Synthesis and dielectric properties of pyrochlore solid solutions in the Bi2O3-ZnO-Nb2O5-TiO2 system

Phase studies of solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ revealed extended regions of pyrochlore formation in the Bi₂O₃-ZnO-TiO₂-Nb₂O₅ system. At room temperature and 1 MHz (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ has a high permittivity ( = 200), low dielectric loss (tan<1·10^–4) and a temperature coefficient of the permittivity, , = –1300ppm/K. Pyrochlore solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be formed with (Bi_2–xZn_x)(Ti_{2– x} Nb_x)O₇ (0.35x1.0) and with (Bi_1.5 Zn_0.5–y/3Ti_1.5+yNb_0.5–2y/3) O₇ (–1.5y0.75). Investigations of the dielectric characteristics showed that the high temperature dependence of the permittivity of (Bi_1.5 Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be significantly modified by changing the composition of the pyrochlore within these regions of solid solubility. Below room temperature several of these compositions also exhibit a diffuse frequency dependent maximum in their permittivity characteristic of a transition to a relaxor type ferroelectric state. A third region of high permittivity pyrochlores with (Bi_1.5+2zZn_0.25–z Ti_2.25–z)O₇ (0.0z<0.15) was also identified in the Bi₂O₃-ZnO-TiO₂ sub system.     

Causes of the occurrence of an “effective” porosity in granular media

The cause of the difference between the total (geometric) and effective _ef (hydrodynamic) porosity of granular media is established.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 21, No. 6, pp. 1017–1024, December, 1971.     

Effect of unsteady natural convection on the structure of a liquid flow in a horizontal mixing chamber

Results are presented of a numerical and experimental investigation of the effect of natural convection on the structure of a liquid flow in a horizontal mixing chamber with changes in the temperature of the liquid at the inlet.Notation t_o, t_in initial temperature and temperature at the inlet to the channel - dimensionless temperature - a heat conductivity - kinematic viscosity - coefficient of cubical expansion - density - P pressure - g acceleration due to gravity - d_equ equivalent diameter of porous body - time - v_o mean velocity at the inlet - X and dimensionless vertical and radial coordinates - U and V dimensionless vertical and horizontal components of velocity - H dimensionless height of channel - R radius of inlet to channel - W velocity of liquid - t temperature drop along channel height - Re = v_oR/ Reynolds number - Pe = v_oR/a Peclet number - Fr = v _o ² /g¦t_in–t_o R Froude number - Ho = v_o/R homochroneous number Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 603–610, October, 1980.     

Characterization of hydroxyapatite powders prepared by ultrasonic spray-pyrolysis technique

The hydroxyapatite (HAp) powder was prepared by the ultrasonic spray-pyrolysis technique; the characterization of the resulting powders was performed. Five kinds of the starting solutions with the Ca/P ratio of 1.67 were prepared by mixing Ca(NO₃)₂, (NH₄)₂HPO₄ and HNO₃; the concentrations of Ca²⁺ and PO₄ ^3– were in the ranges of 0.10 to 0.90 mol·dm^–3 and 0.06 to 0.54 mol·dm^–3, respectively. These solutions were sprayed into the heating zone to prepare the HAp powders. The heating zone was composed of two electric furnaces; the lower furnace was used for the evaporation of the solvent from the droplets (300–500°C) and the upper furnace for the pyrolysis of the precipitated metal salts (750–900°C). The easily sinterable HAp powder was prepared by spray-pyrolysing the solution with Ca²⁺ (0.50 mol·dm^–3) and PO₄ ^3– (0.30 mol·dm^–3) at the temperatures of 800°C (the upper furnaces) and 400°C (the lower furnaces). The resulting powder was composed of the spherical particles with diameters of 1 m or below. Even without the calcination and grinding operations, the relative densities of the compacts fired at 1150 and 1200°C for 5 h attained maxima 95%. The microstructure of the sintered compacts appeared to be uniform; the average grain size was 3 m. The activation energies for the grain growth of the sintered HAp compacts were 120 to 147 kJ · mol^–1 · K^–1.     

