A chiral switchable photovoltaic ferroelectric 1D perovskite

Spin and valley degrees of freedom in materials without inversion symmetry promise previously unknown device functionalities, such as spin-valleytronics. Control of material symmetry with electric fields (ferroelectricity), while breaking additional symmetries, including mirror symmetry, could yield phenomena where chirality, spin, valley, and crystal potential are strongly coupled. Here we report the synthesis of a halide perovskite semiconductor that is simultaneously photoferroelectricity switchable and chiral. Spectroscopic and structural analysis, and first-principles calculations, determine the material to be a previously unknown low-dimensional hybrid perovskite (R)-(−)-1-cyclohexylethylammonium/(S)-(+)-1 cyclohexylethylammonium) PbI3. Optical and electrical measurements characterize its semiconducting, ferroelectric, switchable pyroelectricity and switchable photoferroelectric properties. Temperature dependent structural, dielectric and transport measurements reveal a ferroelectric-paraelectric phase transition. Circular dichroism spectroscopy confirms its chirality. The development of a material with such a combination of these properties will facilitate the exploration of phenomena such as electric field and chiral enantiomer–dependent Rashba-Dresselhaus splitting and circular photogalvanic effects.

Implications of Non-Bingham Rheology

One of the key challenges now facing the US Department of Energy (DOE) is the fate of radioactive waste remaining from World War II and the Cold War, which is stored underground in tanks some 75 feet in diameter and over 30 feet tall. Over time, the waste has segmented into multiple layers with sludges and slurries at the bottom with salt crust layers often at the top and liquid in between. DOE’s current official baseline plans call for remaining sludges and slurries to be removed from the tanks and converted into a stable glass waste form. Minimizing worker exposure to radiation drives DOE to use slurry processing techniques to suspend, mobilize, transport, mix, and process the waste. Therefore, a clear and quantitative understanding of Hanford waste rheology is essential for the success of the DOE mission. Historically much of the waste has been characterized using Eugene Bingham’s century old model that provides a straight line fit to higher shear rate data with the intercept suggesting a yield stress and the slope providing the consistency. Yet, Bingham fits overestimate the shear stress at a given shear rate for low to intermediate shear rates, exactly the range of shear rates typically encountered in pipe flow, where shear rates peak along the pipe wall and vanish in the center. This model produces a fictitious yield stress for some of the wastes that do not exhibit yield phenomena. While overestimating the yield stress may be prudent, safe, and conservative for some applications (e.g., pump sizing to ensure that pumps can handle yield stresses), overestimating the rheology may be inaccurate and non-conservative for other applications (e.g., eroding settled particle beds). Therefore, this paper evaluates the slurry rheology of Hanford and Savannah River wastes using a more modern rheological model that fits the full range of experimental data. Although a bias has been recognized and alternative models proposed, the magnitude of this bias and the implications for tank waste have only been qualitatively suggested. The purpose of this paper is to evaluate quantitatively implications of the poor quality of fit between a Bingham model for rheology and experimental data at modest shear rates. We first demonstrate the magnitude of the bias between the data and the Bingham extrapolation. We then evaluate quantitatively the velocity profile under laminar conditions. This analysis shows that the bias may be large (hundreds of percent or more) at modest shear rates and that modest shear rates dominate pipe velocity profiles.

Flow Stress Behavior and Deformation Characteristics of Ti-22Al-25Nb Alloys at Elevated Temperature

The characteristics of deformation of an orthorhombic phase based alloy, Ti-22Al-25Nb (at%), have been studied by hot compression tests in the temperature range of 940-1150°C with the strain rates of 0.01s-1 and 0.1s-1. A flow curves typically controlled by dynamic recovery were observed in the temperature range of 1090-1150 °C for the strain rate of 0.01s-1 , while discontinuous yield phenomena was found for the strain rate of 0.1s-1. The dynamic recovery can be identified by the microstructure characteristics of the deformation specimens. At 1060°C, the temperature of beta transus, the flow curves and microstructure exhibited the same deformation charateristics as that above the beta transus. At the subtransus temperature, a long period of flow softening followed by the steady-state flow can be observed. During the hot deformation, the hard phases α2 and O elongated, subboundary produced followed by cusp formation in the elongated α2 and O phases, the equiaxed morphology phases occurred by B2 phase penetrating along the subboundaries. The softening mechanism was discussed.

Yield Point Phenomena and Dislocation Velocities Underneath Indentations Into BCC Crystals

In a single material, hardnesses can range from 3σys up to the theoretical strength or approximately E/10 where σys and E are tensile yield strength and modulus. This variation results with decreasing depth of penetration. Such indentation size effects may be associated with surface contamination, passivation films and dislocation phenomena. Even where dislocation nucleation is relatively difficult as in GaAs, the hardness varies from about 1.5 to 15 GPa as the indentation depth decreased from about 100 nm to 10 nm. Similar size effects in BCC metals can give hardnesses which range from about 1 to 30 GPa as the indentations decrease from 1000 nm to 10 nm. Here, there are two types of “yield” phenomena which can be related to an organic contamination film and a dislocation pile-up induced oxide film fracture. As measured in single crystals of Fe, Mo, W, Ta and NiAl, this typically gives lower and upper “yield” points which range from 0.6 to 10 raN. When a dislocation pile-up breaks through the oxide film, velocities can be reasonably large due to the stress at the head of the pile-up. The average dislocation velocity of this avalanche is controlled, to first order, by the Peierls’ energy. A more exhaustive study of NiAl, with a B2/BCC type crystal structure shows that dislocation velocity can be related to the local pile-up stress and a Peierls’ barrier of about 2.2 eV.

Dependence of Segmental Mobility in Polycarbonate on Time and Deformation

Studies are discussed which show that the segmental mobility In polycarbonate (Lexan from General Electric Co.) increases when a static deformation in either extension or simple compression (uniaxial stress) is applied to a specimen, but thereafter the mobility decreases progressively. These changes can be termed erasure (partial) of physical aging (strain softening) and physical aging (hardening), respectively. Because the volume of a specimen deformed in simple compression decreases, reversal of aging (de-aging) cannot be attributed to an increase in free volume, as usually defined. A proposed explanation will be mentioned. A study was also made of the yield stress modified by unconventional methods at several temperatures. The results can be explained In terms of the rate of physical aging and its temperature dependence determined on specimens at 2.6% extension over a broad range of temperatures. These results show that yielding results from progressive de-aging. The yield phenomena obtained by special methods results from both physical aging and de-aging.

Yield Phenomena of Post Biaxially Strained Metals

In sequential of multistep forming operations, the material which has already undergone permanent deformation is expected to behave differently in each stage of deformation than the as-received material. The effect of such changes on (i) the materials ability to distribute strain uniformly, (ii) the strain limits, and (iii) the residual ductility have been studied for a mild steel and for a dual-phase steel. Steels of both materials were balanced biaxially prestrained for different levels of strains and then uniaxial properties and forming limits were determined. Both materials were found to suffer a rapid loss of uniform strain and residual ductility. Forming limits of both steels were found to decrease with prestrain except for stretch-type deformation in the sheet plane. The prestrain produces strain softening1 in mild steel; whereas, this is observed only in 90 deg to the rolling direction in dual-phase steel. In balanced biaxially prestrained (5 percent major and minor strain) dual-phase steel, during hemispherical punch stretching, necking mostly occurs in the rolling direction as opposed to the transverse direction in the as-received material. Such effect can be explained in terms of loss of stability which is observed in uniaxial property of this material.