Session L3: NT.2 Novel Techniques: Temperature

Thermal imaging of Al-CuO thermites

We have performed spatial in-situ temperature measurements of aluminum-copper oxide thermite reactions using high-speed color pyrometry. Electrophoretic deposition was used to create thermite microstructures. Tests were performed with micron- and nano-sized particles at different stoichiometries. The color pyrometry was performed using a high-speed color camera. The color filter array on the image sensor collects light within three spectral bands. Assuming a gray-body emission spectrum a multi-wavelength ratio analysis allows a temperature to be calculated. An advantage of using a two-dimensional image sensor is that it allows heterogeneous flames to be measured with high spatial resolution. Light from the initial combustion of the Al-CuO can be differentiated from the light created by the late time oxidization with atmosphere.   

Er,Yb:ZrO$_{2}$ / Eu:Y$_{2}$O$_{3}$ Core/Shell Assemblies as Potential Temperature Sensors in Explosions

We have recently demonstrated the use of nanophase Eu:Y$_{2}$O$_{3}$ and Eu:ZrO$_{2}$ as temperature sensors in explosions. Initial measurements showed that each of these materials is suitable for a certain temperature range -- Eu:Y$_{2}$O$_{3}$ covers the range from about 500 K to about 900 K, and Eu:ZrO$_{2}$ the range from about 800 K to about 1300 K. In order to have one material that can cover a wider range of temperatures, we have prepared core/shell assemblies of these host materials with different dopants. Here we report on the synthesis and characterization of core/shell assemblies consisting of Er,Yb:ZrO$_{2}$ cores and Eu:Y$_{2}$O$_{3}$ shells. The Er,Yb:ZrO$_{2}$ core was synthesized via forced hydrolysis and the Eu:Y$_{2}$O$_{3}$ shell was synthesized via homogeneous precipitation. Subsequently, these assemblies have been heated by a pyroprobe and a CO$_{2}$ laser for short periods of time. Heat-induced changes in the materials lead to changes in the optical spectra, which can then be correlated with temperature. The Er,Yb:ZrO$_{2}$ core emits upconverted light in the red and green when excited with 970 nm, while the Eu:Y$_{2}$O$_{3}$ shell emits in the red when excited with 532 nm. These spectra can be measured separately allowing us to determine temperatures over a wide range.   

Flash lamp integrating sphere technique for measuring the dynamic reflectance of shocked materials

Accurate reflectance (R) measurements of metals undergoing shock wave compression can benefit high pressure research in several ways. For example, pressure dependent reflectance measurements can be used to deduce electronic band structure, and discrete changes with pressure or temperature may indicate the occurrence of a phase boundary. Additionally, knowledge of the wavelength dependent emissivity (1-R for opaque samples) of the metal surface is essential for accurate pyrometric temperature measurement because the radiance is a function of both the temperature and emissivity. We have developed a method for measuring dynamic reflectance in the visible and near IR spectral regions with nanosecond response time and less than 1.5{\%} uncertainty. The method utilizes an integrating sphere fitted with a xenon flash-lamp illumination source. Because of the integrating sphere, the measurements are insensitive to changes in surface curvature or tilt. The in-situ high brightness of the flash-lamp exceeds the sample's thermal radiance and also enables the use of solid state detectors for recording the reflectance signals with minimal noise. Using the method, we have examined the dynamic reflectance of gallium and tin subjected to shock compression from high explosives. The results suggest significant reflectance changes across phase boundaries for both metals. We have also used the method to determine the spectral emissivity of shock compressed tin at the interface between tin and a LiF window. The results were used to perform emissivity corrections to previous pyrometry data and obtain shock temperatures of the tin/LiF interface with uncertainties of less than 2{\%}. This work was done under Contract No. DE-AC52-06NA25946 with the U.S. Department of Energy, and supported by the Site-Directed Research and Development Program.   

