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Wafer-scale thin films of hexagonal boron nitride have exceptional thermal and mechanical properties, which harness the potential use of these materials in two-dimensional electronic, device applications. Along with unavoidable defects, grains, and wrinkles, which develop during the growth process, underlying substrates influence the physical and mechanical properties of these films. Understanding the interactions of these large-scale films with different substrates is, thus, important for the implementation of this 2D system in device fabrication. MOVPE-grown 2 and 30 nm hBN/sapphire films of size 2 in. diameter are delaminated chemically and transferred on quartz, SiO2/Si, and sapphire substrates. The structural characteristics of these films are investigated by employing Raman spectroscopy. Our results suggest that not only the roughness but also the height modulation at the surface of the substrates play a pivotal role in determining substrate-mediated mechanical strain inhomogeneity in these films. The statistical analysis of the spectral parameters provides us with the overall characteristics of the films. Furthermore, a Stark difference in the thermal evolution of strain in these films depending on substrate materials is observed. It has been demonstrated that not only the differential thermal expansion coefficient of the substrates and the films, but also slippage of the latter during the thermal treatment determines the net strain in the films. The role of the slippage is significantly higher in 2 nm films than in 30 nm films. We believe that the observations provide crucial information on the structural characteristics of the substrate-coupled wafer-scale hBN films for their future use in technology. [ABSTRACT FROM AUTHOR]

• Y 3-x Nd x Al 5 O 12 system studied to mimic substituted garnets for MA transmutation. • Synthesis route enabled stabilization of 33 mol% Nd3+ substituted garnet observed. • Incorporation of Nd3+ causes expansion of the cubic garnet phase. • Stabilization of defect stabilized rhombohedral NdAlO 3 in Nd3+-rich region. • Thermal conductivity/thermal expansion decrease with Nd3+ incorporation in Y 3 Al 5 O 12. The present work reports structural and thermophysical investigation in Nd3+- substituted Y 3 Al 5 O 12 (YAG). In addition to being an interesting system for studying the effect of isovalent substitution on phase/structural evolution in a garnet system, it has direct relevance as an inert matrix for burning minor actinides (MA: Am, Cm, and Np) for reducing the long-term radiotoxicity of nuclear wastes. More importantly, garnet has superior thermal conductivity than the zirconia-based systems which are widely being explored for this application. In order to investigate the phase miscibility and resulting thermophysical behavior, fourteen nominal compositions in Y 3-x Nd x Al 5 O 12 (0.0 ≤ x ≤ 3.0) system (Nd3+: chosen as the surrogate for MA) were synthesized by gel combustion followed by variable temperature annealing. The atomistic-level mixing ensured high solubility (33 mol% at 1273 K) of Nd3+ in YAG retaining the garnet-phase but the solubility reduced (13 mol%) upon annealing at 1773 K. Raman spectroscopy supported the phase evolution with composition and temperature. The microstructure of select compositions was investigated using SEM/EDS. Nd3+ caused dilation of garnet lattice, increase in average thermal expansion, and lowering of thermal conductivity. It must be emphasized that Nd-doped garnet still possessed better thermal conductivity compared ZrO 2 -based systems, a desirable attribute for matrices for MA-transmutation. Other important thermodynamic functions such as molar heat capacity, entropy and molar Gibbs free energy were determined. The study exhibits (a) roles of ionic size, synthesis route, temperature on phase relations upon isovalent substitution in YAG; (b) validation of the thermophysical performance of YAG-type material for MA transmutation. [Display omitted] [ABSTRACT FROM AUTHOR]

In this article, we report negative thermal expansion and spin frustration in hexagonal GdInO3. Rietveld refinements of the x-ray diffraction patterns reveal that the negative thermal expansion in the temperature range of 50-100 K stems from the triangular lattice of Gd3+ ions. The downward deviation of the low-temperature inverse susceptibility (χ-1) versus T plot from the Curie-Weiss law and the large value of the ratio, 0CW|/TN>28, where θCW and TN are respectively Curie-Weiss and Neel temperature, indicate a strong spin frustration, which inhibits long-range magnetic ordering down to 1.8 K. Magnetostriction measurements clearly demonstrate a spin-lattice coupling in the system. Low-temperature anomalous phonon softening, as obtained from temperature-dependent Raman measurements, also reveals the same. Our experimental observations are supported by first-principles density functional theory calculations of the electronic and phonon dispersion in GdInO3. The calculations suggest that the GdInO3 lattice is highly frustrated at low temperature. Further, the calculated normal mode frequencies of the Gd-related G point phonon modes reveal significant magnetoelastic coupling in this system. The competitive role of magnetic interaction energy and thermal stabilization energy in determining the change in interatomic distances is the possible origin for the negative thermal expansion in GdInO3 over a limited range of temperature. [ABSTRACT FROM AUTHOR]