Hydro-solvothermal sintering (HSS) is an Innovative solvent-assisted, uniaxially loaded, low-temperature powder densification process. It has enabled the development of functional materials and multi-materials with a high density of about 90%. These mechano-chemical conditions are similar to those present in the earth's crust, which have allowed geologists to explain the densification phenomena observed by stress dissolution/precipitation processes. While it is known that these processes also occur in hydrothermal sintering, a deeper understanding is needed to optimize this process for each material by also considering the choice of the solvent for a high saturation concentration of the solute. Beyond the material/solvent couple, the initial powder/solvent mass percentage is a key criterion to evaluate the thermodynamic conditions of the pore space (monophasic vapor phase or biphasic liquid/vapor conditions) in order to analyse their impacts on solvation especially at inter-particle contacts. Here, we will address the complexity of the mechano-physico-chemical effects both through modelling (analytical and numerical) and through experimentation based on a new device allowing in-situ measurement of shrinkage, during HSS process.
First, we will review the thermodynamics of interfaces, to show how a pressure jump at the material/solvent interface contributes to shift the saturation of the solute in the solvent. Then, on the basis of this mechano-chemical equilibrium at the interface, a simulation of the solute transport in the solvent and its transfer to the interfaces will be presented to illustrate the kinetic process of the dissolution/transport/precipitation sequence.
Then, we will show using numerical stress calculations that the two-particle shrinkage rate model dedicated to pressure-less solid state sintering can be reconsidered under an applied pressure condition and with the presence of solvent in the pore phase, assuming a sufficiently easy sliding of the particles on each other for 3D compact applications. Within this framework, the conventional anisothermal and non-conventional stepwise isothermal methodologies can be used for the investigation of the sintering mechanism and energetics of the first stage of sintering (after grain rearrangement step).
Two material/solvent couples will be kinetically analysed, (ZnO/acetic acid solution) and (SiO2/NaOH solution) under applied pressures of 320 and 350 MPa, respectively using both methodologies. For each system, we will compare the results extracted from the two methodologies, that is the values of the power-law coefficient of the kinetic equation and of the activation energy characterizing the rate controlling mechanism. On that point, the interest of the stepwise isothermal methodology in obtaining accurate activation energy will be highlighted. Finally, from that kinetic results and the support of Scanning Transmission Electron Microscope images, the most likely mechanism (dissolution reaction or mass transport) and its location will be discussed for ZnO and SiO2 HSS.