Andrew N. Smith

Measurement of Thermal Boundary Conductance of a Series of Metal-Dielectric Interfaces by the Transient Thermoreflectance Technique

Measurement of the thermal boundary conductance (TBC) by use of a nondestructive optical technique, transient thermoreflectance (TTR), is presented. A simple thermal model for the TTR is presented with a discussion of its applicability and sensitivity. A specially prepared sample series of Cr, Al, Au, and Pt on four different substrates (Si, sapphire, GaN, and AlN) were tested at room temperature and the TTR signal fitted to the thermal model. The resulting TBC values vary by more than a factor of 3~0.71 310 8 ‐2.3310 8 W/m 2 K!. It is shown that the diffuse mismatch model (DMM) tended to overpredict the TBC of interfaces with materials having similar phonon spectra, while underpredicting the TBC for interfaces with dissimilar phonon spectra. The DMM only accounts for diffuse elastic scattering. Other scattering mechanisms are discussed which may explain the failure of the DMM at room temperature.@DOI: 10.1115/1.1857944#

Femtosecond pump–probe nondestructive examination of materials (invited)

Ultrashort-pulsed lasers have been demonstrated as effective tools for the nondestructive examination (NDE) of energy transport properties in thin films. After the instantaneous heating of the surface of a 100 nm metal film, it will take ∼100 ps for the influence of the substrate to affect the surface temperature profile. Therefore, direct measurement of energy transport in a thin film sample requires a technique with picosecond temporal resolution. The pump–probe experimental technique is able to monitor the change in reflectance or transmittance of the sample surface as a function of time on a subpicosecond time scale. Changes in reflectance and transmittance can then be used to determine properties of the film. In the case of metals, the change in reflectance is related to changes in temperature and strain. The transient temperature profile at the surface is then used to determine the rate of coupling between the electron and phonon systems as well as the thermal conductivity of the material. In the case of semiconductors, the change in reflectance and transmittance is related to changes in the local electronic states and temperature. Transient thermotransmission experiments have been used extensively to observe electron-hole recombination phenomena and thermalization of hot electrons. Application of the transient thermoreflectance (TTR) and transient thermotransmittance (TTT) technique to the study of picosecond phenomena in metals and semiconductors will be discussed. The pump–probe experimental setup will be described, along with the details of the experimental apparatus in use at the University of Virginia. The thermal model applicable to ultrashort-pulsed laser heating of metals will be presented along with a discussion of the limitations of this model. Details of the data acquisition and interpretation of the experimental results will be given, including a discussion of the reflectance models used to relate the measured changes in reflectance to calculated changes in temperature. Finally, experimental results will be presented that demonstrate the use of the TTR technique for measuring the electron–phonon coupling factor and the thermal conductivity of thin metallic films. The use of the TTT technique to distinguish between different levels of doping and alloying in thin film samples of hydrogenated amorphous silicon will also be discussed briefly.

THERMAL BOUNDARY RESISTANCE MEASUREMENTS USING A TRANSIENT THERMOREFLECTANCE TECHNIQUE

A transient thermoreflectance technique, using a 200-fs laser pulse, is demonstrated as a nondestructive method for measuring the thermal boundary resistance between a thin metallic film and dielectric substrate. Experimental results are presented for Au deposited on silicon and silicon dioxide substrates taken at room temperature and compared to a thermal model. The relevant thermal properties of the metal film and the substrate are known, leaving the thermal boundary resistance as the only free parameter in the least-squares fitting routine. It is shown that the sensitivity of this technique is related directly to the thermal diffusivity of the substrate. A comparison between the diffuse mismatch model, the phonon radiation limit, and the experimental results indicates that the phonon dispersion relations of the materials can be utilized to give a qualitative prediction of the thermal boundary resistance.A transient thermoreflectance technique, using a 200-fs laser pulse, is demonstrated as a nondestructive method for measuring the thermal boundary resistance between a thin metallic film and dielectric substrate. Experimental results are presented for Au deposited on silicon and silicon dioxide substrates taken at room temperature and compared to a thermal model. The relevant thermal properties of the metal film and the substrate are known, leaving the thermal boundary resistance as the only free parameter in the least-squares fitting routine. It is shown that the sensitivity of this technique is related directly to the thermal diffusivity of the substrate. A comparison between the diffuse mismatch model, the phonon radiation limit, and the experimental results indicates that the phonon dispersion relations of the materials can be utilized to give a qualitative prediction of the thermal boundary resistance.

Nonequilibrium heating in metal films : An analytical and numerical analysis

Ultrashort pulsed laser heating of metals at room temperature is commonly studied using the parabolic two step (PTS) model. An analytical solution of the PTS is presented and compared to numerical results. The analytical solution assumes constant thermophysical properties, which is appropriate for applications such as nondestructive testing using low laser fluences. The assumption of constant thermophysical properties is discussed, and appropriate regimes are established. Numerical and analytical solutions of the PTS model are compared to experimental data in order to demonstrate the utility of these solutions for determining the thermal diffusivity of thin films.

