Jurnal f the Krean Ceramic Sciety Vl. 7, N. 1, pp. 9~96, 010. DOI:10.191/KCERS.010.7.1.09 A Scientific Apprach fr Imprving Sensitivity and Selectivity f Miniature, Slid-state, Ptentimetric Carbn Mnxide Gas Sensrs by Differential Electrde Equilibria Mechanism Jun-Yung Park, Jihyun Kim, Ka-Yung Park, and Eric D. Wachsman* Department f Advanced Materials Engineering, Sejng University, Seul 13-77, Krea *Department f Materials Science and Engineering, University f Flrida, Gainesville, Flrida 3611, USA (Received January 9, 010; Accepted January 15, 010) sx ƒ f w yk x p w w Á½ xá ƒ ÁEric D. Wachsman* w œw *v w œw (010 1 9 ; 010 1 15 ) ABSTRACT Based n the differential electrde equilibria apprach, ptentimetric YSZ sensrs with semicnducting xide electrdes fr CO detectin are develped. T imprve the selectivity, sensitivity and respnse-time f the sensr, ur strategy includes (a) selectin f an xide with a semicnducting respnse t CO, (b) additin f ther semicnducting materials, (c) additin f a catalyst (Pd), (d) utilizatin f cmbined p- and n-type electrdes in ne sensr cnfiguratin, and (e) ptimizatin f perating temperatures. Excellent sensing perfrmance is btained by a nvel device structure incrprating La electrdes n ne side and TiO - based electrdes n ppsite substrate faces with Pt cntacts. The resulting respnse prduces additive effects fr the individual La and TiO - based electrdes vltages, thereby realizing an even higher CO sensitivity. The device als is highly selective t CO versus NO with minr sensitivity fr NO cncentratin, cmpared t a ntably large CO sensitivity. Key wrds : Gas sensr, Carbn mnxide, Titania, La 1. ƒ ƒ w š, œ w, w w v w t wù. ƒ j š w»yw ƒ, x, x ƒ. 1,) š w»yw ƒ k ƒ ƒ w š. š 3,) w»yw ƒ ƒ (ptentimetric) ƒ w ƒw ƒ d w d (ampermetric) ƒ j ù. ƒ p w ƒ eƒ v» ù Crrespnding authr : Jun-Yung Park E-mail : jyung@sejng.ac.kr Tel : +8--308-388 Fax : +8--308-3 ƒ. -6)»yw š w» p»» y yw. w ƒ p y g (YSZ) w (Pt), (Au). 7,8) yk (CO) ƒ (O ) w CO ƒ yƒ Au Pt w w sx. Pt CO y O w» Au û ƒ x š, YSZ w P O nn-nernstian ƒ x. 9,10) y nn-nernstian w» w, w ƒ f mixed ptential. 11-13) ƒ w y y sx w ƒ x. 9,10) 9
sx ƒ f w yk x p w w 93, œ» y (NO) ƒ sww» yw YSZ w ƒ w e. O e + O ads (1) NO + O ads NO + e O y NO ƒ y ƒ ƒ. ù NO ƒ O N w sx w. w 1,15)» ƒ ƒ ƒ nn-nernstian x w. ƒ y 15) t xk, -k, š j» w p w mixed ptential. 