arXiv subjects

Alberto Fina

Publications and source records attributed to Alberto Fina.

17 recordsLinked to original sources

arXiv2021

Flexible and highly conductive composites by impregnation of polydimethylsiloxane in graphite nanoplates paper

This paper deals with the preparation and characterization of conductive and flexible sheets based on graphite nanoplates (GNP) and polydimethylsiloxane (PDMS). Highly porous GNP nanopaper were prepared by filtration from a GNP suspension in solvent. Subsequently, the impregnation strategy was pursued to obtain a composite material. By varying the concentration of the solution to be impregnated and the deposition time, PDMS/GNP nanopapers are produced with a range of PDMS content, porosity and density. Thermal diffusivity of the films (both in-plane and cross-plane) were measured and correlated with the structure of the nanopapers. Based on the results found and considering the restraints, three formulations were further investigated for their physical, thermal and mechanical properties. Highly flexible, tough and conductive (up to 25 W/mK) nanopapers were obtained, for possible application in modern electronic devices.

physics.app-ph↗

arXiv2021

Polycaprolactone/graphite nanoplates composite nanopapers

Nanopapers based on graphene and related materials were recently proposed for application in heat spreader applications. To overcome typical limitations in brittleness of such materials, this work addressed the combination of graphite nanoplatelets (GNP) with a soft, tough and crystalline polymer, acting as an efficient binder between nanoplates. With this aim, polycaprolactone (PCL) was selected and exploited in this paper. The crystalline organization of PCL within the nanopaper was studied to investigate the effect of polymer confinement between GNP. Thermomechanical properties were studied by dynamic mechanical analyses at variable temperature and creep measurements at high temperature, demonstrating superior resistance at temperatures well above PCL melting. Finally, the heat conduction properties on the nanopapers were evaluated, resulting in outstanding values above 150 Wm-1K-1.

cond-mat.mtrl-sci↗

arXiv2021

Polymer-embedded molecular junctions between graphene sheets: A Molecular Dynamics study

Thermal conduction in polymer nanocomposites depends on several parameters including the thermal conductivity and geometrical features of the nanoparticles, the particle loading, their degree of dispersion and formation of a percolating networks. To enhance efficiency of thermal contact between free-standing conductive nanoparticles were previously proposed. This work report for the first time the investigation of molecular junctions within a graphene polymer nanocomposite. Molecular dynamics simulations were conducted to investigate the thermal transport efficiency of molecular junctions in polymer tight contact, to quantify the contribution of molecular junctions when graphene and the molecular junctions are surrounded by polydimethylsiloxane (PDMS). A strong dependence of the thermal conductance in PDMS/graphene model was found, with best performances obtained with short and conformationally rigid molecular junctions.

physics.app-ph↗

arXiv2021

Bispyrene functionalization drives self-assembly of graphite nanoplates into highly efficient heat spreader foils

Thermally conductive nanopapers fabricated from graphene and related materials are currently showing a great potential in thermal management applications. However, thermal contacts between conductive plates represent the bottleneck for thermal conductivity of nanopapers prepared in the absence of a high temperature step for graphitization. In this work, the problem of ineffective thermal contacts is addressed by the use of bifunctional polyaromatic molecules designed to drive self-assembly of graphite nanoplates (GnP) and establish thermal bridges between them. To preserve the high conductivity associated to defect-free sp2 structure, non-covalent functionalization with bispyrene compounds, synthesised on purpose with variable tethering chain length, was exploited. Pyrene terminal groups granted for a strong {\pi}-{\pi} interaction with graphene surface, as demonstrated by UV-Vis, fluorescence and Raman spectroscopies. Bispyrene molecular junctions between GnP were found to control GnP organization and orientation within the nanopaper, delivering significant enhancement in both in-plane and cross-plane thermal diffusivity. Finally, nanopapers were validated as heat spreader devices for electronic components, evidencing comparable or better thermal dissipation performance than conventional Cu foil, while delivering over 90% weight reduction.

physics.app-ph↗

arXiv2021

Properties of graphene-related materials controlling thermal conductivity of their polymer nanocomposites

