PUR/PZT/C komposiittivaahtojen dielektriset ominaisuudet ja mittauksen mallinnus

Työssä valmistettiin kolmen- ja neljän faasin vaahtokomposiitteja käyttämällä poluyretaani- (PUR), lyijy zirkonium titanaatti- (PZT), barium titanaatti- (BaTiO₃) ja grafiitti ainesosia sekä näiden yhdistelmiä, joissa ilma toimi yhtenä faasina. Kolmen faasin komposiiteilla PZT- osuus vaihteli välillä 20–80 painoprosenttia (p.%), BaTiO₃-osuus 40–60 p.% sekä grafiitin osuus 0–15 p.%. Valmistetut neljän faasin vaahdot muodostettiin lisäämällä PZT:tä 20–60 p.% grafiitin määrän vaihdellessa välillä 1–15 p.%. Vaahtokomposiittien valmistusmentelmänä käytettiin polyuretaanipohjaisten kahden komponentin eksotermistä reaktiota mihin halutut ferrosähköiset ja johtavat lisäaineet lisättiin. Työn tarkoituksena oli tutkia valmistettujen komposiittien sähköisiä ominaisuuksia sekoitussuhteita vaihtelemalla, minkä perusteella pyrittiin löytämään optimaalinen koostumus neljän faasin komposiiteille sekä havainnoida PZT:n ja grafiitin yhteisvaikutusta. Valmistettujen näytteiden sähköisiä ominaisuuksia tutkittiin myös polaroinnin jälkeen, jonka kautta ominaisuuksia pyrittiin parantamaan. Työn keskeisenä osana oli myös mittapään valmistus puristuksen ja sähköisten ominaisuuksien samanaikaisen mittauksen suorittamiseksi, mittapään kalibrointi sekä mittaustilanteen mallinnus LTSpice-ohjelmalla. Mittaus pohjautui ideaalisen levykondensaattorin kapasitanssin teoreettiseen laskukaavaan, jonka rajoitteita tutkittiin työssä valmistetun ja käytetyn mittapään suhteen. Havaittu epäideaalisuus mittauksien suhteen pyrittiin huomioimaan toteamalla virhekomponentista aiheutuvan virheen suuruus laskennallisesti sekä huomioimalla vaikutuksen suuruus näytekappaleiden mittaustulosten analysoinnissa.In this experiment three and four phase foam composites were prepared using polyurethane (PUR)-, lead zirconium titanate (PZT)-, barium titanate (BaTiO₃)- and grapite components as well as their combinations in which air acted as one phase. In the three phase composite foams the amount of PZT varied between 20–80 weight procent (wt.%), BaTiO₃ between 40–60 wt.% and the amount of graphite between 1–15 wt.%. The four phase foams were formed by adding PZT from 20 to 60 wt.% while graphite’s amount ranged between 1–15 wt.%. The composite foams were made by using polyurethane based two component exothermic reaction in which the wanted ferroelectric- and conducting fillers were added. The goal of the experiment was to examine the electrical properties of the composites in different conditions, to find the optimal mixture for the three phase composites and study the combined effect of PZT and graphite. The electrical properties of the prepared samples were also examined after poling through which the properties were sought to be improved. In a key role was also the preparation of the measuring head for the simultaneous measurement of compression and electrical properties of the sample, calibration of the measuring head and modeling of the device with LTSpice program. Measuring based on the ideal parallel-plate-capacitor’s capacitance’s theoretical calculation formula of which restrictions regarding the measurement head were studied. The deviation from the ideal values regarding the measurements was taken in account by calculating the error components as well as observing the magnitude of the effect by analysing the outcome of the samples

Novel sensor and switch applications for flexible and stretchable electronic materials

