acids and carboxylic acid derivatives in collaboration with the research group of Pro- .... Heterogeneous catalytic hydrogenation of carboxylic acids and.
Master’s Thesis
Design, Fabrication and Testing of a High-Temperature and High-Pressure Micro Batch Reactor N
O N
Christopher Vega Sanchez
A master’s thesis submitted in partial fulfillment of the requirements for the degree of Master of Science of Microsystems Engineering according to the examination regulations at the University of Freiburg for the Master’s degree in Microsystems Engineering of 2009. Laboratory for Design of Microsystems Department of Microsystems Engineering (IMTEK) University of Freiburg Freiburg im Breisgau, Germany
Author
Christopher Vega Sanchez
Thesis period
April 24, 2013 to November 28, 2013
Referees
Prof. Dr.-Ing. Peter Woias, Laboratory for Design of Microsystems Prof. Dr. Holger Reinecke, Laboratory for Process Technology
Supervisor
Dr. Keith Cobry, Laboratory for Design of Microsystems
Title page
From left to right, the figure shows a micro batch reactor in operation, the first stage silicon/glass micro batch reactor and the packaging device.
Declaration
according to the Examination Regulations: I hereby confirm to have written the following thesis on my own, not having used any other sources or resources than those listed. All passages taken over literally or correspondingly from published sources have been marked accordingly. Additionally, this thesis has not been prepared or submitted for another examination, neither partially nor completely. Freiburg, November 21, 2013
Christopher Vega Sanchez
Abstract We present a high-pressure and high-temperature microreactor concept –micro batch reactor with thin silicon membranes as components of an integrated piezoelectricallydriven internal peristaltic pump system–, which provided with the appropriate modular interface packaging, is able to operate, at least, under 49 bar and 120 °C. The development of this project is carried out under the framework of the project Active microfluidic platforms for a novel heterogeneous catalytic hydrogenation of carboxylic acids and carboxylic acid derivatives in collaboration with the research group of Professor Bernhard Breit (Laboratory for Organic Chemistry, University of Freiburg). Several silicon/glass microreactors with KOH etched channels and 50-100 µmthick silicon membranes were tested under applied chamber pressure and short thermal cycling. The silicon membranes are expected to be piezo-electrically driven in future work in order to provide a controllable active mixing. Destructive pressure experiments with nitrogen showed that the microreactor itself is able to withstand pressures up to 4 bar due to the low strength of the silicon membranes. Here we introduce a novel packaging concept which allows the operation of the micro batch reactor with thin silicon membranes under conditions of high pressure and high temperature. This new concept consists in an aluminum housing which uses O-rings to form a compression seal against the microreactor surface, thus establishing an adhesive-free macro-to-micro interconnection. The aluminum housing is equipped with an inner chamber where the silicon/glass microreactor is enclosed in a pressurized atmosphere of nitrogen (or any other fluid). As a result, the differential pressure across the walls of the microreactor is zero. Tests results show that the fluidic connections can be established and removed numerous times without damaging the microreactor. Under this scheme, we proved that the micro batch reactor with thin silicon membranes is able to operate under pressures up to 49 bar and temperatures of 120 °C without problems/failure of the chip or the fluidic connections. However, we believe that higher pressures and temperatures can be reached using the same concept. Thus, we conclude that by using an appropriated packaging, microreactors with low mechanical strength are able to operate under harsh conditions of pressures and temperatures. Keywords: microreactor, high temperature, high pressure, microfluidic packaging, KOH etching, micropumps, micromixers.
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Zusammenfassung In dieser Arbeit wird ein Mikro-Batchreaktorkonzept für den Betrieb bei hohem Druck und hoher Temperatur (bis 49 bar, 120 °C) präsentiert. Der Reaktor enthält Silizium-Membranenum die Integration eines internen peristaltischen Pumpsystems zu ermöglichen, welches durch Piezoaktorenantreibbar ist. Diese Arbeit wurde im Rahmen des Projektes Aktive mikrofluidische Plattform für eine neuartige, heterogene, katalytische Hydrierung von Carbonsäuren und Carbonsäurederivaten in Zusammenarbeit mit dem AK Prof. Berhard Breit, Lehrstuhl für Organische Chemie, Universität Freiburg durchgeführt. Mehrere Silizium/Glas Mikroreaktoren mit KOH-geätzten Kanälen sowie SiliziumMembranen (Stärke 50-100 µm) wurdensowohl unter angelegtem Kammerdruck als auch unter thermischer Wechselbeanspruchung getestet. Die Silizium-Membranen sind für eine künftige Integration von piezoelektrisch-getriebenen Pumpaktoren zur steuerbaren internen Zirkulation und Mischung der Reagenzien vorgesehen. Druckversuche mit Stickstoff haben gezeigt, dass die Mikroreaktoren selbst Drücken bis 4 bar bar standhalten können, wobei die Silizium-Membranendie konstruktiven Schwachstellen darstellen. Ein neuartiges Aufbau- und Verbindungskonzept wird gezeigt, welchesden Betrieb der Mikroreaktoren mit dünnen Membranenbei gleichzeitig hohen Drücken und Temperaturen ermöglicht. Dieses neue Designprinzip verwendet ein Kompressionsgehäuse aus Aluminium, bei dem O-Ringe die Makro-/Mikro-Anschlüsse ohne Kleber bilden. Das Aluminiumgehäuse ist mit einer internen Druckkammer ausgestattet, in welcherder Glas/Silizium-Mikroreaktor unter Hochdruck- Stickstoffatmosphäreeingebaut wird. Andere Fluide anstelle des Stickstoffs könnten ebenso verwendet werden. Durch dieses Aufbaukonzept kann der Druckabfall über die Mikroreaktorwände beseitigt werden. Testergebnisse zeigen, dass die Mikroreaktoren mehrfach fluidisch kontaktiert werden können ohne diese zu beschädigen. Durch den Einbau der Mikroreaktoren in eine Druckkammer konnten diese erfolgreich bei Drücken bis zu 49 bar sowie Temperaturen bis zu 120 °C ohne Versagen der Reaktorchips oder der fluidischen Anschlüsse betrieben werden. Es kann angenommen werden, dass die Mikroreaktoren unter Verwendung des neuen Aufbauprinzips auch bei noch höheren Drücken und Temperaturen betrieben werden können. Es konnte gezeigt werden, dass mit dem beschriebenen Einbau der Mikroreaktoren in eine Druckkammer auch Reaktoren mit niedriger mechanischer Stärke unter harschen Bedingungen betrieben werden können. Stichwörter: Mikroreaktor, Hochdruck, Hochtemperatur, mikrofluidische Aufbauund Verbindungstechnik, Mikropumpen, Mikromischer.
