Unsteady Natural Convection Fluid Flow in a Vertical Microchannel under the Effect of the Dual-Phase-Lag Heat-Conduction Model

The unsteady hydrodynamics and thermal behavior of fluid flow in an open-ended vertical parallel-plate microchannel are investigated semi-analytically under the effect of the dual-phase-lag heat conduction model. The model that combines both the continuum approach and the possibility of slip at the boundary is adopted in the study. The effects of the Knudsen number Kn, the thermal relaxation time τ _q , and the thermal retardation time τ _T on the microchannel hydrodynamics and thermal behavior are investigated using the dual-phase-lag and hyperbolic-heat-conduction models. It is found that as Kn increases the slip in the hydrodynamic and thermal boundary condition increases. Also, the slip in the hydrodynamic behavior increases as τ _T and τ _q decrease, but the effect of τ _T and τ _q on the slip of the thermal behavior is insignificant.     

Simulation of a glass transition in a hot-wire experiment using time-dependent heat capacity

The transient hot-wire method is used for simultaneous measurements of the thermal conductivity λ and the heat capacity per unit volumepc _p and yields a peak in λ and a dip inpc _p near a glass transition. Through simulations, it is shown that these anomalous results arise due to a time dependence inc _p , which is described by a fractional exponential function:c _p (t)=c _p (liquid)+[c _p (glass)-c _p (liquid)c ^-(t,τ)β], where τ is the heat capacity relaxation time and β is a sample dependent parameter (0<β≤1). By a comparison with experimental data for cyclohexanol and glycerol, it is demonstrated that this model can be used to reproduce the peak and the dip as well as the temperature at which these occur. In addition, it is shown that the maximum in λ occurs at τ-0.3 s, whereas τ of the minimum inpc _p is dependent on β and moves from 0.4 to 1 s for a change in β from 1 to 0.5. The difference in τ between the peak and the dip is in agreement with the experimental results. It is concluded that the anomalies reveal glass forming characteristics such as a rough classification in terms of strong and fragile glass formers.     

Photoelectronic properties of HgI2 crystals for nuclear radiation detection

Photoelectronic properties of red mercuric iodide single crystals, grown from its saturated solution in tetrahydrofuran, have been studied for the wavelength range 450–700 nm at temperatures 80,110, 175, 235 and 300 K. Various aspects of the optical generation of charge carriers have been discussed. The computer simulation of the room temperature photoconductivity has generated the optimized values of the mobility-lifetime products μ_eτ_e = 5.67 × 10⁻⁵ cm²/V, μ_hτ_h = 0.18 × 10⁻⁵ cm²/V), and surface recombination velocities (S_e = 3.2 × 10⁵ cm/s, S_h = 4.5 × 10⁵ cm/s) of the charge carriers in these crystals. The estimated values of the electron and hole drift lengths for typical electric fields suggest that, under the negative electrode illumination, THF α-HgI₂ crystals have high potential as regards to their use as photodetectors in most of the scintillation spectrometers.     

Changes in emission characteristics of light-emitting diodes under variation of internal factors and pulse-excitation parameters

A relationship is established between the wavelength λ at the maximum of the near-edge emission band of a light-emitting diode (LED) and the external parameters of the pulse current t_i and Q and the thermal time constant τ_i, which is determined by the product of the thermal resistance and capacitance. A dependence of the shift in λ on the relationship between the LED’s internal and external parameters is discovered. It is shown that for LED operation in the pulsed mode, the shift in λ with variation of t_i. Q, and especially τ_i can reach values on the order of 10 nm (or more), which is a considerable source of errors. Ukraine. Translated from Izmeritel’naya Tekhnika, No. 12, pp. 37–38, December, 1999.     

