Ceramic multilayer based on ZrB2/SiC system for aerospace applications.

The work of this PhD thesis is focused on the processing and characterisation of ZrB2/SiC based multilayer materials, produced by tape casting and sintered without pressure assistance for aerospace applications. The multilayer components were processed in order to be used as external part of a thermal protection system; because of they directly face the atmosphere, in order to withstand high temperature, high heat fluxes and oxidizing environment, they have to show good oxidation and thermal shock resistance, high emissivity, thermal management capability, suitable strength, stiffness and hardness. The development of materials was directed along three main concepts: SiC based multilayer system was developed for the back shield part of the re-entry vehicle which was moderately loaded; ZrB2/SiC composite multilayers showing different ZrB2:SiC ratio were mainly designed for the front shield where the operating condition should be more severe (that means both higher temperature and thermal loads respect to those expected for the back shield). Moreover, in the context of development of reusable TPS to be used for consecutive missions, hybrid multilayer with ZrB2/SiC composite layers in between SiC ones have been prepared: the aim was provide for the degradation of SiC/SiO2 external part inserting an underlying material showing good mechanical and thermal properties at the higher temperature (over 1700°C). These three kinds of ceramic multilayer were processed and characterized from mechanical point of view; short and long term oxidation resistance have been detailed investigated. Tests under re-entry conditions have been performed

Experimental Characterization and Modeling of 3D Printed Continuous Carbon Fibers Composites with Different Fiber Orientation Produced by FFF Process

The development of 3D printed composites showing increased stiffness and strength thanks to the use of continuous carbon fibers has offered new prospects for Fused Filament Fabrication (FFF) technique. This work aims to investigate the microstructure and mechanical properties of 3D printed CCF/PA composites with various layups, and also to apply predictive models. The mechanical properties of the printed parts were directly related to the adopted laminate layup as well as to the microstructure and defects induced by the FFF process. The highest stiffness and strength were reported for longitudinal composites, where the fibers are unidirectionally aligned in the loading direction. In addition, it was found that the reduction in tensile properties obtained for cross-ply and quasi-isotropic laminate layups can be described by using the Angle Minus Longitudinal. A step-like failure with extensive fibers breakage and pull-out was observed for the longitudinal composites. By contrast, the rupture mode of the quasi-isotropic laminates mainly exhibited debonding between beads. Moreover, the predictions obtained using the Volume Average Stiffness method and Classical Laminate Theory were in good agreement with the tensile test results. This work could help engineers to design complex laminates with specific mechanical requirements by tailoring the orientation of continuous carbon fibers

Electrical and Thermal Conductivity of Epoxy-Carbon Filler Composites Processed by Calendaring

Electrical and thermal conductivity of composites which contain carbon-based fillers in an epoxy matrix were investigated. The fillers were dispersed in the liquid matrix by using three roll mill equipment. The filler/matrix mixture was cast in a mold and then cured, thus obtaining composite specimens. Multiwall carbon nanotubes, graphene-like nanoplatelets, and graphite were used as fillers and their effect on conductivity was investigated. Electrical and thermal conductivity were measured at different filler loads. It was found that the formation of percolation paths greatly enhanced electrical conductivity, although they were not so effective in improving thermal conductivity. The behavior of composites containing each single filler was compared with that of hybrid composites containing combinations of two different fillers. Results show that fillers with different aspect ratios displayed a synergetic effect resulting in a noticeable improvement of electrical conductivity. However, only a small effect on thermal conductivity was observed

Mechanical and Thermal Behavior of Ultem® 9085 Fabricated by Fused-Deposition Modeling

Fused-deposition modeling (FDM) is an additive manufacturing technique which is widely used for the fabrication of polymeric end-use products in addition to the development of prototypes. Nowadays, there is an increasing interest in the scientific and industrial communities for new materials showing high performance, which can be used in a wide range of applications. Ultem 9085 is a thermoplastic material that can be processed by FDM; it recently emerged thanks to such good properties as excellent flame retardancy, low smoke generation, and good mechanical performance. A deep knowledge of this material is therefore necessary to confirm its potential use in different fields. The aim of this paper is the investigation of the mechanical and thermal properties of Ultem 9085. Tensile strength and three-point flexural tests were performed on samples with XY, XZ, and ZX building orientations. Moreover, the influence of dierent ageing treatments performed by varying the maximum reached temperature and relative humidity on the mechanical behavior of Ultem 9085 was then investigated. The thermal and thermo-oxidative behavior of this material was also determined through thermal-gravimetric analyses