Dielectric properties of (Ba,Sr)O-(Sm,La)2O3-TiO2 ceramics at microwave frequencies

The dielectric properties of (Ba, Sr)O-(Sm, La)₂O₃-TiO₂ material at microwave frequencies were investigated. By varying the amount of strontium from 0–25 mol% in the 0.15(Ba_1–x Sr _x )O-0.15Sm₂O₃-0.7TiO₂ composition, it was possible to adjust the frequency temperature coefficient, _f, from –13 p.p.m. °C^–1 to + 30 p.p.m. °C^–1. When 7 mol% Sr was substituted for barium, _f=0 p.p.m. °C^–1 was obtained. TiO₂ with rutile phase (_f400 p.p.m. °C^–1) acted as a dominant element in _f variation of the 0.15(Ba_1–x Sr _x )O-0.15(Sm_1–y La _y )₂O₃-0.7TiO₂ (0x0.25, 0y0.6) system. Additionally, increasing the quantity of lanthanum substitution for samarium had a greater positive effect on _f than strontium substitution for barium. When 60 mol% La was substituted for samarium with 7mol% Sr substitution barium, _f of the system reached 95 p.p.m. °C^–1. The effect on microwave dielectric characteristics of the 0.15(Ba_0.93Sr_0.07)O-0.15Sm₂O₃-0.7TiO₂ (BSST) ceramics by varying the calcination and/or sintering conditions or doping additives, were studied. The added SnO₂ acted as a firing agent to lower the sintering temperature, and the dielectricQ(Q _d) value was improved by properly adding CdO. With 1 wt% CdO addition, the highestQ _d value of the BSST resonator, after calcination at 1100 °C/2 h and sintering at 1370 °C/4 h, reached 4180 at 4 GHz with a small _f of –4 p.p.m. °C^–1 and an _r of 80.7 was obtained.     

Dislocation generation at surfaces of tin single crystals

Single crystals of 99.999% purity-tin grown from the melt were shown by X-ray topography to contain dislocations with Burgers vectors of [001] type and of 1/2111 type. Specimen plates cut roughly parallel to (311) were chemically thinned from 1.25 mm to 100m thickness and in two cases characteristic dislocation structures were generated at their surfaces. A specimen thinned in concentrated HCl possessed stress-producing centres distributed on its surfaces with a density of about 75 mm^–2 from which regular helices and coaxial prismatic loops with [001] Burgers vector were generated together with irregular loops of 1/2111 Burgers vector dislocations. In one specimen thinned in a H₃PO₄, CH₃COOH, HF and HNO₃ mixture large arrays of pure edge dislocations grew parallel to the surface at a depth of 2 to 4m below it, the individual dislocations extending at about 1m h^–1 during several weeks. These edge arrays all had that one of the four 1/2111-type Burgers vectors which made the smallest angle (5°) with the surface. The Burgers vector sense, determined by X-ray diffraction contrast, corresponded to a sheet of vacancies lying between the dislocation line and the surface.Visitor to H. H. Wills Physics Laboratory under Royal Society-SSR Cultural Agreement.     

Explanation of Temperature-Dependent Resistivity of Sm-Doped Cuprates by Electron–Phonon Scattering

The resistivity of electron-doped cuprate Sm_1.85Ce_0.15CuO_{4 –} is theoretically analyzed within the framework of electron–phonon i.e., Bloch–Gruneisen (BG) model of resistivity. Characteristic temperatures as the Debye temperature and the Einstein temperature were first derived from an overlap repulsive potential. The optical phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of acoustic phonons above 220 K. While to that, below this temperature, acoustic phonon is a major cause of resistivity. Estimated contribution to resistivity by considering both phonons i.e., _ac (acoustic phonons) and _op (optical phonons), along with the zero limited resistivity, when subtracted from single crystal data, infers a quadratic temperature dependence over most of the temperature range (25 T 300). Power temperature dependence of _diff.{=[ _exp. – ( ₀ + _e-ph(= _ac + _op))]} points the contribution of electron–electron inelastic scattering. The present analysis allows us to infer that the single crystal experimental data is well approximated within the framework of BG electron–phonon model of resistivity. Further calculations of superconducting transition temperature and isotope effect exponent from Kresin's strong coupling theory indicates that the electron–phonon interaction plays an important role in the attractive pairing mechanism.     