Analysis of observations on solid-solution hardening in KBr–KCl single crystals

Available data on the temperature and concentration dependences of the critical resolved shear stress (CRSS) of KBr–KCl solid-solution crystals containing 8–41 mol% KCl in the temperature range 77–230 K have been analysed within the frame work of the kink-pair nucleation model of plastic flow in solid-solution crystals. It is found that CRSS τ decreases with the increase in temperature T in accord with the model relation ln τ = A−BT, where A and B are positive constants. The CRSS τ at a given temperature depends on solute concentration c as τ ∝ c ^p , where exponent p has a value between 0.5 and 1 depending on the temperature at which deformation is carried out. The model parameter W _o, i.e. binding energy between the edge-dislocation segment involved in the unit activation process and the solute atoms close to it (T→0 K), which is inversely proportional to B, increases with solute concentration c monotonically in accord with the model prediction W _o ∝ c ^1/4 upto a critical value c _m = 35 mol% KCl. However, W _o decreases with increase in c beyond c _m , which indicates somewhat ordered distribution of solute in the host lattice of concentrated solid-solutions with c > c _m .     

The effect of metastability on room temperature deformation behavior of β and α + β titanium alloys

The deformation behavior of single-phase metastable β-titanium alloys and two-phase α+metastable-β alloys strongly depends on the degree of stability of the β-phase. Recently, it has been shown that the tensile deformation behavior, as well as the creep deformation behavior at low temperatures (<0.25T _m), is strongly influenced by the degree of metastability. For example, the titanium β-alloy Ti–13.0wt%Mn, which has higher stability than the titanium β-alloy Ti–14.8wt%V, deforms by slip only; whereas the latter deforms by slip and twinning. In addition to the mechanical properties, the deformation mechanisms also depend on the degree of metastability. Further, the deformation mechanisms of a given metastable β-alloy depend on whether the β-phase is present by itself as a single-phase alloy, or in the presence of α-phase in the form of a two-phase alloy. For example, it was found that a metastable Ti–V alloy deforms by slip and twinning when it is in the form of a single-phase alloy, but deforms by slip and martensitic transformation when the same metastable β-phase is present in a two-phase α + β alloy. The mechanical properties of the metastable β alloys in turn depend on these deformation mechanisms. These recent developments are reviewed in this article.     

Shear strength resulting from static friction of some thermoplastics

The effects of normal load and roughness on the static shear strength of three thermoplastics have been investigated for the case of contact between a plastic sample and a very smooth metallic plate. An apparatus which provides a gradual increase in the tangential load was used to measure the minimum force required to shear the adhesion between the two surfaces when subjected to a normal load. This force was set equal to the adhesion component of friction F _f. The real area of contact A _r was also measured using an optical device designed to handle samples and experimental conditions similar to the friction tests. This permitted the evaluation of static shear strength τ as the ratio F _f/A_r. The experimental results for ultra-high molecular weight polyethylene (UHMWPE) show that the relationship between τ and the real contact pressure P can be represented by a regression function of order three. For this material, there is a decrease of τ when the asperities undergo elastic deformations. However, an increase in τ is found when plastic deformation of the asperities occurs. This transition was noted at the region where P is equal to the yield strength S _y of the material. The acetal Delrin also showed a decrease of τ when P<S _y and, although it could not be verified experimentally, it is assumed that it would behave in the same manner as the previous material for higher P. A strong dependence of τ on the initial roughness of the contacting surface was observed for both materials. The results obtained with nylon (PA 66) show an increase of τ with P, even when P is lower than S _y. This difference in behaviour compared to the previous materials is related to the brittleness of the asperities which is a result of its relatively low glass transition temperature. No effect of the roughness on the shear strength was found for this material. The relationship between τ and P can be very well approximated by linear functions whose parameters depend on the type of deformation that occurs at the interface. These parameters are derived and reported.     

Is the two dimensional organic conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuCl2)1+y clean or dirty?

The transport properties of organic τ-type conductors seem to be very clean as well as very dirty depending on what we observe. To clarify this problem, we studied temperature and magnetic field dependence of resistivity, ρ(T, B), in τ-(EDO-S,S-DMEDT-TTF)₂(AuCl₂)_1+y. The properties that favor “probably dirty” are: i) stepwise ρ_a(T) increase below 20 K, which is suppressed by magnetic field, and ii) contrasted difference in ρ(T) as well as magnetization M(T) between field cooled (FC) and zero field cooled (ZFC). On the other hand, Shubnikov de Haas oscillations with very low Dingle temperature (T_D=1.5 K) are typical of clean system. Based on these observations, we conclude this system changes from “dirty” to “clean” system by increase of magnetic field.     

