Thermal transformation of hydrated ferric oxide gel and preparation of ultrafine oxide

Hydrated ferric oxide gel has been prepared by addition of a solution of ferric ammonium sulphate to that of sodium hydroxide kept at room temperature. Thermal analysis of the gel shows an endothermic peak at 130 C and three exothermic peaks at 210, 315 and 430 C. The sequence of transformation has also been investigated by thermogravimetry, X-ray diffraction, infra-red spectroscopy and surface-area measurement. The gel is found to consist of -FeOOH·nH₂O as the primary particle, which crystallizes at around 210 C and subsequently transforms to -Fe₂O₃ at around 315 C. The initial crystallite size of the oxide formed, is about 700 nm. However, it increases significantly ( 3000 nm) accompanied by an exotherm when the oxide is heated beyond 400 C.     

Effect of swaging on the 1000‡C compressive slow plastic flow characteristics of the directionally solidified eutectic alloy γ/γ'′-α

Swaging between 750 and 1050 C has been investigated as a means to introduce work into the directionally solidified eutectic alloy /gg- (Ni-32.3 wt % Mo-6.3 wt % Al) and increase the elevated temperature creep strength. The 1000 C slow plastic compressive flow stress-strain rate properties in air of as-grown, annealed, and worked nominally 10 and 25% materials have been determined. Swaging did not improve the slow plastic behaviour. In fact large reductions tended to degrade the strength and produced a change in the deformation mechanism from uniform flow to one involving intense slip band formation. Comparison of 1000 C tensile and compressive strength-strain rate data reveals that deformation is independent of the stress state.     

Microstructures and microwave dielectric characteristics of ceramics with the composition BaO·Nd2O3·5TiO2

The microstructures and the microwave dielectric characteristics of BaO·Nd₂O₃·5TiO₂ ceramics were determined as a function of the sintering conditions. The ceramics were confirmed to have a ternary orthorhombic major phase with minor amounts of Nd₂Ti₂O₇ and TiO₂, and nearly equiaxed fine structures and homogeneous columnar structures were obtained at sintering temperatures of 1340 C and 1370 C, respectively, while discontinuous and excess grain growth were observed at sintering temperatures 1350–1360 C and 1380 C, due to the formation of a liquid phase. The desired value of Qf was achieved in ceramics with fine-grained and/or homogeneous microstructures, and the discontinuous and excess grain growth was found to be seriously harmful to the dielectric properties.     

Long-term oxidation behaviour of Ni3Al alloys with and without chromium additions

The oxidation behaviour of Ni₃Al alloys with and without chromium additions was studied after long-term exposure in an air furnace over a wide temperature range from 560–1300C. The chromium-containing alloy exhibited a better oxidation resistance below 1150C. Above this temperature the penetration depth of oxides into and along the grain boundaries in the chromium-containing alloy became larger than that in the alloy without chromium. Also the penetration depth increased drastically at 1150C for both alloys. Through energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy analyses, the compositions and structures of the oxide scales for both alloys were also identified.     

Temperature-dependent change of Cu-O bond in La2CuO4 and YBa2Cu3O7

Valence-band spectra of La₂CuO₄ and YBa₂Cu₃O₇ were obtained by using X-ray photoemission varying the temperature of the measurement (35, 260, 300, and 500 C). In La₂CuO₄ the broad band centred around 4eV splits into two peaks at 260 C. At both 35 and 500 C the spectra show almost similar shape. In YBa₂Cu₃O₇ the broad peak centred around 4eV splits into two peaks at 500 C. Below 300 C the spectra show almost similar shape. These splittings of the valence-band spectra may be due to the rearrangement of the crystal structure accompanying the phase transition from the orthorhombic to tetragonal symmetry.     

Wetting of nickel by silver

Sessile drop experiments of molten silver and nickel were performed in air and helium at 970 C. A NiO layer formed at the interface in air; silver formed a 90 contact angle. In helium silver formed a 9 contact angle on nickel. The role of solution reactions in forming these angles is discussed.     

Antimicrobial and antifungal agents derived from clay minerals

Montmorillonite supported by Ag⁺-chelate of hypoxanthine was synthesized and heat-treated at 200 C intervals below 800 C under nitrogen. These samples were subjected to examinations of the structure and properties including antimicrobial activity. Hypoxanthine itself did not possess antimicrobial activity to bothStaphylococcus aureus andEscherichia coli, but montmorillonites supported by the Ag⁺-chelate exhibited activity. Hypoxanthine in montmorillonite decomposed at 400–600 C; and Ag metal particles deposited after heating to 600 C and grew with raising heating temperature. The Ag⁺-chelate montmorillonite showed clear antimicrobial activity even after heating to 800 C.     

