This work investigates a nanoporous aluminum nitride (AlN) layer prepared using an anodic aluminum oxide (AAO) process and its application as a buffer layer for a GaN-based light-emitting diode (LED) fabricated on sapphire substrate. Following this AAO process, the average pore spacing and pore diameter of the nanoporous AlN layer were in the ranges 180-200 nm and 100-150 nm, respectively. The light output power of the GaN-based LED with a nanoporous AlN layer was about 53% higher than that of a GaN-based LED without a nanoporous AlN layer at an injection current of 20 mA. At an injection current of 80 mA, the light output power was increased by about 34%.
In this study, different amounts of tantalum carbide (TaC) powders (10, 20 and 30 mass%) were mixed and added to 440C steel powders. The composite powders were sintered at 1270, 1280 and 1290°C, respectively, for 1 h. The experimental results showed that a suitable amount of TaC particle addition was effective in improving the strength and hardness of the composite materials. Conversely, excess TaC hindered the liquid diffusion of the Fe elements. As a result, the 440C specimens with 30% TaC addition still had 1.3% porosity. The 440C specimens with 10% TaC addition sintered at 1270°C possessed the highest TRS values (2260.3 MPa), while those with 20% TaC addition sintered at 1290°C had the highest hardness values (HRA 85.2). The microstructural evaluation of the 440C specimens with 10% TaC addition revealed that all the rod-shaped M 7 C 3 carbides transformed to M 23 C 6 carbides and precipitated within the grains as a strengthening phase after heat treatment, which was advantageous to the transverse rupture strength (TRS). Consequently, the TRS value of the 440C specimens with 10% TaC addition was significantly increased (2260.3 ¼ 2458.4 MPa) after heat treatment, thus possessing a high hardness (HRA 83.8).
This study utilized two different sizes of 440C pre-alloy powders for vacuum sintering under different sintering temperatures (1 523, 1 533, 1 543 and 1 553 K) to explore the microstructure and mechanical properties of 440C stainless steel. The experimental results indicated that the sintered specimens of the refined alloy powders had good hardness and transverse rupture strength (TRS), due to the smaller grain size of the sintered body and the uniform distribution of the carbides. In addition, two types of carbide precipitates appeared in the microstructure. Fine carbides (M 23 C 6 ) within the grains and rod-shaped types of carbide (M 7 C 3 ) on the grain boundaries were observed after sintering at 1 543 K for 1 h. The research also showed that a suitable amount of carbides is helpful in improving the TRS. However, the fine carbides (M 23 C 6 ) within the grains almost disappeared and the rod-shaped types of carbide (M 7 C 3 ) on the grain boundaries coarsen after sintering at higher temperatures (1 553 K), and so detrimental to the TRS. KEY WORDS: 440C; vacuum sintering; TRS; M 23 C 6 and M 7 C 3 .
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