J.W.C. de Vries

Temperature and thickness dependence of the resistivity of thin polycrystalline aluminium, cobalt, nickel, palladium, silver and gold films

Abstract The resistivity ϱ f of thin aluminium, cobalt, nickel, palladium, silver and gold films is determined as a function of thickness and temperature. The temperature-dependent part of ϱ f turns out to be equal to the bulk value. This is indicative of a dominant grain boundary scattering in these films as follows from simple theoretical considerations. A simple linear relation between the grain boundary scattering function f (α) and the film thickness d is found which extends over a wide range of thicknesses. In the case of the non-transition metals aluminium, silver and gold a value for the scattering coefficient R could be derived.

High Power Characterization of Piezoelectric Materials

Three techniques for measuring high voltage/power piezoelectric properties, which have been developed recently, are compared: a voltage-constant piezoelectric resonance method, a current-constant piezoelectric resonance method, and a pulse drive method. The conventional resonance method with a constant voltage circuit exhibits significant distortion (or a hysteresis) in the resonance frequency spectrum under a high vibration level due to large elastic non-linearity, which limits precise determination of the electromechanical coupling parameters. To the contrary, the resonance method with a constant current circuit (i.e., constant velocity) can determine the coupling parameters more precisely from a perfectly-symmetrical resonance spectrum. The general problem in both resonance methods is heat generation in the sample during the measurement. In order to separate the temperature characteristic from the non-linearity, it is recommended that the pulse method be used in parallel, even though the accuracy is not very high.

Analysis of the critical current density in high-Tc superconducting films

Abstract Current models used to analyse the temperature dependence of the critical current density of high- T c superconductors are reviewed. The relation between the critical current density and the preparation conditions of thin films in the Y-Ba-Cu-O and the Bi(Pb)-Ca-Sr-Cu-O systems is studied. It is found that in thin films made by high-vacuum sputtering, laser ablation or thermal decomposition of metal-organic compounds the critical current can be described by models for granular superconductors. Films made by ultra-high vacuum evaporation are very homogeneous and have a much higher critical current density of 1.8 × 10 11 A/m 2 at 20 K. In this case flux motion determines the critical current density.

Oxygen content, microstructure, and superconductivity of YBa2Cu3O7− x

The influence of preparation conditions and microstructure on the superconductive properties of single-phase poly-crystalline YBa 2 Cu 3 O 7− x was investigated by electron probe microanalysis, transmission electron microscopy (TEM), and x-ray powder diffraction as a function of temperature in various ambients supplemented by resistivity and susceptibility measurements. Leaching of Ba was observed when samples were brought in contact with water. The TEM imaging revealed that individual grains have an extremely defect-rich outer shell and an inner core with a domain structure with a and b axes interchanged. The transition temperature T c was found to decrease with increasing quench temperature in the range 400–900°C. The T c was observed to be linearly proportional to the difference in the orthorhombic cell parameters ( b-a ). Further implications are discussed.

155Gd Mössbauer effect and magnetic properties of some ternary gadolinium intermetallic compounds

Abstract The 155 Gd Mossbauer effect was studied in several ternary intermetallic compounds of different crystal structures and compositions (Fe 2 P, TiNiSi, MgAgAs, Cu 2 MnAl and ThCr 2 Si 2 structure types). The following hyperfine parameters were determined (4.2 K): the effective hyperfine field, the isomer shift, the quadrupole splitting, and the line width. The isomer shift was analysed in terms of charge-transfer effects and s,d electron redistribution according to the model of Miedema and van der Woude. Included in this investigation was a determination of the magnetic properties and the lattice constants.

Preparation, patterning, and properties of thin YBa2Cu3O7−δ films

High TC superconducting thin films were prepared on (100) SrTiO3 substrates by dc triode sputtering and subsequent annealing. In these films Hall‐bar structures having a width down to 5 μm were patterned using a reactive ion etching technique. Superconductivity above 77 K was observed. When compared with the original film there is only a small reduction in TC. The critical current density determined by electrical measurements is substantially reduced. On the other hand, the critical current density in the bulk of the grains as measured by the torque on a film is not reduced by the patterning process. It is suggested that superconductor‐normal metal‐superconductor junctions between the grains account for this difference.

