Exploring Phase-Change Materials for Heat-Storage from First Principles
Exploring Phase-Change Materials for Heat-Storage from First Principles
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DissertationPrüfungsdatum
25.06.2026Datum der Veröffentlichung
26.06.2026Erstgutachter
Bredow, ThomasZweitgutachter
Grimme, StefanMetadaten
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Inhalt
The transition towards a carbon-neutral energy landscape necessitates the development of efficient thermal energy storage systems to bridge the temporal gap between renewable energy supply and thermal demand. The polymorphic ceramic trititanium pentoxide (Ti3O5) has emerged as a promising candidate for latent heat storage, capable of storing thermal energy in a metastable high-temperature phase indefinitely. Here, a comprehensive first-principles investigation of the Ti3O5 heat-storage system is presented, ranging from the electronic structure of the bulk material to the complex thermodynamic and kinetic behavior of doped systems, interfaces, surfaces and nanoparticles.
The initial part of this work establishes a robust theoretical framework for describing the open-shell transition metal oxide. It is demonstrated that the meta-GGA functional r2SCAN, augmented with the D3 dispersion correction, provides an accurate description of the structure of Ti3O5 polymorphs, superior to standard hybrid functionals. In the bulk, the thermodynamic ground state of the β-phase is identified as an antiferromagnetic semiconductor, while the metastable λ-phase is shown to be a ferromagnetic semiconductor. The transition state is characterized by a rotation of a central Ti-dimer, and predicted r2SCAN-D3 phase transition enthalpy and phase transition temperature are in good agreement with experiment.
Building on this foundation, the modulation of heat-storage properties via aliovalent cation doping (Sc, Al, Mg) is explored. The results reveal that doping lowers the phase transition temperature and enthalpy, primarily through local lattice distortions rather than direct electronic effects. Substitution turns the semiconducting bulk materials into metals, with significant electron density accumulation at the defect site. At low dopant concentration the β-Ti3O5 to λ-Ti3O5 barrier remains unchanged while the substitution stabilizes the λ-phases relative to β-Ti3O5. This provides a theoretical basis for tuning the operational temperature window of the material for specific waste-heat recovery applications.
A central challenge in the theoretical description of Ti3O5 has been the discrepancy between calculated and experimental pressures required to induce the phase transition. This thesis resolves this issue by moving beyond bulk models. First, the close degree of lattice matching between β- and λ-phases results in stable optimized phase interfaces along the (100), (010) and (001) grain boundaries between the phases. A more refined picture of the phase transition pathway is obtained by varying the β/λ ratio in large supercell models, revealing a distinct anisotropy in the phase transition pathway involving the (001) interface, which proceeds with a significantly lower barrier as compared to the other considered interfaces, confirming experimental trends. Hydrostatic pressure simulation on these mixed-phase models reveals pressure to destabilize high λ-phase fraction systems, however, these pressures of several GPa still overestimate experimental values by orders of magnitude. Second, the influence of particle size and morphology is quantified. By calculating surface free energies and applying the Wulff construction, it is shown that surface effects stabilize the λ-phase in particles smaller than 43 nm in diameter, providing a thermodynamic explanation for the experimentally observed thermal hysteresis and the persistence of the metastable phase at room temperature. The phase transition temperature is also shown to be influenced by particle size, with nanoparticles exhibiting diameters in the experimentally synthesized regime displaying phase transition temperatures in excellent agreement with experiment.
Finally, the atomistic mechanism of the pressure-induced phase transition is elucidated using machine-learned potentials driven on-the-fly probability enhanced sampling simulations. Simulated annealing simulations reveal a favorable surface reconstruction of the (001) λ-phase surface, which is then subject to repulsive harmonic potentials to model pressure effects. A comprehensive, universal and transferable framework for the translation of the slab compression to a pressure value is introduced. By modeling the uniaxial compression of nanoparticle surfaces rather than hydrostatic bulk compression, the predicted transition threshold of ≈700 bar is brought into better agreement with experimental values (≈600 bar). The mechanism is shown to involve a sequential, system size independent nucleation and a resulting layer-by-layer transformation, which is resolved in detail from direct molecular dynamics simulations under pressure at ambient temperatures.
Collectively, this work bridges the gap between quantum-chemical predictions and experimental observations in Ti3O5. It establishes a validated computational workflow for the discovery and optimization of phase-change materials, highlighting the critical importance of the correct choice of computational method, uncovering the fundamental effects of cation substitution in the modulation of material properties and the use of realistic models accounting for finite-size effects for accurate predictions of thermodynamic material properties.
