Stephan Walter

Bio: Stephan Walter is an academic researcher from RWTH Aachen University. The author has contributed to research in topics: Fiber & Polyvinylidene fluoride. The author has an hindex of 6, co-authored 11 publications receiving 132 citations. Previous affiliations of Stephan Walter include Stora Enso.

Spinnability and Characteristics of Polyvinylidene Fluoride (PVDF)-based Bicomponent Fibers with a Carbon Nanotube (CNT) Modified Polypropylene Core for Piezoelectric Applications.

Abstract: This research explains the melt spinning of bicomponent fibers, consisting of a conductive polypropylene (PP) core and a piezoelectric sheath (polyvinylidene fluoride). Previously analyzed piezoelectric capabilities of polyvinylidene fluoride (PVDF) are to be exploited in sensor filaments. The PP compound contains a 10 wt % carbon nanotubes (CNTs) and 2 wt % sodium stearate (NaSt). The sodium stearate is added to lower the viscosity of the melt. The compound constitutes the fiber core that is conductive due to a percolation CNT network. The PVDF sheath's piezoelectric effect is based on the formation of an all-trans conformation β phase, caused by draw-winding of the fibers. The core and sheath materials, as well as the bicomponent fibers, are characterized through different analytical methods. These include wide-angle X-ray diffraction (WAXD) to analyze crucial parameters for the development of a crystalline β phase. The distribution of CNTs in the polymer matrix, which affects the conductivity of the core, was investigated by transmission electron microscopy (TEM). Thermal characterization is carried out by conventional differential scanning calorimetry (DSC). Optical microscopy is used to determine the fibers' diameter regularity (core and sheath). The materials' viscosity is determined by rheometry. Eventually, an LCR tester is used to determine the core's specific resistance.

Structure, properties, and phase transitions of melt‐spun poly(vinylidene fluoride) fibers

Abstract: Three different experimental techniques were used to study structural phase transitions in melt-spun poly(vinylidene fluoride) fibers, which were produced with different process parameters and processed in the draw-winding process at different temperatures and draw ratios. The fibers are examined with the help of wide-angle X-ray diffraction at elevated temperatures, differential scanning calorimetry with stochastic temperature modulation, and dynamic mechanical analysis. An oriented mesophase and deformed crystal structures can be observed in all fibers and assigned to the mechanical stress occurring in the processes. Furthermore, several phase transitions during melting and two mechanical relaxation processes could be detected. The observed transitions affect the crystal geometry, the orientation distribution, anisotropic thermal expansion, and the mechanic response of the fiber samples. The relaxation processes can be related with an increasing amount of crystalline β-phase in fibers drawn at different temperatures. The detailed information about phase transitions and the related temperatures are used to produce fibers with an extended amount of β-phase crystallites, which are responsible for piezoelectric properties of the material. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Thermal Analysis of Phase Transitions and Crystallization in Polymeric Fibers

Abstract: Each year about 50 Million tons polymer is processed to fibers worldwide [1]. Polymeric fibers are manufactured into all sorts of daily as well as industrial goods [2, 3]. The most prominent materials are thermoplastic among which poly(ethylene terephtalat) (PET), polyamides (PA) and polypropylene (PP) make up the largest fraction [4]. Other thermoplastic polymers such as poly(vinylidene fluoride) (PVDF) belong to niche markets with highly specialized applications [5-7].

Characterisation of piezoelectric PVDF monofilaments

Abstract: The piezoelectric effect of poly(vinylidene fluoride) is demonstrated in monofilaments These fibres produce electrical signal upon mechanical deformation The structure of the monofilaments is analysed by wide angle X-ray diffraction and differential scanning calorimetry, whereas the crystalline fraction mainly consists of the piezoelectric β phase For polarisation and electromechanical characterisation, poly(vinylidene fluoride)-epoxy composites are manufactured, whereas all the filaments are aligned parallel to the composite structure The electric signal is two magnitudes of order larger for the polarised samples compared to the unpolarised ones Furthermore, there is a clear anisotropy, allowing the direction dependent measurement of stress and strain Mechanical stress in the fibre direction produces a much larger signal compared to the same stress in the perpendicular direction

Modification of the mechanical properties of polyamide 6 multifilaments in high-speed melt spinning with nano silicates

Abstract: In the last few years research activities have been focused on the modification of fiber properties with nano-scaled additives. One of the most important fields of research is the alteration of mechanical properties such as the tenacity and the specific breaking load. In this study, we determined the influence of nano-phyllosilicates on the drawability of polyamide 6 multifilament yarns. It was first demonstrated that the drawability of the fibers drastically increased in an industrially relevant high-speed melt spinning process. Structural properties of the material are identified by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Changes in the crystalline properties as well as in the alignment of the silicates are compared with the stress–strain curves of the fibers, and a molecular mechanism for the drawing process is derived from these experiments. In a first step, a significant phase transition in the crystalline structure unaffected by the silicates occurs for low d...

