Dielectric breakdown strength and electrical conductivity of low density polyethylene octylnanosilica composite

One challenge in studying nanodielectric composites is to produce reliable, reproducible samples. A common strategy to suppress aggregation and make the particles more compatible with the polymer matrix is to modify the nanoparticle surface chemistry but, often, evaluation of the effectiveness of the chosen surface functionalization process can prove difficult. In this paper the emphasis is on feasible ways to monitor the production of silane coupled nanosilica low density polyethylene (LDPE) composites, using Fourier transform infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA). The AC-breakdown properties of the resulting composites is studied and the field dependency of the DC-conductivity is measured and also calculated using a space charge limited conduction (SCLC) model together with densities of states obtained from ab initio calculations. For composites containing 13 wt% of nanosilica, breakdown strengths some 18 % higher than that of the unfilled LDPE were obtained. However, the results are not stable over time. This appears to be related to how extensively the composite is dried at elevated temperatures under vacuum

Advanced Data Encryption ​using 2D Materials

Advanced data encryption requires the use of true random number generators (TRNGs) to produce unpredictable sequences of bits. TRNG circuits with high degree of randomness and low power consumption may be fabricated by using the random telegraph noise (RTN) current signals produced by polarized metal/insulator/metal (MIM) devices as entropy source. However, the RTN signals produced by MIM devices made of traditional insulators, i.e., transition metal oxides like HfO and AlO, are not stable enough due to the formation and lateral expansion of defect clusters, resulting in undesired current fluctuations and the disappearance of the RTN effect. Here, the fabrication of highly stable TRNG circuits with low power consumption, high degree of randomness (even for a long string of 2 − 1 bits), and high throughput of 1 Mbit s by using MIM devices made of multilayer hexagonal boron nitride (h-BN) is shown. Their application is also demonstrated to produce one-time passwords, which is ideal for the internet-of-everything. The superior stability of the h-BN-based TRNG is related to the presence of few-atoms-wide defects embedded within the layered and crystalline structure of the h-BN stack, which produces a confinement effect that avoids their lateral expansion and results in stable operation.M.L. acknowledges generous support from the King Abdullah University of Science and Technology. This work was supported by the Ministry of Science and Technology of China (grants no. 2018YFE0100800, 2019YFE0124200), the National Natural Science Foundation of China (grants no. 61874075), the Collaborative Innovation Centre of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the 111 Project from the State Administration of Foreign Experts Affairs of China. A.A. and S.R. acknowledge the project: ModElling Charge and Heat trANsport in 2D-materIals based Composites—MECHANIC reference number: PCI2018-093120 funded by Ministerio de Ciencia, Innovación y Universidades. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). Y.S. acknowledges support from the European Union (Marie Sklodowska-Curie actions (grant no. 894840). The authors acknowledge technical advice from H.-S. Philip Wong from Stanford University and Xiaoming Xie from Chinese Academy of Sciences

An ab initio study on liquid silicon carbide

This work presents for the first time the properties of the liquid phase of silicon carbide using ab initio molecular dynamics simulations based on density-functional theory (DFT). Our DFT scheme employs a plane-wave basis to expand the atomic orbitals, pseudopotentials built with the projector augmented wave method, and the local-density approximation to describe the exchange–correlation interactions. With this approach we we determine a melting temperature of the zinc-blend phase of 2678.54 ( 41.67) K with a pressure of 0.25 ( 0.40) GPa and a density of 3.06 g/cm in good agreement with the experimental normal melting point of 2818.00 ( 40.00) K. At these conditions, the diffusion coefficient of the melt is 6.86 x 10−3 nm/ps which compares well with the estimated value of 2.46 x 10−3 nm/ps in the experiments done at atmospheric pressure. Finally, our model shows that silicon carbide has a negative melting curve that qualitatively agrees with experiments, with a slope of -36.93 K/GPa with pressures between 2.56 and 6.48 GPa, which compares well with the -44 K/GPa reported from the laboratory carried out with pressures of up to 7.7 GPa. This work provides a straightforward methodology based on the popular ’Z-method’ to produce liquid systems of silicon carbide, from which amorphous systems can easily be then produced by quenching.The author acknowledges the support at The Supercomputing Center of Galicia (CESGA) where the calculations have been made.Peer reviewe

Density-functional theory models of Fe(iv)O reactivity in metal–organic frameworks: self-interaction error, spin delocalisation and the role of hybrid exchange

We study the reactivity of Fe(IV)O moieties supported by a metal–organic framework (MOF-74) in the oxidation reaction of methane to methanol using all-electron, periodic density-functional theory calculations. We compare results concerning the electronic properties and reactivity obtained using two hybrid (B3LYP and sc-BLYP) and two standard generalised gradient corrected (PBE and BLYP) semi-local density functional approximations. The semi-local functionals are unable to reproduce the expected reaction profiles and yield a qualitatively incorrect representation of the reactivity. Non-local hybrid functionals provide a substantially more reliable description and predict relatively modest (ca. 60 kJ mol−1) reaction energy barriers for the H-atom abstraction reaction from CH4 molecules. We examine the origin of these differences and we highlight potential means to overcome the limitations of standard semi-local functionals in reactivity calculations in solid-state systems.This research was supported in part by the University of Pittsburgh Center for Research Computing through the resources provided and the Supercomputing Center of Galicia (CESGA), Spain.Peer reviewe

Electronic structure and reactivity of Fe(IV)oxo species in metal–organic frameworks

