SE1930413A1 - Transparent membranes for enabling the optical measurement of properties of soot-containing exhaust gas, and modes of operation for keeping their surface clear from deposits - Google Patents

Abstract

Transparent membranes for enabling the optical measurement of properties of soot-containing exhaust gas, and modes of operation for keeping their surface clear from depositsThe invention relates to a window that is equipped with a mechanism for maintaining transparency over long exposure times to soot-containing exhaust gas, for enabling the measurement of gas properties by optical techniques. Preferred fabrication is in Micro-Electro-Mechanical System (MEMS)- technology. Said window comprises an optically transparent membrane (1) preferably of Silicon-Carbide (SiC) and fabricated out of a coated silicon wafer, which partly remains as support rim (2), with integrated resistive heaters (3) and electrodes (4). A typical system includes several of such windows, mounted on the wall of the exhaust pipe, with some windows providing access of a light beam that is intended for interacting with the exhaust gas, and others for ensuring transmission of the resulting information-carrying light to an external detector.Two modes of operation are available. The first mode involves periodic regeneration (cleaning) of the membrane surface (1), by pulsed heating, using the integrated resistive heater (3), to temperatures that are in principle sufficient for restoring window transparency by oxidizing (burning) soot deposits. In the second mode the membrane (1) is kept transparent by continuous heating, using the same heater (3) up to a temperature slightly higher than that of the exhaust gas, which results in thermophore tic repulsion of airborne soot particles in close proximity, thus preventing their deposition. Mode 2 can be enhanced using electrode patterns (4) on the membrane (1) and the adjacent rim (2), to which such voltages are applied that an electric field results that exerts a repelling (di)electrophoretic force onto airborne particles. The combined operation is most effective; using mode 2 continuously for minimizing deposition and mode 1 at regular intervals for the removal of any remaining soot and oil film deposits.

Description

fšescriptioii of the invention l State-of-the-art (_ prior art) Full rtharacterizatitin of exhau st gas is essential for nierfting increasingly? strict ernissionrequirements. State-of-the-art on exhatist gas sensing is hased on devices in direct contact vviththe gas flovv", such as eltfctrocheniical sensors for the measurement of gas coniposition andelectrical conductarice (conrliictonietric) sors for' particulate rnatter (Ph/i). Ûpticaltechniques are a rnore robust approach in the harsh environnieiit of the soot--containing gas inthe tailpipe ot' tlre cornhiistioii erigirie and vvould eriahle the rroii-ctiritact collection of rnoreinformation as compared to ciirrently' used techniques, provided that optical access isniaintained find deposits on the vt/indow that provides a view' into tlre tailpipe are rirevfeiiterl.The reduced light transmission through the tvindou' over time. due to the 'htiild--iip of light--ahstiirlvirig layers -of contaniiriaiits in its path of prfopagatirirr, has hinrleired tlre iniplernentatirinof otherwise potentially proinisiing optical approaches for rneasuring exhaust gas properties fll.Results of slitirt-teriri exhaust gas coniprisititiri rneasiirerrierit hased on absorption spectrriscopyfhave heen reported using glass fiher prohes for entering light into the exhaust systern iZi-ifšl.An optical vvinriowf is also used as a functional part in a prohtf for Laser-lritlrirtrfd lncaiitiesceiice(flsll) studies lol. l-lovizever, the duration of regaorted tests on actual exhaiust gas does not exceed3G niinutes Ül. Therefore, ensuriiig riiatical transparency river a vvide spectral range is anessential pre-retiuisite for the successlfiul introdiucti on of optical techniques for ilorig-teirrri in situmonitoring of ernissioris as is required hy regulations.

Tlie vvall of the exhaust pipe is the colder part during normal operation of the coinhustionengine. (jrirrihiistioii results in a hot exhanst gas and, crnrseqneritlyf, in ternperature gradients inthe exhaust pipe. with a descending inaxiniiini temperature at the center of any cross-tsectionalarea al ong the axial direction and a irrininiuni at the wall. The :radial temperature gradient is thecause of the therinoplioretic force that pulls the soot particles in the gas flow towards the colderinner vvfall surfzice. Therrririplrriiiesis (or therrno-migration or thermo-diffusion) can heexplained hy referriiig to the temperature dependence of the Broxvnian rnotion of niolecules, vvfliiclr iricreases vvith ternperatuire. ln a. liiglilyf sirnplified. interpretatiorr, gasniolecules in 'hettveeii the particle and the center of the flow channel vihrate more yfigorously' conipaired those in lvetvveeri the particle and tlre cold vvall, thus yielding a net trirce pushingthe paiticle triwfards the svall.

