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Fabrication of Powder-based Ceramic Micro-burners
Procedia Manufacturing Volume 5, 2016, Pages 455–465 44th Proceedings of the North American Manufacturing Research Institution of SME http://www.sme.org/namrc
Fabrication of Powder-based Ceramic Micro-burners Truong Do, Changseop Shin, Junghoon Yeom*, Patrick Kwon* Department of Mechanical Engineering, Michigan State University, East Lansing, MI [email protected], [email protected]
Abstract Miniaturization of chemical system has attracted tremendous interests in chemistry and biology due to many advantages such as enhancement in heat/mass transfer rates, reduction in expensive reagents and hazardous wastes, and facilitation of massive parallelization in reaction/catalyst screening and optimization Fabrication of microchemical systems (ȝCSs) has matured around materials such as silicon, glass and polymers, which are not particularly suitable for harsh environment such as high pressure, high temperature, and corrosive reactants. The main objective of this paper is to introduce a new ceramic-based fabrication framework for such harsh environment. This paper demonstrates the feasibility of the new technique by constructing a micro-burner, which can be used as on-chip heater or sensor. The proposed technique overcomes one major roadblock in fabrication of ceramic-based ȝCSs. The unreliable joining and assembling of simple ceramic components are eliminated by utilizing a fugitive phase machined into an intricate shape of an internal combustion chamber and channels. The main fabrication strategy is to use the advantage of powder, offering the fluidity to flow into a complex cavity or around the solid phase as well as the rigidity to remove the undesirable sections. The paper demonstrates the unique method of fabricating a micro-burner that can sustain an oxyhydrogen flame for an extensive period and under cyclic operations. Keywords: Ceramic powder, compaction, partially sintered ceramics, microchemical system, fugitive phase
1 Introduction A chemical plant produces useful chemicals by either breaking down much more complex chemicals or reacting multiple of chemicals. The examples are polymer, pharmaceutical, power plants, oil refineries, natural gas processing, biochemical plants and water and wastewater treatment. Commonly, using fluidic and chemical reactor systems, chemical operations take place in a large scale where chemicals are fed through the pipes with other chemicals into a reaction chamber for synthesis of new materials or a complex chemical (e.g.: crude oil) is heated to break down into simpler chemicals. Typically these processes are repeated in numerous stages, each stage in a distinct and precise temperature and/or pressure conditions, thus requiring the production facility made of multiple chambers and the channels that connect these chambers.
Selection and peer-review under responsibility of the Scientific Programme Committee of NAMRI/SME c The Authors. Published by Elsevier B.V.
doi:10.1016/j.promfg.2016.08.038
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In the last two decades there have been a great interest in replicating chemical plants in a much smaller scale for the purpose of process developments, portable systems or small-scale power productions (Hasebe, 2004; Hessel & Löwe, 2003; Yao, 2015). This so called microchemical system (ȝCS) exhibits the enhanced chemical kinetics and higher heat and mass transfer rates, facilitating more uniform and faster reaction conditions and thereby improving the process yield and selectivity (Jensen, 2001). Silicon, glass, metal or polymer – the conventional materials for microsystems and microelectromechanical system (MEMS) – have been the popular choice for ȝCS (Jensen, 1999; 2005) but are not suitable for harsh reactions involving high temperature and/or corrosive conditions. The structural elements made of these conventional materials are not thermally stable at high temperature (< 600°C) and chemically unstable under highly reducing or oxidizing environments (Jensen, 2001). Instead, ceramic materials, especially metal oxides, present exceptional high-temperature stability (due to their high melting temperature), structural stability under the cyclic operation (due to their low coefficient of thermal expansion), and chemical stability (due to the fact that they are already oxidized). However, shaping the ceramic materials into the desired structures is not as easy as other materials because ceramics are not amenable to the conventional manufacturing processes like machining, casting/molding, and joining. In fabricating ceramic-based ȝCS components like micro-reactionware, microchannels, and heat exchangers, researchers have employed the processes that are more specific to the ceramic materials. The accuracy attainable through the proposed set of ceramic processing may not match that of the MEMS-based techniques (for silicon or glass materials) due to the shrinkage and non-uniformity of ceramic materials during densification. However, most ȝCSs do not require the extremely high level of accuracy and rather entail the materials and their fabrication processes well suited for high temperature and corrosive environments. Among many ceramic processes, the two main approaches have been frequently utilized for ȝCS components – tape-casting and gel-casting. In tape-casting, each layer of ‘ceramic tape’ consisting of powders and a binder phase is cut and laminated to fabricate small-scale devices (Bau, 1998; Khoong, 2010). The ceramic tapes come with a substantial amount of binder phase, which requires an extensive period of sintering (up to 30 days in some cases) to burn out the binder phase. During the binder burnt-out, very harmful toxic gases are produced, which needed to be exhausted into the atmosphere. Meschke et al. (Meschke, 2005) and Christian et al. (Christian, 2006; 2007) have built ceramic microreactor using the sol-gel technique combined with gel-casting and demonstrated the fabrication of the features smaller than 10 ȝm. The gel-casting technique cannot produce the near full-density components and also fully-enclosed cavity, necessitating additional joining processes for sealed channels and reactors. Knitter et al. (Knitter, 2000; 2001a; 2001b) made ceramic microreactors by utilizing Stereolithography and low pressure injection molding. The abovementioned and other methods of fabricating microreactors have been reviewed by Jensen (Jensen, 2001) and more recently by Marre & Jensen (Marre, 2011). Despite these efforts, the current ceramic fabrication techniques still have some or all of the following challenges: (1) These structures are fabricated as open channel/reactors, requiring a joining process to create a fully enclosed system. (2) It is difficult to join or bond ceramic structures – high temperature adhesive typically used may create thermal mismatch, therefore with temperature cycling the joining areas are susceptible to failure; (3) It is difficult to establish a robust fluidic connection using conventional fitting to ceramic structures; and (4) It is difficult to monolithically integrate functionally diverse structures. In order to address the abovementioned challenges, we propose a novel ceramics fabrication technique that consolidates the metal oxide nanopowders to create fully-enclosed microstructures suitable for ȝCS. The design micro-burner was shown in Figure 1. Without the binder phase in the powder mix, the final chemistry and dimension of the sintered ceramics can be tuned. The key processing steps such as powder compaction and sintering will be investigated to improve the structural quality. Partial sintered state of the powder compact allows us to use the conventional
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machining tools (drilling, milling, etc.) for modifying parts and add/subtracting features. As a proofof-the-concept demonstration, we will utilize the proposed ceramic powder process to fabricate an alumina microcombustor and sustain oxyhydrogen flames in the chamber under the extensive cyclic operation.
Figure 1: Design of micro-burner
2 Materials and Processing Alpha-phase alumina (AKP-50) made by Sumitomo (Tokyo, Japan) was used in the fabrication of the micro-burners. The alumina powder has the purity higher than 99.99% and the particle size between 0.1 and 0.3 micrometer. The simplest shape known to successfully compact the powder in a uniform density is a flat circular disc with the small thickness-to-diameter ratio (~0.1) in a uniaxial die. Even only with the flat cylindrical discs, many fabrication protocols can be explored.
