P. Alex Greaney

University of California, Riverside | UCR · Department of Mechanical Engineering & Materials Science and Engineering Program

In a typical semiconductor material, the majority of the heat is carried by long-wavelength, long-mean-free-path phonons. Nanostructuring strategies to reduce thermal conductivity, a promising direction in the field of thermoelectrics, place scattering centers of size and spatial separation comparable to the mean free paths of the dominant phonons...

The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our...

It remains a challenge to design aqueous electrolytes to secure the complete reversibility of zinc metal anodes. The concentrated water‐in‐salt electrolytes, e.g., 30 m ZnCl2, are promising candidates to address the challenges of the Zn metal anode. However, the pure 30 m ZnCl2 electrolyte fails to deliver a smooth surface morphology and a practica...

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions—25 wt.% LiCl and 62 wt.% H3PO4—cooled to −78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li⁺ ions become less hydrated and pair up with Cl⁻, ice‐like water...

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions—25 wt.% LiCl and 62 wt.% H 3 PO 4 —cooled to −78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li ⁺ ions become less hydrated and pair up with Cl ⁻ , ice‐like...

Predicting nanoscale thermal transport in dielectrics requires models, such as the Boltzmann transport equation (BTE), that account for phonon boundary scattering in structures with complex geometries. Although the BTE has been validated against several key experiments, its computational expense limits its applicability. Here, we demonstrate the us...

Rechargeable aqueous zinc batteries are finding their niche in stationary storage applications where safety, cost, scalability and carbon footprint matter most. However, harnessing this reversible two-electron redox chemistry is plagued by major technical issues, notably hydrogen evolution reaction (HER) at the zinc surface, whose impacts are often...

We present a method for deterministically solving the frequency and temperature dependent phonon radiative transport (PRT) equation in the single-mode relaxation time (SMRT) approximation in the self-adjoint angular flux (SAAF) form. To handle the non-linear coupling between the phonon intensities and the material temperature, we apply a linearizat...

Nanostructured semiconducting alloys obtain ultra-low thermal conductivity as a result of the scattering of phonons with a wide range of mean-free-paths (MFPs). In these materials, long-MFP phonons are scattered at the nanoscale boundaries whereas short-MFP high-frequency phonons are impeded by disordered point defects introduced by alloying. While...

Two new rod‐packing metal–organic frameworks (RPMOF) are constructed by regulating the in situ formation of the capping agent. In CPM‐s7, carboxylate linkers extend 1D manganese‐oxide chains in four additional directions, forming 3D RPMOF. The substitution of Mn²⁺ with a stronger Lewis acidic Co²⁺, leads to an acceleration of the hydrolysis‐prone s...

Silicon carbide nanoparticles with diameters around 8 nm and narrow size distribution have been finely mixed with doped silicon nanopowders and sintered into bulk samples to investigate the influence of nanoinclusions on electrical and thermal transport properties. We have compared the thermoelectric properties of samples ranging from 0%–5% volume...

Sodium ion batteries (NIBs) are an attractive alternative to lithium‐ion batteries in applications that require large‐scale energy storage due to sodium's high natural abundance and low cost. Hard carbon (HC) is the most promising anode material for NIBs; however, there is a knowledge gap in the understanding of the sodium binding mechanism that pr...

The ability to minimize the thermal conductivity of dielectrics with minimal structural intervention that could affect electrical properties is an important capability for engineering thermoelectric efficiency in low-cost materials such as Si. We recently reported the discovery of special arrangements for nanoscale pores in Si that produce a partic...

Nanostructured semiconducting alloys obtain ultra-low thermal conductivity as a result of the scattering of phonons with a wide range of mean-free-paths (MFPs). In these materials, long-MFP phonons are scattered at the nanoscale boundaries whereas short-MFP high-frequency phonons are impeded by disordered point defects introduced by alloying. While...

New acceptor‐type graphite intercalation compounds (GICs) offer candidates of cathode materials for dual‐ion batteries (DIBs), where superhalides represent the emerging anion charge carriers for such batteries. Here, the reversible insertion of [LiCl2]⁻ into graphite from an aqueous deep eutectic solvent electrolyte of 20 m LiCl + 20 m choline chlo...

Nanostructured materials enable high thermal transport tunability, holding promises for thermal management and heat harvesting applications. Predicting the effect that nanostructuring has on thermal conductivity requires models, such as the Boltzmann transport equation (BTE), that capture the non-diffusive transport of phonons. Although the BTE has...

Silicon carbide nanoparticles with diameters around 8 nm and with narrow size distribution have been finely mixed with doped silicon nanopowders and sintered into bulk samples to investigate the influence of nanoinclusions on electrical and thermal transport properties. We have compared the thermoelectric properties of samples ranging from 0-5% vol...

