Experimental and theoretical studies of the influence of surface conditions on radiative properties of opaque materials

Radiative properties of opaque materials strongly depend on their surface condition. The fabrication of superficial cavities of various forms and dimensions modifies the directional spectral emissivities or absorptivities. They are usually increased compared to those of optically smooth material; the gain depends on the material, the type of cavities, as well as the wavelength and the direction of the emitted or incident radiation. When grooves of dimensions larger than are fabricated in a sample, the models, taking into account the successive reflections on their inner sides, give a good agreement with experimental data. But a similar theory does not explain the substantial increase of the infrared emissivity of ballblasted samples.     

Experimental and theoretical studies of palygorskite clays

We have studied the structure and properties of palygorskite clays. A structure analysis was performed using two different models to reproduce the monoclinic and orthorhombic lattices, using the atomic positions and cell parameters of palygorskite structure suggested by several authors, we simulated structures using Molecular Dynamics and Quantum Mechanics. Modifications on the structure and elemental atom changes were made to obtain more stable configurations. X-ray diffraction patterns and high resolution electron microscopy images from simulated structures were compared with experimental results. It could be observed that orthorhombic model shows a better fitting than monoclinic models. These results might help to understand many of the properties of archaeological pigments, such as, Maya Blue, in which palygorskite clay was the main component.     

Experimental study of the influence of transient performance in pressure pulse tubes on isentropic efficiency of rotary jet expanders

A rotary jet expander (RJE) achieves low temperature refrigeration based on the cooling effect of wave expansion after rapidly filling a tube with a compressible fluid. The transient performance in pulse tubes is one of the most important influences on the isentropic efficiency of a RJE, and it depends on the pulse frequency, time matching in one pulse period and transient heat transfer. Work on this transient performance proved that optimization of parameters for high isentropic efficiency in RJE could be achieved.     

Experimental and theoretical studies of the elastic behavior of knitted-fabric composites

This paper presents experimental and theoretical studies of the elastic behavior of knitted-fabric composites. In the experimental studies, two types of weft-knit preforms based upon plain-stitch and rib-stitch fabrics were first fabricated and fabric composites were consolidated by using a hand lay-up process. Tensile and rail shear tests were performed, and Young's moduli along the warp and weft directions and shear modulus determined. In order to correlate the preform microstructure with composite elastic properties, geometric models for plain-stitch and rib-stitch fabric composites were developed. Modeling of the elastic behavior was conducted by using an averaging method. The predicted elastic constants are in reasonably good agreement with experimental values. Finally, the limitation and potential of knitted-fabric composites are discussed.     

Experimental and theoretical studies on phase separations in the Fe-Al-Co ordering alloy system

Phase separations in iron-rich Fe-Al-Co ternary alloys were investigated by means of transmission electron microscopy, differential thermal analysis and magnetization measurement. Two kinds of phase separations have been found at 923 K; A2+B2 and B2+B2*. The former occurs in a tongue-shaped composition region ranging from Fe-12 at % Al-15 at % Co to Fe-35 at % Al-45 at % Co and the latter appears in the small region bordering on the Fe-Al binary side of the A2+B2 field. These two-phase fields have theoretically been evaluated on the basis of the so-called Bragg-Williams-Gorsky model taking account of not only chemical but also magnetic interactions. The magnetic ordering accounts for the expansion of the A2+B2 coexistent region. In the measurement of magnetic properties, large increases in coercive force, H _c, and residual magnetic flux density, B _r, were observed in the two-phase microstructures.     

Experimental investigation of the influence of flow of viscous liquid on transport processes

The authors have made an experimental examination and verification of the molecular separation effect in binary mixtures under the influence of viscous momentum transfer.     

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.     

Experimental and theoretical studies on a resolution enhanced sensing system based on multisensor successive approximation

A new system for highly-accurate sensing is described that uses a multiple of inaccurate sensors. The system is based on principles of successive approximation and statistical estimation. The concept has been implemented in a microcontroller based real-time system. Experimental results for temperature measurement have demonstrated significantly improved accuracy over a conventional single-sensor system. Theoretical analysis supports experimental results. The concept is general and extends to other measurands.     

Experimental and simulation studies on the performance of standing wave thermoacoustic prime mover for pulse tube refrigerator

The thermoacoustic prime mover (TAPM) has gained considerable attention as a pressure wave generator to drive pulse tube refrigerator (PTR) due to no moving parts, reasonable efficiency, use of environmental friendly working fluids etc. To drive PTCs, lower frequencies (f) with larger pressure amplitudes (ΔP) are essential, which are affected by geometric and operating parameters of TAPM as well as working fluids. For driving PTRs, a twin standing wave TAPM is built and studied by using different working fluids such as helium, argon, nitrogen and their binary mixtures. Simulation results of DeltaEc are compared with experimental data wherever possible. DeltaEc predicts slightly increased resonance frequencies, but gives larger ΔP and lower temperature difference ΔT across stack. High mass number working fluid leads to lower frequency with larger ΔP, but higher ΔT. Studies indicate that the binary gas mixture of right composition with lower ΔT can be arrived at to drive TAPM of given geometry.     