The initial part of this work establishes a robust theoretical framework for describing the open-shell transition metal oxide. It is demonstrated that the meta-GGA functional r2SCAN, augmented with the D3 dispersion correction, provides an accurate description of the structure of Ti3O5 polymorphs, superior to standard hybrid functionals. In the bulk, the thermodynamic ground state of the β-phase is identified as an antiferromagnetic semiconductor, while the metastable λ-phase is shown to be a ferromagnetic semiconductor. The transition state is characterized by a rotation of a central Ti-dimer, and predicted r2SCAN-D3 phase transition enthalpy and phase transition temperature are in good agreement with experiment.
Building on this foundation, the modulation of heat-storage properties via aliovalent cation doping (Sc, Al, Mg) is explored. The results reveal that doping lowers the phase transition temperature and enthalpy, primarily through local lattice distortions rather than direct electronic effects. Substitution turns the semiconducting bulk materials into metals, with significant electron density accumulation at the defect site. At low dopant concentration the β-Ti3O5 to λ-Ti3O5 barrier remains unchanged while the substitution stabilizes the λ-phases relative to β-Ti3O5. This provides a theoretical basis for tuning the operational temperature window of the material for specific waste-heat recovery applications.
A central challenge in the theoretical description of Ti3O5 has been the discrepancy between calculated and experimental pressures required to induce the phase transition. This thesis resolves this issue by moving beyond bulk models. First, the close degree of lattice matching between β- and λ-phases results in stable optimized phase interfaces along the (100), (010) and (001) grain boundaries between the phases. A more refined picture of the phase transition pathway is obtained by varying the β/λ ratio in large supercell models, revealing a distinct anisotropy in the phase transition pathway involving the (001) interface, which proceeds with a significantly lower barrier as compared to the other considered interfaces, confirming experimental trends. Hydrostatic pressure simulation on these mixed-phase models reveals pressure to destabilize high λ-phase fraction systems, however, these pressures of several GPa still overestimate experimental values by orders of magnitude. Second, the influence of particle size and morphology is quantified. By calculating surface free energies and applying the Wulff construction, it is shown that surface effects stabilize the λ-phase in particles smaller than 43 nm in diameter, providing a thermodynamic explanation for the experimentally observed thermal hysteresis and the persistence of the metastable phase at room temperature. The phase transition temperature is also shown to be influenced by particle size, with nanoparticles exhibiting diameters in the experimentally synthesized regime displaying phase transition temperatures in excellent agreement with experiment.
Finally, the atomistic mechanism of the pressure-induced phase transition is elucidated using machine-learned potentials driven on-the-fly probability enhanced sampling simulations. Simulated annealing simulations reveal a favorable surface reconstruction of the (001) λ-phase surface, which is then subject to repulsive harmonic potentials to model pressure effects. A comprehensive, universal and transferable framework for the translation of the slab compression to a pressure value is introduced. By modeling the uniaxial compression of nanoparticle surfaces rather than hydrostatic bulk compression, the predicted transition threshold of ≈700 bar is brought into better agreement with experimental values (≈600 bar). The mechanism is shown to involve a sequential, system size independent nucleation and a resulting layer-by-layer transformation, which is resolved in detail from direct molecular dynamics simulations under pressure at ambient temperatures.
Collectively, this work bridges the gap between quantum-chemical predictions and experimental observations in Ti3O5. It establishes a validated computational workflow for the discovery and optimization of phase-change materials, highlighting the critical importance of the correct choice of computational method, uncovering the fundamental effects of cation substitution in the modulation of material properties and the use of realistic models accounting for finite-size effects for accurate predictions of thermodynamic material properties.
