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Nanophase glass ceramics for capacitive energy storage [electronic resource] / by Badri Rangarajan

Author
Rangarajan, Badri, 1980-
Published
[University Park, Pa.] : Pennsylvania State University, 2009.
Physical Description
1 electronic document (182 pages)
Additional Creators
Lanagan, Michael Thomas, 1960- and Shrout, Thomas Richard, 1954-
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Graduate Program
Summary
Glass ceramics are candidate dielectric materials for high energy storage capacitors. Since energy density depends primarily on dielectric permittivity and breakdown strength, glass ceramics with interconnected nano-crystalline particles and low porosity, which leads to high breakdown strength, are expected to have high energy density values. Three glass ceramic systems were investigated. Barium/lead sodium niobate glass ceramics, designated as PNNS (PbO-Na₂O-Nb₂O₅-SiO₂) and BNNS (BaO-Na₂O-Nb₂O₅-SiO₂), and barium titanate silicate glass ceramic, designated as BTS (BaO-TiO₂-SiO₂), belonging to medium ([epsilon]r[subscript] ~ 400-700) and low ([epsilon]r[subscript] ~ 20) permittivity regimes, respectively, were fabricated by roller quenching and controlled crystallization. The overall properties of the glass ceramics were controlled by connectivity and volume fraction of crystallites. PNNS and BNNS developed perovskite and tungsten-bronze phases during crystallization with permittivity values between 400 and 700. Microstructural analysis of PNNS glass ceramic revealed grain sizes of the order of 50 nm. The calculated breakdown strengths were ~0.7 and ~075 MV/cm for PNNS and BNNS respectively. The resulting energy densities at breakdown were ~4.5 and ~6.5 J/cm³ for PNNS and BNNS respectively. However, the disadvantages, such as difficult glass formability, less control over crystallization due to multiphase formation and low dielectric breakdown strength values due to high dielectric contrast between the glass and crystal phases, associated with PNNS and BNNS glass ceramics served as the motivating factor for exploring BTS glass ceramic. The major advantage of studying BTS glass ceramic over the other systems is that a single crystalline phase, fresnoite (Ba₂TiSi₂O₈), grows from the quenched glass and properties can be explored over the whole spectrum ranging from fully amorphous to fully crystalline. Crystallization kinetics of the BTS glass is explored to control the relative volume fractions of amorphous and crystalline phases. The mechanism of crystallization was found to be 3-dimensional interfacial growth as indicated by Avrami parameter values. In addition to the two extremes of fully amorphous and fully crystalline samples, partially crystalline samples were also chosen for electrical property studies. BTS, being an alkali-free system, showed exceptional stability in dielectric properties with temperature until about 400⁰C. The crystalline phase has similar composition and dielectric permittivity as that of the glass phase. It is concluded from impedance results that the glass phase is the major contributor to the composite conductivity. The minimal dielectric contrast between glass ([epsilon]r[subscript] ~ 15) and crystal ([epsilon]r[subscript] ~ 18) phases resulted in comparatively higher breakdown strength values (~1.9 MV/cm) than PNNS and BNNS. The energy density of BTS glass ceramic was calculated to be ~2.5 J/cm³. High field properties were studied to estimate the energy densities of PNNS, BNNS and BTS glass ceramics from polarization-electric field loops and to compare and contrast the amorphous and crystalline BTS samples. Similar to low field impedance results, high field P-E loops indicated amorphous BTS to be more conductive when compared to crystalline BTS.
Other Subject(s)
Dissertation Note
Ph.D. Pennsylvania State University 2009.
Note
Mode of access: World Wide Web.
Thesis advisor: Michael T. Lanagan, Thomas R. Shrout.
Reproduction Note
Microfilm (positive). 1 reel ; 35 mm. (University Microfilms 35-24645)
Technical Details
The full text of the dissertation is available as a Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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