FIELD OF THE INVENTION
This invention relates to a Yttria stabilized Zirconia (YSZ) flat plate electrolyte for solid oxide fuel cells and a method for the manufacture thereof.
BACKGROUND OF THE INVENTION:
A solid oxide fuel cell (SOFC) consist of a cathode, an anode and a solid electrolyte, it operates at high temperature between 850 and 1000°C. While In operation, the anode gets exposed to a reducing environment while the cathode is exposed to an oxidizing environment. Electrolyte is the heart of the cell, which supports the cathode and anode for planar solid oxide fuel cell.
In planar solid oxide fuel cell, electrolyte is the base. Anode and cathode powders are subsequent deposited on electrolyte plate. To make the electrolyte plate from YSZ powders, the easiest techniques are pressing followed by sintering and alternately tape casting followed by sintering. But to make large thin plate, the pressing technique is not feasible, as the handling of green plate will be delicate.
PRIOR ART IN THIS FIELD:
Ceramic plates are made by pressing and sintering. Before pressing the ceramic granules are made my mixing the ceramic powder and some organic binder. These granules are sieved to get a uniform size of powders. In order to do this, a predefined amount of powder is taken in a die and then pressed. After few seconds the pressure Is released and the pressed components is taken out. This is a very good process for thick plate (more than 1-mm thickness), However, this techniques is not useful for making thin plates (<1 mm thickness) because the handling of the plate is very difficult due to Its poor green strength OBJECTS OF THE INVENTION;
It is therefore an object of this invention to propose a Yttria stabilized zirconia flat plate electrolyte which is easy to manufacture and cost-effective.
It is further object of this invention to propose a Yttria stabilized zirconia flat plate electrolyte which does not require the step of pressing during manufacture.
BRIEF DESCRIPTION OF THE INVENTION;
Thus according to this invention is provided Yttria stabilized zirconia (YSZ) flat plate electrolyte for solid oxide fuel cells.
In accordance with this invention, the flat plate electrolyte has a thickness of 200 to 300 microns and the waviness over 60mm length is 25 microns.
According to this invention is further provided a yttria stabilized zircon (YSZ) flat plate electrolyte for solid oxide fuel cell and a process for the manufacture of yttria stabilized zircon (YSZ) flat plate electrolyte comprising the steps of:-
preparing a suspension of YSZ powder in a solvent as herein described and dispersant as herein described;
followed by milling the suspension and adding a binder, plasticizer and homogenizer Triton-X100 thereto to obtain a slurry;
tape casting the slurry;
drying the tape followed by tape stripping and punching;
subjecting the punched tapes to further drying first at a temperature in the range 50°C to 70°C, and thereafter at 70°C to 90°C and finally at 90°C to 110°C and prefiring is conducted by heating the dried tape from room temperature upto 1350°C and back to room temperature followed by a final sintering to obtain electrolyte plates, having a thickness of 200 to 300 microns and a waviness of 10 to 25 microns over a 60 mm length.
In accordance with this invention, YSZ powder is taken in a zirconia pot and a dispersant such as terpineol is added. Thereafter, solvents selected from as toluene, n-butanol, MEK and is opropanol are poured into a pot. The above mixture is kept for milling in a planetary mill for about 6 to 7 hrs. The planetary mill Is kept at low speed so that grinding does not occur but only milling occurs. After milling is over, the binder, piasticizer and homogenizer are added.
Binder: Poly vinyl Butyral (PVB)
Piasticizer: Poly ethylene glycol (PEG) 400, Dt-butyl phthalate, Benzyl butyl
phthalate
Homogenizer.Triton X-100
The above mixture is kept for mixing for a predetermined period. The slurry is ready for casting. Before casting, room temperature Is maintained at about 20°C. The glass plate and doctor blade are cleaned with acetone. Gas mask should be used to avoid inhalation of the evaporated vapour. Casting speed and blade gap should be fixed before tape casting. Once the room is sufficiently cooled, the tape casting can be started. Before tape casting, viscosity of the slurry should be measured. If the viscosity is higher, little solvent can be added or if lower, the slurry can be kept in a vacuum dessicater for solvent evaporation.
After tape casting, the glass plate Is kept in a drying oven for evaporating the solvents. The tape can then be released and turned upside down for drying the bottom side of the tape. At least one day can be allowed for air drying of the tape. Then the tape is ready for punching of required size.
Immediate sintering of green tape may lead to warpage. To reduce warpage in the fire plate, the green tape should be dried very slowly in the oven upto 90 to 110°C. After punching, the tapes are kept on a paper because one tape may stick to the other and both the pieces may be spoiled, in the first step, the tapes are kept in dryer at for eg. 50 to 70°C for 8 hrs. Second step, the tapes are dried