6,11) x w» w, Wachsman f Differential Electrde Equilibria» w w. 15) y -k» y w w s œw. Differential electrde equilibria» (hetergeneus kinetics) w»yw (electrchemical ptential)ƒ.»yw»yw r sx. ƒ y r s³ ƒ. w»yw sx» w w w š,» yw p» w ƒ w ƒ k j w k. 17-)» w, CO ƒ w y ƒ š YSZ ƒ. y (e.g. SnO, ZnO, TiO ) š w x ƒ. 3-6) TiO» ùkü, La () CO ƒ NO x ƒ ùkü š. 0-,7,8) r e p ƒ La yw (mixed inic-electrnic cnductr) NOx ƒ w CO ù CO NO x ƒ w k 350~600 C ƒ w ƒ š š. w ¾ ƒ,9) k,, Fig. 1. Cell cnfiguratin f ptentimetric sensrs using La and TiO -based semicnducting xide electrdes., ƒ, p w wš. w v p w j» w CO ƒ j w y kwš, y ƒw k w j, Pd ƒw w k. w w p- n-k ww ƒ k w jš w. w w ü. w 7,8) La TiO (Cell I) w TiO (Cell II) NO CO w, š p w.. x Fig. 1 La TiO» w YSZ w»yw CO ƒ. ( CO+ N + O )La YSZ TiO ( CO+ N + O ) (Cell I) ( CO+ N + O )Pt, La YSZ TYP, Pt CO+ N + O ( ) (Cell II)» TYP TiO (anatase, 99.9%, Alfa Aesar, USA), Y O 3 (99.99%, Alfa Aesar), Pd(99.99%, Alfa Aesar) 85 :10:5 w. Fig. YSZ. YSZ»q (1 cm cm 0.1 mm Ì) l ve q w Marketech Internatinal (Ceraflex, USA) w š, TiO (anatase), Y O 3, Pd, Alfa Aesar l. La y (aut-ignitin pwder synthesis) w w. 30) La(NO 3 ) 3 5H O, C(NO 3 ) 6H O, š Cu(NO 3 ) 3H O(Alfa Aesar, 99.99%) 7«1y(010)
박준영 김지현 박가영 Eric D. Wachsman 9 Fig.. The device structure f ptentimetric sensrs. Fig.. Fig. 3. Vltage respnse t CO gas at different ranges f O cncentratin as a functin f time at 600C in Cell I. The SEM micrgraph f the thick film depsited n an YSZ substrate. (a) LaCuO electrdes; (b) TiO electrdes. 질산염들은 초순수물에 용해되었고, 0.098의 비율로 구연 산이 적절히 추가되었다. 용액의 온도가 증가하자 질산염 이 녹아있는 용액은 젤(gel) 상태로 바뀌다가 갑작스럽게 열이 발생하면서 재(ash) 상태로 변한다. 결과물은 결정구 조를 이루기 위해 800 C에서 시간 동안 소결되었다. TYP, TiO, La CuO 전극들은 YSZ 전해질 양측에 스크 린프린트 되었고, 15 C 건조된 후 700 C에서 소결되었다. 이후 Pt 선을 각 전극에 Pt 잉크와 함께 붙인 후 전극과 같은 조건으로 열처리 되었다. 생성된 센서 전극 특성을 분석하기 위해 상, 미세구조, 표면적 등이 XRD (Philips APD 370), SEM (JSM-35CF JEOL), BET (Quantachrme Crpratin)에 의해 각각 조사되었다. CO에 대한 센서들의 선택성, 민감성, 응답시간, 환원성 은 다양한 온도에서 CO 농도를 변화 시키면서 연구 되었 다. 3%의 O 와 H O를 함유한 고순도 질소가스 (99.999%) 를 50 sccm의 총 유동속도에서 CO의 농도를 변화시키면 서 운반하는 가스로 사용했다. 센서 신호는 디지털 멀티미 터 (Keithley 000)로 측정되었고 LabView 프로그램을 통 해 컴퓨터에 자동적으로 기록되었다. 3. 결과 및 고찰 한국세라믹학회지 16.7 m g 이하의 표면적으로 매우 미세한 미세구조를 나 타낸다. 또한, 자동-점화 분말합성법을 통해 준비된 La CuO -1 전극내의 초미세 입자들은 각질이 일어난 듯한 형상으로 결합되어 있다. Y O 와 Pd이 첨가된 TYP 복합전극은 미 세구조에 있어서 TiO 전극과 큰 차이가 보이지 않았다. 먼저 YSZ 전해질 전위차 CO 가스센서는 500~600 C에 서 CO와 NO 가스의 민감도와 선택도의 관점에서 각각 연구되었다. Fig. 는 Cell I의 600 C, 10~1,000 ppm CO 농도에서 감지 특성을 보여준다. 그림에서 보는 것처럼 센서는 상당한 가역성과 함께 높은 출력 전압을 보여준 다. 또한 센서의 반응 과 복구 시간은 약 30초 이내로 매 우 자발적이다. 이러한 높은 감지특성은 CO 가스에 대한 La CuO 와 TiO 전극의 초미세 입자 크기로 인한 반응 3-1 Vltage respnse f Cell I (clsed symbl) and II (pen symbl) as a functin f CO and NO cncentratin at 500 and 600C. 센서의 감지특성을 조사하기 전에 감지전극의 물리학 적 특성이 SEM과 BET에 의해 조사되었다. Fig. 3은 스 크린프린팅을 통해 YSZ 기판에 증착된 La CuO 와 TiO 감지 전극의 SEM 미세구조 사진이다. TiO 분말은 0.1~0.3 µm의 크기와 3~5 m g 의 표면적을 갖는 구형 입 자인 반면, La CuO 입자는 0.15 µm 이하의 평균크기와 Fig. 5.
