The challenge of decomposition and melting of gallium nitride under high pressure and high temperature

Gallium nitride (GaN) is considered to be one of the most important semiconductors nowadays. In this report a solution of the long standing puzzle regarding GaN decomposition and melting under high pressure and high temperaturę is presented.This includes the discussion of results obtained so far. The possibility of a consistent parameterisation of pressure (P) evolution of the melting temperaturę (Tm) in basic semiconductors (GaN, germanium, silicon…), independently from signs of dTm/dP is alsopresented

Phase relations near ternary eutectic point in the Ag-In-Sb system

The results of the phase relations near ternary eutectic point in the Ag-In-Sb system are investigated in this paper. Phase equilibrium calculation was done using Thermocalc software and experimental DTA results for the chosen alloys in the isopleths with molar ration of In:Sb = 7:3; 9:1 and 1:1. The structural characteristics of these alloys have been investigated using light optic microscopy and scanning electron microscopy, while hardness measurements have also been done. Solidification path for three ternary alloys located on three different investigated isopleths was calculated using Pandat software

TiC Nanocrystal Formation from Carburization of Laser-Grown Ti/O/C Nanopowders for Nanostructured Ceramics

Refractory carbide ceramics (TiC and ZrC) raise interest as promising materials for high-temperature applications such as structural materials for the future generation of nuclear reactors. In this context, nanostructured ceramics are expected to exhibit improved thermomechanical properties as well as better behavior under irradiation when compared to conventional materials. It is therefore necessary to synthesize carbide nanocrystals of such materials to elaborate the ceramics. We report here the formation study of TiC nanocrystals through the direct carburization of Ti/O/C nanopowders grown by laser pyrolysis. A spray of titanium tetraisopropoxide was laser pyrolyzed with ethylene as the sensitizer, leading to Ti/O/C nanopowders with various C contents controlled by the synthesis conditions. Annealing treatments performed on these nanopowders under an inert atmosphere without any C addition enabled the formation of TiC grains through the carburization of the oxide phase by free C incorporated during the synthesis. The powders were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The final TiC grain size was about 80 nm, and the grains were monocrystalline. The influence of the free C content on the grain growth during the annealing step, together with its effects on the densification of the ceramics after sintering by high-pressure flash sintering, was examined. A 93% densification was finally achieved

Fabrication and Physical Properties of SiC-GaAs Nano-Composites

Nano-composites consisting of primary phase of hard nanocrystalline SiC matrix and the secondary nanocrystalline semiconductor (GaAs) phase were obtained by high-pressure zone infiltration. The synthesis process occurs in three stages: (i) at room temperature the nanopowder of SiC is compacted along with GaAs under high pressure up to 8 GPa, (ii) the temperature is increased above the melting point of GaAs up to 1600~K and, the pores are being filled with liquid, (iii) upon cooling GaAs nanocrystallites grow in the pores. Synthesis of nano-composites was performed using a toroid-type high-pressure apparatus (IHPP of the Polish Academy of Sciences, Warsaw) and six-anvil cubic press (MAX-80 at HASYLAB, Hamburg). X-ray diffraction studies were performed using a laboratory D5000 Siemens diffractometer. Phase composition, grain size, and macrostrains present in the synthesized materials were examined. Microstructure of the composites was characterized using scanning electron microscopy and high resolution transmission electron microscopy. Far-infrared reflectivity measurements were used to determine built-in strain

Elaboration of SiC, TiC and ZrC nanopowders by laser pyrolysis : from nanoparticles to ceramic nanomaterials

Considerable effort is devoted to the synthesis of refractory carbide ceramics as promising materials for high-temperature applications, such as structural materials for the future generation of nuclear reactors. In this context, nanostructured ceramics are expected to exhibit interesting behavior under irradiation as compared with conventional materials. We report on the synthesis of SiC, TiC, and ZrC nanopowders by laser pyrolysis. The powders were characterized by X-ray diffraction, BET specific surface measurements, scanning electron microscopy, and transmission electron microscopy. SiC nanopowders were obtained from the decomposition of a mixture of silane and acetylene and were composed of grains varying in size from 7.2 to 43.0 nm. Liquid precursors were laser pyrolyzed with ethylene as a sensitizer in order to synthesize Ti-C-O or Zr-C-O powders. Annealing treatments in an argon atmosphere enabled the formation of 45-nm TiC grains or 38-nm ZrC grains through the carburization of the oxide phase by free carbon. Nanocrystalline ceramics were elaborated from SiC or TiC powders by very high pressure (8 GPa) sintering without any sintering additives. The densification reached 94% for SiC and 80% for TiC, without significant grain growth

Chapter 13. Microwave-Driven Hydrothermal Synthesis of Oxide Nanopowders for Applications in Optoelectronics

The microwave driven hydrothermal synthesis permits to precisely control the reaction times for this and thus also the grain size of the resulting nanocrystalline powders. An increase of pressure leads to powders with less hydroxide groups comparing to low temperature/pressure synthesis routes. Thus the combination of the two techniques permits to best exploit their advantages: high temperature of the process and fast heating and cooling, in high purity conditions. On the example of ZrO2 doped with Pr it is seen that the luminescence centers in nanocrystalline powders may have a different structure than for bulk materials. Pr3+ ions which would not be stable in a bulk material, are stable on the surface of the powder particles, and influence both their growth rate and luminescence properties. Interaction of the excited states with surfaces leads to very short luminescence decay rates, in the range of 10 nm, which in addition can be controlled by varying the grain size. This opens perspectives for new scintillating materials with short and controlled relaxation times. The nano-powders can be sintered using high pressure techniques and the grain size an be preserved in the nanometer range. The sintered YAG:Nd ceramics displays similar luminescence spectra as single crystals