S. E. Burrows

Combined laser spot imaging thermography and ultrasonic measurements for crack detection

A surface crack, close to a small, well-defined heated spot, impedes lateral heat flow and produces alterations in the shape of the thermal spot image that can be monitored by thermography. Spot heating has been achieved using both cw and pulsed laser beam illumination on stainless steel and titanium samples and the technique has been found to be successful for determining the location of fatigue cracks. When using a pulsed laser beam one can also simultaneously generate wideband ultrasonic signals in the sample; this can be used to detect the presence of surface and sub-surface defects in the sample. Results are presented that have been obtained using a fixed camera and cw laser beam position with the sample being moved through the field of view of the camera. Results are also presented using a fixed camera and sample, with a raster scanned cw or pulsed lasers beam moving across the sample. A demonstration of how a non-contact ultrasonic measurement can be performed simultaneously is presented. Thermal imaging results obtained using cw laser beam heating close to a surface crack are compared with predictions made using a preliminary 2D numerical model.

Detection of cracks in metal sheets using pulsed laser generated ultrasound and EMAT detection

A pulsed Nd:YAG laser with an approximately Gaussian beam shape is directed onto the surface of an aluminium sheet at an energy density below which damage by laser ablation occurs, generating Lamb waves in the sheet. The laser beam is raster scanned across the surface of the sample. The Lamb waves travel radially outwards from the generation point and are detected some distance away by an electromagnetic acoustic transducer with sensitivity to in-plane displacements of the sheet. A number of static EMATs are located around the edges of the sheet, some distance from the generation point. The presence of a crack-like defect on the sheet can be detected by either a sudden change in the ultrasonic waveform or by an enhancement in the frequency content of the waveform when the laser beam illuminates directly onto the crack.

Temperature contour maps at the strain-induced martensitic transition of a Cu–Zn–Al shape-memory single crystal

We study temperature changes at the reverse strain-induced martensitic transformation in a Cu–Zn–Al single crystal. Infrared thermal imaging reveals a markedly inhomogeneous temperature distribution. The evolution of the contour temperature maps enables information to be extracted on the kinetics of the interface motion.

Laser generation of lamb waves for defect detection: Experimental methods and finite element modeling

The propagation of Lamb waves generated by a pulsed laser beam in an aluminum sheet is modeled using finite element analysis, and the interaction with defects is studied and compared to experimental results. The ultrasonic Lamb waves are detected by an electromagnetic acoustic transducer (EMAT). The frequency content of the received wave is shown to be enhanced when the generation point is situated directly over the defect in both the modeled and experimental cases. Time-frequency analysis using a Wigner transform has enabled individual modes to be identified.

Si/SiC bonded wafer: A route to carbon free SiO2 on SiC

This paper describes the thermal oxidation of Si/SiC heterojunction structures, produced using a layer-transfer process, as an alternative solution to fabricating SiC metal-oxide-semiconductor (MOS) devices with lower interface state densities (Dit). Physical characterization demonstrate that the transferred Si layer is relatively smooth, uniform, and essentially monocrystalline. The Si on SiC has been totally or partially thermally oxidized at 900–1150 °C. Dit for both partially and completely oxidized silicon layers on SiC were significantly lower than Dit values for MOS capacitors fabricated via conventional thermal oxidation of SiC. The quality of the SiO2, formed by oxidation of a wafer-bonded silicon layer reported here has the potential to realize a number of innovative heterojunction concepts and devices, including the fabrication of high quality and reliable SiO2 gate oxides.

Shear horizontal (SH) ultrasound wave propagation around smooth corners.

Shear horizontal (SH) ultrasound guided waves are being used in an increasing number of non-destructive testing (NDT) applications. One advantage SH waves have over some wave types, is their ability to propagate around curved surfaces with little energy loss; to understand the geometries around which they could propagate, the wave reflection must be quantified. A 0.83mm thick aluminium sheet was placed in a bending machine, and a shallow bend was introduced. Periodically-poled magnet (PPM) electromagnetic acoustic transducers (EMATs), for emission and reception of SH waves, were placed on the same side of the bend, so that reflected waves were received. Additional bending of the sheet demonstrated a clear relationship between bend angles and the reflected signal. Models suggest that the reflection is a linear superposition of the reflections from each bend segment, such that sharp turns lead to a larger peak-to-peak amplitude, in part due to increased phase coherence.

Si/SiC heterojunctions fabricated by direct wafer bonding

The physical and electrical properties of Si/SiC heterojunctions formed by direct wafer bonding are presented. Atomic force microscopy (AFM) and imaging reveal an improved bonding quality when Si wafers are transferred to on-axis substrates as opposed to off-axis epitaxial layers. AFM analysis of the bonded wafer achieves a smoother surface when compared to molecular beam epitaxy-grown Si layers. A reduced roughness of only 5.8 nm was measured for bonded wafers. Current-voltage measurements were used to extract the rectifying characteristics of Si/SiC heterojunctions. These Si layers could lead to improved high quality and reliable SiO2 gate oxides

Crack Detection by Laser Spot Imaging Thermography

A surface crack, close to a laser heated spot, impedes lateral heat flow and produces alterations in the shape of the laser spot image that can be monitored by thermography. The technique has been found to be successful for determining the location of fatigue cracks. Results are presented that have been obtained using long pulse heating and by flying spot scanning a pulsed laser beam. The results are compared with a preliminary 2D numerical model.

On the Ti3SiC2 metallic phase formation for robust p-type 4H-SiC ohmic contacts

This paper presents a detailed physical and electrical analysis of 4H-SiC ohmic contacts to p-type material, the main aim being to examine their ruggedness under high temperature conditions. XRD, FIB-TEM and SEM are techniques that have been utilized to examine the microstructure and interface properties respectively. A detailed physical study revealed the presence of a crystalline hexagonal Ti layer orientated in the same direction as the 4H-SiC epitaxial layer. This factor seems to be important in terms of electrical performance, having the lowest measured specific contact resistivity of 1x10-6 Ωcm2. We attribute this to the optimized formation of Ti3SiC2 at the metal/SiC interface. An initial high temperature study shows thermionic emission occurring across the metal/semiconductor junction.

Characterization of n-n Ge/SiC heterojunction diodes

In this paper we investigate the physical and electrical properties of germanium deposited on 4H silicon carbide substrates by molecular beam epitaxy. Layers of highly doped and intrinsic germanium were deposited at 300 and 500 °C and compared. Current-voltage measurements reveal low turn-on voltages. The intrinsic samples display ideality factors of 1.1 and a reverse leakage current of 9×10−9 A/cm2, suggesting a high quality electrical interface. X-ray diffraction analysis reveals the polycrystalline nature of the high-temperature depositions, whereas the low-temperature depositions are amorphous. Atomic force microscopy shows that the low-temperature layers have a rms roughness of 3 nm.