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Size effect and dielectric properties of NH4H2PO4 - porous glass composites

Ciżman A., Komar J., Marciniszyn T., Poprawski R., Rysiakiewicz-Pasek E., Sieradzki A.

Materials Science Poland

2012

Vol. 30, No. 2

143--150

NH4H2PO4 nano-composite antiferroelectric materials in porous glass have been studied by means of dielectric and dilatometric investigations. Dielectric spectroscopy measurements in a wide frequency range are reported here for the first time, for both the antiferro- and paraelectric phases of ammonium dihydrogen phosphate (ADP) embedded in a porous matrix. Low frequency relaxation processes above the phase transition temperature were shown to occur. An investigation of the thermal expansion revealed a negative volume jump at the phase transition point. It was found that the phase transition temperature in ADP crystals embedded in porous glass decreased with the decrease of the mean pore size. The experimentally observed shift of the phase transition temperature is caused by a combination of size and pressure effects.

Hydrodynamic squeeze-film characteristic in porous annular disks using the Brinkman model

Lin J. R., Liao W. H.

International Journal of Applied Mechanics and Engineering

2003

Vol. 8, no 3

413-424

Based on the Brinkman model (BM) with the assumption that the pressure gradient across the porous region is an unknown function, the effects of viscous shear stresses upon the squeezing-film motion in porous annular disks are considered. Using the Brinkman equations and applying the continuity conditions at the interface for the velocities, shear stresses and pressures, two coupled modified Reynolds equations governing the squeeze-film pressure are obtained. The film pressure equation is solved and applied to evaluate the load-carrying capacity and the height-time relationship. According to the results obtained, the BM predicts quite a different squeezing action to those derived by the slip-flow model (SFM) and the Darcy model (DM). Comparing with the SFM, the viscous shear effects of the BM increase the load-carrying capacity and the response time. But, these trends are reversed as compared to the DM. On the whole, the effects of viscous shear stresses are more pronounced for moderate-value permeability parameters and a higher-value radius ratio. A design example for porous annular disks is also illustrated for engineering applications.