Update 03/07/2015: I found the right solution - check the last step!
How did it all start you might wonder, so I will let you know ;)
You might have seen my Instructable for the Simple Induction Heater and with my first one I felt the need to find ways of improving the output power.
The elctronically minded people will already know that ferrite is widely used for small tansformers, choke coils and even antennas.
With this wide variety is is only obvious that you will find a lot of differnt types of ferrite designed for a specific job.
Factors like megnetic flux, core saturation and frequency range are just some of the parameters that can be important for proper performance.
What I use for my needs might not even be considered to be ferrite by a proffesional but it does the job quite good.
This Instructable is a work in progress for me as I will update and complete it with pictures "on the go", so every time I make a new batch or try a new recipe you will see the results here - so keep posted for updates.
If my time permits it I will make the next ferrite during the coming weekend.
It will be plaster based and I shall take some pics and a short video as well.
Please post your comments or improvements and I will update the Instructable accordingly.
Step 1: What Is Ferrite?
Ferrite in the commercial sense is a compressed mix of mainly Iron Oxide and binders.
Depending on the necessary properties, Zink Oxide and even rare earth metals are added.
Usually produced under extreme pressures and heat resulting in a ceramic like finnish.
You can look up on Wikipedia for more detailed info as it is not that relevant for this Instructable.
Step 2: Why Can't I Find Anything to Mould My Own Ferrite?
There are only a handful manufactorers that have mouldable or machinable ferrite in their catalogue - and they treat their formulas very good!
Also the prices are not really attractive for individuals like you and me.
Even if find a supplier you have to state the purpose and megnetic properties you need or do your own calculations based on the supplied data sheets.
Step 3: Ok, But What Are the Uses of My Homemade Ferrite?
As said in the beginning, I needed something for my induction heater that does not saturate with the extreme magnetic fields involved and also wanted some form of shielding to prevent interference.
With the current mix (at the end of the steps) I accomplished both.
I can't recommend it for the use of specific HF coils or similar as I have no means of checking the properties and magnetic fields with my limited equippment - sorry for that!
But if you make your own electro magnets, induction coils or have the need to generally direct magnetic field lines it might be just what you need.
Also for general shielding of HF frquencies it should work quite well, making it possible to fully seal a circuit in the ferrite so no leaks can happen (or better: should happen).
A 8mm bolt in my coil with ferrite mantle takes about 90 seconds to get red hot, with a ferrite mantle around the work coil the time is reduced to under 30 seconds.
Step 4: Pro's and Con's
Well, as with all new things they never really perfect ;)
I will start with the negative things first:
* It is very messy to produce, so gloves and outside mixing is highly recomended as you don't want to clean up Iron Oxide dust in your kitchen.
* It is not easy to mix as the Iron Oxide is very fine and tends to ignore all binding at the start of mixing it.
* The mix is not perfect yet, so you might have to test the mix in a small batch to check if it fits your needs.
Now the good bits:
* You can put it into any shape you need.
* It can be sanded or drilled when cured.
* It is a good shield against interference.
* Once mixed it ieasy to handle and form.
* You need no special equippment or expensive ingredients.
* You can easily change the mix to adjust it to your needs.
Step 5: How Is Is Made and What Do I Need?
Tools and things you must have:
* Rubber gloves
* A suitable mixing container - I use the ones to mix plaster
* Spoon, spatula or similar for the mixing
* An area that you clean with a pressure cleaner or that does not mid to get a bit dirty
Things to make it easier:
* Some patience ;)
* Iron Oxide - the blck kind also know as Fe3O4, commonly used to color concrete and quite cheap
* Plaster - Plaster of Paris, wall filler, or some type of resin depending on your needs (this is the binder)
* Water if you use plaster or similar, otherwise check the instruction for your resin
* Zink Oxide and other things you find when searching for Ferrite compositions if you feel the need and get it cheap - totally optional!
How to mix it:
First some explanations;
You want to use as less binder as possible, otherwise the properties of your ferrite might not be as expected.
For high power applications, like an induction coil, you will need a thick layer of ferrite as otherwise the core will saturate or might heat up - it does not harm to use it too thick and you can always add more (around) if saturation is an issue, same for too much as you can sand t down or use a file.
Only mix as much as you can handle within the curing time!
Take a rough estimation of how much in final volume you need for your project and add another 20% be on the save side.
I go for the plaster version as this was my first way of doing it and because it the easiest.
Add the Iron Oxide in your mixing container (put gloves on now if you forgot about them ;) ) followed by about a third in volume of plaster.
Mix well while dry - for bigger batches using a jar with lid saves you a lot of black dust flying around!
Now add the water and keep mixing like you would for mormal plaster so there are no lumps.
Don't worry if you used too much water as you can always add oxide and plaster.
Be aware that this mix dries a bit faster and can produce cracks in thicker layers, so working in small stages with new mixes to build up might be necessary (keep the dry mix and only use with water what you need).
You now can put it into a mould or cover what you need to shield.
Let it set in a moist enviroment to prevent cracks - I simply wrap it in a moist towel (very old one because of the black oxide!).
After about 2 hours you can continue to let it dry in the open.
Final curing time depends on the thickness and humidity!
To for moisture warm it up to about 30° celsius and place into a closed container that was in your freezer to cool down.
If not fully dry you will see a lost of mist condesing on the inside of your container.
Step 6: How to Use It on Moisture Sensitve Stuff or for Sealing in Electronics
There is only one way and this by making sure the moisture won't get to it!
Either seal it first by means of paint, resin and so on, or make a mould for the ferrite and place the part in it once fully cured.
If that is not an option you have to use a castable or mouldable resin instead of the plaster.
