Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2014"

Theoretical description of charge transport in disordered organic semiconductors

Abstract: Twenty years ago Heinz Bassler published in this journal the seminal review article on charge transport in disordered organic semiconductors [Phys. Status Solidi B 175, 15 (1993)], which has become one of the most popular references in this research field. Thanks to this paper, our understanding of charge transport in disordered organic materials has been essentially improved in the past two decades. New theoretical methods have been developed and new results on various phenomena related to charge transport in disordered organic materials have been obtained. The aim of the current review is to present these new theoretical methods and to highlight the most essential results obtained in their framework. While theoretical consideration in the article by Bassler was based on computer simulations, particular attention in the current review is given to the development of analytical theories. Dependences of charge carrier mobility and diffusivity on temperature, electric field, carrier concentration and on material and sample parameters are discussed in detail. Schematic behaviour of charge carriers within the Gaussian density of states (DOS)

Interplay of Rashba/Dresselhaus spin splittings probed by photogalvanic spectroscopy –A review

Abstract: The paper reviews the interplay of Rashba/Dresselhaus spin splittings in various two-dimensional systems made of zinc-blende III–V, wurtzite, and SiGe semiconductors. We discuss the symmetry aspects of the linear and cubic in electron wavevector spin splitting in heterostructures prepared on (001)-, (110)-, (111)-, (113)-, (112)-, and (013)- oriented substrates and address the requirements for suppression of spin relaxation and realization of the persistent spin helix state. In experimental part of the paper, we overview experimental results on the interplay of Rashba/Dresselhaus spin splittings probed by photogalvanic spectroscopy: The method based on the phenomenological equivalence of the linear-in-wavevector spin splitting and several photogalvanic phenomena.

A genomic approach to the stability, elastic, and electronic properties of the MAX phases

Abstract: In this study, we report a comprehensive assessment on the elastic and electronic properties of 792 possible MAX (Mn+1AXn) phases with n = 1–4 using ab initio methods. These crystals are then screened based on their elastic and thermodynamic stability resulting in a large database of 665 viable crystals. All the experimentally verified MAX phases passed the screening. Various correlations among and between them are fully explored. In particular, the key elements in the interdependence between the elastic properties together with mechanical parameter derived from them and the electronic structure are identified. Detailed analysis of various correlation plots shows that there is a clear correspondence between bulk modulus K and total bond order density (TBOD). Calculations show a marked difference between the carbides and nitrides. This database is also used to test the efficacy of data mining algorithms for materials genome. We further identified several thermodynamically stable new MAX phases with unusual mechanical parameters that have never been synthesized in the laboratory or theoretically investigated. The complete database on the elastic and electronic structure together with the mechanical parameters for these 665 MAX phases compounds are included in the Supplementary Materials and fully accessible.

Simple cubic three-dimensional auxetic metamaterials

Abstract: Elastic instability of soft cellular solids plays an increasingly important role in the creation of metamaterials with smart properties. Inspiration for much of this research comes from a planar metamaterial with negative Poisson's ratio behavior induced by elastic instability. Here we extend the concept of buckling induced pattern switch further to the design of a new series of three-dimensional metamaterials with negative Poisson's ratio over a large strain range. The highlight of this work is that our designs are based on very simple initial geometric shapes.

Relaxor ferroelectrics: Cluster glass ground state via random fields and random bonds

Abstract: Quenched random fields (RFs) are well-known to be a basic driving force of the relaxor behavior in disordered ferroelectrics containing either random charges as in PbMg1/3Nb2/3O3 or random cation vacancies as in SrxBa1−xNb2O6. They give rise to the formation of polar nanoregions in the paraelectric regime, which freeze into a nanodomain state at low temperatures thus precluding the ferroelectric phase transition. On the other hand, isovalent relaxors like bond-disordered BaTi1−xZrxO3 prevalently experience random bonds (RBs), which are responsible for glassy features such as polydispersivity and non-ergodic aging and rejuvenation processes. Inspection shows, however, that all of the above characteristics apply generally to relaxor systems albeit with different weight. Subsequent formation of ferroic nanoregions via RFs followed by mesoscopic cluster glassy freezing via RBs upon cooling are universal signatures.

