Browsing by Subject "Thermal conductivity"

Now showing 1 - 14 of 14

Open Access
Computational thermal homogenization of concrete
(2013) Wu, T.; Temizer, I.; Wriggers, P.
Computational thermal homogenization is applied to the microscale and mesoscale of concrete sequentially. Microscale homogenization is based on a 3D micro-CT scan of hardened cement paste (HCP). Mesoscale homogenization is carried out through the analysis of aggregates which are randomly distributed in a homogenized matrix. The thermal conductivity of this matrix is delivered by the homogenization of HCP, thereby establishing the link between micro-mesoscale of concrete. This link is critical to capture the dependence of the overall conductivity of concrete on the internal relative humidity. Therefore, special emphasis is given to the effect of relative humidity changes in micropores on the thermal conductivity of HCP and concrete. Each step of homogenization is compared with available experimental data. © 2012 Elsevier Ltd. All rights reserved.
Open Access
Development of force fields for novel 2D materials for temperature dependent vibrational properties
(2019-09) Mobaraki, Arash
A new era of nanodevice engineering has been started after fabricating graphene. This motivated vast number of researches for predicting, fabricating and utilizing 2D materials. Temperature dependent properties are essential for device applications. Although rigorous density functional theory based approaches are able to predict electronic and mechanical properties accurately, but they are mostly limited to zero temperature and ab initio based molecular dynamics are computationally very demanding. Classical molecular dynamics is a very powerful alternative, however its accuracy is basically depend on the interatomic potential used for describing the considered system and therefore constructing accurate force fields is always an open problem, especially for the emerging 2D materials with extra ordinary properties. Single-layer transition metal dichalcogenides (TMDs) are new class of 2D materials which are shown to be good candidates for thermoelectric applications, flexible electronic and optoelectronic devices. In order to investigate thermal properties of TMDs, Stillinger-Weber type potentials are developed using particle swarm optimization method. These potentials are validated by comparing the resulted phonon dispersion curves and thermal conductivities with available first principle and experimental results. In addition, for understanding the anharmonic effects imposed by the generated force fields the trends of the shifts of the optical phonon frequencies at 􀀀 point with variation in the temperature are compared with available experimental data. In all cases, optimized potentials generate results which are in agreement with the target data. In the second step, spectral energy density method together with phonon mode decomposition is used for obtaining temperature dependent phonon frequencies and lifetimes in entire Brillouin zone. The contribution of each phonon branch in thermal conductivity is predicted utilizing the obtained phonon lifetimes and group velocities within the framework of relaxation time approximation. Eventually, with the aim of constructing transferable potentials for describing 2D and bulk structures, a very fast and reliable optimization method is presented. Combining local and global optimization methods and utilizing the energy curves obtained from first principle method, novel Stillinger-Weber type potentials for graphene, silicene and group III nitrides are developed. The proposed approach provides a solid framework for parameter selection and investigating the role of each parameter in the resulted phonon dispersion curves.
Open Access
Energy based investigation of process parameters while drilling carbon fiber reinforced polymers
(Elsevier B.V., 2016) Karpat, Yiğit; Bahtiyar, O.
Carbon fiber reinforced polymers (CFRPs) are widely used in the aerospace industry due to their light weight, high strength, and low thermal conductivity. Drilling is a critical process that affects the quality of CFRP parts. This work studies the influence of process parameters on delamination and tool wear. Polycrystalline diamond helical drills are used in the experiments. It has been shown that drilling energy calculations can be used to set appropriate feed and speed parameters and for increasing drilling performance of CFRPs. The results also indicate the importance of thermal modeling of CFRP laminate for better understanding of the drilling process. © 2016 The Authors.
Open Access
A novel method for thermal conductivity measurement of two dimensional materials
(2019-09) Çakıroğlu, Onur
Thermal conductivity is a quantity which governs the heat transfer in a material. After increasing importance of efficiency in power generation systems and cooling mechanisms in micro-structures, many measurement methods have been developed to explore the thermal conductivity in micro and nano-sized materials. However, complexity in experimental setups, difficulties in the fabrication of devices required for measurements, and lacking exact solutions to thermal equations limit the usability of the methods to a class of materials. It is particularly challenging to study atomically thin metallic materials. To tackle this challenge, we have developed a new thermal conductivity measurement method based on the temperature dependent electrical resistance change and analyzed our method analytically and numerically by finite element method. We applied our method to 2H-TaS2 and found thermal conductivity as 9.55 1.27 W/m.K. Thermal conductivity value of TaS2, a metallic transition metal dichalcogenide was measured for the first time. This is supported by Wiedemann-Franz law and thermal conductivity of similar materials such as 2H-TaSe2 and 1T-TaS2. The method can be applied to semiconducting thin materials as well and is superior to other methods in various ways.
