Toivo Lepistö

Substrate-dependent crystallization and enhancement of visible photoluminescence in thermal annealing of Si/SiO2 superlattices

We study annealing of Si/SiO2 superlattices on fused quartz and crystalline Si substrates. Under annealing at 1200 °C, the superlattices on Si undergo partial crystallization involving clusterization of Si layers through ultrathin (1 nm) oxide, and visible photoluminescence (∼2.1 eV) strongly increases for the samples with thinner Si layers (⩽2.5 nm). The annealed superlattices on quartz exhibit a higher disorder, tensile stress, and weaker visible photoluminescence. The results do not support assignment of the observed visible photoluminescence to quantum confinement in Si crystallites but rather indicate that it originates from Si=O bonds stabilized in the Si/SiO2 network.

Aluminum phosphate sealed alumina coating: characterization of microstructure

Abstract The microstructure of aluminum phosphate sealed plasma-sprayed alumina coating was characterized by X-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. Microstructural characterization was carried out to identify the phases of the coating and to understand better the strengthening effect of aluminum phosphate sealant in the coating. The main phases in the coating are metastable γ-Al 2 O 3 and stable α-Al 2 O 3 . The overall structure of the coating is lamellar with columnar γ-Al 2 O 3 grains. The aluminum phosphate sealant shows good penetration into the coating to the depth of about 300 μm filling the structural defects such as pores, cracks and gaps between the lamellae. The sealant in the coating has the relative composition of 26 at.% aluminum and 74 at.% phosphorus giving the molar ratio P:Al of 3, which refers to the metaphosphates Al(PO 3 ) 3 . There is also some crystalline aluminum phosphate in the coating, in the form of berlinite-type orthophosphate AlPO 4 , owing to the reaction between the sealant and the alumina coating. Thus, the phosphate bonding in the alumina coating is based both on chemical bonding resulting from the chemical reaction with the alumina coating and on adhesive binding resulting from the formation of the condensed phosphates in the structural defects of the coating.

The effect of solder paste composition on the reliability of SnAgCu joints

Abstract As the electronics industry is moving towards lead-free manufacturing processes, more effort has been put into the reliability study of lead-free solder materials. Various tin–silver–copper-based solders have become widely accepted alternatives for tin–lead solders. In this study, we have tested three different SnAgCu solder compositions. The first consisted of a hypoeutectic 96.5Sn/3.0Ag/0.5Cu solder, the second of a eutectic 95.5Sn/3.8Ag/0.7Cu solder, and the third of a hypereutectic 95.5Sn/4.0Ag/0.5Cu solder. A eutectic SnPb solder was used as a reference. The test boards were temperature-cycled (−40 to +125 °C) until all samples failed. The results of the temperature cycling test were analyzed, and cross-section samples were made of the failed joints. Scanning electron and optical microscopy were employed to analyze the fracture behavior and microstructures of the solder joints. The reliability of lead-free solders and the effect of microstructures on joint reliability are discussed.

Effect of prestraining on Barkhausen noise vs. stress relation

Mild steel specimens were uniaxially strained in tension at strain levels varying from 0.5 to 10% followed by X-ray diffraction residual stress and Barkhausen noise (BN) measurements. After this tension and compression loading was applied to the specimens. BN measurements were carried out during the whole reloading sequence. These measurements showed that prestrain increased the tensile stress value at which the BN saturated. The linear range of the stress vs. BN curve shifted towards the tensile stress range as a function of prestrain. In the linear range the stress sensitivity remained unchanged despite the prestrain. The obtained stress vs. BN relation was used in the residual stress measurements of a welded tube segment. The residual stress values measured by the BN were compared to X-ray diffraction residual stress values. When the BN values were within the linear range of the stress vs. BN curve, the BN residual stress values agreed quite well with the X-ray diffraction residual stress values.

Generation of silver/palladium nanoparticles by liquid flame spray

Ag-Pd alloy nanoparticles have been generated from silver and palladium nitrate precursors using a high temperature aerosol method, the liquid flame spray (LFS) process. In the LFS process, a spray aerosol of precursor liquid is introduced into a high-temperature H 2 -O 2 flame. The primary micron-sized spray droplets evaporate in the flame, and the final particulate product is a result of the nucleation of the pure metal vapors shortly after the flame. In the study, three Ag-Pd molar ratios-10:90, 50:50, and 90:10-were used in the precursor. As a result of the synthesis, metal alloy nanoparticles with practically the same concentration ratios, correspondingly, were produced with the method. In the experiments, metal mass flow rates of 0.01-0.8 g/min were covered. The size of the particles was determined to be in the range of 10-50 nm by aerosol instrumentation. The particles were spherical and slightly agglomerated. It was concluded that the particle size can be controlled via the total precursor mass flow rate, and the composition can be controlled by the molar ratio of Ag and Pd compounds in the precursor liquid.

