Thermally stimulated polarization current in sodium germanate glass

The electrical conductivity of XNa₂O·(100–X)GeO₂ glasses where X is 0.19 and 27.5 mol%, was studied by means of the technique of thermally stimulated polarization current. The compositional dependence of the parameter ₀ T/n ^1/3, where ₀ is the preexponential facor for conductivity and n is the nominal concentration of sodium ions, suggests that these glasses contain some sort of inhomogeneities. This suggestion is supported by the phase diagram.     

Effect of cationic substituents on particle morphology of goethite and the magnetic properties of maghemite derived from substituted goethite

Preparation of nanoparticles with the desired shape and size by the wet-chemical precipitation process is a challenging task. Thus, the effects of different substitutional impurities such as Al, Cr, Co or Ni on the particle morphology and phase stability of goethite have been investigated. Goethite is prepared by air oxidation of Fe(OH)₂ · xH₂O gel under near neutral conditions. Below certain concentration levels of dopants (Al³⁺ 10; Cr³⁺ 5; Co²⁺ 10 and Ni²⁺ 5 at.%) the samples remained monophasic as revealed by XRD, TEM, and IR studies. Above these levels, the substituents produce traces of secondary phases such as lepidocrocite, spinel ferrite and Mⁿ(OH)_n. The goethite structure is stable in spite of the iso- or aliovalent substitutions. The individual additives have divergent influence on the particle morphology; Al³⁺ and Cr³⁺ decreases the particle size to <50 nm and aspect ratio (AR) <2. Co-substitution produces slender particles with AR as high as 25. Whereas, Ni²⁺ does not have any influence on the particle morphology. The attributable factors in morphology control are the increased nucleation rate, restricted growth along needle axes, and the strain induced in the goethite lattice as a result of difference in ionic radii. Maghemite, -Fe₂O_3–, particles are obtained from goethite wherein the topotactic conversion renders the retention of the particle morphology of the precursor. Maghemite with substituted impurities showed substantial differences in magnetic properties. Saturation magnetization (_s) and coercivity (H _c) go down to very low values due to relaxation of spins on the surface atoms as revealed by Mössbauer spectroscopy. Decrease in coercivity is by way of the presence of diamagnetic ion (Al³⁺). Whereas, Co-substituted maghemite has enhanced H _c as a result of high magnetocrystalline anisotropy accompanied by the shape anisotropy.     

Preparation of spherical SnO2 particles by W/O type emulsion method

An attempt to prepare spherical particles was made using a W/O type emulsion as a reaction field. The effects of surfactant content, W/O ratio and stirring conditions for the preparation of emulsions, which determined the size of water drops in the emulsions, were investigated on the size and morphology of the obtained SnO₂ particles. The size and morphology of the obtained particles were largely influenced by a water/surfactant molar ratio (R-value) for the preparation of the emulsions. Particles having relatively high sphericity were obtained at an R-value below 1500. In order to obtain mono-sized SnO₂ particles with narrow distributions, R-value should be adjusted to the narrow range from 250 to 500. Spherical SnO₂ particles showing narrow particle size distributions were obtained at W/O ratio of 1/1 and surfactant content of 11.2 × 10^–2 mol/l or 22.4 × 10^–2 mol/l. Furthermore, the particle size and morphology of SnO₂ depended on the revolution rate of an emulsifier for emulsification. Mono-dispersed spherical particles having narrow size distributions formed at revolution rates of 3000 and 4000 rpm. At extremely low and high revolution rate of the emulsifier, particles showing high sphericity were not obtained, but agglomerates of un-spherical fine particles. The interfacial reaction time determined the internal structures of spherical particles. The reaction for short time yielded hollow spherical SnO₂ particles, and the internal structure of the particles became denser with increasing reaction time.     