Poisson's ratio of rectangular anti-chiral structures with size dispersion of circular nodes

Abstract: Using Finite Element computer simulations, Poisson's ratio (PR) is determined for anti-chiral structures built on rectangular lattices with disorder introduced by stochastic distributions of circular node sizes. The investigated models are parameterized by the lattice anisotropy, the rib thickness, and the radii distribution of circular nodes. Three approaches are developed. The first approach, exact in the limit of infinitely large system and infinitely dense mesh, uses only planar elements (CPS3). Two other approaches are approximate and exploit one-dimensional elements utilizing the Timoshenko beam theory. It is shown that in the case of sufficiently large anisotropy of the studied structures PR can be highly negative, reaching any negative value, including those lower than . Thin ribs and thin-walled circular nodes favor low values of PR. In the case of thick ribs and thick-walled circular nodes PR is higher. In both cases the dispersion of the values of circular nodes radii has a minor effect on the lowest values of PR. A comparison of the results obtained with three different approaches shows that the Timoshenko beam based approximations are valid only in the thin rib limit. The difference between them grows with increasing thickness.

The physics of atomic‐scale friction: Basic considerations and open questions

Abstract: The field of friction is usually associated with its obvious practical importance. This tends to emphasize the engineering aspects of friction and thereby hides the fact that there is a wealth of interesting physics involved, part of which cannot be regarded as fully understood at present. New techniques, such as friction force microscopy, have started to provide access to the phenomenon of friction on the atomic scale. This has given a strong impulse to the field of tribology, pushing it significantly beyond the engineering level and into the regime of the fundamental aspects of frictional energy dissipation. This article reflects the authors’ personal view on matters of interest in the field of atomic-scale friction. Rather than to review important contributions in this field, we have chosen to summarize what has been learned and identify phenomena that may seem familiar to tribologists but actually should be regarded as non-trivial from a physical point of view.

Magnetocaloric effect in “reduced” dimensions: Thin films, ribbons, and microwires of Heusler alloys and related compounds

Abstract: Room temperature magnetic refrigeration is an energy saving and environmentally-friendly technology, which has developed rapidly from a basic idea to prototype devices. The performance of magnetic refrigerators crucially depends on the magnetocaloric properties and the geometry of the employed refrigerants. Here we review the magnetocaloric properties of Heusler alloys and related compounds with a high surface to volume ratio such as films, ribbons, and microwires, and compare them with their bulk counterparts.

3D auxetic warp‐knitted spacer fabrics

Abstract: Auxetic fabrics have gained an increasing attention in the auxetic material area in recent years. Especially, auxetic fabrics fabricated based on both weft-knitted and warp-knitted structures have made some important progress. However, all the knitted auxetic fabrics reported so far are fabricated based on 2D knitted structures, although some of the weft-knitted auxetic fabrics are finally formed into 3D forms. This paper reports a novel kind of 3D auxetic fabric fabricated based on a warp-knitted spacer structure with adoption of a special geometrical configuration which is formed with parallelograms. The auxetic behavior of these fabrics is discussed in terms of tensile direction, geometrical parameter, and repeated extension. The shape fitting ability on a spherical surface is also demonstrated. The results shows that the 3D auxetic fabrics have auxetic behavior in all the fabric plane directions and the highest auxetic effect is obtained when stretched in the weft direction of the fabric. The 3D auxetic fabrics also display an excellent shape fitting ability, which makes them very attractive for various applications where fitting to the human body's shape is highly required, such as bra cups, protectors for knees and elbows, shoes covers, etc.

