Tejabhiram, it sounds to me like you are being exploratory and methodical. I have seen fellow graduate students take one dataset, and from that, make a long list of assumptions. But because they believe it, they take their assumptions to be fact. It is good to think ahead, and project out many various scenarios which describe your data. But you must consider, if you were present before an auditorium of your peers, could you present enough facts, that support your conclusions. For example, would you believe someones testimony of grain size if their data only consisted of XRD data? Given how easy and prevalent SEM is, wouldn't you like to see confirmation? Nature is not what you want it to be, it is what it is. At the very least, the more data you collect, the more you'll know what your material isn't.

Heating make the process of forming collision of particle ....if u make this systym more viscous with thermolabile material as in your case you are heating ......

U can use adsorbents that can be wash further to get nanoparticles

could you please elaborate on this nilesh? Please

Try to heat the sample at a faster rate and cool at much faster rate. If you heating the temperature at high temperature for long duration, then particle will become bigger due Ostwald ripening. So try to reduce the duration of heating.

Use ultra-rapid thermal heating

At that temperature your surfactants will simply burn off and the particles will go through all sorts of changes. Unfortunately given your current process I do not know how to solve your problem.

I'm not sure I totally understand what your starting with and what your ultimate goal is. Without knowing this, it is hard to recommend a solution. Given my background, I'll take the position that your trying to densify size controlled powder crystallites of Nickel Gadolinium ferrites. It appears to me you are (knowingly/unknowingly) attempting to apply pressureless sintering as your method of densification. As you heat a material there are two basic mass transport mechanisms: (1) surface diffusion, (2) volume diffusion. Surface diffusion takes place at 'lower temperatures' and does not contribute to pore closure but does contribute to grain growth. Volume diffusion dominates at 'high temperatures' and does contribute to pore closure. So, if your goal is to fully densify your powders with the least amount of grain growth you need a sintering/densification method with a high heating rate (>100C/min) to get to temperatures where volume diffusion dominates. I use a method called spark plasma sintering to densify powders. Our instrument can reach heating rates as high as 600C/min. So that permits us to reach high temperatures quickly and make fully dense, nano-scale microstructures in our 3D, bulk materials. Another method using microwaves can also have high heating rates.

But, if you are not trying to densify the powders your creating, then why are you applying such high temperatures? I'm also not sure what you mean by "The recrystallization (magnetic) temperature." Are you intending to say 'magnetic temperature' and by that imply a Curie Temperature or are you referring to the onset temperature of grain growth? I have to agree with Palash, any organic agents you use will burn off between 300C-400C. Have you performed something like XRD to verify the purity/composition of your post-heated products? Or backscatter electron microscopy to look for disassociation of your NiGdFeO products? Are you heating your powder in air, vacuum, noble gas, reducing gas?

did you tried autoignition or autocombustion method ?

http://www.ucl.ac.uk/~ucapqap/SHS/CIMTEC/shs.htm

http://www.stobbe.com/library/hydrogen/partec1_Ferrites_4pages_final.pdf

http://www.bioinfo.in/uploadfiles/13258265881_1_1_JAT.pdf

http://www.ias.ac.in/matersci/bmsdec2011/1325.pdf

http://www.nt.ntnu.no/users/skoge/prost/proceedings/aiche-2004/pdffiles/papers/book_011.pdf

The sample heating rate doesnt seem to be helping my sample much (at least with the given furnace that we have) I heard i might need something like a Ark plasma source or a microwave furnce that can reach 2000 degrees in less than a 5 mins.

Dr.Jason. Thank you for the detailed explantion. I was really skeptical about using the term (magnetic recrystallization) when i wrote the questionj. Im sorry but it is indeed misleading.

I mean to say that, After i synthesized my particles using co-precipitation method, i tried to heat them at 200, 300, 400......1200 C. at each of these temperatures, the materials weren't showing ferromagnetic property. But only at 1200C they did. I also observed that there was a shift in their crystal structure (observed through XRD) at 1200C.

So I have to heat my particle to 1200C without increasing the particle size / restrict grain growth.

I donot have access to either a spark plasma or a microwave heater. Is there any other way i can do this?

First, you need to find out the critical particle size of your material, which is between super-paramagnetic and ferromagnetic region. Because even if you can make it dense but the particle size still below that regime, u won't get the ferromagnetic properties.

second, you have to know the growth the crystal vs time and temperature. By controlling that you will get the properties that you want. Also you don't need so high end equipment to do that.

Go to control the stirring speed at critical temperature and pressure, which may enable you to maintain the particle size.

