Gadolinium doped nanocrystalline cerium dioxide (GDC) powder samples were effectively densified using the development process known as "cold sintering" with adjustments for residence time investigated. Ceria was chosen because of its application as an electrolyte in solid oxide fuel cells. 3, 6 and 12 hour tests were completed with the peak residence temperature of 350˚C through the jacket heaters inside a Carver press applying 500 MPa.
The NaOH-KOH mixed molten hydroxide stream facilitated densification to approximately 61, 76, and 79% relative density for sintering durations of 3, 6, and 12 h, respectively. Measurements showed that the average grain size in the 3- and 12-hour samples was larger than the 6-hour one, with large standard deviations in all samples. Traces of flow were found between the delamination layers, and lines heavier in sodium were identified on the 12-hour sample.
Chemical analysis of 3 different duration samples showed that the flux removal plateau was reached after 6 hours of sintering. Sodium and potassium concentrations were higher in the 3-hour sample compared to similar values in the 6- and 12-hour samples.
Introduction
Where 𝜎𝜎0 is a constant, E is the average height of the diffusion barrier, k is the Boltzmann constant and T is the temperature (K) [8]. This lattice expansion can compromise the mechanical integrity of the unit cell if it is at high temperatures and the geometry is not supported [9]. There are four main components of a fuel cell construction: Anode, Cathode, Electrolyte and Junction.
The porous cathode, with its reducing capacity, converts oxygen atoms into ions, which travel through the crystal lattice of the electrolyte and provide ionic conductivity. There must be gas-tight seals around the edges of the stack because of the slight difference in thermal expansion of the components that cause mechanical stress. Therefore, the ideal situation would be a planar stack with sufficient edge sealing with a Gd-doped CeO2 electrolyte.
This structure is not closed, the transient liquid phase will escape as a gas or liquid and be completely removed by the end of the process. The liquid spreads around to cover the surface of the solid particles and partially dissolves their solid surfaces, creating a liquid phase at the interfaces where the particles are in contact. 11 Diffusion rates are higher in liquids than in solids, so we can say that the addition of a liquid phase accelerates mass transport.
Atomic diffusion in the solid state has a much higher energy barrier and is unlikely to occur, a solid reacting with a liquid can change the kinetics of the system and activate diffusion [4]. The process can be thought of as the dissolution of the particle surface which travels to areas of low chemical potential, such as pores, where it reprecipitates and increases in density. The mechanism that drives this densification is called pressure solution creep and occurs as a result of the uniaxial force on the solid with added liquid in an open system.
This would create hydroxides that require relatively high temperatures to convert the oxide phase, which is hypocritical to the premise of cold sintering. These “deep eutectic” solvents will not evaporate, but will melt further from the unsealed system as the material densifies [13]. The fraction used was 30% by weight, and in this particular experiment the pellets were sintered overnight at 200°C to ensure that all water residue had evaporated [15].
Traces of acetate were found to exist in the sintered ceramics, which warrants further investigation [18]. Traces of current were found after CSP, more on the surface than in the bulk [19, 20].
![Figure 2: Diagram of solid oxide fuel cell structure [1].](https://thumb-ap.123doks.com/thumbv2/123dok/10486956.0/11.918.244.706.214.479/figure-2-diagram-solid-oxide-fuel-cell-structure.webp)
Experimental Procedure
Due to the reactive nature of hydroxides in ambient environments with water in the air, the powders were measured in an argon-filled glove box. In the glove box, the NaOH granules and KOH flakes were ground with an agate mortar and pestle, and then thoroughly mixed together before the appropriate amount of GDC powder was added. These powders were mixed for approximately 3 minutes before being transferred outside the glove box and into the 13 mm diameter stainless steel die shown in Figure 9a.
Under and above the powder were circles of Inconel foil (a corrosion-resistant alloy) as a barrier between the sample and the steel die. A heating jacket was attached around the die with a thermocouple that measured the temperature between the jacket and the die. Underneath this matrix containing the sample was an additional heater to ensure sufficient heating at the correct temperature.
Both heating elements are connected to a temperature controller that adjusts the power percentage and records the temperature from the thermocouple. These are stacked in a Carver press, as shown in Figure 9b, with heat-resistant fibers protecting the press plate. The temperature and pressure remained constant throughout the 3, 6, or 12 hour residence time.
Trial and error showed that the best chance to remove the bottom of the die was immediately after the dwell time had begun and the heat source was turned off. The pellet itself was then pressed out of the die chamber and allowed to cool freely in air. The second sample sintered for 12 hours involved Kapton foil in the experimental procedure, intended to make the separation of the mold easier.