Open Access
Optimized stillinger-weber potentials for 1H, 1T and 1T′ phases of WS2 for molecular dynamics studies: thermal transport as an example
(2024-01) Waheed, Alim Mohamed
The advent of graphene has poured numerous amount of research effort into the study 2D materials and utilizing it for device fabrication. Monolayer Transition Metal Dichalcogenides are one such class of polymorphic material with high prospect in versatile device applications due to its unique properties exhibited across the various phases. Classical Molecular Dynamics is a powerful tool that can be utilized to study the thermal and mechanical properties of these phases. Considering this, we optimise Stillinger-Weber type Potential for the seperate 1H, 1T and 1T′ phases of WS2 using Particle Swarm Optimization. These potentials are validated by comparison of phonon dispersion curves, Density Functional Theory (DFT) based target characteristic data and through an accuracy assessment conducted using Non-Equilibrium Molecular Dynamic (NEMD) simulations to evaluate thermal conductivity of the polymorphic structures. Thermal conductivity results obtained for 1H and 1T′ are in good agreement with first principle predictions calculated using Boltzmann Transport Equation. NEMD simulation of 1T phase prove to be challenging due to its dynamic instability with incoherent buckle structure formation along the symmetric directions.
Open Access
Optoelectronic and thermal properties of metallic transition metal dichalcogenides
(2020-11) Mehmood, Naveed
After the successful isolation of graphene monolayer from its bulky counterpart, there has been tremendous advancement in the field of 2D material. Transition metal dichalcogenides(TMDCs) is family of 2D materials comprising of a transition metal atom sandwiched between two chalcogen atoms. Photoresponse of semiconducting TMDCs has been studied extensively in literature. However, photoresponse from metallic TMDCs is unprecedented and hence has not been studied to explore which mechanism might prevail. Among our findings, we discovered that photocurrent generation through metallic TMDCs is possible and has a photo-thermal origin. Using scanning photo-current microscopy, we were able to obtain spatial photocurrent maps for both, zero biased and biased samples. At zero applied bias, the photocurrent generation is localized to metal-metal junction and governed by Seebeck effect. At finite applied bias, photocurrent from the whole crystal is observed and is due to photobolometric effect. As Photo-bolometric effect relies on photo-thermally induced resistance change of the material, we extended our study to extract thermal conductivity of metallic TMDCs via bolometric effect. As contact of crystal with substrate act as a heat sink, we used suspended crystals over a hole to thermally isolate it from any heat sink. Resistance change via laser induced heating is experimentally measured at the center of the suspended part of crystal. Measured resistance change is matched with expected resistance change which is calculated using thermal conductivity(κ) as a fitting parameter via commercially available finite element method package(COMSOL). This way, thermal conductivity of the metallic TMDCs is calculated with very high accuracy and precision.
Open Access
Quantum effects of thermal conductance through atomic chains
(2001) Ozpineci, A.; Çıracı, Salim
We present a formalism for an atomic scale study of phononic heat transfer. The expression of thermal energy current can be cast in the Landauer form and incorporates the transmission coefficient explicitly. Calculation of the thermal conductance of a monoatomic chain of N atoms between two reservoirs shows interesting quantum features. The conductance density appears as Lorentzian type resonances at the eigenfrequencies of the chain. At low-temperature limit the discrete vibrational frequency spectrum of a "soft" chain may reflect on the thermal conductance by giving rise to a sudden increase. At room temperature, the conductance through a "stiff" chain may oscillate with the number of chain atoms. The obtained quantum features are compared with similar effects found in the quantized electrical conductance.
Open Access
Quantum transport through one-dimensional aluminum wires
(American Vacuum Society, 2002) Batra, I. P.; Sen, P.; Çıracı, Salim
Quantum conductance in narrow channels has been well understood by using the two-dimensional electron gas, a model system which has been realized in semiconductor heterojunctions. An essential property of this electron gas is its ability to support a constriction of width comparable to the Fermi wavelength, a property not shared by even thin metal films. The advent of scanning tunneling microscope has made possible the fabrication of metallic wires of atomic widths. We investigate one-dimensional wires consisting of aluminum atoms, to be specific. Using the first-principles density functional calculations, we obtain the optimal structures and report the bonding as deduced from the charge density analysis. With the calculated electronic structure in hand, we discussed the quantum ballistic transport using channel capacity arguments motivated by the Heisenberg’s uncertainty principle. By comparing our results with the detailed pioneering calculations by Lang, we inferred an average value for channel transmitivity and touched upon material specific contact resistance. Finally, the validity of the Wiedemann–Franz law in the quantum domain is established by studying thermal conductance in nanowires.
