Thermal analysis,microstructure and impurity phases evolution in Fe14Cr ferritic steel powders ball-milled in air and under an argon atmosphere

Journal of Thermal Analysis and Calorimetry - Azobenzenes undergo reversible trans–cis photo-isomerization and have been studied extensively as photo-responsive materials. Despite their...     

Preparation and properties of nanocrystalline BNT-BTx piezoelectric ceramics by sol-gel and spark plasma sintering

(Bi_0.5Na_0.5)TiO₃ was doped in situ with 5, 8, and 11 mol% BaTiO₃ (BNT-BT_x; x = 0.05, 0.08 and 0.11) using a sol-gel technique. The resulting powders from gel precursors showed microstructures consisting of nano-sized grains and crystalline perovskite structure. Spark plasma sintering (SPS) technique was used to prepare high densified (98–99%ρ_theor) BNT-BT_x ceramics from these nanopowders. The results confirm the spark plasma sintering method applied to nano-scale powders, obtained by sol-gel, as a viable route in producing nanostructured ceramics. The evolution of the structure and electrical properties of the ceramics with BaTiO₃ concentration (x) was investigated. The permittivity of BNT-BT_0.08 ceramic is higher (ε_r = 2090, at 100 kHz) than that for x = 0.05 (ε_r = 1350) and x = 0.11 (ε_r = 1800). BNT-BT_0.08 ceramic shows maximum values for the frequency constants (N_p, N_t), piezoelectric charge coefficient (d₃₁) and piezoelectric voltage coefficient (g₃₁), and minimum values for the electromechanical coupling factor (k_p) and piezoelectric charge coefficient (d₃₃). The electrical properties of these ceramics are influenced by grains size, oxygen deficiency and non-uniform internal stresses due to these oxygen deficiencies. BNT-BT_x ceramics sintered by SPS seem to be good ceramic resonators with high mechanical quality factor (Q_m).     

Investigations of the non-isothermal crystallization of Bi4Ge3O12 (2:3) glasses

Amorphous Bi₄Ge₃O₁₂ glass samples were produced by melt quenching procedure stating with Bi₄Ge₃O₁₂ (BGO) powder, obtained by solid state reaction between oxides. The kinetics of non-isothermal crystallization of BGO nano-crystals has been investigated. Differential Thermal Analysis (DTA) can give the main parameters of crystallization with an exothermic peak from 813 K to 851 K depending on the heating rate, which was assigned to the crystallization of cubic BGO in the amorphous matrix and compared with the X-Ray Diffraction (XRD) patterns. The nano-crystal dimensions were calculated from the XRD patterns by using the Debye–Scherrer method and were compared with Transmission Electron Microscopy (TEM) images. It was shown that the Ozawa model is most suitable for describing the behavior of non-isothermal crystallization of BGO nano-crystals within the glass matrix. Experimental results suggest a disk-shape type growth mechanism for the Bi₄Ge₃O₁₂ nano-crystallites. The Flynn–Wall–Ozawa method has shown that the average activated energy value is 385 ± 14 kJ/mol which was computed within the same model and agrees very well with the activation energy of the crystallization.     

Antimicrobial Activity of MgB2 Powders Produced via Reactive Liquid Infiltration Method

We report for the first time on the antimicrobial activity of MgB₂ powders produced via the Reactive Liquid Infiltration (RLI) process. Samples with MgB₂ wt.% ranging from 2% to 99% were obtained and characterized, observing different levels of grain aggregation and of impurity phases. Their antimicrobial activity was tested against Staphylococcus aureus ATCC BAA 1026, Enterococcus faecalis ATCC 29212, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Candida albicans ATCC 10231. A general correlation is observed between the antibacterial activity and the MgB₂ wt.%, but the sample microstructure also appears to be very important. RLI-MgB₂ powders show better performances compared to commercial powders against microbial strains in the planktonic form, and their activity against biofilms is also very similar.     

