Pentti Karjalainen

Phase transition revealed by Raman spectroscopy in screen‐printed lead zirconate titanate thick films

The transition from rhombohedral phase to tetragonal in screen‐printed lead zirconate titanate (PZT) thick films was studied using Raman‐scattering spectroscopy, and the results were compared with those of x‐ray‐diffractometry investigations. The unfired films were subjected to rapid firing in an air atmosphere at temperatures ranging from 960 to 1150 °C. During firing the composition of the films changed gradually as lead evaporated, which moved the composition of films to the ZrO2‐PZT region and resulted in precipitation of ZrO2. This caused the original rhombohedral structure to be converted to tetragonal. Correspondingly, the Raman spectra also changed with increasing firing temperature. The variation in Raman spectra was characterized by three frequency regions, denoted as the low‐, intermediate‐, and high‐frequency region, respectively, which are related to three cubic T1u modes. Moreover, it was also observed that the appearance of tetragonal modes was delayed from the formation of tetragonal struc...

Raman spectra in laser ablated lead zirconate titanate thin films near the morphotropic phase boundary

The Raman scattering measurements were carried out in laser ablated lead zirconate titanate (PZT) thin films, with a Ti/Zr ratio of 0.45/0.55 near the morphotropic phase boundary (MPB). The thin films were deposited using a Nd:YAG laser and annealed between 500 and 900 °C. In the specimens annealed at lower temperature, the Raman spectrum was found to be identical to that of the ceramic target, except for some extra peaks originating from certain minor phases. Increasing the annealing temperature resulted in the rhombohedral Raman line around 500 cm−1 being broadened and splitting of the peak was eventually observed at 750 °C. Simultaneously, a broad shoulder appeared near the zero frequency. These phenomena indicate the tetragonal Raman mode to be formed during annealing. A contradictory result was obtained from the x‐ray diffraction investigation, however, showing the formation of the tetragonal phase only above 800 °C. This discrepancy is thought to be due to a degree of ‘‘mismatch’’ between the struct...

Compositional and structural behaviour of screen-printed pzt thick films during rapid sintering

Abstract Abstract Lead zirconate titanate (PZT) thick films with a thickness of 10 μm were subjected to rapid open-air sintering and the compositional and structural characteristics were investigated. The results show that the evaporation of lead not only alters the sintering characteristic of thick films, but also causes the formation of separate phases from the bulk PZT. The activation energy for the evaporation process is about 2 eV which is higher than that for self-diffusion of lead. The rapid open-air sintering is in fact a competitive process in which both sintering and evaporation are present. Moreover, the titanium and zirconium exhibit different diffusion features during rapid sintering: titanium tends to diffuse into alumina substrates but zirconium is rejected by the substrates, which causes the composition of films to shift towards the tetragonal phase. The diffusion of aluminium into PZT was also observed, which caused the formation of an unknown phase above 1200 °C.

Passive Laser Assisted Bending of Ultra-High Strength Steels

Formability of ultra-high strength steels is poor causing problems in bending and stretch forming. The target of this work was to improve the formability of ultra-high strength steel sheets by controlled local laser heat treatments. Three steel grades, a bainitic-martensitic 4 mm DQ960 and two martensitic WR500 with 6 mm and 10 mm thicknesses were heated by controlled thermal cycles using a 4 kW Yb:Yag –laser, followed by self-cooling. Sheets with the thicknesses of 4 and 6 mm were treated on one side only by heating up to the austenitizing temperature. The 10 mm thick WR500 sheet was heat treated separately on the both surfaces by heating to a lower temperature range to produce a shallow tempered layers. The tensile and bendability tests as well as hardness measurements indicated that laser heat treatment can be used to highly improve the bendability locally without significant strength losses.

Evaluation of PZT thin films on Ag coated Si substrates

Fabrication characteristics of hybrid thin film components are investigated. Lead zirconate titanate (PZT) films, thickness 10 μm, are fabricated by using laser ablation on the Ag electrode (about 1 μm thick) which is deposited on 200 μm Si substrates by evaporation. Composition close to the target material is obtained in PZT films even in air and without substrate heating. Low surface energy in the Ag−Si system causes spheroidization of the Ag layer on the fresh Si substrate, but the surface can be modified by grinding and oxidization. Only some cavities exist at the interface. The interface between the Ag electrode and PZT layer is physically continuous, as revealed by electron microscopy. After annealing at 750°C for 2 h, the PZT layer consists of the rhombohedral perovskite phase with a fraction of the pyrochlore phase. Detrimental interdiffusion between Pb and Si occurs during annealing if the PZT thin film is directly on the Si substrate. This is retarded by the presence of the Ag layer.

Enhancing Mechanical Properties and Formability of AISI 301LN Stainless Steel Sheet by Local Laser Heat Treatment

This study demonstrates applying local laser heat treatment to produce ultrafine-grained austenite (UFGA) structures in an AISI 301LN type commercial austenitic steel. Pieces of 50% cold-rolled sheets containing more than 90% strain-induced martensite were heated locally by a laser beam to various peak temperatures to obtain different degrees of martensite reversion to austenite. Mechanical properties and formability of grain-refined and coarse-grained structures were measured by tensile and Erichsen cup tests. In addition to standard Erichsen cup test, additional interrupted tests were carried out, where cups were first stretched close to the critical strain. Drawn cups were then heated locally by a laser beam to revitalize the structure and thereby enhance the formability in the following cupping test until failure. Results showed that local laser heat treatment is suitable for the reversion treatment to refine the austenite grain size. Various structures were produced: completely reverted microstructures (T > 700 °C) with grain sizes 0.9 - 2 µm in addition to partially reverted structure (T < 700 °C) containing nano- and ultrafine-grained austenite (0.6 µm) with some martensite. The grain refinement by local annealing improved the strength properties. The Erichsen cup tests showed that the formability was equal in the completely reverted ultrafine-grained structures to that of the coarse-grained sheets. It was demonstrated that the local laser treatment restored formability of the drawn cups, allowing stretching to be continued. The second forming step after the laser-treatment provided an enhancement of 19 and 14% in the cup depths in coarse-grained and ultrafine-grained steels, respectively, even though the laser-treatment parameters were not optimized yet.

Studies To Prepare Superconducting Thick Films On Aluminium Oxide And Zirconium Oxide Substrates

Thick film pastes were manufactured from YBa2Cu3O7-8 superconducting powders prepared by the mixed oxide route. Thick films were printed onto A1203 and stabilized ZrO2 substrates. Experimentally determined one or two step profiles in the range of 750 to 1000°C in oxygen or air were used to fire the pastes. The two step profile gave better results as the formation of adverse phases was hindered and the adhesion to the substrate was better. X-ray studies revealed that the top layer of the films on A1203 contained ca 90% of YBa2Cu307_8 phase and a minor amount mainly of the Y2BaCuO5 phase. SEM micrographs showed liquid phase sintering to have occurred on A1203 substrates but not on the ZrO2 substrates. SEM micrographs of the cross-section revealed BaAl2O4-phase at the A1203/thick film interface and some CuO and Y2BaCuO5 phases outside of it in addition to the major superconductive YBa2Cu307_5 phase. On ZrO2, the top layer of the films also contained 80 to 90% of YBa2Cu307_δ phase and, when short firing was used, minor amounts of Y2BaCuO5 and Y2Cu2O5. On longer firing the minor phase was Y2BaCuO5. Resistance measurements of films on both substrate materials showed the transition onset above 85 K.