Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Properties of Perfluorobenzene near the Critical Point

The density of vapor and liquid perfluorobenzene along the liquid–vapor coexistence curve has been studied by a gamma-ray attenuation technique over the temperature range from 299 to 517 K. According to measurements, the coordinates of the critical point are T_C = 516.66 ± 0.05 K and ρ _C = 550.5 ± 2 kg · m⁻³. The critical exponent β of the coexistence curve equals 0.343 ± 0.005, which agrees closely with the non-classical value. The results of our measurements were compared with data available in the literature. The height dependence of the density of a two-phase sample was investigated in relation to the temperature and time. These experiments made it possible to determine the isothermal compressibility of liquid and vapor phases near the critical point.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

On the possibility of growth quantum dot arrays in InAs/GaAs system by droplet epitaxy under MOVPE conditions

We have experimentally studied the possibility of obtaining InAs quantum dot arrays on GaAs(100) substrates by droplet-island growth under low-temperature (160–360°C) metalorganic vapor phase epitaxy (MOVPE) conditions. It is established that trimethylindium (In source) exhibits decomposition even at the lower boundary of the indicated temperature interval. The height of In drops formed on the substrate surface was 3–12 nm with a density of ∼(0.4−1.4) × 109 cm⁻² depending on the H-MOVPE conditions. In order to retain the dimensions of InAs nanocrystals formed at the subsequent stage, the process should be carried out at an increased rate of arsine supply.     

Flow Boiling Heat Transfer in Two-Phase Micro Channel Heat Sink at Low Water Mass Flux

Boiling heat transfer at water flow with low mass flux in heat sink which contained rectangular microchannels was studied. The stainless steel heat sink contained ten parallel microchannels with a size of 640 × 2050 μm in cross-section with typical wall roughness of 10–15 μm. The local flow boiling heat transfer coefficients were measured at mass velocity of 17 and 51 kg/m²s, heat flux on 30 to 150 kW/m² and vapor quality of up to 0.8 at pressure in the channels closed to atmospheric one. It was observed that Kandlikar nucleate boiling correlation is in good agreement with the experimental data at mass flow velocity of 85 kg/m²s. At smaller mass flux the Kandlikar model and Zhang, Hibiki and Mishima model demonstrate incorrect trend of heat transfer coefficients variation with vapor quality.     

Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273–343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using α = α_max + (α_min − α_max)/[1 + exp 2(E − E_g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E_g(T) = E_g(273 K) − (8.8 × 10⁻⁴T²)/(483 + T) + 0.088 (Varshni empirical formula) and E_g(T) = E_g(273 K)−0.231447/[exp(362/T)−1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor ∼0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.     

Effects of dynamic strain aging on mechanical properties of SA508 class 3 reactor pressure vessel steel

An as-received reactor pressure vessel (RPV) steel SA508 class 3 (SA508 Cl.3) has been subjected to uniaxial tension tests in the strain-rate range of 6.67 × 10⁻⁵ s⁻¹ to 1.2 × 10⁻² s⁻¹ and the temperature range of 298 K to 673 K to investigate the effects of temperature and strain rate on its mechanical properties. It was found that the region of dynamic strain aging (DSA) was in the temperature range of 523–623 K at a strain rate of 1.2 × 10⁻³ s⁻¹, 473–573 K at 1.2 × 10⁻⁴ s⁻¹, and 473–573 K at 6.67 × 10⁻⁵ s⁻¹, respectively. Serrated stress–strain behaviors, predominately consisting of type A, B, and C, have been observed in these temperatures and strain-rate ranges. The solutes responsible for DSA have been identified to be carbon and nitrogen, and nitrogen atoms play a more important role. The relative DSA mechanisms for this RPV steel are discussed.     

Stability and recrystallization of PbS nanoparticles

The recrystallization and thermal stability of nanocrystalline lead sulfide have been studied by X-ray diffraction and scanning electron microscopy. PbS nanoparticles ranging in size from 10 to 20 nm were prepared by chemical precipitation from aqueous solutions. To assess the thermal stability of the size of PbS nanoparticles, the nanocrystalline powders were annealed in air or under dynamic vacuum (10⁻³ Pa) at a temperature varied from 433 to 930 K in 50-K steps. Annealing at temperatures of up to 700 K increases the particle size only slightly but relieves the lattice strain, suggesting that the nanocrystalline state of lead sulfide is thermally stable in this temperature range. The temperature range 700–900 K, where the particle size increases by a factor of 5–10, corresponds to the secondary recrystallization of nanocrystalline PbS. The temperature 700 K is half the melting temperature of macrocrystalline PbS, indicating that PbS nanoparticles have higher thermal stability in comparison with other nanomaterials.     

