Analysis of the effect of the decomposition of char-forming heat-insulating materials on the free discharge of a gas mixture

The article presents results of a numerical solution of a nonsteady problem on the free discharge of a mixture of gases from a hemispherical volume with allowance for thermal decomposition of heat-insulating materials.Notation V volume - S area - t - P p - T - u v - Q q, dimensional and dimensionless time, pressure, temperature, TIM decomposition rate, and heat flux - adiabatic exponent - R gas constant - density - H specific enthalpy - c specific heat - thermal conductivity - , , _s dimensionless complexes - coefficient expressing the radiative properties of the gas medium and the heat-transfer surface - Stefan-Boltzmann constant Indices 0 initial state and scale factors - s surface - coke - M TIM material - P pyrolysis front - A ablation front - v volatile degradation products - adiabatic conditions - c completion of discharge Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 54, No. 5, pp. 787–793, May, 1988.     

Effect of zinc on the magnetic properties of acicular cobalt-modified γ-Fe2O3 particles

Acicular -FeOOH particles with a particle length of about 0.35 m and an axial ratio of about 7 were synthesized by the coprecipitation method using the reaction of FeCl₂-NaOH. The (Co, Zn)-modified -F₂O₃ particles were produced by absorbing Co²⁺ and Zn²⁺ ions on the surfaces of -FeOOH particles followed by dehydration, reduction and oxidation. The saturation magnetization and thermal stability of the coercivity of (Co, Zn)--Fe₂O₃ particles were all higher than those of Co--F₂O₃ particles. For the same (Co+Zn) content, the saturation magnetization of (Co, Zn)--Fe₂O₃ particles increased with increasing zinc content but the coercivity decreased.     

Precipitation of β particles in a fully lamellar Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si (at.%) alloy

The precipitation of (B2) particles at intermediate temperatures, between 760°C and 1050°C, is investigated in a fully lamellar TiAl-WMoSi alloy. The particles, having a thin-plate shape, usually precipitate on the ₂ side of ₂/ interfaces at low aging temperatures in an uneven two-dimensional growth mode. While those formed at higher aging temperatures, growing extensively within the ₂ plate and into the adjacent lamellae, have an ellipsoid shape. The growth of particles at low aging temperatures yields an activation energy of about 366 kJ/mol. It is suggested that at low aging temperatures the growth of particles proceeds via an ₂ precipitation process controlled by diffusion of W and Mo along the /₂ and / interfaces.     

Mathematical modelling of long ocean waves with a discontinuous density gradient

A mathematical model for studying the propagation of long internal ocean waves of finite amplitude is proposed. The vertical structure of the pressure perturbation is investigated and reduced to a Sturm–Liouville eigenvalue problem. In the continuous stratification case, the pattern of this vertical structure depends on the choice of the characteristic scale of a varying stratification parameter, denoted by . As this parameter asymptotically approaches a critical value (i.e. _cri), the amplitudes of the solution's normal modes increase considerably. The internal waves break and produce an unstable interface, which degenerates into a turbulent mixed layer. These conditions correspond to the critical state of wave existence. When < _cri a three-layer discontinuous gradient model is proposed to resolve the problem. It consists in specifying one solution within a thin intermediary layer and two solutions on either side of this layer. The results show that the use of appropriately matching interfacial conditions allows to obtain generally matching solutions, even for small values of the nonconstant stratification parameter .     

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.     

The process of heat and mass transport at the critical point of pure fluids

The effects of fast isentropic temperature propagation, called the piston effect, or critical speeding up, and slow mass diffusion, called critical slowing down, are investigated. A temperature propagation experiment in a spherical cell filled with pure SF₆ at critical density was performed during the Second German Spacelab Mission D2 in 1993. The results evidently confirm the presence of the piston effect both in the one-phase region and in the two-phase region. The numerical simulations are in remarkable good quantitative agreement with the experimental results.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

The Ni3Al-Ni3Cr-Ni3Ru section of the Ni-Cr-Al-Ru system

An investigation is reported of the 75 at% nickel section of the Ni-Cr-Al-Ru system at 1523 and 1273 K. Constitutional data obtained by electron probe microanalysis, X-ray diffraction and microscopical examination are presented as partial isothermal sections. At 1523 K, the major part of the section consists of phase, while the aluminium-rich region contains a and+ region; the extent of the solid solution of chromium and ruthenium in totals ~ 4 at%. The ruthenium-rich corner of the section shows a two-phase region consisting of + ruthenium-rich solid solution. At 1273 K the,+ and + ruthenium regions increase in extent. The/ mismatch values in the equilibrated alloys studied lie in the range ~ –0.08 to –0.39%. Constitutional features of as-cast alloys are also reported.     