For 2K compositions this means you have to add equal amount of Oxide to both components of the mix.
Although Fe3O4 is not that reactive it can affect the curing of 2K resins, so do a small batch for testing first to make sure it cures properly and does not heat up too much during curing.
It is very hard to give proper mixing ratios as the properties of the various resins differ too much but I work my way down from a 50/50 mix until I notice either mixing becomes a problem or the curing is not good enough.
But usually I don't go under mix of 35% resin to 65% of Oxide.
Step 7: How Can You Help?
You can help to perfect the way of making ferrite by posting your feedback and tested recipies.
I will add more pictures with my next batch and add another step for collected mixtures from feedback for an easy reference for everyone here.
Step 8: Update!
I realised by the amounts of hits that I need to rush things a bit to get this instructable complete.
After experimenting with additives to reduce the risk of cracks forming during the drying process I decided to use this variation for a video on how to make the ferrite.
To give you a head start on the new recipe I will sum it up here so you get a clue what happens in the video.
1. Prepare yourself with gloves and everything you need, you will see the things in the video.
2. Don't try to follow the video straight away, watch it at least twice and make use of the pause button if I am too fast in the video.
3. Don't do it inside as the black dust goes everywhere!
4. Ingredients are:
Plaster of paris - or any other modelling plaster you have at hand (Gypsum).
Black Iron Oxide
A bit of wall paper glue or if not available in your area use wood glue - this helps to keep the mix workable for longer and slows down the drying process also the finnished product does not crack as easy.
Water and some tools for the mixing and modelling - whatever works best for you and the amount you make.
5. The glue should be added to the water first and only in small amounts for wall paper glue, wood glue should be added in a volume of about 15% to the water.
I might skip this bit in the video and prepare the water glue mix upfront as it takes a while with wall paper glue.
Make a dry mix of plaster and oxide, I do this with a spoon.
Depending on the mix you will get a pretty good or preety bad result in terms of mechanical stability.
I try to use 3-5 parts of plaster to 4 parts of Oxide, a 50-50 mix seems to work well but I always try to stay under it to have more ferrite than plaster in the mix.
The more oxide you add the better are the magnetic properties of the finnished product but the harder it is to work with and to cure it without cracks.
Add water (already prepared with glue) followed by your dry mix into a suitable mixing container and mix well until there are o lumps, keep the mix thick enough to work with but not so thin it runs off, unless you do a casting.
Once you start mixing the clock is ticking as you only have limited time until the mix goes hard and becomes unusable.
It is best to work with small batches that you can properly apply before the initial drying starts.
You must use clean tools and containers as even small amounts of cured mix will mess up your next batch!
It will cause the mix to go hard much faster and can cause lumps in the mix when it cures faster than the rest.
I try to wash my tools and containers before the plaster goes fully hard.
As you can see in the video I only make a small amount to get the first layer onto the coil.
I do it this way to allow for a better drying as we want to prevent to capture too much moisture before adding the next layer.
You should take your time to let it dry - either naturally or by the use of a de-hydrator.
If there is too much moisture left before you do the final curing in the oven it result in massive cracks.
Best way is to let the finnished product dry out for a few days before putting it into the oven.
If you cover moisture sensitve parts it is best to add a layer of paint as a barrier, for best results give the paint a quick sanding to get a rough surface as this will help the mix to stick.
In case you are in a hurry try the flexible tile adhesive mixes from your hardware store without the glue in the mix.
But do some small scale tests first to check if your mix produces cracks while drying.
I experimented a bit more with a lot of different (possible) binders.
Nothing we would find at the local hardware store, the kitchen or local pharmacy (at reasonable prices) worked.
But then it hit me!
Grabbed my last few spoons of black oxide and mixed it with Sodium Silicate - Waterglass.
Of course, me being me, I did not take any pics or videos - shoot me...
Anyway I will try to explain:
Sodium Silicate is another "forgotten" chemical in terms of home use.
Some might still know it from the chemical experiment "Chemical Garden".
In the concentrated liquid form it is somewhere between full cream milk and warm honey in the consistency and glass clear.
Once dried it it goes rock hard - a feature used for repairs on wood, china ware and other things like heat resistant tiles.
If you know "Green Sand Casting" you are already familiar with just adding a tiny bit of water to that mix.
I did the same with the black oxide.
Started with a few tablespoons oxide and added the Sodium Silicate in tiny amounts.
Creates a lot of lumps and small balls, so doing this in a little ball mill might be a good idea (apart from the clean up bit).
Anyway, if you check the videos on grenn sand casting you will see the mix looks almost dry but keeps it shape when pressed - I tried the same but in the end just used a block form and small hammer to compact it.
(This reminds me to mention to get the ferrite mix out of the form after this step - I did not and it was impossible to remove the cured stuff from the form).
After this the testpiece went into the oven for about 90 minutes at full heat - this creates a nice and hard "ferrite".
To get is hard enough to be actually used it is placed into a kiln and is slowly heated to a glowing orange.
After that the cooling was done in the oven, preheated to full.
Oven was turned off once the piece was in and allowed to fully cool down over night.
The result was that
1. I was unable to get the cured ferrite out of the metal box I used.
2. It is so hard that I could not drill into it.
3. It does not break or crack.
I will try to find some more time and black oxide and make a short video of the process.
In the meantime, everyone still following can experiment as the only thing that matters is to just get the oxide moist with the Sodium Silicate so it binds together properly.
During the compacting a bit of excess might be pressed out indicating to use even less sodium silicate for the next mix.
The only downside is that you have to make a small furnace, metal melter or kiln so you can fully harden the mix, which is basically like a ceramic once cured.