Papers by Keyword: Phase Transition

Abstract: Entropy analysis method was adopted to analyze and optimize a jacketed pipes-type heat pipe steam generator. In the entropy analysis process, the entropy production resulted from the total temperature difference across the heat exchanger, the entropy production from main heat transfer element – heat pipes and the entropy production caused by the pressure drop due to working media flow on cold and hot sides were taken into consideration. In the calculation, the entropy production in all parts of the heat exchanger was analyzed row by row in a discrete pattern, except for the entropy production due to pressure drop on flue gas side. Analysis results show that for the heat exchanger in this case, the proportions of entropy production in the above-mentioned three parts are respectively 74.79%, 21.75% and 3.46%. On this basis, the article also made optimizing analysis with the cold and hot source temperature, heat pipe length and No. of heat pipe rows as parameters, to provide theoretical basis for further improving the energy efficiency of heat exchangers.

Abstract: The action flux of ions of inert gas on the substratum Si (100) leads to porosity into the crystal lattice and self-organization of these defects. The kinetic stochastic model of the phase transition at the initial stage is applied to find distributions of defects in sizes and on their coordinates in the layers. The accumulation of stress is determined by computer simulation. Layers of pores and cracks precede to solid state epitaxy of silicon carbide.

Abstract: The anelastic behavior of the ferroelectric ceramics (Pb)(Zr/Ti)O₃ (PZT) and (Pb/La)(Zr/Ti)O₃ (PLZT), with Zr/Ti = 65/35, La = 5 at.% and 8 at.%, was investigated in the region of the ferroelectric phase transitions. Anelastic spectroscopy experiments were performed in an acoustic elastometer system, operating in a kilohertz bandwidth, at temperatures rising from 300 K to 770 K, at a heating rate of 1 K/min, under pressure of 10^-5mbar. Anelastic measurements on PZT showed only one anomaly, associated with the occurrence of a ferroelectric-paraelectric phase transition, while the PLZT data showed two anomalies, which were associated with the following transitions: the ferroelectric-paraelectric phase transition and a ferro-ferroelectric phase transition between distinct rhombohedral ferroelectric phases. The behavior of the relative variation of the elastic moduli with temperature, near the phase transitions, which describes the change in the type of coupling between strain and the order parameter in ferroelectric-paraelectric phase transition, with the increase of lanthanum amount and, linear coupling in the strain and order parameter type to PZT ceramic and linear coupling in the strain but quadratic in order parameter type for PLZT ceramics.

Abstract: This paper carries out the research on using the toughening mechanism of phase transition of ZrO₂ to inhibit the WC grain and improve the toughness of the hard alloys. WC–ZrO2–8wt%Co hard metals was mixed with 0-2 wt% nano-ZrO₂ and prepared by conventional sintering (CS) for 2 h at 1440oC to see whether the addition of ZrO₂ could improve densification behavior, the microstructure and mechanical properties of the samples. Experimental results showed that the use of ZrO₂ nanoparticles could decrease the relative density because of the worse wetting effects; it could also inhibit the growth of the grains of WC-Co hardmetal to enhance the hardness of the alloy. The fracture toughness of sample has an increasing tendency as a whole because of the phase transition during the sintering process.

Abstract: Molybdenum oxide (M_oO₃) is a well known transition metal oxide with multi phase properties that permit novel applications in sensors and batter systems. In this paper, we report a new approach to synthesize meta-stable structured molybdenum oxide (M_oO₃) by microwave assisted solution precipitation method using a domestic microwave oven. The reaction is carried out in a short period of 5 min. The XRD results confirms that the powder exhibits single phase hexagonal M_oO₃ with lattice parameter a= 10.53 Å and c = 14.907 Å. SEM images show, the particles are one dimensional (1-D) structure with hexagonal cross section and EDX spectrum confirm that the particles comprise of only molybdenum and oxygen. Further, the thermal and electrical properties are investigated by thermo gravimetric and impedance analysis and the results are discussed.

