I sintered my sample(CaTiO3) at 1350°c (air sintering). I know that in air sintering oxygen vacancies is created. I want to know that what is the range of temperature when oxygen vacancies is create and how temperature affect thae oxygen vacancies? how can protect the oxygen vacancies?
Hello,
The first question is: How do you know that oxygen vacancies are created ? No so easy to detect.
The second point concerns charge compensation. If oxygen vacancies appear, they are here to compensate another defect. In your case, essentially two possibilities: Ca vacancies. Alkaline earth oxides are known to be volatile at high temperature. Especially true for Ba, a bit less for Sr and Ca but still it has to be considered. To avoid this you can make an atmosphere around your sample by putting your pellet in a CaTiO3 + (10% weight) CaCO3 powder mixture. this will help avoiding Ca volatilization.
The other potential defect is Ti3+. In this case a very small concentation is usually easy to detect since the sample becomes darker (bluish). This could be avoided by allowing a longer cooling at least in the high temperature range (1350->1000°C at 100°C/h would be better).
Hoping it helps.
Guilhem
Are you sure that they are oxygen vacancies and not cationic?
I am with similar doubts myself, but I have the feeling that if you give more time for the sample in the oven or let it cool in the oven it will slowly tend to the lowest energy state that is a little bit lower than the energy state of the lacunar phase.
It is just a feeling, based on the general idea that, although oxygen tends to be loosely bound in some structures, the minimum energy is still that of the regular crystaline phase.
thanks..for reply ..
I am not sure but in sintering process possibilities of creation of oxygen vacancies are more in presence of air.
and i usually give 4°/min temperature rate for the sample(both case heating and cooling)
Check if the oxygen vacacies are created due to the reduction of Ti4+ -> Ti3+, as is the case in BaTiO3. Pay careful attention to the stoichiometric ratio of CaCO3 and TiO2 may help.
In this crystal you can expect only a very low concentration of vacancies, due to vacancy equilibrium /Schotky/. That means that if you really have observed vavancy formation,for example by using of UV, ESR etc spectroscopy, some ions in your crystal have been reduced! Ca+ is very low probable,but, Ti3+ is is a good example for reduction in ceramic science during heating!
thanks for reply..
we know that if we fire the sample in presence of air , leads to oxide formation.so sintering is done at vacuum. but vacuum sintering , oxygen vacancies is created ,leaving two electrons that is n -type semiconductor property ie. dielectric quality decrease.
but i try to ask that is it any specific value of temperature ,which is more dominant for o2 vacancies creation in pervoskites .(in ceramic route method )
and i am preparing pure catio3 so we can't doping .
Hello,
The first question is: How do you know that oxygen vacancies are created ? No so easy to detect.
The second point concerns charge compensation. If oxygen vacancies appear, they are here to compensate another defect. In your case, essentially two possibilities: Ca vacancies. Alkaline earth oxides are known to be volatile at high temperature. Especially true for Ba, a bit less for Sr and Ca but still it has to be considered. To avoid this you can make an atmosphere around your sample by putting your pellet in a CaTiO3 + (10% weight) CaCO3 powder mixture. this will help avoiding Ca volatilization.
The other potential defect is Ti3+. In this case a very small concentation is usually easy to detect since the sample becomes darker (bluish). This could be avoided by allowing a longer cooling at least in the high temperature range (1350->1000°C at 100°C/h would be better).
Hoping it helps.
Guilhem
Just try to increase the partial pressure, like oxygen partial pressure
sorry ,actually i don't know ,how i apply oxygen partial pressure in furnace.
Hello,
If you want to increase oxygen partial pressure, you need to have a furnace with possibility of controlled atmosphere (alumina tube with closed ends and gas entry/exit).
If you have this, you can of course increase the pO2 by using pure oxygen or either increasing a bit more by using it Under pressure 1.5-2 bars.
Once more, i would be curious to know how you can be sure to have oxygen vacancies. If the content was more than 1%, it would correspond probably to oxygen vacancies created by Ca volatilization. In this case, the better way to avoid oxygen vacancies is to control Ca evaporation. If it comes form Ti4+->Ti3+, you can control it by increasing pO2 or by annealing at low temperature (800°C in air) for a long time.
Best regards
Guilhem
thanks a lot ..Mr. Dezanneau..
sir ,how can i detect oxygen vacancies which is created in my sample is due to ''Ca or Ti''? IF ANY METHOD to detect this please tell me sir.
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