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1. Ultralow lattice thermal conductivity of chalcogenide perovskite CaZrSe3 contributes to high thermoelectric figure of merit

   https://doi.org/10.1038/s41524-019-0253-5  

   Osei-Agyemang, Eric ; Adu, Challen Enninful ; Balasubramanian, Ganesh ( December 2019 , npj Computational Materials)

   An emerging chalcogenide perovskite, CaZrSe₃, holds promise for energy conversion applications given its notable optical and electrical properties. However, knowledge of its thermal properties is extremely important, e.g. for potential thermoelectric applications, and has not been previously reported in detail. In this work, we examine and explain the lattice thermal transport mechanisms in CaZrSe₃using density functional theory and Boltzmann transport calculations. We find the mean relaxation time to be extremely short corroborating an enhanced phonon–phonon scattering that annihilates phonon modes, and lowers thermal conductivity. In addition, strong anharmonicity in the perovskite crystal represented by the Grüneisen parameter predictions, and low phonon number density for the acoustic modes, results in the lattice thermal conductivity to be limited to 1.17 W m⁻¹ K⁻¹. The average phonon mean free path in the bulk CaZrSe₃sample (N → ∞) is 138.1 nm and nanostructuring CaZrSe₃sample to ~10 nm diminishes the thermal conductivity to 0.23 W m⁻¹ K⁻¹. We also find that p-type doping yields higher predictions of thermoelectric figure of merit than n-type doping, and values ofZT~0.95–1 are found for hole concentrations in the range 10¹⁶–10¹⁷ cm⁻³and temperature between 600 and 700 K.
2. Modulating the thermal conductivity in hexagonal boron nitride via controlled boron isotope concentration

   https://doi.org/10.1038/s42005-019-0145-5  

   Yuan, Chao ; Li, Jiahan ; Lindsay, Lucas ; Cherns, David ; Pomeroy, James W. ; Liu, Song ; Edgar, James H. ; Kuball, Martin ( May 2019 , Communications Physics)

   Hexagonal boron nitride (h-BN) has been predicted to exhibit an in-plane thermal conductivity as high as ~ 550 W m⁻¹K⁻¹at room temperature, making it a promising thermal management material. However, current experimental results (220–420 W m⁻¹K⁻¹) have been well below the prediction. Here, we report on the modulation of h-BN thermal conductivity by controlling the B isotope concentration. For monoisotopic¹⁰B h-BN, an in-plane thermal conductivity as high as 585 W m⁻¹K⁻¹is measured at room temperature, ~ 80% higher than that of h-BN with a disordered isotope concentration (52%:48% mixture of¹⁰B and¹¹B). The temperature-dependent thermal conductivities of monoisotopic h-BN agree well with first principles calculations including only intrinsic phonon-phonon scattering. Our results illustrate the potential to achieve high thermal conductivity in h-BN and control its thermal conductivity, opening avenues for the wide application of h-BN as a next-generation thin-film material for thermal management, metamaterials and metadevices.
3. Light-triggered thermal conductivity switching in azobenzene polymers

   https://doi.org/10.1073/pnas.1817082116  

   Shin, Jungwoo ; Sung, Jaeuk ; Kang, Minjee ; Xie, Xu ; Lee, Byeongdu ; Lee, Kyung Min ; White, Timothy J. ; Leal, Cecilia ; Sottos, Nancy R. ; Braun, Paul V. ; et al ( March 2019 , Proceedings of the National Academy of Sciences)

   Materials that can be switched between low and high thermal conductivity states would advance the control and conversion of thermal energy. Employing in situ time-domain thermoreflectance (TDTR) and in situ synchrotron X-ray scattering, we report a reversible, light-responsive azobenzene polymer that switches between high (0.35 W m⁻¹K⁻¹) and low thermal conductivity (0.10 W m⁻¹K⁻¹) states. This threefold change in the thermal conductivity is achieved by modulation of chain alignment resulted from the conformational transition between planar (trans) and nonplanar (cis) azobenzene groups under UV and green light illumination. This conformational transition leads to changes in the π-π stacking geometry and drives the crystal-to-liquid transition, which is fully reversible and occurs on a time scale of tens of seconds at room temperature. This result demonstrates an effective control of the thermophysical properties of polymers by modulating interchain π-π networks by light.
4. Effects of intervalence charge transfer interaction between π-stacked mixed valent tetrathiafulvalene ligands on the electrical conductivity of 3D metal–organic frameworks

   https://doi.org/10.1039/d1sc04338b  

   Zhang, Shiyu ; Panda, Dillip K. ; Yadav, Ashok ; Zhou, Wei ; Saha, Sourav ( October 2021 , Chemical Science)