Open Access
Substrate and device pattern dependence of the thermal crosstalk in Y Ba2Cu 3O7−δ transition edge bolometer arrays
(Institute of Electrical and Electronics Engineers, 2006-12) Bozbey, A.; Fardmanesh, M.; Schubert, J.; Banzet, M.
Using YBa2CU3O7-delta (YBCO) thin films, pulsed laser deposited on 1-mm-thick LaAlO3 or SrTiO3 substrates, we made 4 x 1 pixel arrays of transition edge bolometers with separations between neighboring pixels ranging from 40 mu m to 170 mu m for testing purposes. We investigated the effects of the YBCO film thickness (200 and 400 nm), substrate material, and back-etching of the substrate, on the crosstalk between the pixels of the arrays. The investigation was based on the analysis of the voltage response of the de current biased bolometers versus the modulation frequency of a near-infrared laser source. We observed that the bolometer arrays made of 400-nm-thick films had less interpixel thermal crosstalk than the 200-nm-thick films. The effect of substrate thickness on the response of the pixels was investigated by up to 500 mu m back-etching of the substrates. The bolometers made on back-etched LaAlO3 substrates had anomalous crosstalk response behavior, which was effective at higher modulation frequencies. In addition, we present an analytical thermal model for explaining the observed effects of the thermal crosstalk on the response characteristics of the pixels of the arrays. We report the measured response and the anticipated thermal crosstalk of the characterized bolometers'. We describe the responses based on the thermal models and discrepancies from the model's predictions.
Open Access
Superconductivity transition dependence of the thermal crosstalk in YBa 2Cu 3O 7-x edge-transition bolometer arrays
(Institute of Electrical and Electronics Engineers, 2006) Bozbey, A.; Fardmanesh, M.; Schubert, J.; Banzet, M.
The effect of the superconductivity transition on the thermal crosstalk in YBa 2Cu 3O 7-x, edge-transition bolometer arrays is investigated for DC to midrange modulation frequency infrared radiation. The bolometers in the arrays were designed with various distances on SrTiO 3 (100) substrates. We have observed a change in the thermal crosstalk between neighbor devices through the superconductivity transition temperature range. Superconductivity transition dependence of the thermal coupling between the devices was measured by utilizing the thermal conductance measurement methods developed for the bulk material. The knee points in the magnitude of the response versus modulation frequency curves of the devices were also determined by illuminating one of the bolometers in the arrays and measuring the response of the neighbor devices. By using the knee frequency and the distance between the bolometers, the modulation frequency criterion for crosstalk-free response at the transition region in various array structures is found and the results of the thermal coupling measurements are further studied and presented here.
Open Access
Temperature-dependent phonon spectrum of transition metal dichalcogenides calculated from the spectral energy density: lattice thermal conductivity as an application
(American Physical Society, 2019) Mobaraki, Arash; Sevik, C.; Yapıcıoğlu, H.; Çakır, D.; Gülseren, Oğuz
Predicting the mechanical and thermal properties of quasi-two-dimensional (2D) transition metal dichalcogenides (TMDs) is an essential task necessary for their implementation in device applications. Although rigorous density-functional-theory–based calculations are able to predict mechanical and electronic properties, mostly they are limited to zero temperature. Classical molecular dynamics facilitates the investigation of temperature-dependent properties, but its performance highly depends on the potential used for defining interactions between the atoms. In this study, we calculated temperature-dependent phonon properties of single-layer TMDs, namely, MoS2, MoSe2, WS2, and WSe2, by utilizing Stillinger-Weber–type potentials with optimized sets of parameters with respect to the first-principles results. The phonon lifetimes and contribution of each phonon mode in thermal conductivities in these monolayer crystals are systematically investigated by means of the spectral-energy-density method based on molecular dynamics simulations. The obtained results from this approach are in good agreement with previously available results from the Green-Kubo method. Moreover, detailed analysis of lattice thermal conductivity, including temperature-dependent mode decomposition through the entire Brillouin zone, shed more light on the thermal properties of these 2D crystals. The LA and TA acoustic branches contribute most to the lattice thermal conductivity, while ZA mode contribution is less because of the quadratic dispersion around the Brillouin zone center, particularly in MoSe2 due to the phonon anharmonicity, evident from the redshift, especially in optical modes, by increasing temperature. For all the considered 2D crystals, the phonon lifetime values are compelled by transition metal atoms, whereas the group velocity spectrum is dictated by chalcogen atoms. Overall, the lattice thermal conductivity is linearly proportional with inverse temperature.