Hello Michael,

At this point of time, i need not worry whether the particles are superparamagnetic or ferromagnetic. There is a crystallographic phase change happening at close to 1200 C which transforms my material from being antiferromagnetic to ferromagnetic. Superparamagnets as u know are still ferromagnetic materials without the hysteresis. So unless that temperature is reached, the particles are neither going to be ferromagnetic or superparamagnetic.

You mentioned, i need not use such high end equipment - could u please elaborate on that mentioning which equipment i can use to do that?

To control the nanoparticle size properly you require suitable heating with cooling rates to the particular temperature where the phase-separation kinetics becomes faster.

Due to temperature, crystalinity of particles increases and not grain size

Tejabhiram, sorry I haven't answered you for a couple of days. Here is what I think. If you are not able to achieve high heating rates then you might consider coating your powder crystallites with an outer shell material or in a matrix with a very high melting temperature and/or the smallest lattice constants possible. The problem with this however is that the shell/matrix may inhibit the 1200C phase transition your seeing. I think people have made microwave sintering devices themselves. The microwave sources extracted from commercial microwave ovens. That could work, but designing, machining, and testing/calibrating your microwave apparatus could take a great deal of time. Unfortunately, if you cannot get to 1200C quickly (high heating rate) you will have long periods of time to allow grain growth. There are SPS units all over the world. Might there be one in a neighboring university? With whom you could collaborate. But I guess, even if you used an SPS, you'll get a densified product. At that point, how do you know the magnetic properties you are seeing are from each grain acting independently or interacting with neighboring grains? That will further complicate things. You might consider restricting your parameter space (ie temperature range) to only include temperatures where the grains do not grow.

You mentioned your material goes from being AFM to FM at 1200C. I'm curious, how is it a pure AFM? The magnetic moments of Fe and Ni are not equal (Ni^2+ is 2 mB and Fe^2 & Fe^3 being 4 & 5 respectively). So, if the individual moments were aligned antiparallel as in an AFM, how are the ions (Gd, Fe, Ni) aligned such that their sum is zero? You'll have to forgive me, I'm unfamiliar with your material system.

In regards to the XRD data, are you sure what you are seeing is truly a phase change, ie identical stoichiometry & different lattice structures (such as a-Fe2O3 [hexagonal] and y-Fe2O3 [cubic]). Or are you seeing chemical decomposition, from say NiFeGdO4 to NiO + FexOy + GdxOy? The new XRD your are seeing could be the composite result of NiO, FexOy, and GdxOy components.

Maybe I'm only helping to complicate matters, but perhaps something I mention here might spark an idea. What do you think?

Hey Jason,

Thank you for the suggestions.. I havent got an XPS on my material and have just given my particles for ICPAES after you raised the question of materials decomposing into NiO, FeO and GdO. But as u say these materials are not AFM, they are Ferrimagnetic with very low magnetic moment. The moment increases phenomenally when i heat it upto 1200C.

I tried to give my samples for Neutron diffraction in order to understand the spin spin interaction inside my sample only to realize that neutron diffraction is not possible in samples containing Gd.

There is another phenomena that ive seen in my samples that has repeated with Co Ferrite, Ni Ferrite and Zn Ferrite. Once the Gd Substitution increases over x=0.6 (in NiGdxFe2-xO4), the magnetization which was decreasing till 0.4, starts increasing and goes over the x=0.2 sample.

When i searched for this phenomena, i found couple of papers confirming a phase transition from tetragonal to orthorhomic structure which improves the magnetic coupling between different planes inside the material. So as ive seen this phenomena in my material too, I thought that my samples are actually forming NiGdFeO. So now that you raised the question, i will check the ICPAES values and get back to you.

About raising it to 1200 C, i need to see who can give me access to microwave furnace here in chennai or anywhere nereby.

Would love to listen to your comments on my thot process.

Fast nucleation of the phase helps to control the particle size. Freshly nucleated phase is fine in size but as the heating progresses the particles grow and the initial rate of growth is very high. So you should know the exact temperature of the phase formation. Try rapid heating to the required temperature and then rapid cooling rate to control the size.

Read attached publication

Article Magnetic Gold Nanoparticles in SERS-Based Sandwich Immunoass...

Tejabhiram, it sounds to me like you are being exploratory and methodical. I have seen fellow graduate students take one dataset, and from that, make a long list of assumptions. But because they believe it, they take their assumptions to be fact. It is good to think ahead, and project out many various scenarios which describe your data. But you must consider, if you were present before an auditorium of your peers, could you present enough facts, that support your conclusions. For example, would you believe someones testimony of grain size if their data only consisted of XRD data? Given how easy and prevalent SEM is, wouldn't you like to see confirmation? Nature is not what you want it to be, it is what it is. At the very least, the more data you collect, the more you'll know what your material isn't.