Ageing characteristics of rapidly solidified Al-Li-Ti alloys at 473 K

The influence of titanium on the precipitate free zones (PFZs) and on the stable phase is investigated in four melt-spun Al-Li-Ti alloys with 2 mass% lithium and 0.10–0.35 mass% titanium. Rapid solidification gives rise to a homogeneous distribution of titanium on the wheel side of the ribbons, and to a cellular distribution of solute atoms in intermediate regions and on the gas side. Heat treatments of up to 1000 h at 473 K do not modify titanium distribution, which remains in solid solution. During ageing, preferential coarsening of particles occurs on cell walls. The growth rate of the PFZs is lower than in other Al-Li alloys and is mainly controlled by the titanium concentration of the alloys. The nucleation and growth of the stable phase is also delayed by the presence of titanium. These results indicate that the effect of titanium in solid solution is to retain vacancies thus reducing lithium diffusion rate.     

Synthesis of acicular iron oxide nanoparticles and their dispersion in a polymer matrix

An alternative forced hydrolysis approach for the synthesis of -FeOOH of varying dimensions was achieved. The -FeOOH particles were converted into -Fe₂O₃ in a colloidal process, which eliminated the agglomeration of -Fe₂O₃ particles. The acicular -Fe₂O₃ nanoparticles could be readily dispersed into an organic solvent. Preliminary studies have demonstrated that the dispersion of the -Fe₂O₃ nanorods in a polymer matrix is feasible leading to an organic-inorganic nanocomposite.     

Microstructural development in cast and aged Ni–Al–Cr-based alloys derived from the B2 type β-NiAl structure

The formation of intermetallic compounds consisting of nickel-rich B2-type NiAl (-phase) ductilized by two-phase A1 ()/L12 () regions provides the possibility of combining ductility and high-temperature performance. Similar microstructures can also form the basis of high-temperature shape memory alloys, due to martensitic transformation of the -phase to an L10-type product. One route by which –/ microstructures can be produced involves the use of chromium as a -stabilizer. However, microstructural development in such a case is complicated by the formation of -Cr precipitates.This paper examines microstructural development and stability in cast Ni-25 at % Al-14 at % Cr, Ni-29 at % Al-22 at % Cr and Ni-27 at % Al-8 at % Cr alloys, together with a more complex material, namely, Ni-20 at % Al-13 at % Co-9 at % Cr-4 at % Ti-1 at % Mo-1 at % V. Both the as-cast condition and samples aged at 850 and 1100 °C for 140 h are examined using transmission electron microscopy. The paper discusses the formation of L10 martensite, intradendritic ', interdendritic /' and -Cr precipitation.     

Preferentially orientated precipitation in MnTiO3 single crystals

Preferentially orientated precipitation of -TiO₂ (rutile) needles in single crystals of MnTiO₃ grown by a floating-zone method under a controlled atmosphere of oxygen fugacity was studied. Optical microscopy and X-ray analysis revealed that the orientation relation between the -TiO₂ precipitates and the MnTiO₂ matrix is: {000 1}MnTiO₃// {111}-TiO₂ and 11¯20MnTiO₃// 110-TiO₂. The precipitation phenomenon was explained by introducing a nucleation and growth mechanism which was experimentally supported by heat treating experiments.     

o′-β′ sialon ceramics

Powders of Si₃N₄, Al₂O₃ and SiO₂ were mixed with Y₂O₃ as sintering aid and hot-pressed to form o- sialon. During sintering, the o phase preferentially precipitated in the temperature region of 1550 to 1600 °C, and the phase precipitated at temperatures of 1700 °C and above. The resultant microstructure consisted of dual phases of o and . The mechanical properties of o- sialon were improved with increasing amount of the phase. When SiO₂ required for the formation of the o phase was all included in the grain boundary phase, o- sialon exhibited a three-point bending strength as much as 1400 MPa. The oxidation resistance of o- sialon is expected to improve due to the presence of the o phase, but actually was not so good due to the presence of the grain-boundary glassy phase and impurities.     

The critical curve in an antiferromagnetic superconductor with nonmagnetic impurities

The critical curve of a transition of the second kind in an antiferromagnetic superconductor (AFS) with nonmagnetic impurities has been studied. The AFS is described by using the mean-field model given by Nass, Levin, and Grest and assuming a one-dimensional electron band. We find that the points on the critical curve satisfy the thermodynamic stability condition for 0₁/₀5.04 and 0.49H_Q/₀1.64.Here ₁ is the inverse lifetime of a conduction electron for nonmagnetic impurity scattering,H _Q is the antiferromagnetic molecular field, ₀ is the zero-temperature order parameter of a superconductor in the absence ofH _Q and impurities. Further, ₁ and H_Q denote the values of these quantities for points on the critical curve. For ₁/₀>5.04 and H_Q/₀>1.64, the phase transition from the superconducting to the normal state is always of the second kind. Some thermal properties of the system near the critical curve have also been investigated and we find that these depends dramatically on the impurity concentration.     

