Rostislav Medlín

The effect of annealing temperature on the properties of powder metallurgy processed Ti-35Nb-2Zr-0.5O alloy

Ti-35Nb-2Zr-0.5O (wt%) alloy was prepared via a powder metallurgy process (cold isostatic pressing of blended elemental powders and subsequent sintering) with the primary aim of using it as a material for bio-applications. Sintered specimens were swaged and subsequently the influence of annealing temperature on the mechanical and structural properties was studied. Specimens were annealed at 800, 850, 900, 950, and 1000°C for 0.5h and water quenched. Significant changes in microstructure (i.e. precipitate dissolution or grain coarsening) were observed in relation to increasing annealing temperature. In correlation with those changes, the mechanical properties were also studied. The ultimate tensile strength increased from 925MPa (specimen annealed at 800°C) to 990MPa (900°C). Also the elongation increased from ~ 13% (800°C) to more than 20% (900, 950, and 1000°C).

Optical properties of zinc titanate perovskite prepared by reactive RF sputtering

Abstract In this paper we report results from optical transmittance spectroscopy complemented with data on structure from XRD measurements to determine optical properties of a series of ZnTiO3 perovskite thin films deposited on glass by reactive magnetron co-sputtering. The members of the series differ by the titanium content that was revealed as an origin of the changes not only in structure but also in dispersive optical properties. Low porosity has been discovered and calculated using the Bruggeman effective medium approximation. An apparent blue-shift of the optical band gap energies with increasing titanium content was observed. The observed band gap engineering is a good prospective for eg optoelectronic and photocatalytic applications of ZnTiO3.

Dispersible cobalt chromite nanoparticles: facile synthesis and size driven collapse of magnetism

Multiferoic oxides have enormous application potential thanks to the mutually coupled magnetic and dielectric response embedded in a standalone material. Among them, the unique case is the cobalt chromite with spin-induced electric polarization locked to the magnetization direction and propagation vector of the spiral magnetic phase. The nature of the ground state magnetic structure gives rise to complex size dependent magnetic behaviour, which collapses when reaching a critical particle size. In our work, we focused on preparation of standalone cobalt chromite nanoparticles (NPs). We introduced hydrothermal decomposition of cobalt(II)/chromium(III) oleates in a water/ethanol system without need of additional thermal treatment, which lead to stable cobalt chromite nanoparticles with diameter of 3.0(1)–4.2(1) nm and log normal size distribution of 12–16%. The size at the edge of the critical limit was tuned by the reaction temperature reaching typically 240 ± 10 °C. The as-prepared NPs are coated with covalently bonded oleic acid and can be easily dispersed in non-polar solvents, which makes them excellent candidates for custom surface modifications. The NPs were studied using a large number of characterization techniques: powder X-ray diffraction, transmission electron microscopy, small-angle X-ray scattering, and vibrational spectroscopies. The impact of the size effect on the magnetic properties was also investigated by temperature and magnetic field dependent magnetization, a.c. susceptibility and diffuse neutron scattering. The onset of the collective glassy state due to the collapse of long range conical magnetic order was observed. The uniform cobalt chromite NPs coated with oleic acid with size of 3–4 nm are excellent prototypes for studying the size effect on magnetic (and feroic) materials, and can be subjected to manifold surface functionalization required for their embedding in smart nanostructures and nanocomposites.

XRD, SAXS, and PALS investigations of three different polymers reinforced with tetraoctylammonium exchanged montmorillonite

ABSTRACT Three different polymer nanocomposites were prepared using clay modified with tetraoctylammonium (4C8) surfactant. The dispersion of clay silicate layers was studied using X-ray diffraction and small-angle scattering. Free-volume cavity sizes were studied with positron annihilation lifetime spectroscopy. Scattering methods confirmed disruption of all polymer lamellae organization upon organoclay addition and the creation of partially intercalated systems for polyamide and polycaprolactone. The presence of organoclay in the polymers enlarges the lower value of average positronium lifetime τ3 of polyamide and reduces the higher value of τ3 for polycaprolactone and polyethylene.

TaS3 Nanofibers: Layered Trichalcogenide for High-Performance Electronic and Sensing Devices

Layered materials, like transition metal dichalcogenides, exhibit broad spectra with outstanding properties with huge application potential, whereas another group of related materials, layered transition metal trichalcogenides, remains unexplored. Here, we show the broad application potential of this interesting structural type of layered tantalum trisulfide prepared in a form of nanofibers. This material shows tailorable attractive electronic properties dependent on the tensile strain applied to it. Structure of this so-called orthorhombic phase of TaS3 grown in a form of long nanofibers has been solved and refined. Taking advantage of these capabilities, we demonstrate a highly specific impedimetric NO gas sensor based on TaS3 nanofibers as well as construction of photodetectors with excellent responsivity and field-effect transistors. Various flexible substrates were used for the construction of a NO gas sensor. Such a device exhibits a low limit of detection of 0.48 ppb, well under the allowed value set by environmental agencies for NOx (50 ppb). Moreover, this NO gas sensor also showed excellent selectivity in the presence of common interferences formed during fuel combustion. TaS3 nanofibers produced in large scale exhibited excellent broad application potential for various types of devices covering nanoelectronic, optoelectronic, and gas-sensing applications.

