Recrystallization Of Amorphous Phase Research Articles

AsTe3: A novel crystalline semiconductor with ultralow thermal conductivity obtained by congruent crystallization from parent glass

The synthesis of novel narrow-band-gap semiconductors with ultralow thermal conductivity opens a pathway to the design of functional materials with high thermoelectric performance or with interesting optical sensitivity in the infrared range. Here, we report on the discovery of the novel crystalline binary AsTe3 (c-AsTe3) prepared by spark plasma sintering (SPS) from full and congruent crystallization of amorphous AsTe3 (a-AsTe3) previously prepared by twin roller quenching. X-ray diffraction suggests that the structure of c-AsTe3 can be described as a superstructure of elementary Te with a specific distribution of As and Te atoms. More specifically, it appears as an intergrowth of a Te subunit (3 atoms) and As2Te5 subunit (6 As + 15 Te atoms) separated with interlayer spaces. The optical transmittance measured on both crystalline and amorphous AsTe3 indicates a maximum transmittance of 22 % over the infrared range 10–25 µm. Transport properties measurements, performed between 5 and 375 K, reveal that AsTe3 behaves as a lightly doped, p-type semiconductor. The complex crystal structure combined with a small-grain-size microstructure of the sample yields extremely low lattice thermal conductivity values of 0.35 W m−1 K−1 near 300 K. This poor ability to conduct heat is the main property that gives rise to an estimated dimensionless thermoelectric figure of merit ZT of ∼0.3 at 375 K. These findings show that the recrystallization of amorphous phases by SPS provides an effective approach for stabilizing novel phases with interesting functional properties.

High temperature oxidation behavior of Inc-738/NiCrAlY/LaMA thermal barrier coating system

The high temperature oxidation behavior of Inc-738/NiCrAlY/LaMgAl11O19 (LaMA) thermal barrier coating system was studied and compared with Inc-738/NiCrAlY/8YSZ coating system. The morphology of coatings, formation of TGO layer and degradation of coatings during oxidation process were investigated. The results indicated that the thermal cycling lifetime of LaMA coating was more than 8YSZ coating. The morphological evaluation of LaMA coating revealed that the recrystallization of amorphous phase was occurred. The 8YSZ ceramic top coat was sintered and microstructure was generated from columnar to equiaxed structure during thermal cycling process. Following the formation of a continuous alumina layer, undesirable oxides created at the TGO/bond coat (BC) interface in the LaMA coating system, whereas, unwanted oxides created at the TGO/top coat (TC) interface in the 8YSZ coating system and also it is believed that the oxidation of coating systems in steady-state stage was occurred under the diffusion-chemical mechanism and chemical mechanism, respectively. Therefore, the different failure mechanisms were observed in both coating systems. In the 8YSZ coating, the cracks nucleated at the TGO/TC interface, while in the LaMA coating, the cracks nucleation were occurred in TGO layer.

Recrystallization of Amorphous Phases via Post-Coating Heat-Treatment of Plasma-Sprayed Hydroxyapatite Coatings

Plasma-sprayed hydroxyapatite coating suffer from amorphous phases which dissolve in body fluid environment. An attempt has been made to reduce these amorphous phases by post-coating heat treatment of plasma-sprayed specimens. The specimens after post-coating heat treatment were analyzed using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The results showed that amorphous calcium phosphates converted into crystalline hydroxyapatite at 500°C, and calcium oxide was present even after heat treatment at 900°C. A very fine crystalline hydroxyapatite phase of around 100 nm in size was formed at 900°C.

The effect of plasma spraying conditions on the spheroidization of zircon and alumina mixtures

Reaction sintering of zircon and alumina is an easy and inexpensive route to obtain homogeneous mullite-zirconia ceramics with enhanced mechanical properties. This paper presents the preparation of ZrO 2 reinforced mullite by plasma spraying a mixtures of zircon and alumina. The results indicate that the plasma sprayed powders consist of zirconia, zircon and alumina. It was found that fine grained, even amorphous and chemically homogeneous composite powders can be obtained by ball milling and plasma spraying. Recrystallization of amorphous phases and the formation of mullite occurred at ∼1000°C in plasma sprayed powders. This value is more than 500°C lower than the formation of mullite in as-milled powders. The effect of power and secondary gas pressure on the spheroidization of zircon and alumina mixtures was investigated. The results showed that both of them were of importance in controlling the morphology and phase composition of the spheroidized powders.

Plasma Spraying of Al2O3 and ZrSiO4 to Form ZrO2- Mullite Composites

Zirconia is effective in improving fracture toughness of a number of ceramics when introduced as a reinforcement either in the form of participates, dispersed phase or whiskers because of its unique tetragonal-monoclinic (t → m) transformation. In this paper, the authors attempt to prepare ZrO2, reinforced mullite by plasma spraying mixtures of zircon and alumina. Pre-mixed powders of zircon and alumina are injected onto a D.C. plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The results indicate that the plasma sprayed powders consist of zirconia, zircon, and alumina. It was found that fine grained, even amorphous and chemically homogeneous composite powders could be obtained by ball milling and plasma spraying. Recrystallization of amorphous phases and formation of mullite occurred at about 1OOO°C in plasma sprayed powders. This value is more than 500°C lower than the formation of mullite in asmilled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders. Preheat treatment of the spheroidized powder promoted dissociation of zircon. Zirconia remained as tetragonal under 1000°C in the sprayed coatings.