The rise of spin noise spectroscopy in semiconductors: From acoustic to GHz frequencies

Abstract: This article gives an overview on the advance of spin noise spectroscopy (SNS) in semiconductors in the past 8 years from the first measurements in bulk n-GaAs [Oestreich et al., Phys. Rev. Lett. 95, 216603 (2005)] up to the recent achievement of optical detection of the intrinsic spin fluctuations of a single hole confined in an individual self-assembled quantum dot [Dahbashi et al., arXiv:1306.3183 (2013)]. We discuss the general technical implementation of optical SNS and the invaluable profit of the introduction of real-time fast Fourier transform analysis into the data acquisition. By now, the full spin dynamic from the milli- to picosecond timescales can be addressed by SNS and the technique quickly strides ahead to enable real quantum non-demolition measurements in semiconductors. Spin noise spectra recorded in 2005 in bulk n-GaAs with approximately 109 electron spins (Oestreich et al.) and 2013 (Dahbashi et al.) for a single hole spin. The integration time for the latter is more than a factor of 40 shorter due to the significant advances in the measurement technique.

Raman spectra for characterization of defective CVD multi‐walled carbon nanotubes

Abstract: Curved graphene fragments can be considered as building blocks of carbon nanotubes. Thus, the size of graphene fragments may be considered as one of the most important characteristics of their structural disorder. In this paper, we have performed a comparative Raman study of CVD multi-walled carbon nanotubes (MWCNTs) and graphene flakes deposited on MWCNTs. Raman data have been considered in combination with HRTEM characterization of nanotubes. The basic attention has been paid to the behavior of D (disorder-induced), G (tangential mode), and 2D (two-phonon scattering) bands in Raman spectra in order to use them for MWCNT characterization. A ratio of intensities of 2D and D bands (I2D/ID) demonstrates almost a linear dependence on the mean diameter of MWCNTs produced with two different types of catalysts (see abstract figure). It should be mentioned that each type of catalyst provides the linear dependence with its own specific slope. The graphene fragments have been proposed to form a mosaic structure of nanotube walls. I2D/ID ratio depends on the amount of graphene flakes deposited on nanotube surface via ethylene decomposition. A dependence of intensity ratio I2D/ID on the diameter of nanotubes produced with different types of catalysts.

Significant lattice thermal conductivity reduction following phase separation of the highly efficient GexPb1-xTe thermoelectric alloys

Abstract: In an attempt to enhance the thermal to electrical conversion efficiency, novel complicated thermoelectric alloys are constantly reported. Most of these reports, correlate the low lattice thermal conductivity values, attributing to the enhanced efficiencies, to nanofeatures apparent in their systems. Yet, since most of the highly efficient thermoelectric materials ever reported are based on complicated alloys, a major reduction of the lattice thermal conductivity can be solely attributed to alloying/disordering effects. The current manuscript, explores by combined experimental and theoretical, using density functional theory and analytical modeling, approaches the lattice thermal conductivity values originated solely by alloying/disordering effects in the highly thermoelectrically efficient p-type GexPb1–xTe alloys. By comparing these calculated results to various reported experimental values following different synthesis routes, it is shown that solution-treated samples fit well to the calculated values while for phase-separated samples, a significant lattice thermal conductivity reduction of ∼50% might be expected.

Localization effects and band gap of GaAsBi alloys

Abstract: The structural and optical properties of GaAs1−xBix alloys for x up to 0.108 have been investigated by high resolution X-ray diffraction and photoluminescence (PL). At room temperature (RT), the PL intensity of the GaAs0.97Bi0.03 sample was found to be ∼300 times higher than a GaAs control sample grown at the same temperature (400 °C). PL measurements carried out at 10 K show that when excitation power, Pex was increased from 0.11 to 1140 W cm−2, the PL peak energy blue-shifts by 80 meV while the full-width-at-half-maximum reduces from 115 to 63 meV. However, the PL peak emission energy becomes independent of the excitation power at RT. The results indicate the presence of localized energy states in the GaAs0.97Bi0.03 sample, which trap carriers at low temperatures and that the majority of the carriers become delocalized at RT. Furthermore, the temperature dependent PL also shows an S-shape behavior, which is a signature of localization effects. A theoretical model, which was derived by solving a rate equation was employed. The model successfully reproduces the observed S-shape behavior and the theory fits well with the experimental data. The RT band gap of GaAs1−xBix for x up to 0.108 has been plotted and compared with the literature.

