A Voyage Intersecting Nano, Heat, and Energy: Professor Gang Chen’s Scientific Contributions

Professor Gang Chen turns 60 this year. He has made seminal contributions to nanoscale heat transfer and energy sciences in his career spanning over three decades. He has achieved both fundamental progresses and technological advancements with a profound impact on the scientific community and society at large. He is also a prolific educator, having mentored over 100 graduate students and postdocs, many of whom now occupy research and teaching positions in top institutions around the world. With this Special Issue in the Journal of Heat and Mass Transfer, we celebrate Professor Gang Chen’s multifaceted contribution to science and the scientific community by compiling research papers reporting recent progress by former members of his group, including one by Professor Chen himself.Gang’s main research area is heat transfer and thermal science and their applications in energy systems. He often summarizes his broad research interest with three intersecting themes: Nano, Heat, and Energy. Taking a fundamental approach, Gang incorporates his deep knowledge in solid-state physics, statistical thermodynamics, and electromagnetism into heat transfer research, pushing its frontier into the smallest length and time scales. In this process, he pioneered many concepts in nanoscale heat transfer that have led to transformative advancements in renewable energy technologies. For example, Gang provided a solution of the phonon Boltzmann transport equation along the cross-plane direction of a superlattice [1], elucidating the role of interfaces in scattering phonons and reducing the thermal conductivity. This work laid the theoretical foundation for the nanostructuring approach of processing thermoelectric materials [2], a breakthrough in thermoelectric research. Gang’s other field-defining contributions include the experimental demonstration of enhanced radiative heat transfer at nanometer gaps [3], theoretical and experimental investigation of superior heat conduction in aligned polymer chains [4,5], coherent phonon transport and phonon localization [6,7], and his more recent works on applied energy systems, such as solar vapor generation [8,9]: the list goes on and on. For these original contributions, Gang is widely recognized as one of the pioneers establishing the field of nanoscale thermal transport. This Editorial briefly reviews Professor Gang Chen’s scientific contributions in five main areas: (1) nondiffusive phonon transport; (2) nanostructured thermoelectrics; (3) nanoscale thermal radiation; (4) thermal transport in polymers; and (5) solar-photovoltaic-thermal energy engineering. By no means do we attempt to cover the entire scope of research documented in Gang’s more than 400 journal publications. Instead, we discuss his representative works and summarize their scientific significance within a broader context, including a survey of the impact of these original results on defining new frontiers in each area.Heat transfer is an old science, whose formal establishment is usually attributed to Fourier’s treatise on the theory of heat conduction in 1822 [10]. In the following two centuries, heat conduction was often referred to as heat diffusion, based on the common understanding that heat conduction is a “diffusive” process. Diffusive transport often implies two features of the underlying microscopic process: (1) the microscopic energy carriers can be modeled as particles, with insignificant wave nature and phase coherence; (2) the microscopic energy carriers experience significant intrinsic scattering events such that their mean free path (MFP) is much shorter than the extrinsic lengthscale associated with the driving temperature difference. In common solids, heat conduction is mediated by electrons and phonons (quantized lattice waves), and both conditions are often met in macroscopic samples. For example, a textbook estimation of the phonon MFP due to intrinsic phonon–phonon scattering in silicon at room temperature is roughly 50 nm [11]. Frequent intrinsic scattering events, such as phonon–phonon and electron–phonon scatterings, facilitate the thermalization of electrons and phonons, whose distribution functions only deviate slightly from local equilibrium distribution functions, a signature for diffusive transport. Therefore, heat conduction in a macroscopic sample can be accurately described using diffusive transport laws, such as Fourier’s law.With the rapid advancement of nanotechnology over the past few decades, however, the notion of “heat diffusion” started to be challenged as the characteristic size of many devices, such as CMOS transistors, becomes comparable and even much smaller than the intrinsic MFP. In this new regime, novel behaviors of heat conduction emerge because of the breakdown of the assumptions underlying diffusive transport processes. Over his career, Gang has made systematic contributions to our fundamental understanding of these emerging nondiffusive heat conduction regimes, such as ballistic phonon transport, hydrodynamic phonon transport, and coherent phonon transport. In this section, we briefly review his seminal work in this direction.When the characteristic device size becomes smaller than the intrinsic MFP of microscopic energy carriers such as electrons and phonons, these