Solubility and dissolution kinetics of GaN are investigated, as they represent essential parameters for ammonothermal crystal growth of GaN. In situ X-ray imaging is applied to monitor the dissolving crystal. Both ammonoacidic and ammonobasic conditions are investigated. Compared to NH4F, the dissolution is generally much slower using NaN3 mineralizer, leading to a much longer time needed to establish a saturated solution. The solubility of GaN at 540 °C and 260 MPa in supercritical ammonia with a molar concentration of NaN3 of 0.72 mmol/ml is determined to be 0.15 ± 0.01 mol%. This suggest a severe refinement of raw gravimetric literature data also for alkali metal based mineralizers, as we reported previously for ammonium halide mineralizers. The order of magnitude is in good agreement with refined gravimetric solubility data (Griffiths et al., 2016). The apparent discrepancy between the literature and this work regarding the temperature range in which retrograde solubility occurs is discussed. A possible reason for the occurrence of retrograde solubility at high temperatures is described. The paper is complemented by a section pointing out and partially quantifying potential, reactor-material-dependent sources of errors.
Dr. Nicolas Alt,
Liquid Organic Hydrogen Carriers (LOHCs) are able to store hydrogen in a dense and safe form at ambient conditions. While storage of electrical energy in these carrier systems is one possible and attractive application, the dynamics of the load profile has been believed to represent a major challenge for this storage technology. Conversely, we report here that storage systems based on the LOHC technology are indeed able to deal with significant variations in power demand. This is due to the significant free volume in the LOHC release unit offering the opportunity to handle dynamic behavior by pressure changes. While pressure changes allow quick adaption of the power release on demand, changes in the reactor temperature lead to slow modification of the power output, as demonstrated in this work for hydrogen release from perhydro-dibenzyltoluene (H18-DBT).
Anna-Carina L. Kimmel,
Dr. Nicolas Alt,
The first gallium-containing nitridosilicate CaGa-SiN3 was synthesized in newly developed high-pressure autoclaves using supercritical ammonia as solvent and nitriding agent. The reaction was conducted in an ammonobasic environment starting from intermetallic CaGaSi with NaN3 as a mineralizer. At 770 K, intermediate compounds were obtained, which were subsequently converted to the crystalline nitride at temperatures up to 1070K (70–150 MPa) .The impact of other mineralizers (e.g., LiN3, KN3, and CsN3) on the product formation was investigated as well. The crystal structure of CaGaSiN3 was analyzed by powder X-ray diffraction and refined by the Rietveld method. The structural results were further corroborated by transmission electron microscopy, 29SiMAS-NMR, and first-principle DFT calculations. CaGaSiN3 crystallizes in the orthorhombic space group Cmc21 (no. 36) with lattice parameters a=9.8855(11), b=5.6595(1), c=5.0810(1) A, (Z=4, Rwp=0.0326), and is iso-structural with CaAlSiN3 (CASN). Eu2+doped samples exhibit red luminescence with an emission maximum of 620 nm and FWHM of 90 nm. Thus, CaGaSiN3:Eu2+ also represents an interesting candidate as a red-emitting material in phosphor-converted light-emitting diodes (pc-LEDs). In addition to the already known substitution of alkaline-earth metals in(Ca,Sr)AlSiN3:Eu2+,inclusion of Ga is a further and promising perspective for luminescence tuning of widely used red-emitting CASN type materials.
Thomas G. Steigerwald,
Dr. Nicolas Alt,
Peter J. Wellmann
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