Structure and Diamagnetic Properties of (Pb0.6SnyCu0.4 ? y)Sr2(Y1 ? xCax)Cu2Oz

The effect of Sn doping in (Pb_0.6Sn _y Cu_{0.4 – y} )Sr₂(Y_{1 – x} Ca _x )Cu₂O _z with 0 y 0.3 and 0 x 0.7 was investigated. It was established that a nearly pure 1212 phase can be obtained at 0 y 0.1 and 0 x 0.3. The obtained XRD patterns as well as the results of the EDX and ICP-AES analyses showed that Sn substitution is possible in the (Pb,Cu)-1212 phase. Superconductivity was observed at 0.4 x 0.7. The onset of the diamagnetic transitions varied from 10 to 30 K. The influence of the strong Pb deficiency on the superconducting properties of the samples was discussed.     

Nucleate boiling

The study deals with the effect of the surface conditions on the nucleate boiling curve. A relation is proposed which describes the complete nucleate boiling curve.Notation q thermal flux - q* thermal flux at which the liquid boils after one-phase convection - q_c thermal flux during one-phase convection - q_cr1, q_cr2 first and the second critical thermal flux - T saturation temperature - T superheat of the heating surface relative to the saturation temperature - T* superheat prior to boiling of the liquid after one-phase convection - T_cr1 superheat during the first boiling crisis - T_cr3min minimum superheat at which the third boiling crisis can occur - P pressure - P_cr critical pressure - heat transfer coefficient during nucleate boiling - R_cr radius of a critical vapor forming nucleus - coefficient of surface tension - r latent heat of evaporation - thermal conductivity of the liquid - kinematic viscosity of the liquid - , densities of the liquid and the vapor - g gravitational constant - k Boltzmann constant - N Avogadro number - h Planck's constant Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 3, pp. 394–401, March, 1981.     

A note on the solution of a differential equation arising in boundary-layer theory

Summary The differential equation f + ff + f² = 0 (where dashes denote differentiation with respect to the independent variable ) subject to the boundary conditions f(0)=0, f()=0 and either f(0)=1 or f(0)=–1 is considered. It is shown that by using pf as dependent variable and =C–f (where C=f()) as independent variable and then expanding in powers of , a very good approximation to the solution can be obtained using only a few terms in the expansion.     

Growth and Properties of V-Doped CdxHg1 – xTe Crystals

Cd _x Hg_{1 – x} TeV (x= 0.9–0.95) crystals were prepared by two versions of Bridgman growth, and their optical homogeneity and transport properties were studied. The electrical resistivity of the crystals was 10⁴to 10⁸ m. From the temperature dependences of the Hall coefficient, the activation energy of the vanadium level in Cd _x Hg_{1 – x} TeV was determined to be 0.73–0.82 eV.     

Boundary layer similarity flow driven by power-law shear

Summary Similarity solution of the Prandtl boundary layer equations describing wallbounded flows and symmetric free-shear flows driven by rotational velocitiesU(y)=y are determined for a range of exponents and amplitudes . Asymptotic analysis of the equations shows that for <–1 no similarity solutions with proper algebraic decay exist. For wall-bounded flow, exact solutions found at =–1/2 and =1 correspond to an Airy function wall jet and uniform planar Couette flow. Numerical integration of the governing similarity equation reveals singular behaviour for wall-bounded flows as ₀ = –2/3, and no solutions are found in the range –1<–2/3. For >–2/3 the shear stressf(0) parameter is determined as a function of and . Symmetric free-shear flow solutions become singular as ₀ = –1/2 and no solutions are found in the range –1<–1/2. For >–1/2 the centerline velocityf(0) is determined as a function of and . An asymptotic analysis of the singular behavior of these two problems as ₀, given in a separate Appendix, shows excellent comparison with the numerical results. Similarity solutions at the critical values ₀ have exponential decay in the far field and correspond to the Glauert wall jet for wall-bounded flow and to the Schlichting/Bickley planar jet for symmetric free-shear flow.     

