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Title: Structural and chemical studies in the niobium oxides and related compounds
Author: Forghany, S. Kamal E.
ISNI:       0000 0001 3474 7223
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1978
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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 Nb2O5 systems. Single crystals of B- and P-Nb2O5. 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 CO2/CO or CO2/H2 gas mixtures. Structural studies of the oxides involved X-ray diffraction, electron diffraction and high resolution transmission electron microscopy (HRTEM). Results of phase equilibria studies in the NbO2 - (B-Nb2O5) system at 750°C 2O5, the existence of which was contradicted by HRTEM results. A new phase Nb43O107 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-Nb2O5 transformed directly into NbO2 on reduction. In the system NbO2 - (P-Nb2O5) at isotherm 750°C and controlled oxygen potentials, Nb12O29 existed as the only stable intermediate phase. Phase equilibria in the system (Ti-Nb-O) were determined by equilibrating TiNb2O7, Ti2Nb10O29 and TiNb24O62 at 1400°C. The oxide-gas equilibrium was attained rapidly (< 1 hr). The (p, x) isotherms showed complete bivariance in the systems (Ti,Nb)O2 - TiNb2O7, (Ti,Nb)O2 - Ti2Nb10O29 and (Ti,Nb)O2 - TiNb24O62. 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: TiNb2O7 → metastable transient Me25O60 → Ti2Nb10O29 was also detected. Associated with the reduction of Ti-Nb oxides is the generation of a rutile phase TixNb1-xO2, the composition of which is dependent upon the oxygen potential. The results are summarised in a ternary phase diagram.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available