A. Rudajevová

Thermal diffusivity and thermal conductivity of MgSc alloys

In this work the temperature and concentration dependence of the thermal diffusivity of MgSc alloys is reported. Measurements were carried out from 20° up to 300°C and from 3.2 up to 19.0 wt.% Sc using the flash method. The thermal conductivity was calculated as the product of density, specific heat and thermal diffusivity. The thermal conductivity of MgSc alloys increases with increasing temperature and decreases with increasing concentration of Sc. The concentration dependence of all thermophysical properties (thermal diffusivity, thermal conductivity, density and specific heat) is also presented. The results are discussed together with the electrical resistivity data from the literature. The electronic and phonon thermal conductivity is calculated using the Wiedemann-Franz law.

Thermal properties of Mg–Li and Mg–Li–Al alloys

Abstract The present work is a study of the thermal properties of Mg–xLi–y Al with x= 4, 8 and 12 wt-% and y= 0, 3 and 5 wt-% as a function of temperature in the range 20–375°C. The thermal diffusivity and coefficient of thermal expansion (CTE) have been measured and the thermal conductivity calculated. The thermal diffusivity of all alloys decreases with an increasing content of lithium. The CTE of the single phase alloys Mg–4Li and Mg–12Li has a linear character, and the CTE of Mg–12Li is higher than that of Mg–4Li. The influence of thermal stresses in the two phase alloy Mg–8Li is perceptible in terms of temperature dependence of the CTE. In Mg–4Li–3Al and Mg–4Li–5Al, an influence of the solution of AlLi phase on all the studied thermal properties has been found.

The influence of compression pressure on transport properties of polyaniline

The transport of charge carriers and the thermal diffusivity were studied on a set of polyaniline (emeraldine) samples that were compressed at various pressures, 50–1000 MPa. Marked correlations between the plots of compression pressure dependence of the electrical resistivity, the thermal diffusivity and the sample density were observed. This study has demonstrated that in order to obtain comparable data on samples prepared by compression of polymeric powders it is necessary to use sufficiently high pressure. In the case of polyaniline powder the pressure should be higher than 400 MPa. The values of the thermal diffusivity measured on polyaniline salt and its respective base indicate that charge carriers are not dominant in a heat-transport mechanism; only less than ten per cent of the thermal diffusion in pressed polyaniline samples can be eventually attributed to charge carriers.

Thermal strain in Mg composites

In this paper, the thermal expansion behaviour of Mg-9 vol.% Saffil fibres, ZC63-25 vol.% Saffil fibres and Mg8Li-12 vol. % Saffil fibres has been studied from room temperature to 380 °C. The thermal expansion of pure Mg, ZC63 alloy and Mg8Li alloy was measured in order to calculate the thermal expansion characteristics of the composites free of thermal strain using the rule of mixtures. Using the measured relative elongation and that obtained from the rule of mixtures, the temperature dependences of the dilatational strains could be estimated. Whereas the permanent changes of the specimen length were found after the first thermal cycle for the as-cast composites, only the reversible changes of the dilatational strain were observed after the second and subsequent cycles.

Changes in the microstructure of QE22 composites estimated by non-destructive methods

Using non-destructive methods changes in the microstructure of QE22 metal matrix composites (MMC) due to thermal cycling have been investigated. Thermal stresses induced in the composites due to a difference between thermal expansion coefficient of matrix and ceramic particles may create new dislocations during the cooling from elevated temperatures. The thermal stresses can achieve the yield stress in the matrix and micro-glide of new created dislocations as well as their annihilation can occur. Thermodynamic processes in the matrix influences these effects.

Thermal expansion characteristic of Ni53·6Mn27·1Ga19·3 alloy with columnar structure

Abstract As cast ingot of the polycrystalline shape memory Ni53·6Mn27·1Ga19·3 alloy was composed of columnar and equiaxed grains. Dilatation characteristics of the samples with columnar grains were studied in the temperature range of 25–370°C and compared with the dilatation characteristics of the samples with equiaxed grains. The results show that shape memory effect strongly depends on the microstructure. Preferred orientation of crystallites in the columnar structure determines its memory strain. The alloy with columnar structure was also examined after predeformation in compression. Two kinds of strains were found in the predeformed samples. Dilatation characteristics of the non-deformed and predeformed martensite show anomalous behaviour.

