Ferroelectrics,multiferroics and artifacts: Lozenge-shaped hysteresis and things that go bump in the night

This review summarizes dielectric studies and related experiments on ferroelectrics and multiferroics about which there has been considerable controversy in the literature, sometimes at unusually impolite and unprofessional levels. In addition it focuses attention on a new anomalous phenomenon – that of ferroelectric hysteresis loops P(E) that are parallelograms with straight sides. In some cases materials have been considered to be multiferroic when the data can be interpreted more simply via other well-known mechanisms. In some cases the systems truly are multiferroic, despite X-ray crystallographic data implying that this is not possible; some properties arise only from the domain walls. And in some cases authors get different results from previous work, simply because they used bulk ceramics instead of very thin films, or epitaxial films instead of randomly oriented ceramics; it is of course preferable if groups exchange specimens before they claim the work of others is simply wrong. Emphasis is on newly discovered lozenge-shaped hysteresis loops, whose parallelepiped geometries are found in four unrelated materials. This review has intentionally not discussed ferroelectric artifacts that appear in atomic force microscopy, since that is actually more extensive in variety and well reviewed this year by Kalinin’s group [1].     

The roles of magnesium in the mineral metabolism of biological apatite for the treatment of arthritis inspired by the deer antler

Nowadays, the treatment of osteoarthritis (OA), a highly prevalent joint disorder, remains a medical challenge because of the lack of understanding of its pathogenesis. In this work, we developed an alternative strategy of OA treatment using magnesium-based materials as potential therapeutic agent towards subchondral bone remodeling. We selected deer antlers as the animal model where calcification behaviors could provide interesting references for the rapid and reproducible endochondral bone growth. Extremely high content of Mg was detected in the antler, which was able to affect the evolutions of biological apatite. Herein, octacalcium phosphate (OCP) and amorphous calcium phosphate (ACP), which are critically involved in the calcification process, were respectively synthesized under the Mg-containing conditions to understand the role of Mg in the evolution of biological apatite. Results showed that the substitution of Mg²⁺ at lower contents stabilized OCP and ACP, while higher contents of Mg inhibited the formation of both phases. The size of both calcium phosphates was also altered significantly by the addition of Mg. The results of cell culture indicated that excess Mg notably accelerated the secretion of extracellular matrix and inhibited the mineralization of chondrocyte matrix. Hence, utilization of Mg-based materials in subchondral bone was supposed to provide a potential therapeutic approach to treat the OA by inhibiting subchondral of ossification process.     

Programming the shape-shifting of flat soft matter

Shape-shifting of flat materials into the desired 3D configuration is an alternative design route for fabrication of complex 3D shapes, which provides many benefits such as access to the flat material surface and the ability to produce well-described motions. The advanced production techniques that primarily work in 2D could then be used to add complex surface features to the flat material. The combination of complex 3D shapes and surface-related functionalities has a wide range of applications in biotechnology, actuators/sensors, and engineering of complex metamaterials. Here, we categorize the different programming strategies that could be used for planning the shape-shifting of soft matter based on the type of stresses generated inside the flat material and present an overview of the ways those mechanisms could be used to achieve the desired 3D shapes. Stress gradients through the thickness of the material, which generate out-of-plane bending moments, and compressive in-plane stresses that result in out-of-plane buckling constitute the major mechanisms through which shape-shifting of the flat matter could be programmed. We review both programming strategies with a focus on the underlying physical principles, which are highly scalable and could be applied to other structures and materials. The techniques used for programming the time sequence of shape-shifting are discussed as well. Such types of so-called “sequential” shape-shifting enable achieving more complex 3D shapes, as the kinematics of the movements could be planned in time to avoid collisions. Ultimately, we discuss what 3D shapes could be achieved through shape-shifting from flat soft matter and identify multiple areas of application.     

Manipulation of matter by electric and magnetic fields: Toward novel synthesis and processing routes of inorganic materials

The use of external electric and magnetic fields for the synthesis and processing of inorganic materials such as metals and ceramics has seen renewed interest in recent years. Electromagnetic energy can be utilized in different ways to improve or accelerate phase formation and stabilization, chemical ordering, densification and coarsening of particle-based materials (pore elimination and grain growth), and mechanical deformation (plasticity and creep). In these new synthesis and processing routes, the resulting microstructures and macroscopic material behavior are determined by the interaction of the applied fields with defects such as single or clustered point defects, dislocation networks, and interfaces. Multiscale experimental investigations and modeling are necessary to unveil the mechanisms underlying this field-assisted manipulation of matter.     

