Planetary Ball Milling
Selection of the milling media
Milling media refers to the milling balls, the milling jar and the liquid. Typical jar/ball materials are polymer (jar only), glass, alumina, zirconia, and tungsten carbite. The material of the milling balls and jar should be selected along the following lines:
- Hardness: The balls and jar should be harder then the powder that is to be milled to reduce contamination by wear.
- Chemical compatibility: Some abrasion will occur in any case. If the jar/ball material is compatible with the powder, this is not an issue.
- Size: The size of the milling jar can be selected according to the required quantity. In genreal, larger batches (>50g) are more reliable than smaller batches (e.g. 20g).
After the material is selected, the size of the balls needs to be selected. In general, smaller balls result in a higher engery input during milling (also a higher temperature during milling!) and a finer milling result. Finally, the liquid for the suspension needs to be selected. The important question is the chemical compatibility with the powder. Some materials have some solubility in water, so that a water-free liquid is needed (iso-propanol or better n-heptane). If water is used, it should be de-ionized to reduce contamination with cations.
Filling the milling jars
First clean the jar and particularly the balls very thoroughly. The milling balls can contain significant amounts of remainings from the previous milling run. Accordingly, it is preferred to have a specific set of milling balls per material/chemical composition.
The milling jars are filled with balls and powder and liquid sucht that the overall filling reaches 2/3 of the volume of the jar. A bit less is possible, while more is not. A good weight ratio of milling balls to powder is 8:1 for the perovskite SrTiO3. The amount of liquid added to the milling media should be chosen such the slurry is not highly viscous. For SrTiO3, this is about 200ml on 800g balls and 100g powder.
If your powder is a mixture of different precursors, carefully make shure that during filling of the jar, no powder is lost so that the stoichiometry is not changed.
Rember to stir the mix of powder with a glas rod, balls and liquid until no air bubbles are hidden somewhere in the mix of balls, powder and liquid. Otherwise you will not be able to judge the viscosity.
The design of the milling run itself is mill-specific. ON the PM400 by Retsch, the following setup and program gives a very fine (well below 1µm) milling result:
- Ball and jar material: YSZ
- Milling Jar: 500ml
- Ball size: 3.5mm
- Milling speed: 400 rpm
- Milling cycle: 5mins milling, 25mins break to prevent overheating
- 15 cycles
Separating the slurry
Use a very clean sieve (sieves are a source of contamination) and a wide and clean glass beaker to separate the slurry and balls. The finer the balls are, the more slurry and powder remains in the balls. To a ceratin amount, it can be drained with additional liquid. If the balls are smaller than 2mm, it is best to use a technical sieve with 150µm mesh. Using a funnel and a glas bottle with vacuum adapter all with appropriate sealing will ease the procedure and minimize the amount of solvent needed to separate the slurry.
After emptying the milling jar, it is worth to check if there is some powder sticking to the bottom of the jar, which might indicate an incomplete milling and mixing due to an air bubble on the bottom of the jar. This will potentially result in a change in stoichiometry of the powder and can be prevented by initial mixing by stiring after filling the jar and before the milling cycle.
If the weight loss of the jar is measured by weighting the dry and unfilled jar and top plate before and after milling, an approximation for the abraision during milling can be obtained. For larger milling balls (where no balls are lost during the entire procedure inc. cleaning), the weight loss can also be measured.
The glass beaker is covered with aluminium foil with a number (10-30) holes to allow evaporation of the liquid. The beaker then can be dried in a laboratory oven at a temperature below the boiling temperature of the liquid. Boiling should be prevented as it can cause contamination of the oven. If the liquid is flammable, the drying stove should be explosion protected and the lab well-vented. Drying usually takes at least one day.
As after drying, the powder is usually agglomerated, it should be sieved on a Vibrating sieve once it is completely dry. Avoid sieving slightly humid powder. Remember that sieves are one of the major sources of contamination so that a separate sieve should be selected for each material.
Select a clean sieve with about 50µm mesh size, put in on a suitable pan with sealing and a clean sieve top. Add about 50 1cm-sized zirconia balls and put the setup on a vibrating sieve. Select the amplitude sich taht the milling balls jump on the sieve, but do not hit the top plate frequently. While more and more powder passes the sieve, the amplitude needs to be reduced. Wait until most of the powder has passed the sieve.
The last remainings in the sieve can be discarded as the sieve is meant to remove large particles in the powder, but remember that this might change your overall stoichiometry.