The effect of temperature on the pulsed current processing behaviour and structural characteristics of porous ZSM-5 and zeolite Y monoliths

Hierachically porous monoliths of ZSM-5 and Y zeolites have been prepared by pulsed current processing (PCP). The densification behaviour and structural characteristics during rapid thermal treatment of the zeolites have been determined and related to the influence of the Si:Al ratio on the thermal stability of the zeolites. Monoliths of macroscopic shape can be prepared with an insignificant loss of surface area and micropore volume when the PCP-treatment was performed at temperatures below a critical maximum PCP temperature (T_PCP). Full-profile fittings of the powder X-ray diffraction patterns showed that the lattice strain of zeolite Y increases rapidly above the critical T_PCP while the ZSM-5 zeolites undergo a phase transition from orthorhombic to monoclinic. The use of a novel ceramic processing route for the production of the zeolite monoliths that do not significantly influence the structural characteristics and surface area of the starting materials has a potential to be of importance in catalysis and gas separation applications.     

La-Si-O-N glasses: Part II: Vickers hardness and refractive index

Vickers hardness and refractive index have been determined for a series of La-Si-O-N oxy-nitride glasses containing 30–62 e/o of La and 9–68 e/o of N. The hardness varies between 7.7 and 11.5 GPa at a load of 1 kg and is dependent of the N content, while the La content does not influence it significantly. The increase of the hardness with N content is, contrary to reported findings for other oxy-nitride glasses, found not to be linear over the whole compositional range. The refractive index varies between 1.8 and 2.3 and increases non-linearly with increasing N content. The compositional variations of hardness and refractive index are compared with previously published results.     

A study of the crystallization process of Na1.6Zn0.8Si1.2O4 glass and the ionic conductivity properties of partially crystallized samples

The crystallization process of Na_1.6Zn_0.8Si_1.2O₄ glass was studied by means of differential scanning calorimetry, X-ray powder diffraction and the platinum/carbon replication technique. Partially crystallized samples were made by rapidly cooling samples from elevated temperatures using the DSC apparatus, and the ionic conductivity of the materials was determined by means of impedance measurements conducted at lower temperatures where the crystallization rate was negligible. The glass was found to crystallize at 830 K by precipitation and three-dimensional grain-growth of a crystobalite-type phase with the same composition as the glass. The overall activation energy for the crystallization process was determined from isothermal DSC measurements to be 340 kJ mol^–1. The bulk ionic conductivity for partially crystallized samples increases smoothly from 9.3 x 10^–5( cm)^–1 at 600 K for the glass to 2.4 x 10^–3( cm)^–1 for the crystallized material.     

La–Si–O–N glasses: Part I. Extension of the glass forming region

The nitrogen-rich part of the glass forming region in the La–Si–O–N system has been the subject of a comprehensive study. Glasses were prepared by heating powder mixtures of La metal, Si₃N₄ and SiO₂ in a nitrogen atmosphere at 1650–1800 °C. By this new synthesis route, glasses containing up to 68 e/o of N and 62 e/o of La were prepared, showing that the glass forming region is significantly larger than previously reported. The glasses were characterized by elemental analysis, differential thermal analysis, X-ray powder diffraction, and scanning electron microscopy. They were found to be X-ray amorphous and homogenous, with the majority of them containing small amounts of crystalline La silicides and elemental Si. Glass transition temperatures (T_g) were found to vary between 900 and 1100 °C and crystallization to occur typically 120 °C above T_g. The forming of the glasses was investigated by characterizing samples taken out at various steps of the heating cycle. The results indicate that the glass formation is strongly dependent on reaction kinetics. A strong exothermal reaction occurs at temperatures 900–1100 °C, leading to the formation of assemblies of amorphous and crystalline (oxy)nitride phases that melt upon further heating at 1650–1800 °C.     

