Anharmonic free energy calculation of dynamically unstable crystalstructures
Başlık çevirisi mevcut değil.
- Tez No: 840464
- Danışmanlar: PROF. DR. JOCHEN M. SCHNEİDER, PROF. DR. JÖRG NEUGEBAUER, DR. MORİTZ TO BABEN
- Tez Türü: Yüksek Lisans
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2021
- Dil: İngilizce
- Üniversite: Rheınısch-Westfalısche Technısche Hochschule Aachen
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
The Calphad method is a crucial tool for materials and industrial process design. Success of the method relies heavily on the quality of the materials data and preparation of the datasets requires a detailed and careful study. In this thesis, computational methods that calculate the finite temperature free energies of crystal structures are introduced and implemented on different phases of elements such as Ca, Ti, Zr, Al and Cu. The well known quasi-harmonic model cannot be applied to mechanically unstable phases such as BCC Al-Cu solution or BCC Ti because of the observed imaginary phonon frequencies. For such kind of phases, soft phonon modes are stabilized by selecting a value of finite displacement that is comparable to the desired temperature. Force constants obtained from such displacements are then used in the reference state of the Thermodynamic Integration (TI) method and anharmonic free energies are approximated. A detailed derivation of thermodynamic integration is provided in order to show the possible pitfalls to the reader. If the true form of the Gibbs free energy curve is known, heat capacity can be easily derived. However, small deviations in the free energy can lead to erroneous results. In addition to thermodynamic integration, alternative statistical models that estimate the pressure and volume dependent anharmonic heat capacities are presented. Unary datasets are the foundation of many multi-component databases and require a critical assessment. Among these, a special attention should be given to energetically unfavourable end-members due to the limited amount of experimental data available. In literature, there exist two distinct heat capacity measurements for pure BCC Calcium. Grabowski et al. (2011) have already shown that the SGTE (Scientific Group Thermodata Europe) unary dataset is not the best selection according to DFT calculations. In this study, this is verified by using empirical potentials. At elevated temperatures, BCC Al-Cu solution is stable at around ∼60-80 at.% Cu. In addition to the unary systems, it is also shown that for the Al-Cu system, the BCC solid solution phase is more stable than the FCC phase at elevated temperatures.
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