National Institute Of Materials Physics - Romania

The annealing of crystalline defects in Si single crystals created by ion implantation at room temperature was investigated. Silicon single crystals were firstly implanted at room temperature with 1.345 MeV Au1+ ions at fluences from 1 x 10(13) to 1 x 10(14) at/cm(2) to induce damage. A second implantation at room temperature was afterwards performed with 10 MeV Co3+ ions at a fluence of 3 x 10(14) at/cm(2). All samples were analyzed afterwards by Rutherford backscattering in random and channeling geometry to assess the crystalline damage present in the surface region. The results showed a significant reduction of the degree of damage or a reduction of the size of damaged region. The morphology and local atomic structure, studied using high -resolution electron microscopy, selected area electron diffraction and high resolution X-ray diffraction confirmed the reduction of damage degree and volume caused by Au implantation after Co implantation.

Oxide dispersion-strengthened ferritic steels (ODSFSs) are promising structural materials for applications in fusion and fission power reactors, but further improvement in their (high-temperature) mechanical properties and ferrite phase stability is required. This work demonstrates that an approach to produce Fe14Cr ODSFSs with a stable ferrite phase and improved strength could involve grain size strengthening by long-term milling with a tiny amount of nitrogen. Fe-14Cr-3W-0.4Ti-0.25Y(2)O(3) powders were ball-milled up to 170 hours under an argon atmosphere. In addition to X-ray diffraction, the change in product quality during milling and upon heating was thoroughly investigated by more sensitive magnetic and thermal analysis by measuring the saturation magnetization sigma(s), coercivity H-c, Curie temperature T-c, and temperature of ferrite-austenite (alpha ->gamma) transition T-alpha ->gamma. A pronounced modification of magnetic and microstructure parameters was observed when milling over 70 hours and upon heating above 800 degrees C and was found to be generated by long-term milling with a tiny amount of nitrogen. Upon heating, the nitrogen, incorporated during milling, developed a transition region, with the decomposition of nitrides precipitated at the earlier stage of heating followed by austenite decomposition, nitrogen degassing, and microstructure refinement to a grain size of a few tenths of a nm (e.g., 28 nm by heating at 910 degrees C of 100-hour milled powder). The resulting ferrite phase with refined grains is highly stable to (further) heating. The powders milled for 70 and 100 hours containing 0.175 and 0.500 wt pct nitrogen, respectively, were consolidated at 1100 degrees C with subsequent annealing at 1050 degrees C and subjected to Vickers hardness and 3-point bending tests. The steel produced from the powder milled for 70 hours shows lower hardness, higher density (close to the theoretical value of 7.8 g/cm(3)), and fracture strain. The ductility of this ODS alloy (0.075 fracture strain) is comparable with Eurofer97 (0.075 fracture strain), whereas its strength (2070 MPa ultimate stress) is significantly higher than that of Eurofer97 (1222 MPa ultimate stress). This improvement was attributed to grain size strengthening-the refined grains (promoted by milling with nitrogen) could be effectively pinned by Y-Ti-O dispersoids.

Multi-layered structures, composed of thin films from materials with different compositions or physical properties, represents a way to obtain enhanced properties or even new functionalities. In this work, lead zirconate titanate PbZrxTi1-xO3 (PZT; x = 0.20, 0.52, 0.80) multilayers were grown by pulsed laser deposition (PLD) on a single crystal strontium titanate (SrTiO3, STO) substrate, using a strontium ruthenate (SrRuO3, SRO) film as buffer layer for epitaxial growth, and also as back electrode. Up and down multi-layers were grown and their physical and structural properties were compared, up being the structure in which Zr concentration was varied from 20% near the STO substrate to 80% at the surface, while down is for the structure in which the Zr concentration starts with 80% near the substrate and ends with 20% at the surface. It was found that the electric and pyroelectric properties of the two graded structures are significantly different. The up structure presents electric properties that are comparable with those of single composition PZT films while the properties of the down structure are deteriorated, especially in terms of the leakage current magnitude. Pyroelectric signal could be measured only for the up structure. These differences were attributed to larger density of structural defects in the down structure compared to the up one. This is due to the different growth sequence: Lop structure starts with tetragonal PZT on cubic substrate (lower lattice mismatch, 1.1%) while down structure starts with rhombohedral PZT on cubic substrate (larger lattice mismatch, almost 5%).

