F. Jimenez

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.

Domain structure of latgs single crystals

Abstract Single crystals of TGS are grown with different proportions of L-alanine. The crystal habit is related with the sense of the polarization and with the polarization of the seed. A static study of the domain structure by the etching technique shows the differences between the surfaces (010) and (010).

Specific heat measurement of LATGS crystals under nonequilibrium conditions

Abstract The specific heat of two LATGS single crystals of different habits, grown from solutions of TGS containing different proportions of L-alanine, has been studied. The measurements were made between 303 K. and 328 K by decreasing and increasing the temperature at various constant rates ranging from 0 to 0.6 K/h, The specific heat depends on decreasing temperature rate. Relaxation processes with a time constant of 14 h have been also observed. This behaviour is qualitatively explained by assuming that viscoelastic effects are involved in the mechanical process which orients and stabilizes the direction of glycine I, as a consequence of the deformation of the unit cell when L-alanine is introduced.

Thermal hysteresis of pyroelectric signal of LATGS crystals

Abstract The pyroelectric coefficient and the dielectric susceptibility of a LATGS crystal are measured. The existence of thermal hysteresis is shown. From the data of the pyroelectric coefficient, the polarization and the dielectric susceptibility we obtain the behaviour of the internal bias field. This is found to be dependent on the temperature near the transition point.

Influence of the experimental conditions on the specific heat critical behaviour of TGS

Abstract Using a conduction calorimeter the specific heat critical behaviour of a TGS crystal has been measured. The ranges of the deviations from the classical behaviour below and above the transition point are 1 and 20 K respectively. These deviations seem to be well related to LK theory predictions. In a range of 0.1 K below the transition point thermal hysteresis appears which does not depend on the measuring temperature rate. The same behaviour is obtained with the sample at atmospheric pressure and under a high vacuum.

Influence of a d.c. electric field on the thermal hysteresis behaviour of specific heat and dielectric susceptibility of latgs crystals

Abstract The influence of a d.c. external field E cx on the thermal hysteresis behaviour of dielectric susceptibility and specific heat of a LATGS crystal is studied. The maximum value of thermal hysteresis is obtained when the reverse external field counteracts the internal bias field B and it disappears for high values of E ex applied in reverse and in the same direction of B. The equivalence of B and E ex does not occur when a reverse electric field smaller than B is applied to the crystal.

Thermal expansion coefficient of latgs single crystals

Abstract The deformation and the thermal expansion coefficient of LATGS crystal is measure. Non-annealed, annealed and polarized samples are studied. It is found that the effective bias field acting in the sample is higher when heating than when cooling. Non-annealed samples show an anomalous behaviour opposite to logarithmic deviations.

Pyroelectric coefficient of α-LiIO3 below room temperature

Abstract The pyroelectric coefficient of TGS single crystal is measured by means an experimental system which is described. Below room temperature the pvroelectric coefficients of two α-LiIO3 single crystal grown from different pH solution are measured. There is evidence of the influence of crystal growth on pvroelectric properties of α-LiIO3.

Influence of particle size on the crystalline structure and both the sintering and electrical properties of barium titanate

The constant rate thermal analysis method has been used to obtain barium titanate (BT) powder of very small grain size (diameter 0.07 mu m). Both a crystallographic and a dielectric constant study were carried out on samples sintered at different temperatures. It was found that the stabilization of the BT cubic phase depends on the coherently diffracting domain size, D, rather than on the particle size. At room temperature the cubic phase is stable for values of D lower than 250 AA.<<ETX>>

Critical behaviour of the thermal expansion coefficient of latgs single crystals

Abstract The thermal expansion coefficient (α) and the deformation of TGS doped with different proportions of L-alanine are measured during cooling and heating. In the paraelectric phase the data can be fitted by the formula α = M. | In t |-P, In the range 0.3<δT<2 K simple logarithmic behaviour is detectable during cooling in experiments where the samples were not short-circuited.