Symposium EN07—Mechano-Thermal and Electrical Coupling in Emerging Energy Materials

Multifunctionality in polymers facilitates their application in emerging energy technologies. Electrical fields are a preferred stimulus because of the speed and ease of application to bulk polymers. While a wide range of electrically triggered actuators have been developed, and electrically controlled adhesion between gels has been demonstrated, modification of bulk mechanical properties via electrical stimuli remains elusive. Such a function could be particularly useful in electrochemical devices such as batteries and low temperature fuel cells that rely on polymer membranes to conduct ions while maintaining mechanical integrity. Fields vary temporarily and tend to deviate locally, for example from dendrite growth, and an electric field adaptive polymer could help avoid short circuits that lead to device failure. Polymers with covalently incorporated ionic charge should be well suited to achieving this goal since the mechanical properties depend on electrostatic interactions and these charges are intrinsically susceptible to electric fields. I will present our work investigating whether electric fields can modulate the mechanical properties of polyelectrolytes and our efforts to understand potential governing mechanisms. I will focus primarily on our molecular dynamics (MD) simulations. These simulations reveal the importance of polymer chain architecture and show how chain orientation and ion clustering relates to bulk mechanical properties. I will also share some of our theoretical, synthetic, and experimental work that builds from these MD simulations towards a physical realization of the idea.

Biomedical implantable and wearable electronic devices are promising tools to improve human wellness by real-time monitoring health indicators and curing disease more effectively. The development of smart contact lenses is of great importance mainly due to their wide range of applications, from vision correction to drug delivery. Accordingly, there are numerous research studies taking place to commercialize smart contact lenses. One of the main roadblocking issues toward its commercialization is the power supply and management for the microelectronics, which should be mounted on the device. Batteries, photovoltaics, and wireless power transfer are the most well-known strategies to supply power to smart contact lenses. However, all of these techniques suffer from several drawbacks. For instance, radio power scavenging either requires large antennas or the ranging distance is lower than 10 cm. Besides, batteries need periodic charging, which is a cumbersome practice for a contact lens user. On the other hand, mechanical to electrical energy conversion can supply power to the smart contact lenses by natural human eye blinking that is an environmentally independent source of energy.
Triboelectricity based on contact electrification and electrostatic induction, piezoelectricity based on applying stress to a non-centrosymmetric crystal structure, and electrostatic generator based on capacitance gradient are among the most famous mechanisms to convert mechanical energy to electricity. However, herein, we demonstrate the performance of our recent mechanical to electrical energy harvester operating based on electrochemical reactions and charge transfer. A 300nm SiO₂ was thermally grown on a 5×5 mm² p-type silicon piece, followed by the spin coating of 2 µm Cytop as a hydrophobic layer. A 5×1 mm² of the SiO₂/Cytop stack was photolithographically patterned and etched utilizing CF₄ and oxygen gases with the flow rate of 50 and 5 sccm, respectively. A silver paste was used as the bottom contact of the device. Employing a linear actuator, the device has a back-and-forth motion (speed = 20 mm.s^-1) on a moisturizing eye drop in contact with a metal foil. In the present study, we have experimentally measured the electrical output of the sliding electrochemical electrifier using three metal foils, including copper (Cu), aluminum (Al), and gold (Au). The voltage outputs were directly detected at the Siglent oscilloscope with the input impedance of 1 MΩ. The current outputs were detected using the SR570 low noise amplifier.
Results demonstrate the maximum voltage of 80, 200, 620 mV while using Cu, Au, and Al metals, respectively. The corresponding maximum currents are 17.5, 8, and 80 µA. Accordingly, we used a 4.7 µF capacitor to experimentally measure the energy and power output of the electrochemical sliding energy harvester. 15 nJ (0.075 µW.cm^-2), 94 nJ (0.47 µW.cm^-2), and 0.9 µJ (3.6 µW.cm^-2) are the energy (power density) output of the device while using Cu, Au, and Al metals. The electrical output was recorded when the eye drop touches the device’s contact (silver paste). The ascending voltage and energy outputs of the harvester is clearly following the electrochemical potentials of the metals. Physically speaking, the electrochemical reactions between the moisturizing eye drop and metals’ surface resulting in redox reactions. Hence, the liquid either loses or gains electrons which will be dragged and transferred to the silver paste using the frequent sliding motion. This phenomenon can be optimized and used to supply power to the smart contact lenses through a natural eye blinking.

Barocaloric materials—which exhibit pressure-induced thermal changes—can be used to drive solid-state cooling cycles that offer an environmentally friendly alternative to vapor compression refrigeration. The development of efficient, scalable barocaloric cooling will benefit from a class of materials featuring a phase transition mechanism that provides large entropy changes, minimal hysteresis, high sensitivity to hydrostatic pressure, and chemical tunability. Here, we describe a mechanism for realizing a new class of materials that feature colossal and tunable barocaloric effects. This mechanism is based on conformational order–disorder transitions of long-chain organic molecules—confined in space—that give rise to large changes in volume and entropy. Additionally, we demonstrate how the synthetic tunability of this system can be leveraged to control structural and chemical factors that influence the thermodynamics and kinetics of pressure-induced phase transitions. Through these efforts, we report a library of materials that feature a combination of colossal entropy changes, high barocaloric coefficients, and ultralow hysteresis. Further, the excellent processability of these materials, in combination with their unique mechanical properties, offers particularly simple strategies for harnessing barocaloric effects in prototype cooling devices across length scales. This work provides new insights into barocaloric effects of metal–organic materials, with a particular emphasis on understanding and manipulating thermodynamics and kinetics of pressure-induced phase transitions.

The ability to valorise waste is a cornerstone of circular economy. Developing
materials from waste biomass requires fundamental understanding of biomass degradation and biobased materials structure-property relationships. In this talk, we will provide an overview of ongoing work and future perspectives on multiscale modelling and simulations of biomass and biobased functional materials, including lignin, nanocellulose, chitin, biocrude oils, and carbon nanoparticles. By coupling modelling and simulations to hydrothermal processing (HTP) experiments we develop fit-for-purpose biobased materials with applications in energy harvesting and storage, infrastructure and precision agriculture, among others. We apply Density Functional Theory (DFT), Molecular Dynamics (MD) simulations, and coarse-grained (CG) modelling to simulate materials behaviour from the
nanoscale to the mesoscale. We simulate chemical reactivity, electron transport,
mechanical properties, and degradation mechanisms.

2D hybrid organic-inorganic perovskites (HOIPs) are low-cost, high-performance semiconductor materials with great potential demonstrated in various fields, including photovoltaics, light-emitting diodes, lasers, and field-effect transistors. During device fabrication and device operation, 2D HOIPs will go through various temperature statuses. Because of the hybrid organic and inorganic nature, the temperature fluctuation will cause a significant influence on the mechanical behavior of 2D HOIPs, which is vital for material processing and device durability, but is largely unknown. Here we measured the in-plane elastic modulus E of a prototypical 2D HOIP, (C₄H₉-NH₃)₂(CH₃NH₃)₂Pb₃I₁₀ as a function of temperature across a range of technological interest (i.e., -15 ^oC to 65 ^oC) by AFM stretching suspended 2D HOIP nanosheets. Compared to the values at room temperature, E varies by more than 20% and exhibits a non-monotonic temperature dependence, which does not follow the general anharmonic expectation of the thermo-mechanical behavior for materials, where a higher temperature usually leads to weaker bonds and lower E. The observed abnormal trend can be explained by temperature-induced melting and stacking of the organic spacer molecules, which modulates the interlayer interactions of the 2D HOIPs at the van der Waals interface. Our results reveal the critical role of the organic layers in determining the mechanical behaviors of 2D HOIPs, unlike the negligible influence of the organic cations on the mechanical properties of their 3D counterparts, and provides indispensable insights into the processing and device applications of 2D HOIPs involving temperature fluctuations

Conventional methods to harvest and supply energy can hardly satisfy the rapid expansion of the Internet of Things (IoT). As an emerging solution, flexible triboelectric nanogenerators (TENGs) have been invented and developed for over a decade to scavenge mechanical energy from the ambient environment. The operation of TENGs mostly depends on contact electrification, which exists between any two materials with electrostatic charge potential difference which makes it a portable solution to energy generation on demand in the future. Much research has focused in recent years on enhancing the output power of TENGs to its maximum limit. To optimize the performance of TENGs, choosing the desirable material pair with high work function difference, increasing contact surface area between materials, and designing an efficient device structure are deemed to be the three most dominant tasks. Even though many strategies have been applied to enhance contact electrification, for instance, adding nanoparticles or nanofiller, plasma treatment, soft lithography, dry-etch, etc., most of them are still restricted by the materials selection, cost of chemical processing, and loss of mechanical properties. Herein, we present a robust and unique route to fabricate a surface-modified triboelectric nanogenerator to harvest wind energy, realized by the phase separation of polymer blend films. Among many nature and organic matter, polyvinylidene difluoride(PVDF) has been experimentally proven as excellent dielectric materials in TENGs and other energy harvesting technologies, owing to its good formability and flexibility, large dipole moment, and low acoustic impedance. Our proposed approach is based on the increase of surface roughness in nanoscale derived from the controlled phase separation of PVDF containing blend film. This strategy is aimed to enhance the performance of PVDF films in TENGs by regulating the contact surface area and the mechanical energy induced by relative friction force. The topographically designed films, along with metal electrodes, fixed on a supporting device is seamlessly assembled by additive manufacturing (3D printing). The fabrication of future TENG devices followed our proposed methods will be cost effective, and target high power density output in the competition for power via next-generation flexible devices and electronics.