Low temperature and strain rate dependence of fracture stress and fracture toughness on thin Fe40Ni40B20 amorphous ribbon

The fracture stress and the critical stress intensity factor of the Fe₄₀Ni₄₀B₂₀ amorphous metallic ribbons 20 μm thick were measured in the temperature range 4.2–300 K and at deformation rates from 3.3×10⁻⁶ to 1.25×10⁻³ m⁻¹ with the aim to obtain more information on the condition for the onset and development of the inhomogeneous plastic deformation and fracture.     

Investigation of the effect of solidification processing parameters on the rod spacings and variation of microhardness with the rod spacing in the Sn–Cu hypereutectic alloy

Sn–3 wt% Cu hypereutectic alloy was prepared in a graphite crucible under the vacuum atmosphere. The samples were directionally solidified upwards under argon atmosphere with different temperature gradients (G = 4.24–8.09 K/mm) at a constant growth rate (V = 7.64 μm/s) and with different growth rates (V = 2.24–133.33 μm/s) at a constant temperature gradient (G = 4.24 K/mm) by using a Bridgman type directional solidification apparatus. The microstructure of directional solidified Sn–3 wt% Cu alloy seems to be rod eutectic structure. The influence of the growth rate (V) and temperature gradient (G) on the rod spacing (λ) and undercooling (ΔT) were analysed. The values of λ²V, λ²G, ΔTλ, ΔTV^−0.5 and ΔTG^−0.5 were determined by using the Jackson–Hunt eutectic theory. The dependence of microhardness (HV) on the rod spacing (λ) was analyzed. According to present results, it has been found that the value of HV increases with the increasing the value of λ.     

Critical thermal load in saturated boiling of liquids

The critical velocity of vapor flow from the heating surface in boiling is found based on the analogy of bubbling and boiling phenomena. The relation for the critical thermal load q_cr is found from the assumption that this quantity corresponds to the maximum vapor velocity. The expression obtained in this work for q_cr describes experimental data on the saturated boiling of water, ethanol, and benzene at different pressures. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 2, pp. 259–263, March–April, 2008.     

Evidence for the Edge Magnetoplasmon “Acoustic” Mode in an Electron Layer over Liquid Helium

The spectrum of new modes of edge magnetoplasmons (EMP) in an electron layer over liquid helium was studied. The experiments were carried out in the temperature range 0.5–0.7 K at frequencies 0.87–1 MHz and electron density (1.5–3.5)·10¹²m⁻² in a magnetic field up to 0.5 T. Besides conventional edge magnetoplasmons and the EMP mode connected with the displacement of the electron sheet boundary, a new “acoustic” edge magnetoplasmon mode was observed in an electron layer over liquid helium for the first time. Spectrum of all three types of the magnetoplasmon modes are compared with the existing theories.     

Characterisation of vacancy-like defects in III–V compound semiconductors using positron annihilation technique

Single crystals of GaP and InSb were irradiated by 3 MeV electrons at 20 K to a total dose of 4 × 10¹⁸ e ⁻/cm². Isochronal annealing in the temperature region 77–650 K followed the irradiation. In GaP, the positron lifetime measurement indicated the presence of irradiation-induced vacancies in the Ga-sublattice. The vacancies disappeared at two stages observed in temperature ranges 200–300 and 450–550 K. In InSb the positron lifetime was found to increase by 8 ps compared to that in as-grown crystals (i.e. 282±2 ps) after irradiation. The increase indicated the presence of irradiation-induced defects; the crystal was found to recover until 350 K with a sharp annealing stage at 250–350 K.     

Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy

Compression properties of a refractory multi-component alloy, Ta₂₀Nb₂₀Hf₂₀Zr₂₀Ti₂₀, were determined in the temperature range of 296–1473 K and strain rate range of 10⁻¹–10⁻⁵ s⁻¹. The properties were correlated with the microstructure developed during compression testing. The alloy was produced by vacuum arc melting, and it was hot isostatically pressed (HIPd) and homogenized at 1473 K for 24 h prior to testing. It had a single-phase body-centered cubic structure with the lattice parameter a = 340.4 pm. The grain size was in the range of 100–200 μm. During compression at a strain rate of έ = 10⁻³ s⁻¹, the alloy had the yield strength of 929 MPa at 296 K, 790 MPa at 673 K, 675 MPa at 873 K, 535 MPa at 1073 K, 295 MPa at 1273 K and 92 MPa at 1473 K. Continuous strain hardening and good ductility (ε ≥ 50%) were observed in the temperature range from 296 to 873 K. Deformation at T = 1073 K and έ ≥ 10⁻³ s⁻¹ was accompanied by intergranular cracking and cavitation, which was explained by insufficient dislocation and diffusion mobility to accommodate grain boundary sliding activated at this temperature. The intergranular cracking and cavitation disappeared with an increase in the deformation temperature to 1273 and 1473 K or a decrease in the strain rate to ~10⁻⁵ s⁻¹. At these high temperatures and/or low-strain rates the alloy deformed homogeneously and showed steady-state flow at a nearly constant flow stress. Partial dynamic recrystallization, leading to formation of fine equiaxed grains near grain boundaries, was observed in the specimens deformed at 1073 and 1273 K and completed dynamic recrystallization was observed at 1473 K.     

Thermal Conductivity of a Simulated Fuel with Dissolved Fission Products

The thermal diffusivity of a simulated fuel with fission products forming a solid solution was measured using the laser-flash method in the temperature range from room temperature to 1673 K. The density and the grain size of the simulated fuel with the solid solutions used in the measurement were 10.49 g · cm⁻³ (96.9% of theoretical density) at room temperature and 9.5 μm, respectively. The diameter and thickness of the specimens were 10 and 1 mm, respectively. The thermal diffusivity decreased from 2.108 m² · s⁻¹ at room temperature to 0.626 m² · s⁻¹ at 1673 K. The thermal conductivity was calculated by combining the thermal diffusivity with the specific heat and density. The thermal conductivity of the simulated fuel with the dissolved fission products decreased from 4.973 W · m⁻¹ · K⁻¹ at 300 K to 2.02 W · m⁻¹ · K⁻¹ at 1673 K. The thermal conductivity of the simulated fuel was lower than that of UO₂ by 34.36% at 300 K and by 15.05% at 1673 K. The difference in the thermal conductivity between the simulated fuel and UO₂ was large at room temperature, and decreased with an increase in temperature. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Thermoelectric and Magnetothermoelectric Properties of In-doped Nano-ZnO Thin Films Prepared by RF Magnetron Sputtering

Nano In-doped ZnO (IZO) films were deposited on glass substrates by RF magnetron sputtering from a powder target (2at% In) at different substrate temperatures. The thermoelectric and magnetothermoelectric properties of the IZO films were investigated. It shows that the prepared IZO films are c-axis oriented, the grain size is about 22–29 nm, and both the thermoeletromotive force (thermo-emf) and the magneto thermo-emf change linearly with temperature difference, implying that a striking thermoelectric (Seebeck) effect and magnetothermoelectric effect can be apparently observed in IZO films. The thermo-emf is negative, the Seebeck coefficient is about −57, −32, −40 and −66 μV/K for samples deposited at the substrate temperature of room temperature, 100, 200, 300°C, respectively. The power factor is (3.11–5.89)×10⁻⁵ W/K² m for our thin films. The absolute value of the magnetothermo-emf is smaller than the thermo-emf without a magnetic field, showing that the magnetic field has a negative effect on the Seebeck coefficient, which can be explained by the magnetoresistive effect substantially.     

Determination of bandgap variations in nondirect-band semiconductors from their edge luminescence spectra

The temperature dependence of the bandgap width (E _g) has been determined for single-crystalline silicon and cubic silicon carbide using a method based on the linear approximation of (i) the slope of the main peak front in the long-wavelength region of the near-edge electroluminescence (EL) spectrum and (ii) the dependence of the EL intensity on the quantum energy in the region preceding the onset of this peak. It is demonstrated that the temperature dependence of E _g determined by this method in the 80–300 K range coincides with the results of optical absorption measurements. For 3C-SiC, the temperature coefficient of the linear variation in E _g in the 285–765 K range is −3.5 × 10⁻⁴ eV/K, which agrees well with the published data of W.J. Choyke.     

