When a green pellet of an oxide is sintered, there is a weight loss corresponding to loss of binder and moisture. When oxide ceramics, say alumina or zirconia, are re-sintered in dry hydrogen, does congruent or non-congruent sublimation of the oxide take place and reflect in the weight loss? The moisture content in hydrogen is said to be good for the refractories, whereas hydrogen is good for the heating element. such as Molybdenum or Tungsten.
Sublimation of the refractory oxide ceramics is important at high temperature. http://www.dtic.mil/dtic/tr/fulltext/u2/623208.pdf
According to reference https://kundoc.com/pdf-synthesis-vaporization-and-thermodynamics-of-ceramic-powders-based-on-the-y2o3zr.html at the temperature (2773 ± 40) K the ceramic powder containing Y2O3:ZrO2:HfO2 as 0.15:0.60:0.25 mole fractions underwent a congruent sublimation.
there is little known on the vaporaization velocities of alumina and zirconia in hydrogen at ultrahigh temperatures. Some data are contained in the paper by Lukin: http://www.dtic.mil/dtic/tr/fulltext/u2/623208.pdf. The idea behing hydrogen sintering of alumina is to make translucent materials. Pores are trapped at the grain boundaries if magnesia is added (see papers by Coble). hydrogen (or oxygen diffuses out leaving fully dense material) Concerning congruent/noncongruent vaporization it can be said that some reduction takes place at high temperature visible in grey color of the alumina so the alumina will become substochiometric. Magnesia is known to vaporize before alumina. In sintering furnaces running over years you will find magnesia deposits in colder corners.
It seems that the composition of the vapor is different than the composition of the solid.
In case of zirconia reduction is even stronger. Probably it depends also on the type and amount of stabilizer.
Hydrogen at high temperatures diffuses easily in the crystal lattice of oxides, and in some cases, sintering of the ceramics can be accompanied by reducing processes that increase the concentration of defects in the material and, as a result, increase the rate and degree of sintering. [Глазачев В.С., Лукин Е.С., Балашев В.А., Боровкова Л.Б. Механизм образования твердых растворов в системах Y2O3 – HfO2 и Y2O3 – ZrO2 // Изв. АН СССР. Неорган. материалы.- 1977.- Т. 13 , № 10.- С. 1800 - 1804.]
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