Моделювання сенсора вібрацій на основі біморфної структури

У дослідженні розроблена математична модель, що описує амплітудно-частотний відгук сенсора або пристрою збору скидної енергії, виготовленого з сегнетоелектричної бідоменної монокристалічної пластини, по відношенню до величини вібраційного збудження. Математична модель дозволяє прогнозувати залежність напруги на електродах від частоти і амплітуди вібраційного збудження, а також резонансної частоти сенсора, що представляє собою прямокутну пластину, в загальному випадку з сейсмічної масою на вільному кінці, який встановлюється на вібраційному столику, параметри коливань якого задаються. Складено відповідне диференційне рівняння, що описує шукані залежності, і отримано його аналітичне рішення. Для перевірки запропонованої моделі був створений монокристалічний біморф за допомогою відпалу підкладки з ніобату літію (LiNbO3) на повітрі для оберненої дифузії літію і формування бідоменної структури, що представляє собою два зустрічно поляризованих домени в одній пластині (так звана структура "голова-до-голови"). Такий кристал аналогічний біморфу, проте на відміну від нього не містить будь-яких міжфазних меж, за винятком міждоменної. Таким чином, виготовлений біморф являє собою не поширену збірну конструкцію, що складається найчастіше з металевої підкладки, до якої прикріплені п'єзоелектричні пластини, як правило, з п'єзокераміки, а однорідне безперервне середовище. Перевага такого біморфу полягає у тому, що, будучи виготовленим з сегнетоелектричного монокристала ніобату (або танталата) літію, сенсор або пристрій збору скидної енергії має великий коефіцієнт перетворення деформації згинання в електричну деформацію, а отже, високу чутливість, а також широкий температурний діапазон застосування та практично повну відсутність гістерезису і старіння. Проведено порівняння результатів моделювання з експериментальними даними, з якого випливає, що запропонована модель добре відповідає результатам експерименту. Показано, що сенсори коливань на бідоменних монокристалічних пластинах мають виключно високу чутливість. Запропонована модель дозволяє оцінювати і прогнозувати параметри сенсорів вібрації, акселерометрів і пристроїв збору скидної енергії на основі бідоменних сегнетоелектричних кристалів.In the current study, we have developed a mathematical model describing the frequency response of a sensor or energy harvester based on a cantilever made of a ferroelectric bidomain single-crystal plate with metal electrodes deposited on opposite faces. The structure is subjected to vibrational excitations. The model allows to predict the dependence of the voltage between the electrodes vs. the vibration frequency and amplitude as well as resonance frequency of the sensor fabricated in form of a rectangular plate, normally with a seismic mass on its free end. The device is placed on a vibration table, whose vibration parameters are set. The relevant differential equation was composed, and an analytic solution describing the required dependencies was obtained. To validate the proposed model, we created a single-crystal bimorph by annealing a lithium niobate (LiNbO3) wafer in air to promote Li out-diffusion and formation of a bidomain ferroelectric structure, i.e. two oppositely polarized domains within the plate (the so called “headto-head” structure). Such a crystal demonstrates a bimorph-like behavior but does not comprise any interface except for an interdomain wall. Thus, our bimorph is not a commonly used structure, typically consisting of two bonded piezoelectric plates (generally made of PZT piezoceramics), but a homogeneous continuous medium. Being made of a lithium niobate (or lithium tantalate) ferroelectric single crystal, the cantilever sensor or energy harvester demonstrates a strong dependence of the voltage between the electrodes on the bending deformations, with almost totally absent hysteresis and ageing in a wide temperature range. The comparison made between the results of the modeling and the experiment shows that the proposed model is in good agreement with the experiment. We have demonstrated that the vibration sensors based on bidomain single-crystal plates possess an exceptionally high sensitivity. The proposed model can be used to estimate and predict the parameters of vibration sensors, accelerometers and waste energy harvesters based on bidomain ferroelectric crystals

