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arXiv2019

Modeling the non-trivial behavior of anisotropic beams: a simple Timoshenko beam with enhanced stress recovery and constitutive relations

This paper analyzes the non-trivial influence of the material anisotropy on the structural behavior of an anisotropic multilayer planar beam. Indeed, analytical results available in literature are limited to homogeneous beams and several aspects has not been addressed yet, impeding an in-depth understanding of the mechanical response of anisotropic structural elements. This paper proposes an effective recovery of stress distribution and an energetically consistent evaluation of constitutive relations to be used within a planar Timoshenko beam model. The resulting structural-analysis tool highlights the following peculiarities of anisotropic beams: (i) the axial stress explicitly depends on transversal internal force, which can weigh up to 30% on the maximal magnitude of axial stress, and (ii) the anisotropy influences the beam displacements more than standard shear deformation and even for extremely slender beams. A rigorous comparison with analytical and accurate 2D Finite Element solutions confirms the accuracy of the proposed approach that leads to errors exceptionally greater than 5%.

physics.app-ph↗

arXiv2019

Probing the adsorption/desorption of amphiphilic polymers at the air-water interface during large interfacial deformations

Hydrophobically modified polymers are good candidates for the stabilization of liquid interfaces thanks to the high anchoring energy of the hydrophobic parts. In this article we probe the interfacial anchoring of a series of home-made hydrophobically modified polymers of controlled degree of grafting by studying their behavior upon large area dilations and compressions. By comparing the measured interfacial tension to the one that we expect in the case of a constant number of adsorbed monomers, we are able to deduce whether desorption or adsorption occurs during area variations. We find that the polymer chains with the longest hydrophobic grafts desorb for larger compressions than the polymers with the shortest grafts, because of their larger desorption energy. Furthermore, we observe more desorption for polymers with the highest grafting densities. We attribute this counter intuitive result to the fact that for high grafting densities, the length of the polymer loops is shorter, hence the elastic penalty upon compression is larger for these layers, leading to a faster desorption. The dilatation experiments reveal that the number of adsorbed anchors remains constant in the case of chains with low grafting density while chains with the highest degree of grafting seem to show some degree of adsorption during the dilatation. Therefore for these highly grafted chains there may be unadsorbed grafts remaining in the vicinity of the interface, which may adsorb quickly to the interface upon dilatation.

physics.app-ph↗

arXiv2019

Stable Operation of AlGaN/GaN HEMTs at 400$^\circ$C in air for 25 hours

In this letter, we report the operation of AlGaN/GaN HEMTs with Pd gates in air over a wide temperature range from 22$^\circ$C to 500$^\circ$C. The variation in the threshold voltage ($V_{th}$) is less than 1$\%$ over the entire temperature range. Moreover, a safe biasing region where the transconductance peak ($g_m$) occurs over the entire temperature range was observed, enabling high-temperature analog circuit design. Furthermore, the operation of the devices over 25 hours was experimentally studied, demonstrating the stability of the DC characteristics and $V_{th}$ at 400$^\circ$C. Finally, the degradation mechanisms of HEMTs at 500$^\circ$C over 25 hours of operation are discussed, and are shown to be associated with the 2DEG sheet density and mobility decrease.

physics.app-ph↗

arXiv2019

Database-driven high-throughput study for hybrid perovskite coating materials

We developed a high-throughput screening scheme to acquire candidate coating materials for hybrid perovskites. From more than 1.8 million entries of an inorganic compound database, we collected 93 binary and ternary materials with promising properties for protectively coating halide-perovskite photoabsorbers in perovskite solar cells. These candidates fulfill a series of criteria, including wide band gaps, abundant and non-toxic elements, water-insoluble, and small lattice mismatch with surface models of halide perovskites.

physics.app-ph↗

arXiv2019

Structural and magnetic properties in sputtered iron oxide epitaxial thin films -- Magnetite Fe$_3$O$_4$ and epsilon ferrite e-Fe$_2$O$_3$