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Dedicated to my aunt Flor Vega, for her endless support.
Acknowledgements I would like to express my gratitude to my supervisor, Dr. Keith Cobry, who came with this novel idea of designing a Micro Batch Chemical Reactor and gave me the opportunity to get involved in this interesting project. I sincerely appreciate all his expertise, professional guidance and patience throughout this master’s thesis. I would like to thank Prof. Peter Woias, head of the Laboratory for Design of Microsystems, who is the final responsible for this thesis and who trusted in my capabilities making me part of his department. Also, I appreciate the help of Prof. Holger Reinecke who gave me the honor to be part of the examiner committee of this work. A very special thanks goes to M.Sc. Reinhard Roth, for his continuous help and kindness along all this time. Thanks for showing interest in my work, for giving me well-founded technical support and for spending long hours in the cleanroom helping me with the project without expecting anything in return. I must also acknowledge the RSC Service Center at IMTEK and its technical personnel, for offering the required facilities to manufacture the Micro Batch Reactor. Specially I would like to mention the invaluable contribution of Dipl.-Ing. Michael Reichel, who, without having any responsibility in this project, became my technical adviser and supervisor for the whole fabrication process. Thanks for his advice, support and, particularly, patience answering my continuous questions and guiding me through the different fabrication techniques. It is a pleasure to thank the German Academic Exchange Service DAAD and Tecnológico de Costa Rica TEC that have provided me with the scholarship for the whole period of study. Also, I recognize that this research would not have been possible without the financial assistance of the Laboratory for Design of Microsystems at IMTEK, which has provisioned the required materials for the fabrication of the chips and has offered me a well-equipped lab station for running the testing experiments. I express my sincere gratitude to those organizations. I thank my family for the support they provided me through my entire life, in particular to my parents, Carmen Sánchez and Marco Vega, for making me a person with defined personal values. To my aunt, Flor Vega, for being an especial source of motivation and self-confidence and who has always encouraged me to study abroad and broaden my horizons. I would like to thank Olga Pimenova for her personal support, encouragement and great patience through the continuous revision process of my final work. Finally, but by no means least, I thank deeply my friends who, during these last years, brought funny and happy moments to my life even when the atmosphere around our studies was a little bit cloudy. Specially, I would like to mention here
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several Latin names: Miguel Ulloa, Saraí Torres and Fralett Suarez. You, parce and comadres, have been those connection lines to my roots, sometimes in a very special way, to such point that once, I felt you as an extension of me when touching a majestic tree. I hope the seed of my naturalistic religion is still growing inside of you. Thanks for giving me the chance to taste something else than just Kartoffelsalat. I treasure especially the pozole, the colombian empanadas de arroz con pollo, and the tacos al pastor. Thanks for sharing with me not only good times, but for supporting me when I did not feel very optimistic. Thanks for making me feel at home even when I am at thousands of kilometers from it. In one way or another, thanks to all those, people and institutions, who helped me to achieve my personal goal and complete the master´s thesis, which fills me with a great personal and professional satisfaction.
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Contents 1. Introduction 1.1. What is microreaction technology? . . . . . . . . . . . . . . . . . . 1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1. Heterogeneous catalytic hydrogenation of carboxylic acids and carboxylic acid derivatives . . . . . . . . . . . . . . . . . . . 1.2.2. High-pressure high-temperature microreactors . . . . . . . . 1.2.3. Why silicon microtechnology? . . . . . . . . . . . . . . . . . 1.3. Scope of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1. Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2. Structure of this work . . . . . . . . . . . . . . . . . . . . . 2. Literature review 2.1. Microreactors . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Advantages due to physical size decrease . . . . . . 2.1.2. Advantages due to numbering-up . . . . . . . . . . 2.1.3. Advantages regarding applications . . . . . . . . . . 2.2. Micropumps . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Micromixers . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Packaging of high-pressure high-temperature microreactors 2.4.1. Integrated interconnectors . . . . . . . . . . . . . . 2.4.2. Modular interface . . . . . . . . . . . . . . . . . . . 3. Micro batch reactor design 3.1. Micro batch reactor concept . . . . . . . . . . . 3.2. Micro batch reactor design . . . . . . . . . . . . 3.2.1. The inlet and outlet channels . . . . . . 3.2.2. The reaction chamber with active mixing 3.3. Chip layout . . . . . . . . . . . . . . . . . . . . 3.3.1. Micro batch reactor layout . . . . . . . . 3.3.2. Pressure samples layout . . . . . . . . . 3.4. Simulation . . . . . . . . . . . . . . . . . . . . . 3.4.1. Boundary conditions . . . . . . . . . . . 3.4.2. Simulation results . . . . . . . . . . . . .
