CAPITULO IV INGENIERIA DEL PROYECTO
MATERIALES QUE INGRESAN MATERIALES QUE SALEN
4.3. GESTIÓN DE SEGURIDAD E HIGIENE INDUSTRIAL.
Extrusion was used to produce porous ceramic tubes of (La0.8Sr0.2)0.95MnO3−δ, or
(La0.8Sr0.2)0.97MnO3−δ LSM (Praxair), which act as a support and current collector for
the reversible SOFC. A simple handheld ram extruder was used initially, and then later a mechanised single screw extruder. The extrusion process was developed to produce tubes with a homogeneous and defect free microstructure, suitable for dip coating to make reversible SOFC.
Handheld ram extruder
The handheld extruder ‘Makin’s Professional Clay Gun,’ (The Polymer Clay Pit) was designed to produce decorative clay parts. Fig. 2.1.3 shows the extruder, which is a manual ram extruder propelled by a screw, with a range of different dies and mandrels, examples of which are shown in fig. 2.1.4. The original die was 0.3 mm long, with an external diameter of 18.58 mm, which was 2.1 mm smaller than the inside diameter of the screw cap, allowing it to move relative to the mandrel. It has an internal diameter of 6.50 mm, giving it a die ratio (length/diameter) of about 0.05. The mandrel consists of a perforated disc which allows the extrusion mix to flow through it, and a central rod 4.48
2.1. FABRICATION METHODS 67
Figure 2.1.3: Handheld ram extruder ‘Makin’s professional clay gun’ (The Polymer Clay Pit).
mm wide, which forms the bore of the tube. This was later replaced by the die assembly shown in fig. 2.1.4. It consists of a mandrel, with a perforated disc as before, with a central rod with diameter of 4.35 mm. The die defines the outside of the tube, and has an inner diameter of 6.50 mm and a length of 5.0 mm, giving a die ratio of 0.77, much greater than the original die ratio. Finally, the extruder cap holds the other components in place. The following components were used in the extrusion recipes: 5% A-site deficient
a) b)
c) d)
Figure 2.1.4: Handheld extruder die assembly: a) ram, b) mandrel, c) mandrel and die, and d) mandrel, die and cap.
LSM (La0.8Sr0.2)0.95MnO3−δ), with particle sizes from the manufacturer of d10 = 0.8 µm,
d50 = 1.2 µm and d90 = 4 µm, density 6.6 g cm−3, surface area 8.96 m2 g−1 (Praxair,
Pi-Kem), or 3% A-site deficient LSM (La0.8Sr0.2)0.97MnO3−δ), d10= 0.8µm, d50= 1.3µm
and d95 = 2.6 µm, density 6.6 g cm−3, surface area 3.62 m2 g−1 (Praxair, Pi-Kem). The
binder used was cellulose (2-hydroxyethylcellulose), average MW= 90,000 (Aldrich), the
lubricants were polyethylene glycol (Aldrich) and propan-1,2-diol , and a solvent, distilled water, was also used. The components were mixed in the proportions in table 2.1.1, and
Recipe LSM LSM cellulose cellulose water water PEG PEG
no. (g) vol.% (g) vol.% (g) vol.% (g) vol.%
H-cell1 45.00 37.2 1.5 13.6 9.00 49.1 - - H-cell2 28.63 39.7 1.00 15.3 - - 5.53 45.0 H-cell3 20.00 56.1 0.436 13.5 1.490 27.7 0.175 2.9 H-cell4 28.000 37.0 0.736 10.7 5.717 49.8 0.319 2.5 P1,2D P1,2D H-cell5 10.00 36.3 0.687 27.4 1.4143 33.9 0.1057 2.4 H-cell6 30.00 27.4 2.0610 20.4 8.3167 50.1 0.3014 1.8 H-cell7 10.000 35.1 0.687 26.5 1.554 36.0 0.1040 2.3 H-cell8 10.000 28.8 0.687 21.7 2.510 47.6 0.1030 1.9 H-cell9’ 15.000 35.3 0.50 12.9 3.341 51.8 - - H-cell10’ 15.000 36.6 0.50 13.4 3.000 48.3 0.1141 1.8 PEG PEG H-cell11* 15.010 24.3 0.50 8.9 6.000 64.0 0.3 2.8 H-cell12 20.005 33.8 1.778 33.1 2.967 33.1 - - H-cell13 20.003 33.8 1.777 33.0 2.971 33.1 - -
Table 2.1.1: Water or polyethylene glycol based recipes for the manual extruder, full table, PEG = polyethylene glycol, P1,2D = propan-1,2-diol. ’ = recipe for LSM squares, * = recipe contains 53 mg CTAB (cetyl tri-ammonium bromide)