Figure 2: Sequence of fabrication process
Figure 2 shows the overview of the processing sequence to attain internal cavities and external features. The first step is to cut the 0.9 mm thick graphite sheet (EDM-3 grade provided by Saturn 457
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Industries) in a CNC machine to achieve the integrated shape of the combustion chamber and the internal channels. These channels will be subsequently connected to the air inlet, the exhaust outlet and the oxygen and hydrogen lines. This graphite piece is the fugitive phase that will burn out during partial sintering (Step f in Figure 2 and leave the integrated cavity for the combustion chamber and the internal channels. To make the cavity, the half of alumina powder (about 2 grams) measured with the balance (Adventurer AR 2140 made by Ohaus Corp. (USA) with 0.1mg resolution) is deposited into the die. Before pouring the alumina powder, the interior of the die, whose inner diameter is 22.2 mm, was lubricated by zinc stearate (C36H70O4Zn) with 12.5-14% of ZnO (Alfa Aesar (Ward Hill, MA)), which makes the release of the powder compact easier. Reducing the friction is important especially when the powder compact with the graphite is much more fragile than the powder compact without the graphite. After placing the half of alumina powder, the punch was pressed lightly (with its own weight about 200 grams) to flatten the surface. Then the machined graphite is placed on the flatten surface of the powder. The placement of the graphite is important later to locate the junctions that must be drilled to connect the hydrogen and oxygen lines as well as the inlet gas. The sizes of the junctions were enlarged to locate the places for drilling. The other half of alumina powder is then poured into the die, where the powder and the embedded graphite are fully compacted on MTS Insight 300 (MTS Systems Corp., USA) at the compaction pressure of 50MPa with the speed set at 1mm/min. The powder compact was then partially sintered in the furnace (Carbolite-HTF1700, UK) at 800oC for 2 hours to burn out the graphite fugitive phase, which become CO2 gas by reacting with oxygen in the furnace atmosphere. The partially sintered sample was then drilled to make the connecting channels with oxygen and hydrogen lines as well as air inlet. The reason for machining partially sintered samples was due to the fact that fully sintered ceramics are brittle and stiff that does not favor the machining. Finally the partially sintered sample was fully sintered at over 1350oC to provide the mechanical integrity of the micro-burner.
3 Results and Discussion 3.1 Process Challenges The compaction is the most critical processing step in the fabrication of micro-burners. The presence of the graphite in a complex shape hinders the flow of the powder, frequently causing nonuniform stress distribution and resulting in cracks in the final component. One remedy is to eliminate any sharp edges and geometric complexities in the machined graphite, and the rounded corners facilitate the powder flow around the graphite. Otherwise, undesirable cracks can be formed during the powder compaction. The edges of the graphite after cut in a CNC machine were smoothened by manual grinding. The features of high aspect ratio or sharp protrusion in powder compact are sensitive to fracture after the full sintering due to the stress concentration. For example, the region intersecting two gas channels to the combustion chamber was found to be susceptible to the formation of cracks. In addition, the powder compaction with the graphite was often fractured if the size of the graphite was too larger in relation to the cross-sectional area of the die. The powder holds its shape after compaction with the friction among the powder. If the graphite covers a large portion of the cross-sectional area of the die, the powder compact cannot hold the shape. Two reasons why the graphite fugitive phase was successful in introducing the integrated cavity are (1) the low coefficient of the thermal expansion of the graphite, which does not exert much force onto the powder compact when it is in a very fragile state and (2) the graphite burns out before the alumina powder started to consolidate around 1000°C. It is well known that during the sintering process, the temperature ramp rate is a critical factor affecting the structure integrity. If the ramp rate is high, the sample itself has temperature gradient. The temperature difference at varied locations causes internal stresses due to thermal expansion. In addition, the temperature ramp rate needs to be 458
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low enough for the graphite to react with oxygen during sintering before alumina powder consolidates. This can be seen in Figure 3a when the sample sintered at the ramp rate of 15oC per minute from room temperature to 1160oC without any soaking step had been cracked. The crack is formed as the consolidation of alumina initiated before the graphite gets oxidized.
Figure 3: (a) Cracked sample due to high rate of temperature ramp without soaking time (b) Sample sintered at optimized temperature program (Scale bar is 1cm)
Figure 4: Thermogravimetric analysis of graphite
In order to determine the appropriate sintering temperature program for micro-burner, a thermogravimetric analysis (TGA) on graphite was performed by TGA Q500 provided by TA Instruments (USA). This test was necessary to evaluate the stability of graphite at elevated temperatures. The mass of the graphite in this test was 0.1221 g and the masses of micro-burner after compaction and after partial sintering at 800oC were 4.0941 g and 3.9496 g, respectively. During the partial sintering process, the total mass loss was 0.1445 g, which is 0.0224 g larger than the mass of graphite. The missing mass of 0.0224 g can be explained by the two-folds. Some ceramics may have been chipped off during handling. The more likely reason of the additional weight loss may be related to the impurities in alumina that were burned out. Based on these results, we concluded that all the graphite was completely oxidized during the partial sintering process. Based on the TGA result of the bare graphite (see Figure 4), we determined that a partial sintering of the powder compact at 800oC with the soaking time of 2 hours was necessary to burn out all the graphite. The partially sintered sample was slowly cooled down to room temperature followed by the machining process to connect the oxygen, hydrogen and air lines with hypodermic tubing.
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3.2 Machining Because the channels and combustion chamber were created after the graphite burn-out, only drilling is needed to establish fluidic connections for the channels. The fully sintered alumina has poor machinability due to its stiffness and brittleness. Machining the green or white compacts (Halcomb, 1982; Klocke, 1997a; Klcoke, 1997b) is easier if the final tolerance is not strict. Green machining is the method of machining powder after compaction that has not been exposed to high temperature yet. White machining is the method taken place on partially sintered samples. In green machining, the powder is usually mixed with a binder phase to achieve the sufficient strength for machining. The presence of graphite in powder compaction sample and its sensitive structure do not allow us to do green machining. Thus, white machining had been chosen after the graphite was burned out at partially sintered temperature around 800°C. In our previous work (Shin et al., 2004), machining was performed at a low feed rate and a high cutting speed, which shows a very promising result on drilling of alumina after partially sintered at 700°C. This condition was implemented in drilling fluidic holes in our micro-burner. In order to determine appropriate conditions for achieving smooth holes, the drilling process were conducted on EMCO CNC milling machine with the alumina samples after partially sintered at 600°C, 800°C and 1000°C. All the samples were prepared (i.e., pressed and sintered) in the same way as fabricating the micro-burner except that the micro-burner has the graphite fugitive phase. The drilling process was done at a feed rate of 1 mm/min and a cutting speed of 1500 rpm. As observed in Figure 5(a), the edge of the hole exhibits extremely rough boundaries, which can be explained by weak connections among alumina particles when pre-sintered at 600°C. During the cutting process, the partially sintered alumina powder was flaked off by the impaction of drill bit on the machined surface. With the experiments with the samples partially sintered at 600°C, 800°C and 1000°C, the sample partially sintered at 800°C had the best surface quality as evident in Figure 5(b). The improvement in surface quality was evident as the partial sintering temperature increased from 600C to 800°C. This is due to the extensive formation of necks among the individual powder particles at the initial stage of sintering (German, 1976; 1978). This consolidation stage provided stronger connections among alumina particles to prevent chipping during drilling. However, when the partial sintering temperature increased to 1000°C, the surface quality became worsened. At the partial sintering temperature of 1000°C, the formation of necks among the particles is too extensive and the fracture on the surface occurs when machining. In the machining of our micro-burner, because the graphite was compacted without exposing to any exterior surface of the powder compact, it is hard to locate the channels for drilling. To ensure the connections, each end of the graphite (total five circular ends) was made large.
