Hi everyone, I got this fumed silica (0.2 um - 0.3 um) from Sigma Aldrich, and I intend to use it for tape casting. During the slip preparation, I noticed that at a very small amount of solid loading (water = 30 g, fumed silica = 1.5 g), it became very viscous and gel-like. I mixed it in a beaker with a magnetic stirrer, after which I transferred it to a ball mill and mixed for 24 hours. After this, it became fluid, and I put it in the desiccator to get rid of the bubbles and then transferred it to the vacuum mixer. After adding the first amount of binder (HPMC), about 0.02 g, it thickened a bit, but increasing the binder amount in the second and third circles, it became very watery and would not thicken, no matter how much binder I added. This is my first time working with fumed silica.
Have you prepared a paste for tape casting using fumed silica? What procedure did you use? What challenges did you face, and how did you solve them?
After adding additional powder to the paste, it thickened, and I cast it.
The following morning I checked the casted samples, and they were all cracked.
Hi Gideon C. Irogbele, the solid loadings for tape casting should be as high as possible. I have worked in the range of 50 to 80 wt% (of powder + solvent), and these can be even higher than that. The effect is represented in when you added extra powder to the slurry and it thickened.
To avoid cracks, you have to optimize binder quantity. Right now, the binder active matter you are using is extremely low. After making HPMC binder solution in DI water (around 1~3 wt% binder active matter), you have to add the binder in powder + water slurry. The additional water from the binder will lower your silica solid loading, so you have to consider it in your calculations and adjust the total silica amount accordingly. Use at least 5 wt% binder active matter with respect to silica solid loading. And then keep increasing the binder quantity if needed.
Also keep in mind that you have to consider tape drying temperature, heating rate (slower is better), and humidity (a low humidity causes cracks and high humidity extends drying time) to avoid cracks during drying process.
Hi @Muhammad Waqas Khalid, thank you for your suggestions and detailed explanation. I will try this out and see how it works. I appreciate your input. Thank you 👍
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