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Quenching has been established as a viable method to increase the depolarization temperature in (100-x) Na1/2Bi1/2TiO3–xBaTiO3 (NBT–xBT). The proposed hypothesis of a stabilized ferroelectric order would entail changes in the polarized volume. To this end, air-quenched and furnace cooled samples of four compositions of NBT–xBT with x = 3, 6, 9, and 12 mol. % BT were studied. Upon quenching, all the compositions demonstrate an increase in the ferroelectric to relaxor transition temperature, TF-R, by 23–44 °C and enhanced lattice distortion. Resonance frequency damping analysis was utilized to measure Young's modulus in the temperature range of 25 °C to 800 °C and to estimate the volume fraction of polar nanoregions using a composite model. Quenching leads to an 8% decrease in Young's modulus, but to an increase in the volume fraction of polar nanoregions by 12% at 300 °C for NBT-6BT. Transmission electron microscopy investigations of quenched NBT-6BT reveal a combination of lamellar domains and more homogenous areas with nanometer-sized domains. The existence of lamellar domains in quenched morphotropic phase boundary compositions together with enhanced lattice distortion and a decrease in dielectric frequency dispersion substantiate the premise of a stabilized ferroelectric order.

Ultrahigh temperature ceramics, so-called UHTCs, represent a class of materials that can operate under extreme conditions such as ultra-high temperatures (i.e., beyond 2000℃). [1][2][3] They have been investigated within the context of aerospace, for example, leading edges and control surfaces for atmospheric re-entry, hypersonic flight, and scramjet propulsion or with respect to nuclear power applications such as fuel cladding materials or non-oxidic fuels. 2,[4][5][6] UHTCs are characterized by tremendously high melting points, high hardness, stiffness and strength even at (ultra)high temperatures as well as high thermal conductivity. 7-10 Despite their highly attractive properties, there are still challenges related to the development of UHTCs, for example, concerning their sluggish self-diffusion which impedes their sintering ability, [11][12][13][14][15][16] or their rather fair ultrahigh temperature oxidation/ corrosion resistance. [17][18][19][20][21][22][23] In order to overcome these issues, secondary phases, typically silicon-containing, are considered for the sintering of UHTCs and to additionally provide an improvement of their oxidation behavior. Typically, 10-20 vol.% of silica former phases such as SiC, Si 3 N 4 or metal silicides, for example, MoSi 2 have

No abstract

Quenching relaxor ferroelectric 0.94(Na 1/2 Bi 1/2 )TiO 3 -0.06BaTiO 3 (NBT-6BT) enhances the depolarization temperature (T d ), linked to the stabilization of ferroelectric order. The thermal evolution of the domain structure and phase assemblage provides insights about the onset of ferroelectric order in quenched materials. Unpoled furnace cooled and quenched NBT-6BT ceramics were studied using in situ temperature-dependent transmission electron microscopy. The rhombohedral to tetragonal structural transition in furnace cooled and quenched samples occurs in a comparable temperature range of 120 • C-220 • C. While the tetragonal phase is characterized by polar nanoregions (PNRs) and no domain contrast in the furnace cooled state, the quenched composition exhibits an increased fraction of lamellar domains, which are partially stable up to 300 • C, thus benefiting the delayed depolarization. This is further corroborated by the dielectric data indicating earlier freezing of PNR dynamics in the quenched state. The reversibility of the phase transition is demonstrated by successive cooling, where quenched NBT-6BT features an increased grainy PNR contrast after the experiment, followed by a kinetically delayed coalescence of PNRs back into lamellar domains. This demonstrates that the stabilized ferroelectric state upon quenching is associated with the conversion of polar units on the nanometer scale into long-range domain structures.