Lattice Thermal Expansion Research Articles

A non-isothermal phase-field crystal model with lattice expansion: analysis and benchmarks

Abstract We introduce a non-isothermal phase-field crystal model including heat flux and thermal expansion of the crystal lattice. The fundamental thermodynamic relation between internal energy and entropy, as well as entropy production, is derived analytically and further verified by numerical benchmark simulations. Furthermore, we examine how the different model parameters control density and temperature evolution during dendritic solidification through extensive parameter studies. Finally, we extend our framework to the modeling of open systems considering external mass and heat fluxes. This work sets the ground for a comprehensive mesoscale model of non-isothermal solidification including thermal expansion within an entropy-producing framework, and provides a benchmark for further meso- to macroscopic modeling of solidification.

Improving radiation tolerance with room temperature annealing of pre-existing defects

Abstract Pre-existing defects in semiconductor devices can act as nucleation sites for radiation damage. Defects generated from mismatches in lattice constant, stiffness and thermal expansion are difficult to eliminate with thermal annealing. We propose a non-thermal stimulus, the electron wind force, to reduce the pre-existing defect concentration in Zener diodes at room temperature in a minute. The pristine and pre-annealed diodes were exposed to 11 MeV Au3+ ions at different fluences to induce damage levels of 0.2, 2, 10 and 20 displacement per atom (dpa). Post irradiation characterization showed up to 10 times improvement in radiation tolerance in the pre-annealed devices.

Epitaxial growth and characterization of gan thin films on 2D MoS2 substrates via plasma-assisted molecular beam epitaxy

Gallium nitride (GaN) is an excellent material for high-performance optoelectronic and power electronic devices due to its superior thermal conductivity and exceptional electron transport properties. However, growing high-quality GaN films is challenging because of lattice mismatch and thermal expansion coefficient differences with conventional substrates like sapphire and silicon, resulting in threading dislocations and structural defects that compromise device performance. This study addresses these issues by utilizing a 2D-MoS₂ buffer layer, which provides an atomically smooth surface and quasi-van der Waals interface to minimize lattice mismatch stress and suppress defect propagation. GaN thin films were grown on MoS₂-deposited c-sapphire substrates using plasma-assisted molecular beam epitaxy (PA-MBE) under optimized conditions. The MoS₂ layer was prepared using a chemical vapor deposition process followed by nitridation. Characterization techniques, including Reflection High-Energy Electron Diffraction, Atomic Force Microscopy, Field Emission Scanning Electron Microscopy, and X-ray Diffraction, confirmed that the GaN films exhibited high crystallinity, phase purity, and preferred c-axis orientation, with minimal structural defects. Surface analysis showed moderate roughness, with a root mean square value of 11.35 nm, attributed to localized growth variations. The MoS₂ buffer layer significantly reduced lattice mismatch-induced stress, as indicated by the absence of secondary phases in XRD analysis and facilitated defect-free epitaxial growth. These findings demonstrate the transformative potential of combining PA-MBE with 2D material buffers to achieve high-quality GaN films. This approach holds promise for optoelectronic applications such as LEDs and laser diodes, as well as high-frequency electronic devices like high-electron-mobility transistors

Insights into strain dependent lattice thermal conductivity of α - and κ -Ga2O3

α- and κ-Ga2O3 are promising candidates for high-performance devices such as high-power electronics, but the low thermal conductivity (TC) severely hinders its application. Strain inevitably exists in practical Ga2O3-based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth, and it significantly influences the thermal properties of α- and κ-Ga2O3. By employing first-principles calculations and the phonon Boltzmann transport equation, we have studied the TC at the induced strain and optimized strain axis in free states and 16 different strain states. The TC at the induced strain and optimized strain axis generally decreases with increasing strain. Under −4% XZ-axes biaxial compressive strain, the kzz of α-Ga2O3 can increase to ∼1.7 times its original value, while under −2% XY-axes biaxial compressive strain, the kxx of κ-Ga2O3 can increase to 2.8 times its original value. The improvement of thermal transport properties is attributed to the increase in phonon group velocity and relaxation time caused by the phonon hardening and decrease in three-phonon scattering channels, respectively. However, we observed an exception: under +4% X-axis tensile strain, kyy of α-Ga2O3 increased by 1.1 times. Moreover, atomic bond analysis revealed that under XY-direction strain, the ICOHP values for α-Ga2O3 are −3.94 eV (at −4% strain), −3.76 eV (unstrained state), and −3.63 eV (+4% strain). This discovery elucidates the origin of phonon hardening under compressive strain, indicating that strengthened bonds enhance phonon transport. This study provides essential insights into the mechanisms of α- and κ-Ga2O3 TC under different strains.