The ability to minimize the thermal conductivity of dielectric materials with a minimal structural intervention that could affect electrical properties is an important tool for engineering good thermoelectric efficiency in low-cost semiconductor materials such as Si. Special arrangements for pores in nanoporous Si have been recently identified in o...

The influence of strain rate across three orders of magnitude (1.70 × 10⁻⁵ /s to 1.43 × 10⁻² /s) along with the effect of the plastic strain accumulation (up to 10%) on the serrated plastic flow were investigated in the nickel-chromium (Ni-Cr) solid solution alloy Nimonic 75 by performing constant-strain-rate tension testing at 600°C. As the strain...

Engineering materials to include nanoscale porosity or other nanoscale structures has become a well-established strategy for enhancing the thermoelectric performance of dielectrics. However, the approach is only considered beneficial for materials where the intrinsic phonon mean-free path is much longer than that of the charge carriers. As such, th...

We report an ab initio study of the effect of rippling on the structural and electronic properties of the hexagonal Boron Nitride (hBN) and graphene two-dimensional (2D) layers and heterostructures created by placing these layers on the Hydrogen-terminated (H-) diamond (100) surface. Surprisingly, in graphene, rippling does not open a band gap at t...

Front cover image: Despite the remarkable progress made on high‐performing cathode materials, the showstoppers of aqueous Zn‐metal batteries are the dendrite problem and poor reversibility of zinc‐metal anode (ZMA). In article number 10.1002/cey2.70, research teams by Xiulei Ji, Jia‐Xing Jiang, Chong Fang and P. Alex Greaney collaborated to develop...

Nanostructured materials enable high thermal transport tunability, holding promises for thermal management and heat harvesting applications. Predicting the effect that nanostructuring has on thermal conductivity requires models, such as the Boltzmann transport equation (BTE), that capture the nondiffusive transport of phonons. Although the BTE has...

Engineering materials to include nanoscale porosity or other nanoscale structures has become a well-established strategy for enhancing the thermoelectric performance of dielectrics. However, the approach is only considered beneficial for materials where the intrinsic phonon mean free path is much longer than that of the charge carriers. As such, th...

We present a method for predicting thermal conductivity by deterministically solving the Boltzmann transport equation for gray phonons by utilizing arbitrary higher-order continuous finite elements on meshes which may also be unstructured and utilize curved surfaces. The self-adjoint angular flux (SAAF) formulation of the gray, steady-state, single...

We present a review, demonstration, and simulation of phonon transport for the purposes of predicting materials performance at the mesoscale. We focus primarily on the development and implementation of a unified methodology to enable predictive heat transport. We report on the current state of the art as it pertains to deterministic phonon transpor...

Prevailing nanostructuring strategies focus on increasing phonon scattering and reducing the mean-free-path of phonons across the spectrum. In nanoporous Si materials, for example, boundary scattering reduces thermal conductivity drastically. In this work, we identify an unusual anticorrelated specular phonon scattering effect which can result in a...

Prevailing nanostructuring strategies focus on increasing phonon scattering and reducing the mean-free-path of phonons across the spectrum. In nanoporous Si materials, for example, boundary scattering reduces thermal conductivity drastically. In this work, we identify an unusual anticorrelated specular phonon scattering effect which can result in a...

We present a method for predicting thermal conductivity by solving the Boltzmann transport equation for grey phonons deterministically by utilizing arbitrary higher-order continuous finite elements on meshes which may also be unstructured and utilize curved surfaces. The self-adjoint angular flux formulation of the grey, steady-state, single relaxa...

In this work, we present an algorithm which solves a version of the energy-conserving, multi-frequency, and temperature-dependent Boltzmann transport equation which is solved by means of a linearization method. The multi-frequency and temperature-dependent phonon transport equation in self-adjoint angular flux form is spatially discretized using hi...

We report a first-principles study of the structural and electronic properties of two-dimensional (2D) layer/hydrogen-terminated diamond (100) heterostructures. Both the 2D layers exhibit weak van-der-Waals (vdW) interactions and develop rippled configurations with the H-diamond (100) substrate to compensate for the induced strain. The adhesion ene...

A great challenge for all aqueous batteries, including Zn‐metal batteries, is the parasitic hydrogen evolution reaction on the low‐potential anode. Herein, we report the formula of a highly concentrated aqueous electrolyte that mitigates hydrogen evolution by transforming water molecules more inert. The electrolyte comprises primarily ZnCl2 and LiC...

