A note on the similarity solutions for free convection on a vertical plate

Summary The similarity solutions for free convection on a vertical plate when the (non-dimensional) plate temperature is x and when the (non-dimensional) surface heat flux is –x are considered. Solutions valid for 1 and 1 are obtained. Further, for the first problem it is shown that there is a value ₀, dependent on the Prandtl number, such that solutions of the similarity equations are possible only for >₀, and for the second problem that solutions are possible only for >–1 (for all Prandtl numbers). In both cases the solutions becomes singular as ₀ and as –1, and the natures of these singularities are discussed.     

Rheology of stimulated whole saliva in a typical pre-orthodontic sample population

The dynamic viscosity () of stimulated whole saliva in a typical pre-orthodontic sample population was characterized as a function of temperature (T). Samples were collected from 30 adolescents or young adults, after screening for factors that are known to have an effect on salivary viscosity. Using a cone and plate viscometer, 1.5 ml of stimulated whole saliva was evaluated at a constant shear rate of 450 s^-1 from T=20°C to T=40°C. Data from the -T plots showed a negative dependence of the form, =a–bT, over a range of from 1.08 to 2.45 centipoise (cps) at 34°C. Most of the samples fell into a narrow envelope, where the mean of the saliva samples ranged from 2.42±0.61 cps at 20°C to 1.57±0.32 cps at 37°C. With regard to sample stability, viscosity-time plots indicated that a small but predictable decrease in occurred during the 3 h period. The -T plots generated from fresh and frozen saliva samples demonstrated an appreciable change in as a result of refrigeration. With regard to sample reproducibility, viscometric data obtained from a typical pre-orthodontic patient over a 1-week period fluctuated within a fairly broad envelope of values.Presented, in part, at the 21st Annual Meeting and Exhibition of the A.A.D.R., Boston, Massachusetts, March, 1992.     

Single-Mode As–S Glass Fibers

Single-mode As–S glass fibers with a core diameter from 3 to 20 m and a clad diameter of 125 m are prepared by the double-crucible method. The cutoff wavelength of the fibers is 0.9–6 m. The lowest transmission losses in the fibers at 2.2–2.3 m are 100 dB/km, and their mean bending strength is 800–1000 MPa.     

Effect of fiber content on the thermoelectric behavior of cement

The effect of discontinuous stainless steel fibers (diameter 60 m) as an admixture in cement paste on the thermoelectric behavior (the Seebeck effect) was systematically studied as a function of fiber volume fraction from 0 to 0.50 vol%. Without fibers, cement paste has an absolute thermoelectric power of +3 V/°C. A fiber content of up to 0.20 vol% makes the absolute thermoelectric power more negative (down to –63 V/°C), whereas a fiber content of 0.20–0.50 vol% makes the absolute thermoelectric power more positive (up to +31 V/°C)—even more positive than the positive value for the steel fiber by itself (+8 V/°C). The value is zero at a steel fiber content of 0.27 vol%. The effects are probably due to carrier scattering rather than conduction.     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

The role of starting powder size on the compressive response of stand-alone plasma-sprayed alumina coatings

Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 m, 19 m and 32 m alumina; the resulting effective lamella diameters were 10 m, 28 m, and 55 m, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 m powder exhibited greater moduli than tubes sprayed from either 19 or 32 m powders. This difference was attributed to the greater percentage of unmelted -Al₂O₃ in the coating.     

Infrared transmission of sintered 3 mol% Y2O3-ZrO2 gel

Translucent ZrO₂ film was successfully prepared by gelling hydrothermally produced nano-ZrO₂ powders. The film (300 m thick) was found to transmit light to 6.5 m (40% transmission) when sintered at 1200 °C, but transmission was totally lost after sintering at 1300 °C for 1 h. Residual organic material such as urea, which was used for preparing the powder, dominated the transmission of the film in the region between 1.3 and 4.5 m when sintered below 1000 °C. When sintered above 1000 °C, the microstructure controlled the transmission. Both organic residuals and the microstructure of the zirconia were found to determine the transmission in 4.5–6.5 m region.     