at for eg 60 to 80°C for 16 hrs. Third step, the tapes are dried at for eg. 70 to 90°C for 8 hrs. Fourth step, the tapes are dried at for eg 80 to 100°C for 16hrs. Fifth step, the tapes are dried at for eg. 90 to 110°C for 8 hrs. After drying, the tapes are loaded into the furnace for preflring The prefiring cycle is given below. The above time period for drying can vary ±30 minutes.
PREFIRING CYCLE
i. Room temperature to 480 to 520°C in 19 to 21 hrs (±1.0 hour)
ii. At 480 to 520°C soaking for about 45 min to 1 hr 45 min.
ill. 480 to 520°C to 1330 to 1370°C in 8 hrs 45 min to 9hrs 15 min.
iv. 1330 to 1370°C soaking for about 2 hrs 45 min to 3 hrs 15 min.
v. 1330 to 1370°C to 980 to 1020°C in 3 hrs 45 min to 4 hrs. 15 min.
vi. At 980 to 1020°C soaking for about 10 to 25 minutes
vii. 980 to 1020°C to room temperature in 5 hrs 45 min to 6 hrs 15 min.
FINAL FIRING CYCLE
Final sintering which is a very important step so far as the flatness of thin sintered plate is concerned. In order to get a good flatness, the alumna setter plates were diamond polished and ensured that setter plates are flat. One over the other, about 3 to 4 pes. Of YSZ presintered plates were loaded. Approximately 200 to 30 gms of weight to zirconia setter plates were put for making the plate straight (fiat).
The final sintering evete Is as given below:
i Room temperature 980 to 1020°C (±20) in about 5 hrs 45 min to 6 hrs 15
min. ii At 980 to 1020°C soaking for about 10 to 20 minutes iii 980 to 1020°C upto 1530 to 1570°C in about 5 hrs 45 min to 6 hrs 15 min.

Iv At 1530 to 1570°C, soaking for about 3 hrs 45 mln to 4 hrs 15 min.
V 1530 to 1570°C to 980 to 1020°C in about 4 hrs 45 min to 5 hrs 15 min.
vi) At 980 to 1020°C soaking for about 15 to 20 min.
vll) 980 to 1020*0 to room temperature In about 5 hrs 45 mln to 6 hrs 15 mln.
In a preferred embodiment the green tape is dried very slowly in the oven upto 100°C. After punching, the tapes are kept on a paper and In the first step, the tapes are kept In dryer at 60°C for 8 hrs. Second step, the tapes are dried at 70°C for 16 hrs. Third step, the tapes are dried at 80°C for 8 hrs. Fourth step, the tapes are dried at 90°C for 16 hrs. Fifth step, the tapes are dried at 100°C for 8 hrs. After drying, the tapes are loaded into the furnace for preflrlng. The preflring cycle is given below.
PREFIRING CYCLE
i Room temperature to 500°C in 20 hrs.
ii At 500°C soaking for about 1 hr.
Ill 500°Cto 1350°C ln 9 hrs.
iv At 1350°C soaking for about 3 hrs.
V 1350°C to 1000°C (about 4 hrs)
vl) At 1000°C soaking for about 10 mln
vll) 1000°C to room temperature (about 6 hrs)
FINAL FIRING CYCLE
The alumina setter plates were diamond polished and ensured that setter plates are flat. One over the other, about 3 to 4 pcs. Of YSZ presintered plates were loaded. Approximately 300 gms of weight of zirconia setter plates were put for making the plate straight (flat).

