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1. Electronic and optical characterization of bulk single crystals of cubic boron nitride (cBN)

   https://doi.org/10.1063/5.0092557  

   Milas, Peker; Mathab, Sheikh; Sam Abraham, John Bishoy; Alam, Jahangir; Chandrashekar, M. V.; Robinson, Adam J.; Vora, Patrick M.; Ozturk, Birol; Spencer, Michael G. (September 2022, AIP Advances)

   Cubic boron nitride (cBN) is a relatively less studied wide bandgap semiconductor despite its many promising mechanical, thermal, and electronic properties. We report on the electronic, structural, and optical characterization of commercial cBN crystal platelets. Temperature dependent transport measurements revealed the charge limited diode behavior of the cBN crystals. The equilibrium Fermi level was determined to be 0.47 eV below the conduction band, and the electron conduction was identified as n-type. Unirradiated dark and amber colored cBN crystals displayed broad photoluminescence emission peaks centered around different wavelengths. RC series zero phonon line defect emission peaks were observed at room temperature from the electron beam irradiated and oxygen ion implanted cBN crystals, making this material a promising candidate for high power microwave devices, next generation power electronics, and future quantum sensing applications. 

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2. Effect of argon flow rate and direct current bias on the growth of boron nitride coating in low-temperature plasma

   https://doi.org/10.1063/5.0102036  

   Chakrabarty, Kallol; Baker, Paul A.; Vijayan, Vineeth M.; Catledge, Shane A. (August 2022, AIP Advances)

   A microwave plasma chemical vapor deposition system was used to synthesize cubic boron nitride (cBN) coatings on diamond seeded silicon substrates using direct current (DC) bias. Effects of the argon (Ar) flow rate and bias voltage on the growth of the cBN coatings were investigated. Hydrogen (H2), argon (Ar), a mixture of diborane in H2 (95% H2, 5% B2H6), and N2 were used in the feed gas. A DC bias system was used for external biasing of the sample, which facilitates the goal of achieving sp3 bonded cBN. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) revealed the existence of sp3-bonded BN in the produced samples. With increasing Ar flow, the cBN content in the coating increases and reaches a maximum at the maximum Ar flow of 400 SCCM used in this study. High-resolution XPS scans for B1s and N1s indicate that the deposited coating contains more than 70% cBN. This study demonstrates that energetic argon ions generated in a microwave-induced plasma significantly increase cBN content in the coating. 

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3. Synthesis and Thermal Oxidation Resistance of Boron-Rich Boron–Carbide Material

   https://doi.org/10.3390/ma16196526  

   Iwan, Seth; Sutton, Wesley; Baker, Paul A.; Sereika, Raimundas; Vohra, Yogesh K. (October 2023, Materials)

   A boron-rich boron–carbide material (B4+δC) was synthesized by spark plasma sintering of a ball-milled mixture of high-purity boron powder and graphitic carbon at a pressure of 7 MPa and a temperature of 1930 °C. This high-pressure, high-temperature synthesized material was recovered and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Vickers hardness measurements, and thermal oxidation studies. The X-ray diffraction studies revealed a single-phase rhombohedral structure (space group R-3m) with lattice parameters in hexagonal representation as a = 5.609 ± 0.007 Å and c = 12.082 ± 0.02 Å. The experimental lattice parameters result in a value of δ = 0.55, or the composition of the synthesized compound as B4.55C. The high-resolution scans of boron binding energy reveal the existence of a B-C bond at 188.5 eV. Raman spectroscopy reveals the existence of a 386 cm−1 vibrational mode representative of C-B-B linear chain formation due to excess boron in the lattice. The measured Vickers microhardness at a load of 200 gf shows a high hardness value of 33.8 ± 2.3 GPa. Thermal gravimetric studies on B4.55C were conducted at a temperature of 1300 °C in a compressed dry air environment, and its behavior is compared to other high-temperature ceramic materials such as high-entropy transition metal boride. The high neutron absorption cross section, high melting point, high mechanical strength, and thermal oxidation resistance make this material ideal for applications in extreme environments. 

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4. Sodium molybdate‐hexagonal boron nitride composites enabled by cold sintering for microwave dielectric substrates

   https://doi.org/10.1111/jace.19254  

   Mena‐Garcia, Javier; Ndayishimiye, Arnaud; Fan, Zhongming; Perini, Steven E.; Li, Wenjie; Poudel, Bed; Priya, Shashank; Foley, Brian; Gaskins, John; Randall, Clive A. (October 2023, Journal of the American Ceramic Society)

   Abstract To fulfill the demands of more bandwidth in 5G and 6G communication technology, new dielectric substrates that can be co‐fired into packages and devices that have low dielectric loss and improved thermal conductivity are desired. The motivation for this study is to design composites with low dielectric loss (tan δ) and high thermal conductivity (κ), while still limiting the electrical conductivity, for microwave applications involving high power and high frequency. This work describes the fabrication of high‐density electroceramic composites with a model dielectric material for cold sintering, namely sodium molybdate (Na₂Mo₂O₇), and fillers with higher thermal conductivity such as hexagonal boron nitride. The physical properties of the composites were characterized as a function of filler vol.%, temperature, and frequency. Understanding the variation in measured properties is achieved through analyzing the respective transport mechanisms. 

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5. Lower mantle geotherms, flux, and power from incorporating new experimental and theoretical constraints on heat transport properties in an inverse model

   https://doi.org/10.5194/ejm-34-149-2022  

   Hofmeister, Anne M. (January 2022, European Journal of Mineralogy)

   Abstract. An inverse method is devised to probe Earth's thermalstate without assuming its mineralogy. This constrains thermal conductivity(κ) in the lower mantle (LM) by combining seismologic models of bulkmodulus (B) and pressure (P) vs. depth (z) with a new result, ∂ln(κ) / ∂P ∼ 7.33/BT, and available high temperature (T) data onκ for lengths exceeding millimeters. Considering large samples accounts forthe recently revealed dependence of heat transport properties onlength scale. Applying separation of variables to seismologic ∂B/∂P vs. depth isolates changes with T. The resulting LM dT / dz dependson ∂2B/∂P2 and ∂B/∂T, whichvary little among dense phases. Because seismic ∂B/∂P isdiscontinuous and model dependent ∼ 200 km above the core,unlike the LM, our results are extrapolated through this tiny layer (D′′).Flux and power are calculated from dT / dz for cases of high (oxide) and low(silicate) κ. Geotherm calculations are independent of κ,and thus of LM mineralogy, but require specifying a reference temperature atsome depth: a wide range is considered. Limitations on deep melting are usedto ascertain which of our geotherm, flux, and power curves best representEarth's interior. Except for an oxide composition with miniscule ∂2B/∂P2, the LM heats the core, causing it to melt. Deepheating is attributed to cyclical stresses from > 1000 km dailyand monthly fluctuations of the barycenter inside the LM. 

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