Schlagwörter
Trititanium pentoxide, Phase-change materials, Thermal energy storage, Solid-state quantum chemistry, Density functional theory, Machine learning interatomic potentials, Cation doping, Nanoparticles
Klassifikation (DDC)
540 ChemieJütten, Stefan: Exploring Phase-Change Materials for Heat-Storage from First Principles. - Bonn, 2026. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-90645
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-90645
@phdthesis{handle:20.500.11811/14239,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-90645,
author = {{Stefan Jütten}},
title = {Exploring Phase-Change Materials for Heat-Storage from First Principles},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = jun,
note = {The transition towards a carbon-neutral energy landscape necessitates the development of efficient thermal energy storage systems to bridge the temporal gap between renewable energy supply and thermal demand. The polymorphic ceramic trititanium pentoxide (Ti3O5) has emerged as a promising candidate for latent heat storage, capable of storing thermal energy in a metastable high-temperature phase indefinitely. Here, a comprehensive first-principles investigation of the Ti3O5 heat-storage system is presented, ranging from the electronic structure of the bulk material to the complex thermodynamic and kinetic behavior of doped systems, interfaces, surfaces and nanoparticles.
The initial part of this work establishes a robust theoretical framework for describing the open-shell transition metal oxide. It is demonstrated that the meta-GGA functional r2SCAN, augmented with the D3 dispersion correction, provides an accurate description of the structure of Ti3O5 polymorphs, superior to standard hybrid functionals. In the bulk, the thermodynamic ground state of the β-phase is identified as an antiferromagnetic semiconductor, while the metastable λ-phase is shown to be a ferromagnetic semiconductor. The transition state is characterized by a rotation of a central Ti-dimer, and predicted r2SCAN-D3 phase transition enthalpy and phase transition temperature are in good agreement with experiment.
Building on this foundation, the modulation of heat-storage properties via aliovalent cation doping (Sc, Al, Mg) is explored. The results reveal that doping lowers the phase transition temperature and enthalpy, primarily through local lattice distortions rather than direct electronic effects. Substitution turns the semiconducting bulk materials into metals, with significant electron density accumulation at the defect site. At low dopant concentration the β-Ti3O5 to λ-Ti3O5 barrier remains unchanged while the substitution stabilizes the λ-phases relative to β-Ti3O5. This provides a theoretical basis for tuning the operational temperature window of the material for specific waste-heat recovery applications.
A central challenge in the theoretical description of Ti3O5 has been the discrepancy between calculated and experimental pressures required to induce the phase transition. This thesis resolves this issue by moving beyond bulk models. First, the close degree of lattice matching between β- and λ-phases results in stable optimized phase interfaces along the (100), (010) and (001) grain boundaries between the phases. A more refined picture of the phase transition pathway is obtained by varying the β/λ ratio in large supercell models, revealing a distinct anisotropy in the phase transition pathway involving the (001) interface, which proceeds with a significantly lower barrier as compared to the other considered interfaces, confirming experimental trends. Hydrostatic pressure simulation on these mixed-phase models reveals pressure to destabilize high λ-phase fraction systems, however, these pressures of several GPa still overestimate experimental values by orders of magnitude. Second, the influence of particle size and morphology is quantified. By calculating surface free energies and applying the Wulff construction, it is shown that surface effects stabilize the λ-phase in particles smaller than 43 nm in diameter, providing a thermodynamic explanation for the experimentally observed thermal hysteresis and the persistence of the metastable phase at room temperature. The phase transition temperature is also shown to be influenced by particle size, with nanoparticles exhibiting diameters in the experimentally synthesized regime displaying phase transition temperatures in excellent agreement with experiment.
Finally, the atomistic mechanism of the pressure-induced phase transition is elucidated using machine-learned potentials driven on-the-fly probability enhanced sampling simulations. Simulated annealing simulations reveal a favorable surface reconstruction of the (001) λ-phase surface, which is then subject to repulsive harmonic potentials to model pressure effects. A comprehensive, universal and transferable framework for the translation of the slab compression to a pressure value is introduced. By modeling the uniaxial compression of nanoparticle surfaces rather than hydrostatic bulk compression, the predicted transition threshold of ≈700 bar is brought into better agreement with experimental values (≈600 bar). The mechanism is shown to involve a sequential, system size independent nucleation and a resulting layer-by-layer transformation, which is resolved in detail from direct molecular dynamics simulations under pressure at ambient temperatures.
