Ferroelectric hafnium
zirconium oxide holds great promise for a
broad spectrum of complementary metal–oxide–semiconductor
(CMOS) compatible and scaled microelectronic applications, including
memory, low-voltage transistors, and infrared sensors, among others.
An outstanding challenge hindering the implementation of this material
is polarization instability during field cycling. In this study, the
nanoscale phenomena contributing to both polarization fatigue and
wake-up are reported. Using synchrotron X-ray diffraction, the conversion
of non-polar tetragonal and polar orthorhombic phases to a non-polar
monoclinic phase while field cycling devices comprising noble metal
contacts is observed. This phase exchange accompanies a diminishing
ferroelectric remanent polarization and provides device-scale crystallographic
evidence of phase exchange leading to ferroelectric fatigue in these
structures. A reduction in the full width at half-maximum of the superimposed
tetragonal (101) and orthorhombic (111) diffraction reflections is
observed to accompany wake-up in structures comprising tantalum nitride
and tungsten electrodes. Combined with polarization and relative permittivity
measurements, the observed peak narrowing and a shift in position
to lower angles is attributed, in part, to a phase exchange of the
non-polar tetragonal to the polar orthorhombic phase during wake-up.
These results provide insight into the role of electrodes in the performance
of hafnium oxide-based ferroelectrics and mechanisms driving wake-up
and fatigue, and demonstrate a non-destructive means to characterize
the phase changes accompanying polarization instabilities.
Temperature limitations in nickel‐base superalloys have resulted in the emergence of SiC‐based ceramic matrix composites as a viable replacement for gas turbine components in aviation applications. Higher operating temperatures allow for reduced fuel consumption but present a materials design challenge related to environmental degradation. Rare‐earth disilicates (RE2Si2O7) have been identified as coatings that can function as environmental barriers and minimize hot component degradation. In this work, single‐ and multiple‐component rare‐earth disilicate powders were synthesized via a sol‐gel method with compositions selected to exist in the monoclinic C 2/m phase (β phase). Phase stability in multiple cation compositions was shown to follow a rule of mixtures and the C 2/m phase could be realized for compositions that contained up to 25% dysprosium, which typically only exists in a triclinic, P 1¯${\rm{\bar{1}}}$, phase. All compositions exhibited phase stability from room temperature to 1200°C as assessed by X‐ray diffraction. The thermal expansion tensors for each composition were determined from high‐temperature synchrotron X‐ray diffraction and accompanying Rietveld refinements. It was observed that ytterbium‐containing compositions had larger changes in the α31 shear component with increasing temperature that led to a rotation of the principal axes. Principal axes rotation of up to 47° were observed for ytterbium disilicate. The results suggest that microstructure design and crystallographic texture may be essential future avenues of investigation to ensure thermo‐mechanical robustness of rare‐earth disilicate environmental barrier coatings.
Lithium zirconium phosphate (LiZr 2 P 3 O 12) thin films have been prepared on platinized silicon substrates via a chemical solution deposition approach with processing temperatures between 700°C and 775°C. Films that were subject to a single high-temperature anneal were found to crystallize at temperatures above 725°C. Crystallization was observed in films annealed after each deposited layer at 700°C and above. In both cases, grain size was found to increase with annealing temperature. Ion conductivity was found to increase with annealing temperature in singly annealed films. In per-layer annealed films ion conductivity was found to initially increase then decrease with increasing annealing temperature. A maximum ion conductivity of 1.6 × 10 −6 S/cm was observed for the singly annealed 775°C condition, while a maximum ion conductivity of 5.8 × 10 −7 S/cm was observed for the 725°C per-layer annealed condition. These results are consistent with an increasing influence of cross-plane, internal interface resistance and vapor phase carrier loss in the per-layer annealed samples. This work demonstrates that post-deposition processing methods can strongly affect the ion conducting properties of LiZr 2 P 3 O 12 thin films.
A combustion synthesis methodology for the preparation of perovskite Li3xLa1/3‐xTaO3 lithium‐ion conductors with x = 0.033 is presented. Bulk ceramic specimens were sintered under combinations of burial powder and cover crucibles to provide different lithium vapor overpressure conditions. A maximum total lithium ion conductivity of 6 × 10‐6 S cm‐1 at room temperature was found for the pellet covered by a crucible whose lip was sealed using parent powder (moderate overpressure), with agreement to the maximum in the intergranular ion conductivity. Intragranular conductivity was maximized at the low overpressure condition. The trend in ion conductivity was found to correspond to the lithium content in the samples through a combination nuclear reaction analysis and energy dispersive X‐ray spectroscopy phase constitution measurements. The mechanism impacting ion conductivity was determined to be changes in the amount of LaTaO4 secondary phase as driven by the processing conditions during sintering.
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