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Contents 4. Fabrication 4.1. Fabrication process concepts . . . . . . . . . . . . . . 4.1.1. Anodic bonding of silicon/glass substrates . . 4.1.2. Anisotropic KOH etching . . . . . . . . . . . 4.2. Process overview . . . . . . . . . . . . . . . . . . . . 4.2.1. Microreactor without silicon membranes . . . 4.2.2. Microreactor with silicon membranes . . . . . 4.3. Fabrication setup . . . . . . . . . . . . . . . . . . . . 4.3.1. KOH etch setup . . . . . . . . . . . . . . . . . 4.3.2. KOH protective wafer holder . . . . . . . . . . 4.4. KOH etching characterization . . . . . . . . . . . . . 4.4.1. Experimental techniques . . . . . . . . . . . . 4.4.2. Etch rate versus concentration . . . . . . . . . 4.4.3. Roughness and surface quality . . . . . . . . . 4.4.4. Uniformity vs concentration . . . . . . . . . . 4.4.5. Undercutting compensation of convex corners 4.5. Fabricated micro batch reactor . . . . . . . . . . . . . 5. Packaging 5.1. Fluidic interconnect . . . . . . . . . . . . . . . 5.1.1. Design Requirements . . . . . . . . . . 5.1.2. Packaging design 1 . . . . . . . . . . . 5.1.3. Packaging design 2 . . . . . . . . . . . 5.1.4. Packaging design 3 . . . . . . . . . . . 5.2. Electrical interconnect . . . . . . . . . . . . . 5.3. Influence of the packaging on the microreactor 5.3.1. Boundary conditions . . . . . . . . . . 5.3.2. Simulation results . . . . . . . . . . . .
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6. Test of high-pressure high-temperature microreactor assembly 6.1. Silicon/glass anodic bonding strength evaluation . . . . . . . . . . 6.1.1. Blister test setup . . . . . . . . . . . . . . . . . . . . . . . 6.1.2. Blister test results . . . . . . . . . . . . . . . . . . . . . . 6.2. Pressure test of microreactors . . . . . . . . . . . . . . . . . . . . 6.2.1. Packaging design 1 . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Packaging design 2 . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Packaging design 3 . . . . . . . . . . . . . . . . . . . . . . 6.2.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Thermal cycling test . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1. Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2. Evaluation technique . . . . . . . . . . . . . . . . . . . . . 6.3.3. Pressure tests results on previously thermally cycled chips 6.4. Pressure test of microreactors at elevated temperature . . . . . . . 6.4.1. Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Nomenclature Latin characters Variable
Meaning
Unit
a D Dh E G h H l L n P R Ra t U v V w x x, y, z
compensation structure size diameter hydraulic diameter Young’s modulus geometrical correction factor height etching depth length cavity length integer number pressure radius arithmetic average height time voltage velocity etch rate width diffusion length cartesian coordinates
m m m Pa – m m m m – Pa m m s V m/s m/s m m m
Greek characters Variable
Meaning
Unit
∆ η γ ν ρ ξ
difference dynamic viscosity surface energy Poisson’s ratio density pressure loss coefficient
– Ns/m2 J/m2
– kg/m3
–
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Nomenclature
Abbrevations Abbrevation
Meaning
De DRIE HF In ICP-RIE KOH LPCVD MEMS OD Out PMMA QDR Re RIE Si
Dean number Deep Reactive Ion Etching Hydrofluoric acid Inlet Inductively Coupled Plasma Reactive Ion Etching Potassium hydroxide Low Pressure Chemical Vapor Deposition Microelectromechanical system Outer Diameter Outlet Poly-methyl methacrylate Quick Dump Rinser Reynolds number Reactive Ion Etching Silicon
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List of Figures 1.1. Proposed work scheme of the project Active microfluidic platform for a novel heterogeneous catalytic hydrogenation of carboxamides. This master’s thesis consists of the two first stages of the project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Work scheme of this master’s thesis. Notice the iterative process between testing and design in order to reach the goals. . . . . . . . . . . . . . . . . . . . . . . . . .
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2.1. Production schemes for conventional reactors and microreactors [23]. . . . . . . . . 2.2. Simplified schemes of the four most used actuation principles in the fabrication of micropumps: (a) piezo electric, (b) thermopneumatic, (c) electrostatic and (d) electromagetic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Valveless micro diaphragm pump with piezoelectric actuation [53]: schematic diagram (top) and pumping principle (bottom). . . . . . . . . . . . . . . . . . . . . . 2.4. Peristaltic pump presented by Smits in 1990 [50]. . . . . . . . . . . . . . . . . . . . 2.5. Pumping sequence of the peristaltic micropump presented by Smits [50]. . . . . . . 2.6. Electrostatically driven peristaltic pump presented by Lee et al., 2013 [50]. . . . . . 2.7. Classification scheme of micromixers [42]. . . . . . . . . . . . . . . . . . . . . . . . 2.8. PMMA parallel lamination micromixer presented by Erbacher [15]. . . . . . . . . . 2.9. Formation of Dean vortices on the hydrodynamic process presented by Mao [37]. . 2.10. Active mixers: (a) serial segmentation, (b) pressure disturbance along the mixing channel, (c) integrated microstirrer in the mixing channel, (d) electrohydrodynamic disturbance, (e) dielectrophoretic disturbance, (f) electrokinetic disturbance in the mixing chamber and (g) electrokinetic disturbance in the mixing channel. [42]. . . 2.11. Major steps involved in creating the microfluidic connection presented by Pattekar [45]. 2.12. (a) Glass microreactor (1) inlet CO2 ; (2) inlet other reactant(s); (3) reaction zone; (4) fluidic resistor; (5) expansion zone; (6) outlet. (b) Glass chip to examine maximum working pressure: (A) inlet/outlet geometry; (B) transition area towards microchannel; (C) microchannel; (D) glue front (meniscus) [59]. . . . . . . . . . . . 2.13. Cross-sectional views of five different in-plane inlet/outlet geometries [59]. . . . . . 2.14. (a) Photo of the socket. (b) Schematic drawings of the cross-section of the socket mounted with a fluidic chip [67]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.15. Scheme of the interconnection concept developed by Bhagat et al. [5]. . . . . . . . 2.16. High-pressure experimental setup, including a high-pressure and high-temperature microreactor, a compression-cooling aluminum interface, two high-pressure syringe pumps, and a back-pressure regulator [38]. . . . . . . . . . . . . . . . . . . . . . . . 3.1. Goal of the new micro reactor concept. . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Micro batch reactor concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Inlet channel. Injection of reactants through the nozzles. The catalyst present in the liquid phase is expected to disolve partially the gaseous reagent. . . . . . . . . 3.4. Reaction chamber equipped with piezo-electrically driven diaphragm . . . . . . . . 3.5. Final chip layout using KOH etching process for the fabrication of microchannels, inlets and membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Micro batch reactor chip dimensions. All dimensions are given in milimeters. . . . 3.7. Samples for the analysis of the mechanical limitations of the silicon and Pyrex® and, as well, the anodic bonding strength. All dimensions are given in millimeters. . . .
