Ralph B. Dinwiddie

The effect of grain size, porosity and yttria content on the thermal conductivity of nanocrystalline zirconia

In order to accommodate the ever increasing inlet temperatures of gas turbines, air plasma sprayed (APS) or electron beam physically vapor deposited (EB-PVD) yttria stabilized zirconia thermal barrier coatings (TBC`s) are used to insulate the metallic surfaces. Because of its historic use as a TBC, the thermal diffusivity and conductivity of single crystal and polycrystalline stabilized zirconia have been the subject of numerous experimental investigations. However, to the knowledge of the authors, the thermal conductivity of nanocrystalline (gain size < 100 nm) zirconia has not yet been determined. To ascertain whether or when grain boundary effects begin to dominate thermal conductivity, k, values for a variety of nanocrystalline zirconias of different densities (60--100%), grain sizes (30--400 nm), and purities (0--15wt.% yttria) are compared in this work. Finally the measured values are compared with the thermal conductivities of commercially available air plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) coatings.

THERMAL PROPERTIES OF ZIRCONIA CO-DOPED WITH TRIVALENT AND PENTAVALENT OXIDES

Zirconia doped with 6-8 wt% (3.2-4.2 mol%) yttria (6-8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb-Y and Ta-Y co-doped zirconia samples containing only substi- tutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6-8YSZ. In these samples, Nb 51 and Td 51 are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta 51 doping than Nb 51 doping. Finally, Nb 51 /Ta 51 - Y 31 doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal 1 cubic mixtures are again as low as that of the conventional 6-8YSZ.

The effect of graphite flake morphology on the thermal diffusivity of gray cast irons used for automotive brake discs

Thermal diffusivity of automotive grade SAE G3000 (d) gray cast iron has been measured as a function of graphite flake morphology, chemical composition and temperature. Cast iron samples used for this investigation were cut from “step block” castings designed to produce iron with different graphite flake morphologies resulting from different cooling rates. Samples were also machined from prototype and commercial brake rotors, as well as from a series of cast iron slugs with slightly varying compositions. Thermal diffusivity was measured at room and elevated temperatures via the flash technique. Graphite flake morphology of the various cast iron samples was quantified stereologically with image analysis techniques. Several geometric features of the graphite flake morphology were quantified. It was found that the thermal diffusivity of these gray cast irons increases with carbon equivalent and has a strong linear correlation to graphite flake length. For gray iron with the same chemical composition, a four fold increase in the graphite flake size results in a 50% increase in thermal diffusivity. Amongst the commercial rotors, room temperature thermal diffusivity varied from 0.156 to 0.200 cm2/s.

Thermographic In-Situ Process Monitoring of the Electron Beam Melting Technology used in Additive Manufacturing

Oak Ridge National Laboratory (ORNL) has been utilizing the ARCAM electron beam melting technology to additively manufacture complex geometric structures directly from powder. Although the technology has demonstrated the ability to decrease costs, decrease manufacturing lead-time and fabricate complex structures that are impossible to fabricate through conventional processing techniques, certification of the component quality can be challenging. Because the process involves the continuous deposition of successive layers of material, each layer can be examined without destructively testing the component. However, in-situ process monitoring is difficult due to metallization on inside surfaces caused by evaporation and condensation of metal from the melt pool. This work describes a solution to one of the challenges to continuously imaging inside of the chamber during the EBM process. Here, the utilization of a continuously moving Mylar film canister is described. Results will be presented related to in-situ process monitoring and how this technique results in improved mechanical properties and reliability of the process.

Reliability of laser flash thermal diffusivity measurements of the thermal barrier coatings

The thermal diffusivity of free standing thermal barrier coatings (TBCs) was measured by the laser flash technique. The combination of low thermal conductivity (1 to 2 W/m K) and small TBC thickness (300 to 600 µm thick) can cause errors in the measurements. Back surface (opposite the laser) temperatures of free standing plasma-sprayed TBCs were measured as a function of time and laser power. The front surface temperatures were calculated using thermal transport equations. In the high power region, thermal diffusivity decreased significantly with increasing laser power. In the moderate power region, thermal diffusivity remained constant. In the low power region, measurement became unreliable because of noise. The detector nonlinearity was believed to be a possible cause of deviation in the high power region. Measurements at different laser power levels should be conducted in order to obtain reliable thermal diffusivity values for TBCs.