Sintering mechanisms and microstructural development of coprecipitated mullite

Coprecipitated mullite precursor powders of the bulk compositions 78 wt% Al₂O₃+22 wt% SiO₂ (high-Al₂O₃ material) and 72 wt% Al₂O₃+28 wt% SiO₂ (low-Al₂O₃ material) have been used as starting materials. The precursor powders were calcined at 600, 950, 1000, 1250, and 1650 C, and test sintering runs were performed at 1550, 1600, 1650, 1700, and 1750 C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (CIPed) powders sintered in air at 1700 C. Therefore, all further sintering experiments were carried out at 1700 C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600 C and 200 bar argon gas pressure. Sintering densifications of low Al₂O₃ materials ranged between 94% and 95.5%. There was no clear dependency between densification and calcination temperature of the starting powders. High-Al₂O₃ compositions displayed sintering densities which increased from 97% at 600 C calcination temperature to 99% at 950 C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to 95.5% (calcination temperature 1250 C) but later the density strongly decreased to a value of 85% (calcination temperature 1650 C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100 C yielded 100% density. At the given sintering temperature of 1700 C, the microstructure of sample specimens was influenced by Al₂O₃/SiO₂ ratios and by calcination temperatures of the starting powders. Homogeneous and dense microstructures consisting of equiaxed mullite plus some minor amount of -Al₂O₃ were produced from high-Al₂O₃ powders calcined between 600 and 1100 C. Low-Al₂O₃ sample specimens sintered from precursor powders calcined between 600 and 1100 C were less dense than high-Al₂O₃ materials. Their microstructure consisted of relatively large and elongated mullite crystals which were embedded in a fine-grained matrix of mullite plus a coexisting glass phase. The different microstructural developments of high- and low-Al₂O₃ compositions may be explained by solid-state and liquid-phase sintering, respectively. The microstructure of HIPed samples was very similar to that of pressureless sintered materials, but without any pores occurring at grain boundaries.     

Formation and characteristics of lead lanthanum zirconium titanate thin films formed by using the r.f. sputtering method

PLZT (lead lanthanum zirconium titanate) thin films were prepared by using the r.f. magnetron sputtering method and post-annealing for crystallization at 650 C. The films which were annealed at 650 C for 10 min consisted of a metastable phase and a stable phase. However, another film which was annealed at 650 C for 20 min had only stable perfect perovskite phase. The stability of the post-annealed thin film and substrate interfaces was observed by using scanning electron microscopy. The longer the annealing time, the more unstable were the interfaces. By analysing the EDX data, the composition difference between the sputtering target and thin films, and the composition variation between as-deposited and post-annealed PLZT were studied. The films annealed at 650 C for 20 min showed good ferroelectric and electrical properties with a remanent polarization (P _r) of 11.5 C cm^–2, and a coercive field (E _c) of 164 kV cm^–1.     

Phase transformations in the thermotropic liquid crystal polymer: 60/40 PABA/PET

Microstructures observed in 60/40 PABA/PET co-polyester in transmitted polarized light are reported. The microstructure changes as a function of temperature. Between 190 and 340 C the optical textures are similar to those seen in small molecule liquid crystals in the smectic C modification; above 340 C the textures are typical of nematic structures. At 420 C the specimen is totally isotropic and begins to degrade. Rapid cooling to below 190 C can quench in the high temperature phases, including the isotropic one. DSC traces show endotherms identifiable with the onset of mobility at 190 C, the transition from smectic C to nematic-like textures at 340 C and the development of the isotropic phase in the range 350 to 420 C. The smectic C to nematic transition in texture is associated with the appearance of a transient microstructure, known as a myelin texture, and reported here for the first time in a liquid crystal polymer.     

Observations on the mechanical properties of nickel oxide scales

The mechanical properties of NiO scales produced by the complete oxidation of high-purity (grade-1) Ni and commercial-purity (grade-A) Ni have been investigated at 700 to 1000 C. The modulus of elasticity of both grades of oxide decreased with increasing temperature, whereas the modulus of rupture for grade-A oxide exhibited a maximum at 850 C and that for grade-1 oxide decreased with increasing temperature. At 700 C, elastic deformation to fracture occurred with both oxides, whereas, at temperatures 850 C, plasticity was also observed. The plasticity of grade-1 oxide was 3 times greater than that of grade-A oxide.Creep behaviour of the oxides was studied at 900 and 1000 C. Primary and secondary creep was observed and, in both oxides, the creep rates increased with increasing temperature and load. The creep rate of grade-1 oxide was 10 to 20 times greater than that for grade-A oxide.     

