Uniaxial stress dependence of the elastic wave propagation near normal—incommensurate phase transition in (NH4)2ZnCl4

The effect of uniaxial σ₂ stress on the velocity and attenuation of longitudinal ultrasonic wave propagating along the [001] direction in an NH₄)₂ZnCl₄ crystal was studied in the vicinity of the orthorhombic — incommensurate phase transition. A stress induced shift of the transition temperature is dT₁/dσ₂= 0.2 K/MPa. Application of stress greater that 5 MPa results in the appearance of additional anomalies of velocity and attenuation, which are interpreted as manifestations of a new phase induced by uniaxial Stress.     

Faraday effect aid birefringence in orthorhombic Li2Ge7O15 near the ferroelectric phase transition

The tensor of the Faraday effect and its temperature derivatives were determined from the temperature dependence of Faraday rotation and indices of refraction of orthorhombic Li₂Ge₇O₁₅ between 120K and 340K, wave length 633nm. A strong anomaly of the Verdet constant V₃₃ and of the index n₃ was observed passing through the transition at 284K. The ratio of the slopes of the Verdet constant V₃₃ above and below the transition is about -2. This value corresponds to a second-order ferroelectric phase transition.     

Elastic anomaly of TIH2, PO4, single crystal near the ferroelectric phase transition

The ferroelectric phase transition of thallium dihydrogen phosphate has been studied by Brillouin scattering spectroscopy. A step-like anomaly has been detected near the transition temperature for longitudinal acoustic phonon propagating perpendicular to the polar axis. This behaviour is compared with other compounds of the KDP family.     

Temperature-dependence of propagation velocity of ultrasonic wave in coals

To study the characteristics of the change of rheological properties of coals on heating, the propagation velocity of an ultrasonic wave of 100 KHz was determined for 27 coals of different rank (C, 71.2–94.5%) in the temperature range 77–523 K. It was difficult to establish a clear relationship between absolute values of the velocity and rank of coals, but the temperature-dependence of the velocity appears to be a function of the carbon content of the coals, similar to the well-known correlation between hardness and rank of coals. The change of propagation velocity in 17 coals was determined continuously while the coals were heated from room temperature up to 673 K at 0.5 Kmin⁻¹. At temperatures below 523 K the velocity decreased almost linearly with increasing temperature; however it was observed that the velocity suddenly decreased at a certain temperature above 523 K, which was specific for each individual coal. This specific temperature was found for both caking coals and non-caking coals and was significantly lower than the initial softening temperature detectable by the conventional plastometer or dilatometer.     

Composite model of ferroelectric phase transition in the quasi-one-dimensional LHP crystal

Origins of the two dielectric contributions observed in dielectric spectra of a quasi-one-dimensional crystal LHP (lead monohydrogen phosphate PbHPO₄) in the phase transition region are studied numerically. We introduce a simple composite model in which randomly growing ferro- and para-phases are approximated by a set of electrical circuits with different parameters. The proposed model describes essential features of the relaxation response of two-relaxator dielectric systems.     

Dielectric lossbs near the phase transition point in DTGSel crystal

It is shown that at low frequencies near the ferroelectric first-order phase transition of DTGSel crystal new physical mechanisms responsible for dielectric losses take place.     

Well behaved temperature dependence of the ferroelectric coercive field near Tc in TGS

A new and simple method is introduced to make experimental observations of the coercive field of a ferroelectric which avoids the ambiguities associated with the amplitude dependence of this quantity. The temperature dependence of the coercive field of TGS in the vecinity of T_c has been measured and found to obey a law E_c = A(T_c - T)^s, with s = 1.45±0.10, in good agreement with the theoretical prediction s = 3/2.     

Spherical ferroelectric PbZr0.52Ti0.48O3 nanoparticles with high permittivity: Switchable dielectric phase transition with temperature

The spherical ferroelectric PbZr_0.52Ti_0.48O₃ (PZT 52/48) nanoparticles are prepared via simple and environment friendly high temperature solid state method. The crystal structure and morphology of these particles are characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscopy (FESEM). XRD analysis and selected area electron diffraction (SAED) pattern of PZT particles revealed its crystalline nature. The energy involved in the synthesis especially during the initiation and termination processes for the formation of PZT particles is found from the high temperature calorimetric study. These particles are spherical in nature with an average diameter of ≤20 nm. The bulk and surface chemical composition of these particles are investigated by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). XPS study reveals that the prepared PZT particles contain titanium ion in two different oxidation states namely Ti³⁺ and Ti⁴⁺. The PZT particles exhibit high permittivity with relatively low dielectric loss. From temperature dependent dielectric analysis, it is seen that there is a switchable dielectric phase transition at or above 80 °C.     

