NH 4 F is demonstrated to be a promising mineralizer for the acidic ammonothermal crystal growth of GaN. In comparison with other acidic mineralizers such as NH 4 Cl, NH 4 Br, and NH 4 I, NH 4 F behaves distinctively different. First, NH 4 F affords a negative temperature gradient for crystal growth of GaN in supercritical NH 3 at a temperature range from 550 to 650 °C. Second, it enables GaN crystal growth in polar (c plane), semipolar, and nonpolar directions (a plane and m plane). Third, NH 4 F remarkably increases both the growth rate and quality of the GaN crystal. With the aid of NH 4 F, self-nucleation of GaN and bulk growth of hexagonal GaN crystals from the self-nucleated seed have been realized.

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Absorption of hydrogen in a high-strength nickel-chromium-molybdenum steel during tensile deformation in 0.5 MPa gaseous hydrogen was examined using a thermal desorption analysis method. The tensile strength of the specimen was varied in the range from 1 214 to 947 MPa by heat treatment. The dislocation density of the specimens was measured by X-ray diffractometry after tensile testing in a hydrogen atmosphere. The hydrogen content absorbed during tensile deformation increased with increasing tensile strain in proportional elastic range until just before yielding. The yield stress was defined as 0.2 % proof stress in this work. At the same tensile strains, the hydrogen content of lower-strength specimens was larger than that of higher-strength specimens. The dislocation density gradually decreased until just before yielding, corresponding to the proportional increase of hydrogen content to the tensile strain. This implies that the hydrogen absorption behavior during tensile deformation in gaseous hydrogen is related to the motion of mobile dislocations initially contained in the specimens. The activation energy for desorption of hydrogen absorbed during tensile deformation did not depend on the strength of the steel. This indicates that the trap sites of hydrogen atoms created through the tensile deformation were the same regardless of the strength levels.

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