Characterising, understanding and predicting the performance of structural power composites

Dramatic improvements in power generation, energy storage, system integration and light-weighting are needed to meet increasingly stringent carbon emissions targets for future aircraft and road vehicles. The electrification of transport could significantly reduce direct CO2 emissions; however, battery energy and power density limitations pose a major technological barrier. The introduction of multifunctional structural power composites (SPCs), which simultaneously provide mechanical load-bearing and electrochemical energy storage, offers new possibilities. By replacing conventional materials with SPCs, electrical performance requirements could be relaxed, and vehicle mass could be reduced; however, for SPCs to outperform monofunctional systems, significant performance and reliability improvements are still required. The use of computational models to support experimental efforts has so far been overlooked, despite wide recognition of the benefits of such a combined approach. The aim of this work was to develop predictive finite element models for structural supercapacitor composites (SSCs), and use them to investigate their mechanical, electrical, and electrochemical behaviour. A unit cell modelling technique was used to generate realistic mesoscale models of the complex microstructure of SSCs. The effects of composite manufacturing processes on the final performance of SSCs were investigated through characterisation and modelling of compaction and manufacturing defects. Numerical predictions of the elastic properties of SSCs were evaluated against data from the literature; and the presence of defects was shown to significantly degrade performance. Motivated by the large series resistance of SSCs, direct conduction models were developed to better understand electrical charge transport. Based on investigations of various current collector geometries, design strategies for the mitigation of resistive losses were proposed. To enable analysis of the combined mechanical-electrochemical behaviour of SSCs, an ion transport user element subroutine was developed but could not be validated. Overall, this work demonstrates that substantial improvements in the mechanical and electrical properties of SSCs are possible through control of the composite microstructure. The models developed in this work provide guidance for the optimisation of manufacturing processes and the design of new SSC architectures, and underpin the future certification and deployment of these emerging materials.Open Acces

