Though
formamidinium lead triiodide (FAPbI3) possesses
a suitable band gap and good thermal stability, the phase transition
from the pure black perovskite phase (α-phase) to the undesirable
yellow nonperovskite polymorph (δ-phase) at room temperature,
especially under humid air, hinders its practical application. Here,
we investigate the intrinsic instability mechanism of the α-phase
at ambient temperature and demonstrate the existence of an anisotropic
strained lattice in the (111) plane that drives phase transformation
into the δ-phase. Methylammonium bromide (MABr) alloying (or
FAPbI3-MABr) was found to cause lattice contraction, thereby
balancing the lattice strain. This led to dramatic improvement in
the stability of α-FAPbI3. Solar cells fabricated
using FAPbI3-MABr demonstrated significantly enhanced stability
under the humid air.
The effect of DC electric field on sintering, and on the electrical conductivity of undoped rutile, TiO 2 (99.99%), has been investigated at fields ranging from 0 V to 1000 V/cm. The experiments were carried out at a constant heating rate of 10°C/min with the furnace temperatures reaching up to 1150°C. The sintering behavior falls into two regimes: at lower fields, up to 150 V/cm, sintering is enhanced, but densification occurs gradually with time (Type A or FAST sintering). At higher fields sintering occurs abruptly, and is accompanied by a highly nonlinear increase in conductivity, which has been called flash sintering (Type B or FLASH sintering). Arrhenius plots of conductivity yield an activation energy of 1.6 eV in Type A and 0.6 eV in Type B behavior; the first is explained as ionic and the second as electronic conductivity. The evolution of grain size under both types of sintering behavior are reported. These results highlight that the dominant mechanism of field-assisted sintering can change with the field strength and temperature. We are in the very early stages of identifying these mechanisms and mapping them in the field, frequency, and temperature space.
A flash sintering experiment can be carried out by applying an electric field and heating the specimen at a constant rate. The flash event occurs at a specific temperature that depends on the strength of the electric field. Alternatively, the furnace can be held at a constant temperature and the voltage applied as a step function; after an incubation time there is a highly non-linear rise in conductivity. This incubation step is called Stage I. The non-linearity is constrained by switching the power supply to current control. This short transient, during which the sample sinters nearly instantaneously, is the second stage. Under current-control, the (essentially dense) sample remains in a highly excited state indefinitely, which we call Stage III. In this state, the samples are often brightly electroluminescent emitting a green glow; unusual phase transformations occur and the rate of chemical reactions is greatly enhanced. We infer that these manifestations are evidence of a defect catastrophe that includes unusual generation of electrons, holes and point defects, which can produce sintering, electronic conductivity, electroluminescence, and phase transformations, all at the same time. We hypothesize that both Joule heating and electric field are necessary for this catastrophe.
We report results from in-situ measurements of lattice expansion during flash sintering of 3 mol% yttria stabilized tetragonal zirconia taken at the Advanced Photon Source, Argonne National Laboratory. The expansion is anisotropic, with the relative expansion of the a-lattice constant exceeding that of the c-lattice constant. The anisotropic expansion cannot be explained by thermal expansion and is consistent with predictions from ab-initio calculations based upon the generation of vacancy-interstitial pairs of zirconium and oxygen.
K E Y W O R D SField Assisted Sintering Technology (FAST), modeling/model, thermal expansion, X-ray methods
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