Factors Influencing the Stability of Low Temperature Tetragonal ZrO2

Various factors that influence the appearance of a tetragonal (t-) ZrO2 polymorph at room temperature have been extensively investigated. Several proposed models emphasize the role of anionic impurities (SO42-, OH-), crystallite size (surface energy), structural similarities between the starting material and t-ZrO2, lattice strains, water vapor, lattice defects (oxygen vacancies), etc. Our investigations, focused on the stability of low temperature t-ZrO2, showed that, regardless of the structural differences in the starting zirconium materials, their thermal decomposition products crystallized into a metastable t-ZrO2. The t-ZrO2 -> m-ZrO2 transforma-tion occurred during the cooling or further calcination in the pres-ence of air at atmospheric pressure. On the other hand, if these processes are performed in vacuum, the metastable phase is pre-served. These observations indicate that a metastable t-ZrO2 appears at room temperature as a result of stabilization caused by introduction of oxygen vacancies, similarly as in the solid Solutions with aliovalent cations. A decrease in the specific surface area of ZrO2 grains or the presence of the substances that enter into strong surface interactions with ZrO2 (SO42-, Cr2O3) prevents the diffusion of oxygen from the atmosphere into the ZrO2 lattice and due to this fact the metastable t-ZrO2 is stabilized. On the other hand, lattice strain and grain size of metastable t-ZrO2 could not be clearly related to its stability

Microstructural Analysis of Boehmite Nanoparticles Prepared by Rapid Hydrolysis of Aluminum Sec-butoxide

Microstructural properties of six samples, prepared by rapid hydrolysis of aluminum secbutoxide, were investigated at RT using field emission scanning electron microscopy (FE SEM), X-ray powder diffraction (XRD) and Fourier transform infrared (FT-IR) spectrometry. The results of structural analysis show that, regardless of a significant difference in the processing parameters (pH, temperature, time of synthesis), all products contain boehmite as the only crystal phase present. The results of FE-SEM analysis indicate a significant difference in the morphology of obtained boehmites (plates, needles, granules). The results of line-broadening analysis of powder diffraction patterns (Le Bail method – program GSAS) indicate the presence of very small anisotropic crystal domains (around 1.5 to 7 nm in the direction 010; around 3 to 16 nm in the direction perpendicular to 010). In all cases the parameters that contribute to the strain broadening of diffraction lines decrease to nearly zero, which suggests that the obtained boehmites are almost strain-free. (doi: 10.5562/cca1884

In situ phase analysis of the thermal decomposition products of zirconium salts

X-ray powder diffraction at high temperature was used to determine the phase composition of the thermal decomposition products of two zirconium salts, Zr(SO4)(2). 4H(2)O and ZrO(NO3)(2). 2 H2O, and of a mixture of zirconium nitrates having Zr(OH)(2)(NO3)(2). 4.7 H2O and ZrO(NO3)(2). 2H(2)O as dominant components. Heating of the samples up to 1200 degrees C was performed inside a high-temperature chamber, attached to a diffractometer, at an air pressure of approximate to 2 x 10(-3) Pa. Regardless of the structural differences in the starting salts, thermal decomposition products crystallized to t-ZrO2 which remained stable up to 1200 degrees C. This result indicated that the structural nature of the starting materials was not the most important factor of metastable t-ZrO2 formation. The thermodynamically stable m-ZrO2 appeared after the cooling of the samples to room temperature. If the cooling was performed at low air pressure, the m-ZrO2 content was small. Introduction of air, even at RT, caused a considerable increase of m-ZrO2, which became the dominant phase in all cases. The important role of oxygen in the t-ZrO2 --> m-ZrO2 transition indicates that the lack of oxygen in the zirconia lattice favours the formation of metastable t-ZrO2

