J. C. McClure

Low-temperature friction-stir welding of 2024 aluminum

Solid state friction-stir welding (FSW) has been demonstrated to involve dynamic recrystallization producing ultra-fine, equiaxed grain structures to facilitate superplastic deformation as the welding or joining mechanism. However, the average residual, equiaxed, grain size in the weld zone has ranged from roughly 0.5 micron to slightly more than 10 micron, and the larger weld zone grain sizes have been characterized as residual or static grain growth as a consequence of the temperatures in the weld zone (where center-line temperatures in the FSW of 6061 Al have been shown to be as high as 480C or -0.8 T(sub M) where T(sub M) is the absolute melting temperature)). In addition, the average residual weld zone grain size has been observed to increase near the top of the weld, and to decrease with distance on either side of the weld-zone centerline, an d this corresponds roughly to temperature variations within the weld zone. The residual grain size also generally decreases with decreasing FSW tool rotation speed. These observations are consistent with the general rules for recrystallization where the recrystallized grain size decreases with increasing strain (or deformation) at constant strain rate, or with increasing strain-rate, or with increasing strain rate at constant strain; especially at lower ambient temperatures, (or annealing temperatures). Since the recrystallization temperature also decreases with increasing strain rate, the FSW process is somewhat complicated because the ambient temperature, the frictional heating fraction, and the adiabatic heating fraction )proportional to the product of strain and strain-rate) will all influence both the recrystallization and growth within the FSW zone. Significantly reducing the ambient temperature of the base metal or work pieces to be welded would be expected to reduce the residual weld-zone grain size. The practical consequences of this temperature reduction would be the achievement of low temperature welding. This study compares the residual grain sizes and microstructures in 2024 Al friction-stir welded at room temperature (about 30C and low temperature (-30C).

A TEM study of precipitation and related microstructures in friction-stir-welded 6061 aluminium

Residual microstructures, including dynamic recrystallization and grain growth structures and a wide range of precipitation phenomena associated with a friction-stir-weld in a thin 6061-T6 aluminium plate have been systematically investigated utilizing light metallography and transmission electron microscopy. In this rather remarkable process, a hard steel head pin rotating at 400 r.p.m. was advanced into a solid 6061-aluminium plate at a traverse velocity of approximately 2 mm s−1 to produce a solid-phase weld in its trailing side. Maximum work-piece temperatures did not exceed 425°C and there was no melt evidence. Dynamic recrystallization associated with the solid-state plastic flow therefore seems to provide the process mechanism. Weld zone hardnesses averaged roughly 55 Vickers hardness number (VHN) in contrast to the base plate or work-piece hardness of 110 VHN. Precipitation microstructures ranged from continuous to discontinuous coherent zones (∼2 nm thick) coincident with {1 0 0} planes, semicoherent and non-coherent needles and plates characteristic of Widmanstatten structures coincident with {1 1 0} planes, and a range of homogeneous precipitate particles often intermixed with these microstructures in the effective heat-affected zone (HAZ′) connecting the friction-stir-weld zone with the unaltered work piece microstructures.

Characteristics of indium tin oxide films deposited by r.f. magnetron sputtering

Abstract Highly conductive indium tin oxide (ITO) films were deposited by r.f. magnetron sputtering using ITO targets. The composition of the ITO targets was 90% indium oxide and 10% tin oxide. ITO films were deposited on 1 mm thick soda lime glass. Films deposited at substrate temperature of 300 °C, exhibited resistivities as low as 1.3 × 10 −4 ohm cm −1 . Annealing of the ITO films in air for 2 h was necessary for achieving low resistivities. X-ray diffraction and transmissivity tests were carried out to study the effects of annealing. Lowest resistivity and highest transmission were found to occur at an annealing temperature of 350 °C. X-ray diffraction measurements revealed that the as deposited film had a strongly (222) oriented cubic structure. Annealing relieved the as deposited tensile strain and increased crystal perfection.

