Aerogel: Freeze-Drying
Week #04: Preparing The Beads For Impact!
Preface ✨
Hello everyone!
My intentions for writing these articles are:
- Explain technical information about aerogels in simple terms (to the public)
- Store information and habits for my future self and others (in <7 minutes)
Coolio? Sweet. Enjoy the series :-)
TL;DR Of Freeze-Drying ❄️
Note: This week’s article will be a short one. Why? We will transition more into the business model and the problem alginate should tackle for the next ten years.
Freeze Drying (FD) is a process in which a completely frozen sample is placed under a vacuum to remove water or other solvents from the sample.
The ice crystals created from the freeze-drying allow a solid chemical/solvent inside of a material to change from solid to gas WITHOUT passing through the liquid phase (also known as sublimation).
This process is ideal because of the long-term preservation properties it provides to the integrity of the sample’s biological/chemical structure and the minimal heat that is required to enter the machine/system.
The Freeze-Drying Process 🆒
The FD Process can be divided into two steps:
- Pre-Freezing (PF)
- Lyophilization (LYO)
In PF, the liquid in the wet gel is either quickly frozen using liquid (the liquid is either deionized water or nitrogen) at -196°C for ten seconds OR slowly frozen over 15–24 hours using a freezer at -18°C.
15 hours of freezing is not necessary for bead filters because the beads are small (5 millimeters).
In LYO, desorption occurs and the moisture content (in the material) is reduced from 7% to 0.5% and 2.0%
The frozen gel is dried via sublimation at -45°C and 15 Pascals for ~48 hours as the nitrogen gas replaces the liquid in the gel, thus generating the porous structure under a vacuum (in the freeze-drying machine).
The FD Method depends on the solvent crystallization temperature at the freezing point.
Your “solvent” for freeze-drying should be deionized distilled water.
Controllable Properties 🎮
Pore size can be controlled by controlling the freezing point of the solvent.
The pore size (holes in the aerogel beads) is increased when the freezing temperature decreases during the drying process.
Observations found that the pore size decreased as the freeze-drying temperature approached -50°C. No decrease in the pore size was observed after the temperature passed -50°C.
Note: The solvent usually forms a dry powder after the freeze-drying process is complete. The powder can be removed by a vacuum or hydrothermal treatment. These filters will use the vacuum method for its efficiency and friendliness to the environment.
Control the freeze-drying temperature and pressure to achieve the desired filter and properties.
© 2022 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.