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

Atomistic study of surface effects in metals

Atomistic simulations are a useful way to study nanoscale metal structures. At the nanoscale, the surface to volume ratio of the objects becomes large and surface effects start to play a critically important role. The internal stress near a surface can reach the GPa range and thus its effects should not be neglected when dealing with nanowires and other nanostructures. Similarly, surface diffusion of atoms is important in the manufacturing process and subsequent stability of nanostructures. In the study of vacuum breakdown on Cu surfaces, dislocation activity and surface atom diffusion are thought to play a role in the formation of field enhancing emitters. This work investigates a possible mechanism of nucleation of a nanofeature on metal surfaces under high electric fields in the presence of a near-surface defect, and the stability of Au nanowires with respect to surface diffusion. The simulation methods of molecular dynamics, kinetic Monte Carlo and finite elements are employed. A subsurface Fe precipitate is used as an example of subsurface extended defects, and the nucleation of dislocations in regions of high stress concentration is simulated. A process of forming a protrusion on the surface near the precipitate due to dislocation propagation is shown, as well as the possibility of forming new voids on the precipitate interface. Since atomistic simulations are heavily limited in size and time scales, larger scale simulations are conducted by using finite element modelling of nanoscale material behavior under external loading. However, such modeling requires the development of an accurate model of surface stress. In this work, a surface stress model is implemented into a continuum finite element model to enable faster calculations of more extensive nanoscale systems, as well as to combine the mechanical model with electrical effects in vacuum breakdown research. The internal stresses given by the model are validated in comparison with molecular dynamics simulations and against an analytical model of dislocation emission from a near-surface void. Kinetic Monte Carlo simulation is a suitable tool to simulate diffusion processes. However, setting up KMC simulations requires a parametrization of atomic migration barriers. A consistent parametrization scheme, called the tethering method, is developed in the current work. The tethering method provides a robust automatic process to calculate migration barriers for on-lattice diffusion simulations. It allows the calculation of barriers for unstable processes, while having a minimal effect on stable barriers. The tethering method is used to create a parametrization for Au, which is used to simulate nanowire junction fragmentation. Nanowire junctions break up in a process similar to Rayleigh instability. In conjunction with experiments, it is shown that junctions fragment at a low temperature when nanowires themselves remain whole. Simulations demonstrate that the breakup can be explained by surface energy minimization due to atom diffusion and that the formation of a fragment at the nanowire crossing point is very reliable.Atomistiset simulaatiot ovat erinomainen tapa tutkia nanokokoisia metallirakenteita. Nanomittakaavassa pinta-alan suhde tilavuuteen on suuri, ja pinnalla tapahtuvat ilmiöt ovat hyvin merkittävässä roolissa. Sisäinen jännitys lähellä pintaa voi olla useita gigapascaleita, joten sen merkitys on huomioitava nanojohtimia ja muita nanorakenteita tutkittaessa. Myös atomien pintadiffuusio on tärkeä ilmiö nanorakenteiden valmistuksen ja vakauden kannalta. Kuparipinnalla tapahtuvien tyhjiövalokaarien tutkimuksessa dislokaatioiden liikkeen ja pinta-atomien diffusion arvellaan vaikuttavan sähkökenttää vahvistavien emitterien muodostumiseen. Tässä työssä tutkitaan nanomuodostelmien nukleaation mekanismia pintadefektien läheisyydessä voimakkaassa sähkökentässä, ja kultananojohtimien vakautta pintadiffuusion suhteen. Tutkimuksessa käytetään molekyylidynaamisia simulaatioita, kineettistä Monte Carloa ja elementtimenetelmää. Pinnanalaista rautasaostumaa käytetään esimerkkitapauksena pinnan alle ulottuvista defekteistä, ja korkean jännityksen alueilla tapahtuvaa dislokaatioiden nukleaatiota simuloidaan. Dislokaatioiden etenemisen näytetään aiheuttavan ulkonemien muodostumista pinnalla lähellä saostumaa. Myös uusien tyhjiöiden muodostumisen saostuman rajapinnalle näytetään olevan mahdollista. Koska atomististen simulaatioiden koko ja aikaskaala ovat hyvin rajallisia, suuremmat simulaatiot tehdään ulkoisen jännityksen alaisten nanomateriaalien elementtimallinnuksen avulla. Tällainen mallinnus kuitenkin vaatii tarkkaa mallia pintajännityksestä. Tässä työssä pintajännitysmalli toteutetaan jatkumoelementtimallissa, jotta suurempia nanosysteemejä voidaan simuloida nopeammin, ja jotta mekaaniset mallit saadaan yhdistettyä tyhjiövalokaaritutkimukseen. Mallin antamia sisäisen jännityksen arvoja verrataan molekyylidynaamisiin simulaatioihin ja pinnan lähellä sijaitsevan tyhiön emittoimien dislokaatioiden analyyttiseen malliin. Kineettinen Monte Carlo on hyvä työkalu diffuusioprosessien simuloimiseen. KMCsimulaatio kuitenkin vaatii parametreina atomististen siirtymien energiavalleja. Tämän työn osana kehitetään johdonmukainen parametrisointijärjestelmä, nimeltään liekamenetelmä. Tämän menetelmän avulla voidaan laskea hilassa tapahtuvien siirtymien energiavallit luotettavasti ja automaattisesti. Myös epävakaiden siirtymien energiavallien laskeminen mahdollistuu, ilman suurta vaikutusta vakaiden siirtymien energiavalleihin. Liekamenetelmällä luodaan parametrisaatio kultasysteemeille, jonka avulla simuloidaan risteävien nanojohtimien pirstoutumista. Nanojohtimien risteyskohdassa tapahtuva pirstoutuminen on Rayleigh’n epävakauden kaltainen prosessi. Yhdessä kokeellisten tutkimusten kanssa näytetään että risteyskohdat pirstoutuvat matalassa lämpötilassa, jossa yksittäiset nanojohtimet vielä pysyvät kokonaisina. Simulaatioiden perusteella pirstoutuminen voidaan selittää atomien diffuusion aiheuttamalla pintaenergian minimoitumisella. Risteyskohtaan muodostuu sirpale hyvin luotettavasti