We acknowledge Kostas Research Institute(KRI) At Northeastern University Grant #555033-78055 for the support of the work.

Developing a stronger understanding of the metal-insulator transition in transition metal oxides (TMOs) is key for modulating the performance of TMO devices. Two TMO materials (NbO₂ and VO₂) exhibit metal-insulator transitions whose thermodynamics have proven difficult to describe using conventional electronic structure calculation methods. We develop a formalism that links electronic transport properties to the state electronic entropy and apply this formalism to calculate the change in the electronic entropy of transition. Our electronic entropy calculations agree with estimates of the vibrational entropy and calorimetrically measured totals. Further, these calculations suggest a method for quantifying the effect of doping on the transition entropy, thereby enabling the engineering of the metal-insulator transition as a function of chemistry. For systems where the thermodynamics of the transition are well understood, our formalism provides estimates of the electronic transport properties, including Seebeck coefficient and resistivity.

The chemo-mechanical effect in solids refers to dimensional change due to change in stoichiometry. Dimensional change due to electrochemically-induced compositional change has been termed the electro-chemo-mechanical (ECM) effect. The mechanical instability inherent in this effect is clearly deleterious for batteries or fuel cells, but, as recently suggested, has potential for use in actuation[1]. The structure of an actuator device that operates on the ECM principle comprises a micrometer thick solid electrolyte (SE) sandwiched between two ECM-active, working body (WB) layers. An electrochemical reaction must occur in these layers, causing them to alternately expand or contract. In order to facilitate the ECM response, the WB layers should have mixed ionic and electronic conductivity and a large chemical expansion coefficient.
We have constructed a 2mm diameter thin film membrane ECM actuator device comprising 20mol% Gd doped CeO₂ (20GDC) as the SE and [TiO_2-δ\20GDC] or [V₂O_5-δ\20GDC] composites as the WBs[2]. Selected area electron diffraction measurements showed the composite to be nanocrystalline, a morphology that promotes interfacial oxygen ion diffusion. Synchrotron X-ray absorption (XAS) measurements detected a mixture of Ce³⁺/Ce⁴⁺ (~[0.4]/[0.6]) and Ti³⁺/Ti⁴⁺ (~[0.1]/[0.9]) oxidation states in the WB. XAS measurements under bias showed changes in the short-range order of Ti and V oxides supporting the presence of a redox reaction. The deformation of the ECM actuator was observed to be in the bending regime producing large vertical displacements (~3 μm) and ~4 MPa stress. The stress/voltage ratio yields a pseudo piezoelectric stress coefficient of e₃₁=1.26 C/m², comparable to common lead-free piezoelectrics such as lithium niobate, lithium tantalate and alkali niobates.

Metals remain the prototypical test-bed for understanding ultrafast energy transfer amongst non-equilibrium carrier populations, such as electron-phonon scattering rates, due to their dynamics being well-described by the two-temperature model (TTM). The majority of experimental studies in this area of research have devoted their efforts to pump-probe measurements of gold films due to its relatively simple electronic band structure and corresponding optical response. Aluminum, on the other hand, has received significantly less attention, despite a number of works demonstrating interesting and unique processes such as preferential phonon scattering times and non-thermal phonon populations that greatly affect the ultrafast dynamics. Additionally, the understanding of ultrafast energy processes in Al is more relevant than ever, particularly at low temperatures, due to its applicability in quantum sciences.
In this work, we perform a series of wavelength-resolved ultrafast thermometry measurements to directly monitor the dynamics of both the lattice and hot electrons near the interband transition threshold of Aluminum thin films. By monitoring the ITT energy as a function of both incident fluence and pump-probe time delay, we extract changes in the Fermi energy, and thus electronic chemical potential, based on varying electron temperatures. Further, by relating the wavelength-dependent response, an accurate model of the electron temperature of Aluminum upon ultrafast heating is extracted, allowing for measure of an average electron-phonon coupling factor.

Flexible and wearable electronics are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. It is of great importance to explore low cost and scalable preparation approaches for high performance flexible and wearable electronics. Carbon materials have combined superiorities such as good electrical conductivity, intrinsic and structural flexibility, light weight, high chemical and thermal stability, ease to be chemically functionalized, as well as potential mass-production, enabling them to be promising candidate materials for flexible and wearable electronics. In the last several years, based on the unique properties of nanocarbon materials, we have been working on the rational design and controlled fabrication of flexible electronics based on carbon nanotubes, graphene, silk and their hybrid materials and explored their applications in human health monitoring and human-machine interfaces. For examples, we have used carbon materials, including carbon nanotubes, graphene, and natural mass derived carbon as the key materials and develop a series of flexible wearable sensors (strain sensors, pressure sensors, temperature sensors, airflow sensors, sweat monitoring patch etc.), wires and flexible energy devices. The hierarchical structures of the carbon materials plays important roles in achieving flexible devices with desired performance. We have also explored the application of silk in flexible sensors by developing a carbonization strategy of silk nanofibers and fabric and by combing nanocarbons with natural silk materials. High performance strain/pressure/temperature sensors, E-skin, silk/graphene E-tattoos, and silk based conductive wires have been developed. We also developed new technique for the 3D printing of nanocarbon-silk-based smart fibers/textiles. Based on the above, the applications of the obtained electronics in human health monitoring (such as pulse, breath, and temperature and environmental risks), human motion tracking, and human-machine interfaces have been demonstrated. These strategy provide new approaches for the low-cost production of high performance flexible and wearable electronics, which may promote the development of next generation smart electronics.

Aligned assembly of carbon nanotubes (CNTs) is the key for high mechanical, electrical and thermal properties for CNT fibers. To overcome the weak van der Waals interactions between the CNTs, thermosetting polymers can be introduced into the fibers because efficient strengthening can be realized after a thermal curing. Here a better curing strategy, namely electro curing, is applied to bismaleimid (BMI)-impregnated CNT fibers to improve the inter-tube thermal transport. As the electric current is conducting along the CNT surface and induces the strongest Joule heating right there, the aromatic BMI resins can be cured into an oriented structure. Under the optimal curing current whose Joule heating is strong without inducing the BMI degradation, the fiber's intrinsic thermal conductivity can be improved from 30 to 177 W m^-1K^-1, and the apparent conductivity can be up to 374 W m^-1K^-1 at a sample length of 12 mm. The effect of thermal radiation is also semi-quantitatively studied by estimating the real surface area that radiates heat in the 3ω method.

Plastic, though once is considered a miracle material with its durability and malleability, now poses a significant environmental problem by creating hazardous wastes, because of its massive application to single-use products and difficulty in degradation in nature. Our team aims to create a substitute of plastic with better mechanical strength, low cost, and biodegradability for fields like agriculture, packaging, and more. Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer that can be used to make non-toxic bags/films which break down over time. Its production keeps increasing and it has been used in food and drug packaging, as well as biomedical fields including cartilage replacements and contact lenses. Its massive use can effectively reduce land and ocean pollution. However, it lacks thermal stability and is easy to absorb water from a humid environment. We are thus motivated to investigate the large-scale molecular structures of PVA and reveal its structure-thermal-mechanics relationship by using molecular dynamics simulation. We obtain the folded PVA structure from a fully extended state and unfold it in steered molecular dynamics to reveal its nanomechanics, as well as how it interfaces to other biomolecules. We find that the PVA structure folded at room temperature is stabilized by the randomly formed hydrogen bonds between hydroxyl groups but there is a lack of ordered structure, making the structure easy to unfold by temperature fluctuation and interaction with water molecules. A smaller amount of crystallinity and less energy dissipation is found by comparing to the PVA sheet-like structures folded through the annealing process. We study the hybrid mixture of PVA and collagen and investigate how the PVA structure and mechanics are stabilized by forming the interface with collagen. Our study provides insights into mixing PVA to collagen to construct a biodegradable composite material with higher thermal and humidity, enabling its durability in use but still friendly to the environment.

