Materials Science Forum Vols. 730-732

Abstract: Aliphatic polyesters, like PGA, PLA, PCL and PDO, among others, are biodegradable materials that find applications in many biomedical devices, from fibers for subcutaneous sutures to other regenerative surgery implants. The main concept among these applications is to use a biodegradable device that temporarily replace the biomechanical functions, avoiding this way the chirurgical procedures to remove the device. However, the dimensioning of these devices is complex, not only because the mechanical properties evolve during degradation, but also because these biodegradable materials cannot be assumed as elastic materials. In more precise terms, the response of an elastic material implies that the loading and unloading paths coincide, the material responds instantaneously to an applied load, its behavior is time-independent and the material returns to its former unloaded configuration upon the removal of external loads. In this work, fibers of non-degraded PLA-PCL were submitted to tensile testing at different rates, to load-unloading cycles at different load levels and with or without delay before reloading, creep and fatigue tests at different levels of load. These results elucidate the viscoelastic/viscoplastic nature of this class of materials. The load-unloading cyclic test results allow determining the different components of the strain: elastic, plastic and viscous. The visco-plastic nature was also reflected on the creep and fatigue results. The findings discussed in this work must be taken into account when designing biomedical devices, to avoid common causes of failure such as laxity or premature rupture.

Abstract: The synthesis and characterisation of various organic-inorganic nanohybrids are reported in this contribution. The host material was Ca₃Fe-LDH prepared by the co-precipitation method. E-cinnamate, E-4-nitrocinnamate or E-2,5-difluorocinnamate anions were the guests. Successful intercalation occurred through the introduction of the guest anion into the interlayer region of the host materials with the dehydration-rehydration method. The obtained nanohybrids were studied by powder X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray fluorescence (EDX) coupled to the SEM instrument and infrared spectroscopy (IR). These methods were used on one hand to prove that intercalation was successful and for the characterisation of the substances on the other. Molecular modelling was used for visualising the possible spatial arrangement of the organic anions.

Abstract: In the present paper the synthesis conditions for fabrication of Bi_1−xNd_xFeO₃ ceramics are reported. The single phase polycrystalline samples of Bi_1−xNd_xFeO3 were prepared by standard solid state reaction method from the mixture of oxides viz.: Bi₂O₃, Fe₂O₃ and Nd₂O₃. The samples were characterized by impedance spectroscopy in the frequency range 10Hz – 1MHz at room temperature.

Abstract: In the present research the tool of impedance spectroscopy was utilized to characterize dielectric behavior of Aurivillius-type ceramics of Bi₆Fe₂Ti₃O₁₈ composition fabricated by hot pressing method from the stoichiometric mixture of oxides Bi₂O₃, TiO₂ and Fe₂O₃. Impedance spectroscopy was applied to characterize dielectric response of bulk, grain boundary, and material/electrode interfaces of the fabricated polycrystalline ceramic material at both room and elevated temperature. The Kramers-Kronig data validation test was employed in the impedance data analysis. Experimental data were fitted using the CNLS fitting method. Agreement between experimental and simulated data was established.

Abstract: Magnesium-aluminium spinel (MgAl₂O₄) in the form of transparent ceramics is applied in shields of infrared detectors or emitters, high-temperature widows and elements of military aircrafts and vehicles. Apart from high transparency for specific wave lengths, such materials should have good mechanical properties, especially hardness. It is the reason for production of the fine-grained materials. It seems that they can be produced from submicron powders consolidated by one of colloidal techniques. The work presents preliminary results of investigations on colloidal consolidation of two fine spinel powders with different particle size by pressure filtration and slip casting. The aim of the studies was consolidation of the fine powders leading to green samples with narrow pore size distribution. The samples were then sintered in isothermal conditions. Microstructure, Vickers hardness and transmission in infrared spectrum of the dense materials were examined.

Abstract: Several compounds with Aurivillius-type structure in the Bi₄Ti₃O₁₂-BiFeO₃ system with the general formula Bi_m+1Fe_m–3Ti₃O_3m+3 showing multiferroic properties arouse an increasing interest due to their wide potential applications in electronics. The powders composed of the Aurivillius phase with m=5, 7 and 9 were successfully prepared by the co-precipitation-calcination route. The powders were ground and sintered, obtaining dense polycrystalline materials. The dielectric properties of the sintered materials were studied over a wide range of temperature and frequencies. A strong softening of the magnetic properties took place at 410°C, which stayed in good correlation with the recorded changes of the tan δ asymmetry and fact that it did not induce any anomaly of the Curie temperature at 670°C, where phase transition of the ferroelectric properties was determined.