Supercooling of In2Bi and InBi Melts

The effect of melt overheating T ⁺ on the critical supercooling T ^– of liquid In₂Bi and InBi is studied by cyclic thermal analysis. It is shown that, the T ^– for In₂Bi is 2.0 K, independent of the melt preheating temperature. In contrast, the T ^– for InBi varies jumpwise with T ⁺: T ^– 1.0–1.6 K at T ⁺ < 5 K, and T ^– 16 K at T ⁺ = 5–300 K, independent of the cooling rate (varied from 0.002 to 8.0 K/s). The solidification behaviors of In₂Bi and InBi are shown to correlate with the structures of their liquid and solid phases.     

On the driving force for fatigue crack formation from inclusions and voids in a cast A356 aluminum alloy

Monotonic and cyclic finite element simulations are conducted on linear-elastic inclusions and voids embedded in an elasto-plastic matrix material. The elasto-plastic material is modeled with both kinematic and isotropic hardening laws cast in a hardening minus recovery format. Three loading amplitudes (/2=0.10%, 0.15, 0.20%) and three load ratios (R=–1, 0, 0.5) are considered. From a continuum standpoint, the primary driving force for fatigue crack formation is assumed to be the local maximum plastic shear strain range, _max, with respect to all possible shear strain planes. For certain inhomogeneities, the _max was as high as ten times the far field strains. Bonded inclusions have _max values two orders of magnitude smaller than voids, cracked, or debonded inclusions. A cracked inclusion facilitates extremely large local stresses in the broken particle halves, which will invariably facilitate the debonding of a cracked particle. Based on these two observations, debonded inclusions and voids are asserted to be the critical inhomogeneities for fatigue crack formation. Furthermore, for voids and debonded inclusions, shape has a negligible effect on fatigue crack formation compared to other significant effects such as inhomogeneity size and reversed loading conditions (R ratio). Increasing the size of an inclusion by a factor of four increases _max by about a factor of two. At low R ratios (–1) equivalent sized voids and debonded inclusions have comparable _max values. At higher R ratios (0, 0.5) debonded inclusions have _max values twice that of voids.     

The constitution of the Ni-Al-Ru system

The constitution of the Ni-Al-Ru system has been investigated in the range 0 to 50 at% Al. Isothermal sections at 1523 and 1273 K have been determined using microstructural observations, electron probe microanalysis and X-ray diffraction. The phases present were: nickel-based solid solution (); (based on Ni₃Al); solid solutions based on NiAl and RuAl, respectively (designated ₁ and ₂), and ruthenium-based solid solution (Ru). The maximum solubility of Ru in was 5 at%. ₁, and ₂ show extensive range of solubilities, namely up to 20at% Ru in ₁ and up to 25 to 35 at% Ni in ₂. Three-phase equilibrium between , ₂ and (Ru) existed at 1523 and 1273 K. Also at 1523 K, three-phase equilibria existed between , and ₁ and ,₁ and ₂, while at 1273 K, the equilibria were between , ₁,₂ and , , ₂ indicating the occurrence of a reaction +₁, +₂ at a temperature between 1523 and 1273 K. Liquidus features have been deduced from data on as-solidified structures. Lattice parameter data and hardnesses are also reported.     

Preparation of micaceous iron oxide by the oxidation of iron with pressurized oxygen in concentrated sodium hydroxide solution at elevated temperatures

Iron powders were oxidized in NaOH solutions of 5–25 mol kg^–1 at 403–563 K and 5 MPa of oxygen partial pressure. Various types and morphologies of iron compounds such as fine particles of Fe₃O₄, micaceous -Fe₂O₃, and coagulated particles of -NaFeO₂ were formed depending on the experimental conditions. The observed critical concentrations of NaOH above which -NaFeO₂ was formed was in good agreement with those thermodynamically calculated for the hydrolysis equilibrium of -NaFeO₂.     

Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of -Si₃N₄ and aluminium iso-propoxide solution. The oxidation behaviour of this -sialon was investigated, comparing it with commercial -sialon containing Y₂O₃ as a sintering aid. Oxidation tests were carried out at 1200 and 1400C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived -sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400C for 200 h was about 800 MN m^–2, almost the same value as before oxidation. The oxide layer of Y₂O₃-doped -sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y₂Si₂O₇). The strength after oxidation at 1200C for 200 h fell to 1/2(440 MN m^–2) because of pit formation in the oxide layer, and at 1400C for 200 h fell to 1/4(200 MN m^–2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived -sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y₂O₃.     

Effects of alloying in the dilute limit on the quantum states of electrons in magnesium

The electron quantum interference phenomenon was used to determine the effects of substitutional alloying upon the quantum-state lifetime and band gapsE _g at the Fermi energy in single crystals of pure Mg. Vapor-grown alloys containing either Zn or Cd in concentrationsC0.3–15 ppm were studied. The magnetic field dependence of the interference oscillation amplitudes for these samples indicates reductions in and significant increases inE _g relative to pure Mg (impurity concentration 10^–8). Within experimental accuracy the observed quantum-state lifetime satisfies the relation ^–1=C, with 2.7×10¹⁰ sec^–1 per ppm of Cd and 2×10¹¹ sec^–1 per ppm of Zn. The band gap corresponding to Bragg reflection from the (0001) plane of the hcp structure (which in pure Mg arises solely from spin-orbit coupling) was found to increase by more than a factor of two upon the addition of only 10 ppm Cd to Mg. These results are discussed within the framework of the pseudopotential theory of alloying. It is shown that there are discrepancies of more than three orders of magnitude between the experimental and theoretical values forE _g and of about one order of magnitude for in these dilute-limit alloys.Work supported by the National Science Foundation.Submitted to the Department of Physics, the University of Chicago in partial fulfillment of the requirements for the degree of Doctor of Philosophy.Fannie and John Hertz Foundation Fellow.     

Heat exchange in two-phase flow about a surface located in a rectangular channel

Similitude equations are obtained on the basis of the principle of superposition of separate effects to calculate heat exchange between surfaces with complexshaped cross sections located in a rectangular channel during their cooling by a two-phase flow.Notation T, q temperature and heat flux - T_w mean surface temperature - I, R current and electrical resistance - V volume of the material - , , anda heat-transfer coefficients, thermal conductivity, and linear expansion of the material - relative functions - =_m; ^* = _m ^{* *} _s ^* ; temperature factor - X relative weight content of liquid phase Indices w surface - f incoming flow - v volume - m two-phase flow - angle of attack - s shape of surface - * pertains to surface with swirl vanes Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 5, pp. 780–786, May, 1980.     

Coarsening of precipitates and dispersoids in aluminium alloy matrices: a consolidation of the available experimental data

Experimental data on the coarsening of precipitates and dispersoids in aluminium-based matrices are reviewed. Available data are tabulated as K=(r ³–r ₀ ³ )/t where r ₀ is the initial particle radius and r is its value after time t at temperature T, and then plotted as log (KT) against 1/T for consolidation and assessment. The considerable body of data for -A₃Li in Li-containing alloys is well represented by K=(K ₀/T) exp (–Q/RT) with K ₀=(1.3 _–0.5 ^+3.0 ) × 10^–13m³Ks^–1 and Q=115±4kJ mol^–1. The relatively limited data for and in Cucontaining alloys are representable by the same relationship with K ₀4 × 10^–8 and — 4 × 10^–10 m³ Ks^–1, respectively, and Q — 140 kJ mol^–1. Available data for coarsening of L1₂ ^– Al₃(Zr, V) and related phases in Zr-containing alloys and of Al₁₂Fe₃Si and related phases in Al-Fe based alloys indicate (i) rates of coarsening at 375 to 475 °C (0.7 to 0.8T_m) five to eight orders of magnitude less than would be expected for , and in this temperature range, and (ii) high activation energies of 300 and 180 kJ mol^–1, respectively.