ground in a pestle and mortar for 5 min. They were extruded immediately, except for recipes H-cell12 and H-cell13 which were aged overnight in a sealed container to allow the cellulose to fully hydrate. The mixture was extruded by forming it by hand into a cylindrical billet, placing it in the extruder, assembling the die, and slowly rotating the handle attached to the extruder screw. A single long tube was extruded, chopped into sections for sintering, and left to dry overnight. Some of the sections were sealed at one end with a spatula by compressing the open tube end to make a seal while the tube was still wet and malleable. LSM squares were extruded using a different die assembly, with a perforated disc, but no mandrel, and a slot die. Cetyl tri-ammonium bromide (CTAB) (Aldrich) was used as a dispersant in one of the extrusion mixes.
Sintering the tubes
The LSM tubes produced were sintered in a horizontal position on an alumina plate, according to the profile: 0.3 ◦C min−1 to 500 ◦C, then 1 ◦C min−1 to 1000 ◦C, then 3◦C
min−1 to 1200 ◦C, with a 1 h dwell, and then 3 ◦C min−1 to room temperature. Some
of the tubes were sintered in a vertical position, supported by alumina rods on a furnace brick.
2.1. FABRICATION METHODS 69
Mechanised screw extruder
The mechanised extruder (Rondol) in fig. 2.1.5a uses a single 12 mm diameter screw to mix, de-air and extrude the extrusion mix.
a)
b) c)
d) e) f)
Figure 2.1.5: Mechanised single screw extruder (Rondol) with die assembly a) extruder, b) 12 mm extruder screw, c) screw inside extruder barrel, d) mandrel (left) and die insert (right) e) mandrel and die ring assembled and f) die assembly with heater band, and 4x die ring adjusting screws at 90◦ intervals around it.
It has two controllable temperature zones; the barrel and the die assembly. A pressure sensor located between the barrel exit and the entrance to the die assembly, records the pressure in bar. It can be seen sticking up from the top left hand side of the extruder in fig. 2.1.5a. The pressure and temperatures can be logged via an RS-485 communications port. The die thermocouple is faulty and so the die temperature cannot be logged, but is measured manually via a thermocouple inserted inside the mandrel which is hollow, so the thermocouple measures the temperature at the centre of the die.
Figure 2.1.6: Mechanised screw extruder: die assembly schematic. 1) Die adapter, 2) Die clamp plate, 3) nozzle clamp ring, 4) man- drel, 5) die insert, 6) die body, and 8) man- drel threads. The heater band is not shown, but surrounds the die body.
The extrusion mix is placed in the alu- minium hopper on top of the extruder, and feeds into the extruder as the screw rotates. The speed of rotation is controllable from 0 to 50 r.p.m with a precision of 0.5 r.p.m., and the motor and electronics are water cooled at a flow rate of 2 dm3 min−1 to
prevent overheating as they are near the barrel. The extruder screw has a taper in its diameter, becoming thicker towards the die. Therefore, the shear force and the pres- sure increase along the screw towards the die. This provides a de-airing mechanism, as air is forced backwards along the screw toward the feed intake. In order to as- semble the extruder, the screw is attached inside the barrel, as shown in fig. 2.1.5c, to which the die assembly is bolted. This consists of a mandrel, and a die insert, shown in fig. 2.1.5d,e which together shape the extruded tube. These are then placed inside outer casing of the die, shown in 2.1.5f, and a nozzle clamp ring (not shown) is added to hold the die ring and mandrel in place. Fig. 2.1.6 shows a schematic of the die assembly.
