Enhanced ablation efficiency for silicon by femtosecond laser microprocessing with GHz bursts in MHz bursts (BiBurst)

Ultrashort laser pulses confine material processing to the laser-irradiated area by suppressing heat diffusion, resulting in precise ablation in diverse materials. However, challenges occur when high speed material removal and higher ablation efficiencies are required. Ultrafast burst mode laser ablation has been proposed as a successful method to overcome these limitations. Following this approach, we studied the influence of combining GHz bursts in MHz bursts, known as BiBurst mode, on ablation efficiency of silicon. BiBurst mode used in this study consists of multiple bursts happening at a repetition rate of 64 MHz, each of which contains multiple pulses with a repetition rate of 5 GHz. The obtained results show differences between BiBurst mode and conventional single pulse mode laser ablation, with a remarkable increase in ablation efficiency for the BiBurst mode, which under optimal conditions can ablate a volume 4.5 times larger than the single pulse mode ablation when delivering the same total energy in the process.

Latest Advances in Common Signal Processing of Pulsed Thermography for Enhanced Detectability: A Review

Non-destructive testing (NDT) is a broad group of testing and analysis techniques used in science and industry to evaluate the properties of a material, structure, or system for characteristic defects and discontinuities without causing damage. Recently, infrared thermography is one of the most promising technologies as it can inspect a large area quickly using a non-contact and non-destructive method. Moreover, thermography testing has proved to be a valuable approach for non-destructive testing and evaluation of structural stability of materials. Pulsed thermography is one of the active thermography technologies that utilizes external energy heating. However, due to the non-uniform heating, lateral heat diffusion, environmental noise, and limited parameters of the thermal imaging system, there are some difficulties in detecting and characterizing defects. In order to improve this limitation, various signal processing techniques have been developed through many previous studies. This review presents the latest advances and exhaustive summary of representative signal processing techniques used in pulsed thermography according to physical principles and thermal excitation sources. First, the basic concept of infrared thermography non-destructive testing is introduced. Next, the principle of conventional pulsed thermography and signal processing technologies for non-destructive testing are reviewed. Then, we review advances and recent advances in each signal processing. Finally, the latest research trends are reviewed.

Effect of Turbulence On Forced Ignition of Jet-A/Air Mixtures

Consistent ignition of reactive mixtures in turbulent conditions continues to be a challenge, particularly for large, multi-component fuels. Prior work has shown that turbulence can affect ignition parameters such as flame speed, mixture temperature, and minimum ignition energy. However, these works have primarily considered small, single-component fuels. This work studies the effect of turbulence on forced ignition of jet-A/air mixtures with f between 0.3 and 0.7. The ignition probability of these mixtures was measured for bulk velocities between 5 and 7 m/s and turbulence intensities between 3% and 9%. A FLIR SC6700 infrared camera was used to measure the radiation intensity emitted by the flame kernels. Increases in turbulence intensity between 3% and 4% cause the probability of ignition to generally increase. This increase is attributed to the negative flame stretch that develops as a result of the turbulence. This observation is significant because it shows that turbulence can facilitate ignition for jet-A/air mixtures. In contrast, increasing turbulence beyond 5% causes ignition probabilities to decrease. This reduction occurs due to the increased role of heat diffusion and the associated reduction in kernel temperature. The sensitivities of ignition behavior to turbulence intensity and fuel chemistry are reasonably captured using the Peclet number. Further agreement in ignition behavior is achieved by considering Pe/TI2. Ignition probability data for two additional fuels were compared using Pe/TI2. Reasonable agreement within a 95% confidence interval was observed for CH4 mixtures but not for C3H8 mixtures.

Effects of Heat and Momentum Gain Differentiation during Gas Detonation Spraying of FeAl Powder Particles into the Water

In this paper, dynamic interactions between the FeAl particles and the gaseous detonation stream during supersonic D-gun spraying (DGS) conditions into the water are discussed in detail. Analytical and numerical models for the prediction of momentum and complex heat exchange, that includes radiative effects of heat transfer between the FeAl particle and the D-gun barrel wall and phase transformations due to melting and evaporation of the FeAl phase, are analyzed. Phase transformations identified during the DGS process impose the limit of FeAl grain size, which is required to maintain a solid state of aggregation during a collision with the substrate material. The identification of the characteristic time values for particle acceleration in the supersonic gas detonation flux, their convective heating and heat diffusion enable to assess the aggregation state of FeAl particles sprayed into water under certain DGS conditions.

