Subsolidus phase relations in the ZnS-In2S3 system: 600 to 1080° C

Eleven ternary compounds were synthesised, of which nine are new. The seven most ZnS-rich compounds form a series. Each has a primitive hexagonal lattice witha=3.85±0.01 Å. Thec-dimension varies from 37.47 Å (39 ZnS8 In₂S₃) to 18.63 Å (18 ZnS8 In₂S₃) with a periodicity of 3.14 Å which can be related to a decrease in the ZnS content. One of the four remaining compounds (12 ZnS8 In₂S₃) is triclinic, but at least two polytypes are present. The more common variety has the parameters:a=3.86 Å,b=15.48 Å,c=3.54 Å,=90° 16,=120° 02 and=89° 47. The structure of the other polytype and of compounds 17 ZnS:8 In₂S₃ and 10 ZnS8 In₂S₃ have not been determined. Stoichiometric ZnIn₂S₄ has a rhombic hexagonal lattice (a=3.86 Å,c=36.95 Å). This ternary compound which has been synthesised previously is the only one to show appreciable solid solution. Approximately 2 mole % ZnS is soluble in ZnIn₂S₄ at 600° C; this increases to 8 mole % at 1080° C.     

Phase equilibrium,microstructure and properties of some magnesite-chromite refractories

The effect of phase equilibrium and microstructure of magnesite-chromite batches containing from 0 to 100% Egyptian chrome ore, with intervals of 10%, on their physical properties was studied. The phase equilibrium data were calculated using the phase relationships within the system M-M₂S-CMS-MR (M=MgO, S=SiO₂, C=CaO, R=R₂O₃). A computerized electron-probe microanalyser was applied to study the microstructure as well as microchemistry of the fired magnesite-chromite co-clinkers. Some physical and technological properties of the co-clinkered briquettes were also investigated by determining densification parameters, spalling resistance and load-bearing capacity.It is concluded that dense, spalling resistant and refractory magnesite, magnesite-chrome and chrome-magnesite refractories could be produced by co-clinkering of magnesite-chromite batches of 1000, 7030 and 3070 weight ratios, respectively, at 1600 °C. The prepared co-clinkers were subsequently graded, moulded and refired up to 1700 °C in order to obtain direct-bonded bricks. Meanwhile, dense chromite refractories with lower refractory quality could be processed by firing the Egyptian chrome ore up to 1600 °C.     

Modelling of dimensional changes during polymer-ceramic conversion for bulk component fabrication

Shrinkage and porosity generation during conversion of polymer-filler systems into ceramic bodies during pyrolysis is examined. In the presence of an inert filler phase such as Si₃N₄ or SiC powder dispersed in an organosilicon polymeric matrix only porous microstructures may be obtained without any shrinkage. By using an active filler phase such as carbide- or nitride-forming transition metals, however, shrinkage of the polymer matrix may be compensated by appropriate expansion of the filler phase. A model is derived to predict the critical volume fractions of various potential active filler systems in inert and reactive gas atmospheres, which can be effective in controlling shrinkage and porosity during the fabrication of ceramic components from polymer-derived precursor materials.Nomenclature P Polymer phase - C Condensed polymer pyrolysis product (ceramic) - G Gaseous polymer decomposition product - F Inert filler phase - T Active filler phase (e.g. transition metal) - M Reaction product of active filler phase (e.g. carbide, nitride) - m Mass - V Volume fraction - V _F,T ^max Maximum packing density of an inert (F) or active (T) filler powder - V _F,t ^* Critical volume fraction of an inert (F) or active (T) filler powder in the starting polymer-filler mixture - V _v Residual porosity in the polymer pyrolysis product - V _v ^pf Residual porosity in the polymer-inert filler system after pyrolysis - Ceramic yield of polymer after pyrolysis - Weight change of active filler phase during reaction pyrolysis - Density ratio of polymer to polymer pyrolysis (ceramic) product - Density ratio of active filler to filler reaction product - ^P Linear shrinkage of the polymer phase during pyrolysis - ^pf Linear shrinkage of a polymer-inert filler system during pyrolysis - ^paf Linear shrinkage of a polymer-active filler system during reaction pyrolysis - Linear shrinkage/expansion of the filler phase during reaction - Density     