Signal analysis and characterization of experimental setup for the transient thermoreflectance technique

The transient thermoreflectance (TTR) technique is a powerful optical pump-probe technique often used to measure thermal properties and monitor ultrafast processes. The technique has been used to measure a range of properties including the thermal conductivities of thin films and electron-phonon coupling factors to mention a few. TTR measurements are sensitive to residual heating and misalignment, which can lead to erroneous analysis of TTR data. To minimize these errors, we have developed a simple phase correction technique to reduce errors associated with residual heating and other background noise. Besides its simple implementation, the technique also requires no previous knowledge of the transient reflectance response. The technique is verified with simulated experimental data. The importance of proper alignment of the pump and probe beams over the entire range of pump-probe time delays of interest is discussed, along with a means of quantifying error associated with misalignment.

THIN-FILM THERMAL CONDUCTIVITY AND THICKNESS MEASUREMENTS USING PICOSECOND ULTRASONICS

A method utilizing picosecond ultrasonics, combined with a pump-probe thermoreflectance technique, for the simultaneous measurement of thin-film thickness and thermal conductivity normal to the surface is presented. This technique is a noncontact method that is useful for determining thermophysical properties of films with thickness gradients or for films in which the thickness is not a known parameter. The thermal conductivity of a thin tungsten film is extrapolated through a curve-fitting routine that compares results with the parabolic two-step (PTS) heat diffusion model, and the thickness measurement is enabled by the observation of ultrasonic waves generated by femtosecond laser heating pulses.

Influence of bore and rail geometry on an electromagnetic naval railgun system

A large-scale railgun is being considered by the U.S. navy as a future long range (>200 nm) naval weapon system. The notional concept includes a 15 kg projectile with a 2.5 km/sec muzzle velocity. The choice of bore and rail geometry for such a weapon can influence key aspects of the total system design. This study explored a range of bore and rail geometries and looked at their effects on key railgun system parameters such as parasitic mass, inductance gradient, linear current density, required pulse forming network (PFN) size, and barrel mass. Preliminary solid modeling and structural analysis of the integrated launch package was performed in order to quantify parasitic mass. Inductance gradient calculations were based on a current density distribution analysis. A PFN/Launcher numerical simulation model was then used to determine linear current density and PFN size. Finally, barrel mass was estimated by structural analysis based on calculated rail repulsive forces. Trends and sensitivities of the different parameters to changes in the bore and rail geometries are presented and conclusions are given.

Heat Generation During the Firing of a Capacitor-Based Railgun System

Railguns use a high-current, high-energy electrical pulse to accelerate projectiles to hypersonic velocities. Pulse-forming networks that employ capacitors as the energy store are typically used to shape the required electrical pulse. A significant fraction of the stored energy (2% to 40% in large caliber railguns) is converted to projectile kinetic energy during launch. After the projectile exits the launcher, the balance of the energy has either been dissipated as heat in the circuit components or is stored in system inductance. If an energy recovery scheme is not employed, the inductor energy will also be dissipated in the resistance of the active circuit components. A circuit analysis has been performed in order to calculate the current profile from the PFN. A higher fidelity solution was achieved by accounting for the temperature dependent resistance of the rails. This information along with individual component resistance and inductance was used to calculate the distribution of energy subsequent to a single pulse. Detailed component heating information is important when considering the overall thermal management of the system. Once this information has been obtained, the components that require external cooling can be identified, and an appropriate thermal management system can in turn be designed

MICROSCALE THERMAL RELAXATION DURING ACOUSTIC PROPAGATION IN AEROGEL AND OTHER POROUS MEDIA

The longitudinal acoustic velocity in silica aerogel is presented as a function of the interstitial gas type and pressure. This was measured using air-coupled ultrasonic transducers configured for differential pulse transit time measurements. The results are interpreted in terms of the thermal relaxation of the acoustic pulse. The microscale temperature oscillations of the gas and solid phases of the aerogel due to the acoustic pulse are not identical if the rate of heat transfer between the two phases is slow compared to the period of the acoustic oscillation. The energy transferred from the gas to the solid phase is lost to the acoustic propagation and, thus, reduces the amplitude and velocity of the acoustic wave. The gas type and pressure may provide independent variables for probing these effects in aerogel.

Measurement of High-Performance Thermal Interfaces Using a Reduced Scale Steady-State Tester and Infrared Microscopy

Thermal interface materials (TIMs) have reached values approaching the measurement uncertainty of standard ASTM D5470 based testers of approximately ±1 × 10−6 m2 K/W. This paper presents a miniature ASTM-type steady-state tester that was developed to address the resolution limits of standard testers by reducing the heat meter bar thickness and using infrared (IR) thermography to measure the temperature gradient along the heat meter bar. Thermal interfacial resistance measurements on the order of 1 × 10−6 m2 K/W with an order of magnitude improvement in the uncertainty of ±1 × 10−7 m2 K/W are demonstrated. These measurements were made on several TIMs with a thermal resistance as low as 1.14 × 10−6 m2 K/W.