Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors
Abstract not provided
Chung, Hyunjoong↗
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Direct evidence for stress-induced transformation between coexisting multiple martensites in a Ni-Mn-Ga multifunctional alloy
The structural response of coexisting multiple martensites to stress field in a Ni-Mn-Ga multifunctional alloy was investigated by the in situ high-energy x-ray diffraction technique. Stress-induced transformation between coexisting multiple martensites was observed at 110 K, at which five-layered modulated (5M), seven-layered modulated (7M) and non-modulated (NM) martensites coexist. We found that a tiny stress of as low as 0.5 MPa could trigger the transformation from 5M and 7M martensites to NM martensite and this transformation is partly reversible. Besides the transformation between coexisting multiple martensites, rearrangement of martensite variants also occurs during loading, at least at high stress levels. The present study is instructive for designing advanced multifunctional alloys with easy actuation.
external-field-induced structural transformation↗
Two-dimensional shape recognition using sparse distributed memory
Researchers propose a method for recognizing two-dimensional shapes (hand-drawn characters, for example) with an associative memory. The method consists of two stages: first, the image is preprocessed to extract tangents to the contour of the shape; second, the set of tangents is converted to a long bit string for recognition with sparse distributed memory (SDM). SDM provides a simple, massively parallel architecture for an associative memory. Long bit vectors (256 to 1000 bits, for example) serve as both data and addresses to the memory, and patterns are grouped or classified according to similarity in Hamming distance. At the moment, tangents are extracted in a simple manner by progressively blurring the image and then using a Canny-type edge detector (Canny, 1986) to find edges at each stage of blurring. This results in a grid of tangents. While the technique used for obtaining the tangents is at present rather ad hoc, researchers plan to adopt an existing framework for extracting edge orientation information over a variety of resolutions, such as suggested by Watson (1987, 1983), Marr and Hildreth (1980), or Canny (1986).
Kanerva, Pentti↗
Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling
The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. Lastly, if the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.
36 MATERIALS SCIENCE↗
Cold Hibernated Elastic Memory (CHEM) Self-Deployable Structures
Experiments have confirmed the feasibility of an innovative, new class of very simple, reliable, low mass, low packaging volume, and low-cost self-deployable structures for space and commercial applications.
Cold↗
Applications of cold hibernated elastic memory (CHEM) structures
Experiments and analyses have confirmed the feasibility of an innovative, new class of very simple, reliable, low mass, low packaging volume, and low-cost self-deployable structures for space and commercial applications.
shape memory polymers open cellular structures exp↗
Understanding the early cycling evolution behaviors for phase change memory application
The RESET current of T-shaped phase change memory cells with 35 nm heating electrodes has been studied to understand the behavior of early cycling evolution. Results show that the RESET current has been significantly reduced after the early cycling evolution (1st RESET) operation. Compared the transmission electron microscope images, it is found that the hexagonal Ge{sub 2}Sb{sub 2}Te{sub 5} (GST) crystal grains are changed into the grains with face centered cubic structure after the early cycling evolution operation, which is taken as the major reason for the reduced RESET current, confirmed by a two-dimensional finite analysis and ab initio calculations.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Mapping of Texture and Phase Fractions in Heterogeneous Stress States during Multiaxial Loading of Biomedical Superelastic NiTi
Thermoelastic deformation mechanisms in polycrystalline biomedical-grade superelastic NiTi are spatially mapped using in situ neutron diffraction during multiaxial loading and heating. The trigonal R-phase is formed from the cubic phase during cooling to room temperature and subsequently deforms in compression, tension, and torsion. The resulting R-phase variant microstructure from the variant reorientation and detwinning processes are equivalent for the corresponding strain in tension and compression, and the variant microstructure is reversible by isothermal loading. The R-phase variant microstructure is consistent between uniaxial and torsional loading when the principal stress directions of the stress state are considered (for the crystallographic directions observed here). The variant microstructure evolution is tracked and the similarity in general behavior between uniaxial and torsional loading, in spite of the implicit heterogeneous stress state associated with torsional loading, pointed to the ability of the reversible thermoelastic transformation in NiTi to accommodate stress and strain mismatch with deformation. This ability of the R-phase, despite its limited variants, to accommodate stress and strain and satisfy strain incompatibility in addition to the existing internal stresses has significance for reducing irrecoverable deformation mechanisms during loading and cycling through the phase transformation.