Atomistic study of surface effects in metals

Atomistic simulations are a useful way to study nanoscale metal structures. At the nanoscale, the surface to volume ratio of the objects becomes large and surface effects start to play a critically important role. The internal stress near a surface can reach the GPa range and thus its effects should not be neglected when dealing with nanowires and other nanostructures. Similarly, surface diffusion of atoms is important in the manufacturing process and subsequent stability of nanostructures. In the study of vacuum breakdown on Cu surfaces, dislocation activity and surface atom diffusion are thought to play a role in the formation of field enhancing emitters. This work investigates a possible mechanism of nucleation of a nanofeature on metal surfaces under high electric fields in the presence of a near-surface defect, and the stability of Au nanowires with respect to surface diffusion. The simulation methods of molecular dynamics, kinetic Monte Carlo and finite elements are employed. A subsurface Fe precipitate is used as an example of subsurface extended defects, and the nucleation of dislocations in regions of high stress concentration is simulated. A process of forming a protrusion on the surface near the precipitate due to dislocation propagation is shown, as well as the possibility of forming new voids on the precipitate interface. Since atomistic simulations are heavily limited in size and time scales, larger scale simulations are conducted by using finite element modelling of nanoscale material behavior under external loading. However, such modeling requires the development of an accurate model of surface stress. In this work, a surface stress model is implemented into a continuum finite element model to enable faster calculations of more extensive nanoscale systems, as well as to combine the mechanical model with electrical effects in vacuum breakdown research. The internal stresses given by the model are validated in comparison with molecular dynamics simulations and against an analytical model of dislocation emission from a near-surface void. Kinetic Monte Carlo simulation is a suitable tool to simulate diffusion processes. However, setting up KMC simulations requires a parametrization of atomic migration barriers. A consistent parametrization scheme, called the tethering method, is developed in the current work. The tethering method provides a robust automatic process to calculate migration barriers for on-lattice diffusion simulations. It allows the calculation of barriers for unstable processes, while having a minimal effect on stable barriers. The tethering method is used to create a parametrization for Au, which is used to simulate nanowire junction fragmentation. Nanowire junctions break up in a process similar to Rayleigh instability. In conjunction with experiments, it is shown that junctions fragment at a low temperature when nanowires themselves remain whole. Simulations demonstrate that the breakup can be explained by surface energy minimization due to atom diffusion and that the formation of a fragment at the nanowire crossing point is very reliable.Atomistiset simulaatiot ovat erinomainen tapa tutkia nanokokoisia metallirakenteita. Nanomittakaavassa pinta-alan suhde tilavuuteen on suuri, ja pinnalla tapahtuvat ilmiöt ovat hyvin merkittävässä roolissa. Sisäinen jännitys lähellä pintaa voi olla useita gigapascaleita, joten sen merkitys on huomioitava nanojohtimia ja muita nanorakenteita tutkittaessa. Myös atomien pintadiffuusio on tärkeä ilmiö nanorakenteiden valmistuksen ja vakauden kannalta. Kuparipinnalla tapahtuvien tyhjiövalokaarien tutkimuksessa dislokaatioiden liikkeen ja pinta-atomien diffusion arvellaan vaikuttavan sähkökenttää vahvistavien emitterien muodostumiseen. Tässä työssä tutkitaan nanomuodostelmien nukleaation mekanismia pintadefektien läheisyydessä voimakkaassa sähkökentässä, ja kultananojohtimien vakautta pintadiffuusion suhteen. Tutkimuksessa käytetään molekyylidynaamisia simulaatioita, kineettistä Monte Carloa ja elementtimenetelmää. Pinnanalaista rautasaostumaa käytetään esimerkkitapauksena pinnan alle ulottuvista defekteistä, ja korkean jännityksen alueilla tapahtuvaa dislokaatioiden nukleaatiota simuloidaan. Dislokaatioiden etenemisen näytetään aiheuttavan ulkonemien muodostumista pinnalla lähellä saostumaa. Myös uusien tyhjiöiden muodostumisen saostuman rajapinnalle näytetään olevan mahdollista. Koska atomististen simulaatioiden koko ja aikaskaala ovat hyvin rajallisia, suuremmat simulaatiot tehdään ulkoisen jännityksen alaisten nanomateriaalien elementtimallinnuksen avulla. Tällainen mallinnus kuitenkin vaatii tarkkaa mallia pintajännityksestä. Tässä työssä pintajännitysmalli toteutetaan jatkumoelementtimallissa, jotta suurempia nanosysteemejä voidaan simuloida nopeammin, ja jotta mekaaniset mallit saadaan yhdistettyä tyhjiövalokaaritutkimukseen. Mallin antamia sisäisen jännityksen arvoja verrataan molekyylidynaamisiin simulaatioihin ja pinnan lähellä sijaitsevan tyhiön emittoimien dislokaatioiden analyyttiseen malliin. Kineettinen Monte Carlo on hyvä työkalu diffuusioprosessien simuloimiseen. KMCsimulaatio kuitenkin vaatii parametreina atomististen siirtymien energiavalleja. Tämän työn osana kehitetään johdonmukainen parametrisointijärjestelmä, nimeltään liekamenetelmä. Tämän menetelmän avulla voidaan laskea hilassa tapahtuvien siirtymien energiavallit luotettavasti ja automaattisesti. Myös epävakaiden siirtymien energiavallien laskeminen mahdollistuu, ilman suurta vaikutusta vakaiden siirtymien energiavalleihin. Liekamenetelmällä luodaan parametrisaatio kultasysteemeille, jonka avulla simuloidaan risteävien nanojohtimien pirstoutumista. Nanojohtimien risteyskohdassa tapahtuva pirstoutuminen on Rayleigh’n epävakauden kaltainen prosessi. Yhdessä kokeellisten tutkimusten kanssa näytetään että risteyskohdat pirstoutuvat matalassa lämpötilassa, jossa yksittäiset nanojohtimet vielä pysyvät kokonaisina. Simulaatioiden perusteella pirstoutuminen voidaan selittää atomien diffuusion aiheuttamalla pintaenergian minimoitumisella. Risteyskohtaan muodostuu sirpale hyvin luotettavasti

