High-speed Pulsed Laser Research Articles

High-speed ESPI and related techniques: overview and its application in the automotive industry

This paper provides an overview of recent developments and applications of high-speed electronic speckle pattern interferometry (ESPI) and related techniques ranging from high-speed pulsed laser techniques to high-speed camera methods. Different methods for extracting the phase distribution from high-speed ESPI fringe patterns are compared. Unique applications in the automotive industry using these techniques are provided. Finally, the outlook for high-speed ESPI and high-speed digital holographic interferometry are discussed.

CUBIC STRUCTURE AND CATION DISORDERING IN YBCO THIN FILM DEPOSITED BY HIGH SPEED PULSED LASER DEPOSITION

We have investigated the crystalline structure of high rate deposited YBa 2 Cu 3 O x thin films prepared by high speed pulsed laser deposition. A cation disordered cubic structure with lattice parameter of 0.39 nm was found in YBCO thin film deposited at 12.2 nm/s deposition rate and 650°C substrate temperature conditions. The short range ordered cubic YBa 2 Cu 3 O x thin film growth at high deposition rate was explained by the short migration length of Y and Ba cation atoms owing to the high incident flux rate.

Experimental Study of the Meltblowing Process

High-speed digital imaging techniques and web measurements were used to investigate the meltblowing (MB) process. We evaluated fiber diameter, fiber orientation, fiber entanglement, fiber velocity and fiber acceleration between the die and collector. Three processing variables were studied: primary air pressure, die-to-collector distance and collector surface speed. Although results of this investigation are somewhat preliminary, they provide fundamental information about the MB process and increase our understanding of it. Introduction Meltblowing (MB) is a fast, chaotic and complicated process. These features make it difficult to study the MB process theoretically as well as experimentally and most researchers have simply studied the effects of resin and process variables on web structure or web properties. Some researchers, however, have reported on-line measurements during MB [1–9]. Bansal and Shambaugh measured fiber temperature during single-hole MB using an infrared camera [1]. Wu and Shambaugh measured fiber velocity using laser Doppler velocimetry during single-hole MB [2]. Shambaugh and others reported experimental measurements of fiber motion and fiber diameter using a single-hole die [1–7]. Multiple-exposed photographs using conventional film were produced with a strobe light in a dark room to study fiber motion and single-exposed photographs were used to estimate fiber diameter. The exposure duration of the strobe light (50 μs), however, was not short enough to eliminate image blur and obtain sharp images so the primary air velocity used during MB was low (17–55 m/s). Milligan and Utsman used a similar film-based photographic technique to investigate MB using a 30-hole die [8]. Bresee and Yan used a video imaging technique to investigate the dynamics of web formation at the collector of a 600-hole MB line [9]. Measurements of the dynamics between the die and collector of a high-speed commercial-like MB process would be expected to be especially desirable for understanding MB. To directly observe dynamic motions during this fast process, it is necessary to use a short exposure time to freeze motion in each image and a high framing rate to resolve fast fiber motions. We used a high-speed digital camera from Vision Research Inc. to acquire images as rapidly as 1,000 frames/s. The camera produced image frames with a spatial resolution as great as 512×512 pixels and 8-bit gray level resolution (256 gray levels). Electronic shuttering of the camera provided exposure times as short as 50 μs/frame. To obtain exposure times shorter than 50 μs or to obtain multiple-exposed images, a high-speed pulsed laser from Oxford Lasers, Inc. was used for illumination. The laser produced 100 watt peak power at 805 nm and pulse durations as short as 1 μs were synchronized with the camera.

Cubic Y–Ba–Cu–O thin films by high speed pulsed laser deposition

The crystal structure and properties of YBa 2Cu 3O x (YBCO) thin films on LaSrGaO 4 (100) substrate have been investigated as a function of deposition rates in the range of 0.1–12.2 nm/s using different repetition rate of pulsed laser. At a given substrate temperature of 700°C, when the film was deposited at low deposition rate of 0.1 nm/s, c-axis oriented orthorhombic YBCO thin film growth was observed. However, at high deposition rate of 12.2 nm/s, cubic YBCO thin film growth was observed. This demonstrates that the shortening of interval time between the vapor pulses is important for the cubic YBCO thin film growth. The cubic YBCO thin film growth under high deposition rates was explained by the cation kinetics.

Rapid solidification and a finite velocity for the propagation of heat

The Fourier thermal conduction model is subject to a pathological anomaly in that it predicts an infinite velocity of propagation for heat. A finite propagation velocity can be introduced by adding a hyperbolic relaxation term to the conventional model. We show that in high-speed pulsed laser melting experiments, ignoring the hyperbolic nature of thermal conduction may lead to the surface temperature being seriously underestimated. We also develop a model of dendritic growth within a hyperbolic conduction framework. It is found that stable dendritic growth is only possible if the growth velocity is less than half the thermal wave velocity, C.