Structural and chemical studies in the niobium oxides and related compounds
<p>The chemistry and thermodynamics of the Nb-O system and the related Ti-Nb-O system were studied to elucidate: 1) Kinetic characteristics of block structure oxides at T>1300°C , 2) Thermo dynamic properties of block structure oxides at T>1300°C and 3) Phase equilibria in the me...
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| Format: | Thesis |
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1978
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| _version_ | 1817932282998030336 |
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| author | Forghani, S |
| author_facet | Forghani, S |
| author_sort | Forghani, S |
| collection | OXFORD |
| description | <p>The chemistry and thermodynamics of the Nb-O system and the related Ti-Nb-O system were studied to elucidate: 1) Kinetic characteristics of block structure oxides at T>1300°C , 2) Thermo dynamic properties of block structure oxides at T>1300°C and 3) Phase equilibria in the metastable Nb<sub>2</sub>O<sub>5</sub> systems.</p> <p>Single crystals of B- and P-Nb<sub>2</sub>O<sub>5</sub>. were prepared by the Chemical Vapour Transport Technique Tinder predominantly convection controlled conditions. Phase equilibria experiments were carried out in a Cahn microbalance system and a newly designed Horizontal Quench Furnace line. Controlled oxygen potentials were generated from CO<sub>2</sub>/CO or CO<sub>2</sub>/H<sub>2</sub> gas mixtures. Structural studies of the oxides involved X-ray diffraction, electron diffraction and high resolution transmission electron microscopy (HRTEM).</p> <p>Results of phase equilibria studies in the NbO<sub>2</sub> - (B-Nb<sub>2</sub>O<sub>5</sub>) system at 750°C <T<900°C indicated that the transformation of B- to the block structure phase takes place readily on the area of contact with the Pt. container, probably catalyzed by the platinum surface. The transformed oxides contained both disordered and ordered fragments. In diffraction studies, the disordered structures simulated the tetragonal symmetry of M-Nb<sub>2</sub>O<sub>5</sub>, the existence of which was contradicted by HRTEM results. A new phase Nb<sub>43</sub>O<sub>107</sub> having monoclinic lattice parameters of a = 20.3 Å, b = 3.8 Å, c = 61.5 Å and β = 117° was detected in the ordered crystals. Its structural relationships with the neighbouring phases in the Nb-O system is discussed. At 700°C and controlled oxygen potentials, B-Nb<sub>2</sub>O<sub>5</sub> transformed directly into NbO<sub>2</sub> on reduction. In the system NbO<sub>2</sub> - (P-Nb<sub>2</sub>O<sub>5</sub>) at isotherm 750°C and controlled oxygen potentials, Nb<sub>12</sub>O<sub>29</sub> existed as the only stable intermediate phase.</p> <p>Phase equilibria in the system (Ti-Nb-O) were determined by equilibrating TiNb<sub>2</sub>O<sub>7</sub>, Ti<sub>2</sub>Nb<sub>10</sub>O<sub>29</sub> and TiNb<sub>24</sub>O<sub>62</sub> at 1400°C. The oxide-gas equilibrium was attained rapidly (< 1 hr). The (p, x) isotherms showed complete bivariance in the systems (Ti,Nb)O<sub>2</sub> - TiNb<sub>2</sub>O<sub>7</sub>, (Ti,Nb)O<sub>2</sub> - Ti<sub>2</sub>Nb<sub>10</sub>O<sub>29</sub> and (Ti,Nb)O<sub>2</sub> - TiNb<sub>24</sub>O<sub>62</sub>. HRTEM results showed that the bivariance was due to coherent intergrowth of neighbouring phases in the host lattice, suggesting that Wadsley defects were thermodynamically stable. An abrupt process: TiNb<sub>2</sub>O<sub>7</sub> → metastable transient Me<sub>25</sub>O<sub>60</sub> → Ti<sub>2</sub>Nb<sub>10</sub>O<sub>29</sub> was also detected. Associated with the reduction of Ti-Nb oxides is the generation of a rutile phase Ti<sub>x</sub>Nb<sub>1-x</sub>O<sub>2</sub>, the composition of which is dependent upon the oxygen potential. The results are summarised in a ternary phase diagram.</p> |
| first_indexed | 2024-03-06T19:55:51Z |
| format | Thesis |
| id | oxford-uuid:258f5e0f-7b3f-4180-8657-08cb19bb82b3 |
| institution | University of Oxford |
| last_indexed | 2024-12-09T03:35:27Z |
| publishDate | 1978 |
| record_format | dspace |