Abstract In this thesis flexible electronics composite materials were developed and utilized in pressure sensors. Additionally, stretchable materials based on piezoresistive structures were fabricated and their feasibility for printed electronics switches and stretchable strain sensors was investigated. In the first part of the thesis two types of composite materials were developed based on polyurethane foam with added carbon powder and on liquid crystal polymer with ceramic powder. The first developed composite was utilized in piezoresistive and capacitive hybrid sensors and the latter one for an additive manufactured piezoelectric sensor strip suitable for operation at elevated temperatures. The formable hybrid sensor achieved a maximum pressure sensitivity of 0.338 kPa-1 with response and recovery times less than 200 ms at pressures over 200 kPa and also showed a linear response. The sensor could be utilized, for example, in wearable electronics and robotics. The new type of piezoelectric material showed piezoelectric coefficients of d33 > 14 pC/N and g33 > 108 mVm/N at pressure below 10 kPa with a wide pressure sensing range up to 4.5 MPa. This was higher than that previously achieved for materials fabricated using traditional printing techniques. The piezoelectric sensor would be suitable for industrial process control at elevated temperatures. In the second part of the thesis the stretchable materials were utilized in a new type of piezoresistive structure to fabricate one of the first stretchable switches and a machine washable self-adherable strain sensor. The developed stretchable switch could be actuated with either stretching or vibration with a minimum movement of < 2 μm. The versatile strain sensor with a tunable resistance-strain characteristic achieved the currently highest reported gauge factor (>105) at > 70% stretching. The strain sensor could be utilized for sensing human body movements and physiological signals.Tiivistelmä Väitöstyössä kehitettiin joustavan elektroniikan komposiittimateriaaleja, joita hyödynnettiin paineantureissa sekä käytettiin venytettäviä materiaaleja painettavan elektroniikan kytkimen ja venymäanturin valmistukseen. Työn ensimmäisessä osassa kehitettiin kahdenlaisia komposiittimateriaaleja, joista ensimmäinen pohjautui polyuretaanivaahtoihin, joihin sisällytettiin hiilijauhetta, sekä toinen nestekidepolymeeriin, johon lisättiin keraamijauhetta. Ensimmäistä kehitettyä komposiittia hyödynnettiin pietsoresistiivisessä ja -kapasitiivisessa hybridianturissa ja jälkimmäistä lisäaine valmistettavassa pietsosähköisessä anturinauhassa, joka soveltui kohotettuihin lämpötiloihin. Muovattavalla hybridianturilla saavutettiin herkkyyden maksimiarvoksi 0.338 kPa-1, alle 200 ms vaste- ja palautumisajat yli 200 kPa paineessa ja lineaarinen vaste. Anturia voitaisiin monipuolisesti hyödyntää mm. puettavassa elektroniikassa ja robotiikassa. Uudenlaisella pietsosähköisellä materiaalilla saavutettiin pietsosähköiset kertoimet (d33 > 14 pC/N ja g33 > 108 mVm/N < 10 kPa paineessa), jotka olivat korkeammat kuin perinteisin tulostusmenetelmin valmistetuilla materiaaleilla. Pietsosähköinen anturi soveltuisi mm. teolliseen prosessivalvontaan kohotetuissa lämpötiloissa. Toisessa osassa hyödynnettiin venytettäviä materiaaleja uudentyyppisissä pietsoresistiivisissä rakenteissa ensimmäisten venytettävän painettavan elektroniikan kytkimen sekä konepestävän itsekiinnityttävän venymäanturin valmistamiseksi. Tulokset on esitetty kahdessa julkaisussa, joista ensimmäinen keskittyi kytkimen valmistamiseen ja toimintaan sekä toinen venymäanturin toimintaan ihmiskehon liikkeen ja signaalien mittaamiseksi. Kehitettyä kytkintä voitiin aktuoida monipuolisesti esim. venytyksen tai värinän avulla alle 2 μm liikkeellä. Monipuolisella venymäanturilla saavutettiin säädettävä resistanssi-venymä suhde korkeimmalla tähän asti ilmoitettu herkkyydellä (>105) yli 70% venytyksellä. Venymäanturia voitiin hyödyntää ihmiskehon liikkeiden ja fysiologisten signaalien mittaamiseen

Stretchable and washable strain sensor based on cracking structure for human motion monitoring

