Hot-wire Surface Research Articles

A multi-sensor based online monitoring system for laser hot-wire surface cladding process

A multi-sensor online monitoring system, including a high-speed camera, a spectrometer, and an infrared camera, was developed to enhance the laser hot wire cladding process. This innovative process feeds a preheated wire, using a hot-wire power supply, into a shallow molten pool formed on the substrate surface by a rectangular diode laser beam. This study assesses the capability of the monitoring system to detect variations in cladding process conditions that significantly influence the geometry and quality of the clad layer. Specifically, the study involved four sets of laser hot-wire cladding experiments, altering substrate surface conditions, substrate thicknesses, laser powers, and scanning speeds. The real-time data from the three sensors were synchronized and analyzed in depth, focusing on the morphology and microhardness of the clad layers. A laser vision sensor was employed to examine the surface profile and roughness of the clads. The findings indicated that the crucial role of surface preparation in influencing the dynamics of the molten pool, subsequently affecting the geometry, dilution depth, and microhardness of the clads. The sensors demonstrated increased sensitivity to changes in surface conditions compared to variations in other conditions like substrate thickness, laser power, and scanning speed. A significant correlation was found between the dilution depth, clad height, surface roughness, and microhardness of the clad layer. These correlations are in response to adjustments in laser power, scanning speed, and surface preparation and can be reliably and rapidly detected by the multi-sensor system.

A kinetic study of the gas-phase reactions of 1-methylsilacyclobutane in hot wire chemical vapor deposition.

The reaction kinetics of the decomposition of 1-methylsilacyclobutane (MSCB) in a hot wire chemical vapor deposition (HWCVD) reactor was investigated. The stable reaction products were monitored using vacuum ultraviolet laser single photon ionization in tandem with time-of-flight mass spectrometry. Steady-state approximation was used to determine the rate constants of three individual decomposition pathways of MSCB, i.e., cycloreversion to form ethene and methylsilene (R1), ring opening to form propene and methylsilylene (R2), and exocyclic Si-CH3 bond cleavage to form ˙CH3 radicals (R3). The activation energies (Ea) for R2 and R3 in a HWCVD reactor were determined to be 86.6 kJ mol-1 and 106 kJ mol-1, respectively. The fact that these Ea values are close to those obtained for the MSCB decomposition on metal surfaces under collision-free conditions indicates that the heterogeneous reactions on the hot wire surface govern the gas-phase reaction kinetics in the HWCVD reactor. In addition, the Ea values obtained from a theoretical study of the decomposition kinetics using ab initio calculations at the CCSD(T)/6-311++G(3d,2p)//MP2/6-311++G(d,p) level were 62.9 kcal mol-1 (i.e., 263 kJ mol-1), 62.0 kcal mol-1 (i.e., 259 kJ mol-1), and 86.2 kcal mol-1 (i.e., 361 kJ mol-1) for R1, R2, and R3, respectively. The much lower experimental Ea values compared with those from the theoretical calculations clearly suggest that the tungsten filament in the HWCVD reactor catalyzed the decomposition.

Heat transfer augmentation in nanofluids via nanofins

Theoretical results derived in this article are combined with experimental data to conclude that, while there is no improvement in the effective thermal conductivity of nanofluids beyond the Maxwell's effective medium theory (J.C. Maxwell, Treatise on Electricity and Magnetism, 1891), there is substantial heat transfer augmentation via nanofins. The latter are formed as attachments on the hot wire surface by yet an unknown mechanism, which could be related to electrophoresis, but there is no conclusive evidence yet to prove this proposed mechanism.

Improvement of indium–tin oxide films on polyethylene terephthalate substrates using hot-wire surface treatment

Indium–tin oxide (ITO) thin films have been studied extensively in flat panel displays because they combine unique transparent and conducting properties. A continuous roll-to-roll sputter system was used to deposit ITO/SiO 2 thin films on polyethylene terephthalate (PET) substrates with no intentional heating. Typical transmittance and resistivity of the ITO/SiO 2/PET sample were 78% (400∼700 nm) and 7.5 × 10 − 3 Ω cm, respectively. Changes in the characteristics of ITO/SiO 2/PET after hot-wire surface treatment in oxygen were examined in terms of resistivity, transmittance, surface roughness and bond configuration. The effects of process parameters like iridium wire temperature, chamber pressure and process duration on the properties of the ITO films are studied in details and compared with the works by plasma or furnace annealing. Under optimum conditions, the resistivity of the ITO/SiO 2/PET sample can achieve a minimum value of 1 × 10 − 3 Ω cm with a transmittance of 82%. These results indicate that the hot-wire system can be utilized to realize better quality film and to overcome the technological limits imposed by the role-to-role sputtering conditions.