Characterization of mono- and diphasic mullite precursor powders prepared by aqueous routes. 27Al and 29Si MAS-NMR spectroscopy investigations

The structural evolution from amorphous to crystalline mullite, for different 3Al₂O₃ · 2SiO₂ mono- and diphasic precursors, has been investigated by ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The crystallization has also been studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chemical composition in the aluminosilicate network of the diphasic precursors and in the crystallized phases has been determined from the ²⁹Si NMR spectra. A close agreement is found with the composition deduced from the lattice parameters measured by XRD. For monophasic precursors the amount of hexa-coordinated aluminium atoms decreases when the temperature increases while Al(IV) and Al(V) increase. Al(VI) practically completely disappears just before the crystallization at 980 °C. An alumina-rich mullite 2Al₂O₃ · SiO₂ (21 mullite) is then formed through a strong exotherm. An enthalpy of 75 kJ per mol is determined for the crystallization of the 21 mullite. At higher temperatures the segregated silica is progressively reincorporated into the mullite lattice. For diphasic precursors the ²⁹Si NMR spectroscopy shows the segregation of silica. The aluminosilicate network is then richer in alumina and the amount of remaining AlO₆ octahedra before the crystallization at 980 °C is higher. Spinel crystallizes and continues to become richer in alumina until it reacts with silica to form the stoichiometric 32 mullite at 1260–1275 °C. The nature of the crystallization is related to the local composition of the amorphous alumino-silicate network and to the amount of AlO₆ octahedra on approaching 980 °C.     

Low-temperature synthesis,pyrolysis and crystallization of tantalum oxide gels

Tantalum oxide gels in the form of transparent monoliths and powders have been prepared from hydrolysis of tantalum pentaethoxide under controlled conditions using different mole ratios of Ta(OC₂H₅)₅C₂H₅OHH₂OHCl. Alcohol acts as the mutual solvent and HCl as the deflocculating agent. For a fixed alkoxide water HCl ratio, the time of gel formation increased with the alcohol to alkoxide molar ratio. Thermal evolution of the physical and structural changes in the gel has been monitored by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. On heating to 400 °C, the amorphous gel crystallized into the low-temperature orthorhombic phase -Ta₂O₅, which transformed into the high-temperature tetragonal phase -Ta₂O₅ when further heated to 1450 °C. The volume fraction of the crystalline phase increased with the firing temperature. The -Ta₂O₅ converted back into the low-temperature phase, -Ta₂O₅, on slow cooling through the transformation temperature of 1360 °C, indicating a slow but reversible transformation.     

The self-propagating high-temperature synthesis (SHS) of ultrafine high-purity TiC powder from TiO2+Mg+C

The self-propagating high-temperature synthesis (SHS) method has been applied to the preparation of TiC powder from mixtures of TiO₂, Mg, and C. The TiC/MgO product is leached by hydrochloric acid to remove the MgO. The optimum mixing ratio was TiO₂MgC=12.21.5. The final product TiC had > 99.9% purity and relatively uniform particle size of 0.3–0.4 m.     

Growth of Mg2TiO4 single crystals by the floating zone method

Single crystals of the peritectic compound Mg₂TiO₄ up to 8 mm diameter and 80 mm long along the [1 0 0] and [1 1 1] directions, were grown by the travelling solvent float zone (TSFZ) technique using a halogen lamp image furnace. The optimum conditions were as follows: sintering temperature of the charge rod, 1500° C, the growth rate, 2 mm h^–1 and the composition of the charge rod in molar ratio, MgOTiO₂=21.01. The chemical composition of the solvent zone was determined by application of the EPMA technique and the peritectic nature of Mg₂TiO₄ was proved. The refractive index of the grown crystal was 2.05.     