Effect of thermomechanical treatments on microstructure and properties of Cu-base leadframe alloy

The effect of thermomechanical treatments (TMT) on the microstructuresand properties of Cu-1.5Ni-0.3Si-0.03P-0.05Mg leadframe alloy wasinvestigated. The Cu-base leadframe alloy was received as hot rolledplates with 8 mm thickness. The hot rolled plates were solutiontreated at 700°C or 800°C for 1 hour, and coldrolled with 40–85% reduction, then followed by aging treatment at450°C. The leadframe alloy solution treated at 800°Cshowed larger grain size of 15 m comparing with the grain size of10 m in leadframe alloy solution treated at 700°C. Theleadframe alloy with smaller grain size of 10 m showed highertensile strength and lower electrical resistivity than that withlarger grain size of 15 m. The dislocation density increased withincreasing reduction ratio of cold rolling from 40% to 85% andresulted in finer Ni₂Si precipitates. Tensile strength increasedand electrical resistivity decreased with increasing reduction ratioof cold rolling due to the formation of finer Ni₂Si precipitates.Two types of thermomechanical treatments were performed to enhance theproperties of leadframe alloy. One type of thermomechanical treatmentis to refine the grain size through the overaging, cold rollingfollowed by recrystallization. The recrystallization process improvedthe tensile strength to 540 MPa and elongation to 15% by reducing thegrain size to 5 m. The other type of thermomechanical treatmentis to refine the precipitate size by two-step aging process. Thetwo-step aging process increased the tensile strength to 640 MPa andreduced the electrical resistivity to1.475 × 10^–8 m by reducing the size of Ni₂Si precipitates to 4 nm.     

Selective laser sintering of gas atomized M2 high speed steel powder

Selective laser sintering of the gas atomized M2 high speed steel powder was performed using laser powers of 2.5–100 W, scan rates of 1–30 mm/s and scan line spacings of 0.15–0.75 mm. With increasing laser power, the sintered surface varied from open/closed pores to a fully dense structure. Large lateral pores were found in the sintered surface of samples using high scan rates. For fully dense samples, smooth surfaces could be achieved using large scan line spacing. The as-supplied and sieved M2 powder particles with size ranging from 0.04 to 400 m and 53 to 150 m, respectively, were found to give better laser sinterability as compared with those powder particles with finer (<38 m) or coarser (>150 m) sizes.     

Porous HA ceramic for bone replacement: Role of the pores and interconnections – experimental study in the rabbit

Hydroxyapatite (HA) porous ceramics are increasingly used in biomedical applications. Their physical characteristics, such as porous volume, require perfect control of the pore shape, as well as the number and the size of their interconnections.The aim of our study was to evaluate a new HA ceramic using polymethylmethacrylate microbeads (PMMA) as the porous agent. Four interconnection sizes (30, 60, 100 and 130 m) with a 175–260 m pore size and three pore sizes (175–260, 260–350 and 350–435 m) for a 130 m interconnection size were tested. Various HA implants were appraised by microscopic evaluation in a 4.6×10 mm rabbit femur cancellous bone defect 12 weeks after implantation. The best osteoconduction result was obtained in the center of the ceramic by means of a 130 m interconnection size and a 175–260 m mean pore size. Bone formation obtained within the pores was double that obtained in our previous study where naphtalen microbeads were used as the porous agents.© 2001 Kluwer Academic Publishers     

Discrete β-Ta2O5 crystallite formation in reactively sputtered amorphous thin films

