Analysis of thermal effect on transparent conductive oxide thin films ablated by UV laser

Transparent conductive oxide thin films are applied to many computer, communication and consumer electronics products including thin film transistor liquid crystal displays, organic light emitting diodes, solar cells, mobile phones, and digital cameras. The laser direct write patterning of the indium tin oxide (ITO) thin film processing technique produces a heat affected zone that has an enormous effect on the electro-optical efficiency of transparent conductive oxide films. This is because direct laser writing patterning in thermal machining process can create debris and micro-cracks in the substrate. Therefore, this study establishes the ultraviolet (UV) laser ablation of temperature model on the polycarbonate and soda-lime glass substrates using the finite element analysis software ANSYS, and measures the temperature field based on the laser micro-patterning process. The meshing model determines the structure of the pre-processors and parameters were set with ANSYS parameter design language. This study also simulates the Gaussian distribution laser irradiation on the pre-processor structure. A UV laser processing system made micro-patterning on ITO thin films to analyze which conditions damaged the substrates. Comparing the simulation and experiment results reveals the minimum laser ablation threshold of the ITO thin films with the melting and vaporization temperatures. Simulation results show that the temperature distribution on PC and soda-lime glass substrates after laser irradiation of 1.05 μs with a laser output power of 0.07 W produces temperatures of approximately 52 °C, 54 °C and 345°Cand 205 °C at the laser output power of 0.46 W. The experiment results show that the patterning region is similar to the simulation results, and the lower laser power does not damage the substrates.     

State of the art of high temperature power electronics

High temperature power electronics has become possible with the recent availability of silicon carbide devices. This material, as other wide-bandgap semiconductors, can operate at temperatures above 500 °C, whereas silicon is limited to 150-200 °C. Applications such as transportation or a deep oil and gas wells drilling can benefit. A few converters operating above 200 °C have been demonstrated, but work is still ongoing to design and build a power system able to operate in harsh environment (high temperature and deep thermal cycling).     

Effect of high temperature annealing on the electrical performance of titanium/platinum thin films

In this study, the influence of post deposition annealing steps (PDA) on the electrical resistivity of evaporated titanium/platinum thin films on thermally oxidised silicon is investigated. Varying parameters are the impact of thermal loading with maximum temperatures up to T_PDA = 700 °C and the platinum top layer thickness ranging from 24 nm to 105 nm. The titanium based adhesive film thickness is fixed to 10 nm. Up to post deposition annealing temperatures of T_PDA = 450 °C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. Modifications in the intrinsic film stress strongly influence the electrical material parameter in this temperature regime. At T_PDA > 600 °C, diffusion of titanium into the platinum top layer and its plastic deformation dominate the electrical behaviour, both causing an increase in film resistivity above average.     

Effect of the variation of temperature on the structural and optical properties of ZnO thin films prepared on Si (1 1 1) substrates using PLD

ZnO thin films were prepared on Si (1 1 1) substrates at various temperatures from 250 to 700 °C using pulsed laser deposition (PLD) technique in order to investigate the structural and optical properties of the films. The structural and morphological properties of the films were investigated by XRD and SEM measurements, respectively. The quality of the films was improved with the increase of the temperature. By XRD patterns, the FWHMs of the (0 0 2) peaks of the ZnO films became narrower when the temperatures were above 500 °C. The FWHMs of the peaks of (0 0 2) of the films were as narrow as about 0.19° when films were grown at 650 and 700 °C. This indicates the superior crystallinity of the films. The optical properties of the films were studied by photoluminescence spectra using a 325 nm He-Cd laser. The two strongest UV peaks were found at 377.9 nm from ZnO films grown at 650 and 700 °C. This result is consistent with that of the XRD investigation. Broad bands in visible region from 450 to 550 nm were also observed. Our works suggest that UV emissions have close relations with not only the crystallinity but also the stoichiometry of the ZnO films.     

