Stefan Kirchberg

Thermal diffusivity and mechanical properties of polymer matrix composites

Polypropylene–iron-silicon (FeSi) composites with spherical particles and filler content from 0 vol. % to 70 vol. % are prepared by kneading and injection molding. Modulus, crystallinity, and thermal diffusivity of samples are characterized with dynamic mechanical analyzer, differential scanning calorimeter, and laser flash method. Modulus as well as thermal diffusivity of the composites increase with filler fraction while crystallinity is not significantly affected. Measurement values of thermal diffusivity are close to the lower bound of the theoretical Hashin-Shtrikman model. A model interconnectivity shows a poor conductive network of particles. From measurement values of thermal diffusivity, the mean free path length of phonons in the amorphous and crystalline structure of the polymer and in the FeSi particles is estimated to be 0.155 nm, 0.450 nm, and 0.120 nm, respectively. Additionally, the free mean path length of the temperature conduction connected with the electrons in the FeSi particles toget...

Fabrication of long-focal-length plano–convex microlens array by combining the micro-milling and injection molding processes

A uniform plano-convex spherical microlens array with a long focal length was fabricated by combining the micromilling and injection molding processes in this work. This paper presents a quantitative study of the injection molding process parameters on the uniformity of the height of the microlenses. The variation of the injection process parameters, i.e., barrel temperature, mold temperature, injection speed, and packing pressure, was found to have a significant effect on the uniformity of the height of the microlenses, especially the barrel temperature. The filling-to-packing switchover point is also critical to the uniformity of the height of the microlenses. The optimal uniformity was achieved when the polymer melts completely filled the mold cavity, or even a little excessively filled the cavity, during the filling stage. In addition, due to the filling resistance, the practical filling-to-packing switchover point can vary with the change of the filling processing conditions and lead to a non-negligible effect on the uniformity of the height of the microlenses. Furthermore, the effect of injection speed on the uniformity of the height of the microlenses was analyzed in detail. The results indicated that the effect of injection speed on the uniformity of the height of the microlenses is mainly attributed to the two functions of injection speed: transferring the filling-to-packing switchover point and affecting the distribution of residual flow stress in the polymer melt.

Nanoparticles in Organic Solvents with Polymers – Stability and Consequences upon Material Synthesis through Spray Drying and Melt Moulding

It is a well accepted fact that nanoparticles and their industrial and commercial use have the potential to improve properties of modern materials substantially. Especially polymeric materials are attractive for incorporating nanoparticles with special properties only apparent in the nanoscale. One main consideration in processing such composite materials is to prevent the nanoparticles from agglomerating or even worse aggregating. In this paper a modular process method is presented based on a mixture of sterically stabilized nanoparticles in an organic solvent with soluble polymers and subsequent spray drying to quickly yet thermally carefully remove the solvent. The method has the potential for large scale production of highly filled nanoparticle-polymer-composites. However the bottleneck of this method is the unknown interaction of polymers and stabilized nanoparticles. We present the impact of depletion flocculation and subsequent phase separation or stabilization by adsorbing polymers on the dispersion of the nanoparticles. It is shown that the processing method is more adequate when compared to traditional melt moulding. The best magnetite nanoparticle stability in dichloromethane is achieved using ricinoleic acid. Besides flocculates we can identify separate primary particles in the composite. The size of the floccules is in the lower micrometer range for nanoparticles 15 nm in size.

Theoretical consideration of experimental data of thermal and magnetic properties of polymer-bonded soft magnetic composites

Spherical iron silicon (FeSi) particles and irregular shaped magnetite (Fe3O4) particles with particle sizes ≤146 μm and varying volume filler fractions up to x = 0.7 (70 vol%) were mixed with polypropylene (PP) matrix. The samples were prepared by kneading and injection moulding and show particle–particle interaction at elevated filler fraction of x ≥ 0.5. Thermal and magnetic properties of the composites were characterized and show a significant increase at filler fractions of x ≥ 0.3. The thermal conductivity of PP (0.176 W/(m K)) was increased up to seven times to 1.239 W/(m K) at x = 0.7. FeSi-filled composites show slightly higher values of thermal conductivity than Fe3O4-filled composites. The magnetic permeability of the composites rise from 1 for PP to a maximum value of 23.1 for PP/FeSi composites at x = 0.7. The nonlinear increase in the thermal conductivity corresponds with the lower boundary of the Hashin–Shtrikman model. The Bruggeman model can be applied to describe the nonlinear increase in the magnetic permeability. Magnetic permeability increases with mean particle diameter as well as magnetic coercivity and loss dissipation increases with inverse mean particle diameter.