Fluidity of mica particle dispersed aluminium alloy

Cast aluminium alloy-mica particle composites were made by dispersing mica particles in a vortex produced by stirring the liquid Al-4 wt% Cu-1.5 wt% Mg alloy and then casting the melt containing the suspended particles into permanent moulds. Spiral fluidity and casting fluidity of the alloy containing mica particles in suspension were determined. Both the spiral fluidity and the casting fluidity of the base alloy were found to decrease with an increase in volume or weight percent of mica particles (of a given size), and with a decrease in particle size (for a given amount of particles). The fluidities of Al-4 wt% Cu-1.5 wt% Mg alloys containing suspended mica particles were found to correlate very well with the surface area of suspended mica particles. The regression equation for spiral fluidity Y (cm) as a function of surface area of mica particles per gram of spiral X (cm² g^–1) at 700° C was found to be Y=42.62–0.42X with a correlation coefficient of 0.9634. The regression equations for casting fluidity Y (cm) as a functiono of surface area of mica particles per gram of fluidity test piece X (cm² g^–1) at 710 and 670° C were found to be Y=19.71–0.17X and Y=13.52–0.105X with correlation coefficients of 0.9194 and 0.9612 respectively. The percentage decrease in casting fluidity of composite melts containing up to 2.5 wt% mica with a drop in temperature is quite similar to the corresponding decrease in the casting fluidity of base alloy melts (without mica). The change in fluidity due to mica dispersions has been discussed in terms of changes in viscosity of the composite melts. However, the fluidities of these composite alloys containing up to 2.5 wt% mica are adequate for making a variety of simple castings including bearings for which these alloys have been developed.     

The transport properties of3He-4He mixtures near the lambda line—A short review and outlook

A short review is given on the present status of our knowledge on several transport properties, both in the normal and the superfluid phase in the vicinity of the superfluid transition. The concentration range extends from very dilute mixturesX (³He) 10^–7 to those withX X _t = 0.67, whereX _t is the concentration at the tricritical point. The influence of the earth's gravity field on these properties is discussed. It occurs via the concentration susceptibility and its temperature spread increases withX. The ongoing program at Duke University on the shear viscosity is briefly described.     

Development and in vitro evaluation of floating rosiglitazone maleate microspheres

Background: Various approaches have been used to retain the dosage form in stomach as a way of increasing the gastric residence time, including floatation systems; high-density systems; mucoadhesive systems; magnetic systems; unfoldable, extensible, or swellable systems; and superporous hydrogel systems. Aim?: The objective of this study was to prepare and evaluate floating microspheres of rosiglitazone maleate for the prolongation of gastric residence time. Method: The microspheres were prepared by solvent diffusion–evaporation method using ethyl cellulose and hydroxypropylmethylcellulose. A full factorial design was applied to optimize the formulation. Results: Preliminary studies revealed that the polymer:drug ratio, concentration of polymer, and stirring speed significantly affected the characteristics of microspheres. The optimum batch exhibited a prolonged drug release, remained buoyant for >12 hours, high entrapment efficiency, and particle size in the order of 350 μm. Conclusion: The results of 32 full factorial design revealed that the concentration of ethylcellulose 7 cps (X₁) and stirring speed (X₂) significantly affected drug entrapment efficiency, percentage release after 8 h and particle size of microspheres.     

Statics and dynamics in binary mixtures near the liquid-vapor critical line

Static and dynamic properties are investigated near the liquid-vapor critical line in fluid binary mixtures. Simple expressions are given for thermodynamic derivatives and transport coefficients in terms of parameters characterizing the coexistence surface in thep-T- space, where is the chemical potential difference. It becomes possible to understand special crossover phenomena occurring when fluids are nearly azeotropic or when the concentration dependence of the critical temperatureT _c (X) [or the critical pressurep _c (X)] is weak. The results are used to explain recent data on³He-⁴He mixtures that apparently contradict previous theories. As a by-product, general relations are found for correlation functions of dynamic variables and thermodynamic derivatives in fluid mixtures.     