High entropy alloys have attracted extensive attention due to the complex formation from several metal elements and their extraordinary performances, such as lightweight, high ductility in extreme working temperatures. In this study, we studied the intergranular creep fracture of FeCoNiCrMn by exploiting micromechanical modeling. We considered a dislocation-based crystal plasticity model for grain interior and a cohesive zone model for grain boundary sliding and separation. We developed an integrated computational framework to simulate the mechanical properties of FeCoNiCrMn by implementing user-defined material (UMAT) for grain interiors and user-defined element (UEL) for grain boundaries in commercial finite element software Abaqus. We first performed a parametrical study to examine the robustness of the model. The simulation results agree well with the experimental research. Furthermore, we discussed the effects of strain rate, grain boundary separation, and grain boundary viscosity on the mechanical behaviors. We found that the stress concentration occurs at the grain boundaries with a larger disorientation angle, making the grain boundary prone to separate from alleviating this severe strain inhomogeneous. Finally, the competition mechanisms of creep fracture for FeCoNiCrMn were studied. The simulation result suggested that the creep fracture is dominated by grain boundary separation, consistent with the experimental observation. Thus, our model can be applied to study grain interior and grain boundary mechanical behavior at high temperature for different high entropy alloys with great potential in automobile, aerospace, and high-performance alloy design.

GaN-based electronic devices has been widely applied in energy, information and vehicle areas. Strain inevitably exists in practical GaN based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth and band engineering with strain, and significantly influence phonon thermal properties of GaN. In this work, phonon properties and thermal conductivity of GaN including contribution from electron-phonon coupling (EPC) as well as the effects of in-plane biaxial strain are systematically investigated using the first principle calculation method and phonon Boltzmann transport equation. Thermal conductivity of free GaN is 263 and 257 W/mK for in-plane and cross-plane directions, respectively, which is consistent better with the maximum experimental values in literature than previous theoretical reports, and shows a nearly negligible anisotropy of thermal conductivity. Under strain state, thermal conductivity changes remarkably both in absolute values and anisotropy. For instance, under +5% tensile strain state, average thermal conductivity at room temperature decreases by 63% while it increases by 53% under -5% compressive strain state, which is mostly attributed to the changes of phonon relaxation time. Besides, anisotropy of thermal conductivity changes under different strain states, which may result from the weakening effect from strain induced piezoelectric polarization. EPC is also calculated from first principles, and it decreases the lattice thermal conductivity significantly. Specifically, the decrease shows significant dependence on strain state, which is due to the relative changes between phonon-phonon and electron-phonon scattering rates, as well as the absolute value of lattice thermal condutivity without including EPC. Under compressive strain state, decreases of lattice thermal conductivity are 15% and 18% for in-plane and cross-plane conditions, respectively, comparable with those under free state. However, the decreases become very small under tensile strain state, especially for cross-plane condition, which results from the decreased electron-phonon scattering rates and increased phonon anharmonicity.

Low-grade heat is abundant in the ambient environment and provides a sustainable energy source for health monitoring devices and wireless sensor network. Liquid thermocells offer a promising solution for converting the widespread low-grade heat directly into electricity due to the advantages of relatively high thermopower, low cost, and high flexibility. Recently, we developed a polarized ternary electrolyte that switches from n-type to p-type when the hot electrode temperature is heated above the gelation temperature of methylcellulose. The ternary electrolyte achieves ultrahigh thermopowers of -8.18 mV K⁻¹ for n-type and 9.62 mV K⁻¹ for p-type. The polarization switching can be attributed to the strong hydrophobic interaction between methylcellulose and I₃⁻ ions, while the giant thermopowers mainly come from the thermogalvanic effect of the I⁻/I₃⁻ redox couple enhanced synergistically by methylcellulose and potassium chloride. With the developed electrolyte, a single p-type liquid thermocell gives rise to an open-circuit voltage of 134 mV and a maximum power of 80.47 mW under a temperature difference of 15°C, corresponding to a normalized maximum power density of 0.36 mW m^-2 K^-2, which is far superior to other reported I⁻/I₃⁻-based liquid thermocells. Our work demonstrates a cost-effective, high-thermopower polarized electrolyte for low-grade heat harvesting.

Two-dimensional (2D) semiconducting materials, such as phosphorene, MoS₂, InSe, et al. have attracted a great deal of interest due to their unusual structures and fascinating physical properties promising for many novel applications. Since these 2D materials are able to sustain a large tensile strain, which can cause significant changes in electronic and thermal properties, strain engineering is an effective approach to tune their physical properties, which can greatly widen their applications. To do so, however, it is both important and necessary to understand and further tune these properties by applying strain.

In this talk, we report our research work on the strain engineering of 2D semiconducting materials to tune their electronic and/or thermal properties by using computational study. Using first-principles calculations and the non-equilibrium Green’s function method, we investigate ballistic thermal transport in 2D monolayer phosphorene. A significant crystallographic orientation dependence of thermal conductance is observed. Furthermore, we find that the thermal conductance anisotropy with the orientation can be tuned by applying strain. In particular, the zigzag-oriented thermal conductance is enhanced when a zigzag-oriented strain is applied but decreases when an armchair-oriented strain is applied; whereas the armchair-oriented thermal conductance always decreases when either a zigzag- or an armchair-oriented strain is applied. The present work suggests that the remarkable thermal transport anisotropy and its strain-modulated effect in single-layer phosphorene may be used for thermal management in phosphorene-based electronics and optoelectronic devices. Next, using first-principles calculations, we design a 2D transition metal compound, Ni-MoS₂, to investigate its HER catalytic performance. We reveal that with strain engineering, the bandgap can be reduced and a semiconductor-to-metal transition can be induced, which lead to an enhancement in the charge transfer and HER performance. Moreover, together with strain engineering, the thermoneutral condition can be achieved at the S vacancy concentration of only ∼2.5%, which is in strong contrast to ∼12.5% required for MoS₂. Finally, we report our work of using 2D InSe to fabricate a flexible and ultrasensitive three-terminal strain sensor. The strain sensor shows a prominent enhancement of sensitivity through gating effect. We also demonstrated InSe based flexible transistor and Boolean inverter circuit, which achieves a high mobility and an on/off current ratio. This work highlights the potential of using InSe as a common material platform for next generation flexible electronics and strain sensors applications.

Emerging metal halide perovskites have attracted attentions in thermoelectric energy conversions due to their ultralow thermal conductivity and excellent carrier transport properties. Using first-principle calculations, we find that halogen mixing (alloy) in CsPb(I_1-xBr_x)₃ not only leads to suppressed thermal conductivity, but also anisotropic carrier transport due to symmetry-breaking induced residue strains. External strain engineering strategies could tailor various thermoelectric properties, including carrier effective mass, carrier lifetime, and lattice thermal conductivity, in a more controllable manner to achieve the optimal thermoelectric figure of merit (ZT). Anisotropic ZT values predicted for CsPb(I_1-xBr_x)₃ upon uniaxial or biaxial loading show a wide range of variations from 20% to 400% compared to the unstrained cases with an enhanced ZT up to ~1.5 at 300 K. In addition, they could be used to reduce the optimal carrier concentration to achieve the maximum ZT by an order of magnitude compared to the unstrained cases, providing a new avenue to tackle the challenge in heavily-doping perovskites.

Artificial muscles are materials or devices producing work or motion under external stimuli. Recently, twisted-filament artificial muscles (TFAMs) have been demonstrated to meet the demands of miniaturization and multi-functionality in smart systems. TFAMs share a two-step process in triggering rotation or contraction: volumetric expansion and tension-torsion coupling. Various actuating mechanisms such as thermal expansion, electrochemical cues, solvents absorption were proposed. To understand the performance of TFAMs, we simulate the actuating processes by numerical simulations using the discrete elastic model (DER). Theoretical analysis is also conducted for models with a symmetric cross section in supplementary to the simulation results, revealing the basic principles that direct the actuating performance. We estimate the energy conversion efficiency as a function of the material properties and microstructural characteristics, which identifies the upper limits for realistic artificial muscles. We conclude that artificial muscles of twisted-microfilaments still have much space to improve in terms of the energy-conversion efficiency and work capacity.