Figure 5: Optical images of holes drilling at different sintering temperatures (Scale bar is 1mm)
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Figure 6: Topography of internal channel of the samples (a) Graphite without polishing (b) polished graphite.
Figure 7: Profiles of random cross sections of topography (a) alumina surface made from the graphite phase without polishing (b) that made from the polished graphite
3.3 Surface Finish The surface finish quality of the channel is important for reactants’ (e.g. H2 and O2/air) flow to remain as a laminar flow during the operation of micro-burner. The most popular internal ceramic fabrication is gel-casting method, where sometime gelation was inhibited due to the presence of oxygen in the ambient air (Ma, 2003; Landham, 1987). This inhibition causes spallation on surfaces of the air exposed gel-casting sample, which affects the dimensional accuracy of final sintered device. Thus, if the gel-casting method is applied to fabricate this micro-burner, the surfaces of internal channel can be roughened by spallation. In fact, the presence of oxygen can be avoided by sintering a gel-casting sample in an inert gas atmosphere (Ha, 2000), which will increase the total cost of device fabrication process. 461
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The fugitive phase method employed in this work enables us to control the surface quality required for each application. The final surface quality is directly dependent on the smoothness of the graphite fugitive phase and the size of powder. To attain the smooth surface, the graphite sheet is polished before placed into the alumina powder with the powder size between 0.1 and 0.3 micrometer. The surface of internal channel walls is significantly smoother. After sintering, the internal surface of open micro-burners was measured by Confocal Laser Scanning Microscope (Zeiss LSM 210, German). The wavelet-based filtering scheme written in MATLAB was used to eliminate the artifacts. Figure 6(a) show the topography of the sample made with the unpolished graphite, where the peaks range between 10 ȝm to 40 ȝm on the surface. On the surface of the sample made by polished graphite, shown in Figure 6(b), we hardly observed any peak above 10 ȝm. For a closer look, three random profiles of cross-section on the measured area were taken to quantify the surface roughness of the resulting ceramic surface. Figure 7 clearly shows the improvement in surface finish of the sample with the polished graphite. We also estimated the average roughness based on the measured data. Three topological profiles of the three random locations were used to evaluate the surface roughness by taking the average values of the distances between the adjacent peaks and valleys. We obtained the average surface roughness of 2.9 ȝm for the alumina surface made from the unpolished graphite sample and 0.7 ȝm for that made from the polished graphite sample.
3.4 X-ray Computed Tomography (CT-scan) In order to show the presence of the cavity created by burning out the graphite on the enclosed micro-burner, a computerized tomography (CT) scan was carried out with GE eXplore Locus RS micro CT that has the highest resolution up to 27 ȝm. Figure 7 shows that the location of the combustion chamber was at the same position of placed graphite. The machined holes are also visible but slightly misaligned with respect to the position of the channels. This is because the holes were drilled with the assumption that the graphite would not move during compaction. It appears that the graphite may have been shifted or rotated during compaction, and the reference coordinate used to drill the holes was therefore inaccurate. This misalignment does not produce any issue as long as there are fluidic continuities by the connections. With the proposed technique, we were able to achieve the complicated internal chamber at a desired location by placing the graphite piece and compacting the powder.
Figure 8: Micro CT image of enclosed micro-burner showing the internal cavity: (a) from the top direction. (b),(c),(d) different layers from the side direction (Scale bar is 10 mm)
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3.5 Micro-burner Testing The backbone of the micro-burner design was reported in the previous work by Kim et al. (Kim, 2012) and is based on the counter-flow diffusion flame under the laminar flow condition. Two configurations of the micro-burners were fabricated: open vs. enclosed structures (see Figure 9(a) and 9(c)). The micro-burner with the transparent window was used to characterize the flame and optimize the flow rates of fuel and oxidant streams. A quartz disc (25-mm in diameter, Quartz Scientific Inc.) was bonded to the sintered alumina layer using an adhesive (1531 DURASEL+TM) which has the thermal stability up to 343°C. Through the quartz window attached on the channel side, an oxyhydrogen flame can be visually observed to determine the presence and location of the flame within the chamber. The holes on the surface of the alumina micro-burner was made to insert the stainless steel (SS) tubing (0.46 mm I.D., 0.90 mm O.D.) for the fluidic connections and fixed with RESBONDTM 907GF which can endure up to 1260°C. Once the adhesives to bond the quartz window and the SS tubing were cured at room temperature for 24 hours, the micro-burner was formed for testing. (b)
(a)
(c)
(d)
Figure 9: Photographic images of (a) an open micro-burner, (b) a diffusion flame from (a) using H2 and O2/air as fuel and oxidant sources, (c) an enclosed micro-burner. (d) 3D plot of the temperature measured at the locations indicated in (c).