Reactive scattering of H2 on Cu(111) at 925K: Effective Hartree potential vs sudden approximation.

We present new quantum dynamical results for the reactive scattering of hydrogen molecules from a Cu(111) surface at a surface temperature of 925K. Reaction, scattering, and diffraction probabilities are compared for results obtained using both an effective Hartree potential (EfHP) and a sudden approximation approach, implemented through the static corrugation model (SCM), to include surface temperature effects. Toward this goal, we show how the SRP48 DFT-functional and an embedded atom potential perform when used to calculate copper lattice constants and thermal expansion coefficients based on lattice dynamics calculations within the quasi-harmonic approximation. The so-calculated phonons are then used in the EfHP approach to replace the normal modes of a fictitious copper cluster used in earlier work. We find that both the EfHP and SCM approaches correctly predict the reaction probability curve broadening effect when the surface temperature is increased. Similarly, results for rovibrationally elastic scattering appear to be improved, predominantly for the SCM model. The behavior of the EfHP results appears to remain much closer to that of a Born-Oppenheimer static surface approach, which excludes any surface temperature effects. Finally, for the diffraction, we show very clear attenuation effects for the SCM approach, significantly decreasing specular diffraction probabilities at 925K surface temperature. These results demonstrate that state-of-the-art theoretical models are able to reproduce strictly quantum mechanical scattering effects with a sudden approximation model and open up interesting opportunities for further comparisons to experimental diffraction results.

Thermal properties of mullite‐type SnAlBO4 and SnGaBO4

AbstractWe report on temperature‐dependent structural and spectroscopic properties of two new members of the mullite‐type ceramics SnAlBO4 and SnGaBO4. In‐situ X‐ray powder diffraction (XRPD) demonstrates positive thermal expansion behavior for all orthorhombic lattice parameters between 13 and 840 K. The lattice thermal expansion is modeled by Grüneisen first‐order approximation, where the vibrational energy is calculated by the Debye–Einstein–anharmonicity (DEA) approach. Although the thermal changes of the metric parameters do not show any discontinuity, the double‐Debye model and low‐temperature thermal analysis leave hints for subtle displacive changes. Splitting of the tin‐doublets of the 119Sn Mössbauer spectra at low temperature is assumed to be associated with structural modulation although the temperature‐dependent Raman spectra could not support these findings. The modulation could either be dynamic which requires much longer thermal equilibration than the speed of the data collection for XRPD and Raman spectroscopy. Selective Raman mode frequencies are analyzed using a modified Klemens model, which helps to understand the thermal anharmonic behaviors of the SnO4, MO6, and BO3 polyhedra as a function of temperature.

Radiative thermal diode driven by the synergy between isotope engineering and lattice thermal expansion

Radiative thermal diode driven by the synergy between isotope engineering and lattice thermal expansion

Laser-initiated electron and heat transport in gold-skutterudite CoSb3 bilayers resolved by pulsed x-ray scattering

Abstract Electron and lattice heat transport have been investigated in bilayer thin films of gold and CoSb3 after photo-excitation of the nanometric top gold layer through picosecond x-ray scattering in a pump-probe setup. The kinetics of heat transfer are detected by thermal lattice expansion and compared to simulations based on the two-temperature model of coupling of electron and phonon degrees of freedom. The unexpected observation of a larger portion of the deposited heat being detected in the underlying CoSb3 layer before the topmost gold layer is heated supports the picture of transport of the photo-excited electrons from gold to the underlying layer to be converted into lattice heat. The change of partition of heat between the gold and CoSb3 layer with laser fluence and wavelength (either exciting intraband transitions or additionally interband transitions) is rooted in the amplitude of electron temperature. Higher electron temperatures result in a longer equilibration time with the lattice and thus a larger proportion of ballistic electron transport across the interface.