The crystallization of thin amorphous TaOx films formed by d.c. reactive sputtering was investigated at temperatures from 500–700 °C. The films remained amorphous for times up to 100 h at 500 °C. The formation of discrete, single crystallites of the orthorhombic -Ta2O5 phase was observed after annealing at 600 °C for times from 8–108 h. The crystallites were 0.35 m×0.35 m after 8 h and grew to approximately 2.5 m×2.0 m after 108 h. A (2 0 0) fibre texture with a 6° spread was observed. More rapid in-plane growth in the [0 1 0] direction resulted in a near-rectangular shape and is attributed to a ledge growth mechanism. Higher temperature anneals at 650 and 700 °C produced less-textured polycrystalline films with remnant amorphous regions. © 1998 Kluwer Academic Publishers     

Investigation of superplasticity by the impression creep technique

Impression creep tests have been carried out on a Zn-22% Al eutectoid alloy in two heattreatment conditions, as a function of applied stress and at temperatures of 25 to 270° C. At small grain sizes ( 0.9m) and in the temperature range 180 to 270° C, the observed deformation response conforms to Regions I, II and III typical of superplastic behaviour. Furthermore, strain-rate index values determined for Region II fall in the range 0.44 to 0.51, consistent with superplasticity. It is concluded that the impression creep technique offers considerable potential for characterization of superplastic alloy systems on small specimen volumes.     

Structure and properties of injection-moulded nylon-6

The microstructure and morphology of injection-moulded nylon-6 has been studied using wide-angle and small-angle X-ray diffraction, optical microscopy and a density measurement technique. A non-spherulitic surface layer consisting of a metastable pseudohexagonal phase surrounds the moulding and the thickness of the layer is sensitive to mould temperature. This outer structure gives way to a monoclinic phase containing spherulites between 3 and 6 m diameter. The central core of the moulding consists almost entirely of the phase with spherulites 6 m diameter, approximately. Exposure of nylon-6 to boiling water had no significant effect on the morphology of the moulding but the phase in the surface layer was transformed to the monoclinic phase.     

Fibrous growth of tricalcium phosphate ceramics

Structural transformation and sintering processes of tricalcium phosphate (TCP) ceramics prepared from defective hydroxyapatite (Ca9HPO4(PO4)5OH) were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). Starting powders with Ca/P ratio 1.5 were obtained by adding 0.5 l of 0.3 M H3PO4 solution to an equal volume of 0.45 M Ca(OH)2. In the prepared ceramics, the onset temperature for transformation of defective hydroxyapatite into TCP (witlokite) agrees with the onset temperature for sintering (800 °C). Sintering occurs through the formation of a fibrous structure, which resembles biological hard tissue. In the 1000–1200°C range, these fibres coalesce into grains of up to 0.6 m in size with a fibrous-laminar morphology. At the end of this sintering stage witlokite transforms into TCP. At about 1450°C, partial decomposition of TCP into Ca2P2O7+Ca4P2O9 is observed. AFM observations suggest that Ca2P2O7 is segregated in the liquid state and increases the velocity of grain growth (up to 12 m).     

Carbothermal production of β′-sialon from alumina,silica and carbon mixture

Mixtures of pure nanometer-sized amorphous silica and -alumina with the atomic ratio SiAl=1 were reduced by a stoichiometric amount of carbon between 1100 and 1450 °C in flowing nitrogen in order to produce -sialon powder. Using aqueous suspensions of starting materials, compacts with different microstructures were prepared for reaction. Silica reduction to SiO occurred at a temperature as low as 1300 °C and part of it was removed with flowing nitrogen. Carbothermal reaction involving nitrogen stated at 1350 °C and Si₂N₂O was found as an intermediate together with SiC, resulting in -sialon formation. Loss of silica from the system led to AlN formation. Decomposition of -sialon into sialon polytypoids (15R, 12H) was observed as a result of sialon and AlN reaction at 1450 °C. The reaction rate of sialon formation was slowed down compared to the carbothermal reduction of kaolin because of the lack of impurities. The microstructure of the reacted pellets influenced the reaction products, and the narrow pore size distribution as well as good homogeneity enhanced -sialon formation.On leave, from Silesian Technical University, Krasiskiego 8, 40-019 Katowice, Poland.     