Extrusion procedure: water based recipe
Table 2.1.2 shows the two water and cellulose based recipes that were extruded. The first was recipe M-cell-W1, where ’M’ stands for mechanised extruder, ’cell’ stands for cellulose and ’W’ stands for water-based extrusion mixture. It was mixed in a pestle and mortar, placed in a sealed plastic bag overnight to allow the cellulose to fully absorb the water, and then extruded at 15 r.p.m. The tubes produced were sintered with a ramp rate of 3
◦C min−1 to 1200 ◦C, with a 1 h dwell, then 3◦C min−1 to room temperature. A picture
was taken of the sintered tubes. Recipe M-cell-W2 was made in the same way. During extrusion, the speed was varied from 15-55 r.p.m and the temperature from 25 ◦C to 50 ◦C.
Recipe LSM LSM cellulose cellulose water water
no. mass% vol.% mass% vol.% mass% vol.%
M-cell-W1 80.5 33.6 6.9 31.8 12.5 34.6
M-cell-W2 80.5 33.6 6.9 31.7 12.6 34.7
Table 2.1.2: Water and cellulose based extrusion recipes for the mechanised extruder. The cellulose is 2-hydroxyethylcellulose.
2.1. FABRICATION METHODS 71
Extrusion procedure: thermoplastic recipe
When extruding LSM tubes, the aim was to try to use the principles of viscous plastic processing, which is a method of processing ceramics, in which organic binders are added to disperse the fine ceramic particles. The mixture is subjected to high shear forces, which break up ceramic powder agglomerates, and the organic binders prevent re-agglomeration. Therefore a stable, finely dispersed and homogeneous, agglomerate free ceramic mixture is produced. This process has been shown to greatly improve the strength of the sintered parts, by reducing the number of microstructural flaws. It can also be optimised to reduce the level of organic components used in the mixture when compared with a conventional process [2]. Table 2.1.3 shows the different extrusion recipes used. The components used are 5% A-site deficient LSM, (La0.8Sr0.2)0.95MnO3−δ, 99.9%, with particle sizes from the
manufacturer of d10 = 0.6 µm, d50 = 1.1 µm and d95 = 4.3 µm, density 6.6 g cm−3 and
surface area 6.79 m2g−1(Praxair, Pi-Kem). The binders are polypropylene (ICO Polymer)
and paraffin wax (Fisons), the lubricant/surfactant is stearic acid (97%, Acros Organics), and the pore formers are 2-hydroxyethylcellulose (Aldrich, M.W. 90,000) or graphite (325 mesh, 99.8%, Alfa Aesar). Recipies M-gra7 - M-gra11 are all included to show that the tubes extruded from these recipes had a consistent composition.
Recipe LSM LSM PP PP PW PW SA cell. cell.
no. mass% vol.% mass% vol.% mass% vol.% mass% mass% vol.%
M-cell1 79.6 32.8 9.0 27.3 7.4 22.8 0.6 3.3 15.0 M-cell2 85.5 42.2 6.1 22.0 5.5 20.2 0.5 2.6 13.9 M-cell3 83.3 38.2 6.9 23.4 6.2 21.5 0.5 3.0 15.0 M-cell4 81.8 35.9 7.0 22.5 6.3 20.7 2.0 3.0 14.3 M-cell5 82.8 37.5 6.9 22.9 6.8 23.0 0.5 3.0 14.7 M-cell6 83.3 38.2 6.9 23.4 6.2 21.5 0.5 3.0 15.0
Recipe LSM LSM PP PP PW PW SA gra. gra.
