고체전해질형 가스센서의 YSZ제조 및 특성연구

김기동, 조영아, 전진석 , 최동수

한국가스공사 연구개발원

Preparation and Properties of Bulk and Thick Film YSZ as Solid Electrolyte Gas Sensors

K.D.KIM, Y.A.CHO, J.S.Jeon, D.Choi

R & D Center, Korea Gas Corporation

1. Introduction

Solid electrolyte gas sensors(SEGS) have been developed to detect environmentally concerned gases such as SOx, CO2, CO, and NOx. Their applications show several major advantages which are their compactness, low cost, and capability of in-situ monitoring etc. Among SEGSs, one of  commercially available solid-electrolytes is doped zirconia mainly as oxygen sensor. Particularly, YSZ(yttria stabilized zirconia) is a fluorite structured oxide which exhibits excellent oxygen ion conductivity. Because of its high selectivity and sensitivity, ease of operation, fast response and simple construction, it has been used as gas sensor for commercial application.

A YSZ based solid electrolyte gas sensor consists of electrode parts and electrolyte section. Three main modes of operating the sensor cell are as follows

a. concentration cell

b. fuel cell

c. oxygen ion pump

Among these, the wide of concentration cell is mainly used with employing YSZ for oxygen sensors. The structure of concentration cells is as below.

Sample Gas | Sensing electrode || Electrolyte || Reference electrode | Reference gas

In an effort to control the combustion of natural gas and to monitor the composition of flue gas, solid electrolyte(YSZ) based gas sensors have been developed. A successful application will be made in various combustion control such as gas boiler and burners. Therefore, this paper includes the preparation of bulk and thick film type ysz gas sensor, the investigation of their measured gas sensing properties.

2. Experimental

Sensing Device System

A gas sensor device consisted of YSZ, alumina tube with both ends open which was 520□(length),10□(outer diameter), and 8□(inner diameter) as shown in Fig. 1. The alumina tube was commercially available and YSZ of which yttria content was fixed with 8% as purchased from Tosho chemicals. The tube was attached with Pt reference electrode on the inside bottom by applying Pt black which increase electrical contact between Pt mesh and solid  electrolyte, and with Pt sensing electrode. Each electrode was connected to Pt lead wire through physical contact.

Tests of gas sensing performance

Oxygen sensing measurements were performed in a horizontally conventional flow system with high temperature furnace. The device was annealed up to 700℃ before performance tests. The O2 partial pressure of the test gas was prepared by mixing O2 with N2 or Ar in dry condition. While the reference electrode contacted air, the sensing electrode experienced various O2 concentration. The experimental parameters are shown as table 1. The e.m.f(electromotive force) of the sensor device was monitored using a HP electrometer with RS-232 data aquisition.

Characterization

The YSZ and electrodes were characterized by XRD for crystallinity, SEM for surface morphology, □-step for film thickness etc. Also the electrochemical characterization of the sensor cell was conducted by a EG & G potentiometer.

3. Results and discussion

The temperature dependence of the emf of the constructed device was shown in Fig. 2. As temperature increased, the e.m.f trended to increase but the measured e.m.f of cell determines as less than expected. Therefore, in the process of working to cut the discrepancy the contacts of the electrodes were adjusted along with controlled the electrode position. Response time on oxygen concentration at constant temperature increased because of the presence of the large void volume in the device. So, the e.m.f data were collected while keeping in equilibrium condition. The oxygen dependence on e.m.f was well related with Nernst equation in range of concerned oxygen concentration. Also, in the device, oxygen ion conductivity and ion mobility were understood in terms of leakage current and cyclic voltammogram of the cell.

Reference

1. Y.Yan, N.Miura, and N.Yamazoe, J.Ele.Soc., Vol.143, No.1, Feb. 1996

2. R.M.A Kocache, Solid state gas sensor

3. J.Vetron,C.Foster and G.Bai Unpublished paper

Table 1. The parameters of gas sensing performance of YSZ

Parameter			material	comment or specification
tube	material		alumina	commercially available(only bulk type)
	length(O.D, I.D)		520□(10□,6□)	cut it by using wheel cutter(only bulk type)
reference electrode			Pt	Bulk type apply Pt black/annealing at 700℃ Thick film type : Ni+NiO cermet
sensing electrode			Pt	Bulk type using Pt mesh/expose to air Thick film type : 500~2000Å
sensing wire			Pt	thickness : 0.1□
auxiliary phase			-
Gas sensing system		flow	100□/min	a few vol % air(N2/Ar balanced)
		O2 conc.	0 ~ 21%	mix with N2
		temperature	up to 600℃
		measurement	D.C voltage	e.m.f
		electrometer	HP	model HP-34401A