Superior wear resistance of diamond and DLC coatings

As the hardest known material, diamond and its coatings continue to generate significant attention for stringent applications involving extreme tribological conditions. Likewise, diamond-like carbon (DLC, especially the tetragonal amorphous carbon, ta-C) coatings have also maintained a high level interest for numerous industrial applications where efficiency, performance, and reliability are of great importance. The strong covalent bonding or sp³-hybridizaiton in diamond and ta-C coatings assures high mechanical hardness, stiffness, chemical and thermal stability that make them well-suited for harsh tribological conditions involving high-speeds, loads, and temperatures. In particular, unique chemical and mechanical nature of diamond and ta-C surfaces plays an important role in their unusual friction and wear behaviors. As with all other tribomaterials, both diamond and ta-C coatings strongly interact with the chemical species in their surroundings during sliding and hence produce a chemically passive top surface layer which ultimately determines the extent of friction and wear. Thick micro-crystalline diamond films are most preferred for tooling applications, while thinner nano/ultranano-crysalline diamond films are well-suited for mechanical devices ranging from nano- (such as NEMS) to micro- (MEMS and AFM tips) as well as macro-scale devices including mechanical pump seals. The ta-C coatings have lately become indispensable for a variety of automotive applications and are used in very large volumes in tappets, piston pins, rings, and a variety of gears and bearings, especially in the Asian market. This paper is intended to provide a comprehensive overview of the recent developments in tribology of super-hard diamond and DLC (ta-C) films with a special emphasis on their friction and wear mechanisms that are key to their extraordinary tribological performance under harsh tribological conditions. Based on the results of recent studies, the paper will also attempt to highlight what lies ahead for these films in tribology and other demanding industrial applications.     

Controllable and reversible tuning of material rigidity for robot applications

Tunable rigidity materials have potentially widespread implications in robotic technologies. They enable morphological shape change while maintaining structural strength, and can reversibly alternate between rigid, load bearing and compliant, flexible states capable of deformation within unstructured environments. In this review, we cover a range of materials with mechanical rigidity that can be reversibly tuned using one of several stimuli (e.g. heat, electrical current, electric field, magnetism, etc.). We explain the mechanisms by which these materials change rigidity and how they have been used for robot tasks. We quantitatively assess the performance in terms of the magnitude of rigidity, variation ratio, response time, and energy consumption, and explore the correlations between these desired characteristics as principles for material design and usage.     

Recent progresses on physics and applications of vanadium dioxide

As a strongly correlated electron material, vanadium dioxide (VO₂) has been a focus of research since its discovery in 1959, owing to its well-known metal–insulator transition coupled with a structural phase transition. Recent years have witnessed both exciting discoveries in our understanding of the physics of VO₂ and developments in new applications of VO₂-related materials. In this article, we review some of these recent progresses on the phase transition mechanism and dynamics, phase diagrams, and imperfection effects, as well as growth and applications of VO₂. Our review not only offers a summary of the properties and applications of VO₂, but also provides insights into future research of this material by highlighting some of the challenges and opportunities.     

Advances in neutron imaging

Imaging techniques based on neutron beams are rapidly developing and have become versatile non-destructive analyzing tools in many research fields. Due to their intrinsic properties, neutrons differ strongly from electrons, protons or X-rays in terms of their interaction with matter: they penetrate deeply into most common metallic materials while they have a high sensitivity to light elements such as hydrogen, hydrogenous substances, or lithium. This makes neutrons perfectly suited probes for research on materials that are used for energy storage and conversion, e.g., batteries, hydrogen storage, fuel cells, etc. Moreover, their wave properties can be exploited to perform diffraction, phase-contrast and dark-field imaging experiments. Their magnetic moment allows for resolving magnetic properties in bulk samples. This review will focus on recent applications of neutron imaging techniques in both materials research and fundamental science illustrated by examples selected from different areas.     