Hydrogen and oxygen were created using a commercial electrolyzer coupled to the custom-made flow manifold to control the flow rates of each stream (Bae, 2012). Because the electrolyzer produces hydrogen and oxygen at a fixed stoichiometric ratio (H2:O2 = 2:1), a miniature pump was added to deliver air to the oxygen line to independently control the fuel-to-oxidant ratio. Figure 9(a) and (b) shows the micro-burner with the quartz window and the oxy-hydrogen flame within the combustion 463
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channel, respectively. The flow rate of H2 and air were set at 40 and 20 mL/min, respectively. Note that 20 mL/min of O2 was also flowing in the oxidant stream produced from the electrolyzer. A flame image was taken using a Nikon DSLR camera (D50, shutter speed: 30 second, ISO 100) in a dark room (Figure 9(b)). Under normal circumstances, an oxy-hydrogen flame is not visible to bare eyes or regular cameras, and therefore the special UV filter with the signal intensifier would be required to image the flame. We introduced a trace amount of organic contaminants (diffusion oil) to visualize the flame. After we identified the H2 and air flow rates that anchored flame inside the combustion channel, we tested the enclosed micro-burner with the range of similar flow conditions. The flame generated within the enclosed micro-burner cannot be observed due to the opaque nature of the alumina walls. Therefore, we indirectly verified the presence of the flame by measuring the temperature of the outer wall of the micro-burner. While the adiabatic flame temperature of the flame is calculated to be between 2210 and 3200°C depending on the O2 to air ratio in the oxidant stream, the surface temperatures on the quartz glass and the alumina wall is much below than the flame temperature. The exterior surface temperatures of micro-burner were measured while the flame was anchored inside the channel at the flow rate condition of 35 mL/min of H2 and a mixture of oxidants (17.5 mL/min of O2 plus 20 mL/min of air). Figure 9(c) shows the locations on the exterior surface of the micro-burner where the temperatures were measured. The maximum surface temperature of alumina surface was measured to be below 75°C. The regions of the highest temperature indicate the location of the flame, which resembles the flame location of the quartz/alumina micro-burner (see Figure 9(d)). The flame was continuously stable without disruption during the two-hour testing. We have tested this microburner on the course of 6 months and observed no mechanical failure or any sign of leak through the system, manifesting the long-term stability.
4 Conclusion In this research, we developed a unique processing method to fabricate internal channel on ceramic material that can be used as flame ionization detector. By optimizing the design of graphite inclusion and sintering process, we made the sample with the completely interior cavity. This method also allows us to achieve better surface finish of the internal channel easily by polishing the graphite before placement into the die. The open micro-burner with attached transparent quartz was connected to hydrogen and oxygen sources. We were able to capture the flame created inside the channel. The flame is stable that can last for around 2 hours if providing oxygen and hydrogen continuously. By adjusting the flow rate of oxygen and hydrogen, we optimized flame shape in order to achieve good signal on electrode assembled on the exhausted channel. We have developed machining technique to drill into the alumina samples partial sintered at 800°C with the acceptable surface finish. The drilling step connects to the hydrogen and oxygen sources for the micro-burner function. Smooth surface finish allows laminar airflow in internal channel.
References Bau, H., Ananthasuresh, G. K., Snatiago-Aviles, J., Zhong, J., Kim, M., Yi, M. & Espinaza-Vallejo, P. (1998) Ceramic Tape-Based Meso Systems Technology. The Proceedings of ASME International Mechanical Engineering Congress and Exposition, Anaheim, CA, ASME International, 15-20. Christian, M. & Kenis, P.J.A. (2006) Ceramic Microreactors for On-site Hydrogen Production from High Temperature Steam Reforming of Propane. Lab on a Chip, 6, 1328-1337. Christian, M. & Kenis, P.J.A. (2007) Fabrication of Ceramic Microscale Structures. Journal of American Ceramic Society, 90(9) 2779-2783. 464
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German, R.M. & Munir, Z.A. (1976) Surface area reduction during isothermal sintering. Journal of the American Ceramic Society, 59 (9-10), 379-383. German, R.M. & Lathrop, J.F. (1978) Simulation of spherical powder sintering by surface diffusion. Journal of Materials Science, 13(5), 921-929. Ha, J.S. (2000) Effect of Atmosphere Type on Gelcasting Behavior of Al2O3 and Evaluation of Green Strength. Ceramic International, 26, 251–254. Halcomb, D.L. & Rey, M.C. (1982) Ceramic cutting tools for machining unsintered compacts of oxide ceramics. American Ceramic Society Bulletin, 61(12), 1131-1314. Hasebe, S. (2004) Design and operation of micro-chemical plants—bridging the gap between nano, micro and macro technologies. Computers & Chemical Engineering, 29(1), 57-64. Hessel, V. & Lowe, H. (2003) Microchemical Engineering: Components, Plant Concepts, User Acceptance – Part II. Chemical Engineering & Technology, 26(4), 391-408. Jensen K. (1999) Microchemical Systems: Status, Challenges, and Opportunities. AIChE Journal, 45(10), 2051-2054. Jensen K. (2001) Microreaction engineering – is small better? Chemical Engineering Science, 56, 293303. Jensen K. (2005) Chapter 1: Silicon-Based Microreactors. Microreactor Technology and Process Intensification, Edited by Wang & Holladay. American Chemical Society Series, Wilely, 2-22. Khoong, L. E., Tan, Y.M., & Lam, Y.S. (2001) Overview on Fabrication of three-dimensional structures in Multi-layer Ceramics Substrate. Journal of the European Ceramic Society, 30, 19731987. Klocke, F. (1997a) Modern Approaches for the Production of Ceramic Components. Journal of the European Ceramic Society, 17(2-3), 457-465. Klocke, E., Gerent, O. & Schippers, C. (1997b) Machining of advanced ceramics in the green state. CFI Ceramic Forum International, 74(6), 288-290. Knitter, R., Gohring, D., Bram, M., Mechnich, P., & Broucek, R. (2000) Ceramic Microreactor for High-Temperature Reactions. 4th International Conference on Microreaction Technology, AIChE Spring Meeting, March 5-9, 2000, Atlanta, Georgia, Topical Conference Proceedings, 455-460. Knitter, R., Gohring, D., Risthaus, P. & Hauȕelt (2001a) Microfabrication of Ceramic Microreactors. Microsystem Technologies, 7, 85-90. Knitter, R., Lurk, R., Rohde, M., Stolz, S. & Winter, V. (2001b) Heating Concepts for Ceramic Microreactors. 5th International Conference on Microreaction Technology, AIChE Spring Meeting, May 27-30, 2001, Strasbourg, France, Springer Verlag, Berlin, 86-93. Landham, R.R., Nahass, P., Leung, D.K., Ungureit, M., & Bowen, W.E. (1987) Potential Use of Polymerizable Solvents and Dispersants for Tape Casting of Ceramics. American Ceramics Society Bulletin, 66, 1513–16. Ma, J., Xie, Z., Miao, H., Zhou, L., & Huang, Y. (2003) Elimination of surface spallation of alumina green bodies prepared by acrylamide-based gelcasting via poly(vinylpyrrolidone). Journal of American Ceramic Society, 86(2), 266–272. Marre, S., Adamo, A., Basak, S., Aymonier, C., & Jensen, K.F. (2010) Design and Packaging of Microreactors for High Pressure and High Temperature Applications. Industrial Engineering and Chemical Research, 49, 11310–11320. Meschke, F., Riebler, G., Hessel, V., Shurer, J. & Baier, T. (2005) Hermetic Gas-tight Ceramic Microreactors. Chemical Engineering & Technology, 28(4), 465–473. Shin, H., Kok, C.K., Kwon, P., & Case, E. (2004) Meso-scale channels on Ceramics by Machining. Journal of Manufacturing Processes, 6(1), 15-23. Yao, X., Zhang, Y., Du, L., Liu, J., & Yao, J. (2015) Review of the applications of microreactors. Renewable and Sustainable Energy Reviews, 47, 519-539.