A precipitate phase in AA2124

A new precipitate phase (designated X phase) other than the S(Al₂CuMg) precipitate phase has been discovered in the 2124 aluminium alloy. Using selected-area electron diffraction analysis, dynamical diffraction simulation, energy dispersion analysis of X-rays and high-resolution electron microscopy, it is suggested that the X phase has an orthorhombic crystal structure witha=0.492 nm,b=0.852 nm andc=0.701 nm. The space group of the phase is Cmmm. There are 10 atoms in a cell with AlCuMg=244. The orientation relationship between the X phase and the matrix is determined as (1–10)_m(010)_x, [–1–1–1]_m [001]_x.     

Surface nucleation and cellular growth kinetics of cordierite glass ceramics containing 3 mol % Y2O3-ZrO2

Cordierite-based glass ceramics of the 2MgO2Al₂O₃5SiO₂ composition with t-ZrO₂ (3 mol% Y₂O₃-ZrO₂) and P₂O₅ addition, was heat-treated isothermally and isochronically for crystallization studies. Major crystalline phases incurred by the heat treatment were t-ZrO₂ and -cordierite. Surface nucleation predominated when edge and corner nucleation in these samples were suppressed regardless of their radii of curvature. Crystallization began with the formation of -quartz S.S. and its transformation to -cordierite was followed by prolonged heating. Cellular growth of -cordierite on the surface of the quenched glass plates, gave a linear kinetics. The activation energy for cellular growth was 410 kJ mol^–1.     

Effect of gel parameters on monolithicity and density of silica aerogels

As part of continuing investigation of the preparation and characterization of silica aerogels, detailed experimental results on monolithicity and density of the aerogels as a function of catalysts and their concentrations, molar ratios of tetraethylorthosilicate (TEOS), ethanol (EtOH) and H₂O, gel pH and gel ageing, are reported. With large concentrations of catalysts, lower and higher aged alcogels have been found to produce opaque, cracked and highdensity silica aerogels; however, good-quality monolithic, transparent and low-density aerogels have been obtained at lower catalyst concentration (0. 01n HCl) with alcogel ageing periods between 9 and 30 days. The best quality silica aerogels, in terms of transparency and monolithicity, have been obtained using TEOSEtOHH₂O in the molar ratio of 158.     

Singular Band Behavior of the Extended Emery Model for the Superconducting Cuprates

Mean field slave-boson approximation is performed on the extended Emery model for the CuO₂ conducting plane. The model is parameterized by Cu–O charge transfer energy _pd , copper–oxygen overlap t ₀, oxygen–oxygen overlap t', and Coulomb interaction U on the copper site taken as infinite. Special emphasis is placed on the role of t in the renormalization trends of the effective band parameters _pf and t, replacing _pd and t ₀, at small doping . It is shown that small, negative t expands the range of stability of the metallic phase, changing, in the second order of the perturbation theory, the nature of the metal–insulator transition point. In the nonperturbative limit, t modifies strongly the renormalization of _pf , making it saturate at the value of 4t. Finite doping suppresses the insulating state approximately symmetrically with respect to its sign. The regime _pf 4t fits very well the ARPES spectra of Y123, Bi2212, and LSCO and also explans, in the latter case, the evolution of the FS with doping accompanied by the spectral weight-transfer from the oxygen to the resonant band.     

