The degradation of adhesion strength between the back sheet and encapsulant due to moisture penetration has been investigated for commercial crystalline silicon photovoltaic mini-modules. The damp-heat tests originating from the International Electrotechnical Commission qualification test were carried out at five different temperature and relative humidity (RH) conditions (95°C/85% RH, 85°C/85% RH, 65°C/85% RH, 85°C/65% RH and 85°C/45% RH). The adhesion strength was measured by 90°peel tests, carried out at specified time intervals during degradation. Several visible defects were observed, including delamination, moisture ingress and bubble formation. The adhesion strength showed a stretched exponential decay with time, and significant influence of test conditions was demonstrated. A humidity dose model was proposed by assuming micro-climates seen by the modules, that is, surface relative humidity of the back sheet as the driving factor for an Arrhenius-based model using temperature as accelerating factor. The correlation between adhesion strength degradation and humidity dose was investigated through linear and exponential models. Results showed that the conventional linear model failed to represent the relationship while the exponential model fitted to this correlation with extracted activation energy (E a ) of around 63 kJ/mol. This provides a model for the estimation of adhesion strength decay in dependence of the humidity conditions.
To improve the performance of the steel for brake discs, like mechanical properties and thermal fatigue resistance, Nb is added to the steel. In this paper, the effects of Nb and tempering time on the microstructure and mechanical properties of the steel have been studied. The investigation of precipitation behaviors is based on Thermo‐Calc software and transmission electron microscopy. The results show there is a negative linear relationship between the yield strength and the logarithm of time, and tempering time should not exceed 2 h. V‐rich M8C7 and NbC occur during austenitizing, while (Mo,V)C, M23C6, and M7C3 precipitate during tempering. (Mo,V)C has a strong strengthening effect at the early stage of tempering, but it grows faster than M8C7. With 0.025% Nb addition, the yield strength and impact energy increase simultaneously due to the refinement of martensite; the addition of 0.045% Nb promotes the tempering stability due to the suppression of precipitation and coarsening of Cr carbides and the precipitation strengthening of M8C7 and NbC, therefore, the addition of 0.025–0.045% Nb is conducive to improving the performance of the steel. Moreover, the suppression of Cr carbides with 0.045% Nb addition makes up for the damage of large NbC to toughness.
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