Left: High Thermal Conductivity BAs; Right: High Thermal Conductivity c-BN
Despite the fact that heat is everywhere and we need to transfer heat from one place to another all the time, nature has not given us many good choices of materials to handle this job. The thermal conductivity of materials spans only five orders of magnitude (excluding vacuum): from the low end of air at 0.026 W/m-K to the high end of diamond ~2000 W/m-K at room temperature. In comparison, electrical conductivity spans over 30 orders of magnitude (excluding superconductivity). For some applications, we need better thermal insulators, while for others, we want to have better heat conductors. We have been working on a variety of materials and structures to meet the task of different applications, combining first principles simulations, modeling with experiments.
The most thermally conductive 3D material is diamond (k~2000 W/m-K). The next to come to mind is often copper around 400 W/m-K, significantly lower than diamond. In 2018, we and our collaborators demonstrated that a new material, boron arsenide (BAs), has a thermal conductivity around 1200 W/m-K at room temperature, making it the second best heat conductor after diamond. In 2019, we demonstrated that isotopically enriched cubic boron nitride (cBN) has a thermal conductivity around 1600 W/m-K at room temperature. Thus, BAs and cBN are now among the best heat known conductors. Furthermore, we predicted via first principles simulations that BAs also has very good electron mobility. With a bandgap of ~2 eV, BAs is a semiconductor with excellent potential. We are now working with our materials collaborators to grow BAs and measure its electrical and thermal properties.
Left: High Thermal Conductivity BAs; Right: High Thermal Conductivity c-BN
Key Publications
Thermoelectric materials can convert heat directly into electrical energy by exploiting the thermal energy carried by charges (electrons and holes). Thermoelectric materials can be used for power generation as well as solid state refrigeration. The efficiency of thermoelectric power generators is determined by the nondimensional figure-of-merit, ZT=S2σT/k, where S is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T the absolute temperature. Over the past two decades, we have exploited different nanoscale physics to improve materials’ ZT, especially reduced thermal conductivity via boundary- and interface-scattering phonons. We have collaborated with material specialists, especially Prof. Zhifeng Ren at the University of Houston and the late MIT Institute Professor Mildred S. Dresselhaus, to advance the field of thermoelectrics. The nanocomposite approach we reported is widely explored by many groups around the world. We have developed strong simulation and experimental capabilities via these efforts and invented new devices and systems. Commercialization of the nanostructured thermoelectric materials was attempted via a startup for potential applications in vehicle water recovery and combined heat and electricity in solar thermal systems and home boilers.
Left: False Color TEM Image of Nanostructured Bi2Te3;Right: Bulk Form of Nanostructure
Key Publications
It is well known that polymers are poor heat conductors. Most polymers have a thermal conductivity <0.5 W/m-K. However, we believe that polymers can be engineered into good thermal conductors. Taking polyethylene as an example, it has carbon-carbon bonds along its molecular chain, yet the best heat conductor diamond is based on carbon-carbon bonds! In fact, inspired by the work of Fermi and his collaborators, we predicted that a single polyethylene molecule can have infinite thermal conductivity. This has not been proven experimentally yet. We have moved on to stretch polyethylene into nanofibers and plastic sheets and have achieved over 100 W/m-K in polyethylene nanofiber and over 60 W/m-K in polyethylene sheets along the stretching direction, making them comparable to or better than most metals in their thermal conductivity values. We also demonstrated an order of magnitude improvement in polymer thin films obtained via chemical vapor deposition. In general, we believe that there is plenty of room to engineer polymers into good heat conductors and are pursuing research in this direction via combined simulations, materials synthesis, and measurements.
Mechanical Stretching Polyethylene
Key Publications
Hydrogels, such as Jell-O, are water-polymer mixtures in quasi-solid form. There has been lots of exciting work in hydrogels for their biomedical applications. Our interests in hydrogel stem from our work on polymer heat conductors and floating solar absorbers for steam evaporation, being further stimulated by reports from others that hydrogels can be leveraged to achieve a much lower “apparent” latent heat of water and as moisture adsorbents. We are now working on understanding heat conduction mechanisms, thermodynamics, and vapor and ion transport in hydrogels.