Investigations on the structure, composition and performance of nanocrystalline thin film thermocouples deposited using anodic vacuum arc

This paper deals with the performance study of nanocrystalline thin film thermocouples (TFTCs) fabricated using anodic vacuum arc plasma aided deposition technique. Various single junction single elemental metal-metal pairs, elemental metal-metal alloy pairs, and metal alloy-metal alloy pairs were developed on glass substrates Elemental metal films were annealed at 10^− 4 Pa for 4 h while metal alloy films were annealed for 5 h. Their thermoelectric response has been studied in ambient air up to a maximum temperature difference of 300 °C between hot junction and cold junction. The phase purity, microstructure and composition of individual layer films were extensively studied. Elemental metal pairs agree well with their wire thermocouple equivalents. Thermoelectric power (TEP) of Cu-Ni and Fe-Ni TFTCs were found to be 17.81 μV/°C and 27.94 μV/°C at 300 °C, respectively. Among metal alloy-metal alloy TFTCs, a TEP of 32.87 μV/°C at 300 °C was obtained for Chromel-Alumel TFTCs which agree fairly well with its wire counterpart. However, Constantan based TFTCs deviated considerably from their wire counterparts. Cu-Constantan, Fe-Constantan and Chromel-Constantan showed a TEP of 26.48 μV/°C, 35.76 μV/°C and 37.41 μV/°C at 300 °C respectively. This deviation in thermoelectric power of Constantan based TFTCs with their wire counterparts were due to the fractionation of the Constantan arm. This fractionation leads to decrease of Ni content in the film which in turn reduces their TEP.     

Effects of preheating temperature on cold cracks,microstructures and properties of high power laser hybrid welded 10Ni3CrMoV steel

Laser hybrid welding has become one of the most promising welding methods for high strength low alloy steels due to combining the advantage of the laser and arc. A novel Y-groove cold cracking test adapted to laser hybrid welding is designed to assess the weldability of 10Ni3CrMoV steels at room temperature and different preheating temperatures. The experimental results show that the orientation of the predominant root cracks generally follows the contour of the fusion line. As the temperature increases from 25 °C to 150 °C, at first the root crack rate decreases and then slightly increases at 150 °C. The root crack rate obtained at 120 °C is the lowest. The fracture model changes from a brittle cleavage fracture to a mixture fracture with quasi-cleavage facets and dimples. The thermal cycle curves of laser hybrid welding obtained by temperature measurement systems are used to evaluate the crack resistance and microstructure transformation. The microstructures of welded joints obtained at different temperatures are analyzed by optical microscope (OM). The results reveal that the microstructures of the coarse grained region and the fusion zone at 120 °C have higher cold crack resistance and good impact toughness. Mechanical properties of the welded joint obtained at 120 °C and 150 °C are comprehensively evaluated by microhardness test, uniaxial tensile test and charpy V-notch impact test with side notches. Fractographs of the impact specimens are studied by scanning electron microscopy (SEM). The test results show that the welded joints obtained at 120 °C have satisfactory mechanical properties that can meet the technical requirements for shipbuilding industry.     

Greatly enhanced infrared normal spectral emissivity of microstructured silicon using a femtosecond laser

The infrared normal spectral emissivity of microstructured silicon prepared by femtosecond laser irradiation in SF₆ was measured for the wavelength range 2.5 μm to 25 μm. Greatly enhanced emissivity compared to that of flat silicon was observed over the entire wavelength range. For a sample with 13-14 μm high spikes, the emissivity at a temperature of 100 °C is approximately 0.96. The emissivity decreases slightly in the wavelength region above 8 μm, but remains higher than 0.9 over most of the measured wavelength range. Also the average emissivity is less than Nextel- Velvet-811-21 Coating, it can be used stably at more wide temperatures from 100 °C to 400 °C. These results show the potential for microstructured silicon to be used as a flat blackbody source or silicon-based pyroelectric and microbolometer devices.     

Comparative study of resistivity characteristics between transparent conducting AZO and GZO thin films for use at high temperatures

This paper compares in detail the resistivity behavior of transparent conducting Al-doped and Ga-doped ZnO (AZO and GZO) thin films for use in an air environment at high temperatures. AZO and GZO thin films with thicknesses in the range from approximately 30 to 100 nm were prepared on glass substrates at a temperature of 200 °C by rf superimposed dc or conventional dc magnetron sputtering deposition, pulsed laser deposition or vacuum arc plasma evaporation techniques. In heat-resistance tests, the resistivity was measured both before and after heat tests for 30 min in air at a temperature up to 400 °C. The resistivity stability of AZO thin films was found to be always lower than that of GZO thin films prepared with the same thickness under the same deposition conditions, regardless of the deposition technique. However, the resistivity of all AZO and GZO thin films prepared with a thickness above approximately 100 nm was stable when heat tested at a temperature up to approximately 250 °C. It was found that the resistivity stability in both GZO and AZO thin films is dominated by different mechanisms determined by whether the thickness is below or above approximately 50 nm. With thicknesses above approximately 100 nm, the increase in resistivity found in GZO and AZO films after heat testing at a temperature up to 400 °C exhibited different characteristics that resulted from a variation in the behavior of Hall mobility.     

