In this contribution suitability of silicon-technology based microreactors for performing research on reactions where temperature control is essential is discussed. The versatility of silicon micromachining technology is elucidated by describing the fabrication and performance of two types of microreactors for studying Rh-catalyzed gas phase reactions. Depending on the design of the microreactor, working temperatures up to 800 degrees C can be obtaine

Gardeniers, Johannes G.E.

Tiggelaar, Roald M.

van den Berg, Albert

English

University of Twente Research Information

chimica oggi • Chemistry Today • Vol 24 nr 2 • March/April 200652MicroreactorsSilicon-based microreactors as research tools in chemistryR.M. TIGGELAARJ.G.E. GARDENIERSA. VAN DEN BERGABSTRACTIn this contribution suitability of silicon-technology basedmicroreactors for performing research on reactions wheretemperature control is essential is discussed. The versatilityof silicon micromachining technology is elucidated bydescribing the fabrication and performance of two typesof microreactors for studying Rh-catalyzed gas phasereactions. Depending on the design of the microreactor,working temperatures up to 800 °C can be obtained. INTRODUCTIONMicrodevices like microreactors have fundamentaladvantages in comparison with conventional macroscalesystems due to their characteristic dimensions in the sub-micrometer to the sub-millimeter range (1, 2). Thesedimensions result in small internal volumes and highsurface-to-volume ratios, which lead to improved heatand mass transfer rates. As a consequence, fast reactioncontrol is possible while only small amounts of chemicalsare needed (2-4), which makes microreactors extremelysuitable for performing and studying in a safe wayreactions under extreme conditions of temperature andpressure or reactions involving flammable or toxicspecies. The high thermal conductivity of silicon and thepossibility of Very Large Scale Integration (VLSI) of controlfunctionality like temperature sensors, heating filamentsand flow sensors, make silicon-based microreactors usefulresearch tools for investigations on reactions wheretemperature control is essential (1, 2, 5-7). This paper willdiscuss the fabrication and characterization of siliconmicroreactors designed for the study of high temperaturerhodium-catalyzed oxidation reactions.MICROREACTOR WITH INTEGRATED HEATERSAND SENSORS ON AN ULTRATHIN SILICONMEMBRANEA silicon-based microreactor was developed on the basisof published layouts (5 ,7), which contains a flow channeletched in silicon capped with an ultrathin compositemembrane consisting of silicon and silicon nitride, with onthe outside thin-film heaters and sensors, and on theinside a thin-film catalyst patch (Figure 1).The thickness and composition of the membrane isimportant, from a thermal and mechanical point-of-view.For fast heating of the catalyst and gasmixture, thisthickness should be minimized. On the other hand, fastcooling of the gasmixture in the eventual case of runawayof exothermic reactions requires also high transverse heattransport through the membrane, which would speak fora thicker membrane. In order to control high temperaturereactions, the characteristic heat transfer time-constants ofthe microreactor have to be substantially smaller than theresidence time of ca. 10 milliseconds (8). The mechanicalstability of the membrane is threatened by therequirement of fast thermal response, which implies largetemperature gradients which may result in buckling orrupture of the membrane (5). A compromise on thethickness and composition of the membrane was found,using a thermo-mechanical model, in a compositemembrane composed of 850 nm of heavily boron-dopedmonocrystalline silicon and 150 nm of low-stress siliconnitride (8). Fabrication The fabrication of such a composite membrane with arelatively large size of 30 mm x 500 µm and a uniformthickness is the real challenge in the development of thismicroreactor. In short, the fabrication process is as follows(9): a 525 µm thick p-type (100)-oriented silicon waferwas doped on one side with boron to obtain a p++-Silayer of 850 nm, on which 150 nm of low-stress silicon-rich silicon nitride (SiRN) was deposited. On the backsideof the wafer a 500 µm deep flow channel was etchedusing reactive ion etching (RIE) followed by 20 minutesimmersion in a 25 wt.% KOH solution at 75 °C. The flowchannel was finished by etching 15 minutes in 31 wt.