Definition and measurement of the normalized electrical impedance of lossy piezoelectric resonators for ultrasonic transducers

Relevant equivalent circuit parameters and values of material constants of a piezoelectric resonator can be determined from measurements of its electrical input impedance as a function of frequency. The complex electrical impedance curves and the associated critical frequencies are the basis of this characterization by the piezoelectric resonance method. In this paper, the previously introduced concept of normalized electrical impedance of the lossy resonator, extended to include piezoelectric losses, is applied to the analysis of the effects of different types of intrinsic losses on peak values, bandwidths and characteristic frequencies. The resulting impedance patterns depend solely on the electromechanical coupling coefficient and the loss tangents, providing a useful tool for the analysis of low-Q resonators. The normalized impedance is experimentally evaluated from the basic data provided by an HP 4194A impedance analyser by means of specifically developed ASP programs.     

Shear-Induced Restructuring of Colloidal Silica Gels

Continuous oscillatory experiments were used to investigate the viscoelastic properties and structural evolution of silica gels obtained from unsheared and presheared sols, at pH 8. The processes controlling structural evolution before, at and after the gel point are discussed. Although the viscoelastic properties of the aged gels are comparable (implying similar structures on length scales of several microns), their chemical properties and microstructures (<1 m) are significantly different. This is directly related to the different aggregate structures formed in the unsheared and presheared systems (50 and 700 nm, respectively) and the associated concentration of surface hydroxyls available for cross-linking and formation of stable siloxane bonds.     

A mathematical model of aging-related and cortisol induced hippocampal dysfunction

Background  

The hippocampus is essential for declarative memory synthesis and is a core pathological substrate for Alzheimer's disease (AD), the most common aging-related dementing disease. Acute increases in plasma cortisol are associated with transient hippocampal inhibition and retrograde amnesia, while chronic cortisol elevation is associated with hippocampal atrophy. Thus, cortisol levels could be monitored and managed in older people, to decrease their risk of AD type hippocampal dysfunction. We generated an in silicomodel of the chronic effects of elevated plasma cortisol on hippocampal activity and atrophy, using the systems biology mark-up language (SBML). We further challenged the model with biologically based interventions to ascertain if cortisol associated hippocampal dysfunction could be abrogated.     

Formation and Gelation of Titania Nanoparticles from AOT Reverse Micelles

Titania nanoparticles have been produced by the controlled hydrolysis of tetraisopropyltitanate (TPT) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Particle formation and aggregation were investigated by static and dynamic light scattering and the chemical species by vibrational spectroscopy. The kinetics of particle formation and aggregation were controlled by varying [H₂O]/[AOT] (w ₀), [H₂O]/[Ti(IV)] and [AOT]/[Ti(IV)]. Nanoparticles, with diameters<10 nm, could be produced at relatively high Ti(IV) concentrations (up to 0.05 M). These nanoparticles aggregated into sols, with colloid sizes of 20 to 200 nm, eventually forming gelatinous precipitates. Different titania phases were produced, depending on the size of the micellar water pool; small pools (w ₀<6) yielded amorphous particles, while larger pools (w ₀>10) produced anatase.