Orientation and temperature dependence of the flow stress in the intermetallic compound Ni3Ge single crystals

Tensile tests on Ni₃Ge single crystals were carried out to make clear the mechanism of the positive temperature dependence of the CRSS and the work-hardening rate as a function of the tensile axis orientation below room temperature. Both the CRSS () and the maximum work hardening rate ( _M) show positive temperature dependence even below room temperature. The increments of and _M are satisfactorily expressed by the Schmid factor ratio of the cube cross slip system to the primary octahedral one,N=(010) [¯101]/(111) [¯101 ]. Then, the positive temperature dependence of both the CRSS and the maximum work-hardening rate is thought to be governed by two mechanisms. One arises from the Kear—Wilsdorf mechanism, which depends on the orientation. The other seems to arise from the orientation-independent factor, although it is obscure at present.     

Is the Two Dimensional Organic Conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuCl2)1+y Clean or Dirty?

The transport properties of organic τ-type conductors seem to be very clean as well as very dirty depending on what we observe. To clarify this problem, we studied temperature and magnetic field dependence of resistivity, ρ(T, B), in τ-(EDO-S,S-DMEDT-TTF)₂(AuCl₂)_1+y. The properties that favor “probably dirty” are: i) stepwise ρ_a(T) increase below 20 K, which is suppressed by magnetic field, and ii) contrasted difference in ρ(T) as well as magnetization M(T) between field cooled (FC) and zero field cooled (ZFC). On the other hand, Shubnikov de Haas oscillations with very low Dingle temperature (T_D = 1.5 K) are typical of clean system. Based on these observations, we conclude this system changes from “dirty” to “clean” system by increase of magnetic field.     

A new class of electrorheological material: synthesis and electrorheological performance of rare earth complexes of phosphate cellulose

A new class of electrorheological (ER) material, rare earth (RE = Ce, Gd, Er and Y) complexes of phosphate cellulose, has been synthesized using microcrystalline cellulose, phosphoric acid, urea and RE(NO₃)₃ solutions as starting materials. The ER properties of suspensions of microcrystalline cellulose, phosphate cellulose [cellulose-P-ONH₄] and the [cellulose(-P-O)₃RE] complex particle materials in silicon oil have been investigated under DC electric field. The formation of rare earth complexes helps to decrease the shear stress and viscosity at zero electric field, and to enhance the ER effect of the materials. The shear stress (τ_E) of the ER fluid (20% weight fraction) of a typical yttrium complex [cellulose(-P-O)₃Y], the yttrium content of which is 0.04 mol/100 g, is 2.3 kPa at 4.2 kV/mm and 300 s⁻¹ with a τ_r value (τ_r = τ_E/τ₀, where τ₀ is the shear stress at no electric field and 300 s⁻¹) of 34.3, which is 18 times higher than that of pure microcrystalline cellulose suspensions. The improvement of dielectric loss tangent of the material, due solely to the formation of rare earth complexes, resulted in an enhancement in the ER effect of the material. In addition, the cellulose(-P-O)₃RE materials possess better thermal stability, and their suspensions are more stable in the anti-sedimentation than that of the cellulose-P-ONH₄ material.     

Deformation behaviour of single crystals of InP in uniaxial compression

The deformation characteristics of indium phosphide (InP) single crystals under uniaxial compression have been examined as a function of strain rate, temperature and orientation. It has been shown that at temperatures below 0.55T _m (T _m=melting point; 1335 K) the material fractures in a brittle manner whereas at higher temperatures, within the range 0.55 to 0.71T _m, plastic deformation occurs by both slip and deformation twinning; above 0.71T _m, slip alone is the operative deformation mechanism. The observed operative slip systems are of the type {1 1 1} 0 1 1 which are characteristic of most Group IIIb-Group Vb compounds. Deformation twinning occurs predominantly on {1 1 1} planes but some activity is also observed on planes of the type {3 4 5}.     