Preparation of superconducting Bi-Sr-Ca-Cu-O ceramics by the sol-gel method

Superconducting Bi-(Pb)-Sr-Ca-Cu-O ceramics were prepared through the sol-gel method using an aqueous solution of metal acetates containing acetic acid and tartaric acid. The conditions of gelation and conversion of gel to Bi₂Sr₂CaCu₂O_x phase were studied by thermal analysis, infrared absorption spectroscopy and X-ray diffraction technique. It was found that transparent gels were obtained from the solutions containing tartaric acid, with a molar ratio of C₄H₆O₆/Cu of about 0.4. The gel matrix was assumed to be composed of carboxylate anions and metal cations. A gel of molar composition BiSrCaCu = 2223 was decomposed to CaCO₃, CaO, CuO and Bi_1-x Sr_xO_y (x = 0.2 to 0.3) at low temperatures of 270 to 600 ° C and produced Bi₂Sr₂CaCu₂O_x phase at 800 ° C via complex intermediate states. A rapid cooling of the heated product increasedT _c (end) in the gel of molar composition BiSrCaCu = 2223, while a slow cooling increasedT _c (end) in the lead-containing gel of molar composition BiPbSrCaCu = 1.850.351.92.03.1. The latter showedT _c (onset) at 115 K andT _c (end) at 105K.     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

Effects of alkali-metal ions on the thermal decomposition of ammonium metavanadate supported on alumina

The thermal decomposition of ammonium metavandate (AMV) supported on -alumina (11 mol) and treated with alkali-metal ions was investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), infrared (i.r.) absorption spectroscopy and X-ray diffraction (XRD). The results revealed that the presence of alkali-metal ions did little to affect the thermal decomposition processes of AMV supported on alumina. However, these ions enhanced creation of V⁴⁺ ions by heating the samples in air at 400 °C. The oxides produced underwent a solid-solid interaction at 500–700 °C, forming alkali-metal vanadates. The formation of these spinels was found to be strongly dependent on the degree of diffusion and the concentration of the alkali-metal ions. Moreover, the addition of a higher concentration of alkali-metal ions inhibits the formation of aluminium vanadate while enhancing the crystallinity of - to -alumina.     

Thermochemical modelling in CO2 laser cutting of carbon steel

A thermochemical heat transfer model in oxygen-assisted laser cutting of carbon steel has been developed in terms of the laser mode pattern, the power density, combustion reaction, kerf width and cutting speed. This model emphasizes the chemical combustion effect as well as the laser mode pattern, which are usually neglected by most existing laser cutting models. Good agreement was obtained between theoretical and experimental results, indicating that approximately 55–70% of the cutting energy is supplied by the combustion reaction of the steel with oxygen, which is consistent with experimental data obtained by other investigators.Nomenclature a Focused laser beam diameter (m) - A Absorptivity - H Heat of combustion (J kg^–1) - I Power density (Wm^–2) - k Thermal diffusivity (m²s^–1) - K Thermal conductivity (Wm^–1K^–1) - K ₀ Modified Bessel function of the second kind and zeroth order - l Workpiece thickness (m) - P Laser power (W) - q Heat rate (W) - q q/l heat rate per unit length (Wm^–1) - R Half the kerf width (m) - s VR/2 normalized cutting speed - T _mp Melting temperature (K) - T _rm Room temperature (K) - T(x, y) Temperature at (x, y) (K) - V Cutting speed (ms^–1) - W 2R kerf width (m) - x, y, z Cartesian co-ordinates - Thermal diffusivity (m² s^–1) - Average thickness of liquid melt film (m) - Combustion efficiency - , Polar co-ordinates - Material density (kgm^–3)     

Formation and characterization of oxynitride glasses in the Si-Ca-Al-O-N and Si-Ca-Al,B-O-N systems

Oxynitride compositions in the Si-Ca-Al-O-N and Si-Ca-Al, B-O-N systems were melted and furnace-cooled in BN crucibles at temperatures from 1650 to 1850° C under dry nitrogen atmosphereS. Glass formation, phase stability and crystallization were studied by characterizing the cooled melts by X-ray diffraction, DTA, and electron microscopy. Oxynitride batches with nitrogen content up to 11 at % formed glasses in the Si-Ca-Al-O-N system. Glasses in the Si-Ca-Al, B-O-N system could be formed only when the B₂O₃ content of the batch was less than 3 wt %. Oxynitride glasses in these boron-containing systems were characteristically inhomogeneous, difficult to process, and prone to crystallization. In both the systems, glasses exhibited glass transitions beginning at 1000° C and crystallization at 1300 to 1500° C. Nitrogen-containing crystalline phases were identified in devitrified glasses via microstructural and micro-mechanical analyses.     