Epitaxial thin film fabrication of iron oxides including magnetite Fe3O4 and epsilon-ferrite epsilon-Fe2O3 with the potential for advancing electromagnetic devices has been investigated, which led to the first ever epsilon-ferrite epitaxial layer being synthesized in the conventional sputtering process. Concerning Fe3O4 (100) / MgO (100) films, a cube-on-cube epitaxial relationship and sharp rocking curves with FWHM of 50 - 350 arcsec were confirmed regardless of the small amount of Ge additions. Sputtering Ar gas pressure PAr heavily influenced their magnetic and transport properties. High PAr = 15 mTorr caused a high magnetization of 6.52 kG for the Ge added sample and the clear Verwey transition at 122 K for the non Ge addition case. Conversion electron Mossbauer spectroscopy (CEMS) measurements revealed that low PAr < 10 mTorr causes Fe/O off-stoichiometry on the oxidizing side for the non Ge addition case and the reductive side for the Ge addition case, respectively. Regarding the epsilon-Fe2O3 (001) / SrTiO3(111) epilayer synthesis, bilayer microstructure composed of an approximately 5nm thick initially grown epsilon-Fe2O3 (001) epilayer and subsequently grown e-Fe2O3 (001) epilayer was confirmed from cross-sectional TEM observations. The coexistence of magnetically hard and soft phases was confirmed from the magnetization measurements. As a possible application of the single nm thick epsilon-Fe2O3 layer, 4-resistive-state multiferroic tunnel junction (MFTJ) is considered.

physics.app-ph↗

arXiv2019

A caloritronics-based Mott neuristor

Machine learning imitates the basic features of biological neural networks to efficiently perform tasks such as pattern recognition. This has been mostly achieved at a software level, and a strong effort is currently being made to mimic neurons and synapses with hardware components, an approach known as neuromorphic computing. CMOS-based circuits have been used for this purpose, but they are non-scalable, limiting the device density and motivating the search for neuromorphic materials. While recent advances in resistive switching have provided a path to emulate synapses at the 10 nm scale, a scalable neuron analogue is yet to be found. Here, we show how heat transfer can be utilized to mimic neuron functionalities in Mott nanodevices. We use the Joule heating created by current spikes to trigger the insulator-to-metal transition in a biased VO2 nanogap. We show that thermal dynamics allow the implementation of the basic neuron functionalities: activity, leaky integrate-and-fire, volatility and rate coding. By using local temperature as the internal variable, we avoid the need of external capacitors, which reduces neuristor size by several orders of magnitude. This approach could enable neuromorphic hardware to take full advantage of the rapid advances in memristive synapses, allowing for much denser and complex neural networks. More generally, we show that heat dissipation is not always an undesirable effect: it can perform computing tasks if properly engineered.

cs.ET↗

arXiv2019

Wideband tunable microwave signal generation in a silicon-based optoelectronic oscillator

Si photonics has an immense potential for the development of compact and low-loss opto-electronic oscillators (OEO), with applications in radar and wireless communications. However, current Si OEO have shown a limited performance. Si OEO relying on direct conversion of intensity modulated signals into the microwave domain yield a limited tunability. Wider tunability has been shown by indirect phase-modulation to intensity-modulation conversion, requiring precise control of the phase-modulation. Here, we propose a new approach enabling Si OEOs with wide tunability and direct intensity-modulation to microwave conversion. The microwave signal is created by the beating between an optical source and single sideband modulation signal, selected by an add-drop ring resonator working as an optical bandpass filter. The tunability is achieved by changing the wavelength spacing between the optical source and resonance peak of the resonator. Based on this concept, we experimentally demonstrate microwave signal generation between 6 GHz and 18 GHz, the widest range for a Si-based OEO. Moreover, preliminary results indicate that the proposed Si OEO provides precise refractive index monitoring, with a sensitivity of 94350 GHz RIU and a potential limit of detection of only 10-8 RIU, opening a new route for the implementation of high-performance Si photonic sensors.

physics.app-ph↗

arXiv2019

Effects of Membrane Morphology on the Efficiency of Direct Contact Membrane Distillation

A computer simulation is used to predict the effects of membrane morphology on the thermal efficiency of direct contact membrane distillation. The mass transfer through the porous microstructure and the heat conduction through the membrane are both related by the membrane morphology. The interrelated tortuosity of the porous structure and solid phase influences the mass transfer and thermal conductivity, respectively. The effects of varying the morphology are elucidated and introducing a lattice structure, which tailors the morphology, can significantly increase thermal efficiency. A three-layer system is also simulated, where the pore size in the middle layer can be increased without significantly increasing the risk of membrane pore wetting. Three-layer systems that possess a lattice morphology are found to result in thermal efficiencies around 20\% higher than random morphologies.