Effects of Dislocation Filtering Layers on Optical Properties of Third Telecom Window Emitting InAs/InGaAlAs Quantum Dots Grown on Silicon Substrates.

Integrating light emitters based on III-V materials with silicon-based electronics is crucial for further increase in data transfer rates in communication systems since the indirect bandgap of silicon prevents its direct use as a light source. We investigate here InAs/InGaAlAs quantum dot (QD) structures grown directly on 5° off-cut Si substrate and emitting light at 1.5 μm, compatible with established telecom platform. Using different dislocation defect filtering layers, exploiting strained superlattices, and supplementary QD layers, we mitigate the effects of lattice constant and thermal expansion mismatches between III-V materials and Si during growth. Complementary optical spectroscopy techniques, i.e. photoreflectance and temperature-, time- and polarization-resolved photoluminescence, allow us to determine the optical quality and application potential of the obtained structures by comparing them to a reference sample-state-of-the-art QDs grown on InP. Experimental findings are supported by calculations of excitonic states and optical transitions by combining multiband k•p and configuration-interaction methods. We show that our design of structures prevents the generation of a considerable density of defects, as intended. The emission of Si-based structures appears to be much broader than for the reference dots, due to the creation of different QD populations which might be a disadvantage in particular laser applications, however, could be favorable for others, e.g., in broadly tunable devices, sensors, or optical amplifiers. Eventually, we identify the overall most promising combination of defect filtering layers and discuss its advantages and limitations and prospects for further improvements.

High-Contrast Thermochromism in Room-Temperature Transparent Layered Perovskite PEA2PbBr4 with a High Temperature-Induced Bandgap Change Rate of 0.8 meV/K.

Two-dimensional organic-inorganic hybrid layered perovskites have emerged as a new generation of optoelectronic materials. However, the thermochromism in organic-inorganic hybrid layered perovskites has been rarely explored in depth. A further understanding of the mechanism is necessary and favorable for the application. Here, transparent centimeter-sized single crystals of the organic-inorganic hybrid layered perovskite (C6H5C2H4NH3)2PbBr4 (PEA2PbBr4) were synthesized using an improved evaporation method. As a typical organic-inorganic hybrid layered perovskite, the PEA2PbBr4 single crystal shows high-contrast and progressive thermochromism exhibiting a change from colorlessness and transparency to lemon yellow in a wide temperature range of 200-450 K. Based on the calculation through the Varshni equation, the temperature-induced bandgap change rate directly associated with the high-contrast thermochromism of PEA2PbBr4 reaching 0.8 meV/K. This value is higher than that of many three-dimensional perovskites and traditional IV-III semiconductors. Furthermore, the temperature-dependent 193 nm photoluminescence spectra suggest that this high temperature-induced bandgap change rate of PEA2PbBr4 is a result of the competitive interaction between lattice thermal expansion and electron-phonon coupling (Fröhlich coupling coefficient ΓLO = 2.215). Based on the characteristics introduced above, PEA2PbBr4 as an organic-inorganic hybrid layered perovskite has a better performance in achieving the balance between high-contrast and high room-temperature transmittance. Therefore, PEA2PbBr4 is a material with great potential in applications like temperature-indicating labels. This work provides valuable insights into the thermochromism of layered perovskites, offering a new material system and approach for developing thermochromic materials with higher sensitivity and efficiency.

Modeling of a Single-Band-Ratiometric Sensor Based on Lattice Positive Thermal Expansion in Eu3+-Activated Halide Perovskite Cs2NaEuCl6.

