Teresa Sibillano

Analytical characterization of laser-generated copper nanoparticles for antibacterial composite food packaging

AbstractA new type of nanomaterial has been developed as antibacterial additive for food packaging applications. This nanocomposite is composed of copper nanoparticles embedded in polylactic acid, combining the antibacterial properties of copper nanoparticles with the biodegradability of the polymer matrix. Metal nanoparticles have been synthesised by means of laser ablation, a rising and easy route to prepare nanostructures without any capping agent in a liquid environment. As prepared, nanoparticle suspensions have been easily mixed to a polymer solution. The resulting hybrid solutions have been deposited by drop casting, thus obtaining self-standing antibacterial packages. All samples have been characterized by UV–Vis spectroscopy, X-ray photoelectron spectroscopy and electro-thermal atomic absorption spectroscopy. Ion release data have been matched with bioactivity tests performed by Japanese Industrial Standard (JIS) method (JIS Z 2801:2000) against Pseudomonas spp., a very common Gram-negative microbial group able to proliferate in processed food. Online abstract figureAnalytical characterization of copper nanoparticles: an XPS spectrum and a TEM image

Role of heat accumulation on the incubation effect in multi-shot laser ablation of stainless steel at high repetition rates

We study the incubation effect during laser ablation of stainless steel with ultrashort pulses to boost the material removal efficiency at high repetition rates. The multi-shot ablation threshold fluence has been estimated for two pulse durations, 650-fs and 10-ps, in a range of repetition rates from 50 kHz to 1 MHz. Our results show that the threshold fluence decreases with the number of laser pulses N due to damage accumulation mechanisms, as expected. Moreover, approaching the MHz regime, the onset of heat accumulation enhances the incubation effect, which is in turn lower for shorter pulses at repetition rates below 600 kHz. A saturation of the threshold fluence value is shown to occur for a significantly high number of pulses, and well fitted by a modified incubation model.

High-resolution monitoring of the hole depth during ultrafast laser ablation drilling by diode laser self-mixing interferometry

We demonstrate that diode laser self-mixing interferometry can be exploited to instantaneously measure the ablation front displacement and the laser ablation rate during ultrafast microdrilling of metals. The proof of concept was obtained using a 50-μm-thick stainless steel plate as the target, a 120 ps/110 kHz microchip fiber laser as the machining source, and an 823 nm diode laser with an integrated photodiode as the probe. The time dependence of the hole penetration depth was measured with a 0.41 µm resolution.

Simultaneous measurement of multiple target displacements by self-mixing interferometry in a single laser diode

We demonstrate that a single all-optical sensor based on laser diode self-mixing interferometry can monitor the independent displacement of individual portions of a surface. The experimental evidence was achieved using a metallic sample in a translatory motion while partly ablated by a ps-pulsed fiber laser. A model based on the Lang-Kobayashi approach gives an excellent explanation of the experimental results.

Plasma plume oscillations monitoring during laser welding of stainless steel by discrete wavelet transform application.

The plasma optical radiation emitted during CO2 laser welding of stainless steel samples has been detected with a Si-PIN photodiode and analyzed under different process conditions. The discrete wavelet transform (DWT) has been used to decompose the optical signal into various discrete series of sequences over different frequency bands. The results show that changes of the process settings may yield different signal features in the range of frequencies between 200 Hz and 30 kHz. Potential applications of this method to monitor in real time the laser welding processes are also discussed.

Closed Loop Control of Penetration Depth during CO2 Laser Lap Welding Processes

In this paper we describe a novel spectroscopic closed loop control system capable of stabilizing the penetration depth during laser welding processes by controlling the laser power. Our novel approach is to analyze the optical emission from the laser generated plasma plume above the keyhole, to calculate its electron temperature as a process-monitoring signal. Laser power has been controlled by using a quantitative relationship between the penetration depth and the plasma electron temperature. The sensor is able to correlate in real time the difference between the measured electron temperature and its reference value for the requested penetration depth. Accordingly the closed loop system adjusts the power, thus maintaining the penetration depth.

Real time ablation rate measurement during high aspect-ratio hole drilling with a 120-ps fiber laser

We report on the instantaneous detection of the ablation rate as a function of depth during ultrafast microdrilling of metal targets. The displacement of the ablation front has been measured with a sub-wavelength resolution using an all-optical sensor based on the laser diode self-mixing interferometry. The time dependence of the laser ablation process within the depth of aluminum and stainless steel targets has been investigated to study the evolution of the material removal rate in high aspect-ratio micromachined holes.

An analysis of the shielding gas flow from a coaxial conical nozzle during high power CO2 laser welding

An experimental and theoretical study on the role of the nitrogen gas stream, exiting from a conventional conical nozzle tip during a laser welding process, has been carried out. A mathematical model has been used, based on the Navier–Stokes equations which express fundamental conservation laws of mass, momentum and energy for a compressible fluid. Numerical simulations of the gas stream colliding onto a plane surface have been performed showing the effects of variations of inlet gas pressure, nozzle exit diameter and standoff distance on the density and Mach number contours, axis pressure of the gas jet and plate pressure produced on the workpiece surface. Laser welding experiments have been performed on carbon and stainless steel specimens, by varying the process parameters in the same range as in the simulations and keeping constant the incident power and the travel speed. Two different gas stream regimes were found, namely sonic and subsonic, which were experimentally verified to produce cutting and welding conditions, respectively.Weld performances have been evaluated in terms of bead width, penetration depth and melted area. Nozzle standoff distance was found to have a negligible influence, while the exit diameter and the flow rate significantly affect the weld results. The numerical predictions allowed an explanation of the experimental results yielding useful suggestions for enhancing the weld quality, acting simply on the shielding gas parameters.

Spectroscopic closed loop control of penetration depth in laser beam welding process

In-process monitoring and feedback control are fundamental actions for stable and good quality laser welding process. In particular, penetration depth is one of the most critical features to be monitored. In this research, overlap welding of stainless steel is investigated to stably reproduce a fixed penetration depth using both CO2 and Nd:YAG lasers. Plasma electron temperatures of Fe(I) and Cr(I) are evaluated as in process monitoring using the measurement of intensities of emission lines with fast spectrometers. The sensor system is calibrated using a quantitative relationship between electron temperature and penetration depth in different welding conditions. Finally closed loop control of the weld penetration depth is implemented by acquiring the electron temperature value and by adjusting the laser power to maintain a pre-set penetration depth. A PI controller is successfully used to stabilize the electron temperature around the set point corresponding to the right penetration depth starting from a wrong value of any initial laser power different than the set point. Optical inspection of the weld surface and macroscopic analyses of cross sections verify the results obtained with the proposed closed-loop system based on a spectroscopic controller and confirms the reliability of our system.

Real-time in-situ measurement of the penetration depth in short pulse laser percussion drilling of metal targets

High-energy ultra-short pulse laser ablation is a fast-growing technology in precision laser micromachining of transparent as well as opaque materials. Accurate in-situ measurements of physical parameters such as the penetration depth and the removal rate are crucial to fully characterize the ultrafast laser-material interactions [1–5]. Nonetheless, the laser drilling is still lacking of a real-time technique able to monitor and control the spatial- and time-dependent evolution of the hole-depth in metallic plates.