Browsing by Author "Meschia, Steven C. L."

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Conductivity across random barrier distribution as origin of large low-frequency dielectric peak in perovskite crystals and ceramics
(1996-10) Schmidt, V. Hugo; Tuthill, G. F.; Tu, Chi-Shun; Schogoleva, T.V.; Meschia, Steven C. L.
Several perovskite crystals and ceramics show very large dielectric (ϵ′) peaks at high temperature T and low frequency ƒ. In some cases these peaks are in the cubic phase far above any ferroelectric transition. Even at the peaks, the lossy part ϵ″ is larger than the real part ϵ′. The ϵ′ vs T curves for different ƒ follow the same d.c. (low-ƒ) envelope down to some T(ƒ) below which the curve for that ƒfalls below the envelope. Similarly, the conductivity (or ϵ″) data show d.c. and a.c. (high-frequency) envelopes for which data at different ƒ overlap. As a first approximation to a crystal with random barriers impeding conductivity, a model with barriers B (in T units) every lattice constant a = 4 Å and barriers B + Δ every distance d is assumed. The model is fit to permittivity and conductivity data for a strontium titanate single crystal, and a good qualitative fit is obtained.
Dielectric permittivity in paraelectric/ferroelectric coexistence region in several proton glasses
(1995) Howell, Francis L.; Fundaun, I. L.; Stadler, S.; Meschia, Steven C. L.; Tu, Chi-Shun; Schmidt, V. Hugo
We present results for the complex permittivities of several proton glasses of the form M/sub 1-x/(NZ/sub 4/)/sub x/Z/sub 2/AO/sub 4/, where M=Rb, Z=H or D, and A=As or P. All measurements were made perpendicular to the ferroelectric axis, so no effects of domain wall motion occurred. Phase coexistence was apparent in all species. The phosphate glasses exhibited a much narrower coexistence composition range than the arsenate glasses.
An electret accelerometer for use in active vibration control systems
(1996-08) Meschia, Steven C. L.; Schmidt, V. Hugo; Taubner, S
An electret accelerometer was designed which can easily be integrated with piezoelectric actuators in order to actively control sub-audio vibrations.
NMR spin-lattice relaxation study of Cs1‑x(NH4)xH2PO4
(1997) Meschia, Steven C. L.; Zidansek, Aleksander; Brandt, Dan; Schmidt, V. Hugo
Spin-lattice relaxation time T1 for 31P was measured as a function of temperature at 28.2 MHz in a powder sample of Cs1-x (NH4) x H2PO4 (CADP) crystallized from a solution with a molar ratio x = 0.2. This crystal has the same structure as CsH2PO4 (CDP), which exhibits a low-temperature pseudo-one-dimensional ferroelectric phase transition at 159 K and a superionic transition at 504 K on heating. The measured temperature dependence is explained in terms of hydrogen bond dynamics.
Random barrier height model for phase shifted conductivity in perovskites
(1997) Schmidt, V. Hugo; Tuthill, G. F.; Tu, Chi-Shun; Schogoleva, T.V.; Meschia, Steven C. L.
A large dielectric permittivity peak which occurs at low frequency and high temperature in many perovskite crystals and ceramics, and an associated difference in dc and ac conductivity, is attributed to phase shifted conductivity resulting from mobility barriers of different heights. A model is developed which has intrinsic barriers, and higher and more widely spaced extrinsic barriers, and special cases are examined. Model predictions show good agreement with experimental results of Stumpe, Wagner, and Bäuerle for an SrTiO3 single crystal 1.02 mm thick. The additional peaks seen by them at higher temperature for a crystal 0.24 mm thick are qualitatively accounted for by adding a third set of still higher barriers with spacing equal to the crystal thickness.