Abstract Stretchable and wearable strain sensors have been intensively studied in recent years for applications in human motion monitoring. However, achieving a high-performance strain sensor with high stretchability, ultra-sensitivity, and functionality, such as tunable sensing ranges and sensitivity to various stimuli, has not yet been reported, even though such sensors have great importance for the future applications of wearable electronics. Herein, a novel and versatile strain sensor based on a cracking (silver ink patterned silicone elastomer)-(silver plated nylon structure) (Ag-DS/CF) has been designed and fabricated. The unique structure combined precisely shaped stretchable conductive fabrics and wrinkled Ag-ink pattern to achieve an excellent electrical performance. The Ag-DS/CF could be used to detect both large and subtle human motions and activities, pressure changes, and physical vibrations by achieving high stretchability up to 75%, ultrahigh sensitivity (gauge factor >104–106), tunable sensing ranges (from 7 to 75%). Excellent durability was demonstrated for human motion monitoring with machine washability. The extremely versatile Ag-DS/CF showed outstanding potential for the future of wearable electronics in real-time monitoring of human health, sports performance, etc

Kirigami-inspired dual-parameter tactile sensor with ultrahigh sensitivity, multimodal and strain-insensitive features

Abstract Soft sensors with strain-insensitive and multimodal features are intriguing due to their high practical relevance. However, incorporating these functionalities into sensors made of soft materials has been challenging. Herein, a Kirigami-inspired dual-parameter tactile sensor was developed with strain-insensitive and multimodal features. The tactile sensor uses piezoresistive and capacitive transduction modes allowing simultaneous detection of dynamic and static tensile strains, proximity and normal pressures. The convenient structural design enables ultrahigh piezoresistive sensitivity ~23 000 kPa-1 in its resistivity-switching threshold region (in high pressure regimes > 50 kPa). It achieves a linear capacitive gauge factor of ~14.48 for uniaxial elongation up to 80% strain and can accurately measure proximity (≥ 0.01 pF/mm) of objects within distances up to 100 mm. The ultrahigh sensitivity in high pressure regimes allows force adjustable lower limit of detection and sensitivity of the sensor by pre-stress enabling real-time measurement of arterial pulsation. The findings of this work support the design of soft sensors for touch recognition applications in the automotive industry, soft robots or self-adjusting grippers requiring a sense of touch and multimodal and strain-insensitive features

Hybrid foam pressure sensor utilising piezoresistive and capacitive sensing mechanisms

Abstract The development of flexible and stretchable sensing for future applications, e.g. strain, force and pressure, requires novel foam and foam-like structures with properties such as fast relaxation times, a wide linear response range, good sensitivity to different stimuli and repeatability, without compromising low-cost, simple and effective manufacturing methods and excellent mechanical properties. We present a new type of hybrid foam pressure sensor that utilises a combination of piezoresistive and capacitive sensor elements. The hybrid foam sensor shows maximum pressure sensitivity of 0.338 kPa⁻¹ and linear response of 0.049 kPa⁻¹ in the piezoresistive and capacitive sensor elements, respectively, in pressure ranges of <5 kPa and 0–240 kPa, respectively. The response and recovery times of both sensor elements were similar, ≤ 200 ms, at various pressures. In addition, the properties of the hybrid foam sensors, e.g. sensitivity and response and recovery times, can be tuned in various ways, such as by changing the thickness of the composition of the foam. Also, the materials are easily accessible and the sensor can be cost-effectively manufactured and changed for different purposes. The relatively high sensitivity of the piezoresistive sensor element enables object manipulation from low pressure (≥ 21 Pa) to high pressure (> 80 kPa). Simultaneously, both sensor elements can be used for impact measurements across a wide range of pressures up to ≥ 240 kPa. Thus, the concept of the hybrid foam sensor could be utilised widely for sensing of pressures, impacts or even bending in various applications, e.g., wearable electronics

Piezoelectric flexible LCP–PZT composites for sensor applications at elevated temperatures

Abstract In this paper fabrication of piezoelectric ceramic–polymer composites is demonstrated via filament extrusion enabling cost-efficient large-scale production of highly bendable pressure sensors feasible for elevated temperatures. These composites are fabricated by utilizing environmentally resistant and stable liquid crystal polymer matrix with addition of lead zirconate titanate at loading levels of 30 vol%. These composites, of approximately 0.99 mm thick and length of  > 50 cm, achieved excellent bendability with minimum bending radius of ~ 6.6 cm. The maximum piezoelectric coefficients d₃₃ and g₃₃ of the composites were > 14 pC/N and > 108 mVm/N at pressure < 10 kPa. In all cases, the piezoelectric charge coefficient (d₃₃) of the composites decreased as a function of pressure. Also, piezoelectric coefficient (d₃₃) further decreased in the case of increased frequency press-release cycle sand pre-stress levels by approximately 37–50%. However, the obtained results provide tools for fabricating novel piezoelectric sensors in highly efficient way for environments with elevated temperatures