Gasdynamic perturbations under conditions of surface microwave discharge in air

An investigation is performed of gasdynamic perturbations arising in the neighborhood of a dielectric antenna, on the external surface of which a microwave discharge is initiated at high pressures of air, where the electron-molecule collision frequency is much higher than the circular frequency of electromagnetic field. The time dependence of gas temperature is obtained for different values of power input, and it is demonstrated that, in the stage of formation of discharge at atmospheric pressure of air, the gas is heated at a rate of 30–70 K/μs. The electron concentration in plasma channels at atmospheric pressure does not exceed 10¹⁵–10¹⁶ cm⁻³. A study is made of the evolution of shock waves arising under conditions of surface micro-wave discharge at different pressures of surrounding gas, different durations of stimulation, and different power inputs to the discharge. It is demonstrated that the shock wave velocity in the vicinity of the antenna reaches a value of 1 km/s.     

Thermal conductivity of amorphous Teflon (AF 1600) at high pressure

The thermal conductivity, λ of amorphous Teflon AF 1600 [poly(1,3-dioxole-4,5-difluoro-2,2-bis(trifluoromethyl)-co-tetrafluoroethylene)] has been measured at pressures up to 2 GPa in the temperature range 93–392 K. At 295 K and atmospheric pressure, we obtained λ=0.116, W·m⁻¹·K⁻¹. The bulk modulus was measured up to 1.0 GPa in the temperature range 150–296 K and the combined data yielded the following values ofg=(∂ln λ ∂lnp) _r :2.8±0.2 at 296 K, 3.0±0.2 at 258 K, 3.0±0.2 at 236 K. 3.4±0.2 at 200 K. and 3.4±0.2 at 150 K.     

The Sound Velocity in an Alloy Steel at High-Temperature Conditions

In this article, results of the measurements of the longitudinal and transverse wave velocities in steel have been presented as a function of temperature. The conducted tests involved two types of corrosion-resistant steel: X14CrMoS17 and X90CrMoV18. The tests were based on the ultrasonic wave transition method using transducers operating at 5.4 MHz for the longitudinal wave and 3.2 MHz for the transverse wave. Measurements of the wave velocity were taken at temperatures from 293 K to 1,173 K. The longitudinal wave velocity in X14CrMoS17 steel varies from 6,002 m·s⁻¹ at 293 K to 5,115 m·s⁻¹ at 1,173 K, while the velocity in the X90CrMoV18 steel changes from 5,975 m·s⁻¹ at 293 K to 5,381 m·s⁻¹ at 1,023 K. The transverse wave velocities vary from 3,239 m·s⁻¹ at 293 K to 2,449 m·s⁻¹ at 1,173 K in X14CrMoS17 steel, and from 3,251 m·s⁻¹ at 293 K to 2,478 m·s⁻¹ at 1,173 K in X90CrMoV18 steel. The obtained results represented a basis for determination of the properties of the steels examined, such as Young’s modulus, Poisson’s ratio, Helmholtz’s modulus of volume elasticity, or Lame’s constants. The results have been verified by comparing the Young’s modulus obtained with the values corresponding to individual steel grades and temperatures (293 K, 373 K, 473 K, 573 K, and 673 K) obtained by traditional methods of measuring mechanical properties as provided in PN-EN 10088-1:2007. The results of this comparison confirmed the reliability of the conducted investigation.     

Anisotropic behaviors in polycrystalline Cd-doped GaSe thin films

GaSe thin films were deposited by thermal evaporation technique with Cd doping. X-ray diffraction analysis showed that Cd-doped films have polycrystalline structure with the preferred orientation along (008) direction. Temperature dependent electrical conductivity measurements were carried out in the temperature range of 100–400 K along perpendicular and parallel directions to the growth direction for the films exhibiting p-type conduction determined by hot probe technique. The room temperature conductivity values of the films were found to be as 1.5 × 10⁻⁸ and 4.9 × 10⁻¹² (Ω cm)⁻¹ due to the measurements along both perpendicular and parallel directions, respectively. The difference in the conductivity values is the indication of electrical anisotropy in the samples. Carrier conduction in the films was provided by the thermionic emission in the high temperature region (310–400 K) with almost the same activation energies in both directions. Space charge limited current analysis at different temperatures reveals the existence of two discrete sets of trap levels for both perpendicular and parallel directions. Calculated trap levels and trap concentrations are 99 meV, 3.5 × 10¹² cm⁻³ and 418 meV, 2.2 × 10⁵ cm⁻³ for perpendicular direction, 58 meV, 2.1 × 10¹⁸ cm⁻³ and 486 meV, 1.4 × 10¹² cm⁻³ for parallel direction. The differences in the values of the trap levels and concentrations for both directions confirm the existence of electrical anisotropy in Cd-doped GaSe thin films, because of the structural anisotropy between and inside the crystallites.