Pavel Cizek

Facile Preparation and Thermoelectric Properties of Bi2Te3 Based Alloy Nanosheet/PEDOT:PSS Composite Films

Bi2Te3 based alloy nanosheet (NS)/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) composite films were prepared separately by spin coating and drop casting techniques. The drop cast composite film containing 4.10 wt % Bi2Te3 based alloy NSs showed electrical conductivity as high as 1295.21 S/cm, which is higher than that (753.8 S/cm) of a dimethyl sulfoxide doped PEDOT:PSS film prepared under the same condition and that (850-1250 S/cm) of the Bi2Te3 based alloy bulk material. The composite film also showed a very high power factor value, ∼32.26 μWm(-1) K(-2). With the content of Bi2Te3 based alloy NSs increasing from 0 to 4.10 wt %, the electrical conductivity and Seebeck coefficient of the composite films increase simultaneously.

A mechanism of ferrite softening in a duplex stainless steel deformed in hot torsion

A mechanism of dynamic softening of ferrite was studied in a 21Cr-10Ni-3Mo austenite/ferrite duplex stainless steel subjected to torsion at a strain rate of 0.7 s−1 at 1200°C. Transmission electron microscopy together with convergent beam electron diffraction were used with major emphasis on the study of misorientations across ferrite/ferrite boundaries. No evidence of discontinuous dynamic recrystallisation involving nucleation and growth of new grains was found within ferrite contrary to some suggestions made in the literature for similar experimental conditions. The softening mechanism has been classified as extended dynamic recovery characterised by a gradual increase in misorientations between neighbouring subgrains that were created by dynamic recovery processes at the earlier stages of deformation. The resulting dislocation substructure was a complex network of subgrain boundaries composed of a mix of higher- and lower-angle walls characterised by misorientation angles not exceeding 20° at a maximum obtained strain of 1.3.

High-Performance Supercapacitor Electrode Materials from Cellulose-Derived Carbon Nanofibers

Nitrogen-functionalized carbon nanofibers (N-CNFs) were prepared by carbonizing polypyrrole (PPy)-coated cellulose NFs, which were obtained by electrospinning, deacetylation of electrospun cellulose acetate NFs, and PPy polymerization. Supercapacitor electrodes prepared from N-CNFs and a mixture of N-CNFs and Ni(OH)2 showed specific capacitances of ∼236 and ∼1045 F g(-1), respectively. An asymmetric supercapacitor was further fabricated using N-CNFs/Ni(OH)2 and N-CNFs as positive and negative electrodes. The supercapacitor device had a working voltage of 1.6 V in aqueous KOH solution (6.0 M) with an energy density as high as ∼51 (W h) kg(-1) and a maximum power density of ∼117 kW kg(-1). The device had excellent cycle lifetime, which retained ∼84% specific capacitance after 5000 cycles of cyclic voltammetry scans. N-CNFs derived from electrospun cellulose may be useful as an electrode material for development of high-performance supercapacitors and other energy storage devices.

Deformation modes and anisotropy in magnesium alloy AZ31

Abstract A strongly textured sheet of magnesium alloy AZ31 has been subjected to tensile testing at temperatures between ambient and 300 °C. Structures have been examined by optical and transmission electron microscopy and also by atomic force microscopy to quantify surface displacements seen at grain boundaries. Plastic anisotropy varies strongly with test temperature as was observed previously by Agnew and Duygulu. The present findings do not support the view that crystallographic becomes a major contributor to deformation at higher temperatures. Rather, the material behaviour reflects an increasing contribution from grain boundary sliding despite the relatively high strain rate (10– 3 s– 1) used in the mechanical tests.

EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steel.

The evolution of crystallographic texture and deformation substructure was studied in a type 316L austenitic stainless steel, deformed in rolling at 900 °C to true strain levels of about 0.3 and 0.7. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. At the lower strain level, the deformation substructure observed by EBSD appeared to be rather poorly developed. There was considerable evidence of a rotation of the pre‐existing twin boundaries from their original orientation relationship, as well as the formation of highly distorted grain boundary regions. In TEM, at this strain level, the substructure was more clearly revealed, although it appeared rather inhomogeneously developed from grain to grain. The subgrains were frequently elongated and their boundaries often approximated to traces of {111} slip planes. The corresponding misorientations were small and largely displayed a non‐cumulative character. At the larger strain, the substructure within most grains became well developed and the corresponding misorientations increased. This resulted in better detection of sub‐boundaries by EBSD, although the percentage of indexing slightly decreased. TEM revealed splitting of some sub‐boundaries to form fine microbands, as well as the localized formation of microshear bands. The substructural characteristics observed by EBSD, in particular at the larger strain, generally appeared to compare well with those obtained using TEM. With increased strain level, the mean subgrain size became finer, the corresponding mean misorientation angle increased and both these characteristics became less dependent on a particular grain orientation. The statistically representative data obtained will assist in the development of physically based models of microstructural evolution during thermomechanical processing of austenitic stainless steels.

