Enhanced Computational Study with Experimental Correlation on I-V Characteristics of Tantalum Oxide (TaOx) Memristor Devices in a 1T1R Configuration.

Memristors, non-volatile switching memory platform, has recently attracted significant interest, offering unique potential to enable the realization of human brain-like neuromorphic computing efficiency. Memristors also demonstrate excellent temperature tolerance, long-term durability, and high tunability with nanosecond pulses, making them highly attractive for neuromorphic computing applications. To better understand the material processing, microstructure, and property relationship of switching mechanisms in memristor devices, computational methodologies, and tools are developed to predict the I-V characteristics of memristor devices based on tantalum oxide (TaOx) resistive random-access memory (ReRAM) integrated with an n-channel metal-oxide-semiconductor (NMOS) transistor. A multiphysics model based on coupled partial differential equations for electrical and thermal transport phenomena is solved for the high- and low-resistance states during the formation, growth, and destruction of a conducting filament through SET and RESET stages. These stages effectively represent the migration of oxygen vacancies within an oxide exchange layer. A series of parametric studies and energy minimization calculations are conducted to determine probable ranges for key material and model parameters accounting for the experimental data. The computational model successfully predicted the measured I-V curves across various gate voltages applied to the NMOS transistor in the one transistor one resistance (1T1R) configuration.

Shape memory polymer composites (SMPCs) using interconnected nanowire network foams as reinforcements.

Shape memory polymers (SMPs), although offer a suite of advantages such as ease of processability and lower density, lag behind their shape memory alloy counterparts, in terms of mechanical properties such as recovery stress and cyclability. Reinforcing SMPs with inorganic nanowires and carbon nanotubes (CNTs) is a sought-after pathway for tailoring their mechanical properties. Here, inorganic nanowires also offer the added advantage of covalently binding the fillers to the surrounding polymer matrices via organic molecules. The SMP composites (SMPCs) thus obtained have well-engineered nanowire-polymer interfaces, which could be used to tune their mechanical properties. A well-known method of fabricating SMPCs involving casting dispersions of nanowires (or CNTs) in mixtures of monomers and crosslinkers typically results in marginal improvements in the mechanical properties of the fabricated SMPCs. This is owed to the constraints imposed by the rule-of-mixture principles. To circumvent this limitation, a new method for SMPC fabrication is designed and presented. This involves infiltrating polymers into pre-fabricated nanowire foams. The pre-fabricated foams were fabricated by consolidating measured quantities of nanowires and a sacrificial material, such as (NH4)2CO3, followed by heating the consolidated mixtures for subliming the sacrificial material. Similar to the case of traditional composites, use of silanes to functionalize the nanowire surfaces allowed for the formation of bonds between both the nanowire-nanowire and the nanowire-polymer interfaces. SMPCs fabricated using TiO2nanowires and SMP composed of neopentyl glycol diglycidyl ether and poly(propylene glycol) bis(2-aminopropyl ether) (Jeffamine D230) in a 2:1 molar ratio exhibited a 300% improvement in the elastic modulus relative to that of the SMP. This increase was significantly higher than SMPC made using the traditional fabrication route. Well-known powder metallurgy techniques employed for the fabrication of these SMPCs make this strategy applicable for obtaining other SMPCs of any desired shape and chemical composition.

Determining and scaling continuous-wave, laser-induced damage thresholds of thin reflectors.

Scalable and repeatable determinations of continuous wave (CW) laser-induced damage thresholds are required to develop materials for applications ranging from deformable mirrors to momentum transfer. Current standards assume sample geometries and beam conditions where CW damage thresholds are constant in linear power density, depend strongly on substrate thermal conductivity, and are insensitive to environmental conditions. In this work, the CW laser response of thin PET films with a reflective Al/MgF2 coating are experimentally assessed over a range of beam diameters and irradiances. The laser-induced damage threshold decreases with increased exposure time down to a temporally-independent irradiance, decreases with increased beam diameter to an irradiance that is independent of spot size, and depends on radiative and convective cooling. Models are used to define the minimum spot size and exposure time required to achieve such constant damage threshold irradiances for thin reflectors.

