Structural and mechanical properties of sol–gel prepared pyrochlore lanthanum niobates

Lanthanum orthoniobate LaNbO₄ (LN) precursors were prepared by a modified polymeric complex sol–gel method using Nb-tartrate in different solvents (ethanol or methanol) with calcination at 750 °C. The monoclinic LaNbO₄ and orthorhombic LaNb₅O₁₄ phases were formed after annealing at 900 °C from precursors synthesized in ethanol (LN_et) and methanol (LN_met), respectively. FTIR and Raman spectra confirmed different structures of LN at 900 °C in both solvents. The microstructures indicate the strong effect of solvents on surface morphology. TEM and SAED patterns verified the monoclinic LaNbO₄ and orthorhombic LaNb₅O₁₄ phases with ferroelastic domains at 750 and 900 °C, respectively. Mechanical characterization was conducted for the first time by nanoindentation on the sol–gel derived LN after annealing at a low temperature. The hardness and elastic modulus of LN_met (LN_et) were measured to be ~2.6(1.5) and ~45(30) GPa, respectively. The modulus and hardness of LN_met were higher in comparison with LN_et. The strong solvent effect on mechanical properties of lanthanum niobate was confirmed.     

Synthesis of zeolite omega in an alcohol-water system

The crystallization of aluminosilicates at 130 ± 1°C from reaction mixtures containing varied amounts of alcohol (methanol, ethanol, butan-1-ol) and water has been studied. Zeolite species were identified by X-ray diffraction and electron microscopy. The type and morphology of the zeolite isolated were dependent upon the chain length and the alcohol:water ratio. The crystallization of zeolite omega was favored by the presence of longer-chain alcohols, ethanol, and butan-1-ol, and, also, a greater tolerance to alcohol with increasing chain length was observed. Zeolite omega produced in the mixed-solvent phase consisted of well-ordered “bundles” of long acicular crystals as compared to the spherulites formed in the aqueous system.     

The orthorhombic/monoclinic transition in single crystals of zeolite ZSM-5

X-ray photographs of single crystals of zeolite H-ZSM-5 (Si_11.96Al_0.04H_0.04O₂₄) at different temperatures are presented. The reversible orthorhombic/monoclinic transition, previously observed with XRD and ²⁹Si m.a.s. n.m.r. on powder samples of H-ZSM-5, is confirmed. Upon cooling, the orthorhombic H-ZSM-5 single crystal changes into an aggregate of monoclinic twin domains. The directions of a and c in the orthorhombic single crystal and in the monoclinic twinned crystal (a ≈ 90.5°) are identical. The deviation of α from 90° can be ascribed to a mutual shift of successive (010) pentasil layers along c, induced by a distortion or rotation of the T₄ and T₆ rings interconnecting the layers. The space group of the monoclinic phase most probably is P₁²n11.     

Mixed oxides of the system MV-TeIV-O2 (M= Nb,Ta, Sb); II. crystal structure of Ta2Te2O9

Single crystals of a Ta^V and Te^IV mixed oxide of composition Ta₂Te₂O₉ have been obtained by transport reaction of TeO₃ and Ta₂O₅ at 700°C. The crystal structure of this material has been determined by single crystal X-ray diffraction technique. The compound has monoclinic symmetry (S.G. P2₁/c) with lattice parameters: a= 7.096(3), b= 7.486(3), and c= 14.614(3) ȧ, β = 103.010(36)°. R= 0.047 for 1974 observed reflexions. The structure is discussed on the basis of the coordination polyhedra of the central atoms and the effect of the “lone pair” characteristic of Te^IV.     

Crystal growth of barium molybdate by flux evaporation

Single crystals of BaMoO₄ have been grown by isothermal evaporation of lithium chloride at 700°C. The morphology and mechanism of growth of the crystals have been discussed. The quality of the crystal has been assessed by selective etching.     

