1,720,989 research outputs found
Active Polymer Nanocomposites: Application in Thermoplastic Polymers and in Polymer Foams
No full textLayered double hydroxides (LDH) are very used as fillers in polymer nanocomposites research. They cannot be used in their pristine form (magnesium and aluminum hydroxycarbonates) because they exhibit strong interlayer electrostatic interactions and tight basal spacings so that they require the intercalation of organic anions into clay galleries. The organic modification of LDH increases the filler-polymer compatibility, enlarges the basal spacing of the lamellae (making easier the insertion of the polymeric chains into the gallery), and promotes the exfoliation of the filler into the polymeric matrix. Organically modified LDH/polymer nanocomposites exhibit improved mechanical, thermal, and chemical properties when they are compared with pristine polymers or conventional composites. Moreover, the incorporation of LDH allows us to obtain materials with specific functionalities. For example, the intercalation of antimicrobial molecules into LDH (such as benzoate or stearate) and the use of the modified nanoparticles as fillers results in nanocomposites with antimicrobial activity. In this paper, several benzoate and/or stearate LDH/polypropylene (PP) nanocomposites have been produced for food packaging applications. The O2 permeability coefficient of the nanocomposite named 1.5% o-benzoate/1.5% o-stearate LDH/PP/3% PP-g-MAH reduces by 20% compared to unfilled PP and 30% compared to unfilled PP/3%PP-g-MAH. It is accepted that in such a nanocomposite, the lamellar structure of LDH creates a more tortuous path for the progress of oxygen molecules reducing the gas diffusion. For the novel nanocomposite material, the physical, morphological, mechanical, and gas barrier properties have been measured and the antimicrobial effect has been evaluated. In this paper, some benzoate LDH/polyurethane foam (PU) nanocomposites have been also produced. The filler dispersion into the PU has been investigated by scanning electronic microscopy. It was observed that organic LDH particles were uniformly dispersed into the walls of cells, forming small aggregates. The morphological properties of the foam have been analyzed by nanotomography. LDH/PU nanocomposite showed antibacterial and improved mechanical properties. © 2016 IEEE
Effect of cobalt and silver nanoparticles and ions on Lumbricus rubellus health and on microbial community of earthworm faeces and soil
No full textThe aim of this study was to investigate the impact of silver and cobalt, supplied both as ions and nanoparticles (Ag+, Co2+, AgNPs, CoNPs) through contaminated food to earthworms (Lumbricus rubellus), on their health as well as on microbial community of both soil and earthworm faeces. Earthworms and microbes were exposed to the contaminants in laboratory microcosms with artificial soil. Contaminants were supplied once a week for 5 weeks by spiking them on horse manure. The accumulation of CoNPs and Co2+ in earthworm tissues was two and three times greater than AgNPs and Ag+, respectively. Except for AgNPs, contaminants significantly affected microbial community structure of earthworm faeces by increasing G- bacteria, thus also increasing the bacteria/fungi ratio while decreasing the G+/G- bacteria ratio. Such shift was also reflected on soil microbial community, thus suggesting a close relationship between microbial community of soil and of earthworm faeces. Neither of the Co treatments affected soil microbial basal respiration whereas they increased the microbial biomass specific respiration or metabolic quotient, suggesting some stress induction on soil microorganisms. Earthworm health was strongly affected as revealed by the reduced fluidity of fatty acids extracted from the body tissues. In addition, the histological investigations, after the depuration period, showed positive results about the NPs toxicity. In particular, TUNEL-positive nuclei in epidermis and in peritoneum, suggest the presence of toxicosis.The ESEM-EDS technique revealed the presence of Ca-P spherules (calcification) between mouth and clitellum of earthworms fed with Co2+ contaminated food. © 2016 Elsevier B.V
Compression of polystyrene and polypropylene foams for energy absorption applications: A combined mechanical and microstructural study
No full textIn many applications, polymeric foams (such as expanded polystyrene or expanded polypropylene) are used for protection from impacts. Standard design requires the foam to maximize the energy absorption, thus achieving large deformations (typically up to 25% and above in compression) while maintaining the stress level below a threshold value. In this work, steam chest-moulded EPS and EPP were characterized in relation to their density, microstructure and applied strain rate. Typical mechanical parameters (elastic moduli and plateau stress in compression) were compared with existing models and data in the literature. The strain-rate dependence was accurately described using Nagy’s phenomenological model. The mechanical behaviour of the foams was then correlated with their microstructure, as investigated using scanning electron microscopy and X-ray micro-tomography. Structural parameters were obtained using both (2D and 3D) techniques and relevant results were compared. © The Author(s) 2018
Volume orientation: a practical solution to analyse the orientation of fibres in composite materials
No full textMechanical properties of fibres reinforced composite materials depend on the type of fibres used, their percentage as well as their arrangement and orientation. As computer technology continues to improve, high-resolution computed tomography has proven to be an ideal instrument to analyse the structure of this kind of materials. In this context, various approaches have been proposed to detect the fibre orientation distribution and the relative degree of anisotropy of these composite materials. Some of these approaches are based on ‘individual’ measurements that isolate and reconstruct each single fibre and measure its properties. On the other hand, other approaches capture the characteristics of the fibre distribution by means of ‘global’ measurements computed on the entire set of tomographic data. The first methods are more precise but also more complex because they demand a procedure able to segment and separate each single fibre in the polymer, whereas the latter are easier to implement and can be applied even if