Dear Ghani,

Did you analyze the micro structure of the pellets? This problem could be related to the incomplete sintering.

Best Regards,

Fatih.

Dear Ghani,

if you already know that you have a pure phase, try to prepare the pellets by the highest pressure possible and use a very fine powder, the reduced voids will certainly produce a dense pellet.

regards,

Fernando

Dear Ghani,

I think you should use the hot isostaric press for the preperation of your specimen to avoid the problems of porosity and brittleness.

with all my best regards.

Baydaa.

The sintering ability of  (Ce-TZP) depends on starting powder characteristics, which differs depending on the synthesis methods and conditions used. Characteristics include purity, homogeneity, particle shape and size, particle size distribution, presence of agglomerates, etc. Fine powders sinter better, i.e., faster and/or at lower temperatures than coarse ones. However, powders of very fine particles, usually synthesized by wet-chemical routes, do not always show good sinterability, because of the formation of agglomerates. Thus uncontrolled agglomeration or the presence of high-strength agglomerates (aggregates) can be detrimental to the properties of final products.

See refferences:

http://www.nanotecnologia.com.br/redecmdmc/lab/arquivos_publicacoes/642_Synthesis%20and%20sintering%20of%20ZrO2-CeO2%20powder.pdf

http://ul.qucosa.de/fileadmin/data/qucosa/documents/19291/diff_fund_8%282008%297.pdf

Cerium undergoes a change of oxidation state from +4 to +3 even in air, at sintering temperatures. In the +3 state Cerium is not a good stabilizer for the high-temperature tetragonal phase od zirconia. Hence, when you have ceria-zirconia mixture, and you want to sinter them, it is a good practice to use furnaces where you can keep a high partial pressure of oxygen, so that, on cooling, Ce ions can go back to the + 4 state.It also depends on the amount of ceria that you want to incorporate in the zirconia but, typically, the aforementioned change of oxidation state is at the base of the structural instability of ceria-zirconia solid solution.(You can easily check if you have Ce+3 ions: if the core of your sintered pellets are somewhat grey, than you can be sure that some ceria ions are in the reduced state. On the topic you can read my old papers. Also Anil Virkar from Utah university has worked a lot on the subject and, if I remember correctly, even I-Wei Chen).

Thank you everyone for your answers

Dear Ghani,

we have encountered similar problems when using zirconia ceria co-milled powders for the first time.You take them out of the furnace at 200°C and you can actually see them crack and jump away when the martensite start temperature is reached. Valter mentioned an important point, if you sinter too high you end up with a considerable fraction of trivalent ceria which is too big to fit very well into the lattice. As in the case of La2O3/ZrO2 the t-field in ce2O3/zrO2 is very narrow. The best is to sinter under flowing oxygen. As ceria diffuses much slower than yttria you must give your co-milled samples time to take up the stabilizer and you should not exceed 1450°C, if your powder is not sinterable enough it does not work in air. If you sinter as fast as it is recommended for coprecipitated 12Ce-TZP powders you will always end up with cracked specimen.

For the thermodynamic situation read : Journal of the European Ceramic Society 23 (2003) 99–106