Piezoelectric polymer nanofibers have emerged as a competitive platform for shape-adaptive energy harvesting and sensing applications because of mechanical flexibility, lightweight, and relatively low-temperature processing. However, a key challenge has been the relatively low piezoelectric coefficients of these organic piezoelectric materials when compared to their inorganic counterparts. Here, we summarize a collection of advances that push the boundaries to achieve a drastic enhancement of mechanical energy harvesting performance by tailoring both material and structural properties of electrospun piezoelectric polymer fibers at multi-scales. Engineering structural morphology of piezoelectric materials from molecular to device scales will be discussed as a route to achieving improvement in the key figures of merit for energy harvesting and sensing, along with our unfolding new understanding of the underlying physics.

Because the best performing temperature range of thermoelectric (TE) materials differ from each other, segmented TE legs stacked with multiple TE materials has been studied as a mean to improve the performance of thermoelectric generators (TEGs). However, due to the inherent nonlinearity that limits the application of conventional optimization approaches, the optimal segmented TEGs (STEGs) had to be sought heuristically. In this study, we propose a systematic approach enabling the efficient exploration and exploitation of vast design space of STEG by leveraging the fast inference of deep learning. First, we train a neural network (NN) model using dataset generated from FEM simulations for a single TE leg stacked with 4 segments out of 18 TE materials by varying segment lengths and external load. We then conduct a genetic optimization algorithm (GOA) using our trained NN model to search for high-performing design candidates. The performance of the new candidates are computed with FEM, which is used to update the neural network by active transfer learning technique. After iteratively performing the procedure, we are able to fine-tune the TE legs with optimal efficiency, optimal power, as well as a given combination of both. Furthermore, we discuss the physical origin of the optimal performing STEGs.

Thermoelectric effects refer to the direct energy conversion between thermal and electrical energies. When a temperature difference is applied on a thermoelectric element, a thermoelectric generator module can generate electrical power with induced electrical voltage and current while the thermoelectric heat current flows with Fourier (diffusion) and Peltier heat.
Traditionally, thermoelectric efficiency is estimated using a simple metric called a dimensionless figure of merit, expressed as zT = (α²/ρκ)T, where α, ρ, κ, and T are the Seebeck coefficient, electrical resistivity, thermal conductivity, and temperature, respectively. In the constant property model (CPM), in which thermoelectric properties are constant, the maximum thermoelectric efficiency η_max is simply given as η_max = (T_h-T_c)/T_h x [(1+zT_m)^0.5–1]/[(1+zT_m)^0.5+T_c/T_h], where T_h and Tc are hot- and cold-side temperatures, respectively, and T_m is (T_h+T_c)/2 [1]. With a monotonic nature of zT_m in efficiency, it has been widely used to guide thermoelectric material discovery.
However, as the thermoelectric conversion module works under a non-zero temperature difference (T_h-T_c > 0), the crucial assumption of CPM that thermoelectric properties are constant no longer holds. Some studies have attempted to find a suitable efficiency metric for wide temperature applications [1–7]. However, many recent studies have shown that discrepancies between the CPM and efficiency are unavoidable [8–14]. Although a few cumulative approaches have been investigated to correct thermoelectric heat current equations [12,13], the accuracy of efficiency prediction remains questionable. In addition, a counterintuitive example between ZT and efficiency highlights the need for a highly accurate efficiency evaluation method [14]. This type of non-negligible mismatch becomes an obstacle for further material optimization and device design for higher efficiency.
In this work, we develop the general efficiency theory of thermoelectric conversion with temperature-dependent thermoelectric properties to overcome the limitation of zT. We derive thermoelectric equations for power conversion and heat transfer, where the thermoelectric differential equation is successfully transformed into the T integral equation. By introducing two additional indices in addition to the proper definitions for device parameters, we formulate heat current equations at boundaries. As a result, we derive an efficiency equation determined by three thermoelectric degrees of freedom (DOFs): Z_gen, tau, and beta, where the existence of non-zero τ and β well explains the origin of the discrepancy between CPM and efficiency. Our thorough data-driven investigation on approximately 300 published materials confirms that our method is sufficiently accurate. Furthermore, we find that efficiency is a monotonic function of each DOF, which indicates the potential use of a DOF as a figure of merit. Finally, with suitable temperature approximation, we derive highly accurate approximate analytical expressions for thermoelectric module performance that can be easily applied to segmented leg design and p/n-leg pair module performance estimation even with interfacial contact resistance.
[1] A.F. Ioffe, Semiconductor Thermoelements and Thermoelectric Cooling (1957).
[2] J.M. Borrego, IEEE Trans Aerospace 2, 4 (1964).
[3] G. Min et al., J. Phys. D 37, 1301 (2004).
[4] G.J. Snyder and A.H. Snyder, EES 10, 2280 (2017).
[6] B. Ryu, Bulletin of the American Physical Society 64, R47.00007 (2019).
[7] P. Ponnusamy et al., Appl. Energy 262, 114587 (2020).
[8] B. Sherman et al., JAP 31, 1 (1960).
[9] B. Ryu et al., ArXiv:1810.11148 (2018).
[10] B. Ryu et al., ArXiv:1910.11132 (2019).
[11] P. Ponnusamy et al., Müller, Entropy 22, 1128 (2020).
[12] A.A. Efremov and A.S. Pushkarsky, Energy Conversion 11, 101 (1971).
[13] H.S. Kim et al., PNAS 112, 8205 (2015).
[14] B. Ryu et al., APL. 116, 193903 (2020).

Hybrid organic-inorganic perovskites have attracted enormous attention for low-cost solution-processable photovoltaics. In addition to the excellent defects-tolerant optoelectronic properties of the metal halide inorganic lattice, hybrid perovskites have peculiar properties associated to the hybrid organic/inorganic and covalent/ionic nature, giving rise to phenomena coupling electronic properties and ionic dynamics, of relevance for sensing, thermoelectricity, spintronics and many others.
We start discussing the ability of classical interatomic force-fields[1], to reproduce the main structural and thermodynamical properties of 3D and 2D hybrid perovskites[2] and we review a selection of properties associated to ionic dynamics: the dielectric response and macroscopic polarization[3]; the ultra-high and anisotropic mobility of ionic defects[4]; the interaction of defects with extended defects[5,6] and their relevance for hysteresis and memory effects. In particular we report evidence of the formation of long-lived electret states associated to the interaction of defects and ferroelastic domains in the tetragonal phase[7].
[1] A. Mattoni, A. Filippetti, and C. Caddeo, “Modeling hybrid perovskites by molecular dynamics,” J. Phys. Condens. Matter, vol. 29, no. 4, p. 043001, Feb. 2017. 10.1088/1361-648X/29/4/043001
[2] Giri, A.; Chen, A. Z.; Mattoni, A.; Aryana, K.; Zhang, D.; Hu, X.; Lee, S.-H.; Choi, J. J.; Hopkins, P. E. Ultralow Thermal Conductivity of Two-Dimensional Metal Halide Perovskites. Nano Lett. 2020, 20 (5), 3331–3337 10.1021/acs.nanolett.0c00214
[3] A. Mattoni and C. Caddeo, “Dielectric function of hybrid perovskites at finite temperature investigated by classical molecular dynamics,” J. Chem. Phys., vol. 152, no. 10, p. 104705, Mar. 2020. 10.1063/1.5133064
[4] P. Delugas, C. Caddeo, A. Filippetti, and A. Mattoni, “Thermally Activated Point Defect Diffusion in Methylammonium Lead Trihalide: Anisotropic and Ultrahigh Mobility of Iodine,” J. Phys. Chem. Lett., vol. 7, no. 13, 2016. 10.1021/acs.jpclett.6b00963
[5] C. Caddeo, A. Filippetti, and A. Mattoni, “The dominant role of surfaces in the hysteretic behavior of hybrid perovskites,” Nano Energy, vol. 67, p. 104162, Jan. 2020. 10.1016/j.nanoen.2019.104162
[6] N. Phung et al., “The Role of Grain Boundaries on Ionic Defect Migration in Metal Halide Perovskites,” Adv. Energy Mater., vol. 1903735, p. 1903735, Apr. 2020. 10.1002/aenm.201903735
[7] A.G. Lehmann et al., “Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CH 3 NH 3 PbBr 3 ferroelastic single crystals, ” Phys. Chem. Chem. Phys. vol. 23, p. 3233–3245. 2021 https://doi.org/10.1039/D0CP05918H.

