Khalid Rahman

Drop-on-Demand Direct Printing of Colloidal Copper Nanoparticles by Electrohydrodynamic Atomization

Electrohydrodynamic inkjet printing technology has recently become attractive in many industrial fabrication fields mainly due to its advantage of smaller drop generation as compared to the diameter of the delivery nozzle. In this article, drop-on-demand (DoD) printing of colloidal solution containing copper nanoparticles through electrohydrodynamic atomization (EHDA) is investigated. By applying a novel forward multistep waveform (FMSW) superimposed on dc biased voltage, charged fluid drops containing copper nanoparticles are deposited on glass substrate. The main focus of this study is to generate uniform droplets of conductive copper ink in a controlled fashion by varying amplitude and frequency of the applied waveform. The deposited patterns show a series of uniform-sized drops with regular spacing. Using this DoD printing technique, it is feasible to print a variety of patterns of dots and conductive continuous or discontinuous lines.

Fine-resolution patterning of copper nanoparticles through electrohydrodynamic jet printing

This paper presents the low-cost, fine-resolution printing of conductive copper patterns on silicon substrate. The colloidal solution containing copper nanoparticles is deposited through electrohydrodynamic printing technology. Conductive copper tracks of different width are printed by varying the operating conditions (applied voltage and flow rate) and controlling the jet diameter. The minimum pattern width achieved was approximately 12 ?m with the average thickness of 82 nm across the width after the sintering process. The achieved pattern width is five times smaller than the capillary used for patterning. The morphology and purity of the printed copper tracks were analyzed through scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray diffraction (XRD). The current?voltage (I?V) characteristic of the printed copper tracks showed linear Ohmic behavior and exhibited resistivity ranging from 5.98???10?8?? m?1?to 2.42???10?7?? m?1.

Electrode configuration effects on the electrification and voltage variation in an electrostatic inkjet printing head

The electrode configuration of an electrostatic inkjet printing head is under study. This paper introduces the development of a new electrostatic inkjet head with an improved electrode configuration as compared to the conventional configuration. Two tungsten electrodes, connected in parallel, are inserted into the electrostatic print head at a certain angle from opposite sides. The aim of this double-side inserted angular electrodes (DSIAEs) head is to intensify the electrification of the fluid inside the head at minimum suitable exposure of the electrode, which results in maximizing surface charge density. The main advantage of the DSIAEs head is to get a very stable meniscus at low applied voltage for printing. This stable meniscus is transformed to a very stable jet by increasing the applied voltage. Therefore, printed patterns obtained with this DSIAEs head are more uniform because of a more stable meniscus and jet as compared to a conventional electrostatic vertically inserted single electrode head. Also, with this DSIAEs configuration, the life of the electrostatic inkjet printing head is increased.

Direct Fabrication of Copper Nanoparticle Patterns through Electrohydrodynamic Printing in Cone-Jet Mode

Recently, there has been growing interest in direct patterning of conductive nanoparticles on various substrates without utilizing complex micro-fabrication lithography process. The direct fabrication process using electrohydrodynamic printing is expected to be a powerful tool of application for direct patterning in the field of printed electronics. The key advantage of printing through electrohydrodynamic jetting in stable cone-jet mode is achievement of high resolution patterns. This article presents the direct patterning of the copper nanoparticles through electrohydrodynamic printing in cone-jet mode. The patterning was performed on glass substrates at different flow-rates by applying the highest value of applied voltage in stable-cone jet operating envelop. The minimum pattern width achieved with electrohydrodynamic printing at flow-rate of 20 µl/hr and applied DC voltage of 2.6 kV is 32 µm with 2 µm standard deviation. The pattern width achieved through electrohydrodynamic printing is 3.125 times smaller than the nozzle diameter.

Electrohydrodynamic Inkjet – Micro Pattern Fabrication for Printed Electronics Applications

In electronic industry the manufacturing of conductive patterning is necessary and ineluctable. Traditionally, lithography is widely used for fabrication of the conductive patterns. However, lithographic processes require the complicated equipments, are time consuming and the area throughput is limited. In order to reduce the material usage, process time and large area fabrication, different fabrication technique is required. Nonlithographic-direct fabrication method (Pique & Chrisey, 2001) such as inkjet (Gans et al., 2004) and roll-to-roll (Gamota et al., 2004) printing (also known as printed electronics) are predominant examples for reasonable resolution and high throughput as compared to lithography techniques. This direct fabrication technology can be further classified into two different technologies depending on the fabrication method as contact (gravure, offset or flexographic etc) and non-contact (inkjet) method. Non-contact inkjet printing method has moved beyond graphic printing as a versatile manufacturing method for functional and structural materials. Commercially available inkjet printer can be divided into two modes based on the ejection of the fluid: Continuous, where jet emerges from the nozzle which breaks in stream of droplets or Drop-on-Demand, the droplet ejects from the nozzle orifice as required (Lee, 2002). Inkjet printing offers the advantages of low cost, large area throughput and high speed processing. The most prominent examples of inkjet printing includes the direct patterning of, printed circuit board, conductive tracks for antenna of radio frequency identification tags (RFID) (Yang et al., 2007), Photovoltaic (Jung et al., 2010), thin film transistors (Arias et al., 2004), micro arrays of the DNA (Goldmann & Gonzalez, 2000), biosensors, etc. In case of continuous inkjet printing, the deflector directs the stream of droplets into a waste collector or onto substrate, for start and stop of the printing. This wastage of the ink issue has been addressed by the introduction drop-on-demand inkjet printing (thermal and piezoelectric). In drop-on-demand, thermal or vibration pulse are used to eject the liquid droplet from the nozzle to the substrate. However, the current printing technologies have constrained due to limitation of the ink viscosity, clogging of small size nozzles, generation of pattern smaller than the nozzle size and limitation of material to be deposited (Le, 1998). In order address these limitations, many researchers are focusing on electrohydrodynamic inkjet printing (continuous and drop-on-demand) (Park et al., 2007). Electrohydrodynamic jet printing uses electric field energy to eject the liquid from

