The sensitivity of piezoelectric/polymer composite materials is inversely proportional to their dielectric permittivity. Introducing a cellular structure into these composites can decrease the permittivity while enhancing their mechanical flexibility. Foaming of highly filled polymer composites is however challenging. Polymers filled with high content of dense additives such as lead zirconate titanate (PZT) exhibit significantly decreased physical foaming ability. This can be attributed to difficulty in gas diffusion, decreased fraction of the matrix available, the reduced number of nucleated cells and the difficulty in cell growth. Here, both CO 2 foaming and Expancel foaming were examined as potential methods to fabricate low-density thermoplastic polyurethane (TPU)/ PZT composite foams. While composites containing up to only 10vol.% PZT could be foamed using CO 2 , Expancel foaming could successfully yield highly-expanded composite foams containing up to 40vol.% (80wt.%) PZT. Dispersed Expancel particles in TPU/PZT composites acted as the blowing agent, activated by subjecting the samples to high temperatures using a hot press. Using Expancel, foams with expansion ratios of up to 9 were achieved. However, expansion ratios of greater than 4 were not of interest due to their poor structural integrity. The density of solid samples ranged from 1.8 to 3.3 g.cm −3 and dropped by a maximum of 80%, even for the highest PZT content, at an expansion ratio of 4. As the expansion increased, the dielectric permittivity of both CO 2 -foamed and Expancel-foamed TPU/PZT composites decreased significantly (up to 7.5 times), while the dielectric loss and electrical conductivity were affected only slightly. This combination of properties is suitable for high-sensitivity and flexible piezoelectric applications.

This study develops highly flexible, adaptable, portable gauges that give real-time measurements in many applications where compression is of interest. They can be embedded into elastomeric foams, and preserve the desirable physical properties of the foams in dispersing impact energy. We anticipate that these novel and inexpensive sensors will enable real-time measurement of human impacts and athletic performance based on data collected in the field, rather than the current standard of trying to replicate these experiences in the lab. In previous work, we have validated the performance of tensile strain sensors based on a similar technology embedded in thin sheets of silicone. These sensors are capable of measuring up to 50% strains in real time with minimal interference in tissue motion. With the addition of the sensors described in the present work, it is possible to measure both tensile and compressive strains.

The use of energy harvesting systems to provide power to low-power electronic devices has the potential to create autonomous, self-powered electronics. While research has been performed to study the harvesting of ambient energy through a wide variety of transduction mechanisms, this paper presents the investigation of a novel material for vibration-based energy harvesting. Piezoelectret foam, a polymer-based electret material exhibiting piezoelectric properties, is investigated for low-power energy generation. An overview of the fabrication and operation of piezoelectret foams is first given. Mechanical testing is then performed to evaluate the tensile properties of the material, where anisotropy in the length direction is found along with Young’s moduli between 0.5–1 GPa and tensile strengths from 35–70 MPa. Dynamic electromechanical characterization is performed in order to measure the piezoelectric d 33 coefficient of the foam over a wide frequency range. The d 33 coefficient is found to be relatively constant at 35 pC/N from 5 Hz – 1 kHz. Lastly, energy harvesting tests are performed to evaluate the ability of piezoelectric foam to harvest vibration energy. Frequency response measurements of foam samples excited along the length direction confirm the anisotropic behavior of the material. Harmonic excitation of a pre-tensioned 15.2 cm × 15.2 cm sample at a frequency of 60 Hz and displacement of ± 73 μm yields an average power of 5.8 μW delivered to a 1 mF storage capacitor through a simple diode bridge rectifier. The capacitor is charged to 4.67 V in 30 minutes, proving the ability of piezoelectret foam to supply power to low-power electronics.

The aim of this study is to evaluate the influence of microstructure, specifically, variations in foam cell size and density, on the compressive stress-strain behavior of polymeric foam material. Foams with varying densities are examined under incremental loadings to different prescribed strain levels. A comparison is made of the maximum stress level attained during deformation and residual strain on complete unloading and following recovery. Additionally, the durability of the foam material on exposure to service environment, namely, exposure to ultra-violet (UV) light and humidity, is considered. Cylindrical compression samples are exposed to Xenon Arc (63°C, 18 minutes water and light/102 minutes light only) and spectral intensity of 0.3 to 0.4 watts/m 2 for 288 cycles. Parameters investigated include changes in modulus, maximum stress, residual strain and linear shape recovery due to conditioning and mechanical cycling.