A critical discussion on the presently available models for the relaxation time of magnetic nanoparticles approaching the superparamagnetic regime in the presence of interparticle dipolar interactions is considered. The direct implications of such interactions for magnetic fluid hyperthermia therapy through susceptibility loss mechanisms give rise to an indirect method for their study via specific absorption rate measurements performed on ferrofluids of different volume fractions. The theoretical support for the specific evolution of the relaxation time constant and the anisotropy energy barrier versus the interparticle interactions in a perturbation approach of the simple Neel expression for the relaxation time is provided via static and time-dependent micromagnetic simulations.

The work describes the development of a flexible, hydrogel embedded pH-sensor that can be integrated in inexpensive wearable and non-invasive devices at epidermal level for electrochemical quantification of H+ ions in sweat. Such a device can be useful for swift, real time diagnosis and for monitoring specific conditions. The sensors' working electrodes are flexible poly(methyl methacrylate) electrospun fibers coated with a thin gold layer and electrochemically functionalized with nanostructured palladium/palladium oxide. The response to H+ ions is investigated by cyclic voltammetry and electrochemical impedance spectroscopy while open circuit potential measurements show a sensitivity of aprox. -59 mV per pH unit. The modification of the sensing interface upon basic and acid treatment is characterized by scanning and transmission electron microscopy and the chemical composition by X-ray photoelectron spectroscopy. In order to demonstrate the functionality of the pH-sensor at epidermal level, as a wearable device, the palladium/palladium oxide working electrode and silver/silver chloride reference electrode are embedded within a pad of polyacrylamide hydrogel and measurements in artificial sweat over a broad pH range were performed. Sensitivity up to -28 mV/pH unit, response time below 30 s, temperature dependence of approx. 1 mV/degrees C as well as the minimum volume to which the sensor responses of 250 nanoliters were obtained for this device. The proposed configuration represents a viable alternative making use of low-cost and fast fabrication processes and materials.

We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H-3(+) and H-2(+) were the dominant ions results in increased photocurrent.

The present work describes a new simple procedure for the direct immobilization of biomolecules on Ni electrodes using magnetic Ni nanoparticles (NiNPs) as biomolecule carriers. Ni electrodes were fabricated by electroplating, and NiNPs were chemically synthesized. The chemical composition, crystallinity, and granular size of Ni electrodes, NiNP, and NiNP-modified Ni electrodes (NiNP/Ni) were determined by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical characterization of Ni electrodes by cyclic voltammetry and electrochemical impedance spectroscopy confirmed the existence of nickel oxides, hydroxides, and oxohydroxide films at the surface of Ni. Magnetic characterization and micromagnetic simulations were performed in order to prove that the magnetic force is responsible for the immobilization process. Further, Ni electrodes were employed as amperometric sensors for the detection of hydrogen peroxide because it is an important performance indicator for a material to be applied in biosensing. The working principle for magnetic immobilization of the enzyme-functionalized NiNP, without the use of external magnetic sources, was demonstrated for glucose oxidase (GOx) as a model enzyme. XPS results enabled to identify the presence of GOx attached to the NiNP (GOx-NiNP) on Ni electrodes. Finally, glucose detection and quantification were evaluated with the newly developed GOx-NiNP/Ni biosensor by amperometry at different potentials, and control experiments at different electrode materials in the presence and absence of NiNP demonstrated their importance in the biosensor architecture.