Optical reflectance as a dynamic temperature diagnostic

Reliable temperature measurements of materials under dynamic compression remain elusive, especially in quasi-isentropic experiments. Optical pyrometry with nanosecond time resolution is essentially impossible for samples below 1000 K---not enough photons are emitted to make satisfactory measurements. Rather than relying on light emission from the sample, one can also infer temperature by the light reflected by the sample. Thermoreflectance measurements are a proven technique in static systems and can readily be applied to dynamic compression experiments. Gold is an ideal candidate for dynamic thermoreflectance measurements. Gold coatings rapidly equilibriate with their surroundings, acting as an embedded gauge that can be probed optically. The optical properties of gold vary in the visible spectrum, and these variations are known to change with temperature, so in principle one can infer temperature from time-resolved reflectivity measurements. Calibration is the largest barrier for using embedded gold gauges because both temperature and pressure contribute to the measurement. This presentation will discuss static and dynamic calibration efforts to establish gold as a dynamic thermoreflectance standard.   

Probing Shock Compressed Silicon Metallization using VIS/NIR Reflectivity

Broadband reflectivity measurements provide detailed information about the optical and electronic properties of shocked matter, complementing other spectroscopic techniques and increasing the accuracy of pyrometric measurements, which is vital for improving models of planetary cores. A time resolved broadband VIS/NIR reflectivity diagnostic was constructed and used to observe the metallization of shock compressed single crystal \textless 111\textgreater \space silicon at Jupiter Laser Facility at Lawrence Livermore National Lab. A 50-100 fs 800 nm pulse was first sent through a pulse stacker and then an intense white light pulse with wavelengths from $\sim$400 nm to $\sim$1200 nm was generated by focusing the stacked pulses into a water cell. The white light pulses were then sent into the chamber and reflected from the target surface. The reflected light was dispersed using a custom spectrometer which was coupled to a streak camera. On transition to the $\beta$-Sn phase a dramatic increase in reflectivity was observed in the NIR, and to a lesser extent in the visible. This is congruent with the decrease in resistivity that accompanies closure of the silicon band gap and metallization.   

Application of Heterodyne Velocimetry and Pyrometry as Diagnostics for Explosive Characterisation

The results of four cylinder tests performed on two batches of an HMX based explosive using a new suite of diagnostics are described. Heterodyne laser velocimetry (hetV) and pyrometry were fielded for the first time on cylinder tests within AWE. Pyrometry gave a measurement of the temperature of the detonating HE of 2600-3000 K. Sixteen channels of HetV were fielded and provided high fidelity expansion data at distances of up to 30 mm. High speed framing camera images were obtained and show no signs of cylinder break up or spalling until distances greater than 35 mm. The expansion data has revealed the elastic pre-cursor in the cylinder wall and made it possible to resolve up to 8 shock reverberations in the wall as it expands. The expansion of the cylinder wall was recorded both before and after steady state detonation has been reached and the results compared. HetV probes were fielded at different angles to the expanding cylinder wall allowing both the vertical and horizontal expansion velocity to be determined. The extra information that these cylinder tests yielded will allow for more accurate code validation and determination of the equation of state.   

Pyrometric temperature measurements of shocked metals with uncertainties of less than 2{\%}

Advances in reflectance measurements have enabled accurate measurements of the emissivity of metals subjected to shock wave compression. Using three spectral bands, we performed reflectance and radiance measurements of shock compressed tin glued to LiF windows and combined them to determine time resolved temperatures with uncertainties of less than 2{\%}. Details of the uncertainty analysis are discussed. The tin samples were shock loaded using high explosives so there is a Taylor wave stress release that follows the shock front. Stress histories of the release were determined from PDV measurements and were combined with the temperatures to obtain temperature-stress release paths for the tin-glue-LiF interface. We discuss the link between the experimental release paths and release isentropes that begin on the principal shock Hugoniot. There is a complex relationship between the measured interface temperatures and the temperatures within the interior of the samples which complicates the analysis. Several of the complications are briefly discussed and interior temperatures are estimated. This work was done by National Security Technologies, LLC, under Contract No. DE-AC52-06NA25946 with the U.S. Department of Energy, and supported by the Site-Directed Research and Development Program.