%KOH saturated with isopropanol at 75 °C to release themembrane of all silicon except the boron doped layer.The result of this etching procedure is a flow channel witha length of 30 mm, a depth of 525 µm and a width of500 µm. Magnetron sputter deposition of 20 nm thickrhodium patches at the inner side of the membrane wasperformed via a 3D self-aligning shadow mask. A Pyrexplate with powderblasted inlets and outlets (circularthroughholes with a diameter of 450 µm) was anodicallybonded to the silicon wafer so that the flow channelbecomes closed. Prior to dicing, thin-film heaters andtemperature sensors composed of 10 nm Ta and 200 nmPt were evaporated on the outer side of the membranethrough another shadow mask. Gas connections to theinlet and outlet are realized by positioning themicroreactor onto a metal plate containing tubings tomass flow controllers. These connections are ensured tobe gastight by embedding Viton O-rings between thealuminum plate and the inlet/outlet and subsequentclamping of the microreactor onto this plate. Electricalconnections to the heaters and sensors are made bymeans of a printed circuit board.ExperimentsRh-catalyzed oxidation of hydrogen was performed asfollows: after controlling the temperature of the membraneat 280 °C using the thin-film heaters, the composition ofchimica oggi • Chemistry Today • Vol 24 nr 2 • March/April 2006 53Microreactorsthe inlet gasmixture waschanged from a non-explosive composition (44percent H2 – 0 percent O2 –56 percent He) to astoichiometric composition of(45 percent H2 – 23 percentO2 – 32 percent He). Duringthis composition change, thetemperature of themembrane increased to 400°C. Figure 2a plots thetemperature of themembrane, measured withthe integrated temperaturesensors, as a function of theamount of convertedhydrogen. In Figure 2b thegasmixtures fed to themicroreactor are displayed ina reactant-space diagram,demonstrating that thesemicroreactors are clearly capable of handling otherwiseexplosive gas conditions. For temperatures up to 400 °C no mechanicalproblems were observed and thermal response timesof ca. 1 millisecond were obtained (9). Oxidationreactions at temperatures above 450 °C, e.g. Rh-catalyzed direct partial oxidation of methane intosynthesis gas, was not possible in thesemicroreactors, due to electrical breakdown of thedielectric silicon nitride layer on the membrane. Thisis a design issue: it can be solved by avoiding thesandwich construction of a dielectricum confinedbetween two conductors, viz. p++-Si and platinumthin-film heaters. MICROREACTOR WITH INTEGRATED HEATERSAND SENSORS IN SILICON NITRIDE TUBESA way to prevent the above-mentioned electricalbreakdown of the dielectric material is to omit the siliconpart of the membrane. However, in order to avoidmembrane rupture due to thermal stresses at temperaturesabove 550 °C (5), mechanical decoupling zones have tobe included in the silicon nitride membrane. Amicroreactor was developed that contained a SiRN-membrane with integrated corrugated zones (Figure 3),making this microreactor suitable for Rh-catalyzed directpartial oxidation of methane to synthesis gas, a reactionthat has an ignition temperature of ca. 600 °C.The heater is used to heat gases and catalyst patches totemperatures above 500 °C. Due to Joule dissipation ofelectrical power in the thin-film heating filament, theheater and membrane expand a little. The corrugatedzones are used to intercept this expansion, therebyavoiding failure of the microreactor due to thermalinduced stresses. Fabrication The basic layout of this microreactor is identical to thethin membrane microreactor: the flow channel contains5 heaters and 12 temperature sensors, eachembedded in their own, separate suspended siliconnitride tube. Fabrication of these tubes was as follows(10): with standard lithography and RIE the pattern ofheaters and temperature sensors is transferred into thetopside of a silicon wafer. Subsequently, an SiO2-layeris grown, and two plasma etching steps (directionalwith CHF3 and isotropic with SF6) are performed,followed by removal of the SiO2 and deposition of 1µm SiRN. The flow channel was defined on thebackside and transferred into the silicon using RIE.When the channel was etched to a specific depth, i.e.325 µm, suspended SiRN-tubes with corrugated zonesresult (Figure 3). After etching, the flow channel has alength of 30 mm, a width of 500 µm and a depth of325 µm. Catalytic patches of rhodium were sputter-deposited on these tubes using a 3D shadow mask andelectron beam evaporation was used to deposit a thin-film in the SiRN-tubes. Finally, a Pyrex plate withpowderblasted inlets and outlets was anodicallybonded to the backside of the silicon wafer to close theflow channel. Gas and electrical connections to thisFigure 1. Picture of microreactor (3.0 x 4.5 x 0.1 cm3) in which the flow channel region is marked. The 3D-scheme shows the heater locations on the membrane. Close-ups of the membrane with heatersand temperature sensors and the catalytic layer are shown as well as a 2D cross-sectional view of the channel, all with typical dimensionsFigure 2. Temperature of membrane of microreactor during