A Study of the Dependence of Electrohydrodynamic Jetting on the Process Parameters and Liquid Physical Properties

The role of the applied voltage and liquid physical properties in the stable cone jet formation has been investigated in this paper. A higher pulse voltage of large duty cycles superimposed on a high bias voltage produced a stable cone jet under constant applied pressure. The critical voltage required for the formation of the stable cone jet decreased with an increase in the applied pressure for solutions having different conductivities. As compared to low viscous and high conductive solutions, high viscosity liquids with low conductivity were unable to form a stable cone jet easily at higher frequencies and low pulse time. The experimental results indicate that there is a strong relationship among these parameters, and a precise control of these parameters is required to achieve high quality electrohydrodynamic (EHD) jetting. Also five dimensionless parameters have been derived using the Buckingham II theorem, and their effect on EHD jetting has been analyzed thoroughly.

Influence of Electrode Position and Electrostatic Forces on the Generation of Meniscus in Dielectric Ink

This paper analyzes and reports the influence of electrodes positioning on dielectric ink for the formation of stable meniscus in the electrostatic inkjet head, experimentally. To introduce the electrostatic inkjet head for the fabrication of different printed devices, it is important to know the best position of electrode in the inkjet head for the optimal electrification of the ink. The paper also examines electrode position and electrically driven liquid handling operations. From experiments, it has been deduced that the head performs best when the electrode position is at orifice in dielectric ink. For better understanding and evaluation of this phenomenon, simulations were also performed. The finite element model of the head with dielectric fluid has been developed to analyze the behavior under electric field. And to optimize the analysis, the research is also supported by the statistical techniques. Furthermore, the meniscus shape and response on applied voltage is studied and analyzed. Paper also elaborates and addresses issues like the meniscus generation using dielectric ink and ejections at low voltage with stability problems.

Effects of process parameters on cross-talk in triangular array multi-nozzle EHD printing head

In this paper, effect of cross-talk between the nozzles in triangular array multi-nozzle for electrohydrodynamic inkjet printing head is investigated on the basis of jetting angle i.e. deflection of meniscus at the boundary nozzles. By changing the process parameter i.e. flow rate of the liquid, applied voltage and space between the capillary nozzles; jetting angle at end capillaries are measured and analyzed. Experimental results show that, by keeping applied voltage constant and increasing the flow rate the jetting angle decreases. While keeping flow rate constant and increasing the applied voltage jetting angle increases. Moreover jetting angle increases with the decrease in space between the nozzles. To better evaluate this technique, electric field simulations are performed and compared with the experimental results. We observed a strong correlation between the simulated and experimental results.

Effects of thermal material properties on precision of transient temperatures in pulsed laser welding of Ti6Al4V alloy

This study investigates the effect of temperature-dependent material properties on the precision of a simulation in pulsed laser beam welding of Ti6Al4V alloy. Ti6Al4V is one of the most extensively used titanium alloys. The precision in transient temperature distributions developed in the thermal modeling part of a sequentially coupled thermo-mechanical simulation is crucial to the end results of structural mechanics. The temperature profile obtained by a finite element model at two distinct locations is validated by experimental results using temperature-dependent material properties. Then, the effect of assuming constant room temperature values for thermal conductivity, specific heat, and density on the temperature distribution is studied at different welding speeds. Temperature distributions are unaffected by the constant density assumption. The constant thermal conductivity assumption underestimates the peak temperatures far from the weld region, whereas the constant specific heat assumption overestimates these temperatures. This effect becomes prominent at low welding speeds. The temperature profile when conductivity and specific heat are assumed to be constant is nearly similar to that in the case of constant conductivity when conductivity and specific heat are assumed constant. Therefore, conductivity is the dominant variable. The constant conductivity assumption also restricts the heat flow from the weld to the edge region, thus increasing the size of the weld pool. This effect also becomes increasingly prominent at low welding speeds.

Comparison of microstructure, mechanical properties, and residual stresses in tungsten inert gas, laser, and electron beam welding of Ti–5Al–2.5Sn titanium alloy:

Electron beam welding (EBW), pulsed Nd:YAG laser beam welding (P-LBW), and pulsed tungsten inert gas (P-TIG) welding of Ti–5Al–2.5Sn alloy were performed in order to prepare full penetration weldments. Owing to relatively high power density of EBW and LBW, the fusion zone width of EBW weldment was approximately equal to P-LBW weldment. The absence of shielding gas due to vacuum environment in EBW was beneficial to the joint quality (low oxide contents). However, less cooling rates were achieved compared to P-LBW as an increase in heat-affected zone width and partial α′ martensitic transformation in fusion zone were observed in EBW weldments. The microstructure in fusion zone in both the EBW and P-TIG weldments comprised of both acicular α and α′ martensite within the prior β grains. Hardness of the fusion zone in EBW was higher than the fusion zone of P-TIG but less than the fusion zone of P-LBW weldments due to the observed microstructural differences. Notch tensile specimen of P-LBW showed higher load capacity, ductility and absorbed energy as compared to P-TIG and EBW specimens due to the presence of high strength α′ martensite phase. Maximum sheet distortions and tensile residual stresses were observed in P-TIG weldments due to high overall heat input. The lowest residual stresses were found in P-LBW weldments, which were tensile in nature. This was owing to high power density and higher cooling rates in P-LBW operation. EBW weldment exhibited the highest compressive residual stresses due to which the service life of EBW weldment is expected to improve.