Near-field Thermophotovoltaic (NTPV) cells have attracted a wide range of research interest due to their enhanced performances compared to far-field counterparts. However, most of the existing studies are focused on the regime where the thermal emitter has very high temperature, e.g., higher than 1000 K, corresponding to solar radiations or heat radiations from a secondary emitter which receives the solar energy. Besides, due to the considerable frequency mismatch between the thermal emitter and the PV cell, the near-field effect becomes ineffective and the performance of the NTPV systems is significantly reduced.
In our previous paper [1], We focus on the situation where the temperature of the thermal emitter is in the range of common industrial waste heat, i.e., 400 K ~ 800 K. To convert such mid infrared thermal radiation into electricity and to make the photon tunneling efficient, we propose to use graphene-h-BN heterostructures as the emitter and the graphene-covered InSb p-n junction as the TPV cell. We show that the optimal output electrical power can reach 76000 Watts per square meters and the best efficiency is achieved with 33% of the Carnot efficiency. These results show that the performances of near-ield thermophotovoltaic systems can be comparable with or even superior to the state-of-the-art thermoelectric devices. The signiecant improvement of the thermophotovoltaic performance is understood as due to the resonant coupling between the emitter and the p-n junction, where the surface plasmons in graphene and surface-phonon polaritons in boron nitride play crucial roles.
It is remarked that our findings are consistent with the study in Ref. [2], where the synergy between near-field thermal radiation and inelastic thermoelectricity is shown to have considerably improved performance even in the linear-transport regime. Apart from this, our work is also based on the previous studies where the near-field effects are shown to improve the heat radiation flux by orders of magnitude via infrared hyperbolic metamaterials [3-4].

Reference papers:
[1] Rongqian Wang, Jincheng Lu and J.-H. Jiang, Enhancing Thermophotovoltaic Performance using Graphene-BN-InSb Near-Field Heterostructures, Phys. Rev. Applied 12, 044038 (2019).
[2] Jian-Hua Jiang and Yoseph Imry, Near-field three-terminal thermoelectric heat engine, Phys. Rev. B 97, 125422 (2018).
[3] Bo Zhao and Zhuomin Zhang, Enhanced photon tunneling by surface plasmon polaritons in graphene/h-BN heterostructures, J. Heat Transfer 139, 022701 (2015).
[4] Bo Zhao, Brahim Guizal, Zhuomin Zhang, Shanhui Fan, and Mauro Antezza, Near-field heat transfer between graphene/hBN multilayers, Phys. Rev. B 95, 245437 (2017).

The search of alternative energy resources, the development of novel technologies and devices to fight against the use of fossil fuel is indeed a subject of interest worldwide. Thermoelectricity which transforms heat energy directly into electricity, can be utilized to covert waste heat into electricity. The thermoelectric generators that consist of an array of p-n junctions made of two different thermoelectric materials (TEMs) that create a potential difference as the result of a temperature difference across the junction. Widely used and the best known TEMs are hazardous in nature such as Bi₂Te₃ and Sb₂Te₃, therefore, there is a need for a search of new non-toxic TEMs. In this study, thermal, electrical as well as thermoelectric properties of organic polymer of polyaniline (PANI) and inorganic ceramic material of Zinc oxide (ZnO) are investigated. The study is extended to fabricate a novel organic/inorganic p-n junction as a hybrid thermoelectric generator using PANI and ZnO, whereas organic p and inorganic n type material respectively using compressed pellets of HCl doped PANI and Al doped ZnO. Electrical conductivity, thermal conductivity and Seebeck coefficient were investigated, and the figure of merits was calculated for PANI, ZnO and Al -doped ZnO. Fourier-transform infrared and XRD spectra confirmed the synthesis of PANI, ZnO and Al-doped ZnO. The pellets of PANI, ZnO and Al-doped ZnO exhibited electrical conductivities of 63.6 (90 °C), 313 (100 °C) and 356 S m^-1 (100 °C) respectively. The thermal conductivities of respective pellets are 7.38 (90 °C), 23.8 (100 °C), 14.0 W m^-1 K^-1 (100 °C) respectively. PANI showed a positive Seebeck coefficient (confirming the p-type nature) of 34.6 µV K^-1 at 90 °C. ZnO and Al-doped ZnO showed negative Seebeck coefficients (confirming the n-type nature) of -165 µV K^-1 and -225 µVK^-1 respectively at 100 °C. The PANI/ZnO junction generator exhibited a voltage of 21.9 mV for a temperature difference of 78 °C across the junction.

Vanadium dioxide (VO2) and W-doped Vanadium dioxide (VO_2-x) are known to be reliable thermochromic materials for their insulating/metal transition temperature, which can be controlled by the doping concentration level (x), in VO₂/V_1-xW_xO₂ . In this paper, the deposition of various thicknesses VO₂ and W doped V1-xO₂ films on R plane substrate, with two different techniques including (a) pulse laser deposition (PLD) and (b) by reactive electron beam deposition was described.
Various parameters including resistance, refractive index, extinction coefficient, as a function of temperature from 25 to 80 degree C and wavelength ranging from 500 to 3000nm using direct TR, are set up in addition to conventional ellipsometry.
Comparison between reflectance and transmission of spectra obtained from doped and un-doped vanadium shows that below the IMT temperature or in the insulating state, the doped VO₂ is more reflective and less transitive. The real and imaginary parts show that the doped materials produce a loss in the results, which has been confirmed by n and k and RT at various temperature and frequencies, a lower doping level and higher oxygen concentration is needed
Moreover, the influence of temperature and dopping on direct and indirect optical has been reported and various phases have been identified by comparing the transition temperature for direct and indirect bandgap data at room and elevated temperature. Comparison of various batches synthesized at different temperatures shows intermediate phases including VO₂(B) and V₂O₅ shows at a lower temperature less than 500 degree C, single phase is produced by heating the film at elevated temperature and adjusting oxygen pressure. Finally, We are able to adjust various parameters to make the film more suitable for practical applications including frequency detector and memory devices for NIR frequencies.

Nanocomposites have tremendous application in many industries due to their superior mechanical, and unique thermal properties. In automobile industry this class of composites are used for door panels, engine cover, tire, and for improving gas mileage. In energy and aerospace sector, it is used to make windmill blades and aircraft components. Low dimensional materials-based nanocomposites are also gaining interest in the scientific community. For all broad applications possible in industry, the quest for composites with superior mechanical and thermal properties are common factors. Low dimensional materials are prime candidates to achieve these properties. This talk focuses on the mechanical and thermal properties of low dimensional materials in nanocomposites and investigates different types, shape, and bonding characteristics of low dimensional materials on metallic substrates.
We use different computational methods to predict the thermal stability of low dimensional materials on nickel and copper substrates. We then studied the mechanical and interface properties of low dimensional materials-based nanocomposites and investigated the use of low dimensional materials as a fiber in nanocomposites. The result of these studies show that the mechanical properties of a metal matrix can be improved with the addition of these low dimensional materials and provide an insight into the design of metal-based nanocomposites.

Recently, with the rapid development of smart society, our demand for new functional materials and hardware is urgent. Thermal functional materials play an important role in thermal management, energy storage and energy conversion. Phonons are lattice vibrational waves, which are the main energy carriers in dielectric solids. After electronic engineering and photonic engineering, the research progress of phonon engineering has attracted much attention. Phonon engineering is the discipline of designing and controlling phonon transport and conversion by using phonon devices with micro/nano structure, which plays a leading role in the design of thermal functional materials. The research objects of phonon engineering include lattice wave quantum transport, thermoelectric conversion, phonon devices and so on. The progress of nanotechnology has also promoted the development of phonon engineering, so there is an urgent need to understand the mechanism of phonon regulation and transport.
Professor Nuo YANG and the Nano Heat Group [1] focus on the fundamental and applied research of heat transfer, thermal control and energy conversion in micro/nano scale. The report will introduce the research progress of the Nano Heat Group in the following aspects: nanoscale heat conduction [2-6], thermal control [7-12], thermoelectric conversion [13-15], etc.