In situ mechanical investigation of carbon nanotube-graphene junction in three-dimensional carbon nanostructures.

Hierarchically organized three-dimensional (3D) carbon nanotubes/graphene (CNTs/graphene) hybrid nanostructures hold great promises in composite and battery applications. Understanding the junction strength between CNTs and graphene is crucial for utilizing such special nanostructures. Here, in situ pulling an individual CNT bundle out of graphene is carried out for the first time using a nanomechanical tester developed in-house, and the measured junction strength of CNTs/graphene is 2.23 ± 0.56 GPa. The post transmission electron microscopy (TEM) analysis of remained graphene after peeling off CNT forest confirms that the failure during pull-out test occurs at the CNT-graphene junction. Such a carefully designed study makes it possible to better understand the interfacial interactions between CNTs and graphene in the 3D CNTs/graphene nanostructures. The coupled experimental and computational effort suggests that the junction between the CNTs and the graphene layer is likely to be chemically bonded, or at least consisting of a mixture of chemical bonding and van der Waals interactions.

Importance of interfaces in governing thermal transport in composite materials: modeling and experimental perspectives.

Thermal management in polymeric composite materials has become increasingly critical in the air-vehicle industry because of the increasing thermal load in small-scale composite devices extensively used in electronics and aerospace systems. The thermal transport phenomenon in these small-scale heterogeneous systems is essentially controlled by the interface thermal resistance because of the large surface-to-volume ratio. In this review article, several modeling strategies are discussed for different length scales, complemented by our experimental efforts to tailor the thermal transport properties of polymeric composite materials. Progress in the molecular modeling of thermal transport in thermosets is reviewed along with a discussion on the interface thermal resistance between functionalized carbon nanotube and epoxy resin systems. For the thermal transport in fiber-reinforced composites, various micromechanics-based analytical and numerical modeling schemes are reviewed in predicting the transverse thermal conductivity. Numerical schemes used to realize and scale the interface thermal resistance and the finite mean free path of the energy carrier in the mesoscale are discussed in the frame of the lattice Boltzmann-Peierls-Callaway equation. Finally, guided by modeling, complementary experimental efforts are discussed for exfoliated graphite and vertically aligned nanotubes based composites toward improving their effective thermal conductivity by tailoring interface thermal resistance.

Metalized nanotube tips improve through thickness thermal conductivity in adhesive joints.

The through-thickness thermal conductivity in conventional adhesive joints (of approximately 0.3 W/m-K) fails to meet the thermal load transfer requirement in numerous applications to enable lean manufacturing and improve system reliability to thermal load. Carbon nanotubes are known to possess extremely high thermal conductivity along the longitudinal axis. According to molecular dynamics simulations, the value can be as high as 3500 W/m-K at room temperature for multi-walled carbon nanotubes (MWCNT). Meanwhile, the transverse thermal conductivity perpendicular to the longitudinal axis of the MWCNTs is known to be relatively low, approximately 10-15 W/m-K. Existing studies of mixing the MWCNTs in polymers for adhesive joints only achieved minimal enhancement in the thermal conductivity and failed to satisfy the thermal property requirement for the adhesive joints. In order to properly utilize the superior axial thermal conductivity of the MWCNTs, vertically aligned MWCNTs have been used in this study and incorporated in the adhesive joint configuration. Analytical parametric study was conducted to identify critical parameters that affect the overall thermal conductivity of the joint and to provide guidelines for the process development. The process development involved growing the vertically aligned MWCNTs on silicon wafers. The aligned nanotube array was partially infused with epoxy adhesive. Selective reactive ion etching of the epoxy revealed the nanotube tips. In order to reduce the impedance mismatch and phonon scattering at the interface between the nanotube tips and the adherends, gold was thermally evaporated on the nanotube tips. The measured thermal conductivity of the adhesive joint device incorporating the MWCNTs was 262 W/m-K, which is significantly larger compared to that of less than 1 W/m-K without the MWCNTs.