Development of CdTe thin films on flexible substrates—a review

In the recent years there has been an increased interest in photovoltaic structures on lightweight flexible substrates. Photovoltaic structures on lightweight substrates have several advantages over the heavy glass-based structures in both terrestrial and space applications. CdTe is one of the leading candidates for the solar cells due to its optimum band gap and the variety of film preparation methods. The development of CdTe thin films on flexible substrates is discussed. The film growth and characterization are reviewed.

Design issues in the fabrication of CdS–CdTe solar cells on molybdenum foil substrates

Abstract Investigations on CdTe–CdS solar cells on molybdenum foil substrates revealed that the depletion layer spans the entire CdS and CdTe film thickness and the cell should be conceived as a single junction device instead of the three separate junctions (Mo–CdTe, CdTe–CdS, CdS–TCO). Higher open circuit voltages were achieved when two CdS layers (separated by an air anneal) were used instead of a single CdS layer. The high series resistance of this solar cell continues to be the limiting factor in cell performance. Modeling and design issues for improving cell performance are presented.

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Abstract Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.

Physical and electrical characterization of CdS films deposited by vacuum evaporation, solution growth and spray pyrolysis

The physical and electrical characteristics of CdS thin films deposited by vacuum evaporation, solution growth and spray pyrolysis were analysed. The effects of the common grain growth promoter CdCl2 and annealing were investigated. Grain size, bulk composition and surface composition were measured by energy-dispersive X-ray fluorescence, Auger spectroscopy and scanning electron microscopy. Schottky diode analysis was performed to study the electrical characteristics of the films, and energy band gap was measured by spectral transmission.

Electrochemical characterization of tungsten carbonyl compound for oxygen reduction reaction

Abstract The results of electrochemical studies carried out on Wx(CO)n electrocatalyst are presented in this article. Wx(CO)n was synthesized from W(CO)6 in xylene at 140°C. The kinetic studies were carried out in 0.5 M H2SO4 solution under a three-electrode configuration. Linear voltametry studies on Wx(CO)n reveal significant catalytic activity of the material for oxygen reduction. Koutecky–Levich analysis of the voltametry data shows that the reaction follows first-order kinetics and the value of the Koutecky–Levich slope indicates a multi-electron charge transfer in the oxygen reduction reaction. The value of Tafel slope is found to be −120 mV / decade from the mass-transfer-corrected Tafel plots. The charge transfer coefficient and exchange current density were found to be 0.66 and 8.4×10 −5 A/cm 2 , respectively.

Light and voltage dependence of the junction transport properties of CdTe/CdS photovoltaics

Abstract The J–V curve of CdTe/CdS photovoltaics does not consist of a simple superposition of a loss current and a light generated current with a considerable loss in conversion efficiency. This paper uses capacitance/voltage measurements and J–V measurements at a variety of temperatures and light levels to develop a model for this non-superposition. It was found that a light dependent tunneling mechanism dominates at low voltages. Moreover, the tunneling takes place from a trap level within the CdTe.

Phosphor currents in ZnS:Mn ac thin film electroluminescent display devices

Conduction current in the phosphor layer of ZnS:Mn ac thin film electroluminescent (ACTFEL) display device was measured as a function of the amplitude and the rise time of the bipolar voltage pulses: electric field in the phosphor was also determined. It was found that the clamping field of an ac thin film electroluminescent device is not a single‐valued device characteristic. The clamping field depended upon the rise time of the applied voltage pulse increasing in value as the rise time became smaller. These data were interpreted in terms of a nonideal breakdown of the insulator‐phosphor interface and a time delay involved in the emission of electrons from this interface. To account for the nonideality in the interface breakdown the ac equivalent circuit of the ACTFEL device was modified by including a voltage‐dependent resistor in parallel with the phosphor capacitance. The phosphor current measurements, transferred charge measurements and tunneling current calculations indicate that in the nonhystereti...