36 MATERIALS SCIENCE↗
A Sustainable Multi-Dimensional Printable Material
Polymeric materials are usually tailored for specific functionality. A single polymer exhibiting multiple simultaneous functionalities often requires intricate molecular architecture, which is difficult to manufacture at scale because of its complex synthesis routes. In this work, a facile, partly renewable composition—prepared via reactive melt processing—that induces tunable functionalities such as 3D printability, shape recovery, and self-healing while exhibiting satisfactory mechanical properties is reported. The system with a hydrogen-bonded 3D network consists of thermally reversible nano-scale agglomerates of sustainable, rigid phenolic oligomers and crystallizable flexible polymer. Local molecular mobility and temperature-dependent relaxation and recovery of the non-equilibrium networked states enable exploiting these simultaneous functionalities. Transitions involving solidification and structure stabilization at ambient temperature spanning several hours after preheating only at 70 °C directly contrast typical thermoplastic or thermoplastic elastomer behaviors. Results from this study can inform the design of future rheology modifiers and materials for soft robotics.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Determining Multi-Component Phase Diagrams with Desired Characteristics Using Active Learning
Herein, we demonstrate how to predict and experimentally validate phase diagrams for multi-component systems from a high-dimensional virtual space of all possible phase diagrams involving several elements based on small existing experimental data. The experimental data for bulk phases for known systems represents a sampling from this space, and screening the space allows multi-component phase diagrams with given design criteria to be built. This approach uses machine learning methods to predict phase diagrams and Bayesian experimental design to minimize experiments for refinement and validation, all within an active learning loop. The approach is proven by predicting and synthesizing the ferroelectric ceramic system (1-ω)(Ba 0.61 Ca 0.28 Sr 0.11 TiO 3 )-ω(BaTi 0.888 Zr 0.0616 Sn 0.0028 Hf 0.0476 O 3 ) with a relatively high transition temperature and triple point, as well as the NiTi-based pseudo-binary phase diagram (1-ω)(Ti 0.309 Ni 0.485 Hf 0.20 Zr 0.006 )-ω(Ti 0.309 Ni 0.485 Hf 0.07 Zr 0.068 Nb 0.068 ) designed for high transition temperature (ω ≤ 1). Each phase diagram is validated and optimized through only three new experiments. The complexity of these compounds is beyond the reach of today’s computational methods.
36 MATERIALS SCIENCE↗
Magnetic and Transport Properties of Mn{sub 2}Ni{sub 1.8}In{sub 0.2} Alloy
The structural, magnetic, and transport properties of the Mn{sub 2}Ni{sub 1.8}In{sub 0.2} alloy have been presented. It was observed to form in tetragonal structure. Multiple magnetic phase transitions have been detected in magnetization data over the temperature range from 2 to 350 K. From the resistivity, ac susceptibility, and heat capacity data, the superconducting transition are observed at T = 5.6 K. The alloy is considered to be one of the potential candidates from the Mn-rich alloys, which has shown both magnetic and superconducting properties, which can be useful from application point of view.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Compressive Deformation Behavior of Open-Cell Cu-Zn-Al Alloy Foam Made Through P/M Route Using Mechanically Alloyed Powder
Cu-Zn-Al foams of varying porosity fractions using mechanical alloyed powder have been made through powder metallurgy route. Here, NH{sub 4} (HCO{sub 3}) was used as a space holder. Mechanically alloyed Cu-Zn-Al is made using a planetary ball mill taking the ratio of Cu/Zn/Al = 70:25:5 (by weight ratio). The ball/powder ratios were varied in the four ranges 10:1, 15:1, 20:1, and 25:1. Green compacts of milled powder and space holder samples were sintered at three stages at three different temperatures 350, 550, and 850 °C for 1 h at each stage. The crystalline size and particle size as a function of ball/powder ratios were examined. The compressive deformation responses of foams are varied with relative density and the ball/powder ratio. The plateau stress and energy absorption of these foams increase with an increase in relative density but decreases with increase in ball/powder ratio, even though crystalline size decreases. This has further been explained on the basis of particle morphology as a function of ball/powder ratio.
36 MATERIALS SCIENCE↗
Superelasticity by reversible variant reorientation in a Ni-Mn-Ga microwire with bamboo grains
The link between microstructure and mechanical properties is investigated for a superelastic Ni–Mn–Ga microwire with 226 μm diameter, created by solidification via the Taylor method. The wire, which consists of bamboo grains with tetragonal martensite matrix and coarse γ precipitates, exhibits fully reversible superelastic behavior up to 4% tensile strain. Upon multiple tensile load–unload cycles, reproducible stress fluctuations of ~3 MPa are measured on the loading superelastic stress plateau of ~50 MPa. During cycles at various temperatures spanning -70 to 55 °C, the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature. Based on in situ synchrotron X-ray diffraction measurements, we conclude that this superelastic behavior is due to reversible martensite variants reorientation (i.e., reversible twinning) with lattice rotation of ~13°. The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the wire. The strain recovery during unloading is attributed to reverse twinning, driven by the internal stress generated on loading between the elastic γ precipitates and the twinning martensite matrix. Lastly, the temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite, as measured by X-ray diffraction.
