This article shows experimentally that an external electric field affects the velocity of the longitudinal acoustic phonons ( v LA ), thermal conductivity (κ), and diffusivity ( D ) in a bulk lead zirconium titanate–based ferroelectric. Phonon conduction dominates κ, and the observations are due to changes in the phonon dispersion, not in the phonon scattering. This gives insight into the nature of the thermal fluctuations in ferroelectrics, namely, phonons labeled ferrons that carry heat and polarization. It also opens the way for phonon-based electrically driven all-solid-state heat switches, an enabling technology for solid-state heat engines. A quantitative theoretical model combining piezoelectric strain and phonon anharmonicity explains the field dependence of v LA , κ, and D without any adjustable parameters, thus connecting thermodynamic equilibrium properties with transport properties. The effect is four times larger than previously reported effects, which were ascribed to field-dependent scattering of phonons.

Thermal management is essential for maintaining the optimal performance of electronic devices. Although covalent–organic frameworks (COFs) have emerged as a platform for gas and energy storage applications, their thermal transport properties are greatly understudied. Herein, we report the thermal conductivities of three benzobisoxazole (BBO)-linked COFs with nanpores ranging from 1.3 to 2.5 nm over a wide temperature range (80–300 K) using the longitudinal, steady-state heat-flow method. In doing so, thermal conductivity values as high as 0.677 W m–1 K–1 at 300 K were obtained, and no relationship between the thermal conductivity and pore size was observed. These results were supported by density functional theory calculations. The thermal conductivities of the BBO-COFs doped with poly­(3-hexylthiophene) were also investigated. The BBO-COFs could be useful as ultralow-k materials for thermal management applications.

The spin-Seebeck effect (SSE) is an advective transport process traditionally studied in bilayers composed of a ferromagnet (FM) and a non-magnetic metal (NM) with strong spin-orbit coupling. In a temperature gradient, the flux of magnons in the FM transfers spin-angular momentum to electrons in the NM, which by the inverse spin-Hall effect generates an SSE voltage. In contrast, the Nernst effect is a bulk transport phenomenon in homogeneous NMs or FMs. These effects share the same geometry, and we show here that they can be added to each other in a new combination of FM/NM composites where synthesis via in-field annealing results in the FM material (MnBi) forming aligned needles inside an NM matrix with strong spin-orbit coupling (SOC) (Bi). Through examination of the materials’ microstructural, magnetic, and transport properties, we searched for signs of enhanced transverse thermopower facilitated by an SSE contribution from MnBi adding to the Nernst effect in Bi. Our results indicate that these two signals are additive in samples with lower MnBi concentrations, suggesting a new way forward in the study of SSE composite materials.

Microstructural characterization workflows suitable for analysis of magnetic composites have become increasingly important due to growing interest in the development of such materials for applications ranging from rare-earth free permanent magnets to energy conversion and storage devices [1-3]. Here, we discuss the use of correlative imaging, spectroscopy, and diffraction techniques in a scanning electron microscope (SEM) to track the microstructural evolution of MnBi-Bi composites synthesized via melt quenching and magnetic field annealing, and then correlate these microstructural changes with the materials' magnetic properties.

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