1.hello.Im working on the Aluminum and graphene composite. First i will tell you about process which i have used. I ball milled 100 grams of aluminum with 0.5,0.75,1.0 weight percentages of graphene nanopowder in presence of ethanol (8.6 ml for each run) for three different milling times of 03,05 and 07 hours. In this way i got 09 different samples in addition to pure aluminum milled for three different milling times. so total samples are 12. It may be noted that milling jars were first vacuumed and argon gas(upto 01 bar pressure) was purged in milling jars. Milling speed was 350 rpm and it was a plantery ball mill. (for images and attachments please see ppt file)
2. Sir second step included cold compaction of milled powder which i carried out by using a unidirectional hydraulic press and compaction pressure was 500 MPa. After compaction i got rectangular samples of 75mm length and 10x10 (width and height).
3. 3rd step included the sintering in argon environment where im facing problem. Max sintering temperature and holding time was 500 centigrade and 04 hours. Heating rate was 15 degree centigrade/min and in 33 minutes 500 temperature was achieved which was maintained for 04 hours followed by furnace cooling which tends to 7-8 degree initially but as temperature drops below 400 it is 1-3 degree/minute. samples from sintering were very brittle and powder was visible showing no diffusion and sintering. So i again sintered the samples and also placed couple of new samples in furnace and followed the same sintering cycle with max temperature of 600 degrees this time. however results were the same and sampleswere brittle. After discussing the problem i was advised to get an Xrd to see the composition of it whether it is aluminum or its oxide, alumina.So I got the Xrd of pure aluminum un-milled , un-sintered powder and 0.5 weight % graphene +aluminum powder milled for 03 hours and sintered at 600 degrees for 04 hours. Xrd peaks showed that it was aluminum and not alumina. Xrd results are attached. 93 no file for pure aluminum and 94 is graphene+aluminum.
4. sir couple of days back i performed third sintering of samples with a different sintering cycle which is attached as attachment no 3
the idea was to initiate nucleation sites at low temperature as in previous cycles max temp was being achieved in 33 and 39 minutes respectively for 500 and 600 sintering temperatures. We cannot change the heating rate of this furnace so introduced step heating and holding at 150,250,400 and 600 degrees temperature. the results of this cycle were a little better and couple of samples with 07 hours milled time showed a comparatively better sintering but not fully. pics are attached for reference(power point ppt)
5. again i performed the sintering at 630 degree centigrade two hours holding time, ramp temp 15 degree/min. before this sintering cycle i polished the surfaces of samples as if oxide layer is formed and it acts as insulator and not permitting the heat transfer to the core but post sintering results were the same sir for now im stuck with sintering process
6. i performed the 4th sintering cycle using a tube furnace with ramp temp of 5 degree per min, max temp 600 degree and holding for 04 hours in argon environment but results were again the same.
7. Sir i showed file no 94 (Xrd result file) to one of friend at NEDUET. He told me that one of the peaks which is at 38.38 is reported as alumina peak in some papers also.
8. in addition milling operation tends to break the oxygen layer around the grains of powder (see the article graphene-sluminum nanocomposites 2011 - composite preparation) and even if it is oxidized then compaction will break the oxygen layer around the grains eventually..reference is article which is attached. (relevant part is highlighted)
Dear Farhan,
first I would try to find out if this is an effect of the graphene, make a reference experiment with plain aluminium and check if it does sinter under the conditions described above. If aluminium gets dense you can rule out the oxide story and the sluggish sintering of the composites must by in some way related to your graphene dispersion.
If it doesn't densify you have a problem with the oxide flm (or with both).
Carbon is not wetted by aluminium and due to the high surface of the graphene it is quite likely that you cover the grain boundaries of your aluminium with graphene and eliminate any diffusion and grain boundary sliding.
in ceramic/ graphene or CNT composite we observe strong sintering inhibition due to the nanocarbon fillers and there is now way to densify without pressure (hot pressing, SPS, HIP).
best regards
Frank
Dear farhan, what s the shape of Al powder? Flake shape is very difficult to sinter as compared to spherical Al. You also might want to reduce the amount of graphene because at high volume, agglomeration is very difficult to control.
I also think the amount of graphene is very high, reduce it by 1 order of magnitude and see what happens. But, first ensure that pure aluminum sample is getting densified or not. If it is completely densified then, reduce the amount of graphene. Otherwise, oxidation could be an issue.
First, How did you chose this sintering profile, is it based on previous work. IF not, you need to perform a sintering profile first which will save your time and give you a good idea about the whole picture. Second, I would suggest density measurements as a way to trace sintering. With density measurements you can get numbers that you can use to quantitatively evaluate your samples. For me it looks like the aluminum oxide is what prevents densification. In this regard, I would recommend you to try Al-Mg powder as Mg is known to form phases that improve sintering through LPS.
#Abdulwahab Ibrahim thankz... i have selected sintering profile on the basis of previous work but later on modified sintering cycle also...plz guide me how to make a sintering profile. I will also measure density but sintered parts are showing loose powder so density is more or less near to green compact. Today i performed sintering with addition of 3 wt % and 5 wt % of magnesium and but again same problem..sintering with magensium addition sintered samples pictures are also attached for an idea.
Dear Farhan, You might need to perform some sintering experiments on pure aluminum to exclude the effect of graphene and also to understand the system before proceeding with the composite system. You should be aiming for the liquid phase sintering to take place in which you need to add low melting point elements, and that is the case you should deal with elemental powders so you can adjust your composition accordingly. Another factor you need to consider is the sintering atmosphere. Other note is to investigate the pressing sage (green body sate) is there enough contact between the particles, and what density were you able to get. In my opinion you need to get some steps back and then proceed with the composite. Good Luck
Can you please clarify the followings:
1. What is the material of the milling jar and milling balls?
2. What I understand of B-94_1.RTF xrd results, the Al&graphene powders were milled, compacted and sintered. Then after sintering, the compact was sent for xrd analysis. Correct me if I am wrong here.
3. What is the purity of argon gas used for sintering process? How do you control the rate of gas flow in and out? Did you purge the furnace before sintering process? Do you check for gas leaking?
Ensure that any residual dispersant solvent or surfactants, i.e., ethanol , SA, are fully evaporated before you raise the temperature to certain limits for re-crystallization during the sintering process. Otherwise you'll see pores, cracks etc.
Dear Farhan,
I am also working on the graphene aluminum nanocomposite synthesis via conventional sintering. Recently I have developed sintered aluminum nanocomposite with different graphene fractions using nitrogen atmosphere. Fortunately, I have got sintered samples with good densification. So seeing your huge efforts, your compaction pressure is fine along with sintering temperatures. Firstly, ensure graphene uniform dispersion must be obtained. Secondly, I recommend using nitrogen atmosphere with fully dried powder.
Good Luck
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