Dielectric properties of silver nanoparticles coated with silica shells of different thicknesses

Core/shell nanoparticles having metallic silver nanoparticle cores of similar to 38 nm in diameter and silica shells of different thicknesses ranging from similar to 3.6-20 nm were prepared. For the silica coating, a slightly modified Stober method was used which allowed preparing grams of core/shell nanoparticles for the first time. The particles were characterized by UV-vis spectroscopy, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray scattering. Their dielectric properties were measured as pellets in parallel-plate capacitors. It was found that the permittivity is much influenced by the silica shell thickness with an increase in permittivity for thinner shells. A shell thickness of 20 +/- 2 nm allowed fabrication of capacitors which have similar characteristics to those of silica, thus, there is no influence of the metal core on the dielectric properties anymore. However, by decreasing the silica shell to 17 +/- 2, 8 +/- 1.5, and 6.6 +/- 1.5 nm the permittivity at high frequencies is increasing from 10, 34, to 41, respectively. The insulator to metal transition was observed for a silica shell thickness of 3.6 +/- 1 nm. Functionalization of the silica surface with a hydrophobic coating removes surface adsorbed water as observed by the flat dielectric permittivity over a large frequency domain

Hydrophobization of silver nanoparticles through surface-initiated atom transfer radical polymerization

This work describes the synthesis of poly(methyl methacrylate) (PMMA)-coated Ag@SiO2 core-shell particles by conducting a Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP). Silver nanoparticles (AgNPs) were first prepared by the polyol synthesis and subjected to preliminary surface functionalization which includes the encapsulation in an insulating SiO2 shell by an optimized Stober method and functionalization of the shell with an ATRP initiator. The growth of an organic PMMA shell yields inorganic/organic core-shell particles with an enhanced dispersibility in non-polar solvents

Synthesis of silicone elastomers containing trifluoropropyl groups and their use in dielectric elastomer transducers

Vinyl end-functionalized polysiloxanes P-x containing varying mol% of trifluoropropyl groups (x) were prepared starting from 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane (F-3) and octamethylcyclotetrasiloxane (D-4) via anionic polymerization in the presence of tetramethylammonium hydroxide (TMAH) and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane end-capping reagent. Their structures were determined by H-1 NMR spectroscopy and their molecular weights and distributions were measured by GPC. The various P-x were cross-linked in thin films via hydrosilylation of the vinyl groups with tetrakis(dimethylsiloxy) silane cross-linker in the presence of Karstedt catalyst. The mechanical, dielectric and electromechanical properties of the prepared films were investigated. An increase in the permittivity (epsilon') with increasing content of polar trifluoropropyl groups was observed with a maximum value of epsilon' = 6.4 for P-58(0). A maximum lateral actuation strain of 5.4% at an electric field as low as 7.8 V mu m(-1) was measured for a material prepared by cross-linking P-53

Conductive silicone elastomers electrodes processable by screen printing

Conductive inks consisting of graphene and carbon black conductive fillers into a polydimethylsiloxane (PDMS) matrix, which can be processed into thin films by screen printing are developed. The influence of filler composition and content on mechanical and electrical properties of the conductive composites is investigated. The best composites were evaluated as electrode material for dielectric elastomer actuators and for piezoelectric sensors. With increasing filler content, the electrical properties of the resulting composites of graphite nanoplates (GNPs) or a binary mixture of GNPs and carbon black (CB) with PDMS (M-w, =139 kg/mol) are enhanced. Hence, PDMS composites filled with GNPs (42 wt.%) or a binary mixture of GNPs/CB (300/150 ratio, 30 wt.% of total filler loading) exhibited constant contact resistance values of 0.5 and 5 Omega determined in life-cycle test, respectively, thus rendering them suitable as electrode materials for piezosensors. On the other hand, dielectric elastomer actuators require more flexible electrode materials, which could be tuned by varying the polymer molecular weight and by reducing the filler content. Therefore, a composite consisting of PDMS (M-w = 692 kg/mol) and a binary filler mixture of GNPs/CB (150/75 ratio, 18 wt.% of total filler loading) was used for producing the electrodes of dielectric elastomer transducers (DETs). The produced DETs with different electrode thicknesses were characterized in terms of their performance. The negligible hysteresis of the electrode materials is favorable for sensor and actuator applications

Dielectric Materials, Design and Realization

It has been the dream of many scientists to create polymeric materials which exhibit simultaneously high dielectric permittivity, low glass transition temperature, and excellent elastic properties. Such materials would be a highly attractive dielectric in electromechanical transducers. Within this topic we are focusing on silicones because of their excellent elastic properties over wide temperature and frequency ranges combined with low glass transition temperatures. To increase their low permittivity, we followed different approaches which include: blending the matrix with highly polarizable conductive and polar nanofillers and chemical modification with polar side groups. This presentation will show the advantages and disadvantages of the two strategies we have been following and will provide an assessment of their future potential

Continuous Production of Tailored Silver Nanoparticles by Polyol Synthesis and Reaction Yield Measured by X-ray Absorption Spectroscopy: Toward a Growth Mechanism