Introducing the Novel Mixed Gaussian-Lorentzian Lineshape in the Analysis of the Raman Signal of Biochar

In this research, an innovative procedure is proposed to elaborate Raman spectra obtained from nanostructured and disordered solids. As a challenging case study, biochar, a bio-derived carbon based material, was selected. The complex structure of biochar (i.e., channeled surface, inorganic content) represents a serious challenge for Raman characterization. As widely reported, the Raman spectra are closely linked to thermal treatments of carbon material. The individual contributions to the Raman spectra are difficult to identify due to the numerous peaks that contribute to the spectra. To tackle this problem, we propose a brand new approach based on the introduction, on sound theoretical grounds, of a mixed Gaussian-–Lorentzian lineshape. As per the experimental part, biochar samples were carbonized in an inert atmosphere at various temperatures and their respective spectra were successfully decomposed using the new lineshape. The evolution of the structure with carbonization temperature was investigated by Raman and XRD analysis. The results of the two techniques fairly well agree. Compared to other approaches commonly reported in the literature this method (i) gives a sounder basis to the lineshape used in disordered materials, and (ii) appears to reduce the number of components, leading to an easier understanding of their origin

Effect of Solution Treatment on Precipitation Behaviors, Age Hardening Response and Creep Properties of Elektron21 Alloy Reinforced by AlN Nanoparticles

In the present study, the solution and ageing treatments behavior of Mg-RE-Zr-Zn alloy (Elektron21) and its nano-AlN reinforced nanocomposites have been evaluated. The properties of the thermal-treated materials were investigated in terms of Vickers hardness, the area fraction of precipitates, microstructure and phase composition. The solution treatments were performed by treating at 520 ◦C, 550 ◦C and 580 ◦C in argon atmosphere. The outcomes show that the hardness of the solutionized alloys was slightly affected by the solution temperature. X-ray diffraction and image analysis revealed that the complete dissolution of precipitates was not possible, neither for Elektron21 (El21) nor for its AlN containing nanocomposites. The ageing treatment of El21 led to a significant improvement in hardness after 20 h, while for longer times, it progressively decreased. The effect of ageing on the hardness of El21–AlN composites was found to be much less than this effect on the hardness of the host alloy. Electron backscatter diffraction (EBSD) analysis of El21 and El21–1%AlN after solution treatment confirm the random orientation of grains with a typical texture of random distribution. The as-cast creep results showed that the incorporation of nanoparticles could effectively improve the creep properties, while the results after solution treatment at 520 ◦C for 12 h followed by ageing treatment at 200 ◦C for 20 h confirmed that the minimum creep rate of T6-El21 was almost equal to the as-cast El21–AlN

Innovative processing route combining fused deposition modelling and laser writing for the manufacturing of multifunctional polyamide/carbon fiber composites

An innovative manufacturing method, based on Fused Deposition Modelling (FDM) and Laser Writing (LW) technologies, was developed to integrate electrically conductive tracks within high strength 3D printed parts. Carbon fibers reinforced polyamide composites were processed by FDM according to three different printing architectures by varying the raster angle in the layer sequence. Laser writing was then performed to obtain conductive tracks on the surface of the printed parts. The process-structure-property relationship of components before and after the writing of conductive tracks was investigated. The tensile behaviour and the track resistance of samples were correlated to the direction of filament extrusion within the layers, and consequently to fibers orientation. Tracks showing good aesthetic quality and the lowest electrical resistance were obtained on samples with [±45] architecture. These tracks displayed the lowest electrical resistance ever reached by laser writing on metal-free polymer composites (0.008 kΩ/cm). Samples with [±45] architecture also showed high strength (72.9 MPa) and stiffness (7.7 GPa). The integration of FDM and LW emerges as a new approach to transform the surface of high strength polymer composites parts into a highly valuable system. These multifunctional components can find applications in several industrial fields such as automotive, aeronautics, and appliances

High-Temperature Annealed Biochar as a Conductive Filler for the Production of Piezoresistive Materials for Energy Conversion Application

In this research work, we develop a prototype that is able to convert mechanical strain into an electrical signal. To reach this scope, we evaluated the electrical properties of a thermally anneale..