With the intention to synthesize ZnO-based varistors I use sintering (under vacuum) of the following oxides (i.e., ZnO, Bi2O3, Sb2O3, MgO and Al2O3) within a temperature range from 1200 to 1400 °C, then, I would like to know if there would be any interaction between them (oxides) and the used crucible which is made of graphite ?
thanks in advance
Dear Dr. Mezrag Samiha
Peace be up you السلام عليكم ورحمة الله
I don't think that there is no chemical interactions between the mixture of the mentioned oxides and the graphite of the used crucible below the mentioned temperatures, where the melting point of graphite is ranged between 1400-1500°C.
You should be careful to use the graphite crucible at the mentioned temperatures. Therefore you must change the crucible type or make the temperature not exceed 800 with adjust the applied vacuum pressure.
Moreover and to make sure, it should examine the obtained oxide samples using X-ray diffraction. XRD will show if there is any traces of graphite crucible.
Best regards
Yours,
Ahmed Saeed Hassanien
Dear Dr. Samiha:
The temperature of sintering of the ZnO based varistor, 1200 oC to 1400 oC, is very high. In this case, the vacuum type atmosphere is functional only to protect the graphite crucible. In this sense, would be possible reaction between Carbon graphite and metal oxides of green compact of ZnO based varistor.
In the interface, sample/crucible can occur reactions that can destroy crucible or crucible and furnace refractory from phenomenon called "reduction by carbon", which seems be being operational at several oxides presents in your composition. This reduction can understanding as carbon removal.
The reaction has as product CO2 (g), see material from crucible is consumed. Another product of this reaction is the metal stemming of metal-oxide. Depending on temperature, metal can reach the liquid state. The nature of this liquid in a complex system as this is non-trivial for analysis but a priori would be reactive at sufficient degree to affect the furnace refractory.
ZnO react with carbon to gives 1 mol of Zn
Bi2O3 react with C forming 2 moles de Bi
2 Al2O3 react with carbon giving 4 moles de Al
The melting temperature of some metals can be finding in the links:
http://www.engineeringtoolbox.com/melting-temperature-metals-d_860.html
http://www.onlinemetals.com/meltpt.cfm
Best regards
Marcos Nobre
Bismuth and arsenic trioxide can be expected to melt and act as flux for the refractory oxides (ZnO, MgO, Al2O3). They can be then expected to disproportionate between the trioxide and both the elemental (Bi, As) and the pentoxide (Bi2O5, As2O5) oxidation states, where the generated pentoxides should lose oxygen at high temperature, regenerating the trioxide, possibly having tetroxide as intermediate. The elemental form can diffuse to some extent beneath the inner graphite crucible surface, to form bismuth and arsenic carbides (BiC, AsC).
Bonjour Samiha,
J'ai pas bien saisi l'objectif exact de ton sujet, es ce que tu cherches à fabriquer une céramique pour l'électronique avec un effet varistance prononcé ou juste
un effet thermique du matériau, Pour te répondre sur un plan physico-chimique j'attends de ta part quelques explications supplémentaire
Dr. Chabane Talhi
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