physics.comp-ph↗

arXiv2019

Superintense Laser-driven Ion Beam Analysis

Ion beam analysis techniques are among the most powerful tools for advanced material characterization. Despite their growing relevance in a widening number of fields, most ion beam analysis facilities still rely on the oldest accelerator technologies, with severe limitations in terms of portability and flexibility. In this work we thoroughly address the potential of superintense laser-driven proton sources for this application. We develop a complete analytical and numerical framework suitable to describe laser-driven ion beam analysis, exemplifying the approach for Proton Induced X-ray/Gamma-ray emission, a technique of widespread interest. This allows us to propose a realistic design for a compact, versatile ion beam analysis facility based on this novel concept. These results can pave the way for ground-breaking developments in the field of hadron-based advanced material characterization.

physics.plasm-ph↗

arXiv2019

An Accurate Circuit Model for Photoconductive Antennas

THz photoconductive antennas are among the most popular devices for THz wave generation and detection. A novel and accurate circuit is proposed to model the performance of such antennas, especially those which are frequency dependant. The suggested equivalent circuit consists of an external source voltage, a source input admittance, a loss resistance related to the electrode loss resistance, a frequency-dependant capacitance associated with capacitive behavior of the gap between the electrodes, and a controlled voltage dependant source representing the screening of photo-carriers. Based on this model, the radiation from a simple frequency dependent antenna, i.e. a dipole antenna, is investigated. Excellent agreement is observed between the obtained results by the proposed equivalent circuit model and those by measurement.

physics.app-ph↗

arXiv2019

Coercivity enhancement of selective laser sintered NdFeB magnets by grain boundary infiltration

Laser powder bed fusion is a well-established additive manufacturing method that can be used for the production of net-shaped Nd-Fe-B sintered magnets. However, low coercivity has been one of the drawbacks in the laser powder bed fusion processed Nd-Fe-B magnets. In this work, we have demonstrated that the grain boundary diffusion process using low-melting Nd-Cu, Nd-Al-Ni-Cu, and Nd-Tb-Cu alloys to the selective laser sintered NdFeB magnets can results in a substantial enhancement of coercivity from 0.65 T to 1.5 T. Detailed microstructure investigations clarified formation of Nd-rich grain boundary phase, introducing Tb-rich shell at the surface of Nd2Fe14B grains, and maintaining the grain size in nano-scale are responsible for the large coercivity enhancement.

physics.app-ph↗

arXiv2019

Efficient Interfacial Solar Steam Generator with Controlled Macromorphology Derived from Flour via "Dough Figurine" Technology

Solar-driven interface steam generator (SISG) is a most promising technology for seawater desalination and wastewater purification. A shape- and size-controlled, low-cost, eco-friendly solar-absorber material is urgently desired for practical application of SISG. Herein, we proposed a facile, sustainable and scalable approach to produce tailored SISG with controlled macromorphology derived from flour via "dough figurine" technology which is originated from the China Han Dynasty. Three kinds of self-floated flour-based absorbers i.e. near-cylindrical (integrated), near-spherical (loose packing) and powdery (dense packing) absorber used as SISGs were discussed, we found that the macromorphology significantly influences water transport and interfacial thermal management of SISG, the integrated absorber has an overwhelming advantage, which possesses a high evaporation efficiency 71.9% at normal solar illumination. The proposed "dough figurine" technology breaks the limitations of the inherent geometry of reported biomass based SISG, which provides an important guidance for SISG use in remote and impoverished areas.

physics.app-ph↗

arXiv2019

Interdigitated flexible supercapacitor using activated carbon synthesized from biomass for wearable energy storage

We have developed a flexible, interdigitated supercapacitor with high energy storing capacity for wearable, flexible electronic application. Locally obtained low cost biomass (banana peel) was impregnated with KOH under high temperature under an inert atmosphere to synthesize activated carbon with significant Brunauer Emmett Teller surface area of 62.03m2/g. The supercapacitor was fabricated by screen printing interdigitated current collector of conducting silver ink on a thin flexible PET substrate and subsequent deposition of activated carbon and drop casting of gel electrolyte. Fabricated supercapacitor exhibits high capacitance of 33.18 mF/cm2 at 1mV/s scan rate and 20.12 mF/cm2 at a discharge current of 1mA and high energy density of 5.87 micro Wh/cm2. The developed flexible supercapacitor retains its energy storing capacity (90%) over several cycles of mechanical bending and repetitive electronic cycling tests (5000 cycles). Locally available biomass based activated carbon and low cost screen printing technique can be used for large scale fabrication of supercapacitor. The flexible supercapacitor demonstrates high energy storing capacity and mechanical durability through multiple bending and charging and discharging through LED. Therefore, it can be used for further developing integrated wearable and printed electronic devices.

