Wayne Huebner

Structure property relationships in core-shell BaTiO3-LiF ceramics

A sintering, microstructural development and dielectric property study of BaTiO 3 –LiF ceramics was performed to assess the potential application of low-fired multilayer capacitors. Not only does LiF allow for sintering below 1000 °C, it also allows for the manipulation of dielectric properties and interfaces within BaTiO 3 –LiF ceramics. Using mixing laws, a model of the dielectric properties of the core-shell microstructures is presented that agrees well with the observed experimental data.

YBa2Cu3O7−x Superconductor powder synthesis by spray pyrolysis of organic acid solutions

Abstract A process based on the spray pyrolysis of an organic acid solution has been developed for the synthesis of spherical YBa 2 Cu 3 O 7− x superconducting powders. Depending on the solution viscosity, which is controlled by the degree of precursor polymerization, ‘solid’ organic YBC precursor particles are formed by spray pyrolysis. During the decomposition stage of spray pyrolysis these organic YBC precursor particles are converted to microporous particles comprised of inorganic nanocrystallites. The desired orthorhombic, superconducting phase is obtained by subsequent calcination at 850°C for 2 h in 10 −2 atm O 2 , followed by annealing at 500°C for 4 h in flowing O 2 . A.C. magnetic susceptibility results on powders and four-point resistance results on sintered pellets clearly show the transition to the superconducting state, confirming the developed process yields powders suitable for the synthesis of superconducting materials and monoliths.

Dielectric and piezoelectric properties of lanthanum-modified lead magnesium niobium-lead titanate ceramics

Abstract Dielectric and piezoelectric properties of relaxor ferroelectrics in the solid solution, formulated (1−x)(Pb1–1.5yLay)(Mg1/3Nb2/3)—x PbTiO3, have been investigated. The dielectric properties show typical first order ferroelectric behavior for PT ≥ 35% and relaxor type below being indicative of the morphotropic phase boundary. Lanthanum doping resulted in reduction of grain size and shifting of the Tc (Curie Temperature) −25 °C/mole% downward in PMN-PT polycrystalline ceramics. The optimum value of piezoelectric constant, d 33, obtained was 560 pC/N for compositions near the morphotropic phase boundary. Based on this investigation, La doped PMN-PT compositions appear to be promising candidates, for fine grain size piezoelectrics, pyroelectric bolometers, and through hot pressing or hot isostatic pressing, for electro-optic applications.

Aging mechanisms in Pb(Mg1/3Nb2/3)O3-based relaxor ferroelectrics

Abstract The complex perovskite Pb(Mg1/3Nb2/3)O3 [PMN] is just one of many relaxor ferroelectncs having the general formula Pb(B1B2)O3. PMN, and solid solutions thereof with PbTiO3 [PT] and Pb(Zn1/3Nb2/3)O3 [PZN], as well as other relaxor compositions, are excellent materials for use in a wide range of applications including electrostrictive actuators,1 multilayer capacitors,2 and pyroelectric bolometers.3 For any of these applications it is imperative that dielectric aging be minimal. Past studies4–13, on aging in relaxor ferroelectrics (primarily PLZT and PMN) have shown the aging rate depends on many variables including processing parameters, microstructure, dopant content, point defects, etc. Though great strides have been made with respect to modelling dielectric properties of relaxors, the lack of detailed information concerning defect structure, conduction, and ordering still hinders theoretical interpretation of aging results.

Phase change materials for thermal stabilization of composite thermistors

The system chosen for study was pentaerythritol incorporated into a carbon black-polyethylene thermistor system. Pentaerythritol exhibits a first order tegragonal to cubic phase transition at 185 o C, with a 1.87 to 3.18 J/ o C•g change in specific heat and a 425 J/cm 3 heat of transition. Composites with room temperature resistivities as low as 0.1 Ω•m, a PTCR effect of up to six orders of magnitude, and reproducible temperature-cycling behavior were developed. The pentaerythritol introduced thermal delays up to 7 min at 185 o C

Investigating Phase Transitions in ABO₃ Perovskites Using Perturbed Angular Correlation Spectroscopy

Abstract Perturbed Angular Correlation (PAC) measurements were performed at several temperatures on BaTiO3, SrTiO3, BaHfO3 ceramic and PbTiO3 powder samples. In the PbTiO3 and BaTiO3 samples, both the tetragonal (ferroelectric) and the cubic (paraelectric) phases were investigated. For the tetragonal phase temperatures, the perturbation functions for PbTiO3 show strongly decreasing electric field gradients (efgs) and increasing linebroadening with increasing temperature. These two features are evidence for an order-disorder phase transition mechanism. Although the data for tetragonal BaTiO3 provide less information, linebroadening is evident, which is not inconsistent with an order-disorder mechanism. Above the Curie temperatures, both compounds show featureless perturbation functions that decay slowly with increasing time. In addition, this interaction shows little temperature sensitivity. Similarly, PAC measurements on cubic BaHfO3 and cubic SrTiO3 show this behavior. The literature reports similar phen...

Sol Gel Processing of Low Permittivity, Fiber-Reinforced SiO2-Polyimide Substrates

Low permittivity ceramic substrates with a sandwich structure consisting of a porous, fiber-reinforced SiO2 core and two thin polymer plates were fabricated by sol-gel processing, and polymer infiltration. The rheological behavior of an aqueous colloidal SiO2 sol, containing short SiO2 fibers was studied as a function of particle loading, fiber loading and gelation time. Short SiO2 fibers were introduced to limit drying shrinkage and thus minimize substrate cracking during drying. After the psuedoplastic sol was tape cast and sintered at 1150°C for 2 h, a polyimide solution was infiltrated into the porous SiO2 core. Permittivities ranging from 1.81 to 3.08 could be obtained by using 32–42% SiO2, 5–50% polyimide and 63 to 8% porosity. The substrate strength was increased from 1.93 MPa to 3.83 MPa after polyimide infiltration.