Seppo Leppävuori

Thermal oxidation of porous silicon: Study on structure

The structural changes of porous silicon (PS) samples during oxidation are investigated and analyzed using various microscopy techniques and x-ray diffraction. It is found that the surface roughness of oxidized PS layers increases with the oxidation at 200–400°C and decreased at 600–800°C. At 800°C a partially fused surface is observed. The oxide formed on the wall of porous silicon skeleton is amorphous. The shifts of Si(400) peaks are observed in the x-ray diffraction patterns, which are correlated to the lattice deformation induced by thermal expansion coefficient mismatch between the grown SiO2 and the residual Si, and to the intrinsic stress caused by the Si–O bonds at the Si–SiO2 interface. These explanations are supported by thermomechanical modeling using three-dimensional finite element method.

Tape casting of ferroelectric, dielectric, piezoelectric and ferromagnetic materials

Tape casting is a feasible method for preparing ceramic tapes with different electrical and magnetic properties for multilayer ceramic devices. This paper describes the tape casting process for several different electroceramic materials (BST, PZT, NZF and ZSB) utilising similar organic additive and solvent systems. The properties of tapes with different ceramic compositions before and after sintering are investigated, including surface roughness, shrinkage and microstructures. The parameters affecting the casting, shrinkage, lamination, thickness and tensile strength of green tape are also presented. This enables process design for tape which can be used in devices with true integration of dielectric and piezoelectric, ferroelectric and ferromagnetic layers in 3-dimensional multilayer structures.

Optical properties of porous silicon.: Part I: Fabrication and investigation of single layers

The wavelength dependent optical parameters such as refractive index n and absorption coefficient a of porous silicon (PS) have been calculated from optical transmission spectra of plan-parallel films having different porosities. Since the effects of manufacturing conditions on film porosity and thickness have been investigated as well, a complete set of data is obtained enabling the design and easy realization of PS layers with certain structural and optical properties in advanced applications for optoelectronics and sensors.

Optical properties of porous silicon. Part II: Fabrication and investigation of multilayer structures

Abstract By knowing the wavelength dependent optical parameters (refractive index n and absorption coefficient α ) and the obtained thickness of porous silicon (PS) layers when anodizing boron-doped Si wafers, one can design and realize periodic structures by alternating the current density during anodization. Such periodicity in the structure––and thus in optical properties within a film––results in a superlattice, which acts as a Bragg grating. By considering the proper Bragg conditions, the fabrication of optical filter/reflector components can be performed in a simple manner. In this paper, both theoretical and experimental aspects of the fabrication procedure are investigated.

BST powder with sol-gel process in tape casting and firing

Abstract Ferroelectric films such as Barium Strontium Titanate (BST) films are potential candidates for several microwave applications. In this work, the suitability of fine BST powder prepared by the sol-gel process for tape casting and firing were studied and compared with one commercial BST powder. The particle size of the sol-gel powder was 0.05–0.1 μm. The effects of different powders and slurry properties on the tape casting procedure, the thickness and tensile strength of the final tapes and their surface roughness were investigated. Furthermore, the sintering properties of different powders were measured using a dilatometer. The tapes made with sol-gel powder were sintered at 1250xa0°C and had large shrinkage.

Roller-type gravure offset printing of conductive inks for high-resolution printing on ceramic substrates

The use of gravure offset printing methods at an industrial level requires a wide and deep knowledge of the printing properties of the inks. Novel hydrocarbon inks were developed with superior printing properties compared to alternative ethyl cellulose-based inks. There are principal behaviour properties of these inks that can be generally described. In particular, the resistance dependence of printed mass for single and multiprints and on the other hand, the effect of multiprinting on printed area smoothness and line height. This article summarizes the results obtained and describes printing properties and defects: line widening, hair formation, ink flow from the gravure grooves, pinholes, image distortions, ribbing, scooping, streaking and gravure groove blocking. Three different printed samples were demonstrated, which are best suited for this manufacturing method: interdigital capacitor, inductor coil and laser soldering substrates.

Displacement, stiffness and load behaviour of laser-cut RAINBOW actuators

Abstract Commercial piezoceramic PZT 5A discs were reduced in air at 750xa0°C to produce RAINBOW actuators using graphite as the reduction substrate and ZrO2 powder for protection. After the reduction process, the RAINBOW actuators were cut to desired shapes with a Nd–YAG laser. The effect of the laser cutting on the load, displacement, stiffness and dielectric constant characteristics of the actuators was studied for different sizes. Load capabilities of the actuators reduced as the amount of active material decreased but almost 30% of the active material could be removed without significant change in load properties. Results showed the possibilities to shape and decrease the size of the actuators and perform application specific modifications in load, displacement and stiffness properties. The effect of the laser cutting on the dielectric constant was insignificant compared with the de-poling effect of high electric field driving. Manufactured actuators were modelled with the ATILA program. Modelling gave accurate results with the original RAINBOW actuator and with sliced actuators 15 and 10 mm in width.