Thermal, Mechanical, and Acoustic Properties of Polydimethylsiloxane Filled with Hollow Glass Microspheres.

Polydimethylsiloxane (PDMS) is the most widely used silicon-based polymer due to its versatility and its various attractive properties. The fabrication of PDMS involves liquid phase cross-linking to obtain hydrophobic and mechanically flexible material in the final solid form. This allows to add various fillers to affect the properties of the resulting material. PDMS has a relatively low Thermal Conductivity (TC), in the order of 0.2 W/mK, which makes it attractive for thermal insulation applications such as sealing in construction. Although a further decrease in the TC of PDMS can be highly beneficial for such applications, most research on the thermal properties of PDMS composites have focused on fillers that increase the TC rather than decrease it. In the present work, we propose a simple and reliable method for making a PDMS-based composite material with significantly improved thermal insulation properties, by adding hollow glass microspheres (HGMs) to the mixture of the liquid base and the cross-linker (10:1 ratio), followed by degassing and heat-assisted crosslinking. We obtained a 31% reduction of thermal conductivity and a 60% increase in the elastic modulus of samples with HGM content of 17% by weight. At the same time, the sound insulation capacity of the PDMS-HGM composite is slightly decreased in comparison to pure PDMS, as a result of its lower density. Finally, the wettability of the samples had no dependence on HGM content.

Complex tribomechanical characterization of ZnO nanowires: nanomanipulations supported by FEM simulations.

In the present work, we demonstrate a novel approach to nanotribological measurements based on the bending manipulation of hexagonal ZnO nanowires (NWs) in an adjustable half-suspended configuration inside a scanning electron microscope. A pick-and-place manipulation technique was used to control the length of the adhered part of each suspended NW. Static and kinetic friction were found by a 'self-sensing' approach based on the strain profile of the elastically bent NW during manipulation and its Young's modulus, which was separately measured in a three-point bending test with an atomic force microscope. The calculation of static friction from the most bent state was completely reconsidered and a novel more realistic crack-based model was proposed. It was demonstrated that, in contrast to assumptions made in previously published models, interfacial stresses in statically bent NW are highly localized and interfacial strength is comparable to the bending strength of NW measured in respective bending tests.

Mechanical properties of sol-gel derived SiO2 nanotubes.

The mechanical properties of thick-walled SiO2 nanotubes (NTs) prepared by a sol-gel method while using Ag nanowires (NWs) as templates were measured by using different methods. In situ scanning electron microscopy (SEM) cantilever beam bending tests were carried out by using a nanomanipulator equipped with a force sensor in order to investigate plasticity and flexural response of NTs. Nanoindentation and three point bending tests of NTs were performed by atomic force microscopy (AFM) under ambient conditions. Half-suspended and three-point bending tests were processed in the framework of linear elasticity theory. Finite element method simulations were used to extract Young's modulus values from the nanoindentation data. Finally, the Young's moduli of SiO2 NTs measured by different methods were compared and discussed.

Shape restoration effect in Ag-SiO2 core-shell nanowires.

The combination of two different materials in a single composite core-shell heterostructure can lead to improved or even completely novel properties. In this work we demonstrate the enhancement of the mechanical properties of silver (Ag) nanowires (NW) achieved by coating them with a silica (SiO2) shell. In situ scanning electron microscope (SEM) nanomechanical tests of Ag-SiO2 core-shell nanowires reveal an improved fracture resistance and an electron-beam induced shape restoration effect. In addition, control experiments are conducted separately on uncoated Ag NWs and on empty SiO2 shells in order to gain deeper insight into the peculiar properties of Ag-SiO2. Test conditions are simulated using finite-element methods; possible mechanisms responsible for the shape restoration and the enhanced fracture resistance are discussed.

Some aspects of formation and tribological properties of silver nanodumbbells.

In this paper, metal nanodumbbells (NDs) formed by laser-induced melting of Ag nanowires (NWs) on an oxidized silicon substrate and their tribological properties are investigated. The mechanism of ND formation is proposed and illustrated with finite element method simulations. Tribological measurements consist in controllable real-time manipulation of NDs inside a scanning electron microscope (SEM) with simultaneous force registration. The geometry of NDs enables to distinguish between different types of motion, i.e. rolling, sliding and rotation. Real contact areas are calculated from the traces left after the displacement of NDs and compared to the contact areas predicted by the contact mechanics and frozen droplet models. PACS: 81.07.-b; 62.25.-g; 62.23.Hj.

Manipulation of nanoparticles of different shapes inside a scanning electron microscope.

In this work polyhedron-like gold and sphere-like silver nanoparticles (NPs) were manipulated on an oxidized Si substrate to study the dependence of the static friction and the contact area on the particle geometry. Measurements were performed inside a scanning electron microscope (SEM) that was equipped with a high-precision XYZ-nanomanipulator. To register the occurring forces a quartz tuning fork (QTF) with a glued sharp probe was used. Contact areas and static friction forces were calculated by using different models and compared with the experimentally measured force. The effect of NP morphology on the nanoscale friction is discussed.

Simultaneous measurement of static and kinetic friction of ZnO nanowires in situ with a scanning electron microscope.

A novel method for in situ measurement of the static and kinetic friction is developed and demonstrated for zinc oxide nanowires (NWs) on oxidised silicon wafers. The experiments are performed inside a scanning electron microscope (SEM) equipped with a nanomanipulator with an atomic force microscope tip as a probe. NWs are pushed by the tip from one end until complete displacement is achieved, while NW bending is monitored by the SEM. The elastic bending profile of a NW during the manipulation process is used to calculate the static and kinetic friction forces.