Different types of graphene-related materials (GRM) are industrially available and have been exploited for thermal conductivity enhancement in polymers. These include materials with very different features, in terms of thickness, lateral size and composition, especially concerning the oxygen to carbon ratio and the possible presence of surface functionalization. Due to the variability of GRM properties, the differences in polymer nanocomposites preparation methods and the microstructures obtained, a large scatter of thermal conductivity performance is found in literature. However, detailed correlations between GRM-based nanocomposites features, including nanoplatelets thickness and size, defectiveness, composition and dispersion, with their thermal conductivity remain mostly undefined. In the present paper, the thermal conductivity of GRM-based polymer nanocomposites, prepared by melt polymerization of cyclic polybutylene terephtalate oligomers, exploiting 13 different GRM grades, was investigated. The selected GRM, covering a wide range of specific surface area, size and defectiveness, secure a sound basis for the understanding of the effect of GRM properties on the thermal conductivity of their relevant polymer nanocomposites. Indeed, the thermal conductivity obtained appeared to depend on the interplay between the above GRM feature. In particular, the combination of low GRM defectiveness and high filler percolation density was found to maximize the nanocomposites thermal conductivity.

physics.app-ph↗

arXiv2019

Chemical-Vapor-Deposited Graphene as a Thermally Conducting Coating

We performed Scanning Thermal Microscopy measurements on single layers of chemical-vapor-deposited -CVD- graphene supported by different substrates, namely SiO2, Al2O3 and PET using a double-scan technique to remove the contribution to the heat flux through the air and the cantilever. Then, by adopting a simple lumped-elements model, we developed a new method that allows determining, through a multi-step numerical analysis, the equivalent thermal properties of thermally conductive coatings of nanometric thickness. In this specific case we found that our CVD graphene is thermally equivalent, for heat injection perpendicular to the graphene planes, to a coating material of conductivity Keff=2.5+-0.3 mK and thickness teff=3.5+-0.3 nm in perfect contact with the substrate. For the SiO2 substrate, we also measured stacks made of 2 and 4 CVD monolayers and we found that the effective thermal conductivity increases with increasing number of layers and, with a technologically achievable number of layers, is expected to be comparable to that of one order of magnitude-thicker metallic thin films. This study provides a powerful method for characterizing the thermal properties of graphene in view of several thermal management applications.

physics.app-ph↗

arXiv2019

A facile and low environmental impact approach to prepare thermally conductive nanocomposites based on polylactide and graphite nanoplatelets

In this work, the preparation of nanocomposites based on poly(L-lactide) PLLA and graphite nanoplatelets (GNP) was assessed, by applying, for the first time, the reactive extrusion (REX) polymerization approach, which is considered a low environmental impact method to prepare polymer systems and which allows an easy scalability. In particular, ad hoc synthesized molecules, constituted by a pyrene end group and a poly (D-lactide) (PDLA) chain (Pyr-D), capable of interacting with the surface of GNP layers as well as forming stereoblocks during the ring opening polymerization (ROP) of L-lactide, were used. The nanocomposites were synthesized by adding to L-lactide the GNP/initiator system, prepared by dispersing the graphite in the acetone/Pyr-D solution, which was dried after the sonication process. DSC and X-ray diffraction measurements evidenced the stereocomplexation of the systems synthesized by using the pyrene-based initiators, whose extent turned out to depend on the PDLA chain length. All the prepared nanocomposites - including those synthesized starting from a classical initiator, that is 1-dodecanol - retained similar electrical conductivity, whereas the thermal conductivity was found to increase in the stereocomplexed samples. Preferential localization of stereocomplexed PLA close to the interface with GNP was demonstrated by Scanning Probe Microscopy (SPM) techniques, supporting for an important role of local crystallinity in the thermal conductivity of the nanocomposites.

physics.app-ph↗

arXiv2019

Aromatic molecular junctions between graphene sheets: a molecular dynamics screening for enhanced thermal conductance