References
[1] http://nanoheat.energy.hust.edu.cn/
[2] Wan, et.al Nano Lett. 19, 3387 (2019)
[3] Deng, et.al Materials Today Phys. 16 (2021) 100305
[4] An, et.al Nano Lett. 17, 5805 (2017)
[5] Xiao, et.al ES Mater. Manuf., (2019), 5, 2
[6] Zhang, et.al arXiv:1905.09183
[7] Deng, et.al Nano Energy 82 (2021) 105749
[8] Wan, et.al Materials Today Physics 20 (2021) 100445
[9] Ma, et.al Materials Today Phys. 8(2019) 56
[10] Yu, et.al Chin. Phys. Lett. 38, (2021) 014401 Express
[11] Ma, et.al Phys. Rev. B 98 (2018), 245420
[12] Yang, et.al Phys. Rev. B 103, 155305 (2021)
[13] Mi, et.al Nano Lett. 15, 5229, (2015)
[14] Ding, et.al J Mat Chem A, 7(2019), 2114
[15] Ji, et.al Materials Today Physics 19 (2021) 100430

Interfacial thermal resistance (ITR) plays an important role in thermal dissipation across different materials and it has been widely investigated in recent years. An example is the integrated electronic circles in which heat dissipating issue has recently become the bottleneck. With further miniaturization of electronics, the accumulated waste heat has increased the operating temperature to a high value where thermal management and heat dissipation become crucial for the performance of devices ^[1]. The severe thermal management problem has prevented the operating frequency from going beyond several GHz when the dissipated waste heat exceeds 100 Wcm^-2. This work is separated into two parts: (a) Chemical vapor deposition (CVD) method was utilized to fabricate monolayer transition metal dichalcogenides (TMDCs) depositing directly onto SiO₂/Si substrate; we demonstrated that the interfacial thermal conductance of TMDCs/oxide was significantly promoted by introducing a phonon vibration bridge at the interface of TMDCs/oxide ^[2-5]. (b) We introduced an additional electron-phonon coupling channel in the thermal transport across graphene-dielectric interfaces and the scanning thermal microscopy technique measurement of graphene electronic devices gave direct evidence of an enhanced heat dissipation by tuning the surface carries concentration ^[6].

References:
[1] X. Xu et al, Adv. Mat. 30, 1705544 (2018); Adv. Funct. Mater. 30, 1904704 (2020)
[2] J. Guo et al. J. Phys. D: Appl. Phys. 52, 385306 (2019)
[3] D. Liu et al. Nat. Comm. 10, 1188 (2019)
[4] X. Li et al. J. Phys. D: Appl. Phys. 50, 104002 (2017)
[5] D. Liu et al. Adv. Electron. Mater. 6, 2000059 (2020)
[6] Zhang et al. ES Energy & Environ. 8, 42-47 (2020)

The particle nature of phonons has been widely studied in the past, and the control of thermal transport is mainly focused on the incoherent scattering mechanisms via various approaches, such as defect, roughness, isotope and etc. Thanks to the advances in the synthesis technology of nanostructures with ultra-fine features, the study on the wave nature of phonons, namely the coherent phonons, is receiving increasing attention recently. In this talk, I shall first discuss the physical mechanism for storing monochromatic and coherent phonons in a host-guest system built by carbon schwarzite structure. Interestingly, the coherent phonons stored in this phonon nanocapacitor can also be released separately via uniaxial strain. Then, I will demonstrate that a phononic rectifier, which has asymmetric thermal transport in two directions, with large rectification efficiency can be realized based on such host-guest system. Finally, I will discuss how to achieve total-reflection and total-transmission of individual phonons in the periodic nanostructures. Our study reveals that the control of coherent phonons offers a new path to regulate the thermal and phonon transport in nanostructures.

Thermal metamaterials have attracted increasing attentions in recent years as they can be used to manipulate heat almost at will by resorting to specific-designed anisotropic thermal properties beyond natural materials. Most thermal metamaterials are designed by the prevailing transformation thermotics, an extended version of transformation optics in the thermal field. However, the tradiational transformed thermal metamaterials only work well without heat sources. To cope with it, we develop the illusion thermotics techniques and proposed some new thermal functionalities such as thermal camouflage, thermal printing, and so on. Both the transformation thermotics and topology optimization were used to achieve illusion thermotics. In this talk, I will introduce the illusion thermotics and the state-of-the-art progress of thermal metamaterials.

Colossal barocaloric effects (CBCEs) have been discovered in a class of disordered solids called plastic crystals as the cooling effects of pressure-induced phase transitions [1]. CBCEs are characteristic of huge entropy changes up to several hundred joules per kilogram per kelvin under a small pressure change, which are attributed to the combination of extensive molecular orientational disorder, giant compressibility, and highly anharmonic lattice dynamics of these materials [1,2].

Following this physical scenario, a diversity of colossal barocaloric materials have been found, which exhibit very distinct performances including thermal conductivity, thermal hysteresis, critical pressure, transition temperature spans under pressure, and so forth. In particular, NH₄SCN has been identified as the first system with inverse CBCEs [3]. A compromise would be made based on the systematical considerations of these factors and practically promising colossal barocaloric materials can be expected. In this regard, our efforts will be reported in detail.

In addition to refrigeration, the application of colossal barocaloric materials in thermal energy storage has been exploited as well. Compared to the conventional phase-change materials, pressure is an extra dimension and enhances the controllability of heat. In the presentation, the concept of barocaloric thermal batteries will be demonstrated.

References:
1) B. Li et al., Nature 567, 506-511 (2019).
2) F. B. Li et al., Nat. Commun. 11, 4190 (2020).
3) Zhe Zhang et al., arXiv:2103.04348.

The design of new systems with specific technical interests through structural properties and bonding promotes the vigorous development of materials informatics. Here, we propose the concept of component reconstruction, which based on combination the structural and bonding characteristics of initial materials. With this strategy, we developed a new two-dimensional graphene-like compound, named as g-B₃N₅, which has light atomic mass but counter-intuitive ultra-low thermal conductivity of 21.08 W/mK. The ultra-low thermal conductivity is attributed to the component reconstruction of g-BN and nitrogene, which will lead to a synergy of electronics and geometry on thermal transport. Thus, the dominant acoustic branches are strongly softened, and simultaneously the scattering absorption and Umklapp process are suppressed, resulting in low thermal conductivity. Furthermore, based on the component reconstruction strategy, we also constructed g-B₃P₅ and g-B₃As₅ with ultra-low thermal conductivity of 2.50 and 1.85 W/mK, respectively. The ultra-low thermal conductivity materials with lightweight atomic mass cater to the need for lightweight development of momentum machinery, such as aerospace vehicles, high-speed rail, and automobiles.

BaTiO₃ (BT) is a well-known ferroelectric and piezoelectric material, widely used in various devices as, e.g., capacitor for electric circuit, stress sensor, and actuator. However, little is known about the atomistic dynamics of ferroelectric domain growth. Depending on the production procedure (e.g., solidification from melt), doping (e.g., La), and oxygen gas pressure, various point defects are expected to form in BT. The oxygen vacancies (V_O) exist at a relatively low oxygen pressure. As for the barium vacancy (V_Ba) or titanium vacancy (V_Ti) that exist in oxygen-rich BT, the experiment and DFT calculations predicted that such negatively charged monovacancies may combine with positively charged V_O to form the divacancy V_Ba-V_O or V_TI-V_O.
In this study, we comprehensively analyze the effects of mono- and di-vacancies on the domain growth of BT by using the MD simulation with the core-shell inter-atomic potential. We consider not only the monovacancies V_Ba, V_Ti, and V_O but also the divacancies of 1st- and 2nd-neighbor V_Ba-V_O and V_Ti-V_O. We also compare the domain growth speed between the parallel and perpendicular directions to the applied electric field.
We found the following: (1) One kind of monovacancy, V_O1, located on the TiO plane perpendicular to the applied electric field direction, acts to hinder the polarization inversion induced by the applied electric field. The monopole electric field produced by V_O1 either hinders or assists the local polarization inversion in accordance with the local intensity of the total electric field. (2) The 1st-neighbor divacancies V_Ba-V_O and V_Ti-V_O, as compared to the 2nd-neighbor divacancies, asymmetrically affect the domain growth with respect to the applied electric field, determining the hysteresis behavior of applied electric field vs. polarization. The domain grows even at a small electric field when the directions of the applied electric field and the divacancy dipole are mutually the same. (3) The domain growth speed towards the applied electric field direction is about 2 orders of magnitude higher than that towards the perpendicular direction.