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List of Figures 3.8. Mechanical constraints taken into account for the simulation. . . . . . . . . . . . . 3.9. (a) Maximum von Mises stress simulated for different channel depths and chamber pressures between 10 and 40 bar. (b) Common von Mises stress distribution for the chip. The glass cover has been removed in order to show the inner chamber. Notice that the maximum stresses are found at the edges. . . . . . . . . . . . . . . . . . . 3.10. Maximum von Mises stress simulated for different silicon substrate thicknesses. Pyrex® thickness is 1100 µm. The inner chamber pressure is varied between 10 bar and 40 bar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11. (a) Stress distribution on the chip for applied pressure of 10 bar. (b) Maximum von Mises stress simulated for different sizes of the post structures. Pyrex® and silicon thicknesses are 1100 µm and 1000 µm, respectively. The inner chamber pressure varies between 10 bar and 40 bar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12. (a)Material deflection on the chip for applied pressure of 10 bar.(b) Maximum material displacement according to the size of the post structures. Pyrex® and silicon thicknesses are 1100 µm and 1000 µm, respectively. The inner chamber pressure is varied between 10 bar and 40 bar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13. Top view of the chip for different posts configurations . (a) Configuration I: Six posts, one single post across each of the main channels. (b) Configuration II: Twelve posts, two posts across each of the main channels. (c) Configuration III: Eight posts, 1-2-1 configuration along the chamber. (d) Configuration IV: Fourteen posts with a side length of 500 µm following a 2-3-2 configuration. . . . . . . . . . . . . . . . . . . . 3.14. Simulation results (a) maximum von Mises stress and (b) maximum displacement depending of the post configuration (see figure 3.13) . . . . . . . . . . . . . . . . . 3.15. Simulation results show a different stress distribution for the design with rounded convex corners. (a) Sharp corners. (b) Rounded corners with a radius of 100 µm. Notice that the color scales in (a) and (b) are matched in order to have a good comparative criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. The mechanism of the silicon-glass anodic bonding [31]. . . . . . . . . . . . . . . . 4.2. Schematic crosscut of a blister configuration [10]. . . . . . . . . . . . . . . . . . . . 4.3. Assumed crack propagation for (a) round and (b) squared blisters. (c) Real crack propagation described by Doll [10]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Observed fracture behavior of several tested samples (by Doll [10]). The surface energy of the bonded area increases from left to right. . . . . . . . . . . . . . . . . 4.5. (a) (100)-Plane, (b) (111)-plane, (c) (110)-plane of silicon in a Cartesian coordinate system. The model (d) results as a combination of (a), (b), (c) and the consideration of the four-fold symmetry of the silicon crystal[22]. . . . . . . . . . . . . . . . . . . 4.6. KOH etching of a (100) surface-orientated silicon wafer. . . . . . . . . . . . . . . . 4.7. Drawing of the etching analysis of the new corner compensation structure [16]. . . 4.8. Comparison of conventional and new design. (a) Conventional corner compensation structure with groove width W1 and overlap length L1 [39]. (b) New corner compensation structure with groove width W2 and overlap length L2 [16]. . . . . . 4.9. Schematic process flow for the fabrication of chips without membranes. . . . . . . 4.10. Two-step KOH etching process used in the fabrication of membranes. . . . . . . . 4.11. First approach scheme for the fabrication of chips with membranes. . . . . . . . . . 4.12. Second approach scheme for the fabrication of chips with membranes, using a wafer holder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13. Schematic setup of the KOH bath and its realization . . . . . . . . . . . . . . . . . 4.14. (a) Exploded view of the protective wafer holder with its parts. (b)Manufactured wafer holder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.15. Etch rate as a function of concentration at different temperatures for stirred KOH solutions (The graphs are shown separated in order to appreciate the curve slope).