Rh-catalyzed H2-oxidation as function of the amount of convertedhydrogen (a) gasmixtures provided to reactor in a reactant-space diagram (b) gray-shaded area: explosive regime)(a)(b)chimica oggi • Chemistry Today • Vol 24 nr 2 • March/April 200654MicroreactorsR.M. TIGGELAAR*, J.G.E. GARDENIERS, A. VAN DEN BERG*Corresponding authorUniversity of Twente MESA+ Institute for Nanotechnology, BIOS Lab-on-a-Chip Group,P.O. Box 217Enschede, 7500 AE, The Netherlandsmicroreactor are obtained in the same way as for thethin membrane microreactor (see section 2.1).ExperimentsThe high-temperature electrical behaviour of thesemicroreactors was explored for 2 metal films, i.e. 200nm/10 nm Pt/Ta and 200 nm Pt without adhesion layer,when exposed to a static air environment. For both filmsresistivity and temperature coefficient of resistance,properties that need to be well-defined for proper controlof the microreactor, are subject to large fluctuations fortemperatures above 500 °C. The origin of these effectsare irreversible physical degradation processes that takeplace in thin-films when subjected to high temperatures(11). It is noted that the degradation processes in thin-films containing Pt are much moreaccelerated/pronounced at high temperatures when thesurrounding environments are ‘reactive’ (e.g. air, oxygen)instead of (relatively) inert environments (e.g. nitrogen,argon or vacuum) (11). Since these processes occur at amuch slower rate in Pt thin-films than in Pt/Ta thin-films, Ptthin-films can be operated for a significantly longer timeat a high temperature, and are therefore more attractiveto use in microreactors. In Figure 4 the reliability of thethin-films is shown in an endurance test in which the timewas determined during which the films dissipate aconstant power corresponding to a heater temperature of560 ± 50 °C. Pt/Ta films failed after 27.5h, whereas thePt films did not fail after non-stop operation of 110h.Visual inspection showed that Pt/Ta heaters failed due tosevere agglomeration of material, such that the thin-filmwas converted into a collection of electrically isolatedislands. This effect was (almost) not visible in the Ptheaters. Additional experiments showed thatmicroreactors with suspended SiRN-tubes containing Ptthin-films could be operated reliably and safely for morethan 20h at temperatures up to 700 °C and for 10h at800 °C. Although no Rh-catalyzed oxidation reactionsare performed yet in these microreactors, it is believedthat these reactors can be used as tool for research on thisreaction or other high temperature reactions.CONCLUSIONSThe fabrication and characterization of microreactorssuitable for studying Rh-catalyzed gas reactions underextreme conditions of temperature is discussed. Bothsilicon-technology based microreactors have fast responsetimes (~1 millisecond). The working temperature rangedepends on the design: thin membrane reactors can beused up to ~400 °C, whereas microreactors withembedded silicon nitride tubes can be operated up to800 °C, showing the versatility of silicon microreactors asresearch tools. More general, silicon-based microreactorsoffer new opportunities to screen different catalyticmaterials in a fast and safe way: the performance (e.g.selectivity, yield and/or conversion) of catalysts can betested as a function of feed gas composition, temperature,residence time and pressure.REFERENCES AND NOTES1. K.F. Jensen, Chem. Eng. Sci., 56, pp. 293-303 (2001)2. W. Ehrfeld et al., Microreactors: new technology for modernchemistry, Wiley VCH Verlag, (2000)3. K.F. Jensen, AIChE J., 45, pp. 2051-2054 (1999)4. V. Hessel et al., Chemical micro process engineering: fundamentals,modelling and reactions, Wiley VCH Verlag (2004)5. R. Srinivasan et al., AIChE J., 43, pp. 3059-3069 (1997)6. M. Roumanie et al., Catal. Today, 110, pp. 164-170 (2005)7. C. Alépée et al., Meas. Control, 33, pp. 265-268 (2000)8. R.M. Tiggelaar et al., Sens. Actuators A, 112, pp. 267-277 (2004)9. R.M. Tiggelaar et al., Sens. Actuators A, 119, pp. 196-205 (2005)10. R.M. Tiggelaar et al., Lab Chip, 5, pp. 326-336 (2005)11. R.M. Tiggelaar et al., to be published.Figure 3. Schematic cross-sectional views of flow channel ofmicroreactor that contains thin-film heaters located in SiRN-tubesthat can withstand high thermally induced stresses due to inclusion of corrugated zones (a,b) - SEM-picture of suspendedSiRN-tubes in the flow channel (c)(b)(a)(c)Figure 4. Normalized heater resistances (Rnormalized,heater) as a function of time for thin-films of Pt/Ta and Pt thin-film duringendurance experiments (Pheater ~ 1.7 W)

Silicon-based microreactors as research tools in chemistry

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Silicon-based microreactors as research tools in chemistry

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