physics.app-ph↗

arXiv2019

Identifying the Geometry of an Object Using Lock-In Thermography

Lock-in Thermography (LIT) is a type of Infrared Thermography (IRT) that can be used as a useful non-destructive testing (NDT) technique for the detection of subsurface anomalies in objects. Currently, LIT fails to estimate the thickness at a point on the tested object. This makes LIT unable to figure out the 3-dimensional geometry of an object. In this project, two techniques of identifying the geometry of an object using LIT are discussed. The main idea of both techniques is to find a relationship between the parameters obtained from LIT and the thickness at each data point. Technique I builds a numerical function that models the relationship between thickness, Lock-In phase, and other parameters. The function is then inverted for thickness estimation. Technique II is a quantitative method, in which a database is created with six dimensions - thickness, Lock-In phase, Lock-In amplitude and three other parameters, based on data obtained from LIT experiments or simulations. Estimated thickness is obtained by retrieving data from the database. The database can be improved based on Principal Component Analysis. Evaluation of the techniques is done by measuring root-mean-square deviation, and calculating successful rate with different tolerances. Moreover, during the application of the techniques, Stochastic Gradient Descent can be used to determine the time when sufficient data have been collected from LIT measurement to generate the estimated geometry accurately.

physics.app-ph↗

arXiv2019

Sound Transmission Through a Finite-Sized Double Panel Cavity with a Micro-Perforated Panel Insertion

This paper proposes a noise insulation cavity composed of two parallel plates and a micro-perforated plate insertion parallel to the plates, which divides the cavity between the plates into two parts. A theoretical model was established that takes into account of all the couplings among the major parts of the structure, namely the two solid plates, the perforated plate, and the air cavity, together with the simply support boundary conditions. Numerical calculations were performed with different parameters of the micro-perforated plate including its position, perforation ratio, plate thickness, and hole diameters. The calculations indicated that the proposed double-panel structure with a micro-perforated plate insertion exhibited significant improvements in the sound transmission loss (STL) in certain frequency range as compared to a double- or triple-panel structure without a micro-perforated plate. Below 200 Hz the improvement in STL is mainly due to the weakening of the resonances by the energy dissipation of the perforated plate, while in the medium to high frequency range the STL enhancement is mostly due to the dissipation by the perforated plate in the broad frequency band. The theoretical results are in good agreement with the experimental results.

physics.app-ph↗

arXiv2019

Ti surface modification for biomedical applications

Micro Arc Oxidation (MAO) is an electrochemical approach for the surface treatment usually applied on so called valve metals such as Al, Mg and Ti. MAO is usually carried out an aqueous electrolyte, which involves a bath cooling and leads to the creation of surface contained components originated in the electrolyte. In current work, we applied a different approach of ceramic surface formation on Ti alloy used in biomedical applications. Here, MAO process conducted in molten nitrate salt at 280 deg C. The developed surface morphology, chemical and phase composition, and corrosion resistance were investigated and described in the work.

physics.app-ph↗

arXiv2019

Enhancing the Electrical and Thermal Conductivities of Polymer Composites via Curvilinear Fibers: An Analytical Study

The new generation of manufacturing technologies such as e.g. additive manufacturing and automated fiber placement has enabled the development of material systems with desired functional and mechanical properties via particular designs of inhomogeneities and their mesostructural arrangement. Among these systems, particularly interesting are materials exhibiting \textbf{Curvilinear Transverse Isotropy} (CTI) in which the inhomogeneities take the form of continuous fibers following curvilinear paths designed to e.g. optimize the electric and thermal conductivity, and the mechanical performance of the system. In this context, the present work proposes a general framework for the exact, closed-form solution of electrostatic problems in materials featuring curvilinear transverse isotropy. First, the general equations for the fiber paths that optimize the electric conductivity are derived leveraging a proper conformal coordinate system. Then, the continuity equation for the curvilinear, transversely isotropic system is derived in terms of electrostatic potential. A general exact, closed-form expression for the electrostatic potential and electric field is derived and validated by Finite Element Analysis. Finally, potential avenues for the development of materials with superior electric conductivity and damage sensing capabilities are discussed.

physics.app-ph↗

arXiv2019

A New Load-based Thermopower Measurement Setup in the Temperature Range of 5 - 330 K

This is the report on new development of an automated precision load-based measurement setup for thermoelectric power of different types of samples in the temperature range 5 - 330 K. The problems in the old spring-based setup have been solved by making load-based setup. This new setup takes nearly 4 h for each run and typical error is within 5 %. A high quality calibration been demonstrated using high purity platinum wires.

physics.app-ph↗