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List of Figures 4.16. Surface quality as a function of concentration at different temperatures for stirred KOH solutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.17. Definition of arithmetic average roughness Ra [21]. . . . . . . . . . . . . . . . . . . 4.18. Surface morphology of two samples. (a) Sample etched with 20 % wt KOH solution at 90 °C. (b) Sample etched with 20 % wt KOH solution at 100 °C. . . . . . . . . . 4.19. 3D visualization of the surface morphology depending on the KOH concentration at 100 °C achieved by optical profilometry. . . . . . . . . . . . . . . . . . . . . . . . . 4.20. Surface topography for different concentrations and temperature. (a) 22 % wt KOH solution at 60 °C. (b) 22 % wt KOH solution at 75 °C. (c) 28 % wt KOH solution at 90 °C. Images taken with an optical microscope through a 10x objective. . . . . . . 4.21. Mask used for KOH etching of the posts with different sizes of compensation structures. 4.22. Generation of the post with different sizes of corner compensation structures. . . . 4.23. Change in the topography and roughness around the convex corners as a result of corner compensation structures introduced on the KOH mask. The leaf-like masking effect is shown at the bottom surface of the microchannel. Image taken with an optical microscope through a 4x objective. . . . . . . . . . . . . . . . . . . . . . . . 4.24. Reduction of the masking effect by adjusting the concentration and temperature from (a) 22 % wt KOH solution at 75 °C to (b) 16 % wt KOH solution at 90 °C. . . 4.25. Photograph of two microreactors with 12 and 8 post structures. . . . . . . . . . . . 4.26. Photograph of the rounded corners achieved by KOH etching undercutting of silicon. 4.27. Photograph of different section views of the microreactor. The (a) inlet/outlet channel, (b) post structures, (c) 102 µm-silicon membrane and (d) silicon island placed in the center of the reaction chamber, are shown. . . . . . . . . . . . . . . . 5.1. Scheme of the microfluidic packaging design. . . . . . . . . . . . . . . . . . . . . . 5.2. The section view shows the internal disposition of the parts and the details of the interconnection performed using NanoTight fittings. . . . . . . . . . . . . . . . . . 5.3. General dimensions of the first packaging design. . . . . . . . . . . . . . . . . . . . 5.4. Schematic illustration of the leakage test configuration. Notice that the silicon element does not have inlets in order to block the interconnection and evaluate the sealing performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Simulation results showing a uniform temperature distribution over the housing geometry. Maximum gradient of temperature is around 3 °C. . . . . . . . . . . . . 5.6. Back-side pressure approach to reduce the magnitude of the differential pressure across thin silicon membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7. Scheme of the second packaging design. . . . . . . . . . . . . . . . . . . . . . . . . 5.8. The section view shows the back side chamber and the sealing gasket. The interconnection is performed with NanoTight fittings. . . . . . . . . . . . . . . . . . . . 5.9. Scheme of the third packaging design. . . . . . . . . . . . . . . . . . . . . . . . . . 5.10. Section view of the internal configuration. . . . . . . . . . . . . . . . . . . . . . . . 5.11. Schematic configuration of the packaging of the electrical interconnection for operation at high pressures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.12. Comparison of (a) the housing without and (b) with internal illumination. (c) LED placed inside the housing (the lid was removed in order to show the internal chamber). (d) Close-up of the microreactor using the internal illumination. . . . . 5.13. Mechanical constraints taken into account for the simulation of the packaging designs. Additionally, for the packaging design 2 a back side pressure load was implemented. Also, for the packaging design 3, a pressure load was defined on all the external walls of the microreactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.14. Mechanical configuration, resulting force and stress according with the differential pressure across the material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
61 62 62 63
64 65 67
68 68 69 69
70 73 74 74
75 76 77 77 78 79 80 82
83
84 86
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List of Figures 5.15. Simulation results of the von Mises stress analysis for the silicon/Pyrex® microreactor for the three tested packaging designs. Note the differing color scales and the reduction of stress achieved along the three designs. . . . . . . . . . . . . . . . . . 6.1. Blister test results: mean burst pressure as a function of the channel width w. . . . 6.2. Fracture behavior of a sample with a channel with of 6 mm. Notice the round shape of the fracture near to the corners. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Top view of a post after the microreactor failure under pressure. Notice that, including the corners, the full top surface of the post is covered with glass. . . . . . . 6.4. Schematic illustration of the setup used for the pressure test carried out using the first housing design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Micro batch reactor chip dimensions. All dimensions are given in milimeters. . . . 6.6. Failure pressure as a function of the lateral size of the post for a chip fabricated with 525 µm-thick silicon wafer and 1100 µm-thick Pyrex® . Channel depth is 265 µm. Vertical bars show the standard deviations of the measurements. . . . . . . . . . . 6.7. Failure pressure as a function of the channel depth. The tested chip were fabricated with 525 µm-thick silicon wafer and 1100 µm-thick Pyrex® . . . . . . . . . . . . . . . 6.8. Failure pressure as a function of the substrate thickness. . . . . . . . . . . . . . . . 6.9. Schematic illustration of the setup using back side pressure to compensate the differential pressure across the membranes. . . . . . . . . . . . . . . . . . . . . . . . . 6.10. Failure pressure of a chip with membranes as a function of the size of the post. As a comparison, the dashed line shows the result obtained by using the first housing design under the same conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11. Scheme of the setup using a pressurized housing (packaging design 3). . . . . . . . 6.12. Photograph of the setup for pressure test using the packaging design 3. . . . . . . 6.13. Test procedure for evaluation of the micro reactor under high pressure conditions. The graph shows the real measurement of the chip 873-A1 realized during the pressure test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.14. Photograph of the setup for thermal cycling. . . . . . . . . . . . . . . . . . . . . . 6.15. Evaluation of the micro reactor under thermal cycling between 20 and 120 °C. The graph shows the real test of the chip 339-1000-CC. . . . . . . . . . . . . . . . . . . 6.16. (a) Photograph of the setup for the pressure test at elevated temperature. (b)LabView user interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.17. Measurements realized during (a) pressure test at 60 °C and 45 bar, (b) pressure test at 97 °C and 45 bar and (c) pressure cycling test between 0 and 45 bar at 97 °C. . . 6.18. Influence of the post structures at (a) high, (b) moderate and (c) low flow rate. (d) a comparison of the assumed velocity profile for each condition. . . . . . . . . . . . 6.19. Loading and unloading procedures of a suspension into the micro batch reactor. Stage 0 corresponds to the initial state (unfilled) and stage 5 to the completely filled chamber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87 90 90 92 93 93
94 95 95 96
97 98 99
101 103 104 106 107 108
110
7.1. Future work for the completion of the project Active microfluidic platforms for a novel heterogeneous catalytic hydrogenation of carboxylic acids and carboxylic acid derivatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 A.1. Masks used for the fabrication of micro batch reactor with different post sizes. (a) Front-side mask. (b) Back-side mask. . . . . . . . . . . . . . . . . . . . . . . . . . . 120 A.2. Masks used for the fabrication of micro batch reactor with different amount of post. (a) Front-side mask. (b) Back-side mask. . . . . . . . . . . . . . . . . . . . . . . . 121 A.3. Masks used for the fabrication of pressure testing samples. . . . . . . . . . . . . . . 122 B.1. Mechanical assembly scheme of the packaging design 1. . . . . . . . . . . . . . . . 123 B.2. Packaging design 1: Mechanical drawing 1 of 2. . . . . . . . . . . . . . . . . . . . . 124
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List of Figures B.3. Packaging design 1: Mechanical drawing 2 of 2. . . . . . B.4. Mechanical assembly scheme of the packaging design 2. B.5. Packaging design 2: Mechanical drawing 1 of 2. . . . . . B.6. Packaging design 2: Mechanical drawing 2 of 2. . . . . . B.7. Mechanical assembly scheme of the packaging design 3. B.8. Packaging design 3: Mechanical drawing 1 of 4. . . . . . B.9. Packaging design 3: Mechanical drawing 2 of 4. . . . . . B.10.Packaging design 3: Mechanical drawing 3 of 4. . . . . . B.11.Packaging design 3: Mechanical drawing 4 of 4. . . . . . B.12.Photograph of the pressure accumulator parts. . . . . . B.13.Pressure accumulator: Mechanical drawing 1 of 4. . . . B.14.Pressure accumulator: Mechanical drawing 2 of 4. . . . B.15.Pressure accumulator: Mechanical drawing 3 of 4. . . . B.16.Pressure accumulator: Mechanical drawing 4 of 4. . . .