36 MATERIALS SCIENCE↗
Tailored thermal expansion alloys
Current approaches to tailoring the thermal expansion coefficient of materials or finding materials with negative thermal expansion rely on careful manipulation of either the material's composition and/or the complex fabrication of composites. By contrast, we report a new principle that enables the precise control of macroscopic thermal expansion response of bulk materials via crystallographic texture manipulation and by taking advantage of anisotropic Coefficients of Thermal Expansion (CTE) in a large class of martensitically transforming materials. Through simple thermo-mechanical processing, it is possible to tailor the thermal expansion response of a single material––without manipulating its composition––over a wide range of positive and negative values. We demonstrate this principle by gradually tuning the macroscopic CTE in a model NiTiPd alloy between a positive (+14.90 × 10 -6 K -1 ) and a negative (-3.06 × 10 -6 K -1 ) value, simply by incrementally increasing tensile plastic deformation in the martensite phase. This surprising response is linked to the large positive, +51.33 × 10 -6 K -1 , and negative, -34.51 × 10 -6 K -1 , CTE anisotropy, at the lattice level, along the different crystal directions in martensite. Similar CTE anisotropy is also shown experimentally in CoNiGa and TiNb alloys. In a model TiNb alloy, giant macroscopic CTEs of +181 × 10 -6 K -1 and -142 × 10 -6 K -1 are measured. A connection between the CTE anisotropy and the martensitic transformation in these and other materials systems such as NiTi, pure uranium, and PbTiO 3 is later made. It is shown that negative or positive thermal expansion crystallographic directions are connected to the crystallographic relationship between the austenite and martensite lattices, and can easily be predicted using the lattice parameters of austenite and martensite phases. Our current observations and analyses suggest that the tunability of the macroscopic CTE through thermo-mechanical processing is universal in materials––both ceramic and metals––that undergo martensitic transformations
36 MATERIALS SCIENCE↗
Cyclic compression response of micropillars extracted from textured nanocrystalline NiTi thin-walled tubes
Compression-compression cyclic deformation of nanocrystalline NiTi tubes intended for medical stents and with an outer diameter of 1 mm and wall thickness of 70 μm was studied using micropillars produced by FIB with the loading axis orthogonal to the tube axis. These micropillars were cycled in a displacement-controlled mode using a nanoindenter equipped with a flat punch to strain levels of 4, 6 and 8% in each cycle and specimens were subjected to several hundred cycles. Furthermore, the cyclic response of two NiTi tubes, one with Af of 17 °C and the other with an Af of -5 °C is compared. The texture of the tube with the Af of -5 °C was measured at the microscopic level using transmission electron microscopy and at the macroscopic level by X-ray diffraction and good agreement was noted. Characteristics such as i) a reduction in the forward transformation stress, ii) increase in maximum stress for a given displacement amplitude, and iii) a reduction in the hysteresis loop area, all with increasing number of cycles, observed typically during cyclic deformation of conventional macroscopic specimens, were captured in the micropillar cyclic tests. Our observations lead to the conclusion that micropillar compression testing in a cyclic mode can enable characterizing the orientation-dependent response in such small dimension components that see complex loading in service, and additionally provide an opportunity for calibrating constitutive equations in micromechanical models.
36 MATERIALS SCIENCE↗
Giant enhancement of the magnetocaloric response in Ni–Co–Mn–Ti by rapid solidification
Magnetocaloric refrigeration is a solid-state cooling approach that assures high energy efficiency and low environmental impact. It remains uncompetitive with conventional vapor-compression technologies due to lack of high-performing materials that exhibit large magnetocaloric effects in low magnetic fields. In this work we report a game-changing enhancement of the magnetocaloric response in a transition-metal-based Ni–Co–Mn–Ti. Mechanically and chemically stable rapidly solidified ribbons exhibit magnetic entropy changes as high as ~27J·kg –1 K –1 for a moderate field change of 2T, comparable to or larger than the best known materials for near-room temperature applications. The ribbons can be easily manufactured in large quantities and the transition temperature can be adjusted by varying Co concentration.
36 MATERIALS SCIENCE↗
Design of a V–Ti–Ni alloy with superelastic nano-precipitates
Stress-induced martensitic transformations enable metastable alloys to exhibit enhanced strain hardening capacity, leading to improved formability and toughness. As is well-known from transformation-induced plasticity (TRIP) steels, however, the resulting martensite can limit ductility and fatigue life due to its intrinsic brittleness. In this work, we explore an alloy design strategy that utilizes stress-induced martensitic transformations but does not retain the martensite phase. This strategy is based on the introduction of superelastic nano-precipitates, which exhibit reverse transformation after initial stress-induced forward transformation. To this end, utilizing ab-initio simulations and thermodynamic calculations we designed and produced a V 45 Ti 30 Ni 25 (at%) alloy. In this alloy, TiNi is present as nano-precipitates uniformly distributed within a ductile V-rich base-centered cubic (bcc) β matrix, as well as being present as a larger matrix phase. We characterized the microstructure of the produced alloy using various scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. Furthermore, the bulk mechanical properties of the alloy are demonstrated through tensile tests, and the reversible transformation in each of the TiNi morphologies were confirmed by in-situ TEM micro-pillar compression experiments, in-situ high-energy diffraction synchrotron cyclic tensile tests, indentation experiments, and differential scanning calorimetry experiments. The observed transformation pathways and variables impacting phase stability are critically discussed.
59 BASIC BIOLOGICAL SCIENCES↗