Comparative structural and optical studies on pellet and powder samples of BaTiO3 near phase transition temperature

In order to investigate the possible effect of inter-granular strain on the physical properties of Barium titanate (BaTiO₃), comparative studies have been carried out on the polycrystalline pellet and its corresponding powder samples. For this purpose, the polycrystalline pellet sample of BaTiO₃ has been prepared via conventional solid-state reaction route and powder is obtained by crushing the part of the prepared BaTiO₃ pellet. The comparative room temperature structural studies, temperature dependent optical and Raman spectroscopy measurements have been carried out on the prepared pellet and powder samples. Room temperature X-ray diffraction and Raman analysis confirms the presence of extra amount of strain in pellet sample compared with that of the powder sample. Temperature dependent Raman analysis also suggests the difference in the high temperature tetragonal to cubic transition temperature in both cases. Temperature dependent optical absorption properties measured in terms of Urbach energy (E_U), Urbach focus (E₀) and Urbach relaxation clearly indicates the significant change in these quantities for both types of samples on BaTiO₃. Present results strongly reveal the difference in structural, optical and vibrational properties of BaTiO₃ especially across phase transition in pellet and its corresponding powder which clearly shows the importance of inter-granular strain at the grain boundaries.     

Effects of air voids on ultrasonic wave propagation in early age cement pastes

The objective of this paper is to investigate effects of air voids on ultrasonic wave propagation in fresh cement pastes, and relate ultrasonic wave parameters to cement setting times. First, Biot's theory was used to analyze wave propagation in poroelastic media containing air bubbles. Then, in the experimental study, both the compressional (P) and shear (S) waves were monitored in cement pastes with different water/cement ratios (w/c = 0.4 and 0.5) and various air void content (0.1%–5.3% by cement paste volume). Experimental results indicated that existence of air bubbles in cement paste significantly decreases the P wave velocity, but has little effect on the shear wave propagation. Further analysis shows that the shear wave velocity corresponding to the Vicat initial setting times is a relatively constant value for the investigated air content range. This study shows the potential of using shear waves to monitor setting and hardening process of cement.     

n-Pentane isomerization in different phase regions near the critical temperature

The isomerization reaction of n-pentane over porous solid acid catalyst was investigated at 198.0 °C, which is slightly higher than the critical temperature of n-pentane. The pressure of the reactant in the reactor was controlled so that the reaction fluid was in gas, near critical, and supercritical (SC) states. The results showed that the conversion of n-pentane increased sharply with increasing pressure near the critical point. The amount of coke deposition depends strongly on the pressure of the reaction fluid. The main reason for the deactivation of the catalyst was coke deposition on the catalyst. Deactivation of catalyst was repressed at the higher pressure because more coke precursors produced in the reaction was dissolved into the fluid, which increases the stability of the catalyst. UV–vis, IR, thermogravimetric analysis and ¹H NMR studies indicated that there were at least two kinds of coke precursors on catalyst surface. The effect of pressure on the properties of the coke was not considerable, although it influences the quantity of the coke deposition on the catalyst significantly. The loss of sulfur can be neglected at reaction temperature and the state of sulfur remains unchanged after reaction. It is advantageous to conduct the isomerization reaction at SC condition.     

Is there a ferroelectric diffuse phase transition in YBa2Cu3Ox?

Experimental results on ultrasonic attenuation in ceramic samples YBa₂Cu₃O_x are reported. The attenuation peaks near 150°C may be due either to a diffuse phase transition or to defects.     

High temperature monoclinic-to-tetragonal phase transition in magnesium doped lanthanum ortho-niobate

Magnesium doped lanthanum ortho-niobate (La_0.98Mg_0.02NbO₄) was prepared by the molten salt synthesis method. X-ray diffraction and dilatometry methods were used to study high temperature behavior of the ceramic material. Special attention was paid to the phase transition between the monoclinic and tetragonal phases. The values of spontaneous strain on the basis of unit cell parameter, obtained by Rietveld refinement, have been calculated as well as Landau order parameter. The thermal expansion coefficient (TEC) of La_0.98Mg_0.02NbO₄ was determined to be 12×10^?6 1/K and 8×10^?6 1/K below and above 500 °C, respectively.     