Abstract: In this paper the micro mechanism of BaTiO₃ ferroelectric phase transition is studied based on the thermodynamic model using electron cloud model, and further the Curie - Weiss law is explained. The results indicate that the contraction of the electron cloud, as the temperature decreased through Curie temperature, causes oxygen ions shift the equilibrium position, and the Coulomb attraction makes Ti ion shift the equilibrium position causing phase transition; With temperature rising, the ion displacement polarization decreases owing to the electron cloud expansion effect, and the variation of dielectric constant with temperature follows the Curie - Weiss law.

Abstract: Pb0.97La0.02(Zr0.95Ti0.05)O3 antiferroelectric thick films were prepared on platinized silicon substrates by sol–gel methods. Films showed polycrystalline perovskite structure with a strong (100) preferred orientation. The antiferroelectric nature of the films was confirmed by the double hysteresis behaviors versus applied field. The temperature dependence of dielectric constant and loss displayed the Curie temperature was 225oC.The current caused by the polarization and depolarization of polar was detected at coupling application of electric field and temperature. The phase transition characterization could be effectively adjusted by electric field and temperature.

Abstract: The modeling method for freezing tunnel with fractured surrounding rock is discussed. Frost weathering of rock in cold regions poses serious threat to the stability of geotechnical engineering. Fracture in the freezing rock plays an important role in the mechanical features of the rock mass. However, most of the previous study on freezing rock considered the rock as continuous media, in which the effects of fracture are not reflected sufficiently. This paper overcomes the above-mentioned insufficient and considers the fracture as an important factor while modeling. The model of frost fracture is built by AutoCAD and then transferred into ANSYS for meshing, and finally imported into FLAC3D for calculating by converting procedure. The method of equivalent coefficient of thermal expansion is used to simulate the expansion of water while freezing. The stress and thermal fields after some steps of calculation are ultimately simulated and the influence of fractures is reflected in the results.

Abstract: Nanocrystalline (n-) Au shows a large internal friction accompanied with the modulus defects above ~200 K. After the creep test, the strong <111> preferred texture changed to rather random one but the mean grain size was unchanged. In situ STM observation indicated that the crystallites can independently move during the creep deformation. Quasi-two phase state composed of solid crystallites and anelastic/viscoelastic grain boundaries (GBs) is proposed to explain these characteristic mechanical properties of n-Au. Further, GBs show the glass-transition-like change at around 200 K, anelastic/viscoelastic transition at ~30 MPa and dynamical state change above ~200 MPa. The high vacancy-type-defect concentration plays an important role on stabilization of the quasi-two phase state in n-Au.

Abstract: The paper presents recent results of ultrasonic investigation of Sn₂P₂S₆ family ferroelectric crystals and their solid solutions in the temperature range 100-300 K. It was shown that in Sn₂P₂(S,Se)₆ crystals the critical ultrasonic velocity slowing down for longitudinal waves propagating along main crystallographic directions is quite sharp and large. The relative change of longitudinal ultrasonic velocity along z-axis at the phase transition gradually increased from 10 % in pure Sn₂P₂S₆ till 25 % for sample with 0.4 content of Se. Such large velocity change causes the large ultrasonic attenuation anomaly. The increase of relaxation time: τ=τ₀/(T_C-T) leads to the increase of attenuation. Prefactor τ₀ was shown to be very small and the critical attenuation anomaly arises in the narrow temperature range close to phase transition. In the 0.4 Se sample the phase transition is of the first order because small thermal hysteresis exists. The ultrasonic velocity behaviour in the ferroelectric phase was described using Landau theory and free energy expansion including sixth order terms. For (Sn,Pb)₂P₂S₆ system the critical ultrasonic anomalies were smaller and the phase transition temperature substantially decreased (for 0.45 Pb sample the phase transition point was at T_c =140 K). The ultrasonic anomalies at phase transition in (Pb_xSn_1-x)₂P₂S₆ have large hysteresis showing that transition is of the first order, far from the critical point.