Static coarsening behaviour of lamellar microstructure in selective laser melted Ti–6Al–4V

Static coarsening mechanism of selective laser melted (SLMed) Ti–6Al–4V with a lamellar microstructure was established at temperatures from 700 °C to 950 °C. Microstructure evolution revealed that high heat treatment temperature facilitated martensite decomposition and promoted lamellae growth. At each temperature, the growth rate decreased with increasing holding time. The static coarsening behaviour of SLMed Ti–6Al–4V can be interpreted by Lifshitz, Slyozov, and Wagner (LSW) theory. The coarsening coefficient were 0.33, 0.33–0.4, 0.4–0.5 for 700–800 °C, 900 °C and 950 °C, respectively. This indicated the coarsening mechanism was bulk diffusion at 700–800 °C, and a combination of bulk diffusion and interface reaction at 900 °C and 950 °C conditions.     

Le systeme SbWO3 et les bronzes de tungstene — Antimoine

Evidence is obtained for a series of tungsten-antimony bronzes, through reaction of antimony with tungsten trioxide at 900°C. Five phases are characterized: the four first derive from WO₃ with the classical increase of symetry: monoclinic, orthorhombic, tetragonal and cubic; the last one is probably a superstructure of the perovskite.     

AlN epilayers and nanostructures growth in a homebuilt alumina hot-wall high temperature chemical vapor deposition system

AlN epilayers and nanostructures were grown in the range from 500 to 1500 °C in a homebuilt alumina hot-wall high temperature chemical vapor deposition system. The results revealed that high quality AlN epilayers can be grown at high temperature beyond 1100 °C and versatile AlN nanostrctures can be grown at low temperature below 900 °C, enabling the system to tailor AlN structures just by changing the growth temperature. High growth temperature as well as low N/Al ratio was preferable to surface mobility of the adatoms and lateral growth, resulting in a series of morphology changes. Meanwhile, the crystal quality improved with the increasing growth temperature, as proved by the decreasing FWHM of (0002) plane rocking curve of the epilayer and narrowing peaks in θ-2θ XRD pattern of the nanostructures. The epitaixal relationship was proven to be AlN (0001) ‖ sapphire (0001) and AlN [1-210] ‖ sapphire [1-100]. The layer was in tensile stress state in several tens of nanometers range near the interface and turned into compressive stress state out of the range. Tens of atoms layers of sapphire interface were substituted for AlN lattice due to nitridation. Low growth temperature produced versatile AlN nanostructures, whose crystal structures varied from amorphous in 500 °C case to defective crystal in 700 °C case and improved crystallinity in 900 °C case.     

Epitaxial growth of MgAl2O4 spinel crystals by solid-state reaction of MgO crystal with molten Al metal

Single crystals of MgAl₂O₄ spinel have been prepared epitaxially by a solid-state reaction of MgO crystal with molten Al metal under vacuum of 10^?5≈10^?6 torr at 1000≈1100°C. The growth rate was estimated to be about 0.1≈0.2 mm/hour in this temperature range. The as-grown crystals were black and opaque with low crystallinity, which was improved by heating above 1350°C. Chemical analysis showed that the crystals were slightly contaminated with Mg metal which was easily oxidized above 900°C in air.     

Vapor growth of hexagonal GeO2 needle crystals

Needle crystals of haxagonal GeO₂ were grown at 750–900°C by the oxidation of GeO vapor which was generated by the reduction of GeO₂ with graphite. The largest crystals were grown at higher temperature, and had a maximum size of 10 μm diameter and 6 mm length after growth at 900°C for 5 hours. The growth direction of the crystals was parallel to the a-axis.     