A metallic cellular materials containing polymer was fabricated by the penetrating polymer into metal foam. The aluminum and stainless steel foams were selected for the metal foam and epoxy resin and polyurethane resin were selected for the penetrated polymer. The mechanical, damping shock absorbing properties of this material were measured. The results of the compressive tests show that this material has different stress-strain curves among the specimens that include different materials in the cells. Also, these results show that this material has high-energy absorption. The internal friction of this material was measured and the result shows that the internal friction of this material is larger than that of pure aluminum closed cellular material without any polymer and change with increasing of temperature. The shock absorbability of this material is larger than that of polymer and smaller than that of metallic cellular material.

Cellular materials have unique thermal, acoustic, damping and energy absorbing properties that can be combined with their structural efficiency. Therefore, many kinds of cellular materials have been developed and tested as energy absorbing and damping materials. Particularly, closed cellular materials are thought to have many favorable properties and applications. In this study, a metallic closed cellular materials containing polymer was fabricated by the penetrating polymer into metal foam. The aluminum and stainless steel foams were selected for the metal foam and epoxy resin and polyurethane resin were selected for the penetrated polymer. The mechanical and damping properties of this material were measured. The results of the compressive tests show that this material has different stress-strain curves among the specimens that include different materials in the cells. Also, These results show that this material has high-energy absorption. The internal friction of this material was measured and the result shows that the internal friction of this material is larger than that of pure aluminum closed cellular material without any polymer and change with increasing of temperature.

Many materials require functionally graded cellular microstructures whose porosity (i.e. ratio of the void to solid volume of a material) is engineered to meet specific requirements. Indeed numerous applications have demonstrated the engineering potential of porous materials (e.g. polymeric foams) in areas ranging from biomaterial science through to structural engineering. Although a huge variety of foams can be manufactured with homogenous porosity, for heterogeneous foams there are no generic processes for controlling the distribution of porosity throughout the resulting matrix. Motivated by the desire to create a flexible process for engineering heterogeneous foams, this paper reports how ultrasound, applied during some of the foaming stages of a polyurethane (PU) melt, affects both the cellular structure and distribution of the pore size. The experimental results allowed an empirical understanding of how the parameters of ultrasound exposure (i.e. frequency and acoustic pressure) influenced the volume and distribution of pores within the final polyurethane matrix: the data demonstrates that porosity (i.e. volume fraction) varies in direct proportion to the acoustic pressure magnitude of the ultrasound signal. The effects of ultrasound on porosity demonstrated by this work offer the prospect of a manufacturing process that can adjust the cellular geometry of foam and hence ensure that the resulting characteristics match the functional requirements.

This study details the synthesis and characterization of composites and composite foams of low-density polyethylene (LDPE) and multi-walled carbon nanotubes (MWCNT). LDPE-MWCNT composites were prepared by melt blending the components in a twin screw compounder and their foams were produced by batch foaming using CO 2 as the blowing agent. The composites were characterized for dispersion using SEM and image results indicate good dispersability of MWCNT in LDPE with the formation of a MWCNT network in the LDPE matrix. Thermal and rheological properties of the composites were characterized and results indicate that even a small amount (1 wt.%) of MWCNTs can significantly affect the crystallization kinetics and the rheological behavior. Batch foaming results of the composites depict MWCNTs as heterogeneous nucleation sites for gas bubbles as indicated by the increase in cell density of the composite foams when compared to LDPE foams.

Uncertainties in material properties, geometry, manufacturing processes, and operational environments are clearly critical at all scales (nano-, micro-, meso-, and macro-scale). Specifically, reliabilty analysis in mesostructured materials can be driven by these uncertainties. The concept of mesostructured materials is motivated by the desire to put material only where it is needed for a specific application. This research develops a reliability-based synthesis method to design mesostructures under uncertainty, which have superior structural compliant performance per weight than parts with bulk material or foams. The efficiency of the proposed framework is achieved with the combination of topology optimization and stochastic approximation which utilizes stochastic local regression and Latin Hypercube Sampling. The effectiveness of the proposed framework was demonstrated using a ground structure topology optimization approach.