Preparation and characterization of ceramic thin film thermocouples

Indium tin oxide (ITO), alumina doped zinc oxide (ZnO) and NiCrCoAlY/alumina nanocomposites were systematically investigated as thermoelements. These ceramic thermoelements were initially tested relative to a platinum reference electrode and the resulting thermoelectric properties were evaluated. Bi-ceramic junctions comprised of the most stable and responsive ceramic thermoelements, i.e. those thermoelements with the largest and most stable Seebeck coefficients relative to platinum, were fabricated and tested. A bi-ceramic junction based on nitrogen-doped ITO:oxygen-doped ITO exhibited excellent high temperature stability and reproducibility, however, this thermocouple pair had a relatively low Seebeck coefficient (6 μV/°C). Alumina doped ZnO:ITO thermocouples generated a very large electromotive force at low temperatures but lacked high temperature stability. When nitrogen-doped ITO was combined with a NiCoCrAlY/alumina nanocomposite, a very large and stable Seebeck coefficient (375 μV/°C) was realized. Ceramic thermocouples based on several candidate materials were demonstrated at temperatures up to 1200 °C and the potential of using these materials in other thermoelectric devices including those for energy harvesting is discussed.     

Structural and morphological transformations of TiO2 nanotube arrays induced by excimer laser treatment

The structural and morphological transformations of TiO₂ nanotube arrays (TNAs) treated by excimer laser annealing (ELA) were investigated as a function of the laser fluence using parallel and tilted modes. Results showed that the crystallinity of the ELA-treated TNAs reached only about 50% relative to that of TNAs treated by furnace anneal at 400 °C for 1 h. The phase transformation starts from the top surface of the TNAs with surface damage resulting from short penetration depth and limited one-dimensional heat transport from the surface to the bottom under extremely short pulse duration (25 ns) of the excimer laser. When a tilted mode was used, the crystallinity of TNAs treated by ELA at 85° was increased to 90% relative to that by the furnace anneal. This can be attributed to the increased area of the laser energy interaction zone and better heat conduction to both ends of the TNAs.     

Some properties of boronized layers on steels with direct diode laser

Boronized layer on steel is known to be formed by thermal diffusion of boron into the surface of steel improving corrosion-erosion resistant properties. Boronizing is carried out at temperatures ranging from 800 °C to 1050 °C and takes from one to several hours. There is one problem in this process, however, that the structure and properties of the base material are influenced considerably by the high temperature and long time of treatment. In order to avoid the aforementioned drawbacks of pack boronizing and laser-assisted boronizing, a better way is to activate the pack boronizing media and the workpiece with a high density power. The laser boronizing processes do not change the properties of the base material. In this study, the effect of laser characteristics was examined on the laser boronizing of carbon steel. After laser boronizing, the microstructure of the boride layer was analysed with an optical microscope and X-ray diffractometer (XRD). The mechanical properties of borided layer are evaluated using Vickers hardness tester and sand erosion tester. Results showed that the boride layer was composed of FeB and Fe₂B with thickness ranging 200-300 μm. The laser boronizing process did not change the properties of the base material.     

Hot corrosion behavior of powder metallurgy Rene95 nickel-based superalloy in molten NaCl–Na2SO4 salts

Hot corrosion behavior of powder metallurgy (PM) Rene95 Ni-based superalloy in molten 25%NaCl + 75%Na₂SO₄ salts at 650 °C, 700 °C and 750 °C are investigated by weight loss measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Experimental results show that hot corrosion kinetics follow a square power law at 650 °C and linear power laws at 700 °C and 750 °C. The corrosion layers on the surface of PM Rene95 superalloy are detected to be mainly composed of Cr₂O₃, NiO, and Ni₃S₂ at each temperature. Besides, small amounts of NiCr₂O₄ at 700 °C and NaCl at 750 °C are observed respectively. Cross-sectional morphologies and corresponding elemental maps indicate that corrosion layers near scale/alloy interface are composed of oxides at 650 °C while duplex oxides and sulfides at 700 °C and 750 °C. According to these results, a cooperating mechanism of oxidation and sulfuration for hot corrosion of PM Rene95 Ni-based superalloy is confirmed.     