Raman spectroscopy of graphite intercalation compounds: Charge transfer, strain, and electron–phonon coupling in graphene layers

Abstract: Graphite intercalation compounds (GICs) are an interesting and highly studied field since 1970’s. It has gained renewed interest since the discovery of superconductivity at high temperature for CaC and the rise of graphene. Intercalation is a technique used to introduce atoms or molecules into the structure of a host material. Intercalation of alkali metals in graphite has shown to be a controllable procedure recently used as a scalable technique for bulk production of graphene, and nano-ribbons by induced exfoliation of graphite. It also creates supra-molecular interactions between the host and the intercalant, inducing changes in the electronic, mechanical, and physical properties of the host. GICs are the mother system of intercalation also employed in fullerenes, carbon nanotubes, graphene, and carbon-composites. We will show how a combination of Raman and calculations of the density and the electronic band structure in GICs can serve as a tool to elucidate the electronic structure, electron–phonon coupling, charge transfer, and lattice parameters of GICs and the graphene layers within. This knowledge of GICs is of high importance to understand superconductivity and to set the basis for applications with GICs, graphene and other nano-carbon based materials like nanocomposites in batteries and nanoelectronic devices.

Strain glass and ferroic glass – Unusual properties from glassy nano‐domains

Abstract: The present article serves as a concise review of strain glass and its broader concept – ferroic glass. Strain glass is a frozen disordered strain state composed of nano-sized strain domains, which is formed due to the frustration created by point defects or dopants. Such frustration creates glassy nano-sized martensite-like domains that do not grow into a macroscopic martensite during cooling and instead show typical glass-freezing dynamics. Strain glass bears much resemblance with the glass phenomena found in other two types of ferroic systems, relaxor ferroelectric, and spin glass. These three ferroics-based glasses are thus called ferroic glasses. Characteristics of strain glass, including recent in situ high-resolution TEM images, are shown. Unusual properties associated with strain glass, such as superelasticity with narrow hysteresis, high-damping, and low modulus, as well as Invar and Elinvar effect in cold-rolled β-Ti alloys are demonstrated.

Lambda transitions in materials science: Recent advances in CALPHAD and first‐principles modelling

Abstract: This paper provides a comprehensive overview of state-of-the-art computational techniques to thermodynamically model magnetic and chemical order–disorder transitions. Recent advances as well as limitations of various approaches to these so-called lambda transitions are examined in detail, focussing on calphad models and first-principles methods based on density functional theory (DFT). On the one hand empirical implementations –based on the Inden–Hillert–Jarl formalism –are investigated, including a detailed interpretation of the relevant parameters, physical limiting cases and potential extensions. In addition, Bragg–Williams-based approaches as well as cluster-variation methods of chemical order–disorder transitions are discussed. On the other hand, it is shown how magnetic contributions can be introduced based on various microscopic model Hamiltonians (Hubbard model, Heisenberg model and beyond) in combination with DFT-computed parameters. As a result of the investigation we were able to indicate similarities between the treatment of chemical and magnetic degrees of freedom as well as the treatment within the calphad and DFT approaches. Potential synergy effects resulting from this overlap have been derived and alternative approaches have been suggested, in order to improve future thermodynamic modelling of lambda transitions.

Computational analysis of sandwich‐structured composites with an auxetic phase

Abstract: A sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin but stiff layers to a lightweight but thick core Composites analyzed in this paper consist of two different materials: auxetic and structural steel The optimization criterion is minimum compliance for the load case where the frame's top boundary is downward loaded Outer layers are made of steel while the middle layer is two-phase solid material composite Only the middle layer is optimized by means of minimization of the objective function defined as the internal strain energy In the first part of this paper we study the application of the solid isotropic material with penalization (SIMP) model to find the optimal distribution of a given amount of materials in sandwich-structured composite In the second part we propose a multilayered composite structure in which internal layers surfaces are wavy In both cases the total energy strain is analyzed

On the properties of real finite‐sized planar and tubular stent‐like auxetic structures

