Hadimani, Ravi

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Differential effect of magnetic alignment on additive manufacturing of magnetocaloric particles

2020-01-27 , Al-Milaji, Karam , Gupta, Shalabh , Pecharsky, Vitalij , Hadimani, Ravi , Barua, Radhika , Zhao, Hong , Hadimani, Ravi , Ames Laboratory , Materials Science and Engineering

Additive manufacturing of materials using magnetic particles as feedstock has attracted tremendous attention during the past decade owing to its ability to tune both shape and magnetocrystalline anisotropy, which can significantly enhance the magnetic characteristics of materials. We demonstrate that the magnetic response of multilayered thin films of Gd5Si4 can be tailored by controlling the external magnetic field during inkjet printing. The external magnetic field aligns the magnetic particles along their magnetic easy axis, enhancing the magnetic anisotropy of the printed films. Our work demonstrates the ability to print thin magnetic films with a defined anisotropy in any chosen direction with the potential to approaching magnetic properties of corresponding single crystalline materials.

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Stability of magnetocaloric La(FexCoySi1-x-y)13 in water and air

2019-03-19 , Javed, Khushar , Gupta, Shalabh , Pecharsky, Vitalij , Hadimani, Ravi , Hadimani, Ravi , Ames Laboratory , Materials Science and Engineering

Stability of cobalt-doped lanthanum iron silicide, La(FexCoySi1-x-y)13 have been investigated under conditions required for magnetocaloric refrigeration. The XRD analysis revealed that both milled and non-milled samples stored in water loose a few Bragg peaks corresponding to the NaZn13 phase of La(FexCoySi1-x-y)13. Samples stored in air show well-defined Bragg peaks similar to that of pristine material. The SEM-EDS of the milled and non-milled samples stored in water and air show an increased concentration of oxygen in the samples, particularly those treated with water. The course non-milled powders stored in air and water show sharp transitions at the Curie temperature TC = 300K without large magnetization above the TC. The milled, fine-particulate sample stored in air shows a slightly broadened transition at TC, and that stored in deionized water for 14 days shows significantly broadened transition from 300K and retains large magnetizations above 400 K. This is indicative of relatively fast hydrolysis and removal of some or all of La, likely as hydroxide, from fine powders, leaving behind La-poor or, potentially, La-free Fe-Co-Si containing ferromagnetic residue with much higher Curie temperature. The non-milled course sample stored in water has sharper magnetic transition and higher magnetization hence it shows the highest entropy change among all 4 type of samples.

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Transcranial Magnetic Stimulation: Design of a Stimulator and a Focused Coil for the Application of Small Animals

2018-06-28 , Hadimani, Ravi , Rastogi, Priyam , Prabhu Gaunkar, Neelam , Hadimani, Ravi , Mina, Mani , Mina, Mani , Ames Laboratory , Electrical and Computer Engineering , Industrial Design

Transcranial magnetic stimulation (TMS) is a non-invasive, safe, effective, and food and drug administration approved treatment for major depressive disorder. TMS relies on time-varying magnetic fields to induce an electric field in the brain, resulting in depolarization or hyperpolarization of the neurons. Recently, there has been extensive research to improve the magnetic coil design, effectiveness of TMS treatment, and improvement in the computer modeling of human brains, yet little development is reported on the TMS pulse generators and coil design for small animals. TMS pulse generators, or stimulators, are the circuits which provides pulse current to drive the inductive coils (TMS coils), used to generate time-varying magnetic fields. Commercial TMS stimulators are expensive and have limitations of using standard and non-customizable coils. These stimulators do not support small inductive loads, which require high-current capabilities. Furthermore, the commercial animal coil stimulates the entire body of a mouse, as they are designed for large animals. In this paper, the authors present the design of a small sized TMS stimulator and a focused coil for the application on small animals such as mice. The proposed TMS stimulator will have the potential of handling small inductive loads enabling stimulation of specific regions within the mouse brain.

