Reversible Transition Temperature Research Articles

A quantitative model for stabilization effect induced by ferroelectric aging

The stabilization effect, an increase of the reverse transition temperature TC with ferroelectric aging time, was recently found in acceptor doped ferroelectrics. Such an effect has found a microscopic explanation in the symmetry-conforming principle of point defects, which also accounts for a similar stabilization effect in ferroelastic/martensite materials. However, such a microscopic model is difficult to use to make a quantitative/semiquantitative evaluation of the aging process. In the present work, we propose a single-domain thermodynamic model that provides a semiquantitative fit to the experimentally observed stabilization kinetics. Furthermore, it predicts a new aspect of ferroelectric aging: the decrease of latent heat ΔH of the reverse transition with aging time. Such a prediction was confirmed via experimental differential scanning calorimetry results.

Low‐temperature Raman spectroscopic studies in NaNbO3

AbstractRaman spectroscopic measurements were carried out in the temperature range 10–300 K to understand the low‐temperature antiferroelectric (AFE)–ferroelectric (FE) phase transition in NaNbO3. Several modes in the low wavenumber range were found to disappear, while some new modes appeared across the transition. The temperature dependence of mode wavenumbers suggests that, during cooling, the AFE–FE phase transition begins to occur at 180 K, while the reverse transition starts at 260 K during heating. During cooling, the two phases were found to coexist in the temperature range of 220–160 K. Upon heating, the FE phase is retained up to 240 K and both FE and AFE phases coexist in the temperature range 240–300 K. In contrast to the earlier reports, the present results suggest a different coexistence region and the reverse transition temperature. The reported relaxor‐type FE behaviour over a broad temperature is consistent with the observed coexistence of phases during cooling and heating cycles. Copyright © 2010 John Wiley & Sons, Ltd.

Aging Effect on Lanthanum Doped Ferroelectric Lead Titanate Ceramics

Ferroelectric devices are widely applied in many fields, such as energy conversion and communication. The aging effect in ferroelectric materials plays a central role in the reliability of the related equipments. But it is very difficult to understand the origin of aging effect in ferroelectrics because these materials possess different defects and exhibit various aging behavior. The reverse transition temperature in lead titanate doped with lanthanum increases during aging at ferroelectric phase was reported. It is well known that lattice defects, such as vacancies and solute atoms, are ubiquitous in crystalline solids. These point defects affect physical properties in ferroelectrics significantly. The abnormal increase of the reverse transition temperature was discussed in terms of diffusion of point defects during aging. Dielectric performance in the material after aging was measured and discussed as well.

Smart materials and the influence of atom sizes on martensite microstructures in copper-based shape memory alloys

A series of alloy systems exhibit a peculiar property which involves the repeated recovery of macroscopic shape of material at different temperatures. The study of such materials, which are often called smart materials due to their capacity of responding to changes in the environment, is a field of research in rapid evolution. The origin of this phenomenon lies in the fact that the material changes its internal crystalline structure with changing temperature. Copper-based β-phase alloys are widely used as a shape memory component in devices. On cooling from high temperature, these alloys undergo a displacive transition which has a close packed-structure following two ordering transitions. This transition is called martensitic transition and responsible for the shape memory effect. The martensitic structures in β-phase alloys are closely related to the austenitic structures and inherit the order in the parent phase due to the displacive character of transition. In case these alloys are deformed in a temperature range in martensitic condition they change in shape and recover the undeformed original austenitic shape on heating over the reverse transition temperature after removing the strain. These materials regain the deformed shape on cooling to the martensitic state and cycle between deformed and undeformed shapes on cooling and heating. Therefore, this property is called reversible shape memory effect. The {1 1 0} β type plane of parent which is the basal plane for martensite is subjected to the hexagonal distortion with martensite formation on which atom sizes have important effect. In case the atoms occupying the lattice sites have the same size, the hexagon becomes regular hexagon otherwise the hexagon undergoes a distortion in case atom sizes are different. Due to this distortion, the spacing differences, Δ d, between particularly selected pairs of diffraction planes providing a special relation between miller indices become different zero and can be a measure of the ordering degree in martensite. The decrease in spacing difference leads to disordering in martensite.

Stabilization effect in ferroelectric materials during aging in ferroelectric state

We report that aging in ferroelectric state results in an increase of reverse transition temperature Tf in both BaTiO3 single crystal and Pb0.84La0.16Ti0.96O3 polycrystal. This indicates that the ferroelectric phase is gradually stabilized during aging below Curie temperature. The evolution of the stabilization with aging time in these two different systems obeys the same kinetic function, but with different relaxation time. This indicates that the stabilization effect of the two systems stems from a common origin. We suggest that the stabilization is due to a short-range ordering of point defects, driven by a symmetry-conforming tendency of point defects.

Control of oligomeric enzyme thermostability by protein engineering.

The ability to control the resistance of an enzyme to inactivation due to exposure to elevated temperatures is essential for the understanding of thermophilic behavior and for developing rational approaches to enzyme stabilization. By means of site-directed mutagenesis, point mutations have been engineered in the dimeric enzyme yeast triosephosphate isomerase that improve its thermostability. Cumulative replacement of asparagine residues at the subunit interface by residues resistant to heat-induced deterioration and approximating the geometry of asparagine (Asn-14----Thr-14 and Asn-78----Ile-78) nearly doubled the half-life of the enzyme at 100 degrees C, pH 6. Moreover, in an attempt to model the deleterious effects of deamidation, we show that replacement of interfacial Asn-78 by an aspartic acid residue increases the rate constant of irreversible thermal inactivation, drastically decreases the reversible transition temperature, and reduces the stability against dilution-induced dissociation.

Magnetic investigation of the effect of small additions of cobalt and manganese on the martensite reversal in an Fe-Ni alloy

Data on the temperature and composition dependence of the magnetic moment and Curie temperature of several Fe-Ni-Co and Fe-Ni-Mn alloys have been obtained. The temperature dependence of the magnetization was obtained for each alloy from 298 to 873 K, following the magnetization change through the transformation from martensite to austenite. The effect of cobalt and manganese additions to an Fe-29.9 at. pct Ni alloy on the reverse transition temperature,A s , the Curie temperature,T c , and the saturation magnetization at absolute zero, ρso, has been determined, Values forA s , T c , and ρso were obtained by fitting a Brillouin function to the respective contributions of austenite and martensite to the total magnetization. This technique represents a very sensitive method of obtaining transition temperatures and the respective amounts of each phase present in the alloys. A theoretical prediction of ρso andT c was in agreement with the experimentally determined values.