Self-reinforcement in li-α-sialon ceramics

Self-reinforcement of Li--sialon ceramics by in situ growth of elongated and -sialon grains has been explored and analysed. Properties of Li--sialon ceramics are mainly determined by the overall starting composition and the crystalline modification of the starting Si₃N₄ powder which in turn determine the final microstructure of the materials. Both the morphology and crystalline phase of the elongated sialon grains have strong effects on the toughening mechanism. The results indicate that -sialons reinforced by elongated -sialon grains have advantages over similar materials reinforced by elongated -sialon grains because of the type of crack deflection toughening mechanism involved.     

Functional derivative ofT c with α2(ω)F(ω) for weak coupling anisotropic superconductors

Approximate analytic calculations of the functional derivative ofT _c with respect to ²()F() for anisotropic superconductors are presented, with the primary purpose of identifying the determining material parameters. The square-well model for the phonon-mediated electron-electron interaction, the weak coupling limit ( _c /2T _c 1), and separable anisotropy are used. The general behavior of T _c /²()F() is the same as that found in numerical calculations for the case of small anisotropy, a ²/(–*)1; the regime of * is also discussed.     

Anisotropy in the hardness and friction of calcium fluoride crystals

Anisotropy in the Knoop indentation hardness and the friction of diamond cones on calcium fluoride crystals has been investigated at experimental temperatures from 20 to 300° C. It is shown that the directions of minimum and maximum indentation hardness, on the (001) plane, are 110 and 100 respectively whilst the 1¯10 are harder than the 11¯2 directions on the (111) plane. Also, the sliding friction in the (001) plane is greatest in the 110 directions and least in the 100 and, on the (111) cleavage plane, [¯1¯12] sliding leads to higher friction than [11¯2]. The nature of anisotropy, for both hardness and friction measurements, does not change over the experimental temperature range covered in this work. Observations on the resultant deformation are made and these anisotropic properties are explained in terms of the effective resolved shear stresses developed on the {100} 011 primary slip systems at all experimental temperatures.     

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.     

Effects of amines on the formation of β-ferric oxide hydroxide

Colloidal particles of -ferric oxide hydroxide (-FeOOH) were prepared by ageing an FeCl₃ solution at 100°C in the presence of various amines with different numbers of nitrogen atoms. The particle and crystallite sizes of the formed -FeOOH decreased with increasing concentration of amines, but the particles produced contained no amine. The influence of triethanolamine on the growth of -FeOOH particles was more marked than those of ethylenediamine, diethylenetriamine and triaminotriethylamine. The adsorption isotherms of nitrogen and water on the formed particles of different size were measured and surface fractal analysis was performed.     

Thermal Conductivity of Sm3S4 System with Mixed Valence Sm Ions

The thermal conductivity () and electrical resistivity () of mixed-valence compound Sm₃S₄ have been measured in the temperature range 5 to 300 K. The present results and those presented previously [1] for the thermal conductivity between 80 to 850 K are interpreted in terms of the temperature-dependent fluctuating valence of Sm ions. Sm₃S₄ crystallizes in the cubic Th₃P₄ structure, and the cations with different valences occupy equivalent lattice sites. Divalent and trivalent Sm ions are randomly distributed in the ratio of 1:2 over all possible crystallographic cation positions (Sm²⁺ ₂Sm³⁺ ₂S^2– ₄). The behavior of the Sm₃S₄ lattice thermal conductivity _ph is extraordinary since valences of Sm ions are fluctuating (Sm³⁺Sm²⁺) with a temperature dependent frequency. In the interval 20 to 50 K (low hopping frequencies), _ph of Sm₃S₄ varies as _ph T ^–1 (it is similar to materials with static distribution of cations with different valences): at 95 to 300 K (average hopping frequencies 10⁷ to 10¹¹ Hz), _ph changes as _ph T ^–0.3 (it is similar to materials with defects). Defects in Sm₃S₄ appear because of local strains in the lattice by the electrons hopping from Sm²⁺ ions (with big ionic radii) to Sm³⁺ ions (with small ionic radii) and back (Sm²⁺Sm³⁺), at T>300 K (high hopping frequencies), _ph becomes similar to materials with homogenous mixed valence states [1].     

A calorimetric study of S′ and θ′ precipitation in Al2O3 particle-reinforced AA2618

Differential scanning calorimetry (DSC) was used to study the kinetics of S and precipitation in AA2618 and its composite containing 15 vol % Al₂O₃ particles . The unreinforced alloy and the composite were fabricated by a proprietary casting method, followed by extrusion. The DSC studies were carried out on as-quenched samples of the test materials. The precipitation of the S and phases in both materials was found to be kinetically controlled and obey the modified Avrami-Johnson-Mehl equation. The growth mechanisms for S and formation in AA2618 seemed unaffected by the addition of Al₂O₃ particles. The growth parameters obtained for the precipitation of these phases in the matrix alloy and the composite were not significantly different.