Synthesis of silicon nitride powder through nitrogen gas atomization

The feasibility of synthesizing silicon nitride powder utilizing reactive atomization processing was analysed. The range of times required for the flight time of particles, the cooling rate of the silicon melt, the reaction time of silicon and nitrogen, and the diffusion of nitrogen through silicon nitride layers were obtained and compared. The results of this study indicated that the production of silicon nitride powder through the reactive atomization process would be limited by diffusion of nitrogen through the nitride (ash) layer, assuming the nitride layer was coherent and the unreacted core model was a valid representation of the liquid silicon-silicon nitride system.Nomenclature k(T) reaction rate constant at temperature, T(s^–1) - k ₀ Arrhenius coefficient - E activation energy (kJ mol^–1) - R gas constant - T temperature (K) - fraction of normalized conversion of -phase in time t - fraction of normalized conversion of -phase in time t - k reaction rate constant for -phase (s^–1) - k reaction rate constant for -phase (s^–1) - k ⁱ intrinsic first-order rate constant for -phase (s^–1) - x conversion fraction of -phase in time t - x conversion fraction of -phase in time t - n reaction order for -phase = 1 - n reaction order for -phase = 0.5 - J diffusion flux (mol m^–2 s^–1) - D diffusivity, or diffusion coefficient (m² s^–1 or cm² s^–1) - dC change in concentration (mol m^–3) - dl change in distance, l (m) - A(_g) gaseous reactant A - B reactant B (may be solid or liquid) - P solid product P - b stoichiometric coefficient of reactant B - p stoichiometric coefficient of product P - t time of reaction passed (s) - time for complete reaction of a particle (s) - X _B conversion fraction - r _c core radius (m) - R _p particle radius (m) - _B molar density of reactant B (mol m^–3) - k _g mass transfer coefficient between fluid and particle (m s^–1) - C _Ag concentration of gaseous reactant A (mol m^–3) - D _e effective diffusion coefficient of gaseous reactant in ash layer (m² s^–1)     

A theoretical model of the manfacture of reaction-bonded silicon nitride with particular emphasis on the effect of ambient reaction temperature and compact size

An analysis of the exothermic, irreversible silicon-nitrogen reaction, based on the particle-pellet model, is presented using mixed type boundary conditions to represent external resistances. The mathematical model incorporates a sharp cut-off in the reaction and takes into account its transient behaviour. The resulting system of partial differential equations is solved numerically using an explicit finite difference scheme. The effects of varying the ambient reaction temperature and compact size on the temperature distribution inside the nitriding compact and on the solid product formation rate, are examined. The results obtained are in acceptable agreement with previous experimental research by other workers, which illustrates how the model adequately represents the silicon-nitrogen reaction.An investigatory report on the validity of the Arrhenius equation for determining the thermal activation energy of this reaction is also presented. a characteristic dimensions of the compact,m - C concentration of the gas within the compact, mol m^–3 - C _A molar concentration of the product in the compact, mol m^–3 - C _AO maximum concentration of the product, 1.6×10⁴ mol m^–3 - C _f concentration of the gas surrounding the compact, mol m^–3 - C _p specific heat of solid reactant, 1250 J kg^–1K^–1 - D _e effective diffusion coefficient, 2×10^–6 m² sec^–1 - E activation energy of reaction, 5.5×10⁵ J mol^–1 - H heat of reaction, –7.5×10⁵ J mol^–1 - h heat transfer coefficient, Wm^–2 K^–1 - K _e effective thermal conductivity, 6.6 Wm^–1 K^–1 - /n derivative normal to the surface of the compact - r(C, T) reaction rate per unit area, kg m^–2 sec^–1 - r _p mean particle size,m - s _g specific area of the compact, 7.142×10² m² kg^–1 - T absolute temperature within the compact, K - T _c temperature at the centre of the compact, K - T _f initial and surrounding temperature, K - T _s temperature at the surface of the compact, K - t time, sec - u dimensionless concentration of the gas in the compact - U _A C _A/C _AO - V ^* dimensionless space - v dimensionless temperature in the compact - X conversion factor, defined by Equation 7 - x dimensionless space variable within the compact - void fraction of the compact, x0.4 - _b bulk density of the compact, 1400 kg m^–3 - dimensionless time     

