Papers by Keyword: Phase Transition

Abstract: In Aluminum/Water Reaction Motor chamber, water injection angle plays an important role on mixture of aluminum particle fuel and water droplets and can affect motor performance further. On the basis of FLUENT software and taking phase transition and reaction between water droplets and aluminum particles into account, numerically simulated cases of different water injection angles by Particle Stochastic Trajectory Model. Computed total evaporation rate of water droplets, reaction rate of aluminum particles and specific impulse for the motor. Furthermore judged injection angle effect from these parameters. By comparison and analysis, it is found that hybrid injection case could get best multi-phase mixture effect and specific impulse performance for the motor. Namely axial injection angle with 45°in the main chamber and tangential injection angle with 60°in the afterburning chamber is the best case. The conclusion could provide a new idea for motor working process design.

Abstract: Barium titanate (BaTiO3) ceramics with grain size varied from 1000 to 8 nm were prepared by two step sintering method (TSS) and spark plasma sintering (SPS), respectively. Mixture structures of BaTiO3 ceramics were proved by in-situ temperature high resolution x-ray diffraction. Multiple ferroelectric domains present in nano-crystalline BaTiO3 ceramics were observed by transmission electron microscope. The evolution of phase transitions supported the existence of intrinsic mechanism. Dielectric loss of fine grain size BaTiO3 was higher than coarse grain size during Curie phase transition due to diffuse phase transition and grain boundary effects.

Abstract: We study the structural and electronic properties of scandium carbide ScC and niobium carbide NbC in both the sodium chloride rock salt (NaCl) and wurtzite structures by means of accurate first principles total energy calculations. The calculations were performed employing the full-potential linearized plane wave method (FP-LAPW). We used the generalized gradient approximation (GGA) of Perdew Burke and Ernzerhof for the exchange and correlation potential. Volume optimization and density of states including spin (DOS) of the systems are presented.

Abstract: The terminal part of the e₊ track (the positron blob) is formed during ionization slowing down and subsequent ion-electron recombinations produced by a positron. It releases up to 1 keV of energy, which is converted into heat within few picoseconds. If a bulk temperature of a medium is below, but close enough to its melting point, some region of a substance may melt, yielding a peculiar temperature dependence of the lifetime (LT) spectra. We have estimated properties of the molten region with a help of macroscopic heat con- duction equation and suggested a model describing temperature dependence of the ortho- positronium lifetime in frozen methanol, ethanol, butanol and water close to their melting points.

Abstract: The present study reports the synthesis of βTCP foam with fully interconnecting pores based on phase transformation of αTCP foam precursor by employing heat treatment. First, the αTCP foam precursor was fabricated by sintering the ceramics slurry-coated polyurethane foam template at 1,500°C. The resultant αTCP foam was again heated below α,β transition temperature for an extended period of times. After heating at 800°C for 150 hours, 900°C for 100 hours and 1,000°C for 300 hours, βTCP foam was obtained. The compressive strength of βTCP foam was approximately 46 kPa and the porosity was approximately 93%. The long heating period as well as heating temperature were the key to the transformation of βTCP phase. βTCP foam could be an ideal bone replacement since the invasion of bone cells into the pores provides optimum bone growth or repair.

Abstract: The Polytetrafluoroethylene (PTFE or Teflon) based propellants may be used in Pulsed Plasma Thruster, laser ablation thruster and other advanced propulsion systems. Because of the complex behaviors and phenomena of PTFE in ablation process, the study on thrusters’ operation process becomes complicated. Thermal and mechanical events are investigated, including phase transition, thermo-chemical and optical property variations, and multi-pulses laser ablation of PTFE. Considering more details including internal absorption of radiation, reflectivity of material, surface emission, a one-dimensional ablation model is developed and implemented numerically using a non-uniform grid, and implicit finite-volume method to gain greater insight into the process of laser ablation. The model is validated against analytical solutions and is in accordance with previous experimental results. The parameters of optical transmittance, reflectance and absorption coefficients are measured in experiments and are used in the numerical simulation. The laser ablation characteristics of PTFE are investigated, including the effects of wavelength and multipulses. It’s indicated that the laser ablation processes are influenced intensively by changing the laser wavelength and the effects of multiple pulses are also significant. The above numerical simulation provides insight into physical mechanisms of laser ablation, and suggests potential ways of improving thruster’s efficiency

Abstract: The brief review of the current state of the theory of first-order phase transitions is given. The basic processes of nucleation and evolution of nanostructure ensembles on crystal surfaces are considered. The general equations describing nanoparticle size distribution, evolution of their average radius and density are deduced. The influence of mechanical pressure on nucleation and property of quantum dots and nanopores is considered. The equations describing new phase nucleation under condition of mechanical pressure caused by distinction in density of an old and new phase are resulted. The kinetic theory of micropore nucleation in solids under loading is described. The kinetic criterion is received of nucleation of micropores and microcracks in fragile solids under the influence of stretching pressure.

Abstract: The first–order metal–sinsulator phase transition in VO₂ is characterized by an ultrafast several orders of magnitude change in optical transmittance and electrical conductivity, which makes VO₂ an attractive candidate for the potential application in thermochromic coatings and functional packaging materials. Herein, W– and Mo–doped VO₂ nanobelts were successfully synthesized by a facile hydrothermal method. The as–obtained products were characterized by X–ray powder diffraction (XRD), X–ray photoelecton spectroscopy (XPS), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT–IR). The results revealed that W or Mo atoms were successfully doped into the crystal lattice of VO₂ matrix. It was found that the phase transition temperature of doped VO₂ could be simply tuned by changing the doping concentration of W or Mo atoms. Furthermore, the variable–temperature infrared spectra disclosed that the doped VO₂(M) had novel optical switching properties, indicating that the as–obtained products can be used as the intelligent packaging materials.

Abstract: The structures and electronic properties of (La_xAl_1-x)₂O₃ are studied by first-principles calculation method. The results show that the composite material (La_xAl_1-x)₂O₃ tend to be in sixfold-coordinated structure when x0.7. (La_xAl_1-x)₂O₃ is in disorder structure and get the minimum band gap when x equals about 0.7. It suggest that (La_xAl_1-x)₂O₃ can be synthesized as high dielectric constant material by doping La2O3 with a lower Al dopant concentrations or by fabricating (La_xAl_1-x)₂O₃ with rich Al content.

Abstract: Based on the irreversible thermodynamics, a irreversible thermodynamic description of domain occurrences in ferroics such as ferroelectrics, ferromagnetics and ferroelastics was given. The ferroic domain structures occur at the ferroic phase transitions from the prototype phases to the ferroic phases. The processes of transition are stationary state processes so that the principle of minimum entropy production is satisfied. The domain occurrences are a consequence of this principle. The time-spatial symmetry related to the domains and their occurrences was also expounded.