carriers experience more frequent scatterings with extrinsic sample boundaries and interfaces than intrinsic thermalizing scatterings, leading to the breakdown of local equilibrium and failure of diffusive transport laws. This scenario of ballistic or quasi-ballistic transport was first explored by Knudsen in rarefied gas flow and later studied by Fuchs and Sondheimer in electrical conduction in thin films. Early investigations of the ballistic transport of heat-carrying phonons in nanoscale recognized the similarity between ballistic phonons and photons as described in radiative heat transfer. For example, Majumdar showed that the governing phonon Boltzmann transport equation has a similar form as the radiative transport equation under certain approximations and, thus, analogies between ballistic phonon transport and radiative transfer can often be drawn [12]. In the early stage of his career, Gang conducted a series of theoretical and experimental studies on the thermal conductivity of thin films (quantum wells) and superlattices [13–17], motivated by the thermal management issues of III–V quantum well lasers and vertical-cavity surface-emitting lasers at the time. Based on the phonon Boltzmann transport equation and models of phonon–boundary scattering, Gang showed that the thermal conductivity in thin films and superlattices with nanoscale thickness (much shorter than intrinsic phonon MFP) is largely suppressed from its bulk value. In these structures, while phonons travel quasi-ballistically between collisions with the boundaries and interfaces, the small distance traveled between collisions effectively shortens the average MFP of heat-carrying phonons, reducing the effective thermal conductivity in the material. This quasi-ballistic phonon conduction bottleneck is now widely recognized as a fundamental challenge in the thermal management of micro-electronic and nano-electronic devices [18].Many of Gang’s early investigations of quasi-ballistic phonon transport laid the foundation for later important developments. For example, in a seminal study, Gang showed that heat conduction from a small heat source, whose size is much smaller than the phonon MFP of the surrounding medium, is significantly reduced from that predicted by using Fourier’s law [19]. This manifestation of ballistic phonon bottleneck was later confirmed using an optical measurement with small optical and as the for the of experimental to phonon MFP distribution in materials In Gang modeled the cross-plane heat conduction in a superlattice by the phonon Boltzmann transport equation with and scatterings In this Gang the role of quasi-ballistic phonon transport within each and phonon transport. He that the thermal in these is no an intrinsic of the interfaces on the thickness and phonon MFP. this work the thickness is small to the phonon the effective thermal conductivity of the superlattice is largely by and scattering at This provided the theoretical for the nanostructuring approach to the thermal conductivity of bulk [2], the of nanoscale boundaries was to phonon in this Gang also explored the of coherent phonon transport in by papers was later in superlattices and be more in important contribution by Gang in this was the of heat conduction that the phonon Boltzmann transport equation with the Fourier’s law in the quasi-ballistic transport these understanding of quasi-ballistic phonon transport, Gang also made contributions to to phonon transport in with Gang pioneered the of equilibrium with the to phonon transport and thermal conductivity in bulk and superlattices showed that with can accurately the thermal conductivity of materials the small size of the to the phonon MFP. and Gang applied to the MFP of phonon in and an contribution to thermal conductivity by phonons with to the distribution of phonon and their contribution to heat conduction This has now a to phonon MFP in showed that the phonon MFP in silicon has a distribution from of to of with of the thermal conductivity from phonons with MFP shorter than was a significant of the textbook estimation of a MFP. This of phonon MFP distribution in materials also important for to thermal transport in such as with and Gang the of phonon transport in materials theory and lattice this are from their equilibrium positions in materials are using from and can be Based on the phonon and phonon–phonon scattering can be from the thermal conductivity can be by the phonon Boltzmann transport to this on for and can more phonon transport this Gang’s phonon and MFP in a of in with with experimental In Gang’s also with from to more and understanding of and models Gang’s and also pioneered experimental to the phonon MFP distribution in and Gang first that the thermal conductivity of by an optical that is to thermal conductivity in was suppressed the optical size was an that at This was of Gang’s early theoretical investigation on the of small heat on quasi-ballistic phonon transport and was attributed to the that the optical size was smaller than the MFP of certain heat-carrying phonons, much at This was confirmed by the phonon MFP distribution from the to that using the with and Gang’s optical to quasi-ballistic phonon transport. In the of the optical can be to be comparable or smaller than the MFP of heat-carrying phonons, reducing the effective thermal conductivity optical the lengthscale