Microstructure of Al-Zn and Zn-Al Alloys

The change of microstructure of the title alloys with concentration, temperature and applied thermal treatment was studied in situ by XRD. The alloys, having the Zn atomic fraction, x(Zn), from 0.03 to 0.62, were subjected to: (i) rapid quenching from a temperature, Tt, higher than the solid-solution tem-perature, Tss, in water at RT (samples WQ); (ii) slow cooling from Tt to RT (samples SC). The WQ\u27s were solid solutions immediately after quenching, up to x(Zn) ≤ 0.44. For short ageing time, the WQ\u27s contained GP zones, rich in Zn; by a prolonged ageing the WQ\u27s were transformed to a quasi-equilibrium state, containing β precipitates, very rich in Zn. The SC\u27s, also containing β precipitates, were closer to the equilibrium state than the aged WQ\u27s, the microstructure of the latter depended on residual strains, quenched-in vacancies and a non-uniform distribution of β precipitates. Both SC\u27s and prolongedly aged WQ\u27s were slowly heated from RT to Tt and cooled back to RT. Several phenomena were observed in the heating run: a decrease of diffraction line intensities due to enhanced atom vibrations, anisotropy of thermal expansion, change in the precipitate shape, partial or complete dissolution of precipitates, phase transitions, formation of solid solution. In the cooling run, the alloys exhibited a temperature hysteresis in reversal phase transitions. The temperature dependence of microstructure for the SC\u27s was different from that of prolongedly aged WQ\u27s. The sequence of phase transitions, found for the alloys with x(Zn) ≥ 0.44, was not in line with the phase diagram of the Al-Zn system, accepted in literature

Rheological, Microstructural and Thermal Properties of Magnetic Poly(Ethylene Oxide)/Iron Oxide Nanocomposite Hydrogels Synthesized Using a One-Step Gamma-Irradiation Method

Magnetic polymer gels are a new promising class of nanocomposite gels. In this work, magnetic PEO/iron oxide nanocomposite hydrogels were synthesized using the one-step γ-irradiation method starting from poly(ethylene oxide) (PEO) and iron(III) precursor alkaline aqueous suspensions followed by simultaneous crosslinking of PEO chains and reduction of Fe(III) precursor. γ-irradiation dose and concentrations of Fe3+, 2-propanol and PEO in the initial suspensions were varied and optimized. With 2-propanol and at high doses magnetic gels with embedded magnetite nanoparticles were obtained, as confirmed by XRD, SEM and Mössbauer spectrometry. The quantitative determination of γ-irradiation generated Fe2+ was performed using the 1, 10-phenanthroline method. The maximal Fe2+ molar fraction of 0.55 was achieved at 300 kGy, pH = 12 and initial 5% of Fe3+. The DSC and rheological measurements confirmed the formation of a well-structured network. The thermal and rheological properties of gels depended on the dose, PEO concentration and initial Fe3+ content (amount of nanoparticles synthesized inside gels). More amorphous and stronger gels were formed at higher dose and higher nanoparticle content. The properties of synthesized gels were determined by the presence of magnetic iron oxide nanoparticles, which acted as reinforcing agents and additional crosslinkers of PEO chains thus facilitating the one-step gel formation

Still a Long Way to Fully Understanding the Molecular Mechanism of Escherichia coli Purine Nucleoside Phosphorylase

The results of several decades of studying the catalytic mechanism of Escherichia colt purine nucleoside phosphorylases (PNP) by solution studies and crystal structure determinations are presented. Potentially PNPs can be used for enzyme-activating prodrug gene therapy against solid tumours because of the differences in specificity between human and E. coli PNPs. Biologically active form of PNP from E. coli is a homohexamer that catalyses the phosphorolytic cleavage of the glycosidic bond of purine nucleosides. Two conformations of the active site are possible after substrate(s) binding: open and closed. A series of determined 3D-structures of PNP binary and ternary complexes facilitated the prediction of the main steps in the catalytic mechanism. For their validation the active site mutants: Arg24Ala, Asp204Ala, Arg217Ala, Asp204Asn and double mutant Asp204Ala/Arg217Ala were prepared, The activity tests confirm that catalysis involves protonation of the purine base at position N7 and give better insight into the cooperativity between subunits in this oligomeric enzyme