Growth of c-GaN films on GaAs(100) using hot-wire CVD

Cubic gallium nitride (GaN) films were grown on nitrided layers of GaAs(100) by hot-wire chemical vapor deposition. The nitrided layer was also formed by NH x radicals generated on a tungsten hot-wire surface. Nitridation conditions for the growth of GaN with a cubic-type structure were investigated. As a result, GaN film with a preponderant cubic phase was grown on the GaAs surface layer nitrided at a substrate temperature of 550 °C, a filament temperature of 1200 °C and an ammonia (NH 3) pressure of 1 Torr.

O método de fio quente: técnica em paralelo e técnica de superfície

A condutividade térmica é uma das propriedades físicas mais importantes de um material. A sua determinação experimental apresenta algumas dificuldades e requer alta precisão na determinação dos fatores necessários para o seu cálculo. O método de medida de condutividade térmica para materiais refratários mais usado hoje em todo o mundo, tanto nos centros de pesquisa, quanto nos laboratórios industriais, é o Método do Fio Quente, onde a condutividade térmica é calculada a partir do transiente de temperatura gerado por uma fonte de calor ideal, infinitamente longa e fina num meio material infinito. Neste trabalho são comparados os resultados obtidos utilizando-se duas técnicas diferentes para a detecção do transiente térmico: numa delas, a temperatura é medida na superfície do fio quente, (técnica de fio quente de superfície), e na outra, a temperatura é medida a uma certa distância do fio quente (técnica de fio quente paralelo). Os resultados experimentais mostram uma grande vantagem da técnica de fio quente de superfície em relação a técnica de fio quente paralelo para materiais com condutividade térmica superior a 10 W/mK. O intervalo de tempo considerado no cálculo é bem maior que aquele que seria utilizado na técnica de fio quente paralelo, obtendo-se assim resultados mais precisos e confiáveis.

Accurate measurements of thermal conductivity and thermal diffusivity of molten carbonates

Thermal conductivity and thermal diffusivity of molten carbonates (Li 2 CO 3 /K 2 CO 3 , Li 2 CO 3 /Na 2 CO 3 , and Li 2 CO 3 /Na 2 CO 3 /K 2 CO 3 ) were measured by the transient short-hot-wire method in the temperature range from 530 to 670 °C. Two types of probes were examined. One was a platinum short-hot-wire probe coated with a thin film of alumina (Al 2 O 3 ) to prevent current leakage and corrosion. The other was a bare gold short-hot-wire probe. In the case of the platinum probe, the quality of the coating deteriorates gradually during the measurements on molten carbonates because of their high corrosiveness. The deterioration of the quality of coating leads to relatively large errors in the measured thermal conductivity and thermal diffusivity, of around 9% and 50%, respectively. In the case of the bare gold probe, corrosion due to electrochemical reaction can be neglected, and the effect of current leakage into the molten carbonates can be estimated. This allows the temperature dependence of electric resistivity to be calibrated accurately and compared with existing data. By using such a gold probe without coating on the hot-wire surface, the thermal conductivity and thermal diffusivity of molten carbonates can be measured within errors of 3% and 12%, respectively. The gold probe can be used repeatedly without any reduction of quality.

Cross-Flow Drying and Curing of Wet-Laid Glass Fiber Veils

Glass fiber veil is produced with a wet forming process, after which the veil is dried in a wire-type cross-flow dryer. The wet nonwoven web structure of glass fibers and binder undergoes the drying process under specific conditions as air velocity, air temperature and air humidity, each of which can be controlled separately. Drying is an important process step, which highly defines the end product properties of the glass fiber veil. A simulation model is presented that give the opportunity for a better understanding of the cross-flow drying process in relation to product and process properties, and also a better controllability and a better energy management of the drying process. Thermocouples placed beneath the supporting transport wire and air humidity sensors in the dryer sections are the key for evaluation of important non-measurable drying air and glass web properties. The model has been validated using correction factors for heat and mass transfer and assuming leaking air flows between the dryer sections. The impact of the underlying work goes further in the direction of understanding some related product and process problems: (1) There is an explicit binder migration process from the wet glass web to the wet upper surface of the supporting transport wire, while flash-drying of the wet web proceeds on the dry and hot wire surface. (2) The drying process of wet glass fibers and wet binder gels proceeds at a specific temperature under which a solid binder film forming process occurs. Control of the temperature and time for solid film forming in the whole fiber web structure can be effectuated in this way. The relevance of the simulation model may be proved by continuous inline measurements at the production line.

