Dip coating

Dip coating is a technique that is normally used to produce thin electrodes or the electrolyte in solid oxide fuel cells. A diluted tape-casting slurry was used as a dip-coating slurry

2.1. FABRICATION METHODS 73 Recipe 8YSZ Butvar Tri-X PEG DBP MEK:EtOH YSZ

(g) (g) (g) (g) (g) ratio 3:2 (g) vol.% 1 30 3.36 0.2 2.49 2.233 8.70:5.80 16.2 Y-7.4A(added) 4.028 0.452 0.027 0.335 0.301 3.52:2.35 7.4 Y-3.4B(added) 2.572 0.288 0.017 0.213 0.191 5.61:3.74 3.4 Table 2.1.4: YSZ tape casting slurry recipe 1, diluted with 3:2 MEK:EtOH to make slurries Y-7.4A(added) and Y-3.4B(added). These are combined with the original Y-7.4A and Y- 3.4B slurries to make slurries Y-7.4A2 and Y-3.4B2, which are used for dip-coating. Tri-X = Triton-X, PEG = polyethylene glycol, DBP = dibutyl phthalate, MEK = methyl ethyl ketone, EtOH = ethanol. The solids loading (vol.% YSZ) is also shown.

recipe [3]. It was used to coat many of the earlier LSM tubes. In order to make the slurry, which is called recipe 1, 8YSZ (30.00 g, Tosoh), methyl ethyl ketone (MEK)(>99%, Fisher Scientific) and ethanol (EtOH) (99.9%, BDH) in a 3:2 ratio by mass (14 g), and Triton-X (0.2 g) were placed in a plastic bottle, with 24 zirconia 10 mm diameter milling balls, and milled for 18 h at 160 r.p.m. on a horizontal roller mill (University of St Andrews). Then, butvar (3.2 g), polyethylene glycol (2.49 g), dibutyl phthalate (2.233 g), and some more MEK:EtOH (3:2) (0.5g) were added, and the mixture was shaken for 10 min on an automatic shaker (Vibro Mischer E1, Chemap AG). Finally, it was milled for a further 4 h on the horizontal roller mill.

The slurry was diluted by adding further MEK:EtOH (3:2) to it in different ratios, and then shaken by hand for 5 min, to make dip-coating slurries Y-7.4A, Y-3.4B, Y-3.4C, Y-5.1D, and Y-7.3E, with YSZ solids loadings of 7.4%, 3.4%, 3.4%, 5.1% and 7.3% by mass respectively. After the first few dip-coatings, a further quantity of recipe 1 YSZ tape casting slurry, along with more MEK:EtOH was added to slurries Y-7.4A and Y-3.4B, as in table 2.1.4, because the level of slurry in the glass vials was insufficient for dip-coating the tubes. It was mixed for 15 min in an ultrasonic bath (Ultrawave Ltd.), and then shaken by hand for 5 min. The combined slurries were called Y-7.4A2 and Y-3.4B2, and had the same YSZ solids loading as the Y-7.4A and Y-3.4B slurries.

A further batch of tape casting slurry was made up, called recipe 2, which had a very similar composition to recipe 1, except that the YSZ used was 8YSZ (Pi-Kem, surface area 6.9 m2 _g−1_{, average particle size 0.21 µm). From it, three YSZ slurries were made}

by diluting with 3:2 MEK:EtOH as before: slurry Y-3.4F which had 3.4 vol.% YSZ, and slurries Y-7.3G and Y-7.3G2, which had 7.3 vol.% YSZ.

Another type of dip-coating slurry was also made, with 3YSZ (Tosoh TZ-3Y-E ‘easy sinter’ powder, surface area 15.1 m2 _g−1_{, primary crystallite size 26 nm). This powder}

(35.6355 g) was ball milled with Triton-X (0.2410 g) at 100 rpm overnight, and then butvar (0.3500 g) was added, and 3:2 MEK:EtOH (6.8 g), and it was milled for a further 4 h at 100 rpm, which gave slurry H, with 15.8% vol.% 3YSZ. This slurry was further diluted with 3:2 MEK:EtOH, to give slurries Y-10.0I, Y-7.5J, and Y-4.9K, with 10 vol.%, 7.5 vol.%, and 4.9 vol.% 3YSZ respectively.

a) b)

Figure 2.1.7: Electric 12 V DC motor used to dip coat tubes with YSZ electrolyte a) connected to power supply b) ready to dip coat the LSM tube in YSZ slurry.