Formation of plate-like lanthanum-β-Aluminate crystal in Ce-TZP matrix

The reaction route and morphology of lanthanum--aluminate (LBA) crystals formed in Ce-TZP matrix were studied by examining the crystal phase changes and the microstructures in relation to the heat-treatment time, heat-treatment temperatures and the particle size of raw Al₂O₃ powders. In the Ce-TZP matrix, the LBA crystal was formed by the reaction between La₂O₃ and Al₂O₃ through the LaAlO₃ phase as the intermediate. La₂Zr₂O₇ forms at 800 °C and remains in the temperature range 800–1500 °C, and LaAlO₃ forms between 1200 and 1400 °C. The LaAlO₃ reacts with Al₂O₃ to form LBA above 1500 °C. The diffusion of La³⁺ through the La₂Zr₂O₇ phase was faster than that of Al³⁺. The morphology of LBA crystals was dependent on the particle size of the starting raw Al₂O₃ particle. When submicrometre size Al₂O₃ (0.4m) particles were used as the starting particles, anisotropic, plate-like LBA crystals, about 10m long, were formed during heat treatments. On the other hand, Al₂O₃ of larger grain sizes (3.6, 10.3m) yield conglomerates of LBA crystals. The size of the conglomerates is similar to that of the raw Al₂O₃ particle. The dependence of the morphology of LBA on the particle size of Al₂O₃ can be attributable to the sintering process of the Ce-TZP matrix, leading to the control of the mechanical properties of Ce-TZP ceramics with LBA crystals.     

Persistent currents and critical velocities in3He-4He mixtures

Using the method of Doppler-shifted fourth sound, we have investigated the angular velocity _c1 for the onset of vorticity, the saturated critical velocity, and the dependence of _s/ onv _n –v _s for⁴He in three different pore sizes and for three different mixtures of ³ He- ⁴ Hein one of the powders used. These are the first measurements of critical velocities in ³ He- ⁴ He mixtures. We observe only a weak dependence of _c1on the pore size and within the limits of the experiment no dependence on concentrationX up toX=0.11. We find, however, that the maximumv _n –v _s attainable increases as the concentration of ³ He increases forX0.17. The observed dependence of the superfluid density onv _n –v _s is approximately two orders of magnitude stronger than predicted.     

Anomalous magnetic features in α-Fe2O3-based ceramic composites containing B2O3 dispersoids

Ceramic composites based on BaO-Fe₂O₃-B₂O₃ starting compositions have been prepared by the solid-state reaction technique. Basically the samples comprise crystalline particles due to the-Fe₂O₃ phase. It is found that an easy control of the particle size and many of the physical properties of the system is possible by modifications of the composition and the sintering. At sintering temperatures below 1200° C the material is essentially non-magnetic. However, it exhibits surprisingly large magnetization (small particle size and low coercive fields) at sintering temperatures above 1200° C. The anomaly is explained by invoking a heat-induced Fe³⁺ Fe²⁺ reduction and considering a modified distribution of the cations in the-Fe₂O₃ precipitated particles. The results are consistent with microstructures and electron paramagnetic resonance measurements of the samples.     

Interpretation of1H NMR results of HD impurities in solid H2. I. Spin-spin relaxation

Previous¹H NMR results on HD impurities in solid H₂ samples of low ortho concentration (X0.01) are reexamined, resulting in a different interpretation of the spin-spin relaxation timeT ₂. The relaxation time is found to be measured by using short rf pulses rather than the long (100–120 µsec) pulses used previously for the spin-echo pulse sequences. This is due to the fact that the nuclear spin dipolar flip-flop interactions between the HD and ortho-H₂ molecules are effectively quenched because of the large intramolecular dipolar splittings of the H₂ molecules. The measured spin-spin relaxation times for the HD impurities in a sample ofX0.01 are found to be consistent with those from statistical theory, revealing no appreciable motional narrowing, which was invoked previously.     

