Room Temperature Fabrication (RTF)

Ceramic materials have been produced since prehistoric times by high-temperature treatment of various powder mixtures. High temperatures enable the transport of ions so that they migrate and occupy positions with lower energy. During such a temperature- driven process, numerous reactions can take place, known as calcination and/or sintering.

The prepared ceramic exhibits lower porosity due to densification, in comparison to starting green body and pronounced mechanical and other functional properties. Since the high-temperature treatment of ceramics which is highly energy demanding many alternative low-temperature densification methods have been developed recently. One of the new methods, which significantly reduces energy and time consumption, is room temperature densification or more generally room temperature fabrication (RTF), which was first described by Finnish researchers in 2016 (1). They investigated the potential of water-soluble Li2MoO4 ceramic compacts by wetting with water and post-processing at room temperature and additionally at 120oC. The RTF process mimics the formation of the Earth’s crust which involves lithification, where sediments are compacted, cemented, and transformed into sedimentary rock consisting of solid particles interspersed with voids (Fig. 1).

Figure 1: Lithification in nature.

Compaction reduces the pore volume and cementation occurs when minerals crystallize in the pores and bind the sediment. In a typical RTF process, two or more components are mixed with a small amount of water, where at least one component being partially aqueous soluble and acting as a binding phase in densified composites. The other component, usually referred to as the filler phase, provides the desired functional properties of the densified composite. The pasty mixture is pressed into the desired shape, whereby the pressing equipment enables the liquid phase to be removed. The pressing causes (i) dissolution of the binding phase, resulting in a supersaturated solution, (ii) rearrangement and accommodation of the filler phase particles of, (iii) transport of the liquid phase from the exposed highly pressed sites into the pores and (iv) recrystallization of the binding phase from the supersaturated solution. These processes lead to the bonding of the composite particles and densification. The formed green body is further treated at a temperature of up to 120oC to allow evaporation of the residual water and final recrystallization of the binding phase.

Technically interesting, RTF composites contain a high loading of functional ceramic filer phase and a low amount of binding phase; therefore, they are called upside-down composites with 0-3 connectivity.

In the K9 department, we have conducted extensive research in the field of composites with interesting dielectric properties based on SrTiO3 (ST) as the functional phase and Li2MoO4 (LMO) as the binding phase using the RTF method. The improvement of the dielectric properties of the composites was achieved by increasing the relative density. This was accomplished by optimizing the size distribution of the starting particle size distribution and other process parameters such as applied pressure, pressing time, treatment of the starting mixture with ultrasound, etc. [2].

Microstructural investigation of the prepared composites revealed that the binding LMO phase is homogeneously distributed between the larger ST particles and exhibits typical 0-3 connectivity Fig. 2. The contact between the LMO phase and the ST particles is predominantly tight.    

Figure 2: EDS mappings of the cross section of the ST-LMO composite containing 6.5 wt% LMO.

The dielectric properties were additionally improved by impregnating of the prepared composite pellets with titanium (IVI isopropoxide. The applied experimental conditions allowed diffusion of the Ti precursor into the pellets and additional exposure of the pellets to water vapor resulting in hydrolysis of theTi-based compound to amorphous titania-titanium tetrahydroxide (ATTH), Fig. 3.

Figure 3: EDS mapping of 3x-impregnated LMO-ST composite complemented with point chemical analysis.

Further investigations included the introduction of the new compounds such as Na2MoO4, Na2WO4, Na2SiO3 and MgSO4 which serve as the binding phase in the ST-based composites. Some of the new binders significantly improve the dielectric properties. Most importantly, the binder Na2SiO3 (NSiO) increases the relative dielectric constant as well as the biaxial strength of the composites by a factor compared to ST-LMO composites, Fig. 4.

Figure 4: Weibull plot shows the probability ff failure of LMO-ST vs NSiO-ST as a function of failure stress. The solid lines represent the best fit of the strength values according to the maximum likelihood method. The shaded areas show the respective 90% confidence interval of each set.