SU-57 – An aluminosilicogermanate with a DFT topology and variable composition

An aluminosilicogermanate with a DFT zeotype framework (denoted as SU-57) was synthesized for the first time by ethanol-assisted hydrothermal synthesis at 160 and 170 °C. The compound was characterized by single crystal and powder X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy (EDS), thermogravimetry (TG) and solid state nuclear magnetic resonance (NMR) spectroscopy. SU-57 crystallizes in space group P4₂/n with estimated a ≈ 10.38–10.46 Å, c ≈ 8.88–8.91 Å, V ≈ 957–975 Å³. It has variable Al–Si–Ge composition with an approximate formula |C₂H₁₀N₂| [Al(Si_xGe_1?x)O₄]₂ (x ≈ 0.3–0.9), which results in a super-structure originated from different cation occupancies of the two unique tetrahedral (T) sites. Single crystal X-ray structure refinements, together with results from X-ray powder diffraction (XRPD) and EDS analysis, showed that (i) the AlO₄ and (Si, Ge)O₄ tetrahedra are only partially ordered over the DFT framework and do not follow a strict alternating manner. (ii) Al resides predominantly on one T site and Si and Ge predominantly on the other. (iii) The Al cation concentration (Al/(Al + Si + Ge)) is nearly constant and slightly less than 50 at%, while the Si and Ge cation concentrations vary over a large range. (iv) Al and Ge occupy both T sites. The cation disorder was confirmed by ²⁷Al and ²⁹Si solid state NMR. TG analysis and in situ XRPD showed that SU-57 was stable up to 375 °C in N₂ atmosphere.     

Triclinic oxy-hydroxyapatite

Partially dehydrated hydroxyapatite was characterised by X-ray powder diffraction (XRPD) using Guinier-Hägg films, diffractometer and high-resolution synchrotron data as a function of the degree of dehydration (DD). It is for the first time shown that the space group symmetry for partially dehydrated hydroxyapatite changes from hexagonal P6₃/m to triclinic when more than ca. 35% of the structurally bound water is removed. With increasing DD the a- and b-axes decrease and the c-axis increases. At the highest DD value attained (78%) before the onset of a decomposition of the apatite, the cell parameters were determined to be a = 9.40023 (3), b = 9.39704 (3), c = 6.89967 (2) Å, = 90.0626 (2)°, = 89.7478 (1)° and = 119.9971 (2)°. A structure refinement for this sample converged with R _F = 3.7% in space group P , using synchrotron data ( = 0.852790 Å), 795 reflections in the 2 range 5–45° and 62positional parameters. The shifts of the atoms from their corresponding positions in hexagonal hydroxyapatite are small. A possible cause for the triclinic distortion is a tilting of the hydroxide ions in the apatite channels away from the channel axis.     

high-resolution electron microscopy of aSr-containing sialon polytypoid phase

A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi_10-xAl_18+xN_32-xO_x (x≈1) has been found in the Sr–Si–Al–O–N system. The phase was characterised by X-ray powder diffraction (XRPD), and its structure was investigated by electron diffraction (ED) and high resolution electron microscopy (HREM). It is considerably disordered, but the average structure has a rhombohedral unit cell with a=5·335(5)≈ ·a_AlN and c=79·1(1)Å≈30·c_AlN. The Sr atoms are located in layers M–Sr–M, M=(Si/Al), at the origin of the unit cell with 12 X=(O,N) atoms around it, at distances of 3 Å, forming a cubo-octahedron. The X atoms that form a hexagon around the Sr atom in the ab plane are corner shared by M=(Si/Al) tetrahedra with opposite polarity in adjacent layers in which 2/3 of the tetrahedra are occupied. The M–Sr–M layers alternate with normally eight-layer-thick AlN type blocks, although the thickness of these blocks frequently varies. The structural model obtained from the HREM images includes a polarity reversal of the tetrahedra in the AlN blocks, similar to that proposed to occur in Si–Al–O–N polytypoid phases. The model with one Sr layer and 10 M=(Si,Al) layers per 1/3 of the repeat unit agrees with the composition of the phase and experimental HREM images.     

Precise control over shape and size of iron oxide nanocrystals suitable for assembly into ordered particle arrays

Here we demonstrate how monodisperse iron oxide nanocubes and nanospheres with average sizes between 5 and 27 nm can be synthesized by thermal decomposition. The relative importance of the purity of the reactants, the ratio of oleic acid and sodium oleate, the maximum temperature, and the rate of temperature increase, on robust and reproducible size and shape-selective iron oxide nanoparticle synthesis are identified and discussed. The synthesis conditions that generate highly monodisperse iron oxide nanocubes suitable for producing large ordered arrays, or mesocrystals are described in detail.