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Fabrication of Powder-based Ceramic Micro-burners Truong Do, Changseop Shin, Junghoon Yeom*, Patrick Kwon* Department of Mechanical Engineering, Michigan State University, East Lansing, MI [email protected], [email protected]
Abstract Miniaturization of chemical system has attracted tremendous interests in chemistry and biology due to many advantages such as enhancement in heat/mass transfer rates, reduction in expensive reagents and hazardous wastes, and facilitation of massive parallelization in reaction/catalyst screening and optimization Fabrication of microchemical systems (ȝCSs) has matured around materials such as silicon, glass and polymers, which are not particularly suitable for harsh environment such as high pressure, high temperature, and corrosive reactants. The main objective of this paper is to introduce a new ceramic-based fabrication framework for such harsh environment. This paper demonstrates the feasibility of the new technique by constructing a micro-burner, which can be used as on-chip heater or sensor. The proposed technique overcomes one major roadblock in fabrication of ceramic-based ȝCSs. The unreliable joining and assembling of simple ceramic components are eliminated by utilizing a fugitive phase machined into an intricate shape of an internal combustion chamber and channels. The main fabrication strategy is to use the advantage of powder, offering the fluidity to flow into a complex cavity or around the solid phase as well as the rigidity to remove the undesirable sections. The paper demonstrates the unique method of fabricating a micro-burner that can sustain an oxyhydrogen flame for an extensive period and under cyclic operations. Keywords: Ceramic powder, compaction, partially sintered ceramics, microchemical system, fugitive phase
1 Introduction A chemical plant produces useful chemicals by either breaking down much more complex chemicals or reacting multiple of chemicals. The examples are polymer, pharmaceutical, power plants, oil refineries, natural gas processing, biochemical plants and water and wastewater treatment. Commonly, using fluidic and chemical reactor systems, chemical operations take place in a large scale where chemicals are fed through the pipes with other chemicals into a reaction chamber for synthesis of new materials or a complex chemical (e.g.: crude oil) is heated to break down into simpler chemicals. Typically these processes are repeated in numerous stages, each stage in a distinct and precise temperature and/or pressure conditions, thus requiring the production facility made of multiple chambers and the channels that connect these chambers.
Selection and peer-review under responsibility of the Scientific Programme Committee of NAMRI/SME c The Authors. Published by Elsevier B.V.
doi:10.1016/j.promfg.2016.08.038
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Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
In the last two decades there have been a great interest in replicating chemical plants in a much smaller scale for the purpose of process developments, portable systems or small-scale power productions (Hasebe, 2004; Hessel & Löwe, 2003; Yao, 2015). This so called microchemical system (ȝCS) exhibits the enhanced chemical kinetics and higher heat and mass transfer rates, facilitating more uniform and faster reaction conditions and thereby improving the process yield and selectivity (Jensen, 2001). Silicon, glass, metal or polymer – the conventional materials for microsystems and microelectromechanical system (MEMS) – have been the popular choice for ȝCS (Jensen, 1999; 2005) but are not suitable for harsh reactions involving high temperature and/or corrosive conditions. The structural elements made of these conventional materials are not thermally stable at high temperature (< 600°C) and chemically unstable under highly reducing or oxidizing environments (Jensen, 2001). Instead, ceramic materials, especially metal oxides, present exceptional high-temperature stability (due to their high melting temperature), structural stability under the cyclic operation (due to their low coefficient of thermal expansion), and chemical stability (due to the fact that they are already oxidized). However, shaping the ceramic materials into the desired structures is not as easy as other materials because ceramics are not amenable to the conventional manufacturing processes like machining, casting/molding, and joining. In fabricating ceramic-based ȝCS components like micro-reactionware, microchannels, and heat exchangers, researchers have employed the processes that are more specific to the ceramic materials. The accuracy attainable through the proposed set of ceramic processing may not match that of the MEMS-based techniques (for silicon or glass materials) due to the shrinkage and non-uniformity of ceramic materials during densification. However, most ȝCSs do not require the extremely high level of accuracy and rather entail the materials and their fabrication processes well suited for high temperature and corrosive environments. Among many ceramic processes, the two main approaches have been frequently utilized for ȝCS components – tape-casting and gel-casting. In tape-casting, each layer of ‘ceramic tape’ consisting of powders and a binder phase is cut and laminated to fabricate small-scale devices (Bau, 1998; Khoong, 2010). The ceramic tapes come with a substantial amount of binder phase, which requires an extensive period of sintering (up to 30 days in some cases) to burn out the binder phase. During the binder burnt-out, very harmful toxic gases are produced, which needed to be exhausted into the atmosphere. Meschke et al. (Meschke, 2005) and Christian et al. (Christian, 2006; 2007) have built ceramic microreactor using the sol-gel technique combined with gel-casting and demonstrated the fabrication of the features smaller than 10 ȝm. The gel-casting technique cannot produce the near full-density components and also fully-enclosed cavity, necessitating additional joining processes for sealed channels and reactors. Knitter et al. (Knitter, 2000; 2001a; 2001b) made ceramic microreactors by utilizing Stereolithography and low pressure injection molding. The abovementioned and other methods of fabricating microreactors have been reviewed by Jensen (Jensen, 2001) and more recently by Marre & Jensen (Marre, 2011). Despite these efforts, the current ceramic fabrication techniques still have some or all of the following challenges: (1) These structures are fabricated as open channel/reactors, requiring a joining process to create a fully enclosed system. (2) It is difficult to join or bond ceramic structures – high temperature adhesive typically used may create thermal mismatch, therefore with temperature cycling the joining areas are susceptible to failure; (3) It is difficult to establish a robust fluidic connection using conventional fitting to ceramic structures; and (4) It is difficult to monolithically integrate functionally diverse structures. In order to address the abovementioned challenges, we propose a novel ceramics fabrication technique that consolidates the metal oxide nanopowders to create fully-enclosed microstructures suitable for ȝCS. The design micro-burner was shown in Figure 1. Without the binder phase in the powder mix, the final chemistry and dimension of the sintered ceramics can be tuned. The key processing steps such as powder compaction and sintering will be investigated to improve the structural quality. Partial sintered state of the powder compact allows us to use the conventional
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Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
machining tools (drilling, milling, etc.) for modifying parts and add/subtracting features. As a proofof-the-concept demonstration, we will utilize the proposed ceramic powder process to fabricate an alumina microcombustor and sustain oxyhydrogen flames in the chamber under the extensive cyclic operation.
Figure 1: Design of micro-burner
2 Materials and Processing Alpha-phase alumina (AKP-50) made by Sumitomo (Tokyo, Japan) was used in the fabrication of the micro-burners. The alumina powder has the purity higher than 99.99% and the particle size between 0.1 and 0.3 micrometer. The simplest shape known to successfully compact the powder in a uniform density is a flat circular disc with the small thickness-to-diameter ratio (~0.1) in a uniaxial die. Even only with the flat cylindrical discs, many fabrication protocols can be explored.