| spelling | oxford-uuid:258f5e0f-7b3f-4180-8657-08cb19bb82b32024-12-01T19:01:08ZStructural and chemical studies in the niobium oxides and related compoundsThesishttp://purl.org/coar/resource_type/c_db06uuid:258f5e0f-7b3f-4180-8657-08cb19bb82b3Polonsky Theses Digitisation Project1978Forghani, S<p>The chemistry and thermodynamics of the Nb-O system and the related Ti-Nb-O system were studied to elucidate: 1) Kinetic characteristics of block structure oxides at T>1300°C , 2) Thermo dynamic properties of block structure oxides at T>1300°C and 3) Phase equilibria in the metastable Nb<sub>2</sub>O<sub>5</sub> systems.</p> <p>Single crystals of B- and P-Nb<sub>2</sub>O<sub>5</sub>. were prepared by the Chemical Vapour Transport Technique Tinder predominantly convection controlled conditions. Phase equilibria experiments were carried out in a Cahn microbalance system and a newly designed Horizontal Quench Furnace line. Controlled oxygen potentials were generated from CO<sub>2</sub>/CO or CO<sub>2</sub>/H<sub>2</sub> gas mixtures. Structural studies of the oxides involved X-ray diffraction, electron diffraction and high resolution transmission electron microscopy (HRTEM).</p> <p>Results of phase equilibria studies in the NbO<sub>2</sub> - (B-Nb<sub>2</sub>O<sub>5</sub>) system at 750°C <T<900°C indicated that the transformation of B- to the block structure phase takes place readily on the area of contact with the Pt. container, probably catalyzed by the platinum surface. The transformed oxides contained both disordered and ordered fragments. In diffraction studies, the disordered structures simulated the tetragonal symmetry of M-Nb<sub>2</sub>O<sub>5</sub>, the existence of which was contradicted by HRTEM results. A new phase Nb<sub>43</sub>O<sub>107</sub> having monoclinic lattice parameters of a = 20.3 Å, b = 3.8 Å, c = 61.5 Å and β = 117° was detected in the ordered crystals. Its structural relationships with the neighbouring phases in the Nb-O system is discussed. At 700°C and controlled oxygen potentials, B-Nb<sub>2</sub>O<sub>5</sub> transformed directly into NbO<sub>2</sub> on reduction. In the system NbO<sub>2</sub> - (P-Nb<sub>2</sub>O<sub>5</sub>) at isotherm 750°C and controlled oxygen potentials, Nb<sub>12</sub>O<sub>29</sub> existed as the only stable intermediate phase.</p> <p>Phase equilibria in the system (Ti-Nb-O) were determined by equilibrating TiNb<sub>2</sub>O<sub>7</sub>, Ti<sub>2</sub>Nb<sub>10</sub>O<sub>29</sub> and TiNb<sub>24</sub>O<sub>62</sub> at 1400°C. The oxide-gas equilibrium was attained rapidly (< 1 hr). The (p, x) isotherms showed complete bivariance in the systems (Ti,Nb)O<sub>2</sub> - TiNb<sub>2</sub>O<sub>7</sub>, (Ti,Nb)O<sub>2</sub> - Ti<sub>2</sub>Nb<sub>10</sub>O<sub>29</sub> and (Ti,Nb)O<sub>2</sub> - TiNb<sub>24</sub>O<sub>62</sub>. HRTEM results showed that the bivariance was due to coherent intergrowth of neighbouring phases in the host lattice, suggesting that Wadsley defects were thermodynamically stable. An abrupt process: TiNb<sub>2</sub>O<sub>7</sub> → metastable transient Me<sub>25</sub>O<sub>60</sub> → Ti<sub>2</sub>Nb<sub>10</sub>O<sub>29</sub> was also detected. Associated with the reduction of Ti-Nb oxides is the generation of a rutile phase Ti<sub>x</sub>Nb<sub>1-x</sub>O<sub>2</sub>, the composition of which is dependent upon the oxygen potential. The results are summarised in a ternary phase diagram.</p> |
| spellingShingle | Forghani, S Structural and chemical studies in the niobium oxides and related compounds |
| title | Structural and chemical studies in the niobium oxides and related compounds |
| title_full | Structural and chemical studies in the niobium oxides and related compounds |
| title_fullStr | Structural and chemical studies in the niobium oxides and related compounds |
| title_full_unstemmed | Structural and chemical studies in the niobium oxides and related compounds |
| title_short | Structural and chemical studies in the niobium oxides and related compounds |
| title_sort | structural and chemical studies in the niobium oxides and related compounds |
| work_keys_str_mv | AT forghanis structuralandchemicalstudiesintheniobiumoxidesandrelatedcompounds |