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125 126 127 128 129 130 131 132 133 134 135 136 137 138
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List of Tables 1.1. Comparison for the direct nitration of aromates chemical reaction with dinitrogen pentoxide carried out in conventional and micro reactors [36]. . . . . . . . . . . . .
2
2.1. A list of macro-to-micro interfaces used in the literature [18]. . . . . . . . . . . . .
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4.1. Space efficiency of new design and conventional design [16]. . . . . . . . . . . . . .
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6.1. Tested chips under pressures between 46-49 bar by using the housing design 3. NA = not applicable. No damage = after pressure test no cracks or signs of damage were found. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Comparison of the pressure test results according with the packaging design utilized for the test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Tested chips under temperature cycling between 20-120 °C. The first housing design(figure 5.1) was used for this purpose. NA = not applicable. No damage = after temperature cycling and leakage test at 3-4 bar, no cracks or signs of any damage were found. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Tested chips under pressure at elevated temperature. The packaging design 3 (figure 5.9) was used for this purpose. NA = Not applicable. No damage = After temperature cycling and leakage test at 3-4 bar, no cracks or signs of any damage were found. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
101 102
105
108
A.1. Clean room process flow for fabrication of micro batch reactors. . . . . . . . . . . . 119 A.2. Clean room process flow for fabrication of pressure testing samples. . . . . . . . . . 119
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1. Introduction This thesis project is developed in the context of chemical microreactors technology. Particularly, high-pressure and high-temperature microreactors are of interest for this work. This chapter intends to set the bases under which this work is accomplished. What is microreaction technology? Why are microreactors important and attractive for the industrial and research chemistry community? And, why is this project important in the frame of high-temperature and high-pressure microreactors? are some of the questions that we try to answer in this introduction.
1.1. What is microreaction technology? Microreaction technology is a novel concept in chemical technology which enables the introduction of new reaction procedures in chemistry, molecular biology and pharmaceutical chemistry, by using miniaturized chemical reactors known as microreactors. Microreactors, in general, have to perform three basic functions [35]: 1. Initiate and facilitate a reaction through mixing of the reactants; 2. Provide time and volume to allow the reaction to finish (residence times); 3. Enable heat exchange. Today, microreactors are, mainly, manufactured with typical channel or chamber widths in the range of tens to hundreds of micrometers. The main advantages offered by microreactors due to the reduction of the characteristic dimensions are: • High surface-to-volume ratios, which may increase the rates of reactions in comparison to macroscopic devices; • High heat and mass transfer rates, which allow reactions to be carried out more aggressively, producing high yields that cannot be achieved with conventional reactors; • High selectivity of the chemical reactions. Small volumes are easier to be controlled under specific process conditions, e.g. pressure, temperature and mixing, which enables the reduction or elimination of by-products (not desired products). • Safety. In case of a process or system failure, the small amount of chemicals released could be contained easily.
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1. Introduction • Greener technology, which means more environmentally friendly. The waste of reagents, catalysts and products is reduced as small amounts of chemicals are used. • By numbering-up strategy (adding microreactor units instead of increasing the size of the reactor) high throughput can be achieved. Additionally, failed units can be replaced easily without interfering with other equipment, which is both a maintenance and economic advantage. In comparison with conventional macroscopic systems, microreactors provide more benefits for several chemical reaction processes. Some common reactions have been carried out in microreactors, e.g. methanol, oxidation, fluorination of toluene, ammonia oxidation, nitration of benzene, ethane epoxidation, and dehydration of methanol to form formaldehyde [47]. Only to give a notion of these advantages to the reader, a comparison between the conventional and micro technologies is made in the table 1.1 for a specific chemical reaction: Direct nitration of aromates (e.g. naphthalene) with dinitrogen pentoxide [65]. Conventional technology
Microreactor technology
Reaction tends to be explosive
Reaction is controllable
Temperature: -50 °C
Temperature: +30 °C
Mixture of mono-, di-, tri-nitro-product
Only mono-nitro-product
Table 1.1.: Comparison for the direct nitration of aromates chemical reaction with dinitrogen pentoxide carried out in conventional and micro reactors [36].
The capabilities of microreactor technology demonstrate the potential to improve upon many conventional techniques using macroscopic systems for many industrially relevant chemical syntheses. Therefore, microreactor technology results to be very attractive for research and industrial applications. In addition to already demonstrated chemical and biological analysis applications, microfabricated chemical systems are expected to have a number of advantages for chemical synthesis, chemical kinetics studies, and process development [24].