Abstract: Auxetics, i.e. systems with a negative Poisson's ratio, exhibit the unexpected property of becoming wider when stretched and narrower when compressed. This property arises from the manner in which the internal geometric units within the system deform when the system is submitted to a stress and may be explained in terms of ‘geometry–deformation mechanism’ based models. This work considers realistic finite implementations of the well known rotating squares system in the form of (i) a finite planar structure and (ii) a tubular conformation, as one typically finds in stents. It shows that although the existing models of the Poisson's ratios and moduli based on periodic systems may be appropriate to model systems where the geometry/deformation mechanism operate at the micro- or nano- (molecular) level where a system may be considered as a quasi infinite system, corrections to the model may need to be made when one considers finite structures with a small number of repeat units and suggests that for finite systems, especially for the 2D systems, the moduli as predicted by the periodic model may be significantly overestimating the moduli of the real system, even sometimes by as much as 200%.

Perspectives on point defect thermodynamics

Abstract: We review and discuss methods for including the role of point defects in calculations of the free energy, composition and phase stability of elements and compounds. Our principle aim is to explain and to reconcile, with examples, the perspectives on this problem that are often strikingly different between exponents of CALPHAD, and others working in the overlapping fields of physics, chemistry and materials science. Current methodologies described here include the compound energy formalism of CALPHAD, besides the rather different but related canonical and grand-canonical formalisms. We show how the calculation of appropriate defect formation energies should be formulated, how they are included in the different formalisms and in turn how these yield equilibrium defect concentrations and their contribution to free energies and chemical potentials. Furthermore, we briefly review the current state-of-the-art and challenges in determining point defect properties from first-principles calculations as well as from experimental measurements.

Thermodynamic modelling of crystalline unary phases

Abstract: Progress in materials science through thermodynamic modelling may rest crucially on access to a database, such as that developed by Scientific Group Thermodata Europe (SGTE) around 1990. It gives the Gibbs energy G(T)of the elements in the form of series as a function of temperature, i.e. essentially a curve fitting to experimental data. In the light of progress in theoretical understanding and first-principles calculation methods, the possibility for an improved database description of the thermodynamics of the elements has become evident. It is the purpose of this paper to provide a framework for such work. Lattice vibrations, which usually give the major contribution to G(T), are treated in some detail with a discussion of neutron scattering studies of anharmonicity in aluminium, first-principles calculations including ab initio molecular dynamics (AIMD), and the strength and weakness of analytic model representations of data. Similarly, electronic contributions to G(T) are treated on the basis of the density of states N(E) for metals, with emphasis on effects at high T. Further, we consider G(T) below 300 K, which is not covered by SGTE. Other parts in the paper discuss metastable and dynamically unstable lattices, G(T) in the region of superheated solids and the requirement on a database in the calculation of phase diagrams.

Experimental studies on the impact properties of auxetic materials

Abstract: This paper reports an experimental investigation on the effect of auxeticity towards impact loads. Both conventional and auxetic polyurethane (PU) foams were subjected to impact drop tests, and the damaged surfaces were visually inspected. Although it is known that auxetic materials resist indentation due to densification along the load line, experimental impact tests on conventional and auxetic PU foams reveal that auxetic foams are not necessarily advantageous over conventional ones for very high impact. This paper proposes a geometrical model that resembles the microstructure of auxetic foams, as well as elucidates both the auxetic behavior and the limiting strength at high impact.

Nanocomposites of epoxy resin with graphene nanoplates and exfoliated graphite: Synthesis and electrical properties

Abstract: Nanocomposites are nowadays one of the most promising materials. Among different fillers, e.g. carbon nanotubes and silicon carbide nanowires (NWSiC), already used with epoxy resin matrices, graphene exfoliated graphite (EG) and graphene nanoplates have some characteristics that make them unique for electromagnetic shielding materials. However, there is still an unresolved problem of proper dispersion that will ensure the homogeneity of samples. To overcome this drawback, inorganic fibres were proposed. An amount of 0.25 phr (parts per hundred; filler content presented as wt.% of the whole polymeric matrix) NWSiC, added to the EG 1 phr/epoxy resin sample, efficiently prevents filler agglomeration. NWSiC were obtained in combustion synthesis and EG was produced from intercalated graphite using microwaves. Finally, nanocomposites with EG from self-propagating high-temperature synthesis were also tested. The properties of the samples have been characterized, revealing an observable improvement of pure resin features. Raw products of combustion synthesis of Si/polytetra-fluoroethylene (PTFE) stoichiometric mixture, carried out in the larger of the presented reactors.