This study presents the synthesis and characterization of lanthanum-modified alumina supported cerium-manganese mixed oxides, which were prepared by three different methods (coprecipitation, impregnation and citrate-based sol-gel method) followed by calcination at 500 degrees C. The physicochemical properties of the synthesized materials were investigated by various characterization techniques, namely: nitrogen adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and H-2-temperature programmed reduction (TPR). This experimental study demonstrated that the role of the catalytic surface is much more important than the bulk one. Indeed, the incipient impregnation of CeO2-MnOx catalyst, supported on an optimized amount of 4 wt.% La2O3-Al2O3, provided the best results of the catalytic combustion of methane on our catalytic micro-convertors. This is mainly due to: (i) the highest pore size dimensions according to the Brunauer-Emmett-Teller (BET) investigations, (ii) the highest amount of Mn4+ or/and Ce4+ on the surface as revealed by XPS, (iii) the presence of a mixed phase (Ce2MnO6) as shown by X-ray diffraction; and (iv) a higher reducibility of Mn4+ or/and Ce4+ species as displayed by H-2-TPR and therefore more reactive oxygen species.

Detection in short-wave infrared (SWIR) has become a very stringent technology requirement for developing fields like hyperspectral imaging or climate changes. In a market dominated by III-V materials, GeSn, a Si compatible semiconductor, has the advantage of cost efficiency and inerrability by using the mature Si technology. Despite the recent progress in material growth, the easy fabrication of crystalline GeSn still remains a major challenge, and different methods are under investigation. We present the formation of GeSn nanocrystals (NCs) embedded in oxide matrix and their SWIR characterization. The simple and cost-effective fabrication method is based on thermal treatment of amorphous (Ge1-xSnx)(y)(SiO2)(1-y) layers deposited by magnetron sputtering. The nanocrystallization for Ge1-xSnx with 9-22 at. % Sn composition in SiO2 matrix with 9% to 15% mole percent was studied under low thermal budget annealing in the 350-450 degrees C temperature range. While the Sn at.% content is the main parameter influencing the band-structure of the NCs, the SWIR sensitivity can be optimized by SiO2 content and H-2 gas component in the deposition atmosphere. Their role is not only changing the crystallization parameters but also to reduce the carrier recombination by passivation of NCs defects. The experiments indicate a limited composition dependent temperature range for GeSn NCs formation before beta-Sn phase segregation occurs. NCs with an average size of 6 nm are uniformly distributed in the film, except the surface region where larger GeSn NCs are formed. Spectral photovoltaic current measured on SiO2 embedded GeSn NCs deposited on p-Si substrate shows extended SWIR sensitivity up to 2.4 mu m for 15 at. % Sn in GeSn NCs. The large extension of the SWIR detection is a result of many factors related to the growth parameters and also to the in situ or ex situ annealing procedures that influence the uniformity and size distribution of NCs.

Among the rare-earth-free systems that are currently investigated in search for novel permanent magnet solutions for various applications, with special emphasis on the magnets required to operate in extreme conditions, the FePt binary system, where the tetragonal hard magnetic L1(0) phase can be formed by suitable microstructure processing via annealing, has been extensively studied. A variation of this system, the ternary FeMnPt system, has been also recently shown to exhibit good permanent magnet behavior due to the suitable formation of the L1(0) phase. In addition to be likely to form the L1(0) phase as its parent binary system, the ternary FeMnPt benefits from the reduced costs due to the reduced amount of Pt and may exhibit particular magnetic structure due to the influence of the antiferromagnetic Mn. In the present work, we have employed a mixed sputtering technique, based on the use of both elemental and compound target for developing L1(0) FeMnPt thin films with specific structural features that triggers better magnetic performances in terms of coercivity and maximum energy products. The as-obtained films have been thermally annealed and characterized by means of transmission electron microscopy, X-ray diffraction, Mossbauer spectroscopy, magneto-optic Kerr effect (MORE) and SQUID magnetometry. The aim is to correlate the Mn induced microstructural and lattice changes with the magnetic properties and to optimize the microstructure for an early formation of the ordered L1(0) phase and increased coercivity compared to the as-prepared, structurally disordered, face centred cubic initial state of the films.