EBSD study of the orientation dependence of substructure characteristics in a model Fe−30wt%Ni alloy subjected to hot deformation

The aim of the present investigation was to determine the orientation dependence of substructure characteristics in an austenitic Fe−30wt%Ni model alloy subjected to hot plane strain compression. Deformation was carried out at a temperature of 950 °C using a strain rate of 10 s−1 to equivalent strain levels of approximately 0.2, 0.4, 0.6 and 0.8. The specimens obtained were analysed using a fully automatic electron backscatter diffraction technique. The crystallographic texture was characterized for all the strain levels studied and the subgrain structure was quantified in detail at a strain of 0.4. The substructure characteristics displayed pronounced orientation dependence. The major texture components, namely the copper, S, brass, Goss and rotated Goss, generally contained one or two prominent families of parallel larger‐angle extended subboundaries, the traces of which on the longitudinal viewing plane appeared systematically aligned along the {111} slip plane traces, bounding long microbands subdivided into slightly elongated subgrains by short lower‐angle transverse subboundaries. Relatively rare cube‐orientated grains displayed pronounced subdivision into coarse deformation bands containing large, low‐misorientated subgrains. The misorientation vectors across subboundaries largely showed a tendency to cluster around the sample transverse direction. Apart from the rotated Goss texture component, the stored energy levels for the remaining components were principally consistent with the corresponding Taylor factor values.

Characteristics of shear bands formed in an austenitic stainless steel during hot deformation

A detailed investigation of the substructural characteristics of shear bands, formed in an austenitic stainless steel during deformation in torsion at 900°C, with particular emphasis on their nucleation stages, has been undertaken. Shear bands forming at large strains in a matrix of pre-existing microbands are composed of well-recovered, slightly elongated cells. These bands propagate along a similar macroscopic path and the cells, present within their substructure, are rotated relative to the surrounding matrix about axes close to a common macroscopic direction. The cell boundaries are frequently non-crystallographic, suggesting that the cells might often form through the operation of multiple slip. Shear bands appear to form through a cooperative nucleation of originally isolated cells that gradually interconnect with each other to form long, thin bands that subsequently thicken via the formation of new cells. When crossing a shear band, cumulative misorientations across consecutive cell boundaries display a typical sigmoidal profile, characterised by a gradual increase of misorientation angles to a peak value followed by a subsequent gradual decrease towards the matrix orientation. The formation of new cells thus appears to be assisted by the stress fields generated by lattice rotations of the previously formed cells.

EBSD investigation of the microstructure and texture characteristics of hot deformed duplex stainless steel

The microstructure and crystallographic texture characteristics were studied in a 22Cr‐6Ni‐3Mo duplex stainless steel subjected to plastic deformation in torsion at a temperature of 1000 °C using a strain rate of 1 s−1. High‐resolution EBSD was successfully used for precise phase and substructural characterization of this steel. The austenite/ferrite ratio and phase morphology as well as the crystallographic texture, subgrain size, misorientation angles and misorientation gradients corresponding to each phase were determined over large sample areas. The deformation mechanisms in each phase and the interrelationship between the two are discussed.

{101¯2} Twinning in magnesium-based lamellar microstructures

The magnesium-based alloy Mg–9Al–1Zn has been extruded and heat treated to produce a dense population of lamellar plate-shaped particles. In compression with a testing orientation well aligned for prolific { 1 0 1 ¯ 2 } twinning, precipitation resulted in a significant increase in the yield point, but there was no change in the volume fraction of twins that were produced. It is proposed that the larger number of smaller twins observed in the aged condition is the result of inhibition of twin growth by the particles.

Comparison of the deformation characteristics of a Ni-30wt% Fe alloy and plain carbon steel

A major barrier confronting researchers studying the hot deformation of plain and low carbon steels is the inability to directly observe the deformation microstructures of hot worked austenite due to the unavoidable transformation to martensite on quenching to room temperature. Various model materials, such as austenitic stainless steels have been used to overcome this difficulty. However, these materials have markedly different stacking fault energies from plain and low carbon steels and this will affect the evolving deformation structure. In this work, a model austenitic Ni-30wt%Fe alloy, calculated to have a stacking fault energy similar to that of low carbon steel, has been tested using hot compression. Stress-strain curves obtained during hot deformation show characteristics similar to those generated during identical tests on a 0.15wt%C steel. This suggests that the two materials behave similarly during deformation under similar experimental conditions. An application of the Ni-Fe alloy in the study of microstructural changes in austenite during hot deformation is demonstrated. A series of hot torsion experiments on a 0.11wt%C steel have been found to produce deformation-induced, intragranular nucleation of ferrite from austenite when a single deformation pulse is applied at 675°C. A similar set of experiments have also been performed on the Ni-Fe alloy at 750°C. Transmission electron microscopy carried out on the Ni-Fe alloy torsion specimens has revealed that likely preferred sites for intragranular ferrite nucleation appear to be microbands produced in the austenite during deformation.