that can be by and this and Gang to nanoscale in to quasi-ballistic phonon transport to nm with and Gang’s also optical to scattering in to nanoscale devices materials and interfaces and interfaces a common approach to thermal transport in solids, was a interest in understanding thermal transport in of thermal transport in two In the first interfaces phonon scattering, in the of phase of This scenario wave and scattering effectively thermal conductivity. In the scattering is phonons to their phase In this wave and the entire new phonon that from the of each material. In such the thermal transport. as to the nature of these Gang’s has made significant contributions to this and on In an early Gang’s a that both coherent and to the experimental thermal conductivity of superlattices a from to coherent phonon transport as the thickness the of both to the experimental the coherent phonon transport was its experimental studies coherent phonons, to a and coherent thermal transport a broad the and the was to for coherent phonon transport in superlattices and In to studies that and thermal conductivity as a of the of By using superlattices with interfaces and scattering, the that thermal conductivity with the of as in coherent phonon transport. at room the as as in the experimental by showed that phonons have mean free much than the and significantly to heat transport their small of also a similar to the thermal conductivity with the of of phonon that the interfaces are effective in thermal transport by this the to effectively phonons and the phonon the thermal conductivity of superlattices with at the interfaces of significantly suppressed thermal conductivity to superlattices that the thermal conductivity of superlattices with at and with the of while superlattices a by of thermal conductivity. attributed these results to the of phonon similar to the localization of using the localization of electrons has the phonon in nanoscale thermal transport in the of ballistic and diffusive Gang’s has that regime, as hydrodynamic phonon transport, be for a understanding of thermal transport in hydrodynamic phonon transport is from the ballistic and diffusive frequent scattering the ballistic In the hydrodynamic regime, scattering is the process, while in the diffusive regime, scattering to be the process. a phonon transport in the hydrodynamic scattering hydrodynamic phonon transport features phonon flow and are to flow and in hydrodynamic phonon transport was studied in the and was to main for this is that hydrodynamic phonon transport has only at and temperature is usually much than in by in by Gang’s studied hydrodynamic phonon transport in In Gang’s predicted for the first time using that hydrodynamic phonon transport can be the of thermal transport in to 100 is than the conditions for in the of phonons, the phonon and the the features of hydrodynamic phonon transport in also showed that the hydrodynamic can be as significant the ballistic and diffusive at room studies also showed that hydrodynamic phonon transport is significant in similar to that a phonon Knudsen to the Knudsen in significant hydrodynamic phonon transport. by the theoretical using Gang’s confirmed hydrodynamic phonon transport at 100 in new experimental for based on the thermal is a from the conducted in the showed in the temperature field a applied for in the and the was to the to from to smaller length and shorter This to the measurement of in at of Gang’s systematic of quasi-ballistic phonon transport was the of nanostructured [2], a rapid in thermoelectric research for the past generation first its in the a with a of to around research was in the early the works of that significantly the transport of carriers and have the to of thermoelectric materials Gang’s studies of suppressed thermal conduction in quantum and superlattices their for thermoelectric with and Gang and his explored the of on the electrical and thermal transport in with a series of seminal works that thermoelectric research. we these contributions and summarize their impact on the recent progress in nanostructured thermoelectric materials to well heat often as phonon is by the of by and are electrical and thermal is to the electrical and heat conduction of a material. For example, the of thermal conductivity can be achieved by the to phonons, in the thermoelectric materials and scattering also significantly electrical transport the in understanding thermal and thermoelectric transport in quantum and superlattices in the and the of using and quantum size in to the phonon thermal conductivity much of the to a is to the of a to of the to the while using nanoscale interfaces to and phonons and the thermal conductivity the interfaces can also was that electrons and phonons can have MFP the of electrical conduction and phonon conduction with confirmed that the MFP is usually much shorter than the phonon MFP in many these Gang and conducted a series of of thermal and thermoelectric of thermoelectric thin films and superlattices using and in these the significant of thermal conductivity. by this also the of thermoelectric materials the the small and of these their important from this was that of superlattices is a for thermal conductivity reduced thermal conductivity in superlattices from the scattering of phonons than the coherent of phonon This led to the of bulk thermoelectric