Soine specific aspects of the state-olÜ-the-art that are directly' relevant to the iriveritioii disclosed here (hrit not part of tlir: clainis) are: l. "llheiririopihoresis is part of the operating niechanisrn of the conductonietric ParticulateMatter (ÉM) sensor. A DC hias voltage of about 45 V is applied across two lnttfr-Digitatrftililectrodes (llïtlšis). Particles in the exhaust gas tllowiiig along the sensor surface are pulledtovvarris the sensor surface, hecau se of tlir: therniophoretic pulliiig frirce (due to the eoldervvall surface on vvlrich the sen s-'orf is niountetl). As a result, particles are deposited and formdendrites (connected strings of paiticles) on the sensor surface, hriclge the two lDEs. Sincecarlioii is electrically' conductisfe, an electricallyf crirrductrve patlr is torirrerl. The resultin gdeerease in resistance is used as a ineasure of PM [ål 'lherniophoresis has actually heen explored in ctintlucttinietric PM serisors as a means for sensitivity control. A reducedstfnsititfity' with incrtfastfd temptfrature vivas confirmerl [Qi t» Resistivfe lieateis ititegraterl vvfitli a sensor are used in the c-'oriducttirrietric PM sensor' .forregularly' heating of the sensor up to a temperature of ahout óíll) ”C for periodicailyoxitlizirig (iiuriiirig) tlie soot, vvliittii is reierifed to as “regeneratiori of the suritatte” [9] 3. Heating of membrane structures fabricated in silicon MEMS technology have beenextensively reported in literature and usually involve a material combination of(poly)silicon, silicon-oxide and silicon-nitride With integrated resistive heaters.Applications are in devices that require only a few degrees of temperature above ambienttemperature, such as thermal floW sensors [l0], While heating is used in thin-film SnOz-based gas sensors for setting the surface temperature for maximum sensitivity [ll]. Hot-Wire blackbody emitters have been fabricated for use in IR optical applications [l2].Silicon-carbide (SiC) has been considered as a robust material that is particularly suitablefor fabrication of hot-Wire emitters operating at high temperatures [l3]. 4, Operation of tlie conductorrietiic Plvl sor is also attfected hy the electric field that is dueto the BC hiasing of the 'The resulting electrophoretic force on chargecl particies ispiroptirtional to the DC liias voltage and an increased deposition rate vvitli voltztge isexperimentallyf confirmed H41.

Self~cleaning vtfindtivvs are described in iaatent literature llíj. This is a passive approacli andinvoives a transparent Window' With a transparent heat harrier thiri~filrn detitisited at tlie innerstirface (the side facing titt: lieat source) and a suitable. tratalytic tliiii-film deposited theitftiii.'lhe lteat harrier serves as a thermal insulattir trwhicli causes a maximum ternperattire ditlferenceacross the vvindovv cluriiig operation of the heat source. The structure is designed in such a tvaythat the ternpeiature ot' the catalyst is sufficieiit tfor oxidizirig any depositetl soot, lience lteetisthe vvindoiv clean during operation. 'lliis patent has no 'hearing on the ciaims formulated here. lïlrior art in terms of patents vtfitli claims that hear some resenihlance to those disclosed here are: l. llse of an integrated resistive heater for constraiiiing gases or liquids to the area of a thinrneniliiarie that is vvithiri the fiptical path of a Transniissititi Electron lvlicrosctipe (TER/l),thus enahliiig monitoring liy this instrument [lol Use of a resistive heater that is co--iiitegrated With a sensing device on a iiieinhrane andexposed to samt-containing exhaust gas, vvith the rilvjectivfe of providing a rneans forprotection of the sensing device by the particle repelliiig thermophoretic force of the heatedrrietnlirane, thus rirevetitiiig deposits vvithoiit the protective pacltagirig that vtfriiiltl he hu? required to ensure proper long-term sensor action [lT/l. 2 iëísclosiire of the inifetitioti Tlie inventioii comprises two components: (i) An opticaliy transparent niernhrane, preferaihiy fabricated in h/iicro-EEilectiti-hfíechanicalSystem tMEhflSi-technologyi, with the purpose of serving as a iilinclon' through ivhich a light tiearn iiasses and used for optieaiiy nieasnriiig properties of a sent-containing exhaust gasin the taiigiipe and is hept free of iieposits rising resistiife heaters and eieetrotie patters.[and (2) Three nieehanisnis for inaintaining tiranspareney over long exposure tinites to soot-containing exhatist gas for enahiiirig the iong~tertn nieastirenieiit of gas properties by opticaitechniques in this harsh environment; (a) heating to oüü-"FGO “C for oxidizing (burning) sootfiepfßsits (inode l), (ti) using tiieriniipiioresis iïoi' preventing depositioii (Initide 2), (c) using(dfleieetroplioresis for preventiiig deposition pins eoniihiiiations of (a) and (b), or (b) and (e),or (a) aiid (b) :uid (e).

