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Title: Structural and microwave dielectric properties of ceramics of Ca(1-x)Nd2x/3TiOs
Author: Lowndes, Robert
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2012
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Abstract:
Ca(1-x)Nd2x/3TiO3 and MgTiO3-Ca0.61Nd0.26TiO3 composite ceramics were prepared by the mixed oxide route and characterised in terms of their structure, microstructure and properties. Ceramics sintered at 1450-1500oC achieved better than 95% of the theoretical density. X-Ray diffraction (XRD) revealed that Ca(1-x)Nd2x/3TiO3 ceramics were single phase for all compositions. For x ≤ 0.39 the structure was Pbnm with lattice parameters of a = b = √2ac and c = 2ac and a tilt system of a-a-c+. Compositions with x ≥ 0.48 could be better described by a C2/m structure with lattice parameters of a = b = c = 2ac. Scanning electron microscopy (SEM) revealed that the ceramics had grain sizes in the 5-70 μm range with abnormal grain growth for Nd3+ rich compositions. Images revealed that the twin domains in CaTiO3 were needle shaped and on addition of Nd3+ the domain morphology becomes more complex. The needle domain morphology returns for Ca0.43Nd0.38TiO3. High resolution electron microscopy (HAADF-STEM and electron diffraction) was used to probe cation-vacancy ordering (CVO) in the lattice. It was found that there was no CVO for x < 0.48 whilst at x = 0.48 there was evidence of a transition to a short range CVO. A transition to long range ordering is almost complete for the Ca0.1Nd0.6TiO3. The structural characteristics of Ca(1-x)Nd2x/3TiO3 ceramics as a function of temperature were investigated using in-situ XRD and Raman spectroscopy. All compositions were found to have the same structure across the entire temperature range. The Raman spectroscopy as a function of temperature indicated a possible transition with similar characteristics to a Curie temperature in a ferroelectric ceramic. The transition temperature was dependent on the cation ordering with the ceramics with greatest degree of disorder having the lowest transition temperature. The microwave dielectric properties of the samples were measured by a cavity resonance method in the 2-4GHz range. The relative permittivity (εr) was found to decrease from 180 for CaTiO3 to approximately 80 for Ca0.1Nd0.6TiO3 with an exponential dependence between the composition and the property. The temperature coefficient of resonant frequency (τf) ranged from +770ppmK-1 for CaTiO3 to +200ppmK-1 for Ca0.1Nd0.6TiO3. The Q x f for CaTiO3 was found to be 6000GHz and this increased to a maximum of 13000GHz for Ca0.7Nd0.2TiO3. After the Ca0.7Nd0.2TiO3 composition, the Q x f decreased to approximately 1100GHz for Ca0.1Nd0.6TiO3. The εr and τf were found to be mainly dependent on the composition of the ceramics whilst the Q x f value was more complex being dependent on the width of the twin domains in the grains. CaTiO3 samples fabricated by spark plasma sintering at 1150oC and above achieved better than 95% of the theoretical density. XRD revealed only a single phase with an orthorhombic Pbnm structure at room temperature and a tilt system of a-a-c+. SEM confirmed that the samples were single phase with grain size between 500nm-5μm. Transmission electron microscopy (TEM) of specimens sintered at 1150oC showed evidence of both (011) and (112) type domains. The τf of the ceramics was shown to be dependent on the volume of the unit cell, in agreement with the Bosman-Havinga equations. The ceramic sintered at 1150oC showed improvement in the Q x f value compared to samples prepared by conventional sintering. The structure, microstructure and properties of composite ceramics based on the MgTiO3-Ca0.61Nd0.26TiO3 system were investigated. Optimum properties were achieved at a composition of 0.8MgTiO3-0.2Ca0.61Nd0.26TiO3 with τf = -0.1ppmK-1, Q x f of 39000GHz and εr of 25.4. XRD revealed the presence of 3 phases including Ca0.61Nd0.26TiO3, MgTiO3 and MgTi2O5. The grain size of the ceramics was typically 5μm. The Q x f value was sensitive to the cooling rate and these changes could be related to changes in the vibrational properties of the lattice through changes in the lattice parameters.
Supervisor: Freer, Robert Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.558053  DOI: Not available
Keywords: perovskite ; microwave dielectric ceramics ; twin domains
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