Recently, single-band ratiometric (SBR) thermometry has emerged as an innovative approach to traditional fluorescence thermometry, overcoming uncertainties associated with emission spectrum overlap or scattering while maintaining high spatial resolution and remote monitoring. This paper presents a novel Cs2NaEuCl6 perovskite prepared through a slow-cooling solution method. Additionally, it proposes a temperature sensor model that relies on the thermal quenching of charge-transfer state absorption. Mechanical studies highlight the role of lattice positive thermal expansion in affecting Eu3+ emission. Conversely, a significant emission enhancement is observed upon excitation corresponding to both the ground state and excited state absorption. The distinct luminescent behavior of this Eu3+-activated halide perovskite model makes it suitable for developing a highly sensitive SBR-type sensor with a relative sensitivity (Sr) exceeding 1.5% K-1 and temperature resolution (𝛿T) below 1 K at room temperature. Furthermore, it demonstrates the thermal stability during multiple heating-cooling cycles. Finally, the practical applicability of the proposed SBR model is demonstrated by employing a self-manufactured film sensor that enables precise real-time temperature detection for electronic components. The work is regarded as a significant stride toward the development of cutting-edge and exquisitely sensitive thermometers based on lanthanide-based halide double perovskites.

Revisiting thermal transport in CuInTe2: Role of temperature-dependent anharmonicity and higher-order phonon–phonon interactions

The temperature-dependent phonon thermal transport in CuInTe2 is investigated by considering lattice thermal expansion, temperature-dependent anharmonicity, higher-order phonon–phonon interactions, and phonon renormalization with input from first-principles based density functional theory calculations. Incorporating these higher-order temperature-dependent effects reveals that the thermal conductivity varies with temperature as T−1.59 consistent with experimental measurements ranging from T−1.62 to T−1.78. Using the lowest-order theory or only temperature-dependent interatomic force constants or four-phonon scattering resulted in a wrong description of thermal transport physics.

Transient heating of Pd nanoparticles studied by x-ray diffraction with time of arrival photon detection.

Pulsed laser heating of an ensemble of Pd nanoparticles, supported by a MgO substrate, is studied by x-ray diffraction. By time-resolved Bragg peak shift measurements due to thermal lattice expansion, the transient temperature of the Pd nanoparticles is determined, which quickly rises by at least 100 K upon laser excitation and then decays within 90 ns. The diffraction experiments were carried out using a Cu x-ray tube, giving continuous radiation, and the hybrid pixel detector Timepix3 operating with single photon counting in a time-of-arrival mode. This type of detection scheme does not require time-consuming scanning of the pump-probe delay. The experimental time resolution is estimated at 15 ± 5 ns, which is very close to the detector's limit and matches with the 7 ns laser pulse duration. Compared to bulk metal single crystals, it is discussed that the maximum temperature reached by the Pd nanoparticles is higher and their cooling rate is lower. These effects are explained by the oxide support having a lower heat conductivity.

Order-disorder structural transition and thermal expansion properties in Ce-substituted Y2Zr2O7 system

In the pursuit of advancing inert matrix fuel (IMF) applications, zirconate pyrochlores have emerged as promising candidates for the incorporation of minor actinides and plutonium. This study employs CeO2 as a nonradioactive surrogate for PuO2 and focuses on the synthesis of cerium-substituted Y2Zr2O7 samples (Y2-xCexZr2O7, 0.0 ≤ x ≤ 2.0) through a solid-state route under both reducing and oxidizing conditions to mimic plutonium incorporation. The impact of cerium content and its valence state, which in turn is influenced by synthesis conditions, on crystal structure and phase stability has been investigated through X-ray diffraction and Raman spectroscopic studies. Samples synthesized under reduced conditions undergo a defect fluorite to pyrochlore phase transition through a biphasic mixture consisting of these two phases upon increasing Ce-substitution. Conversely, the high temperature oxidizing synthesis conditions yield a defect fluorite phase field over a wide composition range (0.0 ≤ x ≤ 1.6) and a tetragonal phase at x = 2.0. Intriguingly, pyrochlore-type phases obtained under reducing conditions transform into the metastable kappa-phases upon mild oxidation at a relatively low temperature (1073 K), as confirmed by Raman spectroscopic studies. A combination of thermo-gravimetric and Raman spectroscopic investigations confirms the complete reduction of Ce4+ to Ce3+ under reducing conditions. The lattice thermal expansion behavior has also been investigated for the defect fluorite phases, synthesized under oxidizing conditions, by means of high temperature XRD spanning the temperature range 298–1273 K. Notably, the thermal expansion of compositions exhibits an increasing trend with increasing cerium content.