As shown in Pig. l The optioally' transparent irieniiirane (i) is preferaiily' iahrieateoi in a iviEiviStechnology iising of a siiicoii vvafer, tvhich partiy reinains as siipport rini (2). tvitii integrated.iesistive iieziteis (3) and eleotrodes ( 1115-) and is inouiitefi as an iiitegrai structural parti otito thevvaii of the exhaust pipe. A typical system inciudes sevferal ivindonfs, vvitli some providingaccess of a light htfam that is intefndtfd for iiiteratïting vafith the exhaust gas, and others tsnsiiringtransmission of the iesuitirig inforrnatioii~oarryirig Eight to externai detectors.

Restoring *vaindovv transparency (nifode i) achieved hy puised (heating of the inernhrane (i)using the integratefd resistive liefatifi' (3) to temperatures that are in principle sufficient: foroxidizing (burning) any deposits. 'iiie inernbrane (i ) is kept transparent (mode 2.) hy continuousheating, using the saine heater (3) to a teiniaefratiire siiglitly' higher than that of the exhaust gas,Which results in a repeiiing therrnopiioietie iforce that pitasfents deposition of soot paiticiesthereiii. This mode of operation ean he enhanced rising eleetrode patterns on and adjaceiit tothe inernixfaiie (4), to vvhicii voltages are appiieti for generatiiig an eieetiiie field that exerts arepelling (düelectrophoretic force onto particles. (ffioniihiiied operation most effective: usinginode 2 oontinuonslyf for rniiiiirii zing fiepositi on :uid niode l at regular intervals 'for the :rent-rivalof any reinaining soot and oil fiini deposits.

The structure is basically composed of an optically transparent membrane. An electricallyconductive layer on top is patterned into a resistor and can be used for heating of the membrane(both in mode l and mode 2). The heater should preferably be optically transparent for effectiveuse of the entire membrane area as Window. Since resistors are normally sensitive totemperature, the same or another resistor fabricated out of the conductive layer can be used formeasuring the local membrane temperature for an improved heater control.

Operation in the harsh environment of the exhaust gas (in terms of temperature and chemicalattack) and the required heating to temperatures in excess of 700 °C is the first reason for thepreferred use of SiC. The dimensions of the membrane result as a compromise between theminimum aperture required for passing a practical light beam, and the maximum diameter thatcan be sustained When considering residual stress in the layers that comprise the membraneand its thickness. Membrane rupture Would result in case of excessive tensile stress, Whilebuckling is generally caused by residual compressive stress. A Wide range of dimensions canbe used. A convenient choice that provides a sufficient membrane strength for reasonabledevice robustness and a sufficient Window area for transmission of practical light beam 3 diameters, is a l-2 um thick circular SiC membrane With a diameter in the range between 50 um and 2000 um.

For application as optical window, the membrane material should be transparent over a widespectrum (ideally between the near-UV (220 nm) and mid-IR (5 nm)). An often-used dielectricmembrane material, such as SiOz, does satisfy this requirement well, while other materials,such as Si3N4, are less suitable and poly-Si can only be considered a transparent material in theIR (and highly absorbing in the visible spectral range). The electrically conductive layers usedas heaters should preferably also be transparent. Low-conductivity (intrinsic) SiC andelectrically-conductive (highly-doped) poly-SiC are both optically transparent over the visiblespectral range (with index of refraction n~ 2.4i0.2 and eXtinction coefficient l<< 0.2). This isthe second reason for considering SiC a preferred material for use as transparent membrane.