Initial permeability studies on A13+ and Cr3+ substituted Ni–Zn ferrites

The chemical formula of the samples investigated is Ni0.7Zn0.3Alx/CrxFe2-xO4, where x=0.00, 0.05, 0.10, 0.15, 0.20 and 0.25. The samples were obtained by the usual ceramic technology from high-purity oxides. The initial permeability was calculated from the inductance measurements with a torroidal core of 100 turns, using the formula L=0.0046 iN2h log d2/d1. The initial permeability i decreases in Ni0.7Zn0.3Alx/CrxFe2-xO4 with increase in Al3+/Cr3+. The decrease in i is attributed to a decrease of grain size D from 4.9 m to 4.4 m with Al3+ and to 1.9 m with Cr3+ and to variations in the anisotropy constant K1. The main contribution to the variation of permeability with content of Al3+/Cr3+ in the system is the effect each of them has on domain wall motion. The trivalent substituents (Al3+/Cr3+) cause impedance to the domain wall motion, which increases as the content of these ions increases. Al3+ has a stronger effect than Cr3+. The initial permeability components and do not exhibit much variation with temperature, except near Tc, where they fall sharply. The maximum of near Tc has been attributed to a damping effect of domain wall motion. © 1998 Chapman & Hall     

Preparation of Si3N4 whiskers by reaction of wheat husks with NH3

-Si₃N₄ whiskers that are 1–10 mm long and 0.5–1.1 m thick were obtained by the reaction of wheat husks with NH₃ at 1250–1450 °C. A maximum whisker yield of about 30% was obtained at 1450 °C with the addition of an iron impurity. Whiskers with 1.3–2.2 m thickness (average 1.6 m) were obtained by the addition of an H₂S impurity. Thin whiskers with periodic thick and thin diameters were also obtained.     

Measurements of chemical potentials in superconducting tin whiskers

A superconductor / normal conductor potential probe pair was used to measure differences between the time-averaged electrochemical potentials _p of Cooper pairs and of quasiparticle excitations caused by a phase-slip center in several experimental situations. The results can only be understood by assuming that at a larger distance from the phase-slip center differs from the proposal of Skocpol, Beasley, and Tinkham by showing a swinging over or swinging below _p . We propose a modified slope of as a function of site. Possible explanations are discussed. The multiple-contact samples used also allow the study of the influence of phase-slip centers already present on the relaxation of nonequilibrium quasiparticles.     

Crystal growth of U2D2 solid3He

We have measured the growth rate of single crystal U2D2 solid³He in the superfluid B phase as a function of chemical potential difference between the solid and liquid for temperatures between 0.4 and 0.9 T_N. The growth rate was not proportional to , with very slow growth rates for less than about 10 erg/cm³. For larger values of the growth rate depended weakly on temperature and the growth was attributed to a quantum growth mechanism due to screw dislocations. The melting rate was too fast to be measured quantitatively, but was slower at higher temperatures.     

The crack tip strain field of AISI 4340 Part I Measurement technique

This is the first paper in a study on the influence of the environment on the crack tip strain field for AISI 4340. A stressing stage for the environmental scanning electron microscope (ESEM) was constructed which was capable of applying loads up to 60 kN to fracture-mechanics samples. The measurement of the crack tip strain field required preparation (by electron lithography or chemical etching) of a system of reference points spaced at 5 m intervals on the sample surface, loading the sample inside an electron microscope, image processing procedures to measure the displacement at each reference point and calculation of the strain field. Two algorithms to calculate strain were evaluated. Possible sources of errors were calculation errors due to the algorithm, errors inherent in the image processing procedure and errors due to the limited precision of the displacement measurements. Estimation of the contribution of each source of error was performed. The technique allows measurement of the crack tip strain field over an area of 50×40 m with a strain precision better than ±0.02 at distances larger than 5 m from the crack tip.