Nanoscale self-healing mechanisms in shape memory ceramics

Abstract

Shape memory (SM) ceramics, such as yttria-stabilized tetragonal zirconia (YSTZ), are a unique family of SM materials that offer unique properties including ultra-high operating temperature, and high resistance to chemical corrosion and oxidation. However, formation of defects is usually observed in SM ceramics during manufacturing and/or by mechanical deformation. To fully take advantage of the SM properties of these ceramics, it is necessary to fully understand the nanostructural evolution of defects under external stimuli. In this study, defect closure behaviors in YSTZ nanopillars are investigated by atomistic simulations. Two characteristic orientations of 011 and [001] are selected to represent the dominant deformation mechanisms of phase transformation and dislocation migration, respectively. With the presence of crack and void, the strength and yield strain of nanopillars are noted to decrease significantly, especially for 011oriented YSTZ nanopillars. Volume expansion associated with the tetragonal to monoclinic phase transformation is observed to promote healing of crack and void. Atom stress analyses reveal stress concentrations along the newly formed monoclinic phase bands. A critical crack width is identified, less than which the crack can be fully closed in compression. Size effect study reveals that an increase in nanopillar size has a positive effect on crack self-healing behavior. For [001]-oriented YSTZ nanopillars, dislocation migration leads to formations of an amorphous phase, which also assist the crack and void closure process. The revealed crack/void healing mechanisms may provide a path for mitigating internal defects that influences the mechanical properties and deformation mechanisms of SM ceramics.

Rapid One‐Step Synthesis and Compaction of High‐Performance n‐Type Mg3Sb2 Thermoelectrics

“…The intrinsic low k L in Mg 3 Sb 2 is mainly related to the large anharmonic scattering rate induced by the soft transverse acoustic phonon modes. [11,53] Comparing with the pure Mg 3 Sb 2 prepared using the same synthesis method, [41] the Te doping in Mg 3 Sb 2 slightly reduces the lattice thermal conductivity. With the proper Sc and Te co-doping, the lattice thermal conductivity at 300 K can be effectively reduced to 1.05 W m À1 K À1 in Mg 3.5 Sc 0.04 Sb 1.97 Te 0.03 , which decreases to 0.54 W m À1 K À1 at 725 K (see Figure 4 d).…”

“…Recently n-type Mg 3 Sb 2 -based compounds have been intensively studied because of their outstanding TE performance as well as the low-cost and abundantly available constituent elements. [11][12][13][14][15][16][17][18][19][20][21][22][23] Exceptional n-type TE properties were first reported in Te-doped Mg 3+d Sb 1.5 Bi 0.5 with a high zT value of about 1.6 at 725 K by Tamaki et al [15] and Zhang et al, [14] in which the experimental composition and synthesis method reported by Zhang et al can be traced back to the original work by Pedersen [12] in 2012. Several reports then focused on tuning the carrier scattering mechanism through a co-doping with the transition metals on the Mg sites and Te on the Sb sites [16,17] or increasing the pressing temperature [14,19,20] in order to improve the carrier mobility at low temperatures.…”

“…Some of the experimental data deviating from theory may be attributed to factors such as the minor impurity phases and doping on the cation sites. [11] The light conductivity effective mass is another electronic origin for the superior electronic transport in n-type Mg 3 Sb 2 , [14,18] which ensures good carrier mobility. As expected, all n-type (Sc, Te)-doped Mg 3 Sb 2 samples generally show high room-temperature mobility values ranging from 51 to 93 cm 2 V À1 s À1 (see Figure 3 b; Supporting Information, Figures S5-S7), which are larger than those of the reported ptype Mg 3 Sb 2 [51] (about 16 cm 2 V À1 s À1 ) and n-type Te-doped Mg 3 Sb 2 [13,28,29] at a comparable carrier concentration.…”

The effect of a pre-existing nanovoid on martensite formation and interface propagation: a phase field study

“…A velocity term was introduced in [82] to describe the vapor transport for modeling the change in void configuration during the migration. A PFA was presented in [5] to predict the nanovoid evolution in the vicinity of an A – M interface by employing the CH–GL and mechanics equations and the effect of the mobility coefficient and the phase dependent energy barrier of nanovoids, the A – M interface width size and the transformation strain on the nanovoid evolution were investigated.…”

“…Voids represent a misfit strain field inside the solid-gas interface. Owing to their geometry, they play the role of stress concentrators, which significantly change the mechanical driving force for different phenomena, such as dislocations generation and growth [4], PTs [5], nanovoid evolution [6], fracture [7] and more. Hence, because of their stress concentration and interfacial energy, the interaction of voids with other defects, interfaces and interphases plays an important role in kinetics, thermodynamics and nanostructures [8][9][10].…”