Devices can be fabricated in most of the state-of-the-art MEMS processes. One example of apossible process is shown in Fig. 2.

As discussed in the prior art section, the three mechanisms tised for iiiaintairiiiig transparencyover' lorig exposure tiines to stiot-crintaining exhaust gris are riot riew. lrlrwvever, these liave notbeen used for the purpose of enahling the long-term rneasurernent of gas properties by' opticaltechniques. Thermal rixidatiorr of deposits has been dernoristratetl as effective tor the cleanin gof the conductometric PM sensor surface, rising a heater integrated at the hackside of thecerarnic riaeltage. Theiniophoresis has lveeri identified. as orie of the riperzttirig niechanisrrrs ofthe eonductoiiietric Phfl sensor. lvloreover, it has heen disclosed in patent WC? 2009/ l5(i20l .ff\2as a niefchanisrn for protection of sensing dtfvices on a rnembrane. without having to resort topackages based on proteetivte rnaterials surrounding the devices ll7l. fftlso, electrophoresis hasbetfn identified as part of the. operating niefchanisin of the conductometric Plvl stfnsor.'lfhereíiore, the noveltyf of the invention is directly related to (is limited to) the application ofenabliiig tiptical nieastirtfniefrit of properties of exhatist gas in the. tailpipe.

Figure 3 seheniatically shows the structure. of the transparent membrane with one possiblearrangerneiit of the integrated heater that can be heated to 700 *lífiï for oxidation of any sootdeposits. Gther arrangernents vtfitli the sanie ftinetionalitfy' are possible. Hoivetfer, niernbiraiieand lieatei' design slionld preferalily' be such that the lateral ternperature profile results in thehigh temperature over a sufficiently large inner circular area of the niernhrane to restoretransrrrissiriri river the area illuiriinetl hy' :irl irnpinging light hearn (the aperture), twith astiffieient rnargin to the edge of the membrane to limit heat loss to the surroundin support rim,so to en sure a reasonable povver efficiency.

Figure ffll- scheiriaticztllyf slioyvs the structure of the transparent rnenihrarie vvitli one tiossililearrangernent of the integrated heater that can lie heatetl to about lllll “C above exhatist gastemperature, for enahling the. gtsneratitin of theiinti~phoretic ftirees for repeflling airhorneparticles in cltise proxiniity. (šther arrangernents vaitli the sanne furictionality are possible.Figure 4 is similar to Pig 3, vvith the exception that less stringeiit requirements are irnposefd to the heatet design (ihaxinttint tetznpeiature about fšüí) ”få and a second resistor eouid optionailybe hnpiernenttfcí as stfnsor for rneasuring the teniptfrature of the exhaiist gast Linder specialeirouihstatiees, the heating and teihperatxire sensing can be itznpieinented ih the same resistiveheatet. ioïïigoure 5 seheniatieaiiy sitiows the structure of the transparent ihetnbrane xvith ohe possihiearrangemeiit of integrated eieetrodes on the menibrantf and surrounding rim anea that ean beusett for the generation of the appropriate electrical fieids foi' eäectropiioietie repuisioit oftfhargtfd aífborne 1aai"tif.:ies in close. proximity. Other tfleetrodtf arrangenitfitts :with the sametiilhetionaiity' are possible.

An eíectrode geontetrjtf eomposed of a sitnaii area eieetrode at the center of the ineihbrahe anda, snrrfmnding ring-simped eieetiffotie oti the iiitïi cart be used tor geherzttiitg an eieetne field vvitha ihaxinïtinï íaterai gradient in fieid strength (such as a pre--doinihantijy fringe fieid) for :ii--eieetropiwretie iepuisioit of neutral (Infnfi-etiargetí) paiticies in close proxirneity, Figxire showvsone exampie of ah eieetrocie arrangetnent that is particularly saitahie for outiiniitg thecivnceptnai idea, but tvtitei' tjenare-practical) eieetrode arifattgettients and ittipientefttzttitvns arepossibie.