Abstract: Phosphogypsum (PG) is a pollutant residue resulting from the production of phosphoric acid in the phosphated fertilizers industry. About 180 millions of tons of PG are generated worldwide per year, which originates storage problems because of the environmental restrictions and the high costs of storage spaces. Taking into account the mineralizer properties of PG it has been studied a way to valorize this residue as an alternative material in the production of Portland cement clinker. The PG and the raw-materials (limestone, marl, sand and iron oxide) were chemical, mineralogical and thermally characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD) and differential thermal analysis and termogravimetric analysis (DTA/TGA). After milling, the phosphogypsum was mixed with the raw-materials in different amounts up to 10% weight. The raw mixtures were submitted to two types of firing schedules, heating up to 1500°C without any holding time or heating up to 1350°C and holding for 20 minutes. After firing, the clinkers were analyzed by optical microscopy, milled and characterized in terms of chemical and mineralogical compositions. The clinkers were used to produce cement mortar according to NP EN 196-1 standard. The resultant test specimens were mechanically tested at 2 and 28 days according to the same standard. The obtained results show a reduction of about 140°C in the clinkerization temperature, when a raw mixture with 5% phosphogypsum was used. Standard clinkers, without phosphogypsum addition, which were fired at 1500°C, originated test specimens with a compressive strength of 48.1MPa at 28 days. Test specimens produced with clinker containing 5% phosphogypsum present higher compressive strength values at 28 days, being 55.1MPa for clinkers produced at 1500°C, and 49.4 MPa for clinkers produced at 1350°C.

Abstract: In the present study Bi₆Fe₂Ti₃O₁₈ (BFTO) ceramics has been fabricated by solid state reaction from the mixture of simple oxides viz. Bi₂O₃, TiO₂ and Fe₂O₃. Stoichiometric mixture of the powders was thermally analyzed so parameters of the thermal treatment were determined. The EDS measurements have shown conservation of the chemical composition of the ceramic powder after calcination. Hot-pressing method was used for final densification of ceramic samples. The crystalline structure of the sintered samples was examined by X-ray diffraction method at room temperature. It was found that BFTO ceramics sintered at T=980 °C adopted the orthorhombic structure of Aba2 (41) space group with the following elementary cell parameters: a=5.4567(2)Å, b=49.418(2) and c=5.4826(2). Details concerning the atom’s positions are presented.

Abstract: La-modified Bi₄Ti₃O₁₂ is a typical ferroelectric, piezoelectric and electro-optic material, having relatively low coercive field, low dielectric constant, high Curie temperature and high breakdown strength. Goal of the present research was to apply mixed oxide method for fabrication of BiTLax ceramics, study its chemical composition, crystalline structure and microstructure. The compound BiTLax for x=0, 0.25, 0.5, 0.75 ceramic powders were prepared from mixture of oxides by a solid state reaction at T=1000°C for t=3h. Ceramic powders of BiTLax were next press and the compacts were sintered by the free sintering at temperature T=1000°C with the soaking time t=2h. A crystal structure Bi-layered structured of ferroelectrics was confirmed by a X-ray diffraction. It was found that La-doped BiT ceramics sintered at T=1000°C adopted the monoclinic symmetry. Influence of La-doping on chemical composition and microstructure of BiTLa ceramics was studied by scanning electron microscopy HITACHI S-4700 and energy dispersion spectroscopy.

Abstract: The Tetrahedrite’s family constitutes a complete solid-solution series, and is among the most frequent complex sulfides in Nature. This kind of structure can be generically expressed by the composition, Cu₁₂Sb₄S₁₃. We have calculated the electronic band structure of Cu₁₂Sb₄S₁₃ and Ag₆Cu₆Sb₄S₁₃ (with band gaps of 1.24 and 1.20 eV, respectively) to demonstrate that different elements occupying certain sites of the crystal structure will make a difference in what concerns the conduction process in Tetrahedrites. We will use this effect and ab initio calculations to show that the electronic properties of these compounds make them promising candidates as solar cells photovoltaic materials since not only they possess a direct band gap but their energy falls within the range of energies of photovoltaics. Moreover, we can optimize these properties by doping and substituting ions furthermore. Mechanical properties were also calculated for both compounds and will be compared.