On the failure of cracks under mixed-mode loads

Fracture of plates containing a crack under mixed-mode, I and II, loading conditions is investigated. Fracture mechanisms are first examined from fracture surface morphology to correlate with the macroscopic fracture behavior. Two distinct features are observed and they are typical of shear and tensile types of failure. From this correlation, a fracture criterion based on the competition of the attainment of a tensile fracturing stress σ_{_C} and a shear fracturing stress τ_{_C} at a fixed distance around the crack tip is proposed. Material ductility is incorporated using τ_{_C}/σ_{_C} determined from classical material failure theories. The type of fracture is predicted by comparing τ_{_max}/σ_{_max} at r=r_{_C} for a given mixed mode loading to the material ductility_{τ_C/σ_C} , i.e. τ_{_max}/σ_{_max})<(τ_{_C} /σ_{_C}) for tensile type of fracture and (τ_{_max}/σ_{_max}) r (τ_{_C}/ σ_{_C}) for shear type of fracture. It is found that, for typical engineering structural metals with certain ductility, (1) crack propagation initiates according to the maximum hoop stress criterion when the the mode mixity is near mode I and according to the maximum shear stress criterion when the mode mixity is near mode II, and (2) the transition of the failure from tensile to shear type can be predicted by the proposed criterion. For brittle materials the maximum hoop (opening) stress always reaches the tensile fracturing stress before the maximum shear stress reaches the shear fracturing stress of the material at a crack tip. Therefore, specimens made of brittle materials tend to fail under the maximum hoop stress criterion, as demonstrated by Erdogan and Sih (1963) and others. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of irradiation by boron ions on the character of realization of twinning and slip under prolonged (more than 60 sec) loading of single crystals of bismuth

The twinning of single crystals of bismuth irradiated by boron ions with an energy of 25 keV and a dose of 10¹⁷ ion/cm², in the case of prolonged (more than 60 sec) loading of a crystal is studied. It is found that prolonged exposure of a crystal to a concentrated load promotes a decrease in the number of twins in the concentrator of external stresses. An increase of temperature up to 400 K stimulates the process of slip in the realization of plastic deformation of a crystal. Mozyr’ State Pedagogical Institute, Mozyr’, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 72, No. 5, pp. 967–970.     

Electrical and photoelectric properties of GeS layered crystals grown by different techniques

Thermally stimulated currents and anisotropic electrical conductivity are studied in GeS layered crystals prepared by the Bridgman-Stockbarger, Pizzarello, and sublimation techniques. All the crystals arep-type, regardless of the growth technique, owing to the presence of Ge vacancies. The conductivity anisotropy in the melt-grown crystals is high compared to the vapor-grown GeS. The anisotropy rises exponentially with temperature. The concentrations and ionization energies of traps in GeS crystals are determined from thermally stimulated current curves. The spectral response of the photocurrent through the crystals prepared by sublimation, whose structural perfection is higher than that of the melt-grown crystals, is governed by the spectral dependence of the absorption coefficient forad ≪ 1 (near-edge region) and by the spectral dependence of reflectivity for αd > 1 (high-α region). Regardless of the growth technique, the 293-K photocurrent spectra of GeS crystals show strongly polarized peaks at 1.65 (E ∥a) and 1.78 eV (E ∥b), which are due to the Λ ₁ ^v → Λ ₁ ^c and Δ ₂ ^v → Δ ₂ ^c optical transitions. The low-temperature photoresponse athv < 1.7 eV is due to absorption by Si impurity.     

Preparation,microstructure and electrorheological property of nano-sized TiO<Subscript>2</Subscript> particle materials doped with metal oxides

New nano-sized TiO₂ electrorheological (ER) materials doped with different metal (M = Na, Zr, Ce, Al, Ca, Zn) oxides have been prepared. Relationships between the composition, microstructure, conductivity, dielectric property and ER effect of these materials have been studied. The results show that doping Na₂O, ZrO₂, Al₂O₃ or CeO₂ can enhance the ER performance of the TiO₂ material, whereas, doping CaO or ZnO would decrease the ER activity of the material. The shear stress (τ_E) of the suspension (25 wt%) of Na-doped TiO₂ in dimethyl silicone oil reaches 1.6 kPa at the electric field strength E = 4.2 kV/mm and shear rate γ = 300 s⁻¹, and its τ_r value of 54.6 (τ_r = τ_E/τ₀, where τ₀ is the shear stress at no electric field) is seven times higher than that of pure TiO₂ suspension. This high τ_r value is very advantageous to the use. The dielectric loss tangent (tanδ) plays a dominant role in influencing the ER performance of a particle material, and the effect of the surface area (pore volume, especially) and grain size should be taken into account.     

