Design, production and testing of PMN-PT electrostrictive transducers.

Lead magnesium niobate ceramics (PMN) are promising materials for application in the field of high power transducers. The advantage of PMN materials are the large strains generated under moderate electric field and the low hysteresis. The electrostrictive effect is non-linear, the corresponding physical constants depend on temperature and frequency and a DC electrical bias is required. These difficulties must be considered at the design stage. A finite element model has been developed and validated in the ATILA code for non-linear static and time-domain analyses. These numerical modelings are used to design and test two Langevin-type electrostrictive transducers. The first transducer is made of PMN-PT-La (90-10-1%) ceramics (TRS Ceramics), the second one of ESCI ceramics (Morgan Matroc). For given static mechanical prestresses, resonance frequencies and effective coupling coefficients are measured at different DC electric fields and temperatures.

Characterization of multilayered piezoelectric ceramics for high power transducers.

In some circumstances, large vibrational displacements at ultrasonic frequency must be generated using a low voltage drive. This result cannot be obtained with monolithic PZT ceramics which require voltages larger than 1000 V to produce displacements of the micrometer order at resonance. The use of multilayered hard lead zirconate titanate ceramics as transduction material in resonant devices is experimentally investigated for Langevin-type transducers. Large amplitudes are obtained under low drive (5 microm under 10 V). Material constant (compliance, losses) variations under large dynamic stress are, at least, one order of magnitude larger than for monolithic ceramics. Depolarization is found to be a critical issue when the transducer is driven continuously. It is demonstrated that this problem can be solved by polishing the interfaces between different parts of the device and applying an electrical DC bias to the transducer.

[Treatment of progressive multiple sclerosis with cyclophosphamide]. / Traitement des formes progressives de sclérose en plaques par le cyclophosphamide.

Intravenous (IV) cyclophosphamide is currently used in secondary progressive (SP) and Primary progressive (PP) Multiple Sclerosis (MS) but its efficacy remains uncertain. Furthermore, it is necessary to determine which MS should be successfully treated with IV cyclophosphamide. We retrospectively investigated 111 consecutive patients with progressive MS (21 PPMS and 90 SPMS) treated in an open label fashion with IV cyclophosphamide. We analysed clinical data (gender, age, duration of progression, primary versus secondary MS). The treatment response was assessed by EDSS change after 6 months and 1 year of treatment. The annual relapse average decreased from 1.92 before treatment to 0.39 during the treatment. Age and gender did not influence response to therapy. We did not find any difference of response between PPMS and SPMS. Duration of the progressive phase in SPMS was not a predictive factor of efficacy. A better response was noted in SPMS patients with surimposed relapses than in patients without relapses during the year before treatment (p<0.05). Furthermore, the better response in SPMS patients with relapses before treatment suggests that it is necessary to treat when MS is still in an inflammatory stage.

Finite-element modeling of lead magnesium niobate electrostrictive materials: dynamic analysis.

A finite-element model is proposed for the time-domain analysis of electrostrictive materials. Hom's material model, developed for lead magnesium niobate (PMN) ceramics, is used. It includes the quadratic dependence of strain with polarization, the saturation of polarization, assumes constant temperature, and excludes hysteresis. The theoretical formulation is justified by the principle of virtual works. The numerical model is obtained after discretization in space and time. The validation is performed by comparing numerical results with semianalytical results for an electrostrictive spherical shell subjected to a step in voltage or in charge. From these results, a method to compute the coupling coefficient of electrostrictive materials, based on Ikeda's definition, is proposed and applied to a bar with parallel electric field.

[Hearing loss in multiple sclerosis: clinical, electrophysiologic and radiological study]. / Surdité et sclérose en plaques: étude clinique, électrophysiologique et radiologique.

Clinical presentation of multiple sclerosis (MS) is highly heterogeneous. Deafness is considered to be rare. The aim of this study was to determine the frequency of acute deafness in MS and to evaluate prognosis. The occurrence of acute deafness was systematically noted in a cohort of 400 consecutive clinically definite MS patients. Acute deafness was observed in 14 patients (3.5 p. cent) with a mean age of 28 years (15 to 48) and after a mean of 2.3 years (0 to 10) of disease duration. In all but two patients, deafness was associated with disease exacerbations, highly suggestive of infratentorial lesions. MRI showed hyperintensities in the pons or the midbrain in 11 cases. Deafness disappeared in all cases excepted for one patient. After a mean follow-up of 7.2 years (1 to 17), all the patients remain in the relapsing-remitting form of the disease with a mean EDSS of 2.4 (0 to 4.5). Acute deafness is observed in 3.5 p. cent of MS cases and occurs more frequently during the first years of the disease. Although follow-up of a larger cohort is necessary, deafness could be of good prognostic factor in MS.

Numerical modeling of high-power ultrasonic systems: current status and future trends

Numerical models of high power ultrasonic systems are usually based on finite element or boundary element methods. The basic physical models are linear and rely upon the theory of elasticity, the constitutive law of piezoelectricity and the theory of linear acoustics. They are only valid at low drive level. Some recent developments include nonlinearities of the transduction mechanism and of the propagation medium. In this paper, standard and advanced numerical models are discussed and illustrated by several application examples of high power ultrasonics.

Acoustic wave scattering from a composed shell reinforced by a single rib: characteristics of peripheral waves

The steady-state axisymmetrical problem of a plane acoustic wave scattering from a composed shell is considered. The shell has a cylindrical part and two hemispherical endcaps. The rib is a ring of rectangular cross-section that divides the shell into two equal parts. The motion of the shell and the rib is described by the equations of elasticity theory, and the liquid is described by the Helmholtz equation. The solution is obtained numerically by a coupled finite element/boundary element model. Two peripheral waves are generated in the shell: the membrane S0 wave and the bending type water-borne A wave. The form function, acoustic spectrogram and dispersion curves of the phase velocities are presented, and the effect of the rib on the peripheral waves is discussed.