Fully Printed Sensors for In Situ Temperature, Heat Flow, and Thermal Conductivity Measurements in Flexible Devices.

Flexible temperature sensors allow temperature monitoring in wearable healthcare devices. A temperature sensor, which can be printed on flexible substrates, is designed and fabricated using a low-cost silver particle ink and a fast and scalable screen-printing process. A high temperature resolution of 10 m°C is reached. The versatility of this temperature sensor design is demonstrated for various applications, including in situ heat flux measurements, where a 2 mW cm-2 resolution is reached, and thermal conductivity measurements on polymer films as thin as 25 µm, with a wide range of accessible values from ∼0.1 to 0.8 W K-1 m-1.

Bio-Based Poly(hydroxy urethane)s for Efficient Organic High-Power Energy Storage.

Fast, low-cost, and efficient energy storage technologies are urgently needed to balance the intermittence of sustainable energy sources. High-power capacitors using organic polymers offer a green and scalable answer. They require dielectrics with high permittivity (εr) and breakdown strength (EB), which bio-based poly(hydroxy urethane)s (PHUs) can provide. PHUs combine high concentrations of hydroxyl and carbamate groups, thus enhancing their εr, and a highly tunable glass transition (Tg), which dictates the regions of low dielectric losses. By reacting erythritol dicarbonate with bio-based diamines, fully bio-based PHUs were synthesized with Tg ∼ 50 °C, εr > 8, EB > 400 MV·m-1, and low losses (tan δ < 0.03). This results in energy storage performance comparable with the flagship petrochemical materials (discharge energy density, Ue > 6 J·cm-3) combined with a remarkably high discharge efficiency, with η = 85% at EB and up to 91% at 0.5 EB. These bio-based PHUs thus represent a highly promising route to green and sustainable energy storage.

Effect of the TrFE Content on the Crystallization and SSA Thermal Fractionation of P(VDF-co-TrFE) Copolymers.

In this contribution, we study the effect of trifluoro ethylene (TrFE) comonomer content (samples with 80/20, 75/25, and 70/30 VDF/TrFE molar ratios were used) on the crystallization in P(VDF-co-TrFE) in comparison with a PVDF (Poly(vinylidene fluoride)) homopolymer. Employing Polarized Light Optical Microscopy (PLOM), the growth rates of spherulites or axialites were determined. Differential Scanning Calorimetry (DSC) was used to determine overall crystallization rates, self-nucleation, and Successive Self-nucleation and Annealing (SSA) thermal fractionation. The ferroelectric character of the samples was explored by polarization measurements. The results indicate that TrFE inclusion can limit the overall crystallization of the copolymer samples, especially for the ones with 20 and 25% TrFE. Self-nucleation measurements in PVDF indicate that the homopolymer can be self-nucleated, exhibiting the classic three Domains. However, the increased nucleation capacity in the copolymers provokes the absence of the self-nucleation Domain II. The PVDF displays a monomodal distribution of thermal fractions after SSA, but the P(VDF-co-TrFE) copolymers do not experience thermal fractionation, apparently due to TrFE incorporation in the PVDF crystals. Finally, the maximum and remnant polarization increases with increasing TrFE content up to a maximum of 25% TrFE content, after which it starts to decrease due to the lower dipole moment of the TrFE defect inclusion within the PVDF crystals.

Limiting Relative Permittivity "Burn-in" in Polymer Ferroelectrics via Phase Stabilization.

VDF-based polymers, such as poly(vinylidene fluoride) (PVDF) and its copolymers, are well-known ferroelectrics of interest for numerous applications, from energy storage to electrocaloric refrigeration. However, their often complex thermal phase behavior that typically leads to a low phase-stability can drastically affect the long-term dielectric properties of this materials family. Here, we demonstrate on the example of the terpolymer P(VDF-ter-TrFE-ter-CFE) (molar ratio: 64/29/7) that by limiting mass transport/segmental chain motion both during solidification and in the solid state, a drastically smaller "burn-in" in relative permittivity, εr, is observed. Indeed, εr decreases little over time and saturates rapidly at 96-97% of its initial value. Mass transport thereby is limited by using highly entangled systems via the selection of a suitable polymer solution concentration and molecular weight. In addition, rapid solvent extraction assists in reducing unwanted relaxation processes. Accordingly, increased control of the phase stability of P(VDF-ter-TrFE-ter-CFE) is gained. Moreover, pathways are opened to reliably identify processing routes for any given VDF-based polymer, with critical information being obtained from thermal analysis and rheometry data only, enabling rapid feedback to material design, including the prediction of required molecular weights without the need for complex characterization methodologies.

Electrocaloric Enhancement Induced by Cocrystallization of Vinylidene Difluoride-Based Polymer Blends.

Active thermal control will be a major challenge of the twenty-first century, which has emphasized the need for the development of energy-efficient refrigeration techniques such as electrocaloric (EC) cooling. Highly polar semicrystalline VDF-based polymers are promising organic EC materials, however, their cooling performance, which is highly structurally dependent, needs further improvement to become competitive. Here, we report a simple method to increase the crystalline coherence of P(VDF-ter-TrFE-ter-CFE) terpolymer in the plane including the polar direction. This is achieved by blending P(VDF-ter-TrFE-ter-CFE) with minute amounts of P(VDF-co-TrFE) copolymer with similar VDF/TrFE unit content. This similarity allows for a cocrystallization of the copolymer chains in the terpolymer crystalline lamellae, preferentially extending the lateral coherence without lamellar thickening, as validated with a wide range of structural characterization. This trend results in a significant dielectric and electrocaloric enhancement, with a remarkable electrocaloric effect, ΔTEC = 5.2 K, confirmed by direct measurements for a moderate electric field of 90 MV·m-1 in a blend with 1 wt % of copolymer.

Tuning the electrocaloric enhancement near the morphotropic phase boundary in lead-free ceramics.

The need for more energy-efficient and environmentally-friendly alternatives in the refrigeration industry to meet global emission targets has driven efforts towards materials with a potential for solid state cooling. Adiabatic depolarisation cooling, based on the electrocaloric effect (ECE), is a significant contender for efficient new solid state refrigeration techniques. Some of the highest ECE performances reported are found in compounds close to the morphotropic phase boundary (MPB). This relationship between performance and the MPB makes the ability to tune the position of the MPB an important challenge in electrocaloric research. Here, we report direct ECE measurements performed on MPB tuned NBT-06BT bulk ceramics with a combination of A-site substitutions. We successfully shift the MPB of these lead-free ceramics closer to room temperature, as required for solid state refrigeration, without loss of the criticality of the system and the associated ECE enhancement.