Real-time investigation of Double magnetic tunnel junction with a switchable Assistance layer for high efficiency STT-MRAM

International audienceThis work reports experimental confirmation of the working principles of a double magnetic tunnel junction (DMTJ) to achieve highly efficient spin-transfer-torque (STT) switching. This concept uses a magnetically switchable assistance layer (ASL) acting as top perpendicular spin polarizer. The STT-induced switching dynamics were described by macrospin simulations while the magnetic and electrical properties of the device were investigated in switching experiments. The reversal dynamics of the storage layer-ASL coupled system were validated by time-resolved measurements of the device resistance during write operation, confirming that the storage layer is subjected to additive STT contributions both from the reference layer and the ASL throughout its parallel-to-antiparallel and antiparallel-to-parallel transitions. The STT efficiency of the ASL-DMTJ was compared to that of single MTJ stack comprising the same storage layer and no assistance layer. The figure of merit ∆ /Ic (stability/critical current) was found to nearly double in devices of 80 nm and 100 nm diameter, with a smaller 30% increase obtained for 50 nm diameter cells

Real-time investigation of Double magnetic tunnel junction with a switchable Assistance layer for high efficiency STT-MRAM

International audienceThis work reports experimental confirmation of the working principles of a double magnetic tunnel junction (DMTJ) to achieve highly efficient spin-transfer-torque (STT) switching. This concept uses a magnetically switchable assistance layer (ASL) acting as top perpendicular spin polarizer. The STT-induced switching dynamics were described by macrospin simulations while the magnetic and electrical properties of the device were investigated in switching experiments. The reversal dynamics of the storage layer-ASL coupled system were validated by time-resolved measurements of the device resistance during write operation, confirming that the storage layer is subjected to additive STT contributions both from the reference layer and the ASL throughout its parallel-to-antiparallel and antiparallel-to-parallel transitions. The STT efficiency of the ASL-DMTJ was compared to that of single MTJ stack comprising the same storage layer and no assistance layer. The figure of merit ∆ /Ic (stability/critical current) was found to nearly double in devices of 80 nm and 100 nm diameter, with a smaller 30% increase obtained for 50 nm diameter cells

Cost-effectiveness and budget impact of long-acting risperidone in Portugal: a modeling exercise

Abstract: Background: Previous analyses have shown that long-acting risperidone (LAR) is cost-effective in several Western countries. In Portugal, however, the costs of key services are lower. Therefore, available evidence in other countries may have limited relevance. Objective: To estimate costs and effects of LAR versus a conventional depot and a short-acting oral atypical antipsyhcotic over a 5-year period in Portugal. Methods: An existing discrete event model was adapted to reflect the Portuguese healthcare setting, based on expert opinion, clinical, epidemiological, and cost data. The model compares three scenarios. In scenario 1, patients start with a conventional depot; in scenario 2, with LAR; and in scenario 3, with oral risperidone. The model simulates individual patient histories while taking into account patient characteristics such as risk to society and side-effects. Subsequently, the model simulates patient histories in terms of outpatient appointments, psychotic episodes, treatment, compliance, symptom scores, lack of ability to take care presenting an actual risk, and treatment setting. Outcomes were number of psychotic episodes, cumulative symptom score and direct medical costs. Univariate sensitivity analyses were carried out. Results: Compared to a conventional depot and an oral atypical, LAR was estimated to save approximately (sic)3603 and (sic)4682 per patient (respectively) and avoid 0.44 and 0.59 relapses per patient in 5 years. Sensitivity analyses showed that the outcome of dominance was only sensitive to estimates about unit costs of hospital/institutionalization, potential risk, and to the reduction in symptoms by use of atypicals. Conclusion: Based on this modeling exercise, it could be expected that LAR may be a cost-effective treatment with limited budget impact in Portugal. However, further studies are required to test the generalizability of the results of the present modeling study to the larger population of Portugal.