Novel “3-D spacer” all fibre piezoelectric textiles for energy harvesting applications

Abstract: The piezoelectric effect in poly(vinylidene fluoride), PVDF, was discovered over four decades ago and since then, significant work has been carried out aiming at the production of high β-phase fibres and their integration into fabric structures for energy harvesting. However, little work has been done in the area of production of “true piezoelectric fabric structures” based on flexible polymeric materials such as PVDF. In this work, we demonstrate “3D spacer” technology based all-fibre piezoelectric fabrics as power generators and energy harvesters. The knitted single-structure piezoelectric generator consists of high β-phase (∼80%) piezoelectric PVDF monofilaments as the spacer yarn interconnected between silver (Ag) coated polyamide multifilament yarn layers acting as the top and bottom electrodes. The novel and unique textile structure provides an output power density in the range of 1.10–5.10 μW cm−2 at applied impact pressures in the range of 0.02–0.10 MPa, thus providing significantly higher power outputs and efficiencies over the existing 2D woven and nonwoven piezoelectric structures. The high energy efficiency, mechanical durability and comfort of the soft, flexible and all-fibre based power generator are highly attractive for a variety of potential applications such as wearable electronic systems and energy harvesters charged from the ambient environment or by human movement.

Tactile-Sensing Based on Flexible PVDF Nanofibers via Electrospinning: A Review

Abstract: The flexible tactile sensor has attracted widespread attention because of its great flexibility, high sensitivity, and large workable range. It can be integrated into clothing, electronic skin, or mounted on to human skin. Various nanostructured materials and nanocomposites with high flexibility and electrical performance have been widely utilized as functional materials in flexible tactile sensors. Polymer nanomaterials, representing the most promising materials, especially polyvinylidene fluoride (PVDF), PVDF co-polymer and their nanocomposites with ultra-sensitivity, high deformability, outstanding chemical resistance, high thermal stability and low permittivity, can meet the flexibility requirements for dynamic tactile sensing in wearable electronics. Electrospinning has been recognized as an excellent straightforward and versatile technique for preparing nanofiber materials. This review will present a brief overview of the recent advances in PVDF nanofibers by electrospinning for flexible tactile sensor applications. PVDF, PVDF co-polymers and their nanocomposites have been successfully formed as ultrafine nanofibers, even as randomly oriented PVDF nanofibers by electrospinning. These nanofibers used as the functional layers in flexible tactile sensors have been reviewed briefly in this paper. The β-phase content, which is the strongest polar moment contributing to piezoelectric properties among all the crystalline phases of PVDF, can be improved by adjusting the technical parameters in electrospun PVDF process. The piezoelectric properties and the sensibility for the pressure sensor are improved greatly when the PVDF fibers become more oriented. The tactile performance of PVDF composite nanofibers can be further promoted by doping with nanofillers and nanoclay. Electrospun P(VDF-TrFE) nanofiber mats used for the 3D pressure sensor achieved excellent sensitivity, even at 0.1 Pa. The most significant enhancement is that the aligned electrospun core-shell P(VDF-TrFE) nanofibers exhibited almost 40 times higher sensitivity than that of pressure sensor based on thin-film PVDF.

Energy harvesting textiles for a rainy day: woven piezoelectrics based on melt-spun PVDF microfibres with a conducting core

Abstract: Recent advances in ubiquitous low-power electronics call for the development of light-weight and flexible energy sources. The textile format is highly attractive for unobtrusive harvesting of energy from e.g., biomechanical movements. Here, we report the manufacture and characterisation of fully textile piezoelectric generators that can operate under wet conditions. We use a weaving loom to realise textile bands with yarns of melt-spun piezoelectric microfibres, that consist of a conducting core surrounded by β-phase poly(vinylidene fluoride) (PVDF), in the warp direction. The core-sheath constitution of the piezoelectric microfibres results in a—for electronic textiles—unique architecture. The inner electrode is fully shielded from the outer electrode (made up of conducting yarns that are integrated in the weft direction) which prevents shorting under wet conditions. As a result, and in contrast to other energy harvesting textiles, we are able to demonstrate piezoelectric fabrics that do not only continue to function when in contact with water, but show enhanced performance. The piezoelectric bands generate an output of several volts at strains below one percent. We show that integration into the shoulder strap of a laptop case permits the continuous generation of four microwatts of power during a brisk walk. This promising performance, combined with the fact that our solution uses scalable materials and well-established industrial manufacturing methods, opens up the possibility to develop wearable electronics that are powered by piezoelectric textiles.

Poling and characterization of piezoelectric polymer fibers for use in textile sensors

Abstract: This study reports on the poling and characteristics of a melt-spun piezoelectric bicomponent fiber with poly(vinylidene fluoride) (PVDF) as its sheath component and a conductive composite with car ...