sx ƒ f w yk x p w w 95 Table 1. Sensitivities fr NO and CO f Ptentimetric Sensrs with La and TiO -based Semicnducting Oxide Electrdes Cell I II ƒ q. w 30,31) p-type La (+) n-type TiO ( ) w z w ƒ ùkù q. w p 7,8) La cuo TYP Cell II. Fig. 5 500~600 C 0~1,000 ppm CO NO w Cell I Cell II. ƒ d ü p ƒ ƒw w, ƒ w p. š CO ƒ ƒ y (catalytic xidatin) w t CO ƒ (adsrptin) p w š, w (cnductin band)» (excited) w q. 6,,9) CO + Electrdes O ads CO + e CO sensitivity (mv/lg [CO,ppm] La / -56. YSZ/TiO -31.5 La / YSZ/TYP -8.5-3.9 NO sensitivity (mv/lg [NO,ppm] -6.0-1.15 8.06 1.58 Temperature ( C) 500 600 500 600 Table 1. 500 C 600 C TiO /La Cell I ƒƒ 56. 31.5 mv/lg [CO, ppm] š, TYP/La Cell II ƒƒ 8.5 3.9 mv/lg [CO, ppm]. Cell I, Cell II û ùkü. NO ƒ w k š, Fig. 5 ùkü. Cell I II CO ƒ w ùkü š, NO ƒ w k. w ƒ ƒ, CO, NO, NO d w ƒ j., TiO, WO 3, La w ƒƒ CO, NO, NO ƒ ƒ w 500~600 C w ƒƒ ƒ d w ý. wù p Cell I w š CO ƒ w NO ƒ w (3) k ùkü ( w ). z j w ƒ v w w ƒ w ƒ j.. y w YSZ š w»yw CO ƒ ƒ. y CO ƒ w TiO» (TiO, TYP) NO x ƒ w ùkü La. ƒ ƒ ƒw w, ƒ w w. 500 C 600 C TiO /La Cell I ƒƒ 56. 31.5 mv/lg [CO, ppm] š, TYP/La Cell II ƒƒ 8.5 3.9 mv/lg [CO, ppm]. w Cell I II CO ƒ w ùkü š, NO ƒ w k. w ƒ ƒ, CO, NO, NO d w ƒ j. Acknwledgments 009 w ü w. REFERENCES 1. P. T. Mseley, Slid State Gas Sensrs, Mea. Sci. Technl., 8 3-37 (1997).. R. Mukundan and F. Garzn, Electrchemical Sensrs fr Energy and Transprtatin, Electrchem. Sc. Interface, 13 30-5 (00). 3. A.-M. Azad, S. A. Akbar, S. G. Mhaisalkar, L. D. Birkefeld, and K. S. Gt, Slid-State Gas Sensrs: A Review, J. Electrchem. Sc., 139 3690-70 (199).. A.-M. Azad and E. D. Wachsman, Slid State Chemical Sensrs fr CO, pp. 55 in Prceedings f the 08th Meeting f the Electrchemical Sciety, Slid State Inic Devices II, Edited. E. D. Wachsman, W. Weppner, E. Traversa, M. Liu, P. Vanysek, and N. Yamaze, The Electrchemical Sciety, Penningtn, NJ, 001. 5. N. Li, T. C. Tan, and H. C. Zheng, High Temperature Carbn Mnxide Ptentimetric Sensr, J. Electrchem. Sc., 10 1068-73 (1993). 6. J. Y, S. Chatterjee, F. M. Van Assche, and E. D. Wachsman, Influence f Adsrptin and Catalytic Reactin n Sensing Prperties f a Ptentimetric La /YSZ/Pt 7«1y(010)
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