Experimental and theoretical studies suggesting the possibility of metallic boron nitride edges in porous nanourchins

We first describe the synthesis of novel and highly porous boron nitride (BN) nanospheres (100-400 nm o.d.) that exhibit a rough surface consisting of open BN nanocones and corrugated BN ribbons. The material was produced by reacting B2O3 with nanoporous carbon spheres under nitrogen at ca. 1750 degrees C. The BN nanospheres were characterized using scanning electron microscopy, high-resolution electron microscopy, and electron energy loss spectroscopy. The porous BN spheres show relatively large surface areas of ca. 290 m2/g and exhibit surprisingly stable field emission properties at low turn-on voltages (e.g., 1-1.3 V/microm). We attribute these outstanding electron emission properties to the presence of finite BN ribbons located at the surface of the nanospheres (exhibiting zigzag edges), which behave like metals as confirmed by first-principles calculations. In addition, our ab initio theoretical results indicate that the work function associated to these zigzag BN ribbons is 1.3 eV lower when compared with BN-bulk material.     

Experimental and theoretical studies on thermo‐mechanical fatigue test for aluminium cast alloy

Evaluation of the thermo‐mechanical behaviour and prediction of the service life of cast aluminium alloys are important for the design of automobile engine cylinder heads. In this study, cast Al alloy specimens are extracted from cylinder heads and subjected to in‐phase thermo‐mechanical cyclic loading. The hysteresis curves related to stress and strain were recorded under the individual thermo‐mechanical loading conditions. The number cycles to failure corresponding to multiple mechanical strain and temperature ranges were obtained. It is found that the cyclic stress amplitude decreases and the cyclic softening rate increases with increasing maximum temperature rise. A modified fatigue‐creep model based on energy conservation has been developed for prediction of the fatigue life of cylinder heads. The proposed method shows good agreement with the well‐established Ostergren model and low standard deviations. In summary, the proposed method described in this study provides an option for prediction of the thermo‐mechanical behaviour of metals.     

Experimental and first-principles theoretical studies on Ag-doped cuprous oxide as photocathode in photoelectrochemical splitting of water

Nanostructured thin films of undoped and Ag-doped cuprous oxide were deposited on indium tin oxide-coated glass substrate using simple spray pyrolysis method for their use as photocathode in photoelectrochemical (PEC) cell for solar energy based water splitting. Combination of experiments and first-principles density functional theory based calculations was used to determine and understand the effect of Ag substitution on electronic structure and PEC performance. Thin films were characterized using XRD, FE-SEM, UV–Vis spectroscopy and PEC measurements. The results of DFT calculations show that the top of valence band and bottom of conduction band of undoped Cu₂O lie at Г point of brillouin zone, respectively, suggesting that pure Cu₂O is a direct band gap material. Minimal changes appear in the band gap and band gap energies in the Ag-doped Cu₂O system, keeping it still a direct band gap material. A defect band appearance can be seen between ?4 and ?5 eV in the valence band consisting mainly of Ag 4d states and can be explained by a stronger interaction between the Ag 4d and O 2p, due to the larger Ag size. Ag-doped samples exhibit improved conductivity and fourfold increase in photocurrent density with respect to undoped samples.     

Experimental and theoretical investigations on the compressibility of nanocrystalline nickel

We investigated nanocrystalline nickel experimentally (x-ray diffraction) and theoretically (cluster ab initio calculations). No phase transition was observed in nano-nickel. The volume, isothermal bulk modulus and its pressure derivative derived experimentally and theoretically are 43.56 (0.13) ų, 228(15) GPa, 4.02(0.51) and 44.00 ų, 217 GPa and 3.20, respectively. We found no appreciable change in the value of bulk modulus for nano-nickel as compared to the bulk-nickel.An erratum to this article can be found at     

Experimental study of the influence of cold heat exchanger geometry on the performance of a co-axial pulse tube cooler

Improving the performance of the pulse tube cooler is one of the important objectives of the current studies. Besides the phase shifters and regenerators, heat exchangers also play an important role in determining the system efficiency and cooling capacity. A series of experiments on a 10 W @ 77 K class co-axial type pulse tube cooler with different cold heat exchanger geometries are presented in this paper. The cold heat exchangers are made from a copper block with radial slots, cut through using electrical discharge machining. Different slot widths varying from 0.12 mm to 0.4 mm and different slot numbers varying from around 20–60 are investigated, while the length of cold heat exchangers are kept the same. The cold heat exchanger geometry is classified into three groups, namely, constant heat transfer area, constant porosity and constant slot width. The study reveals that a large channel width of 0.4 mm (about ten times the thermal penetration depth of helium gas at 77 K, 100 Hz and 3.5 MPa) shows poor performance, the other results show complicated interaction effects between slot width and slot number. These systematic comparison experiments provide a useful reference for selecting a cold heat exchanger geometry in a practical cooler.     

Experimental investigation of cylindrical shells with an opening under an external pressure pulse

The results of experimental investigation of the deformation of a shell with a circular opening under a pulse of uniform external pressure are presented. A multifactor regression equation is derived on the basis of experimental data. The resultant equation is analyzed statistically. Relationships suitable for the practical computation of the residual deflection and critical pulse are given.Translated from Problemy Prochnosti, No. 2, pp. 68–71, February, 1990.