Sintered and 3D-Printed Bulks of MgB2-Based Materials with Antimicrobial Properties

Pristine high-density bulk disks of MgB₂ with added hexagonal BN (10 wt.%) were prepared using spark plasma sintering. The BN-added samples are machinable by chipping them into desired geometries. Complex shapes of different sizes can also be obtained by the 3D printing of polylactic acid filaments embedded with MgB₂ powder particles (10 wt.%). Our present work aims to assess antimicrobial activity quantified as viable cells (CFU/mL) vs. time of sintered and 3D-printed materials. In vitro antimicrobial tests were performed against the bacterial strains Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 25923, Enterococcus faecium DSM 13590, and Enterococcus faecalis ATCC 29212; and the yeast strain Candida parapsilosis ATCC 22019. The antimicrobial effects were found to depend on the tested samples and microbes, with E. faecium being the most resistant and E. coli the most susceptible.     

Thermal analysis and microstructure of oxide dispersion strengthened ferritic steels produced by ball milling with different amounts of process control agent

Journal of Thermal Analysis and Calorimetry - Fe–14Cr–3 W–0.4Ti–0.25Y2O3 ferritic steels were produced by ball milling of initial mixture of elemental powders with...     

Thermal analysis on C<Subscript>6</Subscript>H<Subscript>10</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>-doped MgB<Subscript>2</Subscript>

Additives to MgB₂ can improve the superconducting functional characteristics, such as critical current density (J _c) and irreversibility field (H _irr). Recently, we have shown that repagermanium (C₆H₁₀Ge₂O₇) is an effective additive, enhancing both J _c and H _irr. To look into details of the processes taking place during the reactive sintering, a thermal analysis study (0.167 K s^?1, in Ar) is reported. We used differential scanning calorimetry between 298 and 863 K and simultaneous thermogravimetric—differential thermal analysis between 298 and 1233 K. Samples were mixtures of powders with composition 97 mol% MgB₂ and 3 mol% C₆H₁₀Ge₂O₇. Up to 863 K, repagermanium decomposes by multiple steps and forms amorphous phases. A reaction with MgB₂ is not observed. Above this temperature, partial decomposition of MgB₂ occurs. Crystalline Ge and MgO are detected before formation of Mg₂Ge and MgB₄, when temperature approaches the melting point of Ge (1211 K). Carbon substitution for boron in the crystal lattice of MgB₂ is observed for samples heated above 863 K. The amount of substitutional C does not significantly change with temperature.     

Crystallization and spectroscopic properties of Eu-doped CaF2 nanocrystals in transparent oxyfluoride glass-ceramics

Transparent oxyfluoride glass-ceramic in the system SiO₂–Al₂O₃–CaF₂–EuF₂ containing Eu-doped CaF₂ nanocrystals were produced by using the controlled crystallization of melt-quenched glass. X-ray diffraction and transmission electron microscopy data have revealed the formation of the CaF₂ nanocrystals of about 65 nm size. Photoluminescence spectra have shown an increase of the splitting of the luminescences associated to the Eu³⁺ ion along with annealing time which is consistent with the Eu³⁺ environment evolving from a glassy to a crystalline state.     

Mössbauer spectroscopy on57Fe in highT c superconductor Bi2Sr2Ca1Cu2O y

Mössbauer measurements were performed on polycrystalline⁵⁷Fe: Bi₂Sr₂Ca₁Cu₂O _y , super-conductor in the temperature range of 77–296 K. The samples were obtained in a solid phase synthesis using 0.01, 0.03, 0.1 and 0.5 mol fractions of α-Fe₂O₃ (96% enriched in⁵⁷Fe). A prevailing quadrupole doublet practically independent of temperature and iron concentration characterizes the obtained Mössbauer spectra. The corresponding hyperfine parameters suggest the presence of high spin Fe¹¹¹ ions in a strongly distorted octahedral symmetry which indicates a probable copper substitution by iron in the system.