The proper design and synthesis of molecular junctions for the purpose of establishing percolative networks of conductive nanoparticles represent an opportunity to develop more efficient thermally-conductive nanocomposites, with several potential applications in heat management. In this work, theoretical classical molecular dynamics simulations were conducted to design and evaluate thermal conductance of various molecules serving as thermal bridges between graphene nanosheets. A wide range of molecular junctions was studied, with a focus on the chemical structures that are viable to synthesize at laboratory scale. Thermal conductances were correlated with the length and mechanical stiffness of the chemical junctions. The simulated tensile deformation of the molecular junction revealed that the mechanical response is very sensitive to small differences in the chemical structure. The analysis of the vibrational density of states provided insights into the interfacial vibrational properties. A knowledgedriven design of the molecular junction structures is proposed, aiming at controlling interfacial thermal transport in nanomaterials. This approach may allow for the design of more efficient heat management in nanodevices, including flexible heat spreaders, bulk heat exchangers and heat storage devices.

physics.app-ph↗

arXiv2019

Decoupled trends for electrical and thermal conductivity in phase-confined CNT co-continuous blends

In the present work, the morphology and the electrical and thermal conduction properties of co-continuous poly(vinylidene fluoride) (PVDF), maleated polypropylene (PPgMA) and multiwall carbon nanotubes (CNT) nanostructured blends are investigated. CNT preferentially locates in the PPgMA phase and clearly causes a refinement in the co-continuous structure. Electrical conductivity experiments show that nanocomposites are well above the percolation threshold and evidence for one order of magnitude enhancement in conductivity for the co-continuous nanocomposites compared to the monophasic nanocomposites with the same CNT volume fraction. On the other hand, thermal diffusivity enhancement for the co-continuous blends is found lower than that for the monophasic nanocomposites at the same CNT volume fraction. An explanation is proposed in terms of large interfacial area, causing phonon scattering at the interface between immiscible PVDF and PPgMA domains. Results described in this paper open the way to the preparation of high electrical and low thermal conductivity materials with possible application as thermoelectrics.

physics.app-ph↗

arXiv2019

Stereocomplexation of Poly(Lactic Acid)s on Graphite Nanoplatelets: from Functionalized Nanoparticles to Self-Assembled Nanostructures

The control of nanostructuration of graphene and graphene related materials (GRM) into self-assembled structures is strictly related to the nanoflakes chemical functionalization, which may be obtained via covalent grafting of non-covalent interactions, mostly exploiting {\pi}-stacking. As the non-covalent functionalization does not affect the sp2 carbon structure, this is often exploited to preserve the thermal and electrical properties of the GRM and it is a well-known route to tailor the interaction between GRM and organic media. In this work, non-covalent functionalization of graphite nanoplatelets (GnP) was carried out with ad-hoc synthesized pyrene-terminated oligomers of polylactic acid (PLA), aiming at the modification of GnP nanopapers thermal properties. PLA was selected based on the possibility to self-assemble in crystalline domains via stereocomplexation of complementary poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) enantiomers. Pyrene-initiated PLLA and PDLA were indeed demonstrated to anchor to the GnP surface. Calorimetric and X-ray diffraction investigations highlighted the enantiomeric PLAs adsorbed on the surface of the nanoplatelets self-organize to produce highly crystalline stereocomplex domains. Most importantly, PLLA/PDLA stereocomplexation delivered a significantly higher efficiency in nanopapers heat transfer, in particular through the thickness of the nanopaper. This is explained by a thermal bridging effect of crystalline domains between overlapped GnP, promoting heat transfer across the nanoparticles contacts. This work demonstrates the possibility to enhance the physical properties of contacts within a percolating network of GRM via the self-assembly of macromolecules and opens a new way for the engineering of GRM-based nanostructures.

physics.app-ph↗

arXiv2019

Thermal bridging of graphene nanosheets via covalent molecular junctions. a Non-Equilibrium Green Functions Density Functional Tight-Binding study

Despite the uniquely high thermal conductivity of graphene is well known, the exploitation of graphene into thermally conductive nanomaterials and devices is limited by the inefficiency of thermal contacts between the individual nanosheets. A fascinating yet experimentally challenging route to enhance thermal conductance at contacts between graphene nanosheets is through molecular junctions, allowing covalently connecting nanosheets otherwise interacting only via weak Van der Waals forces. Beside the bare existence of covalent connections, the choice of molecular structures to be used as thermal junctions should be guided by their vibrational properties, in terms of phonon transfer through the molecular junction. In this paper, density functional tight-binding combined with Green functions formalism was applied for the calculation of thermal conductance and phonon spectra of several different aliphatic and aromatic molecular junctions between graphene nanosheets. Effect of molecular junction length, conformation, and aromaticity were studied in detail and correlated with phonon tunnelling spectra. The theoretical insight provided by this work can guide future experimental studies to select suitable molecular junctions in order to enhance the thermal transport by suppressing the interfacial thermal resistances, particularly attractive for various systems, including graphene nanopapers and graphene polymer nanocomposites, as well as related devices. In a broader view, the possibility to design molecular junctions to control phonon transport currently appears as an efficient way to produce phononic devices and controlling heat management in nanostructures.

