Strain driven phase transition and mechanism for Fe/Ir(111) films

By way of introducing heterogeneous interfaces, the stabilization of crystallographic phases is critical to a viable strategy for developing materials with novel characteristics, such as occurrence of new structure phase, anomalous enhancement in magnetic moment, enhancement of efficiency as nanoportals. Because of the different lattice structures at the interface, heterogeneous interfaces serve as a platform for controlling pseudomorphic growth, nanostructure evolution and formation of strained clusters. However, our knowledge related to the strain accumulation phenomenon in ultrathin Fe layers on face-centered cubic (fcc) substrates remains limited. For Fe deposited on Ir(111), here we found the existence of strain accumulation at the interface and demonstrate a strain driven phase transition in which fcc-Fe is transformed to a bcc phase. By substituting the bulk modulus and the shear modulus and the experimental results of lattice parameters in cubic geometry, we obtain the strain energy density for different Fe thicknesses. A limited distortion mechanism is proposed for correlating the increasing interfacial strain energy, the surface energy, and a critical thickness. The calculation shows that the strained layers undergo a phase transition to the bulk structure above the critical thickness. The results are well consistent with experimental measurements. The strain driven phase transition and mechanism presented herein provide a fundamental understanding of strain accumulation at the bcc/fcc interface.

DESIGNING MULTIFUNCTIONAL INTERFACES TO BRIDGE HETEROGENEOUS MATERIALS TO REDUCE THERMAL FATIGUE

Composite interfaces between heterogeneous materials exist in many applications which includes electronics packaging. The interface will affect properties such as mechanical integrity during thermal cycling, heat transport and electrical transport due to the inherent disparate properties such as coefficient of thermal expansion (CTE), atomic structure, interface bonding and fabrication processes. Novel interface engineering will be vital for electronics packaging utilized in extreme environment temperatures beyond the standard ranges such as -55 °C to 150 °C. The failure of standard electronics packaging materials with heterogeneous interfaces under thermal cycle fatigue is investigated. Based on the failure analysis, several multifunctional interfaces are developed to bridge the heterogeneous interface and to provide the desired properties and functionality. Approaches to achieve these heterogeneous structures, the materials choices to preserve the multifunctional properties and the modeling predictions are considered. Test structures are prepared of the candidate interfaces, the morphologies are investigated and testing of the thermal cycle fatigue properties over the range -55 °C to 300 °C is performed and will be discussed.