401. Back to basics: synthesis of metal oxides

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Clement Nicollet, et al, J. Electroceram. (2023)
https://doi.org/10.1007/s10832-023-00340-y

(1) The preparation of oxide samples for further characterization of their properties can follow a two-step protocol: the synthesis of the material in powder form, and the processing of this powder into the desired shape (dense body, porous layers, etc.).
(2) Entropy variations during a chemical reaction boil down to two main effects: changes of the number of phases/constituents, and changes of state (solid, liquid, and gas).
(3) For synthesizing a complex oxide, the first task is to assess whether or not the thermodynamics are favorable, i.e. that the Gibbs free energy of reaction is negative.
(4) Thermal energy serves mainly one purpose: activating mass transport through diffusion, so that elements from the precursors can meet and re-organize into a new crystalline structure: the targeted oxide material.
(5) The better the initial mixing of element, the lower the energy needed for reacting them together and form the targeted oxide.
(6) As the main mechanism to form a new phase is mass transport through diffusion, the formation of the complex oxide, free of leftover reactants, is closely linked to the length of the diffusion pathways.
(7) Citric acid has three carboxylic functions: two are used in the polymerization reaction, and the third one serves as chelating group to maintain the cation distribution.
(8) The principle of the combustion route is to trigger a highly exothermic reaction from a solution of precursors, which will bring enough energy to directly crystallize the desired complex oxide.
(9) A metal cation in an aqueous solution is stable in acidic pH, but will form a solid hydroxide precipitate when the pH is increased above a defined value, given by their thermochemical properties. The precipitation pH is easily accessible by looking at so-called Pourbaix diagrams (E-pH diagrams)
(10) For preparing complex oxides by precipitation, the precipitation pH values for all constituent cation have to be in a relatively narrow window, to avoid segregation of each cation, thus losing the benefit of the wet chemistry approach.
(11) All rare earths will form hydroxides at similar pH.
(12) All synthesis routes require thermal treatments, whether it is for calcination of synthesis leftover organics, or for annealing the produced powder and form the desired complex oxide in its pure, crystallized form.
(13) For ceramic precursors there are two main sources of errors that can come from weighing precursors: hydration level of the precursor, and non-stoichiometry of oxide precursors.
(14) The reaction of sesquioxides with water is relatively slow (one day to several days), which means that the oxide can be directly used as precursor if the hydroxide is not formed yet.
(15) Most current metal salts are nitrates, chlorides, or acetates. The main difficulty when using metal salts is that they bond strongly with water to form stable hydrates.
(16) With manual grinding with a mortar and pestle, homogeneous mixing mainly depends on two factors: time and force applied.
(17) If the precursors are stable in common solvents like ethanol, adding solvent in the mortar will facilitate mixing and yields more homogeneous reaction media.
(18) Incorporated PMMA (polymethylmethacrylate) microspheres during the process as a template for the formation of nanoporous structures.
(19) Opposite to Pechini method, in which almost all the fabrication route occurs in one pot, precipitation routes encompass a series of steps (precipitation, filtering, drying…).
(20) There are four main concerns to be aware of when performing heat treatments in a high temperature furnace: potential pollutions, furnace calibration, heating and cooling ramps, and atmosphere control.
(21) Although the temperature and duration of the heat treatment are the most relevant parameters for controlling the formation of an oxide phase, heating and cooling rates can also have an impact of the final product.
(22) Fast cooling rates can induce thermal stress to the materials, while slow cooling rates can re-stabilize intermediates compounds.