Formation of Cu1-xGex Nanoplatelets Using LPCVD of Ge2Me6 or Ge2Me6/Et4Pb Mixture

Unlike synthesis of nanowires (1D nano-objects) the synthesis of nanoplatelets (2D nano-objects) has not been performed frequently. Herein, we report on the synthesis of Cu–Ge based on nanoplatelets with a high surface-to-volume ratio prepared by low pressure chemical vapor deposition (LPCVD) of Ge2Me6 and a mixture of Ge2Me6/PbEt4. Nanostructured deposits are composed of Cu1-xGex nanoplatelets, Ge nanowires and Ge nanoparticles. The nanoplatelets, which have the lateral size up to several tens of micrometers and thickness of 100–400 nm, belong to the cubic α phase of Cu91Ge9 alloy (Ge admixture in cubic Cu) and hexagonal ζ phase of Cu85Ge15 alloy. Nanowires composed of cubic Ge have a diameter of about 30 nm and length of several tens of micrometers. Lead does not enter any of these phases due to Pb–Cu and Pb–Ge immiscibility; therefore, it was observed as separate nanoparticles.

Transition from A-Si:H to Si 3 N 4 in thin films deposited by PECVD technology from silane diluted with nitrogen

Series of a-SiN:H thin films similar in thickness (380 ± 10 nm) and a-Si:H/a-Si3N4 multi-layered films (515 ± 20 nm) were prepared by PECVD technology from silane mixed with argon (90 % Ar/10 % SiH4) on Corning Eagle 2000 glass, SiO2 and silicon substrates. Deposition of thin films was carried out on a Samco PD 220 NA PECVD system. Multi-layered films were consequently annealed at high temperatures (700-1100°C) in order to obtain silicon nanocrystals embedded in a dielectric matrix suitable for photovoltaic and photonic applications. X-ray diffraction, Raman and FTIR spectroscopies, TEM, SEM, UV Vis spectrophotometry and spectroscopic ellipsometry were used for the evaluation of material properties of the films.

Self-Texture Control of ZnO Films Prepared by Reactive RF Magnetron Sputtering

Zinc Oxide (ZnO) is a wide bandgap semiconductor material which can be successfully used for wide variety of potential applications such as biosensors or acoustic resonator devices. ZnO normally crystallizes in the wurtzite structure with c-axis (001) preferred orientation. However, for bio-sensing in liquids, it is necessary to generate a shear horizontal mode wave, where the wave displacement is within the plane of the crystal surface. For generation of such a shear horizontal wave, a-axis film textures such as the (110) or (100) is necessary. This work is focused on the preferred orientation control of ZnO film prepared by RF magnetron sputtering. It is found that preferred orientation can be controlled by substrate bias and substrate temperature during deposition without the use of expensive crystalline substrates. There are three areas of operating parameters when the structure of the ZnO films is dominated by different preferred orientation. Moreover, the film annealing was performed to enhance the film structure.

Optical and contact non-destructive measurement of the laser re-melting layers

Laser beam of the infrared pulsed Nd:YAG laser was used to re-melting PVD coatings on the steel substrates. Chemical composition of these layers contains carbide Cr3C2 with alloy NiCr or nitrides TiN, TiAlN, TiAlSiN and CrAlSiN. First coatings were prepared by method of high velocity oxygen fuel (HVOF) that protects the machine component surfaces from abrasion, corrosion or ensures thermal isolation, nitrides by PVD (Physical Vapor Deposition). Processing parameters such as pulse energy, pulse length and frequency were optimized in many experiments to achieve the sufficient surface energy density to melting without vaporization of the material. Multimode beam diameters about some millimetres were computed and adjusted in the suitable distance from focus plane. High laser power re-melting decreases their porosity, increases adhesion to basic material. In case of high laser energy gas vapours escape from basic material and cause fissures, re-melted surfaces have to be carefully controlled. New approach to evaluation of the quality surface structure was realized by laser confocal microscopy. Direct measuring or 3D surface model is possible with resolution less than hundred nanometres, depressions along laser beam path or rises on the laser spot edges were determined. Particles and grains with dimensions about one micron in re-melting structures can be observed better then by optical microscopy. Parallel measurements of the surface roughness were realized by the contact inductive profilometer Talysurf, collected data were displayed by software tool Talymap in a plane or spatial pictures.