1.2. Motivation 1.2.1. Heterogeneous catalytic hydrogenation of carboxylic acids and carboxylic acid derivatives This master’s thesis is developed in the context of the project proposal Active microfluidic platforms for a novel heterogeneous catalytic hydrogenation of carboxylic acids and carboxylic acid derivatives. The goal of this project is to perform a new
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1.2. Motivation heterogeneous catalytic hydrogenation of carboxylic acids and carboxylic acid derivatives, as well as other industry-relevant substrates (e.g. deoxygenation of carboxylic acid derivatives, hydrogenation of carboxylic acids) using novel microreactor technology. This catalytic hydrogenation is of great interest in the industry because it has the potential to make the future production of pharmaceuticals economic, safe and environmentally friendly [7]. So far, the mildest conditions to perform this reaction have been found by Stein and Breit [52]. They have shown that promising results can be achieved by carrying out a heterogeneous catalytic hydrogenation using a suspension of catalyst particles in conventional batch reactors at 120 °C and hydrogen at 10 bar. The results showed that the desired yield of 85 % of amine was obtained after 20h. However, in order to have a reliable synthetic protocol and a variety of tertiary and secondary amides, the process must be performed at 160 °C and a hydrogen pressure of 30 bar [52]. Since microreactor technology offers several advantages, such as: low use of expensive reagents, increased heat and mass transfer in microchannels, faster and more complete mixing of the reagents and increased safety with highly toxic or exothermic reactions; it provides high-throughput experimental methods in order to study the catalytic reactions under a wide range of different conditions, e.g. temperature, pressure, heat/mass transfer and composition of the chemical mixture (catalyst, reagents and solvents). This allows optimization and research of such reactions in a cost-effective manner. High temperature, high pressure and relatively long residence time are required for the heterogeneous catalyst hydrogenation of carboxylic acid amides and derivatives. Therefore, we propose the fabrication of a novel micro batch reactor based on silicon microfabrication technology with integrated active mixer actuators, which allows the precise control of the mixing and residence time of the reagents and catalyst in the reaction chamber. Also, this type of reactor may significantly reduce the duration of the reaction, for example from 20 h to some hours (e.g. 5 hours). Although, such residence time is much higher than the common residence time in continuous-flow microreactors, which residence times are in the range from seconds to minutes, it is expected that the active mixing implemented in this novel micro batch reactor will lead to a controllable residence time as longer as required. The current thesis represents the first stage of this work, involving the development of a microreactor packaging scheme that will allow the incorporation of the active mixing capability in future phases of the project.
1.2.2. High-pressure high-temperature microreactors Microreactors have been proven to work in a reliable way under high-pressures and high-temperatures [59] [38] [60] [65]. The size reduction of channels and chambers in microreactors allows operation of these devices under high-pressures in safe way. Channels with widths ranging from 50 to 500 µm offer a very small area in contact with the fluid. As a consequence, even having fluid under high pressures, the resultant applied force on the devices is very small. In other words, the mechanical stress
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1. Introduction on the device is normally much lower than the failure strength of the materials such as steel, silicon, glass and polymers, which are widely used for the fabrication of microreactors. On the other hand, the main limiting factor to perform chemical reactions in microreactors at elevated pressures is the macro-to-micro interconnect (packaging) quality [54]. Different approaches have been proposed in order to overcome this limitation and several examples are presented in chapter 2.4. The operation of microreactors under high temperatures has been also extensively proven [65]. For many chemical reactions, temperature acts as a catalyst. Increasing the temperature leads to faster conversion of reactants into products. High-temperature microreactor have been tested under temperature of 900 °C [3], for example. A wide range of materials used in the fabrication of MEMS devices are suitable for manufacturing high-temperature microreactors, e.g. tungsten (melting point: 3400 °C), platinum (1770 °C), poly-silicon (1100 °C) and monocrystalline silicon (1420 °C) [65]. The main advantage of high-pressure high-temperature microreactors is the possibility to carry out chemical reactions under conditions that are not achievable with conventional reactors. In general, the risk to operate batch reactors at pressures of 20 bar is very high [47]. However, microreactors are able to perform under pressure of 600 bar in a safe way [59]. Additionally, the heat transfer is highly efficient and fast, which means that the heating or cooling times are reduced and, therefore, the side reactions are minimized, i.e. maximizing the selectivity of the reaction. Furthermore, it results in a more cost-effective process.
1.2.3. Why silicon microtechnology? Nowadays many commercial reactors are able to withstand high temperatures and pressures. However, the microstructured channels and reaction chambers are produced with standard mechanical processes, using metal blocks or plates and metal capillary tubes. Hence, those conventional methods allow only passive mixing and heat transfer, which are determined by the flow rate and the channel geometry. Each reactor has to be manufactured individually and its structural sizes are limited by production engineering. Well controlled pressure and temperature are essential guidelines for chemical processes, heterogeneous catalytic hydrogenation of carboxylic acid amides in our case. These parameters have a large impact on the reaction rate, efficiency and quality of the final products. Besides, detailed characterization of fluidic dynamics and mass/heat transfer is an essential concern in chemical processes as it is fundamentally related to the reproducibility of the experiments. Accordingly, material selection, fabrication processes, packaging and interconnections are the key issues in microreactors technology. Silicon microfabrication is a very well-known manufacturing technology whose techniques have been proven to be successful for a long time in MEMS industry. Moreover, high dimensional reproducibility in the fabrication of microstructures
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1.3. Scope of this work have been shown to be reliable, even in the orders of a few micrometers. In addition, silicon microfabrication technology allows the direct integration of actuators, sensors, control and signal processing electronics through a wide range of different techniques. Regarding high temperature and pressure microreactor technology, several different approaches have been tested and very successful results were achieved. Several examples are presented in the section 2.4 and even more can be found in literature. However, having the short overview presented above, silicon technology appears to fulfill the requirements of this project: to build a micro batch reactor chemically resistant (for hydrogenation reactions); capable to work under continuous operation at high pressure (40 bar) and high temperature (120 °C); able to withstand thermal and pressure load cycling tests and to be mechanical reliable and safe. Therefore, all the cleanroom processes used in silicon microfabrication are adopted in this project. However, as it will be discussed later, the implementation of the active mixing by piezo-electrically driven silicon membranes will be considered in future project phases. Here we focus on the fabrication and packaging of a preliminary design of the micro batch reactor.