A process model for active brazing of ceramics

In the present investigation process modelling techniques have been applied to describe reaction layer growth during active brazing of ceramics. As a starting point, the classical solution for parabolic growth of transformation products is considered. Specific computational features are then explicitly built into the model to allow for transient effects during heating and cooling as well as changes in the growth kinetics due to depletion of the active element during brazing. This approach gives considerable scope for optimization of both process and joint properties through adjustment of the filler metal composition and the temperature-time programme under which brazing takes place. The aptness of the process model is illustrated in an accompanying paper (Part II).Nomenclature A, C B reaction products in diffusion couple - C A ceramic component - C reactive element in braze alloy - C _B concentration of element B at a given position within the reaction layer, C B (mole m^–3) - C _B ^b bulk concentration of element B in ceramic, A B (mole m^–3) - C _B ⁱ (1), C _B ⁱ (2) concentration of element B in reaction layer at C B /A B and braze metal/C B interface, respectively (mole m^–3) - C _C concentration of element C at a given position within the reaction layer, C B (mole m^–3) - C _C ⁰ initial concentration of element C in braze metal (mole m^–3) - C _C ^b bulk concentration of element C in braze metal (mole m^–3) - C _C ⁱ (1), C _C ⁱ (2) concentration of element C in reaction layer at braze metal/C B and C B /A interface, respectively (mole m^–3) - D ₀, D ₀ ^* constants in expression for diffusion coefficient (m² s^–1) - D _B intrinsic diffusivity of B in C B (m² s^–1) - D _C intrinsic diffusivity of C in C B (m² s^–1) - J _B molar flux of element B (mole m^–2 s^–1) - J _C molar flux of element C (mole m^–2 s^–1) - k ₀ constant in expression for k_p (m² s^–1) - k ₀ ^* rate constant referring to infinite diffusion couple analogue (m² s^–1) - k _p parabolic growth rate constant (m² s^–1) - L half width of braze metal zone (m) - m proportionality constant (equal to the ratio between C _C ⁱ (1) and C _C ^b ) - Q _app apparent activation energy for diffusion of C in C B (J mole^–1) - Q _app ^* apparent activation energy for diffusion of B in C B (J mole^–1) - R universal gas constant (8.314 J mole^–1 K^–1) - t time (s) - t ₀ incubation time (s) - t ₁, t ₂ limits of integration (s) - t _i isothermal hold time (s) - t _i time increment used in the numerical integration procedure (s) - T absolute temperature (K or °C) - T _c chosen reference temperature (K or °C) - T _i isothermal hold temperature (K or °C) - X thickness of reaction layer (m) - X _c contribution of the cooling leg of the brazing cycle to the total reaction layer thickness (m) - X _h contribution of the heating leg of the brazing cycle to the total reaction layer thickness (m) - X _i contribution of the isothermal hold period to the total reaction layer thickness (m) - X _lim limiting thickness of reaction layer, C B (m) - X _i increase in reaction layer thickness due to a small time increment t _i (m) - y ₁, y ₂, y ₃ molar partitioning factors - , , , , , ) molar stoichiometric factors - molar volume of reaction product, C B (m³ mole^–1)     

Thermal transport properties of helium near the superfluid transition. II. Dilute3He-4He mixtures in the superfluid phase

Measurements of the average thermal conductivity _exp hQ/T and of the thermal relaxation time to reach steady-state equilibrium conditions are reported in the superfluid phase for dilute mixtures of³He in⁴He. Hereh is the cell height,Q is the heat flux, andT is the temperature difference across the fluid layer. The measurements were made over the impurity range 2×10^–9<X(³He)<3×10^–2 and with heat fluxes 0.3<Q<160 µW/cm². Assuming the boundary resistanceR _b , measured forX<10^–5, to be independent ofX over the whole range ofX, a calculation is given for _exp. ForQ smaller than a well-defined critical heat fluxQ _c (X) X ^0.9, _exp is independent of Q and can be identified with the local conductivity _eff, which is found to be independent of the reduced temperature = (T–T)/T for –10^–2. Its extrapolated value at T is found to depart forX10^–3 from the prediction X ^–1 , tending instead to a weaker divergence X ^–a witha0.08. A finite conductivity asX tends to zero is not excluded by the data, however. ForQ >Q _c (X), a nonlinear regime is entered. ForX10^–6, the measurements with the available temperature resolution are limited to the nonlinear conditions, but can be extrapolated into the linear regime forX2×10^–7. The results for _exp(Q),Q _c (X), and _eff(XX) are found to be internally consistent, as shown by comparison with a theory by Behringer based on Khalatnikov's transport equations. Furthermore, the observed relaxation times (X) in the linear regime are found to be consistent forX>10^–5 with the hydrodynamic calculations using the measured _eff(X). ForX<10^–5, a faster relaxation mechanism than predicted seems to dominate. The transport properties in the nonlinear regimes are presented and unexplained observations are discussed.     

Preparation of monodispersed Y-doped ZrO2 powders

3 mol% Y-doped ZrO₂ powders prepared by the controlled hydrolysis of metal alkoxides were monodispersed and grown to 0.5m after 5 h ageing. The as-prepared powder was amorphous and hydrate but transformed into a tetragonal single phase by heating. Furthermore, the Y₂O₃ concentration of each particle was almost the same in all particles. The synthesis conditions such as ageing time, ageing temperature and water concentration greatly affected the particle morphology. The refluxing of the alkoxide solution was particularly necessary to prepare the monodispersed particles. On the basis of the variation of size distribution with ageing time, the mechanism of particle growth was discussed.     