Figure 2: Sequence of fabrication process
Figure 2 shows the overview of the processing sequence to attain internal cavities and external features. The first step is to cut the 0.9 mm thick graphite sheet (EDM-3 grade provided by Saturn 457
Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
Industries) in a CNC machine to achieve the integrated shape of the combustion chamber and the internal channels. These channels will be subsequently connected to the air inlet, the exhaust outlet and the oxygen and hydrogen lines. This graphite piece is the fugitive phase that will burn out during partial sintering (Step f in Figure 2 and leave the integrated cavity for the combustion chamber and the internal channels. To make the cavity, the half of alumina powder (about 2 grams) measured with the balance (Adventurer AR 2140 made by Ohaus Corp. (USA) with 0.1mg resolution) is deposited into the die. Before pouring the alumina powder, the interior of the die, whose inner diameter is 22.2 mm, was lubricated by zinc stearate (C36H70O4Zn) with 12.5-14% of ZnO (Alfa Aesar (Ward Hill, MA)), which makes the release of the powder compact easier. Reducing the friction is important especially when the powder compact with the graphite is much more fragile than the powder compact without the graphite. After placing the half of alumina powder, the punch was pressed lightly (with its own weight about 200 grams) to flatten the surface. Then the machined graphite is placed on the flatten surface of the powder. The placement of the graphite is important later to locate the junctions that must be drilled to connect the hydrogen and oxygen lines as well as the inlet gas. The sizes of the junctions were enlarged to locate the places for drilling. The other half of alumina powder is then poured into the die, where the powder and the embedded graphite are fully compacted on MTS Insight 300 (MTS Systems Corp., USA) at the compaction pressure of 50MPa with the speed set at 1mm/min. The powder compact was then partially sintered in the furnace (Carbolite-HTF1700, UK) at 800oC for 2 hours to burn out the graphite fugitive phase, which become CO2 gas by reacting with oxygen in the furnace atmosphere. The partially sintered sample was then drilled to make the connecting channels with oxygen and hydrogen lines as well as air inlet. The reason for machining partially sintered samples was due to the fact that fully sintered ceramics are brittle and stiff that does not favor the machining. Finally the partially sintered sample was fully sintered at over 1350oC to provide the mechanical integrity of the micro-burner.
3 Results and Discussion 3.1 Process Challenges The compaction is the most critical processing step in the fabrication of micro-burners. The presence of the graphite in a complex shape hinders the flow of the powder, frequently causing nonuniform stress distribution and resulting in cracks in the final component. One remedy is to eliminate any sharp edges and geometric complexities in the machined graphite, and the rounded corners facilitate the powder flow around the graphite. Otherwise, undesirable cracks can be formed during the powder compaction. The edges of the graphite after cut in a CNC machine were smoothened by manual grinding. The features of high aspect ratio or sharp protrusion in powder compact are sensitive to fracture after the full sintering due to the stress concentration. For example, the region intersecting two gas channels to the combustion chamber was found to be susceptible to the formation of cracks. In addition, the powder compaction with the graphite was often fractured if the size of the graphite was too larger in relation to the cross-sectional area of the die. The powder holds its shape after compaction with the friction among the powder. If the graphite covers a large portion of the cross-sectional area of the die, the powder compact cannot hold the shape. Two reasons why the graphite fugitive phase was successful in introducing the integrated cavity are (1) the low coefficient of the thermal expansion of the graphite, which does not exert much force onto the powder compact when it is in a very fragile state and (2) the graphite burns out before the alumina powder started to consolidate around 1000°C. It is well known that during the sintering process, the temperature ramp rate is a critical factor affecting the structure integrity. If the ramp rate is high, the sample itself has temperature gradient. The temperature difference at varied locations causes internal stresses due to thermal expansion. In addition, the temperature ramp rate needs to be 458
Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
low enough for the graphite to react with oxygen during sintering before alumina powder consolidates. This can be seen in Figure 3a when the sample sintered at the ramp rate of 15oC per minute from room temperature to 1160oC without any soaking step had been cracked. The crack is formed as the consolidation of alumina initiated before the graphite gets oxidized.
Figure 3: (a) Cracked sample due to high rate of temperature ramp without soaking time (b) Sample sintered at optimized temperature program (Scale bar is 1cm)
Figure 4: Thermogravimetric analysis of graphite
In order to determine the appropriate sintering temperature program for micro-burner, a thermogravimetric analysis (TGA) on graphite was performed by TGA Q500 provided by TA Instruments (USA). This test was necessary to evaluate the stability of graphite at elevated temperatures. The mass of the graphite in this test was 0.1221 g and the masses of micro-burner after compaction and after partial sintering at 800oC were 4.0941 g and 3.9496 g, respectively. During the partial sintering process, the total mass loss was 0.1445 g, which is 0.0224 g larger than the mass of graphite. The missing mass of 0.0224 g can be explained by the two-folds. Some ceramics may have been chipped off during handling. The more likely reason of the additional weight loss may be related to the impurities in alumina that were burned out. Based on these results, we concluded that all the graphite was completely oxidized during the partial sintering process. Based on the TGA result of the bare graphite (see Figure 4), we determined that a partial sintering of the powder compact at 800oC with the soaking time of 2 hours was necessary to burn out all the graphite. The partially sintered sample was slowly cooled down to room temperature followed by the machining process to connect the oxygen, hydrogen and air lines with hypodermic tubing.
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Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
3.2 Machining Because the channels and combustion chamber were created after the graphite burn-out, only drilling is needed to establish fluidic connections for the channels. The fully sintered alumina has poor machinability due to its stiffness and brittleness. Machining the green or white compacts (Halcomb, 1982; Klocke, 1997a; Klcoke, 1997b) is easier if the final tolerance is not strict. Green machining is the method of machining powder after compaction that has not been exposed to high temperature yet. White machining is the method taken place on partially sintered samples. In green machining, the powder is usually mixed with a binder phase to achieve the sufficient strength for machining. The presence of graphite in powder compaction sample and its sensitive structure do not allow us to do green machining. Thus, white machining had been chosen after the graphite was burned out at partially sintered temperature around 800°C. In our previous work (Shin et al., 2004), machining was performed at a low feed rate and a high cutting speed, which shows a very promising result on drilling of alumina after partially sintered at 700°C. This condition was implemented in drilling fluidic holes in our micro-burner. In order to determine appropriate conditions for achieving smooth holes, the drilling process were conducted on EMCO CNC milling machine with the alumina samples after partially sintered at 600°C, 800°C and 1000°C. All the samples were prepared (i.e., pressed and sintered) in the same way as fabricating the micro-burner except that the micro-burner has the graphite fugitive phase. The drilling process was done at a feed rate of 1 mm/min and a cutting speed of 1500 rpm. As observed in Figure 5(a), the edge of the hole exhibits extremely rough boundaries, which can be explained by weak connections among alumina particles when pre-sintered at 600°C. During the cutting process, the partially sintered alumina powder was flaked off by the impaction of drill bit on the machined surface. With the experiments with the samples partially sintered at 600°C, 800°C and 1000°C, the sample partially sintered at 800°C had the best surface quality as evident in Figure 5(b). The improvement in surface quality was evident as the partial sintering temperature increased from 600C to 800°C. This is due to the extensive formation of necks among the individual powder particles at the initial stage of sintering (German, 1976; 1978). This consolidation stage provided stronger connections among alumina particles to prevent chipping during drilling. However, when the partial sintering temperature increased to 1000°C, the surface quality became worsened. At the partial sintering temperature of 1000°C, the formation of necks among the particles is too extensive and the fracture on the surface occurs when machining. In the machining of our micro-burner, because the graphite was compacted without exposing to any exterior surface of the powder compact, it is hard to locate the channels for drilling. To ensure the connections, each end of the graphite (total five circular ends) was made large.