Study of Undercooling and Recalescence During Solidification of Sn62.5Pb36.5Ag1 and Sn96.5Ag3Cu0.5 Solders in Real Electronic Joints

Undercooling and recalescence were studied using the differential scanning calorimetry (DSC) method on real electronic systems. Two solder pastes, Sn62.5Pb36.5Ag1 and Sn96.5Ag3Cu0.5, were used for preparation of electronic joints. Various combinations of these solders and soldering pads with different surface finishes such as Cu, Cu-Ni-Au, Cu-Sn, and Cu-Sn99Cu1 were used. During melting of both pastes, the Sn and Sn99Cu1 surface finishes immediately dissolved in the solder and the Cu surface coating was exposed to the melt. Therefore, practically the same undercooling was found for the Cu, Cu-Sn, and Cu-Sn99Cu1 coatings. The lowest undercooling was found for the Cu-Ni-Au surface finish for both solder pastes. If two separated electronic joints were made on the sample, two separate peaks were found in the DSC signal during solidification. In the sample with only one joint, only one exothermic peak was found. These findings were observed for all paste/surface finish combinations. These data were analyzed, showing that this effect is a consequence of undercooling and recalescence: Latent heat released during solidification of the joint increases the surrounding temperature and influences all the processes taking place.

Comparison of shear strength of soldered SMD resistors for various solder alloys

Mechanical stability of solder joints has influence on total reliability of electronic products. Mechanical stress affect on properties of solder joints during function, transport and storage of the electronic products. A method which is commonly used in electronic industry to classify the mechanical properties and reliability of solder joints is shear test. This article deals with the comparison of mechanical stability of soldered joints from the point of shear strength of soldered SMD resistors. Three types of solder pastes, one lead Sn62Pb36Ag2 and two lead free Sn42Bi58 and Sn96.5Ag3Cu0.5, were used for solder joint preparation. In addition, influence of solder paste amount on shear strength of soldered SMD component, were compared. The type of solder paste and its applied amount together with the temperature profile are interesting parameters from point of economic aspects, especially in case of mass production, where it is possible to achieve considerable savings. The results shows, that the amount of solder paste has influence on the share strength of solder joins. The best results, highest share strength, have been achieved for Sn96.5Ag3Cu0.5 solder paste.

Study of influence of thermal capacity and flux activity on the solderability

During the surface mount technological process the assembled PCB is exposed to heat when molten solder alloy wets the surfaces and creates future solder joints. The PCB with all components should ideally be exposed to the same temperature profile during the soldering process. The temperatures differ throughout the PCB due to different thermal capacities of used materials and components even though the temperature profile is stable. The aim of the study was to assess the solderability of the copper material with different thermal capacity and to consider the effect of different types of fluxes. The measurement was done by wetting balance method, where the wetting force is measured as a function of time during which the test specimen is immersed into, remains immersed and then it is pulled out from the solder bath. In our experiments we used copper wire of four different diameters (0.7 mm, 1 mm, 1.5 mm), three types of solders - one lead (Sn63Pb37) and two lead free (Sn99Cu1, Sn97Cu3) solders and three types of fluxes.

Interfacial effects on the thermal conductivity of QE22 alloy in SiC/QE22 composites

The thermal conductivity of unreinforced QE22 and reinforced SiC/QE22 alloys was studied in the temperature range from 20 to 300°C. The thermal conductivity of pure QE22 alloy was compared with that of the QE22 matrix in composites made from QE22 with 10, 15 and 25 vol.% SiC particles. The thermal conductivity of the QE22 matrix in QE22/10 vol.% SiC was nearly the same as that of the pure alloy in the whole temperature range studied. The values of the thermal conductivity of the QE22 matrix in QE22/15 vol.% SiC and in QE22/25 vol.% SiC are practically the same as those of pure QE22 alloy, but only up to 130 and 70°C, respectively. Above these temperatures the thermal conductivity of the QE22 matrix is lower than that of the pure alloy. At low temperatures the lattice defects in composites are localized near the interfaces and do not influence the thermal conductivity of the matrix. With the increasing temperature the defects expand into the matrix and reduce its thermal conductivity.