Two complementary topics are discussed: (i) the continuous production of silver nanoparticles (AgNPs) and (ii) the measurement of reaction yield by X-ray absorption near-edge spectroscopy (XANES). The continuous polyol synthesis of AgNPs in grams amount and in the size range of 7-104 nm was carried out in the segmented flow tubular reactor (SFTR). Particle size was tailored by controlling the synthesis parameters such as temperature, reactant concentrations, and polyvinylpyrrolidone (PVP) molecular weight. The continuous production was tested for 4 h, and the product has shown constant particle size distribution over the whole production time. Reliable continuous production of 2.3 g h(-1) of similar to 100 nm AgNPs can be achieved with a lab-scale SFTR. The produced particles were fully characterized with respect to size, size distribution, and chemical purity. To better understand the growth mechanism, synchrotron-radiation high-resolution X-ray diffraction, and X-ray absorption spectroscopy data were collected directly on the AgNPs suspension. In particular, from}CANES experiments, the conversion yield of Ag+ to Ag-0 was measured. The results are consistent with a two-step process where PVP controls the particle nucleation while growth is ensured by thermally induced ethylene glycol oxidation

Enhanced dispersion stability of gold nanoparticles by the physisorption of cyclic poly(ethylene glycol)

Nano-sized metal particles are attracting much interest in industrial and biomedical applications due to the recent progress and development of nanotechnology, and the surface-modifications by appropriate polymers are key techniques to stably express their characteristics. Herein, we applied cyclic poly(ethylene glycol) (c-PEG), having no chemical inhomogeneity, to provide a polymer topology-dependent stabilization for the surface-modification of gold nanoparticles (AuNPs) through physisorption. By simply mixing c-PEG, but not linear counterparts, enables AuNPs to maintain dispersibility through freezing, lyophilization, or heating. Surprisingly, c-PEG endowed AuNPs with even better dispersion stability than thiolated PEG (HS-PEG-OMe). The stronger affinity of c-PEG was confirmed by DLS,.-potential, and FT-IR. Furthermore, the c-PEG system exhibited prolonged blood circulation and enhanced tumor accumulation in mice. Our data suggests that c-PEG induces physisorption on AuNPs, supplying sufficient stability toward bio-medical applications, and would be an alternative approach to the gold-sulfur chemisorption

Stretchable piezoelectric elastic composites for sensors and energy generators

The search for a piezoelectric elastomer that generates an electrical signal when pressed and stretched has increased significantly in the last decade as they hold great promise in harvesting energy from human motion and monitoring human activities. Here, the excellent elasticity of polydimethylsiloxane-based elastomers and the piezoelectric properties of lead zirconate titanate (PZT) were combined and, using a thermally activated poling process, elastic piezoelectric composites were obtained. For this, two polydimethylsiloxane (PDMS) matrices with a molar mass of 139 kDa and 692 kDa and PZT fillers with particle sizes of 2 and 20 μm were used. For the same poling conditions, an increase in the piezoelectric response with increasing amount of filler, filler size and molar mass of the polymer matrix was observed. Overall, d*33 and d*31 values of 2.7–40 pC N−1 and 16–48 pC N−1 were achieved in this work with filler contents ranging from 37–72 vol%. A composite material with a PZT filler content of 38 vol% (20 μm particle size) in a commercially available PDMS with a Mw = 139 kg mol−1 exhibited high flexibility, good elasticity with long-term mechanical stretchability and high longitudinal and transverse piezoelectric coefficients of 3.6 pC N−1 and 30 pC N−1, respectively. The higher transverse piezoelectric constant d*31 can be explained by an additional capacitor effect of the composite film structure. These properties are interesting features for energy conversion from human motion, monitoring human activities, and stretchable electronics. The functionality of the newly developed material is demonstrated in a pressed sensor

Continuous Production of Tailored Silver Nanoparticles by Polyol Synthesis and Reaction Yield Measured by X‑ray Absorption Spectroscopy: Toward a Growth Mechanism

Two complementary topics are discussed: (i) the continuous production of silver nanoparticles (AgNPs) and (ii) the measurement of reaction yield by X-ray absorption near-edge spectroscopy (XANES). The continuous polyol synthesis of AgNPs in grams amount and in the size range of 7–104 nm was carried out in the segmented flow tubular reactor (SFTR). Particle size was tailored by controlling the synthesis parameters such as temperature, reactant concentrations, and polyvinylpyrrolidone (PVP) molecular weight. The continuous production was tested for 4 h, and the product has shown constant particle size distribution over the whole production time. Reliable continuous production of 2.3 g h^–1 of ∼100 nm AgNPs can be achieved with a lab-scale SFTR. The produced particles were fully characterized with respect to size, size distribution, and chemical purity. To better understand the growth mechanism, synchrotron-radiation high-resolution X-ray diffraction, and X-ray absorption spectroscopy data were collected directly on the AgNPs suspension. In particular, from XANES experiments, the conversion yield of Ag⁺ to Ag⁰ was measured. The results are consistent with a two-step process where PVP controls the particle nucleation while growth is ensured by thermally induced ethylene glycol oxidation