As demand rises for flexible electronics, traditionally prepared sintered ceramic sensors must be transformed into fully new sensor materials that can bend and flex in use and integration. Negative temperature coefficient of resistance (NTC) ceramic thermistors are preferred temperature sensors for their high accuracy and excellent stability, yet their high stiffness and high temperature fabrication process limits their use in flexible electronics. Here, a low stiffness thermistor based on NTC ceramic particles of micron size embedded in an epoxy polymer matrix is reported. The effect of particle-to-particle contact on electrical performance is studied by arranging the NTC particles in the composite films in one of three ways: (1) Low particle contact, (2) Improved particle contact perpendicular to the electrodes and (3) dispersing high particle contact agglomerated clumps throughout the polymer. At 50 vol.% of agglomerated NTC particles, the composite films exhibit a β-value of 2069 K and a resistivity, ρ, of 3.3 · 10⁵ Ωm, 4 orders of magnitude lower than a randomly dispersed composite at identical volume. A quantitative analysis shows that attaining a predominantly parallel connectivity of the NTC particles and polymer is a key parameter in determining the electrical performance of the composite film.

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Piezoelectric energy harvesters are at the front of scientific research as enablers of renewable, sustainable energy for autonomous wireless sensor networks. Crucial for this disruptive technology is the achievable output power. Here we show, analytically, that the maximum output energy per unit volume, under a single sinusoidal excitation, is equal to 1/(4 - 2k²) × 1/2dgX², where k² is the electromechanical coupling coefficient, d and g are the piezoelectric charge and voltage coefficient, respectively, and X is the applied stress. The expression derived is validated by the experimentally measured output energy for a variety of piezoelectric materials over an unprecedented range of more than five orders of magnitude. As the prefactor 1/(4 - 2k²) varies only between 1/2 and 1/4 the figure of merit for piezoelectric materials for energy harvesters is not k², as commonly accepted for vibrational harvesters, but dg. The figure of merit does not depend on the compliance, or Young's modulus. Hence we argue that commonly used brittle inorganic piezoelectric ceramics can be replaced by soft, mechanically flexible polymers and composite films, comprising inorganic piezoelectric materials embedded in a polymer matrix.

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The main objective of this thesis was to develop new piezoelectric ceramic polymer composites that maintain the ease of manufacturing of random composites and could function as a human touch sensor, while simultaneously exploring their potential as energy harvesters.@en

A highly sensitive, lead-free, and flexible piezoelectric touch sensor is reported based on composite films of alkaline niobate K_0.485Na_0.485Li_0.03NbO₃ (KNLN) powders aligned in a polydimethylsiloxane (PDMS) matrix. KNLN powder is fabricated by solid-state sintering and consists of microcubes. The particles are dispersed in uncured PDMS and oriented by application of an oscillating dielectrophoretic alignment field. The dielectric constant of the composite film is almost independent of the microstructure, while upon alignment the piezoelectric charge coefficient increases more than tenfold up to 17 pC N^-1. A quantitative analysis shows that the origin is a reduction of the interparticle distance to under 1.0 μm in the aligned bicontinuous KNLN chains. The temperature stable piezoelectric voltage coefficient exhibits a maximum value of 220 mV m N^-1, at a volume fraction of only 10%. This state-of-the-art value outperforms bulk piezoelectric ceramics and composites with randomly dispersed particles, and is comparable to the values reported for the piezoelectric polymers polyvinylidenefluoride and its random copolymer with trifluoroethylene. Optimized composite films are incorporated in flexible piezoelectric touch sensors. The high sensitivity is analyzed and discussed. As the fabrication technology is straightforward and easy to implement, applications are foreseen in flexible electronics such as wireless sensor networks and biodiagnostics.

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Composites of aligned (K,Na,Li)NbO₃ (KNLN) piezoceramic particles in a PDMS polymer matrix are presented as promising materials for flexible sensors and energy harvesters. Their ease of processing is matched with a relatively high damage tolerance and piezoelectric performance at low dielectric loss. To maximize piezoelectric performance, the effective poling conditions for dielectrophoretically structured KNLN-PDMS composites are studied, and compared with random composites. Effective poling is identified at 7.5 kV/mm, for 6 min at 150°C in structured composites. The structured composites demonstrate improved piezoelectric performance, with respect to random composites, while retaining the low stiffness of the PDMS polymer matrix.

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Piezoelectric composites made from soft and hard lead zirconium titanate (PZT) particles as filler and an epoxy as the matrix were prepared by dielectrophoresis and studied for their piezoelectric properties. It was found that the dielectric constant of the piezoelectric filler plays a significant role in determining the final piezoelectric properties of the composites. Composites with lower dielectric constant for the PZT filler material showed better piezoelectric properties compared to the composites with high dielectric constant filler. This can be ascribed to a more efficient poling of the piezoelectric filler particles. The aging behaviour of these composites was compared to that reported for monolithic ceramics.

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In this study, the influence of Li substitution on the piezoelectric performance of lead-free K0.5Na0.5NbO3 (KNN)-epoxy composites is explored. KNN piezoceramic particles modified with 0-12 mol% of Li are prepared via a double calcination technique, resulting in a perovskite particulate which transitions from an orthorhombic to tetragonal crystal structure between 6 and 9 mol% of Li, and contains a minor nonperovskite second phase from 6 mol%. A cuboid particle morphology is evident in all cases, though tetragonal KNN-based particles have formed with serrated edges and fractures. The particles are dispersed at 10 vol% in an epoxy matrix to develop both random and dielectrophoretically structured (K,Na,Li)NbO3-epoxy composites. The dielectric constant of the composites appears almost independent of Li content, while the piezoelectric charge constant of structured composites peaks before the polymorphic phase transition, at 3 mol% of Li. The peak in performance can be attributed to the increased primary particle size of the composition in combination with its single phase orthorhombic crystal structure. The enhancement of the energy harvesting figure of merit, derived from substituting 3 mol% of Li in the KNN particulate, makes these composites an interesting choice for flexible energy generators.@en

A high-voltage coefficient has been found in lead-free piezoelectric particulate composites based on epoxy with lead-free (K_0.50Na_0.50)_0.94Li_0.06NbO₃ (KNLN) piezoceramic particles with a natural cubic morphology. The KNLN powder used in the composites has been prepared using a new solid-state double calcination processing route. These particles were subsequently used to create random and structured KNLN-epoxy composites. Using dielectrophoresis, these natural cubical KNLN particles were structured into one-dimensional chains inside the epoxy matrix. Composites produced with these powders showed piezoelectric properties about a factor of 2 higher than those of composites processed with conventionally calcined KNLN powders. The dielectrophoretically structured KNLN-epoxy composites with optimized particle size and morphology showed excellent piezoelectric properties, which can replace lead containing piezoelectric composites for sensor and energy harvesting applications in future.

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The active and passive piezoelectric response of lead zirconium titanate (PZT)-epoxy particulate composites loaded in shear is studied using analytical models, a finite element model and by experiments. The response is compared to that of the same composites when loaded in simple tension. Analogously to bulk PZT, particulate PZT-polymer composites loaded in shear show higher piezoelectric charge coefficient (d ₁₅) and energy density figure of merit (FOM₁₅) values compared to simple tension (d ₃₃) and (FOM₃₃). This outcome demonstrates the as-yet barely explored potential of piezoelectric particulate composites for optimal strain energy harvesting when activated in shear.

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