Frictional heating in a thick fault zone with empirical slip-weakening friction: implications for slip parameter estimation from temperature observations in deep fault drilling

The strain energy released during an earthquake is consumed by processes related to seismic radiation or dissipation. Deep fault drilling and subsequent temperature measurements in a thick fault zone immediately after an event have provided important insights into this dissipation process. By employing an analytical solution to the heat conduction problem, which involves the sudden injection of an infinitesimally thin heat source into an infinite medium, previous drilling projects have estimated the strength of the heat source and the level of shear stress from observed temperature anomalies. However, it is unclear under what conditions this analytical source solution can be regarded as a good approximation for the thick fault problem, a situation which has led to uncertainty of the approximation error in these previous studies. In this study, I first derived an analytical solution for the thick fault problem that accounted for experimentally derived slip-weakening friction. I then validated the derived solution both analytically and numerically. Using the derived thick solution, I next demonstrated that the thick, planar, and source solutions can be considered equivalent under the typical conditions of the previous drilling projects. Therefore, the slip parameters estimated by using the source solution obtained by these studies are appropriate. These results suggest that coseismic information with spatio-temporal extent, such as shear stress and friction coefficient, are lost due to heat diffusion when the temperature observations are conducted; thus, they cannot be inferred directly from observed temperature anomalies. These results also suggest that for most drilling projects, including future ones, the observed temperature distribution can be well explained by using the source solution instead of the thick solution as long as coseismic slip is not markedly delocalized and the spatial extent of the temperature measurements is not significantly larger than the diffusion length.

Designing and Testing a Closed-loop Magnetically Actuated Laser Scanning System for Tissue Ablation

Biomedical robotic systems continue to hold unlimited potential for surgical procedures. Robotized laser endoscopic tools provide surgeons with increased accuracy in the laser ablation of tissue and tumors. The research here catalogs the design and implementation of a new laser endoscopic tool for tissue ablation. A novel feature of this new device is the inclusion of a feedback loop that measures the position of the laser beam via a photo-detector sensor. The scale of this new device was governed by the dimensions of the photo-detector sensor. The tip of the laser's fiber optic cable is controlled by the torque interaction between permanent magnet rings surrounding the fiber optic and the custom designed solenoid coils. Prior to building the physical test-bed the system was modeled and simulated using COMSOL software. In pre-clinical trials, the physical experimental results showed that the designed prototype laser scanner system accurately track different ablation patterns and gives a consistent output position for the laser beam however, the heat diffusion into the tissue around the desired line of the geometric shape would give wider ablation margins than was desirable.

AGENT-BASED FIRE-SPREADING MODEL IN A DENSE URBAN COMMUNITY

Metro Manila, home to twelve-million residents scattered in densely populated cities, grows its population at a rate of 1.21% annually. Areas of the metro occupied by residents falling under the poverty line have only been increasing in density per year, and have been prone to fire incidents. One such area, Barangay Addition Hills in Mandaluyong City, has fallen victim to two disastrous fires four years apart: in 2016 and 2020. This study aims to accurately model a portion of Barangay Addition Hills when a fire starts in one of the most densely populated blocks while observing firefighters responding to the incident. The agent-based model adapts features from (Wilensky, 2006)’s Fire model and is virtually simulated with the help of two-dimensional satellite images of the area. The fire-spreading algorithm incorporates solving the heat diffusion equation to determine ignition time of combustible materials per unit area. Firefighters have been incorporated into the model with the help of the Bureau of Fire Protection (BFP)’s Operational Procedures Manual to determine their expected behavior when responding to a fire alarm. Simulations were run on a per-incident basis to determine the total affected area, estimated affected families, and time for the fire to be put under control under varying densities, traffic conditions, firefighter response times and manpower.

The Marangoni Convection Effect on Melt Pool Formation during Selective Laser Melting Process

In order to predict the effect of the Marangoni convection and the morphology of melted stainless steel powder, during the selective laser melting (SLM) process, a transient three-dimensional numerical model is developed at the mesoscale. The evolution of the temperature and velocity fields’ is then studied. The initial powder bed distribution is obtained by the discrete element method (DEM) calculation, and the temperature distribution and the molten pool shape deformation are calculated and analyzed by the Ansys-Fluent commercial code. The molten pool shape is obtained by considering the influence of Marangoni convection on the internal flow behavior. The recoil force was not considered in our calculation. As main results, a slight deviation between the position of the maximum temperature of the molten pool and the center of the laser spot is observed. The direction of the heat diffusion is more likely to be horizontal and the flow centrifugal, which causes the melt track to be wide. Finally, the Marangoni convection is the main driver of the flow.