You can use hydrothermal method which reduced the temperature and increasing the pressure

Hi Jason,

wow your message got me thinking. Anyhoo, I would like to tell you that I have not gotten these results in one go.... There were about 20 total sets of experiments that were conducted. In almost every set, I have gotten the same results on XRD. So... thats why I am breaking my head to get this done.. Now I am asking around and most of them are suggesting that I go for a Solvothermal or Hydrothermal synthesis.

Well, Tejabhiram, what are you really trying to get done? Keep grain size down, inhibiting/promoting high temp transition, getting AFM/FM - what is the larger goal that is being affected by these attributes? Are you trying to get the high temp phase (lattice change & magnetic change) transition without the recrystallization?

Ok I think I get your question now.

So I would like to have a material that has an intrinsic ferromagnetic or superparamagnetic property with a size range of 10 to 50nm. This material needs to be a Nickel ferrite with gadolinium substituted into the B site of the spinel structure.

Yup I think, I want the high temp phase without having an increase in the particle size.

Tejabhiram,

The fact that the transition takes place at such a high temperature could be a problem. You might be able to suspend these low temperature crystallites in a high temperature matrix (SiO2 for example). Raise the temp, change phases, then dissolve the matrix and remove the phase changed crystallites. I have used a hydrothermal process to recrystallize dysprosia. It only required temperatures of ~150C to do it, and dysprosia has a very high melting temp. All I can say is that you need to play with the parameters that influence kinetics, like temperature, pressure, and time. Spark plasma sintering (which our group prefers to call Current Activated Pressure Assisted Densification) or microwave sintering are two good methods for heating a material quickly, and cooling quickly. At 600C per minute heating rate, it would take only 2 minutes to reach 1200C and less than a minute to descend below 1000C and 5-10 minutes for the die assembly to be cool enough to remove your sample. But, with this process, you will not have indvidual crystallites anymore. Instead you'll have a fully dense, 3D bulk sample WITH a nanoscale dimension microstructure (nanostructure).

Hello Dr.Jason

I have been following this hydrothermal process for the past 2 weeks.. it was like magic.. i am able to get perfect nickel ferrite and cobalt ferrites at 150C as you got with your samples. So I think I'm gonna stick to this technique for now. I have started my drug loading process also. Hopefully will finish off my other characterization techniques as soon as i can so that i can update you as to how my results are.

Teja

Teja,

Excellent. I'm happy that it is working out. I am interested to see your characterizations.

Hello Dr.Jason,

To start of with i jus got my first SEM taken.. The particles are really really fine and the avg size is about 10-15nm... I wasn't able to get any better look at them. Here are the pics attached. I need to take a TEM on them and then figure out how the magnetism properties are. I'm really excited because I've got confirmation from IIT Bombay to come and conduct hyperthermia experimentation on my particles. Lets hope I get some Curie point measurements can also be done on them. Thanks for all the help Dr.Jason. I'll keep you posted.

That looks good. This is exciting. Well done.

Tejabhrim. I get similar results with my particles as well. Below a micron the images begin to look fuzzy. I have always wondered if the magnetic nature of such particles contribute to this?

Thanks for all, above comments. They are very useful for me since I started to learn bout that subject.

Hi.

I just look at your question. I do not now if you still need an answer on it or not. I just wanted to let you know that you can get phases that normally are stable at higher temperatures, also at room temperature, due to the generation of the small size of the material. For example, I can make cubic ZrO2 nanoparticles (3-4 nm), which are stable at room temperatures (while normally the monoclinic phase is stable).

However, I am wondering if you need nanoparticles as such (no agglomeration) or aggregated/agglomerated nanoparticles for your application. Because when I look to your SEM, I am wondering if you can use that material for an application. This is because in my research I am focussing on nanoparticles that are not agglomerated, by using stabilizing ligands. If you need the latter, I would recommend you to look into hot-injection/heatin-up type of synthesis methods. You can also try microwave solvothermal synthesis. The latter method generally gives rise to smaller particles and a better size distribution in comparison to conventional (autoclave) solvothermal synthesis..

Uniform heating is the best way......

Microwave heating is a good way for obtain small particles in short times of heating (30 s - 30 min).

Dear Indira Torres, Please, do you have physical explanation about how Microwave affect 20 nm particles? I think it is good issue to think about.

Dr.Jason,

I did go to IIT Bombay. I got hyperthermia done at 250KHz and 0.5KAmps current. The samples with nickel ferrite and zinc ferrites did not show much heating properties while the cobalt ferrites heated up like crazy. One of the samples with a conc of 5mg/ml heated up to 90C in a matter of under 3 mins. I came back and have started analyzing my samples with other techniques. I jus finished XRD on them and it is kinda weird again now. After the reitveld refinement, the samples (most of them) match with Fe2O3. They dont match with cobalt ferrite or nickel ferrite or Fe3O4 which is the usual case. All of them are magnetic too (get attracted to a magnet). Do you think I should do a Mossbauer Spec to understand what is happening here?

