J. Del Cerro

The cubic-tetragonal phase transition in strontium titanate: excess specific heat measurements and evidence for a near-tricritical, mean field type transition mechanism

The transition between the cubic and tetragonal phase in shows an excess specific heat of . Comparison between the temperature evolution of the excess entropy and the structural order parameter Q shows within experimental errors . The apparent order parameter exponent was confirmed and analysed using a Landau-type expression for the excess Gibbs free energy with A = 0.70 J , B = 31.22 J , C = 42.17 J , K and K. The closeness to the tricritical point is seen by B < C; all thermodynamic data between 85 K and could be described selfconsistently using this approach although small deviations cannot be excluded in a temperature interval of less than 1 K around and a small tail of excess entropy at .

New measurement method of thermal properties by means of flux calorimetry

The equations for the heat that has passed through each plane from one steady state to another are obtained from the Laplace transform of the differential equation of heat conduction in a solid bounded by two parallel planes. From these equations for the heat flow, a method for the simultaneous determination of specific heat and thermal conductivity of a solid is deduced. This determination is carried out by measuring quantities of heat, and neither measurement of temperature gradient nor comparison with a sample of known thermophysical properties is needed.

Application of the constant rate thermal analysis technique to the microstructure control of BaTiO3 yielded from coprecipitated oxalate

Abstract The microstructure of BaTiO3 synthesized by the Constant Rate Thermal Analysis method has been studied. The mean particle size of this sample is smaller than the corresponding one of that obtained by conventional methods and it is possible to obtain consistent pellets without sintering. In this case, DSC and dielectric susceptibility studies have shown that ferroelectric transition at 399 K disappears while another anomaly appears at room temperature. When the pellet is heated at 1525 K the ferroelectric transition is recovered. Samples yielded by CRTA method seem to improve their dielectric properties with regards to samples obtained by a conventional method.

Flux calorimeter for measuring thermophysical properties of solids: study of TGS

A flux calorimeter for the determination of the thermal properties of solids, useful for the measurement of specific heat, thermal conductivity and pyroelectric coefficient, is described. The specific heat of aluminium oxide between 250 and 350 K is determined to show the precision of the measurements. The specific heat and the thermal conductivity of triglycine sulphate are measured near the ferroelectric transition point. From these data, the thermal diffusivity of the sample is calculated. The data agree with those found in the literature.

Experimental device for measuring the influence of a uniaxial stress on specific heat: Application to the strontium titanate ferroelastic crystal

A conduction calorimeter where the sample is pressed between two identical heat fluxmeters, each having 50 chromel–constantan thermocouples, is described. The fluxmeters were built to obtain linear heat conduction and with enough rigidity to apply a controlled uniaxial stress on the sample. The method of measurement, together with the fact that the sensors themselves apply the stress to the sample, allow us to obtain absolute values of the specific heat. The specific heat of strontium titanate ferroelastic crystal is measured without stress and under an applied uniaxial stress on the face (110). The measurements were carried out while cooling the sample at a constant rate of 0.06 K/h. The data obtained at intervals of 0.02 K are compared with those obtained with an unannealed and an annealed sample, respectively.

Determination of percolation thresholds in matrix-type controlled release systems: Application of a resistance analysis technique

Abstract In this work, matrix tablets have been prepared with binary mixtures of the inert polymer Eudragit ® RS 100 and a soluble, power-conductor model substance, sodium chloride. The sodium chloride content comprised between 20 and 80% w/w. A technique based on the measurement of the resistance of the matrix tablets has been used to achieve easy estimation of the values of the percolation thresholds, as a function of the sodium chloride loadings. The results obtained have been expressed in terms of resistivity and evaluated on the basis of percolation theory. The tablet resistivity has been considered as a critical property of the system. A change in this property and the presence of the percolation threshold were observed to appear simultaneously. From the data obtained, we have determined a first percolation threshold comprising between 30 and 40% (w/w) sodium chloride loading. The tablets made with a sodium chloride charge higher than 70% (w/w) undergo a disintegration process. The results obtained have been corroborated by scanning electron microscopy.

Unmanned aerial vehicle (UAV) modelling based on supervised neural networks

This paper proposes the utilization of hybrid models of supervised neural networks for the modelling of dynamic systems. Particularly, as an example of a system, an autonomous helicopter or UAV is identified in both attitude and position systems. The evaluation of the model is done by comparing the radial basis and multilayer perceptron with the real system

Correlation between synthesis conditions, coherently diffracting domain size and cubic phase stabilization in barium titanate

Abstract The Constant Rate Thermal Analysis method has been used under different experimental conditions to obtain barium titanate powder of very small grain size. Both a crystallographic and a dielectric study ham been carried out on samples sintered at different temperatures. It is deduced that the formation of small coherently diffracting domains is improved by the generation of oxygen vacancies and the cubic phase stabilization depends on the coherently diffracting domain size rather than on the particle size.

The structural phase transition in under uniaxial stress in the [110] direction: a calorimetric study

The excess specific heat of the structural phase transition of has been measured on single crystals under uniaxial stress along the crystallographic [110] direction and for extremely slow cooling rates. The corresponding excess entropy scales as the square of the thermodynamic order parameter. The order parameter exponent is for stress-free samples and increases systematically with increasing stress. At a stress of 5 bar the exponent is and the phase transition follows mean-field behaviour.

Simultaneous measurement of thermal properties in ferroelectric crystals

Simultaneous measurement of the influence of the electric field on specific heat, thermal conductivity and pyroelectric coefficient is described as a summary of previous papers. From these coefficients, which are determined by means of a flux calorimeter, the behaviour of other properties, such as thermal diffusivity and electrocaloric coefficient, is deduced.The data are compared with the findings obtained by other authors with traditional methods.ZusammenfassungIn Zusammenfassung vorhergehender Arbeiten wurde die simultane Messung des Einflusses von elektrischen Feldern auf spezifische Wärme, Wärmeleitfähigkeit und pyroelektrischen Koeffizienten beschrieben. Auf Grundlage dieser Konstanten, die mittels eines Fluxkalorimeters bestimmt wurden, konnte das Verhalten anderer Eigenschaften wie z. B. thermische Diffusionskonstante und elektrokalorischer Koeffizient abgeleitet werden. Die Daten wurden mit durch herkömmliche Methoden erhaltenen Ergebnissen anderer Autoren verglichen.РезюмеОписано одновременн ое измерения влияния электрического поля на теплоемкость, удельную теплопрово дность и пироэлектри ческий коэффициент ферроэл ектриков. На основе полученных величин, о пределенных с помощь ю калориметра в потоке, были установлены и такие параметры, как т ермопроводимость и электротепловой коэ ффициент. Данные были сопоставлены с данны ми других авторов, пол ученными обычными методами.