Nitriding of iron boride to hexagonal boron nitride

A process for preparing hexagonal boron nitride (h-BN) from iron boride (FeB) has been described for the first time. FeB powder was nitrided in ammonia at 700 K for 24 hours yielding non-crystalline BN and -Fe₂N. Subsequent annealing in ammonia for 1 hour at 1273 K resulted in a mixture of crystalline h-BN together with -Fe, -Fe₃N and -Fe₄N. The morphology of the product grains is described. This comprises an iron nitride core loosely encapsulated in a foliated BN layer. The open nature of the product layer may account for the relatively low temperature (700 K) required for the nitriding reaction. These iron containing phases were removed with dilute mineral acid leaving h-BN and trace amounts of FeB₄₉.     

Charged defects-controlled conductivity in Ge-ln-Se glasses

The variation of the d.c. electrical conductivity, , with composition and temperature was investigated for glasses of the Ge-In-Se system. The results indicate a decrease in the activation energy for electrical conductivity, E, and an increase in on introduction of indium into Ge-Se glasses. The changes in E and with composition (selenium content in the glasses) are identical for the Ge_x In₅ Se_95–x and Ge_x In₈Se_92–x families. The results have been traced to the conduction controlled by charged defects in these chalcogenide glasses. The changes in E and have been explained by a shift in the Fermi level, being brought by the introduction of indium.     

The sequence of precipitation in 339 aluminum castings

The precipitation sequences in direct-quenched from the die (DQD) and solution-treated (SOL) 339 aluminum have been determined by a combination of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). DSC scans for the alloy in both conditions exhibit two distinct exothermic peaks, each associated with a unique precipitate. The peak temperatures for precipitation in the DQD and SOL alloys differ by only a few degrees. TEM of samples heated to the lower temperature peak shows that the first precipitate to form in the DQD alloy is S (Al₂CuMg), whereas in the SOL alloy it is (Mg₂Si). The principal precipitate associated with the higher temperature peak in both DQD and SOL alloys is Si. The DSC peak temperature identifies the specific precipitate in 339 Al, but the peak area is not a reliable measure of precipitate density. Nano-indentation of the dendrites shows that the strength provided by the precipitates increases in the sequence Si < S < . However, their thermal stability increases in the reverse order.     

Phases on the “Perovskite Line” in the La-Sr-Cu-O System

Our recent research in the La-Sr-Cu-O system has revealed the existence of two new oxygendeficient compounds. One is (La_1–x Sr _x )₄Cu₄O₁₀ (x=0.135–0.15 4-4-10 phase), which has the same structure as CaMnO_2.5 and transforms to the well-known (La_1–x Sr _x )₈Cu₈O₂₀ (x=0.135–0.30 8-8-20 phase) at higher temperature. The other is (La_1–x Sr _x )₅Cu₅O₁₃ (x=0.125–0.1666 5-5-13 phase), which is synthesized under high oxygen pressure and is isostructural to La₄BaCu₅O₁₃ (4-1-5 phase). We have also studied the strontium-rich compound (La_1–x Sr _x )₈Cu₈O₁₆₊ (x=0.7–0.8 8-8-16+ phase) and isolated the semiconductor-like 8-8-16 phase (0.0) and the metallic 8-8-18 phase (>2.0). Cations (La and Sr) in several of these compounds on the (La+Sr)Cu=11 line, which we call the perovskite line, are distributed in a disordered manner and their crystal structures can be explained by the systematic removal of oxygen atoms from an ideal perovskite structure.