@article{smith_bilbro_nemanich_2007, title={Considerations for a high-performance thermionic energy conversion device based on a negative electron affinity emitter}, volume={76}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.76.245327}, abstractNote={A theory is developed to model the effect a negative electron affinity (NEA) emitter electrode has on the negative space charge effect of a vacuum thermionic energy conversion device (TEC). The theory is derived by treating the electrons in the interelectrode space as a collisionless gas and self-consistently solving the Boltzmann transport equation and Poisson equation. The theory determines the point on the voltage-current characteristic such that the maximum motive due to space charge is at the same level as the conduction band minimum. It is shown that emitter electrodes with an NEA significantly mitigate the negative space charge effect; therefore a TEC employing such an electrode will outperfrom a similar TEC with conventional electrodes in terms of output power. Additionally, it is shown that a TEC with an NEA emitter electrode can have a greater interelectrode spacing than a TEC with conventional electrodes operating under similar conditions where the outputs of both TEC's are comparable.}, number={24}, journal={PHYSICAL REVIEW B}, author={Smith, Joshua Ryan and Bilbro, Griff L. and Nemanich, Robert J.}, year={2007}, month={Dec} }
 @article{smith_nemanich_bilbro_2006, title={The effect of Schottky barrier lowering and nonplanar emitter geometry on the performance of a thermionic energy converter}, volume={15}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2005.12.057}, abstractNote={An extension of the classical model of thermionic emission was developed to include the effects of nonplanar emitter surfaces and Schottky barrier lowering (SBL) on the output of a thermionic energy converter (TEC). Nonplanar emitter geometries along with Schottky barrier lowering may be useful in increasing both the maximum output power and output current of a thermionic energy converter. The finite element method was used to calculate the enhanced normal electric field at the surface of an emitter coated with an ultra-nanocrystalline diamond (UNCD) film and patterned with field enhancing tips. The result was used to determine the local enhanced output current and power. For the geometries considered the increased surface area of the emitter plays a significant role in increasing the output power and output current. Moreover, a calculation of the single electron time of flight shows that electrons traveling through a field enhanced region of the interelectrode space might spend half as long in transit, thus helping to mitigate the negative space charge effect that degrades the performance of vacuum TECs.}, number={4-8}, journal={DIAMOND AND RELATED MATERIALS}, author={Smith, J. R. and Nemanich, R. J. and Bilbro, G. L.}, year={2006}, pages={870–874} }
 @article{smith_bilbro_nemanich_2006, title={Using negative electron affinity diamond emitters to mitigate space charge in vacuum thermionic energy conversion device}, volume={15}, ISSN={["0925-9635"]}, DOI={10.1016/j.diamond.2006.09.011}, abstractNote={A negative electron affinity (NEA) diamond surface is employed as an emitter electrode in a vacuum thermionic energy conversion device in order to mitigate the negative space charge effect. The motive diagram of an NEA device operating at the virtual saturation point is compared to a similar device with a conventional emitter material operating in the space charge limited regime in order to understand how NEA mitigates space charge. Output current characteristics are calculated for various NEA values, and the results are compared to an ideal (no space charge) model. Increasing the value of the NEA causes the output current characteristic to approach that of the ideal model. Motive diagrams for various values of NEA are calculated and used to explain this phenomenon. It is shown that an NEA device can achieve a maximum output power density equal to the maximum output power density of a similar ideal device.}, number={11-12}, journal={DIAMOND AND RELATED MATERIALS}, author={Smith, J. R. and Bilbro, G. L. and Nemanich, R. J.}, year={2006}, pages={2082–2085} }
 @article{smith_seelecke_ounaies_smith_2003, title={A Free Energy Model for Hysteresis in Ferroelectric Materials}, volume={14}, ISSN={1045-389X 1530-8138}, url={http://dx.doi.org/10.1177/1045389x03038841}, DOI={10.1177/1045389X03038841}, abstractNote={ This paper provides a theory for quantifying the hysteresis and constitutive nonlinearities inherent to piezoceramic compounds through a combination of free energy analysis and stochastic homogenization techniques. In the first step of the model development, Helmholtz and Gibbs free energy relations are constructed at the lattice or domain level to quantify the relation between the field and polarization in homogeneous, single crystal compounds which exhibit uniform effective fields. The effects of material nonhomogeneities, polycrystallinity, and variable effective fields are subsequently incorporated through the assumption that certain physical parameters, including the local coercive and effective fields, are randomly distributed and hence manifestations of stochastic density functions associated with the material. Stochastic homogenization in this manner provides low-order macroscopic models with effective parameters that can be correlated with physical properties of the data. This facilitates the identification of parameters for model construction, model updating to accommodate changing operating conditions, and control design utilizing model-based inverse compensators. Attributes of the model, including the guaranteed closure of biased minor loops in quasistatic drive regimes, are illustrated through examples. }, number={11}, journal={Journal of Intelligent Material Systems and Structures}, publisher={SAGE Publications}, author={Smith, Ralph C. and Seelecke, Stefan and Ounaies, Zoubeida and Smith, Joshua}, year={2003}, month={Nov}, pages={719–739} }