Computational study of elastic waves generated by ultrafast demagnetization in fcc Ni

Picosecond ultrasonics is a fast growing and advanced research field with broad application to the imaging and characterization of nanostructured materials as well as at a fundamental level. The aim of this paper is to provide an advanced 3D model based on atomistic spin-lattice simulations of the laser-induced elastic response in ferromagnetically ordered fcc Ni. The advantage of such an approach is the possibility to take into account the laser radiation interaction with the spins and thus characterize the magnetic contribution to the total stress. We analyze the atomic displacements caused both by the ultrafast thermal expansion of the crystal lattice and by the demagnetization process due to the heating of a certain area of the sample by an ultrashort laser pulse. Subsequently, an attempt is made to propose mathematical expressions for describing the corresponding total stress. The lattice and magnetic contributions have been evaluated, whereupon the former is found to be much greater than the latter. Published by the American Physical Society 2024

Study of the Influence of Doping Efficiency of CeO2 Ceramics with a Stabilizing Additive Y2O3 on Changes in the Strength and Thermophysical Parameters of Ceramics under High-Temperature Irradiation with Heavy Ions

The article outlines findings from a comparative analysis of the effectiveness of doping CeO2 ceramics with a stabilizing additive Y2O3 on alterations in the strength and thermophysical parameters of ceramics under high-temperature irradiation with heavy ions comparable in energy to fission fragments of nuclear fuel, which allows, during high-temperature irradiation, to simulate radiation damage that is as similar as possible to the fission processes of nuclear fuel. During the studies, it was found that the addition of a stabilizing additive Y2O3 to the composition of CeO2 ceramics in the case of high-temperature irradiation causes an increase in stability to swelling and softening because of a decrease in the thermal expansion of the crystal lattice by 3–8 times in comparison with unstabilized CeO2 ceramics. It has been determined that the addition of a stabilizing additive Y2O3 leads not only to a rise in the resistance of the crystal structure to deformation distortions and swelling, but also to a decrease in the effect of thermal expansion of the crystal structure, which has an adverse effect on the structural ordering of CeO2 ceramics exposed to irradiation at high temperatures.

Cross-examination of photoinitiated carrier and structural dynamics of black phosphorus at elevated fluences.

Revived attention in black phosphorus (bP) has been tremendous in the past decade. While many photoinitiated experiments have been conducted, a cross-examination of bP's photocarrier and structural dynamics is still lacking. In this article, we provide such analysis by examining time-resolved data acquired using optical transient reflectivity and reflection ultrafast electron diffraction, two complementary methods under the same experimental conditions. At elevated excitation fluences, we find that more than 90% of the photoinjected carriers are annihilated within the first picosecond (ps) and transfer their energy to phonons in a nonthermal, anisotropic fashion. Electronically, the remaining carrier density around the band edges induces a significant interaction that leads to an interlayer lattice contraction in a few ps but soon diminishes as a result of the continuing loss of carriers. Structurally, phonon-phonon scattering redistributes the energy in the lattice and results in the generation of out-of-plane coherent acoustic phonons and thermal lattice expansion. Their onset times at ∼6ps are found to be in good agreement. Later, a thermalized quasi-equilibrium state is reached following a period of about 40-50ps. Hence, we propose a picture with five temporal regimes for bP's photodynamics.