1.3. Scope of this work In order to reach the main goal of the project Active microfluidic platform for a novel heterogeneous catalytic hydrogenation of carboxamides, the work has been divided in five stages (see figure 1.1). It is believed that the completion of all the stages represents a work load similar to that one carried out during a doctoral or post-doctoral work. Therefore, due to time limitations, mainly, this master’s thesis accomplishes the two first stages of the project. As a result, the goal of this master’s project is the design, fabrication and mechanical testing of a high-temperature and high-pressure silicon micro batch reactor platform. Further stages, such as the implementation of the active mixing by piezo-electrically driven silicon membranes and the mixing and fluidic characterization, are not included in this thesis.
1.3.1. Aims Based on the main goal of this master’s thesis- the design, fabrication and mechanical testing of a high-temperature and high-pressure silicon micro batch reactor platform, the aims of this project are: 1. Present a preliminary design of a novel microreactor concept which fulfill the following requirements: a) Connections for injecting and discharging liquid suspensions; b) Closed loop microreaction chamber for the intended integration of piezoelectrically driven silicon membranes for active and dynamically controllable mixing;
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This master's thesis
1. Introduction
Stage 1: Micro batch reactor design and fabrication
Stage 2: Packaging design and fabrication
Stage 3: Active mixing Implementation
Microreactor without silicon membranes
Microreactor with silicon membranes
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Evaluation of the Anodic bonding strength and hermeticity.
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Evaluation of the silicon membranes under high pressure conditions.
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Evaluation of the packaging performance under high pressure and high temperature conditions.
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Selection of piezoelectric elements for the membranes actuation. Evaluation of adhesive at high temperatures.
Different packaging approaches
Piezo-actuators on the silicon membranes
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Stage 4: Mixing and fluidic characterization
Stage 5: Chemical reaction under high pressure at elevated temperature
Development of a fluidic platform for characterization
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Accomplishment of a setup for carrying out the reaction
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Sincronization of the piezo-actuators in order to create a fluid flow. Evaluation of the mixing quality.
Evaluation of the final performance of the microreactor. Accomplishment of chemical reactions.
Figure 1.1.: Proposed work scheme of the project Active microfluidic platform for a novel heterogeneous catalytic hydrogenation of carboxamides. This master’s thesis consists of the two first stages of the project.
c) Microstructures in the micro reaction chamber to produce eddies and turbulent flow in a mixing circuit; d) Operation at high pressure (40 bar) and high temperature (120 °C); e) Transparent window which allows the visual observation of flow in the reaction chamber and provides the possibility to perform spectroscopic measurements. 2. Fabrication of the micro batch reactor according with the preliminary design.
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1.3. Scope of this work 3. Design, fabrication and testing of a high-pressure and high-temperature packaging interface that provides the fluidic macro-to-micro interconnections. 4. Mechanical testing of the micro batch reactor under high pressure (40 bar) and high temperature (120 °C). In the first aim, we decided to refer to the design presented in this thesis as a preliminary design so not to limit further changes. For example, in the geometry or configuration of the microreactor in order to improve the final performance of the device. The intention of the second aim is to provide a fabrication process which allows the manufacture of the micro batch reactor with micromachining processes available at IMTEK. In order to enable the fluidic interconnection (required for testing and, furthermore, for the operation of the micro batch reactor) a packaging interface must be designed and tested as well. Finally, pressure and thermal cycling tests must provide experimental results which indicate that the designed microreactor is able to work under harsh conditions of pressure and temperature in a reliable and safe way. Hence, we expect that the accomplishment of these aims will fulfill satisfactory the goal of this master’s thesis and the realization of the first two stages of the project Active microfluidic platforms for a novel heterogeneous catalytic hydrogenation of carboxylic acids and carboxylic acid derivatives.
1.3.2. Structure of this work The presented work is divided into seven chapters. First, in chapter 2, we introduce a brief review regarding microreactors, micromixers and micropumps. The intention of this literature review is to offer a background of the state of the art in these fields and get in touch with the limitations of high-temperature and high-pressure microreactors. The reader will find that the proposed micro batch reactor design was influenced by some of the concepts introduced in that chapter. Then, the preliminary design of the micro batch reactor is presented in chapter 3. Here we explain the general concept of the micro batch reactor and how we expect to fulfill the established requirements for this new concept (those presented in the first aim of this master’s thesis). Several simulations have been carried out in order to guide the design process and the results are shown in this part of the work. At the end of this chapter, the chip layout for the fabrication of the chip is given. In chapter 4, all the aspects of the fabrication of the microreactor are presented. Fabrication process, such as anodic bonding and potassium hydroxide wet etching, define the limitations of final performance of our microreactor, therefore they are of especial interest in this stage of the work. Thus, a short introduction and characterization of these fabrication concepts are given. Also, we explain the fabrication process schemes used of the final fabrication of the micro batch reactor.
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1. Introduction
Literature Review (Chapter 2)
Packaging Design (Chapter 5)
Micro batch reactor Design (Chapter 3)
Fabrication (Chapter 4)
Testing at High Pressure and High-Temperature (Chapter 6)
Achieved Goals and Future Work (Chapter 7)
Figure 1.2.: Work scheme of this master’s thesis. Notice the iterative process between testing and design in order to reach the goals.
On the other hand, the design and fabrication of the packaging interconnect is carried out in parallel with the fabrication process of the microreactors. Therefore, in chapter 5 we introduce three different approaches used for the fluidic interconnection of the micro batch reactor. Also, we discuss the influence of the packaging device on the final performance of the microreactor. Subsequently, the testing stage is performed. Several micro reactors are tested under pressure and thermal cycling using the three different packaging designs. The testing procedures and the results are shown in chapter 6. Finally, in chapter 7, the most important achievements of this work are summirized and a brief scope of the future work is proposed.
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