Setting Behavior and Phase Evolution on Heat Treatment of Metakaolin-Based Geopolymers Containing Calcium Hydroxide

The setting behavior of geopolymers is affected by the type of source materials, alkali activators, mix formulations, and curing conditions. Calcium hydroxide is known to be an effective additive to shorten the setting period of geopolymers. However, there is still room for improvement in the understanding of the effect of calcium hydroxide on the setting and phase evolution of geopolymers. In this study, the setting behavior and phase evolution of geopolymer containing calcium hydroxide were investigated by XRD analysis. The setting time of the geopolymer was inconsistently shortened as the amount of calcium hydroxide increased. A low calcium hydroxide dose of up to 2% of the total mix weight could contribute to the enhancement of compressive strength of geopolymers besides a fast-setting effect. The C-S-H gel is rapidly precipitated at the early stage of reaction in geopolymers containing high calcium hydroxide with some of the calcium hydroxide remaining intact. The ex-situ high-temperature XRD analysis and Rietveld refinement results revealed that geopolymer and C-S-H gel transformed into Si-rich nepheline and wollastonite, respectively. The wollastonite was also observed in heat-treated geopolymers with a low calcium hydroxide dose. It is believed that C-S-H gel can be precipitated along with geopolymers regardless of how much calcium hydroxide is added.

High Temperature Performance of Spark Plasma Sintered W0.5(TaTiVCr)0.5 Alloy

The phase stability, compressive strength, and tribology of tungsten alloy containing low activation elements, W0.5(TaTiVCr)0.5, at elevated temperature up to 1400 °C were investigated. The spark plasma sintered W0.5(TaTiVCr)0.5 alloy showed body centered cubic (BCC) structure, which was stable up to 1400 °C using in-situ high temperature XRD analysis and did not show formation of secondary phases. The W0.5(TaTiVCr)0.5 alloy showed exceptionally high compressive yield strength of 1136 ± 40 MPa, 830 ± 60 MPa and 425 ± 15 MPa at 1000 °C, 1200 °C and 1400 °C, respectively. The high temperature tribology at 400 °C showed an average coefficient of friction (COF) and low wear rate of 0.55 and 1.37 × 10−5 mm3/Nm, respectively. The superior compressive strength and wear resistance properties were attributed to the solid solution strengthening of the alloy. The low activation composition, high phase stability, superior high temperature strength, and good wear resistance at 400 °C of W0.5(TaTiVCr)0.5 suggest its potential utilization in extreme applications such as plasma facing materials, rocket nozzles and industrial tooling.

Self-Activated Green-Yellow Emitting Gd2CaZnO5 Phosphor for Efficient Ultra-Violet Light-Emitting Diodes

A new self-activated green-yellow emitting Gd2CaZnO5 (GCZO) phosphor was synthesized using solid-state reaction method at high-temperature. XRD analysis confirmed the orthorhombic structure of the sample with the Pbnm space group. SEM micrographs reveal the irregular morphology with micron sized particles. Detailed photoluminescence (PL) analysis revealed that the excitation of the phosphor lies in the UV range (˜377 nm) with the related broad green-yellow emission centered at 530 nm. The broad band emission ranging from ˜450 nm to 650 nm can be attributed to the surface defects and oxygen vacancies. The calculated luminescence decay lifetime for the optimized phosphor was found to be 2.925 μs. Furthermore, the color-coordinate (x, y) were calculated and found to be (0.44, 0.45), which lies in the green-yellow (˜540 nm) region of the electromagnetic spectrum. The values of color coordinates and Color correlated temperature of 3289 K support the synthesized phosphor for the emission of warm white-light. These results perfectly established the suitability of this green-yellow emitting GCZO phosphor for Ultra-Violet Light-Emitting Diodes (LEDs) excited white-LED applications.

Formation and Thermal Stability of ω-Ti(Fe) in α-Phase-Based Ti(Fe) Alloys

In this work, the formation and thermal stability of the ω-Ti(Fe) phase that were produced by the high-pressure torsion (HPT) were studied in two-phase α-Ti + TiFe alloys containing 2 wt.%, 4 wt.% and 10 wt.% iron. The two-phase microstructure was achieved by annealing the alloys at 470 °C for 4000 h and then quenching them in water. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilized to characterize the samples. The thermal stability of the ω-Ti(Fe) phase was investigated using differential scanning calorimetry (DSC) and in situ high-temperature XRD. In the HPT process, the high-pressure ω-Ti(Fe) phase mainly formed from α-Ti. It started to decompose by a cascade of exothermic reactions already at temperatures of 130 °C. The decomposition was finished above ~320 °C. Upon further heating, the phase transformation proceeded via the formation of a supersaturated α-Ti(Fe) phase. Finally, the equilibrium phase assemblage was established at high temperatures. The eutectoid temperature and the phase transition temperatures measured in deformed and heat-treated samples are compared for the samples with different iron concentrations and for samples with different phase compositions prior to the HPT process. Thermodynamic calculations were carried out to predict stable and metastable phase assemblages after heat-treatments at low (α-Ti + TiFe) and high temperatures (α-Ti + β-(Ti,Fe), β-(Ti,Fe)).