Strain modulation of epitaxial h-BN on sapphire: the role of wrinkle formation for large-area two-dimensional materials.

Strain built-in electronic and optoelectronic devices can influence their properties and lifetime. This effect is particularly significant at the interface between two-dimensional materials and substrates. One such material is epitaxial hexagonal boron nitride (h-BN), which is grown at temperatures often exceeding 1000 °C. Due to the high growth temperature, h-BN based devices operating at room temperature can be strongly affected by strain generated during cooling due to the differences in lattice thermal expansion of h-BN and the substrate. Here, we present results of temperature-dependent Raman studies of the in-plane E2ghighphonon mode in the temperature range of 300-1100 K measured for h-BN grown by metalorganic vapor phase epitaxy. We observe a change, by an order of magnitude, in the rate of the temperature-induced frequency shift for temperatures below 900 K, indicating a strong reduction of the effective h-BN/substrate interaction. We attribute this behavior to the creation of h-BN wrinkles which results in strain relaxation. This interpretation is supported by the observation that no change of layer/substrate interaction and no wrinkles are observed for delaminated h-BN films transferred onto silicon. Our findings demonstrate that wrinkle formation is an inherent process for two-dimensional materials on foreign substrates that has to be understood to allow for the successful engineering of devices based on epitaxially grown van der Waals heterostructures.

Chemical pressure as an effective tool for tuning the structural disordering and barocaloric efficiency of complex fluorides (NH4)3MF7 (M: Sn, Ti, Ge, Si)

Double fluoride salts (NH4)3M4+F7 (M4+: Sn, Ti, Ge, Si) demonstrate a high efficiency of using chemical pressure as a tool for control and tuning structural ordering/disordering, sensitivity to hydrostatic pressure, successions of the phase transitions, etc and, as a result, for purposeful variation within a wide range of parameters of barocaloric effect (BCE). The conventional and inverse BCEs near the triple points were found on the T − p phase diagrams, combination of which can be used to construct original cooling cycle in narrow temperature and pressure ranges. Reconstructive transformation between two cubic phases, Pm3ˉm↔Pa3ˉ , realized in (NH4)3SnF7 at atmospheric pressure and in (NH4)3TiF7 at p> 0.4 GPa are characterized by rather low thermal hysteresis, δT0 = 1 K, and a great entropy change, ΔSBCE = 110–152 J (kg · K)−1, depending on the size of the central atom. At above 300–350 K, a contribution to BCE associated with the regular thermal expansion of the crystal lattice becomes comparable to entropy and temperature changes under pressure in the region of the phase transitions. An analysis of the absolute, relative and integral barocaloric characteristics of (NH4)3M4+F7 compounds showed their high competitiveness with respect to other barocaloric materials considered as promising solid-state refrigerants.

Strain accumulation and relaxation on crack formation in epitaxial AlN film on Si (111) substrate

The formation of cracks is often observed in the epitaxial growth of ultrawide-bandgap aluminum nitride (AlN) semiconductor films on economical and versatile silicon (Si) substrates due to the significant differences in in-plane lattice parameters and thermal expansion coefficients between the film and the substrate, which hampers the development of template, buffer layer, and device structure with a relatively thick AlN layer for devices. The present study aims to elucidate the conditions of crack formation through a simple but comprehensive estimation of strain energy accumulation and relaxation by lattice strain, misfit dislocation density, and crack formation. Strain energy in the epitaxial film from lattice and thermal mismatches is evaluated by an elastic strain equation tailored to the epitaxy of the hexagonal crystal structure. The effects of temperature, thickness, and dislocation density on the lattice and dislocation strain energies of the film are also considered. Finally, the comparison in the changes in the total strain energy and cleavage energy with decreasing temperature shows that cleavage energy is higher than strain energy if the film is thinner than 400 nm but becomes lower than the strain energy if the film is thicker than 400 nm during cooldown, suggesting the crack formation, which matches well with experimental observations.