Open Access
Thermal conductivity and phase-change properties of boron nitride–lead oxide nanoparticle-doped polymer nanocomposites
(MDPI AG, 2023-05-16) Ortaç, Bülend; Mutlu, Saliha; Baskan, T.; Savaşkan Yılmaz, Sevil; Yilmaz, A. H.; Erol, B.
Thermally conductive phase-change materials (PCMs) were produced using the crosslinked Poly (Styrene-block-Ethylene Glycol Di Methyl Methacrylate) (PS-PEG DM) copolymer by employing boron nitride (BN)/lead oxide (PbO) nanoparticles. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) methods were used to research the phase transition temperatures, the phase-change enthalpies (melting enthalpy (ΔHm), and crystallization enthalpies (ΔHc)). The thermal conductivities (λ) of the PS-PEG/BN/PbO PCM nanocomposites were investigated. The λ value of PS-PEG/BN/PbO PCM nanocomposite containing BN 13 wt%, PbO 60.90 wt%, and PS-PEG 26.10 wt% was determined to be 18.874 W/(mK). The crystallization fraction (Fc) values of PS-PEG (1000), PS-PEG (1500), and PS-PEG (10,000) copolymers were 0.032, 0.034, and 0.063, respectively. XRD results of the PCM nanocomposites showed that the sharp diffraction peaks at 17.00 and 25.28 °C of the PS-PEG copolymer belonged to the PEG part. Since the PS-PEG/PbO and the PS-PEG/PbO/BN nanocomposites show remarkable thermal conductivity performance, they can be used as conductive polymer nanocomposites for effective heat dissipation in heat exchangers, power electronics, electric motors, generators, communication, and lighting equipment. At the same time, according to our results, PCM nanocomposites can be considered as heat storage materials in energy storage systems.
Open Access
Thermal conductivity measurements in nanosheets via bolometric effect
(IOP Publishing Ltd, 2020) Çakıroğlu, Onur; Mehmood, Naveed; Çiçek, Mert Miraç; Rasouli, Hamid Reza; Durgun, Engin; Kasırga, T. Serkan; Aikebaier, Aizimaiti
Thermal conductivity measurement techniques for materials with nanoscale dimensions require fabrication of very complicated devices or their applicability is limited to a class of materials. Discovery of new methods with high thermal sensitivity are required for the widespread use of thermal conductivity measurements in characterizing materials' properties. We propose and demonstrate a simple non-destructive method with superior thermal sensitivity to measure the in-plane thermal conductivity of nanosheets and nanowires using the bolometric effect. The method utilizes laser beam heating to create a temperature gradient, as small as a fraction of a Kelvin, over the suspended section of the nanomaterial with electrical contacts. Local temperature rise due to the laser irradiation alters the electrical resistance of the device, which can be measured precisely. This resistance change is then used to extract the temperature profile along the nanomaterial using thermal conductivity as a fitting parameter. We measured the thermal conductivity of V2O3 nanosheets to validate the applicability of the method and found an excellent agreement with the literature. Further, we measured the thermal conductivity of metallic 2H-TaS2 for the first time and performed ab initio calculations to support our measurements. Finally, we discussed the applicability of the method on semiconducting nanosheets and performed measurements on WS2 and MoS2 thin flakes.
Open Access
Validation of inter-atomic potential for WS2 and WSe2 crystals through assessment of thermal transport properties
(Elsevier, 2018) Mobaraki, Arash; Kandemir, A.; Yapıcıoğlu, H.; Gülseren, Oğuz; Sevik, C.
In recent years, transition metal dichalcogenides (TMDs) displaying astonishing properties are emerged as a new class of two-dimensional layered materials. The understanding and characterization of thermal transport in these materials are crucial for efficient engineering of 2D TMD materials for applications such as thermoelectric devices or overcoming general overheating issues. In this work, we obtain accurate Stillinger-Weber type empirical potential parameter sets for single-layer WS2 and WSe2 crystals by utilizing particle swarm optimization, a stochastic search algorithm. For both systems, our results are quite consistent with first-principles calculations in terms of bond distances, lattice parameters, elastic constants and vibrational properties. Using the generated potentials, we investigate the effect of temperature on phonon energies and phonon linewidth by employing spectral energy density analysis. We compare the calculated frequency shift with respect to temperature with corresponding experimental data, clearly demonstrating the accuracy of the generated inter-atomic potentials in this study. Also, we evaluate the lattice thermal conductivities of these materials by means of classical molecular dynamics simulations. The predicted thermal properties are in very good agreement with the ones calculated from first-principles.