Fabrics are an important part of a daily life and regulate our body energy exchange with the environment. While vapor transport, conduction, and convection mechanisms are explored extensively in the making of fabrics, thermal radiation is another channel that remains to be explored. We invented an approach to make visible-opaque, infrared-transparent fabrics by exploring the different scattering regimes of fabrics to visible and infrared light. If more body heat can escape through the fabrics, we can dial up our air conditioning temperature and reduce energy consumption in buildings. In addition to fabrics, we are also developing other materials to engineer photon transport. In collaboration with Prof. Evelyn Wang’s group, we developed optically-transparent and infrared-opaque aerogels for solar thermal applications and window retrofitting. We are also working on thermally-regulated roofs and low-cost window retrofittings.
Infrared-Transparent Visible-Opaque Fabrics
Key Publications
Around 2000, researchers observed that some colloids with nanosized particles suspended in liquids have significantly enhanced thermal conductivity that could be explained using effective media theories. We were asked by an industrial sponsor to understand the mechanisms underpinning this phenomenon. Via some innovative experiments, we eventually understood that one major mechanism is the clustering of the nanoparticles and the percolation path the particles form. With this understanding, we decided that the best way to increase thermal conductivity is actually using micron sized thin sheets, such as graphite flakes. We also observed interesting thermal percolation phenomena. Freezing such fluids also leads to the ability to switch thermal and electrical conductivity.
Graphite in PAO
Key Publications
Despite the fact that heat is everywhere and we need to transfer heat from one place to another all the time, nature has not given us many good choices of materials to handle this job. The thermal conductivity of materials spans only five orders of magnitude (excluding vacuum): from the low end of air at 0.026 W/m-K to the high end of diamond ~2000 W/m-K at room temperature. In comparison, electrical conductivity spans over 30 orders of magnitude (excluding superconductivity). For some applications, we need better thermal insulators, while for others, we want to have better heat conductors. We have been working on a variety of materials and structures to meet the task of different applications, combining first principles simulations, modeling with experiments.
The most thermally conductive 3D material is diamond (k~2000 W/m-K). The next to come to mind is often copper around 400 W/m-K, significantly lower than diamond. In 2018, we and our collaborators demonstrated that a new material, boron arsenide (BAs), has a thermal conductivity around 1200 W/m-K at room temperature, making it the second best heat conductor after diamond. In 2019, we demonstrated that isotopically enriched cubic boron nitride (cBN) has a thermal conductivity around 1600 W/m-K at room temperature. Thus, BAs and cBN are now among the best heat known conductors. Furthermore, we predicted via first principles simulations that BAs also has very good electron mobility. With a bandgap of ~2 eV, BAs is a semiconductor with excellent potential. We are now working with our materials collaborators to grow BAs and measure its electrical and thermal properties.
Left: High Thermal Conductivity BAs; Right: High Thermal Conductivity c-BN
Key Publications
Thermoelectric materials can convert heat directly into electrical energy by exploiting the thermal energy carried by charges (electrons and holes). Thermoelectric materials can be used for power generation as well as solid state refrigeration. The efficiency of thermoelectric power generators is determined by the nondimensional figure-of-merit, ZT=S2σT/k, where S is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T the absolute temperature. Over the past two decades, we have exploited different nanoscale physics to improve materials’ ZT, especially reduced thermal conductivity via boundary- and interface-scattering phonons. We have collaborated with material specialists, especially Prof. Zhifeng Ren at the University of Houston and the late MIT Institute Professor Mildred S. Dresselhaus, to advance the field of thermoelectrics. The nanocomposite approach we reported is widely explored by many groups around the world. We have developed strong simulation and experimental capabilities via these efforts and invented new devices and systems. Commercialization of the nanostructured thermoelectric materials was attempted via a startup for potential applications in vehicle water recovery and combined heat and electricity in solar thermal systems and home boilers.