The addition of porosity to thermoelectric materials can significantly increase the figure of merit, ZT, by reducing the thermal conductivity. However, porosity also is detrimental to the thermoelectric power factor in the numerator of ZT. In this manuscript we derive strategies to recoup electrical performance in nanoporous Si by fine tuning the c...

While the low-index planes of Wurtzite ZnO, such as {0001} and {101-0} are well-understood, the high-index crystal surfaces have not yet been thoroughly researched despite possessing structural characteristics that make them suitable for many important surface chemistry processes. The high surface energy of high-index ZnO crystal surfaces makes syn...

TiO2 has been widely studied as a photocatalytic material due to its non-toxicity, chemical inertness, and high photocatalytic activity. Here, we explore the operational behavior of a novel TiO2 micropillars array being developed to use solar radiation to treat recycled wastewater in long-duration space missions. A Light Capture model was developed...

We present a new method for predicting effective thermal conductivity (k) in materials, informed by ab initio material property simulations. Using the Boltzmann transport equation in a self-adjoint angular flux formulation, we performed simulations in silicon at room temperatures over length scales varying from 10 nm to 10 micron and report tempera...

The interactions between charge carriers and electrode structures represent one of the most important considerations in the search for new energy storage devices. Currently, ionic bonding dominates the battery chemistry. Here we report the reversible insertion of a large molecular dication, methyl viologen, into the crystal structure of an aromatic...

K-ion batteries (KIBs) are emerging as an alternative solution for large-scale energy storage. Carbon materials, including graphite and nongraphitic carbons such as soft carbons and hard carbons, have been studied as anode materials for K-ion storage. However, to date, it has been rarely reported that bulk carbon anode materials could reach a capac...

We present a new method for predicting effective thermal conductivity ($\kappa_{\textrm{eff}}$) in materials, informed by \emph{ab initio} material property simulations. Using the Boltzmann transport equation in a Self-Adjoint Angular Flux formulation, we performed simulations in silicon at room temperatures over length scales varying from 10 nm to...

It is accepted that the performance of batteries is dominated by the properties of their electrode materials. However, this maxim is built from the practice that the majority of research efforts have been directed toward batteries that use metal ions as the charge carriers. Herein, we show that the use of NH4⁺ results in battery performance governe...

The design of Faradaic battery electrodes that exhibit high rate capability and long cycle life equivalent to those of the electrodes of electrical double-layer capacitors is a big challenge. Here we report a strategy to fill this performance gap using the concept of Grotthuss proton conduction, in which proton transfer takes place by means of conc...

Plastic deformation proceeds through a sequence of stochastic local slip followed by load redistribution. With continued deformation this builds up complex stress fields and develops a heterogeneous pat- tern of local strength, leading to the emergence of microvoids and cracks. The goal of this research is to develop a coarse grained model for crys...

This paper utilizes DFT calculations to explore amorphous carbon material, in its context as a NIB anode. In the study, simulations are first used to validate the experimentally determined storage mechanism, with those results then being extrapolated to elucidate a theoretical capacity limit. Through investigating a breadth of structures, with mult...

A method for modeling cubically anisotropic elasticity within the discrete element method is presented. The discrete element method (DEM) is an approach originally intended for modeling granular materials (sand, soil, and powders); however, recent developments have usefully extended it to model stochastic mechanical processes in monolithic solids w...

The spin-phonon interaction is the dominant process for spin relaxation in Si, and as thermal transport in Si is dominated by phonons, one would expect spin polarization to influence Si's thermal conductivity. Here we report the experimental evidence of just such a coupling. We have performed concurrent measurements of spin, charge, and phonon tran...

Thermal resistance across the interface between touching surfaces is critical for many industrial applications. We developed a network model to predict the macroscopic thermal resistance of mechanically contacting surfaces. Contacting interfaces are fractally rough, with small islands of locally intimate contact separated by regions with a wider ga...

We report that crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), an organic solid, is highly amenable to host divalent metal ions, i.e., Mg ²⁺ and Ca ²⁺ , in aqueous electrolytes, where the van der Waals structure is intrinsically superior in hosting charge-dense ions. We observe that the divalency nature of Mg ²⁺ causes never-repor...

Energy storage is the absent enabler to facilitate the proliferation of renewable-but-intermittent solar and wind energy. There is one primary metric for stationary storage: levelized energy cost over the life time of devices. It is, thus, a must to rely on earth-abundant elements, which rules out lithium-based devices for the grid-scale storage pu...

We present a method for solving the Boltzmann transport equation (BTE) for phonons by modifying the neutron transport code Rattlesnake which provides a numerically efficient method for solving the BTE in its self-adjoint angular flux (SAAF) form. Using this approach, we have computed the reduction in thermal conductivity of uranium dioxide (UO2) du...