with nanoscale interfaces that the of their associated and often to a nanoscale such as and can be or due to phase can also be achieved such as and of by or a these is the and can be applied to thermoelectric systems. Early of and for processing thermoelectric materials can be to in the size was reduced to the lattice thermal conductivity was by as with that of a predicted that the thermal conductivity of be even the size be a challenge in and Gang this processing challenge by using also as is with a rapid in a few using a for widely to be effective in In and Chen of nanostructured and with to a few nm with significantly enhanced thermoelectric of due to a reduced thermal conductivity. For example, in the of the nanostructured sample showed a thermal conductivity of around leading to an enhanced of at 100 This a significant breakthrough in achieved in a bulk thermoelectric in 50 a new in thermoelectric research. impact of this work is by its over its in the following this bulk nanostructuring approach was by research and applied to a broad of thermoelectric materials For example, with and to a of phonons to a of at In the theoretical and experimental in understanding phonon MFP distribution in an led by Gang’s group, to a understanding of the impact of on phonon and transport, a manifestation of between theory and in Gang’s research. In to suppressed phonon transport in Gang and his also made fundamental contributions to other concepts in thermoelectric research, such as and phonon and his have made contributions to radiative heat transfer. Gang’s work has to the of thermal as in to be the fundamental assumptions underlying these are For example, Gang’s work explored thermal is in nanoscale gaps to heat that the theoretical and experimental work from Gang’s in this regime, the the size between is comparable to or smaller than the explored phonon can in in radiative heat transfer Gang’s explored can in significant contributions to heat transfer in nanoscale films. more recent work from Gang’s has explored the law of thermal is to radiative heat transfer can be in systems. we a of Gang’s work on radiative heat of radiative heat transfer Gang and the functions and to the for or theoretical results applied to the energy in both and In to Gang’s theoretical to in the modeled between of and using a theoretical based on of the equation and from this new theory with the for of small to the however, was that the is often to is for with much than the In to Gang and also explored can be for energy explored in the along can be to the and of Gang and showed that enhanced by can also the and of of heat Gang and pioneered an experimental to the between a and a using a as a temperature In this experimental a with a of and materials was to the of the and in to a was to the of the was to a stage was to the In to the the with the was to the to the by the and Based on this experimental between a and a was for a of materials such as and due to the contributions from was to the by of at a Gang and the between two in small as to a radiative heat a was to the by law of for more than Gang’s seminal work on thermal has a of investigations in heat transfer in the few decades, of can be in these review Gang and a new to materials be of significant interest for Gang’s has also of the of the in the size between is in the nanometer to the in a seminal work between of two by nm to that can significantly from theoretical based on the size between is a in heat transfer at these that in gaps from and to the conduction progress on has and these significant by Gang’s by the of radiative heat Gang and also made contributions that our understanding of heat conduction in is well the heat flow in and is often mediated by electrons and phonons [11]. In to this in Gang and predicted that can also to or even thermal in thin films made of For example, by including the contribution of the thermal conductivity of a silicon was to be at more than the only was that to such is the of the can even a few Therefore, are to facilitate the experimental of the theoretical a by the theoretical Gang and also made an to the of on results that well with theoretical for the at In the few experimental of enhanced thermal conductivity in both and have only for silicon and also for and from experimental have only this field also led to new experimental deviate of from the of attributed to novel by the work to and can significant has also explored the law of thermal that the and the are can be past works have that law is only is and can be under conditions by theoretical work has that can be in devices materials because the of such materials can be to be by a field of of such often of In the of Gang Chen and the of law can be in the of that can be in due to even in the of an This due to the associated with enhanced at the as and of that recent have a of law in materials are no in for interest in thermal transport in polymer chains was from his studies of systems. is a with by polymer For such a a by and showed that a is microscopic can a thermal transport of this of energy a few that is no effective energy in this and the thermal conductivity Gang and first to chains of the polymer and showed that thermal transport in and aligned chains can significantly thermal to bulk materials the for the thermal conductivity is due to the of leading to phonon energy