Piezoelectricity of ferroelectric crystals has been widely utilized in electromechanical devices such as sensors and actuators. It is broadly believed that the smaller the ferroelectric domain size, the higher the piezoelectricity, arising from the commonly assumed larger contributions from the domain walls. Here we theoretically analyze the domain-size dependence of piezoelectric coefficients of prototypical ferroelectric crystals, guided by phase-field simulations. We reveal that the inverse domain-size effect, i.e., the larger the domain size, the higher the piezoelectricity, is just as common. The nature of the domain-size dependence of piezoelectricity is shown to be determined by the propensity of polarization rotation inside the domains instead of the domain wall contributions. We establish a simple, unified analytical model for predicting the domain-size dependence of piezoelectricity, which is valid regardless of the crystalline symmetry, the materials chemistry, and the domain structures of a ferroelectric crystal, and thus can serve as a guiding tool for optimizing piezoelectricity of ferroelectric materials beyond the “nanodomain” engineering. The same strategy can be extended to understand the microstructural size effect of multiple-physical coupling properties in ferroic material systems to achieve a combination of seemingly conflicting properties.

The majority of commonly used electrostrictive ceramics are based on lead manganese niobate. These ceramics display large electrostriction strain coefficients ≈ 10^-16 m²/V² at frequencies up to a few kHz; however they suffer from two major drawbacks: large dielectric constants (>10000), which require high driving currents, and incompatibility with thin-film Si-microfabrication techniques. We have recently reported that aliovalent doped ceria exhibits electrostriction coefficients >100-fold larger than estimated on the basis of Newnham’s scaling law for classical electrostrictors, despite ceria’s large Young’s modulus (~ 200 GPa and low dielectric constant (~ 30). This “non-classical” behavior has been attributed to the formation of highly polarizable, elastic dipoles reorienting under external electric field.
For 10mol% Sm- or Gd-doped ceria, the measured longitudinal electrostriction strain coefficient, |M|, reaches 10^-16 m²/V²; however, relaxation to < 10^-18 m²/V² is observed at frequencies > 1 Hz, well below the technologically important frequency range 100Hz-100 kHz. The introduction of aliovalent lanthanide dopants with smaller radii than that of Gd, such as Lu or Yb, succeeds in increasing |M| at 100Hz to » 10^-17 m²/V². Nevertheless, aliovalent dopants with radii smaller than that of Lu do not continue the trend.
We have found that partially reduced, 10 mol% Zr⁴⁺-doped ceria displays |M| ≈ 10^-16 m²/V² throughout the 0.1-150 Hz frequency range. However, practical application of these ceramics may be hindered by the relatively large, room-temperature electrical conductivity (10^-10 S/m), a result of the formation of Ce³⁺ which can promote electron hopping. Formation of Ce³⁺ also raises the dielectric constant to »200, as measured by impedance spectroscopy. Suppression of Ce³⁺ by co-doping with 0.5mol% Yb produces a dramatically reduced electrostriction strain coefficient (at f = 0.1-150Hz), ~ 4*10^-18 m²/V². If the Yb concentration is raised to 10 mol% , |M| increases to ~ 2*10^-17 m²/V² but is sharply lowered to 2*10^-18 m²/V² at 15 mol% Yb. Co-doping with a large radius, aliovalent lanthanide, e.g. 0.5-10mol% La, returns the measured high frequency |M| to 4*10^-18 m²/V².
Taken together, these results suggest that elastic dipoles induced in ceria ceramics by small aliovalent dopants, give stronger electrostrictive response at high frequencies (>10 Hz) than the larger aliovalent dopants. For the case of isovalent doping with Zr of reduced ceria, the presence of Ce³⁺ seems to be essential for large |M|. Noting that Ce³⁺ is almost as large as La³⁺, we may conclude that the elastic dipoles operating in Zr-doped, reduced ceria are fundamentally different from those observed for aliovalent doping. Our results imply that by systematically adjusting the composition of ceria-based solid solutions, the potential exists for development of technologically useful electrostrictive materials which are, at the same time, fully compatible with Si-microfabrication.

Nanoporous thin films with periodic circular pores have been extensively studied for their potential applications such as thermoelectrics, heat waveguides, thermal cloaks, thermal diodes, and heat imaging [1]. Fundamentally, it is acknowledged that diffusive phonon scattering by pore edges, as classical phonon size effects, is the major mechanism for the observed thermal conductivity reduction. Despite some earlier debates, phononic effects due to coherent phonon transport within a periodic structure are found to be only critical at cryogenic temperatures [2-4].
Based on classical phonon size effects, new nanoporous patterns have been pursued to better tune the thermal conductivity of thin films [5, 6]. As one new pattern, periodic nanoslots can be employed to effectively tune the in-film phonon transport. When phonons travel ballistically through the narrow neck between adjacent nanoslots, a ballistic thermal resistance is introduced to lower the thermal conductivity. As a result, the lattice thermal conductivity can be largely reduced along the direction perpendicular to nanoslot rows, while keeping a much higher thermal conductivity along the direction parallel to nanoslots. The in-plane thermal anisotropy can be further enhanced with offset nanoslot patterns [7], which can largely benefit the thermal management of thin-film devices.
In analytical modeling, an accurate characteristic length can be derived for nanoslot patterns to predict the thermoelectric properties of such films [8, 9], whereas an accurate characteristic length is hard to be found for thin films with periodic circular pores [1]. For representative nanoslot patterns, the thermoelectric properties can be predicted and compared with nanostructured bulk Si [7, 8]. The predicted thermal conductivities agree well with the experimental data on some nanoslot-patterned Si thin films. A bulk-like specific heat is also measured for these thin films, indicating negligible phononic effects to change the phonon dispersion.
In nanoslot-patterned thin films, the in-plane thermal resistance is a strong function of the neck width between nanoslots. By measuring a series of samples with varied neck widths, inverse phonon transport analysis can be used to reconstruct the in-plane phonon MFP distribution down to ~10 nm [9, 10]. In this aspect, pump probe measurements using nanofabricated heating patterns [11-12] are not suitable for suspended thin films because phonon modes and scattering can be remarkably affected with deposited metal patterns [13]. In practice, nanoporous patterns can also be used to achieve two-dimension thermal cloaking or thermal camouflaging [7]. This new application will also be discussed.

References:
1. Xiao et al., ES Materials & Manufacturing, 2019, 5, 2-18.
2. Hao et al., Scientific reports, 2018, 8, 1-9.
3. Maire et al., Science Advances, 2017, 3, e1700027.
4. Lee et al., Nature Communications, 2017, 8, 14054.
5. Romano et al., Applied Physics Letters, 2014, 105, 033116.
6. Romano et al., Applied Physics Letters, 2017, 110, 093104.
7. Xiao et al., International Journal of Heat and Mass Transfer, 2021, 170, 120944.
8. Hao et al., Physical Review Applied, 2020, 13, 064020.
9. Hao et al., Materials Today Physics, 2019, 10, 100126.
10. Anufriev et al., Physical Review B, 2020, 101, 115301.
11. Hu et al., Nature nanotechnology, 2015, 10, 701-706.
12. Zeng et al., Scientific reports, 2015, 5, 17131.
13. Anufriev et al., Nanoscale, 2017, 9, 15083-15088.

Recent advancement in the thermal drawing of multimaterial fibers have enabled unique material properties and functions in a scalable manner. Here we present the electrical and mechanical properties of thermally drawn multimaterial fibers which make the fibers advantageous for biomedical and wearable applications. First, we demonstrate that thermal drawing promotes unidirectional alignment of carbon nanofibers (CNFs) in a polymer composite. The resultant CNF-composite shows a significant increase in the electrical conductivity by two orders of magnitude compared to that before thermal drawing. Therefore, these highly conductive fiber electrodes can enable miniaturized devices for neural recording in the brain. Next, I will introduce the thermal drawing of other polymer composites such as carbon black in thermoplastic elastomers (TPEs). The scalable fabrication of conductive elastomers can be used for strain sensors in ropes or smart textiles. The electrical and mechanical performance of these fibers will be presented. Finally, thermally drawn piezoelectric fibers will be presented which consist of PVDF-TrFE as the sensing layer and two carbon-loaded polymer layers as the electrodes. The piezoelectric performance of these fibers will be discussed. These results show that thermal drawing provides a unique platform for producing flexible and miniatured multifunctional devices with unique electrical and mechanical properties.

