Witold Łojkowski

Pressure influence on the grain boundary wetting phase transition in FeSi alloys

Abstract The influence of hydrostatic pressure up to 1.4 GPa at 905°C on the wetting of grain boundaries in Fe-6 at.% Si and Fe-12 at.% Si bicrystals by a Zn-rich melt has been studied. The dihedral angle θ at the intersection of the grain boundary with the solid/liquid interface has been measured by light microscopy. The transition from complete (θ = 0) to partial (θ > 0) wetting of the grain boundary (dewetting phase transition) was found to occur as the pressure increased. The dewetting transition pressure is higher for special boundaries than for the general boundary studied and decreases with increase in the Si content in the alloy. The pressure effect on the solidus concentration of Zn in FeSi alloys has been determined. This enabled us to construct the surfaces of grain boundary wetting/dewetting phase transitions in three-dimensional phase diagrams in Zn concentration-temperature-pressure and Zn concentration-Si concentration-pressure coordinates. A thermodynamic analysis of the wetting phenomena in binary and ternary systems is given, taking into account the effect of pressure, chemical interactions and structural misfit on the energy of interfaces.

Zirconia Based Nanomaterials for Oxygen Sensors - Generation, Characterisation and Optical Properties

Microwave driven hydrothermal synthesis and hydrothermal synthesis were used to obtain ZrO2 nanopowders. Their production with varying phase composition, the characterisation and selected optical properties concerning their potential use as luminescence oxygen sensors are reported. It was found that the powders obtained by the microwave driven hydrothermal method and annealed at 750 0C in air show experiment repeatability within an accuracy of 6 %.

High-Pressure Induced Structural Decomposition of RE-Doped YAG Nanoceramics

The preparation of transparent nanoceramics from nanocrystalline Y3Al5O12 (YAG) powders doped with rare-earth ions has been described and the results of investigation of the structure and morphology have been presented. Decomposition of YAG nanocrystals into YAlO3 (YAP) was observed. The temperature and pressure for the decomposition was much lower than that reported for larger crystals. The transformation was connected with grain coarsening. The influence of the method of preparation of the YAG nanopowders on the final transparency of the nanoceramic produced was determined. Preliminary results of the dependence of luminescence properties on the structural transformation of the nanograins are presented.

Hydroxyapatite nanopowder synthesis with a programmed resorption rate

A microwave, solvothermal synthesis of hydroxyapatite (HAp) nanopowder with a programmed material resorption rate was developed. The aqueous reaction solution was heated by a microwave radiation field with high energy density. The measurements included powder X-ray diffraction (PXRD) and the density, specific surface area (SSA), and chemical composition as specified by the inductively coupled plasma optical emission spectrometry technique (ICP-OES). The morphology and structure were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A degradation test in accordance with norm ISO 10993-4 was conducted. The developed method enables control of the average grain size and chemical composition of the obtained HAp nanoparticles by regulating the microwave radiation time. As a consequence, it allows programming of the material degradation rate and makes possible an adjustment of the material activity in a human body to meet individual resorption rate needs. The authors synthesized a pure, fully crystalline hexagonal hydroxyapatite nanopowder with a specific surface area from 60 to almost 240m2/g, a Ca/P molar ratio in the range of 1.57-1.67, and an average grain size from 6nm to over 30 nm. A 28-day degradation test indicated that the material solubility ranged from 4 to 20 mg/dm3.

Significance of polymethylmethacrylate (PMMA) modification by zinc oxide nanoparticles for fungal biofilm formation

The objective of this study was to obtain a material composite with antifungal properties for dentures to be used as an alternative protocol in denture stomatitis treatment and prevention. Denture stomatitis is still a clinical problem in patients particularly vulnerable to this disease. Composites of PMMA and doped ZnO-NPs (weight concentrations, 2.5%, 5%, 7.5%) and PMMA with sprayed solvothermal and hydrothermal ZnO-NPs were tested. The following investigations of newly formed biomaterials were undertaken: influence on Candida albicans solution, biofilm staining, XTT analysis and a quantitative analysis of adhered C. albicans. These studies evidenced the antifungal activity of both nanocomposites PMMA-ZnO-NPs and the efficacy of sputtering of zinc oxide nanoparticles on the PMMA. The study of the biofilm deposition on the surface showed that antifungal properties increase with increasing concentration of ZnO-NPs. The XTT assay in conjunction with testing the turbidity of solutions may indicate the mechanism by which ZnO-NPs exert their effect on the increased induction of antioxidative stress in microorganism cells. The denture base made of the aforesaid materials may play a preventive role in patients susceptible to fungal infections. Based on the results obtained a modified treatment of stomatitis Type II (Newtons classification) complicated by fungal infection was proposed.

Magnetic properties of ZnMnO nanopowders solvothermally grown at low temperature from zinc and manganese acetate

The authors demonstrate that nanometer size ZnMnO nanopowders, grown from zinc and manganese (II) acetates at low temperatures under microwave radiation, are free of Mn clusters and the inclusion of Mn oxides. These nanopowders show a strong paramagnetic phase with only a weak antiferromagnetic contribution due to Mn–Mn interactions.