The fabrication of ZnO/MgO multilayer films on Si (1 1 1) by PLD

ZnO/MgO multilayer thin films were fabricated on Si (1 1 1) substrates by pulsed laser deposition (PLD) at 600 °C for 30 min. The oxygen pressure and laser repetition were kept at 20 Pa and 5 Hz, respectively. The PL measurements suggest that the UV peaks have a blue excursion of 4 nm from 379 to 375 nm, compared with ZnO films. The XRD analysis indicates that the position of ZnO (0 0 2) peaks from ZnO/MgO multilayer films have about 0.12° shift from 34.421°, that of ZnO films, to 34.545°. From TEM images, the thickness of the films is about 200 nm. By HRTEM and SAD images, the crystal phases and the polycrystalline state were observed in the multilayer films.     

The carbonization of thin polyaniline films

Polyaniline films on silicon and ceramic supports were prepared in situ during the oxidative polymerization of aniline. The films were heated up to 500 °C in an inert nitrogen atmosphere. The changes in molecular structure during the carbonization have been studied by infrared spectroscopy and Raman scattering using 514, 633 and 785 nm laser excitation lines. The transformation from polyaniline salt to the base form has been detected above 100 °C. The conversion to nitrogen-containing carbon-like material followed above 200 °C. The molecular structure of the films produced during heating to 500 °C contains crosslinked phenazine-like and oxidized quinonoid units. The aniline oligomers deposited on the support in the early stages of aniline oxidation are stable during heating as it has been observed by resonance Raman scattering using 785 nm laser excitation line. The water contact angles changed after carbonization, and the films became more hydrophilic as carbonization progressed.     

Properties of pulsed laser deposited fluorinated hydroxyapatite films on titanium

Fluorinated hydroxyapatite coated titanium was investigated for application as implant coating for bone substitute materials in orthopaedics and dentistry. Pulsed laser deposition technique was used for films preparation. Fluorinated hydroxyapatite target composition, Ca₁₀(PO₄)₆F_1.37(OH)_0.63, was maintained at 2 J/cm² of laser fluence and 500-600 °C of the substrate temperature. Prepared films had a compact microstructure, composed of spherical micrometric-size aggregates. The average surface roughness resulted to be of 3 nm for the film grown at 500 °C and of 10 nm for that grown at 600 °C, showing that the temperature increase did not favour the growth of a more fine granulated surface. The films were polycrystalline with no preferential growth orientation. The films grown at 500-600 °C were about 8 μm thick and possessed a hardness of 12-13 GPa. Lower or higher substrate temperature provides the possibility to obtain coatings with different fine texture and roughness, thus tayloring them for various applications.     

Resorption system with simultaneous heat and cold production

A resorption system with simultaneous cold and heat production was studied. The heat produced could be used for sanitary or process purposes, or to drive another heat-powered machine. The resorption reactors had MnCl₂ and NH₄Cl as reactant (which are impregnated in expanded graphite) and NH₃ as refrigerant. The combined coefficient of performance and amplification (COPA) of this system reached 1.3 when the cooling effect was produced at 0 °C and heating effect at 75 °C with the regeneration temperature of 140 °C. Its COP was 0.35 with a specific cooling power (SCP) of 1.12 MJ kg⁻¹ day⁻¹, and the heat sink in this case remained below the cooling temperature for more than 5 h. Because of the heat production at certain temperature level (from 70 °C to 80 °C) in this study, the released heat could be used to power a silica gel-water adsorption chiller and the overall COP of the combined system would increase dramatically.     

Femtosecond laser deposited zinc oxide film and its optical properties

Zinc oxide (ZnO) thin films have been grown on Si (100) substrates using a femto-second pulsed laser deposition (fsPLD) technique. The effects of substrate temperature and laser energy on the structural, surface morphological and optical properties of the films are discussed. The X-ray diffraction results show that the films are highly c-axis oriented when grown at 80 °C and (103)-oriented at 500 °C. In the laser energy range of 1.0 mJ-2.0 mJ, the c-axis orientation increases and the mean grain size decreases for the films deposited at 80 °C. The field emission scanning electron microscopy indicates that the films have a typical hexagonal structure. The optical transmissivity results show that the transmittance increases with the increasing substrate temperature. In addition, the photoluminescence spectra excited with 325 nm light at room temperature are studied. The structural properties of ZnO films grown using nanosecond (KrF) laser are also discussed.     

