National Institute Of Materials Physics - Romania

Magnetic materials in the CuZnTi ferrite system with the chemical composition Cu1-xZnxTiyFe2-yO4, with 0.5 less than or equal to x less than or equal to 0.62 and 0 less than or equal to y less than or equal to 0.05 were investigated as a function of sintering temperature and time and cooling speed in order to see the influence of these factors upon the Curie temperature and the permeability behaviour around the Curie point. The quality of the magnetic temperature transducers made from such materials is directly connected with the value of the slope of the mu(T) curve. The higher the slope the more sensitive will be the transducer. Slopes as high as 50%degrees C-1 were obtained by a proper cooling of the samples. this value bring the highest reported so far for such materials. The results are discussed in terms of the ionic distribution of the Cu, Zn and Ti ions over the tetrahedral and octahedral sites of the spinel lattice. The materials proved to be stable in time if the temperature does not exceed 200 degrees C, but higher temperatures may induce irreversible change in the material structure due to cation migration and electron transfer from copper ions to iron ions.

An anomalous high signal was observed across lead sulphide-ferroelectric ceramic heterostructures illuminated at room temperature with light having wavelengths between 0.8 and 3.1 mu m. The frequency characteristic of this signal is similar to a high-pass filter and depends on the value of the load resistance in series with the lead sulphide thin film, This signal is only two to three times smaller than the usual photoconductive signal generated by the lead sulphide thin film. Its nature is not pyroelectric because of its frequency dependence. The interface charge released from the PbS-ferroelectric interface during polarization reversal was integrated and a hysteresis loop was obtained. Electrons and hole concentrations were evaluated from this curve. The values obtained are: 4.4 x 10(16) cm(-3) for holes and 3.4 x 10(16) cm(-3) for electrons.

PbS/PbTiO3/Si heterostructures were prepared by successive depositions of a PbTiO3 thin film using the sol-gel method and of a PbS thin film using the coprecipitation method on a bulk silicon wafer. The current-voltage characteristic of the heterostructure is of a diode type. The heterostructure is sensitive over a broad range of wavelengths, from 0.35 to 3 mu m, when illuminated with continuous light or with modulated Light. Three sensitivity peaks are observed, at 2.7, 0.95, and 0.38 mu m. The measured signals are attributed to a photovoltaic effect which appears in the studied heterostructure.

Photoelectric properties of PbTiO3/Si heterostructures were investigated over a broad range of wavelengths. The spectral distribution of the a.c. open-circuit voltage was measured for wavelengths ranging from 0.35 mu m to 4 mu m. Two local peaks are observed at 0.38 mu m and 0.9 mu m which can be attributed to band-to-band excitations in PbTiO3 and Si respectively. The shape of the a.c. signal depends both on the light intensity and on the wavelength. When the heterojunction is reverse-biased, a d.c. photocurrent was observed which has a similar spectral distribution as the a.c. open-circuit voltage.

Thermal analysis of a pyroelectric bimorph structure, performed for a sinusoidally modulate heat flow, reveals the possibility to obtain a larger signal than in the case of a homogenous pyroelectric structure. The ratio between the signal generated from a bimorph structure and the signal generated from a homogenous structure, in the same conditions, depends on the material properties and on the modulation frequency. This ratio, computed for a particular bimorph, is up to four at 1000 Hz.

The growth and properties of PbS thin films, chemically deposited on bull; lead titano-zirconate ceramics, are investigated. Scanning electron microscopy photographs reveal that the medium size of crystallites is much larger for PbS films grown on poled ferroelectric substrates than for the films grown onto glass or onto unpoled ferroelectric substrates. In the first case the medium size of crystallites is about 1-1.5 mu m while in the second case it is less than 0.3 mu m. Spectral distribution of photoconductivity and thermally stimulated currents (TSC) measurements were performed on all samples. Experimental results indicate that the properties of the PbS films can vary, depending on the substrate nature and in the case of poled ferroelectric substrates, on the polarity. The photoconductive signal is usually larger, the dark resistivity is smaller, and the TSC current value is higher for PbS films deposited on the positive face of the ferroelectric substrate than for those deposited on the negative face. (C) 1995 American Institute of Physics.

Optical charging spectroscopy and thermally stimulated depolarization measurements were performed on metal-insulator-semiconductor-insulator-metal structures with a ZnS:Mn active layer. Two groups of trapping centres situated at every insulator-semiconductor interface were in evidence. Their depths were found to be centred at 0.55 eV and 0.875 eV respectively in the forbidden gap of a ZnS:Mn film. A partial thermal healing of these interface states was observed.

Localized levels play a major role in the electro-optic properties of Bi12SiO2O (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2-1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.