is assessed for both clamped-fixed supports and hinged-fixed supports conditions. The phenomenon of “sling-shot buckling” is observed. Results of parameter studies are also presented, which demonstrate the influence that material and geometric parameters of the system have on critical behavior. In particular, thermal expansion coefficient ratio, modulus ratio and thickness ratio all play a non-trivial role in the temperature carrying capacity of the structure for relatively large patches. Because the loading parameters and geometric parameters appear explicitly in the analytical solution and simulations, the model and results will be useful towards the design of micro-electromechanical systems, smart structures and related configurations.

Nonlinear bending of elastically restrained functionally graded graphene nanoplatelet reinforced beams with an open edge crack

This paper investigates the nonlinear bending behaviours of multilayer functionally graded graphene nanoplatelet-reinforced composite (FG-GPLRC) beams elastically restrained at both ends and with an open edge crack. The GPLs are uniformly distributed in each individual layer but follow a layer-wise variation along the thickness direction. The effective Young's modulus is predicted by the modified Halpin-Tsai micromechanics model while both the thermal expansion coefficient and Poisson's ratio are determined by the rule of mixture. The finite element method is employed to discretize the edge cracked beam, with a particular focus on the influences of the location and depth of the open edge crack on the nonlinear bending deflections of FG-GPLRC beams. It is found that adding a higher content of GPLs into the matrix, dispersing more GPLs near the top and bottom surfaces of the beam and increasing the elastic stiffness of the end constraints can effectively strengthen the beam stiffness hence considerably reduce the deflection. An increase in the crack depth ratio (CDR) and temperature rise weaken the structure and consequently lead to bigger deflections. The nonlinear bending performance of the beam is also found to be highly sensitive to the location of the crack.

A Parametric Study on the Elliptical hole Effects of Laminate Composite Plates under Thermal Buckling Load

AbstractIn this study, a thermal buckling analysis of laminate composite square plate was performed with elliptical hole cutout using the finite element method. Graphite/Epoxy laminate plate used in this study is symmetrical stacking sequence plate [(0/90)2]s. This laminate square plate was subjected to temperature loading with clamped support on all edges. Moreover, the parameters considered were fiber orientation (θ), a/b ratio (elliptical hole), elliptical hole inclination (φ), thermal expansion coefficient ratio (α1/α2), and thickness of plates (t). The results showed that as the thermal expansion coefficient ratio changes, the elliptical hole ratio and the elliptical hole inclination have inconsequential effects on the performance of the resistance of laminate composite plates. The maximum values of thermal buckling amplification factor were obtained when the ratio a/b = 1.0, which is a circle cutout,while the minimum values were obtained when the ratio a/b = 0.5, regardless of the thickness of plates. Moreover, the plate with elliptical or circle cutout hole provides about 4% to 9% higher buckling resistance than that of the plate without hole cutout, because when subjected to temperature loading the plate with hole can release stress better than the plate without hole.

Behavior of confined granular beds under cyclic thermal loading

We investigate the mechanical behavior of a confined granular packing of irregular polyhedral particles under repeated heating and cooling cycles by means of numerical simulations with the non-smooth contact dynamics method. Assuming a homogeneous temperature distribution as well as constant temperature rate, we study the effect of the container shape, and coefficients of thermal expansions on the pressure buildup at the confining walls and the density evolution. We observe that small changes in the opening angle of the confinement can lead to a drastic peak pressure reduction. Furthermore, the displacement fields over several thermal cycles are obtained and we discover the formation of toroidal convection cells inside the granular material. The root mean square of the vorticity is then calculated from the displacement fields and a quadratic dependency on the ratio of thermal expansion coefficients is established.

On the drag effect of one fluid driven by another in a vertical channel

The idea of dragging a viscous fluid by another kind of fluid via the shear stress has fascinated the scientists and engineers. The dependence of the drag effect on the physical parameters of the two immiscible fluids is very much desired but still challenging. In this research, three different kinds of fluids are employed to drag a pure fluid between vertical parallel channel walls, that is, the viscous fluid, the non-Newtonian power-law fluid, and the nanofluid. The drag effects of two-layer fluids are investigated by comparing the velocity fields and the mean velocity curve. Essential parameters determining the dragging efficiencies of the driven fluid are studied systematically: the drag effects of the density ratio p, the thermal conductivity ratio k, the thermal expansion coefficient ratio b, and the viscosity ratio m of the two-layer fluids are focused. Both dilatant flows and pseudo-plastic fluids are considered in driving the viscous fluid. When the pure fluid is driven by the nanofluid, the single-phase model is adopted. The example of 47 nm-Al2O3 nanoparticles suspended in water is analyzed for demonstration: the thermal expansion, the effective viscosity, and the effective thermal conductivity are dependent of the concentration of nanofluid, which makes the nanoparticle volume fraction ϕ a major concern in the drag effects. The findings in the paper reveal several potential strategies to promise high effectiveness on fluid driving via interface shear, which we hope will inspire engineers and researchers in relative working fields.