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Investigation of shape, position, and permeability of shielding material in quadruple butterfly coil for focused transcranial magnetic stimulation

2018-01-01 , Hadimani, Ravi , Zhang, Bowen , Tang, Yalun , Jiles, David , Hadimani, Ravi , Jiles, David , Ames Laboratory , Electrical and Computer Engineering , Materials Science and Engineering

Transcranial magnetic stimulation has been gaining popularity in the therapy for several neurological disorders. A time-varying magnetic field is used to generate electric field in the brain. As the development of TMS methods takes place, emphasis on the coil design increases in order to improve focal stimulation. Ideally reduction of stimulation of neighboring regions of the target area is desired. This study, focused on the improvement of the focality of the Quadruple Butterfly Coil (QBC) with supplemental use of different passive shields. Parameters such as shape, position and permeability of the shields have been explored to improve the focus of stimulation. Results have been obtained with the help of computer modelling of a MRI derived heterogeneous head model over the vertex position and the dorsolateral prefrontal cortex position using a finite element tool. Variables such as maximum electric field induced on the grey matter and scalp, volume and area of stimulation above half of the maximum value of electric field on the grey matter, and ratio of the maximum electric field in the brain versus the scalp have been investigated.

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Inkjet Printing of Magnetic Particles Toward Anisotropic Magnetic Properties

2019-11-07 , Al-Milaji, Karam , Hadimani, Ravi , Gupta, Shalabh , Hadimani, Ravi , Pecharsky, Vitalij , Zhao, Hong , Ames Laboratory , Materials Science and Engineering

Unique properties of one-dimensional assemblies of particles have attracted great attention during the past decades, particularly with respect to the potential for anisotropic magnetism. Patterned films can be created using inkjet printing; however, drying of particle-laden colloidal droplets on solid surfaces is usually accompanied by the well-known coffee-ring effect, deteriorating both the uniformity and resolution of the printed configurations. This study examines the effect of externally applied magnetic field on particle deposition patterns. Ferromagnetic Gd5Si4 particles were formulated in terpineol oil and directly deposited via magnetic field-assisted inkjet printing on a photopaper to generate patterned films with suppressed coffee-ring effect. The particle deposition morphology is determined by both solvent imbibition and particle-magnetic field interactions. Three characteristic times are considered, namely, the critical time for solvent imbibition into the substrate (tim), the time it takes for particles to form chains in the presence of the magnetic field (tch), and the time in which the particles reach the substrate in the direction normal to the substrate (tpz). The characteristic time ratios (tpz/tim) and (tpz/tch) determine the final deposition morphology in the presence of magnetic field. The ability to control particle deposition and assembly, thus tuning the magnetic anisotropic properties of nanostructured materials is a promising approach for many engineering applications.

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Gd5Si4-PVDF nanocomposite films and their potential for triboelectric energy harvesting applications

2019-03-13 , Harstad, S. M. , Hadimani, Ravi , Zhao, P. , Soin, N. , El-Gendy, A. A. , Gupta, Shalabh , Pecharsky, Vitalij , Luo, J. , Hadimani, Ravi , Ames Laboratory , Electrical and Computer Engineering , Materials Science and Engineering

The triboelectric energy generators prepared using the combination of self-polarized, high beta-phase nanocomposite films of Gd5Si4-PVDF and polyamide-6 (PA-6) films have generated significantly higher voltage of 425 V, short-circuit current density of 30 mA/m(2) and a charge density of similar to 116.7 C/m(2) as compared to corresponding values of 300 V, 30 mA/m(2) and 94.7 mu C/m(2), respectively for the pristine PVDF-(PA-6) combination. The magnetic measurements of the Gd5Si4-PVDF films display a ferromagnetic behavior as compared to diamagnetic nature of pristine PVDF. The presence of magnetic nanoparticles in the polymeric matrix allows for some control over the microstructural properties during the preparation process. The results open new routes for multiferroic composite films to be suitable for multi-functional magnetic and triboelectric energy harvesting applications.

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Impact of non-brain anatomy and coil orientation on inter- and intra-subject variability in TMS at midline

2018-05-12 , Hadimani, Ravi , Rastogi, Priyam , Hadimani, Ravi , Jiles, David , Camprodon, Joan , Ames Laboratory , Electrical and Computer Engineering , Materials Science and Engineering

Objective: To investigate inter-subject variability with respect to cerebrospinal fluid thickness and brain-scalp distance, and to investigate intra-subject variability with different coil orientations.

