2009, Vol.19, No.3

Journal of the Korean Crystal Growth and Crystal Technology 2023 KCI Impact Factor : 0.19

pISSN : 1225-1429 / eISSN : 2234-5078
https://journal.kci.go.kr/jkcgct

Particular interest in the role of convection in vapor crystal growth has arisen since some single crystals under high gravity acceleration of 10g0 appear considerably larger than those under normal gravity acceleration (1g0). For both ΔT = 60 K and 90 K, the mass flux increases by a factor of 3 with increasing the gravity acceleration from 1g0 up to 10g0. On the other hand, for ΔT = 30 K, the flux is increased by a factor of 1.36 for the range of 1g0 ≤ g ≤ 10g0. The maximum growth rates for 1g0, 4g0, 10g0 appear approximately in the neighborhood of y = 0.5, and the growth rates shows asymmetrical patterns, which indicate the occurrence of either one single or more than one convective cell. The maximum growth rate for 10g0 is nearly greater than that for 1g0 by a factor of 2.0 at PB = 20 Torr. For three different gravity levels of 1g0, 4g0 and 10g0, the maximum growth rates are greater than the minimum rates by a factor of nearly 3.0, based on PB = 20 Torr. The mass flux increases with increasing the gravity acceleration, for 1g0 ≤gy≤ 10g0, and decreases with increasing the partial pressure of component B, xenon (Xe), PB. The |U|max is directly proportional to the gravity acceleration for 20 Torr ≤ PB≤ 300 Torr. As the partial pressure of PB (Torr) decreases from 300 Torr to 20 Torr, the slopes of the |U|maxs versus the gravity accelerations increase from 0.1 sec to 0.17 sec. The mass flux of Hg2Cl2 is exponentially decayed with increasing the partial pressure of component B, PB (Torr) from 20 Torr up to 300 Torr .

Our computational studies for the physical vapor transport crystal growth of Hg2Cl2-Cl2 system evidence suggests that the PVT growth process exhibits the diffusion-dominated behaviors for aspect ratios more than and equal to 10, which would provide purely diffusive transport conditions adequate to microgravity environments less than 10−3g0. Also, the regimes of high temperature difference based on the fixed source temperature of 380 oC, where ΔT is relatively large enough for the crystal growth of mercurous chloride, the transport rates do not keep increasing with ΔT but tend to some constant value of 2.12 mole cm−2s−1. For the aspect ratios of 5, 10, and 20, the transport rate is directly proportional to the total pressure of the system under consideration. For Ar = 5, the rate is increased by a factor of 2.3 with increasing the total pressure from 403 Torr to 935 Torr, i.e., by a factor of 2.3. For both Ar = 10 and 20, the rate is increased by a factor of 1.25 with increasing the total pressure from 403 Torr to 935 Torr.

The preparation and characterization of niobium carbide crystallites were investigated in this study, and in particular, the effect of preparation conditions were studied on the synthesis of niobium carbides crystallites. For this purpose, various characterization techniques including x-ray diffraction, BET surface area, and oxygen uptake measurements were employed to characterize the synthesized niobium carbide crystallites. The niobium carbide crystallites were prepared using niobium oxide and methane gas or methane-hydrogen mixture. Using x-ray diffraction a lattice parameter of 4.45 Å and a crystallite size ranging from 52 Å to 580 Å was found. BET surface areas ranged from 3.2 m2/g to 16.6 m2/g and oxygen uptake values varied from 0.5 μmol/g to 6.1 μmol/g. It was observed that niobium carbide crystallites were active for ammonia decomposition reaction. While the BET surface area increased with increasing the oxygen uptake, the conversion of ammonia decomposition reaction decreased. These results indicated that the ammonia decomposition over these materials was considered to be structure-sensitive.