fibre segmentation and separation is not effective or practicable. In this paper, a global method based on the technique called volume orientation – originally proposed several years ago to study the anisotropy of bone structures – is applied to fibre reinforced composite materials. This new approach does not require data acquired at very high resolution nor very complex procedures for individual segmentation of the fibres, but only binarised data through common thresholding procedures. The effectiveness of the proposed new approach is demonstrated by comparing it to the results obtained from a method based on individual measurements: when resolution and images quality are good enough, the volume orientation method gives results quite similar to the other approach. The analysis of three different case studies demonstrates its flexibility and its validity as an alternative to methods based on the separation of individual fibres, which are not always usable. The samples have been carefully selected in order to range between different attenuation contrast levels and also include a specimen subjected to mechanical testing which can be of great practical interest. Lay Description: Mechanical properties of fibres reinforced composite materials depend on the type of fibres used, their percentage as well as their arrangement and orientation. Today, both destructive and nondestructive techniques can be used in order to assess the fibre orientation. As computer technology continues to improve, high-resolution computed tomography has proven to be an ideal instrument to analyse the structure of this kind of materials, and then the fibre orientation distribution inside the material. In this context, various strategies have been proposed. Some of them require measurements that isolate and reconstruct each single fibre and measure its properties. On the other hand, other approaches capture the characteristics of the fibre distribution by means of ‘global’ measurements computed on the entire set of tomographic data. The first methods are more precise but also more complex because they demand a procedure able to detect and separate each single fibre in the polymer, whereas the latter are easier to implement and can be applied even if fibre segmentation and separation is not effective or practicable. In this paper, a global method based on the technique called volume orientation – originally proposed several years ago to study the microstructure of bone tissues – is applied to fibre reinforced composite materials. The aim of this work is to demonstrate that this new approach is easier to use. As a matter of fact, it does not require data acquired at very high resolution nor very complex procedures for individual segmentation of the fibres, but only binarised data through common thresholding procedures. The effectiveness of the proposed new approach is shown by comparing it to the results obtained from a method based on individual measurements: when spatial resolution and images quality are good enough, the volume orientation method gives results quite similar to the other already used approach. The analysis of three different case studies demonstrates its flexibility and its validity as an alternative to methods based on the separation of individual fibres, which are not always usable. The samples have been carefully selected in order to range between different attenuation contrast levels and different nature of the fibres (mineral, vegetable or synthetic). A specimen subjected to mechanical testing is also included, because of its great practical interest
Scanning Small- and Wide-Angle X-ray Scattering Microscopy Selectively Probes HA Content in Gelatin/Hydroxyapatite Scaffolds for Osteochondral Defect Repair
No full textThis study is aimed at investigating the structure of a scaffold made of bovine gelatin and hydroxyapatite for bone tissue engineering purposes. In particular, the detailed characterization of such a material has a great relevance because of its application in the healing process of the osteochondral defect that consists of a damage of cartilage and injury of the adjacent subchondral bone, significantly compromising millions of patient's quality of life. Two different techniques exploiting X-ray radiation, with table-top setups, are used: microtomography (micro-CT) and microdiffraction. Micro-CT characterizes the microstructure in the three dimensions at the micrometer scale spatial resolution, whereas microdiffraction provides combined structural/morphological information at the atomic and nanoscale, in two dimensional microscopy images with a hundred micrometer spatial resolution. The combination of these two techniques allowed an appropriate structural characterization for the purpose of validating the engineering approach used for the realization of the hydroxyapatite gradient across the scaffold, with properties close to the natural model. © 2016 American Chemical Society
Structural analysis of advanced polymeric foams by means of high resolution X-ray computed tomography
No full textAdvanced polymeric foams with enhanced thermal insulation and mechanical properties are used in a wide range of industrial applications. The properties of a foam strongly depend upon its cell structure. Traditionally, their microstructure has been studied using 2D imaging systems based on optical or electron microscopy, with the obvious disadvantage that only the surface of the sample can be analysed. To overcome this shortcoming, the adoption of X-ray micro-tomography imaging is here suggested to allow for a complete 3D, non-destructive analysis of advanced polymeric foams. Unlike metallic foams, the resolution of the reconstructed structural features is hampered by the low contrast in the images due to weak X-ray absorption in the polymer. In this work an advanced methodology based on high-resolution and low-contrast techniques is used to perform quantitative analyses on both closed and open cells foams. Local structural features of individual cells such as equivalent diameter, sphericity, anisotropy and orientation are statistically evaluated. In addition, thickness and length of the struts are determined, underlining the key role played by the achieved resolution. In perspective, the quantitative description of these structural features will be used to evaluate the results of in situ mechanical and thermal test on foam samples. © 2016 Author(s)
New approach in Auger elemental relative sensitive factor calculation by using TEM-EDS analysis based on bi-layers of pure elements
