I tried to heat (oxidise) the Fe powder at 500C and it got sintered (bonded) together. The bonding is so strong that its not easy to manually grind it to get back the iron oxide powder. XRD shows that all Fe transformed into Fe2O3 and Fe3O4. What is the reason of this bonding? and How to avoid this?
For entertainment, see this webinar:
Adhesion and cohesion http://tinyurl.com/zwb2wlh
It is highly recommended to go through the webinar given by @ Alan F Rawle, I went through it before and it changed my vision regarding the material at the nano scale.
The 500oC is around one-third the melting point of the iron, it is high enough to activate the sintering process between the small particles to reduce the total surface area of the system as well as the curvature of the surface of each particle.
@ Imad, I will definetily go through the webinar.
I don't think that 500C is high enough to make strong diffusion bonding, there is something related to oxidation. because in other experiments which I did in inert atmosphere, no bonding was observed even upto 800C.
Dear @ Akeshwar Singh Yadav
It is well known that oxidation of iron is an exothermic process. Heating the iron powder up to 500oC will surly increase the rate of oxidation and, hence, the heat releasing will increase which, in turn, will activate the sintering process.
Thanks @ Dr. Rawle, webinar is really good.
So, is there any method like thin coating, or slow oxidation (as suggested by @ Dr. Wang) or something else which can be used to oxidized the particles without sintering.
One route is steric stabilization of the iron particles on an inert (?) substrate such as silica. This approach is used in catalysis. In such situations, the Fe can be oxidized without the Fe particles aggregating.
Now, the other thing to note is that the surface (top 5 - 10 atomic layers) will be fully oxidized anyway - from ESCA/XPS the Fe will be in the +3 oxidation state. For similarities in the Ag (Ag2O) system see:
Silver colloids and invisible ink
https://www.brainshark.com/malvern/vu?pi=560497681&b=1&tx=120486&c1=22
If your aim is to oxidize the Fe powder, this was achieved. The reason for the bonding is sintering.
Fine powders usually present high surface area, which is the driving-force for sintering processes via solid state diffusion. So basically, when heated at 500 ºC, your powders did sinter, meaning necks between the particles were likely formed. In addition to that, oxidation also took place. In fact, it is known that α‐Fe2O3 was found on abraded iron specimens which were oxidized at any temperature above merely 130°C (see H. R. Nelson, The Low Temperature Oxidation of Iron, The Journal of Chemical Physics 6, 606 (1938); https://doi.org/10.1063/1.1750132). Of course kinetics at such low temperatures require longer exposure times. You may monitor the reaction process by carrying out TG/DTA analysis and XRD analysis.
The fact that you observed sintering of the iron powder at 500ºC suggests that you should decrease temperature (try 300 ºC) in order to minimise this phenomenon. If that will not be feasible, you can try to dry mill the heat-treated powder, since at such low temperatures, sintering is certainly at its early stages and so it should not be hard to break the bonds between particles. For that you must use a planetary ball mill. You can control the powder particle size distribution by performing laser diffraction measurements on a CILAS or Malvern equipment. Bear in mind that that oxidation of iron is an exothermic reaction, so localized heat released may also enhance thermal diffusivitites, thereby promoting further sintering.
Kind regards,
Fernando Costa Oliveira
Lisboa, 20th September 2018
For small primary particle size (< 100 nm) then cold sintering will occur at room temperature when particles are brought into contact. Also Google 'comminution limit' - top down processes are generally ineffective and consume large amounts of energy.
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