All-around universal and photoelastic self-healing elastomer with high toughness and resilience

Abstract Ultimately soft electronics seek affordable and high mechanical performance universal self-healing materials that can autonomously heal in harsh environments within short times scales. As of now, such features are not found in a single material. Herein, interpenetrated elastomer network with bimodal chain length distribution showing rapid autonomous healing in universal conditions (<7200 s) with high efficiency (up to 97.6 ± 4.8%) is reported. The bimodal elastomer displays strain-induced photoelastic effect and reinforcement which is responsible for its remarkable mechanical robustness (≈5.5 MPa stress at break and toughness ≈30 MJ m−3). The entropy-driven elasticity allows an unprecedented shape recovery efficiency (100%) even after fracturing and 100% resiliency up to its stretching limit (≈2000% strain). The elastomers can be mechanically conditioned leading to a state where they recover their shape extremely quickly after removal of stress (nearly order of magnitude faster than pristine elastomers). As a proof of concept, universal self-healing mechanochromic strain sensor is developed capable of operating in various environmental conditions and of changing its photonic band gap under mechanical stress

Screen-printed mechanical switch based on stretchable PU-foam film

Abstract A screen-printed mechanical switch based on an electrode structure on stretchable polyurethane (PU)-foam film, Platilon U 4021, combined with a piezoelectric actuator, Smart Material MFC M-4010-P1, is proposed. The minimum actuation voltage of the prepared component is 300 V. The measured resistance was 2 Ω while closed and >0.5 TΩ when open. The electrode structure endured on average of up to 15.5 M cycles with movement ≥100 times greater than the ≤1 μm required for actuation. The results suggest that the switch could be advantageous for various e-textile applications

Dielectric properties of novel polyurethane–PZT–graphite foam composites

Abstract Flexible foam composite materials offer multiple benefits to future electronic applications as the rapid development of the electronics industry requires smaller, more efficient, and lighter materials to further develop foldable and wearable applications. The aims of this work were to examine the electrical properties of three- and four-phase novel foam composites in different conditions, find the optimal mixture for four-phase foam composites, and study the combined effects of lead zirconate titanate (PZT) and graphite fillers. The flexible and highly compressible foams were prepared in a room-temperature mixing process using polyurethane, PZT, and graphite components as well as their combinations, in which air acted as one phase. In three-phase foams the amount of PZT varied between 20 and 80 wt% and the amount of graphite, between 1 and 15 wt%. The four-phase foams were formed by adding 40 wt% of PZT while the amount of graphite ranged between 1 and 15 wt%. The presented results and materials could be utilized to develop new flexible and soft sensor applications by means of material technology

Stretchable sensors with tunability and single stimuli-responsiveness through resistivity switching under compressive stress

Abstract The fascinating human somatosensory system with its complex structure is composed of numerous sensory receptors possessing distinct responsiveness to stimuli. It is a continuous source of inspiration for tactile sensors that mimic its functions. However, to achieve single stimulus-responsiveness with mechanical decoupling is particularly challenging in the light of structural design and has not been fully addressed to date. Here we propose a novel structural design inspired by combining the characteristics of electronic skin (e-skin) and electronic textile (e-textile) into a hybrid interface to achieve a stretchable single stimuli-responsive tactile sensor. The stencil printable biocarbon composite/silver-plated nylon hybrid interface possesses an extraordinary resistance switching (ΔR/R0 up to ∼104) under compressive stress which is controllable by the composite film-thickness. It achieves a very high normal pressure sensitivity (up to 60.8 kPa–1) in a wide dynamic range (up to ∼50 kPa) in the piezoresistive operation mode and can effectively decouple stresses induced by stretching or bending. In addition, the device is capable of high accuracy strain sensing in its capacitive operation mode through dimensional change dominant response. Because of these intriguing features, it has potential for the next-generation Internet of Things devices and user-interactive systems capable of providing visual feedback and more advanced robotics or even prosthetics