In situ Phase Analysis of the Thermal Decomposition Products of Zirconium Salts

X-ray powder diffraction at high temperature was used to determine the phase composition of the thermal decomposition products of two zirconium salts, Zr(SO4)2 ⋅ 4 H2O and ZrO(N03)2 ⋅ 2 H2O, and of a mixture of zirconium nitrates having Zr(OH)2(NO3)2 ⋅ 4.7 H2O and ZrO(NO3)2 ⋅ 2 H2O as dominant components. Heating of the samples up to 1200 °C was performed inside a high-temperature chamber, attached to a diffractometer, at an air pressure of ≈ 2 × 10-3 Pa. Regardless of the structural differences in the start-ing salts, thermal decomposition products crystallized to t-ZrO2, which remained stable up to 1200 °C. This result indicated that the structural nature of the starting materials was not the most important factor of metastable t-ZrO2 formation. The thermodynamically stable m-ZrO2 appeared after the cooling of the samples to room temperature. If the cooling was performed at low air pressure, the m-ZrO2 content was small. Introduction of air, even at RT, caused a considerable increase of m-ZrO2, which became the dominant phase in ali cases. The important role of oxygen in the t-ZrO2 → m-ZrO2 transition indicates that the lack of oxygen in the zirconia lattice favours the formation of metastable t-ZrO2

Formation of ZrO2 by the Thermal Decomposition of Zirconium Salts

Hydrated zirconium salts containing Cl- , NO3- or SO42- anions were thermally treated up to 1300 °C. The thermal decomposition products were investigated using the X-ray diffraction (XRD), Fourier transform IR spectroscopy (FT-IR) and laser Raman spectroscopy. The thermal decomposition products of all three zirconium salts, characterized as amorphous material, showed a maximum of X-ray scattering at -16° and also in the corresponding FT-IR spectra a broad band at 450 cm-1. With an increase of the heating temperature of all three salts (400 °C for ZrOCl2 ∙ 8H2O, 400 °C for ZrO(NO3)2 ∙ 2H2O and 700 °C for Zr(SO4)2 ∙ 4H2O), the metastable f-ZrC>2 was formed, which disappeared on further heating to higher temperatures. The yield of f-ZrC>2, measured by XRD, depended on the nature of the starting salt, and the highest value was obtained for ZrOCl2 ∙ 8H2O salt. The t-ZrO2 phase formed from ZrOCl2 ∙ 8H2O was thermally most unstable, while the presence of a small amount of t-ZrO2 was observed in the thermal decomposition product obtained by heating Zr(SO4)2 ∙ 4H2O even at 1300 °C. Metastable t-ZrO2, generated by the thermal decomposition of ZrOCl2 ∙ 8H2O or ZrO(NO3)2 ∙ 2H2O salt, was highly sensitive to mechanical treatment, while metastable t-ZrO2 formed from Zr(SO4)2 ∙ 4H2O was stable during the same process. The nature of the starting salt influenced the formation of metastable t-ZrO2, however, when t-ZrO2 was once formed, its stability depended on the anionic impurities that remained in the oxide material

Preliminary Crystallographic Study of Streptomyces coelicolor Single-stranded DNA-binding Protein

Single-stranded DNA-binding proteins (SSBs) play a crucial role in DNA processing such as replication, repair and recombination in all organisms, from bacteria to human. Streptomyces coelicolor ssb gene was overexpressed in a heterologous host, Escherichia coli NM522. 15 mg of purified protein from 1 dm3 of culture was obtained in one-step procedure applying Ni2+ chelating chromatography. Among bacterial SSBs with the solved crystal structure, the S. coelicolor SSB displayed significant sequence similarity with those from Mycobacterium tuberculosis and Mycobacterium smegmatis, slow growing bacteria with a high GC content. Moreover, conserved amino acid region that forms additional ß strand in mycobacterial SSBs was also found in S. coelicolor SSB. The full-length protein readily crystallises in space group I222 or I212121 with unit-cell parameters a = 100.8, b = 102.1, c = 164.2 Å. The asymmetric unit is expected to contain four monomers with solvent content of 52–55 %