Figure 5: Optical images of holes drilling at different sintering temperatures (Scale bar is 1mm)
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Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
Figure 6: Topography of internal channel of the samples (a) Graphite without polishing (b) polished graphite.
Figure 7: Profiles of random cross sections of topography (a) alumina surface made from the graphite phase without polishing (b) that made from the polished graphite
3.3 Surface Finish The surface finish quality of the channel is important for reactants’ (e.g. H2 and O2/air) flow to remain as a laminar flow during the operation of micro-burner. The most popular internal ceramic fabrication is gel-casting method, where sometime gelation was inhibited due to the presence of oxygen in the ambient air (Ma, 2003; Landham, 1987). This inhibition causes spallation on surfaces of the air exposed gel-casting sample, which affects the dimensional accuracy of final sintered device. Thus, if the gel-casting method is applied to fabricate this micro-burner, the surfaces of internal channel can be roughened by spallation. In fact, the presence of oxygen can be avoided by sintering a gel-casting sample in an inert gas atmosphere (Ha, 2000), which will increase the total cost of device fabrication process. 461
Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
The fugitive phase method employed in this work enables us to control the surface quality required for each application. The final surface quality is directly dependent on the smoothness of the graphite fugitive phase and the size of powder. To attain the smooth surface, the graphite sheet is polished before placed into the alumina powder with the powder size between 0.1 and 0.3 micrometer. The surface of internal channel walls is significantly smoother. After sintering, the internal surface of open micro-burners was measured by Confocal Laser Scanning Microscope (Zeiss LSM 210, German). The wavelet-based filtering scheme written in MATLAB was used to eliminate the artifacts. Figure 6(a) show the topography of the sample made with the unpolished graphite, where the peaks range between 10 ȝm to 40 ȝm on the surface. On the surface of the sample made by polished graphite, shown in Figure 6(b), we hardly observed any peak above 10 ȝm. For a closer look, three random profiles of cross-section on the measured area were taken to quantify the surface roughness of the resulting ceramic surface. Figure 7 clearly shows the improvement in surface finish of the sample with the polished graphite. We also estimated the average roughness based on the measured data. Three topological profiles of the three random locations were used to evaluate the surface roughness by taking the average values of the distances between the adjacent peaks and valleys. We obtained the average surface roughness of 2.9 ȝm for the alumina surface made from the unpolished graphite sample and 0.7 ȝm for that made from the polished graphite sample.
3.4 X-ray Computed Tomography (CT-scan) In order to show the presence of the cavity created by burning out the graphite on the enclosed micro-burner, a computerized tomography (CT) scan was carried out with GE eXplore Locus RS micro CT that has the highest resolution up to 27 ȝm. Figure 7 shows that the location of the combustion chamber was at the same position of placed graphite. The machined holes are also visible but slightly misaligned with respect to the position of the channels. This is because the holes were drilled with the assumption that the graphite would not move during compaction. It appears that the graphite may have been shifted or rotated during compaction, and the reference coordinate used to drill the holes was therefore inaccurate. This misalignment does not produce any issue as long as there are fluidic continuities by the connections. With the proposed technique, we were able to achieve the complicated internal chamber at a desired location by placing the graphite piece and compacting the powder.
Figure 8: Micro CT image of enclosed micro-burner showing the internal cavity: (a) from the top direction. (b),(c),(d) different layers from the side direction (Scale bar is 10 mm)
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Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
3.5 Micro-burner Testing The backbone of the micro-burner design was reported in the previous work by Kim et al. (Kim, 2012) and is based on the counter-flow diffusion flame under the laminar flow condition. Two configurations of the micro-burners were fabricated: open vs. enclosed structures (see Figure 9(a) and 9(c)). The micro-burner with the transparent window was used to characterize the flame and optimize the flow rates of fuel and oxidant streams. A quartz disc (25-mm in diameter, Quartz Scientific Inc.) was bonded to the sintered alumina layer using an adhesive (1531 DURASEL+TM) which has the thermal stability up to 343°C. Through the quartz window attached on the channel side, an oxyhydrogen flame can be visually observed to determine the presence and location of the flame within the chamber. The holes on the surface of the alumina micro-burner was made to insert the stainless steel (SS) tubing (0.46 mm I.D., 0.90 mm O.D.) for the fluidic connections and fixed with RESBONDTM 907GF which can endure up to 1260°C. Once the adhesives to bond the quartz window and the SS tubing were cured at room temperature for 24 hours, the micro-burner was formed for testing. (b)
(a)
(c)
(d)
Figure 9: Photographic images of (a) an open micro-burner, (b) a diffusion flame from (a) using H2 and O2/air as fuel and oxidant sources, (c) an enclosed micro-burner. (d) 3D plot of the temperature measured at the locations indicated in (c).
Hydrogen and oxygen were created using a commercial electrolyzer coupled to the custom-made flow manifold to control the flow rates of each stream (Bae, 2012). Because the electrolyzer produces hydrogen and oxygen at a fixed stoichiometric ratio (H2:O2 = 2:1), a miniature pump was added to deliver air to the oxygen line to independently control the fuel-to-oxidant ratio. Figure 9(a) and (b) shows the micro-burner with the quartz window and the oxy-hydrogen flame within the combustion 463
Fabrication of Powder-based Ceramic Micro-burners
T. Do, C. Shin, J. Yeom & P. Kwon
channel, respectively. The flow rate of H2 and air were set at 40 and 20 mL/min, respectively. Note that 20 mL/min of O2 was also flowing in the oxidant stream produced from the electrolyzer. A flame image was taken using a Nikon DSLR camera (D50, shutter speed: 30 second, ISO 100) in a dark room (Figure 9(b)). Under normal circumstances, an oxy-hydrogen flame is not visible to bare eyes or regular cameras, and therefore the special UV filter with the signal intensifier would be required to image the flame. We introduced a trace amount of organic contaminants (diffusion oil) to visualize the flame. After we identified the H2 and air flow rates that anchored flame inside the combustion channel, we tested the enclosed micro-burner with the range of similar flow conditions. The flame generated within the enclosed micro-burner cannot be observed due to the opaque nature of the alumina walls. Therefore, we indirectly verified the presence of the flame by measuring the temperature of the outer wall of the micro-burner. While the adiabatic flame temperature of the flame is calculated to be between 2210 and 3200°C depending on the O2 to air ratio in the oxidant stream, the surface temperatures on the quartz glass and the alumina wall is much below than the flame temperature. The exterior surface temperatures of micro-burner were measured while the flame was anchored inside the channel at the flow rate condition of 35 mL/min of H2 and a mixture of oxidants (17.5 mL/min of O2 plus 20 mL/min of air). Figure 9(c) shows the locations on the exterior surface of the micro-burner where the temperatures were measured. The maximum surface temperature of alumina surface was measured to be below 75°C. The regions of the highest temperature indicate the location of the flame, which resembles the flame location of the quartz/alumina micro-burner (see Figure 9(d)). The flame was continuously stable without disruption during the two-hour testing. We have tested this microburner on the course of 6 months and observed no mechanical failure or any sign of leak through the system, manifesting the long-term stability.