Relation between residual stress and Barkhausen noise in a duplex steel

Abstract The relation between residual stress and Barkhausen noise (BN) was evaluated in a duplex stainless steel. The residual stresses had been created to the test piece by local heating. Rms value, position of the maximum of the BN burst, its full width half maximum, power spectrum and pulse height distribution were characterised. All the parameters depended to some extent on the residual stresses and none of them was totally independent on the other parameters. The best parameter for the residual stress evaluation was the rms value of the BN amplitude. The rms value varied according to the total stress (macrostress+homogeneous microstress) in the ferrite phase. The stress component perpendicular to the direction of the magnetization had also some influence. Based on the results it is possible to evaluate the residual stresses of the ferrite phase of duplex stainless steels by the BN method.

Thermal analysis of plasma sprayed oxide coatings sealed with aluminium phosphate

Abstract Thermal analysis by thermogravimetry and differential scanning calorimetry was carried out for plasma-sprayed alumina and chromia coatings to study their stability after plasma spraying and for aluminum phosphate sealant to study phosphate reactions during the sealing heat treatment. Thermogravimetric analysis for alumina coating did not show any change in the coating due to the heat treatment, though the phase structure had changed from metastable γ-Al 2 O 3 to stable α-Al 2 O 3 . In the chromia coating thermogravimetric analysis showed 1.7 wt.% weight increase due to the oxidation of the sprayed coating. During plasma-spray process the chromia coating had gained some under-stoichiometry or some of the chromia had decomposed into metallic chromium or other oxides. Thermal analysis for aluminum phosphate sealant showed weight loss of about 27 wt.%. This corresponds well to the formation of metaphosphates via dehydration of aluminum phosphate solution during the sealing heat treatment. Thermal analysis for the mixture of sealant and alumina coating showed slightly different behaviour than plain sealant. The sealant reacted with the alumina coating forming a crystalline phase, berlinite-type orthophosphate AlPO 4 . Thermal analysis for the mixture of sealant and chromia coating showed nearly similar behaviour than plain sealant and no indications of the chemical reactions were detected.

Metal–Plastic Adhesion in Injection-Molded Hybrids

Polymer–metal hybrids are replacing steel structures in many applications. Combining metals and plastics is, however, complicated because they have very different physical and chemical characteristics. This study characterizes plastic–metal adhesion in insert-injection-molded hybrids. Diaminofunctional silane was used as a coupling agent between thermoplastic urethane and stainless steel. Before silane treatment, various surface treatments, including electrolytic polishing and different oxidation treatments, were applied to the steel inserts to understand better the bonding between silane and steel. The effects of the surface treatments and silane application on plastic–metal adhesion were studied by means of contact angle measurements, adhesion tests, and microscopic characterizations. Electrolytic polishing and oxidation of the steel inserts significantly improved the silane bonding to the steel insert, and consequently the plastic adhesion to steel.

Precipitation at faceting grain boundaries of Cu–Ni–Sn alloys

Abstract The effect of recrystallisation heat treatment on the microstructure of copper–nickel–tin alloys Cu–9Ni–6Sn and Cu–6Ni–7Sn was studied. Cold deformed wire material was recrystallised by a short term anneal at 850°C, close to the solidus of the alloy. Non-cold deformed material was heat treated for up to 1 h at the same temperature range ( T =800–900°C). Slow cooling of the alloys resulted in faceted grain boundaries. Planar precipitates of (Cu,Ni) 3 Sn were detected at the facets. The present observations are discussed based on two possible explanations: the non-equilibrium segregation of alloying elements during cooling and the grain boundary shape modifications due to the precipitate growth.

The influence of multiple reflow cycles on solder joint voids for lead‐free PBGAs

Lead‐free soldering is becoming a common practice in the electronics industry because of the growing general opposition to lead‐containing solders. The reliability of lead‐free solders has been studied a lot recently, but knowledge of it is still incomplete and many issues related to them are under heavy debate. This paper presents results from a study of the formation of voids with regard to the number of reflow cycles in three different kinds of solder joints: first the ones prepared with lead‐free solder paste and lead‐free plastic ball grid array (PBGA) components, next the ones prepared with lead‐free solder paste and tin‐lead‐silver PBGA components, and last the ones prepared with tin‐lead solder paste and tin‐lead‐silver PBGA components.