cond-mat.mes-hall↗

arXiv2018

FTIR and GCMS analysis of epoxy resin decomposition products feeding the flame during UL 94 standard flammability test. Application to the understanding of the blowing-out effect in epoxy/polyhedral silsesquioxane formulations

A novel method was developed for the sampling of volatiles produces by polymer decomposition during UL94 standard flammability tests, allowing to collect, separate and analyze the precise composition of the fuel mixture feeding the flame in the real flammability tests. The system was validated on epoxy resin/polyhedral oligomeric silsesquioxanes and found extremely informative for the understanding of the flame retardancy mechanisms of POSS, previously referred to as the blowing-out effect. Collected products were analyzed by infrared spectroscopy and gas chromatography/mass spectroscopy, to identify the gaseous and liquid decomposition products, allowing to depict a comprehensive decomposition pathway for the epoxy resin. Lightweight volatiles, gaseous at room temperature, showed limited differences as a function of DOPO-POSS, whereas mixtures of liquid products evidenced for dramatic changes in the relative concentration as a function of DOPO-POSS. In pristine epoxy resin, the most abundant products were recognized as benzene, phenol, naphthalene and toluene, along with several tens of other aromatic products observed in lower amounts. The presence of DOPO-POSS at low concentration radically changes the composition of the aromatic volatiles mixture, as no significant amount of benzene is produced, while phenol becomes the main product, accounting for about half of the total, isopropyl phenol and bisphenol A, along with several tens of other products in lower concentrations. Such modification of the fuel mixture feeding the flame provided an explanation for the lower flammability of this formulation, as well as for the phenomenology of the blowing out effect.

physics.chem-ph↗

arXiv2018

Layer-by-Layer Assembly of Efficient Flame Retardant Coatings Based On High Aspect Ratio Graphene Oxide and Chitosan Capable of Preventing Ignition of PU Foam

The layer-by-layer (LbL) technique is adopted for the construction of multilayers encompassing chitosan and graphene oxide (GO) platelets capable of improving the flame retardant properties open cell PU foams. The LbL assembly follows a linear growth regime as evaluated by infrared spectroscopy and yields a multilayer structure where GO platelets are embedded within a chitosan continuous matrix. 3 and 6 bi-layers (BL) can efficiently coat the complex 3D structure of the foam and substantially improve its flame retardant properties. 3BL only add 10% to the original mass and can suppress the melt dripping during flammability and reduce both the peak of heat release rate by 54% and the total smoke released by 59% in forced combustion tests. Unprecedented among other LbL assemblies employed for FR purposes, the deposition 6BL is capable of slowing down the release of combustible volatile to the limits of non-ignitability thus preventing ignition in half of the specimens during cone calorimetry tests. This has been ascribed to the formation of a protective coating where the thermally stable char produced by chitosan serves as a continuous matrix embedding GO platelets, which control volatile release while mechanically sustaining the PU foam structure.

physics.chem-ph↗

arXiv2018

Edge-Grafted Molecular Junctions between Graphene Nanoplatelets: Applied Chemistry to Enhance Heat Transfer in Nanomaterials