Modeling auxetic foams through semi‐rigid rotating triangles

Abstract: Auxetic foams have been widely studied in view of their superior properties and many useful applications and various models have been developed to help explain the auxetic behavior in such foams. One such model involves the description of auxetic foams in terms of rotating units (e.g. the joints where different cell walls meet), a mechanism, which has also been observed experimentally. In the models, the rotating units are taken, to a first approximation, to be representable through rotating rigid triangles, which correspond to the 2D projection of these rotating units and although this model has been improved significantly since it was first proposed, current models still do not fully capture all the deformations that may occur in real foams. In this work, we propose an extended model which not only allows for relative rotation of the units (joints), represented by non-equilateral triangular units, but also for differing amount of material at the joints as well as deformation of the joints themselves, a scenario that is more representative of real auxetic foams. This model shows that, by permitting deformation mechanisms other than rotation of the triangles, the predicted extent of auxeticity decreases when compared to the equivalent idealized rotating rigid triangles model, thus resulting in more plausible predictions of the Poisson's ratios. Furthermore, it is shown that in the manufacturing process, a minimum compression factor, which is dependent on the amount of materials at the joints, is required to obtain an auxetic foam from a conventional foam, as one normally observed in experimental work on foams.

Mid‐IR quantum cascade lasers: Device technology and non‐equilibrium Green's function modeling of electro‐optical characteristics

Abstract: In this paper, we report the results of investigation of 9.5 µm AlGaAs/GaAs and strain compensated 4.7 µm AlInAs/InGaAs/InP QCLs. We also show the results for 9.5 µm lasers based on lattice matched AlInAs/InGaAs/InP structures. The developed GaAs/AlGaAs lasers show the record pulse powers of 6 W at 77 K and up to 50 mW at 300 K. This has been achieved by careful optimization of the MBE growth process and by applying a high reflectivity metallic coating to the back facet of the laser. The 9.5 µm AlInAs/InGaAs/InP lasers utilize AlInAs waveguide and were grown exclusively by MBE without MOCVD regrowth. The short wavelength, strain compensated QCLs were grown by MOCVD. They represent state-of-the-art parameters for the devices of their design. For epitaxial process control, the atomic-force microscopy (AFM), high resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM) were used to characterize the morphological and structural properties of the layers. The basic electro-optical characterization of the lasers is provided. We also present results of Green's function modeling of mid-IR QCLs and demonstrate the capability of non-equilibrium Green's function (NEGF) approach for sophisticated but still computationally effective simulation of laser's characteristics.

On the suitability of hexagonal honeycombs as stent geometries

Abstract: It is a well-known fact that the mechanical properties of coronary stents are defined mainly by two components, the constituting material and the design pattern of the stent itself. The latter especially has attracted the interest of entrepreneurs and scientists alike with a plethora of patents being issued for numerous stent designs. Despite this widespread interest, the suitability of said designs are seldom studied. Accordingly, in this work we have investigated the properties of stent designs based on the hexagonal honeycomb geometry. Stent patterns based upon re-entrant, non re-entrant, and hybrid honeycomb geometries were studied with respect to their behaviour at extremely high strains using Finite Element Analysis. The data collected indicates that although the non re-entrant and hybrid geometries may be more suited to stent designs than the re-entrant geometry in terms of tolerance to high strains, none of these systems convey all the ideal properties desired in a stent, even if the former two have the potential of exhibiting some of them.