References [l] J. írítvdgl-gins-zwfi and RP. Tatam, Optical gas seiising: a revievxf, Technol. 24(2013) 012004 (59pp) doi:10.l088/'0957--0233/24/1/012004 [2] G. Dooly, E. Lewis and C. Fitzpatrick, On-board monitoring of vehicle exhaustemissions using an ultraviolet optical fibre based sensor, J. Opt. A: Pure Appl. Opt. 9(2007) S24-S31 doi:10.1088/1464-4258/9/6/S05. [3] X. Chao, JB. Jeffries and R.K. Hanson, Absorption sensor for CO in combustion gasesusing 2.3 um tunable diode lasers, Meas. Sci. Technol. 20, 2009, 115201 [4] R.M. Spearrin, C.S. Goldenstein, J .B. Jeffries and R.K. Hanson, Fiber-coupled 2.7 umlaser absorption sensor for C02 in harsh combustion environments, Meas. Sci. Technol.24, 2013 055107 [5] P. Geiser, New Opportunities in mid-infrared emission control, Sensors 2015, 15, 22724-22736; doi:10.3390/s150922724 [6] Z. Zhang, L. del Re, and R. Fuerhapter, Fast hybrid sensor for soot of production CIengines, SAE Technical Paper 2017-24-0137, 2017, doi:10.4271/2017-24-0137. [7] M. Degner, N. Damaschke, H. Ewald and E. Lewis, Real time eXhaust gas sensor withhigh resolution for onboard sensing of harrnful components, Proc. IEEE Sensors 2008,pp. 973-976. [8] A. Malik, H. Abdulhamid, J. Pagels, J. Rissler, M. Lindskog, P. Nilsson, R. Bjorklund, P.J ozsa, J. Visser, A. Lloyd Spetz and M. Sanati, A potential soot mass determinationmethod from resistivity measurement of thermophoretically deposited soot, Aerosolscience and technology, (45), 2, 284-294, 201 [9] G. Hagen, C. Spannbauer, M. Feulner, J. Kita, A. Müller and R. Moos, ConductometricSoot Sensors: Intemally Caused Therrnophoresis as an Important Undesired Side Effect,Sensors 2018, 18, 3531; doi:10.3390/s18103531 . . id="p-10" id="p-10"

[10] J. Chen and C. Liu. Development and Characterization of surface micromachined, out-of-plane hot-wire anemometer. Journal of Microelectromechanical Systems, 12(6): 979-988, Dec 2003. ISSN 1057-7157. doi: 10.1109/JMEMS.2003.820261. 11. 11. id="p-11" id="p-11"

[11] T.A. Miller, S.D. Bakrania, C. Perez and M.S. Wooldridge, Nanostructured tin-diodematerials for gas sensor applications, in K.E. Geckeler and E. Rosenberg (eds),Eunctional nanomaterials, Am. Sci. publishers, ISBN 1-58883-067-5, 2006. 12. 12. id="p-12" id="p-12"

[12] W-J. Hwang, K-S. Shin, J-H. Roh, D.-S. Lee and S.-H. Choa, Development of micro-heaters with optimized temperature compensation design for gas sensors. Sensors,11(3):2580-2591, 2011. 13. 13. id="p-13" id="p-13"

[13] J. B. Casady and R.W. Johnson, Status of silicon carbide (SiC) as a wide-bandgapsemiconductor for high-temperature applications: A review, Solid-State Electronics39. 10(1996): 1409-1422. 14. 14. id="p-14" id="p-14"

[14] G. Hagen, W.R. Feulner, M. Schubert, A. Müller, G. Rieß, D. Brüggemann and R. Moos,Capacitive soot sensor for diesel exhausts, Sensors and Actuators B: Chemical, 236(2016), 1020-1027, doi.org/10.1016/j.snb.2016.05.006 . . id="p-15" id="p-15"

[15] W.P. Partridge, M.D. Kass, Patent US 7,247,383 Bl, Integrated self-cleaning windowassembly for optical transmission in combustion environments, March 26, 2004. 16. 16. id="p-16" id="p-16"

[16] J. Amiano and S.E. Mick, Patent WO 2008/ 141 147 A1, Microscopy support Structures,May 9, 2008. 17. 17. id="p-17" id="p-17"

[17] N.R. Behne1, A. Krauss, F. Laermer, G. Flik, T. Fuchs, J. Wolff, Micro-e1ectromechanica1 sensor e1ement, Patent WO 2009/ 156201 A2, December 30, 2009.