Methods: Simulations of the induced electric field (E-Field) using a figure-8 coil over the vertex were conducted on 50 unique head models, and varying orientations on 25 models. Metrics exploring stimulation intensity, spread, and localization were used to describe inter-subject variability and effects of non-brain anatomy.

Results: Both brain-scalp distance and CSF thickness were correlated with weaker stimulation intensity, and greater spread. Coil rotations show that for the dorsal portion of the stimulated brain, E-Field intensities are highest when the anterior-posterior axis of the coil is perpendicular to the longitudinal fissure, but highest for the medial portion of the stimulated brain when the coil is oriented parallel to the longitudinal fissure.

Conclusions: Normal anatomical variation in healthy individuals leads to significant differences in the site of TMS, the intensity and the spread. These variables are generally neglected but could explain significant variability in basic and clinical studies.

Significance: This is the first work to show how brain-scalp distance and cerebrospinal fluid thickness influence focality, and to show the disassociation between dorsal and medial TMS.

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Enhancement of microwave absorption bandwidth of polymer blend using ferromagnetic gadolinium silicide nanoparticles

2019-04-13 , Bora, Pritom , Hadimani, Ravi , Gupta, Shalabh , Pecharsky, Vitalij , Vinoy, K. J. , Ramamurthy, Praveen , Hadimani, Ravi , Ames Laboratory , Electrical and Computer Engineering , Materials Science and Engineering

Ferromagnetic gadolinium silicide (Gd5Si4) nanoparticles significantly enhance the microwave absorption bandwidth of a polymer blend (PVB-PEDOT:PSS). These materials are critically needed for various military and civilian applications such as X-band (8.2–12.4 GHz) and Ku-band (12.4–18 GHz) absorption. A single 1.2 mm thick layer of PVB-PEDOT:PSS-Gd5Si4 (PPGS) nanocomposite film shows the most promising bandwidth (8.2–18 GHz) with a minimum reflection loss of -14 dB. Mechanistically, dielectric loss (tan δe ≈2.4) and magnetic loss (tan δm ≈1.1) contributes more efficiently, and standard microwave simulation confirms the stored energy is predominant in PPGS nanocomposite which enhances the bandwidth.

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Gadolinium silicide (Gd5Si4) nanoparticles for tuneable broad band microwave absorption

2019-01-10 , Bora, Pritom , Hadimani, Ravi , Harstad, S. M. , Gupta, Shalabh , Pecharsky, Vitalij , Vinoy, K. J. , Ramamurthy, Praveen , Hadimani, Ravi , Ames Laboratory , Materials Science and Engineering

Soft magnetic Gd5Si4 nanoparticles exhibit excellent microwave absorption in the Ku-band (12.4-18 GHz) when dispersed in poly (dimethyl siloxane), PDMS. The minimum experimentally recorded reflection loss (RL) of Gd5Si4-PDMS nanocomposite is −69 dB, with a large bandwidth for a single 6 mm-thick layer. The bandwidth can be further extended by using a novel design where 1 mm-thick layers of the nanocomposite are arranged into a modified pyramid-shaped absorber. Standard electromagnetic (EM) simulations confirm experimental results.

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Investigating phase transition temperatures of size separated gadolinium silicide magnetic nanoparticles

2018-05-01 , Hunagund, Shivakumar , Hadimani, Ravi , Harstad, Shane , El-Gendy, Ahmed , Gupta, Shalbh , Pecharsky, Vitalij , Hadimani, Ravi , Ames Laboratory , Materials Science and Engineering

Gadolinium silicide (Gd5Si4) nanoparticles (NPs) exhibit different properties compared to their parent bulk materials due to finite size, shape, and surface effects. NPs were prepared by high energy ball-milling of the as-cast Gd5Si4ingot and size separated into eight fractions using time sensitive sedimentation in an applied dc magnetic field with average particle sizes ranging from 700 nm to 82 nm. The largest Gd5Si4 NPs order ferromagnetically at 316 K. A second anomaly observed at 110 K can be ascribed to a Gd5Si3 impurity. As the particle sizes decrease, the volume fraction of Gd5Si3 phase increases at the expense of the Gd5Si4 phase, and the ferromagnetic transition temperature of Gd5Si4 is reduced from 316 K to 310 K, while the ordering of the minor phase is independent of the particle size, remaining at 110 K.