YSZ electrolyte dip coating by hand

Initial dip coating of LSM tubes by YSZ slurries Y-7.4A and Y-3.4B was carried out by hand. An LSM tube was grasped in tweezers and slowly lowered into the dip-coating slurry, held there for a few seconds, and then slowly raised out again. A porous furnace brick, with thin alumina rods stuck into it vertically, was used to hold the samples as they were dried at 80 ◦_{C for 20 min, and during sintering. The tubes were placed on it with the}

alumina rod inside the tube, holding it upright, with the sealed end at the top. They were sintered at 2◦_{C min}−1 _{to 500}◦_{C, 3}◦_{C min}−1 _{to 1350}◦_{C with a 1 h dwell, and 3}◦_{C min}−1

to room temperature.

YSZ electrolyte dip coating with 12V DC motor.

It is important to control the thickness of the YSZ layer more precisely in order to reproduce the same thickness of YSZ electrolyte each time cells are made. Fig. 2.1.7 shows a 12 V DC motor, which was used to raise and lower LSM tubes into the YSZ dip coating slurry. The speed is proportional to the voltage supplied to the motor. A brass drum with a 2 cm diameter was added to the motor, which winds the thread holding the tube. The voltage was set at 12 V, which gives 18 r.p.m. Therefore, the rate at which the thread is wound is 1.88 cm s−1_{. In order to switch between raising and lowering the tube, the polarity of the}

motor was reversed.

The dip-coating procedure is as follows: a thin string was tied around the open end of the LSM tube, and it was lowered into the slurry until only about 5 mm of the tube protruded from the surface of the liquid. It was held there for 5 s, and then the motor polarity was reversed and it was raised up from the slurry. Sometimes, a tube would contact the side of the glass vial as it was being raised, which would scrape the YSZ slurry from the tube, damaging the dip-coated layer. After the tube was fully out of the slurry, the inside lip of the vial was touched to the bottom of the tube for a couple of seconds to allow the excess slurry to drain off. A bent paper clip was inserted into the tube to hold

2.1. FABRICATION METHODS 75 Recipe NiO/YSZ ink IPA Ink:solvent Solids Solids Tubes dipped

no. (g) (g) (mass ratio) (vol.%) (mass%) (LSM recipe no.)

1 6.585 3.228 2.0x 8.1 39.8 4

2 9.516 4.678 2.0x 8.1 39.7 6,7

3 13.588 5.495 2.47x 8.9 42.2 8

Table 2.1.5: NiO/YSZ dip-coating slurry formulation and LSM tubes dipped. IPA = isopropanol.

it, the thread was untied, then the tube was placed on a vertical alumina rod stuck into a furnace brick, and sintered. It was noted that when the YSZ slurry was shaken before dipping, the coating was more homogeneous. Therefore, there is some settling of YSZ in the slurry.

The tubes were dried at 80 ◦_{C in a drying oven for at least 10 min, to evaporate the}

dip-coating solvents. They were then sintered at 1◦_{C min}−1 _{to 500}◦_{C, 3} ◦_{C min}−1 _{to the}

dwell temperature, which was usually 1350◦_{C, but occasionally 1400}◦_{C, with a 1 h dwell,}

and then cooled at 3◦_{C min}−1 _{to room temperature. Many of the dip-coated tubes were}

photographed after dip-coating, and also after sintering.

NiO/YSZ fuel electrode dip coating with 12 V DC motor

A NiO/YSZ screen printing ink was used to dip-coat the earlier LSM tubes. It was made as described in section 2.1.3, was diluted with isopropanol (99.8%, BDH), in a 2:1 ratio by mass, to make recipe 1, table 2.1.5. Further NiO/YSZ ink and isopropanol was added to this composition to make recipes 2 and 3 in the same table. LSM tubes from extrusion recipes 4, and 6 - 8 were dipped in the NiO/YSZ slurries as in table 2.1.5. The tubes were all sintered with the following profile: 1◦_{C or 2}◦_{C min}−1 _{to 80}◦_{C, 0.5 h dwell, 1} ◦_{C or 2} ◦_{C min}−1 _{to 500}◦_{C, 3}◦_{C min}−1 _{to 1350}◦_{C, 2 h dwell, 3}◦_{C min}−1 _{to room temperature.}

The amount of heat received by the samples was monitored with the use of 1130◦_{C - 1400} ◦_{C sintering process control rings, type PTCR - STH (Ferro Electronic Materials BV). The}

rings shrink according to the maximum temperature reached, and the dwell time. They can be used to compare the amount of heat energy that the sample has experienced in one sintering, to another.