Crystallization of amorphous complex oxides: New geometries and new compositions via solid phase epitaxy

The crystallization of amorphous complex oxides via solid phase epitaxy enables a wide range of opportunities in the formation of oxide materials in new geometries and with previously inaccessible compositions. Emerging methods for controlling crystallization from the amorphous form arise from recent advances in the deposition of amorphous oxides, the formation and placement of crystalline seeds, and have built on an expanded understanding of the kinetics of nucleation and crystal growth. Key discoveries include methods for the creation of epitaxial layers in perovskite, spinel, and pyrochlore complex oxides. The creation of nanoscale homoepitaxial and heteroepitaxial seeds has the potential to enable new directions in the integration of complex oxides with semiconductors and in devices based on oxygen ion transport. Future opportunities include the creation of complex oxides in morphologies and compositions exhibiting electronic, thermal, and magnetic phenomena enabling a variety of applications.     

Recent advances in iron-based superconductors toward applications

Iron with a large magnetic moment was widely believed to be harmful to the emergence of superconductivity because of the competition between the static ordering of electron spins and the dynamic formation of electron pairs (Cooper pairs). Thus, the discovery of a high critical temperature (T_c) iron-based superconductor (IBSC) in 2008 was accepted with surprise in the condensed matter community and rekindled extensive study globally. IBSCs have since grown to become a new class of high-T_c superconductors next to the high-T_c cuprates discovered in 1986. The rapid research progress in the science and technology of IBSCs over the past decade has resulted in the accumulation of a vast amount of knowledge on IBSC materials, mechanisms, properties, and applications with the publication of more than several tens of thousands of papers. This article reviews recent progress in the technical applications (bulk magnets, thin films, and wires) of IBSCs in addition to their fundamental material characteristics. Highlights of their applications include high-field bulk magnets workable at 15–25 K, thin films with high critical current density (J_c) > 1 MA/cm² at ～10 T and 4 K, and an average J_c of 1.3 × 10⁴ A/cm² at 10 T and 4 K achieved for a 100-m-class-length wire. These achievements are based on the intrinsically advantageous properties of IBSCs such as the higher crystallographic symmetry of the superconducting phase, higher critical magnetic field, and larger critical grain boundary angle to maintain high J_c. These properties also make IBSCs promising for applications using high magnetic fields.     

Highly enhanced and stable activity of defect-induced titania nanoparticles for solar light-driven CO2 reduction into CH4

Photocatalytic reduction of CO₂ to fuel offers an exciting opportunity for helping to solve current energy and global warming problems. Although a number of solar active catalysts have been reported, most of them suffer from low product yield, instability, and low quantum efficiency. Therefore, the design and fabrication of highly active photocatalysts remains an unmet challenge. In the current work we utilize hydrogen-doped, blue-colored reduced titania for photocatalytic conversion of CO₂ into methane (CH₄). The photocatalyst is obtained by exposure of TiO₂ to NaBH₄ at 350 °C for 0.5 h. Sensitized with Pt nanoparticles, the material promotes solar spectrum photoconversion of CO₂ to CH₄ with an apparent quantum yield of 12.40% and a time normalized CH₄ generation rate of 80.35 μmol g^?1 h^?1, which to the best of our knowledge is a record for photocatalytic-based CO₂ reduction. The material appears intrinsically stable, with no loss in sample performance over five 6 h cycles, with the sample heated in vacuum after each cycle.     

From flat sheets to curved geometries: Origami and kirigami approaches

Transforming flat sheets into three-dimensional structures has emerged as an exciting manufacturing paradigm on a broad range of length scales. Among other advantages, this technique permits the use of functionality-inducing planar processes on flat starting materials, which after shape-shifting, result in a unique combination of macro-scale geometry and surface topography. Fabricating arbitrarily complex three-dimensional geometries requires the ability to change the intrinsic curvature of initially flat structures, while simultaneously limiting material distortion to not disturb the surface features. The centuries-old art forms of origami and kirigami could offer elegant solutions, involving only folding and cutting to transform flat papers into complex geometries. Although such techniques are limited by an inherent developability constraint, the rational design of the crease and cut patterns enables the shape-shifting of (nearly) inextensible sheets into geometries with apparent intrinsic curvature. Here, we review recent origami and kirigami techniques that can be used for this purpose, discuss their underlying mechanisms, and create physical models to demonstrate and compare their feasibility. Moreover, we highlight practical aspects that are relevant in the development of advanced materials with these techniques. Finally, we provide an outlook on future applications that could benefit from origami and kirigami to create intrinsically curved surfaces.     