We investigate the potential use of Fe(IV)oxo species supported on a metal–organic framework in the catalytic hydroxylation of methane to produce methanol. We use periodic density-functional theory calculations at the 6-31G**/B3LYP level of theory to study the electronic structure and chemical reactivity in the hydrogen abstraction reaction from methane in the presence of Fe(IV)O(oxo) supported on MOF-74. Our results indicate that the Fe(IV)O moiety in MOF-74 is characterised by a highly reactive (quintet) ground-state, with a distance between Fe(IV) and O(oxo) of 1.601 Å, consistent with other high-spin Fe(IV)O inorganic complexes in the gas phase and in aqueous solution. Similar to the latter systems, the highly electrophilic character (and thus the reactivity) of Fe(IV)O in MOF-74 is determined by the presence of a low-lying anti-bonding virtual orbital (3σ*), which acts as an electron acceptor in the early stages of the hydrogen atom abstraction from methane. We estimate an energy barrier for hydrogen abstraction of 50.77 kJ mol−1, which is comparable to the values estimated in other gas-phase and hydrated Fe(IV)O-based complexes with the ability to oxidise methane. Our findings therefore suggest that metal–organic frameworks can provide suitable supports to develop new solid-state catalysts for organic oxidation reactions.This work was supported by STFC through a Service Level Agreement with EPSRC and by the HPC Materials Chemistry Consortium (grant EP/L000202). This research was supported in part by the University of Pittsburgh Center for Research Computing through the resources provided.We acknowledge support of the publication fee by the CSIC Open Access Support Initiative through its Unit of Information Resources for Research (URICI)Peer reviewe

Anisotropic Thermal Conductivity in Few-Layer and Bulk Titanium Trisulphide from First Principles

© 2020 by the authors.We study the thermal conductivity of monolayer, bilayer, and bulk titanium trisulphide (TiS 3 ) by means of an iterative solution of the Boltzmann transport equation based on ab-initio force constants. Our results show that the thermal conductivity of these layers is anisotropic and highlight the importance of enforcing the fundamental symmetries in order to accurately describe the quadratic dispersion of the flexural phonon branch near the center of the Brillouin zone.This research received the financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under grant FEDER-MAT2017-90024-P and the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and by the Generalitat de Catalunya under grants no. 2017 SGR 1506. The authors acknowledge the funding received from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 793726 (TELIOTES—Thermal and electronic transport in inorganic-organic thermoelectric superlattices).Peer reviewe

Optimisation of the thermoelectric efficiency of zirconium trisulphide monolayers through unixial and biaxial strain

The goal of this work is to investigate the influence of mechanical deformation on the electronic and thermoelectric properties of ZrS3 monolayers. We employ density functional theory (DFT) calculations at the hybrid HSE06 level to evaluate the response of the electronic band gap and mobilities, as well as the thermopower, the electrical conductivity, the phononic and electronic contributions to the thermal conductivity, and the heat capacity. Direct examination of the electronic band structures reveals that the band gap can be increased by up to 17% under uniaxial strain, reaching a maximum value of 2.32 eV. We also detect large variations in the electrical conductivity, which is multiplied by 3.40 under a 4% compression, but much smaller changes in the Seebeck coefficient. The effects of mechanical deformation on thermal transport are even more significant, with a nearly five-fold reduction of the lattice thermal conductivity under a biaxial strain of −4%. By harnessing a combination of these effects, the thermoelectric figure of merit of strained ZrS3 could be doubled with respect to the unstrained material.We acknowledge the financial support received from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 793726 (TELIOTES – Thermal and electronic transport in inorganic-organic thermoelectric superlattices), from the Ministerio de Economía, Industria y Competitividad (MINECO) under grant FEDER-MAT2017-90024-P and the Severo Ochoa Centres of Excellence Program Grant SEV-2015-0496, and from the Generalitat de Catalunya under grant no. 2017 SGR 1506. We also acknowledge the support from The Supercomputing Center of Galicia (CESGA), where the calculations have been made, and the fruitful discussions with Dr Andrés Castellanos-Gomez.Peer reviewe

Interatomic potential for predicting the thermal conductivity of zirconium trisulfide monolayers with molecular dynamics

We present here a new interatomic potential parameter set to predict the thermal conductivity of zirconium trisulfide monolayers. The generated Tersoff-type force field is parameterized using data collected with first-principles calculations. We use non-equilibrium molecular dynamics simulations to predict the thermal conductivity. The generated parameters result in very good agreement in structural, mechanical, and dynamical parameters. The room temperature lattice thermal conductivity (κ) of the considered crystal is predicted to be κxx = 25.69 W m−1 K−1 and κyy = 42.38 W m−1 K−1, which both agree well with their corresponding first-principles values with a discrepancy of less than 5%. Moreover, the calculated κ variation with temperature (200 and 400 K) are comparable within the framework of the accuracy of both first-principles and molecular dynamics simulationsWe acknowledge the financial support received from the European Union’s Horizon 2020 Research and Innovation Program under Grant Agreement No. 793726 (TELIOTES—Thermal and electronic transport in inorganic–organic thermoelectric superlattices) and the support of the Supercomputing Center of Galicia (CESGA), where the calculations have been made. We also acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under Grant No. FEDER-MAT2017-90024-P, the Severo Ochoa Centres of Excellence Program under Grant No. SEV-2015-0496, and the Generalitat de Catalunya under Grant No. 2017 SGR 1506. The authors acknowledge the fruitful discussions with Dr. Jesús Carrete Montaña at TU Wien.Peer reviewe

Electrical conductivity and moisture uptake studies of low density polyethylene octylnanosilica composite

In this paper, we report on the time dependent DC-conductivity response at constant electric field of unfilled low density polyethylene (LDPE) and its composites containing untreated and octylsilane treated silica in ambient and dry atmospheres. The conductivity of both composites decrease with time under dry conditions and increase with time in ambient atmosphere. The dielectric response of dry and wet silica polyethylene composites is studied by dielectric spectroscopy