Well, the sintering temperature is usually 1/2 to 1/4 of the melting point of the OXIDE. If you are trying to obtain lower temperature of sintering of a ceramic specimen, the oxide metal may or may not work as a liquid phase sintering. It is kind of easy to see the liquid phase by SEM, on grain boundaries on the ceramic matrix.

The cause of liquid phase is usually observed on the eutetic point of an A-B phase diagram. Did you find the phase diagram of your system?

Hi Preethi

1) Which is the chemical composition of the particles you are sintering, to which you add Iron, Cobalt, Strontium, lithium as sintering aid? Are sintering a metal powder, a ceramic powder or a glass powder?

2) If you sinter in oxidizing atmosphere (air, O2) at 1500C: Then the aid elements will be as oxides.

If you use reducing atmosphere (H2, Ar-H2): It is possible you dont have oxides or you have a lower oxidized state , depending of what you are sintering.

3)As Lucas says, these aids may form with your sintering system  low temperature eutetics, which appears in the respective phase diagrams.

Besides low temperature eutectics, you might observe glassy (oxide) phases at the grain boundaries that promote liquid phase sintering.  Especially in the case of sintering of ceramics.

Hi ,

 The samples are ceramic and the samples contain more than three elements. Its hard to find the eutectic point by phase diagram. Is there any other means of enhancing density other than forming low temperature eutectic ?

there are more additives to lower the sintering temperature. for example, to develop a composite SiC -Mullite , chromium oxide is used to not alter SiC fibers (sintering at 800 ° C)

Not sure this will help or not, but basically in the liquid sintering, the added elements will turn into liquid and form a film surrounding the solid phase SrO (at 1500C, all the elements already melted). Since the liquid is there to provide a sort of "transport" for SrO, thus wetting is the 1st requirement in the liquid sintering process. The second requirement is, the solid SrO solubility in the liquid must be high to promote diffusive transport, thus ensure rapid sintering. Does the explanation helpful?

Other factors appart from additives:

1) Particle size, if you have smaller particles you have more surface, and surface diffusion is faster than other diffusion mechanisms (See Physical Ceramics, Kingery)

2) Which is your green density? A good particle size distribution and higher packing pressure, will give you a higher green density. A typical green density value can be 0.65.

In my opinion the best method to introduce sintering additives is a ball milling procedure, as due to common wear processes the materials from your balls and inner surfaces of your mill are distributing very homogeneously in your composition before sintering procedure. Be sure pottery constituent matters are quite good to intensify the densification of all oxide materials during sintering. It is easy and cheap method!

I totally agree with Dr. Shabalin, but you must be carefull with the milling process. Depeding the kind of miller that you will use, you can have some contamination, specially for high energy milling. I am really interested to know the thermal behavior of your powder system by ATG/DTG and DSC.

Dr. Prado showed important features that contribute on the densification of ceramic materials.

I would also like to feature what I usually do in situations like that:

• Know the raw materials (XRD, XFR analysis - search for impurities)
• Know the thermal behaviour of the raw materials (ATG/DTG, DSC analysis)
• Determine the particle size distribution by laser dispertion
• Mill the raw materials to have a modal particle size distribution (laser dispertion analysis again)
• Know the thermal behaviour of the materials already milled and mixed (analysis cited above)
• With a good modal particle distribution (2-5 microns), determine the ideal pressure for obtain a good green body and an ideal green density
• Knowing the thermal behaviour of your system, determine the ideal temperature for sintering
• Burn, burn, burn and burn again samples with different temperatures of sintering, different heat rates, etc etc. You have to see what is right and what is wrong
• Measure the densification of sintered bodies
• Observe by SEM the liquid phase
• In my case, I always quantify the liquid phase and other phases present using XRD analysis with Rietveld refinement

Best of luck!

Hi!

Following paper is very useful for liquid sintering, you can read

German, R. M., Suri, P. & Park, S. J. 2009. Review: liquid phase sintering. Journal of Materials Science 44(1): 1-39.

for solid state sintering, keep the sintering temperature value to 2/3 of the melting point. ( if using different elements, or making an alloy or composite, then keep the 2/3rd of the lowest element).

the sintering temperature valueis determined exactly with dilatometric curves

I would like to agree with Prof H Osmani concerning the fact that shrinkage analysis is extremely important for the correct determination of sintering temperatures.