Synthesis of magnetite nanoparticles by thermal decomposition of ferrous oxalate dihydrate

Two different polymorphs of ferrous oxalate dihydrate were synthesized by precipitation of ferrous ions with oxalic acid: α-Fe(C₂O₄) · 2H₂O with a monoclinic unit cell is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be tailored from prismatic crystals of the α-polymorph over star-like aggregates of α/β-mixtures to non-agglomerated crystallites of β-oxalate. Thermal decomposition in air gives hematite at T ≥ 250 °C; if the thermolysis reaction is performed at low oxygen partial pressures (e.g., T = 500 °C and p _O2 = 10^?25 atm) magnetite is obtained. The synthesized magnetite is stoichiometric as signaled by lattice parameters of a ₀ = 8.39 Å. The thermal decomposition of ferrous oxalate is monitored by thermal analysis, XRD, and IR-spectroscopy. The morphology of the oxalate crystals is preserved during thermal decomposition; the oxalates are transformed into spinel particle aggregates of similar size and shape. The crystallite size of the magnetite particles increases with temperature and is 40 or 55 nm, if synthesized from β-oxalate at 500 °C or 700 °C, respectively. The saturation magnetization of the magnetite particles decreases with decreasing particle size. Since the particles are larger than the critical diameter for superparamagnetic behavior they display hysteresis behavior at room temperature.     

Phase transitions in Li0.13Na0.87NbO3 crystals

Li_0.13Na_0.87NbO₃ crystals were studied by dilatometric, dielectric, acoustic emission, and optical methods on heating in a temperature range from 20 to 700°C. Based on the results of these measurements, the following sequence of phases was established: 20–35°C, ferroelectric orthorhombic Pmm2; 350–40°C, ferroelectric orthorhombic Pmmm; 400–63°C, ferroelectric tetragonal, 14/mmm; above 63°C, cubic Pm3m. The latter two transformations can be classified as second-order ferroelectric phase transitions.     

Tetragonal → twinned hexagonal crystal phase transformation in polybutene-1

The spontaneous transformation of the metastable (Form II) tetragonal crystals into the stable twinned hexagonal (Form I) crystals in polybutene-1 was studied using several techniques. The mechanical properties of the heat moulded material undergo significant changes in the process and the crystalline melting point increases from 112 to 128‡ C. Results from an Avrami analysis suggest the nucleation of the stable crystalline phase occurs shortly after crystallization from the melt and the subsequent growth of the nuclei follows a rod-like geometry. Transmission electron microscopy of melt-grown thin films shows that nucleation occurs at random positions within a spherulite and growth propagates along the radially oriented fibrillar crystals. As a result of multiple nucleation, each tetragonal crystal generates several twinned hexagonal crystallites with different crystallographic orientations. Besides exhibiting multiple nucleation within each individual tetragonal crystal, solution-grown single crystals also reveal twisting of the crystal lattice about thec-axis. The information obtained shows that residual stresses present in the material do not appear essential for the nucleation of the stable phase. Current concepts of the transformation mechanism are examined.     

Preparation and photocatalytic activity of nanoporous zirconia electrospun fiber mats

Nanoporous zirconia electrospun fiber mats (NZEFM) have been prepared by an electrospinning process using nonionic F108 as a pore-forming agent. This nonaqueous synthesis route has been employed to fabricate a stable “building block” porous structure inside the nano-scale fibers. The mats composed of individual fibers proved to be robust after calcination at up to 450 °C or stirring in water. The photocatalytic activity of NZEFM is apparently higher than that of commercial zirconia powder for the degradation of methyl orange in aqueous solution. Moreover, amorphous NZEFM mixed with the tetragonal phase obtained at 450 °C proved to be more efficient than the monoclinic phase NZEFM obtained at 900 °C. Various dyes could be degraded by NZEFM under UV light irradiation. The highly photocatalytic activity of NZEFM could be attributed to its high specific surface area and nanoporous “building block” structure made up of stacked zirconia nanoparticles.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.     