Collectively, this work bridges the gap between quantum-chemical predictions and experimental observations in Ti3O5. It establishes a validated computational workflow for the discovery and optimization of phase-change materials, highlighting the critical importance of the correct choice of computational method, uncovering the fundamental effects of cation substitution in the modulation of material properties and the use of realistic models accounting for finite-size effects for accurate predictions of thermodynamic material properties.},
url = {https://hdl.handle.net/20.500.11811/14239}
}
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-90645,
author = {{Stefan Jütten}},
title = {Exploring Phase-Change Materials for Heat-Storage from First Principles},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = jun,
note = {The transition towards a carbon-neutral energy landscape necessitates the development of efficient thermal energy storage systems to bridge the temporal gap between renewable energy supply and thermal demand. The polymorphic ceramic trititanium pentoxide (Ti3O5) has emerged as a promising candidate for latent heat storage, capable of storing thermal energy in a metastable high-temperature phase indefinitely. Here, a comprehensive first-principles investigation of the Ti3O5 heat-storage system is presented, ranging from the electronic structure of the bulk material to the complex thermodynamic and kinetic behavior of doped systems, interfaces, surfaces and nanoparticles.
The initial part of this work establishes a robust theoretical framework for describing the open-shell transition metal oxide. It is demonstrated that the meta-GGA functional r2SCAN, augmented with the D3 dispersion correction, provides an accurate description of the structure of Ti3O5 polymorphs, superior to standard hybrid functionals. In the bulk, the thermodynamic ground state of the β-phase is identified as an antiferromagnetic semiconductor, while the metastable λ-phase is shown to be a ferromagnetic semiconductor. The transition state is characterized by a rotation of a central Ti-dimer, and predicted r2SCAN-D3 phase transition enthalpy and phase transition temperature are in good agreement with experiment.
Building on this foundation, the modulation of heat-storage properties via aliovalent cation doping (Sc, Al, Mg) is explored. The results reveal that doping lowers the phase transition temperature and enthalpy, primarily through local lattice distortions rather than direct electronic effects. Substitution turns the semiconducting bulk materials into metals, with significant electron density accumulation at the defect site. At low dopant concentration the β-Ti3O5 to λ-Ti3O5 barrier remains unchanged while the substitution stabilizes the λ-phases relative to β-Ti3O5. This provides a theoretical basis for tuning the operational temperature window of the material for specific waste-heat recovery applications.
A central challenge in the theoretical description of Ti3O5 has been the discrepancy between calculated and experimental pressures required to induce the phase transition. This thesis resolves this issue by moving beyond bulk models. First, the close degree of lattice matching between β- and λ-phases results in stable optimized phase interfaces along the (100), (010) and (001) grain boundaries between the phases. A more refined picture of the phase transition pathway is obtained by varying the β/λ ratio in large supercell models, revealing a distinct anisotropy in the phase transition pathway involving the (001) interface, which proceeds with a significantly lower barrier as compared to the other considered interfaces, confirming experimental trends. Hydrostatic pressure simulation on these mixed-phase models reveals pressure to destabilize high λ-phase fraction systems, however, these pressures of several GPa still overestimate experimental values by orders of magnitude. Second, the influence of particle size and morphology is quantified. By calculating surface free energies and applying the Wulff construction, it is shown that surface effects stabilize the λ-phase in particles smaller than 43 nm in diameter, providing a thermodynamic explanation for the experimentally observed thermal hysteresis and the persistence of the metastable phase at room temperature. The phase transition temperature is also shown to be influenced by particle size, with nanoparticles exhibiting diameters in the experimentally synthesized regime displaying phase transition temperatures in excellent agreement with experiment.
Finally, the atomistic mechanism of the pressure-induced phase transition is elucidated using machine-learned potentials driven on-the-fly probability enhanced sampling simulations. Simulated annealing simulations reveal a favorable surface reconstruction of the (001) λ-phase surface, which is then subject to repulsive harmonic potentials to model pressure effects. A comprehensive, universal and transferable framework for the translation of the slab compression to a pressure value is introduced. By modeling the uniaxial compression of nanoparticle surfaces rather than hydrostatic bulk compression, the predicted transition threshold of ≈700 bar is brought into better agreement with experimental values (≈600 bar). The mechanism is shown to involve a sequential, system size independent nucleation and a resulting layer-by-layer transformation, which is resolved in detail from direct molecular dynamics simulations under pressure at ambient temperatures.
Collectively, this work bridges the gap between quantum-chemical predictions and experimental observations in Ti3O5. It establishes a validated computational workflow for the discovery and optimization of phase-change materials, highlighting the critical importance of the correct choice of computational method, uncovering the fundamental effects of cation substitution in the modulation of material properties and the use of realistic models accounting for finite-size effects for accurate predictions of thermodynamic material properties.},
url = {https://hdl.handle.net/20.500.11811/14239}
}
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