Jun Qian

EDM texturing of multicrystalline silicon wafer and EFG ribbon for solar cell application

Abstract This paper presents a novel electrical discharge machining (EDM) texturing method for roughening mc-Si wafers and EFG ribbons for solar cell application. Experiments were carried out on an EDM die-sinker using a specially designed conductive and soft magnetic brush to texture the workpiece. The textured substrates were investigated and analysed using scanning electron microscope, and solar cells were made on textured samples to evaluate the effect of this method. Preliminary experimental results show that the throughput of this method can be over 1000 mm2 minute with a brush of 100 mm diameter. Solar cells made on textured substrates give reasonable output.

Fabrication of Mesoscale Channel by Scanning Micro Electrochemical Flow Cell (SMEFC)

A unique micro electrochemical machining (ECM) method based on a scanning micro electrochemical flow cell (SMEFC), in which the electrolyte is confined beneath the tool electrode instead of spreading on the workpiece surface, has been developed and its feasibility for fabricating mesoscale channels has been investigated. The effects of the surface conditions, the applied current, the feed rate, the concentration of the electrolyte and several geometrical parameters on the machining performance have been investigated through a series of experiments. The cross-sectional profile of the channels, the roughness of the channel bottom, the width and depth of the channel, the microstructures on the machined surface and the morphologies of the moving droplet have been analyzed and compared under different machining conditions. Furthermore, experiments with different overlaps of the electrolyte droplet traces have also been conducted, in which the SMEFC acts as a “milling tool”. The influences of the electrode offset distance (EOD), the current and the feed rate on the machining performance have also been examined through the comparison of the corresponding cross-sectional profiles and microstructures. The results indicate that, in addition to machining individual channels, the SMEFC system is also capable of generating shallow cavities with a suitable superimposed motion of the tool electrode.

A holistic approach for zero-defect micro-EDM milling

Micro-EDM (μEDM) milling is a well-established micro-manufacturing technique, which offers three-dimensional and flexible machining capabilities for structuring electrically conductive difficult-to-machine materials. There has been considerable progress in micro-manufacturing capabilities by μEDM in recent years. Nevertheless there still exist a few challenges in terms of process accuracy and efficiency, and the achievable shape accuracy in micro-EDM milling is limited to 3-4 microns in vertical direction and 2-3 microns in plane. In order to reach zero-defect manufacturing with this micro-sparking technique, a holistic approach for precision micro-EDM milling is pursued at Leuven University. To improve the overall performance of the micro-EDM milling process, various upgrading has been carried out on a SARIX® machine, which includes monitoring and controlling of the stability of the sparking process (gap variation and energy distribution etc.), wear compensation of the tool-electrode, and on-machine metrology. Preliminary experiments have been carried out with promising results, and further system integration and application on industrial demonstrators are in progress.

The electrical behaviour of the dressing process in ELID-grinding

During ELID-grinding, the metallic wheel is kept sharp through an electrolytic dressing process in which a layer is grown on top of the wheel’s surface. The electrolysis process can be modelled electrically using the Helmholtz representation of a boundary layer. Measurements show that at the start of the (pre-)dressing the capacitance of the system is at its highest value, above 10 µF. After a while, it drops significantly to a level below 1 µF. This signifies a change in the metal-electrolyte interface and the grinding wheel is covered with a layer of (hydr-)oxides. The model is refined in two ways. Firstly, the capacitance between the two metallic electrodes is added. Secondly, the self-inductance of the power cables is included. With the proposed model it is possible to properly simulate the electrical behaviour of the growth of the layer. It also provides a fundamental insight into the electrolytic dressing process.

Reproducibility of a Nanometre Accurate Moving-Scale Measurement System

A dedicated experimental setup has been designed and built up for determining the reproducibility of a 1-DOF moving-scale measurement system. A 3-DOF moving-scale system would enable the measurement of the position of a workpiece table moving in three perpendicular directions with minimized Abbe-offset. This allows normal machine tools and CMMs to match or even surpass the accuracy of ultra-precision machines. An uncertainty budget indicates that the 1-DOF moving-scale system can obtain a measurement uncertainty of 18 nm. Thermal drift is the largest contribution to the measurement uncertainty and should be accurately determined.

Micro-Milling of Fully Sintered ZrO2 Ceramics with Diamond Coated End Mills

This work presents an experimental investigation on micro-milling of fully sintered Zirconia (ZrO2) with CVD diamond coated end mills. The experiments were conducted on a Kern MMP 2522 micro-milling centre. Diamond coated WC micro end mills with stiff geometry, are employed. Effects of cutting parameters on surface quality were investigated by means of a mixed full factorial experimental design. Surface roughness was measured by a Talysurf 120L profilometer. Surface topography was examined using a scanning electron microscope. The results show that a smaller feed rate was apt to result lower surface roughness. With the increase of cutting length, brittle damages were observed in some tests.

Overview of Hybrid Machining Processes

This chapter gives an overview of the different types of hybrid machining processes. It starts by introducing the broad classification of the hybrid machining processes. It then proposes the design principle and introduces design methodology for hybrid machining processes. Major hybrid machining processes, i.e. assisted, combined, and controlled application of process mechanisms are reviewed in detail, including working principles, benefits, and limitations. By the end of the chapter the research focus for industrial application is pointed out.

Experimental Investigation on Ductile Mode Micro-Milling of ZrO2 Ceramics with Diamond-Coated End Mills

ZrO2 ceramics are currently used in a broad range of industrial applications. However, the machining of post-sintered ZrO2 ceramic is a difficult task, due to its high hardness and brittleness. In this study, micro-milling of ZrO2 with two kinds of diamond-coated end mills has been conducted on a Kern MMP 2522 micro-milling center (Kern Microtechnik GmbH, Eschenlohe, Germany). To achieve a ductile mode machining of ZrO2, the feed per tooth and depth of cut was set in the range of a few micrometers. Cutting force and machined surface roughness have been measured by a Kistler MiniDynamometer (Kistler Group, Winterthur, Switzerland) and a Talysurf 120 L profilometer (Taylor Hobson Ltd., Leicester, UK), respectively. Machined surface topography and tool wear have been examined under SEM. Experiment results show that the material can be removed in ductile mode, and mirror quality surface with Ra low as 0.02 μm can be achieved. Curled and smooth chips have been collected and observed. The axial cutting force Fz is always bigger than Fx and Fy, and presents a rising trend with increasing of milling length. Tool wear includes delamination of diamond coating and wear of tungsten carbide substrate. Without the protection of diamond coating, the tungsten carbide substrate was worn out quickly, resulting a change of tool tip geometry.

Tool Wear Characters in Micro-Milling of Fully Sintered ZrO2 Ceramics by Diamond Coated End Mills

This work presents an experimental investigation on micro-milling of fully sintered Zirconia (ZrO2) by diamond coated tools. The experiments were conducted on a Kern MMP 2522 micro-milling centre and WC micro end mills, diamond coated by chemical vapour deposition (CVD) and of stiff geometry were employed as cutting tools. The effects of cutting parameters and milling time on tool wear were investigated. The results revealed that the tool wear characters included diamond coating delamination and wear of substrate WC. Both cutting forces and machined surface quality were affected by tool wear with the progress of milling.