Stress- and Strain Induced Phase Transformations in Pearlitic Steels

An overview of the mechanically driven phase transformations taking place in nanocrystalline pearlitic steels in conditions of the severe plastic deformation (SPD), i.e. combination of high pressure and strong shear strains will be given. Conditions of the discussed experiments (room temperature and moderate strain rates) exclude any thermal origin of the observed transformations. One of them is strain induced cementite decomposition, which is a well-documented phenomenon taking place at cold plastic deformation of pearlitic steels. We explain this process taking into account friction forces at the interface between the hard cementite and ferrite. Under the high pressures and stresses higher than the ferrite matrix yield stress, the later one behaves like a viscoelastic fluid. The friction at the precipitate/matrix interface leads to two effects. One is to induce high strains on the precipitates. This leads to shift of thermodynamic equilibrium towards dissolution of the cementite. The second is wear of the cementite phase due to friction at the ferrite/cementite interface and mechanically induced drag of carbon atoms by the ferrite. This had been recently confirmed in 3D AP experiments, which demonstrated that the process of cementite decomposition starts with depleting of carbides with carbon and formation of non-stoichiometric cementite. The existing theories of atom drag by moving dislocations (ballistic models) can be regarded as one of the many possible mechanism of wear discussed by the wear theory. In that respect the process can be called athermal, as temperature indirectly influences wear processes but is not their main cause. We observed also another strain driven transformation in nanocrystalline pearlitic steel during room temperature high pressure torsion. This is a stress induced α→γ transformation, which has never been observed at conventional deformation of coarse grained iron and carbon steels. This was concluded to have occurred due to a reverse martensitic transformation.

Microwave – Hydrothermal Synthesis of Nanocrystalline Pr - Doped Zirconia Powders at Pressures up to 8 MPa

Nanocrystalline praseodymium doped zirconia powders were produced using a microwave driven hydrothermal process under pressures up to 8 GPa. The ai m of the work was to evaluate the effect of synthesis conditions on the phase composition and g rain size of nanopowders of zirconia with Pr in solid solutions having Pr contents of: 0, 0.5, 1, 5 and 10 mol %. Introduction Application of microwaves (MW) in chemical synthesis has attrac ted onsiderable attention [1]. The advantage using MW as an energy source is primarily the possi bility of carrying out the processes at a much higher rate than during conventional heating. Zirc onia is an important ceramic material with useful mechanical, thermal, optical and electrica l properties. Recently much research effort has been dedicated to study nanocrystalline ceramics, since they may display a range of novel properties, including enhanced plasticity [2,3]. Therefore it is of gr eat importance to develop versatile synthesis methods for producing nanocrystalline ceramic powders. The aim of this work was to explore the possibility of synthesizing nanopowders in a MW driven reaction in aqueous solution under pressures of up to 8 MPa. The tests we re performed with nanocrystalline ZrO2 powders doped with Pr, which might be an interesting pigment [4,5] or oxyg en storage material [6]. It’s production by hydrothermal methods has been extensively studied [4,5,7,8]. Experimental methods Powders containing praseodymium in the 0.5–10 mol% range were obtained by addi ng praseodymium(III) nitrate (Pr(NO3)3*H2O, Carlo Erba) to a 0.5M ZrOCl 2 aqueous solution. The solutions were neutralized with NaOH 1 M to pH 10. 40 ml of the solution was poured into the Teflon vessel of the microwave reactor of volume 110 ml. The reacti ons were carried out using a MW reactor from Plazmatronika Ltd. The system operates at 2.45 GH z and can deliver up to 250 W of unpulsed MW power to the reaction fluid. The power level was automati cally adjusted to the maximum pressure, which pre-set for each experiment. The maximum accessible pressure is 10 MPa. The typical ramp time to a pressure of 4 MPa was 5 min and the cooling down time was 10 min. When the reaction was completed the solid phase was separated from the solution by filtering and the solids were washed free of salts with distilled water and isopropanol. One run produced approximately 0.5 g of powder which was annealed in air at 200 C for 0.5h after synthesis. Solid State Phenomena Online: 2003-06-20 ISSN: 1662-9779, Vol. 94, pp 193-196 doi:10.4028/www.scientific.net/SSP.94.193

Optimization of Conditions of Preparation of YAG Nanopowders for Sintering of Translucent Ceramics

The agglomeration of YAG (Yttrium-Aluminum Garnet Y3Al5O12) nanopowders doped with various rare earth ions obtained by the coprecipitation and calcination route is a major problem if it is wished to exploit the nano-size properties such as the transparency of dispersions of the powders or low temperature sintering. Investigations to optimize the preparation process of powders for High Pressure-Low temperature Sintering (HPLS) of semi-translucent pellets was undertaken. The appropriate milling time to decrease the agglomeration was evaluated with the help of ZETA- potential measurement. The dependence of the agglomerate size against the time of hand milling was used to optimize the process.

Hybrid HAp-Maleic Anhydride Copolymer Nanocomposites Obtained by In Situ Functionalisation

The aim of the work is to establish if maleic anhydride copolymer acts as a grain growth modulator and/or as a biocompatible functionalisation agent for hydroxyapatite. Experimental work was developed in three directions: nanocomposites synthesis, nanocomposites characterization and citotoxicity tests on nanocomposites. Maleic anhydride copolymer – HAp nanocomposites were prepared by in situ functionalisation in hydrothermal conditions and were characterized by chemical quantitative analysis, XRD, FT-IR, SEM, specific surface area and picnometric densities. Chemical bonding between the copolymer carboxyl groups and calcium ions of HAp induced a peak of 1577 cm-1 on the FT-IR analysis. Following the evolution of this characteristic peak with the hydrothermal synthesis conditions (different temperatures and pressures) and corroborates the results with XRD and SEM analysis it was pointed out the copolymer grain growth modulator behaviour. Citotoxicity studies in vitro on mice fibroblast cultures were performed. The results proved the biocompatibility of new hybrid –polymer nanocomposites.