Thermal rectification is an asymmetric heat transfer process where directionally dependent transport occurs along a given axis. In this work, geometric parameters that govern thermal rectification in solids composed of various semiconducting materials were investigated utilizing metalattice data for seven materials with pore sizes ranging between 2 and 30 nm. Using numerical simulation, thermal rectification was calculated at different thermal biases in single material systems, including silicon, cubic boron nitride, and diamond, among others. The largest thermal rectification for each material was exhibited in bilayer sample stacks that were thermally matched (i.e., the thermal resistance of each layer in the stack is equal in either forward or reverse direction). Of the materials tested, diamond provided the highest thermal rectification for all cases, with its best case achieving a thermal rectification of 57.2%. This novel thermal functionality will find application in advanced applications for temperature regulation, including resonator systems where thermal effects may significantly alter and/or degrade performance.

Obtaining and improving measurements of mechanical properties at the nanoscale has been made possible through the continuous advancement of atomic force microscopy (AFM) techniques over the past several decades. Among these advancements include implementing multifunctional AFM probes and developing new detection schemes that enable sensitivity to local mechanical properties. In this work, we demonstrate a proof-of-concept for a detection scheme that enables a standard AFM configuration to produce qualitative local mechanical property maps through the use of an optical pump–probe scheme, alleviating a common requirement of incorporating additional piezoelectric actuators. Data from this work are presented for silicon carbide and epitaxially grown graphene on silicon carbide. Through preliminary analysis of resonant frequency maps acquired through dual-frequency resonance tracking, the local stiffness and elastic modulus can be estimated at each point. This work contributes to the field of scanning probe microscopy by providing a new opportunity for AFM systems that are not currently equipped for a mechanical mode to obtain local mechanical property data.

Controlling the thermal conductivity of semiconductors is of practical interest in optimizing the performance of thermoelectric and phononic devices. The insertion of inclusions of nanometer size in a semiconductor is an effective means of achieving such control; it has been proposed that the thermal conductivity of silicon could be reduced to 1 W/m/K using this approach and that a minimum in the heat conductivity would be reached for some optimal size of the inclusions. Yet the experimental verification of this design rule has been limited. In this work, we address this question by studying the thermal properties of silicon metalattices that consist of a periodic distribution of spherical inclusions with radii from 7 to 30 nm, embedded into silicon. Experimental measurements confirm that the thermal conductivity of silicon metalattices is as low as 1 W/m/K for silica inclusions and that this value can be further reduced to 0.16 W/m/K for silicon metalattices with empty pores. A detailed model of ballistic phonon transport suggests that this thermal conductivity is close to the lowest achievable by tuning the radius and spacing of the periodic inhomogeneities. This study is a significant step in elucidating the scaling laws that dictate ballistic heat transport at the nanoscale in silicon and other semiconductors.

Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generation high-power and high-frequency electronic devices. Recent efforts have focused on realizing vertical power device structures such as in situ oxide, GaN interlayer based vertical trench metal-oxide-semiconductor field-effect transistors (OG-FETs). Unfortunately, the higher-power density of GaN electronics inevitably leads to considerable device self-heating which impacts device performance and reliability. Halide vapor-phase epitaxy (HVPE) is currently the most common approach for manufacturing commercial GaN substrates used to build vertical GaN transistors. Vertical device structures consist of GaN layers of diverse doping levels. Hence, it is of crucial importance to measure and understand how the dopant type (Si, Fe, and Mg), doping level, and crystal quality alter the thermal conductivity of HVPE-grown bulk GaN. In this work, a steady-state thermoreflectance (SSTR) technique was used to measure the thermal conductivity of HVPE-grown GaN substrates employing different doping schemes and levels. Structural and electrical characterization methods including X-ray diffraction (XRD), secondary-ion mass spectrometry (SIMS), Raman spectroscopy, and Hall-effect measurements were used to determine and compare the GaN crystal quality, dislocation density, doping level, and carrier concentration. Using this comprehensive suite of characterization methods, the interrelation among structural/electrical parameters and the thermal conductivity of bulk GaN substrates was investigated. While doping is evidenced to reduce the GaN thermal conductivity, the highest thermal conductivity (201 W/mK) is observed in a heavily Si-doped (1-5.00 x 10(18) cm(-3)) substrate with the highest crystalline quality. This suggests that phonon-dislocation scattering dominates over phonon-impurity scattering in the tested HVPE-grown bulk GaN substrates. The results provide useful information for designing thermal management solutions for vertical GaN power electronic devices.

Understanding the percolation characteristics of multi-component conducting suspensions is critical for the development of flowable (semi-solid) electrochemical systems for energy storage and capacitive deionization with optimal electrochemical and rheological performance. Despite its significance, not much is known about the impact of the selected particle morphology on the agglomeration kinetics and the state of dispersion in flowable electrodes. In this study, the impact of the conductive additive morphology on the electrochemical and rheological response of capacitive flowable electrodes has been systematically investigated. Critical viscosity limits have been determined for common carbon additives that offer slurry formulations with improved electrochemical and rheological performance. For instance, at the same electrical conductivity of 60 mS cm(-1), higher aspect ratio particles, such as graphene and carbon nanotubes, offered 4 and 2.4 times lower viscosity compared to carbon black due to the improved packing and conformity of the high aspect ratio particles. On the other hand, thixotropic measurements showed that the flowable electrodes with carbon black exhibit the fastest agglomeration kinetics, offering 25 % less time to recover from the applied shear due to spherical morphology and facile agglomeration kinetics. Overall, our findings show that the particle morphology has a significant impact on the electrochemical and rheological performance of flowable electrodes with up to 40 % difference in capacitance for similar viscosity suspensions. Furthermore, a direct correlation between the rheological and the electrochemical properties was established, offering morphology-independent practical guidelines for formulating slurries with optimal performance. In this manner, particles that can achieve the highest density of packing before the critical limit were found to offer the optimal balance between electrochemical and rheological performance.

In this study, we report on an approach to flow cell design that can enable a significantly improved power output (similar to 10x) for electrochemical flow capacitors (EFCs), even at large flow channel gaps. Reticulated vitreous carbon (RVC) electrodes of various average pore sizes (0.43-2 mm) were integrated into EFC flow cell fixtures with channel gaps of 5 mm. Electrochemical testing under flow conditions showed a 10-fold improvement in the power density with the RVC integration (290 W/m(2), 580 W/kg) for the same slurry composition. This improvement was mostly attributed to the presence of a 3D porous electrode insert (i.e., RVC) that shortened the travel distance of the electrons to the current collectors. Pressure drop in the RVC containing cells was also investigated and found to increase up to 30% depending on the pore size. However, this increase was found to be offset by the increase in the channel depth, yielding almost no change in pressure as compared to conventional narrow-gap (>0.75 mm) flow channels. RVCs having an average pore size of 0.55 mm showed the best performance out of all studied cases with improved coulombic efficiency and good specific capacity (85 F/g) under flowing conditions. (C) 2018 The Electrochemical Society.