Weft‐knitted auxetic textile design

Abstract: This paper examines a novel approach to designing auxetic textiles. Knit design techniques are used to create auxetic weft-knitted textiles. Development of the appropriate fabrics addresses aesthetic, tactile and subjective design qualities alongside questions of auxetic functionality and testing considerations. The auxetic fabrics produced exhibit expansion in the X- or Y-axis (aligning axes to direction of knitting). The results range from neutral Poisson's ratio to those with a marked expansion in relation to their extension. The fabric discussed uses relief stitch structures and transferred stitches. Variant auxetic knitted fabrics have been produced including related stitch structures and spacer fabrics using the same design processes. The auxetic effects are inspired by common auxetic geometries from the literature. This research aims to combine the methodologies used in knit design and knit engineering to create functional auxetic textiles by using the methods innate to a more traditionally aesthetic design subject. Using quantitative methods to measure the auxetic effect in the samples and qualitative personal reflections on samples, results are kept simple in order to promote dissemination amongst practitioners in different fields. Utilising focus groups with practitioners from various design fields it is also possible to assess the visual, tactile and commercial appeal of the auxetic fabrics whilst they are still in the developmental stage. Using these different methods of feedback into the design stage the theoretical outcome remains simple in terms of language and results stated. The importance of knowledge transfer between disciplines of design and engineering is key to this study. Data capture is imperative encompassing qualitative and quantitative perspectives in conjunction with the use of generically specified terminology. The knowledge transfer afforded by these collaborations is a key output to support the methods chosen for fabric development, those which cross disciplines and appeal to a wide market. Along with an open and experimental approach to conceiving product applications, this will help to place auxetic materials in public view in areas that are novel to the field of auxetic research.

Deconstructing the auxetic behavior of paper

Abstract: The objective of this research is to understand more fully the out-of-plane auxetic behavior of paper. In the reported research, initial characterization of the auxetic properties of common, commercially available papers is described. The results of these experiments suggest that the auxetic response seen in paper structures is related to the cellulose fiber network structure, as previously suggested, and is related to inter-fiber bonding and fiber organization in the sheet. These results also indicate that further study is needed to understand how materials and processing variables influence auxetic behavior. A mathematical model, attempting to explain auxetic response in an idealized arrangement of fibers is included. Ultimately, the fundamental understanding resulting from this research should lead to new product development opportunities for forest product-based paper materials as well as to the establishment of predictive structure–property relations for auxetic materials using paper as a model system.

Auxetic properties of cubic metal single crystals

Abstract: The behaviour of the Poisson's ratio is investigated on the basis of derived by the authors explicit equations determining the components of the compliance tensor in an arbitrarily rotated coordinate frame. Clear analytical expressions describing the angular dependencies of the auxetic properties have been found and the crystallographic regions possessing such auxetic behaviour have been revealed for single crystals of elemental metals. This was completed with the straightforward expressions for calculation of the maximum values of auxeticity. The connection between the auxetic properties and the level of anisotropy of single crystals as well as the Cauchy relation has been analysed for various metals with BCC and FCC lattices.

Interacting magnetic cluster-spin glasses and strain glasses in Ni-Mn based Heusler structured intermetallics

Abstract: Magnetic Ni–Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni–Co–Mn–Sn on the right-hand side). The cubic austenitic phase at high temperature shows long-range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non-modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long-range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non-magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape-memory and exchange-bias effects as well as magnetocaloric effects. The ‘ferroic cooling’ is of particular relevance since it avoids the use of ozone-depleting and greenhouse chemicals compared with conventional fluid-compression technology. Experimental phase diagram of for . Here, is the Curie temperature of austenite; at the system transforms to paramagnetic martensite and at to superparamagnetic martensite (SPM) and then to superspin-glass martensite (SSG) at . The possible strain-glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field-cooling/zero-field-cooling experiments.

Thermodynamic modelling of liquids: CALPHAD approaches and contributions from statistical physics

Abstract: We describe current approaches to thermodynamic modelling of liquids for the CALPHAD method, the use of available experimental methods and results in this type of modelling, and considerations in t ...