Enhanced wetting and properties of carbon/carbon-Cu composites with Cr3C2 coatings by Cr-solution immersion method

A facile ammonium-dichromate solution immersion method was introduced to synthesize the copperwettable Cr_3C_2 coating on and inside the carbon-carbon(C/C) preform. The formation mechanism and the microstructures of the Cr_3C_2 coatings were studied. The contact angle between molten copper and the C/C decreased from 140? to 60?, demonstrating the significant improvement in the wettability. The Cr_3C_2-coated C/C-Cu composite with only 4.2% porosity and 3.69 g cm~(-3) density was manufactured through copper infiltration. As a result, the thermal and electrical conductivity of the modified C/C-Cu increased significantly due to the infiltrated copper. Also the mechanical properties of the composites including both the flexural and compressive strengths were enhanced by over 100%. The modified C/C-Cu composite exhibited lower friction coefficients and wear rates for different load levels than those of the commercial C/Cu composite. These results demonstrate the potential of the modified C/C-Cu material for use in electrical contacts.     

Singlet oxygen evolution from layered transition metal oxide cathode materials and its implications for lithium-ion batteries

For achieving higher energy density lithium-ion batteries, the improvement of cathode active materials is crucial. The most promising cathode materials are nickel-rich layered oxides LiNi_xCo_yMn_zO₂ (NCM) and over lithiated NCM (often called HE-NCM). Unfortunately, the full capacity of NCM cannot be utilized due to its limited cycle-life at high state-of-charge (SOC), while HE-NCM requires high voltages. By operando emission spectroscopy, we show for the first time that highly reactive singlet oxygen is released when charging NCM and HE-NCM to an SOC beyond ≈80%. In addition, on-line mass-spectrometry reveals the evolution of CO and CO₂ once singlet oxygen is detected, providing significant evidence for the reaction between singlet oxygen and electrolyte to be a chemical reaction. It is controlled by the SOC rather than by potential, as would be the case for a purely electrochemical electrolyte oxidation. Singlet oxygen formation therefore imposes a severe challenge to the development of high-energy batteries based on layered oxide cathodes, shifting the focus of research from electrochemically stable 5?V-electrolytes to chemical stability toward singlet oxygen.     

Three-dimensional printing of multilayered tissue engineering scaffolds

The field of tissue engineering has produced new therapies for the repair of damaged tissues and organs, utilizing biomimetic scaffolds that mirror the mechanical and biological properties of host tissue. The emergence of three-dimensional printing (3DP) technologies has enabled the fabrication of highly complex scaffolds that offer a more accurate replication of native tissue properties and architecture than previously possible. Of strong interest to tissue engineers is the construction of multilayered scaffolds that target distinct regions of complex tissues. Musculoskeletal and dental tissues in particular, such as the osteochondral unit and periodontal complex, are composed of multiple interfacing tissue types, and thus benefit from the usage of multilayered scaffold fabrication. Traditional 3DP technologies such as extrusion printing and selective laser sintering have been used for the construction of scaffolds with gradient architectures and mixed material compositions. Additionally, emerging bioprinting strategies have been used for the direct printing and spatial patterning of cells and chemical factors, capturing the complex organization found in the body. To better replicate the varied and gradated properties of larger tissues, researchers have created scaffolds composed of multiple materials spanning natural polymers, synthetic polymers, and ceramics. By utilizing high-precision 3DP techniques and judicious material selection, scaffolds can thus be designed to address the regeneration of previously challenging musculoskeletal, dental, and other heterogeneous target tissues. These multilayered 3DP strategies show great promise in the future of tissue engineering.     

Fabrication of soft,stimulus-responsive structures with sub-micron resolution via two-photon polymerization of poly(ionic liquid)s

Soft, stimulus-responsive 3D structures created from crosslinked poly(ionic liquid)s (PILs) have been fabricated at unprecedented sub-micron resolution by direct laser writing (DLW). These structures absorb considerable quantities of solvent (e.g., water, alcohol, and acetone) to produce PIL hydrogels that exhibit stimulus-responsive behavior. Due to their flexibility and soft, responsive nature, these structures are much more akin to biological systems than the conventional, highly crosslinked, rigid structures typically produced using 2-photon polymerization (2-PP). These PIL gels expand/contract due to solvent uptake/release, and, by exploiting inherited properties of the ionic liquid monomer (ILM), thermo-responsive gels that exhibit reversible area change (30?±?3%, n?=?40) when the temperature is raised from 20?°C to 70?°C can be created. The effect is very rapid, with the response indistinguishable from the microcontroller heating rate of 7.4?°C?s^?1. The presence of an endoskeleton-like framework within these structures influences movement arising from expansion/contraction and assists the retention of structural integrity during actuation cycling.