Influence of temperature drop by phase transition on pervaporation processes in vapor phase feed

In this study, pilot pervaporation experiments of ethanol dehydration from the vapor phase feed have been carried out. The dehydration time decreased with increasing of the feed temperature and did not vary with the feed flow rate. The temperature dependence of permeation rate in vapor phase feed was larger than that in liquid phase feed. Contrary to the pilot pervaporation of liquid phase feed, the higher the feed flow rate, the larger the temperature drop is. The variation of temperature drop with permeate flux in vapor phase feed is larger than that in liquid phase owing to the heat loss of the membrane module itself.     

Viscoelastic dissipations in the fracture of glassy polymers near their glass transition temperature

The viscoelastic energy dissipations within the bulk of the test specimen are not taken into account by classical fracture mechanics. In this study it is shown that on approaching the glass transition temperature, the amount of energy dissipated in the bulk of the specimen relative to that required to create two new surfaces can be substantial, and it may be estimated from measurements carried out on specimens with different gauge lengths. Furthermore stress relaxations can take place at the crack tip, even in absence of yielding. These can be sufficiently pronounced to eliminate stress intensification and give rise to constant fracture stress conditions.     

Electrical performance and ferroelectric phase transition characteristic in different oriented 0.73PMN-0.27PT single crystals

The electrical and optical properties of (001)- and (110)-oriented 0.73 Pb(Mg_1/3Nb_2/3)O₃-0.27PbTiO₃ single crystals are systematically investigated at various temperatures, both of which present a series of ferroelectric phase transition processes. Dielectric performance measurements reveal that the ferroelectric phase transition occurs over a temperature range, rather than at one temperature point. By testing the ferroelectric hysteresis P–E curves as well as bipolar and unipolar electric field-induced strain S–E curves, the values of remnant polarization, coercive field, maximum strain, and converse piezoelectric constant d₃₃^* change considerably near the phase transition temperatures. Simultaneously, the 0.73PMN-0.27PT single crystals with (001)- and (110)-orientations under a low electric field show ultrahigh d₃₃^* values of 3540 and 2817 pm/V, respectively, which can be attributed to the electric field-induced monoclinic and orthorhombic phases, respectively. The series of ferroelectric phase transitions upon heating, that is, from rhombohedral ferroelectric to monoclinic/orthorhombic, followed by from monoclinic/orthorhombic to tetragonal, and finally from tetragonal to cubic paraelectric, are further investigated via polarized light microscopy and Raman spectroscopy.     

The instability of the dislocation ensemble near the first-order phase transition points in crystals

We analyse the influence of the nucleating of the new phase along dislocations on several elements of the dislocation ensemble (DE) of a crystal which undergoes the 1st-order phase transition (PT). We have shown that some of these elements lose their stability near PT points and we have estimated the temperature interval in which the instability may take place. According to this a microplasticity of a crystal may occure in the vicinity of the 1st-order PT and anomalies of mechanical properties are possible     

Smoldering wave propagation mechanism. II. Smoldering velocity and temperature in smoldering zone

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 1, pp. 20–26, January–February, 1993.     

On the conceration dependence of the phase transition temperature in solid solutons of a quantum paraelectric and a feroelectric component near the quantum limit

The lattice dynamics of solid solutions with a quantum paraelectric and a ferroelectric component is considered. Using the self-consistent phonon approximation and the coherent potential approximation the phonon spectrum of the disordered lattice is obtained and the dependence of the phase transition temperature T_c on the concentration × of the ferroelectric component is calculated near the quantum limit. The theoretical result T_c ∼(x-x_c)^1/2 is in excellent C agreement with the experimental findings.     

Antiferroelectric‐ferroelectric phase transition in lead‐free AgNbO3 ceramics for energy storage applications

The high‐energy storage density reported in lead‐free AgNbO₃ ceramics makes it a fascinating material for energy storage applications. The phase transition process of AgNbO₃ ceramics plays an important role in its properties and dominates the temperature and electric field dependent behavior. In this work, the phase transition behavior of AgNbO₃ ceramics was investigated by polarization hysteresis and dielectric tunability measurements. It is revealed that the ferrielectric (FIE) phase at room temperature possesses both ferroelectric (FE)‐like and antiferroelectric (AFE)‐like dielectric responses prior to the critical AFE‐FE transition point. A recoverable energy storage density of 2 J/cm³ was achieved at 150 kV/cm due to the AFE‐FE transition. Based on a modified Laudau phenomenological theory, the stabilities among the AFE, FE and FIE phases are discussed, laying a foundation for further optimization of the dielectric properties of AgNbO₃.