First-step Temperature Research Articles

Design of TiC intergranular-dispersed Al2O3 composites via in situ and two-step sintering with enhanced mechanical and electrical properties

To further enhance the mechanical properties of the Al2O3/TiC composites and reduce the percolation threshold induced by the controlling microstructures, composites containing 0–20 % TiC were fabricated in this work using the two-step sintering (TSS) method, in which TiC was in situ synthesized from carbon nanotubes and TiH2. From the acquired results, it was indicated that the TiC particles were completely dispersed at the grain boundaries. As the first-step temperature (T1) increased, the Al2O3 grains grew up, while the second-step temperature (T2) mainly affected the densification of the whole structure. The composites could be listed as follows, 1550/1400 > 1600/1400 > 1600/1300 > 1550/1300, which corresponds to a descending order of the balanced Young’s modulus, Vickers hardness, flexural strength, fracture toughness, and electrical conductivity. Moreover, the largest fracture toughness exhibited by 1600/1400-20 was 5.83 MPa m1/2, which increased by 69 % compared with monolithic Al2O3. The similarity of the Al2O3 grain size and the TiC particles size, as well as the complete grain-boundary dispersion of the relatively large TiC particles significantly contributed to the lower percolation value (10.5 %) of the Al2O3/TiC composites produced via the two-step sintering process compared with the composites fabricated by utilizing the conventional single-step sintering (percolation value: 11.2 %) method. The lower percolation value facilitates the fabrication of conductive Al2O3/TiC composites with excellent mechanical properties.

Effect of First-Step Temperature and Time on the Microstructure and Mechanical Properties of the Two-Step Cu-Alloyed ADI

Effect of First-Step Temperature and Time on the Microstructure and Mechanical Properties of the Two-Step Cu-Alloyed ADI

New insight into precipitation of Al3Zr and correlative effect on recrystallization behavior in a rapidly-solidified Al-Zn-Mg-Cu-Zr alloy

The influence of temperature of first-step pretreatment on the precipitation and crystal structure of Al3Zr in a spray-formed (SFed) Al-Zn-Mg-Cu alloy was systematically explored. With different first-step pretreatments, the microstructure evolutions during hot extrusion and solution treatment were studied. The results show that pretreatment before hot-working is necessary for rapidly-solidified Al-Zn-Mg-Cu alloy and temperature of first-step pretreatment is crucial for stabilizing microstructures. In detailed, the homogenous nucleation of L12 Al3Zr is limited at low first-step temperature (<250°C), while the transformation from L12 Al3Zr to D023 Al3Zr is simulated and high first-step temperature (≥420°C). The crystallographic orientation relationship between D023 Al3Zr precipitate (P) and Al matrix is recognized as: [001]Al//[011]P, (200)Al//(011¯)P, (020)Al//(200)P, which is different from the previous works. When the first-step temperature is 250–350°C, the dispersed η-Mg(Zn,Al,Cu)2 phases formed during spray forming can act as heterogeneous nucleation sites of Al3Zr precipitation. The recrystallization resistance of SFed Al-Zn-Mg-Cu alloy during solution treatment is markedly enhanced by an optimized pretreatment of 250°[email protected] h + 470°[email protected] h.

Formation behavior of oxygen precipitates in silicon wafers subjected to ultra-high-temperature rapid thermal process