Characteristics of the hot-wire CVD reactor on a-Si:H deposition

We prepared a-Si:H films by hot-wire assisted chemical vapor deposition (hot-wire CVD). Considering dissociation rates on the hot-wire surface, with increasing wire temperature, the deposition rates shifted from surface-dissociation-rate limited to diffusion-limited at the hot-wire surface. At temperatures of 2000°C, the measured deposition rates increased with increasing total gas flow rate. Our results for the measured deposition rates agreed with the calculated deposition rates predicted by a model that assumes SiH 4 is uniformly distributed in the reactor and that the SiH 4 dissociation rate is limited by the rate of SiH 4 diffusion to the hot-wire. Furthermore, in the diffusion-controlled regime, the deposition rate can be controlled by the surface area of the hot-wire and by the SiH 4 flow rate at high SiH 4 depletion.

An experimental and numerical investigation on the hot surface ignition of premixed gases under microgravity conditions

There have been many studies of hot surface ignition of premixed gases. However, using the experimentalresults obtained up to this time, it is difficult to understand the basic mechanism of ignition by heated surfaces because these experiments were made under the normal gravity condition, suffering from the effect of natural convection. In the present study, experiments were made on the ignition of mixtures by heated nickel, tungsten,and platinum wires under normal gravity and microgravity conditions. With a methane-oxygen mixture ignited by a heated tungsten wire, a strong gravity effect on the ignition delay is observed. On the contrary, there is almost no gravity effect observed with a methane-air mixture. Results of calculations show that ignition experiments under the microgravity condition are simulated successfully by this numerical model. It is shown from the numerical results that the ignition point in the methane-oxygen case is apart from the hot-wire surface, while, in the methane-air case, this point exists near the hot-wire surface. Therefore, natural convection has little effect on the ignition delay of a methane-air mixture. Experimental results of hydrogen-air mixtures ignited by heated platinum wires show that chemicalspecies are supplied to the platinum wire surface by natural convection, and therefore, the surface reaction is promoted under the normal gravity condition. Experimental and numerical results show that the surface reaction is the most energetic when the equivalence ratio of the mixture is 0.3, which conflicts with the result obtained by Coward and Guest that the rate of catalytic reaction attains a maximum at stoichiometric composition. This contradiction may be due partially to the gravity effect in their experiments.

Alkali cyanides in the iron blast-furnace

There have been many studies of hot surface ignition of premixed gases. However, using the experimentalresults obtained up to this time, it is difficult to understand the basic mechanism of ignition by heated surfaces because these experiments were made under the normal gravity condition, suffering from the effect of natural convection.In the present study, experiments were made on the ignition of mixtures by heated nickel, tungsten,and platinum wires under normal gravity and microgravity conditions. With a methane-oxygen mixture ignited by a heated tungsten wire, a strong gravity effect on the ignition delay is observed. On the contrary, there is almost no gravity effect observed with a methane-air mixture. Results of calculations show that ignition experiments under the microgravity condition are simulated successfully by this numerical model. It is shown from the numerical results that the ignition point in the methane-oxygen case is apart from the hot-wire surface, while, in the methane-air case, this point exists near the hot-wire surface. Therefore, natural convection has little effect on the ignition delay of a methane-air mixture.Experimental results of hydrogen-air mixtures ignited by heated platinum wires show that chemicalspecies are supplied to the platinum wire surface by natural convection, and therefore, the surface reaction is promoted under the normal gravity condition. Experimental and numerical results show that the surface reaction is the most energetic when the equivalence ratio of the mixture is 0.3, which conflicts with the result obtained by Coward and Guest that the rate of catalytic reaction attains a maximum at stoichiometric composition. This contradiction may be due partially to the gravity effect in their experiments.