Triboelectric nanogenerators (TENGs) have received recent interest for converting unconventional mechanical energy to electricity, particularly as a sustainable form of energy generation from sources such as wind and friction. Energy conversion in TENGs is based on the triboelectrification and electrostatic induction effects. In this regard, Polydimethylsiloxane (PDMS) elastomer has been widely used as a good triboelectric active layer in TENG owing to its high electronegativity as ranked in the triboelectric series of different materials. The casting process is simple for PDMS having comparatively low surface energy, and more importantly, their formability is superior to many other counterparts in polymers. The TENGs based on PDMS exhibit a relatively high energy conversion efficiency, however, its output electrical characteristics still need more development. One critical key device parameter to enhancing the output performance of TENG is increasing the contact area between triboelectric active material layers. Patterning the surface, and changing the shape and density of patterns on the surface, is assumed to be an effective strategy for this purpose. However, the origin of the output enhancement by patterning and the role of surface roughness on the charge generation aspects needs further investigation. We present research on what makes the shape of the patterns more effective by their contribution to enhancing surface roughness and contacting area, and thereby higher efficiency, with mechanical pressure and sliding contact. On the other hand, increasing the triboelectric polymer conductivity significantly enhances the current density by facilitating the charge transfer from the contact surface towards electrodes. Many nanoparticles have been used to enhance charge transfer in TENGs owing to having high conductivity. In this research, we investigate this proposed hypothesis to relate pattern size and roughness to the output performance of aluminum/polydimethylsiloxane (Al/PDMS) TENG, modified by carbon-based nanoparticles including carbon nanotube (CNT), reduced graphene oxide (r-GO) and carbon black (CB). The uniform and periodic nanoscale morphology of optical discs (CD and DVD) were replicated onto PDMS films. The procedure is followed by a sequential development of hierarchical low amplitude patterns made by oxygen plasma and ultraviolet ozone wrinkling method, to create a hierarchical pattern as a superposition on the optical disc pattern. The patterning procedure is repeated after replacing PDMS with a nanocomposite of PDMS and polytetrafluoroethylene (PTFE) to take advantage of both the molding capability of PDMS and the higher triboelectric affinity of PTFE. Our hierarchical topographic designed TENG led to a considerably higher current and voltage output, due to the efficient shape and roughness compared to CD and DVD patterns alone, useful to rationally design TENGs with high efficiency for capturing energy from unconventional sources.
We acknowledge DoD Grant #555033-78055 (Contract: Kostas Research Institute (KRI) at Northeastern University) for support of the work.

The thermal conductivity of pseudo-cubic MAPbI₃ was studied using classical molecular dynamics and found to be invariant under compressive strains below 3 %. Such invariance contrasts with the rapid thermal conductivity increase seen in other solids with a similar Young modulus value. Larger compressive strains increase the thermal conductivity as observed in most solids.
Hybrid halide perovskites have attracted a lot of interest due to their excellent energy conversion efficiency. The need for improving its thermoelectric performance in thermoelectrics as well as enhancing its thermal stability in photovoltaic cells has spurred thermal transport investigations. As temperature will unavoidably increase during operation, stress will be induced on a piece of mounted perovskite, changing its properties. Additionally, further enhancement to its solar conversion efficiency by applying structural compression was recently reported. It is, therefore, quintessential to understand how straining a perovskite crystal will affect its thermal transport.
The full phonon dispersion and the respective phonon lifetimes under various pressures were calculated. Stable acoustics phonon branches are obtained at these pressures. The vibration modes in these branches are participated by atoms mainly from the inorganic framework. These modes contribute about 40% to the thermal conductivity in MAPbI₃ at pressures between 0 to 1 GPa, but more than 60% at 30 GPa. The almost invariant thermal conductivity contribution at the lower pressure range arises from the canceling effect between the changes in the heat capacity, group velocity, and lifetime of these acoustic phonons. On the other hand, the increase in both the phonon group velocities and lifetimes are responsible for the larger thermal conductivity contribution at higher pressure.

High-performance thermal management is of great significance to the operational stability of integrated chip and electronic devices, where the heat transfer performance of the core semiconductors plays a key role. In this talk, I would like to introduce our recent work on the performance regulation of heat transfer focusing on few typical semiconductors, such as gallium nitride, boron nitride, boron arsenide, silicene, etc. Based on a concise strategy of accelerating evaluation of converged lattice thermal conductivity, the regulation effects of lots of manners, such as temperature controlling, strain engineering, applying external field, alloying, etc, have been systematically investigated. The fundamental mechanism is revealed based on the deep analysis of the electronic structures. The performance regulation of heat transfer and the underlying mechanism would be helpful on future studies of thermal and electrical coupling in emerging energy materials.

The threats of oil depletion, automotive emission and global warming are on the horizon for many years. Clean and environmentally friendly, electric vehicles have the advantages of energy conservation and emission reduction, which greatly alleviates the dependence on traditional energy sources and are expected to usher in new transportation era in the near future [1, 2]. As the power source of electric vehicles, the lithium-ion battery features salient advantages such as high energy density, high discharge voltage, minimal self-discharge rate and long cycle life, and so on [3, 4]. However, the safety and efficiency of lithium-ion batteries are closely related to their operating temperature. When the batteries work under high load, the battery module and pack produce large amount of heat due to the associated electrochemical exothermic reaction, resulting in the unfavorable rise of battery temperature in the battery. This would lead to the decline in battery capacity, or even cause the battery thermal runaway [5-7]. An effective battery thermal management system (BTMS) is then required to maintain the battery thermal environment, thereby preventing potential operating risks. Existing battery thermal management techniques include air cooling [8-9], liquid cooling [10, 11], phase change material (PCM) cooling [12] and a combination of these cooling methods [13]. Nonetheless, all the above techniques would fail to work if the climatic temperature is close to the battery threshold temperature around 40-50 °C, especially in hot summer days.
Thermoelectric cooling has attracted attention for thermal management of batteries in a module or pack. Thermoelectric cooling is an advanced cooling technique taking advantage of the Peltier effect. Compact in size and silent in operation, the TEC is able to provide sub-ambient cooling as well as heating in cold winter days, which is a potential technique for thermal management of battery module for electric vehicles. In this paper, the thermal characteristics of thermoelectric cooler (TEC) for thermal control of cylindrical battery module is investigated both experimentally and theoretically.
The battery module under investigation consisted of 18650 test batteries in 3x5 matrix embedded in the composite paraffin wax for accelerated heat dissipation. In the experimental test, the transient and steady-state thermal performances were examined based on the thermoelectric cooling in comparison with the natural convection and liquid cooling conditions. Either one lateral side or two lateral sides of the battery module were attached with the thermoelectric cooler to minimize the temperature of the battery module. In comparison, the thermoelectric cooling reduced the battery temperature and prolonged the working time significantly. The optimal current was experimentally obtained to be about 6.0-6.5A based on the highest cooling power or lowest battery temperature. One-dimensional steady-state thermal resistance network is also developed for the battery module with TEC. The theoretical analysis agrees with the optimal current range obtained from the experimental measurements. Further analysis shows that the optimal current is affected markedly by the hot-side thermal resistance, but little by the cold-side thermal resistance. The two-side TEC cooling is also compared with the one-side TEC cooling. The two-side TEC cooling has a better thermal performance, but with an additional liquid cooled heat sink, which may add complexity to system setup. The cooling performance can be further enhanced such as by increasing the pair of TEC arms. Increasing the number of TEC thermoelectric arms has a favorable effect in both minimizing the maximum temperature and increase the coefficient of performance (COP) of the TEC module. Detailed results will be given in the final presentation.

As a promising thermoelectric material, Mg₃Sb₂ has kindled intense research interest in recent years. Since acoustic phonons are the main heat carriers and thermoelectric materials in applications are usually doped at high carrier concentration, coupling of acoustic phonons and carriers determines the thermoelectric performance to a large extent. In this work, acoustic deformation potential coupling constant, the key parameter for quantifying acoustic phonon-carrier interaction, of Mg₃Sb₂ is measured through coherent phonon detection by femtosecond spectroscopy. Agreement between experiment and first-principles calculation results is achieved in terms of acoutic deformation potental quantification. The information about acoustic deformation potental extracted in this work could guide the study on thermal and electrical transport of Mg₃Sb₂ and the performance optimization of this thermoelectric material.