In this study, we investigated the formation behavior of oxygen precipitates in Si wafers after a rapid thermal process (RTP) at 1350 °C in an oxygen atmosphere through experiments and simulations. The oxygen precipitate density varied based on the temperature (750–950 °C) and duration (0.5–16 h) of the first step of the two-step heat treatment after RTP. The highest oxygen precipitate density was achieved when the first-step temperature was set to 850 °C, and the second-step temperature was fixed at 1000 °C. The simulation consisted of the calculation of the depth profiles of the residual vacancy concentrations after RTP and oxygen precipitation during the two-step heat treatment after RTP. An empirical formula, in which the nucleation rate of the oxygen precipitates was assumed to be proportional to the fourth power of the residual vacancy concentration, was used in the temperature range of 600–1000 °C. In addition, we assumed that interstitial Si atoms, which were generated by the growth of the oxygen precipitates, consumed the residual vacancies through pair annihilation, thus stopping oxygen precipitation owing to the depletion of the residual vacancies. The simulation effectively reproduced the variations in the oxygen precipitate densities under different conditions in the first step of the heat treatment. Considering the results of the simulation, we concluded that the resulting oxygen precipitate density was determined by the nucleation rate, growth size, and timing of the terminal point of the nucleation.

Effect of two-step heating on transparency of MgAl2O4 ceramics fabricated by pulsed electric current sintering process

In this work, MgAl2O4 transparent ceramics was fabricated from combustion-synthesized nanopowders by pulsed electric current sintering (PECS) combined with two-step sintering. Ceramic powder was densitified using two-step heating, namely the first-step temperature was 1100 oC with holding time of 60 min and the second-step temperatures in range from 1300 oC to 1450 oC with 20 min duration. The results showed that the transparent ceramic samples sintered at 1100 oC/60 min – 1400 oC/20 min had grain size of 177 ± 8 nm, relative density about 99 %, transmittance up to 80 %, and Vickers hardness of 18 GPa. These values were higher than that of samples obtained by one-step sintering at 1400 oC.

Biocrude oil production and nutrient recovery from algae by two-step hydrothermal liquefaction using a semi-continuous reactor

We evaluated two-step hydrothermal liquefaction in a semi-continuous reactor for recovery of both nutri- ents and biocrude from the alga Coelastrum sp. in direct comparison with a one-step process. The influence of the operating temperature, pressure and water flow rate was investigated by means of a 2 k factorial experimental design and response surface methodology. The two-step process gave a higher total biocrude yield (~36 wt% (daf. basis)) and nutrient recovery level in terms of nitrogen containing compounds (~60 wt% of the protein content in the original algae as ammonium and nitrate ions and protein/polypeptides) than the single-step process. The highest biocrude yield was achieved at first-step temperature of 473 K, second-step temperature of 593 K, pressure of 200 bar and water flow rate of 0.5 mL/min.

Phase separation induced by a two-step temperature jump in polystyrene/poly(2-chlorostyrene) blends: dependence on duration and temperature of the first step

Phase separation induced by a two-step temperature jump was studied by time-resolved light scattering and scanning electron microscopy for polystyrene/poly(2-chlorostyrene) blends. A co-continuous domain structure was formed by the first-step jump, with small droplets being formed in these co-continuous domains by the second-step jump made in the intermediate stage, as well as in the late stage, of phase separation at the first-step temperature. When the duration of the first step was long enough, the growth of co-continuous domains formed after the second jump was well described by the same power function as that found for the corresponding single-step jump if the phase-separation time was shifted; this feature was confirmed for different first-step temperatures.

Growth kinetics of high Tc phase in Bi–Sr–Ca–Cu–O thick films

Thick films of Bi–Sr–Ca–Cu–O on (100) MgO substrates have been prepared by the screen printing technique with the starting composition of 1112. The annealing process involves a two-step heat treatment. The first-step temperature is chosen between 874 °C and 898 °C for 45 min followed by slow cooling (2 °C/min) to 864 °C. In the second step, the films are kept at 864 °C for a time varying from 0 to 104 h. The Tc (R = 0) and intensity of the characteristic peaks of the high Tc and the low Tc phases in the x-ray diffraction pattern are found to be dependent on the first-step temperature and the duration of the second step. The process of crystallization and the growth of plate-like grains as a function of annealing time at the second step are studied using Scanning Electron Microscopy (SEM). Kinetics of the growth of the high Tc phase has been explained in terms of a reaction mechanism involving the first and second annealing steps.