The electric field alignment of short carbon fibres to enhance the toughness of epoxy composites

An investigation is presented on increasing the fracture toughness of epoxy/short carbon fibre (SCF) composites by alignment of SCFs using an externally applied alternating current (AC) electric field. Firstly, the effects of SCF length, SCF content and AC electric field strength on the rotation of the SCFs suspended in liquid (i.e. uncured) epoxy resin are investigated. Secondly, it is shown the mode I fracture toughness of the cured epoxy composites increases with the weight fraction of SCFs up to a limiting value (5 wt%). Thirdly, the toughening effect is greater when the SCFs are aligned in the composite normal to the direction of crack growth. The SCFs increases the fracture toughness by inducing multiple intrinsic and extrinsic toughening mechanisms, which are identified. Based on the identified toughening mechanisms, an analytical model is proposed to predict the enhancement to the fracture toughness due to AC electric field alignment of the SCFs

Materials for in-vessel components

The EUROfusion materials research program for DEMO in-vessel components aligns with the European Fusion Roadmap and comprises the characterization and qualification of the in-vessel baseline materials EUROFER97, CuCrZr and tungsten, advanced structural and high heat flux materials developed for risk mitigation, as well as optical and dielectric functional materials. In support of the future engineering design activities, the focus is primarily to assemble qualified data to supply the design process and generate material property handbooks, material assessment reports, DEMO design criteria and material design limits for DEMO thermal, mechanical and environmental conditions. Highlights are provided on advanced material development including (a) steels optimized towards lower or higher operational windows, (b) heat sink materials (copper alloys or composites) and (c) tungsten based plasma facing materials. The rationale for the down-selection of material choices is also presented. The latter is strongly linked with the results of neutron irradiation campaigns for baseline material characterization (structural, high heat flux and functional materials) and screening of advanced materials. Finally, an outlook on future material development activities to be undertaken during the upcoming Concept Design Phase for DEMO will be provided, which highly depends on an effective interface between materials’ development and components’ design driven by a common technology readiness assessment of the different systems