Unfortunately, the result was no improvement in the ease of removing the sample from the mushroom. The bulk density of each sample was measured using the Archimedean method, which involves weighing the dry, saturated sample after it was drawn under vacuum into a beaker of deionized water. Discs were made by mixing approximately 0.13 g of SpectroBlend powder binder (Chemplex) with a piece of crushed pellet to be of the appropriate size for the instrument.
Results and Discussion
The final stage of liquid phase sintering is controlled by the compaction of the solid particle skeleton network and is significantly slower due to the longer distances for diffusion. It experiences a rebound of the air pressure, which reduces the quality of the final product [24]. This is supported by the observation of the mixture of types in the 3-hour sample where flux had the least time to escape, so that the grain boundary weakening was common and fracture occurred around individual grains.
Some areas in bent line-like shapes on the fracture surface of the 12-hour sample had different color in the backscattered image, and EDS analysis was performed on JEOL JSM-6010PLUS/LA SEM in the form of map shown in Figure 13. stripe pattern that appeared up on top of the breach, was peculiar, and its cause is. K is also an equal part of the flux; However, Na is more volatile and appears to have had more atoms left on the surface, while K dissolved more evenly.
These EDS values are not completely accurate due to the interaction volume of the beam, but give a good estimate of the concentration. Each of the three samples was photographed to obtain a baseline image of compaction and densification visible at this level. 25 If you look at a higher magnification, (Figure 18) the grains and grain boundaries of the samples can be identified.
Two different images from each of the three samples had two vertical, two horizontal, and one diagonal lines drawn, and the length between grain boundaries was measured in nm. The transition liquid phase used in this experiment was a mixture of sodium and potassium hydroxides, which creates a eutectic that melts from the system during densification. Although the samples were annealed for 6 h after the cold sintering process, the amount of flux remaining in the samples afterwards is representative of the previous amount.
In the sample sintered for three hours, the mass percentage of the combination of Na2O and K2O is 7.62. These data are representative of the expected melt evaporation during the sintering process, as the weight fraction decreased by more than 18%. The linear graph clearly shows that after 6 hours of sintering at 350˚C, almost all the flux that will evaporate has disappeared.
The observation was made that there was significantly less flux visible on the outside of the die than in previous runs of all durations. Another idea is that the Kapton foil allowed the system to increase overall heat acting as a case that increased the solution of the flux eventually forced out during annealing.
Conclusions
If the flux was trapped in the delamination pockets during the cold sintering process, it could have easily escaped during the 6-h bake and formed a. Another reason for not seeing as much flux leakage could be that it was left for a long period of time and burned out by the time it was checked.
Future Work
Irshad, M.A., Anwar; Tiwari, Pankaj; Rafiwue, Asia; Ullah, Muhammad Kaleem; Ali, Amjad; Usman, Arslan, A Brief Description of High Temperature Solid Oxide Fuel Cell Operation, Materials, Design, Manufacturing Technologies, and Performance. Maria, J.-P.K., Xiaoyu; Floyd, Richard D.; Dickey, Elizabeth C.; Guo, Hanzheng; Guo, Jing; Baker, Amanda; Funihashi, Shuichi; Randall, Clive A., Cold sintering: Current status and prospects. Ndayishimye, A.S., Mert Y.; Sada, Takao; Dursun, Sinan; Bang, Sun Hwi; Grady, Zane A.; Tsuji, Kosuke; Funahashi, Shuichi; van Duin, Adri C.T.; Randall, Clive A., Roadmap for Densification by Cold Sintering: Chemical Pathways.
Guo, HB, Thorsten JM; Guo, Jing; Baker, Amanda; Randall, Clive A., Cold sintering process for 8mol% Y2O3 stabilized ZrO2 ceramics. Guo, HB, Thorsten JM; Guo, Jing; Baker, Amanda; Randall, Clive A., Current progress and prospects in the application of the cold sintering process to ZrO2-based ceramics. Tsuji, KHDB, Thomas; Ndayishimiye, Arnaud; Wang, Ke; Randall, Clive A., Cold sintering of yttria-stabilized cubic bismuth oxide: conductivity and microstructural evolution of metastable grain boundaries with annealing.
![Figure 5: Tubular SOFC (a) cross-section and (b) stack/series [2].](https://thumb-ap.123doks.com/thumbv2/123dok/10486956.0/14.918.138.797.514.739/figure-5-tubular-sofc-cross-section-stack-series.webp)