Superconductivity above 100 K in Bi(Pb)-Ca-Sr-Cu-O films made by thermal decomposition of metal carboxylates

Abstract Superconducting layers in the Bi(Pb)-Ca-Sr-Cu-O system are prepared by thermal decomposition of metal carboxylates. The films are deposited on MgO single crystal and ceramic substrates using a spin-coating and dip-coating process. The Bi-Ca-Sr-Cu-O films consist mainly of the low- T c phase ( c -axis=3.073 nm), whereas partial substitution of Bi by Pb favours the formation of the high- T c phase ( c -axis=3.707 nm). Films deposited on MgO (100) are strong c -axis preferentially oriented grown. While the Bi-Ca-Sr-Cu-O films show a step in the resistance versus temperature curve ( T cf ⋍80 K) due to the presence of the low- T c and the high- T c phase, the Bi(Pb)-Ca-Sr-Cu-O films have an onset at 110 K and are superconducting at 104 K. The temperature dependence of the critical current indicates that in the Bi-Ca-Sr-Cu-O system weak links of superconductor-isolator-superconductor type are present, while in the Bi(Pb)-Ca-Sr-Cu-O samples the contact is formed by normal-metal barriers. Using magnetic fields up to 5 T, the anisotropy of the resistive transition of the high- T c phase was studied. In Bi(Pb)-Ca-Sr-Cu-O films the anisotropy ratio is about 18, and the corresponding coherence lengths are ξ ab (0)⋍3.6 nm and ξ c (0)⋍0.2 nm. These values are nearly the same as in the low- T c phase.

A Component Level Test Method for Evaluating the Resistance of Pb-free BGA Solder Joints to Brittle Fracture under Shock Impact

A high-speed shear tester has been used to evaluate the interface strength of Pb-free solder balls with varying Ag and Cu content to a BGA laminate interposer with different finishes. The shear rate used is 0.45 m/s. Components with SAC405 and SnPb solder balls have been used as a reference. Results are compared with SAC305, SAC305 with NiGe addition, Castin 258 (2.5Ag-0.8Cu-0.5Sb), SAC105 (1.0Ag-0.5Ag), SAC101 (1.0Ag-0.1Cu-0.02Ni-xIn), SACX (0.3Ag-0.7Cu-0.1Bi), LF35 (1.2Ag-0.5Cu) and Sn3.5Ag. Electroplated NiAu finishes on the BGA laminate interposer studied come from different suppliers. Three failure modes are observed in the high speed shear test: brittle fracture in the intermetallic layer at the ball/interposer interface, ductile fracture in the solder bump and peel off of the solder pad from the interposer. Analysis of the fracture interface shows that almost all Pb-free solder alloys fail either in the intermetallic layer or by pad peel off. Only SAC101 and Sn3.5Ag show ductile fracture in the bump or pad peel off, but no fracture in the intermetallic layer. This is comparable with results for eutectic SnPb under these test conditions. A JEDEC board level drop test on same packaging with SAC101, SAC305 and SnPbAg shows that the number of drops to failure for packages with SAC 101 is also higher than those with SAC 405 and even slightly higher than with eutectic SnPb solder balls. Failure analysis is done to understand the difference in shock resistance with different solder bumps. From the observations in both of the tests, it is evidenced that the high speed shear test, when combined with proper analysis of the failure mode, is a simple but very powerful tool to evaluate the resistance of Pb-free BGA solder joints to brittle fracture under shock impact.

Resistivity of thin Au films as a function of grain diameter and temperature

In thin polycrystalline gold films of four different thicknesses the average grain diameter is varied by different heat treatments. The resistivity as a function of temperature and grain diameter can be analysed satisfactorily in terms of grain-boundary scattering alone. The result is identical with experiments in which the resistivity is measured as a function of the film thickness. Therefore grain-boundary scattering must be the dominant contribution to the excess thin-film resistivity in polycrystalline thin films. Only a rough estimate of the small amount of surface scattering can be given.

Superconductivity in Bi-Sr-Ca-Cu-O bulk samples made by thermal decomposition of metal oxalates

Abstract The preparation and characteristic properties of superconducting powders and ceramics in the system Bi-Sr-Ca-Cu-O were investigated. Powders of 3 different nominal compositions were prepared along the wet-chemical oxalate route. Bulk samples with high amounts of Bi 2 Sr 2 Ca 2 Cu 3 O 10-δ (high- T c phase) and zero resistance up to 105 K could be prepared between 870°C and 880°C. The influence of sintering conditions on the fabrication of superconducting bulk samples and on the formation of the high- T c compound Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ was evaluated.