The final sintering cycle is as given below:
i. Room temperature to 1000°C in about 6 hrs.,
ii. At 1000°G soaking for about 15 min.
III. 1u00°C to 1550°C In about 6 hrs.
iv. At 1550°C soaking for about 4 hrs.
v. 1550°C to 1000°C in about 5 hrs.
vi. At 1000*C soaking for about 15 min.
vti. 1000°C to room temperature In about 6 hrs.
INSPECTIOM AND TESTING
After final stage of sintering, the plates are checked for visual defects like crack, agglomerates, etc, Good plates are segregated for checking flatness, roughness and physical dimension (thickness, length and width), porosity check is carried out on a fractured plate across Its thickness using SEM.
Flatness is measure d by thickness height gauge (M/s Mitutoyo) at nine different points. The fracture surface of the plate are gold coated for SEM analysis. The SEM photograph shows no micro pores (Fig. 1) this indicates that the sintering at 1550°C for 4 hrs. was enough for getting fully dense electrolyte. Few plates are shown in Fig 2.

The flow chart of the manufacturing flat plate is given below.
Flow chart for tape casting and sintering to make flat plate
(Chart Removed)
Though YSZ powder can be pressed and sintered to make flat plate or disc but to make a thin large size, tape casting is the most commonly used technique. It can be cast into tape to any require thickness. Through this technique, the plate can be slotted, punched or any gas passage arrangement can be made because of the flexible nature of the green tape.
Tape casting slurry is composed of several components such as ceramic powder, solvents, dlspersant, binder, plastlclzers and other additives such as homogenizer. The tape casting process was optimized for YSZ tape compositions based on terpineol as a dlspersant. The process and constituents such as binder, etc were optimized to get tape of uniform thickness, easily removable from the base plate after drying. The resultant tapes were free of tiny holes:
The fired plates were investigated for visual cracks, black spot, warpage. Then those plates were examined for flatness, porosity, microstructure, hardness etc. The plates were used for SOFC experiments.

WE CLAIM;
1. A yttria stabilized zircon (YSZ) flat plate electrolyte for solid oxide
fuel cell and a process for the manufacture of yttria stabilized
zircon (YSZ) flat plate electrolyte comprising the steps of:-
preparing a suspension of YSZ powder in a solvent as herein described and dispersant as herein described;
followed by milling the suspension and adding a binder, plasticizer and homogenizer Triton-X100 thereto to obtain a slurry;
tape casting the slurry;
drying the tape followed by tape stripping and punching;
subjecting the punched tapes to further drying first at a temperature in the range 50°C to 70°C, and thereafter at 70°C to 90°C and finally at 90°C to 110°C and prefiring is conducted by heating the dried tape from room temperature upto 1350°C and back to room temperature followed by a final sintering to obtain electrolyte plates, having a thickness of 200 to 300 microns and a waviness of 10 to 25 microns over a 60 mm length.
2. The process as claimed in claim 1, wherein said solvent is selected from toluene, methyl ethyl ketone (MEK), n-butanol or isopropanol.
3. The process as claimed in claim 1, wherein the dispersant is selected from compounds such as terpineol, phosphate ester or fish oil.
4. The process as claimed in claim 1, wherein said milling is effected for a period in the range of 6 to 7 hours.
5. The process as claimed in claim 1, wherein said binder is a ketone compound such as polyvinyl butyral.
6. The process as claimed in claim 1, wherein the plasticizer is selected from compounds such as polyethylene glycol, dibutyl phthalate, benzyl butyl phthalate.
7. The process as claimed in claim 1, wherein the slurry is tape casted at 18 to 22°C.
8. The process as claimed in claim 1, wherein the cast tape is dried at 90 to 110°C.
9. The process as claimed in claim 1, wherein for the sintering cycle, the prefired tape is heated from room temperature upto 1550°C and back to room temperature.