Left: False Color TEM Image of Nanostructured Bi2Te3;Right: Bulk Form of Nanostructure
Key Publications
It is well known that polymers are poor heat conductors. Most polymers have a thermal conductivity <0.5 W/m-K. However, we believe that polymers can be engineered into good thermal conductors. Taking polyethylene as an example, it has carbon-carbon bonds along its molecular chain, yet the best heat conductor diamond is based on carbon-carbon bonds! In fact, inspired by the work of Fermi and his collaborators, we predicted that a single polyethylene molecule can have infinite thermal conductivity. This has not been proven experimentally yet. We have moved on to stretch polyethylene into nanofibers and plastic sheets and have achieved over 100 W/m-K in polyethylene nanofiber and over 60 W/m-K in polyethylene sheets along the stretching direction, making them comparable to or better than most metals in their thermal conductivity values. We also demonstrated an order of magnitude improvement in polymer thin films obtained via chemical vapor deposition. In general, we believe that there is plenty of room to engineer polymers into good heat conductors and are pursuing research in this direction via combined simulations, materials synthesis, and measurements.
Mechanical Stretching Polyethylene
Key Publications
Hydrogels, such as Jell-O, are water-polymer mixtures in quasi-solid form. There has been lots of exciting work in hydrogels for their biomedical applications. Our interests in hydrogel stem from our work on polymer heat conductors and floating solar absorbers for steam evaporation, being further stimulated by reports from others that hydrogels can be leveraged to achieve a much lower “apparent” latent heat of water and as moisture adsorbents. We are now working on understanding heat conduction mechanisms, thermodynamics, and vapor and ion transport in hydrogels.
Fabrics are an important part of a daily life and regulate our body energy exchange with the environment. While vapor transport, conduction, and convection mechanisms are explored extensively in the making of fabrics, thermal radiation is another channel that remains to be explored. We invented an approach to make visible-opaque, infrared-transparent fabrics by exploring the different scattering regimes of fabrics to visible and infrared light. If more body heat can escape through the fabrics, we can dial up our air conditioning temperature and reduce energy consumption in buildings. In addition to fabrics, we are also developing other materials to engineer photon transport. In collaboration with Prof. Evelyn Wang’s group, we developed optically-transparent and infrared-opaque aerogels for solar thermal applications and window retrofitting. We are also working on thermally-regulated roofs and low-cost window retrofittings.
Infrared-Transparent Visible-Opaque Fabrics
Key Publications
Around 2000, researchers observed that some colloids with nanosized particles suspended in liquids have significantly enhanced thermal conductivity that could be explained using effective media theories. We were asked by an industrial sponsor to understand the mechanisms underpinning this phenomenon. Via some innovative experiments, we eventually understood that one major mechanism is the clustering of the nanoparticles and the percolation path the particles form. With this understanding, we decided that the best way to increase thermal conductivity is actually using micron sized thin sheets, such as graphite flakes. We also observed interesting thermal percolation phenomena. Freezing such fluids also leads to the ability to switch thermal and electrical conductivity.
Graphite in PAO
Key Publications
Despite the fact that heat is everywhere and we need to transfer heat from one place to another all the time, nature has not given us many good choices of materials to handle this job. The thermal conductivity of materials spans only five orders of magnitude (excluding vacuum): from the low end of air at 0.026 W/m-K to the high end of diamond ~2000 W/m-K at room temperature. In comparison, electrical conductivity spans over 30 orders of magnitude (excluding superconductivity). For some applications, we need better thermal insulators, while for others, we want to have better heat conductors. We have been working on a variety of materials and structures to meet the task of different applications, combining first principles simulations, modeling with experiments.
The most thermally conductive 3D material is diamond (k~2000 W/m-K). The next to come to mind is often copper around 400 W/m-K, significantly lower than diamond. In 2018, we and our collaborators demonstrated that a new material, boron arsenide (BAs), has a thermal conductivity around 1200 W/m-K at room temperature, making it the second best heat conductor after diamond. In 2019, we demonstrated that isotopically enriched cubic boron nitride (cBN) has a thermal conductivity around 1600 W/m-K at room temperature. Thus, BAs and cBN are now among the best heat known conductors. Furthermore, we predicted via first principles simulations that BAs also has very good electron mobility. With a bandgap of ~2 eV, BAs is a semiconductor with excellent potential. We are now working with our materials collaborators to grow BAs and measure its electrical and thermal properties.
