Because of their characteristics, including a d33 of 10–15 pC/N and high stability up to temperatures over 1000 °C, polar glass–ceramics containing fresnoite crystals can be regarded as highly effective materials for applications requiring piezoelectricity at high temperatures. In the present paper we investigate barium substitutions in an Sr-fresnoite (STS) glass–ceramic. Two aspects are studied: first, the effect of the substitution on the preferential orientation of the crystallization, and second, the ability of the glass–ceramics to generate and propagate surface acoustic waves (SAW) at high temperatures. XRD analyses show that a 10 at.% substitution of Ba allows us to keep a strong preferential orientation of the (00l) planes of the fresnoite crystals down to more than 1 mm below the surfaces. Higher substitution levels (25 and 50 at.%), induce a non-oriented volume crystallization mechanism that competes with the surface mechanism. SAW devices were fabricated from glass–ceramic substrates with 0, 10 and 25 at.% Ba substitutions. Temperature testing reveals the high stability of the frequency and delay for all of these devices. The glass–ceramic with a 10 at.% Ba substitution gives the strongest amplitude of the SAW signal. This is attributed to the high (00l) preferential orientation and the absence of disoriented volume crystallization.

In a European project called CUBISM, humidity sensors based on IDT (Inter Digital Transducer) technology are developed for the generation and detection of surface acoustic waves (SAW).These sensors are designed to operate at high temperature (500°C) for monitoring the drying of refractory concrete.Indeed, this humidity monitoring is important because a sudden evaporation of the water during the achievement of the structure could lead to high pressures in the pores implying consequently the destruction of the structure.Thus, the optimization of the concrete drying cycle must be combined with relevant in-situ physical measurements (humidity, pressure, temperature) and thermomechanical modelling.The real-time availability of this physical data via specific sensors integrated into the concrete is therefore a key to effective drying monitoring.Thus, for this project and its specific constraints, we have chosen the development of SAW humidity sensors because they are the most suitable to meet the specifications.In this study, the optimized parameters include the nature of the humidity-sensitive layer, the nature of the piezoelectric substrate, the architecture of the electrode array and finally the electronic measurement setup.

This paper presents a surface acoustic wave (SAW) device able to operate up to 900 degrees C, based on a piezoelectric non-ferroelectric glass-ceramic containing fresnoite crystals. This material is synthesized by a classic glassmaking technique and isothermal crystallization heat treatment. Its properties are measured at high temperatures. Input and output interdigital transducers (IDT) are realized on its surface to generate and receive the SAW. It creates a temperature device working up to 800 degrees C, that will further be modified in pressure or humidity sensors by applying the appropriate sensitive layer between the input and the output IDT.

This paper discusses a concept of bimorph deformable mirror used in adaptive optics to compensate for manufacturing errors, gravity release and thermal distortion affecting large lightweight mirrors in space telescopes. The mirror consists of a single-crystal Silicon wafer (D=75 mm t=500μm) covered with an optical coating on the front side and an array of 25 independent PZT actuators acting in d31 mode on the back side. The mirror is mounted on an isostatic support with three linear PZT actuators controlling the rigid-body motion. The paper presents the experimental results obtained with this design and a new, more compact alternative.

This work is treated in the framework of CUBISM project funded by INTERREG V program. The purpose of the project is to develop a pressure and humidity SAW sensor, in order to follow the drying of refractory materials under high temperature and pressure conditions. More precisely, we aim to describe and predict the thermo-mechanical behavior of the piezoelectric SAW substrate under such conditions for a full set of geometrical configurations and materials. Besides, we expect to take into account the micro-cracks resulting from thermal expansion mismatch between the substrate and its environment. However, at the microscopic scale, the finite element method is less suitable to describe discontinuities induced by micro-cracks. For that reason, we propose to study the thermo-mechanical behavior using the Discrete Element Method (DEM). This choice is also motivated by the advantage of DEM to describe the crack propagation. This contribution presents significant improvement for DEM to model the 3D thermal-induced damage due to thermal expansion. Furthermore, this study allows to follow the damage level of the material during its lifetime. Thanks to the MULTICOR3D++ code developed in our laboratory, a hybrid particulate-lattice model [1] based on the equivalence between a granular system and a network of cohesive beam elements, is investigated. Our contribution is to introduce the linear thermal expansion at the scale of the contact by modifying the initial free length of each link using the model introduced in 2D by Leclerc et al. [2]. Our objectives are twofold. First, we aim to investigate the suitability of the cohesive beam model, in the context of a thermo-mechanical behavior of heterogeneous continuous media. Consequently, some comparisons are done with finite element calculations in terms of effective coefficient of thermal expansion, and stress and strain fields. In this study, the equivalent stress and strain of each particle are determined using Zhou formulation [3]. Second, we are interested in studying the ability of a DEM to simulate the thermal-induced damage in composite materials. For that purpose, damage effects and interfacial debonding are taken into account, and we distinguish between two cases, according to the temperature variation. In the first case, a rise in temperature leads to cracks initiation and propagation which are modeled by the Removed Discrete Element Failure criterion [2]. In the present contribution, we consider the brittle fracture of fragile materials such as silica and alumina. Due to their strong ability to resist in compression, the failure of fragile materials occurs when submitted to tensile solicitations. In fact, we consider that the fracture occurs when the hydrostatic stress for local tensile solicitations is greater than a given tensile strength limit. In the second case, a drop in temperature generates interfacial debonding which is modeled by the Discrete Damage Zone Model [2]. The idea is to replace the cohesive links connecting two particles belonging to two different phases by spring elements, which has a normal stiffness that tends to zero for high displacements. The results obtained in both cases are in good agreement with theoretical expectations.

We discuss the concept of lightweight segmented bimorph mirrors for adaptive optics. The segment consists of a monocrystal silicon substrate actuated by an array of in-plane piezoceramic (PZT) actuators with honeycomb electrodes. We focus on technological aspects of the segment design that are critical for space applications and describe a single segment demonstrator. The morphing capability of the segment is evaluated experimentally. We also discuss the local deformations (dimples) associated with the shape of the electrodes acting on the PZT array.