We report that crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), an organic solid, is highly amenable to host divalent metal ions, i.e., Mg2+ and Ca2+, in aqueous electrolytes, where the van der Waals structure is intrinsically superior in hosting charge-dense ions. We observe that the divalency nature of Mg2+ causes unique squeezin...

Failure of metallic materials due to plastic and/or creep deformation occur by the emergence of necking, microvoids, and cracks at heterogeneities in the material microstructure. While many traditional deformation modeling approaches have difficulty capturing these emergent phenomena, the discrete element method (DEM) has proven effective for the s...

Hard carbon is the leading candidate anode for commercialization of Na-ion batteries. Hard carbon has a unique local atomic structure, which is composed of nanodomains of layered rumpled sheets that have short-range local order resembling graphene within each layer, but complete disorder along the c-axis between layers. A primary challenge holding...

Graphite is the commercial anode for lithium-ion batteries; however, it fails to extend its success to sodium-ion batteries. Recently, we demonstrated that a low-cost amorphous carbon—soft carbon exhibits remarkable rate performance and stable cycling life of Na-ion storage. However, its Na-ion storage mechanism has remained elusive, which has plag...

The Green-Kubo method is a commonly used approach for predicting transport properties in a system from equilibrium molecular dynamics simulations. The approach is founded on the fluctuation dissipation theorem and relates the property of interest to the lifetime of fluctuations in its thermodynamic driving potential. For heat transport, the lattice...

We demonstrate for the first time that hydronium ions can be reversibly stored in an electrode of crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). PTCDA exhibits a capacity of 85 mAh g(-1) at 1 A g(-1) after an initial conditioning process. Ex situ X-ray diffraction revealed reversible and significant structure dilation upon reduct...

We demonstrate for the first time that hydronium ions can be reversibly stored in an electrode of crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). PTCDA exhibits a capacity of 85 mAh g−1 at 1 A g−1 after an initial conditioning process. Ex situ X-ray diffraction revealed reversible and significant structure dilation upon reduction...

Eine saure wässrige Lösung als Elektrolyt in einer Hydroniumionen-Batterie mit H3O⁺ als Ladungsträger: X. L. Ji et al. bieten in ihrer Zuschrift (DOI: 10.1002/ange.201700148) Belege dafür, dass Hydroniumionen reversibel in 3,4,9,10-Perylentetracarbonsäuredianhydrid (PTCDA) inseriert und aus ihm extrahiert werden können. Die Expansion und Kontraktio...

An acidic aqueous solution acts as an electrolyte for a hydronium-ion battery where the charge carrier is H3O⁺. In their Communication (DOI: 10.1002/anie.201700148), X. L. Ji et al. provide the evidence that hydronium ions can be reversibly inserted into and extracted from 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). The expansion and cont...

For decades silicon has been the workhorse material for electronic devices, and the semiconductor industry has mastered exquisite control of its electronic properties. With this history of development, and due to its relatively slow spin relaxation, Si is also anticipated to be the principal material for the next generation of spintronics devices....

During cycling in a Na/Carbon half-cell, the surface of the Na metal becomes progressively passivated and buried beneath an solid electrolyte interface layer, much like a beach getting covered by a rising tide. This leads to degradation of performance and eventual cell failure. However, performance can be restored to its original values through rep...

Recent advances for fabricating micro-featured architectures such as posts or pillars in fluidic devices provide exciting opportunities for multiphase ow management. Here we describe a novel, multiscale modeling approach for two-phase flows in microfeatured architectures developed within the Shan and Chen Lattice Boltzmann method. In our approach a...

Capacity fading in Na/C half-cells has long been attributed to the continuous formation of the solid electrolyte interphase (SEI). The SEI is often assumed to grow thicker on the carbon anode surface during cycling, eventually rendering it electrochemically inactive. However, the findings here shed new light on this assumption. It is shown that SEI...

This paper presents a method for the systematic and automated design of flexible organic linkers for construction of metal organic-frameworks (MOFs) in which flexibility, compliance, or other mechanically exotic properties originate at the linker level rather than from the framework kinematics. Our method couples a graph grammar method for systemat...

Metal Organic Responsive Frameworks (MORFs) are a proposed new class of smart materials consisting of a Metal Organic Framework (MOF) with photoisomerizing beams (also known as linkers) that fold in response to light. Within a device these new light responsive materials could provide the capabilities such as photo-actuation, photo-tunable rigidity,...

An atomistic level understanding of how varying types and numbers of irradiation induced defects affect thermal resistance in graphite is vital in designing accident tolerant fuels for next-generation nuclear reactors. To this end we performed equilibrium molecular dynamics simulations and computed the change to thermal conductivity due to a series...