Left: High Thermal Conductivity BAs; Right: High Thermal Conductivity c-BN
Key Publications
Thermoelectric materials can convert heat directly into electrical energy by exploiting the thermal energy carried by charges (electrons and holes). Thermoelectric materials can be used for power generation as well as solid state refrigeration. The efficiency of thermoelectric power generators is determined by the nondimensional figure-of-merit, ZT=S2σT/k, where S is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T the absolute temperature. Over the past two decades, we have exploited different nanoscale physics to improve materials’ ZT, especially reduced thermal conductivity via boundary- and interface-scattering phonons. We have collaborated with material specialists, especially Prof. Zhifeng Ren at the University of Houston and the late MIT Institute Professor Mildred S. Dresselhaus, to advance the field of thermoelectrics. The nanocomposite approach we reported is widely explored by many groups around the world. We have developed strong simulation and experimental capabilities via these efforts and invented new devices and systems. Commercialization of the nanostructured thermoelectric materials was attempted via a startup for potential applications in vehicle water recovery and combined heat and electricity in solar thermal systems and home boilers.
Left: False Color TEM Image of Nanostructured Bi2Te3;Right: Bulk Form of Nanostructure
Key Publications
It is well known that polymers are poor heat conductors. Most polymers have a thermal conductivity <0.5 W/m-K. However, we believe that polymers can be engineered into good thermal conductors. Taking polyethylene as an example, it has carbon-carbon bonds along its molecular chain, yet the best heat conductor diamond is based on carbon-carbon bonds! In fact, inspired by the work of Fermi and his collaborators, we predicted that a single polyethylene molecule can have infinite thermal conductivity. This has not been proven experimentally yet. We have moved on to stretch polyethylene into nanofibers and plastic sheets and have achieved over 100 W/m-K in polyethylene nanofiber and over 60 W/m-K in polyethylene sheets along the stretching direction, making them comparable to or better than most metals in their thermal conductivity values. We also demonstrated an order of magnitude improvement in polymer thin films obtained via chemical vapor deposition. In general, we believe that there is plenty of room to engineer polymers into good heat conductors and are pursuing research in this direction via combined simulations, materials synthesis, and measurements.
Mechanical Stretching Polyethylene
Key Publications
Hydrogels, such as Jell-O, are water-polymer mixtures in quasi-solid form. There has been lots of exciting work in hydrogels for their biomedical applications. Our interests in hydrogel stem from our work on polymer heat conductors and floating solar absorbers for steam evaporation, being further stimulated by reports from others that hydrogels can be leveraged to achieve a much lower “apparent” latent heat of water and as moisture adsorbents. We are now working on understanding heat conduction mechanisms, thermodynamics, and vapor and ion transport in hydrogels.
Fabrics are an important part of a daily life and regulate our body energy exchange with the environment. While vapor transport, conduction, and convection mechanisms are explored extensively in the making of fabrics, thermal radiation is another channel that remains to be explored. We invented an approach to make visible-opaque, infrared-transparent fabrics by exploring the different scattering regimes of fabrics to visible and infrared light. If more body heat can escape through the fabrics, we can dial up our air conditioning temperature and reduce energy consumption in buildings. In addition to fabrics, we are also developing other materials to engineer photon transport. In collaboration with Prof. Evelyn Wang’s group, we developed optically-transparent and infrared-opaque aerogels for solar thermal applications and window retrofitting. We are also working on thermally-regulated roofs and low-cost window retrofittings.
Infrared-Transparent Visible-Opaque Fabrics
Key Publications
Around 2000, researchers observed that some colloids with nanosized particles suspended in liquids have significantly enhanced thermal conductivity that could be explained using effective media theories. We were asked by an industrial sponsor to understand the mechanisms underpinning this phenomenon. Via some innovative experiments, we eventually understood that one major mechanism is the clustering of the nanoparticles and the percolation path the particles form. With this understanding, we decided that the best way to increase thermal conductivity is actually using micron sized thin sheets, such as graphite flakes. We also observed interesting thermal percolation phenomena. Freezing such fluids also leads to the ability to switch thermal and electrical conductivity.
Graphite in PAO
Key Publications