Regards

Teja

Dear Okobiah,

the fuzzy look is very common with any magnetic materials. The main reason for it is that the magnetic nature of the sample either moves away or deflects the incoming electron beam causing this fuzzy look to it.

Teja,

What phase of Fe2O3 are you seeing? alpha, gamma, etc.? I am going to guess gamma fe2o3. It is ferrimagnetic with a saturated moment about 90% of that of fe3o4. however gamma is metastable, but the low temps you are at could preserve that phase. as for mossbauer, id like to advise you, but i do not have personal experience with it. but if i remember right mossbauer will tell you iron valences. gamma has fe3+ only. Fe3O4 has both Fe2+ and Fe3+. but you could make more comparisons with you xrd data. it has to match something. you could use raman but you have to be careful that laser power does not induce phase change. how about magnetometry, fc/zfc? look for evidence of verwey transition.

Dr.Jason,

I did check the xrd today, the nickel ferrites and cobalt ferrites match with haematite. the wonderous and weird part is that all the samples are magnetic. I have given them off for VSM MH and MT studies. got to wait for them to come. But them how is it possible sir that i have haematite formed and the paticles are nice and magnetic. Plus all these gave brilliant heating rates in the hyperthermia equipement

Teja,

When you get the MH back, see what the saturation magnetization is. Hematite is very low (0.4 emu/g). Anything above this, you have something else making the magnetic contribution. The magnetism of hematite is low because of the magnetic moments of Fe pairs oppose one another (anti-parallel). If you have Co, or Ni substitution in the hematite lattice, the balance of the magnetism between Fe/Co or Fe/Ni pairs will be non-zero. But, this is supposition. If you want to nail it down, you'll need tools that give you a closer look. In the MvT, look for Verwey (~122K) or Morin (~265K) transitions.

I was thinking the same Dr.Jason. and I have made some assumptions in the past week. This is going into my report to my supervisor.

From the magnetic properties of hematite studied by various scientists, we were able to make the multiple numbers of predictions regarding the magnetic behavior of the samples based on the below available information. Hematite is not just an antiferromagnet but a canted antiferromagnet. The antiferrromagnetic behavior comes from the cyclic spheroidal arrangement of the magnetic spins inside a lattice forming a long range antiferrromagnetic spin wave inside the sample. This results in opposing magnetic spins across the lattice cancelling out any net magnetic moment inside the sample. But this kind of behavior is seen in hematite cooled below 260 K. Any higher temperature, the energy of the system would be high enough to start affecting the antiferrromagnetic arrangement inside the sample due to the canting of the magnetic spins across the spin wave. Due to the canting, the magnetic spins become unequal resulting in a positive magnetic moment. Hence hematite is considered weakly ferromagnetic at room temperature rather than a perfect antiferromagnet. The second fact that needs to be taken into consideration is that, as the size of antiferrromagnetic particle decreases, the formation of an antiferrromagnetic spin cycloid becomes more and more difficult. As the spin cycloid is destroyed, there is a good chance for the material to transform into a ferromagnet rather than remaining an antiferromagnet. Given the fact that the particles in IH, JH and KH series are all less than 20 nanometers in size, the formation of an antiferrromagnetic spin wave is difficult making the material ferromagnetic in nature.

Please lemme know how does it sound!

In response to your detailed answer I'll just say, "sounds right." One more point to add, below the Morin temperature (265K) the magnetic moment alignment is parallel to the c-axis. Above Morin, the magnetic moments lie in the basal plane, and as you said, slightly canted (weak ferromagnetism). That is where the 0.4 emu/g magnetism comes from. You should see a M drop going from room temperature down below Morin.

Will do that Dr.Jason. thank you. I hope my advisor finds this explanation ok. I will keep you posted with the new results as and when they come in.

Hello Yadavalli

Your question is not clear. If I understand your question, you are preparing nanoparticles and recrystallizing them at high temperature.

If so they surely will be sintered and will grow yielding larger particles. Consider the following additional points

To avoid sintering, there are a number of tricks to follow:

1) cool the recerystallized powder (from high temp) quickly to avoid sintering

2) Keep the nanoparticles apart while recrystallizing, to prevent aggregation, by different techniques.

I hope I answered your question

I have opted a quick method to avoid increase in size, Put your sample in the furnace when the temperature have reached its maximum limit. If suppose they got crystallize in 90 minutes, take them out in 80 minutes. In this way you can avoid increase in size with ferrites. Secondly, the temperature you are using will only results in hematite phase of iron oxide.