Claims (1)

1. Ciaíins Ûptieziiiy transparent nierrifirztiie wvith tfie purpose of servin g as a xvioiidrisv tiirough svhieii aEight ihearn passes and iised for optieaiiy' nieasuriiig properties of a soot--ooiitaiiiiiig exhaustgas, and is kept free of deposits using a resistitfe iieatei' and/oi' eieetrride pattern iritegratetiin tiie nieinhraiie. Ûptieztiiy traiispztrent ineniixraiie as in eiaini i, arid eornpiised of Siiietinflarfiiiie (Siff) vvithtiie purpose of servin g as a »vindotv tiirrnigh vtfhich a light hearn riasses and used foroptieaiiy' ineasuring properties of a soot-eontaining exhaiist gas, anti is ktïpt free of deprisitsusing a resistive heater and/rar eieetrode pattern integrated in tiie rnerniiraiie. Üptieaiiy' transparent nitfnihraiie as in eiairn i or eiairn 2, and fahrieattfti in Mieroâitfetro-fi/ieehanieai System (h/iiših/iššyteehrioiogy *with the purpose of serving as a tvindon' throughWhich a iight heani passes and o sed for optieaiiy nieasiiring properties of a soot-eoiitainiiigexhaust gas, and is kept free of deposits using a resistive heater integrated in the nieinhranetiiat is iieatiïti to a ternptfrature suffieieiitiy high for oxidizing (burning) deposits (typieaiiyin the range 609 °C to 7% “fi Ûptieaiiy transparent nieirrhraiie as in eiairn i or 2, and fahrieateci in h/iiero--Eiectrrr-hfieeiiariioai Syfsteni thfiišhfiSi~teohnoir>gy with tfie purpose of sefrvirig as a wfiridtitt/ tirnfotightifhich a Eight ihearn passes and used for optieaiiy ineasuriiig properties of a soot--contaiiiingexhatist and is irept ffree of rieposits using a :resistive heater iritegrated in the rnernhranethat is heated to a temperature iiieirease (differential typieaiiy in the range St) “C to iiií) Û Ü)svith :respect to tfie exiiaust gas terriperattire ttypieatiiy' in the range Éttiii °C to SOU °Cii togenerate a tiierinopiioretie irepuisiwfe force acting on airhorne partieies in eiose proxiniitjy.Optieaiiy transparent inernhrane as in eiairn 4 tvith in addition a irieai temperature, stfnsor,stieh as a tenitierature-dependent resistor, for ineasuririg tiie terriperattire of the exhatist gasat titt: position of the niernhrane, Ûptieaiiy' transparent ineinhrarie as in eiairn i or 2, and fahricated in híiero~išieetro~hfitfehanieai Systern titfiEiviS)áetthnoitigy' tvith the purpose of serving as a wtfindow' throughtwhieii a ii ght hearn passes and used for taptioaiiy rneasuring properties of a sofooeoiitainingrfxiiaust gas, and is kept free of tieíatisits using an eieetrode pattern on the nitfnihraiie andtiie surrounding rini nfitii such tvoitages applied that an eieetrophoretio repuisiwfe foree isgenerated aeting on tfieetrieaiiy eharged airhorne partieitfs in eiose proxiinity, Üptieziiiy transparent rnernhrattie in eiaini i or 2, and fahrieateti in iviiei'ii~i5ieotrr>-ivieehaiiieai System (hfiEiX/iši--teehnoiogy With the purpose of serving as a vøindon' tiirotighvtfiiiofi a iigiit heani aiid used fiir tiptioaiiy nieastirin g tiifopeitioes of a sorihoontaiiiirigexhatist gas, and is kept free of deposits using an eiectrode pattern on the inernhrane andthe surrounding iioin, svith such voitages apriiieti that an di~eieotrritiiitii'etioe repnisisfe force isgenerated aeting on neutraiiy eharged airhorne partieies in eiose proxiinityz Operatin g the tvindiiuf aoeorriin g to ciaini 5, or 5 or 7, with periodie iiiteiiiiptions duringtvhieii the heater is operated in the niode as in eiaini 4 for eieaning of the surface of anyreniaining dtfprisits hy soot rixidation,