Replication, Characterization and Simulation of Micro Injection Molded Microlens Arrays Using Technical Amorphous Polymers

Novel results relating to the characterization of microlens array production by simulation and micro injection molding were examined to produce precise micro-optics from Polymethylmethacrylate (PMMA), Polycarbonate (PC) as well as Polystyrene (PS). The microlens arrays show 100 lenses in a 10×10 array with a lens radius of 271 µm and a lens depth of 45 µm. The pitches between the lenses are fixed at 800 µm. The surface of the ultra-precision diamond end-ball milled microlens array mold was polished to reduce the surface roughness of the injection molded specimen. The injection molding parameters were optimized to get precise lens geometries with high shrinkage uniformity by simulation and experimental methods. The injection molding results show precise injection molded microlens arrays and PC microlens array shows the best geometrical results compared to the microstructured insert.

Effect of compounding principles on thermal, mechanical and magnetic performance of soft magnetic polymethylmethacrylate/Fe3O4 nanocomposites

In this study, the effect of compounding principles on the properties of Polymer Bonded Soft Magnetic Nanocomposites (PBSMNs) was discussed. The polymethylmethacrylate /Fe3O4 magnetic nanocomposites (Fe3O4: 30 wt%) were prepared by the in situ process based on the solution and spray drying method, as well as by the ex situ process based on the kneading machine. As reference, the process combining these two compounding principles was also carried out for the PBSMN preparation, named as in-between process. The morphology structures, thermal, mechanical and magnetic properties of the magnetic nanocomposites achieved with different compounding principles were characterized. The results show that compounding principles have significant influence on the properties of the magnetic polymer nanocomposites. In the end, their contributions to the power electronic applications were discussed as well.

Simulation Research of the Effect of Thermal Reflow Processing Parameters on the Profile of Micro-Spherical Structure

In this paper reflow process is integrated into LIGA process in order to realize three dimensional fabrication. The melting and deformation process of photoresist, which has an initial cylindrical shape of D500μm and h66μm, and the influence of various processing parameters on the height of formed microlens, is simulated with Marc. The optimum processing parameters combination is obtained by orthogonal experiment method and the influence of different processing parameters on the height of micro-spherical structure is studied with single factor experiment method. The results showed that the optimum processing parameters combination was 1°C /s for heating rate, 110°C for heating temperature and 45min for holding time; the significance of processing parameters on the micro-spherical structure’s height can be ordered in holding time＞heating rate＞heating temperature.

Correlation Between Thermal Diffusivity and Dynamic Mechanical Properties of Soft Magnetic Particle Filled Thermoplastic Composites

Measurements of thermal diffusivity α by laser flash method (LFA) and storage modulus E′ by dynamic mechanical analysis (DMA) have been performed on polypropylene-iron silicon (PP/FeSi6.8) composites with filler particle content from 10 vol.-% to 60 vol.-% at temperatures from 300 K to 415 K and 200 K to 425 K, respectively. The thermal diffusivity induces a decline with increased temperature for all examined materials. The drop in thermal diffusivity versus temperature corresponds with the filler fraction. Observing the behavior of the storage modulus of FeSi6.8 filled PP one can show that the larger the filler fraction of particles in the polymer the stronger is the decrease of storage modulus versus temperature. The inflection point of the dynamic mechanical measurement curve at about 360 K corresponds very well with starting increase of thermal conductivity. The achieved results can be explained by the relation of the velocity v of phonons and its free path length l to the thermal diffusivity α according to α = (1/3)vl (Einstein approximation). Moreover, v is correlated to the bulk modulus K (and specific density ρ) via v ≈ (K/ρ)0.5 .© 2011 ASME