Real-time Process Monitoring and Temperature Mapping of a 3D Polymer Printing Process

An extended-range IR camera was used to make temperature measurements of samples as they are being manufactured. The objective is to quantify the temperature variation of the parts as they are being fabricated. The IR camera was also used to map the temperature within the build volume of the oven. The development of the temperature map of the oven provides insight into the global temperature variation within the oven that may lead to understanding variations in the properties of parts as a function of build location within the oven. The observation of the temperature variation of a part during construction provides insight into how the deposition process itself creates temperature distributions, which can lead to failure.

Novel heat spreader coatings for high power electronic devices

A new set of heat spreader coatings consisting of multilayers of diamond/AlN/diamond were deposited on high heat capacity substrates of molybdenum and silicon nitride. Bonding of the heat spreaders to the device wafers using gold-tin eutectic solder was carried out after metallization layers of titanium, gold and copper were deposited on diamond. Prior to bonding, backside of the silicon wafers was also metallized with titanium, gold and copper and the gallium arsenide wafers with titanium, copper-germanium alloy and gold. Characterization of the multilayer diamond films was carried out by Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The bonded wafers were tested for adhesion strength, resistance against peeling due to thermal cycling and failure under stress. Further, the bonded regions were characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray mapping of different elements. The heat spreader characteristics of the single layer diamond and the multilayer diamond substrates were tested by infrared imaging. The results illustrate that the multilayer diamond heat spreader coatings provide better heat dissipation and also possess better adhesion strength and resistance against peeling under thermal cycling. These novel multilayer diamond/AlN/diamond heat spreaders are expected to considerably improve the life of high frequency power devices.

Thermographic Microstructure Monitoring in Electron Beam Additive Manufacturing

To reduce the uncertainty of build performance in metal additive manufacturing, robust process monitoring systems that can detect imperfections and improve repeatability are desired. One of the most promising methods for in situ monitoring is thermographic imaging. However, there is a challenge in using this technology due to the difference in surface emittance between the metal powder and solidified part being observed that affects the accuracy of the temperature data collected. The purpose of the present study was to develop a method for properly calibrating temperature profiles from thermographic data to account for this emittance change and to determine important characteristics of the build through additional processing. The thermographic data was analyzed to identify the transition of material from metal powder to a solid as-printed part. A corrected temperature profile was then assembled for each point using calibrations for these surface conditions. Using this data, the thermal gradient and solid-liquid interface velocity were approximated and correlated to experimentally observed microstructural variation within the part. This work shows that by using a method of process monitoring, repeatability of a build could be monitored specifically in relation to microstructure control.

Thermal transport properties of grey cast irons

Thermal diffusivity and thermal conductivity of grey cast iron have been measured as a function of graphite flake morphology, chemical composition, and position in a finished brake rotor. Cast iron samples used for this investigation were cut from ``step block`` castings designed to produce iron with different graphite flake morphologies resulting from different cooling rates. Samples were also machined from prototype alloys and from production brake rotors representing a variation in foundry practice. Thermal diffusivity was measured at room and elevated temperatures via the flash technique. Heat capacity of selected samples was measured with differential scanning calorimetry, and these results were used to calculate the thermal conductivity. Microstructure of the various cast iron samples was quantified by standard metallography and image analysis, and the chemical compositions were determined by optical emission spectroscopy.

Infrared imaging of the polymer 3D-printing process

Both mid-wave and long-wave IR cameras are used to measure various temperature profiles in thermoplastic parts as they are printed. Two significantly different 3D-printers are used in this study. The first is a small scale commercially available Solidoodle 3 printer, which prints parts with layer thicknesses on the order of 125μm. The second printer used is a “Big Area Additive Manufacturing” (BAAM) 3D-printer developed at Oak Ridge National Laboratory. The BAAM prints parts with a layer thicknesses of 4.06 mm. Of particular interest is the temperature of the previously deposited layer as the new hot layer is about to be extruded onto it. The two layers are expected have a stronger bond if the temperature of the substrate layer is above the glass transition temperature. This paper describes the measurement technique and results for a study of temperature decay and substrate layer temperature for ABS thermoplastic with and without the addition of chopped carbon fibers.