Browsing by Author "Guhathakurta, Jajnabalkya"

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Open Access
Fabrication and electrical characterization of ultra-thin substrate IGBT
(2013) Guhathakurta, Jajnabalkya
Current topics such as electro-mobility and renewable energy demand the development of power devices with high voltage and current ratings along with minimum switching losses. Amongst the power devices in today’s market, IGBTs have gained a lot of significance in this field over its competitors like Power MOSFETS and Thyristors. Today’s industry has recently taken a huge step in this direction to implement the use of thin-wafer technology for fabrication of IGBTs to reduce the on-resistance. However, this comes with a complexity of handling these thin wafers which demands the need of high end robotics. For research work, accesses to such resources are not feasible and if we are not able to fabricate device which the industry is fabricating today, we cannot add improvements to it and hence the research in this domain becomes restricted. The primary objective of this thesis work is to develop a technology for fabrication of Ultra-Thin substrate IGBT which can be achieved at any standard semiconductor research facility and in the course, reducing the on-resistance of the device wherever possible. The most promising solution was the use of Ultra-Thin Si-membranes as a substrate for fabrication. These Si-membranes were fabricated by anisotropic etching of Si wafers by TMAH. IGBT being a four-layered device, the required layers were deposited on the Si-membrane by MBE to achieve very thin and distinct layers which would minimize the on-resistance. Uniquely the Gate terminal of the device was structured on the sidewall of the mesa structure as opposed to conventional trench gate structures. The device with such dimension and structure was initially simulated by the device simulator ATLAS from Silvaco to verify the working of the device and to have a general idea of what kind of a characteristics to expect from the fabricated device. Thereafter the devices were successfully fabricated at our research facility at IHT. Even though there were numerous technical challenges and difficulties, the technology proved to be robust and we were able to fabricate the first IGBT at IHT even in the first run. The electrical characteristics showed a good forward characteristics of the device with good gate response and high on-current to off-current ratio but the reverse characteristics of the device showed unusual characteristics which is also investigated in this thesis work.
Open Access
FL-MISR : fast large-scale multi-image super-resolution for computed tomography based on multi-GPU acceleration
(2021) Sun, Kaicong; Tran, Trung-Hieu; Guhathakurta, Jajnabalkya; Simon, Sven
Multi-image super-resolution (MISR) usually outperforms single-image super-resolution (SISR) under a proper inter-image alignment by explicitly exploiting the inter-image correlation. However, the large computational demand encumbers the deployment of MISR in practice. In this work, we propose a distributed optimization framework based on data parallelism for fast large-scale MISR using multi-GPU acceleration named FL-MISR. The scaled conjugate gradient (SCG) algorithm is applied to the distributed subfunctions and the local SCG variables are communicated to synchronize the convergence rate over multi-GPU systems towards a consistent convergence. Furthermore, an inner-outer border exchange scheme is performed to obviate the border effect between neighboring GPUs. The proposed FL-MISR is applied to the computed tomography (CT) system by super-resolving the projections acquired by subpixel detector shift. The SR reconstruction is performed on the fly during the CT acquisition such that no additional computation time is introduced. FL-MISR is extensively evaluated from different aspects and experimental results demonstrate that FL-MISR effectively improves the spatial resolution of CT systems in modulation transfer function (MTF) and visual perception. Comparing to a multi-core CPU implementation, FL-MISR achieves a more than 50× speedup on an off-the-shelf 4-GPU system.
Open Access
Non-uniform circumferential expansion of cylindrical Li-ion cells - the potato effect
(2021) Hemmerling, Jessica; Guhathakurta, Jajnabalkya; Dettinger, Falk; Fill, Alexander; Birke, Kai Peter
This paper presents the non-uniform change in cell thickness of cylindrical Lithium (Li)-ion cells due to the change of State of Charge (SoC). Using optical measurement methods, with the aid of a laser light band micrometer, the expansion and contraction are determined over a complete charge and discharge cycle. The cell is rotated around its own axis by an angle of α=10° in each step, so that the different positions can be compared with each other over the circumference. The experimental data show that, contrary to the assumption based on the physical properties of electrode growth due to lithium intercalation in the graphite, the cell does not expand uniformly. Depending on the position and orientation of the cell coil, there are different zones of expansion and contraction. In order to confirm the non-uniform expansion around the circumference of the cell in 3D, X-ray computed tomography (CT) scans of the cells are performed at low and at high SoC. Comparison of the high resolution 3D reconstructed volumes clearly visualizes a sinusoidal pattern for non-uniform expansion. From the 3D volume, it can be confirmed that the thickness variation does not vary significantly over the height of the battery cell due to the observed mechanisms. However, a slight decrease in the volume change towards the poles of the battery cells due to the higher stiffness can be monitored.
Open Access
Non-uniform circumferential expansion of cylindrical Li-ion cells : the potato effect
(2021) Hemmerling, Jessica; Guhathakurta, Jajnabalkya; Dettinger, Falk; Fill, Alexander; Birke, Kai Peter
This paper presents the non-uniform change in cell thickness of cylindrical Lithium (Li)-ion cells due to the change of State of Charge (SoC). Using optical measurement methods, with the aid of a laser light band micrometer, the expansion and contraction are determined over a complete charge and discharge cycle. The cell is rotated around its own axis by an angle of α=10° in each step, so that the different positions can be compared with each other over the circumference. The experimental data show that, contrary to the assumption based on the physical properties of electrode growth due to lithium intercalation in the graphite, the cell does not expand uniformly. Depending on the position and orientation of the cell coil, there are different zones of expansion and contraction. In order to confirm the non-uniform expansion around the circumference of the cell in 3D, X-ray computed tomography (CT) scans of the cells are performed at low and at high SoC. Comparison of the high resolution 3D reconstructed volumes clearly visualizes a sinusoidal pattern for non-uniform expansion. From the 3D volume, it can be confirmed that the thickness variation does not vary significantly over the height of the battery cell due to the observed mechanisms. However, a slight decrease in the volume change towards the poles of the battery cells due to the higher stiffness can be monitored.
Open Access
Steered fiber orientation : correlating orientation and residual tensile strength parameters of SFRC
(2022) Medeghini, Filippo; Guhathakurta, Jajnabalkya; Tiberti, Giuseppe; Simon, Sven; Plizzari, Giovanni A.; Mark, Peter
Adding steel fibers to concrete improves the post-cracking tensile strength of the composite material due to fibers bridging the cracks. The residual performance of the material is influenced by fiber type, content and orientation with respect to the crack plane. The latter is a main issue in fiber-reinforced concrete elements, since it significantly influences the structural behavior. The aim of this research is to develop a tailor-made composite material and casting method to orient fibers in order to optimize the performance of the material for structural applications. To this aim, a mechanized concreting device that induces such preferred fiber orientation is designed and fabricated. It uses vibration and a series of narrow channels to guide and orient fibers. A composite with oriented fibers is produced using a hybrid system of macro and micro fibers and high-performance concrete. From the same concrete batch, specimens are cast both with and without the fiber orientation device, obtaining different levels of fiber orientation. Three-point bending tests are performed to measure and compare the residual tensile strength capacities with standard specimens cast according to EN 14651. Elements with favorable fiber orientation show a significant increase in residual tensile strength with respect to the standard beams. Finally, computed tomography and an electromagnetic induction method are employed to better assess the orientation and distribution of fibers in the beams. Their results are in good agreement and enable to link the residual tensile strength parameters with fiber orientation.