Long-Term Mechanical Reliability of Ceramic Thick-Film Circuits and Mechanical Sensors Under Static Load

AbstractIn this work, we study the long-term static fatigue (constant load, room temperature, ∼100% humidity) performance of ceramic thick-film circuits, as a function of substrate and thick-film compositions, comparing standard 96% alumina with high-strength zirconia-toughened alumina (ZTA). Blank ZTA exhibits higher short-term strength and resistance to static fatigue than alumina. However, many thick-film compositions degrade the static fatigue behaviour, with ZTA being in general more affected. This implies that choice of thick-film materials is important for mechanical reliability

Test de collage de billes de verre sur poutre mN

Continuation du Projet de semestre de Maxime Blot. - Socle : optimisation de la composition de la colle pour former le socle - Colle : Essais de colles commerciales H70E et H70E2 (plus visqueuses que H70S précédemment testée) et essais de colles modifiées basées sur H70S

Subsolidus phase equilibria in the PbO-poor part of the TiO2-PbO-SiO2 system and its application in low-temperature thick-film dielectrics

Subsolidus equilibria in the PbO-poor part of the TiO2-PbO-SiO2 diagram were studied with the aim of investigating possible applications for low-temperature thick-film dielectrics. The tie lines are between PbTiO2 and PbSiO3, and between PbTiO3 and SiO2. The results show that the TiO2, when added to low-temperature softening point glasses, reacts with the PbO from the glass, so forming PbTiO3. These results were applied to a low-temperature firing dielectric, consisting of a lead-rich PbO-SiO2-B2O3 glass filled with a TiO2 powder. The conversion of TiO2 to the PbTiO3 crystalline phase was observed above firing temperatures of approximately 600 °C. The kinetics of the reaction depend on the particle size of the TiO

Low-firing thick-film piezoresistive sensors for medical instruments

In this work, a low-firing thick-film materials system allowing fabrication of piezoresistive sensors on surgical alloys is presented in detail, with application to a force-sensing surgical instrument. The system comprises a series of individual thick-film dielectric, conductor, resistive and overglaze compositions based on a lead borosilicate glass matrix. The moderate achieved firing temperature, around 625°C, greatly increases compatibility with metallic substrates, allowing the use of high-strength medical alloys with low thermal degradation. Specific fillers for the dielectric layers increase adhesion on steel substrates and allow thermal matching to austenitic and ferritic / martensitic steels, as well as titanium alloys. The functionality of this materials system is successfully demonstrated here by implementing it into a previously developed ligament-balancing force sensor for total knee arthroplasty (TKA) [1]. Keywords: Medical operations; total knee arthroplasty; thick-film force sensors; medical alloy

Development of low-firing lead-free thick-film materials on steel alloys for piezoresistive sensor applications

Piezoresistive sensors based on steel and other metallic substrates provide higher strain response than on standard ceramic substrates and are more easily packaged. But exposing high-strength steels to the standard high- temperature 850°C thick-film firing cycle affects their mechanical properties. In previous studies, we have developed a range of low-firing thick-film materials based on lead borosilicate glass, which allows processing at low temperatures. However, it is desirable to develop alternatives to potentially toxic lead-based glasses that to not include alkali metals, which degrade high-temperature insulation characteristics of dielectrics. To this end, this work concerns investigations in essentially substituting lead for bismuth, and presents a series of low-melting Bi-B-Zn-Si-Al oxide glasses having good stability against devitrification. However, these glasses, when formulated as thick-film pastes using standard vehicles based on ethylcellulose binders, were found to be quite sensitive to incomplete binder burnout, with strong bubble generation within the layer. Therefore, a novel organic binder based on polypropylene carbonate, featuring clean low temperature burnout, had to be introduced. On this basis, thick-film dielectric compositions have then been developed and tested, aiming to optimise the mechanical strength and their expansion matching with the steel substrates. In the goal of a complete materials system, first tests on compatible conductors and resistors, using the same glasses, are presented as well

High-strain response of piezoresistive thick-film resistors on titanium alloy substrates

We examine, in this work, the integration and high-strain response of piezoresistive thick-film resistors on titanium and titanium alloy, as compared with ferritic stainless steel and the standard alumina substrates. In general, titanium and its alloys are relatively ill suited for use as a thick-film substrate using the standard 850 °C firing process, because of its low oxidation resistance and tendency to dissolve oxygen. This problem can be alleviated by applying suitable thick-film glassy protection layers. Titanium alloys are very elastic, and allow piezoresistive responses well in excess of 4% with little plastic deformation of the substrate, using standard commercial thick-film resistor compositions. In this case, the behaviour of the sensor becomes asymmetric, as failure of the resistor or insulating dielectric under tension––and therefore the mechanical properties of these materials–– becomes the factor limiting the response of the sensor

Assessment of thick-film resistors for manufacturing piezoresistive sensors

A complete line of resistors materials are tested for the manufacture of thick-film piezoresistive sensors and associated simple adjustment / amplification circuits: 10 kΩ compositions for the gauge resistors, together with 100 Ω and 100 kΩ ones for electronics and trimming, as well as PTC compositions for temperature compensation. Several aspects are considered, such as process sensitivity, overglazing and trimming

Signal Stability of LTCC Cantilever Force Sensors

In this work, we study in detail the sensing characteristics of piezoresistive force sensors based on structured LTCC (Low Temperature Co-fired Ceramic) cantilevers carrying a thick-film piezoresistive bridge. These devices show much improved sensitivity compared to classical alumina-based devices, but may exhibit abnormally large signal and drift, which indicates the presence of structural defects originating from fabrication issues or deleterious interactions between materials. To eliminate these effects, the fabrication parameters of the LTCC cantilevers have been studied in detail. By varying materials, layer thicknesses, stacking order and lamination parameters, the respective roles of resistor-termination-tape interactions, plastic deformation of conductor tracks and lamination quality of the LTCC sheets may be elucidated

Manufacturing and trimming of a low-cost industrial thick-film force sensor

This work describes the design, fabrication and laser trimming aspects of a low-cost compressive force sensor with analogue signal conditioning, designed for a medical syringe pump. First, the general design aspects are discussed, in view of facile and reliable batch production and measurement performance. Then, the circuit's trimming procedure is presented, featuring single- pass adjustment of sensor output in its final state, i.e. with all components mounted, without prior passive resistor trimming. Key words: thick-film technology, force sensors, manufacturing, trimmin

Load sensing surgical instruments

Force and pressure sensing technology applied to smart surgical instruments as well as implants allow to give a direct feedback of loads to the surgeon lead to better reliability and success of surgical operations. A common technology used for sensors is low-cost piezoresistive thick-film technology. However, the standard thick-film firing conditions degrade the properties of medical alloys. In order to avoid this problem, the solution is to decrease the firing temperature of thick films. This work presents the development and characterisation of low-firing thick-film systems (dielectrics, resistors and conductors), formulated to achieve chemical and thermal expansion compatibility with an austenitic stainless steel medical alloy. Adherence tests and results on electrical properties of these systems: resistance, temperature coefficient of resistance (TCR) are presented. It was found that the main issue in these systems lies in mastering the materials interactions during firing, especially at the silver-based resistor terminations. The interaction of silver, resistor and dielectric tends to give rise to highly resistive zones at the terminations, affecting reliability. This can be circumvented by post-firing the resistor terminations at a moderate temperature