no. mass% vol.% mass% vol.% mass% vol.% mass% mass% vol.%
M-gra7 76.9 38.1 6.4 23.4 5.8 21.6 0.5 10.3 15.0
M-gra8 76.9 38.1 6.5 23.4 5.8 21.5 0.5 10.3 15.0
M-gra9 76.9 38.1 6.5 23.4 5.8 21.5 0.5 10.3 15.0
M-gra10 76.9 38.1 6.5 23.4 5.8 21.5 0.5 10.3 15.0
M-gra11 76.9 38.1 6.4 23.4 5.8 21.6 0.5 10.3 15.0
Table 2.1.3: LSM thermoplastic extrusion recipes. PP = polypropylene, PW = paraffin wax, SA = stearic acid, cell. = cellulose, gra. = graphite. Recipes 1-6 use cellulose pore former, and recipes 7-11 graphite pore former. The abbreviation ’gra’ stands for graphite pore former.
The extruder was cleaned before recipe 1 was extruded by passing pure polypropylene through it. The extrusion recipes in table 2.1.3 were prepared by grinding the mixture in a pestle and mortar before extrusion. The extruder was then pre-heated to a barrel
temperature of 170 ◦C, and a die temperature of 170 ◦C - 180 ◦C, and the motor was
water cooled with a flow rate of 2 dm3 min−1. For recipes M-cell3 - M-gra11 only, the
mixture was passed twice through the extruder without the die assembly present, i.e. the extruder is as shown in fig. 2.1.5c, in order to homogenise it before extrusion. During extrusion, the 4 screws next to the die ring were adjusted to make the die symmetrical and produce straight tubes. A beaker of water was placed below the die, with the water surface about 5 mm below the die face. The tubes were extruded into water because they are molten when they exit the extruder. They would distort under their own weight if not quickly cooled below the melting point of polypropylene, which is 160 ◦C - 165 ◦C.
As the tube extruded, the bottom end was open. In order to seal the top end, to make a tube suitable to be a reversible cell support, i.e. one end open, one end sealed, scissors were used to cut the tube just below the die face, which dropped into water, and solidified. Extrusion was then stopped, the die face was cleaned by scraping with a metal spatula, and the procedure was repeated. The off-cuts produced were recycled through the extruder. The tubes were then dried at 80 ◦C for 10 min to remove any water. A piece of software
was written using Labview v8.6.1, which recorded the die and barrel temperatures, the pressure just before the die, and the motor current, which is directly proportional to the screw speed.
Solvent extraction of paraffin wax with decane
It was necessary to carry out a ‘de-binding’ process on the extruded tubes, so that they did not crack while sintering. This procedure was carried out for the mixture of recipes M-cell1 and M-cell2, and for recipes M-cell3 - M-gra10 in table 2.1.3. The paraffin wax in the tubes was extracted by immersing them in decane (99%, anhydrous, Aldrich), heated to 60 ◦C - 80 ◦C and magnetically stirred, for 0.5 h - 1 h. The mass loss of the tubes was
measured by removing one from the solution, and drying it for 10 min at 80 ◦C. From
this, the percentage mass and volume loss were calculated. For recipe M-gra11, diethyl ether (99.5%, Sigma Aldrich) heated to 35 ◦C was used instead of decane. The tubes were
weighed as before to confirm that the paraffin wax had been extracted.
Sintering of the tubes
The tubes were placed on an alumina plate covered with a thin layer of LSM powder, coarsened at 1250◦C and ground in a pestle and mortar. They did not adhere to the plate
during sintering. The profile used was 0.3 ◦C min−1 to 500◦C, then 1 ◦C min−1 to 1000 ◦C, and then 3 ◦C min−1 to the dwell temperature, with a 1 h dwell, then 3 ◦C min−1
to room temperature. For recipes M-cell1 - Mcell5 in table 2.1.3, the dwell temperature was 1250 ◦C, for recipe M-cell6, 1330 ◦C, recipe M-gra7, 1280 ◦C, and recipes M-gra8 -
M-gra11, 1330◦C.