The effectiveness of using one variant of the finite-element method for solving nonlinear nonstationary heat-conduction problems

It is established that the use of a previously proposed variant of the finite-element method for solving nonlinear nonstationary heat-conduction problems is efficient (in the sense of computer time expenditures) when the half-width of the band β of the resulting matrix in the system of linear algebraic equations does not exceed a certain value of β_cr. As a result of a numerical experiment for an axially symmetric case we obtain β_cr ≈ 47, which demonstrates the effectiveness of this variant of the finite-element method for solving a wide range of practical problems of heat-conduction theory. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 2, pp. 284–289, March–April, 1997.     

Effect of pre-deformation anneal on the microstructure and texture evolution of Mg–3Al–1Zn–0.7Sr alloy during hot extrusion

The effect of pre-deformation annealing on the microstructure and texture of an AZ31 + 0.74 wt% Sr alloy has been investigated. As-cast samples as well as three samples that have been annealed at 400 °C for 10, 30, and 120 min were extruded at 300 °C. Results indicate that annealing transforms the bulky non-equilibrium Al–Mg–Sr precipitates to stable Al₄Sr spheroids. As the extent of this transformation increases before extrusion, there is seen an increase in the amount of uniformly dispersed intermetallic stringers in the extruded material. Texture measurements reveal the alignment of basal poles with the compression axis (perpendicular to the circular cross section of the extruded bar) and the formation of the basal ring texture in all the samples. However, an increase in the duration of the pre-deformation anneal switches the plane facing the extrusion direction from first order prismatic (10-10) to second order prismatic planes (11-20). Annealing decreases the Al solute concentration in Mg and lowers the lattice resistance against dislocation movement. Consequently, the more favorable (0002)[11-20] slip system is activated in grains that see low basal resolved shear stress (τ). As a result, those grains work harden and are consumed by dynamic recrystallization (DRX). However, the (0002)[-1100] slip system with high τ still avoids basal dislocation movement. Hence, the grains with high τ_{(0002)[-1100]}, which need to move dislocations in the (0002)[-1100] system to fulfill the strain compatibility conditions across the microstructure would be prevented from work hardening and DRX. This specific orientation has a (11-20) plane facing the extrusion direction.     

Plastic deformation of Al2O3 single crystals by indentation at temperatures up to 750° C

Vickers hardness measurements have been made on three differently oriented sapphire single crystals in the temperature range from room temperature to 750° C. The hardness generally decreased with increasing temperature but varied with specimen orientation. In the regions surrounding the indents, twinning lamellae and slip lines have been studied systematically and could be associated with dislocation systems. Rhombohedral {01¯1 2} and basal (0 0 0 1) twinning as well as prismatic {1 1¯ 2 0} and basal (0001) slip have been observed. Plastic deformation occurred during placement of Vickers microhardness indentations. The hardness was markedly influenced by the number and kind of activated systems. A change in the activated systems caused variation and inversion in hardness ratios. Investigating high-index planes gave much more information on anisotropy effects than would have been obtained had only basally and/or prismatically oriented specimens been used. The latter is commonly the case in the literature. It was shown that in brittle materials, such as ceramics, plastic deformation occurs at temperatures below 0.5T/T _m if a stress field with a large hydrostatic component is applied. It is suggested that this is also the case in abrasion.     

Frequency effect on the electrical and dielectric properties of (TlGaS2) 1 ? x(TlInSe2)x (x = 0.005, 0.02) single crystals

The electrical properties (loss tangent (tanδ), real (ɛ) and imaginary (ɛ″) parts of complex dielectric permittivity, and ac conductivity across the layers (σ_ac)) of (TlGaS₂)_{1 − x} (TlInSe₂) _x (x = 0.005, 0.02) layered single crystals have been studied in the frequency range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. The results demonstrate that the dielectric dispersion in the crystals has a relaxation nature. Almost throughout the frequency range studied, their ac conductivity follows the relation σ_ac ∼ f ^0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states (N _F ), the spread of their energies, the mean hop time τ and distance R, and the concentration of deep traps determining the ac conductivity of the crystals (N _t ) have been estimated. With increasing x in (TlGaS₂)_{1 − x} (TlInSe₂) _x , N _F and N _t increase, while τ and R decrease.