James O. Kiggans

Enhanced diffusion in sapphire during microwave heating

The diffusion of oxygen in sapphire was accelerated by heating in a 28 GHz microwave furnace as compared with heating in a conventional furnace. Tracer diffusion experiments were conducted using 18O. Single crystal sapphire wafers with a (1 0 1 2) rhombohedral planar orientation were used as the substrate. Concentration depth profiling was done by proton activation analysis using a 5 MeV Van de Graaff accelerator. The diffusion of 18O was greatly enhanced by microwave heating as compared with conventional heating in the 1500–1800°C range. The apparent activation energy for 18O bulk diffusion was determined to be 390 kJ mol-1 with microwave heating and 650 kJ mol-1 with conventional heating.

Thermoelectric and mechanical properties of multi-walled carbon nanotube doped Bi0.4Sb1.6Te3 thermoelectric material

Since many thermoelectrics are brittle in nature with low mechanical strength, improving their mechanical properties is important to fabricate devices such as thermoelectric power generators and coolers. In this work, multiwalled carbon nanotubes (CNTs) were incorporated into polycrystalline Bi0.4Sb1.6Te3 through powder processing, which increased the flexural strength from 32 MPa to 90 MPa. Electrical and thermal conductivities were both reduced in the CNT containing materials, leading to unchanged figure of merit. Dynamic Youngs and shear moduli of the composites were lower than the base material, while the Poissons ratio was not affected by CNT doping.

Advanced Lithium Battery Cathodes Using Dispersed Carbon Fibers as the Current Collector

To fabricate LiFePO4 battery cathodes, highly conductive carbon fibers of 10-20 m in diameter have been used to replace a conventional aluminum (Al) foil current collector. This disperses the current collector throughout the cathode sheet and increases the contact area with the LiFePO4 (LFP) particles. In addition, the usual organic binder plus carbon-black can be replaced by a high temperature binder of <5 weight % carbonized petroleum pitch (P-pitch). Together these replacements increase the specific energy density and energy per unit area of the electrode. Details of the coating procedure, characterization and approach for maximizing the energy density are discussed. In a side-by-side comparison with conventional cathodes sheets of LFP on Al foil, the carbon fiber composite cathodes have a longer cycle life, higher thermal stability, and high capacity utilization with little sacrifice of the rate performance.

Microwave processing of silicon nitride

Microwave sintering of Si{sub 3}N{sub 4}-based materials showed improved densification as compared to samples heated conventionally under similar conditions. Accelerated nitridation of Si in the microwave furnace to produce Si{sub 3}N{sub 4} was also observed. Dense Si{sub 3}N{sub 4}, annealed by microwave heating, exhibited enhanced grain growth; however preferential coupling of the microwave power to the grain-boundary phases in the present experiments resulted in their degradation. 9 refs., 3 figs., 1 tab.

Microwave Processing of Ceramics: Guidelines Used at the Oak Ridge National Laboratory

To make meaningful comparisons between conventional and microwave processing of materials, one must conduct experiments that are as similar as possible in the two environments. Particular attention must be given to thermal conditions, sample parameters, and furnace environment. Under thermal conditions, one must consider temperature measurement (pyrometer or thermocouple, sheath type, and arcing of thermocouples), thermal history (heating and cooling rates, thermal gradients), and exothermic reactions. Regarding sample parameters, one must consider sample size, and packing powders and insulation systems. With respect to furnaces, one must consider differences in atmosphere, impurities, and uniformity of heating. Examples will be drawn from diffusion, grain growth, sintering, nitridation, and drying experiments conducted at the Oak Ridge National Laboratory (ORNL) over the past six years.

Microwave Sintering of Zirconia-Toughened Alumina Composites

Microwave sintering possesses unique attributes and has the potential to be developed asa new technique for controlling microstructure to improve the properties of advanced ceramics. 1–6 Because microwave radiation penetrates most ceramics, uniform volumetric heating is possible. Thermal gradients, which are produced during conventional sintering because of conductive and radiative heat transfer to and within the part, can be minimized. By eliminating temperature gradients, it is possible to reduce internal stresses, which contribute to cracking of parts during sintering, and to create a more uniform microstructure, which may lead to improved mechanical properties and reliability. With uniform, volumetric temperatures, the generation of nonuniform particle/grain growth due to temperature gradients and associated sintering gradients can be regulated.

Sintered reaction-bonded silicon nitride by microwave heating

Sintered silicon nitride has many desired properties; however, for most applications these materials are too expensive to compete with metal parts. Sintered fraction-bonded silicon nitride (SRBSN) is more economical, with raw material costs <27% those of comparable high-purity materials, making it competitive with metal parts. Conventional processing of SRBSN requires long nitridation times and a two-step firing process. Microwave (M) heating reduces the reaction times and is performed in a one-step process, thereby simplifying the operation. The flexural strength of the M-SRBSN is equivalent to the strength of some materials made from higher-cost powders. Thus, these materials maybe appropriate for a number of applications.

Development of Thermoelectric Fibers for Miniature Thermoelectric Devices

Miniature thermoelectric (TE) devices may be used in a variety of applications such as power sources of small sensors, temperature regulation of precision electronics, etc. Reducing the size of TE elements may also enable design of novel devices with unique form factor and higher device efficiency. Current industrial practice of fabricating TE devices usually involves mechanical removal processes that not only lead to material loss but also limit the geometry of the TE elements. In this project, we explored a powder-processing method for the fabrication of TE fibers with large length-to-area ratio, which could be potentially used for miniature TE devices. Powders were milled from Bi2Te3-based bulk materials and then mixed with a thermoplastic resin dissolved in an organic solvent. Through an extrusion process, flexible, continuous fibers with sub-millimeter diameters were formed. The polymer phase was then removed by sintering. Sintered fibers exhibited similar Seebeck coefficients to the bulk materials. However, their electrical resistivity was much higher, which might be related to the residual porosity and grain boundary contamination. Prototype miniature uni-couples fabricated from these fibers showed a linear I–V behavior and could generate millivolt voltages and output power in the nano-watt range. Further development of these TE fibers requires improvement in their electrical conductivities, which needs a better understanding of the causes that lead to the low conductivity in the sintered fibers.

Characterization of sintered reaction-bonded silicon nitride processed by microwave heating

Sintered reaction-bonded silicon nitride (SRBSN) tiles were fabricated by using microwave and conventional heating. Materials from both processes were analyzed at various stages in their fabrication. Microwave processing resulted in a SRBSN material of higher density and strength than the conventionally processed material.

Experimental study of the maximum resolution and packing density achievable in sintered and non-sintered binder-jet 3D printed steel microchannels

Developing high-resolution 3D printed metallic microchannels is a challenge especially when there is an essential need for high packing density of the primary metal. While high packing density could be achieved by heating the structure to the sintering temperature, some heat sensitive applications require other strategies to improve the packing density of primary metal. In this study the goal is to develop microchannels with high green (bound) or pack densities on the scale of 100–300 microns which have a robust mechanical structure. Binder-jet 3D printing is an additive manufacturing process in which droplets of binder are deposited via inkjet into a bed of powder. By repeatedly spreading thin layers of powder and depositing binder into the appropriate 2D profiles, complex 3D objects can be created one layer at time. Microchannels with features on the order of 500 microns were fabricated via binder jetting of steel powder and then sintered and/or infiltrated with a secondary material. The droplet volume of the inkjet-deposited binder was varied along with the print orientation. The resolution of the process, the subsequent features sizes of the microchannels, and the overall microchannel quality were studied as a function of droplet volume, orientation, and infiltration level.Copyright