Mechanical properties of additive manufactured titanium (Ti–6Al–4V) blocks deposited by a solid-state laser and wire

In this paper, the mechanical properties and chemical composition of additive manufactured Ti–6Al–4V blocks are investigated and compared to plate material and aerospace specifications. Blocks (seven beads wide, seven layers high, 165 mm long) were deposited using a 3.5 kW Nd:YAG laser and Ti–6Al–4V wire. Two different sets of process parameters are used and three different conditions (as-built, 600 °C/4 h, 1200 °C/2 h) of the deposit are investigated. The particular impurity levels of the blocks are considerably below those tolerated according to aerospace material specifications (AMS 4911L). Static tensile samples are extracted from the blocks in the deposition direction and punch samples are extracted in the building direction. The experiments show that as-deposited Ti–6Al–4V can achieve strength and ductility properties that fulfill aerospace specifications of the wrought Ti–6Al–4V material (AMS 4928). The 600 °C/4 h heat treatment leads to a significantly higher strength in the deposition direction, but can also decrease ductility. The 1200 °C/2 h treatment tends to decrease the alloy’s strength.     

Effects of Ag doping on the crystallization properties of Sb-rich GeSb thin films

Ag-doped and un-doped Sb-rich GeSb thin films were deposited by DC magnetron co-sputtering. The electrical, structural, and optical properties of the thin films phase change were investigated using 4-point probe measurement, X-ray diffraction (XRD), transmission electron microscopy (TEM), and a static tester. With increasing Ag doping content, the crystallization temperature and sheet resistance of crystalline state decreased from 325 °C to 283 °C and from 187.33 Ω/□ to 114.62 Ω/□, respectively. XRD patterns of the films showed a Sb hexagonal structure, and the calculated grain size increased from 13.9 nm to 17 nm as the Ag concentration increased. Grain sizes of the Ag-doped thin films were larger than the grain sizes of un-doped thin films, as determined by TEM images. A static tester verified the decreased crystallization speed and optical contrast. Un-doped GeSb crystallization took 160 ns and 16 at.% Ag-doped GeSb crystallization took 200 ns when the laser power was 13 mW. Based on a power-time-effect diagram, the 12.6 at.% Ag-doped GeSb showed good thermal stability in a crystalline state.     

Preparation and characterization of pulsed laser deposited CdTe thin films at higher FTO substrate temperature and in Ar + O2 atmosphere

Pulsed laser deposition (PLD) is one of the promising techniques for depositing cadmium telluride (CdTe) thin films. It has been reported that PLD CdTe thin films were almost deposited at the lower substrate temperatures (<300 °C) under vacuum conditions. However, the poor crystallinity of CdTe films prepared in this way renders them not conducive to the preparation of high-efficiency CdTe solar cells. To obtain high-efficiency solar cell devices, better crystallinity and more suitable grain size are needed, which requires the CdTe layer to be deposited by PLD at high substrate temperatures (>400 °C). In this paper, CdTe layers were deposited by PLD (KrF, λ = 248 nm, 10 Hz) at different higher substrate temperatures (T_s). Excellent performance of CdTe films was achieved at higher substrate temperatures (400 °C, 550 °C) under an atmosphere of Ar mixed with O₂ (1.2 Torr). X-ray diffraction analysis confirmed the formation of CdTe cubic phase with a strong (1 0 0) preferential orientation at all substrates temperatures on 60 mJ laser energy. The optical properties of CdTe were investigated, and the band gaps of CdTe films were 1.51 eV and 1.49 eV at substrate temperatures of 400 °C and 550 °C, respectively. Scanning electron microscopy (SEM) showed an average grain size of 0.3–0.6 μm. Thus, under these conditions of the atmosphere of Ar + O₂ (15 Torr) and at the relatively high T_s (500 °C), an thin-film (FTO/PLD-CdS (100 nm)/PLD-CdTe (~1.5 μm)/HgTe: Cu/Ag) solar cell with an efficiency of 6.68% was fabricated.