Theoretical modeling of molar volume and thermal expansion
Combination of the Calphad method and theoretical calculations provides new possibilities for the study of materials science. This work is a part of the efforts within the CCT project (Centre of Computational Thermodynamics) to combine these methods to facilitate modeling and to extend the thermodyn...
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ndltd-UPSALLA1-oai-DiVA.org-kth-2522013-01-08T13:08:44ZTheoretical modeling of molar volume and thermal expansionengLu, Xiao-GangKTH, Materialvetenskap2005Materials sciencefirst-principles calculationsab initioCalphadDebye-Grüneisen modelthermodynamic propertieselastic modulusvolumethermal expansivitypressureThermo-CalcVASPelementcarbidenitrideMaterialvetenskapMaterials scienceTeknisk materialvetenskapCombination of the Calphad method and theoretical calculations provides new possibilities for the study of materials science. This work is a part of the efforts within the CCT project (Centre of Computational Thermodynamics) to combine these methods to facilitate modeling and to extend the thermodynamic databases with critically assessed volume data. In this work, the theoretical calculations refer to first-principles and Debye-Grüneisen calculations. The first-principles (i.e. ab initio) electronic structure calculations, based on the Density- Functional Theory (DFT), are capable of predicting various physical properties at 0 K, such as formation energy, volume and bulk modulus. The ab initio simulation software, VASP, was used to calculate the binding curves (i.e. equation of state at 0 K) of metallic elements, cubic carbides and nitrides. From the binding curves, the equilibrium volumes at 0 K were calculated for several metastable structures as well as stable structures. The vibrational contribution to the free energy was calculated using the Debye-Grüneisen model combined with first-principles calculations. Two different approximations for the Grüneisen parameter, γ, were used in the Debye-Grüneisen model, i.e. Slater’s and Dugdale-MacDonald’s expressions. The thermal electronic contribution was evaluated from the calculated electronic density of states. The calculated thermal expansivities for metallic elements, cubic carbides and nitrides were compared with Calphad assessments. It was found that the experimental data are within the limits of the calculations using the two approximations for γ. By fitting experimental heat capacity and thermal expansivity around Debye temperatures, we obtained optimal Poisson’s ratio values and used them to evaluate Young’s and Shear moduli. In order to reach a reasonable agreement with the experiments, it is necessary to use the logarithmic averaged mass of the constitutional atoms. The agreements between the calculations and experiments are generally better for bulk modulus and Young’s modulus than that for shear modulus. A new model describing thermodynamic properties at high pressures was implemented in Thermo-Calc. The model is based on an empirical relation between volume and isothermal bulk modulus. Pure Fe and solid MgO were assessed using this model. Solution phases will be considered in a future work to check the model for compositional dependence. Doctoral thesis, comprehensive summaryinfo:eu-repo/semantics/doctoralThesistexthttp://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-252urn:isbn:91-7178-086-0application/pdfinfo:eu-repo/semantics/openAccess |
collection |
NDLTD |
language |
English |
format |
Doctoral Thesis |
sources |
NDLTD |
topic |
Materials science first-principles calculations ab initio Calphad Debye-Grüneisen model thermodynamic properties elastic modulus volume thermal expansivity pressure Thermo-Calc VASP element carbide nitride Materialvetenskap Materials science Teknisk materialvetenskap |
spellingShingle |
Materials science first-principles calculations ab initio Calphad Debye-Grüneisen model thermodynamic properties elastic modulus volume thermal expansivity pressure Thermo-Calc VASP element carbide nitride Materialvetenskap Materials science Teknisk materialvetenskap Lu, Xiao-Gang Theoretical modeling of molar volume and thermal expansion |
description |
Combination of the Calphad method and theoretical calculations provides new possibilities for the study of materials science. This work is a part of the efforts within the CCT project (Centre of Computational Thermodynamics) to combine these methods to facilitate modeling and to extend the thermodynamic databases with critically assessed volume data. In this work, the theoretical calculations refer to first-principles and Debye-Grüneisen calculations. The first-principles (i.e. ab initio) electronic structure calculations, based on the Density- Functional Theory (DFT), are capable of predicting various physical properties at 0 K, such as formation energy, volume and bulk modulus. The ab initio simulation software, VASP, was used to calculate the binding curves (i.e. equation of state at 0 K) of metallic elements, cubic carbides and nitrides. From the binding curves, the equilibrium volumes at 0 K were calculated for several metastable structures as well as stable structures. The vibrational contribution to the free energy was calculated using the Debye-Grüneisen model combined with first-principles calculations. Two different approximations for the Grüneisen parameter, γ, were used in the Debye-Grüneisen model, i.e. Slater’s and Dugdale-MacDonald’s expressions. The thermal electronic contribution was evaluated from the calculated electronic density of states. The calculated thermal expansivities for metallic elements, cubic carbides and nitrides were compared with Calphad assessments. It was found that the experimental data are within the limits of the calculations using the two approximations for γ. By fitting experimental heat capacity and thermal expansivity around Debye temperatures, we obtained optimal Poisson’s ratio values and used them to evaluate Young’s and Shear moduli. In order to reach a reasonable agreement with the experiments, it is necessary to use the logarithmic averaged mass of the constitutional atoms. The agreements between the calculations and experiments are generally better for bulk modulus and Young’s modulus than that for shear modulus. A new model describing thermodynamic properties at high pressures was implemented in Thermo-Calc. The model is based on an empirical relation between volume and isothermal bulk modulus. Pure Fe and solid MgO were assessed using this model. Solution phases will be considered in a future work to check the model for compositional dependence. |
author |
Lu, Xiao-Gang |
author_facet |
Lu, Xiao-Gang |
author_sort |
Lu, Xiao-Gang |
title |
Theoretical modeling of molar volume and thermal expansion |
title_short |
Theoretical modeling of molar volume and thermal expansion |
title_full |
Theoretical modeling of molar volume and thermal expansion |
title_fullStr |
Theoretical modeling of molar volume and thermal expansion |
title_full_unstemmed |
Theoretical modeling of molar volume and thermal expansion |
title_sort |
theoretical modeling of molar volume and thermal expansion |
publisher |
KTH, Materialvetenskap |
publishDate |
2005 |
url |
http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-252 http://nbn-resolving.de/urn:isbn:91-7178-086-0 |
work_keys_str_mv |
AT luxiaogang theoreticalmodelingofmolarvolumeandthermalexpansion |
_version_ |
1716510213788401664 |