Development of Metal Oxide Sensors for Gas Analysis to Monitor Gas Atmospheres in Industrial Furnaces

In many indus­tri­al sec­tors, gas ana­ly­sis is an indis­pensable tool for eco­no­mic­al and safe pro­cess con­trol. In indus­tri­al fur­nace tech­no­lo­gy in par­ti­cu­lar, know­ledge of the cur­rent sta­te of the fur­nace atmo­sphe­re plays a cru­cial role and con­tri­bu­tes to pro­duct qua­li­ty, pro­cess con­trol, and com­bus­ti­on opti­miza­ti­on, as well as to ope­ra­tio­nal moni­to­ring, plant and per­son­nel safe­ty, and envi­ron­men­tal pro­tec­tion.
The rese­arch objec­ti­ve is to adapt and fur­ther deve­lop semi­con­duc­tor sensors—already in use in the auto­mo­ti­ve industry—to make them sui­ta­ble for indus­tri­al fur­nace tech­no­lo­gy. Cost-effec­ti­ve metal oxi­de sen­sors for mea­su­ring H₂O, CO, and NO open up the pos­si­bi­li­ty of imple­men­ting cost-effec­ti­ve moni­to­ring of pro­cess and pro­tec­ti­ve gas atmo­sphe­res through inno­va­ti­ve mea­su­re­ment, con­trol, and auto­ma­ti­on stra­te­gies.
To this end, labo­ra­to­ry pro­to­ty­pes of the humi­di­ty, car­bon mon­oxi­de, and nitro­gen oxi­de sen­sors were manu­fac­tu­red at the Insti­tu­te of Inor­ga­nic Che­mis­try (IAC). The­se pro­to­ty­pes were eva­lua­ted under labo­ra­to­ry con­di­ti­ons at the IAC and elec­tri­cal­ly cha­rac­te­ri­zed.
A gas sen­sor test bench has been set up at the Insti­tu­te for Indus­tri­al Fur­nace Engi­nee­ring (IOB). After being trans­fer­red to the gas sen­sor test bench at the IOB, the humi­di­ty sen­sor was tes­ted in a cham­ber furnace.

Inves­ti­ga­ti­ons of the humi­di­ty sen­sor based on the zeo­li­te H‑ZSM‑5 have shown that

• the H‑ZSM‑5 sen­sor mate­ri­al is ther­mal­ly sta­ble up to 1000°C and is not redu­ced in an H₂ atmosphere,
• the sen­sor exhi­bits a signi­fi­cant increase in con­duc­ti­vi­ty in an H₂ atmosphere,
• no direct influence of the flow velo­ci­ty on the sen­sor impe­dance can be detected,
• at a tem­pe­ra­tu­re of 500°C, mois­tu­re con­cen­tra­ti­ons up to 1600 ppm(V) can be cle­ar­ly detected,
• at an ope­ra­ting tem­pe­ra­tu­re of 750°C, the sen­sor shows no signi­fi­cant chan­ges in impe­dance in respon­se to chan­ges in mois­tu­re concentration.

Inves­ti­ga­ti­ons of the car­bon mon­oxi­de sen­sor have shown that

• gal­li­um oxi­de is the only one of the mate­ri­als stu­di­ed (Ga₂O₃, WO₃, BaS­nO₃) that exhi­bits sen­si­ti­vi­ty to CO at 600°C,
• that gal­li­um oxi­de remains in a nano­par­ti­cu­la­te, sin­gle-pha­se sta­te even after heat tre­at­ment at 1000°C.

Inves­ti­ga­ti­ons of the nitro­gen oxi­de sen­sor have shown that

• the (NO⁺,Na⁺)-Al₂O₃- mem­bra­ne is not sui­ta­ble for use in a tem­pe­ra­tu­re ran­ge of T ≥ 300°C.