Sub-eutectoid aged Mg-PSZ alloy with enhanced thermal up-shock resistance

Mg-PSZ samples prepared in the as-fired condition were strong (550 to 600 MPa) but had poor thermal up-shock resistance in that they shattered during a standard test. However, ageing at 1100 C enabled 70% of their original strength to be retained after thermal up-shock damage. This marked improvement in thermal shock resistance was attributed to the conversion of tetragonal precipitates to the monoclinic state by a process other than particle growth. It is proposed that the precipitate/matrix interface which governs the crystal structure of a precipitate at room temperature was altered by a type of solid state reaction, not previously reported, which occurs during ageing at 1100 C.     

Yttria-stabilized hafnia-zirconia thermal barrier coatings: The influence of hafnia addition on TBC structure and high-temperature behaviour

The search for more reliable and durable thermal barrier systems is a key factor for future aircraft turbine engines success. Hafnia is therefore an attractive ceramic component due to its similarity to zirconia and its elevated structural transformation temperatures. We report here structural and thermomechanical investigations of various plasma-sprayed coatings composed of ZrO₂+x mol% HfO₂ (x=0, 25, 50 and 100), partially stabilized by 4.53 mol% yttria. X-ray diffraction studies show that, a metastable, non-transformable, high yttrium content, tetragonal solid solution is the only phase observed on the as-sprayed samples. This phase is crystallographically equivalent to the t phase described for classical yttrium-partially stabilized zirconia (Y-PSZ) thermal barrier coatings (TBCs). Upon high-temperature annealing in air (T=1200C), however, the return of this t phase to equilibrium differs from the classical tt+c reaction. According to literature data, reactions of the type tt+c+m should prevail at the highest hafnia contents (x50). Indeed, important quantities of monoclinic phase are accordingly being observed upon cooling. Thermal cycling of TBC samples in air has been performed at 1100C. The Young's modulus of the ceramic coating, which progressively increases when hafnia is substituted for zirconia, has a strong influence on TBC thermomechanical resistance.     

Effect of the fluctuation range of formation temperature on the preparation and superconductivity of Bi(Pb)2223 single-phase

Effects of the fluctuation range of formation temperature on preparation and superconducting properties of the Bi(Pb)2223 phase have been investigated in detail. Our results show that knowledge about the temperature distribution and fluctuation in the muffle furnace is necessary for preparation of the Bi(Pb)2223 single-phase. Single-phase samples from Bi_0.17Pb_0.3Sr₂Ca₂Cu₃O_y were obtained by a solid-state reaction in air at 835±5C. The best superconducting property of the as-prepared samples shows a one-step transition of a.c. susceptibility withT _c=109 K. Using this optimum sintering temperature and keeping the temperature fluctuation less than 5C, we have prepared pure Bi(Pb)2223 single-phase samples with good reproducibility.     

Fabrication and characterization of polymer-derived Si2N2O-ZrO2 nanocomposite ceramics

Amorphous Si-Zr-N-O powders, obtained by nitridation in an NH₃ flow of zirconium modified polycarbosilane, have been sintered to full density by hot pressing at 1500C. The resulting ceramic shows an extremely fine-grained microstructure composed of Si₂N₂O and ZrO₂ crystallites 20–30 nm in diameter. Thermal stability measured in air appears excellent up to 1300C for 48 h. Mechanical characterization pointed out good values of flexural strength (330 MPa), fracture toughness (4.1 MPam^0.5) and Weibull modulus.     

Phase stability and oxygen transport characteristics of yttria- and niobia-stabilized bismuth oxide

