Erica L. Stevens

Strong broadband absorption in GaAs nanocone and nanowire arrays for solar cells

We studied the influence of geometric parameters on the optical absorption of gallium arsenide (GaAs) nanocone and nanowire arrays via finite difference time domain simulations. We optimized the structural parameters of the nanocone and nanowire arrays to maximize the ultimate efficiency across a range of lengths from 100 to 1000 nm. Nanocone arrays were found to have improved solar absorption, short-circuit current density, and ultimate efficiencies over nanowire arrays for a wide range of lengths. Detailed simulations reveal that nanocones have superior absorption due to reduced reflection from their smaller tip and reduced transmission from their larger base. Breaking the vertical mirror symmetry of nanowires results in a broader absorption spectrum such that overall efficiencies are enhanced for nanocones. We also evaluated the electric field intensity, carrier generation and angle-dependent optical properties of nanocones and nanowires. The carrier generation in nanocone arrays occurs away from the surface and is more uniform over the entire structure, which should result in less recombination losses than in nanowire arrays.

Designing metal hemispheres on silicon ultrathin film solar cells for plasmonic light trapping

We systematically investigate the design of two-dimensional silver (Ag) hemisphere arrays on crystalline silicon (c-Si) ultrathin film solar cells for plasmonic light trapping. The absorption in ultrathin films is governed by the excitation of Fabry-Perot TEMm modes. We demonstrate that metal hemispheres can enhance absorption in the films by (1) coupling light to c-Si film waveguide modes and (2) exciting localized surface plasmon resonances (LSPRs). We show that hemisphere arrays allow light to couple to fundamental TEm and TMm waveguide modes in c-Si film as well as higher-order versions of these modes. The near-field light concentration of LSPRs also may increase absorption in the c-Si film, though these resonances are associated with significant parasitic absorption in the metal. We illustrate how Ag plasmonic hemispheres may be utilized for light trapping with 22% enhancement in short-circuit current density compared with that of a bare 100 nm thick c-Si ultrathin film solar cell.

Brief data overview of differently heat treated binder jet printed samples made from argon atomized alloy 625 powder.

Powder bed binder jet printing (BJP) is an additive manufacturing method in which powder is deposited layer-by-layer and selectively joined in each layer with binder. The data presented here relates to the characterization of the as-received feedstock powder, BJP processing parameters, sample preparation and sintering profile (“Effect of solutionizing and aging on the microstructure and mechanical properties of powder bed binder jet printed nickel-based superalloy 625” (A. Mostafaei, Y. Behnamian, Y.L. Krimer, E.L. Stevens, J.L. Luo, M. Chmielus, 2016) [1], “Powder bed binder jet printed alloy 625: densification, microstructure and mechanical properties” (A. Mostafaei, E. Stevens, E. Hughes, S. Biery, C. Hilla, M. Chmielus, 2016) [2]). The data presented here relates to the characterization of the as-received feedstock powder, BJP processing parameters, sample preparation and sintering profile. Effect of post heat treatments including solutionizing and aging on the microstructure and mechanical properties of powder bed binder jet printed nickel-based superalloy 625 were compared to that of sintered samples.

Data on the densification during sintering of binder jet printed samples made from water- and gas-atomized alloy 625 powders

Binder jet printing (BJP) is a metal additive manufacturing method that manufactures parts with complex geometry by depositing powder layer-by-layer, selectively joining particles in each layer with a polymeric binder and finally curing the binder. After the printing process, the parts still in the powder bed must be sintered to achieve full densification (A. Mostafaei, Y. Behnamian, Y.L. Krimer, E.L. Stevens, J.L. Luo, M. Chmielus, 2016; A. Mostafaei, E. Stevens, E. Hughes, S. Biery, C. Hilla, M. Chmielus, 2016; A. Mostafaei, Y. Behnamian, Y.L. Krimer, E.L. Stevens, J.L. Luo, M. Chmielus, 2016) [1–3]. The collected data presents the characterization of the as-received gas- and water-atomized alloy 625 powders, BJP processing parameters and density of the sintered samples. The effect of sintering temperatures on the microstructure and the relative density of binder jet printed parts made from differently atomized nickel-based superalloy 625 powders are briefly compared in this paper. Detailed data can be found in the original published papers by authors in (A. Mostafaei, J. Toman, E.L. Stevens, E.T. Hughes, Y.L. Krimer, M. Chmielus, 2017) [4].

An analytical model of the melt pool and single track in coaxial laser direct metal deposition (LDMD) additive manufacturing

An analytical model was developed for the melt pool and single scan track geometry as a function of process parameters. For computational efficiency, the developed model has simple mathematical forms with essential physics taken into account, without the need for complicated numerical simulation. In this research, a non-diverging Gaussian laser beam and coaxial diverging Gaussian powder stream combination is used to represent the coaxial laser direct metal deposition (LDMD) process. Analytical laser-powder interaction model is used to obtain the distribution of attenuated laser intensity and temperature of heated powders at the substrate. On the substrate, the melt pool is calculated by integrating Rosenthal’s point heat source model. An iterative procedure is used to ensure the mass–energy balances and to calculate the melt pool and catchment efficiency. By assuming that the assimilated powder will reshape due to surface tension before solidification, a simple clad geometry model is established. The proposed model is used to simulate the geometry of single track depositions of Ti6Al4V, which shows a good agreement between model prediction and experimental results. This work demonstrates that the developed model has the potential to be used to narrow the parameter space for process optimization.

Microstructural Evaluation of Magnetocaloric Ni-Co-Mn-Sn Produced by Directed Energy Deposition

Magnetocaloric (MC) materials materials that experience temperature changes due to applied magnetic fields have the potential to transform refrigeration and cooling by reducing energy consumption of commercial refrigerators by up to 30% [1]. Some Heusler alloys (ferromagnetic alloys containing no ferromagnetic elements) including Ni-Mn-Sn with Co addition, are important as MC material systems because they do not require rare earth metals. Ni-Co-Mn-Sn experiences a first-order phase transformation from martensite to austenite. When this transformation (which affects magnetic properties) coincides with the Curie temperature, a MC effect is observed [2]. The spatial freedom of additive manufacturing (AM) would provide the ability to print such materials in a variety of geometries to meet a wider variety of device requirements [3]. As a foundation for combining the benefits of this novel materials system and manufacturing process effectively, the microstructure of AM MC materials must be characterized.