Preparation of Sr2CeO4 blue phosphor by ultrasonic spray pyrolysis: effect of NH4NO3 addition

A blue emission Sr₂CeO₄ phosphor powder which has a one-dimensional structure was prepared from a nitrate starting solution by the ultrasonic spray pyrolysis process. To avoid the formation of hollow particles during spray pyrolysis, NH₄NO₃ was employed to change the mechanism of Sr₂CeO₄ formation from droplets, containing Sr(NO₃)₂ and Ce(NO₃)₃ in the required stoichiometric ratio. The mean particle size of as-prepared particle increased from 0.19 to 1.52 m with a change in the overall concentration of the starting solution from 0.005 to 1.5 M. No hollow particles or particle fragments were observed when the ratio of overall concentration of the starting solution to NH₄NO₃ was equal to 1 : 1. The particles that were as-prepared with NH₄NO₃ had spherical morphology and a narrow size distribution. The emission spectrum of post heat-treated particles showed a broad maximum at 470 nm and the CIE Chromaticity coordinates were x = 0.20 and y = 0.27. The decay time of Sr₂CeO₄ phosphor powder was 58 s, which is a long excited-state life time compared with d-f transitions for the Ce³⁺ excited state.     

3He diffusion in liquid isotopic mixtures

Spin echo techniques have been used to study the diffusion of³He in liquid isotopic mixtures with molar concentrations in the range fromX ₃=0.137toX ₃=0.850 over a temperature range from 3.0 to 0.4 K. AboveT the diffusive behavior is very similar to that of pure³He, although there is an increased scattering associated with⁴He. BelowT the diffusion coefficient increases with decreasing temperature and shows a dependence on the³He number density much like that of a classical gas. The excitations of⁴He have little influence on the³He diffusion coefficient, even at temperatures well above 1 K.The Rhodes Trust and the National Research Council of Canada supported one of us (R.B.H.) during the course of this research.     

Preparation of silicon carbide powders by chemical vapour deposition of the SiH4-CH4-H2 system

Chemical vapour deposition (CVD) of the SiH₄ + CH₄ + H₂ system was applied to synthesize-silicon carbide powders in the temperature range 1523 to 1673 K. The powders obtained at 1673 K were single-phase-SiC containing neither free silicon nor free carbon. The powders obtained below 1623 K were composite powders containing free silicon. The carburization ratio (SiC/(SiC + Si)) increased with increasing reaction temperature and total gas flow rate, and with decreasing reactant concentration. The average particle sizes measured by TEM ranged from 46 to 114nm, The particle size increased with the reaction temperature and gas concentration but decreased with gas flow rate. The-SiC particles obtained below 1623 K consisted of a silicon core and a-SiC shell, as opposed to the-SiC particles obtained at 1673 K which were hollow. Infrared absorption peaks were observed at 940 and 810 cm^–1 for particles containing a silicon core; whereas a single peak at about 830 cm^–1 with a shoulder at about 930 cm^–1 was observed for the-SiC hollow particles. The lattice parameter of-SiC having a carburization ratio lower than 70 wt%, was larger than that of bulk-SiC and decreased with the increasing carburization ratio. However, when the carburization ratio exceeded 70 wt%, the lattice parameter became approximately equal to that of bulk-SiC.     

Electrical properties of α-zirconium phosphate and its alkali salts in a humid atmosphere

Humidity dependence of impedance was examined by using the complex impedance analysis for fine crystalline zirconium bis(monohydrogen phosphate) monohydrate and its alkali salts. The value of C ₀ in C _p()=C ₀(j/ ₀)^– was hardly dependent on the relative humidity and monovalent cation species. The resistive component and its activation energy decreased with increase in the relative humidity and the former was expressed as R=R ^* exp (–E/kT ^*) exp (E/kT) The pre-exponential factor R ^*, was hardly influenced by the humidity and decreased when the acidic protons were replaced with monovalent alkali cations. The activation energy of conduction was strongly affected by the degree of orientation and particle size of crystal with layered structure in a whole humidity region and increased by replacing the protons by alkali cations.