As a part of an overall study to explore the potential application of stabilized Bi₂O₃ as oxygen separator in various electrochemical systems, an investigation of the stability and transport characteristics of yttria- and niobia-stabilized bismuth oxide was undertaken. Polycrystalline Bi₂O₃ samples containing 25mol % Y₂O₃ were fabricated by pressureless sintering powder compacts at 1000 C in air. Samples containing 15mol % Nb₂O₅ were also fabricated by pressureless sintering at 900 C in air. The resulting samples were dense and of an equiaxed microstructure with grain size in the range from 28m for the yttria-stabilized and 42m for the niobia-stabilized materials, respectively. X-ray diffraction of the as-sintered specimens showed them to be single phase with CaF₂-type structure. Ionic conductivity was measured by an a.c. technique over a wide range of temperatures. It was observed that the ionic conductivity of the yttria-stabilized bismuth oxide was greater than that of the niobia-stabilized one.The specimens subsequently were annealed over a range of temperatures between 600 C and 700 C for up to several days. X-ray diffraction traces taken on the Y₂O₃-stabilized samples indicated that the original cubic solid solution had decomposed. The decomposition of the yttria-stabilized samples was also accompanied by the occurrence of exaggerated grain growth. The observed decomposition is not in agreement with the phase diagram available in the literature, according to which the cubic phase should be stable over the range of temperatures the samples were annealed in the present study. By contrast, Nb₂O₅-stabilized Bi₂O₃ remained cubic, although it appeared to have dissociated into two cubic solid solutions of slightly differing lattice parameters. There was no perceptible change in the grain size of the niobia-stabilized samples.Several electrolyte tubes made of the yttria- and niobia-stabilized bismuth oxide were electrolytically tested under a d.c. mode with silver electrodes. In tubes made of the yttriastabilized material, the current density decreased with time (under a constant applied voltage) at 650 C and at 700C but did not at 700C consistent with the observation that the material did not decompose at 700 C but did at 650 C. At 600 C, the rate of decrease was slower than at 650 C indicating that the kinetics of phase decomposition is probably slower at 600 C. In the niobia-stabilized tubes the decrease in the current density was lower. This decrease is probably related to the apparent formation of two cubic solid solutions of slightly differing compositions.The present work shows that the published phase diagram of the Y₂O₃-Bi₂O₃ system is incorrect. The present results also suggest that for application to temperatures as low as 650 C (and possibly lower), electrolytes made with Nb₂O₅ as the stabilizer are preferable.     

Deformation in spinel

Stoichiometric MgAl₂O₄ spinel was deformed in compression at temperatures from 1790 to 1895 C and the dislocation structures analysed by transmission electron microscopy. {1 1 1}1 1 0 slip was observed on both the primary and cross-slip systems, and there was much secondary slip as well; all six 1 1 0 Burgers vectors were present in electron micrographs. This secondary slip leads to very high work-hardening rates, approximately/70 at 1790 C, where is the shear modulus. Since it is known that deformation in nonstoichiometric (alumina-rich) spinel crystals occurs by {1 1 0}1 1 0 slip, the electrostatic and geometric aspects of 1/4 110 dislocations moving on {1 1 1} and {1 1 0} planes are considered in some detail. It is porposed that the octahedral cation vacancies present in non-stoichiometric spinel diffuse to dislocations during deformation and thus favour {1 1 0} slip.     

Uniplanar-axial orientation in hot-rolled polymers

The orientations produced by high temperature rolling of polyoxymethylene (126 C) and polytetrafluoroethylene (150 C) and nylon 66 are examined by pole figures. No evidence is found to support the theory of Akahane and Mochizuki that it is the plane of the zig-zag chains that orientates into the rolling plane in nylon 66. The classical rolling texture of Bunn and Garner describes the pole figures except for the (100) pole, which is observed at 28 to the transverse direction; in the texture of Bunn and Garner it should occur at 24 . In hot-rolled polyoxymethylene the texture observed corresponds to a unique orientation of the hexagonal unit cell: (10¯10) planes parallel to the rolling plane and c-axis parallel to rolling direction. The polyoxymethylene texture differs significantly from the pseudo-fibre textures observed in the cold-rolled polymer. The rolling texture of hot-rolled polytetrafluoroethylene is similar to that of hot-rolled polyoxymethylene. These results show the plastic slip system in polyoxymethylene and polytetrafluoroethylene to be (10¯10) [0001].     

Grain boundary sensitization and desensitization during the ageing of 316L(N) austenitic stainless steels

Two casts of type 316L(N) austenitic stainless steel have been solution treated for 1 h at 1070 C, air-cooled, then aged for up to 20 000 h at temperatures between 550 C and 750 C. Grain boundary precipitation of the M₂₃C₆ phase occurs, and the Cr composition profile normal to the grain boundaries has been determined at high resolution by an analytical electron microscope. The data have been fitted firstly to collector plate models, which indicated that some of the material was in the process of desensitization, or healing, indicated by a rise in the boundary Cr content. The data were then fitted to a model of the Cr profile as a function of ageing treatment in the healing regime, and a good correlation was obtained.     

Plastic deformation of polycrystalline zirconium carbide

The compressive yield strength of arc melted, polycrystalline zirconium carbide has been found to vary from 77 kg mm² at 1200C to 19 kg mm² at 1800C. Yield drops were observed with plastic strain-rates greater than 3×10^–3sec^–1 but not with slower strainrates. Strain-rate change experiments yielded values for the strain-rate sensitivity parameterm which range from 6.5 to 1500C to 3.8 at 1800C, and the productm ^*(T) was found to decrease linearly with increasing temperature. The deformation rate results are consistent with the Kelly-Rowcliffe model in which the diffusion of carbon assists the motion of dislocations.