New technique for microwave characterization of material under compressive stress

Abstract

To investigate the high kquency properties of ferrite materials under stress, a new reflection I transmission cell is designed. An uniaxial compressive stress is applied along an hollow cylinder type sample inserted in a modified coaxial cell. This setup allows measurement of permittivity and permeability under stress (values up to 30 hPa) Born IS0 MHz up to IO GHz Specific calculations are used to take into accMmt the modifications in the l i e geometry, Accuracy of the measurement method is then evaluated. IntrodnctionFerrite materials are commonly used as high iiequency magnetic m a t d . The performance of the device is then directly linked to the lasses endlor real permeability at the working hqumcy. But like otha parameters (temperature, static magnetic field,...), stress may modi& the high p@es of the mater* depending on the magnetostridion coefficients. This may significantly affect the performances ofthe material.If the stress effecs on the magnetic response of mataiak have beem widely studied at low frequmcies [I], only a few srrups have been designed to expsimentally measme the high ftequency effects 121. We are desuibiig in this papa a new coaxial cell designed to measure the high fteqency response of farite matesisls unda stress. unlike previous wwk, this setup allows refledion I hansnission measurements, that is both permittivity and permeability under stress determination. Compressive stress is applied along the axis of the hollow cyliida sample. Thm properties are measured perpendicularly to the stress application direction due to the fields in a ooaxial l i e ('EM mode). Experimental W~U D and methodThe measurement setup is based on a coaxial refledion / transmission cell [3]. As &own figure 1, the sample may be inserted betwgn two alumina layers (of known permittivity). Measurement of both SI 1 and S21 is performed using a network a n -. The of& introduced by the alumina layers on both sides of the sample (symmetrical &up) is taken into a m t in the calculations. Permeability and 0-7803-7242-5102/$10.00 82002 IEEE pmittivity of the sample are then obtained (using classical well validated calculations) 141. To create a known state of stress (compressive, uniaxial, controllable level), a new cell is designed : a mechanical &p (see figure 2) lets the alumina layers apply pressure on the parallel sides of the sample while insuriig continuity in the coaxial line structure. This avoids the creation of higher moderThe applied pressure is monitored by a force sensor. The design takes advantage of the high rigidity (my small defamation unda sires) of the ferrite materials to insure good electrical continuity in the cell.As shown figure 2, the external radius of the sample is reduced : lower force is to be applied to obtain the desired level of stress. In that case, standard measurement procedure @T, TEM calculations) gives the dfedive properties of the medium b&weem the alumina layers (air + ferrite). Using static like calculations, the intrinsic properties of the material sample are deduced [5]. 0 sarrp...

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