4 Conclusion In this research, we developed a unique processing method to fabricate internal channel on ceramic material that can be used as flame ionization detector. By optimizing the design of graphite inclusion and sintering process, we made the sample with the completely interior cavity. This method also allows us to achieve better surface finish of the internal channel easily by polishing the graphite before placement into the die. The open micro-burner with attached transparent quartz was connected to hydrogen and oxygen sources. We were able to capture the flame created inside the channel. The flame is stable that can last for around 2 hours if providing oxygen and hydrogen continuously. By adjusting the flow rate of oxygen and hydrogen, we optimized flame shape in order to achieve good signal on electrode assembled on the exhausted channel. We have developed machining technique to drill into the alumina samples partial sintered at 800°C with the acceptable surface finish. The drilling step connects to the hydrogen and oxygen sources for the micro-burner function. Smooth surface finish allows laminar airflow in internal channel.
References Bau, H., Ananthasuresh, G. K., Snatiago-Aviles, J., Zhong, J., Kim, M., Yi, M. & Espinaza-Vallejo, P. (1998) Ceramic Tape-Based Meso Systems Technology. The Proceedings of ASME International Mechanical Engineering Congress and Exposition, Anaheim, CA, ASME International, 15-20. Christian, M. & Kenis, P.J.A. (2006) Ceramic Microreactors for On-site Hydrogen Production from High Temperature Steam Reforming of Propane. Lab on a Chip, 6, 1328-1337. Christian, M. & Kenis, P.J.A. (2007) Fabrication of Ceramic Microscale Structures. Journal of American Ceramic Society, 90(9) 2779-2783. 464
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T. Do, C. Shin, J. Yeom & P. Kwon
German, R.M. & Munir, Z.A. (1976) Surface area reduction during isothermal sintering. Journal of the American Ceramic Society, 59 (9-10), 379-383. German, R.M. & Lathrop, J.F. (1978) Simulation of spherical powder sintering by surface diffusion. Journal of Materials Science, 13(5), 921-929. Ha, J.S. (2000) Effect of Atmosphere Type on Gelcasting Behavior of Al2O3 and Evaluation of Green Strength. Ceramic International, 26, 251–254. Halcomb, D.L. & Rey, M.C. (1982) Ceramic cutting tools for machining unsintered compacts of oxide ceramics. American Ceramic Society Bulletin, 61(12), 1131-1314. Hasebe, S. (2004) Design and operation of micro-chemical plants—bridging the gap between nano, micro and macro technologies. Computers & Chemical Engineering, 29(1), 57-64. Hessel, V. & Lowe, H. (2003) Microchemical Engineering: Components, Plant Concepts, User Acceptance – Part II. Chemical Engineering & Technology, 26(4), 391-408. Jensen K. (1999) Microchemical Systems: Status, Challenges, and Opportunities. AIChE Journal, 45(10), 2051-2054. Jensen K. (2001) Microreaction engineering – is small better? Chemical Engineering Science, 56, 293303. Jensen K. (2005) Chapter 1: Silicon-Based Microreactors. Microreactor Technology and Process Intensification, Edited by Wang & Holladay. American Chemical Society Series, Wilely, 2-22. Khoong, L. E., Tan, Y.M., & Lam, Y.S. (2001) Overview on Fabrication of three-dimensional structures in Multi-layer Ceramics Substrate. Journal of the European Ceramic Society, 30, 19731987. Klocke, F. (1997a) Modern Approaches for the Production of Ceramic Components. Journal of the European Ceramic Society, 17(2-3), 457-465. Klocke, E., Gerent, O. & Schippers, C. (1997b) Machining of advanced ceramics in the green state. CFI Ceramic Forum International, 74(6), 288-290. Knitter, R., Gohring, D., Bram, M., Mechnich, P., & Broucek, R. (2000) Ceramic Microreactor for High-Temperature Reactions. 4th International Conference on Microreaction Technology, AIChE Spring Meeting, March 5-9, 2000, Atlanta, Georgia, Topical Conference Proceedings, 455-460. Knitter, R., Gohring, D., Risthaus, P. & Hauȕelt (2001a) Microfabrication of Ceramic Microreactors. Microsystem Technologies, 7, 85-90. Knitter, R., Lurk, R., Rohde, M., Stolz, S. & Winter, V. (2001b) Heating Concepts for Ceramic Microreactors. 5th International Conference on Microreaction Technology, AIChE Spring Meeting, May 27-30, 2001, Strasbourg, France, Springer Verlag, Berlin, 86-93. Landham, R.R., Nahass, P., Leung, D.K., Ungureit, M., & Bowen, W.E. (1987) Potential Use of Polymerizable Solvents and Dispersants for Tape Casting of Ceramics. American Ceramics Society Bulletin, 66, 1513–16. Ma, J., Xie, Z., Miao, H., Zhou, L., & Huang, Y. (2003) Elimination of surface spallation of alumina green bodies prepared by acrylamide-based gelcasting via poly(vinylpyrrolidone). Journal of American Ceramic Society, 86(2), 266–272. Marre, S., Adamo, A., Basak, S., Aymonier, C., & Jensen, K.F. (2010) Design and Packaging of Microreactors for High Pressure and High Temperature Applications. Industrial Engineering and Chemical Research, 49, 11310–11320. Meschke, F., Riebler, G., Hessel, V., Shurer, J. & Baier, T. (2005) Hermetic Gas-tight Ceramic Microreactors. Chemical Engineering & Technology, 28(4), 465–473. Shin, H., Kok, C.K., Kwon, P., & Case, E. (2004) Meso-scale channels on Ceramics by Machining. Journal of Manufacturing Processes, 6(1), 15-23. Yao, X., Zhang, Y., Du, L., Liu, J., & Yao, J. (2015) Review of the applications of microreactors. Renewable and Sustainable Energy Reviews, 47, 519-539.
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