For Ferroelectric hysteresis loop measurement the sample has to be well sintered and dense pellet. In my case even after sintering the sample is breaking while taking the measurement. So my query is how to choose the temperature and time and also how to check for the well sintered condition to avoid this situation? Can any one please suggest any binder for powder samples which will help ?
The classic system is the DTA (Differential Thermo Analysis). This system measures the shrinkage differential to a reference sample with low thermal expansion during the sintering process. If this is not available you do several sintering trials and measure the specific density (Archimedes principle). Your optimum is the lowest temperature which reaches an optimal density. If a high precision scale (0,0004g) is also not available you can finally look for the rest porosity by metallography. But this is an subjective methode which requires enough experience. Kind regards Christian
You should start with a well compacted sample.
For any form of sintering at least 50% density is required, but rather 60+% is better.
The higher you start with the initial density, the less shrinkage (lower temperature/time) is required. You also want to use a sub-mircon particle size. However, using too small of a particle size will often lead to lower green density; due to the more difficult processing/compaction.
Another trick is to use two or three different particle sizes in order to fill the packing voids between the larges particles with successively smaller particles. This can either be done by using a powder mixture or in some cases you could even infiltrate the green compact a second time with smaller particles.
Goal of this process is the formation of a small volume of porosity with as small as possible pore size.
Finding the best sintering temperature is usually achieved by performing a dilatometry experiment. This will show you the densification behavior. You want to pick a temperature when densification "starts" and then use a "hold-time" at that temperature to complete the densification.
In case densification can not be completed in this fashion or in case the temperature is too high with respect to the phase stability of the material, pressure sintering can be used. Either use a uni-axial pressing or gas pressure sintering (in case you can achieve closed porosity through an initial densification/sintering process.
Your comment/question about the binder is also important. try using as little as possible binder and make sure you don't heat up the sample too fast. The binder will decompose at some point and the resulting gases need to escape from your sample. If done too fast the gas (pressure) can lead to cracks.
It is important that you mention that sintering is performed on a ferroelectric materials. If I presume that it is a complex mixture of oxides …. If the sinter is mechanically fragile, you can detect the sintering temperature needed for the process by recording dilatogram – dimensional change of the compact during heating schedule. That will inform you of the maximal shrinkage rate during heating, (first derivative of the shrinkage obtained by dilatometer device) and the temperature minimal required for the sintering is approximately somewhat higher than maximal shrinkage rate. You also have to determine the isothermal period of holding on the desired temperature. The lowering of the shrinkage rate is subscribed to formation of the new grains in the material and their impingement and further constant dimensions and shrinkage rate is sign of finished sintering process. Then reheating on the sinter obtained during this recording. Reheating will tell you if there are some phase transitions and chemical reactions that are specific for the material but covered with sintering process and distinguish them as atomic events from micro structural sintering changes.
Although, since you are looking for ferroelectric material you have to be aware that a lot of processes that are immanent for the system can be complex due to oxidation of the cations and chemical reactions for complex ferrite formation. Here you must combine DTA/TGA due to oxidation process of most oxidaizable cations since that kind of oxidation can lead to deterioration of the specimen as a ferrite and became a group of mixed oxide phases with low mechanical strength.
For the binder it is most convenient to use poly vinyl alcohol PEG 2% powder mass but it can be sensitive to humidity and to make problems with liquefying and evaporation at lower temperatures than sintering one, leaving large porosity. It can be also palstisized with adding of 0.5 % poly etylen glycol PEG but compaction than can be hindered…
I have added the reference we were working on...
Dilatometry technique can be used to find the temperature and time for the calcination and sintering process.
Please check the following procedure, which is common for almost all materials.
Processing of pellets/Targets
Step-I PVA preparation (10 wt%)
1. 50 ml distilled water + 5 gms of PVA powder
2. Heated and stirred by spatula/spoon (completely dissolved)
Step-II Pellets/Targets preparation
1. Check the dimensions of your pellet.
2. By using green density (~60% of theoretical density) of the material, calculate the approximate mass (m grams) for one pellet.
3. If 10 gms of calcined powders + 1 ml PVA solution (equal to 0.1 gms PVA powder, 1 wt% to 10 gms powder)
4. Powder and PVA should be properly.
5. Use the proper die set to make the Pellet.
6. Sintered this at optimized or reported sintering temperature
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