The edge-functionalization of graphene nanoplatelets (GnP) was carried out exploiting diazonium chemistry, aiming at the synthesis of edge decorated nanoparticles to be used as building blocks in the preparation of engineered nanostructured materials for enhanced heat transfer. Indeed, both phenol functionalized and dianiline-bridged GnP (GnP-OH and E-GnP, respectively) were assembled in nanopapers exploiting the formation of non-covalent and covalent molecular junctions, respectively. Molecular dynamics allowed to estimate the thermal conductance for the two different types of molecular junction, suggesting a factor 6 between conductance of covalent vs. non-covalent junctions. Furthermore, the chemical functionalization was observed to drive the self-organization of the nanoflakes into the nanopapers, leading to a 20% enhancement of the thermal conductivity for GnP-OH and E-GnP while the cross plane thermal conductivity was boosted by 150% in the case of E-GnP. The application of chemical functionalization to the engineering of contact resistance in nanoparticles network was therefore validated as a fascinating route for the enhancement of heat exchange efficiency on nanoparticle networks, with great potential impact in low-temperature heat exchange and recovery applications

cond-mat.mtrl-sci↗

arXiv2017

Thermally and Electrically Conductive Nanopapers from Reduced Graphene Oxide: effect of nanoflakes thermal annealing on the film structure and properties

In this study, we report a different strategy to prepare graphene nanopapers from direct vacuum filtration. Instead of the conventional method, i.e. thermal annealing nanopapers at extremely high temperatures prepared from graphene oxide (GO) or partially reduced GO, we fabricate our graphene nanopapers directly from suspensions of fully reduced graphene oxide (RGO), obtained after RGO and thermally annealing at 1700 °C in vacuum. By using this approach, we studied the effect of thermal annealing on the physical properties of the macroscopic graphene-based papers. Indeed, we demonstrated that the enhancement of the thermal and electrical properties of graphene nanopapers prepared from annealed RGO is strongly influenced by the absence of oxygen functionalities and the morphology of the nanoflakes. Hence, our methodology can be considered as a valid alternative to the classical approach.

cond-mat.mtrl-sci↗

arXiv2017

Molecular junctions for thermal transport between graphene nanoribbons: covalent bonding vs. interdigitated chains

Proper design and manufacturing thermal bridges based on molecular junctions at the contact between graphene platelets or other thermally conductive nanoparticles would provide a fascinating way to produce efficient heat transport networks for the exploitation in heat management applications. In this work, using Non Equilibrium Molecular Dynamics, we calculated thermal conductance of alkyl chains used as molecular junctions between two graphene nanoribbons, both as covalently bound and Van der Waals interdigitated chains. Effect of chain length, grafting density, temperature and chain interdigitation were systematically studied. A clear reduction of conductivity was found with increasing chain length and decreasing grafting density, while lower conductivity was observed for Van der Waals interdigitated chains compared to covalently bound ones. The importance of molecular junctions in enhancing thermal conductance at graphene nanoribbons contacts was further evidenced by calculating the conductance equivalence between a single chain and an overlapping of un-functionalized graphene sheets. As an example, one single pentyl covalently bound chain was found to have a conductance equivalent to the overlapping of an area corresponding to about 152 carbon atoms. These results contribute to the understanding of thermal phenomena occurring within networks of thermally conductive nanoparticles, including graphene nanopapers and graphene-based polymer nanocomposites, which are or high interest for the heat management application in electronics and generally in low-temperature heat exchange and recovery.

cond-mat.mtrl-sci↗

arXiv2017

Super nucleation and orientation of poly (butylene terephthalate) crystals in nanocomposites containing highly reduced graphene oxide

The ring opening polymerization of cyclic butylene terephthalate into poly (butylene terephthalate) (pCBT) in the presence of reduced graphene oxide (RGO) is an effective method for the preparation of polymer nanocomposites. The inclusion of RGO nanoflakes dramatically affects the crystallization of pCBT, shifting crystallization peak temperature to higher temperatures and, overall, increasing the crystallization rate. This was due to a super nucleating effect caused by RGO, which is maximized by highly reduced graphene oxide. Furthermore, combined analyses by differential scanning calorimetry (DSC) experiments and wide angle X-ray diffraction (WAXS) showed the formation of a thick α-crystalline form pCBT lamellae with a melting point of ~250 °C, close to the equilibrium melting temperature of pCBT. WAXS also demonstrated the pair orientation of pCBT crystals with RGO nanoflakes, indicating a strong interfacial interaction between the aromatic rings of pCBT and RGO planes, especially with highly reduced graphene oxide. Such surface self-organization of the polymer onto the RGO nanoflakes may be exploited for the enhancement of interfacial properties in their polymer nanocomposites.

cond-mat.mtrl-sci↗