No full textIn fabrication of microelectronic devices two important steps are often recognized: i) all the processes performed on the wafer in order to build the active part of the devices and, ii) the assembly and packaging processes, typically performed on a chip, in order to fabricate interconnections between active part and exterior. The wafer back side is an active part of power devices and is normally coated with a stack of Ti-Ni-Au or Ti-Ni-Ag layers to ensure the best electrical contact with the frame on which the device is attached prior to the packaging. An important failure mechanism related to this particular process step is related to the diffusion of Ni to the surface of the stack that causes its oxidation on the back metal surface, inhibiting the correct connection to the metallic frame. Auger Electron Spectroscopy (AES) is a powerful analytical technique that can be used to detect this failure mechanism for its very high sensitivity in the characterization of surface layers. Unfortunately, its results are mainly qualitative. Quantitative extrapolations can be inaccurate using library Elemental Relative Sensitive Factor (ERSF) because they are mainly referred to a silicon substrate and could be not valid for a different matrix. A most accurate evaluation of the ERSF is based on the analysis, under identical experimental condition, of standard materials (with known concentration) that should be similar to the unknown sample and having the same matrix. However, the production of this kind of standard is not easy due to the mobility of Ni in Au and Ag. Another commonly used technique is the Energy Dispersive X-ray Spectrometry (EDS) which is less sensitive than the Auger and not sufficiently adequate for a quantitative analysis due to the limitation of the matrix correction methods. Recently, a new method to perform quantitative analysis by using Transmission Electron Microscopy (TEM) EDS was proposed, starting from bi-layers of pure elements. In this work we show how the use of TEM-EDS quantification of Ni in Ag could be a successful method for ERSF evaluation in order to overcome matrix effect in Auger quantification. For this purpose suitable foils of Ag/Al and Ni/Al were used. The validation of the method was performed on a sample with a tri-metal stack of Ti/Ni/Ag previously stimulated by means of a thermal budget to induce Ni migration on Ag surface. The quantitative analysis allowed us to use this characterized sample as AES standard for ERSF calculation. © 201
Quantitative characterisation of low-density, high performance polymeric foams using high resolution X-ray computed tomography and laser confocal microscopy
No full textAdvanced polymeric foams are used in a wide range of industrial and R&D applications. Their properties strongly depend upon the cell structure. Traditionally, their microstructure has been studied using optical or electron microscopy, limiting the investigations to sample's surface only. To overcome this shortcoming, the use of X-ray micro-tomography imaging is here adopted to allow for a complete 3D, non-destructive analysis of advanced polymeric foams. This work brings to fruition high resolution and low-contrast imaging techniques to perform quantitative analyses onto polymeric foams, where the low contrast of reconstructed structural features might hamper accurate quantitative analyses. Local structural features of individual cells are statistically evaluated, showing the key role of the achieved resolution. © 2016 Elsevier Lt
Relationship between the anisotropy tensor calculated through global and object measurements in high-resolution X-ray tomography on cellular and composite materials
No full textStructural anisotropy of two-phase materials can be evaluated through global measurements, as volume orientation or mean-intercept length methods do, or through statistics performed on a set of individual measurements. This last procedure is encouraged by recent improvements in the spatial resolution of conventional X-ray tomography. In this paper, the above-described approaches were compared in three case studies: a foam subjected to an in situ compression test, a second foam with a completely different cell morphology and a plastic material reinforced with short fibres. The approach based on the subdivision into distinguishable objects of the considered material phase has proved to be more sensitive in highlighting small deformations in the structure or small irregularities in an otherwise isotropic structure. On the other hand, the other approach is more general and is always usable. The two methods for calculating the fabric tensor tend to converge as the average anisotropy of individual objects in the statistical population increases. The use of Lambert's cylindrical equal-area projection of cell/fibre directions or local volume orientations is suggested, because the density of points is preserved from the sphere to the plane surface. Finally, a quick vector method to evaluate the anisotropy of the directions distribution has been presented, by defining a coherence index of the average direction
An image-based approach for structure investigation and 3D numerical modelling of polymeric foams
Full text linkPolymeric expanded materials are of great importance in many engineering applications. Despite this, as of today the development of models able to describe the mechanical behaviour of these material as a function of their microstructure is still an open challenge. In this study an image-based approach is proposed for both microstructure characterisation and 3D numerical mechanical simulations. Microstructure is investigated through different algorithms, such as Mean Intercept Length and Autocorrelation function, to determine synthetic parameters able to describe the internal structure. A novel algorithm has been developed to convert the images obtained from computed tomography into a finite element mesh with an optimized number of elements: this method preserves the original structure and can also be used to generate other fictitious structures that can be analysed. The investigation led to the identification of general relationships between foam microstructure and relevant macroscopic physical and mechanical properties. These relationships can serve as a tool to optimize foam morphology or product final properties for several different engineering applications
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