This seminal to common that are thermal and aligned polymer chains can to thermal conductivity that is for thermal management these Gang and a novel to by and was by a solution was drawn from the using a or was to heat the to the temperature of was at a of the has a broad of on the and the temperature Based on a thermal measurement using a the thermal of these to be as as at room is than many by the seminal experimental work by conducted to the thermal transport in and based thermal and thermal and research on thermal conductivity in and have significant interest for thermal management applications the of has by their small size and as well as understanding of the thermal transport in these Gang and a new for aligned polymer films. This a and by the thermal conductivity of to films films achieved a thermal conductivity of with studies and thermal a of and in the the a thermal conductivity on experimental is that is a in the on facilitate thermal transport along polymer or thermal transport between chains This approach has the of both of the for thermal conductivity in Gang and this by thermal conductivity in a thin of the polymer using a vapor This approach both along the polymer and between confirmed the of both of leading to a temperature thermal conductivity of is than that of This the first of both and to thermal transport in this section, we briefly review Professor Gang Chen’s contributions to applied energy and thermoelectric are solid-state devices to solar into by means of a first into and heat into is by a solar to the of a of and thermoelectric that as in the a the heat the the thermoelectric at has to work in an in to a significant of temperature between the and and heat Gang started on around with his students and in with Professor he first a theoretical foundation on this on to the of is important to that that on for both and applications of to to first from Gang on can be in he the theoretical for the estimation of the for these of This work also the fundamental of thermal for the thermoelectric area is much smaller than the This the temperature for a from the solar the of this theoretical by later studies Gang’s was to their first generation of systems, with optical and with optical and This a impact on the thermoelectric community as the of a for thermoelectric This was also by the that the a of around electrical the few Gang and his solar materials and new for optical and heat led to the of the generation of to an of at optical and at optical In the following Gang and also explored and thermoelectric to of the entire solar in from by the of this was to the the thermal conductivity of than that of their be by thermal conductivity of that be by even with the and leading to this under Gang’s first that was the to the thermal conductivity in this with thin by Gang’s as to showed thermal conductivity Gang’s a new thermal the thermal conductivity with phase in a was by optical and studies a temperature was based on a with electrical and thermal of a phase be in energy in the has the for and the solar thermal for at the has a of research Gang’s in materials for this challenge in the Gang and a a thin solar thermal of the with the conduction of the a temperature of under a solar of This temperature the by a of to a due to the of the to the heat at the is seminal work on to a of works within Gang’s group, while a new research by this over the few the research within this field Gang’s to in this For example, generation under was by thermal was also as a for the Gang’s solar of with that the of to the based on in have as an have by the solar and the due to the solar and Gang’s a of in This from solar while the to this achieved a vapor generation of at solar of to its role in reducing thermal in the can also both and radiative heat in the to its solar and thermal conductivity the with an underlying the to a significant of thermal as a solar thermal this Gang’s with to an In their the with the as a of to the underlying the functions as an and to the This a for solar energy in as as over to solar energy at on the for the of solar energy for a of recent Gang has on a fundamental understanding of in to the experimental of the theoretical thermal in Gang has a novel of from the with photons in the is the Gang and have conducted experimental investigations of this in and at a of of at the This can from the of the the that to a local of Gang also a theory for this based on conditions This is a in more theoretical and experimental studies are to the underlying microscopic Gang Chen has made transformative contributions to nanoscale heat transfer and energy science, both fundamental research and technological work has understanding in such as nondiffusive phonon transport, nanostructured nanoscale thermal thermal transport in and solar-photovoltaic-thermal energy systems. Gang’s such as the phonon Boltzmann transport equation and coherent phonon transport, have laid the theoretical for thermoelectric materials and nanoscale energy systems. experimental include to phonon mean free path and thermal conductivity materials contributions have in energy thermal and Gang’s work to frontiers in nanoscale and research. the of energy and Gang’s research to in nanotechnology and energy science, a understanding of heat transfer and their to Professor Gang

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