Synthesis and properties of LaNi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−δ</Subscript> as cathode materials in SOFC

The synthesis of LaNi_{1− x} Fe _x O_3−δ (LNF) perovskites with x = 0.0–1.0, for use as cathode materials for an IT-SOFC, was investigated using four combustion methods, Water Citrate (WC), Modified Water Citrate (MWC), Nitric Citrate (NC), and Modified Nitric Citrate (MNC). The structures and homogeneities of the synthesized powders were examined using an XRD, and the particle sizes were examined using an SEM and a particle size analyzer. All four combustion methods gave the single phase perovskites with the same structure. The main difference was shown in a particle size that the smallest to the largest sizes were obtained from MNC, MWC, NC, and WC, respectively. In this LNF series, as x is 0–0.5, the crystal structure is cubic and rhombohedral at the calcination temperature of 700 and 900 °C, respectively. Further investigation indicated that the cubic structure changed to rhombohedral structure at 900 °C, and was stable up to 1200 °C. As x is 0.6–1.0, the crystal structure is in orthorhombic phase when calcined between 700 and 1000 °C. This orthorhombic phase decomposed above 1100 °C. From the XRD and SEM–EDX results, LaNi_0.6Fe_0.4O_3−δ (LNF64) has a good chemical compatibility with 8YSZ from room temperature up to 900 °C. In addition, its thermal expansion coefficient is 13.2 × 10⁻⁶ K⁻¹ close to that of 8 mol% Y₂O₃ (8YSZ). Therefore, LNF64 also has a good physical compatibility with 8YSZ.     

On the growth of neodymium pentaphosphate crystals for laser action

The growth of Nd_xLa_1?xP₅O₁₄ (0 ≤ x ≤ 1) crystals from phosphoric acid solution has been studied. Crystals have been obtained at temperatures between 300°C and 750°C. Vitreous graphite is a suitable container for the crystal growth. Changes in crystal morphology and crystal quality with growth temperature have been observed. Evaporation at high temperatures (>550°C) in an open system is too rapid to allow the growth of high quality crystals, although large (> 1 cm) crystals may be prepared, and the best quality crystals with respect to optical inhomogeneities are prepared at 450–550°C. Both pulsed and cw laser action have been observed. The Nd³⁺${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ fluorescent lifetime increases from 120 μsec (x = 1) to μsec (sec ≤ 0.05). These values indicate a seven-fold reduction in fluorescence quenching with respect to YAG:Nd. For crystals grown at temperatures below 600°C, however shorter lifetimes are observed and the quenching becomes more severe as the temperature of preparation is lowered. The quenching may be removed by appealing low lifetime crystals at 600–700°C in a P₂O₅ atmosphere. It is proposed on the basis of infrared absorption measurements that the lifetime shortening is related to the presence of hydrogen in crystals grown at lower temperatures. The vibrational characteristics of pentaphosphate crystals have also been investigated by Raman spectroscopy.     

On the transformations of the ψ-AlCuFe icosahedral phase

In the present investigation, different alloy compositions close to the ternary composition values, where the icosahedral phase in AlFeCu is obtained, have been studied. The specimens were obtained using a rapid solidification technique with subsequent thermal treatments of 600°C, 700°C, 800°C and 900°C. The obtained specimens were characterized with X-ray diffraction patterns (XRD) and transmission electron microscopy (TEM). The experimental results show different transformations of the icosahedral phase to crystalline phases between 800°C and 700°C .The cubic β-phase is a solid solution which regulates the formation and decomposition of the ψ-Al₆Cu₂Fe phase to different crystalline phases such as the tetragonal (Al₇Cu₂Fe) and monoclinic (Al₁₃Fe₄) phases.     

Microstructures and high temperature oxidation resistance of alloys from Nb–Cr–Si system

Oxidation behavior of Nb–30Si–(10,20)Cr alloys have been evaluated in air from 700 to 1400 °C by heating for 24 h and furnace cooling them. The lower weight gain per unit area has been observed for 20Cr alloy at 1200, 1300, and 1400 °C. Pesting has been observed at lower temperatures (700, 800, 900 °C). Analysis indicates that the powder formation at 900, 100, 1100 °C may be associated with β form of Nb₂O₅ (base centered monoclinic form). However the m-monoclinic form of Nb₂O₅ evolves at temperatures above 900 °C while o-orthorhombic Nb₂O₅ forms at below this temperature. The phases in the alloys have been calculated using the Pandat^TM software program at different temperatures using calculated Nb–Cr–Si phase diagrams.