Tatiana Petrovsky

Optical properties of CeO2 films prepared from colloidal suspension

Optical absorption of nanocrystalline thin films can be influenced by the presence of both porosity and grain size effects. If both are present simultaneously, their effects are difficult to separate. In this study it is shown that the combination of uv-vis transmittance and reflectance measurements on porous CeO2 films provides enough data to make this separation. The CeO2 films were prepared by deposition of nanosized (∼5 nm) particles from a water colloidal suspension onto sapphire and subjecting these films to sintering temperatures sufficiently high to provide a series of films with a typical thickness of 0.6 μm with a wide range of grain sizes and porosity. X-ray diffraction, scanning electron microscopy, ellipsometry, and profilometry were used to characterize the films and to compare the observed grain sizes and porosity with that obtained from optical measurements. All of the techniques used gave results on porosity and grain size which were in good agreement, from 15% to 50% and 5 to 65 nm, resp...

Electrical Properties of YSZ Films Prepared by Net Shape Technology

The preparation of dense electrolyte films for most electrochemical devices is a crucial technological process. Net shape technology is a new approach, which uses a combination of colloidal suspensions and polymer precursor techniques, to obtain the dense electrolyte layers. It allows the overlapping of the thickness range from 1 to 10 mm in which other preparation techniques experience difficulties. Net shape processing is a low-temperature technology ~preparation temperature can be as low as 400°C! and it eliminates shrinkage of the film during the densification stage, so chemical reactions between the substrate and the film can be minimized. In this study two types of dense substrates were used to confirm these features of the net shape technology: single-crystal sapphire and platinum foil. It was shown that dense yttria-stabilized zirconia ~YSZ! layers can be obtained on both types of substrates at temperatures as low as 400°C. Moreover, further higher annealing temperature does not produce either shrinkage or cracking of the film. Electrical properties of YSZ films were measured in plane~on sapphire! and through the film~on platinum! using impedance spectroscopy and two-probe dc methods.

Low-Temperature Processing of Thin-Film Electrolyte for Electrochemical Devices

Thin-film electrolyte processing has become a key issue for the development of electrochemical devices such as solid oxide fuel cells, because reducing resistance of the electrolyte is critical to enhancing cell performance. In this study, yttrium-stabilized zirconia (YSZ) thin film, 1-2 μm thick, electrolyte has been prepared on dense and porous substrates using a combination of colloidal suspensions and polymer precursors at annealing temperatures <1000°C. Use of processing temperatures <1000°C minimizes interfacial reactions between electrolyte and anode/cathode materials. The conductivity of the thin film annealed at 900°C was measured and was the same as that of bulk YSZ and the processing temperature to obtain such conductivity was as low as 400°C.

Influence of the grain boundaries on conductivity of yttrium stabilized zirconia

The influence of the grain boundaries on the ionic conductivity of yttrium stabilized zirconia (YSZ) was investigated. The initially nanocrystalline samples were prepared using a tape casting process. The samples were annealed at different temperatures in the range from 1000 to 1400°C to overlap the grain size from 100nm to ∼10μm and investigated using impedance spectroscopy. Two distinct semicircles were found on all YSZ samples corresponding to the influence of the grain and grain boundary on the resistance. The activation energies for both resistances are very close (1.00 and 1.03eV correspondingly). The grain resistance does not change significantly during the annealing process, but the grain boundary resistance decreases after high temperature annealing which causes a decrease in the overall resistance of the material. The calculations show that the decrease in the grain boundary resistance is connected only with the increase in the grain size and the specific grain boundary resistance (per unit surface area of grain boundary) does not change with annealing.

Impedance spectroscopy and direct current measurements of YSZ films

In this study the electrical properties of thin films of Ysubstituted zirconia were investigated. The films were prepared using a polymer precursor technique and investigated in the temperature region 250 to 900°C. It was shown, that impedance spectroscopy (IS) and direct current (DC) conductivity measurements results are in good agreement for the films measured in plane for temperatures greater than 400°C. Due to the high resistance resulting from a planar geometry, the DC measurements were found preferable at temperatures Since in planar geometry the films represent a high resistance to the measurement circuit, it is important to minimize sources of electrical leakage, so different sample holders and substrates were investigated. A sapphire substrate and sample holder design using separated alumina single bore tubing for each electrode provided the lowest electrical leakage. The experimental results showed that electrical behavior of all of the films produced at low annealing temperatures (less than 400°C) was similar regardless of Y content. These films have relatively low conductivity and an activation energy of about 1.5eV. The influence of different Y content started to appear after annealing above 600°C. The results of the film conductivity measurements were compared with those for the bulk samples of Y stabilized zirconia prepared from 200nm powder by tape casting. These samples were measured by IS in plane and through the tape. It was shown that electrical properties of bulk and thin film material were similar.

Nanoporous Ceria Films Prepared from Colloidal Suspension

The influence of porosity and densification on optical properties of films on sapphire substrates that were prepared from water colloidal suspensions of small (∼5nm) particles of ceria was investigated. The colloidal ceria films have initially very porous structure (porosity about 50%) and densification starts at about 600°oC accompanied by grain growth. The concurrence of these two processes makes it difficultto interpret the results of the optical spectrophotometry, but the combination of transmittance and reflectance measurements provides enough data to separate these two influences and to calculate the porosity, particlesize and energy band gap separately. XRD, SEM, ellipsometry and mechanical profilometry were used to confirm the results obtained from the specrophotometric measurements. All these methods gave results, which are in good agreement: the change in the porosity from 50% to 15% and the particle size increased from 5 to 65nm in the temperature region from 400 to 1000°oC. An important result of the investigation is the fact that the main optical properties of the coating such as refractive index and band gap energy depend only on the porosity, but not on the grain size. The grain size influences the scattering properties of the coating, which allows the grain size to be estimated from optical measurements.