   Achieving a molecular-level understanding of how the structures and compositions of metal–organic frameworks (MOFs) influence their charge carrier concentration and charge transport mechanism—the two key parameters of electrical conductivity—is essential for the successful development of electrically conducting MOFs, which have recently emerged as one of the most coveted functional materials due to their diverse potential applications in advanced electronics and energy technologies. Herein, we have constructed four new alkali metal (Na, K, Rb, and Cs) frameworks based on an electron-rich tetrathiafulvalene tetracarboxylate (TTFTC) ligand, which formed continuous π-stacks, albeit with different π–π-stacking and S⋯S distances ( d π–π and d S⋯S ). These MOFs also contained different amounts of aerobically oxidized TTFTC˙ + radical cations that were quantified by electron spin resonance (ESR) spectroscopy. Density functional theory calculations and diffuse reflectance spectroscopy demonstrated that depending on the π–π-interaction and TTFTC˙ + population, these MOFs enjoyed varying degrees of TTFTC/TTFTC˙ + intervalence charge transfer (IVCT) interactions, which commensurately affected their electronic and optical band gaps and electrical conductivity. Having the shortest d π–π (3.39 Å) and the largest initial TTFTC˙ + population (∼23%), the oxidized Na-MOF 1-ox displayed the narrowest band gap (1.33 eV) and the highest room temperature electrical conductivitymore »(3.6 × 10 −5 S cm −1 ), whereas owing to its longest d π–π (3.68 Å) and a negligible TTFTC˙ + population, neutral Cs-MOF 4 exhibited the widest band gap (2.15 eV) and the lowest electrical conductivity (1.8 × 10 −7 S cm −1 ). The freshly prepared but not optimally oxidized K-MOF 2 and Rb-MOF 3 initially displayed intermediate band gaps and conductivity, however, upon prolonged aerobic oxidation, which raised the TTFTC˙ + population to saturation levels (∼25 and 10%, respectively), the resulting 2-ox and 3-ox displayed much narrower band gaps (∼1.35 eV) and higher electrical conductivity (6.6 × 10 −5 and 4.7 × 10 −5 S cm −1 , respectively). The computational studies indicated that charge movement in these MOFs occurred predominantly through the π-stacked ligands, while the experimental results displayed the combined effects of π–π-interactions, TTFTC˙ + population, and TTFTC/TTFTC˙ + IVCT interaction on their electronic and optical properties, demonstrating that IVCT interactions between the mixed-valent ligands could be exploited as an effective design strategy to develop electrically conducting MOFs.« less
5. Thermal conductivity of hexagonal BC ₂ P – a first-principles study

   https://doi.org/10.1039/d0ra08444a  

   Muthaiah, Rajmohan ; Tarannum, Fatema ; Annam, Roshan Sameer ; Nayal, Avinash Singh ; Danayat, Swapneel ; Garg, Jivtesh ( November 2020 , RSC Advances)

   In this work, we report a high thermal conductivity ( k ) of 162 W m −1 K −1 and 52 W m −1 K −1 at room temperature, along the directions perpendicular and parallel to the c -axis, respectively, of bulk hexagonal BC 2 P (h-BC 2 P), using first-principles calculations. We systematically investigate elastic constants, phonon group velocities, phonon linewidths and mode thermal conductivity contributions of transverse acoustic (TA), longitudinal acoustic (LA) and optical phonons. Interestingly, optical phonons are found to make a large contribution of 30.1% to the overall k along a direction perpendicular to the c -axis at 300 K. BC 2 P is also found to exhibit high thermal conductivity at nanometer length scales. At 300 K, a high k value of ∼47 W m −1 K −1 is computed for h-BC 2 P at a nanometer length scale of 50 nm, providing avenues for achieving efficient nanoscale heat transfer.