NANOMECHANICAL CHARACTERIZATIONS OF HIGH TEMPERATURE POLYMER MATRIX COMPOSITE RESIN: PMR-15 POLYIMIDE

High Temperature Polymer Matrix Composites (HTPMCs) are widely used by the aerospace industry today because of their high specific strengths, light weight, and the ability to custom tailor their mechanical properties to individual applications. Because of the harsh environmental conditions these materials experience during service use, these composite structures are susceptible to a high rate of thermo-oxidative degradation that ultimately causes premature failure in service. The current knowledge base is lacking in the fundamental spatial variability of the constituent materials upon aging, which precludes composite developers from predicting lifetime mechanical properties of the composites in use. The current study summarizes state of the art techniques in characterizing the thermally oxidized matrix resin system (PMR-15 polyimide), and develops novel techniques in direct mechanical measurement of the spatial variability of mechanical properties. Works to date and future advances in the field with respect to in situ testing are presented

A new mixed model based on the enhanced-Refined Zigzag Theory for the analysis of thick multilayered composite plates

The Refined Zigzag Theory (RZT) has been widely used in the numerical analysis of multilayered and sandwich plates in the last decay. It has been demonstrated its high accuracy in predicting global quantities, such as maximum displacement, frequencies and buckling loads, and local quantities such as through-the-thickness distribution of displacements and in-plane stresses [1,2]. Moreover, the C0 continuity conditions make this theory appealing to finite element formulations [3]. The standard RZT, due to the derivation of the zigzag functions, cannot be used to investigate the structural behaviour of angle-ply laminated plates. This drawback has been recently solved by introducing a new set of generalized zigzag functions that allow the coupling effect between the local contribution of the zigzag displacements [4]. The newly developed theory has been named enhanced Refined Zigzag Theory (en- RZT) and has been demonstrated to be very accurate in the prediction of displacements, frequencies, buckling loads and stresses. The predictive capabilities of standard RZT for transverse shear stress distributions can be improved using the Reissner’s Mixed Variational Theorem (RMVT). In the mixed RZT, named RZT(m) [5], the assumed transverse shear stresses are derived from the integration of local three-dimensional equilibrium equations. Following the variational statement described by Auricchio and Sacco [6], the purpose of this work is to implement a mixed variational formulation for the en-RZT, in order to improve the accuracy of the predicted transverse stress distributions. The assumed kinematic field is cubic for the in-plane displacements and parabolic for the transverse one. Using an appropriate procedure enforcing the transverse shear stresses null on both the top and bottom surface, a new set of enhanced piecewise cubic zigzag functions are obtained. The transverse normal stress is assumed as a smeared cubic function along the laminate thickness. The assumed transverse shear stresses profile is derived from the integration of local three-dimensional equilibrium equations. The variational functional is the sum of three contributions: (1) one related to the membrane-bending deformation with a full displacement formulation, (2) the Hellinger-Reissner functional for the transverse normal and shear terms and (3) a penalty functional adopted to enforce the compatibility between the strains coming from the displacement field and new “strain” independent variables. The entire formulation is developed and the governing equations are derived for cases with existing analytical solutions. Finally, to assess the proposed model’s predictive capabilities, results are compared with an exact three-dimensional solution, when available, or high-fidelity finite elements 3D models. References: [1] Tessler A, Di Sciuva M, Gherlone M. Refined Zigzag Theory for Laminated Composite and Sandwich Plates. NASA/TP- 2009-215561 2009:1–53. [2] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. Assessment of the Refined Zigzag Theory for bending, vibration, and buckling of sandwich plates: a comparative study of different theories. Composite Structures 2013;106:777–92. https://doi.org/10.1016/j.compstruct.2013.07.019. [3] Di Sciuva M, Gherlone M, Iurlaro L, Tessler A. A class of higher-order C0 composite and sandwich beam elements based on the Refined Zigzag Theory. Composite Structures 2015;132:784–803. https://doi.org/10.1016/j.compstruct.2015.06.071. [4] Sorrenti M, Di Sciuva M. An enhancement of the warping shear functions of Refined Zigzag Theory. Journal of Applied Mechanics 2021;88:7. https://doi.org/10.1115/1.4050908. [5] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses, Ibiza, Spain: 2013. [6] Auricchio F, Sacco E. Refined First-Order Shear Deformation Theory Models for Composite Laminates. J Appl Mech 2003;70:381–90. https://doi.org/10.1115/1.1572901