The Anisotropic Thermal Expansion of Non-linear Optical Crystal BaAlBO3F2 Below Room Temperature

Thermal expansion is a crucial factor for the performance of laser devices, since the induced thermal stress by laser irradiation would strongly affect the optical beam quality. For BaAlBO3F2 (BABF), a good non-linear optical (NLO) crystal, due to the highly anisotropic thermal expansion its practical applications are strongly affected by the “tearing” stress with the presence of local overheating area around the laser spot. Recently, the strategy to place the optical crystals in low-temperature environment to alleviate the influence of the thermal effect has been proposed. In order to understand the prospect of BABF for this application, in this work, we investigated its thermal expansion behavior below room temperature. The variable-temperature XRD showed that the ratio of thermal expansion coefficient between along c- and along a(b)- axis is high as 4.5:1 in BABF. The Raman spectrum combined with first-principles phonon analysis revealed that this high thermal expansion anisotropy mainly ascribe to progressive stimulation of the respective vibration phonon modes related with the thermal expansion along a(b)- and c-axis. The good NLO performance in BABF can be kept below room temperature. The work presented in this paper provides an in-depth sight into the thermal expansion behavior in BABF, which, we believe, would has significant implication to the manipulation in atomic scale on the thermal expansion of the materials adopted in strong-field optical facility.

Fully resolved simulation of particulate flows with heat transfer by smoothed profile-lattice Boltzmann method

A fully resolved simulation (FRS) method for particulate flows with heat transfer based on the smoothed profile-lattice Boltzmann method (SP-LBM) is developed. The governing equations of the flow field and the temperature field are solved by the LBM. The fluid-particle interaction is treated by the SPM. Both the no-slip boundary condition for the velocity field and the conjugate interface conditions for the temperature field are satisfied. Unlike many previous models, the present LB model removes the assumption that the particles have the uniform temperature. Motion of circular catalyst particles in an enclosure is simulated to validate our method. The effect of the thermal expansion coefficient ratio on particle motion behaviours is also investigated.

Thermal buckling analysis of laminated composite beams using hyperbolic refined shear deformation theory

In this article, thermal buckling of laminated composite beams, based on hyperbolic refined shear deformation theory, presented for the first time, is formulated using the principle of minimum total potential energy. Navier’s analytical solution is derived to analytically solve the differential equations and the thermal critical buckling is presented in closed-form solution. The effects of temperature distribution, length to thickness ratio, modulus ratio, and thermal expansion coefficient ratio on thermal buckling of isotropic, orthotropic and laminated composite beams are investigated. The accuracy of the numerical model is verified by comparison with the available results in the literature.

Shakedown boundaries of multilayered thermal barrier systems considering interface imperfections

Thermal barrier coatings (TBCs) usually have complex structures and operate in cyclic high temperatures. The cracking phenomenon is usually observed at the interface with microscopic imperfections between the bond and the thermally grown oxide (TGO) layer owing to the cyclic plasticity around the defect interface under the cyclic thermal loading. Shakedown limits of TBCs with interface imperfections are investigated based on the proposed linear matching method considering the effect of temperature-dependent material properties. The influences of geometric parameters ho, R/ho and H/R as well as the thermal expansion coefficient ratio α/αr are discussed in detail. Results present that the thickness of the TGO has no effect and the geometric factor R/ho shows a little influence on shakedown limits of the multilayered systems. However, the geometric parameter H/R and the thermal expansion coefficient ratio α/αr have a remarkable impact on shakedown limits. This indicates that the depth of the defect influences shakedown limits greatly, while the transition radius at the corner of the imperfection shows slight effect. This is obviously different from the common phenomenon that the transition radius usually has an important influence on shakedown limits. Based on the calculated data, the shakedown assessment approach of TBCs considering various factors mentioned above is proposed. It is of great interest that the simulated effective plastic strain occurs at the defect interface between the TGO and the bond layer when the applied load exceeds the shakedown limit. It is the crack failure region of cracked TBCs according to the microscopic morphology observation.

Hoberman-sphere-inspired lattice metamaterials with tunable negative thermal expansion

Materials with engineered thermal expansion coefficients, capable of avoiding failure or irreversible destruction of structures and devices, are important for aerospace, civil, biomedical, optics, and semiconductor applications. In natural materials, thermal expansion usually cannot be adjusted easily and a negative thermal expansion coefficient is still uncommon. Here we propose a novel architected lattice bi-material system, inspired by the Hoberman sphere, showing a wide range of tunable thermal expansion coefficient from negative to positive, −1.04 × 10−3 °C−1 to 1.0 × 10−5 °C−1. Numerical simulations and analytical formulations are implemented to quantify the evolution of the thermal expansion coefficients and reveal the underlying mechanisms responsible for this unusual behavior. We show that the thermal expansion coefficient of the proposed metamaterials depends on the thermal expansion coefficient ratio and the axial stiffness ratio of the constituent materials, as well as the bending stiffness and the topological arrangement of the constitutive elements. The finding reported here provides a new routine to design architected metamaterial systems with tunable negative thermal expansion for a wide range of potential applications.